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 • 258 Effect of γ′ precipitate size on hardness and creep properties of Ni-base single crystal superalloys: Experiment and simulation
    Ali, M.A. and López-Galilea, I. and Gao, S. and Ruttert, B. and Amin, W. and Shchyglo, O. and Hartmaier, A. and Theisen, W. and Steinbach, I.
    Materialia 12 (2020)
    The role and effect of γ′ precipitate size on the mechanical properties of Ni-base single crystal superalloy is investigated. The underlying mechanisms are analyzed on the one hand with the help of experiments including hardness and creep tests, and on the other hand with the help of two different simulation approaches by taking the typical γ/γ′ microstructure into account. Simulations, based on the crystal plasticity finite element method (CPFEM) are carried out for the hardness tests, whereas simulations, based on the crystal plasticity coupled phase-field method (CPPFM) are carried out for the creep tests. The hardness test simulation results show that the hardness of material varies inversely with the size of γ′ precipitates for a given γ′ phase volume fraction and it varies directly with the volume fraction of γ′ precipitates for a given precipitate size. These results are qualitatively consistent with the experimental observations. The creep simulation results show that the refinement of γ′ precipitates with a certain volume fraction of precipitates leads to an improvement of creep resistance by delaying the plastic activity in the material. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.mtla.2020.100692
  • 2020 • 257 On the influence of crystallography on creep of circular notched single crystal superalloy specimens
    Cao, L. and Thome, P. and Agudo Jácome, L. and Somsen, C. and Cailletaud, G. and Eggeler, G.
    Materials Science and Engineering A 782 (2020)
    The present work contributes to a better understanding of the effect of stress multiaxiality on the creep behavior of single crystal Ni-base superalloys. For this purpose we studied the creep deformation and rupture behavior of double notched miniature creep tensile specimens loaded in three crystallographic directions [100], [110] and [111] (creep conditions: 950 °C and 400 MPa net section stress). Crystal plasticity finite element method (CPFEM) was used to analyze the creep stress and strain distributions during creep. Double notched specimens have the advantage that when one notch fails, the other is still intact and allows to study a material state which is close to rupture. No notch root cracking was observed, while microstructural damage (pores and micro cracks) were frequently observed in the center of the notch root region. This is in agreement with the FEM results (high axial stress and high hydrostatic stress in the center of the notched specimen). Twinning was observed in the notch regions of [110] and [111] specimens, and <112> {111} twins were detected and analyzed using orientation imaging scanning electron microscopy. The present work shows that high lattice rotations can be detected in SXs after creep fracture, but they are associated with the high strains accumulated in the final rupture event. © 2020 The Authors
    view abstractdoi: 10.1016/j.msea.2020.139255
  • 2020 • 256 Diffusion, defects and understanding the growth of a multicomponent interdiffusion zone between Pt-modified B2 NiAl bond coat and single crystal superalloy
    Esakkiraja, N. and Gupta, A. and Jayaram, V. and Hickel, T. and Divinski, S.V. and Paul, A.
    Acta Materialia 195 35-49 (2020)
    Composition-dependent diffusion coefficients are determined in B2-Ni(CoPt)Al system following the pseudo-binary and pseudo-ternary diffusion couple methods, which would not be possible otherwise in a quaternary inhomogeneous material fulfilling the conditions to solve the equations developed based on the Onsager formalism. The end-member compositions to produce ideal/near-ideal diffusion profiles are chosen based on thermodynamic details. The pseudo-binary interdiffusion coefficients of Ni and Al decrease in the presence of Co but increase in the presence of Pt. The pseudo-ternary interdiffusion coefficients indicate that the main interdiffusion coefficients increase significantly in the presence of Pt. Marginal changes of the cross interdiffusion coefficients substantiate a minor change of the diffusional interactions between the components. The thermodynamic driving forces show opposite trends with respect to composition as compared to the changes of the interdiffusion coefficients advocating a dominating role of the Pt(Co)-induced modifications of point defect concentrations. DFT-based calculations revealed that Pt alloying increases the Ni vacancy concentration and decreases the activation energy for the triple defect diffusion mechanism. These findings explain the increase in the thickness of the interdiffusion zone between the B2-Ni(Pt)Al bond coat and the single crystal superalloy René N5 because of Pt addition. Furthermore, the EPMA and TEM analyses reveal the growth of refractory elements-enriched precipitates. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.04.016
  • 2020 • 255 Thermal Spray Processes for the Repair of Gas Turbine Components
    Fiebig, J. and Bakan, E. and Kalfhaus, T. and Mauer, G. and Guillon, O. and Vaßen, R.
    Advanced Engineering Materials 22 (2020)
    Gas turbine components are often operated in harsh conditions, which can lead to severe damage. As it is highly desirable from both an economical and an ecological point of view to restore these worn areas instead of manufacturing new components, repair technologies are of huge interest for companies supplying maintenance and overhaul of gas turbines. In this article, two thermal techniques are described that can be used for this application: cold gas spraying (CGS) and vacuum plasma spraying (VPS). The CGS process allows the deposition of metallic coatings with excellent mechanical properties; several examples including γ-TiAl, Inconel (IN) 718, and IN 738 are given. Essential for the deposition of high-performance coatings in CGS is to exceed the so-called critical velocity. This is discussed also with experimental findings. As a final topic, experiments that use VPS for the repair of single-crystal alloys are described. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201901237
  • 2020 • 254 Influence of rafted microstructures on creep in Ni-base single crystal superalloys: A 3D discrete dislocation dynamics study
    Gao, S. and Ali, M.A. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 28 (2020)
    Ni-base single-crystal superalloys exhibit a dynamic evolution of their microstructure during operation at elevated temperatures. The rafting of γ′ precipitates changes the mechanical behavior in a way that was understood insufficiently. In this work, we combine a phase-field method with a discrete dislocation dynamics model to clarify the influence of different rafted microstructures with the same initial dislocation density and configuration on creep behavior. The unrafted and rafted microstructures of Ni-base single crystal superalloys are simulated by a phase-field crystal plasticity method. By introducing these microstructures into a 3D discrete dislocation dynamics (DDD) model, the creep behavior under uniaxial loads of 350 and 250 MPa along [100] direction at 950 °C is studied. Due to the negative lattice mismatch of Ni-base superalloys, the N-type rafting with the formation of plate-like γ′ precipitates occurs under uniaxial tensile loads along {100} direction at high temperatures, while the P-type rafting with the formation of rod-like γ′ precipitates occurs under compressive loads. Taking the cuboidal, N-type rafted and P-type rafted microstructures as the initial and fixed microstructures for the same loading conditions, it is found from DDD simulations that the rafted microstructures result in smaller creep deformation than the cuboidal microstructure. The reason for this is that the coalescence of γ′ precipitates during the rafting diminishes the width of some γ channels, so as to increase the local Orowan stresses which retard the dislocation glide. For tensile loads, the N-type rafted microstructure has the best creep resistance. For a low compressive load, the P-type rafting shows a better creep resistance than N-type rafting. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/ab5e40
  • 2020 • 253 Elastic properties of single crystal Bi12SiO20 as a function of pressure and temperature and acoustic attenuation effects in Bi12 MO20 (M = Si, Ge and Ti)
    Haussühl, E. and Reichmann, H.J. and Schreuer, J. and Friedrich, A. and Hirschle, C. and Bayarjargal, L. and Winkler, B. and Alencar, I. and Wiehl, L. and Ganschow, S.
    Materials Research Express 7 (2020)
    A comprehensive study of sillenite Bi12SiO20 single-crystal properties, including elastic stiffness and piezoelectric coefficients, dielectric permittivity, thermal expansion and molar heat capacity, is presented. Brillouin-interferometry measurements (up to 27 GPa), which were performed at high pressures for the first time, and ab initio calculations based on density functional theory (up to 50 GPa) show the stability of the sillenite structure in the investigated pressure range, in agreement with previous studies. Elastic stiffness coefficients c 11 and c 12 are found to increase continuously with pressure while c 44 increases slightly for lower pressures and remains nearly constant above 15 GPa. Heat-capacity measurements were performed with a quasi-adiabatic calorimeter employing the relaxation method between 2 K and 395 K. No phase transition could be observed in this temperature interval. Standard molar entropy, enthalpy change and Debye temperature are extracted from the data. The results are found to be roughly half of the previous values reported in the literature. The discrepancy is attributed to the overestimation of the Debye temperature which was extracted from high-temperature data. Additionally, Debye temperatures obtained from mean sound velocities derived by Voigt-Reuss averaging are in agreement with our heat-capacity results. Finally, a complete set of electromechanical coefficients was deduced from the application of resonant ultrasound spectroscopy between 103 K and 733 K. No discontinuities in the temperature dependence of the coefficients are observed. High-temperature (up to 1100 K) resonant ultrasound spectra recorded for Bi12 MO20 crystals revealed strong and reversible acoustic dissipation effects at 870 K, 960 K and 550 K for M = Si, Ge and Ti, respectively. Resonances with small contributions from the elastic shear stiffness c 44 and the piezoelectric stress coefficient e 123 are almost unaffected by this dissipation. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/2053-1591/ab6ad6
  • 2020 • 252 On the atomic solute diffusional mechanisms during compressive creep deformation of a Co-Al-W-Ta single crystal superalloy
    He, J. and Zenk, C.H. and Zhou, X. and Neumeier, S. and Raabe, D. and Gault, B. and Makineni, S.K.
    Acta Materialia 184 86-99 (2020)
    We investigated the solute diffusional behavior active during compressive creep deformation at 150 MPa / 975 °C of a Co-Al-W-Ta single crystal superalloy in the [001] orientation. We report the formation of shear-bands that involves re-orientation of γ/γʹ rafts to {111} from {001} planes, referring to as γ/γ′ raft-rotation. In the shear-band regions, we observed abundant micro-twins, stacking faults (SFs), disordered zones within the γʹ termed as ‘γ pockets’ and also few geometrically-close-packed (GCP) phases. We used a correlative approach blending electron microscopy and atom probe tomography to characterize the structure and composition of these features. The SFs were identified as intrinsic and exhibit a W enrichment up to 14.5 at.% and an Al deficiency down to 5.1 at.%, with respect to the surrounding γʹ phase. The micro-twin boundaries show a solute enrichment similar to the SFs with a distinct W compositional profile gradients perpendicular from the boundaries into the twin interior, indicating solute diffusion within the micro-twins. The γ-pockets have a composition close to that of γ but richer in W/Ta. Based on these observations, we propose (i) a solute diffusion mechanism taking place during micro-twinning, (ii) a mechanism for the γ/γʹ raft-rotation process and evaluate their influence on the overall creep deformation of the present Co-based superalloy. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.035
  • 2020 • 251 On the rhenium segregation at the low angle grain boundary in a single crystal Ni-base superalloy
    He, J. and Scholz, F. and Horst, O.M. and Thome, P. and Frenzel, J. and Eggeler, G. and Gault, B.
    Scripta Materialia 185 88-93 (2020)
    Industrial scale single crystal (SX) Ni-base superalloys contain numerous low angle grain boundaries inherited from the solidification process. Here, we demonstrate that low angle grain boundaries in a fully heat-treated SX model Ni-base superalloy are strongly segregated with up to 12 at% Re. Some Re-rich dislocations forming this grain boundary are found located inside γ, others close to a γ/γ′ interface. Although these segregated Re atoms lose their solid-solution strengthening effect, they may enhance the creep resistance by pinning the low angle grain boundaries and slowing down dislocation reactions. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2020.03.063
  • 2020 • 250 Thermoelastic anisotropy in NdScO3 and NdGaO3 perovskites
    Hirschle, C. and Schreuer, J. and Ganschow, S. and Peters, L.
    Materials Chemistry and Physics 254 (2020)
    Single crystal thermal expansion and elastic stiffness of NdGaO3 and NdScO3 were investigated by inductive gauge dilatometry and resonant ultrasound spectroscopy between 103K and 1673K, as they are used extensively as perovskite-type substrates for epitaxial crystal growth. Thermal expansion of NdGaO3 is in agreement with literature data and has very similar magnitude and anisotropy compared to NdScO3. The anisotropy of the elastic stiffness of NdGaO3 is more pronounced and qualitatively different from what is found for NdScO3. It is explained in terms of structural instabilities, which lead to known phase transitions in other perovskites. The anisotropy of the elastic stiffness of NdGaO3 is compatible with what is found for other orthorhombic perovskites that undergo a transition to a rhombohedral structure at high temperatures. The elastic properties of NdScO3 directly follow from the properties of other REScO3. The samples were characterized with regards to their compositions and lattice parameters using electron probe microanalysis and X-ray powder diffraction. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2020.123528
  • 2020 • 249 Femto- To Microsecond Dynamics of Excited Electrons in a Quadruple Cation Perovskite
    Jung, E. and Budzinauskas, K. and Öz, S. and Ünlü, F. and Kuhn, H. and Wagner, J. and Grabowski, D. and Klingebiel, B. and Cherasse, M. and Dong, J. and Aversa, P. and Vivo, P. and Kirchartz, T. and Miyasaka, T. and Van Loosdre...
    ACS Energy Letters 5 785-792 (2020)
    Quadruple cation mixed halide perovskite, GA0.015Cs0.046MA0.152FA0.787Pb(I0.815Br0.185)3, single crystals were grown for the first time using an inverse temperature crystallization process. Solar cell devices in n-i-p stack configuration using thin films of the same materials showed power conversion efficiency above 20%. Complementary time-resolved spectroscopy confirmed that polycrystalline thin films and single crystals identically composed exhibit similar carrier dynamics in the picosecond range. Cooling of excited carriers and bandgap renormalization occur on the same time scale of 200-300 fs. The radiative recombination coefficient (1.2 × 10-9 cm3/s) is comparable to values reported for a GaAs semiconductor. At low excitation density, a long carrier lifetime of 3.2 μs was recorded possibly due to the passivation of recombination centers. This study clarifies discrepancies about the lifetime of hot carriers, the impact of radiative recombination, and the role of recombination centers on solar cell performance. The quadruple cation perovskites displayed short time dynamics with slow recombination of charge carriers. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsenergylett.9b02684
  • 2020 • 248 Metal Complexes of Donor-functionalized Fluorinated β-Ketoiminates – Synthesis, Structure, and CVD Application
    Kaiser, K. and Ganesamoorthy, C. and Wölper, C. and Schulz, S.
    Zeitschrift fur Anorganische und Allgemeine Chemie 646 681-691 (2020)
    Six alkali metal complexes of partly-fluorinated, donor-functionalized β-ketoiminate ligands [L1Li (1), L1Na (2), L1K (3), L1Cs (4), L1 = OC(CF3)CHC(CH3)NCH2CH2OCH3; L2Li (5), L2Na (6), L2 = OC(CF3)CHC(CH3)NCH2CH2N(CH3)2] were prepared and structurally characterized. Reactions of L1Li with PtCl2 gave the homoleptic Pt complex L12Pt (7), which was characterized spectroscopically and by single-crystal X-ray diffraction and whose promising application as CVD precursor (chemical vapor deposition) is shown. Polycrystalline, pure Pt films were grown at 500 °C on SiO2@Si(100) substrates at 10–3 mbar and characterized by XRD, SEM, AFM, EDX and XPS. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/zaac.201900239
  • 2020 • 247 Processing of a single-crystalline CrCoNi medium-entropy alloy and evolution of its thermal expansion and elastic stiffness coefficients with temperature
    Laplanche, G. and Schneider, M. and Scholz, F. and Frenzel, J. and Eggeler, G. and Schreuer, J.
    Scripta Materialia 177 44-48 (2020)
    The equiatomic CrCoNi alloy is regarded as a model single-phase face-centered cubic medium-entropy alloy. A CrCoNi single crystal was grown by a Bridgman technique using a Ni-base superalloy seed. The elastic stiffnesses and thermal expansion coefficient were determined between 100 K and 673 K employing resonant ultrasound spectroscopy and dilatometry, respectively. All data were found to be in excellent agreement with those reported for polycrystalline CrCoNi. A comparison of the normalized Cauchy pressure of CrCoNi with those of other alloys indicates that interatomic bonds become more directional with increasing Cr-concentration while Co and Ni promote a metallic character. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2019.09.020
  • 2020 • 246 A novel algorithm for rate independent small strain crystal plasticity based on the infeasible primal-dual interior point method
    Scheunemann, L. and Nigro, P.S.B. and Schröder, J. and Pimenta, P.M.
    International Journal of Plasticity 124 1-19 (2020)
    Single crystal plasticity plays a major role in the analysis of material anisotropy and texture evolution, treats each crystalline grain individually. The polycrystalline material response is obtained upon considering a structure consisting of various individual grains, often also considering interface effects at the grain boundaries. On the individual grain level, single crystal plasticity can be treated in the mathematical framework of multi-surface plasticity, leading to a constrained optimization problem wherein multiple constraints are defined as yield criteria on the different slip systems. In this work, we present a new algorithm for the solution of the constrained optimization problem based on the Infeasible Primal Dual Interior Point method (IPDIPM). The main motivation herein is the handling of the ill-posed problem without the use of simple perturbation technique, see e.g. Miehe and Schröder [2001]. The proposed algorithm, involving slack variables, is developed for the framework of small strain single crystal plasticity. The use of slack variables therein stabilizes the conventional method and allows for a temporary violation of the constraint condition during the optimization. Moreover, all slip systems are considered simultaneously, omitting an iterative active set search. Several numerical examples are simulated to show the performance of the developed algorithm. © 2019 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijplas.2019.07.020
  • 2020 • 245 On the assessment of creep damage evolution in nickel-based superalloys through correlative HR-EBSD and cECCI studies
    Sulzer, S. and Li, Z. and Zaefferer, S. and Hafez Haghighat, S.M. and Wilkinson, A. and Raabe, D. and Reed, R.
    Acta Materialia 185 13-27 (2020)
    The evolution of dislocation density with creep strain in single-crystal superalloys is studied quantitatively using high-resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging under controlled diffraction conditions (cECCI). Data regarding dislocation density/structure is measured for deformation at 900 °C and 450 MPa up to ≈ 1% plastic strain. Effects of chemical composition are elucidated via three purpose-designed superalloys of differing rhenium and ruthenium contents. The evidence indicates that dislocation avalanching is already prevalent at plastic strains of ≈ 0.1%; thereafter, an exponential decay in the dislocation multiplication rate is indicative of self-hardening due to dislocation constriction within the matrix channels, as confirmed by the imaging. The results are rationalised using discrete dislocation dynamics modelling: a universal dislocation evolution law emerges, which will be useful for alloy design efforts. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.07.018
  • 2020 • 244 Towards an understanding of grain boundary step in diamond cutting of polycrystalline copper
    Wang, Z. and Zhang, J. and Zhang, J. and Li, G. and Zhang, H. and ul Hassan, H. and Hartmaier, A. and Yan, Y. and Sun, T.
    Journal of Materials Processing Technology 276 (2020)
    Microstructural deformation at the grain level has an inherent impact on the achievable ultimate machining accuracy of polycrystalline materials. In the present work, numerical simulations and experiments of diamond cutting of polycrystalline copper are carried out to investigate the formation of surface step at grain boundaries on machined surface. Single crystal diamond cutting tool with straight cutting edge is chosen for experiments to mimic the tool geometry utilized in 2D crystal plasticity finite element simulations. Moreover, the same crystallography configuration of bi-crystal Cu is employed between experiments and simulations. Formation mechanisms of surface steps at grain boundaries are revealed by finite element simulations and corresponding experimental validation, as well as cross-sectional transmission electron microscope characterization. Finally, finite element simulations of orthogonal cutting of bi-crystal Cu are carried out to examine effects of both extrinsic cutting edge radius of diamond cutting tool and intrinsic misorientation angle of grain boundary on the propensity of grain boundary surface step formation. The present work provides theoretical guidelines on the strategy of suppressing grain boundary surface step formation for achieving superior surface finish of polycrystalline materials by diamond cutting. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2019.116400
  • 2020 • 243 Nanocutting mechanism of 6H-SiC investigated by scanning electron microscope online observation and stress-assisted and ion implant-assisted approaches
    Xu, Z. and Liu, L. and He, Z. and Tian, D. and Hartmaier, A. and Zhang, J. and Luo, X. and Rommel, M. and Nordlund, K. and Zhang, G. and Fang, F.
    International Journal of Advanced Manufacturing Technology 106 3869-3880 (2020)
    Nanocutting mechanism of single crystal 6H-SiC is investigated through a novel scanning electron microscope setup in this paper. Various undeformed chip thicknesses on (0001) < 1–100 > orientation are adopted in the nanocutting experiments. Phase transformation and dislocation activities involved in the 6H-SiC nanocutting process are also characterized and analyzed. Two methods of stress-assisted and ion implant-assisted nanocutting are studied to improve 6H-SiC ductile machining ability. Results show that stress-assisted method can effectively decrease the hydrostatic stress and help to activate dislocation motion and ductile machining; ion implant-induced damages are helpful to improve the ductile machining ability from MD simulation and continuous nanocutting experiments under the online observation platform. © 2020, Springer-Verlag London Ltd., part of Springer Nature.
    view abstractdoi: 10.1007/s00170-019-04886-6
  • 2019 • 242 CoTPP molecules deposited on graphene/Ni (111): Quenching of the antiferromagnetic interaction induced by gold intercalation
    Corradini, V. and Candini, A. and Klar, D. and Biagi, R. and De Renzi, V. and Lodi Rizzini, A. and Cavani, N. and Del Pennino, U. and Wende, H. and Otero, E. and Affronte, M.
    Journal of Applied Physics 125 (2019)
    In this work, we investigated the effect of Au-intercalation on the magnetic coupling between a sub-monolayer of Co-Tetraphenylporphyrin molecules and a graphene-covered Ni(111) single crystal. Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element-specific magnetization and its field dependence were probed. Cobalt strongly couples antiferromagnetically to the nickel substrate, also through the graphene layer. The intercalation of graphene with gold leads to a complete removal of this coupling. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5063562
  • 2019 • 241 Structure and Reactivity of 1,8-Bis(naphthalenediyl)dipnictanes
    Dzialkowski, K. and Gehlhaar, A. and Wölper, C. and Auer, A.A. and Schulz, S.
    Organometallics 38 2927-2942 (2019)
    Syntheses and solid-state structures of diarsane Naph2As2 (Naph = 1,8-naphthalenediyl, 1) and (Naph)5Sb4Cl2 3 are reported and the σ-donor capacity of Naph2E2 (E = As 1, Sb 2) was studied in reactions with (coe)Cr(CO)5 (coe = Z-cyclooctene), yielding [Naph2As2][Cr(CO)5]2 (4) and [Naph2E2][Cr(CO)5] (E = As 5, Sb 6). In contrast, reactions of 1 and 3 with Me2SAuCl proceed with oxidation and formation of elemental gold as well as Naph2(AsCl)2 (7) and [NaphSbCl2]2 8. All complexes were characterized by elemental analyses, heteronuclear (1H, 13C) NMR and FT-IR spectroscopy, as well as single crystal X-ray diffraction. Intermolecular E···πinteractions (E = As, Sb), which were observed in 7 and 8, were quantified by use of density functional theory and local coupled cluster electronic structure theory calculations. These allow to assess the nature and relative importance of covalent and noncovalent interactions and illustrate how dispersion interactions change with the electronic structure of the compounds. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.9b00269
  • 2019 • 240 Classic crystal plasticity theory vs crystal plasticity theory based on strong discontinuities—Theoretical and algorithmic aspects
    Fohrmeister, V. and Díaz, G. and Mosler, J.
    International Journal for Numerical Methods in Engineering 117 1283-1303 (2019)
    This paper deals with two different approaches suitable for the description of plasticity in single crystals. The first one is the standard approach that is based on a continuous deformation mapping. Plasticity is driven by a classic Schmid-type relation connecting the shear stresses to the shear strains at a certain slip system. By way of contrast, the second approach is nonstandard. In this novel model, localized plastic deformation at certain slip planes is approximated by a strong discontinuity (discontinuous deformation mapping). Accordingly, a modified Schmid-type model relating the shear stresses to the shear displacements (displacement jump) is considered in this model. Although both models are indeed different, it is shown that they can be characterized by almost the same set of equations, eg, by a multiplicative decomposition of the deformation gradient into an elastic part and a plastic part. This striking analogy eventually leads to a unifying algorithmic formulation covering both models. Since the set of active slip systems is not known in advance, its determination is of utmost importance. This problem is solved here by using the nonlinear complementarity problem (NCP) as advocated by Fischer and Burmeister. While this idea is not new, it is shown that the NCP problem is well posed, independent of the number of active slip systems. To be more explicit, the tangent matrix in the return-mapping scheme is regular even for more than five simultaneously active slip systems. Based on this algorithm, texture evolution in a polycrystal is analyzed by means of both models and the results are compared in detail. © 2018 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/nme.6000
  • 2019 • 239 On the rejuvenation of crept Ni-Base single crystal superalloys (SX)by hot isostatic pressing (HIP)
    Horst, O.M. and Ruttert, B. and Bürger, D. and Heep, L. and Wang, H. and Dlouhý, A. and Theisen, W. and Eggeler, G.
    Materials Science and Engineering A 758 202-214 (2019)
    In the present work, we study the effect of HIP rejuvenation treatments on the creep behavior and residual life of a pre-crept single crystal Ni-base superalloy of type CMSX-4. The present work combines miniature creep experiments of precisely oriented [001]tensile creep specimens with HIP treatments and quantitative analysis of scanning and transmission electron micrographs. A HIP-rejuvenation treatment after 5.0% creep pre-strain significantly improves creep strength. However, the microstructural damage induced by the creep pre-deformation could not be fully removed. In a series of sequential creep/HIP/creep-experiments, increasing levels of pre-deformation result in increasing levels of creep rates even after identical HIP-rejuvenation treatments. The memory effect, which causes this phenomenon, is related to an accumulation of permanent microstructural damage, which is not associated with rafting or cavitation. The mechanical results obtained in the present work are interpreted based on microstructural results on the γ/γʼ-microstructure (γ-channel widths and γʼ-size), on the pore population (number density of pores, pore size distributions and pore area fractions)and dislocation substructures which have formed during creep. The results are discussed in the light of previous results reported in the literature. © 2019 The Authors
    view abstractdoi: 10.1016/j.msea.2019.04.078
  • 2019 • 238 Thermo-mechanical properties of mullite ceramics: New data
    Krenzel, T.F. and Schreuer, J. and Laubner, D. and Cichocki, M. and Schneider, H.
    Journal of the American Ceramic Society 102 416-426 (2019)
    Coefficients of elastic stiffnesses and thermal expansion of hot isostatically pressed, reaction-sintered and technical fused-mullite ceramics were measured between 100 and 1673 K in comparison with single crystal mullite employing resonant ultrasound spectroscopy and dilatometry, respectively. Additionally, chemical and phase compositions and the microstructure of the ceramics were studied using X-ray diffraction techniques and scanning electron microscopy. Our studies revealed that despite polycrystallinity and slight porosity of up to 1.6%, the elastic behavior of the hot isostatically pressed ceramics is near to ideal aggregate elastic properties of mullite single crystal, for example, their bulk moduli fit within 0.7% to B = 170.0 GPa of single crystal mullite. On the other hand, with B = 155 GPa, the reaction-sintered mullite behaves significantly softer. The difference can be explained with more tight grain to grain contacts in hot isostatically pressed ceramics as compared to reaction-sintered materials. The thermal expansion of both types of ceramics almost coincides with the corresponding averaged behavior of single crystal mullite. For example, between 573 and 1273 K, the volume expansion coefficients of all these materials are (18.0 ± 0.3)·10−6 K−1. Obviously, the microstructural features are less important for the macroscopic thermal expansion. Due to heterogeneous microstructure and high α-alumina and zirconia contents, the corresponding properties of fused-mullite refractory deviate strongly from those of the other mullite materials. © 2018 The American Ceramic Society
    view abstractdoi: 10.1111/jace.15925
  • 2019 • 237 Effect of nanometric γ′-particles on the stress-induced martensitic transformation in 〈001〉-oriented Co49Ni21Ga30 shape memory alloy single crystals
    Lauhoff, C. and Reul, A. and Langenkämper, D. and Krooß, P. and Somsen, C. and Gutmann, M.J. and Kireeva, I. and Chumlyakov, Y.I. and Schmahl, W.W. and Niendorf, T.
    Scripta Materialia 168 42-46 (2019)
    The effect of finely dispersed particles on the functional properties and morphology of thermally induced martensite in Co-Ni-Ga shape memory alloys has been already reported in literature, however, still important aspects are not fully understood. The current study focuses on the stress-induced martensitic transformation of solution-annealed, i.e. precipitate-free, and aged 〈001〉-oriented single crystals. In situ optical microscopy and neutron diffraction experiments show a significant influence of γ′-particles on the martensite variant selection and its morphology under pseudoelastic deformation. In addition, the results reveal detwinning upon loading in the presence of nanometric particles, which is experimentally proven for the first time. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2019.04.003
  • 2019 • 236 Tension/Compression asymmetry of a creep deformed single crystal Co-base superalloy
    Lenz, M. and Eggeler, Y.M. and Müller, J. and Zenk, C.H. and Volz, N. and Wollgramm, P. and Eggeler, G. and Neumeier, S. and Göken, M. and Spiecker, E.
    Acta Materialia 166 597-610 (2019)
    The creep behavior of a multinary single crystal Co-base superalloy has been compared for uniaxial tension and compression of 400 MPa applied along [001] at 850 °C. Creep under tensile stress proceeds two times faster than creep under compression. A detailed TEM study shows that already after ∼0.3 % creep strain planar faults are formed in both samples. While extended a/2<112> ribbons with SISF loops embedded in APBs are observed in tension, extrinsic SFs are revealed in compression. At ∼5 % creep strain SISFs confined to the γ′ phase dominate in tension, whereas extrinsic SFs and microtwins extending across both phases are the prevalent planar faults in compression. In addition, dense networks of regular a/2<101> matrix dislocations develop at the γ/γ′ interfaces in both loading scenarios. In tensile creep and early compressive creep the direct contribution of planar faults to plastic deformation is minor and does not exceed 10 % of the measured plastic strain. In contrast, thickening of microtwins appears to become an efficient deformation channel in the later stages of compressive creep. A pronounced asymmetry regarding the rafting kinetics is observed resulting in a P-type rafted and topologically inverted microstructure after ∼5 % creep in tension while hardly any rafting has occurred under compression. The pronounced rafting and related recovery processes are likely responsible for the inferior creep behavior in tension. Finally, two novel diffusion-assisted degradation mechanisms related to microtwins are shown to be active: an expansion of the γ phase into γ′ precipitates along microtwins and the formation of γ phase nuclei at planar fault intersections inside γ′. Both phenomena are hypothesized to be triggered by segregation of γ formers like Co and Cr to planar faults. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.053
  • 2019 • 235 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 • 234 New flat-punch indentation creep testing approach for characterizing the local creep properties at high temperatures
    Matschkal-Amberger, D. and Kolb, M. and Neumeier, S. and Gao, S. and Hartmaier, A. and Durst, K. and Göken, M.
    Materials and Design 183 (2019)
    An indentation creep testing approach has been developed which allows measuring creep properties at high temperatures. In contrast to existing indentation or impression creep experiments, the approach described here allows to achieve a quite high spatial resolution, as flat punch indenters with a diameter of only 20 μm are used. First indentation creep tests have been performed on single crystalline nickel and nickel binary solid solution alloys with Re, Ta or W as alloying elements, respectively. The indentation creep tests have been carried out at a temperature of 650 °C and stress levels in the range of 85 to 400 MPa. Using crystal plasticity finite element modeling, the indentation creep response is converted into equivalent uniaxial creep properties. It is shown that the conversion parameters, evaluated for differently oriented single crystals, can be chosen independently of the creep rate exponent in the power law creep regime. It is found that the indentation creep results agree well with conventional uniaxial creep tests. Furthermore, the results show that Ta is the most effective solid solution strengthener of all tested solid-solution strengtheners at 650 °C because of the large atomic size mismatch, followed by W and Re. © 2019 The Authors
    view abstractdoi: 10.1016/j.matdes.2019.108090
  • 2019 • 233 Effect of heat treatment on the high temperature fatigue life of single crystalline nickel base superalloy additively manufactured by means of selective electron beam melting
    Meid, C. and Dennstedt, A. and Ramsperger, M. and Pistor, J. and Ruttert, B. and Lopez-Galilea, I. and Theisen, W. and Körner, C. and Bartsch, M.
    Scripta Materialia 168 124-128 (2019)
    The high temperature low cycle fatigue behavior of specimens manufactured from a single crystalline nickel base superalloy processed by selective electron beam melting (SEBM) has been investigated with respect to the effect of different heat treatments. The fatigue lifetime of heat treated material was significantly higher than that of as-built material. Applying hot isostatic pressing (HIP) with an integrated heat treatment resulted in even longer fatigue life. Lifetime limiting crack initiation occurred at interfaces of melting layers, at micro-porosity generated during solidification or, in HIP treated samples, at precipitates which formed at the location of collapsed pores. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2019.05.002
  • 2019 • 232 Miniaturization of low cycle fatigue-testing of single crystal superalloys at high temperature for uncoated and coated specimens [Miniaturisierung der Versuchstechnik für Niedrig-Lastwechsel-Ermüdung bei Hochtemperatur an Proben aus einkristallinen Superlegierungen mit und ohne Schutzschichten]
    Meid, C. and Waedt, U. and Subramaniam, A. and Wischek, J. and Bartsch, M. and Terberger, P. and Vaßen, R.
    Materialwissenschaft und Werkstofftechnik 50 777-787 (2019)
    A newly developed miniature specimen and respective fixture for high temperature low cycle fatigue testing of nickel based single crystal superalloys is presented. Miniaturization allows the preparation of test specimens in all main crystallographic orientations of the cubic nickel crystal using laboratory sized material samples and enables excellent utilization of the costly material. The specimen geometry is optimized by means of parameter studies employing numerical calculations such that for the main crystallographic orientations the stress concentration at the fillet between gauge length and specimen head is minimized, and failure is likely to occur within the gauge length. The designed fixture allows easy specimen mounting and provides sufficient support for applying an extensometer for strain measurement. Protective metallic coatings against oxidation can be applied on the specimen by plasma spraying for studying the effect of coatings on the fatigue lifetime. The functionality of the specimen geometry and fixture design for low cycle fatigue testing is demonstrated for temperatures up to 950 °C. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/mawe.201800135
  • 2019 • 231 Stress-induced formation of TCP phases during high temperature low cycle fatigue loading of the single-crystal Ni-base superalloy ERBO/1
    Meid, C. and Eggeler, M. and Watermeyer, P. and Kostka, A. and Hammerschmidt, T. and Drautz, R. and Eggeler, G. and Bartsch, M.
    Acta Materialia 168 343-352 (2019)
    The microstructural evolution in the single crystal Ni-base superalloy ERBO/1 (CMSX 4 type) is investigated after load controlled low cycle fatigue (LCF) at 950 °C (load-ratio: 0.6, tensile stress range: 420–740 MPa, test frequency: 0.25 Hz, fatigue rupture life: about 1000 - 3000 cycles). Bulk topologically close packed (TCP) phase particles precipitated and were analyzed by three-dimensional focus ion beam slice and view imaging and analytical transmission electron microscopy. The particles did not precipitate homogenously but at locations with enhanced levels of local stresses/strains, such as isolated γ-channels subjected to cross channel stresses, shear bands and in front of micro cracks. The influence of stress/strain is furthermore apparent in the spatial arrangement and the shape of the TCP phase particles. Only μ-phase TCP particles were found by electron diffraction. Results of a structure-map analysis suggest that most of these TCP particles observed after LCF testing would not precipitate in thermodynamic equilibrium. In order to rationalize this effect, the atomic volume was analyzed that transition-metal (TM) elements take in unary fcc and in unary μ-phase crystal structures and found that all TM elements except Zr and V take a larger volume in a unary μ phase than in a unary fcc phase. This trend is in line with the observed localized precipitation of TCP phases that are rich in Ni and other late TM elements. The experimental and theoretical findings suggest consistently that formation of TCP particles in LCF tests is considerably influenced by the local tensile stress/strain states. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.02.022
  • 2019 • 230 Atomistic phase field chemomechanical modeling of dislocation-solute-precipitate interaction in Ni–Al–Co
    Mianroodi, J.R. and Shanthraj, P. and Kontis, P. and Cormier, J. and Gault, B. and Svendsen, B. and Raabe, D.
    Acta Materialia 175 250-261 (2019)
    Dislocation-precipitate interaction and solute segregation play important roles in controlling the mechanical behavior of Ni-based superalloys at high temperature. In particular, the increased mobility of solutes at high temperature leads to increased dislocation-solute interaction. For example, atom probe tomography (APT) results [1] for single crystal MC2 superalloy indicate significant segregation of solute elements such as Co and Cr to dislocations and stacking faults in γ′ precipitates. To gain further insight into solute segregation, dislocation-solute interaction, and its effect on the mechanical behavior in such Ni-superalloys, finite-deformation phase field chemomechanics [2] is applied in this work to develop a model for dislocation-solute-precipitate interaction in the two-phase γ-γ′ Ni-based superalloy model system Ni–Al–Co. Identification and quantification of this model is based in particular on the corresponding Ni–Al–Co embedded atom method (EAM) potential [3]. Simulation results imply both Cottrell- and Suzuki-type segregation of Co in γ and γ'. Significant segregation of Co to dislocation cores and faults in γ′ is also predicted, in agreement with APT results. Predicted as well is the drag of Co by γ dislocations entering and shearing γ'. Since solute elements such as Co generally prefer the γ phase, Co depletion in γ′ could be reversed by such dislocation drag. The resulting change in precipitate chemistry may in turn affect its stability and play a role in precipitate coarsening and rafting. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.06.008
  • 2019 • 229 Tunable carrier density and high mobility of two-dimensional hole gases on diamond: The role of oxygen adsorption and surface roughness
    Oing, D. and Geller, M. and Lorke, A. and Wöhrl, N.
    Diamond and Related Materials 97 (2019)
    The transport properties of two-dimensional hole gases (2DHGs) on chemical-vapor-deposition (CVD)-grown diamond are investigated. A hydrogen plasma treatment and exposure to ambient atmosphere are used to establish and tailor the properties of the 2DHG. The transport parameters of the 2DHGs (namely carrier density and mobility) are characterized by temperature-dependent Hall measurements. The importance of the surface oxygen adsorption, determined by X-ray photoelectron spectroscopy (XPS), on the carrier density and mobility is shown. Hall measurements reveal that for oxygen concentrations below 2.2% (relative XPS signal) the carrier density is increasing from 1.4 ∙ 1010 cm−2 to 1.5 ∙ 1013 cm−2 with increasing oxygen adsorption. For oxygen concentrations above 2.2%, the charge carrier density decreases again. The carrier density remains constant over a temperature range between 4.2 K and 325 K. At room temperature, the mobility increases with decreasing carrier concentration. The opposite behavior is observed for 4.2 K. By decreasing the surface roughness to 8.2 nm, we were able to increase the mobility to above 250 cm2/V s at room temperature for a carrier density of 1.2 ∙ 1013 cm−2. This is among the highest values reported for 2DHGs on diamond. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.diamond.2019.107450
  • 2019 • 228 DAMASK – The Düsseldorf Advanced Material Simulation Kit for modeling multi-physics crystal plasticity, thermal, and damage phenomena from the single crystal up to the component scale
    Roters, F. and Diehl, M. and Shanthraj, P. and Eisenlohr, P. and Reuber, C. and Wong, S.L. and Maiti, T. and Ebrahimi, A. and Hochrainer, T. and Fabritius, H.-O. and Nikolov, S. and Friák, M. and Fujita, N. and Grilli, N. and Jan...
    Computational Materials Science 158 420-478 (2019)
    Crystal Plasticity (CP) modeling is a powerful and well established computational materials science tool to investigate mechanical structure–property relations in crystalline materials. It has been successfully applied to study diverse micromechanical phenomena ranging from strain hardening in single crystals to texture evolution in polycrystalline aggregates. However, when considering the increasingly complex microstructural composition of modern alloys and their exposure to—often harsh—environmental conditions, the focus in materials modeling has shifted towards incorporating more constitutive and internal variable details of the process history and environmental factors into these structure–property relations. Technologically important fields of application of enhanced CP models include phase transformations, hydrogen embrittlement, irradiation damage, fracture, and recrystallization. A number of niche tools, containing multi-physics extensions of the CP method, have been developed to address such topics. Such implementations, while being very useful from a scientific standpoint, are, however, designed for specific applications and substantial efforts are required to extend them into flexible multi-purpose tools for a general end-user community. With the Düsseldorf Advanced Material Simulation Kit (DAMASK) we, therefore, undertake the effort to provide an open, flexible, and easy to use implementation to the scientific community that is highly modular and allows the use and straightforward implementation of different types of constitutive laws and numerical solvers. The internal modular structure of DAMASK follows directly from the hierarchy inherent to the employed continuum description. The highest level handles the partitioning of the prescribed field values on a material point between its underlying microstructural constituents and the subsequent homogenization of the constitutive response of each constituent. The response of each microstructural constituent is determined, at the intermediate level, from the time integration of the underlying constitutive laws for elasticity, plasticity, damage, phase transformation, and heat generation among other coupled multi-physical processes of interest. Various constitutive laws based on evolving internal state variables can be implemented to provide this response at the lowest level. DAMASK already contains various CP-based models to describe metal plasticity as well as constitutive models to incorporate additional effects such as heat production and transfer, damage evolution, and athermal transformations. Furthermore, the implementation of additional constitutive laws and homogenization schemes, as well as the integration of a wide class of suitable boundary and initial value problem solvers, is inherently considered in its modular design. © 2018 The Author(s)
    view abstractdoi: 10.1016/j.commatsci.2018.04.030
  • 2019 • 227 Synthesis and structures of gallaarsenes LGaAsGa(X)L featuring a Ga-As double bond
    Schoening, J. and John, L. and Wölper, C. and Schulz, S.
    Dalton Transactions 48 17729-17734 (2019)
    Three equivalents of LGa {L = HC[C(Me)N(2,6-i-Pr2C6H3)]2} react with AsX3 (X = Cl, Br) by insertion into two As-X bonds, followed by the elimination of LGaX2 and formation of LGaAsGa(Cl)L (1) and LGaAsGa(Br)L (2). According to single crystal X-ray analysis, 1 and 2 each exhibit one Ga-As single bond and one Ga-As double bond. The π-bonding contribution (9.71 kcal mol-11 and 9.44 kcal mol-12) was proved by variable temperature (VT) 1H NMR spectroscopy, while the electronic structure of 1′ was studied by quantum chemical calculations. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9dt03998h
  • 2019 • 226 Synthesis and structures of s- and p-block metal complexes containing sterically demanding pentaarylcyclopentadienyl substituents
    Schulte, Y. and Stienen, C. and Wölper, C. and Schulz, S.
    Organometallics 38 2381-2390 (2019)
    The synthesis of alkali metal salts of sterically demanding cyclopentadienyls CpBIGi-PrM (CpBIGi-Pr = Cp(4-i-Pr-Ph)5; M = Li 1, Na 2, K 3, Rb 4, Cs 5), CpBIGn-BuM (CpBIGn-Bu = Cp(4-n-Bu-Ph)5; M = Li 6, Na 7, K 8, Rb 9, Cs 10), and CpBIGt-BuM (CpBIGt-Bu = Cp(4-t-Bu-Ph)5; M = Li 11, Na 12, K 13, Rb 14, Cs 15) and their complete characterization including IR and heteronuclear (1H, 13C, 7Li, 23Na, 87Rb, 133Cs) NMR spectroscopy are reported. In addition, the solid-state structures of 5, 10, 11, and 13 were determined by single-crystal X-ray diffraction, revealing the formation of infinite one-dimensional chain structures in the solid state (5, 10) or solvent-separated ion pairs (11, 13). Salt elimination reactions of CpBIGt-BuK 13 with ECl3 yielded the monosubstituted cyclopentadienyl compounds CpBIGt-BuECl2 (E = P 16, As 17, Sb 18, Bi 19), which were characterized by elemental analysis, IR, and heteronuclear (1H, 13C, 31P) NMR spectroscopy and single-crystal X-ray diffraction (17). © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.9b00203
  • 2019 • 225 Role of Electron-Phonon Coupling in the Thermal Evolution of Bulk Rashba-Like Spin-Split Lead Halide Perovskites Exhibiting Dual-Band Photoluminescence
    Steele, J.A. and Puech, P. and Monserrat, B. and Wu, B. and Yang, R.X. and Kirchartz, T. and Yuan, H. and Fleury, G. and Giovanni, D. and Fron, E. and Keshavarz, M. and Debroye, E. and Zhou, G. and Sum, T.C. and Walsh, A. and Hofk...
    ACS Energy Letters 4 2205-2212 (2019)
    The optoelectronic properties of lead halide perovskites strongly depend on their underlying crystal symmetries and dynamics, sometimes exhibiting a dual photoluminescence (PL) emission via Rashba-like effects. Here we exploit spin- and temperature-dependent PL to study single-crystal APbBr3 (A = Cs and methylammonium; CH3NH3) and evaluate the peak energy, intensity, and line width evolutions of their dual emission. Both perovskites exhibit temperature trends governed by two temperature regimes - above and below approximately 100 K - which impose different carrier scattering and radiative recombination dynamics. With increasing temperature, high-energy optical phonons activate near 100 K to drive energy splitting of the dual bands and induce line width broadening via electron-phonon coupling, with a stronger coupling constant inferred for carriers recombining by the spin-split indirect bands, compared to the direct ones. We find that the unusual thermal evolutions of all-inorganic and hybrid bulk lead bromide perovskites are comparable, suggesting A-site independence and the dominance of dynamic effects, and are best understood within a framework that accounts for Rashba-like effects. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsenergylett.9b01427
  • 2019 • 224 Ni-base superalloy single crystal (SX) mosaicity characterized by the Rotation Vector Base Line Electron Back Scatter Diffraction (RVB-EBSD) method
    Thome, P. and Medghalchi, S. and Frenzel, J. and Schreuer, J. and Eggeler, G.
    Ultramicroscopy 206 (2019)
    In the present work we present the Rotation Vector Base Line Electron Back Scatter Diffraction (RVB-EBSD) method, a new correlative orientation imaging method for scanning electron microscopy (OIM/SEM). The RVB-EBSD method was developed to study crystal mosaicity in as-cast Ni-base superalloy single crystals (SX). The technique allows to quantify small crystallographic deviation angles between individual dendrites and to interpret associated accommodation processes in terms of geometrically necessary dislocations (GNDs). The RVB-EBSD method was inspired by previous seminal approaches which use cross correlation EBSD procedures. It applies Gaussian band pass filtering to improve the quality of more than 500 000 experimental patterns. A rotation vector approximation and a correction procedure, which relies on a base line function, are used. The method moreover features a novel way of intuitive color coding which allows to easily appreciate essential features of crystal mosaicity. The present work describes the key elements of the method and shows examples which demonstrate its potential. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2019.112817
  • 2019 • 223 Three-dimensional character of the Fermi surface in ultrathin LaTiO3/SrTiO3 heterostructures
    Veit, M.J. and Chan, M.K. and Ramshaw, B.J. and Arras, R. and Pentcheva, R. and Suzuki, Y.
    Physical Review B 99 (2019)
    LaTiO3 films on SrTiO3 single crystal substrates exhibit metallic behavior attributed to the LaTiO3 film, the interface as well as part of the SrTiO3. In the limit of ultrathin LaTiO3 films on SrTiO3, the contribution to the metallicity from strain-induced electronic structure modification of the LaTiO3 film is minimized so that the dominant contribution to metallicity is from the interface and part of the SrTiO3 due to charge transfer of 3d electrons from LaTiO3 to SrTiO3. In such a limit, we observe quantum oscillations whose angular dependence indicates a three-dimensional Fermi surface. Such angular dependence is observed in two sets of quantum oscillations - one low frequency and one high frequency - that we have attributed to an inner and outer Fermi surface associated with a Rashba-like spin split hybridized dxz+yz band. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.115126
  • 2019 • 222 Superior cyclic life of thermal barrier coatings with advanced bond coats on single-crystal superalloys
    Vorkötter, C. and Mack, D.E. and Guillon, O. and Vaßen, R.
    Surface and Coatings Technology 361 150-158 (2019)
    Advanced thermal barrier coatings are essential to further increase the efficiency of gas turbine engines. One limiting factor of the TBC lifetime is the temperature dependent formation of the thermally grown oxide (TGO) during thermal exposure resulting in critical stress levels at the top coat-bond coat interface. Oxide dispersion strengthened (ODS) bond coats demonstrated slower oxygen scale growth during thermal exposure in comparison to standard bond coats. In this study for the first time TBC samples on single-crystal substrates (comparable to CMSX4) with thin ODS Co-based flash coats on the same Co-based bond coat (Amdry 995) and a porous atmospherically plasma sprayed (APS) yttria stabilized zirconia (YSZ) topcoat were manufactured by thermal spray techniques and evaluated with respect to their thermal cyclic behavior. As the major performance test cyclic burner rigs, which can establish thermal conditions similar to those in gas turbines, were applied. TBC samples with the new material combination show superior performance compared to previous samples. Cross sections of the samples were analyzed by scanning electron and laser scanning microscopy. Lifetime data and failure mode of the samples are discussed with respect to material properties such as thermal expansion coefficients, microstructural changes and TGO growth. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2019.01.001
  • 2018 • 221 An N-Heterocyclic Carbene Based Silver Precursor for Plasma-Enhanced Spatial Atomic Layer Deposition of Silver Thin Films at Atmospheric Pressure
    Boysen, N. and Hasselmann, T. and Karle, S. and Rogalla, D. and Theirich, D. and Winter, M. and Riedl, T. and Devi, A.
    Angewandte Chemie - International Edition 57 16224-16227 (2018)
    A new N-heterocyclic carbene (NHC)-based silver amide compound, 1,3-di-tert-butyl-imidazolin-2-ylidene silver(I) 1,1,1-trimethyl-N-(trimethylsilyl)silanaminide [(NHC)Ag(hmds)] was synthesized and analyzed by single-crystal X-ray diffraction, 1H and 13C NMR spectroscopy, as well as EI mass spectrometry, and subsequently evaluated for its thermal characteristics. This new halogen- and phosphine-free Ag atomic layer deposition (ALD) precursor was tested successfully for silver thin film growth in atmospheric pressure plasma enhanced spatial (APP-ALD). High-purity conductive Ag thin films with a low sheet resistance of 0.9 Ω/sq (resistivity: 10−5 Ωcm) were deposited at 100 °C and characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, optical transmittance, and Rutherford back-scattering techniques. The carbene-based Ag precursor and the new APP-ALD process are significant developments in the field of precursor chemistry as well as metal ALD processing. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201808586
  • 2018 • 220 Mechanical properties of sprayed overlayers on superalloy substrates, obtained via indentation testing
    Campbell, J.E. and Kalfhaus, T. and Vassen, R. and Thompson, R.P. and Dean, J. and Clyne, T.W.
    Acta Materialia 154 237-245 (2018)
    This paper concerns the use of a recently-developed methodology for inferring stress-strain curves from indentation data, based on iterative FEM simulation of the procedure. A relatively large indenter (2 mm diameter) is used, with deep penetration (to about 25% of the indenter radius). This has been carried out on (polished) free surfaces of sprayed superalloy overlayers on single crystal superalloy substrates. Both load-displacement data and residual indent profiles were obtained, with the overlayers being in two different conditions (as-sprayed and annealed). The overlayers were relatively thick (∼2.5 mm), so it was also possible to carry out uniaxial compression tests on them (in the through-thickness direction). The inferred stress-strain curves were similar in each case when derived from load-displacement data and indent profiles, and also close to the plots obtained by conventional uniaxial testing. The yield stress levels in both cases were around 1000 MPa, but the work hardening rate was significantly higher for the as-sprayed material. This kind of information is of considerable potential value when attempting to optimize the properties of such overlayers. The procedure can be employed, with some confidence, to cases for which uniaxial testing is difficult or impossible. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.05.043
  • 2018 • 219 How evolving multiaxial stress states affect the kinetics of rafting during creep of single crystal Ni-base superalloys
    Cao, L. and Wollgramm, P. and Bürger, D. and Kostka, A. and Cailletaud, G. and Eggeler, G.
    Acta Materialia 158 381-392 (2018)
    Miniature tensile creep specimens are used to investigate the effect of mild circular notches on microstructural evolution during [001] tensile creep of a Ni-base single crystal superalloy. Creep deformed material states from a uniaxial (950 °C, uniaxial stress: 300 MPa) and a circular notched creep specimen (950 °C, net section stress in notch root: 300 MPa) are compared. For both types of tests, creep experiments were interrupted after 81, 169 and 306 h. Quantitative scanning electron microscopy (SEM) is used to assess the evolution of the γ/γ′-microstructure from rafting to topological inversion. Scanning transmission electron microscopy (STEM) was applied to study the evolution of dislocation densities during creep. As a striking new result it is shown that in circular notched specimen, the microstructural evolution is well coupled to the kinetics of the stress redistribution during creep. Rafting, the directional coarsening of the γ′-phase, and the increase of γ-channel dislocation density, start in the notch root before the center of the specimen is affected. When stresses in the circular notched specimens are fully redistributed, the microstructural differences between the notch root and the center of the circular notched specimen disappear. The comparison of the mechanical data and the microstructural findings in uniaxial and circular notched specimens contribute to a better understanding of the role of mild notches, of stress multiaxiality and of strain accumulation in the microstructure evolution of single crystal Ni-base superalloys during creep. The results obtained in the present work are discussed in the light of previous work published in the literature. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.07.061
  • 2018 • 218 Laminate-based modelling of single and polycrystalline ferroelectric materials – application to tetragonal barium titanate
    Dusthakar, D.K. and Menzel, A. and Svendsen, B.
    Mechanics of Materials 117 235-254 (2018)
    The present contribution deals with the development of a laminate-based model designed to study the single and polycrystalline tetragonal ferroelectric material behaviour. Laminate-based models are micromechanically motivated and consider the volume fraction of the distinct ferroelectric variants directly in their formulation. At first, a single crystal laminate-based model is established by considering the average strain and polarisation compatibility conditions. A suitable thermodynamic electric Gibbs energy and a rate-dependent dissipation equation are postulated to capture the dissipative hysteretic material response. The update of the inequality constrained volume fractions is solved by adopting a Fischer–Burmeister-type algorithm in combination with a Newton–Raphson scheme. Following the single crystal formulation, a homogenisation procedure based on random orientation of the individual grains in a polycrystalline aggregate is considered. The material properties and the polarisation switching response of the randomly oriented individual grains are averaged using a finite element framework in order to study the macroscopic polycrystalline behaviour. A parameter fitting procedure based on experimental data for single crystalline response, taken from the literature, is detailed and the material model as well as the algorithmic scheme are verified by solving representative boundary value problems. Moreover, the finite element based simulation results are compared with newly generated experimental hysteresis data for a barium titanate piezoceramic. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.mechmat.2017.10.005
  • 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 A phenomenological creep model for nickel-base single crystal superalloys at intermediate temperatures
    Gao, S. and Wollgramm, P. and Eggeler, G. and Ma, A. and Schreuer, J. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 26 (2018)
    For the purpose of good reproduction and prediction of creep deformation of nickel-base single crystal superalloys at intermediate temperatures, a phenomenological creep model is developed, which accounts for the typical γ/γ′ microstructure and the individual thermally activated elementary deformation processes in different phases. The internal stresses from γ/γ′ lattice mismatch and deformation heterogeneity are introduced through an efficient method. The strain hardening, the Orowan stress, the softening effect due to dislocation climb along γ/γ′ interfaces and the formation of dislocation ribbons, and the Kear-Wilsdorf-lock effect as key factors in the main flow rules are formulated properly. By taking the cube slip in slip systems and twinning mechanisms into account, the creep behavior for [110] and [111] loading directions are well captured. Without specific interaction and evolution of dislocations, the simulations of this model achieve a good agreement with experimental creep results and reproduce temperature, stress and crystallographic orientation dependences. It can also be used as the constitutive relation at material points in finite element calculations with complex boundary conditions in various components of superalloys to predict creep behavior and local stress distributions. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/aabdbe
  • 2018 • 215 Synthesis and solid-state structures of t-Bu3Ga–EPh3 Lewis acid–base adducts
    Heckel, A. and Bendt, G. and John, L. and Wölper, C. and Schulz, S.
    Applied Organometallic Chemistry 32 (2018)
    Three Lewis acid–base adducts t-Bu3Ga–EPh3 (E = P 1, As 2, Sb 3) were synthesized by reactions of Ph3E and t-Bu3Ga and characterized by heteronuclear NMR (1H, 13C (31P)) and IR spectroscopy, elemental analysis and single crystal X-ray diffraction. Their structural parameters are discussed and compared to similar t-Bu3Ga adducts. The strength of the donor-acceptor interactions within 1–3 was investigated in solution by temperature-dependent 1H NMR spectroscopy and by quantum chemical calculations. Copyright © 2018 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/aoc.4430
  • 2018 • 214 Interplay of cation ordering and thermoelastic properties of spinel structure MgGa2O4
    Hirschle, C. and Schreuer, J. and Galazka, Z.
    Journal of Applied Physics 124 (2018)
    The coefficient of thermal expansion and elastic stiffnesses of spinel structure MgGa2O4 were determined from 103 K to 1673 K using dilatometry and resonant ultrasound spectroscopy. The state of cation order was investigated on specimens quenched from temperatures up to 1473 K via single-crystal X-ray diffraction. Even at room-temperature, the material is stiffer than what was expected from DFT simulations at 0 K, however, the stiffness falls within the predicted range based on the stiffness of the constituent oxides of MgGa2O4. The anisotropy of its longitudinal elastic stiffness is low, whereas there is a high anisotropy of the shear resistance compared to other cubic materials. At about 820 K-860 K, the temperature dependences of both thermal expansion and elastic properties change rapidly. Cation reordering also starts in this temperature range; the state of order is static at lower temperatures. Thus, MgGa2O4 undergoes a glass-like transition when heated above 820 K-860 K, where the state of cation order starts relaxing towards equilibrium in laboratory timescales. Landau-theory for nonconvergent cation ordering can describe the observed cation order at elevated temperatures well. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5037786
  • 2018 • 213 On the grain boundary strengthening effect of boron in γ/γ′ Cobalt-base superalloys
    Kolb, M. and Freund, L.P. and Fischer, F. and Povstugar, I. and Makineni, S.K. and Gault, B. and Raabe, D. and Müller, J. and Spiecker, E. and Neumeier, S. and Göken, M.
    Acta Materialia 145 247-254 (2018)
    Boron is an essential solute element for improving the grain boundary strength in several high temperature metallic alloys especially in Ni- and Co-base superalloys although the detailed strengthening mechanisms are still not well understood. In superalloys, boron leads to the formation of borides and precipitate depleted zones around the grain boundaries and alters the bond strength among the grains directly. In this paper, we explore in detail the role of the boron content in ternary γ/γ′ Co-9Al-9W alloys. Local as well as bulk mechanical properties were evaluated using nanoindentation and compression testing and correlated to near-atomic scale microstructure and compositions obtained from electron microscopy and atom probe tomography. The alloy variant with low B content (0.005 at.% B) reveals an increase in yield strength at room temperature and 600 °C and atom probe tomography investigations show that solute B segregates to the grain boundaries. However, in the bulk B exclusively partitions to the γ′ phase. Additionally, the γ′/γ′ grain boundaries are depleted in W and Al with the concentration locally shifted towards the γ composition forming a very thin γ layer at the γ′/γ′ grain boundaries, which supports dislocation mobility in the γ′/γ′ grain boundary region during deformation. Higher content of B (0.04 at.% B) promotes formation of W-rich borides at the grain boundaries that leads to undesirable precipitate depleted zones adjacent to these borides that decrease the strength of the alloy drastically. However, it was also found that a subsequent annealing heat treatment eliminates these detrimental zones by re-precipitating γ′ and thus elevating the strength of the alloy. This result shows that, if a precipitate depleted zone can be avoided, B significantly improves the mechanical properties of polycrystalline Co-base superalloys by strengthening the γ′ phase and by improving grain boundary cohesion. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.12.020
  • 2018 • 212 The effect of chromium and cobalt segregation at dislocations on nickel-based superalloys
    Kontis, P. and Li, Z. and Collins, D.M. and Cormier, J. and Raabe, D. and Gault, B.
    Scripta Materialia 145 76-80 (2018)
    The segregation of solutes at dislocations in a polycrystalline and a single crystal nickel-based superalloy is studied. Our observations confirm the often assumed but yet unproven diffusion along dislocations via pipe diffusion. Direct observation and quantitative, near-atomic scale segregation of chromium and cobalt at dislocations within γ' precipitates and at interfacial dislocations leading to the partial or complete dissolution of γ' precipitates at elevated temperatures is presented. Our results allow us to elucidate the physical mechanism by which pipe diffusion initiates the undesirable dissolution of γ' precipitates. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2017.10.005
  • 2018 • 211 The Role of Oxidized Carbides on Thermal-Mechanical Performance of Polycrystalline Superalloys
    Kontis, P. and Li, Z. and Segersäll, M. and Moverare, J.J. and Reed, R.C. and Raabe, D. and Gault, B.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 4236-4245 (2018)
    Oxidized MC carbides which act as main crack initiation sites in a polycrystalline superalloy under thermal-mechanical fatigue (TMF) conditions at 850 °C were studied. Microstructural observations in the TMF tested specimens were compared to findings from bulk samples exposed isothermally in air at 850 °C for 30 hours in the absence of any external applied load. Carbides were found to oxidize rapidly after exposure at 850 °C for 30 hours resulting in surface eruptions corresponding to oxidation products, from where micro-cracks initiated. Plastic deformation due to volume expansion of the often porous oxidized carbides led to high dislocation densities in the adjacent matrix as revealed by controlled electron channeling contrast imaging. The high dislocation density facilitated the dissolution kinetics of γ′ precipitates by segregation and diffusion of chromium and cobalt along the dislocations via pipe diffusion, resulting in the formation of soft recrystallized grains. Atom probe tomography revealed substantial compositional differences between the recrystallized grains and the adjacent undeformed γ matrix. Similar observations were made for the TMF tested alloy. Our observations provide new insights into the true detrimental role of oxidized MC carbides on the crack initiation performance of polycrystalline superalloys under TMF. © 2018, The Author(s).
    view abstractdoi: 10.1007/s11661-018-4709-x
  • 2018 • 210 Martensite aging in 〈0 0 1〉 oriented Co49Ni21Ga30 single crystals in tension
    Lauhoff, C. and Krooß, P. and Langenkämper, D. and Somsen, C. and Eggeler, G. and Kireeva, I. and Chumlyakov, Y.I. and Niendorf, T.
    Functional Materials Letters 11 (2018)
    Co-Ni-Ga high-temperature shape memory alloys (HT-SMAs) are well-known candidate materials for damping applications at elevated temperatures. Recent studies showed that upon heat treatment in stress-induced martensite under compressive loads transformation temperatures can be increased significantly, qualifying Co-Ni-Ga for HT-actuation. The increase in transformation temperatures is related to a change in chemical order recently validated via neutron diffraction experiments. Since SMAs show distinct tension-compression asymmetry in terms of theoretical transformation strains and bearable stresses, understanding the impact of martensite aging in tension is crucial for future applications. The current results indicate that martensite aging in tension provides for a further improvement in functional properties. © 2018 The Author(s).
    view abstractdoi: 10.1142/S1793604718500248
  • 2018 • 209 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 • 208 On the diffusive phase transformation mechanism assisted by extended dislocations during creep of a single crystal CoNi-based superalloy
    Makineni, S.K. and Kumar, A. and Lenz, M. and Kontis, P. and Meiners, T. and Zenk, C. and Zaefferer, S. and Eggeler, G. and Neumeier, S. and Spiecker, E. and Raabe, D. and Gault, B.
    Acta Materialia 155 362-371 (2018)
    We propose here a deformation-induced diffusive phase transformation mechanism occurring during shearing of γ′ ordered phase in a γ/γ′ single crystalline CoNi-based superalloy. Shearing involved the creation and motion of a high density of planar imperfections. Through correlative electron microscopy and atom probe tomography, we captured a superlattice intrinsic stacking fault (SISF) and its associated moving leading partial dislocation (LPD). The structure and composition of these imperfections reveal characteristic chemical – structural contrast. The SISF locally exhibits a D019 ordered structure coherently embedded in the L12 γ′ and enriched in W and Co. Interestingly, the LPD is enriched with Cr and Co, while the adjoining planes ahead of the LPD are enriched with Al. Quantitative analysis of the three-dimensional compositional field in the vicinity of imperfections sheds light onto a new in-plane diffusion mechanism as the LPD moves on specific {111} planes upon application of stress at high temperature. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.05.074
  • 2018 • 207 Development of Single-Crystal Ni-Base Superalloys Based on Multi-criteria Numerical Optimization and Efficient Use of Refractory Elements
    Markl, M. and Müller, A. and Ritter, N. and Hofmeister, M. and Naujoks, D. and Schaar, H. and Abrahams, K. and Frenzel, J. and Subramanyam, A.P.A. and Ludwig, Al. and Pfetzing-Micklich, J. and Hammerschmidt, T. and Drautz, R. and...
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 4134-4145 (2018)
    The development of new Ni-base superalloys with a complex composition consisting of eight or more alloying elements is a challenging task. The experimental state-of-the-art development cycle is based on the adaption of already existing compositions. Although new alloy compositions with potentially improved material properties are expected to be similar to already known superalloys, this procedure impedes efficiently finding these compositions in the large multi-dimensional design-space of all alloying elements. Modern alloy development combines numerical optimization methods with experimental validation to guide the development towards promising compositions. In this work, an improved numerical multi-criteria optimization tool using CALPHAD calculations and semi-empirical models for alloy development is presented. The model improvements to its predecessor are described and the successful application for the development of rhenium-free single-crystal Ni-base superalloys ERBO/13 and ERBO/15 is revisited. The optimization tool is described and the designed alloys are discussed regarding phase stability. Finally, a possible phase stability model extending the optimization tool and improving the alloy composition predictions is presented. © 2018, The Author(s).
    view abstractdoi: 10.1007/s11661-018-4759-0
  • 2018 • 206 Effect of substrate orientation on local magnetoelectric coupling in bi-layered multiferroic thin films
    Naveed-Ul-Haq, M. and Webers, S. and Trivedi, H. and Salamon, S. and Wende, H. and Usman, M. and Mumtaz, A. and Shvartsman, V.V. and Lupascu, D.C.
    Nanoscale 10 20618-20627 (2018)
    In this study we explore the prospect of strain-mediated magnetoelectric coupling in CoFe2O4-BaTiO3 bi-layers as a function of different interfacial boundary conditions. Pulsed laser deposition fabricated thin films on Nb:SrTiO3(100) and Nb:SrTiO3(111) single crystal substrates were characterized in terms of their peculiarities related to the structure-property relationship. Despite the homogeneous phase formation in both films, transmission electron microscopy showed that the bi-layers on Nb:SrTiO3(100) exhibit a higher number of crystallographic defects when compared to the films on Nb:SrTiO3(111). This signifies an intrinsic relationship of the defects and the substrate orientation. To analyze the consequences of these defects on the overall magnetoelectric coupling of the bi-layered films, piezoresponse force microscopy was performed in situ with an applied magnetic field. The local magnetic field dependence of the piezoresponse was obtained using principal component analysis. A detailed analysis of this dependence led to a conclusion that the bi-layers on Nb:SrTiO3(111) exhibit better strain-transfer characteristics between the magnetic and the piezoelectric layer than those which were deposited on Nb:SrTiO3(100). These strain transfer characteristics correlate well with the interface quality and the defect concentration. This study suggests that in terms of overall magnetoelectric coupling, the Nb:SrTiO3(111) grown bi-layers are expected to outperform their Nb:SrTiO3(100) grown counterparts. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8nr06041j
  • 2018 • 205 Deposition routes of Cs2AgBiBr6 double perovskites for photovoltaic applications
    Pantaler, M. and Fettkenhauer, C. and Nguyen, H.L. and Anusca, I. and Lupascu, D.C.
    MRS Advances 3 1819-1823 (2018)
    The lead free double perovskite Cs2AgBiBr6 is an upcoming alternative to lead based perovskites as absorber material in perovskite solar cells. So far, the majority of investigations on this interesting material have focused on polycrystalline powders and single crystals. We present vapor and solution based approaches for the preparation of Cs2AgBiBr6 thin films. Sequential vapor deposition processes starting from different precursors are shown and their weaknesses are discussed. Single source evaporation of Cs2AgBiBr6 and sequential deposition of Cs3Bi2Br9 and AgBr result in the formation of the double perovskite phase. Additionally, we show the possibility of the preparation of planar Cs2AgBiBr6 thin films by spin coating. © 2018 Materials Research Society.
    view abstractdoi: 10.1557/adv.2018.151
  • 2018 • 204 Crystallographic characterization of laser-generated, polymer-stabilized 4 nm silver-gold alloyed nanoparticles
    Prymak, O. and Jakobi, J. and Rehbock, C. and Epple, M. and Barcikowski, S.
    Materials Chemistry and Physics 207 442-450 (2018)
    Monometallic silver and gold nanoparticles and bimetallic silver-gold (AgAu) nanoparticles were prepared by laser ablation in liquids in the atomic composition range of Ag:Au from 0:100 to 100:0 with steps of 10 at% and colloidally stabilized with poly(N-vinylpyrrolidone) (PVP). As metallic bulk targets for laser ablation, pure silver, pure gold, and alloyed AgAu foils with the desired composition were used. Size separation by centrifugation and freeze-drying gave monodisperse spherical nanoparticles with a diameter of 4 nm as determined by differential centrifugal sedimentation (DCS) and transmission electron microscopy (TEM). A crystallographic characterization of the nanoparticles was carried out by X-ray powder diffraction (XRD) and Rietveld refinement, leading to highly precise cubic lattice parameters (fcc crystal system) and crystallite sizes. For comparison, the same analysis including the determination of the microstrain was carried out for the bulk target materials (AgAu alloys in the full concentration range). Both nanoparticles and bulk target materials obeyed Vegard's rule, with only slight deviations. The fact that the crystallite size as determined by XRD was identical to the hydrodynamic diameter by DCS and the Feret diameter by TEM indicates that the particles consist of only one domain, i.e. they are single crystals. The combination of UV-vis spectroscopy with energy-dispersive X-ray spectroscopy (EDX) as line scan along the nanoparticle showed a homogenous distribution of the gold and silver inside the nanoparticles, indicating solid solution alloys, in contrast to what was observed earlier for chemically prepared AgAu nanoparticles by reduction of metal ions in water. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2017.12.080
  • 2018 • 203 A TEM Investigation of Columnar-Structured Thermal Barrier Coatings Deposited by Plasma Spray-Physical Vapor Deposition (PS-PVD)
    Rezanka, S. and Somsen, C. and Eggeler, G. and Mauer, G. and Vaßen, R. and Guillon, O.
    Plasma Chemistry and Plasma Processing 38 791-802 (2018)
    The plasma spray-physical vapor deposition technique (PS-PVD) is used to deposit various types of ceramic coatings. Due to the low operating pressure and high enthalpy transfer to the feedstock, deposition from the vapor phase is very effective. The particular process conditions allow for the deposition of columnar microstructures when applying thermal barrier coatings (TBCs). These coatings show a high strain tolerance similar to those obtained by electron beam-physical vapor deposition (EB-PVD). But compared to EB-PVD, PS-PVD allows significantly reducing process time and costs. The application-related properties of PS-PVD TBCs have been investigated in earlier work, where the high potential of the process was described and where the good resistance to thermo-mechanical loading conditions was reported. But until now, the elementary mechanisms which govern the material deposition have not been fully understood and it is not clear, how the columnar structure is built up. Shadowing effects and diffusion processes are assumed to contribute to the formation of columnar microstructures in classical PVD processing routes. For such structures, crystallographic textures are characteristic. For PS-PVD, however, no crystallographic textures could initially be found using X-ray diffraction. In this work a more detailed TEM investigations and further XRD measurements of the columnar PS-PVD microstructure were performed. The smallest build units of the columnar TBC structure are referred to as sub-columns. The observed semi-single crystal structure of individual sub-columns was analyzed by means of diffraction experiments. The absence of texture in PS-PVD coatings is confirmed and elementary nucleation and growth mechanisms are discussed. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstractdoi: 10.1007/s11090-018-9898-y
  • 2018 • 202 Rejuvenation of Single-Crystal Ni-Base Superalloy Turbine Blades: Unlimited Service Life?
    Ruttert, B. and Horst, O. and Lopez-galilea, I. and Langenkämper, D. and Kostka, A. and Somsen, C. and Goerler, J.V. and Ali, M.A. and Shchyglo, O. and Steinbach, I. and Eggeler, G. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 1-12 (2018)
    Rejuvenation of the initially hot isostatic pressing (HIP) heat-treated single-crystal Ni-base superalloy (SX) ERBO/1 was examined experimentally and via phase field simulation to establish rejuvenation treatments as a cost-effective alternative for another interval of service life. Creep was performed at 950 °C and 350 MPa, and the specimens were crept to 0.6 pct (creep rate minimum) or 2 pct strain, respectively. A slight coarsening of the γ/γ′ microstructure was observed experimentally and via simulation at 0.6 pct and rafting at 2 pct strain. The damaged microstructures were rejuvenated in a novel hot isostatic press that provides fast quenching rates before the same specimens were recrept under the same initial creep conditions. High-resolution microscopy proves that the rejuvenation re-establishes the original γ/γ′ microstructure in the dendrite core of the precrept specimens (0.6 and 2 pct). However, the interdendritic areas of the 2 pct precrept and rejuvenated specimen still contain elongated γ′ particles enwrapped by interfacial dislocation networks that survived the applied rejuvenation. The subsequent experimental and simulated creep tests after rejuvenation demonstrated that the creep behavior is only reproducible by the proposed rejuvenation for specimens that had crept until the end of the primary creep regime. © 2018 The Minerals, Metals & Materials Society and ASM International
    view abstractdoi: 10.1007/s11661-018-4745-6
  • 2018 • 201 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 • 200 A General Route to Metal-Substituted Dipnictenes of the Type [L(X)M]2E2
    Tuscher, L. and Helling, C. and Wölper, C. and Frank, W. and Nizovtsev, A.S. and Schulz, S.
    Chemistry - A European Journal (2018)
    Two equivalents of LGa (L=HC[C(Me)N(2,6-iPr2C6H3)]2) reacted with PX3 (X=Cl, Br) with insertion into two P-X bonds and formation of [L(X)Ga]2PX (X=Cl 1, Br 2), whereas the analogous reaction with AsCl3 occurred with twofold insertion and subsequent elimination of LGaCl2 and formation of the Ga-substituted diarsene [L(Cl)Ga]2As2 (3). Analogous findings were observed in the reactions with Me2NAsCl2, yielding the unsymmetrically-substituted diarsene [L(Cl)Ga]As=As[Ga(NMe2)L] (4). The reaction of As(NMe2)3 with LGa gave [L(Me2N)Ga]2As2 (5) after heating at 165°C for five days, whereas the reaction with LAl gave [L(Me2N)Al]2As2 (6) after heating at only 80°C for one day. Finally, two equivalents of LGa reacted with Bi(NEt2)3 to give [L(Et2N)Ga]2Bi2 (7). Complexes 1-7 were characterized by NMR spectroscopy (1H, 13C, 31P), elemental analysis, and single-crystal X-ray diffraction (except for 1 and 5). The bonding situations in 4, 6, and 7 were analyzed by quantum chemical calculations. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201705233
  • 2018 • 199 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 • 198 Coupled effect of crystallographic orientation and indenter geometry on nanoindentation of single crystalline copper
    Wang, Z. and Zhang, J. and Hassan, H.U. and Zhang, J. and Yan, Y. and Hartmaier, A. and Sun, T.
    International Journal of Mechanical Sciences 148 531-539 (2018)
    Surface pile-up topography is very significant for property extraction in nanoindentation tests. In the present work, we perform crystal plasticity finite element simulations of Berkovich nanoindentation of single crystalline copper with different crystallographic orientations, which derive quantitatively comparable mechanical properties and surface pile-up topographies with experimental data. Simulation results demonstrate that there is a coupled effect of crystallographic orientation of indented material and indenter geometry on surface pile-up behavior, due to the interaction between intrinsic dislocation slip events and extrinsic discrete stress distribution patterns. Based on the relative spatial orientation between crystallographic orientation of indented material and indenter geometry, a surface pile-up density factor mp is proposed to qualitatively characterize the propensity of surface pile-up behavior in nanoindentation tests of single crystalline copper. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijmecsci.2018.09.007
  • 2018 • 197 In situ TEM observation of rebonding on fractured silicon carbide
    Zhang, Z. and Cui, J. and Wang, B. and Jiang, H. and Chen, G. and Yu, J. and Lin, C. and Tang, C. and Hartmaier, A. and Zhang, J. and Luo, J. and Rosenkranz, A. and Jiang, N. and Guo, D.
    Nanoscale 10 6261-6269 (2018)
    Silicon carbide (SiC) is widely used in harsh environments and under extreme conditions, including at high-power, higherature, high-current, high-voltage and high-frequency. The rebonding and self-matching of stack faults (SFs) is highly desirable to avoid catastrophic failure for SiC devices, especially for specific applications in the aerospace and nuclear power industries. In this study, a novel approach was developed using an eyebrow hair to pick up and transfer nanowires (NWs), in order to obtain in situ transmission electron microscope (TEM) images of the rebonding and self-matching of SFs at atomic resolution. During rebonding and healing, the electron beam was shut off. Rebonding on the fractured surfaces of monocrystalline and amorphous SiC NWs was observed by in situ TEM at room temperature. The fracture strength was 1.7 GPa after crack-healing, restoring 12.9% of that of a single crystal NW. Partial recrystallization along the <111> orientation and the self-matching of SFs are responsible for the rebonding of the monocrystalline NW. In comparison, the fracture strengths were 6.7 and 5.5 GPa for the first and second rebonding, respectively recovering 67% and 55% of that of an amorphous NW. Atomic diffusion contributed enormously to the rebonding on fractured surfaces of an amorphous NW, resulting in a healed surface consisting of an amorphous phase and crystallites. This rebonding function provides new insight into the fabrication of high-performance SiC devices for the aerospace, optoelectronic and semiconductor industries. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8nr00341f
  • 2017 • 196 Weak ferromagnetism and short range polar order in NaMnF3 thin films
    KC, A. and Borisov, P. and Shvartsman, V.V. and Lederman, D.
    Applied Physics Letters 110 (2017)
    The orthorhombically distorted perovskite NaMnF3 has been predicted to become ferroelectric if an a = c distortion of the bulk Pnma structure is imposed. In order to test this prediction, NaMnF3 thin films were grown on SrTiO3 (001) single crystal substrates via molecular beam epitaxy. The best films were smooth and single phase with four different twin domains. In-plane magnetization measurements revealed the presence of antiferromagnetic ordering with weak ferromagnetism below the Néel temperature TN = 66 K. For the dielectric studies, NaMnF3 films were grown on a 30 nm SrRuO3 (001) layer used as a bottom electrode grown via pulsed laser deposition. The complex permittivity as a function of frequency indicated a strong Debye-like relaxation contribution characterized by a distribution of relaxation times. A power-law divergence of the characteristic relaxation time revealed an order-disorder phase transition at 8 K. The slow relaxation dynamics indicated the formation of super-dipoles (superparaelectric moments) that extend over several unit cells, similar to polar nanoregions of relaxor ferroelectrics. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4977421
  • 2017 • 195 Transmission electron microscopy study of the microstructural evolution during higherature and low-stress (011) [11] shear creep deformation of the superalloy single crystal LEK 94
    Agudo Jácome, L. and Göbenli, G. and Eggeler, G.
    Journal of Materials Research 32 4491-4502 (2017)
    The present work describes the shear creep behavior of the superalloy LEK 94 at temperatures between 980 and 1050 °C and shear stresses between 50 and 140 MPa for loading on the macroscopic crystallographic shear system (MCSS) (011) . The strain rate versus strain curves show short primary and extended secondary creep regimes. We find an apparent activation energy for creep of Q app = 466 kJ/mol and a Norton-law stress exponent of n = 6. With scanning transmission electron microscopy, we characterize three material states that differ in temperature, applied stress, and accumulated strain/time. Rafting develops perpendicular to the maximum principal stress direction, γ channels fill with dislocations, superdislocations cut γ′ particles, and dislocation networks form at γ/γ′ interfaces. Our findings are in agreement with previous results for higherature and low-stress [001] and [110] tensile creep testing, and for shear creep testing of the superalloys CMSX-4 and CMSX-6 on the MCSSs (111) and (001)[100]. The parameters that characterize the evolving γ/γ′ microstructure and the evolving dislocation substructures depend on creep temperature, stress, strain, and time. © 2017 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2017.336
  • 2017 • 194 Dielectric Response: Answer to Many Questions in the Methylammonium Lead Halide Solar Cell Absorbers
    Anusca, I. and Balčiunas, S. and Gemeiner, P. and Svirskas, S. and Sanlialp, M. and Lackner, G. and Fettkenhauer, C. and Belovickis, J. and Samulionis, V. and Ivanov, M. and Dkhil, B. and Banys, J. and Shvartsman, V.V. and Lupascu, D.C.
    Advanced Energy Materials (2017)
    Due to the unprecedented rapid increase of their power conversion efficiency, hybrid organic-inorganic perovskites CH3NH3PbX3 (X = I, Br, Cl) can potentially revolutionize the world of solar cells. However, despite tremendous research activity, the origin of the exceptionally large diffusion length of their photogenerated charge carriers, that is, their low recombination rate, remains elusive. Using frequency and temperature-dependent dielectric measurements across the entire frequency spectrum, it is shown that the dielectric constant conserves very high values (&gt;27) for frequencies below 1 THz in all three halides. This efficiently prevents photocarrier trapping and their recombination owing to the strong screening of charged entities. By combining ultrasonic and Raman spectroscopy with dielectric analysis, similarly large contributions to the dielectric constant are attributed to the dipolar disorder of the CH3NH3 + cations as well as lattice dynamics in the gigahertz range yielding dielectric constants of εstat = 62 for the iodide, 58 for the bromide, and about 45 for the chloride below 1 GHz at room temperature. Disorder continuously reduces for decreasing temperature. Dipole dynamics prevail in the intermediate tetragonal phase. The low-temperature orthorhombic state is antipolar. No indications of ferroelectricity are found. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/aenm.201700600
  • 2017 • 193 Syntheses and Structures of Homo- and Heteroleptic Beryllium Complexes Containing N,N'-Chelating Ligands
    Bayram, M. and Naglav, D. and Wölper, C. and Schulz, S.
    Organometallics 36 467-473 (2017)
    Reactions of BeEt2 with S(NSiMe3)2 and carbodiimides RNCNR yielded homoleptic bisdiimidosulfinate [EtS(NSiMe3)2]2Be (1) and amidinate complexes [EtC(NAr)2]2Be (Ar = 2,6-i-Pr2-C6H3 2, SiMe3 3). In addition, the heteroleptic amidinate [t-BuC(NAr)2]BeEt (4) and β-diketiminate [HC(CMeNAr')2]Be(i-Bu) (6) (Ar' = 2,4,6-Me3C6H2) complexes were obtained by alkane elimination reactions of BeEt2 with t-BuC(NSiMe3)N(H)SiMe3 and Be(i-Bu)2 with β-diketimine [Ar'N(H)C(Me)CHC(Me)NAr'], respectively. In addition, 4 was found to react with dry oxygen with formation of the corresponding alkoxide [t-BuC(NAr)2]BeOEt (5). 1-6 were characterized by multinuclear NMR (1H, 9Be, 13C) and IR spectroscopy as well as by single-crystal X-ray diffraction (1, 2, 4, 5, 6). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.6b00865
  • 2017 • 192 Coupled Crystal Plasticity–Phase Field Fracture Simulation Study on Damage Evolution Around a Void: Pore Shape Versus Crystallographic Orientation
    Diehl, M. and Wicke, M. and Shanthraj, P. and Roters, F. and Brueckner-Foit, A. and Raabe, D.
    JOM 69 872-878 (2017)
    Various mechanisms such as anisotropic plastic flow, damage nucleation, and crack propagation govern the overall mechanical response of structural materials. Understanding how these mechanisms interact, i.e. if they amplify mutually or compete with each other, is an essential prerequisite for the design of improved alloys. This study shows—by using the free and open source software DAMASK (the Düsseldorf Advanced Material Simulation Kit)—how the coupling of crystal plasticity and phase field fracture methods can increase the understanding of the complex interplay between crystallographic orientation and the geometry of a void. To this end, crack initiation and propagation around an experimentally obtained pore with complex shape is investigated and compared to the situation of a simplified spherical void. Three different crystallographic orientations of the aluminum matrix hosting the defects are considered. It is shown that crack initiation and propagation depend in a non-trivial way on crystallographic orientation and its associated plastic behavior as well as on the shape of the pore. © 2017, The Author(s).
    view abstractdoi: 10.1007/s11837-017-2308-8
  • 2017 • 191 Crystal plasticity study on stress and strain partitioning in a measured 3D dual phase steel microstructure
    Diehl, M. and An, D. and Shanthraj, P. and Zaefferer, S. and Roters, F. and Raabe, D.
    Physical Mesomechanics 20 311-323 (2017)
    Dual phase steels are advanced high strength alloys typically used for structural parts and reinforcements in car bodies. Their good combination of strength and ductility and their lean composition render them an economically competitive option for realizing multiple lightweight design options in automotive engineering. The mechanical response of dual phase steels is the result of the strain and stress partitioning among the ferritic and martensitic phases and the individual crystallographic grains and subgrains of these phases. Therefore, understanding how these microstructural features influence the global and local mechanical properties is of utmost importance for the design of improved dual phase steel grades. While multiple corresponding simulation studies have been dedicated to the investigation of dual phase steel micromechanics, numerical tools and experiment techniques for characterizing and simulating real 3D microstructures of such complex materials have been emerged only recently. Here we present a crystal plasticity simulation study based on a 3D dual phase microstructure which is obtained by EBdD tomography, also referred to as 3D EBdD (EBdD—electron backscatter diffraction). In the present case we utilized a 3D EBdD serial sectioning approach based on mechanical polishing. Moreover, sections of the 3D microstructure are used as 2D models to study the effect of this simplification on the stress and strain distribution. The simulations are conducted using a phenomenological crystal plasticity model and a spectral method approach implemented in the Düsseldorf Advanced Material Simulation Kit (DAMAdK). © 2017, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S1029959917030079
  • 2017 • 190 Reduction of [Cp*Sb]4 with Subvalent Main-Group Metal Reductants: Syntheses and Structures of [(L1Mg)4(Sb4)] and [(L2Ga)2(Sb4)] Containing Edge-Missing Sb4 Units
    Ganesamoorthy, C. and Krüger, J. and Wölper, C. and Nizovtsev, A.S. and Schulz, S.
    Chemistry - A European Journal 23 2461-2468 (2017)
    [Cp*Sb]4 (Cp*=C5Me5) reacts with [L1Mg]2 and L2Ga with formation of [(L1Mg)4(μ4,η1:2:2:2-Sb4)] (L1=iPr2NC[N(2,6-iPr2C6H3)]2, 1) and [(L2Ga)2(μ,η2:2-Sb4)] (L2=HC[C(Me)N(2,6-iPr2C6H3)]2, 2). The cleavage of the Sb−Sb and Sb−C bonds in [Cp*Sb]4 are the crucial steps in both reactions. The formation of 1 occurred by elimination of the Cp* anion and formation of Cp*MgL1, while 2 was formed by reductive elimination of Cp*2 and oxidative addition of L2Ga to the Sb4 unit. 1 and 2 were characterized by heteronuclear NMR spectroscopy and single-crystal X-ray diffraction, and their bonding situation was studied by quantum chemical calculations. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201605547
  • 2017 • 189 Syntheses and structures of N,C,N-stabilized antimony chalcogenides
    Ganesamoorthy, C. and Wölper, C. and Dostál, L. and Schulz, S.
    Journal of Organometallic Chemistry 845 38-43 (2017)
    The oxidation reactions of ArSb [Ar = 2,6-(HC=N-t-Bu)2C6H3] with S8, grey Se and Te as well as E2Ph2 (E = S, Se, Te) are demonstrated. The reactions of ArSb with elemental sulfur and selenium occurred at elevated temperatures and yielded ArSbE (E = S, 1; Se 2), whereas the reactions with E2Ph2 proceeded at room temperature with subsequent formation of the corresponding insertion complexes ArSb(EPh)2 (E = S 3; Se 4). In addition, ArSb(TePh)2 (5) was formed at very low temperature and showed a temperature-dependent reversible equilibrium with ArSb and Te2Ph2 between -80 °C and 20 °C. The formation and structure of compounds 1-4, which were isolated in good yields, are assigned through multinuclear NMR (1H, 13C, 77Se), IR spectroscopy and microanalyses data. In addition, the molecular structures of 2-4 are further confirmed by single crystal X-ray diffraction studies. © 2017.
    view abstractdoi: 10.1016/j.jorganchem.2017.01.007
  • 2017 • 188 3D discrete dislocation dynamics study of creep behavior in Ni-base single crystal superalloys by a combined dislocation climb and vacancy diffusion model
    Gao, S. and Fivel, M. and Ma, A. and Hartmaier, A.
    Journal of the Mechanics and Physics of Solids 102 209-223 (2017)
    A three-dimensional (3D) discrete dislocation dynamics (DDD) creep model is developed to investigate creep behavior under uniaxial tensile stress along the crystallographic [001] direction in Ni-base single crystal superalloys, which takes explicitly account of dislocation glide, climb and vacancy diffusion, but neglects phase transformation like rafting of γ′ precipitates. The vacancy diffusion model takes internal stresses by dislocations and mismatch strains into account and it is coupled to the dislocation dynamics model in a numerically efficient way. This model is helpful for understanding the fundamental creep mechanisms in superalloys and clarifying the effects of dislocation glide and climb on creep deformation. In cases where the precipitate cutting rarely occurs, e.g. due to the high anti-phase boundary energy and the lack of superdislocations, the dislocation glide in the γ matrix and the dislocation climb along the γ/γ′ interface dominate plastic deformation. The simulation results show that a high temperature or a high stress both promote dislocation motion and multiplication, so as to cause a large creep strain. Dislocation climb accelerated by high temperature only produces a small plastic strain, but relaxes the hardening caused by the filling γ channels and lets dislocations further glide and multiply. The strongest variation of vacancy concentration occurs in the horizontal channels, where more mixed dislocations exit and tend to climb. The increasing internal stresses due to the increasing dislocation density are easily overcome by dislocations under a high external stress that leads to a long-term dislocation glide accompanied by multiplication. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2017.02.010
  • 2017 • 187 New amidinate complexes of indium(III): Promising CVD precursors for transparent and conductive In2O3 thin films
    Gebhard, M. and Hellwig, M. and Kroll, A. and Rogalla, D. and Winter, M. and Mallick, B. and Ludwig, Ar. and Wiesing, M. and Wieck, A.D. and Grundmeier, G. and Devi, A.
    Dalton Transactions 46 10220-10231 (2017)
    For the first time, synthesis of two new amidinate-ligand comprising heteroleptic indium complexes, namely [InCl(amd)2] (1) and [InMe(amd)2] (2), via salt-metathesis and their detailed characterization is reported. For comparison, the earlier reported homoleptic tris-amidinate [In(amd)3] (3) was also synthesized and analyzed in detail especially with respect to the thermal properties and molecular crystal structure analysis which are reported here for the first time. From nuclear magnetic resonance spectroscopy (NMR) and single-crystal X-ray diffraction (XRD), all three compounds were found to be monomeric with C2 (compound 1 and 2) and C3 symmetry (compound 3). Both halide-free compounds 2 and 3 were evaluated regarding their thermal properties using temperature-dependent 1H-NMR, thermogravimetric analysis (TGA) and iso-TGA, revealing suitable volatility and thermal stability for their application as potential precursors for chemical vapor phase thin film deposition methods. Indeed, metalorganic chemical vapor deposition (MOCVD) experiments over a broad temperature range (400 °C-700 °C) revealed the suitability of these two compounds to fabricate In2O3 thin films in the presence of oxygen on Si, thermally grown SiO2 and fused silica substrates. The as-deposited thin films were characterized in terms of their crystallinity via X-ray diffraction (XRD), morphology by scanning electron microscopy (SEM) and composition through complementary techniques such as Rutherford-backscattering spectrometry (RBS) in combination with nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS). From UV/Vis spectroscopy, the deposited In2O3 thin films on fused silica substrates were found to be highly transparent (T &gt; 95% at 560 nm, compound 3). In addition, Hall measurements revealed high charge carrier densities of 1.8 × 1020 cm-3 (2) and 6.5 × 1019 cm-3 (3) with a Hall-mobility of 48 cm2 V-1 s-1 (2) and 74 cm2 V-1 s-1 (3) for the respective thin films, rendering the obtained thin films applicable as a transparent conducting oxide that could be suitable for optoelectronic applications. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7dt01280b
  • 2017 • 186 Topological phase inversion after long-term thermal exposure of nickel-base superalloys: Experiment and phase-field simulation
    Goerler, J.V. and Lopez-Galilea, I. and Mujica Roncery, L. and Shchyglo, O. and Theisen, W. and Steinbach, I.
    Acta Materialia 124 151-158 (2017)
    Ni-base superalloys are materials which are designed to resist extreme thermal and mechanical conditions. In this regard, an essential factor is their microstructure consisting of γ′ precipitates embedded in a γ matrix. The application of superalloys at high temperatures can however induce the topological phase inversion, where the γ′-phase topologically becomes the matrix phase, resulting in subpar material properties. In this work, the topological inversion is analyzed via experiment and phase-field simulation. The evolution of the microstructure has been quantified in the second generation single crystal Ni-base superalloy ERBO/1, which belongs to the family of CMSX-4, submitted to long-term aging at 1100° C for up to 250 h. Phase-field simulations carried out using a multi phase-field approach deliver insight into the microstructure evolution driven by the loss of coherency of the γ′ precipitates, which is induced by the accumulation of dislocations at the γ/γ′ interfaces. The obtained simulation results are in good agreement with the experimental results, and indicate that the mechanisms causing the topological inversion are linked to the accommodation of the lattice misfit, which enables coalescence and ripening of γ′ precipitates. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.10.059
  • 2017 • 185 The shear instability energy: A new parameter for materials design?
    Kanani, M. and Hartmaier, A. and Janisch, R.
    Modelling and Simulation in Materials Science and Engineering 25 (2017)
    Reliable and predictive relationships between fundamental microstructural material properties and observable macroscopic mechanical behaviour are needed for the successful design of new materials. In this study we establish a link between physical properties that are defined on the atomic level and the deformation mechanisms of slip planes and interfaces that govern the mechanical behaviour of a metallic material. To accomplish this, the shear instability energy Γ is introduced, which can be determined via quantum mechanical ab initio calculations or other atomistic methods. The concept is based on a multilayer generalised stacking fault energy calculation and can be applied to distinguish the different shear deformation mechanisms occurring at TiAl interfaces during finite-temperature molecular dynamics simulations. We use the new parameter Γ to construct a deformation mechanism map for different interfaces occurring in this intermetallic. Furthermore, Γ can be used to convert the results of ab initio density functional theory calculations into those obtained with an embedded atom method type potential for TiAl. We propose to include this new physical parameter into material databases to apply it for the design of materials and microstructures, which so far mainly relies on single-crystal values for the unstable and stable stacking fault energy. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/aa865a
  • 2017 • 184 Swift heavy ion track formation in SrTiO3 and TiO2 under random, channeling and near-channeling conditions
    Karlusic, M. and Jaksic, M. and Lebius, H. and Ban-d'Etat, B. and Wilhelm, R. A. and Heller, R. and Schleberger, M.
    Journal of Physics D-applied Physics 50 205302 (2017)
    Conditions for ion track formation in single crystal SrTiO3 and TiO2 (rutile) after irradiations using swift heavy ion beams with specific energies below 1 MeV/amu were investigated in this work. Rutherford backscattering spectroscopy in channeling was used to measure ion tracks in the bulk, while atomic force microscopy was used for observation of ion tracks on the surfaces. Variations in the ion track sizes and respective thresholds were observed after irradiations under random, channeling and near-channeling conditions close to normal incidence. These variations are attributed to the specifics of the electronic stopping power of swift heavy ions under the investigated conditions. In the case of ion channeling, electronic stopping power is reduced and observed ion tracks are smaller. The opposite was found under the near-channeling conditions when lowering of the ion track formation threshold was observed. We attribute this finding to the oscillating electronic stopping power with large peak values. For both materials, thresholds for bulk and surface ion track formation were found to be surprisingly close, around 10 keV nm(-1). Obtained results are compared with predictions of the analytical thermal spike model.
    view abstractdoi: 10.1088/1361-6463/aa678c
  • 2017 • 183 Swift heavy ion track formation in SrTiO3 and TiO2 under random, channeling and near-channeling conditions
    Karlušić, M. and Jakšić, M. and Lebius, H. and Ban-D'Etat, B. and Wilhelm, R.A. and Heller, R. and Schleberger, M.
    Journal of Physics D: Applied Physics 50 (2017)
    Conditions for ion track formation in single crystal SrTiO3 and TiO2 (rutile) after irradiations using swift heavy ion beams with specific energies below 1 MeV/amu were investigated in this work. Rutherford backscattering spectroscopy in channeling was used to measure ion tracks in the bulk, while atomic force microscopy was used for observation of ion tracks on the surfaces. Variations in the ion track sizes and respective thresholds were observed after irradiations under random, channeling and near-channeling conditions close to normal incidence. These variations are attributed to the specifics of the electronic stopping power of swift heavy ions under the investigated conditions. In the case of ion channeling, electronic stopping power is reduced and observed ion tracks are smaller. The opposite was found under the near-channeling conditions when lowering of the ion track formation threshold was observed. We attribute this finding to the oscillating electronic stopping power with large peak values. For both materials, thresholds for bulk and surface ion track formation were found to be surprisingly close, around 10 keV nm-1. Obtained results are compared with predictions of the analytical thermal spike model. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa678c
  • 2017 • 182 Mössbauer spectroscopy evidence of intrinsic non-stoichiometry in iron telluride single crystals
    Kiiamov, A.G. and Lysogorskiy, Y.V. and Vagizov, F.G. and Tagirov, L.R. and Tayurskii, D.A. and Croitori, D. and Tsurkan, V. and Loidl, A.
    Annalen der Physik 529 (2017)
    The FeTe parent compound for iron-superconductor chalcogenides was studied applying Mössbauer spectroscopy accompanied by ab initio calculations of electric field gradients at the iron nuclei. Room-temperature (RT) Mössbauer spectra of single crystals have shown asymmetric doublet structure commonly ascribed to contributions of over-stoichiometric iron or impurity phases. Low-temperature Mössbauer spectra of the magnetically ordered compound could be well described by four hyperfine-split sextets, although no other foreign phases different from Fe1.05Te were detected by XRD and microanalysis within the sensitivity limits of the equipment. Density functional ab initio calculations have shown that over-stoichiometric iron atoms significantly affect electron charge and spin density up to the second coordination sphere of the iron sub-lattice, and, as a result, four non-equivalent groups of iron atoms are formed by their local environment. The resulting four-group model consistently describes the angular dependence of the single crystals Mössbauer spectra as well as intensity asymmetry of the doublet absorption lines in powdered samples at RT. We suppose that our approach could be extended to the entire class of Fe1+ySe1-xTex compounds, which contain excess iron atoms. (Figure presented.). © 2016 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/andp.201600241
  • 2017 • 181 Molecular Engineering of MnII Diamine Diketonate Precursors for the Vapor Deposition of Manganese Oxide Nanostructures
    Maccato, C. and Bigiani, L. and Carraro, G. and Gasparotto, A. and Seraglia, R. and Kim, J. and Devi, A. and Tabacchi, G. and Fois, E. and Pace, G. and Di Noto, V. and Barreca, D.
    Chemistry - A European Journal 23 17954-17963 (2017)
    Molecular engineering of manganese(II) diamine diketonate precursors is a key issue for their use in the vapor deposition of manganese oxide materials. Herein, two closely related β-diketonate diamine MnII adducts with different fluorine contents in the diketonate ligands are examined. The target compounds were synthesized by a simple procedure and, for the first time, thoroughly characterized by a joint experimental–theoretical approach, to understand the influence of the ligand on their structures, electronic properties, thermal behavior, and reactivity. The target compounds are monomeric and exhibit a pseudo-octahedral coordination of the MnII centers, with differences in their structure and fragmentation processes related to the ligand nature. Both complexes can be readily vaporized without premature side decompositions, a favorable feature for their use as precursors for chemical vapor deposition (CVD) or atomic layer deposition applications. Preliminary CVD experiments at moderate growth temperatures enabled the fabrication of high-purity, single-phase Mn3O4 nanosystems with tailored morphology, which hold great promise for various technological applications. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201703423
  • 2017 • 180 Dislocation-twin boundary interaction in small scale Cu bi-crystals loaded in different crystallographic directions
    Malyar, N.V. and Micha, J.-S. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 129 91-97 (2017)
    The mechanical behavior of several 5×5 micron sized Cu bi-crystals with a single coherent Σ3{111} twin boundary (TB) is studied by in situ Laue microdiffraction (μLaue) compression with the aim to unravel the slip transfer mechanisms through TBs. Single crystalline pillars (SCP) are additionally tested and used as reference samples. Engineering stress-strain curves and post mortem scanning electron microscopy (SEM) images were correlated to the local evolution of the TB angle, the storage of geometrically necessary dislocations and crystal orientations investigated by in situ X-ray Laue microdiffraction (μLaue). Both μLaue and post mortem SEM demonstrate multiple transmission events through the TB without significant storage of geometrically necessary dislocations in the crystals or at the boundary, independent on the compression direction. Nevertheless, at engineering strains larger than 5% a small dislocation pile-up was once observed temporarily at the boundary. Upper and lower bounds for the transmission stress are discussed based on the current experimental results. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.02.067
  • 2017 • 179 Strain rate dependence of the slip transfer through a penetrable high angle grain boundary in copper
    Malyar, N.V. and Dehm, G. and Kirchlechner, C.
    Scripta Materialia 138 88-91 (2017)
    Micro pillar compression is used to analyze the strain rate dependence of copper pillars containing a penetrable high-angle grain boundary via in situ compression tests at strain rates ranging from  10− 1 to 10− 4 s− 1. While the grain-boundary containing pillars exhibit a clear strain-rate dependence of m = 0.04 ± 0.02, their single crystal counterparts seem to have a weak strain rate dependence of m = 0.01 ± 0.01. The results strongly suggest that the movement of the dislocation line in the grain boundary, required to change its orientation from the incoming to the outgoing slip plane, is the critical process in deforming this kind of grain-boundary containing pillars. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2017.05.042
  • 2017 • 178 In-situ tensile testing of notched poly-and oligocrystalline 316L wires
    Mitevski, B. and Weis, S. and Fischer, A.
    Materialpruefung/Materials Testing 59 130-135 (2017)
    In-situ testing inside a scanning electron microscope is a helpful tool for detailed analyses of small sized specimens with respect to their mechanical properties and the correlated microstructural alterations. Thus, this test method is used to analyze the tensional properties of thin 316L (1.4441) wires used for microscale components, e. g., like coronary artery stents. Tensile tests were conducted on unnotched and circularly notched 316L wires (∅ 0.95 mm) with a special focus on the number of grains within the cross section as well as the notch geometry. Four combinations of notch width (2 and 4 mm) and notch depth (diameter at notch root: 0.5 and 0.75 mm) were chosen. Notch depth and notch shape were adjusted by means of electrochemical polishing. Previous investigations showed, that oligocrystalline structures exhibit a different mechanical behavior compared to polycrystalline ones or single crystals. There are only a few data available on mechanical testing of oligocrystalline structures with respect to varying notch geometries. Depending on the notch geometry, grain size and, therefore, the number of grains within the notch cross section widely scattering yield-and tensile strength as well as failure elongation values were measured. However, the transition criterion between poly-and oligocrystalline behavior could be quantified to be 6 to 7 grains within the cross section. © Carl Hanser Verlag GmbH &Co. KG.
    view abstractdoi: 10.3139/120.110973
  • 2017 • 177 In-situ tensile testing of notched poly- and oligocrystalline 316L wires
    Mitevski, Bojan and Weiss, Sabine and Fischer, Alfons
    Materials Testing 59 130--135 (2017)
    In-situ testing inside a scanning electron microscope is a helpful tool for detailed analyses of small sized specimens with respect to their mechanical properties and the correlated microstructural alterations. Thus, this test method is used to analyze the tensional properties of thin 316L (1.4441) wires used for microscale components, e. g., like coronary artery stents. Tensile tests were conducted on unnotched and circularly notched 316L wires (phi 0.95 mm) with a special focus on the number of grains within the cross section as well as the notch geometry. Four combinations of notch width (2 and 4 mm) and notch depth (diameter at notch root: 0.5 and 0.75 mm) were chosen. Notch depth and notch shape were adjusted by means of electrochemical polishing. Previous investigations showed, that oligocrystalline structures exhibit a different mechanical behavior compared to polycrystalline ones or single crystals. There are only a few data available on mechanical testing of oligocrystalline structures with respect to varying notch geometries. Depending on the notch geometry, grain size and, therefore, the number of grains within the notch cross section widely scattering yield-and tensile strength as well as failure elongation values were measured. However, the transition criterion between poly-and oligocrystalline behavior could be quantified to be 6 to 7 grains within the cross section.
    view abstractdoi: 10.3139/120.110973
  • 2017 • 176 Structure-property relationships in hydrogen-bonded liquid crystals
    Pfletscher, M. and Hölscher, S. and Wölper, C. and Mezger, M. and Giese, M.
    Chemistry of Materials 29 8462-8471 (2017)
    The structural impact of hydrogen-donating moieties on the liquid crystalline behavior of hydrogen-bonded assemblies (HBAs) is comprehensively investigated. Therefore, a series of phenol derivatives such as phenol, catechol (CA), resorcinol (RE), hydroquinone (HQ), pyrogallol, hydroxyhydroquinone, and phloroglucinol (PHG) were combined with alkoxyazopyridines (Ap-N) yielding 49 new HBAs, which were studied with respect to their mesomorphic properties. The present study revealed significant differences in the liquid crystalline behavior of the structurally diverse assemblies, ranging from the absence of a mesophase to smectic or nematic phases. In contrast to previous studies a comprehensive crystallographic analysis provides insight into the structure-property relationships of the assemblies and proves a correlation between the supramolecular architecture and the macroscopic properties (=liquid crystallinity). More specifically, comparison of the single crystal data with the 2D X-ray diffraction patterns indicates that linear assemblies tend to form crystalline or smectic phases (for the HQ and RE, respectively), while a bent-shaped assembly yields nematic phases (for CA and PHG). Furthermore, our results suggest that segregation of aliphatic and aromatic segments, as observed in the solid state structures, supports the formation of stable mesophases. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.7b03182
  • 2017 • 175 Elasto-viscoplastic phase field modelling of anisotropic cleavage fracture
    Shanthraj, P. and Svendsen, B. and Sharma, L. and Roters, F. and Raabe, D.
    Journal of the Mechanics and Physics of Solids 99 19-34 (2017)
    A finite-strain anisotropic phase field method is developed to model the localisation of damage on a defined family of crystallographic planes, characteristic of cleavage fracture in metals. The approach is based on the introduction of an undamaged configuration, and the inelastic deformation gradient mapping this configuration to a damaged configuration is microstructurally represented by the opening of a set of cleavage planes in the three fracture modes. Crack opening is modelled as a dissipative process, and its evolution is thermodynamically derived. To couple this approach with a physically-based phase field method for brittle fracture, a scalar measure of the overall local damage is introduced, whose evolution is determined by the crack opening rates, and weakly coupled with the non-local phase field energy representing the crack opening resistance in the classical sense of Griffith. A finite-element implementation of the proposed model is employed to simulate the crack propagation path in a laminate and a polycrystalline microstructure. As shown in this work, it is able to predict the localisation of damage on the set of pre-defined cleavage planes, as well as the kinking and branching of the crack resulting from the crystallographic misorientation across the laminate boundary and the grain boundaries respectively. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2016.10.012
  • 2017 • 174 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
  • 2016 • 173 Verification of redox-processes as switching and retention failure mechanisms in Nb:SrTiO3/metal devices
    Baeumer, C. and Raab, N. and Menke, T. and Schmitz, C. and Rosezin, R. and Müller, P. and Andrä, M. and Feyer, V. and Bruchhaus, R. and Gunkel, F. and Schneider, C.M. and Waser, R. and Dittmann, R.
    Nanoscale 8 13967-13975 (2016)
    Nanoscale redox reactions in transition metal oxides are believed to be the physical foundation of memristive devices, which present a highly scalable, low-power alternative for future non-volatile memory devices. The interface between noble metal top electrodes and Nb-doped SrTiO3 single crystals may serve as a prominent but not yet well-understood example of such memristive devices. In this report, we will present experimental evidence that nanoscale redox reactions and the associated valence change mechanism are indeed responsible for the resistance change in noble metal/Nb-doped SrTiO3 junctions with dimensions ranging from the micrometer scale down to the nanometer regime. Direct verification of the valence change mechanism is given by spectromicroscopic characterization of switching filaments. Furthermore, it is found that the resistance change over time is driven by the reoxidation of a previously oxygen-deficient region. The retention times of the low resistance states, accordingly, can be dramatically improved under vacuum conditions as well as through the insertion of a thin Al2O3 layer which prevents this reoxidation. These insights finally confirm the resistive switching mechanism at these interfaces and are therefore of significant importance for the study and application of memristive devices based on Nb-doped SrTiO3 as well as systems with similar switching mechanisms. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6nr00824k
  • 2016 • 172 Synthesis and Structure of Bis(diphenylphosphinimino)methanide and Bis(diphenylphosphinimino)methanediide Beryllium Complexes
    Bayram, M. and Naglav, D. and Wölper, C. and Schulz, S.
    Organometallics 35 2378-2383 (2016)
    Reactions of BeEt2 with bis(diphenylphosphinimino)methanes H2C[PPh2NR]2 yielded the first bis(diphenylphosphinimino)methanide and bis(diphenylphosphinimino)methanediide beryllium complexes [CH(PPh2N-2,6-i-Pr2C6H3)2]BeEt (1), [C(PPh2NSiMe3)2](BeEt)2 (2), and [C(PPh2NPh)2](BeEt)2 (3), respectively. 1-3 were characterized by multinuclear NMR and IR spectroscopy as well as single-crystal X-ray diffraction. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.6b00380
  • 2016 • 171 Unraveling the temperature dependence of the yield strength in single-crystal tungsten using atomistically-informed crystal plasticity calculations
    Cereceda, D. and Diehl, M. and Roters, F. and Raabe, D. and Perlado, J.M. and Marian, J.
    International Journal of Plasticity 78 242-265 (2016)
    We use a physically-based crystal plasticity model to predict the yield strength of body-centered cubic (bcc) tungsten single crystals subjected to uniaxial loading. Our model captures the thermally-activated character of screw dislocation motion and full non-Schmid effects, both of which are known to play critical roles in bcc plasticity. The model uses atomistic calculations as the sole source of constitutive information, with no parameter fitting of any kind to experimental data. Our results are in excellent agreement with experimental measurements of the yield stress as a function of temperature for a number of loading orientations. The validated methodology is employed to calculate the temperature and strain-rate dependence of the yield strength for 231 crystallographic orientations within the standard stereographic triangle. We extract the strain-rate sensitivity of W crystals at different temperatures, and finish with the calculation of yield surfaces under biaxial loading conditions that can be used to define effective yield criteria for engineering design models. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijplas.2015.09.002
  • 2016 • 170 PKU-20: A new silicogermanate constructed from sti and asv layers
    Chen, Y. and Su, J. and Huang, S. and Liang, J. and Lin, X. and Liao, F. and Sun, J. and Wang, Y. and Lin, J. and Gies, H.
    Microporous and Mesoporous Materials 224 384-391 (2016)
    A new silicogermanate (PKU-20) was hydrothermally synthesized using triethylisopropylammonium cation as the structure directing agent in the presence of fluoride. Its structure was determined from a combination of synchrotron single crystal X-ray diffraction and powder X-ray diffraction data. PKU-20 crystallizes in the monoclinic space group C2/m, with the lattice parameters of a = 18.5901(6) Å, b = 13.9118 (4) Å, c = 22.2614 (7) Å and β = 100.1514 (12)°. The framework of PKU-20 is constructed from an alternate stacking of sti and asv layers. The sti layer is exactly the same as that in the STI framework,while the asv layer is a new layer sliced off from the ASV framework parallel to the (112) plane. The take-out scheme of the layer is discussed on the basis of a composite building unit "D4R-lau-D4R". PKU-20 possesses a two-dimensional channel system, where the 10-ring channels parallel to the [010] direction are intercrossed by 12-ring pockets along the [101] direction. © 2016 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2015.12.052
  • 2016 • 169 Pseudomorphic Transformation of Organometal Halide Perovskite Using the Gaseous Hydrogen Halide Reaction
    Chen, K. and Deng, X. and Goddard, R. and Tüysüz, H.
    Chemistry of Materials 28 5530-5537 (2016)
    Halide exchange is a facile method of adjusting the band gap and optimizing the performance of organometal halide perovskite. During the halide exchange processes, preserving the crystallinity and morphology of highly crystalline materials will be desirable for preparing novel materials with outstanding performance. In this study, the gasous hydrogen halides were used as reactants for halide exchange processes. The mutual conversions among three halides for condense films were realized. Moreover, perovskite inverse opals and perovskite single crystals were also adopted as substrates to illustrate the morphology preservation and crystallinity preservation, respectively. Powder X-ray diffraction and UV-vis diffuse reflectance spectra demonstrated the segregation when smaller ions were substituted by larger ions. Scanning electron microscopy displayed the direct evidence for morphology preservation during the transformation. For the first time, single crystal X-ray diffraction confirmed the single-crystal-to-single-crystal transformation from bromide to chloride analogy, which demonstrated that the presented method can preserve the crystalline framework of large-sized perovskite during the halide exchange. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.6b02233
  • 2016 • 168 Lewis acid-base adducts of group 13 elements: Synthesis, structure and reactivity toward benzaldehyde
    Ganesamoorthy, C. and Matthias, M. and Bläser, D. and Wölper, C. and Schulz, S.
    Dalton Transactions 45 11437-11444 (2016)
    Lewis acid-base adducts [LGa-M(C6F5)3] (M = B 1, Al 2, Ga 3) were prepared by the reaction of gallanediyl LGa {L = HC[C(Me)N(2,6-i-Pr2C6H3)]2} with the Lewis acids M(C6F5)3 (M = B, Al, Ga). Benzaldehyde reacts with [LGa-M(C6F5)3] (M = B 1, Al 2) at room temperature with the insertion and formation of [LGa(C6F5){CH(Ph)(OB(C6F5)2)}] (4) and the zwitterionic species [LGa(C6F5){CH(Ph)(OAl(C6F5)2)}] (5), respectively, which was found to decompose at 80 °C with the formation of {(C6F5)2Al(OCH2Ph)}2 (6). Any attempts to isolate the insertion complex of [LGa-Ga(C6F5)3] with benzaldehyde failed and only {(C6F5)2Ga(OCH2Ph)}2 (7) was isolated at elevated temperatures. 2-5 and 7 were structurally characterized by heteronuclear NMR spectroscopy and single crystal X-ray diffraction. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c6dt01688j
  • 2016 • 167 On the rich magnetic phase diagram of (Ni, Co)-Mn-Sn Heusler alloys
    Grünebohm, A. and Herper, H.C. and Entel, P.
    Journal of Physics D: Applied Physics 49 (2016)
    We put a spotlight on the exceptional magnetic properties of the metamagnetic Heusler alloy (Ni, Co)-Mn-Sn by means of first principles simulations. In the energy landscape we find a multitude of local minima, which belong to different ferrimagnetic states and are close in total magnetization and energy. All these magnetic states correspond to the local high spin state of the Mn atoms with different spin alignments and are related to the magnetic properties of Mn. Compared to pure Mn, the magneto-volume coupling is reduced by Ni, Co and Sn atoms in the lattice and no local low-spin Mn states appear. For the cubic phase we find a ferromagnetic ground state whereas the global energy minimum is a tetragonal state with a complicated spin structure and vanishing magnetization which so far has been overlooked in simulations. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/39/395001
  • 2016 • 166 Epitaxial growth of thermally stable cobalt films on Au(111)
    Haag, N. and Laux, M. and Stöckl, J. and Kollamana, J. and Seidel, J. and Großmann, N. and Fetzer, R. and Kelly, L.L. and Wei, Z. and Stadtmüller, B. and Cinchetti, M. and Aeschlimann, M.
    18 (2016)
    Ferromagnetic thin films play a fundamental role in spintronic applications as a source for spin polarized carriers and in fundamental studies as ferromagnetic substrates. However, it is challenging to produce such metallic films with high structural quality and chemical purity on single crystalline substrates since the diffusion barrier across the metal-metal interface is usually smaller than the thermal activation energy necessary for smooth surface morphologies. Here, we introduce epitaxial thin Co films grown on an Au(111) single crystal surface as a thermally stable ferromagnetic thin film. Our structural investigations reveal an identical growth of thin Co/Au(111) films compared to Co bulk single crystals with large monoatomic Co terraces with an average width of 500 Å, formed after thermal annealing at 575 K. Combining our results from photoemission and Auger electron spectroscopy, we provide evidence that no significant diffusion of Au into the near surface region of the Co film takes place for this temperature and that no Au capping layer is formed on top of Co films. Furthermore, we show that the electronic valence band is dominated by a strong spectral contribution from a Co 3d band and a Co derived surface resonance in the minority band. Both states lead to an overall negative spin polarization at the Fermi energy. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/18/10/103054
  • 2016 • 165 On the Crystallography of Silver Nanoparticles with Different Shapes
    Helmlinger, J. and Prymak, O. and Loza, K. and Gocyla, M. and Heggen, M. and Epple, M.
    Crystal Growth and Design 16 3677-3687 (2016)
    The crystallographic properties of silver nanoparticles with different morphologies (two different kinds of spheres, cubes, platelets, and rods) were derived from X-ray powder diffraction and electron microscopy. The size of the metallic particle core was determined by scanning electron microscopy, and the colloidal stability and the hydrodynamic particle diameter were analyzed by dynamic light scattering. The preferred crystallographic orientation (texture) as obtained by X-ray powder diffraction, including pole figure analysis, and high resolution transmission electron microscopy showed the crystallographic nature of the spheres (almost no texture), the cubes (terminated by {100} faces), the platelets (terminated by {111} faces), and the rods (grown from pentagonal twins along [110] and terminated by {100} faces). The crystallite size was determined by Rietveld refinement of X-ray powder diffraction data and agreed well with the transmission electron microscopic data. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.6b00178
  • 2016 • 164 Surface analysis of the Heusler Ni49.7Mn29.1Ga21.2 Alloy: The composition, phase transition, and twinned microstructure of martensite
    Horáková, K. and Cháb, V. and Heczko, O. and Drchal, V. and Fekete, L. and Honolka, J. and Kopeček, J. and Kudrnovský, J. and Polyak, Y. and Sajdl, P. and Vondráček, M. and Lančok, J. and Feyer, V. and Wiemann, C. and Schn...
    Journal of Applied Physics 120 (2016)
    Surface analysis was used to study the dynamics of the martensitic transformation on macro- and mesoscopic scales. The chemical state, morphology, and magnetic and surface structure were monitored at particular stages of the phase transition. At room temperature, the martensitic phase of the Ni49.7Mn29.1Ga21.2 (100) single crystal exhibited macroscopic a/c twinning and a corresponding magnetic domain structure characterized by magnetization vector in and out of the surface plane. Induced by radiation heating, the transformation from martensite to austenite takes place separately at the surface and in the bulk. Its dynamics depend on the history of the sample treatment which affects the crystallographic orientation of twins and minor changes of the surface stoichiometry. The interfaces (twin planes) between twin variants in the martensitic phase were noticeable also in the austenitic phase, thanks to the shape memory effect of this material. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4962648
  • 2016 • 163 Microscale Fracture Behavior of Single Crystal Silicon Beams at Elevated Temperatures
    Jaya, B.N. and Wheeler, J.M. and Wehrs, J. and Best, J.P. and Soler, R. and Michler, J. and Kirchlechner, C. and Dehm, G.
    Nano Letters 16 7597-7603 (2016)
    The micromechanical fracture behavior of Si [100] was investigated as a function of temperature in the scanning electron microscope with a nanoindenter. A gradual increase in KC was observed with temperature, in contrast to sharp transitions reported earlier for macro-Si. A transition in cracking mechanism via crack branching occurs at ∼300 °C accompanied by multiple load drops. This reveals that onset of small-scale plasticity plays an important role in the brittle-to-ductile transition of miniaturized Si. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b03461
  • 2016 • 162 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 • 161 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 • 160 The effect of silicon-substrate orientation on the local piezoelectric characteristics of LiNbO3 films
    Kiselev, D.A. and Zhukov, R.N. and Ksenich, S.V. and Kubasov, I.V. and Temirov, A.A. and Timushkin, N.G. and Bykov, A.S. and Malinkovich, M.D. and Shvartsman, V.V. and Lupascu, D.C. and Parkhomenko, Y.N.
    Journal of Surface Investigation 10 742-747 (2016)
    The domain structure of lithium-niobate thin films grown on Si(111) and Si(100) substrates coated with a native oxide layer with a thickness of no less than 2 nm is investigated by X-ray diffraction, scanning electron microscopy and piezoresponse force microscopy. The films are synthesized by the rf magnetron sputtering of a single-crystal lithium-niobate target. A high degree of grain orientation in the polycrystalline films is demonstrated. The piezoelectric coefficients dzz of the lithium-niobate films on Si(111) and Si(100) substrates are calculated from the measured dependences of the amplitude of the piezoresponse signal on the ac voltage applied between the cantilever tip and the substrate. Piezoelectric hysteresis loops are obtained in the remanent piezoelectric response regime © 2016, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S1027451016040091
  • 2016 • 159 Synthesis of Bi2Te3 and (Bi: XSb1- x)2Te3 nanoparticles using the novel IL [C4mim]3[Bi3I12]
    Loor, M. and Bendt, G. and Hagemann, U. and Wölper, C. and Assenmacher, W. and Schulz, S.
    Dalton Transactions 45 15326-15335 (2016)
    The novel Bi-containing reactive ionic liquid [C4mim]3[Bi3I12], which was synthesized in quantitative yield by equimolar reaction of BiI3 and [C4mim]I, was used as a novel Bi-source for the ionothermal synthesis of Bi2Te3 nanoparticles by reaction with (Et3Si)2Te in the ionic liquid [C4mim]I. The solid state structure of [C4mim]3[Bi3I12] was determined by single crystal X-ray diffraction. In addition, the ionothermal synthesis of the single source precursor (Et2Sb)2Te and [C4mim]3[Bi3I12] yielded the ternary (BixSb1-x)2Te3 (x = 0.25, 0.5, 0.75) nanoparticles. The chemical composition and phase purity of the tetradymite-type materials were determined by EDX and XRD and the surface composition of the nanoparticles was further investigated by IR and XPS. In addition, the morphology of the nanoparticles was investigated by SEM and TEM. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6dt02361d
  • 2016 • 158 The thermal stability of topologically close-packed phases in the single-crystal Ni-base superalloy ERBO/1
    Lopez-Galilea, I. and Koßmann, J. and Kostka, A. and Drautz, R. and Mujica Roncery, L. and Hammerschmidt, T. and Huth, S. and Theisen, W.
    Journal of Materials Science 51 2653-2664 (2016)
    In Ni-base superalloys, the addition of refractory elements such as Cr, Mo, Co, W, and Re is necessary to increase the creep resistance. Nevertheless, these elements induce the formation of different kinds of intermetallic phases, namely, the topologically close-packed (TCP) phases. This work focuses on intermetallic phases present in the second-generation single-crystal (SX) Ni-base superalloy ERBO/1. In the as-cast condition, the typical γ/γ′ structure is accompanied by undesirable intermetallic phases located in the interdendritic regions. The nature of these precipitates as well as their thermal stability between 800 and 1200 °C has been investigated by isothermal heat treatments. The investigation techniques include DSC, SEM, EDX, and TEM. The experimental information is complemented by (1) comparison with a structure map to link the local chemical composition with phase stability, as well as (2) thermodynamic calculations based on the CALPHAD method to determine the occurrence and composition of phases during solidification and in equilibrium conditions. The TCP phases Laves, µ and σ were identified in various temperature/time ranges. © 2015, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-015-9579-7
  • 2016 • 157 Crystal plasticity study of monocrystalline stochastic honeycombs under in-plane compression
    Ma, D. and Eisenlohr, P. and Epler, E. and Volkert, C.A. and Shanthraj, P. and Diehl, M. and Roters, F. and Raabe, D.
    Acta Materialia 103 796-808 (2016)
    We present a study on the plastic deformation of single crystalline stochastic honeycombs under in-plane compression using a crystal plasticity constitutive description for face-centered cubic (fcc) materials, focusing on the very early stage of plastic deformation, and identifying the interplay between the crystallographic orientation and the cellular structure during plastic deformation. We observe that despite the stochastic structure, surprisingly, the slip system activations in the honeycombs are almost identical to their corresponding bulk single crystals at the early stage of the plastic deformation. On the other hand, however, the yield stresses of the honeycombs are nearly independent of their crystallographic orientations. Similar mechanical response is found in compression testing of nanoporous gold micro-pillars aligned with various crystallographic orientations. The macroscopic stress tensors of the honeycombs show the same anisotropy as their respective bulk single crystals. Locally, however, there is an appreciable fluctuation in the local stresses, which are even larger than for polycrystals. This explains why the Taylor/Schmid factor associated with the crystallographic orientation is less useful to estimate the yield stresses of the honeycombs than the bulk single crystals and polycrystals, and why the plastic deformation occurs at smaller strains in the honeycombs than their corresponding bulk single crystals. Besides these findings, the observations of the crystallographic reorientation suggest that conventional orientation analysis tools, such as inverse pole figure and related tools, would in general fail to study the plastic deformation mechanism of monocrystalline cellular materials. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.11.016
  • 2016 • 156 Growth, nanostructure, and optical properties of epitaxial VNX/MgO(001) (0.80 ≤ x ≤ 1.00) layers deposited by reactive magnetron sputtering
    Mei, A.B. and Tuteja, M. and Sangiovanni, D.G. and Haasch, R.T. and Rockett, A. and Hultman, L. and Petrov, I. and Greene, J.E.
    Journal of Materials Chemistry C 4 7924-7938 (2016)
    VNx/MgO(001) films, ∼300 nm thick, with x ranging from 1.00 (stoichiometric) to 0.80 are grown by magnetically-unbalanced reactive magnetron sputter deposition in mixed N2/Ar atmospheres. The combination of lattice-resolution cross-sectional electron microscopy with X-ray diffraction ω - 2θ, φ-scans, pole figures, and high resolution reciprocal space maps show that VNx layers are epitaxial single crystals which grow cube-on-cube with respect to their substrates: (001) VNx ∥(001)MgO and [100] VNx∥[100]MgO. VNx(001) relaxed lattice parameters a0(x) decrease linearly from 0.4134 (x = 1.00) to 0.4098 nm (x = 0.80), in agreement with density functional theory (DFT) calculations. Near-stoichiometric VNx layers (0.95 ≲ x ≤ 1.0) are fully relaxed during growth, while films with lower x values are partially strained as a result of increased anion vacancies impeding dislocation glide. VNx complex dielectric functions ϵ(ω) are determined between 0.7 and 4.5 eV using variable-angle spectroscopic ellipsometry and valence states are probed via ultraviolet photoelectron spectroscopy (UPS) in concert with DFT calculations. VN(001) UPS spectra exhibit a feature at binding energies ranging from the Fermi level to 3 eV, together with two peaks deeper in the valence band. These results are consistent with electronic densities of states computed by scaling Kohn-Sham electronic eigenvalues to account for many-body interactions. Imaginary VN(001) dielectric functions ϵ(ω) determined by ellipsometry also agree with theoretical values obtained within the random-phase approximation using scaled eigenvalues. Analyses of optical matrix element calculations reveal that VNx dielectric responses are controlled by the phase space for interband transitions; band-structure analyses indicate that ϵ2(ω) spectral features in the infrared-visible range arise primarily from the combination of intraband and d-d transitions, while features at higher energies result primarily from p-d interband transitions. The combined nanostructural and spectroscopic analyses establish that, surprisingly, N vacancies are essentially non-interacting in high-quality epitaxial VNx containing vacancy concentrations up to ∼1022 cm-3 (x = 0.80). © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6tc02289h
  • 2016 • 155 Single-shot mega-electronvolt ultrafast electron diffraction for structure dynamic studies of warm dense matter
    Mo, M.Z. and Shen, X. and Chen, Z. and Li, R.K. and Dunning, M. and Sokolowski-Tinten, K. and Zheng, Q. and Weathersby, S.P. and Reid, A.H. and Coffee, R. and Makasyuk, I. and Edstrom, S. and McCormick, D. and Jobe, K. and Hast, C...
    Review of Scientific Instruments 87 (2016)
    We have developed a single-shot mega-electronvolt ultrafast-electron-diffraction system to measure the structural dynamics of warm dense matter. The electron probe in this system is featured by a kinetic energy of 3.2 MeV and a total charge of 20 fC, with the FWHM pulse duration and spot size at sample of 350 fs and 120 μm respectively. We demonstrate its unique capability by visualizing the atomic structural changes of warm dense gold formed from a laser-excited 35-nm freestanding single-crystal gold foil. The temporal evolution of the Bragg peak intensity and of the liquid signal during solid-liquid phase transition are quantitatively determined. This experimental capability opens up an exciting opportunity to unravel the atomic dynamics of structural phase transitions in warm dense matter regime. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4960070
  • 2016 • 154 Nanostructured Ti-Ta thin films synthesized by combinatorial glancing angle sputter deposition
    Motemani, Y. and Khare, C. and Savan, A. and Hans, M. and Paulsen, A. and Frenzel, J. and Somsen, C. and Mücklich, F. and Eggeler, G. and Ludwig, Al.
    Nanotechnology 27 (2016)
    Ti-Ta alloys are attractive materials for applications in actuators as well as biomedical implants. When fabricated as thin films, these alloys can potentially be employed as microactuators, components for micro-implantable devices and coatings on surgical implants. In this study, Ti100-xTa x (x = 21, 30) nanocolumnar thin films are fabricated by glancing angle deposition (GLAD) at room temperature using Ti73Ta27 and Ta sputter targets. Crystal structure, morphology and microstructure of the nanostructured thin films are systematically investigated by XRD, SEM and TEM, respectively. Nanocolumns of ∼150-160 nm in width are oriented perpendicular to the substrate for both Ti79Ta21 and Ti70Ta30 compositions. The disordered α″ martensite phase with orthorhombic structure is formed in room temperature as-deposited thin films. The columns are found to be elongated small single crystals which are aligned perpendicular to the and planes of α″ martensite, indicating that the films' growth orientation is mainly dominated by these crystallographic planes. Laser pre-patterned substrates are utilized to obtain periodic nanocolumnar arrays. The differences in seed pattern, and inter-seed distances lead to growth of multi-level porous nanostructures. Using a unique sputter deposition geometry consisting of Ti73Ta27 and Ta sputter sources, a nanocolumnar Ti-Ta materials library was fabricated on a static substrate by a co-deposition process (combinatorial-GLAD approach). In this library, a composition spread developed between Ti72.8Ta27.2 and Ti64.4Ta35.6, as confirmed by high-throughput EDX analysis. The morphology over the materials library varies from well-isolated nanocolumns to fan-like nanocolumnar structures. The influence of two sputter sources is investigated by studying the resulting column angle on the materials library. The presented nanostructuring methods including the use of the GLAD technique along with pre-patterning and a combinatorial materials library fabrication strategy offer a promising technological approach for investigating Ti-Ta thin films for a range of applications. The proposed approaches can be similarly implemented for other materials systems which can benefit from the formation of a nanocolumnar morphology. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/27/49/495604
  • 2016 • 153 On the Effect of Hot Isostatic Pressing on the Creep Life of a Single Crystal Superalloys
    Mujica Roncery, L. and Lopez-Galilea, I. and Ruttert, B. and Bürger, D. and Wollgramm, P. and Eggeler, G. and Theisen, W.
    Advanced Engineering Materials 18 1381-1387 (2016)
    The creep behavior of a single-crystal Ni-base superalloy in two microstructural states is compared. One is obtained by casting followed by a conventional heat treatment. The other results from the same nominal heat treatment integrated into a hot isostatic pressing process. The microstructure after HIP differed from that in the conventional route in two respects. First, the γ′ particles are smaller and the γ channels are narrower. Second, after HIP, the number density of pores is lower and the pore sizes are smaller. The HIP microstructure improves creep in two respects: the finer γ/γ′-microstructure results in lower minimum creep rates. Moreover, the shrinkage of cast porosity during HIP delays the nucleation and growth of micro cracks and results in higher rupture strains in the low-temperature high stress regime. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201600071
  • 2016 • 152 Influence of temperature, pressure, and cooling rate during hot isostatic pressing on the microstructure of an SX Ni-base superalloy
    Mujica Roncery, L. and Lopez-Galilea, I. and Ruttert, B. and Huth, S. and Theisen, W.
    Materials and Design 97 544-552 (2016)
    This work investigates the application of hot isostatic pressing for heat treatment of the single-crystal Ni-base superalloy ERBO/1. Recent progress regarding incorporation of quenching within hot isostatic pressing enables heat treatments to be performed so that the microstructures can be frozen at a desired point. The influence of the temperature, pressure, and cooling method (quenching, natural convection, and slow cooling) as well as the cooling rate after solutioning-HIP treatment on pore densification and γ/. γ'-morphology was measured. Temperatures above γ'-solvus resulted in a greater efficiency of the porosity reduction. At super-solvus temperatures, pressures above 75. MPa are sufficient enough to annihilate the porosity. The cooling rate after HIP-solutioning treatment has a major influence on the γ'-particle size and shape. Quenching with 45-20. K/s at 100. MPa leads to high number density and monomodal distribution of γ'-particles with sizes around 140. nm. In contrast, slow cooling rate of 0.33. K/s leads to γ'-precipitate sizes of 720. nm. Moreover, an integrated heat treatment at 100. MPa, which consisted of solutioning and aging in the HIP, was successfully applied. It led to smaller γ'-particle sizes and narrower γ-channels compared to the conventionally heat-treated material and also almost no porosity. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2016.02.051
  • 2016 • 151 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 • 150 Considerations on the symmetry of pure silica ITQ-7 zeolite (ISV) derived from 29Si MAS NMR and Rietveld analysis
    Paillaud, J.-L. and Tzanis, L. and Marler, B. and Rigolet, S. and Patarin, J. and Gies, H.
    Microporous and Mesoporous Materials 219 306-310 (2016)
    Pure silica ITQ-7 zeolite of topology ISV synthesized in fluoride media possesses a 3D 12-membered-rings pore system. Originally the structure of ITQ-7 zeolite was solved and refined from a calcined sample in the highest possible space group symmetry P42/mmc. This space group leads to only 5 independent silicon T sites for this structure. However, this high symmetry was not supported by solid state 29Si MAS NMR spectroscopy. In this short communication, we show from a revised 29Si solid state MAS NMR and Rietveld analyses, that lowering of the space group symmetry from P42/mmc to P42 consolidates structure model and experimental data, in particular, the number of non equivalent crystallographic T sites passing from 5 to 16, is in agreement with 29Si solid state NMR spectroscopy. © 2015 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2015.07.002
  • 2016 • 149 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 • 148 Effects of phase stability, lattice ordering, and electron density on plastic deformation in cubic TiWN pseudobinary transition-metal nitride alloys
    Sangiovanni, D.G. and Hultman, L. and Chirita, V. and Petrov, I. and Greene, J.E.
    Acta Materialia 103 823-835 (2016)
    We carry out density functional theory calculations to compare the energetics of layer glide, as well as stress vs. strain curves, for cubic Ti0.5W0.5N pseudobinary alloys and reference B1-structure TiN. Irrespective of the degree of ordering on the metal sublattice, the hardness and stiffness of Ti0.5W0.5N, as estimated by stress-strain results and resistance to layer glide, are comparable to that of the parent binary TiN, while ductility is considerably enhanced. After an initial elastic response to an applied load, the pseudobinary alloy deforms plastically, thus releasing accumulated mechanical stress. In contrast, stress continues to increase linearly with strain in TiN. Layer glide in Ti0.5W0.5N is promoted by a high valence-electron concentration which enables the formation of strong metallic bonds within the slip direction upon deformation. [111]-oriented Ti0.5W0.5N layers characterized by high local metal-sublattice ordering exhibit low resistance to slip along &lt;110&gt; directions due to energetically favored formation of (111) hexagonal stacking faults. This is consistent with the positive formation energy of &lt;111&gt;-ordered Ti0.5W0.5N with respect to mixing of cubic-B1 TiN and hexagonal WC-structure WN. In the cubic pseudobinary alloy, slip occurs parallel, as well as orthogonal, to the resolved applied stress at the interface between layers with the lowest friction. We suggest that analogous structural metastability (mixing cubic and hexagonal TM nitride binary phases) and electronic (high valence electron concentration) effects are responsible for the enhanced toughness recently demonstrated experimentally for cubic single-crystal pseudobinary V0.5W0.5N and V0.5Mo0.5N epitaxial layers. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.10.039
  • 2016 • 147 Orientation of FePt nanoparticles on top of a-SiO2/Si(001), MgO(001) and sapphire(0001): Effect of thermal treatments and influence of substrate and particle size
    Schilling, M. and Ziemann, P. and Zhang, Z. and Biskupek, J. and Kaiser, U. and Wiedwald, U.
    Beilstein Journal of Nanotechnology 7 591-604 (2016)
    Texture formation and epitaxy of thin metal films and oriented growth of nanoparticles (NPs) on single crystal supports are of general interest for improved physical and chemical properties especially of anisotropic materials. In the case of FePt, the main focus lies on its highly anisotropic magnetic behavior and its catalytic activity, both due to the chemically ordered face-centered tetragonal (fct) L10 phase. If the c-axis of the tetragonal system can be aligned normal to the substrate plane, perpendicular magnetic recording could be achieved. Here, we study the orientation of FePt NPs and films on a-SiO2/Si(001), i.e., Si(001) with an amorphous (a-) native oxide layer on top, on MgO(001), and on sapphire(0001) substrates. For the NPs of an approximately equiatomic composition, two different sizes were chosen: "small" NPs with diameters in the range of 2-3 nm and "large" ones in the range of 5-8 nm. The 3 nm thick FePt films, deposited by pulsed laser deposition (PLD), served as reference samples. The structural properties were probed in situ, particularly texture formation and epitaxy of the specimens by reflection high-energy electron diffraction (RHEED) and, in case of 3 nm nanoparticles, additionally by high-resolution transmission electron microscopy (HRTEM) after different annealing steps between 200 and 650 °C. The L10 phase is obtained at annealing temperatures above 550 °C for films and 600 °C for nanoparticles in accordance with previous reports. On the amorphous surface of a-SiO2/Si substrates we find no preferential orientation neither for FePt films nor nanoparticles even after annealing at 630 °C. On sapphire(0001) supports, however, FePt nanoparticles exhibit a clearly preferred (111) orientation even in the as-prepared state, which can be slightly improved by annealing at 600-650 °C. This improvement depends on the size of NPs: Only the smaller NPs approach a fully developed (111) orientation. On top of MgO(001) the effect of annealing on particle orientation was found to be strongest. From a random orientation in the as-prepared state observed for both, small and large FePt NPs, annealing at 650 °C for 30 min reorients the small particles towards a cube-on-cube epitaxial orientation with a minor fraction of (111)-oriented particles. In contrast, large FePt NPs keep their as-prepared random orientation even after doubling the annealing period at 650 °C to 60 min. © 2016 Schilling et al.
    view abstractdoi: 10.3762/bjnano.7.52
  • 2016 • 146 Tunable dielectric properties of KTaO3 single crystals in the terahertz range
    Skoromets, V. and Kadlec, C. and Němec, H. and Fattakhova-Rohlfing, D. and Kužel, P.
    Journal of Physics D: Applied Physics 49 (2016)
    Electric-field tunability of the dielectric properties of potassium tantalate single crystal was studied by terahertz spectroscopy in a broad temperature range (40-250 K). Complex-valued terahertz transmission spectra of samples were measured with an external electric field perpendicular to the sample surface and parallel to the terahertz wave-vector. We found that the ferroelectric soft mode hardening is fully responsible for the observed electric-field-induced changes in the spectra and no signature of a central mode was detected. We determined the anharmonic properties of the soft-mode potential in the mean field approximation. The observed behavior was compared with that previously reported for SrTiO3 single crystals. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/6/065306
  • 2016 • 145 (MeZn)2(μ-η2:η2-N6Ph2): A Powerful Starting Reagent for the Synthesis of Metal Hexazene Complexes
    Stienen, C. and Gondzik, S. and Gehlhaar, A. and Haack, R. and Wölper, C. and Jansen, G. and Schulz, S.
    Organometallics 35 1022-1029 (2016)
    [(MeLDippZn)2(μ-η2:η2-PhN6Ph)] (3), which was synthesized by reaction of MeLDipp 2Zn2 with PhN3, reacts with two equivalents of Me2Zn to give [(MeZn)2(μ-η2:η2-PhN6Ph)] (2). The reaction of 2 with pyridine gave [(MeZn)2(μ-η2:η2-PhN6Ph)(Py)2] (4), while reactions with H-acidic ligands (MeLMesH, MeLPhH) occurred with elimination of methane and formation of [(MeLMesZn)2(μ-η2:η2-PhN6Ph)] (1) and [(MeLPhZn)2(μ-η2:η2-PhN6Ph)] (5). The reaction of 1 with two equivalents of MeLi yielded the heterobimetallic hexazene complex [(MeZn)(μ-η2:η2-PhN6Ph)(Li)], which was found to undergo stepwise reaction with Me2AlCl to give [MeZn(μ-η2:η2-PhN6Ph)AlMe2] and finally [(Me2Al)2(μ-η2:η2-PhN6Ph)(thf)2] (6). Compounds 3-6 were characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray diffraction. Quantum chemical calculations were performed in order to investigate the electronic structure of 4′ and 6′ in more detail and to identify the absorption bands of the hexazene unit. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.6b00116
  • 2016 • 144 Growth, characterization, and transport properties of ternary (Bi1-xSbx)2Te3 topological insulator layers
    Weyrich, C. and Drögeler, M. and Kampmeier, J. and Eschbach, M. and Mussler, G. and Merzenich, T. and Stoica, T. and Batov, I.E. and Schubert, J. and Plucinski, L. and Beschoten, B. and Schneider, C.M. and Stampfer, C. and Grütz...
    Journal of Physics Condensed Matter 28 (2016)
    Ternary (Bi1-xSbx)2Te3 films with an Sb content between 0 and 100% were deposited on a Si(1 1 1) substrate by means of molecular beam epitaxy. X-ray diffraction measurements confirm single crystal growth in all cases. The Sb content is determined by x-ray photoelectron spectroscopy. Consistent values of the Sb content are obtained from Raman spectroscopy. Scanning Raman spectroscopy reveals that the (Bi1-xSbx)2Te3 layers with an intermediate Sb content show spatial composition inhomogeneities. The observed spectra broadening in angular-resolved photoemission spectroscopy (ARPES) is also attributed to this phenomena. Upon increasing the Sb content from x = 0 to 1 the ARPES measurements show a shift of the Fermi level from the conduction band to the valence band. This shift is also confirmed by corresponding magnetotransport measurements where the conductance changes from n- to p-type. In this transition region, an increase of the resistivity is found, indicating a location of the Fermi level within the band gap region. More detailed measurements in the transition region reveals that the transport takes place in two independent channels. By means of a gate electrode the transport can be changed from n- to p-type, thus allowing a tuning of the Fermi level within the topologically protected surface states. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/28/49/495501
  • 2016 • 143 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 • 142 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 • 141 Double minimum creep of single crystal Ni-base superalloys
    Wu, X. and Wollgramm, P. and Somsen, C. and Dlouhy, A. and Kostka, A. and Eggeler, G.
    Acta Materialia 112 242-260 (2016)
    Low temperature (750°C) and high stress (800 MPa) creep curves of single crystal superalloy ERBO/1 tensile specimens loaded in the (001) direction show two creep rate minima. Strain rates decrease towards a first sharp local creep rate minimum at 0.1% strain (reached after 30 min). Then deformation rates increase and reach an intermediate maximum at 1% (reached after 1.5 h). Subsequently, strain rates decrease towards a global minimum at 5% (260 h), before tertiary creep (not considered in the present work) leads to final rupture. We combine high resolution miniature creep testing with diffraction contrast transmission electron microscopy and identify elementary processes which govern this double-minimum type of creep behavior. We provide new quantitative information on the evolution of microstructure during low temperature and high stress creep, focusing on γ-channel dislocation activity and stacking fault shear of the γ′-phase. We discuss our results in the light of previous work published in the literature and highlight areas in need of further work. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.04.012
  • 2016 • 140 On Local Phase Equilibria and the Appearance of Nanoparticles in the Microstructure of Single-Crystal Ni-Base Superalloys
    Yardley, V. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Parsa, A.B. and Kostka, A. and Somsen, C. and Dlouhy, A. and Neuking, K. and George, E.P. and Eggeler, G.
    Advanced Engineering Materials 18 1556-1567 (2016)
    High-resolution characterization techniques are combined with thermodynamic calculations (CALPHAD) to rationalize microstructural features of single crystal Ni-base superalloys. Considering the chemical compositions of dendritic and interdendritic regions one can explain differences in γ′-volume fractions. Using thermodynamic calculations one can explain, why γ-nanoparticles are observed in the central regions of large cuboidal γ′-particles and why tertiary γ′-nanoparticles form in the γ-channels. The chemical compositions of the γ-channels and of the newly formed γ-particles differ because of the Gibbs–Thomson pressure which acts on the small particles. With increasing size of secondary γ′-particles, their shape changes from spherical to cuboidal. Some general thermodynamic aspects including the temperature dependencies of the Gibbs free energy G, the enthalpy H, and the entropy S and site occupancies in the ordered L12 (γ′) phase are considered. The importance of cooling rate after homogenization is discussed. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201600237
  • 2015 • 139 A kinematically-enhanced relaxation scheme for the modeling of displacive phase transformations
    Bartel, T. and Kiefer, B. and Buckmann, K. and Menzel, A.
    Journal of Intelligent Material Systems and Structures 26 701-717 (2015)
    In this contribution, a micro-mechanically motivated, energy relaxation-based constitutive model for phase transformation, martensite reorientation and twin formation in shape memory alloys is proposed. The formulation builds on an idealized parametrization of the austenite-twinned martensite microstructure through first- and second-order laminates. To estimate the effective rank-one convex energy density of the phase mixture, the concept of laminate-based energy relaxation is applied. In this context, the evolution of the energetic and dissipative internal state variables, that describe characteristic microstructural features, is computed via constrained incremental energy minimization. This work also suggests a first step towards the continuous modeling of twin formation within the framework of energy relaxation and can be viewed as a generalization of earlier models suggested by Bartel and Hackl (2009) and Bartel et al. (2011). More specifically, in the current model the orientation of martensitic variants in space is not pre-assigned. Variants are rather left free to arrange themselves relative to the martensite-martensite interface in an energy-minimizing fashion, where, however, it is assumed that they form crystallographically-twinned pairs. The formulation also eliminates the need to introduce specific expressions for the Bain strains in each of the martensitic variants, by relating them to a master variant and utilizing the information about their absolute orientation. The predictive capabilities of the proposed modeling framework are demonstrated in several representative numerical examples. In the first part of the results section, the focus is placed on purely energetic analysis, and the particular influence of the different microstructural degrees of freedom on the relaxed energy densities and the corresponding stress-strain responses is investigated in detail. In the second part, macro-homogeneous uniaxial strain and shear loading cases are analyzed for the dissipative case. It is shown, that the proposed model, which, compared to purely phenomenological macro-scale models, has the advantage of strong micro-mechanical motivation, is capable of qualitatively predicting central features of single crystal shape memory alloy behavior, such as the phase diagram in stress-temperature space, and pseudo-elastic and pseudo-plastic responses, while simultaneously providing valuable insight into the underlying micro-scale mechanisms. © The Author(s) 2014.
    view abstractdoi: 10.1177/1045389X14557507
  • 2015 • 138 Synthesis and X-ray Crystal Structure of Diimidosulfinate Transition Metal Complexes
    Bayram, M. and Bläser, D. and Wölper, C. and Schulz, S.
    Organometallics 34 3421-3427 (2015)
    Bis(trimethylsilyl)sulfurdiimide S(NSiMe3)2 reacts with equimolar amounts of Me2Zn and Cp∗2Zn either with insertion into the metal-carbon bond and formation of the expected S-methyl diimidosulfinate complex [MeZnN(SiMe3)S(Me)NSiMe3]2 1 or the unexpected complex {Me3SiNS[N(SiMe3)SNSiMe3]N(SiMe3)Zn}2 2. Insertion reactions were also observed with Cp∗MMe3 (M = Ti, Zr, Hf), yielding Cp∗(Me)2M[Me3SiNS(Me)NSiMe3] (M = Ti 3, Zr 4, Hf 5), whereas the corresponding Cl-substituted derivatives Cp∗(Cl)2M[(Me3SiNS(Me)NSiMe3] (M = Ti 6, Zr 7, Hf 8) were obtained from salt elimination reactions of Li S-methyl diimidosulfinate (Me3SiN)2S(Me)Li(thf)]2 9 with Cp∗MCl3. Compounds 1-8 were characterized by heteronuclear NMR (1H and 13C) and IR spectroscopy, and the solid state structures of 1-5 and 9 were determined by single crystal X-ray diffraction. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.5b00407
  • 2015 • 137 A quantitative metallographic assessment of the evolution of porosity during processing and creep in single crystal Ni-base super alloys
    Buck, H. and Wollgramm, P. and Parsa, A.B. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 46 577-590 (2015)
    The present work reviews previous research on the evolution of porosity. It presents new results from a detailed study on the evolution of porosity during casting, heat treatment and creep of a single crystal Ni-base superalloy subjected to uniaxial tensile creep at 1050 °C and 160 MPa in [001] and [110] directions. A quantitative metallographic study was performed on carefully polished metallographic cross sections, monitoring sampling fields of 4500 × 1000 μm2 using the back scatter contrast of an analytical scanning electron microscope; evolutions of pore sizes and pore form factors were analyzed and all important details which were previously revealed in a synchrotron study could be reproduced. In addition, it was observed that micro cracks form at larger cast pores. They interlink and thus initiate final rupture. The [110] tensile creep tests showed lower rupture strains than the [001] experiments. In agreement with earlier work, this can be rationalized on the basis of aligned porosity along primary dendrites. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201500379
  • 2015 • 136 Intermartensitic transitions and phase stability in Ni50Mn50-xSnx Heusler alloys
    Çakir, A. and Righi, L. and Albertini, F. and Acet, M. and Farle, M.
    Acta Materialia 99 140-149 (2015)
    Ni-Mn based Heusler alloys are of considerable interest due to their multifunctional properties such as magnetic shape memory, magnetocaloric effect and spintronics. The reason for these multifunctional properties is the presence of a first order martensitic transition and its strong coupling to the magnetization. In this work, one of the outstanding class of martensitic Heuslers, Ni-Mn-Sn, is investigated in relation to magneto-structural phase transitions and the stability of the various crystallographic structures under varying temperature. Temperature-dependent X-ray diffraction, resistance and magnetization measurements on Ni<inf>50</inf>Mn<inf>50-x</inf>Sn<inf>x</inf> alloys are performed in a broad valence electron concentration range 7.91 ≤ (e/a) ≤ 8.34 (5.1 ≤ x ≤ 20.3at.%). The results reveal that in addition to the austenite-martensite transition, further intermartensitic transitions take place with decreasing temperature. Depending on the composition, we observe that the parent martensite phase tends to transform to L1<inf>0</inf> martensite as the ground state phase when the temperature is lowered. A phase diagram of Ni<inf>50</inf>Mn<inf>50-x</inf>Sn<inf>x</inf> is constructed to include intermartensitic phase transition boundaries. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.07.072
  • 2015 • 135 Linking atomistic, kinetic Monte Carlo and crystal plasticity simulations of single-crystal tungsten strength
    Cereceda, D. and Diehl, M. and Roters, F. and Shanthraj, P. and Raabe, D. and Perlado, J.M. and Marian, J.
    GAMM Mitteilungen 38 213-227 (2015)
    Understanding and improving the mechanical properties of tungsten is a critical task for the materials fusion energy program. The plastic behavior in body-centered cubic (bcc) metals like tungsten is governed primarily by screw dislocations on the atomic scale and by ensembles and interactions of dislocations at larger scales. Modeling this behavior requires the application of methods capable of resolving each relevant scale. At the small scale, atomistic methods are used to study single dislocation properties, while at the coarse-scale, continuum models are used to cover the interactions between dislocations. In this work we present a multiscale model that comprises atomistic, kinetic Monte Carlo (kMC) and continuum-level crystal plasticity (CP) calculations. The function relating dislocation velocity to applied stress and temperature is obtained from the kMC model and it is used as the main source of constitutive information into a dislocation-based CP framework. The complete model is used to perform material point simulations of single-crystal tungsten strength. We explore the entire crystallographic orientation space of the standard triangle. Non-Schmid effects are inlcuded in the model by considering the twinning-antitwinning (T/AT) asymmetry in the kMC calculations. We consider the importance of ?111?{110} and 111 {112} slip systems in the homologous temperature range from 0.08T<inf>m</inf> to 0.33T<inf>m</inf>, where T<inf>m</inf> =3680 K is the melting point in tungsten. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/gamm.201510012
  • 2015 • 134 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 • 133 Influence of microstructure on macroscopic elastic properties and thermal expansion of nickel-base superalloys ERBO/1 and LEK94
    Demtröder, K. and Eggeler, G. and Schreuer, J.
    Materialwissenschaft und Werkstofftechnik 46 563-576 (2015)
    In the present work the thermal expansion and the elastic properties of second generation nickel-base superalloy single crystals ERBO/1 (CMSX-4 variation) and LEK94 have been studied between about 100 K and 1273 K using dilatometry and resonant ultrasound spectroscopy, respectively. Inhomogeneity related to the large scale microstructure of the samples can act as a potential source of scatter for the propagation of ultrasonic waves. This can be overcome by choosing samples of sufficient size so that they appear as homogeneous media at the scale of the elastic wave length. Our final results are in good agreement with data reported in literature for similar alloy systems. In particular, the elastic material properties are only weekly affected by moderate variations in chemical composition and microstructure. Taking into account literature data for other superalloys like CMSX-4, we derive general polynomial functions which describe the temperature dependence of the elastic moduli E<inf>〈100〉</inf>, E<inf>〈110〉</inf> and E<inf>〈111〉</inf> in nickel-base superalloys to within about ±3%. It was also observed that the alloys ERBO/1 and LEK94 show weak but significant anomalies in both thermal expansion and temperature coefficients of elastic constants above about 900 K. These anomalies are probably related to the gradual dissolution of the γ′-precipitates at higher temperatures. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201500406
  • 2015 • 132 Microstructure in plasticity, a comparison between theory and experiment
    Dmitrieva, O. and Raabe, D. and Müller, S. and Dondl, P.W.
    Lecture Notes in Applied and Computational Mechanics 78 205-218 (2015)
    We review aspects of pattern formation in plastically deformed single crystals, in particular as described in the investigation of a copper single crystal shear experiment in [DDMR09]. In this experiment, the specimen showed a band-like microstructure consisting of alternating crystal orientations. Such a formation of microstructure is often linked to a lack of convexity in the free energy describing the system. The specific parameters of the observed bands, namely the relative crystal orientation as well as the normal direction of the band layering, are thus compared to the predictions of the theory of kinematically compatible microstructure oscillating between low-energy states of the non-convex energy. We conclude that this theory is suitable to describe the experimentally observed band-like structure. Furthermore, we link these findings to the models used in studies of relaxation and evolution of microstructure. ©Springer International Publishing Switzerland 2015.
    view abstractdoi: 10.1007/978-3-319-18242-1_8
  • 2015 • 131 Comparison of phenomenological and laminate-based models for rate-dependent switching in ferroelectric continua
    Dusthakar, D.K. and Menzel, A. and Svendsen, B.
    GAMM Mitteilungen 38 147-170 (2015)
    The purpose of the current work is the comparison of a phenomenological and a laminate-based model for rate-dependent switching in ferroelectric single crystals. To this end, the phenomenological model formulation of [1] is considered. In this model, the polarization vector is treated as an internal variable. The evolution of the polarization determines the remanent strain; dependence of energy storage on its direction results in generally transverse isotropic material behavior. This is compared here with a laminate-based model in which the volume fractions of ferroelectric variants are treated as internal variables. The evolution of these volume fractions determines in turn the remanent strain and polarization as volume averages of corresponding ferroelectric variant quantities. Besides a comparison of the respective model formulations, the phenomenological and laminate models are compared in the context of numerical simulation examples. It turns out that both modeling frameworks nicely recapture the underlying dissipative and rate-dependent effects, which is represented by means of simulated butterfly curves and hysteresis loops under homogeneous loading conditions as well as by finite element simulations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/gamm.201510008
  • 2015 • 130 Interplay of strain and interdiffusion in Heusler alloy bilayers
    Dutta, B. and Hickel, T. and Neugebauer, J. and Behler, C. and Fähler, S. and Behler, A. and Waske, A. and Teichert, N. and Schmalhorst, J.-M. and Hütten, A.
    Physica Status Solidi - Rapid Research Letters 9 321-325 (2015)
    Combining conventional and inverse magnetocaloric materials promises to enhance solid state refrigeration. As a first step here we present epitaxial Ni-Mn-Ga/Ni-Mn-Sn bilayer films. We examine the dependence of the lateral and normal lattice constants on the deposition sequence by combining experimental and ab initio techniques. Structural properties are determined with X-ray diffraction as well as highresolution transmission electron microscopy, while ab initio calculations explain the interplay of strain, local relaxations and the interdiffusion of atoms. The latter is confirmed by Auger electron spectroscopy and is expected to have a noticeable impact on the functional properties of the Heusler materials. ( © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201510070
  • 2015 • 129 Synergy of atom-probe structural data and quantum-mechanical calculations in a theory-guided design of extreme-stiffness superlattices containing metastable phases
    Friák, M. and Tytko, D. and Holec, D. and Choi, P.-P. and Eisenlohr, P. and Raabe, D. and Neugebauer, J.
    New Journal of Physics 17 (2015)
    A theory-guided materials design of nano-scaled superlattices containing metastable phases is critically important for future development of advanced lamellar composites with application-dictated stiffness and hardness. Our study combining theoretical and experimental methods exemplifies the strength of this approach for the case of the elastic properties of AlN/CrN superlattices that were deposited by reactive radio-frequency magnetron sputtering with a bilayer period of 4 nm. Importantly, CrN stabilizes AlN in a metastable B1 (rock salt) cubic phase only in the form of a layer that is very thin, up to a few nanometers. Due to the fact that B1-AlN crystals do not exist as bulk materials, experimental data for this phase are not available. Therefore, quantum-mechanical calculations have been applied to simulate an AlN/CrN superlattice with a similar bilayer period. The ab initio predicted Young's modulus (428 GPa) along the [001] direction is in excellent agreement with measured nano-indentation values (408 32 GPa). Aiming at a future rapid high-throughput materials design of superlattices, we have also tested predictions obtained within linear-elasticity continuum modeling using elastic properties of B1-CrN and B1-AlN phases as input. Using single-crystal elastic constants from ab initio calculations for both phases, we demonstrate the reliability of this approach to design nano-patterned coherent superlattices with unprecedented and potentially superior properties. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/17/9/093004
  • 2015 • 128 Synthesis of Heterobimetallic Group 13 Compounds via Oxidative Addition Reaction of Gallanediyl LGa and InEt3
    Ganesamoorthy, C. and Blaser, D. and Wolper, C. and Schulz, S.
    Organometallics 34 2991--2996 (2015)
    Equimolar amounts of LGa (L = [(2,6-i-Pr-2-C6H3)NC(Me)](2)CH) and InEt3 were found to react with insertion into the Incarbon bond and formation of LGa(Et)InEt2 (1), while in the presence of the N-heterocyclic carbene It-Bu [C(Nt-Bu2CH)(2)], the base-stabilized compound LGa(Et)Et2In <- It-Bu (2) was formed, which shows an abnormal binding mode of the NHC group. In addition, the reaction of InEt3 with two equivalents of LGa occurred with double insertion and formation of [LGa(Et)](2)InEt (3). 1-3 were characterized by heteronuclear NMR (H-1, C-13) and IR spectroscopy, their solid-state structures were determined by single-crystal X-ray analyses, and their thermal stability was investigated by in situ NMR spectroscopy.
    view abstractdoi: 10.1021/acs.organomet.5b00302
  • 2015 • 127 Te-Te and Te-C bond cleavage reactions using a monovalent gallanediyl
    Ganesamoorthy, C. and Bendt, G. and Bläser, D. and Wölper, C. and Schulz, S.
    Dalton Transactions 44 5153-5159 (2015)
    LGa (L = [(2,6-i-Pr<inf>2</inf>-C<inf>6</inf>H<inf>3</inf>)NC(Me)]<inf>2</inf>CH) reacts with elemental tellurium with formation of the Te-bridged compound [LGa-μ-Te]<inf>2</inf>1, whereas the reactions with Ph<inf>2</inf>Te<inf>2</inf> and i-Pr<inf>2</inf>Te occurred with cleavage of the Te-Te and Te-C bond, respectively, and subsequent formation of LGa(TePh)<inf>2</inf>2 and LGa(i-Pr)Tei-Pr 3. 1-3 were characterized by heteronuclear NMR (1H, 13C, 125Te) and IR spectroscopy and their solid state structures were determined by single crystal X-ray analyses. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5dt00172b
  • 2015 • 126 Primary combination of phase-field and discrete dislocation dynamics methods for investigating athermal plastic deformation in various realistic Ni-base single crystal superalloy microstructures
    Gao, S. and Kumar Rajendran, M. and Fivel, M. and Ma, A. and Shchyglo, O. and Hartmaier, A. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 23 (2015)
    Three-dimensional discrete dislocation dynamics (DDD) simulations in combination with the phase-field method are performed to investigate the influence of different realistic Ni-base single crystal superalloy microstructures with the same volume fraction of γ;precipitates on plastic deformation at room temperature. The phase-field method is used to generate realistic microstructures as the boundary conditions for DDD simulations in which a constant high uniaxial tensile load is applied along different crystallographic directions. In addition, the lattice mismatch between the γand γ;phases is taken into account as a source of internal stresses. Due to the high antiphase boundary energy and the rare formation of superdislocations, precipitate cutting is not observed in the present simulations. Therefore, the plastic deformation is mainly caused by dislocation motion in γ; matrix channels. From a comparison of the macroscopic mechanical response and the dislocation evolution for different microstructures in each loading direction, we found that, for a given γ;phase volume fraction, the optimal microstructure should possess narrow and homogeneous γ; matrix channels. © 2015 IOP Publishing Ltd Printed in the UK.
    view abstractdoi: 10.1088/0965-0393/23/7/075003
  • 2015 • 125 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 • 124 MeL2Zn2(μ-1,6-Ph2-N6)-a building block for new hexazene complexes
    Gondzik, S. and Wölper, C. and Haack, R. and Jansen, G. and Schulz, S.
    Dalton Transactions 44 15703-15711 (2015)
    The zinc hexazene complex MeL<inf>2</inf>Zn<inf>2</inf>(μ-1,6-Ph<inf>2</inf>-N<inf>6</inf>) 1 (MeL = HC[C(Me)N(2,4,6-Me<inf>3</inf>C<inf>6</inf>H<inf>2</inf>)]<inf>2</inf>) is a suitable hexazene transfer reagent in reactions with main group metal and transition metal complexes containing M-Me units. The reactions proceed with elimination of MeLZnMe and the resulting complexes were characterized by NMR and IR spectroscopy and single crystal X-ray diffraction (5, 8). Quantum chemical calculations were performed to investigate the electronic structure of 5′ and 8′ in more detail and to identify the absorption bands of the hexazene unit. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5dt02423d
  • 2015 • 123 Non-innocence of β-diketiminato ligands
    Gondzik, S. and Bläser, D. and Wölper, C. and Schulz, S.
    Journal of Organometallic Chemistry 783 92-95 (2015)
    The homoleptic thio-β-ketiminate zinc complex [MesNC(Me)CH{C(Me)NMes}S]2Zn (1) was obtained from reactions of elemental sulfur with the β-diketiminato zinc complexes [CH{C(Me)NMes}2]ZnMe, [CH{C(Me)NMes}2]2Zn2 and [[CH{C(Me)NMes}2]Zn-TePh]2. 1 and the byproducts were characterized by multinuclear NMR (1H, 13C, 125Te) and IR spectroscopy, elemental analyses as well as by single crystal X-ray diffraction. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jorganchem.2015.02.028
  • 2015 • 122 A variational viscosity-limit approach to the evolution of microstructures in finite crystal plasticity
    Günther, C. and Junker, P. and Hackl, K.
    Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 471 (2015)
    A micromechanical model for finite single crystal plasticity was introduced by Kochmann & Hackl (2011 Contin.Mech. Thermodyn. 23, 63-85 (doi:10.1007/ s00161-010-0714-5)). This model is based on thermodynamic variational principles and leads to a non-convex variational problem. Based on the Lagrange functional, an incremental strategy was outlined to model the time-continuous evolution of a first-order laminate microstructure. Although this model provides interesting results on the material point level, owing to the global minimization in the evolution equations, the calculation time and numerical instabilities may cause problems when applying this model to macroscopic specimens. In this paper, a smooth transition zone between the laminates is introduced to avoid global minimization, which makes the numerical calculations cumbersome compared with the model in Kochmann & Hackl. By introducing a smooth viscous transition zone, the dissipation potential and its numerical treatment have to be adapted. We outline rate-dependent timeevolution equations for the internal variables based on variational techniques and show as first examples single-slip shear and tension/compression tests. © 2015 The Author(s) Published by the Royal Society. All rights reserved.
    view abstractdoi: 10.1098/rspa.2015.0110
  • 2015 • 121 Investigating the influence of crystal orientation on bending size effect of single crystal beams
    Gupta, S. and Ma, A. and Hartmaier, A.
    Computational Materials Science 101 201-210 (2015)
    Influence of crystal orientation on bending size effect has been numerically investigated for single crystal beams. This work is inspired by the experimental observations of Hayashi et al. (2011), where they observed a significant difference in bending size effect for two different crystal orientations. We have used a higher order non-local crystal plasticity model which can account for different hardening contributions by SSDs (statistically stored dislocations) and GNDs (geometrically necessary dislocations) simultaneously. It was found that strain hardening together with an additional kinematic hardening caused by accumulation of GNDs and the number of activated slip systems can be seen as the origin of the orientation dependence of bending size effect. We have also observed a pronounced orientation dependence of spring back size effect, which can be explained on the basis of number of the activated slip systems and equivalent plastic strain. Simulation results showing enhanced or diminished bending size effect for different crystal orientations reveal the importance of crystal orientation for precise micro-bending operations. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2014.12.038
  • 2015 • 120 Combining structural and chemical information at the nanometer scale by correlative transmission electron microscopy and atom probe tomography
    Herbig, M. and Choi, P. and Raabe, D.
    Ultramicroscopy 153 32-39 (2015)
    In many cases, the three-dimensional reconstructions from atom probe tomography (APT) are not sufficiently accurate to resolve crystallographic features such as lattice planes, shear bands, stacking faults, dislocations or grain boundaries. Hence, correlative crystallographic characterization is required in addition to APT at the exact same location of the specimen. Also, for the site-specific preparation of APT tips containing regions of interest (e.g. grain boundaries) correlative electron microscopy is often inevitable. Here we present a versatile experimental setup that enables performing correlative focused ion beam milling, transmission electron microscopy (TEM), and APT under optimized characterization conditions. The setup was designed for high throughput, robustness and practicability. We demonstrate that atom probe tips can be characterized by TEM in the same way as a standard TEM sample. In particular, the use of scanning nanobeam diffraction provides valuable complementary crystallographic information when being performed on atom probe tips. This technique enables the measurement of orientation and phase maps as known from electron backscattering diffraction with a spatial resolution down to one nanometer. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.02.003
  • 2015 • 119 Influence of inclined twin boundaries on the deformation behavior of Cu micropillars
    Imrich, P.J. and Kirchlechner, C. and Dehm, G.
    Materials Science and Engineering A 642 65-70 (2015)
    In situ micromechanical compression tests on Cu pillars were performed to evaluate the influence of twin boundaries on the mechanical behavior. The 1. μm sized Cu samples on a Si substrate prepared by focused ion beam milling were either single crystalline or contained 2-5 twin boundaries that were inclined to the compression direction. The strengths of the pillars vary, depending on the crystal orientation, associated twin boundary inclination and orientation of slip systems. Results show, that multiple slip systems are activated in each pillar. However, slip parallel to the twin boundaries prevails due to the long mean free path for dislocation movement. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.06.064
  • 2015 • 118 Importance of dislocation pile-ups on the mechanical properties and the Bauschinger effect in microcantilevers
    Kapp, M.W. and Kirchlechner, C. and Pippan, R. and Dehm, G.
    Journal of Materials Research 30 791-797 (2015)
    Copper microcantilevers were produced by focused ion beam milling and tested in situ using a scanning electron microscope. To provide different interfaces for piling up dislocations, cantilevers were fabricated to be single crystalline, bicrystalline, or single crystalline with a slit in the region of the neutral axis. The aim of the experiment was to study the influence of dislocation pile-ups on (i) strength and (ii) Bauschinger effects in micrometer-sized, focused ion beam milled bending cantilevers. The samples were loaded monotonically for several times under displacement control. Even though the cantilevers exhibited the same nominal strain gradient the strength varied by 34% within the three cantilever geometries. The Bauschinger effect can be promoted and prohibited by the insertion of different interfaces. © 2015 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2015.49
  • 2015 • 117 Numerical energy relaxation to model microstructure evolution in functional magnetic materials
    Kiefer, B. and Buckmann, K. and Bartel, T.
    GAMM Mitteilungen 38 171-196 (2015)
    This paper proposes energy relaxation-based approaches for the modeling of magnetostriction, with a particular focus on single crystalline magnetic shape memory alloy response. The theoretical development relies on concepts of energy relaxation in the context of nonconvex free energy landscapes whose wells define preferred states of spontaneous straining and magnetization. The constrained theory of magnetoelasticity developed by DeSimone and James [1] represents the point of departure for the model development, and its capabilities, but also limitations, are demonstrated by means of representative numerical examples. The key features that characterize the extended approach are (i) the incorporation of elastic deformations, whose distribution among the individual phases occurs in an energy minimizing fashion, (ii) a finite magnetocrystalline anisotropy energy, that allows magnetization rotations away from easy axes, and (iii) dissipative effects, that are accounted for in an incremental variational setting for standard dissipative materials. In the context of introducing elastic strain energy, two different relaxation concepts, the convexification approach and the rank-one relaxation with respect to first-order laminates, are considered. In this manner, important additional response features, e.g. the hysteretic nature, the linear magnetization response in the pre-variant reorientation regime, and the stress dependence of the maximum field induced strain, can be captured, which are prohibited by the inherent assumptions of the constrained theory. The enhanced modeling capabilities of the extended approach are demonstrated by several representative response simulations and comparison to experimental results taken from literature. These examples particularly focus on the response of single crystals under cyclic magnetic field loading at constant stress and cyclic mechanical loading at constant magnetic field. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/gamm.201510009
  • 2015 • 116 Assessment of geometrically necessary dislocation levels derived by 3D EBSD
    Konijnenberg, P.J. and Zaefferer, S. and Raabe, D.
    Acta Materialia 99 402-414 (2015)
    Existing alternatives for the calculation of geometrically necessary dislocation (GND) densities from orientation fields are discussed. Importantly, we highlight the role of reference frames and consider different sources of error. A well-controlled micro cantilever bending experiment on a copper bicrystal has been analyzed by 3-dimensional electron back scatter diffraction (3D EBSD). The GND density is determined experimentally by two different approaches and assessed theoretically, assuming a homogeneous bending of the cantilever. Experiment and theory agree very well. It is further shown that the deformation is accommodated mainly by GNDs, which carry and store lattice rotation, and not (only) by mobile dislocations that leave a crystal portion inspected, without lattice rotations. A detailed GND analysis reveals a local density minimum close to the grain boundary and a distinct difference in edge to screw ratios for both grains. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.06.051
  • 2015 • 115 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 • 114 Structure-Correlated Exchange Anisotropy in Oxidized Co80Ni20 Nanorods
    Liébana-Viñas, S. and Wiedwald, U. and Elsukova, A. and Perl, J. and Zingsem, B. and Semisalova, A.S. and Salgueiriño, V. and Spasova, M. and Farle, M.
    Chemistry of Materials 27 4015-4022 (2015)
    Rare earth-free permanent magnets for applications in electro-magnetic devices promise better sustainability and availability and lower prices. Exploiting the combination of shape, magnetocrystalline and exchange anisotropy in 3D-metals can pave the way to practical application of nanomagnets. In this context, we study the structural and magnetic properties of Co<inf>80</inf>Ni<inf>20</inf> nanorods with a mean diameter of 6.5 nm and a mean length of 52.5 nm, prepared by polyol reduction of mixed cobalt and nickel acetates. Structural analysis shows crystalline rods with the crystallographic c-axis of the hexagonal close-packed (hcp) phase parallel to the long axis of the Co<inf>80</inf>Ni<inf>20</inf> alloy rods, which appear covered by a thin oxidized face-centered cubic (fcc) shell. The temperature dependence of the surprisingly high coercive field and the exchange bias effect caused by the antiferromagnetic surface oxide indicate a strong magnetic hardening due to alignment of anisotropy axes. We identify a temperature dependent local maximum of the coercive field at T = 250 K, which originates from noncollinear spin orientations in the ferromagnetic core and the antiferromagnetic shell. This might be useful for building four way magnetic switches as a function of temperature. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.5b00976
  • 2015 • 113 On the identification of superdislocations in the γ′-phase of single-crystal Ni-base superalloys - An application of the LACBED method to complex microstructures
    Müller, J. and Eggeler, G. and Spiecker, E.
    Acta Materialia 87 34-44 (2015)
    Ni-base superalloys are used for turbine blades, which operate in the creep range at temperatures above 1000 °C. One of the objectives of modern materials science is to analyze the combination of elementary deformation and microstructural coarsening processes and to identify physically based micromechanical models which allow one to predict the mechanical behavior on the macroscale. High-temperature creep of single-crystal Ni-base superalloys is governed by dislocation plasticity in the well-known γ/γ′-microstructure. For a comprehensive description of plasticity, it is important to understand the nucleation, glide and climb of superdislocations in the γ′-phase. The rate-controlling dislocation processes have to be identified and therefore a reliable Burgers vector analysis of superdislocations is essential. Superdislocations exhibit complex dislocation cores, typically comprising superpartial dislocations and planar defects. Therefore, conventional Burgers vector analysis based on the invisibility criterion often fails, due to the presence of pronounced residual contrast. In the present work, large-angle convergent-beam electron diffraction (LACBED) is employed for Burgers vector determination of two characteristic superdislocations, of the standard <1 1 0> and the more complex <1 0 0> type. LACBED results are compared with results obtained using the conventional invisibility analysis. While both techniques work for the standard superdislocation, the conventional analysis fails to analyze the <1 0 0> superdislocation, which shows pronounced residual contrast even under conditions of g · b = 0 and g · b × u = 0. In contrast, the LACBED technique allows for an unambiguous determination of the Burgers vector, including its magnitude and absolute sense. In the present study, the use of LACBED to identify dislocations in the complex microstructure of an Ni-base superalloy is outlined and the better performance of LACBED as compared to the conventional gb-analysis is discussed. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.12.029
  • 2015 • 112 Surface properties of annealed semiconducting β-Ga2O3 (1 0 0) single crystals for epitaxy
    Navarro-Quezada, A. and Galazka, Z. and Alamé, S. and Skuridina, D. and Vogt, P. and Esser, N.
    Applied Surface Science 349 368-373 (2015)
    We present a detailed study on the surface properties of conductive β-Ga<inf>2</inf>O<inf>3</inf> (1 0 0) single-crystal epiready substrates by means of photoelectron emission spectroscopy. The surface properties are studied prior and after annealing in ultra-high vacuum (UHV). We find that untreated substrates contain a significant amount of adsorbed carbon contaminations at the surface, which can be partly removed by annealing at 800 °C in UHV. Valence band photoemission evidences an upward band bending of about 0.5 eV that increases with annealing, revealing the presence of an electron depletion layer at the near-surface region responsible for the insulating behavior commonly observed for semiconductive β-Ga<inf>2</inf>O<inf>3</inf> single crystals. Our findings become crucial for epitaxial growth, as it is known that carbon modifies the electrical and structural properties of subsequent epitaxial layers. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.04.225
  • 2015 • 111 The effect of cast microstructure and crystallography on rafting, dislocation plasticity and creep anisotropy of single crystal Ni-base superalloys
    Nörtershäuser, P. and Frenzel, J. and Ludwig, Al. and Neuking, K. and Eggeler, G.
    Materials Science and Engineering A 626 305-312 (2015)
    In the present work we investigate three mechanical and microstructural aspects of high temperature and low stress creep of the single crystal superalloy LEK 94. First, we compare the tensile creep behavior of specimens loaded in precise [001] and [110] directions and show that tensile creep specimens with precise [110] directions show significantly lower minimum creep rates. However, small deviations from precise [110] orientations result in a significant increase of creep rate. Second, we use a novel SEM technique to measure dislocation densities. We show that after short periods of creep, dislocation densities in dendritic regions are always higher than in interdendritic regions. This finding is probably associated with wider γ-channels, higher concentrations of W and Re and higher misfit stresses in the γ-channels of dendrites. Finally, we show that internal stresses associated with solidification can drive complex rafting processes during high temperature exposure, which differ between dendrite cores and interdendritic regions. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.12.030
  • 2015 • 110 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 • 109 Ledges and grooves at γ/γ′ interfaces of single crystal superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Kostka, A. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 90 105-117 (2015)
    In the present work we study the formation of grooves and ledges (typical size: <100 nm) at γ/γ′ interfaces of single crystal Ni-base superalloys. We highlight previous work which documents the presence of such interface irregularities and shows that their number and size increases during high temperature exposure and creep. We use diffraction contrast stereo transmission electron microscopy (TEM) to provide new evidence for the presence of ledges and grooves near dislocations at γ/γ′ interfaces after heat treatment and creep. We present a 2D model of the interfacial region which shows how dislocation stress fields alter local chemical potentials and drive diffusional fluxes which result in the formation of a groove. The results of the numerical study yield realistic groove sizes in relevant time scales. The results obtained in the present study suggest that the formation of grooves and ledges represents an elementary process which needs to be considered when rationalizing the kinetics of rafting, the directional coarsening of the γ′ phase. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.02.005
  • 2015 • 108 Advanced scale bridging microstructure analysis of single crystal Ni-base superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Somsen, C. and Kostka, A. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Dlouhy, A. and Müller, J. and Spiecker, E. and Demtroder, K. and Schreuer, J. and Neuking, K. and Eggeler, G.
    Advanced Engineering Materials 17 216-230 (2015)
    In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni-base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the castmicrostructure (dendrites (D) and interdendritic (ID) regions - large scale heterogeneity) and with the well-known γ/γ′ microstructure (small scale heterogeneity). Using electron probe microanalysis (EPMA), we can showthat elements such as Re, Co, andCr partition to the dendrites while ID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the γ′ cubes while g channels show higher concentrations of Co, Cr, Re, andW.We can combine large scale (EPMA) and small-scale analytical methods (APT) to obtain reasonable estimates for γ′ volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140mm x 100mm x 20mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the γ′ phase and the effect of cooling rates after high temperature exposure on the microstructure. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201400136
  • 2015 • 107 Atom probe informed simulations of dislocation-precipitate interactions reveal the importance of local interface curvature
    Prakash, A. and Guénolé, J. and Wang, J. and Müller, J. and Spiecker, E. and Mills, M.J. and Povstugar, I. and Choi, P. and Raabe, D. and Bitzek, E.
    Acta Materialia 92 33-45 (2015)
    The interaction of dislocations with precipitates is an essential strengthening mechanism in metals, as exemplified by the superior high-temperature strength of Ni-base superalloys. Here we use atomistic simulation samples generated from atom probe tomography data of a single crystal superalloy to study the interactions of matrix dislocations with a γ′ precipitate in molecular dynamics simulations. It is shown that the precipitate morphology, in particular its local curvature, and the local chemical composition significantly alter both, the misfit dislocation network which forms at the precipitate interface, and the core structure of the misfit dislocations. Simulated tensile tests reveal the atomic scale details of many experimentally observed dislocation-precipitate interaction mechanisms, which cannot be reproduced by idealized simulation setups with planar interfaces. We thus demonstrate the need to include interface curvature in the study of semicoherent precipitates and introduce as an enabling method atom probe tomography-informed atomistic simulations. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.03.050
  • 2015 • 106 Solution Heat Treatment of the Single Crystal Nickel-Base Superalloy CMSX-4 Fabricated by Selective Electron Beam Melting
    Ramsperger, M. and Mújica Roncery, L. and Lopez-Galilea, I. and Singer, R.F. and Theisen, W. and Körner, C.
    Advanced Engineering Materials 17 1486-1493 (2015)
    Selective electron beam melting (SEBM), which belongs to the additive manufacturing processes, is applied to produce samples from the single crystalline nickel-base superalloy CMSX-4. The influence of the high solidification rates on the microstructure and element distribution is investigated by OM, SEM, DSC, and EMPA. Solution heat treatments at different temperatures and holding times are applied to demonstrate the difference between conventionally cast and SEBM material. The results demonstrate that SEBM is able to produce superalloys with a degree of homogeneity which cannot be realized in conventional processes. Selective electron beam melting (SEBM) manufacturing of CMSX-4 leads to a very fine solidification structure which is two orders of magnitude smaller than in conventional castings (e.g., Bridgman). Microprobe mappings of Re distribution in CMSX-4 can be used to show the differences in homogeneity (a and b). Due to the homogeneity, a solution heat treatment (HT) of 4 min (1320 °C) is already sufficient for homogenization of SEBM CMSX- 4 (c). © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201500037
  • 2015 • 105 Mullite: Crystal Structure and Related Properties
    Schneider, H. and Fischer, R.X. and Schreuer, J.
    Journal of the American Ceramic Society 98 2948-2967 (2015)
    Mullite is certainly one of the most important oxide materials for both conventional and advanced ceramics. Mullite belongs to the compositional series of orthorhombic aluminosilicates with the general composition Al2(Al2+2xSi2-2x)O10-x. Main members are sillimanite (x = 0), stoichiometric 3/2-mullite (x = 0.25), 2/1-mullite (x = 0.40), and the SiO2-free phase ι-alumina (x = 1, crystal structure not known). This study gives an overview on the present state of research regarding single crystal mullite. Following a short introduction, the second part of the review focuses on the crystal structure of mullite. In particular, the characteristic mullite-type structural backbone of parallel chains consisting of edge-sharing MO6 octahedra and their specific cross-linkage by TO4 tetrahedra is explained in detail, the role of cation disorder and structural oxygen vacancies is addressed, and the possibility of cation substitution on different sites is discussed. The third part of the study deals with physical properties being relevant for technical applications of mullite and includes mechanical properties (e.g., elasticity, compressibility, strength, toughness, creep), thermal properties (e.g., thermal expansion, heat capacity, atomic diffusion, thermal conductivity), electrical conductivity, and optical properties. Special emphasis is put on structure-property relationships which allow for interpretation of corresponding experimental data and offer in turn the possibility to tailor new mullite materials with improved properties. Finally, the reported anomalies and discontinuities in the evolution of certain physical properties with temperature are summarized and critically discussed. © 2015 The American Ceramic Society.
    view abstractdoi: 10.1111/jace.13817
  • 2015 • 104 Coordinate-invariant phase field modeling of ferro-electrics, part I: Model formulation and single-crystal simulations
    Schrade, D. and Keip, M.-A. and Thai, H. and Schröder, J. and Svendsen, B. and Müller, R. and Gross, D.
    GAMM Mitteilungen 38 102-114 (2015)
    An electro-mechanically coupled phase field model for ferroelectric domain evolution is introduced. Based on Gurtin's concept of a microforce balance, a generalized Ginzburg-Landau evolution equation is derived from the second law of thermodynamics. The thermodynamic potential is formulated for transversely isotropic material behavior by adopting a coordinateinvariant formulation. The model is reduced to 2D and implemented into a finite element framework. The numerical simulations concern the microstructure evolution in mechanically clamped BaTiO3 single-crystals. In the second part of this contribution Keip et al. [1], the poling behavior of ferroelectric composites and polycrystals is investigated with regard to size effects and the influence of a discontinuous order parameter field across grain boundaries. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/gamm.201510005
  • 2015 • 103 Impact of local order and stoichiometry on the ultrafast magnetization dynamics of Heusler compounds
    Steil, D. and Schmitt, O. and Fetzer, R. and Kubota, T. and Naganuma, H. and Oogane, M. and Ando, Y. and Rodan, S. and Blum, C.G.F. and Balke, B. and Wurmehl, S. and Aeschlimann, M. and Cinchetti, M.
    48 (2015)
    Nowadays, a wealth of information on ultrafast magnetization dynamics of thin ferromagnetic films exists in the literature. Information is, however, scarce on bulk single crystals, which may be especially important for the case of multi-sublattice systems. In Heusler compounds, representing prominent examples for such multi-sublattice systems, off-stoichiometry and degree of order can significantly change the magnetic properties of thin films, while bulk single crystals may be generally produced with a much more well-defined stoichiometry and a higher degree of ordering. A careful characterization of the local structure of thin films versus bulk single crystals combined with ultrafast demagnetization studies can, thus, help to understand the impact of stoichiometry and order on ultrafast spin dynamics. Here, we present a comparative study of the structural ordering and magnetization dynamics for thin films and bulk single crystals of the family of Heusler alloys with composition Co2Fe1 - xMnxSi. The local ordering is studied by 59Co nuclear magnetic resonance (NMR) spectroscopy, while the time-resolved magneto-optical Kerr effect gives access to the ultrafast magnetization dynamics. In the NMR studies we find significant differences between bulk single crystals and thin films, both regarding local ordering and stoichiometry. The ultrafast magnetization dynamics, on the other hand, turns out to be mostly unaffected by the observed structural differences, especially on the time scale of some hundreds of femtoseconds. These results confirm hole-mediated spin-flip processes as the main mechanism for ultrafast demagnetization and the robustness of this demagnetization channel against defect states in the minority band gap as well as against the energetic position of the band gap with respect to the Fermi energy. The very small differences observed in the magnetization dynamics on the picosecond time-scale, on the other hand, can be explained by considering the differences in the electronic structure at the Fermi energy and in the heat diffusion of thin films and bulk crystals. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/16/164016
  • 2015 • 102 Evaluation of the Electrochemical Stability of Model Cu-Pt(111) Near-Surface Alloy Catalysts
    Tymoczko, J. and Calle-Vallejo, F. and Čolić, V. and Schuhmann, W. and Bandarenka, A.S.
    Electrochimica Acta 179 469-474 (2015)
    Better understanding of the factors responsible for the long-term stability of electrocatalysts is of increasing importance for the development of new generations of efficient electrode materials relevant for sustainable energy provision. Therefore, experiments with model, often single-crystal catalytic surfaces are of significance for fundamental electrochemistry and technological applications. Among model electrocatalysts, near-surface alloys (NSAs) of Pt with Cu, Ni and other metals formed via electrochemical deposition and thermal annealing have shown remarkable properties, demonstrating high activity towards a number of important reactions, including the oxygen reduction reaction (ORR) and CO oxidation. However, relatively little is known about the electrochemical stability and mechanisms of degradation of model NSAs. In this work, we employ a simple electrochemical approach, supported by density functional theory calculations, to evaluate the stability of Cu-Pt(111) NSAs in 0.1 M HClO4. Our results show that ∼30% of the Cu atoms initially incorporated into the second atomic layer of Pt are lost within the first 2000 cycles performed between 0.05 V and 1.0 V (RHE). After 5000 cycles, ca. half of the Cu atoms initially placed in the second atomic layer still remained in the subsurface region. The dissolution of Cu has a substantial impact on the measured shift in the average OH-binding energy for the catalyst surface and, consequently, on the ORR activity. Interestingly, after dissolution of Cu from NSAs, voltammetric features, which are characteristic to the Pt(111) facets, are partially restored suggesting the formation of NSA and Pt(111) domains in the resulting surface. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2015.02.110
  • 2015 • 101 Damage evolution in pseudoelastic polycrystalline Co-Ni-Ga high-temperature shape memory alloys
    Vollmer, M. and Krooß, P. and Segel, C. and Weidner, A. and Paulsen, A. and Frenzel, J. and Schaper, M. and Eggeler, G. and Maier, H.J. and Niendorf, T.
    Journal of Alloys and Compounds 633 288-295 (2015)
    Due to its transformation behavior, Co-Ni-Ga represents a very promising high temperature shape memory alloy (HT SMA) for applications at elevated temperatures. Co-Ni-Ga single crystals show a fully reversible pseudoelastic shape change up to temperatures of 400 °C. Unfortunately, polycrystalline Co-Ni-Ga suffers from brittleness and early fracture mainly due to intergranular constraints. In the current study, different thermo-mechanical processing routes produced various microstructures which differ in grain size and texture. A bicrystalline bamboo-like grain structure results in the highest reversible transformation strains and excellent cyclic stability. Moreover, solution-annealed and hot-rolled conditions also showed cyclic stability. Using in situ high-resolution electron microscopy, the elementary processes, which govern the microstructural evolution during pseudoelastic cycling were investigated and the mechanisms that govern structural and functional degradation were identified. The observations documented in the present work suggest that the formation of the ductile γ-phase on and near grain boundaries as well as the activation of multiple martensite variants at grain boundaries are beneficial for improved cyclic performance of polycrystalline Co-Ni-Ga HT SMAs. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.01.282
  • 2015 • 100 Multiscale Simulation of Plasticity in bcc Metals
    Weygand, D. and Mrovec, M. and Hochrainer, T. and Gumbsch, P.
    Annual Review of Materials Research 45 369-390 (2015)
    Significant progress in our understanding of plasticity in body-centered cubic (bcc) metals during the last decade has enabled rigorous multiscale modeling based on quantitative physical principles. Significant advances have been made at the atomistic level in the understanding of dislocation core structures and energetics associated with dislocation glide by using high-fidelity models originating from quantum mechanical principles. These simulations revealed important details about the influence of non-Schmid (nonglide) stresses on the mobility of screw dislocations in bcc metals that could be implemented to mesoscopic discrete dislocation simulations with atomistically informed dislocation mobility laws. First applications of dislocation dynamics simulations to studies of plasticity in small-scale bcc single crystals have been performed. Dislocation dynamics simulations inspired the development of continuum models based on advanced 3D dislocation density measures with evolution equations that naturally track dislocation motion. These advances open new opportunities and perspectives for future quantitative and materials-specific multiscale simulation methods to describe plastic deformation in bcc metals and their alloys. Copyright © 2015 by Annual Reviews. All rights reserved.
    view abstractdoi: 10.1146/annurev-matsci-070214-020852
  • 2015 • 99 On the role of Re in the stress and temperature dependence of creep of Ni-base single crystal superalloys
    Wollgramm, P. and Buck, H. and Neuking, K. and Parsa, A.B. and Schuwalow, S. and Rogal, J. and Drautz, R. and Eggeler, G.
    Materials Science and Engineering A 628 382-395 (2015)
    In the present study we investigate the creep behavior of a Ni-base single crystal superalloy. We evaluate the stress and temperature dependence of the minimum creep rate, which shows a power law type of stress dependence (characterized by a stress exponent n) and an exponential type of temperature dependence (characterized by an apparent activation energy Qapp). Under conditions of high temperature (1323K) and low stress (160MPa) creep, n and Qapp are determined as 5.3 and 529kJ/mol, respectively. For lower temperatures (1123K) and higher stresses (600MPa) the stress exponent n is higher (8.5) while the apparent activation energy of creep is lower (382kJ/mol). We show that there is a general trend: stress exponents n increase with increasing stress and decreasing temperature, while higher apparent activation energies are observed for lower stresses and higher temperatures. We use density functional theory (DFT) to calculate the activation energy of diffusion for Re in a binary Ni-Re alloy with low Re-concentrations. The resulting energy is almost a factor 2 smaller than the apparent activation energy of creep. We explain why it is not straightforward to rationalize the temperature dependence of creep merely on the basis of the diffusion of one alloying element. We show that the evolution of the microstructure also must be considered. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.01.010
  • 2015 • 98 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 • 97 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
  • 2014 • 96 On the nature of γ′ phase cutting and its effect on high temperature and low stress creep anisotropy of Ni-base single crystal superalloys
    Agudo Jácome, L. and Nörtershäuser, P. and Somsen, C. and Dlouhý, A. and Eggeler, G.
    Acta Materialia 69 246-264 (2014)
    The creep anisotropy of the single crystal superalloy LEK 94 deformed in tension along [0 0 1] and [1 1 0] directions at 1293 K and 160 MPa was investigated. Elementary microstructural processes which are responsible for a higher increase in creep rates with strain during [1 1 0] as compared to [0 0 1] tensile loading were identified. [1 1 0] tensile creep is associated with a higher number of γ′ phase cutting events, where two dislocations with equal Burgers vectors of type <1 1 0> jointly shear the γ′ phase. The resulting <2 2 0>-type superdislocation can move by glide. In contrast, during [0 0 1] tensile loading, two dislocations with different <1 1 0>-type Burgers vectors must combine for γ′ phase cutting. The resulting <2 0 0>-type superdislocations can only move by a combination of glide and climb. The evolution of dislocation networks during creep determines the nature of the γ′ phase cutting events. The higher [1 1 0] creep rates at strains exceeding 2% result from a combination of a higher number of cutting events (density of mobile dislocations in γ′) and a higher superdislocation mobility (<2 2 0>glide) in the γ′ phase. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.01.021
  • 2014 • 95 Plastic anisotropy of electro-deposited pure α-iron with sharp crystallographic <1 1 1>// texture in normal direction: Analysis by an explicitly dislocation-based crystal plasticity model
    Alankar, A. and Field, D.P. and Raabe, D.
    International Journal of Plasticity 52 18-32 (2014)
    We present a single crystal plasticity model based on edge and screw dislocation densities for body centered cubic (bcc) crystals. In a bcc crystal screw dislocations experience high lattice friction due to their non-planar core. Hence, they have much slower velocity compared to edge dislocations. This phenomenon is modeled by accounting for the motion of screw dislocations via nucleation and expansion of kink-pairs. The model, embedded as a constitutive law into a crystal plasticity framework, is able to predict the crystallographic texture of a bcc polycrystal subjected to 70%, 80% and 90% thickness reduction. We perform a parametric study based on the velocities of edge and screw dislocations to analyze the effect on plastic anisotropy of electro-deposited pure iron with long needle-shaped grains having sharp crystallographic <1 1 1>//ND texture (ND: normal direction). The model shows a large change in the r-value (Lankford value, planar anisotropy ratio) for pure iron when the texture changes from random to <1 1 1>//ND. For different simulated cases where the crystallites have an orientation deviation of 1, 3 and 5, respectively, from the ideal <1 1 1>//ND axis, the simulations predict r-values between 4.0 and 7.0 which is in excellent agreement with data observed in experiments by Yoshinaga et al. (ISIJ Intern.; 48 (2008) 667-670). For these specific orientations of grains, we also model the effect of long needle shaped grains via a procedure that excludes dislocation annihilation. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijplas.2013.03.006
  • 2014 • 94 Ab initio structure determination of interlayer expanded zeolites by single crystal rotation electron diffraction
    Guo, P. and Liu, L. and Yun, Y. and Su, J. and Wan, W. and Gies, H. and Zhang, H. and Xiao, F.-S. and Zou, X.
    Dalton Transactions 43 10593-10601 (2014)
    Layered solids often form thin plate-like crystals that are too small to be studied by single-crystal X-ray diffraction. Although powder X-ray diffraction (PXRD) is the conventional method for studying such solids, it has limitations because of peak broadening and peak overlapping. We have recently developed a software-based rotation electron diffraction (RED) method for automated collection and processing of 3D electron diffraction data. Here we demonstrate the ab initio structure determination of two interlayer expanded zeolites, the microporous silicates COE-3 and COE-4 (COE-n stands for International Network of Centers of Excellence-n), from submicron-sized crystals by the RED method. COE-3 and COE-4 are built of ferrierite-type layers pillared by (-O-Si(CH 3)2-O-) and (-O-Si(OH)2-O-) linker groups, respectively. The structures contain 2D intersecting 10-ring channels running parallel to the ferrierite layers. Because both COE-3 and COE-4 are electron-beam sensitive, a combination of RED datasets from 2 to 3 different crystals was needed for the structure solution and subsequent structure refinement. The structures were further refined by Rietveld refinement against the PXRD data. The structure models obtained from RED and PXRD were compared. This journal is © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4dt00458b
  • 2014 • 93 Microshear deformation of gold single crystals
    Heyer, J.-K. and Brinckmann, S. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 62 225-238 (2014)
    We perform microshear experiments on Au single crystals, directly imposing shear loading on the microscopic crystallographic h1-10i {111} slip system. We use a focused ion beam machined micro-double shear specimen which we load with a flat punch indenter inside a scanning electron microscope. Our method yields reproducible mechanical data (e.g. critical shear stresses of 63.5 ± 2.5 MPa). We study small-scale plasticity up to high strains (>50%) at constant slip geometry and document localized plastic deformation and sudden plastic deformation events. Strain bursts are observed, which can be related to the formation of new shear bands. Alternatively, they can result from sudden shear strain accumulation events in existing shear bands. Due to the stochastic nature of plastic deformation, the nature and the number of strain bursts can vary. We show and discuss how our in situ test technique captures these effects and how this affects the corresponding load-displacement curves. We discuss the advantages and inconveniences of our microshear test technique compared to other small-scale testing methods and relate our mechanical results to previous results reported for the micromechanical behavior of Au. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.10.002
  • 2014 • 92 Interface properties in lamellar TiAl microstructures from density functional theory
    Kanani, M. and Hartmaier, A. and Janisch, R.
    Intermetallics 54 154-163 (2014)
    The deformability and strength of lamellar two-phase (γ and α2) TiAl alloys strongly depends on the mechanical properties of the different interfaces in such microstructures. We carried out ab-initio density functional theory calculations of interface energy and strength for all known interface variants as well as the corresponding single crystal slip/cleavage planes to obtain a comprehensive database of key mechanical quantities. This data collection can be used for meso-scale simulations of deformation and fracture in TiAl. In spite of the different atomic configurations of the lamellar interfaces and the single crystal planes, the calculated values for the tensile strength are in the same range and can be considered as equal in a meso-scale model. Analysis of generalized stacking fault energy surfaces showed that the shear strength is directional dependent, however, the [112̄] direction is an invariant easy gliding direction in all investigated systems. The probability of different dislocation dissociation reactions as part of a shear deformation mechanism are discussed as well. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.06.001
  • 2014 • 91 Synthesis of WO3 nanoblades by the dealloying of glancing angle deposited W-Fe nanocolumnar thin films
    Khare, C. and Stepanovich, A. and Buenconsejo, P.J.S. and Ludwig, Al.
    Nanotechnology 25 (2014)
    Glancing angle co-deposition of well-separated W-Fe nanocolumns was carried out using a W oblique angle sputter source and a Fe confocal incidence source. As-deposited nanocolumns with an overall composition of W64.6Fe35.4 (at.%) exhibited an average column width w nc of 77 ± 15 nm with predominant growth in the β-W phase. With the aim of synthesizing highly porous nanostructures, the as-deposited precursor W-Fe nanocolumnar thin films were immersed in aqueous HNO3 solution for various dealloying durations (t d ). Formation of nanoflake-, nanocactus-, and nanoblade-like structures were observed during the dealloying treatment, as a result of selective dissolution of Fe from the W-Fe precursor films and simultaneous oxidation of W adatoms. By increasing the dealloying duration, the Fe concentration within the film reduced drastically and the film thickness increased by about three times in comparison to the as-deposited film. The dealloyed film exhibited an overall composition of W95.6Fe4.4, where the effective surface area of the film increased substantially. It was found that W adatom diffusion and subsequent rearrangement are crucially important in determining the resultant thin film morphology. The morphological development, corresponding compositions and crystallographic properties of different nanostructures were found to be significantly dependent on the dealloying duration. For optimized processing parameters, the selective dissolution process led to formation of single crystal monoclinic WO3 nanoblades, with growth along [002] and [020] axes. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/25/20/205606
  • 2014 • 90 Modeling of single crystal magnetostriction based on numerical energy relaxation techniques
    Kiefer, B. and Buckmann, K. and Bartel, T. and Menzel, A.
    ASME 2014 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2014 1 (2014)
    This paper presents an energy relaxation-based approach for the modeling of single crystalline magnetic shape memory alloy response under general two-dimensional magnetomechanical loading. It relies on concepts of energy relaxation in the context of non-convex free energy landscapes whose wells define preferred states of straining and magnetization. The constrained theory of magnetoelasticity developed by DeSimone and James [1] forms the basis for the model development. The key features that characterize the extended approach are (i) dissipative effects, accounted for in an incremental variational setting, and (ii) finite magnetocrystalline anisotropy energy. In this manner, important additional response features, e.g. the hysteretic nature, the linear magnetization response in the prevariant reorientation regime, and the stress dependence of the maximum field induced strain, can be captured, which are prohibited by the inherent assumptions of the constrained theory. The enhanced modeling capabilities of the extended approach are demonstrated by several representative response simulations and comparison to experimental results taken from literature. These examples particularly focus on the response of single crystals under cyclic magnetic field loading at constant stress, and cyclic mechanical loading at constant magnetic field. © 2014 by ASME.
    view abstractdoi: 10.1115/SMASIS20147436
  • 2014 • 89 Cyclic degradation mechanisms in aged FeNiCoAlTa shape memory single crystals
    Krooß, P. and Somsen, C. and Niendorf, T. and Schaper, M. and Karaman, I. and Chumlyakov, Y. and Eggeler, G. and Maier, H.J.
    Acta Materialia 79 126-137 (2014)
    This study focuses on the functional stability of [0 0 1]-oriented Fe 41Ni28Co17Al11.5Ta2.5 (at.%) single crystals. It is shown that functional degradation of aged FeNiCoAlTa, containing fine dispersed γ′-particles ∼5-8 nm in diameter is caused by the interaction of different martensite variants under cyclic loading in tension. Superelastic cycling experiments up to 4.5% total strain resulted in the accumulation of permanent strain mainly caused by the formation of retained martensite. In situ observations were conducted in order to evaluate the local strain evolution and martensite variant interactions on the meso- and microscale. Optical microscopy and transmission electron microscopy observations revealed various differently oriented martensite variants which were retained upon 100 superelastic cycles. In addition, fine martensitic structures remaining in the vicinity of the γ′ precipitates were found after mechanical cycling, which are shown to be important for cyclic degradation in Fe-based shape memory alloys. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.06.019
  • 2014 • 88 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 • 87 Interfacial dislocation motion and interactions in single-crystal superalloys
    Liu, B. and Raabe, D. and Roters, F. and Arsenlis, A.
    Acta Materialia 79 216-233 (2014)
    The early stage of high-temperature low-stress creep in single-crystal superalloys is characterized by the rapid development of interfacial dislocation networks. Although interfacial motion and dynamic recovery of these dislocation networks have long been expected to control the subsequent creep behavior, direct observation and hence in-depth understanding of such processes has not been achieved. Incorporating recent developments of discrete dislocation dynamics models, we simulate interfacial dislocation motion in the channel structures of single-crystal superalloys, and investigate how interfacial dislocation motion and dynamic recovery are affected by interfacial dislocation interactions and lattice misfit. Different types of dislocation interactions are considered: self, collinear, coplanar, Lomer junction, glissile junction, and Hirth junction. The simulation results show that strong dynamic recovery occurs due to the short-range reactions of collinear annihilation and Lomer junction formation. The misfit stress is found to induce and accelerate dynamic recovery of interfacial dislocation networks involving self-interaction and Hirth junction formation, but slow down the steady interfacial motion of coplanar and glissile junction forming dislocation networks. The insights gained from these simulations on high-temperature low-stress creep of single-crystal superalloys are also discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.06.048
  • 2014 • 86 On the influence of isotropic and kinematic hardening caused by strain gradients on the deformation behaviour of polycrystals
    Ma, A. and Hartmaier, A.
    Philosophical Magazine 94 125-140 (2014)
    During the deformation of polycrystals, pronounced strain gradients may occur at grain boundaries between grains whose misorientations lead to a large mismatch in their deformation behaviour. Hence, even under globally uniaxial and homogeneous strains, internal stresses will arise that must be characterized by nonlocal plasticity models. In this work, such a nonlocal constitutive model is formulated based on the concept of densities of geometrically necessary superdislocations in an isotropic elastic-plastic medium. Since the deformation of individual grains is considered, crystal plasticity models are applied that take into account plastic slip on crystallographic planes. This new nonlocal constitutive model is applied to describe the deformation of a polycrystal under the influence of plastic strain gradients caused by isotropic and kinematic strain hardening. It is found that isotropic hardening originating from plastic strain gradients amplifies deformation heterogeneities stemming from different Schmid factors in neighbouring grains. However, the kinematic hardening resulting from plastic strain gradients tends to reduce such deformation heterogeneity. Thus, the capability of a polycrystal to deform uniformly is determined by the competition between isotropic and kinematic hardening. Finally, the model is applied to explain why grain refinement is an efficient way to improve material strength and ductility at the same time. © 2013 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2013.847290
  • 2014 • 85 Relaxor behavior of pure and cerium doped CaxBa 1-xNb2O6
    Pandey, C.S. and Schreuer, J. and Burianek, M. and Muehlberg, M.
    Ferroelectrics 464 80-87 (2014)
    Here we report the relaxor behavior of pure and cerium doped Czochralski grown lead free relaxor ferroelectric single crystals CaxBa 1-xNb2O6 (CBN-x) (0.18 ≤ x ≤ 0.35) using temperature dependent elastic behavior. We observed that the dynamic relaxor behavior strongly varies with the variation of Ca content as well as with doping. Evidence is found for a more pronounced relaxor behavior with increasing Ca content and doping. Characteristic temperature Tz.ast; (temperature at which static behavior of the polar nanoregions begins to appear) found to be unaffected with Ca content variation as well as doping Copyright © Taylor & Francis Group, LLC.
    view abstractdoi: 10.1080/00150193.2014.893152
  • 2014 • 84 Vacancy mobility and interaction with transition metal solutes in Ni
    Schuwalow, S. and Rogal, J. and Drautz, R.
    Journal of Physics Condensed Matter 26 (2014)
    Interaction of Re, Ta, W and Mo solutes with vacancies and their diffusion in fcc Ni is investigated by density-functional theory in combination with kinetic Monte Carlo simulations. Interaction energies are calculated for the first six neighbor shells around the solutes and a complete set of diffusion barriers for these shells is provided. Further, diffusion coefficients for the four elements in Ni as well as for vacancies in the presence of these elements are calculated. The calculated solute diffusion coefficients based on our ab initio data are found to compare favorably to experimental values. The mobility of the vacancies as a key factor in dislocation climb is only minimally influenced by the solute atoms within the dilute limit. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/48/485014
  • 2014 • 83 Modeling the microstructure influence on fatigue life variability in structural steels
    Sharaf, M. and Kucharczyk, P. and Vajragupta, N. and Münstermann, S. and Hartmaier, A. and Bleck, W.
    Computational Materials Science 94 258-272 (2014)
    The endurance and HCF lifetime of multiphase steel components depend mainly on the phase of fatigue microcrack initiation and early propagation. A numerical study, which quantitatively describes the influence of microstructural features on the initiation and growth of cyclic microcracks, is presented within the context of microstructure-sensitive modeling. The implementation of kinematic hardening on each slip system in a crystal plasticity model allows for capturing the local accumulation of plastic microdeformation representing slip irreversibility occurring in the crack incubation phase. A load increasing testing technique with continuous temperature measurement and interrupted cyclic bending experiments deliver information about the endurance strength of a structural steel and allow for metallographic observation of cyclic microcrack propagation and thereby provide the experimental basis for the numerical simulations. The material model is implemented in cyclic computations with statistically representative volume elements, which are based on experimental microstructure description using the electron backscatter diffraction technique (EBSD). The extreme value distributions of the computed accumulation of local dislocation slip are then correlated to the microstructure in an approach to assess and explore the validity extent of microstructure-sensitive modeling using fatigue indicator parameters (FIPs) to correlate to the endurance limit and fatigue life under high-cycle fatigue conditions. The eligibility of consideration of the stresses normal to the planes of localized plastic damage assisting fatigue crack formation into these FIPs is investigated. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2014.05.059
  • 2014 • 82 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 • 81 A model for high temperature creep of single crystal superalloys based on nonlocal damage and viscoplastic material behavior
    Trinh, B.T. and Hackl, K.
    Continuum Mechanics and Thermodynamics 26 551-562 (2014)
    A model for high temperature creep of single crystal superalloys is developed, which includes constitutive laws for nonlocal damage and viscoplasticity. It is based on a variational formulation, employing potentials for free energy, and dissipation originating from plasticity and damage. Evolution equations for plastic strain and damage variables are derived from the well-established minimum principle for the dissipation potential. The model is capable of describing the different stages of creep in a unified way. Plastic deformation in superalloys incorporates the evolution of dislocation densities of the different phases present. It results in a time dependence of the creep rate in primary and secondary creep. Tertiary creep is taken into account by introducing local and nonlocal damage. Herein, the nonlocal one is included in order to model strain localization as well as to remove mesh dependence of finite element calculations. Numerical results and comparisons with experimental data of the single crystal superalloy LEK94 are shown. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00161-013-0317-6
  • 2014 • 80 Position of Cu atoms at the Pt(111) electrode surfaces controls electrosorption of (H)SO4 (2)- from H2SO4 electrolytes
    Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
    ChemElectroChem 1 (2014)
    Selective positioning of monolayer amounts of foreign atoms at the surface and subsurface regions of metal electrodes is a promising way to fine-tune the properties of the electrode/ electrolyte interface. The latter is critical as it largely governs the adsorption of electrolyte components and reaction intermediates and, therefore, controls many key electrocatalytic processes. Using model Pt(111) single-crystal electrodes, we demonstrate how the relative position of Cu atoms at the surface drastically changes the adsorption energies for (bi)sulfate anions. Our measurements involve pseudomorphic overlayers of Cu on Pt(111) as well as Cu-Pt(111) surface and sub-surface alloys, where Cu atoms were located either in the first or in the second atomic layers of Pt, respectively. In the case of Cu- Pt(111) surface alloys, specific adsorption of the anions starts earlier compared to the unmodified Pt(111) surface. In contrast, placing Cu atoms into the second atomic layer weakens the binding between the surface and the anions. Surprisingly, Cu pseudomorphic overlayers do not reveal any specific adsorption of (bi)sulfates (within the region of the overlayer stability). Taking into account that electrified interfaces between Pt(111) electrodes and sulfate-containing electrolytes often play the role of benchmark systems in fundamental physico-chemical and, particularly, electrocatalytic studies, our findings demonstrate a promising and relatively easy route of tuning the properties of these interfaces. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201300107
  • 2014 • 79 Synthesis, characterization, and nanoindentation response of single crystal Fe-Cr-Ni alloys with FCC and BCC structures
    Xia, Y.Z. and Bei, H. and Gao, Y.F. and Catoor, D. and George, E.P.
    Materials Science and Engineering A 611 177-187 (2014)
    Fe-based alloys are used extensively in many structural applications including under irradiation conditions in the nuclear industry. In this study, model Fe-Cr, Fe-Ni and Fe-Cr-Ni alloys that are the basis of many structural steels were synthesized as single crystals and characterized. The compositions investigated were Fe-15Cr, Fe-30Cr, Fe-30Ni and Fe-15Cr-15Ni (at%). Several key mechanical properties were determined which will be useful in further studies of irradiation/deformation-induced defects. Incipient plasticity and slip characteristics were investigated by nanoindentation on (001) and (1-10) surfaces, and hardness, modulus, pop-in behavior and theoretical strength were determined. The slip trace patterns after microindentation were imaged in a microscope. A novel slip trace analysis was developed and the underlying deformation mechanisms identified. The analysis shows that under both (001) and (1-10) indentations, the activated slip system for the BCC alloys is {112} for the FCC alloys the activated slip plane is {111}. These results were confirmed with finite element simulations using a slip-based crystal-plasticity model. Finally, the effects of heterogeneous pop-in mechanisms are discussed in the context of incipient plasticity in the four different alloys. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.05.079
  • 2013 • 78 Specific features of Yb3+ ions in electronic band energy structure and optical functions of RbNd(WO4)2 crystals: Synchrotron ellipsometry measurements and DFT simulations
    Andriyevsky, B. and Piasecki, M. and Dorywalski, K. and Cobet, C. and Esser, N. and Świrkowicz, M. and Majchrowski, A. and Jaroszewicz, L.R. and Kityk, I.V.
    Journal of Alloys and Compounds 577 237-246 (2013)
    The influence of 5% and 10% (in molar units) of ytterbium doping of RbNd(WO4)2 single crystals on the dielectric permittivity e in the spectral range of electronic excitations was studied. The corresponding differences of the pseudo-dielectric functions ≪ε(E)≫ for pure and Yb-doped RbNd(WO4)2 were studied by spectroscopic ellipsometry method including VUV spectral range using synchrotron radiation light source. This method opens an opportunity for spectroscopic diagnostics of the rare earth dopants in crystals using information concerning the electronic inter-band optical transitions. To better understand the experimental results, for the first time the ab initio calculations of band structure, related properties and optical spectra of the centrosymmetric RbNd(WO4) 2 single crystal were performed using the VASP code. The calculated real and imaginary parts of dielectric function dispersions ε 1(E) and ε2(E) agree satisfactorily with our experimental results. It was established that differences of optical functions for pure and doped crystals may serve as a powerful tool for spectroscopic diagnostic of localized rare earth ions with respect to the strong inter-band transitions in the wide band dielectrics. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2013.04.066
  • 2013 • 77 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 • 76 Atomistic aspects of 〈1 1 1〉 screw dislocation behavior in α-iron and the derivation of microscopic yield criterion
    Chen, Z.M. and Mrovec, M. and Gumbsch, P.
    Modelling and Simulation in Materials Science and Engineering 21 (2013)
    The plastic deformation of body-centered cubic iron at low temperatures is governed by slip behavior of 1 2 〈1 1 1〉 screw dislocations. Their non-planar core structure gives rise to a strong temperature dependence of the yield stress and overall plastic behavior that does not follow the Schmid law common to most close-packed metals. In this work,we carry out a systematic study of the screw dislocation behavior in α-Fe by means of atomistic simulations using a state-of-the-art magnetic bond-order potential. Based on the atomistic simulations of the screw dislocations under various external loadings, we formulate an analytical yield criterion that correctly captures the non-Schmid plastic response of iron single crystals under general loading conditions. The theoretical predictions of operative slip systems for uniaxial loadings agree well with available experimental observations, and demonstrate the robustness and reliability of such atomistically based yield criterion. In addition, this bottom-up approach can be directly utilized to formulate dislocation mobility rules in mesoscopic discrete dislocation dynamics simulations. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/21/5/055023
  • 2013 • 75 Ultraviolet vacuum ultraviolet optical functions for SrTiO3 and NdGaO3 crystals determined by spectroscopic ellipsometry
    Dorywalski, K. and Andriyevsky, B. and Piasecki, M. and Lemee, N. and Patryn, A. and Cobet, C. and Esser, N.
    Journal of Applied Physics 114 (2013)
    Complex dielectric functions ε(E) = ε1(E) + ε2(E) were experimentally evaluated within the spectral range E = 2-25 eV and E = 2-20 eV for SrTiO3 and NdGaO3 single crystals, respectively, using synchrotron-based spectroscopic ellipsometry measurements. The ellipsometric spectra were evaluated within a framework of optical layer model taking into account sample surface roughness and anisotropy of NdGaO3. The parameters of Herzinger-Johs oscillator model were fitted to reproduce sufficiently all features of the optical spectra within the spectral range 2-10 eV. Only slight differences were revealed for spectra polarized along b and c crystallographic axes of the NdGaO3, which can confirm weak optical anisotropy. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4816624
  • 2013 • 74 Effect of climb on dislocation mechanisms and creep rates in γ′-strengthened Ni base superalloy single crystals: A discrete dislocation dynamics study
    Hafez Haghighat, S.M. and Eggeler, G. and Raabe, D.
    Acta Materialia 61 3709-3723 (2013)
    Creep of single-crystal superalloys is governed by dislocation glide, climb, reactions and annihilation. Discrete three-dimensional (3D) dislocation dynamics (DDD) simulations are used to study the evolution of the dislocation substructure in a γ/γ′ microstructure of a single-crystal superalloy for different climb rates and loading conditions. A hybrid mobility law for glide and climb is used to map the interactions of dislocations with γ′ cubes. The focus is on the early stages of creep, where dislocation plasticity is confined to narrow γ channels. With enhancing climb mobility, the creep strain increases, even if the applied resolved shear stress is below the critical stress required for squeezing dislocations into the γ channels. The simulated creep microstructure consists of long dislocations and a network near the corners of the γ′ precipitate in the low-stress regime. In the high-stress regime, dislocations squeeze into the γ channels, where they deposit dislocation segments at the γ/γ′ interfaces. These observations are in good agreement with experimentally observed dislocation structures that form during high-temperature and low-stress creep. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.03.003
  • 2013 • 73 Preparation of thin film Cu-Pt(1 1 1) near-surface alloys: One small step towards up-scaling model single crystal surfaces
    Henry, J.B. and Maljusch, A. and Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
    Electrochimica Acta 112 887-893 (2013)
    A method for the preparation of Pt(1 1 1) like thin films and thin film Cu-Pt(1 1 1) near-surface alloys (where Cu is preferentially located in the subsurface region) is reported in detail. Cyclic voltammograms of the resultant Pt(1 1 1)-like thin films in 0.1 M HClO4 demonstrate characteristic "butterfly" peaks attributed to the disorder/order phase transition in the adsorbed*OH layer, typical for a large Pt(1 1 1) crystals. Modification of the film surface with a monolayer of Cu and subsequent annealing in a reducing Ar/H2(5%) atmosphere, the voltammograms resemble those obtained for Cu-Pt(1 1 1) near-surface alloys prepared on commercial bulk single crystals. This method shows promise for the fabrication of extended (1 1 1)-type alloy surfaces of Pt and its alloys and can additionally be used to up-scale model objects for wider industrial and laboratory applications. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2012.11.139
  • 2013 • 72 A Z′ = 6 crystal structure of (E)-N,N′-dicyclohexylacetamidine
    Krasnopolski, M. and Seidel, R.W. and Goddard, R. and Breidung, J. and Winter, M.V. and Devi, A. and Fischer, R.A.
    Journal of Molecular Structure 1031 239-245 (2013)
    The crystal and molecular structure of (E)-N,N′- dicyclohexylacetamidine (1) is described. Crystalline material of 1 was obtained by sublimation. Single-crystal X-ray analysis revealed a centrosymmetric triclinic structure (space group P1̄) with six molecules in the asymmetric unit (Z′ = 6). The six crystallographically distinct molecules all exhibit an E-syn structure, but differ in the orientation of the cyclohexyl groups about the central acetamidine moiety. In the crystal, the molecules form polymeric helices via NH⋯N hydrogen bonds. The crystal structure comprises two crystallographically distinct helices of opposite handedness (P and M form). The characterisation of 1 in the solid-state is augmented by powder X-ray diffraction, infrared spectroscopy and thermal analysis. Density functional theory (DFT) structure optimisation and frequency calculation were performed at the B3LYP/cc-pVTZ level. The DFT results for the isolated molecule are compared with the experimental results for the solid-state. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.molstruc.2012.10.003
  • 2013 • 71 Effect of high pressure and high temperature on the microstructural evolution of a single crystal Ni-based superalloy
    Lopez-Galilea, I. and Huth, S. and Theisen, W. and Fockenberg, T. and Chakraborty, S.
    Journal of Materials Science 48 348-358 (2013)
    The application of high nearly hydrostatic pressures at elevated temperatures on the LEK94 single crystal (SX) nickel-based superalloy directly affects its microstructure. This is due to a combination of the effect of pressure on the Gibbs free energy, on the diffusion coefficients of the alloying elements, on the internal coherent stresses, and on the porosity distribution. The last effect depends at least on the first three. Therefore, based on the theoretical influences of the pressure, the main objective of this work is to understand, by means of an experimental work, the effect of high pressure at elevated temperature during annealing on the evolution of the phases morphology, and porosity of the high-temperature material LEK94. Specifically, pressures up to 4 GPa, temperatures up to 1180 C, and holding times up to 100 h were investigated. The main findings are that, porosity can be considerably reduced without affecting significantly the γ/γ′ microstructure by high pressure annealing and the verification that increasing the external pressure stabilizes the γ′-phase. © 2012 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-012-6752-0
  • 2013 • 70 Formation of carbon nanofilms on single crystal quartz
    Samsonau, S.V. and Dzedzits, E. and Shvarkov, S.D. and Meinerzhagen, F. and Wieck, A.D. and Zaitsev, A.M.
    Sensors and Actuators, B: Chemical 186 610-613 (2013)
    In this work formation of the very first layers of carbon nanofilms on single crystal quartz substrates is studied. Films where grown by molecular beam growth, and have been characterized by Raman spectroscopy and atomic force microscopy. Formation of a non-conductive carbon layer of low crystallinity on the initial stage of the growth process is reported. Ab-initio calculations with an atom-by-atom approach have been performed to explain the experimental data. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.snb.2013.06.023
  • 2013 • 69 Macroscopic and local piezoelectric properties of Pb(Mg 1/3Nb2/3)O3-PbTiO3 single crystals exhibiting giant piezoelectric response
    Shvartsman, V.V. and Kholkin, A.L. and Raevski, I.P. and Raevskaya, S.I. and Savenko, F.I. and Emelyanov, A.S.
    Journal of Applied Physics 113 (2013)
    The temperature and bias field dependences of macroscopic, measured by pulsating load method, and local, measured by piezoresponse force microscopy, longitudinal piezoelectric responses have been studied in (001)-oriented flux-grown (1 - x)Pb(Mg1/3Nb2/3)O3-xPbTiO 3 (0.0 x 0.29) single crystals. Both types of responses exhibit a dramatic enhancement with increasing bias fields. At the same time, their temperature maxima shift from the Vogel-Fulcher temperature to the vicinity of the dielectric permittivity maximum, where the critical point in the E-T phase diagram is located. Both datasets confirm a quasicritical nature of the giant field-induced piezoelectric response in relaxor single crystals. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4801964
  • 2012 • 68 Advanced scanning transmission stereo electron microscopy of structural and functional engineering materials
    Agudo Jácome, L. and Eggeler, G. and Dlouhý, A.
    Ultramicroscopy 122 48-59 (2012)
    Stereo transmission electron microscopy (TEM) provides a 3D impression of the microstructure in a thin TEM foil. It allows to perform depth and TEM foil thickness measurements and to decide whether a microstructural feature lies inside of a thin foil or on its surface. It allows appreciating the true three-dimensional nature of dislocation configurations. In the present study we first review some basic elements of classical stereo TEM. We then show how the method can be extended by working in the scanning transmission electron microscope (STEM) mode of a modern analytical 200. kV TEM equipped with a field emission gun (FEG TEM) and a high angle annular dark field (HAADF) detector. We combine two micrographs of a stereo pair into one anaglyph. When viewed with special colored glasses the anaglyph provides a direct and realistic 3D impression of the microstructure. Three examples are provided which demonstrate the potential of this extended stereo TEM technique: a single crystal Ni-base superalloy, a 9% Chromium tempered martensite ferritic steel and a NiTi shape memory alloy. We consider the effect of camera length, show how foil thicknesses can be measured, and discuss the depth of focus and surface effects. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2012.06.017
  • 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 Single crystal growth and characterization of mullite-type Bi 2Mn 4O 10
    Burianek, M. and Krenzel, T.F. and Schmittner, M. and Schreuer, J. and Fischer, R.X. and Mühlberg, M. and Nénertd, G. and Schneider, H. and Gesing, T.M.
    International Journal of Materials Research 103 449-455 (2012)
    A part of the pseudo-binary join Bi 2O 3-Bi 2Mn 4O 10 of the ternary system Bi 2O 3 -MnO-MnO 2 was examined using thermo-analytical methods. Because Bi 2Mn 4O 10 melts incongruently single crystals of up to 20 mm in diameter were grown by the top seeded solution growth method in the temperature range from about 1223 K to 1173 K. Single crystal neutron diffraction confirmed the principles of the crystal structure of Bi 2Mn 4O 10 but revealed much smaller distortions of the cation coordination polyhedra. In contrast to the anisotropy observed in other mullite-type Bi containing compounds, the linear thermal expansion of Bi 2Mn 4O 10, as studied by means of dilatometry and X-ray powder diffraction techniques, is characterized by α 11 &gt; α;33 &gt; α22 at room temperature. The relatively large expansion along the a-axis can be attributed to the two oxygen atoms bridging two corner shared MnO 5 tetrahedral pyramids which alternate with the structural void between two adjacent Bi 3+ cations. © 2012 Carl Hanser Verlag.
    view abstractdoi: 10.3139/146.110714
  • 2012 • 65 Continuum simulation of the evolution of dislocation densities during nanoindentation
    Engels, P. and Ma, A. and Hartmaier, A.
    International Journal of Plasticity 38 159-169 (2012)
    When nanoindenting dislocation-free regions of single crystals a so-called pop-in phenomenon is commonly observed. Molecular dynamics (MD) studies have revealed homogeneous nucleation of dislocations in a perfect crystal as a mechanism causing such pop-in behavior. In this work we transfer this knowledge gained on the atomic scale into a dislocation nucleation model that is applied within a dislocation density based crystal plasticity description. Furthermore, we develop a non-local formulation of a crystal plasticity model that is devised to yield a valid description of plasticity also in situations where the dislocation density is small or even vanishing and where conventional plasticity models fail. This is accomplished by studying the evolution of statistically stored and geometrically necessary dislocation densities separately. We apply this non-local crystal plasticity model to investigate the evolution of dislocation densities in the early stages of nanoindentation. The results of our continuum model show good agreement with MD simulations for cases where nanoindentation into an initially dislocation-free crystal is studied, i.e. where a pop-in occurs when the critical stress underneath the indenter reaches the critical value for homogeneous dislocation nucleation. After thus validating our model we study the influence of pre-existing homogeneous and local dislocation densities. Both cases show a good qualitative agreement with recent experimental findings and it is concluded that pre-existing local dislocations densities reduce the load at which a pop-in occurs and - more importantly - change the mechanism from homogeneous dislocation nucleation to rapid dislocation multiplication. In general, our results show that continuum plasticity formulations can be extended such that applications to nanoscale volumes become possible. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijplas.2012.05.010
  • 2012 • 64 Dependence of the yield stress of Fe3Al on heat treatment
    Hasemann, G. and Schneibel, J.H. and George, E.P.
    Intermetallics 21 56-61 (2012)
    The room-temperature yield strength of quenched FeAl-based iron aluminides depends strongly on the temperature from which the quench occurs. There is evidence that Fe3Al-based iron aluminides show similar behavior, albeit not as pronounced. The purpose of this work was to carry out quenching and annealing experiments to clearly demonstrate this effect, as well as to study its kinetics. Room-temperature compression tests were performed using cast Fe-28at%Al and Fe-30at%Al after quenching from temperatures in the range 300-1000 °C. Kinetic studies were carried out to assess the reduction of the yield stress by subsequent annealing for various times at relatively low temperatures. Results from the present study are compared with data available in the literature and explanations for the dependence of room-temperature mechanical behavior on annealing history are evaluated. It is concluded that room-temperature strengthening in Fe3Al-based iron aluminides is consistent with behavior expected for quenched-in thermal vacancies and this could be an important strengthening mechanism in Fe3Al. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2011.09.005
  • 2012 • 63 Orientation dependence of shear banding in face-centered-cubic single crystals
    Jia, N. and Eisenlohr, P. and Roters, F. and Raabe, D. and Zhao, X.
    Acta Materialia 60 3415-3434 (2012)
    We present crystal plasticity finite element simulations of plane strain compression of α-Brass single crystals with different initial orientations. The aim is to study the fundamentals of mesoscale structure and texture development in face-centered-cubic (fcc) metals with low stacking fault energy (SFE). Shear banding depends on the initial orientation of the crystals. In Copper and Brass-R-oriented crystals which show the largest tendency to form shear bands, an inhomogeneous texture distribution induced by shear banding is observed. To also understand the influence of the micromechanical boundary conditions on shear band formation, simulations on Copper-oriented single crystals with varying sample geometry and loading conditions are performed. We find that shear banding can be understood in terms of a mesoscopic softening mechanism. The predicted local textures and the shear banding patterns agree well with experimental observations in low SFE fcc crystals. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.03.005
  • 2012 • 62 In-situ tensile testing of single-crystal molybdenum-alloy fibers with various dislocation densities in a scanning electron microscope
    Johanns, K.E. and Sedlmayr, A. and Sudharshan Phani, P. and Mönig, R. and Kraft, O. and George, E.P. and Pharr, G.M.
    Journal of Materials Research 27 508-520 (2012)
    In-situ tensile tests have been performed in a dual beam focused ion beam and scanning electron microscope on as-grown and prestrained single-crystal molybdenum-alloy (Mo-alloy) fibers. The fibers had approximately square cross sections with submicron edge lengths and gauge lengths in the range of 9-41 μm. In contrast to previously observed yield strengths near the theoretical strength of 10 GPa in compression tests of ∼1-3-μm long pillars made from similar Mo-alloy single crystals, a wide scatter of yield strengths between 1 and 10 GPa was observed in the as-grown fibers tested in tension. Deformation was dominated by inhomogeneous plastic events, sometimes including the formation of Lüders bands. In contrast, highly prestrained fibers exhibited stable plastic flow, significantly lower yield strengths of ∼1 GPa, and stress-strain behavior very similar to that in compression. A simple, statistical model incorporating the measured dislocation densities is developed to explain why the tension and compression results for the as-grown fibers are different. © 2012 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2011.298
  • 2012 • 61 Advanced nanomechanics in the TEM: Effects of thermal annealing on FIB prepared Cu samples
    Kiener, D. and Zhang, Z. and Šturm, S. and Cazottes, S. and Imrich, P.J. and Kirchlechner, C. and Dehm, G.
    Philosophical Magazine 92 3269-3289 (2012)
    The effect of focused ion beam (FIB) fabrication on the mechanical properties of miniaturized mechanical tests has recently been realized, but is not well documented. In this study, the effect of post thermal annealing on the plastic properties of FIB fabricated micro- and nanometer-sized Cu samples was studied by means of advanced analytic and in situ transmission electron microscopy. In situ heating experiments on thin films and pillars revealed a reduction of the initially high dislocation density, but never a recovery of the bulk dislocation density. Aberration-corrected atomic imaging documented the recovery of a pristine crystalline surface structure upon annealing, while electron energy-loss spectroscopy showed that the remaining contamination layer consisted of amorphous carbon. These structural observations were combined with the mechanical data from in situ tests of annealed micro- and nanometer-sized tensile and compression samples. The thermal annealing in the micron regime mainly influences the initial yield point, as it reduces the number of suited dislocation sources, while the flow behavior is mostly unaffected. For the submicron samples, the annealed material sustains significantly higher stresses throughout the deformation. This is explained by the high stresses required for surface-mediated dislocation nucleation of the annealed material at the nanoscale. In the present case, the FIB affected the surface near defects and facilitated dislocation nucleation, thereby lowering the material strength. © 2012 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2012.685966
  • 2012 • 60 Atomistically informed crystal plasticity model for body-centered cubic iron
    Koester, A. and Ma, A. and Hartmaier, A.
    Acta Materialia 60 3894-3901 (2012)
    The glide of screw dislocations with non-planar dislocation cores dominates the plastic deformation behavior in body-centered cubic iron. This yields a strong strain rate and temperature dependence of the flow stress, the breakdown of Schmid's law and a dependence of dislocation mobility on stress components that do not contribute to the mechanical driving force of dislocation glide. We developed a constitutive plasticity model that takes all these effects into account. The model is based on the crystal plasticity approach and parameterized by performing molecular statics calculations using a semi-empirical potential. The atomistic studies yield quantitative relations between local stress tensor components and the mobility of dislocations. Together with experimental stress-strain curves obtained for two different orientations of iron single crystals taken from the literature, the constitutive law is completely parameterized. The model is validated by comparing numerical single crystal tension tests for a third orientation to the equivalent experimental data from the literature. We also provide results for the temperature and strain rate dependence of the new atomistically informed constitutive model. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.03.053
  • 2012 • 59 Thermal expansion and elastic properties of mullite-type Bi 2Ga 4O 9 and Bi 2Fe 4O 9 single crystals
    Krenzel, T.F. and Schreuer, J. and Gesing, T.M. and Burianek, M. and Mühlberg, M. and Schneider, H.
    International Journal of Materials Research 103 438-448 (2012)
    Resonant ultrasound spectroscopy was used to characterize the elastic properties of single crystal orthorhombic Bi 2Ga 4O 9 and Bi 2Fe 4O 9 between room temperature and about 1200 K. Additionally, the coefficients of thermal expansion were studied in the range 100 K to 1 280 K using high-resolution dilatometry and X-ray powder diffraction. The elastic constants at 295 K are in GPa c 11 = 143.4(1), c 22 = 161.9(1), c 33 = 224.5(1), c 44 = 68.4(1), c 55 = 49.3(1), c 66 = 76.6(1), c 12 = 74.2(1), c 13 = 62.2(1), c 23 = 70.5(1) for Bi 2Ga 4O 9, and c 11 = 106.7(1), c 22 = 141.2(1), c 33 = 183.7(2), c 44 = 53.7(1), c 55 = 41.9(1), c 66 = 63.8(1), c 12 = 63.5(1), c 13 = 59.8(1), c 23 = 63.4(2) for Bi 2Fe 4O 9. In both mullite-type compounds the strong bond chains built up by edge-sharing coordination octahedra extending parallel to [001] dominate the anisotropy of their elastic and thermoelastic properties. Smaller variations of elastic anisotropy within the (001) plane can be attributed to the specific type of cross-linking of the octahedral chains. The temperature evolution of the c ij shows no hint on any structural instability or glass-like transition that might be related to the suspected ion conductivity at high temperatures. However, in both crystal species characteristic anelastic relaxation phenomena occur in the ultrasonic frequency regime close to room temperature. The smallest thermal expansion is observed in the plane perpendicular to the stiffest octahedral chains. A model is discussed to explain the apparent discrepancy in terms of cross-correlations within the three-dimensional framework of edge- and corner- linked coordination polyhedra. © 2012 Carl Hanser Verlag.
    view abstractdoi: 10.3139/146.110718
  • 2012 • 58 Ab-initio investigation of spin states of sodium cobaltate Na 2/3CoO2
    Lysogorskiy, Y.V. and Nedopekin, O.V. and Krivenko, S.A. and Minisini, B. and Tayurskii, D.A.
    Journal of Physics: Conference Series 394 (2012)
    Resent experiments in the lamellar system NaxCoO2 detected a transition of Co planes into a puzzling metallic state at x ≥ 2/3, which co-exists with a robust arrangement of the 3d cobalt electrons: The triangular Co lattices are disproportionated in the spinless Co3+ sites (Co1), and Co3.44+ sites (Co2) with enhanced magnetism forming conducting sublattices. This textures concur with a tightening of the ferromagnetic (FM) interaction in planes, and emerge when the sodium ions become arranged in layers in between the CoO2 slabs. In the present research we have investigated ab-initio the appearance of such state in Na2/3CoO2. Towards this end in view we studied an interplay between the electronic coupling to the superstructure of the Na+ ions and local correlations of the itinerant d electrons treated within the GGA+U approximation. Employing the exact crystallographic supercell, the electronic organization has been analyzed upon increasing the energy U of the Coulomb repulsion within the 3d shells at T = 0. The metallic ground state, being a spin density wave with the inplane FM and antiferromagnetic interplane correlations, has been obtained and established to posses two regimes. When U &gt; 2 eV, a crossover develops from a uniform state of the d-lattice to the regular phase with the spin/charge disproportionation between the sites. In particular at the representative value U = 5 eV, the Co13+ sites with suppressed magnetism appears, while the spin-active Co4+ holes are accumulated by the Co2 sites. A related formation of an isolated, narrow conduction band at the Fermi level implies a considerable enhancement of the electron correlations in the crystal field imposed by the Na+ patterns.
    view abstractdoi: 10.1088/1742-6596/394/1/012019
  • 2012 • 57 A quick method for the preparation of Pt(111)-like thin films
    Maljusch, A. and Henry, J.B. and Schuhmann, W. and Bondarenko, A.S.
    Electrochemistry Communications 16 88-91 (2012)
    A simple and quick method for forming Pt(111)-like thin films on Si/Ti substrates for electrochemical and/or electrocatalytic experiments is reported. This method involves physical vapour deposition followed by flame annealing, electrochemical cleaning and a short heat treatment under a controlled atmosphere. Careful selection of the substrate, surface preparation and cooling atmosphere allows production of Pt thin films which show voltammetry features typical of large Pt(111) single crystal electrodes in 0.1 M HClO 4. This technique promises a method for the production of Pt(111) type surfaces on a larger scale. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2011.12.004
  • 2012 • 56 Crystal chemistry and properties of mullite-type Bi 2M 4O 9: An overview
    Schneider, H. and Fischer, R.X. and Gesing, T.M. and Schreuer, J. and Mühlberg, M.
    International Journal of Materials Research 103 422-429 (2012)
    Bi 2M 4O 9 (M = Al 3+, Ga 3+, Fe 3+) belongs to the family of mullite-type crystal structures. The phases are orthorhombic with the space group Pbam. The backbones of the isostructural phases are edge-connected, mullite-type octahedral chains. The octahedral chains are linked by dimers of M 2O 7 tetrahedral groups and by BiO polyhedra. The Bi 3+ cations in Bi 2M 4O 9 contain stereo-chemically active 6s 2 lone electron pairs (LEPs) which are essential for the stabilization of the structure. Although the octahedral chains of the closely related Bi 2Mn 4O 10 are similar to those of Bi 2M 4O 9, Bi 2 Mn 4O 10 contains dimers of edge-connected, five-fold coordinated pyramids instead of four-fold coordinated tetrahedra. Also the 6s 2 LEPs of Bi 3+ in Bi 2Mn 4O 10 are not stereo-chemically active. Complete and continuous solid solutions exist for Bi 2(Al 1-xFe x) 4O 9 and Bi 2(Ga 1-x Fe x) 4O 9 (x = 0 - 1). Things are more complex in the case of the Bi 2(Fe 1-xMn x) 4O 9+y mixed crystals, where a miscibility gap occurs between x = 0.25 - 0.75. In the Fe-rich mixed crystals most Mn atoms enter the octahedra as Mn 4+, with part of the tetrahedral dimers being replaced by fivefold coordinated polyhedra, whereas in the Mn-rich compound Fe 3+ favorably replaces Mn 3+ in the pyramids. The crystal structure of Bi 2M 4O 9 directly controls its mechanical properties. The stiffnesses of phases are highest parallel to the strongly bonded octahedral chains running parallel to the crystallographic c-axis. Perpendicular to the octahedral chains little anisotropy is observed. The temperature- induced expansion perpendicular to the octahedral chains is probably superimposed by contractions. As a result the c-axis expansion appears as relatively high and does not display its lowest value parallel to c, as could be inferred. Maximally 6% of Bi 3+ is substituted by Sr 2+ in Bi 2Al 4O 9 corresponding to a composition of (Bi 0.94Sr 0.06) 2Al 4O 8.94. Sr 2+ for Bi 3+ substitution is probably associated with formation of vacancies of oxygen atoms bridging the tetrahedral dimers. Hopping of oxygen atoms towards the vacancies should strongly enhance the oxygen conductivity. Actually the conductivity is rather low (σ = 7 . 10 -2 S m -1 at 1073 K, 800 °C). An explanation could be the low thermal stability of Sr-doped Bi 2Al 4O 9, especially in coexistence with liquid Bi 2O 3. Therefore, Bi 2Al 4O 9 single crystals and polycrystalline ceramics both with significant amounts of M2+ doping (M = Ca 2+, Sr 2+) have not been produced yet. Thus the question whether or not M 2+-doped Bi 2M 4O 9 is an oxygen conducting material is still open. © 2012 Carl Hanser Verlag.
    view abstractdoi: 10.3139/146.110716
  • 2012 • 55 Elastic and piezoelectric constants of tourmaline single crystals at non-ambient temperatures determined by resonant ultrasound spectroscopy
    Shekhar Pandey, C. and Schreuer, J.
    Journal of Applied Physics 111 (2012)
    The full sets of elastic constants (considering piezoelectric effect, i.e., taking into account piezoelectric coupling effects) and piezoelectric stress constants of five natural tourmaline single crystals of different chemical composition have been determined from room temperature up to ∼900 K employing resonant ultrasound spectroscopy. Further, the coefficients of thermal expansion were studied in a wide temperature range from 100 K to 950 K. Electron microprobe analysis showed that the chemical composition of investigated samples varied over a wide range, which allows to study the compositional dependence of thermal, elastic, and piezoelectric properties of tourmalines. At room temperature, the piezoelectric constants of tourmalines were found at-least 1.5 times higher than that for -quartz. Importantly, we find that the application of tourmaline at temperatures up to its decomposition temperature is not limited by phase transitionsor any ultrasound dissipation effects. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.3673820
  • 2012 • 54 Leaf-like dislocation substructures and the decrease of martensitic start temperatures: A new explanation for functional fatigue during thermally induced martensitic transformations in coarse-grained Ni-rich Ti-Ni shape memory alloys
    Zhang, J. and Somsen, C. and Simon, T. and Ding, X. and Hou, S. and Ren, S. and Ren, X. and Eggeler, G. and Otsuka, K. and Sun, J.
    Acta Materialia 60 1999-2006 (2012)
    During repeatedly imposed thermally induced martensitic transformations in Ti-Ni shape memory alloys, the martensite start temperature M s decreases. This has been rationalized on the basis of a scenario where an increasing dislocation density makes it more and more difficult for martensite to form. However, it is not clear why dislocations which form because they accommodate the growth of martensite during the first cooling cycle should act as obstacles during subsequent transformation cycles. In the present work we use diffraction contrast transmission electron microscopy to monitor the formation of unique leaf-like dislocation substructures which form as the martensite start temperature decreases during thermal cycling. We interpret our microstructural results on the basis of a microstructural scenario where dislocations play different roles with respect to the propagation of a big martensite needle in one transformation cycle and the nucleation and growth of new martensite needles in the following cycles. As a consequence, chestnut-leaf-like dislocation arrays spread out in different crystallographic directions. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.12.014
  • 2011 • 53 Field cooling-induced magnetic anisotropy in exchange biased CoO/Fe bilayer studied by ferromagneticresonance
    Akdoǧan, N. and Kazan, S. and Akta, B. and Özdemir, M. and Inam, H. and Obaida, M. and Dudek, J. and Westerholt, K.
    Journal of Magnetism and Magnetic Materials 323 346-350 (2011)
    Exchange-biased CoO/Fe bilayer grown on MgO (0 0 1) substrate by sputtering, studied by variable angle and temperature ferromagnetic resonance. Room temperature in-plane measurements reveal that the Fe layer was epitaxially grown on MgO substrate with a fourfold cubic symmetry. The data also show that the easy axis of magnetization is in the film plane and makes an angle of 45° with the [1 0 0] crystallographic direction of MgO substrate. The low temperature data exhibit a sudden onset of a field cooling-induced and shifted cubic anisotropy below the Nel temperature of CoO. This results in a twofold uniaxial or fourfold cubic symmetry for in-plane magnetic anisotropy depending on a field cooling direction. Low temperature measurements also present a reduction in the resonance fields due to the antiferromagnetic/ferromagnetic coupling. The developed theoretical model perfectly simulates the experimental data of coupled CoO/Fe bilayer. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmmm.2010.09.037
  • 2011 • 52 A dislocation density-based crystal plasticity constitutive model for prismatic slip in α-titanium
    Alankar, A. and Eisenlohr, P. and Raabe, D.
    Acta Materialia 59 7003-7009 (2011)
    A new constitutive plasticity model for prismatic slip in hexagonal α-titanium is developed. In the concept pure edge and screw dislocation densities evolve on the {101̄0}〈12̄10〉 slip systems. The model considers that the screw dislocation segments have a spread out core, leading to a much higher velocity of edge compared with screw dislocations. This enables the model to describe the observed transition in strain hardening from stage I to stage II in single crystals oriented for prismatic slip. Good agreement is found between the experimentally observed and simulated stress-strain behavior. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.07.053
  • 2011 • 51 Spectral ellipsometry study of SBN single crystals in visible and ultraviolet region
    Andriyevsky, B. and Dorywalski, K. and Kityk, I. and Piasecki, M. and Łukasiewicz, T. and Świrkowicz, M. and Patryn, A. and Dec, J. and Esser, N. and Cobet, C.
    Ferroelectrics 417 14-19 (2011)
    The pseudo-dielectric function < ε&gt;(E) = < ε 1&gt;(E) + i< ε 2&gt;(E) of strontium-barium niobate single crystals, Sr xBa 1-xNb 2O 6 (SBN), was studied at room temperature for five nominal compounds indices x (x = 0.40, 0.55, 0.61, 0.65, 0.75) by spectroscopic ellipsometry using the synchrotron light ellipsometer in the photon energy range of 1.5-10 eV. The spectra of < ε 2&gt;(E) obtained appear to be similar to the analogous spectrum of LiNbO 3 crystal, where the NbO 6 octahedral groups are responsible for the features of < ε 2&gt; (E). Differences in the dielectric function of SBN with various x have been found and analyzed. The nonlinear dependences of optical values on the compound index x have been found in SBN. Copyright © Taylor & Francis Group, LLC.
    view abstractdoi: 10.1080/00150193.2011.578460
  • 2011 • 50 Thermodynamic and relaxation-based modeling of the interaction between martensitic phase transformations and plasticity
    Bartel, T. and Menzel, A. and Svendsen, B.
    Journal of the Mechanics and Physics of Solids 59 1004-1019 (2011)
    This paper focuses on the issue plasticity within the framework of a micromechanical model for single-crystal shape-memory alloys. As a first step towards a complete micromechanical formulation of such models, we work with classical J2-von Mises-type plasticity for simplicity. The modeling of martensitic phase transitions is based on the concept of energy relaxation (quasiconvexification) in connection with evolution equations derived from inelastic potentials. Crystallographic considerations lead to the derivation of Bain strains characterizing the transformation kinematics. The model is derived for arbitrary numbers of martensite variants and thus can be applied to any shape-memory material such as CuAlNi or NiTi. The phase transition model captures effects like tension/compression asymmetry and transformation induced anisotropy. Additionally, attention is focused on the interaction between phase transformations and plasticity in terms of the inheritance of plastic strain. The effect of such interaction is demonstrated by elementary numerical studies. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2011.02.006
  • 2011 • 49 Atomistic processes of dislocation generation and plastic deformation during nanoindentation
    Begau, C. and Hartmaier, A. and George, E.P. and Pharr, G.M.
    Acta Materialia 59 934-942 (2011)
    To enable plastic deformation during nanoindentation of an initially defect-free crystal, it is necessary first to produce dislocations. While it is now widely accepted that the nucleation of the first dislocations occurs at the start of the pop-in event frequently observed in experiments, it is unclear how these initial dislocations multiply during the early stages of plastic deformation and produce pop-in displacements that are typically much larger than the magnitude of the Burgers vector. This uncertainty about the complex interplay between dislocation multiplication and strain hardening during nanoindentation makes a direct correlation between force-displacement curves and macroscopic material properties difficult. In this paper, we study the early phase of plastic deformation during nanoindentation with the help of large-scale molecular dynamics simulations. A skeletonization method to simplify defect structures in atomistic simulations enables the direct observation and quantitative analysis of dislocation nucleation and multiplication processes occurring in the bulk as well as at the surface. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.10.016
  • 2011 • 48 Raman spectra of graphene exfoliated on insulating crystalline substrates
    Bukowska, H. and Meinerzhagen, F. and Akcöltekin, S. and Ochedowski, O. and Neubert, M. and Buck, V. and Schleberger, M.
    New Journal of Physics 13 (2011)
    We have investigated single layer, bilayer and few-layer graphene exfoliated on SiO 2 and on single crystal surfaces of SrTiO 3, Al 2O 3 and TiO 2 using Raman spectroscopy. The typical 'fingerprint' 2D peak turns out to be indicative of the number of graphene layers independent of the substrate material. The morphological quality of the graphene is as good as on SiO 2 substrates for all the materials. We find evidence for substrate-induced changes due to doping. With most substrates, hole doping is observed, but with SrTiO 3 we have identified a dielectric substrate with which electron accumulation in graphene can be achieved. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/13/6/063018
  • 2011 • 47 Indirect magnetic coupling of manganese porphyrin to a ferromagnetic cobalt substrate
    Chylarecka, D. and Kim, T.K. and Tarafder, K. and Müller, K. and Gödel, K. and Czekaj, I. and Wäckerlin, C. and Cinchetti, M. and Ali, Md.E. and Piamonteze, C. and Schmitt, F. and Wüstenberg, J.-P. and Ziegler, C. and Nolting,...
    115 1295-1301 (2011)
    The coupling mechanism of magnetic molecules to ferromagnetic surfaces is of scientific interest to design and tune molecular spintronic interfaces utilizing their molecular and surface architecture. Indirect magnetic coupling has been proposed earlier on the basis of density functional theory +U (DFT+U) calculations, for the magnetic coupling of manganese(II) porphyrin (MnP) molecules to thin Co films. Here we provide an experimental X-ray magnetic circular dichroism (XMCD) spectroscopy and scanning tunneling microscopy (STM) study of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) on rough (exhibiting a high density of monatomic steps) and smooth (exhibiting a low density of monatomic steps) thin Co films grown on a Cu(001) single crystal toward the assessment of the magnetic coupling mechanism. After deposition onto the surface, MnTPPCl molecules were found to couple ferromagnetically to both rough and smooth Co substrates. For high molecular coverage, we observed higher XMCD signals at the Mn L-edges on the smooth Co substrate than on the rough Co substrate, as expected for the proposed indirect magnetic coupling mechanism on the basis of its predominance on the flat surface areas. In particular, DFT+U calculations predict a weak ferromagnetic molecule-substrate coupling only if the chloride ion of the MnTPPCl molecule orients away (Co-Mn-Cl) from the Co surface. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp106822s
  • 2011 • 46 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 • 45 Methylated [(arene)(1,3-cyclohexadiene)Ru(0)] complexes as low-melting MOCVD precursor complexes with a controlled follow-up chemistry of the ligands
    Jipa, I. and Danova, K. and Popovska, N. and Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B. and Cremer, T. and Maier, F. and Marbach, H. and Steinrück, H.-P. and Heinemann, F.W. and Zenneck, U.
    Journal of Materials Chemistry 21 3014-3024 (2011)
    [(Benzene)(2-methyl-1,3-cyclohexadiene)Ru(0)] (1), [(1,3-cyclohexadiene) (toluene)Ru(0)] (2), and [(methyl-cyclohexadiene)(toluene)Ru(0)] (3, mixture of isomers) have been prepared and tested as new metal organic ruthenium precursor complexes for chemical vapor deposition (MOCVD) with favorable properties. 1 is a low-melting precursor complex (mp = 29 °C) and the isomeric mixture 3 forms a liquid at room temperature. X-ray diffraction studies of single crystals of complexes 1 and 2 are characteristic for true Ru(0) π-complexes without molecular structure peculiarities or significant intermolecular interactions in the solid state, which could hinder undecomposed evaporation. Differential thermal analysis (DTA), differential scanning calorimetry (DSC) and vapor pressure data qualify the compounds as almost ideal MOCVD precursors. Thin ruthenium films have been deposited successfully on silicon wafers and substrate temperatures between 200 and 450 °C in inert gas atmospheres. Film growth and properties were evaluated by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and four-point probe conductivity measurements. All films consist of polycrystalline metallic ruthenium with a low surface roughness. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c0jm02652b
  • 2011 • 44 Methylated [(benzene)(1,3-butadiene)Ru0] derivatives as novel MOCVD precursors with favorable properties
    Jipa, I. and Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B. and Marbach, H. and Cremer, T. and Maier, F. and Steinrück, H.-P. and Danova, K. and Popovska, N. and Heinemann, F.W. and Zenneck, U.
    Chemical Vapor Deposition 17 15-21 (2011)
    [(Benzene)(2-methyl-1,3-butadiene)Ru0] (1), [(benzene)(2,3- dimethyl-1,3-butadiene)Ru0] (2), and [(2,3-dimethyl-1,3-butadiene) (toluene)Ru0] (3) are prepared and tested as new metal-organic (MO) ruthenium precursor complexes with favorable deposition properties for the CVD of thin ruthenium films. X-ray diffraction (XRD) studies of single crystals of the complexes are characteristic for true Ru0 π-complexes without molecular structure peculiarities or significant intermolecular interactions in the solid state, which can hinder undecomposed evaporation. Differential thermal analysis (DTA) and vapor pressure data qualify the compounds as almost ideal MOCVD precursors. Thin ruthenium films are deposited successfully on silicon wafers at substrate temperatures between 200 and 400°C in a nitrogen gas atmosphere. X-ray photoelectron spectroscopy (XPS), four-point probe conductivity measurements, and atomic force microscopy (AFM) are used to characterize the films. All films consist of polycrystalline metallic ruthenium with a low surface roughness. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201006853
  • 2011 • 43 Work hardening in micropillar compression: In situ experiments and modeling
    Kiener, D. and Guruprasad, P.J. and Keralavarma, S.M. and Dehm, G. and Benzerga, A.A.
    Acta Materialia 59 3825-3840 (2011)
    Experimental measurements and simulation results for the evolution of plastic deformation and hardening in micropillars are compared. The stress-strain response of high-symmetry Cu single crystals is experimentally determined using in situ micropillar compression. Discrete dislocation simulations are conducted within a two-dimensional plane-strain framework with the dislocations modeled as line singularities in an isotropic elastic medium. Physics-based constitutive rules are employed for an adequate representation of hardening. The numerical parameters entering the simulations are directly identified from a subset of experimental data. The experimental measurements and simulation results for the flow stress at various strain levels and the hardening rates are in good quantitative agreement. Both flow strength and hardening rate are size-dependent and increase with decreasing pillar size. The size effect in hardening is mainly caused by the build-up of geometrically necessary dislocations. Their evolution is observed to be size-dependent and more localized for smaller sample volumes, which is also reflected in local crystal misorientations. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.003
  • 2011 • 42 Atomistically informed continuum model for body centered cubic iron
    Koester, A. and Ma, A. and Hartmaier, A.
    Materials Research Society Symposium Proceedings 1296 47-55 (2011)
    Plastic deformation in body centered cubic iron is dominated by glide of screw dislocations with non-planar dislocation cores. This causes a strong strain rate and temperature dependence of flow stress, the breakdown of Schmid's law and a dependence of dislocation mobility on shear stress components that do not contribute to the mechanical driving force for dislocation glide. Based on the framework of crystal plasticity, we developed a constitutive plasticity model that takes all these phenomena into account. To parameterize this continuum plasticity model molecular statics simulations using a semi-empirical potential have been performed. These atomistic calculations yielded quantitative relationships for the influence of all components of the local stress tensor on dislocation mobility. Together with experimental data from the literature on the kinetics of screw dislocations in bcc iron the constitutive relation presented here has been developed. As application example of the model, we calculated the tension compression asymmetry and the strain rate dependence of the hardening behavior within a bcc iron crystal. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/opl.2011.1445
  • 2011 • 41 Structural characterization of Et4Sb2 and Et 4Bi2
    Kuczkowski, A. and Heimann, S. and Weber, A. and Schulz, S. and Bläser, D. and Wölper, C.
    Organometallics 30 4730-4735 (2011)
    The solid-state structures of Et4Sb2 (1) and Et 4Bi2 (2) were determined by single-crystal X-ray diffraction. Single crystals of 1 and 2 were grown in a closed quartz glass capillary under an inert argon atmosphere on the diffractometer using a specific IR-laser-assisted technique. 1 shows short intermolecular Sb••• Sb interactions, whereas the closest Bi•••Bi distances are longer than the sum of the van der Waals radii. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/om2005723
  • 2011 • 40 Planar-defect characteristics and cross-sections of 〈001〉, 〈111〉, and 〈112〉 InAs nanowires
    Li, Z.-A. and Mller, C. and Migunov, V. and Spasova, M. and Farle, M. and Lysov, A. and Gutsche, C. and Regolin, I. and Prost, W. and Tegude, F.-J. and Ercius, P.
    Journal of Applied Physics 109 (2011)
    We report on detailed structural and morphological characterizations of InAs nanowires of 〈001〉, 〈111〉, and 〈112〉 crystallographic directions grown on (001)B InAs wafer substrates using high-resolution transmission electron microscopy. We find that 〈001〉 -oriented InAs nanowires are cubic zincblende-type structure and free of planar defects. The 〈111〉- and 〈112〉-oriented InAs nanowires both have densely twinned (111) planar defects that are perpendicular and parallel to the growth direction, respectively. The cross sections of all three types of InAs nanowires are obtained from 3D reconstructions using electron tomography. The characteristics of the planar defects and the 3D wire shape should provide better estimations of microstructure-relevant physical properties, such as conductivity and Young's modulus of InAs nanowires. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3592186
  • 2011 • 39 Indentation Schmid factor and orientation dependence of nanoindentation pop-in behavior of NiAl single crystals
    Li, T.L. and Gao, Y.F. and Bei, H. and George, E.P.
    Journal of the Mechanics and Physics of Solids 59 1147-1162 (2011)
    Instrumented nanoindentation techniques have been widely used to characterize the small-scale mechanical behavior of materials. The elasticplastic transition during nanoindentation is often indicated by a sudden displacement burst (pop-in) in the measured loaddisplacement curve. In defect-free single crystals, the pop-in is believed to be the result of homogeneous dislocation nucleation because the maximum shear stress corresponding to the pop-in load approaches the theoretical strength of the materials and because the statistical distribution of pop-in stresses is consistent with what is expected for a thermally activated process of homogeneous dislocation nucleation. This paper investigates whether this process is affected by crystallography and stress components other than the resolved shear stress. A Stroh formalism coupled with the two-dimensional Fourier transformation is used to derive the analytical stress fields in elastically anisotropic solids under Hertzian contact, which allows the determination of an indentation Schmid factor, namely, the ratio of maximum resolved shear stress to the maximum contact pressure. Nanoindentation tests were conducted on B2-structured NiAl single crystals with different surface normal directions. This material was chosen because it deforms at room temperature by {1 1 0}〈0 0 1〉 slip and thus avoids the complexity of partial dislocation nucleation. Good agreement is obtained between the experimental data and the theoretically predicted orientation dependence of pop-in loads based on the indentation Schmid factor. Pop-in load is lowest for indentation directions close to 〈1 1 1〉 and highest for those close to 〈0 0 1〉. In nanoindentation, since the stress component normal to the slip plane is typically comparable in magnitude to the resolved shear stress, we find that the pressure sensitivity of homogeneous dislocation nucleation cannot be determined from pop-in tests. Our statistical measurements generally confirm the thermal activation model of homogeneous dislocation nucleation. That is, the extracted dependence of activation energy on resolved shear stress is almost the same for all the indentation directions considered in this study, except for those close to 〈0 0 1〉. Because very high pop-in loads are measured for orientations close to 〈0 0 1〉, which implies a large contact area at pop-in, there is a higher probability of activating pre-existing dislocations in these orientations, which may explain the discrepancy near 〈0 0 1〉. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2011.04.003
  • 2011 • 38 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 • 37 Effect of HIP parameters on the micro-structural evolution of a single crystal Ni-based superalloy
    Lopez-Galilea, I. and Huth, S. and Bartsch, M. and Theisen, W.
    Advanced Materials Research 278 72-77 (2011)
    For reducing the porosity of single crystal (SX) nickel-based superalloys, Hot Isostatic Pressing (HIP) is used. High pressures of about 100-170 MPa lead to local deformation, which close the pores. However, since HIP also requires high temperatures (1000-12007deg;C) it has a pronounced effect on the microstructure and the local distribution of elements. This contribution analyses the effect of different HIP treatments on both the microstructure and the segregation of the SX superalloy LEK94 in the as-precipitation-hardened state. In addition, the effects of rapid or slow cooling are analyzed. To distinguish the effect of pressure from those of temperature, the HIPed samples are compared with specimens annealed at atmospheric pressure. © (2011) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/AMR.278.72
  • 2011 • 36 Simulation of the external pressure influence on the micro-Structural evolution of a Single Crystal Ni-Based superalloy
    Lopez-Galilea, I. and Huth, S. and Fries, S.G. and Steinbach, I. and Theisen, W.
    Advanced Materials Research 278 247-252 (2011)
    The phase field method has been applied to simulate the microstructural evolution of a commercial single crystal Ni-based superalloy during both, HIP and annealing treatments. The effects of applying high isostatic pressure on the microstructural evolution, which mainly retards the diffusion of the alloying elements causing the loss of the orientational coherency between the phases is demonstrated by the simulation and experimental results. © (2011) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/AMR.278.247
  • 2011 • 35 Improved interfacial local structural ordering of epitaxial Fe 3Si(001) thin films on GaAs(001) by a MgO(001) tunneling barrier
    Makarov, S.I. and Krumme, B. and Stromberg, F. and Weis, C. and Keune, W. and Wende, H.
    Applied Physics Letters 99 (2011)
    Although the quasi-Heusler compound Fe3Si is a promising candidate for spintronics applications, its combination with the reactive GaAs surface is problematic, since it deteriorates its beneficial attributes due to a large amount of interdiffusion at the Fe3Si/GaAs interface. Here, we show the epitaxial growth of Fe3Si with low evaporation rates on GaAs(001) and report on improved local structural D03 ordering in epitaxial Fe3Si(001) films grown on GaAs(001) by inserting a MgO buffer layer. Conversion-electron Mssbauer spectroscopy with 57Fe3Si tracer layers reveals that the effect of thermally induced interdiffusion at the Fe3Si/GaAs(001) interface is dramatically reduced by inserting a 30 MgO tunneling barrier between the film and the substrate. The chemical order of Fe3Si is comparable to that of Fe3Si films which are grown directly on MgO(001) single crystals. It is proposed that this preparation method can be useful to achieve high-efficiency spin-polarized electron currents from ferromagnetic Fe 3Si into semiconducting GaAs(001). © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3646390
  • 2011 • 34 Malonate complexes of dysprosium: Synthesis, characterization and application for LI-MOCVD of dysprosium containing thin films
    Milanov, A.P. and Seidel, R.W. and Barreca, D. and Gasparotto, A. and Winter, M. and Feydt, J. and Irsen, S. and Becker, H.-W. and Devi, A.
    Dalton Transactions 40 62-78 (2011)
    A series of malonate complexes of dysprosium were synthesized as potential metalorganic precursors for Dy containing oxide thin films using chemical vapor deposition (CVD) related techniques. The steric bulkiness of the dialkylmalonato ligand employed was systematically varied and its influence on the resulting structural and physico-chemical properties that is relevant for MOCVD was studied. Single crystal X-ray diffraction analysis revealed that the five homoleptic tris-malonato Dy complexes (1-5) are dimers with distorted square-face bicapped trigonal-prismatic geometry and a coordination number of eight. In an attempt to decrease the nuclearity and increase the solubility of the complexes in various solvents, the focus was to react these dimeric complexes with Lewis bases such as 2,2′-biypridyl and pyridine (6-9). This resulted in monomeric tris-malonato mono Lewis base adduct complexes with improved thermal properties. Finally considering the ease of synthesis, the monomeric nature and promising thermal characteristics, the silymalonate adduct complex [Dy(dsml)3bipy] (8) was selected as single source precursor for growing DySixOy thin films by liquid injection metalorganic chemical vapor deposition (LI-MOCVD) process. The as-deposited films were analyzed for their morphology and composition by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, Rutherford backscattering (RBS) analysis and X-ray photoelectron spectroscopy. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0dt00455c
  • 2011 • 33 Correlation between dielectric properties and chemical composition of the tourmaline single crystals
    Pandey, C.S. and Jodlauk, S. and Schreuer, J.
    Applied Physics Letters 99 (2011)
    Dielectric responses were studied on piezoelectric tourmaline single crystals of widely varying chemical composition from different geological origins. The dielectric constants at constants stress, and dissipation factor were measured as a function of frequency (100-1000 kHz) using method of substitution. A correlation between two independent dielectric constants (along and perpendicular to crystallographic c-axis) is observed, and dependence of dielectric constants on chemical composition is presented. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3646912
  • 2011 • 32 Scanning transmission electron microscope observations of defects in as-grown and pre-strained Mo alloy fibers
    Phani, P.S. and Johanns, K.E. and Duscher, G. and Gali, A. and George, E.P. and Pharr, G.M.
    Acta Materialia 59 2172-2179 (2011)
    Compression testing of micro-pillars has recently been of great interest to the small-scale mechanics community. Previous compression tests on single crystal Mo alloy micro-pillars produced by directional solidification of eutectic alloys showed that as-grown pillars yield at strengths close to the theoretical strength while pre-strained pillars yield at considerably lower stresses. In addition, the flow behavior changes from stochastic to deterministic with increasing pre-strain. In order to gain a microstructural insight into this behavior, an aberration corrected scanning transmission electron microscope was used to study the defect structures in as-grown and pre-strained single crystal Mo alloy fibers. The as-grown fibers were found to be defect free over large lengths while the highly pre-strained (16%) fibers had high defect densities that were uniform throughout. Interestingly, the fibers with intermediate pre-strain (4%) exhibited an inhomogeneous defect distribution. The observed defect structures and their distributions are correlated with the previously reported stress-strain behavior. Some of the mechanistic interpretations of Bei et al. are examined in the light of new microstructural observations. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.12.018
  • 2011 • 31 Anomalous elastic behavior of relaxor ferroelectric Ca 0.28Ba 0.72Nb 2O 6:Ce studied by resonant ultrasound spectroscopy
    Shekhar Pandey, C. and Schreuer, J. and Burianek, M. and Mühlberg, M.
    Applied Physics Letters 99 (2011)
    Elastic behavior of tetragonal tungsten bronze uniaxial relaxor ferroelectric cerium doped Ca 0.28Ba 0.72Nb 2O 6 single crystal was investigated employing resonant ultrasound spectroscopy in the temperature range from room temperature up to 1323 K. Doping of cerium lowers the phase transition temperature T c and Burns temperature T b significantly, however, intermediate characteristic temperature T * (between the Burns temperature T b and the temperature of maximum dielectric permittivity T m) remains same as for pure Ca 0.28Ba 0.72Nb 2O 6. All independent elastic constants evolved differently with temperature, reflecting their coupling to different types of the reorientational motion of the polar nanoregions through their interaction with the acoustic waves. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3670323
  • 2011 • 30 Thermally driven solid-phase epitaxy of laser-ablated amorphous AlFe films on (0001)-oriented sapphire single crystals
    Trautvetter, M. and Wiedwald, U. and Paul, H. and Minkow, A. and Ziemann, P.
    Applied Physics A: Materials Science and Processing 102 725-730 (2011)
    Solid-phase epitaxy is demonstrated for the metallic binary alloy AlFe. Stoichiometric thin films are deposited at ambient temperature onto c-cut sapphire by pulsed laser deposition (PLD), resulting in smooth amorphous films as revealed by X-ray diffraction (XRD) and atomic force microscopy (AFM). By annealing at 600°C, still smooth epitaxial AlFe films are obtained exhibiting the B2 phase with the (110) direction parallel to the substrate normal and an in-plane orientation as given by AlFe[001]||Al2O 3[112̄0]. While ferromagnetism is observed for the amorphous phase, the formation of the B2 structure is accompanied by paramagnetic behavior, confirming the high structural quality. © Springer-Verlag 2010.
    view abstractdoi: 10.1007/s00339-010-5972-x
  • 2011 • 29 Determining the activation energies and slip systems for dislocation nucleation in body-centered cubic Mo and face-centered cubic Ni single crystals
    Wang, L. and Bei, H. and Li, T.L. and Gao, Y.F. and George, E.P. and Nieh, T.G.
    Scripta Materialia 65 179-182 (2011)
    Nanoindentation tests were performed on single crystals of Mo and Ni. The critical shear stress for the first pop-in was ∼1/7 of the shear modulus in both crystals. The dependence of pop-in probability on load was understood in terms of a thermally activated dislocation nucleation process. Comparison of the activation energies suggests nucleation of full dislocations in Mo and partial dislocations in Ni. The activation energy analysis also offers information on the specific slip system on which dislocations are nucleated. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2011.03.036
  • 2011 • 28 Photocatalytic activity of bulk TiO2 anatase and rutile single crystals using infrared absorption spectroscopy
    Xu, M. and Gao, Y. and Moreno, E.M. and Kunst, M. and Muhler, M. and Wang, Y. and Idriss, H. and Wöll, C.
    Physical Review Letters 106 (2011)
    A systematic study on the photocatalytic activity of well-defined, macroscopic bulk single-crystal TiO2 anatase and rutile samples has been carried out, which allows us to link photoreactions at surfaces of well-defined oxide semiconductors to an important bulk property with regard to photochemistry, the life time of e-h pairs generated in the bulk of the oxides by photon absorption. The anatase (101) surface shows a substantially higher activity, by an order of magnitude, for CO photo-oxidation to CO2 than the rutile (110) surface. This surprisingly large difference in activity tracks the bulk e-h pair lifetime difference for the two TiO2 modifications as determined by contactless transient photoconductance measurements on the corresponding bulk materials. © 2011 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.106.138302
  • 2011 • 27 Analysis of the plastic anisotropy and pre-yielding of (γ/ α2)-phase titanium aluminide microstructures by crystal plasticity simulation
    Zambaldi, C. and Roters, F. and Raabe, D.
    Intermetallics 19 820-827 (2011)
    The plastic deformation of lamellar microstructures composed of the two phases γ-TiAl and α2-Ti3Al is highly orientation dependent. In this paper we present a homogenized model that takes into account the micromechanical effect of the plate-like morphologies that are often observed in two-phase titanium aluminide alloys. The model is based on crystal elasto-viscoplasticity and 18 deformation systems were implemented that have been identified to govern the plastic flow of the lamellar microstructures. The model is validated against experiments on polysynthetically twinned (PST) crystals and shows good agreement with the data. On a larger length scale, the model is applied to a 64-grain aggregate to investigate the mechanical response of two different kinds of microstructures. Different magnitudes of the kinematic constraints exerted by the densely spaced and highly aligned interfaces are shown to affect the macroscopic flow behavior of the microstructures. The phenomenon of pronounced microplasticity of fully lamellar material as well as the stress variation inside two-phase microstructures are studied quantitatively. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2011.01.012
  • 2010 • 26 3D x-ray microprobe investigation of local dislocation densities and elastic strain gradients in a NiAl-Mo composite and exposed Mo micropillars as a function of prestrain
    Barabash, R.I. and Bei, H. and Gao, Y. and Ice, G.E. and George, E.P.
    Journal of Materials Research 25 199-206 (2010)
    3D spatially-resolved polychromatic microdiffraction was used to nondestructively obtain depth-dependent elastic strain gradients and dislocation densities in the constituent phases of a directionally solidified NiAl-Mo eutectic composite consisting of ~500-800 nm Mo fibers in a NiAl matrix. Measurements were made before and after the composite was compressed by 5% and 11%. The Mo fibers were analyzed both in their embedded state and after the matrix was etched to expose them as pillars. In the as-grown composite, due to differential thermal contraction during cooldown, the Mo phase is under compression and the NiAl phase is in tension. After the prestrains, the situation is reversed with the Mo phase in tension and NiAl matrix in compression. This result can be explained by taking into account the mismatch in yield strains of the constituent phases and the elastic constraints during unloading. The dislocation density in both the Mo and NiAl phases is found to increase after prestraining. Within experimental uncertainty there is little discernible difference in the total dislocation densities in the Mo phase of the 5% and 11% prestrained specimens. However, the density of the geometrically necessary dislocations and the deviatoric strain gradients increase with increasing prestrain in both the Mo and NiAl phases. © 2010 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2010.0043
  • 2010 • 25 Structural, static and dynamic magnetic properties of Co2MnGe thin films on a sapphire a-plane substrate
    Belmeguenai, M. and Zighem, F. and Chauveau, T. and Faurie, D. and Roussigń, Y. and Ch́rif, S.M. and Moch, P. and Westerholt, K. and Monod, P.
    Journal of Applied Physics 108 (2010)
    Magnetic properties of Co2 MnGe thin films of different thicknesses (13, 34, 55, 83, 100, and 200 nm), grown by rf sputtering at 400 °C on single crystal sapphire substrates, were studied using vibrating sample magnetometry and conventional or microstrip line ferromagnetic resonance. Their behavior is described assuming a magnetic energy density showing twofold and fourfold in-plane anisotropies with some misalignment between their principal directions. For all the samples, the easy axis of the fourfold anisotropy is parallel to the c -axis of the substrate while the direction of the twofold anisotropy easy axis varies from sample to sample and seems to be strongly influenced by the growth conditions. Its direction is most probably monitored by the slight unavoidable miscut angle of the Al2 O 3 substrate. The twofold in-plane anisotropy field Hu is almost temperature independent, in contrast with the fourfold field H 4 which is a decreasing function of the temperature. Finally, we study the frequency dependence of the observed line-width of the resonant mode and we conclude to a typical Gilbert damping constant α value of 0.0065 for the 55-nm-thick film. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3475501
  • 2010 • 24 Orientation gradients and geometrically necessary dislocations in ultrafine grained dual-phase steels studied by 2D and 3D EBSD
    Calcagnotto, M. and Ponge, D. and Demir, E. and Raabe, D.
    Materials Science and Engineering A 527 2738-2746 (2010)
    We study orientation gradients and geometrically necessary dislocations (GNDs) in two ultrafine grained dual-phase steels with different martensite particle size and volume fraction (24 vol.% and 38 vol.%). The steel with higher martensite fraction has a lower elastic limit, a higher yield strength and a higher tensile strength. These effects are attributed to the higher second phase fraction and the inhomogeneous transformation strain accommodation in ferrite. The latter assumption is analyzed using high-resolution electron backscatter diffraction (EBSD). We quantify orientation gradients, pattern quality and GND density variations at ferrite-ferrite and ferrite-martensite interfaces. Using 3D EBSD, additional information is obtained about the effect of grain volume and of martensite distribution on strain accommodation. Two methods are demonstrated to calculate the GND density from the EBSD data based on the kernel average misorientation measure and on the dislocation density tensor, respectively. The overall GND density is shown to increase with increasing total martensite fraction, decreasing grain volume, and increasing martensite fraction in the vicinity of ferrite. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.msea.2010.01.004
  • 2010 • 23 Focused ion beam/scanning electron microscopy tomography and conventional transmission electron microscopy assessment of Ni4Ti3 morphology in compression-aged Ni-rich Ni-Ti single crystals
    Cao, S. and Somsen, C. and Croitoru, M. and Schryvers, D. and Eggeler, G.
    Scripta Materialia 62 399-402 (2010)
    The size, morphology and configuration of Ni4Ti3 precipitates in a single-crystal Ni-Ti alloy have been investigated by two-dimensional transmission electron microscopy-based image analysis and three-dimensional reconstruction from slice-and-view images obtained in a focused ion beam/scanning electron microscopy (FIB/SEM) dual-beam system. Average distances between the precipitates measured along the compression direction correlate well between both techniques, while particle shape and configuration data is best obtained from FIB/SEM. Precipitates form pockets of B2 of 0.54 μm in the compression direction and 1 μm perpendicular to the compression direction. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2009.11.040
  • 2010 • 22 Spin Noise of Electrons and Holes in Self-Assembled Quantum Dots
    Crooker, S.A. and Brandt, J. and Sandfort, C. and Greilich, A. and Yakovlev, D.R. and Reuter, D. and Wieck, A.D. and Bayer, M.
    Physical Review Letters 104 (2010)
    We measure the frequency spectra of random spin fluctuations, or "spin noise," in ensembles of (In,Ga)As/GaAs quantum dots (QDs) at low temperatures. We employ a spin noise spectrometer based on a sensitive optical Faraday rotation magnetometer that is coupled to a digitizer and field-programmable gate array, to measure and average noise spectra from 0-1 GHz continuously in real time with subnanoradian/Hz sensitivity. Both electron and hole spin fluctuations generate distinct noise peaks, whose shift and broadening with magnetic field directly reveal their g factors and dephasing rates within the ensemble. A large, energy-dependent anisotropy of the in-plane hole g factor is clearly exposed, reflecting systematic variations in the average QD confinement potential. © 2010 The American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.104.036601
  • 2010 • 21 Bending of single crystal microcantilever beams of cube orientation: Finite element model and experiments
    Demir, E. and Roters, F. and Raabe, D.
    Journal of the Mechanics and Physics of Solids 58 1599-1612 (2010)
    The aim of this work is to investigate the microstructure evolution, stressstrain response and strain hardening behavior of microscale beams. For that purpose, two single crystal cantilever beams in the size dependent regime were manufactured by ion beam milling and beams were bent with an indenter device. A crystal plasticity material model for large deformations was implemented in a finite element framework to further investigate the effect of boundary constraints. Simulations were performed using bulk material properties of single crystal copper without any special treatment for the strain gradients. The difference between the slopes of the experimental and the simulated force displacement curves suggested negligible amount of strain gradient hardening compared to the statistical hardening mechanisms. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2010.07.007
  • 2010 • 20 The mechanical size effect as a mean-field breakdown phenomenon: Example of microscale single crystal beam bending
    Demir, E. and Raabe, D. and Roters, F.
    Acta Materialia 58 1876-1886 (2010)
    Single crystalline copper beams with thicknesses between 0.7 and 5 μm are manufactured with a focused ion beam technique and bent in a nanoindenter. The yield strengths of the beams show a mechanical size effect (smaller-is-stronger). The geometrically necessary dislocation (GND) densities estimated from misorientation maps do not explain the observed size effect. Also, accumulation of GNDs principally requires pre-straining. We hence introduce a mean-field breakdown theory and generalize it to small-scale mechanical tests other than bending. The mean-field breakdown limit is defined in terms of a microstructural correlation measure (characteristic dislocation bow-out length) below which the local availability of dislocation sources and not the density of GNDs dominates the mechanical size effect. This explains why a size dependence can occur for samples that are not pre-strained (by using a very small critical strain to define the yield strength). After pre-straining, when GNDs build up, they can contribute to the flow stress. The mean-field breakdown theory can also explain the large scatter typically observed in small-scale mechanical tests as the availability of sufficiently soft sources at scales around or below the correlation length does not follow statistical laws but highly depends on the position where the probe is taken. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2009.11.031
  • 2010 • 19 Investigation of the internal substructure of microbands in a deformed copper single crystal: Experiments and dislocation dynamics simulation
    Dmitrieva, O. and Svirina, J.V. and Demir, E. and Raabe, D.
    Modelling and Simulation in Materials Science and Engineering 18 (2010)
    We investigate the internal structure of microbands in a shear-deformed copper single crystal. The microstructure is characterized using high-resolution electron backscatter diffraction. The occurrence of microbands is due to the alternation of local orientation, which is characteristic of a deformation laminate. These microbands contain a substructure consisting of further local 1°-orientation alternations. A two-dimensional discrete dislocation dynamics model is used to describe the orientation substructure within the microbands. The boundary conditions for the simulation were estimated from the distribution of the geometrically necessary dislocation density obtained from the orientation map. The dislocation arrangement in the dynamic simulation explains the formation of the experimentally observed substructure. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/18/8/085011
  • 2010 • 18 Combined ab initio and experimental study of structural and elastic properties of Fe3Al-based ternaries
    Friák, M. and Deges, J. and Krein, R. and Frommeyer, G. and Neugebauer, J.
    Intermetallics 18 1310-1315 (2010)
    A combined theoretical and experimental study of thermodynamical, structural, and elastic properties of Fe3Al-based ternary alloys is presented. The theoretical part is based on a scale-bridging, multi-disciplinary combination of (i) thermodynamic aspects of the site preference and (ii) elastic stiffness data for substitutional ternary elements in Fe3Al single crystals, as determined by parameter-free first-principles calculations, and (iii) Hershey's homogenization model for the polycrystalline aggregates within the frame of linear elasticity theory. The approach was employed in order to explore the relation between chemical composition and both structural and elastic properties of Fe3Al ternary alloys containing the selected substituents (Ti, V, W, Cr and Si). The ab initio calculations employ density-functional theory (DFT) and the generalized gradient approximation (GGA). The determined elastic constants are used to calculate the elastic moduli, such as the Young's and bulk modulus. The theoretical results are compared to both literature data and novel impulse excitation measurements. Specifically, for Fe3Al-Ti alloys with low to medium Ti concentrations, an unexpected non-linear compositional dependence of the polycrystalline Young's modulus was found experimentally. The origin of this behavior is analyzed and discussed based on our theoretical results. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.02.025
  • 2010 • 17 Growth of single-crystal mesoporous carbons with im 3̄ m symmetry
    Gu, D. and Bongard, H. and Meng, Y. and Miyasaka, K. and Terasaki, O. and Zhang, F. and Deng, Y. and Wu, Z. and Feng, D. and Fang, Y. and Tu, B. and Schüth, F. and Zhao, D.
    Chemistry of Materials 22 4828-4833 (2010)
    Highly ordered mesoporous carbon FDU-16 rhombic dodecahedral single crystals with body-centered cubic structure (space group Im3̄m) have been successfully synthesized by employing an organic-organic assembly of triblock copolymer Pluronic F127 (EO106PO70EO106) and phenol/formaldehyde resol in basic aqueous solution. Synthetic factors (including reaction time, temperature, and stirring rate) are explored for controlling the formation of rhombic dodecahedral single crystals. The optimal stirring rate and the reaction temperature are 300 ± 10 rpm and ∼66 °C, respectively. High-resolution scanning electron microscopy (HRSEM), scanning transmission electron microscopy (STEM), and ultramicrotomy are applied to study the fine structures of the carbon single crystals. The mesopores are arranged in body-centered cubic symmetry throughout the entire particle. Surface steps are clearly observed in the {110} surface, which suggests a layer-by-layer growth of the mesoporous carbon FDU-16 single crystals. Cryo-SEM results from the reactant solution confirm the formation of resol/F127 unit micelles, further supporting the layer-by-layer growth process. The mesoporous carbon FDU-16 single crystals grow up to the final size of 2-4 μm within 2 days. These findings may have consequences for the growth mechanism of other carbon materials in aqueous solution; moreover, the high-quality single crystals also have potential applications in nanodevice technologies. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/cm101648y
  • 2010 • 16 Aerosol-derived bimetallic alloy powders: Bridging the gap
    Halevi, B. and Peterson, E.J. and Delariva, A. and Jeroro, E. and Lebarbier, V.M. and Wang, Y. and Vohs, J.M. and Kiefer, B. and Kunkes, E. and Havecker, M. and Behrens, M. and Schlögl, R. and Datye, A.K.
    Journal of Physical Chemistry C 114 17181-17190 (2010)
    We present aerosol-derived alloy powders as a uniquely useful platform for studying the contribution of the metal phase to multifunctional supported catalysts. Multimetallic heterogeneous catalysts made by traditional methods are usually nonhomogenous while UHV-based methods, such as mass selected clusters or metal vapor deposited on single crystals, lead to considerably more homogeneous, well-defined samples. However, these well-defined samples have low surface areas and do not lend themselves to catalytic activity tests in flow reactors under industrially relevant conditions. Bimetallic alloy powders derived by aerosol synthesis are homogeneous and single phase and can have surface areas ranging 1-10 m2/g, making them suitable for use in conventional flow reactors. The utility of aerosol-derived alloy powders as model catalysts is illustrated through the synthesis of single phase PdZn which was used to derive the specific reactivity of the L10 tetragonal alloy phase for methanol steam reforming. Turnover frequencies on unsupported PdZn were determined from the experimentally determined metal surface area to be 0.21 molecules of methanol reacted per surface Pd at 250 °C and 0.06 molecules of CO oxidized to CO2 per surface Pd at 185 °C. The experimentally measured activation energies for MSR and CO-oxidation on PdZn are 48 and 87 kJ/mol, respectively. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp103967x
  • 2010 • 15 Determining Burgers vectors and geometrically necessary dislocation densities from atomistic data
    Hua, J. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 18 (2010)
    We describe a novel analysis method to quantify the Burgers vectors of dislocations in atomistic ensembles and to calculate densities of geometrically necessary and statistically stored dislocations. This is accomplished by combining geometrical methods to determine dislocation cores and the slip vector analysis, which yields the relative slip of the atoms in dislocation cores and indicates the Burgers vectors of the dislocations. To demonstrate its prospects, the method is applied to investigate the density of geometrically necessary dislocations under a spherical nanoindentation. It is seen that this local information about dislocation densities provides useful information to bridge the gap between atomistic methods and continuum descriptions of plasticity, in particular for non-local plasticity. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/18/4/045007
  • 2010 • 14 Multi-axial behavior of ferroelectrics with two types of micro-macro mechanical models
    Jayabal, K. and Arockiarajan, A. and Menzel, A. and Sivakumar, S.M.
    IUTAM Bookseries 19 95-102 (2010)
    Ferroelectric ceramics exhibit a significantly different nonlinear behavior with external electric and mechanical fields applied at angles to the initial poled direction. This angle dependent response of the ferroelectric polycrystals are predicted by two types of models based on irreversible thermodynamics and physics of domain switching. The first type is a uniaxial model dealing with simultaneous evolution of three variants at a given instant. The back stress and electric fields, assumed as linear functions of remnant strain and polarization developed by the domain switching process, are introduced in the model to assist or resist further switching process. The second type is a three dimensional model that considers all six variants of a tetragonal crystal in each grain and the dissipation associated with grain boundary constraints are brought into the model through switching criterion. The pressure dependent constraints imposed by the surrounding grains on the grain of interest at its boundary during domain switching process is correlated with the resistance experienced by a ferroelectric single crystal on its boundary during domain switching. Taking all the domain switching possibilities, the volume fractions of each of the variants are tracked and homogenized for macroscopic behavior. Numerical simulations were carried out for the behavior of ferroelectrics using both the models and the outcome was found to be qualitatively comparable with experimental observations given in literature. © 2010 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/978-90-481-3771-8-10
  • 2010 • 13 Comparison of finite element and fast Fourier transform crystal plasticity solvers for texture prediction
    Liu, B. and Raabe, D. and Roters, F. and Eisenlohr, P. and Lebensohn, R.A.
    Modelling and Simulation in Materials Science and Engineering 18 (2010)
    We compare two full-field formulations, i.e. a crystal plasticity fast Fourier transform-based (CPFFT) model and the crystal plasticity finite element model (CPFEM) in terms of the deformation textures predicted by both approaches. Plane-strain compression of a 1024-grain ensemble is simulated with CPFFT and CPFEM to assess the models in terms of their predictions of texture evolution for engineering applications. Different combinations of final textures and strain distributionsare obtained with the CPFFT and CPFEM models for this 1024-grain polycrystal. To further understand these different predictions, the correlation between grain rotations and strain gradients is investigated through the simulation of plane-strain compression of bicrystals. Finally, a study of the influence of the initial crystal orientation and the crystallographic neighborhood on grain rotations and grain subdivisions is carried out by means of plane-strain compression simulations of a 64-grain cluster. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/18/8/085005
  • 2010 • 12 A review of crystallographic textures in chemical vapor-deposited diamond films
    Liu, T. and Raabe, D. and Mao, W.-M.
    Signal, Image and Video Processing 4 1-16 (2010)
    Diamond is one of the most important functional materials for film applications due to its extreme physical and mechanical properties, many of which depend on the crystallographic texture. The influence of various deposition parameters matters to the texture formation and evolution during chemical vapor deposition (CVD) of diamond films. In this overview, the texture evolutions are presented in terms of both simulations and experimental observations. The crystallographic textures in diamond are simulated based on the van der Drift growth selection mechanism. The film morphology and textures associated with the growth parameters α (proportional to the ratio of the growth rate along the 〈100〉 direction to that along the 〈111〉 direction) are presented and determined by applying the fastest growth directions. Thick films with variations in substrate temperature, methane concentration, film thickness, and nitrogen addition were analyzed using high-resolution electron back-scattering diffraction (HR-EBSD) as well as X-ray diffraction (XRD), and the fraction variations of fiber textures with these deposition parameters were explained. In conjunction with the focused ion beam (FIB) technique for specimen preparation, the grain orientations in the beginning nucleation zones were studied using HR-EBSD (50 nm step size) in another two sets of thin films deposited with variations in methane concentration and substrate material. The microstructures, textures, and grain boundary character were characterized. Based on the combination of an FIB unit for serial sectioning and HR-EBSD, diamond growth dynamics was observed using a 3D EBSD technique, with which individual diamond grains were investigated in 3D. Microscopic defects were observed in the vicinity of the high-angle grain boundaries by using the transmission electron microscopy (TEM) technique, and the advances of TEM orientation microscopy make it possible to identify the grain orientations in nano-crystalline diamond. © 2010 Higher Education Press and Springer Berlin Heidelberg.
    view abstractdoi: 10.1007/s11760-008-0099-7
  • 2010 • 11 Detection of Yb impurities in the VUV spectral range of NdGaO3 crystals
    Piasecki, M.M. and Andriyevsky, B.B. and Cobet, C.C. and Esser, N.N. and Kityk, I.V. and Świrkowicz, M.M. and Majchrowski, A.A.
    Optics Communications 283 3998-4003 (2010)
    Substantial influence of the 1 at.% Yb doping of NdGaO3 single crystals on the optical functions ε1(E) and ε 2(E) in the spectral range of electronic excitations is established. The corresponding differences of the optical functions for pure and doped NdGaO3 have been monitored using spectroscopic ellipsometry method and synchrotron radiation light source. This opens an opportunity for the spectroscopic diagnostics of the rare earth dopants in crystals using the spectroscopic information concerning the electronic inter-band optical transitions. To understand better the experimental results obtained, the ab-initio calculations of band structure and optical spectra of the centrosymmetric single crystal NdGaO3 have been performed using the VASP code (Vienna Ab-initio Simulation Package). The calculated dielectric functions ε1(E) and ε2(E) agree satisfactorily with our experimental results obtained with using the spectroscopic ellipsometry method and synchrotron radiation. It was demonstrated that differences of the optical functions for pure and doped crystals using stable synchrotron source may serve as a powerful tool for spectroscopic diagnostic of localised rare earth ions with respect to the strong inter-band transitions. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.optcom.2010.06.004
  • 2010 • 10 The exoskeleton of the American lobster- From texture to anisotropic properties
    Raue, L. and Klein, H. and Raabe, D.
    Solid State Phenomena 160 287-294 (2010)
    The exoskeleton of the crustacean Homarus americanus, the American lobster, is a biological multiphase composite consisting of a crystalline organic matrix (chitin), crystalline biominerals (calcite), amorphous calcium carbonate and proteins. One special structural aspect is the occurrence of pronounced crystallographic orientations and resulting directional anisotropic mechanical properties. The crystallographic textures of chitin and calcite have been measured by wide-angle Bragg diffraction, calculating the Orientation Distribution Function (ODF) from pole figures by using the series expansion method according to Bunge. A general strong relationship can be established between the crystallographic and the resulting mechanical and physical properties. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/SSP.160.287
  • 2010 • 9 Photoemission microscopy study of picosecond magnetodynamics in spin-valve-type thin film elements
    Schneider, C.M. and Kaiser, A. and Wiemann, C. and Tieg, C. and Cramm, S.
    Journal of Electron Spectroscopy and Related Phenomena 181 159-163 (2010)
    Exploring ultimate time scales of magnetic switching processes is an important issue in spin electronics. In spin valves or magnetic tunnelling junctions magnetic anisotropies and coupling phenomena alter the magnetodynamic response of the entire system. Understanding the role of these interactions is a key to the design of optimized devices. We have employed time-resolved X-ray photoemission microscopy to address the magnetodynamics in spin-valve-type model systems in the ns- and ps-regime. In Co/Cr/Fe(0 0 1) single crystal elements we find a strong influence of the magnetocrystalline anisotropy, which tends to suppress rotation processes. In addition, we observe a dynamic "decoupling" of the layers. In low-anisotropy FeNi/Cr/FeCo trilayers, the interlayer coupling character determines the dynamic response. Particularly, rotational processes in the FeNi and FeCo layers are temporarily shifted to each other, which can be related to different coercivities of the individual layers. By contrast, the domain wall motion in both layers closely agrees, caused by an enhanced coupling due to the domain wall stray fields. Our examples demonstrate that the detailed magnetodynamics in coupled magnetic layers is quite complex and depends strongly on the timescale under consideration. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elspec.2010.01.005
  • 2010 • 8 Structural characterization of a completely alkyl-substituted Al-Sb Lewis acid-base adduct
    Schulz, S. and Kuczkowski, A. and Nieger, M. and Saxell, H.
    Journal of Organometallic Chemistry 695 2281-2283 (2010)
    The Lewis acid-base adduct t-Bu3Al-SbMe3 (1), which was synthesized by reaction of equimolar amounts of t-Bu3Al and trimethylstibine SbMe3, was characterized by multinuclear NMR ( 1H, 13C) spectroscopy, elemental analyses as well as by single crystal X-ray diffraction. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.jorganchem.2010.06.022
  • 2010 • 7 On the multiplication of dislocations during martensitic transformations in NiTi shape memory alloys
    Simon, T. and Kröger, A. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 58 1850-1860 (2010)
    In situ and post-mortem diffraction contrast transmission electron microscopy (TEM) was used to study the multiplication of dislocations during a thermal martensitic forward and reverse transformation in a NiTi shape memory alloy single crystal. An analysis of the elongated dislocation loops which formed during the transformation was performed. It is proposed that the stress field of an approaching martensite needle activates an in-grown dislocation segment and generates characteristic narrow and elongated dislocation loops which expand on {1 1 0}B2 planes parallel to {0 0 1}B19′ compound twin planes. The findings are compared with TEM results reported in the literature for NiTi and other shape memory alloys. It is suggested that the type of dislocation multiplication mechanism documented in the present study is generic and that it can account for the increase in dislocation densities during thermal and stress-induced martensitic transformations in other shape memory alloys. © 2010.
    view abstractdoi: 10.1016/j.actamat.2009.11.028
  • 2010 • 6 Crystal orientation effects in scratch testing with a spherical Indenter
    Swadener, J.G. and Bögershausen, H. and Sander, B. and Raabe, D.
    Journal of Materials Research 25 921-926 (2010)
    Spherical scratch tests were conducted in individual grains of a randomly oriented polycrystalline body-centered-cubic (bcc) Ti-Nb alloy. For each grain, scratch tests were conducted at four different levels of normal load, which resulted in varying amounts of plastic strain during indentation. The results show a dependence of the horizontal load component on the crystallographic orientation and on the amount of plastic strain. The component of the horizontal force that resulted from plastic deformation was found to correlate with the active slip systems for the particular grain orientation. © 2010 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2010.0108
  • 2010 • 5 Microplasticity phenomena in thermomechanically strained nickel thin films
    Taylor, A.A. and Oh, S.H. and Dehm, G.
    Journal of Materials Science 45 3874-3881 (2010)
    Magnetron sputtered Ni thin films on both oxidised Si (100) and α-Al2O3 (0001) substrates of thickness 150-1000 nm were tested thermomechanically with a wafer curvature system, as well as in situ in a transmission electron microscope. The films on oxidised Si have a {111}-textured columnar microstructure with a mean grain size similar to the film thickness. On (0001) α-Al2O3 a near single crystal epitaxy with two growth variants is achieved leading to a significantly larger grain size. The thermomechanical testing was analysed in terms of the room temperature/high temperature flow stresses in the films and the observed thermoelastic slopes. It was found that the room temperature flow stresses increased with decreasing film thickness until a plateau of ∼1100 MPa was reached for films thinner than 400 nm. This plateau is attributed to the present experiments exerting insufficient thermal strain to induce yielding in these thinner films. At 500 °C the compressive flow stresses of the films show a competition between dislocation and diffusion mediated plasticity. A size effect is also observed in the thermoelastic slopes of the films, with thinner films coming closer to the slope predicted by mismatch in thermal expansion coefficients. It is put forward here that this is due to a highly inhomogeneous stress distribution in the films arising from the grain size distribution. © 2010 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-010-4445-0
  • 2010 • 4 Studies regarding the homogeneity range of the zirconium phosphide telluride Zr2+δPTe2
    Tschulik, K. and Hoffmann, S. and Fokwa, B.P.T. and Gilleßen, M. and Schmidt, P.
    Solid State Sciences 12 2030-2035 (2010)
    The phosphide tellurides Zr2+δPTe2 (0 ≤ δ ≤ 1) can be synthesized from the elements in a solid state reaction or by thermal decomposition of Z. Zr2PTe2 decomposes under release of Te2(g) + P4(g) forming the homogeneity range Zr2+δPTe2. The growth of single crystals of Zr 2+δPTe2 succeeded by chemical vapour transport using iodine as transport agent from 830 °C in direction of higher temperatures up to 900 °C. Zr2+δPTe2 crystallizes in the rhombohedral space group R3m (no. 166) with lattice parameters a = 383(1)...386(1) pm and c = 2935(4)...2970(4) pm for δ = 0...1, respectively. Single crystal data have been determined for Zr 2.40(2)PTe2 with lattice parameters a = 385.24(4) pm and c = 2967.8(4) pm. The electronic structure and chemical bonding in Zr 2+δPTe2 was investigated by the linear muffin-tin orbital (LMTO) method. Both Zr2PTe2 and Zr 3PTe2 show non-vanishing DOS values at the Fermi level (EF) indicating metallic character. According to COHP bonding analyses, mainly the heteroatomic Zr-P and Zr-Te bonds are responsible for the structural stability of Zr3PTe2. The new Zr2-Te bond, which is not present in Zr2PTe2, is stronger than Zr1-Te and is thought to be responsible for the stability of phases having Zr in excess.
    view abstractdoi: 10.1016/j.solidstatesciences.2010.08.022
  • 2010 • 3 Fast, physically-based algorithms for online calculations of texture and anisotropy during fabrication of steel sheets
    Winning, M. and Raabe, D.
    Advanced Engineering Materials 12 1206-1211 (2010)
    Because of the complex microstructures of crystalline materials exposed to commercial manufacturing processes it is up to now not possible to obtain fast and on-line simulations of crystallographic texture and anisotropy in the course of multiple deformation- and heat treatment procedures. In the present paper a hybrid approach for the on-line texture and anisotropy prediction will be developed for the fabrication of low alloyed ferritic steel sheets during cold rolling and subsequent annealing procedures. Our approach is based on two consecutive models: The first one is an artificial neuronal network (ANN) for the description of the rolling texture evolution. The second one is an analytical, Avrami-based texture component approach for the recrystallization. First results on low carbon steels will be presented. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201000206
  • 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