Prof. Dr. Dierk Raabe

Microstructure Physics and Alloy Design
Max-Planck-Institut für Eisenforschung

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  • Comparative study of hydrogen embrittlement resistance between additively and conventionally manufactured 304L austenitic stainless steels
    Lee, D.-H. and Sun, B. and Lee, S. and Ponge, D. and Jägle, E.A. and Raabe, D.
    Materials Science and Engineering A 803 (2021)
    Hydrogen embrittlement in 304L austenitic stainless steel fabricated by laser powder-bed-fusion (LPBF) was investigated and compared to conventionally produced 304L samples with two different processing histories; casting plus annealing (CA) and CA plus thermomechanical treatment (CA-TMT). Interestingly, no significant difference in the amount of deformation-induced α′ martensite between the LPBF and CA-TMT samples was observed, suggesting that the solidification substructure in the LPBF sample enhanced the strength without promoting the harmful hydrogen embrittlement effect. These results are discussed in terms of the chemical inhomogeneity, hydrogen-assisted cracking behavior, and hydrogen diffusion and trapping in the present 304L samples. © 2020
    view abstract10.1016/j.msea.2020.140499
  • Intercritical annealing to achieve a positive strain-rate sensitivity of mechanical properties and suppression of macroscopic plastic instabilities in multi-phase medium-Mn steels
    Benzing, J.T. and Luecke, W.E. and Mates, S.P. and Ponge, D. and Raabe, D. and Wittig, J.E.
    Materials Science and Engineering A 803 (2021)
    This study investigates the high strain-rate tensile properties of a cold-rolled medium-Mn steel (Fe–12Mn–3Al-0.05C % in mass fraction) designed to have a multi-phase microstructure and positive strain-rate sensitivity. At the intercritical annealing temperature of 585 °C, increasing the annealing time from 0.5 h to 8 h increased the phase volume fraction of ultrafine-grained (UFG) austenite from 2% to 35% by reversion. The remainder of the microstructure was composed of UFG ferrite and recovered α′-martensite (the latter resembles the cold-rolled state). Servo hydraulic tension testing and Kolsky-bar tension testing were used to measure the tensile properties from quasi-static strain rates to dynamic strain rates (ε˙ = 10-4 s-1 to ε˙ = 103 s-1). The strain-rate sensitivities of the yield strength (YS) and ultimate tensile strength (UTS) were positive for both annealing times. Tensile properties and all non-contact imaging modalities (infrared imaging and digital image correlation) indicated an advantageous suppression of Lüders bands and Portevin Le Chatelier (PLC) bands (a critical challenge in multi-phase medium-Mn steel design) due to the unique combination of microstructural constituents and overall composition. Fracture surfaces of specimens annealed for 0.5 h showed some instances of localized cleavage fracture (approximately 30 μm wide areas and lath-like ridges). Specimens annealed for 8 h maintained a greater product of strength and elongation by at least 2.5 GPa % (on average for each strain rate). The relevant processing-structure-property relationships are discussed in the context of recommendations for design strategies concerning multi-phase steels such that homogeneous deformation behavior and positive strain-rate sensitivities can be achieved. © 2020
    view abstract10.1016/j.msea.2020.140469
  • Machine-learning-enhanced time-of-flight mass spectrometry analysis
    Wei, Y. and Varanasi, R.S. and Schwarz, T. and Gomell, L. and Zhao, H. and Larson, D.J. and Sun, B. and Liu, G. and Chen, H. and Raabe, D. and Gault, B.
    Patterns 2 (2021)
    Mass spectrometry is a widespread approach used to work out what the constituents of a material are. Atoms and molecules are removed from the material and collected, and subsequently, a critical step is to infer their correct identities based on patterns formed in their mass-to-charge ratios and relative isotopic abundances. However, this identification step still mainly relies on individual users' expertise, making its standardization challenging, and hindering efficient data processing. Here, we introduce an approach that leverages modern machine learning technique to identify peak patterns in time-of-flight mass spectra within microseconds, outperforming human users without loss of accuracy. Our approach is cross-validated on mass spectra generated from different time-of-flight mass spectrometry (ToF-MS) techniques, offering the ToF-MS community an open-source, intelligent mass spectra analysis. Time-of-flight mass spectrometry (ToF-MS) is a mainstream analytical technique widely used in biology, chemistry, and materials science. ToF-MS provides quantitative compositional analysis with high sensitivity across a wide dynamic range of mass-to-charge ratios. A critical step in ToF-MS is to infer the identity of the detected ions. Here, we introduce a machine-learning-enhanced algorithm to provide a user-independent approach to performing this identification using patterns from the natural isotopic abundances of individual atomic and molecular ions, without human labeling or prior knowledge of composition. Results from several materials and techniques are compared with those obtained by field experts. Our open-source, easy-to-implement, reliable analytic method accelerates this identification process. A wide range of ToF-MS-based applications can benefit from our approach, e.g., hunting for patterns of biomarkers or for contamination on solid surfaces in high-throughput data. A machine-learning application for the accelerated data processing and interpretation of time-of-flight mass spectrometry is presented. The machine learns patterns in a human-label-free manner, making the process easy to implement and the result highly reproducible. © 2020 The Authors
    view abstract10.1016/j.patter.2020.100192
  • Magnetoelectric Tuning of Pinning-Type Permanent Magnets through Atomic-Scale Engineering of Grain Boundaries
    Ye, X. and Yan, F. and Schäfer, L. and Wang, D. and Geßwein, H. and Wang, W. and Chellali, M.R. and Stephenson, L.T. and Skokov, K. and Gutfleisch, O. and Raabe, D. and Hahn, H. and Gault, B. and Kruk, R.
    Advanced Materials 33 (2021)
    Pinning-type magnets with high coercivity at high temperatures are at the core of thriving clean-energy technologies. Among these, Sm2Co17-based magnets are excellent candidates owing to their high-temperature stability. However, despite intensive efforts to optimize the intragranular microstructure, the coercivity currently only reaches 20–30% of the theoretical limits. Here, the roles of the grain-interior nanostructure and the grain boundaries in controlling coercivity are disentangled by an emerging magnetoelectric approach. Through hydrogen charging/discharging by applying voltages of only ≈1 V, the coercivity is reversibly tuned by an unprecedented value of ≈1.3 T. In situ magneto-structural characterization and atomic-scale tracking of hydrogen atoms reveal that the segregation of hydrogen atoms at the grain boundaries, rather than the change of the crystal structure, dominates the reversible and substantial change of coercivity. Hydrogen reduces the local magnetocrystalline anisotropy and facilitates the magnetization reversal starting from the grain boundaries. This study opens a way to achieve the giant magnetoelectric effect in permanent magnets by engineering grain boundaries with hydrogen atoms. Furthermore, it reveals the so far neglected critical role of grain boundaries in the conventional magnetization-switching paradigm of pinning-type magnets, suggesting a critical reconsideration of engineering strategies to overcome the coercivity limits. © 2020 The Authors. Advanced Materials published by Wiley-VCH GmbH
    view abstract10.1002/adma.202006853
  • Nucleation and growth of α phase in a metastable β-Titanium Ti-5Al-5Mo-5V-3Cr alloy: Influence from the nano-scale, ordered-orthorhombic O″ phase and α compositional evolution
    Antonov, S. and Shi, R. and Li, D. and Kloenne, Z. and Zheng, Y. and Fraser, H.L. and Raabe, D. and Gault, B.
    Scripta Materialia 194 (2021)
    We investigated the nucleation and growth of α precipitates in the presence of nano-scale, titanium-rich, ordered-orthorhombic O″ precipitates formed during heating at 5 °C/min to 400 °C in a metastable β-Ti alloy, Ti-5Al-5Mo-5V-3Cr. The Oʺ precipitates are found to assist α nucleation by serving as preferential nucleation sites. In the very initial stages of its precipitation, the α phase is found to have formed via a coupled diffusional-displacive mode as titanium-rich plates. At this stage, aluminum partitions equally between the β matrix and the growing α precipitates, but slowly diffuses up-hill into the α phase during isothermal aging at 400 °C for 15 h. The interplay between such pre-formed metastable phases and α can be exploited to tailor the microstructure, by refining the α distribution, and consequently improve the mechanical properties of β-Ti alloys. Our work paves the way for exploiting this cascade of metastable phases for further microstructural engineering of β-Ti alloys. © 2020
    view abstract10.1016/j.scriptamat.2020.113672
  • Orientation-dependent plastic deformation mechanisms and competition with stress-induced phase transformation in microscale NiTi
    Choi, W.S. and Pang, E.L. and Ko, W.-S. and Jun, H. and Bong, H.J. and Kirchlechner, C. and Raabe, D. and Choi, P.-P.
    Acta Materialia 208 (2021)
    Understanding the orientation-dependent deformation behavior of NiTi shape-memory alloys at small length scales is of importance for designing nano- and micro-electromechanical systems. However, a complete understanding of the orientation- and size-dependent competition between the various modes of slip, deformation twinning, and martensitic transformation in NiTi shape-memory alloys is still lacking, especially in micron-scale specimens. In the present study, we perform micro-compression tests on [001]- and [112]-oriented micro-pillars of a solutionized Ti-49.9at.% Ni alloy. Post-mortem TEM analysis of the deformed pillars reveal that the operating plastic deformation modes are {011}<100> slip and {114}<221¯> deformation twinning, which compete with the martensitic transformation, depending on the crystal orientation. Furthermore, in both experiments and molecular dynamics simulations, we consistently find residual B19′ martensite in a herringbone microstructure composed of finely spaced (001)B19′ compound twins instead of the generally assumed [011]B19′ type II twins common in bulk samples, suggesting that the operative martensitic transformation mode may be size-dependent. Schmid factors in compression are calculated for all commonly reported slip, deformation twinning, and martensitic transformation modes as a function of crystallographic orientation, which rationalize the orientation-dependent competition between these deformation modes. © 2021 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2021.116731
  • Reducing hot tearing by grain boundary segregation engineering in additive manufacturing: example of an AlxCoCrFeNi high-entropy alloy
    Sun, Z. and Tan, X. and Wang, C. and Descoins, M. and Mangelinck, D. and Tor, S.B. and Jägle, E.A. and Zaefferer, S. and Raabe, D.
    Acta Materialia 204 (2021)
    One major hindrance that alloy design for additive manufacturing (AM) faces nowadays is hot tearing. Contrary to the previous works which either try to reduce solidification range or introduce grain refinement, the current work presents a new approach of employing segregation engineering to alter the residual stress states at the interdendritic and grain boundary regions and consequently prevent hot tearing. Here, in situ Al alloying is introduced into an existing hot-cracking susceptible high-entropy alloy CoCrFeNi. It is found that within a certain range of compositions, such as Al0.5CoCrFeNi, the hot crack density was drastically decreased. During the solidification of this specific alloy composition, Al is firstly ejected from the primary dendritic face-centred cubic (FCC) phase and segregates into the interdendritic regions. Spinodal decomposition then occurs in these Al-enriched regions to form the ordered B2 NiAl and disordered body-centred cubic (BCC) Cr phases. Due to the higher molar volume and lower homologous temperatures of these B2/BCC phases, the inherent residual strain is accommodated and transformed from a maximum 0.006 tensile strain in CoCrFeNi to a compressive strain of ~0.001 in Al0.5CoCrFeNi. It is believed that this grain boundary segregation engineering method could provide a new pathway to systematically counteract the hot tearing problem in additive manufacturing of metals and alloys, using available thermodynamic and kinetic database information. © 2020
    view abstract10.1016/j.actamat.2020.116505
  • Revisiting ω phase embrittlement in metastable β titanium alloys: Role of elemental partitioning
    Lai, M.J. and Li, T. and Yan, F.K. and Li, J.S. and Raabe, D.
    Scripta Materialia 193 (2021)
    The role of elemental partitioning between β and ω phase in embrittling an originally ductile ω-containing Ti–12Mo (wt.%) model alloy was studied using transmission electron microscopy and atom probe tomography. It is revealed that the embrittlement of this alloy already occurs after aging at 400 °C for as short as 10 min, when the size, inter-particle spacing and volume fraction of the ω particles remain almost unchanged. The origin of the aging-induced embrittlement is attributed to the significant rejection of Mo (>5 at.%) from the ω particles during aging, which leads to remarkable increase in the shear modulus (>30 GPa) of the ω particles, promoting intense plastic flow localization and facilitating crack nucleation prior to macroscopic yielding. © 2020
    view abstract10.1016/j.scriptamat.2020.10.031
  • Ti-bearing lightweight steel with large high temperature ductility via thermally stable multi-phase microstructure
    Moon, J. and Jo, H.-H. and Park, S.-J. and Kim, S.-D. and Lee, T.-H. and Lee, C.-H. and Lee, M.-G. and Hong, H.-U. and Suh, D.-W. and Raabe, D.
    Materials Science and Engineering A 808 (2021)
    The global demand for lightweight design is increasing to provide sustainable solutions to counteract climate change. We developed a novel Ti-bearing lightweight steel (8% lower mass density than general steels), which exhibits an excellent combination of strength (491 MPa ultimate tensile strength) and tensile ductility (31%) at elevated temperature (600 °C). The developed steel is suitable for parts subjected to high temperature at reduced dynamical load. The composition of the developed steel (Fe–20Mn–6Ti–3Al–0.06C–NbNi (wt%)) lends the alloy a multiphase structure with austenite matrix, partially ordered ferrite, Fe2Ti Laves phase, and fine MC carbides. At elevated temperature (600 °C), the ductility of the new material is at least 2.5 times higher than that of conventional lightweight steels based on the Fe–Mn–Al system, which become brittle at elevated temperatures due to the inter/intragranular precipitation of κ-carbides. This is achieved by the high thermal stability of its microstructure and the avoidance of brittle κ-carbides in this temperature range. © 2021 Elsevier B.V.
    view abstract10.1016/j.msea.2021.140954
  • (Al, Zn)3Zr dispersoids assisted η′ precipitation in anAl-Zn-Mg-Cu-Zr alloy
    Zhao, H. and Chen, Y. and Gault, B. and Makineni, S.K. and Ponge, D. and Raabe, D.
    Materialia 10 (2020)
    The influence of (Al,Zn)3Zr dispersoids on the precipitation of the main strengthening (Mg,Zn)-rich phases was investigated during isothermal aging of a model Al-Zn-Mg-Cu-Zr alloy. Upon homogenization of the alloy, dispersoids of (Al,Zn)3Zr with a L12 structure are present. Isothermal aging at 120 °C for 0.5 h leads to the homogeneous formation of spherical GP zones in the α-Al matrix and heterogeneous nucleation on (Al,Zn)3Zr dispersoids. After 2 h of aging, GP zones remain present in the α-Al matrix while the accelerated transformation of GP zones to plate-shaped (Mg,Zn)-rich ηʹ precipitates is shown on the {111} planes at the interface of the L12 dispersoids. Even at grain boundaries, the similar composite structure comprising ηʹ precipitates on the coarser Zr-dispersoid is observed, along with 10-nm wide precipitate-free zones around them. The composition and structure of pre-existing dispersoids, their role in the formation of the composite structure are discussed. © 2020
    view abstract10.1016/j.mtla.2020.100641
  • A new class of lightweight, stainless steels with ultra-high strength and large ductility
    Moon, J. and Ha, H.-Y. and Kim, K.-W. and Park, S.-J. and Lee, T.-H. and Kim, S.-D. and Jang, J.H. and Jo, H.-H. and Hong, H.-U. and Lee, B.H. and Lee, Y.-J. and Lee, C. and Suh, D.-W. and Han, H.N. and Raabe, D. and Lee, C.-H.
    Scientific Reports 10 (2020)
    Steel is the global backbone material of industrialized societies, with more than 1.8 billion tons produced per year. However, steel-containing structures decay due to corrosion, destroying annually 3.4% (2.5 trillion US$) of the global gross domestic product. Besides this huge loss in value, a solution to the corrosion problem at minimum environmental impact would also leverage enhanced product longevity, providing an immense contribution to sustainability. Here, we report a leap forward toward this aim through the development of a new family of low-density stainless steels with ultra-high strength (> 1 GPa) and high ductility (> 35%). The alloys are based on the Fe–(20–30)Mn–(11.5–12.0)Al–1.5C–5Cr (wt%) system and are strengthened by dispersions of nano-sized Fe3AlC-type κ-carbide. The alloying with Cr enhances the ductility without sacrificing strength, by suppressing the precipitation of κ-carbide and thus stabilizing the austenite matrix. The formation of a protective Al-rich oxide film on the surface lends the alloys outstanding resistance to pitting corrosion similar to ferritic stainless steels. The new alloy class has thus the potential to replace commercial stainless steels as it has much higher strength at similar formability, 17% lower mass density and lower environmental impact, qualifying it for demanding lightweight, corrosion resistant, high-strength structural parts. © 2020, The Author(s).
    view abstract10.1038/s41598-020-69177-7
  • A strong and ductile medium-entropy alloy resists hydrogen embrittlement and corrosion
    Luo, H. and Sohn, S.S. and Lu, W. and Li, L. and Li, X. and Soundararajan, C.K. and Krieger, W. and Li, Z. and Raabe, D.
    Nature Communications 11 (2020)
    Strong and ductile materials that have high resistance to corrosion and hydrogen embrittlement are rare and yet essential for realizing safety-critical energy infrastructures, hydrogen-based industries, and transportation solutions. Here we report how we reconcile these constraints in the form of a strong and ductile CoNiV medium-entropy alloy with face-centered cubic structure. It shows high resistance to hydrogen embrittlement at ambient temperature at a strain rate of 10−4 s−1, due to its low hydrogen diffusivity and the deformation twinning that impedes crack propagation. Moreover, a dense oxide film formed on the alloy’s surface reduces the hydrogen uptake rate, and provides high corrosion resistance in dilute sulfuric acid with a corrosion current density below 7 μA cm−2. The combination of load carrying capacity and resistance to harsh environmental conditions may qualify this multi-component alloy as a potential candidate material for sustainable and safe infrastructures and devices. © 2020, The Author(s).
    view abstract10.1038/s41467-020-16791-8
  • An efficient and robust approach to determine material parameters of crystal plasticity constitutive laws from macro-scale stress-strain curves
    Sedighiani, K. and Diehl, M. and Traka, K. and Roters, F. and Sietsma, J. and Raabe, D.
    International Journal of Plasticity 134 (2020)
    A severe obstacle for the routine use of crystal plasticity models is the effort associated with determining their constitutive parameters. Obtaining these parameters usually requires time-consuming micromechanical tests that allow probing of individual grains. In this study, a novel, computationally efficient, and fully automated approach is introduced which allows the identification of constitutive parameters from macroscopic tests. The approach presented here uses the response surface methodology together with a genetic algorithm to determine an optimal set of parameters. It is especially suited for complex models with a large number of parameters. The proposed approach also helps to develop a quantitative and thorough understanding of the relative influence of the different constitutive parameters and their interactions. Such general insights into parameter relations in complex models can be used to improve constitutive laws and reduce redundancy in parameter sets. The merits of the methodology are demonstrated on the examples of a dislocation-density-based crystal plasticity model for bcc steel, a phenomenological crystal plasticity model for fcc copper, and a phenomenological crystal plasticity model incorporating twinning deformation for hcp magnesium. The approach proposed is, however, model-independent and can be also used to identify parameters of, for instance, fatigue, creep and damage models. The method has been implemented into the Düsseldorf Advanced Material Simulation Kit (DAMASK) and is available as free and open-source software. The capability of translating complex material response into a micromechanical digital twin is an essential precondition for the ongoing digitalization of material property prediction, quality control of semi-finished parts, material response in manufacturing and the long-term behavior of products and materials when in service. © 2020 The Authors.
    view abstract10.1016/j.ijplas.2020.102779
  • Atomic Scale Origin of Metal Ion Release from Hip Implant Taper Junctions
    Balachandran, S. and Zachariah, Z. and Fischer, A. and Mayweg, D. and Wimmer, M.A. and Raabe, D. and Herbig, M.
    Advanced Science 7 (2020)
    Millions worldwide suffer from arthritis of the hips, and total hip replacement is a clinically successful treatment for end-stage arthritis patients. Typical hip implants incorporate a cobalt alloy (Co–Cr–Mo) femoral head fixed on a titanium alloy (Ti-6Al-4V) femoral stem via a Morse taper junction. However, fretting and corrosion at this junction can cause release of wear particles and metal ions from the metallic implant, leading to local and systemic toxicity in patients. This study is a multiscale structural-chemical investigation, ranging from the micrometer down to the atomic scale, of the underlying mechanisms leading to metal ion release from such taper junctions. Correlative transmission electron microscopy and atom probe tomography reveals microstructural and compositional alterations in the subsurface of the titanium alloy subjected to in vitro gross-slip fretting against the cobalt alloy. Even though the cobalt alloy is comparatively more wear-resistant, changes in the titanium alloy promote tribocorrosion and subsequent degradation of the cobalt alloy. These observations regarding the concurrent occurrence of electrochemical and tribological phenomena are vital to further improve the design and performance of taper junctions in similar environments. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/advs.201903008
  • Atomic Structure and Chemical Composition of Planar Fault Structures in Co-Base Superalloys
    Lenz, M. and Wu, M. and He, J. and Makineni, S.K. and Gault, B. and Raabe, D. and Neumeier, S. and Spiecker, E.
    Minerals, Metals and Materials Series (2020)
    We report atomic structures and chemical compositions of defects associated to planar faults in a creep deformed Co-base superalloy and discuss their formation and contribution to plastic deformation. The multinary single crystalline Co-base superalloy was creep deformed under tension along [ 001 ] -direction at 850 °C and 400 MPa. The creep microstructure comprises a high density of planar defects. Solute segregation to superlattice intrinsic stacking faults (SISF) is characterized via EDXS analysis of a statistically relevant number of faults and compared at different creep stages. The amount of solute segregation shows negligible difference at different creep stages indicating that segregation directly occurs during planar fault formation and does not significantly evolve afterward. Based on the observation and analysis of Frank partial dislocations with a/3⟨111⟩ Burgers vectors terminating SISF, we discuss a new route to SISF formation via dislocation climb. Additionally, two more complex fault structures are analyzed, and potential formation mechanisms are discussed. The first of these structures is a terminating end of an SISF where an a/3⟨112⟩ partial dislocation splits up into two closely spaced a/6⟨112⟩ partials separated by an SESF. The second structure consists of two parallel SISFs connected by an anti-phase boundary (APB). All deformation mechanisms described in this study show an involvement of solute segregation directly affecting formation and propagation of creep defects by changing planar fault energies and chemical environments of dislocations. Solute segregation is therefore expected to be a key to future alloy design by enabling control of creep deformation mechanisms in specific temperature and stress regimes. © 2020, The Minerals, Metals & Materials Society.
    view abstract10.1007/978-3-030-51834-9_90
  • Author Correction: A strong and ductile medium-entropy alloy resists hydrogen embrittlement and corrosion (Nature Communications, (2020), 11, 1, (3081), 10.1038/s41467-020-16791-8)
    Luo, H. and Sohn, S.S. and Lu, W. and Li, L. and Li, X. and Soundararajan, C.K. and Krieger, W. and Li, Z. and Raabe, D.
    Nature Communications 11 (2020)
    The original version of this Article contained an error in the author affiliations. The affiliation of Hong Luo and Xiaogang Li with Beijing Advanced Innovation Center for Materials Genome Engineering, Beijing 100083, China was inadvertently omitted. This has now been corrected in both the PDF and HTML versions of the Article. © 2020, The Author(s).
    view abstract10.1038/s41467-020-17295-1
  • Beyond Solid Solution High-Entropy Alloys: Tailoring Magnetic Properties via Spinodal Decomposition
    Rao, Z. and Dutta, B. and Körmann, F. and Lu, W. and Zhou, X. and Liu, C. and da Silva, A.K. and Wiedwald, U. and Spasova, M. and Farle, M. and Ponge, D. and Gault, B. and Neugebauer, J. and Raabe, D. and Li, Z.
    Advanced Functional Materials (2020)
    Since its first emergence in 2004, the high-entropy alloy (HEA) concept has aimed at stabilizing single- or dual-phase multi-element solid solutions through high mixing entropy. Here, this strategy is changed and renders such massive solid solutions metastable, to trigger spinodal decomposition for improving the alloys’ magnetic properties. The motivation for starting from a HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behavior using multiple components. The key idea is to form Fe-Co enriched regions which have an expanded volume (relative to unconstrained Fe-Co), due to coherency constraints imposed by the surrounding HEA matrix. As demonstrated by theory and experiments, this leads to improved magnetic properties of the decomposed alloy relative to the original solid solution matrix. In a prototype magnetic FeCoNiMnCu HEA, it is shown that the modulated structures, achieved by spinodal decomposition, lead to an increase of the Curie temperature by 48% and a simultaneous increase of magnetization by 70% at ambient temperature as compared to the homogenized single-phase reference alloy. The findings thus open a pathway for the development of advanced functional HEAs. © 2020 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
    view abstract10.1002/adfm.202007668
  • Chemical boundary engineering: A new route toward lean, ultrastrong yet ductile steels
    Ding, R. and Yao, Y. and Sun, B. and Liu, G. and He, J. and Li, T. and Wan, X. and Dai, Z. and Ponge, D. and Raabe, D. and Zhang, C. and Godfrey, A. and Miyamoto, G. and Furuhara, T. and Yang, Z. and van der Zwaag, S. and Chen, H.
    Science Advances 6 (2020)
    For decades, grain boundary engineering has proven to be one of the most effective approaches for tailoring the mechanical properties of metallic materials, although there are limits to the fineness and types of microstructures achievable, due to the rapid increase in grain size once being exposed to thermal loads (low thermal stability of crystallographic boundaries). Here, we deploy a unique chemical boundary engineering (CBE) approach, augmenting the variety in available alloy design strategies, which enables us to create a material with an ultrafine hierarchically heterogeneous microstructure even after heating to high temperatures. When applied to plain steels with carbon content of only up to 0.2 weight %, this approach yields ultimate strength levels beyond 2.0 GPa in combination with good ductility (>20%). Although demonstrated here for plain carbon steels, the CBE design approach is, in principle, applicable also to other alloys. Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).
    view abstract10.1126/sciadv.aay1430
  • Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface
    Colombara, D. and Elanzeery, H. and Nicoara, N. and Sharma, D. and Claro, M. and Schwarz, T. and Koprek, A. and Wolter, M.H. and Melchiorre, M. and Sood, M. and Valle, N. and Bondarchuk, O. and Babbe, F. and Spindler, C. and Cojocaru-Miredin, O. and Raabe, D. and Dale, P.J. and Sadewasser, S. and Siebentritt, S.
    Nature Communications 11 (2020)
    The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe2 solar cells. © 2020, The Author(s).
    view abstract10.1038/s41467-020-17434-8
  • Compatible deformation and extra strengthening by heterogeneous nanolayer composites
    Li, J. and Lu, W. and Gibson, J. and Zhang, S. and Korte-Kerzel, S. and Raabe, D.
    Scripta Materialia 179 (2020)
    A topologically heterogeneous microstructure design is introduced in a Cu/Zr nanolayered composite, in which each soft 100 nm Cu or Zr layer is surrounded on both sides by several hard 10 nm Cu/Zr bilayers. This design aims to impose a full geometrical constraint on all of the soft layers. Micropillar compression tests demonstrate that the composite deforms in a compatible fashion among the layers, in which no extrusion of the soft layers occurs. An elevated strength of 730 MPa is achieved in the composite compared with the strength prediction based on the linear rule of mixtures. © 2020
    view abstract10.1016/j.scriptamat.2020.01.006
  • Control of thermally stable core-shell nano-precipitates in additively manufactured Al-Sc-Zr alloys
    Kürnsteiner, P. and Bajaj, P. and Gupta, A. and Wilms, M.B. and Weisheit, A. and Li, X. and Leinenbach, C. and Gault, B. and Jägle, E.A. and Raabe, D.
    Additive Manufacturing 32 (2020)
    Laser Additive Manufacturing (LAM) of light metals such as high-strength Al-based alloys offers tremendous potential for e.g. weight reduction and associated reduced fuel consumptions for the transportation industry. Typically, commercial Sc-containing alloys, such as Scalmalloy®, rely on precipitation hardening to increase their strength. Conventional processing involves controlled ageing during which ordered and coherent Al3Sc precipitates form from a Sc-supersaturated solid solution. Here we show how the intrinsic heat treatment (IHT) of directed energy deposition (DED) can be used to trigger the precipitation of Al3Sc already during the LAM process. High number densities of 1023 nano-precipitates per m3 can be realized through solid-state phase transformation from the supersaturated Al-Sc matrix that results from the fast cooling rate in LAM. Yet, the IHT causes precipitates to coarsen, hence reducing their strengthening effect. We implement alternative solidification conditions to exploit the IHT to form a Zr-rich shell around the Al3Sc precipitates that prevents coarsening. Our approach is applicable to a wide range of precipitation-hardened alloys to trigger in-situ precipitation during LAM. © 2019 Elsevier B.V.
    view abstract10.1016/j.addma.2019.100910
  • Could face-centered cubic titanium in cold-rolled commercially-pure titanium only be a Ti-hydride?
    Chang, Y. and Zhang, S. and Liebscher, C.H. and Dye, D. and Ponge, D. and Scheu, C. and Dehm, G. and Raabe, D. and Gault, B. and Lu, W.
    Scripta Materialia 178 (2020)
    A face-centered cubic (FCC) phase in electro-polished specimens for transmission electron microscopy of commercially pure titanium has sometimes been reported. Here, a combination of atom-probe tomography, scanning transmission electron microscopy and low-loss electron energy loss spectroscopy is employed to study both the crystal structural and chemical composition of this FCC phase. Our results prove that the FCC phase is actually a TiHx (x ≥ 1) hydride, and not a new allotrope of Ti, in agreement with previous reports. The formation of the hydride is discussed. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2019.11.010
  • Crystal–Glass High-Entropy Nanocomposites with Near Theoretical Compressive Strength and Large Deformability
    Wu, G. and Balachandran, S. and Gault, B. and Xia, W. and Liu, C. and Rao, Z. and Wei, Y. and Liu, S. and Lu, J. and Herbig, M. and Lu, W. and Dehm, G. and Li, Z. and Raabe, D.
    Advanced Materials 32 (2020)
    High-entropy alloys (HEAs) and metallic glasses (MGs) are two material classes based on the massive mixing of multiple-principal elements. HEAs are single or multiphase crystalline solid solutions with high ductility. MGs with amorphous structure have superior strength but usually poor ductility. Here, the stacking fault energy in the high-entropy nanotwinned crystalline phase and the glass-forming-ability in the MG phase of the same material are controlled, realizing a novel nanocomposite with near theoretical yield strength (G/24, where G is the shear modulus of a material) and homogeneous plastic strain above 45% in compression. The mutually compatible flow behavior of the MG phase and the dislocation flux in the crystals enable homogeneous plastic co-deformation of the two regions. This crystal–glass high-entropy nanocomposite design concept provides a new approach to developing advanced materials with an outstanding combination of strength and ductility. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adma.202002619
  • Current Challenges and Opportunities in Microstructure-Related Properties of Advanced High-Strength Steels
    Raabe, D. and Sun, B. and Kwiatkowski Da Silva, A. and Gault, B. and Yen, H.-W. and Sedighiani, K. and Thoudden Sukumar, P. and Souza Filho, I.R. and Katnagallu, S. and Jägle, E. and Kürnsteiner, P. and Kusampudi, N. and Stephenson, L. and Herbig, M. and Liebscher, C.H. and Springer, H. and Zaefferer, S. and Shah, V. and Wong, S.-L. and Baron, C. and Diehl, M. and Roters, F. and Ponge, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 51 (2020)
    This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels, etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation, and twinning effects. Recent developments on microalloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modeling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation, and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation. © 2020, The Author(s).
    view abstract10.1007/s11661-020-05947-2
  • Defect Segregation and Its Effect on the Photoelectrochemical Properties of Ti-Doped Hematite Photoanodes for Solar Water Splitting
    Scherrer, B. and Li, T. and Tsyganok, A. and Döbeli, M. and Gupta, B. and Malviya, K.D. and Kasian, O. and Maman, N. and Gault, B. and Grave, D.A. and Mehlman, A. and Visoly-Fisher, I. and Raabe, D. and Rothschild, A.
    Chemistry of Materials 32 (2020)
    Optimizing the photoelectrochemical performance of hematite photoanodes for solar water splitting requires better understanding of the relationships between dopant distribution, structural defects, and photoelectrochemical properties. Here, we use complementary characterization techniques including electron microscopy, conductive atomic force microscopy (CAFM), Rutherford backscattering spectroscopy (RBS), atom probe tomography (APT), and intensity-modulated photocurrent spectroscopy (IMPS) to study this correlation in Ti-doped (1 cat. %) hematite films deposited by pulsed laser deposition (PLD) on F:SnO2 (FTO)-coated glass substrates. The deposition was carried out at 300 °C followed by annealing at 500 °C for 2 h. Upon annealing, Ti was observed by APT to segregate to the hematite/FTO interface and into some hematite grains. Since no other pronounced changes in microstructure and chemical composition were observed by electron microscopy and RBS after annealing, a nonuniform Ti redistribution seems to be the reason for reduced interfacial recombination in the annealed films, as observed by IMPS. This results in a lower onset potential, higher photocurrent, and larger fill factor with respect to the as-deposited state. This work provides atomic-scale insights into the microscopic inhomogeneity in Ti-doped hematite thin films and the role of defect segregation in their electrical and photoelectrochemical properties. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acs.chemmater.9b03704
  • Dependence of hydrogen embrittlement mechanisms on microstructure-driven hydrogen distribution in medium Mn steels
    Sun, B. and Krieger, W. and Rohwerder, M. and Ponge, D. and Raabe, D.
    Acta Materialia 183 (2020)
    The risk of hydrogen embrittlement (HE) is currently one important factor impeding the use of medium Mn steels. However, knowledge about HE in these materials is sparse. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature of deformation-driven phase transformation render systematic studies of HE mechanisms challenging. Here we investigate two austenite-ferrite medium Mn steel samples with very different phase characteristics. The first one has a ferritic matrix (~74 vol.% ferrite) with embedded austenite and a high dislocation density (~1014 m−2) in ferrite. The second one has a well recrystallized microstructure consisting of an austenitic matrix (~59 vol.% austenite) and embedded ferrite. We observe that the two types of microstructures show very different response to HE, due to fundamental differences between the HE micromechanisms acting in them. The influence of H in the first type of microstructure is explained by the H-enhanced local plastic flow in ferrite and the resulting increased strain incompatibility between ferrite and the adjacent phase mixture of austenite and strain-induced α'-martensite. In the second type of microstructure, the dominant role of H lies in its decohesion effect on phase and grain boundaries, due to the initially trapped H at the interfaces and subsequent H migration driven by deformation-induced austenite-to-martensite transformation. The fundamental change in the prevalent HE mechanisms between these two microstructures is related to the spatial distribution of H within them. This observation provides significant insights for future microstructural design towards higher HE resistance of high-strength steels. © 2019
    view abstract10.1016/j.actamat.2019.11.029
  • Effects of Mo on the mechanical behavior of γ/γʹ-strengthened Co-Ti-based alloys
    Im, H.J. and Lee, S. and Choi, W.S. and Makineni, S.K. and Raabe, D. and Ko, W.-S. and Choi, P.-P.
    Acta Materialia 197 (2020)
    We investigated the flow behavior of γ/γʹ-strengthened Co-12Ti and Co-12Ti-4Mo (at.%) alloys at room and elevated temperatures (up to 900°C) by electron microscopy and density functional theory. The Mo-added alloy exhibited an enhanced compressive yield strength and strain hardening behavior as compared to the reference binary alloy. This behavior could be attributed to a ~25% larger γʹ volume fraction and ~7% higher planar fault energies in Co-12Ti-4Mo. Using electron channeling contrast imaging, we observed interrupted slip bands in the Co-12Ti-4Mo alloy deformed to a strain of 6%, which led to enhanced strain hardening, in contrast to extended slip bands along {111} planes in the Co-12Ti alloy. Interrupted slip band formation in Co-12Ti-4Mo could be explained by rapid exhaustion of dislocation sources and a higher energy barrier required to cut the γʹ precipitates. These effects are due to a reduced γ channel width and substantial hardening effect of γʹ-Co3(Ti,Mo) in the ternary alloy as well as due to the large shear modulus difference between γʹ and γ. © 2020
    view abstract10.1016/j.actamat.2020.07.037
  • Electronic structure based design of thin film metallic glasses with superior fracture toughness
    Evertz, S. and Kirchlechner, I. and Soler, R. and Kirchlechner, C. and Kontis, P. and Bednarcik, J. and Gault, B. and Dehm, G. and Raabe, D. and Schneider, J.M.
    Materials and Design 186 (2020)
    High fracture toughness is crucial for the application of metallic glasses as structural materials to avoid catastrophic failure of the material in a brittle manner. One fingerprint for fracture toughness in metallic glasses is the fraction of hybridized bonds, which is affected by alloying Pd57.4Al23.5Y7.8M11.3 with M = Fe, Ni, Co, Cu, Os, Ir, Pt, and Au. It is shown that experimental fracture toughness data is correlated to the fraction of hybridized bonds which scale with the localized bonds at the Fermi level. Thus, the localized bonds at the Fermi level are utilized quantitatively as a measure for fracture toughness. Based on ab initio calculations, the minimum fraction of hybridized bonds was identified for Pd57.4Al23.5Y7.8Ni11.3. According to the ansatz that the crystal orbital overlap population at the Fermi level scales with fracture toughness, for Pd57.4Al23.5Y7.8Ni11.3 a value of around 95 ± 20 MPa·m0.5 is predicted quantitatively for the first time. Consistent with this prediction, in micro-mechanical beam bending experiments Pd57.4Al23.5Y7.8Ni11.3 thin films show pronounced plasticity and absence of crack growth. © 2018 The Authors
    view abstract10.1016/j.matdes.2019.108327
  • Formation mechanism of κ-carbides and deformation behavior in Si-alloyed FeMnAlC lightweight steels
    Wang, Z. and Lu, W. and Zhao, H. and He, J. and Wang, K. and Zhou, B. and Ponge, D. and Raabe, D. and Li, Z.
    Acta Materialia 198 (2020)
    The formation of κ-carbides in austenite Fe-30Mn-9Al-1.2C (wt. %) lightweight steels is tuned via alloying of Si (0, 1, 2 wt. %), an element that can remarkably raise the activities of Al and C based on thermodynamic calculations. Ordered L12 nano-domains (with a size <1 nm), lacking elemental partition, were observed in the solution-treated steel without Si alloying, while with the increase of Si to 2 wt. %, cuboidal L′12 intragranular κ-carbides were well developed with an average size of 11.5 nm and a volume fraction of 25.9 %. These κ-carbides found in the solution-treated steel with 2 wt. % Si follow a different precipitation route from previous pathways that require aging. Also, particle-shaped L′12 intergranular κ0-carbides and DO3 phase were formed at austenite grain boundaries in the steel with 2 wt. % Si. The precipitation of κ-carbides in grain interiors leads to an improvement of the yield strength from ~450 MPa to ~950 MPa as the Si content increases from 0 to 2 wt. %. The primary deformation mechanism is the formation of slip bands in all three steels, which involves the shear of ordered nano-domains or κ-carbides. The uniform distribution of the slip bands is essential for the high strain hardening, provided by the dynamic slip band refinement in the steel without Si. Lower strain hardening is seen in the steel with 2 wt. % Si due to the formation of localized coarse slip bands. These findings offer valuable insights into the design of high-performance lightweight steels. © 2020
    view abstract10.1016/j.actamat.2020.08.003
  • Formation of a 2D Meta-stable Oxide by Differential Oxidation of AgCu Alloys
    Schweinar, K. and Beeg, S. and Hartwig, C. and Rajamathi, C.R. and Kasian, O. and Piccinin, S. and Prieto, M.J. and Tanase, L.C. and Gottlob, D.M. and Schmidt, T. and Raabe, D. and Schlögl, R. and Gault, B. and Jones, T.E. and Greiner, M.T.
    ACS Applied Materials and Interfaces 12 (2020)
    Metal alloy catalysts can develop complex surface structures when exposed to reactive atmospheres. The structures of the resulting surfaces have intricate relationships with a myriad of factors, such as the affinity of the individual alloying elements to the components of the gas atmosphere and the bond strengths of the multitude of low-energy surface compounds that can be formed. Identifying the atomic structure of such surfaces is a prerequisite for establishing structure-property relationships, as well as for modeling such catalysts in ab initio calculations. Here, we show that an alloy, consisting of an oxophilic metal (Cu) diluted into a noble metal (Ag), forms a meta-stable two-dimensional oxide monolayer, when the alloy is subjected to oxidative reaction conditions. The presence of this oxide is correlated with selectivity in the corresponding test reaction of ethylene epoxidation. In the present study, using a combination of in situ, ex situ, and theoretical methods (NAP-XPS, XPEEM, LEED, and DFT), we determine the structure to be a two-dimensional analogue of Cu2O, resembling a single lattice plane of Cu2O. The overlayer holds a pseudo-epitaxial relationship with the underlying noble metal. Spectroscopic evidence shows that the oxide's electronic structure is qualitatively distinct from its three-dimensional counterpart, and because of weak electronic coupling with the underlying noble metal, it exhibits metallic properties. These findings provide precise details of this peculiar structure and valuable insights into how alloying can enhance catalytic properties. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acsami.0c03963
  • Grain boundary segregation, phase formation, and their influence on the coercivity of rapidly solidified SmF e11Ti hard magnetic alloys
    Palanisamy, D. and Ener, S. and Maccari, F. and Schäfer, L. and Skokov, K.P. and Gutfleisch, O. and Raabe, D. and Gault, B.
    Physical Review Materials 4 (2020)
    SmFe11Ti-based alloys have potential as permanent magnet materials; however, until now, crystallographically textured bulk permanent magnets have not yet been produced from this alloy system. This is partly due to the lack of information on the morphology and composition of grain boundary phases present in the Fe-rich Sm-Fe-Ti alloys. Here we investigated the microstructure of a Sm1.25Fe11Ti alloy by using correlative transmission electron microscopy and atom-probe tomography, combined with magneto-optical Kerr effect (MOKE) probing to relate the material's micro- and nanostructure to its properties. The grains of the Sm(Fe,Ti)12 matrix phase are separated by grain boundaries exhibiting a different composition over 3-4 nm width. They contain >75at% of the ferromagnetic element Fe, with an enrichment of Sm of up to 16.6 at% and a depletion in Ti, down to approx. 3.4 at%. We believe that the grain boundary is ferromagnetic at room temperature, which makes the magnetic decoupling of the grains practically impossible, which, in turn, leads to a low coercivity of SmFe11Ti-based alloys. MOKE measurements reveal the strong ferromagnetic coupling across the grain boundary, causing the nucleation of reversal magnetic domains when exposed to low magnetic fields. In a triple-junction area we identified three other ferromagnetic phases: Sm3(Fe,Ti)29,SmFe2, and Fe2Ti. These details bring out the scope of further adjustment of the coercivity in the Sm-Fe-Ti alloy system by grain boundary segregation engineering through the reduction of the presence of ferromagnetic phases to ensure a magnetic decoupling of the micrometer-sized Sm(Fe,Ti)12 grains. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.
    view abstract10.1103/PhysRevMaterials.4.054404
  • High-strength Damascus steel by additive manufacturing
    Kürnsteiner, P. and Wilms, M.B. and Weisheit, A. and Gault, B. and Jägle, E.A. and Raabe, D.
    Nature 582 (2020)
    Laser additive manufacturing is attractive for the production of complex, three-dimensional parts from metallic powder using a computer-aided design model1–3. The approach enables the digital control of the processing parameters and thus the resulting alloy’s microstructure, for example, by using high cooling rates and cyclic re-heating4–10. We recently showed that this cyclic re-heating, the so-called intrinsic heat treatment, can trigger nickel-aluminium precipitation in an iron–nickel–aluminium alloy in situ during laser additive manufacturing9. Here we report a Fe19Ni5Ti (weight per cent) steel tailor-designed for laser additive manufacturing. This steel is hardened in situ by nickel-titanium nanoprecipitation, and martensite is also formed in situ, starting at a readily accessible temperature of 200 degrees Celsius. Local control of both the nanoprecipitation and the martensitic transformation during the fabrication leads to complex microstructure hierarchies across multiple length scales, from approximately 100-micrometre-thick layers down to nanoscale precipitates. Inspired by ancient Damascus steels11–14—which have hard and soft layers, originally introduced via the folding and forging techniques of skilled blacksmiths—we produced a material consisting of alternating soft and hard layers. Our material has a tensile strength of 1,300 megapascals and 10 per cent elongation, showing superior mechanical properties to those of ancient Damascus steel12. The principles of in situ precipitation strengthening and local microstructure control used here can be applied to a wide range of precipitation-hardened alloys and different additive manufacturing processes. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstract10.1038/s41586-020-2409-3
  • Hydrogen resistance of a 1 GPa strong equiatomic CoCrNi medium entropy alloy
    Soundararajan, C.K. and Luo, H. and Raabe, D. and Li, Z.
    Corrosion Science 167 (2020)
    In this work, we study the influence of hydrogen on the deformation behavior and microstructure evolution in an equiatomic CoCrNi medium entropy alloy (MEA) with an ultimate tensile strength of ∼1 GPa. Upon deformation, hydrogen-charged samples exhibit enhanced dislocation activity and nanotwinning. Hydrogen shows both positive and negative effects on the deformation behavior of the CoCrNi MEA. More specifically, it weakens grain boundaries during loading, leading to intergranular cracking. Also, it promotes the formation of twins which enhance the material's resistance to crack propagation. The underlying mechanisms responsible for the hydrogen resistance of the CoCrNi MEA are discussed in detail. © 2020 Elsevier Ltd
    view abstract10.1016/j.corsci.2020.108510
  • In-situ synthesis via laser metal deposition of a lean Cu–3.4Cr–0.6Nb (at%) conductive alloy hardened by Cr nano-scale precipitates and by Laves phase micro-particles
    Kini, A.R. and Maischner, D. and Weisheit, A. and Ponge, D. and Gault, B. and Jägle, E.A. and Raabe, D.
    Acta Materialia 197 (2020)
    Conductive and yet strong copper alloys are essential materials in highly mechanically loaded electrical devices. We demonstrate a novel in-situ synthesis approach via laser metal deposition (LMD) in a lean copper alloy, Cu–3.4Cr–0.6Nb (at%). Strengthening in the lean alloy comes from chromium nano-scale precipitates formed in-situ (4 nm diameter; number density 8 × 1023 m−3) and from Laves phase particles (< 1 µm diameter; 2.2 vol%), dispersed across the microstructure. This dual dispersion, in a nearly pure copper matrix, is achieved through a suited combination of chromium alloying and cooling rate during LMD synthesis. The as-synthesized alloy has a conductivity of 68% IACS (International Annealed Copper Standard) and a Vickers hardness of 146, at room temperature. The latter is 11% above the value reported for the strongest lean reference ternary alloy Cu–8Cr–4Nb (at%). The in-situ synthesis approach averts any heat treatment step, which has been an essential step previously in conventional manufacturing, for realizing the property combination in lean Cu–Cr based system. © 2020
    view abstract10.1016/j.actamat.2020.07.035
  • Interfacial nanophases stabilize nanotwins in high-entropy alloys
    Lu, W. and Liebscher, C.H. and Yan, F. and Fang, X. and Li, L. and Li, J. and Guo, W. and Dehm, G. and Raabe, D. and Li, Z.
    Acta Materialia 185 (2020)
    Nanostructuring metals through nanograins and nanotwins is an efficient strategy for strength increase as the mean free path of dislocations is reduced. Yet, nanostructures are thermally often not stable, so that the material properties deteriorate upon processing or during service. Here, we introduce a new strategy to stabilize nanotwins by an interfacial nanophase design and realize it in an interstitial high-entropy alloy (iHEA). We show that nanotwins in a carbon-containing FeMnCoCrNi iHEA can remain stable up to 900 °C. This is enabled by co-segregation of Cr and C to nanoscale 9R structures adjacent to incoherent nanotwin boundaries, transforming the 9R structures into elongated nano-carbides in equilibrium with the nanotwin boundaries. This nanoscale 9R structures assisted nano-carbide formation leads to an unprecedented thermal stability of nanotwins, enabling excellent combination of yield strength (~1.1 GPa) and ductility (~21%) after exposure to high temperature. Stimulating the formation of nanosized 9R phases by deformation together with interstitial doping establishes a novel interfacial-nanophase design strategy. The resulting formation of nano-carbides at twin boundaries enables the development of strong, ductile and thermally stable bulk nanotwinned materials. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.12.010
  • Interplay of Chemistry and Faceting at Grain Boundaries in a Model Al Alloy
    Zhao, H. and Huber, L. and Lu, W. and Peter, N.J. and An, D. and De Geuser, F. and Dehm, G. and Ponge, D. and Neugebauer, J. and Gault, B. and Raabe, D.
    Physical Review Letters 124 (2020)
    The boundary between two crystal grains can decompose into arrays of facets with distinct crystallographic character. Faceting occurs to minimize the system's free energy, i.e., when the total interfacial energy of all facets is below that of the topologically shortest interface plane. In a model Al-Zn-Mg-Cu alloy, we show that faceting occurs at investigated grain boundaries and that the local chemistry is strongly correlated with the facet character. The self-consistent coevolution of facet structure and chemistry leads to the formation of periodic segregation patterns of 5-10 nm, or to preferential precipitation. This study shows that segregation-faceting interplay is not limited to bicrystals but exists in bulk engineering Al alloys and hence affects their performance. © 2020 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevLett.124.106102
  • Interstitial doping enhances the strength-ductility synergy in a CoCrNi medium entropy alloy
    Moravcik, I. and Hornik, V. and Minárik, P. and Li, L. and Dlouhy, I. and Janovska, M. and Raabe, D. and Li, Z.
    Materials Science and Engineering A 781 (2020)
    An equiatomic CoCrNi medium entropy alloy (MEA) with face-centered cubic (FCC) structure exhibits excellent combination of strength and ductility. Here we employ interstitial doping to enhance its mechanical performance. Interstitial CoCrNi MEAs with two different carbon contents, i.e., 0.5 at. % and 1 at. %, as well as a carbon-free CoCrNi reference MEA have been studied. The results show that up to 1 at. % carbon can be fully dissolved into the homogenized plus water-quenched FCC solid solution structure. Subsequent annealing leads to precipitation of nano-sized M23C6 type carbides which provide dispersion strengthening and enhanced strain hardening. The best combination of ultimate tensile strength of 1180 MPa at an elongation above 60% was obtained in fine grained CoCrNi doped with 0.5 at. % of carbon. Carbon alloying is also shown to significantly increase the lattice friction stress. Dislocation glide and mechanical twinning act as main deformation mechanisms. Thus, the joint contribution of multiple deformation mechanisms in the carbon-doped MEAs leads to significantly enhanced strength-ductility combinations compared to the carbon-free reference alloy, demonstrating that interstitial alloying can enhance the mechanical properties of MEAs. © 2020 Elsevier B.V.
    view abstract10.1016/j.msea.2020.139242
  • Irreversible Structural Changes of Copper Hexacyanoferrate Used as a Cathode in Zn-Ion Batteries
    Lim, J. and Kasiri, G. and Sahu, R. and Schweinar, K. and Hengge, K. and Raabe, D. and La Mantia, F. and Scheu, C.
    Chemistry - A European Journal 26 (2020)
    The structural changes of copper hexacyanoferrate (CuHCF), a Prussian blue analogue, which occur when used as a cathode in an aqueous Zn-ion battery, are investigated using electron microscopy techniques. The evolution of ZnxCu1−xHCF phases possessing wire and cubic morphologies from initial CuHCF nanoparticles are monitored after hundreds of cycles. Irreversible introduction of Zn ions to CuHCF is revealed locally using scanning transmission electron microscopy. A substitution mechanism is proposed to explain the increasing Zn content within the cathode material while simultaneously the Cu content is lowered during Zn-ion battery cycling. The present study demonstrates that the irreversible introduction of Zn ions is responsible for the decreasing Zn ion capacity of the CuHCF cathode in high electrolyte concentration. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/chem.201905384
  • Joint investigation of strain partitioning and chemical partitioning in ferrite-containing TRIP-assisted steels
    Tan, X. and Ponge, D. and Lu, W. and Xu, Y. and He, H. and Yan, J. and Wu, D. and Raabe, D.
    Acta Materialia 186 (2020)
    We applied two types of hot-rolling direct quenching and partitioning (HDQ&P) schemes to a low-C low-Si Al-added steel and obtained two ferrite-containing TRIP-assisted steels with different hard matrix structures, viz, martensite or bainite. Using quasi in-situ tensile tests combined with high-resolution electron back-scattered diffraction (EBSD) and microscopic digital image correlation (µ-DIC) analysis, we quantitatively investigated the TRIP effect and strain partitioning in the two steels and explored the influence of the strain partitioning between the soft and hard matrix structures on the TRIP effect. We also performed an atomic-scale analysis of the carbon partitioning among the different phases using atom probe tomography (APT). The results show that the strain mainly localizes in the ferrite in both types of materials. For the steel with a martensitic hard-matrix, a strong strain contrast exists between ferrite and martensite, with the local strain difference reaching up to about 75% at a global strain of 12.5%. Strain localization bands initiated in the ferrite rarely cross the ferrite/martensite interfaces. The low local strain (2%–10%) in the martensite regions leads to a slight TRIP effect with a transformation ratio of the retained austenite of about 7.5%. However, for the steel with bainitic matrix, the ferrite and bainite undergo more homogeneous strain partitioning, with an average local strain in ferrite and bainite of 15% and 8%, respectively, at a global strain of 12.5%. The strain localization bands originating in the ferrite can cross the ferrite/bainite (F/B) interfaces and increase the local strain in the bainite regions, resulting in an efficient TRIP effect. In that case the transformation ratio of the retained austenite is about 41%. The lower hardness difference between the ferrite and bainite of about 178 HV, compared with that between the ferrite and martensite of about 256 HV, leads to a lower strain contrast at the ferrite/bainite interfaces, thus retarding interfacial fracture. Further microstructure design for TRIP effect optimization should particularly focus on adjusting the strength contrast among the matrix structures and tuning strain partitioning to enhance the local strain partitioning into the retained austenite. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.12.050
  • Microchemistry-dependent simulation of yield stress and flow stress in non-heat treatable Al sheet alloys
    Wong, S.L. and Laptyeva, G. and Brüggemann, T. and Engler, O. and Roters, F. and Raabe, D. and Karhausen, K.-F.
    Modelling and Simulation in Materials Science and Engineering 28 (2020)
    A flow stress model which considers the processing conditions for a given alloy composition as well as the microchemistry of the alloy allows for integrated optimization of alloy composition, thermal treatments and forming operations to achieve the desired properties in the most efficient processing route. In the past, a statistical flow stress model for cell forming metals, 3IVM+ (3 Internal Variable Model), has been used for through process modeling of sheet production. However, this model was restricted to a given alloy in the state in which it was calibrated. In this work, the existing 3IVM+ model is augmented with an analytical solute strengthening model which uses input from ab initio simulations. Furthermore, a new particle strengthening model for non-shearable precipitates has been introduced which takes Orowan looping at low temperatures and dislocation climb at high temperatures into account. Hence, the present modeling approach considers the strengthening contributions from solutes, precipitates and forest dislocations. Three case studies on the alloys AA 1110, AA 3003 and AA 8014 are presented to assess the performance of the model in simulating the yield stress and flow stress of Al alloys over a wide range of temperatures and strain rates. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstract10.1088/1361-651X/ab71c0
  • Microscale plastic anisotropy of basal and pyramidal I slip in pure magnesium tested in shear
    Seok, M.-Y. and Gopalan, H. and Nandy, S. and Zaefferer, S. and Raabe, D. and Kirchlechner, C. and Dehm, G.
    Materialia 14 (2020)
    An optimised micro-shear testing protocol was adopted to measure the critical resolved shear stresses for basal and pyramidal I slip systems in pure magnesium. The micro-shear samples are carefully aligned for basal and pyramidal I slip by electron backscatter diffraction and fabricated by focussed ion beam milling. In situ scanning electron microscopy based shear testing identified that the two different sample orientations lead to activation of basal or 〈c+a〉pyramidal I slip, respectively. The critical resolved shear stress for basal slip was found to be 57 ± 19 MPa, and 371 ± 81 MPa for pyramidal I slip, albeit for slightly different geometric dimensions. Accounting for sample size-dependent flow stress for basal slip, we found that the plastic anisotropy with respect to pyramidal I slip is substantially reduced to a factor of 3 at the microscale compared to nearly a factor of 100 in the bulk. Multiple slip systems are therefore expected to operate in ultra-fine grain sized magnesium offering a pathway for improving ductility. © 2020
    view abstract10.1016/j.mtla.2020.100932
  • Microstructure-based multiscale modeling of large strain plastic deformation by coupling a full-field crystal plasticity-spectral solver with an implicit finite element solver
    Han, F. and Roters, F. and Raabe, D.
    International Journal of Plasticity 125 (2020)
    We present a fully embedded implementation of a full-field crystal plasticity model in an implicit finite element (FE) framework, a combination which realizes a multiscale approach for the simulation of large strain plastic deformation. At each integration point of the macroscopic FE model a spectral solver, based on Fast Fourier Transforms (FFTs), feeds-in the homogenized response from an underlying full-field polycrystalline representative volume element (RVE) model which is solved by using a crystal plasticity constitutive formulation. Both, a phenomenological hardening law and a dislocation density based hardening model, implemented in the open source software DAMASK, have been employed to provide the constitutive response at the mesoscale. The accuracy of the FE-FFT model has been benchmarked by one-element tests of several loading scenarios for an FCC polycrystal including simple tension, simple compression, and simple shear. The multiscale model is applied to simulate four application cases, i.e., plane strain deformation of an FCC plate, compression of an FCC cylinder, four-point bending of HCP bars, and beam bending of a dual-phase steel. The excellent capabilities of the model to predict the microstructure evolution at the mesoscale and the mechanical responses at both macroscale and mesoscale are demonstrated. © 2019 Elsevier Ltd.
    view abstract10.1016/j.ijplas.2019.09.004
  • Moving cracks form white etching areas during rolling contact fatigue in bearings
    Morsdorf, L. and Mayweg, D. and Li, Y. and Diederichs, A. and Raabe, D. and Herbig, M.
    Materials Science and Engineering A 771 (2020)
    White etching cracks (WECs) and the associated white etching areas (WEAs) are responsible for failure of widely spread engineering applications such as bearings and railways. Although the phenomenon is known for more than 100 years, the underlying mechanisms are still a matter of debate. In this work, we thoroughly investigate a 100Cr6 wind turbine gearbox bearing after failure in service operation. Based on our findings from detailed microstructure characterization on multiple length scales we formulate a new consistent explanation for the formation of WEAs during rolling contact fatigue. We propose a mechanism of moving WECs - not only in terms of conventional crack propagation but also as a movement of the crack normal to its plane. During cyclic loading the crack continuously changes its position and leaves behind a severely plastically deformed area consisting of ferritic nano-grains, i.e. the WEAs. The atomic-scale delocalization of the crack plane in a single loading cycle adds up to micron-sized WEAs during repetitive loading/unloading. After the initial formation of a fatigue crack around inclusions, crack face rubbing occurs during compressive loading cycles. This leads to the formation of WEA by local severe plastic deformation. It also leads to partial cohesion of the abutting crack faces and material transport between them. As a result, the WEC opens at a slightly shifted position with respect to its former location during unloading. © 2019 Elsevier B.V.
    view abstract10.1016/j.msea.2019.138659
  • Multi-component chemo-mechanics based on transport relations for the chemical potential
    Shanthraj, P. and Liu, C. and Akbarian, A. and Svendsen, B. and Raabe, D.
    Computer Methods in Applied Mechanics and Engineering 365 (2020)
    A chemo-mechanical model for a finite-strain elasto-viscoplastic material containing multiple chemical components is formulated and an efficient numerical implementation is developed to solve the resulting transport relations. The numerical solution relies on inverting the constitutive model for the chemical potential. In this work, a semi-analytical inversion for a general family of multi-component regular-solution chemical free energy models is derived. This is based on splitting the chemical free energy into a convex contribution, treated implicitly, and a non-convex contribution, treated explicitly. This results in a reformulation of the system transport equations in terms of the chemical potential rather than the composition as the independent field variable. The numerical conditioning of the reformulated system, discretised by finite elements, is shown to be significantly improved, and convergence to the Cahn–Hilliard solution is demonstrated for the case of binary spinodal decomposition. Chemo-mechanically coupled binary and ternary spinodal decomposition systems are then investigated to illustrate the effect of anisotropic elastic deformation and plastic relaxation of the resulting spinodal morphologies in more complex material systems. © 2020 The Author(s)
    view abstract10.1016/j.cma.2020.113029
  • Nanocrystalline Sm-based 1:12 magnets
    Schönhöbel, A.M. and Madugundo, R. and Barandiarán, J.M. and Hadjipanayis, G.C. and Palanisamy, D. and Schwarz, T. and Gault, B. and Raabe, D. and Skokov, K. and Gutfleisch, O. and Fischbacher, J. and Schrefl, T.
    Acta Materialia 200 (2020)
    Recently 1:12 magnets of Sm-(Fe,V) have shown promising coercivities and the potential to be alternative rare-earth-lean permanent magnets. In this work, we investigated the effects of partial substitution of Cu, Mo and Ti for V in the magnets prepared by hot compaction and hot deformation of mechanically milled powders. The microstructure of the Sm-Fe-(V,Cu) and Sm-Fe-(V,Ti) hot-deformed magnets consisted in fine grains with sizes between 50 and 150 nm. The Sm-Fe-(V,Cu) magnet showed the best performance with μ0Hc=0.96 T, μ0Mr=0.49 T, (BH)max=42kJm−3 and TC=362∘C. Atom probe tomography of this magnet revealed the presence of a thin Sm17.5Fe71.5V8Cu3 intergranular phase of 3-6 nm surrounding the 1:12 nanograins. The addition of a small amount of Cu, not only improved the magnetic properties but also hindered the grain growth during hot deformation. Micromagnetic simulations of the magnetization reversal agreed with the experimental values of coercivity. The presence of the intergranular phase reduces the number of grains that switch simultaneously. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.08.075
  • Nanoglass–Nanocrystal Composite—a Novel Material Class for Enhanced Strength–Plasticity Synergy
    Katnagallu, S. and Wu, G. and Singh, S.P. and Nandam, S.H. and Xia, W. and Stephenson, L.T. and Gleiter, H. and Schwaiger, R. and Hahn, H. and Herbig, M. and Raabe, D. and Gault, B. and Balachandran, S.
    Small 16 (2020)
    The properties of a material can be engineered by manipulating its atomic and chemical architecture. Nanoglasses which have been recently invented and comprise nanosized glassy particles separated by amorphous interfaces, have shown promising properties. A potential way to exploit the structural benefits of nanoglasses and of nanocrystalline materials is to optimize the composition to obtain crystals forming within the glassy particles. Here, a metastable Fe-10 at% Sc nanoglass is synthesized. A complex hierarchical microstructure is evidenced experimentally at the atomic scale. This bulk material comprises grains of a Fe90Sc10 amorphous matrix separated by an amorphous interfacial network enriched and likely stabilized by hydrogen, and property-enhancing pure-Fe nanocrystals self-assembled within the matrix. This composite structure leads a yield strength above 2.5 GPa with an exceptional quasi-homogeneous plastic flow of more than 60% in compression. This work opens new pathways to design materials with even superior properties. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/smll.202004400
  • On the assessment of creep damage evolution in nickel-based superalloys through correlative HR-EBSD and cECCI studies
    Sulzer, S. and Li, Z. and Zaefferer, S. and Hafez Haghighat, S.M. and Wilkinson, A. and Raabe, D. and Reed, R.
    Acta Materialia 185 (2020)
    The evolution of dislocation density with creep strain in single-crystal superalloys is studied quantitatively using high-resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging under controlled diffraction conditions (cECCI). Data regarding dislocation density/structure is measured for deformation at 900 °C and 450 MPa up to ≈ 1% plastic strain. Effects of chemical composition are elucidated via three purpose-designed superalloys of differing rhenium and ruthenium contents. The evidence indicates that dislocation avalanching is already prevalent at plastic strains of ≈ 0.1%; thereafter, an exponential decay in the dislocation multiplication rate is indicative of self-hardening due to dislocation constriction within the matrix channels, as confirmed by the imaging. The results are rationalised using discrete dislocation dynamics modelling: a universal dislocation evolution law emerges, which will be useful for alloy design efforts. © 2019
    view abstract10.1016/j.actamat.2019.07.018
  • On the atomic solute diffusional mechanisms during compressive creep deformation of a Co-Al-W-Ta single crystal superalloy
    He, J. and Zenk, C.H. and Zhou, X. and Neumeier, S. and Raabe, D. and Gault, B. and Makineni, S.K.
    Acta Materialia 184 (2020)
    We investigated the solute diffusional behavior active during compressive creep deformation at 150 MPa / 975 °C of a Co-Al-W-Ta single crystal superalloy in the [001] orientation. We report the formation of shear-bands that involves re-orientation of γ/γʹ rafts to {111} from {001} planes, referring to as γ/γ′ raft-rotation. In the shear-band regions, we observed abundant micro-twins, stacking faults (SFs), disordered zones within the γʹ termed as ‘γ pockets’ and also few geometrically-close-packed (GCP) phases. We used a correlative approach blending electron microscopy and atom probe tomography to characterize the structure and composition of these features. The SFs were identified as intrinsic and exhibit a W enrichment up to 14.5 at.% and an Al deficiency down to 5.1 at.%, with respect to the surrounding γʹ phase. The micro-twin boundaries show a solute enrichment similar to the SFs with a distinct W compositional profile gradients perpendicular from the boundaries into the twin interior, indicating solute diffusion within the micro-twins. The γ-pockets have a composition close to that of γ but richer in W/Ta. Based on these observations, we propose (i) a solute diffusion mechanism taking place during micro-twinning, (ii) a mechanism for the γ/γʹ raft-rotation process and evaluate their influence on the overall creep deformation of the present Co-based superalloy. © 2019
    view abstract10.1016/j.actamat.2019.11.035
  • Phase boundary segregation-induced strengthening and discontinuous yielding in ultrafine-grained duplex medium-Mn steels
    Ma, Y. and Sun, B. and Schökel, A. and Song, W. and Ponge, D. and Raabe, D. and Bleck, W.
    Acta Materialia 200 (2020)
    The combination of different phase constituents to realize a mechanical composite effect for superior strength-ductility synergy has become an important strategy in microstructure design in advanced high-strength steels. Introducing multiple phases in the microstructure essentially produces a large number of phase boundaries. Such hetero-interfaces affect the materials in various aspects such as dislocation activity and damage formation. However, it remains a question whether the characteristics of phase boundaries, such as their chemical decoration states, would also have an impact on the mechanical behavior in multiphase steels. Here we reveal a phase boundary segregation-induced strengthening effect in ultrafine-grained duplex medium-Mn steels. We found that the carbon segregation at ferrite-austenite phase boundaries can be manipulated by adjusting the cooling conditions after intercritical annealing. Such phase boundary segregation in the investigated steels resulted in a yield strength enhancement by 100–120 MPa and simultaneously promoted discontinuous yielding. The sharp carbon segregation at the phase boundaries impeded interfacial dislocation emission, thus increasing the stress required to activate such dislocation nucleation process and initiate plastic deformation. This observation suggests that the enrichment of carbon at the phase boundaries can enhance the energy barrier for dislocation emission, which provides a favorable condition for plastic flow avalanches and thus discontinuous yielding. These findings extend the current understanding of the yielding behavior in medium-Mn steels, and more importantly, shed light on utilizing and manipulating phase boundary segregation to improve the mechanical performance of multiphase metallic materials. © 2020
    view abstract10.1016/j.actamat.2020.09.007
  • Probing catalytic surfaces by correlative scanning photoemission electron microscopy and atom probe tomography
    Schweinar, K. and Nicholls, R.L. and Rajamathi, C.R. and Zeller, P. and Amati, M. and Gregoratti, L. and Raabe, D. and Greiner, M. and Gault, B. and Kasian, O.
    Journal of Materials Chemistry A 8 (2020)
    The chemical composition and the electronic state of the surface of alloys or mixed oxides with enhanced electrocatalytic properties are usually heterogeneous at the nanoscale. The non-uniform distribution of the potential across their surface affects both activity and stability. Studying such heterogeneities at the relevant length scale is crucial for understanding the relationships between structure and catalytic behaviour. Here, we demonstrate an experimental approach combining scanning photoemission electron microscopy and atom probe tomography performed at identical locations to characterise the surface's structure and oxidation states, and the chemical composition of the surface and sub-surface regions. Showcased on an Ir-Ru thermally grown oxide, an efficient catalyst for the anodic oxygen evolution reaction, the complementary techniques yield consistent results in terms of the determined surface oxidation states and local oxide stoichiometry. Significant chemical heterogeneities in the sputter-deposited Ir-Ru alloy thin films govern the oxide's chemistry, observed after thermal oxidation both laterally and vertically. While the oxide grains have a composition of Ir0.94Ru0.06O2, the composition in the grain boundary region varies from Ir0.70Ru0.30O2 to Ir0.40Ru0.60O2 and eventually to Ir0.75Ru0.25O2 from the top surface into the depth. The influence of such compositional non-uniformities on the catalytic performance of the material is discussed, along with possible engineering levers for the synthesis of more stable and reactive mixed oxides. The proposed method provides a framework for investigating materials of interest in the field of electrocatalysis and beyond. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c9ta10818a
  • Publisher Correction: Unveiling the Re effect in Ni-based single crystal superalloys (Nature Communications, (2020), 11, 1, (389), 10.1038/s41467-019-14062-9)
    Wu, X. and Makineni, S.K. and Liebscher, C.H. and Dehm, G. and Mianroodi, J.R. and Shanthraj, P. and Svendsen, B. and Bürger, D. and Eggeler, G. and Raabe, D. and Gault, B.
    Nature Communications 11 (2020)
    The original version of this Article contained an error in Fig. 2h, in which the left-side label was given incorrectly as ‘(b3):SISF’, rather than the correct ‘(i):SISF’, and the right-side label was given incorrectly as ‘(c3):SESF’, rather than the correct ‘(j):SESF’. The original version of this Article also contained an error in Fig. 8, in which all three x-axis tick marks were incorrectly labelled ‘200, 250, 300’. The correct x-axis tick marks read ’20, 25, 30’. These errors have now been corrected in both the PDF and the HTML versions of the Article © 2020, The Author(s).
    view abstract10.1038/s41467-020-14820-0
  • Revealing nano-chemistry at lattice defects in thermoelectric materials using atom probe tomography
    Yu, Y. and Zhou, C. and Zhang, S. and Zhu, M. and Wuttig, M. and Scheu, C. and Raabe, D. and Snyder, G.J. and Gault, B. and Cojocaru-Mirédin, O.
    Materials Today 32 (2020)
    The population of all non-equilibrium lattice defects in materials is referred to as microstructure. Examples are point defects such as substitutional and interstitial atoms, and vacancies; line defects such as dislocations; planar defects such as interfaces and stacking faults; or mesoscopic defects such as second-phase precipitates. These types of lattice imperfections are usually described in terms of their structural features, breaking the periodicity of the otherwise regular crystalline structure. Recent analytical probing at the nanoscale has revealed that their chemical features are likewise important and characteristic. The structure of the defects as well as their individual chemical composition, that is their chemical decoration state, which results from elemental partitioning with the adjacent matrix, can significantly influence the electrical and thermal transport properties of thermoelectric materials. The emergence of atom probe tomography (APT) has now made routinely accessible the mapping of three-dimensional chemical composition with sub-nanometer spatial accuracy and elemental sensitivity in the range of tens of ppm. Here, we review APT-based investigations and results related to the local chemical decoration states of various types of lattice defects in thermoelectric materials. APT allows to better understand the interplay between thermoelectric properties and microstructural features, extending the concept of defect engineering to the field of segregation engineering so as to guide the rational design of high-performance thermoelectric materials. © 2019 Elsevier Ltd
    view abstract10.1016/j.mattod.2019.11.010
  • Reversion and re-aging of a peak aged Al-Zn-Mg-Cu alloy
    Zhao, H. and Gault, B. and Ponge, D. and Raabe, D.
    Scripta Materialia 188 (2020)
    High-strength Al-Zn-Mg-Cu alloys are highly susceptible to stress corrosion cracking (SCC) which severely limits their lifetime. Reversion and re-aging (RRA) temper provides a higher SCC resistance at no loss in strength, yet the microstructural origins of these enhanced properties remain elusive. In an Al-Zn-Mg-Cu alloy, we show that the fine precipitate dispersion in the grain interiors is similar in the peak aged and RRA tempers. However, upon RRA, precipitates inside the grains are enriched in Cu, lowering the Cu matrix content, and reducing the relative difference in the Cu precipitate composition between bulk and grain boundaries. This study enriches the current understanding on the critical role of Cu related to SCC resistance in Al-Zn-Mg-Cu alloys. © 2020
    view abstract10.1016/j.scriptamat.2020.07.049
  • Review on Quantum Mechanically Guided Design of Ultra-Strong Metallic Glasses
    Evertz, S. and Schnabel, V. and Köhler, M. and Kirchlechner, I. and Kontis, P. and Chen, Y.-T. and Soler, R. and Jaya, B.N. and Kirchlechner, C. and Music, D. and Gault, B. and Schneider, J.M. and Raabe, D. and Dehm, G.
    Frontiers in Materials 7 (2020)
    Quantum mechanically guided materials design has been used to predict the mechanical property trends in crystalline materials. Thereby, the identification of composition-structure-property relationships is enabled. However, quantum mechanics based design guidelines and material selection criteria for ultra-strong metallic glasses have been lacking. Hence, based on an ab initio model for metallic glasses in conjunction with an experimental high-throughput methodology geared toward revealing the relationship between chemistry, topology and mechanical properties, we propose principles for the design of tough as well as stiff metallic glasses. The main design notion is that a low fraction of hybridized bonds compared to the overall bonding in a metallic glass can be used as a criterion for the identification of damage-tolerant metallic glass systems. To enhance the stiffness of metallic glasses, the bond energy density must be increased as the bond energy density is the origin of stiffness in metallic glasses. The thermal expansion, which is an important glass-forming identifier, can be predicted based on the Debye-Grüneisen model. © Copyright © 2020 Evertz, Schnabel, Köhler, Kirchlechner, Kontis, Chen, Soler, Jaya, Kirchlechner, Music, Gault, Schneider, Raabe and Dehm.
    view abstract10.3389/fmats.2020.00089
  • Role of magnetic ordering for the design of quinary TWIP-TRIP high entropy alloys
    Wu, X. and Li, Z. and Rao, Z. and Ikeda, Y. and Dutta, B. and Körmann, F. and Neugebauer, J. and Raabe, D.
    Physical Review Materials 4 (2020)
    We reveal the impact of magnetic ordering on stacking fault energy (SFE) and its influence on the deformation mechanisms and mechanical properties in a class of nonequiatomic quinary Mn-containing compositional complex alloys or high entropy alloys (HEAs). By combining ab initio simulation and experimental validation, we demonstrate magnetic ordering as an important factor in the activation and transition of deformation modes from planar dislocation slip to TWIP (twinning-induced plasticity) and/or TRIP (transformation-induced plasticity). A wide compositional space of Cr20MnxFeyCo20Niz(x+y+z=60, at. %) was probed by density-functional theory calculations to search for potential alloys displaying the TWIP/TRIP effects. Three selected promising HEA compositions with varying Mn concentrations were metallurgically synthesized, processed, and probed for microstructure, deformation mechanism, and mechanical property evaluation. The differences in the deformation modes of the probed HEAs are interpreted in terms of the computed SFEs and their dependence on the predicted magnetic state, as revealed by ab initio calculations and validated by explicit magnetic measurements. It is found that the Mn content plays a key role in the stabilization of antiferromagnetic configurations which strongly impact the SFEs and eventually lead to the prevalent deformation behavior. © 2020 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevMaterials.4.033601
  • Segregation-assisted spinodal and transient spinodal phase separation at grain boundaries
    Darvishi Kamachali, R. and Kwiatkowski da Silva, A. and McEniry, E. and Ponge, D. and Gault, B. and Neugebauer, J. and Raabe, D.
    npj Computational Materials 6 (2020)
    Segregation to grain boundaries affects their cohesion, corrosion, and embrittlement and plays a critical role in heterogeneous nucleation. In order to quantitatively study segregation and low-dimensional phase separation at grain boundaries, here, we apply a density-based phase-field model. The current model describes the grain-boundary thermodynamic properties based on available bulk thermodynamic data, while the grain-boundary-density profile is obtained using atomistic simulations. To benchmark the performance of the model, Mn grain-boundary segregation in the Fe–Mn system is studied. 3D simulation results are compared against atom probe tomography measurements conducted for three alloy compositions. We show that a continuous increase in the alloy composition results in a discontinuous jump in the segregation isotherm. The jump corresponds to a spinodal phase separation at grain boundary. For alloy compositions above the jump, we reveal an interfacial transient spinodal phase separation. The transient spinodal phenomenon opens opportunities for knowledge-based microstructure design through the chemical manipulation of grain boundaries. The proposed density-based model provides a powerful tool to study thermodynamics and kinetics of segregation and phase changes at grain boundaries. © 2020, The Author(s).
    view abstract10.1038/s41524-020-00456-7
  • Snoek-type damping performance in strong and ductile high-entropy alloys
    Lei, Z. and Wu, Y. and He, J. and Liu, X. and Wang, H. and Jiang, S. and Gu, L. and Zhang, Q. and Gault, B. and Raabe, D. and Lu, Z.
    Science Advances 6 (2020)
    Noise and mechanical vibrations not only cause damage to devices, but also present major public health hazards. High-damping alloys that eliminate noise and mechanical vibrations are therefore required. Yet, low operating temperatures and insufficient strength/ductility ratios in currently available high-damping alloys limit their applicability. Using the concept of high-entropy alloy (HEA), we present a class of high-damping materials. The design is based on refractory HEAs, solid-solutions doped with either 2.0 atomic % oxygen or nitrogen, (Ta0.5Nb0.5HfZrTi)98O2 and (Ta0.5Nb0.5HfZrTi)98N2. Via Snoek relaxation and ordered interstitial complexes mediated strain hardening, the damping capacity of these HEAs is as high as 0.030, and the damping peak reaches up to 800 K. The model HEAs also exhibit a high tensile yield strength of ~1400 MPa combined with a large ductility of ~20%. The high-temperature damping properties, together with superb mechanical properties make these HEAs attractive for applications where noise and vibrations must be reduced. © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).
    view abstract10.1126/sciadv.aba7802
  • Solute hydrogen and deuterium observed at the near atomic scale in high-strength steel
    Breen, A.J. and Stephenson, L.T. and Sun, B. and Li, Y. and Kasian, O. and Raabe, D. and Herbig, M. and Gault, B.
    Acta Materialia 188 (2020)
    Observing solute hydrogen (H) in matter is a formidable challenge, yet, enabling quantitative imaging of H at the atomic-scale is critical to understand its deleterious influence on the mechanical strength of many metallic alloys that has resulted in many catastrophic failures of engineering parts and structures. Here, we report on the APT analysis of hydrogen (H) and deuterium (D) within the nanostructure of an ultra-high strength steel with high resistance to hydrogen embrittlement. Cold drawn, severely deformed pearlitic steel wires (Fe–0.98C–0.31Mn–0.20Si–0.20Cr–0.01Cu–0.006P–0.007S wt%, ε=3.1) contains cementite decomposed during the pre-deformation of the alloy and ferrite. We find H and D within the decomposed cementite, and at some interfaces with the surrounding ferrite. To ascertain the origin of the H/D signal obtained in APT, we explored a series of experimental workflows including cryogenic specimen preparation and cryogenic-vacuum transfer from the preparation into a state-of-the-art atom probe. Our study points to the critical role of the preparation, i.e. the possible saturation of H-trapping sites during electrochemical polishing, how these can be alleviated by the use of an outgassing treatment, cryogenic preparation and transfer prior to charging. Accommodation of large amounts of H in the under-stoichiometric carbide likely explains the resistance of pearlite against hydrogen embrittlement. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.02.004
  • Steels in additive manufacturing: A review of their microstructure and properties
    Bajaj, P. and Hariharan, A. and Kini, A. and Kürnsteiner, P. and Raabe, D. and Jägle, E.A.
    Materials Science and Engineering A 772 (2020)
    Today, a large number of different steels are being processed by Additive Manufacturing (AM) methods. The different matrix microstructure components and phases (austenite, ferrite, martensite) and the various precipitation phases (intermetallic precipitates, carbides) lend a huge variability in microstructure and properties to this class of alloys. This is true for AM-produced steels just as it is for conventionally-produced steels. However, steels are subjected during AM processing to time-temperature profiles which are very different from the ones encountered in conventional process routes, and hence the resulting microstructures differ strongly as well. This includes a very fine and highly morphologically and crystallographically textured microstructure as a result of high solidification rates as well as non-equilibrium phases in the as-processed state. Such a microstructure, in turn, necessitates additional or adapted post-AM heat treatments and alloy design adjustments. In this review, we give an overview over the different kinds of steels in use in fusion-based AM processes and present their microstructures, their mechanical and corrosion properties, their heat treatments and their intended applications. This includes austenitic, duplex, martensitic and precipitation-hardening stainless steels, TRIP/TWIP steels, maraging and carbon-bearing tool steels and ODS steels. We identify areas with missing information in the literature and assess which properties of AM steels exceed those of conventionally-produced ones, or, conversely, which properties fall behind. We close our review with a short summary of iron-base alloys with functional properties and their application perspectives in Additive Manufacturing. © 2019 Elsevier B.V.
    view abstract10.1016/j.msea.2019.138633
  • The impact of grain-scale strain localization on strain hardening of a high-Mn steel: Real-time tracking of the transition from the γ → ε → α’ transformation to twinning
    Souza Filho, I.R. and Dutta, A. and Almeida Junior, D.R. and Lu, W. and Sandim, M.J.R. and Ponge, D. and Sandim, H.R.Z. and Raabe, D.
    Acta Materialia 197 (2020)
    Strain partitioning and localization were investigated in a high-Mn steel (17.1 wt.% Mn) during tensile testing by a correlative probing approach including in-situ synchrotron X-ray diffraction, micro- digital image correlation (μ-DIC) and electron microscopy. By combining Warren's theory with the μ-DIC analysis, we monitored the formation of planar faults (stacking faults and mechanical twins) and correlated them with the local strain partitioning behavior within the microstructure. Starting with an initial microstructure of austenite (γ) and athermally formed ε- and α’-martensite, strain accumulates preferentially near the γ/ε interfaces during tensile straining. The local microscopic von Mises strain (εvM) maps obtained from μ-DIC probing show that these local strain gradients produce local strain peaks approximately twice as high as the imposed macroscopic engineering strain (ε), thus locally triggering formation of ε-martensite already at early yielding. The interior of the remaining austenite, without such interfacial strain peaks, remained nearly devoid of planar faults. The local strain-driven growth of the ε-domains occurs concomitantly with the α’-martensite formation. At intermediate macroscopic applied strains, austenite grain size is considerably reduced to a few nanometers and the associated γ/ε interfacial microscopic strain peaks increase in magnitude. This scenario favors twinning to emerge as a competing strain hardening mechanism at engineering strain levels from ε = 0.075 onwards. At large tensile strains, the γ → ε → α’ transformation rates tend to cease making both twinning and SFs formation to operate as the main strain hardening mechanisms. The findings shed light on the transformation micro-mechanisms in multiphase Mn-TRIP steels by revealing how strain localization among the constituents can directly influence the kinetics of the competing strain hardening mechanisms. © 2020
    view abstract10.1016/j.actamat.2020.07.038
  • Ultrastrong lightweight compositionally complex steels via dual-nanoprecipitation
    Wang, Z. and Lu, W. and Zhao, H. and Liebscher, C.H. and He, J. and Ponge, D. and Raabe, D. and Li, Z.
    Science Advances 6 (2020)
    High-performance lightweight materials are urgently needed, given the pressing quest for weight reduction and the associated energy savings and emission reduction. Here, by incorporating the multi-principal element feature of compositionally complex alloys, we develop the concept of lightweight steels further and propose a new class of compositionally complex steels (CCSs). This approach allows us to use the high solid solution strengthening and shift the alloys' compositions into previously unattainable phase regions where both nanosized shearable κ-carbides and non-shearable B2 particles are simultaneously formed. The achievement of dual-nanoprecipitation in our CCSs leads to materials with ultrahigh specific tensile strength (up to 260 MPa·cm3 g-1) and excellent tensile elongation (13 to 38%), a combination outperforming all other high-strength high-entropy alloys and advanced lightweight steels. Our concept of CCSs is thus useful for guiding the design of ultrastrong lightweight metallic materials. Copyright © 2020 The Authors, some rights reserved.
    view abstract10.1126/sciadv.aba9543
  • Unveiling the mechanism of abnormal magnetic behavior of FeNiCoMnCu high-entropy alloys through a joint experimental-theoretical study
    Rao, Z. and Dutta, B. and Körmann, F. and Ponge, D. and Li, L. and He, J. and Stephenson, L. and Schäfer, L. and Skokov, K. and Gutfleisch, O. and Raabe, D. and Li, Z.
    Physical Review Materials 4 (2020)
    We combined experimental investigations and theoretical calculations to unveil an abnormal magnetic behavior caused by addition of the nonmagnetic element Cu in face-centered-cubic FeNiCoMn-based high-entropy alloys (HEAs). Upon Cu addition, the probed HEAs show an increase of both Curie temperature and saturation magnetization in as-cast and homogenized states. Specifically, the saturation magnetization of the as-cast HEAs at room temperature increases by 77% and 177% at a Cu content of 11 and 20 at. %, respectively, compared to the as-cast equiatomic FeNiCoMn HEA without Cu. The increase in saturation magnetization of the as-cast HEAs is associated with the formation of an Fe-Co rich phase in the dendritic regions. For the homogenized HEAs, the magnetic state at room temperature transforms from paramagnetism to ferromagnetism after 20 at. % Cu addition. The increase of the saturation magnetization and Curie temperature cannot be adequately explained by the formation of Cu enriched zones according to atom probe tomography analysis. Ab initio calculations suggest Cu plays a pivotal role in the stabilization of a ferromagnetic ordering of Fe, and reveal an increase of the Curie temperature caused by Cu addition which agrees well with the experimental results. The underlying mechanism behind this phenomenon lies in a combined change in unit-cell volume and chemical composition and the related energetic stabilization of the magnetic ordering upon Cu alloying as revealed by theoretical calculations. Thus, the work unveils the mechanisms responsible for the Cu effect on the magnetic properties of FeNiCoMn HEAs, and suggests that nonmagnetic elements are also crucial to tune and improve magnetic properties of HEAs. © 2020 American Physical Society.
    view abstract10.1103/PhysRevMaterials.4.014402
  • Unveiling the Re effect in Ni-based single crystal superalloys
    Wu, X. and Makineni, S.K. and Liebscher, C.H. and Dehm, G. and Rezaei Mianroodi, J. and Shanthraj, P. and Svendsen, B. and Bürger, D. and Eggeler, G. and Raabe, D. and Gault, B.
    Nature Communications 11 (2020)
    Single crystal Ni-based superalloys have long been an essential material for gas turbines in aero engines and power plants due to their outstanding high temperature creep, fatigue and oxidation resistance. A turning point was the addition of only 3 wt.% Re in the second generation of single crystal Ni-based superalloys which almost doubled the creep lifetime. Despite the significance of this improvement, the mechanisms underlying the so-called “Re effect” have remained controversial. Here, we provide direct evidence of Re enrichment to crystalline defects formed during creep deformation, using combined transmission electron microscopy, atom probe tomography and phase field modelling. We reveal that Re enriches to partial dislocations and imposes a drag effect on dislocation movement, thus reducing the creep strain rate and thereby improving creep properties. These insights can guide design of better superalloys, a quest which is key to reducing CO2 emissions in air-traffic. © 2020, The Author(s).
    view abstract10.1038/s41467-019-14062-9
  • Using spectral-based representative volume element crystal plasticity simulations to predict yield surface evolution during large scale forming simulations
    Han, F. and Diehl, M. and Roters, F. and Raabe, D.
    Journal of Materials Processing Technology 277 (2020)
    We present a new approach to predict the evolution of anisotropic yield functions by coupling large scale forming simulations with crystal plasticity-spectral based virtual experiments, realizing a multi-scale model for metal forming. Employing a fast spectral method solver enables us to conduct on-the-fly full-field virtual experiments to evolve the yield surface at each integration point of the macroscopic finite element model. As illustrative example, two advanced anisotropic yield functions, namely Yld2000-2D and Yld2004-18p, are used in finite element simulations of deep drawing for a 2090-T3 aluminum alloy sheet. The simulated earing profiles are compared to the experimental ones as well as to simulations with non-evolving yield functions. It is found that the prediction of the earing is improved for the case of the evolving Yld2000-2D yield function. The evolution of the plastic anisotropy during cup drawing is systematically analyzed, showing that the evolution of anisotropy can have considerable effect on the prediction accuracy of the macroscopic simulations. © 2019 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2019.116449
  • Yield strength increase of a CoCrNi medium entropy alloy by interstitial nitrogen doping at maintained ductility
    Moravcik, I. and Hadraba, H. and Li, L. and Dlouhy, I. and Raabe, D. and Li, Z.
    Scripta Materialia 178 (2020)
    We show that interstitial nitrogen doping improves the tensile properties of a CoCrNi alloy. A material with 0.5 at% interstitial nitrogen was compared to a nitrogen-free CoCrNi alloy. The nitrogen-doped variant has a stable, single-phase face-centered cubic (FCC) lattice structure without nitrides, also after different annealing treatments (800–900 °C, 10 and 30 min). The nitrogen caused an increase in yield strength by 24–33% at identical ductility compared to the nitrogen-free material with similar grain size. The strengthening effect of nitrogen was explained in terms of the simultaneous increase of the lattice friction stress and of the Hall-Petch coefficient. © 2019
    view abstract10.1016/j.scriptamat.2019.12.007
  • An Automated Computational Approach for Complete In-Plane Compositional Interface Analysis by Atom Probe Tomography
    Peng, Z. and Lu, Y. and Hatzoglou, C. and Kwiatkowski Da Silva, A. and Vurpillot, F. and Ponge, D. and Raabe, D. and Gault, B.
    Microscopy and Microanalysis 25 (2019)
    We introduce an efficient, automated computational approach for analyzing interfaces within atom probe tomography datasets, enabling quantitative mapping of their thickness, composition, as well as the Gibbsian interfacial excess of each solute. Detailed evaluation of an experimental dataset indicates that compared with the composition map, the interfacial excess map is more robust and exhibits a relatively higher resolution to reveal compositional variations. By field evaporation simulations with a predefined emitter mimicking the experimental dataset, the impact of trajectory aberrations on the measurement of the thickness, composition, and interfacial excess of the decorated interface are systematically analyzed and discussed. Copyright © Microscopy Society of America 2019.
    view abstract10.1017/S1431927618016112
  • Application of atom probe tomography to complex microstructures of laser additively manufactured samples
    Kürnsteiner, P. and Hariharan, A. and Jung, H.Y. and Peter, N. and Wilms, M.B. and Weisheit, A. and Barriobero-Vila, P. and Gault, B. and Raabe, D. and Jägle, E.A.
    Microscopy and Microanalysis 25 (2019)
    view abstract10.1017/S1431927619013308
  • Atomic-scale grain boundary engineering to overcome hot-cracking in additively-manufactured superalloys
    Kontis, P. and Chauvet, E. and Peng, Z. and He, J. and da Silva, A.K. and Raabe, D. and Tassin, C. and Blandin, J.-J. and Abed, S. and Dendievel, R. and Gault, B. and Martin, G.
    Acta Materialia 177 (2019)
    There are still debates regarding the mechanisms that lead to hot cracking in parts build by additive manufacturing (AM) of non-weldable nickel-based superalloys. This lack of in-depth understanding of the root causes of hot cracking is an impediment to designing engineering parts for safety-critical applications. Here, we deploy a near-atomic-scale approach to investigate the details of the compositional decoration of grain boundaries in the coarse-grained, columnar microstructure in parts built from a non-weldable nickel-based superalloy by selective electron-beam melting. The progressive enrichment in Cr, Mo and B at grain boundaries over the course of the AM-typical successive solidification and remelting events, accompanied by solid-state diffusion, causes grain boundary segregation induced liquation. This observation is consistent with thermodynamic calculations. We demonstrate that by adjusting build parameters to obtain a fine-grained equiaxed or a columnar microstructure with grain width smaller than 100 μm enables to avoid cracking, despite strong grain boundary segregation. We find that the spread of critical solutes to a higher total interfacial area, combined with lower thermal stresses, helps to suppress interfacial liquation. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.07.041
  • Atomic-scale investigation of hydrogen distribution in a Ti–Mo alloy
    Yan, F. and Mouton, I. and Stephenson, L.T. and Breen, A.J. and Chang, Y. and Ponge, D. and Raabe, D. and Gault, B.
    Scripta Materialia 162 (2019)
    Ingress of hydrogen is often linked to catastrophic failure of Ti-alloys. Here, we quantify the hydrogen distribution in fully β and α + β Ti–Mo alloys by using atom probe tomography. Hydrogen does not segregate at grain boundaries in the fully β sample but segregates at some α/β phase boundaries with a composition exceeding 20 at.% in the α + β sample. No stable hydrides were observed in either sample. The hydrogen concentration in β phases linearly decreases from ~13 at. % to ~4 at. % with increasing Mo-content, which is ascribed to the suppression of hydrogen uptake by Mo addition. © 2018 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2018.11.040
  • Atomistic phase field chemomechanical modeling of dislocation-solute-precipitate interaction in Ni–Al–Co
    Mianroodi, J.R. and Shanthraj, P. and Kontis, P. and Cormier, J. and Gault, B. and Svendsen, B. and Raabe, D.
    Acta Materialia 175 (2019)
    Dislocation-precipitate interaction and solute segregation play important roles in controlling the mechanical behavior of Ni-based superalloys at high temperature. In particular, the increased mobility of solutes at high temperature leads to increased dislocation-solute interaction. For example, atom probe tomography (APT) results [1] for single crystal MC2 superalloy indicate significant segregation of solute elements such as Co and Cr to dislocations and stacking faults in γ′ precipitates. To gain further insight into solute segregation, dislocation-solute interaction, and its effect on the mechanical behavior in such Ni-superalloys, finite-deformation phase field chemomechanics [2] is applied in this work to develop a model for dislocation-solute-precipitate interaction in the two-phase γ-γ′ Ni-based superalloy model system Ni–Al–Co. Identification and quantification of this model is based in particular on the corresponding Ni–Al–Co embedded atom method (EAM) potential [3]. Simulation results imply both Cottrell- and Suzuki-type segregation of Co in γ and γ'. Significant segregation of Co to dislocation cores and faults in γ′ is also predicted, in agreement with APT results. Predicted as well is the drag of Co by γ dislocations entering and shearing γ'. Since solute elements such as Co generally prefer the γ phase, Co depletion in γ′ could be reversed by such dislocation drag. The resulting change in precipitate chemistry may in turn affect its stability and play a role in precipitate coarsening and rafting. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.06.008
  • Calibration of Atom Probe Tomography Reconstructions Through Correlation with Electron Micrographs
    Mouton, I. and Katnagallu, S. and Makineni, S.K. and Cojocaru-Mirédin, O. and Schwarz, T. and Stephenson, L.T. and Raabe, D. and Gault, B.
    Microscopy and Microanalysis 25 (2019)
    Although atom probe tomography (APT) reconstructions do not directly influence the local elemental analysis, any structural inferences from APT volumes demand a reliable reconstruction of the point cloud. Accurate estimation of the reconstruction parameters is crucial to obtain reliable spatial scaling. In the current work, a new automated approach of calibrating atom probe reconstructions is developed using only one correlative projection electron microscopy (EM) image. We employed an algorithm that implements a 2D cross-correlation of microstructural features observed in both the APT reconstructions and the corresponding EM image. We apply this protocol to calibrate reconstructions in a Cu(In,Ga)Se2-based semiconductor and in a Co-based superalloy. This work enables us to couple chemical precision to structural information with relative ease. Copyright © Microscopy Society of America 2019.
    view abstract10.1017/S1431927618016161
  • Carbon and strain partitioning in a quenched and partitioned steel containing ferrite
    Tan, X. and Ponge, D. and Lu, W. and Xu, Y. and Yang, X. and Rao, X. and Wu, D. and Raabe, D.
    Acta Materialia 165 (2019)
    We applied a hot rolling direct quenching and partitioning (HDQ&P) process to a low-C low-Si Al-added steel and obtained a Q&P steel containing 40 vol % of ferrite. Microstructure characterization was performed by means of SEM, EBSD, TEM and XRD. Atomic-scale characterization of carbon partitioning among the phases was carried out by atom probe tomography (APT). The carbon distribution in the retained austenite and near the martensite/retained austenite interfaces was quantitatively analyzed to study its partitioning behavior. The macroscopic strain distribution evolution across the tensile sample surface was investigated using macro digital image correlation (DIC) analysis. Combining these results with joint micro-DIC and EBSD analysis during quasi in-situ tensile testing, we investigated the strain partitioning among the different phases and the TRIP effect. Coupling of these results enabled us to reveal the relation among carbon partitioning, strain partitioning and the TRIP effect. The large blocky retained austenite with a side length of about 300–600 nm located near the ferrite/martensite (F/M) interfaces has low stability and transforms to martensite during the early deformation stages, i.e. at average strain below 21%. The retained austenite films in the centers of the martensite regions are more stable. The carbon distribution in both, the martensite and the retained austenite are inhomogeneous, with 0.5–2.0 at. % in the martensite and 4.0–7.5 at. % in the retained austenite. Strong carbon concentration gradients of up to 1.1 at. %/nm were observed near the martensite/retained austenite interfaces. The large blocky retained austenite (300–600 nm in side length) near the F/M interfaces has 1.5–2.0 at. % lower carbon content than that in the narrow retained austenite films (20–150 nm in thickness). The ferrite is soft and deforms prior to the martensite. The strain distribution in ferrite and martensite is inhomogeneous, varying by up to 20% within the same phase at an average strain of about 20%. Ferrite deformation is the main origin of ductility of the material. The balance between ferrite fraction and martensite morphology controls the TRIP effect and its efficiency in reaching a suited combination of strength and ductility. Reducing the ferrite volume fraction and softening the martensite by coarsening and polygonization can enhance the strain carried by the martensite, thus promoting more retained austenite in the martensite regions enabling a TRIP effect. The enhancement of the TRIP effect and the decrease of the strain contrast between ferrite and martensite jointly optimize the micromechanical deformation compatibility of the adjacent phases, thus improving the material's ductility. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.12.019
  • Carbon partitioning and microstructure evolution during tempering of an Fe-Ni-C steel
    Harding, I. and Mouton, I. and Gault, B. and Raabe, D. and Kumar, K.S.
    Scripta Materialia 172 (2019)
    Partitioning of C during tempering of quenched Fe-9.6Ni-0.5C-0.6Mn-0.6Mo-0.7Cr-0.1V (at.%) steel is determined by atom probe tomography and the resulting microstructure is described. The precipitated austenite size, together with its C and Ni content control its thermal stability and these can vary differently with tempering time and temperature. Thus, both austenite and strong carbide formers compete for the available C early in the process. Due to widely different transport kinetics, C likely plays a dominant role early but is either fully consumed or its role diminishes by dilution, and Ni partitioning eventually takes over as the austenite stability-controlling species. © 2019 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2019.06.036
  • DAMASK – The Düsseldorf Advanced Material Simulation Kit for modeling multi-physics crystal plasticity, thermal, and damage phenomena from the single crystal up to the component scale
    Roters, F. and Diehl, M. and Shanthraj, P. and Eisenlohr, P. and Reuber, C. and Wong, S.L. and Maiti, T. and Ebrahimi, A. and Hochrainer, T. and Fabritius, H.-O. and Nikolov, S. and Friák, M. and Fujita, N. and Grilli, N. and Janssens, K.G.F. and Jia, N. and Kok, P.J.J. and Ma, D. and Meier, F. and Werner, E. and Stricker, M. and Weygand, D. and Raabe, D.
    Computational Materials Science 158 (2019)
    Crystal Plasticity (CP) modeling is a powerful and well established computational materials science tool to investigate mechanical structure–property relations in crystalline materials. It has been successfully applied to study diverse micromechanical phenomena ranging from strain hardening in single crystals to texture evolution in polycrystalline aggregates. However, when considering the increasingly complex microstructural composition of modern alloys and their exposure to—often harsh—environmental conditions, the focus in materials modeling has shifted towards incorporating more constitutive and internal variable details of the process history and environmental factors into these structure–property relations. Technologically important fields of application of enhanced CP models include phase transformations, hydrogen embrittlement, irradiation damage, fracture, and recrystallization. A number of niche tools, containing multi-physics extensions of the CP method, have been developed to address such topics. Such implementations, while being very useful from a scientific standpoint, are, however, designed for specific applications and substantial efforts are required to extend them into flexible multi-purpose tools for a general end-user community. With the Düsseldorf Advanced Material Simulation Kit (DAMASK) we, therefore, undertake the effort to provide an open, flexible, and easy to use implementation to the scientific community that is highly modular and allows the use and straightforward implementation of different types of constitutive laws and numerical solvers. The internal modular structure of DAMASK follows directly from the hierarchy inherent to the employed continuum description. The highest level handles the partitioning of the prescribed field values on a material point between its underlying microstructural constituents and the subsequent homogenization of the constitutive response of each constituent. The response of each microstructural constituent is determined, at the intermediate level, from the time integration of the underlying constitutive laws for elasticity, plasticity, damage, phase transformation, and heat generation among other coupled multi-physical processes of interest. Various constitutive laws based on evolving internal state variables can be implemented to provide this response at the lowest level. DAMASK already contains various CP-based models to describe metal plasticity as well as constitutive models to incorporate additional effects such as heat production and transfer, damage evolution, and athermal transformations. Furthermore, the implementation of additional constitutive laws and homogenization schemes, as well as the integration of a wide class of suitable boundary and initial value problem solvers, is inherently considered in its modular design. © 2018 The Author(s)
    view abstract10.1016/j.commatsci.2018.04.030
  • Deformation compatibility between nanotwinned and recrystallized grains enhances resistance to interface cracking in cyclic loaded stainless steel
    Li, Q. and Yan, F.K. and Tao, N.R. and Ponge, D. and Raabe, D. and Lu, K.
    Acta Materialia 165 (2019)
    Cracks often initiate at phase boundaries in conventional second phase reinforced alloys during cyclic loading, which limits their fatigue properties. Here, we prepared a nanotwin strengthened 316L stainless steel consisting of nanotwinned and recrystallized grains by using plastic deformation and subsequent partial recrystallization annealing. Fatigue tests revealed that interfaces separating hard nanotwinned grains from soft recrystallized ones exhibited excellent resistance to crack initiation. More than half of the cracks (57% in number fraction) are found in recrystallized grains while a small fraction (11%) is observed at the interfaces between nanotwinned and recrystallized grains. This is ascribed to the elastic homogeneity and cyclic deformation compatibility between nanotwinned and recrystallized grains. At small cumulative cyclic strains (below 4000 cycles at σa = 450 MPa), nanotwinned grains deform compatibly with the recrystallized grains without noticeable strain localization at their interfaces. Nanotwins can accommodate cyclic plastic strains by interaction of dislocations with twin boundaries, especially through the motion of the well-ordered threading dislocations inside the twin lamellae. At large cumulative strains, a moderate strain gradient is developed in recrystallized grains surrounding nanotwinned grains as a function of distance from the interfaces due to the occurrence of localized deformation in nanotwinned grains. The nanotwinned grains show high microstructural stability without notable de-twinnning, thus retarding crack initiation and propagation. Therefore, improved fatigue property with high fatigue limit of ∼350 MPa and high fatigue ratio of ∼0.45 is achieved in the nanotwin strengthened stainless steel, which is better than that of conventional second phase reinforced steels with comparable strength. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.11.033
  • Deformation of Borides in Nickel-based Superalloys: A study of segregation at dislocations
    Lilensten, L. and Antonov, S. and Raabe, D. and Tin, S. and Gault, B. and Kontis, P.
    Microscopy and Microanalysis 25 (2019)
    view abstract10.1017/s1431927619013424
  • Deformation-driven bidirectional transformation promotes bulk nanostructure formation in a metastable interstitial high entropy alloy
    Su, J. and Wu, X. and Raabe, D. and Li, Z.
    Acta Materialia 167 (2019)
    We investigate the mechanisms of deformation-driven forward and reverse (bidirectional) martensitic transformation and the associated nanostructure formation in a metastable carbon-doped high entropy alloy (HEA) upon cold rolling. At thickness reductions below 14%, forward hexagonal-close packed (HCP) martensitic transformation prevails in the single face-centered cubic (FCC) matrix. Surprisingly, at the intersections of two crossing HCP lamellae, deformation-induced reverse transformation from the HCP martensite back to the FCC phase occurs. At higher thickness reductions around 26%–34%, multiple deformation kink bands develop, mainly on the pyramidal habit planes of the HCP martensite, among which reverted FCC phase is also observed resulting in a dual-phase nano-laminated structure. The deformation-induced reverted FCC phase regions exhibit a twin stacking sequence relative to the prior FCC matrix, which is related to the underlying dislocation reactions and rearrangement of the partial dislocations. At 67% thickness reduction, the deformation bands develop further into micro-shear bands consisting of nanosized (sub)grains. For rendering the dual-phase nanostructure back to single-phase FCC, 400 °C/10 min tempering is applied on a 34% cold-rolled specimen. The resulting nanostructure is characterized by nano-(sub)grains and nano-twins. It exhibits an excellent strength-ductility synergy (ultimate tensile strength 1.05 GPa at 35% total elongation) due to the improved work hardening enabled by both, FCC-HCP martensitic transformation in confined regions and mechanical twinning. With this, we show that bulk nanostructured alloys with bidirectional transformation can be designed by tuning the materials’ phase stability to their thermodynamic limits with the aim to trigger sequential athermal forward and reverse transformation under load. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.01.030
  • Degradation of iridium oxides via oxygen evolution from the lattice: Correlating atomic scale structure with reaction mechanisms
    Kasian, O. and Geiger, S. and Li, T. and Grote, J.-P. and Schweinar, K. and Zhang, S. and Scheu, C. and Raabe, D. and Cherevko, S. and Gault, B. and Mayrhofer, K.J.J.
    Energy and Environmental Science 12 (2019)
    Understanding the fundamentals of iridium degradation during the oxygen evolution reaction is of importance for the development of efficient and durable water electrolysis systems. The degradation mechanism is complex and it is under intense discussion whether the oxygen molecule can be directly released from the oxide lattice. Here, we define the extent of lattice oxygen participation in the oxygen evolution and associated degradation of rutile and hydrous iridium oxide catalysts, and correlate this mechanism with the atomic-scale structures of the catalytic surfaces. We combine isotope labelling with atom probe tomography, online electrochemical and inductively coupled plasma mass spectrometry. Our data reveal that, unlike rutile IrO2, Ir hydrous oxide contains -IrIIIOOH species which directly contribute to the oxygen evolution from the lattice. This oxygen evolution mechanism results in faster degradation and dissolution of Ir. In addition, near surface bulk regions of hydrous oxide are involved in the oxygen catalysis and dissolution, while only the topmost atomic layers of rutile IrO2 participate in both reactions. Overall our data provide a contribution to the fundamental understanding of the exceptional stability of Ir-oxides towards the oxygen evolution reaction. The proposed approach to a quantitative assessment of the degree of lattice oxygen participation in the oxygen evolution reaction can be further applied to other oxide catalyst systems. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9ee01872g
  • Density, distribution and nature of planar faults in silver antimony telluride for thermoelectric applications
    Abdellaoui, L. and Zhang, S. and Zaefferer, S. and Bueno-Villoro, R. and Baranovskiy, A. and Cojocaru-Mirédin, O. and Yu, Y. and Amouyal, Y. and Raabe, D. and Snyder, G.J. and Scheu, C.
    Acta Materialia 178 (2019)
    Defects such as planar faults in thermoelectric materials improve their performance by scattering phonons with short and medium mean free paths (3–100 nm), thereby reducing the lattice thermal conductivity,κl. Understanding statistically the microscopic distribution of these extended defects within the grains and in low angle grain boundaries is necessary to tailor and develop materials with optimal thermoelectric performance for waste heat harvesting. Herein, we analyze these defects from the millimeter down to the nanometer scale in a AgSbTe2 thermoelectric material with low angle grain boundaries. The investigations were performed using electron channeling contrast imaging combined with transmission electron microscopy. The microstructure study was complemented by estimating the effect of planar faults on the phonon scattering using the Debye-Callaway model. AgSbTe2 is a promising thermoelectric material, which exhibits extremely low thermal conductivity, κ, of 0.5 Wm−1K−1 at room temperature. In contrast to conventional alloys or intermetallic materials, in the present material small angle grain boundaries are not composed of individual dislocations but of a dense arrangement of stacked planar faults with fault densities up to NPF=1.6⋅108m−1. We explain their abundance based on their low interfacial energy of about 186 mJm−2 calculated ab-initio. The current findings show, that it is possible to reach very high densities of phonon-scattering planar faults by the correct microstructure engineering in AgSbTe2 thermoelectric materials. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.07.031
  • Dislocation mechanism based size-dependent crystal plasticity modeling and simulation of gradient nano-grained copper
    Lu, X. and Zhang, X. and Shi, M. and Roters, F. and Kang, G. and Raabe, D.
    International Journal of Plasticity 113 (2019)
    Overcoming the trade-off between strength and ductility in metallic materials is a grand challenge. Recently, materials with a gradient nano-grained (GNG) surface layer adhering to a ductile coarse-grained (CG) substrate have been proposed to overcome this long-standing dilemma. Constitutive modeling and simulation are crucial to understand the deformation mechanisms controlling the strength and ductility in GNG/CG materials, and to enable theory to guide microstructure optimization for upscaling. Here, we develop a dislocation mechanism based size-dependent crystal plasticity model, where multiple dislocation evolution mechanisms are considered. Furthermore, damage evolution and mechanically driven grain growth during the deformation of GNG/CG materials are incorporated into the constitutive model to study the role of microstructure gradient in the overall plastic response. The developed size-dependent constitutive model was implemented within a finite-strain crystal plasticity finite element framework, and used to predict the tensile mechanical behavior of GNG/CG copper, including yield stress, strain-hardening and ductility with a highly simplified geometrical representation of the microstructure. The simulations reveal some of the underlying deformation mechanisms controlling ductility and strengthening in terms of the spatial distribution and temporal evolution of microstructure and damage. The model was also used to demonstrate optimization of strength and ductility of GNG/CG copper. By manipulating the thickness of the GNG layer and the grain size of the CG substrate, the strength increase is associated with a loss of ductility showing the same linear inverse relationship observed experimentally for GNG/CG copper, which indicates the improvement over the typical nonlinear trade-off between strength and ductility. © 2018 Elsevier Ltd.
    view abstract10.1016/j.ijplas.2018.09.007
  • Elemental re-distribution inside shear bands revealed by correlative atom-probe tomography and electron microscopy in a deformed metallic glass
    Balachandran, S. and Orava, J. and Köhler, M. and Breen, A.J. and Kaban, I. and Raabe, D. and Herbig, M.
    Scripta Materialia 168 (2019)
    A density variation in shear bands visible by electron microscopy is correlated with compositionally altered locations measured by atom-probe tomography in plastically-deformed Al 85.6 Y 7.5 Fe 5.8 metallic-glass ribbons. Two compositionally distinct regions are identified along shear bands, one is Al-rich (~92 at.%), the other is Al-depleted (~82.5 at.%) and both regions show marginal concentration fluctuations of Y and Fe. The elemental re-distribution is observed within shear bands only, and no chemical exchange with the surrounding glassy matrix is observed. © 2019
    view abstract10.1016/j.scriptamat.2019.04.014
  • Elemental site occupancy in the L12 A3B ordered intermetallic phase in Co-based superalloys and its influence on the microstructure
    Pandey, P. and Makineni, S.K. and Samanta, A. and Sharma, A. and Das, S.M. and Nithin, B. and Srivastava, C. and Singh, A.K. and Raabe, D. and Gault, B. and Chattopadhyay, K.
    Acta Materialia 163 (2019)
    We explore the effects of the elemental site occupancy in γ′-A3B (L12) intermetallic phases and their partitioning across the γ/γ′ interface in a class of multicomponent W-free Co-based superalloys. Atom probe tomography and first principles density functional theory calculations (DFT) were used to evaluate the Cr site occupancy behavior in the γ′ phase and its effect on the γ/γ′ partitioning behavior of other solutes in a series of Co-30Ni-10Al-5Mo-2Ta-2Ti-XCr alloys, where x is 0, 2, 5, and 8 at.% Cr, respectively. The increase in Cr content from 0 to 2 to 5 at.% leads to an inversion of the partitioning behavior of the solute Mo from the γ′ phase (KMo>1) into the γ matrix (KMo<1). At 5 at.% Cr, the Cr also has a preference to replace the excess anti-site Co atoms from the B-sites. At 8 at.% Cr, the Cr develops an additional preference to replace Co atoms from the A-sites. These compositional changes in the phases and the site partitioning behavior in the γ′ phase are accompanied by an overall decrease in the lattice misfit (δ) across the γ/γ′ interfaces as measured by high-resolution X-ray diffraction at room temperature. The reduction in misfit triggers a change in morphology of the γ′ phase from cuboidal (δ ∼ +0.48% at 0 at.% Cr) to round-cornered (δ ∼ +0.34% at 5 at.% Cr) to spheroidal shaped (δ ∼ +0.19% at 8 at.% Cr) precipitates. We also observed an increase in the solvus temperature from 1066 °C to 1105 °C when adding 5 at.% Cr to the alloy. These results on the effects of Cr in Co-base superalloys enable tuning the microstructure of these alloys and widening the alloy spectrum for designing improved high temperature alloys. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.09.049
  • Engineering atomic-level complexity in high-entropy and complex concentrated alloys
    Oh, H.S. and Kim, S.J. and Odbadrakh, K. and Ryu, W.H. and Yoon, K.N. and Mu, S. and Körmann, F. and Ikeda, Y. and Tasan, C.C. and Raabe, D. and Egami, T. and Park, E.S.
    Nature Communications 10 (2019)
    Quantitative and well-targeted design of modern alloys is extremely challenging due to their immense compositional space. When considering only 50 elements for compositional blending the number of possible alloys is practically infinite, as is the associated unexplored property realm. In this paper, we present a simple property-targeted quantitative design approach for atomic-level complexity in complex concentrated and high-entropy alloys, based on quantum-mechanically derived atomic-level pressure approximation. It allows identification of the best suited element mix for high solid-solution strengthening using the simple electronegativity difference among the constituent elements. This approach can be used for designing alloys with customized properties, such as a simple binary NiV solid solution whose yield strength exceeds that of the Cantor high-entropy alloy by nearly a factor of two. This study provides general design rules that enable effective utilization of atomic level information to reduce the immense degrees of freedom in compositional space without sacrificing physics-related plausibility. © 2019, The Author(s).
    view abstract10.1038/s41467-019-10012-7
  • Erratum to: Enhanced strength and ductility in a high-entropy alloy via ordered oxygen complexes (Nature, (2018), 563, 7732, (546-550), 10.1038/s41586-018-0685-y)
    Lei, Z. and Liu, X. and Wu, Y. and Wang, H. and Jiang, S. and Wang, S. and Hui, X. and Wu, Y. and Gault, B. and Kontis, P. and Raabe, D. and Gu, L. and Zhang, Q. and Chen, H. and Wang, H. and Liu, J. and An, K. and Zeng, Q. and Nieh, T.-G. and Lu, Z.
    Nature 565 (2019)
    Change history: In this Letter, owing to a production error, all the data points (except the two points for O-2 and N-2, respectively) were missing in Fig. 1b. The figure has been corrected online. © 2018, Springer Nature Limited.
    view abstract10.1038/s41586-018-0834-3
  • Experimental and numerical study of mechanical properties of multi-phase medium-Mn TWIP-TRIP steel: Influences of strain rate and phase constituents
    Benzing, J.T. and Liu, Y. and Zhang, X. and Luecke, W.E. and Ponge, D. and Dutta, A. and Oskay, C. and Raabe, D. and Wittig, J.E.
    Acta Materialia 177 (2019)
    In the current work we investigate the room temperature tensile properties of a medium-Mn twinning- and transformation-induced plasticity (TWIP-TRIP) steel from quasi-static to low-dynamic strain rates (ε˙ = 10−4 s−1 to ε˙ = 102 s−1). The multi-phase microstructure consists of coarse-grained recovered α'-martensite (inherited from the cold-rolled microstructure), multiple morphologies of ultrafine-grained (UFG) austenite (equiaxed, rod-like and plate-like), and equiaxed UFG ferrite. The multi-phase material exhibits a positive strain-rate sensitivity for yield and ultimate tensile strengths. Thermal imaging and digital image correlation allow for in situ measurements of temperature and local strain in the gauge length during tensile testing, but Lüders bands and Portevin Le Chatelier bands are not observed. A finite-element model uses empirical evidence from electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), plus constitutive equations to dissect the microstructural influences of grain size, dislocation density and TWIP-TRIP driving forces on tensile properties. Calibration of tensile properties not only captures the strain rate sensitivity of the multi-phase TWIP-TRIP steel, but also provides opportunity for a complete parametric analysis by changing one variable at a time (phase fraction, grain size, strain-induced twin fraction and strain-induced ε-martensite fraction). An equivalent set of high-rate mechanical properties can be matched by changing either the austenite phase fraction or the ratio of twinning vs. transformation to ε-martensite. This experimental-computational framework enables the prediction of mechanical properties in multi-phase steels beyond the experimental regime by tuning variables that are relevant to the alloy design process. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.07.036
  • Hierarchical microstructure design to tune the mechanical behavior of an interstitial TRIP-TWIP high-entropy alloy
    Su, J. and Raabe, D. and Li, Z.
    Acta Materialia 163 (2019)
    We demonstrate a novel approach of utilizing a hierarchical microstructure design to improve the mechanical properties of an interstitial carbon doped high-entropy alloy (HEA) by cold rolling and subsequent tempering and annealing. Bimodal microstructures were produced in the tempered specimens consisting of nano-grains (∼50 nm) in the vicinity of shear bands and recovered parent grains (10–35 μm) with pre-existing nano-twins. Upon annealing, partial recrystallization led to trimodal microstructures characterized by small recrystallized grains (<1 μm) associated with shear bands, medium-sized grains (1–6 μm) recrystallized through subgrain rotation or coalescence of parent grains and retained large un-recrystallized grains. To reveal the influence of these hierarchical microstructures on the strength-ductility synergy, the underlying deformation mechanisms and the resultant strain hardening were investigated. A superior yield strength of 1.3 GPa was achieved in the bimodal microstructure, more than two times higher than that of the fully recrystallized microstructure, owing to the presence of nano-sized grains and nano-twins. The ductility was dramatically improved from 14% to 60% in the trimodal structure compared to the bimodal structure due to the appearance of a multi-stage work hardening behavior. This important strain hardening sequence was attributed to the sequential activation of transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects as a result of the wide variation in phase stability promoted by the grain size hierarchy. These findings open a broader window for achieving a wide spectrum of mechanical properties for HEAs, making better use of not only compositional variations but also microstructure and phase stability tuning. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.10.017
  • High-entropy alloys
    George, E.P. and Raabe, D. and Ritchie, R.O.
    Nature Reviews Materials 4 (2019)
    Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking. © 2019, This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply.
    view abstract10.1038/s41578-019-0121-4
  • Imaging individual solute atoms at crystalline imperfections in metals
    Katnagallu, S. and Stephenson, L.T. and Mouton, I. and Freysoldt, C. and Subramanyam, A.P.A. and Jenke, J. and Ladines, A.N. and Neumeier, S. and Hammerschmidt, T. and Drautz, R. and Neugebauer, J. and Vurpillot, F. and Raabe, D. and Gault, B.
    New Journal of Physics 21 (2019)
    Directly imaging all atoms constituting a material and, maybe more importantly, crystalline defects that dictate materials' properties, remains a formidable challenge. Here, we propose a new approach to chemistry-sensitive field-ion microscopy (FIM) combining FIM with time-of-flight mass-spectrometry (tof-ms). Elemental identification and correlation to FIM images enabled by data mining of combined tof-ms delivers a truly analytical-FIM (A-FIM). Contrast variations due to different chemistries is also interpreted from density-functional theory (DFT). A-FIM has true atomic resolution and we demonstrate how the technique can reveal the presence of individual solute atoms at specific positions in the microstructure. The performance of this new technique is showcased in revealing individual Re atoms at crystalline defects formed in Ni-Re binary alloy during creep deformation. The atomistic details offered by A-FIM allowed us to directly compare our results with simulations, and to tackle a long-standing question of how Re extends lifetime of Ni-based superalloys in service at high-temperature. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/ab5cc4
  • Influence of composition and precipitation evolution on damage at grain boundaries in a crept polycrystalline Ni-based superalloy
    Kontis, P. and Kostka, A. and Raabe, D. and Gault, B.
    Acta Materialia 166 (2019)
    The microstructural and compositional evolution of intergranular carbides and borides prior to and after creep deformation at 850 °C in a polycrystalline nickel-based superalloy was studied. Primary MC carbides, enveloped within intergranular γ′ layers, decomposed resulting in the formation of layers of the undesirable η phase. These layers have a composition corresponding to Ni3Ta as measured by atom probe tomography and their structure is consistent with the D024 hexagonal structure as revealed by transmission electron microscopy. Electron backscattered diffraction reveals that they assume various misorientations with regard to the adjacent grains. As a consequence, these layers act as brittle recrystallized zones and crack initiation sites. The composition of the MC carbides after creep was altered substantially, with the Ta content decreasing and the Hf and Zr contents increasing, suggesting a beneficial effect of Hf and Zr additions on the stability of MC carbides. By contrast, M5B3 borides were found to be microstructurally stable after creep and without substantial compositional changes. Borides at 850 °C were found to coarsen, resulting in some cases into γ′- depleted zones, where, however, no cracks were observed. The major consequences of secondary phases on the microstructural stability of superalloys during the design of new polycrystalline superalloys are discussed. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.12.039
  • Influence of phase decomposition on mechanical behavior of an equiatomic CoCuFeMnNi high entropy alloy
    MacDonald, B.E. and Fu, Z. and Wang, X. and Li, Z. and Chen, W. and Zhou, Y. and Raabe, D. and Schoenung, J. and Hahn, H. and Lavernia, E.J.
    Acta Materialia 181 (2019)
    Phase decomposition is commonly observed experimentally in single-phase high entropy alloys (HEAs). Hence, it is essential for the consideration of HEAs for structural applications to study and understand the nature of phase decomposition in HEAs, particularly the influence it has on mechanical behavior. This paper describes the phase decomposition in the equiatomic CoCuFeMnNi HEA and how the reported secondary phases influence mechanical behavior. Thermomechanical processing, followed by systematic post deformation annealing treatments, revealed the formation of two distinct secondary phases within the equiatomic face-centered cubic (FCC) matrix phase. Low temperature annealing treatments at 600 °C and below led to the nucleation of Fe-Co rich ordered B2 precipitates that contributed precipitation hardening while sufficiently small in size, on the order of 140 nm in diameter. At temperatures <800 °C Cu segregation, due to its immiscibility with the other constituents, eventually forms a Cu-rich disordered FCC phase that is determined to increase the yield strength of the alloy while reducing the ductility, likely attributable to the presence of additional interfaces. The thermal stability and chemistry of these phases are compared to those predicted on the basis of calculated phase diagram (CALPHAD) analyses. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.09.030
  • Invar effects in FeNiCo medium entropy alloys: From an Invar treasure map to alloy design
    Rao, Z. and Ponge, D. and Körmann, F. and Ikeda, Y. and Schneeweiss, O. and Friák, M. and Neugebauer, J. and Raabe, D. and Li, Z.
    Intermetallics 111 (2019)
    To facilitate the understanding of Invar effects and design of FeNiCo-base Invar alloys characterized by low thermal expansion coefficient (TEC), we investigated the magnetic and thermal expansion behavior of an equiatomic prototype medium entropy alloy FeNiCo and a non-equiatomic (super Invar)Fe63Ni32Co5 (at. %)reference alloy by means of experiments and ab initio calculations. Both alloys consist of a single face-centered cubic phase with fully recrystallized microstructure in the homogenized state. Large spontaneous volume magnetostriction is observed in both alloys below their respective Curie temperatures. The Invar effect in the non-equiatomic Fe63Ni32Co5 alloy is of step-type with nearly zero TEC over a wide temperature range (from room temperature to 120 °C)below its Curie temperature. The equiatomic FeNiCo alloy shows a peak-type Invar effect in a very narrow temperature range (from ∼675 °C to ∼730 °C)with relatively low TECs. The equiatomic FeNiCo alloy shows both higher saturation magnetization and Curie temperature than the non-equiatomic Fe63Ni32Co5 alloy. The relationships among magnetic behavior, spontaneous volume magnetostriction and Invar effects for a wider array of metallic alloys are discussed mainly based on Masumoto's rule combined with Wohlfarth's itinerant electron theory. An Invar alloy search map is constructed based on the present results and available literature data to visualize the relationships among saturation magnetization, Curie temperature and thermal expansion coefficient for a wide range of Invar alloys. Based on this treasure map a design route for further developments of new Invar alloys by tuning their magnetic properties is discussed. © 2019 Elsevier Ltd
    view abstract10.1016/j.intermet.2019.106520
  • Joint contribution of transformation and twinning to the high strength-ductility combination of a FeMnCoCr high entropy alloy at cryogenic temperatures
    He, Z.F. and Jia, N. and Ma, D. and Yan, H.L. and Li, Z.M. and Raabe, D.
    Materials Science and Engineering A 759 (2019)
    The microstructure-mechanical property relationships of a non-equiatomic FeMnCoCr high entropy alloy (HEA), which shows a single face-centered cubic (fcc) structure in the undeformed state, have been systematically investigated at room and cryogenic temperatures. Both strength and ductility increase significantly when reducing the probing temperature from 293 K to 77 K. During tensile deformation at 293 K, dislocation slip and mechanical twinning prevail. At 173 K deformation-driven athermal transformation from the fcc phase to the hexagonal close-packed (hcp) martensite is the dominant mechanism while mechanical twinning occurs in grains with high Schmid factors. At 77 K athermal martensitic transformation continues to prevail in addition to dislocation slip and twinning. The reduction in the mean free path for dislocation slip through the fine martensite bundles and deformation twins leads to the further increased strength. The joint activation of transformation and twinning under cryogenic conditions is attributed to the decreased stacking fault energy and the enhanced flow stress of the fcc matrix with decreasing temperature. These mechanisms lead to an elevated strain hardening capacity and an enhanced strength-ductility combination. The temperature-dependent synergy effects of martensite formation, twinning and dislocation plasticity originate from the metastability alloy design concept. This is realized by relaxing the equiatomic HEA constraints towards reduced Ni and increased Mn contents, enabling a non-equiatomic material with low stacking fault energy. These insights are important for designing strong and ductile Ni-saving alloys for cryogenic applications. © 2019 Elsevier B.V.
    view abstract10.1016/j.msea.2019.05.057
  • Light, strong and cost effective: Martensitic steels based on the Fe – Al – C system
    Springer, H. and Zhang, J.-L. and Szczepaniak, A. and Belde, M. and Gault, B. and Raabe, D.
    Materials Science and Engineering A 762 (2019)
    We introduce a novel alloy design concept for density reduced ultra-high strength steels. It is based on the effects of C to increase martensite strength and the Al-solubility in austenite, in conjunction with Al to increase the martensite start temperature and to reduce density. This alloy combination results in inherently strong but light martensitic microstructures, whose mechanical properties (i.e. strength and ductility) can be readily adjusted over a wide range by applying straightforward and established heat treatments. The concept is validated on an Fe – 8 Al – 1.1C (wt.%) alloy subjected to quench and tempering treatments. The steel exhibits relatively low yield strength (~600 MPa) and reasonable ductility (~ 15% elongation) after hot rolling and soft annealing, but can be brought to a maximum hardness of 62 HRC after quenching and tempering at 250 °C. These attractive mechanical properties are coupled with a low density (6.95 g cm–3) and high elastic stiffness (Young's modulus 199 GPa). These lightweight martensitic steels are cost-effective structural materials for weight-critical applications, promising unprecedented specific strength. Advanced characterisation presented herein allows us to derive the fundamental underpinnings of carbide precipitation and phase transformation, and outline and discuss perspectives for refining the alloy composition and processing parameters. © 2019 Elsevier B.V.
    view abstract10.1016/j.msea.2019.138088
  • Macroscopic to nanoscopic in situ investigation on yielding mechanisms in ultrafine grained medium Mn steels: Role of the austenite-ferrite interface
    Sun, B. and Ma, Y. and Vanderesse, N. and Varanasi, R.S. and Song, W. and Bocher, P. and Ponge, D. and Raabe, D.
    Acta Materialia 178 (2019)
    Ultrafine austenite-ferrite duplex medium Mn steels often show a discontinuous yielding phenomenon, which is not commonly observed in other composite-like multiphase materials. The underlying dislocation-based mechanisms are not understood. Here we show that medium Mn steels with an austenite matrix (austenite fraction ∼65 vol%) can exhibit pronounced discontinuous yielding. A combination of multiple in situ characterization techniques from macroscopic (a few millimeters) down to nanoscopic scale (below 100 nm) is utilized to investigate this phenomenon. We observe that both austenite and ferrite are plastically deformed before the macroscopic yield point. In this microplastic regime, plastic deformation starts in the austenite phase before ferrite yields. The austenite-ferrite interfaces act as preferable nucleation sites for new partial dislocations in austenite and for full dislocations in ferrite. The large total interface area, caused by the submicron grain size, can provide a high density of dislocation sources and lead to a rapid increase of mobile dislocations, which is believed to be the major reason accounting for discontinuous yielding in such steels. We simultaneously study the Lüders banding behavior and the local deformation-induced martensite forming inside the Lüders bands. We find that grain size and the austenite stability against deformation-driven martensite formation are two important microstructural factors controlling the Lüders band characteristics in terms of the number of band nucleation sites and their propagation velocity. These factors thus govern the early yielding stages of medium Mn steels, due to their crucial influence on mobile dislocation generations and local work hardening. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.07.043
  • Magnetic properties of a 17.6 Mn-TRIP steel: Study of strain-induced martensite formation, austenite reversion, and athermal α′-formation
    Souza Filho, I.R. and Sandim, M.J.R. and Cohen, R. and Nagamine, L.C.C.M. and Sandim, H.R.Z. and Raabe, D.
    Journal of Magnetism and Magnetic Materials 473 (2019)
    Strain-induced martensite (SIM) formation was evaluated upon cold-rolling of a 17.6 wt.%Mn-TRIP steel by means of magnetic measurements, X-ray diffraction, and high-resolution electron backscatter diffraction (EBSD). α′-martensite formation was observed to be dependent on the presence of prior ε-martensite. Upon deformation, the coercivity of the ferromagnetic α′-martensite is characterized by strong magnetic shape anisotropy. Austenite (γ) reversion was evaluated by means of in situ magnetic measurements during continuous annealing. The experimental results were compared to thermodynamic simulations. It turned out that γ-reversion was not completed in the regime where a γ-single phase field is expected, which suggests the splitting of α′ → γ transformation into two stages. The Curie temperature of remaining α′-martensite was determined as being ∼620 °C. Magnetic properties presented an annealing time-dependence within the temperature range of 500–600 °C, suggesting long-range diffusional α′ → γ transformation. With the aid of electron channeling contrast image technique (ECCI), we noticed that the formation of γ-nanograins in the early stages of reversion is sufficient to induce strong magnetic shape anisotropy in this steel. After full austenitization at 800 °C, further in situ magnetic measurements were also used to track the magnetic response of the material upon controlled cooling. Athermal formation of α′-martensite within the prior athermal ε-phase was clearly observed for temperatures lower than 100 °C. Using thermodynamic modeling we also calculated the start temperature for ε-formation (Ms ε). Results showed that ε-martensite is indeed expected to form before α′ which was confirmed in all cases by means of EBSD. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmmm.2018.10.034
  • Martensite to austenite reversion in a high-Mn steel: Partitioning-dependent two-stage kinetics revealed by atom probe tomography, in-situ magnetic measurements and simulation
    Souza Filho, I.R. and Kwiatkowski da Silva, A. and Sandim, M.J.R. and Ponge, D. and Gault, B. and Sandim, H.R.Z. and Raabe, D.
    Acta Materialia 166 (2019)
    Austenite (γ) reversion in a cold-rolled 17.6 wt.% Mn steel was tracked by means of dilatometry and in-situ magnetic measurements during slow continuous annealing. A splitting of the γ-reversion into two stages was observed to be a result of strong elemental partitioning between γ and α′-martensite during the low temperature stage between 390 and 575 °C. Atom probe tomography (APT) results enable the characterization of the Mn-enriched reversed-γ and the Mn-depleted remaining α′-martensite. Because of its lower Mn content, the reversion of the remaining α′-martensite into austenite takes place at a higher temperature range between 600 and 685 °C. APT results agree with partitioning predictions made by thermo-kinetic simulations of the continuous annealing process. The critical composition for γ-nucleation was predicted by thermodynamic calculations (Thermo-Calc) and a good agreement was found with the APT data. Additional thermo-kinetic simulations were conducted to evaluate partitioning-governed γ-growth during isothermal annealing at 500 °C and 600 °C. Si partitioning to γ was predicted by DICTRA and confirmed by APT. Si accumulates near the moving interface during γ-growth and homogenizes over time. We used the chemical composition of the remaining α′-martensite from APT data to calculate its Curie temperature (TCurie) and found good agreement with magnetic measurements. These results indicate that elemental partitioning strongly influences not only γ-reversion but also the TCurie of this steel. The results are important to better understand the thermodynamics and kinetics of austenite reversion for a wide range of Mn containing steels and its effect on magnetic properties. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.12.046
  • Metastability alloy design
    Raabe, D. and Li, Z. and Ponge, D.
    MRS Bulletin 44 (2019)
    This article reviews the concept of metastability in alloy design. While most materials are thermodynamically metastable at some stage during synthesis and service, we discuss here cases where metastable phases are not coincidentally inherited from processing, but rather are engineered. Specifically, we aim at compositional (partitioning), thermal (kinetics), and microstructure (size effects and confinement) tuning of metastable phases so that they can trigger athermal transformation effects when mechanically, thermally, or electromagnetically loaded. Such a concept works both at the bulk scale and also at a spatially confined microstructure scale, such as at lattice defects. In the latter case, local stability tuning works primarily through elemental partitioning to dislocation cores, stacking faults, interfaces, and precipitates. Depending on stability, spatial confinement, misfit, and dispersion, both bulk and local load-driven athermal transformations can equip alloys with substantial gain in strength, ductility, and damage tolerance. Examples include self-organized metastable nanolaminates, austenite reversion steels, metastable medium- A nd high-entropy alloys, as well as steels and titanium alloys with martensitic phase transformation and twinning-induced plasticity effects. © 2019 Materials Research Society.
    view abstract10.1557/mrs.2019.72
  • Misorientation-dependent solute enrichment at interfaces and its contribution to defect formation mechanisms during laser additive manufacturing of superalloys
    Hariharan, A. and Lu, L. and Risse, J. and Kostka, A. and Gault, B. and Jägle, E.A. and Raabe, D.
    Physical Review Materials 3 (2019)
    A vital issue during selective laser melting of nonweldable polycrystalline nickel-base superalloys is the formation of microcracks. These are cracks occurring during the last stage of solidification and only at high angle grain boundaries (HAGBs). Solute enrichment to the remaining interdendritic liquid and its partial back-diffusion into the solid contributes to the crack nucleation mechanism. Here we use atom probe tomography coupled with transmission Kikuchi diffraction to determine the misorientation and chemical composition profiles across HAGBs (with and without cracks) and across crack-free low angle grain boundaries (LAGBs). The Gibbsian interfacial excess of solutes (mainly B, C, Si, and Zr) is at least two times higher at the HAGB compared to the LAGB. The chemical profiles show the opposite behavior to established model predictions of the last stage of solidification. Our diffusion calculations elucidate that the chemical profiles are influenced by both microsegregation (of Ti, Nb, and Si) during solidification and solid-state segregation (of B, C, and Zr) during cooling. The chemical profiles in the topmost layer indicate a negligible effect of remelting and reheating. Except for Ti-rich carbides, no secondary phases are found. Additionally, we study an alloy with a reduced content of Zr and Si (by at least 60 wt. %), relative to the standard IN738LC composition. We achieved a 99% reduction in crack length per unit area. However, the grain boundary enrichment of Zr and Si in the modified alloy was similar to the standard alloy. Based on these findings, we critically discuss the contribution of various mechanisms proposed for solidification cracking. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.123602
  • Multi-scale characterization of austenite reversion and martensite recovery in a cold-rolled medium-Mn steel
    Benzing, J.T. and Kwiatkowski da Silva, A. and Morsdorf, L. and Bentley, J. and Ponge, D. and Dutta, A. and Han, J. and McBride, J.R. and Van Leer, B. and Gault, B. and Raabe, D. and Wittig, J.E.
    Acta Materialia 166 (2019)
    A medium-Mn steel (Fe-12Mn-3Al-0.05C wt%) was designed using Thermo-Calc ® simulations to balance the fraction and stacking fault energy of reverted austenite. Intercritical annealing for 0.5, 8 and 48 h was carried out at 585 °C to investigate the microstructural evolution. X-ray diffraction (XRD), electron backscatter diffraction (EBSD), 3-dimensional EBSD, energy-dispersive spectroscopy via scanning-transmission electron microscopy (STEM-EDS) and atom probe tomography (APT) enable characterization of phase fraction, grain area, grain morphology and alloy partitioning. An increase in annealing time from 0.5 h to 48 h increases the amount of ultrafine-grained (UFG) reverted austenite from 3 to 40 vol %. EBSD and TEM reveal multiple morphologies of UFG austenite (equiaxed, rod-like and plate-like). In addition, most of the remaining microstructure consists of recovered α′-martensite that resembles the cold-rolled state, as well as a relatively small fraction of UFG ferrite (i.e., only a small amount of martensite recrystallization occurs). Multi-scale characterization results show that the location within the cold-rolled microstructure has a strong influence on boundary mobility and grain morphology during austenite reversion. Results from APT reveal Mn-decoration of dislocation networks and low-angle lath boundaries in the recovered α′-martensite, but an absence of Mn-decoration of defects in the vicinity of austenite grains, thereby promoting recovery. STEM-EDS and APT reveal Mn depletion zones in the ferrite/recovered α′-martensite near austenite boundaries, whereas gradients of C and Mn co-partitioning are visible within some of the austenite grains after annealing for 0.5 h. Relatively flat C-enriched austenite boundaries are present even after 8 h of annealing and indicate certain boundaries possess low mobility. At later stages the growth of austenite followed the local equilibrium (LE) model such that the driving force between two equilibrium phases moves the mobile interface, as confirmed by DICTRA simulations (a Thermo-Calc ® diffusion module). The sequence of austenite reversion is: (i) formation of Mn- and C-enriched face-centered-cubic nuclei from decorated dislocations and/or particles; (ii) co-partitioning of Mn and C and (iii) growth of austenite controlled by the LE mode. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.01.003
  • Nonbasal Slip Systems Enable a Strong and Ductile Hexagonal-Close-Packed High-Entropy Phase
    Bu, Y. and Li, Z. and Liu, J. and Wang, H. and Raabe, D. and Yang, W.
    Physical Review Letters 122 (2019)
    Linear defects, referred to as dislocations, determine the strength, formability, and toughness of crystalline metallic alloys. The associated deformation mechanisms are well understood for traditional metallic materials consisting of one or two prevalent matrix elements such as steels or aluminum alloys. In the recently developed high-entropy alloys (HEAs) containing multiple principal elements, the relationship between dislocations and the mechanical behavior is less understood. Particularly HEAs with a hexagonal close-packed (hcp) structure can suffer from intrinsic brittleness due to their insufficient number of slip systems. Here we report on the surprisingly high formability of a novel high-entropy phase with hcp structure. Through in situ tensile testing and postmortem characterization by transmission electron microscopy we reveal that the hcp phase in a dual-phase HEA (Fe50Mn30Co10Cr10, at. %) activates three types of dislocations, i.e., a ©, ccopy;, and +a©. Specifically, nonbasal c+a© dislocations occupy a high line fraction of ∼31% allowing for frequent double cross slip which explains the high deformability of this high-entropy phase. The hcp structure has a c/a ratio of 1.616, i.e., below the ideal value of 1.633. This modest change in the structure parameters promotes nonbasal c+a© slip, suggesting that ductile HEAs with hcp structure can be designed by shifting the c/a ratio into regimes where nonbasal slip systems are activated. This simple alloy design principle is particularly suited for HEAs due to their characteristic massive solid solution content which readily allows tuning the c/a ratio of hcp phases into regimes promoting nonbasal slip activation. © 2019 American Physical Society.
    view abstract10.1103/PhysRevLett.122.075502
  • On the compositional partitioning during phase transformation in a binary ferromagnetic MnAl alloy
    Palanisamy, D. and Raabe, D. and Gault, B.
    Acta Materialia 174 (2019)
    We introduce a new perspective on the classical massive mode of solid-state phase transformation enabled by the correlative use of atomic-scale electron microscopy and atom probe tomography. This is demonstrated in a binary MnAl alloy which has Heusler-like characteristics. In this system, the τ phase formed by a massive transformation from the high-temperature ε phase is metastable and ferromagnetic. The transformation results in a high density of micro-twins inside the newly grown τ phase. Atomic-scale compositional analysis across the interface boundaries and atomic structure of the micro-twins reveals the involvement of both structural modification and also the compositional partitioning during the growth of the τ phase. This is assisted by the migrating τ/ε interface boundary during transformation. Finally, the role of micro-twins on nucleating the equilibrium phases and the influence of the defects and phase formation on the magnetic properties are discussed. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.05.037
  • On the influence of the heat treatment on microstructure formation and mechanical properties of near-α Ti-Fe alloys
    Sandlöbes, S. and Korte-Kerzel, S. and Raabe, D.
    Materials Science and Engineering A 748 (2019)
    We study the microstructure formation and mechanical properties of Ti-1Fe (wt%) and Ti-3Fe (wt%) alloys for different heat treatments in the β-phase and α + β-phase regions. By applying different heat treatment routes, we observe different microstructure formation mechanisms causing a wide range of mechanical properties from high strength (1.3 GPa) and low ductility (2%) to intermediate strength (700 MPa) and high ductility (30%) in these simple binary alloys. We performed microstructure characterizsation using scanning electron microscopy, transmission electron microscopy and atom probe tomography to show that the alloying content and heat treatment significantly affect the local martensitic and / or diffusional phase transformations causing the substantial changes in the mechanical behavior. © 2018
    view abstract10.1016/j.msea.2018.12.071
  • On the interaction of precipitates and tensile twins in magnesium alloys
    Liu, C. and Shanthraj, P. and Robson, J.D. and Diehl, M. and Dong, S. and Dong, J. and Ding, W. and Raabe, D.
    Acta Materialia 178 (2019)
    Although magnesium alloys deform extensively through shear strains and crystallographic re-orientations associated with the growth of twins, little is known about the strengthening mechanisms associated with this deformation mode. A crystal plasticity based phase field model for twinning is employed in this work to study the strengthening mechanisms resulting from the interaction between twin growth and precipitates. The full-field simulations reveal in great detail the pinning and de-pinning of a twin boundary at individual precipitates, resulting in a maximum resistance to twin growth when the precipitate is partially embedded in the twin. Furthermore, statistically representative precipitate distributions are used to systematically investigate the influence of key microstructural parameters such as precipitate orientation, volume fraction, size, and aspect ratio on the resistance to twin growth. The results indicate that the effective critical resolved shear stress (CRSS) for twin growth increases linearly with precipitate volume fraction and aspect ratio. For a constant volume fraction of precipitates, reduction of the precipitate size below a critical level produces a strong increase in the CRSS due to the OROWAN-like strengthening mechanism between the twin interface and precipitates. Above this level the CRSS is size independent. The results are quantitatively and qualitatively comparable with experimental measurements and predictions of mean-field strengthening models. Based on the results, guidelines for the design of high strength magnesium alloys are discussed. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.07.046
  • On the mechanism of extraordinary strain hardening in an interstitial high-entropy alloy under cryogenic conditions
    Wang, Z. and Lu, W. and Raabe, D. and Li, Z.
    Journal of Alloys and Compounds (2019)
    We investigate the cryogenic deformation response and underlying mechanisms of a carbon-doped interstitial high-entropy alloy (iHEA) with a nominal composition of Fe49.5Mn30Co10Cr10C0.5 (at. %). Extraordinary strain hardening of the iHEA at 77 K leads to a substantial increase in ultimate tensile strength (∼1300 MPa) with excellent ductility (∼50%) compared to that at room temperature. Prior to loading, iHEAs with coarse (∼100 μm) and fine (∼6 μm) grain sizes show nearly single face-centered cubic (FCC) structure, while the fraction of hexagonal close-packed (HCP) phase reaches up to ∼70% in the cryogenically tensile-fractured iHEAs. Such an unusually high fraction of deformation-induced phase transformation and the associated plasticity (TRIP effect) is caused by the strong driving force supported by the reduced stacking fault energy and increased flow stress at 77 K. The transformation mechanism from the FCC matrix to the HCP phase is revealed by transmission electron microscopy (TEM) observations. In addition to the deformation-induced phase transformation, stacking faults and dislocation slip contribute to the deformation of the FCC matrix phase at low strains and of the HCP phase at medium and large strains, suggesting dynamic strain partitioning among these two phases. The combination of TRIP and dynamic strain partitioning explain the striking strain hardening capability and resulting excellent combination of strength and ductility of iHEAs under cryogenic conditions. The current investigation thus offers guidance for the design of high-performance HEAs for cryogenic applications. © 2018 Elsevier B.V.
    view abstract10.1016/j.jallcom.2018.12.061
  • Quantification Challenges for Atom Probe Tomography of Hydrogen and Deuterium in Zircaloy-4
    Mouton, I. and Breen, A.J. and Wang, S. and Chang, Y. and Szczepaniak, A. and Kontis, P. and Stephenson, L.T. and Raabe, D. and Herbig, M. and Britton, T.B. and Gault, B.
    Microscopy and Microanalysis 25 (2019)
    Analysis and understanding of the role of hydrogen in metals is a significant challenge for the future of materials science, and this is a clear objective of recent work in the atom probe tomography (APT) community. Isotopic marking by deuteration has often been proposed as the preferred route to enable quantification of hydrogen by APT. Zircaloy-4 was charged electrochemically with hydrogen and deuterium under the same conditions to form large hydrides and deuterides. Our results from a Zr hydride and a Zr deuteride highlight the challenges associated with accurate quantification of hydrogen and deuterium, in particular associated with the overlap of peaks at a low mass-to-charge ratio and of hydrogen/deuterium containing molecular ions. We discuss possible ways to ensure that appropriate information is extracted from APT analysis of hydrogen in zirconium alloy systems that are important for nuclear power applications. Copyright © Microscopy Society of America 2019.
    view abstract10.1017/S143192761801615X
  • Quantification of solute deuterium in titanium deuteride by atom probe tomography with both laser pulsing and high-voltage pulsing: Influence of the surface electric field
    Chang, Y.H. and Mouton, I. and Stephenson, L. and Ashton, M. and Zhang, G.K. and Szczpaniak, A. and Lu, W.J. and Ponge, D. and Raabe, D. and Gault, B.
    New Journal of Physics 21 (2019)
    Atom probe tomography (APT) has been increasingly used to investigate hydrogen embrittlement in metals due to its unique capacity for direct imaging of H atoms interacting with microstructural features. The quantitativeness of hydrogen measurements by APT is yet to be established in views of erroneous compositional measurements of bulk hydrides and the influence of spurious hydrogen, e.g. residual gas inside the analysis chamber. Here, we analyzed titanium deuteride (approx. 65.0 at%-66.6 at% D) in lieu of hydride to minimize the overlap with residual gas, both with laser pulsing and high-voltage (HV) pulsing. Strategies were deployed to prevent H pick-up during specimen fabrication, including preparing specimens at cryogenic temperature. The measured composition of deuterium by APT with laser pulsing decreases significantly with the applied laser pulse energy, which is interpreted with regards to the strength of the corresponding surface electrostatic field, as assessed by the evolution of charge-state ratio. In contrast, compositional analyses with HV pulsing are roughly independent of the applied experimental parameters, although approx. 15 at%-20 at% off the nominal composition. Aided by plotting paired mass-to-charge correlations, the mechanisms of composition bias in both pulsing modes are discussed. A special emphasis is placed on the local variations of the measured composition as a function of the local electric field across the specimen's surface, which is not uniform due to asymmetric heat distribution related to the localized laser absorption and the faceted nature of surface caused by the crystallographic structure. Our investigations demonstrate the challenges of quantitative analysis of solute deuterium by APT but nevertheless provide insight to achieving the best possible experimental protocol. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/ab1c3b
  • Quasi-Fermi-Level Splitting of Cu -Poor and Cu -Rich CuIn S2 Absorber Layers
    Lomuscio, A. and Rödel, T. and Schwarz, T. and Gault, B. and Melchiorre, M. and Raabe, D. and Siebentritt, S.
    Physical Review Applied 11 (2019)
    Cu(In,Ga)S2-based solar cells are interesting tandem partners for Si or chalcopyrite solar cells, but suffer from a low open-circuit voltage. Recently, record efficiencies have been achieved by using higher growth temperatures for the absorber. To understand the effect of higher growth temperatures, we investigate the structural and electronic properties of CuInS2 absorbers. By investigating the absorber alone as opposed to complete solar cells, we can separate changes in the absorber from effects of the interface properties. We show that the quasi-Fermi-level splitting, which indicates the maximum open-circuit voltage an absorber is capable of, increases with higher growth temperature. The quasi-Fermi-level splitting is limited by a deep defect, the concentration of which decreases with higher growth temperature and is less prominent in Cu-rich films. Thus, we demonstrate that the open-circuit voltage of CuInS2-based solar cells is limited to below 850 mV by the absorber itself, independent of the interface. In contrast to the changes in the electronic properties, the structural properties are rather independent of temperature within the range investigated but are significantly influenced by the composition. © 2019 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevApplied.11.054052
  • Reconstructing the austenite parent microstructure of martensitic steels: A case study for reduced-activation Eurofer steels
    Almeida Junior, D.R. and Zilnyk, K.D. and Raabe, D. and Sandim, H.R.Z.
    Journal of Nuclear Materials 516 (2019)
    A computer program was employed to reconstruct the parent microstructure from electron backscatter diffraction maps taken from martensite. The reconstruction is based on a given user-selected orientation relationship. Two reduced-activation ferritic-martensitic Eurofer steels were austenitized for several times and cooled down at different rates. Two parameters are proposed to assess the quality of the reconstruction: the relative frequency of orientations in a determined sub-portion of the Euler space and the distribution of the angular deviation from a given theoretical orientation relationship. The number of active martensitic orientation variants during the transformation depends on grain size. Coarser grains enable a greater number of active variants improving the reconstruction quality. The distributions of angular deviations reveal that the Greninger-Troiano orientation relationship is the one that best describes the martensitic transformation in both steel grades. © 2019 Elsevier B.V.
    view abstract10.1016/j.jnucmat.2019.01.022
  • Revealing fracture mechanisms of medium manganese steels with and without delta-ferrite
    Sun, B. and Palanisamy, D. and Ponge, D. and Gault, B. and Fazeli, F. and Scott, C. and Yue, S. and Raabe, D.
    Acta Materialia 164 (2019)
    Medium Mn steels possess a composite like microstructure containing multiple phase constituents like metastable austenite, ferrite, δ-ferrite and α′-martensite with a wide range of fractions for each constituent. The high mechanical contrast among them and the deformation-driven evolution of the microstructure lead to complex fracture mechanisms. Here we investigate tensile fracture mechanisms of medium Mn steels with two typical types of microstructures. One group consists of ferrite (α) plus austenite (γ) and the other one of a layered structure with an austenite-ferrite constituent and δ-ferrite. Samples with the first type of microstructure show a dimple-type fracture due to void formation primarily at the ferrite/strain-induced α′-martensite (α′) interfaces. In contrast, the fracture surface of δ-ferrite containing steels shows a combination of cleavage in δ-ferrite and dimple/quasi-cleavage zones in the γ-α (or γ/α′-α) constituent. The embrittlement of δ-ferrite is due to the formation of B2 ordered phase. Failure of these samples is govern by crack initiation related to δ-ferrite and crack-arresting ability of the γ-α layers. Austenite stability is critical for the alloys' fracture resistance, in terms of influencing void growth and coalescence for the first type of microstructure and crack initiation and termination for the microstructure containing δ-ferrite. This effect is here utilized to increase ductility and toughness. By tailoring austenite stability towards higher fracture resistance, the total elongation of δ-ferrite containing steels increases from ∼13% to ∼33%. This approach opens a new pathway towards an austenite-stability-controlled microstructural design for substantially enhanced damage tolerance in steels containing metastable austenite and δ-ferrite. © 2018
    view abstract10.1016/j.actamat.2018.11.029
  • Role of elemental intermixing at the In 2 S 3 /CIGSe heterojunction deposited using reactive RF magnetron sputtering
    Soni, P. and Raghuwanshi, M. and Wuerz, R. and Berghoff, B. and Knoch, J. and Raabe, D. and Cojocaru-Mirédin, O.
    Solar Energy Materials and Solar Cells 195 (2019)
    In the present work, sputtered In 2 S 3 buffer layers are deposited on Cu(In,Ga)Se 2 absorbers with no or minimal sputter damage. Buffer deposition at slower sputter rates (0.22 Å/s) with H 2 S as a reactive gas improved the interface quality and uniformity. We obtained crystalline In 2 S 3 films at room temperature with the deposition parameters used in this work. Elemental intermixing effect at the In 2 S 3 /CIGSe heterointerface at different annealing temperatures was mapped in 3-dimensions using atom probe tomography (APT). APT results revealed the induced sputter damage during buffer layer deposition, and the effect of diffusion and segregation of elements at the heterointerface. Knowledge of elemental redistribution at the buffer-absorber heterointerface can help better understand the relation of the chemical intermixing with electrical performance of the cell. X-ray photoelectron spectroscopy (XPS) reveal accelerated Cu + doping of the In 2 S 3 buffer at 225 °C. Cu-depletion on CIGSe surface up to a few nanometers improves the cell performance. However, at higher annealing temperature of 275 °C, CuIn 5 S 8 and Cu(In x Ga 1-x ) 5 Se 8 phases are formed which is detrimental for cell performance. Na diffusing from the CIGSe absorber segregates at the In 2 S 3 /CIGSe heterojunction. This actively suppresses the formation of charged antisites defects, lowering the recombination in the space charged region. Cu-depletion at CIGSe surface, Cu and Na diffusion in In 2 S 3 and passivation of interfacial defect states by Na, giving an efficiency of 13.84% (14.83% with CdS-reference) for sputtered In 2 S 3 buffer layers. Consequently, this work elucidates the chemistry of buried hetero-interfaces and their significance to improve the electrical performance of solar cells. © 2019 Elsevier B.V.
    view abstract10.1016/j.solmat.2019.03.026
  • Segregation-driven grain boundary spinodal decomposition as a pathway for phase nucleation in a high-entropy alloy
    Li, L. and Li, Z. and Kwiatkowski da Silva, A. and Peng, Z. and Zhao, H. and Gault, B. and Raabe, D.
    Acta Materialia 178 (2019)
    Elemental segregation to grain boundaries (GBs) can induce structural and chemical transitions at GBs along with significant changes in material properties. The presence of multiple principal elements interacting in high-entropy alloys (HEAs) makes the GB segregation and interfacial phase transformation a rather challenging subject to investigate. Here, we explored the temporal evolution of the chemistry for general high-angle GBs in a typical equiatomic FeMnNiCoCr HEA during aging heat treatment through detailed atom probe tomography (APT) analysis. We found that the five principal elements segregate heterogeneously at the GBs. More specifically, Ni and Mn co-segregate to some regions of the GBs along with the depletion of Fe, Co and Cr, while Cr is enriched in other regions of the GBs where Ni and Mn are depleted. The redistribution of these elements on the GBs follow a periodic characteristic, spinodal-like compositional modulation. The accumulation of elements at the GBs can create local compositions by shifting their state from a solid solution (like in the adjacent bulk region) into a spinodal regime to promote interfacial phase-like transitions as segregation proceeds. These results not only shed light on phase precursor states and the associated nucleation mechanism at GBs in alloy systems with multiple principal elements but also help to guide the microstructure design of advanced HEAs in which formation of embrittling phases at interfaces must be avoided. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.07.052
  • Shape-preserving machining produces gradient nanolaminate medium entropy alloys with high strain hardening capability
    Guo, W. and Pei, Z. and Sang, X. and Poplawsky, J.D. and Bruschi, S. and Qu, J. and Raabe, D. and Bei, H.
    Acta Materialia 170 (2019)
    A high density of grain boundaries can potentially increase structural materials' strength, but at the expense of losing the materials' strain hardening ability at high flow stress levels. However, endowing materials with grain size gradients and a high density of internal interfaces can simultaneously increase the strength and strain hardening ability. This applies particularly for through-thickness gradients of nanoscale interface structures. Here we apply a machining method that produces metals with nanoscale interface gradients. Conventional bulk plastic deformation such as rolling, a process applied annually to about 2 billion tons of material, aims to reduce the metal thickness. We have modified this process by introducing severe strain path changes, realized by leading the sheet through a U-turn while preserving its shape, an approach known as ‘hard turning’. We applied this process at both room temperature and 77 K to a NiCrCo medium entropy alloy. Micropillar compression was conducted to evaluate the mechanical response. After hard turning at room temperature, the surface microstructure obtained a ∼50% increase in yield stress (0.9 GPa) over the original state with homogeneous grain size (0.4 GPa), but the initial strain hardening rate did not show significant improvement. However, after hard turning at 77 k, the gradient nanolaminate structure tripled in yield stress and more than doubled its initial strain hardening rate. The improvements were achieved by introducing a specific microstructure that consists of gradient nanolaminates in the form of nanospaced twins and martensite in the face center cubic (fcc) phase. This microstructure was formed only at cryogenic temperature. It was found after turning at room temperature that only nanospaced twins were present in the fcc phase inside nanolaminates that had formed at the surface. The origin of the enhanced strain hardening mechanism was studied. Joint density functional theory (DFT) and axial next nearest neighbor Ising (ANNNI) models were used to explain the temperature-dependent phase formation of the NiCrCo nanolaminate at the surface of the hard-turned material. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.03.024
  • Site-specific quasi in situ investigation of primary static recrystallization in a low carbon steel
    Diehl, M. and Kertsch, L. and Traka, K. and Helm, D. and Raabe, D.
    Materials Science and Engineering A 755 (2019)
    Low-alloyed steels with body-centered cubic crystal structure are a material class that is widely used for sheet metal forming applications. When having an adequate crystallographic texture and microstructure, their mechanical behavior is characterized by an isotropic in-plane flow behavior in combination with a low yield strength. The decisive processing steps for obtaining these beneficial mechanical properties are cold rolling and subsequent annealing. While for the former the number of passes, the deformation rates, and the total thickness reduction are the main processing parameters, the latter is described mainly by the heating rate and the holding temperature and time. Primary static recrystallization during annealing subsequent to the cold rolling process alters mainly two aspects of the material state: It firstly replaces the elongated and heavily deformed grains of the cold rolled microstructure by small, globular grains with low dislocation density and secondly it changes the crystallographic texture insofar as it typically diminishes the α- and strengthens the γ-fiber texture components. In the present work, the recrystallization behavior of a commercial non-alloyed low carbon steel is studied. A quasi in situ setup that enables site-specific characterization is employed to gain a local picture of the nucleation and recrystallization process. From the Kernel Average Misorientation (KAM) values of the deformation structure, the tendency to be consumed by new grains can be predicted. Crystallographic analysis shows that the most deformed regions have either a γ-fiber orientation or belong to heavily fragmented regions. New grains nucleate especially in such highly deformed regions and inherit often the orientation from the deformation microstructure. © 2019 Elsevier B.V.
    view abstract10.1016/j.msea.2019.02.032
  • Spectral solvers for crystal plasticity and multi-physics simulations
    Shanthraj, P. and Diehl, M. and Eisenlohr, P. and Roters, F. and Raabe, D.
    Handbook of Mechanics of Materials (2019)
    The local and global behavior of materials with internal microstructure is often investigated on a (representative) volume element. Typically, periodic boundary conditions are applied on such “virtual specimens” to reflect the situation in the bulk of the material. Spectral methods based on Fast Fourier Transforms (FFT) have been established as a powerful numerical tool especially suited for this task. Starting from the pioneering work of Moulinec and Suquet, FFT-based solvers have been significantly improved with respect to performance and stability. Recent advancements of using the spectral approach to solve coupled field equations enable also the modeling of multiphysical phenomena such as fracture propagation, temperature evolution, chemical diffusion, and phase transformation in conjunction with the mechanical boundary value problem. The fundamentals of such a multi-physics framework, which is implemented in the Düsseldorf Advanced Materials Simulation Kit (DAMASK), are presented here together with implementation aspects. The capabilities of this approach are demonstrated on illustrative examples. © Springer Nature Singapore Pte Ltd. 2019.
    view abstract10.1007/978-981-10-6884-3_80
  • Sputtering as a viable route for In2S3 buffer layer deposition in high efficiency Cu(In,Ga)Se2 solar cells
    Soni, P. and Raghuwanshi, M. and Wuerz, R. and Berghoff, B. and Knoch, J. and Raabe, D. and Cojocaru-Mirédin, O.
    Energy Science and Engineering 7 (2019)
    Alternative buffer layers in CIGSe are deposited mainly using chemical bath deposition because of its benefits like simplicity, good film quality and surface/step coverage. All the layers in CIGSe cell stack such as back contact, absorber and window layers are deposited by vacuum–deposition methods such as coevaporation, sputtering, and sometimes thermal evaporation, except for the buffer layer. Therefore, in the present work we demonstrate the feasibility to deposit In2S3 by RF magnetron sputtering reaching cell efficiencies of 13.6%, which is the highest value available for sputtered In2S3 in literature to date. Absorber surface damage and nonuniform buffer layer thickness are the primary limitations when using sputtering, and hence need to be eliminated for reaching reasonable cell efficiencies. We studied the extent of sputter induced damage on CIGSe absorber as well as the sputtering– and annealing–induced intermixing phenomenon at the In2S3/Cu(In,Ga)Se2 interface at the subnanometer level using atom probe tomography. We have also shown that a post deposition annealing not only significantly improves the crystallinity of In2S3, but also recovers the surface damage caused by sputter-induced intermixing resulting in an improved p-n Junction quality (as shown by the electron beam induced current investigations), and substantially improves cell efficiency. The present work opens a new way for designing efficient and industry–compatible CIGSe cells using sputter–deposited Cd-free buffer layers. Moreover, this work clearly demonstrates that this novel and fully vacuum–deposited CIGSe cell meets the standard requirements, in terms of chemistry, structure, and electrical performance of a working cell for the PV industry. © 2019 The Authors. Energy Science & Engineering published by the Society of Chemical Industry and John Wiley & Sons Ltd.
    view abstract10.1002/ese3.295
  • Strain hardening mechanisms during cold rolling of a high-Mn steel: Interplay between submicron defects and microtexture
    Souza Filho, I.R. and Sandim, M.J.R. and Ponge, D. and Sandim, H.R.Z. and Raabe, D.
    Materials Science and Engineering A 754 (2019)
    The formation of submicron structural defects within austenite (γ), ε- and α′-martensite during cold rolling was followed in a 17.6 wt.% Mn steel. Several probes, including XRD, EBSD, and ECCI-imaging, were used to reveal the complex superposition of the strain hardening mechanisms of these phases. The maximum amount of ε-martensite is observed at a strain of ε = 0.11. At larger strains, the amount of ε decreases suggesting that it precedes the α′-formation (γ → ε → α′). Stacking faults and twins are the main planar defects noticed in ε-martensite. The remaining γ is finely subdivided by stacking faults and twins up to ε = 0.22. From ε = 0.51 on, twinning and multiplication of dislocations are the principal strain hardening mechanisms in austenite. Deformation is accommodated in α′ by the rearrangement of dislocation tangles into dislocation cells plus shear banding at ε = 1.56. During cold rolling, austenite develops a Brass-type texture component, which can be associated to mechanical twinning. ε-martensite presents its basal planes tilted ∼24° from the normal direction towards the rolling direction. The α′-martensite develops and strengthens both, the bcc α- and γ-texture fibers during cold rolling. © 2019 Elsevier B.V.
    view abstract10.1016/j.msea.2019.03.116
  • Strain partitioning and strain localization in medium manganese steels measured by in situ microscopic digital image correlation
    Dutta, A. and Ponge, D. and Sandlöbes, S. and Raabe, D.
    Materialia 5 (2019)
    In situ microscopic-digital image correlation (µ-DIC) is used to investigate the strain partitioning and strain localization behavior in a medium manganese steel. Continuous yielding results from strain partitioning with higher strain in the reverted austenite (γ R ) islands and less strain in the tempered martensite (α temp ′ ) matrix, both in hot and cold rolled material. µ-DIC experiments are performed to further understand the effects of texture and grain morphology on strain partitioning which cannot be locally resolved through high resolution x-ray or neutron diffraction experiments. Apart from strain partitioning, strain localization is observed in hot rolled samples within colonies of lamellar γ R islands. This localization does not only depend on the crystallographic orientation, but also on the spatial alignment of an austenite island relative to the loading direction. The effects of texture, spatial and colony alignment are interpreted within the concept of a relative grain size effect resulting in different yield stresses in the hot and cold rolled samples showing continuous yielding. Strain partitioning and strain localization based on texture and spatial alignment can be extended to numerous dual phase morphologies with similar texture, colony and spatial alignment effects. © 2019
    view abstract10.1016/j.mtla.2019.100252
  • Strategies for improving the sustainability of structural metals
    Raabe, D. and Tasan, C.C. and Olivetti, E.A.
    Nature 575 (2019)
    Metallic materials have enabled technological progress over thousands of years. The accelerated demand for structural (that is, load-bearing) alloys in key sectors such as energy, construction, safety and transportation is resulting in predicted production growth rates of up to 200 per cent until 2050. Yet most of these materials require a lot of energy when extracted and manufactured and these processes emit large amounts of greenhouse gases and pollution. Here we review methods of improving the direct sustainability of structural metals, in areas including reduced-carbon-dioxide primary production, recycling, scrap-compatible alloy design, contaminant tolerance of alloys and improved alloy longevity. We discuss the effectiveness and technological readiness of individual measures and also show how novel structural materials enable improved energy efficiency through their reduced mass, higher thermal stability and better mechanical properties than currently available alloys. © 2019, Springer Nature Limited.
    view abstract10.1038/s41586-019-1702-5
  • The role of lattice defects, element partitioning and intrinsic heat effects on the microstructure in selective laser melted Ti-6Al-4V
    Haubrich, J. and Gussone, J. and Barriobero-Vila, P. and Kürnsteiner, P. and Jägle, E.A. and Raabe, D. and Schell, N. and Requena, G.
    Acta Materialia 167 (2019)
    The microstructure and phase composition in selective laser melted (SLM) Ti-6Al-4V plays a key role for its mechanical performance. The microstructure evolution in SLM Ti-6Al-4V was studied in the as-built condition and after sub-transus heat treatments between 400 °C and 800 °C focusing on elemental partitioning and the role of lattice defects on precipitation of the β phase. With SLM parameters corresponding to low volume energy density (E V = 77 J/mm 3 ) the as-built microstructure consisted of acicular martensite and showed a higher density of lattice defects than that synthesized under high E V = 145 J/mm 3 condition. High energy X-ray synchrotron diffraction indicated the presence of ∼2 wt.% β-phase at this high E V . Moreover, atom-probe tomography revealed enrichments in β-stabilizers at one- and two-dimensional lattice defects. These fine enriched one-dimensional columnar and two-dimensional features are identified as precursors of β-phase, revealing the role of lattice defects for β-precipitation. Upon annealing at 400 °C and 530 °C, β-films began to fragment into β−platelets and nanoparticles, whereas annealing at 800 °C led to a coarse-lamellar α/β-microstructure. Moreover, α 2 -Ti 3 Al was found in the 400 °C annealed condition. In line with the microstructure changes, Vickers hardness increased upon annealing at temperatures up to 530 °C and dropped when coarsening occurred at higher temperatures. Substantial element partitioning occurred during thermally driven martensite decomposition, which was significantly stronger for Fe than for V. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.01.039
  • The through-process texture analysis of plate rolling by coupling finite element and fast Fourier transform crystal plasticity analysis
    Fujita, N. and Igi, S. and Diehl, M. and Roters, F. and Raabe, D.
    Modelling and Simulation in Materials Science and Engineering 27 (2019)
    The demand for heavy-gauge steel plates for use in the large structures has been increasing steadily. Controlled rolling is an effective method of both increasing the strength of steel plates and improving their toughness. Techniques for controlling the texture of polycrystalline materials are very important because texture distribution has a strong effect not only on the characteristics of steel plates but also on their mechanical and toughness properties. However, it is difficult to quantitatively evaluate the evolution of the plate rolling texture through experimentation because the plate rolling process includes recrystallization and transformation. For this purpose, a recently developed hybrid numerical approach that involves the integrated use of the crystal plasticity (CP) finite element method and the CP fast Fourier transform method was employed in this study. With this hybrid method, the influence of the plate rolling conditions on the deformed/transformed texture evolution and the influence of texture distribution in the thickness direction on the fracture toughness can be captured. The through-process texture analysis presented in this paper provides new insights into the prediction of texture, microstructure, and mechanical response in the industrial processes which are hard to monitor directly. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-651X/ab4143
  • Thermodynamics of grain boundary segregation, interfacial spinodal and their relevance for nucleation during solid-solid phase transitions
    Kwiatkowski da Silva, A. and Kamachali, R.D. and Ponge, D. and Gault, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 168 (2019)
    Grain boundary segregation, embrittlement and phase nucleation are interconnected phenomena that are often treated separately, which is partly due to limitations of the current models to predict grain boundary segregation in non-ideal solid solutions. Here, a simple model is introduced to predict grain boundary segregation in solid solutions by coupling available bulk thermodynamic data with a mean-field description of the grain boundary character. The model is confronted with experimental results obtained in Fe-Mn alloys analysed by atom probe tomography. This model successfully predicts a first order transition or a discontinuous jump in the composition of the grain boundary which kinetically implies the formation of spinodal Mn fluctuations that tend to grow further with time within the segregated region. The increase in solute concentration at the grain boundary leads to an increase of the enthalpy of the boundary and to its embrittlement at lower temperatures. Once austenite is formed, the amount of segregated solute Mn on the grain boundaries is drastically reduced and the toughness of the grain boundary is increased. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.02.005
  • Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials
    Chang, Y. and Lu, W. and Guénolé, J. and Stephenson, L.T. and Szczpaniak, A. and Kontis, P. and Ackerman, A.K. and Dear, F.F. and Mouton, I. and Zhong, X. and Zhang, S. and Dye, D. and Liebscher, C.H. and Ponge, D. and Korte-Kerzel, S. and Raabe, D. and Gault, B.
    Nature Communications 10 (2019)
    Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale. © 2019, The Author(s).
    view abstract10.1038/s41467-019-08752-7
  • Ultrastrong Medium-Entropy Single-Phase Alloys Designed via Severe Lattice Distortion
    Sohn, S.S. and Kwiatkowski da Silva, A. and Ikeda, Y. and Körmann, F. and Lu, W. and Choi, W.S. and Gault, B. and Ponge, D. and Neugebauer, J. and Raabe, D.
    Advanced Materials 31 (2019)
    Severe lattice distortion is a core effect in the design of multiprincipal element alloys with the aim to enhance yield strength, a key indicator in structural engineering. Yet, the yield strength values of medium- and high-entropy alloys investigated so far do not substantially exceed those of conventional alloys owing to the insufficient utilization of lattice distortion. Here it is shown that a simple VCoNi equiatomic medium-entropy alloy exhibits a near 1 GPa yield strength and good ductility, outperforming conventional solid-solution alloys. It is demonstrated that a wide fluctuation of the atomic bond distances in such alloys, i.e., severe lattice distortion, improves both yield stress and its sensitivity to grain size. In addition, the dislocation-mediated plasticity effectively enhances the strength–ductility relationship by generating nanosized dislocation substructures due to massive pinning. The results demonstrate that severe lattice distortion is a key property for identifying extra-strong materials for structural engineering applications. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adma.201807142
  • Understanding the role of cu and clustering on strain hardening and strain rate sensitivity of al-mg-si-cu alloys
    Langille, M. and Diak, B.J. and De Geuser, F. and Guiglionda, G. and Meddeb, S. and Zhao, H. and Gault, B. and Raabe, D. and Deschamps, A.
    Minerals, Metals and Materials Series (2019)
    Increased demand for light-weighting in passenger vehicles has created a need for strong, light, ductile materials to be used in body-in-white applications. The AA6xxx-series of aluminum alloys are suitable candidates meeting most requirements but can fall short of the formability demands of designers, necessitating an understanding of what controls the formability in this alloy series. This work examines the effect of copper alloying in AA6xxx on the pre-ageing and natural ageing responses of the microstructure and mechanical properties. The changes in microstructure observed by differential scanning calorimetry and hardness testing are related to the work-hardening and strain-rate sensitivity parameters for these alloys measured by tensile testing. An observed asymmetry in the measured strain-rate sensitivity associated with increasing versus decreasing strain rate changes suggests that a different mechanism operates for the two conditions. It is postulated how this asymmetry in strain-rate sensitivity will impact the necking and ductility behaviour of these alloys. © 2019, The Minerals, Metals & Materials Society.
    view abstract10.1007/978-3-030-05864-7_20
  • Unraveling the Metastability of Cn 2+ (n = 2-4) Clusters
    Peng, Z. and Zanuttini, D. and Gervais, B. and Jacquet, E. and Blum, I. and Choi, P.-P. and Raabe, D. and Vurpillot, F. and Gault, B.
    Journal of Physical Chemistry Letters 10 (2019)
    Pure carbon clusters have received considerable attention for a long time. However, fundamental questions, such as what the smallest stable carbon cluster dication is, remain unclear. We investigated the stability and fragmentation behavior of Cn 2+ (n = 2-4) dications using state-of-the-art atom probe tomography. These small doubly charged carbon cluster ions were produced by laser-pulsed field evaporation from a tungsten carbide field emitter. Correlation analysis of the fragments detected in coincidence reveals that they only decay to Cn-1 + + C+. During C2 2+ ? C+ + C+, significant kinetic energy release (5.75-7.8 eV) is evidenced. Through advanced experimental data processing combined with ab initio calculations and simulations, we show that the field-evaporated diatomic 12C2 2+ dications are either in weakly bound 3?u and 3Sg - states, quickly dissociating under the intense electric field, or in a deeply bound electronic 5Su - state with lifetimes &gt;180 ps. © Copyright © 2019 American Chemical Society.
    view abstract10.1021/acs.jpclett.8b03449
  • Variable chemical decoration of extended defects in Cu-poor C u2ZnSnS e4 thin films
    Schwarz, T. and Redinger, A. and Siebentritt, S. and Peng, Z. and Gault, B. and Raabe, D. and Choi, P.-P.
    Physical Review Materials 3 (2019)
    We report on atom probe tomography studies of variable chemical decorations at extended defects in Cu-poor and Zn-rich Cu2ZnSnSe4 thin films. For a precursor film, which was co-evaporated at 320C, grain boundaries and dislocations are found enriched with Cu. Furthermore, Na out-diffusion from the soda-lime glass substrate occurs even at such a low temperature, resulting in Na segregation at defects. In contrast, stacking faults in the precursor film show clear Zn enrichment as well as Cu and Sn depletion. After an annealing step at 500C, we detect changes in the chemical composition of grain boundaries as compared to the precursor. Moreover, we measure an increase in the grain boundary excess of Na by one order of magnitude. We show that grain boundaries and dislocations in the annealed Cu2ZnSnSe4 film exhibit no or only slight variations in composition of the matrix elements. Thus, the effect of annealing is a homogenization of the chemical composition. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.035402
  • Advanced data mining in field ion microscopy
    Katnagallu, S. and Gault, B. and Grabowski, B. and Neugebauer, J. and Raabe, D. and Nematollahi, A.
    Materials Characterization (2018)
    Field ion microscopy (FIM) allows to image individual surface atoms by exploiting the effect of an intense electric field. Widespread use of atomic resolution imaging by FIM has been hampered by a lack of efficient image processing/data extraction tools. Recent advances in imaging and data mining techniques have renewed the interest in using FIM in conjunction with automated detection of atoms and lattice defects for materials characterization. After a brief overview of existing routines, we review the use of machine learning (ML) approaches for data extraction with the aim to catalyze new data-driven insights into high electrical field physics. Apart from exploring various supervised and unsupervised ML algorithms in this context, we also employ advanced image processing routines for data extraction from large sets of FIM images. The outcomes and limitations of such routines are discussed, and we conclude with the possible application of energy minimization schemes to the extracted point clouds as a way of improving the spatial resolution of FIM. © 2018 Elsevier Inc.
    view abstract10.1016/j.matchar.2018.02.040
  • Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials
    Yu, Y. and Zhang, S. and Mio, A.M. and Gault, B. and Sheskin, A. and Scheu, C. and Raabe, D. and Zu, F. and Wuttig, M. and Amouyal, Y. and Cojocaru-Mirédin, O.
    ACS Applied Materials and Interfaces 10 (2018)
    Dislocations have been considered to be an efficient source for scattering midfrequency phonons, contributing to the enhancement of thermoelectric performance. The structure of dislocations can be resolved by electron microscopy whereas their chemical composition and decoration state are scarcely known. Here, we correlate transmission Kikuchi diffraction and (scanning) transmission electron microscopy in conjunction with atom probe tomography to investigate the local structure and chemical composition of dislocations in a thermoelectric Ag-doped PbTe compound. Our investigations indicate that Ag atoms segregate to dislocations with a 10-fold excess of Ag compared with its average concentration in the matrix. Yet the Ag concentration along the dislocation line is not constant but fluctuates from ∼0.8 to ∼10 atom % with a period of about 5 nm. Thermal conductivity is evaluated applying laser flash analysis, and is correlated with theoretical calculations based on the Debye-Callaway model, demonstrating that these Ag-decorated dislocations yield stronger phonon scatterings. These findings reduce the knowledge gap regarding the composition of dislocations needed for theoretical calculations of phonon scattering and pave the way for extending the concept of defect engineering to thermoelectric materials. © 2018 American Chemical Society.
    view abstract10.1021/acsami.7b17142
  • An integrated crystal plasticity-phase field model for spatially resolved twin nucleation, propagation, and growth in hexagonal materials
    Liu, C. and Shanthraj, P. and Diehl, M. and Roters, F. and Dong, S. and Dong, J. and Ding, W. and Raabe, D.
    International Journal of Plasticity 106 (2018)
    Typical hexagonal engineering materials, such as magnesium and titanium, deform extensively through shear strains and crystallographic re-orientations associated with the nucleation, propagation, and growth of twins. To accurately predict their deformation behavior it is, therefore, critical for constitutive models to incorporate these mechanisms. In this work an integrated approach for modeling the concurrent dislocation mediated plasticity and heterogeneous twinning behavior in hexagonal materials is presented. A dislocation density-based crystal plasticity model is employed to predict the heterogeneous distribution of stress, strain and dislocation activity and is coupled to a phase field model for the description of the nucleation, propagation, and growth of {1012} tensile twins. A stochastic model is used to nucleate twins at grain boundaries, and their subsequent propagation and growth are driven by the Ginzburg-Landau relaxation of the system free energy which includes the orientation dependent twin interfacial energy and the elastic strain energy. Application of this novel and fully coupled model to the cases of magnesium single crystal, bicrystal, and polycrystal deformation is shown to demonstrate its predictive capability. Numerical simulations predict, in accordance with experimental observations, twin nucleation at grain boundaries followed by twin propagation into the grain interior and subsequent transverse twin thickening. Through this new combination of modeling approaches it is possible to systematically study the twin induced strain fields, the stress distribution along twin boundaries, and the spatial evolution of dislocation density within twins and parent grains. © 2018 Elsevier Ltd.
    view abstract10.1016/j.ijplas.2018.03.009
  • Atomic scale analysis of grain boundary deuteride growth front in Zircaloy-4
    Breen, A.J. and Mouton, I. and Lu, W. and Wang, S. and Szczepaniak, A. and Kontis, P. and Stephenson, L.T. and Chang, Y. and da Silva, A.K. and Liebscher, C.H. and Raabe, D. and Britton, T.B. and Herbig, M. and Gault, B.
    Scripta Materialia 156 (2018)
    Zircaloy-4 (Zr-1.5%Sn-0.2%Fe-0.1%Cr wt%) was electrochemically charged with deuterium to create deuterides and subsequently analysed with atom probe tomography and scanning transmission electron microscopy to understand zirconium hydride formation and embrittlement. At the interface between the hexagonal close packed (HCP) α-Zr matrix and a face centred cubic (FCC) δ deuteride (ZrD1.5–1.65), a HCP ζ phase deuteride (ZrD0.25–0.5) has been observed. Furthermore, Sn is rejected from the deuterides and segregates to the deuteride/α-Zr reaction front. © 2018 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2018.06.044
  • Atomic-scale insights into surface species of electrocatalysts in three dimensions
    Li, T. and Kasian, O. and Cherevko, S. and Zhang, S. and Geiger, S. and Scheu, C. and Felfer, P. and Raabe, D. and Gault, B. and Mayrhofer, K.J.J.
    Nature Catalysis 1 (2018)
    The topmost atomic layers of electrocatalysts determine the mechanism and kinetics of reactions in many important industrial processes, such as water splitting, chlor-electrolysis or fuel cells. Optimizing the performance of electrocatalysts requires a detailed understanding of surface-state changes during the catalytic process, ideally at the atomic scale. Here, we use atom probe tomography to reveal the three-dimensional structure of the first few atomic layers of electrochemically grown iridium oxide, an efficient electrocatalyst for the oxygen evolution reaction. We unveil the formation of confined, non-stoichiometric Ir-O species during oxygen evolution. These species gradually transform to IrO2, providing improved stability but also a decrease in activity. Additionally, electrochemical growth of oxide in deuterated solutions allowed us to trace hydroxy-groups and water molecules present in the regions of the oxide layer that are favourable for the oxygen evolution and iridium dissolution reactions. Overall, we demonstrate how tomography with near-atomic resolution advances the understanding of complex relationships between surface structure, surface state and function in electrocatalysis. © 2018 The Author(s).
    view abstract10.1038/s41929-018-0043-3
  • Boron doped ultrastrong and ductile high-entropy alloys
    Seol, J.B. and Bae, J.W. and Li, Z. and Chan Han, J. and Kim, J.G. and Raabe, D. and Kim, H.S.
    Acta Materialia 151 (2018)
    A new class of materials called high-entropy alloys (HEAs) constitutes multiple principal elements in similar compositional fractions. The equiatomic Fe20Mn20Cr20Co20Ni20 (at%) HEA shows attractive mechanical properties, particularly under cryogenic conditions. Yet, it lacks sufficient yield and ultimate tensile strengths at room temperature. To strengthen these materials, various strategies have been proposed mainly by tuning the composition of the bulk material while no efforts have been made to decorate and strengthen the grain boundaries. Here, we introduce a new HEA design approach that is based on compositionally conditioning the grain boundaries instead of the bulk. We found that as little as 30 ppm of boron doping in single-phase HEAs, more specific in an equiatomic FeMnCrCoNi and in a non-equiatomic Fe40Mn40Cr10Co10 (at%), improves dramatically their mechanical properties, increasing their yield strength by more than 100% and ultimate tensile strength by ∼40% at comparable or even better ductility. Boron decorates the grain boundaries and acts twofold, through interface strengthening and grain size reduction. These effects enhance grain boundary cohesion and retard capillary driven grain coarsening, thereby qualifying boron-induced grain boundary engineering as an ideal strategy for the development of advanced HEAs. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.04.004
  • Characterizing solute hydrogen and hydrides in pure and alloyed titanium at the atomic scale
    Chang, Y. and Breen, A.J. and Tarzimoghadam, Z. and Kürnsteiner, P. and Gardner, H. and Ackerman, A. and Radecka, A. and Bagot, P.A.J. and Lu, W. and Li, T. and Jägle, E.A. and Herbig, M. and Stephenson, L.T. and Moody, M.P. and Rugg, D. and Dye, D. and Ponge, D. and Raabe, D. and Gault, B.
    Acta Materialia 150 (2018)
    Ti and its alloys have a high affinity for hydrogen and are typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and α+β binary model alloys. No hydrides are observed in the α phase in alloys with Al addition or quenched-in Mo supersaturation. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.02.064
  • Combinatorial metallurgical synthesis and processing of high-entropy alloys
    Li, Z. and Ludwig, Al. and Savan, A. and Springer, H. and Raabe, D.
    Journal of Materials Research (2018)
    High-entropy alloys (HEAs) with multiple principal elements open up a practically infinite space for designing novel materials. Probing this huge material universe requires the use of combinatorial and high-throughput synthesis and processing methods. Here, we present and discuss four different combinatorial experimental methods that have been used to accelerate the development of novel HEAs, namely, rapid alloy prototyping, diffusion-multiples, laser additive manufacturing, and combinatorial co-deposition of thin-film materials libraries. While the first three approaches are bulk methods which allow for downstream processing and microstructure adaptation, the latter technique is a thin-film method capable of efficiently synthesizing wider ranges of composition and using high-throughput measurement techniques to characterize their structure and properties. Additional coupling of these high-throughput experimental methodologies with theoretical guidance regarding specific target features such as phase (meta)stability allows for effective screening of novel HEAs with beneficial property profiles. Copyright © Materials Research Society 2018
    view abstract10.1557/jmr.2018.214
  • Competition between formation of carbides and reversed austenite during tempering of a medium-manganese steel studied by thermodynamic-kinetic simulations and atom probe tomography
    Kwiatkowski da Silva, A. and Inden, G. and Kumar, A. and Ponge, D. and Gault, B. and Raabe, D.
    Acta Materialia 147 (2018)
    We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-0.1C-0.5Si medium manganese steel during low temperature tempering. The material was annealed up to 24 h at 450 °C in order to follow the kinetics of precipitation. Using thermodynamics and kinetics simulations, we predicted the growth of M23C6 carbides according to the local-equilibrium negligible partition (LENP) mode where carbide growth is controlled by the diffusion of carbon, while maintaining local chemical equilibrium at the interface. Atom-probe tomography (APT) measurements performed on samples annealed for 1, 6 and 24 h at 450 °C confirmed that LENP is indeed the mode of carbide growth and that Mn segregation is necessary for the nucleation. Additionally, we observed the heterogeneous nucleation of transition carbides with a carbon content between 6 and 8 at% at segregated dislocations and grain boundaries. We describe these carbides as a complex face-centered cubic transition carbide type (CFCC-TmC phase) obtained by the supersaturation of the FCC structure by carbon that will act as precursor to the more stable γ-M23C6 carbide that forms at the dislocations and grain boundaries. The results suggest that the addition of carbon does not directly favor the formation of austenite, since Mn is consumed by the formation of the carbides and the nucleation of austenite is thus retarded to later stages of tempering as every FCC nucleus in the initial stages of tempering is readily converted into a carbide nucleus. We propose the following sequence of transformation: (i) carbon and Mn co-segregation to dislocations and grain boundaries; (ii) formation of FCC transition carbides; (iii) growth controlled according to the LENP mode and (iv) austenite nucleation and growth. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.01.022
  • Compositional evolution of long-period stacking ordered structures in magnesium studied by atom probe tomography
    Kim, J.-K. and Guo, W. and Choi, P.-P. and Raabe, D.
    Scripta Materialia 156 (2018)
    Mg alloys containing long-period stacking ordered (LPSO) structures are strong and ductile compared to conventional Mg alloys. We study here the compositional evolution of LPSO structures in a Mg97Y2Zn1 (at.%) alloy upon annealing at 500 °C using atom probe tomography. In the material annealed for 2.5 h, the Zn/Y ratio of the building blocks in the interdendritic LPSO phase (0.73) is close to the stoichiometric composition of Y8Zn6 L12 clusters while that in plate-type LPSO structures (0.66) slightly deviates from the ideal value. The Y/Zn enrichment in LPSO structures in the α-Mg matrix slightly decreases with increasing annealing time. © 2018 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2018.07.017
  • Correlative Microscopy—Novel Methods and Their Applications to Explore 3D Chemistry and Structure of Nanoscale Lattice Defects: A Case Study in Superalloys
    Makineni, S.K. and Lenz, M. and Kontis, P. and Li, Z. and Kumar, A. and Felfer, P.J. and Neumeier, S. and Herbig, M. and Spiecker, E. and Raabe, D. and Gault, B.
    JOM (2018)
    Nanoscale solute segregation to or near lattice defects is a coupled diffusion and trapping phenomenon that occurs in superalloys at high temperatures during service. Understanding the mechanisms underpinning this crucial process will open pathways to tuning the alloy composition for improving the high-temperature performance and lifetime. Here, we introduce an approach combining atom probe tomography with high-end scanning electron microscopy techniques, in transmission and backscattering modes, to enable direct investigation of solute segregation to defects generated during high-temperature deformation such as dislocations in a heat-treated Ni-based superalloy and planar faults in a CoNi-based superalloy. Three protocols were elaborated to capture the complete structural and compositional nature of the targeted defect in the alloy. © 2018 The Author(s)
    view abstract10.1007/s11837-018-2802-7
  • Correlative transmission Kikuchi diffraction and atom probe tomography study of Cu(In,Ga)Se2 grain boundaries
    Schwarz, T. and Stechmann, G. and Gault, B. and Cojocaru-Mirédin, O. and Wuerz, R. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 26 (2018)
    We combined transmission Kikuchi diffraction and atom probe tomography techniques to investigate the relationship between the structure and chemistry of grain boundaries in Cu(In,Ga)Se2 thin films. Kikuchi patterns with the tetragonal structure of Cu(In,Ga)Se2 were simulated to emphasize the pseudosymmetry issue in this material system and, hence, the orientation determination ambiguity in case of indexing with a cubic zinc-blende structure. We compared these patterns with experimental data. We detect an elemental redistribution at random high-angle grain boundaries but no chemical fluctuations at Σ3 twin boundaries. The atom probe tomography analyses reveal Cu depletion as well as In and Se enrichment at random grain boundaries and, at some random grain boundaries, a slight Ga depletion. This In on Cu scenario is accompanied by cosegregation of Na and K originating from the soda-lime glass substrate. The amount of impurity segregation does vary not only from one grain boundary to another but also along an individual grain boundary. Hence, our results suggest that the degree of passivation of detrimental, nonradiative recombination centers does differ not only between Σ3 twin boundaries and random grain boundaries but also within the same random grain boundary. Copyright © 2017 John Wiley & Sons, Ltd.
    view abstract10.1002/pip.2966
  • Corrosion behavior of an equiatomic CoCrFeMnNi high-entropy alloy compared with 304 stainless steel in sulfuric acid solution
    Luo, H. and Li, Z. and Mingers, A.M. and Raabe, D.
    Corrosion Science (2018)
    The corrosion resistance and passive film properties of an equiatomic CoCrFeMnNi high-entropy alloy (HEA) compared with 304L stainless steel in 0.1 M H2SO4 solution were investigated. The in-situ element-resolved corrosion analysis shows that selective dissolution of elements in the HEA is not evident compared to a 304 L stainless steel during passivation. The passive film formed on the HEA is enriched in Fe and Mn but depleted in Cr. The low content of Cr and the extensive formation of metal hydroxide in the passive film are responsible for the lower anti-corrosion performance of the HEA. © 2018 Elsevier Ltd.
    view abstract10.1016/j.corsci.2018.02.031
  • Crystallographic examination of the interaction between texture evolution, mechanically induced martensitic transformation and twinning in nanostructured bainite
    Morales-Rivas, L. and Archie, F. and Zaefferer, S. and Benito-Alfonso, M. and Tsai, S.-P. and Yang, J.-R. and Raabe, D. and Garcia-Mateo, C. and Caballero, F.G.
    Journal of Alloys and Compounds 752 (2018)
    The deformation mechanisms operating in nanostructured bainite, leading to its excellent combination of strength and ductility, are far from being understood. Its nanocrystalline nature and its multiphase-evolving structure underlie the plastic flow and the strain-hardening behaviour. In this work, the microstructural and crystallographic bulk changes of a high-C nanostructured bainite under tensile testing have been evaluated. The influence of the mechanically-induced transformation of the C-enriched retained austenite into α martensite and other deformation mechanisms on the texture evolution has been analysed by electron backscatter diffraction (EBSD). Additionally, the undeformed and the deformed conditions have been examined by electron channelling contrast imaging (ECCI) and transmission electron microscopy (TEM). Results reveal the presence of plate martensite and suggest a strong variant selection during the transformation, mainly responsible for the texture observed. Mechanical twinning in austenite seems to be basically the mechanism of accommodation of the displacive bainitic transformation, while some direct interaction with the applied stress also appears. © 2018
    view abstract10.1016/j.jallcom.2018.04.189
  • Deformation induced degradation of hot-dip aluminized steel
    Lemmens, B. and Springer, H. and Peeters, M. and De Graeve, I. and De Strycker, J. and Raabe, D. and Verbeken, K.
    Materials Science and Engineering A 710 (2018)
    In this work the fracture and corrosion behaviour of hot-dip aluminized steels is investigated in controlled dipping experiments which allowed to separately study the effects of Si in the Al bath (1–10 wt%) and the intermetallic phase thickness (5–30 µm). The addition of Si had no direct influence on the performance of the coating system for similar thickness values of the IMP seam, which in turn showed to be the dominant factor independent from the amount of Si. Thin intermetallic phase seams (< about 10 µm) exhibited more (about 5–10 per 100 µm interfacial length) but smaller cracks with a fishnet pattern on the outer Al-Si coating, which remained intact and interconnected until a tensile deformation of 15–20%. Thicker intermetallic phase seams resulted in less (about 2 per 100 µm interfacial length) but broader cracks perpendicular to the tensile direction, giving rise to a lamellar pattern on the Al-Si coating, which cracks and uncovers the steel already at strains below 10%, and readily flakes off leaving the steel substrate to accelerated corrosion in chloride environments. Our results indicate that the reduction of the intermetallic phase seam thickness remains the main target to improve the performance of hot-dip aluminized coated steel by combining appropriate Si additions with minimized dipping temperatures and times. © 2017 Elsevier B.V.
    view abstract10.1016/j.msea.2017.10.094
  • Deformation mechanisms, activated slip systems and critical resolved shear stresses in an Mg-LPSO alloy studied by micro-pillar compression
    Chen, R. and Sandlöbes, S. and Zehnder, C. and Zeng, X. and Korte-Kerzel, S. and Raabe, D.
    Materials and Design 154 (2018)
    We study the micro-mechanical behaviour of single-crystalline long-period-stacking ordered (LPSO) structures, α-Mg and bi-crystalline Mg/LPSO micro-pillars, all cut from the same Mg97Y2Zn1 (at.%) alloy. To investigate the deformation and co-deformation mechanisms of Mg-LPSO alloys we performed micro-pillar compression experiments with micro-pillars of an orientation inclined by 7°, 46° and 90° to (0001) orientation, respectively. Electron backscatter diffraction-assisted slip trace analysis and post-mortem transmission electron microscopy analysis showed predominant deformation by basal 〈a〉 dislocation slip in 46°(0001) and 7°(0001) oriented micro-pillars. In 7°(0001) oriented micro-pillars additional non-basal dislocation slip and the formation of micro shear bands along pyramidal planes were activated in the α-Mg and the LPSO structure, respectively. In 90°(0001) oriented micro-pillars 11¯001¯1¯20 prismatic slip was predominantly activated during the early deformation stages. The relative magnitude of the critical stresses depends on the crystal phase as well as the crystallographic orientation, i.e. the activated slip system. Specifically, basal 〈a〉 slip has the lowest critical resolved shear stress in both, α-Mg and the LPSO structure, while the CRSS of prismatic 〈a〉 slip is about 5 times higher than basal 〈a〉 slip in α-Mg and about 15 times higher than basal 〈a〉 slip in LPSO. © 2018 Elsevier Ltd
    view abstract10.1016/j.matdes.2018.05.037
  • Development of high modulus steels based on the Fe – Cr – B system
    Baron, C. and Springer, H. and Raabe, D.
    Materials Science and Engineering A 724 (2018)
    We present a novel alloy design strategy for cost-efficient high modulus steels with an increased stiffness / mass density ratio. The concept is based on the liquid metallurgy synthesis of Fe – Cr – B based alloys, straightforward processability, and well tuneable mechanical properties via plain heat treatments. The base alloy Fe – 18 Cr – 1.6 B (wt%) contained 14–17 vol% of (Cr,Fe)2B particles of ellipsoidal morphology in a ferritic matrix. Hot rolled materials revealed a specific modulus of 32.8 GPa g−1 cm3, exceeding that of conventional Fe-Cr steels by almost 30%. Mechanical properties obtained are comparable to TiB2 based high modulus steels. Addition of 1 wt% Cu to the base alloy did not interact with the formation, fraction, size and morphology of (Cr,Fe)2B particles, and allowed to mildly increase the strength values by ageing treatments, however at the price of a reduction of the specific modulus. C additions of 0.2 wt% did not affect the (Cr,Fe)2B particle microstructure greatly, but free C dissolved in the matrix enables for the first time to utilize the wide range of microstructures and mechanical properties of established C-containing high strength steels also in high modulus steels. © 2018 Elsevier B.V.
    view abstract10.1016/j.msea.2018.03.082
  • Effects of strain rate on mechanical properties and deformation behavior of an austenitic Fe-25Mn-3Al-3Si TWIP-TRIP steel
    Benzing, J.T. and Poling, W.A. and Pierce, D.T. and Bentley, J. and Findley, K.O. and Raabe, D. and Wittig, J.E.
    Materials Science and Engineering A 711 (2018)
    The effects of quasi-static and low-dynamic strain rate (ε̇ = 10−4 /s to ε̇ = 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21 ± 3 mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain. © 2017 Elsevier B.V.
    view abstract10.1016/j.msea.2017.11.017
  • Elemental partitioning and site-occupancy in γ/γ′ forming Co-Ti-Mo and Co-Ti-Cr alloys
    Im, H.J. and Makineni, S.K. and Gault, B. and Stein, F. and Raabe, D. and Choi, P.-P.
    Scripta Materialia 154 (2018)
    We report on the sub-nanometer scale characterization of Co-12Ti-4Mo and Co-12Ti-4Cr (at.%) model alloys. Atom probe tomography reveals that Co and Cr partition to γ, whereas Ti and Mo to γ′. Additions of Mo and Cr to the reference Co-12Ti system lead to strong increases in γ′ volume fraction by about 25% and 12%, respectively. Element-specific spatial distribution maps along the [001] direction of the L12-ordered γ′ phase reveal that both Mo and Cr preferentially replace Ti on its sublattice. The remaining excess Ti is available for formation of additional γ′, resulting in enhanced γ′ volume fractions. © 2018 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2018.05.041
  • Elemental segregation to antiphase boundaries in a crept CoNi-based single crystal superalloy
    Makineni, S.K. and Lenz, M. and Neumeier, S. and Spiecker, E. and Raabe, D. and Gault, B.
    Scripta Materialia 157 (2018)
    We report on the full three-dimensional compositional partitioning among the features of a planar imperfection comprising a superlattice intrinsic stacking fault (SISF) with its leading and trailing partials, as well as the antiphase boundary (APB) in the wake of the trailing partial formed in the L12-ordered γ′ phase of a CoNi-based single crystal superalloy. The partial dislocations and the APB are found to be Cr/Co rich relative to the surrounding γ′ and richer in W/Ta/Ti compared to the γ matrix phase. Solute diffusion mechanisms are derived from the compositional gradients in the vicinity of the imperfection. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2018.07.042
  • Elemental segregation to twin boundaries in a MnAl ferromagnetic Heusler alloy
    Palanisamy, D. and Raabe, D. and Gault, B.
    Scripta Materialia 155 (2018)
    Electron microscopy and atom probe tomography were combined to investigate the crystallography and chemistry of a single twin boundary (TB) in a rare-earth-free ferromagnetic MnAl Heusler alloy. The results establish a significant segregation of Mn along the twin boundaries. An enrichment of approx. ~8 at.% Mn was measured along the twin boundary with a confined depletion outside the twin boundary, suggesting short range solute diffusion occurring during massive transformation. © 2018 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2018.06.037
  • Evolution of dislocation patterns inside the plastic zone introduced by fatigue in an aged aluminium alloy AA2024-T3
    Breitbarth, E. and Zaefferer, S. and Archie, F. and Besel, M. and Raabe, D. and Requena, G.
    Materials Science and Engineering A 718 (2018)
    The formation of dislocations patterns within the plastic zone ahead of a crack tip introduced by fatigue loading in AA2024-T3 sheets is studied by electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). The aim lies in correlating local plastic effects at grain level with digital image correlation (DIC) results obtained at coarser resolution. While dislocation patterns are observed already prior to fatigue loading, the presence of dislocation sources and dislocation entanglements increases towards highly deformed regions near the crack path. Also, deformation veins are revealed by EBSD in grains located along the crack path. Based on ECCI and EBSD the dislocation density was determined and shows a gradient towards the crack tip. The results help linking the process of fatigue deformation at grain-level to the deformation field probed at the meso-scale level. © 2018 Elsevier B.V.
    view abstract10.1016/j.msea.2018.01.068
  • Experimental-numerical study on strain and stress partitioning in bainitic steels with martensite-austenite constituents
    Fujita, N. and Ishikawa, N. and Roters, F. and Tasan, C.C. and Raabe, D.
    International Journal of Plasticity (2018)
    To achieve safety and reliability in pipelines installed in seismic and permafrost regions, it is necessary to use linepipe materials with high strength and ductility. The introduction of dual-phase steels, e.g., with a bainite and dispersed martensite-austenite (MA) constituent, would provide the necessary ingredients for the improvement of the strain capacity (as required by a new strain-based linepipe design approach) and toughness. To fine-tune the alloy design and ensure these dual-phase steels have the required mechanical properties, an understanding of the governing deformation micromechanisms is essential. For this purpose, a recently developed joint numerical-experimental approach that involves the integrated use of microscopic digital image correlation analysis, electron backscatter diffraction, and multiphysics crystal plasticity simulations with a spectral solver was employed in this study. The local strain and stress evolution and microstructure maps of representative microstructural patches were captured with a high spatial resolution using this approach. A comparison of these maps provides new insights into the deformation mechanism in dual-phase microstructures, especially regarding the influence of the bainite and MA grain size and the MA distribution on the strain localization behavior. © 2018 Elsevier Ltd.
    view abstract10.1016/j.ijplas.2018.01.012
  • Finite-deformation phase-field chemomechanics for multiphase, multicomponent solids
    Svendsen, B. and Shanthraj, P. and Raabe, D.
    Journal of the Mechanics and Physics of Solids 112 (2018)
    The purpose of this work is the development of a framework for the formulation of geometrically non-linear inelastic chemomechanical models for a mixture of multiple chemical components diffusing among multiple transforming solid phases. The focus here is on general model formulation. No specific model or application is pursued in this work. To this end, basic balance and constitutive relations from non-equilibrium thermodynamics and continuum mixture theory are combined with a phase-field-based description of multicomponent solid phases and their interfaces. Solid phase modeling is based in particular on a chemomechanical free energy and stress relaxation via the evolution of phase-specific concentration fields, order-parameter fields (e.g., related to chemical ordering, structural ordering, or defects), and local internal variables. At the mixture level, differences or contrasts in phase composition and phase local deformation in phase interface regions are treated as mixture internal variables. In this context, various phase interface models are considered. In the equilibrium limit, phase contrasts in composition and local deformation in the phase interface region are determined via bulk energy minimization. On the chemical side, the equilibrium limit of the current model formulation reduces to a multicomponent, multiphase, generalization of existing two-phase binary alloy interface equilibrium conditions (e.g., KKS). On the mechanical side, the equilibrium limit of one interface model considered represents a multiphase generalization of Reuss-Sachs conditions from mechanical homogenization theory. Analogously, other interface models considered represent generalizations of interface equilibrium conditions consistent with laminate and sharp-interface theory. In the last part of the work, selected existing models are formulated within the current framework as special cases and discussed in detail. © 2017 Elsevier Ltd
    view abstract10.1016/j.jmps.2017.10.005
  • Formation of eta carbide in ferrous martensite by room temperature aging
    Lu, W. and Herbig, M. and Liebscher, C.H. and Morsdorf, L. and Marceau, R.K.W. and Dehm, G. and Raabe, D.
    Acta Materialia 158 (2018)
    For several decades, the formation of carbon(C)-rich domains upon room temperature aging of supersaturated martensite has been a matter of debate. C-rich tweed-like patterns are observed to form after short aging times at room temperature and coarsen upon further aging. Here, we present a systematic atomic-scale investigation of carbide formation in Fe-15Ni-1C (wt.%) martensite after two to three years of isothermal room temperature aging by a combination of atom probe tomography and transmission electron microscopy. Owing to the sub-zero martensite start temperature of −25 °C, a fully austenitic microstructure is maintained at room temperature and the martensitic phase transformation is initiated during quenching in liquid nitrogen. In this way, any diffusion and redistribution of C in martensite is suppressed until heating up the specimen and holding it at room temperature. The microstructural changes that accompany the rearrangement of C atoms have been systematically investigated under controlled isothermal conditions. Our results show that after prolonged room temperature aging nanometer-sized, plate-shaped η-Fe2C carbides form with a macroscopic martensite habit plane close to {521}. The orientation relationship between the η-Fe2C carbides and the parent martensite grain (α′) follows [001]α’//[001]η, (1¯10) α’//(020)η. The observation of η-Fe2C–carbide formation at room temperature is particularly interesting, as transition carbides have so far only been reported to form above 100 °C. After three years of room temperature aging a depletion of Fe is observed in the η carbide while Ni remains distributed homogenously. This implies that the substitutional element Fe can diffuse several nanometers in martensite at room temperature within three years. © 2018
    view abstract10.1016/j.actamat.2018.07.071
  • Hot cracking mechanism affecting a non-weldable Ni-based superalloy produced by selective electron Beam Melting
    Chauvet, E. and Kontis, P. and Jägle, E.A. and Gault, B. and Raabe, D. and Tassin, C. and Blandin, J.-J. and Dendievel, R. and Vayre, B. and Abed, S. and Martin, G.
    Acta Materialia 142 (2018)
    A non weldable nickel-based superalloy was fabricated by powder bed-based selective electron beam melting (S-EBM). The as-built samples exhibit a heterogeneous microstructure along the build direction. A gradient of columnar grain size as well as a significant gradient in the γ′ precipitate size were found along the build direction. Microstructural defects such as gas porosity inherited from the powders, shrinkage pores and cracks inherited from the S-EBM process were identified. The origins of those defects are discussed with a particular emphasis on crack formation. Cracks were consistently found to propagate intergranular and the effect of crystallographic misorientation on the cracking behavior was investigated. A clear correlation was identified between cracks and high angle grain boundaries (HAGB). The cracks were classified as hot cracks based on the observation of the fracture surface of micro-tensile specimens machined from as-built S-EBM samples. The conditions required to trigger hot cracking, namely, presence of a liquid film during the last stage of solidification and thermal stresses are discussed within the framework of additive manufacturing. Understanding the cracking mechanism enables to provide guidelines to obtain crack-free specimens of non-weldable Ni-based superalloys produced by S-EBM. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.09.047
  • Hydrogen embrittlement of an interstitial equimolar high-entropy alloy
    Luo, H. and Li, Z. and Lu, W. and Ponge, D. and Raabe, D.
    Corrosion Science 136 (2018)
    We investigated the hydrogen embrittlement mechanism in an interstitially carbon alloyed equimolar CoCrFeMnNi high-entropy alloy (HEA) through low strain rate tensile testing under in-situ hydrogen charging. The tensile ductility was significantly reduced by hydrogen charging. The failure mode of the interstitial HEA in presence of hydrogen was a combination of intergranular and transgranular fracture as well as microvoid coalescence. Aggregated nano-carbides act as potential sites for crack initiation. These findings show that the carbon alloyed equimolar high-entropy alloy is susceptible to hydrogen embrittlement. © 2018 Elsevier Ltd
    view abstract10.1016/j.corsci.2018.03.040
  • Impact of local electrostatic field rearrangement on field ionization
    Katnagallu, S. and Dagan, M. and Parviainen, S. and Nematollahi, A. and Grabowski, B. and Bagot, P.A.J. and Rolland, N. and Neugebauer, J. and Raabe, D. and Vurpillot, F. and Moody, M.P. and Gault, B.
    Journal of Physics D: Applied Physics 51 (2018)
    Field ion microscopy allows for direct imaging of surfaces with true atomic resolution. The high charge density distribution on the surface generates an intense electric field that can induce ionization of gas atoms. We investigate the dynamic nature of the charge and the consequent electrostatic field redistribution following the departure of atoms initially constituting the surface in the form of an ion, a process known as field evaporation. We report on a new algorithm for image processing and tracking of individual atoms on the specimen surface enabling quantitative assessment of shifts in the imaged atomic positions. By combining experimental investigations with molecular dynamics simulations, which include the full electric charge, we confirm that change is directly associated with the rearrangement of the electrostatic field that modifies the imaging gas ionization zone. We derive important considerations for future developments of data reconstruction in 3D field ion microscopy, in particular for precise quantification of lattice strains and characterization of crystalline defects at the atomic scale. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aaaba6
  • In-situ SEM observation of phase transformation and twinning mechanisms in an interstitial high-entropy alloy
    Wang, M. and Li, Z. and Raabe, D.
    Acta Materialia 147 (2018)
    The recently developed interstitial high-entropy alloys (iHEAs) exhibit an enhanced combination of strength and ductility. These properties are attributed to dislocation hardening, deformation-driven athermal phase transformation from the face-centered cubic (FCC) γ matrix into the hexagonal close-packed (HCP) ε phase, stacking fault formation, mechanical twinning and precipitation hardening. For gaining a better understanding of these mechanisms as well as their interactions direct observation of the deformation process is required. For this purpose, an iHEA with nominal composition of Fe-30Mn-10Co-10Cr-0.5C (at. %) was produced and investigated via in-situ and interrupted in-situ tensile testing in a scanning electron microscope (SEM) combining electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) techniques. The results reveal that the iHEA is deformed by formation and multiplication of stacking faults along {111} microbands. Sufficient overlap of stacking faults within microbands leads to intrinsic nucleation of HCP ε phase and incoherent annealing twin boundaries act as preferential extrinsic nucleation sites for HCP ε formation. With further straining HCP ε nuclei grow into the adjacent deformed FCC γ matrix. γ regions with smaller grain size have higher mechanical stability against phase transformation. Twinning in FCC γ grains with a size of ∼10 μm can be activated at room temperature at a stress below ∼736 MPa. With increasing deformation, new twin lamellae continuously nucleate. The twin lamellae grow in preferred directions driven by the motion of the mobile partial dislocations. Owing to the individual grain size dependence of the activation of the dislocation-mediated plasticity, of the athermal phase transformation and of mechanical twinning at the different deformation stages, desired strain hardening profiles can be tuned and adjusted over the entire deformation regime by adequate microstructure design, providing excellent combinations of strength and ductility. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.01.036
  • Laser beam welding of dual-phase DP1000 steel
    Alves, P.H.O.M. and Lima, M.S.F. and Raabe, D. and Sandim, H.R.Z.
    Journal of Materials Processing Technology 252 (2018)
    Autogeneous laser beam welding is an efficient process to join ferritic-martensitic dual-phase steels without large dimensional distortions. Localized softening may occur in the heat affected zone, particularly in DP1000 steel with a high martensite volume fraction. DP1000 steel was welded in bead-on-plate configuration varying the welding power between 0.4 and 2.0 kW and the welding speed between 20 and 150 mm/s. Light optical microscopy, scanning electron microscopy, and X-ray diffraction were used to perform the microstructural characterization of the welded joints. High-quality laser beam welds of thin sheets of DP1000 steel can be produced using appropriate welding parameters. The optimal welding condition was a nominal laser power of 2.0 kW and a welding speed of 150 mm/s. This condition minimizes the amount of softening of prior martensite and yields a narrow heat affected zone and a small volume fraction of retained austenite in the weld. © 2017 Elsevier B.V.
    view abstract10.1016/j.jmatprotec.2017.10.008
  • Machine-learning-based atom probe crystallographic analysis
    Wei, Y. and Gault, B. and Varanasi, R.S. and Raabe, D. and Herbig, M. and Breen, A.J.
    Ultramicroscopy 194 (2018)
    Atom probe tomography is known for its accurate compositional analysis at the nanoscale. However, the patterns created by successive hits on the single particle detector during experiments often contain complementary information about the specimen's crystallography, including structure and orientation. This information remains in most cases unexploited because it is, up to now, retrieved predominantly manually. Here, we propose an approach combining image analysis techniques for feature selection and deep-learning to automatically interpret the patterns. Application of unsupervised machine learning techniques allows to build and train a deep neural network, based on a library generated from theoretically known crystallographic angular relationships. This approach enables direct interpretation of the detector hit maps, as shown here on the example of numerous pure-Al, and is robust enough to function under various conditions of base temperature, pulsing mode and pulse fraction. We benchmark our approach against recent attempts to automate the pattern identification via Hough-transform and discuss the current limitations of our approach. This new automated approach renders crystallographic atom probe tomography analysis more efficient, feature-sensitive, robust, user-independent and reliable. With that, deep-learning algorithms show a great potential to give access to combined atom probe crystallographic and compositional analysis to a large community of users. © 2018 Elsevier B.V.
    view abstract10.1016/j.ultramic.2018.06.017
  • Manipulation of matter by electric and magnetic fields: Toward novel synthesis and processing routes of inorganic materials
    Guillon, O. and Elsässer, C. and Gutfleisch, O. and Janek, J. and Korte-Kerzel, S. and Raabe, D. and Volkert, C.A.
    Materials Today 21 (2018)
    The use of external electric and magnetic fields for the synthesis and processing of inorganic materials such as metals and ceramics has seen renewed interest in recent years. Electromagnetic energy can be utilized in different ways to improve or accelerate phase formation and stabilization, chemical ordering, densification and coarsening of particle-based materials (pore elimination and grain growth), and mechanical deformation (plasticity and creep). In these new synthesis and processing routes, the resulting microstructures and macroscopic material behavior are determined by the interaction of the applied fields with defects such as single or clustered point defects, dislocation networks, and interfaces. Multiscale experimental investigations and modeling are necessary to unveil the mechanisms underlying this field-assisted manipulation of matter. © 2018 Elsevier Ltd
    view abstract10.1016/j.mattod.2018.03.026
  • Microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure
    Soler, R. and Evirgen, A. and Yao, M. and Kirchlechner, C. and Stein, F. and Feuerbacher, M. and Raabe, D. and Dehm, G.
    Acta Materialia 156 (2018)
    The microstructural and mechanical characterization of an equiatomic YGdTbDyHo high entropy alloy with hexagonal close-packed structure was performed. The phase state and chemical homogeneity of the solid solution were analysed with respect to crystal structure, phase stability, and oxide formation. It was found that Y-rich precipitates form at grain boundaries and that the alloy is prone to oxidation, leading to a homogeneous distribution of ∼10 nm-sized oxides in the grain interiors. The plastic response at the sub-grain level was studied in terms of the activated slip systems, critical resolved shear stresses (CRSS), and strain hardening using micropillar compression tests. We observe plastic slip on the basal <a> system, with a CRSS of 196 ± 14.7 MPa. Particle strengthening and strength dependence on sample size are discussed on the basis of dislocation particle interaction and mechanical size effects. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.06.010
  • Microstructural degradation of polycrystalline superalloys from oxidized carbides and implications on crack initiation
    Kontis, P. and Collins, D.M. and Wilkinson, A.J. and Reed, R.C. and Raabe, D. and Gault, B.
    Scripta Materialia 147 (2018)
    Surface connected carbides in a polycrystalline superalloy oxidized at 750 °C in air were studied as potential crack initiation sites. Lattice rotations measured in the γ/γ′ grains using high-resolution electron backscatter diffraction enabled investigation of the plastic deformation induced solely by the oxidation of carbides. Dislocations were found to enhance γ′ precipitate dissolution kinetics, resulting in soft recrystallized regions in the vicinity of the oxidized carbide with substantial compositional variation compared to the original γ/γ′ microstructure. Ramifications of such deleterious oxidized carbides alongside soft recrystallized regions on the crack initiation life in superalloys are discussed. © 2017 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2017.12.028
  • Misorientation distribution between martensite and austenite in Fe-31 wt%Ni-0.01 wt%C
    Zilnyk, K.D. and Almeida Junior, D.R. and Sandim, H.R.Z. and Rios, P.R. and Raabe, D.
    Acta Materialia 143 (2018)
    We characterized the morphology, substructure and crystallography of lenticular martensite in a Fe-Ni-C alloy by means of electron backscatter diffraction and scanning electron microscopy. Electron backscatter diffraction maps were used to determine the orientation relationship between austenite and martensite across large regions of the microstructure. We employed orientation distribution functions as a statistical representation method for the observed orientation relationships. High-resolution point-to-point scans were used to normalize the effects of the orientation changes in the austenite caused by the plastic deformation during the formation of lenticular martensite. The analysis revealed that most of the transformation in this material follows an orientation relationship close to the one proposed by Greninger and Troiano. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.10.026
  • Molecular statics simulation of CdTe grain boundary structures and energetics using a bond-order potential
    Stechmann, G. and Zaefferer, S. and Raabe, D.
    Modelling and Simulation in Materials Science and Engineering 26 (2018)
    The structure and energetics of coincidence site lattice grain boundaries (GB) in CdTe are investigated by mean of molecular statics simulations, using the Cd-Zn-Te bond-order potential (second iteration) developed by Ward et al (2012 Phys. Rev. B 86 245203; 2013 J. Mol. Modelling 19 5469-77). The effects of misorientation (Σ value) and interface plane are treated separately, complying with the critical need for full five-parameter characterization of GB. In addition, stoichiometric shifts, occurring between the inner interfaces and their adjacent atomic layers, are also predicted, revealing the energetic preference of Te-rich boundaries, opening opportunities for crystallography-based intrinsic interface doping. Our results also suggest that the intuitive assumption that Σ3 boundaries with low-indexed planes are more energetically favorable is often unfounded, except for coherent twins developing on {111} boundary planes. Therefore, Σ5, 7 or 9 boundaries, with lower interface energy than that of twin boundaries lying on different facets, are frequently encountered. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-651X/aaba87
  • Multiple slip dislocation patterning in a dislocation-based crystal plasticity finite element method
    Grilli, N. and Janssens, K.G.F. and Nellessen, J. and Sandlöbes, S. and Raabe, D.
    International Journal of Plasticity 100 (2018)
    Dislocation structures forming during cyclic loading of fcc metals are fatigue damage precursors. Their specific structures are caused by the motion and interactions of dislocations. Depending on the load conditions, the grain orientation, the stacking fault energy, a variety of different dislocation structures appear in the material such as labyrinths, cells, veins and persistent slip bands. We present a continuum dislocation-based model for cyclic fatigue and incorporate it into a crystal plasticity finite element solver. A method for the simulation of dislocation junction formation is introduced, which reproduces the behaviour of discrete objects, such as dislocations, in a continuum framework. The formation of dislocation walls after 50 and 100 deformation cycles at 0.95% and 0.65% strain amplitude starting from an initial random dislocation distribution is predicted for 〈001〉 and 〈11¯0〉 oriented crystals. Simulations and cyclic tension-compression experiments of polycrystalline 316L stainless steel are performed to compare our model with another model based on edge and screw dislocation densities. The simulated dislocation structures and experimental results, obtained with the electron channeling contrast imaging technique, are compared using a 2D orientation distribution function of the dislocation structures. The dominant orientation of dislocation walls is predicted by the new model; it turns out to be perpendicular to the intersection line between the two slip planes involved in their formation and at an angle of around 45o from the loading axis. This agrees well with the experimental observations and represents a step forward for understanding the formation mechanism of these dislocation structures. © 2017 Elsevier Ltd
    view abstract10.1016/j.ijplas.2017.09.015
  • Nano-laminated thin film metallic glass design for outstanding mechanical properties
    Kontis, P. and Köhler, M. and Evertz, S. and Chen, Y.-T. and Schnabel, V. and Soler, R. and Bednarick, J. and Kirchlechner, C. and Dehm, G. and Raabe, D. and Schneider, J.M. and Gault, B.
    Scripta Materialia 155 (2018)
    We report the enhancement of fracture toughness and strength of a cobalt‑tantalum-based metallic glass thin film with increasing boron content. The improvement of the mechanical performance is attributed to the formation of a compositionally lamellar compared to uniform glass microstructure, which becomes thinner with increasing boron content as revealed by transmission electron microscopy. Compositional variations across the lamellar structure are revealed by atom probe tomography. Cobalt- and boron-rich regions alternate sequentially, whereas tantalum exhibits slight variations across the lamellae. Our results can be utilized in future design efforts for metallic glass thin films with outstanding mechanical performance. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2018.06.015
  • On the detection of multiple events in atom probe tomography
    Peng, Z. and Vurpillot, F. and Choi, P.-P. and Li, Y. and Raabe, D. and Gault, B.
    Ultramicroscopy 189 (2018)
    In atom probe tomography (APT), multiple events can arise as a consequence of e.g. correlated field evaporation and molecular ion dissociation. They represent challenging cases for single-particle detectors and can cause compositional as well as spatial inaccuracies. Here, two state-of-the-art atom probe microscopes (Cameca LEAP 5000 XS and 5000 XR) were used to investigate cemented tungsten carbide, which exhibits high amounts of multiple events. By advanced data analysis methods, the natural character of the multiple events, as well as the performance of the APT detectors, are assessed. Accordingly, possible signal loss mechanisms are discussed. © 2018 Elsevier B.V.
    view abstract10.1016/j.ultramic.2018.03.018
  • On the diffusive phase transformation mechanism assisted by extended dislocations during creep of a single crystal CoNi-based superalloy
    Makineni, S.K. and Kumar, A. and Lenz, M. and Kontis, P. and Meiners, T. and Zenk, C. and Zaefferer, S. and Eggeler, G. and Neumeier, S. and Spiecker, E. and Raabe, D. and Gault, B.
    Acta Materialia 155 (2018)
    We propose here a deformation-induced diffusive phase transformation mechanism occurring during shearing of γ′ ordered phase in a γ/γ′ single crystalline CoNi-based superalloy. Shearing involved the creation and motion of a high density of planar imperfections. Through correlative electron microscopy and atom probe tomography, we captured a superlattice intrinsic stacking fault (SISF) and its associated moving leading partial dislocation (LPD). The structure and composition of these imperfections reveal characteristic chemical – structural contrast. The SISF locally exhibits a D019 ordered structure coherently embedded in the L12 γ′ and enriched in W and Co. Interestingly, the LPD is enriched with Cr and Co, while the adjoining planes ahead of the LPD are enriched with Al. Quantitative analysis of the three-dimensional compositional field in the vicinity of imperfections sheds light onto a new in-plane diffusion mechanism as the LPD moves on specific {111} planes upon application of stress at high temperature. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.05.074
  • On the grain boundary strengthening effect of boron in γ/γ′ Cobalt-base superalloys
    Kolb, M. and Freund, L.P. and Fischer, F. and Povstugar, I. and Makineni, S.K. and Gault, B. and Raabe, D. and Müller, J. and Spiecker, E. and Neumeier, S. and Göken, M.
    Acta Materialia 145 (2018)
    Boron is an essential solute element for improving the grain boundary strength in several high temperature metallic alloys especially in Ni- and Co-base superalloys although the detailed strengthening mechanisms are still not well understood. In superalloys, boron leads to the formation of borides and precipitate depleted zones around the grain boundaries and alters the bond strength among the grains directly. In this paper, we explore in detail the role of the boron content in ternary γ/γ′ Co-9Al-9W alloys. Local as well as bulk mechanical properties were evaluated using nanoindentation and compression testing and correlated to near-atomic scale microstructure and compositions obtained from electron microscopy and atom probe tomography. The alloy variant with low B content (0.005 at.% B) reveals an increase in yield strength at room temperature and 600 °C and atom probe tomography investigations show that solute B segregates to the grain boundaries. However, in the bulk B exclusively partitions to the γ′ phase. Additionally, the γ′/γ′ grain boundaries are depleted in W and Al with the concentration locally shifted towards the γ composition forming a very thin γ layer at the γ′/γ′ grain boundaries, which supports dislocation mobility in the γ′/γ′ grain boundary region during deformation. Higher content of B (0.04 at.% B) promotes formation of W-rich borides at the grain boundaries that leads to undesirable precipitate depleted zones adjacent to these borides that decrease the strength of the alloy drastically. However, it was also found that a subsequent annealing heat treatment eliminates these detrimental zones by re-precipitating γ′ and thus elevating the strength of the alloy. This result shows that, if a precipitate depleted zone can be avoided, B significantly improves the mechanical properties of polycrystalline Co-base superalloys by strengthening the γ′ phase and by improving grain boundary cohesion. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.12.020
  • On the nature of twin boundary-associated strengthening in Fe-Mn-C steel
    Choi, W.S. and Sandlöbes, S. and Malyar, N.V. and Kirchlechner, C. and Korte-Kerzel, S. and Dehm, G. and Choi, P.-P. and Raabe, D.
    Scripta Materialia 156 (2018)
    We unravel the nature of twin boundary-associated strengthening in Fe-Mn-C twinning-induced plasticity steel (TWIPs) by micro-pillar compression tests. Dislocation interactions with a coherent twin boundary and their role on strain hardening were investigated. The results indicate that twin-matrix bundles dynamically introduced by deformation twinning and their interaction with dislocations are required for strengthening Fe-Mn-C TWIPs, while single coherent twin boundaries enable dislocation transmission. Correlative studies on orientation dependent deformation mechanisms, detailed dislocation-twin boundary interactions, and the resulting local stress-strain responses suggest that twin boundary-associated strengthening is primarily caused by the reduction of the mean free dislocation path in nano-twinned microstructures. © 2018
    view abstract10.1016/j.scriptamat.2018.07.009
  • On the origin of the improvement of shape memory effect by precipitating VC in Fe–Mn–Si-based shape memory alloys
    Lai, M.J. and Li, Y.J. and Lillpopp, L. and Ponge, D. and Will, S. and Raabe, D.
    Acta Materialia 155 (2018)
    We studied the role of VC precipitation in improving the shape memory effect (SME) of the as-solution treated Fe–Mn–Si-based shape memory alloys by examining the microstructures developed during aging and deformation using transmission electron microscopy and electron channeling contrast imaging. Our results suggest that VC particles are not the only product of aging. Upon aging at 650 °C, the precipitation of VC particles is accompanied by the formation of profuse dislocations (2.26 ± 0.098 × 1013 m−2). In this case, the SME is not improved compared to the as-solution treated reference state. Upon aging at high temperatures (700–900 °C), a number of stacking faults are formed accompanying the VC precipitation and the SME is significantly improved, where the recovery ratios reach almost twice that of the as-solution treated state (&lt;50%). For these high-temperature aged states, in situ straining experiments reveal that the stacking faults rather than the VC particles play an important role in the stress-induced martensitic transformation, leading to the formation of very thin (&lt;3 nm) martensite plates with a single crystallographic variant within each grain. These martensite plates are in contrast to the very thick (from tens to hundreds of nanometers) and multi-variant martensite plates that prevail in the as-solution treated state. By comparing the characteristics of the martensite plates between the as-solution treated and the high-temperature aged states, the reasons for the improvement of SME by precipitating VC were discussed. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.06.008
  • On the role of the collinear dislocation interaction in deformation patterning and laminate formation in single crystal plasticity
    Wang, D. and Diehl, M. and Roters, F. and Raabe, D.
    Mechanics of Materials 125 (2018)
    We investigate the reasons for severe deformation patterning observed in crystal plasticity simulations of an fcc nickel single crystal with initial near-Copper orientation deformed in plane strain compression. The resulting strain partitioning in the form of alternating parallel bands initiates at a very early loading stage, i.e. <0.5% global strain, and sharpens with ongoing deformation. At an applied average strain of 5.5%, the local strains finally deviate by half an order of magnitude in different regions of the initially homogeneous single crystal. We show that this microstructure lamination is the result of a complex interplay between available deformation systems, strain hardening, kinematics, and deformation energetics. Moreover, the boundary conditions play an important role as under the applied load two slip systems—which are collinear with respect to each other—have the same highest Schmid factor and therefore are preferentially activated. During strain hardening, the strong collinear interaction strength causes—depending on the initial deviation from the nominal orientation—the selection of a single prevalent slip system in clearly delimited regions. This behavior is explained by the lower global deformation energy in comparison to a homogeneous double slip behavior. We also reveal that the observed deformation pattern forms only for dislocation interaction strength values in the range predicted by discrete dislocation dynamic simulations. © 2018
    view abstract10.1016/j.mechmat.2018.06.007
  • Parameter free quantitative analysis of atom probe data by correlation functions: Application to the precipitation in Al-Zn-Mg-Cu
    Zhao, H. and Gault, B. and Ponge, D. and Raabe, D. and De Geuser, F.
    Scripta Materialia 154 (2018)
    Atom probe tomography enables precise quantification of the composition of second phase particles from their early stages, leading to improved understanding of the thermodynamic and kinetic mechanisms of phase formation and quantify structure-property relationships. Here we demonstrate how approaches developed for small-angle scattering can be adapted to atom probe tomography. By exploiting nearest-neighbor distributions and radial distribution function, we introduce a parameter free methodology to efficiently extract information such as particle size, composition, volume fraction, number density and inter-particle distance. We demonstrate the strength of this approach in the analysis of a precipitation-hardened model Al-Zn-Mg-Cu high-strength lightweight alloy. © 2018 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2018.05.024
  • Phase nucleation through confined spinodal fluctuations at crystal defects evidenced in Fe-Mn alloys
    Kwiatkowski Da Silva, A. and Ponge, D. and Peng, Z. and Inden, G. and Lu, Y. and Breen, A. and Gault, B. and Raabe, D.
    Nature Communications 9 (2018)
    Analysis and design of materials and fluids requires understanding of the fundamental relationships between structure, composition, and properties. Dislocations and grain boundaries influence microstructure evolution through the enhancement of diffusion and by facilitating heterogeneous nucleation, where atoms must overcome a potential barrier to enable the early stage of formation of a phase. Adsorption and spinodal decomposition are known precursor states to nucleation and phase transition; however, nucleation remains the less well-understood step in the complete thermodynamic sequence that shapes a microstructure. Here, we report near-atomic-scale observations of a phase transition mechanism that consists in solute adsorption to crystalline defects followed by linear and planar spinodal fluctuations in an Fe-Mn model alloy. These fluctuations provide a pathway for austenite nucleation due to the higher driving force for phase transition in the solute-rich regions. Our observations are supported by thermodynamic calculations, which predict the possibility of spinodal decomposition due to magnetic ordering. © 2018 The Author(s).
    view abstract10.1038/s41467-018-03591-4
  • Precipitation hardening effects on extension twinning in magnesium alloys
    Fan, H. and Zhu, Y. and El-Awady, J.A. and Raabe, D.
    International Journal of Plasticity 106 (2018)
    Precipitation is an efficient method to strengthen metallic materials. While precipitation hardening effects on dislocation slip have been studied extensively in the past, the influence of precipitates on twinning mediated plasticity and the development of corresponding hardening models that account for twin-precipitate interactions have received less attention. Here, the interaction of {10-12} extension twin boundaries (TBs) in pure magnesium with precipitates of plate-, sphere- and rod-like shapes is studied using molecular dynamics (MD) simulations. We find that TBs that engulf precipitates are absorbed by the precipitate-matrix interfaces, and the precipitates are neither twinned nor sheared but deform elastically leading to their rotation. TBs can pass small precipitates (length? 20 nm) and remain intact. In contrast when TBs are interacting with large precipitates (length? 50 nm), basal dislocations or stacking faults nucleate from the interfaces, causing local plastic relaxation. The stress field around a plate-like precipitate as calculated in the MD simulations suggests that a strong back-stress is imposed on the TBs. We then coarse grain these mechanisms into an analytical mean field model of precipitation hardening on twinning in magnesium alloys, which is based on the energy conservation during the TB-precipitate interaction. The model is in good agreement with the current MD simulations and published experimental observations. The hardening model shows that spherical precipitates have the strongest hardening effect on twinning, basal and prismatic plate-like precipitates have a medium effect while rod-like precipitates exert the weakest influence. We also find that most types of precipitates show a stronger hardening effect on twinning mediated plasticity than on basal dislocation slip. Finally, prismatic plate-like precipitates are predicted to have reasonable hardening effects on both twinning and basal slip. These results can help guiding the development of magnesium alloys with enhanced strength and ductility. © 2018 Elsevier Ltd.
    view abstract10.1016/j.ijplas.2018.03.008
  • Segregation assisted grain boundary precipitation in a model Al-Zn-Mg-Cu alloy
    Zhao, H. and De Geuser, F. and Kwiatkowski da Silva, A. and Szczepaniak, A. and Gault, B. and Ponge, D. and Raabe, D.
    Acta Materialia 156 (2018)
    Understanding the composition evolution of grain boundaries and grain boundary precipitation at near-atomic scale in aluminum alloys is crucial to tailor mechanical properties and to increase resistance to corrosion and stress corrosion cracking. Here, we elucidate the sequence of precipitation on grain boundaries in comparison to the bulk in a model Al-Zn-Mg-Cu alloy. We investigate the material from the solution heat treated state (475 °C), through the very early stages of aging to the peak aged state at 120 °C and further into the overaged regime at 180 °C. The process starts with solute enrichment on grain boundaries due to equilibrium segregation accompanied by solute depletion in their vicinity, the formation of Guinier–Preston (GP) zones in the solute-enriched grain boundary regions, and GP zones growth and transformation. The equilibrium segregation of solutes to grain boundaries during aging accelerates this sequence compared to the bulk. Analysis of the ∼10 nm wide precipitate-free zones (PFZs) adjacent to the solute-enriched grain boundaries shows that the depletion zones are determined by (i) interface equilibrium segregation; (ii) formation and coarsening of the grain boundary precipitates and (iii) the diffusion range of solutes in the matrix. In addition, we quantify the difference in kinetics between grain boundary and bulk precipitation. The precipitation kinetics, as observed in terms of volume fraction, average radius, and number density, is almost identical next to the depletion zone in the bulk and far inside the bulk grain remote from any grain boundary influence. This observation shows that the region influenced by the grain boundaries does not extend beyond the PFZs. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.07.003
  • Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers
    Colombara, D. and Werner, F. and Schwarz, T. and Cañero Infante, I. and Fleming, Y. and Valle, N. and Spindler, C. and Vacchieri, E. and Rey, G. and Guennou, M. and Bouttemy, M. and Manjón, A.G. and Peral Alonso, I. and Melchiorre, M. and El Adib, B. and Gault, B. and Raabe, D. and Dale, P.J. and Siebentritt, S.
    Nature Communications 9 (2018)
    Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se2 films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe2 grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se2 and Cu(In,Ga)3Se5 phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells. © 2018 The Author(s).
    view abstract10.1038/s41467-018-03115-0
  • Strain-Induced Asymmetric Line Segregation at Faceted Si Grain Boundaries
    Liebscher, C.H. and Stoffers, A. and Alam, M. and Lymperakis, L. and Cojocaru-Mirédin, O. and Gault, B. and Neugebauer, J. and Dehm, G. and Scheu, C. and Raabe, D.
    Physical Review Letters 121 (2018)
    The unique combination of atomic-scale composition measurements, employing atom probe tomography, atomic structure determination with picometer resolution by aberration-corrected scanning transmission electron microscopy, and atomistic simulations reveals site-specific linear segregation features at grain boundary facet junctions. More specific, an asymmetric line segregation along one particular type of facet junction core, instead of a homogeneous decoration of the facet planes, is observed. Molecular-statics calculations show that this segregation pattern is a consequence of the interplay between the asymmetric core structure and its corresponding local strain state. Our results contrast with the classical view of a homogeneous decoration of the facet planes and evidence a complex segregation patterning. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.121.015702
  • Synthesis and stabilization of a new phase regime in a Mo-Si-B based alloy by laser-based additive manufacturing
    Makineni, S.K. and Kini, A.R. and Jägle, E.A. and Springer, H. and Raabe, D. and Gault, B.
    Acta Materialia 151 (2018)
    Mo-Si-B alloys are potential creep resistant materials for accessing harsh loading scenarios beyond Ni-based superalloys due to their excellent mechanical performance at ultra-high temperatures (&gt; 1200 °C). Here, we report on the fabrication through laser additive manufacturing of a Mo rich Mo-Si-B alloy with and without dispersion of oxide (La2O3) particles. The major phase in the solidified material is dendritic α-Mo. The inter-dendritic regions contain a mixture of the Mo5Si3 (T1) + Mo5SiB2 (T2) phases, and not the expected equilibrium Mo3Si + Mo5SiB2 (T2) phases. This combination of phases is shown to yield improved high temperature creep resistance but was only accessible through by addition of Nb, W or Ti that substitute Mo in the intermetallic phases. Whereas here it is attributed to the large undercooling in the small melt pool produced during laser processing. We show that this phase mixture, upon annealing, is stable at 1200 °C for 200 h. We also demonstrate successful dispersion of oxide particles mainly in the inter-dendritic regions leading to a high indentation fracture toughness of ∼18 MPa√m at room temperature. Toughening originates from crack trapping in the ductile α-Mo and the formation of micro-cracks and crack deflection in the vicinity of oxide particles. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.03.037
  • The effect of chromium and cobalt segregation at dislocations on nickel-based superalloys
    Kontis, P. and Li, Z. and Collins, D.M. and Cormier, J. and Raabe, D. and Gault, B.
    Scripta Materialia 145 (2018)
    The segregation of solutes at dislocations in a polycrystalline and a single crystal nickel-based superalloy is studied. Our observations confirm the often assumed but yet unproven diffusion along dislocations via pipe diffusion. Direct observation and quantitative, near-atomic scale segregation of chromium and cobalt at dislocations within γ' precipitates and at interfacial dislocations leading to the partial or complete dissolution of γ' precipitates at elevated temperatures is presented. Our results allow us to elucidate the physical mechanism by which pipe diffusion initiates the undesirable dissolution of γ' precipitates. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2017.10.005
  • The Role of Oxidized Carbides on Thermal-Mechanical Performance of Polycrystalline Superalloys
    Kontis, P. and Li, Z. and Segersäll, M. and Moverare, J.J. and Reed, R.C. and Raabe, D. and Gault, B.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 (2018)
    Oxidized MC carbides which act as main crack initiation sites in a polycrystalline superalloy under thermal-mechanical fatigue (TMF) conditions at 850 °C were studied. Microstructural observations in the TMF tested specimens were compared to findings from bulk samples exposed isothermally in air at 850 °C for 30 hours in the absence of any external applied load. Carbides were found to oxidize rapidly after exposure at 850 °C for 30 hours resulting in surface eruptions corresponding to oxidation products, from where micro-cracks initiated. Plastic deformation due to volume expansion of the often porous oxidized carbides led to high dislocation densities in the adjacent matrix as revealed by controlled electron channeling contrast imaging. The high dislocation density facilitated the dissolution kinetics of γ′ precipitates by segregation and diffusion of chromium and cobalt along the dislocations via pipe diffusion, resulting in the formation of soft recrystallized grains. Atom probe tomography revealed substantial compositional differences between the recrystallized grains and the adjacent undeformed γ matrix. Similar observations were made for the TMF tested alloy. Our observations provide new insights into the true detrimental role of oxidized MC carbides on the crack initiation performance of polycrystalline superalloys under TMF. © 2018, The Author(s).
    view abstract10.1007/s11661-018-4709-x
  • Thermophysical and Mechanical Properties of Advanced Single Crystalline Co-base Superalloys
    Volz, N. and Zenk, C.H. and Cherukuri, R. and Kalfhaus, T. and Weiser, M. and Makineni, S.K. and Betzing, C. and Lenz, M. and Gault, B. and Fries, S.G. and Schreuer, J. and Vaßen, R. and Virtanen, S. and Raabe, D. and Spiecker, E. and Neumeier, S. and Göken, M.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 (2018)
    A set of advanced single crystalline γ′ strengthened Co-base superalloys with at least nine alloying elements (Co, Ni, Al, W, Ti, Ta, Cr, Si, Hf, Re) has been developed and investigated. The objective was to generate multinary Co-base superalloys with significantly improved properties compared to the original Co-Al-W-based alloys. All alloys show the typical γ/γ′ two-phase microstructure. A γ′ solvus temperature up to 1174 °C and γ′ volume fractions between 40 and 60 pct at 1050 °C could be achieved, which is significantly higher compared to most other Co-Al-W-based superalloys. However, higher contents of Ti, Ta, and the addition of Re decrease the long-term stability. Atom probe tomography revealed that Re does not partition to the γ phase as strongly as in Ni-base superalloys. Compression creep properties were investigated at 1050 °C and 125 MPa in 〈001〉 direction. The creep resistance is close to that of first generation Ni-base superalloys. The creep mechanisms of the Re-containing alloy was further investigated and it was found that the deformation is located preferentially in the γ channels although some precipitates are sheared during early stages of creep. The addition of Re did not improve the mechanical properties and is therefore not considered as a crucial element in the design of future Co-base superalloys for high temperature applications. Thermodynamic calculations describe well how the alloying elements influence the transformation temperatures although there is still an offset in the actual values. Furthermore, a full set of elastic constants of one of the multinary alloys is presented, showing increased elastic stiffness leading to a higher Young’s modulus for the investigated alloy, compared to conventional Ni-base superalloys. The oxidation resistance is significantly improved compared to the ternary Co-Al-W compound. A complete thermal barrier coating system was applied successfully. © 2018, The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-018-4705-1
  • Unexpected cyclic stress-strain response of dual-phase high-entropy alloys induced by partial reversibility of deformation
    Niendorf, T. and Wegener, T. and Li, Z. and Raabe, D.
    Scripta Materialia 143 (2018)
    The recently developed dual-phase high-entropy alloys are characterized by pronounced strain hardening and high ductility under monotonic loading owing to the associated transformation induced plasticity effect. Fatigue properties of high-entropy alloys have not been studied in depth so far. The current study focuses on the low-cycle fatigue regime. Cyclic tests were conducted and the microstructure evolution was studied post-mortem. Despite deformation-induced martensitic transformation during cycling at given plastic strain amplitudes, intense strain hardening in the cyclic stress-strain response is not observed. This behavior is attributed to the planar nature of slip and partial reversibility of deformation. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2017.09.013
  • ω phase acts as a switch between dislocation channeling and joint twinning- and transformation-induced plasticity in a metastable β titanium alloy
    Lai, M.J. and Li, T. and Raabe, D.
    Acta Materialia 151 (2018)
    We have investigated the twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) as well as the influence of ω phase on these two phenomena in a metastable β-type Ti–25Nb–0.7Ta–2Zr (at.%) alloy. We set off with two starting states: one is ω-free and the other one contains a high number density (3.20 ± 0.78 × 1024 m−3) of nanometer-sized (∼1.23 nm) ω particles. Deformation experiments demonstrate that the plastic deformation of the ω-free alloy is mediated by stress-induced β → α” martensitic transformation, {332} twinning and dislocation slip, where the former two induce joint TRIP and TWIP effects and the latter one carries the majority of the plastic strain. In the ω-enriched alloy, the ω particles fully suppress the TWIP and TRIP effects and promote localization of dislocation plasticity into specific ω-devoid channels. Atom probe tomography analysis reveals that the elemental partitioning between β and ω results in only subtle enrichment of solutes in the β matrix, which cannot sufficiently stabilize the matrix to prevent martensitic transformation and twinning. A new mechanism based on the shear modulus difference between β and ω is proposed to explain the suppression of TRIP and TWIP effects by ω particles. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.03.053
  • 1 billion tons of nanostructure - Segregation engineering enables confined transformation effects at lattice defects in steels
    Raabe, D. and Ponge, D. and Wang, M.-M. and Herbig, M. and Belde, M. and Springer, H.
    IOP Conference Series: Materials Science and Engineering 219 (2017)
    The microstructure of complex steels can be manipulated by utilising the interaction between the local mechanical distortions associated with lattice defects, such as dislocations and grain boundaries, and solute components that segregate to them. Phenomenologically these phenomena can be interpreted in terms of the classical Gibbs adsorption isotherm, which states that the total system energy can be reduced by removing solute atoms from the bulk and segregating them at lattice defects. Here we show how this principle can be utilised through appropriate heat treatments not only to enrich lattice defects by solute atoms, but also to further change these decorated regions into confined ordered structural states or even to trigger localized decomposition and phase transformations. This principle, which is based on the interplay between the structure and mechanics of lattice defects on the one hand and the chemistry of the alloy's solute components on the other hand, is referred to as segregation engineering. In this concept solute decoration to specific microstructural traps, viz. lattice defects, is not taken as an undesired effect, but instead seen as a tool for manipulating specific lattice defect structures, compositions and properties that lead to beneficial material behavior. Owing to the fairly well established underlying thermodynamic and kinetic principles, such local decoration and transformation effects can be tuned to proceed in a self-organised manner by adjusting (i) the heat treatment temperatures for matching the desired trapping, transformation or reversion regimes, and (ii) the corresponding timescales for sufficient solute diffusion to the targeted defects. Here we show how this segregation engineering principle can be applied to design self-organized nano- and microstructures in complex steels for improving their mechanical properties. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/219/1/012006
  • 100 years public-private partnership in metallurgical and materials science research
    Raabe, D. and Dehm, G. and Neugebauer, J. and Rohwerder, M.
    Materials Today 20 (2017)
    view abstract10.1016/j.mattod.2017.02.005
  • A correlative investigation of grain boundary crystallography and electronic properties in CdTe thin film solar cells
    Stechmann, G. and Zaefferer, S. and Schwarz, T. and Konijnenberg, P. and Raabe, D. and Gretener, C. and Kranz, L. and Perrenoud, J. and Buecheler, S. and Nath Tiwari, A.
    Solar Energy Materials and Solar Cells 166 (2017)
    Evaluating the impact of grain boundaries on the functional properties of CdTe thin films, consistent with processes used in photovoltaic solar cells, requires a direct correlation between their crystallography and electronic behavior. In the present work, we propose a novel comprehensive approach, combining focused ion beam/electron backscatter diffraction tomography (3D-EBSD) and quantitative cathodoluminescence (CL). While the former enables a full five parameter characterization of the interfaces, the latter is used to probe the spatial distribution of recombination centers and their characteristics. In addition, critical issues associated with sample preparation are also discussed. Monte Carlo simulations, together with electron channeling contrast imaging (ECCI), are employed to evaluate the effects of ion-sputtering damage on the CL response of CdTe thin films, as well as to overcome the resolution limit of EBSD characterization. The results obtained show that, at the exception of coherent twin boundaries, all interfaces behave as non-radiative recombination centers, exhibiting significant recombination velocities. Furthermore, there is no direct correlation between the misorientation parameters of the interfaces and their recombination properties. In contrast, trends can be observed when considering the crystallography of the boundary planes. © 2017 Elsevier B.V.
    view abstract10.1016/j.solmat.2017.03.022
  • A novel type of Co–Ti–Cr-base γ/γ′ superalloys with low mass density
    Zenk, C.H. and Povstugar, I. and Li, R. and Rinaldi, F. and Neumeier, S. and Raabe, D. and Göken, M.
    Acta Materialia 135 (2017)
    A γ′ strengthened Co–Ti–Cr superalloy is presented with a mass density ∼14 % below that of typical Co–Al–W-based alloys. The lattice misfit is sufficiently low to form coherent cuboidal γ′ precipitates. Atom probe tomography shows that Cr partitions to the γ phase, but increases the γ′ volume fraction compared to a binary Co-Ti alloy to more than 60 %. The solubility of Cr in the γ′ phase is significantly higher than expected from previously published values. The γ′ solvus temperature is above 1100 °C. The yield strength shows a distinct increase above 600 °C surpassing that of Co–9Al–8W (at.%) and conventional Co-base superalloys, even more so when it is normalized by the mass density. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.06.024
  • A rare-earth free magnesium alloy with improved intrinsic ductility
    Sandlöbes, S. and Friák, M. and Korte-Kerzel, S. and Pei, Z. and Neugebauer, J. and Raabe, D.
    Scientific Reports 7 (2017)
    Metals are the backbone of manufacturing owing to their strength and formability. Compared to polymers they have high mass density. There is, however, one exception: magnesium. It has a density of only 1.7 g/cm3, making it the lightest structural material, 4.5 times lighter than steels, 1.7 times lighter than aluminum, and even slightly lighter than carbon fibers. Yet, the widespread use of magnesium is hampered by its intrinsic brittleness. While other metallic alloys have multiple dislocation slip systems, enabling their well-known ductility, the hexagonal lattice of magnesium offers insufficient modes of deformation, rendering it intrinsically brittle. We have developed a quantum-mechanically derived treasure map which screens solid solution combinations with electronic bonding, structure and volume descriptors for similarity to the ductile magnesium-rare earth alloys. Using this insight we synthesized a surprisingly simple, compositionally lean, low-cost and industry-compatible new alloy which is over 4 times more ductile and 40% stronger than pure magnesium. The alloy contains 1 wt.% aluminum and 0.1 wt.% calcium, two inexpensive elements which are compatible with downstream recycling constraints. © 2017 The Author(s).
    view abstract10.1038/s41598-017-10384-0
  • A TRIP-assisted dual-phase high-entropy alloy: Grain size and phase fraction effects on deformation behavior
    Li, Z. and Tasan, C.C. and Pradeep, K.G. and Raabe, D.
    Acta Materialia 131 (2017)
    We present a systematic microstructure oriented mechanical property investigation for a newly developed class of transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) with varying grain sizes and phase fractions. The DP-HEAs in both, as-homogenized and recrystallized states consist of a face-centered cubic (FCC) matrix containing a high-density of stacking faults and a laminate hexagonal close-packed (HCP) phase. No elemental segregation was observed in grain interiors or at interfaces even down to near-atomic resolution, as confirmed by energy-dispersive X-ray spectroscopy and atom probe tomography. The strength-ductility combinations of the recrystallized DP-HEAs (Fe50Mn30Co10Cr10) with varying FCC grain sizes and HCP phase fractions prior to deformation are superior to those of the recrystallized equiatomic single-phase Cantor reference HEA (Fe20Mn20Ni20Co20Cr20). The multiple deformation micro-mechanisms (including strain-induced transformation from FCC to HCP phase) and dynamic strain partitioning behavior among the two phases are revealed in detail. Both, strength and ductility of the DP-HEAs increase with decreasing the average FCC matrix grain size and increasing the HCP phase fraction prior to loading (in the range of 10–35%) due to the resulting enhanced stability of the FCC matrix. These insights are used to project some future directions for designing advanced TRIP-HEAs through the adjustment of the matrix phase's stability by alloy tuning and grain size effects. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.03.069
  • Ab initio assisted design of quinary dual-phase high-entropy alloys with transformation-induced plasticity
    Li, Z. and Körmann, F. and Grabowski, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 136 (2017)
    We introduce a new class of high-entropy alloys (HEAs), i.e., quinary (five-component) dual-phase (DP) HEAs revealing transformation-induced plasticity (TRIP), designed by using a quantum mechanically based and experimentally validated approach. Ab initio simulations of thermodynamic phase stabilities of Co20Cr20Fe40-xMn20Nix (x = 0–20 at. %) HEAs were performed to screen for promising compositions showing the TRIP-DP effect. The theoretical predictions reveal several promising alloys, which have been cast and systematically characterized with respect to their room temperature phase constituents, microstructures, element distributions and compositional homogeneity, tensile properties and deformation mechanisms. The study demonstrates the strength of ab initio calculations to predict the behavior of multi-component HEAs on the macroscopic scale from the atomistic level. As a prototype example a non-equiatomic Co20Cr20Fe34Mn20Ni6 HEA, selected based on our ab initio simulations, reveals the TRIP-DP effect and hence exhibits higher tensile strength and strain-hardening ability compared to the corresponding equiatomic CoCrFeMnNi alloy. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.07.023
  • Ab initio explanation of disorder and off-stoichiometry in Fe-Mn-Al-C κ carbides
    Dey, P. and Nazarov, R. and Dutta, B. and Yao, M. and Herbig, M. and Friák, M. and Hickel, T. and Raabe, D. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 95 (2017)
    Carbides play a central role for the strength and ductility in many materials. Simulating the impact of these precipitates on the mechanical performance requires knowledge about their atomic configuration. In particular, the C content is often observed to substantially deviate from the ideal stoichiometric composition. In this work, we focus on Fe-Mn-Al-C steels, for which we determined the composition of the nanosized κ carbides (Fe,Mn)3AlC by atom probe tomography in comparison to larger precipitates located in grain boundaries. Combining density functional theory with thermodynamic concepts, we first determine the critical temperatures for the presence of chemical and magnetic disorder in these carbides. Second, the experimentally observed reduction of the C content is explained as a compromise between the gain in chemical energy during partitioning and the elastic strains emerging in coherent microstructures. © 2017 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstract10.1103/PhysRevB.95.104108
  • Abnormal grain growth in Eurofer-97 steel in the ferrite phase field
    Oliveira, V.B. and Sandim, H.R.Z. and Raabe, D.
    Journal of Nuclear Materials 485 (2017)
    Reduced-activation ferritic-martensitic (RAFM) Eurofer-97 steel is a candidate material for structural applications in future fusion reactors. Depending on the amount of prior cold rolling strain and annealing temperature, important solid-state softening reactions such as recovery, recrystallization, and grain growth occur. Eurofer-97 steel was cold rolled up to 70, 80 and 90% reductions in thickness and annealed in the ferrite phase field (below ≈ 800 °C). Changes in microstructure, micro-, and mesotexture were followed by orientation mappings provided by electron backscatter diffraction (EBSD). Eurofer-97 steel undergoes abnormal grain growth above 650 °C and this solid-state reaction seems to be closely related to the high mobility of a few special grain boundaries that overcome pinning effects caused by fine particles. This solid-state reaction promotes important changes in the microstructure and microtexture of this steel. Abnormal grain growth kinetics for each condition was determined by means of quantitative metallography. © 2016 Elsevier B.V.
    view abstract10.1016/j.jnucmat.2016.12.019
  • Atomic diffusion induced degradation in bimetallic layer coated cemented tungsten carbide
    Peng, Z. and Rohwerder, M. and Choi, P.-P. and Gault, B. and Meiners, T. and Friedrichs, M. and Kreilkamp, H. and Klocke, F. and Raabe, D.
    Corrosion Science 120 (2017)
    We investigated the temporal degradation of glass moulding dies, made of cemented tungsten carbide coated with PtIr on an adhesive Cr or Ni interlayer, by electron microscopy and atom probe tomography. During the exposure treatments at 630 °C under an oxygen partial pressure of 1.12 × 10−23 bar, Cr (Ni) was found to diffuse outwards via grain boundaries in the PtIr, altering the surface morphology. Upon dissolution of the interlayer, the WC substrate also started degrading. Extensive interdiffusion processes involving PtIr, Cr (Ni) and WC took place, leading to the formation of intermetallic phases and voids, deteriorating the adhesion of the coating. © 2017 The Authors
    view abstract10.1016/j.corsci.2017.01.007
  • Atomic scale characterization of white etching area and its adjacent matrix in a martensitic 100Cr6 bearing steel
    Li, Y.J. and Herbig, M. and Goto, S. and Raabe, D.
    Materials Characterization 123 (2017)
    Atom probe tomography was employed to characterize the microstructure and C distribution in the white etching area (WEA) of a martensitic 100Cr6 bearing steel subjected to rolling contact fatigue. Different from its surrounding matrix where a plate-like martensitic structure prevails, the WEA exhibits equiaxed grains with a uniform grain size of about 10 nm. Significant C grain boundary enrichment (>7.5at.%) and an overall higher C concentration than the nominal value are observed in the WEA. These results suggest that the formation of WEA results from severe local plastic deformation that causes dissolution of carbides and the redistribution of C. © 2016 Elsevier Inc.
    view abstract10.1016/j.matchar.2016.12.002
  • Bone-like crack resistance in hierarchical metastable nanolaminate steels
    Koyama, M. and Zhang, Z. and Wang, M. and Ponge, D. and Raabe, D. and Tsuzaki, K. and Noguchi, H. and Tasan, C.C.
    Science 355 (2017)
    Fatigue failures create enormous risks for all engineered structures, as well as for human lives, motivating large safety factors in design and, thus, inefficient use of resources. Inspired by the excellent fracture toughness of bone, we explored the fatigue resistance in metastability-assisted multiphase steels. We show here that when steel microstructures are hierarchical and laminated, similar to the substructure of bone, superior crack resistance can be realized. Our results reveal that tuning the interface structure, distribution, and phase stability to simultaneously activate multiple micromechanisms that resist crack propagation is key for the observed leap in mechanical response. The exceptional properties enabled by this strategy provide guidance for all fatigue-resistant alloy design efforts. © 2017, American Association for the Advancement of Science. All rights reserved.
    view abstract10.1126/science.aal2766
  • Cd and Impurity Redistribution at the CdS/CIGS Interface after Annealing of CIGS-Based Solar Cells Resolved by Atom Probe Tomography
    Koprek, A. and Cojocaru-Miredin, O. and Wuerz, R. and Freysoldt, C. and Gault, B. and Raabe, D.
    IEEE Journal of Photovoltaics 7 (2017)
    Cd and impurity redistribution in the vicinity of the CdS/C(In,Ga)Se2 (CIGS) interface is studied by means of atom probe tomography. We find an increase of the Cd content in the CIGS layer and redistribution of O (in form of O+ and OH+) at the CdS/CIGS interface after annealing the samples at 200, 250, and 300 °C. About 0.2 at% of Na impurity is observed at the interface, across the range of heat treatments performed here. Simultaneously, the J-V measurements of the treated samples show a drop in the open-circuit voltage and fill factor, and thus of the cell efficiency, compared with the untreated sample. © 2011-2012 IEEE.
    view abstract10.1109/JPHOTOV.2016.2629841
  • Coarsening of Y-rich oxide particles in 9%Cr-ODS Eurofer steel annealed at 1350 °C
    Sandim, M.J.R. and Souza Filho, I.R. and Bredda, E.H. and Kostka, A. and Raabe, D. and Sandim, H.R.Z.
    Journal of Nuclear Materials 484 (2017)
    Oxide-dispersion strengthened (ODS) Eurofer steel is targeted for structural applications in future fusion nuclear reactors. Samples were cold rolled down to 80% reduction in thickness and annealed at 1350 °C up to 8 h. The microstructural characterization was performed using Vickers microhardness testing, electron backscatter diffraction, scanning and scanning transmission electron microscopies. Experimental results provide evidence of coarsening of the Y-rich oxide particles in ODS-Eurofer steel annealed at 1350 °C within delta ferrite phase field. © 2016 Elsevier B.V.
    view abstract10.1016/j.jnucmat.2016.12.025
  • Combinatorial Alloy Design by Laser Additive Manufacturing
    Knoll, H. and Ocylok, S. and Weisheit, A. and Springer, H. and Jägle, E. and Raabe, D.
    Steel Research International 88 (2017)
    The authors uses laser additive manufacturing (LAM) as a combinatorial method for synthesizing microstructurally and compositionally piecewise graded bulk alloys. Authors fabricate blocks consisting of a sequence of ≈500μm thick tool steel layers, each with different chemical composition, by laser metal deposition where alloy powders are deposited layer-wise on a substrate. The reference materials are a Cr-Mo-V hot working tool steel and a Ni-based maraging steel. The layers between them consist of corresponding blends of the two materials with varying composition from layer to layer (alloy volume fractions 80:20, 60:40, 40:60, and 20:80). The bulk alloy is hot rolled and heat treated. Subsequently each layer is characterized for microstructure, chemical composition and mechanical properties using electron back scatter diffraction, tensile testing, and indentation. The approach is an efficient high-throughput method enabling rapid probing of novel compositional alloy blends. It can be applied for finding new alloys both, by LAM and for LAM. For the tool steel blends synthesized here, authors observe that the Cr-Mo-V tool steel, when mixed with the Ni-base maraging steel, can be continuously tuned for a strength-ductility profile in the range of 800-1650MPa strength and 15-25% tensile elongation. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201600416
  • Comparison of maraging steel micro- and nanostructure produced conventionally and by laser additive manufacturing
    Jägle, E.A. and Sheng, Z. and Kürnsteiner, P. and Ocylok, S. and Weisheit, A. and Raabe, D.
    Materials 10 (2017)
    Maraging steels are used to produce tools by Additive Manufacturing (AM) methods such as Laser Metal Deposition (LMD) and Selective Laser Melting (SLM). Although it is well established that dense parts can be produced by AM, the influence of the AM process on the microstructure-in particular the content of retained and reversed austenite as well as the nanostructure, especially the precipitate density and chemistry, are not yet explored. Here, we study these features using microhardness measurements, Optical Microscopy, Electron Backscatter Diffraction (EBSD), Energy Dispersive Spectroscopy (EDS), and Atom Probe Tomography (APT) in the as-produced state and during ageing heat treatment. We find that due to microsegregation, retained austenite exists in the as-LMD- and as-SLM-produced states but not in the conventionally-produced material. The hardness in the as-LMD-produced state is higher than in the conventionally and SLM-produced materials, however, not in the uppermost layers. By APT, it is confirmed that this is due to early stages of precipitation induced by the cyclic re-heating upon further deposition-i.e., the intrinsic heat treatment associated with LMD. In the peak-aged state, which is reached after a similar time in all materials, the hardness of SLM- and LMD-produced material is slightly lower than in conventionally-produced material due to the presence of retained austenite and reversed austenite formed during ageing. © 2017 by the authors.
    view abstract10.3390/ma10010008
  • Comparison of the quantitative analysis performance between pulsed voltage atom probe and pulsed laser atom probe
    Takahashi, J. and Kawakami, K. and Raabe, D.
    Ultramicroscopy 175 (2017)
    The difference in quantitative analysis performance between the voltage-mode and laser-mode of a local electrode atom probe (LEAP3000X HR) was investigated using a Fe-Cu binary model alloy. Solute copper atoms in ferritic iron preferentially field evaporate because of their significantly lower evaporation field than the matrix iron, and thus, the apparent concentration of solute copper tends to be lower than the actual concentration. However, in voltage-mode, the apparent concentration was higher than the actual concentration at 40 K or less due to a detection loss of matrix iron, and the concentration decreased with increasing specimen temperature due to the preferential evaporation of solute copper. On the other hand, in laser-mode, the apparent concentration never exceeded the actual concentration, even at lower temperatures (20 K), and this mode showed better quantitative performance over a wide range of specimen temperatures. These results indicate that the pulsed laser atom probe prevents both detection loss and preferential evaporation under a wide range of measurement conditions. © 2017 Elsevier B.V.
    view abstract10.1016/j.ultramic.2017.01.015
  • Competitive bcc and fcc crystal nucleation from non-equilibrium liquids studied by phase-field crystal simulation
    Tang, S. and Wang, J.C. and Svendsen, B. and Raabe, D.
    Acta Materialia 139 (2017)
    Crystal nucleation is among the most important processes in the synthesis of materials. Here we study the competitive and multistep nucleation process of body centered cubic (bcc) and face centered cubic (fcc) crystals using phase-field crystal simulations. The initial state is a non-equilibrium liquid. This transforms into an amorphous phase composed of clusters with short-range order (SRO) and medium-range order (MRO). Crystal nucleation begins with the formation of MRO clusters structurally similar to the subsequently nucleated crystal. The formation of bcc and fcc nuclei from MRO clusters involves the following steps: (1) formation of a small thin platelet with MRO; (2) its subsequent growth into 3D MRO clusters; (3) development of crystal embryos from MRO clusters; and (4) crystal embryos transformation into a stable crystal nuclei. In addition, the role of bcc precursors in the formation of fcc nuclei is clarified. In particular, {112} surfaces and steps on {110} surfaces of bcc precursors serve as energetically favorable sites for fcc nucleation. This is also reflected in the resulting orientation relationships, i.e., Pitsch, Nishiyama-Wassermann and Kurdjumov-Sachs, between the bcc precursor and fcc nucleus. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.08.015
  • Complexion-mediated martensitic phase transformation in Titanium
    Zhang, J. and Tasan, C.C. and Lai, M.J. and Dippel, A.-C. and Raabe, D.
    Nature Communications 8 (2017)
    The most efficient way to tune microstructures and mechanical properties of metallic alloys lies in designing and using athermal phase transformations. Examples are shape memory alloys and high strength steels, which together stand for 1,500 million tons annual production. In these materials, martensite formation and mechanical twinning are tuned via composition adjustment for realizing complex microstructures and beneficial mechanical properties. Here we report a new phase transformation that has the potential to widen the application window of Ti alloys, the most important structural material in aerospace design, by nanostructuring them via complexion-mediated transformation. This is a reversible martensitic transformation mechanism that leads to a final nanolaminate structure of α″ (orthorhombic) martensite bounded with planar complexions of athermal ω (a-ω hexagonal). Both phases are crystallographically related to the parent β (BCC) matrix. As expected from a planar complexion, the a-ω is stable only at the hetero-interface. © The Author(s) 2017.
    view abstract10.1038/ncomms14210
  • Confined chemical and structural states at dislocations in Fe-9wt%Mn steels: A correlative TEM-atom probe study combined with multiscale modelling
    Kwiatkowski da Silva, A. and Leyson, G. and Kuzmina, M. and Ponge, D. and Herbig, M. and Sandlöbes, S. and Gault, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 124 (2017)
    We investigated a high-purity cold-rolled martensitic Fe-9wt%Mn alloy. Tensile tests performed at room temperature after tempering for 6 h at 450 °C showed discontinuous yielding. Such static strain ageing phenomena in Fe are usually associated with the segregation of interstitial elements such as C or N to dislocations. Here we show by correlative transmission electron microscopy (TEM)/atom probe tomography (APT) experiments that in this case Mn segregation to edge dislocations associated with the formation of confined austenitic states causes similar effects. The local chemical composition at the dislocation cores was investigated for different tempering temperatures by APT relative to the adjacent bcc matrix. In all cases the Mn partitioning to the dislocation core regions matches to the one between ferrite and austenite in thermodynamic equilibrium at the corresponding tempering temperature. Although a stable structural and chemical confined austenitic state has formed at the dislocation cores these regions do not grow further even upon prolonged tempering. Simulation reveals that the high Mn enrichment along the edge dislocation lines (25 at.%Mn at 450 °C) cannot be described merely as a Cottrell atmosphere formed by segregation driven by size interaction. Thermodynamic calculations based on a multiscale model indicate that these austenite states at the dislocation cores are subcritical and defect-stabilized by the compression stress field of the edge dislocations. Phenomenologically, these states are the 1D equivalent to the so-called complexions which have been extensively reported to be present at 2D defects, hence have been named linear complexions. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.11.013
  • Constitutive modeling of strain induced grain boundary migration via coupling crystal plasticity and phase-field methods
    Jafari, M. and Jamshidian, M. and Ziaei-Rad, S. and Raabe, D. and Roters, F.
    International Journal of Plasticity 99 (2017)
    We have developed a thermodynamically-consistent finite-deformation-based constitutive theory to describe strain induced grain boundary migration due to the heterogeneity of stored deformation energy in a plastically deformed polycrystalline cubic metal. Considering a representative volume element, a mesoscale continuum theory is developed based on the coupling between dislocation density-based crystal plasticity and phase field methods. Using the Taylor model-based homogenization method, a multiscale coupled finite-element and phase-field staggered time integration procedure is developed and implemented into the Abaqus/Standard finite element package via a user-defined material subroutine. The developed constitutive model is then used to perform numerical simulations of strain induced grain boundary migration in polycrystalline tantalum. The simulation results are shown to qualitatively and quantitatively agree with experimental results. © 2017 Elsevier Ltd.
    view abstract10.1016/j.ijplas.2017.08.004
  • Core-shell nanoparticle arrays double the strength of steel
    Seol, J.-B. and Na, S.-H. and Gault, B. and Kim, J.-E. and Han, J.-C. and Park, C.-G. and Raabe, D.
    Scientific Reports 7 (2017)
    Manipulating structure, defects and composition of a material at the atomic scale for enhancing its physical or mechanical properties is referred to as nanostructuring. Here, by combining advanced microscopy techniques, we unveil how formation of highly regular nano-arrays of nanoparticles doubles the strength of an Fe-based alloy, doped with Ti, Mo, and V, from 500 MPa to 1 GPa, upon prolonged heat treatment. The nanoparticles form at moving heterophase interfaces during cooling from the high-temperature face-centered cubic austenite to the body-centered cubic ferrite phase. We observe MoC and TiC nanoparticles at early precipitation stages as well as core-shell nanoparticles with a Ti-C rich core and a Mo-V rich shell at later precipitation stages. The core-shell structure hampers particle coarsening, enhancing the material's strength. Designing such highly organized metallic core-shell nanoparticle arrays provides a new pathway for developing a wide range of stable nano-architectured engineering metallic alloys with drastically enhanced properties. ©The Author(s) 2017.
    view abstract10.1038/srep42547
  • Correlating Atom Probe Tomography with Atomic-Resolved Scanning Transmission Electron Microscopy: Example of Segregation at Silicon Grain Boundaries
    Stoffers, A. and Barthel, J. and Liebscher, C.H. and Gault, B. and Cojocaru-Mirédin, O. and Scheu, C. and Raabe, D.
    Microscopy and Microanalysis (2017)
    In the course of a thorough investigation of the performance-structure-chemistry interdependency at silicon grain boundaries, we successfully developed a method to systematically correlate aberration-corrected scanning transmission electron microscopy and atom probe tomography. The correlative approach is conducted on individual APT and TEM specimens, with the option to perform both investigations on the same specimen in the future. In the present case of a Σ9 grain boundary, joint mapping of the atomistic details of the grain boundary topology, in conjunction with chemical decoration, enables a deeper understanding of the segregation of impurities observed at such grain boundaries. © Microscopy Society of America 2017
    view abstract10.1017/S1431927617000034
  • Coupled Crystal Plasticity–Phase Field Fracture Simulation Study on Damage Evolution Around a Void: Pore Shape Versus Crystallographic Orientation
    Diehl, M. and Wicke, M. and Shanthraj, P. and Roters, F. and Brueckner-Foit, A. and Raabe, D.
    JOM 69 (2017)
    Various mechanisms such as anisotropic plastic flow, damage nucleation, and crack propagation govern the overall mechanical response of structural materials. Understanding how these mechanisms interact, i.e. if they amplify mutually or compete with each other, is an essential prerequisite for the design of improved alloys. This study shows—by using the free and open source software DAMASK (the Düsseldorf Advanced Material Simulation Kit)—how the coupling of crystal plasticity and phase field fracture methods can increase the understanding of the complex interplay between crystallographic orientation and the geometry of a void. To this end, crack initiation and propagation around an experimentally obtained pore with complex shape is investigated and compared to the situation of a simplified spherical void. Three different crystallographic orientations of the aluminum matrix hosting the defects are considered. It is shown that crack initiation and propagation depend in a non-trivial way on crystallographic orientation and its associated plastic behavior as well as on the shape of the pore. © 2017, The Author(s).
    view abstract10.1007/s11837-017-2308-8
  • Crystal plasticity study on stress and strain partitioning in a measured 3D dual phase steel microstructure
    Diehl, M. and An, D. and Shanthraj, P. and Zaefferer, S. and Roters, F. and Raabe, D.
    Physical Mesomechanics 20 (2017)
    Dual phase steels are advanced high strength alloys typically used for structural parts and reinforcements in car bodies. Their good combination of strength and ductility and their lean composition render them an economically competitive option for realizing multiple lightweight design options in automotive engineering. The mechanical response of dual phase steels is the result of the strain and stress partitioning among the ferritic and martensitic phases and the individual crystallographic grains and subgrains of these phases. Therefore, understanding how these microstructural features influence the global and local mechanical properties is of utmost importance for the design of improved dual phase steel grades. While multiple corresponding simulation studies have been dedicated to the investigation of dual phase steel micromechanics, numerical tools and experiment techniques for characterizing and simulating real 3D microstructures of such complex materials have been emerged only recently. Here we present a crystal plasticity simulation study based on a 3D dual phase microstructure which is obtained by EBdD tomography, also referred to as 3D EBdD (EBdD—electron backscatter diffraction). In the present case we utilized a 3D EBdD serial sectioning approach based on mechanical polishing. Moreover, sections of the 3D microstructure are used as 2D models to study the effect of this simplification on the stress and strain distribution. The simulations are conducted using a phenomenological crystal plasticity model and a spectral method approach implemented in the Düsseldorf Advanced Material Simulation Kit (DAMAdK). © 2017, Pleiades Publishing, Ltd.
    view abstract10.1134/S1029959917030079
  • Crystallisation of amorphous Fe – Ti – B alloys as a design pathway for nano-structured high modulus steels
    Aparicio-Fernández, R. and Szczepaniak, A. and Springer, H. and Raabe, D.
    Journal of Alloys and Compounds 704 (2017)
    We systematically studied the transformation temperatures, type and growth kinetics of phases formed during annealing of Fe – 10.10 Ti – 3.86 B alloys, initially amorphous after rapid solidification. With increasing the temperature, four distinct crystallisation steps could be determined. At 530 °C α-Fe (partly supersaturated with Ti) appeared, and at 600 °C small amounts of not identified phases formed in the amorphous matrix. The matrix fully crystallised to α-Fe and contained (Fe,Ti)-B rich particles at 730 °C, which transformed to the equilibrium TiB2 phase at 755 °C. During annealing at 1000 °C both α-Fe and TiB2 exhibited parabolic, apparently diffusion controlled growth at rates of about 2.333 and 0.466 nm s−1/2, respectively. Factors influencing the amorphisation as well as strategies for the fabrication of nano-structured high stiffness low density steels via crystallisation from an amorphous precursor state are outlined and discussed. © 2017 Elsevier B.V.
    view abstract10.1016/j.jallcom.2017.02.077
  • Design of Mg alloys: The effects of Li concentration on the structure and elastic properties in the Mg-Li binary system by first principles calculations
    Pavlic, O. and Ibarra-Hernandez, W. and Valencia-Jaime, I. and Singh, S. and Avendaño-Franco, G. and Raabe, D. and Romero, A.H.
    Journal of Alloys and Compounds 691 (2017)
    First principles calculations have been employed to search for energetically stable structures of the Mg-Li binary system over all possible Mg concentrations. Volume, space group, c/a and b/a ratios, vibrational contribution to the heat capacity, and Debye temperature are reported to investigate the effects of Li concentration on the Mg-Li binary system. Structures of high symmetry and those lying on or close to the convex hull are further explored to search for dynamic and elastic stability. Five ground state structures were found at Li1 Mg13, Li1 Mg2, Li1 Mg1, Li7 Mg2, and Li15 Mg1 compositions. Elastic constants and elastic properties of the selected low energy structures were calculated and analyzed. We also investigate the effect of the phonon band gap found in Li1 Mg1 and Li1 Mg2 on lattice thermal conductivity. © 2016 Elsevier B.V.
    view abstract10.1016/j.jallcom.2016.08.217
  • Designing duplex, ultrafine-grained Fe-Mn-Al-C steels by tuning phase transformation and recrystallization kinetics
    Zhang, J. and Raabe, D. and Tasan, C.C.
    Acta Materialia 141 (2017)
    A novel, lightweight Fe-25.7Mn-10.6Al-1.2C (wt.%) steel is designed by exploiting the concurrent progress of primary recrystallization and phase transformation, in order to produce an ultrafine-grained, duplex microstructure. The microstructure consists of recrystallized austenite grains surrounded by submicron-sized ferrite grains, and recovered austenite regions with preferential nano-κ-carbide precipitation. This partially recrystallized duplex microstructure demonstrates excellent strength-ductility combinations, e.g. a yield strength of 1251 MPa, an ultimate tensile strength of 1387 MPa, and a total elongation of 43%, arising from the composite response by virtue of diverging constituent strength and strain hardening behaviors. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.09.026
  • Diffusional-displacive transformation enables formation of long-period stacking order in magnesium
    Kim, J.-K. and Jin, L. and Sandlöbes, S. and Raabe, D.
    Scientific Reports 7 (2017)
    Mg is the most important lightweight engineering alloy enabling future weight-reduced and fuel-saving engineering solutions. Yet, Mg is soft. Long-period stacking ordered (LPSO) structures in Mg alloys have unique crystal structures, characterized by both complex chemical and stacking order. They are essential for strengthening of Mg alloys. The formation mechanism of these LPSO structures is still under discussion. Here we report that Y/Zn enriched Guinier-Preston (GP) zones observed in a lean Mg-Y-Zn model alloy are precursors of early stage LPSO structures. We provide evidence of a new type of phase transformation mechanism which comprises the diffusional formation of Y/Zn enriched GP zones and their subsequent shear transformation into LPSO building blocks. The mechanism constitutes a new type of coupled diffusional-displacive phase formation sequence which may also be applicable to other alloy systems. © 2017 The Author(s).
    view abstract10.1038/s41598-017-04343-y
  • Dislocation activities at the martensite phase transformation interface in metastable austenitic stainless steel: An in-situ TEM study
    Liu, J. and Chen, C. and Feng, Q. and Fang, X. and Wang, H. and Liu, F. and Lu, J. and Raabe, D.
    Materials Science and Engineering A 703 (2017)
    Understanding the mechanism of martensitic transformation is of great importance in developing advanced high strength steels, especially TRansformation-Induced Plasticity (TRIP) steels. The TRIP effect leads to enhanced work-hardening rate, postponed onset of necking and excellent formability. In-situ transmission electron microscopy has been performed to systematically investigate the dynamic interactions between dislocations and α′ martensite at microscale. Local stress concentrations, e.g. from notches or dislocation pile-ups, render free edges and grain boundaries favorable nucleation sites for α′ martensite. Its growth leads to partial dislocation emission on two independent slip planes from the hetero-interface when the austenite matrix is initially free of dislocations. The kinematic analysis reveals that activating slip systems on two independent {111} planes of austenite are necessary in accommodating the interfacial mismatch strain. Full dislocation emission is generally observed inside of austenite regions that contain high density of dislocations. In both situations, phase boundary propagation generates large amounts of dislocations entering into the matrix, which renders the total deformation compatible and provide substantial strain hardening of the host phase. These moving dislocation sources enable plastic relaxation and prevent local damage accumulation by intense slipping on the softer side of the interfacial region. Thus, finely dispersed martensite distribution renders plastic deformation more uniform throughout the austenitic matrix, which explains the exceptional combination of strength and ductility of TRIP steels. © 2017 Elsevier B.V.
    view abstract10.1016/j.msea.2017.06.107
  • Dislocation interaction and twinning-induced plasticity in face-centered cubic Fe-Mn-C micro-pillars
    Choi, W.S. and Sandlöbes, S. and Malyar, N.V. and Kirchlechner, C. and Korte-Kerzel, S. and Dehm, G. and De Cooman, B.C. and Raabe, D.
    Acta Materialia 132 (2017)
    Deformation twinning contributes to a high work-hardening rate through modification of the dislocation structure and a dynamic Hall-Petch effect in polycrystalline steel. Due to the well-defined compression axis and limited deformation volume of micro-pillars, micro-compression testing is a suitable method to investigate the mechanisms of deformation twinning and the interactions of dislocations with twin boundaries. The material investigated is an austenitic Fe-22 wt%Mn-0.6 wt%C twining-induced plasticity steel. Micro-pillars oriented preferentially for deformation twinning and dislocation glide are compressed and the activated deformation systems are characterized. We observe that deformation twinning induces higher flow stresses and a more unstable work-hardening behavior than dislocation glide, while dislocation glide dominated deformation results in a stable work-hardening behavior. The higher flow stresses and unstable work-hardening behavior in micro-pillars oriented for deformation twinning are assumed to be caused by the activation of secondary slip systems and accumulated plastic deformation. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.04.043
  • Effect of silicon on the microstructure and growth kinetics of intermetallic phases formed during hot-dip aluminizing of ferritic steel
    Lemmens, B. and Springer, H. and De Graeve, I. and De Strycker, J. and Raabe, D. and Verbeken, K.
    Surface and Coatings Technology 319 (2017)
    view abstract10.1016/j.surfcoat.2017.03.040
  • Elasto-viscoplastic phase field modelling of anisotropic cleavage fracture
    Shanthraj, P. and Svendsen, B. and Sharma, L. and Roters, F. and Raabe, D.
    Journal of the Mechanics and Physics of Solids 99 (2017)
    A finite-strain anisotropic phase field method is developed to model the localisation of damage on a defined family of crystallographic planes, characteristic of cleavage fracture in metals. The approach is based on the introduction of an undamaged configuration, and the inelastic deformation gradient mapping this configuration to a damaged configuration is microstructurally represented by the opening of a set of cleavage planes in the three fracture modes. Crack opening is modelled as a dissipative process, and its evolution is thermodynamically derived. To couple this approach with a physically-based phase field method for brittle fracture, a scalar measure of the overall local damage is introduced, whose evolution is determined by the crack opening rates, and weakly coupled with the non-local phase field energy representing the crack opening resistance in the classical sense of Griffith. A finite-element implementation of the proposed model is employed to simulate the crack propagation path in a laminate and a polycrystalline microstructure. As shown in this work, it is able to predict the localisation of damage on the set of pre-defined cleavage planes, as well as the kinking and branching of the crack resulting from the crystallographic misorientation across the laminate boundary and the grain boundaries respectively. © 2016 Elsevier Ltd
    view abstract10.1016/j.jmps.2016.10.012
  • Enhanced strength and ductility in a friction stir processing engineered dual phase high entropy alloy
    Nene, S.S. and Liu, K. and Frank, M. and Mishra, R.S. and Brennan, R.E. and Cho, K.C. and Li, Z. and Raabe, D.
    Scientific Reports 7 (2017)
    The potential of high-entropy alloys (HEAs) to exhibit an extraordinary combination of properties by shifting the compositional regime from the corners towards the centers of phase diagrams has led to worldwide attention by material scientists. Here we present a strong and ductile non-equiatomic HEA obtained after friction stir processing (FSP). A transformation-induced plasticity (TRIP) assisted HEA with composition Fe50Mn30Co10Cr10 (at.%) was severely deformed by FSP and evaluated for its microstructure-mechanical property relationship. The FSP-engineered microstructure of the TRIP HEA exhibited a substantially smaller grain size, and optimized fractions of face-centered cubic (f.c.c., γ) and hexagonal close-packed (h.c.p., ϵ) phases, as compared to the as-homogenized reference material. This results in synergistic strengthening via TRIP, grain boundary strengthening, and effective strain partitioning between the γ and ϵ phases during deformation, thus leading to enhanced strength and ductility of the TRIP-assisted dual-phase HEA engineered via FSP. © 2017 The Author(s).
    view abstract10.1038/s41598-017-16509-9
  • Evaluation of Analysis Conditions for Laser-Pulsed Atom Probe Tomography: Example of Cemented Tungsten Carbide
    Peng, Z. and Choi, P.-P. and Gault, B. and Raabe, D.
    Microscopy and Microanalysis 23 (2017)
    Cemented tungsten carbide has been analyzed using laser-pulsed atom probe tomography (APT). The influence of experimental parameters, including laser pulse energy, pulse repetition rate, and specimen base temperature, on the acquired data were evaluated from different aspects, such as mass spectrum, chemical composition, noise-to-signal ratio, and multiple events. Within all the applied analysis conditions, only 1 MHz pulse repetition rate led to a strong detector saturation effect, resulting in a largely biased chemical composition. A comparative study of the laser energy settings showed that an ~12 times higher energy was required for the less focused green laser of the LEAPTM 3000X HR system to achieve a similar evaporation field as the finer spot ultraviolet laser of the LEAPTM 5000 XS system. © Microscopy Society of America 2016
    view abstract10.1017/S1431927616012654
  • Formation of nanometer-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films and their effect on solar cell efficiency
    Schwarz, T. and Cojocaru-Mirédin, O. and Mousel, M. and Redinger, A. and Raabe, D. and Choi, P.-P.
    Acta Materialia 132 (2017)
    Atom probe tomography and transmission electron microscopy are used to study the formation of nano-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films. For a Cu-rich precursor, which was deposited at 320 °C under Cu- and Zn-rich growth conditions, Cu2-xSe grains at the surface are detected. During annealing the precursor at 500 °C in a SnSe + Se atmosphere most of the Cu2-xSe is transformed to Cu2ZnSnSe4 via the consumption of excessive ZnSe and incorporation of Sn. However, atom probe tomography studies also reveal the formation of various nanometer-sized Cu-Sn-Se particles close to the CdS/Cu2ZnSnSe4 interface. One of those particles has a composition close to the Cu2SnSe3 compound. This phase has a smaller band gap than Cu2ZnSnSe4 and is proposed to lead to a significant drop in the open-circuit voltage and could be the main cause for a detrimental p-n junction and the zero efficiency of the final device. Possible effects of the other phases on solar cell performance and formation mechanisms are discussed as well. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.04.056
  • Grain boundary character distribution in electroplated nanotwinned copper
    Ratanaphan, S. and Raabe, D. and Sarochawikasit, R. and Olmsted, D.L. and Rohrer, G.S. and Tu, K.N.
    Journal of Materials Science 52 (2017)
    The grain boundary character distribution (GBCD) of nanotwinned copper, fabricated by electroplating inside small-scale through-wafer vias, was characterized using a stereological interpretation of electron backscatter diffraction maps. The GBCD of electroplated nanotwinned copper, specified by five macroscopic parameters (three for the lattice misorientation and two for the grain boundary plane inclination), is similar to the GBCD of coarse-grained polycrystalline copper used here as a reference material. The GBCD was compared to calculated grain boundary energies determined from atomistic simulations. We find that the grain boundary population in the electroplated nanotwinned and coarse-grained reference copper is both on average inversely correlated to the grain boundary energies. The slopes of the relationships between grain boundary population and energy for the most highly populated misorientations (Σ3, Σ9, and Σ11) are different. The relationships are strongly influenced by the geometric constraints at the triple junctions and multiple twinning, which enhanced the observed frequencies of Σ9 boundaries. The results suggest that the grain boundary network and the GBCD in the polycrystalline specimens are strongly influenced by the microstructure, grain boundary energy, and multiple twining. © 2016, Springer Science+Business Media New York.
    view abstract10.1007/s10853-016-0670-5
  • Hydrogen effects on microstructural evolution and passive film characteristics of a duplex stainless steel
    Luo, H. and Li, Z. and Chen, Y.-H. and Ponge, D. and Rohwerder, M. and Raabe, D.
    Electrochemistry Communications 79 (2017)
    We revealed the effects of hydrogen on the microstructural evolution and passive film properties of a 2205 duplex stainless steel by the joint use of electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. The microstructural analysis results show that effects of hydrogen on the two phases are different: (i) in austenite, stacking faults are induced by hydrogen, and (ii) in ferrite, hydrogen causes an increase of the dislocation density. The XPS analysis revealed that hydrogen reduced the occurrence of Cr2O3 and nitrogen in the passive film, leading to the reduction of their overall thickness. Furthermore, for the first time we demonstrated that the hydrogen release time plays an important role in the electrochemical behavior of the hydrogen charged steel. © 2017 Elsevier B.V.
    view abstract10.1016/j.elecom.2017.04.013
  • Hydrogen enhances strength and ductility of an equiatomic high-entropy alloy
    Luo, H. and Li, Z. and Raabe, D.
    Scientific Reports 7 (2017)
    Metals are key materials for modern manufacturing and infrastructures as well as transpot and energy solutions owing to their strength and formability. These properties can severely deteriorate when they contain hydrogen, leading to unpredictable failure, an effect called hydrogen embrittlement. Here we report that hydrogen in an equiatomic CoCrFeMnNi high-entropy alloy (HEA) leads not to catastrophic weakening, but instead increases both, its strength and ductility. While HEAs originally aimed at entropy-driven phase stabilization, hydrogen blending acts opposite as it reduces phase stability. This effect, quantified by the alloy's stacking fault energy, enables nanotwinning which increases the material's work-hardening. These results turn a bane into a boon: hydrogen does not generally act as a harmful impurity, but can be utilized for tuning beneficial hardening mechanisms. This opens new pathways for the design of strong, ductile, and hydrogen tolerant materials. © 2017 The Author(s).
    view abstract10.1038/s41598-017-10774-4
  • Hydrogen-assisted failure in Ni-based superalloy 718 studied under in situ hydrogen charging: The role of localized deformation in crack propagation
    Tarzimoghadam, Z. and Ponge, D. and Klöwer, J. and Raabe, D.
    Acta Materialia 128 (2017)
    We investigated hydrogen embrittlement in Ni-based superalloy 718 by tensile testing at slow strain rate (10−4 s−1) under continuous electrochemical hydrogen charging. Hydrogen-assisted cracking mechanisms were studied via electron backscatter diffraction (EBSD) analysis and electron channeling contrast imaging (ECCI). In order to elucidate the effects of stress or strain in the cracking mechanisms, material conditions with different strength levels were investigated, including samples in solution annealed (as water quenched) and 780 °C age-hardened states. The microstructure observations in the vicinity of the cracks enabled us to establish correlations between the microstructure, crack initiation sites, and crack propagation pathways. Fracture in the hydrogen-charged samples was dominated by localized plastic deformation. Strain-controlled transgranular cracking was caused by shear localization due to hydrogen-enhanced localized plasticity (HELP) and void nucleation and coalescence along {111} slip planes in both, the solution annealed and age-hardened materials. Stress-assisted intergranular cracking in the presence of hydrogen was only observed in the high strength age-hardened material, due to slip localization at grain boundaries, grain boundary triple junction cracking, and δ/γ-matrix interface cracking. To investigate the effect of δ-phase in crack propagation along grain boundaries, the over-aged state (aged at 870 °C) with different precipitation conditions for the δ-phase was also investigated. Observations confirmed that presence of δ-phase promotes hydrogen-induced intergranular failure by initializing micro-cracks from δ/γ interfaces. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.02.059
  • Identifying Structure–Property Relationships Through DREAM.3D Representative Volume Elements and DAMASK Crystal Plasticity Simulations: An Integrated Computational Materials Engineering Approach
    Diehl, M. and Groeber, M. and Haase, C. and Molodov, D.A. and Roters, F. and Raabe, D.
    JOM 69 (2017)
    Predicting, understanding, and controlling the mechanical behavior is the most important task when designing structural materials. Modern alloy systems—in which multiple deformation mechanisms, phases, and defects are introduced to overcome the inverse strength–ductility relationship—give raise to multiple possibilities for modifying the deformation behavior, rendering traditional, exclusively experimentally-based alloy development workflows inappropriate. For fast and efficient alloy design, it is therefore desirable to predict the mechanical performance of candidate alloys by simulation studies to replace time- and resource-consuming mechanical tests. Simulation tools suitable for this task need to correctly predict the mechanical behavior in dependence of alloy composition, microstructure, texture, phase fractions, and processing history. Here, an integrated computational materials engineering approach based on the open source software packages DREAM.3D and DAMASK (Düsseldorf Advanced Materials Simulation Kit) that enables such virtual material development is presented. More specific, our approach consists of the following three steps: (1) acquire statistical quantities that describe a microstructure, (2) build a representative volume element based on these quantities employing DREAM.3D, and (3) evaluate the representative volume using a predictive crystal plasticity material model provided by DAMASK. Exemplarily, these steps are here conducted for a high-manganese steel. © 2017, The Author(s).
    view abstract10.1007/s11837-017-2303-0
  • Influence of compositional inhomogeneity on mechanical behavior of an interstitial dual-phase high-entropy alloy
    Li, Z. and Raabe, D.
    Materials Chemistry and Physics (2017)
    In this study we present and discuss the influence of compositional inhomogeneity on the mechanical behavior of an interstitially alloyed dual-phase non-equiatomic high-entropy alloy (Fe49.5Mn30Co10Cr10C0.5). Various processing routes including hot-rolling, homogenization, cold-rolling and recrystallization annealing were performed on the cast alloys to obtain samples in different compositional homogeneity states. Grain sizes of the alloys were also considered. Tensile testing and microstructural investigations reveal that the deformation behavior of the interstitial dual-phase high-entropy alloy samples varied significantly depending on the compositional homogeneity of the specimens probed. In the case of coarse-grains (∼300 μm) obtained for cast alloys without homogenization treatment, ductility and strain-hardening of the material was significantly reduced due to its compositional inhomogeneity. This detrimental effect was attributed to preferred deformation-driven phase transformation occurring in the Fe enriched regions with lower stacking fault energy, promoting early stress-strain localization. The grain-refined alloy (∼4 μm) with compositional heterogeneity which was obtained for recrystallization annealed alloys without homogenization treatment was characterized by almost total loss in work-hardening. This effect was attributed to large local shear strains due to the inhomogeneous planar slip. These insights demonstrate the essential role of compositional homogeneity through applying corresponding processing steps for the development of advanced high-entropy alloys. © 2017 Elsevier B.V.
    view abstract10.1016/j.matchemphys.2017.04.050
  • Influence of rhenium on γ′-strengthened cobalt-base superalloys
    Kolb, M. and Zenk, C.H. and Kirzinger, A. and Povstugar, I. and Raabe, D. and Neumeier, S. and Göken, M.
    Journal of Materials Research 32 (2017)
    The element Re is known to be a very potent strengthener concerning the creep properties of Ni-base superalloys. In this paper the influence of Re on the properties of new γ′-strengthened Co-base superalloys is addressed. Atom probe tomography reveals that Re partitions preferentially to the γ phase, but not as pronounced as in ni-base superalloys. Nanoindentation and micro-pillar compression tests of the γ′ phase indicate an increase of the hardness and the critical resolved shear stress caused by a considerable concentration of Re in the γ′ phase. Creep investigations show that the positive effect of Re is by far not as pronounced as in Ni-base superalloys. Several effects, which can contribute to this behavior, such as the lower Re concentration in γ and hence a slightly reduced effective diffusion coefficient, a smaller diffusion barrier of Re in Co compared to Ni, a slightly lower lattice misfit and γ′ volume fraction of the Re-containing alloy, are discussed. © Materials Research Society 2017.
    view abstract10.1557/jmr.2017.242
  • Interfacial hydrogen localization in austenite/martensite dual-phase steel visualized through optimized silver decoration and scanning Kelvin probe force microscopy
    Nagashima, T. and Koyama, M. and Bashir, A. and Rohwerder, M. and Tasan, C.C. and Akiyama, E. and Raabe, D. and Tsuzaki, K.
    Materials and Corrosion 68 (2017)
    The hydrogen distribution in an austenite-martensite dual-phase steel was investigated using silver decoration and scanning Kelvin probe force microscopy. The silver decoration technique optimized for spacial resolution reveals interfacial segregation of hydrogen along the plate-type martensite-martensite grain boundaries. In addition, the scanning Kelvin probe force microscopy kinetically elucidates that hydrogen preferentially diffused out from the martensite-martensite grain boundaries. These preferential sites of hydrogen desorption correspond to the regions of hydrogen-assisted damage. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/maco.201609104
  • Interstitial atoms enable joint twinning and transformation induced plasticity in strong and ductile high-entropy alloys
    Li, Z. and Tasan, C.C. and Springer, H. and Gault, B. and Raabe, D.
    Scientific Reports 7 (2017)
    High-entropy alloys (HEAs) consisting of multiple principle elements provide an avenue for realizing exceptional mechanical, physical and chemical properties. We report a novel strategy for designing a new class of HEAs incorporating the additional interstitial element carbon. This results in joint activation of twinning- and transformation-induced plasticity (TWIP and TRIP) by tuning the matrix phase's instability in a metastable TRIP-assisted dual-phase HEA. Besides TWIP and TRIP, such alloys benefit from massive substitutional and interstitial solid solution strengthening as well as from the composite effect associated with its dual-phase structure. Nanosize particle formation and grain size reduction are also utilized. The new interstitial TWIP-TRIP-HEA thus unifies all metallic strengthening mechanisms in one material, leading to twice the tensile strength compared to a single-phase HEA with similar composition, yet, at identical ductility. © The Author(s) 2017.
    view abstract10.1038/srep40704
  • Large strain synergetic material deformation enabled by hybrid nanolayer architectures
    Li, J. and Lu, W. and Zhang, S. and Raabe, D.
    Scientific Reports 7 (2017)
    Nanolayered metallic composites are much stronger than pure nanocrystalline metals due to their high density of hetero-interfaces. However, they are usually mechanically instable due to the deformation incompatibility among the soft and hard constituent layers promoting shear instability. Here we designed a hybrid material with a heterogeneous multi-nanolayer architecture. It consists of alternating 10 nm and 100 nm-thick Cu/Zr bilayers which deform compatibly in both stress and strain by utilizing the layers' intrinsic strength, strain hardening and thickness, an effect referred to as synergetic deformation. Micropillar tests show that the 6.4 GPa-hard 10 nm Cu/Zr bilayers and the 3.3 GPa 100 nm Cu layers deform in a compatible fashion up to 50% strain. Shear instabilities are entirely suppressed. Synergetic strengthening of 768 MPa (83% increase) compared to the rule of mixture is observed, reaching a total strength of 1.69 GPa. We present a model that serves as a design guideline for such synergetically deforming nano-hybrid materials. © 2017 The Author(s).
    view abstract10.1038/s41598-017-11001-w
  • Long-term thermal stability of nanoclusters in ODS-Eurofer steel: An atom probe tomography study
    Zilnyk, K.D. and Pradeep, K.G. and Choi, P. and Sandim, H.R.Z. and Raabe, D.
    Journal of Nuclear Materials 492 (2017)
    Oxide-dispersion strengthened materials are important candidates for several high-temperature structural applications in advanced nuclear power plants. Most of the desirable mechanical properties presented by these materials are due to the dispersion of stable nanoparticles in the matrix. Samples of ODS-Eurofer steel were annealed for 4320 h (6 months) at 800 °C. The material was characterized using atom probe tomography in both conditions (prior and after heat treatment). The particles number density, size distribution, and chemical compositions were determined. No significant changes were observed between the two conditions indicating a high thermal stability of the Y-rich nanoparticles at 800 °C. © 2017 Elsevier B.V.
    view abstract10.1016/j.jnucmat.2017.05.027
  • Massive nanoprecipitation in an Fe-19Ni-xAl maraging steel triggered by the intrinsic heat treatment during laser metal deposition
    Kürnsteiner, P. and Wilms, M.B. and Weisheit, A. and Barriobero-Vila, P. and Jägle, E.A. and Raabe, D.
    Acta Materialia 129 (2017)
    Due to the layer-by-layer build-up of additively manufactured parts, the deposited material experiences a cyclic re-heating in the form of a sequence of temperature pulses. In the current work, this “intrinsic heat treatment (IHT)” was exploited to induce the precipitation of NiAl nanoparticles in an Fe-19Ni-xAl (at%) model maraging steel, a system known for rapid clustering. We used Laser Metal Deposition (LMD) to synthesize compositionally graded specimens. This allowed for the efficient screening of effects associated with varying Al contents ranging from 0 to 25 at% and for identifying promising concentrations for further studies. Based on the existence of the desired martensitic matrix, an upper bound for the Al concentration of 15 at% was defined. Owing to the presence of NiAl precipitates as observed by Atom Probe Tomography (APT), a lower bound of 3–5 at% Al was established. Within this concentration window, increasing the Al concentration gave rise to an increase in hardness by 225 HV due to an exceptionally high number density of 1025 NiAl precipitates per m3, as measured by APT. This work demonstrates the possibility of exploiting the IHT of the LMD process for the production of samples that are precipitation strengthened during the additive manufacturing process without need for any further heat treatment. © 2017
    view abstract10.1016/j.actamat.2017.02.069
  • Microstructural stability of a niobium single crystal deformed by equal channel angular pressing
    Bernardi, H.H. and Sandim, H.R.Z. and Zilnyk, K.D. and Verlinden, B. and Raabe, D.
    Materials Research 20 (2017)
    A [211]-oriented niobium single crystal was deformed by equal channel angular pressing (ECAP) at room temperature using the route Bc to a total strain of 9.2. A sharp cube texture develops after ECAP processing. The deformed samples were annealed in vacuum from 400ºC (673 K) to 900ºC (1173 K) for 1 h to evaluate their microstructural stability. Scanning electron microscopy (SEM) was used to image the microstructures of as-deformed and annealed specimens. Electron backscatter diffraction (EBSD) was employed to determine the respective microtextures before and after annealing. Coarsening of the microstructure occurs at a maximum rate at 550ºC (823 K) due to discontinuous recrystallization. Normal grain growth replaces discontinuous recrystallization as the main coarsening mechanism above 700ºC (973 K).
    view abstract10.1590/1980-5373-MR-2017-0288
  • Modulation of plastic flow in metallic glasses via nanoscale networks of chemical heterogeneities
    Kim, J. and Oh, H.S. and Kim, W. and Choi, P.-P. and Raabe, D. and Park, E.S.
    Acta Materialia 140 (2017)
    We systematically investigate the microstructures of metallic glasses with nanoscale networks of chemical heterogeneities introduced by the presence of a metastable miscibility gap, and their effects on modulating plastic flow of the alloys. Microstructural analysis of as-quenched alloys and the associated thermodynamic assessment in Cu-Zr-Al-Y metallic glass-forming system suggest that the existence of a metastable miscibility gap can induce not only phase-separated microstructures with sharp phase interfaces but also compositional fluctuations without a clear interface ranging from atomic scale to a few-nanometer scale in the fully amorphous alloys. The statistical analysis of shear avalanches in such compositionally heterogeneous metallic glasses reveals that chemical heterogeneities extending over a few nanometers promote a relatively large population of shear deformation units jammed before the nucleation of mature shear bands. This leads to the multiple nucleation of shear bands and sluggish deformation behavior along them. However, phase interfaces formed by phase separation inside the miscibility gap promote rapid propagation of shear bands at low flow stress, while compositional fluctuations creating non-sharp interfaces emerging at the outside of miscibility gap have relatively high resistance against shear band propagation. We hence suggest that the optimization of nanoscale compositional fluctuations in metallic glasses in terms of topology, percolation and magnitude can be an effective route for improving the materials’ damage tolerance upon plastic flow. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.08.002
  • Multiscale characterization of White Etching Cracks (WEC) in a 100Cr6 bearing from a thrust bearing test rig
    Danielsen, H.K. and Guzmán, F.G. and Dahl, K.V. and Li, Y.J. and Wu, J. and Jacobs, G. and Burghardt, G. and Fæster, S. and Alimadadi, H. and Goto, S. and Raabe, D. and Petrov, R.
    Wear 370-371 (2017)
    A common cause for premature bearing failures in wind turbine gearboxes are the so-called White Etching Cracks (WEC). These undirected, three-dimensional cracks are bordered by regions of altered microstructure and ultimately lead to a cracking or spalling of the raceway. An accelerated WEC test was carried out on a FE8 test rig using cylindrical roller thrust bearings made of martensitic 100Cr6 steel. The resulting WECs were investigated with several characterisation techniques. Ultrasonic measurements showed the WEC were mainly located in the region of the overrolled surface in which negative slip occurs, which agrees with hypotheses based on an energetic approach for a prognosis. SEM orientation contrast imaging of the area around WEC revealed an inhomogeneous structure with varied grain sizes and a large amount of defects. Microstructure characterization around the WEA using EBSD showed significant grain refinement. Atom probe tomography showed the microstructure in the undamaged zone has a plate-like martensitic structure with carbides, while no carbides were detected in the WEA where the microstructure consisted of equiaxed 10 nm grains. A three dimensional characterisation of WEC network was successfully demonstrated with X-ray computerized tomography, showing crack interaction with unidentified inclusion-like particles. © 2016 Elsevier B.V.
    view abstract10.1016/j.wear.2016.11.016
  • Overview of hydrogen embrittlement in high-Mn steels
    Koyama, M. and Akiyama, E. and Lee, Y.-K. and Raabe, D. and Tsuzaki, K.
    International Journal of Hydrogen Energy (2017)
    Hydrogen and fuels derived from it will serve as the energy carriers of the future. The associated rapidly growing demand for hydrogen energy-related infrastructure materials has stimulated multiple engineering and scientific studies on the hydrogen embrittlement resistance of various groups of high performance alloys. Among these, high-Mn steels have received special attention owing to their excellent strength - ductility - cost relationship. However, hydrogen-induced delayed fracture has been reported to occur in deep-drawn cup specimens of some of these alloys. Driven by this challenge we present here an overview of the hydrogen embrittlement research carried out on high-Mn steels. The hydrogen embrittlement susceptibility of high-Mn steels is particularly sensitive to their chemical composition since the various alloying elements simultaneously affect the material's stacking fault energy, phase stability, hydrogen uptake behavior, surface oxide scales and interstitial diffusivity, all of which affect the hydrogen embrittlement susceptibility. Here, we discuss the contribution of each of these factors to the hydrogen embrittlement susceptibility of these steels and discuss pathways how certain embrittlement mechanisms can be hampered or even inhibited. Examples of positive effects of hydrogen on the tensile ductility are also introduced. © 2017 Hydrogen Energy Publications LLC.
    view abstract10.1016/j.ijhydene.2017.02.214
  • Partial recrystallization of gum metal to achieve enhanced strength and ductility
    Zhang, J.-L. and Tasan, C.C. and Lai, M.J. and Yan, D. and Raabe, D.
    Acta Materialia 135 (2017)
    Here we present a microstructure design approach which leads to partial recrystallization and nano-precipitation within the same single-step heat treatment. This produces a dual-constituent microstructure in Ti-Nb based gum metal, which consists of nano-ω-particle-rich ultrafine recrystallized grain chains embedded in ω-lean subgrain-containing recovered zones. This partially recrystallized microstructure exhibits an improved strength-ductility combination that surpasses the inverse strength-ductility relationship exhibited by materials with similar composition. The strengthening effects due to precipitates and grain refinement were studied by nanoindentation. The deformation mechanisms of the partially recrystallized material were investigated by in-situ scanning electron microscope tensile tests, micro-strain mapping and post-mortem microstructure characterization. The improved mechanical properties are attributed to the high yield strength of the recrystallized grains and the sequential activation of dislocation slip and dislocation channeling. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.06.051
  • Recent progress in microstructural hydrogen mapping in steels: quantification, kinetic analysis, and multi-scale characterisation
    Koyama, M. and Rohwerder, M. and Tasan, C.C. and Bashir, A. and Akiyama, E. and Takai, K. and Raabe, D. and Tsuzaki, K.
    Materials Science and Technology (United Kingdom) (2017)
    This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement. © 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/02670836.2017.1299276
  • Reply to the 'Comments on "dental lessons from past to present: Ultrastructure and composition of teeth from plesiosaurs, dinosaurs, extinct and recent sharks"' by H. Botella: Et al., RSC Adv., 2016, 6, 74384-74388
    Luebke, A. and Loza, K. and Patnaik, R. and Enax, J. and Raabe, D. and Prymak, O. and Fabritius, H.-O. and Gaengler, P. and Epple, M.
    RSC Advances 7 (2017)
    The structure and composition of 13 fossilized tooth and bone samples aged between 3 and 70 million years were analysed. It was found that they all contained high amounts of fluoroapatite. This indicates that originally present hydroxyapatite had been converted to fluoroapatite during the diagenesis. Thus, the chemical analysis allows no conclusion with respect to the original composition of our fossil samples. Our results indicate that the diagenetic transformation of hydroxyapatite into fluoroapatite is at least partially dependent on microstructural characteristics of the original tissue such as the degree of porosity. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c6ra27121a
  • Role of Nanostructuring and Microstructuring in Silver Antimony Telluride Compounds for Thermoelectric Applications
    Cojocaru-Mirédin, O. and Abdellaoui, L. and Nagli, M. and Zhang, S. and Yu, Y. and Scheu, C. and Raabe, D. and Wuttig, M. and Amouyal, Y.
    ACS Applied Materials and Interfaces 9 (2017)
    Thermoelectric (TE) materials are of utmost significance for conversion of heat flux into electrical power in the low-power regime. Their conversion efficiency depends strongly on the microstructure. AgSbTe2-based compounds are high-efficiency TE materials suitable for the mid-temperature range. Herein, we explore an Ag16.7Sb30Te53.3 alloy (at %) subjected to heat treatments at 380 °C for different durations aimed at nucleation and coarsening of Sb2Te3-precipitates. To characterize the Sb2Te3-precipitation, we use a set of methods combining thermal and electrical measurements in concert with transmission electron microscopy and atom probe tomography. We find correlations between the measured TE transport coefficients and the applied heat treatments. Specifically, the lowest electrical and thermal conductivity values are obtained for the as-quenched state, whereas the highest values are observed for alloys aged for 8 h. In turn, long-term heat treatments result in intermediate values of transport coefficients. We explain these findings in terms of interplay between precipitate formation and variations in the matrix composition, highlighting the importance of thermal stability of the material under service conditions. © 2017 American Chemical Society.
    view abstract10.1021/acsami.7b00689
  • Room temperature deformation of LPSO structures by non-basal slip
    Chen, R. and Sandlöbes, S. and Zeng, X. and Li, D. and Korte-Kerzel, S. and Raabe, D.
    Materials Science and Engineering A 682 (2017)
    We investigated the deformation mechanisms of long period stacking ordered (LPSO) structures in an extruded Mg97Y2Zn1 (at%) alloy. Tensile deformation was performed in such a way that basal slip and kink band formation were inhibited. Slip trace analysis and transmission electron microscopy reveal a predominant activity of non-basal < a&gt; slip. © 2016 Elsevier B.V.
    view abstract10.1016/j.msea.2016.11.056
  • Stiff, light, strong and ductile: nano-structured High Modulus Steel
    Springer, H. and Baron, C. and Szczepaniak, A. and Uhlenwinkel, V. and Raabe, D.
    Scientific Reports 7 (2017)
    Structural material development for lightweight applications aims at improving the key parameters strength, stiffness and ductility at low density, but these properties are typically mutually exclusive. Here we present how we overcome this trade-off with a new class of nano-structured steel - TiB2 composites synthesised in-situ via bulk metallurgical spray-forming. Owing to the nano-sized dispersion of the TiB2 particles of extreme stiffness and low density - obtained by the in-situ formation with rapid solidification kinetics - the new material has the mechanical performance of advanced high strength steels, and a 25% higher stiffness/density ratio than any of the currently used high strength steels, aluminium, magnesium and titanium alloys. This renders this High Modulus Steel the first density-reduced, high stiffness, high strength and yet ductile material which can be produced on an industrial scale. Also ideally suited for 3D printing technology, this material addresses all key requirements for high performance and cost effective lightweight design. © The Author(s) 2017.
    view abstract10.1038/s41598-017-02861-3
  • Strengthening and strain hardening mechanisms in a precipitation-hardened high-Mn lightweight steel
    Yao, M.J. and Welsch, E. and Ponge, D. and Haghighat, S.M.H. and Sandlöbes, S. and Choi, P. and Herbig, M. and Bleskov, I. and Hickel, T. and Lipinska-Chwalek, M. and Shanthraj, P. and Scheu, C. and Zaefferer, S. and Gault, B. and Raabe, D.
    Acta Materialia 140 (2017)
    We report on the strengthening and strain hardening mechanisms in an aged high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C, wt.%) studied by electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM), atom probe tomography (APT) and correlative TEM/APT. Upon isothermal annealing at 600 °C, nano-sized κ-carbides form, as characterized by TEM and APT. The resultant alloy exhibits high strength and excellent ductility accompanied by a high constant strain hardening rate. In comparison to the as-quenched κ-free state, the precipitation of κ-carbides leads to a significant increase in yield strength (∼480 MPa) without sacrificing much tensile elongation. To study the strengthening and strain hardening behavior of the precipitation-hardened material, deformation microstructures were analyzed at different strain levels. TEM and correlative TEM/APT results show that the κ-carbides are primarily sheared by lattice dislocations, gliding on the typical face-centered-cubic (fcc) slip system {111}<110>, leading to particle dissolution and solute segregation. Ordering strengthening is the predominant strengthening mechanism. As the deformation substructure is characterized by planar slip bands, we quantitatively studied the evolution of the slip band spacing during straining to understand the strain hardening behavior. A good agreement between the calculated flow stresses and the experimental data suggests that dynamic slip band refinement is the main strain hardening mechanism. The influence of κ-carbides on mechanical properties is discussed by comparing the results with that of the same alloy in the as-quenched, κ-free state. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.08.049
  • Strengthening Fe – TiB2 based high modulus steels by precipitations
    Szczepaniak, A. and Springer, H. and Aparicio-Fernández, R. and Baron, C. and Raabe, D.
    Materials and Design 124 (2017)
    We systematically studied the microstructure, mechanical and physical properties of hyper-eutectic Fe – TiB2 high modulus steels (20 vol% TiB2) with (Si, Mn, Ni) and Cu additions for the formation of G-phase and Cu precipitates during ageing treatments. Alloying with Si, Mn and Ni led predominantly to pronounced solid solution strengthening, reaching tensile strength (UTS) values up to 1100 MPa after quenching. While G-phase formation could be observed in aged materials, its preferential formation on grain boundaries significantly deteriorated ductility. Its effects on strength were partially balanced by a decrease of grain boundary density. Additions of 1 and 2 wt% Cu, respectively, led to lower strength in the as quenched state, but also to significant strengthening via ageing. The peak ageing conditions as well as the Cu particle structure and size are comparable to values reported for Cu strengthened interstitial free steels and Fe-Cu alloys. Both alloying additions slightly lowered the specific elastic modulus of the HMS, most pronounced for Cu addition with a drop of about 3 GPa cm3 g− 1 per wt% and also promoted embrittlement. Microstructure-property relationships and consequences for future alloy design, especially towards more ductile hypo-eutectic HMS, are outlined and discussed. © 2017
    view abstract10.1016/j.matdes.2017.03.042
  • Strong and Ductile Non-equiatomic High-Entropy Alloys: Design, Processing, Microstructure, and Mechanical Properties
    Li, Z. and Raabe, D.
    JOM 69 (2017)
    We present a brief overview on recent developments in the field of strong and ductile non-equiatomic high-entropy alloys (HEAs). The materials reviewed are mainly based on massive transition-metal solute solutions and exhibit a broad spectrum of microstructures and mechanical properties. Three relevant aspects of such non-equiatomic HEAs with excellent strength–ductility combination are addressed in detail, namely phase stability-guided design, controlled and inexpensive bulk metallurgical processing routes for appropriate microstructure and compositional homogeneity, and the resultant microstructure–property relations. In addition to the multiple principal substitutional elements used in these alloys, minor interstitial alloying elements are also considered. We show that various groups of strong and ductile HEAs can be obtained by shifting the alloy design strategy from single-phase equiatomic to dual- or multiphase non-equiatomic compositional configurations with carefully designed phase instability. This design direction provides ample possibilities for joint activation of a number of strengthening and toughening mechanisms. Some potential research efforts which can be conducted in the future are also proposed. © 2017, The Author(s).
    view abstract10.1007/s11837-017-2540-2
  • Superplasticity in a lean Fe-Mn-Al steel
    Han, J. and Kang, S.-H. and Lee, S.-J. and Kawasaki, M. and Lee, H.-J. and Ponge, D. and Raabe, D. and Lee, Y.-K.
    Nature Communications 8 (2017)
    Superplastic alloys exhibit extremely high ductility (>300%) without cracks when tensile-strained at temperatures above half of their melting point. Superplasticity, which resembles the flow behavior of honey, is caused by grain boundary sliding in metals. Although several non-ferrous and ferrous superplastic alloys are reported, their practical applications are limited due to high material cost, low strength after forming, high deformation temperature, and complicated fabrication process. Here we introduce a new compositionally lean (Fe-6.6Mn-2.3Al, wt.%) superplastic medium Mn steel that resolves these limitations. The medium Mn steel is characterized by ultrafine grains, low material costs, simple fabrication, i.e., conventional hot and cold rolling, low deformation temperature (ca. 650 °C) and superior ductility above 1300% at 850 °C. We suggest that this ultrafine-grained medium Mn steel may accelerate the commercialization of superplastic ferrous alloys. © 2017 The Author(s).
    view abstract10.1038/s41467-017-00814-y
  • The effects of prior austenite grain boundaries and microstructural morphology on the impact toughness of intercritically annealed medium Mn steel
    Han, J. and da Silva, A.K. and Ponge, D. and Raabe, D. and Lee, S.-M. and Lee, Y.-K. and Lee, S.-I. and Hwang, B.
    Acta Materialia 122 (2017)
    The effects of prior austenite (γ) grain boundaries and microstructural morphology on the impact toughness of an annealed Fe-7Mn-0.1C-0.5Si medium Mn steel were investigated for two different microstructure states, namely, hot-rolled and annealed (HRA) specimens and cold-rolled and annealed (CRA) specimens. Both types of specimens had a dual-phase microstructure consisting of retained austenite (γR) and ferrite (α) after intercritical annealing at 640 °C for 30 min. The phase fractions and the chemical composition of γR were almost identical in both types of specimens. However, their microstructural morphology was different. The HRA specimens had lath-shaped morphology and the CRA specimens had globular-shaped morphology. We find that both types of specimens showed transition in fracture mode from ductile and partly quasi-cleavage fracture to intergranular fracture with decreasing impact test temperature from room temperature to −196 °C. The HRA specimen had higher ductile to brittle transition temperature and lower low-temperature impact toughness compared to the CRA specimen. This was due to intergranular cracking in the HRA specimens along prior γ grain boundaries decorated by C, Mn and P. In the CRA specimen intergranular cracking occurred along the boundaries of the very fine α and α′ martensite grains. The results reveal that cold working prior to intercritical annealing promotes the elimination of the solute-decorated boundaries of coarse prior γ grains through the recrystallization of αʹ martensite prior to reverse transformation, hence improving the low-temperature impact toughness of medium Mn steel. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.09.048
  • The relaxed-polar mechanism of locally optimal Cosserat rotations for an idealized nanoindentation and comparison with 3D-EBSD experiments
    Fischle, A. and Neff, P. and Raabe, D.
    Zeitschrift fur Angewandte Mathematik und Physik 68 (2017)
    The rotation polar (F) ∈ SO (3) arises as the unique orthogonal factor of the right polar decomposition F=polar(F)U of a given invertible matrix F∈ GL +(3). In the context of nonlinear elasticity Grioli (Boll Un Math Ital 2:252–255, 1940) discovered a geometric variational characterization of polar (F) as a unique energy-minimizing rotation. In preceding works, we have analyzed a generalization of Grioli’s variational approach with weights (material parameters) μ&gt; 0 and μc≥ 0 (Grioli: μ= μc). The energy subject to minimization coincides with the Cosserat shear–stretch contribution arising in any geometrically nonlinear, isotropic and quadratic Cosserat continuum model formulated in the deformation gradient field F: = ∇ φ: Ω → GL +(3) and the microrotation field R: Ω → SO (3). The corresponding set of non-classical energy-minimizing rotations rpolarμ,μc±(F):=arg minR∈SO(3){Wμ,μc(R;F):=μ||sym(RTF-1)||2+μc||skew(RTF-1)||2}represents a new relaxed-polar mechanism. Our goal is to motivate this mechanism by presenting it in a relevant setting. To this end, we explicitly construct a deformation mapping φnano which models an idealized nanoindentation and compare the corresponding optimal rotation patterns rpolar1,0±(Fnano) with experimentally obtained 3D-EBSD measurements of the disorientation angle of lattice rotations due to a nanoindentation in solid copper. We observe that the non-classical relaxed-polar mechanism can produce interesting counter-rotations. A possible link between Cosserat theory and finite multiplicative plasticity theory on small scales is also explored. © 2017, Springer International Publishing AG.
    view abstract10.1007/s00033-017-0834-4
  • Thermo-mechanical stresses within switching contact systems after arcing events
    Mutzel, T. and Ma, D. and Roters, F. and Raabe, D.
    Electrical Contacts, Proceedings of the Annual Holm Conference on Electrical Contacts (2017)
    Silver tin oxide (Ag/SnO2) contact materials are widely used in contactor applications. A general trend in this application field is the steadily ongoing miniaturization of switching devices, resulting in growing energy densities to be handled by the contact system. The higher arcing energy densities in such new designs are inducing increased thermo-mechanical stresses in the contact system. As these stresses cannot be measured, FEM simulation was applied to make them visible for heavy duty break arcs. Focus of the presented studies is the temperature dependent elastic and plastic modeling of the intermediate Ag layer and the braze filler. Based on these simulations, improvements in material stress behavior of the complete system (sub-assembly) should be realized. © 2017 IEEE.
    view abstract10.1109/HOLM.2017.8088077
  • Ultra-stiff metallic glasses through bond energy density design
    Schnabel, V. and Köhler, M. and Music, D. and Bednarcik, J. and Clegg, W.J. and Raabe, D. and Schneider, J.M.
    Journal of Physics Condensed Matter 29 (2017)
    The elastic properties of crystalline metals scale with their valence electron density. Similar observations have been made for metallic glasses. However, for metallic glasses where covalent bonding predominates, such as metalloid metallic glasses, this relationship appears to break down. At present, the reasons for this are not understood. Using high energy x-ray diffraction analysis of melt spun and thin film metallic glasses combined with density functional theory based molecular dynamics simulations, we show that the physical origin of the ultrahigh stiffness in both metalloid and non-metalloid metallic glasses is best understood in terms of the bond energy density. Using the bond energy density as novel materials design criterion for ultra-stiff metallic glasses, we are able to predict a Co33.0Ta3.5B63.5 short range ordered material by density functional theory based molecular dynamics simulations with a high bond energy density of 0.94 eV Å-3 and a bulk modulus of 263 GPa, which is 17% greater than the stiffest Co-B based metallic glasses reported in literature. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/aa72cb
  • Ultrastrong steel via minimal lattice misfit and high-density nanoprecipitation
    Jiang, S. and Wang, H. and Wu, Y. and Liu, X. and Chen, H. and Yao, M. and Gault, B. and Ponge, D. and Raabe, D. and Hirata, A. and Chen, M. and Wang, Y. and Lu, Z.
    Nature (2017)
    Next-generation high-performance structural materials are required for lightweight design strategies and advanced energy applications. Maraging steels, combining a martensite matrix with nanoprecipitates, are a class of high-strength materials with the potential for matching these demands. Their outstanding strength originates from semi-coherent precipitates, which unavoidably exhibit a heterogeneous distribution that creates large coherency strains, which in turn may promote crack initiation under load. Here we report a counterintuitive strategy for the design of ultrastrong steel alloys by high-density nanoprecipitation with minimal lattice misfit. We found that these highly dispersed, fully coherent precipitates (that is, the crystal lattice of the precipitates is almost the same as that of the surrounding matrix), showing very low lattice misfit with the matrix and high anti-phase boundary energy, strengthen alloys without sacrificing ductility. Such low lattice misfit (0.03 ± 0.04 per cent) decreases the nucleation barrier for precipitation, thus enabling and stabilizing nanoprecipitates with an extremely high number density (more than 1024 per cubic metre) and small size (about 2.7 ± 0.2 nanometres). The minimized elastic misfit strain around the particles does not contribute much to the dislocation interaction, which is typically needed for strength increase. Instead, our strengthening mechanism exploits the chemical ordering effect that creates backstresses (the forces opposing deformation) when precipitates are cut by dislocations. We create a class of steels, strengthened by Ni(Al,Fe) precipitates, with a strength of up to 2.2 gigapascals and good ductility (about 8.2 per cent). The chemical composition of the precipitates enables a substantial reduction in cost compared to conventional maraging steels owing to the replacement of the essential but high-cost alloying elements cobalt and titanium with inexpensive and lightweight aluminium. Strengthening of this class of steel alloy is based on minimal lattice misfit to achieve maximal precipitate dispersion and high cutting stress (the stress required for dislocations to cut through coherent precipitates and thus produce plastic deformation), and we envisage that this lattice misfit design concept may be applied to many other metallic alloys. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
    view abstract10.1038/nature22032
  • (Nbx, Zr1-x)4AlC3 MAX Phase Solid Solutions: Processing, Mechanical Properties, and Density Functional Theory Calculations
    Lapauw, T. and Tytko, D. and Vanmeensel, K. and Huang, S. and Choi, P.-P. and Raabe, D. and Caspi, E.N. and Ozeri, O. and To Baben, M. and Schneider, J.M. and Lambrinou, K. and Vleugels, J.
    Inorganic Chemistry 55 (2016)
    The solubility of zirconium (Zr) in the Nb4AlC3 host lattice was investigated by combining the experimental synthesis of (Nbx, Zr1-x)4AlC3 solid solutions with density functional theory calculations. High-purity solid solutions were prepared by reactive hot pressing of NbH0.89, ZrH2, Al, and C starting powder mixtures. The crystal structure of the produced solid solutions was determined using X-ray and neutron diffraction. The limited Zr solubility (maximum of 18.5% of the Nb content in the host lattice) in Nb4AlC3 observed experimentally is consistent with the calculated minimum in the energy of mixing. The lattice parameters and microstructure were evaluated over the entire solubility range, while the chemical composition of (Nb0.85, Zr0.15)4AlC3 was mapped using atom probe tomography. The hardness, Young's modulus, and fracture toughness at room temperature as well as the high-temperature flexural strength and E-modulus of (Nb0.85, Zr0.15)4AlC3 were investigated and compared to those of pure Nb4AlC3. Quite remarkably, an appreciable increase in fracture toughness was observed from 6.6 ± 0.1 MPa/m1/2 for pure Nb4AlC3 to 10.1 ± 0.3 MPa/m1/2 for the (Nb0.85, Zr0.15)4AlC3 solid solution. © 2016 American Chemical Society.
    view abstract10.1021/acs.inorgchem.6b00484
  • 3-Dimensional microstructural characterization of CdTe absorber layers from CdTe/CdS thin film solar cells
    Stechmann, G. and Zaefferer, S. and Konijnenberg, P. and Raabe, D. and Gretener, C. and Kranz, L. and Perrenoud, J. and Buecheler, S. and Tiwari, A.N.
    Solar Energy Materials and Solar Cells 151 (2016)
    The present work reports on a study on the microstructure and its evolution during processing of CdTe absorber layers from CdTe/CdS thin film solar cells grown by low-temperature processes in substrate configuration. Investigations were performed at different stages of the cell manufacturing, from deposition to the final functional solar cell, with the aim to understand the microstructure formation of the photoactive layer. To this end 3-dimensional microstructure characterization was performed using focused ion beam/electron backscatter diffraction tomography ("3D-EBSD") together with conventional 2D-EBSD. The analyses revealed strong microstructural and textural changes developing across the thickness of the absorber material, between the back contact and the p-n junction interfaces. Based on the 3-dimensional reconstruction of the CdTe thin film, a coherent growth model was proposed, emphasizing the microstructural continuity before and after a typical CdCl2-annealing activation treatment. One of the principal results is that the absorber layer is created by two concomitant processes, deposition and recrystallization, which led to different textures and microstructures. Further changes are the result of subsequent annealing treatments, favoring twinning and promoting well-defined texture components. The results open the possibility for a grain boundary engineering approach applied to the design of such cells. © 2016 Elsevier B.V.
    view abstract10.1016/j.solmat.2016.02.023
  • A crystal plasticity model for twinning- and transformation-induced plasticity
    Wong, S.L. and Madivala, M. and Prahl, U. and Roters, F. and Raabe, D.
    Acta Materialia 118 (2016)
    A dislocation density-based crystal plasticity model incorporating both transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) is presented. The approach is a physically-based model which reflects microstructure investigations of ε-martensite, twins and dislocation structures in high manganese steels. Validation of the model was conducted using experimental data for a TRIP/TWIP Fe-22Mn-0.6C steel. The model is able to predict, based on the difference in the stacking fault energies, the activation of TRIP and/or TWIP deformation mechanisms at different temperatures. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.07.032
  • A phase field model for damage in elasto-viscoplastic materials
    Shanthraj, P. and Sharma, L. and Svendsen, B. and Roters, F. and Raabe, D.
    Computer Methods in Applied Mechanics and Engineering 312 (2016)
    A phase field method for brittle fracture is formulated for a finite strain elasto-viscoplastic material using a novel obstacle phase field energy model. The obstacle energy model results in a crack profile with compact support, and thus gives a physically realistic description of the material behaviour at the vicinity of the crack tip. The resulting variational inequality is discretised by a finite element method, and is efficiently solved using a reduced space NEWTON method. The solution accuracy and numerical performance of this method is compared with a conventional phase field energy model for brittle fracture through representative examples, and a significant reduction in the numerical solution cost is demonstrated. © 2016 Elsevier B.V.
    view abstract10.1016/j.cma.2016.05.006
  • A virtual laboratory using high resolution crystal plasticity simulations to determine the initial yield surface for sheet metal forming operations
    Zhang, H. and Diehl, M. and Roters, F. and Raabe, D.
    International Journal of Plasticity 80 (2016)
    We present a virtual laboratory to investigate the anisotropic yield behavior of polycrystalline materials by using high resolution crystal plasticity simulations. Employing a fast spectral method solver enables us to conduct a large number of full-field virtual experiments with different stress states to accurately identify the yield surface of the probed materials. Based on the simulated yield stress points, the parameters for many commonly used yield functions are acquired simultaneously with a nonlinear least square fitting procedure. Exemplarily, the parameters of four yield functions frequently used in sheet metal forming, namely Yld91, Yld2000-2D, Yld2004-18p, and Yld2004-27p are adjusted to accurately describe the yield behavior of an AA3014 aluminum alloy at two material states, namely with a recrystallization texture and a cold rolling texture. The comparison to experimental results proves that the methodology presented, combining accuracy with efficiency, is a promising micromechanics-based tool for probing the mechanical anisotropy of polycrystalline metals and for identifying the parameters of advanced yield functions. © 2016 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijplas.2016.01.002
  • Ab initio-guided design of twinning-induced plasticity steels
    Raabe, D. and Roters, F. and Neugebauer, J. and Gutierrez-Urrutia, I. and Hickel, T. and Bleck, W. and Schneider, J.M. and Wittig, J.E. and Mayer, J.
    MRS Bulletin 41 (2016)
    The twinning-induced plasticity effect enables designing austenitic Fe-Mn-C-based steels with >70% elongation with an ultimate tensile strength >1 GPa. These steels are characterized by high strain hardening due to the formation of twins and complex dislocation substructures that dynamically reduce the dislocation mean free path. Both mechanisms are governed by the stacking-fault energy (SFE) that depends on composition. This connection between composition and substructure renders these steels ideal model materials for theory-based alloy design: Ab initio-guided composition adjustment is used to tune the SFE, and thus, the strain-hardening behavior for promoting the onset of twinning at intermediate deformation levels where the strain-hardening capacity provided by the dislocation substructure is exhausted. We present thermodynamic simulations and their use in constitutive models, as well as electron microscopy and combinatorial methods that enable validation of the strain-hardening mechanisms. Copyright © 2016 Materials Research Society.
    view abstract10.1557/mrs.2016.63
  • Atom probe tomography of intermetallic phases and interfaces formed in dissimilar joining between Al alloys and steel
    Lemmens, B. and Springer, H. and Duarte, M.J. and De Graeve, I. and De Strycker, J. and Raabe, D. and Verbeken, K.
    Materials Characterization 120 (2016)
    While Si additions to Al are widely used to reduce the thickness of the brittle intermetallic seam formed at the interface during joining of Al alloys to steel, the underlying mechanisms are not clarified yet. The developed approach for the site specific atom probe tomography analysis revealed Si enrichments at grain and phase boundaries between the θ (Fe4Al13) and η (Fe2Al5) phase, up to about ten times that of the concentration in Al. The increase in Si concentration could play an important role for the growth kinetics of the intermetallic phases formed for example in hot-dip aluminizing of steel. © 2016 Elsevier Inc.
    view abstract10.1016/j.matchar.2016.09.008
  • Atom probe tomography of metallic nanostructures
    Hono, K. and Raabe, D. and Ringer, S.P. and Seidman, D.N.
    MRS Bulletin 41 (2016)
    This article focuses on four topics that demonstrate the importance of atom probe tomography for obtaining nanostructural information that provides deep insights into the structures of metallic alloys, leading to a better understanding of their properties. First, we discuss the microstructure-coercivity relationship of Nd-Fe-B permanent magnets, essential for developing a higher coercivity magnet. Second, we address equilibrium segregation at grain boundaries with the aim of manipulating their interfacial structure, energies, compositions, and properties, thereby enabling beneficial material behavior. Third, recent progress in the search to extend the performance and practicality of the next generation of advanced high-strength steels is discussed. Finally, a study of the temporal evolution of a Ni-Al-Cr alloy through the stages of nucleation, growth, and coarsening (Ostwald ripening) and its relationship with the predictions of a model for quasi-stationary coarsening is described. This information is critical for understanding high-Temperature mechanical properties of the material. © Copyright Materials Research Society 2016.
    view abstract10.1557/mrs.2015.314
  • Autonomous Filling of Grain-Boundary Cavities during Creep Loading in Fe-Mo Alloys
    Zhang, S. and Fang, H. and Gramsma, M.E. and Kwakernaak, C. and Sloof, W.G. and Tichelaar, F.D. and Kuzmina, M. and Herbig, M. and Raabe, D. and Brück, E. and van der Zwaag, S. and van Dijk, N.H.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 47 (2016)
    We have investigated the autonomous repair of creep damage by site-selective precipitation in a binary Fe-Mo alloy (6.2 wt pct Mo) during constant-stress creep tests at temperatures of 813 K, 823 K, and 838 K (540 °C, 550 °C, and 565 °C). Scanning electron microscopy studies on the morphology of the creep-failed samples reveal irregularly formed deposits that show a close spatial correlation with the creep cavities, indicating the filling of creep cavities at grain boundaries by precipitation of the Fe2Mo Laves phase. Complementary transmission electron microscopy and atom probe tomography have been used to characterize the precipitation mechanism and the segregation at grain boundaries in detail. © 2016, The Author(s).
    view abstract10.1007/s11661-016-3642-0
  • Combinatorial design of transitory constitution steels: Coupling high strength with inherent formability and weldability through sequenced austenite stability
    Springer, H. and Belde, M. and Raabe, D.
    Materials and Design 90 (2016)
    We introduce a novel alloying and processing scheme for high strength steels, which allows for precise and cost-effective cold forming due to high fractions of metastable austenite, and a subsequent low-distortion, coating-preserving strengthening through martensitic transformation induced by low temperature treatments. As the constitution is thus synchronised with the processing requirements, we refer to these materials as Transitory Constitution Steels. Suitable alloy compositions were identified by high throughput screenings through the exemplarily material systems Fe-5Ni-0.3C-(3-15)Mn and Fe-13.5Cr-6Mn-2Cu-0.2C-(0-2)Ni (wt.%) using combinatorial bulk metallurgical methods. The transformation behaviour, mechanical properties and underlying microstructural phenomena were studied in more detail after upscaling of selected compositions. The steel Fe-13.5Cr-6Mn-1.5Cu-0.2C (wt.%) exhibited an increase in yield strength from 300 to 1050. MPa after immersion into liquid nitrogen, as well as an ultimate tensile strength of more than 1700. MPa at a total elongation of more than 9%. Despite the ultra high strength, no embrittlement induced by Laser beam welding was observed, highlighting the inherent weldability of steels synthesised by the alloying and processing scheme presented here. Possibilities for flexible alloy design and processing variations are discussed. © 2015 Elsevier Ltd.
    view abstract10.1016/j.matdes.2015.11.050
  • Combinatorial screening of the microstructure–property relationships for Fe–B–X stiff, light, strong and ductile steels
    Baron, C. and Springer, H. and Raabe, D.
    Materials and Design 112 (2016)
    We systematically screened the mechanical, physical and microstructural properties of the alloy systems Fe–10 B–5 X (at.%; X = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W), in order to identify novel metal matrix composite steels as next generation lightweight materials. The alloys were synthesised and processed by bulk liquid metallurgical techniques, and subsequently analysed for their mechanical and physical properties (i.e. Young's modulus, density, tensile strength and ductility) as well their microstructure and constitution. From the wide variety of observed boride phases and microstructures and resultant different properties, Cr and Zr additions were found to be most effective. Cr qualifies well as the high fraction of M2B borides of spherical morphology allows achieving a similar stiffness/density ratio and mechanical performance as the reference Ti alloyed materials, but at substantially reduced alloy costs. Zr blended composites on the other hand are softer and less ductile, but the alignment of spiky ZrB2 particles during swaging led to a much higher – though most probably anisotropic – specific modulus. Consequences and recommendations for future alloy and processing design are outlined and discussed. © 2016 Elsevier Ltd
    view abstract10.1016/j.matdes.2016.09.065
  • Combined atom probe tomography and density functional theory investigation of the Al off-stoichiometry of κ-carbides in an austenitic Fe-Mn-Al-C low density steel
    Yao, M.J. and Dey, P. and Seol, J.-B. and Choi, P. and Herbig, M. and Marceau, R.K.W. and Hickel, T. and Neugebauer, J. and Raabe, D.
    Acta Materialia 106 (2016)
    We report on the investigation of the off-stoichiometry and site-occupancy of κ-carbide precipitates within an austenitic (γ), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the κ-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L′12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the κ/γ mismatch, which facilitates occupation of Al sites in κ-carbide by Mn atoms (Mnγ Al anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.01.007
  • Composites of fluoroapatite and methylmethacrylate-based polymers (PMMA) for biomimetic tooth replacement
    Lübke, A. and Enax, J. and Wey, K. and Fabritius, H.-O. and Raabe, D. and Epple, M.
    Bioinspiration and Biomimetics 11 (2016)
    Synthetic composite materials that mimic the structure and composition of mammalian tooth enamel were prepared by mixing fluoroapatite rods (diameter 2-3 μm, thickness about 0.5 μm) and methylmethacrylate (MMA), followed by polymerization either during or immediately after ultracentrifugation, using either a tertiary amine/radical initiator for polymerization at room temperature or a radical initiator for thermal polymerization. This led to mineral-rich composites (mineral content between 50 and 75 wt%). To enhance the mechanical stability and the interaction between fluoroapatite and polymer matrix, small amounts of differently functionalized MMA monomers were added to the co-monomer mixture. Another approach was the coating of the fluoroapatite rods with silica and the polymerization in the presence of a siloxane-functionalized MMA monomer. The hardness of the composites was about 0.2-0.4 GPa as determined by Vickers indentation tests, about 2 times higher than the polymer matrix alone. The composites had a good resistance against acids (60 min at pH 3, 37 °C). © 2016 IOP Publishing Ltd.
    view abstract10.1088/1748-3190/11/3/035001
  • Computational modeling of dual-phase steels based on representative three-dimensional microstructures obtained from EBSD data
    Brands, D. and Balzani, D. and Scheunemann, L. and Schröder, J. and Richter, H. and Raabe, D.
    Archive of Applied Mechanics 86 (2016)
    The microstructure of dual-phase steels consisting of a ferrite matrix with embedded martensite inclusions is the main contributor to the mechanical properties such as high ultimate tensile strength, high work hardening rate, and good ductility. Due to the composite structure and the wide field of applications of this steel type, a wide interest exists in corresponding virtual computational experiments. For a reliable modeling, the microstructure should be included. For that reason, in this paper we follow a computational strategy based on the definition of a representative volume element (RVE). These RVEs will be constructed by a set of tomographic measurements and mechanical tests. In order to arrive at more efficient numerical schemes, we also construct statistically similar RVEs, which are characterized by a lower complexity compared with the real microstructure but which represent the overall material behavior accurately. In addition to the morphology of the microstructure, the austenite–martensite transformation during the steel production has a relevant influence on the mechanical properties and is considered in this contribution. This transformation induces a volume expansion of the martensite phase. A further effect is determined in nanoindentation test, where it turns out that the hardness in the ferrite phase increases exponentially when approaching the martensitic inclusion. To capture these gradient properties in the computational model, the volumetric expansion is applied to the martensite phase, and the arising equivalent plastic strain distribution in the ferrite phase serves as basis for a locally graded modification of the ferritic yield curve. Good accordance of the model considering the gradient yield behavior in the ferrite phase is observed in the numerical simulations with experimental data. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00419-015-1044-1
  • Crystal plasticity modeling of size effects in rolled multilayered Cu-Nb composites
    Jia, N. and Raabe, D. and Zhao, X.
    Acta Materialia 111 (2016)
    We present size-dependent crystal plasticity finite element simulations of the deformation microstructure, plastic flow and texture evolution in multilayered Cu-Nb composites during cold rolling. The model is based on a constitutive framework incorporating thermally activated dislocation slip, mechanical twinning and non-crystallographic shear banding. It also accounts for the dislocation density evolution and its dependence on initial grain size. By performing a series of quadricrystal simulations considering characteristic heterophase microstructures, the underlying micromechanics and texture of the composites are explored. Significant shear banding occurs in both phases, primarily determined by their initial orientations. For each phase, the activation of shear banding is also affected by the mechanical properties and orientations of the adjacent phase. For composites with an initial single layer thickness of 35 μm or 4 μm, the layer thickness reduction after rolling is non-uniform and the typical rolling textures for bulk pure metals develop in the respective phases. For the 75 nm initial single layer thickness composite, both phases are reduced uniformly in thickness and the initial orientations prevail. The predictions agree well with experimental observations in cold-rolled Cu-Nb thin films. The simulations reveal that for the composites with initial single layer thickness of micrometer scale, dislocation slip is the dominant deformation mechanism although shear banding increasingly carries the deformation at larger strains. For the samples with initial single layer thickness of a few tens of nanometers, shear banding and dislocation slip are the dominant mechanisms. This transition in deformation characteristics leads to different textures in micrometer- and nanometer-scaled multilayers. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2016.03.055
  • Crystal plasticity study of monocrystalline stochastic honeycombs under in-plane compression
    Ma, D. and Eisenlohr, P. and Epler, E. and Volkert, C.A. and Shanthraj, P. and Diehl, M. and Roters, F. and Raabe, D.
    Acta Materialia 103 (2016)
    We present a study on the plastic deformation of single crystalline stochastic honeycombs under in-plane compression using a crystal plasticity constitutive description for face-centered cubic (fcc) materials, focusing on the very early stage of plastic deformation, and identifying the interplay between the crystallographic orientation and the cellular structure during plastic deformation. We observe that despite the stochastic structure, surprisingly, the slip system activations in the honeycombs are almost identical to their corresponding bulk single crystals at the early stage of the plastic deformation. On the other hand, however, the yield stresses of the honeycombs are nearly independent of their crystallographic orientations. Similar mechanical response is found in compression testing of nanoporous gold micro-pillars aligned with various crystallographic orientations. The macroscopic stress tensors of the honeycombs show the same anisotropy as their respective bulk single crystals. Locally, however, there is an appreciable fluctuation in the local stresses, which are even larger than for polycrystals. This explains why the Taylor/Schmid factor associated with the crystallographic orientation is less useful to estimate the yield stresses of the honeycombs than the bulk single crystals and polycrystals, and why the plastic deformation occurs at smaller strains in the honeycombs than their corresponding bulk single crystals. Besides these findings, the observations of the crystallographic reorientation suggest that conventional orientation analysis tools, such as inverse pole figure and related tools, would in general fail to study the plastic deformation mechanism of monocrystalline cellular materials. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.11.016
  • Decomposition of the single-phase high-entropy alloy CrMnFeCoNi after prolonged anneals at intermediate temperatures
    Otto, F. and Dlouhý, A. and Pradeep, K.G. and Kuběnová, M. and Raabe, D. and Eggeler, G. and George, E.P.
    Acta Materialia 112 (2016)
    Among the vast number of multi-principal-element alloys that are referred to as high-entropy alloys (HEAs) in the literature, only a limited number solidify as single-phase solid solutions. The equiatomic HEA, CrMnFeCoNi, is a face-centered cubic (FCC) prototype of this class and has attracted much attention recently because of its interesting mechanical properties. Here we evaluate its phase stability after very long anneals of 500 days at 500-900 °C during which it is reasonable to expect thermodynamic equilibrium to have been established. Microstructural analyses were performed using complementary analysis techniques including scanning and transmission electron microscopy (SEM/TEM/STEM), energy dispersive X-ray (EDX) spectroscopy, selected area electron diffraction (SAD), and atom probe tomography (APT). We show that the alloy is a single-phase solid solution after homogenization for 2 days at 1200 °C and remains in this state after a subsequent anneal at 900 °C for 500 days. However, it is unstable and forms second-phase precipitates at 700 and 500 °C. A Cr-rich σ phase forms at 700 °C, whereas three different phases (L10-NiMn, B2-FeCo and a Cr-rich body-centered cubic, BCC, phase) precipitate at 500 °C. These precipitates are located mostly at grain boundaries, but also form at intragranular inclusions/pores, indicative of heterogeneous nucleation. Since there is limited entropic stabilization of the solid solution state even in the extensively investigated CrMnFeCoNi alloy, the stability of other HEAs currently thought to be solid solutions should be carefully evaluated, especially if they are being considered for applications in vulnerable temperature ranges. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2016.04.005
  • Deformation-Induced Martensite: A New Paradigm for Exceptional Steels
    Djaziri, S. and Li, Y. and Nematollahi, G.A. and Grabowski, B. and Goto, S. and Kirchlechner, C. and Kostka, A. and Doyle, S. and Neugebauer, J. and Raabe, D. and Dehm, G.
    Advanced Materials 28 (2016)
    view abstract10.1002/adma.201601526
  • Effect of intercritical deformation on microstructure and mechanical properties of a low-silicon aluminum-added hot-rolled directly quenched and partitioned steel
    Tan, X.-D. and Xu, Y.-B. and Ponge, D. and Yang, X.-L. and Hu, Z.-P. and Peng, F. and Ju, X.-W. and Wu, D. and Raabe, D.
    Materials Science and Engineering A 656 (2016)
    Here, we applied hot-rolling in conjunction with direct quenching and partitioning (HDQ&P) processes with different rolling schedules to a low-C low-Si Al-added steel. Ferrite was introduced into the steel by intercritical rolling and air cooling after hot-rolling. The effect of intercritcal deformation on the microstructure evolution and mechanical properties was investigated. The promotion of austenite stabilization and the optimization of the TRIP effect due to a moderate degree of intercritical deformation were systematically explored. The results show that the addition of 1.46 wt% of Al can effectively promote ferrite formation. An intercritical deformation above 800 °C can result in a pronounced bimodal grain size distribution of ferrite and some elongated ferrite grains containing sub-grains. The residual strain states of both austenite and ferrite and the occurrence of bainite transformation jointly increase the retained austenite fraction due to its mechanical stabilization and the enhanced carbon partitioning into austenite from its surrounding phases. An intercritical deformation below 800 °C can profoundly increase the ferrite fraction and promote the recrystallization of deformed ferrite. The formation of this large fraction of ferrite enhances the carbon enrichment in the untransformed austenite and retards the bainite transformation during the partitioning process and finally enhances martensite transformation and decreases the retained austenite fraction. The efficient TRIP effect of retained austenite and the possible strain partitioning of bainite jointly improve the work hardening and formability of the steel and lead to the excellent mechanical properties with relatively high tensile strength (905 MPa), low yield ratio (0.60) and high total elongation (25.2%). © 2016 Elsevier B.V.
    view abstract10.1016/j.msea.2016.01.040
  • Effect of Si on the acceleration of bainite transformation by pre-existing martensite
    Toji, Y. and Matsuda, H. and Raabe, D.
    Acta Materialia 116 (2016)
    Bainite transformation was investigated focusing on the influence of pre-existing martensite on the transformation kinetics, morphology and crystallographic orientation of subsequently formed bainite using EBSD and atom probe tomography. Two 1.1 wt% C-3wt.%Mn steels with and without 2 wt% Si were used to clarify the effect of Si. Steels were rapidly cooled from 900 °C to 300 °C and held at this temperature, or quenched from 900 °C once in water to generate approximately 30 vol% martensite followed by holding at 300 °C. Bainite transformation was clearly accelerated by pre-existing martensite in both Si-containing and Si-free steels. Bainite surrounds the pre-existing martensite in the Si-free steel, whereas it grows to the interior of the austenite grains in the steel containing 2 wt% Si. The major orientation relationship between bainite and adjacent austenite was changed by the presence of martensite from Nishiyama-Wassermann (N-W) to Greninger-Troiano (G-T) regardless of Si content. Clear carbon partitioning from martensite into austenite was observed prior to the bainite transformation in the 2 wt% Si steel, which was not observed in the Si-free steel. We suggest that the dislocations introduced by the martensite transformation act as a primary factor accelerating the bainite transformation when martensite is introduced prior to the bainite transformation. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.06.044
  • Effects of Mn additions on microstructure and properties of Fe–TiB2 based high modulus steels
    Baron, C. and Springer, H. and Raabe, D.
    Materials and Design 111 (2016)
    We studied the effects of Mn additions from 0 to 30 wt.% on microstructure, mechanical and physical properties of liquid metallurgy synthesised high modulus steels in hypo- and hyper-eutectic TiB2 concentrations. While Mn has little effect on density, both Young's modulus and mechanical properties were strongly affected by the achieved wide spectrum of matrix microstructures, ranging from ferrite to martensite, reverted austenite, ε-martensite and austenite. Mn additions of 20 and 30 wt.% did not translate into enhanced mechanical performance despite the higher inherent ductility of the predominantly austenitic matrix, and instead eliminate the intended weight saving potential by significantly reducing the Young's modulus. Martensitic matrices of Mn concentrations of 10 wt.%, on the other hand, are favourable for improved matrix/particle co-deformation without sacrificing too much of the composites' stiffness. In hypo-eutectic Fe – TiB2 based steels, mechanical properties on the level of high strength dual phase steels could be achieved (~ 900 MPa UTS and 20% tensile elongation) but with an enhanced Young's modulus of 217 GPa and reduced density of 7460 kg m− 3. These significantly improved physical and mechanical properties render Mn alloyed high modulus steels promising candidate materials for next generation lightweight structural applications. © 2016 Elsevier Ltd
    view abstract10.1016/j.matdes.2016.09.003
  • Efficient additive manufacturing production of oxide- and nitride-dispersion-strengthened materials through atmospheric reactions in liquid metal deposition
    Springer, H. and Baron, C. and Szczepaniak, A. and Jägle, E.A. and Wilms, M.B. and Weisheit, A. and Raabe, D.
    Materials and Design 111 (2016)
    Despite being extremely attractive compounds for strengthening, oxides and nitride particles have found only limited use in metallic materials design, as obtaining appropriate size and dispersion up to now necessitates production by time- and cost-intensive powder metallurgy processes. Here we present an alternative production method, based on the oxide and nitride formation during liquid-metal-deposition procedures in oxygen and/or nitrogen containing atmospheres. Rapid solidification of the small liquid zone suppresses floatation and agglomeration of particles, while subsequent thermo-mechanical treatments densify the material and aids particle dispersion. The in-situ particle formation coupled to the high deposition rates ensures a drastically shortened production chain. The feasibility of the method is exemplarily demonstrated on austenitic stainless steel and commercially available deposition techniques as used in additive manufacturing, performed without shielding gas but instead at air. Even without substantial optimisation of processes and material, &gt; 2 vol.% of hard and stable Cr2N particles with sizes down to 80 nm could be evenly dispersed, resulting in pronounced strengthening at both room temperature and 700 °C without significant loss in ductility. Future possibilities for creating novel generations of cost effective and lean high strength materials, especially for high temperature applications, are outlined and discussed. © 2016 Elsevier Ltd
    view abstract10.1016/j.matdes.2016.08.084
  • Efficient liquid metallurgy synthesis of Fe-TiB2 high modulus steels via in-situ reduction of titanium oxides
    Baron, C. and Springer, H. and Raabe, D.
    Materials and Design 97 (2016)
    We studied the in-situ reduction of Ti oxides by Al as an alternative and cost effective route for the liquid metallurgical synthesis of low density, high stiffness steels (high modulus steels) containing about 10 vol.% TiB2. TiO2, TiO1.83 and TiO were inserted via iron tubes into Fe-B melts, with Al either premixed with the oxide powders or liquid in the melt. Depending on Ti oxide type and location of the redox partner Al, greatly differing reaction kinetics, slag formation and corresponding microstructures of the high modulus steels were observed. TiO1.83 and TiO premixed with Al showed the highest TiB2 yield in the cast steel and are thus favourable candidates for the cost effective production of high modulus steels. Based on our findings, a novel synthesis process is proposed, based on filling wire injection into a continuous casting process, allowing the utilisation of the additionally formed oxide particles for the further improvement of the property profile of high modulus steels. © 2016 Elsevier Ltd.
    view abstract10.1016/j.matdes.2016.02.076
  • Electronic hybridisation implications for the damage-tolerance of thin film metallic glasses
    Schnabel, V. and Jaya, B.N. and Köhler, M. and Music, D. and Kirchlechner, C. and Dehm, G. and Raabe, D. and Schneider, J.M.
    Scientific Reports 6 (2016)
    A paramount challenge in materials science is to design damage-tolerant glasses. Poisson's ratio is commonly used as a criterion to gauge the brittle-ductile transition in glasses. However, our data, as well as results in the literature, are in conflict with the concept of Poisson's ratio serving as a universal parameter for fracture energy. Here, we identify the electronic structure fingerprint associated with damage tolerance in thin film metallic glasses. Our correlative theoretical and experimental data reveal that the fraction of bonds stemming from hybridised states compared to the overall bonding can be associated with damage tolerance in thin film metallic glasses. © The Author(s) 2016.
    view abstract10.1038/srep36556
  • Elemental partitioning, lattice misfit and creep behaviour of Cr containing gammaprime strengthened Co base superalloys
    Povstugar, I. and Zenk, C.H. and Li, R. and Choi, P.-P. and Neumeier, S. and Dolotko, O. and Hoelzel, M. and Göken, M. and Raabe, D.
    Materials Science and Technology (United Kingdom) 32 (2016)
    Novel Cr containing Co-Al-W base superalloys were studied by atom probe tomography and neutron diffraction. Cr is found to predominantly partition to the gamma matrix and decrease partitioning of W to gammaprime. Furthermore, Cr significantly enhances the gammaprime volume fraction, decreases the gamma/gammaprime lattice misfit and deteriorates the creep resistance. Addition of Ni to the Cr containing alloys affects partitioning of W and Al, further decreases the lattice misfit and results in the formation of irregularly shaped precipitates. Al, W and Cr tend to occupy the ‘B'sublattice in the gammaprime-A3B phase (L12 type), while Co and Ni reside in the ‘A' sublattice. © 2016 Institute of Materials.
    view abstract10.1179/1743284715Y.0000000112
  • Formation of nanosized grain structure in martensitic 100Cr6 bearing steels upon rolling contact loading studied by atom probe tomography
    Li, Y.J. and Herbig, M. and Goto, S. and Raabe, D.
    Materials Science and Technology (United Kingdom) 32 (2016)
    To understand the origin of white etching cracks (WECs), a systematic microstructural characterisation in the regions affected from the near-surface region down to the subsurface layers where WECs occur is necessary. As a starting point, we focus on the near-surface region of an axial thrust bearing, made of martensitic 100Cr6 steel, to study the influence of rolling contact loading on the microstructure and the resulting distributions of the major alloying elements C and Cr using atom probe tomography. We find that upon rolling contact loading the original plate-like martensitic structure evolves into a nanosized equiaxed grain structure with C segregation up to 5 at.-% at the grain boundaries. Cementite particles, located at grain boundaries and triple junctions, undergo spheroidisation. The originally homogeneously distributed Cr becomes enriched in spheroidised cementite particles. The microstructural changes give strong hints that rolling contact loading induces plastic deformation and an increased temperature on the near-surface region. This paper is part of a Themed Issue on Recent developments in bearing steels. © 2016 Institute of Materials, Minerals and Mining.
    view abstract10.1080/02670836.2015.1120458
  • From electronic structure to phase diagrams: A bottom-up approach to understand the stability of titanium–transition metal alloys
    Huang, L.-F. and Grabowski, B. and Zhang, J. and Lai, M.-J. and Tasan, C.C. and Sandlöbes, S. and Raabe, D. and Neugebauer, J.
    Acta Materialia 113 (2016)
    We have computed formation energies for all technologically relevant transition metal solutes in the α, β, and ω phases of Ti, employing ab initio simulations. We analyze and explain their periodic-table trends, and from their differences we derive stabilization energies which provide direct insight into phase stabilization effects of the various solutes with respect to α, β, and ω. This allows us to identify strong β stabilizers in the middle of each electronic d shell in consistency with experimental knowledge. Based on an extension of the stabilization energies to free energies we derive a wide range of Ti-transition metal phase diagrams. A detailed comparison to available experimental martensitic transformation temperatures and to measurements performed in this study shows that, despite some quantitative discrepancies, the qualitative trends can be expected to be correct. An important feature that is displayed by a limited range of the computed phase diagrams is a triple point at which the three phases, α, β, and ω, meet. This insight provides a plausible explanation for the complexity observed in gum metals, a class of Ti alloys with very special materials properties. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.04.059
  • From insect scales to sensor design: Modelling the mechanochromic properties of bicontinuous cubic structures
    Wu, X. and Ma, D. and Eisenlohr, P. and Raabe, D. and Fabritius, H.-O.
    Bioinspiration and Biomimetics 11 (2016)
    Many of the three-dimensional photonic crystals occurring in the scales of insects have bicontinuous cubic structures. Their optical properties have been studied extensively, however little is known about their mechanical properties and their optical response under deformation. We demonstrated a mechanochromic effect by deforming the scales of a weevil and calculated the elastic, optical and mechanochromic (assuming homogeneous deformation) properties of the three types of bicontinuous cubic structures occurring in nature: P-structure (primitive), G-structure (gyroid) and D-structure (diamond). The results show that all investigated properties of these three structure types strongly depend on their geometry, structural parameters such as volume fractions of the two constituting phases and the directions of the incident light or applied stress, respectively. Interestingly, the mechanochromic simulation results predict that these structures may show blue-shift or even red-shift under compression along certain directions. Our results provide design guidelines for mechanochromic sensing materials operating in the elastic regime, including parameters such as sensitivity and direction of spectral shift. © 2016 IOP Publishing Ltd.
    view abstract10.1088/1748-3190/11/4/045001
  • Functional adaptation of crustacean exoskeletal elements through structural and compositional diversity: A combined experimental and theoretical study
    Fabritius, H.-O. and Ziegler, A. and Friák, M. and Nikolov, S. and Huber, J. and Seidl, B.H.M. and Ruangchai, S. and Alagboso, F.I. and Karsten, S. and Lu, J. and Janus, A.M. and Petrov, M. and Zhu, L.-F. and Hemzalová, P. and Hild, S. and Raabe, D. and Neugebauer, J.
    Bioinspiration and Biomimetics 11 (2016)
    The crustacean cuticle is a composite material that covers the whole animal and forms the continuous exoskeleton. Nano-fibers composed of chitin and protein molecules form most of the organic matrix of the cuticle that, at the macroscale, is organized in up to eight hierarchical levels. At least two of them, the exo- and endocuticle, contain a mineral phase of mainly Mg-calcite, amorphous calcium carbonate and phosphate. The high number of hierarchical levels and the compositional diversity provide a high degree of freedom for varying the physical, in particular mechanical, properties of the material. This makes the cuticle a versatile material ideally suited to form a variety of skeletal elements that are adapted to different functions and the eco-physiological strains of individual species. This review presents our recent analytical, experimental and theoretical studies on the cuticle, summarising at which hierarchical levels structure and composition are modified to achieve the required physical properties. We describe our multi-scale hierarchical modeling approach based on the results from these studies, aiming at systematically predicting the structure-composition-property relations of cuticle composites from the molecular level to the macro-scale. This modeling approach provides a tool to facilitate the development of optimized biomimetic materials within a knowledge-based design approach. © 2016 IOP Publishing Ltd.
    view abstract10.1088/1748-3190/11/5/055006
  • Growth of bainitic ferrite and carbon partitioning during the early stages of bainite transformation in a 2 mass% silicon steel studied by in situ neutron diffraction, TEM and APT
    Timokhina, I.B. and Liss, K.D. and Raabe, D. and Rakha, K. and Beladi, H. and Xiong, X.Y. and Hodgson, P.D.
    Journal of Applied Crystallography 49 (2016)
    In situ neutron diffraction, transmission electron microscopy (TEM) and atom probe tomography (APT) have been used to study the early stages of bainite transformation in a 2 mass% Si nano-bainitic steel. It was observed that carbon redistribution between the bainitic ferrite and retained austenite at the early stages of the bainite transformation at low isothermal holding occurred in the following sequence: (i) formation of bainitic ferrite nuclei within carbon-depleted regions immediately after the beginning of isothermal treatment; (ii) carbon partitioning immediately after the formation of bainitic ferrite nuclei but substantial carbon diffusion only after 33 min of bainite isothermal holding; (iii) formation of the carbon-enriched remaining austenite in the vicinity of bainitic laths at the beginning of the transformation; (iv) segregation of carbon to the dislocations near the austenite/ferrite interface; and (v) homogeneous redistribution of carbon within the remaining austenite with the progress of the transformation and with the formation of bainitic ferrite colonies. Bainitic ferrite nucleated at internal defects or bainite/austenite interfaces as well as at the prior austenite grain boundary. Bainitic ferrite has been observed in the form of an individual layer, a colony of layers and a layer with sideplates at the early stages of transformation. © 2016 International Union of Crystallography.
    view abstract10.1107/S1600576716000418
  • Hydrogen-assisted damage in austenite/martensite dual-phase steel
    Koyama, M. and Tasan, C.C. and Nagashima, T. and Akiyama, E. and Raabe, D. and Tsuzaki, K.
    Philosophical Magazine Letters 96 (2016)
    For understanding the underlying hydrogen embrittlement mechanism in transformation-induced plasticity steels, the process of damage evolution in a model austenite/martensite dual-phase microstructure following hydrogenation was investigated through multi-scale electron channelling contrast imaging and in situ optical microscopy. Localized diffusible hydrogen in martensite causes cracking through two mechanisms: (1) interaction between {1 1 0}M localized slip and {1 1 2}M twin and (2) cracking of martensite-martensite grain interfaces. The former resulted in nanovoids along the {1 1 2}M twin. The coalescence of the nanovoids generated plate-like microvoids. The latter caused shear localization on the specific plane where the crack along the martensite/martensite boundary exists, which led to additional martensite/martensite boundary cracking. © 2015 Taylor & Francis.
    view abstract10.1080/09500839.2015.1130275
  • Improving the mechanical properties of Fe – TiB2 high modulus steels through controlled solidification processes
    Zhang, H. and Springer, H. and Aparicio-Fernández, R. and Raabe, D.
    Acta Materialia 118 (2016)
    We investigated novel pathways to improve the mechanical properties of liquid metallurgy produced Fe – TiB2 based high modulus steels (HMS) by controlled solidification kinetics and subsequent thermo-mechanical treatments. The solidification rate was varied by casting of hyper-eutectic alloys (20 vol% TiB2) into moulds with differing internal thickness. Ingots between 5 and 40 mm thickness exhibited irregular particle microstructure consisting of sharp-edged coarse primary particles (increasingly clustered with slower solidification) and closely spaced irregular lamellae. Casting defects can be alleviated by hot rolling, but the mechanical properties remain unsatisfactory. Increasing the solidification rate results only at mould thicknesses of 4 mm and below in a significant refinement of the particle microstructure, necessitating liquid metal deposition techniques to utilise it for obtained improved mechanical performance of HMS. Decreasing the solidification rate causes density-induced floatation of the primary particles, which can be used in block-casting for the production of alloys consisting of small and spheroidised eutectic particles, exhibiting high ductility and superior toughness. Annealing just above the solidus-temperature allows the eutectic zones to liquefy and sink, leaving only primary TiB2 particles behind in the top zone of the alloy. Despite the increased particle fraction up to 24 vol%, both strength, specific modulus and ductility are improved over standard processed HMS alloys with 20 vol% TiB2. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.07.056
  • In-situ metal matrix composite steels: Effect of alloying and annealing on morphology, structure and mechanical properties of TiB2 particle containing high modulus steels
    Aparicio-Fernández, R. and Springer, H. and Szczepaniak, A. and Zhang, H. and Raabe, D.
    Acta Materialia 107 (2016)
    We systematically study the morphology, size and dispersion of TiB2 particles formed in-situ from Fe-Ti-B based melts, as well as their chemical composition, crystal structure and mechanical properties. The effects of 5 wt.% additions of Cr, Ni, Co, Mo, W, Mn, Al, Si, V, Ta, Nb and Zr, respectively, as well as additional annealing treatments, were investigated in order to derive guidelines for the knowledge based alloy design of steels with an increased stiffness/density ratio and sufficiently high ductility. All alloying elements were found to increase the size of the coarse primary TiB2 particles, while Co led to the most homogeneous size distribution. The size of the eutectic TiB2 constituents was decreased by all alloying additions except Ni, while their aspect ratio was little affected. No clear relation between chemical composition, crystal structure and mechanical properties of the particles could be observed. Annealing of the as-cast alloys slightly increased the size of the primary particles, but at the same time strongly spheroidised the eutectics. Additions of Co and Cr appear thus as the best starting point for designing novel in-situ high modulus metal matrix composite steels, while using Mn in concert with thermo-mechanical processing is most suited to adapt the matrix' microstructure and optimise the particle/matrix co-deformation processes. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2016.01.048
  • Lattice Distortions in the FeCoNiCrMn High Entropy Alloy Studied by Theory and Experiment
    Oh, H. S. and Ma, D. and Leyson, G. P. and Grabowski, B. and Park, E. S. and Kormann, F. and Raabe, D.
    Entropy 18 (2016)
    Lattice distortions constitute one of the main features characterizing high entropy alloys. Local lattice distortions have, however, only rarely been investigated in these multi-component alloys. We, therefore, employ a combined theoretical electronic structure and experimental approach to study the atomistic distortions in the FeCoNiCrMn high entropy (Cantor) alloy by means of density-functional theory and extended X-ray absorption fine structure spectroscopy. Particular attention is paid to element-resolved distortions for each constituent. The individual mean distortions are small on average, <1%, but their fluctuations (i.e., standard deviations) are an order of magnitude larger, in particular for Cr and Mn. Good agreement between theory and experiment is found.
    view abstract10.3390/e18090321
  • Localized plastic deformation in a model metallic glass: A survey of free volume and local force distributions
    Hassani, M. and Engels, P. and Raabe, D. and Varnik, F.
    Journal of Statistical Mechanics: Theory and Experiment 2016 (2016)
    Plastic deformation of a model glass is investigated via large scale molecular dynamics simulations. The role of microscopic fluctuations of the structure for the deformation behavior is highlighted by demonstrating that statistically independent samples prepared via an identical protocol develop qualitatively distinct deformation paths. As a quantitative measure, the spatial distribution of the particle based excess volume is monitored via Voronoi tesselation. While the fluctuations of the thus defined single-particle based excess volume do not seem to show any signature of the strain field, a non-local definition of the excess volume clearly correlates with the observed shear deformation field. The distribution of the force acting on individual particles also shows a pattern strongly similar to that of the strain. In line with other studies, these results underline the importance of both the structural heterogeneities as well as the fluctuations of the locally acting forces and stresses for plastic deformation in amorphous solids. © 2016 IOP Publishing Ltd and SISSA Medialab srl.
    view abstract10.1088/1742-5468/2016/08/084006
  • Low cycle fatigue in aluminum single and bi-crystals: On the influence of crystal orientation
    Nellessen, J. and Sandlöbes, S. and Raabe, D.
    Materials Science and Engineering A 668 (2016)
    Aluminum single crystals with three different double-slip orientations and two aluminum bi-crystals - one with a high-angle grain boundary and one with a low-angle grain boundary - were cyclically deformed up to 100 cycles under constant displacement control. The distribution of the local strain and the local strain amplitudes was captured by in-situ digital image correlation (DIC). Dislocation structure analysis was performed by electron channeling contrast imaging (ECCI) and the evolution of local misorientations was recorded by high resolution electron backscatter diffraction (EBSD). The DIC results show a homogeneous strain amplitude distribution in the single crystals while the measured strain amplitude in the low-angle grain boundary bi-crystal sample differs significantly. ECCI observations reveal the presence of dislocation cells elongated along the trace of the primary {111} slip plane in all investigated crystals and the formation of deformation bands parallel to the trace of {110} planes. Deformation bands (DB) were observed in all samples but their frequency and misorientation with respect to the matrix was found to sensitively depend on the crystal orientation and the local strain amplitude. Our results on the bi-crystals show that the grain orientation mainly determines the local stresses and therefore also the formation of the associated dislocation structures rather than the grain boundary character. © 2016 Elsevier B.V.
    view abstract10.1016/j.msea.2016.05.054
  • Magnetic microstructure in a stress-annealed Fe73.5Si15.5B7Nb3Cu1 soft magnetic alloy observed using off-axis electron holography and Lorentz microscopy
    Kovács, A. and Pradeep, K.G. and Herzer, G. and Raabe, D. and Dunin-Borkowski, R.E.
    AIP Advances 6 (2016)
    Fe-Si-B-Nb-Cu alloys are attractive for high frequency applications due to their low coercivity and high saturation magnetization. Here, we study the effect of stress annealing on magnetic microstructure in Fe73.5Si15.5B7Nb3Cu1 using off-axis electron holography and the Fresnel mode of Lorentz transmission electron microscopy. A stress of 50 MPa was applied to selected samples during rapid annealing for 4 s, resulting in uniaxial anisotropy perpendicular to the stress direction. The examination of focused ion beam milled lamellae prepared from each sample revealed a random magnetic domain pattern in the sample that had been rapidly annealed in the absence of stress, whereas a highly regular domain pattern was observed in the stress-annealed sample. We also measured a decrease in domain wall width from ∼ 94 nm in the sample annealed without stress to ∼ 80 nm in the stress-annealed sample. © 2016 Author(s).
    view abstract10.1063/1.4942954
  • Metastable high-entropy dual-phase alloys overcome the strength-ductility trade-off
    Li, Z. and Pradeep, K.G. and Deng, Y. and Raabe, D. and Tasan, C.C.
    Nature 534 (2016)
    Metals have been mankind's most essential materials for thousands of years; however, their use is affected by ecological and economical concerns. Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency. However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength-ductility trade-off. Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned. We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys. In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength. Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility. This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials. This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys.
    view abstract10.1038/nature17981
  • Microstructural analysis in the Fe-30.5Mn-8.0Al-1.2C and Fe-30.5Mn-2.1Al-1.2C steels upon cold rolling
    Souza, F.M. and Padilha, A.F. and Gutierrez-Urrutia, I. and Raabe, D.
    Revista Escola de Minas 69 (2016)
    Electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) were used to examine microstructural changes of the austenitic low-density Fe-30.5Mn-8.0Al-1.2C (8Al) and Fe-30.5Mn-2.1Al-1.2C (2Al) (wt.%) steels during cold rolling. As the strain increased, deformation mechanisms, such as stacking faults, slip, mechanical twinning, and shear banding were activated in both steels cold rolled up to strain of 0.69. Only slip was noted in these steels at low strain (ε=0.11) and slip dominance was detected in the 8Al steel at higher strains. Shear banding became active at higher strain (ε~0.7) in these materials. An inhomogeneous microstructure formed in both alloys at such strain level. More extensive mechanical twinning in the 2Al alloy than that in the 8Al alloy was observed. Fish bone-like structure patterns were revealed in the 8Al steel and a river-like structure in the 2Al steel. Detailed microstructure features as elongated and fragmented grains along the rolling direction (RD) were found for both steels, as already observed in other high-Mn steels. These deformed structures are composed by lamellar packets which can contain mechanical twins or slip lines and shear bands. © 2016, Escola de Minas. All rights reserved.
    view abstract10.1590/0370-44672015690097
  • Multi-scale and spatially resolved hydrogen mapping in a Ni-Nb model alloy reveals the role of the δ phase in hydrogen embrittlement of alloy 718
    Tarzimoghadam, Z. and Rohwerder, M. and Merzlikin, S.V. and Bashir, A. and Yedra, L. and Eswara, S. and Ponge, D. and Raabe, D.
    Acta Materialia 109 (2016)
    We investigated the hydrogen distribution and desorption behavior in an electrochemically hydrogen-charged binary Ni-Nb model alloy to study the role of δ phase in hydrogen embrittlement of alloy 718. We focus on two aspects, namely, (1) mapping the hydrogen distribution with spatial resolution enabling the observation of the relations between desorption profiles and desorption sites; and (2) correlating these observations with mechanical testing results to reveal the degradation mechanisms. The trapping states of hydrogen in the alloy were globally analyzed by Thermal Desorption Spectroscopy (TDS). Additionally, spatially resolved hydrogen mapping was conducted using silver decoration, Scanning Kelvin Probe Force Microscopy (SKPFM) and Secondary Ion Mass Spectrometry (SIMS): The Ag decoration method revealed rapid effusion of hydrogen at room temperature from the γ-matrix. The corresponding kinetics was resolved in both, space and time by the SKPFM measurements. At room temperature the hydrogen release from the γ-matrix steadily decreased until about 100 h and then was taken over by the δ phase from which the hydrogen was released much slower. For avoiding misinterpretation of hydrogen signals stemming from environmental effects we also charged specimens with deuterium. The deuterium distribution in the microstructure was studied by SIMS. The combined results reveal that hydrogen dissolves more preferably inside the γ-matrix and is diffusible at room temperature while the δ phase acts as a deeper trapping site for hydrogen. With this joint and spatially resolving approach we observed the microstructure- and time-dependent distribution and release rate of hydrogen with high spatial and temporal resolution. Correlating the obtained results with mechanical testing of the hydrogen-charged samples shows that hydrogen enhanced decohesion (HEDE) occurring at the δ/matrix interfaces promotes the embrittlement. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2016.02.053
  • Multiple mechanisms of lath martensite plasticity
    Morsdorf, L. and Jeannin, O. and Barbier, D. and Mitsuhara, M. and Raabe, D. and Tasan, C.C.
    Acta Materialia 121 (2016)
    The multi-scale complexity of lath martensitic microstructures requires scale-bridging analyses to better understand the deformation mechanisms activated therein. In this study, plasticity in lath martensite is investigated by multi-field mapping of deformation-induced microstructure, topography, and strain evolution at different spatial resolution vs. field-of-view combinations. These investigations reveal site-specific initiation of dislocation activity within laths, as well as significant plastic accommodation in the vicinity of high angle block and packet boundaries. The observation of interface plasticity raises several questions regarding the role of thin inter-lath austenite films. Thus, accompanying transmission electron microscopy and synchrotron x-ray diffraction experiments are carried out to investigate the stability of these films to mechanical loading, and to discuss alternative boundary sliding mechanisms to explain the observed interface strain localization. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.09.006
  • Multiscale description of carbon-supersaturated ferrite in severely drawn pearlitic wires
    Nematollahi, Gh.A. and Grabowski, B. and Raabe, D. and Neugebauer, J.
    Acta Materialia 111 (2016)
    A multiscale simulation approach based on atomistic calculations and a discrete diffusion model is developed and applied to carbon-supersaturated ferrite, as experimentally observed in severely deformed pearlitic steel. We employ the embedded atom method and the nudged elastic band technique to determine the energetic profile of a carbon atom around a screw dislocation in bcc iron. The results clearly indicate a special region in the proximity of the dislocation core where C atoms are strongly bound, but where they can nevertheless diffuse easily due to low barriers. Our analysis suggests that the previously proposed pipe mechanism for the case of a screw dislocation is unlikely. Instead, our atomistic as well as the diffusion model results support the so-called drag mechanism, by which a mobile screw dislocation is able to transport C atoms along its glide plane. Combining the C-dislocation interaction energies with density-functional-theory calculations of the strain dependent C formation energy allows us to investigate the C supersaturation of the ferrite phase under wire drawing conditions. Corresponding results for local and total C concentrations agree well with previous atom probe tomography measurements indicating that a significant contribution to the supersaturation during wire drawing is due to dislocations. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.03.052
  • On Local Phase Equilibria and the Appearance of Nanoparticles in the Microstructure of Single-Crystal Ni-Base Superalloys
    Yardley, V. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Parsa, A.B. and Kostka, A. and Somsen, C. and Dlouhy, A. and Neuking, K. and George, E.P. and Eggeler, G.
    Advanced Engineering Materials 18 (2016)
    High-resolution characterization techniques are combined with thermodynamic calculations (CALPHAD) to rationalize microstructural features of single crystal Ni-base superalloys. Considering the chemical compositions of dendritic and interdendritic regions one can explain differences in γ′-volume fractions. Using thermodynamic calculations one can explain, why γ-nanoparticles are observed in the central regions of large cuboidal γ′-particles and why tertiary γ′-nanoparticles form in the γ-channels. The chemical compositions of the γ-channels and of the newly formed γ-particles differ because of the Gibbs–Thomson pressure which acts on the small particles. With increasing size of secondary γ′-particles, their shape changes from spherical to cuboidal. Some general thermodynamic aspects including the temperature dependencies of the Gibbs free energy G, the enthalpy H, and the entropy S and site occupancies in the ordered L12 (γ′) phase are considered. The importance of cooling rate after homogenization is discussed. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adem.201600237
  • On the mechanism of {332} twinning in metastable β titanium alloys
    Lai, M.J. and Tasan, C.C. and Raabe, D.
    Acta Materialia 111 (2016)
    {332} twinning, an unusual twinning mode in other body-centered cubic (bcc) metals and alloys, has been demonstrated to be a fundamental deformation mode in bcc metastable β titanium alloys. Recent studies suggest that this twinning mode plays an important role in enhancing the work hardening and thus improving the mechanical properties. Here, we studied the mechanism of this twinning mode in a metastable β Ti-36Nb-2Ta-3Zr (wt.%) alloy. Tensile tests were performed to induce the formation of {332} twins. By using electron backscatter diffraction, transmission electron microscopy and in situ scanning electron microscopy, the surface-to-bulk microstructures and the initiation and propagation of {332} twins were investigated. In addition to the previously reported high densities of straight dislocations within the twin, we have observed that an α″ martensite band is present near the surface adjacent to the twin. During annealing at 900°C, the α″ martensite band transforms into the adjacent twin rather than into the matrix, indicating that {332} twin nucleates within α″ martensite. Further evidence for this is the constitution of the twin in the initial stage of its formation, where the first portion formed consists of α″ martensite. During propagation, the twins propagating to the opposite directions can merge together when their lateral boundaries impinge on each other. Based on the experimental observations, an α″-assisted twinning mechanism is proposed and the origin of the dislocations within {332} twin is discussed accordingly. © 2016 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.03.040
  • On the origin of creep dislocations in a Ni-base, single-crystal superalloy: An ECCI, EBSD, and dislocation dynamics-based study
    Ram, F. and Li, Z. and Zaefferer, S. and Hafez Haghighat, S.M. and Zhu, Z. and Raabe, D. and Reed, R.C.
    Acta Materialia 109 (2016)
    This work investigates the origin of creep dislocations in a Ni-base, single crystal superalloy subject to creep at an intermediate stress and temperature. Employing high angular resolution electron backscatter diffraction (HR-EBSD), electron channeling contrast imaging under controlled diffraction conditions (cECCI) and discrete dislocation dynamics (DDD) modelling, it is shown that low-angle boundaries - which correspond to dendrite boundaries or their residues after annealing - are not the major sources of creep dislocations. At the onset of creep deformation, they are the only active sources. Creep dislocations are emitted from them and percolate into the dislocation-depleted crystal. However, the percolation is very slow. As creep deformation proceeds, before the boundary-originated dislocations move further than a few micrometers away from their source boundary, individual dislocations that are spread throughout the undeformed microstructure become active and emit avalanches of creep dislocations in boundary-free regions, i.e. regions farther than a few micrometer away from boundaries. Upon their activation, the density of creep dislocations in boundary-free regions soars by two orders of magnitude; and the entire microstructure becomes deluged with creep dislocations. The total area of boundary-free regions is several times the total area of regions covered by boundary-originated creep dislocations. Therefore, the main sources of creep dislocations are not low-angle boundaries but individual, isolated dislocations in boundary-free regions. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.02.038
  • Phase selection and nanocrystallization in Cu-free soft magnetic FeSiNbB amorphous alloy upon rapid annealing
    Morsdorf, L. and Pradeep, K.G. and Herzer, G. and Kovács, A. and Dunin-Borkowski, R.E. and Povstugar, I. and Konygin, G. and Choi, P. and Raabe, D.
    Journal of Applied Physics 119 (2016)
    Nucleation of soft magnetic Fe3Si nanocrystals in Cu-free Fe74.5Si15.5Nb3B7 alloy, upon rapid (10 s) and conventional (30 min) annealing, was investigated using x-ray diffraction, transmission electron microscopy, Mössbauer spectroscopy, and atom probe tomography. By employing rapid annealing, preferential nucleation of Fe3Si nanocrystals was achieved, whereas otherwise there is simultaneous nucleation of both Fe3Si and undesired Fe-B compound phases. Analysis revealed that the enhanced Nb diffusivity, achieved during rapid annealing, facilitates homogeneous nucleation of Fe3Si nanocrystals while shifting the secondary Fe-B crystallization to higher temperatures resulting in pure soft magnetic nanocrystallization with very low coercivities of ∼10 A/m. © 2016 AIP Publishing LLC.
    view abstract10.1063/1.4944595
  • Plastic accommodation at homophase interfaces between nanotwinned and recrystallized grains in an austenitic duplex-microstructured steel
    Gutierrez-Urrutia, I. and Archie, F. and Raabe, D. and Yan, F.-K. and Tao, N.-R. and Lu, K.
    Science and Technology of Advanced Materials 17 (2016)
    The plastic co-deformation behavior at the homophase interfaces between the hard nanotwinned grain inclusions and the soft recrystallized matrix grains in a duplex-microstructured AISI 316L austenitic stainless steel is examined through the analysis of long-range orientation gradients within the matrix grains by electron backscatter diffraction and transmission electron microcopy. Our analysis reveals that the mechanical accommodation of homophase interfaces until a macroscopic strain of 22% is realized within a small area of soft grains (about four grains) adjacent to the homophase interface. The activation of deformation twinning in the first two grain layers results in the occurrence of a ‘hump’ in the orientation gradient profile. We ascribe this effect to the role of deformation twinning on the generation of geometrically necessary dislocations. The smooth profile of the orientation gradient amplitude within the first 10 grain layers indicates a gradual plastic accommodation of the homophase interfaces upon straining. As a consequence, damage nucleation at such interfaces is impeded, resulting in an enhanced ductility of the single phase duplex-microstructured steel. © 2016 The Author(s).
    view abstract10.1080/14686996.2016.1140302
  • Precipitation Reactions in Age-Hardenable Alloys During Laser Additive Manufacturing
    Jägle, E.A. and Sheng, Z. and Wu, L. and Lu, L. and Risse, J. and Weisheit, A. and Raabe, D.
    JOM 68 (2016)
    We describe and study the thermal profiles experienced by various age-hardenable alloys during laser additive manufacturing (LAM), employing two different manufacturing techniques: selective laser melting and laser metal deposition. Using scanning electron microscopy and atom probe tomography, we reveal at which stages during the manufacturing process desired and undesired precipitation reactions can occur in age-hardenable alloys. Using examples from a maraging steel, a nickel-base superalloy and a scandium-containing aluminium alloy, we demonstrate that precipitation can already occur during the production of the powders used as starting material, during the deposition of material (i.e. during solidification and subsequent cooling), during the intrinsic heat treatment effected by LAM (i.e. in the heat affected zones) and, naturally, during an ageing post-heat treatment. These examples demonstrate the importance of understanding and controlling the thermal profile during the entire additive manufacturing cycle of age-hardenable materials including powder synthesis. © 2016, The Author(s).
    view abstract10.1007/s11837-015-1764-2
  • Response to "Comment on 'Viscous coalescence of droplets: A lattice Boltzmann study'" [Phys. Fluids 28, 079101 (2016)]
    Gross, M. and Steinbach, I. and Raabe, D. and Varnik, F.
    Physics of Fluids 28 (2016)
    view abstract10.1063/1.4958828
  • Revealing the relationships between chemistry, topology and stiffness of ultrastrong Co-based metallic glass thin films: A combinatorial approach
    Schnabel, V. and Köhler, M. and Evertz, S. and Gamcova, J. and Bednarcik, J. and Music, D. and Raabe, D. and Schneider, J.M.
    Acta Materialia 107 (2016)
    An efficient way to study the relationship between chemical composition and mechanical properties of thin films is to utilize the combinatorial approach, where spatially resolved mechanical property measurements are conducted along a concentration gradient. However, for thin film glasses many properties including the mechanical response are affected by chemical topology. Here a novel method is introduced which enables spatially resolved short range order analysis along concentration gradients of combinatorially synthesized metallic glass thin films. For this purpose a CoZrTaB metallic glass film of 3 μm thickness is deposited on a polyimide foil, which is investigated by high energy X-ray diffraction in transmission mode. Through the correlative chemistry-topology-stiffness investigation, we observe that an increase in metalloid concentration from 26.4 to 32.7 at% and the associated formation of localized (hybridized) metal - metalloid bonds induce a 10% increase in stiffness. Concomitantly, along the same composition gradient, a metalloid-concentration-induced increase in first order metal - metal bond distances of 1% is observed, which infers itinerant (metallic) bond weakening. Hence, the metalloid concentration induced increase in hybridized bonding dominates the corresponding weakening of metallic bonds. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2016.01.060
  • Spectral TRIP enables ductile 1.1 GPa martensite
    Wang, M.-M. and Tasan, C.C. and Ponge, D. and Raabe, D.
    Acta Materialia 111 (2016)
    Introduction of interlath reverted austenite is an effective method to design ductile lath martensitic steels. The challenge in this concept is that all reverted austenite films have similar mechanical stability, hence, they all undergo transformation-induced plasticity (TRIP) at the same strain level. Here we propose a new thermo-mechanical treatment route to activate the TRIP effect over a broad strain regime and refer to it as 'spectral TRIP effect'. It aims at spreading the micro-mechanical stability of reverted austenite grains by widening the austenite nucleation barrier in martensite. To validate the proposed thermo-mechanical treatment route, an as-quenched medium-Mn martensitic steel was cold rolled prior to the reversion treatment at 600 °C. Microstructure characterization was carried out by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). Mechanical tests show that the approach is effective. The spectral TRIP effect improves both, the strength and the ductility due to the well dispersed size distribution and the associated size-dependent deformation and phase transformation behavior of the reverted austenite grains, extending TRIP-related work hardening over a broad strain range. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.03.070
  • Strain hardening by dynamic slip band refinement in a high-Mn lightweight steel
    Welsch, E. and Ponge, D. and Hafez Haghighat, S.M. and Sandlöbes, S. and Choi, P. and Herbig, M. and Zaefferer, S. and Raabe, D.
    Acta Materialia 116 (2016)
    The strain hardening mechanism of a high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C (wt%)) is investigated by electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM). The alloy is characterized by a constant high strain hardening rate accompanied by high strength and high ductility (ultimate tensile strength: 900 MPa, elongation to fracture: 68%). Deformation microstructures at different strain levels are studied in order to reveal and quantify the governing structural parameters at micro- and nanometer scales. As the material deforms mainly by planar dislocation slip causing the formation of slip bands, we quantitatively study the evolution of the slip band spacing during straining. The flow stress is calculated from the slip band spacing on the basis of the passing stress. The good agreement between the calculated values and the tensile test data shows dynamic slip band refinement as the main strain hardening mechanism, enabling the excellent mechanical properties. This novel strain hardening mechanism is based on the passing stress acting between co-planar slip bands in contrast to earlier attempts to explain the strain hardening in high-Mn lightweight steels that are based on grain subdivision by microbands. We discuss in detail the formation of the finely distributed slip bands and the gradual reduction of the spacing between them, leading to constantly high strain hardening. TEM investigations of the precipitation state in the as-quenched state show finely dispersed atomically ordered clusters (size < 2 nm). The influence of these zones on planar slip is discussed. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.06.037
  • Structural and optical properties of (1122) InGaN quantum wells compared to (0001) and (1120)
    Pristovsek, M. and Han, Y. and Zhu, T. and Oehler, F. and Tang, F. and Oliver, R.A. and Humphreys, C.J. and Tytko, D. and Choi, P.-P. and Raabe, D. and Brunner, F. and Weyers, M.
    Semiconductor Science and Technology 31 (2016)
    We benchmarked growth, microstructure and photo luminescence (PL) of (112-2) InGaN quantum wells (QWs) against (0001) and (112-0). In incorporation, growth rate and the critical thickness of (112-2) QWs are slightly lower than (0001) QWs, while the In incorporation on (112-0) is reduced by a factor of three. A small step-bunching causes slight fluctuations of the emission wavelength. Transmission electron microscopy as well as atom probe tomography (APT) found very flat interfaces with little In segregation even for 20% In content. APT frequency distribution analysis revealed some deviation from a random InGaN alloy, but not as severe as for (112-0). The slight deviation of (112-2) QWs from an ideal random alloy did not broaden the 300 K PL, the line widths were similar for (112-2) and (0001) while (112-0) QWs were broader. Despite the high structural quality and narrow PL, the integrated PL signal at 300 K was about 4 lower on (112-2) and more than 10 lower on (112-0). © 2016 IOP Publishing Ltd.
    view abstract10.1088/0268-1242/31/8/085007
  • Structure and dynamics of shear bands in amorphous-crystalline nanolaminates
    Guo, W. and Gan, B. and Molina-Aldareguia, J.M. and Poplawsky, J.D. and Raabe, D.
    Scripta Materialia 110 (2016)
    The velocities of shear bands in amorphous CuZr/crystalline Cu nanolaminates were quantified as a function of strain rate and crystalline volume fraction. A rate-dependent transition in flow response was found in a 100 nm CuZr/10 nm Cu nanolaminates. When increasing the Cu layer thickness from 10 nm to 100 nm, the instantaneous velocity of the shear band in these nanolaminates decreases from 11.2 μm/s to <∼500 nm/s. Atom probe tomography and transmission election microcopy observation revealed that in post-deformed pillars both grain rotation in the crystalline portion and non-diffusive crystallization in the amorphous layer affect the viscosity of shear bands. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2015.07.038
  • The role of metastable LPSO building block clusters in phase transformations of an Mg-Y-Zn alloy
    Kim, J.-K. and Ko, W.-S. and Sandlöbes, S. and Heidelmann, M. and Grabowski, B. and Raabe, D.
    Acta Materialia 112 (2016)
    We present a systematic atomic scale analysis of the structural evolution of long-period-stacking-ordered (LPSO) structures in the (i) α-Mg matrix and in the (ii) interdendritic LPSO phase of an Mg97Y2Zn1 (at. %) alloy annealed at 500°C, using high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Various types of metastable LPSO building block clusters have been observed in both regions. The thermodynamic phase stabilities computed by density-functional-theory calculations explain the diversity of the LPSO structures which are distinguished by their different arrangements of the Y/Zn enriched LPSO building blocks that have a local fcc stacking sequence on the close packed planes. A direct evidence of the transformation from 18R to 14H is presented. This finding suggests that LPSO structures can change their separation distance - quantified by the number of α-Mg layers between them - at a low energy penalty by generating the necessary Shockley partial dislocation on a specific glide plane. Based on our results the most probable transformation sequence of LPSO precipitate plates in the α-Mg matrix is: single building block → various metastable LPSO building block clusters → 14H, and the most probable transformation sequence in the interdendritic LPSO phase is: 18R→ various metastable LPSO building block clusters → 14H. The thermodynamically most stable structures in both the α-Mg matrix and the interdendritic LPSO phase are a mixture of 14H and α-Mg. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.04.016
  • Ultra-strong and damage tolerant metallic bulk materials: A lesson from nanostructured pearlitic steel wires
    Hohenwarter, A. and Völker, B. and Kapp, M.W. and Li, Y. and Goto, S. and Raabe, D. and Pippan, R.
    Scientific Reports 6 (2016)
    Structural materials used for safety critical applications require high strength and simultaneously high resistance against crack growth, referred to as damage tolerance. However, the two properties typically exclude each other and research efforts towards ever stronger materials are hampered by drastic loss of fracture resistance. Therefore, future development of novel ultra-strong bulk materials requires a fundamental understanding of the toughness determining mechanisms. As model material we use today's strongest metallic bulk material, namely, a nanostructured pearlitic steel wire, and measured the fracture toughness on micron-sized specimens in different crack growth directions and found an unexpected strong anisotropy in the fracture resistance. Along the wire axis the material reveals ultra-high strength combined with so far unprecedented damage tolerance. We attribute this excellent property combination to the anisotropy in the fracture toughness inducing a high propensity for micro-crack formation parallel to the wire axis. This effect causes a local crack tip stress relaxation and enables the high fracture toughness without being detrimental to the material's strength. © 2016 The Author(s).
    view abstract10.1038/srep33228
  • Unraveling the temperature dependence of the yield strength in single-crystal tungsten using atomistically-informed crystal plasticity calculations
    Cereceda, D. and Diehl, M. and Roters, F. and Raabe, D. and Perlado, J.M. and Marian, J.
    International Journal of Plasticity 78 (2016)
    We use a physically-based crystal plasticity model to predict the yield strength of body-centered cubic (bcc) tungsten single crystals subjected to uniaxial loading. Our model captures the thermally-activated character of screw dislocation motion and full non-Schmid effects, both of which are known to play critical roles in bcc plasticity. The model uses atomistic calculations as the sole source of constitutive information, with no parameter fitting of any kind to experimental data. Our results are in excellent agreement with experimental measurements of the yield stress as a function of temperature for a number of loading orientations. The validated methodology is employed to calculate the temperature and strain-rate dependence of the yield strength for 231 crystallographic orientations within the standard stereographic triangle. We extract the strain-rate sensitivity of W crystals at different temperatures, and finish with the calculation of yield surfaces under biaxial loading conditions that can be used to define effective yield criteria for engineering design models. © 2015 Elsevier Ltd.
    view abstract10.1016/j.ijplas.2015.09.002
  • Vessel microstructure design: A new approach for site-specific core-shell micromechanical tailoring of TRIP-assisted ultra-high strength steels
    Belde, M. and Springer, H. and Raabe, D.
    Acta Materialia 113 (2016)
    The mechanical performance of multi-phase steel microstructures critically depends on the constituents' chemical and morphological constitutions, which in combination determine the composite hardness, the onset of plasticity, internal load and strain-partitioning, as well as the stability and transformation kinetics of retained austenite in case of TRIP steels. The novel approach of utilising temporary vessel phases, hence termed vessel microstructure design, enables the tuning of constituent phase properties by linking their formation to a controllable landscape of chemical gradients. This approach hinges on the introduction of alloy carbides as a temporary container, or 'vessel' phase, deliberately producing localised enrichment of alloying elements in a structure predetermined by preliminary heat treatments, referred to as conditioning and accumulation stages. These vessel carbides, which act as reservoirs for specific alloying elements, are then partially dissolved through flash heating, leading to a self-organising landscape of alloying elements in the vicinity of the dissolving particles. The resulting three- or multiple phase microstructures then consist of confined laminates incorporating retained carbides, enveloped by retained austenite shells, embedded within a martensitic matrix. Such complex yet entirely self-organized microstructures offer unique opportunities for strain and load partitioning which we refer to as core-shell micromechanics. Different variants of these core-shell composite structures are produced and examined together with reference microstructures by tensile testing, hardness mappings, impact toughness, X-ray measurements, as well as by electron microscopy. It is found that these novel microstructures, when tempered, exhibit ultra-high strength and delayed necking, enabled by a combination of gradual strain-hardening and transformation-induced plasticity that is tuneable via control of the initial carbide structure. © 2016 Acta Materialia Inc. All rights reserved.
    view abstract10.1016/j.actamat.2016.04.051
  • 3D structural and atomic-scale analysis of lath martensite: Effect of the transformation sequence
    Morsdorf, L. and Tasan, C.C. and Ponge, D. and Raabe, D.
    Acta Materialia 95 (2015)
    To improve the fundamental understanding of the multi-scale characteristics of martensitic microstructures and their micro-mechanical properties, a multi-probe methodology is developed and applied to low-carbon lath martensitic model alloys. The approach is based on the joint employment of electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), atom probe tomography (APT) and nanoindentation, in conjunction with high precision and large field-of-view 3D serial sectioning. This methodology enabled us to resolve (i) size variations of martensite sub-units, (ii) associated dislocation sub-structures, (iii) chemical heterogeneities, and (iv) the resulting local mechanical properties. The identified interrelated microstructure heterogeneity is discussed and related to the martensitic transformation sequence, which is proposed to intrinsically lead to formation of a nano-composite structure in low-carbon martensitic steels. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.05.023
  • A novel roll-bonding methodology for the cross-scale analysis of phase properties and interactions in multiphase structural materials
    Springer, H. and Tasan, C. and Raabe, D.
    International Journal of Materials Research 106 (2015)
    We introduce a new thermo-mechanical approach for producing layered bulk samples built-up from the constituent phases of structural materials for the analysis of multiphase co-deformation phenomena. Following a thermo-mechani- cally controlled roll-bonding procedure, the intrinsic properties of the microstructural components as well as their mutual mechanical interaction and interfacial phenomena can be systematically investigated in highly controlled model microstructures of reduced complexity. The effectiveness of the approach is demonstrated on two examples where austenite or martensite layers, respectively, are introduced in a bulk ferritic matrix, representing in either case components of high strength steels. Special emphasis is laid on how the plasticity of martensite within ferrite, as a key parameter required for understanding and optimising dual phase steels, can be investigated following the proposed approach.
    view abstract10.3139/146.111156
  • A study on the geometry of dislocation patterns in the surrounding of nanoindents in a TWIP steel using electron channeling contrast imaging and discrete dislocation dynamics simulations
    Zhang, J.-L. and Zaefferer, S. and Raabe, D.
    Materials Science and Engineering A 636 (2015)
    Electron channeling contrast imaging under controlled diffraction conditions (cECCI) enables observation of crystal defects, especially dislocations, stacking faults and nano-twins, close to the surface of bulk samples. In this work cECCI has been employed to observe defects around nanoindents into the surface of {100}-, {110}-, {111}-oriented grains in a Fe-22Mn-0.65C (wt%) TWIP steel sample (fcc crystal structure, stacking fault energy ~20. mJ/m) using a cone-spherical indenter. The dislocation patterns show four- and two-fold symmetries for the {100}- and {110}-orientation, and a three-fold symmetry for the {111}-orientation which is, however, difficult to observe. Discrete dislocation dynamics (DDD) simulations of the indentation were carried out to complement the static experimental investigations. The simulations were carried out with both, cross-slip disabled and enabled conditions, where the former were found to match to the experimental results better, as may be expected for an fcc material with low stacking fault energy. The 3-dimensional geometry of the dislocation patterns of the different indents was analysed and discussed with respect to pattern formation mechanisms. The force-displacement curves obtained during indentation showed a stronger strain hardening for the {111} oriented crystal than that for the other orientations. This is in contrast to the behaviour of, for example, copper and is interpreted to be due to planar slip. Irrespective of orientation and indentation depth the radius of the plastically deformed area was found to be approximately 4 times larger than that of the indenter contact area. © 2015 Elsevier B.V.
    view abstract10.1016/j.msea.2015.03.078
  • Ab initio study of compositional trends in solid solution strengthening in metals with low Peierls stresses
    Ma, D. and Friák, M. and Von Pezold, J. and Neugebauer, J. and Raabe, D.
    Acta Materialia 98 (2015)
    Abstract We identify and analyze general trends governing solid solution strengthening in binary alloys containing solutes across the Periodic table using quantum-mechanical calculations. Here we present calculations for the model system of Al binary solid solutions. The identified trends originate from an approximately parabolic dependence of two strengthening parameters to quantitatively predict the solid solution strengthening effect, i.e. the volume and slip misfit parameters. The volume misfit parameter shows a minimum (concave-up behavior) as a function of the solute element group number in the periodic table, whereas the slip misfit parameter shows a maximum (concave-down behavior). By analyzing reported data, a similar trend is also found in Ni and Mg (basal slip) binary systems. Hence, these two strengthening parameters are strongly anti-correlated, which can be understood in terms of the Fermi level shift in the framework of free electron model. The chemical trends identified in this study enable a rapid and efficient identification of the solutes that provide optimum solid-solution strengthening. The approach described here may thus serve as basis for ab initio guided metallurgical materials design. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.07.054
  • Ab initio thermodynamics of the CoCrFeMnNi high entropy alloy: Importance of entropy contributions beyond the configurational one
    Ma, D. and Grabowski, B. and Körmann, F. and Neugebauer, J. and Raabe, D.
    Acta Materialia 100 (2015)
    We investigate the thermodynamic properties of the prototype equi-atomic high entropy alloy (HEA) CoCrFeMnNi by using finite-temperature ab initio methods. All relevant free energy contributions are considered for the hcp, fcc, and bcc structures, including electronic, vibrational, and magnetic excitations. We predict the paramagnetic fcc phase to be most stable above room temperature in agreement with experiment. The corresponding thermal expansion and bulk modulus agree likewise well with experimental measurements. A careful analysis of the underlying entropy contributions allows us to identify that the originally postulated dominance of the configurational entropy is questionable. We show that vibrational, electronic, and magnetic entropy contributions must be considered on an equal footing to reliably predict phase stabilities in HEA systems. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.08.050
  • Advanced scale bridging microstructure analysis of single crystal Ni-base superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Somsen, C. and Kostka, A. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Dlouhy, A. and Müller, J. and Spiecker, E. and Demtroder, K. and Schreuer, J. and Neuking, K. and Eggeler, G.
    Advanced Engineering Materials 17 (2015)
    In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni-base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the castmicrostructure (dendrites (D) and interdendritic (ID) regions - large scale heterogeneity) and with the well-known γ/γ′ microstructure (small scale heterogeneity). Using electron probe microanalysis (EPMA), we can showthat elements such as Re, Co, andCr partition to the dendrites while ID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the γ′ cubes while g channels show higher concentrations of Co, Cr, Re, andW.We can combine large scale (EPMA) and small-scale analytical methods (APT) to obtain reasonable estimates for γ′ volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140mm x 100mm x 20mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the γ′ phase and the effect of cooling rates after high temperature exposure on the microstructure. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/adem.201400136
  • Aging in amorphous solids: A study of the first-passage time and persistence time distributions
    Siboni, N.H. and Raabe, D. and Varnik, F.
    EPL 111 (2015)
    The time distribution of relaxation events in an aging system is investigated via molecular-dynamics simulations. The focus is on the distribution functions of the first-passage time, p1(δt), and the persistence time, p(τ ). In contrast to previous reports, both p1 and p are found to evolve with time upon aging. The age dependence of the persistence time distribution is shown to be sensitive to the details of the algorithm used to extract it from particle trajectories. By updating the reference point in event detection algorithm and accounting for the event specific aging time, we uncover the age dependence of p(τ), hidden to previous studies. Moreover, the apparent age dependence of p1 in continuous time random walk with an age-independent p(τ) is shown to result from an implicit synchronization of all the random walkers at the starting time.
    view abstract10.1209/0295-5075/111/48004
  • Alloying effects on microstructure formation of dual phase steels
    Schemmann, L. and Zaefferer, S. and Raabe, D. and Friedel, F. and Mattissen, D.
    Acta Materialia 95 (2015)
    In dual-phase (DP) steels, inherited microstructures and elemental distributions affect the kinetics and morphology of phase transformation phenomena and the mechanical properties of the final material. In order to study the inheritance process, we selected two model materials with the same average DP steel composition but with different initial microstructures, created by coiling at different temperatures after hot rolling. These samples were submitted to a DP-steel heat treatment consisting of a short isothermal annealing in the pure austenite region and a quenching process. The evolution of microstructure, chemical composition and mechanical properties (hardness) during this treatment was investigated. The initial samples had a bainitic-martensitic (B + M) microstructure for the material coiled at lower temperature and a ferritic-pearlitic (P + F) microstructure for that coiled at higher temperature. The P + F microstructure had a much more inhomogeneous distribution of substitutional elements (in particular of Mn) and of carbon. After complete heat treatment, both materials showed a typical DP microstructure (martensite islands embedded in ferrite) but the P + F material showed lower hardness compared to the B + M material. It was found that the inhomogeneous elemental distribution prevailed in the P + F material. The inheritance process was studied by combining measurements of the elemental distribution by Wavelength-Dispersive X-ray spectroscopy (WDX), simulations of the evolution of the elemental composition via the DICTRA (diffusion-controlled reactions) computer programme, dilatometry to observe the kinetics of phase transformation, and observation and quantification of the microstructures by Electron Backscatter Diffraction (EBSD) measurements. For the P + F material it was found that the α-γ transformation during annealing is slowed down in regions of lower Mn content and is therefore not completed. During the subsequent cooling the incompletely autenitized material does not require ferrite nucleation and the γ-α transformation starts at relative high temperatures. For B + M, in contrast, nucleation of ferrite is needed and the transformation starts at lower temperatures. As a result the B + M material develops a higher martensite content as well as a higher density of geometrically necessary dislocations (GNDs). It is speculated that for the B + M material the γ-α transformation occurs through a bainitic (i.e. partly displacive) process while the transformation at higher temperatures in the P + F material proceeds exclusively in a diffusive way. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.05.005
  • An Overview of Dual-Phase Steels: Advances in Microstructure-Oriented Processing and Micromechanically Guided Design
    Tasan, C.C. and Diehl, M. and Yan, D. and Bechtold, M. and Roters, F. and Schemmann, L. and Zheng, C. and Peranio, N. and Ponge, D. and Koyama, M. and Tsuzaki, K. and Raabe, D.
    Annual Review of Materials Research 45 (2015)
    Dual-phase (DP) steel is the flagship of advanced high-strength steels, which were the first among various candidate alloy systems to find application in weight-reduced automotive components. On the one hand, this is a metallurgical success story: Lean alloying and simple thermomechanical treatment enable use of less material to accomplish more performance while complying with demanding environmental and economic constraints. On the other hand, the enormous literature on DP steels demonstrates the immense complexity of microstructure physics in multiphase alloys: Roughly 50 years after the first reports on ferrite-martensite steels, there are still various open scientific questions. Fortunately, the last decades witnessed enormous advances in the development of enabling experimental and simulation techniques, significantly improving the understanding of DP steels. This review provides a detailed account of these improvements, focusing specifically on (a) microstructure evolution during processing, (b) experimental characterization of micromechanical behavior, and (c) the simulation of mechanical behavior, to highlight the critical unresolved issues and to guide future research efforts. Copyright © 2015 by Annual Reviews. All rights reserved.
    view abstract10.1146/annurev-matsci-070214-021103
  • Analytical bounds of in-plane Young's modulus and full-field simulations of two-dimensional monocrystalline stochastic honeycomb structures
    Ma, D. and Eisenlohr, P. and Shanthraj, P. and Diehl, M. and Roters, F. and Raabe, D.
    Computational Materials Science 109 (2015)
    Abstract In this study, we focus on the interplay between the honeycomb structure and the crystallographic orientation. Specifically, the in-plane Young's moduli of monocrystalline stochastic honeycombs are calculated by a numerical and an analytical approach. The in-plane Young's moduli of the honeycombs were calculated numerically using a solution scheme for the full-field mechanical equilibrium based on spectral methods and anisotropic crystal elasticity. The analytical approach formulates two alternative assumptions, i.e. uniform force and uniform strain per strut, considers the elastic anisotropy of the base material, and depends on the two-variable distribution of the strut length and inclination angle as the structural parameters characterizing the stochastic honeycombs. The uniform strain assumption agrees closely with the numerical simulation results and constitutes an improvement compared to analytical solutions proposed in previous studies. © 2015 Elsevier B.V.
    view abstract10.1016/j.commatsci.2015.07.041
  • Assessment of geometrically necessary dislocation levels derived by 3D EBSD
    Konijnenberg, P.J. and Zaefferer, S. and Raabe, D.
    Acta Materialia 99 (2015)
    Existing alternatives for the calculation of geometrically necessary dislocation (GND) densities from orientation fields are discussed. Importantly, we highlight the role of reference frames and consider different sources of error. A well-controlled micro cantilever bending experiment on a copper bicrystal has been analyzed by 3-dimensional electron back scatter diffraction (3D EBSD). The GND density is determined experimentally by two different approaches and assessed theoretically, assuming a homogeneous bending of the cantilever. Experiment and theory agree very well. It is further shown that the deformation is accommodated mainly by GNDs, which carry and store lattice rotation, and not (only) by mobile dislocations that leave a crystal portion inspected, without lattice rotations. A detailed GND analysis reveals a local density minimum close to the grain boundary and a distinct difference in edge to screw ratios for both grains. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.06.051
  • Atom probe informed simulations of dislocation-precipitate interactions reveal the importance of local interface curvature
    Prakash, A. and Guénolé, J. and Wang, J. and Müller, J. and Spiecker, E. and Mills, M.J. and Povstugar, I. and Choi, P. and Raabe, D. and Bitzek, E.
    Acta Materialia 92 (2015)
    The interaction of dislocations with precipitates is an essential strengthening mechanism in metals, as exemplified by the superior high-temperature strength of Ni-base superalloys. Here we use atomistic simulation samples generated from atom probe tomography data of a single crystal superalloy to study the interactions of matrix dislocations with a γ′ precipitate in molecular dynamics simulations. It is shown that the precipitate morphology, in particular its local curvature, and the local chemical composition significantly alter both, the misfit dislocation network which forms at the precipitate interface, and the core structure of the misfit dislocations. Simulated tensile tests reveal the atomic scale details of many experimentally observed dislocation-precipitate interaction mechanisms, which cannot be reproduced by idealized simulation setups with planar interfaces. We thus demonstrate the need to include interface curvature in the study of semicoherent precipitates and introduce as an enabling method atom probe tomography-informed atomistic simulations. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.03.050
  • Atom probe tomography investigation of heterogeneous short-range ordering in the 'komplex' phase state (K-state) of Fe-18Al (at.%)
    Marceau, R.K.W. and Ceguerra, A.V. and Breen, A.J. and Palm, M. and Stein, F. and Ringer, S.P. and Raabe, D.
    Intermetallics 64 (2015)
    We study an Fe-18Al (at.%) alloy after various thermal treatments at different times (24-336 h) and temperatures (250-1100 °C) to determine the nature of the so-called 'komplex' phase state (or "K-state"), which is common to other alloy systems having compositions at the boundaries of known order-disorder transitions and is characterised by heterogeneous short-range-ordering (SRO). This has been done by direct observation using atom probe tomography (APT), which reveals that nano-sized, ordered regions/particles do not exist. Also, by employing shell-based analysis of the three-dimensional atomic positions, we have determined chemically sensitive, generalised multicomponent short-range order (GM-SRO) parameters, which are compared with published pairwise SRO parameters derived from bulk, volume-averaged measurement techniques (e.g. X-ray and neutron scattering, Mössbauer spectroscopy) and combined ab-initio and Monte Carlo simulations. This analysis procedure has general relevance for other alloy systems where quantitative chemical-structure evaluation of local atomic environments is required to understand ordering and partial ordering phenomena that affect physical and mechanical properties. © 2015 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2015.04.005
  • Atom probe tomography reveals options for microstructural design of steels and titanium alloys by segregation engineering
    Raabe, D. and Herbig, M. and Kuzmina, M. and Sandlöbes, S. and Tarzimoghadam, Z. and Ponge, D.
    MATEC Web of Conferences 33 (2015)
    Here we discuss approaches for designing microstructures in steels and titanium alloys by manipulating the segregation content and the structural state of lattice defects. Different mechanisms can be utilized in that context, such as for instance site specific segregation as described by the Gibbs isotherm and the generalized defectant concept, confined phase transformation phenomena and the formation of complexions, i.e. confined chemical and structural states at lattice defects. © Owned by the authors, published by EDP Sciences, 2015.
    view abstract10.1051/matecconf/20153301001
  • Atom probe tomography study of internal interfaces in Cu2ZnSnSe4 thin-films
    Schwarz, T. and Cojocaru-Mirédin, O. and Choi, P. and Mousel, M. and Redinger, A. and Siebentritt, S. and Raabe, D.
    Journal of Applied Physics 118 (2015)
    We report on atom probe tomography studies of the composition at internal interfaces in Cu<inf>2</inf>ZnSnSe<inf>4</inf> thin-films. For Cu<inf>2</inf>ZnSnSe<inf>4</inf> precursors, which are deposited at 320 °C under Zn-rich conditions, grain boundaries are found to be enriched with Cu irrespective of whether Cu-poor or Cu-rich growth conditions are chosen. Cu<inf>2</inf>ZnSnSe<inf>4</inf> grains are found to be Cu-poor and excess Cu atoms are found to be accumulated at grain boundaries. In addition, nanometer-sized ZnSe grains are detected at or near grain boundaries. The compositions at grain boundaries show different trends after annealing at 500 °C. Grain boundaries in the annealed absorber films, which are free of impurities, are Cu-, Sn-, and Se-depleted and Zn-enriched. This is attributed to dissolution of ZnSe at the Cu-enriched grain boundaries during annealing. Furthermore, some of the grain boundaries of the absorbers are enriched with Na and K atoms, stemming from the soda-lime glass substrate. Such grain boundaries show no or only small changes in composition of the matrix elements. Na and K impurities are also partly segregated at some of the Cu<inf>2</inf>ZnSnSe<inf>4</inf>/ZnSe interfaces in the absorber, whereas for the precursors, only Na was detected at such phase boundaries possibly due to a higher diffusivity of Na compared to K. Possible effects of the detected compositional fluctuations on cell performance are discussed. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4929874
  • Atomic scale investigation of non-equilibrium segregation of boron in a quenched Mo-free martensitic steel
    Li, Y.J. and Ponge, D. and Choi, P. and Raabe, D.
    Ultramicroscopy 159 (2015)
    B-added low carbon steels exhibit excellent hardenability. The reason has been frequently attributed to B segregation at prior austenite grain boundaries, which prevents the austenite to ferrite transformation and favors the formation of martensite. The segregation behavior of B at prior austenite grain boundaries is strongly influenced by processing conditions such as austenitization temperatures and cooling rates and by alloying elements such as Mo, Cr, and Nb. Here an local electrode atom probe was employed to investigate the segregation behavior of B and other alloying elements (C, Mn, Si, and Cr) in a Cr-added Mo-free martensitic steel. Similar to our previous results on a Mo-added steel, we found that in both steels B is segregated at prior austenite grain boundaries with similar excess values, whereas B is neither detected in the martensitic matrix nor at martensite-martensite boundaries at the given cooling rate of 30 K/s. These results are in agreement with the literature reporting that Cr has the same effect on hardenability of steels as Mo in the case of high cooling rates. The absence of B at martensite-martensite boundaries suggests that B segregates to prior austenite grain boundaries via a non-equilibrium mechanism. Segregation of C at all boundaries such as prior austenite grain boundaries and martensite-martensite boundaries may occur by an equilibrium mechanism. © 2015 Elsevier B.V.
    view abstract10.1016/j.ultramic.2015.03.009
  • Atomic scale study of CU clustering and pseudo-homogeneous Fe-Si nanocrystallization in soft magnetic FeSiNbB(CU) alloys
    Pradeep, K.G. and Herzer, G. and Raabe, D.
    Ultramicroscopy 159 (2015)
    A local electrode atom probe has been employed to trace the onset of Cu clustering followed by their coarsening and subsequent growth upon rapid (10s) annealing of an amorphous Fe73.5Si15.5Cu1Nb3B7 alloy. It has been found that the clustering of Cu atoms introduces heterogeneities in the amorphous matrix, leading to the formation of Fe rich regions which crystallizes pseudo-homogeneously into Fe-Si nanocrystals upon annealing. In this paper, we present the data treatment method that allows for the visualization of these different phases and to understand their morphology while still quantifying them in terms of their size, number density and volume fraction. The crystallite size of Fe-Si nanocrystals as estimated from the atom probe data are found to be in good agreement with other complementary techniques like XRD and TEM, emphasizing the importance of this approach towards accurate structural analysis. In addition, a composition driven data segmentation approach has been attempted to determine and distinguish nanocrystalline regions from the remaining amorphous matrix. Such an analysis introduces the possibility of retrieving crystallographic information from extremely fine (2-4nm sized) nanocrystalline regions of very low volume fraction (< 5Vol%) thereby providing crucial in-sights into the chemical heterogeneity induced crystallization process of amorphous materials. © 2015 Elsevier B.V.
    view abstract10.1016/j.ultramic.2015.04.006
  • Autonomous Repair Mechanism of Creep Damage in Fe-Au and Fe-Au-B-N Alloys
    Zhang, S. and Kwakernaak, C. and Tichelaar, F.D. and Sloof, W.G. and Kuzmina, M. and Herbig, M. and Raabe, D. and Brück, E. and van der Zwaag, S. and van Dijk, N.H.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 (2015)
    The autonomous repair mechanism of creep cavitation during high-temperature deformation has been investigated in Fe-Au and Fe-Au-B-N alloys. Combined electron-microscopy techniques and atom probe tomography reveal how the improved creep properties result from Au precipitation within the creep cavities, preferentially formed on grain boundaries oriented perpendicular to the applied stress. The selective precipitation of Au atoms at the free creep cavity surface results in pore filling, and thereby, autonomous repair of the creep damage. The large difference in atomic size between the Au and Fe strongly hampers the nucleation of precipitates in the matrix. As a result, the matrix acts as a reservoir for the supersaturated solute until damage occurs. Grain boundaries and dislocations are found to act as fast transport routes for solute gold from the matrix to the creep cavities. The mechanism responsible for the self-healing can be characterized by a simple model for cavity growth and cavity filling. © 2015, The Author(s).
    view abstract10.1007/s11661-015-3169-9
  • Carbon partitioning during quenching and partitioning heat treatment accompanied by carbide precipitation
    Toji, Y. and Miyamoto, G. and Raabe, D.
    Acta Materialia 86 (2015)
    Carbon partitioning from martensite into austenite in the quenching and partitioning (Q&P) process has been suggested to be controlled by the constrained carbon equilibrium (CCE) criterion. It defines an approach for predicting the carbon concentration in austenite under the condition that competing reactions such as carbide formation and bainite transformation are suppressed. Carbide precipitation in martensite is, however, often observed during the partitioning step, even in low-carbon steels as well as in high-carbon steels, even when containing a high amount of Si. Therefore, carbon partitioning from martensite into austenite is studied here, considering carbide precipitation in martensite. Carbon partitioning was investigated by means of a field-emission electron probe micro analysis (FE-EPMA) and atom probe tomography (APT), using 1.07 wt.% and 0.59 wt.% carbon steels with various martensite volume fractions. Carbon partitioning from martensite to austenite was clearly observed in all specimens, even though a considerable amount of carbide precipitated inside the martensite. The austenite carbon concentration after the partitioning step was not influenced by either the martensite volume fraction or the bulk carbon content. A modified model for predicting the austenite carbon concentration after the partitioning step was proposed to explain the experimental results by assuming carbon equilibria between austenite, ferrite and cementite under a constrained condition. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.11.049
  • Cd and impurity redistribution at the p-n junction of CIGS based solar cells resolved by atom-probe tomography
    Koprek, A. and Cojocaru-Miredin, O. and Wuerz, R. and Freysoldt, C. and Raabe, D.
    2015 IEEE 42nd Photovoltaic Specialist Conference, PVSC 2015 (2015)
    Cd and impurity redistribution in the vicinity of CdS/CIGS interface is studied by means of atom probe tomography (APT). We find an increase of the Cd content in the CIGS layer and redistribution of O at the CdS/CIGS interface after annealing the samples at 200 °C, 250 °C, or 300 °C. Very small amounts (∼0.1 at. %) of Na impurity where observed at the p-n junction independent on the heat treatment. Simultaneously, the I-V measurements of the treated samples show a drop in the open circuit voltage and thus of the efficiency compared to the untreated sample. The effect of Cd diffusion in CIGS and of O and Na segregation at the CdS/CIGS interface on the cell performance is discussed. © 2015 IEEE.
    view abstract10.1109/PVSC.2015.7355651
  • Combining structural and chemical information at the nanometer scale by correlative transmission electron microscopy and atom probe tomography
    Herbig, M. and Choi, P. and Raabe, D.
    Ultramicroscopy 153 (2015)
    In many cases, the three-dimensional reconstructions from atom probe tomography (APT) are not sufficiently accurate to resolve crystallographic features such as lattice planes, shear bands, stacking faults, dislocations or grain boundaries. Hence, correlative crystallographic characterization is required in addition to APT at the exact same location of the specimen. Also, for the site-specific preparation of APT tips containing regions of interest (e.g. grain boundaries) correlative electron microscopy is often inevitable. Here we present a versatile experimental setup that enables performing correlative focused ion beam milling, transmission electron microscopy (TEM), and APT under optimized characterization conditions. The setup was designed for high throughput, robustness and practicability. We demonstrate that atom probe tips can be characterized by TEM in the same way as a standard TEM sample. In particular, the use of scanning nanobeam diffraction provides valuable complementary crystallographic information when being performed on atom probe tips. This technique enables the measurement of orientation and phase maps as known from electron backscattering diffraction with a spatial resolution down to one nanometer. © 2015 Elsevier B.V.
    view abstract10.1016/j.ultramic.2015.02.003
  • Complex Nanotwin Substructure of an Asymmetric Σ9 Tilt Grain Boundary in a Silicon Polycrystal
    Stoffers, A. and Ziebarth, B. and Barthel, J. and Cojocaru-Mirédin, O. and Elsässer, C. and Raabe, D.
    Physical Review Letters 115 (2015)
    Grain boundaries in materials have substantial influences on device properties, for instance on mechanical stability or electronic minority carrier lifetime in multicrystalline silicon solar cells. This applies especially to asymmetric, less ordered or faceted interface portions. Here, we present the complex atomic interface structure of an asymmetric Σ9 tilt grain boundary in silicon, observed by high resolution scanning transmission electron microscopy (HR-STEM) and explained by atomistic modeling and computer simulation. Structural optimization of interface models for the asymmetric Σ9 and related symmetrical Σ9 and Σ3 tilt grain boundaries, by means of molecular-statics simulations with empirical silicon potentials in combination with first-principles calculations, results in a faceted asymmetric interface structure, whose grain-boundary energy is so low that it is likely to exist. The simulated local atomic structures match the observed HR-STEM images very well. © 2015 American Physical Society.
    view abstract10.1103/PhysRevLett.115.235502
  • Computationally efficient and quantitatively accurate multiscale simulation of solid-solution strengthening by ab initio calculation
    Ma, D. and Friák, M. and Von Pezold, J. and Raabe, D. and Neugebauer, J.
    Acta Materialia 85 (2015)
    We propose an approach for the computationally efficient and quantitatively accurate prediction of solid-solution strengthening. It combines the 2-D Peierls-Nabarro model and a recently developed solid-solution strengthening model. Solid-solution strengthening is examined with Al-Mg and Al-Li as representative alloy systems, demonstrating a good agreement between theory and experiments within the temperature range in which the dislocation motion is overdamped. Through a parametric study, two guideline maps of the misfit parameters against (i) the critical resolved shear stress, τ0, at 0 K and (ii) the energy barrier, ΔEb, against dislocation motion in a solid solution with randomly distributed solute atoms are created. With these two guideline maps, τ0 at finite temperatures is predicted for other Al binary systems, and compared with available experiments, achieving good agreement. © 2014 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2014.10.044
  • Damage resistance in gum metal through cold work-induced microstructural heterogeneity
    Zhang, J.-L. and Tasan, C.C. and Lai, M.L. and Zhang, J. and Raabe, D.
    Journal of Materials Science 50 (2015)
    Cold-worked alloys exhibit high strength, but suffer from limited ductility. In contrast, Ti-based gum metal was reported to exhibit high strength combined with good ductility upon severe pre-straining. Motivated by this anomaly, we systematically studied the evolution of gum metal microstructure during severe cold working (swaging and rolling) and the resulting deformation and damage micro-mechanical mechanisms during follow-up tensile deformation. To this end, various experimental in situ and post-mortem methodologies are employed, including scanning electron microscopy imaging, high-resolution electron backscatter diffraction mapping and transmission electron microscopy. These observations reveal that intense grain refinement takes place through dislocation plasticity-dominated deformation banding upon cold working. The observed enhancement in crack blunting and failure resistance which prolongs the post-necking ductility of gum metal during follow-up tensile straining can be attributed to the deformation-induced development of local heterogeneities in texture and grain size. © 2015 Springer Science+Business Media New York
    view abstract10.1007/s10853-015-9105-y
  • Deformation induced alloying in crystalline - metallic glass nano-composites
    Guo, W. and Yao, J. and Jägle, E.A. and Choi, P.-P. and Herbig, M. and Schneider, J.M. and Raabe, D.
    Materials Science and Engineering A 628 (2015)
    We study the mechanisms of deformation driven chemical mixing in a metallic nanocomposite model system. More specific, we investigate shear banding at the atomic scale in an amorphous CuZr/ crystalline Cu nanolaminate, deformed by microindentation. Three CuZr/Cu multilayer systems (100 nm Cu/100 nm CuZr, 50 nm Cu/100 nm CuZr, and 10 nm Cu/100 nm CuZr) are fabricated to study the effect of layer thickness on shear band formation and deformation induced alloying. The chemical and structural evolution at different strain levels are traced by atom probe tomography and transmission electron microscopy combined with nano-beam diffraction mapping. The initially pure crystalline Cu and amorphous CuZr layers chemically mix by cross-phase shear banding after reaching a critical layer thickness. The Cu inside the shear bands develops a high dislocation density and can locally undergo transition to an amorphous state when sheared and mixed. We conclude that the severe deformation in the shear bands in the amorphous layer squeeze Zr atoms into the Cu dislocation cores in the Cu layers (thickness <5 nm), resulting in local chemical mixing. © 2015 Elsevier B.V.
    view abstract10.1016/j.msea.2015.01.062
  • Deformation mechanism of ω-enriched Ti-Nb-based gum metal: Dislocation channeling and deformation induced ω-β transformation
    Lai, M.J. and Tasan, C.C. and Raabe, D.
    Acta Materialia 100 (2015)
    Gum metal, a class of multifunctional β titanium alloys, has attracted much attention in the past decade due to its initially-proposed dislocation-free deformation mechanism based on giant faults, i.e., macroscopic planar defects carrying significant plastic strain. Special deformation features were observed in these alloys, such as plastic flow localization, pronounced surface steps, low work hardening, and large elongation. These were all proposed to arise from the special giant fault mechanism activated in the β-Ti matrix, while the initial presence or mechanically-induced formation of other phases was debated in several follow-up studies. Here, we set off with Ti-Nb-based gum metal samples with confirmed presence of large amounts of nanometer-sized hexagonal ω particles. Deformation experiments demonstrate all the features observed in the original reports, mentioned above. However, careful characterization reveals that the deformation bands (similar to giant faults) where plastic flow localized are "dislocation channels" that are depleted of ω phase. These channels are proposed to form by a {1 1 2}<1 1 1> dislocation dissociation mechanism, promoting reverse transformation of the ω phase into the β phase. The deformation induced ω-β transformation and the associated dislocation channeling process can explain the presence of the aforementioned special deformation features in the current gum metal. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.08.047
  • Dental lessons from past to present: Ultrastructure and composition of teeth from plesiosaurs, dinosaurs, extinct and recent sharks
    Lübke, A. and Enax, J. and Loza, K. and Prymak, O. and Gaengler, P. and Fabritius, H.-O. and Raabe, D. and Epple, M.
    RSC Advances 5 (2015)
    Teeth represent the hardest tissue in vertebrates and appear very early in their evolution as an ancestral character of the Eugnathostomata (true jawed vertebrates). In recent vertebrates, two strategies to form and mineralize the outermost functional layer have persisted. In cartilaginous fish, the enameloid is of ectomesenchymal origin with fluoroapatite as the mineral phase. All other groups form enamel of ectodermal origin using hydroxyapatite as the mineral phase. The high abundance of teeth in the fossil record is ideal to compare structure and composition of teeth from extinct groups with those of their recent successors to elucidate possible evolutionary changes. Here, we studied the chemical composition and the microstructure of the teeth of six extinct shark species, two species of extinct marine reptiles and two dinosaur species using high-resolution chemical and microscopic methods. Although many of the ultrastructural features of fossilized teeth are similar to recent ones (especially for sharks where the ultrastructure basically did not change over millions of years), we found surprising differences in chemical composition. The tooth mineral of all extinct sharks was fluoroapatite in both dentin and enameloid, in sharp contrast to recent sharks where fluoroapatite is only found in enameloid. Unlike extinct sharks, recent sharks use hydroxyapatite as mineral in dentin. Most notably and hitherto unknown, all dinosaur and extinct marine reptile teeth contained fluoroapatite as mineral in dentin and enamel. Our results indicate a drastic change in the tooth mineralization strategy especially for terrestrial vertebrates that must have set in after the cretaceous period. Possibly, this is related to hitherto unconsidered environmental changes that caused unfavourable conditions for the use of fluoroapatite as tooth mineral. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5ra11560d
  • Design of a twinning-induced plasticity high entropy alloy
    Deng, Y. and Tasan, C.C. and Pradeep, K.G. and Springer, H. and Kostka, A. and Raabe, D.
    Acta Materialia 94 (2015)
    We introduce a liquid metallurgy synthesized, non-equiatomic Fe<inf>40</inf>Mn<inf>40</inf>Co<inf>10</inf>Cr<inf>10</inf> high entropy alloy that is designed to undergo mechanically-induced twinning upon deformation at room temperature. Microstructure characterization, carried out using SEM, TEM and APT shows a homogeneous fcc structured single phase solid solution in the as-cast, hot-rolled and homogenized states. Investigations of the deformation substructures at specific strain levels with electron channeling contrast imaging (ECCI) combined with EBSD reveal a clear change in the deformation mechanisms of the designed alloy starting from dislocation slip to twinning as a function of strain. Such twinning induced plasticity has only been observed under cryogenic conditions in the equiatomic FeMnNiCoCr high entropy alloy. Thus, despite the decreased contribution of solid solution strengthening, the tensile properties of the introduced lean alloy at room temperature are found to be comparable to that of the well-studied five component FeMnNiCoCr system. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.04.014
  • Detection of Cu2Zn5SnSe8 and Cu2Zn6SnSe9 phases in co-evaporated Cu2ZnSnSe4 thin-films
    Schwarz, T. and Marques, M.A.L. and Botti, S. and Mousel, M. and Redinger, A. and Siebentritt, S. and Cojocaru-Mirédin, O. and Raabe, D. and Choi, P.-P.
    Applied Physics Letters 107 (2015)
    Cu2ZnSnSe4 thin-films for photovoltaic applications are investigated using combined atom probe tomography and ab initio density functional theory. The atom probe studies reveal nano-sized grains of Cu2Zn5SnSe8 and Cu2Zn6SnSe9 composition, which cannot be assigned to any known phase reported in the literature. Both phases are considered to be metastable, as density functional theory calculations yield positive energy differences with respect to the decomposition into Cu2ZnSnSe4 and ZnSe. Among the conceivable crystal structures for both phases, a distorted zinc-blende structure shows the lowest energy, which is a few tens of meV below the energy of a wurtzite structure. A band gap of 1.1 eV is calculated for both the Cu2Zn5SnSe8 and Cu2Zn6SnSe9 phases. Possible effects of these phases on solar cell performance are discussed. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4934847
  • Dynamic strain aging studied at the atomic scale
    Aboulfadl, H. and Deges, J. and Choi, P. and Raabe, D.
    Acta Materialia 86 (2015)
    Dynamic strain aging arises from the interaction between solute atoms and matrix dislocations in strained metallic alloy. It initiates jerky dislocation motion and abrupt softening, causing negative strain rate sensitivity. This effect leads to instable flow phenomena at the macroscopic scale, appearing as a serrated stress-strain response and deformation banding. These macroscopic features are referred to as the Portevin-Le Chatelier effect (PLC). Here we study the atomistic origin of dynamic strain aging in an Al-4.8 at.% Mg alloy using atom probe tomography (APT) and transmission electron microscopy (TEM). Samples were prepared from as-cold rolled (90% thickness reduction), stabilized (120 °C, 20 h) and recrystallized sheets (400°C, 10 min), respectively. In the stabilized state, Mg was found to decorate <1 1 0> aligned dislocations with up to ∼12.5 at.%. Tensile tests in combination with thermographic and laser speckle observations were used to map the deformation bands for the site-specific extraction of APT samples from regions inside the PLC bands. We observed an asymmetrical Mg distribution along some of the dislocations, matching model predictions for high dislocation speeds at peak drag stress by Zhang and Curtin. In this case, the Mg distribution is characterized by depletion in the compressive regime above the dislocation slip plane and enrichment in the dilatation region below the slip plane. Mg also depletes in a tail-like form behind fast-moving dislocations, further promoting slip localization. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.12.028
  • Dynamic strain-induced transformation: An atomic scale investigation
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Springer, H. and Raabe, D.
    Scripta Materialia 109 (2015)
    Phase transformations provide the most versatile access to the design of complex nanostructured alloys in terms of grain size, morphology, local chemical constitution etc. Here we study a special case of deformation induced phase transformation. More specifically, we investigate the atomistic mechanisms associated with dynamic strain-induced transformation (DSIT) in a dual-phased multicomponent iron-based alloy at high temperatures. DSIT phenomena and the associated secondary phase nucleation were observed at atomic scale using atom probe tomography. The obtained local chemical composition was used for simulating the nucleation process which revealed that DSIT, occurring during load exertion, proceeds by a diffusion-controlled nucleation process. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2015.07.010
  • Effect of ruthenium on the precipitation of topologically close packed phases in Ni-based superalloys of 3rd and 4th generation
    Matuszewski, K. and Rettig, R. and Matysiak, H. and Peng, Z. and Povstugar, I. and Choi, P. and Müller, J. and Raabe, D. and Spiecker, E. and Kurzydłowski, K.J. and Singer, R.F.
    Acta Materialia 95 (2015)
    The precipitation of topologically close packed (TCP) phases is detrimental for the high temperature strength of high refractory Ni-based superalloys. The beneficial influence of Ru with respect to this so called instability is nowadays well accepted. In the present paper the precipitation of topologically close packed (TCP) phases is studied quantitatively in two experimental alloys (one Ru-free and one with addition of Ru) to clarify the mechanism of the Ru effect. It is confirmed that the TCP phase precipitates undergo sequential phase transformation with the tetragonal σ-phase precipitating first. Ru retards the phase transformation and leads to decreased equilibrium volume fraction of TCP phases. The results clearly indicate that Ru decreases the driving force for TCP phase precipitation. Investigations of crystallography and chemistry of the TCP/matrix interface point to an additional effect by increase of misfit strain energy. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.05.033
  • Effects of retained austenite volume fraction, morphology, and carbon content on strength and ductility of nanostructured TRIP-assisted steels
    Shen, Y.F. and Qiu, L.N. and Sun, X. and Zuo, L. and Liaw, P.K. and Raabe, D.
    Materials Science and Engineering A 636 (2015)
    With a suite of multi-modal and multi-scale characterization techniques, the present study unambiguously proves that a substantially-improved combination of ultrahigh strength and good ductility can be achieved by tailoring the volume fraction, morphology, and carbon content of the retained austenite (RA) in a transformation-induced-plasticity (TRIP) steel with the nominal chemical composition of 0.19C-0.30Si-1.76Mn-1.52Al (weight percent, wt%). After intercritical annealing and bainitic holding, a combination of ultimate tensile strength (UTS) of 1100. MPa and true strain of 50% has been obtained, as a result of the ultrafine RA lamellae, which are alternately arranged in the bainitic ferrite around junction regions of ferrite grains. For reference, specimens with a blocky RA, prepared without the bainitic holding, yield a low ductility (35%) and a low UTS (800. MPa). The volume fraction, morphology, and carbon content of RA have been characterized using various techniques, including the magnetic probing, scanning electron microscopy (SEM), electron-backscatter-diffraction (EBSD), and transmission electron microscopy (TEM). Interrupted tensile tests, mapped using EBSD in conjunction with the kernel average misorientation (KAM) analysis, reveal that the lamellar RA is the governing microstructure component responsible for the higher mechanical stability, compared to the blocky one. By coupling these various techniques, we quantitatively demonstrate that in addition to the RA volume fraction, its morphology and carbon content are equally important in optimizing the strength and ductility of TRIP-assisted steels. © 2015 Elsevier B.V.
    view abstract10.1016/j.msea.2015.04.030
  • Effects of Ru on elemental partitioning and precipitation of topologically close-packed phases in Ni-based superalloys
    Peng, Z. and Povstugar, I. and Matuszewski, K. and Rettig, R. and Singer, R. and Kostka, A. and Choi, P.-P. and Raabe, D.
    Scripta Materialia 101 (2015)
    Two Ni-based superalloys (one Ru-free and one containing 1 at.% Ru) were comparatively studied by Atom Probe Tomography and complimentary techniques. Ru addition impedes precipitation of the σ phase at 950 °C. Ru partitions nearly equally to the γ and σ phase. Neither reverse elemental partitioning nor destabilization of the γ' phase is detected when adding Ru. We propose an increase in the γ/σ lattice misfit caused by Ru as a major reason for impeded nucleation of the σ phase. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2015.01.014
  • Effects of strain amplitude, cycle number and orientation on low cycle fatigue microstructures in austenitic stainless steel studied by electron channelling contrast imaging
    Nellessen, J. and Sandlöbes, S. and Raabe, D.
    Acta Materialia 87 (2015)
    Substructure analysis on cyclically deformed metals is typically performed by time-consuming transmission electron microscopy probing, thus limiting such studies often to a single parameter. Here, we present a novel approach which consists in combining electron backscatter diffraction (EBSD), digital image correlation and electron channelling contrast imaging (ECCI), enabling us to systematically probe a large matrix of different parameters with the aim of correlating and comparing their interdependence. The main focus here is to identify the influence of cycle number, initial grain orientation and local strain amplitude on the evolving dislocation patterns. Therefore, experiments up to 100 cycles were performed on a polycrystalline austenitic stainless steel with local strain amplitudes between 0.35% and 0.95%. EBSD and ECCI maps reveal the individual influence of each parameter while the others remained constant. We find that the dislocation structures strongly depend on grain orientation. Dislocation structures in grains with double-slip (〈1 1 2〉 // LD, 〈1 2 2〉 // LD and 〈0 1 2〉 // LD) and multiple-slip (〈1 1 1〉 // LD, M 〈0 1 1〉 // LD and 〈0 0 1〉 // LD) orientations with respect to the loading direction (LD) are characterized under the variation of strain amplitude and cycle number. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.12.024
  • Enhancing Hydrogen Embrittlement Resistance of Lath Martensite by Introducing Nano-Films of Interlath Austenite
    Wang, M. and Tasan, C.C. and Koyama, M. and Ponge, D. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 (2015)
    Partial reversion of interlath austenite nano-films is investigated as a potential remedy for hydrogen embrittlement susceptibility of martensitic steels. We conducted uniaxial tensile tests on hydrogen-free and pre-charged medium-Mn transformation-induced plasticity-maraging steels with different austenite film thicknesses. Mechanisms of crack propagation and microstructure interaction are quantitatively analyzed using electron channelling contrast imaging and electron backscatter diffraction, revealing a promising strategy to utilize austenite reversion for hydrogen-resistant martensitic steel design. © 2015, The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-015-3009-y
  • Errautm: From generalized stacking fault energies to dislocation properties: Five-energy-point approach and solid solution effects in magnesium (Physical Review B - Condensed Matter and Materials Physics (2015) 92 (064107))
    Pei, Z. and Ma, D. and Friák, M. and Svendsen, B. and Raabe, D. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    view abstract10.1103/PhysRevB.92.179901
  • Error analysis of the crystal orientations and disorientations obtained by the classical electron backscatter diffraction technique
    Ram, F. and Zaefferer, S. and Jäpel, T. and Raabe, D.
    Journal of Applied Crystallography 48 (2015)
    The fidelity - that is, the error, precision and accuracy - of the crystallographic orientations and disorientations obtained by the classical two-dimensional Hough-transform-based analysis of electron backscatter diffraction patterns (EBSPs) is studied. Using EBSPs simulated based on the dynamical electron diffraction theory, the fidelity analysis that has been previously performed using the patterns simulated based on the theory of kinematic electron diffraction is improved. Using the same patterns, the efficacy of a Fisher-distribution-based analytical accuracy measure for orientation and disorientation is verified.
    view abstract10.1107/S1600576715005762
  • From generalized stacking fault energies to dislocation properties: Five-energy-point approach and solid solution effects in magnesium
    Pei, Z. and Ma, D. and Friák, M. and Svendsen, B. and Raabe, D. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    Using ab initio calculations and symmetrized plane waves, we analyze the basal-plane generalized stacking fault energies in pure Mg and Mg-Y alloys and show that the knowledge of energies of only five specific points is sufficient to accurately predict the core structures and Peierls stresses of (a)-type edge dislocations in these alloys. Our five-point approach substantially reduces the computational cost related to the Peierls-Nabarro (PN) model and allows for a high-throughput application of the PN model to study Peierls stress changes in Mg upon alloying. We employ our approach to study Mg binary alloys containing nine rare-earth (RE) and 11 other solutes. Based on the Peierls stresses of these 20 Mg alloys calculated from the Peierls-Nabarro model, the solutes are divided into three groups: (i) the first group, consisting of Be, Zn, Tl, Tc, Os, Ru, Re, and Co, when added as solutes into Mg, lead to more compact dislocation core structures and larger Peierls stresses than found for pure Mg. (ii) Elements in the second group, including Ti, Nd, Lu, Zr, Hf, La, and Pr change the core widths and Peierls stresses moderately. (iii) The solutes in the third group containing Y, Er, Tm, Ho, and Sc extend the stacking fault width, and the resulting Peierls stresses are generally very low. Based on an error analysis, we conclude that the first group has a clear solute strengthening effect and the third group has a clear solute softening effect, while the effects of the elements in the second group are too small to be resolved by the present approach. © 2015 American Physical Society. ©2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.064107
  • From hard to rubber-like: mechanical properties of resorcinol–formaldehyde aerogels
    Schwan, M. and Naikade, M. and Raabe, D. and Ratke, L.
    Journal of Materials Science 50 (2015)
    Four types of resorcinol–formaldehyde (RF) aerogels, stiff, brittle, low-flexible, and super-flexible are studied in this work. Despite several studies on mechanical properties on RF aerogels their response when exposed to compressive loading and their fracture behavior are not well investigated. Here, we cover aerogels with a very broad density range of 0.08–0.3 g cm−3 and compressive moduli from 0.12 to 28 MPa. We relate the microstructure of the synthesized aerogels and their behavior under uniaxial compression. Additionally, this work is the first, to our knowledge, to implement the usage of digital image correlation for characterizing the deformation of RF aerogels. The comparison of surface strain distribution of four types of aerogels provides an insight to their reaction on compressive loading. © 2015, Springer Science+Business Media New York.
    view abstract10.1007/s10853-015-9094-x
  • From high-entropy alloys to high-entropy steels
    Raabe, D. and Tasan, C.C. and Springer, H. and Bausch, M.
    Steel Research International 86 (2015)
    Inspired by high-entropy alloys, we study the design of steels that are based on high configurational entropy for stabilizing a single-phase solid solution matrix. The focus is placed on the system Fe-Mn-Al-Si-C but we also present trends in the alloy system Fe-Mn-Al-C. Unlike in conventional high-entropy alloys, where five or more equiatomically proportioned components are used, we exploit the flat configurational entropy plateau in transition metal mixtures, stabilizing solid solutions also for lean, non-equiatomic compositions. This renders the high-entropy alloying concept, where none of the elements prevails, into a class of Fe-based materials which we refer to as high-entropy steels. A point that has received little attention in high-entropy alloys is the use of interstitial elements. Here, we address the role of C in face-centered cubic solid solution phases. High-entropy steels reveal excellent mechanical properties, namely, very high ductility and toughness; excellent high rate and low-temperature ductility; high strength of up to 1 GPa; up to 17% reduced mass density; and very high strain hardening. The microstructure stability can be tuned by adjusting the stacking fault energy. This enables to exploit deformation effects such as the TRIP, TWIP, or precipitation determined mechanisms. We present a class of massive solid solution steels with high configurational entropy. Focus is placed on the system Fe-Mn-Al-Si-C, i.e., considering also C interstitials. By exploiting the flat configurational entropy plateau in metal mixtures, solid solutions of lean, non-equiatomic compositions can be stabilized. This renders the high-entropy alloying concept, where none of the elements prevails, into high-entropy steels. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/srin.201500133
  • Grain boundary segregation engineering and austenite reversion turn embrittlement into toughness: Example of a 9 wt.% medium Mn steel
    Kuzmina, M. and Ponge, D. and Raabe, D.
    Acta Materialia 86 (2015)
    We study grain boundary embrittlement in a quenched and tempered Fe-Mn high-purity model martensite alloy using Charpy impact tests and grain boundary characterization by atom probe tomography. We observe that solute Mn directly embrittles martensite grain boundaries while reversion of martensite to austenite at high-angle grain boundaries cleans the interfaces from solute Mn by partitioning the Mn into the newly formed austenite, hence restoring impact toughness. Microalloying with B improves the impact toughness in the quenched state and delays temper embrittlement at 450 °C. Tempering at 600 °C for 1 min significantly improves the impact toughness and further tempering at lower temperature does not cause the embrittlement to return. At higher temperatures, regular austenite nucleation and growth takes place, whereas at lower temperature, Mn directly promotes its growth. ©2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.12.021
  • Grain boundary segregation in Fe-Mn-C twinning-induced plasticity steels studied by correlative electron backscatter diffraction and atom probe tomography
    Herbig, M. and Kuzmina, M. and Haase, C. and Marceau, R.K.W. and Gutierrez-Urrutia, I. and Haley, D. and Molodov, D.A. and Choi, P. and Raabe, D.
    Acta Materialia 83 (2015)
    We report on the characterization of grain boundary (GB) segregation in an Fe-28Mn-0.3C (wt.%) twinning-induced plasticity (TWIP) steel. After recrystallization of this steel for 24 h at 700 °C, ∼50% general grain boundaries (GBs) and ∼35% Σ3 annealing twin boundaries were observed (others were high-order Σ and low-angle GBs). The segregation of B, C and P and traces of Si and Cu were detected at the general GB by atom probe tomography (APT) and quantified using ladder diagrams. In the case of the Σ3 coherent annealing twin, it was necessary to first locate the position of the boundary by density analysis of the atom probe data, then small amounts of B, Si and P segregation and, surprisingly, depletion of C were detected. The concentration of Mn was constant across the interface for both boundary types. The depletion of C at the annealing twin is explained by a local change in the stacking sequence at the boundary, creating a local hexagonal close-packed structure with low C solubility. This finding raises the question of whether segregation/depletion also occurs at Σ3 deformation twin boundaries in high-Mn TWIP steels. Consequently, a previously published APT dataset of the Fe-22Mn-0.6C alloy system, containing a high density of deformation twins due to 30% tensile deformation at room temperature, was reinvestigated using the same analysis routine as for the annealing twin. Although crystallographically identical to the annealing twin, no evidence of segregation or depletion was found at the deformation twins, owing to the lack of mobility of solutes during twin formation at room temperature. © 2014 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2014.09.041
  • Grain boundary segregation in multicrystalline silicon: Correlative characterization by EBSD, EBIC, and atom probe tomography
    Stoffers, A. and Cojocaru-Mirédin, O. and Seifert, W. and Zaefferer, S. and Riepe, S. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 23 (2015)
    This study aims to better understand the influence of crystallographic structure and impurity decoration on the recombination activity at grain boundaries in multicrystalline silicon. A sample of the upper part of a multicrystalline silicon ingot with intentional addition of iron and copper has been investigated. Correlative electron-beam-induced current, electron backscatter diffraction, and atom probe tomography data for different types of grain boundaries are presented. For a symmetric coherent Σ3 twin boundary, with very low recombination activity, no impurities are detected. In case of a non-coherent (random) high-angle grain boundary and higher order twins with pronounced recombination activity, carbon and oxygen impurities are observed to decorate the interface. Copper contamination is detected for the boundary with the highest recombination activity in this study, a random high-angle grain boundary located in the vicinity of a triple junction. The 3D atom probe tomography study presented here is the first direct atomic scale identification and quantification of impurities decorating grain boundaries in multicrystalline silicon. The observed deviations in chemical decoration and induced current could be directly linked with different crystallographic structures of silicon grain boundaries. Hence, the current work establishes a direct correlation between grain boundary structure, atomic scale segregation information, and electrical activity. It can help to identify interface-property relationships for silicon interfaces that enable grain boundary engineering in multicrystalline silicon. Copyright © 2015 John Wiley & Sons, Ltd.
    view abstract10.1002/pip.2614
  • Guided mass spectrum labelling in atom probe tomography
    Haley, D. and Choi, P. and Raabe, D.
    Ultramicroscopy 159 (2015)
    Atom probe tomography (APT) is a valuable near-atomic scale imaging technique, which yields mass spectrographic data. Experimental correctness can often pivot on the identification of peaks within a dataset, this is a manual process where subjectivity and errors can arise. The limitations of manual procedures complicate APT experiments for the operator and furthermore are a barrier to technique standardisation. In this work we explore the capabilities of computer-guided ranging to aid identification and analysis of mass spectra. We propose a fully robust algorithm for enumeration of the possible identities of detected peak positions, which assists labelling. Furthermore, a simple ranking scheme is developed to allow for evaluation of the likelihood of each possible identity being the likely assignment from the enumerated set. We demonstrate a simple, yet complete work-chain that allows for the conversion of mass-spectra to fully identified APT spectra, with the goal of minimising identification errors, and the inter-operator variance within APT experiments. This work chain is compared to current procedures via experimental trials with different APT operators, to determine the relative effectiveness and precision of the two approaches. It is found that there is little loss of precision (and occasionally gain) when participants are given computer assistance. We find that in either case, inter-operator precision for ranging varies between 0 and 2 "significant figures" (2 σ confidence in the first n digits of the reported value) when reporting compositions. Intra-operator precision is weakly tested and found to vary between 1 and 3 significant figures, depending upon species composition levels. Finally it is suggested that inconsistencies in inter-operator peak labelling may be the largest source of scatter when reporting composition data in APT. © 2015 Elsevier B.V.
    view abstract10.1016/j.ultramic.2015.03.005
  • High resolution in situ mapping of microstrain and microstructure evolution reveals damage resistance criteria in dual phase steels
    Yan, D. and Tasan, C.C. and Raabe, D.
    Acta Materialia 96 (2015)
    Microstructures of multi-phase alloys undergo morphological and crystallographic changes upon deformation, corresponding to the associated microstructural strain fields. The multiple length and time scales involved therein create immense complexity, especially when microstructural damage mechanisms are also activated. An understanding of the relationship between microstructure and damage initiation can often not be achieved by post-mortem microstructural characterization alone. Here, we present a novel multi-probe analysis approach. It couples various scanning electron microscopy methods to microscopic-digital image correlation (μ-DIC), to overcome various challenges associated with concurrent mapping of the deforming microstructure along with the associated microstrain fields. For this purpose a contrast- and resolution-optimized μ-DIC patterning method and a selective pattern/microstructure imaging strategy were developed. They jointly enable imaging of (i) microstructure-independent pattern maps and (ii) pattern-independent microstructure maps. We apply this approach here to the study of damage nucleation in ferrite/martensite dual-phase (DP) steel. The analyses provide four specific design guidelines for developing damage-resistant DP steels. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.05.038
  • Homogeneity and composition of AlInGaN: A multiprobe nanostructure study
    Krause, F.F. and Ahl, J.-P. and Tytko, D. and Choi, P.-P. and Egoavil, R. and Schowalter, M. and Mehrtens, T. and Müller-Caspary, K. and Verbeeck, J. and Raabe, D. and Hertkorn, J. and Engl, K. and Rosenauer, A.
    Ultramicroscopy 156 (2015)
    The electronic properties of quaternary AlInGaN devices significantly depend on the homogeneity of the alloy. The identification of compositional fluctuations or verification of random-alloy distribution is hence of grave importance. Here, a comprehensive multiprobe study of composition and compositional homogeneity is presented, investigating AlInGaN layers with indium concentrations ranging from 0 to 17. at% and aluminium concentrations between 0 and 39 at% employing high-angle annular dark field scanning electron microscopy (HAADF STEM), energy dispersive X-ray spectroscopy (EDX) and atom probe tomography (APT). EDX mappings reveal distributions of local concentrations which are in good agreement with random alloy atomic distributions. This was hence investigated with HAADF STEM by comparison with theoretical random alloy expectations using statistical tests. To validate the performance of these tests, HAADF STEM image simulations were carried out for the case of a random-alloy distribution of atoms and for the case of In-rich clusters with nanometer dimensions. The investigated samples, which were grown by metal-organic vapor phase epitaxy (MOVPE), were thereby found to be homogeneous on this nanometer scale. Analysis of reconstructions obtained from APT measurements yielded matching results. Though HAADF STEM only allows for the reduction of possible combinations of indium and aluminium concentrations to the proximity of isolines in the two-dimensional composition space. The observed ranges of composition are in good agreement with the EDX and APT results within the respective precisions. © 2015 Elsevier B.V.
    view abstract10.1016/j.ultramic.2015.04.012
  • Hydrogen diffusion and segregation in α iron ∑ 3 (111) grain boundaries
    Hamza, M. and Hatem, T.M. and Raabe, D. and El-Awady, J.A.
    ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) 9-2015 (2015)
    Polycrystalline material generally exhibits degradation in its mechanical properties and shows more tendency for intergranular fracture due to segregation and diffusion of hydrogen on the grain boundaries (GBs). Understanding the parameters affecting the diffusion and binding of hydrogen within GBs will allow enhancing the mechanical properties of the commercial engineering materials and developing interface dominant materials. In practice during forming processes, the coincidence site lattice (CSL) GBs are experiencing deviations from their ideal configurations. Consequently, this will change the atomic structural integrity by superposition of sub-boundary dislocation networks on the ideal CSL interfaces. For this study, the ideal ∑ 3 111 [11 0] GB structure and its angular deviations in BCC iron within the range of Brandon criterion will be studied comprehensively using molecular statics (MS) simulations. The clean GB energy will be quantified, followed by the GB and free surface segregation energies calculations for hydrogen atoms. Rice-Wang model will be used to assess the embrittlement impact variation over the deviation angles. The results showed that the ideal GB structure is having the greatest resistance to embrittlement prior GB hydrogen saturation, while the 3° deviated GB is showing the highest susceptibility to embrittlement. Upon saturation, the 5° deviated GB appears to have the highest resistance instead due to the lowest stability of hydrogen atoms observed in the free surfaces of its simulation cell. Molecular dynamics (MD) simulations are then applied to calculate hydrogen diffusivity within the ideal and deviated GB structure. It is shown that hydrogen diffusivity decreases significantly in the deviated GB models. In addition, the 5° deviated GB is representing the local minimum for diffusivity results suggesting the existence of the highest atomic disorder and excessive secondary dislocation accommodation within this interface. Copyright © 2015 by ASME.
    view abstract10.1115/IMECE2015-53118
  • Interface engineering and characterization at the atomic-scale of pure and mixed ion layer gas reaction buffer layers in chalcopyrite thin-film solar cells
    Cojocaru-Mirédin, O. and Fu, Y. and Kostka, A. and Sáez-Araoz, R. and Beyer, A. and Knaub, N. and Volz, K. and Fischer, C.-H. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 23 (2015)
    In this work, we investigate the p-n junction region for two different buffer/Cu(In,Ga)(Se,S)<inf>2</inf> (CIGSSe) samples having different conversion efficiencies (the cell with pure In<inf>2</inf>S<inf>3</inf> buffer shows a lower efficiency than the nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffered one). To explain the better efficiency of the sample with nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffer layer, combined transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies were performed. In the pure In<inf>2</inf>S<inf>3</inf> buffered sample, a CuIn<inf>3</inf>Se<inf>5</inf> ordered-defect compound is observed at the CIGSSe surface, whereas in the nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffered sample no such compound is detected. The absence of an ordered-defect compound in the latter sample is explained either by the presence of the ZnS nanodots, which may act as a barrier layer against Cu diffusion in CIGSSe hindering the formation of CuIn<inf>3</inf>Se<inf>5</inf>, or by the presence of Zn at the CIGSSe surface, which may disturb the formation of this ordered-defect compound. In the nano-ZnS/In<inf>2</inf>S<inf>3</inf> sample, Zn was found in the first monolayers of the absorber layer, which may lead to a downward band bending at the surface. This configuration is very stable (Fermi level pinning at the conduction band, as observed for Cd in Cu(In,Ga)Se<inf>2</inf>) and reduces the recombination rate at the interface. This effect may explain why the sample with ZnS nanodots possesses a higher efficiency. This work demonstrates the capability of correlative transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies in investigating buried interfaces. The study provides essential information for understanding and modeling the p-n junction at the nanoscale in CIGSSe solar cells. Copyright © 2014 John Wiley & Sons, Ltd.
    view abstract10.1002/pip.2484
  • Interface engineering and nanoscale characterization of Zn(S,O) alternative buffer layer for CIGS thin film solar cells
    Soni, P. and Cojocaru-Miredin, O. and Raabe, D.
    2015 IEEE 42nd Photovoltaic Specialist Conference, PVSC 2015 (2015)
    The buffer layers in Cu(In,Ga)Se2 solar cells play a crucial role in device performance, although their thickness is only a few tens of nanometers. Moreover, often Zn(S,O) alternative buffer layers have been studied in view of their structure, band alignment, and optical properties, but not much work exists on their nanoscale chemical properties. This work focuses on the chemical characterization of Zn(S,O) using x-ray photoelectron spectroscopy for determination of the Zn(S,O) and Cu(In,Ga)Se2 depth composition, and atom probe tomography for probing the nano-scale chemical fluctuations at the Zn(S,O)/Cu(In,Ga)Se2 interface. The Zn(O,S) buffer layer was deposited by RF magnetron sputtering. The aim is to study the nanoscale concentration changes and atomic interdiffusion between CIGS and Zn(S,O) after sputter deposition at room temperature and after post-deposition heat treatment at 200°C. © 2015 IEEE.
    view abstract10.1109/PVSC.2015.7355889
  • International high manganese steel conference
    Bleck, W. and Raabe, D. and Dong, H.
    Steel Research International 86 (2015)
    view abstract10.1002/srin.201500237
  • Linear complexions: Confined chemical and structural states at dislocations
    Kuzmina, M. and Herbig, M. and Ponge, D. and Sandlöbes, S. and Raabe, D.
    Science 349 (2015)
    For 5000 years, metals have been mankind's most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We report chemical and structural states confined at dislocations. In a body-centered cubic Fe-9 atomic percent Mn alloy, we found Mn segregation at dislocation cores during heating, followed by formation of face-centered cubic regions but no further growth. The regions are in equilibrium with the matrix and remain confined to the dislocation cores with coherent interfaces. The phenomenon resembles interface-stabilized structural states called complexions. A cubic meter of strained alloy contains up to a light year of dislocation length, suggesting that linear complexions could provide opportunities to nanostructure alloys via segregation and confined structural states.
    view abstract10.1126/science.aab2633
  • Linking atomistic, kinetic Monte Carlo and crystal plasticity simulations of single-crystal tungsten strength
    Cereceda, D. and Diehl, M. and Roters, F. and Shanthraj, P. and Raabe, D. and Perlado, J.M. and Marian, J.
    GAMM Mitteilungen 38 (2015)
    Understanding and improving the mechanical properties of tungsten is a critical task for the materials fusion energy program. The plastic behavior in body-centered cubic (bcc) metals like tungsten is governed primarily by screw dislocations on the atomic scale and by ensembles and interactions of dislocations at larger scales. Modeling this behavior requires the application of methods capable of resolving each relevant scale. At the small scale, atomistic methods are used to study single dislocation properties, while at the coarse-scale, continuum models are used to cover the interactions between dislocations. In this work we present a multiscale model that comprises atomistic, kinetic Monte Carlo (kMC) and continuum-level crystal plasticity (CP) calculations. The function relating dislocation velocity to applied stress and temperature is obtained from the kMC model and it is used as the main source of constitutive information into a dislocation-based CP framework. The complete model is used to perform material point simulations of single-crystal tungsten strength. We explore the entire crystallographic orientation space of the standard triangle. Non-Schmid effects are inlcuded in the model by considering the twinning-antitwinning (T/AT) asymmetry in the kMC calculations. We consider the importance of ?111?{110} and 111 {112} slip systems in the homologous temperature range from 0.08T<inf>m</inf> to 0.33T<inf>m</inf>, where T<inf>m</inf> =3680 K is the melting point in tungsten. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/gamm.201510012
  • Martensitic transformation in Eurofer-97 and ODS-Eurofer steels: A comparative study
    Zilnyk, K.D. and Oliveira, V.B. and Sandim, H.R.Z. and Möslang, A. and Raabe, D.
    Journal of Nuclear Materials 462 (2015)
    (Figure Presented). Reduced-activation ferritic-martensitic Eurofer-97 and ODS-Eurofer steels are potential candidates for structural applications in advanced nuclear reactors. Samples of both steel grades in the as-tempered condition were austenitized in vacuum for 1 h from 900 °C to 1300 °C followed by air cooling to room temperature. The microstructure was characterized by dilatometry, electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). Thermodynamic calculations provided by Thermo-Calc software were used to determine their transformation temperatures. Even having similar chemical composition, important changes were observed after martensitic transformation in these steels. Significant austenitic grain growth was observed in Eurofer-97 steel leading to the development of coarser martensitic packets. Contrastingly, austenitic grain growth was prevented in ODS-Eurofer steel due to fine and stable dispersion of Y-based particles. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jnucmat.2014.12.112
  • Mechanisms of subgrain coarsening and its effect on the mechanical properties of carbon-supersaturated nanocrystalline hypereutectoid steel
    Li, Y.J. and Kostka, A. and Choi, P. and Goto, S. and Ponge, D. and Kirchheim, R. and Raabe, D.
    Acta Materialia 84 (2015)
    Carbon-supersaturated nanocrystalline hypereutectoid steels with a tensile strength of 6.35 GPa were produced from severely cold-drawn pearlite. The nanocrystalline material undergoes softening upon annealing at temperatures between 200 and 450°C. The ductility in terms of elongation to failure exhibits a non-monotonic dependence on temperature. Here, the microstructural mechanisms responsible for changes in the mechanical properties were studied using transmission electron microscopy (TEM), TEM-based automated scanning nanobeam diffraction and atom probe tomography (APT). TEM and APT investigations of the nanocrystalline hypereutectoid steel show subgrain coarsening upon annealing, which leads to strength reduction following a Hall-Petch law. APT analyzes of the Mn distribution near subgrain boundaries and in the cementite give strong evidence of capillary-driven subgrain coarsening occurring through subgrain boundary migration. The pronounced deterioration of ductility after annealing at temperatures above 350°C is attributed to the formation of cementite at subgrain boundaries. The overall segregation of carbon atoms at ferrite subgrain boundaries gives the nanocrystalline material excellent thermal stability upon annealing. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.10.027
  • Microstructure design and mechanical properties in a near-α Ti-4Mo alloy
    Tarzimoghadam, Z. and Sandlöbes, S. and Pradeep, K.G. and Raabe, D.
    Acta Materialia 97 (2015)
    Abstract We study the effects of different heat treatment routes on microstructure engineering and the resulting mechanical response in a plain binary Ti-4Mo (wt%) model alloy. We observe a broad variety of microstructure formation mechanisms including diffusion driven allotropic phase transformations as well as shear and/or diffusion dominated modes of martensitic transformations, enabling a wealth of effective microstructure design options even in such a simple binary Ti alloy. This wide variety of microstructures allows tailoring the mechanical properties ranging from low yield strength (350 MPa) and high ductility (30-35% tensile elongation) to very high yield strength (1100 MPa) and medium ductility (10-15% tensile elongation) as well as a variety of intermediate states. Mechanical testing and microstructure characterization using optical microscopy, scanning electron microscopy based techniques, transmission electron microscopy and atom probe tomography were performed revealing that minor variations in the heat treatment cause significant changes in the resulting microstructures (e.g. structural refinement, transition between diffusive and martensitic transformations). The experimental results on microstructure evolution during the applied different heat treatment routes are discussed with respect to the mechanical properties. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.06.043
  • Microstructure in plasticity, a comparison between theory and experiment
    Dmitrieva, O. and Raabe, D. and Müller, S. and Dondl, P.W.
    Lecture Notes in Applied and Computational Mechanics 78 (2015)
    We review aspects of pattern formation in plastically deformed single crystals, in particular as described in the investigation of a copper single crystal shear experiment in [DDMR09]. In this experiment, the specimen showed a band-like microstructure consisting of alternating crystal orientations. Such a formation of microstructure is often linked to a lack of convexity in the free energy describing the system. The specific parameters of the observed bands, namely the relative crystal orientation as well as the normal direction of the band layering, are thus compared to the predictions of the theory of kinematically compatible microstructure oscillating between low-energy states of the non-convex energy. We conclude that this theory is suitable to describe the experimentally observed band-like structure. Furthermore, we link these findings to the models used in studies of relaxation and evolution of microstructure. ©Springer International Publishing Switzerland 2015.
    view abstract10.1007/978-3-319-18242-1_8
  • Microstructure refinement for high modulus in-situ metal matrix composite steels via controlled solidification of the system Fe-TiB2
    Springer, H. and Aparicio Fernandez, R. and Duarte, M.J. and Kostka, A. and Raabe, D.
    Acta Materialia 96 (2015)
    Microstructures of Fe-TiB<inf>2</inf> metal-matrix-composites formed in-situ from Fe-Ti-B melts were investigated for hypo- and hyper-eutectic concentrations down to atomic-scale resolution. Special emphasis is laid on the influence of the solidification rate on particle size, morphology and distribution as well as their relation to mechanical properties. Innovative routes for the cost-effective production of stiff and ductile high modulus steels for lightweight structural applications are discussed, focusing on hyper-eutectic compositions due to their high stiffness/density ratio: firstly, very slow cooling allows the primary particles floating to the top of the cast, from which they can either be easily removed for retaining bulk material containing only fine-dispersed eutectic particles, or be kept and utilised as a wear resistant surface. Secondly, annealing of amorphous matrix material obtained from very fast solidification leads to fine dispersed nano-scaled precipitation of TiB<inf>2</inf> particles. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.06.017
  • Multiphase microstructures via confined precipitation and dissolution of vessel phases: Example of austenite in martensitic steel
    Belde, M. and Springer, H. and Inden, G. and Raabe, D.
    Acta Materialia 86 (2015)
    We present a novel method to locally control the constitution, morphology, dispersion and transformation behavior of multiphase materials. The approach is based on the targeted, site-specific formation and confined dissolution of precipitated carbides or intermetallic phases. These dispersoids act as "vessels" or "containers" for specific alloying elements forming controlled chemical gradients within the microstructure upon precipitation and subsequent (partial) dissolution at elevated temperatures. The basic processing sequence consists of three subsequent steps, namely: (i) matrix homogenization (conditioning step); (ii) nucleation and growth of the vessel phases (accumulation step); and (iii) (partial) vessel dissolution (dissolution step). The vessel phase method offers multiple pathways to create dispersed microstructures by the variation of plain thermomechanical parameters such as time, temperature and deformation. This local microstructure design enables us to optimize the mechanical property profiles of advanced structural materials such as high strength steels at comparatively lean alloy compositions. The approach is demonstrated on a 11.6Cr-0.32C (wt.%) steel, where by using M23C6 carbides as a vessel phase, Cr and C can be locally enriched so that the thus-lowered martensite start temperature allows the formation of a significant quantity of retained austenite (up to 14 vol.%) of fine dispersion and controlled morphology. The effects of processing parameters on the obtained microstructures are investigated, with a focus on the dissolution kinetics of the vessel carbides. The approach is referred to as vessel microstructure design. © 2014 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2014.11.025
  • Nanolaminate transformation-induced plasticity-twinning-induced plasticity steel with dynamic strain partitioning and enhanced damage resistance
    Wang, M.-M. and Tasan, C.C. and Ponge, D. and Dippel, A.-Ch. and Raabe, D.
    Acta Materialia 85 (2015)
    Conventional martensitic steels have limited ductility due to insufficient microstructural strain-hardening and damage resistance mechanisms. It was recently demonstrated that the ductility and toughness of martensitic steels can be improved without sacrificing the strength, via partial reversion of the martensite back to austenite. These improvements were attributed to the presence of the transformation-induced plasticity (TRIP) effect of the austenite phase, and the precipitation hardening (maraging) effect in the martensitic matrix. However, a full micromechanical understanding of this ductilizing effect requires a systematic investigation of the interplay between the two phases, with regards to the underlying deformation and damage micromechanisms. For this purpose, in this work, a Fe-9Mn-3Ni-1.4Al-0.01C (mass%) medium-Mn TRIP maraging steel is produced and heat-treated under different reversion conditions to introduce well-controlled variations in the austenite-martensite nanolaminate microstructure. Uniaxial tension and impact tests are carried out and the microstructure is characterized using scanning and transmission electron microscopy based techniques and post mortem synchrotron X-ray diffraction analysis. The results reveal that (i) the strain partitioning between austenite and martensite is governed by a highly dynamical interplay of dislocation slip, deformation-induced phase transformation (i.e. causing the TRIP effect) and mechanical twinning (i.e. causing the twinning-induced plasticity effect); and (ii) the nanolaminate microstructure morphology leads to enhanced damage resistance. The presence of both effects results in enhanced strain-hardening capacity and damage resistance, and hence the enhanced ductility. © 2014 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2014.11.010
  • Nanostructure of wet-chemically prepared, polymer-stabilized silver-gold nanoalloys (6 nm) over the entire composition range
    Ristig, S. and Prymak, O. and Loza, K. and Gocyla, M. and Meyer-Zaika, W. and Heggen, M. and Raabe, D. and Epple, M.
    Journal of Materials Chemistry B 3 (2015)
    Bimetallic silver-gold nanoparticles were prepared by co-reduction using citrate and tannic acid in aqueous solution and colloidally stabilized with poly(N-vinylpyrrolidone) (PVP). The full composition range of silver:gold from 0:100 to 100:0 (n:n) was prepared with steps of 10 mol%. The nanoparticles were spherical, monodispersed, and had a diameter of ∼6 nm, except for Ag:Au 90:10 nanoparticles and pure Ag nanoparticles which were slightly larger. The size of the nanoalloys was determined by differential centrifugal sedimentation (DCS) and transmission electron microscopy (TEM). By means of X-ray powder diffraction (XRD) together with Rietveld refinement, precise lattice parameters, crystallite size and microstrain were determined. Scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) showed that the particles consisted of a gold-rich core and a silver-rich shell. XRD and DCS indicated that the nanoparticles were not twinned, except for pure Ag and Ag:Au 90:10, although different domains were visible in the TEM. A remarkable negative deviation from Vegard's linear rule of alloy mixtures was observed (isotropic contraction of the cubic unit cell with a minimum at a 50:50 composition). This effect was also found for Ag:Au bulk alloys, but it was much more pronounced for the nanoalloys. Notably, it was much less pronounced for pure silver and gold nanoparticles. The microstrain was increased along with the contraction of the unit cell with a broad maximum at a 50:50 composition. The synthesis is based on aqueous solvents and can be easily scaled up to a yield of several mg of a well dispersed nanoalloy with application potential due to its tuneable antibacterial action (silver) and its optical properties for bioimaging. © The Royal Society of Chemistry 2015.
    view abstract10.1039/c5tb00644a
  • Non-equiatomic high entropy alloys: Approach towards rapid alloy screening and property-oriented design
    Pradeep, K.G. and Tasan, C.C. and Yao, M.J. and Deng, Y. and Springer, H. and Raabe, D.
    Materials Science and Engineering A 648 (2015)
    The high entropy alloy (HEA) concept has triggered a renewed interest in alloy design, even though some aspects of the underlying thermodynamic concepts are still under debate. This study addresses the short-comings of this alloy design strategy with the aim to open up new directions of HEA research targeting specifically non-equiatomic yet massively alloyed compositions. We propose that a wide range of massive single phase solid solutions could be designed by including non-equiatomic variants. It is demonstrated by introducing a set of novel non-equiatomic multi-component CoCrFeMnNi alloys produced by metallurgical rapid alloy prototyping. Despite the reduced configurational entropy, detailed characterization of these materials reveals a strong resemblance to the well-studied equiatomic single phase HEA: The microstructure of these novel alloys exhibits a random distribution of alloying elements (confirmed by Energy-Dispersive Spectroscopy and Atom Probe Tomography) in a single face-centered-cubic phase (confirmed by X-ray Diffraction and Electron Backscatter Diffraction), which deforms through planar slip (confirmed by Electron-Channeling Contrast Imaging) and leads to excellent ductility (confirmed by uniaxial tensile tests). This approach widens the field of HEAs to non-equiatomic multi-component alloys since the concept enables to tailor the stacking fault energy and associated transformation phenomena which act as main mechanisms to design useful strain hardening behavior. © 2015 Elsevier B.V.
    view abstract10.1016/j.msea.2015.09.010
  • On the role of zinc on the formation and growth of intermetallic phases during interdiffusion between steel and aluminium alloys
    Springer, H. and Szczepaniak, A. and Raabe, D.
    Acta Materialia 96 (2015)
    The effect of Zn - both within Al and as a coating on steel - on the intermetallic phase formation and growth was systematically studied in controlled experiments, simulating the interfacial reactions taking place in dissimilar solid/solid and solid/liquid joining procedures. Independent from the reaction temperature, the addition of 1.05 at.% Zn (2.5 wt.%) to Al had no effect on the reaction layers' build-up with the η phase (Al<inf>5</inf>Fe<inf>2</inf>) as the dominant component, but accelerated their parabolic growth up to a factor of 13. While Zn-coatings on steel were found to be beneficial for the regular and even formation of intermetallic reaction zones in solid/liquid joining procedures, their role in solid-state processes was found to be more complex and, if no countermeasures are taken, extremely detrimental to the joint properties. Possible reasons for the Zn-induced growth acceleration are discussed, as well as consequences for possible optimisation steps for reducing harmful effects of Zn in dissimilar joints between Al alloys and steel. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.06.028
  • On the room temperature deformation mechanisms of a Mg-Y-Zn alloy with long-period-stacking-ordered structures
    Kim, J.-K. and Sandlöbes, S. and Raabe, D.
    Acta Materialia 82 (2015)
    We present a transmission electron microscopy study on the room temperature deformation mechanisms in a Mg97Y2Zn1 (at.%) alloy with long-period-stacking-order (LPSO) phase. The alloy consists of α-Mg matrix with platelet-shaped LPSO precipitates 3-5 nm thick and interdendritic LPSO (18R structures) phase grains. The interdendritic LPSO phase was found to deform either by kink-banding in conjunction with basal < a&gt; slip or by basal < a&gt; slip and the formation of dislocation walls. No orientation dependence of these different deformation modes was observed. The α-Mg matrix deforms by basal < a&gt; slip and pyramidal < c + a&gt; slip. No twinning was observed in the α-Mg matrix during room temperature deformation. The elastic modulus mismatch between α-Mg matrix and LPSO plates is suggested to be the main source for activating non-basal dislocations. The combination of the soft α-Mg matrix strengthened by LPSO precipitates and harder "bulk" interdendritic LPSO grains is suggested to contribute to the well-known good mechanical properties of Mg-LPSO alloys at room temperature. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.09.036
  • Orientation dependent deformation by slip and twinning in magnesium during single crystal indentation
    Zambaldi, C. and Zehnder, C. and Raabe, D.
    Acta Materialia 91 (2015)
    We present the orientation dependent indentation response of pure magnesium during single grain indentation. A conical indenter and maximum loads between 50 mN and 900 mN were employed. Indent topographies were acquired by confocal microscopy. The indents were also characterized by electron backscatter orientation microscopy for their microstructures. Pronounced activation of specific twinning systems was observed around the impressions. The resulting data were compiled into the inverse pole figure presentation of indent microstructures and topographies after Zambaldi and Raabe, Acta Mater. (2010). Three-dimensional crystal plasticity finite element simulation of the indentation deformation supports the interpretation of the orientation dependent slip and twinning patterns around the indents. The match between the activation of observed and simulated twinning variants is discussed with respect to the conditions for nucleation and growth of extension twins. Furthermore, the compatibility of the twinning strains with the imposed deformation is discussed based on the expanding cavity model of indentation. The orientation dependent response of magnesium during indentation is compared to the literature data for indentation of alpha-titanium and beryllium. Recommendations are given on how to exploit the characteristic nature of the observed indentation patterns to rapidly assess the relative activity of deformation mechanisms and their critical shear stresses during alloy development. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.01.046
  • Origin of shear induced β to ω transition in Ti-Nb-based alloys
    Lai, M.J. and Tasan, C.C. and Zhang, J. and Grabowski, B. and Huang, L.F. and Raabe, D.
    Acta Materialia 92 (2015)
    Ti-Nb-based alloys are essential materials for biomedical implant and aerospace applications. They reveal complex phase transformation behavior. Here, a {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> twinning induced β (body-centered cubic phase) to ω (hexagonal phase) transition in Ti-Nb-based alloys is demonstrated by transmission electron microscopy and analyzed employing ab initio calculations and the linear elastic inclusion theory. Our theoretical results reveal a distinct energy barrier for the β to ω transition, where the contribution from lattice rearrangement, rather than the elastic contribution associated with lattice parameter mismatch, plays the major role. It is shown that this energy barrier can be overcome by {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> shear, explaining why {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> twinning or, alternatively, the β to α″ (orthorhombic phase) transition promotes local formation of the ω phase. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.03.040
  • Phase stability of non-equiatomic CoCrFeMnNi high entropy alloys
    Ma, D. and Yao, M. and Pradeep, K.G. and Tasan, C.C. and Springer, H. and Raabe, D.
    Acta Materialia 98 (2015)
    Abstract The objective of this study is to experimentally and theoretically investigate the phase stability of non-equiatomic Fe<inf>x</inf>Mn<inf>62-x</inf>Ni<inf>30</inf>Co<inf>6</inf>Cr<inf>2</inf> based high entropy alloys, where x ranges from 22 to 42 at.%. Another aim is to systematically and critically assess the predictive capability of the CALPHAD approach for such high entropy alloy systems. We find that the CALPHAD simulations provide a very consistent assessment of phase stability yielding good agreement with experimental observations. These include the equilibrium phase formation at high temperatures, the constituent phases after non-equilibrium solidification processes, unfavorable segregation profiles inherited from solidification together with the associated nucleation and growth of low temperature phases, and undesired martensitic transformation effects. Encouraged by these consistent theoretical and experimental results, we extend our simulations to other alloy systems with equiatomic compositions reported in the literature. Using these other equiatomic model systems we demonstrate how systematic CALPHAD simulations can improve and accelerate the design of multicomponent alloy systems. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.07.030
  • Quantitative chemical-structure evaluation using atom probe tomography: Short-range order analysis of Fe-Al
    Marceau, R.K.W. and Ceguerra, A.V. and Breen, A.J. and Raabe, D. and Ringer, S.P.
    Ultramicroscopy 157 (2015)
    Short-range-order (SRO) has been quantitatively evaluated in an Fe-18Al (at%) alloy using atom probe tomography (APT) data and by calculation of the generalised multicomponent short-range order (GM-SRO) parameters, which have been determined by shell-based analysis of the three-dimensional atomic positions. The accuracy of this method with respect to limited detector efficiency and spatial resolution is tested against simulated D0<inf>3</inf> ordered data. Whilst there is minimal adverse effect from limited atom probe instrument detector efficiency, the combination of this with imperfect spatial resolution has the effect of making the data appear more randomised. The value of lattice rectification of the experimental APT data prior to GM-SRO analysis is demonstrated through improved information sensitivity. © 2015 Elsevier B.V.
    view abstract10.1016/j.ultramic.2015.05.001
  • Rapid theory-guided prototyping of ductile Mg alloys: From binary to multi-component materials
    Pei, Z. and Friák, M. and Sandlöbes, S. and Nazarov, R. and Svendsen, B. and Raabe, D. and Neugebauer, J.
    New Journal of Physics 17 (2015)
    In order to identify a method allowing for a fast solute assessment without lengthy ab initio calculations, we analyze correlations and anti-correlation between the stacking fault energies (SFEs), which were shown to be related to the macroscopic ductility in Mg alloys, and five material parameters of 18 different elemental solutes. Our analysis reveals that the atomic volume V of pure solutes, their electronegativity ν and bulk modulus B are either linearly or logarithmically related to the SFE. Comparing the impact of solutes with that of yttrium (that increases the ductility in Mg) we propose a single numerical quantity (called yttrium similarity index, YSI) that is based on these inter-relations. Subsequently, we evaluate this new figure of merit for 76 elements from the periodic table of elements in search for solutes reducing the SFE. Limiting ourselves first to binary Mg alloys, we hardly find any alternative solutes providing similar reduction as that due to rare-earth (RE) additions. Therefore, we extended our search to ternary Mg alloys. Assuming that the physical properties of solute combinations can be represented by their average values, 2850 solute combinations were checked and 133 solute pairs (not including any RE elements) have been found to have a YSI larger than 0.85. Quantum-mechanical calculations have been subsequently performed for 11 solute pairs with YSIs higher than 0.95 and they were all found to reduce the in excellent agreement with the predictions based on the YSI. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/17/9/093009
  • Relationship Between Damping Capacity and Variations of Vacancies Concentration and Segregation of Carbon Atom in an Fe-Mn Alloy
    Wen, Y. and Xiao, H. and Peng, H. and Li, N. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 (2015)
    We investigated effects of quenching temperature and ageing on variations of vacancies concentration and segregation of solute atoms and their relationship with damping capacity in an Fe-Mn alloy. The damping capacity can be remarkably improved by lowering vacancies concentration but deteriorated by segregation of carbon atoms. A higher damping capacity can be obtained by furnace cooling or quenching and then ageing in Fe-Mn alloy with lower carbon content or addition of Ti or Nb. © 2015, The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-015-3111-1
  • Segregation of boron at prior austenite grain boundaries in a quenched martensitic steel studied by atom probe tomography
    Li, Y.J. and Ponge, D. and Choi, P. and Raabe, D.
    Scripta Materialia 96 (2015)
    The distribution of B and other alloying elements (C, Cr, Mo) at prior austenite grain boundaries (PAGBs) and in the matrix was quantified by atom probe tomography in a quenched martensitic steel. B and Mo were observed to be segregated only at PAGBs and to be absent at martensite-martensite boundaries. C is segregated both at PAGBs and at martensite-martensite boundaries, whereas Cr is homogeneously distributed in the probed volume. Our results indicate that B undergoes a non-equilibrium segregation. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2014.09.031
  • Self-Assembled Monolayers: Star-Shaped Crystallographic Cracking of Localized Nanoporous Defects (Adv. Mater. 33/2015)
    Renner, F.U. and Ankah, G.N. and Bashir, A. and Ma, D. and Biedermann, P.U. and Shrestha, B.R. and Nellessen, M. and Khorashadizadeh, A. and Losada-Pérez, P. and Duarte, M.J. and Raabe, D. and Valtiner, M.
    Advanced materials (Deerfield Beach, Fla.) 27 (2015)
    On page 4877, F. U. Renner, A. Bashir, M. Valtiner, and co-workers describe a star-like dealloying corrosion morphology that appears during the localized attack of smooth well-prepared Cu-Au surfaces. The surfaces are initially protected by thiol or selenol inhibitior films. Localized dealloying of Cu-Au produces nanoporous gold under stress and crystallographic cracks - thereby opening a new approach combining surface science with nanoscale mechanical testing. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adma.201570223
  • Shear-flow-controlled mode selection in a nonlinear autocatalytic medium
    Ayodele, S.G. and Raabe, D. and Varnik, F.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 91 (2015)
    The effect of shear flow on mode selection and the length scale of patterns formed in a nonlinear autocatalytic reaction-diffusion model is investigated. We predict analytically the existence of transverse and longitudinal modes. The type of the selected mode strongly depends on the difference in the flow rates of the participating species, quantified by the differential flow parameter. Spatial structures are obtained by varying the length scale of individual modes and superposing them via the differential flow parameter. Our predictions are in line with numerical results obtained from lattice Boltzmann simulations. © 2015 American Physical Society.
    view abstract10.1103/PhysRevE.91.022913
  • Size and orientation effects in partial dislocation-mediated deformation of twinning-induced plasticity steel micro-pillars
    Choi, W.S. and De Cooman, B.C. and Sandlöbes, S. and Raabe, D.
    Acta Materialia 98 (2015)
    Abstract Bulk and micro-pillar single crystals were used to investigate the twinning-induced plasticity mechanism in austenitic Fe-22 wt%Mn-0.6 wt%C TWIP steel. Compression of micro-pillars oriented either for deformation-induced twinning or for perfect dislocation glide was carried out for pillars with diameters in the range of 600 nm to 4 μm. The same size dependence of the critical resolved shear stress was observed for both orientations. The critical micro-pillar diameter for size-independent plasticity was approximately 7.6 μm. Partial dislocation-mediated formation of twins and ε-martensite was observed in micro-pillars oriented for twinning by transmission electron microscopy. The elastic-plastic transition in micro-pillars oriented for deformation twinning did not involve twinning, and dislocation-dislocation interactions were a necessary precondition for twin formation. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.06.065
  • Spatially and kinetically resolved mapping of hydrogen in a twinning-induced plasticity steel by use of Scanning Kelvin Probe Force Microscopy
    Koyama, M. and Bashir, A. and Rohwerder, M. and Merzlikin, S.V. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Journal of the Electrochemical Society 162 (2015)
    The hydrogen distribution in a hydrogen-charged Fe-18Mn-1.2C (wt%) twinning-induced plasticity austenitic steel was studied by Scanning Kelvin Probe Force Microscopy (SKPFM). We observed that 1-2 days after the hydrogen-charging, hydrogen showed a higher activity at twin boundaries than inside the matrix. This result indicates that hydrogen at the twin boundaries is diffusible at room temperature, although the twin boundaries act as deeper trap sites compared to typical diffusible hydrogen trap sites such as dislocations. After about 2 weeks the hydrogen activity in the twin boundaries dropped and was indistinguishable from that in the matrix. These SKPFM results were supported by thermal desorption spectrometry and scanning electron microscopic observations of deformation-induced surface cracking parallel to deformation twin boundaries. With this joint approach, two main challenges in the field of hydrogen embrittlement research can be overcome, namely, the detection of hydrogen with high local and chemical sensitivity and the microstructure-dependent and spatially resolved observation of the kinetics of hydrogen desorption. © 2015 The Electrochemical Society.
    view abstract10.1149/2.0131512jes
  • Star-Shaped Crystallographic Cracking of Localized Nanoporous Defects
    Renner, F.U. and Ankah, G.N. and Bashir, A. and Ma, D. and Biedermann, P.U. and Shrestha, B.R. and Nellessen, M. and Khorashadizadeh, A. and Losada-Pérez, P. and Duarte, M.J. and Raabe, D. and Valtiner, M.
    Advanced Materials 27 (2015)
    On self-assembled monolayer-covered Cu-Au substrates, localized volume shrinkage at initial dealloying sites leads to cracks within the attacked regions. It is started from well-controlled surface structures to gain fundamental insights in the driving mechanisms of localized corrosion and crack formation. Both the crack density and the crack morphology are critically dependent on surface orientation, crystallography, and inhibitor molecule species. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adma.201405565
  • Suppression of twinning and phase transformation in an ultrafine grained 2 GPa strong metastable austenitic steel: Experiment and simulation
    Shen, Y.F. and Jia, N. and Wang, Y.D. and Sun, X. and Zuo, L. and Raabe, D.
    Acta Materialia 97 (2015)
    Abstract An ultrafine-grained 304 austenitic 18 wt.% Cr-8 wt.% Ni stainless steel with a grain size of ∼270 nm was synthesized by accumulative rolling (67% total reduction) and annealing (550°C, 150 s). Uniaxial tensile testing at room temperature reveals an extremely high yield strength of 1890 ± 50 MPa and a tensile strength of 2050 ± 30 MPa, while the elongation reaches 6 ± 1%. Experimental characterization on samples with different grain sizes between 270 nm and 35 μm indicates that both, deformation twinning and martensitic phase transformation are significantly retarded with increasing grain refinement. A crystal plasticity finite element model incorporating a constitutive law reflecting the grain size-controlled dislocation slip and deformation twinning captures the micromechanical behavior of the steels with different grain sizes. Comparison of simulation and experiment shows that the deformation of ultrafine-grained 304 steels is dominated by the slip of partial dislocations, whereas for coarse-grained steels dislocation slip, twinning and martensite formation jointly contribute to the shape change. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.06.053
  • Synergy of atom-probe structural data and quantum-mechanical calculations in a theory-guided design of extreme-stiffness superlattices containing metastable phases
    Friák, M. and Tytko, D. and Holec, D. and Choi, P.-P. and Eisenlohr, P. and Raabe, D. and Neugebauer, J.
    New Journal of Physics 17 (2015)
    A theory-guided materials design of nano-scaled superlattices containing metastable phases is critically important for future development of advanced lamellar composites with application-dictated stiffness and hardness. Our study combining theoretical and experimental methods exemplifies the strength of this approach for the case of the elastic properties of AlN/CrN superlattices that were deposited by reactive radio-frequency magnetron sputtering with a bilayer period of 4 nm. Importantly, CrN stabilizes AlN in a metastable B1 (rock salt) cubic phase only in the form of a layer that is very thin, up to a few nanometers. Due to the fact that B1-AlN crystals do not exist as bulk materials, experimental data for this phase are not available. Therefore, quantum-mechanical calculations have been applied to simulate an AlN/CrN superlattice with a similar bilayer period. The ab initio predicted Young's modulus (428 GPa) along the [001] direction is in excellent agreement with measured nano-indentation values (408 32 GPa). Aiming at a future rapid high-throughput materials design of superlattices, we have also tested predictions obtained within linear-elasticity continuum modeling using elastic properties of B1-CrN and B1-AlN phases as input. Using single-crystal elastic constants from ab initio calculations for both phases, we demonstrate the reliability of this approach to design nano-patterned coherent superlattices with unprecedented and potentially superior properties. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/17/9/093004
  • The influence of stacking fault energy on the microstructural and strain-hardening evolution of Fe-Mn-Al-Si steels during tensile deformation
    Pierce, D.T. and Jiménez, J.A. and Bentley, J. and Raabe, D. and Wittig, J.E.
    Acta Materialia 100 (2015)
    Understanding the relationship between the stacking-fault energy (SFE), deformation mechanisms, and strain-hardening behavior is important for alloying and design of high-Mn austenitic transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The present study investigates the influence of SFE on the microstructural and strain-hardening evolution of three TRIP/TWIP alloys (Fe-22/25/28Mn-3Al-3Si wt.%). The SFE is increased by systemically increasing the Mn content from 22 to 28 wt.%. The Fe-22Mn-3Al-3Si alloy, with a SFE of 15 mJ m-2, deforms by planar dislocation glide and strain-induced ε<inf>hcp</inf>-/α<inf>bcc</inf>-martensite formation which occurs from the onset of plastic deformation, resulting in improved work-hardening at low strains but lower total elongation. With an increased SFE of 21 mJ m-2 in the Fe-25Mn-3Al-3Si alloy, both mechanical twinning and ε<inf>hcp</inf>-martensite formation are activated during deformation, and result in the largest elongation of the three alloys. A SFE of 39 mJ m-2 enables significant dislocation cross slip and suppresses ε<inf>hcp</inf>-martensite formation, causing reduced work-hardening during the early stages of deformation in the Fe-28Mn-3Al-3Si alloy while mechanical twinning begins to enhance the strain-hardening after approximately 10% strain. The increase in SFE from 15 to 39 mJ m-2 results in significant changes in the deformation mechanisms and, at low strains, decreased work-hardening, but has a relatively small influence on strength and ductility. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.08.030
  • The nucleation of Mo-rich Laves phase particles adjacent to M23C6 micrograin boundary carbides in 12% Cr tempered martensite ferritic steels
    Isik, M.I. and Kostka, A. and Yardley, V.A. and Pradeep, K.G. and Duarte, M.J. and Choi, P.P. and Raabe, D. and Eggeler, G.
    Acta Materialia 90 (2015)
    We study the nucleation of Mo-rich Laves phase particles during aging and creep of 12 wt.% Cr tempered martensite ferritic steels (TMFS). Recently, in Isik et al. (2014) we reported that Laves phase particles tend to form at micrograin boundaries of TMFSs after Mo and Si had segregated from the ferritic matrix to these internal interfaces. In the present work, we employ transmission electron microscopy (TEM) and atom probe tomography (APT) to study the formation of Laves phase particles. We investigate the preference of Laves phase particles to nucleate next to M23C6 micrograin boundary carbides. Our results suggest that this joint precipitation effect is due to the combined segregation of Mo and Si from the matrix to the micrograin boundaries and Si and P enrichment around the growing carbides.
    view abstract10.1016/j.actamat.2015.01.027
  • The role of grain boundaries in the initial oxidation behavior of austenitic stainless steel containing alloyed Cu at 700°C for advanced thermal power plant applications
    Kim, J.-H. and Kim, B.K. and Kim, D.-I. and Choi, P.-P. and Raabe, D. and Yi, K.-W.
    Corrosion Science 96 (2015)
    The role of grain boundaries during the early stages of oxidation in austenitic stainless steels containing alloyed Cu was investigated using APT, TEM, EBSD, EPMA, and XRD. The oxidation experiments were performed at 700°C in air with 20% water vapor. Within 4μm from the grain boundaries, the oxide layer exhibits a dual-layer structure consisting of a thin Fe-rich spinel oxide on a protective Cr<inf>2</inf>O<inf>3</inf> oxide. Away from the grain boundaries, non-protective spinel oxide layers are formed as the outer and inner oxide layers. A critical grain size that prevents the formation of fast-growing spinel oxides is discussed. © 2015 Elsevier Ltd.
    view abstract10.1016/j.corsci.2015.03.014
  • Thermal dissolution mechanisms of AlN/CrN hard coating superlattices studied by atom probe tomography and transmission electron microscopy
    Tytko, D. and Choi, P.-P. and Raabe, D.
    Acta Materialia 85 (2015)
    AlN/CrN superlattices with a B1 cubic crystal structure and a bilayer period of 4 nm were deposited by reactive radiofrequency magnetron sputtering. The coatings were investigated with respect to their thermal stability and changes in microstructure and chemical composition at 900 °C. The AlN layers show high chemical stability but undergo dissolution by pinching off at grain boundaries. A transformation from cubic to hexagonal AlN with subsequent coarsening at grain boundary triple junctions is observed. In contrast to AlN, the CrN layers show poor chemical stability and their compositions are shifted towards Cr2N upon annealing in a protective argon atmosphere due to nitrogen loss. However, even after establishing Cr2N stoichiometry the crystal structure of the layers remains cubic. © 2014 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2014.11.004
  • A novel approach to measure grain boundary segregation in bulk polycrystalline materials in dependence of the boundaries' five rotational degrees of freedom
    Mandal, S. and Pradeep, K.G. and Zaefferer, S. and Raabe, D.
    Scripta Materialia 81 (2014)
    We demonstrate a simplified nondestructive 3-D electron backscatter diffraction (EBSD) methodology that enables the measurement of all five degrees of freedom of grain boundaries (GBs) combined with segregation analysis using atom probe tomography (APT). The approach is based on two 2-D EBSD measurements on orthogonal surfaces at a sharp edge of the specimen followed by site-specific GB composition analysis using APT. An example of an asymmetric Σ9 boundary exhibiting GB segregation emphasizes the need for complete GB characterization in this context. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2014.02.016
  • A novel high manganese austenitic steel with higher work hardening capacity and much lower impact deformation than Hadfield manganese steel
    Wen, Y.H. and Peng, H.B. and Si, H.T. and Xiong, R.L. and Raabe, D.
    Materials and Design 55 (2014)
    To tackle the problem of poor work hardening capacity and high initial deformation under low load in Hadfield manganese steel, the deformation behavior and microstructures under tensile and impact were investigated in a new high manganese austenitic steel Fe18Mn5Si0.35C (wt.%). The results show that this new steel has higher work hardening capacity at low and high strains than Hadfield manganese steel. Its impact deformation is much lower than that of Hadfield manganese steel. The easy occurrence and rapid increase of the amount of stress-induced ε martensitic transformation account for this unique properties in Fe18Mn5Si0.35C steel. The results indirectly confirm that the formation of distorted deformation twin leads to the anomalous work hardening in Hadfield manganese steel. © 2013 Elsevier Ltd.
    view abstract10.1016/j.matdes.2013.09.057
  • A novel, single phase, non-equiatomic FeMnNiCoCr high-entropy alloy with exceptional phase stability and tensile ductility
    Yao, M.J. and Pradeep, K.G. and Tasan, C.C. and Raabe, D.
    Scripta Materialia 72-73 (2014)
    A non-equiatomic FeMnNiCoCr alloy is introduced and characterized at multiple scales employing various characterization techniques (e.g. atom probe tomography, electron channeling contrast imaging, electron backscatter diffraction, etc.) to elucidate (i) the role of configurational entropy and (ii) the intrinsic tensile ductility of high-entropy alloys. Results reveal that the new material is a true high-entropy alloy with a stable random solid solution despite its comparably low configurational entropy, and that it has excellent tensile ductility irrespective of the substantial lattice distortion. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2013.09.030
  • Alloy Design, Combinatorial Synthesis, and Microstructure–Property Relations for Low-Density Fe-Mn-Al-C Austenitic Steels
    Raabe, D. and Springer, H. and Gutierrez-Urrutia, I. and Roters, F. and Bausch, M. and Seol, J.-B. and Koyama, M. and Choi, P.-P. and Tsuzaki, K.
    JOM 66 (2014)
    We present recent developments in the field of austenitic steels with up to 18% reduced mass density. The alloys are based on the Fe-Mn-Al-C system. Here, two steel types are addressed. The first one is a class of low-density twinning-induced plasticity or single phase austenitic TWIP (SIMPLEX) steels with 25–30 wt.% Mn and <4–5 wt.% Al or even <8 wt.% Al when naturally aged. The second one is a class of κ-carbide strengthened austenitic steels with even higher Al content. Here, κ-carbides form either at 500–600°C or even during quenching for >10 wt.% Al. Three topics are addressed in more detail, namely, the combinatorial bulk high-throughput design of a wide range of corresponding alloy variants, the development of microstructure–property relations for such steels, and their susceptibility to hydrogen embrittlement. © 2014, The Minerals, Metals & Materials Society.
    view abstract10.1007/s11837-014-1032-x
  • Atom probe tomography observation of hydrogen in high-Mn steel and silver charged via an electrolytic route
    Haley, D. and Merzlikin, S.V. and Choi, P. and Raabe, D.
    International Journal of Hydrogen Energy 39 (2014)
    We investigate an electrolytic route for hydrogen charging of metals and its detection in Atom Probe Tomography (APT) experiments. We charge an austenitic Fe-30Mn-8Al-1.2C (wt.%) weight reduced high-Mn steel and subsequently demonstrate the detectability of deuterium in an APT experiment. The experiment is repeated with a deposited Ag film upon an APT tip of a high-Mn steel. It is shown that a detectable deuterium signal can be seen in the high-Mn steel, and a D:H ratio of 0.84 can be reached in Ag films. Additionally, it was found that the predicted time constraint on detectability of D in APT was found to be lower than predicted by bulk diffusion for the high-Mn steel. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2014.05.169
  • Atom probe tomography study of ultrahigh nanocrystallization rates in FeSiNbBCu soft magnetic amorphous alloys on rapid annealing
    Pradeep, K.G. and Herzer, G. and Choi, P. and Raabe, D.
    Acta Materialia 68 (2014)
    Rapid annealing (4-10 s) induced primary crystallization of soft magnetic Fe-Si nanocrystals in a Fe73.5Si15.5Cu1Nb 3B7 amorphous alloy has been systematically studied by atom probe tomography in comparison with conventional annealing (30-60 min). It was found that the nanostructure obtained after rapid annealing is basically the same, irrespective of the different time scales of annealing. This underlines the crucial role of Cu during structure formation. Accordingly, the clustering of Cu atoms starts at least 50 C below the onset temperature of primary crystallization. As a consequence, coarsening of Cu atomic clusters also starts prior to crystallization, resulting in a reduction of available nucleation sites during Fe-Si nanocrystallization. Furthermore, the experimental results explicitly show that these Cu clusters initially induce a local enrichment of Fe and Si in the amorphous matrix. These local chemical heterogeneities are proposed to be the actual nuclei for subsequent nanocrystallization. Nevertheless, rapid annealing in comparison with conventional annealing results in the formation of ∼30% smaller Fe-Si nanocrystals, but of identical structure, volume fraction and chemical composition, indicating the limited influence of thermal treatment on nanocrystallization, owing to the effect of Cu. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.01.031
  • Atomic-scale analysis of carbon partitioning between martensite and austenite by atom probe tomography and correlative transmission electron microscopy
    Toji, Y. and Matsuda, H. and Herbig, M. and Choi, P.-P. and Raabe, D.
    Acta Materialia 65 (2014)
    Carbon partitioning between ferritic and austenitic phases is essential for austenite stabilization in the most advanced steels such as those produced by the quenching and partitioning (Q&P) process. The atomistic analysis of the carbon partitioning in Q&P alloys is, however, difficult owing to the simultaneous occurrence of bainite transformation, which can also contribute to carbon enrichment into remaining austenite and hence overlap with the carbon partitioning from martensite into austenite. Therefore, we provide here a direct atomic-scale evidence of carbon partitioning from martensite into austenite without the presence of bainite transformation. Carbon partitioning is investigated by means of atom probe tomography and correlative transmission electron microscopy. A model steel (Fe-0.59 wt.% C (2.7 at.% C)-2.0 wt.% Si-2.9 wt.% Mn) with martensite finish temperature below room temperature was designed and used in order to clearly separate the carbon partitioning between martensite and austenite from the bainite transformation. The steel was austenitized at 900°C, then water-quenched and tempered at 400°C. Approximately 8 vol.% retained austenite existed in the asquenched state. We confirmed by X-ray diffraction and dilatometry that austenite decomposition via bainite transformation did not occur during tempering. No carbon enrichment in austenite was observed in the as-quenched specimen. On the other hand, clear carbon enrichment in austenite was observed in the 400°C tempered specimens with a carbon concentration inside the austenite of 5-8 at.%. The results hence quantitatively revealed carbon partitioning from martensite to austenite, excluding bainite transformation during the Q&P heat treatment. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.10.064
  • Atomic-scale quantification of grain boundary segregation in nanocrystalline material
    Herbig, M. and Raabe, D. and Li, Y.J. and Choi, P. and Zaefferer, S. and Goto, S.
    Physical Review Letters 112 (2014)
    Grain boundary segregation leads to nanoscale chemical variations that can alter a material's performance by orders of magnitude (e.g., embrittlement). To understand this phenomenon, a large number of grain boundaries must be characterized in terms of both their five crystallographic interface parameters and their atomic-scale chemical composition. We demonstrate how this can be achieved using an approach that combines the accuracy of structural characterization in transmission electron microscopy with the 3D chemical sensitivity of atom probe tomography. We find a linear trend between carbon segregation and the misorientation angle ω for low-angle grain boundaries in ferrite, which indicates that ω is the most influential crystallographic parameter in this regime. However, there are significant deviations from this linear trend indicating an additional strong influence of other crystallographic parameters (grain boundary plane, rotation axis). For high-angle grain boundaries, no general trend between carbon excess and ω is observed; i.e., the grain boundary plane and rotation axis have an even higher influence on the segregation behavior in this regime. Slight deviations from special grain boundary configurations are shown to lead to unexpectedly high levels of segregation. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.112.126103
  • Atomistic simulation of the a0 〈1 0 0〉 binary junction formation and its unzipping in body-centered cubic iron
    Hafez Haghighat, S.M. and Schäublin, R. and Raabe, D.
    Acta Materialia 64 (2014)
    Molecular dynamics simulation is used to study the formation of the a 0 〈1 0 0〉 binary dislocation junction in body-centered cubic Fe. Results show that under an applied strain two intersecting 1/2 a 0 〈1 1 1〉 dislocations, one mobile edge and one immobile screw, form an a0 〈1 0 0〉 binary junction of mixed character in the glide plane of the mobile edge dislocation. It appears, however, that the binary junction does not necessarily lay in one of the three possible {1 1 0} glide planes of the screw dislocation. The binary junction starts to unzip as the impinging edge dislocation bows around and moves away, which results in the formation of a screw dipole along its Burgers vector. The dipole eventually annihilates, completing the unzipping process of the junction, which liberates the edge dislocation. The effects of temperature and strain rate on the unzipping of the junction are quantified by the critical release stress needed to detach the edge dislocation from the screw one. The critical stress decreases when the temperature increases from 10 to 300 K, whereas it increases with increasing applied strain rate, or dislocation speed. The interaction mechanism and strength of the a0 〈1 0 0〉 binary junction as an obstacle to the edge dislocation are compared to that of other types of defect, namely nanosized voids, Cu and Cr precipitates, and dislocation loops in Fe. It appears that the binary junction strength is in the lowest range, comparable to that of a coherent Cr precipitate. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.11.037
  • Composition Dependence of Phase Stability, Deformation Mechanisms, and Mechanical Properties of the CoCrFeMnNi High-Entropy Alloy System
    Tasan, C.C. and Deng, Y. and Pradeep, K.G. and Yao, M.J. and Springer, H. and Raabe, D.
    JOM 66 (2014)
    The proposal of configurational entropy maximization to produce massive solid-solution (SS)-strengthened, single-phase high-entropy alloy (HEA) systems has gained much scientific interest. Although most of this interest focuses on the basic role of configurational entropy in SS formability, setting future research directions also requires the overall property benefits of massive SS strengthening to be carefully investigated. To this end, taking the most promising CoCrFeMnNi HEA system as the starting point, we investigate SS formability, deformation mechanisms, and the achievable mechanical property ranges of different compositions and microstructural states. A comparative assessment of the results with respect to room temperature behavior of binary Fe-Mn alloys reveals only limited benefits of massive SS formation. Nevertheless, the results also clarify that the compositional requirements in this alloy system to stabilize the face-centered cubic (fcc) SS are sufficiently relaxed to allow considering nonequiatomic compositions and exploring improved strength–ductility combinations at reduced alloying costs. © 2014, The Minerals, Metals & Materials Society.
    view abstract10.1007/s11837-014-1133-6
  • Correlations of plasticity in sheared glasses
    Varnik, F. and Mandal, S. and Chikkadi, V. and Denisov, D. and Olsson, P. and Vågberg, D. and Raabe, D. and Schall, P.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 89 (2014)
    In a recent paper [Mandal, Phys. Rev. E 88, 022129 (2013)PLEEE81539-375510. 1103/PhysRevE.88.022129], the nature of spatial correlations of plasticity in hard-sphere glasses was addressed both via computer simulations and in experiments. It was found that the experimentally obtained correlations obey a power law, whereas the correlations from simulations are better fitted by an exponential decay. We here provide direct evidence - via simulations of a hard-sphere glass in two dimensions (2D) - that this discrepancy is a consequence of the finite system size in the 3D simulations. By extending the study to a 2D soft disk model at zero temperature [Durian, Phys. Rev. Lett. 75, 4780 (1995)PRLTAO0031-900710.1103/PhysRevLett.75.4780], the robustness of the power-law decay in sheared amorphous solids is underlined. Deviations from a power law occur when either reducing the packing fraction towards the supercooled regime in the case of hard spheres or changing the dissipation mechanism from contact dissipation to a mean-field-type drag in the case of soft disks. © 2014 American Physical Society.
    view abstract10.1103/PhysRevE.89.040301
  • Crystallization, phase evolution and corrosion of Fe-based metallic glasses: An atomic-scale structural and chemical characterization study
    Duarte, M.J. and Kostka, A. and Jimenez, J.A. and Choi, P. and Klemm, J. and Crespo, D. and Raabe, D. and Renner, F.U.
    Acta Materialia 71 (2014)
    Understanding phase changes, including their formation and evolution, is critical for the performance of functional as well as structural materials. We analyze in detail microstructural and chemical transformations of the amorphous steel Fe50Cr15Mo14C15B6 during isothermal treatments at temperatures ranging from 550 to 800 °C. By combining high-resolution transmission electron microscopy and Rietveld analyses of X-ray diffraction patterns together with the local chemical data obtained by atom probe tomography, this research provides relevant information at the atomic scale about the mechanisms of crystallization and the subsequent phases evolution. During the initial stages of crystallization a stable (Fe,Cr) 23(C,B)6 precipitates as well as two metastable intermediates of M3(C,B) and the intermetallic χ-phase. When full crystallization is reached, only a percolated nano-scale Cr-rich (Fe,Cr) 23(C,B)6 and Mo-rich η-Fe3Mo3C structure is detected, with no evidence to suggest that other phases appear at any subsequent time. Finally, the corrosion behavior of the developed phases is discussed from considerations of the obtained atomic information. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.02.027
  • Deformation mechanisms in an austenitic single-phase duplex microstructured steel with nanotwinned grains
    Yan, F.K. and Tao, N.R. and Archie, F. and Gutierrez-Urrutia, I. and Raabe, D. and Lu, K.
    Acta Materialia 81 (2014)
    A novel type of duplex microstructure is generated in a single-phase austenitic steel (AISI 316L; X2CrNiMo19-12), consisting of plastically compliant recrystallized austenitic grains as the matrix containing coarse non-recrystallized grains with a nanotwinned austenitic (nt-γ) structure as strengthening inclusions. This novel type of single-phase yet duplex microstructured steel exhibits an excellent combination of strength and ductility. We study the plastic co-deformation mechanisms between the nanotwinned and the recrystallized grains under tension using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). At tensile strains below 5%, the nt-γ grains nearly deform homogeneously in conjunction with the surrounding statically recrystallized (SRX) grains without generating notable strain localization near their interfaces. The anisotropic plastic deformation of the nt-γ grains with predominant shear parallel to the twin boundaries results in a higher dislocation density in the neighboring SRX grains. As the strain exceeds 12%, localized deformation occurs within the nt-γ grains in the form of shear banding. A strain gradient is developed in the surrounding SRX grains as a function of distance from the nt-γ/SRX interface. Deformation twinning is observed in the SRX grains near the nt-γ grains, while away from nt-γ grains dislocation slip dominates the deformation. The strengthening effect of the strong and ductile nt-γ grains may offer a novel approach to strengthen austenitic steels and related alloys by generating a nanotwinned/recrystallized duplex microstructure. © 2014 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2014.08.054
  • Designing Heusler nanoprecipitates by elastic misfit stabilization in Fe-Mn maraging steels
    Millán, J. and Sandlöbes, S. and Al-Zubi, A. and Hickel, T. and Choi, P. and Neugebauer, J. and Ponge, D. and Raabe, D.
    Acta Materialia 76 (2014)
    B2 NiMn and Ni2MnAl Heusler nanoprecipitates are designed via elastic misfit stabilization in Fe-Mn maraging steels by combining transmission electron microscopy (TEM) correlated atom probe tomography (APT) with ab initio simulations. Guided by these predictions, the Al content of the alloys is systematically varied, and the influence of the Al concentration on structure stability, size and distribution of precipitates formed during ageing at 450 °C is studied using scanning electron microscopy-electron backscatter diffraction, TEM and APT. Specifically, the Ni2MnAl Heusler nanoprecipitates exhibit the finest sizes and highest dispersion and hence lead to significant strengthening. The formation of the different types of precipitates and their structure, size, dispersion and effect on the mechanical properties of the alloys are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.05.016
  • Designing quadplex (four-phase) microstructures in an ultrahigh carbon steel
    Zhang, H. and Ponge, D. and Raabe, D.
    Materials Science and Engineering A 612 (2014)
    Here we present an approach to design a ferrite-based quadplex microstructure (ferrite/martensite/carbide/austenite) using a lean alloyed Mn-Si-Cr-Al ultrahigh carbon steel. The material has 1500MPa tensile strength and 11% elongation. The thermomechanical processing includes two main steps, namely, first, the formation of a ferrite plus carbide duplex microstructure by warm rolling below Ae1; and second, annealing just above Ae1 for a short time (~20min). The quadplex microstructure consists of 57vol% ultrafine ferrite (mean grain size ~1.5μm), 29vol% martensite, 12vol% spherical carbide and 2vol% austenite. Fracture analysis after tensile deformation reveals a mixed ductile and brittle failure mode without necking. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and dilatometry tests were conducted to map the microstructure characteristics and the contribution of each phase to the overall deformation. © 2014 Elsevier B.V.
    view abstract10.1016/j.msea.2014.06.023
  • Dislocation density distribution around an indent in single-crystalline nickel: Comparing nonlocal crystal plasticity finite-element predictions with experiments
    Reuber, C. and Eisenlohr, P. and Roters, F. and Raabe, D.
    Acta Materialia 71 (2014)
    We present a physics-based constitutive model of dislocation glide in metals that explicitly accounts for the redistribution of dislocations due to their motion. The model parameterizes the complex microstructure by dislocation densities of edge and screw character, which either occur with monopolar properties, i.e. a single dislocation with positive or negative line sense, or with dipolar properties, i.e. two dislocations of opposite line sense combined. The advantage of the model lies in the description of the dislocation density evolution, which comprises the usual rate equations for dislocation multiplication and annihilation, and formation and dissociation of dislocation dipoles. Additionally, the spatial redistribution of dislocations by slip is explicitly accounted for. This is achieved by introducing an advection term for the dislocation density that turns the evolution equations for the dislocation density from ordinary into partial differential equations. The associated spatial gradients of the dislocation slip render the model nonlocal. The model is applied to wedge indentation in single-crystalline nickel. The simulation results are compared to published experiments (Kysar et al., 2010) in terms of the spatial distribution of lattice rotations and geometrically necessary dislocations. In agreement with experiment, the predicted dislocation fluxes lead to accumulation of geometrically necessary dislocations around a vertical geometrical border with a high orientation gradient below the indenter that is decisive for the overall plastic response. A local model variant without dislocation transport is not able to predict the influence of this geometrical transition zone correctly and is shown to behave markedly softer. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.03.012
  • Ductility improvement of Mg alloys by solid solution: Ab initio modeling, synthesis and mechanical properties
    Sandlöbes, S. and Pei, Z. and Friák, M. and Zhu, L.-F. and Wang, F. and Zaefferer, S. and Raabe, D. and Neugebauer, J.
    Acta Materialia 70 (2014)
    The I1 intrinsic stacking fault energy (I1 SFE) serves as an alloy design parameter for ductilizing Mg alloys. In view of this effect we have conducted quantum-mechanical calculations for Mg15X solid-solution crystals (X = Dy, Er, Gd, Ho, Lu, Sc, Tb, Tm, Nd, Pr, Be, Ti, Zr, Zn, Tc, Re, Co, Ru, Os, Tl). We find that Y, Sc and all studied lanthanides reduce the I1 SFE and render hexagonal closed-packed (hcp) and double hcp phases thermodynamically, structurally and elastically similar. Synthesis, experimental testing and characterization of some of the predicted key alloys (Mg-3Ho, Mg-3Er, Mg-3Tb, Mg-3Dy) indeed confirm reduced I1 SFEs and significantly improved room-temperature ductility by up to 4-5 times relative to pure Mg, a finding that is attributed to the higher activity of non-basal dislocation slip. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.02.011
  • Effect of retained beta layer on slip transmission in Ti-6Al-2Zr-1Mo-1V near alpha titanium alloy during tensile deformation at room temperature
    He, D. and Zhu, J. and Zaefferer, S. and Raabe, D.
    Materials and Design 56 (2014)
    Slip is the main plastic deformation mechanism in titanium alloys at room temperature. This is especially so for near alpha titanium alloy like Ti-6Al-2Zr-1Mo-1V, which contains low beta stabilizing and high aluminum (alpha stabilizing) element additions. The effects of retained beta layers on slip transmission across α/β interfaces in Ti-6Al-2Zr-1Mo-1V during tensile deformation have been studied in the current work. High resolution scanning electron microscopy (HR-SEM) and electron backscatter diffraction (EBSD) techniques were used to study the deformation microstructure. The results indicate that the full Burgers crystal orientation relationship (OR) between the α and the thin retained β phase layers facilitates slip transition but is not the necessary requirement/restriction. Some novel slip trace morphologies that are characterized by deflection and bifurcation (fork-like morphology) are revealed in the retained β layers between two abutting α grains. The possible reasons for these different slip transmission patterns are analyzed by EBSD results and a schematic model is proposed. © 2013 Elsevier Ltd.
    view abstract10.1016/j.matdes.2013.12.018
  • Effect of strain rate on twinning in a Zr alloy
    Kapoor, R. and Sarkar, A. and Singh, J. and Samajdar, I. and Raabe, D.
    Scripta Materialia 74 (2014)
    Zr-1Nb alloy uniaxially compressed at room temperature at 10-2 and 103 s-1 exhibited twinning and a three-stage strain-hardening behavior. At 103 s-1 the twin fraction initially increased to 0.3, decreasing to 0.02 at higher strains. Despite the difference in texture at intermediate strains, the final texture was similar at both strain rates. The increasing strain-hardening rate in the second stage was attributed to strengthening from grain boundaries and dislocations, and softening from reorientation due to twinning. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2013.10.025
  • Elemental partitioning and mechanical properties of Ti- and Ta-containing Co-Al-W-base superalloys studied by atom probe tomography and nanoindentation
    Povstugar, I. and Choi, P.-P. and Neumeier, S. and Bauer, A. and Zenk, C.H. and Göken, M. and Raabe, D.
    Acta Materialia 78 (2014)
    Elemental partitioning and hardness in Ti- and Ta-containing Co-base superalloys, strengthened by γ′-Co3(Al, W) precipitates, have been studied by local measurements. Using atom probe tomography, we detect strong partitioning of W (partitioning coefficients from 2.4 to 3.4) and only slight partitioning of Al (partitioning coefficients ≤1.1) to the γ′-Co3(Al, W) phase. Al segregates to the γ/γ′ phase boundaries, whereas W is depleted at the γ side of the boundaries after aging at 900 °C and slow air cooling. This kind of Al segregation and W depletion is much less pronounced when water quenching is applied. As a result, these effects are considered to be absent at high temperatures and therefore should not influence the creep properties. Ti and Ta additions are found to strongly partition to the γ′ phase and greatly increase the γ′ volume fraction. Our results indicate that the alloying elements Al, W, Ti and Ta all occupy the B sublattice of the A 3B structure (L12 type) and affect the partitioning behavior of each other. Nanoindentation measurements show that Ta also increases the hardness of the γ′ phase, while the hardness of the γ channels remains nearly constant in all alloys. The change in hardness of the γ′ phase can be ascribed to the substitution of Al and W atoms by Ti and/or Ta. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.06.020
  • Enhanced superplasticity in an Al-alloyed multicomponent Mn-Si-Cr-C steel
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Choi, P. and Tasan, C.C. and Raabe, D.
    Acta Materialia 63 (2014)
    Excellent superplasticity (elongation ∼720%) is observed in a novel multi-component (Mn-S-Cr-Al alloyed) ultrahigh carbon steel during tensile testing at a strain rate of 2 × 10-3 s-1 and a temperature of 1053 K (just above the equilibrium austenite-pearlite transformation temperature). In order to understand superplasticity in this material and its strong Al dependence, the deformation-induced microstructure evolution is characterized at various length scales down to atomic resolution, using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, energy-dispersive X-ray spectroscopy and atom probe tomography. The results reveal that 1 wt.% Al addition influences various microprocesses during deformation, e.g. it impedes Ostwald ripening of carbides, carbide dissolution, austenite nucleation and growth and void growth. As a result, the size of the austenite grains and voids remains relatively fine (< 10 μm) during superplastic deformation, and fine-grained superplasticity is enabled without premature failure. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.10.034
  • Erratum: Shear-induced mixing governs codeformation of crystalline- amorphous nanolaminates (Physical Review Letters (2014) 113 (035501))
    Guo, W. and Jägle, E.A. and Choi, P.-P. and Yao, J. and Kostka, A. and Schneider, J.M. and Raabe, D.
    Physical Review Letters 113 (2014)
    view abstract10.1103/PhysRevLett.113.069903
  • Grain boundary characterization in multicrystalline silicon using joint EBSD, EBIC, and atom probe tomography
    Stoffers, A. and Cojocaru-Miredin, O. and Breitenstein, O. and Seifert, W. and Zaefferer, S. and Raabe, D.
    2014 Ieee 40th Photovoltaic Specialist Conference (pvsc) (2014)
    The efficiency of multicrystalline silicon solar cells suffers from the presence of extended defects like dislocations and grain boundaries. In fact, the defects themselves do not implicitly have to be harmful, but their interaction with impurities makes them detrimental for the cell efficiencies. Here, we present a systematic method to correlate the grain boundary charge recombination activity with local grain boundary properties and the site specific segregation information. For that, electron beam induced current is used to characterize the recombination activity at the grain boundaries, while electron backscatter diffraction is used to map the grain boundary crystallography. Atom probe tips containing the desired grain boundary are cut by using a novel site-specific sample preparation. Finally, atom probe tomography is used to reveal the 3D distribution of the impurities at the selected grain boundary. In conclusion, this work is one of the first studies based on understanding the correlation between the charge recombination activity and structural as well as chemical properties at grain boundaries in multicrystalline silicon solar cells.
    view abstract10.1109/PVSC.2014.6925089
  • Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces
    Raabe, D. and Herbig, M. and Sandlöbes, S. and Li, Y. and Tytko, D. and Kuzmina, M. and Ponge, D. and Choi, P.-P.
    Current Opinion in Solid State and Materials Science 18 (2014)
    Grain boundaries influence mechanical, functional, and kinetic properties of metallic alloys. They can be manipulated via solute decoration enabling changes in energy, mobility, structure, and cohesion or even promoting local phase transformation. In the approach which we refer here to as 'segregation engineering' solute decoration is not regarded as an undesired phenomenon but is instead utilized to manipulate specific grain boundary structures, compositions and properties that enable useful material behavior. The underlying thermodynamics follow the adsorption isotherm. Hence, matrix-solute combinations suited for designing interfaces in metallic alloys can be identified by considering four main aspects, namely, the segregation coefficient of the decorating element; its effects on interface cohesion, energy, structure and mobility; its diffusion coefficient; and the free energies of competing bulk phases, precipitate phases or complexions. From a practical perspective, segregation engineering in alloys can be usually realized by a modest diffusion heat treatment, hence, making it available in large scale manufacturing. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.cossms.2014.06.002
  • High strength and ductile low density austenitic FeMnAlC steels: Simplex and alloys strengthened by nanoscale ordered carbides
    Gutierrez-Urrutia, I. and Raabe, D.
    Materials Science and Technology (United Kingdom) 30 (2014)
    We introduce the alloy design concepts of high performance austenitic FeMnAlC steels, namely, Simplex and alloys strengthened by nanoscale ordered k-carbides. Simplex steels are characterised by an outstanding strain hardening capacity at room temperature. This is attributed to the multiple stage strain hardening behaviour associated to dislocation substructure refinement and subsequent activation of deformation twinning, which leads to a steadily increase of the strain hardening. Al additions higher that 5 wt-% promote the precipitation of nanoscale L912 ordered precipitates (so called k-carbides) resulting in high strength (yield stress ∼ 1.0 GPa) and ductile (elongation to fracture 7sim; 30%) steels. Novel insights into dislocation-particle interactions in a Fe- 30.5Mn-8.0Al-1.2C (wt-%) steel strengthened by nanoscale k-carbides are discussed. © 2014 Institute of Materials, Minerals and Mining.
    view abstract10.1179/1743284714Y.0000000515
  • Hydrogen embrittlement associated with strain localization in a precipitation-hardened Fe-Mn-Al-C light weight austenitic steel
    Koyama, M. and Springer, H. and Merzlikin, S.V. and Tsuzaki, K. and Akiyama, E. and Raabe, D.
    International Journal of Hydrogen Energy 39 (2014)
    Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2013.12.171
  • Hydrogen-assisted decohesion and localized plasticity in dual-phase steel
    Koyama, M. and Tasan, C.C. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Acta Materialia 70 (2014)
    Hydrogen embrittlement affects high-strength ferrite/martensite dual-phase (DP) steels. The associated micromechanisms which lead to failure have not been fully clarified yet. Here we present a quantitative micromechanical analysis of the microstructural damage phenomena in a model DP steel in the presence of hydrogen. A high-resolution scanning electron microscopy-based damage quantification technique has been employed to identify strain regimes where damage nucleation and damage growth take place, both with and without hydrogen precharging. The mechanisms corresponding to these regimes have been investigated by employing post-mortem electron channeling contrast imaging and electron backscatter diffraction analyses, as well as additional in situ deformation experiments. The results reveal that damage nucleation mechanism (i.e. martensite decohesion) and the damage growth mechanisms (e.g. interface decohesion) are both promoted by hydrogen, while the crack-arresting capability of the ferrite is significantly reduced. The observations are discussed on the basis of the hydrogen-enhanced decohesion and hydrogen-enhanced localized plasticity mechanisms. We discuss corresponding microstructure design strategies for better hydrogen-related damage tolerance of DP steels. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.01.048
  • Impact of nanodiffusion on the stacking fault energy in high-strength steels
    Hickel, T. and Sandlöbes, S. and Marceau, R.K.W. and Dick, A. and Bleskov, I. and Neugebauer, J. and Raabe, D.
    Acta Materialia 75 (2014)
    A key requirement of modern steels - the combination of high strength and high deformability - can best be achieved by enabling a local adaptation of the microstructure during deformation. A local hardening is most efficiently obtained by a modification of the stacking sequence of atomic layers, resulting in the formation of twins or martensite. Combining ab initio calculations with in situ transmission electron microscopy, we show that the ability of a material to incorporate such stacking faults depends on its overall chemical composition and, importantly, the local composition near the defect, which is controlled by nanodiffusion. Specifically, the role of carbon for the stacking fault energy in high-Mn steels is investigated. Consequences for the long-term mechanical properties and the characterisation of these materials are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.04.062
  • In situ observation of collective grain-scale mechanics in Mg and Mg-rare earth alloys
    Wang, F. and Sandlöbes, S. and Diehl, M. and Sharma, L. and Roters, F. and Raabe, D.
    Acta Materialia 80 (2014)
    The microstructure evolution of pure Mg and two Mg-rare-earth alloys (Mg-3 wt.% Dy and Mg-3 wt.% Er) was studied during in situ compression tests by electron backscatter diffraction and electron channelling contrast imaging. Strain localization and the formation of an early stage shear band ("pre-shear band") were observed in pure Mg during compressive deformation below 5% engineering strain. In the experiments percolative grain clusters with prevalent basal slip as a precursor for shear band formation was observed. This collective grain-cluster shear behaviour was analysed in more detail using crystal plasticity simulations, revealing a percolation of intense basal slip activity across grain boundaries as the mechanism for shear band initiation. Plane trace analysis, Schmid factor calculation and deformation transfer analysis at the grain boundaries were performed for the activated twins. It appears that many activated tension twins exhibit pronounced non-Schmid behaviour. Twinning appears to be a process of accommodating local strain rather than a response to macroscopic strain. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.07.048
  • Influence of the dislocation core on the glide of the 〈1 1 〉{1 1 0} edge dislocation in bcc-iron: An embedded atom method study
    Hafez Haghighat, S.M. and Von Pezold, J. and Race, C.P. and Körmann, F. and Friák, M. and Neugebauer, J. and Raabe, D.
    Computational Materials Science 87 (2014)
    Four commonly used embedded atom method potentials for bcc-Fe by Ackland et al. (1997), Mendelev et al. (2003), Chiesa et al. (2009) and Malerba et al. (2010) are critically evaluated with respect to their description of the edge dislocation core structure and its dynamic behavior. Our results allow us to quantify the transferability of the various empirical potentials in the study of the 〈1 1 〉{1 1 0} edge dislocation core structure and kinetics. Specifically, we show that the equilibrium dislocation core structure is a direct consequence of the shape of the extended gamma surface. We further find that there is a strong correlation between the structure of the edge dislocation core and its glide stress. An in depth analysis of the dislocation migration results reveals that the dominant migration mechanism is via progressing straight line segments of the dislocation. This is further confirmed by the excellent qualitative agreement of nudged elastic band calculations of the Peierls barrier with the dynamically determined critical shear stresses. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.commatsci.2014.02.031
  • Integrated experimental-simulation analysis of stress and strain partitioning in multiphase alloys
    Tasan, C.C. and Diehl, M. and Yan, D. and Zambaldi, C. and Shanthraj, P. and Roters, F. and Raabe, D.
    Acta Materialia 81 (2014)
    The mechanical response of multiphase alloys is governed by the microscopic strain and stress partitioning behavior among microstructural constituents. However, due to limitations in the characterization of the partitioning that takes place at the submicron scale, microstructure optimization of such alloys is typically based on evaluating the averaged response, referring to, for example, macroscopic stress-strain curves. Here, a novel experimental-numerical methodology is introduced to strengthen the integrated understanding of the microstructure and mechanical properties of these alloys, enabling joint analyses of deformation-induced evolution of the microstructure, and the strain and stress distribution therein, down to submicron resolution. From the experiments, deformation-induced evolution of (i) the microstructure, and (ii) the local strain distribution are concurrently captured, employing in situ secondary electron imaging and electron backscatter diffraction (EBSD) (for the former), and microscopic-digital image correlation (for the latter). From the simulations, local strain as well as stress distributions are revealed, through 2-D full-field crystal plasticity (CP) simulations conducted with an advanced spectral solver suitable for heterogeneous materials. The simulated model is designed directly from the initial EBSD measurements, and the phase properties are obtained by additional inverse CP simulations of nanoindentation experiments carried out on the original microstructure. The experiments and simulations demonstrate good correlation in the proof-of-principle study conducted here on a martensite-ferrite dual-phase steel, and deviations are discussed in terms of limitations of the techniques involved. Overall, the presented integrated computational materials engineering approach provides a vast amount of well-correlated structural and mechanical data that enhance our understanding as well as the design capabilities of multiphase alloys. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.07.071
  • Interfacial dislocation motion and interactions in single-crystal superalloys
    Liu, B. and Raabe, D. and Roters, F. and Arsenlis, A.
    Acta Materialia 79 (2014)
    The early stage of high-temperature low-stress creep in single-crystal superalloys is characterized by the rapid development of interfacial dislocation networks. Although interfacial motion and dynamic recovery of these dislocation networks have long been expected to control the subsequent creep behavior, direct observation and hence in-depth understanding of such processes has not been achieved. Incorporating recent developments of discrete dislocation dynamics models, we simulate interfacial dislocation motion in the channel structures of single-crystal superalloys, and investigate how interfacial dislocation motion and dynamic recovery are affected by interfacial dislocation interactions and lattice misfit. Different types of dislocation interactions are considered: self, collinear, coplanar, Lomer junction, glissile junction, and Hirth junction. The simulation results show that strong dynamic recovery occurs due to the short-range reactions of collinear annihilation and Lomer junction formation. The misfit stress is found to induce and accelerate dynamic recovery of interfacial dislocation networks involving self-interaction and Hirth junction formation, but slow down the steady interfacial motion of coplanar and glissile junction forming dislocation networks. The insights gained from these simulations on high-temperature low-stress creep of single-crystal superalloys are also discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.06.048
  • Intrinsic and extrinsic size effects in the deformation of amorphous CuZr/nanocrystalline Cu nanolaminates
    Guo, W. and Jägle, E. and Yao, J. and Maier, V. and Korte-Kerzel, S. and Schneider, J.M. and Raabe, D.
    Acta Materialia 80 (2014)
    Introducing a soft crystalline phase into an amorphous alloy can promote the compound's ductility. Here we synthesized multilayered nanolaminates consisting of alternating amorphous Cu54Zr46 and nanocrystalline Cu layers. The Cu layer thickness was systematically varied in different samples. Mechanical loading was imposed by nanoindentation and micropillar compression. Increasing the Cu layer thickness from 10 to 100 nm led to a transition from sharp, cross-phase shear banding to gradual bending and co-deformation of the two layer types (amorphous/nanocrystalline). Specimens with a sequence of 100 nm amorphous Cu54Zr46 and 50 nm Cu layers show a compressive flow stress of 2.57 ± 0.21 GPa, matching the strength of pure CuZr metallic glass, hence exceeding the linear rule of mixtures. In pillar compression, 40% strain without fracture was achieved by the suppression of percolative shear band propagation. The results show that inserting a ductile nanocrystalline phase into a metallic glass prevents catastrophic shear banding. The mechanical response of such nanolaminates can be tuned by adjusting the layer thickness. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.07.027
  • Kikuchi bandlet method for the accurate deconvolution and localization of Kikuchi bands in Kikuchi diffraction patterns
    Ram, F. and Zaefferer, S. and Raabe, D.
    Journal of Applied Crystallography 47 (2014)
    In order to retrieve crystallographic information from an electron backscatter Kikuchi diffraction pattern, its Kikuchi bands have to be localized. One of the main reasons for the limited precision of the present Kikuchi band localization methods is that the diffuse edges of a Kikuchi band are convoluted by many other Kikuchi bands that intersect them. To improve the localization accuracy, Kikuchi bands have to be deconvoluted. In this article, a new method for the deconvolution and localization of Kikuchi bands is presented. The deconvolution is based on the fact that, in a Kikuchi pattern, there are a number of Kikuchi bands that are not parallel to the bands that intersect them. It is performed in Fourier space. After deconvolution, localization is carried out by a quantitative shape analysis of the intensity profiles of the deconvoluted Kikuchi bands. Using the introduced method, for a real electron backscatter Kikuchi diffraction pattern with 45° half capture angle and 0.12°per pixel maximum scale factor, the characteristic hyperbolic features of the Kikuchi bands can be localized with a precision of better than 0.1°in reflection angle. © 2014 International Union of Crystallography.
    view abstract10.1107/S1600576713030446
  • Large recovery strain in Fe-Mn-Si-based shape memory steels obtained by engineering annealing twin boundaries
    Wen, Y.H. and Peng, H.B. and Raabe, D. and Gutierrez-Urrutia, I. and Chen, J. and Du, Y.Y.
    Nature Communications 5 (2014)
    Shape memory alloys are a unique class of materials that can recover their original shape upon heating after a large deformation. Ti-Ni alloys with a large recovery strain are expensive, while low-cost conventional processed Fe-Mn-Si-based steels suffer from a low recovery strain (<3%). Here we show that the low recovery strain results from interactions between stress-induced martensite and a high density of annealing twin boundaries. Reducing the density of twin boundaries is thus a critical factor for obtaining a large recovery strain in these steels. By significantly suppressing the formation of twin boundaries, we attain a tensile recovery strain of 7.6% in an annealed cast polycrystalline Fe-20.2Mn-5.6Si-8.9Cr-5.0Ni steel (weight%). Further attractiveness of this material lies in its low-cost alloying components and simple synthesis-processing cycle consisting only of casting plus annealing. This enables these steels to be used at a large scale as structural materials with advanced functional properties © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms5964
  • Long-term microstructural stability of oxide-dispersion strengthened Eurofer steel annealed at 800 °c
    Zilnyk, K.D. and Sandim, H.R.Z. and Bolmaro, R.E. and Lindau, R. and Möslang, A. and Kostka, A. and Raabe, D.
    Journal of Nuclear Materials 448 (2014)
    Oxide-dispersion strengthened ferritic martensitic steels such as ODS-Eurofer grade are good candidates for structural applications in future fusion power reactors. Long-term annealing treatments in vacuum were carried out in cold-rolled samples (80% reduction in thickness) from 1 h up to 4320 h (6 months) at 800 °C, i.e. the maximum temperature in the ferritic phase field, to follow its softening behavior. The microstructural stability of this steel was mapped using several characterization techniques including scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, Vickers microhardness testing, X-ray diffraction texture measurements, low-temperature electrical resistivity, and magnetic coercive field measurements. ODS-Eurofer steel displays good microstructural stability. Discontinuous recrystallization occurs at the early stages of annealing resulting in a low volume fraction of recrystallized grains. Extended recovery is the predominant softening mechanism at this temperature for longer times. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jnucmat.2014.01.032
  • Microstructure-magnetic property relations in grain-oriented electrical steels: Quantitative analysis of the sharpness of the Goss orientation
    Gutierrez-Urrutia, I. and Böttcher, A. and Lahn, L. and Raabe, D.
    Journal of Materials Science 49 (2014)
    We have investigated microstructure-magnetic property relations in several high-permeability grain-oriented steels by large-area electron backscatter diffraction (EBSD) mapping. The evaluation of the Goss sharpness determined from orientation distribution functions provides a more quantitative and detailed texture analysis than conventional pole figure estimates. Accordingly, it results in a more quantitative treatment of texture-magnetic property relations. The analysis of crystal-orientation distribution by a statistical binning method provides further insights into the occurrence of secondary texture components. Specifically, we have found the formation of two weak secondary components rotated about 4°-6° and 8°-10° from the texture center, respectively. These texture components have a strong influence on the magnetic polarization but a small influence on the core loss. We explain this effect in terms of the magneto-crystalline anisotropy energy of a cubic crystal and magnetic domain-microstructure feature interactions. © 2013 Springer Science+Business Media New York.
    view abstract10.1007/s10853-013-7701-2
  • Modelling of dendritic growth during alloy solidification under natural convection
    Zhu, M. and Sun, D. and Pan, S. and Zhang, Q. and Raabe, D.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    A two-dimensional (2D) lattice Boltzmann method (LBM)-cellular automaton model is presented to investigate the dendritic growth of binary alloys in the presence of natural convection. The kinetic-based LBM is adopted to calculate the transport phenomena by the evolution of distribution functions of moving pseudo-particles. To numerically solve natural convection thermal and solute transport simultaneously, three sets of distribution functions are employed in conjunction with the lattice Bhatnagar-Gross-Krook scheme. Based on the LBM calculated local temperature and concentration at the solid/liquid interface, the kinetics of dendritic growth is determined according to a local solute equilibrium approach. Thus, the physics of a complete time-dependent interaction of natural convection, thermal and solutal transport, and dendritic growth during alloy solidification is embedded in the model. Model validation is performed by comparing the simulated results with literature data and analytical predictions. The model is applied to simulate dendritic growth in binary alloys under the influence of natural convection. The effects of Rayleigh numbers and initial undercooling on dendrite growth are investigated. The results show that natural buoyancy flow, induced by thermal and solutal gradients under gravity, transports the heat and solute from the lower region to the upper region. The dendritic growth is thus accelerated in the downward direction, whereas it is inhibited in the upward direction, yielding asymmetrical dendrite patterns. Increasing the Rayleigh number and undercooling will enhance and reduce, respectively, the influence of natural flow on the dendritic growth. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0965-0393/22/3/034006
  • Multiple reentrant glass transitions in confined hard-sphere glasses
    Mandal, S. and Lang, S. and Gross, M. and Oettel, M. and Raabe, D. and Franosch, T. and Varnik, F.
    Nature Communications 5 (2014)
    Glass-forming liquids exhibit a rich phenomenology upon confinement. This is often related to the effects arising from wall-fluid interactions. Here we focus on the interesting limit where the separation of the confining walls becomes of the order of a few particle diameters. For a moderately polydisperse, densely packed hard-sphere fluid confined between two smooth hard walls, we show via event-driven molecular dynamics simulations the emergence of a multiple reentrant glass transition scenario upon a variation of the wall separation. Using thermodynamic relations, this reentrant phenomenon is shown to persist also under constant chemical potential. This allows straightforward experimental investigation and opens the way to a variety of applications in micro-and nanotechnology, where channel dimensions are comparable to the size of the contained particles. The results are in line with theoretical predictions obtained by a combination of density functional theory and the mode-coupling theory of the glass transition. © 2014 Macmillan Publishers Limited.
    view abstract10.1038/ncomms5435
  • Nano-scale characterization of thin-film solar cells
    Schwarz, T. and Cojocaru-Mirédin, O. and Choi, P.-P. and Lämmle, A. and Würz, R. and Mousel, M. and Redinger, A. and Siebentritt, S. and Botti, S. and Raabe, D.
    Microscopy and Microanalysis 20 (2014)
    view abstract10.1017/S1431927614003699
  • New insights into the austenitization process of low-alloyed hypereutectoid steels: Nucleation analysis of strain-induced austenite formation
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Raabe, D.
    Acta Materialia 80 (2014)
    Austenite formation, which originated from a fined-grained ferrite plus carbide microstructure, was observed during tensile testing at 973 K (60 K below Ae1, the equilibrium austenite-pearlite transformation temperature). Scanning electron microscopy, electron backscatter diffraction and atom probe tomography results reveal the mechanism of austenitic transformation below Ae1. The initial fine-grained microstructure, in combination with the warm deformation process, determines the occurrence of strain-induced austenite formation below Ae1. The initial fine-grained microstructure essentially contains a higher dislocation density to facilitate the formation of Cottrell atmospheres and a larger area fraction of ferrite/carbide interfaces which serve as austenite nucleation sites. The warm deformation promotes the Ostwald ripening process and the increase in dislocation density, and hence promotes the accumulation of local high carbon concentrations in the form of Cottrell atmospheres to reach a sufficiently high thermodynamic driving force for austenite nucleation. The critical carbon concentration required for the nucleation of austenite was calculated using classical nucleation theory, which correlated well with the experimental observations. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.07.073
  • On the spheroidized carbide dissolution and elemental partitioning in high carbon bearing steel 100Cr6
    Song, W. and Choi, P.-P. and Inden, G. and Prahl, U. and Raabe, D. and Bleck, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 45 (2014)
    We report on the characterization of high carbon bearing steel 100Cr6 using electron microscopy and atom probe tomography in combination with multi-component diffusion simulations. Scanning electron micrographs show that around 14 vol pct spheroidized carbides are formed during soft annealing and only 3 vol pct remain after dissolution into the austenitic matrix through austenitization at 1123 K (850 °C) for 300 seconds. The spheroidized particles are identified as (Fe, Cr)3C by transmission electron microscopy. Atom probe analysis reveals the redistribution and partitioning of the elements involved, i.e., C, Si, Mn, Cr, Fe, in both, the spheroidized carbides and the bainitic matrix in the sample isothermally heat-treated at 773 K (500 °C) after austenitization. Homogeneous distribution of C and a Cr gradient were detected within the spheroidized carbides. Due to its limited diffusivity in (Fe, Cr) 3C, Cr exhibits a maximum concentration at the surface of spheroidized carbides (16 at. pct) and decreases gradually from the surface towards the core down to about 2 at. pct. The atom probe results also indicate that the partially dissolved spheroidized carbides during austenitization may serve as nucleation sites for intermediate temperature cementite within bainite, which results in a relatively softer surface and harder core in spheroidized particles. This microstructure may contribute to the good wear resistance and fatigue properties of the steel. Good agreement between DICTRA simulations and experimental composition profiles is obtained by an increase of mobility of the substitutional elements in cementite by a factor of five, compared to the mobility in the database MOBFE2. © The Minerals, Metals & Materials Society and ASM International 2013.
    view abstract10.1007/s11661-013-2048-5
  • Plastic anisotropy of electro-deposited pure α-iron with sharp crystallographic <1 1 1>// texture in normal direction: Analysis by an explicitly dislocation-based crystal plasticity model
    Alankar, A. and Field, D.P. and Raabe, D.
    International Journal of Plasticity 52 (2014)
    We present a single crystal plasticity model based on edge and screw dislocation densities for body centered cubic (bcc) crystals. In a bcc crystal screw dislocations experience high lattice friction due to their non-planar core. Hence, they have much slower velocity compared to edge dislocations. This phenomenon is modeled by accounting for the motion of screw dislocations via nucleation and expansion of kink-pairs. The model, embedded as a constitutive law into a crystal plasticity framework, is able to predict the crystallographic texture of a bcc polycrystal subjected to 70%, 80% and 90% thickness reduction. We perform a parametric study based on the velocities of edge and screw dislocations to analyze the effect on plastic anisotropy of electro-deposited pure iron with long needle-shaped grains having sharp crystallographic <1 1 1>//ND texture (ND: normal direction). The model shows a large change in the r-value (Lankford value, planar anisotropy ratio) for pure iron when the texture changes from random to <1 1 1>//ND. For different simulated cases where the crystallites have an orientation deviation of 1, 3 and 5, respectively, from the ideal <1 1 1>//ND axis, the simulations predict r-values between 4.0 and 7.0 which is in excellent agreement with data observed in experiments by Yoshinaga et al. (ISIJ Intern.; 48 (2008) 667-670). For these specific orientations of grains, we also model the effect of long needle shaped grains via a procedure that excludes dislocation annihilation. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijplas.2013.03.006
  • Precipitation and austenite reversion behavior of a maraging steel produced by selective laser melting
    Jägle, E.A. and Choi, P.-P. and Van Humbeeck, J. and Raabe, D.
    Journal of Materials Research 29 (2014)
    Materials produced by selective laser melting (SLM) experience a thermal history that is markedly different from that encountered by conventionally produced materials. In particular, a very high cooling rate from the melt is combined with cyclical reheating upon deposition of subsequent layers. Using atom-probe tomography (APT), we investigated how this nonconventional thermal history influences the phase-transformation behavior of maraging steels (Fe-18Ni-9Co-3.4Mo-1.2Ti) produced by SLM. We found that despite the "intrinsic heat treatment" and the known propensity of maraging steels for rapid clustering and precipitation, the material does not show any sign of phase transformation in the as-produced state. Upon aging, three different types of precipitates, namely (Fe,Ni,Co)3(Ti,Mo), (Fe,Ni,Co)3(Mo,Ti), and (Fe,Ni,Co)7Mo6 (μ phase), were observed as well as martensite-to-austenite reversion around regions of the retained austenite. The concentration of the newly formed phases as quantified by APT closely matches thermodynamic equilibrium calculations. Copyright © 2014 Materials Research Society.
    view abstract10.1557/jmr.2014.204
  • Quantum-mechanical study of single-crystalline and polycrystalline elastic properties of Mg-substituted calcite crystals
    Friák, M. and Zhu, L.-F. and Lymperakis, L. and Titrian, H. and Aydin, U. and Janus, A.M. and Fabritius, H.-O. and Ziegler, A. and Nikolov, S. and Hemzalová, P. and Raabe, D. and Neugebauer, J.
    Key Engineering Materials 592-593 (2014)
    We use quantum-mechanical calculations to study single-crystalline elastic properties of (Ca,Mg)CO3 crystals with concentrations ranging from calcite CaCO3 to magnesite MgCO3. By analyzing results for a dense set of distributions of Ca and Mg atoms within 30-atom supercells, our theoretical study shows that those atomic configurations, that minimize the total energy for a given concentration, are characterized by elastic constants that either increase with the Mg content or remain nearly constants. Employing these ab initio calculated single-crystalline elastic parameters, the polycrystalline elastic properties of (Ca,Mg)CO3 aggregates are determined using a mean-field self-consistent homogenization method. The computed integral elastic moduli (bulk and shear) show a significant stiffening impact of Mg atoms on calcite crystals. Our analysis also demonstrates that it is not advantageous to use a granular two-phase composite of stoichiometric calcite and magnesite instead of substituting individual Ca and Mg atoms. Such two-phase aggregates are not significantly thermodynamically favorable and do not offer any strong additional stiffening effect. © (2014) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/KEM.592-593.335
  • Recovery and Recrystallization: Phenomena, Physics, Models, Simulation
    Raabe, D.
    Physical Metallurgy: Fifth Edition 1 (2014)
    Recovery, recrystallization and grain growth are among the most important metallurgical heat treatment processes to soften cold worked metals and design desired microstructures and textures. Specifically the reduction in grain size can be efficiently achieved by recrystallization. While plastic cold working increases the stored energy of metals, mainly through dislocation accumulation, recovery and specifically recrystallization lead to it reduction. While recovery describes the gradual re-ordering and annihilation of the stored dislocations, primary recrystallization proceeds discontinuously by the formation and motion of high angle grain boundaries which discontinuously sweep the deformation substructure. Grain growth describes the process of competitive capillary driven coarsening of the average grain size. This chapter reviews the main mechanisms, lattice defects, and driving forces associated with recovery, recrystallization and grain growth and provides an introduction to the simulation of these phenomena. © 2014 Elsevier B.V. All rights reserved..
    view abstract10.1016/B978-0-444-53770-6.00023-X
  • Segregation stabilizes nanocrystalline bulk steel with near theoretical strength
    Li, Y. and Raabe, D. and Herbig, M. and Choi, P.-P. and Goto, S. and Kostka, A. and Yarita, H. and Borchers, C. and Kirchheim, R.
    Physical Review Letters 113 (2014)
    Grain refinement through severe plastic deformation enables synthesis of ultrahigh-strength nanostructured materials. Two challenges exist in that context: First, deformation-driven grain refinement is limited by dynamic dislocation recovery and crystal coarsening due to capillary driving forces; second, grain boundary sliding and hence softening occur when the grain size approaches several nanometers. Here, both challenges have been overcome by severe drawing of a pearlitic steel wire (pearlite: lamellar structure of alternating iron and iron carbide layers). First, at large strains the carbide phase dissolves via mechanical alloying, rendering the initially two-phase pearlite structure into a carbon-supersaturated iron phase. This carbon-rich iron phase evolves into a columnar nanoscaled subgrain structure which topologically prevents grain boundary sliding. Second, Gibbs segregation of the supersaturated carbon to the iron subgrain boundaries reduces their interface energy, hence reducing the driving force for dynamic recovery and crystal coarsening. Thus, a stable cross-sectional subgrain size <10nm is achieved. These two effects lead to a stable columnar nanosized grain structure that impedes dislocation motion and enables an extreme tensile strength of 7 GPa, making this alloy the strongest ductile bulk material known. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.113.106104
  • Shear-induced mixing governs codeformation of crystalline-amorphous nanolaminates
    Guo, W. and Jägle, E.A. and Choi, P.-P. and Yao, J. and Kostka, A. and Schneider, J.M. and Raabe, D.
    Physical Review Letters 113 (2014)
    Deformation of ductile crystalline-amorphous nanolaminates is not well understood due to the complex interplay of interface mechanics, shear banding, and deformation-driven chemical mixing. Here we present indentation experiments on 10 nm nanocrystalline Cu-100 nm amorphous CuZr model multilayers to study these mechanisms down to the atomic scale. By using correlative atom probe tomography and transmission electron microscopy we find that crystallographic slip bands in the Cu layers coincide with noncrystallographic shear bands in the amorphous CuZr layers. Dislocations from the crystalline layers drag Cu atoms across the interface into the CuZr layers. Also, crystalline Cu blocks are sheared into the CuZr layers. In these sheared and thus Cu enriched zones the initially amorphous CuZr layer is rendered into an amorphous plus crystalline nanocomposite. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.113.035501
  • Smaller is less stable: Size effects on twinning vs. transformation of reverted austenite in TRIP-maraging steels
    Wang, M.-M. and Tasan, C.C. and Ponge, D. and Kostka, A. and Raabe, D.
    Acta Materialia 79 (2014)
    Steels containing reverted nanoscale austenite (γRN) islands or films dispersed in a martensitic matrix show excellent strength, ductility and toughness. The underlying microstructural mechanisms responsible for these improvements are not yet understood, but are observed to be strongly connected to the γRN island or film size. Two main micromechanical effects are conceivable in this context, namely: (i) interaction of γRN with microcracks from the matrix (crack blunting or arresting); and (ii) deformation-induced phase transformation of γRN to martensite (TRIP effect). The focus here is on the latter phenomenon. To investigate size effects on γRN transformation independent of other factors that can influence austenite stability (composition, crystallographic orientation, defect density, surrounding phase, etc.), a model (TRIP-maraging steel) microstructure is designed with support from diffusion simulations (using DICTRA software) to have the same, homogeneous chemical composition in all γRN grains. Characterization is conducted by in-situ tension and bending experiments in conjunction with high-resolution electron backscatter diffraction mapping and scanning electron microscopy imaging, as well as post-mortem transmission electron microscopy and synchrotron X-ray diffraction analysis. Results reveal an unexpected "smaller is less stable" effect due to the size-dependent competition between mechanical twinning and deformation-induced phase transformation. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.07.020
  • Strain localization and damage in dual phase steels investigated by coupled in-situ deformation experiments and crystal plasticity simulations
    Tasan, C.C. and Hoefnagels, J.P.M. and Diehl, M. and Yan, D. and Roters, F. and Raabe, D.
    International Journal of Plasticity 63 (2014)
    Ferritic-martensitic dual phase (DP) steels deform spatially in a highly heterogeneous manner, i.e. with strong strain and stress partitioning at the micro-scale. Such heterogeneity in local strain evolution leads in turn to a spatially heterogeneous damage distribution, and thus, plays an important role in the process of damage inheritance and fracture. To understand and improve DP steels, it is important to identify connections between the observed strain and damage heterogeneity and the underlying microstructural parameters, e.g. ferrite grain size, martensite distribution, martensite fraction, etc. In this work we pursue this aim by conducting in-situ deformation experiments on two different DP steel grades, employing two different microscopic-digital image correlation (μDIC) techniques to achieve microstructural strain maps of representative statistics and high-resolution. The resulting local strain maps are analyzed in connection to the observed damage incidents (identified by image post-processing) and to local stress maps (obtained from crystal plasticity (CP) simulations of the same microstructural area). The results reveal that plasticity is typically initiated within "hot zones" with larger ferritic grains and lower local martensite fraction. With increasing global deformation, damage incidents are most often observed in the boundary of such highly plastified zones. High-resolution μDIC and the corresponding CP simulations reveal the importance of martensite dispersion: zones with bulky martensite are more susceptible to macroscopic localization before the full strain hardening capacity of the material is consumed. Overall, the presented joint analysis establishes an integrated computational materials engineering (ICME) approach for designing advanced DP steels. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijplas.2014.06.004
  • Structure of rapidly quenched (Cu0.5Zr0.5) 100-xAgx alloys (x = 0-40 at.%)
    Mattern, N. and Han, J.H. and Pradeep, K.G. and Kim, K.C. and Park, E.M. and Kim, D.H. and Yokoyama, Y. and Raabe, D. and Eckert, J.
    Journal of Alloys and Compounds 607 (2014)
    The influence of Ag addition on the microstructure of rapidly quenched (Cu0.5Zr0.5)100-xAg x melts was investigated (x = 0-40 at.%). Fully glassy alloys were obtained for 0 ≤ x ≤ 20 at.% Ag, which are characterized by a homogeneous microstructure without any indication of phase separation. For 30 ≤ x ≤ 40 at.% Ag a composite structure is formed consisting of fcc-Ag nano-crystallites 5 nm in size and an amorphous matrix phase Cu40Zr40Ag 20. With higher Ag-content the volume fraction of the fcc-Ag phase becomes increased mainly due to crytal growth during quenching. The primary formation of fcc-Ag for 30 ≤ x ≤ 40 at.% Ag is confirmed by the analysis of the microstructure of mold cast bulk samples which were fully crystalline. From the experimental results we conclude that the miscibility gap of the liquid phase of the ternary Ag-Cu-Zr system may occur only for x &gt; 40 at.% Ag. For the bulk glass forming quaternary Cu40Zr40Al 10Ag10 alloy a homogeneous element distribution is observed in accordance with the microstructure of ternary (Cu 0.5Zr0.5)100-xAg x glasses (x = 10, 20 at.%). © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2014.04.047
  • Superplastic Mn-Si-Cr-C duplex and triplex steels: Interaction of microstructure and void formation
    Zhang, H. and Ponge, D. and Raabe, D.
    Materials Science and Engineering A 610 (2014)
    Duplex and triplex microstructures consisting initially of ferrite plus carbide or of martensite, ferrite plus carbide, respectively, can undergo strain induced austenite formation during superplastic deformation at 30K below Ae1 (Ae1: equilibrium pearlite-austenite transformation temperature) and low strain rate (e.g. 2×10-3s-1). The effect leads to excellent superplasticity of the materials (elongation ~500%, flow stress < 50MPa) through fine austenite grains (~10μm). Using a deformation temperature just below Ae1 leads to a weak driving force for both, carbide dissolution and austenite formation. Thereby a sufficient volume fraction of carbides (1-2μm, 15vol%) is located at austenite grain boundaries suppressing austenite grain growth during superplastic deformation. Also, void nucleation and growth in the superplastic regime are slowed down within the newly transformed austenite plus carbide microstructure. In contrast, austenite grains and voids grow fast at a high deformation temperature (120K above Ae1). At a low deformation temperature (130K below Ae1), strain induced austenite formation does not occur and the nucleation of multiple voids at the ferrite-carbide interfaces becomes relevant. The fast growth of grains and voids as well as the formation of multiple voids can trigger premature failure during tensile testing in the superplastic regime. EBSD is used to analyze the microstructure evolution and void formation during superplastic deformation, revealing optimum microstructural and forming conditions for superplasticity of Mn-Si-Cr-C steels. The study reveals that excellent superplasticity can be maintained even at 120K above Ae1 by designing an appropriate initial duplex ferrite plus carbide microstructure. © 2014 Elsevier B.V.
    view abstract10.1016/j.msea.2014.05.061
  • Texture and microstructure evolution during non-crystallographic shear banding in a plane strain compressed Cu-Ag metal matrix composite
    Jia, N. and Raabe, D. and Zhao, X.
    Acta Materialia 76 (2014)
    We studied the texture and microstructure evolution in a plane strain compressed Cu-Ag metal matrix composite (MMC) with a heterophase microstructure using crystal plasticity finite element simulations. Lattice reorientations induced by both crystallographic (dislocation slip and twinning) and non-crystallographic (shear banding) mechanisms are addressed. First, simulations on a polycrystalline composite are made. Quite similar texture trends are observed for the composites and for the individual single-phase materials, namely, copper-type texture components in the Cu phase and brass-type texture components in the Ag phase. This result differs from experimental data that show less copper-type and more brass-type textures in both phases for the composite materials. To explore co-deformation mechanisms that lead to the specific crystallographic textures in the MMC, bicrystal simulations for the composite with specific initial orientation combinations are performed. The bicrystal simulations reproduce the experimentally observed trends of texture evolution in the respective phases of the composite, indicating that the localized stress and strain fields as well as the co-deformation mechanisms within the actual heterophase microstructure are well captured. The modeling shows that to accommodate plastic deformation between adjacent phases in the bicrystals, pronounced shear bands are triggered by stress concentration at the hetero-interfaces. With further deformation the bands penetrate through the phase boundaries and lead to larger lattice rotations. The simulations confirm that the shear banding behavior in heterophase composites is different from that in single-phase metals and the texture evolution in composite materials is strongly influenced by the starting texture, the local constraints exerted from the phase boundaries and the constitutive properties of the abutting phases. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.05.036
  • Texture evolution as determined by in situ neutron diffraction during annealing of iron deformed by equal channel angular pressing
    Sandim, H.R.Z. and Bolmaro, R.E. and Renzetti, R.A. and Sandim, M.J.R. and Hartwig, K.T. and Vogel, S.C. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 45 (2014)
    In situ neutron diffraction experiments were performed to follow the annealing behavior of iron deformed by equal-channel angular pressing at room temperature using route B <inf>c</inf> to a total von Mises strain of ε <inf>vM</inf> = 9.2. The temperature was varied from room temperature to 1223 K (950 °C), while neutron diffraction data for quantitative texture analysis were collected at a given temperature when holding for 5 minutes. Pole figures and orientation distribution function maps from neutron diffraction and electron backscatter diffraction measurements were used to follow the changes in crystallographic texture and grain size during annealing. In situ neutron diffraction experiments allowed understanding and identifying texture-related changes that occur during recrystallization, grain growth, and phase transformation in iron. © 2014 The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-014-2401-3
  • The influence of manganese content on the stacking fault and austenite/ε-martensite interfacial energies in Fe-Mn-(Al-Si) steels investigated by experiment and theory
    Pierce, D.T. and Jiménez, J.A. and Bentley, J. and Raabe, D. and Oskay, C. and Wittig, J.E.
    Acta Materialia 68 (2014)
    The stacking fault and interfacial energies of three transformation- and twinning-induced plasticity steels (TRIP/TWIP) (Fe-22/25/28Mn-3Al-3Si wt.%) were determined by experimental and theoretical methods. Analysis of Shockley partial dislocation configurations in the three alloys using weak-beam dark-field transmission electron microscopy yielded stacking fault energy (SFE) values of 15 ± 3, 21 ± 3 and 39 ± 5 mJ m-2 for alloys with 22, 25 and 28 wt.% Mn, respectively. The experimental SFE includes a coherency strain energy of ∼1-4 mJ m-2, determined by X-ray diffraction, which arises from the contraction in volume of the stacking fault upon the face-centered cubic (fcc) to hexagonal close-packed (hcp) phase transformation. The ideal SFE, computed as the difference between the experimental SFE and the coherency strain energy, is equal to14 ± 3, 19 ± 3 and 35 ± 5 mJ m-2, respectively. These SFE values were used in conjunction with a thermodynamic model developed in the present work to calculate the free energy difference of the fcc and hcp phases and to determine a probable range for the fcc/hcp interfacial energy in the three Fe-Mn-(Al-Si) steels investigated. In addition, the interfacial energies of three Fe-18Mn-0.6C-0/1.5(Al/Si) TWIP and five Fe-16/18/20/22/25Mn binary alloys were also determined from experimental data in the literature. The interfacial energy ranged from 8 to 12 mJ m-2 in the TRIP/TWIP steels and from 15 to 33 mJ m-2 in the binary Fe-Mn alloys. The interfacial energy exhibits a strong dependence on the difference in Gibbs energy of the individual fcc and hcp phases. Accordingly, an empirical description of this parameter is proposed to improve the accuracy of thermodynamic SFE calculations.© 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.01.001
  • The maximum separation cluster analysis algorithm for atom-probe tomography: Parameter determination and accuracy
    Jägle, E.A. and Choi, P.-P. and Raabe, D.
    Microscopy and Microanalysis 20 (2014)
    Atom-probe tomography is a materials characterization method ideally suited for the investigation of clustering and precipitation phenomena. To distinguish the clusters from the surrounding matrix, the maximum separation algorithm is widely employed. However, the results of the cluster analysis strongly depend on the parameters used in the algorithm and hence, a wrong choice of parameters leads to erroneous results, e.g., for the cluster number density, concentration, and size. Here, a new method to determine the optimum value of the parameter d max is proposed, which relies only on information contained in the measured atom-probe data set. Atom-probe simulations are employed to verify the method and to determine the sensitivity of the maximum separation algorithm to other input parameters. In addition, simulations are used to assess the accuracy of cluster analysis in the presence of trajectory aberrations caused by the local magnification effect. In the case of Cu-rich precipitates (Cu concentration 40-60 at% and radius 0.25-1.0 nm) in a bcc Fe-Si-Cu matrix, it is shown that the error in concentration is below 10 at% and the error in radius is <0.15 nm for all simulated conditions, provided that the correct value for d max, as determined with the newly proposed method, is employed. © Microscopy Society of America 2014.
    view abstract10.1017/S1431927614013294
  • Ultrastructural organization and micromechanical properties of shark tooth enameloid
    Enax, J. and Janus, A.M. and Raabe, D. and Epple, M. and Fabritius, H.-O.
    Acta Biomaterialia 10 (2014)
    The outer part of shark teeth is formed by the hard and mineral-rich enameloid that has excellent mechanical properties, which makes it a very interesting model system for the development of new bio-inspired dental materials. We characterized the microstructure, chemical composition and resulting local mechanical properties of the enameloid from teeth of Isurus oxyrinchus (shortfin mako shark) by performing an in-depth analysis using various high-resolution analytical techniques, including scanning electron microscopy, qualitative energy-dispersive X-ray spectroscopy and nanoindentation. Shark tooth enameloid reveals an intricate hierarchical arrangement of thin (50-80 nm) and long (>1 μm) crystallites of fluoroapatite with a high degree of structural anisotropy, which leads to exceptional mechanical properties. Both stiffness and hardness are surprisingly homogeneous in the shiny layer as well as in the enameloid: although both tooth phases differ in structure and composition, they show almost no orientation dependence with respect to the loading direction of the enameloid crystallites. The results were used to determine the structural hierarchy of shark teeth, which can be used as a base for establishing design criteria for synthetic bio-inspired and biomimetic dental composites. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actbio.2014.04.028
  • Ab initio and atomistic study of generalized stacking fault energies in Mg and Mg-Y alloys
    Pei, Z. and Zhu, L.-F. and Friák, M. and Sandlöbes, S. and Von Pezold, J. and Sheng, H.W. and Race, C.P. and Zaefferer, S. and Svendsen, B. and Raabe, D. and Neugebauer, J.
    New Journal of Physics 15 (2013)
    Magnesium-yttrium alloys show significantly improved room temperature ductility when compared with pure Mg. We study this interesting phenomenon theoretically at the atomic scale employing quantum-mechanical (so-called ab initio) and atomistic modeling methods. Specifically, we have calculated generalized stacking fault energies for five slip systems in both elemental magnesium (Mg) and Mg-Y alloys using (i) density functional theory and (ii) a set of embedded-atom-method (EAM) potentials. These calculations predict that the addition of yttrium results in a reduction in the unstable stacking fault energy of basal slip systems. Specifically in the case of an I2 stacking fault, the predicted reduction of the stacking fault energy due to Y atoms was verified by experimental measurements. We find a similar reduction for the stable stacking fault energy of the non-basal slip system. On the other hand, other energies along this particular γ-surface profile increase with the addition of Y. In parallel to our quantum-mechanical calculations, we have also developed a new EAM Mg-Y potential and thoroughly tested its performance. The comparison of quantum-mechanical and atomistic results indicates that the new potential is suitable for future large-scale atomistic simulations. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/15/4/043020
  • Ab Initio Guided Design of Materials
    Friák, M. and Raabe, D. and Neugebauer, J.
    Structural Materials and Processes in Transportation (2013)
    view abstract10.1002/9783527649846.ch15
  • Ab initio identified design principles of solid-solution strengthening in Al
    Ma, D. and Friák, M. and Pezold, J.V. and Raabe, D. and Neugebauer, J.
    Science and Technology of Advanced Materials 14 (2013)
    Solid-solution strengthening in six Al-X binary systems is investigated using first-principle methods. The volumetric mismatch parameter and the solubility enthalpy per solute were calculated. We derive three rules for designing solid-solution strengthened alloys: (i) the solubility enthalpy per solute is related to the volumetric mismatch by a power law; (ii) for each annealing temperature, there exists an optimal solute-volume mismatch to achieve maximum strength; and (iii) the strengthening potential of high volumetric mismatch solutes is severely limited by their low solubility. Our results thus show that the thermodynamic properties of the system (here Al-X alloys) set clear upper bounds to the achievable strengthening effects owing to the reduced solubility with increasing volume mismatch. © 2013 National Institute for Materials Science.
    view abstract10.1088/1468-6996/14/2/025001
  • Ab initio study of single-crystalline and polycrystalline elastic properties of Mg-substituted calcite crystals
    Zhu, L.-F. and Friák, M. and Lymperakis, L. and Titrian, H. and Aydin, U. and Janus, A.M. and Fabritius, H.-O. and Ziegler, A. and Nikolov, S. and Hemzalová, P. and Raabe, D. and Neugebauer, J.
    Journal of the Mechanical Behavior of Biomedical Materials 20 (2013)
    We employ ab initio calculations and investigate the single-crystalline elastic properties of (Ca,Mg)CO3 crystals covering the whole range of concentrations from pure calcite CaCO3 to pure magnesite MgCO3. Studying different distributions of Ca and Mg atoms within 30-atom supercells, our theoretical results show that the energetically most favorable configurations are characterized by elastic constants that nearly monotonously increase with the Mg content. Based on the first principles-derived single-crystalline elastic anisotropy, the integral elastic response of (Ca,Mg)CO3 polycrystals is determined employing a mean-field self-consistent homogenization method. As in case of single-crystalline elastic properties, the computed polycrystalline elastic parameters sensitively depend on the chemical composition and show a significant stiffening impact of Mg atoms on calcite crystals in agreement with the experimental findings. Our analysis also shows that it is not advantageous to use a higher-scale two-phase mix of stoichiometric calcite and magnesite instead of substituting Ca atoms by Mg ones on the atomic scale. Such two-phase composites are not significantly thermodynamically favorable and do not provide any strong additional stiffening effect. © 2013 Elsevier Ltd.
    view abstract10.1016/j.jmbbm.2013.01.030
  • Ab initio study of thermodynamic, electronic, magnetic, structural, and elastic properties of Ni4N allotropes
    Hemzalová, P. and Friák, M. and Šob, M. and Ma, D. and Udyansky, A. and Raabe, D. and Neugebauer, J.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    We have employed parameter-free density functional theory calculations to study the thermodynamic stability and structural parameters as well as elastic and electronic properties of Ni4N in eight selected crystallographic phases. In agreement with the experimental findings, the cubic structure with Pearson symbol cP5, space group Pm3̄m (221) is found to be the most stable and it is also the only thermodynamically stable structure at T=0 K with respect to decomposition to the elemental Ni crystal and N2 gas phase. We determine structural parameters, bulk moduli, and their pressure derivatives for all eight allotropes. The thermodynamic stability and bulk modulus is shown to be anticorrelated. Comparing ferromagnetic and nonmagnetic states, we find common features between the magnetism of elemental Ni and studied ferromagnetic Ni4N structures. For the ground-state Ni4N structure and other two Ni4N cubic allotropes, we predict a complete set of single-crystalline elastic constants (in the equilibrium and under hydrostatic pressure), the Young and area moduli, as well as homogenized polycrystalline elastic moduli obtained by different homogenization methods. We demonstrate that the elastic anisotropy of the ground-state Ni4N is qualitatively opposite to that in the elemental Ni, i.e., these materials have hard and soft crystallographic directions interchanged. Moreover, one of the studied metastable cubic phases is found auxetic, i.e., exhibiting negative Poisson ratio. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.174103
  • Abnormal grain growth in ferritic-martensitic Eurofer-97 steel
    de Oliveira, V.B. and Padilha, A.F. and Möslang, A. and Raabe, D. and Zschommler Sandim, H.R.
    Materials Science Forum 753 (2013)
    Ferritic-martensitic steels like Eurofer-97 are candidate structural materials for future fusion reactors. In the tempered state, this steel contains fine particles dispersed in the ferritic matrix. The aim of this work is to investigate abnormal grain growth in Eurofer-97 steel. The microstructural evolution was followed by isothermal annealing between 200 and 800°C (ferritic phase field) after cold rolling to 70, 80, and 90% reductions. Representative samples were characterized by scanning electron microscopy in the backscattered electron mode. Microtexture was evaluated by electron backscattered diffraction. We propose a mechanism based on the size advantage acquired by nuclei with misorientation angles above 45° relative to their nearest neighbors to explain abnormal grain growth. Abnormal grain growth textures have components belonging to the α- and γ-fibers with predominance of {111}<110>, {111}<112>, and {100}<110>. © (2013) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.753.333
  • Atom probe study of Cu2ZnSnSe4 thin-films prepared by co-evaporation and post-deposition annealing
    Schwarz, T. and Cojocaru-Mirédin, O. and Choi, P. and Mousel, M. and Redinger, A. and Siebentritt, S. and Raabe, D.
    Applied Physics Letters 102 (2013)
    We use atom probe tomography (APT) for resolving nanometer scale compositional fluctuations in Cu2ZnSnSe4 (CZTSe) thin-films prepared by co-evaporation and post-deposition annealing. We detect a complex, nanometer-sized network of CZTSe and ZnSe domains in these films. Some of the ZnSe domains contain precipitates having a Cu- and Sn-rich composition, where the composition cannot be assigned to any of the known equilibrium phases. Furthermore, Na impurities are found to be segregated at the CZTSe/ZnSe interface. The insights given by APT are essential for understanding the growth of CZTSe absorber layers for thin-film solar cells and for optimizing their optoelectronic properties. © 2013 American Institute of Physics.
    view abstract10.1063/1.4788815
  • Atom Probe Tomography Studies on the Cu(In,Ga)Se-2 Grain Boundaries
    Cojocaru-Miredin, O. and Schwarz, T. and Choi, P. P. and Herbig, M. and Wuerz, R. and Raabe, D.
    Jove-journal of Visualized Experiments (2013)
    Compared with the existent techniques, atom probe tomography is a unique technique able to chemically characterize the internal interfaces at the nanoscale and in three dimensions. Indeed, APT possesses high sensitivity (in the order of ppm) and high spatial resolution (sub nm). Considerable efforts were done here to prepare an APT tip which contains the desired grain boundary with a known structure. Indeed, sitespecific sample preparation using combined focused-ion-beam, electron backscatter diffraction, and transmission electron microscopy is presented in this work. This method allows selected grain boundaries with a known structure and location in Cu(In, Ga)Se-2 thin-films to be studied by atom probe tomography. Finally, we discuss the advantages and drawbacks of using the atom probe tomography technique to study the grain boundaries in Cu(In,Ga)Se-2 thin-film solar cells.
    view abstract10.3791/50376
  • Atomic scale investigation of redistribution of alloying elements in pearlitic steel wires upon cold-drawing and annealing
    Li, Y.J. and Choi, P. and Goto, S. and Borchers, C. and Raabe, D. and Kirchheim, R.
    Ultramicroscopy 132 (2013)
    A local electrode atom probe has been employed to analyze the redistribution of alloying elements including Si, Mn, and Cr in pearlitic steel wires upon cold-drawing and subsequent annealing. It has been found that the three elements undergo mechanical mixing upon cold-drawing at large strains, where Mn and Cr exhibit a nearly homogeneous distribution throughout both ferrite and cementite, whereas Si only dissolves slightly in cementite. Annealing at elevated temperatures leads to a reversion of the mechanical alloying. Si atoms mainly segregate at well-defined ferrite (sub)grain boundaries formed during annealing. Cr and Mn are strongly concentrated in cementite adjacent to the ferrite/cementite interface due to their lower diffusivities in cementite than in ferrite. © 2012.
    view abstract10.1016/j.ultramic.2012.10.010
  • Atomic-scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high-entropy alloy using atom probe tomography
    Pradeep, K.G. and Wanderka, N. and Choi, P. and Banhart, J. and Murty, B.S. and Raabe, D.
    Acta Materialia 61 (2013)
    We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600 C for 15 min at 650 MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12 vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60 vol.% bcc and 40 vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc-bcc solid solution but instead a composite of bcc structured Ni-Al-, Cr-Fe- and Fe-Cr-based regions and of fcc Cu-Zn-based regions. The Cu-Zn-rich phase has 30 at.% Zn α-brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr- and Fe-rich bcc regions were presumably formed by spinodal decomposition of a Cr-Fe phase that was inherited from the hot compacted state. The Ni-Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20-30 nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870 ± 10 HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high-entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high-entropy alloys is encouraged as a pathway towards exploiting the solid-solution and stability idea inherent in this concept. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.04.059
  • Atomic-scale investigation of ε and θ precipitates in bainite in 100Cr6 bearing steel by atom probe tomography and ab initio calculations
    Song, W. and Von Appen, J. and Choi, P. and Dronskowski, R. and Raabe, D. and Bleck, W.
    Acta Materialia 61 (2013)
    Carbide precipitation during upper and lower bainite formation in high-carbon bearing steel 100Cr6 is characterized using transmission electron microscopy and atom probe tomography. The results reveal that both ε and θ carbides precipitate in lower bainite isothermally held at 260 C and only θ precipitates form in upper bainite isothermally held at 500 C. ε and θ precipitate under paraequilibrium condition at 260 C in lower bainite and θ precipitates under negligible partitioning local equilibrium condition in upper bainite at 500 C. In order to theoretically study ε and θ precipitation and the ε → θ transition in bainite, thermodynamic calculations have been carried out using ab initio techniques. We find that ε and θ carbides in ferrite have almost identical thermodynamic stability, and hence have similar formation probability. In austenite, however, cementite formation is clearly preferred: it is favored by 5 kJ mol-1 at room temperature and still by 4 kJ mol-1 at 500 C. Hence, the thermodynamic predictions agree well with the atom probe tomography results. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.08.051
  • Basal and non-basal dislocation slip in Mg–Y
    Sandlöbes, S. and Friák, M. and Neugebauer, J. and Raabe, D.
    Materials Science and Engineering A 576 (2013)
    The activation of non-basal slip systems is of high importance for the ductility in hcp Mg and its alloys. In particular, for Mg–Y alloys where a higher activation of pyramidal dislocation slip causes an increased ductility detailed characterization of the activated slip systems is essential to understand and describe plasticity in these alloys. In this study a detailed analysis of the activated dislocations and slip systems via post-mortem TEM and SEM-EBSD based slip band analysis in 3% deformed Mg–3 wt% Y is presented. The analysis reveals a substantial activity of pyramidal <c+a> dislocations with different Burgers vectors. The obtained dislocation densities and active slip systems are discussed with respect to atomistic simulations of non-basal dislocations in hcp Mg. © 2013 Elsevier B.V.
    view abstract10.1016/j.msea.2013.03.006
  • Bulk combinatorial design of ductile martensitic stainless steels through confined martensite-to-austenite reversion
    Springer, H. and Belde, M. and Raabe, D.
    Materials Science and Engineering A 582 (2013)
    The effect of local martensite-to-austenite reversion on microstructure and mechanical properties was studied with the aim of designing ductile martensitic steels. Following a combinatorial screening with tensile and hardness testing on a matrix of six alloys (0-5. wt% Mn, 0-2. wt% Si, constant 13.5. wt% Cr and 0.45. wt% C) and seven martensite tempering conditions (300-500. °C, 0-30. min), investigations were focussed on martensite-to-austenite reversion during tempering as function of chemical composition and its correlation with the mechanical properties. While Mn additions promoted austenite formation (up to 35. vol%) leading to a martensitic-austenitic TRIP steel with optimum mechanical properties (1.5. GPa ultimate tensile strength and 18% elongation), Si led to brittle behaviour despite even larger austenite contents. Combined additions of Mn and Si broadened the temperature range of austenite reversion, but also significantly lowered hardness and yield strength at limited ductility. These drastically diverging mechanical properties of the probed steels are discussed in light of microstructure morphology, dispersion and transformation kinetics of the austenite, as a result of the composition effects on austenite retention and reversion. © 2013 Elsevier B.V.
    view abstract10.1016/j.msea.2013.06.036
  • Characterization of crocodile teeth: Correlation of composition, microstructure, and hardness
    Enax, J. and Fabritius, H.-O. and Rack, A. and Prymak, O. and Raabe, D. and Epple, M.
    Journal of Structural Biology 184 (2013)
    Structure and composition of teeth of the saltwater crocodile Crocodylus porosus were characterized by several high-resolution analytical techniques. X-ray diffraction in combination with elemental analysis and infrared spectroscopy showed that the mineral phase of the teeth is a carbonated calcium-deficient nanocrystalline hydroxyapatite in all three tooth-constituting tissues: Dentin, enamel, and cementum. The fluoride content in the three tissues is very low (<0.1. wt.%) and comparable to that in human teeth. The mineral content of dentin, enamel, and cementum as determined by thermogravimetry is 71.3, 80.5, and 66.8. wt.%, respectively. Synchrotron X-ray microtomography showed the internal structure and allowed to visualize the degree of mineralization in dentin, enamel, and cementum. Virtual sections through the tooth and scanning electron micrographs showed that the enamel layer is comparably thin (100-200 μm). The crystallites in the enamel are oriented perpendicularly to the tooth surface. At the dentin-enamel-junction, the packing density of crystallites decreases, and the crystallites do not display an ordered structure as in the enamel. The microhardness was 0.60 ± 0.05. GPa for dentin, 3.15 ± 0.15. GPa for enamel, 0.26 ± 0.08. GPa for cementum close to the crown, and 0.31 ± 0.04. GPa for cementum close to the root margin. This can be explained with the different degree of mineralization of the different tissue types and is comparable with human teeth. © 2013 Elsevier Inc.
    view abstract10.1016/j.jsb.2013.09.018
  • Coupling of electron channeling with EBSD: Toward the quantitative characterization of deformation structures in the sem
    Gutierrez-Urrutia, I. and Zaefferer, S. and Raabe, D.
    JOM 65 (2013)
    The coupling of electron channeling contrast imaging (ECCI) with electron backscatter diffraction (EBSD) provides an efficient and fast approach to perform ECCI of crystal defects, such as dislocations, cells, and stacking faults, under controlled diffraction conditions with enhanced contrast. From a technical point of view, the ECCI technique complements two of the main electron microscopy techniques, namely, EBSD and conventional diffraction-based transmission electron microscopy. In this review, we provide several application examples of the EBSD-based ECCI approach on microstructure characterization, namely, characterization of single dislocations, measurement of dislocation densities, and characterization of dislocation substructures in deformed bulk materials. We make use of a two-beam Bloch wave approach to interpret the channeling contrast associated with crystal defects. The approach captures the main features observed in the experimental contrast associated with stacking faults and dislocations. © 2013 TMS.
    view abstract10.1007/s11837-013-0678-0
  • Crossover from tumbling to tank-treading-like motion in dense simulated suspensions of red blood cells
    Krüger, T. and Gross, M. and Raabe, D. and Varnik, F.
    Soft Matter 9 (2013)
    Via computer simulations, we provide evidence that the shear rate induced red blood cell tumbling-to-tank-treading transition also occurs at quite high volume fractions, where collective effects are important. The transition takes place as the ratio of effective suspension stress to the characteristic cell membrane stress exceeds a certain value and does not explicitly depend on volume fraction or cell deformability. This value coincides with that for a transition from an orientationally less ordered to a highly ordered phase. The average cell deformation does not show any signature of transition, but rather follows a simple scaling law independent of volume fraction. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c3sm51645h
  • Direct evidence for the formation of ordered carbides in a ferrite-based low-density Fe-Mn-Al-C alloy studied by transmission electron microscopy and atom probe tomography
    Seol, J.-B. and Raabe, D. and Choi, P. and Park, H.-S. and Kwak, J.-H. and Park, C.-G.
    Scripta Materialia 68 (2013)
    We study the structure and chemical composition of the κ-carbide formed as a result of isothermal transformation in an Fe-3.0Mn-5.5Al-0.3C alloy using transmission electron microscopy and atom probe tomography. Both methods reveal the evolution of κ-particle morphology as well as the partitioning of solutes. We propose that the κ-phase is formed by a eutectoid reaction associated with nucleation growth. The nucleation of κ-carbide is controlled by both the ordering of Al partitioned to austenite and the carbon diffusion at elevated temperatures.© 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2012.08.013
  • Effect of climb on dislocation mechanisms and creep rates in γ′-strengthened Ni base superalloy single crystals: A discrete dislocation dynamics study
    Hafez Haghighat, S.M. and Eggeler, G. and Raabe, D.
    Acta Materialia 61 (2013)
    Creep of single-crystal superalloys is governed by dislocation glide, climb, reactions and annihilation. Discrete three-dimensional (3D) dislocation dynamics (DDD) simulations are used to study the evolution of the dislocation substructure in a γ/γ′ microstructure of a single-crystal superalloy for different climb rates and loading conditions. A hybrid mobility law for glide and climb is used to map the interactions of dislocations with γ′ cubes. The focus is on the early stages of creep, where dislocation plasticity is confined to narrow γ channels. With enhancing climb mobility, the creep strain increases, even if the applied resolved shear stress is below the critical stress required for squeezing dislocations into the γ channels. The simulated creep microstructure consists of long dislocations and a network near the corners of the γ′ precipitate in the low-stress regime. In the high-stress regime, dislocations squeeze into the γ channels, where they deposit dislocation segments at the γ/γ′ interfaces. These observations are in good agreement with experimentally observed dislocation structures that form during high-temperature and low-stress creep. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.03.003
  • Element-resolved corrosion analysis of stainless-type glass-forming steels
    Duarte, M.J. and Klemm, J. and Klemm, S.O. and Mayrhofer, K.J.J. and Stratmann, M. and Borodin, S. and Romero, A.H. and Madinehei, M. and Crespo, D. and Serrano, J. and Gerstl, S.S.A. and Choi, P.P. and Raabe, D. and Renner, F.U.
    Science 341 (2013)
    Ultrathin passive films effectively prevent the chemical attack of stainless steel grades in corrosive environments; their stability depends on the interplay between structure and chemistry of the constituents iron, chromium, and molybdenum (Fe-Cr-Mo). Carbon (C), and eventually boron (B), are also important constituents of steels, although in small quantities. In particular, nanoscale inhomogeneities along the surface can have an impact on material failure but are still poorly understood. Addressing a stainless-type glass-forming Fe50Cr15Mo14C15B 6 alloy and using a combination of complementary high-resolution analytical techniques, we relate near-atomistic insights into increasingly inhomogeneous nanostructures with time- and element-resolved dissolution behavior. The progressive elemental partitioning on the nanoscale determines the degree of passivation. A detrimental transition from Cr-controlled passivity to Mo-controlled breakdown is dissected atom by atom, demonstrating the importance of nanoscale knowledge for understanding corrosion.
    view abstract10.1126/science.1230081
  • Extreme optical properties tuned through phase substitution in a structurally optimized biological photonic polycrystal
    Wu, X. and Erbe, A. and Raabe, D. and Fabritius, H.-O.
    Advanced Functional Materials 23 (2013)
    Biological photonic structures evolved by insects provide inspiring examples for the design and fabrication of synthetic photonic crystals. The small scales covering the beetle Entimus imperialis are subdivided into irregularly shaped domains that mostly show striking colors, yet some appear colorless. The colors originate from photonic crystals consisting of cuticular material and air, which are geometrically separated by a triply periodic D-surface (diamond). The structure and orientation of the photonic crystals are charactized and it is shown that in colorless domains SiO2 substitutes the air. The experimental results are incorporated into a precise D-surface structure model used to simulate the photonic band structure. The study shows that the structural parameters in colored domains are optimized for maximum reflectivity by maximizing the stop gap width. The colorless domains provide a biological example of how the optical appearance changes through alteration of the refractive index contrast between the constituting phases. The cuticular photonic polycrystals formed by the beetle Entimus imperialis are a perfect example of a natural diamond-type triply periodic bicontinuous cubic structure with structural parameters that are optimized to open up the largest possible photonic stop gaps. Depending on whether the cuticular network is complemented by air or SiO2, the optical properties of individual domains vary from bright coloration to no coloration and transparency. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201203597
  • Flow heterogeneity and correlations in a sheared hard sphere glass: Insight from computer simulations
    Mandal, S. and Gross, M. and Raabe, D. and Varnik, F.
    AIP Conference Proceedings 1518 (2013)
    Understanding the origin of flow heterogeneity in glassy systems is of high interest both due to its importance from theoretical standpoint as well as due to its occurrence in a large number of practical situations such as the flow of the so-called soft-glassy materials (foams, colloidal suspensions, granular media, etc). Detailed experimental investigations do indeed confirm that the flow of driven amorphous solids is not homogeneous, even if the macroscopic stress is constant across the system. We study this issue via large scale event driven molecular dynamics simulations of a hard sphere glass. We observe significant fluctuations of the velocity profile with a time scale of the order of a few hundreds percent strain. Furthermore, there appears to be a correlation between the fluctuations of the local volume fraction and the fluctuations of the local shear rate. The time scales of the fluctuations of density and shear rate are practically identical. These observations motivate an interpretation of our results via the shear concentration coupling (SCC) theory. A detailed comparison, however, reveals serious inconsistencies. In particular, the amplitude of the fluctuations of the shear rate seems to be decoupled from that of density, a feature which is rather unexpected within the SCC picture. An alternative interpretation of our observations is also discussed invoking dynamic heterogeneity. © 2013 American Institute of Physics.
    view abstract10.1063/1.4794578
  • Grain boundary segregation in a bronze-route Nb3Sn superconducting wire studied by atom probe tomography
    Sandim, M.J.R. and Tytko, D. and Kostka, A. and Choi, P. and Awaji, S. and Watanabe, K. and Raabe, D.
    Superconductor Science and Technology 26 (2013)
    Atom probe tomography was used to characterize the A15 phase in a bronze-route Nb3Sn superconducting wire with a bronze matrix composition of Cu-8Sn-0.3Ti (in at.%). We observed depletion of niobium and segregation of Cu and Ti atoms at Nb3Sn grain boundaries. While the Nb depletion is about 15% relative to the grain interior, the average ratio between Cu and Ti excess values is 9 to 2. Segregation extends to a distance d ∼ 9 Å from the point of maximum Cu and Ti concentrations. Such local variation in the stoichiometry at the grain boundary region can be an additional source of flux-pinning in the Nb3Sn phase. Other microstructural parameters, such as the grain size and chemical composition of the Nb 3Sn layer, were investigated by electron backscatter diffraction and transmission electron microscopy. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0953-2048/26/5/055008
  • High-power InP quantum dot based semiconductor disk laser exceeding 1.3 W
    Schwarzback, T. and Bek, R. and Hargart, F. and Kessler, C. A. and Kahle, H. and Koroknay, E. and Jetter, M. and Michler, P.
    Applied Physics Letters 102 (2013)
    We demonstrate an optically pumped semiconductor disk laser (OP-SDL) using InP quantum dots (QDs) as active material fabricated by metal-organic vapor-phase epitaxy. The QDs are grown within [(Al0.1Ga0.9)(0.52)In-0.48](0.5)P-0.5 (abbr. Al0.1GaInP) barriers in order to achieve an emission wavelength around 655 nm. We present optical investigations of the active region showing typical QD behavior like blue shift with increasing excitation power and single emission lines, which show anti-bunching in an intensity auto-correlation measurement. We report maximum output powers of the OP-SDL of 1.39 W at low emission wavelength of similar to 654 nm with a slope efficiency of eta(diff) = 25.4%. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793299]
    view abstract10.1063/1.4793299
  • Hydrogen-assisted failure in a twinning-induced plasticity steel studied under in situ hydrogen charging by electron channeling contrast imaging
    Koyama, M. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Acta Materialia 61 (2013)
    We investigated the hydrogen embrittlement of a Fe-18Mn-1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7 × 10-6 s -1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe-Mn-C twining-induced plasticity steel. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.04.030
  • Influence of Al content and precipitation state on the mechanical behavior of austenitic high-Mn low-density steels
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 68 (2013)
    We investigate the strain hardening of two austenitic high-Mn low density steels, namely, Fe-30.5Mn-2.1Al-1.2C and Fe-30.5Mn-8.0Al-1.2C (wt.%), containing different precipitation states. The strain hardening of the alloy with low Al content is attributed to dislocation and twin substructures. The precipitation of intergranular M3C-type carbides strongly influences the fracture mode. We associate the strain hardening behavior of the alloy with high Al content to the precipitation of shearable nanosized κ-carbides and their role in the development of planar dislocation substructures.© 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2012.08.038
  • Influence of supersaturated carbon on the diffusion of Ni in ferrite determined by atom probe tomography
    Kresse, T. and Li, Y.J. and Boll, T. and Borchers, C. and Choi, P. and Al-Kassab, T. and Raabe, D. and Kirchheim, R.
    Scripta Materialia 69 (2013)
    In patented and cold-drawn pearlitic steel wires dissociation of cementite occurs during mechanical deformation. In this study the influence of the carbon decomposition on the diffusion of nickel in ferrite is investigated by means of atom probe tomography. In the temperature range 423-523 K we observed a much smaller activation energy of Ni diffusion than for self-diffusion in body-centered cubic iron, indicating an increased vacancy density owing to enhanced formation of vacancy-carbon complexes. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2013.05.039
  • Inheritance of dislocations and crystallographic texture during martensitic reversion into austenite
    Nakada, N. and Fukagawa, R. and Tsuchiyama, T. and Takaki, S. and Ponge, D. and Raabe, D.
    ISIJ International 53 (2013)
    A novel type of heat treatment to control austenite stability by precipitation and dissolution of carbide was applied to a 18%Ni-0.6%C steel. The new method enables to study the inherent characteristics of austenite formed by martensitic reversion. The material selected for this study is a hypereutectoid steel with a large carbon solubility gap depending on the austenitizing temperature. The precipitated carbide immediately dissolves into the austenite matrix again after martensitic reversion in order to restore the Ms and Mf temperatures to their initial values. The crystallographic orientation of each phase was mapped by means of the electron back scattering diffraction (EBSD) method using a field emission scanning electron microscope. A full fcc-bcc-fcc reversible martensitic transformations cycle was observed. As a result of the specific heat treatment, the austenite that was formed by martensitic reversion remains stable at ambient temperature.
    view abstract10.2355/isijinternational.53.1286
  • Interaction between recrystallization and phase transformation during intercritical annealing in a cold-rolled dual-phase steel: A cellular automaton model
    Zheng, C. and Raabe, D.
    Acta Materialia 61 (2013)
    The concurrent ferrite recrystallization and austenitic transformation during intercritical annealing of cold-rolled DP steels is investigated by cellular automaton (CA) modeling. The simulations provide insight into the microstructural phenomena that result from the interaction of primary recrystallization and phase transformation. We find that the interaction between ferrite recrystallization and austenite formation affects not only the transformation kinetics but also the morphology and spatial distribution of the austenite. From this we can interpret experimental data of the observed temperature-dependent hardness and its dependence on the two metallurgical processes. The influence of the initial heating rate on subsequent isothermal transformation kinetics and the microstructure evolution is also obtained by the model. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.05.040
  • Interface-directed spinodal decomposition in TiAlN/CrN multilayer hard coatings studied by atom probe tomography
    Povstugar, I. and Choi, P.-P. and Tytko, D. and Ahn, J.-P. and Raabe, D.
    Acta Materialia 61 (2013)
    Microstructural and compositional changes in TiAlN/CrN multilayered films occurring at temperatures up to 1000 C were studied at different length scales by a combination of atom probe tomography, transmission electron microscopy and X-ray diffraction. We observe the onset of decomposition of the multilayer structure at 700 C via the mechanism of interface-directed spinodal decomposition of TiAlN layers, where Al atoms preferentially move toward the nearest interface and segregate there. The interface-directed mechanism later transforms into isotropic spinodal decomposition and is accompanied by intense interdiffusion between the constituting layers. Distinct compositional gradients across columnar grain boundaries (extending perpendicular to the multilayers) are detected at this stage of decomposition. Drastic differences in decomposition behavior across the film depth were observed at elevated temperatures (800-1000 C): the layered structure completely dissolves in the near-surface part but persists in the regions distant from the surface. The influence of residual stresses caused by the sputter deposition process on the thermally induced evolution of the multilayer thin films is discussed. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.08.028
  • Lattice Boltzmann modeling of advection-diffusion-reaction equations: Pattern formation under uniform differential advection
    Ayodele, S.G. and Raabe, D. and Varnik, F.
    Communications in Computational Physics 13 (2013)
    A lattice Boltzmann model for the study of advection-diffusion-reaction (ADR) problems is proposed. Via multiscale expansion analysis, we derive from the LB model the resulting macroscopic equations. It is shown that a linear equilibrium distribution is sufficient to produce ADR equations within error terms of the order of the Mach number squared. Furthermore, we study spatially varying structures arising fromthe interaction of advective transportwith a cubic autocatalytic reaction-diffusion process under an imposed uniform flow. While advecting all the present species leads to trivial translation of the Turing patterns, differential advection leads to flow induced instability characterized with traveling stripes with a velocity dependent wave vector parallel to the flow direction. Predictions from a linear stability analysis of the model equations are found to be in line with these observations. © 2013 Global-Science Press.
    view abstract10.4208/cicp.441011.270112s
  • Maintaining the equipartition theorem in small heterogeneous molecular dynamics ensembles
    Siboni, N.H. and Raabe, D. and Varnik, F.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 87 (2013)
    It has been reported recently that the equipartition theorem is violated in molecular dynamics simulations with periodic boundary condition. This effect is associated with the conservation of the total momentum. Here, we propose a fluctuating center of mass molecular dynamics approach to solve this problem. Using the analogy to a system exchanging momentum with its surroundings, we work out - and validate via simulations - an expression for the rate at which fluctuations shall be added to the system. It is shown that the proposed method maintains equipartition both at equilibrium and beyond equilibrium in the linear response regime. © 2013 American Physical Society.
    view abstract10.1103/PhysRevE.87.030101
  • Measurement of the indium concentration in high indium content InGaN layers by scanning transmission electron microscopy and atom probe tomography
    Mehrtens, T. and Schowalter, M. and Tytko, D. and Choi, P. and Raabe, D. and Hoffmann, L. and Jönen, H. and Rossow, U. and Hangleiter, A. and Rosenauer, A.
    Applied Physics Letters 102 (2013)
    A method for determining concentrations from high-angle annular dark field-scanning transmission electron microscopy images is presented. The method is applied to an InGaN/GaN multi-quantum well structure with high In content, as used for the fabrication of light emitting diodes and laser diodes emitting in the green spectral range. Information on specimen thickness and In concentration is extracted by comparison with multislice calculations. Resulting concentration profiles are in good agreement with a comparative atom probe tomography analysis. Indium concentrations in the quantum wells ranging from 26 at. to 33 at. are measured in both cases. © 2013 American Institute of Physics.
    view abstract10.1063/1.4799382
  • Measuring composition in InGaN from HAADF-STEM images and studying the temperature dependence of Z-contrast
    Mehrtens, T. and Schowalter, M. and Tytko, D. and Choi, P. and Raabe, D. and Hoffmann, L. and Jönen, H. and Rossow, U. and Hangleiter, A. and Rosenauer, A.
    Journal of Physics: Conference Series 471 (2013)
    In this contribution, the indium concentration profile of an In xGa1-xN/GaN five-fold multi quantum well structure is measured from high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images. The results are compared with an atom probe tomography study. Indium concentrations in the range of 26 at.% to 33 at.% are measured in the centre of the quantum wells. An additional indium layer of 14 at.% has been found on top of the quantum wells. In the second part, the temperature dependence of measured intensities in GaN is investigated. Here, multislice calculations in the frozen lattice approximation are carried out in dependence of specimen thickness and compared to experimental data. An increase of intensity with specimen temperature is found.
    view abstract10.1088/1742-6596/471/1/012009
  • Microbanding mechanism in an Fe-Mn-C high-Mn twinning-induced plasticity steel
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 69 (2013)
    We study the microbanding mechanism in an Fe-22Mn-0.6C (wt.%) twinning-induced plasticity steel. Dislocation substructures were examined by electron channeling contrast imaging and electron backscatter diffraction. We observe a pronounced effect of the strain path on microbanding, which is explained in terms of Schmid's law. Microbands created under shear loading have a non-crystallographic character. This is attributed to the microbanding mechanism and its relation with the dislocation substructure. Further insights into the dislocation configuration of microbands are provided.© 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2013.03.010
  • Multi-scale correlative microscopy investigation of both structure and chemistry of deformation twin bundles in Fe-Mn-C steel
    Marceau, R.K.W. and Gutierrez-Urrutia, I. and Herbig, M. and Moore, K.L. and Lozano-Perez, S. and Raabe, D.
    Microscopy and Microanalysis 19 (2013)
    A multi-scale investigation of twin bundles in Fe-22Mn-0.6C (wt%) twinning-induced plasticity steel after tensile deformation has been carried out by truly correlative means; using electron channelling contrast imaging combined with electron backscatter diffraction, high-resolution secondary ion mass spectrometry, scanning transmission electron microscopy, and atom probe tomography on the exact same region of interest in the sample. It was revealed that there was no significant segregation of Mn or C to the twin boundary interfaces. Copyright © 2013 Microscopy Society of America.
    view abstract10.1017/S1431927613013494
  • Nanocrystalline Fe-C alloys produced by ball milling of iron and graphite
    Chen, Y.Z. and Herz, A. and Li, Y.J. and Borchers, C. and Choi, P. and Raabe, D. and Kirchheim, R.
    Acta Materialia 61 (2013)
    A series of nanocrystalline Fe-C alloys with different carbon concentrations (xtot) up to 19.4 at.% (4.90 wt.%) are prepared by ball milling. The microstructures of these alloys are characterized by transmission electron microscopy and X-ray diffraction, and partitioning of carbon between grain boundaries and grain interiors is determined by atom probe tomography. It is found that the segregation of carbon to grain boundaries of α-ferrite can significantly reduce its grain size to a few nanometers. When the grain boundaries of ferrite are saturated with carbon, a metastable thermodynamic equilibrium between the matrix and the grain boundaries is approached, inducing a decreasing grain size with increasing xtot. Eventually the size reaches a lower limit of about 6 nm in alloys with x tot &gt; 6.19 at.% (1.40 wt.%); a further increase in xtot leads to the precipitation of carbon as Fe3C. The observed presence of an amorphous structure in 19.4 at.% C (4.90 wt.%) alloy is ascribed to a deformation-driven amorphization of Fe3C by severe plastic deformation. By measuring the temperature dependence of the grain size for an alloy with 1.77 at.% C additional evidence is provided for a metastable equilibrium reached in the nanocrystalline alloy. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.02.006
  • On dislocation involvement in Ti-Nb gum metal plasticity
    Plancher, E. and Tasan, C.C. and Sandloebes, S. and Raabe, D.
    Scripta Materialia 68 (2013)
    The excellent mechanical properties of the Ti-Nb-based gum metal were originally proposed to arise from a "dislocation-free" giant fault mechanism; however, the involvement of lattice dislocations in the process is still under debate. To address this issue, gum metal deformation mechanisms are investigated systematically on cast specimens, employing postmortem and in situ analysis techniques. The results demonstrate that a giant fault mechanism (which appears to be a phase-transformation-assisted nanotwinning mechanism) governs gum metal plasticity without direct assistance from dislocations during the process. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2013.01.034
  • Particle stimulated nucleation in coarse-grained ferritic stainless steel
    De Siqueira, R.P. and Sandim, H.R.Z. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 44 (2013)
    Particle-stimulated nucleation (PSN) is investigated in Nb-containing ferritic stainless steel. Coarse-grained sheets were cold rolled to 80 pct thickness reduction and annealed from 973 K to 998 K (700 C to 725 C) to obtain partially recrystallized microstructures. Electron backscatter diffraction was performed around coarse niobium carbonitride particles (larger than 1 μm) within coarse grains (~1 mm), with different host orientations in both deformed and annealed states. In the deformed state, the deformation zones around both spherical and rectangular particles were investigated. The local lattice rotations about the transverse direction necessary to accommodate the particle-matrix strain incompatibility were observed in all grains investigated. After annealing, recrystallization occurs preferentially around coarse particles at the initial stages of recrystallization. Based on a total number of 130 grains nucleated via PSN, we observe both, randomly oriented and minor {111}110 oriented texture components. The results also reveal that PSN in this material is not associated with a specific host orientation. © 2012 The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-012-1408-x
  • Revealing the strain-hardening behavior of twinning-induced plasticity steels: Theory, simulations, experiments
    Steinmetz, D.R. and Jäpel, T. and Wietbrock, B. and Eisenlohr, P. and Gutierrez-Urrutia, I. and Saeed-Akbari, A. and Hickel, T. and Roters, F. and Raabe, D.
    Acta Materialia 61 (2013)
    We present a multiscale dislocation density-based constitutive model for the strain-hardening behavior in twinning-induced plasticity (TWIP) steels. The approach is a physics-based strain rate- and temperature-sensitive model which reflects microstructural investigations of twins and dislocation structures in TWIP steels. One distinct advantage of the approach is that the model parameters, some of which are derived by ab initio predictions, are physics-based and known within an order of magnitude. This allows more complex microstructural information to be included in the model without losing the ability to identify reasonable initial values and bounds for all parameters. Dislocation cells, grain size and twin volume fraction evolution are included. Particular attention is placed on the mechanism by which new deformation twins are nucleated, and a new formulation for the critical twinning stress is presented. Various temperatures were included in the parameter optimization process. Dissipative heating is also considered. The use of physically justified parameters enables the identification of a universal parameter set for the example of an Fe-22Mn-0.6C TWIP steel. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.09.064
  • Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite
    Raabe, D. and Sandlöbes, S. and Millán, J. and Ponge, D. and Assadi, H. and Herbig, M. and Choi, P.-P.
    Acta Materialia 61 (2013)
    In an Fe-9 at.% Mn maraging alloy annealed at 450 C reversed allotriomorphic austenite nanolayers appear on former Mn decorated lath martensite boundaries. The austenite films are 5-15 nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation) in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.06.055
  • Self-consistent scale-bridging approach to compute the elasticity of multi-phase polycrystalline materials
    Titrian, H. and Aydin, U. and Friák, M. and Ma, D. and Raabe, D. and Neugebauer, J.
    Materials Research Society Symposium Proceedings 1524 (2013)
    A necessary prerequisite for a successful theory-guided up-scale design of materials with application-driven elastic properties is the availability of reliable homogenization techniques. We report on a new software tool that enables us to probe and analyze scale-bridging structure-property relations in the elasticity of materials. The newly developed application, referred to as SC-EMA (Self-consistent Calculations of Elasticity of Multi-phase Aggregates) computes integral elastic response of randomly textured polycrystals. The application employs a Python modular library that uses single-crystalline elastic constants Cij as input parameters and calculates macroscopic elastic moduli (bulk, shear, and Young's) and Poisson ratio of both single-phase and multi-phase aggregates. Crystallites forming the aggregate can be of cubic, tetragonal, hexagonal, orthorhombic, or trigonal symmetry. For cubic polycrystals the method matches the Hershey homogenization scheme. In case of multi-phase polycrystalline composites, the shear moduli are computed as a function of volumetric fractions of phases present in aggregates. Elastic moduli calculated using the analytical self-consistent method are computed together with their bounds as determined by Reuss, Voigt and Hashin-Shtrikman homogenization schemes. The library can be used as (i) a toolkit for a forward prediction of macroscopic elastic properties based on known single-crystalline elastic characteristics, (ii) a sensitivity analysis of macro-scale output parameters as function of input parameters, and, in principle, also for (iii) an inverse materials-design search for unknown phases and/or their volumetric ratios. © 2013 Materials Research Society.
    view abstract10.1557/opl.2013.41
  • Simulation of shear banding in heterophase co-deformation: Example of plane strain compressed Cu-Ag and Cu-Nb metal matrix composites
    Jia, N. and Roters, F. and Eisenlohr, P. and Raabe, D. and Zhao, X.
    Acta Materialia 61 (2013)
    The co-deformation and shear localization in heterophase alloys is studied using two-dimensional crystal plasticity finite element simulations on plane strain compressed Cu-Ag and Cu-Nb metal matrix composites. The aim is to study the fundamentals of micromechanics, co-deformation and shear banding in materials with heterophase interfaces. It is observed that, depending on the initial orientations of the crystals, co-deformation of the constituent heterophases often proceeds via collective mechanisms, i.e. by pronounced shear banding triggered by stress concentration at the interfaces. This phenomenon leads to highly localized strains within the bands, exceeding the average strain in part by two orders of magnitude. Shear band development is related to the inherent mechanical properties of each crystal and also to the properties of the abutting crystals. The predicted topology and nature of the cross-phase shear bands, i.e. the extreme local strains, significant bending of the interface regions, and sharp strain localization that propagates across the interfaces, agree well with experimental observations in cold-rolled composites. The simulations reveal that cross-phase shear banding leads to large and highly localized values of stress and strain at heterophase interfaces. Such information is essential for a better understanding of the micromechanical boundary conditions inside co-deformed composites and the associated shear-induced chemical mixing. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2013.04.029
  • Single-particle fluctuations and directional correlations in driven hard-sphere glasses
    Mandal, S. and Chikkadi, V. and Nienhuis, B. and Raabe, D. and Schall, P. and Varnik, F.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 88 (2013)
    Via event-driven molecular dynamics simulations and experiments, we study the packing-fraction and shear-rate dependence of single-particle fluctuations and dynamic correlations in hard-sphere glasses under shear. At packing fractions above the glass transition, correlations increase as shear rate decreases: the exponential tail in the distribution of single-particle jumps broadens and dynamic four-point correlations increase. Interestingly, however, upon decreasing the packing fraction, a broadening of the exponential tail is also observed, while dynamic heterogeneity is shown to decrease. An explanation for this behavior is proposed in terms of a competition between shear and thermal fluctuations. Building upon our previous studies, we further address the issue of anisotropy of the dynamic correlations. © 2013 American Physical Society.
    view abstract10.1103/PhysRevE.88.022129
  • Spatial Distributions of Alloying Elements Obtained from Atom Probe Tomography of the Amorphous Ribbon Fe75C11Si2B8Cr4
    Shin, J. and Yi, S. and Pradeep, K.G. and Choi, P.P. and Raabe, D.
    Korean Journal of Materials Research 23 (2013)
    Spatial distributions of alloying elements of an Fe-based amorphous ribbon with a nominal composition of Fe75C11Si2B8Cr4we re analyzed through the atom probe tomography method. The amorphous ribbon was prepared through the melt spinning method. The macroscopic amorphous natures were confirmed using an X-ray diffractometer (XRD) and a differential scanning calorimeter (DSC). Atom Probe (Cameca LEAP 3000X HR) analyses were carried out in pulsed voltage mode at a specimen base temperature of about 60 K, a pulse to base voltage ratio of 15 %, and a pulse frequency of 200 kHz. The target detection rate was set to 5 ions per 1000 pulses. Based on a statistical analyses of the data obtained from the volume of 59 x 59 x 33 nm3, homogeneous distributions of alloying elements in nano-scales were concluded. Even with high carbon and strong carbide forming element contents, nano-scale segregation zones of alloying elements were not detected within the Fe-based amorphous ribbon. However, the existence of small sub-nanometer scale clusters due to short range ordering cannot be completely excluded. © Materials Research Society of Korea, All rights reserved.
    view abstract10.3740/MRSK.2013.23.3.190
  • Thermodynamics of carbon solubility in ferrite and vacancy formation in cementite in strained pearlite
    Nematollahi, G.A. and Von Pezold, J. and Neugebauer, J. and Raabe, D.
    Acta Materialia 61 (2013)
    In order to investigate the thermodynamic driving force for the experimentally observed accumulation of C in ferritic layers of severely plastically deformed pearlitic wires, the stabilities of C interstitials in ferrite and of C vacancies in cementite are investigated as a function of uniaxial stain, using density-functional theory. In the presence of an applied strain along [1 1 0] or [1 1 1], the C interstitial in ferrite is significantly stabilized, while the C vacancy in cementite is moderately destabilized by the corresponding strain states in cementite [1 0 0] and ([0 1 0]). The enhanced stabilization of the C interstitial gives rise to an increase in the C concentration within the ferritic layers by up to two orders of magnitude. Our results thus suggest that in addition to the generally assumed non-equilibrium, dislocation-based mechanism, there is also a strain-induced thermodynamic driving force for the experimentally observed accumulation of C in ferrite. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.12.001
  • Transition from diffusive to displacive austenite reversion in low-alloy steel
    Nakada, N. and Tsuchiyama, T. and Takaki, S. and Ponge, D. and Raabe, D.
    ISIJ International 53 (2013)
    For understanding the transition from diffusive to displacive austenite reversion mechanism in steel, the effect of heating rate on austenite reversion behavior was investigated in 0.15%C- 5%Mn steel. Austenite reversion temperature first increased gradually with the heating rate owing to the superheating effect and then remained at a constant temperature above a critical heating rate. In response, the austenite formed by rapid heating exhibited a coarse prior austenite grain structure, indicating the occurrence of displacive reversion even in low-alloy steel. © 2013 ISIJ.
    view abstract10.2355/isijinternational.53.2275
  • Understanding the detection of carbon in austenitic high-Mn steel using atom probe tomography
    Marceau, R.K.W. and Choi, P. and Raabe, D.
    Ultramicroscopy 132 (2013)
    A high-Mn TWIP steel having composition Fe-22Mn-0.6C (wt%) is considered in this study, where the need for accurate and quantitative analysis of clustering and short-range ordering by atom probe analysis requires a better understanding of the detection of carbon in this system. Experimental measurements reveal that a high percentage of carbon atoms are detected as molecular ion species and on multiple hit events, which is discussed with respect to issues such as optimal experimental parameters, correlated field evaporation and directional walk/migration of carbon atoms at the surface of the specimen tip during analysis. These phenomena impact the compositional and spatial accuracy of the atom probe measurement and thus require careful consideration for further cluster-finding analysis. © 2013 Elsevier B.V.
    view abstract10.1016/j.ultramic.2013.01.010
  • Viscous coalescence of droplets: A lattice Boltzmann study
    Gross, M. and Steinbach, I. and Raabe, D. and Varnik, F.
    Physics of Fluids 25 (2013)
    The coalescence of two resting liquid droplets in a saturated vapor phase is investigated by Lattice Boltzmann simulations in two and three dimensions. We find that, in the viscous regime, the bridge radius obeys a t1/2-scaling law in time with the characteristic time scale given by the viscous time. Our results differ significantly from the predictions of existing analytical theories of viscous coalescence as well as from experimental observations. While the underlying reason for these deviations is presently unknown, a simple scaling argument is given that describes our results well. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4803178
  • Adiabatic temperature increase associated with deformation twinning and dislocation plasticity
    Eisenlohr, A. and Gutierrez-Urrutia, I. and Raabe, D.
    Acta Materialia 60 (2012)
    We studied local deformation and temperature effects associated with mechanical twinning in Fe-3 wt.% Si at room temperature. During tensile testing, two large stress drops occurred. They were accompanied by local strain and temperature bursts, which we mapped via simultaneous displacement and temperature field characterization. To identify the microstructural origin of these phenomena, we performed high resolution electron backscatter scanning diffraction and electron channeling contrast imaging measurements. The microstructure at the positions where strong adiabatic heating occurred was characterized by the formation of primary twins and high dislocation activity within a range of about 10 μm around the twin-matrix interface. We suggest that the local temperature and strain jumps result from the formation and dissipative motion of lattice dislocations that accommodate twinning. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.03.008
  • Advanced methods and tools for reconstruction and analysis of grain boundaries from 3D-EBSD data sets
    Konijnenberg, P.J. and Zaefferer, S. and Lee, S.-B. and Rollett, A.D. and Rohrer, G.S. and Raabe, D.
    Materials Science Forum 702-703 (2012)
    We report the recent development of a 3D orientation data post-processing software, which we refer to as QUBE. Amongst other functionalities, it offers the possibility to specify the spatial and orientational distribution of boundary normals. We describe a method to reconstruct a voxel-accurate and smooth 3D boundary triangle mesh by algorithmic means. A proof of concept is given by a benchmark on a generic dataset and we demonstrate a first result with the description of selected grain boundaries in an Fe-28%Ni sample. © (2012) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.702-703.475
  • Atomic scale effects of alloying, partitioning, solute drag and austempering on the mechanical properties of high-carbon bainitic-austenitic TRIP steels
    Seol, J.-B. and Raabe, D. and Choi, P.-P. and Im, Y.-R. and Park, C.-G.
    Acta Materialia 60 (2012)
    Understanding alloying and thermal processing at an atomic scale is essential for the optimal design of high-carbon (0.71 wt.%) bainitic-austenitic transformation-induced plasticity (TRIP) steels. We investigate the influence of the austempering temperature, chemical composition (especially the Si:Al ratio) and partitioning on the nanostructure and mechanical behavior of these steels by atom probe tomography. The effects of the austempering temperature and of Si and Al on the compositional gradients across the phase boundaries between retained austenite and bainitic ferrite are studied. We observe that controlling these parameters (i.e. Si, Al content and austempering temperature) can be used to tune the stability of the retained austenite and hence the mechanical behavior of these steels. We also study the atomic scale redistribution of Mn and Si at the bainitic ferrite/austenite interface. The observations suggest that either para-equilibrium or local equilibrium-negligible partitioning conditions prevail depending on the Si:Al ratio during bainite transformation. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.07.064
  • Characterization of thin anodic oxides of Ti-Nb alloys by electrochemical impedance spectroscopy
    Woldemedhin, M.T. and Raabe, D. and Hassel, A.W.
    Electrochimica Acta 82 (2012)
    Electrochemical impedance spectroscopy was used to study the interface between the anodic oxide formed on Ti-Nb alloys with specific compositions of Ti-10 wt.% Nb and Ti-20 wt.% Nb and the electrolyte. The anodic oxides were grown in an acetate buffer of pH 6.0 by using cyclic voltammetry electrochemical technique in which the potential is scanned at a rate of 100 mV s -1. The potential applied starts from 0 V and increasing at steps of 1 V till 8 V which allows to study the mechanism and the kinetics involved during the oxide growth. The electrochemical impedance measurements were started prior to applying any potential so that the electronic properties of the native oxide on the Ti-Nb alloys can be determined. The electrochemical measurements were then carried out after each oxide growth so that the electronic properties of the previously grown oxide can also be determined. The variation of the capacitance of the respective oxides determined from the impedance measurements with the applied potential enables the calculation of the relative permittivity of the respective oxides on the two alloys. Moreover the semiconducting properties of the oxides were determined by using Mott-Schottky analysis. The Mott-Schottky analysis involves electrochemical impedance measurements at fixed frequency with increasing applied bias potential so that the variation of the capacitance of the space charge region with the applied potential can be followed. The oxides from both alloys showed an n-type semiconducting property with 7.5 × 10 18 cm -3 and 2.4 × 10 19 cm -3 donor concentration for Ti-10 wt.% Nb and Ti-20 wt.% Nb alloys respectively. © 2012 Elsevier Ltd.
    view abstract10.1016/j.electacta.2012.06.029
  • Correlation of structure, composition and local mechanical properties in the dorsal carapace of the edible crab cancer pagurus
    Fabritius, H.-O. and Karsten, E.S. and Balasundaram, K. and Hild, S. and Huemer, K. and Raabe, D.
    Zeitschrift fur Kristallographie 227 (2012)
    The exoskeleton of crustaceans is formed by the cuticle, a chitin-protein-based nano-composite with hierarchical organization over at least eight levels. On the molecular level, it consists of chitin associated with proteins forming fibres, which are organized in the form of twisted plywood. On the higher levels, the twisted plywood organization is modified and forms skeletal elements with elaborate functions. The load-bearing parts of crustacean cuticle are reinforced with both crystalline and amorphous biominerals. During evolution, all parts of the exoskeleton were optimized to fulfill different functions according to different ecophysiological strains faced by the animals. This is achieved by modifications in microstructure and chemical composition. In order to understand the relationship between structure, composition, mechanical properties and function we structurally characterized cuticle from the dorsal carapace of the edible crab Cancer pagurus using light and scanning electron microscopy (SEM). The local chemical composition was investigated using energy dispersive X-ray spectroscopy (EDX) and confocal m-Raman spectroscopy. Nanoindentation tests were performed to study the resulting local mechanical properties. The results show local differences in structure on several levels of the structural hierarchy in combination with a very heterogeneous mineralization. The distal exocuticle is mineralized with calcite, followed by a layer containing a magnesium, phosphate and carbonate rich phase and ACC in the proximal part. The endocuticle contains magnesian calcite and ACC in special regions below the exocuticle. Structure and mineral phase are reflected in the local stiffness and hardness of the respective cuticle regions. The heterogeneity of structural organization and mechanical properties suggests remarkable consequences for the mechanical behaviour of the bulk material.
    view abstract10.1524/zkri.2012.1532
  • DAMASK: The Düsseldorf advanced material simulation kit for studying crystal plasticity using an fe based or a spectral numerical solver
    Roters, F. and Eisenlohr, P. and Kords, C. and Tjahjanto, D.D. and Diehl, M. and Raabe, D.
    Procedia IUTAM 3 (2012)
    The solution of a continuum mechanical boundary value problem requires a constitutive response that connects deformation and stress at each material point. Such connection can be regarded as three separate hierarchical problems. At the top-most level, partitioning of the (mean) boundary values of the material point among its microstructural constituents and the associated homogenization of their response is required, provided there is more than one constituent present. Second, based on an elastoplastic decomposition of (finite strain) deformation, these responses follow from explicit or implicit time integration of the plastic deformation rate per constituent. Third, to establish the latter, a state variable-based constitutive law needs to be interrogated and its state updated. The D̈üsseldorf Advanced MAterial Simulation Kit (DAMASK) reflects this hierarchy as it is built in a strictly modular way. This modular structure makes it easy to add additional constitutive models as well as homogenization schemes. Moreover it interfaces with a number of FE solvers as well as a spectral solver using an FFT. We demonstrate the versatility of such a modular framework by considering three scenarios: Selective refinement of the constitutive material description within a single geometry, component-scale forming simulations comparing different homogenization schemes, and comparison of representative volume element simulations based on the FEM and the spectral solver. © 2012 Published by Elsevier B.V.
    view abstract10.1016/j.piutam.2012.03.001
  • Dislocation density measurement by electron channeling contrast imaging in a scanning electron microscope
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 66 (2012)
    We have measured the average dislocation density by electron channeling contrast imaging (ECCI) in a scanning electron microscope under controlled diffraction conditions in a Fe-3 wt.% Si alloy tensile deformed to a macroscopic stress of 500 MPa. Under optimal diffraction conditions, ECCI provides an average dislocation density close to that obtained by bright-field transmission electron microscopy. This result confirms that ECCI is a powerful technique for determining dislocation densities in deformed bulk metals. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2011.11.027
  • Elastic properties of face-centred cubic Fe-Mn-C studied by nanoindentation and ab initio calculations
    Reeh, S. and Music, D. and Gebhardt, T. and Kasprzak, M. and Jäpel, T. and Zaefferer, S. and Raabe, D. and Richter, S. and Schwedt, A. and Mayer, J. and Wietbrock, B. and Hirt, G. and Schneider, J.M.
    Acta Materialia 60 (2012)
    We have studied experimentally and theoretically the influence of C and Mn content on the Young's modulus of Fe-Mn-C alloys. Combinatorial thin film and bulk samples were characterized regarding their structure, texture and Young's modulus. The following chemical composition range was investigated: 1.5-3.0 at.% C, 28.0-37.5 at.% Mn and 60.6-69.8 at.% Fe. The experimental lattice parameters change marginally within 3.597-3.614 Å with the addition of C and are consistent with ab initio calculations. The Young's modulus data are in the range of 185 ± 12-251 ± 59 GPa for the bulk samples and the thin film, respectively. C has no significant effect on the Young's modulus of these alloys within the composition range studied here. The ab initio calculations are 15-22% larger than the average Young's modulus values of the as-deposited and polished thin film at 3 at.% C. The comparison of thin film and bulk samples results reveals similar elastic properties for equivalent compositions, indicating that the applied research strategy consisting of the combinatorial thin film approach in conjunction with ab initio calculations is useful to study the composition dependence of the structure and elastic properties of Fe-Mn-C alloys. The very good agreement between the presented calculations and the experimentally determined lattice parameters and Young's modulus values implies that the here-adopted simulation strategy yields a reliable description of carbon in Fe-Mn alloys, important for future alloy design. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.07.038
  • Erratum: Dislocation density measurement by electron channeling contrast imaging in a scanning electron microscope (Scripta Materialia (2012) 66 (343-346))
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 66 (2012)
    view abstract10.1016/j.scriptamat.2012.02.001
  • Evolution of strength and microstructure during annealing of heavily cold-drawn 6.3 GPa hypereutectoid pearlitic steel wire
    Li, Y.J. and Choi, P. and Goto, S. and Borchers, C. and Raabe, D. and Kirchheim, R.
    Acta Materialia 60 (2012)
    Hypereutectoid steel wires with 6.35 GPa tensile strength after a cold-drawing true strain of 6.02 were annealed between 300 and 723 K. The ultrahigh strength remained upon annealing for 30 min up to a temperature of 423 K but dramatically decreased with further increasing temperature. The reduction of tensile strength mainly occurred within the first 2-3 min of annealing. Atom probe tomography and transmission electron microscopy reveal that the lamellar structure remains up to 523 K. After annealing at 673 K for 30 min, coarse hexagonal ferrite (sub)grains with spheroidized cementite, preferentially located at triple junctions, were observed in transverse cross-sections. C and Si segregated at the (sub)grain boundaries, while Mn and Cr enriched at the ferrite/cementite phase boundaries due to their low mobility in cementite. No evidence of recrystallization was found even after annealing at 723 K for 30 min. The stability of the tensile strength for low-temperature annealing (<473 K) and its dramatic drop upon high-temperature annealing (>473 K) are discussed based on the nanostructural observations. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.03.006
  • Exploring the p-n junction region in Cu(In,Ga)Se 2 thin-film solar cells at the nanometer-scale
    Cojocaru-Mirédin, O. and Choi, P. and Wuerz, R. and Raabe, D.
    Applied Physics Letters 101 (2012)
    In this work we study the CdS/Cu(In,Ga)Se 2 p-n junction region in Cu(In,Ga)Se 2 thin-film solar cells using atom probe tomography. A Cu-, Ga-depleted, and Cd-doped region of about 1 nm thickness is detected at the Cu(In,Ga)Se 2 side of the CdS/Cu(In,Ga)Se 2 interface. Furthermore, Cd is also found to be enriched at Cu(In,Ga)Se 2 grain boundaries connected to the CdS layer. Na and O impurities decorate the CdS/CIGS interface, where Na-rich clusters are preferentially located in CdS regions abutting to Cu(In,Ga)Se 2 grain boundaries. The experimental findings of this work demonstrate the capability of atom probe tomography in studying buried interfaces and yield vital information for understanding and modeling the p-n junction band structure in Cu(In,Ga)Se 2 solar cells. © 2012 American Institute of Physics.
    view abstract10.1063/1.4764527
  • Grain size effect on strain hardening in twinning-induced plasticity steels
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 66 (2012)
    We investigate the influence of grain size on the strain hardening of two Fe-22Mn-0.6C (wt.%) twinning-induced plasticity steels with average grain sizes of 3 and 50 μm, respectively. The grain size has a significant influence on the strain hardening through the underlying microstructure. The dislocation substructure formed in the early deformation stages determines the density of nucleation sites for twins per unit grain boundary area which controls the developing twin substructure. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2012.01.037
  • Grain structure and irreversibility line of a bronze route CuNb reinforced NbNb3Sn multifilamentary wire
    Sandim, M.J.R. and Stamopoulos, D. and Aristomenopoulou, E. and Zaefferer, S. and Raabe, D. and Awaji, S. and Watanabe, K.
    Physics Procedia 36 (2012)
    High-resolution electron backscatter diffraction (EBSD) technique and DC magnetization were used to characterize a Cu-Nb reinforced bronze route Nb3Sn superconducting multifilamentary wire. The results of DC-magnetization show an extended regime of magnetic reversibility in the operational magnetic field-temperature phase diagram. This observation is discussed in terms of microstructure characteristics of the A15 phase such as grain size, grain boundary misorientation angle distribution, tin gradient across the filaments and residual strain, in connection to the literature. © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.
    view abstract10.1016/j.phpro.2012.06.122
  • Heterogeneous shear in hard sphere glasses
    Mandal, S. and Gross, M. and Raabe, D. and Varnik, F.
    Physical Review Letters 108 (2012)
    There is growing evidence that the flow of driven amorphous solids is not homogeneous, even if the macroscopic stress is constant across the system. Via event-driven molecular dynamics simulations of a hard sphere glass, we provide the first direct evidence for a correlation between the fluctuations of the local volume fraction and the fluctuations of the local shear rate. Higher shear rates do preferentially occur at regions of lower density and vice versa. The temporal behavior of fluctuations is governed by a characteristic time scale, which, when measured in units of strain, is independent of shear rate in the investigated range. Interestingly, the correlation volume is also roughly constant for the same range of shear rates. A possible connection between these two observations is discussed. © 2012 American Physical Society.
    view abstract10.1103/PhysRevLett.108.098301
  • Hydrogen-induced cracking at grain and twin boundaries in an Fe-Mn-C austenitic steel
    Koyama, M. and Akiyama, E. and Sawaguchi, T. and Raabe, D. and Tsuzaki, K.
    Scripta Materialia 66 (2012)
    Hydrogen embrittlement was observed in an Fe-18Mn-1.2C (wt.%) steel. The tensile ductility was drastically reduced by hydrogen charging during tensile testing. The fracture mode was mainly intergranular fracture, though transgranular fracture was also partially observed. The transgranular fracture occurred parallel to the primary and secondary deformation twin boundaries, as confirmed by electron backscattering diffraction analysis and orientation-optimized electron channeling contrast imaging. The microstructural observations indicate that cracks are initiated at grain boundaries and twin boundaries. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2011.12.015
  • Improved single- and multi-contact life-time testing of dental restorative materials using key characteristics of the human masticatory system and a force/position-controlled robotic dental wear simulator
    Raabe, D. and Harrison, A. and Ireland, A. and Alemzadeh, K. and Sandy, J. and Dogramadzi, S. and Melhuish, C. and Burgess, S.
    Bioinspiration & Biomimetics 7 (2012)
    This paper presents a new in vitro wear simulator based on spatial parallel kinematics and a biologically inspired implicit force/position hybrid controller to replicate chewing movements and dental wear formations on dental components, such as crowns, bridges or a full set of teeth. The human mandible, guided by passive structures such as posterior teeth and the two temporomandibular joints, moves with up to 6 degrees of freedom (DOF) in Cartesian space. The currently available wear simulators lack the ability to perform these chewing movements. In many cases, their lack of sufficient DOF enables them only to replicate the sliding motion of a single occlusal contact point by neglecting rotational movements and the motion along one Cartesian axis. The motion and forces of more than one occlusal contact points cannot accurately be replicated by these instruments. Furthermore, the majority of wear simulators are unable to control simultaneously the main wear-affecting parameters, considering abrasive mechanical wear, which are the occlusal sliding motion and bite forces in the constraint contact phase of the human chewing cycle. It has been shown that such discrepancies between the true in vivo and the simulated in vitro condition influence the outcome and the quality of wear studies. This can be improved by implementing biological features of the human masticatory system such as tooth compliance realized through the passive action of the periodontal ligament and active bite force control realized though the central nervous system using feedback from periodontal preceptors. The simulator described in this paper can be used for single-and multi-occlusal contact testing due to its kinematics and ability to exactly replicate human translational and rotational mandibular movements with up to 6 DOF without neglecting movements along or around the three Cartesian axes. Recorded human mandibular motion and occlusal force data are the reference inputs of the simulator. Experimental studies of wear using this simulator demonstrate that integrating the biological feature of combined force/position hybrid control in dental material testing improves the linearity and reduces the variability of results. In addition, it has been shown that present biaxially operated dental wear simulators are likely to provide misleading results in comparative in vitro/in vivo one-contact studies due to neglecting the occlusal sliding motion in one plane which could introduce an error of up to 49% since occlusal sliding motion D and volumetric wear loss V-loss are proportional.
    view abstract10.1088/1748-3182/7/1/016002
  • Influence of grain boundary mobility on microstructure evolution during recrystallisation
    Winning, M. and Raabe, D.
    Materials Science Forum 715-716 (2012)
    The paper introduces first investigations on how low angle grain boundaries can influence the recrystallisation behaviour of crystalline metallic materials. For this purpose a threedimensional cellular automaton model was used. The approach in this study is to allow even low angle grain boundaries to move during recrystallisation. The effect of this non-zero mobility of low angle grain boundaries will be analysed for the recrystallisation of deformed Al single crystals with Cube orientation. It will be shown that low angle grain boundaries indeed influence the kinetics as well as the texture evolution of metallic materials during recrystallisation. © (2012) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.715-716.191
  • Influences of deformation strain, strain rate and cooling rate on the Burgers orientation relationship and variants morphology during β→α phase transformation in a near α titanium alloy
    He, D. and Zhu, J.C. and Zaefferer, S. and Raabe, D. and Liu, Y. and Lai, Z.L. and Yang, X.W.
    Materials Science and Engineering A 549 (2012)
    High temperature compression deformation studies of Ti-6Al-2Zr-1Mo-1V titanium alloy in full β phase region with different strains/strain rates and then with subsequent varied cooling rates were performed to understand the microstructure evolution. Crystal orientation information and microstructure morphology of all tested samples were investigated by electron backscatter diffraction (EBSD) measurements. The crystal orientations of prior high temperature β grains were estimated by reconstructing the retained β phase at room temperature. The theoretical crystal orientations of all possible α variants within an investigated prior β grain were calculated according to the Burgers orientation relationship (OR) between parent and product phase. The calculated and experimental results were then compared and analyzed. The influences of deformation strain, strain rate and cooling rate on the Burgers OR between prior β matrix and precipitated α phase were investigated. Full discussions have been conducted by combination of crystal plasticity finite element method (CP-FEM) grain-scale simulation results. The results indicate that external factors (such as deformation strain, strain rate and cooling rate) have a slight influence on the obeying of Burgers OR rule during β → α phase transformation. However, strain rate and cooling rate have a significant effect on the morphology of precipitated α phase. © 2012 Elsevier B.V.
    view abstract10.1016/j.msea.2012.03.110
  • Microstructural evolution of a Ni-based superalloy (617B) at 700 °c studied by electron microscopy and atom probe tomography
    Tytko, D. and Choi, P.-P. and Klöwer, J. and Kostka, A. and Inden, G. and Raabe, D.
    Acta Materialia 60 (2012)
    We report on the microstructural evolution of a polycrystalline Ni-based superalloy (Alloy 617B) for power plant applications at a service temperature of 700 °C. The formation of secondary M 23C 6-carbides close to grain boundaries (GBs) and around primary Ti(C,N) particles is observed upon annealing at 700 °C, where γ′ is found to nucleate heterogeneously at M 23C 6 carbides. Using atom probe tomography, elemental partitioning to the phases and composition profiles across phase and grain boundaries are determined. Enrichments of B at γ/M 23C 6 and γ′/M 23C 6 interfaces as well as at grain boundaries are detected, while no B enrichment is found at γ/γ′ interfaces. It is suggested that segregation of B in conjunction with γ′ formation stabilizes a network of secondary M 23C 6 precipitates near GBs and thus increases the creep rupture life of Alloy 617B. Calculations of the equilibrium phase compositions by Thermo-Calc confirm the chemical compositions measured by atom probe tomography. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.11.020
  • Microstructure evolution during recrystallization in dual-phase steels
    Peranio, N. and Roters, F. and Raabe, D.
    Materials Science Forum 715-716 (2012)
    The microstructure and texture of rolled and annealed dual-phase steels with 0.147 wt. % C, 1.868 wt. % Mn, and 0.403 wt. % Si were analyzed using SEM, EDX, and EBSD. Hot rolled sheets showed a ferritic-pearlitic microstructure with a pearlite volume fraction of about 40 % and ferrite grain size of about 6 μm. Ferrite and pearlite were heterogeneously distributed at the surface and distributed in bands at the center of the sheets. The hot rolled sheets revealed a throughthickness texture inhomogeneity with a plane-strain texture with strong α-fiber and γ-fiber at the center and a shear texture at the surface. After cold rolling, the ferrite grains showed elongated morphology and larger orientation gradients, the period of the ferrite-pearlite band structure at the center of the sheets was decreased, and the plane-strain texture components were strengthened in the entire sheet. Recrystallization, phase transformation, and the competition of both processes were of particular interest with respect to the annealing experiments. For this purpose, various annealing techniques were applied, i.e., annealing in salt bath, conductive annealing, and industrial hot-dip coating. The sheets were annealed in the ferritic, intercritical, and austenitic temperature regimes in a wide annealing time range including variation of heating and cooling rates. © (2012) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.715-716.13
  • Modelling of dendritic growth and bubble formation
    Wu, W. and Zhu, M.F. and Sun, D.K. and Dai, T. and Han, Q.Y. and Raabe, D.
    IOP Conference Series: Materials Science and Engineering 33 (2012)
    A two-dimensional lattice Boltzmann method (LBM)-cellular automaton (CA) model is developed for the simulation of dendritic growth and bubble formation during alloy solidification. In the model, a kinetic LBM, which describes flow dynamics through the evolution of distribution functions of moving pseudo-particles, is adopted to numerically solve the gas-liquid two-phase flow based on the Shan-Chen multiphase scheme. The kinetics of dendritic growth is determined according to a local solute equilibrium approach. The present model takes into account the effect of liquid-solid phase transformation on the nucleation and growth of bubbles. The interaction mechanism between dendrites and bubbles is also embedded in the model. The wettability of a bubble on a smooth solid surface is simulated. The simulated contact angles with various interaction coefficients agree well with the data calculated from an empirical formula derived from the Young's equation. The proposed model is applied to simulate dendritic growth and bubble formation under directional solidification conditions. The simulated results are compared with those observed experimentally during solidification of a transparent organic material. The simulation results reveal some dynamic features of bubble nucleation, growth, and motion, as well as the interaction between the dendritic growth and bubble formation during solidification. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1757-899X/33/1/012103
  • Modification of pineapple leaf fibers and graft copolymerization of acrylonitrile onto modified fibers
    Maniruzzaman, M. and Rahman, M.A. and Gafur, M.A. and Fabritius, H. and Raabe, D.
    Journal of Composite Materials 46 (2012)
    Raw pineapple leaf fibers (PALFs) were chemically modified by scouring, NaOH treatment, and bleaching (NaClO2). The graft copolymerization of synthetic acrylonitrile monomer onto bleached PALFs was carried out in aqueous medium using potassium persulfate (K2S2O8/FeSO4) as a redox initiator. The maximum grafting level at optimum conditions, namely, monomer concentration, initiator concentration, catalyst concentration, reaction time, and temperature have been determined. The main objective of this study is to decrease the amorphous region of lignocellulose in PALFs and improve its hydrophobic nature by incorporation of synthetic polymer of polyacrylonitrile and mechanical properties. The modified and grafted fibers were characterized by Fourier transform infrared spectroscopy, scanning electron microscope, thermogravimetric analysis, and X-ray diffraction study techniques. The moisture content and tensile strength properties were also evaluated for their environmental and mechanical performances. © The Author(s) 2011.
    view abstract10.1177/0021998311410486
  • Multistage strain hardening through dislocation substructure and twinning in a high strength and ductile weight-reduced Fe-Mn-Al-C steel
    Gutierrez-Urrutia, I. and Raabe, D.
    Acta Materialia 60 (2012)
    We investigate the kinetics of the deformation structure evolution and its contribution to the strain hardening of a Fe-30.5Mn-2.1Al-1.2C (wt.%) steel during tensile deformation by means of transmission electron microscopy and electron channeling contrast imaging combined with electron backscatter diffraction. The alloy exhibits a superior combination of strength and ductility (ultimate tensile strength of 1.6 GPa and elongation to failure of 55%) due to the multiple-stage strain hardening. We explain this behavior in terms of dislocation substructure refinement and subsequent activation of deformation twinning. The early hardening stage is fully determined by the size of the dislocation substructure, namely, Taylor lattices, cell blocks and dislocation cells. The high carbon content in solid solution has a pronounced effect on the evolving dislocation substructure. We attribute this effect to the reduction of the dislocation cross-slip frequency by solute carbon. With increasing applied stress, the cross-slip frequency increases. This results in a gradual transition from planar (Taylor lattices) to wavy (cells, cell blocks) dislocation configurations. The size of such dislocation substructures scales inversely with the applied resolved stress. We do not observe the so-called microband-induced plasticity effect. In the present case, due to texture effects, microbanding is not favored during tensile deformation and, hence, has no effect on strain hardening. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.07.018
  • Nanoscale austenite reversion through partitioning, segregation and kinetic freezing: Example of a ductile 2 GPa Fe-Cr-C steel
    Yuan, L. and Ponge, D. and Wittig, J. and Choi, P. and Jiménez, J.A. and Raabe, D.
    Acta Materialia 60 (2012)
    Austenite reversion during tempering of a Fe-13.6 Cr-0.44 C (wt.%) martensite results in an ultra-high-strength ferritic stainless steel with excellent ductility. The austenite reversion mechanism is coupled to the kinetic freezing of carbon during low-temperature partitioning at the interfaces between martensite and retained austenite and to carbon segregation at martensite-martensite grain boundaries. An advantage of austenite reversion is its scalability, i.e. changing tempering time and temperature tailors the desired strength-ductility profiles (e.g. tempering at 400 °C for 1 min produces a 2 GPa ultimate tensile strength (UTS) and 14% elongation while 30 min at 400 °C results in a UTS of ∼1.75 GPa with an elongation of 23%). The austenite reversion process, carbide precipitation and carbon segregation have been characterized by X-ray diffraction, electron back-scatter diffraction, transmission electron microscopy and atom probe tomography in order to develop the structure-property relationships that control the material's strength and ductility. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.01.045
  • New insights on quantitative microstructure characterization by electron channeling contrast imaging under controlled diffraction conditions in SEM
    Gutierrez-Urrutia, I. and Raabe, D.
    Microscopy and Microanalysis 18 (2012)
    view abstract10.1017/S1431927612005284
  • Non-crystallographic shear banding in crystal plasticity FEM simulations: Example of texture evolution in α-brass
    Jia, N. and Roters, F. and Eisenlohr, P. and Kords, C. and Raabe, D.
    Acta Materialia 60 (2012)
    We present crystal plasticity finite element simulations of the texture evolution in α-brass polycrystals under plane strain compression. The novelty is a non-crystallographic shear band mechanism [Anand L, Su C. J Mech Phys Solids 2005;53:1362] that is incorporated into the constitutive model in addition to dislocation and twinning. Non-crystallographic deformation associated with shear banding leads to weaker copper and S texture components and to a stronger brass texture compared to simulations enabling slip and twinning only. The lattice rotation rates are reduced when shear banding occurs. This effect leads to a weaker copper component. Also, the initiation of shear banding promotes brass-type components. In summary the occurrence of non-crystallographic deformation through shear bands shifts face-centered-cubic deformation textures from the copper type to the brass type. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.10.047
  • Numerical simulation of dynamic strain-induced austenite-ferrite transformation and post-dynamic kinetics in a low carbon steel
    Zheng, C. and Raabe, D. and Li, D.
    Materials Science Forum 706-709 (2012)
    2-D cellular automaton model was developed to simulate the dynamic strain-induced transformation (DSIT) from austenite (?) to ferrite (a) and the post-dynamic kinetic behavior in a low carbon steel with the purpose of developing a methodology of mesoscopic computer simulation for an improved understanding of the formation of ultra-fine ferrite (UFF) in DSIT and the conservation of this microstructure during the post-deformation period. The predicted microstructure obtained after DSIT was compared with a quenched dual-phase steel. Its microstructure, consisting of fine-grained ferrite and fine islands of retained austenite dispersed in the matrix, were found to be in good agreement with the predictions. The simulated results indicate that the refinement of ferrite grains produced via DSIT can be interpreted in terms of unsaturated nucleation and limited growth mechanisms. It is also revealed that continuing transformation from retained austenite to ferrite and the reverse transformation both could take place simultaneously during the post-deformation isothermal holding. A competition between them exists at the early stage of the post-dynamic transformation. © 2012 Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.706-709.1592
  • On the correlation between thermal cycle and formation of intermetallic phases at the interface of laser-welded aluminum-steel overlap joints
    Szczepaniak, A. and Fan, J. and Kostka, A. and Raabe, D.
    Advanced Engineering Materials 14 (2012)
    A laser beam welding process via heat conduction was applied to join DC01 steel with aluminum (Al) in overlap configuration without filler wire. The effect of the applied laser power (1.7, 1.8, 2.1, and 2.4 kW) on the formation and evolution of the interfaces between steel and Al was analyzed. Two intermetallic compounds were found at the interface, namely, one adjacent to the steel layer (Al 5Fe 2) and one close to the solidified Al (Al 13Fe 4). The thickness of the intermetallic reaction layer increases with laser power, while the morphology of its individual components evolves due to differences in accumulated thermal cycles. Correlations between simulations and measurements show that the peak temperature has significantly stronger influence on the thickness of the intermetallic reaction layer than cooling time and the integral of temperature over the time. Shear/tensile strength tests reveal that all the specimens fail in the Al heat affected zone. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201200075
  • On the effect of manganese on grain size stability and hardenability in ultrafine-grained ferrite/martensite dual-phase steels
    Calcagnotto, M. and Ponge, D. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 (2012)
    Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). The Mn distribution in ferrite and martensite is analyzed by high-resolution electron backscatter diffraction (EBSD) combined with energy dispersive X-ray spectroscopy (EDX). The experimental findings are supported by the calculated phase diagrams, equilibrium phase compositions, and the estimated diffusion distances using Thermo-Calc (Thermo-Calc Software, McMurray, PA) and Dictra (Thermo-Calc Software). Mn substantially enhances the grain size stability during intercritical annealing and the ability of austenite to undergo martensitic phase transformation. The first observation is explained in terms of the alteration of the phase transformation temperatures and the grain boundary mobility, while the second is a result of the Mn enrichment in cementite during large strain warm deformation, which is inherited by the newly formed austenite and increases its hardenability. The latter is the main reason why the ultrafine-grained material exhibits a hardenability that is comparable with the hardenability of the coarse-grained reference material. © 2011 The Minerals, Metals & Materials Society and ASM International.
    view abstract10.1007/s11661-011-0828-3
  • Orientation dependence of shear banding in face-centered-cubic single crystals
    Jia, N. and Eisenlohr, P. and Roters, F. and Raabe, D. and Zhao, X.
    Acta Materialia 60 (2012)
    We present crystal plasticity finite element simulations of plane strain compression of α-Brass single crystals with different initial orientations. The aim is to study the fundamentals of mesoscale structure and texture development in face-centered-cubic (fcc) metals with low stacking fault energy (SFE). Shear banding depends on the initial orientation of the crystals. In Copper and Brass-R-oriented crystals which show the largest tendency to form shear bands, an inhomogeneous texture distribution induced by shear banding is observed. To also understand the influence of the micromechanical boundary conditions on shear band formation, simulations on Copper-oriented single crystals with varying sample geometry and loading conditions are performed. We find that shear banding can be understood in terms of a mesoscopic softening mechanism. The predicted local textures and the shear banding patterns agree well with experimental observations in low SFE fcc crystals. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.03.005
  • Orientation informed nanoindentation of α-titanium: Indentation pileup in hexagonal metals deforming by prismatic slip
    Zambaldi, C. and Yang, Y. and Bieler, T.R. and Raabe, D.
    Journal of Materials Research 27 (2012)
    This study reports on the anisotropic indentation response of α-titanium. Coarse-grained titanium was characterized by electron backscatter diffraction. Sphero-conical nanoindentation was performed for a number of different crystallographic orientations. The grain size was much larger than the size of the indents to ensure quasi-single-crystal indentation. The hexagonal c-axis was determined to be the hardest direction. Surface topographies of several indents were measured by atomic force microscopy. Analysis of the indent surfaces, following Zambaldi and Raabe (Acta Mater. 58(9), 3516-3530), revealed the orientation-dependent pileup behavior of α-titanium during axisymmetric indentation. Corresponding crystal plasticity finite element (CPFE) simulations predicted the pileup patterns with good accuracy. The constitutive parameters of the CPFE model were identified by a nonlinear optimization procedure, and reproducibly converged toward easy activation of prismatic glide systems. The calculated critical resolved shear stresses were 150 ± 4, 349 ± 10, and 1107 ± 39 MPa for prismatic and basal «a»-glide and pyramidal «c + a»-glide, respectively. © 2011 Materials Research Society.
    view abstract10.1557/jmr.2011.334
  • Polycrystal model of the mechanical behavior of a Mo-TiC 30 vol.% metal-ceramic composite using a three-dimensional microstructure map obtained by dual beam focused ion beam scanning electron microscopy
    Cédat, D. and Fandeur, O. and Rey, C. and Raabe, D.
    Acta Materialia 60 (2012)
    The mechanical behavior of a Mo-TiC 30 vol.% ceramic-metal composite was investigated over a wide temperature range (25-700 °C). High-energy X-ray tomography was used to reveal percolation of the hard titanium carbide phase through the composite. Using a polycrystal approach for a two-phase material, finite-element simulations were performed on a real three-dimensional (3-D) aggregate of the material. The 3-D microstructure, used as the starting configuration for the predictions, was obtained by serial sectioning in a dual beam focused ion beam scanning electron microscope coupled to an electron backscattered diffraction system. The 3-D aggregate consists of a molybdenum matrix and a percolating TiC skeleton. As for most body-centered cubic (bcc) metals, the molybdenum matrix phase is characterized by a change in plasticity mechanism with temperature. We used a polycrystal model for bcc materials which was extended to two phases (TiC and Mo). The model parameters of the matrix were determined from experiments on pure molydenum. For all temperatures investigated the TiC particles were considered to be brittle. Gradual damage to the TiC particles was treated, based on an accumulative failure law that is approximated by evolution of the apparent particle elastic stiffness. The model enabled us to determine the evolution of the local mechanical fields with deformation and temperature. We showed that a 3-D aggregate representing the actual microstructure of the composite is required to understand the local and global mechanical properties of the composite studied. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.11.055
  • Precipitation behavior of v and/or cu bearing middle carbon steels
    Iwasa, N. and Sakata, N. and Takahashi, J. and Raabe, D. and Takemoto, Y. and Senuma, T.
    Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan 98 (2012)
    In this study, the precipitation behavior of middle carbon steels with single and multiple additions of 0.3mass%V and 2mass%Cu has been investigated. The precipitation treatment was carried out isothermally at 600°C.In the 0.3mass%V bearing steel, precipitates were observed in rows, indicating the occurrence of the interphase precipitation. On the other hand, precipitates observed in the 2mass%Cu bearing steel were randomly dispersed. In the V and Cu bearing steel, Cu precipitates were not randomly dispersed and mainly observed attached with VCN which were precipitated in rows. In the paper, the different precipitation behavior of the three steels is discussed in detail and is explained from a viewpoint of the thermodynamics and kinetics using a metallurgical model.
    view abstract10.2355/tetsutohagane.98.434
  • Prediction of post-dynamic austenite-to-ferrite transformation and reverse transformation in a low-carbon steel by cellular automaton modeling
    Zheng, C. and Raabe, D. and Li, D.
    Acta Materialia 60 (2012)
    The post-dynamic transformation that takes place during the subsequent isothermal holding for the case when dynamic strain-induced transformation (DSIT) from austenite to ferrite occurs during hot deformation is investigated by cellular automaton modeling. The simulation provides a better understanding of carbon diffusion in retained austenite and the resulting microstructure evolution during the post-dynamic transformation. The predictions reveal that continuing transformation from retained austenite to ferrite and the reverse transformation can occur simultaneously in the same microstructure during post-deformation isothermal holding owing to the locally acting chemical equilibrium conditions. Competition between forward and reverse transformation exists during the early stage of post-dynamic heat treatment. It is also revealed that increasing the final strain of DSIT might promote the reverse transformation, whereas the continuous austenite-to-ferrite transformation yields a diminishing effect. The influence of the DSIT final strain on the grain size of ferrite and the characteristics of the resultant microstructure is also discussed. © 2012 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstract10.1016/j.actamat.2012.06.007
  • Rapid alloy prototyping: Compositional and thermo-mechanical high throughput bulk combinatorial design of structural materials based on the example of 30Mn-1.2C-xAl triplex steels
    Springer, H. and Raabe, D.
    Acta Materialia 60 (2012)
    We introduce a new experimental approach to the compositional and thermo-mechanical design and rapid maturation of bulk structural materials. This method, termed rapid alloy prototyping (RAP), is based on semi-continuous high throughput bulk casting, rolling, heat treatment and sample preparation techniques. 45 Material conditions, i.e. 5 alloys with systematically varied compositions, each modified by 9 different ageing treatments, were produced and investigated within 35 h. This accelerated screening of the tensile, hardness and microstructural properties as a function of chemical and thermo-mechanical parameters allows the highly efficient and knowledge-based design of bulk structural alloys. The efficiency of the approach was demonstrated on a group of Fe-30Mn-1.2C-xAl steels which exhibit a wide spectrum of structural and mechanical characteristics, depending on the respective Al concentration. High amounts of Al addition (>8 wt.%) resulted in pronounced strengthening, while low concentrations (<2 wt.%) led to embrittlement of the material during ageing. © 2012 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstract10.1016/j.actamat.2012.05.017
  • Recrystallization and grain growth in ultra fine grained materials produced by high pressure torsion
    Khorashadizadeh, A. and Winning, M. and Zaefferer, S. and Raabe, D.
    Recrystallization and Grain Growth Iv 715-716 (2012)
    view abstract10.4028/www.scientific.net/MSF.715-716.373
  • Recrystallization during Annealing of a Cold Rolled Lean Duplex Stainless Steel
    Herrera, C. and Ponge, D. and Raabe, D.
    Recrystallization and Grain Growth Iv 715-716 (2012)
    view abstract10.4028/www.scientific.net/MSF.715-716.550
  • Shear-induced anisotropic decay of correlations in hard-sphere colloidal glasses
    Chikkadi, V. and Mandal, S. and Nienhuis, B. and Raabe, D. and Varnik, F. and Schall, P.
    EPL 100 (2012)
    Spatial correlations of microscopic fluctuations are investigated via real-space experiments and computer simulations of colloidal glasses under steady shear. It is shown that while the distribution of one-particle fluctuations is always isotropic regardless of the relative importance of shear as compared to thermal fluctuations, their spatial correlations show a marked sensitivity to the competition between shear-induced and thermally activated relaxation. Correlations are isotropic in the thermally dominated regime, but develop strong anisotropy as shear dominates the dynamics of microscopic fluctuations. We discuss the relevance of this observation for a better understanding of flow heterogeneity in sheared amorphous solids. © Copyright EPLA, 2012.
    view abstract10.1209/0295-5075/100/56001
  • Simulation of dislocation penetration through a general low-angle grain boundary
    Liu, B. and Eisenlohr, P. and Roters, F. and Raabe, D.
    Acta Materialia 60 (2012)
    The interaction of dislocations with low-angle grain boundaries (LAGBs) is considered one important contribution to the mechanical strength of metals. Although LAGBs have been frequently observed in metals, little is known about how they interact with free dislocations that mainly carry the plastic deformation. Using discrete dislocation dynamics simulations, we are able to quantify the resistance of a LAGB - idealized as three sets of dislocations that form a hexagonal dislocation network - against lattice dislocation penetration, and examine the associated dislocation processes. Our results reveal that such a coherent internal boundary can massively obstruct and even terminate dislocation transmission and thus make a substantial contribution to material strength. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.05.002
  • Structure, composition, and mechanical properties of shark teeth
    Enax, J. and Prymak, O. and Raabe, D. and Epple, M.
    Journal of Structural Biology 178 (2012)
    The teeth of two different shark species (Isurus oxyrinchus and Galeocerdo cuvier) and a geological fluoroapatite single crystal were structurally and chemically characterized. In contrast to dentin, enameloid showed sharp diffraction peaks which indicated a high crystallinity of the enameloid. The lattice parameters of enameloid were close to those of the geological fluoroapatite single crystal. The inorganic part of shark teeth consisted of fluoroapatite with a fluoride content in the enameloid of 3.1 wt.%, i.e., close to the fluoride content of the geological fluoroapatite single crystal (3.64 wt.%). Scanning electron micrographs showed that the crystals in enameloid were highly ordered with a special topological orientation (perpendicular towards the outside surface and parallel towards the center). By thermogravimetry, water, organic matrix, and biomineral in dentin and enameloid of both shark species were determined. Dentin had a higher content of water, organic matrix, and carbonate than enameloid but contained less fluoride. Nanoindentation and Vicker's microhardness tests showed that the enameloid of the shark teeth was approximately six times harder than the dentin. The hardness of shark teeth and human teeth was comparable, both for dentin and enamel/enameloid. In contrast, the geological fluoroapatite single crystal was much harder than both kinds of teeth due to the absence of an organic matrix. In summary, the different biological functions of the shark teeth (" tearing" for Isurus and "cutting" for Galeocerdo) are controlled by the different geometry and not by the chemical or crystallographic composition. © 2012 Elsevier Inc.
    view abstract10.1016/j.jsb.2012.03.012
  • Study of deformation twinning and planar slip in a TWIP steel by electron channeling contrast imaging in a SEM
    Gutierrez-Urrutia, I. and Raabe, D.
    Materials Science Forum 702-703 (2012)
    We study the dislocation and twin substructures in a high manganese twinning-induced-plasticity steel (TWIP) by means of electron channeling contrast imaging. At low strain (true strain below 0.1) the dislocation substructure shows strong orientation dependence. It consists of dislocation cells and planar dislocation arrangements. This dislocation substructure is replaced by a complex dislocation/twin substructure at high strain (true strain of 0.3-0.4). The twin substructure also shows strong orientation dependence. We identify three types of dislocation/twin substructures. Two of these substructures, those which are highly favorable or unfavorable oriented for twinning, exhibit a Schmid behavior. The other twin substructure does not fulfill Schmid's law. © (2012) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.702-703.523
  • Temperature dependence of Z-Contrast for InGaN
    Mehrtens, T. and Schowalter, M. and Tytko, D. and Choi, P.-P. and Raabe, D. and Hoffmann, L. and Hangleiter, A. and Rosenauer, A.
    Microscopy and Microanalysis 18 (2012)
    view abstract10.1017/S143192761201094X
  • Tensile deformation characteristics of bulk ultrafine-grained austenitic stainless steel produced by thermal cycling
    Ravi Kumar, B. and Raabe, D.
    Scripta Materialia 66 (2012)
    Deformation microstructures of bulk ultrafine-grained austenitic AISI 304L stainless steels were analyzed by electron backscatter diffraction. Samples with grain sizes below 500 nm showed transition from grain-scale deformation to the collective phenomenon of shear banding. This was assisted by strain-induced grain rotation and coalescence. This phenomenon was suppressed in samples with a bimodal grain size distribution (maxima at ∼650 and ∼1400 nm) due to deformation-induced martensite formation, yielding high tensile strength and ductility (1348 MPa ultimate tensile strength at 0.36 max. true strain). © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2012.01.052
  • The relation between ductility and stacking fault energies in Mg and Mg-Y alloys
    Sandlöbes, S. and Friák, M. and Zaefferer, S. and Dick, A. and Yi, S. and Letzig, D. and Pei, Z. and Zhu, L.-F. and Neugebauer, J. and Raabe, D.
    Acta Materialia 60 (2012)
    The underlying mechanisms that are responsible for the improved room-temperature ductility in Mg-Y alloys compared to pure Mg are investigated by transmission electron microscopy and density functional theory. Both methods show a significant decrease in the intrinsic stacking fault I 1 energy (I 1 SFE) with the addition of Y. The influence of the SFE on the relative activation of different competing deformation mechanisms (basal, prismatic, pyramidal slip) is discussed. From this analysis we suggest a key mechanism which explains the transition from primary basal slip in hexagonal close-packed Mg to basal plus pyramidal slip in solid solution Mg-Y alloys. This mechanism is characterized by enhanced nucleation of 〈c + a〉 dislocations where the intrinsic stacking fault I 1 (ISF 1) acts as heterogeneous source for 〈c + a〉 dislocations. Possible electronic and geometric reasons for the modification of the SFE by substitutional Y atoms are identified and discussed. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.02.006
  • Theory-guided materials design of multi-phase Ti-Nb alloys with bone-matching elastic properties
    Friák, M. and Counts, W.A. and Ma, D. and Sander, B. and Holec, D. and Raabe, D. and Neugebauer, J.
    Materials 5 (2012)
    We present a scale-bridging approach for modeling the integral elastic response of polycrystalline composite that is based on a multi-disciplinary combination of (i) parameter-free first-principles calculations of thermodynamic phase stability and single-crystal elastic stiffness; and (ii) homogenization schemes developed for polycrystalline aggregates and composites. The modeling is used as a theory-guided bottom-up materials design strategy and applied to Ti-Nb alloys as promising candidates for biomedical implant applications. The theoretical results (i) show an excellent agreement with experimental data and (ii) reveal a decisive influence of the multi-phase character of the polycrystalline composites on their integral elastic properties. The study shows that the results based on the density functional theory calculations at the atomistic level can be directly used for predictions at the macroscopic scale, effectively scale-jumping several orders of magnitude without using any empirical parameters. © 2012 by the authors; licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma5101853
  • A (S)TEM and atom probe tomography study of InGaN
    Mehrtens, T. and Bley, S. and Schowalter, M. and Sebald, K. and Seyfried, M. and Gutowski, J. and Gerstl, S.S.A. and Choi, P.-P. and Raabe, D. and Rosenauer, A.
    Journal of Physics: Conference Series 326 (2011)
    In this work we show how the indium concentration in high indium content InxGa1-xN quantum wells, as they are commonly used in blue and green light emitting diodes, can be deduced from high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. This method bases on introducing normalized intensities which can be compared with multislice simulations to determine the specimen thickness or the indium concentration. The evaluated concentrations are compared with atom probe tomography measurements. It is also demonstrated how the quality of focused ion beam prepared TEM-lamellas can be improved by an additional etching with low energy ions.
    view abstract10.1088/1742-6596/326/1/012029
  • A dislocation density-based crystal plasticity constitutive model for prismatic slip in α-titanium
    Alankar, A. and Eisenlohr, P. and Raabe, D.
    Acta Materialia 59 (2011)
    A new constitutive plasticity model for prismatic slip in hexagonal α-titanium is developed. In the concept pure edge and screw dislocation densities evolve on the {101̄0}〈12̄10〉 slip systems. The model considers that the screw dislocation segments have a spread out core, leading to a much higher velocity of edge compared with screw dislocations. This enables the model to describe the observed transition in strain hardening from stage I to stage II in single crystals oriented for prismatic slip. Good agreement is found between the experimentally observed and simulated stress-strain behavior. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.07.053
  • A phase-field model for incoherent martensitic transformations including plastic accommodation processes in the austenite
    Kundin, J. and Raabe, D. and Emmerich, H.
    Journal of the Mechanics and Physics of Solids 59 (2011)
    If alloys undergo an incoherent martensitic transformation, then plastic accommodation and relaxation accompany the transformation. To capture these mechanisms we develop an improved 3D microelasticplastic phase-field model. It is based on the classical concepts of phase-field modeling of microelastic problems (Chen, L.Q.; Wang Y.; Khachaturyan, A.G.; 1992. Philos. Mag. Lett. 65, 1523). In addition to these it takes into account the incoherent formation of accommodation dislocations in the austenitic matrix, as well as their inheritance into the martensitic plates based on the crystallography of the martensitic transformation. We apply this new phase-field approach to the butterfly-type martensitic transformation in a Fe30 wt%Ni alloy in direct comparison to recent experimental data (Sato, H.; Zaefferer, S.; 2009. Acta Mater. 57, 19311937). It is shown that the therein proposed mechanisms of plastic accommodation during the transformation can indeed explain the experimentally observed morphology of the martensitic plates as well as the orientation between martensitic plates and the austenitic matrix. The developed phase-field model constitutes a general simulations approach for different kinds of phase transformation phenomena that inherently include dislocation based accommodation processes. The approach does not only predict the final equilibrium topology, misfit, size, crystallography, and aspect ratio of martensiteaustenite ensembles resulting from a transformation, but it also resolves the associated dislocation dynamics and the distribution, and the size of the crystals itself. © 2011 Elsevier Ltd.
    view abstract10.1016/j.jmps.2011.07.001
  • Analysis of the plastic anisotropy and pre-yielding of (γ/ α2)-phase titanium aluminide microstructures by crystal plasticity simulation
    Zambaldi, C. and Roters, F. and Raabe, D.
    Intermetallics 19 (2011)
    The plastic deformation of lamellar microstructures composed of the two phases γ-TiAl and α2-Ti3Al is highly orientation dependent. In this paper we present a homogenized model that takes into account the micromechanical effect of the plate-like morphologies that are often observed in two-phase titanium aluminide alloys. The model is based on crystal elasto-viscoplasticity and 18 deformation systems were implemented that have been identified to govern the plastic flow of the lamellar microstructures. The model is validated against experiments on polysynthetically twinned (PST) crystals and shows good agreement with the data. On a larger length scale, the model is applied to a 64-grain aggregate to investigate the mechanical response of two different kinds of microstructures. Different magnitudes of the kinematic constraints exerted by the densely spaced and highly aligned interfaces are shown to affect the macroscopic flow behavior of the microstructures. The phenomenon of pronounced microplasticity of fully lamellar material as well as the stress variation inside two-phase microstructures are studied quantitatively. © 2011 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2011.01.012
  • Annealing effects on microstructure and coercive field of ferritic-martensitic ODS Eurofer steel
    Renzetti, R.A. and Sandim, H.R.Z. and Sandim, M.J.R. and Santos, A.D. and Möslang, A. and Raabe, D.
    Materials Science and Engineering A 528 (2011)
    Oxide dispersion strengthened reduced-activation ferritic-martensitic steels are promising candidates for applications in future fusion power plants. Samples of a reduced activation ferritic-martensitic 9wt.%Cr-oxide dispersion strengthened Eurofer steel were cold rolled to 80% reduction in thickness an