Scientific Output

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

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  • 2020 • 269 Improvement of Fatigue Strength in Lightweight Selective Laser Melted Alloys by In-Situ and Ex-Situ Composition and Heat Treatment
    Awd, M. and Johannsen, J. and Chan, T. and Merghany, M. and Emmelmann, C. and Walther, F.
    Minerals, Metals and Materials Series 115-126 (2020)
    Selective laser melting is a powder-bed-fusion process that is applied to different alloys. Thus, it is essential to study what are the different process variables that affect the static, quasi-static, and cyclic mechanical properties. In this contribution, two examples of alloys are introduced: AlSi (AlSi12, AlSi10Mg) and Ti-6Al-4V. The influence of controlled cooling and degassing mechanisms of residual gases is investigated by structural analysis in electron microscopy and X-ray computed tomography. Controlled cooling through platform heating or multi-exposure treatments increased the dendritic width in AlSi alloys and decomposed alpha prime in Ti-6Al-4V. The alteration was a cause for enhanced ductility and slowing of crack propagation. The cyclic deformation is tracked during mechanical testing and is simulated in FE software using a high-throughput methodology to calculate Woehler curves based on Fatemi-Socie damage parameters. The cyclic deformation simulation is in agreement with the experimental data and quantified cyclic damage using Fatemi-Socie parameters. © 2020, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/978-3-030-36296-6_11
  • 2020 • 268 Computational shape optimisation for a gradient-enhanced continuum damage model
    Guhr, F. and Sprave, L. and Barthold, F.-J. and Menzel, A.
    Computational Mechanics 65 1105-1124 (2020)
    An isotropic gradient-enhanced damage model is applied to shape optimisation in order to establish a computational optimal design framework in view of optimal damage distributions. The model is derived from a free Helmholtz energy density enriched by the damage gradient contribution. The Karush–Kuhn–Tucker conditions are solved on a global finite element level by means of a Fischer–Burmeister function. This approach eliminates the necessity of introducing a local variable, leaving only the global set of equations to be iteratively solved. The necessary steps for the numerical implementation in the sense of the finite element method are established. The underlying theory as well as the algorithmic treatment of shape optimisation are derived in the context of a variational framework. Based on a particular finite deformation constitutive model, representative numerical examples are discussed with a focus on and application to damage optimised designs. © 2020, The Author(s).
    view abstractdoi: 10.1007/s00466-019-01810-3
  • 2020 • 267 Dislocation-induced breakthrough of strength and ductility trade-off in a non-equiatomic high-entropy alloy
    Guo, W. and Su, J. and Lu, W. and Liebscher, C.H. and Kirchlechner, C. and Ikeda, Y. and Körmann, F. and Liu, X. and Xue, Y. and Dehm, G.
    Acta Materialia 185 45-54 (2020)
    In conventional metallic materials, strength and ductility are mutually exclusive, referred to as strength-ductility trade-off. Here, we demonstrate an approach to improve the strength and ductility simultaneously by introducing micro-banding and the accumulation of a high density of dislocations in single-phase high-entropy alloys (HEAs). We prepare two compositions (Cr10Mn50Fe20Co10Ni10 and Cr10Mn10Fe60Co10Ni10) with distinctive different stacking fault energies (SFEs) as experimental materials. The strength and ductility of the Cr10Mn50Fe20Co10Ni10 HEA are improved concurrently by grain refinement from 347.5 ± 216.1 µm to 18.3 ± 9.3 µm. The ultimate tensile strength increases from 543 ± 4 MPa to 621 ± 8 MPa and the elongation to failure enhances from 43±2% to 55±1%. To reveal the underlying deformation mechanisms responsible for such a strength-ductility synergy, the microstructural evolution upon loading is investigated by electron microscopy techniques. The dominant deformation mechanism observed for the Cr10Mn50Fe20Co10Ni10 HEA is the activation of micro-bands, which act both as dislocation sources and dislocation barriers, eventually, leading to the formation of dislocation cell structures. By decreasing grain size, much finer dislocation cell structures develop, which are responsible for the improvement in work hardening rate at higher strains (>7%) and thus for the increase in both strength and ductility. In order to drive guidelines for designing advanced HEAs by tailoring their SFE and grain size, we compute the SFEs of Cr10MnxFe70–xCo10Ni10 (10 ≤ x ≤ 60) based on first principles calculations. Based on these results the overall changes on deformation mechanism can be explained by the influence of Mn on the SFE. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.11.055
  • 2020 • 266 A generalized micromorphic approach accounting for variation and dispersion of preferred material directions
    von Hoegen, M. and Skatulla, S. and Schröder, J.
    Computers and Structures 232 (2020)
    Materials exhibiting a heterogeneous and non-uniform composition in terms of elastic and anisotropic properties such as biological tissues require special efforts to accurately describe their constitutive behavior. In contrast to classical models, micromorphic formulations can predict the macroscopically observable material response as originated from distinct scale-dependent micro-structural deformation mechanisms. This is facilitated by additional independent degrees of freedom and associated additional strain and stress quantities. Here, a generalized continuum is mathematically constructed from a macro-continuum and a micro-continuum which are both adequately coupled on kinematics and constitutive levels as well as by micro-boundary conditions. In view of biomechanical modeling, the potential of the formulation is studied for a number of academic examples characterized by an anisotropic material composition to elucidate the micromorphic material response as compared with the one obtained using a classical continuum mechanics approach. The results demonstrate the ability of the generalized continuum approach to address non-affine elastic reorientation of the preferred material direction in the macro-space and its dispersion in the micro-space as affecting deformation, strain and stress on the macroscopic level. In particular, if the anisotropy in the micromorphic formulation is solely linked to the extra degrees of freedom and associated strain and stress measures, the deformation for small and large strains is shown to be distinctly different to the classical response. Together with the ability to implicitly account for scale-dependent higher-order deformation effects in the constitutive law the proposed generalized micromorphic formulation provides an advanced description, especially for fibrous biological materials. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.compstruc.2017.11.013
  • 2020 • 265 Elasto-plastic large deformation analysis of multi-patch thin shells by isogeometric approach
    Huynh, G.D. and Zhuang, X. and Bui, H.G. and Meschke, G. and Nguyen-Xuan, H.
    Finite Elements in Analysis and Design 173 (2020)
    This paper studies elasto-plastic large deformation behaviour of thin shell structures using the isogeometric computational approach with the main focus on the efficiency in modelling the multi-patches and arbitrary material formulation. In terms of modelling, we employ the bending strip method to connect the patches in the structure. The incorporation of bending strips allows to eliminate the strict demand of the C1 continuity condition, which is postulated in the Kirchhoff-Love theory for thin shell, and therefore it enables us to use the standard multi-patch structure even with C0 continuity along the patch boundaries. Furthermore, arbitrary nonlinear material models such as hyperelasticity and finite strain plasticity are embedded in the shell formulation, from which a unified thin shell formulation can be achieved. In terms of analysis, the Bézier decomposition concept is used to retain the local support of the traditional finite element. The performance of the presented approach is verified through several numerical benchmarks. © 2020
    view abstractdoi: 10.1016/j.finel.2020.103389
  • 2020 • 264 Joining dissimilar thin-walled tubes by Magnetic Pulse Welding
    Lueg-Althoff, J. and Bellmann, J. and Hahn, M. and Schulze, S. and Gies, S. and Tekkaya, A.E. and Beyer, E.
    Journal of Materials Processing Technology 279 (2020)
    Welding dissimilar metal tubes attracts interest for a wide range of automotive, aeronautical, and plant engineering applications as well as other consumables. Hybrid driveshafts or structural elements can meet mechanical requirements at a reduced weight. However, joining materials with strongly different thermo-physical properties is a challenge for conventional fusion welding processes. In Magnetic Pulse Welding (MPW), the weld formation is based on the high-velocity collision between the joining partners, without additional heat input. This allows for the fabrication of sound “cold” welds. MPW of tubular parts is usually realized by the radial electromagnetic compression of the outer “flyer” part and the subsequent impact on the inner “parent” part. This impact represents a harsh loading for the parent, which therefore is usually designed as a thick-walled or solid part to avoid damage or unwanted deformations. To further increase the lightweight potential, the objective of the present manuscript is the comprehensive analysis of MPW with thin-walled parent parts. Experimental and analytical investigations are presented, which enable to reduce the parent thickness without affecting the joint strength. The approaches comprise the observation of the impact and deformation behavior by inline laser-based measurement technology as well as the development of adequate, re-usable mandrels to support the parent parts. The focus is on aluminum flyer parts, which are welded to steel and copper parent parts. Critical values for the parent wall thickness are deduced and recommendations for the process design of MPW with thin-walled tubes are given. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2019.116562
  • 2020 • 263 Crystal structure and composition dependence of mechanical properties of single-crystalline NbCo2 Laves phase
    Luo, W. and Kirchlechner, C. and Zavašnik, J. and Lu, W. and Dehm, G. and Stein, F.
    Acta Materialia 184 151-163 (2020)
    Extended diffusion layers of the cubic C15 and hexagonal C14 and C36 NbCo2 Laves phases with concentration gradients covering their entire homogeneity ranges were produced by the diffusion couple technique. Single-phase and single-crystalline micropillars of the cubic and hexagonal NbCo2 Laves phases were prepared in the diffusion layers by focused ion beam (FIB) milling. The influence of chemical composition, structure type, orientation and pillar size on the deformation behavior and the critical resolved shear stress (CRSS) was studied by micropillar compression tests. The pillar orientation influences the activated slip systems, but the deformation behavior and the CRSS are independent of orientation. The deformation of the smallest NbCo2 micropillars (0.8 µm in top diameter) appears to be dislocation nucleation controlled and the CRSS approaches the theoretical shear stress for dislocation nucleation. The CRSS of the 0.8 µm-sized NbCo2 micropillars is nearly constant from 26 to 34 at.% Nb where the C15 structure is stable. It decreases as the composition approaches the Co-rich and Nb-rich boundaries of the homogeneity range where the C15 structure transforms to the C36 and the C14 structure, respectively. The decrease in the CRSS at these compositions is related to the reduction of shear modulus and stacking fault energy. As the pillar size increases, stochastic deformation behavior and large scatter in the CRSS values occur and obscure the composition effect on the CRSS. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.036
  • 2020 • 262 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 • 261 Experimental and analytical analysis on the stacking sequence of composite pressure vessels
    Nebe, M. and Asijee, T.J. and Braun, C. and van Campen, J.M.J.F. and Walther, F.
    Composite Structures 247 (2020)
    The industrialization of fuel cell electric vehicles demands cost efficient storage solutions for hydrogen. While gaseous storage in type IV pressure vessels is currently the most mature technology, further structure optimization needs to be undertaken in order to meet cost requirements. This research investigates the effects of stacking sequence of composite pressure vessels regarding laminate quality, structural deformation and finally burst pressure. Therefore, a known laminate is studied on a subscale vessel geometry with changing stacking sequences. The specimens are pressurized in a specially designed chamber up to burst pressures of 166.11 MPa. Through a multisensor arrangement of stereometric systems, the deformation is tracked up to burst by using 3D digital image correlation. The experimental results show a difference of 67% in burst pressure between the investigated stacking sequences. Experimental cylinder strains and burst pressures are compared to results derived from 3D elasticity theory with implemented first ply failure criterion. Additionally, using X-ray computed tomography and acid digestion tests, insights about the distribution of fiber volume fraction and porosity are provided. For the investigated sequences in this research, the results show the considerable influence of stacking sequence on the laminate quality, the structural deformation and finally the burst pressure of composite pressure vessels. Moreover, it is shown that while the used 3D elasticity approach proved to be a useful tool for the prediction of strains and failure in the cylindrical section, discrepancies between prediction and experiment can arise based on preliminary failure occuring at the cylinder-dome transition. The results therefore emphasize the need for analytical and numerical analysis strategies to consider transition-related effects between cylinder and dome. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.compstruct.2020.112429
  • 2020 • 260 Analysis of elastic rolling stand deformation and interstand tension effects on section faults of hot rolled wire rod and bars Untersuchung der Einflüsse der elastischen Gerüstauffederung und der Längsspannungen auf die Querschnittsabweichungen beim Warmwalzen von Draht und Stabstahl
    Overhagen, C. and Braun, R. and Deike, R.
    Technisches Messen 87 343-348 (2020)
    The present work aims at the modelling and simulation of the hot rolling process for wire rod and bars. After the fundamentals of plasticity, which are essential for the understanding of the process characteristics have been described, typical section deviations that can be expected in wire rod and bar mills are calculated with help of a numerical simulation model. The model allows the calculation of section shapes under the influence of elastic rolling stand deformations and interstand tensions. From this computational assessment of section faults, the necessity of inline measurement and process control for wire rod and bar mills is shown. This work is part of the PIREF project which incorporates the development of sensors, control systems and process models in order to control the dimensional accuracy of hot rolled wire rod and bars. The metal forming process model, as described here is used internally as a model for the static and kinematic interactions in the rolling process inside of the control model. © 2020 Walter de Gruyter GmbH, Berlin/Boston.
    view abstractdoi: 10.1515/teme-2019-0130
  • 2020 • 259 Orientation-dependent deformation behavior of 316L steel manufactured by laser metal deposition and casting under local scratch and indentation load
    Pöhl, F. and Hardes, C. and Scholz, F. and Frenzel, J.
    Materials 13 (2020)
    This study analyzes the local deformation behavior of austenitic stainless steel 316L, manufactured conventionally by casting and additively by laser metal deposition (LMD). We produced directionally solidified 316L specimens with most grains showing (001) orientations parallel to the longitudinal specimen axis. We conducted nanoindentation and scratch experiments for local mechanical characterization and topography measurements (atomic force microscopy and confocal laser scanning microscopy) of indentation imprints and residual scratch grooves for the analysis of the deformation behavior and, in particular, of the pile-up behavior. The local mechanical properties and deformation behavior were correlated to the local microstructure investigated by scanning electron microscopy with energy dispersive X-ray spectroscopy and electron backscatter diffraction analysis. The results show that the local mechanical properties, deformation behavior, and scratch resistance strongly depend on the crystallographic orientation. Nearly (001)-oriented grains parallel to the surface show the lowest hardness, followed by an increasing hardness of nearly (101)-and (111)-oriented grains. Consequently, scratch depth is the greatest for nearly (001)-oriented grains followed by (101) and (111) orientations. This tendency is seen independently of the analyzed manufacturing route, namely Bridgman solidification and laser metal deposition. In general, the laser metal deposition process leads to a higher strength and hardness, which is mainly attributed to a higher dislocation density. Under the investigated loading conditions, the cellular segregation substructure is not found to significantly and directly change the local deformation behavior during indentation and scratch testing. © 2020 by the authors.
    view abstractdoi: 10.3390/MA13071765
  • 2020 • 258 Deformation mechanisms in a superelastic NiTi alloy: An in-situ high resolution digital image correlation study
    Polatidis, E. and Šmíd, M. and Kuběna, I. and Hsu, W.-N. and Laplanche, G. and Van Swygenhoven, H.
    Materials and Design 191 (2020)
    An in-situ high resolution digital image correlation investigation during uniaxial tensile deformation reveals the recoverable and the non-recoverable strain mechanisms in a Ni51Ti49 alloy with a mean grain size of 35 μm. Recoverable strain is due to the martensitic transformation, for which more than one variant per grain can be activated. The majority of the activated variants exhibit high Schmid factor. The variant selection can be influenced by shear transmission across grain boundaries, when the geometrical compatibility between the neighboring habit plane variants is favourable; in these cases variants that do not have the highest Schmid factor, with respect to the macroscopically applied load, are activated. The experimentally determined transformation strains agree well with theoretical calculations for single crystals. The non-recoverable strain is due to deformation slip in austenite, twinning in martensite and residual martensite. The results are discussed in view of possible twinning modes that can occur in austenite resulting in significant non-recoverable strain. © 2020 The Authors
    view abstractdoi: 10.1016/j.matdes.2020.108622
  • 2020 • 257 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 • 256 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 • 255 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 • 254 Role of magnetic ordering for the design of quinary TWIP-TRIP high entropy alloys
    Wu, X. and Li, Z. and Rao, Z. and Ikeda, Y. and Dutta, B. and Körmann, F. and Neugebauer, J. and Raabe, D.
    Physical Review Materials 4 (2020)
    We reveal the impact of magnetic ordering on stacking fault energy (SFE) and its influence on the deformation mechanisms and mechanical properties in a class of nonequiatomic quinary Mn-containing compositional complex alloys or high entropy alloys (HEAs). By combining ab initio simulation and experimental validation, we demonstrate magnetic ordering as an important factor in the activation and transition of deformation modes from planar dislocation slip to TWIP (twinning-induced plasticity) and/or TRIP (transformation-induced plasticity). A wide compositional space of Cr20MnxFeyCo20Niz(x+y+z=60, at. %) was probed by density-functional theory calculations to search for potential alloys displaying the TWIP/TRIP effects. Three selected promising HEA compositions with varying Mn concentrations were metallurgically synthesized, processed, and probed for microstructure, deformation mechanism, and mechanical property evaluation. The differences in the deformation modes of the probed HEAs are interpreted in terms of the computed SFEs and their dependence on the predicted magnetic state, as revealed by ab initio calculations and validated by explicit magnetic measurements. It is found that the Mn content plays a key role in the stabilization of antiferromagnetic configurations which strongly impact the SFEs and eventually lead to the prevalent deformation behavior. © 2020 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.4.033601
  • 2019 • 253 Viscoplastic deformation behaviour of preloaded stainless steel connections
    Afzali, N. and Stranghöner, N. and Pilhagen, J. and Manninen, T. and Schedin, E.
    Journal of Constructional Steel Research 152 225-234 (2019)
    Preloaded bolted connections made of stainless steel are not commonly used in stainless steel structures as their application is not allowed by the execution standard EN 1090–2 and the design standard EN 1993-1-4 for stainless steel structures unless otherwise specified, respectively unless their acceptability for a particular application has been demonstrated from test results. This restriction is mainly caused by three facts: firstly, it is feared that due to the viscoplastic deformation behaviour of stainless steel, severe preload losses have to be expected, secondly, neither stainless steel bolting assemblies for preloading nor tightening procedures exist on which could have been relied and thirdly, galling and seizure of stainless steel bolting assemblies lead to problems on site. These three questions, beside others, were treated in the frame of the European RFCS-research project “Execution and reliability of slip resistant connections for steel structures using CS and SS” SIROCO. Some of the results are presented in this contribution with the main focus on the loss of preload of preloaded stainless steel bolted connections. The main conclusion is that preloaded stainless steel bolted connections can be treated similar to those made of carbon steel with regard to preload losses as they show comparable magnitudes of preload losses. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.jcsr.2018.07.004
  • 2019 • 252 Soliton solutions in geometrically nonlinear Cosserat micropolar elasticity with large deformations
    Böhmer, C.G. and Lee, Y. and Neff, P.
    Wave Motion 84 110-124 (2019)
    We study the fully nonlinear dynamical Cosserat micropolar elasticity problem in three dimensions with various energy functionals dependent on the microrotation [Formula presented] and the deformation gradient tensor [Formula presented]. We derive a set of coupled nonlinear equations of motion from first principles by varying the complete energy functional. We obtain a double sine–Gordon equation and construct soliton solutions. We show how the solutions can determine the overall deformational behaviours and discuss the relations between wave numbers and wave velocities thereby identifying parameter values where the soliton solution does not exist. © 2018 The Authors
    view abstractdoi: 10.1016/j.wavemoti.2018.10.005
  • 2019 • 251 The brittle-to-ductile transition in cold rolled tungsten plates: Impact of crystallographic texture, grain size and dislocation density on the transition temperature
    Bonnekoh, C. and Jäntsch, U. and Hoffmann, J. and Leiste, H. and Hartmaier, A. and Weygand, D. and Hoffmann, A. and Reiser, J.
    International Journal of Refractory Metals and Hard Materials 78 146-163 (2019)
    The aim of this paper is to elucidate the mechanisms controlling the brittle-to-ductile transition (BDT) in pre-deformed, textured, polycrystalline body-centred cubic (bcc) metals by the example of cold rolled tungsten (W). For this purpose, five sheets were rolled out from one and the same sintered ingot, by various levels, representing degrees of deformation of 1.8, 2.5, 3.0, 3.4, and 4.1 (this refers to 83.5%, 91.8%, 95.0%, 96.7%, and 98.3% in the technical notation). Toughness tests show that the BDT temperature decreases with increasing degree of deformation from 115 °C ± 15 °C (388 K ± 15 K) down to −65 °C ± 15 °C (208 K ± 15 K). This is an improvement of >600 K compared with a sintered ingot. In this paper we perform an in-depth analysis of the microstructure of the five sheets mentioned above. This analysis includes the assessment of (i) crystallographic texture, (ii) grain size and (iii) dislocation density. A comparison between microstructural features and experimental data confirms our working hypothesis which states that the BDT is controlled by the glide of screw dislocations and that the transition temperature decreases with decreasing spacing, λ of dislocation sources along the crack front. Sources for dislocations may be the intersection points of grain boundaries with the crack front (BDT-temperature-grain-size-relation) or dislocation multiplication processes such as e.g., the expansion of open and closed loops (impact of dislocation density). © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijrmhm.2018.09.010
  • 2019 • 250 Nonbasal Slip Systems Enable a Strong and Ductile Hexagonal-Close-Packed High-Entropy Phase
    Bu, Y. and Li, Z. and Liu, J. and Wang, H. and Raabe, D. and Yang, W.
    Physical Review Letters 122 (2019)
    Linear defects, referred to as dislocations, determine the strength, formability, and toughness of crystalline metallic alloys. The associated deformation mechanisms are well understood for traditional metallic materials consisting of one or two prevalent matrix elements such as steels or aluminum alloys. In the recently developed high-entropy alloys (HEAs) containing multiple principal elements, the relationship between dislocations and the mechanical behavior is less understood. Particularly HEAs with a hexagonal close-packed (hcp) structure can suffer from intrinsic brittleness due to their insufficient number of slip systems. Here we report on the surprisingly high formability of a novel high-entropy phase with hcp structure. Through in situ tensile testing and postmortem characterization by transmission electron microscopy we reveal that the hcp phase in a dual-phase HEA (Fe50Mn30Co10Cr10, at. %) activates three types of dislocations, i.e., a ©, ccopy;, and +a©. Specifically, nonbasal c+a© dislocations occupy a high line fraction of ∼31% allowing for frequent double cross slip which explains the high deformability of this high-entropy phase. The hcp structure has a c/a ratio of 1.616, i.e., below the ideal value of 1.633. This modest change in the structure parameters promotes nonbasal c+a© slip, suggesting that ductile HEAs with hcp structure can be designed by shifting the c/a ratio into regimes where nonbasal slip systems are activated. This simple alloy design principle is particularly suited for HEAs due to their characteristic massive solid solution content which readily allows tuning the c/a ratio of hcp phases into regimes promoting nonbasal slip activation. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.122.075502
  • 2019 • 249 A multicomponent flow model in deformable porous media
    Detmann, B. and Krejčí, P.
    Mathematical Methods in the Applied Sciences 42 1894-1906 (2019)
    We propose a model for multicomponent flow of immiscible fluids in a deformable porous medium accounting for capillary hysteresis. Oil, water, and air in the soil pores offer a typical example of a real situation occurring in practice. We state the problem within the formalism of continuum mechanics as a slow diffusion process in Lagrange coordinates. The balance laws for volumes, masses, and momentum lead to a degenerate parabolic PDE system. In the special case of a rigid solid matrix material and three fluid components, we prove under further technical assumptions that the system is mathematically well posed in a small neighborhood of an equilibrium. © 2019 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/mma.5482
  • 2019 • 248 Site-specific quasi in situ investigation of primary static recrystallization in a low carbon steel
    Diehl, M. and Kertsch, L. and Traka, K. and Helm, D. and Raabe, D.
    Materials Science and Engineering A 755 295-306 (2019)
    Low-alloyed steels with body-centered cubic crystal structure are a material class that is widely used for sheet metal forming applications. When having an adequate crystallographic texture and microstructure, their mechanical behavior is characterized by an isotropic in-plane flow behavior in combination with a low yield strength. The decisive processing steps for obtaining these beneficial mechanical properties are cold rolling and subsequent annealing. While for the former the number of passes, the deformation rates, and the total thickness reduction are the main processing parameters, the latter is described mainly by the heating rate and the holding temperature and time. Primary static recrystallization during annealing subsequent to the cold rolling process alters mainly two aspects of the material state: It firstly replaces the elongated and heavily deformed grains of the cold rolled microstructure by small, globular grains with low dislocation density and secondly it changes the crystallographic texture insofar as it typically diminishes the α- and strengthens the γ-fiber texture components. In the present work, the recrystallization behavior of a commercial non-alloyed low carbon steel is studied. A quasi in situ setup that enables site-specific characterization is employed to gain a local picture of the nucleation and recrystallization process. From the Kernel Average Misorientation (KAM) values of the deformation structure, the tendency to be consumed by new grains can be predicted. Crystallographic analysis shows that the most deformed regions have either a γ-fiber orientation or belong to heavily fragmented regions. New grains nucleate especially in such highly deformed regions and inherit often the orientation from the deformation microstructure. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2019.02.032
  • 2019 • 247 On the role of nitrogen on hydrogen environment embrittlement of high-interstitial austenitic CrMnC(N) steels
    Egels, G. and Fussik, R. and Weber, S. and Theisen, W.
    International Journal of Hydrogen Energy 44 32323-32331 (2019)
    This work investigates the susceptibility of high-interstitial CrMn austenitic stainless steel CN0.96 to hydrogen environment embrittlement. In this context, an N-free model alloy of CN0.96 steel was designed, produced, and characterized. Both steels were subjected to tensile tests in air and in a high-pressure hydrogen gas atmosphere. Both steels undergo severe hydrogen embrittlement. The CN0.96 steel shows trans- and intergranular failure in hydrogen, whereas the N-free model alloy shows exclusively intergranular failure. The different failure modes could be related to different deformation modes that are induced by the presence or absence of N, respectively. In the CN0.96 steel, N promotes planar dislocation slip. Due to the absence of N in the model alloy, localized slip is less pronounced and mechanical twinning is a more preferred deformation mechanism. The embrittlement of the model alloy could therefore be related to mechanisms that are known from hydrogen embrittlement of twinning-induced plasticity steels. © 2019 Hydrogen Energy Publications LLC
    view abstractdoi: 10.1016/j.ijhydene.2019.10.109
  • 2019 • 246 Magnetostriction in magnetic gels and elastomers as a function of the internal structure and particle distribution
    Fischer, L. and Menzel, A.M.
    Journal of Chemical Physics 151 (2019)
    Magnetic gels and elastomers are promising candidates to construct reversibly excitable soft actuators, triggered from outside by magnetic fields. These magnetic fields induce or alter the magnetic interactions between discrete rigid particles embedded in a soft elastic polymeric matrix, leading to overall deformations. It is a major challenge in theory to correctly predict from the discrete particle configuration the type of deformation resulting for a finite-sized system. Considering an elastic sphere, we here present such an approach. The method is in principle exact, at least within the framework of linear elasticity theory and for large enough interparticle distances. Different particle arrangements are considered. We find, for instance, that regular simple cubic configurations show elongation of the sphere along the magnetization if oriented along a face or space diagonal of the cubic unit cell. Contrariwise, with the magnetization along the edge of the cubic unit cell, they contract. The opposite is true in this geometry for body- and face-centered configurations. Remarkably, for the latter configurations but the magnetization along a face or space diagonal of the unit cell, contraction was observed to revert to expansion with decreasing Poisson ratio of the elastic material. Randomized configurations were considered as well. They show a tendency of elongating the sphere along the magnetization, which is more pronounced for compressible systems. Our results can be tested against actual experiments for spherical samples. Moreover, our approach shall support the search of optimal particle distributions for a maximized effect of actuation. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5118875
  • 2019 • 245 A fully-relaxed variationally-consistent framework for inelastic micro-sphere models: Finite viscoelasticity
    Govindjee, S. and Zoller, M.J. and Hackl, K.
    Journal of the Mechanics and Physics of Solids 127 1-19 (2019)
    The micro-sphere modeling framework provides a popular means by which one-dimensional mechanical models can easily and quickly be generalized into three-dimensional stress-strain models. The essential notion of the framework, similar to homogenization theory, is that one allows the microstructural kinematic fields to relax subject to a constraint connected to a macroscopic deformation measure. In its standard presentation, the micro-sphere modeling framework is strictly applicable to elastic materials. Presentations considering inelastic phenomena invariably, and inconsistently, assume an affine relation between inelastic macroscopic and microscopic phenomena. In this work we present a methodology by which one can lift this modeling restriction using two formally different approaches. In particular, we show how one can construct and apply a homogenization with Biot theory to generate fully-relaxed variationally-consistent macroscopic models for inelastic materials within the context of the micro-sphere model. The primary application example will be finite deformation viscoelasticity. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2019.02.014
  • 2019 • 244 Analytical prediction of wall thickness reduction and forming forces during the radial indentation process in Incremental Profile Forming
    Grzancic, G. and Löbbe, C. and Ben Khalifa, N. and Tekkaya, A.E.
    Journal of Materials Processing Technology 267 68-79 (2019)
    Incremental Profile Forming (IPF) is a recently introduced flexible tube forming technology, which allows the manufacture of tubular structures with varying cross-sectional geometries along the longitudinal axis of the part. The process is characterized mainly by the operation of several tools, laterally moving, indenting and deforming the initial tubular workpiece. In kinematic IPF the use of universal tools with hemispheric tool shapes allow the flexible manufacture of highly complex parts since its geometry is mainly defined by the tool motions. Thinning of the tube material in the tool contact region is typical for kinematic IPF forming processes. In order to predict the forming behavior, an analytical model is developed taking the tube dimensions, the tool geometry as well as the tube material into account. Based on the predicted forming behavior, the process force during the indentation process is also determined analytically. The validation of the analytical model is performed by experimental and numerical investigations. After the geometrical analysis of the tool contact region and the tube deformations, the plastic strain distribution in the forming zone is described, in order to predict the reduction of the wall thickness. Furthermore, the analytical model allows the prediction of the forming force course over the indenting depth for various process parameters. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2018.12.003
  • 2019 • 243 Joint contribution of transformation and twinning to the high strength-ductility combination of a FeMnCoCr high entropy alloy at cryogenic temperatures
    He, Z.F. and Jia, N. and Ma, D. and Yan, H.L. and Li, Z.M. and Raabe, D.
    Materials Science and Engineering A 759 437-447 (2019)
    The microstructure-mechanical property relationships of a non-equiatomic FeMnCoCr high entropy alloy (HEA), which shows a single face-centered cubic (fcc) structure in the undeformed state, have been systematically investigated at room and cryogenic temperatures. Both strength and ductility increase significantly when reducing the probing temperature from 293 K to 77 K. During tensile deformation at 293 K, dislocation slip and mechanical twinning prevail. At 173 K deformation-driven athermal transformation from the fcc phase to the hexagonal close-packed (hcp) martensite is the dominant mechanism while mechanical twinning occurs in grains with high Schmid factors. At 77 K athermal martensitic transformation continues to prevail in addition to dislocation slip and twinning. The reduction in the mean free path for dislocation slip through the fine martensite bundles and deformation twins leads to the further increased strength. The joint activation of transformation and twinning under cryogenic conditions is attributed to the decreased stacking fault energy and the enhanced flow stress of the fcc matrix with decreasing temperature. These mechanisms lead to an elevated strain hardening capacity and an enhanced strength-ductility combination. The temperature-dependent synergy effects of martensite formation, twinning and dislocation plasticity originate from the metastability alloy design concept. This is realized by relaxing the equiatomic HEA constraints towards reduced Ni and increased Mn contents, enabling a non-equiatomic material with low stacking fault energy. These insights are important for designing strong and ductile Ni-saving alloys for cryogenic applications. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2019.05.057
  • 2019 • 242 Modified mixed least-squares finite element formulations for small and finite strain plasticity
    Igelbüscher, M. and Schwarz, A. and Steeger, K. and Schröder, J.
    International Journal for Numerical Methods in Engineering 117 141-160 (2019)
    In this contribution, we propose mixed least-squares finite element formulations for elastoplastic material behavior. The resulting two-field formulations depending on displacements and stresses are given through the (Formula presented.) -norm minimization of the residuals of the first-order system of differential equations. The residuals are the balance of momentum and the constitutive equation. The advantage of using mixed methods for an elastoplastic material description lies in the direct approximation of the stresses as an unknown variable. In addition to the standard least-squares formulation, an extension of the least-squares functional as well as a modified formulation is done. The modification by means of a varied first variation of the functional is necessary to guarantee a continuous weak form, which is not automatically given within the elastoplastic least-squares approach. For the stress approximation, vector-valued Raviart-Thomas functions are chosen. On the other hand, standard Lagrange polynomials are taken into account for the approximation of the displacements. We consider classical J2 plasticity for a small and a large deformation model for the proposed formulations. For the description of the elastic material response, we choose for the small strain model Hooke's law and for finite deformations a hyperelastic model of Neo-Hookean type. The underlying plastic material response is defined by an isotropic von Mises yield criterion with linear hardening. © 2018 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/nme.5951
  • 2019 • 241 Hydrogels – a macroscopic approach based on microscopic physics
    Keller, K. and Ricken, T. and Wallmersperger, 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 274-278 (2019)
    As a smart material, hydrogels react to different stimuli. These could be – for example – of chemical, electrical, mechanical or thermal nature. The hydrogels consist of a more or less crosslinked polyelectrolytic polymer network, depending on the conditions, components, and time spent for its fabrication. The reaction of these stimulated hydrogels is to uptake or deliver ions with hydration shells or solvent, followed by e.g. an elongation or bending deformation of the gels. Gels can reach enormous swelling ratios and are applicable as actuators, energy converters or sensors. In the present approach, the underlying physical phenomena of the chemical, electrical as well as of the mechanical field are incorporated in a homogenized way by using different partial differential equations. Local effects, e.g. the osmotic pressure differences in the mechanical field, are derived over reference and local concentrations. Depending on the type of the stimulus, the hydrogel reaction is more or less sensitive. For example, the mechanical reaction under chemical stimulation is far beyond the reaction under electrical stimulation. The applied coupled multi-field formulation is capable of providing local concentrations, electric potential distributions and displacements. The hydrogel model of a finger gripper is formulated by the use of finite elements. For the simulation the Newton-Raphson method and implicit Euler time integration are applied. Here only small volume changes, corresponding swelling ratios and deformations are considered. The results to different kinds of stimulation will be presented. © 2019 Taylor & Francis Group, London, UK.
    view abstractdoi: 10.1201/9780429426506-46
  • 2019 • 240 Dislocation mechanism based size-dependent crystal plasticity modeling and simulation of gradient nano-grained copper
    Lu, X. and Zhang, X. and Shi, M. and Roters, F. and Kang, G. and Raabe, D.
    International Journal of Plasticity 113 52-73 (2019)
    Overcoming the trade-off between strength and ductility in metallic materials is a grand challenge. Recently, materials with a gradient nano-grained (GNG) surface layer adhering to a ductile coarse-grained (CG) substrate have been proposed to overcome this long-standing dilemma. Constitutive modeling and simulation are crucial to understand the deformation mechanisms controlling the strength and ductility in GNG/CG materials, and to enable theory to guide microstructure optimization for upscaling. Here, we develop a dislocation mechanism based size-dependent crystal plasticity model, where multiple dislocation evolution mechanisms are considered. Furthermore, damage evolution and mechanically driven grain growth during the deformation of GNG/CG materials are incorporated into the constitutive model to study the role of microstructure gradient in the overall plastic response. The developed size-dependent constitutive model was implemented within a finite-strain crystal plasticity finite element framework, and used to predict the tensile mechanical behavior of GNG/CG copper, including yield stress, strain-hardening and ductility with a highly simplified geometrical representation of the microstructure. The simulations reveal some of the underlying deformation mechanisms controlling ductility and strengthening in terms of the spatial distribution and temporal evolution of microstructure and damage. The model was also used to demonstrate optimization of strength and ductility of GNG/CG copper. By manipulating the thickness of the GNG layer and the grain size of the CG substrate, the strength increase is associated with a loss of ductility showing the same linear inverse relationship observed experimentally for GNG/CG copper, which indicates the improvement over the typical nonlinear trade-off between strength and ductility. © 2018 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijplas.2018.09.007
  • 2019 • 239 Advances in in situ nanomechanical testing
    Minor, A.M. and Dehm, G.
    MRS Bulletin 44 438-442 (2019)
    In situ nanomechanical testing provides critical insight into the fundamental processes that lead to deformation phenomena in materials. Often, in situ tests are performed in relevant conditions such as high or low temperatures, tribological contact, gas environments, or under radiation exposure. Modern diffraction and imaging methods of materials under load provide high spatial resolution and enable extraction of quantitative mechanical data from local microstructure components or nano-sized objects. The articles in this issue cover recent advances in different types of in situ nanomechanical testing methods, spanning from dedicated nanomechanical testing platforms and microelectromechanical systems devices to deformation analyses via in situ diffraction and imaging methods. This includes scanning electron microscopy, advanced scanning transmission electron microscopy, electron diffraction, x-ray diffraction, and synchrotron techniques. Emerging areas such as in situ tribology enable novel insights into the origin of deformation mechanisms, while the evolution of microelectromechanical systems for controlled in situ testing provide opportunities for advanced control and loading strategies. Discussion on the current state of the art for in situ nanomechanical testing and future opportunities in imaging, strain sensing, and testing environments are also addressed. Copyright © Materials Research Society 2019.
    view abstractdoi: 10.1557/mrs.2019.127
  • 2019 • 238 Experimental characterization of the structural deformation of type IV pressure vessels subjected to internal pressure
    Nebe, M. and Maraite, D. and Braun, C. and Hülsbusch, D. and Walther, F.
    Key Engineering Materials 809 KEM 47-52 (2019)
    The investigations deal with the experimental characterization of the structural deformation of type IV pressure vessels subjected to internal pressure. For the widespread use of hydrogen technology in transport industries, the development of cost-effective storage systems is a crucial step. State of the art in the field of hydrogen storage are type IV pressure vessels, which consist of a polymeric liner and an enforcing winding of carbon fiber-reinforced plastic (CFRP). For the development of material-optimized and high-safety pressure vessels, the acquisition of reliable experimental data in order to validate numerical simulations is a necessity. In a specially designed test chamber subscale vessels are clamped and subjected to internal pressure. At defined pressure stages the vessel’s deformation is recorded and analyzed. Consequently, the overall structural deformation is assessed with regard to the used structural mass, the burst pressure and the resulting failure. The results can be used for structure optimization purposes as well as for the optimization of numerical simulation models. © 2019 Trans Tech Publications Ltd, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.809.47
  • 2019 • 237 Model order reduction with Galerkin projection applied to nonlinear optimization with infeasible primal-dual interior point method
    Nigro, P.S.B. and Simões, E.T. and Pimenta, P.M. and Schröder, J.
    International Journal for Numerical Methods in Engineering 120 1310-1348 (2019)
    It is not new that model order reduction (MOR) methods are employed in almost all fields of engineering to reduce the processing time of complex computational simulations. At the same time, interior point methods (IPMs), a method to deal with inequality constraint problems (which is little explored in engineering), can be applied in many fields such as plasticity theory, contact mechanics, micromechanics, and topology optimization. In this work, a MOR based in Galerkin projection is coupled with the infeasible primal-dual IPM. Such research concentrates on how to develop a Galerkin projection in one field with the interior point method; the combination of both methods, coupled with Schur complement, permits to solve this MOR similar to problems without constraints, leading to new approaches to adaptive strategies. Moreover, this research develops an analysis of error from the Galerkin projection related to the primal and dual variables. Finally, this work also suggests an adaptive strategy to alternate the Galerkin projection operator, between primal and dual variable, according to the error during the processing of a problem. © 2019 The Authors. International Journal for Numerical Methods in Engineering Published by John Wiley & Sons Ltd.
    view abstractdoi: 10.1002/nme.6181
  • 2019 • 236 On the implementation of finite deformation gradient-enhanced damage models
    Ostwald, R. and Kuhl, E. and Menzel, A.
    Computational Mechanics 64 847-877 (2019)
    We introduce a comprehensive framework for the efficient implementation of finite deformation gradient-regularised damage formulations in existing finite element codes. The numerical implementation is established within a thermo-mechanically fully coupled finite element formulation, where the heat equation solution capabilities are utilised for the damage regularisation. The variationally consistent, gradient-extended and geometrically non-linear damage formulation is based on an overall free energy function, where the standard local free energy contribution is additively extended by two non-local terms. The first additional term basically contains the referential gradient of the non-local damage variable. Secondly, a penalty term is added to couple the local damage variable—the evolution of which is governed by an ordinary differential equation—and the non-local damage field variable that is governed by an additional balance equation of elliptic type. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s00466-019-01684-5
  • 2019 • 235 Deformation behavior and dominant abrasion micro mechanisms of tempering steel with varying carbon content under controlled scratch testing
    Pöhl, F. and Hardes, C. and Theisen, W.
    Wear 422-423 212-222 (2019)
    The objective of this paper is the investigation of the abrasive wear behavior of tempering steel by the analysis of its deformation behavior and dominant abrasion micro mechanisms under controlled single scratch testing. We analyzed single scratches induced by a sphero-conical diamond tip with constant and progressively increasing load in tempering steel with varying carbon content in the as quenched condition. Among scratch and hardness testing the analysis and characterization included the optical determination of the deformation behavior (SEM, CLSM, AFM). The results show that the deformation behavior strongly depends on applied normal load and strength (yield stress, ultimate tensile strength) as it varies with carbon content. It was shown that an increasing load leads to the transition of predominant abrasion micro mechanisms from ideal micro ploughing to wedge and chip formation. The scratch resistance increased with increasing carbon content until for higher carbon contents a saturation tendency due to the presence of retained austenite is observed. An increasing carbon content also shifted the aforementioned transition of dominant micro mechanisms not only to higher applied normal loads, but also to higher scratch depths. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2019.01.073
  • 2019 • 234 Local quasi-static and cyclic deformation behaviour of brazed AISI 304L/BAu-4 joints characterised by digital image correlation
    Schmiedt, A. and Manka, M. and Tillmann, W. and Walther, F.
    Welding in the World 63 501-509 (2019)
    For a reliable design of brazed components, the degradation of mechanical properties due to the corrosive attack by aggressive operating environments has to be considered. In this study, the effect of a condensate corrosion, which is performed according to VDA test sheet 230-214 up to 6 weeks, on the mechanical behaviour of brazed AISI 304L/BAu-4 stainless steel joints is investigated. A time-dependent reduction of the tensile and fatigue strength values down to 42% of the as-received condition is determined. As standard strain measurements are not appropriate to characterise the local strain distributions of heterogeneous material systems, the optical digital image correlation technique is used to evaluate the local quasi-static and cyclic deformation behaviour of the 50 μm wide brazing seam. A novel triggered image acquisition enables measurements in fatigue tests at a frequency of 10 Hz. The reduction of the virtual gauge length from 12.5 down to 0.5 mm leads to an increase of the total strain and ratcheting strain values, which is more pronounced for higher stresses and enhanced for pre-corroded brazed joints. For a microstructure-related analysis of the damage processes, scanning electron microscopy was used. © 2019, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-018-00693-x
  • 2019 • 233 Fatigue and corrosion fatigue behaviour of brazed stainless steel joints AISI 304L/BAu-4 in synthetic exhaust gas condensate
    Schmiedt-Kalenborn, A. and Lingnau, L.A. and Manka, M. and Tillmann, W. and Walther, F.
    Materials 12 (2019)
    As brazed stainless steel components in service often have to withstand cyclic loads in corrosive environments, the corrosion fatigue properties of brazed joints have to be characterised. Application-relevant corrosion fatigue tests in corrosive media are extremely rare for brazed joints and cyclic deformation curves are barely investigated. In this study, fatigue tests of brazed AISI 304L/BAu-4 joints were performed in air and synthetic exhaust gas condensate K2.2 according to VDA 230-214. The fatigue behaviour of the brazed joints was compared to properties of the austenitic base material. Strain, electrical, magnetic, temperature and electrochemical measurement techniques were applied within fatigue and corrosion fatigue tests to characterise the cyclic deformation and damage behaviour of the brazed joints. It was found that the fatigue strength of 397 MPa at 2 × 106 cycles was reduced down to 51% due to the superimposed corrosive loading. Divergent microstructure-related damage mechanisms were identified for corrosion fatigue loadings and fatigue loadings of specimens in the as-received and pre-corroded conditions. The investigations demonstrate the important role of corrosive environments for the mechanical performance of brazed stainless steel joints. © 2019 by the authors.
    view abstractdoi: 10.3390/ma12071040
  • 2019 • 232 In Situ Monitoring Mesoscopic Deformation of Nanostructured Porous Titania Films Caused by Water Ingression
    Song, L. and Rawolle, M. and Hohn, N. and Gutmann, J.S. and Frielinghaus, H. and Müller-Buschbaum, P.
    ACS Applied Materials and Interfaces 11 32552-32558 (2019)
    Nanostructured porous titania films are used in many energy-related applications. In this work, the temporal evolution of the mesoscopic deformation of mesoporous titania films synthesized via block copolymer-assisted sol-gel chemistry is investigated with in situ grazing incidence small-angle neutron scattering (GISANS) during exposure to D2O vapor. Two types of mesoporous titania films are compared, which have a different degree of structural stability, depending on the applied annealing temperature (400 °C vs 600 °C) in a nitrogen atmosphere. Water ingression causes a gradual structure deformation in terms of decreasing center-to-center distances and broadening of the size distribution of the titania nanostructures. Based on the evolution of the mesopore size obtained from in situ GISANS measurements, the results show that structures synthesized at lower temperature undergo a stronger deformation because of the lower elastic modulus originating from larger pores, despite having a higher degree of order. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsami.9b10750
  • 2019 • 231 Deformation-driven bidirectional transformation promotes bulk nanostructure formation in a metastable interstitial high entropy alloy
    Su, J. and Wu, X. and Raabe, D. and Li, Z.
    Acta Materialia 167 23-39 (2019)
    We investigate the mechanisms of deformation-driven forward and reverse (bidirectional) martensitic transformation and the associated nanostructure formation in a metastable carbon-doped high entropy alloy (HEA) upon cold rolling. At thickness reductions below 14%, forward hexagonal-close packed (HCP) martensitic transformation prevails in the single face-centered cubic (FCC) matrix. Surprisingly, at the intersections of two crossing HCP lamellae, deformation-induced reverse transformation from the HCP martensite back to the FCC phase occurs. At higher thickness reductions around 26%–34%, multiple deformation kink bands develop, mainly on the pyramidal habit planes of the HCP martensite, among which reverted FCC phase is also observed resulting in a dual-phase nano-laminated structure. The deformation-induced reverted FCC phase regions exhibit a twin stacking sequence relative to the prior FCC matrix, which is related to the underlying dislocation reactions and rearrangement of the partial dislocations. At 67% thickness reduction, the deformation bands develop further into micro-shear bands consisting of nanosized (sub)grains. For rendering the dual-phase nanostructure back to single-phase FCC, 400 °C/10 min tempering is applied on a 34% cold-rolled specimen. The resulting nanostructure is characterized by nano-(sub)grains and nano-twins. It exhibits an excellent strength-ductility synergy (ultimate tensile strength 1.05 GPa at 35% total elongation) due to the improved work hardening enabled by both, FCC-HCP martensitic transformation in confined regions and mechanical twinning. With this, we show that bulk nanostructured alloys with bidirectional transformation can be designed by tuning the materials’ phase stability to their thermodynamic limits with the aim to trigger sequential athermal forward and reverse transformation under load. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.01.030
  • 2019 • 230 Carbon and strain partitioning in a quenched and partitioned steel containing ferrite
    Tan, X. and Ponge, D. and Lu, W. and Xu, Y. and Yang, X. and Rao, X. and Wu, D. and Raabe, D.
    Acta Materialia 165 561-576 (2019)
    We applied a hot rolling direct quenching and partitioning (HDQ&P) process to a low-C low-Si Al-added steel and obtained a Q&P steel containing 40 vol % of ferrite. Microstructure characterization was performed by means of SEM, EBSD, TEM and XRD. Atomic-scale characterization of carbon partitioning among the phases was carried out by atom probe tomography (APT). The carbon distribution in the retained austenite and near the martensite/retained austenite interfaces was quantitatively analyzed to study its partitioning behavior. The macroscopic strain distribution evolution across the tensile sample surface was investigated using macro digital image correlation (DIC) analysis. Combining these results with joint micro-DIC and EBSD analysis during quasi in-situ tensile testing, we investigated the strain partitioning among the different phases and the TRIP effect. Coupling of these results enabled us to reveal the relation among carbon partitioning, strain partitioning and the TRIP effect. The large blocky retained austenite with a side length of about 300–600 nm located near the ferrite/martensite (F/M) interfaces has low stability and transforms to martensite during the early deformation stages, i.e. at average strain below 21%. The retained austenite films in the centers of the martensite regions are more stable. The carbon distribution in both, the martensite and the retained austenite are inhomogeneous, with 0.5–2.0 at. % in the martensite and 4.0–7.5 at. % in the retained austenite. Strong carbon concentration gradients of up to 1.1 at. %/nm were observed near the martensite/retained austenite interfaces. The large blocky retained austenite (300–600 nm in side length) near the F/M interfaces has 1.5–2.0 at. % lower carbon content than that in the narrow retained austenite films (20–150 nm in thickness). The ferrite is soft and deforms prior to the martensite. The strain distribution in ferrite and martensite is inhomogeneous, varying by up to 20% within the same phase at an average strain of about 20%. Ferrite deformation is the main origin of ductility of the material. The balance between ferrite fraction and martensite morphology controls the TRIP effect and its efficiency in reaching a suited combination of strength and ductility. Reducing the ferrite volume fraction and softening the martensite by coarsening and polygonization can enhance the strain carried by the martensite, thus promoting more retained austenite in the martensite regions enabling a TRIP effect. The enhancement of the TRIP effect and the decrease of the strain contrast between ferrite and martensite jointly optimize the micromechanical deformation compatibility of the adjacent phases, thus improving the material's ductility. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.019
  • 2019 • 229 On the onset of deformation twinning in the CrFeMnCoNi high-entropy alloy using a novel tensile specimen geometry
    Thurston, K.V.S. and Hohenwarter, A. and Laplanche, G. and George, E.P. and Gludovatz, B. and Ritchie, R.O.
    Intermetallics 110 (2019)
    Deformation-induced nanoscale twinning is one of the mechanisms responsible for the excellent combination of strength and fracture toughness of the single-phase, face-centered cubic CrMnFeCoNi (Cantor)alloy, especially at cryogenic temperatures. Here, we use a novel, modified dogbone geometry that permits the sampling of varying stress and strain regions within a single tensile specimen to characterize the onset of twinning in CrMnFeCoNi at 293 K, 198 K and 77 K. Electron backscatter diffraction (EBSD)and backscattered electron (BSE)imaging revealed the presence of deformation nano-twins in regions of the samples that had experienced plastic strains of ∼25% at 293 K, ∼16% at 198 K, and ∼8% at 77 K, which are similar to the threshold strains described by Laplanche et al. (Acta Mater. 118, 2016, 152–163). From these strains we estimate that the critical tensile stress for the onset of twinning in this alloy is on the order of 750 MPa. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.intermet.2019.04.012
  • 2019 • 228 Crystal plasticity finite element simulation and experiment investigation of nanoscratching of single crystalline copper
    Wang, Z. and Zhang, H. and Li, Z. and Li, G. and Zhang, J. and Zhang, J. and Hassan, H.U. and Yan, Y. and Hartmaier, A. and Sun, T.
    Wear 430-431 100-107 (2019)
    Mechanical properties of crystalline materials strongly correlate with deformation behaviour at the grain level. In the present work, we establish a 3D crystal plasticity finite element model of nanoscratching of single crystalline copper using a Berkovich probe, which is capable of addressing the crystallography influence. In particular, nanoindentation experiments and high resolution electron back-scatter diffraction characterization are jointly carried out to precisely calibrate parameters used in the crystal plasticity finite element model. Subsequent finite element simulations of nanoscratching are performed to reveal fundamental deformation behaviour of single crystalline copper in terms of mechanical response and surface pile-up topography, as well as their dependence on crystallographic orientation. Furthermore, nanoscratching experiments with the same parameters used in the finite element simulations are carried out, the results of which are further compared with predication results by the finite element simulations. Simulation data and experimental results jointly demonstrate the strong anisotropic characteristics of single crystalline copper under nanoscratching, due to the crystallographic orientation dependent coupled effects of intrinsic dislocation slip and extrinsic discrete stress distribution by probe geometry. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2019.04.024
  • 2019 • 227 On the mechanism of extraordinary strain hardening in an interstitial high-entropy alloy under cryogenic conditions
    Wang, Z. and Lu, W. and Raabe, D. and Li, Z.
    Journal of Alloys and Compounds 734-743 (2019)
    We investigate the cryogenic deformation response and underlying mechanisms of a carbon-doped interstitial high-entropy alloy (iHEA) with a nominal composition of Fe49.5Mn30Co10Cr10C0.5 (at. %). Extraordinary strain hardening of the iHEA at 77 K leads to a substantial increase in ultimate tensile strength (∼1300 MPa) with excellent ductility (∼50%) compared to that at room temperature. Prior to loading, iHEAs with coarse (∼100 μm) and fine (∼6 μm) grain sizes show nearly single face-centered cubic (FCC) structure, while the fraction of hexagonal close-packed (HCP) phase reaches up to ∼70% in the cryogenically tensile-fractured iHEAs. Such an unusually high fraction of deformation-induced phase transformation and the associated plasticity (TRIP effect) is caused by the strong driving force supported by the reduced stacking fault energy and increased flow stress at 77 K. The transformation mechanism from the FCC matrix to the HCP phase is revealed by transmission electron microscopy (TEM) observations. In addition to the deformation-induced phase transformation, stacking faults and dislocation slip contribute to the deformation of the FCC matrix phase at low strains and of the HCP phase at medium and large strains, suggesting dynamic strain partitioning among these two phases. The combination of TRIP and dynamic strain partitioning explain the striking strain hardening capability and resulting excellent combination of strength and ductility of iHEAs under cryogenic conditions. The current investigation thus offers guidance for the design of high-performance HEAs for cryogenic applications. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2018.12.061
  • 2018 • 226 An algorithm based on incompatible modes for the global tracking of strong discontinuities in shear localization analyses
    Alsahly, A. and Callari, C. and Meschke, G.
    Computer Methods in Applied Mechanics and Engineering 330 33-63 (2018)
    Numerical methods for predicting localized shear failure in elasto-plastic solids have experienced considerable advancements in the last decades. Among these approaches, the so-called “Embedded Strong Discontinuity (ESD)” method is often successfully used to accurately simulate the post-localization response with negligible dependence on the finite element discretization. However, it was observed that the employed discontinuity tracking strategy plays a crucial role in the successful localization analysis. In this contribution, we propose a novel strategy for the global tracking of discontinuity surfaces. It is based on exploiting information obtained from the enhanced parameters employed in Enhanced Assumed Strain (EAS) formulations. It is well known, that enhanced strain element formulations are able to better capture localized shear deformations as compared to standard finite elements. This can be explained as a consequence of the improved performance in bending. We observed, that the approximation of the strain jumps delimiting the shear band is connected with a deformation field characterized by opposite bending curvatures across these two discontinuities. Hence, in view of the relations existing between the kinematics of strong and weak discontinuities, we formulate a proper scalar function of the enhanced parameters to identify potential strong discontinuity surfaces, which are evaluated in each step of the analysis with negligible computational cost. This proposed approach has a global character, as it is based upon evaluating discontinuity surfaces defined in the complete analysis domain that are, by construction, continuous across elements. We demonstrate that the tracking algorithm correctly identifies the potential strong discontinuity surface already in early loading stages, even before a localization condition is fulfilled. In those elements which are crossed by the potential failure surface and which also satisfy the localization condition, the kinematics of embedded strong discontinuities is activated to capture the shear failure surface. The performance of the new tracking algorithm is demonstrated by means of several numerical shear localization analyses using associative and non-associative Drucker–Prager elastoplastic models to simulate 2-D and 3-D benchmarkanalyses. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.cma.2017.10.014
  • 2018 • 225 Effects of strain rate on mechanical properties and deformation behavior of an austenitic Fe-25Mn-3Al-3Si TWIP-TRIP steel
    Benzing, J.T. and Poling, W.A. and Pierce, D.T. and Bentley, J. and Findley, K.O. and Raabe, D. and Wittig, J.E.
    Materials Science and Engineering A 711 78-92 (2018)
    The effects of quasi-static and low-dynamic strain rate (ε̇ = 10−4 /s to ε̇ = 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21 ± 3 mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.11.017
  • 2018 • 224 A physics-based micromechanical model for electroactive viscoelastic polymers
    Brighenti, R. and Menzel, A. and Vernerey, F.J.
    Journal of Intelligent Material Systems and Structures 29 2902-2918 (2018)
    Electroactive polymers with time-dependent behavior are considered in the present paper by way of a new physics-based micromechanical model; such viscoelastic response is described by the internal evolution of the polymer network, providing a new viewpoint on the stress relaxation occurring in elastomers. The main peculiarity of such internally rearranging materials is their capacity to locally reset their reference stress-free state, leading to a mechanical behavior that relaxes out (eases off) an induced stress state and that can thus be assimilated to a sort of internal self-healing process. Such high deformability and recoverability displayed by dynamically cross-linked polymers can be conveniently exploited when they are coupled in electromechanical problems; the deformation induced by an electric field can be easily tuned by the intensity of the electric field itself and the obtained shape can be maintained without any electric influence once the material microstructure has rearranged after a sufficient curing time. In the present paper, both features of the polymeric material, that is, internal remodeling and electromechanical coupled response, are considered and a theoretical framework is established to simulate representative boundary value problems. © The Author(s) 2018.
    view abstractdoi: 10.1177/1045389X18781036
  • 2018 • 223 Deformation mechanisms, activated slip systems and critical resolved shear stresses in an Mg-LPSO alloy studied by micro-pillar compression
    Chen, R. and Sandlöbes, S. and Zehnder, C. and Zeng, X. and Korte-Kerzel, S. and Raabe, D.
    Materials and Design 154 203-216 (2018)
    We study the micro-mechanical behaviour of single-crystalline long-period-stacking ordered (LPSO) structures, α-Mg and bi-crystalline Mg/LPSO micro-pillars, all cut from the same Mg97Y2Zn1 (at.%) alloy. To investigate the deformation and co-deformation mechanisms of Mg-LPSO alloys we performed micro-pillar compression experiments with micro-pillars of an orientation inclined by 7°, 46° and 90° to (0001) orientation, respectively. Electron backscatter diffraction-assisted slip trace analysis and post-mortem transmission electron microscopy analysis showed predominant deformation by basal 〈a〉 dislocation slip in 46°(0001) and 7°(0001) oriented micro-pillars. In 7°(0001) oriented micro-pillars additional non-basal dislocation slip and the formation of micro shear bands along pyramidal planes were activated in the α-Mg and the LPSO structure, respectively. In 90°(0001) oriented micro-pillars 11¯001¯1¯20 prismatic slip was predominantly activated during the early deformation stages. The relative magnitude of the critical stresses depends on the crystal phase as well as the crystallographic orientation, i.e. the activated slip system. Specifically, basal 〈a〉 slip has the lowest critical resolved shear stress in both, α-Mg and the LPSO structure, while the CRSS of prismatic 〈a〉 slip is about 5 times higher than basal 〈a〉 slip in α-Mg and about 15 times higher than basal 〈a〉 slip in LPSO. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2018.05.037
  • 2018 • 222 On the nature of twin boundary-associated strengthening in Fe-Mn-C steel
    Choi, W.S. and Sandlöbes, S. and Malyar, N.V. and Kirchlechner, C. and Korte-Kerzel, S. and Dehm, G. and Choi, P.-P. and Raabe, D.
    Scripta Materialia 156 27-31 (2018)
    We unravel the nature of twin boundary-associated strengthening in Fe-Mn-C twinning-induced plasticity steel (TWIPs) by micro-pillar compression tests. Dislocation interactions with a coherent twin boundary and their role on strain hardening were investigated. The results indicate that twin-matrix bundles dynamically introduced by deformation twinning and their interaction with dislocations are required for strengthening Fe-Mn-C TWIPs, while single coherent twin boundaries enable dislocation transmission. Correlative studies on orientation dependent deformation mechanisms, detailed dislocation-twin boundary interactions, and the resulting local stress-strain responses suggest that twin boundary-associated strengthening is primarily caused by the reduction of the mean free dislocation path in nano-twinned microstructures. © 2018
    view abstractdoi: 10.1016/j.scriptamat.2018.07.009
  • 2018 • 221 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 • 220 Energy Structure of an Individual Mn Acceptor in GaAs : Mn
    Dimitriev, G.S. and Krainov, I.V. and Sapega, V.F. and Averkiev, N.S. and Debus, J. and Lähderanta, E.
    Physics of the Solid State 60 1568-1577 (2018)
    The energy structure of the Mn acceptor, which is a complex of Mn2+ ion plus valence band hole, is investigated in the external magnetic field and under presence of an uniaxial stress has been studied. The spin-flip Raman spectra are studied under resonant excitation of exciton bound to the Mn acceptor. The gfactors of the ground F = 1 and the first excited F = 2 states are determined and selection rules for the optical transitions between the acceptor states are described. The value of the random field (stress or electric field) acting on manganese acceptor and the deformation potential for the exchange interaction constant of the Mn2+ + hole complex are obtained. A theoretical model is developed that takes into account the influence of random internal and uniaxial external stress and magnetic field. The proposed model describes well the lines of spin-flip Raman scattering of Mn acceptor. © 2018, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S106378341808005X
  • 2018 • 219 Cyclic deformation behavior of a damage tolerant CrMnNi TRIP steel produced by electron beam melting
    Droste, M. and Günther, J. and Kotzem, D. and Walther, F. and Niendorf, T. and Biermann, H.
    International Journal of Fatigue 114 262-271 (2018)
    A high alloy CrMnNi TRIP steel has been processed by electron beam melting, a powder-bed based additive manufacturing (AM) technology, to investigate its fatigue properties. The material was characterized by average grain sizes of 32 μm in the as-built and 106 μm in the solution annealed state. Total strain controlled fatigue tests with strain amplitudes in the range of 0.25% ≤ Δεt/2 ≤ 1.2% were performed revealing a similar cyclic deformation behavior and α′-martensite evolution compared to a hot pressed reference material. Moreover, the fatigue lives of the EBM states were surprisingly high in consideration of severe process-induced lack of fusion defects of more than 500 μm revealed by investigations of the fracture surfaces. Thus, the impact of these inhomogeneities was substantially alleviated by the outstanding damage tolerance of the present TRIP steel induced by its high ductility and remarkable hardening capacity. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijfatigue.2018.05.031
  • 2018 • 218 Development of W-coating with functionally graded W/EUROFER-layers for protection of First-Wall materials
    Emmerich, T. and Qu, D. and Vaßen, R. and Aktaa, J.
    Fusion Engineering and Design 128 58-67 (2018)
    To protect First-Wall components, made of reduced activation ferritic martensitic steel, against the plasma of future fusion reactors, tungsten coatings are a feasible option. The difference in coefficient of thermal expansion between the coating and the steel substrate can be compensated using functionally graded material layers. Such layers were successfully produced by vacuum plasma spraying. This technique reduces, however, the hardness of the substrate surface near zone. Modified spraying parameters moderate the hardness loss. The parameters may, though, affect also the layer bonding toughness which is evaluated in this work by four point bending tests. Furthermore, the layers behavior on First-Wall Mock‐ups and under different thermal loads is investigated by finite element simulations. The measurement of the layer adhesion indicates that the layer adhesion decreases only for modified spraying parameters that do not reduce the substrate hardness. It follows also from the toughness calculation that without layer residual stresses the toughness values depend on coating thickness. In regard to the Mock‐up behavior the simulations show that intermediate steps are necessary during heating and cooling to prevent artificial stresses and inelastic deformation. It is, however, not possible to avoid stresses and inelastic deformation completely as they originate from the residual stresses. © 2018
    view abstractdoi: 10.1016/j.fusengdes.2018.01.047
  • 2018 • 217 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 • 216 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 • 215 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 • 214 Deformation induced degradation of hot-dip aluminized steel
    Lemmens, B. and Springer, H. and Peeters, M. and De Graeve, I. and De Strycker, J. and Raabe, D. and Verbeken, K.
    Materials Science and Engineering A 710 385-391 (2018)
    In this work the fracture and corrosion behaviour of hot-dip aluminized steels is investigated in controlled dipping experiments which allowed to separately study the effects of Si in the Al bath (1–10 wt%) and the intermetallic phase thickness (5–30 µm). The addition of Si had no direct influence on the performance of the coating system for similar thickness values of the IMP seam, which in turn showed to be the dominant factor independent from the amount of Si. Thin intermetallic phase seams (< about 10 µm) exhibited more (about 5–10 per 100 µm interfacial length) but smaller cracks with a fishnet pattern on the outer Al-Si coating, which remained intact and interconnected until a tensile deformation of 15–20%. Thicker intermetallic phase seams resulted in less (about 2 per 100 µm interfacial length) but broader cracks perpendicular to the tensile direction, giving rise to a lamellar pattern on the Al-Si coating, which cracks and uncovers the steel already at strains below 10%, and readily flakes off leaving the steel substrate to accelerated corrosion in chloride environments. Our results indicate that the reduction of the intermetallic phase seam thickness remains the main target to improve the performance of hot-dip aluminized coated steel by combining appropriate Si additions with minimized dipping temperatures and times. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.10.094
  • 2018 • 213 An integrated crystal plasticity-phase field model for spatially resolved twin nucleation, propagation, and growth in hexagonal materials
    Liu, C. and Shanthraj, P. and Diehl, M. and Roters, F. and Dong, S. and Dong, J. and Ding, W. and Raabe, D.
    International Journal of Plasticity 106 203-227 (2018)
    Typical hexagonal engineering materials, such as magnesium and titanium, deform extensively through shear strains and crystallographic re-orientations associated with the nucleation, propagation, and growth of twins. To accurately predict their deformation behavior it is, therefore, critical for constitutive models to incorporate these mechanisms. In this work an integrated approach for modeling the concurrent dislocation mediated plasticity and heterogeneous twinning behavior in hexagonal materials is presented. A dislocation density-based crystal plasticity model is employed to predict the heterogeneous distribution of stress, strain and dislocation activity and is coupled to a phase field model for the description of the nucleation, propagation, and growth of {1012} tensile twins. A stochastic model is used to nucleate twins at grain boundaries, and their subsequent propagation and growth are driven by the Ginzburg-Landau relaxation of the system free energy which includes the orientation dependent twin interfacial energy and the elastic strain energy. Application of this novel and fully coupled model to the cases of magnesium single crystal, bicrystal, and polycrystal deformation is shown to demonstrate its predictive capability. Numerical simulations predict, in accordance with experimental observations, twin nucleation at grain boundaries followed by twin propagation into the grain interior and subsequent transverse twin thickening. Through this new combination of modeling approaches it is possible to systematically study the twin induced strain fields, the stress distribution along twin boundaries, and the spatial evolution of dislocation density within twins and parent grains. © 2018 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijplas.2018.03.009
  • 2018 • 212 Hyperelastic bodies under homogeneous Cauchy stress induced by three-dimensional non-homogeneous deformations
    Mihai, L.A. and Neff, P.
    Mathematics and Mechanics of Solids 23 606-616 (2018)
    In isotropic finite elasticity, unlike in linear elastic theory, a homogeneous Cauchy stress may be induced by non-homogeneous strains. To illustrate this, we identify compatible non-homogeneous three-dimensional deformations producing a homogeneous Cauchy stress on a cuboid geometry, and provide an example of an isotropic hyperelastic material that is not rank-one convex, and for which the homogeneous stress and associated non-homogeneous strains are given explicitly on a domain similar to those analysed. © 2016, © The Author(s) 2016.
    view abstractdoi: 10.1177/1081286516682556
  • 2018 • 211 Crystallographic examination of the interaction between texture evolution, mechanically induced martensitic transformation and twinning in nanostructured bainite
    Morales-Rivas, L. and Archie, F. and Zaefferer, S. and Benito-Alfonso, M. and Tsai, S.-P. and Yang, J.-R. and Raabe, D. and Garcia-Mateo, C. and Caballero, F.G.
    Journal of Alloys and Compounds 752 505-519 (2018)
    The deformation mechanisms operating in nanostructured bainite, leading to its excellent combination of strength and ductility, are far from being understood. Its nanocrystalline nature and its multiphase-evolving structure underlie the plastic flow and the strain-hardening behaviour. In this work, the microstructural and crystallographic bulk changes of a high-C nanostructured bainite under tensile testing have been evaluated. The influence of the mechanically-induced transformation of the C-enriched retained austenite into α martensite and other deformation mechanisms on the texture evolution has been analysed by electron backscatter diffraction (EBSD). Additionally, the undeformed and the deformed conditions have been examined by electron channelling contrast imaging (ECCI) and transmission electron microscopy (TEM). Results reveal the presence of plate martensite and suggest a strong variant selection during the transformation, mainly responsible for the texture observed. Mechanical twinning in austenite seems to be basically the mechanism of accommodation of the displacive bainitic transformation, while some direct interaction with the applied stress also appears. © 2018
    view abstractdoi: 10.1016/j.jallcom.2018.04.189
  • 2018 • 210 Numerical and Experimental Study of the Spatial Stress Distribution on the Cornea Surface During a Non-Contact Tonometry Examination
    Muench, S. and Roellig, M. and Spoerl, E. and Balzani, D.
    Experimental Mechanics (2018)
    The determination of biomechanical properties of the cornea by a non-contact tonometry (NCT) examination requires a precise knowledge of the air puff generated in the device, which is applied to the cornea surface. In this study, a method is proposed to identify the resulting stress profile on the surface, which may be used to numerically solve an inverse problem to obtain the material properties. This method is based on an experimental characterization of the air puff created by the Corvis ST in combination with computational fluid dynamic (CFD) simulations, which are adjusted to the experimental data. The identified nozzle inlet pressure of approximately 25 kPa (188.5mmHg) is then used for a numerical influence study of the interaction between the air puff and the cornea deformation. Therefore, eleven cornea deformation states based on measurements are implemented in the CFD model. A more realistic model is also analyzed by the geometrical reproduction of the human face, which is used for a further influence study. The outcomes showed a dependence between the cornea deformation and the pressure as well as the shear stress distribution. However, quantitatively, the shear stress component can be considered of minor importance being approximately one hundred times smaller than the pressure. The examination with consideration of the human face demonstrates that the pressure and shear stress distributions are not rotationally symmetric in measurements on real humans, which indicates the requirement to include more complex stress distributions on the eye. We present the detailed stress distribution on the cornea during a non-contact tonometry examination, which is made accessible for further investigations in the future by analytical nonlinear functions. © 2018, Society for Experimental Mechanics.
    view abstractdoi: 10.1007/s11340-018-00449-0
  • 2018 • 209 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 • 208 Inherent toughness and fracture mechanisms of refractory transition-metal nitrides via density-functional molecular dynamics
    Sangiovanni, D.G.
    Acta Materialia 151 11-20 (2018)
    Hard refractory transition-metal nitrides possess unique combinations of outstanding mechanical and physical properties, but are typically brittle. Recent experimental results demonstrated that single-crystal NaCl-structure (B1) V0.5Mo0.5N pseudobinary solid solutions are both hard (∼20 GPa) and ductile; that is, they exhibit toughness, which is unusual for ceramics. However, key atomic-scale mechanisms underlying this inherent toughness are unknown. Here, I carry out density-functional ab initio molecular dynamics (AIMD) simulations at room temperature to identify atomistic processes and associated changes in the electronic structure which control strength, plasticity, and fracture in V0.5Mo0.5N, as well as reference B1 TiN, subject to &lt;001&gt; and &lt;110&gt; tensile deformation. AIMD simulations reveal that V0.5Mo0.5N is considerably tougher than TiN owing to its ability to (i) isotropically redistribute mechanical stresses within the elastic regime, (ii) dissipate the accumulated strain energy by activating local structural transformations beyond the yield point. In direct contrast, TiN breaks in brittle manner when applied stresses reach its tensile strength. Charge transfer maps show that the adaptive mechanical response of V0.5Mo0.5N originates from highly populated d-d metallic-states, which allow for counterbalancing the destabilization induced via tensile deformation by enabling formation of new chemical bonds. The high ionic character and electron-localization in TiN precludes the possibility of modifying bonding geometries to accommodate the accumulated stresses, thus suddenly causing material's fracture for relatively low strain values. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.03.038
  • 2018 • 207 Frequency-dependent fatigue and corrosion fatigue assessment of brazed AISI 304L/BNi-2 joints in air and synthetic exhaust gas condensate
    Schmiedt, A. and Lücker, L. and Manka, M. and Tillmann, W. and Walther, F.
    Fatigue and Fracture of Engineering Materials and Structures 41 2338-2349 (2018)
    Since brazed components are often cyclically loaded in corrosive environments, the corrosion fatigue behaviour of the joints has to be investigated. Fatigue tests of brazed AISI 304L/BNi-2 joints, relevant for exhaust gas heat exchangers, were performed with specimens in the as-received condition and after pre-corrosion according to VDA 230-214. Additionally, the superimposed corrosion fatigue loading in an exhaust gas condensate was realised using a corrosion cell. Corrosion-induced and deformation-induced microstructural changes were metallographically evaluated. The influence of the test frequency from 1 to 150 Hz on the cyclic deformation and damage behaviour was characterised. In a statistical analysis, the fatigue strength of 210 MPa at 2·107 cycles was determined for the as-received condition with a 50% failure probability. The pre-corrosion as well as the superimposed loading lead to a reduction of the fatigue strength down to 22%. A novel test strategy is suitable for precise fatigue and corrosion fatigue assessments. © 2018 Wiley Publishing Ltd.
    view abstractdoi: 10.1111/ffe.12902
  • 2018 • 206 Different approaches for mixed LSFEMs in hyperelasticity: Application of logarithmic deformation measures
    Schwarz, A. and Steeger, K. and Igelbüscher, M. and Schröder, J.
    International Journal for Numerical Methods in Engineering 115 1138-1153 (2018)
    We present geometrically nonlinear formulations based on a mixed least-squares finite element method. The L2-norm minimization of the residuals of the given first-order system of differential equations leads to a functional, which is a two-field formulation dependent on displacements and stresses. Based thereon, we discuss and investigate two mixed formulations. Both approaches make use of the fact that the stress symmetry condition is not fulfilled a priori due to the row-wise stress approximation with vector-valued functions belonging to a Raviart-Thomas space, which guarantees a conforming discretization of H(div). In general, the advantages of using the least-squares finite element method lie, for example, in an a posteriori error estimator without additional costs or in the fact that the choice of the polynomial interpolation order is not restricted by the Ladyzhenskaya-Babuška-Brezzi condition (inf-sup condition). We apply a hyperelastic material model with logarithmic deformation measures and investigate various benchmark problems, adaptive mesh refinement, computational costs, and accuracy. Copyright © 2018 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/nme.5838
  • 2018 • 205 Deformation of Mesoporous Titania Nanostructures in Contact with D2O Vapor
    Song, L. and Rawolle, M. and Hohn, N. and Gutmann, J.S. and Frielinghaus, H. and Müller-Buschbaum, P.
    Small 14 (2018)
    For many applications, mesoporous titania nanostructures are exposed to water or need to be backfilled via infiltration with an aqueous solution, which can cause deformations of the nanostructure by capillary forces. In this work, the degree of deformation caused by water infiltration in two types of mesoporous, nanostructured titania films exposed to water vapor is compared. The different types of nanostructured titania films are prepared via a polymer template assisted sol–gel synthesis in conjunction with a polymer-template removal at high-temperatures under ambient conditions versus nitrogen atmosphere. Information about surface and inner morphology is extracted by scanning electron microscopy and grazing incidence small-angle neutron scattering (GISANS) measurements, respectively. Furthermore, complementary information on thin film composition and porosity are probed via X-ray reflectivity. The backfilling induced deformation of near surface structures and structures inside the mesoporous titania films is determined by GISANS before and after D2O infiltration. The respective atmosphere used for template removal influences the details of the titania nanostructure and strongly impacts the degree of water induced deformation. Drying of the films shows reversibility of the deformation. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/smll.201801461
  • 2018 • 204 Finite-deformation phase-field chemomechanics for multiphase, multicomponent solids
    Svendsen, B. and Shanthraj, P. and Raabe, D.
    Journal of the Mechanics and Physics of Solids 112 619-636 (2018)
    The purpose of this work is the development of a framework for the formulation of geometrically non-linear inelastic chemomechanical models for a mixture of multiple chemical components diffusing among multiple transforming solid phases. The focus here is on general model formulation. No specific model or application is pursued in this work. To this end, basic balance and constitutive relations from non-equilibrium thermodynamics and continuum mixture theory are combined with a phase-field-based description of multicomponent solid phases and their interfaces. Solid phase modeling is based in particular on a chemomechanical free energy and stress relaxation via the evolution of phase-specific concentration fields, order-parameter fields (e.g., related to chemical ordering, structural ordering, or defects), and local internal variables. At the mixture level, differences or contrasts in phase composition and phase local deformation in phase interface regions are treated as mixture internal variables. In this context, various phase interface models are considered. In the equilibrium limit, phase contrasts in composition and local deformation in the phase interface region are determined via bulk energy minimization. On the chemical side, the equilibrium limit of the current model formulation reduces to a multicomponent, multiphase, generalization of existing two-phase binary alloy interface equilibrium conditions (e.g., KKS). On the mechanical side, the equilibrium limit of one interface model considered represents a multiphase generalization of Reuss-Sachs conditions from mechanical homogenization theory. Analogously, other interface models considered represent generalizations of interface equilibrium conditions consistent with laminate and sharp-interface theory. In the last part of the work, selected existing models are formulated within the current framework as special cases and discussed in detail. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2017.10.005
  • 2018 • 203 On the role of the collinear dislocation interaction in deformation patterning and laminate formation in single crystal plasticity
    Wang, D. and Diehl, M. and Roters, F. and Raabe, D.
    Mechanics of Materials 125 70-79 (2018)
    We investigate the reasons for severe deformation patterning observed in crystal plasticity simulations of an fcc nickel single crystal with initial near-Copper orientation deformed in plane strain compression. The resulting strain partitioning in the form of alternating parallel bands initiates at a very early loading stage, i.e. <0.5% global strain, and sharpens with ongoing deformation. At an applied average strain of 5.5%, the local strains finally deviate by half an order of magnitude in different regions of the initially homogeneous single crystal. We show that this microstructure lamination is the result of a complex interplay between available deformation systems, strain hardening, kinematics, and deformation energetics. Moreover, the boundary conditions play an important role as under the applied load two slip systems—which are collinear with respect to each other—have the same highest Schmid factor and therefore are preferentially activated. During strain hardening, the strong collinear interaction strength causes—depending on the initial deviation from the nominal orientation—the selection of a single prevalent slip system in clearly delimited regions. This behavior is explained by the lower global deformation energy in comparison to a homogeneous double slip behavior. We also reveal that the observed deformation pattern forms only for dislocation interaction strength values in the range predicted by discrete dislocation dynamic simulations. © 2018
    view abstractdoi: 10.1016/j.mechmat.2018.06.007
  • 2018 • 202 In-situ SEM observation of phase transformation and twinning mechanisms in an interstitial high-entropy alloy
    Wang, M. and Li, Z. and Raabe, D.
    Acta Materialia 147 236-246 (2018)
    The recently developed interstitial high-entropy alloys (iHEAs) exhibit an enhanced combination of strength and ductility. These properties are attributed to dislocation hardening, deformation-driven athermal phase transformation from the face-centered cubic (FCC) γ matrix into the hexagonal close-packed (HCP) ε phase, stacking fault formation, mechanical twinning and precipitation hardening. For gaining a better understanding of these mechanisms as well as their interactions direct observation of the deformation process is required. For this purpose, an iHEA with nominal composition of Fe-30Mn-10Co-10Cr-0.5C (at. %) was produced and investigated via in-situ and interrupted in-situ tensile testing in a scanning electron microscope (SEM) combining electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) techniques. The results reveal that the iHEA is deformed by formation and multiplication of stacking faults along {111} microbands. Sufficient overlap of stacking faults within microbands leads to intrinsic nucleation of HCP ε phase and incoherent annealing twin boundaries act as preferential extrinsic nucleation sites for HCP ε formation. With further straining HCP ε nuclei grow into the adjacent deformed FCC γ matrix. γ regions with smaller grain size have higher mechanical stability against phase transformation. Twinning in FCC γ grains with a size of ∼10 μm can be activated at room temperature at a stress below ∼736 MPa. With increasing deformation, new twin lamellae continuously nucleate. The twin lamellae grow in preferred directions driven by the motion of the mobile partial dislocations. Owing to the individual grain size dependence of the activation of the dislocation-mediated plasticity, of the athermal phase transformation and of mechanical twinning at the different deformation stages, desired strain hardening profiles can be tuned and adjusted over the entire deformation regime by adequate microstructure design, providing excellent combinations of strength and ductility. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.01.036
  • 2018 • 201 Spheroidal and conical shapes of ferrofluid-filled capsules in magnetic fields
    Wischnewski, C. and Kierfeld, J.
    Physical Review Fluids 3 (2018)
    We investigate the deformation of soft spherical elastic capsules filled with a ferrofluid in external uniform magnetic fields at fixed volume by a combination of numerical and analytical approaches. We develop a numerical iterative solution strategy based on nonlinear elastic shape equations to calculate the stretched capsule shape numerically and a coupled finite element and boundary element method to solve the corresponding magnetostatic problem and employ analytical linear response theory, approximative energy minimization, and slender-body theory. The observed deformation behavior is qualitatively similar to the deformation of ferrofluid droplets in uniform magnetic fields. Homogeneous magnetic fields elongate the capsule and a discontinuous shape transition from a spheroidal shape to a conical shape takes place at a critical field strength. We investigate how capsule elasticity modifies this hysteretic shape transition. We show that conical capsule shapes are possible but involve diverging stretch factors at the tips, which gives rise to rupture for real capsule materials. In a slender-body approximation we find that the critical susceptibility above which conical shapes occur for ferrofluid capsules is the same as for droplets. At small fields capsules remain spheroidal and we characterize the deformation of spheroidal capsules both analytically and numerically. Finally, we determine whether wrinkling of a spheroidal capsule occurs during elongation in a magnetic field and how it modifies the stretching behavior. We find the nontrivial dependence between the extent of the wrinkled region and capsule elongation. Our results can be helpful in quantitatively determining capsule or ferrofluid material properties from magnetic deformation experiments. All results also apply to elastic capsules filled with a dielectric liquid in an external uniform electric field. © 2018 American Physical Society.
    view abstractdoi: 10.1103/PhysRevFluids.3.043603
  • 2018 • 200 Strong Deformation of Ferrofluid-Filled Elastic Alginate Capsules in Inhomogenous Magnetic Fields
    Wischnewski, C. and Zwar, E. and Rehage, H. and Kierfeld, J.
    Langmuir 34 13534-13543 (2018)
    We present a new system based on alginate gels for the encapsulation of a ferrofluid drop, which allows us to create millimeter-sized elastic capsules that are highly deformable by inhomogeneous magnetic fields. We use a combination of experimental and theoretical work in order to characterize and quantify the deformation behavior of these ferrofluid-filled capsules. We introduce a novel method for the direct encapsulation of unpolar liquids by sodium alginate. By adding 1-hexanol to the unpolar liquid, we can dissolve sufficient amounts of CaCl2 in the resulting mixture for ionotropic gelation of sodium alginate. The addition of polar alcohol molecules allows us to encapsulate a ferrofluid as a single phase rather than an emulsion without impairing ferrofluid stability. This encapsulation method increases the amount of encapsulated magnetic nanoparticles resulting in high deformations of approximately 30% (in height-to-width ratio) in inhomogeneous magnetic field with magnetic field variations of 50 mT over the size of the capsule. This offers possible applications of capsules as actuators, switches, or valves in confined spaces like microfluidic devices. We determine both elastic moduli of the capsule shell, Young's modulus and Poisson's ratio, by employing two independent mechanical methods, spinning capsule measurements and capsule compression between parallel plates. We then show that the observed magnetic deformation can be fully understood from magnetic forces exerted by the ferrofluid on the capsule shell if the magnetic field distribution and magnetization properties of the ferrofluid are known. We perform a detailed analysis of the magnetic deformation by employing a theoretical model based on nonlinear elasticity theory. Using an iterative solution scheme that couples a finite element/boundary element method for the magnetic field calculation to the solution of the elastic shape equations, we achieve quantitative agreement between theory and experiment for deformed capsule shapes using the Young modulus from mechanical characterization and the surface Poisson ratio as a fit parameter. This detailed analysis confirms the results from mechanical characterization that the surface Poisson ratio of the alginate shell is close to unity, that is, deformations of the alginate shell are almost area conserving. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.8b02357
  • 2018 • 199 Production, deformation and mechanical investigation of magnetic alginate capsules
    Zwar, E. and Kemna, A. and Richter, L. and Degen, P. and Rehage, H.
    Journal of Physics Condensed Matter 30 (2018)
    In this article we investigated the deformation of alginate capsules in magnetic fields. The sensitivity to magnetic forces was realised by encapsulating an oil in water emulsion, where the oil droplets contained dispersed magnetic nanoparticles. We solved calcium ions in the aqueous emulsion phase, which act as crosslinking compounds for forming thin layers of alginate membranes. This encapsulating technique allows the production of flexible capsules with an emulsion as the capsule core. It is important to mention that the magnetic nanoparticles were stable and dispersed throughout the complete process, which is an important difference to most magnetic alginate-based materials. In a series of experiments, we used spinning drop techniques, capsule squeezing experiments and interfacial shear rheology in order to determine the surface Young moduli, the surface Poisson ratios and the surface shear moduli of the magnetically sensitive alginate capsules. In additional experiments, we analysed the capsule deformation in magnetic fields. In spinning drop and capsule squeezing experiments, water droplets were pressed out of the capsules at elevated values of the mechanical load. This phenomenon might be used for the mechanically triggered release of water-soluble ingredients. After drying the emulsion-filled capsules, we produced capsules, which only contained a homogeneous oil phase with stable suspended magnetic nanoparticles (organic ferrofluid). In the dried state, the thin alginate membranes of these particles were rather rigid. These dehydrated capsules could be stored at ambient conditions for several months without changing their properties. After exposure to water, the alginate membranes rehydrated and became flexible and deformable again. During this swelling process, water diffused back in the capsule. This long-term stability and rehydration offers a great spectrum of different applications as sensors, soft actuators, artificial muscles or drug delivery systems. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aaa6f5
  • 2017 • 198 A finite deformation continuum modelling framework for curvature effects in fibre-reinforced nanocomposites
    Asmanoglo, T. and Menzel, A.
    Journal of the Mechanics and Physics of Solids 107 411-432 (2017)
    Motivated by experimental findings on one-dimensional nano-materials, this contribution focusses on the elaboration of a fibre curvature based higher-order gradient contribution to the stored energy function in a finite deformation setting. The presented approach is based on the fundamental theoretical developments for fibre-reinforced composites presented by Spencer and Soldatos (2007), which take into account the fibre-bending stiffness in addition to the directional dependency induced by the fibres. A mixed-type finite element formulation is then used for the solution of the resulting system of coupled partial differential equations. A specific form of the stored energy function is introduced such that well-interpretable contributions to the stress- and the couple stress tensor are obtained. It is shown that this framework may, in principle, account for fibres of different diameters and induces a natural length scale into the model. Such continuum theory covering size-effects is of special interest since experiments for different materials suggest significant size-effects at small length scales. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2017.06.012
  • 2017 • 197 Room temperature deformation of LPSO structures by non-basal slip
    Chen, R. and Sandlöbes, S. and Zeng, X. and Li, D. and Korte-Kerzel, S. and Raabe, D.
    Materials Science and Engineering A 682 354-358 (2017)
    We investigated the deformation mechanisms of long period stacking ordered (LPSO) structures in an extruded Mg97Y2Zn1 (at%) alloy. Tensile deformation was performed in such a way that basal slip and kink band formation were inhibited. Slip trace analysis and transmission electron microscopy reveal a predominant activity of non-basal < a&gt; slip. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2016.11.056
  • 2017 • 196 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 • 195 The relaxed-polar mechanism of locally optimal Cosserat rotations for an idealized nanoindentation and comparison with 3D-EBSD experiments
    Fischle, A. and Neff, P. and Raabe, D.
    Zeitschrift fur Angewandte Mathematik und Physik 68 (2017)
    The rotation polar (F) ∈ SO (3) arises as the unique orthogonal factor of the right polar decomposition F=polar(F)U of a given invertible matrix F∈ GL +(3). In the context of nonlinear elasticity Grioli (Boll Un Math Ital 2:252–255, 1940) discovered a geometric variational characterization of polar (F) as a unique energy-minimizing rotation. In preceding works, we have analyzed a generalization of Grioli’s variational approach with weights (material parameters) μ&gt; 0 and μc≥ 0 (Grioli: μ= μc). The energy subject to minimization coincides with the Cosserat shear–stretch contribution arising in any geometrically nonlinear, isotropic and quadratic Cosserat continuum model formulated in the deformation gradient field F: = ∇ φ: Ω → GL +(3) and the microrotation field R: Ω → SO (3). The corresponding set of non-classical energy-minimizing rotations rpolarμ,μc±(F):=arg minR∈SO(3){Wμ,μc(R;F):=μ||sym(RTF-1)||2+μc||skew(RTF-1)||2}represents a new relaxed-polar mechanism. Our goal is to motivate this mechanism by presenting it in a relevant setting. To this end, we explicitly construct a deformation mapping φnano which models an idealized nanoindentation and compare the corresponding optimal rotation patterns rpolar1,0±(Fnano) with experimentally obtained 3D-EBSD measurements of the disorientation angle of lattice rotations due to a nanoindentation in solid copper. We observe that the non-classical relaxed-polar mechanism can produce interesting counter-rotations. A possible link between Cosserat theory and finite multiplicative plasticity theory on small scales is also explored. © 2017, Springer International Publishing AG.
    view abstractdoi: 10.1007/s00033-017-0834-4
  • 2017 • 194 Analysis of dislocation structures in ferritic and dual phase steels regarding continuous and discontinuous loading paths
    Gerstein, G. and Clausmeyer, T. and Gutknecht, F. and Tekkaya, A.E. and Nürnberger, F.
    Minerals, Metals and Materials Series Part F6 203-210 (2017)
    In sheet-bulk metal forming processes the hardening behavior of the material depends on the sequence of deformation steps and the type of deformation. Loading path changes induce transient hardening phenomena. These phenomena are linked to the formation and interaction of oriented dislocation structures. The aim of this study is to investigate the effect of continuous and discontinuous loading path changes on the dislocation microstructure in ferritic and ferritic-martensitic dual-phase steel, respectively. For the experiments a biaxial test stand was used, which permits to continuously change the load from tension to shear. In the ferrite single-phase steel transmission-electron microscopy reveals a reduced evolution of oriented dislocation structures for continuous loading path changes compared to discontinuous loading path changes. This evolution is further decreased in dual-phase steel compared to the ferritic steel. Microstructural results for the ferritic steel are accompanied by simulation results with a transient hardening model. © The Minerals, Metals & Materials Society 2017.
    view abstractdoi: 10.1007/978-3-319-51493-2_20
  • 2017 • 193 Effect of workpiece deformation on Joule heat losses in electromagnetic forming coils
    Gies, S. and Tekkaya, A.E.
    Procedia Engineering 207 341-346 (2017)
    Coils in electromagnetic forming operations are exposed to mechanical as well as thermal loads. Especially in case of high volume production the thermal loading due to Joule heating needs to be considered in the coil and process design to prevent thermal overstressing. An analytical approach for the calculation of Joule heat losses in electromagnetic forming coils considering the changing gap width between coil and workpiece is presented. An electromagnetic sheet metal forming application using a straight one-turn coil is used as reference case to prove the accuracy of the model. A comparison of the analytical calculation and the numerical results based on a simulation with a coupling of structural, thermal, and electromagnetic effects is provided. It is proved that the coil heating in case of a deforming workpiece ranges between the heating without workpiece and the heating with rigid workpiece. It is shown that with increasing workpiece velocities the heat losses in the coil tend more and more towards the lower bound represented by a coil without workpiece.
    view abstractdoi: 10.1016/j.proeng.2017.10.785
  • 2017 • 192 Consistent elastoplastic cohesive zone model at finite deformations – Variational formulation
    Heitbreder, T. and Ottosen, N.S. and Ristinmaa, M. and Mosler, J.
    International Journal of Solids and Structures 106-107 284-293 (2017)
    Cohesive zone models based on classical interface-type formulations at finite deformations are subjected to fundamental physical principles such as thermodynamical consistency, balance equations and material frame indifference. However, these restraints are often ignored and in that respect such formulations are inconsistent. By way of contrast, a consistent cohesive zone framework suitable for the analysis of localized elastoplastic deformations which only depends on the displacement jump was recently advocated in Ottosen et al. (2015). A certain subclass of this consistent framework is analyzed here, further extended and finally, an efficient numerical implementation is proposed. Conceptually, the considered cohesive zone model is a fiber-like model where the fiber direction is defined by the direction of the displacement discontinuity. A novel unloading model is advocated where the key idea is to assign a vanishing bending stiffness to the fibers and they therefore buckle when compressive stresses are initiated. Following ideas known from wrinkling in membranes, it is shown that the resulting framework can be rewritten into a variationally consistent format such that all unknowns follow jointly from minimizing a time-dependent potential whose discretization leads to an efficient implementation in terms of an efficient variational constitutive update. The physical properties of the final constitutive framework are analyzed by means of numerical examples. This analysis shows that although the framework is based on elastoplasticity, it predicts for the L-shaped structure investigated a mechanical response similar to that of damage theory even during unloading. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijsolstr.2016.10.027
  • 2017 • 191 A generalized micromorphic approach accounting for variation and dispersion of preferred material directions
    Horn-Von Hoegen, M. and Skatulla, S. and Schröder, J.
    Computers and Structures (2017)
    Materials exhibiting a heterogeneous and non-uniform composition in terms of elastic and anisotropic properties such as biological tissues require special efforts to accurately describe their constitutive behavior. In contrast to classical models, micromorphic formulations can predict the macroscopically observable material response as originated from distinct scale-dependent micro-structural deformation mechanisms. This is facilitated by additional independent degrees of freedom and associated additional strain and stress quantities. Here, a generalized continuum is mathematically constructed from a macro-continuum and a micro-continuum which are both adequately coupled on kinematics and constitutive levels as well as by micro-boundary conditions. In view of biomechanical modeling, the potential of the formulation is studied for a number of academic examples characterized by an anisotropic material composition to elucidate the micromorphic material response as compared with the one obtained using a classical continuum mechanics approach. The results demonstrate the ability of the generalized continuum approach to address non-affine elastic reorientation of the preferred material direction in the macro-space and its dispersion in the micro-space as affecting deformation, strain and stress on the macroscopic level. In particular, if the anisotropy in the micromorphic formulation is solely linked to the extra degrees of freedom and associated strain and stress measures, the deformation for small and large strains is shown to be distinctly different to the classical response. Together with the ability to implicitly account for scale-dependent higher-order deformation effects in the constitutive law the proposed generalized micromorphic formulation provides an advanced description, especially for fibrous biological materials. © 2017 Elsevier Ltd.
    view abstractdoi: 10.1016/j.compstruc.2017.11.013
  • 2017 • 190 Comparative characterization of quasi-static and cyclic deformation behavior of glass fiber-reinforced polyurethane (GFR-PU) and epoxy (GFR-EP)
    Hülsbusch, D. and Jamrozy, M. and Frieling, G. and Mueller, Y. and Barandun, G.A. and Niedermeier, M. and Walther, F.
    Materialpruefung/Materials Testing 59 109-117 (2017)
    Glass fiber-reinforced polymers (GFRP) are highly suitable for use in transportation industry in order to achieve the targets of energy and resource efficiency. In this context, due to its high specific strength, GFR-epoxy (GFR-EP) has already been implemented in a wide range of applications. However, in cases of energy efficiency and damage tolerance, GFR-EP shows disadvantages compared to GFR-polyurethane (GFR-PU). The aim of this study is the comparative characterization of the quasi-static and cyclic deformation behavior of GFR-PU and GFR-EP with similar layer setup. The mechanical properties have been investigated in instrumented tensile, interlaminar shear strength and compression after impact tests. In addition, the tests were combined with varying temperatures (-30 °C, RT, +70 °C) with respect to aerospace applications to determine the material property development under low and elevated temperatures. In cyclic investigations, the fatigue properties have been estimated by resource-efficient multiple step tests and validated in constant amplitude tests. Hysteresis and temperature measurements were applied in order to investigate the damage processes. It could be shown that polyurethane exhibits improved damage tolerance by significantly reducing delamination area under impact loading, whereas epoxy leads to optimized properties under elevated temperature. Furthermore, epoxy generally underlines higher capabilities under cyclic loading, which is due to void content of polyurethane. © Carl Hanser Verlag GmbH &Co. KG.
    view abstractdoi: 10.3139/120.110972
  • 2017 • 189 Mechanism-oriented characterization of the fatigue behavior of glass fiber-reinforced polyurethane based on hysteresis and temperature measurements
    Hülsbusch, D. and Jamrozy, M. and Mrzljak, S. and Walther, F.
    Key Engineering Materials 742 KEM 629-635 (2017)
    In order to optimize resource efficiency, glass fiber-reinforced polymers (GFRP) have been implemented in recent years in a wide range of applications in transportation industry. In this context, GFR-epoxy (GFR-EP) is currently being used mainly because of their sufficiently investigated properties and production processes. Polyurethane (PU), however, shows advantages in terms of energy efficiency and damage tolerance. The aim of this study is the characterization of the fatigue behavior of GFR-PU by stepwise exploration of damage development on microscopic level. Therefore, multiple amplitude and constant amplitude tests have been carried out. Hysteresis and temperature measurements were applied in order to investigate the damage processes and correlated with in situ computed tomography (CT) in intermitting tests. The damage development and mechanisms could be characterized and separated. The results confirm known GFRP damage characteristics, whereas also material-specific peculiarities regarding crack development could be revealed. © 2017 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.742.629
  • 2017 • 188 Constitutive modeling of strain induced grain boundary migration via coupling crystal plasticity and phase-field methods
    Jafari, M. and Jamshidian, M. and Ziaei-Rad, S. and Raabe, D. and Roters, F.
    International Journal of Plasticity 99 19-42 (2017)
    We have developed a thermodynamically-consistent finite-deformation-based constitutive theory to describe strain induced grain boundary migration due to the heterogeneity of stored deformation energy in a plastically deformed polycrystalline cubic metal. Considering a representative volume element, a mesoscale continuum theory is developed based on the coupling between dislocation density-based crystal plasticity and phase field methods. Using the Taylor model-based homogenization method, a multiscale coupled finite-element and phase-field staggered time integration procedure is developed and implemented into the Abaqus/Standard finite element package via a user-defined material subroutine. The developed constitutive model is then used to perform numerical simulations of strain induced grain boundary migration in polycrystalline tantalum. The simulation results are shown to qualitatively and quantitatively agree with experimental results. © 2017 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijplas.2017.08.004
  • 2017 • 187 Pre- and post-buckling behavior of bi-crystalline micropillars: Origin and consequences
    Kirchlechner, C. and Toth, F. and Rammerstorfer, F.G. and Fischer, F.D. and Dehm, G.
    Acta Materialia 124 195-203 (2017)
    Compression of micropillars is routinely used to measure the material response under uniaxial load. In bi-crystalline pillars an S-shaped grain-boundary together with an S-shaped pillar is often observed after deformation raising the question of its origin and consequences for stress-strain materials data. In addition to dislocation and grain-boundary based mechanisms, this observation can be caused by buckling and subsequent post-buckling deformation. Deviations from the classical pre- and post-buckling deformation behavior are assigned to imperfections, which are categorized in extrinsic and intrinsic imperfections in this work. In the present paper, the S-shaped actual deformation state is particularly promoted by an intrinsic imperfection, caused by a material heterogeneity (due to the bi-crystal arrangement). This kind of deformation behavior is investigated by micro-compression experiments on 7 × 7 × 21 μm3 sized bi-crystal copper pillars with nearly elastic (axial Young's modulus) homogeneity and identical Schmid factors for both grain orientations. Complementary finite element simulations are performed, in which also the role of friction and of an extrinsic imperfection in the form of initial misalignment of the loading on the S-shape are considered. There, a material model describing the flow stress distribution caused by a dislocation pile-up at the grain-boundary is applied. Finally, suggestions to prevent buckling and, thus, transversal post-buckling displacements during micropillar compression tests are given with the goal to extract engineering stress-strain curves. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.10.075
  • 2017 • 186 Integrability conditions between the first and second Cosserat deformation tensor in geometrically nonlinear micropolar models and existence of minimizers
    Lankeit, J. and Neff, P. and Osterbrink, F.
    Zeitschrift fur Angewandte Mathematik und Physik 68 (2017)
    In this note, we extend integrability conditions for the symmetric stretch tensor U in the polar decomposition of the deformation gradient ∇φ=F=RU to the nonsymmetric case. In doing so, we recover integrability conditions for the first Cosserat deformation tensor. Let (Formula presented.). Then, (Formula presented.), giving a connection between the first Cosserat deformation tensor U¯ and the second Cosserat tensor K. (Here, Anti denotes an isomorphism between R3 × 3 and So(3):={A∈R3×3×3|A.u∈so(3)∀u∈R3}). The formula shows that it is not possible to prescribe U¯ and K independent from each other. We also propose a new energy formulation of geometrically nonlinear Cosserat models which completely separate the effects of nonsymmetric straining and curvature. For very weak constitutive assumptions (no direct boundary condition on rotations, zero Cosserat couple modulus, quadratic curvature energy), we show existence of minimizers in Sobolev spaces. © 2016, Springer International Publishing.
    view abstractdoi: 10.1007/s00033-016-0755-7
  • 2017 • 185 Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi
    Laplanche, G. and Kostka, A. and Reinhart, C. and Hunfeld, J. and Eggeler, G. and George, E.P.
    Acta Materialia 128 292-303 (2017)
    The tensile properties of CrCoNi, a medium-entropy alloy, have been shown to be significantly better than those of CrMnFeCoNi, a high-entropy alloy. To understand the deformation mechanisms responsible for its superiority, tensile tests were performed on CrCoNi at liquid nitrogen temperature (77 K) and room temperature (293 K) and interrupted at different strains. Microstructural analyses by transmission electron microscopy showed that, during the early stage of plasticity, deformation occurs by the glide of 1/2&lt;110&gt; dislocations dissociated into 1/6&lt;112&gt; Shockley partials on {111} planes, similar to the behavior of CrMnFeCoNi. Measurements of the partial separations yielded a stacking fault energy of 22 ± 4 mJ m−2, which is ∼25% lower than that of CrMnFeCoNi. With increasing strain, nanotwinning appears as an additional deformation mechanism in CrCoNi. The critical resolved shear stress for twinning in CrCoNi with 16 μm grain size is 260 ± 30 MPa, roughly independent of temperature, and comparable to that of CrMnFeCoNi having similar grain size. However, the yield strength and work hardening rate of CrCoNi are higher than those of CrMnFeCoNi. Consequently, the twinning stress is reached earlier (at lower strains) in CrCoNi. This in turn results in an extended strain range where nanotwinning can provide high, steady work hardening, leading to the superior mechanical properties (ultimate strength, ductility, and toughness) of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.02.036
  • 2017 • 184 A TRIP-assisted dual-phase high-entropy alloy: Grain size and phase fraction effects on deformation behavior
    Li, Z. and Tasan, C.C. and Pradeep, K.G. and Raabe, D.
    Acta Materialia 131 323-335 (2017)
    We present a systematic microstructure oriented mechanical property investigation for a newly developed class of transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) with varying grain sizes and phase fractions. The DP-HEAs in both, as-homogenized and recrystallized states consist of a face-centered cubic (FCC) matrix containing a high-density of stacking faults and a laminate hexagonal close-packed (HCP) phase. No elemental segregation was observed in grain interiors or at interfaces even down to near-atomic resolution, as confirmed by energy-dispersive X-ray spectroscopy and atom probe tomography. The strength-ductility combinations of the recrystallized DP-HEAs (Fe50Mn30Co10Cr10) with varying FCC grain sizes and HCP phase fractions prior to deformation are superior to those of the recrystallized equiatomic single-phase Cantor reference HEA (Fe20Mn20Ni20Co20Cr20). The multiple deformation micro-mechanisms (including strain-induced transformation from FCC to HCP phase) and dynamic strain partitioning behavior among the two phases are revealed in detail. Both, strength and ductility of the DP-HEAs increase with decreasing the average FCC matrix grain size and increasing the HCP phase fraction prior to loading (in the range of 10–35%) due to the resulting enhanced stability of the FCC matrix. These insights are used to project some future directions for designing advanced TRIP-HEAs through the adjustment of the matrix phase's stability by alloy tuning and grain size effects. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.03.069
  • 2017 • 183 Influence of compositional inhomogeneity on mechanical behavior of an interstitial dual-phase high-entropy alloy
    Li, Z. and Raabe, D.
    Materials Chemistry and Physics (2017)
    In this study we present and discuss the influence of compositional inhomogeneity on the mechanical behavior of an interstitially alloyed dual-phase non-equiatomic high-entropy alloy (Fe49.5Mn30Co10Cr10C0.5). Various processing routes including hot-rolling, homogenization, cold-rolling and recrystallization annealing were performed on the cast alloys to obtain samples in different compositional homogeneity states. Grain sizes of the alloys were also considered. Tensile testing and microstructural investigations reveal that the deformation behavior of the interstitial dual-phase high-entropy alloy samples varied significantly depending on the compositional homogeneity of the specimens probed. In the case of coarse-grains (∼300 μm) obtained for cast alloys without homogenization treatment, ductility and strain-hardening of the material was significantly reduced due to its compositional inhomogeneity. This detrimental effect was attributed to preferred deformation-driven phase transformation occurring in the Fe enriched regions with lower stacking fault energy, promoting early stress-strain localization. The grain-refined alloy (∼4 μm) with compositional heterogeneity which was obtained for recrystallization annealed alloys without homogenization treatment was characterized by almost total loss in work-hardening. This effect was attributed to large local shear strains due to the inhomogeneous planar slip. These insights demonstrate the essential role of compositional homogeneity through applying corresponding processing steps for the development of advanced high-entropy alloys. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2017.04.050
  • 2017 • 182 Dislocation activities at the martensite phase transformation interface in metastable austenitic stainless steel: An in-situ TEM study
    Liu, J. and Chen, C. and Feng, Q. and Fang, X. and Wang, H. and Liu, F. and Lu, J. and Raabe, D.
    Materials Science and Engineering A 703 236-243 (2017)
    Understanding the mechanism of martensitic transformation is of great importance in developing advanced high strength steels, especially TRansformation-Induced Plasticity (TRIP) steels. The TRIP effect leads to enhanced work-hardening rate, postponed onset of necking and excellent formability. In-situ transmission electron microscopy has been performed to systematically investigate the dynamic interactions between dislocations and α′ martensite at microscale. Local stress concentrations, e.g. from notches or dislocation pile-ups, render free edges and grain boundaries favorable nucleation sites for α′ martensite. Its growth leads to partial dislocation emission on two independent slip planes from the hetero-interface when the austenite matrix is initially free of dislocations. The kinematic analysis reveals that activating slip systems on two independent {111} planes of austenite are necessary in accommodating the interfacial mismatch strain. Full dislocation emission is generally observed inside of austenite regions that contain high density of dislocations. In both situations, phase boundary propagation generates large amounts of dislocations entering into the matrix, which renders the total deformation compatible and provide substantial strain hardening of the host phase. These moving dislocation sources enable plastic relaxation and prevent local damage accumulation by intense slipping on the softer side of the interfacial region. Thus, finely dispersed martensite distribution renders plastic deformation more uniform throughout the austenitic matrix, which explains the exceptional combination of strength and ductility of TRIP steels. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.06.107
  • 2017 • 181 Insights into the deformation behavior of the CrMnFeCoNi high-entropy alloy revealed by elevated temperature nanoindentation
    Maier-Kiener, V. and Schuh, B. and George, E.P. and Clemens, H. and Hohenwarter, A.
    Journal of Materials Research 32 2658-2667 (2017)
    A CrMnFeCoNi high-entropy alloy was investigated by nanoindentation from room temperature to 400 °C in the nanocrystalline state and cast plus homogenized coarse-grained state. In the latter case a âŒ100)-orientated grain was selected by electron back scatter diffraction for nanoindentation. It was found that hardness decreases more strongly with increasing temperature than Young's modulus, especially for the coarse-grained state. The modulus of the nanocrystalline state was slightly higher than that of the coarse-grained one. For the coarse-grained sample a strong thermally activated deformation behavior was found up to 100-150 °C, followed by a diminishing thermally activated contribution at higher testing temperatures. For the nanocrystalline state, different temperature dependent deformation mechanisms are proposed. At low temperatures, the governing processes appear to be similar to those in the coarse-grained sample, but with increasing temperature, dislocation-grain boundary interactions likely become more dominant. Finally, at 400 °C, decomposition of the nanocrystalline alloy causes a further reduction in thermal activation. This is rationalized by a reduction of the deformation controlling internal length scale by precipitate formation in conjunction with a diffusional contribution. © 2017 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2017.260
  • 2017 • 180 Size effect in bi-crystalline micropillars with a penetrable high angle grain boundary
    Malyar, N.V. and Micha, J.S. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 129 312-320 (2017)
    The implications of various size effects on the deformation behavior of and near grain boundaries is not yet fully understood. In this manuscript, slip transfer mechanisms through a general high angle grain boundary (HAGB) allowing for easy transfer are investigated in order to understand the size dependence of the dislocation-grain-boundary interaction. Complementary in situ micro compression tests on copper single and bi-crystals in the scanning electron microscope and with x-ray Laue microdiffraction were used to correlate the mechanical response with the evolving microstructure. It is shown that no dislocation pile-up is formed at the boundary. The lack of pile-up stresses results in a deformation process which is dominated by the initial dislocation source statistics. This is evidenced by similar size scaling of the single and bi-crystalline samples with the grain size being the characteristic length scale. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.03.003
  • 2017 • 179 Reactive Liquid-Flow Simulation of Micromixers Based on Grid Deformation Techniques
    Mierka, O. and Munir, M. and Spille, C. and Timmermann, J. and Schlüter, M. and Turek, S.
    Chemical Engineering and Technology 40 1408-1417 (2017)
    Process intensification of engineering applications in the framework of reacting flows in micromixer devices attracts the attention of engineers and scientists from various fields. With the steadily increasing available computational resources, the traditional experimentally supported investigations may be extended by computational ones. For this purpose, a simulation framework based on state-of-the-art numerical techniques extended with special grid deformation techniques has been developed. Its validation in terms of comparison with computational and experimental results in reacting as well as in non-reacting frameworks has been performed on the basis of the T-mixer and SuperFocus mixer, respectively. The computational efficiency of the developed tool is shown to be applicable for optimization tasks, such as reverse engineering purposes. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ceat.201600686
  • 2017 • 178 Hyperelastic bodies under homogeneous Cauchy stress induced by non-homogeneous finite deformations
    Mihai, L.A. and Neff, P.
    International Journal of Non-Linear Mechanics 89 93-100 (2017)
    We discuss whether homogeneous Cauchy stress implies homogeneous strain in isotropic nonlinear elasticity. While for linear elasticity the positive answer is clear, we exhibit, through detailed calculations, an example with inhomogeneous continuous deformation but constant Cauchy stress. The example is derived from a non rank-one convex elastic energy. © 2016 The Authors
    view abstractdoi: 10.1016/j.ijnonlinmec.2016.12.003
  • 2017 • 177 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 • 176 A non-affine electro-viscoelastic microsphere model for dielectric elastomers: Application to VHB 4910 based actuators
    Thylander, S. and Menzel, A. and Ristinmaa, M.
    Journal of Intelligent Material Systems and Structures 28 627-639 (2017)
    Dielectric elastomers belong to a larger group of materials, the so-called electroactive polymers, which have the capability of transforming electric energy to mechanical energy through deformation. VHB 4910 is one of the most popular materials for applications of dielectric elastomers and therefore one of the most investigated. This paper includes a new micromechanically motivated constitutive model for dielectric elastomers that incorporates the nearly incompressible and viscous time-dependent behaviour often found in this type of material. A non-affine microsphere framework is used to transform the microscopic constitutive model to a macroscopic continuum counterpart. Furthermore the model is calibrated, through both homogeneous deformation examples and more complex finite element analysis, to VHB 4910. The model is able to capture both the purely elastic, the viscoelastic and the electro-viscoelastic properties of the elastomer and demonstrates the power and applicability of the electromechanically coupled microsphere framework. © SAGE Publications.
    view abstractdoi: 10.1177/1045389X16651157
  • 2017 • 175 Compressed Bi-crystal micropillars showing a sigmoidal deformation state – A computational study
    Toth, F. and Kirchlechner, C. and Fischer, F.D. and Dehm, G. and Rammerstorfer, F.G.
    Materials Science and Engineering A 700 168-174 (2017)
    It is the aim of this paper to show the mechanisms behind the experimental observations of rather smooth sigmoidal deformations in bi-crystal micropillar tests (in contrast to single crystal micro-compression tests) and to point out that the appearance of such deformation modes are a further reason for being careful when interpreting the force-axial displacement behavior in terms of stress-strain curves. Instabilities, i.e., buckling and subsequent post-buckling deformations, inhomogeneous strain fields and substantial deformations of the base as well as pronounced free surface effects are considered. The influences of imperfections and of friction as well as a possible clearance in the guidance of the loading device are taken into account, too. From these studies, the experimenter may get information how and with which limitations material parameters can be obtained from such compression tests in combination with simulations. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.05.098
  • 2017 • 174 Strengthening and strain hardening mechanisms in a precipitation-hardened high-Mn lightweight steel
    Yao, M.J. and Welsch, E. and Ponge, D. and Haghighat, S.M.H. and Sandlöbes, S. and Choi, P. and Herbig, M. and Bleskov, I. and Hickel, T. and Lipinska-Chwalek, M. and Shanthraj, P. and Scheu, C. and Zaefferer, S. and Gault, B. an...
    Acta Materialia 140 258-273 (2017)
    We report on the strengthening and strain hardening mechanisms in an aged high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C, wt.%) studied by electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM), atom probe tomography (APT) and correlative TEM/APT. Upon isothermal annealing at 600 °C, nano-sized κ-carbides form, as characterized by TEM and APT. The resultant alloy exhibits high strength and excellent ductility accompanied by a high constant strain hardening rate. In comparison to the as-quenched κ-free state, the precipitation of κ-carbides leads to a significant increase in yield strength (∼480 MPa) without sacrificing much tensile elongation. To study the strengthening and strain hardening behavior of the precipitation-hardened material, deformation microstructures were analyzed at different strain levels. TEM and correlative TEM/APT results show that the κ-carbides are primarily sheared by lattice dislocations, gliding on the typical face-centered-cubic (fcc) slip system {111}<110>, leading to particle dissolution and solute segregation. Ordering strengthening is the predominant strengthening mechanism. As the deformation substructure is characterized by planar slip bands, we quantitatively studied the evolution of the slip band spacing during straining to understand the strain hardening behavior. A good agreement between the calculated flow stresses and the experimental data suggests that dynamic slip band refinement is the main strain hardening mechanism. The influence of κ-carbides on mechanical properties is discussed by comparing the results with that of the same alloy in the as-quenched, κ-free state. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.08.049
  • 2017 • 173 Interaction between phase transformations and dislocations at incipient plasticity of monocrystalline silicon under nanoindentation
    Zhang, J. and Zhang, J. and Wang, Z. and Hartmaier, A. and Yan, Y. and Sun, T.
    Computational Materials Science 131 55-61 (2017)
    Structural phase transformation and dislocation slip are two important deformation modes of monocrystalline silicon. In the present work, we elucidate mechanisms of inhomogeneous elastic-plastic transition in spherical nanoindentation of monocrystalline silicon by means of molecular dynamics simulations. The Stillinger-Weber potential is utilized to present simultaneous phase transformations and dislocation activities in the silicon nanoindentation. And a bond angle analysis-based method is proposed to quantitatively clarify silicon phases. The influence of crystallographic orientation on the silicon nanoindentation is further addressed. Our simulation results indicate that prior to the “Pop-In” event, Si(0 1 0) undergoes inelastic deformation accompanied by the phase transformation from the Si-I to the Si-III/Si-XII, which is not occurred in Si(1 1 0) and Si(1 1 1). While the phase transformation from the Si-I to the bct-5 is the dominant mechanism of incipient plasticity for each crystallographic orientation, dislocation nucleation is also an operating deformation mode in the elastic-plastic transition of Si(0 1 0). Furthermore, interactions between phase transformations and dislocations are more pronounced in Si(0 1 0) than the other two crystallographic orientations. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2017.01.043
  • 2016 • 172 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 • 171 The second Sandia Fracture Challenge: predictions of ductile failure under quasi-static and moderate-rate dynamic loading
    Boyce, B.L. and Kramer, S.L.B. and Bosiljevac, T.R. and Corona, E. and Moore, J.A. and Elkhodary, K. and Simha, C.H.M. and Williams, B.W. and Cerrone, A.R. and Nonn, A. and Hochhalter, J.D. and Bomarito, G.F. and Warner, J.E. and ...
    International Journal of Fracture 198 5-100 (2016)
    Ductile failure of structural metals is relevant to a wide range of engineering scenarios. Computational methods are employed to anticipate the critical conditions of failure, yet they sometimes provide inaccurate and misleading predictions. Challenge scenarios, such as the one presented in the current work, provide an opportunity to assess the blind, quantitative predictive ability of simulation methods against a previously unseen failure problem. Rather than evaluate the predictions of a single simulation approach, the Sandia Fracture Challenge relies on numerous volunteer teams with expertise in computational mechanics to apply a broad range of computational methods, numerical algorithms, and constitutive models to the challenge. This exercise is intended to evaluate the state of health of technologies available for failure prediction. In the first Sandia Fracture Challenge, a wide range of issues were raised in ductile failure modeling, including a lack of consistency in failure models, the importance of shear calibration data, and difficulties in quantifying the uncertainty of prediction [see Boyce et al. (Int J Fract 186:5–68, 2014) for details of these observations]. This second Sandia Fracture Challenge investigated the ductile rupture of a Ti–6Al–4V sheet under both quasi-static and modest-rate dynamic loading (failure in (Formula presented.) 0.1 s). Like the previous challenge, the sheet had an unusual arrangement of notches and holes that added geometric complexity and fostered a competition between tensile- and shear-dominated failure modes. The teams were asked to predict the fracture path and quantitative far-field failure metrics such as the peak force and displacement to cause crack initiation. Fourteen teams contributed blind predictions, and the experimental outcomes were quantified in three independent test labs. Additional shortcomings were revealed in this second challenge such as inconsistency in the application of appropriate boundary conditions, need for a thermomechanical treatment of the heat generation in the dynamic loading condition, and further difficulties in model calibration based on limited real-world engineering data. As with the prior challenge, this work not only documents the ‘state-of-the-art’ in computational failure prediction of ductile tearing scenarios, but also provides a detailed dataset for non-blind assessment of alternative methods. © 2016, The Author(s).
    view abstractdoi: 10.1007/s10704-016-0089-7
  • 2016 • 170 Towards a physics-based multiscale modelling of the electro-mechanical coupling in electro-active polymers
    Cohen, N. and Menzel, A. and DeBotton, G.
    Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 472 (2016)
    Owing to the increasing number of industrial applications of electro-active polymers (EAPs), there is a growing need for electromechanical models which accurately capture their behaviour. To this end, we compare the predicted behaviour of EAPs undergoing homogeneous deformations according to three electromechanical models. The first model is a phenomenological continuumbased model composed of the mechanical Gent model and a linear relationship between the electric field and the polarization. The electrical and the mechanical responses according to the second model are based on the physical structure of the polymer chain network. The third model incorporates a neo-Hookean mechanical response and a physically motivated microstructurally based long-chains model for the electrical behaviour. In the microstructural-motivated models, the integration from the microscopic to the macroscopic levels is accomplished by the micro-sphere technique. Four types of homogeneous boundary conditions are considered and the behaviours determined according to the three models are compared. For the microstructurally motivated models, these analyses are performed and compared with the widely used phenomenological model for the first time. Some of the aspects revealed in this investigation, such as the dependence of the intensity of the polarization field on the deformation, highlight the need for an in-depth investigation of the relationships between the structure and the behaviours of the EAPs at the microscopic level and their overall macroscopic response. © 2016 The Author(s) Published by the Royal Society. All rights reserved.
    view abstractdoi: 10.1098/rspa.2015.0462
  • 2016 • 169 Coarse graining of force fields for metal-organic frameworks
    Dürholt, J.P. and Galvelis, R. and Schmid, R.
    Dalton Transactions 45 4370-4379 (2016)
    We have adapted our genetic algorithm based optimization approach, originally developed to generate force field parameters from quantum mechanic reference data, to derive a first coarse grained force field for a MOF, taking the atomistic MOF-FF as a reference. On the example of the copper paddle-wheel based HKUST-1, a maximally coarse grained model, using a single bead for each three and four coordinated vertex, was developed as a proof of concept. By adding non-bonded interactions with a modified Buckingham potential, the resulting MOF-FF-CGNB is able to predict local deformation energies of the building blocks as well as bulk properties like the tbo vs. pto energy difference or elastic constants in a semi-quantitative way. As expected, the negative thermal expansion of HKUST-1 is not reproduced by the maximally coarse grained model. At the expense of atomic resolution, substantially larger systems (up to tens of nanometers in size) can be simulated with respect to structural and mechanical properties, bridging the gap to the mesoscale. As an example the deformation of the [111] surface of HKUST-1 by a "tip" could be computed without artifacts from periodic images. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c5dt03865k
  • 2016 • 168 Plastic accommodation at homophase interfaces between nanotwinned and recrystallized grains in an austenitic duplex-microstructured steel
    Gutierrez-Urrutia, I. and Archie, F. and Raabe, D. and Yan, F.-K. and Tao, N.-R. and Lu, K.
    Science and Technology of Advanced Materials 17 29-36 (2016)
    The plastic co-deformation behavior at the homophase interfaces between the hard nanotwinned grain inclusions and the soft recrystallized matrix grains in a duplex-microstructured AISI 316L austenitic stainless steel is examined through the analysis of long-range orientation gradients within the matrix grains by electron backscatter diffraction and transmission electron microcopy. Our analysis reveals that the mechanical accommodation of homophase interfaces until a macroscopic strain of 22% is realized within a small area of soft grains (about four grains) adjacent to the homophase interface. The activation of deformation twinning in the first two grain layers results in the occurrence of a ‘hump’ in the orientation gradient profile. We ascribe this effect to the role of deformation twinning on the generation of geometrically necessary dislocations. The smooth profile of the orientation gradient amplitude within the first 10 grain layers indicates a gradual plastic accommodation of the homophase interfaces upon straining. As a consequence, damage nucleation at such interfaces is impeded, resulting in an enhanced ductility of the single phase duplex-microstructured steel. © 2016 The Author(s).
    view abstractdoi: 10.1080/14686996.2016.1140302
  • 2016 • 167 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 • 166 Stacking fault based analysis of shear mechanisms at interfaces in lamellar TiAl alloys
    Kanani, M. and Hartmaier, A. and Janisch, R.
    Acta Materialia 106 208-218 (2016)
    The interfaces in lamellar TiAl alloys have a strong influence on the strength and deformability of the microstructure. It is widely accepted that their number and spacing can be used to tune these properties. However, the results of molecular dynamics simulations of sliding at γ/γ interfaces in lamellar TiAl alloys presented here suggest that important factors, namely the sequence of different interface types as well as the orientation of in-plane directions with respect to the loading axis, have to be included into meso-scale models. Simulations of bicrystal shear show significant differences in the deformation behavior of the different interfaces, as well as pronounced in-plane anisotropy of the shear strength of the individual interfaces. The critical stresses derived from bicrystal shear simulations are of the same order of magnitude as the one for nucleation and motion of twins in a γ-single crystal, showing that these mechanisms are competitive. In total four different deformation mechanisms, interface migration, twin nucleation and migration, dislocation nucleation, and rigid grain boundary sliding are observed. Their occurrence can be understood based on a multilayer generalized stacking fault energy analysis. This link between physical properties, geometry and deformation mechanism can provide guidelines for future alloy development. © 2016 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.11.047
  • 2016 • 165 On the mechanism of {332} twinning in metastable β titanium alloys
    Lai, M.J. and Tasan, C.C. and Raabe, D.
    Acta Materialia 111 173-186 (2016)
    {332} twinning, an unusual twinning mode in other body-centered cubic (bcc) metals and alloys, has been demonstrated to be a fundamental deformation mode in bcc metastable β titanium alloys. Recent studies suggest that this twinning mode plays an important role in enhancing the work hardening and thus improving the mechanical properties. Here, we studied the mechanism of this twinning mode in a metastable β Ti-36Nb-2Ta-3Zr (wt.%) alloy. Tensile tests were performed to induce the formation of {332} twins. By using electron backscatter diffraction, transmission electron microscopy and in situ scanning electron microscopy, the surface-to-bulk microstructures and the initiation and propagation of {332} twins were investigated. In addition to the previously reported high densities of straight dislocations within the twin, we have observed that an α″ martensite band is present near the surface adjacent to the twin. During annealing at 900°C, the α″ martensite band transforms into the adjacent twin rather than into the matrix, indicating that {332} twin nucleates within α″ martensite. Further evidence for this is the constitution of the twin in the initial stage of its formation, where the first portion formed consists of α″ martensite. During propagation, the twins propagating to the opposite directions can merge together when their lateral boundaries impinge on each other. Based on the experimental observations, an α″-assisted twinning mechanism is proposed and the origin of the dislocations within {332} twin is discussed accordingly. © 2016 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.03.040
  • 2016 • 164 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 • 163 Importance of inclusion of the effect of s electrons into bond-order potentials for transition bcc metals with d-band mediated bonding
    Lin, Y.-S. and Mrovec, M. and Vitek, V.
    Modelling and Simulation in Materials Science and Engineering 24 (2016)
    In bond-order potentials (BOPs) for transition metals only the bonding mediated by the d electrons is included explicitly and the covalent part of the cohesive energy is evaluated using Slater-Koster dd bond integrals. However, the effect of s electrons with orbitals centered on atoms neighboring the corresponding dd bond is not necessarily negligible. As shown in Nguyen-Manh et al (2000 Phys. Rev. Lett. 85 4136) this can be taken into account via screening of the dd bond integrals. In a recent paper (Lin et al 2014 Model. Simul. Mater. Sci. Eng. 22 034002) the dd bond integrals were determined using a projection scheme utilizing atomic orbitals that give the best representation of the electronic wave functions in the calculations based on the density functional theory (DFT) (Madsen et al 2011 Phys. Rev. B 83 4119) and it was inferred that in this case the effect of s electrons was already included. In this paper we analyze this hypothesis by comparing studies employing BOPs with both unscreened and screened dd bond integrals. In all cases results are compared with calculations based on DFT and/or experiments. Studies of structures alternate to the bcc lattice, transformation paths that connect the bcc structure with fcc, simple cubic (sc), body centered tetragonal (bct) and hcp structures via continuously distorted configurations and calculations of γ-surfaces were all found to be insensitive to the screening of bond integrals. On the other hand, when the bond integrals are screened, formation energies of vacancies are improved and calculated phonon dispersion spectra reproduce the experimentally observed ones much better. Most importantly, dislocation core structure and dislocation glide are significantly different without and with screening of dd bond integrals. The latter lead to a much better agreement with available experiments. These findings suggest that the effect of s electrons on dd bonds, emulated by the screening of corresponding bond integrals, is the least significant when the lattice is distorted away from the ideal bcc structure homogeneously even if such distortion is large. On the other hand, when the distortion is local and inhomogeneous the impact of screening of the dd bond integrals is significant. In the studies presented in this paper such local inhomogeneities occur when phonons propagate through the lattice, at point defects and in the cores of dislocations. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/24/8/085001
  • 2016 • 162 Hybrid calculations of bending deformations for 20 roll cold rolling mill
    Mauer, P. and Weyh, B. and Mauk, P.J.
    Key Engineering Materials 716 856-863 (2016)
    In multi-high mills, the slim fixed floating work rolls are supported by primary and secondary intermediate rolls and several supporting rolls mounted pairs-wise on supporting shafts. Over the eccentric adjustment of saddle assemblies and hydraulic adjustment device the roll gap adjusts the specific elastic bending deformation of the support system. Based on a hybrid calculation model the influence and limits of an active adjustment device for 20 roll mill will be presented. Thereby the bending deformations as well as the contact deformations of the complete system including the forming process in settings of the elementary plasticity theory (EPT) will be considered. The modeling of the bending deformations of the roll systems are based on FE-Beam elements. The flattening in the contact zone between the rolls will be formulated by a modified non-linear approach, according to Hertz-Johnson. The contact of the deformed zone is aligned in the discretized EPT-roll model by Alexander with Hitchcock-flattening. The iterative analysis of the compact models leads to "force-deformation-relations", which provides the influence of the variation of the gap. The force-deformation-relations will be detailed discussed in parameter studies of a real 20 roll mill. © 2016 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.716.856
  • 2016 • 161 The Exponentiated Hencky Strain Energy in Modeling Tire Derived Material for Moderately Large Deformations
    Montella, G. and Govindjee, S. and Neff, P.
    Journal of Engineering Materials and Technology, Transactions of the ASME 138 (2016)
    This work presents a hyperviscoelastic model, based on the Hencky-logarithmic strain tensor, to model the response of a tire derived material (TDM) undergoing moderately large deformations. The TDM is a composite made by cold forging a mix of rubber fibers and grains, obtained by grinding scrap tires, and polyurethane binder. The mechanical properties are highly influenced by the presence of voids associated with the granular composition and low tensile strength due to the weak connection at the grain-matrix interface. For these reasons, TDM use is restricted to applications involving a limited range of deformations. Experimental tests show that a central feature of the response is connected to highly nonlinear behavior of the material under volumetric deformation which conventional hyperelastic models fail in predicting. The strain energy function presented here is a variant of the exponentiated Hencky strain energy, which for moderate strains is as good as the quadratic Hencky model and in the large strain region improves several important features from a mathematical point of view. The proposed form of the exponentiated Hencky energy possesses a set of parameters uniquely determined in the infinitesimal strain regime and an orthogonal set of parameters to determine the nonlinear response. The hyperelastic model is additionally incorporated in a finite deformation viscoelasticity framework that accounts for the two main dissipation mechanisms in TDMs, one at the microscale level and one at the macroscale level. The new model is capable of predicting different deformation modes in a certain range of frequency and amplitude with a unique set of parameters with most of them having a clear physical meaning. This translates into an important advantage with respect to overcoming the difficulties related to finding a unique set of optimal material parameters as are usually encountered fitting the polynomial forms of strain energies. Moreover, by comparing the predictions from the proposed constitutive model with experimental data we conclude that the new constitutive model gives accurate prediction. © 2016 by ASME.
    view abstractdoi: 10.1115/1.4032749
  • 2016 • 160 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 • 159 Low cycle fatigue in aluminum single and bi-crystals: On the influence of crystal orientation
    Nellessen, J. and Sandlöbes, S. and Raabe, D.
    Materials Science and Engineering A 668 166-179 (2016)
    Aluminum single crystals with three different double-slip orientations and two aluminum bi-crystals - one with a high-angle grain boundary and one with a low-angle grain boundary - were cyclically deformed up to 100 cycles under constant displacement control. The distribution of the local strain and the local strain amplitudes was captured by in-situ digital image correlation (DIC). Dislocation structure analysis was performed by electron channeling contrast imaging (ECCI) and the evolution of local misorientations was recorded by high resolution electron backscatter diffraction (EBSD). The DIC results show a homogeneous strain amplitude distribution in the single crystals while the measured strain amplitude in the low-angle grain boundary bi-crystal sample differs significantly. ECCI observations reveal the presence of dislocation cells elongated along the trace of the primary {111} slip plane in all investigated crystals and the formation of deformation bands parallel to the trace of {110} planes. Deformation bands (DB) were observed in all samples but their frequency and misorientation with respect to the matrix was found to sensitively depend on the crystal orientation and the local strain amplitude. Our results on the bi-crystals show that the grain orientation mainly determines the local stresses and therefore also the formation of the associated dislocation structures rather than the grain boundary character. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2016.05.054
  • 2016 • 158 Framework for non-coherent interface models at finite displacement jumps and finite strains
    Ottosen, N.S. and Ristinmaa, M. and Mosler, J.
    Journal of the Mechanics and Physics of Solids 90 124-141 (2016)
    This paper deals with a novel constitutive framework suitable for non-coherent interfaces, such as cracks, undergoing large deformations in a geometrically exact setting. For this type of interface, the displacement field shows a jump across the interface. Within the engineering community, so-called cohesive zone models are frequently applied in order to describe non-coherent interfaces. However, for existing models to comply with the restrictions imposed by (a) thermodynamical consistency (e.g., the second law of thermodynamics), (b) balance equations (in particular, balance of angular momentum) and (c) material frame indifference, these models are essentially fiber models, i.e. models where the traction vector is collinear with the displacement jump. This constraints the ability to model shear and, in addition, anisotropic effects are excluded. A novel, extended constitutive framework which is consistent with the above mentioned fundamental physical principles is elaborated in this paper. In addition to the classical tractions associated with a cohesive zone model, the main idea is to consider additional tractions related to membrane-like forces and out-of-plane shear forces acting within the interface. For zero displacement jump, i.e. coherent interfaces, this framework degenerates to existing formulations presented in the literature. For hyperelasticity, the Helmholtz energy of the proposed novel framework depends on the displacement jump as well as on the tangent vectors of the interface with respect to the current configuration - or equivalently - the Helmholtz energy depends on the displacement jump and the surface deformation gradient. It turns out that by defining the Helmholtz energy in terms of the invariants of these variables, all above-mentioned fundamental physical principles are automatically fulfilled. Extensions of the novel framework necessary for material degradation (damage) and plasticity are also covered. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2016.02.034
  • 2016 • 157 A fast three-dimensional model for strip rolling
    Overhagen, C. and Mauk, P.J.
    Key Engineering Materials 716 566-578 (2016)
    Rolling Models have come a long way from the first empirical relations about forward slip and bite conditions to their current state, which allows local quantities to be calculated in two and three dimensions. In this paper, state-of-The-Art of analytical modelling of the rolling process is shown with a fully three-dimensional rolling model for hot and cold strip rolling with stress distributions in the longitudinal, vertical and lateral directions. For this purpose, von Karman's strip approach is extended to account for the stress gradient in lateral direction, as was already shown in different papers. The stress gradient in the vertical (through-Thickness) direction is introduced by a modern implementation of Orowan's inhomogeneous deformation theory. The local stress distributions are compared to results from Finite-Element Calculations obtained with modern FEM codes. It will be shown, under which circumstances expensive FEM calculations can be replaced by simpler models like the one proposed here, which are more time and cost-effective without a significant loss in result precision. The rolling model is extended with a Finite Element Beam Model for work and backup roll deformation, as well as local work roll flattening and thermal crown for hot rolling. The Effects of those features on stress distribution and exit strip profile are shown for hot and cold rolling. © 2016 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/KEM.716.566
  • 2016 • 156 Simulation of balloon angioplasty in residually stressed blood vessels—Application of a gradient-enhanced fibre damage model
    Polindara, C. and Waffenschmidt, T. and Menzel, A.
    Journal of Biomechanics 49 2341-2348 (2016)
    In this contribution we study the balloon angioplasty in a residually stressed artery by means of a non-local gradient-enhanced fibre damage model. The balloon angioplasty is a common surgical intervention used to extend or reopen narrowed blood vessels in order to restore the continuous blood flow in, for instance, atherosclerotic arteries. Inelastic, i.e. predominantly damage-related and elastoplastic processes are induced in the artery during its inflation resulting in an irreversible deformation. As a beneficial consequence, provided that the inelastic deformations do not exceed a specific limit, higher deformations can be obtained within the same pressure level and a continuous blood flow can be guaranteed. In order to study the mechanical response of the artery in this scenario, we make use of the non-local gradient-enhanced model proposed in Waffenschmidt et al. (2014). In this contribution, we extend this model to make use of an incompressible format in connection with a Q1Q1P0 finite element implementation. The residual stresses in the artery are also taken into account following the framework presented in Waffenschmidt (2015). From the results it becomes apparent that, when the artery is subjected to radial stresses beyond the physiological range, damage evolution is triggered in the collagen fibres. The impact of the residual stresses on the structural response and on the circumferential stress distribution along the thickness of the arterial wall is also studied. It is observed that the residual stresses have a beneficial effect on the mechanical response of the arterial wall. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.jbiomech.2016.01.037
  • 2016 • 155 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 • 154 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 • 153 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 • 152 Towards control of viscous effects in acrylic-based actuator applications
    Thylander, S. and Menzel, A. and Ristinmaa, M.
    Smart Materials and Structures 25 (2016)
    Dielectric elastomers offer clear advantages over more traditional and conventional materials when soft, lightweight, noiseless actuator applications with large deformations are considered. However, the viscous time-dependent behaviour associated with most elastomers limit the number of possible applications. For this purpose, the possibility of controlling the viscous response by regulating the applied electric potential is explored. The constitutive model chosen is calibrated to fit the electro-viscoelastic response of an acrylic elastomer often used in dielectric elastomer actuators. The response of both homogeneous deformation examples and inhomogeneous finite element boundary value problems, chosen to mimic existing applications, are presented. Control of both force and displacement quantities are successfully achieved. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0964-1726/25/9/095034
  • 2016 • 151 Ab Initio Study of Deformation Influence on the Electronic Properties of Graphene Structures Containing One-Dimensional Topological Defects
    Valishina, A.A. and Lysogorskiy, Y.V. and Nedopekin, O.V. and Tayurskii, D.A.
    Journal of Low Temperature Physics 185 712-716 (2016)
    The band structures of single and bilayer graphene with one-dimensional topological defects were calculated along the defect line, and appearance of the flat band near the Fermi level was observed. In addition, the influence of deformation (compression/expansion) on the flat band was studied. It was shown that compression across the grain boundary leads to disappearance of the flat band near the Fermi level, while the stretching along this direction does not significantly change the band structure. However, neither compression nor stretching along the grain boundary destroys the flat band. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10909-016-1664-z
  • 2016 • 150 Assessment of strain hardening in copper single crystals using in situ SEM microshear experiments
    Wieczorek, N. and Laplanche, G. and Heyer, J.-K. and Parsa, A.B. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 113 320-334 (2016)
    The effect of a pre-strain on the plasticity of copper single crystals subjected to in situ microshear deformation in a scanning electron microscope (SEM) is investigated. Pre-strains of 6.5 and 20% are imposed using [1 0 0] tensile testing. During tensile pre-deformation, several slip systems are activated and irregularly spaced slip bands form. A trace analysis revealed the presence of several slip bands on the tensile specimen near the grips while one family of slip bands parallel to the (1 1 1) crystallographic plane were detected in the middle of the tensile specimen. From the middle of the pre-deformed tensile specimens double microshear samples were prepared using focused ion beam (FIB) machining such that the [0 -1 -1] (1 -1 1) slip system could be directly activated. The results show how microshear behavior reacts to different levels of tensile pre-deformation. Sudden deformation events (SDEs) are observed during microshear testing. The critical stress associated with the first SDE is shown to increase with increasing pre-deformation as a result of an increasing number of slip bands introduced during pre-deformation per shear zone. The results allow also to obtain information on the interaction between dislocations activated during microshearing ([0 -1 -1] (1 -1 1)) and those which were introduced during tensile pre-deformation ([1 0 -1] (1 1 1) and [1 -1 0] (1 1 1)). When these slip systems interact glissile junctions and Lomer-Cottrell locks are likely to form. In the light of this analysis, we rationalize the occurrence of sudden deformation events based on piled up dislocation assemblies which overcome Lomer-Cottrell lock barriers. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.04.055
  • 2016 • 149 The effect of stress, temperature and loading direction on the creep behaviour of Ni-base single crystal superalloy miniature tensile specimens
    Wollgramm, P. and Bürger, D. and Parsa, A.B. and Neuking, K. and Eggeler, G.
    Materials at High Temperatures 33 346-360 (2016)
    In the present work, we use a miniature test procedure to investigate the tensile creep behaviour of the single crystal superalloy ERBO1. We test precisely oriented [0 0 1], [1 1 0] and [1 1 1] creep specimens and determine the stress and the temperature dependence of characteristic creep rates in limited stress and temperature regimes, where the stress and temperature dependence of characteristic creep rates can be well described by power law and Arrhenius type of relations, with stress exponents n and apparent activation energies Qapp. n-values increase with stress and decrease with temperature. Qapp-values, on the other hand, increase with increasing temperature and decrease with increasing stress. Creep curve shapes gradually evolve from the high temperature low stress to the low temperature high stress (LTHS) regime. This implies that there is a gradual change in elementary deformation and softening mechanisms, which is qualitatively confirmed using transmission electron microscopy. While at high temperatures different loading directions only have a moderate influence on creep, there is a very strong effect of loading direction at low temperatures. The [1 1 0] tests show the fastest deformation rates and the shortest rupture times. In the LTHS creep regime, we confirm the double minimum (DM) type of creep behaviour, which was previously reported but never explained. Further work is required to rationalise DM-creep. The implications of this type of creep behaviour on scatter and on extrapolation of creep data is discussed in the light of previous results published in the literature. © 2016 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/09603409.2016.1186414
  • 2016 • 148 A crystal plasticity model for twinning- and transformation-induced plasticity
    Wong, S.L. and Madivala, M. and Prahl, U. and Roters, F. and Raabe, D.
    Acta Materialia 118 140-151 (2016)
    A dislocation density-based crystal plasticity model incorporating both transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) is presented. The approach is a physically-based model which reflects microstructure investigations of ε-martensite, twins and dislocation structures in high manganese steels. Validation of the model was conducted using experimental data for a TRIP/TWIP Fe-22Mn-0.6C steel. The model is able to predict, based on the difference in the stacking fault energies, the activation of TRIP and/or TWIP deformation mechanisms at different temperatures. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.07.032
  • 2016 • 147 From insect scales to sensor design: Modelling the mechanochromic properties of bicontinuous cubic structures
    Wu, X. and Ma, D. and Eisenlohr, P. and Raabe, D. and Fabritius, H.-O.
    Bioinspiration and Biomimetics 11 (2016)
    Many of the three-dimensional photonic crystals occurring in the scales of insects have bicontinuous cubic structures. Their optical properties have been studied extensively, however little is known about their mechanical properties and their optical response under deformation. We demonstrated a mechanochromic effect by deforming the scales of a weevil and calculated the elastic, optical and mechanochromic (assuming homogeneous deformation) properties of the three types of bicontinuous cubic structures occurring in nature: P-structure (primitive), G-structure (gyroid) and D-structure (diamond). The results show that all investigated properties of these three structure types strongly depend on their geometry, structural parameters such as volume fractions of the two constituting phases and the directions of the incident light or applied stress, respectively. Interestingly, the mechanochromic simulation results predict that these structures may show blue-shift or even red-shift under compression along certain directions. Our results provide design guidelines for mechanochromic sensing materials operating in the elastic regime, including parameters such as sensitivity and direction of spectral shift. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1748-3190/11/4/045001
  • 2015 • 146 Dynamic strain aging studied at the atomic scale
    Aboulfadl, H. and Deges, J. and Choi, P. and Raabe, D.
    Acta Materialia 86 34-42 (2015)
    Dynamic strain aging arises from the interaction between solute atoms and matrix dislocations in strained metallic alloy. It initiates jerky dislocation motion and abrupt softening, causing negative strain rate sensitivity. This effect leads to instable flow phenomena at the macroscopic scale, appearing as a serrated stress-strain response and deformation banding. These macroscopic features are referred to as the Portevin-Le Chatelier effect (PLC). Here we study the atomistic origin of dynamic strain aging in an Al-4.8 at.% Mg alloy using atom probe tomography (APT) and transmission electron microscopy (TEM). Samples were prepared from as-cold rolled (90% thickness reduction), stabilized (120 °C, 20 h) and recrystallized sheets (400°C, 10 min), respectively. In the stabilized state, Mg was found to decorate <1 1 0> aligned dislocations with up to ∼12.5 at.%. Tensile tests in combination with thermographic and laser speckle observations were used to map the deformation bands for the site-specific extraction of APT samples from regions inside the PLC bands. We observed an asymmetrical Mg distribution along some of the dislocations, matching model predictions for high dislocation speeds at peak drag stress by Zhang and Curtin. In this case, the Mg distribution is characterized by depletion in the compressive regime above the dislocation slip plane and enrichment in the dilatation region below the slip plane. Mg also depletes in a tail-like form behind fast-moving dislocations, further promoting slip localization. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.12.028
  • 2015 • 145 Modelling of Viscoelastic Dielectric Elastomers with Deformation Dependent Electric Properties
    Ask, A. and Menzel, A. and Ristinmaa, M.
    Procedia IUTAM 12 134-144 (2015)
    This work deals with electro-viscoelastic modelling and simulation of dielectric elastomer actuators (DEA), including the case of deformation dependent electromechanical coupling. A large deformation modelling framework is adopted, and specific thermo- dynamically consistent material models are established. The general framework is applied to VHB49 polyacrylic polymers which are commonly used in DEA applications. The effects of viscosity and deformation dependent electric permittivity are studied with regards to the stability behaviour and also in view of predicting experimentally observed electromechanical behaviour using numerical simulations. © 2014 The Authors.
    view abstractdoi: 10.1016/j.piutam.2014.12.015
  • 2015 • 144 Investigations on the thermal workpiece distortion in MQL deep hole drilling of an aluminium cast alloy
    Biermann, D. and Iovkov, I.
    CIRP Annals - Manufacturing Technology 64 85-88 (2015)
    Dry machining is frequently applied in cutting operations, in order to reduce the energy consumption and the production costs. In deep hole drilling operations minimum quantity lubrication (MQL) is used to obtain a reliable chip evacuation, since completely dry machining is not feasible. Due to the low cooling effect of MQL, the drilling process generates a high thermal load on the workpiece, which leads to thermally induced workpiece deformations. This paper presents fundamental experimental investigations on the workpiece temperature, the resulting in-process deformations and the achievable straightness accuracy of the borehole. The investigations focus on two different strategies for enhancing the deep hole drilling using MQL. Initially, a high-feed process guiding is introduced, in order to obtain a higher productivity and to reduce the heat input into the workpiece. The second approach is a novel radial spindle compensation, which performs a directional control of the straightness deviation of the deep hole. © 2015 CIRP.
    view abstractdoi: 10.1016/j.cirp.2015.04.072
  • 2015 • 143 Simulation of MQL deep hole drilling for predicting thermally induced workpiece deformations
    Biermann, D. and Blum, H. and Frohne, J. and Iovkov, I. and Rademacher, A. and Rosin, K.
    Procedia CIRP 31 148-153 (2015)
    The resulting thermomechanical load on the workpiece in deep hole drilling operations using minimum quantity lubrication (MQL) induces a strong in-process deflection of the machined component and can cause an insufficient accuracy of the produced hole. Also subsequent machining operations can be affected by the thermoelastic component of this deformation, which remains within the workpiece after the drilling process. Due to the comparatively long main time of typical deep hole drilling operations the thermomechanical simulation of commonly complex machined parts is challenging. In this paper, a fast finite-element approach using massive parallel solution methods is presented and validated for different wall thickness situations. © 2015 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2015.03.038
  • 2015 • 142 Large deformation framework for phase-field simulations at the mesoscale
    Borukhovich, E. and Engels, P.S. and Mosler, J. and Shchyglo, O. and Steinbach, I.
    Computational Materials Science 108 367-373 (2015)
    Abstract A large-strain plasticity framework is set up for phase-field simulations at the mesoscopic scale. The approach is based on an Eulerian setting with remeshing after each time step to keep a fixed structured mesh. Rotations, as evaluated from the antisymmetric part of the deformation gradient tensor, are integrated to capture the process history. Special emphasis is also given to the homogenization of the diffuse interface region to ensure the Hadamard jump condition and 2-dimensional scaling of the interface. The approach is applied to deformation of a polycrystal. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2015.06.021
  • 2015 • 141 Size and orientation effects in partial dislocation-mediated deformation of twinning-induced plasticity steel micro-pillars
    Choi, W.S. and De Cooman, B.C. and Sandlöbes, S. and Raabe, D.
    Acta Materialia 98 391-404 (2015)
    Abstract Bulk and micro-pillar single crystals were used to investigate the twinning-induced plasticity mechanism in austenitic Fe-22 wt%Mn-0.6 wt%C TWIP steel. Compression of micro-pillars oriented either for deformation-induced twinning or for perfect dislocation glide was carried out for pillars with diameters in the range of 600 nm to 4 μm. The same size dependence of the critical resolved shear stress was observed for both orientations. The critical micro-pillar diameter for size-independent plasticity was approximately 7.6 μm. Partial dislocation-mediated formation of twins and ε-martensite was observed in micro-pillars oriented for twinning by transmission electron microscopy. The elastic-plastic transition in micro-pillars oriented for deformation twinning did not involve twinning, and dislocation-dislocation interactions were a necessary precondition for twin formation. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.06.065
  • 2015 • 140 Design of a twinning-induced plasticity high entropy alloy
    Deng, Y. and Tasan, C.C. and Pradeep, K.G. and Springer, H. and Kostka, A. and Raabe, D.
    Acta Materialia 94 124-133 (2015)
    We introduce a liquid metallurgy synthesized, non-equiatomic Fe<inf>40</inf>Mn<inf>40</inf>Co<inf>10</inf>Cr<inf>10</inf> high entropy alloy that is designed to undergo mechanically-induced twinning upon deformation at room temperature. Microstructure characterization, carried out using SEM, TEM and APT shows a homogeneous fcc structured single phase solid solution in the as-cast, hot-rolled and homogenized states. Investigations of the deformation substructures at specific strain levels with electron channeling contrast imaging (ECCI) combined with EBSD reveal a clear change in the deformation mechanisms of the designed alloy starting from dislocation slip to twinning as a function of strain. Such twinning induced plasticity has only been observed under cryogenic conditions in the equiatomic FeMnNiCoCr high entropy alloy. Thus, despite the decreased contribution of solid solution strengthening, the tensile properties of the introduced lean alloy at room temperature are found to be comparable to that of the well-studied five component FeMnNiCoCr system. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.04.014
  • 2015 • 139 Influence of misfit stresses on dislocation glide in single crystal superalloys: A three-dimensional discrete dislocation dynamics study
    Gao, S. and Fivel, M. and Ma, A. and Hartmaier, A.
    Journal of the Mechanics and Physics of Solids 76 276-290 (2015)
    In the characteristic γ/γ′ microstructure of single crystal superalloys, misfit stresses occur due to a significant lattice mismatch of those two phases. The magnitude of this lattice mismatch depends on the chemical composition of both phases as well as on temperature. Furthermore, the lattice mismatch of γ and γ′ phases can be either positive or negative in sign. The internal stresses caused by such lattice mismatch play a decisive role for the micromechanical processes that lead to the observed macroscopic athermal deformation behavior of these high-temperature alloys. Three-dimensional discrete dislocation dynamics (DDD) simulations are applied to investigate dislocation glide in γ matrix channels and shearing of γ′ precipitates by superdislocations under externally applied uniaxial stresses, by fully taking into account internal misfit stresses. Misfit stress fields are calculated by the fast Fourier transformation (FFT) method and hybridized with DDD simulations. For external loading along the crystallographic [001] direction of the single crystal, it was found that the different internal stress states for negative and positive lattice mismatch result in non-uniform dislocation movement and different dislocation patterns in horizontal and vertical γ matrix channels. Furthermore, positive lattice mismatch produces a lower deformation rate than negative lattice mismatch under the same tensile loading, but for an increasing magnitude of lattice mismatch, the deformation resistance always diminishes. Hence, the best deformation performance is expected to result from alloys with either small positive, or even better, vanishing lattice mismatch between γ and γ′ phase. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2014.11.015
  • 2015 • 138 Deformation induced alloying in crystalline - metallic glass nano-composites
    Guo, W. and Yao, J. and Jägle, E.A. and Choi, P.-P. and Herbig, M. and Schneider, J.M. and Raabe, D.
    Materials Science and Engineering A 628 269-280 (2015)
    We study the mechanisms of deformation driven chemical mixing in a metallic nanocomposite model system. More specific, we investigate shear banding at the atomic scale in an amorphous CuZr/ crystalline Cu nanolaminate, deformed by microindentation. Three CuZr/Cu multilayer systems (100 nm Cu/100 nm CuZr, 50 nm Cu/100 nm CuZr, and 10 nm Cu/100 nm CuZr) are fabricated to study the effect of layer thickness on shear band formation and deformation induced alloying. The chemical and structural evolution at different strain levels are traced by atom probe tomography and transmission electron microscopy combined with nano-beam diffraction mapping. The initially pure crystalline Cu and amorphous CuZr layers chemically mix by cross-phase shear banding after reaching a critical layer thickness. The Cu inside the shear bands develops a high dislocation density and can locally undergo transition to an amorphous state when sheared and mixed. We conclude that the severe deformation in the shear bands in the amorphous layer squeeze Zr atoms into the Cu dislocation cores in the Cu layers (thickness <5 nm), resulting in local chemical mixing. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.01.062
  • 2015 • 137 ICME for Crashworthiness of TWIP Steels: From Ab Initio to the Crash Performance
    Güvenç, O. and Roters, F. and Hickel, T. and Bambach, M.
    JOM 67 120-128 (2015)
    During the last decade, integrated computational materials engineering (ICME) emerged as a field which aims to promote synergetic usage of formerly isolated simulation models, data and knowledge in materials science and engineering, in order to solve complex engineering problems. In our work, we applied the ICME approach to a crash box, a common automobile component crucial to passenger safety. A newly developed high manganese steel was selected as the material of the component and its crashworthiness was assessed by simulated and real drop tower tests. The crashworthiness of twinning-induced plasticity (TWIP) steel is intrinsically related to the strain hardening behavior caused by the combination of dislocation glide and deformation twinning. The relative contributions of those to the overall hardening behavior depend on the stacking fault energy (SFE) of the selected material. Both the deformation twinning mechanism and the stacking fault energy are individually well-researched topics, but especially for high-manganese steels, the determination of the stacking-fault energy and the occurrence of deformation twinning as a function of the SFE are crucial to understand the strain hardening behavior. We applied ab initio methods to calculate the stacking fault energy of the selected steel composition as an input to a recently developed strain hardening model which models deformation twinning based on the SFE-dependent dislocation mechanisms. This physically based material model is then applied to simulate a drop tower test in order to calculate the energy absorption capacity of the designed component. The results are in good agreement with experiments. The model chain links the crash performance to the SFE and hence to the chemical composition, which paves the way for computational materials design for crashworthiness. © 2014, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1192-8
  • 2015 • 136 Spatially and kinetically resolved mapping of hydrogen in a twinning-induced plasticity steel by use of Scanning Kelvin Probe Force Microscopy
    Koyama, M. and Bashir, A. and Rohwerder, M. and Merzlikin, S.V. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Journal of the Electrochemical Society 162 C638-C647 (2015)
    The hydrogen distribution in a hydrogen-charged Fe-18Mn-1.2C (wt%) twinning-induced plasticity austenitic steel was studied by Scanning Kelvin Probe Force Microscopy (SKPFM). We observed that 1-2 days after the hydrogen-charging, hydrogen showed a higher activity at twin boundaries than inside the matrix. This result indicates that hydrogen at the twin boundaries is diffusible at room temperature, although the twin boundaries act as deeper trap sites compared to typical diffusible hydrogen trap sites such as dislocations. After about 2 weeks the hydrogen activity in the twin boundaries dropped and was indistinguishable from that in the matrix. These SKPFM results were supported by thermal desorption spectrometry and scanning electron microscopic observations of deformation-induced surface cracking parallel to deformation twin boundaries. With this joint approach, two main challenges in the field of hydrogen embrittlement research can be overcome, namely, the detection of hydrogen with high local and chemical sensitivity and the microstructure-dependent and spatially resolved observation of the kinetics of hydrogen desorption. © 2015 The Electrochemical Society.
    view abstractdoi: 10.1149/2.0131512jes
  • 2015 • 135 Characterization of dislocation structures and deformation mechanisms in as-grown and deformed directionally solidified NiAl-Mo composites
    Kwon, J. and Bowers, M.L. and Brandes, M.C. and McCreary, V. and Robertson, I.M. and Phani, P.S. and Bei, H. and Gao, Y.F. and Pharr, G.M. and George, E.P. and Mills, M.J.
    Acta Materialia 89 315-326 (2015)
    Directionally solidified (DS) NiAl-Mo eutectic composites were strained to plastic strain values ranging from 0% to 12% to investigate the origin of the previously observed stochastic versus deterministic mechanical behaviors of Mo-alloy micropillars in terms of the development of dislocation structures at different pre-strain levels. The DS composites consist of long, [1 0 0] single-crystal Mo-alloy fibers with approximately square cross-sections embedded in a [1 0 0] single-crystal NiAl matrix. Scanning transmission electron microscopy (STEM) and computational stress state analysis were conducted for the current study. STEM of the as-grown samples (without pre-straining) reveal no dislocations in the investigated Mo-alloy fibers. In the NiAl matrix, on the other hand, a〈1 0 0〉-type dislocations exist in two orthogonal orientations: along the [1 0 0] Mo fiber axis, and wrapped around the fiber axis. They presumably form to accommodate the different thermal contractions of the two phases during cool down after eutectic solidification. At intermediate pre-strain levels (4-8%), a/2〈1 1 1〉-type dislocations are present in the Mo-alloy fibers and the pre-existing dislocations in the NiAl matrix seem to be swept toward the interphase boundary. Some of the dislocations in the Mo-alloy fibers appear to be transformed from a〈1 0 0〉-type dislocations present in the NiAl matrix. Subsequently, the transformed dislocations in the fibers propagate through the NiAl matrix as a〈1 1 1〉 dislocations and aid in initiating additional slip bands in adjacent fibers. Thereafter, co-deformation presumably occurs by 〈1 1 1〉 slip in both phases. With a further increase in the pre-strain level (>10%), multiple a/2〈1 1 1〉-type dislocations are observed in many locations in the Mo-alloy fibers. Interactions between these systems upon subsequent deformation could lead to stable junctions and persistent dislocation sources. The transition from stochastic to deterministic, bulk-like behavior in sub-micron Mo-alloy pillars may therefore be related to an increasing number of multiple a〈1 1 1〉 dislocation systems within the Mo fibers with increasing pre-strain, considering that the bulk-like behavior is governed by the forest hardening of these junctions. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.01.059
  • 2015 • 134 Deformation mechanism of ω-enriched Ti-Nb-based gum metal: Dislocation channeling and deformation induced ω-β transformation
    Lai, M.J. and Tasan, C.C. and Raabe, D.
    Acta Materialia 100 290-300 (2015)
    Gum metal, a class of multifunctional β titanium alloys, has attracted much attention in the past decade due to its initially-proposed dislocation-free deformation mechanism based on giant faults, i.e., macroscopic planar defects carrying significant plastic strain. Special deformation features were observed in these alloys, such as plastic flow localization, pronounced surface steps, low work hardening, and large elongation. These were all proposed to arise from the special giant fault mechanism activated in the β-Ti matrix, while the initial presence or mechanically-induced formation of other phases was debated in several follow-up studies. Here, we set off with Ti-Nb-based gum metal samples with confirmed presence of large amounts of nanometer-sized hexagonal ω particles. Deformation experiments demonstrate all the features observed in the original reports, mentioned above. However, careful characterization reveals that the deformation bands (similar to giant faults) where plastic flow localized are "dislocation channels" that are depleted of ω phase. These channels are proposed to form by a {1 1 2}<1 1 1> dislocation dissociation mechanism, promoting reverse transformation of the ω phase into the β phase. The deformation induced ω-β transformation and the associated dislocation channeling process can explain the presence of the aforementioned special deformation features in the current gum metal. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.08.047
  • 2015 • 133 The influence of a brittle Cr interlayer on the deformation behavior of thin Cu films on flexible substrates: Experiment and model
    Marx, V.M. and Toth, F. and Wiesinger, A. and Berger, J. and Kirchlechner, C. and Cordill, M.J. and Fischer, F.D. and Rammerstorfer, F.G. and Dehm, G.
    Acta Materialia 89 278-289 (2015)
    Thin metal films deposited on polymer substrates are used in flexible electronic devices such as flexible displays or printed memories. They are often fabricated as complicated multilayer structures. Understanding the mechanical behavior of the interface between the metal film and the substrate as well as the process of crack formation under global tension is important for producing reliable devices. In the present work, the deformation behavior of copper films (50-200 nm thick), bonded to polyimide directly or via a 10 nm chromium interlayer, is investigated by experimental analysis and computational simulations. The influence of the various copper film thicknesses and the usage of a brittle interlayer on the crack density as well as on the stress magnitude in the copper after saturation of the cracking process are studied with in situ tensile tests in a synchrotron and under an atomic force microscope. From the computational point of view, the evolution of the crack pattern is modeled as a stochastic process via finite element based cohesive zone simulations. Both, experiments and simulations show that the chromium interlayer dominates the deformation behavior. The interlayer forms cracks that induce a stress concentration in the overlying copper film. This behavior is more pronounced in the 50 nm than in the 200 nm copper films. © Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2015.01.047
  • 2015 • 132 High-precision deformation and damage development assessment of composite materials by high-speed camera, high-frequency impulse and digital image correlation techniques
    Myslicki, S. and Ortlieb, M. and Frieling, G. and Walther, F.
    Materialpruefung/Materials Testing 57 933-941 (2015)
    Although composite materials like wood, vulcanized fiber and carbon reinforced plastic (CFRP) are already investigated by means of their mechanical properties, the abrupt fracture mechanism as well as the deformation behavior right before and after fracture has not been investigated. However, it is marginally investigated for CFRP because of the quite high fracture speed. The knowledge about the damage evolution as the crack start and propagation can help to improve the strength and sensitivity to fracture by improving the materials structure and to utilize these materials for structural applications. For the investigated materials, fracture happens abruptly as it is the nature of composites and the detailed fracture mechanisms could not be detected by conventional measurement techniques. Therefore, an innovative combination of testing devices is presented which is able to fill this gap. Tensile tests were performed to receive conventional stress-strain curves. At the fracture stage, a high-speed camera recorded the fracture process. This information could be combined with digital image correlation (DIC) to visualize the deformation behavior. At the same time acoustic emission (AE) was used to detect the spectrum of mechanical vibrations which gives information about the released energy due to fracture. The challenging triggering of the high-speed camera was solved for each material individually. By using improved light sources, the recording speed could be set up to 2 million frames per second (Mfps). The investigations show different fracture mechanisms for each composite. Wood and vulcanized fiber were also investigated in different directions due to their anisotropy. © Carl Hanser Verlag, München.
    view abstractdoi: 10.3139/120.110813
  • 2015 • 131 An energy-barrier-based computational micro-sphere model for phase-transformations interacting with plasticity
    Ostwald, R. and Bartel, T. and Menzel, A.
    Computer Methods in Applied Mechanics and Engineering 293 232-265 (2015)
    We extend a newly introduced framework for the simulation of shape memory alloys undergoing martensite-austenite phase-transformations by allowing for the evolution of individual plastic deformations in each phase considered. The goal is to obtain a generalised model which will facilitate the reflection of the characteristic macroscopic behaviour of SMA as well as TRIP steels. Particularly, we show that the incorporation of plasticity effects interacting with phase-transformations allows to capture the typical multi-cyclic stress-strain responses. As a basis, we use a scalar-valued phase-transformation model where a Helmholtz free energy function depending on volumetric and deviatoric strain measures is assigned to each phase. The incorporation of plasticity phenomena is established by enhancing the deviatoric contributions of the Helmholtz free energy functions of the material phases considered, where the plastic driving forces acting in each phase are derived from the overall free energy potential of the mixture. The resulting energy landscape of the constitutive model is obtained from the contributions of the individual constituents, where the actual energy barriers are computed by minimising parametric intersection curves of elliptic paraboloids. With the energy barriers at hand, we use a statistical physics based approach to determine the resulting evolution of volume fractions due to acting thermo-mechanical loads. Though the model allows to take into account an arbitrary number of solid phases of the underlying material, we restrict the investigations to the simulation of phase-transformations between an austenitic parent phase and a martensitic tension and compression phase. The scalar-valued model is embedded into a computational micro-sphere formulation in order to simulate three-dimensional boundary value problems. The systems of evolution equations are solved in a staggered manner, where a newly proposed, physically motivated plasticity inheritance law accounts for the inheritance of plastic deformations due to evolving phases. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.cma.2015.04.008
  • 2015 • 130 Interplanar potential for tension-shear coupling at grain boundaries derived from ab initio calculations
    Pang, X.Y. and Janisch, R. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 24 (2015)
    Based on ab initio density functional theory (DFT) calculations we derive an analytical expression for the interplanar potential of grain boundaries and single crystals as a function of coupled tensile and shear displacements. This energy function captures even details of the grain boundary behaviour, such as the tension-softening of the shear instability of aluminium grain boundaries, with good accuracy. The good agreement between the analytical model and the DFT calculations is achieved by introducing two new characteristic parameters, namely the position of the generalised unstable stacking fault with respect to the stable stacking fault, and the ratio of stable and unstable generalised stacking fault energies. One of the potentials' parameters also serves as a criterion to judge if a grain boundary deforms via crack propagation or dislocation nucleation. We suggest this potential function for application in continuum models, where constitutive relationships for grain boundaries need to be derived from a sound physical model. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/24/1/015007
  • 2015 • 129 Analytical investigation of structurally stable configurations in shape memory alloy-actuated plates
    Peraza Hernandez, E.A. and Kiefer, B. and Hartl, D.J. and Menzel, A. and Lagoudas, D.C.
    International Journal of Solids and Structures 69-70 442-458 (2015)
    Strains produced by active materials embedded in plates have been extensively used to manipulate the shape of surface-like engineering structures. Shape memory alloys (SMAs) are active materials that provide a significant amount of strain under large stresses, a characteristic of great utility in such morphing structures. In this work, an analytical approach to approximate the deformation of plates with SMA constituents is developed via the Rayleigh-Ritz method. An additive set of kinematically admissible displacement fields with unknown coefficients is used to describe the plate displacement field. The total potential energy is then calculated using the displacement field, loading conditions, and constitutive relations for the plate layer(s) composed of SMA wire meshes, dense SMA films, and/or elastic material. The unknown coefficients are then found via minimization of the total potential energy. This approach provides closed-form expressions for the approximate deformation of the plates including multistable configurations. The response of circular SMA-based plates is studied herein. The results show that temperature fields with a linear variation in the radial direction induce multistable configurations in which the plate Gaussian curvature is determined by the direction of the temperature gradient. An alternative inversion of the proposed approach is used to directly compute the temperature field required to morph a plate towards a prescribed goal shape. The obtained closed-form expressions show good agreement with detailed non-linear finite element analysis simulations. The proposed approach provides analysts with a low computational cost and relatively simple implementation to assess the potentially stable configurations of SMA-based plates under given loading conditions. Knowledge of such stable configurations is very valuable in the design of SMA-based morphing structures. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijsolstr.2015.05.007
  • 2015 • 128 Comparing small scale plasticity of copper-chromium nanolayered and alloyed thin films at elevated temperatures
    Raghavan, R. and Harzer, T.P. and Chawla, V. and Djaziri, S. and Phillipi, B. and Wehrs, J. and Wheeler, J.M. and Michler, J. and Dehm, G.
    Acta Materialia 93 175-186 (2015)
    Abstract The yield strengths and deformation mechanisms of Cu-Cr nanolayered and alloyed thin films were studied by microcompression testing at elevated temperatures. The mechanical response of the films with alternating layers of Cu and Cr with sub-100 nm interlayer thicknesses and alloyed films of the same average composition was compared to determine the role of the interfaces on deformation. Higher resistance to plastic flow at elevated temperatures was exhibited by the nanolayered films with smaller interlayer thickness among the layered films, while the alloyed film revealed an anomalous increase in strength with temperature exhibiting a deformation mechanism similar to the pure Cr film. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.04.008
  • 2015 • 127 Heat input modeling and calibration in dry NC-milling processes
    Schweinoch, M. and Joliet, R. and Kersting, P. and Zabel, A.
    Production Engineering 9 495-504 (2015)
    Due to friction and material deformation in the shear zone, workpieces in NC-milling processes are subjected to heat input and thermal loading. Ongoing geometric changes as well as time-varying contact and cutting conditions result in an inhomogeneous temperature field that is constantly in flux. Such thermally loaded workpieces often exhibit complex and transient thermomechanical deformations, which may result in erroneous material removal with respect to the desired shape. In order to meet critical manufacturing tolerances, it is therefore necessary to avoid and compensate these effects. Predicting the deformation exhibited by a thermally loaded workpiece is a problem of linear thermoelasticity, which can be solved by use of the finite element (FE) method. A prerequisite to this is the accurate calculation of the temperature field that results within the workpiece material during the course of the milling process. Although the FE method may be used for this as well, the practical application to realistic milling processes is limited due to the required computational resources. This paper presents a fast geometric process simulation for the prediction of cutting forces, heat input and thermal loading in dry NC milling. The temperature field of the workpiece is continuously updated, such that it is possible to determine the temperature of any material point at any point in time of the milling process. Individual models comprising the simulation system are described in detail, along with the experiments that are required to calibrate them. The accuracy of the geometric process simulation is validated by comparison with experimental data for a non-trivial milling process. © 2015, German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-015-0621-z
  • 2015 • 126 Predicting thermal loading in NC milling processes
    Schweinoch, M. and Joliet, R. and Kersting, P.
    Production Engineering 9 179-186 (2015)
    In dry NC milling, a significant amount of heat is introduced into the workpiece due to friction and material deformation in the shear zone. Time-varying contact conditions, relative tool–workpiece movement and continuous geometric change of the workpiece due to material removal lead to a perpetually changing inhomogeneous temperature distribution within the workpiece. This in turn subjects the workpiece to ongoing complex thermomechanical deformations. Machining such a thermally loaded and deformed workpiece to exact specifications may result in unacceptable shape deviations and thermal errors, which become evident only after dissipation of the introduced heat. This paper presents a hybrid simulation system consisting of a geometric multiscale milling simulation and a finite element method kernel for solving problems of linear thermoelasticity. By combination and back-coupling, the described system is capable of accurately modeling heat input, thermal dispersion, transient thermomechanical deformation and resulting thermal errors as they occur in NC milling processes. A prerequisite to accurately predicting thermomechanical errors is the correct simulation of the temperature field within the workpiece during the milling process. Therefore, this paper is subjected to the precise prediction of the transient temperature distribution inside the workpiece. © 2014, German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-014-0598-z
  • 2015 • 125 Suppression of twinning and phase transformation in an ultrafine grained 2 GPa strong metastable austenitic steel: Experiment and simulation
    Shen, Y.F. and Jia, N. and Wang, Y.D. and Sun, X. and Zuo, L. and Raabe, D.
    Acta Materialia 97 305-315 (2015)
    Abstract An ultrafine-grained 304 austenitic 18 wt.% Cr-8 wt.% Ni stainless steel with a grain size of ∼270 nm was synthesized by accumulative rolling (67% total reduction) and annealing (550°C, 150 s). Uniaxial tensile testing at room temperature reveals an extremely high yield strength of 1890 ± 50 MPa and a tensile strength of 2050 ± 30 MPa, while the elongation reaches 6 ± 1%. Experimental characterization on samples with different grain sizes between 270 nm and 35 μm indicates that both, deformation twinning and martensitic phase transformation are significantly retarded with increasing grain refinement. A crystal plasticity finite element model incorporating a constitutive law reflecting the grain size-controlled dislocation slip and deformation twinning captures the micromechanical behavior of the steels with different grain sizes. Comparison of simulation and experiment shows that the deformation of ultrafine-grained 304 steels is dominated by the slip of partial dislocations, whereas for coarse-grained steels dislocation slip, twinning and martensite formation jointly contribute to the shape change. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.06.053
  • 2015 • 124 Deformation, orientation and bursting of microcapsules in simple shear flow: Wrinkling processes, tumbling and swinging motions
    Unverfehrt, A. and Rehage, H.
    Procedia IUTAM 16 12-21 (2015)
    In a series of experiments we systematically investigated the orientation and deformation behavior of non-spherical capsules in simple shear flow. We observed a continuous capsule rotation at low shear rates, denoted as tumbling mode. A swinging mode at elevated shear rates was characterized by oscillations of the inclination angle around positive values, superimposed by a tank-treading motion of the capsule membrane. The transition between these different modes occurred via an intermittent regime. It turned out that the capsule deformation greatly influenced the orientation dynamics. In several experiments, we also observed shear-induced membrane wrinkling processes. © 2015 The Authors.
    view abstractdoi: 10.1016/j.piutam.2015.03.003
  • 2015 • 123 Deformation, orientation and bursting of microcapsules in simple shear flow: Wrinkling processes, tumbling and swinging motions
    Unverfehrt, A. and Koleva, I. and Rehage, H.
    Journal of Physics: Conference Series 602 (2015)
    In a series of experiments we studied the deformation and orientation behaviour of microcapsules in simple shear flow. For a large number of capsules we observed folding processes which were induced by the bending resistance, by membrane pre-stresses or the mechanical asymmetry of the surrounding viscoelastic wall materials. Periodic oscillations of the inclination angle were detected for non-spherical particles. At low shear rates a tumbling motion occurred in which the capsule turned around its axis. A swinging mode at evaluated shear rates was accompanied by tank-treading motions, a rotation of the membrane around the capsule core. Between these two well-known motions we also observed an intermittent regime. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/602/1/012002
  • 2015 • 122 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 • 121 Micro-tension study of miniaturized cu lines at variable temperatures
    Wimmer, A. and Heinz, W. and Leitner, A. and Detzel, T. and Robl, W. and Kirchlechner, C. and Dehm, G.
    Acta Materialia 92 243-254 (2015)
    In this study, tension experiments on Cu micro-samples at temperatures between 143 and 873 K were performed in order to analyze the influence of grain size, temperature and strain rate on the mechanical properties and fracture mode. The activation energy and evolution of the dislocation density have been analyzed to identify the deformation mechanisms. A transition from bulk-like to stochastic, small-scale behavior has been found with increasing grain size. Furthermore, dependent on the grain size and temperature a change from dislocation based plasticity to diffusion controlled deformation was observed. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.03.056
  • 2015 • 120 Deformation-induced spatiotemporal fluctuation, evolution and localization of strain fields in a bulk metallic glass
    Wu, Y. and Bei, H. and Wang, Y.L. and Lu, Z.P. and George, E.P. and Gao, Y.F.
    International Journal of Plasticity 71 136-145 (2015)
    Deformation behavior and local strain evolutions upon loading and unloading of a bulk metallic glass (BMG) were systematically investigated by in situ digital image correlation (DIC). Distinct fluctuations and irreversible local strains were observed before the onset of macroscopic yielding. Statistical analysis shows that these fluctuations might be related to intrinsic structural heterogeneities, and that the evolution history and characteristics of local strain fields play an important role in the subsequent initiation of shear bands. Effects of sample size, pre-strain, and loading conditions were systematically analyzed in terms of the probability distributions of the resulting local strain fields. It is found that a higher degree of local shear strain heterogeneity corresponds to a more ductile stress-strain curve. Implications of these findings are discussed for the design of new materials. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijplas.2015.05.006
  • 2015 • 119 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 • 118 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 • 117 Damage resistance in gum metal through cold work-induced microstructural heterogeneity
    Zhang, J.-L. and Tasan, C.C. and Lai, M.L. and Zhang, J. and Raabe, D.
    Journal of Materials Science 50 (2015)
    Cold-worked alloys exhibit high strength, but suffer from limited ductility. In contrast, Ti-based gum metal was reported to exhibit high strength combined with good ductility upon severe pre-straining. Motivated by this anomaly, we systematically studied the evolution of gum metal microstructure during severe cold working (swaging and rolling) and the resulting deformation and damage micro-mechanical mechanisms during follow-up tensile deformation. To this end, various experimental in situ and post-mortem methodologies are employed, including scanning electron microscopy imaging, high-resolution electron backscatter diffraction mapping and transmission electron microscopy. These observations reveal that intense grain refinement takes place through dislocation plasticity-dominated deformation banding upon cold working. The observed enhancement in crack blunting and failure resistance which prolongs the post-necking ductility of gum metal during follow-up tensile straining can be attributed to the deformation-induced development of local heterogeneities in texture and grain size. © 2015 Springer Science+Business Media New York
    view abstractdoi: 10.1007/s10853-015-9105-y
  • 2014 • 116 Adaptive computational simulation of TBM-soil interactions during machine-driven tunnel construction in saturated soft soils
    Alsahly, A. and Stascheit, J. and Meschke, G.
    Geotechnical Special Publication 769-779 (2014)
    In soft, partially or fully saturated ground conditions, machine-driven tunnel construction causes short- and long-term ground deformations resulting from the disturbance of the virgin stress state of the soil and changes in the pore water conditions. These variations are, in turn, influenced by the heading face support, shield skin friction and by the gap grouting. Realistic large-scale 3D simulations are, therefore, increasingly required to investigate the interaction between machine-driven tunnel construction and the surrounding soil in order to provide reliable estimates of the expected settlements and associated risks of damage for existing structures, respectively, in particular in urban tunneling projects. If performed properly, these simulations involve complex interactions between individual components of the numerical model. The presented paper is concerned with recent advances in the process-oriented adaptive computational simulation of the excavation and steering processes in mechanized tunneling in soft soils using the finite element method. A novel automated adaptive mesh-refinement procedure is proposed to allow a refined resolution of the region of interest in the vicinity of the tunnel face during the TBM advancement. This procedure allows for an accurate assessment of the tunnel face stability and for the investigation of the immediate soil deformation and pore pressure changes around the tunnel. Furthermore, selected aspects of the numerical treatment - such as the stabilization of low order, two-phase, finite elements and the sub-stepping schemes inherent in the numerical integration of elasto-plastic models -,are also addressed in the presentation. © 2014 American Society of Civil Engineers.
    view abstractdoi: 10.1061/9780784413449.075
  • 2014 • 115 On the modelling of electro-viscoelastic response of electrostrictive polyurethane elastomers
    Ask, A. and Menzel, A. and Ristinmaa, M.
    IOP Conference Series: Materials Science and Engineering 10 (2014)
    Electroactive polymers (EAP) deform under electric fields. This effect in fact generates various new fields of engineering applications of high technological interest. As an advantage, EAP may undergo deformations much larger than those capable by electroactive ceramics - however, to the price of acting at comparatively low forces. As common for polymers, EAP exhibit time-dependent material behaviour. The model proposed in this contribution, on the one hand, captures these electro-viscoelastic effects and, on the other hand, also nicely fits into iterative finite element formulations in order to simulate general boundary value problems. While the deformation itself as well as the electric potential are introduced as global degrees of freedom, the internal variables accounting for the viscous response are incorporated at the so-called local integration point level. Apart form calibrating the model against experimental data, a simple coupled finite element example is studied to show the applicability of the finite deformation electro-viscoelastic formulation proposed. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/10/1/012101
  • 2014 • 114 Large strain elasto-plasticity for diffuse interface models
    Borukhovich, E. and Engels, P.S. and Böhlke, T. and Shchyglo, O. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    Most solid-state phase transformations are accompanied by large deformations, stemming either from external load, transformation strains or plasticity. The consideration of such large deformations will affect the numerical treatment of such transformations. In this paper, we present a new scheme to embed large deformations in an explicit phase-field scheme and its implementation in the open-source framework OpenPhase. The suggested scheme combines the advantages of a spectral solver to calculate the mechanical boundary value problem in a small strain limit and an advection procedure to transport field variables over the calculation grid. Since the developed approach should be used for various sets of problems, e.g. simulations of thermodynamically driven phase transformations, the mechanic formulation is kept general. However, to ensure compatibility with phase-field methods using the concept of diffuse interface, the latter is treated with special care in the present work. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/034008
  • 2014 • 113 Multiscale modeling of nanoindentation: From Atomistic to continuum models
    Engels, P.S. and Begau, C. and Gupta, S. and Schmaling, B. and Ma, A. and Hartmaier, A.
    Solid Mechanics and its Applications 203 285-322 (2014)
    Nanoindentation revealed a number of effects, like pop-in behavior or indentation size effects, that are very different from the classical mechanical behavior of bulk materials and that have therefore sparked a lot of research activities. In this contribution a multiscale approach is followed to understand the mechanisms behind this peculiar material behavior during nanoindentation. Atomistic simulations reveal the mechanisms of dislocation nucleation and multiplication during the very start of plastic deformation. From mesoscale dislocation density based models we gain advanced insight into how plastic zones develop and spread through materials with heterogeneous dislocation microstructures. Crystal plasticity models on the macroscale, finally, are able to reproduce load-indentation curves and remaining imprint topologies in a way that is directly comparable to experimental results and, thus, allows for the determination of true material properties by inverse methods. The complex interplay of the deformation mechanisms occurring on different length scales is described and the necessity to introduce the knowledge about fundamental deformation mechanisms into models on higher length scales is highlighted. © Springer Science+Business Media Dordrecht 2014.
    view abstractdoi: 10.1007/978-94-007-6919-9_15
  • 2014 • 112 Thermal loads of working coils in electromagnetic sheet metal forming
    Gies, S. and Löbbe, C. and Weddeling, C. and Tekkaya, A.E.
    Journal of Materials Processing Technology 214 2537-2544 (2014)
    One basic problem of electromagnetic forming is the limited tool life. Besides the mechanical loads especially thermal loads acting on the tool coil affect its lifetime. In electromagnetic forming, about 50% of the deployed electrical energy is lost because of joule heating in the working coil. In case of high volume production, an accumulation of this heat promotes failure of the coil and reduces the coil lifetime. Despite this importance of the thermal loads only insufficient information about the coil temperature and its influencing parameters is available. Focus of this paper is on the determination of the temperature distribution in case of long-term discharge sequences. Experimental investigations using an infrared camera were performed to measure the coil surface temperature. Numerical process simulation is used to gather information about the temperature inside the working coil. The results prove that the coil reaches an equilibrium temperature after several discharges. For the analyzed range of input power the maximum coil surface temperature and the maximum coil winding temperature reached values of 92 °C and 178 °C, respectively. These temperatures exceed the weakening temperature of most reinforcement and insulation materials. The derived knowledge about the parameters influencing the coil temperature can be used for an improved process design to avoid thermal overstressing of the coil. A comparison of experiments with and without workpiece deformation revealed that the temperature in case of prevented deformation is always higher, and thus, represents an upper bound for the coil temperature. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2014.05.005
  • 2014 • 111 The generalized Hill model: A kinematic approach towards active muscle contraction
    Göktepe, S. and Menzel, A. and Kuhl, E.
    Journal of the Mechanics and Physics of Solids 72 20-39 (2014)
    Excitation-contraction coupling is the physiological process of converting an electrical stimulus into a mechanical response. In muscle, the electrical stimulus is an action potential and the mechanical response is active contraction. The classical Hill model characterizes muscle contraction though one contractile element, activated by electrical excitation, and two non-linear springs, one in series and one in parallel. This rheology translates into an additive decomposition of the total stress into a passive and an active part. Here we supplement this additive decomposition of the stress by a multiplicative decomposition of the deformation gradient into a passive and an active part. We generalize the one-dimensional Hill model to the three-dimensional setting and constitutively define the passive stress as a function of the total deformation gradient and the active stress as a function of both the total deformation gradient and its active part. We show that this novel approach combines the features of both the classical stress-based Hill model and the recent active-strain models. While the notion of active stress is rather phenomenological in nature, active strain is micro-structurally motivated, physically measurable, and straightforward to calibrate. We demonstrate that our model is capable of simulating excitation-contraction coupling in cardiac muscle with its characteristic features of wall thickening, apical lift, and ventricular torsion. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmps.2014.07.015
  • 2014 • 110 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 • 109 Shear-induced mixing governs codeformation of crystalline-amorphous nanolaminates
    Guo, W. and Jägle, E.A. and Choi, P.-P. and Yao, J. and Kostka, A. and Schneider, J.M. and Raabe, D.
    Physical Review Letters 113 (2014)
    Deformation of ductile crystalline-amorphous nanolaminates is not well understood due to the complex interplay of interface mechanics, shear banding, and deformation-driven chemical mixing. Here we present indentation experiments on 10 nm nanocrystalline Cu-100 nm amorphous CuZr model multilayers to study these mechanisms down to the atomic scale. By using correlative atom probe tomography and transmission electron microscopy we find that crystallographic slip bands in the Cu layers coincide with noncrystallographic shear bands in the amorphous CuZr layers. Dislocations from the crystalline layers drag Cu atoms across the interface into the CuZr layers. Also, crystalline Cu blocks are sheared into the CuZr layers. In these sheared and thus Cu enriched zones the initially amorphous CuZr layer is rendered into an amorphous plus crystalline nanocomposite. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.113.035501
  • 2014 • 108 High strength and ductile low density austenitic FeMnAlC steels: Simplex and alloys strengthened by nanoscale ordered carbides
    Gutierrez-Urrutia, I. and Raabe, D.
    Materials Science and Technology (United Kingdom) 30 1099-1104 (2014)
    We introduce the alloy design concepts of high performance austenitic FeMnAlC steels, namely, Simplex and alloys strengthened by nanoscale ordered k-carbides. Simplex steels are characterised by an outstanding strain hardening capacity at room temperature. This is attributed to the multiple stage strain hardening behaviour associated to dislocation substructure refinement and subsequent activation of deformation twinning, which leads to a steadily increase of the strain hardening. Al additions higher that 5 wt-% promote the precipitation of nanoscale L912 ordered precipitates (so called k-carbides) resulting in high strength (yield stress ∼ 1.0 GPa) and ductile (elongation to fracture 7sim; 30%) steels. Novel insights into dislocation-particle interactions in a Fe- 30.5Mn-8.0Al-1.2C (wt-%) steel strengthened by nanoscale k-carbides are discussed. © 2014 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1179/1743284714Y.0000000515
  • 2014 • 107 Effect of retained beta layer on slip transmission in Ti-6Al-2Zr-1Mo-1V near alpha titanium alloy during tensile deformation at room temperature
    He, D. and Zhu, J. and Zaefferer, S. and Raabe, D.
    Materials and Design 56 937-942 (2014)
    Slip is the main plastic deformation mechanism in titanium alloys at room temperature. This is especially so for near alpha titanium alloy like Ti-6Al-2Zr-1Mo-1V, which contains low beta stabilizing and high aluminum (alpha stabilizing) element additions. The effects of retained beta layers on slip transmission across α/β interfaces in Ti-6Al-2Zr-1Mo-1V during tensile deformation have been studied in the current work. High resolution scanning electron microscopy (HR-SEM) and electron backscatter diffraction (EBSD) techniques were used to study the deformation microstructure. The results indicate that the full Burgers crystal orientation relationship (OR) between the α and the thin retained β phase layers facilitates slip transition but is not the necessary requirement/restriction. Some novel slip trace morphologies that are characterized by deflection and bifurcation (fork-like morphology) are revealed in the retained β layers between two abutting α grains. The possible reasons for these different slip transmission patterns are analyzed by EBSD results and a schematic model is proposed. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2013.12.018
  • 2014 • 106 Impact of nanodiffusion on the stacking fault energy in high-strength steels
    Hickel, T. and Sandlöbes, S. and Marceau, R.K.W. and Dick, A. and Bleskov, I. and Neugebauer, J. and Raabe, D.
    Acta Materialia 75 147-155 (2014)
    A key requirement of modern steels - the combination of high strength and high deformability - can best be achieved by enabling a local adaptation of the microstructure during deformation. A local hardening is most efficiently obtained by a modification of the stacking sequence of atomic layers, resulting in the formation of twins or martensite. Combining ab initio calculations with in situ transmission electron microscopy, we show that the ability of a material to incorporate such stacking faults depends on its overall chemical composition and, importantly, the local composition near the defect, which is controlled by nanodiffusion. Specifically, the role of carbon for the stacking fault energy in high-Mn steels is investigated. Consequences for the long-term mechanical properties and the characterisation of these materials are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.04.062
  • 2014 • 105 Differences in deformation behavior of bicrystalline Cu micropillars containing a twin boundary or a large-angle grain boundary
    Imrich, P.J. and Kirchlechner, C. and Motz, C. and Dehm, G.
    Acta Materialia 73 240-250 (2014)
    Micrometer-sized compression pillars containing a grain boundary are investigated to better understand under which conditions grain boundaries have a strengthening effect. The compression experiments were performed on focused ion beam fabricated micrometer-sized bicrystalline Cu pillars including either a large-angle grain boundary (LAGB) or a coherent twin boundary (CTB) parallel to the compression axis and additionally on single-crystalline reference samples. Pillars containing a LAGB show increased strength, stronger hardening and smaller load drops compared to single crystals and exhibit a bent boundary and pillar shape. Samples with a CTB show no major difference in stress-strain data compared to the corresponding single-crystalline samples. This is due to the special orientation and symmetry of the twin boundary and is reflected in a characteristic pillar shape after deformation. The experimental findings can be related to the dislocation-boundary interactions at the different grain boundaries and are compared with three-dimensional discrete dislocation dynamics simulations. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.04.022
  • 2014 • 104 A procedure for the evaluation and compensation of form errors by means of global isometric registration with subsequent local reoptimization
    Klein, L. and Wagner, T. and Buchheim, C. and Biermann, D.
    Production Engineering 8 81-89 (2014)
    Stresses remaining in the component after sheet metal forming processes can result in complex form errors, such as springback and torsions. In order to compensate these process-induced deformations, the local and global deformations have to be analyzed. Hence, an appropriate comparison between the actually manufactured and the target design is required. For this purpose, the surface of the actual workpiece is scanned and the so-obtained scan points have to be assigned to corresponding points of the target shape defined by the workpiece model. From these correspondences, a field of deformation vectors can be computed which represents the basis for the compensation strategy. The task of finding appropriate correspondences is called registration. It is usually solved using rigid transformations, i.e., translation and rotation. Due to the locality, strength and complexity of the deformations, rigid transformations are usually not sufficient. As a more flexible alternative, a procedure for non-rigid registration is presented in this paper. Therein, isometry, i.e., the conservation of distances between corresponding points within an appropriate neighborhood structure, is defined as the objective function. The procedure consists of three steps: definition of the neighborhood structure, global registration, and local reoptimization. The main focus of the paper is set to the latter, where an adapted gradient descent method also allowing projections into the triangles of the target shape is presented and experimentally validated. With these three steps, an assignment between both shapes can be calculated, even for strong local deformations and coarse triangular meshes representing the workpiece model. © 2013 German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-013-0510-2
  • 2014 • 103 Mechanical properties of Al-Cu-Fe quasicrystalline and crystalline phases: An analogy
    Laplanche, G. and Bonneville, J. and Joulain, A. and Gauthier-Brunet, V. and Dubois, S.
    Intermetallics 50 54-58 (2014)
    The mechanical properties of the ω-Al7Cu2Fe crystalline phase have been investigated over a large temperature range (650-1000 K). Despite of its antinomic structure with the icosahedral Al-Cu-Fe quasicrystalline phase, i.e. periodic vs non-periodic, its mechanical properties are very similar to those of the quasicrystalline phase, which strongly suggest similar deformation mechanisms. Consequently, as for the quasicrystalline structure, we propose that dislocation climb might control the plastic deformation of the ω-phase. However, in the present case, the specificities of the quasicrystalline structure cannot be invoked to justify the predominance of dislocation climb, which questions the role of quasiperiodicity on dislocation mobility. We suggest that this deformation mode certainly results from specific non-planar extensions of the dislocation core. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.02.004
  • 2014 • 102 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 • 101 A Riemannian approach to strain measures in nonlinear elasticity
    Neff, P. and Eidel, B. and Osterbrink, F. and Martin, R.
    Comptes Rendus - Mecanique 342 254-257 (2014)
    The isotropic Hencky strain energy appears naturally as a distance measure of the deformation gradient to the set SO(n) of rigid rotations in the canonical left-invariant Riemannian metric on the general linear group GL(n). Objectivity requires the Riemannian metric to be left-GL(n)-invariant, isotropy requires the Riemannian metric to be right-O(n)-invariant. The latter two conditions are only satisfied for a three-parameter family of Riemannian metrics on the tangent space of GL(n). Surprisingly, the final result is basically independent of the chosen parameters.In deriving the result, geodesics on GL(n) have to be parameterized and a novel minimization problem, involving the matrix logarithm for non-symmetric arguments, has to be solved. © 2014.
    view abstractdoi: 10.1016/j.crme.2013.12.005
  • 2014 • 100 Influence of grain orientation on the local deformation mode induced by cavitation erosion in a CuSnNi alloy
    Stella, J. and Pohl, M. and Bock, C. and Kunze, U.
    Wear 316 1-5 (2014)
    The local deformation induced by vibratory cavitation erosion in a CuSnNi alloy was studied employing electron back-scattering diffraction and three-dimensional profiling. The study focused on evaluating initial plastic deformation corresponding to a group of 57 grains in order to correlate crystal orientation and local deformation morphologies. While detectable slip markings developed in grains oriented close to the 〈111〉 and 〈101〉 crystallographic directions during the incubation phase of the cavitation test, grains oriented close to 〈001〉 exhibited no visible slip markings and extensive formation of craters and hills. Furthermore, an analysis based on the average Schmid factors calculated for each grain satisfactorily reflects the transition for the mentioned deformation modes. The quantitative evaluation of the Schmid factor for all slip systems suggests a novel interpretation of the degradation phenomena observed in face-centered cubic materials exposed to a cavitating liquid. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2014.04.010
  • 2014 • 99 Robust numerical calculation of tangent moduli at finite strains based on complex-step derivative approximation and its application to localization analysis
    Tanaka, M. and Fujikawa, M. and Balzani, D. and Schröder, J.
    Computer Methods in Applied Mechanics and Engineering 269 454-470 (2014)
    An extremely robust and efficient numerical approximation of material and spatial tangent moduli at finite strains is presented that can be easily implemented within standard FEM software. This method is based on the complex-step derivative approximation (CSDA) approach. The CSDA is proved to be of second order accurate and it does not suffer from roundoff errors in floating point arithmetics that limit the accuracy of other classical numerical approaches as e.g. finite difference approximation. Therefore, the CSDA can provide approximations extremely similar to analytical solutions when perturbation values are chosen close to machine precision. Implementation details of the robust numerical approximation of tangent moduli from stress calculations using the CSDA are given and their performance is illustrated through representative examples involving finite deformations. In addition to that, we focus on the determination of material instabilities. Therefore, an accompanying localization analysis is performed, where the acoustic tensor is directly computed from the approximation of the moduli. It is shown that classical numerical approximations are sensitive with respect to the perturbation value such that material instabilities may be artificially detected just as a result of slightly changing the perturbation. On the other hand, the CSDA approach provides high-accurate and robust approximations within a wide range of perturbation values such that the material instabilities can be detected precisely. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.cma.2013.11.005
  • 2014 • 98 Composition Dependence of Phase Stability, Deformation Mechanisms, and Mechanical Properties of the CoCrFeMnNi High-Entropy Alloy System
    Tasan, C.C. and Deng, Y. and Pradeep, K.G. and Yao, M.J. and Springer, H. and Raabe, D.
    JOM 66 1993-2001 (2014)
    The proposal of configurational entropy maximization to produce massive solid-solution (SS)-strengthened, single-phase high-entropy alloy (HEA) systems has gained much scientific interest. Although most of this interest focuses on the basic role of configurational entropy in SS formability, setting future research directions also requires the overall property benefits of massive SS strengthening to be carefully investigated. To this end, taking the most promising CoCrFeMnNi HEA system as the starting point, we investigate SS formability, deformation mechanisms, and the achievable mechanical property ranges of different compositions and microstructural states. A comparative assessment of the results with respect to room temperature behavior of binary Fe-Mn alloys reveals only limited benefits of massive SS formation. Nevertheless, the results also clarify that the compositional requirements in this alloy system to stabilize the face-centered cubic (fcc) SS are sufficiently relaxed to allow considering nonequiatomic compositions and exploring improved strength–ductility combinations at reduced alloying costs. © 2014, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1133-6
  • 2014 • 97 Strain localization and damage in dual phase steels investigated by coupled in-situ deformation experiments and crystal plasticity simulations
    Tasan, C.C. and Hoefnagels, J.P.M. and Diehl, M. and Yan, D. and Roters, F. and Raabe, D.
    International Journal of Plasticity 63 198-210 (2014)
    Ferritic-martensitic dual phase (DP) steels deform spatially in a highly heterogeneous manner, i.e. with strong strain and stress partitioning at the micro-scale. Such heterogeneity in local strain evolution leads in turn to a spatially heterogeneous damage distribution, and thus, plays an important role in the process of damage inheritance and fracture. To understand and improve DP steels, it is important to identify connections between the observed strain and damage heterogeneity and the underlying microstructural parameters, e.g. ferrite grain size, martensite distribution, martensite fraction, etc. In this work we pursue this aim by conducting in-situ deformation experiments on two different DP steel grades, employing two different microscopic-digital image correlation (μDIC) techniques to achieve microstructural strain maps of representative statistics and high-resolution. The resulting local strain maps are analyzed in connection to the observed damage incidents (identified by image post-processing) and to local stress maps (obtained from crystal plasticity (CP) simulations of the same microstructural area). The results reveal that plasticity is typically initiated within "hot zones" with larger ferritic grains and lower local martensite fraction. With increasing global deformation, damage incidents are most often observed in the boundary of such highly plastified zones. High-resolution μDIC and the corresponding CP simulations reveal the importance of martensite dispersion: zones with bulky martensite are more susceptible to macroscopic localization before the full strain hardening capacity of the material is consumed. Overall, the presented joint analysis establishes an integrated computational materials engineering (ICME) approach for designing advanced DP steels. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijplas.2014.06.004
  • 2014 • 96 Experimental investigation and numerical simulation of the mechanical and thermal behavior of a superelastic shape memory alloy beam during bending
    Ullrich, J. and Schmidt, M. and Schütze, A. and Wieczorek, A. and Frenzel, J. and Eggeler, G. and Seelecke, S.
    ASME 2014 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2014 2 (2014)
    Superelastic Shape Memory Alloys (SMA) are typically used in applications where the martensitic phase transformation is exploited for its reversible, large deformation such as medical applications (e.g. stents). In this work, we focus on the mechanical and thermal behavior of a Nickel-Titanium SMA strip in bending mode. One possible application of this mode is to provide a restoring force when used in joints of SMA wire actuator systems making the need for an antagonistic SMA actuator redundant. In these applications mentioned above, typically only the mechanical properties are of interest while the temperature is considered constant, even though the martensitic phase transformation in SMA is a thermomechanically coupled process. As a part of the DFG (German Research Association) Priority Programme SPP1599 "Ferroic Cooling" which aims at advancing the development of solid state cooling devices, we have an equally large interest for the thermal evolution of Nickel-Titanium SMA during deformation and its induced phase transformation. In this paper we investigate the thermal and the mechanical response of a SMA beam during bending experiments in which the deformation is induced by holding one end of a SMA strip fixed while the other end is subject to a prescribed deflection. Sensors and high speed thermal cameras are used to capture reaction forces, deformations and temperature changes. We compare these experimental results with numerical simulation results obtained from Finite Element simulations where a thermo-mechanically coupled SMA model is implemented into a finite deformation framework. © 2014 by ASME.
    view abstractdoi: 10.1115/SMASIS20147619
  • 2014 • 95 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 • 94 A gradient-enhanced large-deformation continuum damage model for fibre-reinforced materials
    Waffenschmidt, T. and Polindara, C. and Menzel, A. and Blanco, S.
    Computer Methods in Applied Mechanics and Engineering 268 801-842 (2014)
    A non-local gradient-based damage formulation within a geometrically non-linear setting is presented. The hyperelastic constitutive response at local material point level is governed by a strain energy which is additively composed of an isotropic matrix and of an anisotropic fibre-reinforced material, respectively. The inelastic constitutive response is governed by a scalar [1d]-type damage formulation, where only the anisotropic elastic part is assumed to be affected by the damage. Following the concept in Dimitrijević and Hackl [28], the local free energy function is enhanced by a gradient-term. This term essentially contains the gradient of the non-local damage variable which, itself, is introduced as an additional independent variable. In order to guarantee the equivalence between the local and non-local damage variable, a penalisation term is incorporated within the free energy function. Based on the principle of minimum total potential energy, a coupled system of Euler-Lagrange equations, i.e., the balance of linear momentum and the balance of the non-local damage field, is obtained and solved in weak form. The resulting coupled, highly non-linear system of equations is symmetric and can conveniently be solved by a standard incremental-iterative Newton-Raphson-type solution scheme. Several three-dimensional displacement- and force-driven boundary value problems-partially motivated by biomechanical application-highlight the mesh-objective characteristics and constitutive properties of the model and illustratively underline the capabilities of the formulation proposed. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.cma.2013.10.013
  • 2014 • 93 A gradient-enhanced continuum damage model for residually stressed fibre-reinforced materials at finite strains
    Waffenschmidt, T. and Polindara, C. and Menzel, A.
    Lecture Notes in Applied and Computational Mechanics 74 19-40 (2014)
    Themodelling of damage effects inmaterials constitutes a major challenge in various engineering-related disciplines. However, the assumption of purely local continuum damage formulations may lead to ill-posed boundary value problems and—with regard to numerical methods such as the finite element method—to meshdependent solutions, a vanishing localised damage zone upon mesh refinement, and hence physically questionable results. In order to circumvent these deficiencies, we present a non-local gradient-enhanced damage model at finite strains. We additively compose the hyperelastic constitutive response at local material point level of an isotropic matrix and of an anisotropic fibre-reinforced material. The inelastic constitutive response is characterised by a scalar [1– d]-damage model, where we assume only the anisotropic elastic part to damage. Furthermore, we enhance the local free energy by a gradient-term. This term essentially contains the gradient of the non-local damage variable which we introduce as an additional global field variable. In order to guarantee the equivalence between the local and non-local damage variable, we incorporate a penalisation term within the free energy. Based on the principle of minimum total potential energy, we obtain a coupled system of variational equations. The associated non-linear system of equations is symmetric and can conveniently be solved by standard incremental-iterative Newton-Raphson schemes or arc-length-based solution methods. As a further key aspect, we incorporate residual stresses by means of a multiplicative composition of the deformation gradient. As a three-dimensional finite element example, we study the material degradation of a fibre-reinforced tube subjected to internal pressure. This highlights the meshobjective and constitutive properties of the model and illustratively underlines the capabilities of the formulation with regard to biomechanical application such as the simulation of arteries. © Springer International Publishing Switzerland 2014.
    view abstractdoi: 10.1007/978-3-319-10981-7_2
  • 2014 • 92 Smaller is less stable: Size effects on twinning vs. transformation of reverted austenite in TRIP-maraging steels
    Wang, M.-M. and Tasan, C.C. and Ponge, D. and Kostka, A. and Raabe, D.
    Acta Materialia 79 268-281 (2014)
    Steels containing reverted nanoscale austenite (γRN) islands or films dispersed in a martensitic matrix show excellent strength, ductility and toughness. The underlying microstructural mechanisms responsible for these improvements are not yet understood, but are observed to be strongly connected to the γRN island or film size. Two main micromechanical effects are conceivable in this context, namely: (i) interaction of γRN with microcracks from the matrix (crack blunting or arresting); and (ii) deformation-induced phase transformation of γRN to martensite (TRIP effect). The focus here is on the latter phenomenon. To investigate size effects on γRN transformation independent of other factors that can influence austenite stability (composition, crystallographic orientation, defect density, surrounding phase, etc.), a model (TRIP-maraging steel) microstructure is designed with support from diffusion simulations (using DICTRA software) to have the same, homogeneous chemical composition in all γRN grains. Characterization is conducted by in-situ tension and bending experiments in conjunction with high-resolution electron backscatter diffraction mapping and scanning electron microscopy imaging, as well as post-mortem transmission electron microscopy and synchrotron X-ray diffraction analysis. Results reveal an unexpected "smaller is less stable" effect due to the size-dependent competition between mechanical twinning and deformation-induced phase transformation. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.07.020
  • 2014 • 91 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 • 90 A novel high manganese austenitic steel with higher work hardening capacity and much lower impact deformation than Hadfield manganese steel
    Wen, Y.H. and Peng, H.B. and Si, H.T. and Xiong, R.L. and Raabe, D.
    Materials and Design 55 798-804 (2014)
    To tackle the problem of poor work hardening capacity and high initial deformation under low load in Hadfield manganese steel, the deformation behavior and microstructures under tensile and impact were investigated in a new high manganese austenitic steel Fe18Mn5Si0.35C (wt.%). The results show that this new steel has higher work hardening capacity at low and high strains than Hadfield manganese steel. Its impact deformation is much lower than that of Hadfield manganese steel. The easy occurrence and rapid increase of the amount of stress-induced ε martensitic transformation account for this unique properties in Fe18Mn5Si0.35C steel. The results indirectly confirm that the formation of distorted deformation twin leads to the anomalous work hardening in Hadfield manganese steel. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2013.09.057
  • 2014 • 89 Phase-specific deformation behavior of a relatively tough NiAl-Cr(Mo) lamellar composite
    Yu, D. and Bei, H. and Chen, Y. and George, E.P. and An, K.
    Scripta Materialia 84-85 59-62 (2014)
    A NiAl-Cr(Mo) nanolayered composite exhibits improved room-temperature toughness in compression compared to its constituent phases, NiAl and Cr solid solution (Crss). Real-time in situ neutron diffraction and post-test microstructural observations show that the Crss layers with thickness of ∼400 nm can bear very high stresses and deform plastically before fracture, unlike in bulk form, where the Cr solid solution fractures in a relatively brittle fashion at significantly lower stresses, which contribute to the much higher toughness of the composite. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2014.04.025
  • 2014 • 88 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 • 87 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
  • 2013 • 86 Incipient plasticity and deformation mechanisms in single-crystal Mg during spherical nanoindentation
    Catoor, D. and Gao, Y.F. and Geng, J. and Prasad, M.J.N.V. and Herbert, E.G. and Kumar, K.S. and Pharr, G.M. and George, E.P.
    Acta Materialia 61 2953-2965 (2013)
    Incipient plasticity in Mg single crystals was investigated using the pop-ins generated during spherical nanoindentation on (0 0 0 1), (1 0 -1 2) and (1 0 -1 0) surfaces. Representative deformed regions extracted from underneath indents by means of focused ion beam machining were examined by transmission electron microscopy (TEM) to identify the deformation mechanisms. Anisotropic elastic Hertzian contact theory was used to calculate indentation Schmid factors and the relevant resolved shear stresses at pop-in from the load-displacement curves. The pop-in statistics in conjunction with the TEM analysis showed that the most likely deformation mechanism responsible for pop-in is slip via 〈a〉 dislocations even in the case of indentation along the c-axis. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.01.055
  • 2013 • 85 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 • 84 Chain deformation in translocation phenomena
    Farahpour, F. and Maleknejad, A. and Varnik, F. and Ejtehadi, M.R.
    Soft Matter 9 2750-2759 (2013)
    Deformation of single stranded DNA in a translocation process before reaching the pore is investigated. By solving the Laplace equation in a suitable coordinate system and with appropriate boundary conditions, an approximate solution for the electric field inside and outside a narrow pore is obtained. With an analysis based on the "electrohydrodynamic equivalence" we determine the possibility of the extension of a charged polymer due to the presence of an electric field gradient in the vicinity of the pore entrance. With a multi-scale hybrid simulation (LB-MD), it is shown that an effective deformation before reaching the pore occurs, which facilitates the process of finding the entrance for the end monomers. We also highlight the role of long range hydrodynamic interactions via comparison of the LB-MD results with those obtained using a Langevin thermostat instead of the LB solver. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2sm27416g
  • 2013 • 83 High-temperature deformation and recrystallization: A variational analysis and its application to olivine aggregates
    Hackl, K. and Renner, J.
    Journal of Geophysical Research: Solid Earth 118 943-967 (2013)
    We develop a framework for a variational analysis of microstructural evolution during inelastic high-temperature deformation accommodated by dislocation mechanisms and diffusive mass transport. A polycrystalline aggregate is represented by a distribution function characterizing the state of individual grains by three variables, dislocation density, grain size, and elastic strain. The aggregate's free energy comprises elastic energy and energies of lattice distortions due to dislocations and grain boundaries. The work performed by the external loading is consumed by changes in the number of defects and their migration leading to inelastic deformation. The variational approach minimizes the rate of change of free energy with the evolution of the state variables under constraints on the aggregate volume, on a relation between changes in plastic strain and dislocation density, and on the form of the dissipation functionals for defect processes. The constrained minimization results in four basic evolution equations, one each for the evolution in grain size and dislocation density and flow laws for dislocation and diffusion creep. Analytical steady state scaling relations between stress and dislocation density and grain size (piezometers) are derived for quasi-homogeneous materials characterized by a unique relation between grain size and dislocation density. Our model matches all currently available experimental observations regarding high-temperature deformation of olivine aggregates with plausible values for the involved micromechanical model parameters. The relation between strain rate and stress for olivine aggregates maintaining a steady state microstructure is distinctly nonlinear in stark contrast to the majority of geodynamical modeling relying on linear relations, i.e., Newtonian behavior. Key Points Analytical derivation of steady-state piezometers using variational analysis Matches observations for olivine rocks with plausible micromechanical parameters Provides insight into rheology of olivine aggregates, e.g., lifetime of grains ©2013. American Geophysical Union. All Rights Reserved.
    view abstractdoi: 10.1002/jgrb.50125
  • 2013 • 82 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 • 81 Simulation of shear banding in heterophase co-deformation: Example of plane strain compressed Cu-Ag and Cu-Nb metal matrix composites
    Jia, N. and Roters, F. and Eisenlohr, P. and Raabe, D. and Zhao, X.
    Acta Materialia 61 4591-4606 (2013)
    The co-deformation and shear localization in heterophase alloys is studied using two-dimensional crystal plasticity finite element simulations on plane strain compressed Cu-Ag and Cu-Nb metal matrix composites. The aim is to study the fundamentals of micromechanics, co-deformation and shear banding in materials with heterophase interfaces. It is observed that, depending on the initial orientations of the crystals, co-deformation of the constituent heterophases often proceeds via collective mechanisms, i.e. by pronounced shear banding triggered by stress concentration at the interfaces. This phenomenon leads to highly localized strains within the bands, exceeding the average strain in part by two orders of magnitude. Shear band development is related to the inherent mechanical properties of each crystal and also to the properties of the abutting crystals. The predicted topology and nature of the cross-phase shear bands, i.e. the extreme local strains, significant bending of the interface regions, and sharp strain localization that propagates across the interfaces, agree well with experimental observations in cold-rolled composites. The simulations reveal that cross-phase shear banding leads to large and highly localized values of stress and strain at heterophase interfaces. Such information is essential for a better understanding of the micromechanical boundary conditions inside co-deformed composites and the associated shear-induced chemical mixing. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.029
  • 2013 • 80 Validation of a heat input model for the prediction of thermomechanical deformations during NC milling
    Joliet, R. and Byfut, A. and Kersting, P. and Schröder, A. and Zabel, A.
    Procedia CIRP 8 403-408 (2013)
    During roughing in NC milling, heat is introduced into the workpiece. For the manufacturing of large structural components, a constantly changing temperature field is created due to the rapid movement and the varying contact conditions between tool and workpiece. Therefore, significant deformations can cause form errors that lead to rejects in the production process. In this paper, a simulation system for the prediction of transient workpiece temperatures is presented. In order to calibrate the system, simple experiments have been conducted, and a model for the introduction of energy into the workpiece via cutting has been developed. The newly developed cutting-energy input model makes it possible to perform fast simulations. Therefore, it can be used to perform simulations of the thermoelastic workpiece deformations during milling of complex shaped parts. Copyright © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2013.06.124
  • 2013 • 79 Simulation of electromagnetic forming of a cross-shaped cup by means of a viscoplasticity model coupled with damage at finite strains
    Kiliclar, Y. and Demir, O.K. and Vladimirov, I.N. and Kwiatkowski, L. and Reese, S. and Tekkaya, A.E.
    Key Engineering Materials 554-557 2363-2368 (2013)
    In the field of sheet metal forming conventional forming processes are well established. However, a quasi-static forming process combined with a high speed forming process can enhance the forming limits of a single one. In this paper, the investigation of the process chain quasi-static deep drawing - electromagnetic forming by means of a new coupled damage-viscoplasticity model for large deformations is performed. The finite strain constitutive model, used in the finite element simulation combines nonlinear kinematic and isotropic hardening and is derived in a thermodynamically consistent setting. This anisotropic viscoplastic model is based on the multiplicative decomposition of the deformation gradient in the context of hyperelasticity. The kinematic hardening component represents a continuum extension of the classical rheological model of Armstrong-Frederick kinematic hardening. The coupling of damage and plasticity is carried out in a constitutive manner according to the effective stress concept. The constitutive equations of the material model are integrated in an explicit manner and implemented as a user material subroutine in the commercial finite element package LS-DYNA with the electromagnetical module. The aim of the work is to show the increasing formability of the sheet by combining quasistatic deep drawing processes with high speed electromagnetic forming. Copyright © 2013 Trans Tech Publications Ltd.
    view abstractdoi: 10.4028/www.scientific.net/KEM.554-557.2363
  • 2013 • 78 Hydrogen-assisted failure in a twinning-induced plasticity steel studied under in situ hydrogen charging by electron channeling contrast imaging
    Koyama, M. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Acta Materialia 61 4607-4618 (2013)
    We investigated the hydrogen embrittlement of a Fe-18Mn-1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7 × 10-6 s -1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe-Mn-C twining-induced plasticity steel. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.030
  • 2013 • 77 Crossover from tumbling to tank-treading-like motion in dense simulated suspensions of red blood cells
    Krüger, T. and Gross, M. and Raabe, D. and Varnik, F.
    Soft Matter 9 9008-9015 (2013)
    Via computer simulations, we provide evidence that the shear rate induced red blood cell tumbling-to-tank-treading transition also occurs at quite high volume fractions, where collective effects are important. The transition takes place as the ratio of effective suspension stress to the characteristic cell membrane stress exceeds a certain value and does not explicitly depend on volume fraction or cell deformability. This value coincides with that for a transition from an orientationally less ordered to a highly ordered phase. The average cell deformation does not show any signature of transition, but rather follows a simple scaling law independent of volume fraction. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3sm51645h
  • 2013 • 76 Cyclic deformation and lifetime of Alloy 617B during thermo-mechanical fatigue
    Maier, G. and Riedel, H. and Nieweg, B. and Somsen, C. and Eggeler, G. and Klöwer, J. and Mohrmann, R.
    Materials at High Temperatures 30 27-35 (2013)
    Different heats of the nickel-base Alloy 617B are tested under in-phase and out-of-phase thermo-mechanical fatigue (TMF) conditions at temperatures between 50 and 900 °C. During one of the TMF tests the growth of microcracks is observed using the replica technique. After the tests, some of the specimens are inspected by scanning electron microscopy in order to analyse the prevailing damage mechanisms compared with those observed in isothermal low-cycle fatigue (LCF) tests. In addition, a Chaboche-type model and a fracture-mechanics-based lifetime model are employed to describe the cyclic viscoplastic deformation and fatigue lifetime. The Chaboche model adjusted to isothermal data is found to reasonably predict the cyclic viscoplastic behaviour of thermo-mechanically loaded specimens. Lifetime data of TMF tests fall into a common scatter band with LCF tests at temperatures above 400 °C if the test results are analysed based on the introduced lifetime model.
    view abstractdoi: 10.3184/096034013X13636905345685
  • 2013 • 75 Cyclic deformation and lifetime of Alloy 617B during isothermal low cycle fatigue
    Maier, G. and Riedel, H. and Somsen, C.
    International Journal of Fatigue 55 126-135 (2013)
    Isothermal low cycle fatigue tests are carried out on the nickel-base Alloy 617B in the solution-annealed, stabilized and long-term aged conditions at temperatures between room temperature and 900 C. In addition, fatigue microcrack growth is measured using the replica technique. Transmission electron microscopy studies suggest that the observed differences in cyclic hardening between the different heat treatments result from the precipitation of fine carbides. Scanning electron microscope observations indicate a change in fracture mode for the solution-annealed and long-term aged material with temperature. The Chaboche model is able to describe the time and temperature dependent cyclic plasticity of the three material conditions. The measured lifetimes and crack growth rates can be described using a fracture mechanics based lifetime model. However, the data for room temperature and for temperatures above 400 C fall into two different scatter bands due to differences in crack growth rates. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2013.06.001
  • 2013 • 74 Parallelized computational modeling of pile-soil interactions in mechanized tunneling
    Meschke, G. and Ninić, J. and Stascheit, J. and Alsahly, A.
    Engineering Structures 47 35-44 (2013)
    The construction of tunnels in soft ground causes short and long term ground deformations resulting from the disturbance of the virgin stress state of the soil and the changing pore water conditions. In particular in urban tunneling, in each stage of the construction process, interactions between the construction process, the soil and existing building infrastructure need to be evaluated to limit the risk of damage on existing buildings and to decide on appropriate mitigation measures. Besides conventional tunneling, mechanized tunneling is a well established and flexible technology in particular in urban areas, which allows for tunnel advances in a wide range of soils and difficult conditions. The paper presents a finite element model for the simulation of interactions between mechanized tunnel construction, the surrounding soil and existing buildings resting on pile foundations in the framework of a process-oriented simulation model for mechanized tunneling. The performance of the model is demonstrated by means of selected prototype analyses. As a consequence of the high computational demand connected with this type of spatio-temporal simulations, problem specific parallelization techniques are investigated to increase the numerical efficiency of the numerical analyses. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.engstruct.2012.07.001
  • 2013 • 73 The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy
    Otto, F. and Dlouhý, A. and Somsen, C. and Bei, H. and Eggeler, G. and George, E.P.
    Acta Materialia 61 5743-5755 (2013)
    An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4-160 μm. Quasi-static tensile tests at an engineering strain rate of 10-3 s-1 were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to ∼2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1}〈1 1 0〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈1 1 0〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈1 1 2〉 Shockley partials. At later stages (∼20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing ("dynamic Hall-Petch"), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.06.018
  • 2013 • 72 Iterative, simulation-based shape modification by free-form deformation of the NC programs
    Sacharow, A. and Odendahl, S. and Peuker, A. and Biermann, D. and Surmann, T. and Zabel, A.
    Advances in Engineering Software 56 63-71 (2013)
    In production engineering, there are several applications where the geometry of a designed workpiece has to be modified, e.g., optimization of forming tools during springback compensation in sheet metal forming. In general, the modified shape of the workpiece is given as a mesh and has to be converted to a parametric representation by surface reconstruction before manufacturing. In this paper, a new approach for obtaining small shape modifications by direct deformation of the NC programs is presented. In an iterative process, the CAM data is modified by a free-form deformation and is verified by a milling simulation so that the modified workpiece can be manufactured directly on the basis of the original CAD/CAM data without surface reconstruction. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.advengsoft.2012.11.007
  • 2013 • 71 Basal and non-basal dislocation slip in Mg–Y
    Sandlöbes, S. and Friák, M. and Neugebauer, J. and Raabe, D.
    Materials Science and Engineering A 576 61-68 (2013)
    The activation of non-basal slip systems is of high importance for the ductility in hcp Mg and its alloys. In particular, for Mg–Y alloys where a higher activation of pyramidal dislocation slip causes an increased ductility detailed characterization of the activated slip systems is essential to understand and describe plasticity in these alloys. In this study a detailed analysis of the activated dislocations and slip systems via post-mortem TEM and SEM-EBSD based slip band analysis in 3% deformed Mg–3 wt% Y is presented. The analysis reveals a substantial activity of pyramidal <c+a> dislocations with different Burgers vectors. The obtained dislocation densities and active slip systems are discussed with respect to atomistic simulations of non-basal dislocations in hcp Mg. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2013.03.006
  • 2013 • 70 Mechanisms of anisotropic friction in nanotwinned Cu revealed by atomistic simulations
    Zhang, J.J. and Hartmaier, A. and Wei, Y.J. and Yan, Y.D. and Sun, T.
    Modelling and Simulation in Materials Science and Engineering 21 (2013)
    The nature of nanocrystalline materials determines that their deformation at the grain level relies on the orientation of individual grains. In this work, we investigate the anisotropic response of nanotwinned Cu to frictional contacts during nanoscratching by means of molecular dynamics simulations. Nanotwinned Cu samples containing embedded twin boundaries parallel, inclined and perpendicular to scratching surfaces are adopted to address the effects of crystallographic orientation and inclination angle of aligned twin boundaries cutting the scratching surface. The transition in deformation mechanisms, the evolution of friction coefficients and the friction-induced microstructural changes are analyzed in detail and are related to the loading conditions and the twinned microstructures of the materials. Furthermore, the effect of twin spacing on the frictional behavior of Cu samples is studied. Our simulation results show that the crystallographic orientation strongly influences the frictional response in different ways for samples with different twin spacing, because the dominant deformation mode varies upon scratching regions of different orientations. A critical inclination angle of 26.6° gives the lowest yield strength and the highest friction coefficient, at which the plasticity is dominated by twin boundary migration and detwinning. It is demonstrated that the anisotropic frictional response of nanotwinned Cu originates from the heterogeneous localized deformation, which is strongly influenced by crystallographic orientation, twin boundary orientation and loading condition. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/21/6/065001
  • 2012 • 69 Phenomenological modeling of viscous electrostrictive polymers
    Ask, A. and Menzel, A. and Ristinmaa, M.
    International Journal of Non-Linear Mechanics 47 156-165 (2012)
    A common usage for electroactive polymers (EAPs) is in different types of actuators, where advantage is taken of the deformation of the polymer due to an electric field. It turns out that time-dependent effects are present in these applications. One of these effects is the viscoelastic behavior of the polymer material. In view of the modeling and simulation of applications for EAP within a continuum mechanics setting, a phenomenological framework for an electro-viscoelastic material model is elaborated in this work. The different specific models are fitted to experimental data available in the literature. While the experimental data used for inherent electrostriction is restricted to small strains, a large strain setting is used for the model in order to account for possible applications where the polymers undergo large deformations, such as in pre-strained actuators. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijnonlinmec.2011.03.020
  • 2012 • 68 A structural optimisation viewpoint on growth phenomena
    Barthold, F.-J.
    Bulletin of the Polish Academy of Sciences: Technical Sciences 60 247-252 (2012)
    Evolutionary solid bodies undergoing changes of mass, of properties, and of shapes are considered in models of growth and adaptation and similarily in structural optimisation. A fundamental separation of different growth phenomena and a subsequent parametrisation using independent design variables for the amount of substance as well as for molar mass and molar volume facilitates an efficient formulation of the design space. Thus, the effects of design variations, i.e. change of amount of substance, on the variations of the structural response, i.e. The deformation in physical space, can be clearly described. Overall, a novel treatment of growth processes based on an evolution of the amount of substance is outlined. The parallelism of variations in physical and design space are highlighted and compared with the multiplicative decomposition of the deformation gradient into a growth and an elastic part incorporating an incompatible intermediate configuration. This drawback is overcome by a compatible manifold based on material points modelling the amount of substance outside of any geometrical space.
    view abstractdoi: 10.2478/v10175-012-0033-6
  • 2012 • 67 A novel approach to study dislocation density tensors and lattice rotation patterns in atomistic simulations
    Begau, C. and Hua, J. and Hartmaier, A.
    Journal of the Mechanics and Physics of Solids 60 711-722 (2012)
    Crystal plasticity caused by the nucleation and interaction of dislocations is an important aspect in crystal deformation. Recent nanoindentation experiments in single crystals of copper or aluminum revealed large deviations in the lattice rotation and an inhomogeneous distribution of the dislocation density in the plastic zone under the indenter tip. Molecular dynamics simulations offer the possibility to study the origin of these phenomena on an atomistic scale, but require sophisticated analysis routines in order to deal with the massive amount of generated data. Here a new efficient approach to analyze atomistic data on the fly during the simulation is introduced. This approach allows us to identify the dislocation network including Burgers vectors on the timescale of picoseconds and below. This data does not only reveal the evolution of dislocation structures, but it offers the possibility to quantify local dislocation density tensors calculated on an atomic level. The numerical results are compared with experimental data from the literature. The presented approach provides useful insight into the active deformation mechanisms during plastic deformation that will help us to bridge simulations on atomic scales and continuum descriptions. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2011.12.005
  • 2012 • 66 Influence of surface characteristics on fatigue behaviour of laser sintered plastics
    Blattmeier, M. and Witt, G. and Wortberg, J. and Eggert, J. and Toepker, J.
    Rapid Prototyping Journal 18 161-171 (2012)
    Purpose - The purpose of this paper is to provide macromechanical insight into the fatigue behaviour of laser sintered parts and to understand the influence of the laser sintered surface structure on this behaviour. Design/methodology/approach - A background on the technological maturity of manufacturing processes and the demand for structural and aesthetic properties of laser sintered plastic products is given. As the contribution of surface structure on part quality was the focus, laser sintered specimens with and without surface finishes, as well as injection moulded specimens were used. The latter simply served as a comparison and was not intended to qualify injection moulding. The study comprises the determination of short-term tensile properties, the load increase method for investigating fracture and deformation behaviours, and fatigue crack propagation analysis. Findings - According to the test results, the contribution of laser sintered surface structures to relevant mechanical properties can be neglected. Under dynamic loading conditions, laser sintered specimens achieved a longer lifetime but showed less deformation capabilities in contrast to injection moulded specimens. In general, laser sintered specimens presented considerable resistance to crack initiation and propagation. Research limitations/implications - Because of the long-term approach of the research, the number of tests conducted per lot was limited. Thus, the effects of different process settings and the reproducibility could not be fully analysed. Practical implications - The studied fatigue behaviour of laser sintered specimens has implications for the functional testing of parts or components, for the product and process design as well as for the general compatibility of laser sintering as a manufacturing technology of end-customer products. Originality/value - The value of this paper lies in the better understanding of deformation and fracture behaviours of laser sintered polymers. © 2012 Emerald Group Publishing Limited.
    view abstractdoi: 10.1108/13552541211212140
  • 2012 • 65 On the effect of manganese on grain size stability and hardenability in ultrafine-grained ferrite/martensite dual-phase steels
    Calcagnotto, M. and Ponge, D. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 37-46 (2012)
    Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). The Mn distribution in ferrite and martensite is analyzed by high-resolution electron backscatter diffraction (EBSD) combined with energy dispersive X-ray spectroscopy (EDX). The experimental findings are supported by the calculated phase diagrams, equilibrium phase compositions, and the estimated diffusion distances using Thermo-Calc (Thermo-Calc Software, McMurray, PA) and Dictra (Thermo-Calc Software). Mn substantially enhances the grain size stability during intercritical annealing and the ability of austenite to undergo martensitic phase transformation. The first observation is explained in terms of the alteration of the phase transformation temperatures and the grain boundary mobility, while the second is a result of the Mn enrichment in cementite during large strain warm deformation, which is inherited by the newly formed austenite and increases its hardenability. The latter is the main reason why the ultrafine-grained material exhibits a hardenability that is comparable with the hardenability of the coarse-grained reference material. © 2011 The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-011-0828-3
  • 2012 • 64 Dislocation starvation and exhaustion hardening in Mo alloy nanofibers
    Chisholm, C. and Bei, H. and Lowry, M.B. and Oh, J. and Syed Asif, S.A. and Warren, O.L. and Shan, Z.W. and George, E.P. and Minor, A.M.
    Acta Materialia 60 2258-2264 (2012)
    The evolution of defects in Mo alloy nanofibers with initial dislocation densities ranging from 0 to ∼1.6 × 10 14 m -2 were studied using an in situ "push-to-pull" device in conjunction with a nanoindenter in a transmission electron microscope. Digital image correlation was used to determine stress and strain in local areas of deformation. When they had no initial dislocations the Mo alloy nanofibers suffered sudden catastrophic elongation following elastic deformation to ultrahigh stresses. At the other extreme fibers with a high dislocation density underwent sustained homogeneous deformation after yielding at much lower stresses. Between these two extremes nanofibers with intermediate dislocation densities demonstrated a clear exhaustion hardening behavior, where the progressive exhaustion of dislocations and dislocation sources increases the stress required to drive plasticity. This is consistent with the idea that mechanical size effects ("smaller is stronger") are due to the fact that nanostructures usually have fewer defects that can operate at lower stresses. By monitoring the evolution of stress locally we find that exhaustion hardening causes the stress in the nanofibers to surpass the critical stress predicted for self-multiplication, supporting a plasticity mechanism that has been hypothesized to account for the rapid strain softening observed in nanoscale bcc materials at high stresses. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.12.027
  • 2012 • 63 In-Situ TEM Straining Experiments: Recent Progress in Stages and Small-Scale Mechanics
    Dehm, G. and Legros, M. and Kiener, D.
    In-Situ Electron Microscopy: Applications in Physics, Chemistry and Materials Science 227-254 (2012)
    doi: 10.1002/9783527652167.ch10
  • 2012 • 62 Adiabatic temperature increase associated with deformation twinning and dislocation plasticity
    Eisenlohr, A. and Gutierrez-Urrutia, I. and Raabe, D.
    Acta Materialia 60 3994-4004 (2012)
    We studied local deformation and temperature effects associated with mechanical twinning in Fe-3 wt.% Si at room temperature. During tensile testing, two large stress drops occurred. They were accompanied by local strain and temperature bursts, which we mapped via simultaneous displacement and temperature field characterization. To identify the microstructural origin of these phenomena, we performed high resolution electron backscatter scanning diffraction and electron channeling contrast imaging measurements. The microstructure at the positions where strong adiabatic heating occurred was characterized by the formation of primary twins and high dislocation activity within a range of about 10 μm around the twin-matrix interface. We suggest that the local temperature and strain jumps result from the formation and dissipative motion of lattice dislocations that accommodate twinning. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.03.008
  • 2012 • 61 Multistage strain hardening through dislocation substructure and twinning in a high strength and ductile weight-reduced Fe-Mn-Al-C steel
    Gutierrez-Urrutia, I. and Raabe, D.
    Acta Materialia 60 5791-5802 (2012)
    We investigate the kinetics of the deformation structure evolution and its contribution to the strain hardening of a Fe-30.5Mn-2.1Al-1.2C (wt.%) steel during tensile deformation by means of transmission electron microscopy and electron channeling contrast imaging combined with electron backscatter diffraction. The alloy exhibits a superior combination of strength and ductility (ultimate tensile strength of 1.6 GPa and elongation to failure of 55%) due to the multiple-stage strain hardening. We explain this behavior in terms of dislocation substructure refinement and subsequent activation of deformation twinning. The early hardening stage is fully determined by the size of the dislocation substructure, namely, Taylor lattices, cell blocks and dislocation cells. The high carbon content in solid solution has a pronounced effect on the evolving dislocation substructure. We attribute this effect to the reduction of the dislocation cross-slip frequency by solute carbon. With increasing applied stress, the cross-slip frequency increases. This results in a gradual transition from planar (Taylor lattices) to wavy (cells, cell blocks) dislocation configurations. The size of such dislocation substructures scales inversely with the applied resolved stress. We do not observe the so-called microband-induced plasticity effect. In the present case, due to texture effects, microbanding is not favored during tensile deformation and, hence, has no effect on strain hardening. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.018
  • 2012 • 60 A new method for determining dynamic grain structure evolution during hot aluminum extrusion
    Güzel, A. and Jäger, A. and Parvizian, F. and Lambers, H.-G. and Tekkaya, A.E. and Svendsen, B. and Maier, H.J.
    Journal of Materials Processing Technology 212 323-330 (2012)
    In this paper, a new method for analyzing the microstructure evolution of aluminum during deformation at elevated temperatures by extrusion is presented, which is entirely separated from secondary restoration effects viz. static recrystallization and grain growth. In order to observe the development of grains and their orientation under severe plastic deformation, a small-scale forward extrusion setup was designed which allows quenching the extrusion butt together with the die and the container immediately after extrusion to preserve the grain structure evolved during the deformation. The forming path and the forming history of a selected material point were calculated by numerical simulation. The evolution of the microstructure along the forming path was analyzed using electron backscatter diffraction. A database for the development of physically based phenomenological models for predicting and simulating the evolution of microstructure during the hot deformation of EN AW-6082 alloy is provided. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2011.09.018
  • 2012 • 59 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 • 58 Efficient modeling of microstructure evolution in magnesium by energy minimization
    Homayonifar, M. and Mosler, J.
    International Journal of Plasticity 28 1-20 (2012)
    The description of the complex interplay between deformation-induced twinning and dislocation slip, typical for metals showing an hcp structure such as magnesium, is of utmost importance for understanding their deformation behavior. In the present paper, an incremental energy principle is presented for that purpose. Within this principle, dislocation slip is modeled by crystal plasticity theory, while the phase decomposition associated with twinning is considered by a mixture theory. This mixture theory naturally avoids the decomposition of the twinning process into so-called pseudo-dislocations followed by a reorientation of the total crystal. By way of contrast, the proposed model captures the transformation of the crystal lattice due to twinning in a continuous fashion by simultaneously taking dislocation slip within both, possibly co-existent, phases into account. The shear strain induced by twinning as well as the deformation history are consistently included within the twinned domain by an enhanced multiplicative decomposition of the deformation gradient. Kinematic compatibility between the different phases is enforced by a Hadamard-type compatibility condition, while compatibility with respect to the boundary conditions requires the introduction of a boundary layer. The evolution of all state variables such as the twinning volume and the plastic strains associated with dislocation slip follow jointly and conveniently from minimizing the stress power of the total crystal. This canonical variational principle is closely related to the postulate of maximum dissipation and guarantees thermodynamical consistency of the resulting model. Particularly, the second law of thermodynamics is fulfilled. In contrast to previous models suitable for the analysis of the deformation systems in magnesium, the Helmholtz energy of the twinning interfaces and that of the aforementioned boundary layer are considered. Analogously, the energy due to twinning nucleation and that related to twinning growth are accounted for by suitable dissipation functionals. By doing so, the number of twinning laminates becomes an additional unknown within the minimization principle and thus, the thickness of the lamellas can be computed. Interestingly, by interpreting this thickness as the mean free path of dislocations, a size effect of Hall-Petch-type can naturally be included within the novel model. The predictive capabilities of the resulting approach are finally demonstrated by analyzing the channel die test. For that purpose, a certain rank-two laminate structure is considered. However, it bears emphasis that the proposed framework is very general and consequently, it can also be applied to other materials. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijplas.2011.05.011
  • 2012 • 57 Non-crystallographic shear banding in crystal plasticity FEM simulations: Example of texture evolution in α-brass
    Jia, N. and Roters, F. and Eisenlohr, P. and Kords, C. and Raabe, D.
    Acta Materialia 60 1099-1115 (2012)
    We present crystal plasticity finite element simulations of the texture evolution in α-brass polycrystals under plane strain compression. The novelty is a non-crystallographic shear band mechanism [Anand L, Su C. J Mech Phys Solids 2005;53:1362] that is incorporated into the constitutive model in addition to dislocation and twinning. Non-crystallographic deformation associated with shear banding leads to weaker copper and S texture components and to a stronger brass texture compared to simulations enabling slip and twinning only. The lattice rotation rates are reduced when shear banding occurs. This effect leads to a weaker copper component. Also, the initiation of shear banding promotes brass-type components. In summary the occurrence of non-crystallographic deformation through shear bands shifts face-centered-cubic deformation textures from the copper type to the brass type. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.10.047
  • 2012 • 56 Expected and unexpected plastic behavior at the micron scale: An in situ μlaue tensile study
    Kirchlechner, C. and Imrich, P.J. and Grosinger, W. and Kapp, M.W. and Keckes, J. and Micha, J.S. and Ulrich, O. and Thomas, O. and Labat, S. and Motz, C. and Dehm, G.
    Acta Materialia 60 1252-1258 (2012)
    The study of mechanical properties in micron- and submicron-sized metal crystals raises fundamental questions about the influence of size on different aspects of plasticity. In situ characterization of the microstructure evolution during loading is necessary to understand the physics underlying crystal deformation. In situ μLaue diffraction is able to provide unique statistical information on the evolution of type and density of stored dislocations. Here we show macroscopically expected and unexpected plastic behavior at low strains, observed during in situ μLaue tensile tests on micron-sized, single slip oriented Cu samples. Regardless of the initial behavior, a steady state is reached which qualifies a technical yield criterion at the micron scale. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.10.058
  • 2012 • 55 The multiscale approach to the curing of polymers incorporating viscous and shrinkage effects
    Klinge, S. and Bartels, A. and Steinmann, P.
    International Journal of Solids and Structures 49 3883-3900 (2012)
    This contribution deals with the modeling of viscoelastic and shrinkage effects accompanying the curing of polymers at multiple length scales. For the modeling of viscous effects, the deformation at the microlevel is decomposed into an elastic and a viscoelastic part, and a corresponding energy density consisting of equilibrium and non-equilibrium parts is proposed. The former is related to the total deformation; it has the form of a convolution integral and depends on the time evolution of the material parameters. The non-equilibrium part depends on the elastic part of deformations only. The material parameters are constant in time, thus an integral form is not necessary. In contrast to the viscous effects, the modeling of shrinkage effects does not require any further extension of the expression for the energy density, but an additional decomposition of the deformation into a shrinkage and a mechanical part. Since the material compressibility is taken into consideration, a multifield formulation is applied for the equilibrium as well as for the non-equilibrium energy part. As a final aspect, the paper includes a study of macroheterogenous polymers for whose modeling the multiscale finite element method is applied. Within this numerical approach, a macroscopic body is treated as a homogeneous body whose effective properties are evaluated on the basis of the simulations which are carried out at the level of the representative volume element. The application of the model proposed is illustrated on the basis of examples studying the influence of individual parameters on the stress state as well as the influence of the volume fraction of different phases at the microscale on the effective material behavior. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijsolstr.2012.08.016
  • 2012 • 54 A comparison of different experimental methods for investigating the mechanical properties of plane polysiloxane membranes and capsule walls
    Koleva, I. and Rehage, H.
    Soft Matter 8 7672-7682 (2012)
    In this publication we systematically compared the surface rheological properties of different polysiloxane capsule membranes with planar cross-linked films of the same chemical composition. We performed interfacial rheological measurements and different experiments of the capsule deformation in shear and centrifugal fields. In a series of experiments we determined the surface shear and Young's moduli of the polysiloxane membranes. These investigations allowed us to calculate the two-dimensional Poisson ratio of the viscoelastic polysiloxane shells. We also compared the yield values of the capsule deformation Dmax and the maximal shear strain γmax, which describe the limits of the linear viscoelastic response. In the regime of large centrifugal forces, we observed a new capsule bursting mechanism. For small deformations, where the membranes showed linear-viscoelastic properties we found a very good agreement between measurements and theoretical predictions. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2sm25720c
  • 2012 • 53 Mechanisms of crazing in glassy polymers revealed by molecular dynamics simulations
    Mahajan, D.K. and Hartmaier, A.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 86 (2012)
    Mechanisms leading to initiation of crazing type failure in a glassy polymer are not clearly understood. This is mainly due to the difficulty in characterizing the stress state and polymer configuration sufficiently locally at the craze initiation site. Using molecular dynamics simulations, we have now been able to access this information and have shown that the local heterogeneous deformation leads to craze initiation in glassy polymers. We found that zones of high plastic activity are constrained by their neighborhood and become unstable, initiating crazing from these sites. Furthermore, based on the constant flow stresses observed in the unstable zones, we conclude that microcavitation is the essential local deformation mode to trigger crazing in glassy polymers. Our results demonstrate the basic difference in the local deformation mode as well as the conditions that lead to either shear-yielding or crazing type failures in glassy polymers. We anticipate our paper to help in devising a new criterion for craze initiation that not only considers the stress state, but also considers local deformation heterogeneities that form the necessary condition for crazing in glassy polymers. © 2012 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.86.021802
  • 2012 • 52 High quality extrudates from aluminum chips by new billet compaction and deformation routes
    Misiolek, W.Z. and Haase, M. and Ben Khalifa, N. and Tekkaya, A.E. and Kleiner, M.
    CIRP Annals - Manufacturing Technology 61 239-242 (2012)
    The effects of different billet preparation techniques as well as selection of various deformation routes and their influence on the final mechanical properties in chip extrusion was studied. The AA6060 chips were compacted into billets using various techniques and then extruded through the flat-face, porthole and ECAP dies to create different deformation routes. The microstructures and the mechanical properties of the chip extruded profiles were compared to cast billets extruded through the flat-face die under the same conditions. The proposed technology shows very promising results in terms of energy savings and production of the high quality engineered aluminum profiles. © 2012 CIRP.
    view abstractdoi: 10.1016/j.cirp.2012.03.113
  • 2012 • 51 Variational constitutive updates for microstructure evolution in hcp metals
    Mosler, J. and Homayonifar, M.
    GAMM Mitteilungen 35 43-58 (2012)
    Magnesium and its alloys are promising materials for lightweight applications. Unfortunately, the macroscopic formability of such materials is relatively poor at room temperature and these metals are characterized by a complex mechanical response. This response is a result of the interplay between different deformation modes at the microscale. Since magnesium is a material showing a hexagonal close-packed (hcp) structure of the underlying atomic lattice, plasticity caused by dislocations and deformation-induced twinning are the most relevant deformation modes. Within the present paper, two different recently advocated modeling approaches suitable for capturing such modes at the microscale are analyzed. It is shown that both models can be rewritten into a variationally consistent format where every aspect is naturally driven by energy minimization. In addition to this already known feature, it turns out that both models are based on the same minimization problem. The difference between the models results from different constraints enforced within the variational principle. For getting further insight into the interaction between dislocations and twinning interfaces, accompanying atomistic simulations based on molecular dynamics are also performed. The results of such simulations enter the micromechanical model through the initial plastic deformation within the twinned phase. ©c 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/gamm.201210004
  • 2012 • 50 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 • 49 Finite element-fictitious boundary methods (FEM-FBM) for 3D particulate flow
    Münster, R. and Mierka, O. and Turek, S.
    International Journal for Numerical Methods in Fluids 69 294-313 (2012)
    In this paper we discuss numerical simulation techniques using a finite element approach in combination with the fictitious boundary method (FBM) for rigid particulate flow configurations in 3D. The flow is computed with a multigrid finite element solver (FEATFLOW), the solid particles are allowed to move freely through the computational mesh which can be static or adaptively aligned by a grid deformation method allowing structured as well as unstructured meshes. We explain the details of how we can use the FBM to simulate flows with complex geometries that are hard to describe analytically. Stationary and time-dependent numerical examples, demonstrating the use of such geometries are provided. Our numerical results include well-known benchmark configurations showing that the method can accurately and efficiently handle prototypical particulate flow situations in 3D with particles of different shape and size. © 2011 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/fld.2558
  • 2012 • 48 Orientation dependence of stress-induced phase transformation and dislocation plasticity in NiTi shape memory alloys on the micro scale
    Pfetzing-Micklich, J. and Ghisleni, R. and Simon, T. and Somsen, C. and Michler, J. and Eggeler, G.
    Materials Science and Engineering A 538 265-271 (2012)
    NiTi shape memory alloys can be used as micro actuators and small scale pseudoelastic components. Therefore there is a need to characterize their mechanical properties on the micro scale. In several previous studies, such tests (nanoindentation, pillar compression) were performed for different NiTi alloys. However, no consistent results concerning the coupling between plastic deformation and martensitic transformation were obtained. Moreover it is unclear whether the material's response to loading on the micro scale reflects its large scale mechanical anisotropy. In this study, we investigate a binary, solution annealed precipitate free NiTi alloy and compress small pillars in <0. 0. 1>-, <1. 0. 1>- and <1. 1. 1>-directions. Mechanical results are analyzed in the light of SEM and post-mortem TEM investigations. We identify deformation mechanisms and show that there is deformation anisotropy. We show that micro pillar testing yields results which are in good qualitative agreement with previous work from macroscopic investigations. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2012.01.042
  • 2012 • 47 Tensile deformation characteristics of bulk ultrafine-grained austenitic stainless steel produced by thermal cycling
    Ravi Kumar, B. and Raabe, D.
    Scripta Materialia 66 634-637 (2012)
    Deformation microstructures of bulk ultrafine-grained austenitic AISI 304L stainless steels were analyzed by electron backscatter diffraction. Samples with grain sizes below 500 nm showed transition from grain-scale deformation to the collective phenomenon of shear banding. This was assisted by strain-induced grain rotation and coalescence. This phenomenon was suppressed in samples with a bimodal grain size distribution (maxima at ∼650 and ∼1400 nm) due to deformation-induced martensite formation, yielding high tensile strength and ductility (1348 MPa ultimate tensile strength at 0.36 max. true strain). © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2012.01.052
  • 2012 • 46 DAMASK: The Düsseldorf advanced material simulation kit for studying crystal plasticity using an fe based or a spectral numerical solver
    Roters, F. and Eisenlohr, P. and Kords, C. and Tjahjanto, D.D. and Diehl, M. and Raabe, D.
    Procedia IUTAM 3 3-10 (2012)
    The solution of a continuum mechanical boundary value problem requires a constitutive response that connects deformation and stress at each material point. Such connection can be regarded as three separate hierarchical problems. At the top-most level, partitioning of the (mean) boundary values of the material point among its microstructural constituents and the associated homogenization of their response is required, provided there is more than one constituent present. Second, based on an elastoplastic decomposition of (finite strain) deformation, these responses follow from explicit or implicit time integration of the plastic deformation rate per constituent. Third, to establish the latter, a state variable-based constitutive law needs to be interrogated and its state updated. The D̈üsseldorf Advanced MAterial Simulation Kit (DAMASK) reflects this hierarchy as it is built in a strictly modular way. This modular structure makes it easy to add additional constitutive models as well as homogenization schemes. Moreover it interfaces with a number of FE solvers as well as a spectral solver using an FFT. We demonstrate the versatility of such a modular framework by considering three scenarios: Selective refinement of the constitutive material description within a single geometry, component-scale forming simulations comparing different homogenization schemes, and comparison of representative volume element simulations based on the FEM and the spectral solver. © 2012 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.piutam.2012.03.001
  • 2012 • 45 Extrusion benchmark 2011: Evaluation of different design strategies on process conditions, die deflection and seam weld quality in hollow profiles
    Selvaggio, A. and Segatori, A. and Guzel, A. and Donati, L. and Tomesani, L. and Tekkaya, A.E.
    Key Engineering Materials 491 1-10 (2012)
    In the paper experimental investigations aimed at allowing a detailed and accurate comparison of different FEM codes were presented and discussed. Two hollow profiles within the same die were characterized by different thicknesses within the profile, two welding chambers and critical tongues (one fully supported and one partially supported). The material flow balance was performed by means of feeder size and position on a profile and by means of bearings on the other one. Accurate monitoring of process parameters was carried out by using a self-calibrating pyrometer for profile temperature, six thermocouples for die thermal monitoring, a laser velocitymeter for profile speed and two laser sensors for die deflection on critical tongues. AA6082 alloy was used as deforming material, while H-11 hot-work tool steel was selected for the die material. The experiments were repeated at least three times under the same conditions in order to provide a nearly steady state statistical distribution of the acquired data. These are used as a reference for the 2011 edition of the extrusion benchmark. © (2012) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/KEM.491.1
  • 2012 • 44 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
  • 2011 • 43 An efficient algorithm for the inverse problem in elasticity imaging by means of variational r-adaption
    Arnold, A. and Bruhns, O.T. and Mosler, J.
    Physics in Medicine and Biology 56 4239-4265 (2011)
    A novel finite element formulation suitable for computing efficiently the stiffness distribution in soft biological tissue is presented in this paper. For that purpose, the inverse problem of finite strain hyperelasticity is considered and solved iteratively. In line with Arnold et al (2010 Phys. Med. Biol. 55 2035), the computing time is effectively reduced by using adaptive finite element methods. In sharp contrast to previous approaches, the novel mesh adaption relies on an r-adaption (re-allocation of the nodes within the finite element triangulation). This method allows the detection of material interfaces between healthy and diseased tissue in a very effective manner. The evolution of the nodal positions is canonically driven by the same minimization principle characterizing the inverse problem of hyperelasticity. Consequently, the proposed mesh adaption is variationally consistent. Furthermore, it guarantees that the quality of the numerical solution is improved. Since the proposed r-adaption requires only a relatively coarse triangulation for detecting material interfaces, the underlying finite element spaces are usually not rich enough for predicting the deformation field sufficiently accurately (the forward problem). For this reason, the novel variational r-refinement is combined with the variational h-adaption (Arnold et al 2010) to obtain a variational hr-refinement algorithm. The resulting approach captures material interfaces well (by using r-adaption) and predicts a deformation field in good agreement with that observed experimentally (by using h-adaption). © 2011 Institute of Physics and Engineering in Medicine.
    view abstractdoi: 10.1088/0031-9155/56/14/004
  • 2011 • 42 Deformation resistance in the transition from coarse-grained to ultrafine-grained Cu by severe plastic deformation up to 24 passes of ECAP
    Blum, W. and Li, Y.J. and Zhang, Y. and Wang, J.T.
    Materials Science and Engineering A 528 8621-8627 (2011)
    Pure Cu was subjected to severe plastic predeformation by p=1, 2, 4, 8, 16 and 24 passes of equal channel angular pressing (ECAP) on route BC at ambient temperature and subsequently tested in uniaxial compression parallel to the extrusion direction at constant rate or constant stress and temperatures from ambient temperature up to 418K. The maximum compressive strength of the ECAPed Cu varies in a systematic fashion with p, until a steady state is finally reached between p=8 and 16 where the rate sensitivity of flow stress is maximal. The results are quantitatively interpreted in terms of the boundary structure, considering the superposition of hardening due to refinement of low-angle boundaries and softening due to enhanced thermal recovery at high-angle boundaries. Beyond the maximum the compressive strength declines with strain for relatively low rate and/or elevated temperature of compression. This is explained by dynamic grain coarsening towards the new steady state developing in compression. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.08.010
  • 2011 • 41 Accurate deep drawing simulation by combining analytical approaches
    Cwiekala, T. and Brosius, A. and Tekkaya, A.E.
    International Journal of Mechanical Sciences 53 374-386 (2011)
    The basic contribution of this work is the description of the development of an analytical simulation method for deep drawing processes. By considering multiple deformation steps, this method takes time dependent process parameters and non-linear deformation paths into account. Contrary to existing analytical approaches, this method allows an accurate strain prediction and, thus, a prediction of formability. Compared to numerical onestep solvers, the developed method is much faster, and due to a better consideration of deformation paths, also a higher accuracy is reached in simulating axisymmetric and prismatic parts. Due to its efficient combination of computation speed and accuracy, this method allows an application in fast process optimizations or online process control systems, where existing approaches are either too slow in case of numerical simulation or too inaccurate in case of analytical simulation. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijmecsci.2011.02.007
  • 2011 • 40 The collagen fibril architecture in the lamina cribrosa and peripapillary sclera predicted by a computational remodeling approach
    Grytz, R. and Meschke, G. and Jonas, J.B.
    Biomechanics and Modeling in Mechanobiology 10 371-382 (2011)
    The biomechanics of the optic nerve head is assumed to play an important role in ganglion cell loss in glaucoma. Organized collagen fibrils form complex networks that introduce strong anisotropic and nonlinear attributes into the constitutive response of the peripapillary sclera (PPS) and lamina cribrosa (LC) dominating the biomechanics of the optic nerve head. The recently presented computational remodeling approach (Grytz and Meschke in Biomech Model Mechanobiol 9:225-235, 2010) was used to predict the micro-architecture in the LC and PPS, and to investigate its impact on intraocular pressure-related deformations. The mechanical properties of the LC and PPS were derived from a microstructure-oriented constitutive model that included the stretch-dependent stiffening and the statistically distributed orientations of the collagen fibrils. Biomechanically induced adaptation of the local micro-architecture was captured by allowing collagen fibrils to be reoriented in response to the intraocular pressure-related loading conditions. In agreement with experimental observations, the remodeling algorithm predicted the existence of an annulus of fibrils around the scleral canal in the PPS, and a predominant radial orientation of fibrils in the periphery of the LC. The peripapillary annulus significantly reduced the intraocular pressure-related expansion of the scleral canal and shielded the LC from high tensile stresses. The radial oriented fibrils in the LC periphery reinforced the LC against transversal shear stresses and reduced LC bending deformations. The numerical approach presents a novel and reasonable biomechanical explanation of the spatial orientation of fibrillar collagen in the optic nerve head. © 2010 Springer-Verlag.
    view abstractdoi: 10.1007/s10237-010-0240-8
  • 2011 • 39 Inverse method for identification of initial yield locus of sheet metals utilizing inhomogeneous deformation fields
    Güner, A. and Yin, Q. and Soyarslan, C. and Brosius, A. and Tekkaya, A.E.
    International Journal of Material Forming 4 121-128 (2011)
    Accurate finite element simulation of sheet metal forming processes requires among others accurate description of plastic behaviour of materials. This is achieved by utilization of sophisticated yield criteria having several material parameters. This work proposes a procedure which makes use of the distribution of strains to identify the initial yield locus of sheet metals by the help of inverse analysis. For this purpose a flat specimen having a varying cross-section is introduced, which is capable of revealing different deformation states in one test. Numerical simulations are performed with 2 representative materials for steel and aluminium, using the material model Yld2000-2d. The results of these simulations are treated as experimentally obtained results and with the inverse methods it is tried to obtain the given yield locus. The relation between the supplied input and the outcome of the inverse algorithm is studied by examining different objective function definitions. The numerical studies show that inclusion of the strain distribution in the definition of objective function is a key issue in identification of the yield locus. The orientation of the specimen with respect to the rolling direction also determines the amount and quality of the information used for parameter identification. Consequently the circumstances, under which the inverse method can predict the initial yield locus, are defined. © 2010 Springer-Verlag France.
    view abstractdoi: 10.1007/s12289-010-1009-4
  • 2011 • 38 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 • 37 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 • 36 Design of a novel Mn-based 1 GPa duplex stainless TRIP steel with 60% ductility by a reduction of austenite stability
    Herrera, C. and Ponge, D. and Raabe, D.
    Acta Materialia 59 4653-4664 (2011)
    We report on the microstructure, texture and deformation mechanisms of a novel ductile lean duplex stainless steel (Fe-19.9Cr-0.42Ni-0.16N-4.79Mn-0.11C- 0.46Cu-0.35Si, wt.%). The austenite is stabilized by Mn, C, and N (instead of Ni). The microstructure is characterized by electron channeling contrast imaging (ECCI) for dislocation mapping and electron backscattering diffraction (EBSD) for texture and phase mapping. The material has 1 GPa ultimate tensile strength and an elongation to fracture of above 60%. The mechanical behavior is interpreted in terms of the strength of both the starting phases, austenite and ferrite, and the amount, dispersion, and transformation kinetics of the mechanically induced martensite (TRIP effect). Transformation proceeds from austenite to hexagonal martensite to near cubic martensite (γ → → α′). The -martensite forms in the austenite with an orientation relationship close to Shoji-Nishiyama. The α′-martensite nucleates at the intersections of deformation bands, especially -bands, with Kurdjumov-Sachs and Nishiyama-Wassermann relationships. The ferrite deforms by dislocation slip and contains cell substructures. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.04.011
  • 2011 • 35 On the coupling of plastic slip and deformation-induced twinning in magnesium: A variationally consistent approach based on energy minimization
    Homayonifar, M. and Mosler, J.
    International Journal of Plasticity 27 983-1003 (2011)
    The present paper is concerned with the analysis of the deformation systems in single crystal magnesium at the micro-scale and with the resulting texture evolution in a polycrystal representing the macroscopic mechanical response. For that purpose, a variationally consistent approach based on energy minimization is proposed. It is suitable for the modeling of crystal plasticity at finite strains including the phase transition associated with deformation-induced twinning. The method relies strongly on the variational structure of crystal plasticity theory, i.e.; an incremental minimization principle can be derived which allows to determine the unknown slip rates by computing the stationarity conditions of a (pseudo) potential. Phase transition associated with twinning is modeled in a similar fashion. More precisely, a solid-solid phase transition corresponding to twinning is assumed, if this is energetically favorable. Mathematically speaking, the aforementioned transition can be interpreted as a certain rank-one convexification. Since such a scheme is computationally very expensive and thus, it cannot be applied to the analysis of a polycrystal, a computationally more efficient approximation is elaborated. Within this approximation, the deformation induced by twinning is decomposed into the reorientation of the crystal lattice and simple shear. The latter is assumed to be governed by means of a standard Schmid-type plasticity law (pseudo-dislocation), while the reorientation of the crystal lattice is considered, when the respective plastic shear strain reaches a certain threshold value. The underlying idea is in line with experimental observations, where dislocation slip within the twinned domain is most frequently seen, if the twin laminate reaches a critical volume. The resulting model predicts a stress-strain response in good agreement with that of a rank-one convexification method, while showing the same numerical efficiency as a classical Taylor-type approximation. Consequently, it combines the advantages of both limiting cases. The model is calibrated for single crystal magnesium by means of the channel die test and finally applied to the analysis of texture evolution in a polycrystal. Comparisons of the predicted numerical results to their experimental counterparts show that the novel model is able to capture the characteristic mechanical response of magnesium very well. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijplas.2010.10.009
  • 2011 • 34 Dynamic plumbing system beneath volcanoes revealed by kinetic modeling, and the connection to monitoring data: An example from Mt. Etna
    Kahl, M. and Chakraborty, S. and Costa, F. and Pompilio, M.
    Earth and Planetary Science Letters 308 11-22 (2011)
    Our ability to monitor volcanoes (using seismic signals, ground deformation, gas fluxes, or other ground and satellite based observations) as well as our understanding of melt reservoirs that feed eruptions have evolved tremendously in recent years. The complex plumbing systems that are thought to feed eruptions are, however, difficult to relate to the monitoring signals. Here we show that the record preserved in compositional zoning of erupted minerals may be used to reconstruct sections of the plumbing system. Kinetic modeling of such zoning can yield information on the residence time of magma in different segments of the plumbing systems. This allows a more nuanced evaluation of the link between observed monitoring signals or eruption styles and the magmatic processes and movement of batches of melts at depth. The approach is illustrated through a study of the compositional zoning recorded in olivine crystals from the 1991-1993 SE-flank eruption products of Mt. Etna (Sicily). The zoning patterns in crystals reveal that the plumbing system of the volcano consisted of at least three different magmatic environments between which magma was transported and mixed in the year or two preceding the start of eruption. Quantification of this history indicates that two main pathways of melt migration and three timescales dominated the dynamics of the system. Combination of this information with the timing of observation of various monitoring signals allows a reconstruction of the dynamic evolution of this section of the plumbing system during the early stages of the 1991-1993 eruption. It is seen, for example, how the migration of melt through the same sections of the plumbing system can cause pre-eruptive triggering, enhance Strombolian activity, and through the ensuing eruption cleanse and flush the plumbing system. Different kinds of mixing occur simultaneously at different sections of the plumbing system on different timescales (a few days up to two years). © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.epsl.2011.05.008
  • 2011 • 33 In situ μlaue: Instrumental setup for the deformation of micron sized samples
    Kirchlechner, C. and Keckes, J. and Micha, J.-S. and Dehm, G.
    Advanced Engineering Materials 13 837-844 (2011)
    μLaue diffraction sheds light onto the deformation behavior of miniaturized samples. Here we present a new instrumental setup for the in situ deformation of micron sized specimens at BM32 of the ESRF synchrotron source. Furthermore, a compression test of a 7 μm sized single slip oriented copper pillar is presented, showing the activation of an unpredicted slip system due to misalignment and the formation of several sub-grains. The results of the compressed pillar as well as possibilities and crucial points for measuring and data evaluation are discussed. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201000286
  • 2011 • 32 Dislocation storage in single slip-oriented Cu micro-tensile samples: New insights via X-ray microdiffraction
    Kirchlechner, C. and Kiener, D. and Motz, C. and Labat, S. and Vaxelaire, N. and Perroud, O. and Micha, J.-S. and Ulrich, O. and Thomas, O. and Dehm, G. and Keckes, J.
    Philosophical Magazine 91 1256-1264 (2011)
    Synchrotron X-ray microdiffraction was used to characterize the deformation structure of single crystalline Cu micro-tensile specimens which were oriented for single slip. The 3-m thick samples were strained in situ in a scanning electron microscope (SEM). Electron microscopy observations revealed glide steps at the surface indicating single slip. While the slip steps at the surface must have formed by the predominant activation of the primary glide system, analysis of Laue peak streaking directions revealed that, even at low strains, dislocations had been activated and stored on an unpredicted slip system. Furthermore, the Laue scans showed that multiple slip takes over at a later state of deformation. © 2011 Taylor & Francis.
    view abstractdoi: 10.1080/14786431003785639
  • 2011 • 31 The evolution of laminates in finite crystal plasticity: A variational approach
    Kochmann, D.M. and Hackl, K.
    Continuum Mechanics and Thermodynamics 23 63-85 (2011)
    The analysis and simulation of microstructures in solids has gained crucial importance, virtue of the influence of all microstructural characteristics on a material's macroscopic, mechanical behavior. In particular, the arrangement of dislocations and other lattice defects to particular structures and patterns on the microscale as well as the resultant inhomogeneous distribution of localized strain results in a highly altered stress-strain response. Energetic models predicting the mechanical properties are commonly based on thermodynamic variational principles. Modeling the material response in finite strain crystal plasticity very often results in a non-convex variational problem so that the minimizing deformation fields are no longer continuous but exhibit small-scale fluctuations related to probability distributions of deformation gradients to be calculated via energy relaxation. This results in fine structures that can be interpreted as the observed microstructures. In this paper, we first review the underlying variational principles for inelastic materials. We then propose an analytical partial relaxation of a Neo-Hookean energy formulation, based on the assumption of a first-order laminate microstructure, thus approximating the relaxed energy by an upper bound of the rank-one-convex hull. The semi-relaxed energy can be employed to investigate elasto-plastic models with a single as well as multiple active slip systems. Based on the minimization of a Lagrange functional (consisting of the sum of energy rate and dissipation potential), we outline an incremental strategy to model the time-continuous evolution of the laminate microstructure, then present a numerical scheme by means of which the microstructure development can be computed, and show numerical results for particular examples in single- and double-slip plasticity. We discuss the influence of hardening and of slip system orientations in the present model. In contrast to many approaches before, we do not minimize a condensed energy functional. Instead, we incrementally solve the evolution equations at each time step and account for the actual microstructural changes during each time step. Results indicate a reduction in energy when compared to those theories based on a condensed energy functional. © 2010 Springer-Verlag.
    view abstractdoi: 10.1007/s00161-010-0174-5
  • 2011 • 30 Efficient and accurate simulations of deformable particles immersed in a fluid using a combined immersed boundary lattice Boltzmann finite element method
    Krüger, T. and Varnik, F. and Raabe, D.
    Computers and Mathematics with Applications 61 3485-3505 (2011)
    The deformation of an initially spherical capsule, freely suspended in simple shear flow, can be computed analytically in the limit of small deformations [D. Barths-Biesel, J.M. Rallison, The time-dependent deformation of a capsule freely suspended in a linear shear flow, J. Fluid Mech. 113 (1981) 251267]. Those analytic approximations are used to study the influence of the mesh tessellation method, the spatial resolution, and the discrete delta function of the immersed boundary method on the numerical results obtained by a coupled immersed boundary lattice Boltzmann finite element method. For the description of the capsule membrane, a finite element method and the Skalak constitutive model [R. Skalak, A. Tozeren, R.P. Zarda, S. Chien, Strain energy function of red blood cell membranes, Biophys. J. 13 (1973) 245264] have been employed. Our primary goal is the investigation of the presented model for small resolutions to provide a sound basis for efficient but accurate simulations of multiple deformable particles immersed in a fluid. We come to the conclusion that details of the membrane mesh, as tessellation method and resolution, play only a minor role. The hydrodynamic resolution, i.e., the width of the discrete delta function, can significantly influence the accuracy of the simulations. The discretization of the delta function introduces an artificial length scale, which effectively changes the radius and the deformability of the capsule. We discuss possibilities of reducing the computing time of simulations of deformable objects immersed in a fluid while maintaining high accuracy. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.camwa.2010.03.057
  • 2011 • 29 Alginate/poly-l-lysine capsules: Mechanical properties and drug release characteristics
    Leick, S. and Kemper, A. and Rehage, H.
    Soft Matter 7 6684-6694 (2011)
    In this paper we studied the mechanical stability and the release kinetics of different types of liquid-filled calcium alginate/poly-l-lysine capsules. The aqueous cores of these particles were filled with anthocyanins which have antioxidant abilities and may, as additives in foods, provide several benefits of health. By means of spinning capsule experiments it was possible to measure the deformation of the gel particles at various centrifugal forces. These investigations provided insight into the elastic properties of the capsule membranes. In a series of experiments we measured the capsule deformation as a function of the poly-l-lysine concentration and the adsorption time. From these data we calculated the surface Young moduli. In addition, the use of a pH-dependent UV/VIS-absorption spectroscopy method gained access to the diffusive drug-release performance of the encapsulated anthocyanins. From these kinetic measurements we could evaluate the effective diffusion constants of the encapsulated compounds. The performed experiments showed that the mechanical properties of liquid-filled alginate capsules could be changed and adjusted selectively by the addition of poly-l-lysine. The drug release properties, however, did not change significantly for different compositions of the multi-component capsules. In addition, it could be shown that a high amount of anthocyanin molecules was immobilized in the capsules. This phenomenon could be explained by adsorption or polymerization processes of the colored ingredients. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1sm05676j
  • 2011 • 28 Atomic-scale mechanisms of deformation-induced cementite decomposition in pearlite
    Li, Y.J. and Choi, P. and Borchers, C. and Westerkamp, S. and Goto, S. and Raabe, D. and Kirchheim, R.
    Acta Materialia 59 3965-3977 (2011)
    Pearlitic steel can exhibit tensile strengths higher than 5 GPa after severe plastic deformation, where the deformation promotes a refinement of the lamellar structure and cementite decomposition. However, a convincing correlation between deformation and cementite decomposition in pearlite is still absent. In the present work, a local electrode atom probe was used to characterize the microstructural evolution of pearlitic steel, cold-drawn with progressive strains up to 5.4. Transmission electron microscopy was also employed to perform complementary analyses of the microstructure. Both methods yielded consistent results. The overall carbon content in the detected volumes as well as the carbon concentrations in ferrite and cementite were measured by atom probe. In addition, the thickness of the cementite filaments was determined. In ferrite, we found a correlation of carbon concentration with the strain, and in cementite, we found a correlation of carbon concentration with the lamella thickness. Direct evidence for the formation of cell/subgrain boundaries in ferrite and segregation of carbon atoms at these defects was found. Based on these findings, the mechanisms of cementite decomposition are discussed in terms of carbon-dislocation interaction. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.022
  • 2011 • 27 Influence of dislocation density on the pop-in behavior and indentation size effect in CaF2 single crystals: Experiments and molecular dynamics simulations
    Lodes, M.A. and Hartmaier, A. and Göken, M. and Durst, K.
    Acta Materialia 59 4264-4273 (2011)
    In this work, the indentation size effect and pop-in behavior are studied for indentations in undeformed and locally pre-deformed CaF2 single crystals, using both nanoindentation experiments and molecular dynamics simulations. To study the influence of dislocation density on the indentation behavior, small-scale indentations are carried out inside the plastic zone of larger indentations. This experiment is mimicked in the simulations by indenting a small sphere into the center of the residual impression of a larger sphere. The undeformed material shows the well-known pop-in behavior followed by the indentation size effect. Pre-deforming the material leads to a reduction in the indentation size effect both for experiments and simulations, which is in accordance with the Nix-Gao theory. Furthermore, the pop-in load is reduced in the experiments, whereas a smooth transition from elastic to plastic deformation is found in the simulations. There, plasticity is initiated by the movement of pre-existing dislocation loops in the vicinity of the plastic zone. The simulations thus give a detailed insight into the deformation mechanism during indentation and highlight the importance of the dislocation microstructure for the indentation size effect and dislocation nucleation. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.050
  • 2011 • 26 Efficient modeling of localized material failure by means of a variationally consistent embedded strong discontinuity approach
    Mosler, J. and Stanković, L. and Radulović, R.
    International Journal for Numerical Methods in Engineering 88 1008-1041 (2011)
    This paper is concerned with a novel embedded strong discontinuity approach suitable for the analysis of material failure at finite strains. Focus is on localized plastic deformation particularly relevant for slip bands. In contrast to already existing models, the proposed implementation allows to consider several interacting discontinuities in each finite element. Based on a proper re-formulation of the kinematics, an efficient parameterization of the deformation gradient is derived. It permits to compute the strains explicitly that improves the performance significantly. However, the most important novel contribution of the present paper is the advocated variational constitutive update. Within this framework, every aspect is naturally driven by energy minimization, i.e. all unknown variables are jointly computed by minimizing the stress power. The proposed update relies strongly on an extended principle of maximum dissipation. This framework provides enough flexibility for different failure types and for a broad class of non-associative evolution equations. By discretizing the aforementioned continuous variational principle, an efficient numerical implementation is obtained. It shows, in addition to its physical and mathematical elegance, several practical advantages. For instance, the physical minimization principle itself specifies automatically and naturally the set of active strong discontinuities. © 2011 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/nme.3210
  • 2011 • 25 A micropolar continuum model for large deformation caused by magnetic or electric fields
    Münch, I. and Neff, P. and Wagner, W.
    Proceedings of SPIE - The International Society for Optical Engineering 7978 (2011)
    An appropriate continuum theory to predict the behavior of flexible magnetic or electrically polarized materials undergoing large deformations is explained. The formulation treats the angular momentum as an explicit complementary principle including net-couples from magnetic resp. electric fields. As a consequence non-symmetric Cauchy stresses are mandatory for equilibrium, which is unlike in classical theories. However, the micropolar model is in accordance with classical phenomenological modeling parameters but with the feature to cover large deformations and non-classical types of loading. The formulation considers rotational degrees of freedom to appear in the kinematical equations as exact rotations in SO(3). This is a source of nonlinearity in the model but allows easily for large deformation as well as for net-couples. A simple example is the torque of a compass needle to explain the effect of materials with remanent magnetization within a magnetic field. The twisting moment becomes a maximum for remanent magnetization being perpendicular to an outer magnetic field. It vanishes if both fields are parallel. We investigate magnetic structures using finite element simulations. The development of active materials on the micro-level is in the focus. © 2011 SPIE.
    view abstractdoi: 10.1117/12.880568
  • 2011 • 24 Transversely isotropic material: Nonlinear Cosserat versus classical approach
    Münch, I. and Neff, P. and Wagner, W.
    Continuum Mechanics and Thermodynamics 23 27-34 (2011)
    We consider a specific case of unidirectional reinforced material under applied tensile load. The reinforcement of the material is inclined with 45° to the direction of the tensile resultant. Different approaches are discussed: one experiment and three computational models. Two models use the classical Cauchy continuum theory whereas the third computational model is based on a Cosserat continuum. It is well known that test specimen with inclination between unidirectional reinforcement and tensile direction show, besides Poissons effect, additional deformation perpendicular to the load direction. The classical transversely isotropic continuum theory predicts this deformation as typical S-shape. In the Cosserat continuum the orientation of the inner structure is incorporated. Thus, structural parameters influence the deformation. With the proposed geometrically non-linear Cosserat model classical and non-classical behaviour can be modelled. In the non-classical case, the transverse deformation is not described by one S-shape but by multiple S-shaped modes. The additional rotational parameters in the Cosserat continuum are responsible for the non-classical behaviour which is due to non-symmetric strain. © 2010 Springer-Verlag.
    view abstractdoi: 10.1007/s00161-010-0150-0
  • 2011 • 23 Grout and bentonite flow around a TBM: Computational modeling and simulation-based assessment of influence on surface settlements
    Nagel, F. and Meschke, G.
    Tunnelling and Underground Space Technology 26 445-452 (2011)
    Adequate consideration of the various interactions between the Tunnel Boring Machine (TBM) and the surrounding underground is a pre-requisite for reliable prognoses in shield supported tunneling based upon numerical analysis. In addition to face support and the grouting of the annular gap the contact conditions along the shield skin between the moving TBM and the surrounding, deforming soil constitute the most relevant component of TBM-soil interactions in mechanized tunneling. This paper is concerned with the analysis of the interface conditions between the shield skin and the soil and its adequate numerical representation in the context of a process-oriented numerical simulation model for mechanized tunneling. The situation around the shield skin is influenced by the design of the Tunnel Boring Machine, the deformational behavior of the surrounding underground and by a possible inflow of process liquids into the steering gap. A novel simulation method is proposed which allows to model the viscous flow of the process liquids into the steering gap and its interactions with the face support, the tail void grouting, the deforming soil and the moving TBM. The proposed numerical model for the TBM-soil interaction is part of a recently developed three-dimensional, process-oriented finite element model for shield tunneling (Nagel et al., 2010). It allows to investigate the effects of the inflow of process liquids into the steering gap during TBM advance considering realistic machine-related and geological conditions. It is, in particular, capable to compute the pressure distribution within the developing liquid film in association with the face support and grouting conditions and to predict its influence on the surface settlements and the overall TBM-soil interaction affecting, e.g. the hydraulic jack forces or shield deformations. © 2010 Elsevier Ltd.
    view abstractdoi: 10.1016/j.tust.2010.12.001
  • 2011 • 22 Strain-induced effects on the electronic structure and N K-edge ELNES of wurtzite A1N and AlxGa1-xN
    Petrov, M. and Holec, D. and Lymperakis, L. and Neugebauer, J. and Humphreys, C.J.
    Journal of Physics: Conference Series 326 (2011)
    Analysis of the electron energy loss near edge structure (ELNES) provides an experimental tool to probe the density of unoccupied states. Here we present a first principles study on the projected density of states (PDOS) of AlN, GaN, and AlxGa1-xN alloys in order to investigate the impact of strain on the N K-edge ELNES. Uni-axial and bi-axial strain, volume conserving, and bi-axial stress deformation modes are calculated for the whole compositional range from AlN to GaN. Our results show that only the strain along the c-axis has a pronounced impact on the PDOS. Furthermore, we find that bi-axial stress in the basal plane, which is present in pseudomorphic polar heteroepitaxial layers, does not significantly influence the N K-edge spectra. However, strain-induced changes may appear for different deformation modes and/or specimen geometries.
    view abstractdoi: 10.1088/1742-6596/326/1/012016
  • 2011 • 21 Micro-shear deformation of pure copper
    Pfetzing-Micklich, J. and Brinckmann, S. and Dey, S.R. and Otto, F. and Hartmaier, A. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 42 219-223 (2011)
    In this paper a new micro-shear experiment is introduced using a double shear specimen machined by a focused ion beam technique. The micro-shear specimen is structured from pure copper promoting (111) [101] slip. Comparing scanning electron microscopy images before and after deformation provides evidence for localized shear. Load-displacement data identify a load plateau and characterize the localized shear process (critical shear-stress for activation of (111) [101] slip: 170 MPa). Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/mawe.201100715
  • 2011 • 20 Deformation mechanisms in micron-sized PST TiAl compression samples: Experiment and model
    Rester, M. and Fischer, F.D. and Kirchlechner, C. and Schmoelzer, T. and Clemens, H. and Dehm, G.
    Acta Materialia 59 3410-3421 (2011)
    Titanium aluminides are the most promising intermetallics for use in aerospace and automotive applications. Consequently, it is of fundamental interest to explore the deformation mechanisms occurring in this class of materials. One model material which is extensively used for such studies are polysynthetically twinned (PST) TiAl crystals, which consist predominantly of parallel γ-TiAl and, fewer, α2-Ti3Al lamellae. In the present study, PST TiAl crystals with a nominal composition of Ti-50 at.% Al were machined by means of the focused ion beam (FIB) technique into miniaturized compression samples with a square cross-section of approximately 9 μm × 9 μm. Compression tests on the miniaturized samples were performed in situ inside a scanning electron microscope using a microindenter equipped with a diamond flat punch. After deformation, thin foils were cut from the micro-compression samples and thinned to electron transparency using a FIB machine in order to study the deformation structure by transmission electron microscopy (TEM). The TEM studies reveal mechanical twinning as the main deformation mechanism at strains of 5.4%, while at strains of 8.3% dislocation glide becomes increasingly important. The experimentally observed twins scale in size with the width of the γ-TiAl lamella. A kinematic and thermodynamic model is developed to describe the twin-related length change of the micro-compression sample at small strains as well as the relationship of an increase of twin width with increasing γ-TiAl lamella thickness. The developed twin model predicts a width of the twins in the range of a few nanometers, which is in agreement with experimental findings. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.02.016
  • 2011 • 19 Non-rigid isometric ICP: A practical registration method for the analysis and compensation of form errors in production engineering
    Sacharow, A. and Balzer, J. and Biermann, D. and Surmann, T.
    CAD Computer Aided Design 43 1758-1768 (2011)
    The unprecedented success of the iterative closest point (ICP) method for registration in geometry processing and related fields can be attributed to its efficiency, robustness, and wide spectrum of applications. Its use is however quite limited as soon as the objects to be registered arise from each other by a transformation significantly different from a Euclidean motion. We present a novel variant of ICP, tailored for the specific needs of production engineering, which registers a triangle mesh with a second surface model of arbitrary digital representation. Our method inherits most of ICP's practical advantages but is capable of detecting medium-strength bendings i.e. isometric deformations. Initially, the algorithm assigns to all vertices in the source their closest point on the target mesh and then iteratively establishes isometry, a process which, very similar to ICP, requires intermediate re-projections. A NURBS-based technique for applying the resulting deformation to arbitrary instances of the source geometry, other than the very mesh used for correspondence estimation, is described before we present numerical results on synthetic and real data to underline the viability of our approach in comparison with others. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.cad.2011.07.007
  • 2011 • 18 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 • 17 Wetting morphologies and their transitions in grooved substrates
    Seemann, R. and Brinkmann, M. and Herminghaus, S. and Khare, K. and Law, B.M. and McBride, S. and Kostourou, K. and Gurevich, E. and Bommer, S. and Herrmann, C. and Michler, D.
    Journal of Physics Condensed Matter 23 (2011)
    When exposed to a partially wetting liquid, many natural and artificial surfaces equipped with complex topographies display a rich variety of liquid interfacial morphologies. In the present article, we focus on a few simple paradigmatic surface topographies and elaborate on the statics and dynamics of the resulting wetting morphologies. It is demonstrated that the spectrum of wetting morphologies increases with increasing complexity of the groove structure. On elastically deformable substrates, additional structures in the liquid morphologies can be observed, which are caused by deformations of the groove geometry in the presence of capillary forces. The emergence of certain liquid morphologies in grooves can be actively controlled by changes in wettability and geometry. For electrically conducting solid substrates, the apparent contact angle can be varied by electrowetting. This allows, depending on groove geometry, a reversible or irreversible transport of liquid along surface grooves. In the case of irreversible liquid transport in triangular grooves, the dynamics of the emerging instability is sensitive to the apparent hydrodynamic slip at the substrate. On elastic substrates, the geometry can be varied in a straightforward manner by stretching or relaxing the sample. The imbibition velocity in deformable grooves is significantly reduced compared to solid grooves, which is a result of the microscopic deformation of the elastic groove material close to the three phase contact line. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/18/184108
  • 2010 • 16 Direct free-form deformation of NC programs for surface reconstruction and form-error compensation
    Biermann, D. and Sacharow, A. and Surmann, T. and Wagner, T.
    Production Engineering 4 501-507 (2010)
    In this paper a new approach for manufacturing modified workpieces by milling is presented. In course of product development, several optimization iterations are often required, in which the shape of the workpiece is modified. Conventional method for manufacturing modified workpieces includes a time-consuming and error-prone step of reverse engineering, where the new CAD/CAM data is generated with respect to the measurement data of the manufactured workpiece. The new approach generates a continuous deformation function in order to approximate the discrete displacement vectors between the initial and the modified shapes, and applies this function on the original NC programs of the workpiece. Hence, it is possible to directly manufacture the modified shape. The process of reverse engineering can be eliminated so that manufacturing costs and the time from workpiece design to the production decrease significantly. © 2010 German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-010-0260-3
  • 2010 • 15 Stimulated deformation of polysiloxane capsules in external electric fieldsa
    Degen, P. and Chen, Z. and Rehage, H.
    Macromolecular Chemistry and Physics 211 434-442 (2010)
    Due to their pronounced viscoelastic properties polymer microcapsules are able to undergo mechanical deformations when they are stimulated by external signals. The aim of our work was to prepare new types of electric switchable microcapsules based on the interfacial polycondensation of octadecyltrichlorosilane (OTS) and to investigate their deformation behavior in externally electric fields. In a series of experiments we explored the influence of different additives on the deformation behavior of the capsules in electric fields as well as on the shear rheological properties of the polysiloxane networks at the planar interface. We could show that the additives had a significant influence on the rheological membrane properties, but we observed only small changes in the deformation of the whole capsules. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/macp.200900452
  • 2010 • 14 Numerical analysis and implementational aspects of a new multilevel grid deformation method
    Grajewski, M. and Köster, M. and Turek, S.
    Applied Numerical Mathematics 60 767-781 (2010)
    Recently, we introduced and mathematically analysed a new method for grid deformation (Grajewski et al., 2009) [15] we call basic deformation method (BDM) here. It generalises the method proposed by Liao et al. (Bochev et al., 1996; Cai et al., 2004; Liao and Anderson, 1992) [4,6,20]. In this article, we employ the BDM as core of a new multilevel deformation method (MDM) which leads to vast improvements regarding robustness, accuracy and speed. We achieve this by splitting up the deformation process in a sequence of easier subproblems and by exploiting grid hierarchy. Being of optimal asymptotic complexity, we experience speed-ups up to a factor of 15 in our test cases compared to the BDM. This gives our MDM the potential for tackling large grids and time-dependent problems, where possibly the grid must be dynamically deformed once per time step according to the user's needs. Moreover, we elaborate on implementational aspects, in particular efficient grid searching, which is a key ingredient of the BDM. © 2010 IMACS.
    view abstractdoi: 10.1016/j.apnum.2010.03.017
  • 2010 • 13 The effect of grain size and grain orientation on deformation twinning in a Fe-22wt.% Mn-0.6wt.% C TWIP steel
    Gutierrez-Urrutia, I. and Zaefferer, S. and Raabe, D.
    Materials Science and Engineering A 527 3552-3560 (2010)
    We investigate the effect of grain size and grain orientation on deformation twinning in a Fe-22wt.% Mn-0.6wt.% C TWIP steel using microstructure observations by electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Samples with average grain sizes of 3μm and 50μm were deformed in tension at room temperature to different strains. The onset of twinning concurs in both materials with yielding which leads us to propose a Hall-Petch-type relation for the twinning stress using the same Hall-Petch constant for twinning as that for glide. The influence of grain orientation on the twinning stress is more complicated. At low strain, a strong influence of grain orientation on deformation twinning is observed which fully complies with Schmid's law under the assumption that slip and twinning have equal critical resolved shear stresses. Deformation twinning occurs in grains oriented close to 〈1. 1. 1〉//tensile axis directions where the twinning stress is larger than the slip stress. At high strains (0.3 logarithmic strain), a strong deviation from Schmid's law is observed. Deformation twins are now also observed in grains unfavourably oriented for twinning according to Schmid's law. We explain this deviation in terms of local grain-scale stress variations. The local stress state controlling deformation twinning is modified by local stress concentrations at grain boundaries originating, for instance, from incoming bundles of deformation twins in neighboring grains. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.02.041
  • 2010 • 12 Framework for deformation induced anisotropy in glassy polymers
    Harrysson, M. and Ristinmaa, M. and Wallin, M. and Menzel, A.
    Acta Mechanica 211 195-213 (2010)
    In this paper a constitutive model for glassy polymers is developed. Glassy polymers consist of a number of polymer chains that at a microscopic level form a network. If the distribution of the polymer chains shows some preferred direction, the mechanical response at a global macroscopic level will be anisotropic. To incorporate the orientational distribution of the polymer chains, a homogenization procedure involving a chain orientation distribution function was undertaken. When polymers are exposed to external loading, the chains at the microscopic level orient in a certain manner, leading to an evolution of the macroscopic anisotropic properties. This phenomenon was modeled by use of evolution equations for the chains at a microscopic level and are then-by using the orientation distribution function-transformed to the macroscopic level. The theories involved are developed in a large strain setting in which a multiplicative split of the deformation gradient for the elastic-viscoplastic response is adopted. Various numerical experiments were conducted to evaluate the model that was developed. © 2009 Springer-Verlag.
    view abstractdoi: 10.1007/s00707-009-0232-x
  • 2010 • 11 Numerical investigation of room-temperature deformation behavior of a duplex type γtiAl alloy using a multi-scale modeling approach
    Kabir, M.R. and Chernova, L. and Bartsch, M.
    Acta Materialia 58 5834-5847 (2010)
    Room-temperature deformation of a niobium-rich TiAl alloy with duplex microstructure has been numerically investigated. The model links the microstructural features at micro- and meso-scale by the two-level (FE 2) multi-scale approach. The deformation mechanisms of the considered phases were described in the micro-mechanical crystal-plasticity model. Initial material parameters for the model were taken from the literature and validated using tensile experiments at macro-scale. For the niobium-rich TiAl alloy further adaptation of the crystal plasticity parameters is proposed. Based on these model parameters, the influences of the grain orientation, grain size, and texture on the global mechanical behavior have been investigated. The contributions of crystal deformation modes (slips and dislocations in the phases) to the mechanical response are also analyzed. The results enable a quantitative prediction of relationships between microstructure and mechanical behavior on global and local scale, including an assessment of possible crack initiation sites. The model can be used for microstructure optimization to obtain better material properties. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.06.058
  • 2010 • 10 Low cycle lifetime assessment of Al2024 alloy
    Khan, S. and Vyshnevskyy, A. and Mosler, J.
    International Journal of Fatigue 32 1270-1277 (2010)
    The 2024-T351 aluminum alloy is extensively used for fabricating aircraft parts. This alloy shows a relatively low ductility at room temperature and is generally heat treated in various conditions to suit particular applications. The present study experimentally and numerically analyzes the damage mechanism of an Al2024-T351 plate (short transverse direction) subjected to multi-axial stress states. The purpose of this work is to predict the cyclic lifetime of the considered alloy, based on the local approach of damage evolution using continuum damage modeling (CDM). The experimental program involves different kinds of specimens and loading conditions. Monotonic and cyclic tests have been conducted in order to measure the mechanical response and also to perform micromechanical characterization of damage and fracture processes. The cyclic plasticity behavior has been characterized by means of smooth cylindrical specimens. For analyzing the evolution of plastic deformation and damage under multi-axial stress conditions, cyclic loading tests in the low cycle regime have been conducted on different round notched bars. The predictions of the CDM were compared to the experimentally observed mechanical response and to the micromechanical characterization of damage. Emphasis was placed on the prediction of the number of cycles to failure. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2010.01.014
  • 2010 • 9 Orientation dependence of local lattice rotations at precipitates: Example of κ-Fe3AlC carbides in a Fe3Al-based alloy
    Kobayashi, S. and Zambaldi, C. and Raabe, D.
    Acta Materialia 58 6672-6684 (2010)
    Local lattice rotations and in-grain orientation gradients at κ precipitates in matrix grains with orientations near the 45° rotated cube {0 0 1}〈1 1 0〉 (RC) and the γ-fiber components {1 1 1}〈1 1 2〉 were investigated in a Fe3Al alloy warm-rolled to reductions of between 10% and 60%. Near-RC grains showed larger local lattice rotations at precipitates than γ-fiber grains. In RC-oriented grains the local lattice rotations about the transverse direction (TD) were dominant at low reductions, but rotations about the rolling direction (RD) also occurred at higher strains. In the γ-fiber grains the axes of the in-grain lattice rotations were scattered between TD and RD. The rotations around the particles and their orientation dependence were analyzed using 3-D crystal plasticity finite-element simulations of a spherical inclusion in a plane strain deformed matrix of different orientations, namely RC, {1 1 1}〈1 1 2〉 and {1 1 1}〈0 1 1〉. © 2010 AWE and Crown Copyright. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.08.030
  • 2010 • 8 Integrity of micro-hotplates during high-temperature operation monitored by digital holographic microscopy
    Lai, Y.W. and Koukourakis, N. and Gerhardt, N.C. and Hofmann, M.R. and Meyer, R. and Hamann, S. and Ehmann, M. and Hackl, K. and Darakis, E. and Ludwig, Al.
    Journal of Microelectromechanical Systems 19 1175-1179 (2010)
    An investigation on the integrity of micro-hotplates using in situ digital holographic microscopy is reported. The surface topography and surface evolution of the devices during high-temperature operation (heating/cooling cycles) is measured with nanometer-scale resolution. A localized permanent out-of-plane surface deformation of 40% of the membrane thickness caused by the top measurement electrodes occurring after the first cycle is observed. The integrity-related issues caused by such a permanent deformation are discussed. © 2006 IEEE.
    view abstractdoi: 10.1109/JMEMS.2010.2067442
  • 2010 • 7 Microstructures and mechanical properties of Al-base composite materials reinforced by Al-Cu-Fe particles
    Laplanche, G. and Joulain, A. and Bonneville, J. and Schaller, R. and El Kabir, T.
    Journal of Alloys and Compounds 493 453-460 (2010)
    In this study, we produced four composite materials with Al-based matrix reinforced by Al-Cu-Fe particles initially of the quasicrystalline (QC) phase. The processing route was a gas-pressure infiltration of QC particle preforms by molten commercial Al and Al alloys. The resulting composites were investigated by scanning electron microscopy (SEM) working in the energy dispersive spectroscopy (EDS) mode and by X-ray diffraction (XRD). It is shown that such a synthesis technique leads to the formation of various phases resulting from specific diffusion processes. Compression tests were performed at constant strain rate in the temperature range 290-770 K. The stress-strain curves look similar to those of Al-Cu-Fe poly-quasicrystals and show the yield point, the origin of which is however of very different nature. Composite deformation is recognised to occur through the rupture of a hard phase skeleton and localised plastic deformation in the matrix. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2009.12.124
  • 2010 • 6 Extrusion benchmark 2009 experimental analysis of deflection in extrusion dies
    Pietzka, D. and Khalifa, N.B. and Donati, L. and Tomesani, L. and Tekkaya, A.E.
    Key Engineering Materials 424 19-26 (2010)
    In this paper experimental investigations aimed at measuring the die deformations during aluminum extrusion process is presented and discussed. A two-holes die generating two U-shape profiles with different supporting legs was produced and tested under strictly monitored conditions. The influence of die deformation on the speed, temperature distribution and distortion of the two profiles is reported and analyzed. AA6082 alloy was used as deforming material while H-13 hotwork tool steel was selected as die material. The experiments were repeated at least three times in the same conditions in order to achieve a statistical distribution of the acquired data: such data are then used as a reference for the 2009 edition of the extrusion benchmark. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/KEM.424.19
  • 2010 • 5 Localization events and microstructural evolution in ultra-fine grained NiTi shape memory alloys during thermo-mechanical loading
    Schaefer, A. and Wagner, M.F.-X. and Pelegrina, J.L. and Olbricht, J. and Eggeler, G.
    Advanced Engineering Materials 12 453-459 (2010)
    Subjecting a thin NiTi specimen to uniaxial tension often leads to a localized martensitic transformation: macroscopic transformation bands form and propagate through the specimen, separating it into regions of fully transformed martensite and original austenite. In the present study, the alternating current potential drop (ACPD) technique is used to analyze the change in electrical resistance of ultra-fine grained NiTi wires subjected to a broad range of thermo-mechanical load cases: (i) uniaxial tensile straining at constant temperatures (pseudoelastic deformation); (ii) cooling and heating through the transformation range at constant load (actuator load case); (iii) a combination of mechanical and thermal loading. We monitor the ACPD signals in several zones along the gauge length of specimens, and we demonstrate that a localized type of transformation is a generic feature of pseudoelastic as well as of shape memory deformation. Moreover, the ACPD signals allow to differentiate between temperature-induced martensite (formed during cooling at no or relatively small loads), stress-induced martensite, and reoriented martensite (formed under load at low temperatures). © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201000063
  • 2010 • 4 Monotonic and cyclic deformation behaviour of the SiC particle-reinforced aluminium matrix composite AMC225xe
    Smaga, M. and Walther, F. and Eifler, D.
    Advanced Engineering Materials 12 262-268 (2010)
    The monotonic and cyclic deformation behaviour of the aluminium matrix composite AMC225xe - i.e., the aerospace grade aluminium alloy AA 2124 reinforced with 25vol.-% ultrafine SiC particles - is characterised in detail on the basis of mechanical stress-strain hysteresis curves as well as temperature and electrical resistance measurements. A pronounced difference in plastic strain response is observed between tension and compression under monotonic and cyclic loading. ln fully reversed stress-controlled constant amplitude tests, negative plastic mean strains developed. The cyclic deformation behaviour ofAMC225xe is characterised by pronounced initial cyclic hardening. The endurance limit is reliably estimated in continuous load increase tests. ln particular, electrical resistance data are used as input parameters for fatigue life calculations analogous to the Basquin equation. Microstructural details are investigated by light and scanning electron microscopy. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.200900345
  • 2010 • 3 Numerical simulation and benchmarking of a monolithic multigrid solver for fluid-structure interaction problems with application to hemodynamics
    Turek, S. and Hron, J. and Mádlík, M. and Razzaq, M. and Wobker, H. and Acker, J.F.
    Lecture Notes in Computational Science and Engineering 73 LNCSE 193-220 (2010)
    An Arbitrary Lagrangian-Eulerian (ALE) formulation is applied in a fully coupled monolithic way, considering the fluid-structure interaction (FSI) problem as one continuum. The mathematical description and the numerical schemes are designed in such a way that general constitutive relations (which are realistic for biomechanics applications) for the fluid as well as for the structural part can be easily incorporated. We utilize the LBB-stable finite element pairs Q 2 P 1 and P 2 + P 1 for discretization in space to gain high accuracy and perform as time-stepping the 2nd order Crank-Nicholson, respectively, a new modified Fractional-Step-θ-scheme for both solid and fluid parts. The resulting discretized nonlinear algebraic system is solved by a Newton method which approximates the Jacobian matrices by a divided differences approach, and the resulting linear systems are solved by direct or iterative solvers, preferably of Krylov-multigrid type. For validation and evaluation of the accuracy and performance of the proposed methodology, we present corresponding results for a new set of FSI benchmark configurations which describe the self-induced elastic deformation of a beam attached to a cylinder in laminar channel flow, allowing stationary as well as periodically oscillating deformations. Then, as an example of FSI in biomedical problems, the influence of endovascular stent implantation on cerebral aneurysm hemodynamics is numerically investigated. The aim is to study the interaction of the elastic walls of the aneurysm with the geometrical shape of the implanted stent structure for prototypical 2D configurations. This study can be seen as a basic step towards the understanding of the resulting complex flow phenomena so that in future aneurysm rupture shall be suppressed by an optimal setting of the implanted stent geometry. © 2011 Springer.
    view abstractdoi: 10.1007/978-3-642-14206-2_8
  • 2010 • 2 Crystal plasticity modelling and experiments for deriving microstructure-property relationships in γ-TiAl based alloys
    Zambaldi, C. and Raabe, D.
    Journal of Physics: Conference Series 240 (2010)
    Single-crystals of γ-TiAl cannot be grown for the compositions present inside the two-phase γ/α 2-microstructures that show good mechanical properties. Therefore the single crystal constitutive behaviour of γ-TiAl was studied by nanoindentation experiments in single phase regions of these microstructures. The experiments were extensively characterized by a combined experimental approach to clarify the orientation dependent mechanical response during nanoindentation. They further were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behaviour of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. On the length-scale of multi-grain aggregates two kinds of microstructures were investigated. The lamellar microstructure was analyzed in terms of kinematic constraints perpendicular to densely spaced lamellar boundaries which lead to pronounced plastic anisotropy. Secondly, the mechanical behaviour of massively transformed microstructures was modelled by assuming a lower degree of kinematic constraints. This resulted in less plastic anisotropy on a single grain scale and lower compatibility stresses in a 64-grain aggregate. On the macroscopic length scale, the results could possibly explain the pre-yielding of lamellar microstructures. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/240/1/012140
  • 2010 • 1 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