Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

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  • 2020 • 240 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 abstractdoi: 10.1126/sciadv.aay1430
  • 2020 • 239 The effect of downstream laser fragmentation on the specific surface area and photoelectrochemical performance of barium tantalum oxynitride
    Haydous, F. and Waag, F. and Si, W. and Li, F. and Barcikowski, S. and Gökce, B. and Lippert, T.
    Applied Surface Science 510 (2020)
    One approach to improve the photoelectrochemical solar water splitting performance of photoanodes based on oxynitride perovskite particles is through increasing the active surface area which allows the generation of more electron-hole pairs that contribute in the water reduction and oxidation reactions. In this study, we explore the pros and cons of downstream laser fragmentation as a method to increase the specific surface area of oxynitride particles and highlight the important issues that must be considered for effective solar water splitting. The synthesis of particles with a high surface area of up to 32.4 m2 g−1 is demonstrated. Furthermore, the fragmented oxynitrides revealed lower absorbance values, a blue shift in the absorption edge and a higher background absorbance. These observations, in addition to the lower crystalline quality of the fragmented oxynitrides, were attributed to the loss of N content during fragmentation and the formation of secondary phases. The photoanodes based on the fragmented particles showed lower photocurrents than those prepared from the un-fragmented particles even though the surface area was increased. The decrease in photoactivity was ascribed to the presence of more grain boundaries in the fragmented oxynitride photoanodes which leads to more recombinations of the photogenerated carriers. Interestingly, after seven fragmentation passages, the photocurrent starts to increase again due to the formation of an amorphous layer which improves the transport of the photogenerated carriers. © 2020
    view abstractdoi: 10.1016/j.apsusc.2020.145429
  • 2020 • 238 On the rhenium segregation at the low angle grain boundary in a single crystal Ni-base superalloy
    He, J. and Scholz, F. and Horst, O.M. and Thome, P. and Frenzel, J. and Eggeler, G. and Gault, B.
    Scripta Materialia 185 88-93 (2020)
    Industrial scale single crystal (SX) Ni-base superalloys contain numerous low angle grain boundaries inherited from the solidification process. Here, we demonstrate that low angle grain boundaries in a fully heat-treated SX model Ni-base superalloy are strongly segregated with up to 12 at% Re. Some Re-rich dislocations forming this grain boundary are found located inside γ, others close to a γ/γ′ interface. Although these segregated Re atoms lose their solid-solution strengthening effect, they may enhance the creep resistance by pinning the low angle grain boundaries and slowing down dislocation reactions. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2020.03.063
  • 2020 • 237 Size dependent strength, slip transfer and slip compatibility in nanotwinned silver
    Kini, M.K. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 184 120-131 (2020)
    Perfect slip transfer through single coherent Σ3 twin boundaries is known to be a cross-slip-like mechanism occurring at low stresses, which is expected to strongly depend on material properties like stacking fault energy. In the present study, we extend the argument of perfect slip transfer to (i) multiple closely spaced coherent twin boundaries in a nanotwinned thin film and (ii) to materials with very low stacking fault energy. The slip transfer is indicated by the continuity of slip steps and observed across up to 100 coherent Σ3 boundaries. The study addresses size scaling due to multiple weak obstacles for dislocation motion and discusses the underlying deformation mechanisms. The importance of strain compatibility is further extended to incoherent twin boundaries. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.042
  • 2020 • 236 Correlative chemical and structural investigations of accelerated phase evolution in a nanocrystalline high entropy alloy
    Li, Y.J. and Kostka, A. and Savan, A. and Ludwig, Al.
    Scripta Materialia 183 122-126 (2020)
    Based on our recently-developed combinatorial processing platforms for accelerated investigations of phase evolution in multinary alloys, a novel correlative atom probe tomography and transmission electron microscopy approach is proposed to study phase stability in a nanocrystalline CrMnFeCoNi alloy. We observed that the material can decompose at 250 °C for 5 h or 300 °C for 1 h, having the same decomposed products as in its coarse-grained counterpart after annealing at 500 °C for 500 days. A low apparent activation energy for the diffusion of Ni in the nanocrystalline alloy is derived and explains the fast kinetics of phase decomposition in nanocrystalline alloys. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2020.03.016
  • 2020 • 235 Tantalum and zirconium induced structural transitions at complex [111] tilt grain boundaries in copper
    Meiners, T. and Duarte, J.M. and Richter, G. and Dehm, G. and Liebscher, C.H.
    Acta Materialia 190 93-104 (2020)
    Alloying nanocrystalline copper (Cu) with immiscible elements, such as tantalum (Ta) and zirconium (Zr), is a promising technique to manipulate grain boundary properties and by this suppress grain growth at elevated temperatures. However, insights on the atomistic origins on the influence of impurity elements on grain boundaries are lacking. In this study, the atomistic effects of Ta and Zr on [111] tilt grain boundaries in Cu are investigated by high resolution scanning transmission electron microscopy techniques. In case of Ta, the formation of spherical, nano-scale precipitates in close vicinity to the grain boundaries is observed, but no sign of segregation. The particles induce a repelling force to migrating boundaries and act as local pinning points. The segregation of Zr is observed to occur either at confined grain boundary steps or homogeneously along the boundaries without steps. In both cases a strong disordering of the defect or grain boundary structure is revealed. Furthermore, at low Zr concentrations it induces structural grain boundary transitions and partial atomic reordering of the grain boundary structural units. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.02.064
  • 2020 • 234 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 ""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 abstractdoi: 10.1103/PhysRevMaterials.4.054404
  • 2020 • 233 Deformation mechanisms in a superelastic NiTi alloy: An in-situ high resolution digital image correlation study
    Polatidis, E. and Šmíd, M. and Kuběna, I. and Hsu, W.-N. and Laplanche, G. and Van Swygenhoven, H.
    Materials and Design 191 (2020)
    An in-situ high resolution digital image correlation investigation during uniaxial tensile deformation reveals the recoverable and the non-recoverable strain mechanisms in a Ni51Ti49 alloy with a mean grain size of 35 μm. Recoverable strain is due to the martensitic transformation, for which more than one variant per grain can be activated. The majority of the activated variants exhibit high Schmid factor. The variant selection can be influenced by shear transmission across grain boundaries, when the geometrical compatibility between the neighboring habit plane variants is favourable; in these cases variants that do not have the highest Schmid factor, with respect to the macroscopically applied load, are activated. The experimentally determined transformation strains agree well with theoretical calculations for single crystals. The non-recoverable strain is due to deformation slip in austenite, twinning in martensite and residual martensite. The results are discussed in view of possible twinning modes that can occur in austenite resulting in significant non-recoverable strain. © 2020 The Authors
    view abstractdoi: 10.1016/j.matdes.2020.108622
  • 2020 • 232 Role of inclination dependence of grain boundary energy on the microstructure evolution during grain growth
    Salama, H. and Kundin, J. and Shchyglo, O. and Mohles, V. and Marquardt, K. and Steinbach, I.
    Acta Materialia 188 641-651 (2020)
    The role of inclination dependence of grain boundary energy on the microstructure evolution and the orientation distribution of grain boundary planes during grain growth in polycrystalline materials is investigated by three-dimensional phase-field simulations. The anisotropic grain boundary energy model uses the description of the faceted surface structure of the individual crystals and constructs an anisotropic energy of solid-solid interface. The energy minimization occurs by the faceting of the grain boundary due to inclination dependence of the grain boundary energy. The simulation results for a single grain show the development of equilibrium shapes (faceted grain morphologies) with different families of facets which agrees well with the theoretical predictions. The results of grain growth simulations with isotropic and anisotropic grain boundary energy for cubic symmetry show that inclination dependence of grain boundary energy has a significant influence on the grain boundary migration, grain growth kinetics and the grain boundary plane distribution. It has been shown that the model essentially reproduces the experimental studies reported for NaCl and MgO polycrystalline systems where the anisotropic distribution of grain boundary planes has a peak for the low-index {100} type boundaries. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.02.043
  • 2020 • 231 Structure zone investigation of multiple principle element alloy thin films as optimization for nanoindentation measurements
    Savan, A. and Allermann, T. and Wang, X. and Grochla, D. and Banko, L. and Kalchev, Y. and Kostka, A. and Pfetzing-Micklich, J. and Ludwig, Al.
    Materials 13 (2020)
    Multiple principal element alloys, also often referred to as compositionally complex alloys or high entropy alloys, present extreme challenges to characterize. They show a vast, multidimensional composition space that merits detailed investigation and optimization to identify compositions and to map the composition ranges where useful properties are maintained. Combinatorial thin film material libraries are a cost-effective and efficient way to create directly comparable, controlled composition variations. Characterizing them comes with its own challenges, including the need for high-speed, automated measurements of dozens to hundreds or more compositions to be screened. By selecting an appropriate thin film morphology through predictable control of critical deposition parameters, representative measured values can be obtained with less scatter, i.e., requiring fewer measurement repetitions for each particular composition. In the present study, equiatomic CoCrFeNi was grown by magnetron sputtering in different locations in the structure zone diagram applied to multinary element alloys, followed by microstructural and morphological characterizations. Increasing the energy input to the deposition process by increased temperature and adding high-power impulse magnetron sputtering (HiPIMS) plasma generators led to denser, more homogeneous morphologies with smoother surfaces until recrystallization and grain boundary grooving began. Growth at 300 ffiC, even without the extra particle energy input of HiPIMS generators, led to consistently repeatable nanoindentation load-displacement curves and the resulting hardness and Young's modulus values. © 2020 by the authors.
    view abstractdoi: 10.3390/ma13092113
  • 2020 • 230 A novel algorithm for rate independent small strain crystal plasticity based on the infeasible primal-dual interior point method
    Scheunemann, L. and Nigro, P.S.B. and Schröder, J. and Pimenta, P.M.
    International Journal of Plasticity 124 1-19 (2020)
    Single crystal plasticity plays a major role in the analysis of material anisotropy and texture evolution, treats each crystalline grain individually. The polycrystalline material response is obtained upon considering a structure consisting of various individual grains, often also considering interface effects at the grain boundaries. On the individual grain level, single crystal plasticity can be treated in the mathematical framework of multi-surface plasticity, leading to a constrained optimization problem wherein multiple constraints are defined as yield criteria on the different slip systems. In this work, we present a new algorithm for the solution of the constrained optimization problem based on the Infeasible Primal Dual Interior Point method (IPDIPM). The main motivation herein is the handling of the ill-posed problem without the use of simple perturbation technique, see e.g. Miehe and Schröder [2001]. The proposed algorithm, involving slack variables, is developed for the framework of small strain single crystal plasticity. The use of slack variables therein stabilizes the conventional method and allows for a temporary violation of the constraint condition during the optimization. Moreover, all slip systems are considered simultaneously, omitting an iterative active set search. Several numerical examples are simulated to show the performance of the developed algorithm. © 2019 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijplas.2019.07.020
  • 2020 • 229 Analysis of strengthening due to grain boundaries and annealing twin boundaries in the CrCoNi medium-entropy alloy
    Schneider, M. and George, E.P. and Manescau, T.J. and Záležák, T. and Hunfeld, J. and Dlouhý, A. and Eggeler, G. and Laplanche, G.
    International Journal of Plasticity 124 155-169 (2020)
    CrCoNi exhibits the best combination of strength and ductility among all the equiatomic single-phase FCC subsets of the CrMnFeCoNi high-entropy alloy. Here, its yield strength was determined in compression as a function of grain size and temperature. Yield strength was also plotted as a function of "crystallite" size, which takes into account both annealing twin boundaries and grain boundaries. The resulting Hall-Petch slopes were straight lines but with different slopes that depend on the number of twin boundaries per grain. Scanning transmission electron microscopy of deformed specimens revealed the formation of dislocation pile-ups at grain and annealing twin boundaries indicating that the latter also act as obstacles to slip and contribute to strength. Using a simple pile-up model, the strengths of the grain and twin boundaries were estimated to lie in the range 900-1250 »MPa. Assuming that they have the same strength, in the case of twin boundaries this strength corresponds roughly to the stress required to constrict Shockley partials, which suggests that dissociated dislocations have to become compact before they can cross the annealing twin boundaries. © 2019 The Authors.
    view abstractdoi: 10.1016/j.ijplas.2019.08.009
  • 2020 • 228 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 8 388-400 (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 abstractdoi: 10.1039/c9ta10818a
  • 2020 • 227 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 abstractdoi: 10.1016/j.corsci.2020.108510
  • 2020 • 226 Study of grain boundary self-diffusion in iron with different atomistic models
    Starikov, S. and Mrovec, M. and Drautz, R.
    Acta Materialia 188 560-569 (2020)
    We studied grain boundary (GB) self-diffusion in body-centered cubic iron using ab initio calculations and molecular dynamics simulations with various interatomic potentials. A combination of different models allowed us to determine the principal characteristics of self-diffusion along different types of GBs. In particular, we found that atomic self-diffusion in symmetric tilt GBs is mostly driven by self-interstitial atoms. In contrast, in general GBs atoms diffuse predominantly via an exchange mechanism that does not involve a particular defect but is similar to diffusion in a liquid. Most observed mechanisms lead to a significant enhancement of self-diffusion along GBs as compared to diffusion in the bulk. The results of simulations are verified by comparison with available experimental data. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.02.027
  • 2020 • 225 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 183 313-328 (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 abstractdoi: 10.1016/j.actamat.2019.11.029
  • 2020 • 224 Towards an understanding of grain boundary step in diamond cutting of polycrystalline copper
    Wang, Z. and Zhang, J. and Zhang, J. and Li, G. and Zhang, H. and ul Hassan, H. and Hartmaier, A. and Yan, Y. and Sun, T.
    Journal of Materials Processing Technology 276 (2020)
    Microstructural deformation at the grain level has an inherent impact on the achievable ultimate machining accuracy of polycrystalline materials. In the present work, numerical simulations and experiments of diamond cutting of polycrystalline copper are carried out to investigate the formation of surface step at grain boundaries on machined surface. Single crystal diamond cutting tool with straight cutting edge is chosen for experiments to mimic the tool geometry utilized in 2D crystal plasticity finite element simulations. Moreover, the same crystallography configuration of bi-crystal Cu is employed between experiments and simulations. Formation mechanisms of surface steps at grain boundaries are revealed by finite element simulations and corresponding experimental validation, as well as cross-sectional transmission electron microscope characterization. Finally, finite element simulations of orthogonal cutting of bi-crystal Cu are carried out to examine effects of both extrinsic cutting edge radius of diamond cutting tool and intrinsic misorientation angle of grain boundary on the propensity of grain boundary surface step formation. The present work provides theoretical guidelines on the strategy of suppressing grain boundary surface step formation for achieving superior surface finish of polycrystalline materials by diamond cutting. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2019.116400
  • 2020 • 223 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 abstractdoi: 10.1103/PhysRevLett.124.106102
  • 2020 • 222 (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 abstractdoi: 10.1016/j.mtla.2020.100641
  • 2019 • 221 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 178 135-145 (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 abstractdoi: 10.1016/j.actamat.2019.07.031
  • 2019 • 220 Electron-Beam-Induced Current Measurements of Thin-Film Solar Cells
    Abou-Ras, D. and Kirchartz, T.
    ACS Applied Energy Materials 2 6127-6139 (2019)
    The present tutorial review provides a practical guide to the analysis of semiconductor devices using electron-beam-induced currents (EBICs). The authors focus on cross-sectional EBIC measurements that provide an experimental assay of the efficiency of charge carrier collection in a semiconductor diode. The tutorial covers the fundamental physics of the technique, specimen preparation, data acquisition, and numerical simulation and analysis of the experimental data. A key focus is put on application cases from the field of thin-film photovoltaics as well as specific pitfalls that may occur, such as effects occurring under high-level injection and at grain boundaries of polycrystalline materials. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b01172
  • 2019 • 219 Studying grain boundary strengthening by dislocation-based strain gradient crystal plasticity coupled with a multi-phase-field model
    Amin, W. and Ali, M.A. and Vajragupta, N. and Hartmaier, A.
    Materials 12 (2019)
    One ambitious objective of Integrated Computational Materials Engineering (ICME) is to shorten the materials development cycle by using computational materials simulation techniques at different length scales. In this regard, the most important aspects are the prediction of the microstructural evolution during material processing and the understanding of the contributions of microstructural features to the mechanical response of the materials. One possible solution to such a challenge is to apply the Phase Field (PF) method because it can predict the microstructural evolution under the influence of different internal or external stimuli, including deformation. To accomplish this, it is necessary to take into account plasticity or, specifically, non-homogeneous plastic deformation, which is particularly important for investigating the size effects in materials emerging at the micron length scale. In this work, we present quasi-2D simulations of plastic deformation in a face centred cubic system using a finite strain formulation. Our model consists of dislocation-based strain gradient crystal plasticity implemented into a PF code. We apply this model to study the influence of grain size on the mechanical behavior of polycrystals, which includes dislocation storage and annihilation. Furthermore, the initial state of the material before deformation is also considered. The results show that a dislocation-based strain gradient crystal plasticity model can capture the Hall-Petch effect in many aspects. The model reproduced the correct functional dependence of the flow stress of the polycrystal on grain size without assigning any special properties to the grain boundaries. However, the predicted Hall-Petch coefficients are significantly smaller than those found typically in experiments. In any case, we found a good qualitative agreement between our findings and experimental results. © 2019 by the authors.
    view abstractdoi: 10.3390/ma12182977
  • 2019 • 218 Influence of Microstructural Features on the Strain Hardening Behavior of Additively Manufactured Metallic Components
    Biswas, A. and Prasad, M.R.G. and Vajragupta, N. and ul Hassan, H. and Brenne, F. and Niendorf, T. and Hartmaier, A.
    Advanced Engineering Materials 21 (2019)
    Additive manufacturing (AM) has recently become one of the key manufacturing processes in the era of Industry 4.0 because of its highly flexible production scheme. Due to complex thermal cycles during the manufacturing process itself and special solidification conditions, the microstructure of AM components often exhibits elongated grains together with a pronounced texture. These microstructural features significantly contribute to an anisotropic mechanical behavior. In this work, the microstructure and mechanical properties of additively manufactured samples of 316L stainless steel are characterized experimentally and a micromechanical modeling approach is employed to predict the macroscopic properties. The objective of this work is to study the effects of texture and microstructural morphology on yield strength and strain hardening behavior of face-centered cubic additively manufactured metallic components. To incorporate the texture in synthetic representative volume elements (RVE), the proposed approach considers both the crystallographic and grain boundary textures. The mechanical behavior of these RVEs is modeled using crystal plasticity finite element method, which incorporates size effects through the implementation of strain gradients. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201900275
  • 2019 • 217 The brittle-to-ductile transition in cold rolled tungsten plates: Impact of crystallographic texture, grain size and dislocation density on the transition temperature
    Bonnekoh, C. and Jäntsch, U. and Hoffmann, J. and Leiste, H. and Hartmaier, A. and Weygand, D. and Hoffmann, A. and Reiser, J.
    International Journal of Refractory Metals and Hard Materials 78 146-163 (2019)
    The aim of this paper is to elucidate the mechanisms controlling the brittle-to-ductile transition (BDT) in pre-deformed, textured, polycrystalline body-centred cubic (bcc) metals by the example of cold rolled tungsten (W). For this purpose, five sheets were rolled out from one and the same sintered ingot, by various levels, representing degrees of deformation of 1.8, 2.5, 3.0, 3.4, and 4.1 (this refers to 83.5%, 91.8%, 95.0%, 96.7%, and 98.3% in the technical notation). Toughness tests show that the BDT temperature decreases with increasing degree of deformation from 115 °C ± 15 °C (388 K ± 15 K) down to −65 °C ± 15 °C (208 K ± 15 K). This is an improvement of >600 K compared with a sintered ingot. In this paper we perform an in-depth analysis of the microstructure of the five sheets mentioned above. This analysis includes the assessment of (i) crystallographic texture, (ii) grain size and (iii) dislocation density. A comparison between microstructural features and experimental data confirms our working hypothesis which states that the BDT is controlled by the glide of screw dislocations and that the transition temperature decreases with decreasing spacing, λ of dislocation sources along the crack front. Sources for dislocations may be the intersection points of grain boundaries with the crack front (BDT-temperature-grain-size-relation) or dislocation multiplication processes such as e.g., the expansion of open and closed loops (impact of dislocation density). © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijrmhm.2018.09.010
  • 2019 • 216 Real-time nanoscale observation of deformation mechanisms in CrCoNi-based medium- to high-entropy alloys at cryogenic temperatures
    Ding, Q. and Fu, X. and Chen, D. and Bei, H. and Gludovatz, B. and Li, J. and Zhang, Z. and George, E.P. and Yu, Q. and Zhu, T. and Ritchie, R.O.
    Materials Today 25 21-27 (2019)
    Technologically important mechanical properties of engineering materials often degrade at low temperatures. One class of materials that defy this trend are CrCoNi-based medium- and high-entropy alloys, as they display enhanced strength, ductility, and toughness with decreasing temperature. Here we show, using in situ straining in the transmission electron microscope at 93 K (−180 °C)that their exceptional damage tolerance involves a synergy of deformation mechanisms, including twinning, glide of partials and full dislocations, extensive cross-slip, and multiple slip activated by dislocation and grain-boundary interactions. In particular, massive cross-slip occurs at the early stages of plastic deformation, thereby promoting multiple slip and dislocation interactions. These results indicate that the reduced intensity of thermal activation of defects at low temperatures and the required increase of applied stress for continued plastic flow, together with high lattice resistance, play a pivotal role in promoting the concurrent operation of multiple deformation mechanisms, which collectively enable the outstanding mechanical properties of these alloys. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.mattod.2019.03.001
  • 2019 • 215 Development of Multilayer Sinter Cladding of Cold Work Tool Steel on Hadfield Steel Plates for Wear-Resistant Applications
    Farayibi, P.K. and Blüm, M. and Theisen, W. and Weber, S.
    Journal of Materials Engineering and Performance 28 1833-1847 (2019)
    Machinery components used for mining and mineral processing activities are often subjected to high impact loads and wear which have placed demands for the development of materials with high resistance to dynamic loads and aggressive wear conditions. In this study, a multilayered cladding of high alloyed cold work tool steel (X245VCrMo9-4), interlayered with Hadfield steel (X120Mn12) plates, which was also used as substrate using super-solidus liquid-phase sintering technique was investigated. A stack of the cold work tool steel powder was prepared with interlayered X120Mn12 steel plates in an alumina crucible at tap density with the substrate placed on it and was sintered in a vacuum furnace at 1250 °C at a heating rate of 10 K/min, held for 30 min under a nitrogen atmosphere at 0.08 MPa and furnace-cooled. Sample from the as-sintered cladding was subjected to austenization at 1000 °C, quenched in oil and tempered at 150 °C for 2 h. Samples were subjected to microstructural examination using optical and scanning electron microscopy. The microstructural investigations were supplemented by hardness and impact wear tests. Computational thermodynamics was used to support experimental findings. The results revealed that a near-net densification of the sintered X245 was achieved with 99.93 ± 0.01% density. The sintered X245 was characterized by a dispersion of vanadium carbonitride precipitates, especially at the grain boundaries. The heat-treated X245 sample had the highest hardness of 680 ± 7 HV30 due to the matrix of tempered martensitic microstructure when compared to as-sintered with hardness of 554 ± 2 HV30. The X245/X120 interface was characterized by diffusion of Cr, Mo, Mn and C, which resulted in metallurgical bonding between the cladded materials. The impact wear resistance of the sintered X245 was eight times that of the X120; hence, a tough and wear-resistant tool is anticipated when the X120 work hardened in service. © 2019, ASM International.
    view abstractdoi: 10.1007/s11665-019-03942-2
  • 2019 • 214 Fermi-level pinning in methylammonium lead iodide perovskites
    Gallet, T. and Grabowski, D. and Kirchartz, T. and Redinger, A.
    Nanoscale 11 16828-16836 (2019)
    Hybrid organic inorganic perovskites are ideal candidates for absorber layers in next generation thin film photovoltaics. The polycrystalline nature of these layers imposes substantial complications for the design of high efficiency devices since the optoelectronic properties can vary on the nanometre scale. Here we show via scanning tunnelling microscopy and spectroscopy that different grains and grain facets exhibit variations in the local density of states. Modeling of the tunneling spectroscopy curves allows us to quantify the density and fluctuations of surface states and estimate the variations in workfunction on the nanometre scale. The simulations corroborate that the high number of surface states leads to Fermi-level pinning of the methylammonium lead iodide surfaces. We do not observe a variation of the local density of states at the grain boundaries compared to the grain interior. These results are in contrast to other reported SPM measurements in literature. Our results show that most of the fluctuations of the electrical properties in these polycrystalline materials arise due to grain to grain variations and not due to distinct electronic properties of the grain boundaries. The measured workfunction changes at the different grains result in local variations of the band alignment with the carrier selective top contact and the varying number of surface states influence the recombination activity in the devices. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9nr02643f
  • 2019 • 213 Initiation and stagnation of room temperature grain coarsening in cyclically strained gold films
    Glushko, O. and Dehm, G.
    Acta Materialia 169 99-108 (2019)
    Despite the large number of experiments demonstrating that grains in a metallic material can grow at room temperature due to applied mechanical load, the mechanisms and the driving forces responsible for mechanically induced grain coarsening are still not understood. Here we present a systematic study of room temperature grain coarsening induced by cyclic strain in thin polymer-supported gold films. By means of detailed electron backscatter diffraction analysis we were able to capture both the growth of individual grains and the evolution of the whole microstructure on the basis of statistical data over thousands of grains. The experimental data are reported for three film thicknesses with slightly different microstructures and three different amplitudes of cyclic mechanical loading. Although different kinds of grain size evolution with increasing cycle number are observed depending on film thickness and strain amplitude, a single model based on a thermodynamic driving force is shown to be capable to explain initiation and stagnation of grain coarsening in all cases. The main implication of the model is that the grains having lower individual yield stress are coarsening preferentially. Besides, it is demonstrated that the existence of local shear stresses imposed on a grain boundary is not a necessary requirement for room-temperature grain coarsening. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.03.004
  • 2019 • 212 Oxygen-mediated deformation and grain refinement in Cu-Fe nanocrystalline alloys
    Guo, J. and Duarte, M.J. and Zhang, Y. and Bachmaier, A. and Gammer, C. and Dehm, G. and Pippan, R. and Zhang, Z.
    Acta Materialia 166 281-293 (2019)
    Light elements play a crucial role on the microstructure and properties of conventional alloys and steels. Oxygen is one of the light elements which is inevitably introduced into nanocrystalline alloys during manufacturing. Here, we report that severe plastic deformation can fragment the oxides formed in powder processing and eventually cause oxygen dissolution in the matrix. A comparative investigation on Cu-Fe nanocrystalline alloys generated from different initial materials, blended powders and arc-melted bulk materials which have different oxygen contents, reveals that fragmented oxides at grain boundaries effectively decrease the grain boundary mobility, markedly facilitating grain refinement. In contrast, those oxygen atoms dissolved as interstitials in the Cu-Fe matrix lead to lattice expansion and significant decrease of stacking fault energy locally as validated by density functional theory. Such oxygen-mediated microstructure gives rise to enhanced strength and superior structural stability. The remarkable tailoring effect of oxygen can be employed to engineer nanocrystalline materials with desired properties for different applications. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.040
  • 2019 • 211 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 170 176-186 (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 abstractdoi: 10.1016/j.actamat.2019.03.024
  • 2019 • 210 Influence of bulk energy and triple junction mobility on interface kinetics - A tool for interpretation of experiments
    Hackl, K. and Khan, A.U. and Fischer, F.D. and Svoboda, J.
    Acta Materialia 174 310-318 (2019)
    A material system consisting of a lamellar grain structure adjacent to a large single grain is investigated. The system evolution is driven by changing of interface energy of the lamellar structure as well as by difference in bulk energies stored in the single grain and the lamellar grains. The triple junctions and the grain boundaries are assumed to have finite mobilities representing kinetic material parameters of system. A complete analysis of the kinetics of this system is provided, which involves several possible scenarios depending on the values of the geometrical and material parameters of the system. The scenarios are fully classified. Moreover, the analysis offers a way, how the values of the material parameters (interface energy densities, difference in bulk energies and mobilities) can be extracted from the measured system kinetics and geometry. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.05.005
  • 2019 • 209 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 abstractdoi: 10.1103/PhysRevMaterials.3.123602
  • 2019 • 208 On pinning-depinning and microkink-flow in solid state dewetting: Insights by in-situ ESEM on Al thin films
    Hieke, S.W. and Willinger, M.-G. and Wang, Z.-J. and Richter, G. and Chatain, D. and Dehm, G. and Scheu, C.
    Acta Materialia 165 153-163 (2019)
    The dynamics of solid state dewetting phenomena of a 50 nm thick, mazed bicrystalline Al film on single crystalline α-Al2O3 (sapphire) substrates was studied in-situ using an environmental scanning electron microscope (ESEM). The bicrystalline Al thin films served as a model system where the influence of grain boundaries and texture effects are well determined compared to polycrystalline films. The experiments were performed in controlled oxidizing and reducing atmospheres at 773 K and 823 K, respectively, to shed light on the differences in dewetting mechanisms and dynamics. While the reducing atmosphere led to spontaneous dewetting at 823 K after an incubation time of a few minutes, a hierarchical dewetting process was observed for the sluggish dewetting under oxidizing conditions. Voids initiated at (substrate or surface) defects and expanded trying to maintain a hexagonal shape. Pinning and depinning processes led to a discontinuous void growth and irregular void shapes including finger instabilities. As a consequence, the void growth followed a variety of power law exponents between 0.10 and 0.55. A new microkink-flow mechanism was discovered at the terminating Al planes at the void. © 2018
    view abstractdoi: 10.1016/j.actamat.2018.11.028
  • 2019 • 207 Partitioning of interstitial segregants during decohesion: A DFT case study of the ∑3 symmetric tilt grain boundary in ferritic steel
    Huang, X. and Janisch, R.
    Materials 12 (2019)
    The effect of hydrogen atoms at grain boundaries in metals is usually detrimental to the cohesion of the interface. This effect can be quantified in terms of the strengthening energy, which is obtained following the thermodynamic model of Rice and Wang. A critical component of this model is the bonding or solution energy of the atoms to the free surfaces that are created during decohesion. At a grain boundary in a multicomponent system, it is not immediately clear how the different species would partition and distribute on the cleaved free surfaces. In this work, it is demonstrated that the choice of partitioning pattern has a significant effect on the predicted influence of H and C on grain boundary cohesion. To this end, the ∑3(112)[110] symmetric tilt grain boundary in bcc Fe with different contents of interstitial C and H was studied, taking into account all possible distributions of the elements, as well as surface diffusion effects. H as a single element has a negative influence on grain boundary cohesion, independent of the details of the H distribution. C, on the other hand, can act both ways, enhancing or reducing the cohesion of the interface. The effect of mixed H and C compositions depends on the partition pattern. However, the general trend is that the number of detrimental cases increases with increasing H content. A decomposition of the strengthening energy into chemical and mechanical contributions shows that the elastic contribution dominates at high C contents, while the chemical contribution sets the trend for high H contents. © 2019 by the authors.
    view abstractdoi: 10.3390/ma12182971
  • 2019 • 206 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 166 158-167 (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 abstractdoi: 10.1016/j.actamat.2018.12.039
  • 2019 • 205 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 177 209-221 (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 abstractdoi: 10.1016/j.actamat.2019.07.041
  • 2019 • 204 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 168 109-120 (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 abstractdoi: 10.1016/j.actamat.2019.02.005
  • 2019 • 203 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 1-9 (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 abstractdoi: 10.1016/j.actamat.2019.07.052
  • 2019 • 202 Evolution of porosity, crack density, and CMAS penetration in thermal barrier coatings subjected to burner rig testing
    Mack, D.E. and Laquai, R. and Müller, B. and Helle, O. and Sebold, D. and Vaßen, R. and Bruno, G.
    Journal of the American Ceramic Society 102 6163-6175 (2019)
    Degradation of thermal barrier coatings (TBCs) in gas-turbine engines due to calcium–magnesium–aluminosilicate (CMAS) glassy deposits from various sources has been a persistent issue since many years. In this study, state of the art electron microscopy was correlated with X-ray refraction techniques to elucidate the intrusion of CMAS into the porous structure of atmospheric plasma sprayed (APS) TBCs and the formation and growth of cracks under thermal cycling in a burner rig. Results indicate that the sparse nature of the infiltration as well as kinetics in the burner rig are majorly influenced by the wetting behavior of the CMAS. Despite the obvious attack of CMAS on grain boundaries, the interaction of yttria-stabilized zirconia (YSZ) with intruded CMAS has no immediate impact on structure and density of internal surfaces. At a later stage the formation of horizontal cracks is observed in a wider zone of the TBC layer. © 2019 The American Ceramic Society
    view abstractdoi: 10.1111/jace.16465
  • 2019 • 201 Synthesis and mechanical testing of grain boundaries at the micro and sub-micro scale
    Malyar, N.V. and Springer, H. and Wichert, J. and Dehm, G. and Kirchlechner, C.
    Materialpruefung/Materials Testing 61 5-18 (2019)
    The important role of grain boundaries for the mechanical properties of polycrystalline materials has been recognized for many decades. Up to now, the underlying deformation mechanisms at the nano- and micro scale are not understood quantitatively. An overview of the synthesis and subsequent mechanical testing of specific grain boundaries at the micro and sub-micro scale is discussed in the present contribution, including various methods for producing one or multiple specific, crystallographically well-defined grain boundaries. Furthermore, established micromachining methods for isolating and measuring local dislocation-grain boundary interactions are portrayed. Examples of the techniques described are shown with to the aid of copper grain boundaries. © Carl Hanser Verlag, München
    view abstractdoi: 10.3139/120.111286
  • 2019 • 200 Predicting grain boundary structure and energy in BCC metals by integrated atomistic and phase-field modeling
    Qiu, D. and Zhao, P. and Shen, C. and Lu, W. and Zhang, D. and Mrovec, M. and Wang, Y.
    Acta Materialia 164 799-809 (2019)
    We predict structure and energy of low-angle (11¯0) pure twist grain boundaries (GBs) in five BCC transition metals (β-titanium, molybdenum, niobium, tungsten, and tantalum) using a combination of atomistic and microscopic phase-field (MPF) modeling. The MPF model takes as inputs solely the generalized stacking fault energy surfaces (i.e., the γ-surface) and elastic constants obtained from the atomistic simulations. Being an energy-based method, the MPF model lifts the degeneracy of the geometric models in predicting GB structures. For example, the multiple indefinite solutions offered by the Frank-Bilby equation are shown to converge to exactly the same equilibrium structure. It predicts a transition of the equilibrium GB structure from a pure screw hexagonal network (Mo and W) to mixed hexagonal networks (Nb and Ta) to a rhombus network (β-Ti) of dislocations. Parametric simulation studies and detailed analyses of the underlying dislocation reactions that are responsible for the formation of the rhombus and hexagonal structures reveal a close correlation between material properties (including the elastic anisotropic ratio and the local curvature on the γ-surface) and the GB structure and energy in BCC metals. This integrated approach allows one to explore, through high throughput calculations, the potential to tailor the structure and energy of special GBs in BCC metals by alloying. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.11.023
  • 2019 • 199 Top-down fabrication and transformation properties of vanadium dioxide nanostructures
    Rastjoo, S. and Wang, X. and Ludwig, Al. and Kohl, M.
    Journal of Applied Physics 125 (2019)
    The fabrication of nanostructures of vanadium dioxide (VO 2)-based films with critical dimensions down to 100 nm and the characterization of their phase transformation properties are presented. Starting materials are VO 2 and V 0.99 Mo 0.01 O 2 films that are deposited by magnetron sputtering. For nanofabrication, two top-down processes are investigated, in which the substrate is nanomachined either before or after film deposition. Electrical resistance measurements on V 0.99 Mo 0.01 O 2 bridge nanostructures exhibit a semiconductor-metal transition similar to reference films. A detailed analysis of phase transition temperatures does not reveal any significant width-dependence as it may be expected when approaching the grain size of 100 nm. The absolute electrical resistance in the semiconducting state scales inversely proportional to the width reflecting homogeneous material characteristics. Yet, the resistance change at the semiconductor-metal transition tends to increase for decreasing width indicating reduced carrier scattering as the absolute number of grain boundaries decreases. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5085322
  • 2019 • 198 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 abstractdoi: 10.1103/PhysRevMaterials.3.035402
  • 2019 • 197 Tungsten carbide as a deoxidation agent for plasma-facing tungsten-based materials
    Šestan, A. and Zavašnik, J. and Kržmanc, M.M. and Kocen, M. and Jenuš, P. and Novak, S. and Čeh, M. and Dehm, G.
    Journal of Nuclear Materials 524 135-140 (2019)
    Tungsten (W) and various composites are being considered as the primary plasma-facing materials for fusion reactors. Like all engineering materials, they contain certain levels of impurities, which can have an important impact on mechanical properties. In the present work, oxygen was identified as a major impurity in our starting tungsten powder. At elevated temperatures, the presence of interstitial elements such as oxygen leads to the formation of an oxide-rich tungsten phase at the tungsten grain boundaries. In this study, we determined the capacity of tungsten carbide (WC) nanoparticles to remove the oxide impurities from a tungsten body. Tungsten composites with 0.05, 0.25 and 0.51 wt. % carbon (C) in the form of WC were sintered using a field-assisted sintering technique (FAST) at 1900 °C for 5 min. The sintered samples were characterized using field-emission scanning and transmission electron microscopy. Thermodynamic and kinetic considerations allowed us to determine the optimum theoretical amount of WC to prevent the in-situ formation of WO2. © 2019 Andreja Šestan, Janez Zavašnik, Marjeta Maček Kržmanc, Matej Kocen, Petra Jenuš, Saša Novak, Miran Čeh, Gerhard Dehm
    view abstractdoi: 10.1016/j.jnucmat.2019.06.030
  • 2019 • 196 Spatially resolved investigation of the defect states in methylammonium lead iodide perovskite bicrystals
    Sirotinskaya, S. and Fettkenhauer, C. and Okada, D. and Yamamoto, Y. and Lupascu, D.C. and Schmechel, R. and Benson, N.
    Journal of Materials Chemistry C 7 13156-13160 (2019)
    Organic-inorganic halide perovskites are one of the most promising novel materials for photovoltaic applications. One of the most common and well-researched compounds in this material class is methylammonium lead iodide (MAPI). However, the formation of different kinds of defects in the polycrystalline MAPI thin films and their influence on the electrical performance of solar cells is still a matter of debate. In this work, we use single MAPI crystals grown by inverse temperature crystallization as a model system for the systematic experimental evaluation of defects as suggested by current theoretical work on MAPI thin films. The macroscopic nature of these crystallites enables an experimental approach that allows the separate evaluation of the grain boundary and crystallite bulk. For this purpose, we have employed a combination of μ-PL and μ-XPS measurements to probe the defect types suggested by the literature to determine the most likely defect types in the MAPI crystallites/thin film areas. © The Royal Society of Chemistry 2019.
    view abstractdoi: 10.1039/c8tc06622a
  • 2019 • 195 Oxide Dispersion Strengthened Bond Coats with Higher Alumina Content: Oxidation Resistance and Influence on Thermal Barrier Coating Lifetime
    Vorkötter, C. and Hagen, S.P. and Pintsuk, G. and Mack, D.E. and Virtanen, S. and Guillon, O. and Vaßen, R.
    Oxidation of Metals 92 167-194 (2019)
    The oxidation resistance of the bond coat in thermal barrier coating systems has significant influence on thermal cycling performance of the protective coating. In this study, the influence of varying the alumina content of plasma-sprayed oxide dispersion strengthened bond coats with CoNiCrAlY matrix material on the oxidation resistance was analysed by thermogravimetric analysis, SEM and TEM. Yttrium ions at the alumina scale grain boundaries and the grain size in the scale appear as major factors influencing oxidation properties. The ODS material with 2, 10 and 30 wt% alumina content was applied in TBC systems as an additional thin bond coat. The thermal cycling performance of those advanced TBC systems, in burner rig tests, was evaluated with respect to the ODS material properties. Thermal cycling behaviour is in good correlation with the isothermal oxidation resistance. All results indicate that TBC systems with 10 wt% alumina content in the ODS bond coat have a superior thermal cycling performance, as compared to ODS bond coats with lower or higher alumina content. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstractdoi: 10.1007/s11085-019-09931-z
  • 2019 • 194 Effect of Nb on improving the impact toughness of Mo-containing low-alloyed steels
    Wang, H.C. and Somsen, C. and Li, Y.J. and Fries, S.G. and Detemple, E. and Eggeler, G.
    Journal of Materials Science 54 7307-7321 (2019)
    The microalloying of low-alloyed steels with Nb can improve the strength-to-toughness balance. Such an effect of Nb is usually ascribed to the refinement of the grain structure occurring in the austenite regime during hot forming. In the present work, we report that Nb enhances the impact toughness of a low-alloyed Cr–Mo steel by a mechanism which has not been appreciated so far. The lower impact toughness in the Nb-free Cr–Mo steel is due to segregation of Mo to boundaries, which facilitates the formation of fine Mo-rich ξ-phase carbides lining up along the boundaries. This further promotes the nucleation and propagation of microcracks. The addition of Nb leads to the formation of Mo-enriched NbC particles. The interfaces between these particles and the matrix supply new preferential sites for precipitation of Mo-rich ξ-phase carbides upon subsequent tempering. In this way, Nb additions result in a decrease of Mo segregation to boundaries, significantly reducing the precipitation of ξ-phase carbides on grain boundaries, thus leading to improved impact toughness. In addition to the classical microstructural explanation (grain size effect), this chemical role of Nb sheds new light on the design strategies of advanced low-alloyed steels with optimized strength-to-toughness ratios. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstractdoi: 10.1007/s10853-019-03374-2
  • 2019 • 193 Crystal plasticity finite element modeling and simulation of diamond cutting of polycrystalline copper
    Wang, Z. and Zhang, J. and Xu, Z. and Zhang, J. and Hassan, H.U. and Li, G. and Zhang, H. and Hartmaier, A. and Fang, F. and Yan, Y. and Sun, T.
    Journal of Manufacturing Processes 38 187-195 (2019)
    Microstructural-related deformation behavior leads to anisotropic machining characteristics of polycrystalline materials. In the present work, we develop a crystal plasticity finite element model of ultra-precision diamond cutting of polycrystalline copper, aiming to evaluate the influence of grain boundaries on the correlation between microscopic deformation behavior of the material and macroscopic machining results. The crystal plasticity dealing with the anisotropy of polycrystalline copper is implemented in a user subroutine (UMAT), and an efficient element deletion technique based on the Johnson-Cook damage model is adopted to describe material removal and chip formation. The effectiveness of as-established crystal plasticity finite element model is verified by experiments of nanoindentation, nanoscratching and in-situ diamond microcutting. Subsequent crystal plasticity finite element simulation of diamond cutting across a high angle grain boundary demonstrates significant anisotropic machining characteristics in terms of machined surface quality, chip profile and cutting force, due to heterogeneous plastic deformation behavior in the grain level. © 2019
    view abstractdoi: 10.1016/j.jmapro.2019.01.007
  • 2019 • 192 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 162 321-325 (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 abstractdoi: 10.1016/j.scriptamat.2018.11.040
  • 2018 • 191 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 42-46 (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 abstractdoi: 10.1016/j.scriptamat.2018.06.044
  • 2018 • 190 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...
    Acta Materialia 150 273-280 (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 abstractdoi: 10.1016/j.actamat.2018.02.064
  • 2018 • 189 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 82-94 (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 abstractdoi: 10.1016/j.actamat.2017.09.047
  • 2018 • 188 Overview on micro- and nanomechanical testing: New insights in interface plasticity and fracture at small length scales
    Dehm, G. and Jaya, B.N. and Raghavan, R. and Kirchlechner, C.
    Acta Materialia 142 248-282 (2018)
    Micro- and nanomechanical testing has seen a rapid development over the last decade with miniaturized test rigs and MEMS-based devices providing access to the mechanical properties and performance of materials from the micrometer down to the tenths of nanometer length scale. In this overview, we summarize firstly the different testing concepts with excursions into recent imaging and diffraction developments, which turn micro- and nanomechanical testing into “quantitative mechanical microscopy” by resolving the underlying material physics and simultaneously providing mechanical properties. A special focus is laid on the pitfalls of micro-compression testing with its stringent boundary conditions often hampering reliable experiments. Additionally, the challenges of instrumented micro- and nanomechanical testing at elevated temperature are summarized. From the wide variety of research topics employing micro- and nanomechanical testing of materials we focus here on miniaturized samples and test rigs and provide three examples to elucidate the state-of-the-art of the field: (i) probing the “strength” of individual grain boundaries in metals, (ii) temperature dependent deformation mechanisms in metallic nanolayered and -alloyed structures, and (iii) the prospects and challenges of fracture studies employing micro- and nanomechanical testing of brittle and ductile monolithic materials, and materials containing interfaces. Proven concepts and new endeavors are reported for the topics discussed in this overview. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.06.019
  • 2018 • 187 Microstructural features of dynamic recrystallization in alloy 625 friction surfacing coatings
    Hanke, S. and Sena, I. and Coelho, R.S. and dos Santos, J.F.
    Materials and Manufacturing Processes 33 270-276 (2018)
    In friction surfacing (FS), material is deposited onto a substrate in the plasticized state, using frictional heat and shear stresses. The coating material remains in the solid state and undergoes severe plastic deformation (SPD) at high process temperatures (≈0.8 Tmelt), followed by high cooling rates in the range of 30 K/s. Dynamic recrystallization and the thermal cycle determine the resulting microstructure. In this study, Ni-based alloy 625 was deposited onto 42CrMo4 substrate, suitable, for instance, for repair welding of corrosion protection layers. Alloy 625 is known to undergo discontinuous dynamic recrystallization under SPD, and the resulting grain size depends on the strain rate. The coating microstructure was studied by microscopy and electron backscatter diffraction (EBSD). The coatings exhibit a fully recrystallized microstructure with equiaxed grains (0.5–12 µm) and a low degree of grain average misorientation. Flow lines caused by a localized decrease in grain size and linear alignment of grain boundaries are visible. Grain nucleation and growth were found to be strongly affected by localized shear and nonuniform material flow, resulting in varying amounts of residual strain, twins and low-angle grain boundaries in different regions within a single coating layer’s cross section. FS can be used to study dynamic recrystallization at high temperatures, strains and strain rates, while at the same time materials with a recrystallization grain size sensitive to the strain rate can be used to study the material flow during the process. © 2017 Taylor & Francis.
    view abstractdoi: 10.1080/10426914.2017.1291947
  • 2018 • 186 Grain boundary-constrained reverse austenite transformation in nanostructured Fe alloy: Model and application
    Huang, L. and Lin, W. and Wang, K. and Song, S. and Guo, C. and Chen, Y. and Li, Y. and Liu, F.
    Acta Materialia 154 56-70 (2018)
    Reverse austenite transformation (RAT) is critical for designing advanced high-strength steels (AHSS), which, however, has not been sufficiently studied in nanostructured (NS) steels or Fe alloys, and hence not fully understood yet. Herein, the RAT (e.g. ferrite to austenite) kinetics in the NS Fe alloy upon continuous heating was experimentally and theoretically investigated, where, the ultrafine austenite characterized by a sluggish growth velocity and a high thermal stability, and additionally, an appreciable solute partitioning detected using atom probe microscopy, indicate the diffusion-controlled mechanism of RAT. The double-edged role of grain boundaries (GBs) in the NS alloy is elucidated, i.e. enhancing the diffusivity due to the type-A kinetics, and simultaneously, facilitating the formation of constrained diffusion field mainly due to the segmented effect of GB nucleation. On this basis, a modified diffusion model incorporating the effect of GBs is implemented to understand the GB-constrained austenite growth and the associated partitioning behavior, and further complemented with Cahn model, an austenite growth model is applied to predict the overall kinetics of RAT in the NS Fe alloy. It then follows that a strategy by combination of diffusion-controlled growth model and microstructure model could serve as a framework to predict the kinetics of RAT in the NS alloys. Regarding the RAT kinetics in the NS alloys, the present work uncovers the ‘GB-constrained’ mechanism, which is expected to offer the potential application for nanostructure manipulation in the development of AHSS. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.05.021
  • 2018 • 185 Why Tin-Doping Enhances the Efficiency of Hematite Photoanodes for Water Splitting—The Full Picture
    Hufnagel, A.G. and Hajiyani, H. and Zhang, S. and Li, T. and Kasian, O. and Gault, B. and Breitbach, B. and Bein, T. and Fattakhova-Rohlfing, D. and Scheu, C. and Pentcheva, R.
    Advanced Functional Materials 28 (2018)
    The beneficial effects of Sn(IV) as a dopant in ultrathin hematite (α-Fe 2 O 3 ) photoanodes for water oxidation are examined. Different Sn concentration profiles are prepared by alternating atomic layer deposition of Fe 2 O 3 and SnO x . Combined data from spectrophotometry and intensity-modulated photocurrent spectroscopy yields the individual process efficiencies for light harvesting, charge separation, and charge transfer. The best performing photoanodes are Sn-doped both on the surface and in the subsurface region and benefit from enhanced charge separation and transfer. Sn-doping throughout the bulk of the hematite photoanode causes segregation at the grain boundaries and hence a lower overall efficiency. Fe 2 O 3 (0001) and terminations, shown to be dominant by microstructural analysis, are investigated by density functional theory (DFT) calculations. The energetics of surface intermediates during the oxygen evolution reaction (OER) reveal that while Sn-doping decreases the overpotential on the (0001) surface, the Fe 2 O 3 orientation shows one of the lowest overpotentials reported for hematite so far. Electronic structure calculations demonstrate that Sn-doping on the surface also enhances the charge transfer efficiency by elimination of surface hole trap states (passivation) and that subsurface Sn-doping introduces a gradient of the band edges that reinforces the band bending at the semiconductor/electrolyte interface and thus boosts charge separation. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201804472
  • 2018 • 184 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 145 247-254 (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 abstractdoi: 10.1016/j.actamat.2017.12.020
  • 2018 • 183 Phase-field modeling of pores and precipitates in polycrystalline systems
    Kundin, J. and Sohaib, H. and Schiedung, R. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 26 (2018)
    In this work, we develop an efficient phase-field approach to simulate the grain growth in polycrystalline ceramic materials in the presence of pores with various mobilities and diffusion coefficients. The multi-phase-field model is coupled to the Cahn-Hilliard equation for pore dynamics by interaction functions which describe the interaction of pores with grain boundaries. Two types of the model are suggested with one and two order parameters responsible for the pores. We also show that the model can be applied to the simulation of the interaction of the grain boundaries with coherent and non-coherent particles. The parameters of the model allow us to reproduce the equilibrium dihedral angle in the triple-junction of a pore or a particle and a grain boundary. A drag velocity of the grain boundary in the presence of pores or precipitates was also measured for various diffusion coefficients and grain boundary mobilities. The effects of the pore dynamics on the grain size evolution in ceramic materials was investigated and compared with reported theoretical predictions and experimental data. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/aacb94
  • 2018 • 182 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 147 165-175 (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 abstractdoi: 10.1016/j.actamat.2018.01.022
  • 2018 • 181 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 abstractdoi: 10.1103/PhysRevLett.121.015702
  • 2018 • 180 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 106 203-227 (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 abstractdoi: 10.1016/j.ijplas.2018.03.009
  • 2018 • 179 Multiscale Characterization of Microstructure in Near-Surface Regions of a 16MnCr5 Gear Wheel After Cyclic Loading
    Medghalchi, S. and Jamebozorgi, V. and Bala Krishnan, A. and Vincent, S. and Salomon, S. and Basir Parsa, A. and Pfetzing, J. and Kostka, A. and Li, Y. and Eggeler, G. and Li, T.
    JOM 1-7 (2018)
    The dependence of the microstructure on the degree of deformation in near-surface regions of a 16MnCr5 gear wheel after 2.1 × 106 loading cycles has been investigated by x-ray diffraction analysis, transmission electron microscopy, and atom probe tomography. Retained austenite and large martensite plates, along with elongated lamella-like cementite, were present in a less deformed region. Comparatively, the heavily deformed region consisted of a nanocrystalline structure with carbon segregation up to 2 at.% at grain boundaries. Spheroid-shaped cementite, formed at the grain boundaries and triple junctions of the nanosized grains, was enriched with Cr and Mn but depleted with Si. Such partitioning of Cr, Mn, and Si was not observed in the elongated cementite formed in the less deformed zone. This implies that rolling contact loading induced severe plastic deformation as well as a pronounced annealing effect in the active contact region of the toothed gear during cyclic loading. © 2018 The Minerals, Metals & Materials Society
    view abstractdoi: 10.1007/s11837-018-2931-z
  • 2018 • 178 Sulfur – induced embrittlement in high-purity, polycrystalline copper
    Meiners, T. and Peng, Z. and Gault, B. and Liebscher, C.H. and Dehm, G.
    Acta Materialia 156 64-75 (2018)
    Tensile tests were carried out in high-purity, polycrystalline copper alloys with three concentrations of sulfur impurities (14, 27 and 7920 at ppm) at temperatures between 20 °C and 400 °C. The ductility drops with increasing sulfur concentration and temperature while the ultimate tensile strength increases. The alloys exhibit a grain size of several millimeters and contain mostly random grain boundaries (GBs). The microstructure and composition is investigated by transmission electron microscopy (TEM) and atom probe tomography (APT). The microstructure of the samples with sulfur contents of 14 and 27 ppm consists of globular grains and neither of the microanalytical techniques employed reveals the formation of Cu-sulfides or sulfur segregation to GBs. Even after annealing at 500 °C, no sulfide formation or sulfur segregation to GBs was detected. In the alloy with a sulfur content of 7920 ppm, a dendritic structure is observed and in the interdendritic region monoclinic Cu2S precipitates with a size range from 5 nm to several μm are observed at GBs and also within the grains. The influence of S on the ductility is discussed considering the TEM and APT results. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.06.013
  • 2018 • 177 Analysis of hydrogen diffusion and trapping in ultra-high strength steel grades
    Schaffner, T. and Hartmaier, A. and Kokotin, V. and Pohl, M.
    Journal of Alloys and Compounds 746 557-566 (2018)
    The transport behavior of hydrogen in ultra-high strength steel grades (UHSS) has been analyzed by several test and evaluation methods. In particular, permeation and desorption measurements have been performed to evaluate material specific parameters such as the effective diffusion coefficient, the reversible trap density and the reversible trap activation energy. Subjects of this study were a dual phase steel grade (DP) with a ferritic-martensitic microstructure and a martensitic steel grade (MS). The results of the permeation measurements indicate that the influence of irreversible traps might be negligible for the investigated UHSS compared to other impact factors. The evaluated reversible trap densities were some orders of magnitude higher than those known for pure iron reflecting the more complex microstructure. The major influence on hydrogen trapping is attributed to reversible traps like grain boundaries and dislocations based on the results of desorption measurements. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2018.02.264
  • 2018 • 176 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 151 366-376 (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 abstractdoi: 10.1016/j.actamat.2018.04.004
  • 2018 • 175 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 86-96 (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 abstractdoi: 10.1016/j.actamat.2018.06.010
  • 2018 • 174 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 abstractdoi: 10.1088/1361-651X/aaba87
  • 2018 • 173 An examination of interactions between temperature, pressure, sintering time, and Si/W ratio on the sintering behavior of CrSiW
    Tillmann, W. and Fehr, A. and Ferreira, M. and Stangier, D.
    International Journal of Refractory Metals and Hard Materials 73 146-156 (2018)
    A Statistic Design of Experiments (DoE) was implemented with the aim to systematically investigate sintering parameter interactions of hot pressed 80Cr10Si10W. By varying the temperature between 999 and 1200 °C at pressures ranging from 2 to 8 MPa for 6.6 to 23.4 min, an initial examination of the ternary system is realized. The overall objective of this study is to minimize the porosity and to foster a diffusion of the elements. The investigations revealed that high temperatures (&gt;1000 °C) and pressures (&gt;6 MPa) support the diffusion between chromium and silicon while tungsten particles accumulate at the grain boundaries of silicon. XRD analyses confirmed the existence of a c-CrSi3 phase. The setup of the DoE, which promises the highest densification, was subsequently used to examine the influence of the silicon and tungsten content on the porosity according to the pattern 80Cr(20-x)Si(x)W for 0 ≥ x ≥ 20 (in wt%). This approach generated the lowest porosity (3.2 ± 0.93 area%) for 80Cr20W and led to homogenous particle distributions. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijrmhm.2018.02.012
  • 2018 • 172 Grain boundaries in bcc-Fe: A density-functional theory and tight-binding study
    Wang, J. and Madsen, G.K.H. and Drautz, R.
    Modelling and Simulation in Materials Science and Engineering 26 (2018)
    Grain boundaries (GBs) have a significant influence on material properties. In the present paper, we calculate the energies of eleven low-σ (σ > 13) symmetrical tilt GBs and two twist GBs in ferromagnetic bcc iron using firstprinciples density functional theory (DFT) calculations. The results demonstrate the importance of a sufficient sampling of initial rigid body translations in all three directions. We show that the relative GB energies can be xplained by the miscoordination of atoms at the GB region. While the main features of the studied GB structures were captured by previous empirical interatomic potential calculations, it is shown that the absolute values of GB energies calculated were substantially underestimated. Based on DFT-calculated GB structures and energies, we construct a new d-band orthogonal tight-binding (TB) model for bcc iron. The TB model is validated by its predictive power on all the studied GBs. We apply the TB model to block boundaries in lath martensite and demonstrate that the experimentally observed GB character distribution can be explained from the viewpoint of interface energy. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/aa9f81
  • 2018 • 171 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 156 318-329 (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 abstractdoi: 10.1016/j.actamat.2018.07.003
  • 2017 • 170 Thermal resistance of twist boundaries in silicon nanowires by nonequilibrium molecular dynamics
    Bohrer, J.K. and Schröer, K. and Brendel, L. and Wolf, D.E.
    AIP Advances 7 (2017)
    The thermal boundary resistance (Kapitza resistance) of (001) twist boundaries in silicon is investigated by nonequilibrium molecular dynamics simulations. In order to enable continuous adjustment of the mismatch angle, a cylindrical geometry with fixed atomic positions at the boundaries is devised. The influence of the boundary conditions on the Kapitza resistance is removed by means of a finite size analysis. Due to the diamond structure of silicon, twist boundaries with mismatch angles ϕ and 90°−ϕ are not equivalent, whereas those with ±ϕ or with 90°±ϕ are. The Kapitza resistance increases with mismatch angle up to 45°, where it reaches a plateau around 1.56±0.05Km2/GW. Between 80° and the 90°Σ1 grain boundary it drops by about 30%. Surprisingly, lattice coincidence at other angles (Σ5,Σ13,Σ27,Σ25) has no noticable effect on the Kapitza resistance. However, there is a clear correlation between the Kapitza resistance and the width of a non-crystalline layer at the twist boundaries. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4979982
  • 2017 • 169 Stress intensity factor dependence on anisotropy and geometry during micro-fracture experiments
    Brinckmann, S. and Kirchlechner, C. and Dehm, G.
    Scripta Materialia 127 76-78 (2017)
    Miniaturized fracture beam experiments are often used to identify the fracture toughness of single phases and particular grain boundaries because large-scale experiments reveal only homogenized material properties. The evaluation of the microscale toughness is based on isotropic 2D models although the majority of materials are anisotropic. Moreover, the thickness influences the fracture toughness because the crack driving force is maximum in the beam center. This study quantifies the influence of anisotropy, Poisson's ratio and beam geometry using thousands of 3D simulations. We give guidelines for micro-cantilever design and quantify the changes in fracture toughness, if the guidelines cannot be fulfilled. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2016.08.027
  • 2017 • 168 Efficient sampling in materials simulation - Exploring the parameter space of grain boundaries
    Dette, H. and Gösmann, J. and Greiff, C. and Janisch, R.
    Acta Materialia 125 145-155 (2017)
    In the framework of materials design there is the demand for extensive databases of specific materials properties. In this work we suggest an improved strategy for creating future databases, especially for extrinsic properties that depend on several material parameters. As an example we choose the energy of grain boundaries as a function of their geometric degrees of freedom. The construction of many existing databases of grain boundary energies in face-centred and body centred cubic metals relied on the a-priori knowledge of the location of important cusps and maxima in the five-dimensional energy landscape, and on an as-densely-as-possible sampling strategy. We introduce two methods to improve the current state of the art. Based on an existing energy model the location and number of the energy minima along which the hierarchical sampling takes place is predicted from existing data points without any a-priori knowledge, using a predictor function. Furthermore we show that in many cases it is more efficient to use a sequential sampling in a “design of experiment” scheme, rather than sampling all observations homogeneously in one batch. This sequential design exhibits a smaller error than the simultaneous one, and thus can provide the same accuracy with fewer data points. The new strategy should be particularly beneficial in the exploration of grain boundary energies in new alloys and/or non-cubic structures. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.11.044
  • 2017 • 167 Analysis of the ductility dip cracking in the nickel-base alloy 617mod
    Eilers, A. and Nellesen, J. and Zielke, R. and Tillmann, W.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    While testing steam leading power plant components made of the nickel-base alloy A617mod at elevated temperatures (700 °C), ductility dip cracking (DDC) was observed in welding seams and their surroundings. In order to clarify the mechanism of crack formation, investigations were carried out on welded specimens made of A617mod. Interrupted tensile tests were performed on tensile specimens taken from the area of the welding seam. To simulate the conditions, the tensile tests were conducted at a temperature of 700 °C and with a low strain rate. Local strain fields at grain boundaries and inside single grains were determined at different deformation states by means of two-dimensional digital image correlation (DIC). Besides the strain fields, local hardnesses (nanoindentation), energy dispersive X-Ray spectroscopy (EDX), and electron backscatter diffraction (EBSD) measurements were performed. Besides information concerning the grain orientation, the EBSD measurement provides information on the coincidence site lattice (CSL) at grain boundaries as well as the Schmid factor of single grains. All results of the analysis methods mentioned above were correlated and compared to each other and related to the crack formation. Among other things, correlations between strain fields and Schmid factors were determined. The investigations show that the following influences affect the crack formation: orientation of the grain boundaries to the direction of the loading, the orientation of the grains to each other (CSL), and grain boundary sliding. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/181/1/012020
  • 2017 • 166 Experimental analysis of anisotropic damage in dual-phase steel by resonance measurement
    Gerstein, G. and Clausmeyer, T. and Isik, K. and Nürnberger, F. and Tekkaya, A.E. and Bruchanov, A.A. and Maier, H.J.
    International Journal of Damage Mechanics 26 1147-1169 (2017)
    The ductile damage in deformed dual-phase steel sheets (DP600) was investigated based on measurements of the degradation of the direction-dependent Young's modulus. The study focuses on the material-induced damage anisotropy in such advanced high-strength steel. The elastic properties in the direction of applied loading of the deformed sheets were determined by measuring the resonance frequency of rectangular samples. The material was investigated in the as-delivered condition and after annealing at 220 for 48 h. Tensile strains of up to 10% were applied after annealing. Tensile tests were performed in different directions with respect to the rolling direction to determine the evolution of damage in different directions. The comparison of the obtained results with the electron micrographs shows that the damage in the steel sheets occurs in the form of nano and micro damages near the grain boundary and interfaces of phases. The maximum decrease of the Young's modulus in the transverse direction was observed for the largest applied deformation of 10% tensile strain in the transverse direction. An efficient calculation method to obtain information on the distribution of anisotropy in the plane of the sheet was applied. This calculation method relies on an efficient representation of the material's texture. In order to assess the influence of texture, the texture was determined experimentally. © 2017 SAGE Publications.
    view abstractdoi: 10.1177/1056789516650245
  • 2017 • 165 Nanostructure of and structural defects in a Mo2BC hard coating investigated by transmission electron microscopy and atom probe tomography
    Gleich, S. and Fager, H. and Bolvardi, H. and Achenbach, J.-O. and Soler, R. and Pradeep, K.G. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Journal of Applied Physics 122 (2017)
    In this work, the nanostructure of a Mo2BC hard coating was determined by several transmission electron microscopy methods and correlated with the mechanical properties. The coating was deposited on a Si (100) wafer by bipolar pulsed direct current magnetron sputtering from a Mo2BC compound target in Ar at a substrate temperature of 630 °C. Transmission electron microscopy investigations revealed structural features at various length scales: bundles (30 nm to networks of several micrometers) consisting of columnar grains (∼10 nm in diameter), grain boundary regions with a less ordered atomic arrangement, and defects including disordered clusters (∼1.5 nm in diameter) as well as stacking faults within the grains. The most prominent defect with a volume fraction of ∼0.5% is the disordered clusters, which were investigated in detail by electron energy loss spectroscopy and atom probe tomography. The results provide conclusive evidence that Ar is incorporated into the Mo2BC film as disordered Ar-rich Mo-B-C clusters of approximately 1.5 nm in diameter. Hardness values of 28 ± 1 GPa were obtained by nanoindentation tests. The Young's modulus of the Mo2BC coating exhibits a value of 462 ± 9 GPa, which is consistent with ab initio calculations for crystalline and defect free Mo2BC and measurements of combinatorically deposited Mo2BC thin films at a substrate temperature of 900 °C. We conclude that a reduction of the substrate temperature of 270 °C has no significant influence on hardness and Young's modulus of the Mo2BC hard coating, even if its nanostructure exhibits defects. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4999304
  • 2017 • 164 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 122 199-206 (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 abstractdoi: 10.1016/j.actamat.2016.09.048
  • 2017 • 163 Comparative study of severe plastic deformation at elevated temperatures of two aluminium alloys during friction surfacing
    Hanke, S. and dos Santos, J.F.
    Journal of Materials Processing Technology 247 257-267 (2017)
    Aluminium alloys 5083 and 6082 were deposited by Friction Surfacing (FS) under the same process conditions. Process characteristics including torque and forces, temperatures and the deposit microstructure were compared. The observed differences are discussed with regard to material strength, thermal softening rate and recrystallization mechanisms. AA 6082 plasticises faster, reaching ≈30 K higher temperatures, thicker and wider coatings and a higher material efficiency. The specific energy required for plastification is in the same order of magnitude as the activation energy for self-diffusion, emphasising the influence of dynamic recrystallization (DRX) mechanisms. A tendency for lower grain size and larger variations in grain boundary misorientation observed for AA 5083 points towards a shift in the steady-state DRX balance towards dislocation generation, due to the higher Mg content of this alloy. This corresponds to the lower process speeds required for AA 5083. AA 6082 may undergo more localized shear because of its high thermal softening rate and additional loss of strength through dissolution of Mg2Si with increasing temperature. This may contribute to a higher energy and material efficiency for plastification and deposition of AA 6082 by FS. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2017.04.021
  • 2017 • 162 Stability, phase separation and oxidation of a supersaturated nanocrystalline Cu-33 at.% Cr thin film alloy
    Harzer, T.P. and Dehm, G.
    Thin Solid Films 623 48-58 (2017)
    A binary nanocrystalline Cu67Cr33 thin film alloy consisting of columnar grains was synthesized via co-evaporation of the constituent elements under non-equilibrium ultra-high vacuum conditions using molecular beam epitaxy. In the as-deposited state, the alloy film is a supersaturated solid solution with a single-phase body-centered cubic structure. In order to study the thermal stability of the microstructure and phase separation behavior towards the two phase equilibrium structure, isothermal annealing experiments in a temperature range of 150 °C – 500 °C were conducted inside a transmission electron microscope and compared to data obtained by X-ray diffraction under protective N2 atmosphere. It is shown that the single-phase nature of the alloy film is maintained for annealing temperatures of ≤ 300 °C, whereas heat treatment at temperatures of ≥ 400 °C results in the formation of a second phase, i.e. the equilibrium face-centered cubic phase of Cu. Phase separation proceeds predominantly by a spinodal-type decomposition process but a simultaneous diffusion of Cr along the columnar grain boundaries to the surface of the alloy film is observed as well. Temperature dependent diffusion coefficients for volume and grain boundary diffusion along with the activation energy for volume diffusion of Cr within the crystal lattice of the alloy film in a temperature range between 400 °C – 500 °C are determined from analytical in situ transmission electron microscopy experiments. Moreover, grain boundary diffusion of Cr leads to the growth of an external Cr-rich oxide scale. It is found that the growth kinetics of this oxide scale exhibits a transition from a linear to a nearly parabolic growth rate. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2016.12.048
  • 2017 • 161 In–situ TEM study of diffusion kinetics and electron irradiation effects on the Cr phase separation of a nanocrystalline Cu–4 at.% Cr thin film alloy
    Harzer, T.P. and Duarte, M.J. and Dehm, G.
    Journal of Alloys and Compounds 695 1583-1590 (2017)
    The Cr phase separation process and kinetics of a metastable Cu96Cr4 alloy film were investigated by isothermal annealing at different temperatures of up to 500 °C using transmission electron microscopy. It is shown that the Cr phase separation proceeds predominantly via enrichment of Cr at grain boundaries and grain boundary diffusion. Temperature dependent diffusion coefficients and the activation energy for grain boundary diffusion of Cr in face–centered cubic Cu are determined from analytical in–situ transmission electron microscopy experiments. In addition, the influence of electron beam irradiation on the diffusion kinetics is considered. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2016.10.302
  • 2017 • 160 Microstructural evolution and solid state dewetting of epitaxial Al thin films on sapphire (α-Al2O3)
    Hieke, S.W. and Breitbach, B. and Dehm, G. and Scheu, C.
    Acta Materialia 133 356-366 (2017)
    Solid state dewetting can be used for targeted patterning, but also causes degradation or failure of thin film devices. In this work the temperature-induced changes of a tetracrystalline model system with inhibited surface diffusion are studied. This is accomplished by growing Al thin films by molecular beam epitaxy on single crystalline (0001) oriented sapphire substrates. The as-deposited Al films form two orientation relationships (OR I and OR II) both subdivided in two twin-related growth variants leading to a tetracrystalline microstructure. Two processes evolve during annealing at 600 °C. Grain growth and texture evolution towards OR II occur in addition to the formation of drum-like voids in the Al film covered by a thin membrane. The surface oxide suppresses Al surface diffusion and in contrast to classical solid state dewetting interface and grain boundary diffusion dominate. High energy grain boundaries were identified as initial points of the void formation. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.05.026
  • 2017 • 159 Effect of test atmosphere composition on high-temperature oxidation behaviour of CoNiCrAlY coatings produced from conventional and ODS powders
    Huang, T. and Bergholz, J. and Mauer, G. and Vassen, R. and Naumenko, D. and Quadakkers, W.J.
    Materials at High Temperatures 1-11 (2017)
    The oxidation behaviour of free-standing CoNiCrAlY coatings produced by low-pressure plasma spraying using conventional powder and oxide dispersion strengthened (ODS) powder containing 2 wt. % Al-oxide dispersion was investigated. Thermogravimetric experiments at 1100 °C in Ar-20%O2 and Ar-4%H2-2%H2O showed lower oxidation rates of the ODS than the conventional coating. In the latter material the scale growth was enhanced by extensive Y-incorporation of Y/Al-mixed oxide precipitates in the scale and apparently by Y-segregation to oxide grain boundaries. In the ODS coating the alumina dispersion bonded Y in the form of Y-aluminate thereby effectively suppressing scale ‘overdoping’. SEM/EBSD studies of all alumina scales revealed a columnar grain structure with the lateral grain size increasing approximately linearly with depth from the oxide/gas interface. For both coatings the alumina scale growth was slower in Ar–H2–H2O than in Ar–O2. The result is believed to be related to a lower oxygen potential gradient and to slower grain boundary diffusion in the scale forming in H2/H2O containing gas. © 2017 Informa UK Limited, trading as Taylor & Francis Group
    view abstractdoi: 10.1080/09603409.2017.1389422
  • 2017 • 158 Ab initio modelling of solute segregation energies to a general grain boundary
    Huber, L. and Grabowski, B. and Militzer, M. and Neugebauer, J. and Rottler, J.
    Acta Materialia 132 138-148 (2017)
    We apply a quantum mechanical/molecular mechanical (QM/MM) multiscale approach to calculate the segregation energies of Mg and Pb to two kinds of grain boundaries in Al. The first boundary, a symmetric (310)[001] Σ5 tilt boundary, is also tractable using traditional QM calculations, and serves as a validation for the QM/MM method. The second boundary is a general, low-symmetry tilt boundary that is completely inaccessible to pure QM calculations. QM/MM results for both of these boundaries are used to evaluate the accuracy of empirical (EAM) potentials for the Al-Mg and Al-Pb alloy systems. Based on these results we develop a physical model for the segregation energy based on elastic interaction and bond breaking terms. Both MM calculations with the EAM potentials and the model work quantitatively well for describing Mg-GB interaction across a wide range of local environments. For Pb, MM performance is weaker and the model provides only qualitative insight, demonstrating the utility of a QM/MM approach. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.04.024
  • 2017 • 157 Modelling of grain boundary dynamics using amplitude equations
    Hüter, C. and Neugebauer, J. and Boussinot, G. and Svendsen, B. and Prahl, U. and Spatschek, R.
    Continuum Mechanics and Thermodynamics 29 895-911 (2017)
    We discuss the modelling of grain boundary dynamics within an amplitude equations description, which is derived from classical density functional theory or the phase field crystal model. The relation between the conditions for periodicity of the system and coincidence site lattices at grain boundaries is investigated. Within the amplitude equations framework, we recover predictions of the geometrical model by Cahn and Taylor for coupled grain boundary motion, and find both (Formula presented.) and (Formula presented.) coupling. No spontaneous transition between these modes occurs due to restrictions related to the rotational invariance of the amplitude equations. Grain rotation due to coupled motion is also in agreement with theoretical predictions. Whereas linear elasticity is correctly captured by the amplitude equations model, open questions remain for the case of nonlinear deformations. © 2015 Springer-Verlag Berlin Heidelberg
    view abstractdoi: 10.1007/s00161-015-0424-7
  • 2017 • 156 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 19-42 (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 abstractdoi: 10.1016/j.ijplas.2017.08.004
  • 2017 • 155 Fracture behavior of nanostructured heavily cold drawn pearlitic steel wires before and after annealing
    Jaya, B.N. and Goto, S. and Richter, G. and Kirchlechner, C. and Dehm, G.
    Materials Science and Engineering A 707 164-171 (2017)
    In situ micro-cantilever fracture testing is used to demonstrate changes in fracture behavior of nanostructured, heavily cold drawn pearlitic steel wires as a function of drawing strain and annealing conditions. It is shown that these steels exhibit a sharp transition in fracture behavior between a drawing strain of 320% and 520% with a drop in fracture toughness from 7.5 to 4 MPam1/2. This is confirmed from the nature of fracture which is stable with some degree of plasticity at drawing strains below 320% and changes to catastrophic cleavage fracture at drawing strains of 420% and above. This transition and associated brittleness is attributed to structural (cementite decomposition and strain induced increase in tetragonality) and microstructural (increasing nanocrystallinity and dislocation density) evolution that these steels undergo at higher drawing strains. On heat treating the 420% strained sample, brittle cleavage fracture continues for low temperature (200 °C) annealing with no visible changes in microstructure, while crack growth is suppressed and large-scale plasticity is recovered for high temperature (500 °C) annealing with accompanying grain coarsening, and re-precipitation of spherodized cementite at grain boundaries. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.09.010
  • 2017 • 154 The shear instability energy: A new parameter for materials design?
    Kanani, M. and Hartmaier, A. and Janisch, R.
    Modelling and Simulation in Materials Science and Engineering 25 (2017)
    Reliable and predictive relationships between fundamental microstructural material properties and observable macroscopic mechanical behaviour are needed for the successful design of new materials. In this study we establish a link between physical properties that are defined on the atomic level and the deformation mechanisms of slip planes and interfaces that govern the mechanical behaviour of a metallic material. To accomplish this, the shear instability energy Γ is introduced, which can be determined via quantum mechanical ab initio calculations or other atomistic methods. The concept is based on a multilayer generalised stacking fault energy calculation and can be applied to distinguish the different shear deformation mechanisms occurring at TiAl interfaces during finite-temperature molecular dynamics simulations. We use the new parameter Γ to construct a deformation mechanism map for different interfaces occurring in this intermetallic. Furthermore, Γ can be used to convert the results of ab initio density functional theory calculations into those obtained with an embedded atom method type potential for TiAl. We propose to include this new physical parameter into material databases to apply it for the design of materials and microstructures, which so far mainly relies on single-crystal values for the unstable and stable stacking fault energy. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/aa865a
  • 2017 • 153 Pre- and post-buckling behavior of bi-crystalline micropillars: Origin and consequences
    Kirchlechner, C. and Toth, F. and Rammerstorfer, F.G. and Fischer, F.D. and Dehm, G.
    Acta Materialia 124 195-203 (2017)
    Compression of micropillars is routinely used to measure the material response under uniaxial load. In bi-crystalline pillars an S-shaped grain-boundary together with an S-shaped pillar is often observed after deformation raising the question of its origin and consequences for stress-strain materials data. In addition to dislocation and grain-boundary based mechanisms, this observation can be caused by buckling and subsequent post-buckling deformation. Deviations from the classical pre- and post-buckling deformation behavior are assigned to imperfections, which are categorized in extrinsic and intrinsic imperfections in this work. In the present paper, the S-shaped actual deformation state is particularly promoted by an intrinsic imperfection, caused by a material heterogeneity (due to the bi-crystal arrangement). This kind of deformation behavior is investigated by micro-compression experiments on 7 × 7 × 21 μm3 sized bi-crystal copper pillars with nearly elastic (axial Young's modulus) homogeneity and identical Schmid factors for both grain orientations. Complementary finite element simulations are performed, in which also the role of friction and of an extrinsic imperfection in the form of initial misalignment of the loading on the S-shape are considered. There, a material model describing the flow stress distribution caused by a dislocation pile-up at the grain-boundary is applied. Finally, suggestions to prevent buckling and, thus, transversal post-buckling displacements during micropillar compression tests are given with the goal to extract engineering stress-strain curves. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.10.075
  • 2017 • 152 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 123 349-353 (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 abstractdoi: 10.1016/j.matchar.2016.12.002
  • 2017 • 151 Magnetic Skyrmion Formation at Lattice Defects and Grain Boundaries Studied by Quantitative Off-Axis Electron Holography
    Li, Z.-A. and Zheng, F. and Tavabi, A.H. and Caron, J. and Jin, C. and Du, H. and Kovács, A. and Tian, M. and Farle, M. and Dunin-Borkowski, R.E.
    Nano Letters 17 1395-1401 (2017)
    We use in situ Lorentz microscopy and off-axis electron holography to investigate the formation and characteristics of skyrmion lattice defects and their relationship to the underlying crystallographic structure of a B20 FeGe thin film. We obtain experimental measurements of spin configurations at grain boundaries, which reveal inversions of crystallographic and magnetic chirality across adjacent grains, resulting in the formation of interface spin stripes at the grain boundaries. In the absence of material defects, we observe that skyrmions lattices possess dislocations and domain boundaries, in analogy to atomic crystals. Moreover, the distorted skyrmions can flexibly change their size and shape to accommodate local geometry, especially at sites of dislocations in the skyrmion lattice. Our findings provide a detailed understanding of the elasticity of topologically protected skyrmions and their correlation with underlying material defects. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b04280
  • 2017 • 150 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 703 236-243 (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 abstractdoi: 10.1016/j.msea.2017.06.107
  • 2017 • 149 Insights into the deformation behavior of the CrMnFeCoNi high-entropy alloy revealed by elevated temperature nanoindentation
    Maier-Kiener, V. and Schuh, B. and George, E.P. and Clemens, H. and Hohenwarter, A.
    Journal of Materials Research 32 2658-2667 (2017)
    A CrMnFeCoNi high-entropy alloy was investigated by nanoindentation from room temperature to 400 °C in the nanocrystalline state and cast plus homogenized coarse-grained state. In the latter case a âŒ100)-orientated grain was selected by electron back scatter diffraction for nanoindentation. It was found that hardness decreases more strongly with increasing temperature than Young's modulus, especially for the coarse-grained state. The modulus of the nanocrystalline state was slightly higher than that of the coarse-grained one. For the coarse-grained sample a strong thermally activated deformation behavior was found up to 100-150 °C, followed by a diminishing thermally activated contribution at higher testing temperatures. For the nanocrystalline state, different temperature dependent deformation mechanisms are proposed. At low temperatures, the governing processes appear to be similar to those in the coarse-grained sample, but with increasing temperature, dislocation-grain boundary interactions likely become more dominant. Finally, at 400 °C, decomposition of the nanocrystalline alloy causes a further reduction in thermal activation. This is rationalized by a reduction of the deformation controlling internal length scale by precipitate formation in conjunction with a diffusional contribution. © 2017 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2017.260
  • 2017 • 148 Size effect in bi-crystalline micropillars with a penetrable high angle grain boundary
    Malyar, N.V. and Micha, J.S. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 129 312-320 (2017)
    The implications of various size effects on the deformation behavior of and near grain boundaries is not yet fully understood. In this manuscript, slip transfer mechanisms through a general high angle grain boundary (HAGB) allowing for easy transfer are investigated in order to understand the size dependence of the dislocation-grain-boundary interaction. Complementary in situ micro compression tests on copper single and bi-crystals in the scanning electron microscope and with x-ray Laue microdiffraction were used to correlate the mechanical response with the evolving microstructure. It is shown that no dislocation pile-up is formed at the boundary. The lack of pile-up stresses results in a deformation process which is dominated by the initial dislocation source statistics. This is evidenced by similar size scaling of the single and bi-crystalline samples with the grain size being the characteristic length scale. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.03.003
  • 2017 • 147 Strain rate dependence of the slip transfer through a penetrable high angle grain boundary in copper
    Malyar, N.V. and Dehm, G. and Kirchlechner, C.
    Scripta Materialia 138 88-91 (2017)
    Micro pillar compression is used to analyze the strain rate dependence of copper pillars containing a penetrable high-angle grain boundary via in situ compression tests at strain rates ranging from  10− 1 to 10− 4 s− 1. While the grain-boundary containing pillars exhibit a clear strain-rate dependence of m = 0.04 ± 0.02, their single crystal counterparts seem to have a weak strain rate dependence of m = 0.01 ± 0.01. The results strongly suggest that the movement of the dislocation line in the grain boundary, required to change its orientation from the incoming to the outgoing slip plane, is the critical process in deforming this kind of grain-boundary containing pillars. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2017.05.042
  • 2017 • 146 Hydrogen behaviour at twist {110} grain boundaries in α-Fe
    McEniry, E.J. and Hickel, T. and Neugebauer, J.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 375 (2017)
    The behaviour of hydrogen at structural defects such as grain boundaries plays a critical role in the phenomenon of hydrogen embrittlement. However, characterization of the energetics and diffusion of hydrogen in the vicinity of such extended defects using conventional ab initio techniques is challenging due to the relatively large system sizes required when dealing with realistic grain boundary geometries. In order to be able to access the required system sizes, as well as high-Throughput testing of a large number of configurations, while remaining within a quantum-mechanical framework, an environmental tight-binding model for the iron-hydrogen system has been developed. The resulting model is applied to study the behaviour of hydrogen at a class of low-energy {110}-Terminated twist grain boundaries in α-Fe. We find that, for particular Σ values within the coincidence site lattice description, the atomic geometry at the interface plane provides extremely favourable trap sites for H, which also possess high escape barriers for diffusion. By contrast, via simulated tensile testing, weakly trapped hydrogen at the interface plane of the bulk-like Σ3 boundary acts as a 'glue' for the boundary, increasing both the energetic barrier and the elongation to rupture. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
    view abstractdoi: 10.1098/rsta.2016.0402
  • 2017 • 145 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 68 306-310 (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 abstractdoi: 10.1002/maco.201609104
  • 2017 • 144 High-throughput study of binary thin film tungsten alloys
    Nikolić, V. and Wurster, S. and Savan, A. and Ludwig, Al. and Pippan, R.
    International Journal of Refractory Metals and Hard Materials 69 40-48 (2017)
    Combinatorial magnetron co-sputtering from elemental sources was applied to produce W-alloy thin film composition spread materials libraries with well-defined, continuous composition gradients (film thicknesses between 1 and 2.5 μm). Three systems were studied: W-Fe (0–7 at.%), W-Ti (0–15 at.%) and W-Ir (0–12 at.%). High-throughput characterization of the materials libraries comprised of chemical, morphological and microstructural analyses. Scanning electron microscope investigations revealed that the films have a columnar structure of inverted cone-like units separated by voided boundaries, with a strong correlation to the alloying element content. Significant morphological changes occurred with an increase in the amount of the added element; W films with lower at.% of the alloying element had higher density and tighter grain boundaries, altering towards an increased amount of voids as the concentration of the alloying element increased. Electron backscatter diffraction scanning was used to determine microstructural components (grain size, grain shape, texture evolution), in dependence on the concentration of the alloying element. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijrmhm.2017.07.017
  • 2017 • 143 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 23-38 (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 abstractdoi: 10.1016/j.jnucmat.2016.12.019
  • 2017 • 142 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 1-13 (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 abstractdoi: 10.1016/j.corsci.2017.01.007
  • 2017 • 141 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 abstractdoi: 10.1088/1757-899X/219/1/012006
  • 2017 • 140 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 4070-4085 (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 abstractdoi: 10.1007/s10853-016-0670-5
  • 2017 • 139 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 19-34 (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 abstractdoi: 10.1016/j.jmps.2016.10.012
  • 2017 • 138 Microstructure and texture evolution during severe plastic deformation of CrMnFeCoNi high-entropy alloy
    Skrotzki, W. and Pukenas, A. and Joni, B. and Odor, E. and Ungar, T. and Hohenwarter, A. and Pippan, R. and George, E.P.
    IOP Conference Series: Materials Science and Engineering 194 (2017)
    An equiatomic high-entropy alloy CrMnFeCoNi was severely deformed at room temperature by high pressure torsion up to shear strains of about 170. Its microstructure and texture were analyzed by X-ray diffraction (X-ray line profile analysis and X-ray microdiffraction, respectively). It is shown that at a shear strain of about 20 a steady state domain/grain size of 24 nm and a dislocation density of 3 × 1016 m-2 is reached, while the twin density goes over a maximum of 2% at this strain. The texture developed is typical for sheared face-centred cubic metals, but it is extremely weak. The results are discussed in terms of the mechanisms of deformation, including dislocation slip, twinning and grain boundary sliding. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/194/1/012028
  • 2017 • 137 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 166 108-120 (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 abstractdoi: 10.1016/j.solmat.2017.03.022
  • 2017 • 136 Microstructure and thermoelectric properties of Si-WSi2 nanocomposites
    Stoetzel, J. and Schneider, T. and Mueller, M.M. and Kleebe, H.-J. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Acta Materialia 125 321-326 (2017)
    Nanocomposites of n-doped Si/WSi2 were prepared and morphologically and thermoelectrically investigated. The composites were densified by spark-plasma-sintering of doped Si nanoparticles with WSi2 nanoinclusions. The nanoparticles were synthesized in a gas-phase process. The microstructure of the bulk nanocomposite shows an inhomogeneous distribution of the WSi2 nanoinclusions in form of WSi2-rich and -depleted regions. This inhomogeneity is not present in the starting material and is assigned to a self-organizing process during sintering. The inhomogeneities are in the micrometer range and may act as scattering centers for long-wavelength phonons. The WSi2 nanoinclusions grow during sintering from originally 3–7 nm up to 30–143 nm depending on the total W content and might act as scattering centers for the medium wavelength range of phonons. Further, the growth of Si grains is suppressed by the WSi2 inclusions, which leads to an enhanced grain boundary density. Adding 1 at% W reduces lattice thermal conductivity by almost 35% within the temperature range from 300 K to 1250 K compared to pure, nanocrystalline silicon (doped). By addition of 6 at% W a reduction of 54% in lattice thermal conductivity is achieved. Although little amounts of W slightly reduce the power factor an enhancement of the thermoelectric figure of merit of 50% at 1250 K compared to a tungsten-free reference was realized. © 2016
    view abstractdoi: 10.1016/j.actamat.2016.11.069
  • 2017 • 135 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 124 183-193 (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 abstractdoi: 10.1016/j.matdes.2017.03.042
  • 2017 • 134 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 128 365-374 (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 abstractdoi: 10.1016/j.actamat.2017.02.059
  • 2016 • 133 Compositional and electrical properties of line and planar defects in Cu(In,Ga)Se2 thin films for solar cells - a review
    Abou-Ras, D. and Schmidt, S.S. and Schäfer, N. and Kavalakkatt, J. and Rissom, T. and Unold, T. and Mainz, R. and Weber, A. and Kirchartz, T. and Simsek Sanli, E. and van Aken, P.A. and Ramasse, Q.M. and Kleebe, H.-J. and Azulay,...
    Physica Status Solidi - Rapid Research Letters 10 363-375 (2016)
    The present review gives an overview of the various reports on properties of line and planar defects in Cu(In,Ga)(S,Se)2 thin films for high-efficiency solar cells. We report results from various analysis techniques applied to characterize these defects at different length scales, which allow for drawing a consistent picture on structural and electronic defect properties. A key finding is atomic reconstruction detected at line and planar defects, which may be one mechanism to reduce excess charge densities and to relax deep-defect states from midgap to shallow energy levels. On the other hand, nonradiative Shockley-Read-Hall recombination is still enhanced with respect to defect-free grain interiors, which is correlated with substantial reduction of luminescence intensities. Comparison of the microscopic electrical properties of planar defects in Cu(In,Ga)(S,Se)2 thin films with two-dimensional device simulations suggest that these defects are one origin of the reduced open-circuit voltage of the photovoltaic devices. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) This review gives an overview on the current understanding of line and planar defects in Cu(In,Ga)Se2 thin films and their impacts on the corresponding solar-cell devices. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201510440
  • 2016 • 132 The impact of carbon and oxygen in alpha-titanium: Ab initio study of solution enthalpies and grain boundary segregation
    Aksyonov, D.A. and Hickel, T. and Neugebauer, J. and Lipnitskii, A.G.
    Journal of Physics Condensed Matter 28 (2016)
    The solution, grain boundary (GB) segregation, and co-segregation of carbon and oxygen atoms in α-titanium are studied using density functional theory. For five titanium tilt boundaries, including T1, T2, and C1 twin systems, we determine the GB structure, as well as GB energy and excess volume. The segregation energies and volumes of carbon and oxygen are calculated for 23 inequivalent interstitial voids, while for co-segregation 75 configurations are considered. It is obtained that depending on the type of the segregation void both a positive and a negative segregation process is possible. The physical reasons of segregation are explained in terms of the analysis of the void atomic geometry, excess volume and features of the electronic structure at the Fermi level. Although carbon and oxygen show qualitatively similar properties in α-Ti, several distinctions are observed for their segregation behavior and mutual interactions. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/28/38/385001
  • 2016 • 131 Microstructure design of tempered martensite by atomistically informed full-field simulation: From quenching to fracture
    Borukhovich, E. and Du, G. and Stratmann, M. and Boeff, M. and Shchyglo, O. and Hartmaier, A. and Steinbach, I.
    Materials 9 (2016)
    Martensitic steels form a material class with a versatile range of properties that can be selected by varying the processing chain. In order to study and design the desired processing with the minimal experimental effort, modeling tools are required. In this work, a full processing cycle from quenching over tempering to mechanical testing is simulated with a single modeling framework that combines the features of the phase-field method and a coupled chemo-mechanical approach. In order to perform the mechanical testing, the mechanical part is extended to the large deformations case and coupled to crystal plasticity and a linear damage model. The quenching process is governed by the austenite-martensite transformation. In the tempering step, carbon segregation to the grain boundaries and the resulting cementite formation occur. During mechanical testing, the obtained material sample undergoes a large deformation that leads to local failure. The initial formation of the damage zones is observed to happen next to the carbides, while the final damage morphology follows the martensite microstructure. This multi-scale approach can be applied to design optimal microstructures dependent on processing and materials composition. © 2016 by the authors.
    view abstractdoi: 10.3390/ma9080673
  • 2016 • 130 Atom probe tomography of metallic nanostructures
    Hono, K. and Raabe, D. and Ringer, S.P. and Seidman, D.N.
    MRS Bulletin 41 23-29 (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 abstractdoi: 10.1557/mrs.2015.314
  • 2016 • 129 A QM/MM approach for low-symmetry defects in metals
    Huber, L. and Grabowski, B. and Militzer, M. and Neugebauer, J. and Rottler, J.
    Computational Materials Science 118 259-268 (2016)
    Concurrent multiscale coupling is a powerful tool for obtaining quantum mechanically (QM) accurate material behavior in a small domain while still capturing long range stress fields using a molecular mechanical (MM) description. We outline an improved scheme for QM/MM coupling in metals which permits the QM treatment of a small region chosen from a large, arbitrary MM domain to calculate total system energy and relaxed geometry. In order to test our improved method, we compute solute-vacancy binding in bulk Al as well as the binding of Mg and Pb to a symmetric Σ5 grain boundary. Results are calculated with and without our improvement to the QM/MM scheme and compared to periodic QM results for the same systems. We find that our scheme accurately and efficiently reproduces periodic QM target values in these test systems and therefore can be expected to perform well using more general geometries. © 2016 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2016.03.028
  • 2016 • 128 Stacking fault based analysis of shear mechanisms at interfaces in lamellar TiAl alloys
    Kanani, M. and Hartmaier, A. and Janisch, R.
    Acta Materialia 106 208-218 (2016)
    The interfaces in lamellar TiAl alloys have a strong influence on the strength and deformability of the microstructure. It is widely accepted that their number and spacing can be used to tune these properties. However, the results of molecular dynamics simulations of sliding at γ/γ interfaces in lamellar TiAl alloys presented here suggest that important factors, namely the sequence of different interface types as well as the orientation of in-plane directions with respect to the loading axis, have to be included into meso-scale models. Simulations of bicrystal shear show significant differences in the deformation behavior of the different interfaces, as well as pronounced in-plane anisotropy of the shear strength of the individual interfaces. The critical stresses derived from bicrystal shear simulations are of the same order of magnitude as the one for nucleation and motion of twins in a γ-single crystal, showing that these mechanisms are competitive. In total four different deformation mechanisms, interface migration, twin nucleation and migration, dislocation nucleation, and rigid grain boundary sliding are observed. Their occurrence can be understood based on a multilayer generalized stacking fault energy analysis. This link between physical properties, geometry and deformation mechanism can provide guidelines for future alloy development. © 2016 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.11.047
  • 2016 • 127 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 1100-1105 (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 abstractdoi: 10.1080/02670836.2015.1120458
  • 2016 • 126 Progress on effect of processes and microelements on liquation cracking of weld heat-affected zone of nickel-based alloy
    Li, Z. and Wang, H. and Li, Y. and Kim, H.J. and Tillmann, W.
    Jixie Gongcheng Xuebao/Journal of Mechanical Engineering 52 37-45 (2016)
    Nickel-based alloy with excellent high temperature resistance and corrosion resistance, has been widely applied to aerospace, nuclear power and offshore oil industry. Based on the importance of nickel-based alloy, studying the liquation cracking of weld heat-affected zone (HAZ) is of great significance to resolve the above problem. Welding process has a direct influence on welding performance of nickel-based alloy, so it is one of the important factors to lead to HAZ liquation cracking. By heat treatment processes, the susceptibility to liquation cracking can be reduced because of expected microstructure and properties obtained. Meanwhile, the microelements in base metal affect the non-equilibrium segregation behavior of grain boundaries, so it can leads to the generation of HAZ liquation cracking. The progress on liquation cracking of weld heat-affected zone of nichel-based alloy are reviewed from welding processes, heat treatment processes and micro-elements, and the test method of liquation cracking is summarized. The future research trends are prospected. © 2016 Journal of Mechanical Engineering.
    view abstractdoi: 10.3901/JME.2016.06.037
  • 2016 • 125 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 668 166-179 (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 abstractdoi: 10.1016/j.msea.2016.05.054
  • 2016 • 124 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 40-52 (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 abstractdoi: 10.1016/j.actamat.2016.04.005
  • 2016 • 123 Beam-induced atomic migration at Ag-containing nanofacets at an asymmetric Cu grain boundary
    Peter, N.J. and Liebscher, C.H. and Kirchlechner, C. and Dehm, G.
    Journal of Materials Research 32 968-982 (2016)
    Besides the high spatial resolution achieved in aberration-corrected scanning transmission microscopy, beam-induced dynamic effects have to be considered for quantitative chemical characterization on the level of single atomic columns. The present study investigates the influence of imaging conditions in an aberration-corrected scanning transmission electron microscope on the beam-induced atomic migration at a complex Ag-segregated, nanofaceted Cu grain boundary. Three distinct imaging conditions including static single image and serial image acquisition have been utilized. Chemical information on the Ag column occupation of single atomic columns at the grain boundary was extracted by the evolution of peak intensity ratios and compared to idealized scanning transmission electron microscopy image simulations. The atomic column occupation is underestimated when using conventional single frame acquisition due to an averaging of Ag atomic migration events during acquisition. Possible migration paths for the beam-induced atomic motion at a complex Cu grain boundary are presented. Copyright © Materials Research Society 2016
    view abstractdoi: 10.1557/jmr.2016.398
  • 2016 • 122 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 109 151-161 (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 abstractdoi: 10.1016/j.actamat.2016.02.038
  • 2016 • 121 Phase-field study of zener drag and pinning of cylindrical particles in polycrystalline materials
    Schwarze, C. and Darvishi Kamachali, R. and Steinbach, I.
    Acta Materialia 106 59-65 (2016)
    Zener drag and pinning in composites reinforced with cylindrical particles is investigated using three-dimensional phase-field simulations. Detailed systematic studies clarify the effect of relative orientation of the particle and length/diameter ratio on the kinetics of drag. It is shown that a combination of local equilibrium at junctions in contact with the particles, initial driving force of the migrating grain boundaries, and configuration of the particles within the polycrystalline matrix determine the intensity and persistence of drag and pinning effects. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.10.045
  • 2016 • 120 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 66-80 (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 abstractdoi: 10.1016/j.solmat.2016.02.023
  • 2016 • 119 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 49 399-414 (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 abstractdoi: 10.1107/S1600576716000418
  • 2016 • 118 Ab Initio Study of Deformation Influence on the Electronic Properties of Graphene Structures Containing One-Dimensional Topological Defects
    Valishina, A.A. and Lysogorskiy, Y.V. and Nedopekin, O.V. and Tayurskii, D.A.
    Journal of Low Temperature Physics 185 712-716 (2016)
    The band structures of single and bilayer graphene with one-dimensional topological defects were calculated along the defect line, and appearance of the flat band near the Fermi level was observed. In addition, the influence of deformation (compression/expansion) on the flat band was studied. It was shown that compression across the grain boundary leads to disappearance of the flat band near the Fermi level, while the stretching along this direction does not significantly change the band structure. However, neither compression nor stretching along the grain boundary destroys the flat band. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10909-016-1664-z
  • 2016 • 117 First-principles study of carbon segregation in bcc iron symmetrical tilt grain boundaries
    Wang, J. and Janisch, R. and Madsen, G.K.H. and Drautz, R.
    Acta Materialia 115 259-268 (2016)
    Segregation of light elements can profoundly affect the energies and cohesive properties of grain boundaries. First-principles calculations have been performed to determine the carbon solution energies and cohesive properties of three different grain boundaries in presence of carbon. It is demonstrated that the most stable segregation sites possess the greatest coordination number and maximum Fe-C nearest neighbor distance. Thereby a geometric criterion for predicting the segregation sites is suggested. Open grain boundary structures are shown to be more attractive to C atoms than the compact grain boundary structure, vacancies and dislocations, and C segregation at open grain boundaries decreases the grain boundary energy. The theoretical fracture strength of grain boundaries increases with C concentration and tend to similar values for certain areal concentrations irrespective of the grain boundary structures. This implies that the maximum fracture strength of a grain boundary depends on the maximum C areal concentration it can accommodate. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.04.058
  • 2016 • 116 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 4831-4844 (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 abstractdoi: 10.1007/s11661-016-3642-0
  • 2015 • 115 First-principles investigation of hydrogen trapping and diffusion at grain boundaries in nickel
    Di Stefano, D. and Mrovec, M. and Elsässer, C.
    Acta Materialia 98 306-312 (2015)
    Abstract In this work, the interaction of hydrogen with high-angle GBs in nickel has been investigated by means of density functional theory simulations. Two distinct types of GBs have been considered: the Σ3(111)[1¯10] with a close-packed interface structure and the Σ5(210)[001] with a less dense interface structure consisting of open structural units. Our calculations reveal that these two GBs have a markedly different interaction behavior with atomic hydrogen. The close-packed Σ3 GB neither traps H nor enhances its diffusion, but instead acts as a two-dimensional diffusion barrier. In contrast, the Σ5 GB provides numerous trapping sites for H within the open structural units as well as easy migration pathways for H diffusion along the GB plane that can enhance the H diffusivity by about two orders of magnitude compared to bulk Ni. The obtained results are analysed in detail and compared with available experimental and other theoretical data. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.031
  • 2015 • 114 Thermoelectric transport properties of boron-doped nanocrystalline diamond foils
    Engenhorst, M. and Fecher, J. and Notthoff, C. and Schierning, G. and Schmechel, R. and Rosiwal, S.M.
    Carbon 81 650-662 (2015)
    Natural diamond is known for its outstanding thermal conductivity and electrical insulation. However, synthetic production allows for doping and tailoring microstructural and transport properties. Despite some motivation in the literature and the ongoing search for abundant and non-toxic thermoelectric materials, the first experimental study on a set of eight substrate-free boron-doped nanocrystalline diamond foils is presented herein. All transport coefficients were determined in the same direction within the same foils over a broad temperature range up to 900 °C. It is found that nanostructuring reduces the thermal conductivity by two orders of magnitude, but the mobility decreases significantly to around 1 cm2 V-1 s-1, too. Although degenerate transport can be concluded from the temperature dependence of the Seebeck coefficient, charge carriers notably scatter at grain boundaries where sp2-carbon modifications and amorphous boron-rich phases form during synthesis. A detailed analysis of doping efficiency yields an acceptor fraction of only 8-18 at%, meaning that during synthesis excess boron thermodynamically prefers electrically inactive sites. Decent power factors above 10-4Wm-1 K-2 at 900 °C are found despite the low mobility, and a Jonker-type analysis grants a deeper insight into this issue. Together with the high thermal conductivity, the thermoelectric figure of merit zT does not exceed 0.01 at 900 °C. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2014.10.002
  • 2015 • 113 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 abstractdoi: 10.1115/IMECE2015-53118
  • 2015 • 112 Sliding wear behaviour of a Cr-base alloy after microstructure alterations induced by friction surfacing
    Hanke, S. and Fischer, A. and dos Santos, J.F.
    Wear 338-339 332-338 (2015)
    Friction surfacing is a method suitable to generate a wide variety of metallic coatings by means of frictional heating and severe shear deformation. It is a solid-state joining method, and therefore may be applied to non-fusion weldable as well as non-deformable brittle materials, as Cr-based alloys are. In the present study coatings of Cr60Ni40 alloy are generated onto Nimonic 80A substrates. Microstructural investigations of the coating material are carried out and compared to the usual cast state. The wear behaviour of the coatings as well as the cast material is examined under reciprocating sliding against 52100 ball bearing steel by means of a ball-on-flat test rig, lubricated with silicone oil to prevent oxidation. In this tribological system, wear takes place by abrasion with microploughing being the predominant submechanism, surface fatigue as well as adhesion by materials transfer of Cr60Ni40 from the flats to the steel balls. White etching layers form on Cr60Ni40 underneath the worn surfaces, which show cracks and delaminations. The amount of wear of all coatings is within the same magnitude compared to the cast state but slightly smaller. This can be explained by the distinctly finer microstructure (grain boundary strengthening) and a high degree of supersaturation of the solid solutions (solid solution strengthening) within the coatings. The results of this study show that it is possible to generate coatings of brittle alloys like Cr60Ni40 by friction surfacing, which show a slightly better wear behaviour under reciprocating sliding. Thus, in combination with a ductile substrate, these coatings are likely to extend the range of applicability of such high-temperature wear and corrosion resistant alloys. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2015.07.010
  • 2015 • 111 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 32-39 (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 abstractdoi: 10.1016/j.ultramic.2015.02.003
  • 2015 • 110 In Situ TEM Microcompression of Single and Bicrystalline Samples: Insights and Limitations
    Imrich, P.J. and Kirchlechner, C. and Kiener, D. and Dehm, G.
    JOM 67 1704-1712 (2015)
    In situ micromechanical compression experiments in a transmission electron microscope enable the study and analysis of small-scale deformation behavior. The implementation of instrumented indenter systems allows measuring the force and displacement, providing additionally insights on sample strength and flow behavior. Using focused ion beam sample preparation, single- and bicrystalline specimens can be fabricated to study the influence of individual grain boundaries on the mechanical behavior. Taperless single crystalline and bicrystalline Cu compression pillars including a coherent twin boundary were deformed in scanning and conventional transmission electron microscopy mode to study the applicability of both techniques for examining dislocation dynamics and interaction with the boundary. Based on experimental results, possibilities and limitations of such experiments are critically discussed, including sample preparation, in situ annealing to remove ion beam-induced defects, imaging of dislocations, and acquisition of stress–strain data. Finally, an outlook is given on the potential of micromechanical in situ transmission electron microscopic experiments for analyzing the influence of grain boundaries on mechanical behavior. © 2015, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-015-1440-6
  • 2015 • 109 Influence of inclined twin boundaries on the deformation behavior of Cu micropillars
    Imrich, P.J. and Kirchlechner, C. and Dehm, G.
    Materials Science and Engineering A 642 65-70 (2015)
    In situ micromechanical compression tests on Cu pillars were performed to evaluate the influence of twin boundaries on the mechanical behavior. The 1. μm sized Cu samples on a Si substrate prepared by focused ion beam milling were either single crystalline or contained 2-5 twin boundaries that were inclined to the compression direction. The strengths of the pillars vary, depending on the crystal orientation, associated twin boundary inclination and orientation of slip systems. Results show, that multiple slip systems are activated in each pillar. However, slip parallel to the twin boundaries prevails due to the long mean free path for dislocation movement. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.06.064
  • 2015 • 108 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 52-66 (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 abstractdoi: 10.1016/j.corsci.2015.03.014
  • 2015 • 107 Assessment of geometrically necessary dislocation levels derived by 3D EBSD
    Konijnenberg, P.J. and Zaefferer, S. and Raabe, D.
    Acta Materialia 99 402-414 (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 abstractdoi: 10.1016/j.actamat.2015.06.051
  • 2015 • 106 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 86 182-192 (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 abstractdoi: 10.1016/j.actamat.2014.12.021
  • 2015 • 105 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 159 240-247 (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 abstractdoi: 10.1016/j.ultramic.2015.03.009
  • 2015 • 104 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 13-16 (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 abstractdoi: 10.1016/j.scriptamat.2014.09.031
  • 2015 • 103 Effects of boron on the fracture behavior and ductility of cast Ti-6Al-4V alloys
    Luan, J.H. and Jiao, Z.B. and Heatherly, L. and George, E.P. and Chen, G. and Liu, C.T.
    Scripta Materialia 100 90-93 (2015)
    Minor amounts of boron additions have been found to greatly enhance the ductility of cast Ti-6Al-4V alloys, which was considered to be due to the grain-size refinement. In this paper, we report our interesting finding that the beneficial effect of boron on the ductility of the cast titanium alloys is due not only to the grain-size refinement but the enhancement of the prior-β grain-boundary cohesion by boron segregation at the grain boundaries, as evidenced by Auger electron microscopy. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2014.12.022
  • 2015 • 102 Cyclic degradation of titanium-tantalum high-temperature shape memory alloys - The role of dislocation activity and chemical decomposition
    Niendorf, T. and Krooß, P. and Somsen, C. and Rynko, R. and Paulsen, A. and Batyrshina, E. and Frenzel, J. and Eggeler, G. and Maier, H.J.
    Functional Materials Letters 8 (2015)
    Titanium-tantalum shape memory alloys (SMAs) are promising candidates for actuator applications at elevated temperatures. They may even succeed in substituting ternary nickel-titanium high temperature SMAs, which are either extremely expensive or difficult to form. However, titanium-tantalum alloys show rapid functional and structural degradation under cyclic thermo-mechanical loading. The current work reveals that degradation is not only governed by the evolution of the ω-phase. Dislocation processes and chemical decomposition of the matrix at grain boundaries also play a major role. © 2015 The Author(s).
    view abstractdoi: 10.1142/S1793604715500629
  • 2015 • 101 Interplanar potential for tension-shear coupling at grain boundaries derived from ab initio calculations
    Pang, X.Y. and Janisch, R. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 24 (2015)
    Based on ab initio density functional theory (DFT) calculations we derive an analytical expression for the interplanar potential of grain boundaries and single crystals as a function of coupled tensile and shear displacements. This energy function captures even details of the grain boundary behaviour, such as the tension-softening of the shear instability of aluminium grain boundaries, with good accuracy. The good agreement between the analytical model and the DFT calculations is achieved by introducing two new characteristic parameters, namely the position of the generalised unstable stacking fault with respect to the stable stacking fault, and the ratio of stable and unstable generalised stacking fault energies. One of the potentials' parameters also serves as a criterion to judge if a grain boundary deforms via crack propagation or dislocation nucleation. We suggest this potential function for application in continuum models, where constitutive relationships for grain boundaries need to be derived from a sound physical model. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/24/1/015007
  • 2015 • 100 From wetting to melting along grain boundaries using phase field and sharp interface methods
    Sai Pavan Kumar Bhogireddy, V. and Hüter, C. and Neugebauer, J. and Shchyglo, O. and Steinbach, I. and Spatschek, R.
    Computational Materials Science 108 293-300 (2015)
    Abstract We investigate the ability of a multi-order parameter phase field model with obstacle potentials to describe grain boundary premelting in equilibrium situations. In agreement with an energetic picture we find that the transition between dry and wet grain boundaries at the bulk melting point is given by the threshold 2σsl=σgb, with σsl being the solid-melt interfacial energy and σgb the energy of a dry grain boundary. The predictions for premelting are confirmed by simulations using the phase field package OpenPhase. For the prediction of the kinetics of melting along grain boundaries in pure materials, taking into account the short ranged interactions which are responsible for the grain boundary premelting, a sharp interface theory is developed. It confirms that for overheated grain boundaries the melting velocity is reduced (increased) for non-wetting (wetting) grain boundaries. Numerical steady state predictions are in agreement with a fully analytical solution in a subset of the parameter space. Phase field simulations confirm the predictions of the sharp interface theory. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2015.02.040
  • 2015 • 99 Coordinate-invariant phase field modeling of ferro-electrics, part I: Model formulation and single-crystal simulations
    Schrade, D. and Keip, M.-A. and Thai, H. and Schröder, J. and Svendsen, B. and Müller, R. and Gross, D.
    GAMM Mitteilungen 38 102-114 (2015)
    An electro-mechanically coupled phase field model for ferroelectric domain evolution is introduced. Based on Gurtin's concept of a microforce balance, a generalized Ginzburg-Landau evolution equation is derived from the second law of thermodynamics. The thermodynamic potential is formulated for transversely isotropic material behavior by adopting a coordinateinvariant formulation. The model is reduced to 2D and implemented into a finite element framework. The numerical simulations concern the microstructure evolution in mechanically clamped BaTiO3 single-crystals. In the second part of this contribution Keip et al. [1], the poling behavior of ferroelectric composites and polycrystals is investigated with regard to size effects and the influence of a discontinuous order parameter field across grain boundaries. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/gamm.201510005
  • 2015 • 98 Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation
    Schuh, B. and Mendez-Martin, F. and Völker, B. and George, E.P. and Clemens, H. and Pippan, R. and Hohenwarter, A.
    Acta Materialia 96 258-268 (2015)
    An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50 nm. As a result, strength increased significantly to 1950 MPa, and hardness to ∼520 HV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1 h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of ∼630 HV at about 450 °C before softening set in at higher temperatures. The isothermal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910 HV after 100 h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA. After times as short as 5 min at 450 °C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites. © 2015 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2015.06.025
  • 2015 • 97 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 abstractdoi: 10.1063/1.4929874
  • 2015 • 96 Multiferroic grain boundaries in oxygen-deficient ferroelectric lead titanate
    Shimada, T. and Wang, J. and Ueda, T. and Uratani, Y. and Arisue, K. and Mrovec, M. and Elsä Sser, C. and Kitamura, T.
    Nano Letters 15 27-33 (2015)
    Ultimately thin multiferroics arouse remarkable interest, motivated by the diverse utility of coexisting ferroelectric and (anti)ferromagnetic order parameters for novel functional device paradigms. However, the ferroic order is inevitably destroyed below a critical size of several nanometers. Here, we demonstrate a new path toward realization of atomically thin multiferroic monolayers while resolving a controversial origin for unexpected "-dilute ferromagnetism" emerged in nanocrystals of nonmagnetic ferroelectrics PbTiO3. The state-of-the-art hybrid functional of Hartree-Fock and density functional theories successfully identifies the origin and underlying physics; oxygen vacancies interacting with grain boundaries (GBs) bring about (anti)ferromagnetism with localized spin moments at the neighboring Ti atoms. This is due to spin-polarized defect states with broken orbital symmetries at GBs. In addition, the energetics of oxygen vacancies indicates their self-assembling nature at GBs resulting in considerably high concentration, which convert the oxygen-deficient GBs into multiferroic monolayers due to their atomically thin interfacial structure. This synthetic concept that realizes multiferroic and multifunctional oxides in a monolayered geometry through the self-assembly of atomic defects and grain boundary engineering opens a new avenue for promising paradigms of novel functional devices. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nl502471a
  • 2015 • 95 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 abstractdoi: 10.1103/PhysRevLett.115.235502
  • 2015 • 94 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 1742-1753 (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 abstractdoi: 10.1002/pip.2614
  • 2015 • 93 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 32-41 (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 abstractdoi: 10.1016/j.actamat.2014.11.004
  • 2015 • 92 Damage evolution in pseudoelastic polycrystalline Co-Ni-Ga high-temperature shape memory alloys
    Vollmer, M. and Krooß, P. and Segel, C. and Weidner, A. and Paulsen, A. and Frenzel, J. and Schaper, M. and Eggeler, G. and Maier, H.J. and Niendorf, T.
    Journal of Alloys and Compounds 633 288-295 (2015)
    Due to its transformation behavior, Co-Ni-Ga represents a very promising high temperature shape memory alloy (HT SMA) for applications at elevated temperatures. Co-Ni-Ga single crystals show a fully reversible pseudoelastic shape change up to temperatures of 400 °C. Unfortunately, polycrystalline Co-Ni-Ga suffers from brittleness and early fracture mainly due to intergranular constraints. In the current study, different thermo-mechanical processing routes produced various microstructures which differ in grain size and texture. A bicrystalline bamboo-like grain structure results in the highest reversible transformation strains and excellent cyclic stability. Moreover, solution-annealed and hot-rolled conditions also showed cyclic stability. Using in situ high-resolution electron microscopy, the elementary processes, which govern the microstructural evolution during pseudoelastic cycling were investigated and the mechanisms that govern structural and functional degradation were identified. The observations documented in the present work suggest that the formation of the ductile γ-phase on and near grain boundaries as well as the activation of multiple martensite variants at grain boundaries are beneficial for improved cyclic performance of polycrystalline Co-Ni-Ga HT SMAs. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.01.282
  • 2015 • 91 Atomistic investigation of wear mechanisms of a copper bi-crystal
    Zhang, J. and Begau, C. and Geng, L. and Hartmaier, A.
    Wear 332-333 941-948 (2015)
    In the present work, we investigate the wear mechanisms of a Cu bi-crystal containing a random high angle grain boundary by means of molecular dynamics simulations. The underlying deformation behavior of the material is analyzed and further related to the observed characteristics of mechanical response and resulting morphology of the worn surface. In particular, the grain boundary-associated mechanisms are characterized by advanced analysis techniques for lattice defects. Our simulation results indicate that in addition to dislocation slip and dislocation-grain boundary interactions, grain boundary migration plays an important role in the plastic deformation of Cu bi-crystal. It is found that the wear behavior of Cu depends on the crystallographic orientation of the worn surface and can be altered quite significantly by the presence of a grain boundary. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2015.02.023
  • 2015 • 90 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 5656-5670 (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 abstractdoi: 10.1007/s11661-015-3169-9
  • 2014 • 89 Phase-field modeling of grain-boundary premelting using obstacle potentials
    Bhogireddy, V.S.P.K. and Hüter, C. and Neugebauer, J. and Steinbach, I. and Karma, A. and Spatschek, R.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 90 (2014)
    We investigate the multiorder parameter phase field model of Steinbach and Pezzolla [Physica D 134, 385 (1999)PDNPDT0167-278910.1016/S0167-2789(99)00129-3] concerning its ability to describe grain boundary premelting. For a single order parameter situation solid-melt interfaces are always attractive, which allows us to have (unstable) equilibrium solid-melt-solid coexistence above the bulk melting point. The temperature-dependent melt layer thickness and the disjoining potential, which describe the interface interaction, are affected by the choice of the thermal coupling function and the measure to define the amount of the liquid phase. Due to the strictly finite interface thickness the interaction range also is finite. For a multiorder parameter model we find either purely attractive or purely repulsive finite-ranged interactions. The premelting transition is then directly linked to the ratio of the grain boundary and solid-melt interfacial energy. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.90.012401
  • 2014 • 88 Sigma phase evolution in Co-Re-Cr-based alloys at 1100 C
    Depka, T. and Somsen, C. and Eggeler, G. and Mukherji, D. and Rösler, J.
    Intermetallics 48 54-61 (2014)
    Co-Re-based alloys have been introduced as a novel metallic system that possesses a wide exploitable composition range and high melting temperatures. The poor oxidation resistance of the binary system can be improved by alloying chromium. However, adding chromium also leads to the occurrence of the sigma phase of type Cr2Re3. In the present study, we investigate the evolution of the sigma phase during creep and aging at 1100 C for three selected alloys based on the ternary composition Co-17Re-23Cr (at.%). In all alloys, sigma phase populates the grain boundaries of the hexagonally close-packed (HCP) matrix phase in a blocky morphology. Additionally, a fine dispersion of lamellar sigma phase in the grain interiors has formed during the initial processing or forms during thermal exposure. This precipitation takes place by a cellular reaction that transforms a supersaturated HCP phase into alternating lamellae of a near-equilibrium HCP phase and the sigma phase. The process therefore has the character of a discontinuous precipitation. Using orientation imaging microscopy, we observe an orientation relationship between the lamellae, which describes the basal plane of the HCP phase to form a coherent interface with the base layer of atoms of the tetragonal sigma phase. After long-term thermal exposure to 1100 C, overaging of the lamellar structure results in spheroidization of the sigma lamellae and subsequent Ostwald ripening. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2013.11.003
  • 2014 • 87 Parameterized electronic description of carbon cohesion in iron grain boundaries
    Hatcher, N. and Madsen, G.K.H. and Drautz, R.
    Journal of Physics Condensed Matter 26 (2014)
    We employ a recently developed iron-carbon orthogonal tight-binding model in calculations of carbon in iron grain boundaries. We use the model to evaluate the properties of carbon near and on the Σ5 (3 1 0)[0 0 1] symmetric tilt grain boundary (GB) in iron, and calculations show that a carbon atom lowers the GB energy by 0.29 eV/atom in accordance with DFT. Carbon segregation to the GB is analyzed, and we find an energy barrier of 0.92 eV for carbon to segregate to the carbon-free interface while segregation to a fully filled interface is disfavored. Local volume (via Voronoi tessellation), magnetic, and electronic effects are correlated with atomic energy changes, and we isolate two different mechanisms governing carbon's behavior in iron: a volumetric strain which increases the energy of carbon in interstitial α iron and a non-strained local bonding which stabilizes carbon at the GB. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/14/145502
  • 2014 • 86 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 abstractdoi: 10.1103/PhysRevLett.112.126103
  • 2014 • 85 Ab initio based understanding of the segregation and diffusion mechanisms of hydrogen in steels
    Hickel, T. and Nazarov, R. and McEniry, E.J. and Leyson, G. and Grabowski, B. and Neugebauer, J.
    JOM 66 1399-1405 (2014)
    A microscopic understanding of the processes that lead to hydrogen embrittlement is of critical importance for developing new generations of high-strength steels. With this article, we provide an overview of insights that can be gained from ab initio based methods when investigating the segregation and diffusion mechanisms of hydrogen in steels. We first discuss the solubility and diffusion behavior of hydrogen in the ferrite, austenite, and martensite phases. We consider not only defect-free bulk phases but also the influence of alloying elements and geometric defects such as vacancies and grain boundaries. In the second part, the behavior of hydrogen in the presence of precipitates, the solubility, the surface absorption, and the influence of hydrogen on the interface cohesion are studied. Finally, we provide simulation results for the interaction of hydrogen with dislocations. For all these applications, we will comment on advantages and shortcomings of ab initio methods and will demonstrate how the obtained data and insights can complement experimental approaches to extract general trends and to identify causes of hydrogen embrittlement. © 2014 The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1055-3
  • 2014 • 84 Scale bridging between atomistic and mesoscale modelling: Applications of amplitude equation descriptions
    Hüter, C. and Nguyen, C.-D. and Spatschek, R. and Neugebauer, J.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    Amplitude equations are discussed as an extension of phase field models, which contain atomic resolution and allow one to describe polycrystalline structures, lattice deformations and defects. The interaction of adjacent grains, which are separated by a thin melt layer, leads to structural interactions if the grains are slightly misplaced, similar to the concept of γ-surfaces. We are able to predict these interactions essentially analytically, leading to a superposition of short-ranged interaction terms related to the individual density waves. Deviations from the analytical predictions are found only at short distances between the grains and are most pronounced in situations with different ranges of the contributions. Furthermore, we demonstrate the ability of the amplitude equation model to predict dislocation pairing transitions at high temperatures, which supports earlier findings using molecular dynamics and phase field crystal simulations. To effectively perform the numerical simulations, we present a way to implement the model on graphics cards. An enormous acceleration of the code in comparison to a single CPU code by up to two orders of magnitude is reached. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/034001
  • 2014 • 83 Differences in deformation behavior of bicrystalline Cu micropillars containing a twin boundary or a large-angle grain boundary
    Imrich, P.J. and Kirchlechner, C. and Motz, C. and Dehm, G.
    Acta Materialia 73 240-250 (2014)
    Micrometer-sized compression pillars containing a grain boundary are investigated to better understand under which conditions grain boundaries have a strengthening effect. The compression experiments were performed on focused ion beam fabricated micrometer-sized bicrystalline Cu pillars including either a large-angle grain boundary (LAGB) or a coherent twin boundary (CTB) parallel to the compression axis and additionally on single-crystalline reference samples. Pillars containing a LAGB show increased strength, stronger hardening and smaller load drops compared to single crystals and exhibit a bent boundary and pillar shape. Samples with a CTB show no major difference in stress-strain data compared to the corresponding single-crystalline samples. This is due to the special orientation and symmetry of the twin boundary and is reflected in a characteristic pillar shape after deformation. The experimental findings can be related to the dislocation-boundary interactions at the different grain boundaries and are compared with three-dimensional discrete dislocation dynamics simulations. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.04.022
  • 2014 • 82 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 72-75 (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 abstractdoi: 10.1016/j.scriptamat.2013.10.025
  • 2014 • 81 Phase-field modeling for 3D grain growth based on a grain boundary energy database
    Kim, H.-K. and Kim, S.G. and Dong, W. and Steinbach, I. and Lee, B.-J.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    A 3D phase-field model for grain growth combined with a grain boundary (GB) energy database is proposed. The phase-field model is applied to a grain growth simulation of polycrystalline bcc Fe to investigate the effect of anisotropic GB energy on the microstructural evolution and its kinetics. It is found that the anisotropy in the GB energy results in different microstructures and slower kinetics, especially when the portion of low-angle, low-energy GBs is large. We discuss the applicability of the proposed phase-field simulation technique, based on the GB or interfacial energy database to simulations for microstructural evolution, including abnormal grain growth, phase transformations, etc., in a wider range of polycrystalline materials. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/034004
  • 2014 • 80 Influence of formation and coarsening of the laves phase on the mechanical properties of heat-resistant ferritic steels
    Klein, S. and Nabiran, N. and Weber, S. and Theisen, W.
    Steel Research International 85 851-862 (2014)
    Three newly designed heat-resistant ferritic alloys containing the intermetallic Laves phase were investigated with respect to an annealing dwell time of up to 1440 h at 900C and were compared with commercially available steels. A detailed characterization of the microstructure evolution in dependence of the annealing dwell time was performed. In order to estimate the influence of Laves phase formation and coarsening on the strength, ductility and toughness, the results of the microstructural analysis were correlated with tensile tests at room temperature and with Charpy-V impact tests. Precipitates of the Laves phase were observed in the recrystallized state with a mean particle diameter about 0.25 μm. The Laves phase in all investigated alloys showed rapid growth and coarsening with increasing annealing time. In spite of this behavior, the strength and ductility of the newly designed alloys were conserved, even after annealing for 1440 h. However, the toughness decreased with coarsening of the Laves phase, which is expressed by a shift of the ductile-to-brittle transition temperature to a higher temperature. Overall, it was shown that the influence of grain growth on the mechanical properties is more significant than the presence of the Laves phase. Precipitation of Laves phase lowers the mobility of the grain boundaries so that grain growth can be avoided. To improve mechanical properties of ferritic heat-resistant steels at high temperature, steels were developed that contain a certain amount of Laves phase. In the present work, the effect of its precipitation and coarsening on mechanical properties at room temperature was investigated. It was found, that the Laves phase rises DBTT, but retards grain coarsening and therefore stabilizes mechanical properties during annealing. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201300257
  • 2014 • 79 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 4634-4646 (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 abstractdoi: 10.1016/j.ijhydene.2013.12.171
  • 2014 • 78 Precipitation of the α-phase in an ultrafine grained beta-titanium alloy processed by severe plastic deformation
    Li, T. and Kent, D. and Sha, G. and Dargusch, M.S. and Cairney, J.M.
    Materials Science and Engineering A 605 144-150 (2014)
    A fine and uniform distribution of α phase at grain boundaries is expected to improve the mechanical properties and thermal stability of beta Ti alloys. To design high strength alloys, a key factor is the volume fraction of α, which is related to the concentration of the α phase. In this study, α-phase precipitates were characterized in an ultrafine-grained Ti-15Nb-2Mo-2Zr-1Sn (at%) alloy processed by severe plastic deformation in two different ways (hot drawing and cold rolling in conjunction with annealing). A combination of transmission Kikuchi diffraction, transmission electron microscopy and atom-probe tomography revealed that ultra-fine α precipitates precipitate at grain boundaries in hot-drawn material or at sub-grain boundaries in the cold-rolled samples. The Nb concentrations of α phases formed were not those expected for an equilibrium state, which highlights the importance of understanding the chemistry of the α precipitates for engineering microstructures in advanced Ti alloys. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.03.044
  • 2014 • 77 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 abstractdoi: 10.1103/PhysRevLett.113.106104
  • 2014 • 76 On the influence of isotropic and kinematic hardening caused by strain gradients on the deformation behaviour of polycrystals
    Ma, A. and Hartmaier, A.
    Philosophical Magazine 94 125-140 (2014)
    During the deformation of polycrystals, pronounced strain gradients may occur at grain boundaries between grains whose misorientations lead to a large mismatch in their deformation behaviour. Hence, even under globally uniaxial and homogeneous strains, internal stresses will arise that must be characterized by nonlocal plasticity models. In this work, such a nonlocal constitutive model is formulated based on the concept of densities of geometrically necessary superdislocations in an isotropic elastic-plastic medium. Since the deformation of individual grains is considered, crystal plasticity models are applied that take into account plastic slip on crystallographic planes. This new nonlocal constitutive model is applied to describe the deformation of a polycrystal under the influence of plastic strain gradients caused by isotropic and kinematic strain hardening. It is found that isotropic hardening originating from plastic strain gradients amplifies deformation heterogeneities stemming from different Schmid factors in neighbouring grains. However, the kinematic hardening resulting from plastic strain gradients tends to reduce such deformation heterogeneity. Thus, the capability of a polycrystal to deform uniformly is determined by the competition between isotropic and kinematic hardening. Finally, the model is applied to explain why grain refinement is an efficient way to improve material strength and ductility at the same time. © 2013 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2013.847290
  • 2014 • 75 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 16-19 (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 abstractdoi: 10.1016/j.scriptamat.2014.02.016
  • 2014 • 74 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 18 253-261 (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 abstractdoi: 10.1016/j.cossms.2014.06.002
  • 2014 • 73 Growth mechanism of Al2Cu precipitates during in situ TEM heating of a HPT deformed Al-3wt.%Cu alloy
    Rashkova, B. and Faller, M. and Pippan, R. and Dehm, G.
    Journal of Alloys and Compounds 600 43-50 (2014)
    The microstructural evolution of Al2Cu precipitates in an ultrafine-grained Al-3wt.% Cu model alloy produced by high-pressure torsion (HPT) was studied by in situ transmission electron microscopy (TEM). The precipitation growth was systematically investigated by isothermal heating experiments in the temperature range of 120 C to 170 C. The experimental data is analysed with respect of the diffusion kinetics and activation energy to determine the most prominent diffusion path: lattice or grain boundary diffusion. The results imply that grain boundary diffusion is the relevant mechanism for Al2Cu growth in the HPT deformed material. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2014.02.090
  • 2014 • 72 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) 42--46 (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 abstractdoi: 10.1109/PVSC.2014.6925089
  • 2014 • 71 Hydrogen embrittlement of a carbon segregated σ5 (310) [001] symmetrical tilt grain boundary in α-Fe
    Tahir, A.M. and Janisch, R. and Hartmaier, A.
    Materials Science and Engineering A 612 462-467 (2014)
    The physical and mechanical properties of a σ5 (310) [001] symmetrical tilt grain boundary (STGB) in body centred cubic (bcc) Fe are investigated by means of ab initio calculations with respect to the effect of a varying number of C and H atoms at the grain boundary. The obtained results show that with increasing number of C atoms the grain boundary energy is lowered, and the segregation energy remains negative up to a full coverage of the grain boundary with C. Thus, in a bcc Fe-C system with a sufficient amount of interstitial C, the C segregated state should be considered as the ground state of this interface. Ab initio uni-axial tensile tests of the grain boundary reveal that the work of separation as well as the theoretical strength of the σ5 (310) [001] STGB increases significantly with increasing C content. The improved cohesion due to C is mainly a chemical effect, but the mechanical contribution is also cohesion enhancing. The presence of hydrogen changes the cohesion enhancing mechanical contribution of C to an embrittling contribution, and also reduces the beneficial chemical contribution to the cohesion. When hydrogen is present together with C at the grain boundary, the reduction in strength amounts to almost 20% for the co-segregated case and to more than 25% if C is completely replaced by H. Compared to the strength of the STGB in pure iron, however, the influence of H is negligible. Hence, H embrittlement can only be understood in the three component Fe-C-H system. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.06.071
  • 2014 • 70 Micromechanical investigations and modelling of a Copper-Antimony-Alloy under creep conditions
    Vöse, M. and Otto, F. and Fedelich, B. and Eggeler, G.
    Mechanics of Materials 69 41-62 (2014)
    In many practical applications, creep damage is the limiting factor of a component's lifetime. A micromechanical model of creep induced grain boundary damage is proposed, which allows for the simulation of creep damage in a polycrystal within the framework of finite element analysis. The model considers grain boundary cavitation and sliding according to a micromechanically motivated cohesive zone model while creep deformation of the grains is described following the slip system theory. The model can be applied to idealised polycrystalline structures, such as a Voronoi tessellation or, like demonstrated here, to real grain structures of miniature creep specimens. Creep tests with pure Cu single crystals and with a coarse-grained polycrystalline Cu-1 wt.% Sb alloy at 823 K have been performed and used to calibrate the polycrystal model. The grain structure of the polycrystalline Cu-Sb specimens has been revealed by the EBSD method. Extensive grain boundary sliding and cavitation has been observed in the crept specimens. Grain boundary sliding has been found to promote wedge-type damage at grain boundary triple junctions and to contribute significantly to the total creep strain. Furthermore, the assumed stress sensitivity of the models grain boundary cavity nucleation rate strongly influences the development of wedge-type damage. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.mechmat.2013.09.013
  • 2014 • 69 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 77-93 (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 abstractdoi: 10.1016/j.actamat.2014.07.048
  • 2014 • 68 Temperature dependent transition of intragranular plastic to intergranular brittle failure in electrodeposited Cu micro-tensile samples
    Wimmer, A. and Smolka, M. and Heinz, W. and Detzel, T. and Robl, W. and Motz, C. and Eyert, V. and Wimmer, E. and Jahnel, F. and Treichler, R. and Dehm, G.
    Materials Science and Engineering A 618 398-405 (2014)
    Smaller grain sizes are known to improve the strength and ductility of metals by the Hall-Petch effect. Consequently, metallic thin films and structures which must sustain mechanical loads in service are deposited under processing conditions that lead to a fine grain size. In this study, we reveal that at temperatures as low as 473. K the failure mode of 99.99. at% pure electro-deposited Cu can change from ductile intragranular to brittle intergranular fracture. The embrittlement is accompanied by a decrease in strength and elongation to fracture. Chemical analyses indicate that the embrittlement is caused by impurities detected at grain boundaries. In situ micromechanical experiments in the scanning electron microscope and atomistic simulations are performed to study the underlying mechanisms. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.09.029
  • 2014 • 67 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 610 355-369 (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 abstractdoi: 10.1016/j.msea.2014.05.061
  • 2013 • 66 Compressive behavior of Ti3AlC2 and Ti 3Al0.8Sn0.2C2 MAX phases at room temperature
    Bei, G.-P. and Laplanche, G. and Gauthier-Brunet, V. and Bonneville, J. and Dubois, S.
    Journal of the American Ceramic Society 96 567-576 (2013)
    In this study, we report on the compressive behavior of Ti 3AlC2 and Ti3Al0.8Sn 0.2C2 MAX phases at room temperature. We found that these two phases could be classified as Kinking Nonlinear Elastic (KNE) solids. The cyclic compressive stress-strain loops for Ti3AlC2 and Ti3Al0.8Sn0.2C2 are typical hysteretic and fully reversible. At failure, both compositions fracture in shear with maximum stresses of 545 MPa for Ti3AlC2 and 839 MPa for Ti3Al0.8Sn0.2C2. Consequently, the macroshear stresses for failure, τc, are 185 MPa and 242 MPa for Ti3AlC2 and Ti3Al0.8Sn 0.2C2, respectively. In addition to the grain size effects, the presence of a ductile TixAly intermetallic distributed in the grain boundaries plays an important role in the enhancement of the ultimate compressive and macroshear stresses for Ti3Al 0.8Sn0.2C2. SEM observations reveal that these two MAX phases exhibit crack deflections, intragranular fractures, kink band formation and delaminations, grain push-in and pull-out. © 2012 The American Ceramic Society.
    view abstractdoi: 10.1111/jace.12092
  • 2013 • 65 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 3172-3185 (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 abstractdoi: 10.1016/j.actamat.2013.02.006
  • 2013 • 64 A variational approach to grooving and wetting
    Hackl, K. and Fischer, F.D. and Klevakina, K. and Renner, J. and Svoboda, J.
    Acta Materialia 61 1581-1591 (2013)
    Two bodies, e.g. grains with a certain surface contour, are assumed to be in contact at a plane interface, e.g. a common grain boundary with an arbitrary inclination relatively to the surface and with zero mobility and diffusivity. A groove appears due to surface diffusion along the triple line, i.e. the intersection line of the two surfaces and the grain boundary. The thermodynamic extremum principle is applied to derive the evolution equations for the surfaces of both bodies as well as the contact conditions at the triple line. Applications to grooving and wetting are demonstrated and compared with the results from the literature. The simulations indicate that the groove root angle can be significantly different from the value of the dihedral angle calculated from the equilibrium condition for the specific grain boundary and surface energies. Moreover, it is demonstrated that the groove angle is dependent on the kinetic parameters, e.g. surface diffusion coefficients of individual grains. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.11.035
  • 2013 • 63 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 4607-4618 (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 abstractdoi: 10.1016/j.actamat.2013.04.030
  • 2013 • 62 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 132 233-238 (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 abstractdoi: 10.1016/j.ultramic.2012.10.010
  • 2013 • 61 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 7534-7542 (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 abstractdoi: 10.1016/j.actamat.2013.08.028
  • 2013 • 60 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 4696-4706 (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 abstractdoi: 10.1016/j.actamat.2013.04.059
  • 2013 • 59 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 6132-6152 (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 abstractdoi: 10.1016/j.actamat.2013.06.055
  • 2013 • 58 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 abstractdoi: 10.1088/0953-2048/26/5/055008
  • 2013 • 57 Lattice dependent motion of voids during electromigration
    Sindermann, S.P. and Latz, A. and Dumpich, G. and Wolf, D.E. and Meyer zu Heringdorf, F.-J.
    Journal of Applied Physics 113 (2013)
    The influence of the crystal lattice configuration to electromigration processes, e.g., void formation and propagation, is investigated in suitable test structures. They are fabricated out of self-assembled, bi-crystalline Ag islands, grown epitaxially on a clean Si(111) surface. The μm-wide and approximately 100 nm thick Ag islands are a composition of a Ag(001) and a Ag(111) part. By focused ion beam milling, they are structured into wires with a single grain boundary, the orientation of which can be chosen arbitrarily. In-situ scanning electron microscopy (SEM) allows to capture an image sequence during electrical stressing and monitors the development of voids and hillocks in time. To visualize the position and motion of voids, we calculate void maps using a threshold algorithm. Most of the information from the SEM image sequence is compressed into one single image. Our present electromigration studies are based on in-situ SEM investigations for three different lattice configurations: Ag(001) (with electron current flow in [110] direction), Ag(111) (with electron current flow in [112] direction), and additionally 90°rotated Ag(111) (with electron current flow in [110] direction). Our experimental results show that not only the formation and shape but also the motion direction of voids strongly depends on the crystal orientation. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4798367
  • 2013 • 56 Ab initio calculation of traction separation laws for a grain boundary in molybdenum with segregated C impurites
    Tahir, A.M. and Janisch, R. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 21 (2013)
    We have determined the influence of carbon on mechanical properties such as grain boundary energy, work of separation (WoS) and fracture strength of the Σ5(3 1 0)[0 0 1] symmetrical tilt grain boundary (STGB) in molybdenum with ab initio methods. From our ab initio results, we derived traction-separation laws that can be used in continuum simulations of fracture employing cohesive zones. Our results show that with an increasing number of C atoms at the grain boundary, the energy of the grain boundary is lowered, indicating a strong driving force for segregation. Uni-axial tensile tests of the grain boundary reveal that there is only a small effect of segregated C atoms on the cohesive energy or WoS of the grain boundary, while the strength of the Σ5(3 1 0)[0 0 1] STGB increases by almost 30% for a complete monolayer of C. This increase in strength is accompanied by an increase in grain boundary stiffness and a decrease of the interface excess volume. The characteristic parameters are combined in the concentration-dependent traction-separation laws. A study of the scaling behaviour of the different investigated systems shows that the energy-displacement curves can be well described by the universal binding energy relationship even for different C concentrations. These findings open the way for significant simplification of the calculation of ab initio traction separation laws for grain boundaries with and without impurities. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/21/7/075005
  • 2012 • 55 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 37-46 (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 abstractdoi: 10.1007/s11661-011-0828-3
  • 2012 • 54 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 abstractdoi: 10.1063/1.4764527
  • 2012 • 53 Modelling the kinetics of a triple junction
    Fischer, F.D. and Svoboda, J. and Hackl, K.
    Acta Materialia 60 4704-4711 (2012)
    The grain structure in a one phase system involves grain boundaries (surfaces), three-grain junctions (lines) and four-grain junctions (points). A certain Gibbs energy and mobility can be assigned to each object. System evolution, driven by a decrease in the total Gibbs energy, occurs by migration of objects constrained by rather complex contact conditions. A system with cylindrical symmetry is assumed, where three grain boundaries with different mobilities and different specific Gibbs energies are in contact at a triple junction line of given mobility. The equations of evolution of the system are derived by means of the thermodynamic extremal principle. New general contact conditions at the triple junction are derived, including the mobilities of all objects and the energies, contact angles and curvatures of the grain boundaries. Special contact conditions are also provided for the cases where the triple junction mobility is infinite and/or in the case of one of the grain boundaries having zero mobility. The model is demonstrated by several examples. © 2012 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2012.05.018
  • 2012 • 52 Grain size effect on strain hardening in twinning-induced plasticity steels
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 66 992-996 (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 abstractdoi: 10.1016/j.scriptamat.2012.01.037
  • 2012 • 51 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 702-703 475-478 (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 abstractdoi: 10.4028/
  • 2012 • 50 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 66 459-462 (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 abstractdoi: 10.1016/j.scriptamat.2011.12.015
  • 2012 • 49 Influence of impurity elements on the nucleation and growth of Si in high purity melt-spun Al-Si-based alloys
    Li, J.H. and Zarif, M.Z. and Dehm, G. and Schumacher, P.
    Philosophical Magazine 92 3789-3805 (2012)
    The nucleation and growth of Si has been investigated by TEM in a series of high purity melt spun Al-5Si (wt%)-based alloys with a trace addition of Fe and Sr. In the as-melt-spun condition, some twinned Si particles were found to form directly from the liquid along the grain boundary. The addition of Sr into Al-5Si-based alloys promotes the twinning of Si particles on the grain boundary and the formation of Si precipitates in the α-Al matrix. The majority of plate-shaped and truncated pyramid-shaped Si precipitates were also found to nucleate and grow along {111}-Al planes from supersaturated solid solution in the α-Al matrix. In contrast, controlled slow cooling decreased the amount of Si precipitates, while the size of the Si precipitates increased. The orientation relationship between these Si precipitates and the α-Al matrix still remained cube to cube. The β-Al5 FeSi intermetallic was also observed, depending on subsequent controlled cooling. © 2012 Copyright Taylor and Francis Group, LLC.
    view abstractdoi: 10.1080/14786435.2012.687840
  • 2012 • 48 Characterization of oxidation and reduction of Pt-Ru and Pt-Rh-Ru alloys by atom probe tomography and comparison with Pt-Rh
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Tsang, S.C.E. and Smith, G.D.W.
    Journal of Physical Chemistry C 116 17633-17640 (2012)
    Pt-based alloys containing Rh and Ru are effective catalysts in a range of applications, including pollution control and low-temperature fuel cells. As the Pt group metals are generally rare and expensive, minimizing the loading of them while also increasing the efficiency of catalyst materials is a continual challenge in heterogeneous catalysis. A smart method to "nanoengineer" the surface of the nanocatalyst particles would greatly aid this goal. In our study, the oxidation of a Pt-8.9 at. % Ru alloy between 773 and 973 K and the oxidation and oxidation/reduction behavior of a Pt-23.9 at. % Rh-9.7 at. % Ru alloy at 873 K for various exposure times were studied using atom probe tomography. The surface of the Pt-Ru alloy is enriched with Ru after oxidation at 773 K, whereas it is depleted in Ru at 873 K, and at 973 K. The surface oxide layer vanishes at higher temperatures, leaving behind a Pt-rich surface. In the case of the Pt-Rh-Ru alloy, oxidation initiates from the grain boundaries, forming an oxide with a stoichiometry of MO 2. As the oxidation time increases, this oxide evolves into a twophase nanostructure, involving a Rh-rich oxide phase (Rh, Ru) 2O 3 and a Ru-rich oxide phase (Ru, Rh)O 2. When this two-phase oxide is reduced in hydrogen at low temperatures, separate Rh-rich and Ru-rich nanoscale regions remain. This process could, therefore, be useful for synthesizing complex island structures on Pt-Rh-Ru nanoparticle catalysts. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp304359m
  • 2012 • 47 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 4005-4016 (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 abstractdoi: 10.1016/j.actamat.2012.03.006
  • 2012 • 46 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 5380-5390 (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 abstractdoi: 10.1016/j.actamat.2012.05.002
  • 2012 • 45 Component interactions after long-term operation of an SOFC stack with LSM cathode
    Malzbender, J. and Batfalsky, P. and Vaßen, R. and Shemet, V. and Tietz, F.
    Journal of Power Sources 201 196-203 (2012)
    The reliable long-term operation of stacks with a low degradation rate is a prerequisite for the commercialization of solid oxide fuel cell (SOFC) technology. A detailed post-test analysis of stacks is of major importance in understanding degradation mechanisms. Here the results are reported of a post-test analysis of an SOFC stack with anode supported cells with Ni/YSZ anode, 8YSZ electrolyte, and a lanthanum strontium manganite (LSM) cathode operated under steady-state conditions for 19,000 h. In particular, the microstructural and chemical analyses of the relevant metallic and ceramic components are reported. The interconnects were coated with a (Mn,Co,Fe) 3O 4 spinel by atmospheric plasma spraying, which prevented Cr evaporating into the cathode compartment. The diffusion of Mn from the (La,Sr)MnO 3 cathode into the 8YSZ electrolyte led to local enrichment at grain boundaries, which might have been responsible for the degradation via electronic pathways leading to partial short-circuiting across the electrolyte. However, the ultimate failure of the stack was the result of a weakening and fracture of the 8YSZ electrolyte along grain boundaries due to the local Mn enrichment. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2011.10.117
  • 2012 • 44 Dislocation engineering and its effect on the oxidation behaviour
    Naraparaju, R. and Christ, H.-J. and Renner, F.U. and Kostka, A.
    Materials at High Temperatures 29 116-122 (2012)
    Shot-peening of the surface of steel prior to oxidation can have a beneficial effect. Shot-peening can improve the oxidation resistance by introducing a localised plastic deformation in the near surface region resulting in an increase of the dislocation density. These dislocations can act in Cr-containing steels as fast diffusion paths for Cr promoting the formation of protective Cr-oxides. However, the effect of shot-peening has some limitations such as working temperature and microstructure. It has different effects on austenitic steels and ferritic martensitic steels. The effect of shot-peening can become futile due to recovery and recrystallisation of the alloy when subjected to higher temperatures for longer periods. In the present work, the main emphasis is put on the type of dislocation arrangement promoting the positive effect on the oxidation behaviour. Dislocation engineering was applied on shot-peened samples by means of some pre-annealing procedures resulting in a recovery process. During the process, dislocations were assumed to rearrange and form certain combinations nearer to the alloy grain boundaries. These arrays of dislocations can result in different oxidation behaviour. In the present study, 18 wt% Cr and 12 wt% Cr steels were shot-peened and vacuum annealed at 750°C for 1 h, 2 h, 3 h, 5 h and 15 h. Subsequently these steels were oxidised at 750°C. The mass gain in all cases is different for both steels, and in the case of both 12 wt% Cr and 18 wt% Cr steels the best oxidation resistance was achieved for the shot-peened 1 h pre-annealed sample.
    view abstractdoi: 10.3184/096034012X13322687148749
  • 2012 • 43 On the effect of grain boundary segregation on creep and creep rupture
    Otto, F. and Viswanathan, G.B. and Payton, E.J. and Frenzel, J. and Eggeler, G.
    Acta Materialia 60 2982-2998 (2012)
    The present work investigates the effect of grain boundary chemistry and crystallography on creep and on creep damage accumulation in Cu-0.008 wt.% Bi and Cu-0.92 wt.% Sb at stresses ranging from 10 to 20 MPa and temperatures between 773 and 873 K. Small additions of Bi and Sb significantly reduce the rupture strain and rupture time during creep of Cu. High stress exponents (Cu-Bi) and high apparent activation energies for creep (Cu-Bi and Cu-Sb) are obtained. Sb promotes creep cavitation on random high-angle grain boundaries. Bi, on the other hand, causes brittle failure when small crack-like cavities cause decohesion. Both elements suppress dynamic recrystallization, which occurs during creep of Cu at high stresses and temperatures. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.02.004
  • 2012 • 42 The effectiveness of coincidence site lattice criteria in predicting creep cavitation resistance
    Otto, F. and Payton, E.J. and Frenzel, J. and Eggeler, G.
    Journal of Materials Science 47 2915-2927 (2012)
    The coincidence site lattice (CSL) concept is often used in microstructural characterization by researchers studying grain boundary engineering as a method for improving the performance of polycrystalline materials. It is assumed that a high degree of shared lattice sites in the boundary between two grains will result in improved mechanical properties. For practical application of the CSL concept to experimental results, a maximum deviation from ideal CSL orientation relationships must be defined to distinguish potential CSL boundaries from random boundaries that are not likely to exhibit "special" properties. Several different maximum deviation criteria have been proposed in the literature. In this study, four of these criteria are investigated for their effectiveness in predicting the creep cavitation resistance of boundaries of different CSL character in three model alloys: pure Cu, Cu-Bi, and Cu-Sb. Bi and Sb strongly segregate to Cu grain boundaries and are detrimental to creep life. The experimental observations are compared to simulation results for a non-textured polycrystal. It is observed that only boundaries related to cubic annealing twins (∑3 and ∑9) exhibit special resistance to creep cavitation, that boundaries with ∑ > 3 are affected by the presence of segregants, and that the fraction of non-∑(3,9) boundaries tracks closely with what would be expected from a random polycrystal. It is shown that more restrictive criteria result in more reliable characterization of the fraction of cavitation-resistant boundaries only because they exclude more non-∑(3,9) boundaries from the analysis. © 2011 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-011-6124-1
  • 2012 • 41 The mechanical shear behavior of Al single crystals and grain boundaries
    Pang, X. and Ahmed, N. and Janisch, R. and Hartmaier, A.
    Journal of Applied Physics 112 (2012)
    To investigate the mechanical shear properties of interfaces in metals, we have determined the γ-surfaces of different special tilt and twist grain boundaries in aluminum by means of ab initio calculations. From the γ-surfaces, we obtained minimum energy paths and barriers, as well as the theoretical shear strength. For the [110] tilt grain boundaries, there is a pronounced easy-sliding direction along the tilt axis. The theoretical shear strength scales with the height of the slip barrier and exhibits a relation with the misorientation angle: the closer the angle to 90°, the higher the shear stress. There is no simple relationship with the periodicity of the grain boundary, i.e., the Σ value or the grain boundary energy. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4736525
  • 2012 • 40 Effect of welding parameters on the heat-affected zone of AISI409 ferritic stainless steel
    Ranjbarnodeh, E. and Hanke, S. and Weiss, S. and Fischer, A.
    International Journal of Minerals, Metallurgy and Materials 19 923-929 (2012)
    One of the main problems during the welding of ferritic stainless steels is severe grain growth within the heat-affected zone (HAZ). In the present study, the microstructural characteristics of tungsten inert gas (TIG) welded AISI409 ferritic stainless steel were investigated by electron backscattered diffraction (EBSD), and the effects of welding parameters on the grain size, local misorientation, and low-angle grain boundaries were studied. A 3-D finite element model (FEM) was developed to predict the effects of welding parameters on the holding time of the HAZ above the critical temperature of grain growth. It is found that the base metal is not fully recrystallized. During the welding, complete recrystallization is followed by severe grain growth. A decrease in the number of low-angle grain boundaries is observed within the HAZ. FEM results show that the final state of residual strains is caused by competition between welding plastic strains and their release by recrystallization. Still, the decisive factor for grain growth is heat input. © 2012 University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s12613-012-0648-5
  • 2012 • 39 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 1504-1509 (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 abstractdoi: 10.1016/j.phpro.2012.06.122
  • 2012 • 38 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 1731-1740 (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 abstractdoi: 10.1016/j.actamat.2011.11.020
  • 2012 • 37 Influence of grain boundary mobility on microstructure evolution during recrystallisation
    Winning, M. and Raabe, D.
    Materials Science Forum 715-716 191-196 (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 abstractdoi: 10.4028/
  • 2012 • 36 Yield stress influenced by the ratio of wire diameter to grain size - A competition between the effects of specimen microstructure and dimension in micro-sized polycrystalline copper wires
    Yang, B. and Motz, C. and Rester, M. and Dehm, G.
    Philosophical Magazine 92 3243-3256 (2012)
    Polycrystalline copper wires with diameters of 25, 30 and 50m were annealed at temperatures between 200°C and 900°C, resulting in different microstructures with ratios of wire diameter to grain size between 1.1 and 15.6. The microstructure evolution and tensile behavior were studied systematically. In comparison with experimental data available in the literature, the results revealed that the tensile yield stresses of these micro-sized wires are influenced not only by the grain size but also by the ratio of wire diameter to grain size. This is clearly seen when comparing identical grain sizes but different wire diameters where thinner wires reveal smaller flow stress values. A model is proposed to explain the smaller is softer phenomenon, taking into account the higher strengthening effect of grain boundaries compared to the free surface. © 2012 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2012.693215
  • 2012 • 35 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 2790-2804 (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 abstractdoi: 10.1016/j.actamat.2012.01.045
  • 2011 • 34 Mechanisms of grain boundary softening and strain-rate sensitivity in deformation of ultrafine-grained metals at high temperatures
    Ahmed, N. and Hartmaier, A.
    Acta Materialia 59 4323-4334 (2011)
    Two-dimensional dislocation dynamics and diffusion kinetics simulations are employed to study the different mechanisms of plastic deformation of ultrafine-grained (UFG) metals at different temperatures. Besides conventional plastic deformation by dislocation glide within the grains, we also consider grain boundary (GB)-mediated deformation and recovery mechanisms based on the absorption of dislocations into GBs. The material is modeled as an elastic continuum that contains a defect microstructure consisting of a pre-existing dislocation population, dislocation sources and GBs. The mechanical response of the material to an external load is calculated with this model over a wide range of temperatures. We find that at low homologous temperatures, the model material behaves in agreement with the classical Hall-Petch law. At high homologous temperatures, however, a pronounced GB softening and, moreover, a high strain-rate sensitivity of the model material is found. Qualitatively, these numerical results agree well with experimental results known from the literature. Thus, we conclude that dynamic recovery processes at GBs and GB diffusion are the rate-limiting processes during plastic deformation of UFG metals. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.056
  • 2011 • 33 Microstructure evolution and mechanical properties of an intermetallic Ti-43.5Al-4Nb-1Mo-0.1B alloy after ageing below the eutectoid temperature
    Cha, L. and Clemens, H. and Dehm, G.
    International Journal of Materials Research 102 703-708 (2011)
    Intermetallic γ-TiAl based alloys with a chemical composition of Ti-(42-45)Al-(3-5)Nb-(0.1-2)Mo-(0.1-0.2)B (in atom percent) are termed TNM ™ alloys. They exhibit several distinct characteristics, including excellent hot-workability and balanced mechanical properties. In this study, the relationship between microstructure and mechanical behavior in a Ti-43.5Al-4Nb-1Mo-0.1B alloy after two different heat treatments was investigated. One of the analyzed microstructures consisted of lamellar γ-TiAl/α2-Ti3Al colonies with a small volume fraction of globular γ-TiAl and β0-TiAl grains at their grain boundaries, whereas the second microstructure basically exhibited the same arrangement of the microstructural constituents, but a fraction of the lamellar colonies was altered by a cellular reaction. The prevailing microstructures have been analyzed by means of scanning electron microscopy and transmission electron microscopy. Macro-and micro-hardness measurements as well as room temperature tensile tests have revealed that the sample with both cellular and lamellar features show lower yield stress and hardness than the ones exhibiting undisturbed lamellar microstructures. The strength and hardness properties are primarily connected to the lamellar spacing within the colonies, where strength increases with decreasing lamellar spacing. The appearance of a cellular reaction leads to a refinement of the lamellar colonies which in turn influences positively the plastic fracture strain at room temperature. © Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110526
  • 2011 • 32 Comparative atom probe study of Cu(In,Ga)Se 2 thin-film solar cells deposited on soda-lime glass and mild steel substrates
    Choi, P.-P. and Cojocaru-Mirédin, O. and Wuerz, R. and Raabe, D.
    Journal of Applied Physics 110 (2011)
    We report on a comparative study of Cu(In,Ga)Se 2 solar cells deposited on soda-lime glass and mild steel substrates, using atom probe tomography in conjunction with secondary ion mass spectrometry, x-ray fluorescence, current density-voltage, and external quantum efficiency measurements. Cu(In,Ga)Se 2 films deposited on soda-lime glass substrates and on steel substrates with a NaF precursor layer on top of the Mo back contact contain a significant amount of Na impurities and yield an enhanced open circuit voltage and fill factor. Using atom probe tomography, Na atoms are found to be segregated at grain boundaries and clustered in both bulk and grain boundaries. The atom probe data indicate that Na Cu point defects are most likely formed at grain boundaries, reducing the number of compensating In Cu point defects and thus contributing to an enhanced cell efficiency. However, for steel substrates the positive effect of Na on the cell performance is counterbalanced by the incorporation of Fe impurities into the Cu(In,Ga)Se 2 film. Fe atoms are homogeneously distributed inside the grains suggesting that Fe introduces point defects in the bulk © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3665723
  • 2011 • 31 Thermal stability of TiAIN/CrN multilayer coatings studied by atom probe tomography
    Choi, P.-P. and Povstugar, I. and Ahn, J.-P. and Kostka, A. and Raabe, D.
    Ultramicroscopy 111 518-523 (2011)
    This study is about the microstructural evolution of TiAlN/CrN multilayers (with a Ti:Al ratio of 0.75:0.25 and average bilayer period of 9 nm) upon thermal treatment. Pulsed laser atom probe analyses were performed in conjunction with transmission electron microscopy and X-ray diffraction. The layers are found to be thermally stable up to 600 °C. At 700 °C TiAlN layers begin to decompose into Ti- and Al-rich nitride layers in the out-of-plane direction. Further increase in temperature to 1000 °C leads to a strong decomposition of the multilayer structure as well as grain coarsening. Layer dissolution and grain coarsening appear to begin at the surface. Domains of AlN and TiCrN larger than 100 nm are found, together with smaller nano-sized AlN precipitates within the TiCrN matrix. Fe and V impurities are detected in the multilayers as well, which diffuse from the steel substrate into the coating along columnar grain boundaries. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.11.012
  • 2011 • 30 Characterization of grain boundaries in Cu(In,Ga)Se 2 films using atom-probe tomography
    Cojocaru-Mirédin, O. and Choi, P.-P. and Abou-Ras, D. and Schmidt, S.S. and Caballero, R. and Raabe, D.
    IEEE Journal of Photovoltaics 1 207-212 (2011)
    This paper discusses the advantages of pulsed laser atom-probe tomography (APT) to analyze Cu(In,Ga)Se 2-based solar cells. Electron backscatter diffraction (EBSD) was exploited for site-specific preparation of APT samples at selected Cu(In,Ga)Se 2 grain boundaries. This approach is very helpful not only to determine the location of grain boundaries but also to classify them as well. We demonstrate that correlative transmission electron microscopy (TEM) analyses on atom-probe specimens enable the atom-probe datasets to be reconstructed with high accuracy. Moreover, EBSD and TEM can be very useful to obtain complementary information about the crystal structure in addition to the compositional analyses. The local chemical compositions at grain boundaries of a solar grade Cu(In,Ga)Se 2 film are presented here. Na, K, and O impurities are found to be segregated at grain boundaries. These impurities most likely diffuse from the soda lime glass substrate into the absorber layer during cell fabrication and processing. Based on the experimental results, we propose that Na, K, and O play an important role in the electrical properties of grain boundaries in Cu(In,Ga)Se 2 thin films for solar cells. © 2011 IEEE.
    view abstractdoi: 10.1109/JPHOTOV.2011.2170447
  • 2011 • 29 Atomic-scale distribution of impurities in cuinse2-based thin-film solar cells
    Cojocaru-Miredin, O. and Choi, P. and Wuerz, R. and Raabe, D.
    Ultramicroscopy 111 552-556 (2011)
    Atom Probe Tomography was employed to investigate the distribution of impurities, in particular sodium and oxygen, in a cuinse2-based thin-film solar cell. It could be shown that sodium, oxygen, and silicon diffuse from the soda lime glass substrate into the cuinse2 film and accumulate at the grain boundaries. Highly dilute concentrations of sodium and oxygen were measured in the bulk. Selenium was found to be depleted at the grain boundaries. These observations could be confirmed by complementary energy dispersive X-ray spectroscopy studies. Our results support the model proposed by Kronik et al. (1998) [1], which explains the enhanced photovoltaic efficiency of sodium containing cuinse2 solar cells by the passivation of selenium vacancies at grain boundaries. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.12.034
  • 2011 • 28 Nanoscaled alloy formation from self-assembled elemental Co nanoparticles on top of Pt films
    Han, L. and Wiedwald, U. and Biskupek, J. and Fauth, K. and Kaiser, U. and Ziemann, P.
    Beilstein Journal of Nanotechnology 2 473-485 (2011)
    The thermally activated formation of nanoscale CoPt alloys was investigated, after deposition of self-assembled Co nanoparticles on textured Pt(111) and epitaxial Pt(100) films on MgO(100) and SrTiO3(100) substrates, respectively. For this purpose, metallic Co nanoparticles (diameter 7 nm) were prepared with a spacing of 100 nm by deposition of precursor-loaded reverse micelles, subsequent plasma etching and reduction on flat Pt surfaces. The samples were then annealed at successively higher temperatures under a H2 atmosphere, and the resulting variations of their structure, morphology and magnetic properties were characterized. We observed pronounced differences in the diffusion and alloying of Co nanoparticles on Pt films with different orientations and microstructures. On textured Pt(111) films exhibiting grain sizes (20-30 nm) smaller than the particle spacing (100 nm), the formation of local nanoalloys at the surface is strongly suppressed and Co incorporation into the film via grain boundaries is favoured. In contrast, due to the absence of grain boundaries on high quality epitaxial Pt(100) films with micron-sized grains, local alloying at the film surface was established. Signatures of alloy formation were evident from magnetic investigations. Upon annealing to temperatures up to 380 °C, we found an increase both of the coercive field and of the Co orbital magnetic moment, indicating the formation of a CoPt phase with strongly increased magnetic anisotropy compared to pure Co. At higher temperatures, however, the Co atoms diffuse into a nearby surface region where Pt-rich compounds are formed, as shown by element-specific microscopy. © 2011 Han et al.
    view abstractdoi: 10.3762/bjnano.2.51
  • 2011 • 27 Electrical properties and structure of grain boundaries in n-conducting BaTiO3 ceramics
    Hou, J. and Zhang, Z. and Preis, W. and Sitte, W. and Dehm, G.
    Journal of the European Ceramic Society 31 763-771 (2011)
    The electrical properties of positive temperature coefficient (PTC) ceramics are expected to strongly correlate with the potential barrier height at grain boundaries, which in turn may be influenced by the grain boundary structure and chemistry. In this study, n-conducting BaTiO3 ceramics co-doped by La and Mn were prepared, and the electrical properties were determined by impedance spectroscopy and dc four-point van der Pauw measurements. Detailed analysis of the grain boundary structure was performed by electron microscopy techniques across different length scales. The study revealed that the randomly oriented polycrystalline microstructure was dominated by large angle grain boundaries, which in the present case were dry although a secondary crystalline and glass phase formed at triple junctions. The relationship between the observed grain boundary atomic structures and electrical properties is briefly discussed. © 2010 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jeurceramsoc.2010.11.016
  • 2011 • 26 Solubility of carbon in α-iron under volumetric strain and close to the Σ5(3 1 0)[0 0 1] grain boundary: Comparison of DFT and empirical potential methods
    Hristova, E. and Janisch, R. and Drautz, R. and Hartmaier, A.
    Computational Materials Science 50 1088-1096 (2011)
    The solubility of carbon in α-Fe as a function of lattice strain and in the vicinity of the ∑5(310)[001] symmetrical tilt grain boundary is calculated with ab initio methods based on density-functional theory (DFT). The results are compared to four different empirical potentials: the embedded-atom method (EAM) potentials of Lau et al. [1], Ruda et al. [2] and Hepburn et al. [3], and the modified embedded-atom method (MEAM) potential of Lee [4]. The results confirm that the solubility of carbon in body-centered-cubic (bcc) Fe increases under local volume expansion and provide quantitative data for the excess enthalpy to be used in thermodynamic databases. According to our study the excess enthalpy obtained from DFT is more strain-sensitive than the ones obtained from the tested empirical potentials. The comparison of the applied methods furthermore reveals that among the empirical potentials the MEAM is most appropriate to describe the solubility of C in bcc Fe under strain. The differences between the four empirical potentials stem from different parameterizations of the EAM potentials and, in the case of the MEAM, from the altogether different formalism that also includes angular dependent terms in the binding energy. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2010.11.006
  • 2011 • 25 Recrystallization and grain growth in ultrafine-grained materials produced by high pressure torsion
    Khorashadizadeh, A. and Raabe, D. and Winning, M. and Pippan, R.
    Advanced Engineering Materials 13 245-250 (2011)
    Ultrafine-grained (UFG) materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of UFG materials. For this purpose a Cu-0.17 wt%Zr alloy was subjected to high pressure torsion (HPT) with a maximal pressure of 4.8GPa at room temperature. The microstructures of the specimens were characterized using electron back scatter (EBSD) measurements, transmission electron microscopy (TEM), and hardness measurements. During annealing of the samples, dispersoids were formed which improved the thermal stability of the alloy. At higher strain levels the fraction of high angle grain boundaries (HAGBs) increased above 70% of the total grain boundaries. Ultrafine-grained materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of such materials. For this purpose a Cu-0.17 wt%Zr alloy was subjected to high pressure torsion. The microstructures of the specimens were characterized in the deformed state as well as after annealing using EBSD and hardness measurements. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201000253
  • 2011 • 24 Five-parameter grain boundary analysis by 3D EBSD of an ultra fine grained CuZr alloy processed by equal channel angular pressing
    Khorashadizadeh, A. and Raabe, D. and Zaefferer, S. and Rohrer, G.S. and Rollett, A.D. and Winning, M.
    Advanced Engineering Materials 13 237-244 (2011)
    The 3D grain boundary character distribution (GBCD) of a sample subjected to equal channel angular pressing (ECAP) after eight passes and successive annealing at 650°C for about 10min is analyzed. The experiments are conducted using a dual beam system, which is a combination of a focused ion beam and a scanning electron microscope to collect a series of electron backscatter diffraction (EBSD) maps of the microstructure (3D EBSD). The data set was aligned and reconstructed to a 3D microstructure. The crystallographic character of the grain boundary planes was determined using three different methods, namely, the line segment method, the stereological method, and the triangular surface mesh method. The line segment and triangular surface mesh methods produce consistent data sets, both yielding approximately a 7% area fraction of coherent twins. These results starkly contrast that of the statistical stereological method, which produced a 44% area fraction of coherent twins. The 3D grain boundary character distribution (GBCD) of a sample subjected to equal channel angular pressing (ECAP) after eight passes and successive annealing at 650°C for about 10min is analyzed. The crystallographic character of the grain boundary planes was determined using three different methods, namely, the line segment method, the stereological method, and the triangular surface mesh method. The line segment and triangular surface mesh methods produce consistent results. These results starkly contrast that of the statistical stereological method. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201000259
  • 2011 • 23 Atomic-scale mechanisms of deformation-induced cementite decomposition in pearlite
    Li, Y.J. and Choi, P. and Borchers, C. and Westerkamp, S. and Goto, S. and Raabe, D. and Kirchheim, R.
    Acta Materialia 59 3965-3977 (2011)
    Pearlitic steel can exhibit tensile strengths higher than 5 GPa after severe plastic deformation, where the deformation promotes a refinement of the lamellar structure and cementite decomposition. However, a convincing correlation between deformation and cementite decomposition in pearlite is still absent. In the present work, a local electrode atom probe was used to characterize the microstructural evolution of pearlitic steel, cold-drawn with progressive strains up to 5.4. Transmission electron microscopy was also employed to perform complementary analyses of the microstructure. Both methods yielded consistent results. The overall carbon content in the detected volumes as well as the carbon concentrations in ferrite and cementite were measured by atom probe. In addition, the thickness of the cementite filaments was determined. In ferrite, we found a correlation of carbon concentration with the strain, and in cementite, we found a correlation of carbon concentration with the lamella thickness. Direct evidence for the formation of cell/subgrain boundaries in ferrite and segregation of carbon atoms at these defects was found. Based on these findings, the mechanisms of cementite decomposition are discussed in terms of carbon-dislocation interaction. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.022
  • 2011 • 22 Dislocation interactions and low-angle grain boundary strengthening
    Liu, B. and Raabe, D. and Eisenlohr, P. and Roters, F. and Arsenlis, A. and Hommes, G.
    Acta Materialia 59 7125-7134 (2011)
    The transmission of an incoming dislocation through a symmetrical low-angle tilt grain boundary (GB) is studied for {1 1 0}〈1 1 1〉 slip systems in body-centered cubic metals using discrete dislocation dynamics (DD) simulations. The transmission resistance is quantified in terms of the different types of interactions between the incoming and GB dislocations. Five different dislocation interaction types are considered: collinear, mixed-symmetrical junction, mixed-asymmetrical junction, edge junction, and coplanar. Mixed-symmetrical junction formation events are found not only to cause a strong resistance against the incident dislocation penetration, but also to transform the symmetrical low-angle tilt GB into a hexagonal network (a general low-angle GB). The interactions between the incident dislocation and the GB dislocations can form an array of 〈1 0 0〉 dislocations (binary junctions) in non-coplanar interactions, or a single 〈1 0 0〉 dislocation in coplanar interaction. We study how the transmission resistance depends on the mobility of 〈1 0 0〉 dislocations. 〈1 0 0〉 dislocations have usually been treated as immobile in DD simulations. In this work, we discuss and implement the mobility law for 〈1 0 0〉 dislocations. As an example, we report how the mobility of 〈1 0 0〉 dislocations affects the equilibrium configuration of a ternary dislocation interaction. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.07.067
  • 2011 • 21 Effect of boron on the fracture behavior and grain boundary chemistry of Ni3Fe
    Liu, Y. and Liu, C.T. and Heatherly, L. and George, E.P.
    Scripta Materialia 64 303-306 (2011)
    The effect of B on the fracture behavior of Ni3Fe alloys (24 and 26 at.% Fe) was studied after cathodic charging with hydrogen. In contrast to its disordered state, ordered Ni3Fe underwent brittle intergranular fracture at room temperature. Boron addition changed its fracture mode to predominantly ductile transgranular. The grain boundary chemistry of ordered Ni3Fe was analyzed by Auger electron spectroscopy. Boron was found to segregate to the grain boundaries of both Ni-24Fe and Ni-26Fe and reduce the hydrogen-induced embrittlement of these alloys in the ordered state. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2010.08.027
  • 2011 • 20 Effect of shot-peening on the oxidation behaviour of boiler steels
    Naraparaju, R. and Christ, H.-J. and Renner, F.U. and Kostka, A.
    Oxidation of Metals 76 233-245 (2011)
    The presence of short diffusion paths is very important for rapid diffusion processes which are involved in forming protective oxide layers against high temperature corrosion, e.g. on boiler steels. Rapid diffusion paths can be produced by applying cold work such as shot-peening to the surface of the boiler steels prior to oxidation. The effect of shot-peening on oxidation behaviour was tested experimentally on 12 wt% Cr martensitic steel and 18 wt% Cr austenitic steel. Isothermal oxidation tests were performed at 700 and 750 °C. The surface treatment proved to be very effective in improving oxidation protection at 700 °C. Shot-peening the surface prior to the oxidation has an influential effect in changing the diffusion mechanisms of the elements involved in oxidation and changes the oxidation kinetics substantially at the applied conditions in this study. © 2011 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s11085-011-9250-x
  • 2011 • 19 On the influence of small quantities of Bi and Sb on the evolution of microstructure during swaging and heat treatments in copper
    Otto, F. and Frenzel, J. and Eggeler, G.
    Journal of Alloys and Compounds 509 4073-4080 (2011)
    In the present work, the influence of small amounts of Bi and Sb on the microstructural evolution of Cu during an ingot metallurgy processing route is investigated. Both elements are known to segregate to grain boundaries in Cu. Cu ingots with an outer diameter of 40 mm containing 0.008 wt.% Bi and 0.92 wt.% Sb, respectively, were vacuum induction melted, cast, and gradually swaged down to a final diameter of 11.7 mm with several intermediate annealing steps. Subsequent annealing treatments were conducted to investigate the microstructural evolution of the swaged bars. Optical microscopy, hardness testing and orientation imaging microscopy were used to characterize the deformation and recrystallization behavior, as well as the evolution of texture in the alloys. The results are then compared to those obtained for pure Cu. It is shown that even small amounts of alloying elements significantly alter the hardening behavior and suppress recrystallization at low temperatures. At higher temperatures, recrystallization in Cu, Cu-Bi and Cu-Sb leads to different textures. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2010.12.178
  • 2011 • 18 Sintering kinetics study of mechanically alloyed nanocrystalline Mo-30 wt.% W
    Paul, B. and Jain, D. and Chakraborty, S.P. and Sharma, I.G. and Pillai, C.G.S. and Suri, A.K.
    Thermochimica Acta 512 134-141 (2011)
    The paper details the results of sintering kinetics studies conducted on nanocrystalline Mo-30 wt.% W alloy powders prepared through mechanical alloying route. Both, constant rate of heating method as well as Stepwise Isothermal Dilatometry (SID) technique were used for studying the sintering kinetics. Measured step isothermal shrinkage data were analyzed by Mekipritti-Meng method. The shrinkage data was found to fit well with the rate equation proposed in this method and its validity was established for mechanically alloyed systems. Kinetic parameters were evaluated and sintering was found to occur through two major mechanisms operative successively, which are grain boundary diffusion and lattice diffusion with corresponding energies of activation as 230 kJ/mol and 480 kJ/mol, respectively. The results have been well supported by micro structural evaluation of specimens at different stages of sintering. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tca.2010.09.015
  • 2011 • 17 Epitaxial Ag wires with a single grain boundary for electromigration
    Sindermann, S. and Witt, C. and Spoddig, D. and Horn-Von Hoegen, M. and Dumpich, G. and Meyer zu Heringdorf, F.-J.
    Review of Scientific Instruments 82 (2011)
    Test structures for electromigration with defined grain boundary configurations can be fabricated using focused ion beam (FIB). We present a novel approach of combining epitaxial growth of Ag islands with FIB milling. Depending on the growth parameters, bi-crystalline Ag islands can be grown on Si(111) surfaces and can be structured into wires by FIB. To avoid doping effects of the used Ga FIB, silicon on insulator (SOI) substrates are used. By cutting through the device layer of the SOI substrate with deep trenches, the Ag wire can be electrically separated from the rest of the substrate. In this way, Ag wires with one isolated grain boundary of arbitrary direction can be assembled. Using scanning electron microscopy we demonstrate the feasibility of our approach. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3671802
  • 2011 • 16 Artificially nanostructured n-type SiGe bulk thermoelectrics through plasma enhanced growth of alloy nanoparticles from the gas phase
    Stein, N. and Petermann, N. and Theissmann, R. and Schierning, G. and Schmechel, R. and Wiggers, H.
    Journal of Materials Research 26 1872-1878 (2011)
    SiGe alloys belong to the class of classic high temperature thermoelectric materials. By the means of nanostructuring, the performance of this well-known material can be further enhanced. Additional grain boundaries and point defects added to the alloy structure result in a strong decrease in thermal conductivity because of reduced lattice contribution to the overall thermal conductivity. Hence, the figure of merit can be increased. To obtain a nanostructured bulk material, a nanosized raw material is essential. In this work, a new approach toward nanostructured SiGe alloys is presented where alloyed nanoparticles are synthesized from a homogeneous mixture of the respective precursors in a microwave plasma reactor. As-prepared nanoparticles are compacted to a dense bulk material by a field assisted sintering technique. A figure of merit of zT = 0.5 ± 0.09 at 450 °C and a peak zT of 0.8 ± 0.15 at 1000 °C could be achieved for a nanostructured, 0.8% phosphorus-doped Si 80Ge20 alloy without any further optimization. Copyright © Materials Research Society 2011.
    view abstractdoi: 10.1557/jmr.2011.117
  • 2011 • 15 Grain boundary electrochemistry of β-type Nb-Ti alloy using a scanning droplet cell
    Woldemedhin, M.T. and Raabe, D. and Hassel, A.W.
    Physica Status Solidi (A) Applications and Materials Science 208 1246-1251 (2011)
    Localized oxide spots were grown at the grain boundaries of a technically relevant 30at.% Nb-Ti β-type titanium alloy to study the local electrochemical response. The grain boundaries selected were combinations of grains having different orientations and grain boundary angle. Crystallographic information of the grains and boundary angles were revealed by electron back scattering diffraction (EBSD) technique. Cyclic voltammetry is the electrochemical technique used to grow the oxides starting from 0V and increasing the potential in steps of 1V till 8V at a scan rate of 100mVs -1 in an acetate buffer of pH 6.0. Electrochemical impedance spectroscopy was used to investigate the electrical properties of the oxide/electrolyte interface in the frequency range between 100kHz and 100mHz. Important oxide parameters such as formation factor and dielectric number were determined from these measurements. Significant differences were observed for different grain boundaries. The semiconducting properties of the oxides at the grain boundaries were assessed by using Mott-Schottky analysis on a potentiostatically grown oxide. All the oxides showed n-type semiconducting properties where the donor concentration varies with the grain boundaries mentioned above. A flat band potential -0.25 ±0.02V versus standard hydrogen electrode is more or less the same for all the boundaries studied. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201000991
  • 2011 • 14 Superplastic martensitic Mn-Si-Cr-C steel with 900% elongation
    Zhang, H. and Bai, B. and Raabe, D.
    Acta Materialia 59 5787-5802 (2011)
    High-strength (1.2-1.5)C-(2-2.5)Mn-(1.5-2)Si-(0.8-1.5)Cr steels (mass%) consisting of martensite and carbides exhibit excellent superplastic properties (e.g. strain rate sensitivity m ≈ 0.5, elongation ≈900% at 1023 K). A homogeneous martensitic starting microstructure is obtained through thermomechanical processing (austenitization plus 1.2 true strain, followed by quenching). Superplastic forming leads to a duplex structure consisting of ferrite and spherical micro-carbides. Through 1.5-2% Si alloying, carbides precipitate at hetero-phase interfaces and martensite blocks at the beginning of superplastic forming. Via Ostwald ripening, these interface carbides grow at the expense of carbides precipitating at martensite laths, thereby promoting ferrite dynamic recrystallization. Simultaneously, carbides at ferrite grain boundaries retard the growth of recrystallized ferrite grains. Due to 2-2.5% Mn and 0.8-1.5% Cr alloying, carbide coarsening is suppressed owing to the slow diffusion of these elements. As a result, fine and homogeneous ferrite plus spherical carbide duplex microstructures with a ferrite grain size of ∼1.5 μm are obtained after superplastic forming. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.05.055
  • 2010 • 13 A two-dimensional dislocation dynamics model of the plastic deformation of polycrystalline metals
    Ahmed, N. and Hartmaier, A.
    Journal of the Mechanics and Physics of Solids 58 2054-2064 (2010)
    Two-dimensional dislocation dynamics (2D-DD) simulations under fully periodic boundary conditions are employed to study the relation between microstructure and strength of a material. The material is modeled as an elastic continuum that contains a defect microstructure consisting of a preexisting dislocation population, dislocation sources, and grain boundaries. The mechanical response of such a material is tested by uniaxially loading it up to a certain stress and allowing it to relax until the strain rate falls below a threshold. The total plastic strain obtained for a certain stress level yields the quasi-static stressstrain curve of the material. Besides assuming FrankRead-like dislocation sources, we also investigate the influence of a pre-existing dislocation density on the flow stress of the model material. Our results show that despite its inherent simplifications the 2D-DD model yields material behavior that is consistent with the classical theories of Taylor and HallPetch. Consequently, if set up in a proper way, these models are suited to study plastic deformation of polycrystalline materials. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2010.09.005
  • 2010 • 12 Creep properties beyond 1100°C and microstructure of Co-Re-Cr alloys
    Brunner, M. and Hüttner, R. and Bölitz, M.-C. and Völkl, R. and Mukherji, D. and Rösler, J. and Depka, T. and Somsen, C. and Eggeler, G. and Glatzel, U.
    Materials Science and Engineering A 528 650-656 (2010)
    The melting point of a novel Co-17Re-23Cr alloy (numbers given in at.%) could be increased by 250 °C as compared to established Ni-based superalloys, by optimising the content of Re. Samples were produced by vacuum arc-melting in order to evaluate the creep behaviour at temperatures beyond 1100 °C and for microstructural analysis. Three alloys (the Co-17Re-23Cr-based material, and the carbide strengthened alloys Co-17Re-23Cr-2.6C and Co-17Re-23Cr-2.6C-1.2Ta) were investigated. Creep properties, especially the minimum creep rate and the Larson-Miller plots, were compared. The Co-17Re-23Cr-2.6C-1.2Ta alloy has a higher minimum creep rate than Co-17Re-23Cr at 1200 °C but it has a lower minimum creep rate than Co-17Re-23Cr at 1100 °C. TaC coarsening, detected via transmission electron microscope (TEM) measurements may explain this effect. The overall creep behaviour of Co-17Re-23Cr-2.6C at 1200 °C is better than that of Co-17Re-23Cr-2.6C-1.2Ta, but worse than that of Co-17Re-23Cr.Microstructural investigations by scanning electron microscopy and TEM reveal a hexagonal closed-packed (hcp) matrix and σ-phases. The microhardness of the σ-phase was about 1570. HV (load: 1. g) and around 800. HV for the matrix. Pores and cracks occur along the brittle σ-phases and grain boundaries in the Co-Re-Cr alloys. A Norton exponent n in between 1.4 and 3.0 points to grain boundary dominated creep mechanisms. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.09.035
  • 2010 • 11 Influence of heat treatment and microstructure on the tensile pseudoelastic response of an Ni-rich NiTi shape memory alloy
    Bujoreanu, L.-G. and Young, M.L. and Gollerthan, S. and Somsen, C. and Eggeler, G.
    International Journal of Materials Research 101 623-630 (2010)
    The influence of microstructure on the stress-strain behavior of an Ni-rich NiTi shape memory alloy is examined. Specimens cut from a large-diameter bar of Ni50.7Ti49.3 shape memory alloy were analyzed in two states: (i) annealed and (ii) annealed and aged. The annealed state shows a fully austenitic structure with no precipitates and no distortions caused by residual stresses. The annealed and aged state has coherent Ni 4Ti3 particles precipitated in the proximity of the austenite grain boundaries. The size of the precipitates increases moving away from the grain boundaries toward the grain interiors. The evolution of the two states in the stress-strain-temperature space has been analyzed using tensile specimens with special geometry. Due to the complex effects of the coherent precipitates, the specimens in the aged state exhibited lower stress plateaus in the tensile loading-unloading curves, which enabled the occurrence of transformation pseudoelasticity from room temperature to 333 K. © 2010 Carl Hanser Verlag.
    view abstractdoi: 10.3139/146.110317
  • 2010 • 10 The effect of grain size and grain orientation on deformation twinning in a Fe-22wt.% Mn-0.6wt.% C TWIP steel
    Gutierrez-Urrutia, I. and Zaefferer, S. and Raabe, D.
    Materials Science and Engineering A 527 3552-3560 (2010)
    We investigate the effect of grain size and grain orientation on deformation twinning in a Fe-22wt.% Mn-0.6wt.% C TWIP steel using microstructure observations by electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Samples with average grain sizes of 3μm and 50μm were deformed in tension at room temperature to different strains. The onset of twinning concurs in both materials with yielding which leads us to propose a Hall-Petch-type relation for the twinning stress using the same Hall-Petch constant for twinning as that for glide. The influence of grain orientation on the twinning stress is more complicated. At low strain, a strong influence of grain orientation on deformation twinning is observed which fully complies with Schmid's law under the assumption that slip and twinning have equal critical resolved shear stresses. Deformation twinning occurs in grains oriented close to 〈1. 1. 1〉//tensile axis directions where the twinning stress is larger than the slip stress. At high strains (0.3 logarithmic strain), a strong deviation from Schmid's law is observed. Deformation twins are now also observed in grains unfavourably oriented for twinning according to Schmid's law. We explain this deviation in terms of local grain-scale stress variations. The local stress state controlling deformation twinning is modified by local stress concentrations at grain boundaries originating, for instance, from incoming bundles of deformation twins in neighboring grains. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.02.041
  • 2010 • 9 Multi-axial behavior of ferroelectrics with two types of micro-macro mechanical models
    Jayabal, K. and Arockiarajan, A. and Menzel, A. and Sivakumar, S.M.
    IUTAM Bookseries 19 95-102 (2010)
    Ferroelectric ceramics exhibit a significantly different nonlinear behavior with external electric and mechanical fields applied at angles to the initial poled direction. This angle dependent response of the ferroelectric polycrystals are predicted by two types of models based on irreversible thermodynamics and physics of domain switching. The first type is a uniaxial model dealing with simultaneous evolution of three variants at a given instant. The back stress and electric fields, assumed as linear functions of remnant strain and polarization developed by the domain switching process, are introduced in the model to assist or resist further switching process. The second type is a three dimensional model that considers all six variants of a tetragonal crystal in each grain and the dissipation associated with grain boundary constraints are brought into the model through switching criterion. The pressure dependent constraints imposed by the surrounding grains on the grain of interest at its boundary during domain switching process is correlated with the resistance experienced by a ferroelectric single crystal on its boundary during domain switching. Taking all the domain switching possibilities, the volume fractions of each of the variants are tracked and homogenized for macroscopic behavior. Numerical simulations were carried out for the behavior of ferroelectrics using both the models and the outcome was found to be qualitatively comparable with experimental observations given in literature. © 2010 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/978-90-481-3771-8-10
  • 2010 • 8 Multiscale simulations on the grain growth process in nanostructured materials
    Kamachali, R.D. and Hua, J. and Steinbach, I. and Hartmaier, A.
    International Journal of Materials Research 101 1332-1338 (2010)
    In this work, multi-phase field and molecular dynamics simulations have been used to investigate nanoscale grain growth mechanisms. Based on experimental observations, the combination of grain boundary expansion and vacancy diffusion has been considered in the multi-phase field model. The atomistic mechanism of boundary movement and the free volume redistribution during the growth process have been investigated using molecular dynamics simulations. According to the multi-phase field results, linear grain growth in nanostructured materials at low temperature can be explained by vacancy diffusion in the stress field around the grain boundaries. Molecular dynamics simulations confirm the observation of linear grain growth for nanometresized grains. The activation energy of grain boundary motion in this regime has been determined to be of the order of onetenth of the self-diffusion activation energy, which is consistent with experimental data. Based on the simulation results, the transition from linear to normal grain growth is discussed in detail and a criterion for this transition is proposed. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110419
  • 2010 • 7 A review of crystallographic textures in chemical vapor-deposited diamond films
    Liu, T. and Raabe, D. and Mao, W.-M.
    Signal, Image and Video Processing 4 1-16 (2010)
    Diamond is one of the most important functional materials for film applications due to its extreme physical and mechanical properties, many of which depend on the crystallographic texture. The influence of various deposition parameters matters to the texture formation and evolution during chemical vapor deposition (CVD) of diamond films. In this overview, the texture evolutions are presented in terms of both simulations and experimental observations. The crystallographic textures in diamond are simulated based on the van der Drift growth selection mechanism. The film morphology and textures associated with the growth parameters α (proportional to the ratio of the growth rate along the 〈100〉 direction to that along the 〈111〉 direction) are presented and determined by applying the fastest growth directions. Thick films with variations in substrate temperature, methane concentration, film thickness, and nitrogen addition were analyzed using high-resolution electron back-scattering diffraction (HR-EBSD) as well as X-ray diffraction (XRD), and the fraction variations of fiber textures with these deposition parameters were explained. In conjunction with the focused ion beam (FIB) technique for specimen preparation, the grain orientations in the beginning nucleation zones were studied using HR-EBSD (50 nm step size) in another two sets of thin films deposited with variations in methane concentration and substrate material. The microstructures, textures, and grain boundary character were characterized. Based on the combination of an FIB unit for serial sectioning and HR-EBSD, diamond growth dynamics was observed using a 3D EBSD technique, with which individual diamond grains were investigated in 3D. Microscopic defects were observed in the vicinity of the high-angle grain boundaries by using the transmission electron microscopy (TEM) technique, and the advances of TEM orientation microscopy make it possible to identify the grain orientations in nano-crystalline diamond. © 2010 Higher Education Press and Springer Berlin Heidelberg.
    view abstractdoi: 10.1007/s11760-008-0099-7
  • 2010 • 6 Atomistic simulations of lattice defects in tungsten
    Mrovec, M. and Elsässer, C. and Gumbsch, P.
    International Journal of Refractory Metals and Hard Materials 28 698-702 (2010)
    The mechanical behavior of materials is ultimately determined by events occurring at the atomic scale. The onset of plastic yield corresponds to triggering of dislocation motion. Subsequent hardening is mainly controlled by interaction of gliding dislocations with other lattice defects such as forest dislocations, grain boundaries, interfaces and surfaces. Finally, material failure is influenced by processes at the tip of a crack propagating in a crystal lattice. In this work we review atomistic simulations of lattice defects in tungsten. We show that these studies are able to provide not only a detailed understanding of defect properties but also reveal how the fundamental processes at the atomic scale are linked to macroscopic material behavior. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijrmhm.2010.05.007
  • 2010 • 5 Microstructural characterization of lamellar features in TiAl by FIB imaging
    Peter, D. and Eggeler, G. and Wagner, M.F.-X.
    Advanced Engineering Materials 12 447-452 (2010)
    A novel experimental procedure is introduced to determine phase fractions and the distribution of individual phases of TiAl-based two-phase alloys using the focused ion beam (FIB) technique. Two γ-titanium aluminide alloys with a fine-grained duplex and a nearly lamellar microstructure are examined. The special FIB-based preparation procedure results in high contrast ion beam-induced images for all investigated alloys and allows to quantify the phase contents easily by automated microstructural analysis. Fine two-phase structures, e.g. lamellar colonies in γ-TiAl, can be imaged in high resolution with respect to different phases. To validate the FIB-derived data, we compare them to results obtained with another method to determine phase fractions, electron back-scatter diffraction (EBSD). This direct comparison shows that the FIB-based technique generally provides slightly higher α2-fractions, and thus helps to overcome the limited lateral resolution near grain boundaries and interfaces associated with the conventional EBSD approach. Our study demonstrates that the FIB-based technique is a simple, fast, and more exact way to determine high resolution microstructural characteristics with respect to different phase constitutions in two-phase TiAl alloys and other such materials with fine, lamellar microstructures. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.200900339
  • 2010 • 4 Annealing behavior of ferritic-martensitic 9%Cr-ODS-Eurofer steel
    Sandim, H.R.Z. and Renzetti, R.A. and Padilha, A.F. and Raabe, D. and Klimenkov, M. and Lindau, R. and Möslang, A.
    Materials Science and Engineering A 527 3602-3608 (2010)
    Oxide dispersion strengthened ferritic-martensitic steels are potential candidates for applications in future fusion power plants. High creep resistance, good oxidation resistance, reduced neutron activation and microstructural long-term stability at temperatures of about 650-700°C are required in this context. In order to evaluate its thermal stability in the ferritic phase field, samples of the reduced activation ferritic-martensitic 9%Cr-ODS-Eurofer steel were cold rolled to 50% and 80% reductions and further annealed in vacuum from 300 to 800°C for 1h. The characterization in the annealed state was performed by scanning electron microscopy in the backscattered electron mode, high-resolution electron backscatter diffraction and transmission electron microscopy. Results show that the fine dispersion of Y-based particles (about 10nm in size) is effective to prevent recrystallization. The low recrystallized volume fraction (< 0.1) is associated to the nuclei found at prior grain boundaries and around large M23C6 particles. Static recovery was found to be the predominant softening mechanism of this steel in the investigated temperature range. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.02.051
  • 2010 • 3 Investigation of the ferroelectric-relaxor transition in PbMg 1/3Nb2/3O3-PbTiO3 ceramics by piezoresponse force microscopy
    Shvartsman, V.V. and Kholkin, A.L.
    Journal of Applied Physics 108 (2010)
    The spontaneous transition between the ferroelectric and relaxor states was investigated in 0.86PbMg1/3Nb2/3O3 -0.14 PbTiO3 ceramics using piezoresponse force microscopy (PFM). Macroscopically, the transition from the ferroelectric to relaxor phases manifests itself by an anomaly in the temperature dependences of the dielectric permittivity and by a sharp decline of the remanent polarization. Alternatively, PFM reveals a decay of the ferroelectric micron-size domains at the macroscopic Curie temperature, TC. Simultaneously, smaller domains of submicron sizes are observed at temperatures appreciably above TC, being concentrated near grain boundaries. It is argued that the particular mechanical and electrical conditions at the grain boundaries promote nucleation of the ferroelectric phase. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3474962
  • 2010 • 2 Phase-field model with plastic flow for grain growth in nanocrystalline material
    Steinbach, I. and Song, X. and Hartmaier, A.
    Philosophical Magazine 90 485-499 (2010)
    A phase-field model is presented which considers the accumulation of structural defects in grain boundaries by an isotropic eigenstrain associated with the grain boundaries. It is demonstrated that the elastic energy caused by dilatation of the grain boundary with respect to the bulk crystal contributes largely to the grain boundary energy. The sign of this contribution can be both positive and negative dependent on the local stress state in the grain boundary. Self-diffusion of atoms is taken into account to relax the stress caused by the dilatation of the grain boundary. Application of the model to discontinuous grain growth in pure nanocrystalline cobalt material is presented. Linear grain growth is found in the nanocrystalline state, which is explained by the interpretation of grain boundary motion as a diffusive process defining an upper limit of the grain boundary velocity independent of the grain boundary curvature but dependent on temperature. The transition to regular grain growth at a critical temperature, as observed experimentally, is explained by the drop of theoretical grain boundary velocity due to its mean curvature during coarsening of the nanograin structure below the maximum velocity.
    view abstractdoi: 10.1080/14786430903074763
  • 2010 • 1 Modeling size effects on fracture toughness by dislocation dynamics
    Zeng, X.H. and Hartmaier, A.
    Acta Materialia 58 301-310 (2010)
    The effects of grain size and of crack-tip blunting radius on the fracture toughness of tungsten polycrystals are studied by using a combined dislocation dynamics/cohesive zone model (CZM). Two-dimensional dislocation dynamics are employed to analyze crack-tip plasticity and crack propagation is characterized by a CZM. The geometry of the crack and the corresponding boundary conditions are described by means of a boundary element method with dislocation dipoles as fundamental solution. Grain boundaries are introduced as obstacles for dislocation motion. Numerical experiments reveal that the fracture toughness decreases with grain size, because grain boundaries confine the plastic zone. This effect is particularly pronounced at small loading rates, where the unconfined plastic zone is large. Our results also show that fracture toughness scales with the tip radius as the stress concentration at the crack tip is reduced and the plastic zone is enlarged. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2009.09.005