Scientific Output

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

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  • 2021 • 184 Orientation-dependent plastic deformation mechanisms and competition with stress-induced phase transformation in microscale NiTi
    Choi, W.S. and Pang, E.L. and Ko, W.-S. and Jun, H. and Bong, H.J. and Kirchlechner, C. and Raabe, D. and Choi, P.-P.
    Acta Materialia 208 (2021)
    Understanding the orientation-dependent deformation behavior of NiTi shape-memory alloys at small length scales is of importance for designing nano- and micro-electromechanical systems. However, a complete understanding of the orientation- and size-dependent competition between the various modes of slip, deformation twinning, and martensitic transformation in NiTi shape-memory alloys is still lacking, especially in micron-scale specimens. In the present study, we perform micro-compression tests on [001]- and [112]-oriented micro-pillars of a solutionized Ti-49.9at.% Ni alloy. Post-mortem TEM analysis of the deformed pillars reveal that the operating plastic deformation modes are {011}<100> slip and {114}<221¯> deformation twinning, which compete with the martensitic transformation, depending on the crystal orientation. Furthermore, in both experiments and molecular dynamics simulations, we consistently find residual B19′ martensite in a herringbone microstructure composed of finely spaced (001)B19′ compound twins instead of the generally assumed [011]B19′ type II twins common in bulk samples, suggesting that the operative martensitic transformation mode may be size-dependent. Schmid factors in compression are calculated for all commonly reported slip, deformation twinning, and martensitic transformation modes as a function of crystallographic orientation, which rationalize the orientation-dependent competition between these deformation modes. © 2021 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2021.116731
  • 2021 • 183 Elemental segregation to lattice defects in the CrMnFeCoNi high-entropy alloy during high temperature exposures
    Heczko, M. and Mazánová, V. and Gröger, R. and Záležák, T. and Hooshmand, M.S. and George, E.P. and Mills, M.J. and Dlouhý, A.
    Acta Materialia 208 (2021)
    The influence of small plastic pre-strains on the elevated-temperature stability and microstructure of the equiatomic CrMnFeCoNi FCC solid solution is investigated. Particular attention is given to whether any of the alloy elements segregate to individual dislocations. To that end, CrMnFeCoNi samples were first deformed in tension at room temperature to plastic strains of 0.2 and 2.3%, and subsequently annealed at 973 K for 800 hours. The pre-strains activated planar slip of 1/2<110>-type dislocations on {111}-type glide planes. Interactions of this planar slip with special Σ3 grain boundaries formed a large number of dislocation segments with a <110>-type crystallographic orientation suitable for a credible end-on analysis of dislocation cores in HR-STEM. The cores of the 1/2<110> dislocations pushed up against the investigated grain boundaries were found to be close to the compact configuration. Within the sensitivity of the Super-X EDS mapping, no concentration gradient was detected near dislocations that would indicate enrichment at dislocation cores of any of the elemental constituents of the alloy after the pre-deformation and annealing. However, a Cr-rich tetragonal sigma phase nucleated and grew at grain boundary triple junctions during this anneal, processes that were not accelerated by the enhanced dislocation density present after pre-strain. A clear chromium gradient was observed in the Cr-depleted zones near grain boundaries suggesting that Cr transport occurred by relatively slow diffusion from the bulk to the grain boundaries and then by relatively fast diffusion along the grain boundaries to the precipitates. Accompanying the Cr depletion near grain boundaries is a simultaneous Ni and Mn enrichment, which promotes formation of the L10 NiMn phase that is observed on the grain boundaries after prolonged annealing. © 2021
    view abstractdoi: 10.1016/j.actamat.2021.116719
  • 2021 • 182 Ab initio based models for temperature-dependent magnetochemical interplay in bcc Fe-Mn alloys
    Schneider, A. and Fu, C.-C. and Waseda, O. and Barreteau, C. and Hickel, T.
    Physical Review B 103 (2021)
    Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behavior from both experiments and 0 K electronic-structure calculations, which is due to the half-filled 3d band of Mn. We propose effective interaction models for these alloys, which contain both atomic-spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of noncollinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fitting. Employing these models in atomic Monte Carlo simulations enables the prediction of magnetic and thermodynamic properties of the Fe-Mn alloys, and their coupling, as functions of temperature. This includes the decrease of Curie temperature with increasing Mn concentration, the temperature evolution of the mixing enthalpy, and its correlation with the alloy magnetization. Also, going beyond the defect-free systems, we determined the binding free energy between a vacancy and a Mn atom, which is a key parameter controlling the atomic transport in Fe-Mn alloys. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.024421
  • 2021 • 181 Limited Elemental Mixing in Nanoparticles Generated by Ultrashort Pulse Laser Ablation of AgCu Bilayer Thin Films in a Liquid Environment: Atomistic Modeling and Experiments
    Shih, C.-Y. and Chen, C. and Rehbock, C. and Tymoczko, A. and Wiedwald, U. and Kamp, M. and Schuermann, U. and Kienle, L. and Barcikowski, S. and Zhigilei, L.V.
    Journal of Physical Chemistry C (2021)
    Pulsed laser ablation in liquids (PLAL) is a promising technique for the generation of colloidal alloy nanoparticles that are of high demand in a broad range of fields, including catalysis, additive manufacturing, and biomedicine. Many of the applications have stringent requirements on the nanoparticle composition and size distributions, which can only be met through innovations in the PLAL technique guided by a clear understanding of the nanoparticle formation mechanisms. In this work, we undertake a combined computational and experimental study of the nanoparticle formation mechanisms in ultrashort PLAL of Ag/Cu and Cu/Ag bilayer thin films. Experimental probing of the composition of individual nanoparticles and predictions from large-scale atomistic simulations provide consistent evidence of limited mixing between the two components from bilayer films by PLAL. The simulated and experimental distributions of nanoparticle compositions exhibit an enhanced abundance of Ag-rich and Cu-rich nanoparticles, as well as a strongly depressed population of well-mixed alloy nanoparticles. The surprising observation that the nanoscale phase separation of the two components in the bilayer films manifests itself in the sharp departure from the complete quantitative mixing in the colloidal nanoparticles is explained by the complex dynamic interaction between the ablation plume and liquid environment revealed in the simulations of the initial stage of the ablation process. The simulations predict that rapid deceleration of the ablation plume by the liquid environment results in the formation of a transient hot and dense metal region at the front of the plume, which hampers the mixing of the two components and, at the same time, contributes to the stratification of the plume in the emerging cavitation bubble. As a result, nanoparticles of different sizes and compositions are produced in different parts of the emerging cavitation bubble during the first nanoseconds of the ablation process. Notably, the diameters of the largest nanoparticles generated in the simulations of the initial stage of the ablation process are more than twice larger than the thickness of the original bilayer films. This observation provides a plausible scenario for the formation of large nanoparticles observed in the experiments. The conclusion on limited elemental mixing in the nanoparticles is validated in simulations of bilayers with different spatial order of Cu and Ag layers, even though the two systems exhibit some notable quantitative differences mainly related to the different strength of electron-phonon coupling in Cu and Ag. Overall, the results of this study provide new insights into the formation mechanism of bimetallic nanoparticles in ultrashort PLAL from thin bilayer targets and suggest that the formation of alloy nanoparticles from immiscible elements may be hampered for targets featuring distinctive elemental segregation. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.0c09970
  • 2021 • 180 On the synthesis and structural evolution of artificial CrN/TiN nanocomposites
    Tillmann, W. and Kokalj, D. and Stangier, D. and Fu, Q. and Kruis, F.E. and Kesper, L. and Berges, U. and Westphal, C.
    Applied Surface Science 535 (2021)
    The synthesis of nanocomposites is limited to thermodynamically immiscible phases or to phase separation by exceeding the limits of solution. Hence, the formation of nanocomposites based on transition metals, revealing a nanocrystalline Metal-Nitride/nanocrystalline Metal-Nitride structure, is restricted. These restrictions can be overruled by a spatially separated synthesis of the two phases and a recombination during the deposition. With this approach, the limits of current systems can be expanded, enabling the synthesis of artificial nanocomposites based on a variety of materials. We demonstrate the synthesis of a composite of two nanocrystalline phases of the miscible transition metal-nitrides CrN and TiN. TiN nanoparticles were synthesized using an atmospheric-pressure arc reactor and in-situ injected into a growing CrN thin film. The thin films are analyzed regarding their physical- and microstructure using two-dimensional GIXRD, XPS based on synchrotron radiation and TEM. The CrTiN thin film reveals a two-phase structure consisting of nanocrystalline CrN and TiN phases with crystallite sizes of 9 nm and 4 nm according to GIXRD. XPS indicates bonding of Cr-N, Cr-Cr, and Ti-N. No hint for Cr-Ti bonding was found, excluding (Cr,Ti)N solid solution formation. Based on the TEM-investigations, TiN nanoparticles are embedded as agglomerates in the CrN matrix. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2020.147736
  • 2020 • 179 In situ X-ray emission and high-resolution X-ray absorption spectroscopy applied to Ni-based bimetallic dry methane reforming catalysts
    Beheshti Askari, A. and Al Samarai, M. and Hiraoka, N. and Ishii, H. and Tillmann, L. and Muhler, M. and Debeer, S.
    Nanoscale 12 15185-15192 (2020)
    The promoting effect of cobalt on the catalytic activity of a NiCoO Dry Methane Reforming (DMR) catalyst was studied by a combination of in situ Kβ X-ray Emission Spectroscopy (XES) and Kβ-detected High Energy Resolution Fluorescence Detected X-ray absorption spectroscopy (HERFD XAS). Following the calcination process, Ni XES and Kβ-detected HERFD XAS data revealed that the NiO coordination in the NiCoO catalyst has a higher degree of symmetry and is different than that of pure NiO/γ-Al2O3. Following the reductive activation, it was found that the NiCoO/γ-Al2O3 catalyst required a relatively higher temperature compared to the monometallic NiO/γ-Al2O3 catalyst. This finding suggests that Co is hampering the reduction of Ni in the NiCoO catalyst by modulation of its electronic structure. It has also been previously shown that the addition of Co enhances the DMR activity. Further, the Kβ XES spectrum of the partly reduced catalysts at 450 °C reveals that the Ni sites in the NiCoO catalyst are electronically different from the NiO catalyst. The in situ X-ray spectroscopic study demonstrates that reduced metallic Co and Ni are the primary species present after reduction and are preserved under DMR conditions. However, the NiCo catalyst appears to always be somewhat more oxidized than the Ni-only species, suggesting that the presence of cobalt modulates the Ni electronic structure. The electronic structural modulations resulting from the presence of Co may be the key to the increased activity of the NiCo catalyst relative to the Ni-only catalyst. This study emphasizes the potential of in situ X-ray spectroscopy experiments for probing the electronic structure of catalytic materials during activation and under operating conditions. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0nr01960g
  • 2020 • 178 In Situ X-ray Microscopy Reveals Particle Dynamics in a NiCo Dry Methane Reforming Catalyst under Operating Conditions
    Beheshti Askari, A. and Al Samarai, M. and Morana, B. and Tillmann, L. and Pfänder, N. and Wandzilak, A. and Watts, B. and Belkhou, R. and Muhler, M. and Muhler, M. and Debeer, S.
    ACS Catalysis 10 6223-6230 (2020)
    Herein, we report the synthesis of a γ-Al2O3-supported NiCo catalyst for dry methane reforming (DMR) and study the catalyst using in situ scanning transmission X-ray microscopy (STXM) during the reduction (activation step) and under reaction conditions. During the reduction process, the NiCo alloy particles undergo elemental segregation with Co migrating toward the center of the catalyst particles and Ni migrating to the outer surfaces. Under DMR conditions, the segregated structure is maintained, thus hinting at the importance of this structure to optimal catalytic functions. Finally, the formation of Ni-rich branches on the surface of the particles is observed during DMR, suggesting that the loss of Ni from the outer shell may play a role in the reduced stability and hence catalyst deactivation. These findings provide insights into the morphological and electronic structural changes that occur in a NiCo-based catalyst during DMR. Further, this study emphasizes the need to study catalysts under operating conditions in order to elucidate material dynamics during the reaction. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.9b05517
  • 2020 • 177 An approach to use sub-surface characteristics for the prediction of process forces during cutting operations
    Bergmann, J.A. and Kimm, J. and Theisen, W. and Wiederkehr, P.
    Procedia CIRP 88 276-281 (2020)
    During machining operations, complex engagement situations between the tool and workpiece lead to varying amplitudes and directions of cutting forces. Correlations between material properties, process parameters and resulting forces have to be analyzed to ensure the predictability of machining processes. To model the material removal, a detailed analysis of the material behavior during the engagement is needed. In this work, a geometric physically-based simulation system is extended to take the material behavior into account for improving the prediction accuracy of process forces. A detailed analysis of the low-alloyed steel 42CrMo4 and validation experiments are presented. © 2020 The Authors.
    view abstractdoi: 10.1016/j.procir.2020.05.048
  • 2020 • 176 Local and Nonlocal Electron Dynamics of Au/Fe/MgO (001) Heterostructures Analyzed by Time-Resolved Two-Photon Photoemission Spectroscopy
    Beyazit, Y. and Beckord, J. and Zhou, P. and Meyburg, J.P. and Kühne, F. and Diesing, D. and Ligges, M. and Bovensiepen, U.
    Physical Review Letters 125 (2020)
    Employing femtosecond laser pulses in front and back side pumping of Au/Fe/MgO(001) combined with detection in two-photon photoelectron emission spectroscopy, we analyze local relaxation dynamics of excited electrons in buried Fe, injection into Au across the Fe-Au interface, and electron transport across the Au layer at 0.6 to 2.0 eV above the Fermi energy. By analysis as a function of Au film thickness we obtain the electron lifetimes of bulk Au and Fe and distinguish the relaxation in the heterostructure's constituents. We also show that the excited electrons propagate through Au in a superdiffusive regime and conclude further that electron injection across the epitaxial interface proceeds ballistically by electron wave packet propagation. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.125.076803
  • 2020 • 175 Trends in elastic properties of Ti-Ta alloys from first-principles calculations
    Chakraborty, T. and Rogal, J.
    Journal of Physics Condensed Matter 33 (2020)
    The martensitic start temperature (M s) is a technologically fundamental characteristic of high-temperature shape memory alloys. We have recently shown [Chakraborty et al 2016 Phys. Rev. B 94 224104] that the two key features in describing the composition dependence of M s are the T = 0 K phase stability and the difference in vibrational entropy which, within the Debye model, is directly linked to the elastic properties. Here, we use density functional theory together with special quasi-random structures to study the elastic properties of disordered martensite and austenite Ti-Ta alloys as a function of composition. We observe a softening in the tetragonal shear elastic constant of the austenite phase at low Ta content and a non-linear behavior in the shear elastic constant of the martensite. A minimum of 12.5% Ta is required to stabilize the austenite phase at T = 0 K. Further, the shear elastic constants and Young's modulus of martensite exhibit a maximum for Ta concentrations close to 30%. Phenomenological, elastic-constant-based criteria suggest that the addition of Ta enhances the strength, but reduces the ductile character of the alloys. In addition, the directional elastic stiffness, calculated for both martensite and austenite, becomes more isotropic with increasing Ta content. The reported trends in elastic properties as a function of composition may serve as a guide in the design of alloys with optimized properties in this interesting class of materials. © 2020 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/abba67
  • 2020 • 174 Influence of lattice misfit on the deformation behaviour of α2/γ lamellae in TiAl alloys
    Chauniyal, A. and Janisch, R.
    Materials Science and Engineering A 796 (2020)
    Interfaces play a significant role in the deformation behaviour of lamellar two-phase TiAl alloys and contribute to their increased strength. We study the deformation behaviour of α2/γ bilayers with either coherent or semicoherent interfaces, using atomistic simulations. We identify the nucleation sites for dislocations and decouple the effects of the microstructural parameters volume fraction and layer thickness on the yield stress and strain. Uniaxial tensile tests are carried out on bi-layer specimens with α2 and γ phases along directions parallel and perpendicular to the interface. Coherent α2∕γ bi-layers show residual stresses due to lattice mismatch which are linearly related to the volume fractions of the phases. These residual stresses, superimposed with tensile stresses during loading, lead to early yielding of the γ phase. In contrast, a semi-coherent interface leads to negligible residual stresses, but contains misfit dislocations which create localized stresses within the γ layer and thus contributes to dislocation nucleation. We show that along loading directions parallel to the interface, the layer thickness does not affect the deformation behaviour, irrespective of the type of interface, instead volume fraction is the governing parameter. When loading perpendicular to the interface, the absolute layer thickness does not affect the deformation behaviour of a bi-layer with a coherent interface, but determines the yield stress and strain in case of a semi coherent interface. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2020.140053
  • 2020 • 173 Sintering and biocompatibility of blended elemental Ti-xNb alloys
    Chen, Y. and Han, P. and Dehghan-Manshadi, A. and Kent, D. and Ehtemam-Haghighi, S. and Jowers, C. and Bermingham, M. and Li, T. and Cooper-White, J. and Dargusch, M.S.
    Journal of the Mechanical Behavior of Biomedical Materials 104 (2020)
    Titanium-niobium (Ti–Nb) alloys have great potential for biomedical applications due to their superior biocompatibility and mechanical properties that match closely to human bone. Powder metallurgy is an ideal technology for efficient manufacture of titanium alloys to generate net-shape, intricately featured and porous components. This work reports on the effects of Nb concentrations on sintered Ti-xNb alloys with the aim to establish an optimal composition in respect to mechanical and biological performances. Ti-xNb alloys with 33, 40, 56 and 66 wt% Nb were fabricated from elemental powders and the sintering response, mechanical properties, microstructures and biocompatibility assessed and compared to conventional commercial purity titanium (CPTi). The sintered densities for all Ti-xNb compositions were around 95%, reducing slightly with increasing Nb due to increasing open porosity. Higher Nb levels retarded sintering leading to more inhomogeneous phase and pore distributions. The compressive strength decreased with increasing Nb, while all Ti-xNb alloys displayed higher strengths than CPTi except the Ti–66Nb alloy. The Young's moduli of the Ti-xNb alloys with ≥40 wt% Nb were substantially lower (30–50%) than CPTi. In-vitro cell culture testing revealed excellent biocompatibility for all Ti-xNb alloys comparable or better than tissue culture plate and CPTi controls, with the Ti–40Nb alloy exhibiting superior cell-material interactions. In view of its mechanical and biological performance, the Ti–40Nb composition is most promising for hard tissue engineering applications. © 2020
    view abstractdoi: 10.1016/j.jmbbm.2020.103691
  • 2020 • 172 A model for grain boundary thermodynamics
    Darvishi Kamachali, R.
    RSC Advances 10 26728-26741 (2020)
    Systematic microstructure design requires reliable thermodynamic descriptions of each and all microstructure elements. While such descriptions are well established for most bulk phases, thermodynamic assessment of microstructure defects is challenging because of their individualistic nature. In this paper, a model is devised for assessing grain boundary thermodynamics based on available bulk thermodynamic data. We propose a continuous relative atomic density field and its spatial gradients to describe the grain boundary region with reference to the homogeneous bulk and derive the grain boundary Gibbs free energy functional. The grain boundary segregation isotherm and phase diagram are computed for a regular binary solid solution, and qualitatively benchmarked for the Pt-Au system. The relationships between the grain boundary's atomic density, excess free volume, and misorientation angle are discussed. Combining the current density-based model with available bulk thermodynamic databases enables constructing databases, phase diagrams, and segregation isotherms for grain boundaries, opening possibilities for studying and designing heterogeneous microstructures. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ra04682e
  • 2020 • 171 Effect of Oxygen on High-temperature Phase Equilibria in Ternary Ti-Al-Nb Alloys
    Distl, B. and Dehm, G. and Stein, F.
    Zeitschrift fur Anorganische und Allgemeine Chemie 646 1151-1156 (2020)
    Alloys based on titanium aluminides received a lot of attention because of their capability to substitute Ni-based superalloys in high-temperature applications. However, the phase equilibria between the main microstructure constituents (αTi), (βTi), γ (TiAl) and α2(Ti3Al) can be shifted significantly by impurities such as oxygen especially at high temperatures. This behavior is investigated on the tie-triangle (αTi) + (βTi) + γ (TiAl) in the ternary Ti-Al-Nb system at 1300 °C. An explanation for this behavior could be the occupation of octahedral voids by impurities in certain phases. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/zaac.202000098
  • 2020 • 170 Magnetic response of FeRh to static and dynamic disorder
    Eggert, B. and Schmeink, A. and Lill, J. and Liedke, M.O. and Kentsch, U. and Butterling, M. and Wagner, A. and Pascarelli, S. and Potzger, K. and Lindner, J. and Thomson, T. and Fassbender, J. and Ollefs, K. and Keune, W. and Bal...
    RSC Advances 10 14386-14395 (2020)
    Atomic scale defects generated using focused ion as well as laser beams can activate ferromagnetism in initially non-ferromagnetic B2 ordered alloy thin film templates. Such defects can be induced locally, confining the ferromagnetic objects within well-defined nanoscale regions. The characterization of these atomic scale defects is challenging, and the mechanism for the emergence of ferromagnetism due to sensitive lattice disordering is unclear. Here we directly probe a variety of microscopic defects in systematically disordered B2 FeRh thin films that are initially antiferromagnetic and undergo a thermally-driven isostructural phase transition to a volatile ferromagnetic state. We show that the presence of static disorder i.e., the slight deviations of atoms from their equilibrium sites is sufficient to induce a non-volatile ferromagnetic state at room temperature. A static mean square relative displacement of 9 × 10-4 Å-2 is associated with the occurrence of non-volatile ferromagnetism and replicates a snapshot of the dynamic disorder observed in the thermally-driven ferromagnetic state. The equivalence of static and dynamic disorder with respect to the ferromagnetic behavior can provide insights into the emergence of ferromagnetic coupling as well as achieving tunable magnetic properties through defect manipulations in alloys. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ra01410a
  • 2020 • 169 Magneto-structural correlations in a systematically disordered B2 lattice
    Ehrler, J. and Sanyal, B. and Grenzer, J. and Zhou, S. and Böttger, R. and Eggert, B. and Wende, H. and Lindner, J. and Fassbender, J. and Leyens, C. and Potzger, K. and Bali, R.
    New Journal of Physics 22 (2020)
    Ferromagnetism in certain B2 ordered alloys such as Fe60Al40 can be switched on, and tuned, via antisite disordering of the atomic arrangement. The disordering is accompanied by a ∼1 % increase in the lattice parameter. Here we performed a systematic disordering of B2 Fe60Al40 thin films, and obtained correlations between the order parameter (S), lattice parameter (a 0), and the induced saturation magnetization (M s). As the lattice is gradually disordered, a critical point occurs at 1-S = 0.6 and a 0 = 2.91 Å, where a sharp increase of the M s is observed. DFT calculations suggest that below the critical point the system magnetically behaves as it would still be fully ordered, whereas above, it is largely the increase of a 0 in the disordered state that determines the M s. The insights obtained here can be useful for achieving tailored magnetic properties in alloys through disordering. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/ab944a
  • 2020 • 168 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 • 167 Competition of defect ordering and site disproportionation in strained LaCoO3 on SrTiO3 (001)
    Geisler, B. and Pentcheva, R.
    Physical Review B 101 (2020)
    The origin of the 3×1 reconstruction observed in epitaxial LaCoO3 films on SrTiO3(001) is assessed by first-principles calculations including a Coulomb repulsion term. We compile a phase diagram as a function of the oxygen pressure, which shows that (3×1)-ordered oxygen vacancies (LaCoO2.67) are favored under commonly used growth conditions, while stoichiometric films emerge under oxygen-rich conditions. Growth of further reduced LaCoO2.5 brownmillerite films is impeded by phase separation. We report two competing ground-state candidates for stoichiometric films: a semimetallic phase with 3×1 low-spin/intermediate-spin/intermediate-spin (LS/IS/IS) magnetic order and a semiconducting phase with IS/IS/IS magnetic order. This demonstrates that tensile strain induces ferromagnetism even in the absence of oxygen vacancies. Both phases exhibit an intriguing (3×1)-reconstructed octahedral rotation pattern and accordingly modulated La-La distances. In particular, charge and bond disproportionation and concomitant orbital order of the t2g hole emerge at the Co sites that are also observed for unstrained bulk LaCoO3 in the IS state and explain structural data obtained by x-ray diffraction at elevated temperature. Site disproportionation drives a metal-to-semiconductor transition that reconciles the IS state with the experimentally observed low conductivity during spin-state crossover without the presence of Jahn-Teller distortions. © 2020 American Physical Society. ©2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.165108
  • 2020 • 166 The steady-state kinetics of CO hydrogenation to higher alcohols over a bulk Co-Cu catalyst
    Göbel, C. and Schmidt, S. and Froese, C. and Bujara, T. and Viktor Scherer and Muhler, M.
    Journal of Catalysis (2020)
    The kinetics of higher alcohol synthesis was investigated using a hydrotalcite-derived Co-Cu-based catalyst aiming at a deeper understanding of the complex reaction network. At steady state similar chain growth probabilities of about 0.4 according to the Anderson-Schulz-Flory distribution were observed for alcohols, hydrocarbons and olefins indicating common intermediates. Alkanes were found to be formed consecutively from primarily formed olefins. The observed decrease of the selectivities to alcohols with increasing CO conversion at higher temperatures and higher residence times is ascribed to an increased availability of adsorbed atomic hydrogen, which decreases the saturated coverage of CO-derived CxHyOz species favoring hydrocarbon formation. Correspondingly, reaction orders of 0 and 0.8 for CO and H2, respectively, were derived based on a power-law approach including an apparent activation energy of 140 kJ mol−1. A reaction network based on the CO insertion factor was established, in which the competing reactions β-hydrogen elimination, chain growth and CO insertion proceed from common adsorbed CxHy intermediates. Selective higher alcohol formation was favored at low temperatures and short residence times, high pressures and a moderate H2:CO ratio of 1 requiring a compromise between conversion and selectivity. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2020.10.017
  • 2020 • 165 Structural evolution of bimetallic Co-Cu catalysts in CO hydrogenation to higher alcohols at high pressure
    Göbel, C. and Schmidt, S. and Froese, C. and Fu, Q. and Chen, Y.-T. and Pan, Q. and Muhler, M.
    Journal of Catalysis 383 33-41 (2020)
    Bimetallic Co-Cu catalysts are widely applied in higher alcohol synthesis (HAS), but the formation of the final active structure has not yet been fully clarified, especially for Co-rich catalysts. We investigated the structural evolution of a Co-Cu catalyst (Co:Cu = 2) from the hydrotalcite precursor containing additional Al3+ and Zn2+ to the final active state after 80 h under reaction conditions at 280 °C and 60 bar. The reconstruction of the bimetallic Co-Cu nanoparticles obtained by H2 reduction was induced by the feed gas consisting of an equimolar H2 and CO syngas mixture resulting in fast phase separation and sintering of metallic Cu0 and Co0 in the first 2 h time on stream (TOS) and a continuous carbidization of Co0 forming Co2C and its sintering until steady state was reached after 40 h TOS. An intergrowth of metallic Cu0 nanoparticles with Co2C nanoparticles was observed to occur under reaction conditions. The high selectivity to oxygenates amounting to 41% compared with 29% to hydrocarbons is ascribed to the multi-functional Co2C/Cu0 interface enabling dissociative CO adsorption, hydrogenation and CO insertion. The formation of hydrogenated carbon species (CxHy) originating from dissociative CO chemisorption is assumed to be favored by hydrogen spillover from Cu0 to Co2C. The adsorption sites for molecular CO provided by both Cu0 and Co2C facilitate its insertion into the CxHy intermediates thus leading to a higher selectivity to alcohols following the Anderson-Schulz-Flory distribution. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2020.01.004
  • 2020 • 164 Effects of Mo on the mechanical behavior of γ/γʹ-strengthened Co-Ti-based alloys
    Im, H.J. and Lee, S. and Choi, W.S. and Makineni, S.K. and Raabe, D. and Ko, W.-S. and Choi, P.-P.
    Acta Materialia 197 69-80 (2020)
    We investigated the flow behavior of γ/γʹ-strengthened Co-12Ti and Co-12Ti-4Mo (at.%) alloys at room and elevated temperatures (up to 900°C) by electron microscopy and density functional theory. The Mo-added alloy exhibited an enhanced compressive yield strength and strain hardening behavior as compared to the reference binary alloy. This behavior could be attributed to a ~25% larger γʹ volume fraction and ~7% higher planar fault energies in Co-12Ti-4Mo. Using electron channeling contrast imaging, we observed interrupted slip bands in the Co-12Ti-4Mo alloy deformed to a strain of 6%, which led to enhanced strain hardening, in contrast to extended slip bands along {111} planes in the Co-12Ti alloy. Interrupted slip band formation in Co-12Ti-4Mo could be explained by rapid exhaustion of dislocation sources and a higher energy barrier required to cut the γʹ precipitates. These effects are due to a reduced γ channel width and substantial hardening effect of γʹ-Co3(Ti,Mo) in the ternary alloy as well as due to the large shear modulus difference between γʹ and γ. © 2020
    view abstractdoi: 10.1016/j.actamat.2020.07.037
  • 2020 • 163 Micro-, macromechanical and aeroelastic investigation of glass - fiber based, lightweight turbomachinery components
    Iseni, S. and Prasad, M.R.G. and Hartmaier, A. and Holeczek, K. and Modeler, N. and di Mare, F.
    Proceedings of the ASME Turbo Expo 10A-2020 (2020)
    A major technical challenge for modern aero engines is the development of designs which reduce noise and emission whilst increasing aerodynamic efficiency and ensuring aeroelastic stability of low-temperature engine components such as fans and low-pressure compressors. Composites are used in aviation due to their excellent stiffness and strength properties, which also enable additional flexibility in the design process. The weight reduction of the turbomachine components, due to composite materials and lighter engines, is especially relevant for the design and developments of hybrid-electric or distributed propulsion systems [1]. To accomplish this, a representative volume element (RVE) of a glass-fiber reinforced polymer is created, describing the geometrical arrangement of the textile reinforcement structure within the polymer matrix. For both phases, realistic linear elastic properties are assumed. This RVE will be investigated with the finite element method under various loading conditions to assess its anisotropic elastic properties and also its damping behaviour for elastic waves. To study the influence of delamination on the mechanical properties, small defects will be introduced into the model at the interface between reinforcement and matrix. Based on this micromechanical approach, a constitutive model for the composite will be formulated that describes the anisotropic properties as well as the damping behaviour. This constitutive model is then used to describe the material response in a macro-mechanical model, which serves as the basis for an aeroelastic analysis of a 1/3-scaled high-speed fan using a conventional (Ti-6Al -4V) and fiber composite material. Copyright © 2020 ASME
    view abstractdoi: 10.1115/GT2020-14951
  • 2020 • 162 Thermodynamic modelling of the Ni–Zr system
    Jana, A. and Sridar, S. and Fries, S.G. and Hammerschmidt, T. and Kumar, K.C.H.
    Intermetallics 116 (2020)
    In this work, we report the thermodynamic modelling of the Ni–Zr system using the Calphad method combined with ab initio calculations. Density functional theory (DFT) is employed to calculate the enthalpy of formation of the intermediate phases. The calculated enthalpies of formation are in close agreement with the experimental data. An approach based on special quasirandom structures (SQS) was used for calculating the enthalpy of mixing of the fcc solid solution. The vibrational contribution to the heat capacities of NiZr, NiZr2, Ni3Zr and Ni7Zr2 phases were calculated using the quasiharmonic approximation (QHA) and the corresponding electronic contribution was obtained using an approach based on Mermin statistics. The total heat capacities for these phases were fitted to appropriate expressions and integrated to obtain the Gibbs energy functions valid down to 0 K. The calculated thermochemical properties along with critically selected experimental constitutional and thermochemical data served as input for the thermodynamic optimisation of the system. The calculated phase equilibria and the thermodynamic properties using the optimised Gibbs energy functions are in good agreement with the input data. The calculated congruent melting points of NiZr and NiZr2 phases are close to the recent experimental data. The Ni10Z7 phase forms by a peritectic reaction, which is also in agreement with the experimental data. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.intermet.2019.106640
  • 2020 • 161 Composition and structure of magnetic high-temperature-phase, stable Fe-Au core-shell nanoparticles with zero-valent bcc Fe core
    Kamp, M. and Tymoczko, A. and Popescu, R. and Schürmann, U. and Nadarajah, R. and Gökce, B. and Rehbock, C. and Gerthsen, D. and Barcikowski, S. and Kienle, L.
    Nanoscale Advances 2 3912-3920 (2020)
    Advanced quantitative TEM/EDXS methods were used to characterize different ultrastructures of magnetic Fe-Au core-shell nanoparticles formed by laser ablation in liquids. The findings demonstrate the presence of Au-rich alloy shells with varying composition in all structures and elemental bcc Fe cores. The identified structures are metastable phases interpreted by analogy to the bulk phase diagram. Based on this, we propose a formation mechanism of these complex ultrastructures. To show the magnetic response of these magnetic core nanoparticles protected by a noble metal shell, we demonstrate the formation of nanostrands in the presence of an external magnetic field. We find that it is possible to control the lengths of these strands by the iron content within the alloy nanoparticles. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0na00514b
  • 2020 • 160 Nanoglass–Nanocrystal Composite—a Novel Material Class for Enhanced Strength–Plasticity Synergy
    Katnagallu, S. and Wu, G. and Singh, S.P. and Nandam, S.H. and Xia, W. and Stephenson, L.T. and Gleiter, H. and Schwaiger, R. and Hahn, H. and Herbig, M. and Raabe, D. and Gault, B. and Balachandran, S.
    Small 16 (2020)
    The properties of a material can be engineered by manipulating its atomic and chemical architecture. Nanoglasses which have been recently invented and comprise nanosized glassy particles separated by amorphous interfaces, have shown promising properties. A potential way to exploit the structural benefits of nanoglasses and of nanocrystalline materials is to optimize the composition to obtain crystals forming within the glassy particles. Here, a metastable Fe-10 at% Sc nanoglass is synthesized. A complex hierarchical microstructure is evidenced experimentally at the atomic scale. This bulk material comprises grains of a Fe90Sc10 amorphous matrix separated by an amorphous interfacial network enriched and likely stabilized by hydrogen, and property-enhancing pure-Fe nanocrystals self-assembled within the matrix. This composite structure leads a yield strength above 2.5 GPa with an exceptional quasi-homogeneous plastic flow of more than 60% in compression. This work opens new pathways to design materials with even superior properties. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/smll.202004400
  • 2020 • 159 Chip formation and phase transformation in orthogonal machining of NiTi shape memory alloy: microstructure-based modelling and experimental validation
    Kaynak, Y. and Manchiraju, S. and Jawahir, I.S. and Biermann, D.
    CIRP Annals 69 85-88 (2020)
    Phase transformation and shape memory response of NiTi alloys are sensitive to the variation of temperature and stress. Thus, the phase transformation of NiTi alloys becomes more complex during machining process. This study presents findings from a major study involving modelling of machining-induced phase transformation of NiTi alloys performed by modifying Helmholtz free energy-based microstructure model. Orthogonal cutting tests were performed to validate the predicted outputs from the simulation, such as cutting forces, temperatures and chip morphology. This work provides a strong evidence that the developed new model can accurately predict the experimentally recorded outputs in machining of NiTi alloys. © 2020
    view abstractdoi: 10.1016/j.cirp.2020.04.025
  • 2020 • 158 Vertical bonding distances and interfacial band structure of PTCDA on a Sn-Ag surface alloy
    Knippertz, J. and Kelly, L.L. and Franke, M. and Kumpf, C. and Cinchetti, M. and Aeschlimann, M. and Stadtmüller, B.
    Physical Review B 102 (2020)
    Molecular materials enable a vast variety of functionalities for novel electronic and spintronic devices. The unique possibility to alter organic molecules or metallic substrates offers the opportunity to optimize interfacial properties for almost any desired field of application. For this reason, we extend the successful approach to control metal-organic interfaces by surface alloying. We present a comprehensive characterization of the structural and electronic properties of the interface formed between the prototypical molecule PTCDA and a Sn-Ag surface alloy grown on an Ag(111) single crystal surface. We monitor the changes of adsorption height of the surface alloy atoms and electronic valence band structure upon adsorption of one layer of PTCDA using the normal incidence X-ray standing wave technique in combination with momentum-resolved photoelectron spectroscopy. We find that the vertical buckling and the surface band structure of the SnAg2 surface alloy is not altered by the adsorption of one layer of PTCDA, in contrast to our recent study of PTCDA on a PbAg2 surface alloy [B. Stadtmüller, Phys. Rev. Lett. 117, 096805 (2016)PRLTAO0031-900710.1103/PhysRevLett.117.096805]. In addition, the vertical adsorption geometry of PTCDA and the interfacial energy level alignment indicate the absence of any chemical interaction between the molecule and the surface alloy. We attribute the different interactions at these PTCDA/surface alloy interfaces to the presence or absence of local σ-bonds between the PTCDA oxygen atoms and the surface atoms. Combining our findings with results from literature, we are able to propose an empiric rule for engineering the surface band structure of alloys by adsorption of organic molecules. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.075447
  • 2020 • 157 Zn- or Cu-containing CaP-based coatings formed by micro-arc oxidation on titanium and Ti-40Nb Alloy: Part I-Microstructure, composition and properties
    Komarova, E.G. and Sharkeev, Y.P. and Sedelnikova, M.B. and Prosolov, K.A. and Khlusov, I.A. and Prymak, O. and Epple, M.
    Materials 13 (2020)
    Zn- and Cu-containing CaP-based coatings, obtained by micro-arc oxidation process, were deposited on substrates made of pure titanium (Ti) and novel Ti-40Nb alloy. The microstructure, phase, and elemental composition, as well as physicochemical and mechanical properties, were examined for unmodified CaP and Zn- or Cu-containing CaP coatings, in relation to the applied voltage that was varied in the range from 200 to 350 V. The unmodified CaP coatings on both types of substrates had mainly an amorphous microstructure with a minimal content of the CaHPO4 phase for all applied voltages. The CaP coatings modified with Zn or Cu had a range from amorphous to nano- and microcrystalline structure that contained micro-sized CaHPO4 and Ca(H2PO4)2·H2O phases, as well as nano-sized β-Ca2P2O7, CaHPO4, TiO2, and Nb2O5 phases. The crystallinity of the formed coatings increased in the following order: CaP/TiNb < Zn-CaP/TiNb < Cu-CaP/TiNb < CaP/Ti < Zn-CaP/Ti < Cu-CaP/Ti. The increase in the applied voltage led to a linear increase in thickness, roughness, and porosity of all types of coatings, unlike adhesive strength that was inversely proportional to an increase in the applied voltage. The increase in the applied voltage did not affect the Zn or Cu concentration (~0.4 at%), but led to an increase in the Ca/P atomic ratio from 0.3 to 0.7. © 2020 by the authors.
    view abstractdoi: 10.3390/ma13184116
  • 2020 • 156 Zn- or Cu-containing CaP-Based Coatings Formed by Micro-Arc Oxidation on Titanium and Ti-40Nb Alloy: Part II—Wettability and Biological Performance
    Komarova, E.G. and Sharkeev, Y.P. and Sedelnikova, M.B. and Prymak, O. and Epple, M. and Litvinova, L.S. and Shupletsova, V.V. and Malashchenko, V.V. and Yurova, K.A. and Dzyuman, A.N. and Kulagina, I.V. and Mushtovatova, L.S. and...
    Materials 13 1-23 (2020)
    This work describes the wettability and biological performance of Zn-and Cu-containing CaP-based coatings prepared by micro-arc oxidation on pure titanium (Ti) and novel Ti-40Nb alloy. Good hydrophilic properties of all the coatings were demonstrated by the low contact angles with liquids, not exceeding 45◦ . An increase in the applied voltage led to an increase of the coating roughness and porosity, thereby reducing the contact angles to 6◦ with water and to 17◦ with glycerol. The free surface energy of 75 ± 3 mJ/m2 for all the coatings were determined. Polar component was calculated as the main component of surface energy, caused by the presence of strong polar PO43− and OH− bonds. In vitro studies showed that low Cu and Zn amounts (~0.4 at.%) in the coatings promoted high motility of human adipose-derived multipotent mesenchymal stromal cells (hAMMSC) on the implant/cell interface and subsequent cell ability to differentiate into osteoblasts. In vivo study demonstrated 100% ectopic bone formation only on the surface of the CaP coating on Ti. The Zn-and Cu-containing CaP coatings on both substrates and the CaP coating on the Ti-40Nb alloy slightly decreased the incidence of ectopic osteogenesis down to 67%. The MAO coatings showed antibacterial efficacy against Staphylococcus aureus and can be arranged as follows: Zn-CaP/Ti > Cu-CaP/TiNb, Zn-CaP/TiNb > Cu-CaP/Ti. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ma13194366
  • 2020 • 155 Influence of precipitation hardening on the high-temperature sliding wear resistance of an aluminium alloyed iron-nickel base alloy
    Krell, J. and Röttger, A. and Ziesing, U. and Theisen, W.
    Tribology International 148 (2020)
    This work investigates the effects of precipitation hardening on hot hardness and high-temperature sliding wear resistance of an iron-nickel base alloy. Three variants of a carbide rich alloy containing 0, 2 and 3 wt.-% aluminium were manufactured and aged for 24 h at 650, 700 and 750 °C. Hot hardness (20–800 °C) and sliding wear tests (600 °C) were conducted for each condition. The addition of aluminium has little effect on the type or volume content of the carbides. Ageing caused the precipitation of NiAl in the aluminium-containing alloys. The precipitation-hardened samples show higher hot hardness and better hot wear resistance. The lower wear loss can mainly be attributed to the improved support of the carbides by the precipitation strengthened matrix. © 2020
    view abstractdoi: 10.1016/j.triboint.2020.106342
  • 2020 • 154 Stepwise Bi-Bi Bond Formation: From a Bi-centered Radical to Bi4Butterfly and Bi8Cuneane-Type Clusters
    Krüger, J. and Wölper, C. and Schulz, S.
    Inorganic Chemistry 59 11142-11151 (2020)
    In contrast to their lighter homologues (P, As, Sb), the synthesis of polybismuthane clusters is still restricted to classical solid-state approaches. We herein report on systematic reduction reactions of different bismuth precursors with Ga(I) and Mg(I) complexes. This study not only yielded the first metal-coordinated tetrabismuthane ([{L1(Cl)Ga}2-μ,η1:1-Bi4] 3, L1 = HC[C(Me)N(2,6-i-Pr2C6H3)]2) and realgar-type bismuth cluster ([(L2Mg)4(μ4,η2:2:2:2-Bi8)] 4, L2 = HC[C(Me)N(2,4,6-Me3C6H2)]2) in addition to the bismuth-centered radical [L1Ga(Cl)]2Bi• 1 and dibismuthene [L1(Cl)GaBi]2 2, but clearly demonstrates the crucial role of the substituents and the oxidation state of the bismuth precursor as well as the specific reduction potential of the main group metal reductants on the product formation. Compounds 3 and 4 were spectroscopically characterized (1H, 13C NMR, IR), and the structures of 1-4 were determined by single-crystal X-ray diffraction. Computational calculations gave deeper insights into the electronic structures of 1′, 3′, and 4′. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.inorgchem.0c01657
  • 2020 • 153 Control of thermally stable core-shell nano-precipitates in additively manufactured Al-Sc-Zr alloys
    Kürnsteiner, P. and Bajaj, P. and Gupta, A. and Wilms, M.B. and Weisheit, A. and Li, X. and Leinenbach, C. and Gault, B. and Jägle, E.A. and Raabe, D.
    Additive Manufacturing 32 (2020)
    Laser Additive Manufacturing (LAM) of light metals such as high-strength Al-based alloys offers tremendous potential for e.g. weight reduction and associated reduced fuel consumptions for the transportation industry. Typically, commercial Sc-containing alloys, such as Scalmalloy®, rely on precipitation hardening to increase their strength. Conventional processing involves controlled ageing during which ordered and coherent Al3Sc precipitates form from a Sc-supersaturated solid solution. Here we show how the intrinsic heat treatment (IHT) of directed energy deposition (DED) can be used to trigger the precipitation of Al3Sc already during the LAM process. High number densities of 1023 nano-precipitates per m3 can be realized through solid-state phase transformation from the supersaturated Al-Sc matrix that results from the fast cooling rate in LAM. Yet, the IHT causes precipitates to coarsen, hence reducing their strengthening effect. We implement alternative solidification conditions to exploit the IHT to form a Zr-rich shell around the Al3Sc precipitates that prevents coarsening. Our approach is applicable to a wide range of precipitation-hardened alloys to trigger in-situ precipitation during LAM. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2019.100910
  • 2020 • 152 Influence of Ti3Ni4 precipitates on the indentation-induced two-way shape-memory effect in Nickel-Titanium
    Laursen, C.M. and Peter, N.J. and Gerstein, G. and Maier, H.J. and Dehm, G. and Frick, C.P.
    Materials Science and Engineering A 792 (2020)
    Nickel Titanium (NiTi) alloys have been used for many years based on their unique ability to exhibit the shape-memory and pseudoelastic effects. The indentation-induced two-way shape memory effect (TWSME) is a specific sub-capability of this alloy such that a repeatably switchable surface can be created by “training” the material through mechanical indentation and activated through temperature transitions between the austenitic and martensitic phases. This study sought to observe the effect Ti3Ni4 precipitate aging would have on the indentation-induced TWSME. Ti3Ni4 has previously been shown as an effective method to alter NiTi transformation temperatures, yet it was unclear what effect localized stress fields around precipitates would have on the TWSME. The results presented here indicate that growth of precipitates in the alloy before training suppresses the resultant indentation-induced TWSME, and small precipitates, which cause minimal lattice mismatch to the matrix (i.e. highest coherency), have the strongest role in suppressing the effect. It is suggest that lattice coherency acts to inhibit plastic deformation, suppressing the creation of the preferred microstructure under the indent required to guide the TWSME. Therefore, precipitate aging is not a recommended alternative to precise alloying in order to alter transformation temperatures with the goal of maximizing the indentation-induced TWSME effect within a targeted temperate transformation regime. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2020.139373
  • 2020 • 151 Electroenzymatic Nitrogen Fixation Using a MoFe Protein System Immobilized in an Organic Redox Polymer
    Lee, Y.S. and Ruff, A. and Cai, R. and Lim, K. and Schuhmann, W. and Minteer, S.D.
    Angewandte Chemie - International Edition 59 16511-16516 (2020)
    We report an organic redox-polymer-based electroenzymatic nitrogen fixation system using a metal-free redox polymer, namely neutral-red-modified poly(glycidyl methacrylate-co-methylmethacrylate-co-poly(ethyleneglycol)methacrylate) with a low redox potential of −0.58 V vs. SCE. The stable and efficient electric wiring of nitrogenase within the redox polymer matrix enables mediated bioelectrocatalysis of N3−, NO2− and N2 to NH3 catalyzed by the MoFe protein via the polymer-bound redox moieties distributed in the polymer matrix in the absence of the Fe protein. Bulk bioelectrosynthetic experiments produced 209±30 nmol NH3 nmol MoFe−1 h−1 from N2 reduction. 15N2 labeling experiments and NMR analysis were performed to confirm biosynthetic N2 reduction to NH3. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstractdoi: 10.1002/anie.202007198
  • 2020 • 150 Composition dependence of hardness and elastic modulus of the cubic and hexagonal NbCo2 Laves phase polytypes studied by nanoindentation
    Luo, W. and Kirchlechner, C. and Li, J. and Dehm, G. and Stein, F.
    Journal of Materials Research 35 185-195 (2020)
    Regarding the effect of composition on the mechanical properties of intermetallic phases such as Laves phases, there is conflicting information in the literature. Some authors observed defect hardening when deviating from stoichiometric Laves phase composition, whereas others find defect softening. Here, we present a systematic investigation of the defect state, hardness, and elastic modulus of cubic and hexagonal NbCo2 Laves phases as a function of crystal structure and composition. For this purpose, diffusion couples were prepared which exhibit diffusion layers of the cubic C15 and hexagonal C14 and C36 NbCo2 Laves phases, with concentration gradients covering their entire homogeneity ranges from 24 to 37 at.% Nb. Direct observations of dislocations and stacking faults in the diffusion layers as a function of composition were performed by electron channeling contrast imaging, and the hardness and elastic modulus were probed in the diffusion layers along the concentration gradients by nanoindentation. © 2020 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2019.384
  • 2020 • 149 Crystal structure and composition dependence of mechanical properties of single-crystalline NbCo2 Laves phase
    Luo, W. and Kirchlechner, C. and Zavašnik, J. and Lu, W. and Dehm, G. and Stein, F.
    Acta Materialia 184 151-163 (2020)
    Extended diffusion layers of the cubic C15 and hexagonal C14 and C36 NbCo2 Laves phases with concentration gradients covering their entire homogeneity ranges were produced by the diffusion couple technique. Single-phase and single-crystalline micropillars of the cubic and hexagonal NbCo2 Laves phases were prepared in the diffusion layers by focused ion beam (FIB) milling. The influence of chemical composition, structure type, orientation and pillar size on the deformation behavior and the critical resolved shear stress (CRSS) was studied by micropillar compression tests. The pillar orientation influences the activated slip systems, but the deformation behavior and the CRSS are independent of orientation. The deformation of the smallest NbCo2 micropillars (0.8 µm in top diameter) appears to be dislocation nucleation controlled and the CRSS approaches the theoretical shear stress for dislocation nucleation. The CRSS of the 0.8 µm-sized NbCo2 micropillars is nearly constant from 26 to 34 at.% Nb where the C15 structure is stable. It decreases as the composition approaches the Co-rich and Nb-rich boundaries of the homogeneity range where the C15 structure transforms to the C36 and the C14 structure, respectively. The decrease in the CRSS at these compositions is related to the reduction of shear modulus and stacking fault energy. As the pillar size increases, stochastic deformation behavior and large scatter in the CRSS values occur and obscure the composition effect on the CRSS. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.036
  • 2020 • 148 Cold Gas Spraying of Nickel-Titanium Coatings for Protection Against Cavitation
    Mauer, G. and Rauwald, K.-H. and Sohn, Y.J. and Weirich, T.E.
    Journal of Thermal Spray Technology (2020)
    Cavitation erosion is a sever wear mechanism that takes place in hydrodynamic systems. Examples are turbine vanes of hydropower plants or components of valves and pumps in hydraulic systems. Nickel-titanium shape memory alloys (NiTi) are attractive materials for cavitation-resistant coatings because of their pronounced intrinsic damping mitigating cavitation-induced erosion. In this work, NiTi coatings were produced by cold gas spraying. The phase transformation behaviors of the powder feedstock and the as-sprayed coatings were investigated. Regarding the obtained transformation temperatures, the measured substrate temperatures during spraying rule out that either the shape memory effect or the pseudoelasticity of NiTi could affect the deposition efficiency under the applied conditions of cold gas spraying. Another potential effect is stress-induced amorphization which could occur at the particle–substrate interfaces and impair particle bonding by stress relaxation. Moreover, also oxide formation can be significant. Thus, the presence of amorphous phases and oxides in the near-surface zone of particles bounced off after impact was investigated. Oxidation could be confirmed, but no indication of amorphous phase was found. Besides, also the evolution of local microstrains implies that the substrate temperatures affect the deposition efficiency. These temperatures were significantly influenced by the spray gun travel speed. © 2020, The Author(s).
    view abstractdoi: 10.1007/s11666-020-01139-x
  • 2020 • 147 The sum is more than its parts: stability of MnFe oxide nanoparticles supported on oxygen-functionalized multi-walled carbon nanotubes at alternating oxygen reduction reaction and oxygen evolution reaction conditions
    Morales, D.M. and Kazakova, M.A. and Purcel, M. and Masa, J. and Schuhmann, W.
    Journal of Solid State Electrochemistry 24 2901-2906 (2020)
    Successful design of reversible oxygen electrocatalysts does not only require to consider their activity towards the oxygen reduction (ORR) and the oxygen evolution reactions (OER), but also their electrochemical stability at alternating ORR and OER operating conditions, which is important for potential applications in reversible electrolyzers/fuel cells or metal/air batteries. We show that the combination of catalyst materials containing stable ORR active sites with those containing stable OER active sites may result in a stable ORR/OER catalyst if each of the active components can satisfy the current demand of their respective reaction. We compare the ORR/OER performances of oxides of Mn (stable ORR active sites), Fe (stable OER active sites), and bimetallic Mn0.5Fe0.5 (reversible ORR/OER catalyst) supported on oxidized multi-walled carbon nanotubes. Despite the instability of Mn and Fe oxide for the OER and the ORR, respectively, Mn0.5Fe0.5 exhibits high stability for both reactions. © 2020, The Author(s).
    view abstractdoi: 10.1007/s10008-020-04667-2
  • 2020 • 146 Unfolding the complexity of phonon quasi-particle physics in disordered materials
    Mu, S. and Olsen, R.J. and Dutta, B. and Lindsay, L. and Samolyuk, G.D. and Berlijn, T. and Specht, E.D. and Jin, K. and Bei, H. and Hickel, T. and Larson, B.C. and Stocks, G.M.
    npj Computational Materials 6 (2020)
    The concept of quasi-particles forms the theoretical basis of our microscopic understanding of emergent phenomena associated with quantum-mechanical many-body interactions. However, the quasi-particle theory in disordered materials has proven difficult, resulting in the predominance of mean-field solutions. Here, we report first-principles phonon calculations and inelastic X-ray and neutron-scattering measurements on equiatomic alloys (NiCo, NiFe, AgPd, and NiFeCo) with force-constant dominant disorder—confronting a key 50-year-old assumption in the Hamiltonian of all mean-field quasi-particle solutions for off-diagonal disorder. Our results have revealed the presence of a large, and heretofore unrecognized, impact of local chemical environments on the distribution of the species-pair-resolved force-constant disorder that can dominate phonon scattering. This discovery not only identifies a critical analysis issue that has broad implications for other elementary excitations, such as magnons and skyrmions in magnetic alloys, but also provides an important tool for the design of materials with ultralow thermal conductivities. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41524-020-0271-3
  • 2020 • 145 Picosecond laser-induced surface structures on alloys in liquids and their influence on nanoparticle productivity during laser ablation
    Nadarajah, R. and Barcikowski, S. and Gökce, B.
    Optics Express 28 2909-2924 (2020)
    The productivity of nanoparticles formed by laser ablation of gold-silver and iron-gold alloy as well as copper and iron-nickel alloy targets in water is correlated with the formation of laser-induced surface structures. At a laser fluence optimized for maximum nanoparticle productivity, it is found that a binary alloy with an equimolar ratio forms laser-induced periodic surface structures (LIPSS) after ablation, if one of the constituent metals also form LIPSS. The ablation rate of nanoparticles linearly depends on the laser fluence if LIPSS is not formed, while a logarithmic trend and a decrease in productivity is evident when LIPSS is formed. To cancel LIPSS formation and recover from this decrease, a change to circularly polarized light is performed and an increase in nanoparticle productivity of more than 30% is observed. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
    view abstractdoi: 10.1364/OE.28.002909
  • 2020 • 144 Anisotropic failure behavior of ordered intermetallic TiAl alloys under pure mode-I loading
    Neogi, A. and Alam, M. and Hartmaier, A. and Janisch, R.
    Modelling and Simulation in Materials Science and Engineering 28 (2020)
    Whether a metallic material fractures by brittle cleavage or by ductile rupture is primarily governed by the competition between cleavage and dislocation emission at the crack tip. The linear elastic fracture mechanics (LEFM) based criterion of Griffith, respectively the one for dislocation emission of Rice, are sufficiently reliable for determining the possible crack tip propagation mechanisms in isotropic crystalline metals. However, the applicability of these criteria is questionable when non-cubic, anisotropic solids are considered, as e.g. ordered intermetallic TiAl phases, where slip systems are limited and elastic anisotropy is pronounced. We study brittle versus ductile failure mechanisms in face-centered tetragonal TiAl and hexagonal Ti3Al using large-scale atomistic simulations and compare our findings to the predictions of LEFM-based criteria augmented by elastic anisotropy. We observe that the augmented Griffith and Rice criteria are reliable for determining the direction dependent crack tip mechanisms, if all the available dislocation slip systems are taken into account. Yet, atomistic simulations are necessary to understand crack blunting due to mixed mechanisms, or shear instabilities other than dislocation emission. The results of our systematic study can be used as basis for modifications of the Griffith/Rice criteria in order to incorporate such effects. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/aba738
  • 2020 • 143 Deformation mechanisms in a superelastic NiTi alloy: An in-situ high resolution digital image correlation study
    Polatidis, E. and Šmíd, M. and Kuběna, I. and Hsu, W.-N. and Laplanche, G. and Van Swygenhoven, H.
    Materials and Design 191 (2020)
    An in-situ high resolution digital image correlation investigation during uniaxial tensile deformation reveals the recoverable and the non-recoverable strain mechanisms in a Ni51Ti49 alloy with a mean grain size of 35 μm. Recoverable strain is due to the martensitic transformation, for which more than one variant per grain can be activated. The majority of the activated variants exhibit high Schmid factor. The variant selection can be influenced by shear transmission across grain boundaries, when the geometrical compatibility between the neighboring habit plane variants is favourable; in these cases variants that do not have the highest Schmid factor, with respect to the macroscopically applied load, are activated. The experimentally determined transformation strains agree well with theoretical calculations for single crystals. The non-recoverable strain is due to deformation slip in austenite, twinning in martensite and residual martensite. The results are discussed in view of possible twinning modes that can occur in austenite resulting in significant non-recoverable strain. © 2020 The Authors
    view abstractdoi: 10.1016/j.matdes.2020.108622
  • 2020 • 142 Beyond Solid Solution High-Entropy Alloys: Tailoring Magnetic Properties via Spinodal Decomposition
    Rao, Z. and Dutta, B. and Körmann, F. and Lu, W. and Zhou, X. and Liu, C. and da Silva, A.K. and Wiedwald, U. and Spasova, M. and Farle, M. and Ponge, D. and Gault, B. and Neugebauer, J. and Raabe, D. and Li, Z.
    Advanced Functional Materials (2020)
    Since its first emergence in 2004, the high-entropy alloy (HEA) concept has aimed at stabilizing single- or dual-phase multi-element solid solutions through high mixing entropy. Here, this strategy is changed and renders such massive solid solutions metastable, to trigger spinodal decomposition for improving the alloys’ magnetic properties. The motivation for starting from a HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behavior using multiple components. The key idea is to form Fe-Co enriched regions which have an expanded volume (relative to unconstrained Fe-Co), due to coherency constraints imposed by the surrounding HEA matrix. As demonstrated by theory and experiments, this leads to improved magnetic properties of the decomposed alloy relative to the original solid solution matrix. In a prototype magnetic FeCoNiMnCu HEA, it is shown that the modulated structures, achieved by spinodal decomposition, lead to an increase of the Curie temperature by 48% and a simultaneous increase of magnetization by 70% at ambient temperature as compared to the homogenized single-phase reference alloy. The findings thus open a pathway for the development of advanced functional HEAs. © 2020 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/adfm.202007668
  • 2020 • 141 Unveiling the mechanism of abnormal magnetic behavior of FeNiCoMnCu high-entropy alloys through a joint experimental-theoretical study
    Rao, Z. and Dutta, B. and Körmann, F. and Ponge, D. and Li, L. and He, J. and Stephenson, L. and Schäfer, L. and Skokov, K. and Gutfleisch, O. and Raabe, D. and Li, Z.
    Physical Review Materials 4 (2020)
    We combined experimental investigations and theoretical calculations to unveil an abnormal magnetic behavior caused by addition of the nonmagnetic element Cu in face-centered-cubic FeNiCoMn-based high-entropy alloys (HEAs). Upon Cu addition, the probed HEAs show an increase of both Curie temperature and saturation magnetization in as-cast and homogenized states. Specifically, the saturation magnetization of the as-cast HEAs at room temperature increases by 77% and 177% at a Cu content of 11 and 20 at. %, respectively, compared to the as-cast equiatomic FeNiCoMn HEA without Cu. The increase in saturation magnetization of the as-cast HEAs is associated with the formation of an Fe-Co rich phase in the dendritic regions. For the homogenized HEAs, the magnetic state at room temperature transforms from paramagnetism to ferromagnetism after 20 at. % Cu addition. The increase of the saturation magnetization and Curie temperature cannot be adequately explained by the formation of Cu enriched zones according to atom probe tomography analysis. Ab initio calculations suggest Cu plays a pivotal role in the stabilization of a ferromagnetic ordering of Fe, and reveal an increase of the Curie temperature caused by Cu addition which agrees well with the experimental results. The underlying mechanism behind this phenomenon lies in a combined change in unit-cell volume and chemical composition and the related energetic stabilization of the magnetic ordering upon Cu alloying as revealed by theoretical calculations. Thus, the work unveils the mechanisms responsible for the Cu effect on the magnetic properties of FeNiCoMn HEAs, and suggests that nonmagnetic elements are also crucial to tune and improve magnetic properties of HEAs. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.4.014402
  • 2020 • 140 Electrochemical dealloying as a tool to tune the porosity, composition and catalytic activity of nanomaterials
    Rurainsky, C. and Manjón, A.G. and Hiege, F. and Chen, Y.-T. and Scheu, C. and Tschulik, K.
    Journal of Materials Chemistry A 8 19405-19413 (2020)
    Electrochemical dealloying as a post-Treatment can greatly improve the catalytic activity of nanoparticles. To date, selecting suitable conditions to reach desired porosity, composition and catalytic activity is based on trial-And-error-Attempts, due to insufficient understanding of the electrochemically induced morphological and compositional changes of the nanoparticles. These changes are elucidated here by combining electrochemistry with identical location electron microscopy analyses and linking them to the electrocatalytic properties of the obtained nanocatalysts. Using AgAu alloy nanoparticles and the hydrogen evolution reaction as a model system, the influence of cyclic voltammetry parameters on the catalytic activity upon electrochemical dealloying is investigated. Increasing the number of cycles initially results in a decreased Ag content and a sharp improvement in activity. Additional dealloying increases the nanoparticle porosity, while marginally altering their composition, due to surface motion of atoms. Since this is accompanied by particle aggregation, a decrease in catalytic activity results upon extensive cycling. This transition between porosity formation and particle aggregation marks the optimum for nanocatalyst post-production. The gained insights may aid speeding up the development of new materials by electrochemical dealloying as an easy-To-control post-processing route to tune the properties of existing nanoparticles, instead of having to alter usually delicate synthesis routes as a whole. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ta04880a
  • 2020 • 139 Formation of a 2D Meta-stable Oxide by Differential Oxidation of AgCu Alloys
    Schweinar, K. and Beeg, S. and Hartwig, C. and Rajamathi, C.R. and Kasian, O. and Piccinin, S. and Prieto, M.J. and Tanase, L.C. and Gottlob, D.M. and Schmidt, T. and Raabe, D. and Schlögl, R. and Gault, B. and Jones, T.E. and Gr...
    ACS Applied Materials and Interfaces 12 23595-23605 (2020)
    Metal alloy catalysts can develop complex surface structures when exposed to reactive atmospheres. The structures of the resulting surfaces have intricate relationships with a myriad of factors, such as the affinity of the individual alloying elements to the components of the gas atmosphere and the bond strengths of the multitude of low-energy surface compounds that can be formed. Identifying the atomic structure of such surfaces is a prerequisite for establishing structure-property relationships, as well as for modeling such catalysts in ab initio calculations. Here, we show that an alloy, consisting of an oxophilic metal (Cu) diluted into a noble metal (Ag), forms a meta-stable two-dimensional oxide monolayer, when the alloy is subjected to oxidative reaction conditions. The presence of this oxide is correlated with selectivity in the corresponding test reaction of ethylene epoxidation. In the present study, using a combination of in situ, ex situ, and theoretical methods (NAP-XPS, XPEEM, LEED, and DFT), we determine the structure to be a two-dimensional analogue of Cu2O, resembling a single lattice plane of Cu2O. The overlayer holds a pseudo-epitaxial relationship with the underlying noble metal. Spectroscopic evidence shows that the oxide's electronic structure is qualitatively distinct from its three-dimensional counterpart, and because of weak electronic coupling with the underlying noble metal, it exhibits metallic properties. These findings provide precise details of this peculiar structure and valuable insights into how alloying can enhance catalytic properties. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsami.0c03963
  • 2020 • 138 Probing catalytic surfaces by correlative scanning photoemission electron microscopy and atom probe tomography
    Schweinar, K. and Nicholls, R.L. and Rajamathi, C.R. and Zeller, P. and Amati, M. and Gregoratti, L. and Raabe, D. and Greiner, M. and Gault, B. and Kasian, O.
    Journal of Materials Chemistry A 8 388-400 (2020)
    The chemical composition and the electronic state of the surface of alloys or mixed oxides with enhanced electrocatalytic properties are usually heterogeneous at the nanoscale. The non-uniform distribution of the potential across their surface affects both activity and stability. Studying such heterogeneities at the relevant length scale is crucial for understanding the relationships between structure and catalytic behaviour. Here, we demonstrate an experimental approach combining scanning photoemission electron microscopy and atom probe tomography performed at identical locations to characterise the surface's structure and oxidation states, and the chemical composition of the surface and sub-surface regions. Showcased on an Ir-Ru thermally grown oxide, an efficient catalyst for the anodic oxygen evolution reaction, the complementary techniques yield consistent results in terms of the determined surface oxidation states and local oxide stoichiometry. Significant chemical heterogeneities in the sputter-deposited Ir-Ru alloy thin films govern the oxide's chemistry, observed after thermal oxidation both laterally and vertically. While the oxide grains have a composition of Ir0.94Ru0.06O2, the composition in the grain boundary region varies from Ir0.70Ru0.30O2 to Ir0.40Ru0.60O2 and eventually to Ir0.75Ru0.25O2 from the top surface into the depth. The influence of such compositional non-uniformities on the catalytic performance of the material is discussed, along with possible engineering levers for the synthesis of more stable and reactive mixed oxides. The proposed method provides a framework for investigating materials of interest in the field of electrocatalysis and beyond. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ta10818a
  • 2020 • 137 Bistability and oscillations in cooperative microtubule and kinetochore dynamics in the mitotic spindle
    Schwietert, F. and Kierfeld, J.
    New Journal of Physics 22 (2020)
    In the mitotic spindle microtubules attach to kinetochores via catch bonds during metaphase, and microtubule depolymerization forces give rise to stochastic chromosome oscillations. We investigate the cooperative stochastic microtubule dynamics in spindle models consisting of ensembles of parallel microtubules, which attach to a kinetochore via elastic linkers. We include the dynamic instability of microtubules and forces on microtubules and kinetochores from elastic linkers. A one-sided model, where an external force acts on the kinetochore is solved analytically employing a mean-field approach based on Fokker-Planck equations. The solution establishes a bistable force-velocity relation of the microtubule ensemble in agreement with stochastic simulations. We derive constraints on linker stiffness and microtubule number for bistability. The bistable force-velocity relation of the one-sided spindle model gives rise to oscillations in the two-sided model, which can explain stochastic chromosome oscillations in metaphase (directional instability). We derive constraints on linker stiffness and microtubule number for metaphase chromosome oscillations. Including poleward microtubule flux into the model we can provide an explanation for the experimentally observed suppression of chromosome oscillations in cells with high poleward flux velocities. Chromosome oscillations persist in the presence of polar ejection forces, however, with a reduced amplitude and a phase shift between sister kinetochores. Moreover, polar ejection forces are necessary to align the chromosomes at the spindle equator and stabilize an alternating oscillation pattern of the two kinetochores. Finally, we modify the model such that microtubules can only exert tensile forces on the kinetochore resulting in a tug-of-war between the two microtubule ensembles. Then, induced microtubule catastrophes after reaching the kinetochore are necessary to stimulate oscillations. The model can reproduce experimental results for kinetochore oscillations in PtK1 cells quantitatively. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/ab7ede
  • 2020 • 136 Structural evolution and magnetic properties of high-entropy CuCrFeTiNi alloys prepared by high-energy ball milling and spark plasma sintering
    Shkodich, N.F. and Spasova, M. and Farle, M. and Kovalev, D.Y. and Nepapushev, A.A. and Kuskov, K.V. and Vergunova, Y.S. and Scheck, Y.B. and Rogachev, A.S.
    Journal of Alloys and Compounds 816 (2020)
    A powder of equiatomic CuCrFeTiNi high-entropy alloy (HEA) was prepared by short-term (30 min) high-energy ball milling (HEBM). Our structural and chemical analysis showed that micron sized particles of bcc CuCrFeTiNi consisting of nanosized crystalline grains (∼6 nm) could be obtained after 30 min of HEBM. The influence of milling time (30 ÷ 240 min) on structural and magnetic transformations of CuCrFeTiNi powder mixture was investigated. The HEA powders were thermally stable up to 500°С based on DSC results. The HEA powder was subsequently consolidated by spark plasma sintering at 700 °C resulting in a consolidated bulk HEA with co-existing bcc and fcc phases. The as-milled CuCrFeTiNi powder blend contained a solid solution with bcc (Im3m) structure. Annealing at 600°С (t = 180 min) increased the crystallinity of the α-phase (bcc) and gave rise to formation of the γ-phase (fcc, Fm3m) whose amount grew with increasing dwell time. Between 800 and 1000°С, a tetragonal intermetallic σ-phase – most likely FeCr - appeared and subsequently vanished. At 1000°С, the final product was found to contain two solid solutions based on the γ-phase (fcc). The Vickers hardness HvHEBM = 7.7 GPa of the SPS consolidated CuCrFeTiNi alloy (milled for t = 180 min) was markedly higher than the one of SPS-produced ones without HEBM (Hv = 2.1 GPa). Paramagnetic behavior at room temperature with a small ferromagnetic contribution at low fields was observed for as-milled powder after 180 min of HEBM. A small magnetic hysteresis was observed at 5 K and 300 K with a coercive field of around 16 kA/m. Above 100 K, the inverse susceptibility of a HEA powder ball-milled for t = 240 min showed a clear paramagnetic response. The Curie temperature TC ∼50 K was found. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2019.152611
  • 2020 • 135 Atomic scale configuration of planar defects in the Nb-rich C14 Laves phase NbFe2
    Šlapáková, M. and Zendegani, A. and Liebscher, C.H. and Hickel, T. and Neugebauer, J. and Hammerschmidt, T. and Ormeci, A. and Grin, J. and Dehm, G. and Kumar, K.S. and Stein, F.
    Acta Materialia 183 362-376 (2020)
    Laves phases belong to the group of tetrahedrally close-packed intermetallic phases, and their crystal structure can be described by discrete layer arrangements. They often possess extended homogeneity ranges and the general notion is that deviations from stoichiometry are accommodated by anti-site atoms or vacancies. The present work shows that excess Nb atoms in a Nb-rich NbFe2 C14 Laves phase can also be incorporated in various types of planar defects. Aberration-corrected scanning transmission electron microscopy and density functional theory calculations are employed to characterize the atomic configuration of these defects and to establish stability criteria for them. The planar defects can be categorized as extended or confined ones. The extended defects lie parallel to the basal plane of the surrounding C14 Laves phase and are fully coherent. They contain the characteristic Zr4Al3-type (O) units found in the neighboring Nb6Fe7 µ phase. An analysis of the chemical bonding reveals that the local reduction of the charge transfer is a possible reason for the preference of this atomic arrangement. However, the overall layer stacking deviates from that of the perfect µ phase. The ab initio calculations establish why these exceptionally layered defects can be more stable configurations than coherent nano-precipitates of the perfect µ phase. The confined defects are observed with pyramidal and basal habit planes. The pyramidal defect is only ~1 nm thick and resembles the perfect µ phase. In contrast, the confined basal defect can be regarded as only one single O unit and it appears as if the stacking sequence is disrupted. This configuration is confirmed by ab initio calculations to be metastable. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.004
  • 2020 • 134 Deactivating deformation twinning in medium-entropy CrCoNi with small additions of aluminum and titanium
    Slone, C.E. and LaRosa, C.R. and Zenk, C.H. and George, E.P. and Ghazisaeidi, M. and Mills, M.J.
    Scripta Materialia 178 295-300 (2020)
    High strain-hardening rates in equiatomic CrCoNi and other multi-principal element alloys have been attributed to deformation twinning. This work shows that small additions of Al and Ti to a CrCoNi alloy deactivate deformation twinning with only minor changes to uniform elongation and ultimate tensile strength. The initial microstructure is free of chemically ordered (Al,Ti)-rich precipitates after solutionizing and quenching. Tensile properties for the alloy are reported and compared to equiatomic CrCoNi, and the post-deformation microstructure is assessed. Density functional theory calculations indicate that energetically unfavorable Al-Al bonds may discourage shearing via partial dislocations, which are necessary for twinning to occur. © 2019
    view abstractdoi: 10.1016/j.scriptamat.2019.11.053
  • 2020 • 133 In situ synthesis of a binary Ti–10at% Nb alloy by electron beam melting using a mixture of elemental niobium and titanium powders
    Surmeneva, M.A. and Koptyug, A. and Khrapov, D. and Ivanov, Y.F. and Mishurova, T. and Evsevleev, S. and Prymak, O. and Loza, K. and Epple, M. and Bruno, G. and Surmenev, R.A.
    Journal of Materials Processing Technology 282 (2020)
    This study reports the results of the preliminary assessment to fabricate Ti-10at% Nb alloy by electron beam melting (EBM®) from a blend of elemental Nb and Ti powders. The microstructure of the EBM-manufactured Ti-10at% Nb alloys is sensitive to the following factors: different sintering properties of Nb and Ti powders, powder particle properties, material viscosities at varying melt pool temperatures, β-stabilizer element content and the EBM® process parameters. Three phases were observed in as-manufactured Ti-10at% Nb alloy: μm-size Nb phase, a Nb-rich β-solid solution surrounding Nb phase, lamellar structured α-phase and β-solid solution with different distribution and volume fraction. Thus, the combination of powder particle characteristics, very short time material spends in molten condition and sluggish kinetics of mixing and diffusional process in Ti-Nb alloy results in heterogeneous microstructures depending on the local Nb content in the powder blend and the EBM® process conditions. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2020.116646
  • 2020 • 132 Nitrogen doping of MoSx thin films sputtered by reactive High Power Impulse Magnetron Sputtering
    Tillmann, W. and Wittig, A. and Moldenhauer, H. and Thomann, C.-A. and Debus, J. and Aurich, D. and Bruemmer, A.
    Thin Solid Films 713 (2020)
    Incorporating nitrogen into non-stoichiometric molybdenum disulfide (MoSx) thin films is a promising approach in order to improve the mechanical properties. Nevertheless, the adhesion between the film and the substrate is still challenging and the interaction between the mechanical and the tribological properties is not fully understood yet. Subsequently, reactive High Power Impulse Magnetron Sputtering (HiPIMS) is used to deposit nitrogen doped MoSx thin films with different nitrogen amounts on 16MnCr5 steel. The interaction between the structural changes, the mechanical properties and the tribological behavior depending on the nitrogen amount is investigated. The results prove that an increasing amount of nitrogen significantly affects the structure and the tribo-mechanical properties of the thin films. X-ray diffraction analysis reveals a transformation from crystalline to amorphous with an increasing amount of nitrogen from (7.1 ± 0.3) at.-% to (19.5 ± 0.5) at.-%. This transformation is related to a suppression of the columnar microstructure as well as an increasing hardness and Young‘s modulus from (0.14 ± 0.02) GPa, and (5.28 ± 0.32) GPa for the undoped film, to (5.12 ± 0.32) GPa and (92.5 ± 6.2) GPa, for the film with the highest nitrogen amount. The results of the Rockwell indentation tests show that the films with a small amount of nitrogen exhibit an improved adhesion behavior. The wear coefficient can be reduced to a quarter of the value of the undoped MoSx film, whereas coefficients of friction are at similar level of 0.2 in ambient air. Reactive HiPIMS has proven to be promising to deposit nitrogen doped MoSx thin films on steel substrates, which reveal improved mechanical properties and an excellent transfer film built-up during the tribo-tests without failures. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2020.138267
  • 2020 • 131 Comparison of cryogenic deformation of the concentrated solid solutions CoCrFeMnNi, CoCrNi and CoNi
    Tirunilai, A.S. and Hanemann, T. and Reinhart, C. and Tschan, V. and Weiss, K.-P. and Laplanche, G. and Freudenberger, J. and Heilmaier, M. and Kauffmann, A.
    Materials Science and Engineering A 783 (2020)
    The current work compares the deformation behavior of CoCrFeMnNi and CoCrNi in the temperature interval between 295 K and 8 K through a series of quasi-static tensile tests. Temperature-dependent yield stress variation was found to be similarly high in these two alloys. Previous investigations only extended down to 77 K and showed that a small amount of ε-martensite was formed in CoCrNi while this phase was not observed in CoCrFeMnNi. The present study extends these investigations down to 8 K where similar low levels of ε-martensite were presently detected. Based on this result, a rough assessment has been made estimating the importance of deformation twinning to the strength. The relative work hardening rates of CoCrFeMnNi and CoCrNi were comparable in value despite the differences in ε-martensite formation during deformation. CoCrFeMnNi deforms by dislocation slip and deformation twinning while deformation in CoCrNi is also accommodated by the formation of ε-martensite at cryogenic temperatures. Additionally, CoNi, a solid solution from the Co–Cr–Fe–Mn–Ni system with low strength, was used for comparison, showing contrasting deformation behavior at cryogenic temperatures. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2020.139290
  • 2020 • 130 Microhardness and microabrasion behaviour of NiTi shape memory alloy after femtosecond laser shock peening without coating in air
    Wang, H. and Gurevich, E.L. and Ostendorf, A.
    Proceedings of SPIE - The International Society for Optical Engineering 11273 (2020)
    The experiment study presents the influence of femtosecond laser shock peening (FsLSP) without a protective layer in the air on the surface hardness and surface mechanical property of NiTi shape memory alloy. Femtosecond laser shock peening is a new possibility of direct laser ablation without any protective layer under atmospheric conditions, which can produce intense shock waves with low pulse energy in the air. The average surface roughness values of the NiTi alloy samples were measured, because the surface roughness may affect its friction resistance. The results showed that the surface roughness of NiTi increased after femtosecond laser shock peening treatment. In comparison with the initial state, the coefficient of friction decreased and surface microhardness increased after femtosecond laser shock peening treatment with different FsLSP parameters. This improvement of wear properties may be attributed to the enhancement of surface microhardness and surface titanium oxide layer induced by the shock wave and laser ablation during FsLSP treatment. © 2020 SPIE.
    view abstractdoi: 10.1117/12.2543550
  • 2020 • 129 Femtosecond laser shock peening on the surface of NiTi shape memory alloy
    Wang, H. and Gurevich, E.L. and Ostendorf, A.
    Procedia CIRP 94 910-913 (2020)
    Laser shock peening with a femtosecond laser system was presented in this research work. The NiTi shape memory alloy was processed by the femtosecond laser shock peening (FsLSP) treatment without a protective layer in the air. Femtosecond laser shock peening is a new surface technology, which can induce an intense shock wave with low single laser pulse energy under atmospheric conditions. The surface topography, roughness, microhardness, and wear resistance were measured on the surface of NiTi alloy before and after femtosecond laser peening treatment. The results showed that the surface roughness and microhardness could be increased after femtosecond laser shock peening, which may be due to the laser ablation and micro-plastic deformation induced by the shock wave. The wear property of NiTi alloy was improved, which may be attributed to the FsLSPed surface texturing and enhancement of surface microhardness. © 2020 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2020.09.071
  • 2020 • 128 Corrosion behavior of NiTi alloy subjected to femtosecond laser shock peening without protective coating in air environment
    Wang, H. and Jürgensen, J. and Decker, P. and Hu, Z. and Yan, K. and Gurevich, E.L. and Ostendorf, A.
    Applied Surface Science 501 (2020)
    Laser shock peening with femtosecond laser was used to improve the corrosion resistance of biomedical NiTi alloy without protective coating in the air environment. The energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) based analysis showed that the laser ablation could produce titanium oxide layer and femtosecond laser shock peening (FsLSP) can generate residual stress in the surface layer of NiTi alloy. The FsLSP improved the corrosion resistance of NiTi in 3.5% NaCl solution and Hank's solution and also prevented the formation of corrosion cracks and pits during corrosion testing. The reasons for the improvement of corrosion behavior may be the generation of residual stress and titanium oxide film during the laser surface treatment. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2019.144338
  • 2020 • 127 Surface modification of NiTi alloy by ultrashort pulsed laser shock peening
    Wang, H. and Kalchev, Y. and Wang, H. and Yan, K. and Gurevich, E.L. and Ostendorf, A.
    Surface and Coatings Technology 394 (2020)
    This research paper presents the attempt at ultrashort pulsed laser shock peening with absence of absorptive layer and confining medium which could enhance surface microhardness and the abrasion property of NiTi shape memory alloy. The average roughness values of NiTi specimen were measured on the surface, because the roughness would affect the friction resistance. The microhardness and Young's modulus were investigated at different position of single laser spot by nanoindentation technique. The pin-on-plate sliding abrasion testing were performed with different load-force (0.5 N and 2 N) for different testing time. Results showed that ultrashort pulsed laser shock peening treatment would cause a significant improvement on friction coefficient and abrasion property, which was attributed to the change of surface modification, such as roughness, microhardness, microstructure and titanium oxide layer, but the ultrashort pulsed laser shock peening treatment did not enhance its tensile strength during present research. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.125899
  • 2020 • 126 Structural stability of Co–V intermetallic phases and thermodynamic description of the Co–V system
    Wang, P. and Hammerschmidt, T. and Kattner, U.R. and Olson, G.B.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 68 (2020)
    The Co–V system has been reviewed. Density functional theory (DFT) calculations using the generalized gradient approximation (GGA) were used to obtain the energies for the end-members for all three intermediate phases, Co3V, σ and CoV3. Results from DFT calculations considering spin polarization were used to evaluate the CALPHAD (Calculation of phase diagrams) model parameters. The method to evaluate the contribution of the magnetism to the energies of Co-rich compounds that was introduced in our previous work is presented in more detail in the present work. For the description of the σ phase, the magnetic part of the total energy is included in the description of the pure Co end-member compound resulting in a non-linear description of the magnetic contribution over composition. The calculated phase diagram obtained from the present CALPHAD description is in good agreement with the experimental data. The metastable FCC-L12 phase diagram was calculated and compared with experimental data. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.calphad.2019.101729
  • 2020 • 125 Trivalent iron rich CoFe layered oxyhydroxides for electrochemical water oxidation
    Weiß, S. and Ertl, M. and Varhade, S.D. and Radha, A.V. and Schuhmann, W. and Breu, J. and Andronescu, C.
    Electrochimica Acta 350 (2020)
    Layered double hydroxides (LDHs) are presently among the best-performing oxygen evolution reaction (OER) electrocatalysts in alkaline media. The high activity of LDHs is due to synergistic effects between two transition metals as well as the layered structure which facilitates electron transfer. Because of a perfect match with the size of interlayer carbonate a ratio of 2:1 for the di- and tri-valent octahedral cations is energetically preferred. Here we present a strategy, where first mixed valent (Co2+ 1-zFe2+ z)4 Fe3+ 2 - LDHs, with z values between 0 and 0.75 are synthesized, which are subsequently oxidized to Co2+Fe3+ LDH-type layered (oxy)hydroxides with an unusual high trivalent Fe content. Characterization of the chemically oxidized materials using bulk and surface techniques demonstrated the successful synthesis of LDH-like trivalent iron rich (Co2+)4-4z (Fe3+)2+4z (oxy)hydroxides with a final Fe content ranging from 33.3 to 83.3%. Current densities of up to 200 mA cm−2 were obtained at potentials lower than 1.7 V vs. RHE for (Co2+)4-4z (Fe3+)2+4z (oxy)hydroxides containing a maximum of 80% Fe. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2020.136256
  • 2020 • 124 The importance of nanoscale confinement to electrocatalytic performance
    Wordsworth, J. and Benedetti, T.M. and Alinezhad, A. and Tilley, R.D. and Edwards, M.A. and Schuhmann, W. and Gooding, J.J.
    Chemical Science 11 1233-1240 (2020)
    Electrocatalytic nanoparticles that mimic the three-dimensional geometric architecture of enzymes where the reaction occurs down a substrate channel isolated from bulk solution, referred to herein as nanozymes, were used to explore the impact of nano-confinement on electrocatalytic reactions. Surfactant covered Pt-Ni nanozyme nanoparticles, with Ni etched from the nanoparticles, possess a nanoscale channel in which the active sites for electrocatalysis of oxygen reduction are located. Different particle compositions and etching parameters allowed synthesis of nanoparticles with different average substrate channel diameters that have varying amounts of nano-confinement. The results showed that in the kinetically limited regime at low overpotentials, the smaller the substrate channels the higher the specific activity of the electrocatalyst. This is attributed to higher concentrations of protons, relative to bulk solution, required to balance the potential inside the nano-confined channel. However, at higher overpotentials where limitation by mass transport of oxygen becomes important, the nanozymes with larger substrate channels showed higher electrocatalytic activity. A reaction-diffusion model revealed that the higher electrocatalytic activity at low overpotentials with smaller substrate channels can be explained by the higher concentration of protons. The model suggests that the dominant mode of mass transport to achieve these high concentrations is by migration, exemplifying how nano-confinement can be used to enhance reaction rates. Experimental and theoretical data show that under mass transport limiting potentials, the nano-confinement has no effect and the reaction only occurs at the entrance of the substrate channel at the nanoparticle surface. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9sc05611d
  • 2019 • 123 Magnetically and optically tunable terahertz radiation from Ta/NiFe/Pt spintronic nanolayers generated by femtosecond laser pulses
    Adam, R. and Chen, G. and Bürgler, D.E. and Shou, T. and Komissarov, I. and Heidtfeld, S. and Hardtdegen, H. and Mikulics, M. and Schneider, C.M. and Sobolewski, R.
    Applied Physics Letters 114 (2019)
    We generate terahertz (THz) transients by illuminating a few-nanometer-thick Ta/NiFe/Pt nanolayers with a train of linearly polarized 100-fs-wide laser pulses. The transients are ∼1-ps-wide free-space propagating bursts of electromagnetic radiations with amplitudes that are magnetically and optically tunable. Their spectral frequency content extends up to 5 THz, and the 3-dB cutoff is at 0.85 THz. The observed transient electromagnetic signals originate from the NiFe/Pt bilayer, and their amplitude dependence on the external magnetic field, applied in the sample plane, very closely follows the static magnetization versus magnetic field dependence of the NiFe film. For the same laser power, excitation with highly energetic, blue light generates THz transients with amplitudes approximately three times larger than the ones resulting from excitation by infrared light. In both cases, the transients exhibit the same spectral characteristics and are linearly polarized in the perpendicular direction to the sample magnetization. The polarization direction can be tuned by rotation of the magnetic field around the laser light propagation axis. The characteristics of our THz spintronic emitter signals confirm that THz transient generation is due to the inverse spin Hall effect in the Pt layer and demonstrate that ferromagnet/metal nanolayers excited by femtosecond laser pulses can serve as efficient sources of magnetically and optically tunable, polarized transient THz radiation. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5099201
  • 2019 • 122 Thermally stable iridium contacts to highly doped p-In0:53Ga0:47As for indium phosphide double heterojunction bipolar transistors
    Brahem, M. and Mogilatenko, A. and Stoppel, D. and Berger, D. and Hochheim, S. and Rentner, D. and Ostermay, I. and Reiner, M. and Boppel, S. and Nosaeva, K. and Weimann, N.
    Microelectronic Engineering 215 (2019)
    We report on surface pretreatment for ohmic contacts to p-doped In0.53Ga0.47 As with improved thermal stability. It is found that the cleaning of In0.53Ga0.47 As surface by ammonium sulfide or sulfuric acid offers the optimum surface treatment prior to metal deposition. Contacts using an iridium contact layer and palladium diffusion barrier were fabricated and compared to a conventional platinum-based contact Pt/Ti/Pt/Au. Pt-based metal stack suffered from void formation and high reactivity with the semiconductor when annealed at 240 °C for a few hours, as examined by transmission electron microscopy. As a result, the Pt-based stack exhibited strong deterioration of the resistivity. On the other hand, the Ir contact maintained its integrity during thermal stress. The improved contact exhibited a void and reaction-free microstructure and offered stable resistivity values with annealing. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.mee.2019.111017
  • 2019 • 121 Bimetallic silver-platinum nanoparticles with combined osteo-promotive and antimicrobial activity
    Breisch, M. and Grasmik, V. and Loza, K. and Pappert, K. and Rostek, A. and Ziegler, N. and Ludwig, Al. and Heggen, M. and Epple, M. and Tiller, J.C. and Schildhauer, T.A. and Köller, M. and Sengstock, C.
    Nanotechnology 30 (2019)
    Bimetallic alloyed silver-platinum nanoparticles (AgPt NP) with different metal composition from Ag10Pt90 to Ag90Pt10 in steps of 20 mol% were synthesized. The biological effects of AgPt NP, including cellular uptake, cell viability, osteogenic differentiation and osteoclastogenesis as well as the antimicrobial activity towards Staphylococcus aureus and Escherichia coli were analyzed in comparison to pure Ag NP and pure Pt NP. The uptake of NP into human mesenchymal stem cells was confirmed by cross-sectional focused-ion beam preparation and observation by scanning and transmission electron microscopy in combination with energy-dispersive x-ray analysis. Lower cytotoxicity and antimicrobial activity were observed for AgPt NP compared to pure Ag NP. Thus, an enhanced Ag ion release due to a possible sacrificial anode effect was not achieved. Nevertheless, a Ag content of at least 50 mol% was sufficient to induce bactericidal effects against both Staphylococcus aureus and Escherichia coli. In addition, a Pt-related (≥50 mol% Pt) osteo-promotive activity on human mesenchymal stem cells was observed by enhanced cell calcification and alkaline phosphatase activity. In contrast, the osteoclastogenesis of rat primary precursor osteoclasts was inhibited. In summary, these results demonstrate a combinatory osteo-promotive and antimicrobial activity of bimetallic Ag50Pt50 NP. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/ab172b
  • 2019 • 120 Influence of SMA-induced stress on shape memory alloy metal matrix composites manufactured by continuous composite extrusion
    Dahnke, C. and Pottmeyer, F. and Pinter, P. and Weidenmann, K.A. and Tekkaya, A.E.
    Smart Materials and Structures 28 (2019)
    In shape memory alloy metal matrix composites manufactured by continuous composite extrusion the strategies of active property tuning and active strain energy tuning are used for the improvement of the mechanical properties of the components. Due to the thermal activation of the embedded NiTi wires (SM495), compressive stresses are transferred to the surrounding aluminum matrix (AA6060). At elevated temperatures tensile tests, three-point-bending tests and notch impact test show the influence of temperature, prestrain and reinforcing volume on the component performance. In tensile testing, a simultaneous increase of strength and ductility can be found, leading to an increase of the energy abortion capacity. Results of the three-point-bending test and notch impact test also show an increase of the required work depending on the thermomechanical treatment of the specimens. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-665X/ab2d6a
  • 2019 • 119 Thermomechanical behavior of shape memory alloy metal matrix composite actuator manufactured by composite extrusion
    Dahnke, C. and Reeb, A. and Pottmeyer, F. and Weidenmann, K.A. and Tekkaya, A.E.
    Smart Materials and Structures 28 (2019)
    Continuous composite extrusion represents a new possibility for the manufacturing of shape memory alloy metal matrix composites (SMA-MMC). During the process SMA wires are embedded into aluminum or magnesium profiles by means of modified porthole dies. Due to the high flexibility regarding the profile geometry, the materials as well as number, thickness and position of the SMA elements, the process can be used for the generation of a profile integrated bending function. The bending function of the actuator profile depends on the temperature and is thermally activated. The parameters influencing the behavior of the manufactured composited actuators are experimentally investigated. It is found that the radius of curvature mainly depends on the recovery stress and the eccentric position of the SMA wire as well as on the bending stiffness of the actuator profile. The bending mechanism and the experimental results are described by the use of an analytical model as well as a finite element analysis. Based on the results the analytical model is used for the targeted design of a profile with multiple embedded NiTi wires, which is able to perform a repeatable, pure elastic deflection within a defined temperature range between room temperature and 75 °C. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-665X/ab0ef5
  • 2019 • 118 On the role of nitrogen on hydrogen environment embrittlement of high-interstitial austenitic CrMnC(N) steels
    Egels, G. and Fussik, R. and Weber, S. and Theisen, W.
    International Journal of Hydrogen Energy 44 32323-32331 (2019)
    This work investigates the susceptibility of high-interstitial CrMn austenitic stainless steel CN0.96 to hydrogen environment embrittlement. In this context, an N-free model alloy of CN0.96 steel was designed, produced, and characterized. Both steels were subjected to tensile tests in air and in a high-pressure hydrogen gas atmosphere. Both steels undergo severe hydrogen embrittlement. The CN0.96 steel shows trans- and intergranular failure in hydrogen, whereas the N-free model alloy shows exclusively intergranular failure. The different failure modes could be related to different deformation modes that are induced by the presence or absence of N, respectively. In the CN0.96 steel, N promotes planar dislocation slip. Due to the absence of N in the model alloy, localized slip is less pronounced and mechanical twinning is a more preferred deformation mechanism. The embrittlement of the model alloy could therefore be related to mechanisms that are known from hydrogen embrittlement of twinning-induced plasticity steels. © 2019 Hydrogen Energy Publications LLC
    view abstractdoi: 10.1016/j.ijhydene.2019.10.109
  • 2019 • 117 Discovery of ω -free high-temperature Ti-Ta- X shape memory alloys from first-principles calculations
    Ferrari, A. and Paulsen, A. and Langenkämper, D. and Piorunek, D. and Somsen, C. and Frenzel, J. and Rogal, J. and Eggeler, G. and Drautz, R.
    Physical Review Materials 3 (2019)
    The rapid degradation of the functional properties of many Ti-based alloys is due to the precipitation of the ω phase. In the conventional high-temperature shape memory alloy Ti-Ta, the formation of this phase compromises completely the shape memory effect, and high (>100°C) transformation temperatures cannot be maintained during cycling. A solution to this problem is the addition of other elements to form Ti-Ta-X alloys, which often modifies the transformation temperatures; due to the largely unexplored space of possible compositions, very few elements are known to stabilize the shape memory effect without decreasing the transformation temperatures below 100°C. In this study, we use transparent descriptors derived from first-principles calculations to search for new ternary Ti-Ta-X alloys that combine stability and high temperatures. We suggest four alloys with these properties, namely Ti-Ta-Sb, Ti-Ta-Bi, Ti-Ta-In, and Ti-Ta-Sc. Our predictions for the most promising of these alloys, Ti-Ta-Sc, are subsequently fully validated by experimental investigations, the alloy Ti-Ta-Sc showing no traces of ω phase after cycling. Our computational strategy is transferable to other materials and may contribute to suppress ω phase formation in a large class of alloys. ©2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.103605
  • 2019 • 116 First-principles characterization of reversible martensitic transformations
    Ferrari, A. and Sangiovanni, D.G. and Rogal, J. and Drautz, R.
    Physical Review B 99 (2019)
    Reversible martensitic transformations (MTs) are the origin of many fascinating phenomena, including the famous shape memory effect. In this work, we present a fully ab initio procedure to characterize MTs in alloys and to assess their reversibility. Specifically, we employ ab initio molecular dynamics data to parametrize a Landau expansion for the free energy of the MT. This analytical expansion makes it possible to determine the stability of the high- and low-temperature phases, to obtain the Ehrenfest order of the MT, and to quantify its free energy barrier and latent heat. We apply our model to the high-temperature shape memory alloy Ti-Ta, for which we observe remarkably small values for the metastability region (the interval of temperatures in which the high- and low-temperature phases are metastable) and for the barrier: these small values are necessary conditions for the reversibility of MTs and distinguish shape memory alloys from other materials. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.094107
  • 2019 • 115 Synthesis, microstructure, and hardness of rapidly solidified Cu-Cr alloys
    Garzón-Manjón, A. and Christiansen, L. and Kirchlechner, I. and Breitbach, B. and Liebscher, C.H. and Springer, H. and Dehm, G.
    Journal of Alloys and Compounds 794 203-209 (2019)
    Cu-Cr alloys with ∼32 at.% Cr were rapidly solidified by splat quenching or laser melting techniques with the intention to form a very fine grained, non-equilibrium nanostructure similar to those obtained by severe plastic deformation or thin film deposition. The rapidly solidified Cu-Cr alloys were analyzed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Both synthesis techniques lead to a similar two-phase microstructure with a nearly pure fcc Cu matrix with μm grain sizes and bcc Cr particles highly supersaturated with Cu. Splat quenching provides finer bcc particles with dimensions of less than 50 nm compared to laser melting with particle sizes of 100–2000 nm. In case of laser melting, (14 ± 2) at.% Cu are contained in the bcc phase, while splat quenching freezes (20 ± 2) at.% Cu in the bcc particles. The microstructures are discussed and compared to the non-equilibrium microstructures reported in literature using severe plastic deformation and thin films deposition. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2019.04.209
  • 2019 • 114 Cold spray deposition of Cr2AlC MAX phase for coatings and bond-coat layers
    Go, T. and Sohn, Y.J. and Mauer, G. and Vaßen, R. and Gonzalez-Julian, J.
    Journal of the European Ceramic Society 39 860-867 (2019)
    Highly pure Cr2AlC powders were synthesized and deposited for the first time by cold spray technology on stainless steel substrates. The Cr2AlC coatings were relative dense, up to 91%, and present high purity (> 98%) since only small traces of Cr2Al, Al2O3 and Cr2O3 were detected by XRD, SEM and EDX. The microstructure of the coatings is homogeneous, although some preferential orientation in the basal plane was observed by XRD pole figures. The adhesion between the coating and the substrate is strong, and compressive residual stresses up to 300 MPa in the coating were determined by XRD. Furthermore, a conventional YSZ Thermal Barrier Coating (TBCs) was deposited by Atmospheric Plasma Spray (APS) on top of the cold sprayed Cr2AlC coating in order to demonstrate the processing feasibility of Cr2AlC MAX phases as a bond-coat layer. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.jeurceramsoc.2018.11.035
  • 2019 • 113 Oxygen-mediated deformation and grain refinement in Cu-Fe nanocrystalline alloys
    Guo, J. and Duarte, M.J. and Zhang, Y. and Bachmaier, A. and Gammer, C. and Dehm, G. and Pippan, R. and Zhang, Z.
    Acta Materialia 166 281-293 (2019)
    Light elements play a crucial role on the microstructure and properties of conventional alloys and steels. Oxygen is one of the light elements which is inevitably introduced into nanocrystalline alloys during manufacturing. Here, we report that severe plastic deformation can fragment the oxides formed in powder processing and eventually cause oxygen dissolution in the matrix. A comparative investigation on Cu-Fe nanocrystalline alloys generated from different initial materials, blended powders and arc-melted bulk materials which have different oxygen contents, reveals that fragmented oxides at grain boundaries effectively decrease the grain boundary mobility, markedly facilitating grain refinement. In contrast, those oxygen atoms dissolved as interstitials in the Cu-Fe matrix lead to lattice expansion and significant decrease of stacking fault energy locally as validated by density functional theory. Such oxygen-mediated microstructure gives rise to enhanced strength and superior structural stability. The remarkable tailoring effect of oxygen can be employed to engineer nanocrystalline materials with desired properties for different applications. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.040
  • 2019 • 112 Ultra-fast sintered functionally graded Fe/W composites for the first wall of future fusion reactors
    Heuer, S. and Lienig, T. and Mohr, A. and Weber, T. and Pintsuk, G. and Coenen, J.W. and Gormann, F. and Theisen, W. and Linsmeier, C.
    Composites Part B: Engineering 164 205-214 (2019)
    Aiming at the realisation of nuclear fusion reactors, the joining of W and steel parts is currently examined. Based on proposals to implement functionally graded steel/W materials (FGMs) in the joint to cope with thermal stresses, the present contribution introduces a novel fabrication method for steel/W FGMs. Electro Discharge Sintering (EDS) was used to consolidate Fe/W powders within milliseconds at atmosphere. Due to the short process time, the formation of detrimental intermetallic Fe-W precipitates is limited compared to established fabrication methods. The current work first presents results of the Fe/W powder processing, then a feasibility study regarding the fabrication of homogeneous and graded Fe/W composites with W volume fractions of 0, 25, 50 and 75 % via EDS is presented. Lastly, the composites are characterised microstructurally, thermo-physically, and mechanically in detail. © 2018
    view abstractdoi: 10.1016/j.compositesb.2018.11.078
  • 2019 • 111 Synthesis of Mixed AuZn Nanoparticles by Spark Discharge Technique
    Kala, S. and Kruis, F.E.
    MRS Advances 4 1621-1629 (2019)
    In this study, feasibility of spark discharge technique to generate mixed metal nanoparticles is demonstrated. Two immiscible metals Au and Zn are selected to prepare AuZn mixed nanoparticles. Ignition of spark between Au and Zn electrodes under normal pressure, in the presence of carrier gas, leads to formation of mixed nanoparticles by condensation and nucleation. Online particle size-distribution is monitored by a scanning mobility particle sizer on changing carrier gas flow rate and capacitor charging current during co-sparking between Au and Zn electrodes. The technique provides flexibility to generate binary mixture of AuZn nanoparticles in the size range of 10-80 nm. Distribution of Au and Zn in the prepared mixed nanoparticles is mapped by scanning electron microscopy and high resolution electron microscopy. © Materials Research Society 2019.
    view abstractdoi: 10.1557/adv.2019.221
  • 2019 • 110 Templated Dealloying: Designing Ultrastructures by Memory Effect
    Kamp, M. and Tymoczko, A. and Schürmann, U. and Jakobi, J. and Rehbock, C. and Barcikowski, S. and Kienle, L.
    Crystal Growth and Design 19 4957-4963 (2019)
    Tailoring the morphology of nanoporous structures widens the scope of applications in catalysis and sensing. The synthesis of versatile nanoporous morphologies with the spatial distribution of porosity is permitted by the dealloying of unique, metastable Au-Fe alloy template nanoparticles generated by laser ablation in liquids. This approach opens the door to a novel process, which involves a special transformation mechanism, including oxidation and Kirkendall effect, which is decisive for the stabilization of hollow structures with the spatial distribution of porosity and represents a memory effect of morphology. Within this work, nanoporous Au particles, hollow nanoporous Au shells with the spatial distribution of porosity, and yolk-shell-like Au nanoparticles encapsulated in ultrathin Au shells are synthesized. A distinct variation of crystallinity and an increased lattice strain is observed, which implies an improved catalytic activity for oxidation reactions. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.9b00175
  • 2019 • 109 Imaging individual solute atoms at crystalline imperfections in metals
    Katnagallu, S. and Stephenson, L.T. and Mouton, I. and Freysoldt, C. and Subramanyam, A.P.A. and Jenke, J. and Ladines, A.N. and Neumeier, S. and Hammerschmidt, T. and Drautz, R. and Neugebauer, J. and Vurpillot, F. and Raabe, D. ...
    New Journal of Physics 21 (2019)
    Directly imaging all atoms constituting a material and, maybe more importantly, crystalline defects that dictate materials' properties, remains a formidable challenge. Here, we propose a new approach to chemistry-sensitive field-ion microscopy (FIM) combining FIM with time-of-flight mass-spectrometry (tof-ms). Elemental identification and correlation to FIM images enabled by data mining of combined tof-ms delivers a truly analytical-FIM (A-FIM). Contrast variations due to different chemistries is also interpreted from density-functional theory (DFT). A-FIM has true atomic resolution and we demonstrate how the technique can reveal the presence of individual solute atoms at specific positions in the microstructure. The performance of this new technique is showcased in revealing individual Re atoms at crystalline defects formed in Ni-Re binary alloy during creep deformation. The atomistic details offered by A-FIM allowed us to directly compare our results with simulations, and to tackle a long-standing question of how Re extends lifetime of Ni-based superalloys in service at high-temperature. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/ab5cc4
  • 2019 • 108 Fe/Co/Ni mixed oxide nanoparticles supported on oxidized multi-walled carbon nanotubes as electrocatalysts for the oxygen reduction and the oxygen evolution reactions in alkaline media
    Kazakova, M.A. and Morales, D.M. and Andronescu, C. and Elumeeva, K. and Selyutin, A.G. and Ishchenko, A.V. and Golubtsov, G.V. and Dieckhöfer, S. and Schuhmann, W. and Masa, J.
    Catalysis Today (2019)
    Fabrication of efficient and cost-effective bifunctional oxygen electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) remains a challenge for the development of rechargeable metal-air batteries and unitized regenerative fuel cells technologies. Herein, we report high-performance bifunctional ORR/OER electrocatalysts consisting of mixed transition metal (Fe, Co, Ni) oxide nanoparticles supported on oxidized multi-walled carbon nanotubes (MWCNT). Investigation of the ORR and OER activity of samples with different metal compositions showed that trimetallic/MWCNT composites having Fe:Ni:Co = x:x:(1-2x) ratios, with 0.25 ≤ x ≤ 0.4, exhibit highest bifunctional activity in terms of the reversible ORR/OER overvoltage at a given current density. Moreover, the trimetallic catalysts exhibited improved selectivity with respect to the reduction of O 2 to OH − compared to the bimetallic Fe-Ni, Fe-Co and Co-Ni catalysts, thus revealing synergistic interactions among the metal oxide components. Correlation of the electrocatalytic activity with the structure of the composites is discussed for the most representative cases. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2019.02.047
  • 2019 • 107 Current-induced domain wall oscillations in a nanowire imaged by time-resolved photoemission electron microscopy
    Khan, M.I. and Cramm, S. and Bürgler, D.E. and Parlak, U. and Nemšák, S. and Gottlob, D.M. and Hackl, J. and Doğanay, H. and Schneider, C.M.
    Journal of Magnetism and Magnetic Materials 476 538-545 (2019)
    We study reversible domain wall motion in half-ring Ni 80 Fe 20 nanowires on a nanosecond (ns) timescale in a truly current-induced pump-probe experiment using an energy filtered, aberration-corrected photoemission electron microscope. The X-ray magnetic circular dichroism signal is probed at different time delays before, during and after the current pulse in a stroboscopic mode with circularly polarized synchrotron radiation in the energy range of the Fe L 3 -edge (707 eV). We observe lateral domain wall oscillations with a frequency of ∼0.4 GHz. Comparing the results to a proposed string model, we find that the domain wall oscillations can be described as string-like asymmetric oscillations. © 2019
    view abstractdoi: 10.1016/j.jmmm.2019.01.003
  • 2019 • 106 Influence of composition and precipitation evolution on damage at grain boundaries in a crept polycrystalline Ni-based superalloy
    Kontis, P. and Kostka, A. and Raabe, D. and Gault, B.
    Acta Materialia 166 158-167 (2019)
    The microstructural and compositional evolution of intergranular carbides and borides prior to and after creep deformation at 850 °C in a polycrystalline nickel-based superalloy was studied. Primary MC carbides, enveloped within intergranular γ′ layers, decomposed resulting in the formation of layers of the undesirable η phase. These layers have a composition corresponding to Ni3Ta as measured by atom probe tomography and their structure is consistent with the D024 hexagonal structure as revealed by transmission electron microscopy. Electron backscattered diffraction reveals that they assume various misorientations with regard to the adjacent grains. As a consequence, these layers act as brittle recrystallized zones and crack initiation sites. The composition of the MC carbides after creep was altered substantially, with the Ta content decreasing and the Hf and Zr contents increasing, suggesting a beneficial effect of Hf and Zr additions on the stability of MC carbides. By contrast, M5B3 borides were found to be microstructurally stable after creep and without substantial compositional changes. Borides at 850 °C were found to coarsen, resulting in some cases into γ′- depleted zones, where, however, no cracks were observed. The major consequences of secondary phases on the microstructural stability of superalloys during the design of new polycrystalline superalloys are discussed. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.039
  • 2019 • 105 Comparative study of different anisotropy and potential formulations of phase-field models for dendritic solidification
    Kundin, J. and Steinbach, I.
    Computational Materials Science 170 (2019)
    Phase-field model formulations with double well and double obstacle potentials, and different anisotropy models are investigated with respect to their potential to simulate (i) tip growth on a quantitative level, (ii) well resolved side-branching. The dilute binary alloy Al-4 at%Cu is used as a model alloy. The effects of the numerical resolution (the ratio of the capillary length to the grid spacing) on the growth velocity are studied by means of convergence tests for isothermal and directional solidification in comparison to the theoretical values calculated by the Green-function method (A. Karma, W.J. Rappel, Phys. Rev. E 57 (1998) 4323). An interface stability parameter is introduced as a measure for the estimation of the maximum value of the grid spacing for effective simulations. We show that predominantly the side-branching occurs at numerical resolution lower than the limit value needed to produce correct results in accordance to the convergence analysis. The best results for dendrite growth at a relevant numerical resolution are obtained for the double well potential. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2019.109197
  • 2019 • 104 Thermodynamics of grain boundary segregation, interfacial spinodal and their relevance for nucleation during solid-solid phase transitions
    Kwiatkowski da Silva, A. and Kamachali, R.D. and Ponge, D. and Gault, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 168 109-120 (2019)
    Grain boundary segregation, embrittlement and phase nucleation are interconnected phenomena that are often treated separately, which is partly due to limitations of the current models to predict grain boundary segregation in non-ideal solid solutions. Here, a simple model is introduced to predict grain boundary segregation in solid solutions by coupling available bulk thermodynamic data with a mean-field description of the grain boundary character. The model is confronted with experimental results obtained in Fe-Mn alloys analysed by atom probe tomography. This model successfully predicts a first order transition or a discontinuous jump in the composition of the grain boundary which kinetically implies the formation of spinodal Mn fluctuations that tend to grow further with time within the segregated region. The increase in solute concentration at the grain boundary leads to an increase of the enthalpy of the boundary and to its embrittlement at lower temperatures. Once austenite is formed, the amount of segregated solute Mn on the grain boundaries is drastically reduced and the toughness of the grain boundary is increased. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.02.005
  • 2019 • 103 Quasi-Fermi-Level Splitting of Cu -Poor and Cu -Rich CuIn S2 Absorber Layers
    Lomuscio, A. and Rödel, T. and Schwarz, T. and Gault, B. and Melchiorre, M. and Raabe, D. and Siebentritt, S.
    Physical Review Applied 11 (2019)
    Cu(In,Ga)S2-based solar cells are interesting tandem partners for Si or chalcopyrite solar cells, but suffer from a low open-circuit voltage. Recently, record efficiencies have been achieved by using higher growth temperatures for the absorber. To understand the effect of higher growth temperatures, we investigate the structural and electronic properties of CuInS2 absorbers. By investigating the absorber alone as opposed to complete solar cells, we can separate changes in the absorber from effects of the interface properties. We show that the quasi-Fermi-level splitting, which indicates the maximum open-circuit voltage an absorber is capable of, increases with higher growth temperature. The quasi-Fermi-level splitting is limited by a deep defect, the concentration of which decreases with higher growth temperature and is less prominent in Cu-rich films. Thus, we demonstrate that the open-circuit voltage of CuInS2-based solar cells is limited to below 850 mV by the absorber itself, independent of the interface. In contrast to the changes in the electronic properties, the structural properties are rather independent of temperature within the range investigated but are significantly influenced by the composition. © 2019 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevApplied.11.054052
  • 2019 • 102 Influence of phase decomposition on mechanical behavior of an equiatomic CoCuFeMnNi high entropy alloy
    MacDonald, B.E. and Fu, Z. and Wang, X. and Li, Z. and Chen, W. and Zhou, Y. and Raabe, D. and Schoenung, J. and Hahn, H. and Lavernia, E.J.
    Acta Materialia 181 25-35 (2019)
    Phase decomposition is commonly observed experimentally in single-phase high entropy alloys (HEAs). Hence, it is essential for the consideration of HEAs for structural applications to study and understand the nature of phase decomposition in HEAs, particularly the influence it has on mechanical behavior. This paper describes the phase decomposition in the equiatomic CoCuFeMnNi HEA and how the reported secondary phases influence mechanical behavior. Thermomechanical processing, followed by systematic post deformation annealing treatments, revealed the formation of two distinct secondary phases within the equiatomic face-centered cubic (FCC) matrix phase. Low temperature annealing treatments at 600 °C and below led to the nucleation of Fe-Co rich ordered B2 precipitates that contributed precipitation hardening while sufficiently small in size, on the order of 140 nm in diameter. At temperatures <800 °C Cu segregation, due to its immiscibility with the other constituents, eventually forms a Cu-rich disordered FCC phase that is determined to increase the yield strength of the alloy while reducing the ductility, likely attributable to the presence of additional interfaces. The thermal stability and chemistry of these phases are compared to those predicted on the basis of calculated phase diagram (CALPHAD) analyses. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.09.030
  • 2019 • 101 Role of Boron and Phosphorus in Enhanced Electrocatalytic Oxygen Evolution by Nickel Borides and Nickel Phosphides
    Masa, J. and Andronescu, C. and Antoni, H. and Sinev, I. and Seisel, S. and Elumeeva, K. and Barwe, S. and Marti-Sanchez, S. and Arbiol, J. and Roldan Cuenya, B. and Muhler, M. and Schuhmann, W.
    ChemElectroChem 6 235-240 (2019)
    The modification of nickel with boron or phosphorus leads to significant enhancement of its electrocatalytic activity for the oxygen evolution reaction (OER). However, the precise role of the guest elements, B and P, in enhancing the OER of the host element (Ni) remains unclear. Herein, we present insight into the role of B and P in enhancing electrocatalysis of oxygen evolution by nickel borides and nickel phosphides. The apparent activation energy, Ea*, of electrocatalytic oxygen evolution on Ni2P was 78.4 kJ/mol, on Ni2B 65.4 kJ/mol, and on Ni nanoparticles 94.0 kJ/mol, thus revealing that both B and P affect the intrinsic activity of nickel. XPS data revealed shifts of −0.30 and 0.40 eV in the binding energy of the Ni 2p3/2 peak of Ni2B and Ni2P, respectively, with respect to that of pure Ni at 852.60 eV, thus indicating that B and P induce opposite electronic effects on the surface electronic structure of Ni. The origin of enhanced activity for oxygen evolution cannot, therefore, be attributed to such electronic modification or ligand effect. Severe changes induced on the nickel lattice, specifically, the Ni-Ni atomic order and interatomic distances (strain effect), by the presence of the guest atoms seem to be the dominant factors responsible for enhanced activity of oxygen evolution in nickel borides and nickel phosphides. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201800669
  • 2019 • 100 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 • 99 On the effects of microstructure on the mechanical properties of open-pore Al–11Zn foams
    Matz, A.M. and Matz, B.S. and Parsa, A.B. and Jost, N. and Eggeler, G.
    Materials Science and Engineering A 759 552-564 (2019)
    The mechanical properties of investment casted open-pore metal foams have been investigated on the example of the binary alloy Al–11Zn. The samples were subjected to different cooling conditions subsequent to casting and to different homogenization and ageing treatments. Variation in cooling was done either by quenching the mold in water or slowly cooling it in air. Homogenization and ageing varied in terms of temperature and time. The effects of the different treatments were investigated through microstructural and mechanical characterization methods. Using TEM, we found that the presence of GP zones and their morphological arrangement are the main factors dominating the mechanical performance. Micro- and nanoindentation testing of single foam struts reveal maximum hardness H when room temperature ageing was applied. Ageing at a temperature of 150 °C results in the lowest H in the present study; that is approximately 2/3 of the hardness achieved when ageing at room temperature. This can also be confirmed by the strength of non-porous bulk material obtained by tensile tests, which further show an increase in ductility up to a factor of 5 due to ageing at elevated temperatures. By compression testing of open-pore Al–11Zn foams, we notice that the presence of the microstructural effects varies in extent as a function of the strain ε. At low strains, we observe differences in mechanical performance to a high extent, becoming less with increasing compaction of the samples until they behave as non-porous bulk material. Based on these findings, we deduce a strong interaction of the structural morphology of the foam and its microstructure that determines the mechanical properties dominated by strength and ductility of the base material. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2019.05.087
  • 2019 • 98 Influence of Ag on antibacterial performance, microstructure and phase transformation of NiTi shape memory alloy coatings
    Momeni, S. and Tillmann, W.
    Vacuum 164 242-245 (2019)
    Shape memory binary NiTi and ternary NiTiAg coatings were deposited by means of magnetron sputtering technique. The results show how simultaneous sputtering of Ag can affect the microstructure, phase transformation behavior and antibacterial properties of NiTi coatings. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.vacuum.2019.02.051
  • 2019 • 97 On the role of cobalt carbidization in higher alcohol synthesis over hydrotalcite-based Co-Cu catalysts
    Nebel, J. and Schmidt, S. and Pan, Q. and Lotz, K. and Kaluza, S. and Muhler, M.
    Chinese Journal of Catalysis 40 1731-1740 (2019)
    Co-Cu-based catalysts are widely applied in higher alcohol synthesis (HAS) from synthesis gas. Although the nature of the active sites is still not fully understood, the formation of Co2C under HAS conditions seems to play a major role. A CO pretreatment procedure was developed allowing a systematic investigation of the influence of cobalt carbidization on the structural properties and catalytic performance of the catalysts. By exposing the catalyst to a CO-containing atmosphere prior to HAS, Co enrichment of the catalyst surface occurred followed by carbide formation. This surface modification decreased the formation of hydrocarbons and enhanced the formation of C2+OH. The catalyst pretreated with CO at 20 bar achieved the highest selectivity to ethanol and the lowest hydrocarbon selectivity. © 2019 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences
    view abstractdoi: 10.1016/S1872-2067(19)63344-9
  • 2019 • 96 Iron Aluminides
    Palm, M. and Stein, F. and Dehm, G.
    Annual Review of Materials Research 49 297-326 (2019)
    The iron aluminides discussed here are Fe-Al-based alloys, in which the matrix consists of the disordered bcc (Fe,Al) solid solution (A2) or the ordered intermetallic phases FeAl (B2) and Fe3Al (D03). These alloys possess outstanding corrosion resistance and high wear resistance and are lightweight materials relative to steels and nickel-based superalloys. These materials are evoking new interest for industrial applications because they are an economic alternative to other materials, and substantial progress in strengthening these alloys at high temperatures has recently been achieved by applying new alloy concepts. Research on iron aluminides started more than a century ago and has led to many fundamental findings. This article summarizes the current knowledge of this field in continuation of previous reviews. © 2019 by Annual Reviews. All rights reserved.
    view abstractdoi: 10.1146/annurev-matsci-070218-125911
  • 2019 • 95 On the evolution of dislocation cell structures in two Al-alloys (Al-5Mg and Al-11Zn) during reciprocal sliding wear at high homologous temperatures
    Parsa, A.B. and Walter, M. and Theisen, W. and Bürger, D. and Eggeler, G.
    Wear 1-12 (2019)
    The formation of dislocation substructures in up to 10 µm deep subsurface regions of two aluminium alloys, Al-5Mg and Al-11Zn, was investigated under conditions of high homologous temperature reciprocal sliding wear (HT-RSW). Under creep conditions, Al-5Mg shows a solid solution type of inverse primary creep. In contrast, Al-11Zn creeps obstacle controlled and exhibits normal primary creep. These two materials were subjected to reciprocal sliding wear at 200 and 300 °C for 100 and 1000 cycles. Flat polished disks were exposed to the 1 mm reciprocal movements of a spherical aluminium oxide counterbody under normal forces of 5 and 10 N at an oscillation frequency of 1 Hz. Using focused ion beam (FIB) micromachining thin electron transparent foils were prepared from the surface regions of the as received and worn material states. Transmission electron microscopy (TEM) was used to study the evolution of nano and micro grain sizes in the surface regions. Despite the different creep behavior, the two materials behave similar under conditions of reciprocal sliding wear. The results obtained in the present work show that subgrain sizes decrease with increasing numbers of wear cycles and increasing normal forces. Subgrain sizes also increase with increasing temperature. At 300 °C, dynamic recrystallization was observed in both Al-alloys. The results of the present work are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2018.10.018
  • 2019 • 94 X-ray powder diffraction to analyse bimetallic core-shell nanoparticles (gold and palladium; 7-8 nm)
    Rostek, A. and Loza, K. and Heggen, M. and Epple, M.
    RSC Advances 9 26628-26636 (2019)
    A comparative X-ray powder diffraction study on poly(N-vinyl pyrrolidone) (PVP)-stabilized palladium and gold nanoparticles and bimetallic Pd-Au nanoparticles (both types of core-shell nanostructures) was performed. The average diameter of Au and Pd nanoparticles was 5 to 6 nm. The two types of core-shell particles had a core diameter of 5 to 6 nm and an overall diameter of 7 to 8 nm, i.e. a shell thickness of 1 to 2 nm. X-ray powder diffraction on a laboratory instrument was able to distinguish between a physical mixture of gold and palladium nanoparticles and bimetallic core-shell nanoparticles. It was also possible to separate the core from the shell in both kinds of bimetallic core-shell nanoparticles due to the different domain size and because it was known which metal was in the core and which was in the shell. The spherical particles were synthesized by reduction with glucose in aqueous media. After purification by multiple centrifugation steps, the particles were characterized with respect to their structural, colloid-chemical, and spectroscopic properties, i.e. particle size, morphology, and internal elemental distribution. Dynamic light scattering (DLS), differential centrifugal sedimentation (DCS), atomic absorption spectroscopy (AAS), ultraviolet-visible spectroscopy (UV-vis), high-angle annular dark field imaging (HAADF), and energy-dispersed X-ray spectroscopy (EDX) were applied for particle characterization. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ra05117a
  • 2019 • 93 On the influence of the heat treatment on microstructure formation and mechanical properties of near-α Ti-Fe alloys
    Sandlöbes, S. and Korte-Kerzel, S. and Raabe, D.
    Materials Science and Engineering A 748 301-312 (2019)
    We study the microstructure formation and mechanical properties of Ti-1Fe (wt%) and Ti-3Fe (wt%) alloys for different heat treatments in the β-phase and α + β-phase regions. By applying different heat treatment routes, we observe different microstructure formation mechanisms causing a wide range of mechanical properties from high strength (1.3 GPa) and low ductility (2%) to intermediate strength (700 MPa) and high ductility (30%) in these simple binary alloys. We performed microstructure characterizsation using scanning electron microscopy, transmission electron microscopy and atom probe tomography to show that the alloying content and heat treatment significantly affect the local martensitic and / or diffusional phase transformations causing the substantial changes in the mechanical behavior. © 2018
    view abstractdoi: 10.1016/j.msea.2018.12.071
  • 2019 • 92 Electron-phonon coupling and superconductivity-induced distortion of the phonon lineshape in V3Si
    Sauer, A. and Zocco, D.A. and Said, A.H. and Heid, R. and Böhmer, A. and Weber, F.
    Physical Review B 99 (2019)
    Phonon measurements in the A15-type superconductors were complicated in the past because of the unavailability of large single crystals for inelastic neutron scattering, e.g., in the case of Nb3Sn, or unfavorable neutron scattering properties in the case of V3Si. Hence, only few studies of the lattice dynamical properties with momentum resolved methods were published, in particular below the superconducting transition temperature Tc. Here, we overcome these problems by employing inelastic x-ray scattering and report a combined experimental and theoretical investigation of lattice dynamics in V3Si with the focus on the temperature-dependent properties of low-energy acoustic phonon modes in several high-symmetry directions. We paid particular attention to the evolution of the soft phonon mode of the structural phase transition observed in our sample at Ts=18.9K, i.e., just above the measured superconducting phase transition at Tc=16.8K. Theoretically, we predict lattice dynamics including electron-phonon coupling based on density-functional-perturbation theory and discuss the relevance of the soft phonon mode with regard to the value of Tc. Furthermore, we explain superconductivity-induced anomalies in the lineshape of several acoustic phonon modes using a model proposed by Allen et al, [Phys. Rev. B 56, 5552 (1997)10.1103/PhysRevB.56.5552]. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.134511
  • 2019 • 91 Segregation tendency of Heusler alloys
    Sokolovskiy, V.V. and Gruner, M.E. and Entel, P. and Acet, M. and Çaklr, A. and Baigutlin, D.R. and Buchelnikov, V.D.
    Physical Review Materials 3 (2019)
    Segregation in a series of Ni2Mn1+x(In,Sn,Ga,Al)1-x and Mn2Ni1+x(Ga, Al) as well as Ni2+xMn1-xGa Heusler alloys is studied by first-principles calculations. We show that Mn-rich Ni2Mn1+x(In,Sn,Al)1-x compounds are at low temperatures unstable in the whole concentration range against decomposition into a dual-phase system consisting of an L21-cubic Ni2MnX phase with ferromagnetic order and an L10-tetragonal NiMn phase ordered antiferromagnetically. In contrast, Ni2Mn1+xGa1-x and Mn2Ni1+x(Ga,Al)1-x are stable in the narrow concentration range near the 2-1-1 stoichiometry. This concentration range depends on the presence of a martensitic transformation and becomes wider with increasing energy difference between austenite and martensite phases in a parent system. We find that ferromagnetic Ni-rich Ni2+xMn1-xGa is stable in the concentration range 0<x≤0.6. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.084413
  • 2019 • 90 Ab initio phase stabilities of Ce-based hard magnetic materials and comparison with experimental phase diagrams
    Sözen, H.Ä. and Ener, S. and MacCari, F. and Skokov, K.P. and Gutfleisch, O. and Körmann, F. and Neugebauer, J. and Hickel, T.
    Physical Review Materials 3 (2019)
    Recent developments in electrical transportation and renewable energies have significantly increased the demand of hard magnetic materials with a reduced critical rare-earth content, but with properties comparable to (Nd,Dy)-Fe-B permanent magnets. Though promising alternative compositions have been identified in high-throughput screenings, the thermodynamic stability of these phases against decomposition into structures with much less favorable magnetic properties is often unclear. In the case of Ce-Fe-Ti alloys, we have used finite temperature ab initio methods to provide this missing information. Employing state-of-the-art approaches for vibrational, electronic, and magnetic entropy contributions, the Helmholtz free energy, F(T,V), is calculated for the desired hard magnetic CeFe11Ti phase and all relevant competing phases. The latter have been confirmed experimentally by employing reactive crucible melting (RCM) and energy-dispersive x-ray spectroscopy (EDS). Our ab initio based free energy calculations reveal that the presence of the CeFe2 Laves phase suppresses the formation of CeFe11Ti up to 700 K. The result is in agreement with RCM, in which CeFe11Ti is only observed above 1000 K, while the CeFe2 and Ce2Fe17 phases are stable at lower temperatures. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.084407
  • 2019 • 89 Facile Protocol for Alkaline Electrolyte Purification and Its Influence on a Ni-Co Oxide Catalyst for the Oxygen Evolution Reaction
    Spanos, I. and Tesch, M.F. and Yu, M. and Tüysüz, H. and Zhang, J. and Feng, X. and Müllen, K. and Schlögl, R. and Mechler, A.K.
    ACS Catalysis 9 8165-8170 (2019)
    We report a simple and effective electrochemical method to remove Fe impurities from commercial KOH electrolyte. We therefore utilize a MoS2 catalyst deposited on porous Ni foam as both the anode and cathode in a two-electrode electrolysis setup. After 12 h of constant galvanostatic electrolysis at 100 mA, the Fe impurities from the KOH electrolyte were successfully removed, as confirmed by means of inductively coupled plasma optical emission spectroscopy analysis. In the purified KOH, a Ni-Co3O4 composite oxide catalyst showed no Fe-induced activation. In contrast, we directly observed the uptake of Fe on the Ni-Co3O4 catalyst from the nontreated electrolyte during catalyst operation using a coupled spectroelectrochemical setup. Interestingly, we further identified an influence on the dissolution behavior of Ni and Co in the presence of Fe impurities. Whereas hitherto mainly the activation effect of Fe impurities has been discussed, we hereby show that they additionally suppress corrosion under reaction conditions. Using our fast and low-cost method for the purification of large amounts of electrolyte, catalyst materials can be widely studied without these additional effects induced by Fe impurities in commercial KOH. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.9b01940
  • 2019 • 88 Altering the stability of nanoislands through core-shell supports
    Sprodowski, C. and Morgenstern, K.
    Nanoscale 11 10314-10319 (2019)
    We follow the decay of two-dimensional Ag nanoclusters, called islands, on Cu-Ag core-shell supports by variable low temperature scanning tunneling microscopy in the temperature range between 160 and 260 K. We reveal two qualitatively different types of decay mechanisms, either linear in time, indicative of an interface-limited decay, or non-linear in time, indicative of diffusion-limited decay. In contrast to conventional decay on monometallic supports, the decay exponent of the diffusion-limited decay depends on temperature; it varies by one order of magnitude. Moreover, the decay rate decreases with increasing temperature. This unusual behaviour is traced back to the temperature-dependent shell of the core-shell support. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9nr00529c
  • 2019 • 87 Temperature and load-ratio dependent fatigue-crack growth in the CrMnFeCoNi high-entropy alloy
    Thurston, K.V.S. and Gludovatz, B. and Yu, Q. and Laplanche, G. and George, E.P. and Ritchie, R.O.
    Journal of Alloys and Compounds 794 525-533 (2019)
    Multiple-principal element alloys known as high-entropy alloys have rapidly been gaining attention for the vast variety of compositions and potential combinations of properties that remain to be explored. Of these alloys, one of the earliest, the ‘Cantor alloy’ CrMnFeCoNi, displays excellent damage-tolerance with tensile strengths of ∼1 GPa and fracture toughness values in excess of 200 MPa√m; moreover, these mechanical properties tend to further improve at cryogenic temperatures. However, few studies have explored its corresponding fatigue properties. Here we expand on our previous study to examine the mechanics and mechanisms of fatigue-crack propagation in the CrMnFeCoNi alloy (∼7 μm grain size), with emphasis on long-life, near-threshold fatigue behavior, specifically as a function of load ratio at temperatures between ambient and liquid-nitrogen temperatures (293 K–77 K). We find that ΔKth fatigue thresholds are decreased with increasing positive load ratios, R between 0.1 and 0.7, but are increased at decreasing temperature. These effects can be attributed to the role of roughness-induced crack closure, which was estimated using compliance measurements. Evidence of deformation twinning at the crack tip during fatigue-crack advance was not apparent at ambient temperatures but seen at higher stress intensities (ΔK ∼ 20 MPa√m) at 77 K by post mortem microstructural analysis for tests at R = 0.1 and particularly at 0.7. Overall, the fatigue behavior of this alloy was found to be superior, or at least comparable, to conventional cryogenic and TWIP steels such as 304 L or 316 L steels and Fe-Mn steels; these results coupled with the remarkable strength and fracture toughness of the Cantor alloy at low temperatures indicate significant promise for the utility of this material for applications at cryogenic environments. © 2019
    view abstractdoi: 10.1016/j.jallcom.2019.04.234
  • 2019 • 86 Powder metallurgic fabricated plug targets for the synthesis of AlCrSiWN multicomponent coating systems
    Tillmann, W. and Fehr, A. and Stangier, D.
    International Journal of Refractory Metals and Hard Materials 85 (2019)
    Monolithic and alloyed targets, conventionally produced by means of melt metallurgy, have been established as conventional target material designs for physical vapor depositions (PVD). However, integrating refractory metals into the sputtering material, leads to restrictions concerning the solubility and phase formation in the target compound. In this context, plug targets are commonly used to deposit multinary coatings with a desired chemical composition. However, producing plugs by means of melt metallurgy restricts the types and amounts of integrated elements. Since current PVD coating concepts aim at an extension of the functionality spectrum by element doping, new target concepts are required. The use of several monolithic targets is one method to produce coatings combining various elements within one coating. Yet depending on the target setup, this approach can result in a nanolaminar coating deposition. To circumvent this, a new production route, which ensures the integration of sintered CrSiW plugs in a monolithic aluminum target, is examined in this study. Two coating deposititons, each with an Al(CrSiW)20 and an Al(CrSiW)48 plug target, were performed by means of direct current (DC) magnetron sputtering and compared with a reference coating, which was synthesized using an AlCr20 target. The dense morphology of AlCrN was significantly changed to a more columnar structure due to slight additions of silicon and tungsten. High aluminum contents in AlCrN and AlCrSiWN, in turn, resulted in a distinct enhancement of the mechanical properties. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijrmhm.2019.105081
  • 2019 • 85 Effects of AlN and BCN thin film multilayer design on the reaction time of Ni/Ni-20Cr thin film thermocouples on thermally sprayed Al2O3
    Tillmann, W. and Kokalj, D. and Stangier, D. and Schöppner, V. and Malatyali, H.
    Sensors (Switzerland) 19 (2019)
    Thin film thermocouples are widely used for local temperature determinations of surfaces. However, depending on the environment in which they are used, thin film thermocouples need to be covered by a wear or oxidation resistant top layer. With regard to the utilization in wide-slit nozzles for plastic extrusion, Ni/Ni-20Cr thin film thermocouples were manufactured using directcurrent (DC) magnetron sputtering combined with Aluminiumnitride (AlN) and Boron- Carbonitride (BCN) thin films. On the one hand, the deposition parameters of the nitride layers were varied to affect the chemical composition and morphology of the AlN and BCN thin films. On the other hand, the position of the nitride layers (below the thermocouple, above the thermocouple, around the thermocouple) was changed. Both factors were investigated concerning the influence on the Seebeck coefficient and the reaction behaviour of the thermocouples. Therefore, the impact of the nitride thin films on the morphology, physical structure, crystallite size, electrical resistance and hardness of the Ni and Ni-20Cr thin films is analysed. The investigations reveal that the Seebeck coefficient is not affected by the different architectures of the thermocouples. Nevertheless, the reaction time of the thermocouples can be significantly improved by adding a thermal conductive top coat over the thin films, whereas the top coat should have a coarse structure and low nitrogen content. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/s19153414
  • 2019 • 84 Improved adhesion of a-C and a-C:H films with a CrC interlayer on 16MnCr5 by HiPIMS-pretreatment
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Maus-Friedrichs, W. and Gustus, R. and Thomann, C.A. and Moldenhauer, H. and Debus, J.
    Surface and Coatings Technology 375 877-887 (2019)
    Ensuring a high adhesion of amorphous carbon films to steel substrates remains a challenging task, sustaining continuous research efforts to improve the adhesion strength. Besides the interlayer system and the substrate material, surface pretreatments have a significant impact on the adhesion behavior. Within this context, the influence of the High Power Impulse Magnetron Sputtering (HiPIMS) pretreatment on the adhesion of magnetron sputtered hydrogenfree (a-C) and hydrogenated (a-C:H) amorphous carbon films with a chromium carbide (CrC) interlayer on 16MnCr5 steel is investigated. The plasma treatment consisted of 30 min Ar ion etching as well as a sequential 5 min of HiPIMS-pretreatment with a Cr cathode. Subsequently this pretreatment was compared to a procedure without utilizing the HiPIMS technique. The impact of the HiPIMS-pretreatment on the structure of the film was systematically analyzed by taking the CrC interlayer as well as the entire film structure into consideration. The adhesion strength of the a-C and a-C:H films is significantly improved by the formation of a Cr HiPIMS-nanolayer in the substrate/film interface. In scratch tests, the critical load Lc3 for a total film delamination increases from 43 ± 4 to 59 ± 3 N and from 48 ± 2 to 64 ± 3 N for the a-C and a-C:H film. The improved adhesion behavior of the carbon films is ascribed to the increased adhesion of the CrC interlayer, which did not delaminate when scratched with a load up to 159 ± 18 N. Complementary Rockwell indentation tests reveal that the HiPIMS-pretreatment improves the adhesion class from HF6 to HF4 and from HF5 to HF3 for a-C and a-C:H. The enhanced adhesion is essential to exploit the properties of a-C and a-C:H films in applications with high loads. In conclusion, the HiPIMS-pretreatment has proven to be a promising technique to increase the adhesion strength of carbon films. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2019.07.076
  • 2019 • 83 One-step synthesis of Fe-Au core-shell magnetic-plasmonic nanoparticles driven by interface energy minimization
    Tymoczko, A. and Kamp, M. and Rehbock, C. and Kienle, L. and Cattaruzza, E. and Barcikowski, S. and Amendola, V.
    Nanoscale Horizons 4 1326-1332 (2019)
    Directing the assembly of atoms into core-shell particles generally requires elegant but sophisticated procedures. Here we show how the thermodynamic driving force to minimization of surface and interface energy can be exploited to produce colloidal Fe-Au core-shell nanoparticles in one step and with a yield approaching 99.7% in mass. This is obtained by laser ablation with nanosecond pulses of thin bimetallic films immersed in acetone. The Fe-Au core-shell nanoparticles show magnetic and plasmonic properties, and a surface available to bioconjugation and analytical assays. This laser assisted synthetic method represents a step forward in the facile preparation of core-shell nanospheres with multiple appealing functionalities. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9nh00332k
  • 2019 • 82 Effects of femtosecond laser shock peening in distilled water on the surface characterizations of NiTi shape memory alloy
    Wang, H. and Pöhl, F. and Yan, K. and Decker, P. and Gurevich, E.L. and Ostendorf, A.
    Applied Surface Science 471 869-877 (2019)
    NiTi shape memory alloy was processed by femtosecond laser shock peening (FLSP) without protective coating in distilled water to modify its surface characterizations. The surface topography, microhardness, microstructure and scratch testing were studied before and after FLSP treatment. The experimental results show that FLSP with different laser scanning speeds and passes can change the surface roughness and microhardness due to laser ablation and high pressure of shock wave. The average microhardness value of the specimens FLSPed in distilled water increased. Grain refinement was observed in the surface layer of FLSPed NiTi alloy. The scratch testing revealed that FLSP process can decrease the frictional force and coefficient of friction, and it also demonstrated that the FLSP technique is beneficial to enhance the surface wear property of NiTi alloy. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.12.087
  • 2019 • 81 Thermodynamic assessment of the Co-Ta system
    Wang, P. and Koßmann, J. and Kattner, U.R. and Palumbo, M. and Hammerschmidt, T. and Olson, G.B.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 64 205-212 (2019)
    The Co-Ta system has been reviewed and the thermodynamic description was re-assessed in the present work. DFT (density functional theory) calculations considering spin polarization were performed to obtain the energies for all end-member configurations of the C14, C15, C36 and μ phases for the evaluation of the Gibbs energies of these phases. The phase diagram calculated with the present description agrees well with the experimental and theoretical data. Considering the DFT results was essential for giving a better description of the μ phase at lower temperatures. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.calphad.2018.12.002
  • 2019 • 80 Synergistic Effects of Mo2C-NC@CoxFey Core–Shell Nanoparticles in Electrocatalytic Overall Water Splitting Reaction
    Wang, S. and Bendt, G. and Saddeler, S. and Schulz, S.
    Energy Technology 7 (2019)
    Transition metals (TMs) are highly investigated as nonprecious electrocatalysts for hydrogen evolution (HER) and oxygen evolution (OER) reactions. There is a strong demand for highly efficient and inexpensive catalysts for overall water splitting. Herein, the bimetallic CoxFey alloy nanoparticles encapsulated in a N-doped graphene shell containing molybdenum carbide (Mo2C) nanoparticles are synthesized by the pyrolysis of cobalt ferrite (CoxFe3−xO4) nanoparticles coated by melamine-formaldehyde resin cross-linked with molybdic acid. Molybdic acid not only serves as precursor for the formation of highly dispersed Mo2C nanoparticles in the N-doped graphene shell but also enhances the thermal stability of the organic shell, resulting in the formation of smaller CoxFey cores. The formation of Mo2C nanoparticles in the graphene shell is promoted by the CoxFe3−xO4 core. Interestingly, the synergistic presence of Mo2C nanoparticles not only enhances the HER activity of the material but also renders a partial breakage of the graphene shell, which increases the surface concentration of OER-active Co and therefore enhances the OER activity. The as-prepared TM-based materials serve as bifunctional catalysts for the overall water splitting and exhibit improved electrocatalytic performances compared to standard cells based on precious metals, with the potentials of 1.53 and 1.60 V at 10 and 20 mA cm−2 in alkaline media, respectively. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ente.201801121
  • 2019 • 79 Strength of hydrogen-free and hydrogen-doped Ni50Ti50 shape memory platelets
    Weiser, A. and Buršíková, V. and Jarý, M. and Dymáček, P. and Dugáček, J. and Frenzel, J. and Čermák, J. and Dlouhý, A.
    Scripta Materialia 162 151-155 (2019)
    Small-punch and nano-indentation tests were used for the first time to probe strength of 500 μm thin Ni50Ti50 shape memory platelets in their hydrogen-free and hydrogen-doped states. Results show excellent reproducibility and suggest that hydrogen penetrates the alloy more efficiently during the cathodic charging at ambient temperatures as compared to heat treatments in a controlled hydrogen atmosphere. Hydrogen content exceeding 100 wtppm results in a retransformation from the B19′ martensite to the R lattice and causes a systematic drop of the rupture strength. The retransformation events in thin surface lamellae were documented by the transmission electron microscopy. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.10.044
  • 2019 • 78 Strength of hydrogen-free and hydrogen-doped Ni 50 Ti 50 shape memory platelets
    Weiser, A. and Buršíková, V. and Jarý, M. and Dymáček, P. and Dugáček, J. and Frenzel, J. and Čermák, J. and Dlouhý, A.
    Scripta Materialia 162 151-155 (2019)
    Small-punch and nano-indentation tests were used for the first time to probe strength of 500 μm thin Ni 50 Ti 50 shape memory platelets in their hydrogen-free and hydrogen-doped states. Results show excellent reproducibility and suggest that hydrogen penetrates the alloy more efficiently during the cathodic charging at ambient temperatures as compared to heat treatments in a controlled hydrogen atmosphere. Hydrogen content exceeding 100 wtppm results in a retransformation from the B19′ martensite to the R lattice and causes a systematic drop of the rupture strength. The retransformation events in thin surface lamellae were documented by the transmission electron microscopy. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.10.044
  • 2019 • 77 Phase Formation during Heat Treatment of Zn- and ZnAlMg-Coated Steels at 400 and 750 °C
    Windmann, M. and Barthen, M. and Opitz, T. and Hahn, I. and Röttger, A. and Theisen, W.
    Steel Research International (2019)
    In contrast to a cold-forming process, a tempered forming process is able to deform high-strength steel used for manufacturing automotive bodyworks in a more economic manner. Cold-formed steel sheets are commonly coated with a Zn or ZnAlMg layer for cathodic corrosion protection. The tempering process would lead to diffusion processes at the steel/coating interface, which is accompanied by the formation of new phases in the coatings. This publication focuses on phase formation in Zn and ZnAlMg coatings on steel sheets, which are heat-treated at 400 and 750 °C. the authors find that the pure Zn coating remains in the solid state and transforms into the intermetallic δ phase (FeZn 10 ) during heat treatment at 400 °C. The coating melts during heating to 750 °C, but remains in the solid state after transformation into the Γ phase (Fe 4 Zn 9 ) and α-Fe. In the ZnAlMg coating, minor iron diffusion occurs at a temperature of 400 °C. Within a dwell time of 600 s, intermetallic Fe–Zn phases are not formed. During heat treatment at 750 °C, phase formation in the ZnAlMg coating is very similar to that in the pure Zn coating, during which Γ (Fe 4 Zn 9 ) and α-Fe are formed. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/srin.201800588
  • 2019 • 76 Epitaxial strain adaptation in chemically disordered FeRh thin films
    Witte, R. and Kruk, R. and Wang, D. and Schlabach, S. and Brand, R.A. and Gruner, M.E. and Wende, H. and Hahn, H.
    Physical Review B 99 (2019)
    Strain and strain adaptation mechanisms in modern functional materials are of crucial importance for their performance. Understanding these mechanisms will advance innovative approaches for material properties engineering. Here we study the strain adaptation mechanism in a thin film model system as a function of epitaxial strain. Chemically disordered FeRh thin films are deposited on W-V buffer layers, which allow for large variation of the preset lattice constants, e.g., epitaxial boundary condition. It is shown by means of high-resolution x-ray reciprocal space maps and transmission electron microscopy that the system reacts with a tilting mechanism of the structural units in order to adapt to the lattice constants of the buffer layer. This response is explained by density functional theory calculations, which evidence an energetic minimum for structures with a distortion of c/a≈0.87. The experimentally observed tilting mechanism is induced by this energy gain and allows the system to remain in the most favorable structure. In general, it is shown that the use of epitaxial model heterostructures consisting of alloy buffer layers of fully miscible elements and the functional material of interest allows to study strain adaptation behaviors in great detail. This approach makes even small secondary effects observable, such as the directional tilting of the structural domains identified in the present case study. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.134109
  • 2019 • 75 Large thermopower anisotropy in PdCo O2 thin films
    Yordanov, P. and Sigle, W. and Kaya, P. and Gruner, M.E. and Pentcheva, R. and Keimer, B. and Habermeier, H.-U.
    Physical Review Materials 3 (2019)
    Motivated by recent theoretical studies predicting a large thermopower anisotropy in the layered delafossite PdCoO2, we have used pulsed laser deposition to synthesize thin films on (0001)-oriented and offcut Al2O3 substrates. By combining transport measurements on films with different offcut angles, tensor rotation relations, and an iterative fit procedure for the transport parameters, we have determined the resistivity and the thermopower along the main crystallographic axes in the temperature range 300-815 K. The data reveal a small positive Seebeck coefficient along the delafossite planes and a large negative Seebeck coefficient perpendicular to the planes, in excellent agreement with density functional calculations in the presence of moderate Coulomb correlations. The methodology introduced here is generally applicable for measurements of the thermoelectric properties of materials with highly anisotropic electronic structures. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.085403
  • 2019 • 74 Enhanced propylene oxide selectivity for gas phase direct propylene epoxidation by lattice expansion of silver atoms on nickel nanoparticles
    Yu, B. and Ayvalı, T. and Raine, E. and Li, T. and Li, M.M.-J. and Zheng, J. and Wu, S. and Bagabas, A.A. and Tsang, S.C.E.
    Applied Catalysis B: Environmental 243 304-312 (2019)
    A series of surfactant-free nickel-core and silver-shell (Ni@Ag) nanoparticles encapsulated within the mesopores of SBA-15 were synthesized and tested as catalysts for direct propylene oxidation by molecular oxygen. The influences of temperature, Gas Hour Space Velocity (GHSV) and Ni/Ag ratio on catalytic activity were systematically investigated. Among the prepared samples, Ni1Ag0.4/SBA-15 exhibited the best catalytic performance with selectivity of 70.7% and PO production rate of 4.4 nmol/g/s under 1 bar at 220 °C with GHSV of 192 h−1. High selectivity was attributed to longer Ag-Ag interatomic distance obtained by careful engineering the thickness of Ag shell over preformed Ni nanoparticles. In addition, all prepared new Ni@Ag core-shell catalysts presented excellent stability, which could maintain the conversion and selectivity for at least 10 h. These results suggest that new designs based on Ag surface atoms tailoring might pave the way to highly efficient and robust Ag catalysts for direct propylene oxidation using molecular oxygen as sole oxidant. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2018.10.061
  • 2019 • 73 Selective acid leaching: A simple way to engineer cobalt oxide nanostructures for the electrochemical oxygen evolution reaction
    Yu, M. and Belthle, K.S. and Tüysüz, C. and Tüysüz, H.
    Journal of Materials Chemistry A 7 23130-23139 (2019)
    Developing a simple and cost-effective strategy to construct earth-abundant catalysts is in high demand for diverse applications. Herein, a general and facile strategy is developed to engineer cobalt oxide nanostructures via selective acid leaching for the electrochemical oxygen evolution reaction (OER). A leaching process is implemented to selectively remove CoMoO4 by treating mixed Co-Mo oxides in diluted hydrochloric acid solution, resulting in the formation of sub-5 nm particles and a threefold increase in the specific surface area (up to 150 m2 g-1). The leached oxides exhibit superior OER activity to pristine oxides as a result of (i) a larger surface area, (ii) phase purification to expose more active Co3O4 species to the reactant, and (iii) faster charge transfer kinetics for the OER. This strategy can be also applied to a broader range of earth-abundant metals, where a second metal (Li, Ca, and Mg) is selectively leached out, which results in a material with a larger surface area and enhanced catalytic performance for the OER. Moreover, various metal oxides with a high surface area, such as NiO and Fe2O3, can be prepared via this simple synthetic method. This work will pave a new practical way for the production of high surface area catalysts for diverse applications. © The Royal Society of Chemistry 2019.
    view abstractdoi: 10.1039/c9ta07835e
  • 2019 • 72 Realizing facile regeneration of spent NaBH4 with Mg-Al alloy
    Zhong, H. and Ouyang, L. and Zeng, M. and Liu, J. and Wang, H. and Shao, H. and Felderhoff, M. and Zhu, M.
    Journal of Materials Chemistry A 7 10723-10728 (2019)
    The regeneration of sodium borohydride (NaBH4) is crucial to form a closed cycle after it either supplies hydrogen energy via a hydrolysis process or provides energy through electron transfer at the anode of direct borohydride fuel cells (DBFCs). In both of these cases, the spent fuels are NaB(OH)4 from NaBO2 aqueous solution. However, the current regeneration process from (NaB(OH)4)·xH2O to form NaBH4 by reduction reaction and calcination at high temperature with metal hydrides as reducing agents is very expensive. In this work, we developed a simple regeneration process via ball milling with Mg-Al alloys as the reducing agent for NaB(OH)4 under an argon atmosphere. Under optimized conditions, a high yield of about 72% of NaBH4 could be obtained. Mechanistic study showed that all the hydrogen atoms from NaB(OH)4 remain in NaBH4 and no additional hydrogen sources are needed for the reduction process. The inexpensive Mg-Al alloy works as a reducing agent transforming the H+ to H- in NaBH4. This approach demonstrates a ∼20-fold cost reduction compared with the method using metal hydrides. This opens the door to the commercial implementation of simple ball milling processes for the regeneration of spent NaBH4 from NaB(OH)4 with cheap reducing agents. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ta00769e
  • 2018 • 71 Antibacterial Efficacy of Sacrifical Anode Thin Films Combining Silver with Platinum Group Elements within a Bacteria-Containing Human Plasma Clot
    Abuayyash, A. and Ziegler, N. and Gessmann, J. and Sengstock, C. and Schildhauer, T.A. and Ludwig, Al. and Köller, M.
    Advanced Engineering Materials 20 (2018)
    Silver (Ag) dots arrays (64 and 400 dots per mm2) are fabricated on a continuous platinum (Pt), palladium (Pd), or iridium (Ir) thin film (sacrifical anode systems for Ag) and for comparison on titanium (Ti) film (non-sacrifical anode system for Ag) by sputter deposition and photolithographic patterning. The samples are embedded within a tissue-like plasma clot matrix containing Staphylococcus aureus (S. aureus), cultivated for 24 h. Bacterial growth is analyzed by fluorescence microscopy. Among platinum group sacrifical anode elements and a dense Ag sample, only the high Ag ion releasing Ag–Ir system is able to inhibit the bacterial growth within the adjacent plasma clot matrix. This study demonstrates that the antibacterial efficiency of Ag coatings is reduced under tissue-like conditions. However, the new sacrificial anode based Ag–Ir system can overcome this limitation. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201700493
  • 2018 • 70 Influence of Temperature and Electrolyte Concentration on the Structure and Catalytic Oxygen Evolution Activity of Nickel–Iron Layered Double Hydroxide
    Andronescu, C. and Seisel, S. and Wilde, P. and Barwe, S. and Masa, J. and Chen, Y.-T. and Ventosa, E. and Schuhmann, W.
    Chemistry - A European Journal 24 13773-13777 (2018)
    NiFe layered double hydroxide (LDH) is inarguably the most active contemporary catalyst for the oxygen evolution reaction under alkaline conditions. However, the ability to sustain unattenuated performance under challenging industrial conditions entailing high corrosivity of the electrolyte (≈30 wt. % KOH), high temperature (&gt;80 °C) and high current densities (&gt;500 mA cm−2) is the ultimate criterion for practical viability. This work evaluates the chemical and structural stability of NiFe LDH at conditions akin to practical electrolysis, in 30 % KOH at 80 °C, however, without electrochemical polarization, and the resulting impact on the OER performance of the catalyst. Post-analysis of the catalyst by means of XRD, TEM, FT-IR, and Raman spectroscopy after its immersion into 7.5 m KOH at 80 °C for 60 h revealed a transformation of the structure from NiFe LDH to a mixture of crystalline β-Ni(OH)2 and discrete predominantly amorphous FeOOH containing minor non-homogeneously distributed crystalline domains. These structural and compositional changes led to a drastic loss of the OER activity. It is therefore recommended to study catalyst stability at industrially relevant conditions. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201803165
  • 2018 • 69 Development of high modulus steels based on the Fe – Cr – B system
    Baron, C. and Springer, H. and Raabe, D.
    Materials Science and Engineering A 724 142-147 (2018)
    We present a novel alloy design strategy for cost-efficient high modulus steels with an increased stiffness / mass density ratio. The concept is based on the liquid metallurgy synthesis of Fe – Cr – B based alloys, straightforward processability, and well tuneable mechanical properties via plain heat treatments. The base alloy Fe – 18 Cr – 1.6 B (wt%) contained 14–17 vol% of (Cr,Fe)2B particles of ellipsoidal morphology in a ferritic matrix. Hot rolled materials revealed a specific modulus of 32.8 GPa g−1 cm3, exceeding that of conventional Fe-Cr steels by almost 30%. Mechanical properties obtained are comparable to TiB2 based high modulus steels. Addition of 1 wt% Cu to the base alloy did not interact with the formation, fraction, size and morphology of (Cr,Fe)2B particles, and allowed to mildly increase the strength values by ageing treatments, however at the price of a reduction of the specific modulus. C additions of 0.2 wt% did not affect the (Cr,Fe)2B particle microstructure greatly, but free C dissolved in the matrix enables for the first time to utilize the wide range of microstructures and mechanical properties of established C-containing high strength steels also in high modulus steels. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2018.03.082
  • 2018 • 68 Electrocatalytic Nanoparticles That Mimic the Three-Dimensional Geometric Architecture of Enzymes: Nanozymes
    Benedetti, T.M. and Andronescu, C. and Cheong, S. and Wilde, P. and Wordsworth, J. and Kientz, M. and Tilley, R.D. and Schuhmann, W. and Gooding, J.J.
    Journal of the American Chemical Society 140 13449-13455 (2018)
    Enzymes are characterized by an active site that is typically embedded deeply within the protein shell thus creating a nanoconfined reaction volume in which high turnover rates occur. We propose nanoparticles with etched substrate channels as a simplified enzyme mimic, denominated nanozymes, for electrocatalysis. We demonstrate increased electrocatalytic activity for the oxygen reduction reaction using PtNi nanoparticles with isolated substrate channels. The PtNi nanoparticles comprise an oleylamine capping layer that blocks the external surface of the nanoparticles participating in the catalytic reaction. Oxygen reduction mainly occurs within the etched channels providing a nanoconfined reaction volume different from the bulk electrolyte conditions. The oxygen reduction reaction activity normalized by the electrochemically active surface area is enhanced by a factor of 3.3 for the nanozymes compared to the unetched nanoparticles and a factor of 2.1 compared to mesoporous PtNi nanoparticles that possess interconnecting pores. © Copyright 2018 American Chemical Society.
    view abstractdoi: 10.1021/jacs.8b08664
  • 2018 • 67 A variable pole magnet
    Çakir, A. and Krenke, T. and Farle, M. and Acet, M.
    Journal of Physics Condensed Matter 30 (2018)
    The off-stoichiometric antiferromagnetic Heusler alloy Fe50Mn45Ga5 decomposes and forms ferromagnetic Fe50Mn25Ga25 precipitates embedded in an antiferromagnetic Fe50Mn50 matrix when temper-annealed at temperatures T &gt; 550 K. The ferromagnetism of the precipitates is soft so that the magnetization direction of the non-interacting precipitates in a macroscopic material can be manipulated by locally applied fields so that even two similar poles can form at the ends of a centimeter-long bar. The cause for the soft magnetic behavior is due to the weak AF exchange anisotropy of the cubic Fe50Mn50 matrix and the precipitate. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aaa5f5
  • 2018 • 66 Characterizing solute hydrogen and hydrides in pure and alloyed titanium at the atomic scale
    Chang, Y. and Breen, A.J. and Tarzimoghadam, Z. and Kürnsteiner, P. and Gardner, H. and Ackerman, A. and Radecka, A. and Bagot, P.A.J. and Lu, W. and Li, T. and Jägle, E.A. and Herbig, M. and Stephenson, L.T. and Moody, M.P. and...
    Acta Materialia 150 273-280 (2018)
    Ti and its alloys have a high affinity for hydrogen and are typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and α+β binary model alloys. No hydrides are observed in the α phase in alloys with Al addition or quenched-in Mo supersaturation. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.02.064
  • 2018 • 65 Hedgehog Spin-Vortex Crystal Antiferromagnetic Quantum Criticality in CaK (Fe1-xNix)4As4 Revealed by NMR
    Ding, Q.-P. and Meier, W.R. and Cui, J. and Xu, M. and Böhmer, A.E. and Bud'Ko, S.L. and Canfield, P.C. and Furukawa, Y.
    Physical Review Letters 121 (2018)
    Two ordering states, antiferromagnetism and nematicity, have been observed in most iron-based superconductors (SCs). In contrast to those SCs, the newly discovered SC CaK(Fe1-xNix)4As4 exhibits an antiferromagnetic (AFM) state, called hedgehog spin-vortex crystal (SVC) structure, without nematic order, providing the opportunity for the investigation into the relationship between spin fluctuations and SC without any effects of nematic fluctuations. Our As75 nuclear magnetic resonance studies on CaK(Fe1-xNix)4As4 (0≤x≤0.049) revealed that CaKFe4As4 is located close to a hidden hedgehog SVC AFM quantum-critical point (QCP). The magnetic QCP without nematicity in CaK(Fe1-xNix)4As4 highlights the close connection of spin fluctuations and superconductivity in iron-based SCs. The advantage of stoichiometric composition also makes CaKFe4As4 an ideal platform for further detailed investigation of the relationship between magnetic QCP and superconductivity in iron-based SCs without disorder effects. © 2018 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.121.137204
  • 2018 • 64 Glycerol Oxidation Using MgO- and Al2O3-supported Gold and Gold–Palladium Nanoparticles Prepared in the Absence of Polymer Stabilizers
    Dodekatos, G. and Abis, L. and Freakley, S.J. and Tüysüz, H. and Hutchings, G.J.
    ChemCatChem 10 1351-1359 (2018)
    Au and AuPd nanoparticles supported on MgO and Al2O3 were employed for the selective aqueous phase oxidation of glycerol under basic conditions. Catalysts were prepared by sol-immobilization without the addition of a stabilizing agent such as polyvinyl alcohol (PVA), which is generally added to stabilize the noble metal sol prior to immobilization. The obtained materials prepared with and without stabilizing agent were active for glycerol oxidation and showed similar catalytic performances—implying that the stabilizing polymer is not required to obtain active materials. Depending on the support used, it was possible to tailor the selectivity towards the desired oxidation products by using catalysts prepared with or without stabilizing agent. PVA-free Au/γ-Al2O3 exhibited a remarkably high selectivity towards tartronic acid (40 % at 97 % conversion), which was not observed for Au/γ-Al2O3 prepared with PVA (27 % at isoconversion). Selective glycerol oxidation performed under base-free conditions over AuPd/MgO catalysts also corroborated the previous results that the presence of a stabilizing polymer is not required to prepare active catalysts by sol-immobilization. Thus, a facile way to circumvent the inherent drawbacks encountered by the use of polymer stabilizers during catalyst preparation is presented herein. Experimental results suggest that the presence of the polymer stabilizers can affect the reaction pathways and control selectivity. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201800074
  • 2018 • 63 On the Electropolishing Mechanism of Nickel Titanium in Methanolic Sulfuric acid − An Electrochemical Impedance Study
    Fushimi, K. and Neelakantan, L. and Eggeler, G. and Hassel, A.W.
    Physica Status Solidi (A) Applications and Materials Science 215 (2018)
    Electropolishing of NiTi shape memory alloys is possible in methanolic 3 m H2SO4. The electro-dissolution behavior of NiTi in methanolic 3 m H2SO4 is ascertained in terms of Nyquist plots using electrochemical impedance spectroscopy (EIS) under limiting current flow (mass transfer control) condition. The electro-dissolution behavior is studied under convective conditions using a rotating disc electrode. The influence of changes in rotation rate, applied potential, and temperature are determined. This study demonstrates that electro-dissolution under mass transfer condition follows a compact salt-film mechanism. In order to quantitatively characterize the salt film formed during electropolishing, EIS is performed under stationary conditions. The increase in applied voltage causes an increase in polarization resistance and decrease in capacitance of the interface film. © 2018 The Authors. Published by Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/pssa.201800011
  • 2018 • 62 Coherently strained [Fe-Co(C)/Au-Cu]n multilayers: A path to induce magnetic anisotropy in Fe-Co films over large thicknesses
    Giannopoulos, G. and Salikhov, R. and Varvaro, G. and Psycharis, V. and Testa, A.M. and Farle, M. and Niarchos, D.
    Journal of Physics D: Applied Physics 51 (2018)
    Among novel critical-element-free materials for permanent magnets, the nearly equiatomic Fe-Co alloy has recently attracted a great deal of attention as a large magneto-crystalline anisotropy can be induced by straining the Fe-Co unit cell. In thin film systems, the use of a suitable underlayer allows a tetragonal reconstruction of the Fe-Co to be triggered up to a critical thickness of few nanometers, above which the crystal structure relaxes to the magnetically soft cubic phase. Scaling-up the thickness of the metastable tetragonal Fe-Co phase is of crucial significance for different nanoscale applications, such as magnetic micro- and nano-electromechanical systems. To suppress the strain relaxation occurring at high thicknesses, we explored a novel approach based on Fe-Co(C)/Au-Cu multilayer films, where both Au-Cu interlayers and carbon (C) doping were used to stabilize the strained Fe-Co tetragonal phase over large thicknesses. Both doped and un-doped multilayer structures show a coherently strained regime, persisting up to a thickness of 60 nm, which leads, possibly in combination with the surface anisotropy induced at the Au-Cu interfaces, to the appearance of a large out-of-plane anisotropy (up to 0.4 MJ m-3), thus suggesting the potential of such an approach to develop critical-element-free thin film permanent magnets for a variety of nanoscale applications. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aaa41c
  • 2018 • 61 Fabrication of perovskite-based porous nanotubes as efficient bifunctional catalyst and application in hybrid lithium-oxygen batteries
    Gong, H. and Wang, T. and Guo, H. and Fan, X. and Liu, X. and Song, L. and Xia, W. and Gao, B. and Huang, X. and He, J.
    Journal of Materials Chemistry A 6 16943-16949 (2018)
    The design of efficient oxygen electrocatalysts is extremely important and urgent for much energy storage and conversion equipment. Among these, the high energy densities of lithium-oxygen batteries (LOBs) have driven us to explore bifunctional catalysts. Compared with non-aqueous LOBs, which have been blamed for poor cycling stability due to their undesirable side reaction, hybrid LOBs have been considered an alternative solution due to their high electrochemical reversibility and safeness. Here, one-dimensional hierarchical mesoporous/macroporous LaMn0.7Co0.3O3-x nanotubes were synthesized through an electrospinning method combined with an annealing treatment. With the suitable heat treatment and rational doping with elemental Co, the LMCO-800 sample shows a well-designed hierarchical porous nanotube structure and possess great bifunctional electrocatalytic performance. The linear sweep voltammetry (LSV) curves show that the half-wave potential (E1/2) of the LMCO-800 sample is 0.72 V (vs. RHE) and the average electron transfer number (n) is calculated to be 3.8. Moreover, the successful doping of elemental Co into the LMCO-800 nanotubes can shorten the average distance of the Mn-Mn atoms and promote the formation of O-O bonds, contributing to the enhanced OER performance. The high specific surface area and one-dimensional nanotubes can greatly benefit oxygen diffusion, facilitate electrolyte infiltration and improve electron transfer. Consequently, the as-assembled hybrid lithium-oxygen batteries with an LMCO-800 cathode exhibit superior cycling stability. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8ta04599b
  • 2018 • 60 CuPd Mixed-Metal HKUST-1 as a Catalyst for Aerobic Alcohol Oxidation
    Guo, P. and Froese, C. and Fu, Q. and Chen, Y.-T. and Peng, B. and Kleist, W. and Fischer, R.A. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 122 21433-21440 (2018)
    Metal-organic frameworks (MOFs) featuring isolated coordinatively unsaturated metal sites (CUS) have enormous potential as single-site catalysts. In particular, mixed-metal MOFs may exhibit unique catalytic properties compared to their monometallic counterparts. Herein, we report a thorough fundamental study on the mixed-metal CuPd-HKUST-1 ([Cu 3-x Pd x (BTC) 2 ] n ; BTC = 1,3,5-benzenetricarboxylate) including the two-step synthesis, characterization, and catalytic performance evaluation. The combined results from a multitechnique approach provide solid evidence that the chemical properties of HKUST-1 can be tuned via successful incorporation of Pd-CUS into the framework, leading to the formation of new Cu-Pd and/or Pd-Pd dimers. The introduction of Pd occurs exclusively at the metal nodes in a controlled manner while retaining the structural integrity. The incorporated Pd ions have an oxidation state of +2, whereas no PdO or metallic Pd nanoparticles embedded inside MOFs are detected. These mixed-metal CuPd-MOFs exhibit superior catalytic activity and selectivity for the aerobic oxidation of benzyl alcohol to benzaldehyde, and the doped Pd 2+ -CUS species are identified as isolated single-active sites. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b05882
  • 2018 • 59 Elemental partitioning and site-occupancy in γ/γ′ forming Co-Ti-Mo and Co-Ti-Cr alloys
    Im, H.J. and Makineni, S.K. and Gault, B. and Stein, F. and Raabe, D. and Choi, P.-P.
    Scripta Materialia 154 159-162 (2018)
    We report on the sub-nanometer scale characterization of Co-12Ti-4Mo and Co-12Ti-4Cr (at.%) model alloys. Atom probe tomography reveals that Co and Cr partition to γ, whereas Ti and Mo to γ′. Additions of Mo and Cr to the reference Co-12Ti system lead to strong increases in γ′ volume fraction by about 25% and 12%, respectively. Element-specific spatial distribution maps along the [001] direction of the L12-ordered γ′ phase reveal that both Mo and Cr preferentially replace Ti on its sublattice. The remaining excess Ti is available for formation of additional γ′, resulting in enhanced γ′ volume fractions. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.05.041
  • 2018 • 58 Temperature-Dependent Ultrastructure Transformation of Au-Fe Nanoparticles Investigated by in Situ Scanning Transmission Electron Microscopy
    Kamp, M. and Tymoczko, A. and Schürmann, U. and Jakobi, J. and Rehbock, C. and Rätzke, K. and Barcikowski, S. and Kienle, L.
    Crystal Growth and Design 18 5434-5440 (2018)
    Three-dimensional morphology changes of bimetallic nanoparticles (NPs) with nominal composition Au50Fe50 and Au20Fe80, generated by pulsed laser ablation in liquid, are monitored in situ and ex situ via scanning transmission electron microscopy and electron tomography. The samples are made up of a chemically segregated core-shell (CS) NPs structure, with an Au-rich shell and Fe-rich core, and solid solution (SS) NPs in the pristine state. Further, the examinations reveal information about a sequence of characteristic changes from the pristine metastable and intermediate ultrastructures up to thermodynamically stable products. In the case of the Au20Fe80 sample, a metastable spherical CS morphology is transformed at equilibrium conditions into a cube-shaped Fe-rich core faceted by truncated Au-rich pyramids. For the Au50Fe50 sample, the Au-rich shell is solved into the Fe-rich core, and chemically homogeneous (SS) NPs are formed. Interestingly, this transformation was proven to occur via an intermediate ultrastructure with lamellar segregation, not previously reported as a transient state during in situ heating. On the basis of these observations, a correlation between the composition and the morphology at equilibrium is suggested, in accordance with the bulk phase diagram of Au-Fe. At the same time, our examinations directly prove that laser ablation synthesis creates nonequilibrium NP morphologies, frozen in metastable, spherical core-shell particles. Copyright © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.8b00809
  • 2018 • 57 Understanding the Effect of Au in Au-Pd Bimetallic Nanocrystals on the Electrocatalysis of the Methanol Oxidation Reaction
    Kelly, C.H.W. and Benedetti, T.M. and Alinezhad, A. and Schuhmann, W. and Gooding, J.J. and Tilley, R.D.
    Journal of Physical Chemistry C 122 21718-21723 (2018)
    Pd or Pt alloyed with a secondary metal are the typical catalysts at the anode for the direct oxidation of methanol. The secondary metal is employed to diminish deactivation commonly ascribed to CO poisoning. Here we investigate the origin of the improved performance of Au-Pd core-shell and alloy nanocrystals as electrocatalysts for the methanol oxidation reaction (MOR), relative to Pd alone. Monodisperse Au-Pd core-shell nanocrystals were synthesized using H2 as a mild reducing agent followed by annealing under a 5% H2 atmosphere to produce the Au-Pd alloys. The nanocrystals were characterized using high-resolution electron microscopy to confirm their structures. The core-shell and alloy nanocrystals showed an improvement in specific activity with respect to pure Pd nanocrystals. Importantly, the stability was also improved by the inclusion of Au for both nanocrystals, being 2.7× higher for the alloy than for the core-shell after 30 min, while the activity is completely lost for the Pd nanocrystals within 10 min. We show that there is no evidence of CO formation for any of the Pd-based catalysts in an alkaline environment. The origin of the improvement in terms of both activity and stability results from positive shifts in the PdO formation/reduction potential caused by the presence of Au, which results in more Pd sites available for the MOR. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b05407
  • 2018 • 56 Compositional evolution of long-period stacking ordered structures in magnesium studied by atom probe tomography
    Kim, J.-K. and Guo, W. and Choi, P.-P. and Raabe, D.
    Scripta Materialia 156 55-59 (2018)
    Mg alloys containing long-period stacking ordered (LPSO) structures are strong and ductile compared to conventional Mg alloys. We study here the compositional evolution of LPSO structures in a Mg97Y2Zn1 (at.%) alloy upon annealing at 500 °C using atom probe tomography. In the material annealed for 2.5 h, the Zn/Y ratio of the building blocks in the interdendritic LPSO phase (0.73) is close to the stoichiometric composition of Y8Zn6 L12 clusters while that in plate-type LPSO structures (0.66) slightly deviates from the ideal value. The Y/Zn enrichment in LPSO structures in the α-Mg matrix slightly decreases with increasing annealing time. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.07.017
  • 2018 • 55 NiTi-Based Elastocaloric Cooling on the Macroscale: From Basic Concepts to Realization
    Kirsch, S.-M. and Welsch, F. and Michaelis, N. and Schmidt, M. and Wieczorek, A. and Frenzel, J. and Eggeler, G. and Schütze, A. and Seelecke, S.
    Energy Technology 6 1567-1587 (2018)
    Solid-state cooling is an environmentally friendly, no global warming potential alternative to vapor compression-based systems. Elastocaloric cooling based on NiTi shape memory alloys exhibits excellent cooling capabilities. Due to the high specific latent heats activated by mechanical loading/unloading, large temperature changes can be generated in the material. The small required work input enables a high coefficient of performance. An overview of elastocaloric cooling from basic principles, such as elastocaloric cooling cycles, material characterization, modeling, and optimization, to the design of elastocaloric cooling devices is presented. Current work performed within the DFG (Deutsche Forschungsgemeinschaft) Priority Program SPP 1599 “Ferroic Cooling”, which is focused on the development and realization of a continuously operating elastocaloric cooling device, is highlighted. The cooling device operates in a rotatory mode with wires under tensile loading. The design allows maximization of cooling power by suitable wire diameter scaling as well as efficiency optimization by implementing a novel drive concept. Finally, computer-aided design (CAD) models of the discussed solid-state air cooling device are presented. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ente.201800152
  • 2018 • 54 Control of Cooperativity through a Reversible Structural Phase Transition in MoMo-Methyl/Cu(111)
    Kollamana, J. and Wei, Z. and Lyu, L. and Zimmer, M. and Dietrich, F. and Eul, T. and Stöckl, J. and Maniraj, M. and Ponzoni, S. and Cinchetti, M. and Stadtmüller, B. and Gerhards, M. and Aeschlimann, M.
    Advanced Functional Materials 28 (2018)
    The bimetallic molecular compound Dimolybdenum tetraacetate (MoMo-Methyl) is grown on a Cu(111) surface with submonolayer coverage. Scanning tunneling microscopy experiments reveal that the compound forms two different structural phases on the Cu surface, whose ratio can be reversibly controlled by changing the sample temperature. The so-called chain-phase is characterized by tilted MoMo dimers bonded to the Cu surface via the methyl groups. In the so-called mesh-phase, on the other hand, the molecules adsorb in a flat lying adsorption configuration with one of the Mo-atoms in direct contact with the Cu surface. Crucially, the different structural properties of the two phases reflect the different inter- and intramolecular interactions between the Mo metal centers, as well as the different interactions between Mo and the Cu surface atoms. In this way, the structural changes result in a modification of the cooperative effects in the system. Therefore, it is proposed that the observed reversible structural phase transition could be used to control the strength of cooperative effects in MoMo-Methyl on Cu(111). © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201703544
  • 2018 • 53 Influence of composition and crystal structure on the fracture toughness of NbCo2 Laves phase studied by micro-cantilever bending tests
    Luo, W. and Kirchlechner, C. and Fang, X. and Brinckmann, S. and Dehm, G. and Stein, F.
    Materials and Design 145 116-121 (2018)
    Cubic and hexagonal NbCo2 Laves phases are known to have composition dependent hardness and yield strength. However, it is unknown whether this dependence is also reflected in their fracture toughness values. In order to elucidate the fracture behavior, single-crystalline micro-cantilevers of the cubic and hexagonal NbCo2 Laves phases having different compositions were fabricated in the diffusion layers grown by the diffusion couple technique. Micro-cantilever bending tests were performed to study the composition- and crystal-structure-dependence of the fracture toughness. To exclude the influence of micro-cantilever geometry, pentagonal and rectangular beams were tested and found to result in the same fracture toughness value. The present results reveal that neither a change of the crystal structure nor a change in chemical composition has a significant influence on the fracture toughness of NbCo2 Laves phase. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2018.02.045
  • 2018 • 52 Multiscale Characterization of Microstructure in Near-Surface Regions of a 16MnCr5 Gear Wheel After Cyclic Loading
    Medghalchi, S. and Jamebozorgi, V. and Bala Krishnan, A. and Vincent, S. and Salomon, S. and Basir Parsa, A. and Pfetzing, J. and Kostka, A. and Li, Y. and Eggeler, G. and Li, T.
    JOM 1-7 (2018)
    The dependence of the microstructure on the degree of deformation in near-surface regions of a 16MnCr5 gear wheel after 2.1 × 106 loading cycles has been investigated by x-ray diffraction analysis, transmission electron microscopy, and atom probe tomography. Retained austenite and large martensite plates, along with elongated lamella-like cementite, were present in a less deformed region. Comparatively, the heavily deformed region consisted of a nanocrystalline structure with carbon segregation up to 2 at.% at grain boundaries. Spheroid-shaped cementite, formed at the grain boundaries and triple junctions of the nanosized grains, was enriched with Cr and Mn but depleted with Si. Such partitioning of Cr, Mn, and Si was not observed in the elongated cementite formed in the less deformed zone. This implies that rolling contact loading induced severe plastic deformation as well as a pronounced annealing effect in the active contact region of the toothed gear during cyclic loading. © 2018 The Minerals, Metals & Materials Society
    view abstractdoi: 10.1007/s11837-018-2931-z
  • 2018 • 51 Operando Raman spectroscopy on CO2 methanation over alumina-supported Ni, Ni3Fe and NiRh0.1 catalysts: Role of carbon formation as possible deactivation pathway
    Mutz, B. and Sprenger, P. and Wang, W. and Wang, D. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 556 160-171 (2018)
    The methanation of CO2, as a part of the power-to-gas concept, was studied under various industrially relevant feed compositions with a focus on the formation and influence of carbonaceous species. For this purpose, 5 wt.% Ni/Al2O3, 5 wt.% Ni3Fe/Al2O3 and 3.4 wt.% NiRh0.1/Al2O3 catalysts were prepared and characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). During the methanation of CO2, the Ni3Fe catalyst emerged as the most active and selective catalyst in the mid-temperature regime (300–350 °C). At 400 °C, all three tested catalysts showed high conversion of CO2 (67–75%; Ni &gt; Ni3Fe &gt; NiRh0.1) and selectivity towards CH4 (95–98%). Operando Raman spectroscopy was applied to elucidate the possible influence of carbonaceous species on the performance of the catalysts. Notably, no carbon deposition was observed under various feed compositions, even in CO2 or CO2/CH4 mixtures, e.g. as provided by biogas plants. Only in pure CH4 atmosphere an intensive carbon deposition with graphitic structure occurred as uncovered by operando Raman spectroscopy. Experiments in the lab-scale reactor and a spectroscopic microreactor could be correlated and revealed a strong catalytic deactivation of the carbon covered catalysts including a pronounced shift of the selectivity towards CO. The initial activity could be recovered after reactivation in H2 at elevated temperatures, which led to a removal of the deposits especially from the metal particles. Raman spectroscopy, supported by the results from high-resolution transmission electron microscopy (HRTEM) and EELS, revealed that carbon remained on the support material. The latter did not have any significant influence on the catalytic activity and could be removed in an oxidizing atmosphere. © 2018
    view abstractdoi: 10.1016/j.apcata.2018.01.026
  • 2018 • 50 Microstructure and mechanical properties in the thin film system Cu-Zr
    Oellers, T. and Raghavan, R. and Chakraborty, J. and Kirchlechner, C. and Kostka, A. and Liebscher, C.H. and Dehm, G. and Ludwig, Al.
    Thin Solid Films 645 193-202 (2018)
    A composition-spread Cu-Zr thin film library with Zr contents from 2.5 up to 6.5 at.% was synthesized by magnetron sputtering on a thermally oxidized Si wafer. The compositional and microstructural variations of the Cu-Zr thin film across the composition gradient were examined using energy dispersive X-ray spectroscopy, X-ray diffraction, and high-resolution scanning and transmission electron microscopy of cross-sections fabricated by focused ion beam milling. Composition-dependent hardness and elastic modulus values were obtained by nanoindentation for measurement areas with discrete Zr contents along the composition gradient. Similarly, the electrical resistivity was investigated by 4-point resistivity measurements to study the influence of Zr composition and microstructural changes in the thin film. Both, the mechanical and electrical properties reveal a significant increase in hardness and resistivity with increasing Zr content. The trends of the mechanical and functional properties are discussed with respect to the local microstructure and composition of the thin film library. © 2017
    view abstractdoi: 10.1016/j.tsf.2017.10.030
  • 2018 • 49 Elemental segregation to twin boundaries in a MnAl ferromagnetic Heusler alloy
    Palanisamy, D. and Raabe, D. and Gault, B.
    Scripta Materialia 155 144-148 (2018)
    Electron microscopy and atom probe tomography were combined to investigate the crystallography and chemistry of a single twin boundary (TB) in a rare-earth-free ferromagnetic MnAl Heusler alloy. The results establish a significant segregation of Mn along the twin boundaries. An enrichment of approx. ~8 at.% Mn was measured along the twin boundary with a confined depletion outside the twin boundary, suggesting short range solute diffusion occurring during massive transformation. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.06.037
  • 2018 • 48 Segregation Phenomena in Size-Selected Bimetallic CuNi Nanoparticle Catalysts
    Pielsticker, L. and Zegkinoglou, I. and Divins, N.J. and Mistry, H. and Chen, Y.-T. and Kostka, A. and Boscoboinik, J.A. and Cuenya, B.R.
    Journal of Physical Chemistry B 122 919-926 (2018)
    Surface segregation, restructuring, and sintering phenomena in size-selected copper-nickel nanoparticles (NPs) supported on silicon dioxide substrates were systematically investigated as a function of temperature, chemical state, and reactive gas environment. Using near-ambient pressure (NAP-XPS) and ultrahigh vacuum X-ray photoelectron spectroscopy (XPS), we showed that nickel tends to segregate to the surface of the NPs at elevated temperatures in oxygen- or hydrogen-containing atmospheres. It was found that the NP pretreatment, gaseous environment, and oxide formation free energy are the main driving forces of the restructuring and segregation trends observed, overshadowing the role of the surface free energy. The depth profile of the elemental composition of the particles was determined under operando CO2 hydrogenation conditions by varying the energy of the X-ray beam. The temperature dependence of the chemical state of the two metals was systematically studied, revealing the high stability of nickel oxides on the NPs and the important role of high valence oxidation states in the segregation behavior. Atomic force microscopy (AFM) studies revealed a remarkable stability of the NPs against sintering at temperatures as high as 700 °C. The results provide new insights into the complex interplay of the various factors which affect alloy formation and segregation phenomena in bimetallic NP systems, often in ways different from those previously known for their bulk counterparts. This leads to new routes for tuning the surface composition of nanocatalysts, for example, through plasma and annealing pretreatments. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.7b06984
  • 2018 • 47 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 • 46 Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications
    Scheibel, F. and Gottschall, T. and Taubel, A. and Fries, M. and Skokov, K.P. and Terwey, A. and Keune, W. and Ollefs, K. and Wende, H. and Farle, M. and Acet, M. and Gutfleisch, O. and Gruner, M.E.
    Energy Technology 6 1397-1428 (2018)
    Magnetic refrigeration relies on a substantial entropy change in a magnetocaloric material when a magnetic field is applied. Such entropy changes are present at first-order magnetostructural transitions around a specific temperature at which the applied magnetic field induces a magnetostructural phase transition and causes a conventional or inverse magnetocaloric effect (MCE). First-order magnetostructural transitions show large effects, but involve transitional hysteresis, which is a loss source that hinders the reversibility of the adiabatic temperature change ΔTad. However, reversibility is required for the efficient operation of the heat pump. Thus, it is the mastering of that hysteresis that is the key challenge to advance magnetocaloric materials. We review the origin of the large MCE and of the hysteresis in the most promising first-order magnetocaloric materials such as Ni–Mn-based Heusler alloys, FeRh, La(FeSi)13-based compounds, Mn3GaC antiperovskites, and Fe2P compounds. We discuss the microscopic contributions of the entropy change, the magnetic interactions, the effect of hysteresis on the reversible MCE, and the size- and time-dependence of the MCE at magnetostructural transitions. © 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/ente.201800264
  • 2018 • 45 Computationally Efficient Phase-field Simulation Studies Using RVE Sampling and Statistical Analysis
    Schwarze, C. and Darvishi Kamachali, R. and Kühbach, M. and Mießen, C. and Tegeler, M. and Barrales-Mora, L. and Steinbach, I. and Gottstein, G.
    Computational Materials Science 147 204-216 (2018)
    For large-scale phase-field simulations, the trade-off between accuracy and computational cost as a function of the size and number of simulations was studied. For this purpose, a large reference representative volume element (RVE) was incrementally subdivided into smaller solitary samples. We have considered diffusion-controlled growth and early ripening of δ′ (Al3Li) precipitate in a model Al-Li system. The results of the simulations show that decomposition of reference RVE can be a valuable computational technique to accelerate simulations without a substantial loss of accuracy. In the current case study, the precipitate number density was found to be the key controlling parameter. For a pre-set accuracy, it turned out that large-scale simulations of the reference RVE can be replaced by simulating a combination of smaller solitary samples. This shortens the required simulation time and improves the memory usage of the simulation considerably, and thus substantially increases the efficiency of massive parallel computation for phase-field applications. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2018.02.005
  • 2018 • 44 An examination of interactions between temperature, pressure, sintering time, and Si/W ratio on the sintering behavior of CrSiW
    Tillmann, W. and Fehr, A. and Ferreira, M. and Stangier, D.
    International Journal of Refractory Metals and Hard Materials 73 146-156 (2018)
    A Statistic Design of Experiments (DoE) was implemented with the aim to systematically investigate sintering parameter interactions of hot pressed 80Cr10Si10W. By varying the temperature between 999 and 1200 °C at pressures ranging from 2 to 8 MPa for 6.6 to 23.4 min, an initial examination of the ternary system is realized. The overall objective of this study is to minimize the porosity and to foster a diffusion of the elements. The investigations revealed that high temperatures (&gt;1000 °C) and pressures (&gt;6 MPa) support the diffusion between chromium and silicon while tungsten particles accumulate at the grain boundaries of silicon. XRD analyses confirmed the existence of a c-CrSi3 phase. The setup of the DoE, which promises the highest densification, was subsequently used to examine the influence of the silicon and tungsten content on the porosity according to the pattern 80Cr(20-x)Si(x)W for 0 ≥ x ≥ 20 (in wt%). This approach generated the lowest porosity (3.2 ± 0.93 area%) for 80Cr20W and led to homogenous particle distributions. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijrmhm.2018.02.012
  • 2018 • 43 Optimization of the deposition parameters of Ni-20Cr thin films on thermally sprayed Al2O3 for sensor application
    Tillmann, W. and Kokalj, D. and Stangier, D.
    Surface and Coatings Technology 344 223-232 (2018)
    Ni-Cr (80–20 at.-%) thin films were synthesized by means of DC magnetron sputtering on thermally sprayed Al2O3 insolating coatings. In view of the application as a thin-film thermocouple, the ceramic layer provides an electrical as well a thermal insulation. The thin film must feature a good adhesion as well as a good electrical conductivity to be suitable as a thin-film thermocouple. Design of Experiments was utilized to determine if deposition parameters including the bias-voltage, heating power, and chamber pressure have a significant influence regarding critical scratch load, hardness to Young's modulus ratio, and electrical resistance. It was found out that the bias-voltage is the most important influencing factor. In order to extract the significant parameters, the morphology and topography were analyzed by means of SEM. The structural properties were obtained by means of XRD on an atomic level, using synchrotron radiation. Mechanical properties, such as the hardness and Young's modulus, as well the scratch load were evaluated using nanoindentation and a scratch-tester. To verify the findings, an optimized thin film was deposited, which showed an increased layer adhesion and a significantly lower electrical resistance. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2018.03.029
  • 2018 • 42 Influence of Cr-Content on the thermoelectric and mechanical properties of NiCr thin film thermocouples synthesized on thermally sprayed Al2O3
    Tillmann, W. and Kokalj, D. and Stangier, D. and Schöppner, V. and Benis, H.B. and Malatyali, H.
    Thin Solid Films 663 148-158 (2018)
    Ni/Ni-Cr thin film thermocouples were deposited by means of DC magnetron sputtering for the application in plastic extrusion dies. In order to ensure the application of the thermocouples on complex surfaces, a thermally sprayed Al2O3 layer was used between the substrate and the thin films for thermal insulation and electrical isolation. The study includes the deposition, characterization, calibration, and validation of thin film thermocouples. The influence of the composition (10 at.-%, 20 at.-% and 30 at.-% Cr) was analyzed concerning mechanical and electrical properties. It was found out that the electrical resistance increases with the temperature and the Cr-content due to a decreasing crystallite size. By means of a scratch-tester and nanoindentation, a higher scratch load and hardness were observed for larger Cr-contents on the thermally sprayed Al2O3 coating. Despite the composition, the influence of the size as well as the thickness of the thermocouples were analyzed regarding the Seebeck coefficient. Independent from the size and Cr-content, all thermocouples feature a comparable Seebeck coefficient of about 40 μV/K. This high coefficient is similar to that of wire thermocouples, showing a low number of defects in the thin films. The ceramic coating features a thermal insulation of about 20 °C at 200 °C, allowing a more accurate measurement of the ambient medium. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2018.08.023
  • 2018 • 41 How the crystal structure and phase segregation of Au-Fe alloy nanoparticles are ruled by the molar fraction and size
    Tymoczko, A. and Kamp, M. and Prymak, O. and Rehbock, C. and Jakobi, J. and Schürmann, U. and Kienle, L. and Barcikowski, S.
    Nanoscale 10 16434-16437 (2018)
    The application of an Au-Fe nanoalloy is determined by its internal phase structure. Our experimental and theoretical findings explain how the prevalence of either a core-shell or a disordered solid solution structure is ruled by the target composition and the particle diameter. Furthermore, we found metastable phases not predefined by the bulk phase diagram. © The Royal Society of Chemistry 2018.
    view abstractdoi: 10.1039/c8nr03962c
  • 2017 • 40 Thermally activated lightweight actuator based on hot extruded shape memory metal matrix composites (SMA-MMC)
    Dahnke, C. and Shapovalov, A. and Tekkaya, A.E.
    Procedia Engineering 207 1511-1516 (2017)
    Based on modified porthole dies, continuous composite extrusion allows the manufacturing of shape memory alloy metal matrix composites (SMA-MMC). In order to extend the functionality of SMA-MMC, the present work shows a new actuator concept based on integrated NiTi wires within an aluminum matrix. Due to an eccentric positioning of the wires as well as a prestraining, a bending moment is generated within the profile when the temperature is increased to a critical value. Depending on the process parameters, the occurring bending moment is able to cause an elastic or elastic-plastic deformation of the profile or specimen. © 2017 The Authors. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2017.10.1083
  • 2017 • 39 Iron-Induced Activation of Ordered Mesoporous Nickel Cobalt Oxide Electrocatalyst for the Oxygen Evolution Reaction
    Deng, X. and Öztürk, S. and Weidenthaler, C. and Tüysüz, H.
    ACS Applied Materials and Interfaces 9 21225-21233 (2017)
    Herein, ordered mesoporous nickel cobalt oxides prepared by the nanocasting route are reported as highly active oxygen evolution reaction (OER) catalysts. By using the ordered mesoporous structure as a model system and afterward elevating the optimal catalysts composition, it is shown that, with a simple electrochemical activation step, the performance of nickel cobalt oxide can be significantly enhanced. The electrochemical impedance spectroscopy results indicated that charge transfer resistance increases for Co3O4 spinel after an activation process, while this value drops for NiO and especially for CoNi mixed oxide significantly, which confirms the improvement of oxygen evolution kinetics. The catalyst with the optimal composition (Co/Ni 4/1) reaches a current density of 10 mA/cm2 with an overpotential of a mere 336 mV and a Tafel slope of 36 mV/dec, outperforming benchmarked and other reported Ni/Co-based OER electrocatalysts. The catalyst also demonstrates outstanding durability for 14 h and maintained the ordered mesoporous structure. The cyclic voltammograms along with the electrochemical measurements in Fe-free KOH electrolyte suggest that the activity boost is attributed to the generation of surface Ni(OH)2 species that incorporate Fe impurities from the electrolyte. The incorporation of Fe into the structure is also confirmed by inductively coupled plasma optical emission spectrometry. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b02571
  • 2017 • 38 Electron-induced photon emission above the quantum cutoff due to time-energy uncertainty
    Ekici, E. and Kapitza, P. and Bobisch, C.A. and Möller, R.
    Optics Letters 42 4585-4588 (2017)
    The light emission from a tunneling junction induced by tunneling electrons has been studied around the cutoff at hν eVt. The emitted photons are found to exceed the excitation energy provided by the energy of the tunneling electrons. The experiments have been performed by a low-temperature scanning tunneling microscope at 80 K for an Ag(111) surface and an Ag-covered PtIr tip. A detailed analysis of the emission spectra reveals that the findings cannot be explained by the thermal broadening of the electron Fermi distribution alone. However, a correct description is found if a finite lifetime of the excited states in the range of 30–80 fs is included. © 2017 Optical Society of America.
    view abstractdoi: 10.1364/OL.42.004585
  • 2017 • 37 Unraveling compositional effects on the light-induced oxygen evolution in Bi(V-Mo-X)O4 material libraries
    Gutkowski, R. and Khare, C. and Conzuelo, F. and Kayran, Y.U. and Ludwig, Al. and Schuhmann, W.
    Energy and Environmental Science 10 1213-1221 (2017)
    The influence of co-deposited transition metals X (X = Ta, W, Nb) with various relative concentrations on the photoelectrochemical performance of BiVO4 is investigated. Thin film material libraries with well-defined composition gradients of Bi, V and two transition metals are fabricated by combinatorial sputter co-deposition. Materials with the highest photoelectrochemical performance are identified by high-throughput characterization of the Bi(V-Mo-X)O4 material libraries using an optical scanning droplet cell. Bi(V-Mo-W)O4 and Bi(V-Mo-Nb)O4 material libraries show the highest improvement in the photocurrent, with ten times higher photocurrents of up to 1 mA cm-2 compared to a BiVO4 reference material library. Deviations from the V:Bi equiatomic ratio lead to a decrease in the photocurrent for pristine monoclinic BiVO4. By the addition of transition metals this effect is minimized and no significant decrease in the photocurrent occurs up to 10 at% variation from the equiatomic V:Bi ratio. Excellent photoelectrochemical performance is reached under these conditions in regions with a V:Bi atomic ratio of 70:30 and co-deposited Nb concentrations of &gt;10 at%. Scanning photoelectrochemical microscopy allows the evaluation of the correlation between the generated oxygen at a photoanode and the measured photocurrent. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ee00287d
  • 2017 • 36 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 • 35 Electronic and molecular structure relations in diiron compounds mimicking the [FeFe]-hydrogenase active site studied by X-ray spectroscopy and quantum chemistry
    Kositzki, R. and Mebs, S. and Schüth, N. and Leidel, N. and Schwartz, L. and Karnahl, M. and Wittkamp, F. and Daunke, D. and Grohmann, A. and Apfel, U.-P. and Gloaguen, F. and Ott, S. and Haumann, M.
    Dalton Transactions 46 12544-12557 (2017)
    Synthetic diiron compounds of the general formula Fe2(μ-S2R)(CO)n(L)6-n (R = alkyl or aromatic groups; L = CN- or phosphines) are versatile models for the active-site cofactor of hydrogen turnover in [FeFe]-hydrogenases. A series of 18 diiron compounds, containing mostly a dithiolate bridge and terminal ligands of increasing complexity, was characterized by X-ray absorption and emission spectroscopy in combination with density functional theory. Fe K-edge absorption and Kβ main-line emission spectra revealed the varying geometry and the low-spin state of the Fe(i) centers. Good agreement between experimental and calculated core-to-valence-excitation absorption and radiative valence-to-core-decay emission spectra revealed correlations between spectroscopic and structural features and provided access to the electronic configuration. Four main effects on the diiron core were identified, which were preferentially related to variation either of the dithiolate or of the terminal ligands. Alteration of the dithiolate bridge affected mainly the Fe-Fe bond strength, while more potent donor substitution and ligand field asymmetrization changed the metal charge and valence level localization. In contrast, cyanide ligation altered all relevant properties and, in particular, the frontier molecular orbital energies of the diiron core. Mutual benchmarking of experimental and theoretical parameters provides guidelines to verify the electronic properties of related diiron compounds. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7dt02720f
  • 2017 • 34 Micromagnetic Simulations for Coercivity Improvement Through Nano-Structuring of Rare-Earth-Free L10-FeNi Magnets
    Kovacs, A. and Fischbacher, J. and Oezelt, H. and Schrefl, T. and Kaidatzis, A. and Salikhov, R. and Farle, M. and Giannopoulos, G. and Niarchos, D.
    IEEE Transactions on Magnetics 53 (2017)
    In this paper, we investigate the potential of tetragonal L10-ordered FeNi as the candidate phase for rare-earth-free permanent magnets considering anisotropy values from recently synthesized, partially ordered FeNi thin films. In particular, we estimate the maximum energy product (BH)max of L10-FeNi nanostructures using micromagnetic simulations. The maximum energy product is limited due to the small coercive field of partially ordered L10-FeNi. Nano-structured magnets consisting of 128 equi-axed, platelet-like, and columnar-shaped grains show a theoretical maximum energy product of 228, 208, and 252 kJm $^{-3}$ , respectively. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2017.2701418
  • 2017 • 33 Potential of an alumina-supported Ni3Fe catalyst in the methanation of CO2: Impact of alloy formation on activity and stability
    Mutz, B. and Belimov, M. and Wang, W. and Sprenger, P. and Serrer, M.A. and Wang, D. and Pfeifer, P. and Kleist, W. and Grunwaldt, J.-D.
    ACS Catalysis 7 6802-6814 (2017)
    A promising bimetallic 17 wt % Ni3Fe catalyst supported on γ-Al2O3 was prepared via homogeneous deposition-precipitation for the application in the methanation of CO2 to gather more detailed insight into the structure and performance of the catalyst compared to state-of-the-art methanation systems. X-ray diffraction (XRD) analysis, detailed investigations using scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy analysis (EDX) of single particles as well as larger areas, high-resolution transmission electron microscopy (HRTEM) imaging, temperature-programmed reduction (H2-TPR), and in-depth interpretation of Raman bands led to the conclusion that a high fraction of the Ni and Fe formed the desired Ni3Fe alloy resulting in small and well-defined nanoparticles with 4 nm in size and a dispersion of 24%. For comparison, a monometallic catalyst with similar dispersion using the same preparation method and analysis was prepared. Using a fixed-bed reactor, the Ni3Fe catalyst showed better low-temperature performance compared to a monometallic Ni reference catalyst, especially at elevated pressures. Longterm experiments in a microchannel packed bed reactor under industrially relevant reaction conditions in competition with a commercial Ni-based methanation catalyst revealed an improved performance of the Ni3Fe system at 358°C and 6 bar involving enhanced conversion of CO2 to 71%, selectivity to CH4 &gt; 98%, and most notably a high stability. Deactivation occurred only at lower temperatures, which was related to carbon deposition due to an increased CO production. Kinetic measurements were compared with literature models derived for Ni/Al2O3 catalysts, which fit well but underestimate the performance of the Ni3Fe system, emphasizing the synergetic effect of Ni and Fe. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01896
  • 2017 • 32 High-throughput study of binary thin film tungsten alloys
    Nikolić, V. and Wurster, S. and Savan, A. and Ludwig, Al. and Pippan, R.
    International Journal of Refractory Metals and Hard Materials 69 40-48 (2017)
    Combinatorial magnetron co-sputtering from elemental sources was applied to produce W-alloy thin film composition spread materials libraries with well-defined, continuous composition gradients (film thicknesses between 1 and 2.5 μm). Three systems were studied: W-Fe (0–7 at.%), W-Ti (0–15 at.%) and W-Ir (0–12 at.%). High-throughput characterization of the materials libraries comprised of chemical, morphological and microstructural analyses. Scanning electron microscope investigations revealed that the films have a columnar structure of inverted cone-like units separated by voided boundaries, with a strong correlation to the alloying element content. Significant morphological changes occurred with an increase in the amount of the added element; W films with lower at.% of the alloying element had higher density and tighter grain boundaries, altering towards an increased amount of voids as the concentration of the alloying element increased. Electron backscatter diffraction scanning was used to determine microstructural components (grain size, grain shape, texture evolution), in dependence on the concentration of the alloying element. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijrmhm.2017.07.017
  • 2017 • 31 Gold-Palladium Bimetallic Catalyst Stability: Consequences for Hydrogen Peroxide Selectivity
    Pizzutilo, E. and Freakley, S.J. and Cherevko, S. and Venkatesan, S. and Hutchings, G.J. and Liebscher, C.H. and Dehm, G. and Mayrhofer, K.J.J.
    ACS Catalysis 7 5699-5705 (2017)
    During application, electrocatalysts are exposed to harsh electrochemical conditions, which can induce degradation. This work addresses the degradation of AuPd bimetallic catalysts used for the electrocatalytic production of hydrogen peroxide (H2O2) by the oxygen reduction reaction (ORR). Potential-dependent changes in the AuPd surface composition occur because the two metals have different dissolution onset potentials, resulting in catalyst dealloying. Using a scanning flow cell (SFC) with an inductively coupled plasma mass spectrometer (ICP-MS), simultaneous Pd and/or Au dissolution can be observed. Thereafter, three accelerated degradation protocols (ADPs), simulating different dissolution regimes, are employed to study the catalyst structure degradation on the nanoscale with identical location (IL) TEM. When only Pd or both Au and Pd dissolve, the composition changes rapidly and the surface becomes enriched with Au, as observed by cyclic voltammetry and elemental mapping. Such changes are mirrored by the evolution of electrocatalytic performances toward H2O2 production. Our experimental findings are finally summarized in a dissolution/structure/selectivity mechanism, providing a clear picture of the degradation of bimetallic catalyst used for H2O2 synthesis. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01447
  • 2017 • 30 The Space Confinement Approach Using Hollow Graphitic Spheres to Unveil Activity and Stability of Pt-Co Nanocatalysts for PEMFC
    Pizzutilo, E. and Knossalla, J. and Geiger, S. and Grote, J.-P. and Polymeros, G. and Baldizzone, C. and Mezzavilla, S. and Ledendecker, M. and Mingers, A. and Cherevko, S. and Schüth, F. and Mayrhofer, K.J.J.
    Advanced Energy Materials 7 (2017)
    The performance of polymer electrolyte fuel cells is strongly correlated to the electrocatalytic activity and stability. In particular, the latter is the result of an interplay between different degradation mechanisms. The essential high stability, demanded for real applications, requires the synthesis of advanced electrocatalysts that withstand the harsh operation conditions. In the first part of this study, the synthesis of oxygen reduction electrocatalysts consisting of Pt-Co (i.e., Pt5Co, Pt3Co, and PtCo) alloyed nanoparticles encapsulated in the mesoporous shell of hollow graphitic spheres (HGS) is reported. The mass activities of the activated catalysts depend on the initial alloy composition and an activity increase on the order of two to threefold, compared to pure Pt@HGS, is achieved. The key point of the second part is the investigation of the degradation of PtCo@HGS (showing the highest activity). Thanks to pore confinement, the impact of dissolution/dealloying and carbon corrosion can be studied without the interplay of other degradation mechanisms that would induce a substantial change in the particle size distribution. Therefore, impact of the upper potential limit and the scan rates on the dealloying and electrochemical surface area evolution can be examined in detail. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/aenm.201700835
  • 2017 • 29 The roles of Co-precipitation pH, phase-purity and alloy formation for the ammonia decomposition activity of Ga-promoted Fe/MgO catalysts
    Rein, D. and Friedel Ortega, K. and Weidenthaler, C. and Bill, E. and Behrens, M.
    Applied Catalysis A: General 548 52-61 (2017)
    A series of mesoporous MgFe1.75Ga0.25O4 mixed spinel oxides obtained upon calcination of hydrotalcite-like precursors was investigated in the ammonia decomposition reaction at 1 atm after reduction in H2 atmosphere. The corresponding precursors were synthesized from metal salt solutions at five constant pH values in the range between 8.5 and 10.5 by co-precipitation in aqueous media to study the impact of pH variation on the catalyst's structure and activity. N2 physisorption, thermogravimetric analysis, powder X-ray diffraction, Mössbauer spectroscopy, and temperature programmed techniques (H2-TPR and NH3-TPD) were applied to gather information about the textural, (micro-)structural, and adsorption properties of the samples. While phase purity in the precursor and oxide stages is only observed for pH = 10, undesired by-phases (MgFe2O4 and/or Fe3O4) are additionally formed during co-precipitation at the remaining pH values. This is partly related to an incomplete precipitation of Mg2+ cations in less alkaline environments. In situ XRD measurements during reduction revealed that Fe-Ga alloys are formed between 500 and 600 °C. The absence of by-phases avoids the formation of α-Fe, thus improving the structural and compositional homogeneity of the nitridated samples. This beneficial effect is reflected by the low activation energy (70 kJ/mol) and the enhanced low temperature activity (&lt;450 °C) of the phase pure material in the catalytic decomposition of ammonia. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2017.09.004
  • 2017 • 28 Spinel-Structured ZnCr2O4 with Excess Zn Is the Active ZnO/Cr2O3 Catalyst for High-Temperature Methanol Synthesis
    Song, H. and Laudenschleger, D. and Carey, J.J. and Ruland, H. and Nolan, M. and Muhler, M.
    ACS Catalysis 7 7610-7622 (2017)
    A series of ZnO/Cr2O3 catalysts with different Zn:Cr ratios was prepared by coprecipitation at a constant pH of 7 and applied in methanol synthesis at 260-300 °C and 60 bar. The X-ray diffraction (XRD) results showed that the calcined catalysts with ratios from 65:35 to 55:45 consist of ZnCr2O4 spinel with a low degree of crystallinity. For catalysts with Zn:Cr ratios smaller than 1, the formation of chromates was observed in agreement with temperature-programmed reduction results. Raman and XRD results did not provide evidence for the presence of segregated ZnO, indicating the existence of Zn-rich nonstoichiometric Zn-Cr spinel in the calcined catalyst. The catalyst with Zn:Cr = 65:35 exhibits the best performance in methanol synthesis. The Zn:Cr ratio of this catalyst corresponds to that of the Zn4Cr2(OH)12CO3 precursor with hydrotalcite-like structure obtained by coprecipitation, which is converted during calcination into a nonstoichiometric Zn-Cr spinel with an optimum amount of oxygen vacancies resulting in high activity in methanol synthesis. Density functional theory calculations are used to examine the formation of oxygen vacancies and to measure the reducibility of the methanol synthesis catalysts. Doping Cr into bulk and the (10-10) surface of ZnO does not enhance the reducibility of ZnO, confirming that Cr:ZnO cannot be the active phase. The (100) surface of the ZnCr2O4 spinel has a favorable oxygen vacancy formation energy of 1.58 eV. Doping this surface with excess Zn charge-balanced by oxygen vacancies to give a 60% Zn content yields a catalyst composed of an amorphous ZnO layer supported on the spinel with high reducibility, confirming this as the active phase for the methanol synthesis catalyst. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b01822
  • 2017 • 27 On the competition between the stress-induced formation of martensite and dislocation plasticity during crack propagation in pseudoelastic NiTi shape memory alloys
    Ungár, T. and Frenzel, J. and Gollerthan, S. and Ribárik, G. and Balogh, L. and Eggeler, G.
    Journal of Materials Research 1-10 (2017)
    The present work addresses the competition between dislocation plasticity and stress-induced martensitic transformations in crack affected regions of a pseudoelastic NiTi miniature compact tension specimen. For this purpose X-ray line profile analysis was performed after fracture to identify dislocation densities and remnant martensite volume fractions in regions along the crack path. Special emphasis was placed on characterizing sub fracture surface zones to obtain depth profiles. The stress affected zone in front of the crack-tip is interpreted in terms of a true plastic zone associated with dislocation plasticity and a pseudoelastic zone where stress-induced martensite can form. On unloading, most of the stress-induced martensite transforms back to austenite but a fraction of it is stabilized by dislocations in both, the irreversible martensite and the surrounding austenite phase. The largest volume fraction of the irreversible or remnant martensite along with the highest density of dislocations in this phase was found close to the primary crack-tip. With increasing distance from the primary crack-tip both, the dislocation density and the volume fraction of irreversible martensite decrease to lower values. Copyright © Materials Research Society 2017
    view abstractdoi: 10.1557/jmr.2017.267
  • 2017 • 26 Doubling of the magnetic energy product in ferromagnetic nanowires at ambient temperature by capping their tips with an antiferromagnet
    Wang, F.Z. and Salikhov, R. and Spasova, M. and Liébana-Viñas, S. and Bran, C. and Chen, Y.-S. and Vazquez, M. and Farle, M. and Wiedwald, U.
    Nanotechnology 28 (2017)
    We present an approach to prepare free-standing tips of micrometer-long nanowires electrodeposited in anodic aluminum oxide nanopores. Such open tips can be further processed, e.g. for vertical interconnects of functional layers or for tailoring the magnetization reversal of ferromagnetic nanowires. The magnetic switching of nanowires is usually initiated by vortex or domain formation at the nanowire tips. We show that coating the tips of Fe30Co70 nanowires (diameter 40 nm, length 16 μm) with thin antiferromagnetic Fe50Mn50 capping layers (thickness ≈10 nm) leads to magnetic hardening with a more than doubled energy product at ambient temperature. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/aa77b7
  • 2017 • 25 Elastically confined martensitic transformation at the nano-scale in a multifunctional titanium alloy
    Wang, H.L. and Hao, Y.L. and He, S.Y. and Li, T. and Cairney, J.M. and Wang, Y.D. and Wang, Y. and Obbard, E.G. and Prima, F. and Du, K. and Li, S.J. and Yang, R.
    Acta Materialia 135 330-339 (2017)
    A martensitic transformation (MT) is a typical first-order diffusionless crystal structural change with strong autocatalysis like avalanche at a speed of sound propagation. This unique characteristic, however, is undetectable in some multifunctional titanium alloys. Recently, a nano-scale elastically confined MT mechanism was proposed because a nano-scale Nb modulation in a Ti-Nb based alloy was observed. Here we analyze the elastic confinement in details and its induced novel properties in a wide temperature range. The statistical analyses of atom probe tomography (APT) data confirm the existence of the nano-scale Nb concentration modulation. The synchrotron X-ray diffraction (SXRD) profiles demonstrate that the nano-scale Nb modulation causes weak diffuse scattering, as evidenced by the extreme broad diffraction bands. The tensile tests find a critical temperature of ∼150 K, where the critical stress to induce the MT and Young's modulus reach the minimum and the superelastic strain reaches the maximum (∼4.5%) and keeps constant as the temperature decreases further to <4.2 K. To reveal these abnormal behaviors of the MT, the Born criterion governing the elastic stability of cubic crystal is modified by introducing an elastic confinement term and a new Clausius-Clapeyron relationship is established to evaluate the elastically confined MT. The results are consistent with the experimental findings, including the solely stress-induced (no thermally induced) reversibility. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.06.040
  • 2017 • 24 Material Testing and Chip Formation Simulation for Different Heat Treated Workpieces of 51CrV4 Steel
    Zabel, A. and Rödder, T. and Tiffe, M.
    Procedia CIRP 58 181-186 (2017)
    The heat treatment has a major impact on the mechanical properties of steel alloys and therefore on the condition of a machining processes. In this paper, the low alloy steel 51CrV4 with different heat treatments is investigated in terms of its mechanical properties under high dynamic conditions using a Split Hopkinson Pressure Bar (SHPB) and by means of orthogonal cutting tests. The latter provide a detailed insight in the ongoing processes during chip formation by analyzing the present microstructure of the generated chips. Furthermore, the obtained data from the SHPB tests is used as an input for material models applied for the simulation of chip formation with the Finite-Element-Method. The results reveal fundamental differences in the chip formation mechanisms between the differently heat treated workpiece materials. © 2017 The Authors.
    view abstractdoi: 10.1016/j.procir.2017.03.218
  • 2017 • 23 A novel type of Co–Ti–Cr-base γ/γ′ superalloys with low mass density
    Zenk, C.H. and Povstugar, I. and Li, R. and Rinaldi, F. and Neumeier, S. and Raabe, D. and Göken, M.
    Acta Materialia 135 244-251 (2017)
    A γ′ strengthened Co–Ti–Cr superalloy is presented with a mass density ∼14 % below that of typical Co–Al–W-based alloys. The lattice misfit is sufficiently low to form coherent cuboidal γ′ precipitates. Atom probe tomography shows that Cr partitions to the γ phase, but increases the γ′ volume fraction compared to a binary Co-Ti alloy to more than 60 %. The solubility of Cr in the γ′ phase is significantly higher than expected from previously published values. The γ′ solvus temperature is above 1100 °C. The yield strength shows a distinct increase above 600 °C surpassing that of Co–9Al–8W (at.%) and conventional Co-base superalloys, even more so when it is normalized by the mass density. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.06.024
  • 2017 • 22 Partial recrystallization of gum metal to achieve enhanced strength and ductility
    Zhang, J.-L. and Tasan, C.C. and Lai, M.J. and Yan, D. and Raabe, D.
    Acta Materialia 135 400-410 (2017)
    Here we present a microstructure design approach which leads to partial recrystallization and nano-precipitation within the same single-step heat treatment. This produces a dual-constituent microstructure in Ti-Nb based gum metal, which consists of nano-ω-particle-rich ultrafine recrystallized grain chains embedded in ω-lean subgrain-containing recovered zones. This partially recrystallized microstructure exhibits an improved strength-ductility combination that surpasses the inverse strength-ductility relationship exhibited by materials with similar composition. The strengthening effects due to precipitates and grain refinement were studied by nanoindentation. The deformation mechanisms of the partially recrystallized material were investigated by in-situ scanning electron microscope tensile tests, micro-strain mapping and post-mortem microstructure characterization. The improved mechanical properties are attributed to the high yield strength of the recrystallized grains and the sequential activation of dislocation slip and dislocation channeling. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.06.051
  • 2017 • 21 In-situ tracking the structural and chemical evolution of nanostructured CuCr alloys
    Zhang, Z. and Guo, J. and Dehm, G. and Pippan, R.
    Acta Materialia 138 42-51 (2017)
    We report the thermal stability of supersaturated CuCr nanocrystallines alloys at the atomic resolution using modern spherical aberration-corrected transmission electron microscopy (TEM) via performing in-situ structural and spectroscopy experiments. It is found that CuCr nanocrystallines are not only subjected to a structural change but also undergo a chemical evolution upon annealing. Chemical destabilization of supersaturated CuCr nanocrystallines occurs at a quite low temperature. Heating triggers a rapid separation of Cu and Cr grains at the forced intermixing zone, accompanied by an obvious decrease of average interface width whereas the grain growth is not significant. Elemental profiles and images recorded in real time reveal that the local compositions vary with heating, which in turn permits to derive the concentration of excess vacancy generated by deformation and observe its evolution with temperature, further to analyze the dynamic behavior in nanocrystalline materials. Electronic structure changes at the interface forced intermixing zone are revealed by the fine structure analysis. The study uncovers the interplay between the thermal stability and chemical decomposition process of bulk nanostructured materials in real-time. © 2017
    view abstractdoi: 10.1016/j.actamat.2017.07.039
  • 2016 • 20 Mg-based compounds for hydrogen and energy storage
    Crivello, J.-C. and Denys, R.V. and Dornheim, M. and Felderhoff, M. and Grant, D.M. and Huot, J. and Jensen, T.R. and de Jongh, P. and Latroche, M. and Walker, G.S. and Webb, C.J. and Yartys, V.A.
    Applied Physics A: Materials Science and Processing 122 1-17 (2016)
    Magnesium-based alloys attract significant interest as cost-efficient hydrogen storage materials allowing the combination of high gravimetric storage capacity of hydrogen with fast rates of hydrogen uptake and release and pronounced destabilization of the metal–hydrogen bonding in comparison with binary Mg–H systems. In this review, various groups of magnesium compounds are considered, including (1) RE–Mg–Ni hydrides (RE = La, Pr, Nd); (2) Mg alloys with p-elements (X = Si, Ge, Sn, and Al); and (3) magnesium alloys with d-elements (Ti, Fe, Co, Ni, Cu, Zn, Pd). The hydrogenation–disproportionation–desorption–recombination process in the Mg-based alloys (LaMg12, LaMg11Ni) and unusually high-pressure hydrides synthesized at pressures exceeding 100 MPa (MgNi2H3) and stabilized by Ni–H bonding are also discussed. The paper reviews interrelations between the properties of the Mg-based hydrides and p–T conditions of the metal–hydrogen interactions, chemical composition of the initial alloys, their crystal structures, and microstructural state. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-016-9601-1
  • 2015 • 19 The crystallographic template effect assisting the formation of stable α-Al2O3 during low temperature oxidation of Fe-Al alloys
    Brito, P. and Pinto, H. and Kostka, A.
    Corrosion Science (2015)
    The role of thermally grown α-Fe2O3 on the nucleation of α-Al2O3 during oxidation of binary Fe-Al alloys with 15 and 26 at.%Al at 700°C was investigated. Surface morphology of the oxide scales indicated direct nucleation of α-Al2O3 preferentially instead of conversion from metastable Al2O3 polymorphs. Oxide scale development over time was also monitored by use of synchrotron X-ray diffraction and Raman spectroscopy. The results showed that the α-Fe2O3 crystal lattice decreases in volume as oxidation progresses, which was found to be consistent with an Al3+ enrichment of α-Fe2O3 as confirmed by the change in relative intensity of α-Fe2O3 Raman peaks. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.corsci.2016.01.007
  • 2015 • 18 On the widths of the hysteresis of mechanically and thermally induced martensitic transformations in Ni-Ti-based shape memory alloys
    Jaeger, S. and Maaß, B. and Frenzel, J. and Schmidt, M. and Ullrich, J. and Seelecke, S. and Schütze, A. and Kastner, O. and Eggeler, G.
    International Journal of Materials Research 106 1029-1039 (2015)
    It is well known that a good crystallographic compatibility between austenite and martensite in Ni-Ti-based shape memory alloys results in narrow thermal hystereses (e. g. Ball and James, Arch. Ration. Mech. Anal., 1987). The present work suggests that a good crystallographic fit is moreover associated with a small mechanical hysteresis width, observed during a forward and reverse stress-induced transformation. Furthermore, shape memory alloys with a good crystallographic fit show smaller transformation strains. The results obtained in the present study suggest that these correlations are generic and apply to binary Ni-Ti (with varying Ni contents) and quaternary Ni-Ti-Cu-X (X = Cr, Fe, V) alloys. For binary Ni-Ti, it was observed that Ni-rich compositions (good lattice fit) show a lower accummulation of irreversible strains during pseudoelastic cycling. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.111284
  • 2015 • 17 Structural stability of Fe-based topologically close-packed phases
    Ladines, A.N. and Hammerschmidt, T. and Drautz, R.
    Intermetallics 59 59-67 (2015)
    Precipitates of topologically close-packed (TCP) phases play an important role in hardening mechanisms of high-performance steels. We analyze the influence of atomic size, electron count, magnetism and external stress on TCP phase stability in Fe-based binary transition metal alloys. Our density-functional theory calculations of structural stability are complemented by an analysis with an empirical structure map for TCP phases. The structural stability and lattice parameters of the Fe-Nb/Mo/V compounds are in good agreement with experiment. The average magnetic moments follow the Slater-Pauling relation to the average number of valence-electrons and can be rationalized in terms of the electronic density of states. The stabilizing effect of the magnetic energy, estimated by additional non-magnetic calculations, increases as the magnetic moment increases with band filling for the binary systems of Fe and early transition metals. For the case of Fe2Nb, we demonstrate that the influence of magnetism and external stress is sufficiently large to alter the energetic ordering of the closely competing Laves phases C14, C15 and C36. We find that the A15 phase is not stabilized by atomic-size differences, while the stability of C14 is increasing with increasing difference in atomic size. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.12.009
  • 2015 • 16 Ab initio study of compositional trends in solid solution strengthening in metals with low Peierls stresses
    Ma, D. and Friák, M. and Von Pezold, J. and Neugebauer, J. and Raabe, D.
    Acta Materialia 98 367-376 (2015)
    Abstract We identify and analyze general trends governing solid solution strengthening in binary alloys containing solutes across the Periodic table using quantum-mechanical calculations. Here we present calculations for the model system of Al binary solid solutions. The identified trends originate from an approximately parabolic dependence of two strengthening parameters to quantitatively predict the solid solution strengthening effect, i.e. the volume and slip misfit parameters. The volume misfit parameter shows a minimum (concave-up behavior) as a function of the solute element group number in the periodic table, whereas the slip misfit parameter shows a maximum (concave-down behavior). By analyzing reported data, a similar trend is also found in Ni and Mg (basal slip) binary systems. Hence, these two strengthening parameters are strongly anti-correlated, which can be understood in terms of the Fermi level shift in the framework of free electron model. The chemical trends identified in this study enable a rapid and efficient identification of the solutes that provide optimum solid-solution strengthening. The approach described here may thus serve as basis for ab initio guided metallurgical materials design. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.054
  • 2015 • 15 Rapid theory-guided prototyping of ductile Mg alloys: From binary to multi-component materials
    Pei, Z. and Friák, M. and Sandlöbes, S. and Nazarov, R. and Svendsen, B. and Raabe, D. and Neugebauer, J.
    New Journal of Physics 17 (2015)
    In order to identify a method allowing for a fast solute assessment without lengthy ab initio calculations, we analyze correlations and anti-correlation between the stacking fault energies (SFEs), which were shown to be related to the macroscopic ductility in Mg alloys, and five material parameters of 18 different elemental solutes. Our analysis reveals that the atomic volume V of pure solutes, their electronegativity ν and bulk modulus B are either linearly or logarithmically related to the SFE. Comparing the impact of solutes with that of yttrium (that increases the ductility in Mg) we propose a single numerical quantity (called yttrium similarity index, YSI) that is based on these inter-relations. Subsequently, we evaluate this new figure of merit for 76 elements from the periodic table of elements in search for solutes reducing the SFE. Limiting ourselves first to binary Mg alloys, we hardly find any alternative solutes providing similar reduction as that due to rare-earth (RE) additions. Therefore, we extended our search to ternary Mg alloys. Assuming that the physical properties of solute combinations can be represented by their average values, 2850 solute combinations were checked and 133 solute pairs (not including any RE elements) have been found to have a YSI larger than 0.85. Quantum-mechanical calculations have been subsequently performed for 11 solute pairs with YSIs higher than 0.95 and they were all found to reduce the in excellent agreement with the predictions based on the YSI. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/17/9/093009
  • 2015 • 14 Incorporating the CALPHAD sublattice approach of ordering into the phase-field model with finite interface dissipation
    Zhang, L. and Stratmann, M. and Du, Y. and Sundman, B. and Steinbach, I.
    Acta Materialia 88 156-169 (2015)
    A new approach to incorporate the sublattice models in the CALPHAD (CALculation of PHAse Diagram) formalism directly into the phase-field formalism is developed. In binary alloys, the sublattice models can be classified into two types (i.e., "Type I" and "Type II"), depending on whether a direct one-to-one relation between the element site fraction in the CALPHAD database and the phase concentration in the phase-field model exists (Type I), or not (Type II). For "Type II" sublattice models, the specific site fractions, corresponding to a given mole fraction, have to be established via internal relaxation between different sublattices. Internal minimization of sublattice occupancy and solute evolution during microstructure transformation leads, in general, to a solution superior to the separate solution of the individual problems. The present coupling technique is validated for Fe-C and Ni-Al alloys. Finally, the model is extended into multicomponent alloys and applied to simulate the nucleation process of VC monocarbide from austenite matrix in a steel containing vanadium. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.11.037
  • 2014 • 13 The thermodynamic assessment of the Au-In-Ga system
    Ghasemi, M. and Sundman, B. and Fries, S.G. and Johansson, J.
    Journal of Alloys and Compounds 600 178-185 (2014)
    The Au-In-Ga ternary phase diagram is of importance for understanding the involved thermodynamic processes during the growth of Au-seeded III-V heterostructure nanowires containing In and Ga (e.g. Au-seeded InAs/GaAs nanowires). In this work the Au-In-Ga system has been thermodynamically modeled using the CALPHAD technique based on a recent experimental investigation of the phase equilibria in the system. As a result, a set of self-consistent interaction parameters have been optimized that can reproduce most of the experimental results. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2014.02.071
  • 2014 • 12 Effect of ternary element addition on the corrosion behaviour of NiTi shape memory alloys
    Kassab, E. and Neelakantan, L. and Frotscher, M. and Swaminathan, S. and Maaß, B. and Rohwerder, M. and Gomes, J. and Eggeler, G.
    Materials and Corrosion 65 18-22 (2014)
    The goal of this study is to compare the corrosion behaviour of selected ternary nickel titanium (NiTi)-based alloys (Ni45Ti 50Cu5, Ni47Ti50Fe3 and Ni39Ti50Pd11) with a binary, pseudoelastic Ni50.7Ti49.3 alloy. We examine the influence of the ternary elements on the corrosion behaviour using standard electrochemical techniques. All measurements were done in a physiological solution (0.9% NaCl) simulating a body temperature of 37 ± 1 °C. The influence of Cu and Pd addition on the surface oxide film was characterised by X-ray photoelectron spectroscopy (XPS). The results revealed that, the localised corrosion resistance of these ternary alloys is lower than the binary NiTi alloy. By comparing the different NiTi-based alloys, the following relation has been proposed for their localised corrosion resistances: NiTiCu < NiTiFe < NiTiPd < NiTi. Depth profiling by XPS showed that the surface oxide film on all the investigated NiTi-based alloys is mainly of TiO2, however, the NiTiPd and NiTiCu alloys showed metallic ternary element distributed within TiO2 layer. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/maco.201206587
  • 2014 • 11 On the functional degradation of binary titanium-tantalum high-temperature shape memory alloys - A new concept for fatigue life extension
    Niendorf, T. and Krooß, P. and Batyrsina, E. and Paulsen, A. and Frenzel, J. and Eggeler, G. and Maier, H.J.
    Functional Materials Letters 7 (2014)
    High-temperature shape memory alloys are promising candidates for actuator applications at elevated temperatures. Ternary nickel-titanium-based alloys either contain noble metals which are very expensive, or suffer from poor workability. Titanium-tantalum shape memory alloys represent a promising alternative if one can avoid the cyclic degradation due to the formation of the omega phase. The current study investigates the functional fatigue behavior of Ti-Ta and introduces a new concept providing for pronounced fatigue life extension. © 2014 The Authors.
    view abstractdoi: 10.1142/S1793604714500428
  • 2014 • 10 Composition Dependence of Phase Stability, Deformation Mechanisms, and Mechanical Properties of the CoCrFeMnNi High-Entropy Alloy System
    Tasan, C.C. and Deng, Y. and Pradeep, K.G. and Yao, M.J. and Springer, H. and Raabe, D.
    JOM 66 1993-2001 (2014)
    The proposal of configurational entropy maximization to produce massive solid-solution (SS)-strengthened, single-phase high-entropy alloy (HEA) systems has gained much scientific interest. Although most of this interest focuses on the basic role of configurational entropy in SS formability, setting future research directions also requires the overall property benefits of massive SS strengthening to be carefully investigated. To this end, taking the most promising CoCrFeMnNi HEA system as the starting point, we investigate SS formability, deformation mechanisms, and the achievable mechanical property ranges of different compositions and microstructural states. A comparative assessment of the results with respect to room temperature behavior of binary Fe-Mn alloys reveals only limited benefits of massive SS formation. Nevertheless, the results also clarify that the compositional requirements in this alloy system to stabilize the face-centered cubic (fcc) SS are sufficiently relaxed to allow considering nonequiatomic compositions and exploring improved strength–ductility combinations at reduced alloying costs. © 2014, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1133-6
  • 2014 • 9 Recovery, recrystallization, grain growth and phase stability of a family of FCC-structured multi-component equiatomic solid solution alloys
    Wu, Z. and Bei, H. and Otto, F. and Pharr, G.M. and George, E.P.
    Intermetallics 46 131-140 (2014)
    The equiatomic high-entropy alloy FeNiCoCrMn is known to crystallize as a single phase with the face-centered cubic (FCC) crystal structure. To better understand this quinary solid solution alloy, we investigate various binary, ternary and quaternary alloys made from its constituent elements. Our goals are twofold: (i) to investigate which of these lower order systems also form solid solution alloys consisting of a single FCC phase, and (ii) to characterize their phase stability and recovery, recrystallization, and grain growth behaviors. X-ray diffraction (XRD) and scanning electron microscopy with backscattered electron images showed that three of the five possible quaternaries (FeNiCoCr, FeNiCoMn and NiCoCrMn), five of the ten possible ternaries (FeNiCo, FeNiCr, FeNiMn, NiCoCr, and NiCoMn), and two of the ten possible binaries (FeNi and NiCo) were single-phase FCC solid solutions in the cast and homogenized condition, whereas the others either had different crystal structures or were multi-phase. The single-phase FCC quaternary, FeNiCoCr, along with its equiatomic ternary and binary subsidiaries, were selected for further investigations of phase stability and the thermomechanical processing needed to obtain equiaxed grain structures. Only four of these subsidiary alloys - two binaries (FeNi and NiCo) and two ternaries (FeNiCo and NiCoCr) - were found to be single-phase FCC after rolling at room temperature followed by annealing for 1 h at temperatures of 300-1100 C. Pure Ni, which is FCC and one of the constituents of the quinary high-entropy alloy (FeNiCoCrMn), was also investigated for comparison with the higher order alloys. Among the materials investigated after thermomechanical processing (FeNiCoCr, FeNiCo, NiCoCr, FeNi, NiCo, and Ni), FeNiCo and Ni showed abnormal grain growth at relatively low annealing temperatures, while the other four showed normal grain growth behavior. The grain growth exponents for all five of the equiatomic alloys were found to be ∼0.25 (compared to ∼0.5 for unalloyed Ni), suggesting that solute drag may control grain growth in the alloys. For all five alloys, as well as for pure Ni, microhardness increases as the grain size decreases in a Hall-Petch type way. The ternary alloy NiCoCr was the hardest of the alloys investigated in this study, even when compared to the quaternary FeNiCoCr alloy. This suggests that solute hardening in equiatomic alloys depends not just on the number of alloying elements but also their type. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2013.10.024
  • 2014 • 8 Modelling of dendritic growth during alloy solidification under natural convection
    Zhu, M. and Sun, D. and Pan, S. and Zhang, Q. and Raabe, D.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    A two-dimensional (2D) lattice Boltzmann method (LBM)-cellular automaton model is presented to investigate the dendritic growth of binary alloys in the presence of natural convection. The kinetic-based LBM is adopted to calculate the transport phenomena by the evolution of distribution functions of moving pseudo-particles. To numerically solve natural convection thermal and solute transport simultaneously, three sets of distribution functions are employed in conjunction with the lattice Bhatnagar-Gross-Krook scheme. Based on the LBM calculated local temperature and concentration at the solid/liquid interface, the kinetics of dendritic growth is determined according to a local solute equilibrium approach. Thus, the physics of a complete time-dependent interaction of natural convection, thermal and solutal transport, and dendritic growth during alloy solidification is embedded in the model. Model validation is performed by comparing the simulated results with literature data and analytical predictions. The model is applied to simulate dendritic growth in binary alloys under the influence of natural convection. The effects of Rayleigh numbers and initial undercooling on dendrite growth are investigated. The results show that natural buoyancy flow, induced by thermal and solutal gradients under gravity, transports the heat and solute from the lower region to the upper region. The dendritic growth is thus accelerated in the downward direction, whereas it is inhibited in the upward direction, yielding asymmetrical dendrite patterns. Increasing the Rayleigh number and undercooling will enhance and reduce, respectively, the influence of natural flow on the dendritic growth. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/034006
  • 2013 • 7 Atomic engineering of platinum alloy surfaces
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Edman Tsang, S.C. and Smith, G.D.W.
    Ultramicroscopy 132 205-211 (2013)
    A major practical challenge in heterogeneous catalysis is to minimize the loading of expensive platinum group metals (PGMs) without degrading the overall catalytic efficiency. Gaining a thorough atomic-scale understanding of the chemical/structural changes occurring during catalyst manufacture/operation could potentially enable the design and production of "nano-engineered" catalysts, optimized for cost, stability and performance. In the present study, the oxidation behavior of a Pt-31 at% Pd alloy between 673-1073. K is investigated using atom probe tomography (APT). Over this range of temperatures, three markedly different chemical structures are observed near the surface of the alloy. At 673. K, the surface oxide formed is enriched with Pd, the concentration of which rises further following oxidation at 773. K. During oxidation at 873. K, a thick, stable oxide layer is formed on the surface with a stoichiometry of PdO, beneath which a Pd-depleted (Pt-rich) layer exists. Above 873. K, the surface composition switches to enrichment in Pt, with the Pt content increasing further with increasing oxidation temperature. This treatment suggests a route for tuning the surfaces of Pt-Pd nanoparticles to be either Pd-rich or Pt-rich, simply by adjusting the oxidation temperatures in order to form two different types of core-shell structures. In addition, comparison of the oxidation behavior of Pt-Pd with Pt-Rh and Pd-Rh alloys demonstrates markedly different trends under the same conditions for these three binary alloys. © 2012.
    view abstractdoi: 10.1016/j.ultramic.2012.10.012
  • 2013 • 6 The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy
    Otto, F. and Dlouhý, A. and Somsen, C. and Bei, H. and Eggeler, G. and George, E.P.
    Acta Materialia 61 5743-5755 (2013)
    An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4-160 μm. Quasi-static tensile tests at an engineering strain rate of 10-3 s-1 were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to ∼2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1}〈1 1 0〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈1 1 0〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈1 1 2〉 Shockley partials. At later stages (∼20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing ("dynamic Hall-Petch"), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.06.018
  • 2011 • 5 Achieving small structures in thin NiTi sheets for medical applications with water jet and micro machining: A comparison
    Frotscher, M. and Kahleyss, F. and Simon, T. and Biermann, D. and Eggeler, G.
    Journal of Materials Engineering and Performance 20 776-782 (2011)
    NiTi shape memory alloys (SMA) are used for a variety of applications including medical implants and tools as well as actuators, making use of their unique properties. However, due to the hardness and strength, in combination with the high elasticity of the material, the machining of components can be challenging. The most common machining techniques used today are laser cutting and electrical discharge machining (EDM). In this study, we report on the machining of small structures into binary NiTi sheets, applying alternative processing methods being well-established for other metallic materials. Our results indicate that water jet machining and micro milling can be used to machine delicate structures, even in very thin NiTi sheets. Further work is required to optimize the cut quality and the machining speed in order to increase the cost-effectiveness and to make both methods more competitive. © ASM International.
    view abstractdoi: 10.1007/s11665-010-9789-8
  • 2011 • 4 Molecular dynamics simulation study of microstructure evolution during cyclic martensitic transformations
    Kastner, O. and Eggeler, G. and Weiss, W. and Ackland, G.J.
    Journal of the Mechanics and Physics of Solids 59 1888-1908 (2011)
    Shape memory alloys (SMA) exhibit a number of features which are not easily explained by equilibrium thermodynamics, including hysteresis in the phase transformation and reverse shape memory in the high symmetry phase. Processing can change these features: repeated cycling can train the reverse shape memory effect, while changing the amount of hysteresis and other functional properties. These effects are likely to be due to formations of localised defects and these can be studied by atomistic methods. Here we present a molecular dynamics simulation study of such behaviour employing a two-dimensional, binary LennardJones model. Our atomistic model exhibits a symmetry breaking, displacive phase transition from a high temperature, entropically stabilised, austenite-like phase to a low temperature martensite-like phase. The simulations show transformations in this model material proceed by non-diffusive nucleation and growth processes and produce distinct microstructures. We observe the generation of persistent lattice defects during forward-and-reverse transformations which serve as nucleation centres in subsequent transformation processes. These defects interfere the temporal and spatial progression of transformations and thereby affect subsequent product morphologies. During cyclic transformations we observe accumulations of lattice defects so as to establish new microstructural elements which represent a memory of the previous morphologies. These new elements are self-organised and they provide a basis of the reversible shape memory effect in the model material. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmps.2011.05.009
  • 2011 • 3 On the stress-induced formation of R-phase in ultra-fine-grained Ni-rich NiTi shape memory alloys
    Olbricht, J. and Yawny, A. and Pelegrina, J.L. and Dlouhy, A. and Eggeler, G.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 42 2556-2574 (2011)
    Phase transformations in binary ultra-fine-grained (UFG) pseudoelastic NiTi wires were studied in a wide temperature range using mechanical loading/unloading experiments, resistance measurements, differential scanning calorimetry (DSC), thermal infrared imaging, and transmission electron microscopy (TEM). The formation of R-phase can be detected in the mechanical experiments. It is shown that the stress-strain response of the R-phase can be isolated from the overall stress-strain data. The R-phase always forms prior to B19' when good pseudoelastic properties are observed. The stress-induced B2 to R-phase transition occurs in a homogeneous manner, contrary to the localized character of the B2/R to B19' transformations. The temperature dependence of the critical stress values for the formation of the martensitic phases shows a Clausius Clapeyron type of behavior with constants close to 6 MPa/K (B19') and 18 MPa/K (R-phase). A stress-temperature map is suggested that summarizes the experimentally observed sequences of elementary transformation/deformation processes. © The Minerals, Metals & Materials Society and ASM International 2011.
    view abstractdoi: 10.1007/s11661-011-0679-y
  • 2011 • 2 TCP phase predictions in Ni-based superalloys: Structure maps revisited
    Seiser, B. and Drautz, R. and Pettifor, D.G.
    Acta Materialia 59 749-763 (2011)
    The traditional methods for predicting the occurrence of deleterious topologically close-packed (TCP) phases in Ni-based superalloys have been based on the PHACOMP and newPHACOMP methodologies. These schemes use the average number of holes Nh or the centre of gravity of the elemental d-bands Md to predict whether or not a given multicomponent alloy will be prone to TCP formation. However, as both these one-dimensional methodologies are well-known to fail with respect to new generations of alloys, a novel two-dimensional structure map (N,ΔV/V) is introduced where N is the average electron concentration and ΔV/V is a composition-dependent size-factor difference. This map is found to separate the experimental data on the TCP phases of binary A-B transition metal alloys into well-defined but sometimes overlapping regions corresponding to different structure types such as A15, σ, χ, R, P, δ, μ, M and Laves. Detailed investigations of ternary phase diagrams and multicomponent systems show that TCP phases, regardless of the number of constituents, are located in the same regions of the structure map that are favoured by the binary compounds of the same structure type. The structure map is then used in conjunction with CALPHAD computations of σ phase stability to show that the predictive power of newPHACOMP for the seven component Ni-Co-Cr-Ta-W-Re-Al system studied recently by Reed et al. [24] is indeed poor. This supports a growing consensus that robust methods of TCP phase prediction in multicomponent alloys will require the inclusion of reliable first-principles thermodynamic databases within the semi-empirical CALPHAD scheme. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.10.013
  • 2010 • 1 L21-ordered Fe-Al-Ti alloys
    Krein, R. and Friak, M. and Neugebauer, J. and Palm, M. and Heilmaier, M.
    Intermetallics 18 1360-1364 (2010)
    Fe-Al-Ti alloys with the ordered L21-structure (Heusler phase) belong to the few Fe-Al-based alloys which show comparably high-strength at high temperatures, e.g. at 800 °C. However, like many other high-temperature materials based on intermetallics they show limited ductility even at high temperatures. In order to further explore the possibilities in increasing their strength and ductility, alloys with four different microstructures, i.e. single-phase L21, L21 with incoherent precipitates of TiB2 or Laves phase, and coherent L21 + A2, were produced. Also, the influence of alloying with Cr and B has been investigated. The Young's modulus of Fe-25Al-20Ti-4Cr (at.%) in dependence of temperature up to 900 °C has been determined and results of the compressive flow stress, creep strength and brittle-toductile transition temperatures (BDTT) are summarised and compared to those of binary Fe3Al (D03), Fe-Al-Ti-based alloys, and some commercial alloys. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2009.12.036