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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  • 2021 • 469 Automated image analysis for quantification of materials microstructure evolution
    Ahmed, M. and Horst, O.M. and Obaied, A. and Steinbach, I. and Roslyakova, I.
    Modelling and Simulation in Materials Science and Engineering 29 (2021)
    In this work, an automated image analysis procedure for the quantification of microstructure evolution during creep is proposed for evaluating scanning electron microscopy micrographs of a single crystal Ni-based superalloy before and after creep at 950 °C and 350 MPa. scanning electron microscopy-micrographs of γ/γ′ microstructures are transformed into binary images. Image analysis, which involves pixel by pixel classification and feature extraction, is then combined with a supervised machine learning algorithm to improve the binarization and the quality of the results. The binarization of the gray scale images is not always straight forward, especially when the difference in gray levels between the γ-channels and the γ′-phase is small. To optimize feature extraction, we utilized a series of bilateral filters as well as a machine learning algorithm, known as the gradient boosting method, that was used for training and classifying the micrograph pixels. After testing the two methods, the gradient boosting method was identified as the most effective. Subsequently, a Python routine was written and implemented for the automated quantification of the γ′ area fraction and the γ channel width. Our machine learning method is documented and the results of the automatic procedure are discussed based on results which we previously reported in the literature. © 2021 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/abfd1a
  • 2021 • 468 Chemical heterogeneity enhances hydrogen resistance in high-strength steels
    Sun, B. and Lu, W. and Gault, B. and Ding, R. and Makineni, S.K. and Wan, D. and Wu, C.-H. and Chen, H. and Ponge, D. and Raabe, D.
    Nature Materials (2021)
    The antagonism between strength and resistance to hydrogen embrittlement in metallic materials is an intrinsic obstacle to the design of lightweight yet reliable structural components operated in hydrogen-containing environments. Economical and scalable microstructural solutions to this challenge must be found. Here, we introduce a counterintuitive strategy to exploit the typically undesired chemical heterogeneity within the material’s microstructure that enables local enhancement of crack resistance and local hydrogen trapping. We use this approach in a manganese-containing high-strength steel and produce a high dispersion of manganese-rich zones within the microstructure. These solute-rich buffer regions allow for local micro-tuning of the phase stability, arresting hydrogen-induced microcracks and thus interrupting the percolation of hydrogen-assisted damage. This results in a superior hydrogen embrittlement resistance (better by a factor of two) without sacrificing the material’s strength and ductility. The strategy of exploiting chemical heterogeneities, rather than avoiding them, broadens the horizon for microstructure engineering via advanced thermomechanical processing. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41563-021-01050-y
  • 2021 • 467 Columnar Thermal Barrier Coatings Produced by Different Thermal Spray Processes
    Kumar, N. and Gupta, M. and Mack, D.E. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology (2021)
    Suspension plasma spraying (SPS) and plasma spray-physical vapor deposition (PS-PVD) are the only thermal spray technologies shown to be capable of producing TBCs with columnar microstructures similar to the electron beam-physical vapor deposition (EB-PVD) process but at higher deposition rates and relatively lower costs. The objective of this study was to achieve fundamental understanding of the effect of different columnar microstructures produced by these two thermal spray processes on their insulation and lifetime performance and propose an optimized columnar microstructure. Characterization of TBCs in terms of microstructure, thermal conductivity, thermal cyclic fatigue lifetime and burner rig lifetime was performed. The results were compared with TBCs produced by the standard thermal spray technique, atmospheric plasma spraying (APS). Bondcoats deposited by the emerging high-velocity air fuel (HVAF) spraying were compared to the standard vacuum plasma-sprayed (VPS) bondcoats to investigate the influence of the bondcoat deposition process as well as topcoat–bondcoat interface topography. The results showed that the dense PS-PVD-processed TBC had the highest lifetime, although at an expense of the highest thermal conductivity. The reason for this behavior was attributed to the dense intracolumnar structure, wide intercolumnar gaps and high column density, thus improving the strain tolerance and fracture toughness. © 2021, The Author(s).
    view abstractdoi: 10.1007/s11666-021-01228-5
  • 2021 • 466 Constitutive modeling of cyclic plasticity at elevated temperatures for a nickel-based superalloy
    Shahmardani, M. and Hartmaier, A.
    International Journal of Fatigue 151 (2021)
    During the operation of turbines in jet engines or in power plants, high thermal and intermittent mechanical loads appear, which can lead to high-temperature fatigue failure if thermal and mechanical loads vary at the same time. Since fatigue testing is a time-consuming process, it is important to develop realistic material models with predictive capabilities that are able to extrapolate the limited experimental results for cyclic plasticity within a wide range of temperatures. To accomplish this, an approach based on a representative volume element (RVE), mimicking the typical γ/γ′ microstructure of a Ni-based single crystal superalloy, is adopted for cyclic loading conditions. With the help of this RVE, the temperature- and deformation-dependent internal stresses in the microstructure can be taken into account in a realistic manner, which proves to be essential in understanding the fatigue behavior of this material. The material behavior in the elastic regime is described by temperature-dependent anisotropic elastic constants. The flow rule for plastic deformation is governed by the thermal activation of various slip systems in the γ matrix, the γ′ precipitate and also by cube slip along the γ/γ′ microstructure. This phenomenological crystal plasticity/creep model takes different mechanisms into account, including thermally activated dislocation slip, the internal stresses due to inhomogeneous strains in different regions of γ matrix channels and in γ′ precipitates, the softening effect due to dislocation climb, the formation of 〈112〉 dislocation ribbons for precipitate shearing, and Kear-Wilsdorf locks. This constitutive law is parameterized based on experimental data for the CMSX-4 single-crystal superalloy by applying an inverse analysis to identify the material parameters based on many low cycle fatigue tests in the intermediate temperature and high stress regime. The identified material parameters could predict cyclic plasticity and low cycle fatigue behavior at different temperatures. The model does not only reliably reproduce the experimental results along different crystallographic loading directions, but it also reveals the relative importance of the different deformation mechanisms for the fatigue behavior under various conditions. © 2021 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijfatigue.2021.106353
  • 2021 • 465 Influence of Direct Splat-Affecting Parameters on the Splat-Type Distribution, Porosity, and Density of Segmentation Cracks in Plasma-Sprayed YSZ Coatings
    Tillmann, W. and Khalil, O. and Baumann, I.
    Journal of Thermal Spray Technology 30 1015-1027 (2021)
    The integrity and properties of ceramic coatings produced by atmospheric plasma spraying are highly controlled by the splat morphology and splat interconnection. In this study, the influence of selected parameters (spray angle, surface velocity of the spray gun, and substrate temperature) on splat morphology and coating microstructure was investigated. A favorite set of spray gun parameters, of which their effects on splat morphology and coating microstructure have been verified by previous experiments, were used to conduct the experiments for the present work. It was found that depositing fully molten particles on a hot substrate increases the fraction of disk-like splats by about 60% at the expense of the fraction of irregular splats. Preheating the substrate also increases the pore count and level of coating porosity, while it does not influence the density of segmentation cracks. In contrast, the surface velocity of the spray gun does not affect the splat morphology while a slow speed decreases the coating porosity and plays a significant role in generating segmentation cracks. Shifting the spray angle by 15° distorts up to 20% of disk-like splats and slightly decreases the porosity level. However, changing the spray angle does not affect the generation of segmentation cracks. © 2021, The Author(s).
    view abstractdoi: 10.1007/s11666-021-01180-4
  • 2021 • 464 Intercritical annealing to achieve a positive strain-rate sensitivity of mechanical properties and suppression of macroscopic plastic instabilities in multi-phase medium-Mn steels
    Benzing, J.T. and Luecke, W.E. and Mates, S.P. and Ponge, D. and Raabe, D. and Wittig, J.E.
    Materials Science and Engineering A 803 (2021)
    This study investigates the high strain-rate tensile properties of a cold-rolled medium-Mn steel (Fe–12Mn–3Al-0.05C % in mass fraction) designed to have a multi-phase microstructure and positive strain-rate sensitivity. At the intercritical annealing temperature of 585 °C, increasing the annealing time from 0.5 h to 8 h increased the phase volume fraction of ultrafine-grained (UFG) austenite from 2% to 35% by reversion. The remainder of the microstructure was composed of UFG ferrite and recovered α′-martensite (the latter resembles the cold-rolled state). Servo hydraulic tension testing and Kolsky-bar tension testing were used to measure the tensile properties from quasi-static strain rates to dynamic strain rates (ε˙ = 10-4 s-1 to ε˙ = 103 s-1). The strain-rate sensitivities of the yield strength (YS) and ultimate tensile strength (UTS) were positive for both annealing times. Tensile properties and all non-contact imaging modalities (infrared imaging and digital image correlation) indicated an advantageous suppression of Lüders bands and Portevin Le Chatelier (PLC) bands (a critical challenge in multi-phase medium-Mn steel design) due to the unique combination of microstructural constituents and overall composition. Fracture surfaces of specimens annealed for 0.5 h showed some instances of localized cleavage fracture (approximately 30 μm wide areas and lath-like ridges). Specimens annealed for 8 h maintained a greater product of strength and elongation by at least 2.5 GPa % (on average for each strain rate). The relevant processing-structure-property relationships are discussed in the context of recommendations for design strategies concerning multi-phase steels such that homogeneous deformation behavior and positive strain-rate sensitivities can be achieved. © 2020
    view abstractdoi: 10.1016/j.msea.2020.140469
  • 2021 • 463 Internal Diameter Coating by Warm Spraying of Fine WC-12Co Powders (− 10 + 2 µm) with Very Short Spray Distances up to 10 mm
    Baumann, I. and Tillmann, W. and Schaak, C. and Schmidt, K. and Hagen, L. and Zajaczkowski, J. and Schmidtmann, G. and Matthäus, G. and Luo, W.
    Journal of Thermal Spray Technology (2021)
    The internal diameter (ID) coating by means of thermal spraying is currently experiencing growing interest in science and industry. In contrast to the well-established plasma- and arc-based spray techniques, there is a lack of knowledge concerning kinetic processes such as HVOF, HVAF and warm spray (WS). A major challenge represents the necessity of short spray distances and the compact design of novel ID spray guns with reduced combustion power. Conventional WC-Co powders (− 45 + 15 µm) are not able to achieve a sufficient heat and momentum transfer. The use of fine powders < 15 µm offers an approach to overcome this drawback as they feature a larger surface-to-volume ratio and a lower mass. However, the processing of fine powders requires suitable spray equipment and a sensitive parameter adjustment. In this study, warm spraying of fine WC-12Co powders (− 10 + 2 µm) with a novel ID spray gun (HVOF + N2) “ID RED” (Thermico Engineering GmbH, Germany) was investigated. First, the flame profile as well as the in-flight behavior of the particles along the spray jet (spray distances SD = 10-80 mm) was analyzed at different nitrogen flows NF = 15-115 L/min to find suitable spray parameter intervals. Subsequently, planar steel samples were coated with SD = 10-50 mm and constant NF = 90 L/min. Analyses regarding the microstructure, the mechanical properties and the phase evolution of the coatings were performed. The aim was to study spraying with the novel ID gun and to scrutinize shortest feasible spray distances. Finally, steel tubes (internal diameter of 81.6 mm and a wall thickness of 10.0 mm) were coated with SD = 20 mm and NF = 90 L/min to investigate in how far the results can be transferred to ID parts. Correlations between the particle behavior, the microstructure and the coating properties were made. © 2021, The Author(s).
    view abstractdoi: 10.1007/s11666-021-01195-x
  • 2021 • 462 Laser metal deposition of Al0.6CoCrFeNi with Ti & C additions using elemental powder blends
    Asabre, A. and Wilms, M.B. and Kostka, A. and Gemagami, P. and Weisheit, A. and Laplanche, G.
    Surface and Coatings Technology 418 (2021)
    Laser metal deposition (LMD) was used to in-situ alloy a crack-free Al0.6CoCrFeNi compositionally complex alloy (CCA) with 3 at.% Ti and 0.25 at.% C additions on an initially ferritic H10 tool steel from an elemental powder blend. After LMD, the material was annealed at 900 °C for 30 min to induce martensitic hardening in the substrate. The CCA in both as-deposited and annealed states exhibited a lamellar microstructure consisting of four phases: a matrix of interwoven disordered and ordered body-centered cubic phases, titanium carbides distributed randomly within the microstructure, and disordered face-centered cubic (FCC) plates that precipitated at the grain boundaries and grew towards the center of the grains. Chemical analyses along the build-up direction of the coating revealed a compositional gradient, similar in both as-deposited and annealed states, due to the intermixing between the substrate and the CCA. Despite a strong variation of the Fe-content, the hardness and the microstructure remain roughly constant in the major part of the as-deposited coating, which contains a large fraction of FCC plates that are beneficial to increase ductility and ensure a good compatibility with the substrate. In contrast, the upper part of the as-deposited coating, corresponding to the last solidified melt pool after LMD, has a much lower FCC fraction with an enhanced hardness. After annealing, the hardness of the tool steel substrate significantly increased and the FCC volume fraction in the coating increased from ~16% (as-deposited) to ~58%. Overall the microstructure of the coating became more homogeneous while its hardness decreased only by 10–15%. These results demonstrate that the CCA can be employed as a protective coating on a less expensive tool steel to improve its lifetime during service. © 2021 The Author(s)
    view abstractdoi: 10.1016/j.surfcoat.2021.127233
  • 2021 • 461 Microstructure and Fatigue Damage Evolution in Additive-Manufactured Metals Using Enhanced Measurement Techniques and Modeling Approaches
    Awd, M. and Walther, F. and Siddique, S. and Fatemi, A.
    Minerals, Metals and Materials Series 5 753-762 (2021)
    Process-induced microstructures have a high impact on the fatigue strength of engineering materials. Advanced materials testing builds the base for the design and manufacturing of reliable, high-performance products for various technical applications. Combining modern analytical and intermittent testing strategies with applied enhanced measurement techniques, i.e., physical instrumentation of testing specimens during loading, allows the characterization of process-structure-property relationships in various fatigue damage stages. Further, in situ mechanical testing in analytical devices like micro-computed tomography (µ-CT) enables the immediate correlation of material’s physical reactions with the applied loading conditions. The focus of the presented studies. Using the proposed technique, the characterization of fatigue damage evolution and progression before failure depending on environmental as well as material specific microstructural characteristics is carried out. Investigations on additively manufactured Al alloys revealed the interaction between porosity and microstructure under very high-cycle fatigue (VHCF) loading conditions. Measurement-based fatigue damage tracking during testing of SLM aluminum alloys revealed the interaction between porosity and microstructure under loading in the very high-cycle fatigue (VHCF) regime. The grain boundary strengthening of the microstructure increased VHCF strength by 33%. © 2021, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/978-3-030-65261-6_68
  • 2021 • 460 Microstructure formation and mechanical properties of ODS steels built by laser additive manufacturing of nanoparticle coated iron-chromium powders
    Doñate-Buendia, C. and Kürnsteiner, P. and Stern, F. and Wilms, M.B. and Streubel, R. and Kusoglu, I.M. and Tenkamp, J. and Bruder, E. and Pirch, N. and Barcikowski, S. and Durst, K. and Schleifenbaum, J.H. and Walther, F. and G...
    Acta Materialia 206 (2021)
    Oxide dispersion strengthened (ODS) steels are known for their enhanced mechanical performance at high temperatures or under radiation exposure. Their microstructure depends on the manufacturing process, from the nanoparticle addition to the base steel powder, to the processing of the nanoparticle enriched powder. The optimization and control of the processing steps still represent a challenge to establish a clear methodology for the additive manufacturing of ODS steels. Here, we evaluate the microstructure, nanoparticle evolution, and mechanical properties of ODS steels prepared by dielectrophoretic controlled adsorption of 0.08 wt% laser-synthesized yttrium oxide (Y2O3) on an iron-chromium ferritic steel powder (PM2000). The influence of the ODS steel fabrication technique is studied for two standard additive manufacturing techniques, directed energy deposition (DED) and laser powder bed fusion (LPBF). The compressive strength of the ODS steels at 600 °C is increased by 21% and 29% for the DED and LPBF samples, respectively, compared to the DED and LPBF steels manufactured without Y2O3 nanoparticle addition. The Martens hardness is enhanced by 9% for the LPBF ODS steel while no significant change is observed in the DED ODS steel. The microstructure and nanoparticle composition and distribution are evaluated by electron backscatter diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and atom probe tomography, to compare the microstructural features of DED and LPBF manufactured parts. Smaller grain size and more homogeneous distribution with lower agglomeration of Y-O nanoparticles in the LPBF sample are found to be key factors for enhanced mechanical response at 600 °C. The enhanced mechanical properties of the LPBF-processed sample and the more homogeneous nanoparticle dispersion can be linked to results obtained by finite element methods simulations of the melt pool that show two orders of magnitude faster cooling rates for LPBF than for DED. Therefore, this work presents and validates a complete laser-based methodology for the preparation and processing of an ODS steel, proving the modification of the microstructure and enhancement of the high-temperature strength of the as-built parts. © 2020
    view abstractdoi: 10.1016/j.actamat.2020.116566
  • 2021 • 459 Processing of a newly developed nitrogen-alloyed ferritic-austenitic stainless steel by laser powder bed fusion – Microstructure and properties
    Becker, L. and Röttger, A. and Boes, J. and Weber, S. and Theisen, W.
    Additive Manufacturing 46 (2021)
    In this work, a novel alloy design of a stainless steel with a ferritic-austenitic microstructure is derived for PBF-LB/M (powder bed fusion-laser beam/metal). The alloy was developed based on X2CrNiMo17-12-2 steel, for which an austenite volume content of approx. 54 vol% in the PBF-LB/M state was achieved using a reduced Ni equivalent. Partial substitution of Ni by Mn increases the N solubility of the alloy. By melting and further gas-atomizing this melt in an N2 atmosphere, an N content of 0.27 mass% was set in the produced steel powder. This leads to both high strength and high corrosion resistance of the PBF-LB/M-processed steel. However, microstructural investigations in the PBF-LB/M state confirm a microstructure consisting of ferrite, austenite, and Mo- and Cr-rich nitrides of M2N type. The nitrides were not completely eliminated by a subsequent heat treatment of the PBF-LB/M samples. As a result of the solution annealing, the microstructure approaches the thermodynamic equilibrium so that the austenite volume content increases from 54.2 vol% to 92.7 vol%. The higher Cr and N contents result in a higher corrosion resistance of the investigated steel compared to PBF-LB/M-processed X2CrNiMo17-12-2, regarded as the reference material. In addition, the measured strengths are significantly higher due to the larger amounts of austenite/ferrite interfaces and the N-induced solid-solution strengthening effect compared to X2CrNiMo17-12-2. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2021.102185
  • 2021 • 458 Progress on effects of alloying elements on bainite formation and strength and toughness of high strength steel weld metal
    Zhang, T. and Yu, H. and Li, Z. and Kou, S. and Kim, H.J. and Tillmann, W.
    Materials Research Express 8 (2021)
    High-strength steel has excellent mechanical properties and develops rapidly, but the toughness of weld metal cannot be well solved, which hinders the large-scale application of high-strength steel to a certain extent. Thus it is urgent to improve the strengthening and toughening mechanism of high-strength steel weld metal and develop the corresponding welding consumables. This paper summarizes current main research methods of the influence of alloying elements on the microstructure transformation and mechanical properties, and the effects of alloying elements on the strength and toughness of high-strength steel weld metals. It briefly analyzes the influence mechanism of alloying elements on microstructure transformation and the relationship between alloy composition, microstructure transformation and mechanical properties. It is found that the multiple phase microstructure composed of bainite and acicular ferrite can make the weld metal obtain good toughness. In addition, the paper also discusses future development trend of high-strength steel welding, providing the guidance on the research and application of high-strength steel welding consumables. © 2021 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/2053-1591/abea58
  • 2021 • 457 Teeth of Past and Present Elephants: Microstructure and Composition of Enamel in Fossilized Proboscidean Molars and Implications for Diagenesis
    Białas, N. and Prymak, O. and Singh, N.P. and Paul, D. and Patnaik, R. and Epple, M.
    Geochemistry, Geophysics, Geosystems 22 (2021)
    Enamel as hardest biological tissue remains unaltered for millions of years and is therefore an excellent archive for studies on paleodiet, paleoecology, paleoclimate, paleoenvironment, biomechanical, and evolutionary studies. However, diagenetic alterations can influence such interpretations and therefore we analyzed the microstructure and composition (elemental and stable isotopic) of fossil and extant proboscidean teeth to study the extent of diagenesis in them. We report for the first time on the enamel microstructure data of the Indian elephantiformes Anancus, Stegodon, Elephas, and Palaeoloxodon besides analyzing Gomphotherium and Deinotherium from new formations. Furthermore, we compare their microstructure with those of the primitive African taxa of Moeritherium and Palaeomastodon. Our results from depth-related elemental composition and oxygen isotope ratios of enamel phosphate and carbonate indicate no or only negligible modification. There is also a lack of age-dependency of these minor alterations within the fossils collected from Siwaliks of the Himalayan Foreland Basin. Overall, our study indicates that diagenesis has not played any significant role on the samples studied here and are therefore well suited for chemical and paleontological studies and proxy for paleoclimate and paleoenvironment reconstruction. © 2021. The Authors.
    view abstractdoi: 10.1029/2020GC009557
  • 2021 • 456 Twin-boundary assisted crack tip plasticity and toughening in lamellar γ-TiAl
    Neogi, A. and Janisch, R.
    Acta Materialia 213 (2021)
    The internal twin-boundaries in lamellar γ-TiAl alloys, namely true-twin (TT), rotational boundary (RB), and pseudo-twin (PT), are known to be effective in strengthening the TiAl microstructures. Nevertheless, for designing microstructures with optimised mechanical properties, a better understanding of the role of these boundaries on fracture behavior is still required. To this end, we study how and to what degree crack advancement is affected by the local lattice orientation and atomic structure at the various twin boundaries. Molecular statics simulations were performed in conjunction with a linear elastic fracture mechanics based analysis, to understand the inter-lamellar and as well as trans-lamellar crack advancement at a TT, RB, and PT interface. The fracture toughness as well as the crack advancement mechanisms of the inter-lamellar cracks depend critically on the propagation direction. For instance, cracks along 〈112¯] in the TT, RB, and PT plane always emit dislocations at the crack tip, while the cracks along the opposite direction are brittle in nature. When it comes to trans-lamellar crack advancement, the crack tip shows significant plastic deformation and toughening for all interfaces. However, at a TT, a brittle crack is able to penetrate through the interface at a higher applied load, and propagates in the adjacent γ′ phase, while in the case of RB and PT, the crack tip is blunted and arrested at or near the boundary, resulting in dislocation emission and crack tip toughening. This suggests that a variation of the sequence of the different rotational boundaries could be a possibility to tune the crack tip plasticity and toughening in lamellar TiAl. © 2021 The Author(s)
    view abstractdoi: 10.1016/j.actamat.2021.116924
  • 2020 • 455 A Computational Approach to the Microstructural Design of High-Speed Steels
    Egels, G. and Wulbieter, N. and Weber, S. and Theisen, W.
    Steel Research International 91 (2020)
    Increasing requirements concerning the operational conditions and durability of tools create a demand for the optimization of tool steels. High-speed steels (HSSs), for example, contain high amounts of carbides embedded in a secondary hardenable martensitic matrix. The wear behavior and the mechanical properties of HSS can be optimized for a certain application by adjusting the type and amount of carbides, as well as their compositions and the composition of the matrix. Computational thermodynamics based on the calculation of phase diagrams method allow the estimation of arising phases as well as phase compositions during the solidification or the heat treatment of a steel. However, in complex alloy systems, for example, HSS, the relationships between the content of alloying elements and the stability and the composition of phases can be complicated and nonlinear. Therefore, it can be difficult to find alloy compositions that are suitable to achieve a desired microstructure with iterative calculations. To handle this difficulty, a computational tool is developed, which determines compositions to obtain predefined HSS microstructures. The computational tool is based on a neural network that was previously trained with a thermodynamically calculated database. The efficiency of this approach is experimentally verified by producing and investigating laboratory melts of different HSS. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/srin.201900455
  • 2020 • 454 A model for grain boundary thermodynamics
    Darvishi Kamachali, R.
    RSC Advances 10 26728-26741 (2020)
    Systematic microstructure design requires reliable thermodynamic descriptions of each and all microstructure elements. While such descriptions are well established for most bulk phases, thermodynamic assessment of microstructure defects is challenging because of their individualistic nature. In this paper, a model is devised for assessing grain boundary thermodynamics based on available bulk thermodynamic data. We propose a continuous relative atomic density field and its spatial gradients to describe the grain boundary region with reference to the homogeneous bulk and derive the grain boundary Gibbs free energy functional. The grain boundary segregation isotherm and phase diagram are computed for a regular binary solid solution, and qualitatively benchmarked for the Pt-Au system. The relationships between the grain boundary's atomic density, excess free volume, and misorientation angle are discussed. Combining the current density-based model with available bulk thermodynamic databases enables constructing databases, phase diagrams, and segregation isotherms for grain boundaries, opening possibilities for studying and designing heterogeneous microstructures. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ra04682e
  • 2020 • 453 Adaptive Concurrent Topology Optimization of Coated Structures with Nonperiodic Infill for Additive Manufacturing
    Hoang, V.-N. and Tran, P. and Nguyen, N.-L. and Hackl, K. and Nguyen-Xuan, H.
    CAD Computer Aided Design 129 (2020)
    The present research develops a direct multiscale topology optimization method for additive manufacturing (AM) of coated structures with nonperiodic infill by employing an adaptive mapping technique of adaptive geometric components (AGCs). The AGCs consist of a framework of macro-sandwich bars that represent the macrostructure with the solid coating and a network of micro-solid bars that represent the nonperiodic infill at the microstructural scale. The macrostructure including the coating skin and the internal architecture of the microstructures of cellular structures is simultaneously optimized by straightforwardly searching optimal geometries of the AGCs. Compared with most existing methods, the proposed method does not require material homogenization technique at the microscale; the continuity of microstructures and structural porosities are ensured without additional constraints; Finite element analysis (FEA) and geometric parameter updates are required only once for each optimization iteration. AGCs allow us to model coated structures with porosity infill on a coarse finite element mesh. The adaptive mapping technique may reduce mapping time by up to 50%. Besides, it is easy to control the length scales of the coating and infill as desired to make it possible with AM. This investigation also explores the ability to realize concurrent designs of coated structures with nonperiodic infill patterns using 3D printing techniques. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.cad.2020.102918
  • 2020 • 452 An Integrated HIP Heat-Treatment of a Single Crystal Ni-Base Superalloy
    Ruttert, B. and Lopez-Galilea, I. and Theisen, W.
    Minerals, Metals and Materials Series 391-399 (2020)
    The heat-treatment of a second generation single crystal Ni-base superalloy was implemented in a hot isostatic press providing fast quenching rates. Thus, it is possible to homogenize chemical heterogeneities, close porosity, and to set a fine and uniform γ/γ′-microstructure via fast quenching and subsequent aging in one processing step. The microstructural evolution in dependence of parameters such as temperature, pressure, and quenching is investigated on different length scales using diverse characterization methods. A virtually defect-free microstructure is the outcome of this unique integrated supersolvus HIP heat-treatment. © 2020, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/978-3-030-51834-9_38
  • 2020 • 451 Analysis of the functional properties in the bore sub-surface zone during BTA deep-hole drilling
    Schmidt, R. and Strodick, S. and Walther, F. and Biermann, D. and Zabel, A.
    Procedia CIRP 88 318-323 (2020)
    The BTA deep hole drilling process is analysed to determine the relationships between process-structure and residual stress as a function of surface conditioning during the machining process. The residual stresses, the hardness and structural changes in the microstructure are used as indicators for a following process control in order to generate an improved component service life or reliability. In the first tests, the deep-drilled samples are examined with regard to residual stresses, bore tolerances, microstructure and hardness in the sub-surface zone. Furthermore, the specimens will be subjected to a heat treatment to establish a stress-free microstructure as a reference state for the magnetic residual stress measurement. © 2020 The Authors.
    view abstractdoi: 10.1016/j.procir.2020.05.055
  • 2020 • 450 Analysis of the nanoparticle dispersion and its effect on the crystalline microstructure in carbon-additivated PA12 feedstock material for laser powder bed fusion
    Hupfeld, T. and Sommereyns, A. and Riahi, F. and Doñate-Buendía, C. and Gann, S. and Schmidt, M. and Gökce, B. and Barcikowski, S.
    Materials 13 (2020)
    Driven by the rapid development of additive manufacturing technologies and the trend towards mass customization, the development of new feedstock materials has become a key aspect. Additivation of the feedstock with nanoparticles is a possible route for tailoring the feedstock material to the printing process and to modify the properties of the printed parts. This study demonstrates the colloidal additivation of PA12 powder with laser-synthesized carbon nanoparticles at >95% yield, focusing on the dispersion of the nanoparticles on the polymer microparticle surface at nanoparticle loadings below 0.05 vol%. In addition to the descriptors "wt%" and "vol%", the descriptor "surf%" is discussed for characterizing the quantity and quality of nanoparticle loading based on scanning electron microscopy. The functionalized powders are further characterized by confocal dark field scattering, differential scanning calorimetry, powder rheology measurements (avalanche angle and Hausner ratio), and regarding their processability in laser powder bed fusion (PBF-LB). We find that heterogeneous nucleation is induced even at a nanoparticle loading of just 0.005 vol%. Finally, analysis of the effect of low nanoparticle loadings on the final parts' microstructure by polarization microscopy shows a nanoparticle loading-dependent change of the dimensions of the lamellar microstructures within the printed part. © 2020 by the authors.
    view abstractdoi: 10.3390/ma13153312
  • 2020 • 449 Application of the eutectic high entropy alloy Nb0.73CoCrFeNi2.1 for high temperature joints
    Tillmann, W. and Wojarski, L. and Stangier, D. and Manka, M. and Timmer, C.
    Welding in the World 64 1597-1604 (2020)
    The eutectic high entropy alloy Nb0.73CoCrFeNi2.1 was manufactured by means of arc smelting and the obtained ingots were cut into 300-μm-thick foils, which were used as filler alloys to braze Crofer 22 APU to Hf-metallized yttria-stabilized zirconia (3YSZ). The brazing process was conducted in a vacuum furnace at 1200 °C for 5 min at a vacuum of 4.3·10–4 mbar. In order to minimize the intense diffusion and erosion of the steel substrate, a heating and cooling rate of 50 K/min was applied. Sound joints without any pores or flaws were obtained. The microstructure of the joints consisted of an HfO2 reaction layer at the ceramic interface and the same eutectic structure consisting of a Laves phase and a solid solution that was already detected in the smelted foil. The average hardness of the microstructure in the joint seam amounted to 352 ± 17 HV0.01 and the joints reached strength values up to 97 ± 7 MPa while the fracture area was always located at the ceramic interface in the HfO2 layer. Comparable joints, with AgCuTi3 as filler metal, brazed at 920 °C, only reached a shear strength of ~ 52 ± 2 MPa. © 2020, The Author(s).
    view abstractdoi: 10.1007/s40194-020-00944-w
  • 2020 • 448 Application Progress of Annealing Treatment Process in the Study of Nano-multilayer Films [退火处理工艺在纳米多层膜材料研究中的应用进展]
    Li, H. and Xing, Z. and Hodúlová, E. and Hu, A. and Tillmann, W.
    Cailiao Daobao/Materials Reports 34 03099-03105 (2020)
    Compared with traditional bulk materials, nano-multilayer films exhibit unique optical, magnetic, electrical, mechanical and thermal properties due to their small-size effects, surface effects, quantum size effects, and quantum tunneling effects. Therefore, nano-multilayer films have been widely used in the areas of optical devices, semiconductors, electromagnetic protection, processing and manufacturing, surface protection and electronic packaging as optical absorbing materials, electromagnetic absorbing materials, magnetic recording materials, photovoltaic materials and low-temperature joining materials. There exists intrinsic size dependence in the physical and mechanical properties with the microstructure of nano-multilayer films. Due to the limitation of the preparation process, defects such as vacancies and dislocations can cause difficulty in fully meeting the requirements of heat resistance, wear resistance and corrosion resistance in the complex service environment, which limits the further development of nano-multilayer films. In the field of concentrating circuits and chip fabrication, nano-multilayer films devices are often working in a severe environment deviating from the normal temperature. However, metastable nano-multilayer films with high surface free energy tend to reach a state of low-energy and form a stable structure by interdiffusion of immiscible dual phases, interlayer detachment and interface evolution under heat. It might result in the extinction of melting point depression property, superhardness property and so on due to the destructions of the nano-multilayer structure. Therefore, studying on the microstructure evolution, thermal stability and failure mechanism of nano-multilayer films is particularly important for increasing the service life and reliability of nano-multilayer systems. As a common heat treatment method, the annealing process is widely used to eliminate defects in metals, so as to achieve to modify the properties. For nano-multilayer films operating at high temperatures, the annealing process is also an effective means of extending its service life. At present, the main directions of annealing process in nano-multilayer films research and application are: (i) improving nano-multilayer film performance by adopting different annealing temperature, holding time and cooling rate; (ii) investigating the effect of annealing temperature on the thermal stability of nano-multilayer films by increasing the annealing upper limit temperature and obtain a critical temperature that maintains stability of the interface of nano-multilayer. It is found that the appropriate annealing process can refine the nano-multilayer films grain structure, increase the density, decrease the defect density, induce the formation of special structures, reinforcing the interaction of atoms and dislocations. Therefore, the light transmittance of the film is increased with improvement of optical properties, as well as the magnetic, electrical and mechanical properties are significantly improved; (iii) in addition, the nano-multilayer film is annealed in a certain temperature range to observe the bilayer interface evolution, atomic diffusion and new phase formation using TEM, XRD and other means. Thus the structural stability, chemical stability and mechanical stability of nano-multilayer film can be studied. In this paper, the current progress and challenges of annealing process in nano-multilayer films modification and thermal stability research are reviewed. The influence of annealing parameters on the enhancement of nano-multilayer properties including optical properties, magnetic properties, electrical properties, mechanical properties is elaborated. Furthermore, it mainly focuses on the influencing mechanism of elevated temperature annealing on the thermal stability and microstructure evolution of immiscible nano-multilayer system. At last, the further development of annealing process for designing and preparing of high-strength and thermally stable nano-multilayer films are prospected, which has important theoretical significance and application value in materials welding/joining, integrated circuits, cutting tools, absorbing coatings, etc. © 2020, Materials Review Magazine. All right reserved.
    view abstractdoi: 10.11896/cldb.19010159
  • 2020 • 447 Biomimetic scaffold fabricated with a mammalian trabecular bone template
    Bulygina, I. and Senatov, F. and Choudhary, R. and Kolesnikov, E. and Kaloshkin, S. and Scholz, R. and Knyazeva, M. and Walther, F. and Anisimova, N. and Kiselevskiy, M.
    Polymer Degradation and Stability 172 (2020)
    This study proposes the method of ultra-high molecular weight polyethylene (UHMWPE) biomimetic scaffold fabrication. Anisotropy is considered to be a distinctive feature of native bone but basically only a 3D-fabricated scaffold structure may be anisotropic, while 3D-printing is not applicable to UHMWPE. We proposed a novel method that suggested a template of native mammalian bone to be used as a negative for UHMWPE scaffold fabrication. This method allows direct replication of the bone's structural features on the micro- and macro-scale. Bone scaffolds obtained using the specified method showed anisotropic structure; the pores' average proportions for scaffold and bone were 770 and 470, and 700 and 500 μm, respectively. According to SEM and CT investigations, the scaffolds' macro- and microstructure mimicked the native bone architecture; this feature distinguishes the proposed method from the other UHMWPE scaffold fabrication techniques. The combination of the hydrophilic surface and the nanorelief affected the adhesion and proliferation of cells: the adhesion of multipotent mesenchymal stromal cells (MMSC) amounted to 40% after 4 h; the proliferation of MMSC was 75% after 48 h. The proposed novel method of fabricating biomimetic scaffolds can be used to obtain bone implants of the complex microstructure and anisotropy from high-melt viscosity polymers which cannot be 3D-printed to be further applied in bone reconstruction. The FT-IR analysis confirmed the occurrence of carboxyl oxidation when the surface of UHMWPE sample was treated with chromic acid. The oxidation index (OI) of the samples was found in the order of etching in chromic acid > sterilization > hot moulding respectively. It can be suggested that the oxidative degradation of UHMWPE can be reduced by optimizing manufacturing conditions and further selection of an appropriate processing method. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.polymdegradstab.2020.109076
  • 2020 • 446 Biomimetic UHMWPE/HA scaffolds with rhBMP-2 and erythropoietin for reconstructive surgery
    Senatov, F. and Amanbek, G. and Orlova, P. and Bartov, M. and Grunina, T. and Kolesnikov, E. and Maksimkin, A. and Kaloshkin, S. and Poponova, M. and Nikitin, K. and Krivozubov, M. and Strukova, N. and Manskikh, V. and Anisimova, ...
    Materials Science and Engineering C 111 (2020)
    A promising direction for the replacement of expanded bone defects is the development of bioimplants based on synthetic biocompatible materials impregnated with growth factors that stimulate bone remodeling. Novel biomimetic highly porous ultra-high molecular weight polyethylene (UHMWPE)/40% hydroxyapatite (HA) scaffold for reconstructive surgery with the porosity of 85 ± 1% vol. and a diameter of pores in the range of 50–800 μm was developed. The manufacturing process allowed the formation of trabecular-like architecture without additional solvents and thermo-oxidative degradation. Biomimetic UHMWPE/HA scaffold was biocompatible and provided effective tissue ingrowth on a model of critical-sized cranial defects in mice. The combined use of UHMWPE/HA with Bone Morphogenetic Protein-2 (BMP-2) demonstrated intensive mineralized bone formation as early as 3 weeks after surgery. The addition of erythropoietin (EPO) significantly enhanced angiogenesis in newly formed tissues. The effect of EPO of bacterial origin on bone tissue defect healing was demonstrated for the first time. The developed biomimetic highly porous UHMWPE/HA scaffold can be used separately or in combination with rhBMP-2 and EPO for reconstructive surgery to solve the problems associated with difference between implant architecture and trabecular bone, low osteointegration and bioinertness. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2020.110750
  • 2020 • 445 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 • 444 Chemical complexity, microstructure and martensitic transformation in high entropy shape memory alloys
    Piorunek, D. and Frenzel, J. and Jöns, N. and Somsen, C. and Eggeler, G.
    Intermetallics 122 (2020)
    High entropy shape memory alloys (HESMAs) represent a relatively young class of functional materials. They show a reversible martensitic phase transformation which allows to exploit shape memory effects at relatively high temperatures. HESMAs represent ordered complex solid-solutions. Their high temperature phase is of B2 type, and various elements (e.g. Ni, Cu, Ti, Zr, Hf) occupy sites in specific sub-lattices. In the present work, we study the processing and the functional properties of HESMAs. We study effects of chemical complexity on solidification microstructures and martensitic transformations. Binary, ternary, quaternary, quinary and senary model alloys were investigated using advanced microstructural and thermal characterization methods. The results show that element partitioning during solidification results in a redistribution of individual alloy elements in dendritic/interdendritic regions. Surprisingly, the atomic ratios of the two groups of elements which occupy the Ni- (first group: Ni, Cu and Pd) and Ti-sub-lattice (second group: Ti, Zr, Hf) are maintained. This allows the material to form martensite throughout its heterogeneous microstructure. The effect of chemical complexity/composition on martensite start temperatures, MS, is discussed on the basis of valence electron concentrations, cV. Some of the alloys fall into MS(cV)-regimes which are uncommon for classical Ni-Ti-based shape memory alloys. In the present work, a new HESMA of type NiCuPdTiZrHf was identified which has the potential to provide maximum shape memory strains close to 15%. © 2020
    view abstractdoi: 10.1016/j.intermet.2020.106792
  • 2020 • 443 Chip formation and phase transformation in orthogonal machining of NiTi shape memory alloy: microstructure-based modelling and experimental validation
    Kaynak, Y. and Manchiraju, S. and Jawahir, I.S. and Biermann, D.
    CIRP Annals 69 85-88 (2020)
    Phase transformation and shape memory response of NiTi alloys are sensitive to the variation of temperature and stress. Thus, the phase transformation of NiTi alloys becomes more complex during machining process. This study presents findings from a major study involving modelling of machining-induced phase transformation of NiTi alloys performed by modifying Helmholtz free energy-based microstructure model. Orthogonal cutting tests were performed to validate the predicted outputs from the simulation, such as cutting forces, temperatures and chip morphology. This work provides a strong evidence that the developed new model can accurately predict the experimentally recorded outputs in machining of NiTi alloys. © 2020
    view abstractdoi: 10.1016/j.cirp.2020.04.025
  • 2020 • 442 Coatings with Columnar Microstructures for Thermal Barrier Applications
    Mauer, G. and Vaßen, R.
    Advanced Engineering Materials 22 (2020)
    Columnar-structured thermal barrier coatings (TBCs) manufactured by electron beam-physical vapor deposition (EB-PVD) are well known to exhibit high strain tolerance. However, as EB-PVD is a high-vacuum process, it is expensive. Suspension plasma spraying (SPS) and plasma spray-physical vapor deposition (PS-PVD) are alternatives for the manufacture of similar microstructures. Herein, the state of the art of manufacturing columnar-structured TBCs by SPS and PS-PVD is outlined. Both processes have been investigated and further developed at Forschungszentrum Jülich for many years. The mechanisms leading to the formation of columnar-structured coatings are described and differentiated from EB-PVD. Examples are given for SPS and PS-PVD columnar microstructures and their life performance. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201900988
  • 2020 • 441 Combinatorial Synthesis and High-Throughput Characterization of Microstructure and Phase Transformation in Ni–Ti–Cu–V Quaternary Thin-Film Library
    Al Hasan, N.M. and Hou, H. and Sarkar, S. and Thienhaus, S. and Mehta, A. and Ludwig, Al. and Takeuchi, I.
    Engineering 6 637-643 (2020)
    Ni–Ti–based shape memory alloys (SMAs) have found widespread use in the last 70 years, but improving their functional stability remains a key quest for more robust and advanced applications. Named for their ability to retain their processed shape as a result of a reversible martensitic transformation, SMAs are highly sensitive to compositional variations. Alloying with ternary and quaternary elements to fine-tune the lattice parameters and the thermal hysteresis of an SMA, therefore, becomes a challenge in materials exploration. Combinatorial materials science allows streamlining of the synthesis process and data management from multiple characterization techniques. In this study, a composition spread of Ni–Ti–Cu–V thin-film library was synthesized by magnetron co-sputtering on a thermally oxidized Si wafer. Composition-dependent phase transformation temperature and microstructure were investigated and determined using high-throughput wavelength dispersive spectroscopy, synchrotron X-ray diffraction, and temperature-dependent resistance measurements. Of the 177 compositions in the materials library, 32 were observed to have shape memory effect, of which five had zero or near-zero thermal hysteresis. These compositions provide flexibility in the operating temperature regimes that they can be used in. A phase map for the quaternary system and correlations of functional properties are discussed with respect to the local microstructure and composition of the thin-film library. © 2020 THE AUTHORS
    view abstractdoi: 10.1016/j.eng.2020.05.003
  • 2020 • 440 Comparison of cryogenic deformation of the concentrated solid solutions CoCrFeMnNi, CoCrNi and CoNi
    Tirunilai, A.S. and Hanemann, T. and Reinhart, C. and Tschan, V. and Weiss, K.-P. and Laplanche, G. and Freudenberger, J. and Heilmaier, M. and Kauffmann, A.
    Materials Science and Engineering A 783 (2020)
    The current work compares the deformation behavior of CoCrFeMnNi and CoCrNi in the temperature interval between 295 K and 8 K through a series of quasi-static tensile tests. Temperature-dependent yield stress variation was found to be similarly high in these two alloys. Previous investigations only extended down to 77 K and showed that a small amount of ε-martensite was formed in CoCrNi while this phase was not observed in CoCrFeMnNi. The present study extends these investigations down to 8 K where similar low levels of ε-martensite were presently detected. Based on this result, a rough assessment has been made estimating the importance of deformation twinning to the strength. The relative work hardening rates of CoCrFeMnNi and CoCrNi were comparable in value despite the differences in ε-martensite formation during deformation. CoCrFeMnNi deforms by dislocation slip and deformation twinning while deformation in CoCrNi is also accommodated by the formation of ε-martensite at cryogenic temperatures. Additionally, CoNi, a solid solution from the Co–Cr–Fe–Mn–Ni system with low strength, was used for comparison, showing contrasting deformation behavior at cryogenic temperatures. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2020.139290
  • 2020 • 439 Composition and structure of magnetic high-temperature-phase, stable Fe-Au core-shell nanoparticles with zero-valent bcc Fe core
    Kamp, M. and Tymoczko, A. and Popescu, R. and Schürmann, U. and Nadarajah, R. and Gökce, B. and Rehbock, C. and Gerthsen, D. and Barcikowski, S. and Kienle, L.
    Nanoscale Advances 2 3912-3920 (2020)
    Advanced quantitative TEM/EDXS methods were used to characterize different ultrastructures of magnetic Fe-Au core-shell nanoparticles formed by laser ablation in liquids. The findings demonstrate the presence of Au-rich alloy shells with varying composition in all structures and elemental bcc Fe cores. The identified structures are metastable phases interpreted by analogy to the bulk phase diagram. Based on this, we propose a formation mechanism of these complex ultrastructures. To show the magnetic response of these magnetic core nanoparticles protected by a noble metal shell, we demonstrate the formation of nanostrands in the presence of an external magnetic field. We find that it is possible to control the lengths of these strands by the iron content within the alloy nanoparticles. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0na00514b
  • 2020 • 438 Comprehensive investigation of the microstructure-property relationship of differently manufactured Co–Cr–C alloys at room and elevated temperature
    Krell, J. and Röttger, A. and Theisen, W.
    Wear 444-445 (2020)
    The purpose of this study was to investigate the influence of the microstructure on sliding wear and hardness of four different Co–Cr–C alloys at room and elevated temperature. Different microstructures were produced by applying three different processes. The hardness, hot hardness and wear loss at room temperature of these alloys correlate strongly with the carbide volume content. In sliding wear tests against an Al2O3 ball, abrasive wear occurs at room temperature. The size or geometric arrangement of the carbides or metal matrix plays a minor role at room temperature. At 600 °C the wear behaviour changes due to the softening matrix. In alloys with small free matrix path lengths, the highest wear rates occur due to micro-fatigue and micro-cracking. In hypoeutectic alloys with a high free matrix path length, the carbides lose their effectiveness due to the lack of support by the matrix. In these alloys, wear is dominated by the properties of the matrix. A hypereutectic casting alloy with large primary carbides shows the best wear results, as the carbides support themselves due to their size and retain their wear-reducing effect. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2019.203138
  • 2020 • 437 Current Challenges and Opportunities in Microstructure-Related Properties of Advanced High-Strength Steels
    Raabe, D. and Sun, B. and Kwiatkowski Da Silva, A. and Gault, B. and Yen, H.-W. and Sedighiani, K. and Thoudden Sukumar, P. and Souza Filho, I.R. and Katnagallu, S. and Jägle, E. and Kürnsteiner, P. and Kusampudi, N. and Stephen...
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 51 5517-5586 (2020)
    This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels, etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation, and twinning effects. Recent developments on microalloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modeling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation, and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation. © 2020, The Author(s).
    view abstractdoi: 10.1007/s11661-020-05947-2
  • 2020 • 436 Deactivating deformation twinning in medium-entropy CrCoNi with small additions of aluminum and titanium
    Slone, C.E. and LaRosa, C.R. and Zenk, C.H. and George, E.P. and Ghazisaeidi, M. and Mills, M.J.
    Scripta Materialia 178 295-300 (2020)
    High strain-hardening rates in equiatomic CrCoNi and other multi-principal element alloys have been attributed to deformation twinning. This work shows that small additions of Al and Ti to a CrCoNi alloy deactivate deformation twinning with only minor changes to uniform elongation and ultimate tensile strength. The initial microstructure is free of chemically ordered (Al,Ti)-rich precipitates after solutionizing and quenching. Tensile properties for the alloy are reported and compared to equiatomic CrCoNi, and the post-deformation microstructure is assessed. Density functional theory calculations indicate that energetically unfavorable Al-Al bonds may discourage shearing via partial dislocations, which are necessary for twinning to occur. © 2019
    view abstractdoi: 10.1016/j.scriptamat.2019.11.053
  • 2020 • 435 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 • 434 Development of high entropy alloys for brazing applications
    Tillmann, W. and Ulitzka, T. and Wojarski, L. and Manka, M. and Ulitzka, H. and Wagstyl, D.
    Welding in the World 64 201-208 (2020)
    High entropy alloys are novel and innovative metallic materials, which have intensively moved into the focus of research over the last decade. The high entropy effect in those multi-component alloys promotes the formation of a characteristic crystal structure, the random solid solution, which features unique material properties, and reduces the number of possible brittle phases. In this publication, the influence of gallium as a melting point depressant on the melting range and the microstructure of the two-phased equimolar CoCrCoFeNi were determined. In order to integrate the vacuum brazing process into the solutioning heat treatment of the nickel-based super alloy Mar-M 247 between 1180 and 1270 °C, the liquidus temperature of CoCrCoFeNi was aimed to be below 1270 °C. The changes in the melting ranges due to the modified compositions were predicted by CALPHAD simulations and verified by differential thermal analysis measurements. The promising multi-component filler alloy CoCrCuFeNiGa was determined for further investigations. For this purpose, the microstructures of the filler metal itself and the brazement were conducted. A shear strength of 388 ± 73 MPa was achieved for a brazing gap of 200 μm. The crack, which led to joint failure propagated through high-entropic, fcc-structured phases in the brazing seam. © 2019, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-019-00824-y
  • 2020 • 433 Early stage phase separation of AlCoCr0.75Cu0.5FeNi high-entropy powder at the nanoscale
    Peter, N.J. and Duarte, M.J. and Liebscher, C.H. and Srivastava, V.C. and Uhlenwinkel, V. and Jägle, E.A. and Dehm, G.
    Journal of Alloys and Compounds 820 (2020)
    High entropy alloys are generally considered to be single phase material. This state is, however, typically a non-equilibrium state after fabrication at high cooling rates. Phase constitution after fabrication or heat treatment is mostly known for isothermal annealing only and for casts as well as rapidly quenched alloys. Knowledge on early phase separation stages of high entropy alloys and their mechanisms are missing so far. Here, we present results on phase separation at intermediate cooling rates, by characterization of gas atomized powder of the AlCoCr0.75Cu0.5FeNi alloy. Although investigation by X-ray diffraction and Electron Backscatter Diffraction indicates a single-phase nature of the powder particles, aberration-corrected scanning transmission electron microscopy and atom probe tomography reveal a nanoscale phase separation into Ni–Al-rich B2 and Fe–Cr-rich A2 regions as well as a high number density of 3.1 × 1024 Cu-rich clusters per m3 in the B2 matrix. The observed phase separation and cluster formation are linked to spinodal decomposition and nucleation processes, respectively. The study highlights that adequate characterization techniques need to be chosen when making statements about phase stability and structural evolution in compositionally complex alloys. © 2019 The Authors
    view abstractdoi: 10.1016/j.jallcom.2019.153149
  • 2020 • 432 Effect of Grain Statistics on Micromechanical Modeling: The Example of Additively Manufactured Materials Examined by Electron Backscatter Diffraction
    Biswas, A. and Prasad, M.R.G. and Vajragupta, N. and Kostka, A. and Niendorf, T. and Hartmaier, A.
    Advanced Engineering Materials 22 (2020)
    Micromechanical modeling is one of the prominent numerical tools for the prediction of mechanical properties and the understanding of deformation mechanisms of metals. As input parameters, it uses data obtained from microstructure characterization techniques, among which the electron backscatter diffraction (EBSD) technique allows us to understand the nature of microstructural features, that are usually described by statistics. Because of these advantages, the EBSD dataset is widely used for synthetic microstructure generation. However, for the statistical description of microstructural features, the population of input data must be considered. Preferably, the EBSD measurement area must be sufficiently large to cover an adequate number of grains. However, a comprehensive study of this measurement area with a crystal plasticity finite element method (CPFEM) framework is still missing although it would considerably facilitate information exchange between experimentalists and simulation experts. Herein, the influence of the EBSD measurement area and the number of grains on the statistical description of the microstructural features and studying the corresponding micromechanical simulation results for 316L stainless steel samples produced by selective laser melting is investigated. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201901416
  • 2020 • 431 Effect of nanoparticle additivation on the microstructure and microhardness of oxide dispersion strengthened steels produced by laser powder bed fusion and directed energy deposition
    Doñate-Buendia, C. and Streubel, R. and Kürnsteiner, P. and Wilms, M.B. and Stern, F. and Tenkamp, J. and Bruder, E. and Barcikowski, S. and Gault, B. and Durst, K. and Schleifenbaum, J.H. and Walther, F. and Gökce, B.
    Procedia CIRP 94 41-45 (2020)
    In this contribution, the effect of nanoparticle additivation on the microstructure and microhardness of oxide dispersion strengthened steels (ODS) manufactured by laser powder bed fusion (L-PBF) and directed energy deposition (DED) additive manufacturing (AM) is studied. The powder composites are made of micrometer-sized iron-chromium-alloy based powder which are homogenously decorated with Y2O3 nanoparticles synthesized by pulsed laser fragmentation in water. Consolidated by L-PBF and DED, an enhanced microhardness of the AM-built ODS sample is found. This increase is related to the significant microstructural differences found between the differently processed samples. © 2020 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2020.09.009
  • 2020 • 430 Effect of phase formation due to holding time of vacuum brazed AISI 304L/NiCrSiB joints on corrosion fatigue properties
    Otto, J.L. and Penyaz, M. and Schmiedt-Kalenborn, A. and Knyazeva, M. and Ivannikov, A. and Kalin, B. and Walther, F.
    Journal of Materials Research and Technology 9 10550-10558 (2020)
    Whether in turbine components or exhaust gas heat exchangers, vacuum-brazed nickel-based joints are subjected to varying cyclical loads during their applications, often in corrosive environments. The microstructure of the brazed seam, which is determined by the alloy composition and the brazing process parameters, is essential for the service life. In this experimental study a modified BNi-5a foil was produced and used to braze cylindrical AISI 304L butt joints with two different holding times. Using energy dispersive spectroscopy analyses, a direct correlation of the element distribution at the brazing seam with the holding time was detected as a result of diffusion processes. Individual phases were identified, and it could be shown that the longer holding time led to a reduction of borides and silicides as well as to a more even microhardness curve through the seam. The effect of the microstructure on the corrosion fatigue properties was evaluated using multiple amplitude tests by a stepwise increase of the maximum stress amplitude in synthetic exhaust gas condensate. Thereby, improved corrosion fatigue and cyclic deformation behaviors were achieved for the more homogeneous microstructure. Afterwards, topography analyses of the fracture surfaces enabled an understanding of microstructure-dependent damage mechanisms including fatigue crack initiation and propagation. © 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
    view abstractdoi: 10.1016/j.jmrt.2020.07.047
  • 2020 • 429 Electro-discharge sintering of nanocrystalline NdFeB magnets: process parameters, microstructure, and the resulting magnetic properties
    Leich, L. and Röttger, A. and Kuchenbecker, R. and Theisen, W.
    Journal of Materials Science: Materials in Electronics 31 20431-20443 (2020)
    This study investigates the compaction of nanocrystalline NdFeB magnet powder by electro-discharge sintering (EDS). On this account, process parameters, microstructure, and the associated magnetic properties of the EDS-densified nanocrystalline NdFeB specimens were investigated by varying the discharge energy EEDS and compression load pEDS. Although optimized process parameters could be evaluated, three different microstructures (fully densified zone, insufficiently densified zone, and melted zone) are present in the EDS-compacted specimens. Thereby, volume fractions of these formed three different microstructures determine the resulting mechanical and magnetic properties of the specimens. For all specimens, the intrinsic coercivity Hc,J deteriorates with increasing discharge energy, as the generated Joule heat leads to microstructural changes (grain growth, dissolution of magnetic phases), which reduces the magnetic properties. The compression load has less influence on the coercivity Hc,J, as it only affects the initial resistance of the pre-compacted powder loose. The residual induction Br deteriorates with increasing the discharge energy due to microstructural changes. An increase in the compression load pEDS results in an increase in the specimens’ density and thus promotes the residual induction Br. © 2020, The Author(s).
    view abstractdoi: 10.1007/s10854-020-04562-6
  • 2020 • 428 Elevated temperature microstructure evolution of a medium-entropy CrCoNi superalloy containing Al,Ti
    Slone, C.E. and George, E.P. and Mills, M.J.
    Journal of Alloys and Compounds 817 (2020)
    A new medium-entropy superalloy was produced based on the compositions of equiatomic CrCoNi and Ni-base superalloy Inconel 740H. Initial alloy design was performed using Thermo-Calc. The aging response and microstructural stability were assessed following heat treatment at temperatures between 600 and 900 °C and durations up to 100 h. Aging from a fully recrystallized state resulted in negligible grain growth and produced γ’ and σ phases. The same phases were present after aging from a cold-rolled state, but partially recrystallized microstructures resulted in multi-modal size distributions and heterogeneous spatial arrangements. Room temperature hardness measurements were used to correlate aging conditions with quantitative precipitate measurements and mechanical properties. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2019.152777
  • 2020 • 427 Exploring the fundamentals of Ni-based superalloy single crystal (SX) alloy design: Chemical composition vs. microstructure
    Horst, O.M. and Adler, D. and Git, P. and Wang, H. and Streitberger, J. and Holtkamp, M. and Jöns, N. and Singer, R.F. and Körner, C. and Eggeler, G.
    Materials and Design 195 (2020)
    The present work contributes to a better understanding of the basic assumptions and principles behind the design of Ni-based single crystal superalloys (SXs). For this purpose, we cast and heat-treat four Ni-based single crystal superalloys (SXs) and compare their creep performances: ERBO/1 (with Re) and three ERBO/15 variants (no Re but increased levels of Ti, Mo and W). We show that Re can be replaced by other elements without losing creep strength. To come to this conclusion one has to consider both, alloy composition and microstructure. We analyze the mechanical, microstructural and chemical results (creep rates, γʼ-volume fractions, average γʼ-particle sizes, average γ-channel widths and the chemistry of γ- and γʼ-phases) obtained for ERBO/15 and its two leaner variants (less Mo: ERBO/15-Mo and less W: ERBO/15-W). ERBO/15-Mo and ERBO/15-W show higher creep rates than ERBO/15, because they exhibit lower Mo and W concentrations in the γ-channels. This results in higher diffusion rates, accelerated rafting and faster dislocation climb at γ/γʼ-interfaces. © 2020 The Authors
    view abstractdoi: 10.1016/j.matdes.2020.108976
  • 2020 • 426 High entropy alloys: A focused review of mechanical properties and deformation mechanisms
    George, E.P. and Curtin, W.A. and Tasan, C.C.
    Acta Materialia 188 435-474 (2020)
    The high-entropy alloy (HEA) concept was based on the idea that high mixing entropy can promote formation of stable single-phase microstructures. During the past 15 years, various alloy systems have been explored to identify HEA systems with improved property combinations, leading to an extraordinary growth of this field. In the large pool of alloys with varying characteristics, the first single-phase HEA with good tensile properties, the equiatomic CrMnFeCoNi alloy has become the benchmark material, and it forms the basis of much of our current fundamental understanding of HEA mechanical behavior. As the field is evolving to the more broadly defined complex concentrated alloys (CCAs) and the available data in the literature increase exponentially, a fundamental question remains unchanged: how special are these new materials? In the first part of this review, select mechanical properties of HEAs and CCAs are compared with those of conventional engineering alloys. This task is difficult because of the limited tensile data available for HEAs and CCAs. Additionally, the wider suite of mechanical properties needed to assess structural materials is woefully lacking. Nonetheless, our evaluations have not revealed many HEAs or CCAs with properties far exceeding those of conventional engineering alloys, although specific alloys can show notable enhancements in specific properties. Consequently, it is reasonable to first approach the understanding of HEAs and CCAs through the assessment of how the well-established deformation mechanisms in conventional alloys operate or are modified in the presence of the high local complexity of the HEAs and CCAs. The second part of the paper provides a detailed review of the deformation mechanisms of HEAs with the FCC and BCC structures. For the former, we chose the CrMnFeCoNi (Cantor) alloy because it is the alloy on which the most rigorous and thorough investigations have been performed and, for the latter, we chose the TiZrHfNbTa (Senkov) alloy because this is one of the few refractory HEAs that exhibits any tensile ductility at room temperature. As expected, our review shows that the fundamental deformation mechanisms in these systems, and their dependence on basic physical properties, are broadly similar to those of conventional FCC and BCC metals. The third part of this review examines the theoretical and modeling efforts to date that seek to provide either qualitative or quantitative understanding of the mechanical performance of FCC and BCC HEAs. Since experiments reveal no fundamentally new mechanisms of deformation, this section starts with an overview of modeling perspectives and fundamental considerations. The review then turns to the evolution of modeling and predictions as compared to recent experiments, highlighting both successes and limitations. Finally, in spite of some significant successes, important directions for further theory development are discussed. Overall, while the individual deformation mechanisms or properties of the HEAs and CCAs are not, by and large, “special” relative to conventional alloys, the present HEA rush remains valuable because the compositional freedom that comes from the multi-element space will allow exploration of whether multiple mechanisms can operate sequentially or simultaneously, which may yet lead to the creation of new alloys with a spectrum of mechanical properties that are significantly superior to those of current engineering alloys. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.12.015
  • 2020 • 425 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 • 424 Influence of Hot Hardness and Microstructure of High-Alloyed Powder Metallurgical Tool Steels on Abrasive Wear Behavior at Elevated Temperatures
    Wulbieter, N. and Pöhl, F. and Theisen, W.
    Steel Research International 91 (2020)
    Herein, the abrasive wear behavior of different high-alloyed powder metallurgical (PM) tool steels is investigated at elevated temperatures (400–600 °C) in a dry-pot wear tester containing Al2O3 particles. To identify the influence of the microstructure, PM tool steels with different hot hardnesses, carbide types, and carbide volume contents are selected. Wear tracks are analyzed by scanning electron microscopy (SEM) to clarify wear mechanisms. The results show that there is no direct correlation between wear resistance and only one material property such as hot hardness, carbide content, or carbide type. More important seems to be the best possible compromise between a sufficient hot hardness of the metallic matrix and a high volume content of carbides that are harder than the attacking abrasive particles at the respective temperature. When the test temperatures surpass the tempering temperature of the investigated steels, there is a pronounced change in wear behavior due to the stronger embedding of abrasive particles into the wear surface. It is thus necessary to discuss the microstructural properties as a function of temperature, considering interactions with the abrasive particles. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/srin.201900461
  • 2020 • 423 Influence of Pore Characteristics on Anisotropic Mechanical Behavior of Laser Powder Bed Fusion–Manufactured Metal by Micromechanical Modeling
    R. G. Prasad, M. and Biswas, A. and Geenen, K. and Amin, W. and Gao, S. and Lian, J. and Röttger, A. and Vajragupta, N. and Hartmaier, A.
    Advanced Engineering Materials 22 (2020)
    In recent times, additive manufacturing (AM) has proven to be an indispensable technique for processing complex 3D parts because of the versatility and ease of fabrication it offers. However, the generated microstructures show a high degree of complexity due to the complex solidification process of the melt pool. In this study, micromechanical modeling is applied to gain deeper insight into the influence of defects on plasticity and damage of 316L stainless steel specimens produced by a laser powder bed fusion (L-PBF) process. With the statistical data obtained from microstructure characterization, the complex AM microstructures are modeled by a synthetic microstructure generation tool. A damage model in combination with an element deletion technique is implemented into a nonlocal crystal plasticity model to describe anisotropic mechanical behavior, including damage evolution. The element deletion technique is applied to effectively model the growth and coalescence of microstructural pores as described by a damage parameter. Numerical simulations show that the shape of the pores not only affects the yielding and hardening behavior but also influences the porosity evolution itself. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/adem.202000641
  • 2020 • 422 Influence of Process Parameters on the Aerosol Deposition (AD) of Yttria-Stabilized Zirconia Particles
    Mishra, T.P. and Singh, R. and Mücke, R. and Malzbender, J. and Bram, M. and Guillon, O. and Vaßen, R.
    Journal of Thermal Spray Technology (2020)
    Aerosol deposition (AD) is a novel deposition process for the fabrication of dense and rather thick oxide films at room temperature. The bonding of the deposited ceramic particles is based on a shock-loading consolidation, resulting from the impact of the ceramic particles on the substrate. However, the deposition mechanism is not fully understood. In addition, many technical challenges have been observed for achieving a successful deposition of the oxides with higher efficiency. In this work, the influence of different processing parameters on the properties of the deposited layer is studied. Proof of concept was done using 8 mol.% yttria-stabilized zirconia (8YSZ) powder as starting material. The window of deposition with respect to carrier gas flows for successful deposition was identified. The influence of this carrier gas flow, the substrate materials and the carrier gas species on the coating thickness, interface quality and coating microstructure was systematically investigated. The derived mechanical characteristics revealed an unexpected behavior related to a gradient microstructure. This study supports understanding of the mechanism of room-temperature impact consolidation and its effect on the mechanical properties of the deposited layer. © 2020, ASM International.
    view abstractdoi: 10.1007/s11666-020-01101-x
  • 2020 • 421 Influence of rafted microstructures on creep in Ni-base single crystal superalloys: A 3D discrete dislocation dynamics study
    Gao, S. and Ali, M.A. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 28 (2020)
    Ni-base single-crystal superalloys exhibit a dynamic evolution of their microstructure during operation at elevated temperatures. The rafting of γ′ precipitates changes the mechanical behavior in a way that was understood insufficiently. In this work, we combine a phase-field method with a discrete dislocation dynamics model to clarify the influence of different rafted microstructures with the same initial dislocation density and configuration on creep behavior. The unrafted and rafted microstructures of Ni-base single crystal superalloys are simulated by a phase-field crystal plasticity method. By introducing these microstructures into a 3D discrete dislocation dynamics (DDD) model, the creep behavior under uniaxial loads of 350 and 250 MPa along [100] direction at 950 °C is studied. Due to the negative lattice mismatch of Ni-base superalloys, the N-type rafting with the formation of plate-like γ′ precipitates occurs under uniaxial tensile loads along {100} direction at high temperatures, while the P-type rafting with the formation of rod-like γ′ precipitates occurs under compressive loads. Taking the cuboidal, N-type rafted and P-type rafted microstructures as the initial and fixed microstructures for the same loading conditions, it is found from DDD simulations that the rafted microstructures result in smaller creep deformation than the cuboidal microstructure. The reason for this is that the coalescence of γ′ precipitates during the rafting diminishes the width of some γ channels, so as to increase the local Orowan stresses which retard the dislocation glide. For tensile loads, the N-type rafted microstructure has the best creep resistance. For a low compressive load, the P-type rafting shows a better creep resistance than N-type rafting. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/ab5e40
  • 2020 • 420 Influence of rf plasma jet surface treatment on wetting behavior of yttria stabilized zirconia sps coatings
    Komarov, P. and Jech, D. and Čelko, L. and Pijáková, B. and Zhou, D. and Vaßen, R.
    Defect and Diffusion Forum 405 DDF 423-429 (2020)
    According to the value of water contact angle (WCA), the surfaces can be roughly defined as hydrophilic (with WCA less than 90°) or as hydrophobic (with WCA higher than 90°). Water wetting behavior plays important role and surfaces with special wettability (hydrophobic-superhydrophobic; hydrophilic-superhydrophilic) can be used both in the daily life (solar cells, smartphones, car windows, etc.) and in the industry (corrosion resistance, self-cleaning, anti-icing properties, etc.). Nowadays, the development of hydrophobic surface treatment that may be applied in the industry is very interesting topic. Therefore, it was decided to estimate the influence of radiofrequency (RF) plasma jet in atmosphere on wetting behavior of ceramic plasma sprayed coatings. As the initial material for surface treatment, yttria stabilized zirconia suspension plasma sprayed coatings were used. The influence of RF plasma jet on suspension plasma sprayed coatings was estimated on both hydrophilic and hydrophobic surfaces, and resulted water contact angle and free surface energy of modified samples were measured by sessile droplet method. Microstructure, phase composition and topography investigation were carried out by means of light microscopy, X-ray diffraction techniques and non-contact profilometry. © 2020 Trans Tech Publications Ltd, Switzerland
    view abstractdoi: 10.4028/www.scientific.net/DDF.405.423
  • 2020 • 419 Influence of the cathode microstructure on the stability of inverted planar perovskite solar cells
    Sirotinskaya, S. and Schmechel, R. and Benson, N.
    RSC Advances 10 23653-23661 (2020)
    One of the main challenges for perovskite solar cells (PSC) is their environmental stability, as oxygen and water induced aging may result in mobile decomposition compounds, which can enhance the recombination rate and react with charge carrier extraction layers or the contact metallization. In this contribution the importance of the microstructure of the contact metallization on the environmental cell stability is investigated. For this purpose, the storage stability of inverted planar methylammonium lead iodide (MAPI)-based perovskite solar cells without encapsulation is tested, using the metals aluminum (Al), silver (Ag), gold (Au) and nickel (Ni) as representative cathode materials. For this study, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis of the different electrodes as well as the perovskite is correlated with PSC device current-voltage (J-V) and impedance measurements. Our findings substantiate that the metal microstructure has a significant influence on the PSC aging properties. While a strong perovskite decomposition and iodide diffusion to the contacts were detected for devices using Al, Ag or Au cathodes with a polycrystalline microstructure, these effects were strongly reduced when Ni metallization was employed, where a nanocrystalline microstructure was exhibited under the chosen process conditions. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ra00195c
  • 2020 • 418 Interstitial doping enhances the strength-ductility synergy in a CoCrNi medium entropy alloy
    Moravcik, I. and Hornik, V. and Minárik, P. and Li, L. and Dlouhy, I. and Janovska, M. and Raabe, D. and Li, Z.
    Materials Science and Engineering A 781 (2020)
    An equiatomic CoCrNi medium entropy alloy (MEA) with face-centered cubic (FCC) structure exhibits excellent combination of strength and ductility. Here we employ interstitial doping to enhance its mechanical performance. Interstitial CoCrNi MEAs with two different carbon contents, i.e., 0.5 at. % and 1 at. %, as well as a carbon-free CoCrNi reference MEA have been studied. The results show that up to 1 at. % carbon can be fully dissolved into the homogenized plus water-quenched FCC solid solution structure. Subsequent annealing leads to precipitation of nano-sized M23C6 type carbides which provide dispersion strengthening and enhanced strain hardening. The best combination of ultimate tensile strength of 1180 MPa at an elongation above 60% was obtained in fine grained CoCrNi doped with 0.5 at. % of carbon. Carbon alloying is also shown to significantly increase the lattice friction stress. Dislocation glide and mechanical twinning act as main deformation mechanisms. Thus, the joint contribution of multiple deformation mechanisms in the carbon-doped MEAs leads to significantly enhanced strength-ductility combinations compared to the carbon-free reference alloy, demonstrating that interstitial alloying can enhance the mechanical properties of MEAs. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2020.139242
  • 2020 • 417 Laser additive manufacturing of hot work tool steel by means of a starting powder containing partly spherical pure elements and ferroalloys
    Taruttis, A. and Hardes, C. and Röttger, A. and Uhlenwinkel, V. and Chehreh, A.B. and Theisen, W. and Walther, F. and Zoch, H.W.
    Procedia CIRP 94 46-51 (2020)
    Until now, additive manufacturing of high-performance materials such as martensitic hardenable tool steels is rarely investigated. This work addresses the introduction of an alternative alloying strategy for hot work tool steel powder, provided for laser powder bed fusion (L-PBF). The focus is on the question whether a powder mixture of spherical iron powder mixed with mechanically crushed ferroalloy particles can be processed by L-PBF, instead of using cost-intensive pre-alloyed gas-atomized powder, and to investigate the material properties associated with it. The particle morphology, packing density and flowability of this L-PBF powder feedstock is compared to gas-atomized spherical pre-alloyed steel powder and the results are correlated to the defect density, the resulting microstructure and the chemical homogeneity. Finally the resulting surface hardness is compared to a conventionally casted material as a reference state. It shows that the L-PBF fabrication of high-dense parts by means of both starting powders is technically feasible. Even though the alternative alloying concept promotes local chemical inhomogeneities within the microstructure, the overall porosity and the appearance of micro cracks are reduced. © 2020 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2020.09.010
  • 2020 • 416 Limits of Effective Material Properties in the Context of an Electromagnetic Tissue Model
    Jerbic, K. and Neumann, K. and Svejda, J.T. and Sievert, B. and Rennings, A. and Erni, D.
    IEEE Access 8 223806-223826 (2020)
    Most calibration schemes for reflection-based tissue spectroscopy in the mm-wave/ THz-frequency range are based on homogenized, frequency-dependent tissue models where macroscopic material parameters have either been determined by measurement or calculated using effective material theory. However, as the resolution of measurement at these frequencies captures the underlying microstructure of the tissue, we will investigate the validity limits of such effective material models over a wide frequency range (10 MHz - 200 GHz). Embedded in a parameterizable virtual workbench, we implemented a numerical homogenization method using a hierarchical multiscale approach to capture both the dispersive and tensorial electromagnetic properties of the tissue, and determined at which frequency this homogenized model deviated from a full-wave electromagnetic reference model within the framework of a Monte-Carlo analysis. Simulations were carried out using a generic hypodermal tissue that emulated the morphology of the microstructure. Results showed that the validity limit occurred at surprisingly low frequencies and thus contradicted the traditional usage of homogenized tissue models. The reasons for this are explained in detail and thus it is shown how both the lower 'allowed' and upper 'forbidden' frequency ranges can be used for frequency-selective classification/identification of specific material and structural properties employing a supervised machine-learning approach. Using the implemented classifier, we developed a method to identify specific frequency bands in the forbidden frequency range to optimize the reliability of material classification. © 2013 IEEE.
    view abstractdoi: 10.1109/ACCESS.2020.3045327
  • 2020 • 415 Microstructural damage and fracture mechanisms of selective laser melted Al-Si alloys under fatigue loading
    Awd, M. and Siddique, S. and Walther, F.
    Theoretical and Applied Fracture Mechanics 106 (2020)
    Employment of selective laser melted materials for industrial applications has grown in the recent years owing to improvements in machine systems and scanning techniques to result in near full-density parts; however, there are still limitations when their industrial application, particularly under cyclic loading, is concerned. This study investigates the chain from processing to post-process stress-relief to structural investigations in terms of microstructure and three-dimensional defect analysis by micro-computed tomography, and their corresponding behavior under quasi-static, high-cycle fatigue as well as very high-cycle fatigue for AlSi12 and AlSi10Mg alloys. Microstructure and damage mechanisms have been investigated as a function of in-process and post-process thermal treatments. Their corresponding influence on mechanical behavior revealed that there do exist differences in damage mechanisms in high-cycle fatigue and very high-cycle fatigue where the role of microstructure and small porosity respectively determines the damage mechanism in the two regions. The process parameters determining the required set of material behavior under quasi-static and cyclic loading have been identified. © 2020
    view abstractdoi: 10.1016/j.tafmec.2020.102483
  • 2020 • 414 Microstructure and mechanical properties of 316L austenitic stainless steel processed by different SLM devices
    Röttger, A. and Boes, J. and Theisen, W. and Thiele, M. and Esen, C. and Edelmann, A. and Hellmann, R.
    International Journal of Advanced Manufacturing Technology 108 769-783 (2020)
    In this work, we examined the influence of different types of selective laser melting (SLM) devices on the microstructure and the associated material properties of austenitic 316L stainless steel. Specimens were built using powder from the same powder batch on four different SLM machines. For the specimen build-up, optimized parameter sets were used, as provided by the manufacturers for each individual SLM machine. The resulting microstructure was investigated by means of scanning electron microscopy, which revealed that the different samples possess similar microstructures. Differences between the microstructures were found in terms of porosity, which significantly influences the material properties. Additionally, the build-up direction of the specimens was found to have a strong influence on the mechanical properties. Thus, the defect density defines the material’s properties so that the ascertained characteristic values were used to determine a Weibull modulus for the corresponding values in dependence on the build-up direction. Based on these findings, characteristic averages of the mechanical properties were determined for the SLM-manufactured samples, which can subsequently be used as reference parameters for designing industrially manufactured components. © 2020, The Author(s).
    view abstractdoi: 10.1007/s00170-020-05371-1
  • 2020 • 413 Microstructure and properties of high-strength C + N austenitic stainless steel processed by laser powder bed fusion
    Boes, J. and Röttger, A. and Theisen, W.
    Additive Manufacturing 32 (2020)
    In the developing field of laser powder bed fusion (L-PBF), austenitic stainless steels, such as AISI 316L, have gained great importance owing to their excellent processability. However, the moderate strength of these steels limits their applicability. This can be counteracted by the use of nitrogen as an alloying element to improve both strength and corrosion resistance. In this work, nitrogen-alloyed high-strength austenitic stainless steel X40MnCrMoN19-18-1 was processed by L-PBF, and the resulting microstructural and mechanical properties were investigated. The same material was also processed by hot isostatic pressing (HIP), which was used as a reference state. In the L-PBF process, argon and nitrogen were used as process gases to investigate the influence of process atmosphere on the microstructure and on changes in the chemical composition during processing. The results show a minor decrease in the nitrogen content of the steel after L-PBF, independently of the process gas, whereby argon resulted in a slightly higher specimen density. The microstructure after L-PBF processing contained small precipitates that could be removed by a short solution-annealing treatment. The tensile properties of the L-PBF-built steel are comparable to those of the steel produced by hot isostatic pressing in terms of ultimate tensile strength, but had lower elongation to fracture values. The ductility of the material was enhanced by solution annealing without significant impairment of the ultimate tensile strength. This work demonstrates that nitrogen-alloyed stainless steels can be processed by means of L-PBF and can extend the variety of appropriate steels towards applications with high requirements for the material strength and chemical resistance. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2020.101081
  • 2020 • 412 Microstructure-based multiscale modeling of large strain plastic deformation by coupling a full-field crystal plasticity-spectral solver with an implicit finite element solver
    Han, F. and Roters, F. and Raabe, D.
    International Journal of Plasticity 125 97-117 (2020)
    We present a fully embedded implementation of a full-field crystal plasticity model in an implicit finite element (FE) framework, a combination which realizes a multiscale approach for the simulation of large strain plastic deformation. At each integration point of the macroscopic FE model a spectral solver, based on Fast Fourier Transforms (FFTs), feeds-in the homogenized response from an underlying full-field polycrystalline representative volume element (RVE) model which is solved by using a crystal plasticity constitutive formulation. Both, a phenomenological hardening law and a dislocation density based hardening model, implemented in the open source software DAMASK, have been employed to provide the constitutive response at the mesoscale. The accuracy of the FE-FFT model has been benchmarked by one-element tests of several loading scenarios for an FCC polycrystal including simple tension, simple compression, and simple shear. The multiscale model is applied to simulate four application cases, i.e., plane strain deformation of an FCC plate, compression of an FCC cylinder, four-point bending of HCP bars, and beam bending of a dual-phase steel. The excellent capabilities of the model to predict the microstructure evolution at the mesoscale and the mechanical responses at both macroscale and mesoscale are demonstrated. © 2019 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijplas.2019.09.004
  • 2020 • 411 Microstructures, Heat Treatment, and Properties of Boron-Alloyed Tool Steels
    Lentz, J. and Röttger, A. and Theisen, W.
    Steel Research International 91 (2020)
    To enable the development of novel Fe–C–B–Cr and Fe–C–B–Cr–Mo cold work tool steels, the microstructures and hardness-tempering behaviors of hypoeutectic laboratory melts are investigated. The results show that increasing Cr content enhances the thermodynamic stability of the ultrahard M2B borides. The formation of carboborides is suppressed by adjusting the B/(C + B) ratio, Cr content, and austenitization temperature. A secondary hardenability at 500 °C is achieved by Mo addition. In addition, Mo stabilizes the M23(C,B)6 phase and at higher contents the M3B2 boride. Based on these investigations, Fe0.4C1B–Cr alloys are designed which, inspired by the microstructure of the steel X153CrMoV12-1, feature a α′-Fe hardenable matrix but 15 vol% of eutectic M2B borides instead of M7C3 for wear protection. The Fe0.4C1B–Cr steels are produced by casting and hot rolling as well as powder metallurgy and hot isostatic pressing. The (tribo-) mechanical properties are investigated and compared with X153CrMoV12-1. Fracture toughness, bending strength, wear resistance, and hardness of the novel Fe0.4C1B–Cr alloys are found to be similar or superior to the steel X153CrMoV12-1, at decreased material cost. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/srin.201900416
  • 2020 • 410 Morphology, microstructure, coordinative unsaturation, and hydrogenation activity of unsupported MoS2: How idealized models fail to describe a real sulfide material
    Bekx-Schürmann, S. and Mangelsen, S. and Breuninger, P. and Antoni, H. and Schürmann, U. and Kienle, L. and Muhler, M. and Bensch, W. and Grünert, W.
    Applied Catalysis B: Environmental 266 (2020)
    Polycrystalline MoS2 from (NH4)2MoS4 thermolysis was activated in dilute H2 at 523 K < TR < 873 K and studied by XRD, total scattering analysis, XPS, HRTEM, and chemisorption to explain, why coordinative unsaturation decreases with growing TR contrary to expectations from the MoS2 structure. Hydrogenation rates were measured for identifying active sites. With increasing TR, activity and chemisorption peaked at different TR,peak. Below TR,peak, increasing activity was not paralleled by changes in MoS2 microstructure. Decreasing chemisorption above TR,peak was assigned to saturation of vacancies by sulfide from internal defects and to inclusion of vacancies in the interior of aggregates. Upon high-temperature reduction, stacks grew anisotropically (basal planes extended), retaining defects like bending, turbostratic disorder. Preferential exposure of stack bases in aggregate surfaces resulted in enhanced decrease of chemisorption. Correlations between activity, edge area and (b)rim length estimated from a morphological model localized active sites in the (b)rim region. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2020.118623
  • 2020 • 409 On the role of chemical heterogeneity in phase transformations and mechanical behavior of flash annealed quenching & partitioning steels
    Liu, G. and Li, T. and Yang, Z. and Zhang, C. and Li, J. and Chen, H.
    Acta Materialia 201 266-277 (2020)
    The microstructure of advanced high-strength steels (AHSSs) is usually designed via adjusting austenite decomposition behavior upon cooling, while relatively less attention was paid to austenite formation upon heating. Here we explore the potentials of flash heating in tuning the microstructure and mechanical behavior of Quenching & Partitioning (Q&P) steels with an emphasis on the role of chemical heterogeneity. Besides substantially refining intercritical austenite grains (e.g. austenite formed during intercritical annealing), it was interestingly found that flash heating can also allow intercritical austenite to inherit Mn heterogeneity in the original pearlite-ferrite microstructure due to the kinetic mismatch between the sluggish diffusion of Mn and the rapid austenite formation. Chemical heterogeneity can to a large extent alter the decomposition of intercritical austenite and carbon partitioning upon cooling, and plays a notable role in enhancing thermal stability of austenite. The role of chemical heterogeneity in austenite decomposition and carbon partitioning behavior was explained via phase field simulations. The flash treated Q&P (FQP) steels have a broad range of tensile strength (from 980 MPa to 1180 MPa) and good ductility, which outperforms the conventional Q&P (CQP) steels. The current study demonstrates that flash heating opens alternative routes to create unique microstructures and improve the mechanical performance of AHSSs. © 2020
    view abstractdoi: 10.1016/j.actamat.2020.10.007
  • 2020 • 408 Pattern-forming nanoprecipitates in NiTi-related high entropy shape memory alloys
    Hinte, C. and Barienti, K. and Steinbrücker, J. and Gerstein, G. and Swider, M.A. and Herbst, S. and Eggeler, G. and Maier, H.J.
    Scripta Materialia 186 132-135 (2020)
    The microstructure and the fracture behavior of TiZrHfCoNiCu high entropy shape memory alloys with two different compositions were investigated. An unusual microstructure featuring pattern-forming nanoprecipitates was observed in dendritic and the interdendritic regions of both alloys. The unique higher-level order of these precipitates does not follow concentration gradients but is influenced by homogeneity and mechanical stress. The results also demonstrate that high entropy alloys are not necessarily homogeneous single-phase solid solutions. Moreover, it appears that solid solution strengthening as the primary mechanism also has to be questioned. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2020.05.007
  • 2020 • 407 Performance of YSZ and Gd2Zr2O7/YSZ double layer thermal barrier coatings in burner rig tests
    Vaßen, R. and Bakan, E. and Mack, D. and Schwartz-Lückge, S. and Sebold, D. and Jung Sohn, Y. and Zhou, D. and Guillon, O.
    Journal of the European Ceramic Society 40 480-490 (2020)
    Double layer thermal barrier coatings (TBCs) consisting of a Gd2Zr2O7 (GZO) top and an ytrria stabilized zirconia (YSZ) interlayer have been tested in a burner rig facility and the results compared to the ones of conventional YSZ single layers. In order to gain insight in the high temperature capability of the alternative TBC material, high surface temperatures of up to 1550 °C have been chosen while keeping the bond coat temperature similar. It turned out that the performance of all systems is largely depending on the microstructure of the coatings especially reduced porosity levels of GZO being detrimental. In addition, it was more difficult in GZO than in YSZ coatings to obtain highly porous and still properly bonded microstructures. Another finding was the reduced lifetime with increasing surface temperatures, the amount of reduction is depending on the investigated system. The reasons for this behavior are analyzed and discussed in detail. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jeurceramsoc.2019.10.021
  • 2020 • 406 Phase boundary segregation-induced strengthening and discontinuous yielding in ultrafine-grained duplex medium-Mn steels
    Ma, Y. and Sun, B. and Schökel, A. and Song, W. and Ponge, D. and Raabe, D. and Bleck, W.
    Acta Materialia 200 389-403 (2020)
    The combination of different phase constituents to realize a mechanical composite effect for superior strength-ductility synergy has become an important strategy in microstructure design in advanced high-strength steels. Introducing multiple phases in the microstructure essentially produces a large number of phase boundaries. Such hetero-interfaces affect the materials in various aspects such as dislocation activity and damage formation. However, it remains a question whether the characteristics of phase boundaries, such as their chemical decoration states, would also have an impact on the mechanical behavior in multiphase steels. Here we reveal a phase boundary segregation-induced strengthening effect in ultrafine-grained duplex medium-Mn steels. We found that the carbon segregation at ferrite-austenite phase boundaries can be manipulated by adjusting the cooling conditions after intercritical annealing. Such phase boundary segregation in the investigated steels resulted in a yield strength enhancement by 100–120 MPa and simultaneously promoted discontinuous yielding. The sharp carbon segregation at the phase boundaries impeded interfacial dislocation emission, thus increasing the stress required to activate such dislocation nucleation process and initiate plastic deformation. This observation suggests that the enrichment of carbon at the phase boundaries can enhance the energy barrier for dislocation emission, which provides a favorable condition for plastic flow avalanches and thus discontinuous yielding. These findings extend the current understanding of the yielding behavior in medium-Mn steels, and more importantly, shed light on utilizing and manipulating phase boundary segregation to improve the mechanical performance of multiphase metallic materials. © 2020
    view abstractdoi: 10.1016/j.actamat.2020.09.007
  • 2020 • 405 Phase Transformation-Induced Changes in Microstructure and Residual Stresses in Thermally Sprayed MnCoFeO4 Protective Coatings
    Back, H.C. and Gibmeier, J. and Vaßen, R.
    Journal of Thermal Spray Technology 29 1242-1255 (2020)
    The contribution comprises the investigation of the microstructure and residual stresses in thermally sprayed Mn1.0Co1.9Fe0.1O4.0 (MCF) protective coatings for interconnectors of SOFC stacks, deposited on ferritic steel Crofer 22 APU via atmospheric plasma spraying (APS). The coatings are designated to prevent Cr evaporation during high operation temperature of the SOFCs. The local microstructure, pore distributions and pore shapes, phase fractions, micro-hardness, Youngs’ modulus and residual stresses through the coating thickness were characterized in as-sprayed state and compared with longtime (10-100 h) heat-treated samples (700 and 850 °C). The results show that the long-term thermal aging treatment causes a successive high sintering of the coatings characterized by a reduction in pore density, by phase transformation from the metastable rock salt structure that gradually transformed to a spinel structure and by a slight relaxation of the process-induced tensile residual stresses in the coating. For SOFC application of the MCF coating, this indicates an improvement in the coatings integrity. During operation, a self-repair proceeds leading to dense and gas-proof coatings, while the mechanical properties are mainly retained. © 2020, The Author(s).
    view abstractdoi: 10.1007/s11666-020-00997-9
  • 2020 • 404 Processing of X65MoCrWV3-2 Cold Work Tool Steel by Laser Powder Bed Fusion
    Boes, J. and Röttger, A. and Theisen, W.
    Steel Research International 91 (2020)
    Laser powder bed fusion (L-PBF) of forming tools has become of major interest in the tooling industry because of the high geometrical flexibility of this process. During L-PBF, a metallic powder bed is melted selectively by a laser beam, enabling the layer-wise manufacturing of parts from 3D computer-aided design data. The process is characterized by a locally and temporally unsteady heat flow in the solidified part and in the melt pool, causing nonequilibrium solidification and phase transformations. In addition, rapid heating and cooling occur, promoting the formation of microstructural defects, cold cracks, and distortion. Because of the high tendency to form cold cracks, processing of martensitic tool steels is still a challenging task. Tool steel X65MoCrWV3-2 is processed by L-PBF and the resulting microstructure and the associated local properties are investigated by microhardness measurements, nanoindentation, and scanning electron microscopy. It is gathered from the investigations that regions of different microstructures and mechanical properties on both micro- and macroscale are present in the L-PBF-densified steel. The different microstructures and properties are the result of the alternating heat insert at different temperature regimes, forming heat-affected zones in which the tempering processes are triggered and strongly varying properties are generated. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/srin.201900445
  • 2020 • 403 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 • 402 Short SiC fiber/Ti3SiC2 MAX phase composites: Fabrication and creep evaluation
    Dash, A. and Malzbender, J. and Vaßen, R. and Guillon, O. and Gonzalez-Julian, J.
    Journal of the American Ceramic Society 103 7072-7081 (2020)
    The compressive creep of silicon carbide fiber reinforced Ti3SiC2 MAX phase with both fine and coarse microstructure was investigated in the temperature range of 1000-1300°C. Comparison of only steady-state creep was done to understand the response of fabricated composite materials toward creep deformation. It was demonstrated that the fibers are more effective in reducing the creep rates for the coarse microstructure by an increase in activation energy compared to the variant with a finer microstructure, being partly a result of the enhanced creep rates for the microstructure with larger grain size. Grain boundary sliding along with fiber fracture appears to be the main creep mechanism for most of the tested temperature range. However, there are indications for a changed creep mechanism for the fine microstructure for the lowest testing temperature. Local pores are formed to accommodate differences in strain related to creeping matrix and predominantly elastically deformed fibers during creep. Microstructural analysis was done on the material before and after creep to understand the deformation mechanics. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)
    view abstractdoi: 10.1111/jace.17337
  • 2020 • 401 Spinodal decomposition versus classical γ′ nucleation in a nickel-base superalloy powder: An in-situ neutron diffraction and atomic-scale analysis
    Collins, D.M. and D'Souza, N. and Panwisawas, C. and Papadaki, C. and West, G.D. and Kostka, A. and Kontis, P.
    Acta Materialia 200 959-970 (2020)
    Contemporary powder-based polycrystalline nickel-base superalloys inherit microstructures and properties that are heavily determined by their thermo-mechanical treatments during processing. Here, the influence of a thermal exposure to an alloy powder is studied to elucidate the controlling formation mechanisms of the strengthening precipitates using a combination of atom probe tomography and in-situ neutron diffraction. The initial powder comprised a single-phase supersaturated γ only; from this, the evolution of γ′ volume fraction and lattice misfit was assessed. The initial powder notably possessed elemental segregation of Cr and Co and elemental repulsion between Ni, Al and Ti with Cr; here proposed to be a precursor for subsequent γ to γ′ phase transformations. Subsolvus heat treatments yielded a unimodal γ′ distribution, formed during heating, with evidence supporting its formation to be via spinodal decomposition. A supersolvus heat treatment led to the formation of this same γ′ population during heating, but dissolves as the temperature increases further. The γ′ then reprecipitates as a multimodal population during cooling, here forming by classical nucleation and growth. Atom probe characterisation provided intriguing precipitate characteristics, including clear differences in chemistry and microstructure, depending on whether the γ′ formed during heating or cooling. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.09.055
  • 2020 • 400 The brittle-to-ductile transition in cold-rolled tungsten sheets: the rate-limiting mechanism of plasticity controlling the BDT in ultrafine-grained tungsten
    Bonnekoh, C. and Reiser, J. and Hartmaier, A. and Bonk, S. and Hoffmann, A. and Rieth, M.
    Journal of Materials Science 55 12314-12337 (2020)
    Conventionally produced tungsten (W) sheets are brittle at room temperature. In contrast to that, severe deformation by cold rolling transforms W into a material exhibiting room-temperature ductility with a brittle-to-ductile transition (BDT) temperature far below room temperature. For such ultrafine-grained (UFG) and dislocation-rich materials, the mechanism controlling the BDT is still the subject of ongoing debates. In order to identify the mechanism controlling the BDT in room-temperature ductile W sheets with UFG microstructure, we conducted campaigns of fracture toughness tests accompanied by a thermodynamic analysis deducing Arrhenius BDT activation energies. Here, we show that plastic deformation induced by rolling reduces the BDT temperature and also the BDT activation energy. A comparison of BDT activation energies with the trend of Gibbs energy of kink-pair formation revealed a strong correlation between both quantities. This demonstrates that out of the three basic processes, nucleation, glide, and annihilation, crack tip plasticity in UFG W is still controlled by the glide of dislocations. The glide is dictated by the mobility of the screw segments and therefore by the underlying process of kink-pair formation. Reflecting this result, a change of the rate-limiting mechanism for plasticity of UFG W seems unlikely, even at deformation temperatures well below room temperature. As a result, kink-pair formation controls the BDT in W over a wide range of microstructural length scales, from single crystals and coarse-grained specimens down to UFG microstructures. © 2020, The Author(s).
    view abstractdoi: 10.1007/s10853-020-04801-5
  • 2020 • 399 The impact of grain-scale strain localization on strain hardening of a high-Mn steel: Real-time tracking of the transition from the γ → ε → α’ transformation to twinning
    Souza Filho, I.R. and Dutta, A. and Almeida Junior, D.R. and Lu, W. and Sandim, M.J.R. and Ponge, D. and Sandim, H.R.Z. and Raabe, D.
    Acta Materialia 197 123-136 (2020)
    Strain partitioning and localization were investigated in a high-Mn steel (17.1 wt.% Mn) during tensile testing by a correlative probing approach including in-situ synchrotron X-ray diffraction, micro- digital image correlation (μ-DIC) and electron microscopy. By combining Warren's theory with the μ-DIC analysis, we monitored the formation of planar faults (stacking faults and mechanical twins) and correlated them with the local strain partitioning behavior within the microstructure. Starting with an initial microstructure of austenite (γ) and athermally formed ε- and α’-martensite, strain accumulates preferentially near the γ/ε interfaces during tensile straining. The local microscopic von Mises strain (εvM) maps obtained from μ-DIC probing show that these local strain gradients produce local strain peaks approximately twice as high as the imposed macroscopic engineering strain (ε), thus locally triggering formation of ε-martensite already at early yielding. The interior of the remaining austenite, without such interfacial strain peaks, remained nearly devoid of planar faults. The local strain-driven growth of the ε-domains occurs concomitantly with the α’-martensite formation. At intermediate macroscopic applied strains, austenite grain size is considerably reduced to a few nanometers and the associated γ/ε interfacial microscopic strain peaks increase in magnitude. This scenario favors twinning to emerge as a competing strain hardening mechanism at engineering strain levels from ε = 0.075 onwards. At large tensile strains, the γ → ε → α’ transformation rates tend to cease making both twinning and SFs formation to operate as the main strain hardening mechanisms. The findings shed light on the transformation micro-mechanisms in multiphase Mn-TRIP steels by revealing how strain localization among the constituents can directly influence the kinetics of the competing strain hardening mechanisms. © 2020
    view abstractdoi: 10.1016/j.actamat.2020.07.038
  • 2020 • 398 The phenomenon of bitumen ‘bee' structures–bulk or surface layer–a closer look
    Ganter, D. and Franzka, S. and Shvartsman, V.V. and Lupascu, D.C.
    International Journal of Pavement Engineering (2020)
    Bitumen is the primary binder of asphalt covering most of the roads in the world. The origin of the primary oil, the refinery treatment, the specific chemical structure, and the natural or induced chemical modifications determine the mechanical properties of bitumen. Aging affects the time evolution of its thermo-rheological behaviour. Numerous studies have suggested that the particular local microstructure of bitumen affects its rheological properties including ‘bee’–patterned structures. We have used scanning probe microscopy to distinguish between surface effects and bulk properties. Using scanning probe microscopy we conclude that the ‘bee’ structures exist at the surface only and do not occur in the bulk. In particular, they are not observed on freshly fractured surfaces. A special technique was used to chisel off the bitumen surface. The material relaxes and the ‘bee’ structures disappear from the surface. This proves that the ‘bees’ are formed as a thin surface layer. Improved recycling will thus have to rely on chemical treatment of aged and used bitumen rather than on intentional modifications to the surface microstructure which is merely a surface effect. The bitumen surface microstructure can nevertheless be seen as a fingerprint of the overall bitumen properties to a certain degree. © 2020 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/10298436.2020.1823390
  • 2020 • 397 Thermal barrier coatings with novel architectures for diesel engine applications
    Uczak de Goes, W. and Markocsan, N. and Gupta, M. and Vaßen, R. and Matsushita, T. and Illkova, K.
    Surface and Coatings Technology 396 (2020)
    The increased demands for higher efficiency and environmentally friendly diesel engines have led to a continuous search for new coating processing routes and new ceramic materials that can provide the required properties when applied on engine components such as pistons and exhaust manifolds. Although successful in gas turbine applications, thermal barrier coatings (TBCs) produced by suspension plasma spraying (SPS) processes have not been employed so far in the automotive industry. This work aims to achieve a better understanding of the role of thermal conductivity and thermal effusivity on the durability of SPS TBCs applied to pistons of diesel engines. Three different coating architectures were considered for this study. The first architecture was yttria-stabilized zirconia (YSZ) lamellar top coat deposited by APS (Atmospheric Plasma Spray) and used as a reference sample in this study. The second architecture was a columnar SPS top coat of either YSZ or gadolinium zirconate (GZO) while the third architecture was an SPS columnar top coat, “sealed” with a dense sealing layer deposited on the top coat. Two types of sealing layers were used, a metallic (M) or a ceramic thermal spray layer (C). Laser Flash Analysis (LFA) was used to determine the thermal conductivity and thermal effusivity of the coatings. Two different thermal cyclic tests were used to test the TBCs behavior under cyclic thermal loads. Microstructure analysis before and after the thermal cyclic tests were performed using SEM in different microstructures and materials. The thermal cyclic test results were correlated with coatings microstructure and thermophysical properties. It was observed that the columnar coatings produced by SPS had an enhanced service life in the thermal cyclic tests as compared to the APS coatings. © 2020 The Authors
    view abstractdoi: 10.1016/j.surfcoat.2020.125950
  • 2020 • 396 Thermomechanical modeling of microstructure evolution caused by strain-induced crystallization
    Aygün, S. and Klinge, S.
    Polymers 12 1-30 (2020)
    The present contribution deals with the thermomechanical modeling of the strain-induced crystallization in unfilled polymers. This phenomenon significantly influences mechanical and thermal properties of polymers and has to be taken into consideration when planning manufacturing processes as well as applications of the final product. In order to simultaneously capture both kinds of effects, the model proposed starts by introducing a triple decomposition of the deformation gradient and furthermore uses thermodynamic framework for material modeling based on the Coleman–Noll procedure and minimum principle of the dissipation potential, which requires suitable assumptions for the Helmholtz free energy and the dissipation potential. The chosen setup yields evolution equations which are able to simulate the formation and the degradation of crystalline regions accompanied by the temperature change during a cyclic tensile test. The boundary value problem corresponding to the described process includes the balance of linear momentum and balance of energy and serves as a basis for the numerical implementation within an FEM code. The paper closes with the numerical examples showing the microstructure evolution and temperature distribution for different material samples. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/polym12112575
  • 2020 • 395 Torsion plastometer trials to investigate the effect of non-proportional loading paths in caliber rolling on damage and performance of metal parts
    Wang, S. and Dunlap, A. and Möhring, K. and Lohmar, J. and Schwedt, A. and Aretz, A. and Walther, F. and Hirt, G.
    Production Engineering 14 17-32 (2020)
    The non-proportional loading path describes a strain-dependent development of stress triaxiality and Lode parameter during metal forming processes. Existing studies suggest a strong dependence of damage evolution on the non-proportional loading path. This work focuses on investigating the influence of non-proportional loading paths observed in hot caliber rolling of the case-hardening steel 16MnCrS5 using laboratory scale experiments. The applied torsion plastometer is highly flexible as it can apply combined loading types (tension, compression and torsion) on notched round specimen and enables deformation at elevated temperature. In this study, two characteristic non-proportional loading paths in caliber rolling and the maximal achievable non-proportional loading path variation were recreated in the torsion plastometer based on both FE simulations and experiments. After deformation, the specimen were further analyzed using Scanning Electron Microscopy (SEM) to quantify the damage. The results indicate an influence of the non-proportional loading path on damage evolution. Furthermore, fatigue tests were employed to characterize the fatigue performance of the deformed specimens. In the torsion plastometer trials carried out no clear correlation of performance and damage was observed. This is most likely due to differences in residual dislocation density after static recrystallization and deviations in the microstructure after hot working. Thus, the superposition of microstructure evolution and damage needs to be considered carefully when testing at elevated temperature. © 2020, The Author(s).
    view abstractdoi: 10.1007/s11740-019-00949-5
  • 2020 • 394 Tribo-mechanical properties and adhesion behavior of DLC coatings sputtered onto 36NiCrMo16 produced by selective laser melting
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Hagen, L. and Schaper, M. and Hengsbach, F. and Hoyer, K.-P.
    Surface and Coatings Technology 394 (2020)
    The combination of selective laser melted nickel-chromium-molybdenum alloyed steel and diamond-like carbon (DLC) coatings is an aspiring approach to produce light-weight components with improved tribological properties. In order to assure the tribo-mechanical properties of the DLC coatings, good adhesion on the additively manufactured substrates under high loads is essential. 36NiCrMo16, produced by Selective Laser Melting (SLM) and wrought material techniques, served as substrate material for hydrogen-free a-C and hydrogenated a-C:H coatings with a chromium carbide interlayer. The structural and mechanical properties of 36NiCrMo16 were examined in order to consider their effects on the tribo-mechanical properties and adhesion of the DLC coatings deposited by magnetron sputtering. In x-ray diffraction analyses, retained austenite was identified for the 36NiCrMo16 substrate processed by SLM. This increased austenite content in the SLM built steel is attributed to the pre-heating temperature of 200 °C during fabrication. Additionally, a relative density of 99.98% was detected for the SLM manufactured tempering steel by using an x-ray microscope. In contrast to the conventional 36NiCrMo16, the SLM substrates exhibited a higher surface roughness, which is ascribed to the different phase composition and microstructure. In nanoindentation tests, the a-C and a-C:H coatings deposited onto 36NiCrMo16 exhibited a hardness of ~22 and ~19–20 GPa, respectively. In general, the DLC coatings revealed a good adhesion to the conventional and SLM 36NiCrMo16 in Rockwell C indentation tests. Local, limited spalling failures were identified for a-C and a-C:H on the SLM substrate, while no delamination was observed for the coatings on conventional steel. The differences in microstructure, such as the retained austenite and higher surface roughness, do appear to affect the adhesion of the DLC coatings. In tribometer tests against 100Cr6 counterparts, the DLC coatings significantly reduced the friction and increased the wear resistance of 36NiCrMo16. Therefore, the DLC coatings are promising to enhance the tribo-mechanical properties of SLM 36NiCrMo16 substrates, but it is crucial to consider the surface integrity of SLM steel on the adhesion behavior of DLC coatings. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.125748
  • 2020 • 393 Tuning the Microstructure and Thickness of Ceramic Layers with Advanced Coating Technologies Using Zirconia as an Example
    Guillon, O. and Dash, A. and Lenser, C. and Uhlenbruck, S. and Mauer, G.
    Advanced Engineering Materials 22 (2020)
    The properties of ceramic layers are not only related to the coating material but also—to a very high degree—the processing technology used. In particular, microstructure and thickness are key to the successful implementation of functional layers in application. This will be shown using yttria-stabilized zirconia (YSZ) as an example, a highly versatile compound with high fracture toughness, high chemical and thermal stability, high biological compatibility, and high oxygen ion conductivity. For each application, specific microstructures are required, which can only be obtained by suitable processing. Herein, coating technologies for layers with thicknesses spanning the nanometer range up to several hundred micrometers, and from full density to tailored open porosity are focused. Wet processing routes, thin-film deposition from the gas phase as well as thermal and plasma spraying are presented along with the resulting YSZ layers. © 2020 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/adem.202000529
  • 2020 • 392 Use of (nano-)additives in Laser Powder Bed Fusion of Al powder feedstocks: Research directions within the last decade
    Kusoglu, I.M. and Gökce, B. and Barcikowski, S.
    Procedia CIRP 94 11-16 (2020)
    An exponential increase for the number of Scientific Citation Index (SCI) expanded articles in the field of Laser Powder Bed Fusion (L-PBF) of metal powder feedstocks are generated within the last decade. Al powder feedstocks have become one of the most cited metal alloys in this field. By in-depth analyzing the experimental research data provided within SCI-expanded articles, it is obvious that material properties and laser process properties have a high impact to produce as-built parts with crack free microstructure, high density, high mechanical properties and high repetition rates of as-built parts. As a future research trend of using additive powders in Al powder feedstocks are found to be promising candidates to develop L-PBF of as-built Al parts. This study quantitatively evaluates the effect of compositions and mass fractions on several Al alloy powder feedstocks by extracting research data given in the SCI-expanded articles. The latest research directions showed that several additives are useful to refine microstructure and to enhance the mechanical properties of as-built Al parts. The effect of additives on processability, as-built density, tensile properties, and flexural properties of several Al alloy parts are discussed. © 2020 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2020.09.003
  • 2020 • 391 Zn- or Cu-containing CaP-based coatings formed by micro-arc oxidation on titanium and Ti-40Nb Alloy: Part I-Microstructure, composition and properties
    Komarova, E.G. and Sharkeev, Y.P. and Sedelnikova, M.B. and Prosolov, K.A. and Khlusov, I.A. and Prymak, O. and Epple, M.
    Materials 13 (2020)
    Zn- and Cu-containing CaP-based coatings, obtained by micro-arc oxidation process, were deposited on substrates made of pure titanium (Ti) and novel Ti-40Nb alloy. The microstructure, phase, and elemental composition, as well as physicochemical and mechanical properties, were examined for unmodified CaP and Zn- or Cu-containing CaP coatings, in relation to the applied voltage that was varied in the range from 200 to 350 V. The unmodified CaP coatings on both types of substrates had mainly an amorphous microstructure with a minimal content of the CaHPO4 phase for all applied voltages. The CaP coatings modified with Zn or Cu had a range from amorphous to nano- and microcrystalline structure that contained micro-sized CaHPO4 and Ca(H2PO4)2·H2O phases, as well as nano-sized β-Ca2P2O7, CaHPO4, TiO2, and Nb2O5 phases. The crystallinity of the formed coatings increased in the following order: CaP/TiNb < Zn-CaP/TiNb < Cu-CaP/TiNb < CaP/Ti < Zn-CaP/Ti < Cu-CaP/Ti. The increase in the applied voltage led to a linear increase in thickness, roughness, and porosity of all types of coatings, unlike adhesive strength that was inversely proportional to an increase in the applied voltage. The increase in the applied voltage did not affect the Zn or Cu concentration (~0.4 at%), but led to an increase in the Ca/P atomic ratio from 0.3 to 0.7. © 2020 by the authors.
    view abstractdoi: 10.3390/ma13184116
  • 2019 • 390 Achieving ultra-high strength and ductility in equiatomic CrCoNi with partially recrystallized microstructures
    Slone, C.E. and Miao, J. and George, E.P. and Mills, M.J.
    Acta Materialia 165 496-507 (2019)
    Despite having otherwise outstanding mechanical properties, many single-phase medium and high entropy alloys are limited by modest yield strengths. Although grain refinement offers one opportunity for additional strengthening, it requires significant and undesirable compromises to ductility. This work therefore explores an alternative, simple processing route to achieve strength by cold-rolling and annealing an equiatomic CrCoNi alloy to produce heterogeneous, partially recrystallized microstructures. Tensile tests reveal that our approach dramatically increases the yield strength (to ∼1100 MPa) while retaining good ductility (total elongation ∼23%) in the single-phase CrCoNi alloy. Scanning and transmission electron microscopy indicate that the strengthening is due to the non-recrystallized grains retaining their deformation-induced twins and very high dislocation densities. Load-unload-reload tests and grain-scale digital image correlation are also used to study the accumulation of plastic deformation in our highly heterogeneous microstructures. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.015
  • 2019 • 389 Additive manufacturing of CMSX-4 Ni-base superalloy by selective laser melting: Influence of processing parameters and heat treatment
    Lopez-Galilea, I. and Ruttert, B. and He, J. and Hammerschmidt, T. and Drautz, R. and Gault, B. and Theisen, W.
    Additive Manufacturing 30 (2019)
    Selective laser melting (SLM) provides an economic approach to manufacturing Ni-base superalloy components for high-pressure gas turbines as well as repairing damaged blade sections during operation. In this study, two advanced processing routes are combined: SLM, to fabricate small specimens of the nonweldable CMSX-4, and hot isostatic pressing (HIP) with a rapid cooling rate as post-processing to heal defects while the target γ/γ´ microstructure is developed. An initial parametric study is carried out to investigate the influence of the SLM process parameters on the microstructure and defects occurring during SLM. Special emphasis is placed on understanding and characterizing the as-built SLM microstructures by means of high-resolution characterization techniques. The post-processing heat treatment is then optimized with respect to segregation and the γ/γ´ microstructure. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2019.100874
  • 2019 • 388 Carbon partitioning and microstructure evolution during tempering of an Fe-Ni-C steel
    Harding, I. and Mouton, I. and Gault, B. and Raabe, D. and Kumar, K.S.
    Scripta Materialia 172 38-42 (2019)
    Partitioning of C during tempering of quenched Fe-9.6Ni-0.5C-0.6Mn-0.6Mo-0.7Cr-0.1V (at.%) steel is determined by atom probe tomography and the resulting microstructure is described. The precipitated austenite size, together with its C and Ni content control its thermal stability and these can vary differently with tempering time and temperature. Thus, both austenite and strong carbide formers compete for the available C early in the process. Due to widely different transport kinetics, C likely plays a dominant role early but is either fully consumed or its role diminishes by dilution, and Ni partitioning eventually takes over as the austenite stability-controlling species. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2019.06.036
  • 2019 • 387 Cavitation erosion resistance of 316L austenitic steel processed by selective laser melting (SLM)
    Hardes, C. and Pöhl, F. and Röttger, A. and Thiele, M. and Theisen, W. and Esen, C.
    Additive Manufacturing 29 (2019)
    Every SLM-fabricated component typically possesses a process-specific microstructure that fundamentally differs from any conventionally fabricated specimen. This publication addresses the evaluation of microstructure-related influencing factors on the resistance against cavitation erosion. We exemplarily compared the findings to a cast and hot rolled reference sample. Due to careful adjustment of the process parameters, the overall cavitation erosion resistance of both SLM-processed and conventionally fabricated 316L are very much alike in the investigated case. The incubation period of intact surface areas is improved by the greater hardness and yield strength of the SLM specimen, which is attributable to an increased dislocation density and a smaller grain size. Nevertheless, processing and powder feeding during SLM-fabrication occasionally results in microstructural defects, at which pronounced mass loss during cavitation was registered. X-ray measurements of the residual stresses reveal the development of severe compressive stresses that emerge after a few seconds of cavitation. This compressive stress state delays the immediate propagation of SLM-inherent micro cracks. Moreover, investigations of the microstructure in combination with examination of the ongoing surface deformation highlighted the emergence of coarse grains that grew towards the temperature gradient. This effect leads to a temporarily high surface roughness, local stress concentrations and an increased probability of cavitation impacts. Furthermore, parallel cracks appear perpendicular to the scan tracks that are traced back to formerly protruded slip bands. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2019.100786
  • 2019 • 386 Columnar to equiaxed transition and grain refinement of cast CrCoNi medium-entropy alloy by microalloying with titanium and carbon
    Liu, X.W. and Laplanche, G. and Kostka, A. and Fries, S.G. and Pfetzing-Micklich, J. and Liu, G. and George, E.P.
    Journal of Alloys and Compounds 775 1068-1076 (2019)
    Thermomechanical processing has been used to control the grain size/shape of the equiatomic CrCoNi medium-entropy alloy (MEA) and obtain excellent strength and ductility. However, in the cast state, the alloy has coarse columnar grains with average widths and lengths of approximately 120 and 1000 μm, respectively, resulting in inferior mechanical properties. To overcome this deficiency, here we microalloyed with Ti and C and successfully changed the grain shape (from columnar to equiaxed) and refined the grain size. The degree to which the microstructure changes depends on the amount of Ti and C added, with the best results obtained at 0.4 at.% each. In the optimal alloy [(CrCoNi)99.2Ti0.4C0.4], the as-cast grains were nearly equiaxed with a uniform size of ∼75 μm. Associated with this change in grain shape/size was a significant improvement of yield strength, ultimate tensile strength and elongation to fracture at both 293 and 77 K. The columnar to equiaxed transition is attributed to the strong mutual affinity of C and Ti, which leads to their build-up ahead of the solid-liquid interface and, in turn, to enhanced constitutional undercooling. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2018.10.187
  • 2019 • 385 Combined phase-field crystal plasticity simulation of P- and N-type rafting in Co-based superalloys
    Wang, C. and Ali, M.A. and Gao, S. and Goerler, J.V. and Steinbach, I.
    Acta Materialia 175 21-34 (2019)
    We combine a phase-field model with a crystal plasticity model to simulate the microstructural evolution during creep in the Co-based superalloy ERBOCo-2Ta. Three-dimensional simulations of tensile and compressive creep tests in [100] direction were performed to study the rafting behavior in Co-based superalloys. The loss of coherency between γ matrix and γ′ precipitate, which is essential for the understanding of rafted structures, is modeled in relation to the dislocation activity in the γ-channels. Special attention is given to the interplay between creep deformation and microstructure stability. Appropriate constitutive modeling is applied to simulate realistic microstructure evolution under creep conditions. Thus, with the removal of the misfit stress, γ′ precipitates lose their cuboidal shape and form rafts. During N-type rafting more γ′ precipitates coalesce than during P-type rafting. The γ′ volume fraction during rafting increases under tensile stress but decreases under compressive stress. The morphological evolution of γ′ precipitates under tensile and compressive stresses in Co-based superalloy is consistent with the rafting characteristics in experimental observations. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.05.063
  • 2019 • 384 Corrosion and corrosion fatigue properties of additively manufactured magnesium alloy WE43 in comparison to titanium alloy Ti-6Al-4V in physiological environment
    Wegner, N. and Kotzem, D. and Wessarges, Y. and Emminghaus, N. and Hoff, C. and Tenkamp, J. and Hermsdorf, J. and Overmeyer, L. and Walther, F.
    Materials 12 (2019)
    Laser powder bed fusion (L-PBF) of metals enables the manufacturing of highly complex geometries which opens new application fields in the medical sector, especially with regard to personalized implants. In comparison to conventional manufacturing techniques, L-PBF causes different microstructures, and thus, new challenges arise. The main objective of this work is to investigate the influence of different manufacturing parameters of the L-PBF process on the microstructure, process-induced porosity, as well as corrosion fatigue properties of the magnesium alloy WE43 and as a reference on the titanium alloy Ti-6Al-4V. In particular, the investigated magnesium alloy WE43 showed a strong process parameter dependence in terms of porosity (size and distribution), microstructure, corrosion rates, and corrosion fatigue properties. Cyclic tests with increased test duration caused an especially high decrease in fatigue strength for magnesium alloy WE43. It can be demonstrated that, due to high process-induced surface roughness, which supports locally intensified corrosion, multiple crack initiation sites are present, which is one of the main reasons for the drastic decrease in fatigue strength. © 2019 by the authors.
    view abstractdoi: 10.3390/ma12182892
  • 2019 • 383 Creep properties of single crystal Ni-base superalloys (SX): A comparison between conventionally cast and additive manufactured CMSX-4 materials
    Bürger, D. and Parsa, A.B. and Ramsperger, M. and Körner, C. and Eggeler, G.
    Materials Science and Engineering A 762 (2019)
    The present work compares the microstructures and the creep properties of two types of single crystal Ni-base superalloy CMSX-4 materials (SXs). One was produced by conventional directional solidification Bridgman processing. The other was manufactured by selective electron beam melting (SEBM). The microstructures of the two types of materials are compared with emphasis placed on the large (dendritic/interdendritic regions) and small scale (γ-matrix/γ′-precipitates) microstructural heterogeneities, which characterize SX microstructures and their evolution during processing, heat treatment and creep. It is shown that heat treated SEBM materials have creep properties, which match or even outperform those of conventionally processed SX materials. Creep properties were assessed using a miniature creep test technique where [001] miniature tensile creep specimens were tested in the high temperature/low stress (1050 °C, 160 MPa) and in the low temperature/high stress (850 °C, 600 MPa) creep regimes. The creep behavior is interpreted based on microstructural results, which were obtained using analytical scanning and transmission electron microscopy (SEM and TEM). © 2019 The Authors
    view abstractdoi: 10.1016/j.msea.2019.138098
  • 2019 • 382 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 • 381 Determination of the properties and loading efficiency of encapsulated BSA-FITC and dexamethasone for drug delivery systems
    Chudinova, E. and Surmeneva, M. and Koptyug, A. and Sokolova, V. and Prymak, O. and Bouckercha, S. and Epple, M. and Surmenev, R.
    IOP Conference Series: Materials Science and Engineering 597 (2019)
    In this work porous microparticles of calcium carbonate were synthesized with bovine serum albumin - fluorescein isothiocyanate conjugate (BSA-FITC) and dexamethasone, and then used for encapsulation in polymer microcapsules by means of layer-by-layer assembly (LbL). The properties of the obtained microcapsules were characterized by scanning electron microscopy, dynamic light scattering, infrared-, ultraviolet- and visible spectroscopy. According to the performed DLS measurements, an average hydrodynamic diameter ranged from 4 to 8 m and zeta-potential for all types of capsules was determined as -18 and -21 mV. BSA-FITC was encapsulated using this approach yielded a loading efficiency of 49 % protein. This value calculated for dexamethasone was of 38%. The microcapsules filled with an encapsulated drug may find applications in the field of biotechnology, biochemistry, and medicine. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/597/1/012056
  • 2019 • 380 Development of a thermodynamically consistent model towards biogeochemical processes within antarctic sea ice microstructure within the extended theory of porous media (Etpm)
    Thom, A. and Ricken, T.
    Advances in Engineering Materials, Structures and Systems: Innovations, Mechanics and Applications - Proceedings of the 7th International Conference on Structural Engineering, Mechanics and Computation, 2019 292-296 (2019)
    According to NASA (National Aeronautics and Space Administration), Antarctic sea ice reached its lowest extent ever recorded by satellites at the end of summer 2017 after decades of moderate sea ice expansion. Besides a strong influence on the global climate, linking the exchange of energy and gases between the atmosphere and the ocean, changes in sea ice also have a biological impact on the structure and function of the ecosystem. These effects are strongly related to the physical and mechanical properties of the sea ice structure. Seawater is trapped in so-called brine pockets during the growth of sea ice, providing a natural habitat for sea ice microorganisms. The microorganisms are supplied with nutrients they need for primary production from the seawater. A small-scale modeling of the porosity of the sea ice and its inclusions and the solid/brine multiphase microstructure, respectively, thermodynamics of air-sea interactions as well as sea ice-biological linkages is a necessary tool to better understand the heterogeneous nature of sea ice. Based on the extended Theory of Porous Media (eTPM), the development of a multiphasic, multi-component model which enables the continuum mechanical description of transport and phase transition phenomena in sea ice at a homogenized pore scale is developed. The foundation builds a biphasic (ice and brine) model, in which the brine is composed of freshwater and salt. © 2019 Taylor & Francis Group, London, UK.
    view abstractdoi: 10.1201/9780429426506-50
  • 2019 • 379 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 • 378 Effect of Al, Ti and C additions on Widmanstätten microstructures and mechanical properties of cast Al0.6CoCrFeNi compositionally complex alloys
    Asabre, A. and Kostka, A. and Stryzhyboroda, O. and Pfetzing-Micklich, J. and Hecht, U. and Laplanche, G.
    Materials and Design 184 (2019)
    The cast microstructure of the Al0.6CoCrFeNi compositionally complex alloy was successfully refined with small additions of Al, Ti and C and its mechanical properties were optimized. In the as-cast state, this alloy has a Widmanstätten microstructure with coarse grains (∼110 μm) of a strong BCC/B2 matrix and soft FCC plates (∼65 vol.%) with large widths (∼1.3 μm). The addition of 0.25 at.% C to Al0.6CoCrFeNi stabilizes the FCC phase and favors the formation of a coarse dendritic microstructure making this alloy unsuitable for structural applications. In contrast, alloying of either 3 at.% Al, Ti, or 3% Ti and 0.25% C to Al0.6CoCrFeNi refined its Widmanstätten microstructure, i.e. the thickness of the FCC plates and/or the size of the prior BCC/B2 grains were significantly reduced. As a result of these microstructural changes, Al and Ti containing alloys show an outstanding strength (twice higher than that of Al0.6CoCrFeNi) and ductilities ≤5% at 20 °C. These properties are retained at 400 °C but at 700 °C, the strength and ductility of almost all alloys decrease. However, Ti containing alloys exhibit much larger ductilities (∼50%) at 700 °C due to their high density of grain boundaries which accommodate plastic deformation through grain boundary sliding. © 2019 The Authors
    view abstractdoi: 10.1016/j.matdes.2019.108201
  • 2019 • 377 Effect of heat treatment on the high temperature fatigue life of single crystalline nickel base superalloy additively manufactured by means of selective electron beam melting
    Meid, C. and Dennstedt, A. and Ramsperger, M. and Pistor, J. and Ruttert, B. and Lopez-Galilea, I. and Theisen, W. and Körner, C. and Bartsch, M.
    Scripta Materialia 168 124-128 (2019)
    The high temperature low cycle fatigue behavior of specimens manufactured from a single crystalline nickel base superalloy processed by selective electron beam melting (SEBM) has been investigated with respect to the effect of different heat treatments. The fatigue lifetime of heat treated material was significantly higher than that of as-built material. Applying hot isostatic pressing (HIP) with an integrated heat treatment resulted in even longer fatigue life. Lifetime limiting crack initiation occurred at interfaces of melting layers, at micro-porosity generated during solidification or, in HIP treated samples, at precipitates which formed at the location of collapsed pores. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2019.05.002
  • 2019 • 376 Effect of the dwell time on the microstructure and tensile strength of vacuum-brazed tool steels using BNi-2 filler metal
    Tillmann, W. and Henning, T. and Boretius, M.
    Welding in the World (2019)
    Nickel-based brazing alloys are widely used to vacuum braze numerous base materials such as tool steels, heat resistant steels, austenitic steels, and nickel-based alloys. The formation of intermetallic phases like Ni3Si, Ni3B, or CrxBy can cause a significant embrittlement of the joint. A sufficient diffusion of the melting point depressants boron and silicon will avoid such phase formations and can be primarily affected by the temperature-time-cycle. The process parameters required to achieve an entire nickel solid solution microstructure can be thermodynamically predicted, but usually exceeds the specifications of the heat treatment of the base material by far or necessitate hardly practicable small brazing gaps. This research is focused on the microstructure and the properties of vacuum-brazed joints, using a lower process temperature compared to thermodynamically optimized brazing parameters of AISI H11/BNi-2 joints. In order to investigate the influence of the dwell time, the tool steels AISI H11 and AISI 420 were brazed at 1050 °C for 25 and for 90 min with a BNi-2 amorphous foil (50.8 μm). The extended dwell time has mainly led to a higher Fe/Ni ratio within the brazed metal. Therefore, the average tensile strength was improved from 668 to 1304 MPa for AISI H11 joints as well as from 815 to 1351 MPa for AISI 420 joints. Furthermore, the fracture path was located at the interface brazed metal/diffusion area and could be attributed to a high disparity of the microhardness as well as a weakening by Kirkendall porosity. © 2019, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-019-00734-z
  • 2019 • 375 Effects of ta substitution on the microstructure and transport properties of hf-doped nbfesb half-heusler thermoelectric materials
    Farahi, N. and Stiewe, C. and Truong, D.Y.N. and Shi, Y. and Salamon, S. and Landers, J. and Eggert, B. and Wende, H. and De Boor, J. and Kleinke, H. and Müller, E.
    ACS Applied Energy Materials 2 8244-8252 (2019)
    This investigation demonstrates the effect of partial substitutions of Nb by refractory Ta on the microstructure and thermoelectric properties of Hf-doped NbFeSb materials. All the synthesized samples show a heavily doped semiconducting character with the electrical conductivity higher than 3500 ω-1 cm-1 at 326 K. Furthermore, the samples containing Ta display consistently lower (∼10-13%) thermal conductivity of ∼7 W m-1 K-1 at 350 K compared to a value of ∼8 W m-1 K-1 at the same temperature for the nonsubstituted sample. The vivid impact of Ta substitutions on reducing the lattice thermal conductivity of NbFeSb based materials is chiefly due to the lattice disorder originating from the mass difference between Ta and Nb atoms, resulting in ∼28% reduction in lattice thermal conductivity of the Nb0.73Hf0.12Ta0.15FeSb sample at 350 K compared to the nonsubstituted sample. The results of our Mößbauer spectroscopy measurements exclude the possibility of mixed Fe occupancies. Although magnetic properties were not strongly modified by the Ta substitution, Nb0.83Hf0.12Ta0.05FeSb shows a magnetic phase transition at ∼150 K, which is not observed in Nb0.88Hf0.12FeSb. This could be caused by extrinsic defects and microstructure induced by Ta addition. All samples exhibit a similar maximum dimensionless figure of merit value, zTmax, of ∼0.75 at 800 K, which is comparable to the high efficiency materials previously reported in this system and makes them potential candidates to be utilized as p-type legs in half-Heusler based thermoelectric generators (TEG). Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b01711
  • 2019 • 374 Effects of selenization time and temperature on the growth of Cu2ZnSnSe4 thin films on a metal substrate for flexible solar cells
    Stanchik, A.V. and Gremenok, V.F. and Juskenas, R. and Tyukhov, I.I. and Tivanov, M.S. and Fettkenhauer, C. and Shvartsman, V.V. and Giraitis, R. and Hagemann, U. and Lupascu, D.C.
    Solar Energy 142-149 (2019)
    Thin film Cu2ZnSnSe4 (CZTSe) solar cells can be grown on flexible and lightweight metal substrates allowing their direct integration on bendable surfaces and where the weight of solar cell is an important criterion. Flexible substrates make it possible to use the roll-to-roll technology of solar cells, which leads to an additional reduction in the cost of production and final cost of solar cells. The CZTSe thin films were fabricated by selenization of electrodeposited metallic precursors onto tantalum (Ta) flexible substrates at different temperature and time. The results of the effect of selenization temperature and time on the morphology, structural, and optical property of the CZTSe films are presented in this work. It was found that the morphology of the CZTSe thin films depend on their elemental composition and time of selenization. Experimental data indicate that composition of the CZTSe films selenized within 10 and 20 min at 560 °C have the CZTSe basic phase and secondary phases (CuSe, SnSe and ZnSe). In contrast, the increase in selenization temperature and/or time leads to disappearing of the secondary phases (CuSe, SnSe) and better crystallization of the CZTSe films. It was found that films selenized at 560 and 580 °C within the same time have similar characteristics. Depending on selenization time and temperature of the CZTSe, thin films exhibited a shift in band gap from 1.16 to 1.19 and to 1.22 eV, respectively. The change of band gap of the CZTSe thin films is associated with changes of elemental and phase compositions, and thickness of the film. These results showed that the received CZTSe films on Ta foil can be used for fabrication of thin film solar cells. © 2018
    view abstractdoi: 10.1016/j.solener.2018.12.025
  • 2019 • 373 Elemental site occupancy in the L12 A3B ordered intermetallic phase in Co-based superalloys and its influence on the microstructure
    Pandey, P. and Makineni, S.K. and Samanta, A. and Sharma, A. and Das, S.M. and Nithin, B. and Srivastava, C. and Singh, A.K. and Raabe, D. and Gault, B. and Chattopadhyay, K.
    Acta Materialia 163 140-153 (2019)
    We explore the effects of the elemental site occupancy in γ′-A3B (L12) intermetallic phases and their partitioning across the γ/γ′ interface in a class of multicomponent W-free Co-based superalloys. Atom probe tomography and first principles density functional theory calculations (DFT) were used to evaluate the Cr site occupancy behavior in the γ′ phase and its effect on the γ/γ′ partitioning behavior of other solutes in a series of Co-30Ni-10Al-5Mo-2Ta-2Ti-XCr alloys, where x is 0, 2, 5, and 8 at.% Cr, respectively. The increase in Cr content from 0 to 2 to 5 at.% leads to an inversion of the partitioning behavior of the solute Mo from the γ′ phase (KMo>1) into the γ matrix (KMo<1). At 5 at.% Cr, the Cr also has a preference to replace the excess anti-site Co atoms from the B-sites. At 8 at.% Cr, the Cr develops an additional preference to replace Co atoms from the A-sites. These compositional changes in the phases and the site partitioning behavior in the γ′ phase are accompanied by an overall decrease in the lattice misfit (δ) across the γ/γ′ interfaces as measured by high-resolution X-ray diffraction at room temperature. The reduction in misfit triggers a change in morphology of the γ′ phase from cuboidal (δ ∼ +0.48% at 0 at.% Cr) to round-cornered (δ ∼ +0.34% at 5 at.% Cr) to spheroidal shaped (δ ∼ +0.19% at 8 at.% Cr) precipitates. We also observed an increase in the solvus temperature from 1066 °C to 1105 °C when adding 5 at.% Cr to the alloy. These results on the effects of Cr in Co-base superalloys enable tuning the microstructure of these alloys and widening the alloy spectrum for designing improved high temperature alloys. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.09.049
  • 2019 • 372 Gradients of microstructure, stresses and mechanical properties in a multi-layered diamond thin film revealed by correlative cross-sectional nano-analytics
    Gruber, D.P. and Todt, J. and Wöhrl, N. and Zalesak, J. and Tkadletz, M. and Kubec, A. and Niese, S. and Burghammer, M. and Rosenthal, M. and Sternschulte, H. and Pfeifenberger, M.J. and Sartory, B. and Keckes, J.
    Carbon 144 666-674 (2019)
    Thin diamond films deposited by chemical vapour deposition (CVD) usually feature cross-sectional gradients of microstructure, residual stress and mechanical properties, which decisively influence their functional properties. This work introduces a novel correlative cross-sectional nano-analytics approach, which is applied to a multi-layered CVD diamond film grown using microwave plasma-enhanced CVD and consisting of a ∼8 μm thick nanocrystalline (NCD) base and a ∼14.5 μm thick polycrystalline (PCD) top diamond sublayers. Complementary cross-sectional 30 nm beam synchrotron X-ray diffraction, depth-resolved micro-cantilever and hardness testing and electron microscopy analyses reveal correlations between microstructure, residual stress and mechanical properties. The NCD sublayer exhibits a 1.5 μm thick isotropic nucleation region with the highest stresses of ∼1.3 GPa and defect-rich nanocrystallites. With increasing sublayer thickness, a 110 fibre texture evolves gradually, accompanied by an increase in crystallite size and a decrease in stress. At the NCD/PCD sublayer interface, texture, stresses and crystallite size change abruptly and the PCD sublayer exhibits the presence of Zone T competitive grain growth microstructure. NCD and PCD sublayers differ in fracture stresses of ∼14 and ∼31 GPa, respectively, as well as in elastic moduli and hardness, which are correlated with their particular microstructures. In summary, the introduced nano-analytics approach provides complex correlations between microstructure, stresses, functional properties and deposition conditions. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.carbon.2018.12.093
  • 2019 • 371 Influence of Ag on antibacterial performance, microstructure and phase transformation of NiTi shape memory alloy coatings
    Momeni, S. and Tillmann, W.
    Vacuum 164 242-245 (2019)
    Shape memory binary NiTi and ternary NiTiAg coatings were deposited by means of magnetron sputtering technique. The results show how simultaneous sputtering of Ag can affect the microstructure, phase transformation behavior and antibacterial properties of NiTi coatings. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.vacuum.2019.02.051
  • 2019 • 370 Influence of Introducing an Organic Pore-Forming Agent on the Porosity and Microstructure of Alumina Coatings Produced by the Atmospheric Plasma Spray Process
    Tillmann, W. and Khalil, O. and Abdulgader, M.
    Journal of Thermal Spray Technology 28 1919-1932 (2019)
    Controlling the amount of porosity that can be added to ceramic coatings produced by means of atmospheric plasma spraying has always been a challenging task, especially, when coatings are produced with high adhesion/cohesion strengths combined with higher deposition efficiency. However, coating porosity can be varied in a limited fashion between 2 and 25% by changing the process parameters. The use of pore-forming agent is, therefore, an adequate alternative to add porosity by using the optimal spraying parameters. In this work, baking flour was vertically inserted into the plasma plume at different positions and with different feed rates while depositing alumina particles, as the basic feedstock powder, using optimized spray parameters. According to the spray conditions used in this work, flour particles have successfully contributed to the obtained coating porosity; however, the effectiveness is higher when flour is inserted at about half of the spray distance and depends on the feed rate of the flour. In comparison with flour-free alumina coatings, the porosity of flour-alumina’s coatings increased by approx. 50 area% and 170 area% when the flour’s weight fraction was equal 10 and 20 wt.% of the feedstock powder, respectively. Adhesion/cohesion strengths, hardness and thickness of coatings were strongly affected. © 2019, ASM International.
    view abstractdoi: 10.1007/s11666-019-00934-5
  • 2019 • 369 Lanthanum tungstate membranes for H 2 extraction and CO 2 utilization: Fabrication strategies based on sequential tape casting and plasma-spray physical vapor deposition
    Ivanova, M.E. and Deibert, W. and Marcano, D. and Escolástico, S. and Mauer, G. and Meulenberg, W.A. and Bram, M. and Serra, J.M. and Vaßen, R. and Guillon, O.
    Separation and Purification Technology 100-112 (2019)
    In the context of energy conversion efficiency and decreasing greenhouse gas emissions from power generation and energy-intensive industries, membrane technologies for H 2 extraction and CO 2 capture and utilization become pronouncedly important. Mixed protonic-electronic conducting ceramic membranes are hence attractive for the pre-combustion integrated gasification combined cycle, specifically in the water gas shift and H 2 separation process, and also for designing catalytic membrane reactors. This work presents the fabrication, microstructure and functional properties of Lanthanum tungstates (La 28−x W 4+x O 54+δ , LaWO) asymmetric membranes supported on porous ceramic and porous metallic substrates fabricated by means of the sequential tape casting route and plasma spray-physical vapor deposition (PS-PVD). Pure LaWO and W site substituted LaWO were employed as membrane materials due to the promising combination of properties: appreciable mixed protonic-electronic conductivity at intermediate temperatures and reducing atmospheres, good sinterability and noticeable chemical stability under harsh operating conditions. As substrate materials porous LaWO (non-substituted), MgO and Crofer22APU stainless steel were used to support various LaWO membrane layers. The effect of fabrication parameters and material combinations on the assemblies’ microstructure, LaWO phase formation and gas tightness of the functional layers was explored along with the related fabrication challenges for shaping LaWO layers with sufficient quality for further practical application. The two different fabrication strategies used in the present work allow for preparing all-ceramic and ceramic-metallic assemblies with LaWO membrane layers with thicknesses between 25 and 60 μm and H 2 flux of ca. 0.4 ml/min cm 2 measured at 825 °C in 50 vol% H 2 in He dry feed and humid Ar sweep configuration. Such a performance is an exceptional achievement for the LaWO based H 2 separation membranes and it is well comparable with the H 2 flux reported for other newly developed dual phase cer-cer and cer-met membranes. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2019.03.015
  • 2019 • 368 Lanthanum tungstate membranes for H2 extraction and CO2 utilization: Fabrication strategies based on sequential tape casting and plasma-spray physical vapor deposition
    Ivanova, M.E. and Deibert, W. and Marcano, D. and Escolástico, S. and Mauer, G. and Meulenberg, W.A. and Bram, M. and Serra, J.M. and Vaßen, R. and Guillon, O.
    Separation and Purification Technology 219 100-112 (2019)
    In the context of energy conversion efficiency and decreasing greenhouse gas emissions from power generation and energy-intensive industries, membrane technologies for H2 extraction and CO2 capture and utilization become pronouncedly important. Mixed protonic-electronic conducting ceramic membranes are hence attractive for the pre-combustion integrated gasification combined cycle, specifically in the water gas shift and H2 separation process, and also for designing catalytic membrane reactors. This work presents the fabrication, microstructure and functional properties of Lanthanum tungstates (La28−xW4+xO54+δ, LaWO) asymmetric membranes supported on porous ceramic and porous metallic substrates fabricated by means of the sequential tape casting route and plasma spray-physical vapor deposition (PS-PVD). Pure LaWO and W site substituted LaWO were employed as membrane materials due to the promising combination of properties: appreciable mixed protonic-electronic conductivity at intermediate temperatures and reducing atmospheres, good sinterability and noticeable chemical stability under harsh operating conditions. As substrate materials porous LaWO (non-substituted), MgO and Crofer22APU stainless steel were used to support various LaWO membrane layers. The effect of fabrication parameters and material combinations on the assemblies’ microstructure, LaWO phase formation and gas tightness of the functional layers was explored along with the related fabrication challenges for shaping LaWO layers with sufficient quality for further practical application. The two different fabrication strategies used in the present work allow for preparing all-ceramic and ceramic-metallic assemblies with LaWO membrane layers with thicknesses between 25 and 60 μm and H2 flux of ca. 0.4 ml/min cm2 measured at 825 °C in 50 vol% H2 in He dry feed and humid Ar sweep configuration. Such a performance is an exceptional achievement for the LaWO based H2 separation membranes and it is well comparable with the H2 flux reported for other newly developed dual phase cer-cer and cer-met membranes. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2019.03.015
  • 2019 • 367 Metastability alloy design
    Raabe, D. and Li, Z. and Ponge, D.
    MRS Bulletin 44 266-272 (2019)
    This article reviews the concept of metastability in alloy design. While most materials are thermodynamically metastable at some stage during synthesis and service, we discuss here cases where metastable phases are not coincidentally inherited from processing, but rather are engineered. Specifically, we aim at compositional (partitioning), thermal (kinetics), and microstructure (size effects and confinement) tuning of metastable phases so that they can trigger athermal transformation effects when mechanically, thermally, or electromagnetically loaded. Such a concept works both at the bulk scale and also at a spatially confined microstructure scale, such as at lattice defects. In the latter case, local stability tuning works primarily through elemental partitioning to dislocation cores, stacking faults, interfaces, and precipitates. Depending on stability, spatial confinement, misfit, and dispersion, both bulk and local load-driven athermal transformations can equip alloys with substantial gain in strength, ductility, and damage tolerance. Examples include self-organized metastable nanolaminates, austenite reversion steels, metastable medium- A nd high-entropy alloys, as well as steels and titanium alloys with martensitic phase transformation and twinning-induced plasticity effects. © 2019 Materials Research Society.
    view abstractdoi: 10.1557/mrs.2019.72
  • 2019 • 366 Micromechanical modelling of the influence of strain ratio on fatigue crack initiation in a martensitic steel-a comparison of different fatigue indicator parameters
    Schäfer, B.J. and Sonnweber-Ribic, P. and ul Hassan, H. and Hartmaier, A.
    Materials 12 (2019)
    Micromechanical fatigue lifetime predictions, in particular for the high cycle fatigue regime, require an appropriate modelling of mean stress effects in order to account for lifetime reducing positive mean stresses. Focus of this micromechanical study is the comparison of three selected fatigue indicator parameters (FIPs), with respect to their applicability to different total strain ratios. In this work, investigations are performed on the modelling and prediction of the fatigue crack initiation life of the martensitic high-strength steel SAE 4150 for two different total strain ratios. First, multiple martensitic statistical volume elements (SVEs) are generated by multiscale Voronoi tessellations. Micromechanical fatigue simulations are then performed on these SVEs by means of a crystal plasticity model to obtain microstructure dependent fatigue responses. In order to account for the material specific fatigue damage zone, a non-local homogenisation scheme for the FIPs is introduced for lath martensitic microstructures. The numerical results of the different non-local FIPs are compared with experimental fatigue crack initiation results for two different total strain ratios. It is concluded that the multiaxial fatigue criteria proposed by Fatemi-Socie is superior for predicting fatigue crack initiation life to the energy dissipation criteria and the accumulated plastic slip criteria for the investigated total strain ratios. © 2019 by the authors.
    view abstractdoi: 10.3390/ma12182852
  • 2019 • 365 Microstructural Analysis of Powder Metallurgy Tool Steels in the Context of Abrasive Wear Behavior: A New Computerized Approach to Stereology
    Benito, S. and Wulbieter, N. and Pöhl, F. and Theisen, W.
    Journal of Materials Engineering and Performance 28 2919-2936 (2019)
    The present work describes a new methodology designed to characterize the microstructures of tool steels containing carbide hard phases, with the focus set on their abrasive wear resistance. A series of algorithms were designed and implemented in MATLAB® to (i) recognize each of the features of interest, (ii) measure relevant quantities and (iii) characterize each of the phases and the alloy in function of attributes usually neglected in wear description applications: size distribution, shape and contiguity of the hard phases. The new framework incorporates new parameters to describe each one of these attributes, as observed in SEM micrographs. All three aforementioned stages contain novel contributions that can be potentially beneficial to the field of materials design in general and to the field of alloy design for severely abrasive environments in particular. Models of known geometry and micrographs of different powder metallurgy steels were analyzed, and the obtained results were compared with the obtained by the linear intercept method. The relation between the new parameters and the ones available in the scientific literature is also discussed. © 2019, ASM International.
    view abstractdoi: 10.1007/s11665-019-04036-9
  • 2019 • 364 Microstructure and mechanical properties of as-built and heat-treated electron beam melted Ti–6Al–4V
    Syed, A.K. and Awd, M. and Walther, F. and Zhang, X.
    Materials Science and Technology (United Kingdom) 35 653-660 (2019)
    This paper investigates the effect of post-deposition heat treatment on porosity, microstructure, and mechanical properties of Ti–6Al–4V produced via an Electron Beam Melting process. Samples were studied in the conditions of as-built and heat treated at 920°C and 1030°C. The as-built samples were characterised by columnar β grains consists of α+β microstructure with Widmanstätten and colony morphologies were found. Heat treatment resulted in increased α lath width. The yield strength and ultimate tensile strength was greater in the as-built condition than wrought material. Porosity re-growth occurred after heat treatment but it did not affect the tensile properties. Greater ductility after heat treatment was attributed to the larger α lath width which increases effective slip length. © 2019, © 2019 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1080/02670836.2019.1580434
  • 2019 • 363 Microstructure, mechanical, and tribological properties of M3:2 high-speed steel processed by selective laser melting, hot-isostatic pressing, and casting
    Geenen, K. and Röttger, A. and Feld, F. and Theisen, W.
    Additive Manufacturing 28 585-599 (2019)
    In this work, the influence of different manufacturing techniques of M3:2 high-speed steel on the resulting microstructure and the associated material properties was investigated. Therefore, microstructure as well as the mechanical and tribological properties of cast steel (with subsequent hot-forming) and steel powder processed by two techniques: hot-isostatic pressing (HIP) and selective laser melting (SLM) were compared. A detailed SLM parameter analysis revealed that the porosity of SLM specimens can be decreased towards a smaller point distance and a longer exposure time (high energy input). A rise in preheating temperature is associated with a reduction in the crack density or the complete avoidance of cracks. In this context, the high-speed steel showed outstanding densification behavior by SLM, even though this steel is considered to be hardly processable by SLM due to its high content of carbon and hard phase-forming elements. In addition, the reusability of steel powder for SLM processing was investigated. The results indicated that multiple reuse is possible, but only in combination with powder processing (mechanical sieving) after each SLM cycle. The microstructure of SLM-densified high-speed steel consists of a cellular, fine dendritic subgrain segregation structure (submicro level) that is not significantly affected by preheating the base plate. The mechanical and tribological properties were examined in relation to the manufacturing technique and the subsequent heat treatment. Our investigations revealed promising behavior with respect to hardness tempering (position of the secondary hardness peak) and tribology of the M3:2 steel processed by SLM compared to the HIP and cast conditions. © 2019
    view abstractdoi: 10.1016/j.addma.2019.05.028
  • 2019 • 362 Microstructure-related Stoneley waves and their effect on the scattering properties of a 2D Cauchy/relaxed-micromorphic interface
    Aivaliotis, A. and Daouadji, A. and Barbagallo, G. and Tallarico, D. and Neff, P. and Madeo, A.
    Wave Motion 90 99-120 (2019)
    In this paper we set up the full two-dimensional plane wave solution for scattering from an interface separating a classical Cauchy medium from a relaxed micromorphic medium. Both media are assumed to be isotropic and semi-infinite to ease the semi-analytical implementation of the associated boundary value problem. Generalized macroscopic boundary conditions are presented (continuity of macroscopic displacement, continuity of generalized tractions and, eventually, additional conditions involving purely microstructural constraints), which allow for the effective description of the scattering properties of an interface between a homogeneous solid and a mechanical metamaterial. The associated “generalized energy flux” is introduced so as to quantify the energy which is transmitted at the interface via a simple scalar, macroscopic quantity. Two cases are considered in which the left homogeneous medium is “stiffer” and “softer” than the right metamaterial and the transmission coefficient is obtained as a function of the frequency and of the direction of propagation of the incident wave. We show that the contrast of the macroscopic stiffnesses of the two media, together with the type of boundary conditions, strongly influence the onset of Stoneley (or evanescent)waves at the interface. This allows for the tailoring of the scattering properties of the interface at both low and high frequencies, ranging from zones of complete transmission to zones of zero transmission well beyond the band-gap region. © 2019
    view abstractdoi: 10.1016/j.wavemoti.2019.04.003
  • 2019 • 361 Multi-scale characterization of austenite reversion and martensite recovery in a cold-rolled medium-Mn steel
    Benzing, J.T. and Kwiatkowski da Silva, A. and Morsdorf, L. and Bentley, J. and Ponge, D. and Dutta, A. and Han, J. and McBride, J.R. and Van Leer, B. and Gault, B. and Raabe, D. and Wittig, J.E.
    Acta Materialia 166 512-530 (2019)
    A medium-Mn steel (Fe-12Mn-3Al-0.05C wt%) was designed using Thermo-Calc ® simulations to balance the fraction and stacking fault energy of reverted austenite. Intercritical annealing for 0.5, 8 and 48 h was carried out at 585 °C to investigate the microstructural evolution. X-ray diffraction (XRD), electron backscatter diffraction (EBSD), 3-dimensional EBSD, energy-dispersive spectroscopy via scanning-transmission electron microscopy (STEM-EDS) and atom probe tomography (APT) enable characterization of phase fraction, grain area, grain morphology and alloy partitioning. An increase in annealing time from 0.5 h to 48 h increases the amount of ultrafine-grained (UFG) reverted austenite from 3 to 40 vol %. EBSD and TEM reveal multiple morphologies of UFG austenite (equiaxed, rod-like and plate-like). In addition, most of the remaining microstructure consists of recovered α′-martensite that resembles the cold-rolled state, as well as a relatively small fraction of UFG ferrite (i.e., only a small amount of martensite recrystallization occurs). Multi-scale characterization results show that the location within the cold-rolled microstructure has a strong influence on boundary mobility and grain morphology during austenite reversion. Results from APT reveal Mn-decoration of dislocation networks and low-angle lath boundaries in the recovered α′-martensite, but an absence of Mn-decoration of defects in the vicinity of austenite grains, thereby promoting recovery. STEM-EDS and APT reveal Mn depletion zones in the ferrite/recovered α′-martensite near austenite boundaries, whereas gradients of C and Mn co-partitioning are visible within some of the austenite grains after annealing for 0.5 h. Relatively flat C-enriched austenite boundaries are present even after 8 h of annealing and indicate certain boundaries possess low mobility. At later stages the growth of austenite followed the local equilibrium (LE) model such that the driving force between two equilibrium phases moves the mobile interface, as confirmed by DICTRA simulations (a Thermo-Calc ® diffusion module). The sequence of austenite reversion is: (i) formation of Mn- and C-enriched face-centered-cubic nuclei from decorated dislocations and/or particles; (ii) co-partitioning of Mn and C and (iii) growth of austenite controlled by the LE mode. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.01.003
  • 2019 • 360 Non-destructive testing derived parameters for microstructure-based residual service life assessment of aging metallic materials in nuclear engineering
    Acosta, R. and Boller, C. and Starke, P. and Jamrozy, M. and Knyazeva, M. and Walther, F. and Heckmann, K. and Sievers, J. and Schopf, T. and Weihe, S.
    Materialpruefung/Materials Testing 61 1029-1038 (2019)
    Metallic components in nuclear engineering are exposed to extensive loads such as pressurization and temperature changes which can affect the properties of the material significantly depending on the load spectrum applied. In view of developing a procedure to evaluate the residual service life of metallic components in nuclear power plants aged during service, metastable austenitic steel AISI 347 (German designation: X6CrNiNb18-10) has been considered as an example. To this purpose, total strain-controlled fatigue tests were carried out under different environmental conditions and monitored by continuously measuring thermometric, resistometric, electromagnetic and electrochemical parameters. These parameters provide an information gain in terms of material characterization when compared to conventional strain measurements. Based on these parameters, the short time evaluation procedure StrainLife has been developed, which allows the determination of local S-N curves with a significantly reduced effort as compared with traditional procedures. This method has been implemented into the structural simulation program PROST for the integrity assessment of the components while considering local fatigue properties. This very effective method allows for the determination of local fatigue properties including the strain-specific local scatter of the metallic microstructure properties of the material which has not been possible by traditional means. © Carl Hanser Verlag GmbH & Co. KG
    view abstractdoi: 10.3139/120.111417
  • 2019 • 359 Numerical Study on Particle–Gas Interaction Close to the Substrates in Thermal Spray Processes with High-Kinetic and Low-Pressure Conditions
    Mauer, G.
    Journal of Thermal Spray Technology 28 27-39 (2019)
    In thermal spray processes, the interaction between the gas jet and the particulate feedstock can affect the coating build-up mechanisms considerably. In particular under high-kinetic and low-pressure conditions, small particles are subjected to rapid deflection and velocity changes close to the substrate. In this work, numerical studies were carried out to investigate the interaction between gas and particles in the substrate boundary layers (BL). Typical conditions for suspension plasma spraying (SPS), plasma spray-physical vapor deposition (PS-PVD), and aerosol deposition (AD) were taken as a basis. Particular importance was attached to the consideration of rarefaction and compressibility effects on the drag force. Typical Stokes numbers for the different thermal spray processes were calculated and compared. Possible effects on the resulting coating build-up mechanisms and microstructure formation are discussed. The results show that just for larger particles in the SPS process the laminar flow attached to the particles begins to separate so that the drag coefficients have to be corrected. Furthermore, slip effects occur in all the investigated processes and must be considered. The comparison of calculated Stokes numbers with critical values shows that there is a disposition to form columnar microstructures or stacking effects depending on the particle size for PS-PVD and SPS, but not for AD. © 2018, ASM International.
    view abstractdoi: 10.1007/s11666-018-0810-3
  • 2019 • 358 On the effects of microstructure on the mechanical properties of open-pore Al–11Zn foams
    Matz, A.M. and Matz, B.S. and Parsa, A.B. and Jost, N. and Eggeler, G.
    Materials Science and Engineering A 759 552-564 (2019)
    The mechanical properties of investment casted open-pore metal foams have been investigated on the example of the binary alloy Al–11Zn. The samples were subjected to different cooling conditions subsequent to casting and to different homogenization and ageing treatments. Variation in cooling was done either by quenching the mold in water or slowly cooling it in air. Homogenization and ageing varied in terms of temperature and time. The effects of the different treatments were investigated through microstructural and mechanical characterization methods. Using TEM, we found that the presence of GP zones and their morphological arrangement are the main factors dominating the mechanical performance. Micro- and nanoindentation testing of single foam struts reveal maximum hardness H when room temperature ageing was applied. Ageing at a temperature of 150 °C results in the lowest H in the present study; that is approximately 2/3 of the hardness achieved when ageing at room temperature. This can also be confirmed by the strength of non-porous bulk material obtained by tensile tests, which further show an increase in ductility up to a factor of 5 due to ageing at elevated temperatures. By compression testing of open-pore Al–11Zn foams, we notice that the presence of the microstructural effects varies in extent as a function of the strain ε. At low strains, we observe differences in mechanical performance to a high extent, becoming less with increasing compaction of the samples until they behave as non-porous bulk material. Based on these findings, we deduce a strong interaction of the structural morphology of the foam and its microstructure that determines the mechanical properties dominated by strength and ductility of the base material. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2019.05.087
  • 2019 • 357 On the influence of the heat treatment on microstructure formation and mechanical properties of near-α Ti-Fe alloys
    Sandlöbes, S. and Korte-Kerzel, S. and Raabe, D.
    Materials Science and Engineering A 748 301-312 (2019)
    We study the microstructure formation and mechanical properties of Ti-1Fe (wt%) and Ti-3Fe (wt%) alloys for different heat treatments in the β-phase and α + β-phase regions. By applying different heat treatment routes, we observe different microstructure formation mechanisms causing a wide range of mechanical properties from high strength (1.3 GPa) and low ductility (2%) to intermediate strength (700 MPa) and high ductility (30%) in these simple binary alloys. We performed microstructure characterizsation using scanning electron microscopy, transmission electron microscopy and atom probe tomography to show that the alloying content and heat treatment significantly affect the local martensitic and / or diffusional phase transformations causing the substantial changes in the mechanical behavior. © 2018
    view abstractdoi: 10.1016/j.msea.2018.12.071
  • 2019 • 356 On the numerical modeling of nucleation and growth of microstructurally short cracks in polycrystals under cyclic loading
    Boeff, M. and Hassan, H.U. and Hartmaier, A.
    Journal of Materials Research (2019)
    In the scope of this work, a micromechanical model based on the crystal plasticity finite element method is proposed and applied to describe the nucleation and growth of microstructurally short fatigue cracks in polycrystalline materials under cyclic loads. The microstructure is generated in the form of a representative volume element of a polycrystalline material with equiaxed grains having columnar structure along thickness and random crystallographic texture. With this model, we investigate the influence of loading amplitude on the crack growth behavior. It is shown that for smaller strain amplitudes, a single crack nucleates and propagates, while for larger strain amplitudes several independent crack nucleation sites form, from which microcracks start propagating. It is also observed that the global plastic strain amplitude decreases from the initial to the final cycle, during total strain-controlled loading. However, this can even increase the crack growth rate because the crack advance is governed by the local plastic slip which accumulates at the crack tip over the number of cycles. With this work, it is shown that micromechanical modeling can strongly improve our understanding of the mechanisms of short-crack nucleation and growth under fatigue loading. © 2019 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2019.270
  • 2019 • 355 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 • 354 Phase-field simulation of martensite microstructure in low-carbon steel
    Shchyglo, O. and Du, G. and Engels, J.K. and Steinbach, I.
    Acta Materialia 175 415-425 (2019)
    We present three-dimensional phase-field simulations of martensite microstructure formation in low-carbon steel. In this study, a full set of 24 Kurdjumov-Sachs symmetry variants of martensite is considered. Three different carbon compositions are investigated in order to reveal the effect of carbon content on the martensite microstructure formation. The simulations are performed using the finite strain framework which allows considering real martensite transformation strains. Using Neuber elasto-plastic approximation to the mechanical equilibrium solution, realistic stresses and strains can be obtained during martensite formation resulting in realistic mechanical driving forces for the transformation. The simulated microstructures are compared to experimental results for three carbon compositions. Good agreement between simulated and experimental results is achieved. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.06.036
  • 2019 • 353 Processing of gas-nitrided AISI 316L steel powder by laser powder bed fusion – Microstructure and properties
    Boes, J. and Röttger, A. and Becker, L. and Theisen, W.
    Additive Manufacturing 30 (2019)
    This work investigated the processing of high nitrogen-alloyed austenitic stainless steels by laser powder bed fusion (L-PBF). Prior to L-PBF processing, the AISI 316 L steel powder was nitrided at a temperature of 675°C in a 3 bar nitrogen atmosphere, thus achieving a N content of 0.58 mass-%. By mixing nitrided 316 L powder with untreated 316 L powder, two different powder mixtures were obtained with 0.065 mass-% and 0.27 mass-% nitrogen, respectively. After nitriding and mixing, the powder was characterized in terms of its flow properties and chemical composition. The nitrided steel powder was then processed by L-PBF, and the microstructure as well as the chemical composition were investigated by means of scanning electron microscopy and carrier gas hot extraction. It was shown that nitriding of steel powders in an N2 atmosphere can be used to significantly increase the nitrogen content of the powder without impairing its flow properties. With increasing nitrogen content of the powder, the porosity within the L-PBF built specimens increased. However, both the yield strength and the tensile strength were greatly improved without a marked reduction in the elongation at fracture of the respective steels. This work shows that nitrogen-alloyed austenitic stainless steels can be processed by L-PBF and the mechanical properties can be improved. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2019.100836
  • 2019 • 352 Quantification of uncertain macroscopic material properties resulting from variations of microstructure morphology based on statistically similar volume elements: application to dual-phase steel microstructures
    Miska, N. and Balzani, D.
    Computational Mechanics 64 1621-1637 (2019)
    A method to quantify uncertain macroscopic material properties resulting from variations of a material’s microstructure morphology is proposed. Basis is the computational homogenization of virtual experiments as part of a Monte-Carlo simulation to obtain the associated uncertain macroscopic material properties. A new general approach is presented to construct a set of artificial microstructures, which exhibits a statistically similar variation of the morphology as the real material’s microstructure. The individual artificial microstructures are directly constructed in a way that a lower discretization effort is required compared to real microstructures. The costs to perform the computational homogenization for all considered SSVEs are reduced by an adapted form of the Finite Cell concept and by applying the multilevel Monte-Carlo method. As an illustrative example, the proposed method is applied to a real Dual-Phase steel microstructure. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s00466-019-01738-8
  • 2019 • 351 Relaxed micromorphic model of transient wave propagation in anisotropic band-gap metastructures
    Barbagallo, G. and Tallarico, D. and D'Agostino, M.V. and Aivaliotis, A. and Neff, P. and Madeo, A.
    International Journal of Solids and Structures 162 148-163 (2019)
    In this paper, we show that the transient waveforms arising from several localised pulses in a micro-structured material can be reproduced by a corresponding generalised continuum of the relaxed micromorphic type. Specifically, we compare the dynamic response of a bounded micro-structured material to that of bounded continua with special kinematic properties: (i) the relaxed micromorphic continuum and (ii) an equivalent Cauchy linear elastic continuum. We show that, while the Cauchy theory is able to describe the overall behaviour of the metastructure only at low frequencies, the relaxed micromorphic model goes far beyond by giving a correct description of the pulse propagation in the frequency band-gap and at frequencies intersecting the optical branches. In addition, we observe a computational time reduction associated with the use of the relaxed micromorphic continuum, compared to the sensible computational time needed to perform a transient computation in a micro-structured domain. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijsolstr.2018.11.033
  • 2019 • 350 Repair of Ni-based single-crystal superalloys using vacuum plasma spray
    Kalfhaus, T. and Schneider, M. and Ruttert, B. and Sebold, D. and Hammerschmidt, T. and Frenzel, J. and Drautz, R. and Theisen, W. and Eggeler, G. and Guillon, O. and Vassen, R.
    Materials and Design 168 (2019)
    Turbine blades in aviation engines and land based gas-turbines are exposed to extreme environments. They suffer damage accumulation associated with creep, oxidation and fatigue loading. Therefore, advanced repair methods are of special interest for the gas-turbine industry. In this study, CMSX-4 powder is sprayed by Vacuum Plasma Spray (VPS) on single-crystalline substrates with similar compositions. The influence of the substrate temperature is investigated altering the temperature of the heating stage between 850 °C to 1000 °C. Different spray parameters were explored to identify their influence on the microstructure. Hot isostatic pressing (HIP) featuring fast quenching rates was used to minimize porosity and to allow for well-defined heat-treatments of the coatings. The microstructure was analysed by orientation imaging scanning electron microscopy (SEM), using electron backscatter diffraction (EBSD). The effects of different processing parameters were analysed regarding their influence on porosity and grain size. The results show that optimized HIP heat-treatments can lead to dense coatings with optimum γ/γ′ microstructure. The interface between the coating and the substrate is oxide free and shows good mechanical integrity. The formation of fine crystalline regions as a result of fast cooling was observed at the single-crystal surface, which resulted in grain growth during heat-treatment in orientations determined by the crystallography of the substrate. © 2019
    view abstractdoi: 10.1016/j.matdes.2019.107656
  • 2019 • 349 Revealing fracture mechanisms of medium manganese steels with and without delta-ferrite
    Sun, B. and Palanisamy, D. and Ponge, D. and Gault, B. and Fazeli, F. and Scott, C. and Yue, S. and Raabe, D.
    Acta Materialia 164 683-696 (2019)
    Medium Mn steels possess a composite like microstructure containing multiple phase constituents like metastable austenite, ferrite, δ-ferrite and α′-martensite with a wide range of fractions for each constituent. The high mechanical contrast among them and the deformation-driven evolution of the microstructure lead to complex fracture mechanisms. Here we investigate tensile fracture mechanisms of medium Mn steels with two typical types of microstructures. One group consists of ferrite (α) plus austenite (γ) and the other one of a layered structure with an austenite-ferrite constituent and δ-ferrite. Samples with the first type of microstructure show a dimple-type fracture due to void formation primarily at the ferrite/strain-induced α′-martensite (α′) interfaces. In contrast, the fracture surface of δ-ferrite containing steels shows a combination of cleavage in δ-ferrite and dimple/quasi-cleavage zones in the γ-α (or γ/α′-α) constituent. The embrittlement of δ-ferrite is due to the formation of B2 ordered phase. Failure of these samples is govern by crack initiation related to δ-ferrite and crack-arresting ability of the γ-α layers. Austenite stability is critical for the alloys' fracture resistance, in terms of influencing void growth and coalescence for the first type of microstructure and crack initiation and termination for the microstructure containing δ-ferrite. This effect is here utilized to increase ductility and toughness. By tailoring austenite stability towards higher fracture resistance, the total elongation of δ-ferrite containing steels increases from ∼13% to ∼33%. This approach opens a new pathway towards an austenite-stability-controlled microstructural design for substantially enhanced damage tolerance in steels containing metastable austenite and δ-ferrite. © 2018
    view abstractdoi: 10.1016/j.actamat.2018.11.029
  • 2019 • 348 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 • 347 Simulation of magnetised microstructure evolution based on a micromagnetics-inspired FE framework: application to magnetic shape memory behaviour
    Buckmann, K. and Kiefer, B. and Bartel, T. and Menzel, A.
    Archive of Applied Mechanics 89 1085-1102 (2019)
    Microstructure evolution in magnetic materials is typically a non-local effect, in the sense that the behaviour at a material point depends on the magnetostatic energy stored within the demagnetisation field in the entire domain. To account for this, we propose a finite element framework in which the internal state variables parameterising the magnetic and crystallographic microstructure are treated as global fields, optimising a global potential. Contrary to conventional micromagnetics, however, the microscale is not spatially resolved and exchange energy terms are neglected in this approach. The influence of microstructure evolution is rather incorporated in an effective manner, which allows the computation of meso- and macroscale problems. This approach necessitates the development and implementation of novel mixed finite element formulations. It further requires the enforcement of inequality constraints at the global level. To handle the latter, we employ Fischer–Burmeister complementarity functions and introduce the associated Lagrange multipliers as additional nodal degrees-of-freedom. As a particular application of this general methodology, a recently established energy-relaxation-based model for magnetic shape memory behaviour is implemented and tested. Special cases—including ellipsoidal specimen geometries—are used to verify the magnetisation and field-induced strain responses obtained from finite element simulations by comparison to calculations based on the demagnetisation factor concept. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s00419-018-1482-7
  • 2019 • 346 Sintering behavior of columnar thermal barrier coatings deposited by axial suspension plasma spraying (SPS)
    Zhou, D. and Malzbender, J. and Sohn, Y.J. and Guillon, O. and Vaßen, R.
    Journal of the European Ceramic Society 39 482-490 (2019)
    During the last decade, Suspension Plasma Spraying (SPS) attracted a lot of interest as an alternative process to produce columnar Thermal Barrier Coatings (TBCs). In this study, columnar TBCs were deposited with SPS. After spraying, samples were isothermally annealed at 1373 K for 1 h, 3 h, 10 h and 50 h, respectively. Microstructures and mechanical properties of the ceramic coatings were investigated as a function of annealing time. Annealing resulted in healing of micro-cracks, coarsening of pores, growth of domain size, companied with a decrease of porosity within columns. The change of coating microstructure led to change of mechanical properties. In addition, residual stress in SPS coatings was also investigated. Furthermore, as-sprayed coatings and pre-annealed coatings were subjected to burner rig tests. Short time pre-annealing allowed to enhance thermal cycling lifetime of such SPS coatings. The thermal cycling results were related to microstructure modifications of coatings. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.jeurceramsoc.2018.09.020
  • 2019 • 345 Structural and mechanical properties of carbon incorporation in DC/HiPIMS CrAlN coatings
    Tillmann, W. and Stangier, D. and Roese, P. and Shamout, K. and Berges, U. and Westphal, C. and Debus, J.
    Surface and Coatings Technology 374 774-783 (2019)
    Incorporating carbon into ternary nitride coatings to tune the mechanical and tribological properties of thin films is of great interest in order to improve the performance of tools and components. Especially, the approach to tailor CrAlN coatings by doping transition metals has been extensively studied in recent years. Nevertheless, the microstructural changes, induced by carbon incorporation, especially into Al-rich CrAlN coatings, are not yet fully understood. Thus, detailed investigations of the microstructure, performed by means of synchrotron radiation, using x-ray diffraction and x-ray photoelectron spectroscopy and Raman scattering with different laser excitation (355 nm and 532 nm), were conducted to understand the evolution of the mechanical properties of CrAlCN coatings depending on the carbon content. The results prove that an increasing carbon content significantly influences the microstructure, residual stresses, as well as the mechanical properties of the coatings. The presence of C[dbnd]C and C[dbnd]N bonds was proven by investigating the C 1s orbital. Furthermore, the increasing amount of carbon forms amorphous Cr[sbnd]C structures, which were detected by analyzing the Cr 3p orbital. These results were confirmed for the amorphous phases by Raman scattering additionally indicating the formation of nanocomposite structures due to the formation of carbon nano-onoin like structures. The investigations of the crystalline structure using XRD reveal the existence of a fcc structure for the CrAlN phase as well as small amounts of hexagonal AlN in the CrAlCN coating with the highest carbon content. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2019.06.053
  • 2019 • 344 Structure and mechanical properties of hafnium nitride films deposited by direct current, mid-frequency, and high-power impulse magnetron sputtering
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Tolan, M. and Paulus, M.
    Thin Solid Films 669 65-71 (2019)
    The structural properties of hafnium nitride films are mainly influenced by the deposition conditions, which are affected by the sputtering technique. A suitable use of the different sputtering modes allows to control the structural development of the films and thus to adjust the profile of the properties. NaCl-type hafnium nitride films were deposited using direct current magnetron sputtering (dcMS), mid-frequency magnetron sputtering (mfMS), and high-power impulse magnetron sputtering (HiPIMS). dcMS produces films with a columnar microstructure, whereas a fully-dense morphology is achieved by mfMS and HiPIMS. X-ray diffraction patterns show that films sputtered in dcMS mode have a (200) orientation, whereas mfMS and HiPIMS favor an orientation with the (111) plane parallel to the samples’ surface. mfMS leads to films with the largest crystal sizes and lowest stresses, which is ascribed to recrystallization mechanisms during the film growth. Hafnium films with an overstoichiometric composition show the highest hardness values. In this context, the dcMS-Hf49.8N50.2, mfMS-Hf50.4N49.6, and HiPIMS-Hf49.0N51.0 have a hardness of 28.2 ± 2.1, 32.4 ± 3.4, and 30.4 ± 3.1 GPa, respectively. In summary, the sputtering technique has a crucial role on the properties of the film and can be suitable used to adjust the structure and hardness of HfN films. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2018.10.035
  • 2019 • 343 Synthesis, microstructure, and hardness of rapidly solidified Cu-Cr alloys
    Garzón-Manjón, A. and Christiansen, L. and Kirchlechner, I. and Breitbach, B. and Liebscher, C.H. and Springer, H. and Dehm, G.
    Journal of Alloys and Compounds 794 203-209 (2019)
    Cu-Cr alloys with ∼32 at.% Cr were rapidly solidified by splat quenching or laser melting techniques with the intention to form a very fine grained, non-equilibrium nanostructure similar to those obtained by severe plastic deformation or thin film deposition. The rapidly solidified Cu-Cr alloys were analyzed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Both synthesis techniques lead to a similar two-phase microstructure with a nearly pure fcc Cu matrix with μm grain sizes and bcc Cr particles highly supersaturated with Cu. Splat quenching provides finer bcc particles with dimensions of less than 50 nm compared to laser melting with particle sizes of 100–2000 nm. In case of laser melting, (14 ± 2) at.% Cu are contained in the bcc phase, while splat quenching freezes (20 ± 2) at.% Cu in the bcc particles. The microstructures are discussed and compared to the non-equilibrium microstructures reported in literature using severe plastic deformation and thin films deposition. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2019.04.209
  • 2019 • 342 Temperature dependence of elastic moduli in a refractory HfNbTaTiZr high-entropy alloy
    Laplanche, G. and Gadaud, P. and Perrière, L. and Guillot, I. and Couzinié, J.P.
    Journal of Alloys and Compounds 799 538-545 (2019)
    The equiatomic HfNbTaTiZr solid solution is currently regarded as a model disordered body-centered cubic high-entropy alloy. Therefore, the temperature dependence of its elastic moduli is of prime importance to improve our understanding of the mechanical properties of this refractory alloy. In this study, the alloy was found to be single phase, fully recrystallized with a slight texture along the normal direction after thermomechanical processing at room temperature. Elastic moduli were determined over the temperature range [293 K-1100 K]. © 2019 The Authors
    view abstractdoi: 10.1016/j.jallcom.2019.05.322
  • 2019 • 341 Understanding the role of cu and clustering on strain hardening and strain rate sensitivity of al-mg-si-cu alloys
    Langille, M. and Diak, B.J. and De Geuser, F. and Guiglionda, G. and Meddeb, S. and Zhao, H. and Gault, B. and Raabe, D. and Deschamps, A.
    Minerals, Metals and Materials Series 143-151 (2019)
    Increased demand for light-weighting in passenger vehicles has created a need for strong, light, ductile materials to be used in body-in-white applications. The AA6xxx-series of aluminum alloys are suitable candidates meeting most requirements but can fall short of the formability demands of designers, necessitating an understanding of what controls the formability in this alloy series. This work examines the effect of copper alloying in AA6xxx on the pre-ageing and natural ageing responses of the microstructure and mechanical properties. The changes in microstructure observed by differential scanning calorimetry and hardness testing are related to the work-hardening and strain-rate sensitivity parameters for these alloys measured by tensile testing. An observed asymmetry in the measured strain-rate sensitivity associated with increasing versus decreasing strain rate changes suggests that a different mechanism operates for the two conditions. It is postulated how this asymmetry in strain-rate sensitivity will impact the necking and ductility behaviour of these alloys. © 2019, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/978-3-030-05864-7_20
  • 2019 • 340 Very high-cycle fatigue properties and microstructural damage mechanisms of selective laser melted AlSi10Mg alloy
    Awd, M. and Siddique, S. and Johannsen, J. and Emmelmann, C. and Walther, F.
    International Journal of Fatigue 124 55-69 (2019)
    The influence of cooling rates in selective laser melted AlSi10Mg on fatigue properties is studied. The failure mechanisms during quasistatic and fatigue loading were analyzed using electron microscopy, X-ray computed tomography and ultrasonic fatigue testing systems. It was found that even when densification mechanism was not significantly enhanced, the morphology of defects was adopting more regularly spherical shape which is less critical under structural loading. The controlled cooling introduced microstructural homogeneity and further property enhancement of the microstructure which contributed to higher quasistatic and fatigue strength in this study and compared to the literature on the same alloy. Elimination of semi-coherent Si agglomerates on the melt pool boundaries impeded crack propagation and forced it to adjust to a perpendicular orientation to the columnar dendrites formed instead. The strengthening mechanism was shown effective in quasistatic, servohydraulic and ultrasonic fatigue tests in very high-cycle fatigue. The influence of testing at ultrasonic frequencies was analyzed through the fatigue data and influence was not significant. Analysis of the interrupted fatigue specimens in the X-ray computed tomography revealed a pore-crack interaction mechanism which was hypothesized earlier in the literature. Although impedance of crack propagation and theoretical drop of stress intensity factors at crack tips was expected, it was implied that recommence of crack propagation and the end life of the specimen will still be dependent on the local strength of the microstructure in the vicinity of the pore. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijfatigue.2019.02.040
  • 2018 • 339 A method for the in-situ study of solid-state joining techniques using synchrotron radiation - observation of phase transformations in Ti-6Al-4V after friction surfacing
    Hanke, S. and Staron, P. and Fischer, T. and Fitseva, V. and dos Santos, J.F.
    Surface and Coatings Technology 335 355-367 (2018)
    The solid-state deposition process Friction Surfacing (FS) was applied to Ti-6Al-4V alloy on portable welding equipment at a high-energy synchrotron beamline. The heat input and coating thickness were altered by varying the deposition speed. X-ray diffraction was carried out in-situ during the deposition process and the cooling of the coated samples. Phase transformations were evaluated and correlated with thermal cycles determined by thermocouples and an infrared camera. SEM investigation of the coating microstructure was conducted to examine the morphology of the α phase. During FS the coating material is severely deformed and dynamically recrystallized in the β phase state at temperatures > 1300 °C. Small changes in the β grain size were observed within the first 2 s after deposition only. Depending on the cooling rate it transforms into different types of α phase during cooling. Phase transformation rates were found to correlate well with the differences in α morphology. The two faster translational speeds showed transformation rates > 45 vol%/s and a partially martensitic microstructure. When a thick coating is deposited at low translational speed, α → β transformation continues for several seconds after deposition, followed by a slow cooling rate resulting in martensite free coatings containing α m from massive transformation. The potential gain and the deficiencies of this complex in-situ study of a technical process, instead of simplified model experiments, for the understanding of fundamental mechanisms involved in FS are discussed. © 2017
    view abstractdoi: 10.1016/j.surfcoat.2017.12.049
  • 2018 • 338 A TEM Investigation of Columnar-Structured Thermal Barrier Coatings Deposited by Plasma Spray-Physical Vapor Deposition (PS-PVD)
    Rezanka, S. and Somsen, C. and Eggeler, G. and Mauer, G. and Vaßen, R. and Guillon, O.
    Plasma Chemistry and Plasma Processing 38 791-802 (2018)
    The plasma spray-physical vapor deposition technique (PS-PVD) is used to deposit various types of ceramic coatings. Due to the low operating pressure and high enthalpy transfer to the feedstock, deposition from the vapor phase is very effective. The particular process conditions allow for the deposition of columnar microstructures when applying thermal barrier coatings (TBCs). These coatings show a high strain tolerance similar to those obtained by electron beam-physical vapor deposition (EB-PVD). But compared to EB-PVD, PS-PVD allows significantly reducing process time and costs. The application-related properties of PS-PVD TBCs have been investigated in earlier work, where the high potential of the process was described and where the good resistance to thermo-mechanical loading conditions was reported. But until now, the elementary mechanisms which govern the material deposition have not been fully understood and it is not clear, how the columnar structure is built up. Shadowing effects and diffusion processes are assumed to contribute to the formation of columnar microstructures in classical PVD processing routes. For such structures, crystallographic textures are characteristic. For PS-PVD, however, no crystallographic textures could initially be found using X-ray diffraction. In this work a more detailed TEM investigations and further XRD measurements of the columnar PS-PVD microstructure were performed. The smallest build units of the columnar TBC structure are referred to as sub-columns. The observed semi-single crystal structure of individual sub-columns was analyzed by means of diffraction experiments. The absence of texture in PS-PVD coatings is confirmed and elementary nucleation and growth mechanisms are discussed. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstractdoi: 10.1007/s11090-018-9898-y
  • 2018 • 337 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 • 336 Combinatorial metallurgical synthesis and processing of high-entropy alloys
    Li, Z. and Ludwig, Al. and Savan, A. and Springer, H. and Raabe, D.
    Journal of Materials Research 1-14 (2018)
    High-entropy alloys (HEAs) with multiple principal elements open up a practically infinite space for designing novel materials. Probing this huge material universe requires the use of combinatorial and high-throughput synthesis and processing methods. Here, we present and discuss four different combinatorial experimental methods that have been used to accelerate the development of novel HEAs, namely, rapid alloy prototyping, diffusion-multiples, laser additive manufacturing, and combinatorial co-deposition of thin-film materials libraries. While the first three approaches are bulk methods which allow for downstream processing and microstructure adaptation, the latter technique is a thin-film method capable of efficiently synthesizing wider ranges of composition and using high-throughput measurement techniques to characterize their structure and properties. Additional coupling of these high-throughput experimental methodologies with theoretical guidance regarding specific target features such as phase (meta)stability allows for effective screening of novel HEAs with beneficial property profiles. Copyright © Materials Research Society 2018
    view abstractdoi: 10.1557/jmr.2018.214
  • 2018 • 335 Computationally Efficient Phase-field Simulation Studies Using RVE Sampling and Statistical Analysis
    Schwarze, C. and Darvishi Kamachali, R. and Kühbach, M. and Mießen, C. and Tegeler, M. and Barrales-Mora, L. and Steinbach, I. and Gottstein, G.
    Computational Materials Science 147 204-216 (2018)
    For large-scale phase-field simulations, the trade-off between accuracy and computational cost as a function of the size and number of simulations was studied. For this purpose, a large reference representative volume element (RVE) was incrementally subdivided into smaller solitary samples. We have considered diffusion-controlled growth and early ripening of δ′ (Al3Li) precipitate in a model Al-Li system. The results of the simulations show that decomposition of reference RVE can be a valuable computational technique to accelerate simulations without a substantial loss of accuracy. In the current case study, the precipitate number density was found to be the key controlling parameter. For a pre-set accuracy, it turned out that large-scale simulations of the reference RVE can be replaced by simulating a combination of smaller solitary samples. This shortens the required simulation time and improves the memory usage of the simulation considerably, and thus substantially increases the efficiency of massive parallel computation for phase-field applications. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2018.02.005
  • 2018 • 334 Damage mechanisms and mechanical properties of high-strength multiphase steels
    Heibel, S. and Dettinger, T. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    Materials 11 (2018)
    The usage of high-strength steels for structural components and reinforcement parts is inevitable for modern car-body manufacture in reaching lightweight design as well as increasing passive safety. Depending on their microstructure these steels show differing damage mechanisms and various mechanical properties which cannot be classified comprehensively via classical uniaxial tensile testing. In this research, damage initiation, evolution and final material failure are characterized for commercially produced complex-phase (CP) and dual-phase (DP) steels in a strength range between 600 and 1000 MPa. Based on these investigations CP steels with their homogeneous microstructure are characterized as damage tolerant and hence less edge-crack sensitive than DP steels. As final fracture occurs after a combination of ductile damage evolution and local shear band localization in ferrite grains at a characteristic thickness strain, this strain measure is introduced as a new parameter for local formability. In terms of global formability DP steels display advantages because of their microstructural composition of soft ferrite matrix including hard martensite particles. Combining true uniform elongation as a measure for global formability with the true thickness strain at fracture for local formability the mechanical material response can be assessed on basis of uniaxial tensile testing incorporating all microstructural characteristics on a macroscopic scale. Based on these findings a new classification scheme for the recently developed high-strength multiphase steels with significantly better formability resulting of complex underlying microstructures is introduced. The scheme overcomes the steel designations using microstructural concepts, which provide no information about design and production properties. © 2018 by the authors.
    view abstractdoi: 10.3390/ma11050761
  • 2018 • 333 Decreasing the Actuation Voltage in Electrowetting on Dielectric with Thin and Micro-Structured Dielectric
    Turk, S. and Verheyen, E. and Viga, R. and Allani, S. and Jupe, A. and Vogt, H.
    PRIME 2018 - 14th Conference on Ph.D. Research in Microelectronics and Electronics 205-208 (2018)
    This work presents the analysis of the minimum actuation voltage Vmin for droplet actuation with electrowetting on dielectric (EWOD). First, the fundamentals of electrowetting are described. In the second chapter, the impact on the actuation voltage in EWOD is shown by a dielectric deposited with atomic layer deposition (ALD) and micro-structured surface. In the last part, results of a simulation with COMSOL MultiphysicsOR are presented to verify the hypothesis and a short discussion about the results is given. © 2018 IEEE.
    view abstractdoi: 10.1109/PRIME.2018.8430346
  • 2018 • 332 Densification of nanocrystalline NdFeB magnets processed by electro-discharge sintering – Microstructure, magnetic, and mechanical properties
    Leich, L. and Röttger, A. and Theisen, W. and Krengel, M.
    Journal of Magnetism and Magnetic Materials 460 454-460 (2018)
    This work investigates the densification process of nanocrystalline NdFeB powder by electro-discharge sintering (EDS) and the associated magnetic properties. The EDS technique is used as a fast and energy-saving compaction process for metal powders. A large current is discharged from capacitors into a pre-compacted loose powder, thus resulting in complete compaction. In this study, the microstructure, magnetic, and mechanical properties of the compacted, hard magnetic NdFeB specimens were investigated under variation of the energy EEDS and compression load pEDS. For all specimens, the intrinsic coercivity HcJ decreases on increasing the discharge energy. However, the compaction load has apparently no influence on the coercivity HcJ, whereas the residual induction Br decreases only with increasing discharge energy. An increase in the compression load pEDS causes an increase in the specimens’ density and thus promotes residual induction Br. The applied EDS parameters led to the formation of three different microstructures (insufficiently densified zone, fully densified zone, and remelted zone) along the cross-section of the EDS-densified specimens. Volume fractions of the three different microstructures that form during the EDS process determine the resulting mechanical and magnetic properties of the specimens. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2018.04.035
  • 2018 • 331 Development of high modulus steels based on the Fe – Cr – B system
    Baron, C. and Springer, H. and Raabe, D.
    Materials Science and Engineering A 724 142-147 (2018)
    We present a novel alloy design strategy for cost-efficient high modulus steels with an increased stiffness / mass density ratio. The concept is based on the liquid metallurgy synthesis of Fe – Cr – B based alloys, straightforward processability, and well tuneable mechanical properties via plain heat treatments. The base alloy Fe – 18 Cr – 1.6 B (wt%) contained 14–17 vol% of (Cr,Fe)2B particles of ellipsoidal morphology in a ferritic matrix. Hot rolled materials revealed a specific modulus of 32.8 GPa g−1 cm3, exceeding that of conventional Fe-Cr steels by almost 30%. Mechanical properties obtained are comparable to TiB2 based high modulus steels. Addition of 1 wt% Cu to the base alloy did not interact with the formation, fraction, size and morphology of (Cr,Fe)2B particles, and allowed to mildly increase the strength values by ageing treatments, however at the price of a reduction of the specific modulus. C additions of 0.2 wt% did not affect the (Cr,Fe)2B particle microstructure greatly, but free C dissolved in the matrix enables for the first time to utilize the wide range of microstructures and mechanical properties of established C-containing high strength steels also in high modulus steels. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2018.03.082
  • 2018 • 330 Effect of porosity and eutectics on the high-temperature low-cycle fatigue performance of a nickel-base single-crystal superalloy
    Ruttert, B. and Meid, C. and Mujica Roncery, L. and Lopez-Galilea, I. and Bartsch, M. and Theisen, W.
    Scripta Materialia 155 139-143 (2018)
    This work investigates the separate influence of porosity and γ/γ′-eutectics on the low-cycle fatigue life of a single-crystal Ni-base superalloy at high temperatures. A conventional vacuum furnace heat-treatment but also integrated heat-treatments in a hot isostatic press are applied to produce different material variants of the same alloy. High-resolution electron microscopy revealed that both pores and γ/γ′-eutectics act as crack starters, thus initiating early failure. Moreover, the results indicate that remaining γ/γ′-eutectics can weaken the fatigue resistance even more than pores. Furthermore, the results confirm the beneficial effect of proper integrated hot isostatic pressing heat-treatments on the fatigue performance. © 2018
    view abstractdoi: 10.1016/j.scriptamat.2018.06.036
  • 2018 • 329 Effect of tool wear evolution on chip formation during dry machining of Ti-6Al-4V alloy
    Dargusch, M.S. and Sun, S. and Kim, J.W. and Li, T. and Trimby, P. and Cairney, J.
    International Journal of Machine Tools and Manufacture 126 13-17 (2018)
    The complex microstructure of segmented chips and the changing deformation mechanisms during the machining of the Ti-6Al-4V alloy for a given cutting tool have been explored. Chip geometry and microstructure were investigated for increasing volumes of material removed at a cutting speed at which the tool characteristically develops gradual flank wear. The degree of chip segmentation and deformation mode changed significantly as machining progressed from using a new tool towards a worn tool. Chip formation processes when machining near the end of the cutting tool life is characterised by increasing amounts of twinning formed through both tension and compression. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijmachtools.2017.12.003
  • 2018 • 328 Elastic capsules at liquid-liquid interfaces
    Hegemann, J. and Boltz, H.-H. and Kierfeld, J.
    Soft Matter 14 5665-5685 (2018)
    We investigate the deformation of elastic microcapsules adsorbed at liquid-liquid interfaces. An initially spherical elastic capsule at a liquid-liquid interface undergoes circumferential stretching due to the liquid-liquid surface tension and becomes lens- or discus-shaped, depending on its bending rigidity. The resulting elastic capsule deformation is qualitatively similar, but distinct from the deformation of a liquid droplet into a liquid lens at a liquid-liquid interface. We discuss the deformed shapes of droplets and capsules adsorbed at liquid-liquid interfaces for a whole range of different surface elasticities: from droplets (only surface tension) deforming into liquid lenses, droplets with a Hookean membrane (finite stretching modulus, zero bending modulus) deforming into elastic lenses, to microcapsules (finite stretching and bending modulus) deforming into rounded elastic lenses. We calculate capsule shapes at liquid-liquid interfaces numerically using shape equations from nonlinear elastic shell theory. Finally, we present theoretical results for the contact angle (or the capsule height) and the maximal capsule curvature at the three phase contact line. These results can be used to infer information about the elastic moduli from optical measurements. During capsule deformation into a lens-like shape, surface energy of the liquid-liquid interface is converted into elastic energy of the capsule shell giving rise to an overall adsorption energy gain by deformation. Soft hollow capsules exhibit a pronounced increase of the adsorption energy as compared to filled soft particles and, thus, are attractive candidates as foam and emulsion stabilizers. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8sm00316e
  • 2018 • 327 Experimental-numerical study on strain and stress partitioning in bainitic steels with martensite-austenite constituents
    Fujita, N. and Ishikawa, N. and Roters, F. and Tasan, C.C. and Raabe, D.
    International Journal of Plasticity (2018)
    To achieve safety and reliability in pipelines installed in seismic and permafrost regions, it is necessary to use linepipe materials with high strength and ductility. The introduction of dual-phase steels, e.g., with a bainite and dispersed martensite-austenite (MA) constituent, would provide the necessary ingredients for the improvement of the strain capacity (as required by a new strain-based linepipe design approach) and toughness. To fine-tune the alloy design and ensure these dual-phase steels have the required mechanical properties, an understanding of the governing deformation micromechanisms is essential. For this purpose, a recently developed joint numerical-experimental approach that involves the integrated use of microscopic digital image correlation analysis, electron backscatter diffraction, and multiphysics crystal plasticity simulations with a spectral solver was employed in this study. The local strain and stress evolution and microstructure maps of representative microstructural patches were captured with a high spatial resolution using this approach. A comparison of these maps provides new insights into the deformation mechanism in dual-phase microstructures, especially regarding the influence of the bainite and MA grain size and the MA distribution on the strain localization behavior. © 2018 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijplas.2018.01.012
  • 2018 • 326 Fracture toughness of Mo2BC thin films: Intrinsic toughness versus system toughening
    Soler, R. and Gleich, S. and Kirchlechner, C. and Scheu, C. and Schneider, J.M. and Dehm, G.
    Materials and Design 154 20-27 (2018)
    The fracture behaviour and microstructure evolution of sputtered Mo2BC films as a function of their deposition temperature is studied. Bipolar pulsed direct current magnetron sputtering was used to deposit Mo2BC thin films onto Si (100) wafers at substrate temperatures ranging from 380 to 630 °C. Microstructural characterization by transmission electron microscopy revealed that increasing the deposition temperature induces larger and more elongated grains, and a higher degree of crystallinity, transitioning from a partially amorphous to a fully crystalline film. The intrinsic fracture toughness of the Mo2BC films was studied by focussed ion beam milled micro-cantilever bending tests. A mild dependency of the intrinsic fracture toughness on the substrate deposition temperature was found. Fractograph analysis showed that the fracture behaviour was dominated by intergranular fracture or by fracture within the amorphous regions. Additionally, nanoindentation based fracture toughness measurements were used to probe the fracture behaviour of the Mo2BC/Si system, where residual stresses define the ‘apparent’ fracture toughness of the system. Depending on the substrate deposition temperature either compressive or tensile residual stresses developed in the films. This causes a relative change in the system toughness by up to one order of magnitude. The fracture experiments clearly reveal that notched cantilevers provide intrinsic toughness values of a material, while nanoindentation probes the toughness of the entire coating-substrate system. The combination of both techniques provides valuable design information for enhancing fracture resistance of Mo2BC films. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2018.05.015
  • 2018 • 325 General investigations on processing tool steel X40CrMoV5-1 with selective laser melting
    Krell, J. and Röttger, A. and Geenen, K. and Theisen, W.
    Journal of Materials Processing Technology 255 679-688 (2018)
    The X40CrMoV5-1 (H13) hot work tool steel was densified by selective laser melting (SLM) using different laser parameters and preheating temperatures. The porosity and crack densities of the processed specimen were determined, the resulting microstructure characterized, tempering hardness diagrams recorded and the reusability of the powder assessed. The X40CrMoV5-1 steel showed a good densification behaviour. Relative densities of above 99.5% were obtained. After SLM densification, the specimen showed a fine-grained microstructure, with a cellular arrangement consisting of ferrite and austenite. Although the microstructure did not change with preheating temperature, a decrease in crack density could be observed for higher preheating temperatures. By combining microstructural observations with some simulations, a new model describing the microstructural evolution of SLM-densified X40CrMoV5-1 is suggested. The peak in secondary hardness after tempering SLM-densified X40CrMoV5-1 was observed at higher temperatures compared to the cast reference steel in the same heat treatment condition. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2018.01.012
  • 2018 • 324 High-performance elastocaloric materials for the engineering of bulk- and micro-cooling devices
    Frenzel, J. and Eggeler, G. and Quandt, E. and Seelecke, S. and Kohl, M.
    MRS Bulletin 43 280-284 (2018)
    Pseudoelastic NiTi-based shape-memory alloys (SMAs) have recently received attention as candidate materials for solid-state refrigeration. The elastocaloric effect in SMAs exploits stress-induced martensitic transformation, which is associated with large latent heat. Most importantly, cyclic mechanical loading/unloading provides large adiabatic temperature drops exceeding 25 K at high process efficiencies. This article summarizes the underlying principles, important material parameters and process requirements, and reviews recent progress in the development of pseudoelastic SMAs with large coefficients of performance, as well as very good functional fatigue resistance. The application potential of SMA film and bulk materials is demonstrated for the case of cyclic tensile loading/unloading in prototypes ranging from miniature-scale devices to large-scale cooling units. Copyright © Materials Research Society 2018.
    view abstractdoi: 10.1557/mrs.2018.67
  • 2018 • 323 How the colloid chemistry of precursor electrocatalyst dispersions is related to the polymer electrolyte membrane fuel cell performance
    Bredol, M. and Szydło, A. and Radev, I. and Philippi, W. and Bartholomäus, R. and Peinecke, V. and Heinzel, A.
    Journal of Power Sources 402 15-23 (2018)
    Polymer electrolyte membrane fuel cells (PEMFCs) operating at low temperature (60–80 °C, up to 110 °C) are mostly limited in their performance by the kinetics of the oxygen reduction reaction (ORR), leading to high loadings of platinum (Pt) in the cathode. Pt catalysts are without alternative in numerous industrial applications, and since Pt resources are limited, the associated high costs for low temperature fuel cells are hindering among other factors their commercialization. In order to increase the fraction of electrocatalytically available Pt towards ORR, this work is devoted to the factors responsible for the microstructure of the PEMFC cathodes. Typically, the active layers are coated by processes like spraying, doctor blading, printing etc. Therefore, the final structure actually is strongly dependent on the coating process and the physicochemical properties of the catalyst dispersions used. Selecting commercially available electrocatalysts from Johnson-Matthey and Tanaka as active material and ultrasonically assisted spraying as the coating method, systematic variations of the surface chemistry of the catalyst particles and their influence on catalyst layer morphology and therefore electrical and electrochemical properties of resulting membrane electrode assemblies (MEA) have been investigated. It could be shown, that the colloid–chemical properties of the catalyst dispersions have a profound influence not only on the microstructure of the MEAs but also on the performance under operating conditions. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2018.09.005
  • 2018 • 322 Imprinting and column damage on CoCrMo head taper surfaces in total hip replacements
    Hall, D.J. and McCarthy, S.M. and Ehrich, J. and Urban, R.M. and Fischer, A. and Jacobs, J.J. and Lundberg, H.J. and Pourzal, R.
    ASTM Special Technical Publication STP 1606 131-155 (2018)
    Material degradation within taper junctions of modular total hip replacements remains of great concern. Imprinting and column damage are two damage modes that frequently occur on head taper surfaces. Both can cover large areas and therefore can be associated with significant material loss. It was the purpose of this study to determine the prevalence of imprinting and column damage on a group of retrievals collected at our medical center and to identify damage pathways on selected cases. We asked two research questions: (1) How do CoCrMo stems imprint into CoCrMo heads? (2) Does alloy microstructure influence the column damage pattern in CoCrMo heads? In order to answer these questions, we conducted a retrieval study on moderately to severely worn femoral head taper surfaces that were paired with stems of different materials. All components were viewed under a stereo-light microscope to determine the presence of imprinting and column damage. Selected cases were further studied by means of scanning electron microscope, interferometry, and metallography to determine damage mode and the potential role of alloy microstructure. Our results demonstrated that imprinting is independent of the stem material but highly dependent on its topography. The imprinting process is at least initially driven by fretting and the generation of oxide particles. Column damage on the other hand is highly dependent on the microstructure of wrought CoCrMo alloys, which can exhibit banding resulting from slight alloy segregations that were characterized by molybdenum depletion. Therefore, column damage may be prevented by avoiding banding of the alloy during the thermomechanical processing. This study demonstrates that it is important to consider differences among the occurring degradation mechanisms and to understand how they relate to material and design factors. Copyright © 2018 by ASTM International.
    view abstractdoi: 10.1520/STP160620I70121
  • 2018 • 321 Influence of Feedstock Powder Modification by Heat Treatments on the Properties of APS-Sprayed Al2O3-40% TiO2 Coatings
    Berger, L.-M. and Sempf, K. and Sohn, Y.J. and Vaßen, R.
    Journal of Thermal Spray Technology 27 654-666 (2018)
    The formation and decomposition of aluminum titanate (Al2TiO5, tialite) in feedstock powders and coatings of the binary Al2O3-TiO2 system are so far poorly understood. A commercial fused and crushed Al2O3-40%TiO2 powder was selected as the feedstock for the experimental series presented in this paper, as the composition is close to that of Al2TiO5. Part of that powder was heat-treated in air at 1150 and 1500 °C in order to modify the phase composition, while not influencing the particle size distribution and processability. The powders were analyzed by thermal analysis, XRD and FESEM including EDS of metallographically prepared cross sections. Only a maximum content of about 45 wt.% Al2TiO5 was possible to obtain with the heat treatment at 1500 °C due to inhomogeneous distribution of Al and Ti in the original powder. Coatings were prepared by plasma spraying using a TriplexPro-210 (Oerlikon Metco) with Ar-H2 and Ar-He plasma gas mixtures at plasma power levels of 41 and 48 kW. Coatings were studied by XRD, SEM including EDS linescans of metallographically prepared cross sections, and microhardness HV1. With the exception of the powder heat-treated at 1500 °C an Al2TiO5-Ti3O5 (tialite–anosovite) solid solution Al2−xTi1+xO5 instead of Al2TiO5 existed in the initial powder and the coatings. © 2018, ASM International.
    view abstractdoi: 10.1007/s11666-018-0716-0
  • 2018 • 320 Influence of the deep hole drilling process and sulphur content on the fatigue strength of AISI 4140 steel components
    Nickel, J. and Baak, N. and Biermann, D. and Walther, F.
    Procedia CIRP 71 209-214 (2018)
    When using quenched and tempered steels in the automotive industry or other industrial applications the fatigue behavior is of elementary importance. The surface and subsurface integrity of machined parts is known as one key factor related to the fatigue strength of components and is strongly influenced by the machining process. Especially for the deep hole single lip drilling, a large part of the cutting force is transferred to the bore wall and the surface and subsurface zone are influenced by the interaction between the tool and the workpiece material. In this study, the approach of inserting residual stresses into the bore wall and influencing the microstructure of the bores subsurface in a way, that the components can withstand a higher mechanical and dynamic load, is investigated in detail. In a close cooperation between the authors the parameters of the single-lip drilling process and the workpiece material are correlated with the produced surface quality and subsurface microstructure as well as the resulting fatigue strength of the components. For the comparative characterization of the fatigue behavior, established test procedures of destructive material testing in combination with new non-destructive test methods, such as the Barkhausen noise analysis, are applied and developed further. The results of the fatigue tests in correlation with the results of the Barkhausen noise analysis are used to design a model for predicting the fatigue strength of drilled components. © 2018 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.procir.2018.05.069
  • 2018 • 319 Influence of the microstructure on the cyclic stress-strain behaviour and fatigue life in hypo-eutectic Al-Si-Mg cast alloys
    Tenkamp, J. and Koch, A. and Knorre, S. and Krupp, U. and Michels, W. and Walther, F.
    MATEC Web of Conferences 165 (2018)
    Aluminium alloys are promising candidates for energy-and cost-efficient components in automotive and aerospace industries, due to their excellent strength-to-weight ratio and relatively low cost compared to titanium alloys. As modern cast processing and post-processing, e.g. hot isostatic pressing, result in decreased frequency and size of defects, the weakest link depends on microstructural characteristics, e.g. secondary dendrite arm spacing (SDAS), Si eutectic morphology and α-Al solid solution hardness. Hereby, fatigue investigations of the effect of the microstructure characteristics on the cyclic stress-strain behaviour as well as fatigue mechanisms in the low cycle and high cycle fatigue regime are performed. For this purpose, samples of the aluminium cast alloy EN AC-AlSi7Mg0.3 with different Si eutectic morphology and α-Al solid solution hardness were investigated. To compare the monotonic and cyclic stress-strain curves, quasistatic tensile tests and incremental step tests were performed on two microstructure conditions. The results show that the cyclic loading leads to a hardening of the material compared to monotonic loading. Based on damage parameter Woehler curves, it is possible to predict the damage progression and fatigue life for monotonic and cyclic loading in hypo-eutectic Al-Si-Mg cast alloys by one power law. © The Authors, published by EDP Sciences, 2018.
    view abstractdoi: 10.1051/matecconf/201816515004
  • 2018 • 318 Interaction between laser radiation and metallic powder of 316L austenitic steel during selective laser melting
    Mutke, C. and Geenen, K. and Röttger, A. and Theisen, W.
    Materials Characterization 145 337-346 (2018)
    During selective laser melting (SLM), a complex heat state develops that leads to a characteristic crystallography and microstructure of the processed materials. Depending on the geometry of the processed part, most scan tracks of a new layer, so-called hatches, are located above a dense substrate or already solidified structures whereas others are located above loose powder. This is, inter alia, the case for overhanging structures. Attributable to the lower thermal conductivity of loose powder, temperature gradients and cooling rates of the melt pool differentiate in these areas, resulting in a different microstructural build-up. In this work, the microstructure and the crystallographic orientation of grade 316L austenitic stainless steel processed by SLM was investigated to understand the interaction between the laser radiation and the metallic powder during SLM-processing and to investigate the remelting of a track on a previous SLM-densified track. Single and multiple tracks on a loose bulk powder substrate, as well as single tracks on a dense substrate plate, were generated. A parameter study revealed that high energy densities are necessary to build continuous tracks on a loose bulk powder substrate. In addition, the amount of adhered particles, which are sintered on the fully melted and solidified tracks, is determined in comparison to the melted powder because the sintered particles strongly influence the surface roughness. To understand the microstructure development and, particularly, the influence of adjacent hatches during SLM-processing, investigations on the resulting microstructure and crystallographic orientation of a single track and two connected multiple tracks were carried out. During SLM processing of the tracks, the substrate plate and the solidified structures influence the temperature gradient and cooling rate of the melt pool, thus directionally solidified and elongated grains occur. Furthermore, the solidification is characterized by an epitaxial growth due to a distinct thermal gradient between the melt pool and the surrounding elements. © 2018 Elsevier Inc.
    view abstractdoi: 10.1016/j.matchar.2018.08.061
  • 2018 • 317 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 • 316 Microstructure and mechanical properties in the thin film system Cu-Zr
    Oellers, T. and Raghavan, R. and Chakraborty, J. and Kirchlechner, C. and Kostka, A. and Liebscher, C.H. and Dehm, G. and Ludwig, Al.
    Thin Solid Films 645 193-202 (2018)
    A composition-spread Cu-Zr thin film library with Zr contents from 2.5 up to 6.5 at.% was synthesized by magnetron sputtering on a thermally oxidized Si wafer. The compositional and microstructural variations of the Cu-Zr thin film across the composition gradient were examined using energy dispersive X-ray spectroscopy, X-ray diffraction, and high-resolution scanning and transmission electron microscopy of cross-sections fabricated by focused ion beam milling. Composition-dependent hardness and elastic modulus values were obtained by nanoindentation for measurement areas with discrete Zr contents along the composition gradient. Similarly, the electrical resistivity was investigated by 4-point resistivity measurements to study the influence of Zr composition and microstructural changes in the thin film. Both, the mechanical and electrical properties reveal a significant increase in hardness and resistivity with increasing Zr content. The trends of the mechanical and functional properties are discussed with respect to the local microstructure and composition of the thin film library. © 2017
    view abstractdoi: 10.1016/j.tsf.2017.10.030
  • 2018 • 315 Microstructure and mechanical properties of Al0.7CoCrFeNi high-entropy-alloy prepared by directional solidification
    Liu, G. and Liu, L. and Liu, X. and Wang, Z. and Han, Z. and Zhang, G. and Kostka, A.
    Intermetallics 93 93-100 (2018)
    The high-entropy-alloy Al0.7CoCrFeNi (molar ratio) was prepared by vacuum arc melting followed by directional solidification (DS) with <001> oriented seed. The unique lamellar-dendrite microstructure was obtained over a wide cooling rate range. During solidification, Fe and Co are prone to segregate to the dendrite, while Cr and Al segregate to interdendrite. The solute pile-up of Cr and Al at the solid/liquid interface leads to the dendritic solidification. During the following cooling process, the BCC phase precipitates from the FCC dendrite to form the lamellar structure, while the ordered B2 phase precipitates from the interdendrite. Moreover, the lamellar spacing is significantly refined with increasing cooling rate, resulting in the higher hardness and compressive yield strength. Directional solidification is proved to be an efficient way to improve the mechanical properties of multi-phases high-entropy alloys. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.intermet.2017.11.019
  • 2018 • 314 Modeling phononic crystals via theweighted relaxed micromorphic model with free and gradient micro-inertia
    Madeo, A. and Collet, M. and Miniaci, M. and Billon, K. and Ouisse, M. and Neff, P.
    Journal of Elasticity 130 59-83 (2018)
    In this paper the relaxed micromorphic continuum model with weighted free and gradient micro-inertia is used to describe the dynamical behavior of a real two-dimensional phononic crystal for a wide range of wavelengths. In particular, a periodic structure with specific micro-structural topology and mechanical properties, capable of opening a phononic band-gap, is chosen with the criterion of showing a low degree of anisotropy (the bandgap is almost independent of the direction of propagation of the traveling wave). A Bloch wave analysis is performed to obtain the dispersion curves and the corresponding vibrational modes of the periodic structure. A linear-elastic, isotropic, relaxed micromorphic model including both a free micro-inertia (related to free vibrations of the microstructures) and a gradient micro-inertia (related to the motions of the microstructure which are coupled to the macro-deformation of the unit cell) is introduced and particularized to the case of plane wave propagation. The parameters of the relaxed model, which are independent of frequency, are then calibrated on the dispersion curves of the phononic crystal showing an excellent agreement in terms of both dispersion curves and vibrational modes. Almost all the homogenized elastic parameters of the relaxed micromorphic model result to be determined. This opens the way to the design of morphologically complex meta-structures which make use of the chosen phononic material as the basic building block and which preserve its ability of “stopping” elastic wave propagation at the scale of the structure. © Springer Science+Business Media Dordrecht 2017.
    view abstractdoi: 10.1007/s10659-017-9633-6
  • 2018 • 313 Monte Carlo simulation of column growth in plasma spray physical vapor deposition process
    Wang, P. and He, W. and Mauer, G. and Mücke, R. and Vaßen, R.
    Surface and Coatings Technology 335 188-197 (2018)
    Plasma spray-physical vapor deposition is used to produce columnar microstructure coatings under particular operating parameters. Simulations of the growth of columns were carried out through a two-dimensional Monte Carlo model. The modeling was performed using inclined vapor flux impinging onto a substrate due to shadowing effects. An incoming particle travels along a straight line and attaches itself to already deposited particles. Furthermore, the newly deposited particle will relax to a stable surrounding position along the incoming velocity direction. The modeling results predicted the linking of an oblique vapor flux and column orientation. The numerical simulations were validated in three ways. Firstly, the porosity of simulated columns was predicted and compared to that obtained in the experimental columnar microstructure of coatings. Secondly, the morphology of simulated structures is compared to that of experimental coatings produced by plasma spray physical vapor deposition. Finally, the simulated orientation of columns is compared to the experimental one. © 2017
    view abstractdoi: 10.1016/j.surfcoat.2017.12.023
  • 2018 • 312 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 • 311 Numerical Benchmark of Phase-Field Simulations with Elastic Strains: Precipitation in the Presence of Chemo-Mechanical Coupling
    Darvishi Kamachali, R. and Schwarze, C. and Lin, M. and Diehl, M. and Shanthraj, P. and Prahl, U. and Steinbach, I. and Raabe, D.
    Computational Materials Science 155 541-553 (2018)
    Phase-field studies of solid-state precipitation under strong chemo-mechanical coupling are performed and benchmarked against the existing analytical solutions. The open source software packages OpenPhase and DAMASK are used for the numerical studies. Solutions for chemical diffusion and static mechanical equilibrium are investigated individually followed by a chemo-mechanical coupling effect arising due to composition dependence of the elastic constants. The accuracy of the numerical solutions versus the analytical solutions is quantitatively discussed. For the chemical diffusion benchmark, an excellent match, with a deviation <0.1%, was obtained. For the static mechanical equilibrium benchmark Eshelby problem was considered where a deviation of 5% was observed in the normal component of the stress, while the results from the diffuse interface (OpenPhase) and sharp interface (DAMASK) models were slightly different. In the presence of the chemo-mechanical coupling, the concentration field around a static precipitate was benchmarked for different coupling coefficients. In this case, it is found that the deviation increases proportional to the coupling coefficient that represents the strength of coupling concentration and elastic constants. Finally, the interface kinetics in the presence of the considered chemo-mechanical coupling were studied using OpenPhase and a hybrid OpenPhase–DAMASK implementation, replacing the mechanical solver of OpenPhase with DAMASK's. The observed deviations in the benchmark studies are discussed to provide guidance for the use of these results in studying further phase transformation models and implementations involving diffusion, elasticity and chemo-mechanical coupling effect. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2018.09.011
  • 2018 • 310 Numerically efficient microstructure-based calculation of internal stresses in superalloys
    Gao, S. and Gogilan, U. and Ma, A. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 26 (2018)
    According to the classical Eshelby inclusion problem, we introduce a new linear relation to calculate internal stresses in γ/γ′ microstructures of superalloys via an effective stiffness method. To accomplish this, we identify regions with almost uniform deformation behavior within the microstructure. Assigning different eigenstrains to these regions results in a characteristic internal stress state. The linear relation between eigenstrains and internal stresses, as proposed by Eshelby for simpler geometries, is shown to be a valid approximation to the solution for complex microstructures. The fast Fourier transformation method is chosen as a very efficient numerical solver to determine the effective stiffness matrix. Numerical validation shows that this generalized method with the effective stiffness matrix is efficient to obtain appropriate internal stresses and that it can be used to consider the influence of internal stresses on plasticity and creep kinetics in superalloys. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/aa9ba3
  • 2018 • 309 Phase stability and kinetics of σ-phase precipitation in CrMnFeCoNi high-entropy alloys
    Laplanche, G. and Berglund, S. and Reinhart, C. and Kostka, A. and Fox, F. and George, E.P.
    Acta Materialia 161 338-351 (2018)
    Although the phase stability of high-entropy alloys in the Cr-Mn-Fe-Co-Ni system has received considerable attention recently, knowledge of their thermodynamic equilibrium states and precipitation kinetics during high-temperature exposure is limited. In the present study, an off-equiatomic Cr26Mn20Fe20Co20Ni14 high-entropy alloy was solutionized and isothermally aged at temperatures between 600 °C and 1000 °C for times to 1000 h. In the original single-phase fcc matrix, an intermetallic σ phase was found to form at all investigated temperatures. Its morphology and composition were determined and the precipitation kinetics analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a time-temperature-transformation diagram (TTT diagram) is constructed for this off-equiatomic alloy. We combine our findings with theories of precipitation kinetics developed for traditional polycrystalline fcc alloys to calculate a TTT diagram for the equiatomic CrMnFeCoNi HEA. The results of our investigation may serve as a guide to predict precipitation kinetics in other complex alloys in the Cr-Mn-Fe-Co-Ni system. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.09.040
  • 2018 • 308 Phase nucleation through confined spinodal fluctuations at crystal defects evidenced in Fe-Mn alloys
    Kwiatkowski Da Silva, A. and Ponge, D. and Peng, Z. and Inden, G. and Lu, Y. and Breen, A. and Gault, B. and Raabe, D.
    Nature Communications 9 (2018)
    Analysis and design of materials and fluids requires understanding of the fundamental relationships between structure, composition, and properties. Dislocations and grain boundaries influence microstructure evolution through the enhancement of diffusion and by facilitating heterogeneous nucleation, where atoms must overcome a potential barrier to enable the early stage of formation of a phase. Adsorption and spinodal decomposition are known precursor states to nucleation and phase transition; however, nucleation remains the less well-understood step in the complete thermodynamic sequence that shapes a microstructure. Here, we report near-atomic-scale observations of a phase transition mechanism that consists in solute adsorption to crystalline defects followed by linear and planar spinodal fluctuations in an Fe-Mn model alloy. These fluctuations provide a pathway for austenite nucleation due to the higher driving force for phase transition in the solute-rich regions. Our observations are supported by thermodynamic calculations, which predict the possibility of spinodal decomposition due to magnetic ordering. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03591-4
  • 2018 • 307 Qualification of selective laser-melted Al alloys against fatigue damage by means of measurement and modeling techniques
    Awd, M. and Johannsen, J. and Siddique, S. and Emmelmann, C. and Walther, F.
    MATEC Web of Conferences 165 (2018)
    Aluminum alloys processed through selective laser melting possess unique features of microstructure, defect morphology and mechanical properties. Constitution of fine cellular dendrites results from the high-cooling rate of the melt pool during the consolidation process. Investigation of the microstructure by scanning electron microscopy identifies supersaturation of Si particles as a secondary strengthening mechanism. On the contrary, platform heating that induces coarser microstructure leads to migration of Si particles from the Al matrix to the eutectic phase. As a result, tensile strength is reduced by ∼3%, while fracture strain is increased by ∼17%. Fine-grained structures exhibit a lower amount of plastic damage accumulation as well as delayed crack initiation as determined by the applied measurement techniques. Finite element models of the investigated configurations are obtained using scans of computed tomography under consideration of process-induced defects. Comparison of modeling and experimental results concluded that dominant fatigue damage mechanisms are related to the loading regime from low-cycle (LCF) to very-high-cycle fatigue (VHCF). Thus, process-inherent features of microstructure and porosity have different quantitative effects concerning the applied load. In VHCF, a material configuration with platform heating possesses an improved fatigue strength by ∼33% at 1E9 cycles, concerning the material configuration without platform heating. © The Authors, published by EDP Sciences, 2018.
    view abstractdoi: 10.1051/matecconf/201816502001
  • 2018 • 306 Simulation of steatosis zonation in liver lobule—a continuummechanical bi-scale, tri-phasic, multi-component approach
    Ricken, T. and Waschinsky, N. and Werner, D.
    Lecture Notes in Applied and Computational Mechanics 84 15-33 (2018)
    The human liver is an important metabolic organ which regulates metabolism of the body in a complex time depending and non-linear coupled functionperfusion-mechanism. Harmful microstructure failure strongly affects the viability of the organ. The excessive accumulation of fat in the liver tissue, known as a fatty liver, is one of the most common liver micro structure failures, especially in western countries. The growing fat has a high impact on the blood perfusion and thus on the functionality of the organ. This interaction between perfusion, growth of fat and functionality on the hepatic microcirculation is poorly understood and many biological aspects of the liver are still subject of discussion. The presented computational model consists of a bi-scale, tri-phasic, multi-component approach based on the theory of porous media. The model includes the stress and strain state of the liver tissue, the transverse isotropic blood perfusion in the sinusoidal micro perfusion system. Furthermore, we describe the glucose metabolism in a two-scale PDE-ODE approach whereas the fat metabolism is included via phenomenological functions. Different inflow boundary conditions are tested against the influence on fat deposition and zonation in the liver lobules. With this example we can discuss biological assumptions and get a better understanding of the coupled function-perfusion ability of the liver. © Springer International Publishing AG 2018.
    view abstractdoi: 10.1007/978-3-319-59548-1_2
  • 2018 • 305 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 • 304 Tailoring columnar microstructure of axial suspension plasma sprayed TBCs for superior thermal shock performance
    Ganvir, A. and Joshi, S. and Markocsan, N. and Vassen, R.
    Materials and Design 144 192-208 (2018)
    This paper investigates the thermal shock behavior of thermal barrier coatings (TBCs) produced by axial suspension plasma spraying (ASPS). TBCs with different columnar microstructures were subjected to cyclic thermal shock testing in a burner rig. Failure analysis of these TBCs revealed a clear relationship between lifetime and porosity. However, tailoring the microstructure of these TBCs for enhanced durability is challenging due to their inherently wide pore size distribution (ranging from few nanometers up to few tens of micrometers). This study reveals that pores with different length scales play varying roles in influencing TBC durability. Fracture toughness shows a strong correlation with the lifetime of various ASPS TBCs and is found to be the prominent life determining factor. Based on the results, an understanding-based design philosophy for tailoring of the columnar microstructure of ASPS TBCs for enhanced durability under cyclic thermal shock loading is proposed. © 2018 The Authors
    view abstractdoi: 10.1016/j.matdes.2018.02.011
  • 2018 • 303 Temperature-Dependent Ultrastructure Transformation of Au-Fe Nanoparticles Investigated by in Situ Scanning Transmission Electron Microscopy
    Kamp, M. and Tymoczko, A. and Schürmann, U. and Jakobi, J. and Rehbock, C. and Rätzke, K. and Barcikowski, S. and Kienle, L.
    Crystal Growth and Design 18 5434-5440 (2018)
    Three-dimensional morphology changes of bimetallic nanoparticles (NPs) with nominal composition Au50Fe50 and Au20Fe80, generated by pulsed laser ablation in liquid, are monitored in situ and ex situ via scanning transmission electron microscopy and electron tomography. The samples are made up of a chemically segregated core-shell (CS) NPs structure, with an Au-rich shell and Fe-rich core, and solid solution (SS) NPs in the pristine state. Further, the examinations reveal information about a sequence of characteristic changes from the pristine metastable and intermediate ultrastructures up to thermodynamically stable products. In the case of the Au20Fe80 sample, a metastable spherical CS morphology is transformed at equilibrium conditions into a cube-shaped Fe-rich core faceted by truncated Au-rich pyramids. For the Au50Fe50 sample, the Au-rich shell is solved into the Fe-rich core, and chemically homogeneous (SS) NPs are formed. Interestingly, this transformation was proven to occur via an intermediate ultrastructure with lamellar segregation, not previously reported as a transient state during in situ heating. On the basis of these observations, a correlation between the composition and the morphology at equilibrium is suggested, in accordance with the bulk phase diagram of Au-Fe. At the same time, our examinations directly prove that laser ablation synthesis creates nonequilibrium NP morphologies, frozen in metastable, spherical core-shell particles. Copyright © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.8b00809
  • 2018 • 302 Unexpected cyclic stress-strain response of dual-phase high-entropy alloys induced by partial reversibility of deformation
    Niendorf, T. and Wegener, T. and Li, Z. and Raabe, D.
    Scripta Materialia 143 63-67 (2018)
    The recently developed dual-phase high-entropy alloys are characterized by pronounced strain hardening and high ductility under monotonic loading owing to the associated transformation induced plasticity effect. Fatigue properties of high-entropy alloys have not been studied in depth so far. The current study focuses on the low-cycle fatigue regime. Cyclic tests were conducted and the microstructure evolution was studied post-mortem. Despite deformation-induced martensitic transformation during cycling at given plastic strain amplitudes, intense strain hardening in the cyclic stress-strain response is not observed. This behavior is attributed to the planar nature of slip and partial reversibility of deformation. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2017.09.013
  • 2018 • 301 Zinc stannate by reactive laser sintering
    Mackert, V. and Gebauer, J.S. and Notthoff, C. and Winterer, M.
    Applied Surface Science 457 1174-1180 (2018)
    A novel procedure for producing polycrystalline zinc stannate (Zn 2 SnO 4 , ZTO) films is presented in this paper. Nanocrystals of zinc oxide (ZnO) and tin dioxide (SnO 2 ) are prepared by chemical vapor synthesis (CVS) and processed into stable aqueous dispersions including mixed colloids. These colloids are transformed into nanostructured films via electrophoretic deposition where the mixed colloid forms a homogeneous, nanoscaled composite. Ultraviolet (UV) laser sintering of these codeposited ZnO-SnO 2 nanocrystals generates the inverse cubic spinel Zn 2 SnO 4 phase by chemical reaction on the area of interest. The effects of UV laser sintering at a wavelength of 325 nm on the nanoscaled microstructure of pure deposited films are investigated by variation of laser power and scanning speed. The microstructure of composite films is compared to a film obtained by classical reactive sintering in a furnace. High-resolution scanning electron microscopy and energy dispersive X-ray spectroscopy are used to investigate film morphology and chemical composition. Structural characterization is performed by X-ray diffraction. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.06.304
  • 2017 • 300 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 • 299 A Study on Microstructural Parameters for the Characterization of Granular Porous Ceramics Using a Combination of Stochastic and Mechanical Modeling
    Kulosa, M. and Neumann, M. and Boeff, M. and Gaiselmann, G. and Schmidt, V. and Hartmaier, A.
    International Journal of Applied Mechanics 9 (2017)
    To correlate the mechanical properties of granular porous materials with their microstructure, typically porosity is being considered as the dominant parameter. In this work, we suggest the average coordination number, i.e., the average number of connections that each grain of the porous material has to its neighboring grains, as additional - and possibly even more fundamental - microstructural parameter. In this work, a combination of stochastic and mechanical modeling is applied to study microstructural influences on the elastic properties of porous ceramics. This is accomplished by generating quasi-two-dimensional (2D) and fully three-dimensional (3D) representative volume elements (RVEs) with tailored microstructural features by a parametric stochastic microstructure model. In the next step, the elastic properties of the RVEs are characterized by finite element analysis. The results reveal that the average coordination number exhibits a very strong correlation with the Young's modulus of the material in both 2D and 3D RVEs. Moreover, it is seen that quasi-2D RVEs with the same average coordination number, but largely different porosities, only differ very slightly in their elastic properties such that the correlation is almost unique. This finding is substantiated and discussed in terms of the load distribution in microstructures with different porosities and average coordination numbers. © 2017 World Scientific Publishing Europe Ltd.
    view abstractdoi: 10.1142/S1758825117500697
  • 2017 • 298 A variant of the linear isotropic indeterminate couple-stress model with symmetric local force-stress, symmetric nonlocal force-stress, symmetric couple-stresses and orthogonal boundary conditions
    Ghiba, I.-D. and Neff, P. and Madeo, A. and Münch, I.
    Mathematics and Mechanics of Solids 22 1221-1266 (2017)
    In this paper we venture a new look at the linear isotropic indeterminate couple-stress model in the general framework of second-gradient elasticity and we propose a new alternative formulation which obeys Cauchy-Boltzmann's axiom of the symmetry of the force-stress tensor. For this model we prove the existence of solutions for the equilibrium problem. Relations with other gradient elastic theories and the possibility of switching from a fourth-order (gradient elastic) problem to a second-order micromorphic model are also discussed with the view of obtaining symmetric force-stress tensors. It is shown that the indeterminate couple-stress model can be written entirely with symmetric force-stress and symmetric couple-stress. The difference of the alternative models rests in specifying traction boundary conditions of either rotational type or strain type. If rotational-type boundary conditions are used in the integration by parts, the classical anti-symmetric nonlocal force-stress tensor formulation is obtained. Otherwise, the difference in both formulations is only a divergence-free second-order stress field such that the field equations are the same, but the traction boundary conditions are different. For these results we employ an integrability condition, connecting the infinitesimal continuum rotation and the infinitesimal continuum strain. Moreover, we provide the orthogonal boundary conditions for both models. © SAGE Publications.
    view abstractdoi: 10.1177/1081286515625535
  • 2017 • 297 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 • 296 Adaptation of TiC hard particles properties and morphology in metal matrix composites by refractory elements
    Mohr, A. and Röttger, A. and Theisen, W.
    Key Engineering Materials 742 KEM 99-105 (2017)
    High mechanical loads, corrosion, and abrasion decrease the lifetime of tools. One way to increase the wear resistance of tool materials can be achieved by adding hard particles to the metal matrix such as titanium carbide, which protect the softer metal matrix against abrasive particles. This material concept is designated as metal matrix composite (MMC). Ferro-Titanit® is such MMC material, possessing high wear and a simultaneously high corrosion resistance, for which reason this material is used in the polymers industry. The material concept is based on a corrosion-resistant Fe-base matrix with up to 45 vol% titanium carbide (TiC) as a hard particle addition to improve the wear resistance against abrasion. These TiC hard particles must be adapted to the present tribological system in terms of hardness, size and morphology. This study shows how the size and morphology of TiC hard particles can be influenced by the refractory element niobium (Nb). Therefore, the element Nb was added with 2 and 4 mass% to the soft-martensitic Ferro-Titanit® Grade Nikro128. The investigated materials were compacted by sintering, and the densified microstructure was further characterized by scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), and optical image analyses. Furthermore, microstructure and properties of the compacted Nb-alloyed samples were compared to the reference material Nikro128. The results show that the addition of Nb influences the morphology, size and chemical composition of the TiC hard particle. These changes in the hard phase characteristics also influence the materials properties. It was shown that the phase niobium carbide (NbC) is formed around the TiC during the densification process, leading to a change in morphology and size of the TiC. © 2017 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.742.99
  • 2017 • 295 Antibacterial activity of microstructured sacrificial anode thin films by combination of silver with platinum group elements (platinum, palladium, iridium)
    Köller, M. and Bellova, P. and Javid, S.M. and Motemani, Y. and Khare, C. and Sengstock, C. and Tschulik, K. and Schildhauer, T.A. and Ludwig, Al.
    Materials Science and Engineering C 74 536-541 (2017)
    Five different Ag dots arrays (16 to 400dots/mm2) were fabricated on a continuous platinum, palladium, or iridium thin film and for comparison also on titanium film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Staphylococcus aureus (S. aureus) were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards . S. aureus was induced by Ag dot arrays on each of the platinum group thin film (sacrificial anode system for Ag) in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag). Among platinum group elements the Ir-Ag system exerted the highest antibacterial activity which was accompanied by most advanced dissolution of the Ag dots and Ag ion release compared to Ag dots on Pt or Pd. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2016.12.075
  • 2017 • 294 Characterization of recycled TiC and its influence on the microstructural, tribological, and corrosion properties of a TiC-reinforced metal matrix composites
    Mohr, A. and Röttger, A. and Theisen, W.
    Journal of Composite Materials 51 3611-3621 (2017)
    Ferro-Titanit® is a metal matrix composite (MMC) with a high wear and corrosion resistance. It contains TiC as hard particles on account of their high hardness, good corrosion resistance, and low density. This wear- and corrosion-resistant material is amenable to machining in the soft-annealed state, which gives rise to chips containing a large amount of the expensive TiC hard particles. Due to the cost of TiC, there is great interest in recycling the TiC from these chips so that it can be reused in the production of further Ferro-Titanit® materials. In this study, the recycled TiC [(Ti,X)C] is investigated with regard to morphology, particle size, chemical composition, and phases, and the results were compared to industrially produced TiC. In the next step, the (Ti,X)C was reused in the production of new Ferro-Titanit®. The Ferro-Titanit® reinforced with (Ti,X)C was also characterized with respect to microstructure, wear behavior, and corrosion resistance. Our investigations identified a change in the chemical composition of the TiC as a result of diffusion processes and a decrease in TiC particle size with respect to the initial state. The change in morphology and size of TiC during the recycling process influences the microstructure and the material behavior of the MMC containing recycled TiC. © 2017, © The Author(s) 2017.
    view abstractdoi: 10.1177/0021998317692032
  • 2017 • 293 Comparison of microstructure and mechanical properties of Scalmalloy® produced by selective laser melting and laser metal deposition
    Awd, M. and Tenkamp, J. and Hirtler, M. and Siddique, S. and Bambach, M. and Walther, F.
    Materials 11 (2017)
    The second-generation aluminum-magnesium-scandium (Al-Mg-Sc) alloy, which is often referred to as Scalmalloy®, has been developed as a high-strength aluminum alloy for selective laser melting (SLM). The high-cooling rates of melt pools during SLM establishes the thermodynamic conditions for a fine-grained crack-free aluminum structure saturated with fine precipitates of the ceramic phase Al3-Sc. The precipitation allows tensile and fatigue strength of Scalmalloy® to exceed those of AlSi10Mg by ~70%. Knowledge about properties of other additive manufacturing processes with slower cooling rates is currently not available. In this study, two batches of Scalmalloy® processed by SLM and laser metal deposition (LMD) are compared regarding microstructure-induced properties. Microstructural strengthening mechanisms behind enhanced strength and ductility are investigated by scanning electron microscopy (SEM). Fatigue damage mechanisms in low-cycle (LCF) to high-cycle fatigue (HCF) are a subject of study in a combined strategy of experimental and statistical modeling for calculation of Woehler curves in the respective regimes. Modeling efforts are supported by non-destructive defect characterization in an X-ray computed tomography (μ-CT) platform. The investigations show that Scalmalloy® specimens produced by LMD are prone to extensive porosity, contrary to SLM specimens, which is translated to ~30% lower fatigue strength. © 2017 by the author.
    view abstractdoi: 10.3390/ma11010017
  • 2017 • 292 Complexion-mediated martensitic phase transformation in Titanium
    Zhang, J. and Tasan, C.C. and Lai, M.J. and Dippel, A.-C. and Raabe, D.
    Nature Communications 8 (2017)
    The most efficient way to tune microstructures and mechanical properties of metallic alloys lies in designing and using athermal phase transformations. Examples are shape memory alloys and high strength steels, which together stand for 1,500 million tons annual production. In these materials, martensite formation and mechanical twinning are tuned via composition adjustment for realizing complex microstructures and beneficial mechanical properties. Here we report a new phase transformation that has the potential to widen the application window of Ti alloys, the most important structural material in aerospace design, by nanostructuring them via complexion-mediated transformation. This is a reversible martensitic transformation mechanism that leads to a final nanolaminate structure of α″ (orthorhombic) martensite bounded with planar complexions of athermal ω (a-ω hexagonal). Both phases are crystallographically related to the parent β (BCC) matrix. As expected from a planar complexion, the a-ω is stable only at the hetero-interface. © The Author(s) 2017.
    view abstractdoi: 10.1038/ncomms14210
  • 2017 • 291 Crystal plasticity study on stress and strain partitioning in a measured 3D dual phase steel microstructure
    Diehl, M. and An, D. and Shanthraj, P. and Zaefferer, S. and Roters, F. and Raabe, D.
    Physical Mesomechanics 20 311-323 (2017)
    Dual phase steels are advanced high strength alloys typically used for structural parts and reinforcements in car bodies. Their good combination of strength and ductility and their lean composition render them an economically competitive option for realizing multiple lightweight design options in automotive engineering. The mechanical response of dual phase steels is the result of the strain and stress partitioning among the ferritic and martensitic phases and the individual crystallographic grains and subgrains of these phases. Therefore, understanding how these microstructural features influence the global and local mechanical properties is of utmost importance for the design of improved dual phase steel grades. While multiple corresponding simulation studies have been dedicated to the investigation of dual phase steel micromechanics, numerical tools and experiment techniques for characterizing and simulating real 3D microstructures of such complex materials have been emerged only recently. Here we present a crystal plasticity simulation study based on a 3D dual phase microstructure which is obtained by EBdD tomography, also referred to as 3D EBdD (EBdD—electron backscatter diffraction). In the present case we utilized a 3D EBdD serial sectioning approach based on mechanical polishing. Moreover, sections of the 3D microstructure are used as 2D models to study the effect of this simplification on the stress and strain distribution. The simulations are conducted using a phenomenological crystal plasticity model and a spectral method approach implemented in the Düsseldorf Advanced Material Simulation Kit (DAMAdK). © 2017, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S1029959917030079
  • 2017 • 290 Degradation in concrete structures due to cyclic loading and its effect on transport processes—Experiments and modeling
    Przondziono, R. and Timothy, J.J. and Weise, F. and Krütt, E. and Breitenbücher, R. and Meschke, G. and Hofmann, M.
    Structural Concrete 18 519-527 (2017)
    According to the objectives of the research group 1498, this paper deals with degradation effects in concrete structures that are caused by cyclic flexural loading. The goal is to determine their influence on the fluid transport processes within the material on the basis of experimental results and numerical simulations. The overall question was, to which extent the ingress of externally supplied alkalis and subsequently an alkali-silica reaction are affected by such modifications in the microstructure. Degradation in the concrete microstructure is characterized by ultrasonic wave measurements as well as by microscopic crack analysis. Furthermore, experiments on the penetration behavior of water into the investigated materials were performed. The penetration behavior into predamaged concrete microstructures was examined by the classical Karsten tube experiment, nuclear magnetic resonance method, and time domain reflectometry techniques. In order to create an appropriate model of the material's degradation on the water transport, the Darcy law was applied to describe the flow in partially saturated concrete. Material degradation is taken into account by an effective permeability that is dependent on the state of degradation. This effective permeability is obtained by the micromechanical homogenisation of the flow in an Representative Elementary Volume (REV) with distributed ellipsoidal microcracks embedded in a porous medium. The data gained in the microscopic crack analysis is used as input for the micromechanical model. Finite element simulations for unsaturated flow using the micromechanical model were compared with the experimental results showing good qualitative and quantitative agreement. © 2017 fib. International Federation for Structural Concrete
    view abstractdoi: 10.1002/suco.201600180
  • 2017 • 289 Designing duplex, ultrafine-grained Fe-Mn-Al-C steels by tuning phase transformation and recrystallization kinetics
    Zhang, J. and Raabe, D. and Tasan, C.C.
    Acta Materialia 141 374-387 (2017)
    A novel, lightweight Fe-25.7Mn-10.6Al-1.2C (wt.%) steel is designed by exploiting the concurrent progress of primary recrystallization and phase transformation, in order to produce an ultrafine-grained, duplex microstructure. The microstructure consists of recrystallized austenite grains surrounded by submicron-sized ferrite grains, and recovered austenite regions with preferential nano-κ-carbide precipitation. This partially recrystallized duplex microstructure demonstrates excellent strength-ductility combinations, e.g. a yield strength of 1251 MPa, an ultimate tensile strength of 1387 MPa, and a total elongation of 43%, arising from the composite response by virtue of diverging constituent strength and strain hardening behaviors. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.09.026
  • 2017 • 288 Development of electrically conductive microstructures based on polymer/CNT nanocomposites via two-photon polymerization
    Staudinger, U. and Zyla, G. and Krause, B. and Janke, A. and Fischer, D. and Esen, C. and Voit, B. and Ostendorf, A.
    Microelectronic Engineering 179 48-55 (2017)
    Femtosecond laser-induced two-photon polymerization (2PP) of carbon nanofiller doped polymers was utilized to produce electrically conductive microstructures, which are expected to be applicable as microelectronic components or micro-electromechanical systems in sensors. The nanocomposites were processed by compounding an inorganic-organic hybrid material with two different types (short and long) of single walled carbon nanotubes (SWCNTs). Different SWCNT contents were dispersed in the polymer by sonication to adjust the electrical conductivity of the nanocomposites. Low surface resistivity values of ~ 4.6 × 105 Ω/sq. could be measured for coated reference films with a thickness of 30 μm having an exceptionally low SWCNT content of 0.01 wt% of the long type of SWCNTs. In contrast, a higher minimum resistivity of 1.5 × 106 Ω/sq. was exhibited for composites with a higher content, 2 wt%, of short SWCNTs. The structural quality of the microstructures processed by 2PP was mainly influenced by the dispersion quality of the SWCNTs. To characterize the electrical conductivity, conductive atomic force microscopy was applied for the first time. In microstructures with 0.05 wt% of the long type of SWCNTs, a contact current could be detected over a wide range of the measured area visualizing the electrical conductive CNT network, which has not been reported before. © 2017
    view abstractdoi: 10.1016/j.mee.2017.04.024
  • 2017 • 287 Duplex stainless steel fabricated by selective laser melting - Microstructural and mechanical properties
    Hengsbach, F. and Koppa, P. and Duschik, K. and Holzweissig, M.J. and Burns, M. and Nellesen, J. and Tillmann, W. and Tröster, T. and Hoyer, K.-P. and Schaper, M.
    Materials and Design 133 136-142 (2017)
    In the scope of the present study, microstructural and mechanical characterizations of duplex stainless steel UNS S31803 processed by selective laser melting (SLM) are conducted. The findings shed light on the phase arrangement evolving in the as-built condition and in several heat-treated conditions. In the as-built condition, austenite formation is almost suppressed due to process-related high cooling rates. Therefore, several heat treatments ranging from 900 °C to 1200 °C for 5 min each were performed in order to adjust to the desired austenitic-ferritic microstructure. Results generated by transmission electron microscopy (TEM) reveal a high dislocation density induced during SLM fabrication, such that a recrystallized microstructure prevails after the heat treatment. Tensile tests display the severe impact of the heat treatment on the resulting mechanical response. The nearly complete ferritic as-built specimens obtain a higher ultimate tensile strength and a reduced elongation at fracture compared to the heat-treated specimens. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2017.07.046
  • 2017 • 286 Erosion Performance of Gadolinium Zirconate-Based Thermal Barrier Coatings Processed by Suspension Plasma Spray
    Mahade, S. and Curry, N. and Björklund, S. and Markocsan, N. and Nylén, P. and Vaßen, R.
    Journal of Thermal Spray Technology 26 108-115 (2017)
    7-8 wt.% Yttria-stabilized zirconia (YSZ) is the standard thermal barrier coating (TBC) material used by the gas turbines industry due to its excellent thermal and thermo-mechanical properties up to 1200 °C. The need for improvement in gas turbine efficiency has led to an increase in the turbine inlet gas temperature. However, above 1200 °C, YSZ has issues such as poor sintering resistance, poor phase stability and susceptibility to calcium magnesium alumino silicates (CMAS) degradation. Gadolinium zirconate (GZ) is considered as one of the promising top coat candidates for TBC applications at high temperatures (>1200 °C) due to its low thermal conductivity, good sintering resistance and CMAS attack resistance. Single-layer 8YSZ, double-layer GZ/YSZ and triple-layer GZdense/GZ/YSZ TBCs were deposited by suspension plasma spray (SPS) process. Microstructural analysis was carried out by scanning electron microscopy (SEM). A columnar microstructure was observed in the single-, double- and triple-layer TBCs. Phase analysis of the as-sprayed TBCs was carried out using XRD (x-ray diffraction) where a tetragonal prime phase of zirconia in the single-layer YSZ TBC and a cubic defect fluorite phase of GZ in the double and triple-layer TBCs was observed. Porosity measurements of the as-sprayed TBCs were made by water intrusion method and image analysis method. The as-sprayed GZ-based multi-layered TBCs were subjected to erosion test at room temperature, and their erosion resistance was compared with single-layer 8YSZ. It was shown that the erosion resistance of 8YSZ single-layer TBC was higher than GZ-based multi-layered TBCs. Among the multi-layered TBCs, triple-layer TBC was slightly better than double layer in terms of erosion resistance. The eroded TBCs were cold-mounted and analyzed by SEM. © 2016, ASM International.
    view abstractdoi: 10.1007/s11666-016-0479-4
  • 2017 • 285 Excavation tool concepts for TBMs – Understanding the material-dependent response to abrasive wear
    Küpferle, J. and Röttger, A. and Theisen, W.
    Tunnelling and Underground Space Technology 68 22-31 (2017)
    Wear of cutting tools for tunneling applications can lead to decreased advance rates and unscheduled downtimes that are associated with increased tunneling times and project costs. During the planning phase, wear of tools and their associated lifetime and replacement times are estimated on the basis of the ground that is to be excavated. However, from the viewpoint of materials technology, this procedure is insufficient because it is essential to take account of the interactions between tool material, ground, and the acting wear mechanisms on the microscopic scale, such as abrasion, fatigue, or forced fracture. The respective tool materials feature different tribomechanical properties and thus different wear mechanisms and rates that depend on the ground to be mined. Low wear rates can only be achieved using an optimized tool material concept that is adapted to the acting ground and the associated tribological system. This requires a comprehensive understanding of the wear behavior of the respective materials. This article focuses on the different, commonly used tool concepts and their microstructure. Interactions of the microstructure of these materials with the abrasive particles and the associated microwear mechanisms are analyzed. The results provide a deeper understanding of the wear process of excavation tools depending on the respective tool and the material concept. The discussed correlations are illustrated by results from the RUB Tunneling Device and nanoscratch tests, which are used to map the tribological TBM tool system on the macroscopic and microscopic scales. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.tust.2017.05.013
  • 2017 • 284 Formation of nanometer-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films and their effect on solar cell efficiency
    Schwarz, T. and Cojocaru-Mirédin, O. and Mousel, M. and Redinger, A. and Raabe, D. and Choi, P.-P.
    Acta Materialia 132 276-284 (2017)
    Atom probe tomography and transmission electron microscopy are used to study the formation of nano-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films. For a Cu-rich precursor, which was deposited at 320 °C under Cu- and Zn-rich growth conditions, Cu2-xSe grains at the surface are detected. During annealing the precursor at 500 °C in a SnSe + Se atmosphere most of the Cu2-xSe is transformed to Cu2ZnSnSe4 via the consumption of excessive ZnSe and incorporation of Sn. However, atom probe tomography studies also reveal the formation of various nanometer-sized Cu-Sn-Se particles close to the CdS/Cu2ZnSnSe4 interface. One of those particles has a composition close to the Cu2SnSe3 compound. This phase has a smaller band gap than Cu2ZnSnSe4 and is proposed to lead to a significant drop in the open-circuit voltage and could be the main cause for a detrimental p-n junction and the zero efficiency of the final device. Possible effects of the other phases on solar cell performance and formation mechanisms are discussed as well. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.04.056
  • 2017 • 283 Hardness and Microstructure of a Newly Developed Stainless Steel after Ausforming
    Seifert, M. and Botzet, M. and Theisen, W.
    Steel Research International (2017)
    In this work, ausforming is applied to a newly developed stainless steel. This process consists of austenitisation, quenching to a deformation temperature above room temperature, deformation of the metastable austenitic microstructure without the formation of martensite, and subsequent quenching in liquid nitrogen. The investigated steel is explicitly developed to be processed by ausforming and manufactured as a laboratory size test melt. The aim is to achieve a steel having a high hardness as well as a high corrosion resistance. Instead of conventional quenching and tempering, conventional processing is followed by ausforming. A parameter study incorporating the austenitisation temperature and time, deformation temperature, deformation speed, and degree of deformation is performed to achieve maximum hardness. Furthermore, the influence of soft annealing prior to ausforming is also investigated. The hardness of ausformed specimens is measured and correlated to the parameters used for processing. The microstructure of selected specimens is also investigated. Surprisingly, small amounts of martensite are found after ausforming, although a hardness of about 600 HV10 is achieved. In fact, a highly deformed austenitic microstructure is found predominantly. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201700010
  • 2017 • 282 Hydrogen effects on microstructural evolution and passive film characteristics of a duplex stainless steel
    Luo, H. and Li, Z. and Chen, Y.-H. and Ponge, D. and Rohwerder, M. and Raabe, D.
    Electrochemistry Communications 79 28-32 (2017)
    We revealed the effects of hydrogen on the microstructural evolution and passive film properties of a 2205 duplex stainless steel by the joint use of electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. The microstructural analysis results show that effects of hydrogen on the two phases are different: (i) in austenite, stacking faults are induced by hydrogen, and (ii) in ferrite, hydrogen causes an increase of the dislocation density. The XPS analysis revealed that hydrogen reduced the occurrence of Cr2O3 and nitrogen in the passive film, leading to the reduction of their overall thickness. Furthermore, for the first time we demonstrated that the hydrogen release time plays an important role in the electrochemical behavior of the hydrogen charged steel. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.elecom.2017.04.013
  • 2017 • 281 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
  • 2017 • 280 Identifying Structure–Property Relationships Through DREAM.3D Representative Volume Elements and DAMASK Crystal Plasticity Simulations: An Integrated Computational Materials Engineering Approach
    Diehl, M. and Groeber, M. and Haase, C. and Molodov, D.A. and Roters, F. and Raabe, D.
    JOM 69 848-855 (2017)
    Predicting, understanding, and controlling the mechanical behavior is the most important task when designing structural materials. Modern alloy systems—in which multiple deformation mechanisms, phases, and defects are introduced to overcome the inverse strength–ductility relationship—give raise to multiple possibilities for modifying the deformation behavior, rendering traditional, exclusively experimentally-based alloy development workflows inappropriate. For fast and efficient alloy design, it is therefore desirable to predict the mechanical performance of candidate alloys by simulation studies to replace time- and resource-consuming mechanical tests. Simulation tools suitable for this task need to correctly predict the mechanical behavior in dependence of alloy composition, microstructure, texture, phase fractions, and processing history. Here, an integrated computational materials engineering approach based on the open source software packages DREAM.3D and DAMASK (Düsseldorf Advanced Materials Simulation Kit) that enables such virtual material development is presented. More specific, our approach consists of the following three steps: (1) acquire statistical quantities that describe a microstructure, (2) build a representative volume element based on these quantities employing DREAM.3D, and (3) evaluate the representative volume using a predictive crystal plasticity material model provided by DAMASK. Exemplarily, these steps are here conducted for a high-manganese steel. © 2017, The Author(s).
    view abstractdoi: 10.1007/s11837-017-2303-0
  • 2017 • 279 Influence of rhenium on γ′-strengthened cobalt-base superalloys
    Kolb, M. and Zenk, C.H. and Kirzinger, A. and Povstugar, I. and Raabe, D. and Neumeier, S. and Göken, M.
    Journal of Materials Research 32 2551-2559 (2017)
    The element Re is known to be a very potent strengthener concerning the creep properties of Ni-base superalloys. In this paper the influence of Re on the properties of new γ′-strengthened Co-base superalloys is addressed. Atom probe tomography reveals that Re partitions preferentially to the γ phase, but not as pronounced as in ni-base superalloys. Nanoindentation and micro-pillar compression tests of the γ′ phase indicate an increase of the hardness and the critical resolved shear stress caused by a considerable concentration of Re in the γ′ phase. Creep investigations show that the positive effect of Re is by far not as pronounced as in Ni-base superalloys. Several effects, which can contribute to this behavior, such as the lower Re concentration in γ and hence a slightly reduced effective diffusion coefficient, a smaller diffusion barrier of Re in Co compared to Ni, a slightly lower lattice misfit and γ′ volume fraction of the Re-containing alloy, are discussed. © Materials Research Society 2017.
    view abstractdoi: 10.1557/jmr.2017.242
  • 2017 • 278 Influence of rotational speed in the friction surfacing of titanium grade 1 on Ti-6Al-4V
    Do Vale, N.L. and Fitseva, V. and Hanke, S. and Filho, S.L.U. and Dos Santos, J.F.
    Materials Research 20 830-835 (2017)
    Titanium Grade 1 was deposited on Ti-6Al-4V, 2 mm thickness, by Friction Surfacing. The process parameters were rotational speed, deposition speed and consumption rate. Only the rotational speed was varied in order to evaluate the influence of this parameter on the coatings generated. The applicability of the process has been described for a large number of materials, although the depositions of titanium alloys are still not widely studied. The objective is to investigate the effects of the rotational speed on the coatings' geometry and microstructural evolution. This investigation has shown that Titanium Grade 1 coatings can be deposited onto a Ti-6Al-4V by Friction Surfacing depending on the rotational speed. The coatings' surface homogeneity was influenced by the rotational speed, being inhomogeneous for the lowest speed. The coatings' thickness and width increased with enhancing this speed. The heat affected zone in the substrate corresponded to the complete thickness under the depositions. © 2017 Universidade Federal de Sao Carlos. All rights reserved.
    view abstractdoi: 10.1590/1980-5373-MR-2016-1011
  • 2017 • 277 Influence of rotational speed on process characteristics in friction surfacing of Ti-6Al-4V
    Fitseva, V. and Hanke, S. and dos Santos, J.F.
    Materials and Manufacturing Processes 32 557-563 (2017)
    Friction surfacing process is employed to deposit metallic coatings, whereby similar and dissimilar material combinations can be realized. The process can be applied as a local repair technology, or the coating material can locally modify the surfaces. One advantage of this process is that the coatings are deposited in solid state without reaching the melting range of materials, thereby avoiding dilution with the substrate. The involved severe plastic deformation under high temperatures alters the microstructure of the coating material, leaving it fully dynamically recrystallized. The current work focuses on deposition of Ti-6Al-4V coatings. For that material, the process parameter rotational speed plays a major role in the material’s response during processing. Two different regimes with a threshold at 2000 min−1exist, upon which the flow behavior of Ti-6Al-4V significantly differs, affecting among others the coating dimensions. Microstructural analysis reveals that the material is deformed in a high temperature β phase, and the high cooling rates (46.4 Ks−1) lead to martensitic transformation. The β grain size differs in the low and high rotational speed regimes. This study shows that metallurgical processes play an important role in friction surfacing, since they influence all relevant process characteristics, including microstructure, material efficiency and process forces. © 2017 Taylor & Francis.
    view abstractdoi: 10.1080/10426914.2016.1257799
  • 2017 • 276 Kinetics and crystallization path of a Fe-based metallic glass alloy
    Duarte, M.J. and Kostka, A. and Crespo, D. and Jimenez, J.A. and Dippel, A.-C. and Renner, F.U. and Dehm, G.
    Acta Materialia 127 341-350 (2017)
    The thermal stability and the quantification of the different transformation processes involved in the overall crystallization of the Fe50Cr15Mo14C15B6 amorphous alloy were investigated by several characterization techniques. Formation of various metastable and stable phases during the devitrification process in the sequence α-Fe, χ-Cr6Fe18Mo5, M23(C,B)6, M7C3, η-Fe3Mo3C and FeMo2B2 (with M = Fe, Cr, Mo), was observed by in-situ synchrotron high energy X-ray diffraction and in-situ transmission electron microscopy. By combining these techniques with differential scanning calorimetry data, the crystallization states and their temperature range of stability under continuous heating were related with the evolution of the crystallized fraction and the phase sequence as a function of temperature, revealing structural and chemical details of the different transformation mechanisms. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.01.031
  • 2017 • 275 Micromechanical modeling approach to derive the yield surface for BCC and FCC steels using statistically informed microstructure models and nonlocal crystal plasticity
    Vajragupta, N. and Ahmed, S. and Boeff, M. and Ma, A. and Hartmaier, A.
    Physical Mesomechanics 20 343-352 (2017)
    In order to describe irreversible deformation during metal forming processes, the yield surface is one of the most important criteria. Because of their simplicity and efficiency, analytical yield functions along with experimental guidelines for parameterization become increasingly important for engineering applications. However, the relationship between most of these models and microstructural features are still limited. Hence, we propose to use micromechanical modeling, which considers important microstructural features, as a part of the solution to this missing link. This study aims at the development of a micromechanical modeling strategy to calibrate material parameters for the advanced analytical initial yield function Barlat YLD 2004-18p. To accomplish this, the representative volume element is firstly created based on a method making use of the statistical description of microstructure morphology as input parameter. Such method couples particle simulations to radical Voronoi tessellations to generate realistic virtual microstructures as representative volume elements. Afterwards, a nonlocal crystal plasticity model is applied to describe the plastic deformation of the representative volume element by crystal plasticity finite element simulation. Subsequently, an algorithm to construct the yield surface based on the crystal plasticity finite element simulation is developed. The primary objectives of this proposed algorithm are to automatically capture and extract the yield loci under various loading conditions. Finally, a nonlinear least square optimization is applied to determine the material parameters of Barlat YLD 2004-18p initial yield function of representative volume element, mimicking generic properties of bcc and fcc steels from the numerical simulations. © 2017, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S1029959917030109
  • 2017 • 274 Microstructural characterization and simulation of damage for geared sheet components
    Gerstein, G. and Isik, K. and Gutknecht, F. and Sieczkarek, P. and Ewert, J. and Tekkaya, A.E. and Clausmeyer, T. and Nürnberger, F.
    Journal of Physics: Conference Series 896 (2017)
    The evolution of damage in geared components manufactured from steel sheets was investigated, to analyse the influence of damage caused by the sheet-bulk-metal forming. Due to the inhomogeneous and multi-axial deformation in the investigated parts, different aspects such as the location-dependent shape and size of voids are analysed by means of various microscopic methods. In particular, a method to characterize the state of damage evolution, i. e. void nucleation, growth and coalescence using scanning electron microscopy (SEM) is applied. The investigations reveal a strong dependence of the void area fraction, shape of voids and thus damage evolution on the loading mode. The microstructural analysis is complemented with FEM simulations using material models which consider the characteristics of the void evolution. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/896/1/012076
  • 2017 • 273 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 • 272 Microstructural stability of a niobium single crystal deformed by equal channel angular pressing
    Bernardi, H.H. and Sandim, H.R.Z. and Zilnyk, K.D. and Verlinden, B. and Raabe, D.
    Materials Research 20 1238-1247 (2017)
    A [211]-oriented niobium single crystal was deformed by equal channel angular pressing (ECAP) at room temperature using the route Bc to a total strain of 9.2. A sharp cube texture develops after ECAP processing. The deformed samples were annealed in vacuum from 400ºC (673 K) to 900ºC (1173 K) for 1 h to evaluate their microstructural stability. Scanning electron microscopy (SEM) was used to image the microstructures of as-deformed and annealed specimens. Electron backscatter diffraction (EBSD) was employed to determine the respective microtextures before and after annealing. Coarsening of the microstructure occurs at a maximum rate at 550ºC (823 K) due to discontinuous recrystallization. Normal grain growth replaces discontinuous recrystallization as the main coarsening mechanism above 700ºC (973 K).
    view abstractdoi: 10.1590/1980-5373-MR-2017-0288
  • 2017 • 271 Microstructure and mechanical properties of the heat-affected zone in laser-welded/brazed steel 22MnB5–AA6016 aluminum/AZ31 magnesium alloy
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W.
    Journal of Materials Processing Technology 247 11-18 (2017)
    The martensitic microstructure of the steel 22MnB5 was tempered during laser welding/brazing. The strength of the HAZ greatly decreased from 1500 MPa to 800–1100 MPa, depending on the heat input. The lowest strength always occurred in the area with the highest heat input directly beside the welding zone. The strength of the aluminum alloy was slightly reduced from 233 MPa to 212 MPa. The strongest decrease in the strength did occur in the area with a critical temperature range of 400–500 °C due to the coarsening of GP zones. The short heat input in the laser welding/brazing process did not lead to a significant change in the material strength and microstructure in the HAZ of the AZ31 magnesium alloy. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2017.04.008
  • 2017 • 270 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 • 269 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 • 268 Microstructure evolution in refill friction stir spot weld of a dissimilar Al–Mg alloy to Zn-coated steel
    Suhuddin, U.F.H. and Fischer, V. and Kostka, A. and dos Santos, J.F.
    Science and Technology of Welding and Joining 1-8 (2017)
    In the present study, dissimilar welds of an Al–Mg–Mn alloy and a Zn-coated high-strength low-alloy steel were welded by refill friction stir spot welding. The maximum shear load recorded was approximately 7.8 kN, obtained from the weld produced with a 1600 rev min−1 tool rotational speed. Microstructural analyses showed the formation of a solid–liquid structure of an Al solid solution in Mg–Al-rich Zn liquid, which gives rise to the formation of Zn-rich Al region and microfissuring in some regions during welding. Exposure of steel surface to Mg–Al-rich Zn liquid led to the formation of Fe2Al5 and Fe4Al13 intermetallics. The presence of defective Zn-rich Al regions and Fe–Al intermetallics at the faying surface affects the weld strength. © 2017 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute
    view abstractdoi: 10.1080/13621718.2017.1300744
  • 2017 • 267 Microstructure-based assessment of the corrosion fatigue behavior of the creep-resistant DieMag422 and AE42 magnesium alloys
    Klein, M. and Buhr, P. and Walther, F.
    Solid State Phenomena 258 SSP 530-533 (2017)
    Magnesium alloys offer high potential for lightweight constructions, e.g. in automotive applications. However, their application range is limited due to their low corrosion resistance. In the present study, the influence of corrosion on the microstructure and the depending mechanical properties under cyclic loading were characterized for the creep-resistant DieMag422 (Mg-4Al-2Ba-2Ca) and AE42 magnesium alloys. In this context, fatigue properties in distilled water and sodium chloride solutions were assessed in constant amplitude tests. The results were correlated with corrosion properties of the alloys, which were evaluated by immersion tests. Corrosion- and deformation-induced microstructural changes were observed by light and scanning electron microscopy (SEM), yielding a structure-property-relationship for a comprehensive understanding of mechanical and corrosive deterioration mechanisms. © 2017 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/SSP.258.530
  • 2017 • 266 Modeling Phononic Crystals via the Weighted Relaxed Micromorphic Model with Free and Gradient Micro-Inertia
    Madeo, A. and Collet, M. and Miniaci, M. and Billon, K. and Ouisse, M. and Neff, P.
    Journal of Elasticity 1-25 (2017)
    In this paper the relaxed micromorphic continuum model with weighted free and gradient micro-inertia is used to describe the dynamical behavior of a real two-dimensional phononic crystal for a wide range of wavelengths. In particular, a periodic structure with specific micro-structural topology and mechanical properties, capable of opening a phononic band-gap, is chosen with the criterion of showing a low degree of anisotropy (the band-gap is almost independent of the direction of propagation of the traveling wave). A Bloch wave analysis is performed to obtain the dispersion curves and the corresponding vibrational modes of the periodic structure. A linear-elastic, isotropic, relaxed micromorphic model including both a free micro-inertia (related to free vibrations of the microstructures) and a gradient micro-inertia (related to the motions of the microstructure which are coupled to the macro-deformation of the unit cell) is introduced and particularized to the case of plane wave propagation. The parameters of the relaxed model, which are independent of frequency, are then calibrated on the dispersion curves of the phononic crystal showing an excellent agreement in terms of both dispersion curves and vibrational modes. Almost all the homogenized elastic parameters of the relaxed micromorphic model result to be determined. This opens the way to the design of morphologically complex meta-structures which make use of the chosen phononic material as the basic building block and which preserve its ability of “stopping” elastic wave propagation at the scale of the structure. © 2017 Springer Science+Business Media Dordrecht
    view abstractdoi: 10.1007/s10659-017-9633-6
  • 2017 • 265 Multiphase Characterization of Cu-In-Sn Alloys with 17 at.% Cu and Comparison with Calculated Phase Equilibria
    Tumminello, S. and Del Negro, N. and Carrascal, C. and Fries, S.G. and Alonso, P.R. and Sommadossi, S.
    Journal of Phase Equilibria and Diffusion 38 276-287 (2017)
    Cu-In-Sn alloys are among the suggested materials to replace Pb-Sn alloys traditionally used in joining processes by the electronic industry. Thorough thermodynamic understanding is required for the selection/design of adequate and efficient alloys for specific applications. Understanding the effects that high cost elements such as In have on microstructure and phase stability is imperative for industrial use. In this work ternary alloys were prepared by melting high purity elements (5N) for selected compositions of the 17 at.% Cu isopleth, and cooling down to reproduce process conditions. Chemical composition was determined using scanning electron microscopy equipped with electron probe microanalysis. Measurements of transition temperatures were done by heat-flux differential scanning calorimetry. We present a comprehensive comparison between our experimental results and phase diagram calculations using Liu et al. (J Electron Mater 30:1093, 2001) thermodynamic description based in the CALPHAD method, available in the literature. © 2017 ASM International
    view abstractdoi: 10.1007/s11669-017-0538-7
  • 2017 • 264 Multiscale characterization of White Etching Cracks (WEC) in a 100Cr6 bearing from a thrust bearing test rig
    Danielsen, H.K. and Guzmán, F.G. and Dahl, K.V. and Li, Y.J. and Wu, J. and Jacobs, G. and Burghardt, G. and Fæster, S. and Alimadadi, H. and Goto, S. and Raabe, D. and Petrov, R.
    Wear 370-371 73-82 (2017)
    A common cause for premature bearing failures in wind turbine gearboxes are the so-called White Etching Cracks (WEC). These undirected, three-dimensional cracks are bordered by regions of altered microstructure and ultimately lead to a cracking or spalling of the raceway. An accelerated WEC test was carried out on a FE8 test rig using cylindrical roller thrust bearings made of martensitic 100Cr6 steel. The resulting WECs were investigated with several characterisation techniques. Ultrasonic measurements showed the WEC were mainly located in the region of the overrolled surface in which negative slip occurs, which agrees with hypotheses based on an energetic approach for a prognosis. SEM orientation contrast imaging of the area around WEC revealed an inhomogeneous structure with varied grain sizes and a large amount of defects. Microstructure characterization around the WEA using EBSD showed significant grain refinement. Atom probe tomography showed the microstructure in the undamaged zone has a plate-like martensitic structure with carbides, while no carbides were detected in the WEA where the microstructure consisted of equiaxed 10 nm grains. A three dimensional characterisation of WEC network was successfully demonstrated with X-ray computerized tomography, showing crack interaction with unidentified inclusion-like particles. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2016.11.016
  • 2017 • 263 On the numerical evaluation of local curvature for diffuse interface models of microstructure evolution
    Vakili, S. and Steinbach, I. and Varnik, F.
    Procedia Computer Science 108 1852-1862 (2017)
    Within diffuse interface models for multiphase problems, the curvature of the phase boundary can be expressed as the difference of two terms, a Laplacian and a second, gradient, term of the diffuse interface variable, ø. In phase field simulations of microstructure evolution, the second term is often replaced by f'(ø) = df/dø, where f(ø) is the potential function in the free energy functional of the underlying physical model. We show here that this replacement systematically deteriorates the accuracy in local curvature evaluation as compared to a discretized evaluation of the second term. Analytic estimates reveal that the discretization errors in the Laplacian and in the second term have roughly the same spatial dependence across the interface, thus leading to a cancellation of errors in k. This is confirmed in a test case, where the discretization error can be determined via comparison to the exact solution. If, however, the second term is replaced by a quasi exact expression, the error in δø enters k without being compensated and can obscure the behavior of the local curvature. Due to the antisymmetric variations of this error term, approaches using the average curvature, as obtained from an integral along the interface normal, are immune to this problem. © 2017 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.procs.2017.05.256
  • 2017 • 262 Parallel multiphase field simulations with OpenPhase
    Tegeler, M. and Shchyglo, O. and Kamachali, R.D. and Monas, A. and Steinbach, I. and Sutmann, G.
    Computer Physics Communications 215 173-187 (2017)
    The open-source software project OpenPhase allows the three-dimensional simulation of microstructural evolution using the multiphase field method. The core modules of OpenPhase and their implementation as well as their parallelization for a distributed-memory setting are presented. Especially communication and load-balancing strategies are discussed. Synchronization points are avoided by an increased halo-size, i.e. additional layers of ghost cells, which allow multiple stencil operations without data exchange. Load-balancing is considered via graph-partitioning and sub-domain decomposition. Results are presented for performance benchmarks as well as for a variety of applications, e.g. grain growth in polycrystalline materials, including a large number of phase fields as well as Mg–Al alloy solidification. Program summary Program Title: OpenPhase Program Files doi: http://dx.doi.org/10.17632/2mnv2fvkkk.1 Licensing provisions: GPLv3 Programming language: C++ Nature of problem: OpenPhase[1] allows the simulation of microstructure evolution during materials processing using the multiphase field method. In order to allow an arbitrary number of phase fields active parameter tracking is used, which can cause load imbalances in parallel computations. Solution method: OpenPhase solves the phase field equations using an explicit finite difference scheme. The parallel version of OpenPhase provides load-balancing using over-decomposition of the computational domain and graph-partitioning. Adaptive sub-domain sizes are used to minimize the computational overhead of the over-decomposition, while allowing appropriate load-balance. Additional comments including Restrictions and Unusual features: The distributed-memory parallelism in OpenPhase uses MPI. Shared-memory parallelism is implemented using OpenMP. The library uses C++11 features and therefore requires GCC version 4.7 or higher. [1] www.openphase.de © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.cpc.2017.01.023
  • 2017 • 261 Partial recrystallization of gum metal to achieve enhanced strength and ductility
    Zhang, J.-L. and Tasan, C.C. and Lai, M.J. and Yan, D. and Raabe, D.
    Acta Materialia 135 400-410 (2017)
    Here we present a microstructure design approach which leads to partial recrystallization and nano-precipitation within the same single-step heat treatment. This produces a dual-constituent microstructure in Ti-Nb based gum metal, which consists of nano-ω-particle-rich ultrafine recrystallized grain chains embedded in ω-lean subgrain-containing recovered zones. This partially recrystallized microstructure exhibits an improved strength-ductility combination that surpasses the inverse strength-ductility relationship exhibited by materials with similar composition. The strengthening effects due to precipitates and grain refinement were studied by nanoindentation. The deformation mechanisms of the partially recrystallized material were investigated by in-situ scanning electron microscope tensile tests, micro-strain mapping and post-mortem microstructure characterization. The improved mechanical properties are attributed to the high yield strength of the recrystallized grains and the sequential activation of dislocation slip and dislocation channeling. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.06.051
  • 2017 • 260 Prediction of plasticity and damage initiation behaviour of C45E + N steel by micromechanical modelling
    Wu, B. and Vajragupta, N. and Lian, J. and Hangen, U. and Wechsuwanmanee, P. and Münstermann, S.
    Materials and Design 121 154-166 (2017)
    For large-scale engineering applications, macroscopic phenomenological damage mechanics models with less complexity are usually applied due to their high computational efficiency and simple implementation procedures in finite element simulations. Compared with micromechanical models, however, they also have a significant disadvantage, namely the lack of microstructure sensitivity. This work aims to develop a method to integrate the influence of microstructural features into the parameter calibration of a stress-state-dependent damage mechanics model (the modified Bai-Wierzbicki model) for a C45E + N steel. For this purpose, virtual experiments are performed on an artificial microstructure model to derive the plasticity and damage initiation behaviour for the investigated material. A crystal plasticity model for ferrite along with an empirical strain hardening law for pearlite are assigned to the corresponding constituents in the artificial microstructure model to define their material properties. Nanoindentation tests and numerical analysis are used to calibrate the parameters of the crystal plasticity model. By applying different boundary conditions to the artificial microstructure model, both the plasticity and the damage initiation behaviour under different stress states are calibrated by the virtual experiments. In addition, this approach is also applied to investigating the influence of microstructure on plasticity and damage initiation. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2017.02.032
  • 2017 • 259 Production-and microstructurebased fatigue assessment of metallic AISI 304/430 multilayer materials produced by hot pack rolling
    Schmiedt, A. and Luecker, L. and Kolesnikov, A.G. and Plokhikh, A. and Walther, F.
    Materialpruefung/Materials Testing 59 123-129 (2017)
    Metallic multilayer materials consisting of hundreds or thousands of layers offer a high potential for broad applications in modern technology. A uniform and gradual thinning of layers can be realized by using alloys with different crystal structures in combination with the efficient and high-performance technology of hot pack rolling. However, investigations on fatigue properties, especially to evaluate the influence of the number of layers, are still missing. In the present study, the fatigue behavior of metallic multilayer materials consisting of austenitic and ferritic stainless steels AISI 304 and AISI 430 with 100 and 1400 layers are characterized by applying a time-efficient load increase procedure. Therefore, instrumented stepwise load increase tests were performed to define suitable loading parameters for a convenient comparison of fatigue properties in constant amplitude tests. A benefit of the complex production process leading to 1400 layers was verified concerning the investigated load level in the range of low cycle fatigue with a significant improvement by the factor of 3.5. The alternating current potential drop method for measurements of change in voltage was determined to be most suitable to detect microstructural changes at an early state of fatigue damage for multilayer materials. Microstructures as well as fractured surfaces were investigated using light and scanning electron microscopy to evaluate the results of the two technological manufacturing routes as well as the crack and failure behavior. © Carl Hanser Verlag GmbH &Co. KG.
    view abstractdoi: 10.3139/120.110976
  • 2017 • 258 Recent progress in microstructural hydrogen mapping in steels: quantification, kinetic analysis, and multi-scale characterisation
    Koyama, M. and Rohwerder, M. and Tasan, C.C. and Bashir, A. and Akiyama, E. and Takai, K. and Raabe, D. and Tsuzaki, K.
    Materials Science and Technology (United Kingdom) 1-16 (2017)
    This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement. © 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/02670836.2017.1299276
  • 2017 • 257 Rejuvenation of creep resistance of a Ni-base single-crystal superalloy by hot isostatic pressing
    Ruttert, B. and Bürger, D. and Roncery, L.M. and Parsa, A.B. and Wollgramm, P. and Eggeler, G. and Theisen, W.
    Materials and Design 134 418-425 (2017)
    Ni-base single-crystal turbine blades are exposed to a combination of high temperatures and high stresses during their service life in high-pressure turbines of aero engines or stationary gas turbines. This unavoidably leads to various internal microstructural changes such as rafting and the formation of cavities. This study introduces a creep-rejuvenation-creep test cycle using one miniature Ni-base single-crystal creep specimen. A novel hot isostatic press providing high quenching rates was applied to rejuvenate the damaged microstructure of the specimen after the first high-temperature creep degradation before the same specimen was repeatedly creep-tested under the same initial creep conditions. After rejuvenating, microstructural results obtained from high-resolution microscopy prove that the creep cavities were closed, dislocation densities were re-set, and the original but now slightly finer γ/γ′-microstructure was restored without any recrystallization. The subsequent creep test of the rejuvenated specimen demonstrated that the proposed rejuvenation procedure in this work is a suitable method to reproduce the initial creep behavior and to thus prolong the lifetime of an already crept Ni-base single-crystal specimen. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2017.08.059
  • 2017 • 256 Simulation of the effect of the porous support on flux through an asymmetric oxygen transport membrane
    Unije, U. and Mücke, R. and Niehoff, P. and Baumann, S. and Vaßen, R. and Guillon, O.
    Journal of Membrane Science 524 334-343 (2017)
    Asymmetric membranes provide a low ionic resistance of the functional separation layer together with a high mechanical stability. However, the microstructure of the porous support in the membrane assembly affects the overall flux significantly. This effect was studied by applying the binary friction model (BFM) for the support together with a modified Wagner equation for the dense membrane using transport relevant parameters obtained from micro computed tomography data of a tape cast Ba0.5Sr0.5Co0.8Fe0.2O3– δ support. The influence of different pore diameters and thicknesses of the support were compared for different feed gases (oxygen and air) and flow configurations (3-end, 4-end, assembly orientation). The effect of the support at large pore diameters (&gt;35 µm) for the 3-end mode transport process using oxygen as feed gas, was negligible. This was not the case for the 4-end mode irrespective of the feed gas, and for the 3-end mode using air as feed gas. This was attributed to the binary diffusion term in the BFM. Thin small-pored supports yield the same flux as thick large-pored supports considering a non-linear relationship between thickness and pore size. This can be used for the optimization of the support's microstructure with regards to mechanical strength and permeability. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2016.10.037
  • 2017 • 255 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 • 254 Study of stability of microstructure and residual strain after thermal loading of plasma sprayed YSZ by through surface neutron scanning
    Gibmeier, J. and Back, H.C. and Mutter, M. and Vollert, F. and Rebelo-Kornmeier, J. and Mücke, R. and Vaßen, R.
    Physica B: Condensed Matter (2017)
    Yttria stabilized zirconia (YSZ) is often applied as thermal barrier coating on metal parts as e.g. turbine blades made of nickel base super alloys. The coating process in combination with the preconditioning of the substrate material induces characteristic residual stress distributions in the coating system consisting of topcoat, bondcoat and the substrate material. Knowledge about the residual stress depth distribution in the coating and at the interfaces down to the substrate material is essential for the assessment of the mechanical integrity and the reliability of the coating. In this regard the stability of the microstructure and the residual stresses is of particular interest; hence this forms the scope of our investigations. Yttria (8 wt.%) stabilized zirconia with a NiCoCrAlY bondcoat was deposited by atmospheric plasma spraying (APS) at different spray conditions on a nickel base super alloy substrate material. The coatings were subjected to different heat-treatment processes, i.e. static aging and cyclic thermal loadings. Through surface scanning using neutron diffraction was carried out for the as sprayed condition and for the thermally loaded samples. Based on the measured diffraction data the stability of the microstructure (phases) and the residual strain/stresses through the depths of the coating system were assessed. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.physb.2017.12.014
  • 2017 • 253 Tailoring microstructure, mechanical and tribological properties of NiTi thin films by controlling in-situ annealing temperature
    Momeni, S. and Biskupek, J. and Tillmann, W.
    Thin Solid Films 628 13-21 (2017)
    Magnetron sputtered near equiatomic NiTi thin films were deposited on Si (100) and hot work tool steel substrates. The deposited thin films were in-situ annealed at four different temperatures viz., 80 °C, 305 °C, 425 °C, and 525 °C. The effect of the in-situ annealing temperature on the microstructure of the film, the morphology, as well as mechanical and tribological properties was studied using X-ray diffraction, synchrotron diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, ball-on-disc, scratch test, and three dimensional optical microscopy. The obtained results revealed how the variation of in-situ annealing temperature affects the crystallization, microstructure evolution, as well as mechanical and tribological properties of NiTi thin films. © 2017
    view abstractdoi: 10.1016/j.tsf.2017.02.052
  • 2017 • 252 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 • 251 Topological phase inversion after long-term thermal exposure of nickel-base superalloys: Experiment and phase-field simulation
    Goerler, J.V. and Lopez-Galilea, I. and Mujica Roncery, L. and Shchyglo, O. and Theisen, W. and Steinbach, I.
    Acta Materialia 124 151-158 (2017)
    Ni-base superalloys are materials which are designed to resist extreme thermal and mechanical conditions. In this regard, an essential factor is their microstructure consisting of γ′ precipitates embedded in a γ matrix. The application of superalloys at high temperatures can however induce the topological phase inversion, where the γ′-phase topologically becomes the matrix phase, resulting in subpar material properties. In this work, the topological inversion is analyzed via experiment and phase-field simulation. The evolution of the microstructure has been quantified in the second generation single crystal Ni-base superalloy ERBO/1, which belongs to the family of CMSX-4, submitted to long-term aging at 1100° C for up to 250 h. Phase-field simulations carried out using a multi phase-field approach deliver insight into the microstructure evolution driven by the loss of coherency of the γ′ precipitates, which is induced by the accumulation of dislocations at the γ/γ′ interfaces. The obtained simulation results are in good agreement with the experimental results, and indicate that the mechanisms causing the topological inversion are linked to the accommodation of the lattice misfit, which enables coalescence and ripening of γ′ precipitates. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.10.059
  • 2017 • 250 Towards prediction of springback in deep drawing using a micromechanical modeling scheme
    Vajragupta, N. and Ul Hassan, H. and Hartmaier, A.
    Procedia Engineering 207 60-65 (2017)
    Deep drawing is one of the most commonly used sheet metal forming processes, which can produce metal parts at a high rate. One of the major problems in deep drawing is springback, which is mainly elastic deformation occurring when the tool is removed. The focus of this work is the prediction of springback in deep drawing for DC04 steel using a micromechanical modeling scheme. A novel method is used for the characterization of material that leads to cyclic stress-strain curve. Simulations are performed with the Yoshida Uemori (YU) model for the prediction of springback for a U draw-bend geometry. The maximum deviation between the geometries of experiment and the springback simulation for hat geometry is 2.2 mm. It is shown that this micromechanical modeling scheme allows us to relate the influence of the microstructure to the springback prediction. © 2017 The Authors. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2017.10.739
  • 2017 • 249 Transmission electron microscopy study of the microstructural evolution during higherature and low-stress (011) [11] shear creep deformation of the superalloy single crystal LEK 94
    Agudo Jácome, L. and Göbenli, G. and Eggeler, G.
    Journal of Materials Research 32 4491-4502 (2017)
    The present work describes the shear creep behavior of the superalloy LEK 94 at temperatures between 980 and 1050 °C and shear stresses between 50 and 140 MPa for loading on the macroscopic crystallographic shear system (MCSS) (011) . The strain rate versus strain curves show short primary and extended secondary creep regimes. We find an apparent activation energy for creep of Q app = 466 kJ/mol and a Norton-law stress exponent of n = 6. With scanning transmission electron microscopy, we characterize three material states that differ in temperature, applied stress, and accumulated strain/time. Rafting develops perpendicular to the maximum principal stress direction, γ channels fill with dislocations, superdislocations cut γ′ particles, and dislocation networks form at γ/γ′ interfaces. Our findings are in agreement with previous results for higherature and low-stress [001] and [110] tensile creep testing, and for shear creep testing of the superalloys CMSX-4 and CMSX-6 on the MCSSs (111) and (001)[100]. The parameters that characterize the evolving γ/γ′ microstructure and the evolving dislocation substructures depend on creep temperature, stress, strain, and time. © 2017 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2017.336
  • 2016 • 248 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 • 247 Ab initio-guided design of twinning-induced plasticity steels
    Raabe, D. and Roters, F. and Neugebauer, J. and Gutierrez-Urrutia, I. and Hickel, T. and Bleck, W. and Schneider, J.M. and Wittig, J.E. and Mayer, J.
    MRS Bulletin 41 320-325 (2016)
    The twinning-induced plasticity effect enables designing austenitic Fe-Mn-C-based steels with >70% elongation with an ultimate tensile strength >1 GPa. These steels are characterized by high strain hardening due to the formation of twins and complex dislocation substructures that dynamically reduce the dislocation mean free path. Both mechanisms are governed by the stacking-fault energy (SFE) that depends on composition. This connection between composition and substructure renders these steels ideal model materials for theory-based alloy design: Ab initio-guided composition adjustment is used to tune the SFE, and thus, the strain-hardening behavior for promoting the onset of twinning at intermediate deformation levels where the strain-hardening capacity provided by the dislocation substructure is exhausted. We present thermodynamic simulations and their use in constitutive models, as well as electron microscopy and combinatorial methods that enable validation of the strain-hardening mechanisms. Copyright © 2016 Materials Research Society.
    view abstractdoi: 10.1557/mrs.2016.63
  • 2016 • 246 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 • 245 Atomistically informed extended Gibbs energy description for phase-field simulation of tempering of martensitic steel
    Shchyglo, O. and Hammerschmidt, T. and Čak, M. and Drautz, R. and Steinbach, I.
    Materials 9 (2016)
    In this study we propose a unified multi-scale chemo-mechanical description of the BCT (Body-Centered Tetragonal) to BCC (Body-Centered Cubic) order-disorder transition in martensitic steel by adding the mechanical degrees of freedom to the standard CALPHAD (CALculation of PHAse Diagrams) type Gibbs energy description. The model takes into account external strain, the effect of carbon composition on the lattice parameter and elastic moduli. The carbon composition effect on the lattice parameters and elastic constants is described by a sublattice model with properties obtained from DFT (Density Functional Theory) calculations; the temperature dependence of the elasticity parameters is estimated from available experimental data. This formalism is crucial for studying the kinetics of martensite tempering in realistic microstructures. The obtained extended Gibbs energy description opens the way to phase-field simulations of tempering of martensitic steel comprising microstructure evolution, carbon diffusion and lattice symmetry change due to the ordering/disordering of carbon atoms under multiaxial load. © 2016 by the authors.
    view abstractdoi: 10.3390/ma9080669
  • 2016 • 244 Characterisation of the mechanical and corrosive properties of newly developed glass-steel composites
    Lyubimova, O. and Gridasova, E. and Gridasov, A. and Frieling, G. and Klein, M. and Walther, F.
    Materiali in Tehnologije 50 95-100 (2016)
    This paper presents a preliminary research about a newly developed glass-steel composite created with diffusion welding of glass (C49-1) and carbon steel (St3sp). The main conclusions on the process of forming a diffusion zone during the welding of glass and steel are made. The results of quasi-static and cyclic mechanical tests and corrosion investigations are presented and interpreted on the basis of the microstructure developed during diffusion welding and described in this article.
    view abstractdoi: 10.17222/mit.2014.305
  • 2016 • 243 Combinatorial screening of the microstructure–property relationships for Fe–B–X stiff, light, strong and ductile steels
    Baron, C. and Springer, H. and Raabe, D.
    Materials and Design 112 131-139 (2016)
    We systematically screened the mechanical, physical and microstructural properties of the alloy systems Fe–10 B–5 X (at.%; X = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W), in order to identify novel metal matrix composite steels as next generation lightweight materials. The alloys were synthesised and processed by bulk liquid metallurgical techniques, and subsequently analysed for their mechanical and physical properties (i.e. Young's modulus, density, tensile strength and ductility) as well their microstructure and constitution. From the wide variety of observed boride phases and microstructures and resultant different properties, Cr and Zr additions were found to be most effective. Cr qualifies well as the high fraction of M2B borides of spherical morphology allows achieving a similar stiffness/density ratio and mechanical performance as the reference Ti alloyed materials, but at substantially reduced alloy costs. Zr blended composites on the other hand are softer and less ductile, but the alignment of spiky ZrB2 particles during swaging led to a much higher – though most probably anisotropic – specific modulus. Consequences and recommendations for future alloy and processing design are outlined and discussed. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2016.09.065
  • 2016 • 242 Comparison of fatigue life assessment by analytical, experimental and damage accumulation modelling approach for steel SAE 1045
    Imran, M. and Siddique, S. and Guchinsky, R. and Petinov, S. and Walther, F.
    Fatigue and Fracture of Engineering Materials and Structures 39 1138-1149 (2016)
    Fatigue life assessment for two-phase steel SAE 1045 has been carried out by experimental and simulation techniques. Analytical approach, termed as fatigue lifetime calculation, was employed making use of a load increase testing procedure and constant amplitude tests equipped with measurement techniques – plastic strain amplitude, change in temperature and change in electrical potential difference. The predicted fatigue life has been validated by constant amplitude tests and compared with fatigue life estimation by microstructure-based simulation. Simulation has been carried out over the complete cross section of the specimen. The simulation uses damage accumulation in the gage section of the specimen culminating in the macro-crack propagation, taking into account the inhomogeneous fatigue resistance of the material element. The results show that at the initial intervals of high cycle fatigue range at relatively higher stress amplitudes, the experimental and simulation results are in agreement; whereas in the (high cycle fatigue) region at relatively low stress amplitudes, the simulation results were found more optimistic and the corresponding fatigue scatter is also increased. Each scatter is attributed to the relatively small number of analysed models of the material structure. Scanning electron microscope was used to determine volume fraction of the microstructure for simulation. Fatigue fracture surface analysis shows that crack initiated from internal defect of material and crack propagation is driven by silicon oxide inclusion. © 2016 Wiley Publishing Ltd.
    view abstractdoi: 10.1111/ffe.12426
  • 2016 • 241 Comparison of microstructure and mechanical properties of 316 L austenitic steel processed by selective laser melting with hot-isostatic pressed and cast material
    Röttger, A. and Geenen, K. and Windmann, M. and Binner, F. and Theisen, W.
    Materials Science and Engineering A 678 365-376 (2016)
    Besides the chemical composition, the manufacturing route primarily determines a material's properties. In this work, the influence of the manufacturing process of the 316 L grade austenitic steel on the microstructure and the resulting material properties were investigated. Thus, the microstructure and mechanical properties of cast and solution annealed, as well as steel powder densified by hot-isostatic pressing (HIP), selective laser melting (SLM) and SLM+HIP, were compared. A SLM parameter study illustrates that the porosity of SLM-densified specimens can be reduced with direction of a higher exposure time and a smaller point distance. With an additional treatment by HIP, the porosity scarcely changes, while cracks are reduced. The mechanical properties were investigated depending on the manufacturing process, and the influence of the sample build up by SLM was examined. High mechanical values have been obtained; in particular, the yield strength in the SLM-densified condition is much higher than in cast or HIP condition, as a result of the smaller grain size. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2016.10.012
  • 2016 • 240 Computational modeling of dual-phase steels based on representative three-dimensional microstructures obtained from EBSD data
    Brands, D. and Balzani, D. and Scheunemann, L. and Schröder, J. and Richter, H. and Raabe, D.
    Archive of Applied Mechanics 86 575-598 (2016)
    The microstructure of dual-phase steels consisting of a ferrite matrix with embedded martensite inclusions is the main contributor to the mechanical properties such as high ultimate tensile strength, high work hardening rate, and good ductility. Due to the composite structure and the wide field of applications of this steel type, a wide interest exists in corresponding virtual computational experiments. For a reliable modeling, the microstructure should be included. For that reason, in this paper we follow a computational strategy based on the definition of a representative volume element (RVE). These RVEs will be constructed by a set of tomographic measurements and mechanical tests. In order to arrive at more efficient numerical schemes, we also construct statistically similar RVEs, which are characterized by a lower complexity compared with the real microstructure but which represent the overall material behavior accurately. In addition to the morphology of the microstructure, the austenite–martensite transformation during the steel production has a relevant influence on the mechanical properties and is considered in this contribution. This transformation induces a volume expansion of the martensite phase. A further effect is determined in nanoindentation test, where it turns out that the hardness in the ferrite phase increases exponentially when approaching the martensitic inclusion. To capture these gradient properties in the computational model, the volumetric expansion is applied to the martensite phase, and the arising equivalent plastic strain distribution in the ferrite phase serves as basis for a locally graded modification of the ferritic yield curve. Good accordance of the model considering the gradient yield behavior in the ferrite phase is observed in the numerical simulations with experimental data. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00419-015-1044-1
  • 2016 • 239 Corrosion fatigue assessment of creep-resistant magnesium alloy Mg-4Al-2Ba-2Ca in aqueous sodium chloride solution
    Wittke, P. and Klein, M. and Dieringa, H. and Walther, F.
    International Journal of Fatigue 83 59-65 (2016)
    Low corrosion resistance of magnesium alloys strongly limits their application range. This study aims at the investigation of corrosion influence on microstructure and depending mechanical properties of newly developed magnesium alloy Mg-4Al-2Ba-2Ca. The fatigue properties of this creep-resistant magnesium alloy were investigated under three corrosive environments: double distilled water, 0.01 and 0.1 mol L-1 NaCl solutions. Potentiodynamic polarization measurements and immersion tests were performed to estimate the corrosion behaviour. Specimen surfaces were observed using light and scanning electron microscopy for microstructure-related assessment of corrosion mechanisms. The corrosion fatigue behaviour was characterized in continuous load increase tests using plastic strain and electrochemical measurements. Continuous load increase tests allow estimating the fatigue limit and determining the failure stress amplitude with one single specimen. Fatigue results showed a significant decrease in the estimated fatigue limit and determined failure stress amplitude with increasing corrosion impact of the environments. This corrosion-structure-property relation was quantitatively described by means of model-based correlation approaches and failure hypotheses. Plastic strain amplitude and deformation-induced changes in electrochemical measurands can be equivalently applied for precise corrosion fatigue assessment. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijfatigue.2015.04.001
  • 2016 • 238 Crystal plasticity modeling of size effects in rolled multilayered Cu-Nb composites
    Jia, N. and Raabe, D. and Zhao, X.
    Acta Materialia 111 116-128 (2016)
    We present size-dependent crystal plasticity finite element simulations of the deformation microstructure, plastic flow and texture evolution in multilayered Cu-Nb composites during cold rolling. The model is based on a constitutive framework incorporating thermally activated dislocation slip, mechanical twinning and non-crystallographic shear banding. It also accounts for the dislocation density evolution and its dependence on initial grain size. By performing a series of quadricrystal simulations considering characteristic heterophase microstructures, the underlying micromechanics and texture of the composites are explored. Significant shear banding occurs in both phases, primarily determined by their initial orientations. For each phase, the activation of shear banding is also affected by the mechanical properties and orientations of the adjacent phase. For composites with an initial single layer thickness of 35 μm or 4 μm, the layer thickness reduction after rolling is non-uniform and the typical rolling textures for bulk pure metals develop in the respective phases. For the 75 nm initial single layer thickness composite, both phases are reduced uniformly in thickness and the initial orientations prevail. The predictions agree well with experimental observations in cold-rolled Cu-Nb thin films. The simulations reveal that for the composites with initial single layer thickness of micrometer scale, dislocation slip is the dominant deformation mechanism although shear banding increasingly carries the deformation at larger strains. For the samples with initial single layer thickness of a few tens of nanometers, shear banding and dislocation slip are the dominant mechanisms. This transition in deformation characteristics leads to different textures in micrometer- and nanometer-scaled multilayers. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.03.055
  • 2016 • 237 Damage characterization of high-strength multiphase steels
    Heibel, S. and Nester, W. and Clausmeyer, T. and Tekkaya, A.E.
    IOP Conference Series: Materials Science and Engineering 159 (2016)
    High-strength steels show an entirely different material behavior than conventional deep-drawing steels. This fact is caused among others by the multiphase nature of their structure. The Forming Limit Diagram as the classic failure criterion in forming simulation is only partially suitable for this class of steels. An improvement of the failure prediction can be obtained by using damage mechanics. Therefore, an exact knowledge of the material-specific damage is essential for the application of various damage models. In this paper the results of microstructure analysis of a dual-phase steel and a complex-phase steel with a tensile strength of 1000 MPa are shown comparatively at various stress conditions. The objective is to characterize the basic damage mechanisms and based on this to assess the crack sensitivity of both steels. First a structural analysis with regard to non-metallic inclusions, the microstructural morphology, phase identification and the difference in microhardness between the structural phases is carried out. Subsequently, the development of the microstructure at different stress states between uniaxial and biaxial tension is examined. The damage behavior is characterized and quantified by the increase in void density, void size and the quantity of voids. The dominant damage mechanism of the dual-phase steel is the void initiation at phase boundaries, within harder structural phases and at inclusions. In contrast the complex-phase steel shows a significant growth of a smaller amount of voids which initiate only at inclusions. To quantify the damage tolerance and the susceptibility of cracking the criterion of the fracture forming limit line (FFL) is used. The respective statements are supported by results of investigations regarding the edge-crack sensitivity. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/159/1/012013
  • 2016 • 236 Determination of the young modulus of Ti-TiAl3 metallic intermetallic laminate composites by nano-indentation
    Yener, T. and Güler, S. and Siddique, S. and Walther, F. and Zeytin, S.
    Acta Physica Polonica A 129 604-606 (2016)
    Nano-indentation is an important technique to determine the Young modulus of multiphase materials where normal tensile tests are not appropriate. In this work, Ti-TiAl3 metallic-intermetallic laminate composites have been fabricated successfully in open atmosphere using commercial purity Al and Ti foils with 250 μm and 500 μm initial thicknesses, respectively. Sintering process was performed at 700 °C under 2 MPa pressure for 7.5 h. Mechanical properties including the Young modulus were determined after manufacturing. The Young moduli of metallic and intermetallic phases were determined as 89 GPa and 140 GPa, respectively. Microstructure analyses showed that aluminum foil was almost consumed by forming a titanium aluminide intermetallic compound. Titanium aluminides grow up through spherical shaped islands and metallic-intermetallic interface is a wavy form in Ti-Al system. Thus, the final microstructure consists of alternating layers of intermetallic compound and unreacted Ti metal. Microstructure and phase characterizations were performed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Hardness of test samples was determined as 600 HV for intermetallic zone and 130 HV for metallic zone by the Vickers indentation method.
    view abstractdoi: 10.12693/APhysPolA.129.604
  • 2016 • 235 Direct metal deposition of refractory high entropy alloy MoNbTaW
    Dobbelstein, H. and Thiele, M. and Gurevich, E.L. and George, E.P. and Ostendorf, A.
    Physics Procedia 83 624-633 (2016)
    Alloying of refractory high entropy alloys (HEAs) such as MoNbTaW is usually done by vacuum arc melting (VAM) or powder metallurgy (PM) due to the high melting points of the elements. Machining to produce the final shape of parts is often needed after the PM process. Casting processes, which are often used for aerospace components (turbine blades, vanes), are not possible. Direct metal deposition (DMD) is an additive manufacturing technique used for the refurbishment of superalloy components, but generating these components from the bottom up is also of current research interest. MoNbTaW possesses high yield strength at high temperatures and could be an alternative to state-of-the-art materials. In this study, DMD of an equimolar mixture of elemental powders was performed with a pulsed Nd:YAG laser. Single wall structures were built, deposition strategies developed and the microstructure of MoNbTaW was analyzed by back scattered electrons (BSE) and energy dispersive X-ray (EDX) spectroscopy in a scanning electron microscope. DMD enables the generation of composition gradients by using dynamic powder mixing instead of pre-alloyed powders. However, the simultaneous handling of several elemental or pre-alloyed powders brings new challenges to the deposition process. The influence of thermal properties, melting point and vapor pressure on the deposition process and chemical composition will be discussed. © 2016 The Authors.
    view abstractdoi: 10.1016/j.phpro.2016.08.065
  • 2016 • 234 Effect of intercritical deformation on microstructure and mechanical properties of a low-silicon aluminum-added hot-rolled directly quenched and partitioned steel
    Tan, X.-D. and Xu, Y.-B. and Ponge, D. and Yang, X.-L. and Hu, Z.-P. and Peng, F. and Ju, X.-W. and Wu, D. and Raabe, D.
    Materials Science and Engineering A 656 200-215 (2016)
    Here, we applied hot-rolling in conjunction with direct quenching and partitioning (HDQ&P) processes with different rolling schedules to a low-C low-Si Al-added steel. Ferrite was introduced into the steel by intercritical rolling and air cooling after hot-rolling. The effect of intercritcal deformation on the microstructure evolution and mechanical properties was investigated. The promotion of austenite stabilization and the optimization of the TRIP effect due to a moderate degree of intercritical deformation were systematically explored. The results show that the addition of 1.46 wt% of Al can effectively promote ferrite formation. An intercritical deformation above 800 °C can result in a pronounced bimodal grain size distribution of ferrite and some elongated ferrite grains containing sub-grains. The residual strain states of both austenite and ferrite and the occurrence of bainite transformation jointly increase the retained austenite fraction due to its mechanical stabilization and the enhanced carbon partitioning into austenite from its surrounding phases. An intercritical deformation below 800 °C can profoundly increase the ferrite fraction and promote the recrystallization of deformed ferrite. The formation of this large fraction of ferrite enhances the carbon enrichment in the untransformed austenite and retards the bainite transformation during the partitioning process and finally enhances martensite transformation and decreases the retained austenite fraction. The efficient TRIP effect of retained austenite and the possible strain partitioning of bainite jointly improve the work hardening and formability of the steel and lead to the excellent mechanical properties with relatively high tensile strength (905 MPa), low yield ratio (0.60) and high total elongation (25.2%). © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2016.01.040
  • 2016 • 233 Effects of Mn additions on microstructure and properties of Fe–TiB2 based high modulus steels
    Baron, C. and Springer, H. and Raabe, D.
    Materials and Design 111 185-191 (2016)
    We studied the effects of Mn additions from 0 to 30 wt.% on microstructure, mechanical and physical properties of liquid metallurgy synthesised high modulus steels in hypo- and hyper-eutectic TiB2 concentrations. While Mn has little effect on density, both Young's modulus and mechanical properties were strongly affected by the achieved wide spectrum of matrix microstructures, ranging from ferrite to martensite, reverted austenite, ε-martensite and austenite. Mn additions of 20 and 30 wt.% did not translate into enhanced mechanical performance despite the higher inherent ductility of the predominantly austenitic matrix, and instead eliminate the intended weight saving potential by significantly reducing the Young's modulus. Martensitic matrices of Mn concentrations of 10 wt.%, on the other hand, are favourable for improved matrix/particle co-deformation without sacrificing too much of the composites' stiffness. In hypo-eutectic Fe – TiB2 based steels, mechanical properties on the level of high strength dual phase steels could be achieved (~ 900 MPa UTS and 20% tensile elongation) but with an enhanced Young's modulus of 217 GPa and reduced density of 7460 kg m− 3. These significantly improved physical and mechanical properties render Mn alloyed high modulus steels promising candidate materials for next generation lightweight structural applications. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2016.09.003
  • 2016 • 232 Impact of hot isostatic pressing on microstructures of CMSX-4 Ni-base superalloy fabricated by selective electron beam melting
    Ruttert, B. and Ramsperger, M. and Mujica Roncery, L. and Lopez-Galilea, I. and Körner, C. and Theisen, W.
    Materials and Design 110 720-727 (2016)
    Selective electron beam melting (SEBM) is a powder-bed-based additive manufacturing process that was used to produce cylindrical and columnar-grained parts made of Ni-base superalloy CMSX-4 from pre-alloyed and atomized powder. SEBM is characterized by high temperature gradients during solidification, which results in a very fine microstructure that is several orders of magnitude smaller than in conventionally cast material. This opens up new perspectives regarding time-consuming solution heat treatment. Nevertheless, microstructural heterogeneities, such as segregation and porosity, are still present on a much smaller scale, and also the high susceptibility to cracking of this alloy class during welding has to be taken into account. Since the latest generation of hot isostatic presses (HIP) are able to simultaneously heat-treat and eliminate porosity owing to their quenching capability, such a HIP is used in this work. The influence of different HIPing-heat-treatment-strategies with variation of temperature and time at a constant pressure on the SEBM-microstructure was investigated with emphasis primarily on segregation and porosity. The results demonstrate that only a few minutes of solutioning are sufficient to dissolve segregations and to close pores. The initial degree of homogeneity of the SEBM-material is responsible for the short solutioning-time. © 2016
    view abstractdoi: 10.1016/j.matdes.2016.08.041
  • 2016 • 231 Improving the mechanical properties of Fe – TiB2 high modulus steels through controlled solidification processes
    Zhang, H. and Springer, H. and Aparicio-Fernández, R. and Raabe, D.
    Acta Materialia 118 187-195 (2016)
    We investigated novel pathways to improve the mechanical properties of liquid metallurgy produced Fe – TiB2 based high modulus steels (HMS) by controlled solidification kinetics and subsequent thermo-mechanical treatments. The solidification rate was varied by casting of hyper-eutectic alloys (20 vol% TiB2) into moulds with differing internal thickness. Ingots between 5 and 40 mm thickness exhibited irregular particle microstructure consisting of sharp-edged coarse primary particles (increasingly clustered with slower solidification) and closely spaced irregular lamellae. Casting defects can be alleviated by hot rolling, but the mechanical properties remain unsatisfactory. Increasing the solidification rate results only at mould thicknesses of 4 mm and below in a significant refinement of the particle microstructure, necessitating liquid metal deposition techniques to utilise it for obtained improved mechanical performance of HMS. Decreasing the solidification rate causes density-induced floatation of the primary particles, which can be used in block-casting for the production of alloys consisting of small and spheroidised eutectic particles, exhibiting high ductility and superior toughness. Annealing just above the solidus-temperature allows the eutectic zones to liquefy and sink, leaving only primary TiB2 particles behind in the top zone of the alloy. Despite the increased particle fraction up to 24 vol%, both strength, specific modulus and ductility are improved over standard processed HMS alloys with 20 vol% TiB2. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.07.056
  • 2016 • 230 Influence of temperature, pressure, and cooling rate during hot isostatic pressing on the microstructure of an SX Ni-base superalloy
    Mujica Roncery, L. and Lopez-Galilea, I. and Ruttert, B. and Huth, S. and Theisen, W.
    Materials and Design 97 544-552 (2016)
    This work investigates the application of hot isostatic pressing for heat treatment of the single-crystal Ni-base superalloy ERBO/1. Recent progress regarding incorporation of quenching within hot isostatic pressing enables heat treatments to be performed so that the microstructures can be frozen at a desired point. The influence of the temperature, pressure, and cooling method (quenching, natural convection, and slow cooling) as well as the cooling rate after solutioning-HIP treatment on pore densification and γ/. γ'-morphology was measured. Temperatures above γ'-solvus resulted in a greater efficiency of the porosity reduction. At super-solvus temperatures, pressures above 75. MPa are sufficient enough to annihilate the porosity. The cooling rate after HIP-solutioning treatment has a major influence on the γ'-particle size and shape. Quenching with 45-20. K/s at 100. MPa leads to high number density and monomodal distribution of γ'-particles with sizes around 140. nm. In contrast, slow cooling rate of 0.33. K/s leads to γ'-precipitate sizes of 720. nm. Moreover, an integrated heat treatment at 100. MPa, which consisted of solutioning and aging in the HIP, was successfully applied. It led to smaller γ'-particle sizes and narrower γ-channels compared to the conventionally heat-treated material and also almost no porosity. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2016.02.051
  • 2016 • 229 Investigation of the brazing characteristics of a new iron-based brazing filler metal
    Tillmann, W. and Wojarski, L. and Manka, M. and Trelenberg, A.
    Welding in the World 60 869-875 (2016)
    High temperature applications of new class of iron-based filler metals provide brazements with high corrosion resistance and mechanical properties. These brazements are cost-effective alternative to those made of the conventional brazing alloys. However, a wiser usage demands a deeper understanding of the wetting as well as gap filling behavior in conjunction with the resulting microstructure, which is mainly influenced by the applied brazing cycles. Therefore, this paper presents results of the investigation of specific brazing fundamentals for the new iron-based brazing alloy Fe-24Cr-20Ni-10Cu-7P-5Mn-5Si. Followed by DTA/DSC measurements, the spreading and gap filling behavior were examined by using stainless steel AISI 304 as base material. In wetting tests and wedge-gap experiments, the influence of the applied brazing temperature and the dwell time were investigated for vacuum brazing processes. The resulting microstructure was evaluated using a scanning electron microscope (SEM), equipped with an energy dispersive X-ray spectroscopy (EDS). Finally, strength tests were conducted in order to determine the influence of the brazing parameters on the mechanical properties of the joint. © 2016, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-016-0346-4
  • 2016 • 228 Investigation of the self-healing sliding wear characteristics of NiTi-based PVD coatings on tool steel
    Momeni, S. and Tillmann, W.
    Wear 368-369 53-59 (2016)
    Excellent damping capacity and superelasticity of the bulk NiTi shape memory alloy (SMA) makes it a suitable material of choice for tools in machining process as well as tribological systems. Although thin film of NiTi SMA has a same damping capacity as NiTi bulk alloys, it has a poor mechanical properties and undesirable tribological performance. This study aims at eliminating these application limitations for NiTi thin films. In order to achieve this goal, NiTi thin films were magnetron sputtered as an interlayer between reactively sputtered hard TiCN coatings and hot work tool steel substrates. The microstructure, composition, crystallographic phases, mechanical and tribological properties of the deposited thin films were analyzed by using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), nanoindentation, ball-on-disc, scratch test, and three dimensional (3D) optical microscopy. It was found that under a specific coating architecture, the superelasticity of NiTi inter-layer can be combined with high hardness and wear resistance of TiCN protective layers. The obtained results revealed that the thickness of NiTi interlayers is an important factor controlling mechanical and tribological performance of bilayer composite coating systems. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2016.08.004
  • 2016 • 227 Investigation on femto-second laser irradiation assisted shock peening of medium carbon (0.4% C) steel
    Majumdar, J.D. and Gurevich, E.L. and Kumari, R. and Ostendorf, A.
    Applied Surface Science 364 133-140 (2016)
    In the present study, the effect of femtosecond laser irradiation on the peening behavior of 0.4% C steel has been evaluated. Laser irradiation has been conducted with a 100 μJ and 300 fs laser with multiple pulses under varied energy. Followed by laser irradiation, a detailed characterization of the processed zone was undertaken by scanning electron microscopy, and X-ray diffraction technique. Finally, the residual stress distribution, microhardness and wear resistance properties of the processed zone were also evaluated. Laser processing leads to shock peening associated with plasma formation and its expansion, formation of martensite and ferrito-pearlitic phase in the microstructure. Due to laser processing, there is introduction of residual stress on the surface which varies from high tensile (140 MPa) to compressive (-335 MPa) as compared to 152 MPa of the substrate. There is a significant increase in microhardness to 350-500 VHN as compared to 250 VHN of substrate. The fretting wear behavior against hardened steel ball shows a significant reduction in wear depth due to laser processing. Finally, a conclusion of the mechanism of wear has been established. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.12.058
  • 2016 • 226 Magnetic microstructure in a stress-annealed Fe73.5Si15.5B7Nb3Cu1 soft magnetic alloy observed using off-axis electron holography and Lorentz microscopy
    Kovács, A. and Pradeep, K.G. and Herzer, G. and Raabe, D. and Dunin-Borkowski, R.E.
    AIP Advances 6 (2016)
    Fe-Si-B-Nb-Cu alloys are attractive for high frequency applications due to their low coercivity and high saturation magnetization. Here, we study the effect of stress annealing on magnetic microstructure in Fe73.5Si15.5B7Nb3Cu1 using off-axis electron holography and the Fresnel mode of Lorentz transmission electron microscopy. A stress of 50 MPa was applied to selected samples during rapid annealing for 4 s, resulting in uniaxial anisotropy perpendicular to the stress direction. The examination of focused ion beam milled lamellae prepared from each sample revealed a random magnetic domain pattern in the sample that had been rapidly annealed in the absence of stress, whereas a highly regular domain pattern was observed in the stress-annealed sample. We also measured a decrease in domain wall width from ∼ 94 nm in the sample annealed without stress to ∼ 80 nm in the stress-annealed sample. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4942954
  • 2016 • 225 Microcapsule Buckling Triggered by Compression-Induced Interfacial Phase Change
    Salmon, A.R. and Parker, R.M. and Groombridge, A.S. and Maestro, A. and Coulston, R.J. and Hegemann, J. and Kierfeld, J. and Scherman, O.A. and Abell, C.
    Langmuir 32 10987-10994 (2016)
    There is an emerging trend toward the fabrication of microcapsules at liquid interfaces. In order to control the parameters of such capsules, the interfacial processes governing their formation must be understood. Here, poly(vinyl alcohol) films are assembled at the interface of water-in-oil microfluidic droplets. The polymer is cross-linked using cucurbit[8]uril ternary supramolecular complexes. It is shown that compression-induced phase change causes the onset of buckling in the interfacial film. On evaporative compression, the interfacial film both increases in density and thickens, until it reaches a critical density and a phase change occurs. We show that this increase in density can be simply related to the film Poisson ratio and area compression. This description captures fundamentals of many compressive interfacial phase changes and can also explain the observation of a fixed thickness-to-radius ratio at buckling, (TR)buck. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.6b03011
  • 2016 • 224 Metastable high-entropy dual-phase alloys overcome the strength-ductility trade-off
    Li, Z. and Pradeep, K.G. and Deng, Y. and Raabe, D. and Tasan, C.C.
    Nature 534 227-230 (2016)
    Metals have been mankind's most essential materials for thousands of years; however, their use is affected by ecological and economical concerns. Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency. However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength-ductility trade-off. Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned. We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys. In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength. Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility. This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials. This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys.
    view abstractdoi: 10.1038/nature17981
  • 2016 • 223 Microstructural analysis in the Fe-30.5Mn-8.0Al-1.2C and Fe-30.5Mn-2.1Al-1.2C steels upon cold rolling
    Souza, F.M. and Padilha, A.F. and Gutierrez-Urrutia, I. and Raabe, D.
    Revista Escola de Minas 69 167-173 (2016)
    Electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) were used to examine microstructural changes of the austenitic low-density Fe-30.5Mn-8.0Al-1.2C (8Al) and Fe-30.5Mn-2.1Al-1.2C (2Al) (wt.%) steels during cold rolling. As the strain increased, deformation mechanisms, such as stacking faults, slip, mechanical twinning, and shear banding were activated in both steels cold rolled up to strain of 0.69. Only slip was noted in these steels at low strain (ε=0.11) and slip dominance was detected in the 8Al steel at higher strains. Shear banding became active at higher strain (ε~0.7) in these materials. An inhomogeneous microstructure formed in both alloys at such strain level. More extensive mechanical twinning in the 2Al alloy than that in the 8Al alloy was observed. Fish bone-like structure patterns were revealed in the 8Al steel and a river-like structure in the 2Al steel. Detailed microstructure features as elongated and fragmented grains along the rolling direction (RD) were found for both steels, as already observed in other high-Mn steels. These deformed structures are composed by lamellar packets which can contain mechanical twins or slip lines and shear bands. © 2016, Escola de Minas. All rights reserved.
    view abstractdoi: 10.1590/0370-44672015690097
  • 2016 • 222 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 • 221 Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high-entropy alloy
    Laplanche, G. and Kostka, A. and Horst, O.M. and Eggeler, G. and George, E.P.
    Acta Materialia 118 152-163 (2016)
    At low homologous temperatures (down to cryogenic temperatures), the CrMnFeCoNi high-entropy alloy possesses good combination of strength, work hardening rate (WHR), ductility, and fracture toughness. To improve understanding of the deformation mechanisms responsible for its mechanical properties, tensile tests were performed at liquid nitrogen and room temperature (77 K and 293 K) and interrupted at different strains to quantify the evolution of microstructure by transmission electron microscopy. Dislocation densities, and twin widths, their spacings, and volume fractions were determined. Nanotwins were first observed after true strains of ∼7.4% at 77 K and ∼25% at 293 K; at lower strains, deformation occurs by dislocation plasticity. The tensile stress at which twinning occurs is 720 ± 30 MPa, roughly independent of temperature, from which we deduce a critical resolved shear stress for twinning of 235 ± 10 MPa. In the regime where deformation occurs by dislocation plasticity, the shear modulus normalized WHR decreases with increasing strain at both 77 K and 293 K. Beyond ∼7.4% true strain, the WHR at 77 K remains constant at a high value of G/30 because twinning is activated, which progressively introduces new interfaces in the microstructure. In contrast, the WHR at room temperature continues to decrease with increasing strain because twinning is not activated until much later (close to fracture). Thus, the enhanced strength-ductility combination at 77 K compared to 293 K is primarily due to twinning starting earlier in the deformation process and providing additional work hardening. Consistent with this, when tensile specimens were pre-strained at 77 K to introduce nanotwins, and subsequently tested at 293 K, flow stress and ductility both increased compared to specimens that were not pre-strained. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.07.038
  • 2016 • 220 Modeling of low-alloyed trip-steels based on direct micro-macro simulations
    Prüger, S. and Gandhi, A. and Balzani, D.
    ECCOMAS Congress 2016 - Proceedings of the 7th European Congress on Computational Methods in Applied Sciences and Engineering 2 2280-2291 (2016)
    Low-alloyed TRIP steels are often used in the automotive industry due to their favorable mechanical properties such as high ductility and strength and their moderate production costs. These steels possess a heterogeneous multiphase microstructure, initially consisting of ferrite, bainite and retained austenite which is responsible for the mechanical properties. Upon deformation, a diffusionless, stress-induced, martensitic phase transformation from austenite to martensite is observed, enhancing ductility and strength. We focus on multi-scale methods in the sense of FE2 to describe the macroscopic behavior of low-alloyed TRIP-steels, because this approach allows for a straightforward inclusion of various influencing factors such as residual stress distribution, graded material properties which can hardly included in phenomenological descriptions of these heterogeneous multiphase materials. In order to allow for efficient computations, a simplified microstructure is used in an illustrative direct micro-macro simulation. The inelastic processes in the austenitic inclusions involve the phase transformation from austenite to martensite and the inelastic deformation of these two phases. The isotropic, rate-independent, hyperelastic-plastic material model of Hallberg et al. (IJP, 23, pp. 1213-1239, 2007), originally proposed for high-alloyed TRIP steel, is adopted here for the inclusion phase. Minor modifications of the model are proposed to improve its implementation and performance. The influence of various material parameters associated with the phase transformation on the evolution of retained austenite is studied for different homogeneous deformation states. The non-monotonic stress-state dependence observed in experiments is clearly captured by the model. A numerical two-scale calculation is carried out to enlighten the ductility enhancement in low-alloyed TRIP-steels due to the martensitic phase transformation.
    view abstractdoi: 10.7712/100016.1959.7726
  • 2016 • 219 Modeling of Microstructure Evolution with Dynamic Recrystallization in Finite Element Simulations of Martensitic Steel
    Baron, T.J. and Khlopkov, K. and Pretorius, T. and Balzani, D. and Brands, D. and Schröder, J.
    Steel Research International 87 37-45 (2016)
    A metallurgical material description of the flow behavior for finite element (FE) simulations was developed. During hot compression tests, the dynamic microstructure evolution is modeled on the example of high-strength martensitic steel MS-W 1200. Compression tests at 900-1000 °C with a strain rate of 0.1 s-1 on fine-grain and coarse-grain samples were performed. An analysis of the flow behavior identified a strong correlation between the dynamic recrystallization kinetics and the initial microstructure. The regression analysis has been used to determine correction factors of the new model to describe the dynamic recrystallization. A good agreement between FE simulation and measurement shows the validity of the new model. A metallurgical material description of the flow behavior for finite element (FE) simulations is developed. During hot compression tests, the dynamic microstructure evolution is modeled on the example of high-strength martensitic steel MS-W 1200. An analysis of the flow behavior identifies a strong correlation between the dynamic recrystallization kinetics and the initial microstructure. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201400576
  • 2016 • 218 Multiple mechanisms of lath martensite plasticity
    Morsdorf, L. and Jeannin, O. and Barbier, D. and Mitsuhara, M. and Raabe, D. and Tasan, C.C.
    Acta Materialia 121 202-214 (2016)
    The multi-scale complexity of lath martensitic microstructures requires scale-bridging analyses to better understand the deformation mechanisms activated therein. In this study, plasticity in lath martensite is investigated by multi-field mapping of deformation-induced microstructure, topography, and strain evolution at different spatial resolution vs. field-of-view combinations. These investigations reveal site-specific initiation of dislocation activity within laths, as well as significant plastic accommodation in the vicinity of high angle block and packet boundaries. The observation of interface plasticity raises several questions regarding the role of thin inter-lath austenite films. Thus, accompanying transmission electron microscopy and synchrotron x-ray diffraction experiments are carried out to investigate the stability of these films to mechanical loading, and to discuss alternative boundary sliding mechanisms to explain the observed interface strain localization. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.09.006
  • 2016 • 217 Multi-scale and spatially resolved hydrogen mapping in a Ni-Nb model alloy reveals the role of the δ phase in hydrogen embrittlement of alloy 718
    Tarzimoghadam, Z. and Rohwerder, M. and Merzlikin, S.V. and Bashir, A. and Yedra, L. and Eswara, S. and Ponge, D. and Raabe, D.
    Acta Materialia 109 69-81 (2016)
    We investigated the hydrogen distribution and desorption behavior in an electrochemically hydrogen-charged binary Ni-Nb model alloy to study the role of δ phase in hydrogen embrittlement of alloy 718. We focus on two aspects, namely, (1) mapping the hydrogen distribution with spatial resolution enabling the observation of the relations between desorption profiles and desorption sites; and (2) correlating these observations with mechanical testing results to reveal the degradation mechanisms. The trapping states of hydrogen in the alloy were globally analyzed by Thermal Desorption Spectroscopy (TDS). Additionally, spatially resolved hydrogen mapping was conducted using silver decoration, Scanning Kelvin Probe Force Microscopy (SKPFM) and Secondary Ion Mass Spectrometry (SIMS): The Ag decoration method revealed rapid effusion of hydrogen at room temperature from the γ-matrix. The corresponding kinetics was resolved in both, space and time by the SKPFM measurements. At room temperature the hydrogen release from the γ-matrix steadily decreased until about 100 h and then was taken over by the δ phase from which the hydrogen was released much slower. For avoiding misinterpretation of hydrogen signals stemming from environmental effects we also charged specimens with deuterium. The deuterium distribution in the microstructure was studied by SIMS. The combined results reveal that hydrogen dissolves more preferably inside the γ-matrix and is diffusible at room temperature while the δ phase acts as a deeper trapping site for hydrogen. With this joint and spatially resolving approach we observed the microstructure- and time-dependent distribution and release rate of hydrogen with high spatial and temporal resolution. Correlating the obtained results with mechanical testing of the hydrogen-charged samples shows that hydrogen enhanced decohesion (HEDE) occurring at the δ/matrix interfaces promotes the embrittlement. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.02.053
  • 2016 • 216 Nanostructured Ti-Ta thin films synthesized by combinatorial glancing angle sputter deposition
    Motemani, Y. and Khare, C. and Savan, A. and Hans, M. and Paulsen, A. and Frenzel, J. and Somsen, C. and Mücklich, F. and Eggeler, G. and Ludwig, Al.
    Nanotechnology 27 (2016)
    Ti-Ta alloys are attractive materials for applications in actuators as well as biomedical implants. When fabricated as thin films, these alloys can potentially be employed as microactuators, components for micro-implantable devices and coatings on surgical implants. In this study, Ti100-xTa x (x = 21, 30) nanocolumnar thin films are fabricated by glancing angle deposition (GLAD) at room temperature using Ti73Ta27 and Ta sputter targets. Crystal structure, morphology and microstructure of the nanostructured thin films are systematically investigated by XRD, SEM and TEM, respectively. Nanocolumns of ∼150-160 nm in width are oriented perpendicular to the substrate for both Ti79Ta21 and Ti70Ta30 compositions. The disordered α″ martensite phase with orthorhombic structure is formed in room temperature as-deposited thin films. The columns are found to be elongated small single crystals which are aligned perpendicular to the and planes of α″ martensite, indicating that the films' growth orientation is mainly dominated by these crystallographic planes. Laser pre-patterned substrates are utilized to obtain periodic nanocolumnar arrays. The differences in seed pattern, and inter-seed distances lead to growth of multi-level porous nanostructures. Using a unique sputter deposition geometry consisting of Ti73Ta27 and Ta sputter sources, a nanocolumnar Ti-Ta materials library was fabricated on a static substrate by a co-deposition process (combinatorial-GLAD approach). In this library, a composition spread developed between Ti72.8Ta27.2 and Ti64.4Ta35.6, as confirmed by high-throughput EDX analysis. The morphology over the materials library varies from well-isolated nanocolumns to fan-like nanocolumnar structures. The influence of two sputter sources is investigated by studying the resulting column angle on the materials library. The presented nanostructuring methods including the use of the GLAD technique along with pre-patterning and a combinatorial materials library fabrication strategy offer a promising technological approach for investigating Ti-Ta thin films for a range of applications. The proposed approaches can be similarly implemented for other materials systems which can benefit from the formation of a nanocolumnar morphology. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/27/49/495604
  • 2016 • 215 Novel back-reflector architecture with nanoparticle based buried light-scattering microstructures for improved solar cell performance
    Desta, D. and Ram, S.K. and Rizzoli, R. and Bellettato, M. and Summonte, C. and Jeppesen, B.R. and Jensen, P.B. and Tsao, Y.-C. and Wiggers, H. and Pereira, R.N. and Balling, P. and Larsen, A.N.
    Nanoscale 8 12035-12046 (2016)
    A new back-reflector architecture for light-management in thin-film solar cells is proposed that includes a morphologically smooth top surface with light-scattering microstructures buried within. The microstructures are pyramid shaped, fabricated on a planar reflector using TiO2 nanoparticles and subsequently covered with a layer of Si nanoparticles to obtain a flattened top surface, thus enabling growth of good quality thin-film solar cells. The optical properties of this back-reflector show high broadband haze parameter and wide angular distribution of diffuse light-scattering. The n-i-p amorphous silicon thin-film solar cells grown on such a back-reflector show enhanced light absorption resulting in improved external quantum efficiency. The benefit of the light trapping in those solar cells is evidenced by the gains in short-circuit current density and efficiency up to 15.6% and 19.3% respectively, compared to the reference flat solar cells. This improvement in the current generation in the solar cells grown on the flat-topped (buried pyramid) back-reflector is observed even when the irradiation takes place at large oblique angles of incidence. Finite-difference-time-domain simulation results of optical absorption and ideal short-circuit current density values agree well with the experimental findings. The proposed approach uses a low cost and simple fabrication technique and allows effective light manipulation by utilizing the optical properties of micro-scale structures and nanoscale constituent particles. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6nr00259e
  • 2016 • 214 On some fundamental misunderstandings in the indeterminate couple stress model. A comment on recent papers of A.R. Hadjesfandiari and G.F. Dargush
    Neff, P. and Münch, I. and Ghiba, I.-D. and Madeo, A.
    International Journal of Solids and Structures 81 233-243 (2016)
    In a series of papers which are either published [Hadjesfandiari, A., Dargush, G. F., 2011a. Couple stress theory for solids. Int. J. Solids Struct. 48 (18), 2496-2510; Hadjesfandiari, A., Dargush, G. F., 2013. Fundamental solutions for isotropic size-dependent couple stress elasticity. Int. J. Solids Struct. 50 (9), 1253-1265.] or available as preprints [Hadjesfandiari, A., Dargush, G. F., 2010. Polar continuum mechanics. Preprint arXiv:1009.3252; Hadjesfandiari, A. R., Dargush, G. F., 2011b. Couple stress theory for solids. Int. J. Solids Struct. 48, 2496-2510; Hadjesfandiari, A. R., 2013. On the skew-symmetric character of the couple-stress tensor. Preprint arXiv:1303.3569; Hadjesfandiari, A. R., Dargush, G. F., 2015a. Evolution of generalized couple-stress continuum theories: a critical analysis. Preprint arXiv:1501.03112; Hadjesfandiari, A. R., Dargush, G. F., 2015b. Foundations of consistent couple stress theory. Preprint arXiv:1509.06299] Hadjesfandiari and Dargush have reconsidered the linear indeterminate couple stress model. They are postulating a certain physically plausible split in the virtual work principle. Based on this postulate they claim that the second-order couple stress tensor must always be skew-symmetric. Since they do not consider that the set of boundary conditions intervening in the virtual work principle is not unique, their statement is not tenable and leads to some misunderstandings in the indeterminate couple stress model. This is shown by specifying their development to the isotropic case. However, their choice of constitutive parameters is mathematically possible and we show that it still yields a well-posed boundary value problem. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijsolstr.2015.11.028
  • 2016 • 213 On the Effect of Hot Isostatic Pressing on the Creep Life of a Single Crystal Superalloys
    Mujica Roncery, L. and Lopez-Galilea, I. and Ruttert, B. and Bürger, D. and Wollgramm, P. and Eggeler, G. and Theisen, W.
    Advanced Engineering Materials 18 1381-1387 (2016)
    The creep behavior of a single-crystal Ni-base superalloy in two microstructural states is compared. One is obtained by casting followed by a conventional heat treatment. The other results from the same nominal heat treatment integrated into a hot isostatic pressing process. The microstructure after HIP differed from that in the conventional route in two respects. First, the γ′ particles are smaller and the γ channels are narrower. Second, after HIP, the number density of pores is lower and the pore sizes are smaller. The HIP microstructure improves creep in two respects: the finer γ/γ′-microstructure results in lower minimum creep rates. Moreover, the shrinkage of cast porosity during HIP delays the nucleation and growth of micro cracks and results in higher rupture strains in the low-temperature high stress regime. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201600071
  • 2016 • 212 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 • 211 Optical screw-wrench for interlocking 2PP-microstructures
    Köhler, J. and Zyla, G. and Ksouri, S.I. and Esen, C. and Ostendorf, A.
    Proceedings of SPIE - The International Society for Optical Engineering 9764 (2016)
    Two-photon polymerization (2PP) has emerged as a powerful platform for processing three-dimensional microstructures with high resolution. Furthermore, by adding nanoparticles of different materials to the photopolymer the microstructures can be functionalized, e.g. magnetic or electric properties can be adjusted. However, to combine different functions within one microstructure or to manufacture complex microsystems, assembling techniques for multiple 2PP written building blocks are required. In this paper a qualitative approach for assembling microstructures utilizing optical forces is presented. Therefore, screw and nut shaped microstructures are produced by 2PP-technique and screwed together using a holographic optical tweezer (HOT). The interlocking structures are trapped and rotated into each other to cause connection. In this paper the used parameters and possible designs of the interlocking connection are discussed. These findings provide not only the assembling of building blocks to complex microstructures, rather different functionalized 2PP-microstructures can be combined by simply screwing them together with the use of optical forces. © 2016 SPIE.
    view abstractdoi: 10.1117/12.2209325
  • 2016 • 210 Plasmon assisted 3D microstructuring of gold nanoparticle-doped polymers
    Jonušauskas, L. and Lau, M. and Gruber, P. and Gökce, B. and Barcikowski, S. and Malinauskas, M. and Ovsianikov, A.
    Nanotechnology 27 (2016)
    3D laser lithography of a negative photopolymer (zirconium/silicon hybrid solgel SZ2080) doped with gold nanoparticles (Au NPs) is performed with a 515 nm and 300 fs laser system and the effect of doping is explored. By varying the laser-generated Au NP doping concentration from 4.8 • 10-6 wt% to 9.8 • 10-3 wt% we find that the fabricated line widths are enlarged by up to 14.8% compared to structures achieved in pure SZ2080. While implicating a positive effect on the photosensitivity, the doping has no adverse impact on the mechanical quality of intricate 3D microstructures produced from the doped nanocompound. Additionally, we found that SZ2080 increases the long term (∼months) colloidal stability of Au NPs in isopropanol. By discussing the nanoparticle-light interaction in the 3D polymer structures we provide implications that our findings might have on other fields, such as biomedicine and photonics. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/27/15/154001
  • 2016 • 209 Setting mechanical properties of high strength steels for rapid hot forming processes
    Löbbe, C. and Hering, O. and Hiegemann, L. and Tekkaya, A.E.
    Materials 9 (2016)
    Hot stamping of sheet metal is an established method for the manufacturing of light weight products with tailored properties. However, the generally-applied continuous roller furnace manifests two crucial disadvantages: The overall process time is long and a local setting of mechanical properties is only feasible through special cooling techniques. Hot forming with rapid heating directly before shaping is a new approach, which not only reduces the thermal intervention in the zones of critical formability and requested properties, but also allows the processing of an advantageous microstructure characterized by less grain growth, additional fractions (e.g., retained austenite), and undissolved carbides. Since the austenitization and homogenization process is strongly dependent on the microstructure constitution, the general applicability for the process relevant parameters is unknown. Thus, different austenitization parameters are analyzed for the conventional high strength steels 22MnB5, Docol 1400M, and DP1000 in respect of the mechanical properties. In order to characterize the resulting microstructure, the light optical and scanning electron microscopy, micro and macro hardness measurements, and the X-ray diffraction are conducted subsequent to tensile tests. The investigation proves not only the feasibility to adjust the strength and ductility flexibly, unique microstructures are also observed and the governing mechanisms are clarified. © 2016 by the authors.
    view abstractdoi: 10.3390/ma9040229
  • 2016 • 208 Silicon-based nanocomposites for thermoelectric application
    Schierning, G. and Stoetzel, J. and Chavez, R. and Kessler, V. and Hall, J. and Schmechel, R. and Schneider, T. and Petermann, N. and Wiggers, H. and Angst, S. and Wolf, D.E. and Stoib, B. and Greppmair, A. and Stutzmann, M. and B...
    Physica Status Solidi (A) Applications and Materials Science 213 497-514 (2016)
    Here we present the realization of efficient and sustainable silicon-based thermoelectric materials from nanoparticles. We employ a gas phase synthesis for the nanoparticles which is capable of producing doped silicon (Si) nanoparticles, doped alloy nanoparticles of silicon and germanium (Ge), SixGe1-x, and doped composites of Si nanoparticles with embedded metal silicide precipitation phases. Hence, the so-called "nanoparticle in alloy" approach, theoretically proposed in the literature, forms a guideline for the material development. For bulk samples, a current-activated pressure-assisted densification process of the nanoparticles was optimized in order to obtain the desired microstructure. For thin films, a laser annealing process was developed. Thermoelectric transport properties were characterized on nanocrystalline bulk samples and laser-sintered-thin films. Devices were produced from nanocrystalline bulk silicon in the form of p-n junction thermoelectric generators, and their electrical output data were measured up to hot side temperatures of 750°C. In order to get a deeper insight into thermoelectric properties and structure forming processes, a 3D-Onsager network model was developed. This model was extended further to study the p-n junction thermoelectric generator and understand the fundamental working principle of this novel device architecture. Gas phase synthesis of composite nanoparticles; nanocrystalline bulk with optimized composite microstructure; laser-annealed thin film. The authors fabricated thermoelectric nanomaterials from doped silicon and silicon and germanium alloy nanoparticles, as well as composites of Si nanoparticles with embedded metal silicide nanoparticles. Processing was performed applying a current-activated pressure-assisted densification process for bulk samples and a laser annealing process for thin film samples. Devices were produced in the form of pn junction thermoelectric generators. A 3D-Onsager network model was used to understand the fundamental working principle of this novel device architecture. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532602
  • 2016 • 207 The role of process temperature and rotational speed in the microstructure evolution of Ti-6Al-4V friction surfacing coatings
    Fitseva, V. and Hanke, S. and Santos, J.F.D. and Stemmer, P. and Gleising, B.
    Materials and Design 110 112-123 (2016)
    Friction surfacing is a solid state technique for depositing metallic coatings. Coating materials are thermo-mechanically processed at high temperatures during deposition. The high degree of deformation involved leads to a dynamically recrystallised fine grained microstructure. For Ti-6Al-4V, the microstructure and mechanical properties of coatings generated by friction surfacing have not been studied yet. The current work focuses on investigating effects of rotational speed on microstructure, grain size evolution and mechanical properties of the coating material. Various rotational speeds in a wide range, exceeding the range of deformation used in many other severe plastic deformation processes, were used to generate Ti-6Al-4V coatings by friction surfacing. Their influence on the thermal cycle and consequently on microstructure formation was revealed. The β grain size is related to the rotational speed and thermal cycle. Grain refinement at low rotational speed was observed, while higher rotational speeds and corresponding increase in maximum temperature led to grain coarsening. Although the peak temperature dominates the grain size evolution, dynamic recrystallisation during friction surfacing counteracts this effect, reducing the grain size by one order of magnitude. The coatings exhibit a hardness ascent about 15% due to martensite formation, high dislocation density and oxide precipitations. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2016.07.132
  • 2016 • 206 The role of silicon, vacancies, and strain in carbon distribution in low temperature bainite
    Sampath, S. and Rementeria, R. and Huang, X. and Poplawsky, J.D. and Garcia-Mateo, C. and Caballero, F.G. and Janisch, R.
    Journal of Alloys and Compounds 673 289-294 (2016)
    We investigated the phenomenon of carbon supersaturation and carbon clustering in bainitic ferrite with atom probe tomography (APT) and ab-initio density functional theory (DFT) calculations. The experimental results show a homogeneous distribution of silicon in the microstructure, which contains both ferrite and retained austenite. This distribution is mimicked well by the computational approach. In addition, an accumulation of C in certain regions of the bainitic ferrite with C concentrations up to 13 at % is observed. Based on the DFT results, these clusters are explained as strained, tetragonal regions in the ferritic bainite, in which the solution enthalpy of C can reach large, negative values. It seems that Si itself only has a minor influence on this phenomenon. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2016.02.151
  • 2016 • 205 Tribological development of TiCN coatings by adjusting the flowing rate of reactive gases
    Tillmann, W. and Momeni, S.
    Journal of Physics and Chemistry of Solids 90 45-53 (2016)
    TiCN coatings were deposited by means of direct current magnetron sputtering of Ti targets in presence of N2 and C2H2 reactive gases. The microstructure, composition, mechanical and tribological properties of the deposited thin films were analyzed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nanoindentation, ball-on-disc, scratch test, and three dimensional (3D) optical microscopy. The obtained results presents a reproducible processing route for tailoring microstructure, mechanical and tribological behavior of TiCN coatings by controlling flowing rate of the reactive gases. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jpcs.2015.11.009
  • 2016 • 204 Two-Step Annealing Leading to Refined Bi2Te3-In2Te3 Lamellar Structures for Tuning of Thermoelectric Properties
    Liu, D. and Li, X. and Schmechel, R. and Rettenmayr, M.
    Journal of Electronic Materials 45 1654-1660 (2016)
    A two-step annealing process was applied to control the morphology of Bi2Te3-In2Te3 composite materials via precipitation of In2Te3 from supersaturated (Bi,In)2Te3. Finer lamellae were obtained via two-step as compared with single-step isothermal annealing. The microstructure was optimized by exploiting thermodynamic and kinetic effects during nucleation and growth of In2Te3. The relationship between the morphologies and thermoelectric properties was analyzed. With preannealing at a lower temperature, refined morphologies lead to an enhanced power factor and zT in the temperature range from room temperature to ∼100°C. The enhancement is mainly caused by an increased Seebeck coefficient, most probably due to energy-dependent scattering processes. However, the thermal conductivity is dominated by bipolar thermal transport that compensates the low lattice thermal conductivity completely. © 2015, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11664-015-4151-4
  • 2016 • 203 Vessel microstructure design: A new approach for site-specific core-shell micromechanical tailoring of TRIP-assisted ultra-high strength steels
    Belde, M. and Springer, H. and Raabe, D.
    Acta Materialia 113 19-31 (2016)
    The mechanical performance of multi-phase steel microstructures critically depends on the constituents' chemical and morphological constitutions, which in combination determine the composite hardness, the onset of plasticity, internal load and strain-partitioning, as well as the stability and transformation kinetics of retained austenite in case of TRIP steels. The novel approach of utilising temporary vessel phases, hence termed vessel microstructure design, enables the tuning of constituent phase properties by linking their formation to a controllable landscape of chemical gradients. This approach hinges on the introduction of alloy carbides as a temporary container, or 'vessel' phase, deliberately producing localised enrichment of alloying elements in a structure predetermined by preliminary heat treatments, referred to as conditioning and accumulation stages. These vessel carbides, which act as reservoirs for specific alloying elements, are then partially dissolved through flash heating, leading to a self-organising landscape of alloying elements in the vicinity of the dissolving particles. The resulting three- or multiple phase microstructures then consist of confined laminates incorporating retained carbides, enveloped by retained austenite shells, embedded within a martensitic matrix. Such complex yet entirely self-organized microstructures offer unique opportunities for strain and load partitioning which we refer to as core-shell micromechanics. Different variants of these core-shell composite structures are produced and examined together with reference microstructures by tensile testing, hardness mappings, impact toughness, X-ray measurements, as well as by electron microscopy. It is found that these novel microstructures, when tempered, exhibit ultra-high strength and delayed necking, enabled by a combination of gradual strain-hardening and transformation-induced plasticity that is tuneable via control of the initial carbide structure. © 2016 Acta Materialia Inc. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.04.051
  • 2015 • 202 3D structural and atomic-scale analysis of lath martensite: Effect of the transformation sequence
    Morsdorf, L. and Tasan, C.C. and Ponge, D. and Raabe, D.
    Acta Materialia 95 366-377 (2015)
    To improve the fundamental understanding of the multi-scale characteristics of martensitic microstructures and their micro-mechanical properties, a multi-probe methodology is developed and applied to low-carbon lath martensitic model alloys. The approach is based on the joint employment of electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), atom probe tomography (APT) and nanoindentation, in conjunction with high precision and large field-of-view 3D serial sectioning. This methodology enabled us to resolve (i) size variations of martensite sub-units, (ii) associated dislocation sub-structures, (iii) chemical heterogeneities, and (iv) the resulting local mechanical properties. The identified interrelated microstructure heterogeneity is discussed and related to the martensitic transformation sequence, which is proposed to intrinsically lead to formation of a nano-composite structure in low-carbon martensitic steels. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.05.023
  • 2015 • 201 A novel roll-bonding methodology for the cross-scale analysis of phase properties and interactions in multiphase structural materials
    Springer, H. and Tasan, C. and Raabe, D.
    International Journal of Materials Research 106 3-14 (2015)
    We introduce a new thermo-mechanical approach for producing layered bulk samples built-up from the constituent phases of structural materials for the analysis of multiphase co-deformation phenomena. Following a thermo-mechani- cally controlled roll-bonding procedure, the intrinsic properties of the microstructural components as well as their mutual mechanical interaction and interfacial phenomena can be systematically investigated in highly controlled model microstructures of reduced complexity. The effectiveness of the approach is demonstrated on two examples where austenite or martensite layers, respectively, are introduced in a bulk ferritic matrix, representing in either case components of high strength steels. Special emphasis is laid on how the plasticity of martensite within ferrite, as a key parameter required for understanding and optimising dual phase steels, can be investigated following the proposed approach.
    view abstractdoi: 10.3139/146.111156
  • 2015 • 200 A structure zone diagram obtained by simultaneous deposition on a novel step heater: A case study for Cu2O thin films
    Stein, H. and Naujoks, D. and Grochla, D. and Khare, C. and Gutkowski, R. and Grützke, S. and Schuhmann, W. and Ludwig, Al.
    Physica Status Solidi (A) Applications and Materials Science 212 2798-2804 (2015)
    In thin film deposition processes, the deposition temperature is one of the crucial process parameters for obtaining films with desired properties. Usually the optimum deposition temperature is found by conducting several depositions sequentially in a time consuming process. This paper demonstrates a facile and rapid route of the simultaneous thin film deposition at six different deposition temperatures ranging from 100 to 1000 °C. Cuprite (Cu2O) was chosen for the study as this material is of interest for energy applications. The thin films are assessed for their crystallographic, microstructural, Raman scattering, and photoelectrochemical properties. The results show that the utilization of a step heater leads to the rapid optimization of thin film microstructures of an absorber material used in photoelectrochemistry. This results in a structure zone diagram for Cu2O. For a substrate temperature of 600 °C, an optimum between crystallinity and morphology occurs. © 2015 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532384
  • 2015 • 199 A variational viscosity-limit approach to the evolution of microstructures in finite crystal plasticity
    Günther, C. and Junker, P. and Hackl, K.
    Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 471 (2015)
    A micromechanical model for finite single crystal plasticity was introduced by Kochmann & Hackl (2011 Contin.Mech. Thermodyn. 23, 63-85 (doi:10.1007/ s00161-010-0714-5)). This model is based on thermodynamic variational principles and leads to a non-convex variational problem. Based on the Lagrange functional, an incremental strategy was outlined to model the time-continuous evolution of a first-order laminate microstructure. Although this model provides interesting results on the material point level, owing to the global minimization in the evolution equations, the calculation time and numerical instabilities may cause problems when applying this model to macroscopic specimens. In this paper, a smooth transition zone between the laminates is introduced to avoid global minimization, which makes the numerical calculations cumbersome compared with the model in Kochmann & Hackl. By introducing a smooth viscous transition zone, the dissipation potential and its numerical treatment have to be adapted. We outline rate-dependent timeevolution equations for the internal variables based on variational techniques and show as first examples single-slip shear and tension/compression tests. © 2015 The Author(s) Published by the Royal Society. All rights reserved.
    view abstractdoi: 10.1098/rspa.2015.0110
  • 2015 • 198 Advanced scale bridging microstructure analysis of single crystal Ni-base superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Somsen, C. and Kostka, A. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Dlouhy, A. and Müller, J. and Spiecker, E. and Demtroder, K. and Schreuer, J. and Neuking, K. and Eggeler, G.
    Advanced Engineering Materials 17 216-230 (2015)
    In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni-base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the castmicrostructure (dendrites (D) and interdendritic (ID) regions - large scale heterogeneity) and with the well-known γ/γ′ microstructure (small scale heterogeneity). Using electron probe microanalysis (EPMA), we can showthat elements such as Re, Co, andCr partition to the dendrites while ID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the γ′ cubes while g channels show higher concentrations of Co, Cr, Re, andW.We can combine large scale (EPMA) and small-scale analytical methods (APT) to obtain reasonable estimates for γ′ volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140mm x 100mm x 20mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the γ′ phase and the effect of cooling rates after high temperature exposure on the microstructure. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201400136
  • 2015 • 197 Alloying effects on microstructure formation of dual phase steels
    Schemmann, L. and Zaefferer, S. and Raabe, D. and Friedel, F. and Mattissen, D.
    Acta Materialia 95 386-398 (2015)
    In dual-phase (DP) steels, inherited microstructures and elemental distributions affect the kinetics and morphology of phase transformation phenomena and the mechanical properties of the final material. In order to study the inheritance process, we selected two model materials with the same average DP steel composition but with different initial microstructures, created by coiling at different temperatures after hot rolling. These samples were submitted to a DP-steel heat treatment consisting of a short isothermal annealing in the pure austenite region and a quenching process. The evolution of microstructure, chemical composition and mechanical properties (hardness) during this treatment was investigated. The initial samples had a bainitic-martensitic (B + M) microstructure for the material coiled at lower temperature and a ferritic-pearlitic (P + F) microstructure for that coiled at higher temperature. The P + F microstructure had a much more inhomogeneous distribution of substitutional elements (in particular of Mn) and of carbon. After complete heat treatment, both materials showed a typical DP microstructure (martensite islands embedded in ferrite) but the P + F material showed lower hardness compared to the B + M material. It was found that the inhomogeneous elemental distribution prevailed in the P + F material. The inheritance process was studied by combining measurements of the elemental distribution by Wavelength-Dispersive X-ray spectroscopy (WDX), simulations of the evolution of the elemental composition via the DICTRA (diffusion-controlled reactions) computer programme, dilatometry to observe the kinetics of phase transformation, and observation and quantification of the microstructures by Electron Backscatter Diffraction (EBSD) measurements. For the P + F material it was found that the α-γ transformation during annealing is slowed down in regions of lower Mn content and is therefore not completed. During the subsequent cooling the incompletely autenitized material does not require ferrite nucleation and the γ-α transformation starts at relative high temperatures. For B + M, in contrast, nucleation of ferrite is needed and the transformation starts at lower temperatures. As a result the B + M material develops a higher martensite content as well as a higher density of geometrically necessary dislocations (GNDs). It is speculated that for the B + M material the γ-α transformation occurs through a bainitic (i.e. partly displacive) process while the transformation at higher temperatures in the P + F material proceeds exclusively in a diffusive way. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.05.005
  • 2015 • 196 An Overview of Dual-Phase Steels: Advances in Microstructure-Oriented Processing and Micromechanically Guided Design
    Tasan, C.C. and Diehl, M. and Yan, D. and Bechtold, M. and Roters, F. and Schemmann, L. and Zheng, C. and Peranio, N. and Ponge, D. and Koyama, M. and Tsuzaki, K. and Raabe, D.
    Annual Review of Materials Research 45 391-431 (2015)
    Dual-phase (DP) steel is the flagship of advanced high-strength steels, which were the first among various candidate alloy systems to find application in weight-reduced automotive components. On the one hand, this is a metallurgical success story: Lean alloying and simple thermomechanical treatment enable use of less material to accomplish more performance while complying with demanding environmental and economic constraints. On the other hand, the enormous literature on DP steels demonstrates the immense complexity of microstructure physics in multiphase alloys: Roughly 50 years after the first reports on ferrite-martensite steels, there are still various open scientific questions. Fortunately, the last decades witnessed enormous advances in the development of enabling experimental and simulation techniques, significantly improving the understanding of DP steels. This review provides a detailed account of these improvements, focusing specifically on (a) microstructure evolution during processing, (b) experimental characterization of micromechanical behavior, and (c) the simulation of mechanical behavior, to highlight the critical unresolved issues and to guide future research efforts. Copyright © 2015 by Annual Reviews. All rights reserved.
    view abstractdoi: 10.1146/annurev-matsci-070214-021103
  • 2015 • 195 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 • 194 Compact low-cost lensless digital holographic microscope for topographic measurements of microstructures in reflection geometry
    Adinda-Ougba, A. and Kabir, B. and Koukourakis, N. and Mitschker, F. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9628 (2015)
    Digital holography is capable of providing surface profiles of samples with axial resolution in the nanometer range. Lensless digital holography is a well-established microscopic method providing diffraction limited resolution of the order of the wavelength of the used light source. It is based on inline holography and usually allows imaging only in transmission geometry. In this contribution we propose a compact low cost lensless digital holographic microscope capable of performing measurements on reflective microstructures. The novelty of the system consists on a direct use of a laser diode without any need of coupling optics as light source. This simplifies the setup and provides sufficient magnification to measure microstructures. We evaluate our setup by imaging reflective microstructures. We have achieved ∼ 6 mm2 field of view amplitude images with ∼ 2.5μm lateral resolution and phase images with axial resolution in nanometer range. The phase image provides a full-field profile measurement of the sample in nanometer range. © 2015 SPIE.
    view abstractdoi: 10.1117/12.2191073
  • 2015 • 193 Construction of statistically similar RVEs
    Scheunemann, L. and Balzani, D. and Brands, D. and Schröder, J.
    Lecture Notes in Applied and Computational Mechanics 78 219-256 (2015)
    In modern engineering, micro-heterogeneous materials are designed to satisfy the needs and challenges in a wide field of technical applications. The effective mechanical behavior of these materials is influenced by the inherent microstructure and therein the interaction and individual behavior of the underlying phases. Computational homogenization approaches, such as the FE2 method have been found to be a suitable tool for the consideration of the influences of the microstructure. However, when real microstructures are considered, high computational costs arise from the complex morphology of the microstructure. Statistically similar RVEs (SSRVEs) can be used as an alternative, which are constructed to possess similar statistical properties as the realmicrostructure but are defined by a lower level of complexity. These SSRVEs are obtained from a minimization of differences of statistical measures and mechanical behavior compared with a real microstructure in a staggered optimization scheme, where the inner optimization ensures statistical similarity and the outer optimization problem controls themechanical comparativity of the SSRVE and the real microstructure. The performance of SSRVEs may vary with the utilized statistical measures and the parameterization of the microstructure of the SSRVE.With regard to an efficient construction of SSRVEs, it is necessary to consider statistical measures which can be computed in reasonable time and which provide sufficient information of the real microstructure.Minkowski functionals are analyzed as possible basis for statistical descriptors of microstructures and compared with other well-known statistical measures to investigate the performance. In order to emphasize the general importance of considering microstructural features by more sophisticated measures than basic ones, i.e. volume fraction, an analysis of upper bounds on the error of statistical measures and mechanical response is presented. © Springer International Publishing Switzerland 2015.
    view abstractdoi: 10.1007/978-3-319-18242-1_9
  • 2015 • 192 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 • 191 Correlations between microstructure and room temperature tensile behavior of a duplex TNB alloy for systematically heat treated samples
    Kabir, M.R. and Bartsch, M. and Chernova, L. and Kelm, K. and Wischek, J.
    Materials Science and Engineering A 635 13-22 (2015)
    The mechanical properties of TiAl alloys are very sensitive to the inherent microstructure. For an in-depth understanding of microstructural influences on mechanical properties a duplex type TNB (Nb-containing TiAl) alloy has been investigated. For varying the microstructure of this alloy controlled heat treatments (HT) have been performed with eight distinct maximum temperatures, ranging from 1230. °C to 1300. °C with a 10. °C temperature increment. The series of annealing processes resulted in duplex microstructures with a gradual change of the ratio of globular grains and lamellar colonies, keeping the global chemical composition unchanged. Microstructure of each sample was characterized using SEM and TEM before and after mechanical testing to correlate the morphology and microstructure features to the tensile properties. Quantitative data analysis from these results revealed how the evolution of duplex microstructures influences the room temperature tensile properties: i.e. the elastic stiffness, room temperature ductility, work hardening, fracture stress, and fracture strain. The results are discussed with respect to deformation mechanisms as understood from the tensile test results and fracture surface investigations. From the observed correlations between microstructure and properties an optimized constellation of globular and lamellar microstructure for relevant properties can be predicted. Furthermore, the required heat-treatment window for properties targeted can be defined. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.03.041
  • 2015 • 190 Cycling Performance of a Columnar-Structured Complex Perovskite in a Temperature Gradient Test
    Schlegel, N. and Sebold, D. and Sohn, Y.J. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology 24 1205-1212 (2015)
    To increase the efficiency of turbines for the power generation and the aircraft industry, advanced thermal barrier coatings (TBCs) are required. They need to be long-term stable at temperatures higher than 1200 °C. Nowadays, yttria partially stabilized zirconia (YSZ) is applied as standard TBC material. But its long-term application at temperatures higher than 1200 °C leads to detrimental phase changes and sintering effects. Therefore, new materials have to be investigated, for example, complex perovskites. They provide high melting points, high thermal expansion coefficients and thermal conductivities of approx. 2.0 W/(m K). In this work, the complex perovskite La(Al1/4Mg1/2Ta1/4)O3 (LAMT) was investigated. It was deposited by the suspension plasma spraying (SPS) process, resulting in a columnar microstructure of the coating. The coatings were tested in thermal cycling gradient tests and they show excellent results, even though some phase decomposition was found. © 2015, ASM International.
    view abstractdoi: 10.1007/s11666-015-0254-y
  • 2015 • 189 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 • 188 Dental lessons from past to present: Ultrastructure and composition of teeth from plesiosaurs, dinosaurs, extinct and recent sharks
    Lübke, A. and Enax, J. and Loza, K. and Prymak, O. and Gaengler, P. and Fabritius, H.-O. and Raabe, D. and Epple, M.
    RSC Advances 5 61612-61622 (2015)
    Teeth represent the hardest tissue in vertebrates and appear very early in their evolution as an ancestral character of the Eugnathostomata (true jawed vertebrates). In recent vertebrates, two strategies to form and mineralize the outermost functional layer have persisted. In cartilaginous fish, the enameloid is of ectomesenchymal origin with fluoroapatite as the mineral phase. All other groups form enamel of ectodermal origin using hydroxyapatite as the mineral phase. The high abundance of teeth in the fossil record is ideal to compare structure and composition of teeth from extinct groups with those of their recent successors to elucidate possible evolutionary changes. Here, we studied the chemical composition and the microstructure of the teeth of six extinct shark species, two species of extinct marine reptiles and two dinosaur species using high-resolution chemical and microscopic methods. Although many of the ultrastructural features of fossilized teeth are similar to recent ones (especially for sharks where the ultrastructure basically did not change over millions of years), we found surprising differences in chemical composition. The tooth mineral of all extinct sharks was fluoroapatite in both dentin and enameloid, in sharp contrast to recent sharks where fluoroapatite is only found in enameloid. Unlike extinct sharks, recent sharks use hydroxyapatite as mineral in dentin. Most notably and hitherto unknown, all dinosaur and extinct marine reptile teeth contained fluoroapatite as mineral in dentin and enamel. Our results indicate a drastic change in the tooth mineralization strategy especially for terrestrial vertebrates that must have set in after the cretaceous period. Possibly, this is related to hitherto unconsidered environmental changes that caused unfavourable conditions for the use of fluoroapatite as tooth mineral. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5ra11560d
  • 2015 • 187 Design of 3D statistically similar Representative Volume Elements based on Minkowski functionals
    Scheunemann, L. and Balzani, D. and Brands, D. and Schröder, J.
    Mechanics of Materials 90 185-201 (2015)
    In this paper an extended optimization procedure is proposed for the construction of statistically similar RVEs (SSRVEs) which are defined as artificial microstructures showing a lower complexity than the associated real microstructures. This enables a computationally efficient discretization required for numerical calculations of microscopic boundary value problems and leads therefore to more efficient computational two-scale schemes. The optimization procedure is staggered and consists of an outer and an inner optimization problem. The outer problem treats different types of morphology parameterizations, different sets of statistical measures and different sets of weighting factors needed in the inner problem to minimize differences of mechanical errors that compare the response of the SSRVE with a target (real) microstructure. The inner problem minimizes differences of statistical measures describing the microstructure morphology for fixed parameterization type, statistical measures and weighting factors. The main contribution here is the analysis of new microstructure descriptors based on tensor-valued Minkowski functionals, whose numerical calculation requires less time compared to e.g. lineal-path functions. Thereby, a more efficient inner optimization problem can be realized and thus, an automated solution of the outer optimization problem becomes more practicable. Representative examples demonstrate the performance of the proposed method. It turns out that the evaluation of objective functions formulated in terms of the Minkowski functionals is almost 2000 times faster than functions taking into account lineal-path functions. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.mechmat.2015.03.005
  • 2015 • 186 Evolution of microstructure and mechanical properties of coated Co-base superalloys during heat treatment and thermal exposure
    Webler, R. and Ziener, M. and Neumeier, S. and Terberger, P.J. and Vaßen, R. and Göken, M.
    Materials Science and Engineering A 628 374-381 (2015)
    New γ'-strengthened Co-base superalloys show an interesting potential for high temperature applications. However, protective coatings are needed as for Ni-base superalloys to ensure sufficient oxidation and corrosion resistance. Therefore the properties of a commercial coating on a multinary new γ'-strengthened Co-base superalloy have been studied. Especially the influence of the coating process on the substrate also after long term annealing is discussed. It was found that the highly deformed areas at the coating-substrate interface indicated by a high local misorientation and caused by the sandblasting process led to a recrystallization of the interdiffusion zone during the age hardening heat treatment. A chemical gradient of γ and γ' promoting elements was found in the interdiffusion zone causing a change in hardness as measured by nanoindentation. Depending on the composition two separate recrystallized regions formed in the interdiffusion zone, one with single phase γ-(Co,Ni) and the other with a cellular two phase microstructure of discontinuously grown γ and γ'. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.01.060
  • 2015 • 185 Formulation of nonlocal damage models based on spectral methods for application to complex microstructures
    Boeff, M. and Gutknecht, F. and Engels, P.S. and Ma, A. and Hartmaier, A.
    Engineering Fracture Mechanics 147 373-387 (2015)
    The increasing interest in modelling local deformations and damage evolution within materials with complex microstructures leads to an increasing demand for efficient numerical methods. A method designed to study damage evolution within the microstructure should be able to deal with complex geometries and to capture system sizes that are large enough to rectify the assumptions made when naming them representative volume elements (RVEs). We introduce a nonlocal damage model into the framework of a spectral solver and study initiation and evolution of damage on the microstructural scale, where regions susceptible to damage are identified. © 2015.
    view abstractdoi: 10.1016/j.engfracmech.2015.06.030
  • 2015 • 184 High resolution in situ mapping of microstrain and microstructure evolution reveals damage resistance criteria in dual phase steels
    Yan, D. and Tasan, C.C. and Raabe, D.
    Acta Materialia 96 399-409 (2015)
    Microstructures of multi-phase alloys undergo morphological and crystallographic changes upon deformation, corresponding to the associated microstructural strain fields. The multiple length and time scales involved therein create immense complexity, especially when microstructural damage mechanisms are also activated. An understanding of the relationship between microstructure and damage initiation can often not be achieved by post-mortem microstructural characterization alone. Here, we present a novel multi-probe analysis approach. It couples various scanning electron microscopy methods to microscopic-digital image correlation (μ-DIC), to overcome various challenges associated with concurrent mapping of the deforming microstructure along with the associated microstrain fields. For this purpose a contrast- and resolution-optimized μ-DIC patterning method and a selective pattern/microstructure imaging strategy were developed. They jointly enable imaging of (i) microstructure-independent pattern maps and (ii) pattern-independent microstructure maps. We apply this approach here to the study of damage nucleation in ferrite/martensite dual-phase (DP) steel. The analyses provide four specific design guidelines for developing damage-resistant DP steels. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.05.038
  • 2015 • 183 Industrial Benchmark 2015: Process Monitoring and Analysis of Hollow EN AW-6063 Extruded Profile
    Gamberoni, A. and Donati, L. and Reggiani, B. and Haase, M. and Tomesani, L. and Tekkaya, A.E.
    Materials Today: Proceedings 2 4714-4725 (2015)
    The paper reports information in term of simulation settings and output results related to the Industrial benchmark 2015: extrusion benchmark is an event where participants from software houses, industries and academia are requested to simulate an extrusion process case which main experimental data are initially unknown and disclosed only after the submission of simulation results. The industrial benchmark 2015 is focused on the extrusion of a multi-cavities hollow profile with EN AW-6063 aluminum alloy. Thermal field was monitored by means of contactless pyrometers installed on the press and five thermosensors were inserted in key positions in the die. Several extrusion data were continuously acquired including the profile speed, the puller force and the extrusion load. After extrusion the profiles are analyzed in order to determine the position of the seam weld and the microstructure inside the profile cross section after air or water quenching. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matpr.2015.10.004
  • 2015 • 182 Influence of initial microstructure on thermomechanical fatigue behavior of Cu films on substrates
    Heinz, W. and Robl, W. and Dehm, G.
    Microelectronic Engineering 137 5-10 (2015)
    During a switch event in a power semiconductor device temperature changes of up to 300 K can occur in the Cu layer. Repeated switching operations causes cyclic thermal cycling which may finally lead to thermomechanical fatigue with severe microstructural changes. In this study, the influence of the starting microstructure and film thickness (600 nm and 5000 nm) on thermomechanical fatigue was investigated for epitaxial and polycrystalline Cu films for up to 1000 thermal cycles. Severe surface roughening and a texture change (crystal rotation) are detected during thermal cycling for the polycrystalline Cu films, while the epitaxial films maintain their microstructure. Controlling the initial microstructure of a Cu layer in a device exposed to cyclic thermomechanical straining is a route to delay surface damage. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.mee.2014.10.024
  • 2015 • 181 Influence of in-situ and postannealing technique on tribological performance of NiTi SMA thin films
    Tillmann, W. and Momeni, S.
    Surface and Coatings Technology 276 286-295 (2015)
    Magnetron sputtered NiTi thin films were crystallized through two convenient techniques: (i) postannealing and (ii) in-situ annealing during the deposition. The annealing parameters (temperature and time) were kept constant by employing each technique. The microstructure, morphology, phase transformation behavior, mechanical and tribological properties of these thin films were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), 4-point probe resistivity measurement, nanoindentation test, pin-on-disc, scratch test and three dimensional (3D) optical microscopy. The results show how postannealing and in-situ annealing techniques can differently affect properties of NiTi thin films in spite of employing similar annealing temperature and time. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.surfcoat.2015.07.012
  • 2015 • 180 Influence of microstructure on macroscopic elastic properties and thermal expansion of nickel-base superalloys ERBO/1 and LEK94
    Demtröder, K. and Eggeler, G. and Schreuer, J.
    Materialwissenschaft und Werkstofftechnik 46 563-576 (2015)
    In the present work the thermal expansion and the elastic properties of second generation nickel-base superalloy single crystals ERBO/1 (CMSX-4 variation) and LEK94 have been studied between about 100 K and 1273 K using dilatometry and resonant ultrasound spectroscopy, respectively. Inhomogeneity related to the large scale microstructure of the samples can act as a potential source of scatter for the propagation of ultrasonic waves. This can be overcome by choosing samples of sufficient size so that they appear as homogeneous media at the scale of the elastic wave length. Our final results are in good agreement with data reported in literature for similar alloy systems. In particular, the elastic material properties are only weekly affected by moderate variations in chemical composition and microstructure. Taking into account literature data for other superalloys like CMSX-4, we derive general polynomial functions which describe the temperature dependence of the elastic moduli E<inf>〈100〉</inf>, E<inf>〈110〉</inf> and E<inf>〈111〉</inf> in nickel-base superalloys to within about ±3%. It was also observed that the alloys ERBO/1 and LEK94 show weak but significant anomalies in both thermal expansion and temperature coefficients of elastic constants above about 900 K. These anomalies are probably related to the gradual dissolution of the γ′-precipitates at higher temperatures. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201500406
  • 2015 • 179 Influence of process-induced microstructure and imperfections on mechanical properties of AlSi12 processed by selective laser melting
    Siddique, S. and Imran, M. and Wycisk, E. and Emmelmann, C. and Walther, F.
    Journal of Materials Processing Technology 221 205-213 (2015)
    Selective laser melting (SLM) offers high potential for manufacturing complex geometries and custom-made parts due to its unique layer-wise production process. A series of samples of AlSi12 have been manufactured by SLM process to study the effect of process parameters and post-build heat treatment on the microstructure and the corresponding mechanical properties. Optical microscope, scanning electron microscope, quasistatic tests, continuous load increase fatigue tests and constant amplitude fatigue tests have been employed for characterization. A remarkable eutectic microstructure, with dendritic width changing with SLM process parameters, has been observed. Relationship between SLM process parameters, resulting microstructure and the consequent changes in mechanical properties has been discussed. Base plate heating has been found critical in controlling the in-process microstructure. Mechanical properties of SLM parts outperform those of conventionally manufactured alloy, and can be varied as per requirement, by altering the build rate, keeping the process costs in control. Fatigue scatter can also be controlled by heating the base plate during the process. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2015.02.023
  • 2015 • 178 Isothermal aging of a γ'-strengthened Co-Al-W alloy coated with vacuum plasma-sprayed MCrAlY bond coats
    Terberger, P.J. and Sebold, D. and Webler, R. and Ziener, M. and Neumeier, S. and Klein, L. and Virtanen, S. and Göken, M. and Vaßen, R.
    Surface and Coatings Technology 276 360-367 (2015)
    Cobalt-based superalloys with a γ/γ' microstructure were discovered in 2006 and are currently being investigated as an alternative to nickel-based superalloys for high-temperature, high-load applications in gas turbine blades. They promise a better castability combined with a similar creep strength. Superalloy turbine blades are commonly coated with oxidation resistant bond coats. For this reason their compatibility needs to be studied. Co-9Al-9W specimens with a γ/γ' microstructure were coated with either a nickel-based or cobalt-based MCrAlY bond coat using vacuum plasma spraying. After aging at 900. °C in air for up to 500. h no decomposition of the γ' phase was found in the bulk superalloy. The interdiffusion zone shows several different W-rich topologically close-packed phases arising from the dissolution of the γ' phase in this region. The W-rich phases are identified to be μ phase for both bond coats and R phase for the nickel-based bond coat only. Their total volume is higher for the nickel-based bond coat. Therefore the cobalt-based bond coat is better suited for the Co-based superalloy substrate. Room temperature hardness and Young's modulus were measured using nanoindentation in the initial state and after heat treatment. A significantly higher Young's modulus was found for the cobalt-based bond coat. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.surfcoat.2015.06.048
  • 2015 • 177 Large scale Molecular Dynamics simulation of microstructure formation during thermal spraying of pure copper
    Wang, T. and Begau, C. and Sutmann, G. and Hartmaier, A.
    Surface and Coatings Technology 280 72-80 (2015)
    Thermal spray processes are widely used for the manufacture of advanced coating systems, e.g. metallic coatings for wear and corrosion protection. The desired coating properties are closely related to the microstructure, which is highly influenced by the processing parameters, such as temperature, size and velocity of the sprayed particles. In this paper, large scale Molecular Dynamics simulations are conducted to investigate the microstructure formation mechanisms during the spraying process of hot nano-particles onto a substrate at room temperature using pure copper as a benchmark material representing for a wider class of face-centered-cubic metals. To evaluate the influence of processing parameters on the coating morphology, a number of simulations are performed in which the initial temperature, size and velocity of copper particles are systematically varied in order to investigate the thermal and microstructural evolution during impaction. Two distinct types of microstructural formation mechanisms, resulting in different coating morphologies, are observed in the present investigation, which are either governed by plastic deformation or by the process of melting and subsequent solidification. Furthermore, a thermodynamically motivated model as a function of the particle temperature and velocity is developed, which predicts the microstructural mechanisms observed in the simulations. The results provide an elementary insight into the microstructure formation mechanisms on an atomistic scale, which can serve as basic input for continuum modeling of thermal spray process. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2015.08.034
  • 2015 • 176 Laser beam welding of aluminum to Al-base coated high-strength steel 22MnB5
    Windmann, M. and Röttger, A. and Kügler, H. and Theisen, W. and Vollertsen, F.
    Journal of Materials Processing Technology 217 88-95 (2015)
    The microstructure of aluminum-Al-coated steel laser beam welding joints was analyzed with respect to the welding energy. Quantitative and qualitative analysis of the welding microstructure were used to measure the weld width as well as the thickness of the resulting intermetallic layer at the 22MnB5/aluminum interface in relation to the welding parameters. Weldability of Al-coated steel could be improved by removing brittle coating particles and oxides on the steel surface by sandblasting. Adhesion of aluminum filler material to the 22MnB5 steel sheet could be enhanced by inductive preheating of the steel surface during laser welding. This produced welded 22MnB5/aluminum joints that exhibited a linear mechanical resistance of 220 MPa and which failed away from the brittle intermetallic layer on the aluminum side under a tensile load. The shear strength of the intermetallic layer on the 22MnB5/aluminum interface was evaluated to 74 ± 21 MPa. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2014.10.026
  • 2015 • 175 Martensite aging - Avenue to new high temperature shape memory alloys
    Niendorf, T. and Krooß, P. and Somsen, C. and Eggeler, G. and Chumlyakov, Y.I. and Maier, H.J.
    Acta Materialia 89 298-304 (2015)
    High-temperature shape memory alloys are attractive for efficient solid state actuation. A key criterion for shape memory alloys is the martensite start temperature. The current study introduces a concept for increasing this temperature of alloys initially not suited for high-temperature actuation. Aging of stress-induced martensite, referred to as SIM-aging in the current work, is able to increase the martensite start temperature by about 130 °C as demonstrated in the present study for a Co-Ni-Ga shape memory alloy. The increase of transformation temperatures can be explained based on the concept of symmetry-conforming short-range order. Following SIM-aging the Co-Ni-Ga alloy shows cyclic actuation stability at elevated temperatures. While martensite aging has always been viewed as detrimental in the past, it can actually be exploited to design new classes of high-temperature shape memory alloys with excellent properties. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.01.042
  • 2015 • 174 Mechanical and microstructural analysis of ultrasonically assisted induction-brazed TiAl6V4 joints
    Tillmann, W. and Zimpel, M. and Dias, N.F.L. and Pfeiffer, J. and Wojarski, L. and Xu, Z.
    Welding in the World 59 901-909 (2015)
    This paper focuses on the process of ultrasonically assisted induction brazing with regard to titanium brazing. The titanium alloy TiAl6V4 was brazed using an aluminum-based filler alloy (AlMg2.5Cr0.3). It was apparent that the layer thickness of the brazing foil as well as the brazing temperature and the intensity of the ultrasound are significant influencing factors of the combined brazing process and microstructure. It is the aim of this paper to draw conclusions from the microstructural and mechanical investigations of the brazed joint about the process parameters, which are crucial for the properties and quality of the joint. The evaluation of the microstructure of the joint was conducted by means of metallographic investigations and results obtained by means of scanning electron microscopy. Besides mechanical microhardness measurements, strength investigations were conducted in order to evaluate the quality of the joint. Furthermore, the results of conventional vacuum brazing processes were correlated in order to be able to better facilitate and understand the adapted induction brazing process. © 2015, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-015-0260-1
  • 2015 • 173 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 • 172 Microsegregation and precipitates of an as-cast Co-based superalloy—microstructural characterization and phase stability modelling
    Koßmann, J. and Zenk, C.H. and Lopez-Galilea, I. and Neumeier, S. and Kostka, A. and Huth, S. and Theisen, W. and Göken, M. and Drautz, R. and Hammerschmidt, T.
    Journal of Materials Science 50 6329-6338 (2015)
    The demand for increased efficiency of industrial gas turbines and aero engines drives the search for the next generation of materials. Promising candidates for such new materials are Co-based superalloys. We characterize the microsegregation and solidification of a multi-component Co-based superalloy and compare it to a ternary Co–Al–W compound and to two exemplary Ni-based superalloys by combining the experimental characterization of the as-cast microstructures with complementary modelling of phase stability. On the experimental side, we characterize the microstructure and precipitates by electron microscopy and energy-dispersive X-ray spectroscopy and determine the element distributions and microsegregation coefficients by electron probe microanalysis (EPMA). On the modelling side, we carry out solidification simulations and a structure map analysis in order to relate the local chemical composition with phase stability. We find that the microsegregation coefficients for the individual elements are very similar in the investigated Co-based and Ni-based superalloys. By interpreting the local chemical composition from EPMA with the structure map, we effectively unite the set of element distribution maps to compound maps with very good contrast of the dendritic microstructure. The resulting compound maps of the microstructure in terms of average band filling and atomic-size difference explain the formation of topologically close-packed phases in the interdendritic regions. We identify B2, C14, and D0<inf>24</inf> precipitates with chemical compositions that are in line with the structure map. © 2015, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-015-9177-8
  • 2015 • 171 Microstructural evolution in a Ti-Ta hightemperature shape memory alloy during creep
    Rynko, R. and Marquardt, A. and Paulsen, A. and Frenzel, J. and Somsen, C. and Eggeler, G.
    International Journal of Materials Research 106 331-341 (2015)
    Alloys based on the titanium-tantalum system are considered for application as high-temperature shape memory alloys due to their martensite start temperatures, which can surpass 200 °C. In the present work we study the evolution of microstructure and the influence of creep on the phase transformation behavior of a Ti70Ta30 (at.%) high-temperature shape memory alloy. Creep tests were performed in a temperature range from 470 to 530 °C at stresses between 90 and 150 MPa. The activation energy for creep was found to be 307 kJ mol-1 and the stress exponent n was determined as 3.7. Scanning and transmission electron microscopy investigations were carried out to characterize the microstructure before and after creep. It was found that the microstructural evolution during creep suppresses subsequent martensitic phase transformations. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.111189
  • 2015 • 170 Microstructural evolution of a CoCrFeMnNi high-entropy alloy after swaging and annealing
    Laplanche, G. and Horst, O. and Otto, F. and Eggeler, G. and George, E.P.
    Journal of Alloys and Compounds 647 548-557 (2015)
    Abstract The processing parameters which govern the evolution of microstructure and texture during rotary swaging and subsequent heat treatments were studied in an equiatomic single-phase CoCrFeMnNi high-entropy alloy. After vacuum induction melting and casting, the diameter of the 40 mm cast ingot was reduced at room temperature to a final diameter of 16.5 mm by rotary swaging (diameter reduction of 60%/area reduction of 80%) and the alloy was then annealed at different temperatures for 1 h. The resulting microstructures were analyzed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction and correlated with results of microhardness measurements. It was found that the microhardness first increases slightly upon annealing below the recrystallization temperature but then drops steeply at higher annealing temperatures due to the onset of recrystallization. Special emphasis was placed on how the microstructure evolves with respect to the radial and longitudinal position in the rod. Finally, a combination of swaging and heat treatment parameters were identified that can produce CoCrFeMnNi high-entropy alloys with a homogeneous composition and grain size and almost no texture. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2015.05.129
  • 2015 • 169 Microstructure design and mechanical properties in a near-α Ti-4Mo alloy
    Tarzimoghadam, Z. and Sandlöbes, S. and Pradeep, K.G. and Raabe, D.
    Acta Materialia 97 291-304 (2015)
    Abstract We study the effects of different heat treatment routes on microstructure engineering and the resulting mechanical response in a plain binary Ti-4Mo (wt%) model alloy. We observe a broad variety of microstructure formation mechanisms including diffusion driven allotropic phase transformations as well as shear and/or diffusion dominated modes of martensitic transformations, enabling a wealth of effective microstructure design options even in such a simple binary Ti alloy. This wide variety of microstructures allows tailoring the mechanical properties ranging from low yield strength (350 MPa) and high ductility (30-35% tensile elongation) to very high yield strength (1100 MPa) and medium ductility (10-15% tensile elongation) as well as a variety of intermediate states. Mechanical testing and microstructure characterization using optical microscopy, scanning electron microscopy based techniques, transmission electron microscopy and atom probe tomography were performed revealing that minor variations in the heat treatment cause significant changes in the resulting microstructures (e.g. structural refinement, transition between diffusive and martensitic transformations). The experimental results on microstructure evolution during the applied different heat treatment routes are discussed with respect to the mechanical properties. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.06.043
  • 2015 • 168 Microstructure in plasticity, a comparison between theory and experiment
    Dmitrieva, O. and Raabe, D. and Müller, S. and Dondl, P.W.
    Lecture Notes in Applied and Computational Mechanics 78 205-218 (2015)
    We review aspects of pattern formation in plastically deformed single crystals, in particular as described in the investigation of a copper single crystal shear experiment in [DDMR09]. In this experiment, the specimen showed a band-like microstructure consisting of alternating crystal orientations. Such a formation of microstructure is often linked to a lack of convexity in the free energy describing the system. The specific parameters of the observed bands, namely the relative crystal orientation as well as the normal direction of the band layering, are thus compared to the predictions of the theory of kinematically compatible microstructure oscillating between low-energy states of the non-convex energy. We conclude that this theory is suitable to describe the experimentally observed band-like structure. Furthermore, we link these findings to the models used in studies of relaxation and evolution of microstructure. ©Springer International Publishing Switzerland 2015.
    view abstractdoi: 10.1007/978-3-319-18242-1_8
  • 2015 • 167 Microstructure refinement for high modulus in-situ metal matrix composite steels via controlled solidification of the system Fe-TiB2
    Springer, H. and Aparicio Fernandez, R. and Duarte, M.J. and Kostka, A. and Raabe, D.
    Acta Materialia 96 47-56 (2015)
    Microstructures of Fe-TiB<inf>2</inf> metal-matrix-composites formed in-situ from Fe-Ti-B melts were investigated for hypo- and hyper-eutectic concentrations down to atomic-scale resolution. Special emphasis is laid on the influence of the solidification rate on particle size, morphology and distribution as well as their relation to mechanical properties. Innovative routes for the cost-effective production of stiff and ductile high modulus steels for lightweight structural applications are discussed, focusing on hyper-eutectic compositions due to their high stiffness/density ratio: firstly, very slow cooling allows the primary particles floating to the top of the cast, from which they can either be easily removed for retaining bulk material containing only fine-dispersed eutectic particles, or be kept and utilised as a wear resistant surface. Secondly, annealing of amorphous matrix material obtained from very fast solidification leads to fine dispersed nano-scaled precipitation of TiB<inf>2</inf> particles. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.06.017
  • 2015 • 166 Microstructure, Shape Memory Effect and Functional Stability of Ti67Ta33 Thin Films
    Motemani, Y. and Kadletz, P.M. and Maier, B. and Rynko, R. and Somsen, C. and Paulsen, A. and Frenzel, J. and Schmahl, W.W. and Eggeler, G. and Ludwig, Al.
    Advanced Engineering Materials 17 1425-1433 (2015)
    Ti-Ta based alloys are an interesting class of high-temperature shape memory materials. When fabricated as thin films, they can be used as high-temperature micro-actuators with operation temperatures exceeding 100 °C. In this study, microstructure, shape memory effect and thermal cycling stability of room-temperature sputter deposited Ti<inf>67</inf>Ta<inf>33</inf> thin films are investigated. A disordered α martensite (orthorhombic) phase is formed in the as-deposited Ti<inf>67</inf>Ta<inf>33</inf> films. The films show a columnar morphology with the columns being oriented perpendicular to the substrate surface. They are approximately 200 nm in width. XRD texture analysis reveals a martensite fiber texture with {120} and {102} fiber axes. The XRD results are confirmed by TEM analysis, which also shows columnar grains with long axes perpendicular to the {120} and {102} planes of α martensite. The shape memory effect is analyzed in the temperature range of -10 to 240 °C using the cantilever deflection method, with special emphasis placed on cyclic stability. Ti<inf>67</inf>Ta<inf>33</inf> thin films undergo a forward martensitic transformation at M<inf>s</inf> ≈ 165 °C, with a stress relaxation of approximately 33 MPa during the transformation. The actuation response of the film actuators degrades significantly during thermal cycling. TEM analysis shows that this degradation is related to the formation of nanoscale ω precipitates (5-13 nm) which form above the austenite finish temperature. These precipitates suppress the martensitic transformation, as they act as obstacles for the growth of martensite variants. Ti-Ta thin films can be used as high-temperature micro-actuators. In this study, microstructure, shape memory effect, and functional stability of room-temperature sputter deposited Ti<inf>67</inf>Ta<inf>33</inf> thin films are systematically investigated. The actuation response of the film actuators degrades significantly during thermal cycling. This degradation is related to the formation of nanoscale ω precipitates (5-13 nm) which form above the austenite finish temperature. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201400576
  • 2015 • 165 Multiphase microstructures via confined precipitation and dissolution of vessel phases: Example of austenite in martensitic steel
    Belde, M. and Springer, H. and Inden, G. and Raabe, D.
    Acta Materialia 86 1-14 (2015)
    We present a novel method to locally control the constitution, morphology, dispersion and transformation behavior of multiphase materials. The approach is based on the targeted, site-specific formation and confined dissolution of precipitated carbides or intermetallic phases. These dispersoids act as "vessels" or "containers" for specific alloying elements forming controlled chemical gradients within the microstructure upon precipitation and subsequent (partial) dissolution at elevated temperatures. The basic processing sequence consists of three subsequent steps, namely: (i) matrix homogenization (conditioning step); (ii) nucleation and growth of the vessel phases (accumulation step); and (iii) (partial) vessel dissolution (dissolution step). The vessel phase method offers multiple pathways to create dispersed microstructures by the variation of plain thermomechanical parameters such as time, temperature and deformation. This local microstructure design enables us to optimize the mechanical property profiles of advanced structural materials such as high strength steels at comparatively lean alloy compositions. The approach is demonstrated on a 11.6Cr-0.32C (wt.%) steel, where by using M23C6 carbides as a vessel phase, Cr and C can be locally enriched so that the thus-lowered martensite start temperature allows the formation of a significant quantity of retained austenite (up to 14 vol.%) of fine dispersion and controlled morphology. The effects of processing parameters on the obtained microstructures are investigated, with a focus on the dissolution kinetics of the vessel carbides. The approach is referred to as vessel microstructure design. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.11.025
  • 2015 • 164 Nanolaminate transformation-induced plasticity-twinning-induced plasticity steel with dynamic strain partitioning and enhanced damage resistance
    Wang, M.-M. and Tasan, C.C. and Ponge, D. and Dippel, A.-Ch. and Raabe, D.
    Acta Materialia 85 216-228 (2015)
    Conventional martensitic steels have limited ductility due to insufficient microstructural strain-hardening and damage resistance mechanisms. It was recently demonstrated that the ductility and toughness of martensitic steels can be improved without sacrificing the strength, via partial reversion of the martensite back to austenite. These improvements were attributed to the presence of the transformation-induced plasticity (TRIP) effect of the austenite phase, and the precipitation hardening (maraging) effect in the martensitic matrix. However, a full micromechanical understanding of this ductilizing effect requires a systematic investigation of the interplay between the two phases, with regards to the underlying deformation and damage micromechanisms. For this purpose, in this work, a Fe-9Mn-3Ni-1.4Al-0.01C (mass%) medium-Mn TRIP maraging steel is produced and heat-treated under different reversion conditions to introduce well-controlled variations in the austenite-martensite nanolaminate microstructure. Uniaxial tension and impact tests are carried out and the microstructure is characterized using scanning and transmission electron microscopy based techniques and post mortem synchrotron X-ray diffraction analysis. The results reveal that (i) the strain partitioning between austenite and martensite is governed by a highly dynamical interplay of dislocation slip, deformation-induced phase transformation (i.e. causing the TRIP effect) and mechanical twinning (i.e. causing the twinning-induced plasticity effect); and (ii) the nanolaminate microstructure morphology leads to enhanced damage resistance. The presence of both effects results in enhanced strain-hardening capacity and damage resistance, and hence the enhanced ductility. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.11.010
  • 2015 • 163 Nanotribology in austenite: Plastic plowing and crack formation
    Brinckmann, S. and Dehm, G.
    Wear 338-339 436-440 (2015)
    Especially during the run-in phase of metal friction, plasticity develops in the contact zone. If well controlled, the plasticity will lead to an improved microstructure that exhibits low wear rates during machine operation. We investigate the plasticity due to a single stroke of a micrometer-sized asperity to understand fundamentally tribology induced plasticity and microstructure formation. We find that the local crystal orientation has a significant influence on the development and spread of plasticity. Additionally, the complex three-dimensional stress state results in the formation of non-obvious plastic slip patterns. Finally, we observe crack formation in the scratch track even during the single stroke experiments. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.wear.2015.05.001
  • 2015 • 162 Novel opportunities for thermal spray by PS-PVD
    Mauer, G. and Jarligo, M.O. and Rezanka, S. and Hospach, A. and Vaßen, R.
    Surface and Coatings Technology 268 52-57 (2015)
    Plasma spray-physical vapor deposition (PS-PVD) is a novel coating process based on plasma spraying. In contrast to conventional methods, deposition takes place not only from liquid splats but also from nano-sized clusters and from the vapor phase. This offers new opportunities to obtain advanced microstructures and thus to comply with the growing demands on modern functional coatings. Thin and dense ceramic coatings as well as highly porous columnar structures can be achieved, offering novel opportunities for the application of thermal spray technology. This study describes process conditions, which are relevant for the formation of particular microstructures in the PS-PVD process. Following the structure of the process, the feedstock treatment close to the plasma source, plasma particle interaction in the open jet and the formation of coating microstructures on the substrate are covered. Calculated results on the plasma particle interaction under PS-PVD process conditions were found to be in good agreement with OES results and microstructural observations. They show that the feedstock treatment along the very first trajectory segment between injector and jet expansion plays a key role. Varying the plasma parameters, feedstock treatment can be controlled to a broad extent. Consequently, the manifold nature of the feedstock species arriving on the substrate enables to achieve various coating microstructures. As examples, application specific features of PS-PVD coatings are reported for strain-tolerant thermal barrier coatings as well as for gas-tight oxygen transport membranes with high mixed electronic-ionic conductivity. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2014.06.002
  • 2015 • 161 Numerical energy relaxation to model microstructure evolution in functional magnetic materials
    Kiefer, B. and Buckmann, K. and Bartel, T.
    GAMM Mitteilungen 38 171-196 (2015)
    This paper proposes energy relaxation-based approaches for the modeling of magnetostriction, with a particular focus on single crystalline magnetic shape memory alloy response. The theoretical development relies on concepts of energy relaxation in the context of nonconvex free energy landscapes whose wells define preferred states of spontaneous straining and magnetization. The constrained theory of magnetoelasticity developed by DeSimone and James [1] represents the point of departure for the model development, and its capabilities, but also limitations, are demonstrated by means of representative numerical examples. The key features that characterize the extended approach are (i) the incorporation of elastic deformations, whose distribution among the individual phases occurs in an energy minimizing fashion, (ii) a finite magnetocrystalline anisotropy energy, that allows magnetization rotations away from easy axes, and (iii) dissipative effects, that are accounted for in an incremental variational setting for standard dissipative materials. In the context of introducing elastic strain energy, two different relaxation concepts, the convexification approach and the rank-one relaxation with respect to first-order laminates, are considered. In this manner, important additional response features, e.g. the hysteretic nature, the linear magnetization response in the pre-variant reorientation regime, and the stress dependence of the maximum field induced strain, can be captured, which are prohibited by the inherent assumptions of the constrained theory. The enhanced modeling capabilities of the extended approach are demonstrated by several representative response simulations and comparison to experimental results taken from literature. These examples particularly focus on the response of single crystals under cyclic magnetic field loading at constant stress and cyclic mechanical loading at constant magnetic field. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/gamm.201510009
  • 2015 • 160 On the identification of superdislocations in the γ′-phase of single-crystal Ni-base superalloys - An application of the LACBED method to complex microstructures
    Müller, J. and Eggeler, G. and Spiecker, E.
    Acta Materialia 87 34-44 (2015)
    Ni-base superalloys are used for turbine blades, which operate in the creep range at temperatures above 1000 °C. One of the objectives of modern materials science is to analyze the combination of elementary deformation and microstructural coarsening processes and to identify physically based micromechanical models which allow one to predict the mechanical behavior on the macroscale. High-temperature creep of single-crystal Ni-base superalloys is governed by dislocation plasticity in the well-known γ/γ′-microstructure. For a comprehensive description of plasticity, it is important to understand the nucleation, glide and climb of superdislocations in the γ′-phase. The rate-controlling dislocation processes have to be identified and therefore a reliable Burgers vector analysis of superdislocations is essential. Superdislocations exhibit complex dislocation cores, typically comprising superpartial dislocations and planar defects. Therefore, conventional Burgers vector analysis based on the invisibility criterion often fails, due to the presence of pronounced residual contrast. In the present work, large-angle convergent-beam electron diffraction (LACBED) is employed for Burgers vector determination of two characteristic superdislocations, of the standard <1 1 0> and the more complex <1 0 0> type. LACBED results are compared with results obtained using the conventional invisibility analysis. While both techniques work for the standard superdislocation, the conventional analysis fails to analyze the <1 0 0> superdislocation, which shows pronounced residual contrast even under conditions of g · b = 0 and g · b × u = 0. In contrast, the LACBED technique allows for an unambiguous determination of the Burgers vector, including its magnitude and absolute sense. In the present study, the use of LACBED to identify dislocations in the complex microstructure of an Ni-base superalloy is outlined and the better performance of LACBED as compared to the conventional gb-analysis is discussed. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.12.029
  • 2015 • 159 Orientation dependent deformation by slip and twinning in magnesium during single crystal indentation
    Zambaldi, C. and Zehnder, C. and Raabe, D.
    Acta Materialia 91 267-288 (2015)
    We present the orientation dependent indentation response of pure magnesium during single grain indentation. A conical indenter and maximum loads between 50 mN and 900 mN were employed. Indent topographies were acquired by confocal microscopy. The indents were also characterized by electron backscatter orientation microscopy for their microstructures. Pronounced activation of specific twinning systems was observed around the impressions. The resulting data were compiled into the inverse pole figure presentation of indent microstructures and topographies after Zambaldi and Raabe, Acta Mater. (2010). Three-dimensional crystal plasticity finite element simulation of the indentation deformation supports the interpretation of the orientation dependent slip and twinning patterns around the indents. The match between the activation of observed and simulated twinning variants is discussed with respect to the conditions for nucleation and growth of extension twins. Furthermore, the compatibility of the twinning strains with the imposed deformation is discussed based on the expanding cavity model of indentation. The orientation dependent response of magnesium during indentation is compared to the literature data for indentation of alpha-titanium and beryllium. Recommendations are given on how to exploit the characteristic nature of the observed indentation patterns to rapidly assess the relative activity of deformation mechanisms and their critical shear stresses during alloy development. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.01.046
  • 2015 • 158 Porosity-Property Relationships of Plasma-Sprayed Gd2Zr2O7/YSZ Thermal Barrier Coatings
    Bakan, E. and Mack, D.E. and Mauer, G. and Mücke, R. and Vaßen, R.
    Journal of the American Ceramic Society 98 2647-2654 (2015)
    During the past decade, gadolinium zirconate (Gd<inf>2</inf>Zr<inf>2</inf>O<inf>7</inf>, GZO) has attracted interest as an alternative material to partially yttria-stabilized zirconia (YSZ) for thermal barrier coatings (TBCs). Despite the well-known benefits of GZO, such as lower thermal conductivity and superior temperature capability compared to YSZ, processing of GZO via atmospheric plasma spraying (APS) still remains a challenge. Here, we report on APS experiments which were performed to investigate the influence of processing on GZO microstructure and lifetime of GZO/YSZ double-layer TBCs. Different microstructures of GZO were produced and characterized in terms of porosity, stoichiometry, Young′s modulus, and their effects on the lifetime of YSZ/GZO double-layer TBCs were discussed. Particle diagnostics were utilized for the optimization of the process parameters with respect to different microstructures of GZO and stoichiometry. It was found that both cumulative porosity of GZO and pore size distribution, which alter the Young′s modulus significantly, govern the lifetime of double layers. In addition, it was shown that the deviation in GZO stoichiometry due to gadolinia evaporation in the investigated range does not display any critical effect on lifetime. © 2015 The American Ceramic Society.
    view abstractdoi: 10.1111/jace.13611
  • 2015 • 157 Primary combination of phase-field and discrete dislocation dynamics methods for investigating athermal plastic deformation in various realistic Ni-base single crystal superalloy microstructures
    Gao, S. and Kumar Rajendran, M. and Fivel, M. and Ma, A. and Shchyglo, O. and Hartmaier, A. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 23 (2015)
    Three-dimensional discrete dislocation dynamics (DDD) simulations in combination with the phase-field method are performed to investigate the influence of different realistic Ni-base single crystal superalloy microstructures with the same volume fraction of γ;precipitates on plastic deformation at room temperature. The phase-field method is used to generate realistic microstructures as the boundary conditions for DDD simulations in which a constant high uniaxial tensile load is applied along different crystallographic directions. In addition, the lattice mismatch between the γand γ;phases is taken into account as a source of internal stresses. Due to the high antiphase boundary energy and the rare formation of superdislocations, precipitate cutting is not observed in the present simulations. Therefore, the plastic deformation is mainly caused by dislocation motion in γ; matrix channels. From a comparison of the macroscopic mechanical response and the dislocation evolution for different microstructures in each loading direction, we found that, for a given γ;phase volume fraction, the optimal microstructure should possess narrow and homogeneous γ; matrix channels. © 2015 IOP Publishing Ltd Printed in the UK.
    view abstractdoi: 10.1088/0965-0393/23/7/075003
  • 2015 • 156 Processing of NiTi shape memory sheets - Microstructural heterogeneity and evolution of texture
    Laplanche, G. and Kazuch, A. and Eggeler, G.
    Journal of Alloys and Compounds 651 333-339 (2015)
    In the present paper we study the evolution of microstructure and texture during processing of Ni<inf>51</inf>Ti<inf>49</inf> shape memory sheets using electron backscatter diffraction. Hot rolling results in a heterogeneous microstructure which reflects a temperature gradient in the sheet. Equiaxed and randomly oriented grains are observed close to the surface of the hot rolled sheet while the sheet interior shows a strong texture containing two main texture components {111}<110> and {110}<110> with grains elongated along the rolling direction. In contrast, cold rolling in combination with a recrystallization heat treatment produces a more homogeneous microstructure in terms of grain morphology and grain size. It also promotes a random grain orientation along the rolling and transverse directions while the normal direction shows a strong γ-fiber {111}<uvw> texture. To get a better understanding of the elementary deformation mechanisms which control the texture evolution during rolling, textures assessed in the present study are compared with simulations reported in the literature. © 2015, Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.08.127
  • 2015 • 155 Processing, Microstructure and Mechanical Properties of the CrMnFeCoNi High-Entropy Alloy
    Gludovatz, B. and George, E.P. and Ritchie, R.O.
    JOM 67 2262-2270 (2015)
    Equiatomic multi-component alloys, referred to variously as high-entropy alloys, multi-component alloys, or compositionally complex alloys in the literature, have recently received significant attention in the materials science community. Some of these alloys can display a good combination of mechanical properties. Here, we review recent work on the processing, microstructure and mechanical properties of one of the first and most studied high-entropy alloys, namely the single-phase, face-centered cubic alloy CrMnFeCoNi, with emphasis on its excellent damage tolerance (strength with toughness) in the temperature range from room temperature down to liquid nitrogen temperature. © 2015, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-015-1589-z
  • 2015 • 154 Quantitative phase imaging by wide field lensless digital holographic microscope
    Adinda-Ougba, A. and Koukourakis, N. and Essaidi, A. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9529 (2015)
    Wide field, lensless microscopes have been developed for telemedicine and for resource limited setting [1]. They are based on in-line digital holography which is capable to provide amplitude and phase information resulting from numerical reconstruction. The phase information enables achieving axial resolution in the nanometer range. Hence, such microscopes provide a powerful tool to determine three-dimensional topologies of microstructures. In this contribution, a compact, low-cost, wide field, lensless microscope is presented, which is capable of providing topological profiles of microstructures in transparent material. Our setup consist only of two main components: a CMOSsensor chip and a laser diode without any need of a pinhole. We use this very simple setup to record holograms of microobjects. A wide field of view of ~24 mm, and a lateral resolution of ~2 μm are achieved. Moreover, amplitude and phase information are obtained from the numerical reconstruction of the holograms using a phase retrieval algorithm together with the angular spectrum propagation method. Topographic information of highly transparent micro-objects is obtained from the phase data. We evaluate our system by recording holograms of lines with different depths written by a focused laser beam. A reliable characterization of laser written microstructures is crucial for their functionality. Our results show that this system is valuable for determination of topological profiles of microstructures in transparent material. © 2015 SPIE.
    view abstractdoi: 10.1117/12.2184471
  • 2015 • 153 Rate-independent versus viscous evolution of laminate microstructures in finite crystal plasticity
    Günther, C. and Kochmann, D.M. and Hackl, K.
    Lecture Notes in Applied and Computational Mechanics 78 63-88 (2015)
    In this chapter we investigate the variationalmodeling of the evolution of inelastic microstructures by the example of finite crystal plasticity with one active slip system. For this purpose we describe the microstructures by laminates of first order.We propose an analytical partial relaxation of an incompressible neo-Hookean energy formulation, keeping the internal variables and geometric microstructure parameters fixed, thus approximating the relaxed energy by an upper bound of the rank-one-convex hull. Based on the minimization of a Lagrange functional, consisting of the sum of rate of energy and dissipation potential, we derive an incremental strategy to model the time-continuous evolution of the laminate microstructure. Special attention is given to the three distinct cases of microstructure evolution, initiation, rotation, and continuous change. We compare a rate-independent approach with another one that employs viscous regularization which has certain advantages concerning the numerical implementation. Simple shear and tension/compression tests will be shown to demonstrate the differences between both approaches and to show the physical implications of the models introduced. © Springer International Publishing Switzerland 2015.
    view abstractdoi: 10.1007/978-3-319-18242-1_3
  • 2015 • 152 Site-Directed, On-Surface Assembly of DNA Nanostructures
    Meyer, R. and Saccà, B. and Niemeyer, C.M.
    Angewandte Chemie - International Edition 54 12039-12043 (2015)
    Two-dimensional DNA lattices have been assembled from DNA double-crossover (DX) motifs on DNA-encoded surfaces in a site-specific manner. The lattices contained two types of single-stranded protruding arms pointing into opposite directions of the plane. One type of these protruding arms served to anchor the DNA lattice on the solid support through specific hybridization with surface-bound, complementary capture oligomers. The other type of arms allowed for further attachment of DNA-tethered probe molecules on the opposite side of the lattices exposed to the solution. Site-specific lattice assembly and attachment of fluorophore-labeled oligonucleotides and DNA-protein conjugates was demonstrated using DNA microarrays on flat, transparent mica substrates. Owing to their programmable orientation and addressability over a broad dynamic range from the nanometer to the millimeter length scale, such supramolecular architecture might be used for presenting biomolecules on surfaces, for instance, in biosensor applications. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201505553
  • 2015 • 151 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 • 150 Ternary and quaternary Cr or Ga-containing ex-LDH catalysts - Influence of the additional oxides onto the microstructure and activity of Cu/ZnAl2O4 catalysts
    Kühl, S. and Schumann, J. and Kasatkin, I. and Hävecker, M. and Schlögl, R. and Behrens, M.
    Catalysis Today 246 92-100 (2015)
    The stepwise substitution of Al by Cr and Ga leads to quaternary LDH precursors for Cu/ZnM2O4 (M = Al, Ga, Cr) catalysts. With the substitution of Al by Cr the interaction of the Cu phase with the oxide matrix is gradually weakened, which is caused by the participation of the chromium oxide phase in the redox processes during catalyst preparation. Such reactive Cr oxide matrix is less efficient than the inert Al oxide matrix in stabilizing the special microstructure of Cu/ZnM2O4 catalysts. These weakened interactions led to a lowering of the Cu particle embedment, coinciding with a pronounced Cu crystallite growth during reduction. Both effects partially compensate each other and a maximum in Cu surface area is observed for intermediate Cr contents. In the Ga-substituted catalysts, two distinct Cu species were found for high Ga contents. This is attributed to the presence of partially crystalline spinel and the resulting different strength of interface interaction of the CuO phase with the crystalline and the amorphous oxide. After reduction Cu catalysts with similar average Cu particle sizes as well as Cu surface areas were obtained. In both sample series, the catalytic activity in methanol synthesis does not scale with the Cu surface area and the experiments show that a strong interaction to the oxide is necessary to gain stability and activity of the Cu phase. Al substitution thus confirms that interface interactions between Cu and the oxide seem to beneficially affect the activity of the Cu particles and the optimal catalyst requires a compromise of exposed surface and interface. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2014.08.029
  • 2015 • 149 The influence of partitioning on the growth of intragranular α in near-β Ti alloys
    Li, T. and Ahmed, M. and Sha, G. and Shi, R. and Casillas, G. and Yen, H.-W. and Wang, Y. and Pereloma, E.V. and Cairney, J.M.
    Journal of Alloys and Compounds 643 212-222 (2015)
    Abstract We report on partitioning of alloying elements during the formation of fine intragranular α plates in a Ti-55521 alloy after thermo-mechanical processing (TMP) and isothermal ageing at 923 K. The microstructures were characterised using atom probe tomography and high-resolution transmission electron microscopy. The partitioning of Mo, V and Al are strongly affected by their diffusivities and their mutual interaction. This leads to a deviation of the measured contents of alloying elements in the two phases from the predicted equilibrium values. The alloying elements at the broad faces and tips of α plates were found to exhibit different pile-up and segregation behaviours, which is thought to affect the lengthening and thickening kinetics of the α plates. As a result, the aspect ratio of α plates decreased rapidly with increasing ageing time. This study suggests that careful selection of alloying elements could be an effective way in controlling the growth anisotropy of α plates and thus α + β microstructures in near-β Ti alloys. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.04.143
  • 2015 • 148 The relaxed linear micromorphic continuum: Existence, uniqueness and continuous dependence in dynamics
    Ghiba, I.-D. and Neff, P. and Madeo, A. and Placidi, L. and Rosi, G.
    Mathematics and Mechanics of Solids 20 1171-1197 (2015)
    We study well-posedness for the relaxed linear elastic micromorphic continuum model with symmetric Cauchy force-stresses and curvature contribution depending only on the micro-dislocation tensor. In contrast to classical micromorphic models our free energy is not uniformly pointwise positive definite in the control of the independent constitutive variables. Another interesting feature concerns the prescription of boundary values for the micro-distortion field: only tangential traces may be determined which are weaker than the usual strong anchoring boundary condition. There, decisive use is made of new coercive inequalities recently proved by Neff, Pauly and Witsch, and by Bauer, Neff, Pauly and Starke. The new relaxed micromorphic formulation can be related to dislocation dynamics, gradient plasticity and seismic processes of earthquakes. © The Author(s) 2014.
    view abstractdoi: 10.1177/1081286513516972
  • 2015 • 147 The relaxed linear micromorphic continuum: Well-posedness of the static problem and relations to the gauge theory of dislocations
    Neff, P. and Ghiba, I.D. and Lazar, M. and Madeo, A.
    Quarterly Journal of Mechanics and Applied Mathematics 68 53-84 (2015)
    We consider the equilibrium problem in the relaxed linear model of micromorphic elastic materials. The basic kinematical fields of this extended continuum model are the displacement u ε R3 and the non-symmetric micro-distortion density tensor P ε R3×3. In this relaxed theory, a symmetric force-stress tensor arises despite the presence of microstructure and the curvature contribution depends solely on the micro-dislocation tensor Curl P. However, the relaxed model is able to fully describe rotations of the microstructure and to predict non-polar size-effects. In contrast to classical linear micromorphic models, we allow the usual elasticity tensors to become positive-semidefinite. We prove that, nevertheless, the equilibrium problem has a unique weak solution in a suitable Hilbert space. The mathematical framework also settles the question of which boundary conditions to take for the micro-distortion. Similarities and differences between linear micromorphic elasticity and dislocation gauge theory are discussed and pointed out. © The Author, 2015.
    view abstractdoi: 10.1093/qjmam/hbu027
  • 2015 • 146 Thermoelectric properties of pulsed current sintered nanocrystalline Al-doped ZnO by chemical vapour synthesis
    Gautam, D. and Engenhorst, M. and Schilling, C. and Schierning, G. and Schmechel, R. and Winterer, M.
    Journal of Materials Chemistry A 3 189-197 (2015)
    ZnO is a promising n-type oxide thermoelectric material, which is stable in air at elevated temperatures. In the present study, we report the bottom-up approach to create Al-doped ZnO nanocomposites from nanopowders, which are prepared by chemical vapour synthesis. With our synthesis route, we are able to create highly doped Al-containing ZnO nanocomposites that exhibit bulk-like electrical conductivity. Moreover, the impact of the microstructure of the nanocomposites on their thermal conductivity is enormous, with a value of 1.0 W m-1 K-1 for 1% Al-ZnO at room temperature, which is one of the lowest values reported, to date, on ZnO nanocomposites. The optimization of the Al-doping and microstructure with respect to the transport properties of bulk Al-ZnO nanocomposites leads to a zT value of about 0.24 at 950 K, underlining the potential of our technique. This journal is © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4ta04355c
  • 2014 • 145 A fracture-resistant high-entropy alloy for cryogenic applications
    Gludovatz, B. and Hohenwarter, A. and Catoor, D. and Chang, E.H. and George, E.P. and Ritchie, R.O.
    Science 345 1153-1158 (2014)
    High-entropy alloys are equiatomic, multi-element systems that can crystallize as a single phase, despite containing multiple elements with different crystal structures. A rationale for this is that the configurational entropy contribution to the total free energy in alloys with five or more major elements may stabilize the solid-solution state relative to multiphase microstructures. We examined a five-element high-entropy alloy, CrMnFeCoNi, which forms a single-phase face-centered cubic solid solution, and found it to have exceptional damage tolerance with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m1/2. Furthermore, its mechanical properties actually improve at cryogenic temperatures; we attribute this to a transition from planar-slip dislocation activity at room temperature to deformation by mechanical nanotwinning with decreasing temperature, which results in continuous steady strain hardening. © 2014, American Association for the Advancement of Science. All rights reserved.
    view abstractdoi: 10.1126/science.1254581
  • 2014 • 144 A method to quantitatively upscale the damage initiation of dual-phase steels under various stress states from microscale to macroscale
    Lian, J. and Yang, H. and Vajragupta, N. and Münstermann, S. and Bleck, W.
    Computational Materials Science 94 245-257 (2014)
    The aim of this paper is to develop a micromechanical model to quantitatively upscale the damage initiation of dual-phase steels under various stress states from micro to macro and reveal the underlying mechanisms of the damage initiation dependency on stress states from a microstructural level. Finite element (FE) model based on the real microstructure of a DP600 steel sheet is employed by representative volume element (RVE) method. Several numerical aspects are also discussed, such as mesh size and discretisation feature of the phase boundary. The plastic strain localisation theory is applied to the RVE modelling without any other damage models or imperfections. Three typical stress states, uniaxial tension, plane-strain tension and equibiaxial tension, are considered to investigate the influence of the stress state on damage initiation. The quantitative evaluation of the damage initiation for three stress states obtained from the RVE simulation shows the dependency on both stress triaxiality and Lode angle. The results are further compared to the experimentally calibrated damage initiation locus (DIL) and a fairly good agreement is achieved. From this study, the general physical understanding of the effect of stress states on damage initiation is explored and the method for quantitative analysis of the damage initiation in a microstructural level is also established. The microstructure heterogeneity is considered as the key factor that contributes to the damage initiation behaviour of the dual-phase steel. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2014.05.051
  • 2014 • 143 A unifying perspective: The relaxed linear micromorphic continuum
    Neff, P. and Ghiba, I.-D. and Madeo, A. and Placidi, L. and Rosi, G.
    Continuum Mechanics and Thermodynamics 26 639-681 (2014)
    We formulate a relaxed linear elastic micromorphic continuum model with symmetric Cauchy force stresses and curvature contribution depending only on the micro-dislocation tensor. Our relaxed model is still able to fully describe rotation of the microstructure and to predict nonpolar size effects. It is intended for the homogenized description of highly heterogeneous, but nonpolar materials with microstructure liable to slip and fracture. In contrast to classical linear micromorphic models, our free energy is not uniformly pointwise positive definite in the control of the independent constitutive variables. The new relaxed micromorphic model supports well-posedness results for the dynamic and static case. There, decisive use is made of new coercive inequalities recently proved by Neff, Pauly and Witsch and by Bauer, Neff, Pauly and Starke. The new relaxed micromorphic formulation can be related to dislocation dynamics, gradient plasticity and seismic processes of earthquakes. It unifies and simplifies the understanding of the linear micromorphic models. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00161-013-0322-9
  • 2014 • 142 Alloy Design, Combinatorial Synthesis, and Microstructure–Property Relations for Low-Density Fe-Mn-Al-C Austenitic Steels
    Raabe, D. and Springer, H. and Gutierrez-Urrutia, I. and Roters, F. and Bausch, M. and Seol, J.-B. and Koyama, M. and Choi, P.-P. and Tsuzaki, K.
    JOM 66 1845-1856 (2014)
    We present recent developments in the field of austenitic steels with up to 18% reduced mass density. The alloys are based on the Fe-Mn-Al-C system. Here, two steel types are addressed. The first one is a class of low-density twinning-induced plasticity or single phase austenitic TWIP (SIMPLEX) steels with 25–30 wt.% Mn and <4–5 wt.% Al or even <8 wt.% Al when naturally aged. The second one is a class of κ-carbide strengthened austenitic steels with even higher Al content. Here, κ-carbides form either at 500–600°C or even during quenching for >10 wt.% Al. Three topics are addressed in more detail, namely, the combinatorial bulk high-throughput design of a wide range of corresponding alloy variants, the development of microstructure–property relations for such steels, and their susceptibility to hydrogen embrittlement. © 2014, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1032-x
  • 2014 • 141 Chemical-Mechanical Characterization of the Creep-Resistant Mg-Al-Ca Alloy DieMag422 Containing Barium - Quasistatic and Cyclic Deformation Behavior in Different Defined Corrosion Conditions
    Wittke, P. and Klein, M. and Walther, F.
    Materials Testing 56 16-23 (2014)
    The influence of corrosion on the microstructure and the depending mechanical properties was investigated for the creep-resistant Mg-Al-Ca alloy DieMag422 containing barium. In order to investigate the corrosion behavior, potentio-dynamic polarization measurements and immersion tests were performed in pH7 without and with sodium chloride. Specimens in defined corrosion conditions were investigated by SEM for microstructure-related assessment of corrosion mechanisms. Strength and strain properties of non-corroded and corroded specimens were compared in tensile tests, underlining a significant decrease of tensile strength and fracture strain with increasing corrosion grade. The fatigue behaviour of the DieMag422 alloy in different corrosion conditions was characterized in multiple step and single step tests by means of mechanical stress-strain-hysteresis, temperature and electrical resistance measurements. Load increase tests allow to estimate the endurance limit and to determine the stress amplitude leading to fracture with one specimen. Fatigue results also showed a significant decrease in the estimated endurance limit and the failure stress with increasing corrosion grade. The applied physical measurement techniques can be equivalently used for the characterization of the fatigue behavior and representation of the actual fatigue state. The thermal and electrical materials responses were proportional to cyclic plastic deformation and provide the opportunity to evaluate the actual fatigue state of components under service loading in terms of condition monitoring. © Carl Hanser Verlag.
    view abstractdoi: 10.3139/120.110519
  • 2014 • 140 Construction of statistically similar representative volume elements - Comparative study regarding different statistical descriptors
    Scheunemann, L. and Schröder, J. and Balzani, D. and Brands, D.
    Procedia Engineering 81 1360-1365 (2014)
    Advanced high strength steels, such as dual-phase steel (DP steel), provide advantages for engineering applications compared to conventional high strength steel. The main constituents of DP steel on the microscopic level are martensitic inclusions embedded in a ferritic matrix. A way to include these heterogeneities on the microscale into the modeling of the material is the FE2- method. Herein, in every integration point of a macroscopic finite element problem a microscopic boundary value problem is attached, which consists of a representative volume element (RVE) often defined as a segment of a real microstructure. From this representation, high computational costs arise due to the complexity of the discretization which can be circumvented by the use of a Statistically Similar RVE (SSRVE), which is governed by similar statistical features as the real target microstructure but shows a lower complexity. For the construction of such SSRVEs, an optimization problem is constructed which consists of a least-square functional taking into account the differences of statistical measures evaluated for the real microstructure and the SSRVE. This functional is minimized to identify the SSRVE for which the similarity in a statistical sense is optimal. The choice of the statistical measures considered in the least-square functional however play an important role. We focus on the construction of SSRVEs based on the volume fraction, lineal-path function and spectral density and check the performance in virtual tests. Here the response of the individual SSRVEs is compared with the real target microstructure. Further higher order measures, some specific Minkowski functionals, are investigated regarding their applicability and efficiency in the optimization process. © 2014 The Authors. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2014.10.157
  • 2014 • 139 Construction of two- and three-dimensional statistically similar RVEs for coupled micro-macro simulations
    Balzani, D. and Scheunemann, L. and Brands, D. and Schröder, J.
    Computational Mechanics 54 1269-1284 (2014)
    In this paper a method is presented for the construction of two- and three-dimensional statistically similar representative volume elements (SSRVEs) that may be used in computational two-scale calculations. These SSRVEs are obtained by minimizing a least-square functional defined in terms of deviations of statistical measures describing the microstructure morphology and mechanical macroscopic quantities computed for a random target microstructure and for the SSRVE. It is shown that such SSRVEs serve as lower bounds in a statistical sense with respect to the difference of microstructure morphology. Moreover, an upper bound is defined by the maximum of the least-square functional. A staggered optimization procedure is proposed enabling a more efficient construction of SSRVEs. In an inner optimization problem we ensure that the statistical similarity of the microstructure morphology in the SSRVE compared with a target microstructure is as high as possible. Then, in an outer optimization problem we analyze mechanical stress–strain curves. As an example for the proposed method two- and three-dimensional SSRVEs are constructed for real microstructure data of a dual-phase steel. By comparing their mechanical response with the one of the real microstructure the performance of the method is documented. It turns out that the quality of the SSRVEs improves and converges to some limit value as the microstructure complexity of the SSRVE increases. This converging behavior gives reason to expect an optimal SSRVE at the limit for a chosen type of microstructure parameterization and set of statistical measures. © 2014, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00466-014-1057-6
  • 2014 • 138 Corrosion fatigue behaviour of creep-resistant magnesium alloy mg-4al-2ba-2ca
    Wittke, P. and Klein, M. and Walther, F.
    Procedia Engineering 74 78-83 (2014)
    Low corrosion resistance of magnesium alloys strongly limits their application range. This study aims at the investigation of corrosion influence on microstructure and depending mechanical properties of newly developed magnesium alloy Mg-4Al-2Ba- 2Ca. The fatigue properties of this creep-resistant magnesium alloy were investigated under three corrosive environments: double distilled water, 0.01 and 0.1 mol L-1 NaCl solutions. Potentiodynamic polarization measurements and immersion tests were performed to estimate the corrosion behaviour. Specimen surfaces were observed using light and scanning electron microscopy for microstructure-related assessment of corrosion mechanisms. The corrosion fatigue behaviour was characterized in continuous load increase tests using plastic strain and electrochemical measurements. Continuous load increase tests allow estimating the fatigue limit and determining the failure stress amplitude with a single specimen. Fatigue results showed a significant decrease in the estimated fatigue limit and determined failure stress amplitude with increasing corrosion impact of the environments. This corrosion-structure-property relation was quantitatively described by means of model-based correlation approaches and failure hypotheses. Plastic strain amplitude and deformation-induced changes in electrochemical measurands can be equivalently applied for precise corrosion fatigue assessment. © 2014 The Authors. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2014.06.228
  • 2014 • 137 Cyclic plasticity and lifetime of the nickel-based Alloy C-263: Experiments, models and component simulation
    Maier, G. and Hübsch, O. and Riedel, H. and Somsen, C. and Klöwer, J. and Mohrmann, R.
    MATEC Web of Conferences 14 (2014)
    The present work deals with the thermomechanical fatigue and low-cycle fatigue behavior of C-263 in two different material conditions. Microstructural characteristics and fracture modes are investigated with light and electron microscopy. The experimental results indicate that viscoplastic deformations depend on the heat treatment or rather on the current state of the microstructure. The measured data are used to adjust the parameters of a Chaboche type model and a fracture-mechanics based model for fatigue lifetime prediction. The Chaboche model is able to describe the essential phenomena of time and temperature dependent cyclic plasticity including the complex cyclic hardening during thermo-cyclic loading of both material conditions with a unique set of material parameters. This could be achieved by including an additional internal variable into the Chaboche model which accounts for changes in the precipitation microstructure during high temperature loading. Furthermore, the proposed lifetime model is well suited for a common fatigue life prediction of both investigated heats. The deformation and lifetime models are implemented into a user defined material routine. In this work, the material routine is applied for the lifetime prediction of a critical power plant component using the finite element method. © 2014 Owned by the authors, published by EDP Sciences.
    view abstractdoi: 10.1051/matecconf/20141416006
  • 2014 • 136 Designing quadplex (four-phase) microstructures in an ultrahigh carbon steel
    Zhang, H. and Ponge, D. and Raabe, D.
    Materials Science and Engineering A 612 46-53 (2014)
    Here we present an approach to design a ferrite-based quadplex microstructure (ferrite/martensite/carbide/austenite) using a lean alloyed Mn-Si-Cr-Al ultrahigh carbon steel. The material has 1500MPa tensile strength and 11% elongation. The thermomechanical processing includes two main steps, namely, first, the formation of a ferrite plus carbide duplex microstructure by warm rolling below Ae1; and second, annealing just above Ae1 for a short time (~20min). The quadplex microstructure consists of 57vol% ultrafine ferrite (mean grain size ~1.5μm), 29vol% martensite, 12vol% spherical carbide and 2vol% austenite. Fracture analysis after tensile deformation reveals a mixed ductile and brittle failure mode without necking. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and dilatometry tests were conducted to map the microstructure characteristics and the contribution of each phase to the overall deformation. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.06.023
  • 2014 • 135 Development and analysis of microstructures for the transplantation of thermally sprayed coatings
    Freiburg, D. and Biermann, D. and Peuker, A. and Kersting, P. and Maier, H.-J. and Möhwald, K. and Knödler, P. and Otten, M.
    Procedia CIRP 14 245-250 (2014)
    Thermally sprayed coatings and tribological surfaces are a point of interest in many industrial sectors. They are used for better wear resistance of lightweight materials or for oil retention on surfaces. Lightweight materials are often used in the automotive industry as a weight-saving solution in the production of engine blocks. For this, it is necessary to coat the cylinder liners to ensure wear resistance. In most cases, the coating is sprayed directly onto the surface. Previous research has shown that it is possible to transfer these coatings inversely onto other surfaces [1]. This was achieved with plasma sprayed coatings which were transplanted onto pressure-casted surfaces. These transplanted surfaces exhibited better adhesive strength, smoother surfaces, and lower form deviation compared to directly coated surfaces. Additionally, it was shown that even microstructures of a surface coated by plasma spraying can be transferred to pressure-casted surfaces. This paper presents the development and micromilling of different microstructures for transferring thermally sprayed coatings onto pressure-casted surfaces. In the development process, microstructures with different shapes and aspect ratios as well as thin tribological surfaces are designed in order to evaluate the advantages and limitations of the transplantation process. In subsequent experiments, the micromilling process and a simulation of the coating transplantation are presented and analyzed. © 2014 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2014.03.054
  • 2014 • 134 Dislocation density distribution around an indent in single-crystalline nickel: Comparing nonlocal crystal plasticity finite-element predictions with experiments
    Reuber, C. and Eisenlohr, P. and Roters, F. and Raabe, D.
    Acta Materialia 71 333-348 (2014)
    We present a physics-based constitutive model of dislocation glide in metals that explicitly accounts for the redistribution of dislocations due to their motion. The model parameterizes the complex microstructure by dislocation densities of edge and screw character, which either occur with monopolar properties, i.e. a single dislocation with positive or negative line sense, or with dipolar properties, i.e. two dislocations of opposite line sense combined. The advantage of the model lies in the description of the dislocation density evolution, which comprises the usual rate equations for dislocation multiplication and annihilation, and formation and dissociation of dislocation dipoles. Additionally, the spatial redistribution of dislocations by slip is explicitly accounted for. This is achieved by introducing an advection term for the dislocation density that turns the evolution equations for the dislocation density from ordinary into partial differential equations. The associated spatial gradients of the dislocation slip render the model nonlocal. The model is applied to wedge indentation in single-crystalline nickel. The simulation results are compared to published experiments (Kysar et al., 2010) in terms of the spatial distribution of lattice rotations and geometrically necessary dislocations. In agreement with experiment, the predicted dislocation fluxes lead to accumulation of geometrically necessary dislocations around a vertical geometrical border with a high orientation gradient below the indenter that is decisive for the overall plastic response. A local model variant without dislocation transport is not able to predict the influence of this geometrical transition zone correctly and is shown to behave markedly softer. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.03.012
  • 2014 • 133 Efficient Large-Scale Coating Microstructure Formation Using Realistic CFD Models
    Wiederkehr, T. and Müller, H.
    Journal of Thermal Spray Technology 24 283-295 (2014)
    For the understanding of physical effects during the formation of thermally sprayed coating layers and the deduction of the macroscopic properties of a coating, microstructure modeling and simulation techniques play an important role. In this contribution, a coupled simulation framework consisting of a detailed, CFD-based single splat simulation, and a large-scale coating build-up simulation is presented that is capable to compute large-scale, three-dimensional, porous microstructures by sequential drop impingement of more than 10,000 individual particles on multicore workstation hardware. Due to the geometry-based coupling of the two simulations, the deformation, cooling, and solidification of every particle is sensitive to the hit surface area and thereby pores develop naturally in the model. The single splat simulation employs the highly parallel Lattice-Boltzmann method, which is well suited for GPU-acceleration. In order to save splat calculations, the coating simulation includes a database-driven approach that re-uses already computed splats for similar underground shapes at the randomly chosen impact sites. For a fast database search, three different methods of efficient pre-selection of candidates are described and compared against each other. © 2014, ASM International.
    view abstractdoi: 10.1007/s11666-014-0194-y
  • 2014 • 132 Enhanced superplasticity in an Al-alloyed multicomponent Mn-Si-Cr-C steel
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Choi, P. and Tasan, C.C. and Raabe, D.
    Acta Materialia 63 232-244 (2014)
    Excellent superplasticity (elongation ∼720%) is observed in a novel multi-component (Mn-S-Cr-Al alloyed) ultrahigh carbon steel during tensile testing at a strain rate of 2 × 10-3 s-1 and a temperature of 1053 K (just above the equilibrium austenite-pearlite transformation temperature). In order to understand superplasticity in this material and its strong Al dependence, the deformation-induced microstructure evolution is characterized at various length scales down to atomic resolution, using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, energy-dispersive X-ray spectroscopy and atom probe tomography. The results reveal that 1 wt.% Al addition influences various microprocesses during deformation, e.g. it impedes Ostwald ripening of carbides, carbide dissolution, austenite nucleation and growth and void growth. As a result, the size of the austenite grains and voids remains relatively fine (< 10 μm) during superplastic deformation, and fine-grained superplasticity is enabled without premature failure. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.10.034
  • 2014 • 131 Fabrication of a Mo based high temperature TZM alloy by non-consumable arc melting technique
    Chakraborty, S.P. and Krishnamurthy, N.
    Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV 749-752 (2014)
    High temperature structural materials are in great demand for power, chemical and nuclear industries which can perform beyond 1000°C as super alloys usually fail. In this regard, Mo based TZM alloy is capable of retaining strength up to 1500°C with excellent corrosion compatibility against molten alkali metals. Hence, currently this alloy is considered an important candidate material for high temperature compact nuclear and fusion reactors. Due to reactive nature of Mo and having high melting point, manufacturing this alloy by conventional process is unsuitable. Powder metallurgy technique has limited success due to restriction in quantity and purity. This paper deals with fabrication of TZM alloy by non-consumable tungsten arc melting technique. Initially a ternary master alloy of Mo-Ti-Zr was prepared which subsequently by dilution method, was converted into TZM alloy gradually by external addition of Mo and C in various proportions. A number of melting trials were conducted to optimize the process parameters like current, voltage and time to achieve desired alloy composition. The alloy was characterized with respect to composition, elemental distribution profile, microstructure, hardness profile and phase analysis. Well consolidated alloy button was obtained having desired composition, negligible material loss and having microstructure as comparable to standard TZM alloy. © 2014 IEEE.
    view abstractdoi: 10.1109/DEIV.2014.6961791
  • 2014 • 130 Functional and structural fatigue of titanium tantalum high temperature shape memory alloys (HT SMAs)
    Niendorf, T. and Krooß, P. and Batyrsina, E. and Paulsen, A. and Motemani, Y. and Ludwig, Al. and Buenconsejo, P. and Frenzel, J. and Eggeler, G. and Maier, H.J.
    Materials Science and Engineering A 620 359-366 (2014)
    Due to their high work output and good mechanical properties, actuators made from shape memory alloys (SMAs) are used in numerous applications. Unfortunately, SMAs such as nickel-titanium (Ni-Ti) can only be employed at temperatures up to about 100°C. Lately, high-temperature shape memory alloys (HT SMAs) have been introduced to overcome this limitation. Ternary systems based on Ni-Ti have been intensively characterized and alloys are available that can operate at elevated temperatures. However, these alloys either contain substantial amounts of expensive noble elements like platinum and palladium, or the materials are brittle. The titanium-tantalum (Ti-Ta) system has been developed to overcome these issues. Binary Ti-Ta provides relatively high MS temperature combined with excellent workability, but it suffers from fast cyclic degradation. By alloying with third elements this drawback can be overcome: The ternary Ti-Ta-Al alloy shows overall promising properties as will be shown in the present work. In-situ thermo-mechanical cycling experiments were conducted and allowed for evaluation of the factors affecting the functional and structural fatigue of this alloy. Functional fatigue is dominated by ω-phase evolution, while structural fatigue is triggered by an interplay of ω-phase induced embrittlement and deformation constraints imposed by unsuitable texture. In addition, a concept for fatigue life extension proposed very recently for binary Ti-Ta, is demonstrated to be also applicable for the ternary Ti-Ta-Al. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.10.038
  • 2014 • 129 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 • 128 Infrared emitting nanostructures for highly efficient microhotplates
    Müller, L. and Käpplinger, I. and Biermann, S. and Brode, W. and Hoffmann, M.
    Journal of Micromechanics and Microengineering 24 (2014)
    A highly emissive Si-based microhotplate based on self-organizing nanostructures is presented. The silicon was structured by a self-masking deep reactive ion etching process resulting in needle-like non-periodical microstructures. Evaporated platinum settles in a kind of glancing angle deposition as well-defined nanocrystals on the silicon microstructures. Finite-difference time-domain simulation allowed the evaluation of the ideal platinum thickness for maximized infrared absorption and emission. We measured the hemispherical spectral transmittance and reflectivity of the fabricated surfaces and found the hemispherical spectral absorbance to be up to 0.97 in the investigated wavelength range. To demonstrate the advantages of these micro-nano-structures, we present the fabrication and characterization of a thermal infrared hotplate-emitter. With integrated Pt-on-Si-needles, the emitter shows a 2.6 times higher IR emission without wavelength-dependent interference patterns as compared to an uncoated Si-based emitter at the same membrane temperature. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0960-1317/24/3/035014
  • 2014 • 127 Interface properties in lamellar TiAl microstructures from density functional theory
    Kanani, M. and Hartmaier, A. and Janisch, R.
    Intermetallics 54 154-163 (2014)
    The deformability and strength of lamellar two-phase (γ and α2) TiAl alloys strongly depends on the mechanical properties of the different interfaces in such microstructures. We carried out ab-initio density functional theory calculations of interface energy and strength for all known interface variants as well as the corresponding single crystal slip/cleavage planes to obtain a comprehensive database of key mechanical quantities. This data collection can be used for meso-scale simulations of deformation and fracture in TiAl. In spite of the different atomic configurations of the lamellar interfaces and the single crystal planes, the calculated values for the tensile strength are in the same range and can be considered as equal in a meso-scale model. Analysis of generalized stacking fault energy surfaces showed that the shear strength is directional dependent, however, the [112̄] direction is an invariant easy gliding direction in all investigated systems. The probability of different dislocation dissociation reactions as part of a shear deformation mechanism are discussed as well. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.06.001
  • 2014 • 126 Large scale 3-D phase-field simulation of coarsening in Ni-base superalloys
    Rajendran, M.K. and Shchyglo, O. and Steinbach, I.
    MATEC Web of Conferences 14 (2014)
    In this study we present a large scale numerical simulation of γ-γ′ microstructure evolution in Ni-base superalloy using the multi-phase field method in three dimensions. We numerically simulated precipitation hardening heat treatment cycles. Large scale three dimensional simulations are necessary in order to get sufficient statistics for predicting the morphological evolution, average γ′ precipitate size, precipitates size distribution over time and ripening exponent for a given temperature and composition. A detailed analysis of obtained result is presented emphasising the effect of elastic interaction on the coarsening kinetics in Ni-base superalloy. The study is performed using the phase-field modelling library "OpenPhase" which is based on a multi-phase field multi-component model. © 2014 Owned by the authors, published by EDP Sciences.
    view abstractdoi: 10.1051/matecconf/20141411001
  • 2014 • 125 Microstructural and defect analysis of metal nanoparticles in functional catalysts by diffraction and electron microscopy: The Cu/ZnO catalyst for methanol synthesis
    Kandemir, T. and Kasatkin, I. and Girgsdies, F. and Zander, S. and Kühl, S. and Tovar, M. and Schlögl, R. and Behrens, M.
    Topics in Catalysis 57 188-206 (2014)
    The application of different methods for a microstructural analysis of functional catalysts is reported for the example of different Cu/ZnO-based methanol synthesis catalysts. Transmission electron microscopy and diffraction were used as complementary techniques to extract information on the size and the defect concentration of the Cu nano-crystallites. The results, strengths and limitations of the two techniques and of different evaluation methods for line profile analysis of diffraction data including Rietveld-refinement, Scherrer- and (modified) Williamson-Hall-analyses, single peak deconvolution and whole powder pattern modeling are compared and critically discussed. It was found that in comparison with a macrocrystalline pure Cu sample, the catalysts were not only characterized by a smaller crystallite size, but also by a high concentration of lattice defects, in particular stacking faults. Neutron diffraction was introduced as a valuable tool for such analysis, because of the larger number of higher-order diffraction peaks that can be detected with this method. An attempt is reported to quantify the different types of defects for a selected catalyst. © 2013 Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11244-013-0175-2
  • 2014 • 124 Microstructure-oriented fatigue assessment of construction materials and joints using short-time load increase procedure
    Walther, F.
    Materialpruefung/Materials Testing 56 519-527 (2014)
    Stress-strain-hysteresis, change in deformation-induced temperature and change in DC-based electrical resistance measurements are applied for the detailed characterization of structural-mechanical processes in construction materials and joints under multiple-step and single-step fatigue loading. Results concerning the influence of joining technologies on austenitic steel AISI304, carbon-fiber reinforced polymers (CFRP) and beech wood materials, of environmental media on magnesium alloys Mg-4Al-2Ba-2Ca (DieMag422) and Mg-10Gd-lNd, and of manufacturing processes on titanium alloy Ti-6A1-4V and wood-plastic composites (WPC) are discussed. The load- and cycle-dependent change in microstructure was investigated by light and electron microscopy and correlated with fatigue properties, to reach a preferably precise description of process structure property relationship in a qualitative and quantitative manner. The time-efficient load increase procedure applied for evaluation of joining, environmental and manufacturing influence on fatigue performance is suitable for production-accompanied usage. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/120.110592
  • 2014 • 123 Modeling the microstructure influence on fatigue life variability in structural steels
    Sharaf, M. and Kucharczyk, P. and Vajragupta, N. and Münstermann, S. and Hartmaier, A. and Bleck, W.
    Computational Materials Science 94 258-272 (2014)
    The endurance and HCF lifetime of multiphase steel components depend mainly on the phase of fatigue microcrack initiation and early propagation. A numerical study, which quantitatively describes the influence of microstructural features on the initiation and growth of cyclic microcracks, is presented within the context of microstructure-sensitive modeling. The implementation of kinematic hardening on each slip system in a crystal plasticity model allows for capturing the local accumulation of plastic microdeformation representing slip irreversibility occurring in the crack incubation phase. A load increasing testing technique with continuous temperature measurement and interrupted cyclic bending experiments deliver information about the endurance strength of a structural steel and allow for metallographic observation of cyclic microcrack propagation and thereby provide the experimental basis for the numerical simulations. The material model is implemented in cyclic computations with statistically representative volume elements, which are based on experimental microstructure description using the electron backscatter diffraction technique (EBSD). The extreme value distributions of the computed accumulation of local dislocation slip are then correlated to the microstructure in an approach to assess and explore the validity extent of microstructure-sensitive modeling using fatigue indicator parameters (FIPs) to correlate to the endurance limit and fatigue life under high-cycle fatigue conditions. The eligibility of consideration of the stresses normal to the planes of localized plastic damage assisting fatigue crack formation into these FIPs is investigated. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2014.05.059
  • 2014 • 122 New insights into the austenitization process of low-alloyed hypereutectoid steels: Nucleation analysis of strain-induced austenite formation
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Raabe, D.
    Acta Materialia 80 296-308 (2014)
    Austenite formation, which originated from a fined-grained ferrite plus carbide microstructure, was observed during tensile testing at 973 K (60 K below Ae1, the equilibrium austenite-pearlite transformation temperature). Scanning electron microscopy, electron backscatter diffraction and atom probe tomography results reveal the mechanism of austenitic transformation below Ae1. The initial fine-grained microstructure, in combination with the warm deformation process, determines the occurrence of strain-induced austenite formation below Ae1. The initial fine-grained microstructure essentially contains a higher dislocation density to facilitate the formation of Cottrell atmospheres and a larger area fraction of ferrite/carbide interfaces which serve as austenite nucleation sites. The warm deformation promotes the Ostwald ripening process and the increase in dislocation density, and hence promotes the accumulation of local high carbon concentrations in the form of Cottrell atmospheres to reach a sufficiently high thermodynamic driving force for austenite nucleation. The critical carbon concentration required for the nucleation of austenite was calculated using classical nucleation theory, which correlated well with the experimental observations. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.07.073
  • 2014 • 121 On the functional degradation of binary titanium-tantalum high-temperature shape memory alloys - A new concept for fatigue life extension
    Niendorf, T. and Krooß, P. and Batyrsina, E. and Paulsen, A. and Frenzel, J. and Eggeler, G. and Maier, H.J.
    Functional Materials Letters 7 (2014)
    High-temperature shape memory alloys are promising candidates for actuator applications at elevated temperatures. Ternary nickel-titanium-based alloys either contain noble metals which are very expensive, or suffer from poor workability. Titanium-tantalum shape memory alloys represent a promising alternative if one can avoid the cyclic degradation due to the formation of the omega phase. The current study investigates the functional fatigue behavior of Ti-Ta and introduces a new concept providing for pronounced fatigue life extension. © 2014 The Authors.
    view abstractdoi: 10.1142/S1793604714500428
  • 2014 • 120 Oxygen transport through supported Ba0.5Sr0.5Co 0.8Fe0.2O3-δ membranes
    Niehoff, P. and Baumann, S. and Schulze-Küppers, F. and Bradley, R.S. and Shapiro, I. and Meulenberg, W.A. and Withers, P.J. and Vaßen, R.
    Separation and Purification Technology 121 60-67 (2014)
    The oxygen transport through supported membranes made of Ba 0.5Sr0.5Co0.8Fe0.2O 3-δ is investigated. For this, disc shaped membranes were manufactured by means of tape casting, consisting of a gastight layer with varying thickness (0.9 mm-20 μm) and a porous support with varying porosity (34%/41%). The sample's microstructure was analyzed using SEM and X-ray computer tomography and by this means characteristic values (i.e., porosity, tortuosity, and specific surface area) were determined. A modeling concept was developed based on literature approaches, extending the Wagner equation for bulk transfer with a geometrical factor β for the characteristic thickness accounting active supports and different surface microstructures. The results were compared with permeation measurements of samples under varying operation conditions (i.e., sweep flow rate, and feed gas). As a result, good agreement between model and measurement in case of a constant porosity is found for characteristic thicknesses Lc as reported in literature. However, calculations with varying porosity show indistinguishable results, indicating an underestimate of the geometric factor versus the influence of the characteristic thickness L c. Also, significant limitations of the oxygen permeation due to surface exchange and concentration polarization in the support is shown. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.seppur.2013.07.002
  • 2014 • 119 Recycling of metallic chips by electro-discharge sintering
    Mohr, A. and Röttger, A. and Windmann, M. and Theisen, W.
    Materialwissenschaft und Werkstofftechnik 45 552-560 (2014)
    Composite material Ferro-Titanit® is produced powder-metallurgical by Deutsche Edelstahlwerke GmbH (DEW) and is commonly used for wear and corrosion resistant component parts. Materials properties can be attributed to the microstructure which consists of a corrosion resistant metallic matrix and a huge amount of approx. 50 vol.% of hard Ti-monocarbides. Although Ferro-Titanit® possesses a high amount of hard particles, the material can be machined by turning and drilling in solution annealed condition. Due to the alloying content (Mo, Cr, TiC) of Ferro-Titanit®, there is a high motivation to recover those elements by a recycling process of the chips, thus expensive and limited resources can be saved. On idea of a recycling process can be found in the redensification of those chips by electro discharge sintering (EDS). In this work, chips of the material Ferro-Titanit® were densified by EDS technique and the resulting microstructure was investigated by optical and scanning electron microscopy. Furthermore, microstructure and hardness of the EDS densified specimens was discussed with regard to the microstructure of conventionally sintered Ferro-Titanit®-samples in laboratory conditions. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201400266
  • 2014 • 118 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
  • 2014 • 117 Temperature dependencies of the elastic moduli and thermal expansion coefficient of an equiatomic, single-phase CoCrFeMnNi high-entropy alloy
    Laplanche, G. and Gadaud, P. and Horst, O. and Otto, F. and Eggeler, G. and George, E.P.
    Journal of Alloys and Compounds 623 348-353 (2014)
    The equiatomic CoCrFeMnNi alloy is now regarded as a model face-centered cubic single-phase high-entropy alloy. Therefore, determination of its intrinsic properties such as the temperature dependencies of elastic moduli and thermal expansion coefficient are important to improve understanding of this new class of material. These temperature dependencies were measured over a large temperature range (200-1270 K) in this study. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2014.11.061
  • 2014 • 116 Tertiary dendritic instability in late stage solidification of Ni-based superalloys
    Franke, M.M. and Singer, R.F. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    Derivatives of the commercial alloy CMSX-4 were directionally solidified and characterized with respect to their final dendrite microstructure. The results indicate that Ni-based superalloys with high segregation levels show significant instability in secondary dendrite arms and an increased tendency for tertiary arm formation, respectively. Phase-field simulations were used to explore the impact of chemical composition on morphological instability and tertiary arm formation during the directional solidification of Ni-based superalloys. It is found that an increase in specific alloying elements in the overall alloy composition leads to pronounced segregation at the end of solidification. This causes strong growth restriction of the secondary arms and triggers tertiary arm formation. The proposed mechanism explains experimental microstructures found in modifications of the base alloy CMSX-4. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/2/025026
  • 2014 • 115 The evolution of tribolayers during high temperature sliding wear
    Rynio, C. and Hattendorf, H. and Klöwer, J. and Eggeler, G.
    Wear 315 1-10 (2014)
    High temperature reciprocating sliding wear experiments of a Ni-based superalloy pin against a cast iron disc were performed at 600 and 800. °C (load: 20. N, frequency: 20. Hz, stroke: 1. mm). The evolution of tribolayers was investigated using scanning and transmission electron microscopy (SEM and TEM) and energy dispersive X-ray spectroscopy (EDX). Four distinct subsurface zones are identified and discussed in terms of plastic strain accumulation and microstructure evolution. The development of protective nanocrystalline oxide-layers (glaze-layers) on top of the wear surfaces leads to very low wear rates due to a suppression of the direct metal-metal contact between the pin and the disc. The nanohardness, microstructure and chemical composition of the glaze-layers are reported. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2014.03.007
  • 2014 • 114 The modeling scheme to evaluate the influence of microstructure features on microcrack formation of DP-steel: The artificial microstructure model and its application to predict the strain hardening behavior
    Vajragupta, N. and Wechsuwanmanee, P. and Lian, J. and Sharaf, M. and Münstermann, S. and Ma, A. and Hartmaier, A. and Bleck, W.
    Computational Materials Science 94 198-213 (2014)
    Due to the existence of constituents with strong distinction in mechanical properties, dual phase steels exhibit remarkably high-energy absorption along with excellent combination of strength and ductility. Furthermore, these constituents also affect deformation and microcrack formation in which various mechanisms can be observed. Thus, a reliable microstructure-based simulation approach for describing these deformations and microcrack initiation is needed. Under this framework of modeling scheme development, several work packages have been carried out. These work packages includes algorithm to generate the artificial microstructure model, a procedure to derive plasticity parameters for each constituent, and characterization of the microcrack formation and initiation criteria determination. However, due to the complexity of topic and in order to describe each work package in detail, this paper focused only on the approach to generate the artificial microstructure model and its application to predict the strain hardening behavior. The approach was based on the quantitative results of metallographic microstructure analysis and their statistical representation. The dual phase steel was first characterized by EBSD analysis to identify individual phase grain size distribution functions. The results were then input into a multiplicatively weighted Voronoi tessellation based algorithm to generate artificial microstructure geometry models. Afterwards, nanoindentation was performed to calibrate crystal plasticity parameters of ferrite and empirical approach based on local chemical composition was used to approximate flow curve of martensite. By assigning the artificial microstructure model with plasticity description of each constituent, strain-hardening behavior of DP-steel was then predicted. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2014.04.011
  • 2014 • 113 Thermal cycling behavior of an aged FeNiCoAlTa single-crystal shape memory alloy
    Krooß, P. and Holzweissig, M.J. and Niendorf, T. and Somsen, C. and Schaper, M. and Chumlyakov, Y.I. and Maier, H.J.
    Scripta Materialia 81 28-31 (2014)
    In this study the thermal cycling behavior of differently aged [1 0 0]-oriented Fe-28Ni-17Co-11.5Al-2.5Ta (at.%) shape memory single crystals was investigated. The strain-temperature response determined from thermal cycling experiments revealed a strong dependency on the precipitate morphology, which was adjusted by aging heat treatments. Specifically, a high precipitate density in the microstructure leads to small phase transformation-induced strains and low stresses necessary for activation of the martensitic phase transformation. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2014.02.020
  • 2014 • 112 TLP brazing of aluminum to steel using PVD-deposited interlayer
    Wojarski, L. and Tillmann, W.
    Welding in the World 58 673-680 (2014)
    The demand for hybrid material concepts is steadily growing, and especially dissimilar joints between aluminum and steel are, due to their wide dissemination, of major importance. The main obstacle for the fabrication of aluminum-steel joints using thermal processes is the embrittlement of the fusion area. In order to prevent direct contact between aluminum and iron and thus to suppress the formation of brittle iron aluminides, 3-μm thick diffusion barrier coatings, consisting of Ni or Ti, were applied onto the steel surface. Pure copper with a thickness of 3 and 6 μm, respectively, was used as a filler material, and the samples were brazed in a TLP process in a vacuum at 580 °C at varying dwell times (10...50 min). The samples brazed with Ni diffusion barriers showed a considerable formation of Fe2Al 5 even at low dwell times. Furthermore, additional complex ternary phase bands have generated due to the existence of diffusion barrier elements and were detected in the interfacial area. The application of Ti showed a significant decrease of iron aluminides, and no Fe2Al5 could be detected at low dwell times, resulting in a shear strength of 42 MPa for the optimized parameters. © 2014 International Institute of Welding.
    view abstractdoi: 10.1007/s40194-014-0143-x
  • 2014 • 111 Two-scale modeling of DP steel incorporating distributed properties inside micro-constituents
    Schröder, J. and Gandhi, A. and Balzani, D.
    Procedia Engineering 81 1390-1395 (2014)
    Advanced High Strength Steels (AHSS) are increasingly used in the industry due to their excellent strength and formability properties enabling weight savings. In this wide class of steel we restrict ourselves to the modeling of Dual Phase (DP) steels which are, at the microscale, characterized by a hard martensitic inclusion phase embedded in a soft ferritic matrix phase. During the production process the martensite transforms from austenite by rapidly cooling down the material and thereby causing a volume jump leading to initial plastic strains associated with eigenstresses of higher order. A technique to incorporate theses distributed properties in the ferrite matrix is proposed and implemented using the direct micro-macro transition approach. © 2014 The Authors. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2014.10.162
  • 2014 • 110 Wrinkling of random and regular semiflexible polymer networks
    Müller, P. and Kierfeld, J.
    Physical Review Letters 112 (2014)
    We investigate wrinkling of two-dimensional random and triangular semiflexible polymer networks under shear. Both types of semiflexible networks exhibit wrinkling above a small critical shear angle, which scales with an exponent of the bending modulus between 1.9 and 2.0. Random networks exhibit hysteresis at the wrinkling threshold. Wrinkling lowers the total elastic energy by up to 20% and strongly affects the elastic properties of all semiflexible networks such as the crossover between bending and stretching dominated behavior. In random networks, we also find evidence for metastable wrinkled configurations. While the disordered microstructure of random networks affects the scaling behavior of wrinkle amplitudes, it has little effect on wrinkle wavelength. Therefore, wrinkles represent a robust, microstructure-independent assay of shear strain or elastic properties. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.112.094303
  • 2013 • 109 3D analysis of micro-deformation in VHCF-loaded nodular cast iron by μCT
    Fischer, G. and Nellesen, J. and Anar, N.B. and Ehrig, K. and Riesemeier, H. and Tillmann, W.
    Materials Science and Engineering A 577 202-209 (2013)
    The impact of very high cycle fatigue (VHCF) load conditions on the microstructure of specimens consisting of nodular cast iron is analyzed by means of micro-computed tomography (μCT) utilizing both monochromatic synchrotron radiation and polychromatic X-ray tube radiation. Using 3D μCT, the microstructure in the region of the smallest cross-sections of shouldered round specimens is imaged in different stages of the VHCF loading. By digital image correlation (DIC) of these tomograms strain fields are analyzed three-dimensionally. Strain levels in the range of a few percent were detected. It is proven that a localization of strain allows to predict the site of the crack which precedes and induces the macroscopic failure of the specimens. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2013.04.057
  • 2013 • 108 A modular assembling platform for manufacturing of microsystems by optical tweezers
    Ksouri, S.I. and Aumann, A. and Ghadiri, R. and Prüfer, M. and Baer, S. and Ostendorf, A.
    Proceedings of SPIE - The International Society for Optical Engineering 8810 (2013)
    Due to the increased complexity in terms of materials and geometries for microsystems new assembling techniques are required. Assembling techniques from the semiconductor industry are often very specific and cannot fulfill all specifications in more complex microsystems. Therefore, holographic optical tweezers are applied to manipulate structures in micrometer range with highest flexibility and precision. As is well known non-spherical assemblies can be trapped and controlled by laser light and assembled with an additional light modulator application, where the incident laser beam is rearranged into flexible light patterns in order to generate multiple spots. The complementary building blocks are generated by a two-photon-polymerization process. The possibilities of manufacturing arbitrary microstructures and the potential of optical tweezers lead to the idea of combining manufacturing techniques with manipulation processes to microrobotic processes. This work presents the manipulation of generated complex microstructures with optical tools as well as a storage solution for 2PP assemblies. A sample holder has been developed for the manual feeding of 2PP building blocks. Furthermore, a modular assembling platform has been constructed for an -all-in-one' 2PP manufacturing process as a dedicated storage system. The long-term objective is the automation process of feeding and storage of several different 2PP micro-assemblies to realize an automated assembly process. © 2013 SPIE.
    view abstractdoi: 10.1117/12.2025273
  • 2013 • 107 An advanced energy relaxation scheme for the modeling of displacive phase transformations
    Bartel, T. and Buckmann, K. and Kiefer, B. and Menzel, A.
    ASME 2013 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2013 1 (2013)
    In this contribution, a micro-mechanically motivated constitutive model for phase transformation, martensite reorientation and twin formation in shape memory alloys is proposed. The formulation builds on an effective parametrization of the austenite-twinned martensite microstructure through first- And second-order laminates. To define the effective energy density of the phase mixture, the concept of energy relaxation is applied. The values of the dissipative internal state variables that describe the microstructure evolution are computed via constrained incremental energy minimization. This work also suggests a first step towards the continuous modeling of twin formation embedded into the concept of energy relaxation and can be viewed as a generalization of earlier models suggested in [1-3]. More specifically, in the current model the orientation of martensitic variants in space is not pre-assigned. Variants are rather left free to arrange in an energy-minimizing fashion and are only distinguished by their rotation in reference to a master variant. Finally, macro-homogeneous uniaxial strain and pure shear loading cases are analyzed to demonstrate the capabilities of the proposed modeling framework. Copyright © 2013 by ASME.
    view abstractdoi: 10.1115/SMASIS2013-3041
  • 2013 • 106 Bulk combinatorial design of ductile martensitic stainless steels through confined martensite-to-austenite reversion
    Springer, H. and Belde, M. and Raabe, D.
    Materials Science and Engineering A 582 235-244 (2013)
    The effect of local martensite-to-austenite reversion on microstructure and mechanical properties was studied with the aim of designing ductile martensitic steels. Following a combinatorial screening with tensile and hardness testing on a matrix of six alloys (0-5. wt% Mn, 0-2. wt% Si, constant 13.5. wt% Cr and 0.45. wt% C) and seven martensite tempering conditions (300-500. °C, 0-30. min), investigations were focussed on martensite-to-austenite reversion during tempering as function of chemical composition and its correlation with the mechanical properties. While Mn additions promoted austenite formation (up to 35. vol%) leading to a martensitic-austenitic TRIP steel with optimum mechanical properties (1.5. GPa ultimate tensile strength and 18% elongation), Si led to brittle behaviour despite even larger austenite contents. Combined additions of Mn and Si broadened the temperature range of austenite reversion, but also significantly lowered hardness and yield strength at limited ductility. These drastically diverging mechanical properties of the probed steels are discussed in light of microstructure morphology, dispersion and transformation kinetics of the austenite, as a result of the composition effects on austenite retention and reversion. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2013.06.036
  • 2013 • 105 Cavitation erosion of Cr60Ni40 coatings generated by friction surfacing
    Hanke, S. and Beyer, M. and Silvonen, A. and dos Santos, J.F. and Fischer, A.
    Wear 301 415-423 (2013)
    CrNi-alloys with high Cr-content generally are quite brittle and, therefore, only available as castings and regarded as neither weldable nor deformable. The process of friction surfacing offers a possibility to generate Cr60Ni40 coatings e.g. on steel or Ni-base substrates. Cavitation tests were carried out using an ultrasonic vibratory test rig (~ASTM G32) with cast specimens and friction surfaced coatings. The coatings show less deformation and smaller disruptions, and wear rates in steady state were found to be three times higher for the cast and heat treated samples than for the coatings, caused by a highly wear resistant Cr-rich phase. The results of this study show that it is possible to generate defect free coatings of Cr60Ni40 with a thickness of about 250. μm by friction surfacing, which under cavitation show a better wear behavior than the cast material. Thus, in combination with a ductile substrate, these coatings are likely to extend the range of applicability of such high-temperature corrosion resistant alloys. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2012.11.016
  • 2013 • 104 Combined experimental and numerical approach for linking microstructure and mechanical properties on different length scales for near γ-TiAl alloys
    Kabir, M.R. and Bartsch, M. and Chernova, L. and Schneider, J. and Kelm, K.
    Materials Science Forum 750 76-79 (2013)
    At room temperature the macroscopic tensile behavior of TiAl alloys is extremely microstructure sensitive. In general the microstructures of TiAl alloys are heterogeneous at micro and meso scale. The materials micromechanisms that occur at different length scale have to be linked for a proper understanding of the macroscopic response. In order to explore those micromechanisms, methodologies combining advanced experimental and computational analysis have been proposed. Linking microstructure and properties using a two-scale numerical model we are able to explain the stress-strain and hardening behavior of this alloy. © (2013) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/MSF.750.76
  • 2013 • 103 Design of next generation thermal barrier coatings - Experiments and modelling
    Gupta, M. and Curry, N. and Nylén, P. and Markocsan, N. and Vaßen, R.
    Surface and Coatings Technology 220 20-26 (2013)
    Thermal barrier coating (TBC) systems have been used in the gas turbine industry since the 1980s. The future needs both the air and land based turbine industry involve higher operating temperatures with longer lifetime on the component so as to increase power and efficiency of gas turbines. The aim of this study was to meet these future needs by further development of zirconia coatings. The intention was to design a coating system which could be implemented in industry within the next 3. years. Different morphologies of ceramic topcoat were evaluated; using dual layer systems and polymers to generate porosity. Dysprosia stabilised zirconia was also included in this study as a topcoat material along with the state-of-the-art yttria stabilised zirconia (YSZ). High purity powders were selected in this work. Microstructure was assessed with scanning electron microscope and an in-house developed image analysis routine was used to characterise porosity content. Evaluations were carried out using the laser flash technique to measure thermal conductivity. Lifetime was assessed using thermo-cyclic fatigue testing. Finite element analysis was utilised to evaluate thermal-mechanical material behaviour and to design the morphology of the coating with the help of an artificial coating morphology generator through establishment of relationships between microstructure, thermal conductivity and stiffness. It was shown that the combined empirical and numerical approach is an effective tool for developing high performance coatings. The results show that large globular pores and connected cracks inherited within the coating microstructure result in a coating with best performance. A low thermal conductivity coating with twice the lifetime compared to the industrial standard today was fabricated in this work. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2012.09.015
  • 2013 • 102 Determination of average dislocation densities in metals by analysis of digitally processed transmission-electron microscopy images
    Husser, E. and Clausmeyer, T. and Gershteyn, G. and Bargmann, S.
    Materialwissenschaft und Werkstofftechnik 44 541-546 (2013)
    This paper describes an effective and simple procedure to derive information about the dislocation density distribution in metals by applying standard techniques of digital image processing on gray scaled microstructure images obtained from transmission-electron microscopy. In a representative transmission-electron microscopy image, two local dislocation density values were investigated by classical methods and were used as input parameter for further processing. A correlation between dislocations and image intensities is assumed such that dark areas in microstructural images are seen as a dense concentration of dislocations. Then, the contrast is increased for each transmission-electron microscopy photography. In the next step, posterization, a gradation of tone, is applied to these images. From this, a pixel weighted average distribution of gray level correlated dislocation densities is obtained as well as an average value for a given set of images. The implementation of the several processing steps is done in Matlab employing graphical user interfaces. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201300027
  • 2013 • 101 Duplex Steels: Part I: Genesis, Formation, Structure
    Knyazeva, M. and Pohl, M.
    Metallography, Microstructure, and Analysis 2 113-121 (2013)
    The ferritic-austenitic duplex steels are equipped with a mechanical-technological combination of properties, which is advantageous compared to the features of stainless completely ferritic or completely austenite steels. The duplex steels crystallize by fully ferritic or ferritic-austenite solidification with the austenite precipitation due to the solid solution reactions during the further cooling. To adjust the ferrite-austenite ratio, the steels must be heat treated by temperatures above the field of precipitation stability, followed by water quenching. The temperature and the time of the heat treatment effect the element distribution according to their higher solubility in the ferritic or austenitic phases. The typical microstructure of the duplex stainless steels can only be realized due to deformation and recrystallisation processing. © 2013 Springer Science+Business Media New York and ASM International.
    view abstractdoi: 10.1007/s13632-013-0066-8
  • 2013 • 100 Effect of climb on dislocation mechanisms and creep rates in γ′-strengthened Ni base superalloy single crystals: A discrete dislocation dynamics study
    Hafez Haghighat, S.M. and Eggeler, G. and Raabe, D.
    Acta Materialia 61 3709-3723 (2013)
    Creep of single-crystal superalloys is governed by dislocation glide, climb, reactions and annihilation. Discrete three-dimensional (3D) dislocation dynamics (DDD) simulations are used to study the evolution of the dislocation substructure in a γ/γ′ microstructure of a single-crystal superalloy for different climb rates and loading conditions. A hybrid mobility law for glide and climb is used to map the interactions of dislocations with γ′ cubes. The focus is on the early stages of creep, where dislocation plasticity is confined to narrow γ channels. With enhancing climb mobility, the creep strain increases, even if the applied resolved shear stress is below the critical stress required for squeezing dislocations into the γ channels. The simulated creep microstructure consists of long dislocations and a network near the corners of the γ′ precipitate in the low-stress regime. In the high-stress regime, dislocations squeeze into the γ channels, where they deposit dislocation segments at the γ/γ′ interfaces. These observations are in good agreement with experimentally observed dislocation structures that form during high-temperature and low-stress creep. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.03.003
  • 2013 • 99 Effect of high pressure and high temperature on the microstructural evolution of a single crystal Ni-based superalloy
    Lopez-Galilea, I. and Huth, S. and Theisen, W. and Fockenberg, T. and Chakraborty, S.
    Journal of Materials Science 48 348-358 (2013)
    The application of high nearly hydrostatic pressures at elevated temperatures on the LEK94 single crystal (SX) nickel-based superalloy directly affects its microstructure. This is due to a combination of the effect of pressure on the Gibbs free energy, on the diffusion coefficients of the alloying elements, on the internal coherent stresses, and on the porosity distribution. The last effect depends at least on the first three. Therefore, based on the theoretical influences of the pressure, the main objective of this work is to understand, by means of an experimental work, the effect of high pressure at elevated temperature during annealing on the evolution of the phases morphology, and porosity of the high-temperature material LEK94. Specifically, pressures up to 4 GPa, temperatures up to 1180 C, and holding times up to 100 h were investigated. The main findings are that, porosity can be considerably reduced without affecting significantly the γ/γ′ microstructure by high pressure annealing and the verification that increasing the external pressure stabilizes the γ′-phase. © 2012 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-012-6752-0
  • 2013 • 98 Experimental characterization of microstructure development during loading path changes in bcc sheet steels
    Clausmeyer, T. and Gerstein, G. and Bargmann, S. and Svendsen, B. and Van Den Boogaard, A.H. and Zillmann, B.
    Journal of Materials Science 48 674-689 (2013)
    Interstitial free sheet steels show transient work hardening behavior, i.e., the Bauschinger effect and cross hardening, after changes in the loading path. This behavior affects sheet forming processes and the properties of the final part. The transient work hardening behavior is attributed to changes in the dislocation structure. In this work, the morphology of the dislocation microstructure is investigated for uniaxial and plane strain tension, monotonic and forward to reverse shear, and plane strain tension to shear. Characteristic features such as the thickness of cell walls and the shape of cells are used to distinguish microstructural patterns corresponding to different loading paths. The influence of the crystallographic texture on the dislocation structure is analyzed. Digital image processing is used to create a "library" of schematic representations of the dislocation microstructure. The dislocation microstructures corresponding to uniaxial tension, plane strain tension, monotonic shear, forward to reverse shear, and plane strain tension to shear can be distinguished from each other based on the thickness of cell walls and the shape of cells. A statistical analysis of the wall thickness distribution shows that the wall thickness decreases with increasing deformation and that there are differences between simple shear and uniaxial tension. A change in loading path leads to changes in the dislocation structure. The knowledge of the specific features of the dislocation structure corresponding to a loading path may be used for two purposes: (i) the analysis of the homogeneity of deformation in a test sample and (ii) the analysis of a formed part. © 2012 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-012-6780-9
  • 2013 • 97 Friction surfacing of a cold work tool steel-Microstructure and sliding wear behavior
    Hanke, S. and Beyer, M. and Dos Santos, J.F. and Fischer, A.
    Wear 308 180-185 (2013)
    Friction surfacing is a solid-state joining process, during which process temperatures below melting, a high cooling rate, and a high degree of deformation lead to a very fine microstructure and exceptional mechanical properties of the coating material. In the presented study, the friction surfacing process was used to apply self-mating layers onto cold work tool steels, which e.g. are used for deep-drawing dies in the automotive industry. Such dies are subject to wear during operation and the repair of the dies by arc-welding includes many process steps besides the final hard surfacing. By the use of friction surfacing, hard tool steel coatings can be generated in one process step before the final machining operation. A martensitic microstructure and high hardness up to 900 HV10 can be reached within the coatings. Boundary lubricated reciprocating sliding wear tests (ball-on-flat) were conducted on cast and hardened material as well as the coatings. The results showed that over a wide range of loading the coated samples perform as well as the original die material, showing tribochemical reactions and very small wear volumes. After more than 500,000 sliding passes, both the coatings' and the original tool material's wear is dominated by surface fatigue. © 2013.
    view abstractdoi: 10.1016/j.wear.2013.06.017
  • 2013 • 96 Grain size evolution simulation in aluminium alloys AA 6082 and AA 7020 during hot forward extrusion process
    Foydl, A. and Segatori, A. and Ben Khalifa, N. and Donati, L. and Brosius, A. and Tomesani, L. and Tekkaya, A.E.
    Materials Science and Technology (United Kingdom) 29 100-110 (2013)
    The present paper investigates the grain size evolution in aluminium alloys AA 6082 and AA 7020 during hot forward extrusion process. The aim of the present work is the definition and implementation of a predictive algorithm that is able to compute the evolution of the grain shape during the process within the finite element method code Deform. Extrusion experiments were performed at two levels: at reduced scale for investigating and identifying the predictive equations and at industrial scale for validating the developed algorithm. At small scale extrusion, a complete factorial plan was performed for two alloys at three different temperatures, three extrusion ratios and two ram speeds: the discards and extrudates from the experiments were quenched immediately in order to avoid any potential recrystallisation, hence allowing measurements of transitional processing steps. At the industrial scale, instead, the 7020 alloy was extruded with two different die designs, thus producing a 20 mm diameter round bar under different extrusion ratios and strain paths. Finite element simulations were initially validated over visioplastic investigations in order to establish an accurate computation of the material flow, then experimental and numerical results were coupled, thus allowing the definition of the grain evolution model that was successfully integrated and validated on industrial scale trials. © 2013 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1179/1743284712Y.0000000132
  • 2013 • 95 Influence of fiber alignment on creep in directionally solidified NiAl-10Mo in-situ composites
    Seemüller, C. and Heilmaier, M. and Haenschke, T. and Bei, H. and Dlouhy, A. and George, E.P.
    Intermetallics 35 110-115 (2013)
    A NiAl-Mo eutectic having a nominal composition of Ni-45Al-10Mo was directionally solidified in a floating-zone furnace at two different growth rates, 20 and 80 mm/h. At the slower growth rate, the Mo fibers in the composite are well-aligned with the growth direction, whereas at the higher growth rate cellular microstructures are observed. Creep testing at 900°C showed that the minimum creep rate is much higher for cellular than for well-aligned structures. In the cellular case a "soft" cell boundary consisting primarily of binary NiAl surrounding "hard" eutectic cell interiors seems to facilitate cell boundary sliding and therefore results in low creep strength. A microstructure-based composite model is used to explain the effects of fiber alignment on creep resistance. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2012.12.007
  • 2013 • 94 Interaction between recrystallization and phase transformation during intercritical annealing in a cold-rolled dual-phase steel: A cellular automaton model
    Zheng, C. and Raabe, D.
    Acta Materialia 61 5504-5517 (2013)
    The concurrent ferrite recrystallization and austenitic transformation during intercritical annealing of cold-rolled DP steels is investigated by cellular automaton (CA) modeling. The simulations provide insight into the microstructural phenomena that result from the interaction of primary recrystallization and phase transformation. We find that the interaction between ferrite recrystallization and austenite formation affects not only the transformation kinetics but also the morphology and spatial distribution of the austenite. From this we can interpret experimental data of the observed temperature-dependent hardness and its dependence on the two metallurgical processes. The influence of the initial heating rate on subsequent isothermal transformation kinetics and the microstructure evolution is also obtained by the model. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.05.040
  • 2013 • 93 Inverse determination of modeling parameters to consider inhomogeneities of semicrystalline thermoplastics in structure simulations
    Kaiser, J.-M. and Stommel, M.
    Archive of Applied Mechanics 83 889-897 (2013)
    Two semicrystalline thermoplastics, an isotactic polypropylene (iPP, LynedllBasell Moplen HP501L) and a polyethylene-high-density (PE-HD, LynedllBasell Hostalen GC7260), were selected to approve a new approach. The developed approach allows the inverse determination of the amorphous and crystalline mechanical as well as the crystalline geometric constituents' properties. Commonly, these properties are unknown in structure simulations, and hence, the application of micromechanical models to the inhomogeneous microstructure of semicrystalline thermoplastics is restricted. Rather, a homogenous microstructure is assumed, and only one Young's modulus and Poisson's ratio are used in calculations. Thus, the quality and reliability of simulations are limited. In the current study, a new approach was exemplarily conducted for the inverse determination of the required properties by combining a Mori-Tanaka mean field approach with a genetic optimization algorithm. Conclusive results were achieved for both polymers. According to the results, the attained geometric parameters for the crystalline constituents resemble the aspect ratio of the spherulite diameter and the long period of the real crystalline microstructure, and the mechanical properties of the amorphous and crystalline constituents are located within reasonable bounds. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00419-012-0724-3
  • 2013 • 92 Layered WO3/TiO2 nanostructures with enhanced photocurrent densities
    Khare, C. and Sliozberg, K. and Meyer, R. and Savan, A. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 38 15954-15964 (2013)
    Layered WO3/TiO2 nanostructures, fabricated by magnetron sputtering, demonstrate significantly enhanced photocurrent densities compared to individual TiO2 and WO3 layers. First, a large quantity of compositions having different microstructures and thicknesses were fabricated by a combinatorial approach: diverse WO3 microstructures were obtained by adjusting sputtering pressures and depositing the films in form of wedges; later layers of TiO2 nanocolumns were fabricated thereon by the oblique angle deposition. The obtained photocurrent densities of individual WO3 and TiO2 films show thickness and microstructure dependence. Among individual WO3 layers, porous films exhibit increased photocurrent densities as compared to the dense layer. TiO2 nanocolumns show length-dependent characteristics, where the photocurrent increases with increasing film thickness. However, by combining a WO3-wedge type layer with a layer of TiO2 nanocolumns, PEC properties strikingly improve, by about two orders of magnitude as compared to individual WO3 layers. The highest photocurrent that is measured in the combinatorial library of porous WO3/TiO2 films is as high as 0.11 mA/cm2. Efficient charge-separation and charge carrier transfer processes increase the photoconversion efficiency for such films. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights.
    view abstractdoi: 10.1016/j.ijhydene.2013.09.142
  • 2013 • 91 Micromechanical modelling of damage and failure in dual phase steels
    Lian, J. and Vajragupta, N. and Münstermann, S.
    Key Engineering Materials 554-557 2369-2374 (2013)
    Dual phase (DP) steels consisting of two phases, ferrite and dispersed martensite, offer an attractive combination of strength and stretchability, which is a result of the strong distinctions of these constituents in mechanical properties. However, the damage behaviour in DP steels exhibits a rather complex scenario: voids are generated by the debonding of the hard phase from the matrix and the inner cracking of the hard phase in addition to by inclusions. The target of this study is to describe the initiation and evolution of damage in DP steel and develop a microstructure-based model which is capable of reflecting the underlying damage mechanisms. Both uniaxial and biaxial tensile tests are performed and the subsequent metallographic investigations are executed to reveal the mechanisms of damage initiation and evolution under different stress state condition and attention will be paid on the influence of various microstructural features on the initiation of damage. In finite element (FE) simulations, the microstructural features are taken into account by the representative volume elements (RVE). Different treatments of the constitutive behaviour of each constituent including isotropic hardening rule and crystallographically dependent configuration with crystal plasticity finite element method are investigated. Several numerical aspects are also discussed, such as RVE size, mesh size, element type, and boundary connections. In the end, the study is attempting to achieve a quantitative assessment of the cold formability of the investigated steel in a microscopic level based on microstructure information of material as well as to understand the damage mechanisms under different stress states condition which cause the macroscopic failure during plastic deformation. Copyright © 2013 Trans Tech Publications Ltd.
    view abstractdoi: 10.4028/www.scientific.net/KEM.554-557.2369
  • 2013 • 90 Microstructural characterization of porous thermal barrier coatings by IR gas porosimetry and sintering forecasts
    Cernuschi, F. and Golosnoy, I.O. and Bison, P. and Moscatelli, A. and Vassen, R. and Bossmann, H.-P. and Capelli, S.
    Acta Materialia 61 248-262 (2013)
    It is known that the thermal diffusivity of plasma sprayed coatings is quite sensitive to the operating conditions, namely the composition and pressure of the operating atmosphere. This makes it possible, in principle, to obtain information, in a non-destructive way, on the microstructure of thermal barrier coatings (TBCs) from thermal diffusivity measurements. An experimental assessment of this idea is presented in this paper. The microstructure of as-sprayed and sintered free-standing atmospheric plasma sprayed YPSZ TBC samples have been characterized by a new technique, named infrared (IR) gas porosimetry, as well as by mercury intrusion porosimetry and image analysis. The results from these different techniques have been compared. Furthermore, the microstructure and thermal diffusivity of sintered samples have been compared with the sintering forecasts produced by Cipitria's sintering code coupled with the Bruggeman model for thermal diffusivity estimation. Sample-to-sample variations have been discussed and uncertainties in experimental techniques have been analysed. Conditions for accurate microstructural estimations have been obtained and suggestions on the reliability of the statistical evaluation are provided. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.09.055
  • 2013 • 89 Microstructure of retrievals made from standard cast HC-CoCrMo alloys
    Stemmer, P. and Pourzal, R. and Liao, Y. and Marks, L. and Morlock, M. and Jacobs, J.J. and A.wimmer, M. and Fischer, A.
    ASTM Special Technical Publication 1560 STP 251-267 (2013)
    During the past decade, self-mating metal bearings based on cobalt- chromium-molybdenum (CoCrMo) alloys have become very popular in total hip replacements and hip resurfacings. This led to a market share of more than 35 % for metal-on-metal (MoM) bearings in the United States before several cases of high wear with biologic consequences led to a sharp drop in popularity. In part, these failures are a result of a very shallow understanding of the wear mechanisms in MoM joints and their relation to the microstructure. In order to find such a relation, one has to keep in mind that the microstructures of metallic materials depend distinctly on the entire production sequence. In addition, they change markedly under tribological stresses. This paper does not discuss the wear of any specific retrieval or even try to relate that to the specific microstructure, because such a task would be impossible based on the unknown loading history of such retrievals. Thus, we depict only the possible range of microstructures from standardized high carbon (HC)-CoCrMo retrievals. These reveal different types of hard phases: carbides and/or intermetallic phases. Some are fine (<10μm) and homogeneously distributed, whereas others appear as thin (<1 μm) and brittle cord-shaped arrangements at grain or dendrite boundaries. Coarser (>30 μm) types of mixed hard phases, which consist of carbides and interme-tallic phases, often show microcracks already below the articulating surfaces. Such subsurface microcracks are known to destabilize the gradient below the surface and the balance between tribochemical reactions and surface fatigue. In this paper, the microstructures of retrievals manufactured from standard cast CoCrMo alloys are shown and evaluated. Copyright © 2013 by ASTM International.
    view abstractdoi: 10.1520/STP156020120033
  • 2013 • 88 Microstructure, mechanical and biological properties of zirconium alloyed with niobium after severe plastic deformation
    Sharkeev, Y.P. and Eroshenko, A.Y. and Kulyashova, K.S. and Fortuna, S.V. and Suvorov, K.A. and Epple, M. and Prymak, O. and Sokolova, V. and Chernousova, S.
    Materialwissenschaft und Werkstofftechnik 44 198-204 (2013)
    A comparative investigation of microstructure, mechanical and biological properties for zirconium alloyed with niobium in coarse-grained and ultra-fine grained states is presented. The temperature and deformation regimes of multi-stage abc-pressing resulted in ultra-fine grained states with an average size of the structural elements in the range of 0.28-0.55 μm, depending on the accumulated strain during pressing. The increase of the accumulated strain at each stage of pressing increased the uniformity of the structure. The microhardness increased by 50% with increased accumulated strain during the severe plastic deformation. Between the microhardness and the average size of the structural elements, a linear dependence was found, indicating a Hall-Petch relationship. The alloy had a good biocompatibility as shown by an MTT test with osteoblasts (MG-63 cell line). The good mechanical properties (microhardness) of zirconium alloyed with niobium in the ultra-fine grained state make it suitable for medical applications, e. g. as implant material. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201300113
  • 2013 • 87 Molybdenum-based catalysts for the decomposition of ammonia: In situ X-ray diffraction studies, microstructure, and catalytic properties
    Tagliazucca, V. and Schlichte, K. and Schüth, F. and Weidenthaler, C.
    Journal of Catalysis 305 277-289 (2013)
    The ammonia decomposition reaction over molybdenum-based catalysts is an example for the complex influence of different factors, such as phase composition, size of crystalline domains, or defect concentration, on the catalytic behavior of a material. In situ powder diffraction allows the direct analysis of how catalysts change during a reaction with respect to the atomic structure or microstructure in terms of defects or size changes. In this article, the influence of catalyst treatment such as pre-reduction or ball milling on the catalytic properties is discussed in detail. © 2013 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2013.05.011
  • 2013 • 86 Phase-field model for microstructure evolution at the mesoscopic scale
    Steinbach, I.
    Annual Review of Materials Research 43 89-107 (2013)
    This review presents a phase-field model that is generally applicable to homogeneous and heterogeneous systems at the mesoscopic scale. Reviewed first are general aspects about first- and second-order phase transitions that need to be considered to understand the theoretical background of a phase field. The mesoscopic model equations are defined by a coarse-graining procedure from a microscopic model in the continuum limit on the atomic scale. Special emphasis is given to the question of how to separate the interface and bulk contributions to the generalized thermodynamic functional, which forms the basis of all phase-field models. Numerical aspects of the discretization are discussed at the lower scale of applicability. The model is applied to spinodal decomposition and ripening in Ag-Cu with realistic thermodynamic and kinetic data from a database. © Copyright © 2013 by Annual Reviews. All rights reserved.
    view abstractdoi: 10.1146/annurev-matsci-071312-121703
  • 2013 • 85 Process conditions and microstructures of ceramic coatings by gas phase deposition based on plasma spraying
    Mauer, G. and Hospach, A. and Zotov, N. and Vaßen, R.
    Journal of Thermal Spray Technology 22 83-89 (2013)
    Plasma spraying at very low pressure (50-200 Pa) is significantly different from atmospheric plasma conditions (APS). By applying powder feedstock, it is possible to fragment the particles into very small clusters or even to evaporate the material. As a consequence, the deposition mechanisms and the resulting coating microstructures could be quite different compared to conventional APS liquid splat deposition. Thin and dense ceramic coatings as well as columnar-structured strain-tolerant coatings with low thermal conductivity can be achieved offering new possibilities for application in energy systems. To exploit the potential of such a gas phase deposition from plasma spray-based processes, the deposition mechanisms and their dependency on process conditions must be better understood. Thus, plasma conditions were investigated by optical emission spectroscopy. Coating experiments were performed, partially at extreme conditions. Based on the observed microstructures, a phenomenological model is developed to identify basic growth mechanisms. © 2012 ASM International.
    view abstractdoi: 10.1007/s11666-012-9838-y
  • 2013 • 84 Process development and coating characteristics of plasma spray-PVD
    Mauer, G. and Hospach, A. and Vaßen, R.
    Surface and Coatings Technology 220 219-224 (2013)
    Plasma spray physical vapor deposition (PS-PVD) was developed with the aim of depositing uniform and relatively thin coatings with large area coverage. At high power input (~. 150. kW) and very low pressure (~. 100. Pa) the plasma jet properties change considerably compared to conventional plasma spraying and it is even possible to evaporate the powder feedstock material enabling advanced microstructures of the deposits. This relatively new technique bridges the gap between conventional plasma spraying and physical vapor deposition (PVD). Moreover, the resulting microstructures are unique and can hardly be obtained by other processes.In this paper, plasma characteristics of different gas mixtures are investigated. The measurements and calculations provide indications of the growth modes and help to explain the resulting microstructures and coating chemistries. Coatings sprayed from different ceramic powders are discussed. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2012.08.067
  • 2013 • 83 Re effects on phase stability and mechanical properties of Mo SS+Mo3Si+Mo5SiB2 alloys
    Yang, Y. and Bei, H. and Tiley, J. and George, E.P.
    Journal of Alloys and Compounds 556 32-38 (2013)
    In this paper, we investigate the effects of Re additions on the microstructure and mechanical properties of a ternary alloy with the composition Mo-12.5Si-8.5B (at.%). This alloy has a three-phase microstructure consisting of Mo solid-solution (MoSS), Mo3Si, and Mo 5SiB2 and our results show that up to 8.4 at.% Re can be added to it without changing its microstructure or forming any brittle σ phase at 1600 °C. Three-point bend tests using chevron-notched specimens showed that Re did not improve fracture toughness of the three-phase alloy. Nanoindentation performed on the MoSS phase in the three-phase alloy showed that Re increases Young's modulus, but does not lower hardness as in some Mo solid solution alloys. Based on our thermodynamic calculations and microstructural analyses, the lack of a Re softening effect is attributed to the increased Si levels in the Re-containing MoSS phase since Si is known to increase its hardness. This lack of softening is possibly why there is no Re-induced improvement in fracture toughness. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2012.12.047
  • 2013 • 82 The influence of heat treatment and resulting microstructures on the thermophysical properties of martensitic steels
    Wilzer, J. and Lüdtke, F. and Weber, S. and Theisen, W.
    Journal of Materials Science 48 8483-8492 (2013)
    This research study investigates the influence of heat treatment on the thermal conductivities of three different tool steels at room temperature. The results reveal not only that tempering plays a decisive role in their thermophysical properties, but also that the changes in thermal conductivity due to heat treatment are dependent on the degree of alloying. Isobaric heat capacities cp are found to be less dependent on heat treatment than thermal diffusivities a. The results are discussed with respect to the resulting microstructures and, for high-tempered conditions, under consideration of Calphad calculations. The results are relevant for the thermal design of tools, in particular, for tailored tempering of tools used for press hardening. © 2013 The Author(s).
    view abstractdoi: 10.1007/s10853-013-7665-2
  • 2013 • 81 Time- and space-resolved high-throughput characterization of stresses during sputtering and thermal processing of Al-Cr-N thin films
    Grochla, D. and Siegel, A. and Hamann, S. and Buenconsejo, P.J.S. and Kieschnick, M. and Brunken, H. and König, D. and Ludwig, Al.
    Journal of Physics D: Applied Physics 46 (2013)
    (Al100-xCrx)N thin-film materials libraries (x = 31-79 at%) were fabricated on micro-machined cantilever arrays, in order to simultaneously investigate the evolution of stresses during film growth as well as during thermal processing by analysing the changes in cantilever curvature. The issue of the dependence of stress in the growing films on composition, at comparable film thicknesses, was investigated. Among the various experimental parameters studied, it was found that the applied substrate bias has the strongest influence on stress evolution and microstructure formation. The compositions of the films, as well as the applied substrate bias, have a pronounced effect on the lattice parameter and the coherence length. For example, applying a substrate bias in general leads to compressive residual stress, increases the lattice parameter and decreases the coherence length. Moreover, bias can change the film texture from [1 1 1] orientation to [2 0 0]. Further detailed analysis using x-ray diffraction and transmission electron microscopy clearly revealed the presence of a [1 1 1] highly textured face centred cubic (B1 type) Al-Cr-N phase in the as-deposited state as well as the coexistence of the hexagonal [1 1 0] textured Cr2N phase, which forms in the Cr-rich region. These results show that the combinatorial approach provides insight into how stresses and compositions are related to phases and microstructures of different Al-Cr-N compositions fabricated in the form of materials libraries. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/8/084011
  • 2013 • 80 Transmission electron microscopy characterization of CrN films on MgO(001)
    Harzer, T.P. and Daniel, R. and Mitterer, C. and Dehm, G. and Zhang, Z.L.
    Thin Solid Films 545 154-160 (2013)
    Two CrN(001) films with different thickness were grown on MgO(001) substrates using unbalanced d.c.magnetron sputtering.The morphology and interfacial structure of the films are characterized by using conventional transmission electron microscopy, weak-beam dark-field microscopy and spherical aberration (CS)-corrected high-resolution transmission electron microscopy.The microscopy studies revealed the well-known cube-on-cube orientation relationship.While an interface dislocation network with b→=1/2aCrN< 100&gt; edge dislocations was identified, only part of the lattice mismatch is relaxed.The misfit dislocation structure and growth defects are analyzed and discussed based on the weak-beam dark-field and high-resolution transmission electron microscopy results.© 2013 Elsevier B.V.All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2013.07.064
  • 2012 • 79 A micromechanical damage simulation of dual phase steels using XFEM
    Vajragupta, N. and Uthaisangsuk, V. and Schmaling, B. and Münstermann, S. and Hartmaier, A. and Bleck, W.
    Computational Materials Science 54 271-279 (2012)
    As a result of their microstructures being made up by constituents with strong distinctions in mechanical properties, multiphase steels exhibit high energy absorption as well as an excellent combination of strength and ductility. Furthermore, the microstructural composition influences the failure behaviour of such kind of steels because of the occurrence of different fracture mechanisms in parallel. When the failure behaviour of dual phase (DP) steels is investigated, several types of failures are typically observed, such as the ductile failure of ferrite, the brittle failure of martensite and the interface debonding between phases. Hence, a reliable microstructure-based simulation approach must be developed that describes material deformation and failure under any given loading condition. In this work, two different damage mechanics methods were employed to study the interaction between failure modes in DP steels by means of a representative volume element (RVE). In order to consider the characteristics of a real microstructure, all involved phases were modelled with a precise volume fraction. Firstly, the extended finite element method (XFEM) was used to study the damage onset and progression in martensitic regions without prescribing the crack path. Secondly, a damage curve was derived and employed for the ductile ferritic phase. By combining these two damage models in the RVE model on microscopic scale, development of different failures modes in DP steels could be investigated. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2011.10.035
  • 2012 • 78 Application of the multiscale fem to the modeling of nonlinear composites with a random microstructure
    Klinge, S. and Hackl, K.
    International Journal for Multiscale Computational Engineering 10 213-227 (2012)
    In this contribution the properties and application of the multiscale finite element program MSFEAP are presented. This code is developed on basis of coupling the homogenization theory with the finite element method. According to this concept, the investigation of an appropriately chosen representative volume element yields the material parameters needed for the simulation of a macroscopic body. The connection of scales is based on the principle of volume averaging and the Hill-Mandel macrohomogeneity condition. The latter leads to the determination of different types of boundary conditions for the representative volume element and in this way to the postulation of a well-posed problem at this level. The numerical examples presented in the contribution investigate the effective material behavior of microporous media. An isotropic and a transversally anisotropic microstructure are simulated by choosing an appropriate orientation and geometry of the representative volume element in each Gauss point. The results are verified by comparing them with Hashin-Shtrikman's analytic bounds. However, the chosen examples should be understood as simply an illustration of the program application, while its main feature is a modular structure suitable for further development. © 2012 by Begell House, Inc.
    view abstractdoi: 10.1615/IntJMultCompEng.2012002059
  • 2012 • 77 Characteristics of ceramic coatings made by thin film low pressure plasma spraying (LPPS-TF)
    Hospach, A. and Mauer, G. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 21 435-440 (2012)
    The thin film low pressure plasma spray process (LPPS-TF) has been developed with the aim of efficient depositing uniform and thin coatings with large area coverage by plasma spraying. At high power input (∼150 kW) and very low pressure (∼100 Pa) the plasma jet properties change considerably and it is even possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This relatively new technique bridges the gap between conventional plasma spraying and physical vapor deposition. In addition, the resulting microstructures are unique and can hardly be obtained by other processes. In this paper, microstructures made by LPPS-TF are shown and the columnar layer growth by vapor deposition is demonstrated. In addition to the ceramic materials TiO 2, Al2 O3 or MgAl2O4, the focus of the research was placed on partially yttria-stabilized zirconia. Variations of the microstructures are shown and discussed concerning potential coating applications. © ASM International.
    view abstractdoi: 10.1007/s11666-012-9748-z
  • 2012 • 76 Deposition of La 1-xSr xFe 1-yCo yO 3-δ coatings with different phase compositions and microstructures by low-pressure plasma spraying-thin film (LPPS-TF) processes
    Zotov, N. and Hospach A. and Mauer G. and Sebold D. and Vaßen, R.
    Journal of Thermal Spray Technology 21 441-447 (2012)
    Perovskite-type materials with the general chemical formula A 1-xÁ xB́ 1-yB́ yO 3δ have received considerable attention as candidates for oxygen separation membranes. Preparation of La 1-xSr xFe 1-yCo yO 3-δ (LSFC) coatings by low-pressure plasma spraying-thin film processes using different plasma spray parameters is reported and discussed. Deposition with Ar-He plasma leads to formation of coatings containing a mixture of cubic LSFC perovskite, SrLaFeO4, FeCo, and metal oxides. Coatings deposited at higher oxygen partial pressures by pumping oxygen into the vacuum chamber contain more than 85% perovskite and only a few percent Fe32xCoxO4, and/or CoO. The microstructures of the investigated LSFC coatings depend sensitively on the oxygen partial pressure, the substrate temperature, the plasma jet velocities, and the deposition rate. Coatings deposited with Ar-rich plasma, relatively low net torch power, and with higher plasma jet velocities are most promising for applications as oxygen permeation membranes. © ASM International.
    view abstractdoi: 10.1007/s11666-012-9768-8
  • 2012 • 75 Electrodeposition of separated metallic structures in superimposed magnetic gradient fields
    Tschulik, K. and Cierpka, Ch. and Uhlemann, M. and Gebert, A. and Schultz, L.
    ECS Transactions 41 9-16 (2012)
    Structuring of Cu deposits was reported to occur during electrodeposition in magnetic gradient fields, recently. This work aims on increasing the knowledge about the convection-induced structuring mechanism. Therefore, in-situ velocity measurements by Astigmatism Particle Tracking Velocimetry were performed. The information gained by this study is used to choose pulse-reverse-plating parameters suitable for deposition of separated Cu structures. Additionally, it is demonstrated that the structure shape can be changed by varying the superimposed magnetic field gradients. The structure height can be adjusted by the number of pulse-reverse-plating cycles. The structuring mechanism is discussed for potentiostatic as well as for pulse-reverse-plating conditions with respect to the acting magnetic forces. ©The Electrochemical Society.
    view abstractdoi: 10.1149/1.3696825
  • 2012 • 74 Friction-stir dissimilar welding of aluminium alloy to high strength steels: Mechanical properties and their relation to microstructure
    Coelho, R.S. and Kostka, A. and dos Santos, J.F. and Kaysser-Pyzalla, A.
    Materials Science and Engineering A 556 175-183 (2012)
    The use of light-weight materials for industrial applications is a driving force for the development of joining techniques. Friction stir welding (FSW) inspired joints of dissimilar materials because it does not involve bulk melting of the basic components. Here, two different grades of high strength steel (HSS), with different microstructures and strengths, were joined to AA6181-T4 Al alloy by FSW. The purpose of this study is to clarify the influence of the distinct HSS base material on the joint efficiency. The joints were produced using the same welding parameter/setup and characterised regarding microstructure and mechanical properties. Both joints could be produced without any defects. Microstructure investigations reveal similar microstructure developments in both joints, although there are differences e.g. in the size and amount of detached steel particles in the aluminium alloy (heat and thermomechanical affected zone). The weld strengths are similar, showing that the joint efficiency depends foremost on the mechanical properties of the heat and the thermomechanical affected zone of the aluminium alloy. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2012.06.076
  • 2012 • 73 Generation and evolution of inelastic microstructures - An overview
    Hackl, K. and Hoppe, U. and Kochmann, D.M.
    GAMM Mitteilungen 35 91-106 (2012)
    In this paper we give an overview on the modeling of inelastic microstructures using variational methods. We start by discussing the underlying variational principles for inelastic materials, derive evolution equations for internal variables, and introduce the concept of condensed energy. As a mathematical prerequisite we review the variational calculus of nonconvex potentials and the notion of relaxation. We use these instruments in order to study the initiation of plastic microstructures. Here we focus on a model of single-slip crystal plasticity. Afterwards we move on to model the evolution of microstructures. We introduce the concept of essential microstructures and the corresponding relaxed energies and dissipation potentials, and derive evolution equations for microstructure parameters. We then present a numerical scheme by means of which the microstructure development can be computed, and show numerical results for particular examples. ©c 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/gamm.201210007
  • 2012 • 72 Improving mechanical properties of chip-based aluminum extrudates by integrated extrusion and equal channel angular pressing (iECAP)
    Haase, M. and Ben Khalifa, N. and Tekkaya, A.E. and Misiolek, W.Z.
    Materials Science and Engineering A 539 194-204 (2012)
    In order to improve the mechanical properties of profiles extruded from aluminum chips, a four turn equal channel angular pressing tool was integrated into an extrusion die (iECAP die). AA6060 aluminum alloy turning chips were cold pre-compacted to chip-based billets and hot extruded through the iECAP die on a conventional forward extrusion press. Mechanical properties and microstructure of the chip-based billets extruded through the iECAP die were investigated and compared to those extruded through a conventional flat-face die and a porthole die. To evaluate the performance of the iECAP processed chip-based profiles, conventional cast billets were extruded through the flat-face die as a reference material. To investigate the influence of temperature on mechanical properties and microstructure of chip-based profiles, the extrusion was performed at 450. °C and 550. °C.Tensile tests revealed superior mechanical properties of the chip-based billets extruded through the iECAP die in comparison to chip-based billets extruded through the flat-face and the porthole die as well as to cast billets extruded through the flat-face die. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2012.01.081
  • 2012 • 71 Influence of handling parameters on coating characteristics in order to produce near-net-shape wear resistant coatings
    Tillmann, W. and Krebs, B.
    Journal of Thermal Spray Technology 21 644-650 (2012)
    The present study investigates the influence of spray torch handling parameters such as the spray angle, spray distance, track pitch, and gun velocity on the deposition rate and the microstructure of atmospheric plasma sprayed WC-12Co coatings as well as twin wire arc sprayed WSC-Fe coatings. Similarities as well as fundamental differences in the sensitivity of the two spray processes, regarding changes in handling parameters are discussed, using results of light microscopic analyses. Both coating systems show distinct changes of the deposition rate when varying the handling parameters. An empirical model could be determined to describe the coating deposition. This model enables an optimization of path planning processes by reducing the number of optimization loops. However, the coatings show visible changes in the microstructure, which have to be taken into consideration in order to guarantee the production of high quality coatings. © ASM International.
    view abstractdoi: 10.1007/s11666-012-9735-4
  • 2012 • 70 Influences of deformation strain, strain rate and cooling rate on the Burgers orientation relationship and variants morphology during β→α phase transformation in a near α titanium alloy
    He, D. and Zhu, J.C. and Zaefferer, S. and Raabe, D. and Liu, Y. and Lai, Z.L. and Yang, X.W.
    Materials Science and Engineering A 549 20-29 (2012)
    High temperature compression deformation studies of Ti-6Al-2Zr-1Mo-1V titanium alloy in full β phase region with different strains/strain rates and then with subsequent varied cooling rates were performed to understand the microstructure evolution. Crystal orientation information and microstructure morphology of all tested samples were investigated by electron backscatter diffraction (EBSD) measurements. The crystal orientations of prior high temperature β grains were estimated by reconstructing the retained β phase at room temperature. The theoretical crystal orientations of all possible α variants within an investigated prior β grain were calculated according to the Burgers orientation relationship (OR) between parent and product phase. The calculated and experimental results were then compared and analyzed. The influences of deformation strain, strain rate and cooling rate on the Burgers OR between prior β matrix and precipitated α phase were investigated. Full discussions have been conducted by combination of crystal plasticity finite element method (CP-FEM) grain-scale simulation results. The results indicate that external factors (such as deformation strain, strain rate and cooling rate) have a slight influence on the obeying of Burgers OR rule during β → α phase transformation. However, strain rate and cooling rate have a significant effect on the morphology of precipitated α phase. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2012.03.110
  • 2012 • 69 Lamina cribrosa thickening in early glaucoma predicted by a microstructure motivated growth and remodeling approach
    Grytz, R. and Sigal, I.A. and Ruberti, J.W. and Meschke, G. and Crawford Downs, J.
    Mechanics of Materials 44 99-109 (2012)
    Glaucoma is among the leading causes of blindness worldwide. The ocular disease is characterized by irreversible damage of the retinal ganglion cell axons at the level of the lamina cribrosa (LC). The LC is a porous, connective tissue structure whose function is believed to provide mechanical support to the axons as they exit the eye on their path from the retina to the brain. Early experimental glaucoma studies have shown that the LC remodels into a thicker, more posterior structure which incorporates more connective tissue after intraocular pressure (IOP) elevation. The process by which this occurs is unknown. Here we present a microstructure motivated growth and remodeling (G&R) formulation to explore a potential mechanism of these structural changes. We hypothesize that the mechanical strain experienced by the collagen fibrils in the LC stimulates the G&R response at the micro-scale. The proposed G&R algorithm controls collagen fibril synthesis/degradation and adapts the residual strains between collagen fibrils and the surrounding tissue to achieve biomechanical homeostasis. The G&R algorithm was applied to a generic finite element model of the human eye subjected to normal and elevated IOP. The G&R simulation underscores the biomechanical need for a LC at normal IOP. The numerical results suggest that IOP elevation leads to LC thickening due to an increase in collagen fibril mass, which is in good agreement with experimental observations in early glaucoma monkey eyes. This is the first study to demonstrate that a biomechanically-driven G&R mechanism can lead to the LC thickening observed in early experimental glaucoma. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.mechmat.2011.07.004
  • 2012 • 68 Mechanical properties of (20-30)Mn12Cr(0.56-0.7)CN corrosion resistant austenitic TWIP steels
    Mújica Roncery, L. and Weber, S. and Theisen, W.
    Steel Research International 83 307-314 (2012)
    New developed (20-30)Mn12Cr(0.56-0.7)CN TWIP steels developed from thermodynamic calculations exhibit great mechanical properties, such as high strength (1800MPa UTS), deformability (80-100% elongation), toughness (300 J ISO-V), and impact wear resistance equivalent to that of Hadfield steel. In addition, they exhibit corrosion resistance by passivation in aqueous acidic media. Microstructure examination by SEM and EBSD at different degrees of deformation reveals that twinning takes place and is responsible for the high cold-work hardening of the steels. Stacking fault energy measurement of three different developed steels locates them in the range of 30-40mJm -2, being highly dependent on the N and Mn contents. Measurements carried out with digital image correlation indicate that at room temperature dynamic strain aging or Portevin-LeChatelier effect takes place. Measurements of impact toughness indicate that the steels have ductile to brittle transition at cryogenic temperatures as a consequence of the effect of nitrogen on the deformation mechanisms, resulting in a quasi-cleavage fracture along the {111} planes at -196°C. New Fe-Cr-Mn-C-N TWIP steels developed from thermodynamic calculations exhibit great mechanical properties, such as high strength (1800MPa UTS), deformability (80-100% elongation), toughness (300J ISO-V), high impact wear resistance, and corrosion resistance by passivation in aqueous acidic media. This work examines the microstructure, stacking fault energy, and dynamic strain aging to understand the tensile behavior and toughness of these materials. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201100316
  • 2012 • 67 Microstructure evolution during recrystallization in dual-phase steels
    Peranio, N. and Roters, F. and Raabe, D.
    Materials Science Forum 715-716 13-22 (2012)
    The microstructure and texture of rolled and annealed dual-phase steels with 0.147 wt. % C, 1.868 wt. % Mn, and 0.403 wt. % Si were analyzed using SEM, EDX, and EBSD. Hot rolled sheets showed a ferritic-pearlitic microstructure with a pearlite volume fraction of about 40 % and ferrite grain size of about 6 μm. Ferrite and pearlite were heterogeneously distributed at the surface and distributed in bands at the center of the sheets. The hot rolled sheets revealed a throughthickness texture inhomogeneity with a plane-strain texture with strong α-fiber and γ-fiber at the center and a shear texture at the surface. After cold rolling, the ferrite grains showed elongated morphology and larger orientation gradients, the period of the ferrite-pearlite band structure at the center of the sheets was decreased, and the plane-strain texture components were strengthened in the entire sheet. Recrystallization, phase transformation, and the competition of both processes were of particular interest with respect to the annealing experiments. For this purpose, various annealing techniques were applied, i.e., annealing in salt bath, conductive annealing, and industrial hot-dip coating. The sheets were annealed in the ferritic, intercritical, and austenitic temperature regimes in a wide annealing time range including variation of heating and cooling rates. © (2012) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/MSF.715-716.13
  • 2012 • 66 Non-destructive subsurface damage monitoring in bearings failure mode using fractal dimension analysis
    Holweger, W. and Walther, F. and Loos, J. and Wolf, M. and Schreiber, J. and Dreher, W. and Kern, N. and Lutz, S.
    Industrial Lubrication and Tribology 64 132-137 (2012)
    Purpose: Bearings in field applications with high dynamic loading, e.g. wind energy plants, suffer from sudden failure initiated by subsurface material transformation, known as white etching cracks in a typical scale of μm, preferably around the maximum Hertzian stress zone. Despite many investigations in this field no precise knowledge about the root cause of those failures is available, due to the fact that failure under real service conditions of wind energy plants differs from what is known from test rig results in terms of contact loading, lubrication or dynamics. The purpose of this paper is to apply Barkhausen noise measurement to a full bearing test ring running under conditions of elastohydrodynamic lubrication (EHL) with high radial preload. Design/methodology/approach: Full bearing tests are carried out by use of DGBB (Deep Grove Ball Bearings) with 6206 specification, material set constant as 100Cr6, martensitic hardening, 10-12 percent maximum retained austenite and radial preload of 3500 MPa. Speed is set 9000 rpm, temperature is self setting at 80°C by test conditions. For tests, synthetic hydrocarbon base oil (Poly-α-Olefine) with a 1 percent amount of molydenum dithiophosphate (organic chain given as 2-ethylhexyl) was used. Findings: Non-destructive fractal dimension analyses by use of Barkhausen noise measurements is of versatile value in terms of recording bearing manufacturing processes, but can also be part of non-destructive condition monitoring of bearings in field applications, where predictive reactive maintenance is crucial for availability of the plant. Research limitations/implications: Barkhausen noise signal recording may also be valuable for case studies related to microstructure changes of steel under operation conditions. Bearings are exposed in plenty of conditions to phenomena such as straying currents, subsequently straying magnetic fields. Hardly anything is known about how microstructure of bearing steel is susceptible to such conditions. This will be part of further studies. Originality/value: Results given in the paper show that sudden bearing failure, according to formation of subsurface material property changes might be driven by activities of dislocations. Since those activities start with sequences of stress field-induced formation of domains, later by formation of low-angle subgrains, and at least phase transformation, recording of the Barkhausen signal would lead to real predictive condition monitoring in applications where a highly dynamic loading of the contact, even with low nominal contact pressure leads to sudden failure induced by white etching. © Emerald Group Publishing Limited.
    view abstractdoi: 10.1108/00368791211218650
  • 2012 • 65 Numerical simulation of dynamic strain-induced austenite-ferrite transformation and post-dynamic kinetics in a low carbon steel
    Zheng, C. and Raabe, D. and Li, D.
    Materials Science Forum 706-709 1592-1597 (2012)
    2-D cellular automaton model was developed to simulate the dynamic strain-induced transformation (DSIT) from austenite (?) to ferrite (a) and the post-dynamic kinetic behavior in a low carbon steel with the purpose of developing a methodology of mesoscopic computer simulation for an improved understanding of the formation of ultra-fine ferrite (UFF) in DSIT and the conservation of this microstructure during the post-deformation period. The predicted microstructure obtained after DSIT was compared with a quenched dual-phase steel. Its microstructure, consisting of fine-grained ferrite and fine islands of retained austenite dispersed in the matrix, were found to be in good agreement with the predictions. The simulated results indicate that the refinement of ferrite grains produced via DSIT can be interpreted in terms of unsaturated nucleation and limited growth mechanisms. It is also revealed that continuing transformation from retained austenite to ferrite and the reverse transformation both could take place simultaneously during the post-deformation isothermal holding. A competition between them exists at the early stage of the post-dynamic transformation. © 2012 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/MSF.706-709.1592
  • 2012 • 64 On the nature of internal interfaces in a tempered martensite ferritic steel and their evolution during long-term creep
    Payton, E.J. and Aghajani, A. and Otto, F. and Eggeler, G. and Yardley, V.A.
    Scripta Materialia 66 1045-1048 (2012)
    Orientation relationships between ferrite micrograins in a 12% Cr tempered martensite ferritic steel are characterized before and at intermediate times during long-term creep (120 MPa at 550 °C) up to ∼140,000 h. Orientations inherited from the martensitic microstructure during tempering deviate significantly from the well-known/idealized orientation relationships. The observed relationships between micrograins persist throughout creep. The spatial densities of former sub-block, block, and packet boundaries all decrease during creep, suggesting that coarsening takes place on all microstructural scales. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2012.02.042
  • 2012 • 63 Partially relaxed energy potentials for the modelling of microstructures - Application to shape memory alloys
    Bartel, T. and Menzel, A.
    GAMM Mitteilungen 35 59-74 (2012)
    Energy relaxation is well-established by several researchers - especially in the field of the modelling of solid-solid phase transformations. Nevertheless, critics still counter this concept by considering it as a purely mathematical tool with poor physical significance. In this contribution we aim at emphasising the significance of energy relaxation methods for the modelling of dissipative solids and especially microstructure formation and its further evolution. In particular, we shall point out aspects and advantages of this concept which are not straight forward to achieve within alternative schemes. ©c 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/gamm.201210005
  • 2012 • 62 Pearlite revisited
    Steinbach, I. and Plapp, M.
    Continuum Mechanics and Thermodynamics 24 665-673 (2012)
    Zener's model of pearlite transformation in steels can be viewed as the prototype of many microstructure evolution models in materials science. It links principles of thermodynamics and kinetics to the scale of the microstructure. In addition it solves a very practical problem: How the hardness of steel is correlated to the conditions of processing. Although the model is well established since the 1950s, quantitative explanation of growth kinetics was missing until very recently. The present paper will shortly review the classical model of pearlite transformation. Zener's conjecture of maximum entropy production will be annotated by modern theoretical and experimental considerations of a band of stable (sometimes oscillating) states around the state of maximum entropy production. Finally, an explanation of the growth kinetics observed in experiments is proposed based on diffusion fluxes driven by stress gradients due to large transformation strain. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00161-011-0204-y
  • 2012 • 61 Polycrystal model of the mechanical behavior of a Mo-TiC 30 vol.% metal-ceramic composite using a three-dimensional microstructure map obtained by dual beam focused ion beam scanning electron microscopy
    Cédat, D. and Fandeur, O. and Rey, C. and Raabe, D.
    Acta Materialia 60 1623-1632 (2012)
    The mechanical behavior of a Mo-TiC 30 vol.% ceramic-metal composite was investigated over a wide temperature range (25-700 °C). High-energy X-ray tomography was used to reveal percolation of the hard titanium carbide phase through the composite. Using a polycrystal approach for a two-phase material, finite-element simulations were performed on a real three-dimensional (3-D) aggregate of the material. The 3-D microstructure, used as the starting configuration for the predictions, was obtained by serial sectioning in a dual beam focused ion beam scanning electron microscope coupled to an electron backscattered diffraction system. The 3-D aggregate consists of a molybdenum matrix and a percolating TiC skeleton. As for most body-centered cubic (bcc) metals, the molybdenum matrix phase is characterized by a change in plasticity mechanism with temperature. We used a polycrystal model for bcc materials which was extended to two phases (TiC and Mo). The model parameters of the matrix were determined from experiments on pure molydenum. For all temperatures investigated the TiC particles were considered to be brittle. Gradual damage to the TiC particles was treated, based on an accumulative failure law that is approximated by evolution of the apparent particle elastic stiffness. The model enabled us to determine the evolution of the local mechanical fields with deformation and temperature. We showed that a 3-D aggregate representing the actual microstructure of the composite is required to understand the local and global mechanical properties of the composite studied. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.11.055
  • 2012 • 60 Prediction of post-dynamic austenite-to-ferrite transformation and reverse transformation in a low-carbon steel by cellular automaton modeling
    Zheng, C. and Raabe, D. and Li, D.
    Acta Materialia 60 4768-4779 (2012)
    The post-dynamic transformation that takes place during the subsequent isothermal holding for the case when dynamic strain-induced transformation (DSIT) from austenite to ferrite occurs during hot deformation is investigated by cellular automaton modeling. The simulation provides a better understanding of carbon diffusion in retained austenite and the resulting microstructure evolution during the post-dynamic transformation. The predictions reveal that continuing transformation from retained austenite to ferrite and the reverse transformation can occur simultaneously in the same microstructure during post-deformation isothermal holding owing to the locally acting chemical equilibrium conditions. Competition between forward and reverse transformation exists during the early stage of post-dynamic heat treatment. It is also revealed that increasing the final strain of DSIT might promote the reverse transformation, whereas the continuous austenite-to-ferrite transformation yields a diminishing effect. The influence of the DSIT final strain on the grain size of ferrite and the characteristics of the resultant microstructure is also discussed. © 2012 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2012.06.007
  • 2012 • 59 Studies of the contribution of alternating electromagnetic fields toward material fatigue in 100Cr6
    Holweger, W. and Wolf, M. and Walther, F. and Trojahn, W. and Mütze, A. and Kunzmann, J. and Schreiber, J. and Mayer, J. and Reichelt, M.
    Industrial Lubrication and Tribology 64 247-252 (2012)
    The purpose of this paper is to show how controlled exposure of electromagnetic fields toward bearing steel vulnerates the microstructure. The ability of Barkhausen Noise signal processing is used for detecting phenomena such as dislocation and subgrain formation processes as the beginning of later failures. A Barkhausen noise signal measurement equipment is used for detecting subsurface distress of 100Cr6 as a function of the applied electromagnetic and mechanical stress. Barkhausen noise signal is mathematically processed by use of fractal dimension analysis. The paper cleary reveals significant impact of electromagnetic field in junction with mechanical loading. Electromagnetic impact depends on the magnitude of the field. Research limitations are given by the fact that in real field applications, e.g. wind power plants, bearings are exposed by multiple influences and the methodology is not applicable to those conditions. The methodology can be applied to real field applications in condition monitoring systems. Up to now, no reasonable on-line measurement is in use determining sub surface fatigue phenomena. The paper hence, reveals the possibility to raise condition monitoring into a new perspective. The use of Barkhausen noise signal processing, as presented here, is original with respect to real field applications, such as wind power plants with a high demand in condition monitoring, especially off-shore plants. © 2012, Emerald Group Publishing Limited
    view abstractdoi: 10.1108/00368791211249629
  • 2012 • 58 Supersolidus liquid-phase sintering of ultrahigh-boron high-carbon steels for wear-protection applications
    Röttger, A. and Weber, S. and Theisen, W.
    Materials Science and Engineering A 532 511-521 (2012)
    Powder metallurgy (PM) represents an alternative to conventional casting processes for the production of wear-resistant materials. PM hard alloys for wear-protection applications feature both higher strength and fracture toughness compared to cast hard alloys due to their more finely grained microstructure. However, densification by hot-isostatic pressing (HIP), the conventional PM-compaction method, is relatively expensive and thus partially counteracts low-cost processing. To increase the economic efficiency of the processing route, supersolidus liquid-phase sintering (SLPS) was investigated. In addition, expensive Ni- and Co-base hard alloys were substituted by boron-rich Fe-base hard-facing alloys.In this study, three ultrahigh-boron hard-facing alloy powders were densified by SLPS and HIP. The sintering temperatures were optimized by means of sintering experiments that were supported by thermodynamic calculations. Both densification states were investigated and compared with respect to the microstructure and the tribological and mechanical properties of the compacted hard-facing alloys. It was shown that the mechanical and tribological properties are strongly influenced by the microstructure. Although the microstructure is affected by the chemical composition, it can also be adapted by the densification process. SLPS-densified hard-facing alloys have a coarse microstructure that imparts not only a high wear resistance but also a detrimental effect on the mechanical properties. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.10.118
  • 2012 • 57 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
  • 2011 • 56 Analysis of the mechanical properties of an arc-sprayed WC-FeCSiMn coating: Nanoindentation and simulation
    Tillmann, W. and Klusemann, B. and Nebel, J. and Svendsen, B.
    Journal of Thermal Spray Technology 20 328-335 (2011)
    The characterization of thermal-sprayed coatings is often limited to microstructural analysis to evaluate the coatings morphology. Indentation is commonly used to determine the mechanical properties of different kinds of engineering materials. However, due to the complex structure of thermal-sprayed coatings, few results have been obtained so far. In this article, experimental nanoindentation tests and simulation results are compared. The experimental indentation tests show scattering in the force-deformation data due to the complex structure of the arc-sprayed coating which is investigated by means of an indentation test simulation. Based on the results for single constituent parts of the coating, the Young's modulus as well as further mechanical properties are identified. A general procedure is presented to predict the effective mechanical properties based on the microstructure, porosity, chemical composition, and properties of the coating after thermal spraying. © 2010 ASM International.
    view abstractdoi: 10.1007/s11666-010-9550-8
  • 2011 • 55 Analysis of the plastic anisotropy and pre-yielding of (γ/ α2)-phase titanium aluminide microstructures by crystal plasticity simulation
    Zambaldi, C. and Roters, F. and Raabe, D.
    Intermetallics 19 820-827 (2011)
    The plastic deformation of lamellar microstructures composed of the two phases γ-TiAl and α2-Ti3Al is highly orientation dependent. In this paper we present a homogenized model that takes into account the micromechanical effect of the plate-like morphologies that are often observed in two-phase titanium aluminide alloys. The model is based on crystal elasto-viscoplasticity and 18 deformation systems were implemented that have been identified to govern the plastic flow of the lamellar microstructures. The model is validated against experiments on polysynthetically twinned (PST) crystals and shows good agreement with the data. On a larger length scale, the model is applied to a 64-grain aggregate to investigate the mechanical response of two different kinds of microstructures. Different magnitudes of the kinematic constraints exerted by the densely spaced and highly aligned interfaces are shown to affect the macroscopic flow behavior of the microstructures. The phenomenon of pronounced microplasticity of fully lamellar material as well as the stress variation inside two-phase microstructures are studied quantitatively. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2011.01.012
  • 2011 • 54 Annealing effects on microstructure and coercive field of ferritic-martensitic ODS Eurofer steel
    Renzetti, R.A. and Sandim, H.R.Z. and Sandim, M.J.R. and Santos, A.D. and Möslang, A. and Raabe, D.
    Materials Science and Engineering A 528 1442-1447 (2011)
    Oxide dispersion strengthened reduced-activation ferritic-martensitic steels are promising candidates for applications in future fusion power plants. Samples of a reduced activation ferritic-martensitic 9wt.%Cr-oxide dispersion strengthened Eurofer steel were cold rolled to 80% reduction in thickness and annealed in vacuum for 1h from 200 to 1350°C to evaluate its thermal stability. Vickers microhardness testing and electron backscatter diffraction (EBSD) were used to characterize the microstructure. The microstructural changes were also followed by magnetic measurements, in particular the corresponding variation of the coercive field (Hc), as a function of the annealing treatment. Results show that magnetic measurements were sensitive to detect the changes, in particular the martensitic transformation, in samples annealed above 850°C (austenitic regime). © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.10.051
  • 2011 • 53 Approximation of random microstructures by periodic statistically similar representative volume elements based on lineal-path functions
    Schröder, J. and Balzani, D. and Brands, D.
    Archive of Applied Mechanics 81 975-997 (2011)
    For the direct incorporation of micromechanical information into macroscopic boundary value problems, the FE2-method provides a suitable numerical framework. Here, an additional microscopic boundary value problem, based on evaluations of representative volume elements (RVEs), is attached to each Gauss point of the discretized macrostructure. However, for real random heterogeneous microstructures the choice of a "large" RVE with a huge number of inclusions is much too time-consuming for the simulation of complex macroscopic boundary value problems, especially when history-dependent constitutive laws are adapted for the description of individual phases of the mircostructure. Therefore, we propose a method for the construction of statistically similar RVEs (SSRVEs), which have much less complexity but reflect the essential morphological attributes of the microscale. If this procedure is prosperous, we arrive at the conclusion that the SSRVEs can be discretized with significantly less degrees of freedom than the original microstructure. The basic idea for the design of such SSRVEs is to minimize a least-square functional taking into account suitable statistical measures, which characterize the inclusion morphology. It turns out that the combination of the volume fraction and the spectral density seems not to be sufficient. Therefore, a hybrid reconstruction method, which takes into account the lineal-path function additionally, is proposed that yields promising realizations of the SSRVEs. In order to demonstrate the performance of the proposed procedure, we analyze several representative numerical examples. © 2010 Springer-Verlag.
    view abstractdoi: 10.1007/s00419-010-0462-3
  • 2011 • 52 Columnar-structured thermal barrier coatings (TBCs) by thin film low-pressure plasma spraying (LPPS-TF)
    Hospach, A. and Mauer, G. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 20 116-120 (2011)
    The very low-pressure plasma Spray (VLPPS) process has been developed with the aim of depositing uniform and thin coatings with coverage of a large area by plasma spraying. At typical pressures of 100-200 Pa, the characteristics of the plasma jet change compared to conventional low-pressure plasma-spraying processes (LPPS) operating at 5-20 kPa. The combination of plasma spraying at low pressures with enhanced electrical input power has led to the development of the LPPS-TF process (TF = thin film). At appropriate parameters, it is possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This technique offers new possibilities for the manufacturing of thermal barrier coatings (TBCs). Besides the material composition, the microstructure is an important key to reduce thermal conductivity and to increase strain tolerance. In this regard, columnar microstructures deposited from the vapor phase show considerable advantages. Therefore, physical vapor deposition by electron beam evaporation (EB-PVD) is applied to achieve such columnar-structured TBCs. However, the deposition rate is low, and the line-of-sight nature of the process involves specific restrictions. In this article, the deposition of TBCs by the LPPS-TF process is shown. How the evaporation of the feedstock powder could be improved and to what extent the deposition rates could be increased were investigated. © 2010 ASM International.
    view abstractdoi: 10.1007/s11666-010-9549-1
  • 2011 • 51 Development and characterization of novel corrosion-resistant TWIP steels
    Mujica, L. and Weber, S. and Hunold, G. and Theisen, W.
    Steel Research International 82 26-31 (2011)
    Austenitic steels exhibiting twinning induced plasticity (TWIP) are materials with exceptional mechanical properties. In this work, the development of new grades of TWIP steels exhibiting corrosion resistance is presented. The alloy development was supported by thermodynamic and diffusion calculations within the (Fe-Mn-Cr)-(C-N) alloy system. For the calculations ambient pressure and primary austenitic solidification were considered as necessary to avoid nitrogen degassing in all processing steps. Manganese is used as an austenite stabilizer, chromium to increase nitrogen solubility and provide corrosion resistance, while carbon and nitrogen provide mechanical strength. Diffusion calculations were used in order to predict the extent of micro segregations and additionally to evaluate the effect of diffusion annealing treatments. The material was cast in a laboratory scale with a nominal composition of Fe-20Mn-12Cr-0.25C-0.3N. Diffusion annealing was followed by hot rolling and solution annealing resulting in a fully austenitic microstructure. Tensile tests at room temperature were performed, exhibiting yield strengths of 430 MPa and elongation to fracture of 93%. In addition, not only the mechanical properties but also the weldability was studied, focussing on the characterization of the microstructure of bead on plate welds obtained by laser and TIG welding. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201000219
  • 2011 • 50 Dislocation and twin substructure evolution during strain hardening of an Fe-22 wt.% Mn-0.6 wt.% C TWIP steel observed by electron channeling contrast imaging
    Gutierrez-Urrutia, I. and Raabe, D.
    Acta Materialia 59 6449-6462 (2011)
    We study the kinetics of the substructure evolution and its correspondence to the strain hardening evolution of an Fe-22 wt.% Mn-0.6 wt.% C TWIP steel during tensile deformation by means of electron channeling contrast imaging (ECCI) combined with electron backscatter diffraction (EBSD). The contribution of twin and dislocation substructures to strain hardening is evaluated in terms of a dislocation mean free path approach involving several microstructure parameters, such as the characteristic average twin spacing and the dislocation substructure size. The analysis reveals that at the early stages of deformation (strain below 0.1 true strain) the dislocation substructure provides a high strain hardening rate with hardening coefficients of about G/40 (G is the shear modulus). At intermediate strains (below 0.3 true strain), the dislocation mean free path refinement due to deformation twinning results in a high strain rate with a hardening coefficient of about G/30. Finally, at high strains (above 0.4 true strain), the limited further refinement of the dislocation and twin substructures reduces the capability for trapping more dislocations inside the microstructure and, hence, the strain hardening decreases. Grains forming dislocation cells develop a self-organized and dynamically refined dislocation cell structure which follows the similitude principle but with a smaller similitude constant than that found in medium to high stacking fault energy alloys. We attribute this difference to the influence of the stacking fault energy on the mechanism of cell formation. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.07.009
  • 2011 • 49 Enhanced photoelectrochemical properties of WO3 thin films fabricated by reactive magnetron sputtering
    Vidyarthi, V.S. and Hofmann, M. and Savan, A. and Sliozberg, K. and König, D. and Beranek, R. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 36 4724-4731 (2011)
    Polycrystalline WO3 thin films were fabricated by reactive magnetron sputtering at a substrate temperature of 350 °C under different Ar/O2 gas pressures. In order to study the thickness dependence of photoelectrochemical (PEC) behavior of WO3, the thickness-gradient films were fabricated and patterned using a micro-machined Si-shadow mask during the deposition process. The variation of the sputter pressure leads to the evolution of different microstructures of the thin films. The films fabricated at 2 mTorr sputter pressure are dense and show diminished PEC properties, while the films fabricated at 20 mTorr and 30 mTorr are less dense and exhibit enhanced water photooxidation efficiency. The enhanced photooxidation is attributed to the coexistence of porous microstructure and space charge region enabling improved charge carrier transfer to the electrolyte and back contact. A steady-state photocurrent as high as 2.5 mA cm-2 at 1 V vs. an Ag/AgCl (3 M KCl) reference electrode was observed. For WO3 films fabricated at 20 mTorr and 30 mTorr, the photocurrent increases continuously up to a thickness of 600 nm. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2011.01.087
  • 2011 • 48 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 • 47 From nanoparticles to nanocrystalline bulk: Percolation effects in field assisted sintering of silicon nanoparticles
    Schwesig, D. and Schierning, G. and Theissmann, R. and Stein, N. and Petermann, N. and Wiggers, H. and Schmechel, R. and Wolf, D.E.
    Nanotechnology 22 (2011)
    Nanocrystalline bulk materials are desirable for many applications as they combine mechanical strength and specific electronic transport properties. Our bottom-up approach starts with tailored nanoparticles. Compaction and thermal treatment are crucial, but usually the final stage sintering is accompanied by rapid grain growth which spoils nanocrystallinity. For electrically conducting nanoparticles, field activated sintering techniques overcome this problem. Small grain sizes have been maintained in spite of consolidation. Nevertheless, the underlying principles, which are of high practical importance, have not been fully elucidated yet. In this combined experimental and theoretical work, we show how the developing microstructure during sintering correlates with the percolation paths of the current through the powder using highly doped silicon nanoparticles as a model system. It is possible to achieve a nanocrystalline bulk material and a homogeneous microstructure. For this, not only the generation of current paths due to compaction, but also the disintegration due to Joule heating is required. The observed density fluctuations on the micrometer scale are attributed to the heat profile of the simulated powder networks. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/13/135601
  • 2011 • 46 Hierarchical microstructure of explosive joints: Example of titanium to steel cladding
    Song, J. and Kostka, A. and Veehmayer, M. and Raabe, D.
    Materials Science and Engineering A 528 2641-2647 (2011)
    The microstructure of explosive cladding joints formed among parallel Ti and steel plates was examined by electron microscopy. The bonding interface and the bulk materials around it form pronounced hierarchical microstructures. This hierarchy is characterized by the following features: at the mesoscopic scale of the hierarchy a wavy course of the interface characterizes the interface zone. This microstructure level is formed by heavy plastic shear waves (wavelength≈0.5mm) which expand within the two metal plates during the explosion parallel to the bonding interface. At the micro-scale range, intermetallic inclusions (size≈100-200μm) are formed just behind the wave crests on the steel side as a result of partial melting. Electron diffraction revealed FeTi and metastable Fe9.64Ti0.36. Most of the observed phases do not appear in the equilibrium Fe-Ti phase diagram. These intermetallic inclusions are often accompanied by micro-cracks of similar dimension. At the smallest hierarchy level we observe a reaction layer of about 100-300nm thickness consisting of nano-sized grains formed along the entire bonding interface. Within that complex hierarchical micro- and nanostructure, the mesoscopic regime, more precisely the type and brittleness of the intermetallic zones, seems to play the dominant role for the mechanical behavior of the entire compound. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.11.092
  • 2011 • 45 Integration of 3-D cell cultures in fluidic microsystems for biological screenings
    Witte, H. and Stubenrauch, M. and Fröber, U. and Fischer, R. and Voges, D. and Hoffmann, M.
    Engineering in Life Sciences 11 140-147 (2011)
    A life support system for the cultivation of adherent 2-D and scaffold-based 3-D cell cultures in a microfluidic device, a Bio-Micro-Electro-Mechanical System (BioMEMS) is presented. The miniaturization level and system set-up allow incubator-independent operation modes and long-term experiments with real-time microscope observation. A dedicated seeding procedure for adherent cells into the microstructures is one key issue of the BioMEMS developed. Several seeding methods for the cells were evaluated. Biocompatibility of all materials, surfaces and methods could be demonstrated. First experiments with several cell types show the feasibility of the approach employing standard laboratory protocols. At present, the modular design and set-up offer a broad application spectrum as well as its future extension to e.g. cultivation of other cell types, coupled cultivation chambers and the implementation of other manipulation or analysis components. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/elsc.201000045
  • 2011 • 44 Long room-temperature electron spin lifetimes in bulk cubic GaN
    Buß, J.H. and Rudolph, J. and Schupp, T. and As, D.J. and Lischka, K. and Hägele, D.
    Proceedings of SPIE - The International Society for Optical Engineering 7937 (2011)
    We report on very long electron spin lifetimes in cubic GaN measured by time-resolved Kerr-rotation-spectroscopy. The spin coherence times with and without external magnetic field exceed 500 ps at room temperature, despite a high n-type doping level of more than 1019 cm-3 in the bulk sample under investigation. Our findings are therefore highly relevant for spin optoelectronics in the blue wavelength regime. The spin lifetimes are found to be almost temperature independent in accord with a prediction for degenerate electron gases of Dyakonov and Perel from 1972. These results are discussed also in comparison to wurtzite GaN, which shows much shorter spin lifetimes and a dependence of spin lifetimes on the spin orientation. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstractdoi: 10.1117/12.873395
  • 2011 • 43 Manufacturing of dies from hardened tool steels by 3-axis micromilling
    Biermann, D. and Baschin, A. and Krebs, E. and Schlenker, J.
    Production Engineering 5 209-217 (2011)
    In this paper the results of an experimental investigation to analyze the machinability of a hardened, carbide-rich cold-work tool steel 1.2379 (approx.62 HRC) with coated micro end-milling cutters are discussed. Fundamental experiments were performed to determine a cutting-parameter set, which enables an economic manufacturing of dies by 3-axis micromilling with commercially available cemented-carbide tools. The evaluation of the applicability of different tool types is conducted by analyzing the process forces, the tool wear, the surface quality, the material removal rate, and the entire chip volume. Design of experiments was used to significantly reduce the number of experiments and to model the active and passive forces. Concerning the design of tools for the micromilling of such difficult-to-machine materials, it is shown that cemented-carbide tools with robust cutting edges are applicable for this kind of machining. Furthermore, test microstructures were manufactured with the intention of validating the determined cutting-parameter set in combination with the selected tool types. In addition, the dimension and shape accuracy of the microstructures are analyzed. © 2011 German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-010-0293-7
  • 2011 • 42 Microfabrication by optical tweezers
    Ghadiri, R. and Weigel, T. and Esen, C. and Ostendorf, A.
    Proceedings of SPIE - The International Society for Optical Engineering 7921 (2011)
    A new method to fabricate microstructures built by polymer microparticles using a bottom-up technique is presented. The microstructures find broad application in micro-fluidics technology, photonics and tissue-engineering. The handling of the particles is realized by a holographic optical tweezers setup, ensuring the precise allocation of the particles to the desired structure. A biochemical technique ensures that the structure remains stable independent of the laser source. We show that with this method complex two-dimensional durable structures can be assembled and cannot be separated by optical forces. The structures are extendable during the entire fabrication process and can be linked to further particles and structures as desired. © 2011 SPIE.
    view abstractdoi: 10.1117/12.887264
  • 2011 • 41 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 • 40 Microstructure-based description of the deformation of metals: Theory and application
    Helm, D. and Butz, A. and Raabe, D. and Gumbsch, P.
    JOM 63 26-33 (2011)
    Aiming for an integrated approach to computational materials engineering in an industrial context poses big challenges in the development of suitable materials descriptions for the different steps along the processing chain. The first key component is to correctly describe the microstructural changes during the thermal and mechanical processing of the base material into a semi-finished product. Explicit representations of the microstructure are most suitable there. The final processing steps and particularly component assessment then has to describe the entire component which requires homogenized continuum mechanical representations. A key challenge is the step in between, the determination of the (macroscopic) materials descriptions from microscopic structures. This article describes methods to include microstructure into descriptions of the deformation of metal, and demonstrates the central steps of the simulation along the processing chain of an automotive component manufactured from a dual phase steel. © 2011 TMS.
    view abstractdoi: 10.1007/s11837-011-0056-8
  • 2011 • 39 Microstructures and properties of laser cladding NiTi alloy with W for biomedical applications
    Yan, X.J. and Gugel, H. and Huth, S. and Theisen, W.
    Materials Letters 65 2934-2936 (2011)
    The feasibility of improving the radiopacity of NiTi by means of Nd:YAG laser cladding is investigated in the present study. Fine elementary tungsten powder was pasted on NiTi sheets and then melted using laser. The resulting microstructure was analyzed by a scanning electron microscope (SEM) equipped with energy-dispersive X-ray (EDX) spectrometry. The results show that the solubility of tungsten in the NiTi-matrix is low and the excessive tungsten forms fine precipitates and W-rich compounds, which result in the average W content in the fusion zone up to 8 at.%. The laser-clad sample failed near the end of the plateau with a tensile strength of about 410 MPa. © 2011 Elsevier B.V. All Rights Reserved.
    view abstractdoi: 10.1016/j.matlet.2011.06.040
  • 2011 • 38 Molecular dynamics simulation study of microstructure evolution during cyclic martensitic transformations
    Kastner, O. and Eggeler, G. and Weiss, W. and Ackland, G.J.
    Journal of the Mechanics and Physics of Solids 59 1888-1908 (2011)
    Shape memory alloys (SMA) exhibit a number of features which are not easily explained by equilibrium thermodynamics, including hysteresis in the phase transformation and reverse shape memory in the high symmetry phase. Processing can change these features: repeated cycling can train the reverse shape memory effect, while changing the amount of hysteresis and other functional properties. These effects are likely to be due to formations of localised defects and these can be studied by atomistic methods. Here we present a molecular dynamics simulation study of such behaviour employing a two-dimensional, binary LennardJones model. Our atomistic model exhibits a symmetry breaking, displacive phase transition from a high temperature, entropically stabilised, austenite-like phase to a low temperature martensite-like phase. The simulations show transformations in this model material proceed by non-diffusive nucleation and growth processes and produce distinct microstructures. We observe the generation of persistent lattice defects during forward-and-reverse transformations which serve as nucleation centres in subsequent transformation processes. These defects interfere the temporal and spatial progression of transformations and thereby affect subsequent product morphologies. During cyclic transformations we observe accumulations of lattice defects so as to establish new microstructural elements which represent a memory of the previous morphologies. These new elements are self-organised and they provide a basis of the reversible shape memory effect in the model material. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmps.2011.05.009
  • 2011 • 37 Passively mode-locked two section laser diode with intracavity dispersion control
    Schlauch, T. and Balzer, J.C. and Hofmann, M.R. and Klehr, A. and Erbert, G. and Tränkle, G.
    Proceedings of SPIE - The International Society for Optical Engineering 7937 (2011)
    Ultrashort laser pulses with a duration of 200 fs were obtained from a passively modelocked external cavity diode laser at 830 nm emission wavelength. By intracavity dispersion control the spectral bandwidth is increased and the emitted pulses are compressed externaly by a grating compressor. A tapered amplifier is used to achieve peak powers of up to 2.5 kW. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstractdoi: 10.1117/12.873831
  • 2011 • 36 Phase-field modelling of microstructure evolution in solids: Perspectives and challenges
    Steinbach, I. and Shchyglo, O.
    Current Opinion in Solid State and Materials Science 15 87-92 (2011)
    Phase-field modelling is maturing to become a universal tool for modelling microstructure evolution in materials science. In solidification applications it has been proven to give quantitative predictions. In solid state, however, the mechanisms of phase transformation and microstructure evolution in are much more involved due to the existence of mechanical interactions, high interface anisotropies, large densities of defects, and retarded kinetics of diffusion and growth. The paper gives an overview of actual developments in phase-field modelling of solid-state microstructure evolution and highlights necessary directions of future development in order to meet the challenge of quantitative predictions. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.cossms.2011.01.001
  • 2011 • 35 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 • 34 Role of oxygen on microstructure and thermoelectric properties of silicon nanocomposites
    Schierning, G. and Theissmann, R. and Stein, N. and Petermann, N. and Becker, A. and Engenhorst, M. and Kessler, V. and Geller, M. and Beckel, A. and Wiggers, H. and Schmechel, R.
    Journal of Applied Physics 110 (2011)
    Phosphorus-doped silicon nanopowder from a gas phase process was compacted by DC-current sintering in order to obtain thermoelectrically active, nanocrystalline bulk silicon. A density between 95 and 96 compared to the density of single crystalline silicon was achieved, while preserving the nanocrystalline character with an average crystallite size of best 25 nm. As a native surface oxidation of the nanopowder usually occurs during nanopowder handling, a focus of this work is on the role of oxygen on microstructure and transport properties of the nanocomposite. A characterization with transmission electron microscopy (TEM) showed that the original core/shell structure of the nanoparticles was not found within the sintered nanocomposites. Two different types of oxide precipitates could be identified by energy filtered imaging technique. For a detailed analysis, 3-dimensional tomography with reconstruction was done using a needle-shaped sample prepared by focused ion beam (FIB). The 3-dimensional distribution of silicon dioxide precipitates confirmed that the initial core/shell structure breaks down and precipitates are formed. It is further found that residual pores are exclusively located within oxide precipitates. Thermoelectric characterization was done on silicon nanocomposites sintered between 960 C and 1060 C with varying oxygen content between room temperature and 950 C. The higher sintering temperature led to a better electrical activation of the phosphorus dopant. The oxidic precipitates support densification and seem to be able to reduce the thermal conductivity therefore enhancing thermoelectric properties. A peak figure of merit, zT, of 0.5 at 950 C was measured for a sample sintered at 1060 C with a mean crystallite size of 46 nm. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3658021
  • 2011 • 33 Solid particle erosion of thermal spray and physical vapour deposition thermal barrier coatings
    Cernuschi, F. and Lorenzoni, L. and Capelli, S. and Guardamagna, C. and Karger, M. and Vaßen, R. and von Niessen, K. and Markocsan, N. and Menuey, J. and Giolli, C.
    Wear 271 2909-2918 (2011)
    Thermal barrier coatings (TBC) are used to protect hot path components of gas turbines from hot combustion gases. For a number of decades, in the case of aero engines TBCs are usually deposited by electron beam physical vapour deposition (EB-PVD). EB-PVD coatings have a columnar microstructure that guarantees high strain compliance and better solid particle erosion than PS TBCs. The main drawback of EB-PVD coating is the deposition cost that is higher than that of air plasma sprayed (APS) TBC. The major scientific and technical objective of the UE project TOPPCOAT was the development of improved TBC systems using advanced bonding concepts in combination with additional protective functional coatings. The first specific objective was to use these developments to provide a significant improvement to state-of-the-art APS coatings and hence provide a cost-effective alternative to EB-PVD. In this perspective one standard porous APS, two segmented APS, one EB-PVD and one PS-PVD™ were tested at 700°C in a solid particle erosion jet tester, with EB-PVD and standard porous APS being the two reference systems.Tests were performed at impingement angles of 30° and 90°, representative for particle impingement on trailing and leading edges of gas turbine blades and vanes, respectively. Microquartz was chosen as the erodent being one of the main constituents of sand and fly volcanic ashes. After the end of the tests, the TBC microstructure was investigated using electron microscopy to characterise the failure mechanisms taking place in the TBC.It was found that PS-PVD™ and highly segmented TBCs showed erosion rates comparable or better than EB-PVD samples. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2011.06.013
  • 2011 • 32 Spin properties of interfaces with organic semiconductors studied by spin- and time-resolved two-photon photoemission
    Cinchetti, M. and Koffler, K. and Steil, S. and Großmann, N. and Aeschlimann, M.
    7937 (2011)
    We review our recent work in the field of organic spintronics, and show that the spin- and time-resolved two-photon photoemission (STR-2PPE) technique can be used to obtain quantitative information about the spin-dependent properties of hybrid organic-inorganic interfaces, as well as about the spin-dependent transport in organic semiconductors. In addition, we present STR-2PPE measurements performed on the Co-copper phthalocyanine (CuPc) system at different temperatures to investigate the microscopic processes repsonsible for the spin-polarization decay in organic semiconductors and at interfaces with such materials. We found no significant temperature dependence of the spin-injection efficiency across the Co-CuPc interface as well as of the spin-polarization decay length in CuPc. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstractdoi: 10.1117/12.876459
  • 2011 • 31 Studies on synthesis and characterization of mo based in situ composite by silicothermy co-reduction process
    Paul, B. and Chakraborty, S.P. and Kishor, J. and Sharma, I.G. and Suri, A.K.
    Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science 42 700-710 (2011)
    The results of an in situ synthesis of refractory metal-intermetallic composite (RMIC), Mo-16Cr-4Si (wt pct) multiphase alloy and its characterization, are presented in this study. The alloy was prepared from the oxides of molybdenum and chromium by their co-reduction with Si metal powder as a reductant. The exothermic nature of these reactions resulted in the formation of consolidated composite as a product in a single step. The thermodynamic aspects of exothermic reactions were studied by thermogravimetry/differential thermal analyzer. As-reduced alloys were remelted by arc melting and heat treated to obtain a homogenous microstructure. The evolution of phases and microstructures qA studied by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectrum analysis. The multiphase alloy consisted of Mo3Si and discontinuous (Mo, Cr) (ss) phase with a volume percentage of 28 pct. The synthesized alloys were characterized with respect to composition, phases, microstructure, hardness, and oxidation behavior. © The Minerals Metals & Materials Society and ASM International 2011.
    view abstractdoi: 10.1007/s11663-011-9513-6
  • 2011 • 30 The impact of processing on microstructure, single-phase properties and wear resistance of MMCs
    Hill, H. and Weber, S. and Huth, S. and Niederhofer, P. and Theisen, W.
    Wear 271 1895-1902 (2011)
    High mechanical loads and abrasive wear are boundary conditions for many tooling materials used in modern economy. One of the standard materials used in high abrasive environments is tool steel, or more specifically cold work tool steel. Wear resistance can be increased by adding hard phases like titanium carbides (TiC) to obtain a particle-reinforced metal-matrix composite (MMC) that can be produced either by hot isostatic pressing (HIP) or super solidus liquid-phase sintering (SLPS). Starting from the same raw materials, these two processes lead to different microstructures. The amount and dispersion of the hard phases as well as their hardness and fracture toughness control the wear resistance. In this study the hardness, Young's modulus and the fracture toughness of titanium carbides in Fe-base MMCs have been investigated by nanoindentation measurements. Owing to the small size of the TiC particles, the matrix properties have a pronounced effect on the results. This effect was studied in dependence on the indentation depth to enable the discussion of both the apparent and the real changes in the properties resulting from the production process and the heat treatment. This contribution also discusses the link between the microstructural properties and the macroscopic wear behavior. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2010.11.031
  • 2011 • 29 The relation between shear banding, microstructure and mechanical properties in Mg and Mg-Y alloys
    Sandlöbes, S. and Schestakow, I. and Yi, S. and Zaefferer, S. and Chen, J. and Friák, M. and Neugebauer, J. and Raabe, D.
    Materials Science Forum 690 202-205 (2011)
    The formation of deformation-induced shear bands plays an important role for the room temperature deformation of both, Mg and Mg-Y alloys, but the formation and structure of shear bands is distinctively different in the two materials. Due to limited deformation modes in pure Mg, the strain is localized in few shear bands leading to an early failure of the material during cold deformation. Contrarily, Mg-RE (RE: rare earth) alloys exhibit a high density of homogeneously distributed local shear bands during deformation at room temperature. A study of the microstructure of the shear bands by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) at different strains was performed. These investigations give insight into the formation of shear bands and their effects on the mechanical behaviour of pure Mg and Mg-3Y. Since in pure Mg mainly extension twinning and basal <a> dislocation slip are active, high stress fields at grain resp. twin boundaries in shear bands effect fast growth of the shear bands. In Mg-RE alloys additionally contraction and secondary twinning and pyramidal <c+a> dislocation slip are active leading to the formation of microscopic shear bands which are limited to the boundary between two grains. The effects of shear bands on the mechanical behaviour of pure Mg and Mg-RE alloys are discussed with respect to their formation and growth. © (2011) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/MSF.690.202
  • 2011 • 28 Ultrafast few-fermion dynamics in single self-assembled InGaAs/GaAs quantum dots
    Betz, M. and Zecherle, M. and Ruppert, C. and Clark, E.C. and Abstreiter, G. and Finley, J.J.
    Proceedings of SPIE - The International Society for Optical Engineering 7937 (2011)
    We review a comprehensive study of the ultrafast optoelectronic properties of a single self-assembled InGaAs/GaAs quantum dot. While manipulation of the artificial atom relies on two widely and independently tunable picoseconds pulse trains, sensitive readout is achieved via the ~pA photocurrent of the diode device. In particular, the absorption changes after occupation of an s-shell exciton reveal a biexciton absorption line as well as previously unobserved p-shell transitions in the presence of s-shell population. In addition, time-resolved data directly maps the picosecond tunneling times of electrons and holes out of the dot. Beyond these incoherent phenomena, we also realize coherent QD manipulations. Those comprise well-known excitonic Rabi-oscillations as well as single-pulse biexciton generation and conditional Rabi-oscillations of the exciton-biexciton transition after deterministic exciton preparation. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstractdoi: 10.1117/12.873249
  • 2010 • 27 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 • 26 Analysis of local microstructure after shear creep deformation of a fine-grained duplex γ-TiAl alloy
    Peter, D. and Viswanathan, G.B. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 58 6431-6443 (2010)
    The present work characterizes the microstructure of a hot-extruded Ti-45Al-5Nb-0.2B-0.2C (at.%) alloy with a fine-grained duplex microstructure after shear creep deformation (temperature 1023 K; shear stress 175 MPa; shear deformation 20%). Diffraction contrast transmission electron microscopy (TEM) was performed to identify ordinary dislocations, superdislocations and twins. The microstructure observed in TEM is interpreted taking into account the contribution of the applied stress and coherency stresses to the overall local stress state. Two specific locations in the lamellar part of the microstructure were analyzed, where either twins or superdislocations provided c-component deformation in the L10 lattice of the γ phase. Lamellar γ grains can be in soft and hard orientations with respect to the resolved shear stress provided by the external load. The presence of twins can be rationalized by the superposition of the applied stress and local coherency stresses. The presence of superdislocations in hard γ grains represents indirect evidence for additional contributions to the local stress state associated with stress redistribution during creep. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.08.005
  • 2010 • 25 Axial-torsional thermomechanical fatigue of a near-γ TiAl-alloy
    Brookes, S.P. and Kühn, H.-J. and Skrotzki, B. and Klingelhöffer, H. and Sievert, R. and Pfetzing, J. and Peter, D. and Eggeler, G.
    Materials Science and Engineering A 527 3829-3839 (2010)
    The uniaxial, torsional and axial-torsional thermomechanical fatigue (TMF) behavior of the near-γ TiAl-alloy TNB-V5 was investigated. TMF tests were performed at 400-800°C with mechanical strain amplitudes ranging from 0.15% to 0.7%. The tests were conducted thermomechanically in-phase (IP) and out-of-phase (OP).For the same lifetimes, uniaxial IP tests required the highest strain amplitudes, while OP test conditions were most damaging and needed the lowest strain amplitudes. The Mises equivalent mechanical strain amplitudes of pure torsional tests were found in between uniaxial in-phase and out-of-phase tests for the same lifetimes. The non-proportional multiaxial out-of-phase test showed a lower lifetime at the same equivalent mechanical strain amplitude compared to the other types of tests.The microstructure has been characterized applying electron microscopy and microstructural parameters such as fraction of twinned grains, grain size, lamellar distance and dislocation density have been quantified. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.03.073
  • 2010 • 24 Can local hot spots induce α2/γ lamellae during incomplete massive transformation of γ-TiAl alloys?
    Fischer, F.D. and Cha, L. and Dehm, G. and Clemens, H.
    Intermetallics 18 972-976 (2010)
    Oil quenching is applied to a Ti-45Al-7.5b alloy (in at%) in order to investigate the occurrence of local "hot spots" during rapid cooling from the single α-phase region. After quenching the microstructure consists of large α2-Ti3Al grains and a small volume fraction of massively transformed γm-TiAl particles. The majority of the γm-particles show a featureless α2/γm interface, whereas in the immediate vicinity of some γm-particles lamellar α2/γ-structures exist, exhibiting a Blackburn orientation relationship. The extension into the α2-parent grain is in the range of 1-10 μm. In this study it is investigated, if the diffusion-controlled formation of the γ-laths is triggered by the release of latent heat during the α → γm transformation, which leads to a local increase in temperature by a hot spot effect. A theoretical model is presented, which describes the temperature conditions around the hot spot. The model predicts that the life time of the hot spot is much too short to generate γ-laths observed with a length of 1-10 μm. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.01.017
  • 2010 • 23 Combinatorial investigation of Hf-Ta thin films and their anodic oxides
    Mardare, A.I. and Ludwig, Al. and Savan, A. and Wieck, A.D. and Hassel, A.W.
    Electrochimica Acta 55 7884-7891 (2010)
    A co-sputtering technique was used for the fabrication of a thin film combinatorial library (Hf-21 at.% Ta to 91 at.% Ta) based on alloying of Hf and Ta. The microstructure and crystallography of individual metallic alloy compositions were analyzed using SEM and XRD mapping, respectively. Three different zones of microstructure were identified within the range of alloys, going from hexagonal to tetragonal through an intermediate amorphous region. The local oxidation of Hf-Ta parent metal alloys at different compositions was investigated in steps of 1 at.% using an automated scanning droplet cell in the confined droplet mode. Potentiodynamic anodisation cycles combined with in situ impedance spectroscopy provide basic knowledge regarding the oxide formation and corresponding electrical properties. Dielectric constants were mapped for the entire composition range and XPS depth profiles allowed investigation of the oxide compositions. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2010.03.066
  • 2010 • 22 Configurational forces and couples in fracture mechanics accounting for microstructures and dissipation
    Stumpf, H. and Makowski, J. and Hackl, K.
    International Journal of Solids and Structures 47 2380-2389 (2010)
    Configurational forces and couples acting on a dynamically evolving fracture process region as well as their balance are studied with special emphasis to microstructure and dissipation. To be able to investigate fracture process regions preceding cracks of mode I, II and III we choose as underlying continuum model the polar and micropolar, respectively, continuum with dislocation motion on the microlevel. As point of departure balance of macroforces, balance of couples and balance of microforces acting on dislocations are postulated. Taking into account results of the second law of thermodynamics the stress power principle for dissipative processes is derived. Applying this principle to a fracture process region evolving dynamically in the reference configuration with variable rotational and crystallographic structure, the configurational forces and couples are derived generalizing the well-known Eshelby tensor. It is shown that the balance law of configurational forces and couples reflects the structure of the postulated balance laws on macro- and microlevel: the balance law of configurational forces and configurational couples are coupled by field variable, while the balance laws of configurational macro- and microforces are coupled only by the form of the free energy. They can be decoupled by corresponding constitutive assumption. Finally, it is shown that the second law of thermodynamics leads to the result that the generalized Eshelby tensor for micropolar continua with dislocation motion consists of a non-dissipative part, derivable from free and kinetic energy, and a dissipative part, derivable from a dissipation pseudo-potential. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijsolstr.2010.04.032
  • 2010 • 21 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 • 20 Design and characterization of novel wear resistant multilayer CVD coatings with improved adhesion between Al2O3 and Ti(C,N)
    Garcia, J. and Pitonak, R. and Weißenbacher, R. and Köpf, A. and Soldera, F. and Suarez, S. and Miguel, F. and Pinto, H. and Kostka, A. and Mücklich, F.
    Advanced Engineering Materials 12 929-934 (2010)
    Multilayer CVD coatings for high speed cutting applications were designed to achieve high wear and heat resistance during machining of steel alloys. In this work the microstructure and cutting performance of these novel multilayer CVD coatings are investigated and compared with standard CVD multilayer coatings. 3D-FIB tomography is used to characterize the microstructure of the layers, especially the transition between the Ti(C,N) and the Al 2O3 layer. The 3D reconstruction of the surface of the Ti(C,N) layer shows the formation of protruded Ti(C,N) grains with a very particular architecture, which penetrate into the Al2O3 top-layer, providing a mechanical anchoring between both layers. Cemented carbides coated with the novel CVD multilayer present reduced crater and flank wear as well as improved adherence between the Al2O3 top-layer and the Ti(C,N) layer leading to a dramatic improvement of cutting performance. The microstructure and cutting performance of multilayer CVD coatings with a novel transition between the Ti(C,N) and the Al 2O3 layer are investigated. 3D-FIB tomography shows the formation of protruded Ti(C,N) grains with a particular architecture, which penetrate into the Al2O3 top-layer, providing a mechanical anchoring between both layers. Cutting tools coated with the novel CVD multilayer show dramatic improvement of cutting performance, due to reduced crater and flank wear and improved adherence between the Ti(C,N) and the Al 2O3 top-layer. © 2010 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201000130
  • 2010 • 19 Diffusivities of an Al-Fe-Ni melt and their effects on the microstructure during solidification
    Zhang, L. and Du, Y. and Steinbach, I. and Chen, Q. and Huang, B.
    Acta Materialia 58 3664-3675 (2010)
    A systematical investigation of the diffusivities in an Al-Fe-Ni melt was presented. Based on the experimental and theoretical data about diffusivities, the temperature- and composition-dependent atomic mobilities were evaluated for the elements in Al-Ni, Al-Fe, Fe-Ni and Al-Fe-Ni melts via an effective approach. Most of the reported diffusivities can be reproduced well by the obtained atomic mobilities. In particular, for the first time the ternary diffusivity of the liquid in a ternary system is described in conjunction with the established atomic mobilities. The effect of the atomic mobilities in a liquid on microstructure and microsegregation during solidification was demonstrated with one Al-Ni binary alloy. The simulation results indicate that accurate databases of mobilities in the liquid phase are much needed for the quantitative simulation of microstructural evolution during solidification by using various approaches, including DICTRA and the phase-field method. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.03.002
  • 2010 • 18 Effect of plasma treatment on adhesion of DLC layers to steels
    Tillmann, W. and Vogli, E. and Momeni, S.
    Materials Science Forum 638-642 812-817 (2010)
    Diamond like carbon layers play a key role in industrial applications. However the layers quality deteriorates often due to insufficient interfacial adhesion. In this research work a prior plasma treatment of steel substrates was employed aimed to improve the interfacial adhesion of DLC-layers to steel substrates. Three different kinds of steels were employed and their microstructures as well as their compositions before and after plasma treatment were analyzed. The interfacial adhesion of DLC layers on the non-nitrided and nitrided steels was observed and the influence of the steel microstructure on the interfacial adhesion was studied. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/MSF.638-642.812
  • 2010 • 17 Effects of annealing on the microstructure and the mechanical properties of EB-PVD thermal barrier coatings
    Zotov, N. and Bartsch, M. and Chernova, L. and Schmidt, D.A. and Havenith, M. and Eggeler, G.
    Surface and Coatings Technology 205 452-464 (2010)
    The effects of thermal annealing at 1000°C in air on the microstructure and the mechanical properties (Young's modulus and hardness) of thermal barrier coatings consisting of a 4mol% Y2O3 partially stabilized ZrO2 top coat and a NiCoCrAlY bond coat, deposited by electron beam physical vapour deposition on nickel-based superalloy IN 625, have been investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), image analysis and nanoindentation. During annealing, the ceramic top coat undergoes sintering and recrystallization. These processes lead to stress relaxation, an increase of the intra-columnar porosity and the number of large pores as measured by image analysis of SEM micrographs. An increase of the grain size of the γ-phase in the bond coat, accompanied by changes in the morphology of γ-grains with annealing time, is also observed. Correlations between these microstructural changes in the top coat and the bond coat and their mechanical properties are established and discussed. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2010.07.008
  • 2010 • 16 Existence, uniqueness and stability in linear cosserat elasticity for weakest curvature conditions
    Jeong, J. and Neff, P.
    Mathematics and Mechanics of Solids 15 78-95 (2010)
    We investigate the weakest possible constitutive assumptions on the curvature energy in linear Cosserat models still providing for existence, uniqueness and stability. The assumed curvature energy is μL2 c ∥dev sym ∇axl A∥2 where axl A is the axial vector of the skewsymmetric microrotation A ∈ so(3) and dev is the orthogonal projection on the Lie-algebra sl(3) of trace free matrices. The proposed Cosserat parameter values coincide with values adopted in the experimental literature by R. S. Lakes. It is observed that unphysical stiffening for small samples is avoided in torsion and bending while size effects are still present. The number of Cosserat parameters is reduced from six to four. One Cosserat coupling parameter μc &gt; 0 and only one length scale parameter L c &gt; 0. Use is made of a new coercive inequality for conformal Killing vectorfields. An interesting point is that no (controversial) essential boundary conditions on the microrotations need to be specified; thus avoiding boundary layer effects. Since the curvature energy is the weakest possible consistent with non-negativity of the energy, it seems that the Cosserat couple modulus μc &gt; 0 remains a material parameter independent of the sample size which is impossible for stronger curvature expressions. © 2010 SAGE Publications.
    view abstractdoi: 10.1177/1081286508093581
  • 2010 • 15 FE 2-simulation of microheterogeneous steels based on statistically similar RVEs
    Balzani, D. and Schröder, J. and Brands, D.
    IUTAM Bookseries 21 15-28 (2010)
    A main problem of direct homogenization methods is the high computational cost, when we have to deal with large random microstructures. This leads to a large number of history variables which needs a large amount of memory, and moreover a high computation time. We focus on random microstructures consisting of a continuous matrix phase with a high number of embedded inclusions. In this contribution a method is presented for the construction of statistically similar representative volume elements (SSRVEs) which are characterized by a much less complexity than usual random RVEs in order to obtain an efficient simulation tool. The basic idea of the underlying procedure is to find a simplified SSRVE, whose selected statistical measures under consideration are as close as possible to the ones of the original microstructure. © 2010 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/978-90-481-9195-6-2
  • 2010 • 14 Formation, stability and crystal structure of the r phase in Mo-Re-Si alloys
    Bei, H. and Yang, Y. and Viswanathan, G.B. and Rawn, C.J. and George, E.P. and Tiley, J. and Chang, Y.A.
    Acta Materialia 58 6027-6034 (2010)
    The formation, stability and crystal structure of the σ phase in Mo-Re-Si alloys were investigated. Guided by thermodynamic calculations, six critically selected alloys were arc melted and annealed at 1600 °C for 150 h. Their as-cast and annealed microstructures, including phase fractions and distributions, the compositions of the constituent phases and the crystal structure of the r phase were analyzed by thermodynamic modeling coupled with experimental characterization by scanning electron microscopy, electron probe microanalysis, X-ray diffraction and transmission electron microscopy. Two key findings resulted from this work. One is the large homogeneity range of the r phase region, extending from binary Mo-Re to ternary Mo-Re-Si. The other is the formation of a r phase in Mo-rich alloys either through the peritectic reaction of liquid + Moss → σ or primary solidification. These findings are important in understanding the effects of Re on the microstructure and providing guidance on the design of Mo-Re-Si alloys. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.07.020
  • 2010 • 13 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 • 12 Influence of the microstructure of gold-zirconia yolk-shell catalysts on the CO oxidation activity
    Pandey, A.D. and Güttel, R. and Leoni, M. and Schüth, F. and Weidenthaler, C.
    Journal of Physical Chemistry C 114 19386-19394 (2010)
    The gold-zirconia yolk-shell system is an interesting catalyst for CO oxidation. The size distribution of the gold nanoparticles is very narrow, and they are well separated from each other also after treatment at high temperature, which is due to their encapsulation in crystalline zirconia hollow spheres. Because this allows thermal and chemical treatment without affecting the size distribution, different defect structures of the gold nanoparticles can be induced, and the effect on catalytic activity can be investigated. Line profile analysis of the powder diffraction data based on the whole powder pattern modeling approach was used to determine the domain size distribution and lattice defects present in this two-phase system. The influence of different diffractometer setups on the results of the line profile analysis was also investigated. Variation of the chemical and thermal treatment procedures allowed altering the microstructure of the system. The resulting catalysts showed substantial variation in the activity for CO oxidation. Lower dislocation densities and less stacking faults result in decreased catalytic activity. These contributions to activity could be studied without any superimposed size effect due to the constant gold particle sizes. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp106436h
  • 2010 • 11 Mechanical and microstructural single-crystal Bauschinger effects: Observation of reversible plasticity in copper during bending
    Demir, E. and Raabe, D.
    Acta Materialia 58 6055-6063 (2010)
    We study the Bauschinger effect on a bent and straightened micro-sized single-crystal copper beam (width: 8.64 lm; thickness: 7.05 lm) over three consecutive cycles. The reverse yield strengths (straightening step) are much smaller than those in forward loading (bending step). An upper bound estimate shows a load drop of 73% (1st cycle), 76% (2nd cycle) and 83% (3rd cycle) relative to the forward yield stress. Electron backscatter diffraction reveals a dramatic reduction in the bending-induced misorientation gradients upon load reversal (straightening), documenting an unexpected form of microstructure reversibility. The observed Bauschinger softening is interpreted in terms of two effects. The first consists of internal backstresses that support load reversal. They are created by polarized dislocation arrays that are accumulated during forward bending. The second effect is the reduced requirement to activate dislocation sources during reverse loading as the dislocations that were stored during bending did not participate much in cross-hardening and, hence, serve as mobile dislocations upon reverse loading. After straightening the misorientation gradients are largely removed but the non-polarized dislocations remain. We therefore introduce a revised terminology, namely the "mechanical Bauschinger effect" and the "microstructural Bauschinger effect". The former term describes a yield stress drop and the latter one the degree of microstructure reversibility upon load path changes. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.07.023
  • 2010 • 10 Microstructure and wear properties of novel sintered cold work steel and related particle reinforced composite materials
    Weber, S. and Li, J.R. and Theisen, W.
    Materials Science and Technology 26 1494-1502 (2010)
    In the field of wear resistant materials it is known, that for certain applications steel based composites produced by powder metallurgy are beneficial due to a higher wear resistance compared to conventional cast materials. Early developments of these high wear resistant MMC were dependent on hot isostatic pressing but latest experimental findings nowadays also allow for a production by liquid phase sintering. Several materials systems have already been investigated which, however, lack of a sufficient hardness in the as sintered state. Especially for wear resistant coatings it would be beneficial to avoid a separate hardening of a coated component. The low hardness in the as sintered state is related to the transformation kinetics of the metallic matrix of the coating materials, typically leading to a comparatively soft pearlitic microstructure, if the cooling rate is too low. The use of a nickel alloyed PM cold work steel as matrix material avoids this restriction, as the formation of pearlite and bainite is delayed significantly, improving the hardenability of the steel. Adding coarse hard particles of chromium carbide (Cr 3C2), aluminium zirconium oxide (AlZrO) or titanium carbide (TiC) to the steel powder, composite materials with a high abrasive wear resistance can be obtained by liquid phase sintering. The development of these materials, supported by thermodynamic calculations, is presented here together with results of the microstructural investigation and wear tests. © 2010 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1179/026708309X12506933872982
  • 2010 • 9 On the characterization of recrystallized fraction using electron backscatter diffraction: A direct comparison to local hardness in an IF steel using nanoindentation
    Dziaszyk, S. and Payton, E.J. and Friedel, F. and Marx, V. and Eggeler, G.
    Materials Science and Engineering A 527 7854-7864 (2010)
    Recrystallized fraction was characterized in a Ti-stabilized interstitial-free (IF) steel by electron backscatter diffraction (EBSD), optical metallography, and hardness-based techniques. EBSD and nanoindentation were performed on overlapping areas to assess the agreement between standard methods of EBSD analysis of deformation microstructures and local hardness. The results of the study indicate that carefully implemented misorientation-based techniques may be used to effectively determine recrystallization fraction, to better agreement with the local recrystallization state than EBSD image quality, optical metallography, or microhardness-based techniques. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.08.063
  • 2010 • 8 Phase-pure Cu,Zn,Al hydrotalcite-like materials as precursors for copper rich Cu/ZnO/Al2O3 catalysts
    Behrens, M. and Kasatkin, I. and Kühl, S. and Weinberg, G.
    Chemistry of Materials 22 386-397 (2010)
    A series of hydrotalcite-like (htl) compounds of the general composition (Cu,Zn)1-xAlx(OH)2-(CO3) x/2 · m H2O was prepared with a fixed Cu:Zn ratio of 70:30. Phase pure samples could be obtained for 0.3 ≤ x ≥ 0.4. The htl precursors thermally decompose in multiple steps. After dehydration and dehydroxylation amorphous materials were obtained at 330 °C. Phase segregation during this mild calcination was only observed for samples with a Zn: Al ratio deviating strongly from 1:2. A mechanism for this low-temperature segregation process basing on the preformation of the ZnAl2O 4 phase within the amorphous material is proposed. Samples with Zn: Al ratios near 1:2 form an amorphous carbonate-modified mixed oxide "(CuO)x(ZnAl2O4)y" of homogeneous microstructure. Crystallization occurs upon carbonate decomposition at temperatures higher than 500 °C. Despite the small size of the Cu nanoparticles (around 7 nm) formed upon reduction, the accessible Cu surface area is below 5 m2g-1. This can be explained by the unfavorable microstructure of the resulting Cu/ZnAl2O4 catalyst: The Cu particles are to a large extent embedded in a compact oxide matrix. The applicability as Cu/ZnO/Al2O3 catalysts and the role of htl precursor phases in course of industrial catalyst preparation are discussed. © 2009 American Chemical Society.
    view abstractdoi: 10.1021/cm9029165
  • 2010 • 7 Plastic anisotropy of γ-TiAl revealed by axisymmetric indentation
    Zambaldi, C. and Raabe, D.
    Acta Materialia 58 3516-3530 (2010)
    Single crystals of γ-TiAl cannot be grown in the near-stoichiometric compositions that are present inside two-phase γ / α2-microstructures with attractive mechanical properties. Therefore, the single-crystal constitutive behavior of γ-TiAl was studied by nanoindentation experiments in single-phase regions of these γ / α2-microstructures. The experiments were characterized by orientation microscopy and atomic force microscopy to quantify the orientation-dependent mechanical response during nanoindentation. Further, they were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behavior of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. A convention was defined to unambiguously relate any indentation axis to a crystallographic orientation. Experiments and simulations were combined to study the orientation-dependent surface pile-up. The characteristic pile-up topographies were simulated throughout the unit triangle of γ-TiAl and represented graphically in the newly introduced inverse pole figure of pile-up patterns. Through this approach, easy activation of ordinary dislocation glide in stoichiometric γ-TiAl was confirmed independently from dislocation observation by transmission electron microscopy. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.02.025
  • 2010 • 6 Process design and monitoring for plasma sprayed abradable coatings
    Steinke, T. and Mauer, G. and Vaßen, R. and Stöver, D. and Roth-Fagaraseanu, D. and Hancock, M.
    Journal of Thermal Spray Technology 19 756-764 (2010)
    Abradable coatings in compressor and high-pressure stages of gas turbines must provide specific hardness and porosity values to achieve an optimal cut-in of the blade tips. A fractional factorial experimental plan was designed to investigate the influence of the plasma spraying parameters argon flow rate, current, spraying distance and powder feed rate on these properties of magnesia spinel. Based on the results, magnesia spinel coatings with low (∼400 HV 0.5), medium (∼600 HV 0.5) and high hardness (∼800 HV 0.5) could be reliably manufactured. Further incursion rig tests confirmed the dependence of the rub-in behavior and abradability on the coating characteristics and process parameters, respectively. Process monitoring was also applied during plasma spraying of magnesia spinel abradables on batches of turbine components. The recorded particle characteristics and coating properties showed a good reproducibility of the spraying process. © ASM International.
    view abstractdoi: 10.1007/s11666-010-9468-1
  • 2010 • 5 Stress, sheet resistance, and microstructure evolution of electroplated cu films during self-annealing
    Huang, R. and Robl, W. and Ceric, H. and Detzel, T. and Dehm, G.
    IEEE Transactions on Device and Materials Reliability 10 47-54 (2010)
    Electroplated copper films are known to change their microstructure due to the self-annealing effect. The self-annealing effect of electroplated copper films was investigated by measuring the time dependence of the film stress and sheet resistance for different layer thicknesses between 1.5 and 20 μm. While the sheet resistance was found to decrease as time elapsed, a size-dependent change in film stress was observed. Films with the thickness of 5 μm; and below decrease in stress, while thicker films initially reveal an increase in film stress followed by a stress relaxation at a later stage. This behavior is explained by the superposition of grain growth and grain-size-dependent yielding. © 2010 IEEE.
    view abstractdoi: 10.1109/TDMR.2009.2032768
  • 2010 • 4 Study of nanometer-scaled lamellar microstructure in a Ti-45Al-7.5Nb alloy - Experiments and modeling
    Fischer, F.D. and Waitz, T. and Scheu, Ch. and Cha, L. and Dehm, G. and Antretter, T. and Clemens, H.
    Intermetallics 18 509-517 (2010)
    Quenching of Ti-45 at%Al-7.5 at%Nb from the single α-phase region to room temperature followed by aging below the eutectoid temperature leads to the precipitation of ultra-fine γ-TiAl lamellae. In addition to an extensive experimental program, reported by Cha et al. in Intermetallics 16 (2008) 868-875, in this work a micromechanical and thermodynamical model is presented for the formation of γ-TiAl lamellae within the α2-Ti3Al parent phase. A global transformation condition allows to predict a thickness to length ratio in accordance with experimental observations. Furthermore, a local transformation condition offers the basis for a kinetic law. The modeling concept can be applied to similar problems of combined diffusive and displacive phase transformations. © 2009 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2009.09.012
  • 2010 • 3 Synthesis of defect-free single-phase bars of high-melting Laves phases through modified cold crucible levitation melting
    Voß, S. and Stein, F. and Palm, M. and Raabe, D.
    Materials Science and Engineering A 527 7848-7853 (2010)
    Laves phases are the most abundant intermetallic phases. However, their mechanical properties are only poorly understood due to difficulties in producing defect-free samples which are large enough for mechanical testing. By using a modified cold crucible levitation melting technique with in situ heat treatment and subsequent defined cooling, massive and large bars of several hundreds of grams of brittle and high-melting Laves phases were produced. Metallographic investigation revealed single- or near single-phase microstructures and a homogeneous chemical composition within the cast ingots with a diameter of 15. mm and of more than 100. mm length. With these high-quality bars it is now possible to prepare large samples that can be used for obtaining mechanical data of these phases. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.08.066
  • 2010 • 2 Toward process optimization in laser welding of metal to polymer
    Tillmann, W. and Elrefaey, A. and Wojarski, L.
    Materialwissenschaft und Werkstofftechnik 41 879-883 (2010)
    The joining technology of dissimilar lightweight materials between metals and polymer is essential for realizing cars with hybrid structures and for other engineering applications. These types of joints are still difficult to generate and their behaviour is not fully understood. Laser welding offers specific process advantages over conventional technologies, such as short process times, while providing optically and qualitatively valuable weld seams and imposing minimal thermal stress. Furthermore, the process is compatible with automation. This paper summarizes the efforts to attain suitable joint strengths with the stainless steel plate type S30400 and a Polyethylene Terephtalate Glycol (PETG) plastic sheet. The study considers the optimization of two important process parameters, namely laser power, and welding speed. Microstructure features, test of tensile shear strength, investigation of the fracture location, and morphology were used to evaluate the joint performance. The result indicates that there is an optimum value for laser power, which achieves a sufficient melting and heat transfer to the joint without decomposing the plastic sheet and hence, enables to obtain high joint strength. Moreover, a low welding speed is preferable in most combinations of welding parameters since it achieves an adequate melting and wetting of the polymer to the steel surface. Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201000674
  • 2010 • 1 Variational concepts with applications to microstructural evolution
    Fischer, F.D. and Svoboda, J. and Hackl, K.
    IUTAM Bookseries 21 79-90 (2010)
    In systems at elevated temperature the development of the microstructure of a material is controlled by diffusional and interface migration processes. As first step the description of the microstructure is reduced to a finite number of time-dependent characteristic parameters (CPs). Then the Thermodynamic Extremal Principle (TEP) is engaged to develop the evolution equations for these characteristic parameters. This treatment is demonstrated on a bamboo-structured material system predicting the spatial and time distribution of chemical composition as well as the deformation state. © 2010 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/978-90-481-9195-6-6