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.

Below, you can either scroll through the complete list of our annually published material, or search for a specific author or term via the free text search to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.

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  • 2023 • 273 Enhanced activity and coke resistivity of NiCoFe nanoalloy catalyst in CO2 reforming of methane
    Das, Subhasis and Tillmann, Lukas and Xia, Wei and Muhler, Martin
    Journal of the Indian Chemical Society 100 (2023)
    NiCo nanoalloy catalysts were prepared from hydrotalcite precursors and used in CO2 reforming of methane (DRM) under atmospheric and 2 MPa pressure in a fixed-bed reactor at 700-850 °C. The Ni6Co1 catalyst with a molar ratio of Ni/Co to 6 showed the highest stability and activity in DRM under atmospheric pressure. This was due to the homogeneous dispersion of nanoalloy particles (∼14 nm) on the MgAl(O) support, which had a strong metal-support interaction. Nonetheless, a slow and continuous deactivation was spotted under 2 MPa pressure due to the coke deposition. Further modification of Ni6Co1 with optimum amount of Fe (in Ni6Co0.5Fe0.5) formed ternary NiCoFe nanoalloy with improved metal-support interaction and reduced alloy size (∼10 nm). The presence of Fe significantly improved the coke resistance capability and provided high stability under 2 MPa pressure. © 2023 Indian Chemical Society
    view abstractdoi: 10.1016/j.jics.2023.101049
  • 2022 • 272 A mechanically strong and ductile soft magnet with extremely low coercivity
    Han, L. and Maccari, F. and Souza Filho, I.R. and Peter, N.J. and Wei, Y. and Gault, B. and Gutfleisch, O. and Li, Z. and Raabe, D.
    Nature 608 310-316 (2022)
    Soft magnetic materials (SMMs) serve in electrical applications and sustainable energy supply, allowing magnetic flux variation in response to changes in applied magnetic field, at low energy loss1. The electrification of transport, households and manufacturing leads to an increase in energy consumption owing to hysteresis losses2. Therefore, minimizing coercivity, which scales these losses, is crucial3. Yet meeting this target alone is not enough: SMMs in electrical engines must withstand severe mechanical loads; that is, the alloys need high strength and ductility4. This is a fundamental design challenge, as most methods that enhance strength introduce stress fields that can pin magnetic domains, thus increasing coercivity and hysteresis losses5. Here we introduce an approach to overcome this dilemma. We have designed a Fe–Co–Ni–Ta–Al multicomponent alloy (MCA) with ferromagnetic matrix and paramagnetic coherent nanoparticles (about 91 nm in size and around 55% volume fraction). They impede dislocation motion, enhancing strength and ductility. Their small size, low coherency stress and small magnetostatic energy create an interaction volume below the magnetic domain wall width, leading to minimal domain wall pinning, thus maintaining the soft magnetic properties. The alloy has a tensile strength of 1,336 MPa at 54% tensile elongation, extremely low coercivity of 78 A m−1 (less than 1 Oe), moderate saturation magnetization of 100 A m2 kg−1 and high electrical resistivity of 103 μΩ cm. © 2022, The Author(s).
    view abstractdoi: 10.1038/s41586-022-04935-3
  • 2022 • 271 Conservation of Nickel Ion Single-Active Site Character in a Bottom-Up Constructed π-Conjugated Molecular Network
    Baranowski, D. and Cojocariu, I. and Sala, A. and Africh, C. and Comelli, G. and Schio, L. and Tormen, M. and Floreano, L. and Feyer, V. and Schneider, C.M.
    Angewandte Chemie - International Edition 61 (2022)
    On-surface chemistry holds the potential for ultimate miniaturization of functional devices. Porphyrins are promising building-blocks in exploring advanced nanoarchitecture concepts. More stable molecular materials of practical interest with improved charge transfer properties can be achieved by covalently interconnecting molecular units. On-surface synthesis allows to construct extended covalent nanostructures at interfaces not conventionally available. Here, we address the synthesis and properties of covalent molecular network composed of interconnected constituents derived from halogenated nickel tetraphenylporphyrin on Au(111). We report that the π-extended two-dimensional material exhibits dispersive electronic features. Concomitantly, the functional Ni cores retain the same single-active site character of their single-molecule counterparts. This opens new pathways when exploiting the high robustness of transition metal cores provided by bottom-up constructed covalent nanomeshes. © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.
    view abstractdoi: 10.1002/anie.202210326
  • 2022 • 270 High stress twinning in a compositionally complex steel of very high stacking fault energy
    Wang, Z. and Lu, W. and An, F. and Song, M. and Ponge, D. and Raabe, D. and Li, Z.
    Nature Communications 13 (2022)
    Deformation twinning is rarely found in bulk face-centered cubic (FCC) alloys with very high stacking fault energy (SFE) under standard loading conditions. Here, based on results from bulk quasi-static tensile experiments, we report deformation twinning in a micrometer grain-sized compositionally complex steel (CCS) with a very high SFE of ~79 mJ/m2, far above the SFE regime for twinning (<~50 mJ/m2) reported for FCC steels. The dual-nanoprecipitation, enabled by the compositional degrees of freedom, contributes to an ultrahigh true tensile stress up to 1.9 GPa in our CCS. The strengthening effect enhances the flow stress to reach the high critical value for the onset of mechanical twinning. The formation of nanotwins in turn enables further strain hardening and toughening mechanisms that enhance the mechanical performance. The high stress twinning effect introduces a so far untapped strengthening and toughening mechanism, for enabling the design of high SFEs alloys with improved mechanical properties. © 2022, The Author(s).
    view abstractdoi: 10.1038/s41467-022-31315-2
  • 2022 • 269 Indentation behavior of creep-feed grinding induced gradient microstructures in single crystal nickel-based superalloy
    Miao, Q. and Ding, W. and Kuang, W. and Fu, Y. and Yin, Z. and Dai, C. and Cao, L. and Wang, H.
    Materials Letters 306 (2022)
    The gradient microstructures of surface layer in single crystal nickel-based superalloy were produced by creep-feed grinding. The mechanical properties (i.e., hardness, elastic modulus) and room-temperature (RT) creep behavior of such structures were evaluated using a nano-indentation technique. Results show that the gradient structures along depth from ground surface consisted of nanograins, submicron grains and lamellar-shape structures, and dislocation structures. Furthermore, it was found that the hardness and elastic modulus of gradient structures were higher by 8–10% than that of bulk material on average. However, the regions containing nanograins showed a remarkable increase in creep depth compared to bulk material, implying that the creep behavior of ground layer was changed unfavorably. The obtained stress exponents of gradient structures suggested that dislocation activities were the main mechanism for indentation creep deformation. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.matlet.2021.130956
  • 2022 • 268 Introducing Stacking Faults into Three-Dimensional Branched Nickel Nanoparticles for Improved Catalytic Activity
    Ramadhan, Z.R. and Poerwoprajitno, A.R. and Cheong, S. and Webster, R.F. and Kumar, P.V. and Cychy, S. and Gloag, L. and Benedetti, T.M. and Marjo, C.E. and Muhler, M. and Wang, D.-W. and Gooding, J.J. and Schuhmann, W. and Tilley, R.D.
    Journal of the American Chemical Society 144 11094-11098 (2022)
    Creating high surface area nanocatalysts that contain stacking faults is a promising strategy to improve catalytic activity. Stacking faults can tune the reactivity of the active sites, leading to improved catalytic performance. The formation of branched metal nanoparticles with control of the stacking fault density is synthetically challenging. In this work, we demonstrate that varying the branch width by altering the size of the seed that the branch grows off is an effective method to precisely tune the stacking fault density in branched Ni nanoparticles. A high density of stacking faults across the Ni branches was found to lower the energy barrier for Ni2+/Ni3+oxidation and result in enhanced activity for electrocatalytic oxidation of 5-hydroxylmethylfurfural. These results show the ability to synthetically control the stacking fault density in branched nanoparticles as a basis for enhanced catalytic activity. © 2022 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/jacs.2c04911
  • 2022 • 267 Microstructure property classification of nickel-based superalloys using deep learning
    Nwachukwu, U. and Obaied, A. and Horst, O.M. and Ali, M.A. and Steinbach, I. and Roslyakova, I.
    Modelling and Simulation in Materials Science and Engineering 30 (2022)
    Nickel-based superalloys have a wide range of applications in high temperature and stress domains due to their unique mechanical properties. Under mechanical loading at high temperatures, rafting occurs, which reduces the service life of these materials. Rafting is heavily affected by the loading conditions associated with plastic strain; therefore, understanding plastic strain evolution can help understand these material's service life. This research classifies nickel-based superalloys with respect to creep strain with deep learning techniques, a technique that eliminates the need for manual feature extraction of complex microstructures. Phase-field simulation data that displayed similar results to experiments were used to build a model with pre-trained neural networks with several convolutional neural network architectures and hyper-parameters. The optimized hyper-parameters were transferred to scanning electron microscopy images of nickel-based superalloys to build a new model. This fine-tuning process helped mitigate the effect of a small experimental dataset. The built models achieved a classification accuracy of 97.74% on phase-field data and 100% accuracy on experimental data after fine-tuning. © 2022 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/ac3217
  • 2022 • 266 Selective Anodic Oxidation of Solketal as Acetal-Protected Glycerol over Nickel Boride in Alkaline Media to Glyceric Acid**
    Cychy, S. and Lechler, S. and Muhler, M.
    ChemElectroChem 9 (2022)
    Alkaline anodic oxidation of glycerol suffers from facile C−C bond cleavage, especially when using non-precious metal electrocatalysts, which limits the yield of more valuable C3 oxygenates. Usually, a high C3 selectivity is a tradeoff with conversion for most catalysts. Thus, we used solketal as the reactant, which is acetal-protected glycerol with acetone. CV experiments showed that solketal is oxidized over nickel boride (NixB) at potentials where NiOOH is formed. Electrolysis over NixB in a thin-film spectroelectrochemical flow cell at 1.58 V vs. RHE to avoid pronounced oxygen evolution showed a stable current density of ca. 6 mA cm−2. Simultaneously recorded ATR-FTIR spectra revealed solketal conversion to solketalate and formate. Indeed, 59 % conversion and 77 % selectivity to glyceric acid were determined by HPLC after acidic cleavage of the acetal, resulting in a yield of 45 %. Therefore, solketal is a promising reactant for the selective electrosynthesis of glyceric acid. © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH.
    view abstractdoi: 10.1002/celc.202101214
  • 2022 • 265 Very high cycle fatigue durability of an additively manufactured single-crystal Ni-based superalloy
    Bortoluci Ormastroni, L.M. and Lopez-Galilea, I. and Pistor, J. and Ruttert, B. and Körner, C. and Theisen, W. and Villechaise, P. and Pedraza, F. and Cormier, J.
    Additive Manufacturing 54 (2022)
    A single crystalline (SX) nickel-based superalloy additively manufactured (AM) by electron beam-based powder bed fusion (PBF-E) was investigated under very high cycle fatigue (VHCF) at 1,000 °C in fully reversed conditions (Rε = −1). Specimens processed using a classical Bridgman solidification route and the impact of a hot isostatic pressing (HIP) treatment were also considered. It is shown that the fatigue lifetime of the AM specimens is higher or in the same range of the Bridgman processed ones with the same chemical composition. All defect-free AM samples fail by surface initiation with very long VHCF lives. In the absence of metallurgical defects such as grain boundaries or pores, the superalloy chemical stability against oxidation governs VHCF failure. © 2022 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2022.102759
  • 2021 • 264 Atomic scale understanding of phase stability and decomposition of a nanocrystalline CrMnFeCoNi Cantor alloy
    Li, Y.J. and Savan, A. and Ludwig, A.
    Applied Physics Letters 119 (2021)
    High entropy alloys (HEAs) provide superior mechanical and functional properties. However, these advantages may disappear when a metastable single-phase solid solution decomposes at low temperatures upon long-term annealing. Therefore, understanding the underlying phase separation mechanisms is important for the design of new HEAs with controlled properties. In the current work, the thermal stability of a nanocrystalline CrMnFeCoNi HEA was investigated at different annealing conditions using a combinatorial processing platform, involving fast and parallel synthesis of nanocrystalline thin films, short annealing time for a rapid phase evolution, and direct characterization by atom probe tomography. The microstructural features of the decomposed CrMnFeCoNi alloy as well as its decomposition process were analyzed in terms of elemental distributions at the near-atomic scale. The results show that the segregation of Ni and Mn to grain boundaries in the initial single-phase alloy is a prerequisite and is observed to be the only occurring physical process at the early stage of phase decomposition. When the concentrations of Ni and Mn reach a certain value, phase decomposition starts and a MnNi-rich phase forms at grain boundaries. Next, two Cr-rich phases form at the interface between the MnNi-rich phase and the matrix. Meanwhile, a FeCo-rich phase forms in the grain interior. Based on these observations, the underlying mechanisms involving nucleation, diffusivity as well as thermodynamic considerations were discussed. © 2021 Author(s).
    view abstractdoi: 10.1063/5.0069107
  • 2021 • 263 Chemical Vapor Deposition of Cobalt and Nickel Ferrite Thin Films: Investigation of Structure and Pseudocapacitive Properties
    Zywitzki, D. and Schaper, R. and Ciftyürek, E. and Wree, J.-L. and Taffa, D.H. and Baier, D.M. and Rogalla, D. and Li, Y. and Meischein, M. and Ludwig, A. and Li, Z. and Schierbaum, K. and Wark, M. and Devi, A.
    Advanced Materials Interfaces 8 (2021)
    Transition metal ferrites, such as CoFe2O4 (CFO) and NiFe2O4 (NFO), have gained increasing attention as potential materials for supercapacitors. Since chemical vapor deposition (CVD) offers advantages like interface quality to the underlying substrates and the possibility for coverage of 3D substrates, two CVD processes are reported for CFO and NFO. Growth rates amount to 150 to 200 nm h−1 and yield uniform, dense, and phase pure spinel ferrite films according to X-ray diffraction (XRD), Raman spectroscopy, Rutherford backscattering spectrometry and nuclear reaction analysis (RBS/NRA) and scanning electron microscopy (SEM). Atom probe tomography (APT) and synchrotron X-ray photoelectron spectroscopy (XPS) give insights into the vertical homogeneity and oxidation states in the CFO films. Cation disorder of CFO is analyzed for the first time from synchrotron-based XPS. NFO is analyzed via lab-based XPS. Depositions on conducting Ni and Ti substrates result in electrodes with pseudocapacitive behavior, as evidenced by cyclovoltammetry (CV) experiments. The interfacial capacitances of the electrodes are up to 185 µF cm−2. © 2021 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/admi.202100949
  • 2021 • 262 Creep feed grinding induced gradient microstructures in the superficial layer of turbine blade root of single crystal nickel-based superalloy
    Miao, Q. and Ding, W. and Xu, J. and Cao, L. and Wang, H. and Yin, Z. and Dai, C. and Kuang, W.
    International Journal of Extreme Manufacturing 3 (2021)
    The service performance of the turbine blade root of an aero-engine depends on the microstructures in its superficial layer. This work investigated the surface deformation structures of turbine blade root of single crystal nickel-based superalloy produced under different creep feed grinding conditions. Gradient microstructures in the superficial layer were clarified and composed of a severely deformed layer (DFL) with nano-sized grains (48-67 nm) at the topmost surface, a DFL with submicron-sized grains (66-158 nm) and micron-sized laminated structures at the subsurface, and a dislocation accumulated layer extending to the bulk material. The formation of such gradient microstructures was found to be related to the graded variations in the plastic strain and strain rate induced in the creep feed grinding process, which were as high as 6.67 and 8.17 × 107 s-1, respectively. In the current study, the evolution of surface gradient microstructures was essentially a transition process from a coarse single crystal to nano-sized grains and, simultaneously, from one orientation of a single crystal to random orientations of polycrystals, during which the dislocation slips dominated the creep feed grinding induced microstructure deformation of single crystal nickel-based superalloy. © 2021 IOP Publishing Ltd.
    view abstractdoi: 10.1088/2631-7990/ac1e05
  • 2021 • 261 Electrocatalytic Oxidation of Glycerol Using Solid-State Synthesised Nickel Boride: Impact of Key Electrolysis Parameters on Product Selectivity
    Brix, A.C. and Morales, D.M. and Braun, M. and Jambrec, D. and Junqueira, J.R.C. and Cychy, S. and Seisel, S. and Masa, J. and Muhler, M. and Andronescu, C. and Schuhmann, W.
    ChemElectroChem 8 2336-2342 (2021)
    Water electrolysis is a promising technology for sustainable hydrogen production; however, its commercialisation is limited by sluggish kinetics of the oxygen evolution reaction (OER). A potential alternative to the OER is hence required and is seen in the electrocatalytic glycerol oxidation reaction (GOR) as it offers concomitant value-added product generation from a cheap and abundant feedstock. Here, we show a facile solid-state synthesis method to obtain Ni-boride, a non-noble metal-based catalyst subsequently used in an in-depth study of the GOR product distribution as a function of key electrolysis parameters. Highly crystalline, mixed-phase Ni borides were obtained, and their synthesis was successfully optimised regarding GOR activity. Long-term chronoamperometry was conducted in a circular flow-through cell and samples were analysed by HPLC. It is shown that the formation of lactic acid, one of the most valuable GOR products, can be enhanced by optimising the electrolyte composition and the applied potential. © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/celc.202100739
  • 2021 • 260 Engendering Unprecedented Activation of Oxygen Evolution via Rational Pinning of Ni Oxidation State in Prototypical Perovskite: Close Juxtaposition of Synthetic Approach and Theoretical Conception
    Pittkowski, R. and Divanis, S. and Klementová, M. and Nebel, R. and Nikman, S. and Hoster, H. and Mukerjee, S. and Rossmeisl, J. and Krtil, P.
    ACS Catalysis 11 985-997 (2021)
    Rational optimization of the OER activity of catalysts based on LaNiO3 oxide is achieved by maximizing the presence of trivalent Ni in the surface structure. DFT investigations of the LaNiO3 catalyst and surface structures related to it predict an improvement in the OER activity for these materials to levels comparable with the top of the OER volcano if the La content is minimized while the oxidation state of Ni is maintained. These theoretically predicted structures of high intrinsic OER activity can be prepared by a templated spray-freeze freeze-drying synthesis followed by a simple postsynthesis exfoliation-like treatment in acidic media. These nanocrystalline LaNiO3-related materials confirm the theoretical predictions, showing a dramatic improvement in OER activity. The exfoliated surfaces remain stable in OER catalysis, as shown by an in-operando ICP-OES study. The unprecedented OER activation of the synthesized LaNiO3-based materials is related to a close juxtaposition of the theoretical conception of ideal structural motifs and the ability to engender such motifs using a unique synthetic procedure, both principally related to stabilization and pinning of the Ni oxidation state within the local coordination environment of the perovskite structure. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.0c04733
  • 2021 • 259 Highly Efficient and Selective Aerobic Oxidation of Cinnamyl Alcohol under Visible Light over Pt-Loaded NaNbO3Enriched with Oxygen Vacancies by Ni Doping
    Zhao, G. and Bonke, S.A. and Schmidt, S. and Wang, Z. and Hu, B. and Falk, T. and Hu, Y. and Rath, T. and Xia, W. and Peng, B. and Schnegg, A. and Weng, Y. and Muhler, M.
    ACS Sustainable Chemistry and Engineering 9 5422-5429 (2021)
    NaNbO3 enriched with oxygen vacancies by Ni doping was successfully synthesized via a polymerized complex method and applied as a photocatalyst in the oxidation of cinnamyl alcohol (CA) to cinnamaldehyde in air. Reaction rates as high as 45 μmol h-1 were achieved under visible light with a high apparent quantum efficiency of 67.2% and excellent chemoselectivity larger than 99%. UV-vis, electron paramagnetic resonance, and attenuated total reflectance infrared spectroscopy results indicate that the CA molecules preferentially adsorb at the oxygen vacancies, thus enabling electron transfer between coordinatively bound CA and NaNbO3 under visible light, inducing CA oxidation. The photocatalytic aerobic oxidation of CA is assumed to proceed via the one-photon pathway with H2O2 as the coupled product. The photodeposited Pt nanoparticles on the surface not only enhanced the oxidation rate but also improved the selectivity to cinnamaldehyde substantially because of the fast decomposition of formed H2O2, in this way avoiding its consecutive oxidation by H2O2. The oxygen vacancies on the surface generated by Ni doping are identified to play a decisive role in the chemisorption of cinnamyl alcohol and the interface charge transfer. © 2021 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/acssuschemeng.1c00460
  • 2021 • 258 Influence of temperature on void collapse in single crystal nickel under hydrostatic compression
    Prasad, M.R.G. and Neogi, A. and Vajragupta, N. and Janisch, R. and Hartmaier, A.
    Materials 14 (2021)
    Employing atomistic simulations, we investigated the void collapse mechanisms in single crystal Ni during hydrostatic compression and explored how the atomistic mechanisms of void collapse are influenced by temperature. Our results suggest that the emission and associated mutual interactions of dislocation loops around the void is the primary mechanism of void collapse, irrespective of the temperature. The rate of void collapse is almost insensitive to the temperature, and the process is not thermally activated until a high temperature (1200-1500 K) is reached. Our simulations reveal that, at elevated temperatures, dislocation motion is assisted by vacancy diffusion and consequently the void is observed to collapse continuously without showing appreciable strain hardening around it. In contrast, at low and ambient temperatures (1 and 300 K), void collapse is delayed after an initial stage of closure due to significant strain hardening around the void. Furthermore, we observe that the dislocation network produced during void collapse remains the sample even after complete void collapse, as was observed in a recent experiment of nickel-base superalloy after hot isostatic pressing. © 2021 by the authors.
    view abstractdoi: 10.3390/ma14092369
  • 2021 • 257 Influence of the Fe : Ni Ratio in FexNi9-xS8 (x=3–6) on the CO2 Electroreduction
    Tetzlaff, D. and Pellumbi, K. and Puring, K.J. and Siegmund, D. and Polet, W.S.K. and Checinski, M.P. and Apfel, U.-P.
    ChemElectroChem 8 3161-3167 (2021)
    The electrochemical CO2 reduction (CO2R) is a promising approach to decrease the amount of CO2 in the atmosphere by producing commodity chemicals or fuels using renewable energies. Herein, the development of non-noble metal electrocatalysts is regarded as a key point for achieving the transition of CO2R to industrial scales. Transition metal chalcogenides of the pentlandite structure (M9X8) have emerged as promising electrocatalysts to produce syngas. In this line, we present the electrochemical CO2R of FexNi9-xS8 (x=3–6) with variable Fe : Ni ratios. All materials can reduce H2O/CO2 mixtures to CO or H2 respectively with varying efficiency depending on the Fe : Ni ratio and the water content. While CO2R in proton-rich organic electrolytes was mainly accompanied by hydrogen evolution, the CO2R activity climaxed with F.E. of 3.6 % for CO and 0.3 % for methane using Fe3Ni6S8. Using electrolytes with low water content, CO production with F.E. close to 90 % was demonstrated. Counterintuitively, the variation of the Fe : Ni ratio led only to small alterations in the CO2R activity. Quantum mechanical studies were performed to get further information on the observed trends and provide further insight into structure/activity relationships for the Fe/Ni pentlandite system and its CO2R activity opening the path towards the development of more active and robust CO2R electrocatalysts. © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/celc.202100930
  • 2021 • 256 Interplay of structural and dynamical heterogeneity in the nucleation mechanism in nickel
    Díaz Leines, G. and Michaelides, A. and Rogal, J.
    Faraday Discussions 235 406-415 (2021)
    Gaining a fundamental understanding of crystal nucleation processes in metal alloys is crucial for the development and design of high-performance materials with targeted properties. Yet, crystallization is a complex non-equilibrium process and, despite having been studied for decades, the microscopic aspects that govern the crystallization mechanism of a material remain elusive to date. Recent evidence shows that the spatial heterogeneity in the supercooled liquid, characterised by extended regions with distinctive mobility and order, may be a key microscopic factor that determines the mechanism of crystal nucleation. These findings have advanced our view of the fundamental nature of crystallization, as most research has assumed that crystal clusters nucleate from random fluctuations in a ‘homogeneous’ liquid. Here, by analysing transition path sampling trajectories, we show that dynamical heterogeneity plays a key role in the mechanism of crystal nucleation in an elemental metal, nickel. Our results demonstrate that crystallization occurs preferentially in regions of low mobility in the supercooled liquid, evidencing the collective dynamical nature of crystal nucleation in Ni. In addition, our results show that low mobility regions form before and spatially overlap with pre-ordered domains that act as precursors to the crystal phase that subsequently emerges. Our results show a clear link between dynamical and structural heterogeneity in the supercooled liquid and its impact on the nucleation mechanism, revealing microscopic descriptors that could pave a novel way to control crystallization processes in metals. © 2022 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d1fd00099c
  • 2021 • 255 Nickel nanoparticles supported on nitrogen–doped carbon nanotubes are a highly active, selective and stable CO2 methanation catalyst
    Gödde, J. and Merko, M. and Xia, W. and Muhler, M.
    Journal of Energy Chemistry 54 323-331 (2021)
    CO2 methanation using nickel-based catalysts has attracted large interest as a promising power-to-gas route. Ni nanoparticles supported on nitrogen-doped CNTs with Ni loadings in the range from 10 wt% to 50 wt% were synthesized by impregnation, calcination and reduction and characterized by elemental analysis, X-ray powder diffraction, H2 temperature-programmed reduction, CO pulse chemisorption and transmission electron microscopy. The Ni/NCNT catalysts were highly active in CO2 methanation at atmospheric pressure, reaching over 50% CO2 conversion and over 95% CH4 selectivity at 340 °C and a GHSV of 50,000 mL g−1 h−1 under kinetically controlled conditions. The small Ni particle sizes below 10 nm despite the high Ni loading is ascribed to the efficient anchoring on the N-doped CNTs. The optimum loading of 30 wt%–40 wt% Ni was found to result in the highest Ni surface area, the highest degree of conversion and the highest selectivity to methane. A constant TOF of 0.3 s−1 was obtained indicating similar catalytic properties of the Ni nanoparticles in the range from 10 wt% to 50 wt% Ni loading. Long-term experiments showed that the Ni/NCNT catalyst with 30 wt% Ni was highly stable for 100 h time on stream. © 2020 Science Press
    view abstractdoi: 10.1016/j.jechem.2020.06.007
  • 2021 • 254 Processing of a newly developed nitrogen-alloyed ferritic-austenitic stainless steel by laser powder bed fusion – Microstructure and properties
    Becker, L. and Röttger, A. and Boes, J. and Weber, S. and Theisen, W.
    Additive Manufacturing 46 (2021)
    In this work, a novel alloy design of a stainless steel with a ferritic-austenitic microstructure is derived for PBF-LB/M (powder bed fusion-laser beam/metal). The alloy was developed based on X2CrNiMo17-12-2 steel, for which an austenite volume content of approx. 54 vol% in the PBF-LB/M state was achieved using a reduced Ni equivalent. Partial substitution of Ni by Mn increases the N solubility of the alloy. By melting and further gas-atomizing this melt in an N2 atmosphere, an N content of 0.27 mass% was set in the produced steel powder. This leads to both high strength and high corrosion resistance of the PBF-LB/M-processed steel. However, microstructural investigations in the PBF-LB/M state confirm a microstructure consisting of ferrite, austenite, and Mo- and Cr-rich nitrides of M2N type. The nitrides were not completely eliminated by a subsequent heat treatment of the PBF-LB/M samples. As a result of the solution annealing, the microstructure approaches the thermodynamic equilibrium so that the austenite volume content increases from 54.2 vol% to 92.7 vol%. The higher Cr and N contents result in a higher corrosion resistance of the investigated steel compared to PBF-LB/M-processed X2CrNiMo17-12-2, regarded as the reference material. In addition, the measured strengths are significantly higher due to the larger amounts of austenite/ferrite interfaces and the N-induced solid-solution strengthening effect compared to X2CrNiMo17-12-2. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.addma.2021.102185
  • 2021 • 253 Revealing atomic-scale vacancy-solute interaction in nickel
    Morgado, F.F. and Katnagallu, S. and Freysoldt, C. and Klaes, B. and Vurpillot, F. and Neugebauer, J. and Raabe, D. and Neumeier, S. and Gault, B. and Stephenson, L.T.
    Scripta Materialia 203 (2021)
    It is widely accepted that the different types of crystalline imperfections, such as vacancies or dislocations, greatly influence a material's physical and mechanical properties. However, imaging individual vacancies in solids and revealing their atomic neighborhood remains one of the frontiers of microscopy and microanalysis. Here, we study a creep-deformed binary Ni-2 at.% Ta alloy. Atom probe tomography reveals a random distribution of Ta. Field ion microscopy, with contrast interpretation supported by density-functional theory and time-of-flight mass spectrometry, evidences a positive correlation of Ta with vacancies, supporting positive solute-vacancy interactions previously predicted by atomistic simulations. © 2021
    view abstractdoi: 10.1016/j.scriptamat.2021.114036
  • 2021 • 252 Tracer diffusion in the σ phase of the CoCrFeMnNi system
    Zhang, J. and Muralikrishna, G.M. and Asabre, A. and Kalchev, Y. and Müller, J. and Butz, B. and Hilke, S. and Rösner, H. and Laplanche, G. and Divinski, S.V. and Wilde, G.
    Acta Materialia 203 (2021)
    A single Cr-rich σ-phase alloy with a composition of Co17Cr46Fe16.3Mn15.2Ni5.5 (at.%) and a tetragonal lattice structure was produced. The tracer diffusion coefficients of Ni and Fe were measured by secondary electron mass spectroscopy using the highly enriched 64Ni and 58Fe natural isotopes. On the homologous temperature scale, Ni and Fe diffuse in the σ phase faster as compared to the corresponding diffusion rates in the equiatomic and face-centered cubic CoCrFeMnNi alloy. In contrast, on the absolute temperature scale, these elements diffuse roughly at the same rates in both materials. Factors influencing element diffusion and phase stability of the σ phase compared to the equiatomic alloy are discussed. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.116498
  • 2020 • 251 A strong and ductile medium-entropy alloy resists hydrogen embrittlement and corrosion
    Luo, H. and Sohn, S.S. and Lu, W. and Li, L. and Li, X. and Soundararajan, C.K. and Krieger, W. and Li, Z. and Raabe, D.
    Nature Communications 11 (2020)
    Strong and ductile materials that have high resistance to corrosion and hydrogen embrittlement are rare and yet essential for realizing safety-critical energy infrastructures, hydrogen-based industries, and transportation solutions. Here we report how we reconcile these constraints in the form of a strong and ductile CoNiV medium-entropy alloy with face-centered cubic structure. It shows high resistance to hydrogen embrittlement at ambient temperature at a strain rate of 10−4 s−1, due to its low hydrogen diffusivity and the deformation twinning that impedes crack propagation. Moreover, a dense oxide film formed on the alloy’s surface reduces the hydrogen uptake rate, and provides high corrosion resistance in dilute sulfuric acid with a corrosion current density below 7 μA cm−2. The combination of load carrying capacity and resistance to harsh environmental conditions may qualify this multi-component alloy as a potential candidate material for sustainable and safe infrastructures and devices. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-020-16791-8
  • 2020 • 250 Grain boundary energy effect on grain boundary segregation in an equiatomic high-entropy alloy
    Li, L. and Kamachali, R.D. and Li, Z. and Zhang, Z.
    Physical Review Materials 4 (2020)
    Grain boundary (GB) segregation has a substantial effect on the microstructure evolution and properties of polycrystalline alloys. The mechanism of nanoscale segregation at the various GBs in multicomponent alloys is of great challenge to reveal and remains elusive so far. To address this issue, we studied the GB segregation in a representative equiatomic FeMnNiCoCr high-entropy alloy (HEA) aged at 450 °C. By combining transmission Kikuchi diffraction, atom probe tomography analysis and a density-based thermodynamics modeling, we uncover the nanoscale segregation behavior at a series of well-characterized GBs of different characters. No segregation occurs at coherent twin boundaries; only slight nanoscale segregation of Ni takes place at the low-angle GBs and vicinal ς29b coincidence site lattice GBs. Ni and Mn show cosegregation of high levels at the general high-angle GBs with a strong depletion in Fe, Cr, and Co. Our density-based thermodynamic model reveals that the highly negative energy of mixing Ni and Mn is the main driving force for nanoscale cosegregation to the GBs. This is further assisted by the opposite segregation of Ni and Cr atoms with a positive enthalpy of mixing. It is also found that GBs of higher interfacial energy, possessing lower atomic densities (higher disorder and free volume), show higher segregation levels. By clarifying the origins of GB segregations in the FeMnNiCoCr HEA, the current work provides fundamental ideas on nanoscale segregation at crystal defects in multicomponent alloys. © 2020 authors.
    view abstractdoi: 10.1103/PhysRevMaterials.4.053603
  • 2020 • 249 Hard X-ray-based techniques for structural investigations of CO2methanation catalysts prepared by MOF decomposition
    Prinz, N. and Schwensow, L. and Wendholt, S. and Jentys, A. and Bauer, M. and Kleist, W. and Zobel, M.
    Nanoscale 12 15800-15813 (2020)
    Thermal decomposition of metal-organic framework (MOF) precursors is a recent method to create well-dispersed metal centers within active catalyst materials with enhanced stability, as required for dynamic operation conditions in light of challenges caused by the renewable energy supply. Here, we use a hard X-ray-based toolbox of pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) analysis combined with X-ray diffraction and catalytic activity tests to investigate structure-activity correlations of methanation catalysts obtained by thermal decomposition of a Ni(BDC)(PNO) MOF precursor. Increasing the decomposition temperature from 350 to 500 °C resulted in Nifcc nanoparticles with increasing particle sizes, alongside a decrease in Ni2+ species and strain-induced peak broadening. For lower temperatures and inert atmosphere, Ni3C and NiO phases co-existed. A graphitic shell stabilized the Ni particles. Compared to an inert atmosphere, reducing conditions led to larger particles and a faster decomposition of the MOF precursor. Catalytic studies revealed that the decomposition at an intermediate temperature of 375 °C in 5% H2/He is the best set of parameters to obtain high specific surface areas while maintaining particle sizes that feature many active Ni centers for the formation of CH4. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0nr01750g
  • 2020 • 248 High-strength Damascus steel by additive manufacturing
    Kürnsteiner, P. and Wilms, M.B. and Weisheit, A. and Gault, B. and Jägle, E.A. and Raabe, D.
    Nature 582 515-519 (2020)
    Laser additive manufacturing is attractive for the production of complex, three-dimensional parts from metallic powder using a computer-aided design model1–3. The approach enables the digital control of the processing parameters and thus the resulting alloy’s microstructure, for example, by using high cooling rates and cyclic re-heating4–10. We recently showed that this cyclic re-heating, the so-called intrinsic heat treatment, can trigger nickel-aluminium precipitation in an iron–nickel–aluminium alloy in situ during laser additive manufacturing9. Here we report a Fe19Ni5Ti (weight per cent) steel tailor-designed for laser additive manufacturing. This steel is hardened in situ by nickel-titanium nanoprecipitation, and martensite is also formed in situ, starting at a readily accessible temperature of 200 degrees Celsius. Local control of both the nanoprecipitation and the martensitic transformation during the fabrication leads to complex microstructure hierarchies across multiple length scales, from approximately 100-micrometre-thick layers down to nanoscale precipitates. Inspired by ancient Damascus steels11–14—which have hard and soft layers, originally introduced via the folding and forging techniques of skilled blacksmiths—we produced a material consisting of alternating soft and hard layers. Our material has a tensile strength of 1,300 megapascals and 10 per cent elongation, showing superior mechanical properties to those of ancient Damascus steel12. The principles of in situ precipitation strengthening and local microstructure control used here can be applied to a wide range of precipitation-hardened alloys and different additive manufacturing processes. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41586-020-2409-3
  • 2020 • 247 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 • 246 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 • 245 On the assessment of creep damage evolution in nickel-based superalloys through correlative HR-EBSD and cECCI studies
    Sulzer, S. and Li, Z. and Zaefferer, S. and Hafez Haghighat, S.M. and Wilkinson, A. and Raabe, D. and Reed, R.
    Acta Materialia 185 13-27 (2020)
    The evolution of dislocation density with creep strain in single-crystal superalloys is studied quantitatively using high-resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging under controlled diffraction conditions (cECCI). Data regarding dislocation density/structure is measured for deformation at 900 °C and 450 MPa up to ≈ 1% plastic strain. Effects of chemical composition are elucidated via three purpose-designed superalloys of differing rhenium and ruthenium contents. The evidence indicates that dislocation avalanching is already prevalent at plastic strains of ≈ 0.1%; thereafter, an exponential decay in the dislocation multiplication rate is indicative of self-hardening due to dislocation constriction within the matrix channels, as confirmed by the imaging. The results are rationalised using discrete dislocation dynamics modelling: a universal dislocation evolution law emerges, which will be useful for alloy design efforts. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.07.018
  • 2020 • 244 Spinodal decomposition versus classical γ′ nucleation in a nickel-base superalloy powder: An in-situ neutron diffraction and atomic-scale analysis
    Collins, D.M. and D'Souza, N. and Panwisawas, C. and Papadaki, C. and West, G.D. and Kostka, A. and Kontis, P.
    Acta Materialia 200 959-970 (2020)
    Contemporary powder-based polycrystalline nickel-base superalloys inherit microstructures and properties that are heavily determined by their thermo-mechanical treatments during processing. Here, the influence of a thermal exposure to an alloy powder is studied to elucidate the controlling formation mechanisms of the strengthening precipitates using a combination of atom probe tomography and in-situ neutron diffraction. The initial powder comprised a single-phase supersaturated γ only; from this, the evolution of γ′ volume fraction and lattice misfit was assessed. The initial powder notably possessed elemental segregation of Cr and Co and elemental repulsion between Ni, Al and Ti with Cr; here proposed to be a precursor for subsequent γ to γ′ phase transformations. Subsolvus heat treatments yielded a unimodal γ′ distribution, formed during heating, with evidence supporting its formation to be via spinodal decomposition. A supersolvus heat treatment led to the formation of this same γ′ population during heating, but dissolves as the temperature increases further. The γ′ then reprecipitates as a multimodal population during cooling, here forming by classical nucleation and growth. Atom probe characterisation provided intriguing precipitate characteristics, including clear differences in chemistry and microstructure, depending on whether the γ′ formed during heating or cooling. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.09.055
  • 2020 • 243 Tracking the ultrafast nonequilibrium energy flow between electronic and lattice degrees of freedom in crystalline nickel
    Maldonado, P. and Chase, T. and Reid, A.H. and Shen, X. and Li, R.K. and Carva, K. and Payer, T. and Horn-von Hoegen, M. and Sokolowski-Tinten, K. and Wang, X.J. and Oppeneer, P.M. and Dürr, H.A.
    Physical Review B 101 (2020)
    Femtosecond laser excitation of solid-state systems creates out-of-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and ultimately to lattice vibrations of the solid. By combining ab initio calculations with ultrafast diffuse electron scattering, we gain a detailed understanding of the complex nonequilibrium energy transfer between electrons and phonons in laser-excited Ni metal. Our experimental results show that the wave-vector-resolved population dynamics of phonon modes is distinctly different throughout the Brillouin zone and are in remarkable agreement with our theoretical results. We find that zone-boundary phonon modes become occupied first. As soon as the energy in these modes becomes larger than the average electron energy, a backflow of energy from lattice to electronic degrees of freedom occurs. Subsequent excitation of lower-energy phonon modes drives the thermalization of the whole system on the picosecond time scale. We determine the evolving nonequilibrium phonon occupations, which we find to deviate markedly from thermal occupations. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "" Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by "" Bibsam.
    view abstractdoi: 10.1103/PhysRevB.101.100302
  • 2020 • 242 Ultrafast Construction of Oxygen-Containing Scaffold over Graphite for Trapping Ni2+into Single Atom Catalysts
    Liu, Z. and Li, S. and Yang, J. and Tan, X. and Yu, C. and Zhao, C. and Han, X. and Huang, H. and Wan, G. and Liu, Y. and Tschulik, K. and Qiu, J.
    ACS Nano 14 11662-11669 (2020)
    Ultrafast construction of oxygen-containing scaffold over graphite for trapping Ni2+ into single atom catalysts (SACs) was developed and presented by a one-step electrochemical activation technique. The present method for Ni SACs starts with graphite foil and is capable of achieving ultrafast preparation (1.5 min) and mass production. The defective oxygen featuring the strong electronegativity enables primarily attracting Ni2+ ions and stabilizing Ni atoms via Ni-O6 coordination instead of conventional metal-C or metal-N. In addition, the oxygen defects for trapping are tunable through altering the applied voltage or electrolyte, further altering the loading of Ni atoms, indicative of enhanced oxygen evolution activity. This simple and ultrafast electrochemical synthesis is promising for the mass and controllable production of oxygen-coordinated Ni SACs, which exhibit good performance for oxygen evolution reaction. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.0c04210
  • 2020 • 241 Unveiling the Re effect in Ni-based single crystal superalloys
    Wu, X. and Makineni, S.K. and Liebscher, C.H. and Dehm, G. and Rezaei Mianroodi, J. and Shanthraj, P. and Svendsen, B. and Bürger, D. and Eggeler, G. and Raabe, D. and Gault, B.
    Nature Communications 11 (2020)
    Single crystal Ni-based superalloys have long been an essential material for gas turbines in aero engines and power plants due to their outstanding high temperature creep, fatigue and oxidation resistance. A turning point was the addition of only 3 wt.% Re in the second generation of single crystal Ni-based superalloys which almost doubled the creep lifetime. Despite the significance of this improvement, the mechanisms underlying the so-called “Re effect” have remained controversial. Here, we provide direct evidence of Re enrichment to crystalline defects formed during creep deformation, using combined transmission electron microscopy, atom probe tomography and phase field modelling. We reveal that Re enriches to partial dislocations and imposes a drag effect on dislocation movement, thus reducing the creep strain rate and thereby improving creep properties. These insights can guide design of better superalloys, a quest which is key to reducing CO2 emissions in air-traffic. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-019-14062-9
  • 2019 • 240 Ablation target cooling by maximizing the nanoparticle productivity in laser synthesis of colloids
    Waag, F. and Gökce, B. and Barcikowski, S.
    Applied Surface Science 466 647-656 (2019)
    Even if ultrashort laser pulses are used during the laser synthesis of colloids, a significant amount of laser energy is converted into thermal energy, which results in heating the ablation target and the colloid. To date, little attention has been paid to these heating effects in the literature. This study was focused on measurements of the process temperature during the high-power, ultrashort-pulsed laser ablation of a nickel target in a continuous water flow setup. Time-resolved monitoring of the temperature of the ablation target and of the colloid indicated that there was an initial rapid uptake of thermal energy followed by a thermally-stable state in which there was very little additional heating. Shifting the focal plane from behind the target onto its surface and further into the fluid provided insight concerning the different mechanisms of heat generation, dissipation, and transfer in the laser synthesis of colloids. It even was possible to distinguish the fluence effects and the colloid re-irradiation effects. New possibilities of process control were identified by correlating the productivity of laser ablation at different focal plane shifts with the measured thermal data. Counterintuitively, the temperature of the target was minimized via ablation cooling when the productivity of the process was maximized. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.10.030
  • 2019 • 239 Atomic-scale grain boundary engineering to overcome hot-cracking in additively-manufactured superalloys
    Kontis, P. and Chauvet, E. and Peng, Z. and He, J. and da Silva, A.K. and Raabe, D. and Tassin, C. and Blandin, J.-J. and Abed, S. and Dendievel, R. and Gault, B. and Martin, G.
    Acta Materialia 177 209-221 (2019)
    There are still debates regarding the mechanisms that lead to hot cracking in parts build by additive manufacturing (AM) of non-weldable nickel-based superalloys. This lack of in-depth understanding of the root causes of hot cracking is an impediment to designing engineering parts for safety-critical applications. Here, we deploy a near-atomic-scale approach to investigate the details of the compositional decoration of grain boundaries in the coarse-grained, columnar microstructure in parts built from a non-weldable nickel-based superalloy by selective electron-beam melting. The progressive enrichment in Cr, Mo and B at grain boundaries over the course of the AM-typical successive solidification and remelting events, accompanied by solid-state diffusion, causes grain boundary segregation induced liquation. This observation is consistent with thermodynamic calculations. We demonstrate that by adjusting build parameters to obtain a fine-grained equiaxed or a columnar microstructure with grain width smaller than 100 μm enables to avoid cracking, despite strong grain boundary segregation. We find that the spread of critical solutes to a higher total interfacial area, combined with lower thermal stresses, helps to suppress interfacial liquation. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.07.041
  • 2019 • 238 Effect of heat treatment on the high temperature fatigue life of single crystalline nickel base superalloy additively manufactured by means of selective electron beam melting
    Meid, C. and Dennstedt, A. and Ramsperger, M. and Pistor, J. and Ruttert, B. and Lopez-Galilea, I. and Theisen, W. and Körner, C. and Bartsch, M.
    Scripta Materialia 168 124-128 (2019)
    The high temperature low cycle fatigue behavior of specimens manufactured from a single crystalline nickel base superalloy processed by selective electron beam melting (SEBM) has been investigated with respect to the effect of different heat treatments. The fatigue lifetime of heat treated material was significantly higher than that of as-built material. Applying hot isostatic pressing (HIP) with an integrated heat treatment resulted in even longer fatigue life. Lifetime limiting crack initiation occurred at interfaces of melting layers, at micro-porosity generated during solidification or, in HIP treated samples, at precipitates which formed at the location of collapsed pores. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2019.05.002
  • 2019 • 237 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 • 236 Inducing n - And p -Type Thermoelectricity in Oxide Superlattices by Strain Tuning of Orbital-Selective Transport Resonances
    Geisler, B. and Pentcheva, R.
    Physical Review Applied 11 (2019)
    By combining first-principles simulations including an on-site Coulomb-repulsion term and Boltzmann theory, we demonstrate how the interplay of quantum confinement and epitaxial strain allows one to selectively design n- and p-type thermoelectric response in (LaNiO3)3/(LaAlO3)1(001) superlattices. In particular, varying strain from -4.9% to 2.9% tunes the Ni orbital polarization at the interfaces from -6% to 3%. This is caused by an electron redistribution among Ni 3dx2-y2- and 3dz2-derived quantum-well states, which respond differently to strain. Owing to this charge transfer, the position of emerging cross-plane-transport resonances can be tuned relative to the Fermi energy. For moderate compressive strains of -1.5% and -2.8%, the cross-plane Seebeck coefficient reaches approximately -60 and 100μV/K at around room temperature, respectively. This provides a compelling mechanism to tailor thermoelectric materials. Finally, we demonstrate the robustness of the proposed concept with respect to oxygen-vacancy formation. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevApplied.11.044047
  • 2019 • 235 Miniaturization of low cycle fatigue-testing of single crystal superalloys at high temperature for uncoated and coated specimens [Miniaturisierung der Versuchstechnik für Niedrig-Lastwechsel-Ermüdung bei Hochtemperatur an Proben aus einkristallinen Superlegierungen mit und ohne Schutzschichten]
    Meid, C. and Waedt, U. and Subramaniam, A. and Wischek, J. and Bartsch, M. and Terberger, P. and Vaßen, R.
    Materialwissenschaft und Werkstofftechnik 50 777-787 (2019)
    A newly developed miniature specimen and respective fixture for high temperature low cycle fatigue testing of nickel based single crystal superalloys is presented. Miniaturization allows the preparation of test specimens in all main crystallographic orientations of the cubic nickel crystal using laboratory sized material samples and enables excellent utilization of the costly material. The specimen geometry is optimized by means of parameter studies employing numerical calculations such that for the main crystallographic orientations the stress concentration at the fillet between gauge length and specimen head is minimized, and failure is likely to occur within the gauge length. The designed fixture allows easy specimen mounting and provides sufficient support for applying an extensometer for strain measurement. Protective metallic coatings against oxidation can be applied on the specimen by plasma spraying for studying the effect of coatings on the fatigue lifetime. The functionality of the specimen geometry and fixture design for low cycle fatigue testing is demonstrated for temperatures up to 950 °C. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/mawe.201800135
  • 2019 • 234 Segregation-driven grain boundary spinodal decomposition as a pathway for phase nucleation in a high-entropy alloy
    Li, L. and Li, Z. and Kwiatkowski da Silva, A. and Peng, Z. and Zhao, H. and Gault, B. and Raabe, D.
    Acta Materialia 178 1-9 (2019)
    Elemental segregation to grain boundaries (GBs) can induce structural and chemical transitions at GBs along with significant changes in material properties. The presence of multiple principal elements interacting in high-entropy alloys (HEAs) makes the GB segregation and interfacial phase transformation a rather challenging subject to investigate. Here, we explored the temporal evolution of the chemistry for general high-angle GBs in a typical equiatomic FeMnNiCoCr HEA during aging heat treatment through detailed atom probe tomography (APT) analysis. We found that the five principal elements segregate heterogeneously at the GBs. More specifically, Ni and Mn co-segregate to some regions of the GBs along with the depletion of Fe, Co and Cr, while Cr is enriched in other regions of the GBs where Ni and Mn are depleted. The redistribution of these elements on the GBs follow a periodic characteristic, spinodal-like compositional modulation. The accumulation of elements at the GBs can create local compositions by shifting their state from a solid solution (like in the adjacent bulk region) into a spinodal regime to promote interfacial phase-like transitions as segregation proceeds. These results not only shed light on phase precursor states and the associated nucleation mechanism at GBs in alloy systems with multiple principal elements but also help to guide the microstructure design of advanced HEAs in which formation of embrittling phases at interfaces must be avoided. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.07.052
  • 2019 • 233 Shell-ferromagnetism and decomposition in off-stoichiometric Ni 50 Mn 50-x Sb x Heuslers
    Wanjiku, Z. and Çaklr, A. and Scheibel, F. and Wiedwald, U. and Farle, M. and Acet, M.
    Journal of Applied Physics 125 (2019)
    Off stoichiometric Heuslers in the form Ni 50Mn 50 - x Z x, where Z can be a group 13-15 element of the periodic system, decompose at about 650 K into a ferromagnetic full Heusler Ni 50Mn 25 Z 25 and an antiferromagnetic Ni 50Mn 50 component. We study here the case for Z as Sb and report on shell-ferromagnetic properties as well as thermal instabilities. Unlike the case for other Z-elements, in Ni 50Mn 50 - xSb x, the minimum decomposition temperature corresponds to a temperature lying within the austenite state so that it is possible to observe the change in the martensitic transition temperature while annealing, thus providing further information on the change of composition during annealing. Scherrer analysis performed on emerging peaks related to the cubic full-Heusler shows that the precipitate size for shell-FM properties to become observable is around 5-10 nm. Other than vertical shifts in the field-dependence of the magnetization, which are also observed in compounds with Z other than Sb, concurrent exchange-bias effects are observed in the case with Z as Sb. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5057763
  • 2019 • 232 Shell-ferromagnetism and decomposition in off-stoichiometric Ni50Mn50-xSbx Heuslers
    Wanjiku, Z. and Çaklr, A. and Scheibel, F. and Wiedwald, U. and Farle, M. and Acet, M.
    Journal of Applied Physics 125 (2019)
    Off stoichiometric Heuslers in the form Ni 50Mn 50 - x Z x, where Z can be a group 13-15 element of the periodic system, decompose at about 650 K into a ferromagnetic full Heusler Ni 50Mn 25 Z 25 and an antiferromagnetic Ni 50Mn 50 component. We study here the case for Z as Sb and report on shell-ferromagnetic properties as well as thermal instabilities. Unlike the case for other Z-elements, in Ni 50Mn 50 - xSb x, the minimum decomposition temperature corresponds to a temperature lying within the austenite state so that it is possible to observe the change in the martensitic transition temperature while annealing, thus providing further information on the change of composition during annealing. Scherrer analysis performed on emerging peaks related to the cubic full-Heusler shows that the precipitate size for shell-FM properties to become observable is around 5-10 nm. Other than vertical shifts in the field-dependence of the magnetization, which are also observed in compounds with Z other than Sb, concurrent exchange-bias effects are observed in the case with Z as Sb. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5057763
  • 2019 • 231 Strategies for improving the sustainability of structural metals
    Raabe, D. and Tasan, C.C. and Olivetti, E.A.
    Nature 575 64-74 (2019)
    Metallic materials have enabled technological progress over thousands of years. The accelerated demand for structural (that is, load-bearing) alloys in key sectors such as energy, construction, safety and transportation is resulting in predicted production growth rates of up to 200 per cent until 2050. Yet most of these materials require a lot of energy when extracted and manufactured and these processes emit large amounts of greenhouse gases and pollution. Here we review methods of improving the direct sustainability of structural metals, in areas including reduced-carbon-dioxide primary production, recycling, scrap-compatible alloy design, contaminant tolerance of alloys and improved alloy longevity. We discuss the effectiveness and technological readiness of individual measures and also show how novel structural materials enable improved energy efficiency through their reduced mass, higher thermal stability and better mechanical properties than currently available alloys. © 2019, Springer Nature Limited.
    view abstractdoi: 10.1038/s41586-019-1702-5
  • 2019 • 230 The Role of Non-Metallic and Metalloid Elements on the Electrocatalytic Activity of Cobalt and Nickel Catalysts for the Oxygen Evolution Reaction
    Masa, J. and Schuhmann, W.
    ChemCatChem 11 5842-5854 (2019)
    Compounds and alloys of cobalt and nickel with some nonmetals (N, P, S, Se) and metalloids (C, B, C, As and Te) have emerged as very promising noble metal-free pre-catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, the exact role played by the non-metals and metalloids in promoting the OER is not well understood. A holistic understanding of the origin of the OER activity enhancement in these compounds is vital for their exploitation as models to inspire knowledge-guided design of improved OER catalysts. In this review, we elucidate the factors that govern the activity and stability of OER catalysts derived from MX compounds (M=Co or Ni, and X=nonmetal or metalloid), including the impact of surface electronic structure, M : X stoichiometry, material composition, structure and crystallinity, as well as the role of oxoanions on the properties of the electrochemical double layer and interaction energies of the reaction intermediates. Finally, we outline a few perspectives and research directions towards a deeper understanding of the role of the nonmetal and metalloid elements and design of improved OER catalysts. ©2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cctc.201901151
  • 2019 • 229 Tuning the magnetic anisotropy of niptmnga by substitution and epitaxial strain
    Herper, H.C. and Grunebohm, A.
    IEEE Transactions on Magnetics 55 (2019)
    Large magnetocrystalline anisotropy (MCA) is of high technical relevance, in particular for magnetic actuators, permanent magnets, and memory devices with high density. Large MCAs have been reported for the low temperature L10 phase of Ni2MnGa. Both, Mn and Pt substitution can stabilize this phase at and above room temperature. Despite the larger spin-orbit coupling in the heavy 5d-element Pt, it has been reported that Pt substitution may result in degeneration of the MCA. In this paper, we study the MCA for a combination of epitaxial strain and Mn and Pt substitution based on density functional theory methods. We show that one can stabilize both large uniaxial and easy-plane anisotropies depending on the value of strain. In particular, small changes of the applied strain may allow to switch between low- and high-anisotropy states or even switch the direction of the easy-axis magnetization direction. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2018.2856461
  • 2018 • 228 A phenomenological creep model for nickel-base single crystal superalloys at intermediate temperatures
    Gao, S. and Wollgramm, P. and Eggeler, G. and Ma, A. and Schreuer, J. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 26 (2018)
    For the purpose of good reproduction and prediction of creep deformation of nickel-base single crystal superalloys at intermediate temperatures, a phenomenological creep model is developed, which accounts for the typical γ/γ′ microstructure and the individual thermally activated elementary deformation processes in different phases. The internal stresses from γ/γ′ lattice mismatch and deformation heterogeneity are introduced through an efficient method. The strain hardening, the Orowan stress, the softening effect due to dislocation climb along γ/γ′ interfaces and the formation of dislocation ribbons, and the Kear-Wilsdorf-lock effect as key factors in the main flow rules are formulated properly. By taking the cube slip in slip systems and twinning mechanisms into account, the creep behavior for [110] and [111] loading directions are well captured. Without specific interaction and evolution of dislocations, the simulations of this model achieve a good agreement with experimental creep results and reproduce temperature, stress and crystallographic orientation dependences. It can also be used as the constitutive relation at material points in finite element calculations with complex boundary conditions in various components of superalloys to predict creep behavior and local stress distributions. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/aabdbe
  • 2018 • 227 Annealing-time and annealing-temperature dependencies of the size of Ni-Mn-In shell-ferromagnetic nano-precipitates by Scherrer analysis
    Dincklage, L. and Scheibel, F. and Çaklr, A. and Farle, M. and Acet, M.
    AIP Advances 8 (2018)
    Shell-ferromagnetic effects are observed in Ni-Mn-based off-stoichiometric Heuslers decomposed into ferromagnetic precipitates embedded in an antiferromagnetic matrix when the surface-to-volume ratio of the precipitates are sufficiently large. However, since the size of the precipitates have until now not been determined, it is not known which ratios are involved. Here we carry out a Scherrer analysis on decomposed specimens to determine the precipitate-size as a function of decomposition temperature and time. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5018851
  • 2018 • 226 Atomic Layer Deposition of Nickel on ZnO Nanowire Arrays for High-Performance Supercapacitors
    Ren, Q.-H. and Zhang, Y. and Lu, H.-L. and Wang, Y.-P. and Liu, W.-J. and Ji, X.-M. and Devi, A. and Jiang, A.-Q. and Zhang, D.W.
    ACS Applied Materials and Interfaces 10 468-476 (2018)
    A novel hybrid core-shell structure of ZnO nanowires (NWs)/Ni as a pseudocapacitor electrode was successfully fabricated by atomic layer deposition of a nickel shell, and its capacitive performance was systemically investigated. Transmission electron microscopy and X-ray photoelectron spectroscopy results indicated that the NiO was formed at the interface between ZnO and Ni where the Ni was oxidized by ZnO during the ALD of the Ni layer. Electrochemical measurement results revealed that the Ti/ZnO NWs/Ni (1500 cycles) electrode with a 30 nm thick Ni-NiO shell layer had the best supercapacitor properties including ultrahigh specific capacitance (∼2440 F g-1), good rate capability (80.5%) under high current charge-discharge conditions, and a relatively better cycling stability (86.7% of the initial value remained after 750 cycles at 10 A g-1). These attractive capacitive behaviors are mainly attributed to the unique core-shell structure and the combined effect of ZnO NW arrays as short charge transfer pathways for ion diffusion and electron transfer as well as conductive Ni serving as channel for the fast electron transport to Ti substrate. This high-performance Ti/ZnO NWs/Ni hybrid structure is expected to be one of a promising electrodes for high-performance supercapacitor applications. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b13392
  • 2018 • 225 Catalyst Support Effect on the Activity and Durability of Magnetic Nanoparticles: Toward Design of Advanced Electrocatalyst for Full Water Splitting
    Davodi, F. and Mühlhausen, E. and Tavakkoli, M. and Sainio, J. and Jiang, H. and Gökce, B. and Marzun, G. and Kallio, T.
    ACS Applied Materials and Interfaces 10 31300-31311 (2018)
    Earth-abundant element-based inorganic-organic hybrid materials are attractive alternatives for electrocatalyzing energy conversion reactions. Such material structures do not only increase the surface area and stability of metal nanoparticles (NPs) but also modify the electrocatalytic performance. Here, we introduce, for the first time, multiwall carbon nanotubes (MWNTs) functionalized with nitrogen-rich emeraldine salt (ES) (denoted as ES-MWNT) as a promising catalyst support to boost the electrocatalytic activity of magnetic maghemite (γ-Fe2O3) NPs. The latter component has been synthesized by a simple and upscalable one-step pulsed laser ablation method on Ni core forming the core-shell Niγ-Fe2O3 NPs. The catalyst (Niγ-Fe2O3/ES-MWNT) is formed via self-assembly as strong interaction between ES-MWNT and Niγ-Fe2O3 results in NPs' encapsulation in a thin C-N shell. We further show that Ni does not directly function as an active site in the electrocatalyst but it has a crucial role in synthesizing the maghemite shell. The strong interaction between the NPs and the support improves notably the NPs' catalytic activity toward oxygen evolution reaction (OER) in terms of both onset potential and current density, ranking it among the most active catalysts reported so far. Furthermore, this material shows a superior durability to most of the current excellent OER electrocatalysts as the activity, and the structure, remains almost intact after 5000 OER stability cycles. On further characterization, the same trend has been observed for hydrogen evolution reaction, the other half-reaction of water splitting. Copyright © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acsami.8b08830
  • 2018 • 224 Coffee-Waste Templating of Metal Ion-Substituted Cobalt Oxides for the Oxygen Evolution Reaction
    Yu, M. and Chan, C.K. and Tüysüz, H.
    ChemSusChem 11 605-611 (2018)
    A facile and scalable method using coffee waste grounds as a hard template has been developed to fabricate nanostructured Co3O4 for the oxygen evolution reaction (OER). Co3O4 incorporating metals with different valences (M/Co=1:4; M=Cu, Ni, Fe, Cr, and W) were also prepared with similar sheet-like structures comprising nanosized crystallites. After detailed characterization by X-ray diffraction, electron microscopy, and nitrogen sorption, the oxides were employed as OER electrocatalysts. Substitution of octahedral and tetrahedral sites of the spinel structure with divalent and trivalent transition metals (Cu, Ni, Fe, and Cr) increased the activity of Co3O4 for the OER, whereas incorporation of hexavalent W led to formation of a second crystal phase and significantly higher electrocatalytic performance. Furthermore, this method is easily scaled up for mass production of Co3O4 with the same nanostructure, which is highly desirable for large-scale application. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201701877
  • 2018 • 223 Composition-Dependent Effect of the Calcination of Cobalt-, Nickel-, and Gallium-Based Layered Double Hydroxides to Mixed Metal Oxides in the Oxygen Evolution Reaction
    Chakrapani, K. and Özcan, F. and Ortega, K.F. and Machowski, T. and Behrens, M.
    ChemElectroChem 5 93-100 (2018)
    Mixed cobalt and nickel based layered double hydroxides (LDHs) with Ga as the third cation and the mixed metal oxides (MMOs) resulting from their thermal decomposition were synthesized in various compositions through constant pH co-precipitation and calcination. The structural and textural properties of the catalysts with variable Co/Ni ratios were assessed by N2 physisorption, powder X-ray diffraction, and electron microscopy. The obtained materials exhibit electrocatalytic activity for the oxygen evolution reaction in alkaline solution. The highest activity was found for catalysts containing both transition-metal cations, Co and Ni. However, comparison of the LDH precursors and the calcined MMOs revealed a composition-dependent effect of calcination. Co-rich LDH tends to lose activity when calcined, whereas Ni-rich LDH gains activity. The optimal cation composition of the LDH was Co1.5Ni0.5Ga with an overpotential of 382 mV. The highest performance among the MMOs, on the other hand, has been encountered for the Co0.5Ni1.5Ga composition, reaching a similar overpotential. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201700936
  • 2018 • 222 Dry Reforming of Methane at High Pressure in a Fixed-Bed Reactor with Axial Temperature Profile Determination
    Tillmann, L. and Schulwitz, J. and van Veen, A. and Muhler, M.
    Catalysis Letters 148 2256-2262 (2018)
    Abstract: A continuously operated flow setup with fixed-bed reactor and online gas analysis enabled kinetic investigations of catalysts for the carbon dioxide reforming of methane under industrially relevant conditions at temperatures up to 1000 °C and at pressures up to 20 bar. A coaxial reactor design consisting of an inner- and an outer highly alloyed steel tube allowed obtaining axial temperature profiles by means of a moveable thermocouple. A NiAl2O4-based catalyst was tested at 820 °C and pressures of 1, 10 or 20 bar and compared to a conventional Ni catalyst used for steam reforming of methane. A significant cold spot was detected even when using only 10 mg of catalysts diluted in 1 g of silicon carbide. The specifically designed NiAl2O4/Al2O3 dry reforming catalyst with a high dispersion of the active Ni0 phase was found to be far superior to the conventional steam reforming catalyst. Graphical Abstract: [Figure not available: see fulltext.] © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstractdoi: 10.1007/s10562-018-2453-x
  • 2018 • 221 Dual properties of a hydrogen oxidation Ni-catalyst entrapped within a polymer promote self-defense against oxygen /639/638/77/886 /639/638/161/893 /639/638/675 /120 /128 /140/131 article
    Oughli, A.A. and Ruff, A. and Boralugodage, N.P. and Rodríguez-Maciá, P. and Plumeré, N. and Lubitz, W. and Shaw, W.J. and Schuhmann, W. and Rüdiger, O.
    Nature Communications 9 (2018)
    The Ni(P2N2)2 catalysts are among the most efficient non-noble-metal based molecular catalysts for H2 cycling. However, these catalysts are O2 sensitive and lack long term stability under operating conditions. Here, we show that in a redox silent polymer matrix the catalyst is dispersed into two functionally different reaction layers. Close to the electrode surface is the "active" layer where the catalyst oxidizes H2 and exchanges electrons with the electrode generating a current. At the outer film boundary, insulation of the catalyst from the electrode forms a "protection" layer in which H2 is used by the catalyst to convert O2 to H2O, thereby providing the "active" layer with a barrier against O2. This simple but efficient polymer-based electrode design solves one of the biggest limitations of these otherwise very efficient catalysts enhancing its stability for catalytic H2 oxidation as well as O2 tolerance. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03011-7
  • 2018 • 220 Effect of porosity and eutectics on the high-temperature low-cycle fatigue performance of a nickel-base single-crystal superalloy
    Ruttert, B. and Meid, C. and Mujica Roncery, L. and Lopez-Galilea, I. and Bartsch, M. and Theisen, W.
    Scripta Materialia 155 139-143 (2018)
    This work investigates the separate influence of porosity and γ/γ′-eutectics on the low-cycle fatigue life of a single-crystal Ni-base superalloy at high temperatures. A conventional vacuum furnace heat-treatment but also integrated heat-treatments in a hot isostatic press are applied to produce different material variants of the same alloy. High-resolution electron microscopy revealed that both pores and γ/γ′-eutectics act as crack starters, thus initiating early failure. Moreover, the results indicate that remaining γ/γ′-eutectics can weaken the fatigue resistance even more than pores. Furthermore, the results confirm the beneficial effect of proper integrated hot isostatic pressing heat-treatments on the fatigue performance. © 2018
    view abstractdoi: 10.1016/j.scriptamat.2018.06.036
  • 2018 • 219 Electrocatalytic Oxidation of 5-(Hydroxymethyl)furfural Using High-Surface-Area Nickel Boride
    Barwe, S. and Weidner, J. and Cychy, S. and Morales, D.M. and Dieckhöfer, S. and Hiltrop, D. and Masa, J. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 11460-11464 (2018)
    The electrochemical oxidation of the biorefinery product 5-(hydroxymethyl)furfural (HMF) to 2,5-furandicarboxylic acid (FDCA), an important platform chemical for the polymer industry, is receiving increasing interest. FDCA-based polymers such as polyethylene 2,5-furandicarboxylate (PEF) are sustainable candidates for replacing polyethylene terephthalate (PET). Herein, we report the highly efficient electrocatalytic oxidation of HMF to FDCA, using Ni foam modified with high-surface-area nickel boride (NixB) as the electrode. Constant potential electrolysis in combination with HPLC revealed a high faradaic efficiency of close to 100 % towards the production of FDCA with a yield of 98.5 %. Operando electrochemistry coupled to ATR-IR spectroscopy indicated that HMF is oxidized preferentially via 5-hydroxymethyl-2-furancarboxylic acid rather than via 2,5-diformylfuran, which is in agreement with HPLC results. This study not only reports a low-cost active electrocatalyst material for the electrochemical oxidation of HMF to FDCA, but additionally provides insight into the reaction pathway. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201806298
  • 2018 • 218 Formation of nickel nanoparticles and magnetic matrix in nickel phthalocyanine by doping with potassium
    Manukyan, A.S. and Avakyan, L.A. and Elsukova, A.E. and Zubavichus, Y.V. and Sulyanov, S.N. and Mirzakhanyan, A.A. and Kolpacheva, N.A. and Spasova, M. and Kocharian, A.N. and Farle, M. and Bugaev, L.A. and Sharoyan, E.G.
    Materials Chemistry and Physics 214 564-571 (2018)
    A method for synthesis of nickel nanoparticles in a magnetic nickel phthalocyanine anions matrix has been developed. The method is based on intercalation of potassium atoms to the nickel phthalocyanine (NiPc) polycrystalline powder at 300 °C. The structure of (K2NiPc) was investigated by using high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) spectroscopes. Magnetic properties were studied by SQUID magnetometry and magnetic resonances methods. It is revealed that the resultant compound contains of 1 wt% Ni nanoparticles with the average size of 15 nm. The measured values of the magnetization and absorption of the ferromagnetic resonance considerably exceed the magnetism which can be attributed to metallic Ni nanoparticles. The obtained results indicate the presence of room temperature molecular ferromagnetism caused by anionic molecules of NiPc. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2018.04.068
  • 2018 • 217 Hot cracking mechanism affecting a non-weldable Ni-based superalloy produced by selective electron Beam Melting
    Chauvet, E. and Kontis, P. and Jägle, E.A. and Gault, B. and Raabe, D. and Tassin, C. and Blandin, J.-J. and Dendievel, R. and Vayre, B. and Abed, S. and Martin, G.
    Acta Materialia 142 82-94 (2018)
    A non weldable nickel-based superalloy was fabricated by powder bed-based selective electron beam melting (S-EBM). The as-built samples exhibit a heterogeneous microstructure along the build direction. A gradient of columnar grain size as well as a significant gradient in the γ′ precipitate size were found along the build direction. Microstructural defects such as gas porosity inherited from the powders, shrinkage pores and cracks inherited from the S-EBM process were identified. The origins of those defects are discussed with a particular emphasis on crack formation. Cracks were consistently found to propagate intergranular and the effect of crystallographic misorientation on the cracking behavior was investigated. A clear correlation was identified between cracks and high angle grain boundaries (HAGB). The cracks were classified as hot cracks based on the observation of the fracture surface of micro-tensile specimens machined from as-built S-EBM samples. The conditions required to trigger hot cracking, namely, presence of a liquid film during the last stage of solidification and thermal stresses are discussed within the framework of additive manufacturing. Understanding the cracking mechanism enables to provide guidelines to obtain crack-free specimens of non-weldable Ni-based superalloys produced by S-EBM. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.09.047
  • 2018 • 216 Impact of chemical inhomogeneities on local material properties and hydrogen environment embrittlement in AISI 304L steels
    Egels, G. and Mujica Roncery, L. and Fussik, R. and Theisen, W. and Weber, S.
    International Journal of Hydrogen Energy (2018)
    This study investigated the influence of segregations on hydrogen environment embrittlement (HEE) of AISI 304L type austenitic stainless steels. The microstructure of tensile specimens, that were fabricated from commercially available AISI 304L steels and tested by means of small strain-rate tensile tests in air as well as hydrogen gas at room temperature, was investigated by means of combined EDS and EBSD measurements. It was shown that two different austenitic stainless steels having the same nominal alloy composition can exhibit different susceptibilities to HEE due to segregation effects resulting from different production routes (continuous casting/electroslag remelting). Local segregation-related variations of the austenite stability were evaluated by thermodynamic and empirical calculations. The alloying element Ni exhibits pronounced segregation bands parallel to the rolling direction of the material, which strongly influences the local austenite stability. The latter was revealed by generating and evaluating two-dimensional distribution maps for the austenite stability. The formation of deformation-induced martensite was shown to be restricted to segregation bands with a low Ni content. Furthermore, it was shown that the formation of hydrogen induced surface cracks is strongly coupled with the existence of surface regions of low Ni content and accordingly low austenite stability. In addition, the growth behavior of hydrogen-induced cracks was linked to the segregation-related local austenite stability. © 2018 The Author(s).
    view abstractdoi: 10.1016/j.ijhydene.2018.01.062
  • 2018 • 215 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 (>80 °C) and high current densities (>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 • 214 Influence of the Fe:Ni Ratio and Reaction Temperature on the Efficiency of (FexNi1-x)9S8 Electrocatalysts Applied in the Hydrogen Evolution Reaction
    Piontek, S. and Andronescu, C. and Zaichenko, A. and Konkena, B. and Junge Puring, K. and Marler, B. and Antoni, H. and Sinev, I. and Muhler, M. and Mollenhauer, D. and Roldan Cuenya, B. and Schuhmann, W. and Apfel, U.-P.
    ACS Catalysis 8 987-996 (2018)
    Inspired by our recent finding that Fe4.5Ni4.5S8 rock is a highly active electrocatalyst for HER, we set out to explore the influence of the Fe:Ni ratio on the performance of the catalyst. We herein describe the synthesis of (FexNi1-x)9S8 (x = 0-1) along with a detailed elemental composition analysis. Furthermore, using linear sweep voltammetry, we show that the increase in the iron or nickel content, respectively, lowers the activity of the electrocatalyst toward HER. Electrochemical surface area analysis (ECSA) clearly indicates the highest amount of active sites for a Fe:Ni ratio of 1:1 on the electrode surface pointing at an altered surface composition of iron and nickel for the other materials. Specific metal-metal interactions seem to be of key importance for the high electrocatalytic HER activity, which is supported by DFT calculations of several surface structures using the surface energy as a descriptor of catalytic activity. In addition, we show that a temperature increase leads to a significant decrease of the overpotential and gain in HER activity. Thus, we showcase the necessity to investigate the material structure, composition and reaction conditions when evaluating electrocatalysts. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b02617
  • 2018 • 213 Local Surface Structure and Composition Control the Hydrogen Evolution Reaction on Iron Nickel Sulfides
    Bentley, C.L. and Andronescu, C. and Smialkowski, M. and Kang, M. and Tarnev, T. and Marler, B. and Unwin, P.R. and Apfel, U.-P. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 4093-4097 (2018)
    In order to design more powerful electrocatalysts, developing our understanding of the role of the surface structure and composition of widely abundant bulk materials is crucial. This is particularly true in the search for alternative hydrogen evolution reaction (HER) catalysts to replace platinum. We report scanning electrochemical cell microscopy (SECCM) measurements of the (111)-crystal planes of Fe4.5Ni4.5S8, a highly active HER catalyst. In combination with structural characterization methods, we show that this technique can reveal differences in activity arising from even the slightest compositional changes. By probing electrochemical properties at the nanoscale, in conjunction with complementary structural information, novel design principles are revealed for application to rational material synthesis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201712679
  • 2018 • 212 Martensite aging in 〈0 0 1〉 oriented Co49Ni21Ga30 single crystals in tension
    Lauhoff, C. and Krooß, P. and Langenkämper, D. and Somsen, C. and Eggeler, G. and Kireeva, I. and Chumlyakov, Y.I. and Niendorf, T.
    Functional Materials Letters 11 (2018)
    Co-Ni-Ga high-temperature shape memory alloys (HT-SMAs) are well-known candidate materials for damping applications at elevated temperatures. Recent studies showed that upon heat treatment in stress-induced martensite under compressive loads transformation temperatures can be increased significantly, qualifying Co-Ni-Ga for HT-actuation. The increase in transformation temperatures is related to a change in chemical order recently validated via neutron diffraction experiments. Since SMAs show distinct tension-compression asymmetry in terms of theoretical transformation strains and bearable stresses, understanding the impact of martensite aging in tension is crucial for future applications. The current results indicate that martensite aging in tension provides for a further improvement in functional properties. © 2018 The Author(s).
    view abstractdoi: 10.1142/S1793604718500248
  • 2018 • 211 Misorientation distribution between martensite and austenite in Fe-31 wt%Ni-0.01 wt%C
    Zilnyk, K.D. and Almeida Junior, D.R. and Sandim, H.R.Z. and Rios, P.R. and Raabe, D.
    Acta Materialia 143 227-236 (2018)
    We characterized the morphology, substructure and crystallography of lenticular martensite in a Fe-Ni-C alloy by means of electron backscatter diffraction and scanning electron microscopy. Electron backscatter diffraction maps were used to determine the orientation relationship between austenite and martensite across large regions of the microstructure. We employed orientation distribution functions as a statistical representation method for the observed orientation relationships. High-resolution point-to-point scans were used to normalize the effects of the orientation changes in the austenite caused by the plastic deformation during the formation of lenticular martensite. The analysis revealed that most of the transformation in this material follows an orientation relationship close to the one proposed by Greninger and Troiano. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.10.026
  • 2018 • 210 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 • 209 On the nucleation of planar faults during low temperature and high stress creep of single crystal Ni-base superalloys
    Wu, X. and Dlouhy, A. and Eggeler, Y.M. and Spiecker, E. and Kostka, A. and Somsen, C. and Eggeler, G.
    Acta Materialia 144 642-655 (2018)
    The present work studies the nucleation of planar faults in the early stages of low temperature (750 °C) and high stress (800 MPa) creep of a Ni-base single crystal superalloy (SX). Two families of 60° dislocations with different Burgers vectors were detected in the transmission electron microscope (TEM). These can react and form a planar fault in the γ′ phase. A 2D discrete dislocation model helps to rationalize a sequence of events which lead to the nucleation of a planar fault. First, one 60° channel dislocation approaches another 60° interface dislocation with a different Burgers vector. At a distance of 5 nm, it splits up into two Shockley partials. The interface dislocation is pushed into the γ′-phase where it creates a small antiphase boundary. It can only move on when the leading Shockley partial joins it and creates an overall 1/3<112> superdislocation. This process is fast and therefore is difficult to observe. The results obtained in the present work contribute to a better understanding of the processes which govern the early stages of low temperature and high stress primary creep of SX. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.09.063
  • 2018 • 208 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 • 207 Probing magnetic coupling between LnPc2 (Ln = Tb, Er) molecules and the graphene/Ni (111) substrate with and without Au-intercalation: Role of the dipolar field
    Corradini, V. and Candini, A. and Klar, D. and Biagi, R. and De Renzi, V. and Lodi Rizzini, A. and Cavani, N. and Del Pennino, U. and Klyatskaya, S. and Ruben, M. and Velez-Fort, E. and Kummer, K. and Brookes, N.B. and Gargiani, P...
    Nanoscale 10 277-283 (2018)
    Lanthanides (Ln) bis-phthalocyanine (Pc), the so-called LnPc2double decker, are a promising class of molecules with a well-defined magnetic anisotropy. In this work, we investigate the magnetic properties of LnPc2 molecules UHV-deposited on a graphene/Ni(111) substrate and how they modify when an Au layer is intercalated between Ni and graphene. X-ray absorption spectroscopy (XAS), and linear and magnetic circular dichroism (XLD and XMCD) were used to characterize the systems and probe the magnetic coupling between LnPc2 molecules and the Ni substrate through graphene, both gold-intercalated and not. Two types of LnPc2 molecules (Ln = Tb, Er) with a different magnetic anisotropy (easy-axis for Tb, easy-plane for Er) were considered. XMCD shows an antiferromagnetic coupling between Ln and Ni(111) even in the presence of the graphene interlayer. Au intercalation causes the vanishing of the interaction between Tb and Ni(111). In contrast, in the case of ErPc2, we found that the gold intercalation does not perturb the magnetic coupling. These results, combined with the magnetic anisotropy of the systems, suggest the possible importance of the magnetic dipolar field contribution for determining the magnetic behaviour. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7nr06610d
  • 2018 • 206 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 • 205 Surface Termination and Composition Control of Activity of the CoxNi1- xFe2O4(001) Surface for Water Oxidation: Insights from DFT+ U Calculations
    Hajiyani, H. and Pentcheva, R.
    ACS Catalysis 8 11773-11782 (2018)
    Using density functional theory calculations with an on-site Hubbard term (DFT+U), we explore the effect of surface termination and cation substitution on the performance of the CoxNi1-xFe2O4(001) surface (x = 0.0, 0.5, 1.0) as an anode material in the oxygen evolution reaction (OER). Different reaction sites (Fe, Co, Ni, and an oxygen vacancy) were investigated at three terminations: the B-layer with octahedrally coordinated Co/Ni and with an additional half and full monolayer of Fe (0.5A and A-layer, respectively). Ni substitution with an equal concentration of Co and Ni (x = 0.5) reduces the overpotential over the end members for the majority of reaction sites. Surface Co cations are identified as the active sites and the ones at the A-layer termination for x = 0.5 exhibit one of the lowest theoretically reported overpotentials of 0.26 V. The effect of the additional iron layer on the active site modification is 2-fold: analysis of the electronic properties and spin densities indicates that the additional Fe layer stabilizes a bulk-like oxidation state of +2 for Co and Ni at the A-layer termination, whereas at the B-layer termination, they are oxidized to 3+. Moreover, the unusual relaxation pattern enables the formation of a hydrogen bond of the OOH intermediate to a neighboring surface oxygen that lowers the reaction free energy of this formerly rate-limiting step, leading to a deviation from the scaling relationship and almost equidistant reaction free-energy steps of intermediates. This renders an example of how a selective surface modification can result in a significant improvement of OER performance. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.8b00574
  • 2018 • 204 Testing of Ni-base superalloy single crystals with circular notched miniature tensile creep (CNMTC) specimens
    Cao, L. and Bürger, D. and Wollgramm, P. and Neuking, K. and Eggeler, G.
    Materials Science and Engineering A 712 223-231 (2018)
    The present work introduces a novel circular notched miniature tensile creep (CNMTC) specimen which is used to study the influence of notches on creep and of multiaxial stress states on microstructural evolution in Ni-based single crystal (SX) superalloys. It is briefly discussed how mild circular notches affect the stress state in the notch root of a tensile bar during elastic loading. Then the stress redistribution under creep conditions is calculated using the finite element method (FEM), assuming isotropic material behavior. Two series of interrupted creep experiments with the Ni-based single crystal superalloy ERBO1 (CMSX-4 type) were then performed at 950 °C, with flat uniaxial miniature tensile creep (FUMTC) and CNMTC <100> specimens, respectively. The evolution of cavities and microcracks in both types of specimens was carefully analyzed after 81, 169, and 306 h. In the uniaxial experiments, a growth of cast pores and the formation of new creep cavities were observed. These degradation processes were much less pronounced in the circular notched specimens. The results of the present work are discussed in the light of previous findings on the influence of multiaxial stress states on creep in single crystal superalloys. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.11.102
  • 2018 • 203 The effect of chromium and cobalt segregation at dislocations on nickel-based superalloys
    Kontis, P. and Li, Z. and Collins, D.M. and Cormier, J. and Raabe, D. and Gault, B.
    Scripta Materialia 145 76-80 (2018)
    The segregation of solutes at dislocations in a polycrystalline and a single crystal nickel-based superalloy is studied. Our observations confirm the often assumed but yet unproven diffusion along dislocations via pipe diffusion. Direct observation and quantitative, near-atomic scale segregation of chromium and cobalt at dislocations within γ' precipitates and at interfacial dislocations leading to the partial or complete dissolution of γ' precipitates at elevated temperatures is presented. Our results allow us to elucidate the physical mechanism by which pipe diffusion initiates the undesirable dissolution of γ' precipitates. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2017.10.005
  • 2018 • 202 The influence of the nitrogen/nickel-ratio on the cyclic behavior of austenitic high strength steels with twinning-induced plasticity and transformation-induced plasticity effects [Einfluss des Stickstoff-/Nickel-Verhältnisses auf das zyklische Verhalten austenitischer hochfester Stähle mit durch Zwillingsbildung und Umwandlung induzierter Plastizität]
    Güler, S. and Schymura, M. and Fischer, A. and Droste, M. and Biermann, H.
    Materialwissenschaft und Werkstofftechnik 49 61-72 (2018)
    Austenitic high nitrogen (AHNS) and austenitic high interstitial steels (AHIS) are of interest for mechanical engineering applications because of their unique combination of mechanical (strength, ductility), chemical (corrosion resistance) and physical (non-ferromagnetic) properties. But despite their high strength values e. g. after cold deformation up to 2 GPa in combination with an elongation to fracture of 30 %, which is based on twinning-induced plasticity (TWIP) mechanisms and transformation-induced plasticity (TRIP) mechanisms, the fatigue limit remains relatively small. While for chromium-nickel steels the fatigue limit rises with about 0.5-times the elastic limit it does not at all for austenitic high-nitrogen steels or only to a much smaller extent for nickel-free austenitic high-interstitial steels. The reasons are still not fully understood but this behavior can roughly be related to the tendency for planar or wavy slip. Now the latter is hindered by nitrogen and promoted by nickel. This contribution shows the fatigue behavior of chromium-manganese-carbon-nitrogen (CrMnCn) steels with carbon+nitrogen-contents up to 1.07 wt.%. Beside the governing influence of these interstitials on fatigue this study displays, how the nitrogen/nickel-ratio might be another important parameter for the fatigue behavior of such steels. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/mawe.201700107
  • 2017 • 201 3D discrete dislocation dynamics study of creep behavior in Ni-base single crystal superalloys by a combined dislocation climb and vacancy diffusion model
    Gao, S. and Fivel, M. and Ma, A. and Hartmaier, A.
    Journal of the Mechanics and Physics of Solids 102 209-223 (2017)
    A three-dimensional (3D) discrete dislocation dynamics (DDD) creep model is developed to investigate creep behavior under uniaxial tensile stress along the crystallographic [001] direction in Ni-base single crystal superalloys, which takes explicitly account of dislocation glide, climb and vacancy diffusion, but neglects phase transformation like rafting of γ′ precipitates. The vacancy diffusion model takes internal stresses by dislocations and mismatch strains into account and it is coupled to the dislocation dynamics model in a numerically efficient way. This model is helpful for understanding the fundamental creep mechanisms in superalloys and clarifying the effects of dislocation glide and climb on creep deformation. In cases where the precipitate cutting rarely occurs, e.g. due to the high anti-phase boundary energy and the lack of superdislocations, the dislocation glide in the γ matrix and the dislocation climb along the γ/γ′ interface dominate plastic deformation. The simulation results show that a high temperature or a high stress both promote dislocation motion and multiplication, so as to cause a large creep strain. Dislocation climb accelerated by high temperature only produces a small plastic strain, but relaxes the hardening caused by the filling γ channels and lets dislocations further glide and multiply. The strongest variation of vacancy concentration occurs in the horizontal channels, where more mixed dislocations exit and tend to climb. The increasing internal stresses due to the increasing dislocation density are easily overcome by dislocations under a high external stress that leads to a long-term dislocation glide accompanied by multiplication. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.jmps.2017.02.010
  • 2017 • 200 Analysis of the ductility dip cracking in the nickel-base alloy 617mod
    Eilers, A. and Nellesen, J. and Zielke, R. and Tillmann, W.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    While testing steam leading power plant components made of the nickel-base alloy A617mod at elevated temperatures (700 °C), ductility dip cracking (DDC) was observed in welding seams and their surroundings. In order to clarify the mechanism of crack formation, investigations were carried out on welded specimens made of A617mod. Interrupted tensile tests were performed on tensile specimens taken from the area of the welding seam. To simulate the conditions, the tensile tests were conducted at a temperature of 700 °C and with a low strain rate. Local strain fields at grain boundaries and inside single grains were determined at different deformation states by means of two-dimensional digital image correlation (DIC). Besides the strain fields, local hardnesses (nanoindentation), energy dispersive X-Ray spectroscopy (EDX), and electron backscatter diffraction (EBSD) measurements were performed. Besides information concerning the grain orientation, the EBSD measurement provides information on the coincidence site lattice (CSL) at grain boundaries as well as the Schmid factor of single grains. All results of the analysis methods mentioned above were correlated and compared to each other and related to the crack formation. Among other things, correlations between strain fields and Schmid factors were determined. The investigations show that the following influences affect the crack formation: orientation of the grain boundaries to the direction of the loading, the orientation of the grains to each other (CSL), and grain boundary sliding. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/181/1/012020
  • 2017 • 199 Atomic diffusion induced degradation in bimetallic layer coated cemented tungsten carbide
    Peng, Z. and Rohwerder, M. and Choi, P.-P. and Gault, B. and Meiners, T. and Friedrichs, M. and Kreilkamp, H. and Klocke, F. and Raabe, D.
    Corrosion Science 120 1-13 (2017)
    We investigated the temporal degradation of glass moulding dies, made of cemented tungsten carbide coated with PtIr on an adhesive Cr or Ni interlayer, by electron microscopy and atom probe tomography. During the exposure treatments at 630 °C under an oxygen partial pressure of 1.12 × 10−23 bar, Cr (Ni) was found to diffuse outwards via grain boundaries in the PtIr, altering the surface morphology. Upon dissolution of the interlayer, the WC substrate also started degrading. Extensive interdiffusion processes involving PtIr, Cr (Ni) and WC took place, leading to the formation of intermetallic phases and voids, deteriorating the adhesion of the coating. © 2017 The Authors
    view abstractdoi: 10.1016/j.corsci.2017.01.007
  • 2017 • 198 Atomistic insight into the non-classical nucleation mechanism during solidification in Ni
    Díaz Leines, G. and Drautz, R. and Rogal, J.
    Journal of Chemical Physics 146 (2017)
    Nucleation is a key step during crystallization, but a complete understanding of the fundamental atomistic processes remains elusive. We investigate the mechanism of nucleation during solidification in nickel for various undercoolings using transition path sampling simulations. The temperature dependence of the free energy barriers and rate constants that we obtain is consistent with the predictions of classical nucleation theory and experiments. However, our analysis of the transition path ensemble reveals a mechanism that deviates from the classical picture of nucleation: the growing solid clusters have predominantly non-spherical shapes and consist of face-centered-cubic and random hexagonal-close-packed coordinated atoms surrounded by a cloud of prestructured liquid. The nucleation initiates in regions of supercooled liquid that are characterized by a high orientational order with structural features that predetermine the polymorph selection. These results provide atomistic insight not only into the nucleation mechanism of nickel but also into the role of the preordered liquid regions as precursors for crystallization. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4980082
  • 2017 • 197 Combinatorial Alloy Design by Laser Additive Manufacturing
    Knoll, H. and Ocylok, S. and Weisheit, A. and Springer, H. and Jägle, E. and Raabe, D.
    Steel Research International 88 1600416 (2017)
    The authors uses laser additive manufacturing (LAM) as a combinatorial method for synthesizing microstructurally and compositionally piecewise graded bulk alloys. Authors fabricate blocks consisting of a sequence of ≈500μm thick tool steel layers, each with different chemical composition, by laser metal deposition where alloy powders are deposited layer-wise on a substrate. The reference materials are a Cr-Mo-V hot working tool steel and a Ni-based maraging steel. The layers between them consist of corresponding blends of the two materials with varying composition from layer to layer (alloy volume fractions 80:20, 60:40, 40:60, and 20:80). The bulk alloy is hot rolled and heat treated. Subsequently each layer is characterized for microstructure, chemical composition and mechanical properties using electron back scatter diffraction, tensile testing, and indentation. The approach is an efficient high-throughput method enabling rapid probing of novel compositional alloy blends. It can be applied for finding new alloys both, by LAM and for LAM. For the tool steel blends synthesized here, authors observe that the Cr-Mo-V tool steel, when mixed with the Ni-base maraging steel, can be continuously tuned for a strength-ductility profile in the range of 800-1650MPa strength and 15-25% tensile elongation. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201600416
  • 2017 • 196 Degradation mechanisms of pcBN tool material during Friction Stir Welding of Ni-base alloy 625
    Hanke, S. and Lemos, G.V.B. and Bergmann, L. and Martinazzi, D. and dos Santos, J.F. and Strohaecker, T.R.
    Wear 376-377 403-408 (2017)
    In Friction Stir Welding (FSW), interactions between the plasticized stirred material and the tool significantly affect resulting weld properties. When welding metals with high strength and melting point, the tribological load on the tool is severe, and poses the main limiting factor for the technology's industrial exploitation. Since tool materials are loaded to their limits, it is essential to understand the interactions of specific tool material and welded metal combinations. In the present study 3.2 mm alloy 625 sheets were joined using a pcBN tool with W-Re binder phase. Wear lead to a change in tool geometry followed by tool fracture. In SEM investigations the welds revealed typical banded structures, composed of small grains and non-metallic phases containing W from the tool material. The tool surface is extensively covered by adhering sheet metal. Further, BN grain pull-outs and appearances of diffusive wear are visible on the worn tool surface. Tool wear is mainly caused by detachment of BN grains due to thermal softening of the metallic binder phase and dissolution of BN in the hot material in the stirred zone. Using low rotational speeds resulting in lower process temperatures reduces tool wear and results in a homogeneous stirred zone. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2017.01.070
  • 2017 • 195 Electronic structure and magnetism of epitaxial Ni-Mn-Ga(-Co) thin films with partial disorder: A view across the phase transition
    Schleicher, B. and Klar, D. and Ollefs, K. and Diestel, A. and Walecki, D. and Weschke, E. and Schultz, L. and Nielsch, K. and Fähler, S. and Wende, H. and Gruner, M.E.
    Journal of Physics D: Applied Physics 50 (2017)
    The influence of Co-doping in off-stoichiometric Ni-Mn-Ga and Ni-Mn-Ga-Co thin films on the magnetic coupling of the atoms is investigated with x-ray magnetic circular dichroism in both the martensitic as well as austenitic phase, respectively. Additionally, first principles calculations were performed to compare the experimentally obtained absorption spectra with theoretical predictions. Calculated exchange constants and density of states for the different atomic sites underline the large influence of chemical and magnetic order on the magnetocaloric properties of the material. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa8e7c
  • 2017 • 194 High-Throughput Structural and Functional Characterization of the Thin Film Materials System Ni-Co-Al
    Decker, P. and Naujoks, D. and Langenkämper, D. and Somsen, C. and Ludwig, Al.
    ACS Combinatorial Science 19 618-624 (2017)
    High-throughput methods were used to investigate a Ni-Co-Al thin film materials library, which is of interest for structural and functional applications (superalloys, shape memory alloys). X-ray diffraction (XRD) measurements were performed to identify the phase regions of the Ni-Co-Al system in its state after annealing at 600 °C. Optical, electrical, and magneto-optical measurements were performed to map functional properties and confirm XRD results. All results and literature data were used to propose a ternary thin film phase diagram of the Ni-Co-Al thin film system. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscombsci.6b00176
  • 2017 • 193 Hydrogen-assisted failure in Ni-based superalloy 718 studied under in situ hydrogen charging: The role of localized deformation in crack propagation
    Tarzimoghadam, Z. and Ponge, D. and Klöwer, J. and Raabe, D.
    Acta Materialia 128 365-374 (2017)
    We investigated hydrogen embrittlement in Ni-based superalloy 718 by tensile testing at slow strain rate (10−4 s−1) under continuous electrochemical hydrogen charging. Hydrogen-assisted cracking mechanisms were studied via electron backscatter diffraction (EBSD) analysis and electron channeling contrast imaging (ECCI). In order to elucidate the effects of stress or strain in the cracking mechanisms, material conditions with different strength levels were investigated, including samples in solution annealed (as water quenched) and 780 °C age-hardened states. The microstructure observations in the vicinity of the cracks enabled us to establish correlations between the microstructure, crack initiation sites, and crack propagation pathways. Fracture in the hydrogen-charged samples was dominated by localized plastic deformation. Strain-controlled transgranular cracking was caused by shear localization due to hydrogen-enhanced localized plasticity (HELP) and void nucleation and coalescence along {111} slip planes in both, the solution annealed and age-hardened materials. Stress-assisted intergranular cracking in the presence of hydrogen was only observed in the high strength age-hardened material, due to slip localization at grain boundaries, grain boundary triple junction cracking, and δ/γ-matrix interface cracking. To investigate the effect of δ-phase in crack propagation along grain boundaries, the over-aged state (aged at 870 °C) with different precipitation conditions for the δ-phase was also investigated. Observations confirmed that presence of δ-phase promotes hydrogen-induced intergranular failure by initializing micro-cracks from δ/γ interfaces. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.02.059
  • 2017 • 192 Influence of Ni to Co ratio in mixed Co and Ni phosphides on their electrocatalytic oxygen evolution activity
    Barwe, S. and Andronescu, C. and Vasile, E. and Masa, J. and Schuhmann, W.
    Electrochemistry Communications 79 41-45 (2017)
    Prompted by the impact of Ni-based support materials on the intrinsic activity of electrocatalysts, we investigated the influence of partial Co substitution by Ni during the reductive thermal synthesis of cobalt-cobalt phosphide nanoparticles from triphenylphosphine complexes. The obtained catalysts were characterised by X-ray diffraction and electrochemistry. Increasing the amount of Ni in the precursor complexes leads to materials with lower overpotential for the OER at low current densities, and lower Tafel slopes. Co nanoparticles, which are only formed in materials with low Ni content, increase the intrinsic material conductivity and reduce the OER overpotential at high current densities. © 2017
    view abstractdoi: 10.1016/j.elecom.2017.04.014
  • 2017 • 191 Influence of rhenium on γ′-strengthened cobalt-base superalloys
    Kolb, M. and Zenk, C.H. and Kirzinger, A. and Povstugar, I. and Raabe, D. and Neumeier, S. and Göken, M.
    Journal of Materials Research 32 2551-2559 (2017)
    The element Re is known to be a very potent strengthener concerning the creep properties of Ni-base superalloys. In this paper the influence of Re on the properties of new γ′-strengthened Co-base superalloys is addressed. Atom probe tomography reveals that Re partitions preferentially to the γ phase, but not as pronounced as in ni-base superalloys. Nanoindentation and micro-pillar compression tests of the γ′ phase indicate an increase of the hardness and the critical resolved shear stress caused by a considerable concentration of Re in the γ′ phase. Creep investigations show that the positive effect of Re is by far not as pronounced as in Ni-base superalloys. Several effects, which can contribute to this behavior, such as the lower Re concentration in γ and hence a slightly reduced effective diffusion coefficient, a smaller diffusion barrier of Re in Co compared to Ni, a slightly lower lattice misfit and γ′ volume fraction of the Re-containing alloy, are discussed. © Materials Research Society 2017.
    view abstractdoi: 10.1557/jmr.2017.242
  • 2017 • 190 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 • 189 Laser synthesis, structure and chemical properties of colloidal nickel-molybdenum nanoparticles for the substitution of noble metals in heterogeneous catalysis
    Marzun, G. and Levish, A. and Mackert, V. and Kallio, T. and Barcikowski, S. and Wagener, P.
    Journal of Colloid and Interface Science 489 57-67 (2017)
    Platinum and iridium are rare and expensive noble metals that are used as catalysts for different sectors including in heterogeneous chemical automotive emission catalysis and electrochemical energy conversion. Nickel and its alloys are promising materials to substitute noble metals. Nickel based materials are cost-effective with good availability and show comparable catalytic performances. The nickel-molybdenum system is a very interesting alternative to platinum in water electrolysis. We produced ligand-free nickel-molybdenum nanoparticles by laser ablation in water and acetone. Our results show that segregated particles were formed in water due to the oxidation of the metals. X-ray diffraction shows a significant change in the lattice parameter due to a diffusion of molybdenum atoms into the nickel lattice with increasing activity in the electrochemical oxygen evolution reaction. Even though the solubility of molecular oxygen in acetone is higher than in water, there were no oxides and a more homogeneous metal distribution in the particles in acetone as seen by TEM-EDX. This showed that dissolved molecular oxygen does not control oxide formation. Overall, the laser ablation of pressed micro particulate mixtures in liquids offers a combinational synthesis approach that allows the screening of alloy nanoparticles for catalytic testing and can convert micro-mixtures into nano-alloys. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2016.09.014
  • 2017 • 188 Low Resistive Edge Contacts to CVD-Grown Graphene Using a CMOS Compatible Metal
    Shaygan, M. and Otto, M. and Sagade, A.A. and Chavarin, C.A. and Bacher, G. and Mertin, W. and Neumaier, D.
    Annalen der Physik (2017)
    doi: 10.1002/andp.201600410
  • 2017 • 187 Massive nanoprecipitation in an Fe-19Ni-xAl maraging steel triggered by the intrinsic heat treatment during laser metal deposition
    Kürnsteiner, P. and Wilms, M.B. and Weisheit, A. and Barriobero-Vila, P. and Jägle, E.A. and Raabe, D.
    Acta Materialia 129 52-60 (2017)
    Due to the layer-by-layer build-up of additively manufactured parts, the deposited material experiences a cyclic re-heating in the form of a sequence of temperature pulses. In the current work, this “intrinsic heat treatment (IHT)” was exploited to induce the precipitation of NiAl nanoparticles in an Fe-19Ni-xAl (at%) model maraging steel, a system known for rapid clustering. We used Laser Metal Deposition (LMD) to synthesize compositionally graded specimens. This allowed for the efficient screening of effects associated with varying Al contents ranging from 0 to 25 at% and for identifying promising concentrations for further studies. Based on the existence of the desired martensitic matrix, an upper bound for the Al concentration of 15 at% was defined. Owing to the presence of NiAl precipitates as observed by Atom Probe Tomography (APT), a lower bound of 3–5 at% Al was established. Within this concentration window, increasing the Al concentration gave rise to an increase in hardness by 225 HV due to an exceptionally high number density of 1025 NiAl precipitates per m3, as measured by APT. This work demonstrates the possibility of exploiting the IHT of the LMD process for the production of samples that are precipitation strengthened during the additive manufacturing process without need for any further heat treatment. © 2017
    view abstractdoi: 10.1016/j.actamat.2017.02.069
  • 2017 • 186 Metal–Organic Framework Derived Carbon Nanotube Grafted Cobalt/Carbon Polyhedra Grown on Nickel Foam: An Efficient 3D Electrode for Full Water Splitting
    Aijaz, A. and Masa, J. and Rösler, C. and Xia, W. and Weide, P. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    ChemElectroChem 4 188-193 (2017)
    The growth of metal–organic framework (ZIF-67) nanocrystals on nickel foam (NF), followed by carbonization in diluted H2, leads to a nitrogen-doped carbon-nanotube-grafted cobalt/carbon polyhedra film on NF. The obtained material serves as a highly active binder-free electrocatalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), enabling high-performance alkaline (0.1 m KOH) water electrolysis with potentials of 1.62 and 0.24 V, respectively, at OER and HER current densities of 10 mA cm−2. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201600452
  • 2017 • 185 Nanoporous Nitrogen-Doped Graphene Oxide/Nickel Sulfide Composite Sheets Derived from a Metal-Organic Framework as an Efficient Electrocatalyst for Hydrogen and Oxygen Evolution
    Jayaramulu, K. and Masa, J. and Tomanec, O. and Peeters, D. and Ranc, V. and Schneemann, A. and Zboril, R. and Schuhmann, W. and Fischer, R.A.
    Advanced Functional Materials (2017)
    Engineering of controlled hybrid nanocomposites creates one of the most exciting applications in the fields of energy materials and environmental science. The rational design and in situ synthesis of hierarchical porous nanocomposite sheets of nitrogen-doped graphene oxide (NGO) and nickel sulfide (Ni7S6) derived from a hybrid of a well-known nickel-based metal-organic framework (NiMOF-74) using thiourea as a sulfur source are reported here. The nanoporous NGO/MOF composite is prepared through a solvothermal process in which Ni(II) metal centers of the MOF structure are chelated with nitrogen and oxygen functional groups of NGO. NGO/Ni7S6 exhibits bifunctional activity, capable of catalyzing both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with excellent stability in alkaline electrolytes, due to its high surface area, high pore volume, and tailored reaction interface enabling the availability of active nickel sites, mass transport, and gas release. Depending on the nitrogen doping level, the properties of graphene oxide can be tuned toward, e.g., enhanced stability of the composite compared to commonly used RuO2 under OER conditions. Hence, this work opens the door for the development of effective OER/HER electrocatalysts based on hierarchical porous graphene oxide composites with metal chalcogenides, which may replace expensive commercial catalysts such as RuO2 and IrO2. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201700451
  • 2017 • 184 Operando Phonon Studies of the Protonation Mechanism in Highly Active Hydrogen Evolution Reaction Pentlandite Catalysts
    Zegkinoglou, I. and Zendegani, A. and Sinev, I. and Kunze, S. and Mistry, H. and Jeon, H.S. and Zhao, J. and Hu, M.Y. and Alp, E.E. and Piontek, S. and Smialkowski, M. and Apfel, U.-P. and Körmann, F. and Neugebauer, J. and Hicke...
    Journal of the American Chemical Society 139 14360-14363 (2017)
    Synthetic pentlandite (Fe4.5Ni4.5S8) is a promising electrocatalyst for hydrogen evolution, demonstrating high current densities, low overpotential, and remarkable stability in bulk form. The depletion of sulfur from the surface of this catalyst during the electrochemical reaction has been proposed to be beneficial for its catalytic performance, but the role of sulfur vacancies and the mechanism determining the reaction kinetics are still unknown. We have performed electrochemical operando studies of the vibrational dynamics of pentlandite under hydrogen evolution reaction conditions using 57Fe nuclear resonant inelastic X-ray scattering. Comparing the measured Fe partial vibrational density of states with density functional theory calculations, we have demonstrated that hydrogen atoms preferentially occupy substitutional positions replacing pre-existing sulfur vacancies. Once all vacancies are filled, the protonation proceeds interstitially, which slows down the reaction. Our results highlight the beneficial role of sulfur vacancies in the electrocatalytic performance of pentlandite and give insights into the hydrogen adsorption mechanism during the reaction. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b07902
  • 2017 • 183 Optimizing Ni–Ti-based shape memory alloys for ferroic cooling
    Wieczorek, A. and Frenzel, J. and Schmidt, M. and Maaß, B. and Seelecke, S. and Schütze, A. and Eggeler, G.
    Functional Materials Letters 10 1740001 (2017)
    Due to their large latent heats, pseudoelastic Ni–Ti-based shape memory alloys (SMAs) are attractive candidate materials for ferroic cooling, where elementary solid-state processes like martensitic transformations yield the required heat effects. The present work aims for a chemical and microstructural optimization of Ni–Ti for ferroic cooling. A large number of Ni–Ti-based alloy compositions were evaluated in terms of phase transformation temperatures, latent heats, mechanical hysteresis widths and functional stability. The aim was to identify material states with superior properties for ferroic cooling. Different material states were prepared by arc melting, various heat treatments and thermo-mechanical processing. The cooling performance of selected materials was assessed by differential scanning calorimetry, uniaxial tensile loading/unloading, and by using a specially designed ferroic cooling demonstrator setup. A Ni(Formula presented.)Ti(Formula presented.)Cu5V(Formula presented.) SMA was identified as a potential candidate material for ferroic cooling. This material combines extremely stable pseudoelasticity at room temperature and a very low hysteresis width. The ferroic cooling efficiency of this material is four times higher than in the case of binary Ni–Ti. © 2017 World Scientific Publishing Company
    view abstractdoi: 10.1142/S179360471740001X
  • 2017 • 182 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 • 181 Progress in friction stir welding of Ni alloys
    Lemos, G.V.B. and Hanke, S. and Dos Santos, J.F. and Bergmann, L. and Reguly, A. and Strohaecker, T.R.
    Science and Technology of Welding and Joining 22 643-657 (2017)
    In recent years, interest has been increasing in application of Nickel alloys in the oil industry. For subsea engineering, the possibility to weld high-strength materials in an effective manner is essential. Friction Stir Welding (FSW) is alternative to join several materials retaining their properties or even improving them. This fact is relevant for Corrosion-Resistant Alloys (CRA) used in deep-water exploitation of hydrocarbons. Publications up to now have focused on FSW of Inconel® series as alloy 600, 625, and 718. To provide a solid basis for development, this review discusses the crucial points for FSW. The tool materials are described, as well as the joint microstructure and properties achieved. Furthermore, the basics of the corrosion resistance and the early corrosion studies of FSW joints are presented. It is concluded that FSW is a promising process for Ni alloys, but depends on upcoming research regarding tool technology and corrosion investigations. © 2017 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute.
    view abstractdoi: 10.1080/13621718.2017.1288953
  • 2017 • 180 Promoting Photocatalytic Overall Water Splitting by Controlled Magnesium Incorporation in SrTiO3 Photocatalysts
    Han, K. and Lin, Y.-C. and Yang, C.-M. and Jong, R. and Mul, G. and Mei, B.
    ChemSusChem 10 4510-4516 (2017)
    SrTiO3 is a well-known photocatalyst inducing overall water splitting when exposed to UV irradiation of wavelengths &lt;370 nm. However, the apparent quantum efficiency of SrTiO3 is typically low, even when functionalized with nanoparticles of Pt or Ni@NiO. Here, we introduce a simple solid-state preparation method to control the incorporation of magnesium into the perovskite structure of SrTiO3. After deposition of Pt or Ni@NiO, the photocatalytic water-splitting efficiency of the Mg:SrTiOx composites is up to 20 times higher compared to SrTiO3 containing similar catalytic nanoparticles, and an apparent quantum yield (AQY) of 10 % can be obtained in the wavelength range of 300–400 nm. Detailed characterization of the Mg:SrTiOx composites revealed that Mg is likely substituting the tetravalent Ti ion, leading to a favorable surface–space–charge layer. This originates from tuning of the donor density in the cubic SrTiO3 structure by Mg incorporation and enables high oxygen-evolution rates. Nevertheless, interfacing with an appropriate hydrogen evolution catalyst is mandatory and non-trivial to obtain high-performance in water splitting. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201701794
  • 2017 • 179 Rejuvenation of creep resistance of a Ni-base single-crystal superalloy by hot isostatic pressing
    Ruttert, B. and Bürger, D. and Roncery, L.M. and Parsa, A.B. and Wollgramm, P. and Eggeler, G. and Theisen, W.
    Materials and Design 134 418-425 (2017)
    Ni-base single-crystal turbine blades are exposed to a combination of high temperatures and high stresses during their service life in high-pressure turbines of aero engines or stationary gas turbines. This unavoidably leads to various internal microstructural changes such as rafting and the formation of cavities. This study introduces a creep-rejuvenation-creep test cycle using one miniature Ni-base single-crystal creep specimen. A novel hot isostatic press providing high quenching rates was applied to rejuvenate the damaged microstructure of the specimen after the first high-temperature creep degradation before the same specimen was repeatedly creep-tested under the same initial creep conditions. After rejuvenating, microstructural results obtained from high-resolution microscopy prove that the creep cavities were closed, dislocation densities were re-set, and the original but now slightly finer γ/γ′-microstructure was restored without any recrystallization. The subsequent creep test of the rejuvenated specimen demonstrated that the proposed rejuvenation procedure in this work is a suitable method to reproduce the initial creep behavior and to thus prolong the lifetime of an already crept Ni-base single-crystal specimen. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2017.08.059
  • 2017 • 178 Rock Salt Ni/Co Oxides with Unusual Nanoscale-Stabilized Composition as Water Splitting Electrocatalysts
    Fominykh, K. and Tok, G.C. and Zeller, P. and Hajiyani, H. and Miller, T. and Döblinger, M. and Pentcheva, R. and Bein, T. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 27 (2017)
    The influence of nanoscale on the formation of metastable phases is an important aspect of nanostructuring that can lead to the discovery of unusual material compositions. Here, the synthesis, structural characterization, and electrochemical performance of Ni/Co mixed oxide nanocrystals in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is reported and the influence of nanoscaling on their composition and solubility range is investigated. Using a solvothermal synthesis in tert-butanol ultrasmall crystalline and highly dispersible Ni x Co1− x O nanoparticles with rock salt type structure are obtained. The mixed oxides feature non-equilibrium phases with unusual miscibility in the whole composition range, which is attributed to a stabilizing effect of the nanoscale combined with kinetic control of particle formation. Substitutional incorporation of Co and Ni atoms into the rock salt lattice has a remarkable effect on the formal potentials of NiO oxidation that shift continuously to lower values with increasing Co content. This can be related to a monotonic reduction of the work function of (001) and (111)-oriented surfaces with an increase in Co content, as obtained from density functional theory (DFT+U) calculations. Furthermore, the electrocatalytic performance of the Ni x Co1− x O nanoparticles in water splitting changes significantly. OER activity continuously increases with increasing Ni contents, while HER activity shows an opposite trend, increasing for higher Co contents. The high electrocatalytic activity and tunable performance of the nonequilibrium Ni x Co1− x O nanoparticles in HER and OER demonstrate great potential in the design of electrocatalysts for overall water splitting. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201605121
  • 2017 • 177 Role of Composition and Size of Cobalt Ferrite Nanocrystals in the Oxygen Evolution Reaction
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Schwarzrock, I. and Lunkenbein, T. and Salamon, S. and Landers, J. and Wende, H. and Schlögl, R. and Pentcheva, R. and Behrens, M. and Schulz, S.
    ChemCatChem 9 2988-2995 (2017)
    Sub-10 nm CoFe2O4 nanoparticles with different sizes and various compositions obtained by (partial) substitution of Co with Ni cations have been synthesized by using a one-pot method from organic solutions by the decomposition of metal acetylacetonates in the presence of oleylamine. The electrocatalytic activity of CoFe2O4 towards the oxygen evolution reaction (OER) is clearly enhanced with a smaller size (3.1 nm) of the CoFe2O4 nanoparticles (compared with 4.5 and 5.9 nm). In addition, the catalytic activity is improved by partial substitution of Co with Ni, which also leads to a higher degree of inversion of the spinel structure. Theoretical calculations attribute the positive catalytic effect of Ni owing to the lower binding energy differences between adsorbed O and OH compared with pure cobalt or nickel ferrites, resulting in higher OER activity. Co0.5Ni0.5Fe2O4 exhibited a low overpotential of approximately 340 mV at 10 mA cm−2, a smaller Tafel slope of 51 mV dec−1, and stability over 30 h. The unique tunability of these CoFe2O4 nanocrystals provides great potential for their application as an efficient and competitive anode material in the field of electrochemical water splitting as well as for systematic fundamental studies aiming at understanding the correlation of composition and structure with performance in electrocatalysis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201700376
  • 2017 • 176 Room-temperature five-tesla coercivity of a rare-earth-free shell-ferromagnet
    Scheibel, F. and Spoddig, D. and Meckenstock, R. and Gottschall, T. and Çakır, A. and Krenke, T. and Farle, M. and Gutfleisch, O. and Acet, M.
    Applied Physics Letters 110 (2017)
    Ni2MnX-based Heusler (X: main group element), when enriched with Mn, will decompose into ferromagnetic Ni2MnX and antiferromagnetic NiMn when temper-annealed around 650 K. When the starting material is chosen such that the X-composition is about 5 at. % and the annealing takes place in the presence of a magnetic field of about 1 T, the resulting material is a composite of nanoprecipitate strongly pinned shell-ferromagnets with a soft ferromagnetic core embedded in the antiferromagnetic matrix. We show that the shells of the precipitates are so strongly pinned that the estimated field required to fully reorient the spins is in the order of 20 T. We examine in a Ni50.0Mn45.1In4.9 sample the pinning and the magnetic interactions of the precipitate and the matrix with magnetization and ferromagnetic resonance studies carried out in fields ranging up to 14 and 12 T, respectively. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4983199
  • 2017 • 175 Self-sharpening-effect of nickel-diamond coatings sprayed by HVOF
    Tillmann, W. and Brinkhoff, A. and Schaak, C. and Zajaczkowski, J.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    The durability of stone working and drilling tools is an increasingly significant requirement in industrial applications. These tools are mainly produced by brazing diamond metal matrix composites inserts to the tool body. These inserts are produced by sintering diamonds and metal powder (e.g. nickel). If the wear is too high, the diamonds will break out of the metal matrix and other diamonds will be uncovered. This effect is called self-sharpening. But diamonds are difficult to handle because of their thermal sensitivity. Due to their high thermal influence, manufacturing costs, and complicate route of manufacturing (first sintering, then brazing), there is a great need for alternative production methods for such tools. One alternative to produce wear-resistant and self-sharpening coatings are thermal spray processes as examined in this paper. An advantage of thermal spray processes is their smaller thermal influence on the diamond, due to the short dwelling time in the flame. To reduce the thermal influence during spraying, nickel coated diamonds were used in the HVOF-process (high velocity oxygen fuel process). The wear resistance was subsequently investigated by means of a standardized ball-on-disc test. Furthermore, a SEM (scanning electron microscope) was used to gain information about the wear-mechanism and the self-sharpening effect of the coating. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/181/1/012016
  • 2017 • 174 Shape memory micro-and nanowire libraries for the high-throughput investigation of scaling effects
    Oellers, T. and König, D. and Kostka, A. and Xie, S. and Brugger, J. and Ludwig, Al.
    ACS Combinatorial Science 19 574-584 (2017)
    The scaling behavior of Ti-Ni-Cu shape memory thin-film micro- and nanowires of different geometry is investigated with respect to its influence on the martensitic transformation properties. Two processes for the highthroughput fabrication of Ti-Ni-Cu micro- to nanoscale thin film wire libraries and the subsequent investigation of the transformation properties are reported. The libraries are fabricated with compositional and geometrical (wire width) variations to investigate the influence of these parameters on the transformation properties. Interesting behaviors were observed: Phase transformation temperatures change in the range from 1 to 72 °C (austenite finish, (Af), 13 to 66 °C (martensite start, Ms) and the thermal hysteresis from -3.5 to 20 K. It is shown that a vanishing hysteresis can be achieved for special combinations of sample geometry and composition. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscombsci.7b00065
  • 2017 • 173 Shell-ferromagnetic precipitation in martensitic off-stoichiometric Ni-Mn-In Heusler alloys produced by temper-annealing under magnetic field
    Çakır, A. and Acet, M. and Wiedwald, U. and Krenke, T. and Farle, M.
    Acta Materialia 127 117-123 (2017)
    The variety of the multifunctional properties of martensitic Ni-Mn based Heusler alloys are related to the presence of a magnetostructural transition. We report here on a new functionality based on a newly observed property. The observed property is that all off-stoichiometric Ni-Mn-based Heuslers, here in the form Ni50Mn50−xInx with 0< x< 25, decompose into predominantly cubic ferromagnetic Ni50Mn25In25 and tetragonal antiferromagnetic NiMn components when temper-annealed. The new functionality is based on magnetic field assisted temper-annealing of a compound with stoichiometry x=5, whereby precipitates of Ni50Mn25In25 with a ferromagnetic shell are formed with spins in the field direction, strongly pinned by the surrounding antiferromagnetic anisotropy, even at temperatures as high as 500 K. The remanent pinning at high temperatures survives any thermal cycling between lowest temperatures and the annealing temperature and any magnetic field cycling between −9 and +9 T. The resulting product can serve as a thermally stable, magnetic-field-proof memory. © 2017
    view abstractdoi: 10.1016/j.actamat.2017.01.027
  • 2017 • 172 Shell-ferromagnetism in a Ni-Mn-In off-stoichiometric Heusler studied by ferromagnetic resonance
    Scheibel, F. and Spoddig, D. and Meckenstock, R. and Çakir, A. and Farle, M. and Acet, M.
    AIP Advances 7 (2017)
    Next to the multifunctional properties of Ni-Mn-based Heusler alloys new functionalities related to shell-ferromagnetism are emerging. To understand in more detail the properties of shell-ferromagnetism we examine a decomposed Ni50.0Mn45.1In4.9 off-stoichiometric compound using magnetic resonance techniques which provides details on magnetic interactions. We find that the ferromagnetic resonance profile of the shell-ferromagnetic state is symmetric for positive and negative fields and is independent of the direction of the field-sweep except for the hysteresis observed at small fields. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4976335
  • 2017 • 171 Site-Specific SERS Assay for Survivin Protein Dimer: From Ensemble Experiments to Correlative Single-Particle Imaging
    Wissler, J. and Bäcker, S. and Feis, A. and Knauer, S.K. and Schlücker, S.
    Small 13 (2017)
    An assay for Survivin, a small dimeric protein which functions as modulator of apoptosis and cell division and serves as a promising diagnostic biomarker for different types of cancer, is presented. The assay is based on switching on surface-enhanced Raman scattering (SERS) upon incubation of the Survivin protein dimer with Raman reporter-labeled gold nanoparticles (AuNP). Site-specificity is achieved by complexation of nickel-chelated N-nitrilo-triacetic acid (Ni-NTA) anchors on the particle surface by multiple histidines (His6-tag) attached to each C-terminus of the centrosymmetric protein dimer. Correlative single-particle analysis using light sheet laser microscopy enables the simultaneous observation of both elastic and inelastic light scattering from the same sample volume. Thereby, the SERS-inactive AuNP-protein monomers can be directly discriminated from the SERS-active AuNP-protein dimers/oligomers. This information, i.e. the percentage of SERS-active AuNP in colloidal suspension, is not accessible from conventional SERS experiments due to ensemble averaging. The presented correlative single-particle approach paves the way for quantitative site-specific SERS assays in which site-specific protein recognition by small chemical and in particular supramolecular ligands can be tested. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/smll.201700802
  • 2017 • 170 Spontaneous Si-C bond cleavage in (TriphosSi)-nickel complexes
    Petuker, A. and Mebs, S. and Schüth, N. and Gerschel, P. and Reback, M.L. and Mallick, B. and Van Gastel, M. and Haumann, M. and Apfel, U.-P.
    Dalton Transactions 46 907-917 (2017)
    Herein, we report on the versatile reactions of CH3C(CH2PPh2)3 as well as CH3Si(CH2PPh2)3 derived Ni-complexes. While Ni[CH3C(CH2PPh2)3] complexes reveal high stability, the Ni[CH3Si(CH2PPh2)3] analogs show rapid decomposition at room temperature and afford the unprecedented pseudo-tetrahedral phosphino methanide complex 5. We provide a detailed electronic structure of 5 from X-ray absorption and emission spectroscopy data analysis in combination with DFT calculations, as well as from comparison with structurally related complexes. A mechanistic study for the formation of complex 5 by reaction with BF4 − is presented, based on a comparison of experimental data with quantum chemical calculations. We also show a simple route towards isolable Ni(i)-complexes on the gram scale. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6dt04048a
  • 2017 • 169 Strengthening Fe – TiB2 based high modulus steels by precipitations
    Szczepaniak, A. and Springer, H. and Aparicio-Fernández, R. and Baron, C. and Raabe, D.
    Materials and Design 124 183-193 (2017)
    We systematically studied the microstructure, mechanical and physical properties of hyper-eutectic Fe – TiB2 high modulus steels (20 vol% TiB2) with (Si, Mn, Ni) and Cu additions for the formation of G-phase and Cu precipitates during ageing treatments. Alloying with Si, Mn and Ni led predominantly to pronounced solid solution strengthening, reaching tensile strength (UTS) values up to 1100 MPa after quenching. While G-phase formation could be observed in aged materials, its preferential formation on grain boundaries significantly deteriorated ductility. Its effects on strength were partially balanced by a decrease of grain boundary density. Additions of 1 and 2 wt% Cu, respectively, led to lower strength in the as quenched state, but also to significant strengthening via ageing. The peak ageing conditions as well as the Cu particle structure and size are comparable to values reported for Cu strengthened interstitial free steels and Fe-Cu alloys. Both alloying additions slightly lowered the specific elastic modulus of the HMS, most pronounced for Cu addition with a drop of about 3 GPa cm3 g− 1 per wt% and also promoted embrittlement. Microstructure-property relationships and consequences for future alloy design, especially towards more ductile hypo-eutectic HMS, are outlined and discussed. © 2017
    view abstractdoi: 10.1016/j.matdes.2017.03.042
  • 2017 • 168 Study of stability of microstructure and residual strain after thermal loading of plasma sprayed YSZ by through surface neutron scanning
    Gibmeier, J. and Back, H.C. and Mutter, M. and Vollert, F. and Rebelo-Kornmeier, J. and Mücke, R. and Vaßen, R.
    Physica B: Condensed Matter (2017)
    Yttria stabilized zirconia (YSZ) is often applied as thermal barrier coating on metal parts as e.g. turbine blades made of nickel base super alloys. The coating process in combination with the preconditioning of the substrate material induces characteristic residual stress distributions in the coating system consisting of topcoat, bondcoat and the substrate material. Knowledge about the residual stress depth distribution in the coating and at the interfaces down to the substrate material is essential for the assessment of the mechanical integrity and the reliability of the coating. In this regard the stability of the microstructure and the residual stresses is of particular interest; hence this forms the scope of our investigations. Yttria (8 wt.%) stabilized zirconia with a NiCoCrAlY bondcoat was deposited by atmospheric plasma spraying (APS) at different spray conditions on a nickel base super alloy substrate material. The coatings were subjected to different heat-treatment processes, i.e. static aging and cyclic thermal loadings. Through surface scanning using neutron diffraction was carried out for the as sprayed condition and for the thermally loaded samples. Based on the measured diffraction data the stability of the microstructure (phases) and the residual strain/stresses through the depths of the coating system were assessed. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.physb.2017.12.014
  • 2017 • 167 The FMR Behaviour of Li–Ni Ferrite Prepared by Hydrothermal Method
    Al-Shakarchi, E.K. and Lafta, S.H. and Musa, A. and Farle, M. and Salikhov, R.
    Journal of Superconductivity and Novel Magnetism 30 2575-2579 (2017)
    Magnetic hysteresis loop and ferromagnetic resonance (FMR) behaviour had been studied by considering the variation of the molar ratio of Li0.5−0.5xNixFe2.5−0.5xO4 ferrite nanostructure prepared by the hydrothermal method. The parameters of the ferrite nanostructure include coercivity (Hc), saturation magnetisation (Ms) and magnetic susceptibility. There was low coercivity at x = 0.3, high saturation magnetisation and high susceptibility (χi) at (x = 0.5). These parameters showed an improvement in their values compared with the samples prepared by conventional methods. The FMR analysis indicated that these samples were lossy materials due to having a broad line width ranging from 800 to 925 G. The FMR lines had asymmetric shapes due to anisotropic broadening and conduction mechanism. © 2017 Springer Science+Business Media New York
    view abstractdoi: 10.1007/s10948-017-4058-9
  • 2017 • 166 Topological phase inversion after long-term thermal exposure of nickel-base superalloys: Experiment and phase-field simulation
    Goerler, J.V. and Lopez-Galilea, I. and Mujica Roncery, L. and Shchyglo, O. and Theisen, W. and Steinbach, I.
    Acta Materialia 124 151-158 (2017)
    Ni-base superalloys are materials which are designed to resist extreme thermal and mechanical conditions. In this regard, an essential factor is their microstructure consisting of γ′ precipitates embedded in a γ matrix. The application of superalloys at high temperatures can however induce the topological phase inversion, where the γ′-phase topologically becomes the matrix phase, resulting in subpar material properties. In this work, the topological inversion is analyzed via experiment and phase-field simulation. The evolution of the microstructure has been quantified in the second generation single crystal Ni-base superalloy ERBO/1, which belongs to the family of CMSX-4, submitted to long-term aging at 1100° C for up to 250 h. Phase-field simulations carried out using a multi phase-field approach deliver insight into the microstructure evolution driven by the loss of coherency of the γ′ precipitates, which is induced by the accumulation of dislocations at the γ/γ′ interfaces. The obtained simulation results are in good agreement with the experimental results, and indicate that the mechanisms causing the topological inversion are linked to the accommodation of the lattice misfit, which enables coalescence and ripening of γ′ precipitates. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.10.059
  • 2017 • 165 Ultrathin High Surface Area Nickel Boride (NixB) Nanosheets as Highly Efficient Electrocatalyst for Oxygen Evolution
    Masa, J. and Sinev, I. and Mistry, H. and Ventosa, E. and de la Mata, M. and Arbiol, J. and Muhler, M. and Roldan Cuenya, B. and Schuhmann, W.
    Advanced Energy Materials (2017)
    The overriding obstacle to mass production of hydrogen from water as the premium fuel for powering our planet is the frustratingly slow kinetics of the oxygen evolution reaction (OER). Additionally, inadequate understanding of the key barriers of the OER is a hindrance to insightful design of advanced OER catalysts. This study presents ultrathin amorphous high-surface area nickel boride (NixB) nanosheets as a low-cost, very efficient and stable catalyst for the OER for electrochemical water splitting. The catalyst affords 10 mA cm-2 at 0.38 V overpotential during OER in 1.0 m KOH, reducing to only 0.28 V at 20 mA cm-2 when supported on nickel foam, which ranks it among the best reported nonprecious catalysts for oxygen evolution. Operando X-ray absorption fine-structure spectroscopy measurements reveal prevalence of NiOOH, as well as Ni-B under OER conditions, owing to a Ni-B core at nickel oxyhydroxide shell (Ni-B at NiOxH) structure, and increase in disorder of the NiOxH layer, thus revealing important insight into the transient states of the catalyst during oxygen evolution. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/aenm.201700381
  • 2016 • 164 A Three-Electrode, Battery-Type Swagelok Cell for the Evaluation of Secondary Alkaline Batteries: The Case of the Ni-Zn Battery
    Garcia, G. and Schuhmann, W. and Ventosa, E.
    ChemElectroChem 3 592-597 (2016)
    Three-electrode cells are essential in understanding battery materials under operando conditions. A three-electrode, battery-type Swagelok cell for electrochemical studies of secondary alkaline batteries, in particular Ni-Zn batteries, is presented. The relevance of the three-electrode battery-type cell (i.e. sealed and non-flooded) configuration is demonstrated as analytical tool with three observations: 1)The Ni electrode is shown to limit the system in the first cycles, while the Zn electrode becomes limiting in subsequent cycles. 2)Non-woven separators (NWSs) clearly improve the performance of the battery. Besides the known fact of hindering the dendritic growth of Zn, NWSs inhibit the evolution of oxygen and hydrogen at the positive and negative electrodes. 3)The kinetics of the Ni electrode is much slower than that of the Zn electrode, as derived from the charge-transfer resistance of the Ni electrode, which is substantially larger than that of the Zn electrode. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500474
  • 2016 • 163 Ab initio Prediction of Martensitic and Intermartensitic Phase Boundaries in Ni-Mn-Ga
    Dutta, B. and Çaklr, A. and Giacobbe, C. and Al-Zubi, A. and Hickel, T. and Acet, M. and Neugebauer, J.
    Physical Review Letters 116 (2016)
    Despite the importance of martensitic transformations of Ni-Mn-Ga Heusler alloys for their magnetocaloric and shape-memory properties, the martensitic part of their phase diagrams is not well determined. Using an ab initio approach that includes the interplay of lattice and vibrational degrees of freedom we identify an intermartensitic transformation between a modulated and a nonmodulated phase as a function of excess Ni and Mn content. Based on an evaluation of the theoretical findings and experimental x-ray diffraction data for Mn-rich alloys, we are able to predict the phase diagram for Ni-rich alloys. In contrast to other mechanisms discussed for various material systems in the literature, we herewith show that the intermartensitic transformation can be understood solely using thermodynamic concepts. © 2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.116.025503
  • 2016 • 162 Atomic layer deposition and high-resolution electron microscopy characterization of nickel nanoparticles for catalyst applications
    Dashjav, E. and Lipińska-Chwałek, M. and Grüner, D. and Mauer, G. and Luysberg, M. and Tietz, F.
    Surface and Coatings Technology 307 428-435 (2016)
    Ni nanoparticles (diameter <  10 nm) are deposited on Si and ceramic substrates of porous lanthanum-substituted strontium titanate/yttrium-stabilized zirconia (LST/YSZ) composites by a two-step process. First, NiO films are produced by atomic layer deposition at 200 °C using bis(methylcyclopentadienyl)nickel(II) (Ni(MeCp)2) and H2O as precursors. In the second step, the NiO films are reduced in H2 atmosphere at 400–800 °C. The size of the resulting Ni nanoparticles is controlled by the temperature. The largest particles with a diameter of about 7 nm are obtained at 800 °C. NiO film and Ni nanoparticles deposited on Si substrates are characterized by high-resolution electron microscopy. It was found that the Ni(MeCp)2 precursor reacts with the substrate, leading to the formation of NiSi2 precipitates beneath the surface of the Si wafer and amorphization of the surrounding area, resulting in a 10 nm thick top layer of the Si wafer. After reductive annealing, NiSi2 precipitates are preserved but Si recrystallizes and the amorphous NiO film transforms into crystalline Ni nanoparticles well distributed on the wafer surface. Process parameters were optimized for Si substrates and transfer of the process to ceramic LST/YSZ substrates is possible in principle. However, a much higher number of ALD cycles (1200 compared to 100 for Si) are necessary to obtain Ni nanoparticles of similar size and the number density of particles is lower than observed for Si substrates. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.08.074
  • 2016 • 161 Atomic-layer-controlled deposition of TEMAZ/O2-ZrO2 oxidation resistance inner surface coatings for solid oxide fuel cells
    Keuter, T. and Mauer, G. and Vondahlen, F. and Iskandar, R. and Menzler, N.H. and Vaßen, R.
    Surface and Coatings Technology 288 211-220 (2016)
    Solid oxide fuel cells (SOFCs) directly convert the chemical energy of fuels into electrical energy with high efficiency. Under certain conditions oxygen can diffuse to the Ni/8 mol% Y2O3-doped ZrO2 substrate of anode-supported SOFCs, then the nickel re-oxidizes, leading to cracks in the electrolyte and cell failure thus limiting the durability of SOFCs. In order to improve the stability of SOFCs with respect to oxidation, the inner surface of the porous substrate is coated with a ZrO2 oxidation resistance layer using atomic layer deposition (ALD) with the precursors tetrakis(ethylmethylamino)zirconium (TEMAZ) and molecular oxygen. This TEMAZ/O2-ZrO2 ALD process has not yet been reported in the literature and hence, the development of the process is described in this paper. The inner surface of the porous substrate is coated with ZrO2 and the film thickness is compared with theoretical predictions, verifying the ALD model. Furthermore, the coating depth can be estimated using a simple analytical equation. The ALD ZrO2 film protects the nickel in the substrate against oxidation for at least 17 re-oxidation/re-reduction cycles. The ZrO2 inner surface coating is a highly promising candidate for enhancing the resistance of SOFCs to re-oxidation because of the excellent oxidation resistance and good cycling stability of the film. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.01.026
  • 2016 • 160 Characterisation of bifunctional electrocatalysts for oxygen reduction and evolution by means of SECM
    Chen, X. and Botz, A.J.R. and Masa, J. and Schuhmann, W.
    Journal of Solid State Electrochemistry 20 1019-1027 (2016)
    Electrocatalysts that can reversibly reduce oxygen and oxidise water are of prime importance for the advancement of new emerging electrochemical energy storage and conversion systems. We present in this work the application of scanning electrochemical microscopy (SECM) for characterisation of bifunctional catalysts. By using model bifunctional catalysts based on oxides of cobalt (CoxOy) and nickel (NixOy) embedded in nitrogen-doped carbon (NC), we specifically show the unique ability of using SECM to determine a range of the important electrocatalytic parameters including the selectivity of the oxygen reduction reaction (ORR), the initial mechanistic steps during the oxygen evolution reaction (OER), and the onset potential for both ORR and OER in a single experiment. We were able to observe directly that prior to oxygen evolution, local depletion of oxygen occurs at the SECM tip during redox transition accompanying most likely metal oxyhydroxide formation thus enabling direct in situ observation of the initial mechanistic steps of the OER. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s10008-015-3028-z
  • 2016 • 159 Characterization of mechanical properties of hydroxyapatite-silicon-multi walled carbon nano tubes composite coatings synthesized by EPD on NiTi alloys for biomedical application
    Khalili, V. and Khalil-Allafi, J. and Sengstock, C. and Motemani, Y. and Paulsen, A. and Frenzel, J. and Eggeler, G. and Köller, M.
    Journal of the Mechanical Behavior of Biomedical Materials 59 337-352 (2016)
    Release of Ni1+ ions from NiTi alloy into tissue environment, biological response on the surface of NiTi and the allergic reaction of atopic people towards Ni are challengeable issues for biomedical application. In this study, composite coatings of hydroxyapatite-silicon multi walled carbon nano-tubes with 20 wt% Silicon and 1 wt% multi walled carbon nano-tubes of HA were deposited on a NiTi substrate using electrophoretic methods. The SEM images of coated samples exhibit a continuous and compact morphology for hydroxyapatite-silicon and hydroxyapatite-silicon-multi walled carbon nano-tubes coatings. Nano-indentation analysis on different locations of coatings represents the highest elastic modulus (45.8 GPa) for HA-Si-MWCNTs which is between the elastic modulus of NiTi substrate (66.5 GPa) and bone tissue (≈30 GPa). This results in decrease of stress gradient on coating-substrate-bone interfaces during performance. The results of nano-scratch analysis show the highest critical distance of delamination (2.5 mm) and normal load before failure (837 mN) as well as highest critical contact pressure for hydroxyapatite-silicon-multi walled carbon nano-tubes coating. The cell culture results show that human mesenchymal stem cells are able to adhere and proliferate on the pure hydroxyapatite and composite coatings. The presence of both silicon and multi walled carbon nano-tubes (CS3) in the hydroxyapatite coating induce more adherence of viable human mesenchymal stem cells in contrast to the HA coated samples with only silicon (CS2). These results make hydroxyapatite-silicon-multi walled carbon nano-tubes a promising composite coating for future bone implant application. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2016.02.007
  • 2016 • 158 Combinatorial design of transitory constitution steels: Coupling high strength with inherent formability and weldability through sequenced austenite stability
    Springer, H. and Belde, M. and Raabe, D.
    Materials and Design 90 1100-1109 (2016)
    We introduce a novel alloying and processing scheme for high strength steels, which allows for precise and cost-effective cold forming due to high fractions of metastable austenite, and a subsequent low-distortion, coating-preserving strengthening through martensitic transformation induced by low temperature treatments. As the constitution is thus synchronised with the processing requirements, we refer to these materials as Transitory Constitution Steels. Suitable alloy compositions were identified by high throughput screenings through the exemplarily material systems Fe-5Ni-0.3C-(3-15)Mn and Fe-13.5Cr-6Mn-2Cu-0.2C-(0-2)Ni (wt.%) using combinatorial bulk metallurgical methods. The transformation behaviour, mechanical properties and underlying microstructural phenomena were studied in more detail after upscaling of selected compositions. The steel Fe-13.5Cr-6Mn-1.5Cu-0.2C (wt.%) exhibited an increase in yield strength from 300 to 1050. MPa after immersion into liquid nitrogen, as well as an ultimate tensile strength of more than 1700. MPa at a total elongation of more than 9%. Despite the ultra high strength, no embrittlement induced by Laser beam welding was observed, highlighting the inherent weldability of steels synthesised by the alloying and processing scheme presented here. Possibilities for flexible alloy design and processing variations are discussed. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2015.11.050
  • 2016 • 157 Cyclic degradation in bamboo-like Fe-Mn-Al-Ni shape memory alloys - The role of grain orientation
    Vollmer, M. and Krooß, P. and Kriegel, M.J. and Klemm, V. and Somsen, C. and Ozcan, H. and Karaman, I. and Weidner, A. and Rafaja, D. and Biermann, H. and Niendorf, T.
    Scripta Materialia 114 156-160 (2016)
    In the present study the cyclic deformation behavior within differently oriented grains in Fe-34.8Mn-13.5Al-7.4Ni (at.%) shape memory polycrystals featuring a bamboo-like structure was investigated. In cyclic tensile tests up to 50 cycles, the degree of degradation in pseudoelasticity was evaluated and contributing elementary mechanisms are discussed. The results reveal rapid cyclic degradation in the bamboo-like samples. The unexpected stabilization of parent phase in reverse transformed areas and the proceeding activation of new martensite variants in subsequent cycles were found to be the prevailing degradation mechanisms. Dislocation activity is found to be the most detrimental factor. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2015.12.007
  • 2016 • 156 Diffusion of solutes in fcc Cobalt investigated by diffusion couples and first principles kinetic Monte Carlo
    Neumeier, S. and Rehman, H.U. and Neuner, J. and Zenk, C.H. and Michel, S. and Schuwalow, S. and Rogal, J. and Drautz, R. and Göken, M.
    Acta Materialia 106 304-312 (2016)
    The interdiffusivity of Al and the transition metal solutes Ti, V, Cr, Mn, Fe, Nb, Mo, Ru, Ta, W, and Re in fcc Co is characterized at 1373 K, 1473 K and 1573 K by binary diffusion couples. The experimental results are complemented by first-principles calculations in combination with kinetic Monte Carlo simulations to investigate the diffusion of Re, W, Mo and Ta in fcc Co. The interdiffusion coefficients of alloying elements in fcc Co are generally smaller than in fcc Ni, but the correlation between interdiffusion coefficients and the atomic number of metal solutes is comparable in Co and Ni. With increasing atomic number and decreasing atomic radii the interdiffusion coefficients of the investigated elements, except for Mn and Fe, decrease strongly. The trends in the diffusivity determined by experiment and simulation are in excellent agreement. Re is the slowest diffusing element in fcc Co among the investigated elements. The electronic structure calculations indicate that this is caused by strong directional bonds between Re and neighboring Co atoms. The overall lower diffusivity of solute atoms in Co as compared to Ni suggests that diffusion controlled processes could be slower in Co-base superalloys. © 2016 Acta Materialia Inc. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.028
  • 2016 • 155 Double minimum creep of single crystal Ni-base superalloys
    Wu, X. and Wollgramm, P. and Somsen, C. and Dlouhy, A. and Kostka, A. and Eggeler, G.
    Acta Materialia 112 242-260 (2016)
    Low temperature (750°C) and high stress (800 MPa) creep curves of single crystal superalloy ERBO/1 tensile specimens loaded in the (001) direction show two creep rate minima. Strain rates decrease towards a first sharp local creep rate minimum at 0.1% strain (reached after 30 min). Then deformation rates increase and reach an intermediate maximum at 1% (reached after 1.5 h). Subsequently, strain rates decrease towards a global minimum at 5% (260 h), before tertiary creep (not considered in the present work) leads to final rupture. We combine high resolution miniature creep testing with diffraction contrast transmission electron microscopy and identify elementary processes which govern this double-minimum type of creep behavior. We provide new quantitative information on the evolution of microstructure during low temperature and high stress creep, focusing on γ-channel dislocation activity and stacking fault shear of the γ′-phase. We discuss our results in the light of previous work published in the literature and highlight areas in need of further work. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.04.012
  • 2016 • 154 Effect of Ni Content on Structural and Magnetic Properties of Li-Ni Ferrites Nanostructure Prepared by Hydrothermal Method
    Al-Shakarchi, E.K. and Lafta, S.H. and Musa, A.M. and Farle, M. and Salikov, R.
    Journal of Superconductivity and Novel Magnetism 29 923-929 (2016)
    Li-Ni ferrite with chemical formula Li0.5−0.5xNixFe2.5−0.5xO4 was prepared by hydrothermal method with different Ni contents (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) using metal chlorides, ferrous sulfate and sodium hydroxide as oxidants. The hydrothermal treatment was accomplished at (155 ∘ C) for (3 h). The required analyses of XRD, FTIR, SEM, TEM, EDX and magnetic hysteresis loop were performed to characterize the complete behavior as a function of x. It was found that lattice constant has a small increase as x increased. Crystallite size had a minimum value of about 13 nm at x = 1.0 and maximum value of about 33 nm at x = 0.3. It was also found that XRD density increased as x was increased. The particle size distributions showed that the maximum value is around 22 nm. FTIR analysis showed the presence of two main peaks with some shifting. Nanospheres were the predominant particles beside the presence of low nanorod concentration. M-H loops had super paramagnetic shape. The coercivity had a minimum value at x = 0.5. The magnetic saturation had a maximum value at x = 0.3, and the initial susceptibility χi had a maximum value at x = 0.5. © 2015, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10948-015-3334-9
  • 2016 • 153 Effect of Ni Incorporation into Malachite Precursors on the Catalytic Properties of the Resulting Nanostructured CuO/NiO Catalysts
    Garcia, Y. and Su, B.-L. and Ortega, K.F. and Hüttner, A. and Heese, J. and Behrens, M.
    European Journal of Inorganic Chemistry 2016 2063-2071 (2016)
    Synthetic nickelian malachite nanopowders (Cu1-xNix)2(OH)2CO3 with x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1 were prepared by constant-pH coprecipitation. N2 sorption isotherms confirmed a steady increase of the BET surface area with increasing Ni content for the as-synthesized and calcined mesoporous materials. Powder XRD patterns for x ≤ 0.1 indicate the formation of single-phase materials with an anisotropic contraction of the unit cell. This is related to the gradual decrease of the Jahn-Teller distortion in the malachite structure. An XRD-amorphous hydroxide-rich phase is formed for x &gt; 0.1, which appears as spongelike regions in SEM images. Thermogravimetric analysis showed that nickel lowers the onset of thermal decomposition. Powder XRD patterns of the calcined samples evidence the formation of a tenorite structure despite the presence of Ni. Heterogeneous Fenton-like decomposition of Bismarck Brown Y with H2O2 showed that a Cu/Ni ratio of 92:8 in the nanostructured oxide leads to the highest reaction rate constant derived from a pseudo-first-order kinetic rate law expression. Temperature-programmed CO oxidation experiments revealed that pure CuO achieved the highest activity. Similar performance was observed for the binary system obtained through calcination of the precursor prepared with a Cu/Ni ratio of 96:4. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ejic.201501425
  • 2016 • 152 Elemental partitioning, lattice misfit and creep behaviour of Cr containing gammaprime strengthened Co base superalloys
    Povstugar, I. and Zenk, C.H. and Li, R. and Choi, P.-P. and Neumeier, S. and Dolotko, O. and Hoelzel, M. and Göken, M. and Raabe, D.
    Materials Science and Technology (United Kingdom) 32 220-225 (2016)
    Novel Cr containing Co-Al-W base superalloys were studied by atom probe tomography and neutron diffraction. Cr is found to predominantly partition to the gamma matrix and decrease partitioning of W to gammaprime. Furthermore, Cr significantly enhances the gammaprime volume fraction, decreases the gamma/gammaprime lattice misfit and deteriorates the creep resistance. Addition of Ni to the Cr containing alloys affects partitioning of W and Al, further decreases the lattice misfit and results in the formation of irregularly shaped precipitates. Al, W and Cr tend to occupy the ‘B'sublattice in the gammaprime-A3B phase (L12 type), while Co and Ni reside in the ‘A' sublattice. © 2016 Institute of Materials.
    view abstractdoi: 10.1179/1743284715Y.0000000112
  • 2016 • 151 Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures
    Gludovatz, B. and Hohenwarter, A. and Thurston, K.V.S. and Bei, H. and Wu, Z. and George, E.P. and Ritchie, R.O.
    Nature Communications 7 (2016)
    High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centred cubic solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature, the alloy shows tensile strengths of almost 1 GPa, failure strains of ∼70% and KJIc fracture-toughness values above 200 MPa m1/2; at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc values of 275 MPa m1/2. Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning. © 2016, Nature Publishing Group. All rights reserved.
    view abstractdoi: 10.1038/ncomms10602
  • 2016 • 150 First-principles modeling of energetic and mechanical properties of Ni-Cr, Ni-Re and Cr-Re random alloys
    Breidi, A. and Fries, S.G. and Palumbo, M. and Ruban, A.V.
    Computational Materials Science 117 45-53 (2016)
    We apply the exact-muffin-tin-orbitals (EMTO) method to investigate structural properties, formation enthalpies, mechanical stability and polycrystalline moduli in Ni-Re, Ni-Cr and Cr-Re disordered fcc, bcc and hcp phases. Substitutional disorder is treated by using the coherent potential approximation (CPA). We predict the alloy lattice parameters in good agreement with the experiment. We find a continuous softening, as a function of Cr composition, of the tetragonal shear modulus C′ in fcc Ni-Cr phase indicating mechanical instability in Cr-rich Ni-Cr alloys. On the other hand, we show that the mechanical stability of fcc Ni-Re alloys persists through the whole composition range. We observe an intriguing behaviour of the Young's modulus vs. the intrinsic ductility curve in Ni-rich Ni-Re fcc phase. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2016.01.020
  • 2016 • 149 High-Temperature Stable Ni Nanoparticles for the Dry Reforming of Methane
    Mette, K. and Kühl, S. and Tarasov, A. and Willinger, M.G. and Kröhnert, J. and Wrabetz, S. and Trunschke, A. and Scherzer, M. and Girgsdies, F. and Düdder, H. and Kähler, K. and Ortega, K.F. and Muhler, M. and Schlögl, R. an...
    ACS Catalysis 6 7238-7248 (2016)
    Dry reforming of methane (DRM) has been studied for many years as an attractive option to produce synthesis gas. However, catalyst deactivation by coking over nonprecious-metal catalysts still remains unresolved. Here, we study the influence of structural and compositional properties of nickel catalysts on the catalytic performance and coking propensity in the DRM. A series of bulk catalysts with different Ni contents was synthesized by calcination of hydrotalcite-like precursors NixMg0.67-xAl0.33(OH)2(CO3)0.17·mH2O prepared by constant-pH coprecipitation. The obtained Ni/MgAl oxide catalysts contain Ni nanoparticles with diameters between 7 and 20 nm. High-resolution transmission electron microscopy (HR-TEM) revealed a nickel aluminate overgrowth on the Ni particles, which could be confirmed by Fourier transform infrared (FTIR) spectroscopy. In particular, catalysts with low Ni contents (5 mol %) exhibit predominantly oxidic surfaces dominated by Ni2+ and additionally some isolated Ni0 sites. These properties, which are determined by the overgrowth, effectively diminish the formation of coke during the DRM, while the activity is preserved. A large (TEM) and dynamic (microcalorimetry) metallic Ni surface at high Ni contents (50 mol %) causes significant coke formation during the DRM. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b01683
  • 2016 • 148 Impact of hot isostatic pressing on microstructures of CMSX-4 Ni-base superalloy fabricated by selective electron beam melting
    Ruttert, B. and Ramsperger, M. and Mujica Roncery, L. and Lopez-Galilea, I. and Körner, C. and Theisen, W.
    Materials and Design 110 720-727 (2016)
    Selective electron beam melting (SEBM) is a powder-bed-based additive manufacturing process that was used to produce cylindrical and columnar-grained parts made of Ni-base superalloy CMSX-4 from pre-alloyed and atomized powder. SEBM is characterized by high temperature gradients during solidification, which results in a very fine microstructure that is several orders of magnitude smaller than in conventionally cast material. This opens up new perspectives regarding time-consuming solution heat treatment. Nevertheless, microstructural heterogeneities, such as segregation and porosity, are still present on a much smaller scale, and also the high susceptibility to cracking of this alloy class during welding has to be taken into account. Since the latest generation of hot isostatic presses (HIP) are able to simultaneously heat-treat and eliminate porosity owing to their quenching capability, such a HIP is used in this work. The influence of different HIPing-heat-treatment-strategies with variation of temperature and time at a constant pressure on the SEBM-microstructure was investigated with emphasis primarily on segregation and porosity. The results demonstrate that only a few minutes of solutioning are sufficient to dissolve segregations and to close pores. The initial degree of homogeneity of the SEBM-material is responsible for the short solutioning-time. © 2016
    view abstractdoi: 10.1016/j.matdes.2016.08.041
  • 2016 • 147 In-situ metal matrix composite steels: Effect of alloying and annealing on morphology, structure and mechanical properties of TiB2 particle containing high modulus steels
    Aparicio-Fernández, R. and Springer, H. and Szczepaniak, A. and Zhang, H. and Raabe, D.
    Acta Materialia 107 38-48 (2016)
    We systematically study the morphology, size and dispersion of TiB2 particles formed in-situ from Fe-Ti-B based melts, as well as their chemical composition, crystal structure and mechanical properties. The effects of 5 wt.% additions of Cr, Ni, Co, Mo, W, Mn, Al, Si, V, Ta, Nb and Zr, respectively, as well as additional annealing treatments, were investigated in order to derive guidelines for the knowledge based alloy design of steels with an increased stiffness/density ratio and sufficiently high ductility. All alloying elements were found to increase the size of the coarse primary TiB2 particles, while Co led to the most homogeneous size distribution. The size of the eutectic TiB2 constituents was decreased by all alloying additions except Ni, while their aspect ratio was little affected. No clear relation between chemical composition, crystal structure and mechanical properties of the particles could be observed. Annealing of the as-cast alloys slightly increased the size of the primary particles, but at the same time strongly spheroidised the eutectics. Additions of Co and Cr appear thus as the best starting point for designing novel in-situ high modulus metal matrix composite steels, while using Mn in concert with thermo-mechanical processing is most suited to adapt the matrix' microstructure and optimise the particle/matrix co-deformation processes. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.048
  • 2016 • 146 Influence of temperature, pressure, and cooling rate during hot isostatic pressing on the microstructure of an SX Ni-base superalloy
    Mujica Roncery, L. and Lopez-Galilea, I. and Ruttert, B. and Huth, S. and Theisen, W.
    Materials and Design 97 544-552 (2016)
    This work investigates the application of hot isostatic pressing for heat treatment of the single-crystal Ni-base superalloy ERBO/1. Recent progress regarding incorporation of quenching within hot isostatic pressing enables heat treatments to be performed so that the microstructures can be frozen at a desired point. The influence of the temperature, pressure, and cooling method (quenching, natural convection, and slow cooling) as well as the cooling rate after solutioning-HIP treatment on pore densification and γ/. γ'-morphology was measured. Temperatures above γ'-solvus resulted in a greater efficiency of the porosity reduction. At super-solvus temperatures, pressures above 75. MPa are sufficient enough to annihilate the porosity. The cooling rate after HIP-solutioning treatment has a major influence on the γ'-particle size and shape. Quenching with 45-20. K/s at 100. MPa leads to high number density and monomodal distribution of γ'-particles with sizes around 140. nm. In contrast, slow cooling rate of 0.33. K/s leads to γ'-precipitate sizes of 720. nm. Moreover, an integrated heat treatment at 100. MPa, which consisted of solutioning and aging in the HIP, was successfully applied. It led to smaller γ'-particle sizes and narrower γ-channels compared to the conventionally heat-treated material and also almost no porosity. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2016.02.051
  • 2016 • 145 Interface effects on the magnetic properties of layered Ni2MnGa/Ni2MnSn alloys: A first-principles investigation
    Dutta, B. and Opahle, I. and Hickel, T.
    Functional Materials Letters 9 (2016)
    The effect of interfaces on the magnetic properties of multilayers is analyzed forNi2MnGa/Ni2MnSn system using density functional theory. The Ni spin moments at the interface change by about 30% compared to the bulk value, whereas the effect on the Mn spin moments is much less pronounced. A similar strong effect is also observed for the Ni orbital moments at the interface. The magneto-crystalline anisotropy of the multilayer systems can be understood by the additive contribution of the respective values of strained bulk materials. © 2016 World Scientific Publishing Company.
    view abstractdoi: 10.1142/S1793604716420108
  • 2016 • 144 Intermediate Co/Ni-base model superalloys - Thermophysical properties, creep and oxidation
    Zenk, C.H. and Neumeier, S. and Engl, N.M. and Fries, S.G. and Dolotko, O. and Weiser, M. and Virtanen, S. and Göken, M.
    Scripta Materialia 112 83-86 (2016)
    The mechanical properties of γ′-strengthened Co-Ni-Al-W-Cr model superalloys extending from pure Ni-base to pure Co-base superalloys have been assessed. Differential scanning calorimetry measurements and thermodynamic calculations match well and show that the γ′ solvus temperature decreases with increasing Co-content. The γ/γ′ lattice misfit is negative on the Ni- and positive on the Co-rich side. High Ni-contents decelerate the oxidation kinetics up to a factor of 15. The creep strength of the Ni-base alloy increases by an order of magnitude with additions of Co before it deteriorates strongly upon higher additions despite an increasing γ′ volume fraction. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2015.09.018
  • 2016 • 143 Light Localization and Magneto-Optic Enhancement in Ni Antidot Arrays
    Rollinger, M. and Thielen, P. and Melander, E. and Östman, E. and Kapaklis, V. and Obry, B. and Cinchetti, M. and García-Martín, A. and Aeschlimann, M. and Papaioannou, E.Th.
    16 2432-2438 (2016)
    We reveal an explicit strategy to design the magneto-optic response of a magneto-plasmonic crystal by correlating near- and far-fields effects. We use photoemission electron microscopy to map the spatial distribution of the electric near-field on a nanopatterned magnetic surface that supports plasmon polaritons. By using different photon energies and polarization states of the incident light we reveal that the electric near-field is either concentrated in spots forming a hexagonal lattice with the same symmetry as the Ni nanopattern or in stripes oriented along the Γ-K direction of the lattice and perpendicular to the polarization direction. We show that the polarization-dependent near-field enhancement on the patterned surface is directly correlated to both the excitation of surface plasmon polaritons on the patterned surface as well as the enhancement of the polar magneto-optical Kerr effect. We obtain a relationship between the size of the enhanced magneto-optical behavior and the polarization and wavelength of optical excitation. The engineering of the magneto-optic response based on the plasmon-induced modification of the optical properties introduces the concept of a magneto-plasmonic meta-structure. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.5b05279
  • 2016 • 142 Magnetic proximity effect and shell-ferromagnetism in metastable Ni50Mn45Ga5
    Krenke, T. and Çakır, A. and Scheibel, F. and Acet, M. and Farle, M.
    Journal of Applied Physics 120 (2016)
    The present study on magnetic and structural properties of Ni50Mn45Ga5 confirms that structural metastability is an inherent property of Ni50Mn50- xXx Heusler alloys with X as In, Ga, and Sn. The ternary alloy transforms during temper-annealing into a dual-phase composite alloy. The two phases are identified to be cubic L21, Ni50Mn25Ga25, and tetragonal L10 Ni50Mn50. Depending on the annealing temperature, the magnetic-proximity effect giving rise to shell-ferromagnetism has been observed when annealing is carried out under an external magnetic field. The upper and lower remanence values MR+ and MR− have the same sign even at high temperatures. Such alloys can be promising candidates for heat- and magnetic-field-resistant magnetic recording media. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4972480
  • 2016 • 141 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
  • 2016 • 140 Multi-scale and spatially resolved hydrogen mapping in a Ni-Nb model alloy reveals the role of the δ phase in hydrogen embrittlement of alloy 718
    Tarzimoghadam, Z. and Rohwerder, M. and Merzlikin, S.V. and Bashir, A. and Yedra, L. and Eswara, S. and Ponge, D. and Raabe, D.
    Acta Materialia 109 69-81 (2016)
    We investigated the hydrogen distribution and desorption behavior in an electrochemically hydrogen-charged binary Ni-Nb model alloy to study the role of δ phase in hydrogen embrittlement of alloy 718. We focus on two aspects, namely, (1) mapping the hydrogen distribution with spatial resolution enabling the observation of the relations between desorption profiles and desorption sites; and (2) correlating these observations with mechanical testing results to reveal the degradation mechanisms. The trapping states of hydrogen in the alloy were globally analyzed by Thermal Desorption Spectroscopy (TDS). Additionally, spatially resolved hydrogen mapping was conducted using silver decoration, Scanning Kelvin Probe Force Microscopy (SKPFM) and Secondary Ion Mass Spectrometry (SIMS): The Ag decoration method revealed rapid effusion of hydrogen at room temperature from the γ-matrix. The corresponding kinetics was resolved in both, space and time by the SKPFM measurements. At room temperature the hydrogen release from the γ-matrix steadily decreased until about 100 h and then was taken over by the δ phase from which the hydrogen was released much slower. For avoiding misinterpretation of hydrogen signals stemming from environmental effects we also charged specimens with deuterium. The deuterium distribution in the microstructure was studied by SIMS. The combined results reveal that hydrogen dissolves more preferably inside the γ-matrix and is diffusible at room temperature while the δ phase acts as a deeper trapping site for hydrogen. With this joint and spatially resolving approach we observed the microstructure- and time-dependent distribution and release rate of hydrogen with high spatial and temporal resolution. Correlating the obtained results with mechanical testing of the hydrogen-charged samples shows that hydrogen enhanced decohesion (HEDE) occurring at the δ/matrix interfaces promotes the embrittlement. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.02.053
  • 2016 • 139 On Local Phase Equilibria and the Appearance of Nanoparticles in the Microstructure of Single-Crystal Ni-Base Superalloys
    Yardley, V. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Parsa, A.B. and Kostka, A. and Somsen, C. and Dlouhy, A. and Neuking, K. and George, E.P. and Eggeler, G.
    Advanced Engineering Materials 18 1556-1567 (2016)
    High-resolution characterization techniques are combined with thermodynamic calculations (CALPHAD) to rationalize microstructural features of single crystal Ni-base superalloys. Considering the chemical compositions of dendritic and interdendritic regions one can explain differences in γ′-volume fractions. Using thermodynamic calculations one can explain, why γ-nanoparticles are observed in the central regions of large cuboidal γ′-particles and why tertiary γ′-nanoparticles form in the γ-channels. The chemical compositions of the γ-channels and of the newly formed γ-particles differ because of the Gibbs–Thomson pressure which acts on the small particles. With increasing size of secondary γ′-particles, their shape changes from spherical to cuboidal. Some general thermodynamic aspects including the temperature dependencies of the Gibbs free energy G, the enthalpy H, and the entropy S and site occupancies in the ordered L12 (γ′) phase are considered. The importance of cooling rate after homogenization is discussed. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201600237
  • 2016 • 138 On the Effect of Hot Isostatic Pressing on the Creep Life of a Single Crystal Superalloys
    Mujica Roncery, L. and Lopez-Galilea, I. and Ruttert, B. and Bürger, D. and Wollgramm, P. and Eggeler, G. and Theisen, W.
    Advanced Engineering Materials 18 1381-1387 (2016)
    The creep behavior of a single-crystal Ni-base superalloy in two microstructural states is compared. One is obtained by casting followed by a conventional heat treatment. The other results from the same nominal heat treatment integrated into a hot isostatic pressing process. The microstructure after HIP differed from that in the conventional route in two respects. First, the γ′ particles are smaller and the γ channels are narrower. Second, after HIP, the number density of pores is lower and the pore sizes are smaller. The HIP microstructure improves creep in two respects: the finer γ/γ′-microstructure results in lower minimum creep rates. Moreover, the shrinkage of cast porosity during HIP delays the nucleation and growth of micro cracks and results in higher rupture strains in the low-temperature high stress regime. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201600071
  • 2016 • 137 On the origin of creep dislocations in a Ni-base, single-crystal superalloy: An ECCI, EBSD, and dislocation dynamics-based study
    Ram, F. and Li, Z. and Zaefferer, S. and Hafez Haghighat, S.M. and Zhu, Z. and Raabe, D. and Reed, R.C.
    Acta Materialia 109 151-161 (2016)
    This work investigates the origin of creep dislocations in a Ni-base, single crystal superalloy subject to creep at an intermediate stress and temperature. Employing high angular resolution electron backscatter diffraction (HR-EBSD), electron channeling contrast imaging under controlled diffraction conditions (cECCI) and discrete dislocation dynamics (DDD) modelling, it is shown that low-angle boundaries - which correspond to dendrite boundaries or their residues after annealing - are not the major sources of creep dislocations. At the onset of creep deformation, they are the only active sources. Creep dislocations are emitted from them and percolate into the dislocation-depleted crystal. However, the percolation is very slow. As creep deformation proceeds, before the boundary-originated dislocations move further than a few micrometers away from their source boundary, individual dislocations that are spread throughout the undeformed microstructure become active and emit avalanches of creep dislocations in boundary-free regions, i.e. regions farther than a few micrometer away from boundaries. Upon their activation, the density of creep dislocations in boundary-free regions soars by two orders of magnitude; and the entire microstructure becomes deluged with creep dislocations. The total area of boundary-free regions is several times the total area of regions covered by boundary-originated creep dislocations. Therefore, the main sources of creep dislocations are not low-angle boundaries but individual, isolated dislocations in boundary-free regions. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.02.038
  • 2016 • 136 On the rich magnetic phase diagram of (Ni, Co)-Mn-Sn Heusler alloys
    Grünebohm, A. and Herper, H.C. and Entel, P.
    Journal of Physics D: Applied Physics 49 (2016)
    We put a spotlight on the exceptional magnetic properties of the metamagnetic Heusler alloy (Ni, Co)-Mn-Sn by means of first principles simulations. In the energy landscape we find a multitude of local minima, which belong to different ferrimagnetic states and are close in total magnetization and energy. All these magnetic states correspond to the local high spin state of the Mn atoms with different spin alignments and are related to the magnetic properties of Mn. Compared to pure Mn, the magneto-volume coupling is reduced by Ni, Co and Sn atoms in the lattice and no local low-spin Mn states appear. For the cubic phase we find a ferromagnetic ground state whereas the global energy minimum is a tetragonal state with a complicated spin structure and vanishing magnetization which so far has been overlooked in simulations. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/39/395001
  • 2016 • 135 Pentlandite rocks as sustainable and stable efficient electrocatalysts for hydrogen generation
    Konkena, B. and Puring, K.J. and Sinev, I. and Piontek, S. and Khavryuchenko, O. and Dürholt, J.P. and Schmid, R. and Tüysüz, H. and Muhler, M. and Schuhmann, W. and Apfel, U.-P.
    Nature Communications 7 (2016)
    The need for sustainable catalysts for an efficient hydrogen evolution reaction is of significant interest for modern society. Inspired by comparable structural properties of [FeNi]-hydrogenase, here we present the natural ore pentlandite (Fe 4.5 Ni 4.5 S 8) as a direct rock' electrode material for hydrogen evolution under acidic conditions with an overpotential of 280 mV at 10 mA cm -2. Furthermore, it reaches a value as low as 190 mV after 96 h of electrolysis due to surface sulfur depletion, which may change the electronic structure of the catalytically active nickel-iron centres. The rock' material shows an unexpected catalytic activity with comparable overpotential and Tafel slope to some well-developed metallic or nanostructured catalysts. Notably, the rock' material offers high current densities (≤650 mA cm -2) without any loss in activity for approximately 170 h. The superior hydrogen evolution performance of pentlandites as rock' electrode labels this ore as a promising electrocatalyst for future hydrogen-based economy.
    view abstractdoi: 10.1038/ncomms12269
  • 2016 • 134 Progress on effect of processes and microelements on liquation cracking of weld heat-affected zone of nickel-based alloy
    Li, Z. and Wang, H. and Li, Y. and Kim, H.J. and Tillmann, W.
    Jixie Gongcheng Xuebao/Journal of Mechanical Engineering 52 37-45 (2016)
    Nickel-based alloy with excellent high temperature resistance and corrosion resistance, has been widely applied to aerospace, nuclear power and offshore oil industry. Based on the importance of nickel-based alloy, studying the liquation cracking of weld heat-affected zone (HAZ) is of great significance to resolve the above problem. Welding process has a direct influence on welding performance of nickel-based alloy, so it is one of the important factors to lead to HAZ liquation cracking. By heat treatment processes, the susceptibility to liquation cracking can be reduced because of expected microstructure and properties obtained. Meanwhile, the microelements in base metal affect the non-equilibrium segregation behavior of grain boundaries, so it can leads to the generation of HAZ liquation cracking. The progress on liquation cracking of weld heat-affected zone of nichel-based alloy are reviewed from welding processes, heat treatment processes and micro-elements, and the test method of liquation cracking is summarized. The future research trends are prospected. © 2016 Journal of Mechanical Engineering.
    view abstractdoi: 10.3901/JME.2016.06.037
  • 2016 • 133 Relay-Like Exchange Mechanism through a Spin Radical between TbPc2 Molecules and Graphene/Ni(111) Substrates
    Marocchi, S. and Candini, A. and Klar, D. and Van Den Heuvel, W. and Huang, H. and Troiani, F. and Corradini, V. and Biagi, R. and De Renzi, V. and Klyatskaya, S. and Kummer, K. and Brookes, N.B. and Ruben, M. and Wende, H. and De...
    ACS Nano 10 9353-9360 (2016)
    We investigate the electronic and magnetic properties of TbPc2 single ion magnets adsorbed on a graphene/Ni(111) substrate, by density functional theory (DFT), ab initio complete active space self-consistent field calculations, and X-ray magnetic circular dichroism (XMCD) experiments. Despite the presence of the graphene decoupling layer, a sizable antiferromagnetic coupling between Tb and Ni is observed in the XMCD experiments. The molecule-surface interaction is rationalized by the DFT analysis and is found to follow a relay-like communication pathway, where the radical spin on the organic Pc ligands mediates the interaction between Tb ion and Ni substrate spins. A model Hamiltonian which explicitly takes into account the presence of the spin radical is then developed, and the different magnetic interactions at play are assessed by first-principle calculations and by comparing the calculated magnetization curves with XMCD data. The relay-like mechanism is at the heart of the process through which the spin information contained in the Tb ion is sensed and exploited in carbon-based molecular spintronics devices. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.6b04107
  • 2016 • 132 Spent Tea Leaf Templating of Cobalt-Based Mixed Oxide Nanocrystals for Water Oxidation
    Deng, X. and Chan, C.K. and Tüysüz, H.
    ACS Applied Materials and Interfaces 8 32488-32495 (2016)
    The facile synthesis of nanostructured cobalt oxides using spent tea leaves as a hard template is reported. Following an impregnation-calcination and template removal pathway, sheetlike structures containing nanosized crystallites of Co3O4 are obtained. Co3O4 incorporated with Cu, Ni, Fe, and Mn (M/Co = 1/8 atomic ratio) are also prepared, and the materials are thoroughly characterized using X-ray diffraction, electron microscopy, and N2 sorption. The method is applicable to several commercial tea leaves and is successfully scaled up to prepare over 7 g of Co3O4 with the same nanostructure. The oxides are then tested for electrochemical water oxidation, and Cu, Ni, and Fe incorporations show beneficial effect on the catalytic activity of Co3O4, achieving performance comparable to levels from benchmark electrocatalysts. These data suggest that tea leaf templating can be utilized as a facile and promising approach to prepare nanostructured functional catalyst. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsami.6b12005
  • 2016 • 131 Structure-Activity-Stability Relationships for Space-Confined PtxNiy Nanoparticles in the Oxygen Reduction Reaction
    Mezzavilla, S. and Baldizzone, C. and Swertz, A.-C. and Hodnik, N. and Pizzutilo, E. and Polymeros, G. and Keeley, G.P. and Knossalla, J. and Heggen, M. and Mayrhofer, K.J.J. and Schüth, F.
    ACS Catalysis 6 8058-8068 (2016)
    This study focuses on the synthesis and electrochemical performance (i.e, activity and stability) of advanced electrocatalysts for the oxygen reduction reaction (ORR), made of Pt-Ni nanoparticles embedded in hollow graphitic spheres (HGS). The mechanism of the confined space alloying, that is, the controlled alloying of bimetallic precursors with different compositions (i.e., Pt3Ni, PtNi, and PtNi3) within the HGS mesoporous shell, was examined in detail. It was found that the presence of platinum during the reduction step, as well as the application of high annealing temperatures (at least 850°C for 3.5h in Ar), are necessary conditions to achieve the complete encapsulation and the full stability of the catalysts. The evolution of the activity, the electrochemical surface area, and the residual alloy composition of the Pt-Ni@HGS catalysts was thoroughly monitored (at the macro- and nanoscale level) under different degradation conditions. After the initial activation, the embedded Pt-Ni nanoparticles (3-4 nm in size) yield mass activities that are 2- to 3.5-fold higher than that of pure Pt@HGS (depending on the alloy composition). Most importantly, it is demonstrated that under the normal operation range of an ORR catalyst in PEM-FCs (potential excursions between 0.4 and 1.0 VRHE) both the nanoparticle-related degradation pathways (particle agglomeration) and dealloying phenomena are effectively suppressed, irrespectively of the alloy composition. Thus, the initial enhanced activity is completely maintained over an extended degradation protocol. In addition, owing to the peculiar configuration of the catalysts consisting of space-confined nanoparticles, it was possible to elucidate the impact of the dealloying process (as a function of alloy composition and severity of the degradation protocols) separately from other parallel phenomena, providing valuable insight into this elusive degradation mechanism. (Graph Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b02221
  • 2016 • 130 Surface analysis of the Heusler Ni49.7Mn29.1Ga21.2 Alloy: The composition, phase transition, and twinned microstructure of martensite
    Horáková, K. and Cháb, V. and Heczko, O. and Drchal, V. and Fekete, L. and Honolka, J. and Kopeček, J. and Kudrnovský, J. and Polyak, Y. and Sajdl, P. and Vondráček, M. and Lančok, J. and Feyer, V. and Wiemann, C. and Schn...
    Journal of Applied Physics 120 (2016)
    Surface analysis was used to study the dynamics of the martensitic transformation on macro- and mesoscopic scales. The chemical state, morphology, and magnetic and surface structure were monitored at particular stages of the phase transition. At room temperature, the martensitic phase of the Ni49.7Mn29.1Ga21.2 (100) single crystal exhibited macroscopic a/c twinning and a corresponding magnetic domain structure characterized by magnetization vector in and out of the surface plane. Induced by radiation heating, the transformation from martensite to austenite takes place separately at the surface and in the bulk. Its dynamics depend on the history of the sample treatment which affects the crystallographic orientation of twins and minor changes of the surface stoichiometry. The interfaces (twin planes) between twin variants in the martensitic phase were noticeable also in the austenitic phase, thanks to the shape memory effect of this material. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4962648
  • 2016 • 129 The effect of stress, temperature and loading direction on the creep behaviour of Ni-base single crystal superalloy miniature tensile specimens
    Wollgramm, P. and Bürger, D. and Parsa, A.B. and Neuking, K. and Eggeler, G.
    Materials at High Temperatures 33 346-360 (2016)
    In the present work, we use a miniature test procedure to investigate the tensile creep behaviour of the single crystal superalloy ERBO1. We test precisely oriented [0 0 1], [1 1 0] and [1 1 1] creep specimens and determine the stress and the temperature dependence of characteristic creep rates in limited stress and temperature regimes, where the stress and temperature dependence of characteristic creep rates can be well described by power law and Arrhenius type of relations, with stress exponents n and apparent activation energies Qapp. n-values increase with stress and decrease with temperature. Qapp-values, on the other hand, increase with increasing temperature and decrease with increasing stress. Creep curve shapes gradually evolve from the high temperature low stress to the low temperature high stress (LTHS) regime. This implies that there is a gradual change in elementary deformation and softening mechanisms, which is qualitatively confirmed using transmission electron microscopy. While at high temperatures different loading directions only have a moderate influence on creep, there is a very strong effect of loading direction at low temperatures. The [1 1 0] tests show the fastest deformation rates and the shortest rupture times. In the LTHS creep regime, we confirm the double minimum (DM) type of creep behaviour, which was previously reported but never explained. Further work is required to rationalise DM-creep. The implications of this type of creep behaviour on scatter and on extrapolation of creep data is discussed in the light of previous results published in the literature. © 2016 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/09603409.2016.1186414
  • 2016 • 128 The thermal stability of topologically close-packed phases in the single-crystal Ni-base superalloy ERBO/1
    Lopez-Galilea, I. and Koßmann, J. and Kostka, A. and Drautz, R. and Mujica Roncery, L. and Hammerschmidt, T. and Huth, S. and Theisen, W.
    Journal of Materials Science 51 2653-2664 (2016)
    In Ni-base superalloys, the addition of refractory elements such as Cr, Mo, Co, W, and Re is necessary to increase the creep resistance. Nevertheless, these elements induce the formation of different kinds of intermetallic phases, namely, the topologically close-packed (TCP) phases. This work focuses on intermetallic phases present in the second-generation single-crystal (SX) Ni-base superalloy ERBO/1. In the as-cast condition, the typical γ/γ′ structure is accompanied by undesirable intermetallic phases located in the interdendritic regions. The nature of these precipitates as well as their thermal stability between 800 and 1200 °C has been investigated by isothermal heat treatments. The investigation techniques include DSC, SEM, EDX, and TEM. The experimental information is complemented by (1) comparison with a structure map to link the local chemical composition with phase stability, as well as (2) thermodynamic calculations based on the CALPHAD method to determine the occurrence and composition of phases during solidification and in equilibrium conditions. The TCP phases Laves, µ and σ were identified in various temperature/time ranges. © 2015, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-015-9579-7
  • 2016 • 127 Timing photoemission-Final state matters
    Bovensiepen, U. and Ligges, M.
    Science 353 28-29 (2016)
    doi: 10.1126/science.aag1090
  • 2016 • 126 X-ray diffraction (XRD)-studies on the temperature dependent interface reactions on hafnium, zirconium, and nickel coated monocrystalline diamonds used in grinding segments for stone and concrete machining
    Tillmann, W. and Tolan, M. and Pinho Ferreira, M. and Paulus, M. and Becke, M. and Stangier, D.
    Materialwissenschaft und Werkstofftechnik 47 1193-1201 (2016)
    Diamond impregnated metal matrix composites are the state of the art solution for the machining of mineral materials. The type of interface reactions between the metal matrix and diamond surface has an essential influence on the tool performance and durability. To improve the diamond retention, the diamonds can be coated by physical vapour deposition with metallic materials, which enforce interface reactions. Hence, this paper focuses on the investigation of the interfacial area on metal-coated monocrystalline diamonds. Hafnium and zirconium, both known as carbide forming elements, are used as coating materials. The third coating, which is used to determine its catalytic influences when applied as a physical vapour deposition (PVD)-layer, is nickel. Additionally, the coated diamond samples were heat-treated to investigate the starting point of the formation of new phases. X-ray diffraction-analyses revealed the assumed carbide formation on hafnium and zirconium coated samples. The formation temperature was identified between 800 °C and 1000 °C for hafnium and zirconium coatings. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/mawe.201600713
  • 2015 • 125 A quantitative metallographic assessment of the evolution of porosity during processing and creep in single crystal Ni-base super alloys
    Buck, H. and Wollgramm, P. and Parsa, A.B. and Eggeler, G.
    Materialwissenschaft und Werkstofftechnik 46 577-590 (2015)
    The present work reviews previous research on the evolution of porosity. It presents new results from a detailed study on the evolution of porosity during casting, heat treatment and creep of a single crystal Ni-base superalloy subjected to uniaxial tensile creep at 1050 °C and 160 MPa in [001] and [110] directions. A quantitative metallographic study was performed on carefully polished metallographic cross sections, monitoring sampling fields of 4500 × 1000 μm2 using the back scatter contrast of an analytical scanning electron microscope; evolutions of pore sizes and pore form factors were analyzed and all important details which were previously revealed in a synchrotron study could be reproduced. In addition, it was observed that micro cracks form at larger cast pores. They interlink and thus initiate final rupture. The [110] tensile creep tests showed lower rupture strains than the [001] experiments. In agreement with earlier work, this can be rationalized on the basis of aligned porosity along primary dendrites. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201500379
  • 2015 • 124 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 • 123 Adsorption phenomena of cubane-type tetranuclear Ni(II) complexes with neutral, thioether-functionalized ligands on Au(111)
    Heß, V. and Matthes, F. and Bürgler, D.E. and Monakhov, K.Y. and Besson, C. and Kögerler, P. and Ghisolfi, A. and Braunstein, P. and Schneider, C.M.
    Surface Science 641 210-215 (2015)
    Abstract The controlled and intact deposition of molecules with specific properties onto surfaces is an emergent field impacting a wide range of applications including catalysis, molecular electronics, and quantum information processing. One strategy is to introduce grafting groups functionalized to anchor to a specific surface. While thiols and disulfides have proven to be quite effective in combination with gold surfaces, other S-containing groups have received much less attention. Here, we investigate the surface anchoring and organizing capabilities of novel charge-neutral heterocyclic thioether groups as ligands of polynuclear nickel(II) complexes. We report on the deposition of a cubane-type {Ni<inf>4</inf>} (= [Ni(μ<inf>3</inf>-Cl)Cl(HL·S)]<inf>4</inf>) single-molecule magnet from dichloromethane solution on a Au(111) surface, investigated by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction, both immediately after deposition and after subsequent post-annealing. The results provide strong evidence for partial decomposition of the coordination complex upon deposition on the Au(111) surface that, however, leaves the magnetic {Ni<inf>4</inf>Cl<inf>4n</inf>} (n = 1 or 2) core intact. Only post-annealing above 480 K induces further decomposition and fragmentation of the {Ni<inf>4</inf>Cl<inf>4n</inf>} core. The detailed insight into the chemisorption-induced decomposition pathway not only provides guidelines for the deposition of thioether-functionalized Ni(II) complexes on metallic surfaces but also reveals opportunities to use multidentate organic ligands decorated with thioether groups as transporters for highly unstable inorganic structures onto conducting surfaces, where they are stabilized retaining appealing electronic and magnetic properties. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2015.06.026
  • 2015 • 122 Advanced scale bridging microstructure analysis of single crystal Ni-base superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Somsen, C. and Kostka, A. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Dlouhy, A. and Müller, J. and Spiecker, E. and Demtroder, K. and Schreuer, J. and Neuking, K. and Eggeler, G.
    Advanced Engineering Materials 17 216-230 (2015)
    In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni-base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the castmicrostructure (dendrites (D) and interdendritic (ID) regions - large scale heterogeneity) and with the well-known γ/γ′ microstructure (small scale heterogeneity). Using electron probe microanalysis (EPMA), we can showthat elements such as Re, Co, andCr partition to the dendrites while ID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the γ′ cubes while g channels show higher concentrations of Co, Cr, Re, andW.We can combine large scale (EPMA) and small-scale analytical methods (APT) to obtain reasonable estimates for γ′ volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140mm x 100mm x 20mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the γ′ phase and the effect of cooling rates after high temperature exposure on the microstructure. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201400136
  • 2015 • 121 Atom probe informed simulations of dislocation-precipitate interactions reveal the importance of local interface curvature
    Prakash, A. and Guénolé, J. and Wang, J. and Müller, J. and Spiecker, E. and Mills, M.J. and Povstugar, I. and Choi, P. and Raabe, D. and Bitzek, E.
    Acta Materialia 92 33-45 (2015)
    The interaction of dislocations with precipitates is an essential strengthening mechanism in metals, as exemplified by the superior high-temperature strength of Ni-base superalloys. Here we use atomistic simulation samples generated from atom probe tomography data of a single crystal superalloy to study the interactions of matrix dislocations with a γ′ precipitate in molecular dynamics simulations. It is shown that the precipitate morphology, in particular its local curvature, and the local chemical composition significantly alter both, the misfit dislocation network which forms at the precipitate interface, and the core structure of the misfit dislocations. Simulated tensile tests reveal the atomic scale details of many experimentally observed dislocation-precipitate interaction mechanisms, which cannot be reproduced by idealized simulation setups with planar interfaces. We thus demonstrate the need to include interface curvature in the study of semicoherent precipitates and introduce as an enabling method atom probe tomography-informed atomistic simulations. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.03.050
  • 2015 • 120 Austenitic high interstitial steels vs. CoCrMo - Comparison of fatigue behavior
    Gueler, S. and Schymura, M. and Fischer, A.
    International Journal of Fatigue 75 145-152 (2015)
    The favourable combination of strength and ductility as well as excellent corrosion resistance of CrNiCN-austenitic steels is the reason for their use in biomedical engineering. However, Ni was attributed to cause allergies within a certain number of patients. Thus Ni-free alternatives like CoCrMo- and Ti-base alloys are often preferred. Other possible alternatives might be austenitic high nitrogen steels (AHNS) and austenitic high interstitial steels (AHIS). The aim of this study is to compare the fatigue properties of Ni-free austenitic CrMn(Mo)CN-steels with 0.85-1.07 wt% C + N, CoCrMo-alloy with up to 0.25 wt% C and conventional CrNiCN-steels with 0.01-0.42 wt% C + N. The comparison was conducted using total strain controlled axial LCF and HCF fatigue tests at room temperature on solution-annealed samples. After mechanical testing the samples were investigated by means of SEM including EBSD and TEM in order to analyse the distribution of plastic strains. In contrast to the CrNiCN-steels, which show cyclic hardening before the generation of the first crack, the CrMn(Mo)CN as well as the CoCrMo-alloys only showed cyclic softening. Still within the HCF regime the Co-base alloys showed the best fatigue behavior in terms of numbers of cycles to fracture Nf for a given total strain amplitude, while the Ni-free CrMn(Mo)CN-steels were either as good or slightly worse. The results indicate a distinct correlation between slip behavior, stacking fault energy and the amount of interstitials in solid solution. © 2015 Elsevier Ltd All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2015.02.015
  • 2015 • 119 Carbon Monoxide-Induced Stability and Atomic Segregation Phenomena in Shape-Selected Octahedral PtNi Nanoparticles
    Ahmadi, M. and Cui, C. and Mistry, H. and Strasser, P. and Roldan Cuenya, B.
    ACS Nano 9 10686-10694 (2015)
    The chemical and morphological stability of size- and shape-selected octahedral PtNi nanoparticles (NP) were investigated after different annealing treatments up to a maximum temperature of 700 °C in a vacuum and under 1 bar of CO. Atomic force microscopy was used to examine the mobility of the NPs and their stability against coarsening, and X-ray photoelectron spectroscopy to study the surface composition, chemical state of Pt and Ni in the NPs, and thermally and CO-induced atomic segregation trends. Exposing the samples to 1 bar of CO at room temperature before annealing in a vacuum was found to be effective at enhancing the stability of the NPs against coarsening. In contrast, significant coarsening was observed when the sample was annealed in 1 bar of CO, most likely as a result of Ni(CO)4 formation and their enhanced mobility on the support surface. Sample exposure to CO at room temperature prior to annealing led to the segregation of Pt to the NP surface. Nevertheless, oxidic PtOx and NiOx species still remained at the NP surface, and, irrespective of the initial sample pretreatment, Ni surface segregation was observed upon annealing in a vacuum at moderate temperature (T < 300 °C). Interestingly, a distinct atomic segregation trend was detected between 300 and 500 °C for the sample pre-exposed to CO; namely, Ni surface segregation was partially hindered. This might be attributed to the higher bonding energy of CO to Pt as compared to Ni. Annealing in the presence of 1 bar CO also resulted in the initial surface segregation of Ni (T < 400 °C) as long as PtOx and NiOx species were available on the surface as a result of the higher affinity of Ni for oxygen. Above 500 °C, and regardless of the sample pretreatment, the diffusion of Pt atoms to the NP surface and the formation of a Ni-Pt alloy are observed. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.5b01807
  • 2015 • 118 Characteristics of 5M modulated martensite in Ni-Mn-Ga magnetic shape memory alloys
    Çaklr, A. and Acet, M. and Righi, L. and Albertini, F. and Farle, M.
    AIP Advances 5 (2015)
    The applicability of the magnetic shape memory effect in Ni-Mn-based martensitic Heusler alloys is closely related to the nature of the crystallographically modulated martensite phase in these materials. We study the properties of modulated phases as a function of temperature and composition in three magnetic shape memory alloys Ni49.8Mn25.0Ga25.2, Ni49.8Mn27.1Ga23.1 and Ni49.5Mn28.6Ga21.9. The effect of substituting Ga for Mn leads to an anisotropic expansion of the lattice, where the b-parameter of the 5M modulated structure increases and the a and c-parameters decrease with increasing Ga concentration. The modulation vector is found to be both temperature and composition dependent. The size of the modulation vector corresponds to an incommensurate structure for Ni49.8Mn25.0Ga25.2 at all temperatures. For the other samples the modulation is incommensurate at low temperatures but reaches a commensurate value of q 0.400 close to room temperature. The results show that commensurateness of the 5M modulated structure is a special case of incommensurate 5M at a particular temperature. © 2015 Author(s).
    view abstractdoi: 10.1063/1.4932233
  • 2015 • 117 Damage evolution in pseudoelastic polycrystalline Co-Ni-Ga high-temperature shape memory alloys
    Vollmer, M. and Krooß, P. and Segel, C. and Weidner, A. and Paulsen, A. and Frenzel, J. and Schaper, M. and Eggeler, G. and Maier, H.J. and Niendorf, T.
    Journal of Alloys and Compounds 633 288-295 (2015)
    Due to its transformation behavior, Co-Ni-Ga represents a very promising high temperature shape memory alloy (HT SMA) for applications at elevated temperatures. Co-Ni-Ga single crystals show a fully reversible pseudoelastic shape change up to temperatures of 400 °C. Unfortunately, polycrystalline Co-Ni-Ga suffers from brittleness and early fracture mainly due to intergranular constraints. In the current study, different thermo-mechanical processing routes produced various microstructures which differ in grain size and texture. A bicrystalline bamboo-like grain structure results in the highest reversible transformation strains and excellent cyclic stability. Moreover, solution-annealed and hot-rolled conditions also showed cyclic stability. Using in situ high-resolution electron microscopy, the elementary processes, which govern the microstructural evolution during pseudoelastic cycling were investigated and the mechanisms that govern structural and functional degradation were identified. The observations documented in the present work suggest that the formation of the ductile γ-phase on and near grain boundaries as well as the activation of multiple martensite variants at grain boundaries are beneficial for improved cyclic performance of polycrystalline Co-Ni-Ga HT SMAs. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.01.282
  • 2015 • 116 Effect of ruthenium on the precipitation of topologically close packed phases in Ni-based superalloys of 3rd and 4th generation
    Matuszewski, K. and Rettig, R. and Matysiak, H. and Peng, Z. and Povstugar, I. and Choi, P. and Müller, J. and Raabe, D. and Spiecker, E. and Kurzydłowski, K.J. and Singer, R.F.
    Acta Materialia 95 274-283 (2015)
    The precipitation of topologically close packed (TCP) phases is detrimental for the high temperature strength of high refractory Ni-based superalloys. The beneficial influence of Ru with respect to this so called instability is nowadays well accepted. In the present paper the precipitation of topologically close packed (TCP) phases is studied quantitatively in two experimental alloys (one Ru-free and one with addition of Ru) to clarify the mechanism of the Ru effect. It is confirmed that the TCP phase precipitates undergo sequential phase transformation with the tetragonal σ-phase precipitating first. Ru retards the phase transformation and leads to decreased equilibrium volume fraction of TCP phases. The results clearly indicate that Ru decreases the driving force for TCP phase precipitation. Investigations of crystallography and chemistry of the TCP/matrix interface point to an additional effect by increase of misfit strain energy. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.05.033
  • 2015 • 115 Effect of substitution on elastic stability, electronic structure and magnetic property of Ni-Mn based Heusler alloys: An ab initio comparison
    Roy, T. and Gruner, M.E. and Entel, P. and Chakrabarti, A.
    Journal of Alloys and Compounds 632 822-829 (2015)
    First-principles density functional theory based calculations have been used to predict the bulk mechanical properties of magnetic shape memory Heusler alloy Ni2MnGa substituted by copper (Cu), platinum (Pt), palladium (Pd) and manganese (Mn) at the Ni site. The elastic constants of Ni2MnGa alloy with and without substitution are calculated. We analyze and compare in detail the bulk mechanical properties for these alloys, in particular, the ratio between the calculated bulk and shear modulii, as well as the Poisson's ratio and Young's modulii. This analysis further based on an empirical relation, indicates that Pt2MnGa may inherently be the least brittle material, among the above-mentioned alloys. Interesting difference has been observed between the shear modulii calculated from Voigt's and Reuss's method. This has been explained in terms of the values of the tetragonal shear constant C′ of the materials. Study of Heisenberg exchange coupling parameters and Curie temperature as well as density of states of the materials shows the effect of substitution at the Ni site on the magnetic and electronic properties, respectively. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.01.255
  • 2015 • 114 Effects of Ru on elemental partitioning and precipitation of topologically close-packed phases in Ni-based superalloys
    Peng, Z. and Povstugar, I. and Matuszewski, K. and Rettig, R. and Singer, R. and Kostka, A. and Choi, P.-P. and Raabe, D.
    Scripta Materialia 101 44-47 (2015)
    Two Ni-based superalloys (one Ru-free and one containing 1 at.% Ru) were comparatively studied by Atom Probe Tomography and complimentary techniques. Ru addition impedes precipitation of the σ phase at 950 °C. Ru partitions nearly equally to the γ and σ phase. Neither reverse elemental partitioning nor destabilization of the γ' phase is detected when adding Ru. We propose an increase in the γ/σ lattice misfit caused by Ru as a major reason for impeded nucleation of the σ phase. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2015.01.014
  • 2015 • 113 Elastocaloric cooling with ni-Ti based alloys - material characterization and process variation
    Schmidt, M. and Ullrich, J. and Wieczorek, A. and Frenzel, J. and Schötze, A. and Eggeler, G. and Seelecke, S.
    ASME 2015 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2015 1 (2015)
    Solid state refrigeration processes, such as magnetocaloric and electrocaloric refrigeration, have recently shown to be a promising alternative to conventional compression refrigeration. A new solid state elastocaloric refrigeration process using the latent heats within Shape Memory Alloys (SMA) could also hold potential in this field. This work investigates the elastocaloric effects in Ni-Ti-based superelastic Shape Memory Alloy (SMA) systems for use in an elastocaloric cooling processes. Ni-Ti alloys exhibits large latent heats and a small mechanical hysteresis, which may potentially lead to the development of an efficient environmentally friendly solid-state cooling system, without the need for ozone-depleting refrigerants. A systematic investigation of the SMA is conducted using a novel custombuilt scientific testing platform specifically designed to measure cooling process related phenomena. This testing system is capable of performing tensile tests at high rates as well as measuring and controlling the solid-state heat transfer between SMA and heat source/heat sink. Tests are conducted following a cooling process related training cycle where the material has achieved stabilized behavior. First, a characterization of the elastocaloric material properties is performed followed by an investigation of the material under cooling process conditions. A comprehensive monitoring of the mechanical and thermal parameters enables the observation of temperature changes during mechanical cycling of the SMA at high strain rates. These observations can be used to study the rate dependent efficiency of the elastocaloric material. The measurement of the temperature of both the heat source/heat sink and the SMA itself, as well as the required mechanical work during a running cooling process, reveals the influence of the operating conditions on the elastocaloric effect of the material. Furthermore investigations of the process efficiency at different thermal boundary conditions (temperature of heat source/heat sink), indicates that the process is dependent on the boundary conditions which have to be controlled in order to optimize the efficiency. © Copyright 2015 by ASME.
    view abstractdoi: 10.1115/SMASIS2015-8944
  • 2015 • 112 Evolution of microstructure and mechanical properties of coated Co-base superalloys during heat treatment and thermal exposure
    Webler, R. and Ziener, M. and Neumeier, S. and Terberger, P.J. and Vaßen, R. and Göken, M.
    Materials Science and Engineering A 628 374-381 (2015)
    New γ'-strengthened Co-base superalloys show an interesting potential for high temperature applications. However, protective coatings are needed as for Ni-base superalloys to ensure sufficient oxidation and corrosion resistance. Therefore the properties of a commercial coating on a multinary new γ'-strengthened Co-base superalloy have been studied. Especially the influence of the coating process on the substrate also after long term annealing is discussed. It was found that the highly deformed areas at the coating-substrate interface indicated by a high local misorientation and caused by the sandblasting process led to a recrystallization of the interdiffusion zone during the age hardening heat treatment. A chemical gradient of γ and γ' promoting elements was found in the interdiffusion zone causing a change in hardness as measured by nanoindentation. Depending on the composition two separate recrystallized regions formed in the interdiffusion zone, one with single phase γ-(Co,Ni) and the other with a cellular two phase microstructure of discontinuously grown γ and γ'. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.01.060
  • 2015 • 111 Experimental investigations on single-lip deep hole drilling of superalloy Inconel 718 with small diameters
    Biermann, D. and Kirschner, M.
    Journal of Manufacturing Processes 20 332-339 (2015)
    Recent trends in downsizing, as well as miniaturization of components, increase the importance of deep hole drilling with small diameters in various industrial applications. For the manufacturing of deep holes, many processes are used. In addition to mechanical cutting, processes based on thermal material removal mechanisms like electrical discharge machining (EDM), laser drilling and electron-beam drilling are established. Whereas non-mechanical processes can be used for the machining of extremely hard and high-strength materials, substantial disadvantages are the restrictions in the realizable hole diameter and depth dimensions, the long production times, the requirement for special-purpose machinery, as well as the limited bore hole quality. In the aerospace industry, where nickel-based superalloys are widespread, high requirements lie on the surface integrity due to high stress loads during operation. Hence, the major drawback of these non-mechanical processes is the alteration of the subsurface zone, which could affect the part safety. In contrast, the mechanical machining of nickel-based superalloys is extremely difficult due to material properties such as high strength, high tendency to work hardening and low thermal conductivity which results in increased mechanical tool loads and heavy abrasive and adhesive tool wear. An additional challenge is constituted by the chip removal and the unfavorable ratio of cutting edge rounding and undeformed chip thickness caused by the limitation in the feed rates, if deep hole drilling with small diameters and high-length-to-diameter ratios of difficult-to-cut materials like nickel-based superalloys is required. Up to now, the use of standard tool geometries in small diameter single-lip deep hole drilling of Inconel 718 has not been productive and leads to limited process stability and drilling lengths as well as insufficient bore hole quality. Concerning this matter, this paper presents a process adaption using a tailored cutting edge design to accomplish the predominant challenges. Substantial analysis on gun drilling in Inconel 718 regarding mechanical tool loads, chip formation, the tool wear, as well as bore hole quality will be presented. © 2015 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmapro.2015.06.001
  • 2015 • 110 First-Principles Calculations of Magnetic Properties of Cr-Doped Ni45Co5Mn37In13 Heusler Alloys
    Sokolovskiy, V.V. and Buchelnikov, V.D. and Gruner, M.E. and Entel, P.
    IEEE Transactions on Magnetics 51 (2015)
    The magnetic and electronic properties of Co- and Cr-doped Ni50Mn37In13 Heusler alloys with a substitution of 5 at.% Co for Ni and 5 at.% Cr for Ni, Mn, or In are investigated in the framework of the density functional theory method. The chemical disorder in the off-stoichiometric Ni-Co-Mn-Cr-In systems was treated in the coherent potential approximation. Three different ferrimagnetic and one ferromagnetic (FM) spin states for austenite and martensite were considered in ab initio calculations. It is found that for both structures, the intersublattice interactions (MnY(Z)-Co, MnY(Z)-Ni, MnY(Z)-MnZ(Y), MnY(Z)-Cr, and Cr-Co) provide the largest contribution to the exchange due to the shorter distance compared with the intrasublattice interactions (MnY(Z)-MnY(Z), Co-Co, Ni-Ni, and Cr-Cr). Besides, the MnY-MnZ and MnY(Z)-Cr exchanges in the first shell become five times larger in martensite compared with austenite. The largest anti-FM interaction is observed between MnY(Z)-Cr atoms in martensite. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2015.2439391
  • 2015 • 109 First-principles investigation of hydrogen trapping and diffusion at grain boundaries in nickel
    Di Stefano, D. and Mrovec, M. and Elsässer, C.
    Acta Materialia 98 306-312 (2015)
    Abstract In this work, the interaction of hydrogen with high-angle GBs in nickel has been investigated by means of density functional theory simulations. Two distinct types of GBs have been considered: the Σ3(111)[1¯10] with a close-packed interface structure and the Σ5(210)[001] with a less dense interface structure consisting of open structural units. Our calculations reveal that these two GBs have a markedly different interaction behavior with atomic hydrogen. The close-packed Σ3 GB neither traps H nor enhances its diffusion, but instead acts as a two-dimensional diffusion barrier. In contrast, the Σ5 GB provides numerous trapping sites for H within the open structural units as well as easy migration pathways for H diffusion along the GB plane that can enhance the H diffusivity by about two orders of magnitude compared to bulk Ni. The obtained results are analysed in detail and compared with available experimental and other theoretical data. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.031
  • 2015 • 108 Gamma-channel stabilization mechanism in Ni-base superalloys
    Goerler, J.V. and Brinckmann, S. and Shchyglo, O. and Steinbach, I.
    Philosophical Magazine Letters 95 519-525 (2015)
    A mechanism is presented which opposes coalescence of γ′-precipitates in Ni-base superalloys. The mechanism is based on the non-linear behaviour of the elastic energy in γ-channels, caused by the misfit strain between matrix and precipitate, as a function of the channel width. Variation of the channel width causes a disjoining pressure dependent on the density of misfit dislocations. © 2015 Taylor & Francis.
    view abstractdoi: 10.1080/09500839.2015.1109716
  • 2015 • 107 High-Throughput Investigation of the Oxidation and Phase Constitution of Thin-Film Ni-Al-Cr Materials Libraries
    König, D. and Eberling, C. and Kieschnick, M. and Virtanen, S. and Ludwig, Al.
    Advanced Engineering Materials 17 1365-1373 (2015)
    Thin-film materials libraries of the intermetallic model system Ni-Al-Cr were fabricated and their oxidation behavior was studied by compositional, optical, electrical, and structural high-throughput characterization methods. The study reveals the compositional regions of the binary and ternary compositions which withstand longest to annealing in air (up to 700 C), and are, therefore, resistant to oxidation and delamination under these conditions. A complete ternary thin-film phase diagram for the Ni-Al-Cr system in its state after 9 h annealing in air at 500 C was determined. Optical high-throughput characterization is shown to be valid for rapid identification of oxidizing phases. Generally, the initially metallic phases show different oxidation behavior in air. We find that the ternary compositions are more resistant to oxidation than the binary phases. Compositions around Ni<inf>25</inf>Al<inf>12.5</inf>Cr<inf>62.5</inf> were found to show very good oxidation resistance. These results were supported by additional information from corresponding electrical and optical property investigations. The presented high-throughput approach is generic for the efficient study of multinary thin-film materials in harsh environments. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201500023
  • 2015 • 106 Hybridization of an Aβ-specific antibody fragment with aminopyrazole-based β-sheet ligands displays striking enhancement of target affinity
    Hellmert, M. and Müller-Schiffmann, A. and Peters, M.S. and Korth, C. and Schrader, T.
    Organic and Biomolecular Chemistry 13 2974-2979 (2015)
    Determining Aβ levels in body fluids remains a powerful tool in the diagnostics of Alzheimer's disease. This report delineates a new supramolecular strategy which increases the affinity of antibodies towards Aβ to make diagnostic procedures more sensitive. A monoclonal antibody IC16 was generated to an N-terminal epitope of Aβ and the variable regions of the heavy and light chains were cloned as a recombinant protein (scFv). A 6 × histidine tag was fused to the C-terminus of IC16-scFv allowing hybridization with a small organic β-sheet binder via Ni-NTA complexation. On the other hand, a multivalent nitrilotriacetic acid (NTA)-equipped trimeric aminopyrazole (AP) derivative was synthesized based on a cyclam platform; and experimental evidence was obtained for efficient Ni2+-mediated complex formation with the histidine-tagged antibody species. In a proof of principle experiment the hybrid molecule showed a strong increase in affinity towards Aβ. Thus, the specific binding power of recombinant antibody fragments to their β-sheet rich targets can be conveniently enhanced by non-covalent hybridization with small organic β-sheet binders. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4ob02411g
  • 2015 • 105 Influence of film thickness and composition on the martensitic transformation in epitaxial Ni-Mn-Sn thin films
    Teichert, N. and Auge, A. and Yüzüak, E. and Dincer, I. and Elerman, Y. and Krumme, B. and Wende, H. and Yildirim, O. and Potzger, K. and Hütten, A.
    Acta Materialia 86 279-285 (2015)
    Two series of epitaxial Ni-Mn-Sn thin films of different thickness are investigated for the thickness and composition dependence of the martensitic transformation. Thin films ranging in thickness from 20 to 200 nm (series A) and 10 to 100 nm (series B) were prepared by magnetron co-sputtering and deposited on heated MgO(0 0 1) substrates. The structural characterization was done by temperature-dependent X-ray diffraction measurements. Magnetization and resistivity measurements were performed to investigate the transformation characteristics. We find a strong influence of the film thickness on the relative amount of material undergoing the martensitic transformation, the temperature range of the transformation, and the transformation temperatures. The main contribution originates from the rigid substrate which delays the transformation of the Ni-Mn-Sn near the interface and even leads to a layer of residual austenite at low temperatures. Another issue are size effects which presumably broaden the martensitic transformation and decrease the transformation temperatures. By variation of the thin film composition we find changes of the substrate influence due to a different mismatch between the lattice of MgO and austenite. A better phase compatibility between martensite and austenite, denoted by λ2, not only results in a smaller hysteresis but is also beneficial for the transformation of material close to the substrate. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.12.019
  • 2015 • 104 Influence of microstructure on macroscopic elastic properties and thermal expansion of nickel-base superalloys ERBO/1 and LEK94
    Demtröder, K. and Eggeler, G. and Schreuer, J.
    Materialwissenschaft und Werkstofftechnik 46 563-576 (2015)
    In the present work the thermal expansion and the elastic properties of second generation nickel-base superalloy single crystals ERBO/1 (CMSX-4 variation) and LEK94 have been studied between about 100 K and 1273 K using dilatometry and resonant ultrasound spectroscopy, respectively. Inhomogeneity related to the large scale microstructure of the samples can act as a potential source of scatter for the propagation of ultrasonic waves. This can be overcome by choosing samples of sufficient size so that they appear as homogeneous media at the scale of the elastic wave length. Our final results are in good agreement with data reported in literature for similar alloy systems. In particular, the elastic material properties are only weekly affected by moderate variations in chemical composition and microstructure. Taking into account literature data for other superalloys like CMSX-4, we derive general polynomial functions which describe the temperature dependence of the elastic moduli E<inf>〈100〉</inf>, E<inf>〈110〉</inf> and E<inf>〈111〉</inf> in nickel-base superalloys to within about ±3%. It was also observed that the alloys ERBO/1 and LEK94 show weak but significant anomalies in both thermal expansion and temperature coefficients of elastic constants above about 900 K. These anomalies are probably related to the gradual dissolution of the γ′-precipitates at higher temperatures. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201500406
  • 2015 • 103 Intermartensitic transitions and phase stability in Ni50Mn50-xSnx Heusler alloys
    Çakir, A. and Righi, L. and Albertini, F. and Acet, M. and Farle, M.
    Acta Materialia 99 140-149 (2015)
    Ni-Mn based Heusler alloys are of considerable interest due to their multifunctional properties such as magnetic shape memory, magnetocaloric effect and spintronics. The reason for these multifunctional properties is the presence of a first order martensitic transition and its strong coupling to the magnetization. In this work, one of the outstanding class of martensitic Heuslers, Ni-Mn-Sn, is investigated in relation to magneto-structural phase transitions and the stability of the various crystallographic structures under varying temperature. Temperature-dependent X-ray diffraction, resistance and magnetization measurements on Ni<inf>50</inf>Mn<inf>50-x</inf>Sn<inf>x</inf> alloys are performed in a broad valence electron concentration range 7.91 ≤ (e/a) ≤ 8.34 (5.1 ≤ x ≤ 20.3at.%). The results reveal that in addition to the austenite-martensite transition, further intermartensitic transitions take place with decreasing temperature. Depending on the composition, we observe that the parent martensite phase tends to transform to L1<inf>0</inf> martensite as the ground state phase when the temperature is lowered. A phase diagram of Ni<inf>50</inf>Mn<inf>50-x</inf>Sn<inf>x</inf> is constructed to include intermartensitic phase transition boundaries. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.07.072
  • 2015 • 102 Iron-doped nickel oxide nanocrystals as highly efficient electrocatalysts for alkaline water splitting
    Fominykh, K. and Chernev, P. and Zaharieva, I. and Sicklinger, J. and Stefanic, G. and Döblinger, M. and Müller, A. and Pokharel, A. and Böcklein, S. and Scheu, C. and Bein, T. and Fattakhova-Rohlfing, D.
    ACS Nano 9 5180-5188 (2015)
    Efficient electrochemical water splitting to hydrogen and oxygen is considered a promising technology to overcome our dependency on fossil fuels. Searching for novel catalytic materials for electrochemical oxygen generation is essential for improving the total efficiency of water splitting processes. We report the synthesis, structural characterization, and electrochemical performance in the oxygen evolution reaction of Fe-doped NiO nanocrystals. The facile solvothermal synthesis in tert-butanol leads to the formation of ultrasmall crystalline and highly dispersible Fe<inf>x</inf>Ni<inf>1-x</inf>O nanoparticles with dopant concentrations of up to 20%. The increase in Fe content is accompanied by a decrease in particle size, resulting in nonagglomerated nanocrystals of 1.5-3.8 nm in size. The Fe content and composition of the nanoparticles are determined by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy measurements, while Mössbauer and extended X-ray absorption fine structure analyses reveal a substitutional incorporation of Fe(III) into the NiO rock salt structure. The excellent dispersibility of the nanoparticles in ethanol allows for the preparation of homogeneous ca. 8 nm thin films with a smooth surface on various substrates. The turnover frequencies (TOF) of these films could be precisely calculated using a quartz crystal microbalance. Fe<inf>0.1</inf>Ni<inf>0.9</inf>O was found to have the highest electrocatalytic water oxidation activity in basic media with a TOF of 1.9 s-1 at the overpotential of 300 mV. The current density of 10 mA cm-2 is reached at an overpotential of 297 mV with a Tafel slope of 37 mV dec-1. The extremely high catalytic activity, facile preparation, and low cost of the single crystalline Fe<inf>x</inf>Ni<inf>1-x</inf>O nanoparticles make them very promising catalysts for the oxygen evolution reaction. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.5b00520
  • 2015 • 101 Isothermal aging of a γ'-strengthened Co-Al-W alloy coated with vacuum plasma-sprayed MCrAlY bond coats
    Terberger, P.J. and Sebold, D. and Webler, R. and Ziener, M. and Neumeier, S. and Klein, L. and Virtanen, S. and Göken, M. and Vaßen, R.
    Surface and Coatings Technology 276 360-367 (2015)
    Cobalt-based superalloys with a γ/γ' microstructure were discovered in 2006 and are currently being investigated as an alternative to nickel-based superalloys for high-temperature, high-load applications in gas turbine blades. They promise a better castability combined with a similar creep strength. Superalloy turbine blades are commonly coated with oxidation resistant bond coats. For this reason their compatibility needs to be studied. Co-9Al-9W specimens with a γ/γ' microstructure were coated with either a nickel-based or cobalt-based MCrAlY bond coat using vacuum plasma spraying. After aging at 900. °C in air for up to 500. h no decomposition of the γ' phase was found in the bulk superalloy. The interdiffusion zone shows several different W-rich topologically close-packed phases arising from the dissolution of the γ' phase in this region. The W-rich phases are identified to be μ phase for both bond coats and R phase for the nickel-based bond coat only. Their total volume is higher for the nickel-based bond coat. Therefore the cobalt-based bond coat is better suited for the Co-based superalloy substrate. Room temperature hardness and Young's modulus were measured using nanoindentation in the initial state and after heat treatment. A significantly higher Young's modulus was found for the cobalt-based bond coat. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.surfcoat.2015.06.048
  • 2015 • 100 Large magnetocaloric effects in magnetic intermetallics: First-principles and Monte Carlo studies
    Entel, P. and Gruner, M.E. and Ogura, M. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Grünebohm, A. and Arróyave, R. and Uebayashi, K. and Singh, N. and Talapatra, A. and Duong, T. and Acet, M. and Çakir, A.
    MATEC Web of Conferences 33 (2015)
    We have performed ab initio electronic structure calculations and Monte Carlo simulations of frustrated ferroic materials where complex magnetic configurations and chemical disorder lead to rich phase diagrams. With lowering of temperature, we find a ferromagnetic phase which transforms to an antiferromagnetic phase at the magnetostructural (martensitic) phase transition and to a cluster spin glass at still lower temperatures. The Heusler alloys Ni-(Co)-Mn-(Cr)-(Ga, Al, In, Sn, Sb) are of particular interest because of their large inverse magnetocaloric effect associated with the magnetostructural transition and the influence of Co/Cr doping. Besides spin glass features, strain glass behavior has been observed in Ni-Co-Mn-In. The numerical simulations allow a complete characterization of the frustrated ferroic materials including the Fe-Rh-Pd alloys. © Owned by the authors, published by EDP Sciences, 2015.
    view abstractdoi: 10.1051/matecconf/20153302001
  • 2015 • 99 Ledges and grooves at γ/γ′ interfaces of single crystal superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Kostka, A. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 90 105-117 (2015)
    In the present work we study the formation of grooves and ledges (typical size: <100 nm) at γ/γ′ interfaces of single crystal Ni-base superalloys. We highlight previous work which documents the presence of such interface irregularities and shows that their number and size increases during high temperature exposure and creep. We use diffraction contrast stereo transmission electron microscopy (TEM) to provide new evidence for the presence of ledges and grooves near dislocations at γ/γ′ interfaces after heat treatment and creep. We present a 2D model of the interfacial region which shows how dislocation stress fields alter local chemical potentials and drive diffusional fluxes which result in the formation of a groove. The results of the numerical study yield realistic groove sizes in relevant time scales. The results obtained in the present study suggest that the formation of grooves and ledges represents an elementary process which needs to be considered when rationalizing the kinetics of rafting, the directional coarsening of the γ′ phase. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.02.005
  • 2015 • 98 Magnetic States of the Ni1.75Co0.25Mn1.25Cr0.25In0.5 Heusler Alloy
    Buchelnikov, V.D. and Sokolovskiy, V.V. and Gruner, M.E. and Entel, P.
    IEEE Transactions on Magnetics 51 (2015)
    The equilibrium magnetic and structural reference states of Co- and Cr-doped Ni2Mn1.5In0.5 Heusler alloy are investigated by means of the first-principles method using a supercell approach. Three different ferrimagnetic and one ferromagnetic (FM) spin configurations, as well as two supercells with different distributions of excess Mn and In atoms, were considered. It is found that for supercell #1, the FM spin state in austenite is stable, where the martensite with different spin configurations is unstable, while in the case of supercell #2, a ferrimagnetic configuration for both austenite and martensite is favorable. The different trends for martensitic transformation were studied by c/a calculations for the tetragonal and orthorombic distortions of supercells along the z -axis and the y -axis, showing martensitic variants for supercell #2 at a ratio c/a &gt; 1 and c/a < 1. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2015.2438953
  • 2015 • 97 Martensite aging - Avenue to new high temperature shape memory alloys
    Niendorf, T. and Krooß, P. and Somsen, C. and Eggeler, G. and Chumlyakov, Y.I. and Maier, H.J.
    Acta Materialia 89 298-304 (2015)
    High-temperature shape memory alloys are attractive for efficient solid state actuation. A key criterion for shape memory alloys is the martensite start temperature. The current study introduces a concept for increasing this temperature of alloys initially not suited for high-temperature actuation. Aging of stress-induced martensite, referred to as SIM-aging in the current work, is able to increase the martensite start temperature by about 130 °C as demonstrated in the present study for a Co-Ni-Ga shape memory alloy. The increase of transformation temperatures can be explained based on the concept of symmetry-conforming short-range order. Following SIM-aging the Co-Ni-Ga alloy shows cyclic actuation stability at elevated temperatures. While martensite aging has always been viewed as detrimental in the past, it can actually be exploited to design new classes of high-temperature shape memory alloys with excellent properties. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.01.042
  • 2015 • 96 Microscopic analysis of the composition driven spin-reorientation transition in NixPd1-x/Cu(001)
    Gottlob, D.M. and Doğanay, H. and Nickel, F. and Cramm, S. and Krug, I.P. and Nemšák, S. and Schneider, C.M.
    Ultramicroscopy 159 503-507 (2015)
    The spin-reorientation transition (SRT) in epitaxial NixPd1-x/Cu(001) is studied by photoemission microscopy utilizing the X-ray magnetic circular dichroism effect at the Ni L2,3 edge. In a composition/thickness wedged geometry, a composition driven SRT could be observed between 37ML and 60ML, and 0 and 38% of Pd. Microspectroscopy in combination with azimuthal sample rotation confirms a magnetization preference changing from the [001] to an in-plane easy axis. At this increased thickness, the domain patterns arrange comparable to SRTs in ultrathin films. The images document domains equivalent to a canted state SRT, at which an additional effect of in-plane anisotropies could be identified. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.05.022
  • 2015 • 95 Microsegregation and precipitates of an as-cast Co-based superalloy—microstructural characterization and phase stability modelling
    Koßmann, J. and Zenk, C.H. and Lopez-Galilea, I. and Neumeier, S. and Kostka, A. and Huth, S. and Theisen, W. and Göken, M. and Drautz, R. and Hammerschmidt, T.
    Journal of Materials Science 50 6329-6338 (2015)
    The demand for increased efficiency of industrial gas turbines and aero engines drives the search for the next generation of materials. Promising candidates for such new materials are Co-based superalloys. We characterize the microsegregation and solidification of a multi-component Co-based superalloy and compare it to a ternary Co–Al–W compound and to two exemplary Ni-based superalloys by combining the experimental characterization of the as-cast microstructures with complementary modelling of phase stability. On the experimental side, we characterize the microstructure and precipitates by electron microscopy and energy-dispersive X-ray spectroscopy and determine the element distributions and microsegregation coefficients by electron probe microanalysis (EPMA). On the modelling side, we carry out solidification simulations and a structure map analysis in order to relate the local chemical composition with phase stability. We find that the microsegregation coefficients for the individual elements are very similar in the investigated Co-based and Ni-based superalloys. By interpreting the local chemical composition from EPMA with the structure map, we effectively unite the set of element distribution maps to compound maps with very good contrast of the dendritic microstructure. The resulting compound maps of the microstructure in terms of average band filling and atomic-size difference explain the formation of topologically close-packed phases in the interdendritic regions. We identify B2, C14, and D0<inf>24</inf> precipitates with chemical compositions that are in line with the structure map. © 2015, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-015-9177-8
  • 2015 • 94 Modeling thermally induced martensitic transformations in nickel titanium shape memory alloys
    Jaeger, S. and Eggeler, G. and Kastner, O.
    Continuum Mechanics and Thermodynamics 27 461-481 (2015)
    During stress-free thermal analysis with differential scanning calorimetry (DSC), nickel titanium (NiTi) shape memory alloys show a thermal hysteresis which is affected by cooling/heating rates. Moreover, the Ni content of near equiatomic alloys governs the phase transition temperatures. This contribution aims at establishing a constitutive equation which can account for these effects, building on earlier work by Müller, Achenbach and Seelecke (MAS). To be specific, we discuss our new method with a focus on NiTi alloys. As in the original MAS model, our approach is rooted in a non-convex free energy representation and rate equations are utilized to incorporate history dependence during non-equilibrium processes. The relaxation times of these rate equations are determined by characteristic transformation probabilities which in turn are governed by the free energy landscape of our system. We show how the model can be parameterized to rationalize experimental DSC data observed for NiTi samples of variable composition and measured at variable cooling/heating rates. The good agreement between model predictions and experimental results suggests that thermal hystereses are not only related to interfacial strain energy effects but also affected by the transient character of the transformation process incorporating specific thermal relaxation times. Our analysis shows that we observe strong hysteretic effects when the cooling/heating rates exceed these characteristic relaxation rates. © 2014, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00161-014-0375-4
  • 2015 • 93 Nanoindentation studies of the mechanical properties of the μ phase in a creep deformed Re containing nickel-based superalloy
    Rehman, H.U. and Durst, K. and Neumeier, S. and Parsa, A.B. and Kostka, A. and Eggeler, G. and Göken, M.
    Materials Science and Engineering A 634 202-208 (2015)
    Addition of Re in nickel-based superalloys results in an increase of the creep life. However, Re is also known to segregate to the dendrite core and to promote the formation of topologically closed packed (TCP) phases. In the present work, the local segregation of Re was studied in the heat treated and creep deformed state of a nickel-based superalloy. Tensile creep deformation at 1050°C resulted in the formation of TCP phases in the dendritic regions. Characterization using TEM confirmed the presence of μ phase that grows on {111} planes. Measurements with a nanoindenting AFM show that the μ phase is harder and shows less work hardening than both the γ and the γ' phases. Furthermore, in the creep deformed state the hardness of the matrix phase is very similar in the dendrite core and in interdendritic areas, although Re is still enriched in the dendrite core. It is shown that Re is consumed in the dendrite core by the TCP phases. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.03.045
  • 2015 • 92 Nanoscale origins of the damage tolerance of the high-entropy alloy CrMnFeCoNi
    Zhang, Z. and Mao, M.M. and Wang, J. and Gludovatz, B. and Zhang, Z. and Mao, S.X. and George, E.P. and Yu, Q. and Ritchie, R.O.
    Nature Communications 6 (2015)
    Damage tolerance can be an elusive characteristic of structural materials requiring both high strength and ductility, properties that are often mutually exclusive. High-entropy alloys are of interest in this regard. Specifically, the single-phase CrMnFeCoNi alloy displays tensile strength levels of ∼1 GPa, excellent ductility (∼60-70%) and exceptional fracture toughness (KJIc &gt;200 MPa √m). Here through the use of in situ straining in an aberration-corrected transmission electron microscope, we report on the salient atomistic to micro-scale mechanisms underlying the origin of these properties. We identify a synergy of multiple deformation mechanisms, rarely achieved in metallic alloys, which generates high strength, work hardening and ductility, including the easy motion of Shockley partials, their interactions to form stacking-fault parallelepipeds, and arrest at planar slip bands of undissociated dislocations. We further show that crack propagation is impeded by twinned, nanoscale bridges that form between the near-tip crack faces and delay fracture by shielding the crack tip.
    view abstractdoi: 10.1038/ncomms10143
  • 2015 • 91 Nitrogen uptake of nickel free austenitic stainless steel powder during heat treatment-an XPS study
    Weddeling, A. and Lefor, K. and Hryha, E. and Huth, S. and Nyborg, L. and Weber, S. and Theisen, W.
    Surface and Interface Analysis 47 413-422 (2015)
    In austenitic stainless steel nitrogen stabilizes the austenitic phase improves the mechanical properties and increases the corrosion resistance. Nitrogen alloying enables to produce austenitic steels without the element nickel which is high priced and classified as allergy inducing. A novel production route is nitrogen alloying of CrMn-prealloyed steel powder via the gas phase. This is beneficial as the nitrogen content can be adjusted above the amount that is reached during conventional casting. A problem which has to be overcome is the oxide layer present on the powder surface which impedes both the sintering process and the uptake of nitrogen. This study focuses on whether heat treatment under pure nitrogen is an appropriate procedure to enable sintering and nitrogen uptake by reduction of surface oxides. X-ray photoelectron spectroscopy (XPS) in combination with scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) are used to investigate the surface of powdered FeMn19Cr17C0.4N heat treated under nitrogen atmosphere. The analyses showed reduction of iron oxides already at 500 °C leading to oxide-free metallic surface zones. Mn and Cr oxides are reduced at higher temperatures. Distinct nitrogen uptake was registered, and successful subsequent sintering was reached. Copyright © 2014 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.5730
  • 2015 • 90 Non-aqueous semi-solid flow battery based on Na-ion chemistry. P2-type NaxNi0.22Co0.11Mn0.66O2-NaTi2(PO4)3
    Ventosa, E. and Buchholz, D. and Klink, S. and Flox, C. and Chagas, L.G. and Vaalma, C. and Schuhmann, W. and Passerini, S. and Morante, J.R.
    Chemical Communications 51 7298-7301 (2015)
    We report the first proof of concept for a non-aqueous semi-solid flow battery (SSFB) based on Na-ion chemistry using P2-type Na<inf>x</inf>Ni<inf>0.22</inf>Co<inf>0.11</inf>Mn<inf>0.66</inf>O<inf>2</inf> and NaTi<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> as positive and negative electrodes, respectively. This concept opens the door for developing a new low-cost type of non-aqueous semi-solid flow batteries based on the rich chemistry of Na-ion intercalating compounds. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4cc09597a
  • 2015 • 89 On the effect of alloy composition on martensite start temperatures and latent heats in Ni-Ti-based shape memory alloys
    Frenzel, J. and Wieczorek, A. and Opahle, I. and Maaß, B. and Drautz, R. and Eggeler, G.
    Acta Materialia 90 213-231 (2015)
    In the present work we explain the concentration dependence of the martensite start temperature (MS) in Ni-Ti-based shape memory alloys (SMAs). We briefly review the present level of understanding and show that there is a need for further work. We then investigate the strong dependence of MS on alloy composition in binary Ni-Ti, ternary Ni-Ti-X (X = Cr, Cu, Hf, Pd, V, Zr) and quaternary Ni-Ti-Cu-Y (Y = Co, Pd) SMAs. For binary Ni-Ti, we combine differential scanning calorimetry experiments with insight gained through the application of the density functional theory (DFT) to show that heats of transformation ΔH decrease as Ni concentrations increase from 50.0 to 51.2 at.%. This causes a shift in the Gibbs free energy curves of austenite GA(T) and martensite GM(T), which in turn results in a lower MS temperature. Our DFT results suggest that the strong decrease of ΔH is caused by a stabilization of the B2 phase by structural relaxations around Ni antisite atoms, together with a gradual destabilization of B19′. The martensite start temperatures and the latent heats of transformation for binary, ternary and quaternary Ni-Ti-based SMAs are closely related. We observe smaller latent heats when the geometrical differences between the crystal structures of austenite and martensite decrease. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.02.029
  • 2015 • 88 On the identification of superdislocations in the γ′-phase of single-crystal Ni-base superalloys - An application of the LACBED method to complex microstructures
    Müller, J. and Eggeler, G. and Spiecker, E.
    Acta Materialia 87 34-44 (2015)
    Ni-base superalloys are used for turbine blades, which operate in the creep range at temperatures above 1000 °C. One of the objectives of modern materials science is to analyze the combination of elementary deformation and microstructural coarsening processes and to identify physically based micromechanical models which allow one to predict the mechanical behavior on the macroscale. High-temperature creep of single-crystal Ni-base superalloys is governed by dislocation plasticity in the well-known γ/γ′-microstructure. For a comprehensive description of plasticity, it is important to understand the nucleation, glide and climb of superdislocations in the γ′-phase. The rate-controlling dislocation processes have to be identified and therefore a reliable Burgers vector analysis of superdislocations is essential. Superdislocations exhibit complex dislocation cores, typically comprising superpartial dislocations and planar defects. Therefore, conventional Burgers vector analysis based on the invisibility criterion often fails, due to the presence of pronounced residual contrast. In the present work, large-angle convergent-beam electron diffraction (LACBED) is employed for Burgers vector determination of two characteristic superdislocations, of the standard <1 1 0> and the more complex <1 0 0> type. LACBED results are compared with results obtained using the conventional invisibility analysis. While both techniques work for the standard superdislocation, the conventional analysis fails to analyze the <1 0 0> superdislocation, which shows pronounced residual contrast even under conditions of g · b = 0 and g · b × u = 0. In contrast, the LACBED technique allows for an unambiguous determination of the Burgers vector, including its magnitude and absolute sense. In the present study, the use of LACBED to identify dislocations in the complex microstructure of an Ni-base superalloy is outlined and the better performance of LACBED as compared to the conventional gb-analysis is discussed. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.12.029
  • 2015 • 87 On the role of Re in the stress and temperature dependence of creep of Ni-base single crystal superalloys
    Wollgramm, P. and Buck, H. and Neuking, K. and Parsa, A.B. and Schuwalow, S. and Rogal, J. and Drautz, R. and Eggeler, G.
    Materials Science and Engineering A 628 382-395 (2015)
    In the present study we investigate the creep behavior of a Ni-base single crystal superalloy. We evaluate the stress and temperature dependence of the minimum creep rate, which shows a power law type of stress dependence (characterized by a stress exponent n) and an exponential type of temperature dependence (characterized by an apparent activation energy Qapp). Under conditions of high temperature (1323K) and low stress (160MPa) creep, n and Qapp are determined as 5.3 and 529kJ/mol, respectively. For lower temperatures (1123K) and higher stresses (600MPa) the stress exponent n is higher (8.5) while the apparent activation energy of creep is lower (382kJ/mol). We show that there is a general trend: stress exponents n increase with increasing stress and decreasing temperature, while higher apparent activation energies are observed for lower stresses and higher temperatures. We use density functional theory (DFT) to calculate the activation energy of diffusion for Re in a binary Ni-Re alloy with low Re-concentrations. The resulting energy is almost a factor 2 smaller than the apparent activation energy of creep. We explain why it is not straightforward to rationalize the temperature dependence of creep merely on the basis of the diffusion of one alloying element. We show that the evolution of the microstructure also must be considered. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.01.010
  • 2015 • 86 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 • 85 Polycrystalline elastic moduli of a high-entropy alloy at cryogenic temperatures
    Haglund, A. and Koehler, M. and Catoor, D. and George, E.P. and Keppens, V.
    Intermetallics 58 62-64 (2015)
    CrMnCoFeNi is a FCC high-entropy alloy (HEA) that exhibits strong temperature dependence of strength at low homologous temperatures in sharp contrast to pure FCC metals like Ni that show weak temperature dependence. To understand this behavior, elastic constants were determined as a function of temperature. From 300 K down to 55 K, the shear modulus (G) of the HEA changes by only 8%, increasing from 80 to 86 GPa. This temperature dependence is weaker than that of FCC Ni, whose G increases by 12% (81-91 GPa). Therefore, the uncharacteristic temperature-dependence of the strength of the HEA is not due to the temperature dependence of its shear modulus. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.11.005
  • 2015 • 84 Primary combination of phase-field and discrete dislocation dynamics methods for investigating athermal plastic deformation in various realistic Ni-base single crystal superalloy microstructures
    Gao, S. and Kumar Rajendran, M. and Fivel, M. and Ma, A. and Shchyglo, O. and Hartmaier, A. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 23 (2015)
    Three-dimensional discrete dislocation dynamics (DDD) simulations in combination with the phase-field method are performed to investigate the influence of different realistic Ni-base single crystal superalloy microstructures with the same volume fraction of γ;precipitates on plastic deformation at room temperature. The phase-field method is used to generate realistic microstructures as the boundary conditions for DDD simulations in which a constant high uniaxial tensile load is applied along different crystallographic directions. In addition, the lattice mismatch between the γand γ;phases is taken into account as a source of internal stresses. Due to the high antiphase boundary energy and the rare formation of superdislocations, precipitate cutting is not observed in the present simulations. Therefore, the plastic deformation is mainly caused by dislocation motion in γ; matrix channels. From a comparison of the macroscopic mechanical response and the dislocation evolution for different microstructures in each loading direction, we found that, for a given γ;phase volume fraction, the optimal microstructure should possess narrow and homogeneous γ; matrix channels. © 2015 IOP Publishing Ltd Printed in the UK.
    view abstractdoi: 10.1088/0965-0393/23/7/075003
  • 2015 • 83 Solution Heat Treatment of the Single Crystal Nickel-Base Superalloy CMSX-4 Fabricated by Selective Electron Beam Melting
    Ramsperger, M. and Mújica Roncery, L. and Lopez-Galilea, I. and Singer, R.F. and Theisen, W. and Körner, C.
    Advanced Engineering Materials 17 1486-1493 (2015)
    Selective electron beam melting (SEBM), which belongs to the additive manufacturing processes, is applied to produce samples from the single crystalline nickel-base superalloy CMSX-4. The influence of the high solidification rates on the microstructure and element distribution is investigated by OM, SEM, DSC, and EMPA. Solution heat treatments at different temperatures and holding times are applied to demonstrate the difference between conventionally cast and SEBM material. The results demonstrate that SEBM is able to produce superalloys with a degree of homogeneity which cannot be realized in conventional processes. Selective electron beam melting (SEBM) manufacturing of CMSX-4 leads to a very fine solidification structure which is two orders of magnitude smaller than in conventional castings (e.g., Bridgman). Microprobe mappings of Re distribution in CMSX-4 can be used to show the differences in homogeneity (a and b). Due to the homogeneity, a solution heat treatment (HT) of 4 min (1320 °C) is already sufficient for homogenization of SEBM CMSX- 4 (c). © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201500037
  • 2015 • 82 Structure-Correlated Exchange Anisotropy in Oxidized Co80Ni20 Nanorods
    Liébana-Viñas, S. and Wiedwald, U. and Elsukova, A. and Perl, J. and Zingsem, B. and Semisalova, A.S. and Salgueiriño, V. and Spasova, M. and Farle, M.
    Chemistry of Materials 27 4015-4022 (2015)
    Rare earth-free permanent magnets for applications in electro-magnetic devices promise better sustainability and availability and lower prices. Exploiting the combination of shape, magnetocrystalline and exchange anisotropy in 3D-metals can pave the way to practical application of nanomagnets. In this context, we study the structural and magnetic properties of Co<inf>80</inf>Ni<inf>20</inf> nanorods with a mean diameter of 6.5 nm and a mean length of 52.5 nm, prepared by polyol reduction of mixed cobalt and nickel acetates. Structural analysis shows crystalline rods with the crystallographic c-axis of the hexagonal close-packed (hcp) phase parallel to the long axis of the Co<inf>80</inf>Ni<inf>20</inf> alloy rods, which appear covered by a thin oxidized face-centered cubic (fcc) shell. The temperature dependence of the surprisingly high coercive field and the exchange bias effect caused by the antiferromagnetic surface oxide indicate a strong magnetic hardening due to alignment of anisotropy axes. We identify a temperature dependent local maximum of the coercive field at T = 250 K, which originates from noncollinear spin orientations in the ferromagnetic core and the antiferromagnetic shell. This might be useful for building four way magnetic switches as a function of temperature. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.5b00976
  • 2015 • 81 Synthesis of γ-valerolactone by hydrogenation of levulinic acid over supported nickel catalysts
    Hengst, K. and Schubert, M. and Carvalho, H.W.P. and Lu, C. and Kleist, W. and Grunwaldt, J.-D.
    Applied Catalysis A: General 502 18-26 (2015)
    Ni/Al2O3 catalysts were tested for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) as an important bio-based platform molecule for chemical products based on renewable feedstocks. The catalysts were prepared by wet impregnation, incipient wetness impregnation, precipitation, and flame spray pyrolysis; both the influence of different solvents (monovalent alcohols and water) as well as solvent free reaction conditions were screened in batch autoclaves. Whereas alcohols led to a number of side reactions that could only be suppressed by high hydrogen pressures (&gt;20 bar), water as solvent resulted in a GVL selectivity of 100%. The GVL yields reached 57%. Further improvement was achieved without any solvent, whereby the GVL yield increased to 92% at 100% LA conversion. Reuse of the Ni catalysts resulted in a significant drop in activity. The catalysts were thoroughly characterized by temperature programmed reduction (TPR), X-ray diffraction (XRD), linear combination analysis of X-ray absorption near edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS). The results indicated that incorporated Ni2+, as present in flame-derived catalysts, was less active for GVL synthesis compared to supported Ni particles, as present in the wet impregnated catalyst. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2015.05.007
  • 2015 • 80 The effect of cast microstructure and crystallography on rafting, dislocation plasticity and creep anisotropy of single crystal Ni-base superalloys
    Nörtershäuser, P. and Frenzel, J. and Ludwig, Al. and Neuking, K. and Eggeler, G.
    Materials Science and Engineering A 626 305-312 (2015)
    In the present work we investigate three mechanical and microstructural aspects of high temperature and low stress creep of the single crystal superalloy LEK 94. First, we compare the tensile creep behavior of specimens loaded in precise [001] and [110] directions and show that tensile creep specimens with precise [110] directions show significantly lower minimum creep rates. However, small deviations from precise [110] orientations result in a significant increase of creep rate. Second, we use a novel SEM technique to measure dislocation densities. We show that after short periods of creep, dislocation densities in dendritic regions are always higher than in interdendritic regions. This finding is probably associated with wider γ-channels, higher concentrations of W and Re and higher misfit stresses in the γ-channels of dendrites. Finally, we show that internal stresses associated with solidification can drive complex rafting processes during high temperature exposure, which differ between dendrite cores and interdendritic regions. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.12.030
  • 2015 • 79 The metamagnetic behavior and giant inverse magnetocaloric effect in Ni-Co-Mn-(Ga, In, Sn) Heusler alloys
    Entel, P. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Ogura, M. and Gruner, M.E. and Grünebohm, A. and Comtesse, D. and Akai, H.
    Journal of Magnetism and Magnetic Materials 385 193-197 (2015)
    The magnetic and magnetocaloric properties of Ni-Co-Mn-(Ga, In, Sn) Heusler intermetallics are discussed on the basis of ab initio and Monte Carlo calculations. The main emphasis is on the different reference spin states and magnetic exchange coupling constants of high-temperature austenite and low-temperature martensite which are very important for the calculation of magnetocaloric effect. The origin of metamagnetic behavior is considered in the framework of orbital resolved magnetic exchange parameters of austenite and martensite. The decomposition of exchange constants on orbital contributions has shown that a strong ferromagnetic interaction of magnetic moments in austenite is caused by the more itinerant d-electrons with t2g states while a strong antiferromagnetic interaction in martensite is associated with the more localized eg states. In addition, the appearance of a paramagnetic gap between magnetically weak martensite and ferromagnetically ordered austenite can be realized because of strong competition of magnetic exchange interactions. As a result, large magnetization drop and giant inverse magnetocaloric effect can be achieved across the magnetostructural phase transition. ©2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmmm.2015.03.003
  • 2015 • 78 X-ray photoelectron spectroscopy investigations of the surface reaction layer and its effects on the transformation properties of nanoscale Ti51Ni38Cu11 shape memory thin films
    König, D. and Naujoks, D. and De Los Arcos, T. and Grosse-Kreul, S. and Ludwig, Al.
    Advanced Engineering Materials 17 669-673 (2015)
    The depth-dependent chemical constitution of Ti<inf>51</inf>Ni<inf>38</inf>Cu<inf>11</inf> thin films of different total film thickness from 400 to 50-nm was characterized using X-ray photoelectron spectroscopy (XPS). It was analyzed how reaction layers, which form on the surface of the film significantly change the chemical composition of the transforming phase, which leads in turn to altered phase transformation properties. For thinner films, the deviation from the nominal chemical composition increases. For a film thickness of 50-nm, a Ti loss of ≈9-at% is observed. The Ni content is increased by ≈5-at%, whereas the Cu content stays relatively constant for films of different thickness. The results are summarized in a layer model, which supports designing nanoscale shape memory thin films. Ti<inf>51</inf>Ni<inf>38</inf>Cu<inf>11</inf> thin films of different film thickness are investigated regarding the influence of the reaction layers on the chemical composition of the transforming phase and the corresponding functional properties. A model is proposed describing the different reaction layers on the surface of the thin film and at the substrate/thin film interface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201400317
  • 2014 • 77 Ab Initio Predicted Impact of Pt on Phase Stabilities in Ni-Mn-Ga Heusler Alloys
    Dutta, B. and Hickel, T. and Entel, P. and Neugebauer, J.
    Journal of Phase Equilibria and Diffusion 35 695-700 (2014)
    The paper discusses the stabilization of the martensite in Ni2MnGa at finite temperatures that is caused by the substitution of Ni by Pt. For this purpose a recently developed ab initio based formalism employing density functional theory is applied. The free energies of the relevant austenite and martensite phases of Ni1.75Pt0.25MnGa are determined incorporating quasiharmonic phonons and fixed-spin magnons. In addition the dependence of the transition temperatures on the Pt concentration is investigated. Though our results are in qualitative agreement with estimates based on ground-state energies, they clearly demonstrate that a proper treatment of finite temperature contributions is important to predict the martensitic transition quantitatively. © 2014, ASM International.
    view abstractdoi: 10.1007/s11669-014-0342-6
  • 2014 • 76 Carbon monoxide-assisted size confinement of bimetallic alloy nanoparticles
    Cui, C. and Gan, L. and Neumann, M. and Heggen, M. and Roldan Cuenya, B. and Strasser, P.
    Journal of the American Chemical Society 136 4813-4816 (2014)
    Colloid-based chemical synthesis methods of bimetallic alloy nanoparticles (NPs) provide good monodispersity, yet generally show a strong variation of the resulting mean particle size with alloy composition. This severely compromises accurate correlation between composition of alloy particles and their size-dependent properties. To address this issue, a general CO adsorption-assisted capping ligand-free solvothermal synthesis method is reported which provides homogeneous bimetallic NPs with almost perfectly constant particle size over an unusually wide compositional range. Using Pt-Ni alloy NPs as an example, we show that variation of the reaction temperature between 160 and 240 °C allows for precise control of the resulting alloy particle bulk composition between 15 and 70 atomic % Ni, coupled with a constant mean particle size of ∼4 nm. The size-confining and Ni content-controlling role of CO during the nucleation and growth processes are investigated and discussed. Data suggest that size-dependent CO surface chemisorption and reversible Ni-carbonyl formation are key factors for the achievement of a constant particle size and temperature-controlled Ni content. To demonstrate the usefulness of the independent control of size and composition, size-deconvoluted relations between composition and electrocatalytic properties are established. Refining earlier reports, we uncover intrinsic monotonic relations between catalytic activity and initial Ni content, as expected from theoretical considerations. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja4124658
  • 2014 • 75 Ceramic materials for H2 transport membranes applicable for gas separation under coal-gasification-related conditions
    van Holt, D. and Forster, E. and Ivanova, M.E. and Meulenberg, W.A. and Müller, M. and Baumann, S. and Vaßen, R.
    Journal of the European Ceramic Society 34 2381-2389 (2014)
    This work focuses on the synthesis, characterization and testing of mixed protonic-electronic conducting membrane materials for H2 separation from gas mixtures capable of operating in a membrane reactor at temperatures higher than 600°C. La5.5WO12-δ and selected substituted barium zirconates with stoichiometries BaCe0.5Zr0.4Y0.1O3-δ and BaCe0.2Zr0.7Yb0.08Ni0.02O3-δ were therefore characterized and tested under coal-gasification-related conditions at 600-900°C. Sintered samples of the synthesized substituted barium zirconates were characterized by measuring the total conductivity and the thermal expansion coefficients. Also particle size distributions, BET surface-areas and elemental analysis of the starting powders, including commercial La5.5WO12-δ were specified. The compounds were exposed to syngas with steam, as well as to an atmosphere mainly consisting of CO2. The microstructure and phase composition of the membrane materials were studied by SEM, EDX and XRD before and after exposure. BaCe0.2Zr0.7Yb0.08Ni0.02O3-δ shows a very promising chemical stability from 600°C to 900°C and La5.5WO12-δ at 900°C. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jeurceramsoc.2014.03.001
  • 2014 • 74 Composition-structure-function diagrams of Ti-Ni-Au thin film shape memory alloys
    Buenconsejo, P.J.S. and Ludwig, Al.
    ACS Combinatorial Science 16 678-685 (2014)
    Ti-Ni-Au thin film materials libraries were prepared from multilayer precursors by combinatorial sputtering. The materials libraries were annealed at 500, 600, and 700 °C for 1 h and then characterized by high-throughput methods to investigate the relations between composition, structure and functional properties. The identified relations were visualized in functional phase diagrams. The goal is to identify composition regions that are suitable as high temperature shape memory alloys. Phase transforming compositions were identified by electrical resistance measured during thermal cycles in the range of -20 and 250 °C. Three phase transformation paths were confirmed: (1) B2-R, (2) B2-R-B19', and (3) B2-B19. For the materials library annealed at 500 °C only the B2-R transformation was observed. For the materials libraries annealed at 600 and 700 °C, all transformation paths were observed. High transformation temperatures (Ms ≈100 °C) were only obtained by annealing at 600 or 700 °C, and with compositions of Ti ≈ 50 at. % and Au &gt; 20 at. %. This is the composition range that undergoes B2-B19 transformation. The phase transformation behaviors were explained according to the compositional and annealing temperature dependence of phase/structure formation, as revealed by X-ray diffraction analysis of the materials libraries. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/co5000745
  • 2014 • 73 Designing Heusler nanoprecipitates by elastic misfit stabilization in Fe-Mn maraging steels
    Millán, J. and Sandlöbes, S. and Al-Zubi, A. and Hickel, T. and Choi, P. and Neugebauer, J. and Ponge, D. and Raabe, D.
    Acta Materialia 76 94-105 (2014)
    B2 NiMn and Ni2MnAl Heusler nanoprecipitates are designed via elastic misfit stabilization in Fe-Mn maraging steels by combining transmission electron microscopy (TEM) correlated atom probe tomography (APT) with ab initio simulations. Guided by these predictions, the Al content of the alloys is systematically varied, and the influence of the Al concentration on structure stability, size and distribution of precipitates formed during ageing at 450 °C is studied using scanning electron microscopy-electron backscatter diffraction, TEM and APT. Specifically, the Ni2MnAl Heusler nanoprecipitates exhibit the finest sizes and highest dispersion and hence lead to significant strengthening. The formation of the different types of precipitates and their structure, size, dispersion and effect on the mechanical properties of the alloys are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.05.016
  • 2014 • 72 Dislocation density distribution around an indent in single-crystalline nickel: Comparing nonlocal crystal plasticity finite-element predictions with experiments
    Reuber, C. and Eisenlohr, P. and Roters, F. and Raabe, D.
    Acta Materialia 71 333-348 (2014)
    We present a physics-based constitutive model of dislocation glide in metals that explicitly accounts for the redistribution of dislocations due to their motion. The model parameterizes the complex microstructure by dislocation densities of edge and screw character, which either occur with monopolar properties, i.e. a single dislocation with positive or negative line sense, or with dipolar properties, i.e. two dislocations of opposite line sense combined. The advantage of the model lies in the description of the dislocation density evolution, which comprises the usual rate equations for dislocation multiplication and annihilation, and formation and dissociation of dislocation dipoles. Additionally, the spatial redistribution of dislocations by slip is explicitly accounted for. This is achieved by introducing an advection term for the dislocation density that turns the evolution equations for the dislocation density from ordinary into partial differential equations. The associated spatial gradients of the dislocation slip render the model nonlocal. The model is applied to wedge indentation in single-crystalline nickel. The simulation results are compared to published experiments (Kysar et al., 2010) in terms of the spatial distribution of lattice rotations and geometrically necessary dislocations. In agreement with experiment, the predicted dislocation fluxes lead to accumulation of geometrically necessary dislocations around a vertical geometrical border with a high orientation gradient below the indenter that is decisive for the overall plastic response. A local model variant without dislocation transport is not able to predict the influence of this geometrical transition zone correctly and is shown to behave markedly softer. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.03.012
  • 2014 • 71 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 • 70 Elemental composition of particulate matter and the association with lung function
    Eeftens, M. and Hoek, G. and Gruzieva, O. and Mölter, A. and Agius, R. and Beelen, R. and Brunekreef, B. and Custovic, A. and Cyrys, J. and Fuertes, E. and Heinrich, J. and Hoffmann, B. and De Hoogh, K. and Jedynska, A. and Keuke...
    Epidemiology 25 648-657 (2014)
    BACKGROUND: Negative effects of long-term exposure to particulate matter (PM) on lung function have been shown repeatedly. Spatial differences in the composition and toxicity of PM may explain differences in observed effect sizes between studies. METHODS: We conducted a multicenter study in 5 European birth cohorts - BAMSE (Sweden), GINIplus and LISAplus (Germany), MAAS (United Kingdom), and PIAMA (The Netherlands) - for which lung function measurements were available for study subjects at the age of 6 or 8 years. Individual annual average residential exposure to copper, iron, potassium, nickel, sulfur, silicon, vanadium, and zinc within PM smaller than 2.5 μm (PM2.5) and smaller than 10 μm (PM10) was estimated using land-use regression models. Associations between air pollution and lung function were analyzed by linear regression within cohorts, adjusting for potential confounders, and then combined by random effects meta-analysis. RESULTS: We observed small reductions in forced expiratory volume in the first second, forced vital capacity, and peak expiratory flow related to exposure to most elemental pollutants, with the most substantial negative associations found for nickel and sulfur. PM10 nickel and PM10 sulfur were associated with decreases in forced expiratory volume in the first second of 1.6% (95% confidence interval = 0.4% to 2.7%) and 2.3% (-0.1% to 4.6%) per increase in exposure of 2 and 200 ng/m, respectively. Associations remained after adjusting for PM mass. However, associations with these elements were not evident in all cohorts, and heterogeneity of associations with exposure to various components was larger than for exposure to PM mass. CONCLUSIONS: Although we detected small adverse effects on lung function associated with annual average levels of some of the evaluated elements (particularly nickel and sulfur), lower lung function was more consistently associated with increased PM mass. Copyright © 2014 by Lippincott Williams & Wilkins.
    view abstractdoi: 10.1097/EDE.0000000000000136
  • 2014 • 69 Experimental investigation and numerical simulation of the mechanical and thermal behavior of a superelastic shape memory alloy beam during bending
    Ullrich, J. and Schmidt, M. and Schütze, A. and Wieczorek, A. and Frenzel, J. and Eggeler, G. and Seelecke, S.
    ASME 2014 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2014 2 (2014)
    Superelastic Shape Memory Alloys (SMA) are typically used in applications where the martensitic phase transformation is exploited for its reversible, large deformation such as medical applications (e.g. stents). In this work, we focus on the mechanical and thermal behavior of a Nickel-Titanium SMA strip in bending mode. One possible application of this mode is to provide a restoring force when used in joints of SMA wire actuator systems making the need for an antagonistic SMA actuator redundant. In these applications mentioned above, typically only the mechanical properties are of interest while the temperature is considered constant, even though the martensitic phase transformation in SMA is a thermomechanically coupled process. As a part of the DFG (German Research Association) Priority Programme SPP1599 "Ferroic Cooling" which aims at advancing the development of solid state cooling devices, we have an equally large interest for the thermal evolution of Nickel-Titanium SMA during deformation and its induced phase transformation. In this paper we investigate the thermal and the mechanical response of a SMA beam during bending experiments in which the deformation is induced by holding one end of a SMA strip fixed while the other end is subject to a prescribed deflection. Sensors and high speed thermal cameras are used to capture reaction forces, deformations and temperature changes. We compare these experimental results with numerical simulation results obtained from Finite Element simulations where a thermo-mechanically coupled SMA model is implemented into a finite deformation framework. © 2014 by ASME.
    view abstractdoi: 10.1115/SMASIS20147619
  • 2014 • 68 Fabrication of a Ni-Cu thin film material library using pulsed electrodeposition
    Srinivas, P. and Hamann, S. and Wambach, M. and Ludwig, Al. and Dey, S.R.
    Journal of the Electrochemical Society 161 D504-D509 (2014)
    A thin film composition gradient library of the Ni-Cu alloy system is generated through an electrodeposition technique using a complexing citrate electrolyte bath in a modified Hull cell. Energy dispersive X-ray spectroscopy, scanning electron microscopy and automated X-ray diffraction are performed to assess composition, surface morphology, and crystallographic structure of the deposited film as a function of the lateral position on the materials library. The results confirmed deposition of single phase polycrystalline f.c.c. Ni-Cu alloy system with varied lateral composition and lattice parameter, afcc as well. © 2014 The Electrochemical Society. All rights reserved.
    view abstractdoi: 10.1149/2.0451410jes
  • 2014 • 67 Ferromagnetic exchange coupling between Fe phthalocyanine and Ni(111) surface mediated by the extended states of graphene
    Candini, A. and Bellini, V. and Klar, D. and Corradini, V. and Biagi, R. and De Renzi, V. and Kummer, K. and Brookes, N.B. and Del Pennino, U. and Wende, H. and Affronte, M.
    Journal of Physical Chemistry C 118 17670-17676 (2014)
    The interface spin coupling mechanism is studied in a hybrid structure made of Fe phthalocyanine molecules sublimed in ultrahigh vacuum on graphene grown on the magnetic substrate Ni(111). By using synchrotron X-ray magnetic circular dichroism, the field-dependent magnetization of the isolated FePc molecules and of the Ni substrate has been measured at low temperature (8 K). Along with density functional theory calculations, the role of the graphene interlayer in transmitting the magnetic coupling is addressed. Both experiments and theory show a ferromagnetic coupling between the molecules and the substrate which is weakened by the insertion of graphene. DFT calculations indicate that the key role is played by the π orbitals of graphene, which hybridize with the underlying magnetic Ni, giving rise to a sizable spin polarized continuum at the molecular interface. The resulting overlap with the Fe orbitals favors a direct coupling of ferromagnetic nature, as evidenced by our spin density distribution plots. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp5041663
  • 2014 • 66 First-principles studies on graphene-supported transition metal clusters
    Sahoo, S. and Gruner, M.E. and Khanna, S.N. and Entel, P.
    Journal of Chemical Physics 141 (2014)
    Theoretical studies on the structure, stability, and magnetic properties of icosahedral TM13 (TM = Fe, Co, Ni) clusters, deposited on pristine (defect free) and defective graphene sheet as well as graphene flakes, have been carried out within a gradient corrected density functional framework. The defects considered in our study include a carbon vacancy for the graphene sheet and a five-membered and a seven-membered ring structures for graphene flakes (finite graphene chunks). It is observed that the presence of defect in the substrate has a profound influence on the electronic structure and magnetic properties of graphene-transition metal complexes, thereby increasing the binding strength of the TM cluster on to the graphene substrate. Among TM 13 clusters, Co13 is absorbed relatively more strongly on pristine and defective graphene as compared to Fe13 and Ni 13 clusters. The adsorbed clusters show reduced magnetic moment compared to the free clusters. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4893328
  • 2014 • 65 First-principles-based phase diagrams and thermodynamic properties of TCP phases in Re-X systems (X = Ta, V, W)
    Palumbo, M. and Fries, S.G. and Hammerschmidt, T. and Abe, T. and Crivello, J.-C. and Breidi, A.A.H. and Joubert, J.-M. and Drautz, R.
    Computational Materials Science 81 433-445 (2014)
    The structural stability of topologically close-packed phases in binary transition metal alloys is investigated with a combination of first-principles calculations based on density-functional theory and the Bragg-Williams-Gorsky approximation for the description of the configurational entropy. For a variety of different (i) exchange-correlation functionals, (ii) pseudopotentials, and (iii) relaxation schemes, for the relevant phases in Re-X (X = Ta, V, W) binary systems, we compare the energy of formation at T = 0 K, as well as the phase diagrams and site occupancies at finite temperatures. We confirm previous findings that the configurational entropy plays a stabilising role for complex phases in these systems at elevated temperatures. Small differences in the calculated energy of formation for different exchange-correlation functionals, pseudopotentials and relaxation schemes are expected, but give rise to qualitatively different phase diagrams. We employ these differences in order to estimate the order of magnitude of the standard deviation necessary in the qualitatively-reliable calculation of phase diagrams and site occupancies. In an attempt to determine the accuracy that is required to assure a qualitatively correct prediction of phase diagrams, we modify our first-principles results numerically by random variations with the determined standard deviation as maximum amplitude. Taking the order of site occupancies and the set of stable phases as simple criteria for a qualitatively correct prediction, we find that the accuracy required for the energy of formation of the individual configurations in these systems is approximately 5 meV/atom (≈0.5 kJ/mol at). © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.commatsci.2013.08.051
  • 2014 • 64 High-temperature creep and oxidation behavior of Mo-Si-B alloys with high Ti contents
    Schliephake, D. and Azim, M. and Von Klinski-Wetzel, K. and Gorr, B. and Christ, H.-J. and Bei, H. and George, E.P. and Heilmaier, M.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 45 1102-1111 (2014)
    Multiphase alloys in the Mo-Si-B system are potential high-temperature structural materials due to their good oxidation and creep resistance. Since they suffer from relatively high densities, the current study focuses on the influence of density-reducing Ti additions on creep and oxidation behavior at temperatures above 1273 K (1000 C). Two alloys with compositions of Mo-12.5Si-8.5B-27.5Ti and Mo-9Si-8B-29Ti (in at. pct) were synthesized by arc melting and then homogenized by annealing in vacuum for 150 hours at 1873 K (1600 C). Both alloys show similar creep behavior at stresses of 100 to 300 MPa and temperatures of 1473 K and 1573 K (1200 C and 1300 C), although they possess different intermetallic volume fractions. They exhibit superior creep resistance and lower density than a state-of-the-art Ni-base superalloy (single-crystalline CMSX-4) as well as other Mo-Si-B alloys. Solid solution strengthening due to Ti was confirmed by Vickers hardness measurements and is believed to be the reason for the significant increase in creep resistance compared to Mo-Si-B alloys without Ti, but with comparable microstructural length scales. The addition of Ti degrades oxidation resistance relative to a Mo-9Si-8B reference alloy due to the formation of a relatively porous duplex layer with titania matrix enabling easy inward diffusion of oxygen. © 2013 The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-013-1944-z
  • 2014 • 63 Influence of the PM-Processing Route and Nitrogen Content on the Properties of Ni-Free Austenitic Stainless Steel
    Lefor, K. and Walter, M. and Weddeling, A. and Hryha, E. and Huth, S. and Weber, S. and Nyborg, L. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 1154-1167 (2014)
    Ni-free austenitic steels alloyed with Cr and Mn are an alternative to conventional Ni-containing steels. Nitrogen alloying of these steel grades is beneficial for several reasons such as increased strength and corrosion resistance. Low solubility in liquid and δ-ferrite restricts the maximal N-content that can be achieved via conventional metallurgy. Higher contents can be alloyed by powder-metallurgical (PM) production via gas–solid interaction. The performance of sintered parts is determined by appropriate sintering parameters. Three major PM-processing routes, hot isostatic pressing, supersolidus liquid phase sintering (SLPS), and solid-state sintering, were performed to study the influence of PM-processing route and N-content on densification, fracture, and mechanical properties. Sintering routes are designed with the assistance of thermodynamic calculations, differential thermal analysis, and residual gas analysis. Fracture surfaces were studied by X-ray photoelectron spectroscopy, secondary electron microscopy, and energy dispersive X-ray spectroscopy. Tensile tests and X-ray diffraction were performed to study mechanical properties and austenite stability. This study demonstrates that SLPS process reaches high densification of the high-Mn-containing powder material while the desired N-contents were successfully alloyed via gas–solid interaction. Produced specimens show tensile strengths >1000 MPa combined with strain to fracture of 60 pct and thus overcome the other tested production routes as well as conventional stainless austenitic or martensitic grades. © 2014, The Author(s).
    view abstractdoi: 10.1007/s11661-014-2701-7
  • 2014 • 62 Investigation of coking during dry reforming of methane by means of thermogravimetry
    Tarasov, A. and Düdder, H. and Mette, K. and Kühl, S. and Kähler, K. and Schlögl, R. and Muhler, M. and Behrens, M.
    Chemie-Ingenieur-Technik 86 1916-1924 (2014)
    Coking dynamics of Ni-based and Ni-free catalysts were studied in a magnetic suspension thermobalance under methane dry reforming conditions. Ni-rich catalysts undergo strong coking featured with a surface saturation point where the coking rate is drastically reduced. Catalyst resistance towards coking may be enhanced by using noble-metal-based Ni-free precursors or decreasing the Ni content in the catalytic system. The post reaction performed temperature-programmed oxidation experiment of the coked catalyst is diffusion-limited due to large amounts of formed carbon. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cite.201400092
  • 2014 • 61 Investigation of the thin-film phase diagram of the Cr-Ni-Re system by high-throughput experimentation
    Janghorban, A. and Pfetzing-Micklich, J. and Frenzel, J. and Ludwig, Al.
    Advanced Engineering Materials 16 588-593 (2014)
    The Cr-Ni-Re system was investigated over the whole composition range using combinatorial fabrication methods combined with high-throughput characterization techniques in order to establish its thin film phase diagram. After annealing at 940 and 1100°C, the phase equilibrium was reached in the Ni-rich part of the ternary in agreement with the published bulk phase diagram. Annealing the materials library at 940°C is not sufficient to achieve the equilibrium state in the Re-rich part of the system, however by annealing the materials library at 1100°C the formation of expected phases (three solid-solutions and a topologically close packed compound) could be observed. As a result of this study, a thin film phase diagram of the complete Cr-Ni-Re at 1100°C was established, which is well comparable to the bulk phase diagram. This shows that the combinatorial thin film phase diagram approach is feasible and especially promising for materials systems with expensive and/or high melting point constituents. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201300430
  • 2014 • 60 Large scale 3-D phase-field simulation of coarsening in Ni-base superalloys
    Rajendran, M.K. and Shchyglo, O. and Steinbach, I.
    MATEC Web of Conferences 14 (2014)
    In this study we present a large scale numerical simulation of γ-γ′ microstructure evolution in Ni-base superalloy using the multi-phase field method in three dimensions. We numerically simulated precipitation hardening heat treatment cycles. Large scale three dimensional simulations are necessary in order to get sufficient statistics for predicting the morphological evolution, average γ′ precipitate size, precipitates size distribution over time and ripening exponent for a given temperature and composition. A detailed analysis of obtained result is presented emphasising the effect of elastic interaction on the coarsening kinetics in Ni-base superalloy. The study is performed using the phase-field modelling library "OpenPhase" which is based on a multi-phase field multi-component model. © 2014 Owned by the authors, published by EDP Sciences.
    view abstractdoi: 10.1051/matecconf/20141411001
  • 2014 • 59 Metal-free catalysts for oxygen reduction in alkaline electrolytes: Influence of the presence of Co, Fe, Mn and Ni inclusions
    Masa, J. and Zhao, A. and Wei, X. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 128 271-278 (2014)
    Metal-free nitrogen modified carbon catalysts (NC) are very closely related to MNC catalysts which contain a transition metal(s) (M), usually Fe or Co as an essential constituent. We investigated the influence of metal inclusions on the activity of nitrogen-doped carbon black in the electrocatalysis of the oxygen reduction reaction (ORR). A reference metal-free NC catalyst was prepared by pyrolysis of a polypyrrole/Vulcan XC72 composite at 800 °C for 2 h under helium. Controlled amounts of Co, Fe, Mn and Ni in low concentrations were then introduced into NC by impregnating it with the corresponding meso-tetra(4-pyridyl) porphyrin metal complex followed by further pyrolysis at 650 °C for 2 h under helium. The resulting catalysts were investigated for ORR using rotating disk electrode and rotating-ring disk electrode voltammetry in 0.1 M KOH. Additionally, the rate of decomposition of hydrogen peroxide by the different catalysts was determined in order to probe the influence of the metal inclusions on the mechanism and selectivity of the ORR. The results show that Fe, Co and Mn inclusions cause a substantial decrease of the overpotential of the reaction and enhance the catalytic current, whereas the presence of Ni has a poisoning effect on ORR. In the presence of Fe, the catalysts apparently reduce oxygen selectively to OH- in a direct four electron transfer process as opposed to the two-step, two electron pathway involving hydrogen peroxide as an intermediate for the case of the NC catalyst. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2013.11.026
  • 2014 • 58 Metallurgical aspects on the fatigue of solution-annealed austenitic high interstitial steels
    Schymura, M. and Fischer, A.
    International Journal of Fatigue 61 1-9 (2014)
    Austenitic stainless steels have been used for over 100 years for their combination of strength and ductility. In order to further improve the mechanical and chemical properties of austenitic high nitrogen steels (AHNS) were developed. Ni reduces the solubility of N and, therefore, was substituted by Mn in order to allow for up to 1 weight-% N to be alloyed. AHNS show an even higher strength for the solution annealed state, which can be increased further by cold working. Unfortunately the endurance limit did not follow this trend as it is known to for cold-worked Ni-containing steels. The solution annealed Ni-containing austenites allow for wavy slip and the generation of dislocation cells while the Mn-alloyed AHNS only show planar slip with twins and stacking faults. While the stacking fault energy was thought to be the main reason for planar slip, early results showed that there must be other near-field effects. The density of free electrons, which is mainly influenced by the sum and the ratio of C and N, might be responsible. Strain-controlled fatigue tests were carried out in CrMn-alloyed austenitic steels with different sums (C + N: 0.65-1.2) and ratios (C/N: 0.13-).©2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2013.11.018
  • 2014 • 57 New production technologies in aerospace industry - 5th machining innovations conference (MIC 2014) drilling of inconel 718 with geometry-modified twist drills
    Beer, N. and Özkaya, E. and Biermann, D.
    Procedia CIRP 24 49-55 (2014)
    The drilling process of Inconel 718, a nickel-based superalloy, is very challenging due to the material properties, the operating conditions and the high quality requirements. Carbides within the material matrix cause an excessive amount of abrasive tool wear. Moreover, a large amount of the heat caused by the machining process, especially in drilling, has to be dissipated by the tool and the coolant, due to the low thermal conductivity of Inconel 718. This high thermal load also restricts the cutting speed. The combination of all attributes limits productivity and economic efficiency when drilling Inconel 718 with cemented carbide twist drills. This paper presents a method to adapt twist drills considering the mentioned demands by using geometry-modified tools. The aim is to increase the resistance against abrasive wear and to reduce the thermal loads; so that, tool life and bore quality can be improved. The analysis of the new tool geometry was realized by advanced Computational-Fluid-Dynamics (CFD) simulations. The simulations provide detailed information about the coolant flow and consequently the improved cooling of tool regions which are, on the suggested geometry, exposed to very high thermal loads. Experiments showed that the tool life can be increased by up to 50% in contradiction to a standard twist drill. The improvement on the bore quality was shown by determining the roundness deviation and the average surface roughness. In addition, micro hardness tests and metallurgy preparations were conducted to investigate the surface integrity of the bore surface layer. Although the presented geometry only represents a prototype status, the results are impressive. The tool life and the bore quality have been improved, and the simulations showed clearly that there is a significantly better coolant flow, when using the new geometry. © 2014 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.procir.2014.07.124
  • 2014 • 56 On the nature of γ′ phase cutting and its effect on high temperature and low stress creep anisotropy of Ni-base single crystal superalloys
    Agudo Jácome, L. and Nörtershäuser, P. and Somsen, C. and Dlouhý, A. and Eggeler, G.
    Acta Materialia 69 246-264 (2014)
    The creep anisotropy of the single crystal superalloy LEK 94 deformed in tension along [0 0 1] and [1 1 0] directions at 1293 K and 160 MPa was investigated. Elementary microstructural processes which are responsible for a higher increase in creep rates with strain during [1 1 0] as compared to [0 0 1] tensile loading were identified. [1 1 0] tensile creep is associated with a higher number of γ′ phase cutting events, where two dislocations with equal Burgers vectors of type <1 1 0> jointly shear the γ′ phase. The resulting <2 2 0>-type superdislocation can move by glide. In contrast, during [0 0 1] tensile loading, two dislocations with different <1 1 0>-type Burgers vectors must combine for γ′ phase cutting. The resulting <2 0 0>-type superdislocations can only move by a combination of glide and climb. The evolution of dislocation networks during creep determines the nature of the γ′ phase cutting events. The higher [1 1 0] creep rates at strains exceeding 2% result from a combination of a higher number of cutting events (density of mobile dislocations in γ′) and a higher superdislocation mobility (<2 2 0>glide) in the γ′ phase. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.01.021
  • 2014 • 55 Optimizing the magnetocaloric effect in Ni-Mn-Sn by substitution: A first-principles study
    Grünebohm, A. and Comtesse, D. and Hucht, A. and Gruner, M.E. and Maslovskaya, A. and Entel, P.
    IEEE Transactions on Magnetics 50 (2014)
    We optimize the magnetic and structural properties of Ni(Co,Cu)MnSn Heusler alloys for the magnetocaloric effect (MCE) by means of density functional theory combined with Monte Carlo simulations of a classical Heisenberg model. NiMnSn alloys show a drop of magnetization at the martensitic phase transition, which leads to the inverse MCE. We find either disordered or frustrated magnetic configurations directly below the martensitic transition temperature. However, the jump of magnetization at the magnetostructural transition is small as the austenite is in a ferrimagnetic state and not fully magnetized. For Co and Cu substitution, the structural phase transition temperature shifts to lower temperatures. In particular, Co substitution is promising, as the magnetization of the austenite increases by additional ferromagnetic interactions, which enhances the jump of magnetization. © 2014 IEEE.
    view abstractdoi: 10.1109/TMAG.2014.2330845
  • 2014 • 54 Precipitation and austenite reversion behavior of a maraging steel produced by selective laser melting
    Jägle, E.A. and Choi, P.-P. and Van Humbeeck, J. and Raabe, D.
    Journal of Materials Research 29 2072-2079 (2014)
    Materials produced by selective laser melting (SLM) experience a thermal history that is markedly different from that encountered by conventionally produced materials. In particular, a very high cooling rate from the melt is combined with cyclical reheating upon deposition of subsequent layers. Using atom-probe tomography (APT), we investigated how this nonconventional thermal history influences the phase-transformation behavior of maraging steels (Fe-18Ni-9Co-3.4Mo-1.2Ti) produced by SLM. We found that despite the "intrinsic heat treatment" and the known propensity of maraging steels for rapid clustering and precipitation, the material does not show any sign of phase transformation in the as-produced state. Upon aging, three different types of precipitates, namely (Fe,Ni,Co)3(Ti,Mo), (Fe,Ni,Co)3(Mo,Ti), and (Fe,Ni,Co)7Mo6 (μ phase), were observed as well as martensite-to-austenite reversion around regions of the retained austenite. The concentration of the newly formed phases as quantified by APT closely matches thermodynamic equilibrium calculations. Copyright © 2014 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2014.204
  • 2014 • 53 Rapid identification of areas of interest in thin film materials libraries by combining electrical, optical, X-ray diffraction, and mechanical high-throughput measurements: A case study for the system ni-al
    Thienhaus, S. and Naujoks, D. and Pfetzing-Micklich, J. and König, D. and Ludwig, Al.
    ACS Combinatorial Science 16 686-694 (2014)
    The efficient identification of compositional areas of interest in thin film materials systems fabricated by combinatorial deposition methods is essential in combinatorial materials science. We use a combination of compositional screening by EDX together with high-throughput measurements of electrical and optical properties of thin film libraries to determine efficiently the areas of interest in a materials system. Areas of interest are compositions which show distinctive properties. The crystallinity of the thus determined areas is identified by X-ray diffraction. Additionally, by using automated nanoindentation across the materials library, mechanical data of the thin films can be obtained which complements the identification of areas of interest. The feasibility of this approach is demonstrated by using a Ni-Al thin film library as a reference system. The obtained results promise that this approach can be used for the case of ternary and higher order systems. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/co5000757
  • 2014 • 52 Reactive transient liquid phase bonding of ceramic to steel using Zr-Cu-Zr- and Zr-Ni-Cu-Zr-interlayers for high temperature applications
    Tillmann, W. and Pfeiffer, J. and Wojarski, L. and Indacochea, J.-E.
    Materialwissenschaft und Werkstofftechnik 45 512-521 (2014)
    Joints manufactured by transient liquid phase bonding feature comparable properties as diffusion weldements, but considerably lower process temperatures and pressures have to be applied. The liquid phase, which is hereby used, occurs due to interdiffusion between the base and/or the filler materials at a constant temperature, which lies below the melting temperature of the substrates. An essential requirement for this diffusion-based melting is that the involved materials have low melting alloy-constitution areas, such as eutectics. The aim of the study, presented in this contribution, is to evaluate an approach, in which an active transient liquid is created by suitable interlayers, in order to facilitate the wetting of ceramics. The potential of this attempt will be illustrated on zirconia/stainless-steel-joints for high temperature applications, such as solid oxide fuel cells. In such applications, the used materials have to withstand harsh conditions, e.g. high operating temperatures, oxidizing or reducing environments, which represent a demanding challenge for joining technologies, even at the latest state of research. In this study interlayers, consisting of Zirconium, as the active element, in combination with Copper and/or Nickel, have been investigated. These systems exhibit a wide range of alloy-constitutions with low melting temperatures, which can be used for the formation of the transient liquid phase. For the application of the interlayers, physical vapor deposition as well as 75 μm-thick Nickel-foils have been used. The joining was carried out in high vacuum with changing holding times and temperatures. Additionally, the ratio of the thickness of the used interlayers was changed. Results of microstructural investigations, nano-hardness measurements of the joining area as well as shear strength and fractography are presented. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201400267
  • 2014 • 51 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 • 50 Redox dynamics of Ni catalysts in CO2 reforming of methane
    Mette, K. and Kühl, S. and Tarasov, A. and Düdder, H. and Kähler, K. and Muhler, M. and Schlögl, R. and Behrens, M.
    Catalysis Today 101-110 (2014)
    The influence of redox dynamics of a Ni/MgAl oxide catalyst for dry reforming of methane (DRM) at high temperature was studied to correlate structural stability with catalytic activity and coking propensity. Structural aging of the catalyst was simulated by repeated temperature-programmed reduction/oxidation (TPR/TPO) cycles. Despite a very high Ni loading of 55.4 wt.%, small Ni nanoparticles of 11 nm were obtained from a hydrotalcite-like precursor with a homogeneous distribution. Redox cycling gradually changed the interaction of the active Ni phase with the oxide support resulting in a crystalline Ni/MgAl<inf>2</inf>O<inf>4</inf>-type catalyst. After cycling the average particle size increased from 11 to 21 nm - while still a large fraction of small particles was present - bringing about a decrease in Ni surface area of 72%. Interestingly, the redox dynamics and its strong structural and chemical consequences were found to have only a moderate influence on the activity in DRM at 900 °C, but lead to a stable attenuation of carbon formation due to a lower fraction of graphitic carbon after DRM in a fixed-bed reactor. Supplementary DRM experiments in a thermobalance revealed that coke formation as a continuous process until a carbon limit is reached and confirmed a higher coking rate for the cycled catalyst. © 2014 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2014.06.011
  • 2014 • 49 Site occupation of Nb atoms in ternary Ni-Ti-Nb shape memory alloys
    Shi, H. and Frenzel, J. and Martinez, G.T. and Van Rompaey, S. and Bakulin, A. and Kulkova, S. and Van Aert, S. and Schryvers, D.
    Acta Materialia 74 85-95 (2014)
    Nb occupancy in the austenite B2-NiTi matrix and Ti2Ni phase in Ni-Ti-Nb shape memory alloys was investigated by aberration-corrected scanning transmission electron microscopy and precession electron diffraction. In both cases, Nb atoms were found to prefer to occupy the Ti rather than Ni sites. A projector augmented wave method within density functional theory was used to calculate the atomic and electronic structures of the austenitic B2-NiTi matrix phase and the Ti2Ni precipitates both with and without addition of Nb. The obtained formation energies and analysis of structural and electronic characteristics explain the preference for Ti sites for Nb over Ni sites. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.03.062
  • 2014 • 48 Stable performance of Ni catalysts in the dry reforming of methane at high temperatures for the efficient conversion of CO2 into syngas
    Mette, K. and Kühl, S. and Düdder, H. and Kähler, K. and Tarasov, A. and Muhler, M. and Behrens, M.
    ChemCatChem 6 100-104 (2014)
    The catalytic performance of a Ni/MgAlOx catalyst was investigated in the high temperature CO2 reforming of CH4. The catalyst was developed using a Ni, Mg, Al hydrotalcite-like precursor obtained by co-precipitation. Despite the high Ni loading of 55 wt%, the synthesized Ni/MgAlOx catalyst possessed a thermally stable microstructure up to 900 °C with Ni nanoparticles of 9 nm. This stability is attributed to the embedding nature of the oxide matrix, and allows increasing the reaction temperature without losing active Ni surface area. To evaluate the effect of the reaction temperature on the reforming performance and the coking behavior, two different reaction temperatures (800 and 900 °C) were investigated. At both temperatures the prepared catalyst showed high rates of CH4 consumption. The higher temperature promotes the stability of the catalyst performance due to mitigation of the carbon formation. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201300699
  • 2014 • 47 Synthesis, characterization, and nanoindentation response of single crystal Fe-Cr-Ni alloys with FCC and BCC structures
    Xia, Y.Z. and Bei, H. and Gao, Y.F. and Catoor, D. and George, E.P.
    Materials Science and Engineering A 611 177-187 (2014)
    Fe-based alloys are used extensively in many structural applications including under irradiation conditions in the nuclear industry. In this study, model Fe-Cr, Fe-Ni and Fe-Cr-Ni alloys that are the basis of many structural steels were synthesized as single crystals and characterized. The compositions investigated were Fe-15Cr, Fe-30Cr, Fe-30Ni and Fe-15Cr-15Ni (at%). Several key mechanical properties were determined which will be useful in further studies of irradiation/deformation-induced defects. Incipient plasticity and slip characteristics were investigated by nanoindentation on (001) and (1-10) surfaces, and hardness, modulus, pop-in behavior and theoretical strength were determined. The slip trace patterns after microindentation were imaged in a microscope. A novel slip trace analysis was developed and the underlying deformation mechanisms identified. The analysis shows that under both (001) and (1-10) indentations, the activated slip system for the BCC alloys is {112} for the FCC alloys the activated slip plane is {111}. These results were confirmed with finite element simulations using a slip-based crystal-plasticity model. Finally, the effects of heterogeneous pop-in mechanisms are discussed in the context of incipient plasticity in the four different alloys. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.05.079
  • 2014 • 46 Tertiary dendritic instability in late stage solidification of Ni-based superalloys
    Franke, M.M. and Singer, R.F. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    Derivatives of the commercial alloy CMSX-4 were directionally solidified and characterized with respect to their final dendrite microstructure. The results indicate that Ni-based superalloys with high segregation levels show significant instability in secondary dendrite arms and an increased tendency for tertiary arm formation, respectively. Phase-field simulations were used to explore the impact of chemical composition on morphological instability and tertiary arm formation during the directional solidification of Ni-based superalloys. It is found that an increase in specific alloying elements in the overall alloy composition leads to pronounced segregation at the end of solidification. This causes strong growth restriction of the secondary arms and triggers tertiary arm formation. The proposed mechanism explains experimental microstructures found in modifications of the base alloy CMSX-4. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/2/025026
  • 2014 • 45 The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane
    Düdder, H. and Kähler, K. and Krause, B. and Mette, K. and Kühl, S. and Behrens, M. and Scherer, V. and Muhler, M.
    Catalysis Science and Technology 4 3317-3328 (2014)
    Highly stable Ni catalysts with varying Ni contents up to 50 mol% originating from hydrotalcite-like precursors were applied in the dry reforming of methane at 800 and 900 °C. The integral specific rate of methane conversion determined after 10 h on stream was 3.8 mmol s-1 g cat -1 at 900 °C. Due to the outstanding high activity, a catalyst mass of just 10 mg had to be used to avoid operating the reaction in thermodynamic equilibrium. The resulting WHSV was as high as 1.44 × 106 ml gcat -1 h-1. The observed axial temperature distribution with a pronounced cold spot was analyzed by computational fluid dynamics simulations to verify the strong influence of this highly endothermic reaction. Transmission electron microscopy and temperature-programmed oxidation experiments were used to probe the formation of different carbon species, which was found to depend on the catalyst composition and the reaction temperature. Among the formed carbon species, multi-walled carbon nanofibers were detrimental to the long-term stability at 800 °C, whereas their formation was suppressed at 900 °C. The formation of graphitic carbon at 900 °C originating from methane pyrolysis played a minor role. The methane conversion after 100 h of dry reforming at 900 °C compared to the initial one amounted to 98% for the 25 mol% Ni catalyst. The oxidative regeneration of the catalyst was achieved in the isothermal mode using only carbon dioxide in the feed. © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4cy00409d
  • 2014 • 44 TLP brazing of aluminum to steel using PVD-deposited interlayer
    Wojarski, L. and Tillmann, W.
    Welding in the World 58 673-680 (2014)
    The demand for hybrid material concepts is steadily growing, and especially dissimilar joints between aluminum and steel are, due to their wide dissemination, of major importance. The main obstacle for the fabrication of aluminum-steel joints using thermal processes is the embrittlement of the fusion area. In order to prevent direct contact between aluminum and iron and thus to suppress the formation of brittle iron aluminides, 3-μm thick diffusion barrier coatings, consisting of Ni or Ti, were applied onto the steel surface. Pure copper with a thickness of 3 and 6 μm, respectively, was used as a filler material, and the samples were brazed in a TLP process in a vacuum at 580 °C at varying dwell times (10...50 min). The samples brazed with Ni diffusion barriers showed a considerable formation of Fe2Al 5 even at low dwell times. Furthermore, additional complex ternary phase bands have generated due to the existence of diffusion barrier elements and were detected in the interfacial area. The application of Ti showed a significant decrease of iron aluminides, and no Fe2Al5 could be detected at low dwell times, resulting in a shear strength of 42 MPa for the optimized parameters. © 2014 International Institute of Welding.
    view abstractdoi: 10.1007/s40194-014-0143-x
  • 2014 • 43 Vacancy mobility and interaction with transition metal solutes in Ni
    Schuwalow, S. and Rogal, J. and Drautz, R.
    Journal of Physics Condensed Matter 26 (2014)
    Interaction of Re, Ta, W and Mo solutes with vacancies and their diffusion in fcc Ni is investigated by density-functional theory in combination with kinetic Monte Carlo simulations. Interaction energies are calculated for the first six neighbor shells around the solutes and a complete set of diffusion barriers for these shells is provided. Further, diffusion coefficients for the four elements in Ni as well as for vacancies in the presence of these elements are calculated. The calculated solute diffusion coefficients based on our ab initio data are found to compare favorably to experimental values. The mobility of the vacancies as a key factor in dislocation climb is only minimally influenced by the solute atoms within the dilute limit. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/48/485014
  • 2013 • 42 A robust nickel catalyst for cyanomethylation of aldehydes: Activation of acetonitrile under base-free conditions
    Chakraborty, S. and Patel, Y.J. and Krause, J.A. and Guan, H.
    Angewandte Chemie - International Edition 52 7523-7526 (2013)
    Nick of time: The nickel cyanomethyl complex 1 catalyzes the room temperature coupling of aldehydes with acetonitrile under base-free conditions. The catalytic system is long-lived and remarkably efficient with high turnover numbers (TONs) and turnover frequencies (TOFs) achieved. The mild reaction conditions allow a wide variety of aldehydes, including base-sensitive ones, to catalytically react with acetonitrile. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201302613
  • 2013 • 41 Additive manufacturing of shape memory devices and pseudoelastic components
    Haberland, C. and Elahinia, M. and Walker, J. and Meier, H. and Frenzel, J.
    ASME 2013 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2013 1 (2013)
    Processing of Nickel-Titanium shape memory alloys (NiTi) is by no means easy because all processing steps can strongly affect the properties of the material. Hence, near-net-shaping technologies are very attractive for processing NiTi due to reduction of the processing route. Additive Manufacturing (AM) provides especially promising alternatives to conventional processing because it offers unparalleled freedom of design. In the last 5 years AM of NiTi received little attention from academics and researchers and, therefore, is far from being established for processing NiTi today. This work is to highlight the current state of the art of using the AM technique Selective Laser Melting (SLM) for processing high quality NiTi parts. For this reason, fundamentals for SLM processing of NiTi are described. It is shown in detail that a careful control of process parameters is of great importance. Furthermore, this work characterizes structural and functional properties like shape recovery, referring to the shape memory effect in Ti-rich SLM NiTi, or pseudoelasticy in Ni-rich SLM NiTi. It is shown that both types of shape memory effects can be adjusted in SLM NiTi by the choice of the raw material and processing strategy. By comparing the properties of SLM NiTi to those of conventionally processed NiTi, this work clearly shows that SLM is an attractive manufacturing method for production of high quality NiTi parts. Copyright © 2013 by ASME.
    view abstractdoi: 10.1115/SMASIS2013-3070
  • 2013 • 40 Antiferromagnetic coupling of TbPc2 molecules to ultrathin Ni and Co films
    Klar, D. and Klyatskaya, S. and Candini, A. and Krumme, B. and Kummer, K. and Ohresser, P. and Corradini, V. and de Renzi, V. and Biagi, R. and Joly, L. and Kappler, J.-P. and Pennino, U.D. and Affronte, M. and Wende, H. and Ruben, M.
    Beilstein Journal of Nanotechnology 4 320-324 (2013)
    The magnetic and electronic properties of single-molecule magnets are studied by X-ray absorption spectroscopy and X-ray magnetic circular dichroism. We study the magnetic coupling of ultrathin Co and Ni films that are epitaxially grown onto a Cu(100) substrate, to an in situ deposited submonolayer of TbPc2 molecules. Because of the element specificity of the X-ray absorption spectroscopy we are able to individually determine the field dependence of the magnetization of the Tb ions and the Ni or Co film. On both substrates the TbPc2 molecules couple antiferromagnetically to the ferromagnetic films, which is possibly due to a superexchange interaction via the phthalocyanine ligand that contacts the magnetic surface. © 2013 Klar et al.
    view abstractdoi: 10.3762/bjnano.4.36
  • 2013 • 39 Athermal nature of the martensitic transformation in Heusler alloy Ni-Mn-Sn
    Zheng, H. and Wang, W. and Wu, D. and Xue, S. and Zhai, Q. and Frenzel, J. and Luo, Z.
    Intermetallics 36 90-95 (2013)
    Martensitic transformations are generally classified into two groups, namely athermal and isothermal, according to their kinetics. In case of athermal transformations, the amount of the product phase only depends on temperature, and not on time. However, much debate rises about this issue due to unexpected experimental observations of isothermal effects in typically athermal transformations. Considering that the wide applications of Heusler Ni-Mn based materials are based on martensitic transformations, it is of importance to clarify the nature of their martensitic transformation. In this paper, we made an effort to study isothermal effects in a Ni-Mn-Sn alloy using differential scanning calorimetry (DSC). It is proposed that the martensitic transformation of Ni-Mn based materials is athermal in nature although a time-depending effect is observed through DSC interrupted measurements. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2013.01.012
  • 2013 • 38 Atomic-scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high-entropy alloy using atom probe tomography
    Pradeep, K.G. and Wanderka, N. and Choi, P. and Banhart, J. and Murty, B.S. and Raabe, D.
    Acta Materialia 61 4696-4706 (2013)
    We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600 C for 15 min at 650 MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12 vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60 vol.% bcc and 40 vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc-bcc solid solution but instead a composite of bcc structured Ni-Al-, Cr-Fe- and Fe-Cr-based regions and of fcc Cu-Zn-based regions. The Cu-Zn-rich phase has 30 at.% Zn α-brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr- and Fe-rich bcc regions were presumably formed by spinodal decomposition of a Cr-Fe phase that was inherited from the hot compacted state. The Ni-Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20-30 nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870 ± 10 HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high-entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high-entropy alloys is encouraged as a pathway towards exploiting the solid-solution and stability idea inherent in this concept. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.059
  • 2013 • 37 Cavitation erosion of Cr60Ni40 coatings generated by friction surfacing
    Hanke, S. and Beyer, M. and Silvonen, A. and dos Santos, J.F. and Fischer, A.
    Wear 301 415-423 (2013)
    CrNi-alloys with high Cr-content generally are quite brittle and, therefore, only available as castings and regarded as neither weldable nor deformable. The process of friction surfacing offers a possibility to generate Cr60Ni40 coatings e.g. on steel or Ni-base substrates. Cavitation tests were carried out using an ultrasonic vibratory test rig (~ASTM G32) with cast specimens and friction surfaced coatings. The coatings show less deformation and smaller disruptions, and wear rates in steady state were found to be three times higher for the cast and heat treated samples than for the coatings, caused by a highly wear resistant Cr-rich phase. The results of this study show that it is possible to generate defect free coatings of Cr60Ni40 with a thickness of about 250. μm by friction surfacing, which under cavitation show a better wear behavior than the cast material. Thus, in combination with a ductile substrate, these coatings are likely to extend the range of applicability of such high-temperature corrosion resistant alloys. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2012.11.016
  • 2013 • 36 Complex magnetic ordering as a driving mechanism of multifunctional properties of Heusler alloys from first principles
    Entel, P. and Siewert, M. and Gruner, M.E. and Herper, H.C. and Comtesse, D. and Arróyave, R. and Singh, N. and Talapatra, A. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Albertini, F. and Righi, L. and Chernenko, V.A.
    European Physical Journal B 86 (2013)
    First-principles calculations are used to study the structural, electronic and magnetic properties of (Pd, Pt)-Mn-Ni-(Ga, In, Sn, Sb) alloys, which display multifunctional properties like the magnetic shape-memory, magnetocaloric and exchange bias effect. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior arising from competing ferro- and antiferromagnetic interactions with increasing number of Mn excess atoms in the unit cell. This information allows to optimize, for example, the magnetocaloric effect by using the strong influence of compositional changes on the magnetic interactions. Thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. We present guidelines of how to improve the functional properties. For Pt-Ni-Mn-Ga alloys, a shape memory effect with 14% strain can be achieved in an external magnetic field. © 2013 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1140/epjb/e2012-30936-9
  • 2013 • 35 Composition-dependent crystal structure and martensitic transformation in Heusler Ni-Mn-Sn alloys
    Zheng, H. and Wang, W. and Xue, S. and Zhai, Q. and Frenzel, J. and Luo, Z.
    Acta Materialia 61 4648-4656 (2013)
    In the present work, modulated four- and five-layered orthorhombic, seven-layered monoclinic (4O, 10M and 14M) and unmodulated double tetragonal (L10) martensites are characterized in Heusler Ni-Mn-Sn alloys using X-ray diffraction, high-resolution transmission electron microscopy, electron diffraction techniques and thermal analysis. All modulated layered martensites exhibit twins and stacking faults, while the L10 martensite shows fewer structural defects. The substitution of Sn with Mn in Ni 50Mn37+xSn13-x (x = 0, 2, 4) enhances the martensitic transition temperatures, while the transition temperatures decrease with increasing Mn content for constant Sn levels in Ni50-yMn37+ySn13 (y = 0, 2, 4). The compositional dependence of the martensitic transition temperatures is mainly attributed to the valence electron concentration (e/a) and the unit-cell volume of the high-temperature phase. With increasing transition temperatures (or e/a), the resultant martensitic crystal structure evolves in a sequence of 4O → 10M → 14M → L10 in bulk Ni-Mn-Sn alloys. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.035
  • 2013 • 34 Design and fabrication of a bending rotation fatigue test rig for in situ electrochemical analysis during fatigue testing of NiTi shape memory alloy wires
    Neelakantan, L. and Zglinski, J.K. and Frotscher, M. and Eggeler, G.
    Review of Scientific Instruments 84 (2013)
    The current investigation proposes a novel method for simultaneous assessment of the electrochemical and structural fatigue properties of nickel-titanium shape memory alloy (NiTi SMA) wires. The design and layout of an in situ electrochemical cell in a custom-made bending rotation fatigue (BRF) test rig is presented. This newly designed test rig allows performing a wide spectrum of experiments for studying the influence of fatigue on corrosion and vice versa. This can be achieved by performing ex situ andor in situ measurements. The versatility of the combined electrochemicalmechanical test rig is demonstrated by studying the electrochemical behavior of NiTi SMA wires in 0.9 NaCl electrolyte under load. The ex situ measurements allow addressing various issues, for example, the influence of pre-fatigue on the localized corrosion resistance, or the influence of hydrogen on fatigue life. Ex situ experiments showed that a pre-fatigued wire is more susceptible to localized corrosion. The synergetic effect can be concluded from the polarization studies and specifically from an in situ study of the open circuit potential (OCP) transients, which sensitively react to the elementary repassivation events related to the local failure of the oxide layer. It can also be used as an indicator for identifying the onset of the fatigue failure. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4793488
  • 2013 • 33 Em characterization of precipitates in as-cast and annealed Ni45.5Ti45.5Nb9 shape memory alloys
    Shi, H. and Frenzel, J. and Schryvers, D.
    Materials Science Forum 738-739 113-117 (2013)
    Nb-rich precipitates in the matrix of as-cast and annealed Ni45.5Ti45.5Nb9alloys are investigated by scanning and scanning transmission electron microscopy, including slice-and-view and geometric phase analysis (GPA). The Nb-rich bcc nano-precipitates in the as-cast alloy have a 10% lattice parameter difference with the B2 matrix and reveal compensating interface dislocations. The 3D reconstruction of the configuration of small Nb-rich precipitates in the annealed alloy reveals a wall-like distribution of precipitates, which may increase the thermal hysteresis of the material. © (2013) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2013 • 32 Environmental tight-binding modeling of nickel and cobalt clusters
    McEniry, E.J. and Drautz, R. and Madsen, G.K.H.
    Journal of Physics Condensed Matter 25 (2013)
    Tight-binding models derived from density functional theory potentially provide a systematic approach to the development of accurate and transferable models of multicomponent systems. We introduce a systematic methodology for environmental tight binding in which both the overlap and environmental contributions to the electronic structure are included. The parameters of the model are determined directly from ab initio considerations, thus providing a formal conceptual link to density functional approaches. In order to test the validity of the approach, the model is applied to small clusters of Ni and Co, whose electronic structure is largely determined by the interplay of tightly bound d-valent states and the disperse s-states. We numerically illustrate that it is essential to include environmental contributions in the tight-binding approach in order to reliably reproduce the electronic structure of such clusters. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/11/115502
  • 2013 • 31 Extended investigation of intermartensitic transitions in Ni-Mn-Ga magnetic shape memory alloys: A detailed phase diagram determination
    Çakir, A. and Righi, L. and Albertini, F. and Acet, M. and Farle, M. and Aktürk, S.
    Journal of Applied Physics 114 (2013)
    Martensitic transitions in shape memory Ni-Mn-Ga Heusler alloys take place between a high temperature austenite and a low temperature martensite phase. However, intermartensitic transformations have also been encountered that occur from one martensite phase to another. To examine intermartensitic transitions in magnetic shape memory alloys in detail, we carried out temperature dependent magnetization, resistivity, and x-ray diffraction measurements to investigate the intermartensitic transition in Ni50Mn50- xGax in the composition range 12 ≤ x ≤ 25 at. %. Rietveld refined x-ray diffraction results are found to be consistent with magnetization and resistivity data. Depending on composition, we observe that intermartensitic transitions occur in the sequences 7 M → L 1 0, 5 M → 7 M, and 5 M → 7 M → L 1 0 with decreasing temperature. The L1 0 non-modulated structure is most stable at low temperature. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4831667
  • 2013 • 30 High-throughput ab initio screening of binary solid solutions in olivine phosphates for Li-ion battery cathodes
    Hajiyani, H.R. and Preiss, U. and Drautz, R. and Hammerschmidt, T.
    Modelling and Simulation in Materials Science and Engineering 21 (2013)
    A promising approach to improving the performance of iron-phosphate FePO4 cathode materials for Li-ion batteries is to partly or fully substitute Fe with other metals. Here, we use high-throughput density-functional theory (DFT) calculations to investigate binary mixtures of metal atoms M and M′ in (Li)MyM'1-yPO4 olivine phosphates. We determine the formation energy for various stoichiometries of different binary combinations of metals for the cases of full lithiation and delithiation. Systematic screening of all combinations of Fe and Mn with elements of the 3d transition-metal (TM) series allows us to identify trends with average band filling and atomic size. We also included compounds that verify the observed relations or that were discussed as cathode materials, particularly Ni-Co, V-Cu and V-Ni, as well as combinations with 4d TMs (Fe-Zr, Fe-Mo, Fe-Ag) and with Mg (Fe-Mg and Ni-Mg). Based on our DFT calculations for each compound, we estimate the volume change during intercalation, the intercalation voltage, the energy density and the thermal stability with respect to reaction with oxygen. Our calculations indicate that the energy density of the binary TM phosphates increases with average band filling while the thermal stability of the compounds decreases. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/21/7/074004
  • 2013 • 29 Influence of supersaturated carbon on the diffusion of Ni in ferrite determined by atom probe tomography
    Kresse, T. and Li, Y.J. and Boll, T. and Borchers, C. and Choi, P. and Al-Kassab, T. and Raabe, D. and Kirchheim, R.
    Scripta Materialia 69 424-427 (2013)
    In patented and cold-drawn pearlitic steel wires dissociation of cementite occurs during mechanical deformation. In this study the influence of the carbon decomposition on the diffusion of nickel in ferrite is investigated by means of atom probe tomography. In the temperature range 423-523 K we observed a much smaller activation energy of Ni diffusion than for self-diffusion in body-centered cubic iron, indicating an increased vacancy density owing to enhanced formation of vacancy-carbon complexes. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2013.05.039
  • 2013 • 28 Long-range segregation phenomena in shape-selected bimetallic nanoparticles: Chemical state effects
    Ahmadi, M. and Behafarid, F. and Cui, C. and Strasser, P. and Cuenya, B.R.
    ACS Nano 7 9195-9204 (2013)
    A study of the morphological and chemical stability of shape-selected octahedral Pt0.5Ni0.5 nanoparticles (NPs) supported on highly oriented pyrolytic graphite (HOPG) is presented. Ex situ atomic force microscopy (AFM) and in situ X-ray photoelectron spectroscopy (XPS) measurements were used to monitor the mobility of Pt0.5Ni0.5 NPs and to study long-range atomic segregation and alloy formation phenomena under vacuum, H2, and O2 environments. The chemical state of the NPs was found to play a pivotal role in their surface composition after different thermal treatments. In particular, for these ex situ synthesized NPs, Ni segregation to the NP surface was observed in all environments as long as PtOx species were present. In the presence of oxygen, an enhanced Ni surface segregation was observed at all temperatures. In contrast, in hydrogen and vacuum, the Ni outward segregation occurs only at low temperature (< 200-270 C), while PtOx species are still present. At higher temperatures, the reduction of the Pt oxide species results in Pt diffusion toward the NP surface and the formation of a Ni-Pt alloy. A consistent correlation between the NP surface composition and its electrocatalytic CO oxidation activity was established. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nn403793a
  • 2013 • 27 Nano- and microcrystal investigations of precipitates, interfaces and strain fields in Ni-Ti-Nb by various TEM techniques
    Schryvers, D. and Shi, H. and Martinez, G.T. and Van Aert, S. and Frenzel, J. and Van Humbeeck, J.
    Materials Science Forum 738-739 65-71 (2013)
    In the present contribution several advanced electron microscopy techniques are employed in order to describe chemical and structural features of the nano- and microstructure of a Ni45.5Ti45.5Nb9 alloy. A line-up of Nb-rich nano-precipitates is found in the Ni-Ti-rich austenite of as-cast material. Concentration changes of the matrix after annealing are correlated with changes in the transformation temperatures. The formation of rows and plates of larger Nb-rich precipitates and particles is described. The interaction of a twinned martensite plate with a Nb-rich nano-precipitate is discussed and the substitution of Nb atoms on the Ti-sublattice in the matrix is confirmed. © (2013) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2013 • 26 New coating systems for temperature monitoring in turning processes
    Biermann, D. and Kirschner, M. and Pantke, K. and Tillmann, W. and Herper, J.
    Surface and Coatings Technology 215 376-380 (2013)
    High temperature loads in cutting processes can cause high tool wear and damages in the subsurface zone of the workpiece. Especially, the interaction between different cutting parameters affects the thermal loads in the cutting zone. Hence, the knowledge of temperatures in cutting processes is an important factor, and it is the main focus of current investigations. Therefore, this paper deals with an in-process monitoring system for the resulting temperatures in a turning process. In contrast to the investigations performed hitherto, this research deals with a new tool sensor system for temperature measurement. This sensor system is realized by a PVD coating of a Nickel and a Nickel-Chromium layer on the rake face of cutting inserts. On the junction points of this layer system, three thermocouples are deposited. The development of the coating system and the resulting measurement is shown. Additionally, the results are discussed in comparison to thermal imaging system and conventional thermocouples. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2012.08.086
  • 2013 • 25 Observation of current-driven oscillatory domain wall motion in Ni 80Fe20/Co bilayer nanowire
    Zhang, W. and Wong, P.K.J. and Yan, P. and Wu, J. and Morton, S.A. and Wang, X.R. and Hu, X.F. and Xu, Y.B. and Scholl, A. and Young, A. and Barsukov, I. and Farle, M. and Van Der Laan, G.
    Applied Physics Letters 103 (2013)
    Direct observation of current-driven oscillatory domain wall motion above the Walker breakdown by x-ray magnetic circular dichroism in photoemission electron microscopy is reported in Ni80Fe20/Co nanowire, showing micrometer-scale displacement at ∼13 MHz. We identify two key factors that enhance the oscillatory motion: (i) increase of the hard-axis magnetic anisotropy field value Ḣ and (ii) increase of the ratio between non-adiabatic spin-transfer parameter to the Gilbert damping coefficient, β/α, which is required to be larger than 1. These findings point to an important route to tune the long-scale oscillatory domain wall motion using appropriate geometry and materials. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4816359
  • 2013 • 24 Optimization of smart Heusler alloys from first principles
    Entel, P. and Siewert, M. and Gruner, M.E. and Chakrabarti, A. and Barman, S.R. and Sokolovskiy, V.V. and Buchelnikov, V.D.
    Journal of Alloys and Compounds 577 S107-S112 (2013)
    The strong magnetoelastic interaction in ternary X2YZ Heusler alloys is reponsible for the appearance of magnetostructural phase transitions and related functional properties such as the magnetocaloric and magnetic shape-memory effects. Here, X and Y are transition metal elements and Z is usually an element from the III-V group. In order to discuss possibilities to optimize the multifunctional effects, we use density functional theory calculations from which the martensitic driving forces of the magnetic materials can be derived. We find that the electronic contribution arising from the band Jahn-Teller effect is one of the major driving forces. The ab initio calculations also give a hint of how to design new intermetallics with higher martensitic transformation temperatures compared to the prototype alloy system Ni-Mn-Ga. As an example, we discuss quarternary PtxNi 2-xMnGa alloys which have properties very similar to Ni-Mn-Ga but exhibit a higher maximal eigenstrain of 14%. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2012.03.005
  • 2013 • 23 Pincer-ligated nickel hydridoborate complexes: The dormant species in catalytic reduction of carbon dioxide with boranes
    Chakraborty, S. and Zhang, J. and Patel, Y.J. and Krause, J.A. and Guan, H.
    Inorganic Chemistry 52 37-47 (2013)
    Nickel pincer complexes of the type [2,6-(R2PO) 2C6H3]NiH (R = tBu, 1a; R = iPr, 1b; R = cPe, 1c) react with BH3·THF to produce borohydride complexes [2,6-(R2PO)2C 6H3]Ni(η2-BH4) (2a-c), as confirmed by NMR and IR spectroscopy, X-ray crystallography, and elemental analysis. The reactions are irreversible at room temperature but reversible at 60 C. Compound 1a exchanges its hydrogen on the nickel with the borane hydrogen of 9-BBN or HBcat, but does not form any observable adduct. The less bulky hydride complexes 1b and 1c, however, yield nickel dihydridoborate complexes reversibly at room temperature when mixed with 9-BBN and HBcat. The dihydridoborate ligand in these complexes adopts an η2- coordination mode, as suggested by IR spectroscopy and X-ray crystallography. Under the catalytic influence of 1a-c, reduction of CO2 leads to the methoxide level when 9-BBN or HBcat is employed as the reducing agent. The best catalyst, 1a, involves bulky substituents on the phosphorus donor atoms. Catalytic reactions involving 1b and 1c are less efficient because of the formation of dihydridoborate complexes as the dormant species as well as partial decomposition of the catalysts by the boranes. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ic300587b
  • 2013 • 22 Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys
    Otto, F. and Yang, Y. and Bei, H. and George, E.P.
    Acta Materialia 61 2628-2638 (2013)
    High configurational entropies have been hypothesized to stabilize solid solutions in equiatomic, multi-element alloys which have attracted much attention recently as "high-entropy" alloys with potentially interesting properties. To evaluate the usefulness of configurational entropy as a predictor of single-phase (solid solution) stability, we prepared five new equiatomic, quinary alloys by replacing individual elements one at a time in a CoCrFeMnNi alloy that was previously shown to be single-phase [1]. An implicit assumption here is that, if any one element is replaced by another, while keeping the total number of elements constant, the configurational entropy of the alloy is unchanged; therefore, the new alloys should also be single-phase. Additionally, the substitute elements that we chose, Ti for Co, Mo or V for Cr, V for Fe, and Cu for Ni, had the same room temperature crystal structure and comparable size/electronegativity as the elements being replaced to maximize solid solubility consistent with the Hume-Rothery rules. For comparison, the base CoCrFeMnNi alloy was also prepared. After three-day anneals at elevated temperatures, multiple phases were observed in all but the base CoCrFeMnNi alloy, suggesting that, by itself, configurational entropy is generally not able to override the competing driving forces that also govern phase stability. Thermodynamic analyses were carried out for each of the constituent binaries in the investigated alloys (Co-Cr, Fe-Ni, Mo-Mn, etc.). Our experimental results combined with the thermodynamic analyses suggest that, in general, enthalpy and non-configurational entropy have greater influences on phase stability in equiatomic, multi-component alloys. Only when the alloy microstructure is a single-phase, approximately ideal solid solution does the contribution of configurational entropy to the total Gibbs free energy become dominant. Thus, high configurational entropy provides a way to rationalize, after the fact, why a solid solution forms (if it forms), but it is not a useful a priori predictor of which of the so-called high-entropy alloys will form thermodynamically stable single-phase solid solutions.
    view abstractdoi: 10.1016/j.actamat.2013.01.042
  • 2013 • 21 Synthesis of one-dimensional hierarchical NiO hollow nanostructures with enhanced supercapacitive performance
    Zhang, G. and Yu, L. and Hoster, H.E. and Lou, X.W.
    Nanoscale 5 877-881 (2013)
    One-dimensional hierarchical hollow nanostructures composed of NiO nanosheets are successfully synthesized through a facile carbon nanofiber directed solution method followed by a simple thermal annealing treatment. With the advantages of high electro-active surface area, carbon nanofiber supported robust structure and short ion and electron transport pathways, the hierarchical hybrid nanostructures deliver largely enhanced capacitance with excellent cycling stability when evaluated as electrode materials for supercapacitors. More specifically, a high capacitance of 642 F g-1 is achieved when the charge-discharge current density is 3 A g-1 and the total capacitance loss is only 5.6% after 1000 cycles. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2nr33326k
  • 2013 • 20 The biocompatibility of dense and porous Nickel-Titanium produced by selective laser melting
    Habijan, T. and Haberland, C. and Meier, H. and Frenzel, J. and Wittsiepe, J. and Wuwer, C. and Greulich, C. and Schildhauer, T.A. and Köller, M.
    Materials Science and Engineering C 33 419-426 (2013)
    Nickel-Titanium shape memory alloys (NiTi-SMA) are of biomedical interest due to their unusual range of pure elastic deformability and their elastic modulus, which is closer to that of bone than any other metallic or ceramic material. Newly developed porous NiTi, produced by Selective Laser Melting (SLM), is currently under investigation as a potential carrier material for human mesenchymal stem cells (hMSC). SLM enables the production of highly complex and tailor-made implants for patients on the basis of CT data. Such implants could be used for the reconstruction of the skull, face, or pelvis. hMSC are a promising cell type for regenerative medicine and tissue engineering due to their ability to support the regeneration of critical size bone defects. Loading porous SLM-NiTi implants with autologous hMSC may enhance bone growth and healing for critical bone defects. The purpose of this study was to assess whether porous SLM-NiTi is a suitable carrier for hMSC. Specimens of varying porosity and surface structure were fabricated via SLM. hMSC were cultured for 8 days on NiTi specimens, and cell viability was analyzed using two-color fluorescence staining. Viable cells were detected on all specimens after 8 days of cell culture. Cell morphology and surface topography were analyzed by scanning electron microscopy (SEM). Cell morphology and surface topology were dependent on the orientation of the specimens during SLM production. The Nickel ion release can be reduced significantly by aligned laser processing conditions. The presented results clearly attest that both dense SLM-NiTi and porous SLM-NiTi are suitable carriers for hMSC. Nevertheless, before carrying out in vivo studies, some work on optimization of the manufacturing process and post-processing is required. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2012.09.008
  • 2013 • 19 Thickness dependent exchange bias in martensitic epitaxial Ni-Mn-Sn thin films
    Behler, A. and Teichert, N. and Dutta, B. and Waske, A. and Hickel, T. and Auge, A. and Hütten, A. and Eckert, J.
    AIP Advances 3 (2013)
    A thickness dependent exchange bias in the low temperature martensitic state of epitaxial Ni-Mn-Sn thin films is found. The effect can be retained down to very small thicknesses. For a Ni50Mn32Sn18 thin film, which does not undergo a martensitic transformation, no exchange bias is observed. Our results suggest that a significant interplay between ferromagnetic and antiferromagnetic regions, which is the origin for exchange bias, is only present in the martensite. The finding is supported by ab initio calculations showing that the antiferromagnetic order is stabilized in the phase. © 2013 Author(s).
    view abstractdoi: 10.1063/1.4849795
  • 2013 • 18 Universal method for protein immobilization on chemically functionalized germanium investigated by ATR-FTIR difference spectroscopy
    Schartner, J. and Güldenhaupt, J. and Mei, B. and Rögner, M. and Muhler, M. and Gerwert, K. and Kötting, C.
    Journal of the American Chemical Society 135 4079-4087 (2013)
    Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy allows a detailed analysis of surface attached molecules, including their secondary structure, orientation, and interaction with small molecules in the case of proteins. Here, we present a universal immobilization technique on germanium for all oligo-histidine-tagged proteins. For this purpose, new triethoxysilane derivates were developed: we synthesized a linker-silane with a succinimidyl ester as amine-reactive headgroup and a matrix-silane with an unreactive ethylene glycol group. A new methodology for the attachment of triethoxysilanes on germanium was established, and the surface was characterized by ATR-FTIR and X-ray photoelectron spectroscopy. In the next step, the succinimidyl ester was reacted with aminonitrilotriacetic acid. Subsequently, Ni2+ was coordinated to form Ni-nitrilotriacetic acid for His-tag binding. The capability of the functionalized surface was demonstrated by experiments using the small GTPase Ras and photosystem I (PS I). The native binding of the proteins was proven by difference spectroscopy, which probes protein function. The function of Ras as molecular switch was demonstrated by a beryllium trifluoride anion titration assay, which allows observation of the "on" and "off" switching of Ras at atomic resolution. Furthermore, the activity of immobilized PS I was proven by light-induced difference spectroscopy. Subsequent treatment with imidazole removes attached proteins, enabling repeated binding. This universal technique allows specific attachment of His-tagged proteins and a detailed study of their function at the atomic level using FTIR difference spectroscopy. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja400253p
  • 2013 • 17 Vacuum brazing titanium using thin nickel layer deposited by PVD technique
    Elrefaey, A. and Wojarski, L. and Janczak-Rusch, J. and Tillmann, W.
    Materials Science and Engineering A 565 180-186 (2013)
    In this study, the evolution of the interfacial microstructure, hardness distribution, and the joint strength of vacuum brazed commercially pure titanium were evaluated. A thin nickel layer, with different thicknesses, was deposited by PVD technique to serve as the brazing filler metal. Test joints were processed at temperatures of 910°C and 960°C using a soaking time of 15 and 90min. The experimental results showed that sound joints with a good wetting quality as well as lack of pores and cracks can be achieved at a brazing temperature of 960°C. A Ti2Ni intermetallic compound was formed at the interfacial area at a soaking time of 15min and with a deposition rate of 90AH which was detrimental to the joint mechanical properties. Meanwhile, at a soaking time of 90min, intermetallic compound was not detected and the diffusion of nickel was completed at all deposition rates which improve the shear strength of the joints. © 2012 Elsevier B.V..
    view abstractdoi: 10.1016/j.msea.2012.12.028
  • 2012 • 16 A first-principles investigation of the compositional dependent properties of magnetic shape memory heusler alloys
    Siewert, M. and Gruner, M.E. and Hucht, A. and Herper, H.C. and Dannenberg, A. and Chakrabarti, A. and Singh, N. and Arróyave, R. and Entel, P.
    Advanced Engineering Materials 14 530-546 (2012)
    The interplay of structural and magnetic properties of magnetic shape memory alloys is closely related to their composition. In this study the influence of the valence electron concentration on the tetragonal transformation in Ni 2Mn 1 + xZ 1 - x (Z = Ga, In, Sn, Sb) and Co 2Ni 1 + xGa 1 - x is investigated by means of ab initio calculations. While the type of magnetic interaction is different for the two series, the trends of the total energy changes under a tetragonal transformation are very similar. We find that tetragonal structures become energetically preferred with respect to the cubic one as the valence electron concentration e/a is increased regardless of the system under consideration. In particular, the energy difference between the austenite and martensite structures increases linearly with e/a, which is in part responsible for the linear increase of the matensite transformation temperature. The substitution of nickel by platinum increases even further the transformation temperature. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201200063
  • 2012 • 15 Advanced scanning transmission stereo electron microscopy of structural and functional engineering materials
    Agudo Jácome, L. and Eggeler, G. and Dlouhý, A.
    Ultramicroscopy 122 48-59 (2012)
    Stereo transmission electron microscopy (TEM) provides a 3D impression of the microstructure in a thin TEM foil. It allows to perform depth and TEM foil thickness measurements and to decide whether a microstructural feature lies inside of a thin foil or on its surface. It allows appreciating the true three-dimensional nature of dislocation configurations. In the present study we first review some basic elements of classical stereo TEM. We then show how the method can be extended by working in the scanning transmission electron microscope (STEM) mode of a modern analytical 200. kV TEM equipped with a field emission gun (FEG TEM) and a high angle annular dark field (HAADF) detector. We combine two micrographs of a stereo pair into one anaglyph. When viewed with special colored glasses the anaglyph provides a direct and realistic 3D impression of the microstructure. Three examples are provided which demonstrate the potential of this extended stereo TEM technique: a single crystal Ni-base superalloy, a 9% Chromium tempered martensite ferritic steel and a NiTi shape memory alloy. We consider the effect of camera length, show how foil thicknesses can be measured, and discuss the depth of focus and surface effects. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2012.06.017
  • 2012 • 14 Catalytic properties of nickel bis(phosphinite) pincer complexes in the reduction of CO 2 to methanol derivatives
    Chakraborty, S. and Patel, Y.J. and Krause, J.A. and Guan, H.
    Polyhedron 32 30-34 (2012)
    A new nickel bis(phosphinite) pincer complex [2,6-(R 2PO) 2C 6H 3]NiCl (L RNiCl, R = cyclopentyl) has been prepared in one pot from resorcinol, ClP(C 5H 9) 2, NiCl 2, and 4-dimethylaminopyridine. The reaction of this pincer compound with LiAlH 4 produces a nickel hydride complex, which is capable of reducing CO 2 rapidly at room temperature to give a nickel formate complex. X-ray structures of two related nickel formate complexes L RNiOCHO (R = cyclopentyl and isopropyl) have shown an "in plane" conformation of the formato group with respect to the coordination plane. The stoichiometric reaction of nickel formate complexes L RNiOCHO (R = cyclopentyl, isopropyl, and tert-butyl) with catecholborane has suggested that the reaction is favored by a bulky R group. L RNiOCHO (R = tert-butyl) does not react with PhSiH 3 at room temperature; however, it reacts with 9-borabicyclo[3.3.1]nonane and pinacolborane to generate a methanol derivative and a boryl formate species, respectively. The catalytic reduction of CO 2 with catecholborane is more effectively catalyzed by a more sterically hindered nickel pincer hydride complex with bulky R groups on the phosphorus donor atoms. The nickel pincer hydride complexes are inactive catalysts for the hydrosilylation of CO 2 with PhSiH 3. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.poly.2011.04.030
  • 2012 • 13 Cytotoxicity and ion release of alloy nanoparticles
    Hahn, A. and Fuhlrott, J. and Loos, A. and Barcikowski, S.
    Journal of Nanoparticle Research 14 (2012)
    It is well-known that nanoparticles could cause toxic effects in cells. Alloy nanoparticles with yet unknown health risk may be released from cardiovascular implants made of Nickel-Titanium or Cobalt-Chromium due to abrasion or production failure. We show the bio-response of human primary endothelial and smooth muscle cells exposed to different concentrations of metal and alloy nanoparticles. Nanoparticles having primary particle sizes in the range of 5-250 nm were generated using laser ablation in three different solutions avoiding artificial chemical additives, and giving access to formulations containing nanoparticles only stabilized by biological ligands. Endothelial cells are found to be more sensitive to nanoparticle exposure than smooth muscle cells. Cobalt and Nickel nanoparticles caused the highest cytotoxicity. In contrast, Titanium, Nickel- Iron, and Nickel-Titanium nanoparticles had almost no influence on cells below a nanoparticle concentration of 10 lM. Nanoparticles in cysteine dissolved almost completely, whereas less ions are released when nanoparticles were stabilized in water or citrate solution. Nanoparticles stabilized by cysteine caused less inhibitory effects on cells suggesting cysteine to form metal complexes with bioactive ions in media. © Springer Science+Business Media B.V. 2012.
    view abstractdoi: 10.1007/s11051-011-0686-3
  • 2012 • 12 On the properties of Ni-rich NiTi shape memory parts produced by selective laser melting
    Haberland, C. and Meier, H. and Frenzel, J.
    ASME 2012 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2012 1 97-104 (2012)
    Processing of Nickel-Titanium (NiTi) shape memory alloys (SMAs) is challenging because smallest compositional variances and all types of microstructural features strongly affect the elementary processes of the martensitic transformation and thus the functional properties of the material. Against this background, powder metallurgical near net shape methods are attractive for the production of NiTi components. Especially additive manufacturing technologies (AM) seem to provide high potential, although they have received only little attention for processing NiTi so far. This work is the first to report on pseudoelastic properties of additive manufactured Ni-rich NiTi. We show how to establish pseudoelasticity in NiTi samples prepared by the additive manufacturing technique Selective Laser Melting (SLM). Therefore, we analyze phase transformation behavior, mechanical characteristics and functional properties of our materials subjected to different heat treatments. The obtained results are compared to the behavior of conventional NiTi. The presented results clearly indicate that SLM provides a promising processing route for the fabrication of high quality NiTi parts. Copyright © 2012 by ASME.
    view abstractdoi: 10.1115/SMASIS2012-8040
  • 2012 • 11 Probing the timescale of the exchange interaction in a ferromagnetic alloy
    Mathias, S. and La-O-Vorakiat, C. and Grychtol, P. and Granitzka, P. and Turgut, E. and Shaw, J.M. and Adam, R. and Nembach, H.T. and Siemens, M.E. and Eich, S. and Schneider, C.M. and Silva, T.J. and Aeschlimann, M. and Murnane, ...
    Proceedings of the National Academy of Sciences of the United States of America 109 4792-4797 (2012)
    The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies.
    view abstractdoi: 10.1073/pnas.1201371109
  • 2012 • 10 Single-crystalline NiCo 2O 4 nanoneedle arrays grown on conductive substrates as binder-free electrodes for high-performance supercapacitors
    Zhang, G.Q. and Wu, H.B. and Hoster, H.E. and Chan-Park, M.B. and Lou, X.W.
    Energy and Environmental Science 5 9453-9456 (2012)
    In this work, we have successfully grown single-crystalline nanoneedle arrays of NiCo 2O 4 on conductive substrates such as Ni foam and Ti foil through a simple solution method together with a post-annealing treatment. Remarkably, the NiCo 2O 4-Ni foam binder-free electrode exhibits greatly improved electrochemical performance with very high capacitance and excellent cycling stability. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2ee22572g
  • 2012 • 9 Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current
    Rudolf, D. and La-O-Vorakiat, C. and Battiato, M. and Adam, R. and Shaw, J.M. and Turgut, E. and Maldonado, P. and Mathias, S. and Grychtol, P. and Nembach, H.T. and Silva, T.J. and Aeschlimann, M. and Kapteyn, H.C. and Murnane, M...
    Nature Communications 3 (2012)
    Uncovering the physical mechanisms that govern ultrafast charge and spin dynamics is crucial for understanding correlated matter as well as the fundamental limits of ultrafast spin-based electronics. Spin dynamics in magnetic materials can be driven by ultrashort light pulses, resulting in a transient drop in magnetization within a few hundred femtoseconds. However, a full understanding of femtosecond spin dynamics remains elusive. Here we spatially separate the spin dynamics using Ni/Ru/Fe magnetic trilayers, where the Ni and Fe layers can be ferro-or antiferromagnetically coupled. By exciting the layers with a laser pulse and probing the magnetization response simultaneously but separately in Ni and Fe, we surprisingly find that optically induced demagnetization of the Ni layer transiently enhances the magnetization of the Fe layer when the two layer magnetizations are initially aligned parallel. Our observations are explained by a laser-generated superdiffusive spin current between the layers. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms2029
  • 2011 • 8 Can human mesenchymal stem cells survive on a NiTi implant material subjected to cyclic loading?
    Habijan, T. and Glogowski, T. and Kühn, S. and Pohl, M. and Wittsiepe, J. and Greulich, C. and Eggeler, G. and Schildhauer, T.A. and Köller, M.
    Acta Biomaterialia 7 2733-2739 (2011)
    Nickel-titanium shape memory alloys (NiTi-SMAs) exhibit mechanical and chemical properties which make them attractive candidate materials for various types of biomedical applications. However, the high nickel content of NiTi-SMAs may result in adverse tissue reactions, especially when they are considered for load-bearing implants. It is generally assumed that a protective titanium oxide layer separates the metallic alloy from its environment and that this explains the good biocompatibility of NiTi. Cyclic loading may result in failure of the protective oxide layer. The scientific objective of this work was to find out whether cyclic dynamic strain, in a range relevant for orthopedic implants, diminishes the biocompatibility of NiTi-SMAs. In order to analyze the biocompatibility of NiTi-SMA surfaces subjected to cyclic loading, NiTi-SMA tensile specimens were preloaded with mesenchymal stem cells, transferred to a sterile cell culture system and fixed to the pull rods of a tensile testing machine. Eighty-six thousand and four hundred strain cycles at 2% pseudoelastic strain were performed for a period of 24 h or 7 days. Cytokines (IL-6, IL-8 and VEGF) and nickel ion release were determined within the cell culture medium. Adherent cells on the tensile specimens were stained with calcein-AM and propidium iodide to determine cell viability. Dynamic loading of the tensile specimens did not influence the viability of adherent human mesenchymal stem cells (hMSCs) after 24 h or 7 days compared with the non-strained control. Dynamic cycles of loading and unloading did not affect nickel ion release from the tensile specimens. The release of IL-6 from hMSCs cultured under dynamic conditions was significantly higher after mechanical load (873 pg ml -1) compared with static conditions (323 pg ml-1). The present work demonstrates that a new type of mechanical in vitro cell culture experiment can provide information which previously could only be obtained in large animal experiments. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2011.02.022
  • 2011 • 7 Development of graded Ni-YSZ composite coating on Alloy 690 by Pulsed Laser Deposition technique to reduce hazardous metallic nuclear waste inventory
    Sengupta, P. and Rogalla, D. and Becker, H.W. and Dey, G.K. and Chakraborty, S.
    Journal of Hazardous Materials 192 208-221 (2011)
    Alloy 690 based 'nuclear waste vitrification furnace' components degrade prematurely due to molten glass-alloy interactions at high temperatures and thereby increase the volume of metallic nuclear waste. In order to reduce the waste inventory, compositionally graded Ni-YSZ (Y 2O 3 stabilized ZrO 2) composite coating has been developed on Alloy 690 using Pulsed Laser Deposition technique. Five different thin-films starting with Ni80YSZ20 (Ni 80wt%+YSZ 20wt%), through Ni60YSZ40 (Ni 60wt%+YSZ 40wt%), Ni40YSZ60 (Ni 40wt%+YSZ 60wt%), Ni20YSZ80 (Ni 20wt%+YSZ 80wt%) and Ni0YSZ100 (Ni 0wt%+YSZ 100wt%), were deposited successively on Alloy 690 coupons. Detailed analyses of the thin-films identify them as homogeneous, uniform, pore free and crystalline in nature. A comparative study of coated and uncoated Alloy 690 coupons, exposed to sodium borosilicate melt at 1000°C for 1-6h suggests that the graded composite coating could substantially reduced the chemical interactions between Alloy 690 and borosilicate melt. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.jhazmat.2011.05.006
  • 2010 • 6 An efficient nickel catalyst for the reduction of carbon dioxide with a borane
    Chakraborty, S. and Zhang, J. and Krause, J.A. and Guan, H.
    Journal of the American Chemical Society 132 8872-8873 (2010)
    Nickel hydride with a diphosphinite-based ligand catalyzes the highly efficient reduction of CO2 with catecholborane, and the hydrolysis of the resulting methoxyboryl species produces CH3OH in good yield. The mechanism involves a nickel formate, formaldehyde, and a nickel methoxide as different reduced stages for CO2. The reaction may also be catalyzed by an air-stable nickel formate. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja103982t
  • 2010 • 5 An in situ tensile tester for studying electrochemical repassivation behavior: Fabrication and challenges
    Neelakantan, L. and Schönberger, B. and Eggeler, G. and Hassel, A.W.
    Review of Scientific Instruments 81 (2010)
    An in situ tensile rig is proposed, which allows performing electrochemical (repassivation) experiments during dynamic mechanical testing of wires. Utilizing the basic components of a conventional tensile tester, a custom-made minitensile rig was designed and fabricated. The maximal force that can be measured by the force sensor is 80 N, with a sensitivity of 0.5 mV/V. The maximum travel range of the crosshead induced by the motor is 10 mm with a minimum step size of 0.5 nm. The functionality of the tensile test rig was validated by investigating Cu and shape memory NiTi wires. Wires of lengths between 40 and 50 mm with varying gauge lengths can be tested. An interface between wire and electrochemical setup (noncontact) with a smart arrangement of electrodes facilitated the electrochemical measurements during tensile loading. Preliminary results on the repassivation behavior of Al wire are reported. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3292685
  • 2010 • 4 Biocompatibility of nanoactuators: Stem cell growth on laser-generated nickel-titanium shape memory alloy nanoparticles
    Barcikowski, S. and Hahn, A. and Guggenheim, M. and Reimers, K. and Ostendorf, A.
    Journal of Nanoparticle Research 12 1733-1742 (2010)
    Nanoactuators made from nanoparticulate NiTi shape memory alloy show potential in the mechanical stimulation of bone tissue formation from stem cells. We demonstrate the fabrication of Ni, Ti, and NiTi shape memory alloy nanoparticles and their biocompatibility to human adipose-derived stem cells. The stoichiometry and phase transformation property of the bulk alloy is preserved during attrition by femtosecond laser ablation in liquid, giving access to colloidal nanoactuators. No adverse effect on cell growth and attachment is observed in proliferation assay and environmental electron scanning microscopy, making this material attractive for mechanical stimulation of stem cells.
    view abstractdoi: 10.1007/s11051-009-9834-4
  • 2010 • 3 Insight in structures of superbulky metallocenes with the CpBIG ligand: Theoretical considerations of decaphenyl metallocenes
    Kuchenbecker, D. and Harder, S. and Jansen, G.
    Zeitschrift fur Anorganische und Allgemeine Chemie 636 2257-2261 (2010)
    The structures of a series of decaphenyl metallocenes (Ph 5Cp)2M, which model superbulky metallocenes, are calculated by means of density functional theory including a semi-empirical correction for dispersion interactions (DFT+D). Through a detailed investigation of the calcocene it is shown that the interactions between the phenyl substituents of the two cyclopentadienyl ligands lead to a preference of S10 symmetrical structures and that dispersion interactions contribute to the overall stability of superbulky metallocenes. Whereas the Ph substituents of the two ligands bend away from each other with standard DFT, inclusion of the dispersion correction reproduces the experimentally observed slight inclination towards each other. The experimentally observed linear correlation between the out-of-plane bending angle of the phenyl substituents and the size of the metal atom M (M = Fe, Ni, Cr, W, Ca, Yb, Sn, Sm, Sr, Ba) is also confirmed at the DFT+D level. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/zaac.201000192
  • 2010 • 2 Internal exposure, effect monitoring, and lung function in welders after acute short-term exposure to welding fumes from different welding processes
    Brand, P. and Gube, M. and Gerards, K. and Bertram, J. and Kaminski, H. and John, A.C. and Kuhlbusch, T. and Wiemann, M. and Eisenbeis, C. and Winkler, R. and Kraus, T.
    Journal of Occupational and Environmental Medicine 52 887-892 (2010)
    Objective: In this study, the effect of short-term exposure to welding fumes emitted by different welding techniques on workers was investigated. Methods: In a 3-fold crossover study, six welders used three different welding techniques for 3 hours. Before and after welding, blood and urine samples were collected to perform biomonitoring of metals. Breath condensate was collected to assess inflammatory reactions, and lung function measurements were performed. Results: Welding led to a significant increase of chromium and nickel in blood and urine and of nitrate and nitrite in exhaled breath condensate. These increases were higher for manual metal arc welding with alloyed material (MAW-a). Several lung function parameters decreased after welding. This decrease was significantly higher after MAW-a. Conclusions: In respect to biological effects, MAW-a seems to be more important than other welding techniques. © 2010 by American College of Occupational and Environmental Medicine.
    view abstractdoi: 10.1097/JOM.0b013e3181f09077
  • 2010 • 1 The release of nickel from nickel-titanium (NiTi) is strongly reduced by a sub-micrometer thin layer of calcium phosphate deposited by rf-magnetron sputtering
    Surmenev, R.A. and Ryabtseva, M.A. and Shesterikov, E.V. and Pichugin, V.F. and Peitsch, T. and Epple, M.
    Journal of Materials Science: Materials in Medicine 21 1233-1239 (2010)
    Thin calcium phosphate coatings were deposited on NiTi substrates (plates) by rf-magnetron sputtering. The release of nickel upon immersion in water or in saline solution (0.9% NaCl in water) was measured by atomic absorption spectroscopy (AAS) for 42 days. The coating was analyzed before and after immersion by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). After an initial burst during the first 7 days that was observed for all samples, the rate of nickel release decreased 0.4-0.5 ng cm-2 d-1 for a 0.5 μm-thick calcium phosphate coating (deposited at 290 W). This was much less than the release from uncoated NiTi (3.4-4.4 ng cm-2 d-1). Notably, the nickel release rate was not significantly different in pure water and in aqueous saline solution. © 2010 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10856-010-3989-5