Dr. Aleksander Kostka

Institute for Materials
Ruhr-Universität Bochum

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  • Crystallographic Orientation Analysis of Nanocrystalline Tungsten Thin Film Using TEM Precession Electron Diffraction and SEM Transmission Kikuchi Diffraction
    Jeong, J. and Jang, W.-S. and Kim, K.H. and Kostka, A. and Gu, G. and Kim, Y.-M. and Oh, S.H.
    Microscopy and Microanalysis 27 (2021)
    Two advanced, automated crystal orientation mapping techniques suited for nanocrystalline materials - precession electron diffraction (PED) in transmission electron microscopy (TEM) and on-axis transmission Kikuchi diffraction (TKD) in scanning electron microscopy (SEM) - are evaluated by comparing the orientation maps obtained from the identical location on a 30 nm-thick nanocrystalline tungsten (W) thin film. A side-by-side comparison of the orientation maps directly showed that the large-scale orientation features are almost identical. However, there are differences in the fine details, which arise from the fundamentally different nature of the spot pattern and Kikuchi line pattern in terms of the excitation volume and the angular resolution. While TEM-PED is more reliable to characterize grains oriented along low-index zone axes, the high angular resolution of SEM-TKD allows the detection of small misorientation between grains and thus yields better quantification and statistical analysis of grain orientation. Given that both TEM-PED and SEM-TKD orientation mapping techniques are complementary tools for nanocrystalline materials, one can be favorably selected depending on the requirements of the analysis, as they have competitive performance in terms of angular resolution and texture quantification. Copyright © The Author(s), 2021. Published by Cambridge University Press on behalf of the Microscopy Society of America.
    view abstract10.1017/S1431927621000027
  • Influence of low Bi contents on phase transformation properties of VO2studied in a VO2:Bi thin film library
    Wang, X. and Rogalla, D. and Kostka, A. and Ludwig, Al.
    RSC Advances 11 (2021)
    A thin-film materials library in the system V-Bi-O was fabricated by reactive co-sputtering. The composition of Bi relative to V was determined by Rutherford backscattering spectroscopy, ranging from 0.06 to 0.84 at% along the library. The VO2phase M1 was detected by X-ray diffraction over the whole library, however a second phase was observed in the microstructure of films with Bi contents > 0.29 at%. The second phase was determined by electron diffraction to be BiVO4, which suggests that the solubility limit of Bi in VO2is only ∼0.29 at%. For Bi contents from 0.08 to 0.29 at%, the phase transformation temperatures of VO2:Bi increase from 74.7 to 76.4 °C by 8 K per at% Bi. With X-ray photoemission spectroscopy, the oxidation state of Bi was determined to be 3+. The V5+/V4+ratio increases with increasing Bi content from 0.10 to 0.84 at%. The similarly increasing tendency of the V5+/V4+ratio andTcwith Bi content suggests that although the ionic radius of Bi3+is much larger than that of V4+, the charge doping effect and the resulting V5+are more prominent in regulating the phase transformation behavior of Bi-doped VO2 © The Royal Society of Chemistry 2021.
    view abstract10.1039/d0ra09654g
  • Laser metal deposition of Al0.6CoCrFeNi with Ti & C additions using elemental powder blends
    Asabre, A. and Wilms, M.B. and Kostka, A. and Gemagami, P. and Weisheit, A. and Laplanche, G.
    Surface and Coatings Technology 418 (2021)
    Laser metal deposition (LMD) was used to in-situ alloy a crack-free Al0.6CoCrFeNi compositionally complex alloy (CCA) with 3 at.% Ti and 0.25 at.% C additions on an initially ferritic H10 tool steel from an elemental powder blend. After LMD, the material was annealed at 900 °C for 30 min to induce martensitic hardening in the substrate. The CCA in both as-deposited and annealed states exhibited a lamellar microstructure consisting of four phases: a matrix of interwoven disordered and ordered body-centered cubic phases, titanium carbides distributed randomly within the microstructure, and disordered face-centered cubic (FCC) plates that precipitated at the grain boundaries and grew towards the center of the grains. Chemical analyses along the build-up direction of the coating revealed a compositional gradient, similar in both as-deposited and annealed states, due to the intermixing between the substrate and the CCA. Despite a strong variation of the Fe-content, the hardness and the microstructure remain roughly constant in the major part of the as-deposited coating, which contains a large fraction of FCC plates that are beneficial to increase ductility and ensure a good compatibility with the substrate. In contrast, the upper part of the as-deposited coating, corresponding to the last solidified melt pool after LMD, has a much lower FCC fraction with an enhanced hardness. After annealing, the hardness of the tool steel substrate significantly increased and the FCC volume fraction in the coating increased from ~16% (as-deposited) to ~58%. Overall the microstructure of the coating became more homogeneous while its hardness decreased only by 10–15%. These results demonstrate that the CCA can be employed as a protective coating on a less expensive tool steel to improve its lifetime during service. © 2021 The Author(s)
    view abstract10.1016/j.surfcoat.2021.127233
  • Partitioning of Solutes at Crystal Defects in Borides After Creep and Annealing in a Polycrystalline Superalloy
    Lilensten, L. and Kostka, A. and Lartigue-Korinek, S. and Gault, B. and Tin, S. and Antonov, S. and Kontis, P.
    JOM (2021)
    We have investigated the partitioning of solutes at crystal defects in intergranular Cr-rich M2B borides after creep at 850°C/185MPa and annealing at 850°C for approximately 3000 h in a polycrystalline nickel-based superalloy. Borides were found to coarsen in both cases, with the borides after creep being the thickest (800–1100 nm), compared to borides annealed in the absence of an external applied load (400–600 nm). Transmission electron microscopy revealed that the coarsened borides have either a tetragonal I4/mcm structure, or an orthorhombic Fddd, with those two structures coexisting in a single particle. The presence of a very high density of planar faults was systematically observed within the coarsened borides. The faults were correlated with chemical fluctuations of B and Cr, revealed by atom probe tomography. In addition, partitioning of Ni and Co was observed at dislocations within the borides after creep, providing insights into the deformation of borides. © 2021, The Minerals, Metals & Materials Society.
    view abstract10.1007/s11837-021-04736-5
  • A new metalorganic chemical vapor deposition process for MoS2with a 1,4-diazabutadienyl stabilized molybdenum precursor and elemental sulfur
    Wree, J.-L. and Ciftyurek, E. and Zanders, D. and Boysen, N. and Kostka, A. and Rogalla, D. and Kasischke, M. and Ostendorf, A. and Schierbaum, K. and Devi, A.
    Dalton Transactions 49 (2020)
    Molybdenum disulfide (MoS2) is known for its versatile properties and hence is promising for a wide range of applications. The fabrication of high quality MoS2either as homogeneous films or as two-dimensional layers on large areas is thus the objective of intense research. Since industry requirements on MoS2thin films can hardly be matched by established exfoliation fabrication methods, there is an enhanced need for developing new chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes where a rational precursor selection is a crucial step. In this study, a new molybdenum precursor, namely 1,4-di-tert-butyl-1,4-diazabutadienyl-bis(tert-butylimido)molybdenum(vi) [Mo(NtBu)2(tBu2DAD)], is identified as a potential candidate. The combination of imido and chelating 1,4-diazadieneyl ligand moieties around the molybdenum metal center results in a monomeric compound possessing adequate thermal characteristics relevant for vapor phase deposition applications. Hexagonal MoS2layers are fabricated in a metalorganic CVD (MOCVD) process with elemental sulfur as the co-reactant at temperatures between 600 °C and 800 °C. The structure and composition of the films are investigated by X-ray diffraction, high resolution transmission electron microscopy, synchrotron X-ray photoelectron spectroscopy and Raman spectroscopy revealing crystalline and stoichiometric MoS2films. The new MOCVD process developed for MoS2is highly promising due to its moderate process conditions, scalability and controlled targeted composition. © The Royal Society of Chemistry 2020.
    view abstract10.1039/d0dt02471f
  • Correlation of Microstructure and Properties of Cold Gas Sprayed INCONEL 718 Coatings
    Vaßen, R. and Fiebig, J. and Kalfhaus, T. and Gibmeier, J. and Kostka, A. and Schrüfer, S.
    Journal of Thermal Spray Technology (2020)
    In the cold gas spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at temperatures well below their melting point. The high particle impact velocities and corresponding peening effects can lead to high compressive residual stresses in cold spray coatings. This can be advantageous with regard to mechanical properties as fatigue life and hence, cold spray is an ideal process for repair applications. In this study, INCONEL 718 particles were cold sprayed by using nitrogen as propellant gas. The deposited coatings with different thicknesses were characterized using electron microscopy techniques to study grain refinement and precipitates in the coating. In addition, depth-resolved residual stress measurements have been performed by the incremental hole drilling method. The residual stress depth profiles in the coatings indicate compressive residual stresses of several hundred MPa which are hardly influenced by the coating thickness. In addition, large compressive stress levels are found in surface-near regions of the substrate due to the grit blasting process. Furthermore, a post-heat treatment analysis was performed to investigate its influence on residual stresses and bonding strength. These findings are used to develop a consistent explanation of the dependence of strength values on thickness. © 2020, The Author(s).
    view abstract10.1007/s11666-020-00988-w
  • Correlative chemical and structural investigations of accelerated phase evolution in a nanocrystalline high entropy alloy
    Li, Y.J. and Kostka, A. and Savan, A. and Ludwig, Al.
    Scripta Materialia 183 (2020)
    Based on our recently-developed combinatorial processing platforms for accelerated investigations of phase evolution in multinary alloys, a novel correlative atom probe tomography and transmission electron microscopy approach is proposed to study phase stability in a nanocrystalline CrMnFeCoNi alloy. We observed that the material can decompose at 250 °C for 5 h or 300 °C for 1 h, having the same decomposed products as in its coarse-grained counterpart after annealing at 500 °C for 500 days. A low apparent activation energy for the diffusion of Ni in the nanocrystalline alloy is derived and explains the fast kinetics of phase decomposition in nanocrystalline alloys. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2020.03.016
  • Effect of Grain Statistics on Micromechanical Modeling: The Example of Additively Manufactured Materials Examined by Electron Backscatter Diffraction
    Biswas, A. and Prasad, M.R.G. and Vajragupta, N. and Kostka, A. and Niendorf, T. and Hartmaier, A.
    Advanced Engineering Materials 22 (2020)
    Micromechanical modeling is one of the prominent numerical tools for the prediction of mechanical properties and the understanding of deformation mechanisms of metals. As input parameters, it uses data obtained from microstructure characterization techniques, among which the electron backscatter diffraction (EBSD) technique allows us to understand the nature of microstructural features, that are usually described by statistics. Because of these advantages, the EBSD dataset is widely used for synthetic microstructure generation. However, for the statistical description of microstructural features, the population of input data must be considered. Preferably, the EBSD measurement area must be sufficiently large to cover an adequate number of grains. However, a comprehensive study of this measurement area with a crystal plasticity finite element method (CPFEM) framework is still missing although it would considerably facilitate information exchange between experimentalists and simulation experts. Herein, the influence of the EBSD measurement area and the number of grains on the statistical description of the microstructural features and studying the corresponding micromechanical simulation results for 316L stainless steel samples produced by selective laser melting is investigated. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adem.201901416
  • Effect of off-stoichiometric compositions on microstructures and phase transformation behavior in Ni-Cu-Pd-Ti-Zr-Hf high entropy shape memory alloys
    Piorunek, D. and Oluwabi, O. and Frenzel, J. and Kostka, A. and Maier, H.J. and Somsen, C. and Eggeler, G.
    Journal of Alloys and Compounds (2020)
    High entropy shape memory alloys (HE-SMAs) show reversible martensitic phase transformations at elevated temperatures. HE-SMAs were derived from binary NiTi, to which the elements Cu, Pd, Zr and Hf are added. They represent ordered complex solid solutions. Their high temperature phase is of B2 type, where the added elements occupy sites in the Ni-(Cu, Pd) and Ti-sub-lattices (Zr, Hf). In the present study, advanced microstructural and thermal characterization methods were used to study the effects of the additional alloy elements on microstructures and phase transformations. The ratios of Ni-equivalent (Ni, Cu, Pd) and Ti-equivalent (Ti, Zr, Hf) elements in HE-SMAs were varied to establish systems that correspond to stoichiometric, under- and over-stoichiometric binary alloys. It is shown that basic microstructural features of cast and heat-treated HE-SMAs are inherited from the nine binary X–Y subsystems (X: Ni, Cu, Pd; Y: Ti, Zr, Hf). The phase transition temperatures that characterize the martensitic forward and reverse transformations depend on the concentrations of all alloy elements. The data obtained demonstrate how martensite start temperatures are affected by deviations from the composition of an ideal stoichiometric B2 phase. The findings are discussed in the light of previous work on the concentration dependence of SMA transformation temperatures, and directions for the development of new shape memory alloy compositions are proposed. © 2020 The Authors
    view abstract10.1016/j.jallcom.2020.157467
  • Experimental and Theoretical Investigation on Phase Formation and Mechanical Properties in Cr-Co-Ni Alloys Processed Using a Novel Thin-Film Quenching Technique
    Naujoks, D. and Schneider, M. and Salomon, S. and Pfetzing-Micklich, J. and Subramanyam, A.P.A. and Hammerschmidt, T. and Drautz, R. and Frenzel, J. and Kostka, A. and Eggeler, G. and Laplanche, G. and Ludwig, Al.
    ACS Combinatorial Science 22 (2020)
    The Cr-Co-Ni system was studied by combining experimental and computational methods to investigate phase stability and mechanical properties. Thin-film materials libraries were prepared and quenched from high temperatures up to 700 °C using a novel quenching technique. It could be shown that a wide A1 solid solution region exists in the Cr-Co-Ni system. To validate the results obtained using thin-film materials libraries, bulk samples of selected compositions were prepared by arc melting, and the experimental data were additionally compared to results from DFT calculations. The computational results are in good agreement with the measured lattice parameters and elastic moduli. The lattice parameters increase with the addition of Co and Cr, with a more pronounced effect for the latter. The addition of ∼20 atom % Cr results in a similar hardening effect to that of the addition of ∼40 atom % Co. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acscombsci.9b00170
  • High-throughput characterization of Ag–V–O nanostructured thin-film materials libraries for photoelectrochemical solar water splitting
    Kumari, S. and Helt, L. and Junqueira, J.R.C. and Kostka, A. and Zhang, S. and Sarker, S. and Mehta, A. and Scheu, C. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 45 (2020)
    Ag–V–O thin-film materials libraries, with both composition (Ag22-77V23-78Ox) and thickness (123–714 nm) gradients were fabricated using combinatorial reactive magnetron co-sputtering aiming on establishing relations between composition, structure, and functional properties. As-deposited libraries were annealed in air at 300 °C for 10 h. High-throughput characterization methods of composition, structure and functional properties were used to identify photoelectrochemically active regions. The phases AgV6O15, Ag2V4O11, AgVO3, and Ag4V2O7 were observed throughout the composition gradient. The photoelectrochemical properties of Ag–V–O films are dependent on composition and morphology. An enhanced photocurrent density (~300–554 μA/cm2) was obtained at 30 to 45 at.% Ag along the thickness gradient. Thin films of these compositions show a nanowire morphology, which is an important factor for the enhancement of photoelectrochemical performance. The photoelectrochemically active regions were further investigated by high-throughput synchrotron-X-ray diffraction and transmission electron microscopy (Ag32V68Ox) which confirmed the presence of Ag2V4O11 as the dominating phase along with the minor phases AgV6O15 and AgVO3. This enhanced photoactive region shows bandgap values of ~2.30 eV for the direct and ~1.87 eV for the indirect bandgap energies. The porous nanostructured films improve charge transport and are hence of interest for photoelectrochemical water splitting. © 2020 Hydrogen Energy Publications LLC
    view abstract10.1016/j.ijhydene.2020.02.154
  • Laser metal deposition of refractory high-entropy alloys for high-throughput synthesis and structure-property characterization
    Dobbelstein, H. and George, E.P. and Gurevich, E.L. and Kostka, A. and Ostendorf, A. and Laplanche, G.
    International Journal of Extreme Manufacturing 3 (2020)
    Progress in materials development is often paced by the time required to produce and evaluate a large number of alloys with different chemical compositions. This applies especially to refractory high-entropy alloys (RHEAs), which are difficult to synthesize and process by conventional methods. To evaluate a possible way to accelerate the process, high-throughput laser metal deposition was used in this work to prepare a quinary RHEA, TiZrNbHfTa, as well as its quaternary and ternary subsystems by in-situ alloying of elemental powders. Compositionally graded variants of the quinary RHEA were also analyzed. Our results show that the influence of various parameters such as powder shape and purity, alloy composition, and especially the solidification range, on the processability, microstructure, porosity, and mechanical properties can be investigated rapidly. The strength of these alloys was mainly affected by the oxygen and nitrogen contents of the starting powders, while substitutional solid solution strengthening played a minor role. © 2020 The Author(s). Published by IOP Publishing Ltd
    view abstract10.1088/2631-7990/abcca8
  • Phase decomposition in a nanocrystalline CrCoNi alloy
    Li, Y.J. and Kostka, A. and Savan, A. and Ludwig, Al.
    Scripta Materialia 188 (2020)
    Phase stability of a nanocrystalline CrCoNi alloy is investigated using the combinatorial processing platform approach, which enables synthesis, processing and direct atomic-scale characterizations of alloys by atom probe tomography and transmission electron microscopy. Phase decomposition with formation of CoNi-rich phase occurs faster in the smaller (10 nm) grain-sized region than the larger one (20 nm), both being present in the same sample. Chemical analyses indicate that diffusion of Co and Cr plays an important role in phase decomposition. Comparison of phase stability between CrMnFeCoNi and CrCoNi implies that elemental segregation may promote phase decomposition by providing an additional chemical driving force for it. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2020.07.054
  • 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 (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 abstract10.1016/j.actamat.2020.09.055
  • Structure zone investigation of multiple principle element alloy thin films as optimization for nanoindentation measurements
    Savan, A. and Allermann, T. and Wang, X. and Grochla, D. and Banko, L. and Kalchev, Y. and Kostka, A. and Pfetzing-Micklich, J. and Ludwig, Al.
    Materials 13 (2020)
    Multiple principal element alloys, also often referred to as compositionally complex alloys or high entropy alloys, present extreme challenges to characterize. They show a vast, multidimensional composition space that merits detailed investigation and optimization to identify compositions and to map the composition ranges where useful properties are maintained. Combinatorial thin film material libraries are a cost-effective and efficient way to create directly comparable, controlled composition variations. Characterizing them comes with its own challenges, including the need for high-speed, automated measurements of dozens to hundreds or more compositions to be screened. By selecting an appropriate thin film morphology through predictable control of critical deposition parameters, representative measured values can be obtained with less scatter, i.e., requiring fewer measurement repetitions for each particular composition. In the present study, equiatomic CoCrFeNi was grown by magnetron sputtering in different locations in the structure zone diagram applied to multinary element alloys, followed by microstructural and morphological characterizations. Increasing the energy input to the deposition process by increased temperature and adding high-power impulse magnetron sputtering (HiPIMS) plasma generators led to denser, more homogeneous morphologies with smoother surfaces until recrystallization and grain boundary grooving began. Growth at 300 ffiC, even without the extra particle energy input of HiPIMS generators, led to consistently repeatable nanoindentation load-displacement curves and the resulting hardness and Young's modulus values. © 2020 by the authors.
    view abstract10.3390/ma13092113
  • Columnar to equiaxed transition and grain refinement of cast CrCoNi medium-entropy alloy by microalloying with titanium and carbon
    Liu, X.W. and Laplanche, G. and Kostka, A. and Fries, S.G. and Pfetzing-Micklich, J. and Liu, G. and George, E.P.
    Journal of Alloys and Compounds 775 (2019)
    Thermomechanical processing has been used to control the grain size/shape of the equiatomic CrCoNi medium-entropy alloy (MEA) and obtain excellent strength and ductility. However, in the cast state, the alloy has coarse columnar grains with average widths and lengths of approximately 120 and 1000 μm, respectively, resulting in inferior mechanical properties. To overcome this deficiency, here we microalloyed with Ti and C and successfully changed the grain shape (from columnar to equiaxed) and refined the grain size. The degree to which the microstructure changes depends on the amount of Ti and C added, with the best results obtained at 0.4 at.% each. In the optimal alloy [(CrCoNi)99.2Ti0.4C0.4], the as-cast grains were nearly equiaxed with a uniform size of ∼75 μm. Associated with this change in grain shape/size was a significant improvement of yield strength, ultimate tensile strength and elongation to fracture at both 293 and 77 K. The columnar to equiaxed transition is attributed to the strong mutual affinity of C and Ti, which leads to their build-up ahead of the solid-liquid interface and, in turn, to enhanced constitutional undercooling. © 2018 Elsevier B.V.
    view abstract10.1016/j.jallcom.2018.10.187
  • Composition of the nanosized orthorhombic O′ phase and its direct transformation to fine α during ageing in metastable β-Ti alloys
    Li, T. and Lai, M. and Kostka, A. and Salomon, S. and Zhang, S. and Somsen, C. and Dargusch, M.S. and Kent, D.
    Scripta Materialia 170 (2019)
    The structure and chemistry of the orthorhombic O′ phase after quenching of a Ti-23 at.%Nb-2 at.%O was measured using aberration-corrected transmission electron microscopy and atom probe tomography. The nanosized O′ phase, formed in the vicinity of ω, is enriched with oxygen and slightly depleted in Nb. Upon annealing, ω dissolves and the O′ phase develops in β up to 350 °C, above which temperature it transforms to colonies of fine α phase. Another needle-like form of α with lower Nb content is thought to nucleate from Nb-lean regions related to spinodal decomposition of β. © 2019 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2019.06.008
  • Data regarding the influence of Al, Ti, and C additions to as-cast Al0.6CoCrFeNi compositionally complex alloys on microstructures and mechanical properties
    Asabre, A. and Pfetzing-Micklich, J. and Stryzhyboroda, O. and Kostka, A. and Hecht, U. and Laplanche, G.
    Data in Brief 27 (2019)
    This brief paper contains raw data of X-ray diffraction (XRD) measurements, microstructural characterization, chemical compositions, and mechanical properties describing the influence of Al, Ti, and C on as-cast Al0.6CoCrFeNi compositionally complex alloys (CCAs). The presented data are related to the research article in reference [1] and therefore this article can be referred to as for the interpretation of the data. X-ray diffraction data presented in this paper are measurements of 2θ versus intensities for each studied alloy. A Table lists the obtained lattice parameters of each identified phase determined by Rietveld analysis. Microstructural-characterization data reported here include backscattered electron (BSE) micrographs taken at different magnifications in a scanning electron microscope (SEM) of Widmanstätten and dendritic microstructures and microstructural parameters such as phase volume fractions, thickness of face-centered cubic (FCC) plates, and prior grain sizes. The compositions of the identified individual phases determined by energy-dispersive X-ray spectroscopy (EDX) in the transmission electron microscope (TEM) are listed as well. Finally, mechanical data including engineering stress-strain curves obtained at different temperatures (room temperature, 400 °C, and 700 °C) for all CCAs are reported. © 2019 The Authors
    view abstract10.1016/j.dib.2019.104742
  • Effect of Al, Ti and C additions on Widmanstätten microstructures and mechanical properties of cast Al0.6CoCrFeNi compositionally complex alloys
    Asabre, A. and Kostka, A. and Stryzhyboroda, O. and Pfetzing-Micklich, J. and Hecht, U. and Laplanche, G.
    Materials and Design 184 (2019)
    The cast microstructure of the Al0.6CoCrFeNi compositionally complex alloy was successfully refined with small additions of Al, Ti and C and its mechanical properties were optimized. In the as-cast state, this alloy has a Widmanstätten microstructure with coarse grains (∼110 μm) of a strong BCC/B2 matrix and soft FCC plates (∼65 vol.%) with large widths (∼1.3 μm). The addition of 0.25 at.% C to Al0.6CoCrFeNi stabilizes the FCC phase and favors the formation of a coarse dendritic microstructure making this alloy unsuitable for structural applications. In contrast, alloying of either 3 at.% Al, Ti, or 3% Ti and 0.25% C to Al0.6CoCrFeNi refined its Widmanstätten microstructure, i.e. the thickness of the FCC plates and/or the size of the prior BCC/B2 grains were significantly reduced. As a result of these microstructural changes, Al and Ti containing alloys show an outstanding strength (twice higher than that of Al0.6CoCrFeNi) and ductilities ≤5% at 20 °C. These properties are retained at 400 °C but at 700 °C, the strength and ductility of almost all alloys decrease. However, Ti containing alloys exhibit much larger ductilities (∼50%) at 700 °C due to their high density of grain boundaries which accommodate plastic deformation through grain boundary sliding. © 2019 The Authors
    view abstract10.1016/j.matdes.2019.108201
  • Effects of the Ion to Growth Flux Ratio on the Constitution and Mechanical Properties of Cr1- x-Alx-N Thin Films
    Banko, L. and Ries, S. and Grochla, D. and Arghavani, M. and Salomon, S. and Pfetzing-Micklich, J. and Kostka, A. and Rogalla, D. and Schulze, J. and Awakowicz, P. and Ludwig, Al.
    ACS Combinatorial Science 21 (2019)
    Cr-Al-N thin film materials libraries were synthesized by combinatorial reactive high power impulse magnetron sputtering (HiPIMS). Different HiPIMS repetition frequencies and peak power densities were applied altering the ion to growth flux ratio. Moreover, time-resolved ion energy distribution functions were measured with a retarding field energy analyzer (RFEA). The plasma properties were measured during the growth of films with different compositions within the materials library and correlated to the resulting film properties such as phase, grain size, texture, indentation modulus, indentation hardness, and residual stress. The influence of the ion to growth flux ratio on the film properties was most significant for films with high Al-content (xAl = 50 at. %). X-ray diffraction with a 2D detector revealed hcp-AlN precipitation starting from Al-concentration xAl ≥ 50 at. %. This precipitation might be related to the kinetically enhanced adatom mobility for a high ratio of ions per deposited atoms, leading to strong intermixing of the deposited species. A structure zone transition, induced by composition and flux ratio JI/JG, from zone T to zone Ic structure was observed which hints toward the conclusion that the combination of increasing flux ratio and Al-concentration lead to opposing trends regarding the increase in homologous temperature. © 2019 American Chemical Society.
    view abstract10.1021/acscombsci.9b00123
  • Influence of composition and precipitation evolution on damage at grain boundaries in a crept polycrystalline Ni-based superalloy
    Kontis, P. and Kostka, A. and Raabe, D. and Gault, B.
    Acta Materialia 166 (2019)
    The microstructural and compositional evolution of intergranular carbides and borides prior to and after creep deformation at 850 °C in a polycrystalline nickel-based superalloy was studied. Primary MC carbides, enveloped within intergranular γ′ layers, decomposed resulting in the formation of layers of the undesirable η phase. These layers have a composition corresponding to Ni3Ta as measured by atom probe tomography and their structure is consistent with the D024 hexagonal structure as revealed by transmission electron microscopy. Electron backscattered diffraction reveals that they assume various misorientations with regard to the adjacent grains. As a consequence, these layers act as brittle recrystallized zones and crack initiation sites. The composition of the MC carbides after creep was altered substantially, with the Ta content decreasing and the Hf and Zr contents increasing, suggesting a beneficial effect of Hf and Zr additions on the stability of MC carbides. By contrast, M5B3 borides were found to be microstructurally stable after creep and without substantial compositional changes. Borides at 850 °C were found to coarsen, resulting in some cases into γ′- depleted zones, where, however, no cracks were observed. The major consequences of secondary phases on the microstructural stability of superalloys during the design of new polycrystalline superalloys are discussed. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.12.039
  • Misorientation-dependent solute enrichment at interfaces and its contribution to defect formation mechanisms during laser additive manufacturing of superalloys
    Hariharan, A. and Lu, L. and Risse, J. and Kostka, A. and Gault, B. and Jägle, E.A. and Raabe, D.
    Physical Review Materials 3 (2019)
    A vital issue during selective laser melting of nonweldable polycrystalline nickel-base superalloys is the formation of microcracks. These are cracks occurring during the last stage of solidification and only at high angle grain boundaries (HAGBs). Solute enrichment to the remaining interdendritic liquid and its partial back-diffusion into the solid contributes to the crack nucleation mechanism. Here we use atom probe tomography coupled with transmission Kikuchi diffraction to determine the misorientation and chemical composition profiles across HAGBs (with and without cracks) and across crack-free low angle grain boundaries (LAGBs). The Gibbsian interfacial excess of solutes (mainly B, C, Si, and Zr) is at least two times higher at the HAGB compared to the LAGB. The chemical profiles show the opposite behavior to established model predictions of the last stage of solidification. Our diffusion calculations elucidate that the chemical profiles are influenced by both microsegregation (of Ti, Nb, and Si) during solidification and solid-state segregation (of B, C, and Zr) during cooling. The chemical profiles in the topmost layer indicate a negligible effect of remelting and reheating. Except for Ti-rich carbides, no secondary phases are found. Additionally, we study an alloy with a reduced content of Zr and Si (by at least 60 wt. %), relative to the standard IN738LC composition. We achieved a 99% reduction in crack length per unit area. However, the grain boundary enrichment of Zr and Si in the modified alloy was similar to the standard alloy. Based on these findings, we critically discuss the contribution of various mechanisms proposed for solidification cracking. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.123602
  • PEALD of HfO2 Thin Films: Precursor Tuning and a New Near-Ambient-Pressure XPS Approach to in Situ Examination of Thin-Film Surfaces Exposed to Reactive Gases
    Zanders, D. and Ciftyurek, E. and Subaşl, E. and Huster, N. and Bock, C. and Kostka, A. and Rogalla, D. and Schierbaum, K. and Devi, A.
    ACS Applied Materials and Interfaces 11 (2019)
    A bottom-up approach starting with the development of new Hf precursors for plasma-enhanced atomic layer deposition (PEALD) processes for HfO2 followed by in situ thin-film surface characterization of HfO2 upon exposure to reactive gases via near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is reported. The stability of thin films under simulated operational conditions is assessed, and the successful implementation of HfO2 dielectric layers in metal-insulator-semiconductor (MIS) capacitors is demonstrated. Among the series of newly synthesized mono-guanidinato-tris-dialkyl-amido class of Hf precursors, one of them, namely, [Hf{2-(iPrN)2CNEtMe}(NEtMe)3], was representatively utilized with oxygen plasma, resulting in a highly promising low-temperature PEALD process at 60 °C. The new precursors were synthesized in the multigram scale and thoroughly characterized by thermogravimetric analyses, revealing high and tunable volatility reflected by appreciable vapor pressures and accompanied by thermal stability. Typical ALD growth characteristics in terms of linearity, saturation, and a broad ALD window with constant growth of 1.06 Å cycle-1 in the temperature range of 60-240 °C render this process very promising for fabricating high-purity smooth HfO2 layers. For the first time, NAP-XPS surface studies on selected HfO2 layers are reported upon exposure to reactive H2, O2, and H2O atmospheres at temperatures of up to 500 °C revealing remarkable stability against degradation. This can be attributed to the absence of surface defects and vacancies. On the basis of these promising results, PEALD-grown HfO2 films were used as dielectric layers in the MIS capacitor device fabrication exhibiting leakage current densities less than 10-7 A cm-2 at 2 MV cm-1 and permittivities of up to 13.9 without postannealing. © 2019 American Chemical Society.
    view abstract10.1021/acsami.9b07090
  • Stress-induced formation of TCP phases during high temperature low cycle fatigue loading of the single-crystal Ni-base superalloy ERBO/1
    Meid, C. and Eggeler, M. and Watermeyer, P. and Kostka, A. and Hammerschmidt, T. and Drautz, R. and Eggeler, G. and Bartsch, M.
    Acta Materialia 168 (2019)
    The microstructural evolution in the single crystal Ni-base superalloy ERBO/1 (CMSX 4 type) is investigated after load controlled low cycle fatigue (LCF) at 950 °C (load-ratio: 0.6, tensile stress range: 420–740 MPa, test frequency: 0.25 Hz, fatigue rupture life: about 1000 - 3000 cycles). Bulk topologically close packed (TCP) phase particles precipitated and were analyzed by three-dimensional focus ion beam slice and view imaging and analytical transmission electron microscopy. The particles did not precipitate homogenously but at locations with enhanced levels of local stresses/strains, such as isolated γ-channels subjected to cross channel stresses, shear bands and in front of micro cracks. The influence of stress/strain is furthermore apparent in the spatial arrangement and the shape of the TCP phase particles. Only μ-phase TCP particles were found by electron diffraction. Results of a structure-map analysis suggest that most of these TCP particles observed after LCF testing would not precipitate in thermodynamic equilibrium. In order to rationalize this effect, the atomic volume was analyzed that transition-metal (TM) elements take in unary fcc and in unary μ-phase crystal structures and found that all TM elements except Zr and V take a larger volume in a unary μ phase than in a unary fcc phase. This trend is in line with the observed localized precipitation of TCP phases that are rich in Ni and other late TM elements. The experimental and theoretical findings suggest consistently that formation of TCP particles in LCF tests is considerably influenced by the local tensile stress/strain states. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2019.02.022
  • Validation of a Terminally Amino Functionalized Tetra-Alkyl Sn(IV) Precursor in Metal–Organic Chemical Vapor Deposition of SnO2 Thin Films: Study of Film Growth Characteristics, Optical, and Electrical Properties
    Zanders, D. and Ciftyurek, E. and Hoppe, C. and de los Arcos, T. and Kostka, A. and Rogalla, D. and Grundmeier, G. and Schierbaum, K.D. and Devi, A.
    Advanced Materials Interfaces 6 (2019)
    Tin(IV) oxide is a promising semiconductor material with leading-edge properties toward chemical sensing and other applications. For the growth of its thin films, metal–organic chemical vapor deposition (MOCVD) routes are advantageous due to their excellent scalability and potential to tune processing temperatures by careful choice of the reactants. Herein, a new and highly efficient MOCVD process for the deposition of tin(IV) oxide thin films employing a terminally amino alkyl substituted tin(IV) tetra-alkyl compound is reported for the first time. The liquid precursor, tetrakis-[3-(N,N-dimethylamino)propyl] tin(IV), [Sn(DMP)4], is thermally characterized in terms of stability and vapor pressure, yielding highly pure, polycrystalline tin(IV) oxide thin films with tunable structural and morphological features in the presence of oxygen. Detailed X-ray photoelectron spectroscopy (XPS) analysis reveals the presence of oxygen vacancies and high amounts of chemisorbed oxygen species. Based on these promising features, the MOCVD process is optimized toward downscaling the thickness of tin(IV) oxide films from 25 to 50 nm to study the impact of incipient surface morphological changes occurring after initial thin-film formation on the electrical properties as investigated by van der Pauw (vdP) resistivity measurements. Optical bandgaps of thin films with varying thicknesses are estimated using ultraviolet–visible (UV–vis) spectroscopy. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/admi.201801540
  • Accelerated atomic-scale exploration of phase evolution in compositionally complex materials
    Li, Y.J. and Savan, A. and Kostka, A. and Stein, H.S. and Ludwig, Al.
    Materials Horizons 5 (2018)
    Combining nanoscale-tip arrays with combinatorial thin film deposition and processing as well as direct atomic-scale characterization (APT and TEM) enables accelerated exploration of the temperature- and environment-dependent phase evolution in multinary materials systems. Results from nanocrystalline CrMnFeCoNi show that this alloy is unstable and already decomposes after 1 h at low temperatures of around 300 °C. The combinatorial processing platform approach is extendible to explore oxidation and corrosion in complex structural and functional materials on the atomic scale. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c7mh00486a
  • Atomic-scale investigation of fast oxidation kinetics of nanocrystalline CrMnFeCoNi thin films
    Li, Y.J. and Kostka, A. and Savan, A. and Ludwig, Al.
    Journal of Alloys and Compounds 766 (2018)
    Atom probe tomography was combined with transmission electron microscopy to characterize in atomic detail the structure of nanocrystalline high entropy alloy CrMnFeCoNi thin films before and after exposure to air at 500 °C for 5 min. Mn and Cr oxide scales were observed on the surface of the sample. These results on the nanoscale for short experimental time agree with literature reports on bulk counterparts after oxidation at 900 °C for 100 h, which means that the oxidation performance of complex materials can be tested in an accelerated way. Moreover, oxidation-related sub-surface depletion of Mn and Cr together with partial decomposition of the initial Cantor phase yield a FeCo-B2 sub-surface phase. In comparison to coarse-grained bulk material, the faster oxidation of the nanocrystalline material was attributed to an enhanced outwards diffusion of Cr and Mn along grain boundaries and within the newly formed B2 phase. © 2018 Elsevier B.V.
    view abstract10.1016/j.jallcom.2018.07.048
  • Combinatorial Synthesis and High-Throughput Characterization of Fe-V-O Thin-Film Materials Libraries for Solar Water Splitting
    Kumari, S. and Gutkowski, R. and Junqueira, J.R.C. and Kostka, A. and Hengge, K. and Scheu, C. and Schuhmann, W. and Ludwig, Al.
    ACS Combinatorial Science 20 (2018)
    The search for suitable materials for solar water splitting is addressed with combinatorial material science methods. Thin film Fe-V-O materials libraries were synthesized using combinatorial reactive magnetron cosputtering and subsequent annealing in air. The design of the libraries comprises a combination of large compositional gradients (from Fe10V90Ox to Fe79V21Ox) and thickness gradients (from 140 to 425 nm). These material libraries were investigated by high-throughput characterization techniques in terms of composition, structure, optical, and photoelectrochemical properties to establish correlations between composition, thickness, crystallinity, microstructure, and photocurrent density. Results show the presence of the Fe2V4O13 phase from ∼11 to 42 at. % Fe (toward low-Fe region) and the FeVO4 phase from ∼37 to 79 at. % Fe (toward Fe-rich region). However, as a third phase, Fe2O3 is present throughout the compositional gradients (from low-Fe to Fe-rich region). Material compositions with increasing crystallinity of the FeVO4 phase show enhanced photocurrent densities (∼160 to 190 μA/cm2) throughout the thickness gradients whereas compositions with the Fe2V4O13 phase show comparatively low photocurrent densities (∼28 μA/cm2). The band gap energies of Fe-V-O films were inferred from Tauc plots. The highest photocurrent density of ∼190 μA/cm2 was obtained for films with ∼54 to 66 at. % Fe for the FeVO4 phase with ∼2.04 eV for the indirect and ∼2.80 eV for the direct band gap energies. © 2018 American Chemical Society.
    view abstract10.1021/acscombsci.8b00030
  • How evolving multiaxial stress states affect the kinetics of rafting during creep of single crystal Ni-base superalloys
    Cao, L. and Wollgramm, P. and Bürger, D. and Kostka, A. and Cailletaud, G. and Eggeler, G.
    Acta Materialia 158 (2018)
    Miniature tensile creep specimens are used to investigate the effect of mild circular notches on microstructural evolution during [001] tensile creep of a Ni-base single crystal superalloy. Creep deformed material states from a uniaxial (950 °C, uniaxial stress: 300 MPa) and a circular notched creep specimen (950 °C, net section stress in notch root: 300 MPa) are compared. For both types of tests, creep experiments were interrupted after 81, 169 and 306 h. Quantitative scanning electron microscopy (SEM) is used to assess the evolution of the γ/γ′-microstructure from rafting to topological inversion. Scanning transmission electron microscopy (STEM) was applied to study the evolution of dislocation densities during creep. As a striking new result it is shown that in circular notched specimen, the microstructural evolution is well coupled to the kinetics of the stress redistribution during creep. Rafting, the directional coarsening of the γ′-phase, and the increase of γ-channel dislocation density, start in the notch root before the center of the specimen is affected. When stresses in the circular notched specimens are fully redistributed, the microstructural differences between the notch root and the center of the circular notched specimen disappear. The comparison of the mechanical data and the microstructural findings in uniaxial and circular notched specimens contribute to a better understanding of the role of mild notches, of stress multiaxiality and of strain accumulation in the microstructure evolution of single crystal Ni-base superalloys during creep. The results obtained in the present work are discussed in the light of previous work published in the literature. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.07.061
  • Microstructure and mechanical properties in the thin film system Cu-Zr
    Oellers, T. and Raghavan, R. and Chakraborty, J. and Kirchlechner, C. and Kostka, A. and Liebscher, C.H. and Dehm, G. and Ludwig, Al.
    Thin Solid Films 645 (2018)
    A composition-spread Cu-Zr thin film library with Zr contents from 2.5 up to 6.5 at.% was synthesized by magnetron sputtering on a thermally oxidized Si wafer. The compositional and microstructural variations of the Cu-Zr thin film across the composition gradient were examined using energy dispersive X-ray spectroscopy, X-ray diffraction, and high-resolution scanning and transmission electron microscopy of cross-sections fabricated by focused ion beam milling. Composition-dependent hardness and elastic modulus values were obtained by nanoindentation for measurement areas with discrete Zr contents along the composition gradient. Similarly, the electrical resistivity was investigated by 4-point resistivity measurements to study the influence of Zr composition and microstructural changes in the thin film. Both, the mechanical and electrical properties reveal a significant increase in hardness and resistivity with increasing Zr content. The trends of the mechanical and functional properties are discussed with respect to the local microstructure and composition of the thin film library. © 2017
    view abstract10.1016/j.tsf.2017.10.030
  • Microstructure and mechanical properties of Al0.7CoCrFeNi high-entropy-alloy prepared by directional solidification
    Liu, G. and Liu, L. and Liu, X. and Wang, Z. and Han, Z. and Zhang, G. and Kostka, A.
    Intermetallics 93 (2018)
    The high-entropy-alloy Al0.7CoCrFeNi (molar ratio) was prepared by vacuum arc melting followed by directional solidification (DS) with <001> oriented seed. The unique lamellar-dendrite microstructure was obtained over a wide cooling rate range. During solidification, Fe and Co are prone to segregate to the dendrite, while Cr and Al segregate to interdendrite. The solute pile-up of Cr and Al at the solid/liquid interface leads to the dendritic solidification. During the following cooling process, the BCC phase precipitates from the FCC dendrite to form the lamellar structure, while the ordered B2 phase precipitates from the interdendrite. Moreover, the lamellar spacing is significantly refined with increasing cooling rate, resulting in the higher hardness and compressive yield strength. Directional solidification is proved to be an efficient way to improve the mechanical properties of multi-phases high-entropy alloys. © 2017 Elsevier Ltd
    view abstract10.1016/j.intermet.2017.11.019
  • Multiscale Characterization of Microstructure in Near-Surface Regions of a 16MnCr5 Gear Wheel After Cyclic Loading
    Medghalchi, S. and Jamebozorgi, V. and Bala Krishnan, A. and Vincent, S. and Salomon, S. and Basir Parsa, A. and Pfetzing, J. and Kostka, A. and Li, Y. and Eggeler, G. and Li, T.
    JOM (2018)
    The dependence of the microstructure on the degree of deformation in near-surface regions of a 16MnCr5 gear wheel after 2.1 × 106 loading cycles has been investigated by x-ray diffraction analysis, transmission electron microscopy, and atom probe tomography. Retained austenite and large martensite plates, along with elongated lamella-like cementite, were present in a less deformed region. Comparatively, the heavily deformed region consisted of a nanocrystalline structure with carbon segregation up to 2 at.% at grain boundaries. Spheroid-shaped cementite, formed at the grain boundaries and triple junctions of the nanosized grains, was enriched with Cr and Mn but depleted with Si. Such partitioning of Cr, Mn, and Si was not observed in the elongated cementite formed in the less deformed zone. This implies that rolling contact loading induced severe plastic deformation as well as a pronounced annealing effect in the active contact region of the toothed gear during cyclic loading. © 2018 The Minerals, Metals & Materials Society
    view abstract10.1007/s11837-018-2931-z
  • 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 (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 abstract10.1016/j.actamat.2017.09.063
  • Phase stability and kinetics of σ-phase precipitation in CrMnFeCoNi high-entropy alloys
    Laplanche, G. and Berglund, S. and Reinhart, C. and Kostka, A. and Fox, F. and George, E.P.
    Acta Materialia 161 (2018)
    Although the phase stability of high-entropy alloys in the Cr-Mn-Fe-Co-Ni system has received considerable attention recently, knowledge of their thermodynamic equilibrium states and precipitation kinetics during high-temperature exposure is limited. In the present study, an off-equiatomic Cr26Mn20Fe20Co20Ni14 high-entropy alloy was solutionized and isothermally aged at temperatures between 600 °C and 1000 °C for times to 1000 h. In the original single-phase fcc matrix, an intermetallic σ phase was found to form at all investigated temperatures. Its morphology and composition were determined and the precipitation kinetics analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a time-temperature-transformation diagram (TTT diagram) is constructed for this off-equiatomic alloy. We combine our findings with theories of precipitation kinetics developed for traditional polycrystalline fcc alloys to calculate a TTT diagram for the equiatomic CrMnFeCoNi HEA. The results of our investigation may serve as a guide to predict precipitation kinetics in other complex alloys in the Cr-Mn-Fe-Co-Ni system. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2018.09.040
  • Rejuvenation of Single-Crystal Ni-Base Superalloy Turbine Blades: Unlimited Service Life?
    Ruttert, B. and Horst, O. and Lopez-galilea, I. and Langenkämper, D. and Kostka, A. and Somsen, C. and Goerler, J.V. and Ali, M.A. and Shchyglo, O. and Steinbach, I. and Eggeler, G. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science (2018)
    Rejuvenation of the initially hot isostatic pressing (HIP) heat-treated single-crystal Ni-base superalloy (SX) ERBO/1 was examined experimentally and via phase field simulation to establish rejuvenation treatments as a cost-effective alternative for another interval of service life. Creep was performed at 950 °C and 350 MPa, and the specimens were crept to 0.6 pct (creep rate minimum) or 2 pct strain, respectively. A slight coarsening of the γ/γ′ microstructure was observed experimentally and via simulation at 0.6 pct and rafting at 2 pct strain. The damaged microstructures were rejuvenated in a novel hot isostatic press that provides fast quenching rates before the same specimens were recrept under the same initial creep conditions. High-resolution microscopy proves that the rejuvenation re-establishes the original γ/γ′ microstructure in the dendrite core of the precrept specimens (0.6 and 2 pct). However, the interdendritic areas of the 2 pct precrept and rejuvenated specimen still contain elongated γ′ particles enwrapped by interfacial dislocation networks that survived the applied rejuvenation. The subsequent experimental and simulated creep tests after rejuvenation demonstrated that the creep behavior is only reproducible by the proposed rejuvenation for specimens that had crept until the end of the primary creep regime. © 2018 The Minerals, Metals & Materials Society and ASM International
    view abstract10.1007/s11661-018-4745-6
  • 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 (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 abstract10.1021/acs.jpcb.7b06984
  • Understanding precipitate evolution during friction stir welding of Al-Zn-Mg-Cu alloy through in-situ measurement coupled with simulation
    dos Santos, J.F. and Staron, P. and Fischer, T. and Robson, J.D. and Kostka, A. and Colegrove, P. and Wang, H. and Hilgert, J. and Bergmann, L. and Hütsch, L.L. and Huber, N. and Schreyer, A.
    Acta Materialia 148 (2018)
    Friction Stir Welding (FSW) imparts both heat and deformation to the metal being joined, producing profound microstructural changes that determine the weld properties. In the case of welding of aerospace aluminium alloys, the most important change is the modification of the size, nature, and fraction of strengthening precipitates. To understand these changes requires the ability to measure the microstructural evolution during the welding process. This paper describes a new tool, the FlexiStir system, a portable friction stir unit designed for use in a high-energy synchrotron beamline that enables in-situ studies of microstructural evolution during FSW. FlexiStir has been used to measure precipitate evolution during FSW of aluminium alloy 7449-TAF and provide time-resolved measurement of precipitate size and volume fraction via small angle X-ray scattering (SAXS). These measurements have been interpreted with the aid of a previously developed microstructural model. The model predictions and SAXS measurements are in good qualitative agreement and demonstrate the complex precipitate transformation, dissolution, and reprecipitation events that occur during welding. © 2018
    view abstract10.1016/j.actamat.2018.01.020
  • Coarsening of Y-rich oxide particles in 9%Cr-ODS Eurofer steel annealed at 1350 °C
    Sandim, M.J.R. and Souza Filho, I.R. and Bredda, E.H. and Kostka, A. and Raabe, D. and Sandim, H.R.Z.
    Journal of Nuclear Materials 484 (2017)
    Oxide-dispersion strengthened (ODS) Eurofer steel is targeted for structural applications in future fusion nuclear reactors. Samples were cold rolled down to 80% reduction in thickness and annealed at 1350 °C up to 8 h. The microstructural characterization was performed using Vickers microhardness testing, electron backscatter diffraction, scanning and scanning transmission electron microscopies. Experimental results provide evidence of coarsening of the Y-rich oxide particles in ODS-Eurofer steel annealed at 1350 °C within delta ferrite phase field. © 2016 Elsevier B.V.
    view abstract10.1016/j.jnucmat.2016.12.025
  • Kinetics and crystallization path of a Fe-based metallic glass alloy
    Duarte, M.J. and Kostka, A. and Crespo, D. and Jimenez, J.A. and Dippel, A.-C. and Renner, F.U. and Dehm, G.
    Acta Materialia 127 (2017)
    The thermal stability and the quantification of the different transformation processes involved in the overall crystallization of the Fe50Cr15Mo14C15B6 amorphous alloy were investigated by several characterization techniques. Formation of various metastable and stable phases during the devitrification process in the sequence α-Fe, χ-Cr6Fe18Mo5, M23(C,B)6, M7C3, η-Fe3Mo3C and FeMo2B2 (with M = Fe, Cr, Mo), was observed by in-situ synchrotron high energy X-ray diffraction and in-situ transmission electron microscopy. By combining these techniques with differential scanning calorimetry data, the crystallization states and their temperature range of stability under continuous heating were related with the evolution of the crystallized fraction and the phase sequence as a function of temperature, revealing structural and chemical details of the different transformation mechanisms. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.01.031
  • Microstructure evolution in refill friction stir spot weld of a dissimilar Al–Mg alloy to Zn-coated steel
    Suhuddin, U.F.H. and Fischer, V. and Kostka, A. and dos Santos, J.F.
    Science and Technology of Welding and Joining (2017)
    In the present study, dissimilar welds of an Al–Mg–Mn alloy and a Zn-coated high-strength low-alloy steel were welded by refill friction stir spot welding. The maximum shear load recorded was approximately 7.8 kN, obtained from the weld produced with a 1600 rev min−1 tool rotational speed. Microstructural analyses showed the formation of a solid–liquid structure of an Al solid solution in Mg–Al-rich Zn liquid, which gives rise to the formation of Zn-rich Al region and microfissuring in some regions during welding. Exposure of steel surface to Mg–Al-rich Zn liquid led to the formation of Fe2Al5 and Fe4Al13 intermetallics. The presence of defective Zn-rich Al regions and Fe–Al intermetallics at the faying surface affects the weld strength. © 2017 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute
    view abstract10.1080/13621718.2017.1300744
  • Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi
    Laplanche, G. and Kostka, A. and Reinhart, C. and Hunfeld, J. and Eggeler, G. and George, E.P.
    Acta Materialia 128 (2017)
    The tensile properties of CrCoNi, a medium-entropy alloy, have been shown to be significantly better than those of CrMnFeCoNi, a high-entropy alloy. To understand the deformation mechanisms responsible for its superiority, tensile tests were performed on CrCoNi at liquid nitrogen temperature (77 K) and room temperature (293 K) and interrupted at different strains. Microstructural analyses by transmission electron microscopy showed that, during the early stage of plasticity, deformation occurs by the glide of 1/2&lt;110&gt; dislocations dissociated into 1/6&lt;112&gt; Shockley partials on {111} planes, similar to the behavior of CrMnFeCoNi. Measurements of the partial separations yielded a stacking fault energy of 22 ± 4 mJ m−2, which is ∼25% lower than that of CrMnFeCoNi. With increasing strain, nanotwinning appears as an additional deformation mechanism in CrCoNi. The critical resolved shear stress for twinning in CrCoNi with 16 μm grain size is 260 ± 30 MPa, roughly independent of temperature, and comparable to that of CrMnFeCoNi having similar grain size. However, the yield strength and work hardening rate of CrCoNi are higher than those of CrMnFeCoNi. Consequently, the twinning stress is reached earlier (at lower strains) in CrCoNi. This in turn results in an extended strain range where nanotwinning can provide high, steady work hardening, leading to the superior mechanical properties (ultimate strength, ductility, and toughness) of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.02.036
  • 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 (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 abstract10.1021/acscombsci.7b00065
  • Deformation-Induced Martensite: A New Paradigm for Exceptional Steels
    Djaziri, S. and Li, Y. and Nematollahi, G.A. and Grabowski, B. and Goto, S. and Kirchlechner, C. and Kostka, A. and Doyle, S. and Neugebauer, J. and Raabe, D. and Dehm, G.
    Advanced Materials 28 (2016)
    view abstract10.1002/adma.201601526
  • 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 (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 abstract10.1016/j.actamat.2016.04.012
  • Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high-entropy alloy
    Laplanche, G. and Kostka, A. and Horst, O.M. and Eggeler, G. and George, E.P.
    Acta Materialia 118 (2016)
    At low homologous temperatures (down to cryogenic temperatures), the CrMnFeCoNi high-entropy alloy possesses good combination of strength, work hardening rate (WHR), ductility, and fracture toughness. To improve understanding of the deformation mechanisms responsible for its mechanical properties, tensile tests were performed at liquid nitrogen and room temperature (77 K and 293 K) and interrupted at different strains to quantify the evolution of microstructure by transmission electron microscopy. Dislocation densities, and twin widths, their spacings, and volume fractions were determined. Nanotwins were first observed after true strains of ∼7.4% at 77 K and ∼25% at 293 K; at lower strains, deformation occurs by dislocation plasticity. The tensile stress at which twinning occurs is 720 ± 30 MPa, roughly independent of temperature, from which we deduce a critical resolved shear stress for twinning of 235 ± 10 MPa. In the regime where deformation occurs by dislocation plasticity, the shear modulus normalized WHR decreases with increasing strain at both 77 K and 293 K. Beyond ∼7.4% true strain, the WHR at 77 K remains constant at a high value of G/30 because twinning is activated, which progressively introduces new interfaces in the microstructure. In contrast, the WHR at room temperature continues to decrease with increasing strain because twinning is not activated until much later (close to fracture). Thus, the enhanced strength-ductility combination at 77 K compared to 293 K is primarily due to twinning starting earlier in the deformation process and providing additional work hardening. Consistent with this, when tensile specimens were pre-strained at 77 K to introduce nanotwins, and subsequently tested at 293 K, flow stress and ductility both increased compared to specimens that were not pre-strained. © 2016 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2016.07.038
  • 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 (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 abstract10.1002/adem.201600237
  • Palladium Nanoparticles Supported on Nitrogen-Doped Carbon Nanotubes as a Release-and-Catch Catalytic System in Aerobic Liquid-Phase Ethanol Oxidation
    Dong, W. and Chen, P. and Xia, W. and Weide, P. and Ruland, H. and Kostka, A. and Köhler, K. and Muhler, M.
    ChemCatChem 8 (2016)
    Pd nanoparticles supported on carbon nanotubes were applied in the selective oxidation of ethanol in the liquid phase. The characterization of the surface and bulk properties combined with the catalytic tests indicated the dissolution and redeposition of Pd under the reaction conditions. A dynamic interplay within the Pd life cycle was identified to be responsible for the overall reactivity. Nitrogen-doped carbon nanotubes were found to act as an excellent support for the Pd catalyst system by efficiently stabilizing and recapturing the Pd species, which resulted in high activity and selectivity to acetic acid. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201501379
  • Tempering behavior of a low nitrogen boron-added 9%Cr steel
    Fedorova, I. and Kostka, A. and Tkachev, E. and Belyakov, A. and Kaibyshev, R.
    Materials Science and Engineering A 662 (2016)
    The effect of tempering temperature on microstructure and mechanical properties was studied in a low-nitrogen, high-boron, 9%Cr steel. After normalizing and low-temperature tempering, cementite platelets precipitated within the martensitic matrix. This phase transformation has no distinct effect on mechanical properties. After tempering at 500 °C, M23C6 carbides appeared in the form of layers and particles with irregular shapes along the high-angle boundaries. Approximately, 6% of the retained austenite was observed after normalizing, which reduced to 2% after tempering at 550 °C. This is accompanied by reduction in toughness from 40 J/cm2 to 8.5 J/cm2. Further increase of the tempering temperature led to spheroidization and coagulation of M23C6 particles that is followed by a significant increase in toughness to 250 J/cm2 at 750 °C. Three-phase separations of M(C,N) carbonitrides to particles enriched with V, Nb and Ti were detected after high-temperature tempering. © 2016 Elsevier B.V.
    view abstract10.1016/j.msea.2016.03.092
  • 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 (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 abstract10.1007/s10853-015-9579-7
  • Transmission electron microscopy of a CMSX-4 Ni-base superalloy produced by selective electron beam melting
    Parsa, A.B. and Ramsperger, M. and Kostka, A. and Somsen, C. and Körner, C. and Eggeler, G.
    Metals 6 (2016)
    In this work, the microstructures of superalloy specimens produced using selective electron beam melting additive manufacturing were characterized. The materials were produced using a CMSX-4 powder. Two selective electron beam melting processing strategies, which result in higher and lower effective cooling rates, are described. Orientation imaging microscopy, scanning transmission electron microscopy and conventional high resolution transmission electron microscopy are used to investigate the microstructures. Our results suggest that selective electron beam melting processing results in near equilibrium microstructures, as far as γ′ volume fractions, the formation of small amounts of TCP phases and the partitioning behavior of the alloy elements are concerned. As expected, higher cooling rates result in smaller dendrite spacings, which are two orders of magnitude smaller than observed during conventional single crystal casting. During processing, columnar grains grow in <100> directions, which are rotated with respect to each other. There are coarse γ/γ′ microstructures in high angle boundary regions. Dislocation networks form low angle boundaries. A striking feature of the as processed selective electron beam melting specimens is their high dislocation density. From a fundamental point of view, this opens new possibilities for the investigation of elementary dislocation processes which accompany solidification. © 2016 by the authors; licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met6110258
  • 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 (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 abstract10.1002/adem.201400136
  • CNT-TiO2-δ composites for improved co-catalyst dispersion and stabilized photocatalytic hydrogen production
    Chen, P. and Wang, L. and Wang, P. and Kostka, A. and Wark, M. and Muhler, M. and Beranek, R.
    Catalysts 5 (2015)
    Composites consisting of carbon nanotubes (CNTs) grown directly on oxygen-deficient anatase TiO2 (TiO2-δ) were synthesized by a two-step chemical vapor deposition (CVD) method and applied in photocatalytic hydrogen production from aqueous methanol solutions using photodeposited Pt as the co-catalyst. Thermogravimetry coupled with mass spectroscopy, X-ray diffraction, scanning electron microscopy, photocurrent analysis, X-ray photoelectron spectroscopy, and (scanning) transmission electron microscopy were performed to investigate the physical and (photo)chemical properties of the synthesized CNT-TiO2-δ composites before and after photocatalytic methanol reforming. The initial photocatalytic activity of TiO2 was found to be significantly improved in the presence of oxygen vacancies. An optimized amount (~7.2 wt%) of CNTs grown on the TiO2-δ surface led to a highly effective stabilization of the photocatalytic performance of TiO2 -δ, which is attributed to the improved dispersion and stability of the photodeposited Pt co-catalyst nanoparticles and enhanced separation efficiency of photogenerated electron-hole pairs, rendering the photocatalysts less prone to deactivation. © 2015, MDPI AG. All rights reserved.
    view abstract10.3390/catal5010270
  • Design of a twinning-induced plasticity high entropy alloy
    Deng, Y. and Tasan, C.C. and Pradeep, K.G. and Springer, H. and Kostka, A. and Raabe, D.
    Acta Materialia 94 (2015)
    We introduce a liquid metallurgy synthesized, non-equiatomic Fe<inf>40</inf>Mn<inf>40</inf>Co<inf>10</inf>Cr<inf>10</inf> high entropy alloy that is designed to undergo mechanically-induced twinning upon deformation at room temperature. Microstructure characterization, carried out using SEM, TEM and APT shows a homogeneous fcc structured single phase solid solution in the as-cast, hot-rolled and homogenized states. Investigations of the deformation substructures at specific strain levels with electron channeling contrast imaging (ECCI) combined with EBSD reveal a clear change in the deformation mechanisms of the designed alloy starting from dislocation slip to twinning as a function of strain. Such twinning induced plasticity has only been observed under cryogenic conditions in the equiatomic FeMnNiCoCr high entropy alloy. Thus, despite the decreased contribution of solid solution strengthening, the tensile properties of the introduced lean alloy at room temperature are found to be comparable to that of the well-studied five component FeMnNiCoCr system. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.04.014
  • 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 (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 abstract10.1016/j.scriptamat.2015.01.014
  • Interface engineering and characterization at the atomic-scale of pure and mixed ion layer gas reaction buffer layers in chalcopyrite thin-film solar cells
    Cojocaru-Mirédin, O. and Fu, Y. and Kostka, A. and Sáez-Araoz, R. and Beyer, A. and Knaub, N. and Volz, K. and Fischer, C.-H. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 23 (2015)
    In this work, we investigate the p-n junction region for two different buffer/Cu(In,Ga)(Se,S)<inf>2</inf> (CIGSSe) samples having different conversion efficiencies (the cell with pure In<inf>2</inf>S<inf>3</inf> buffer shows a lower efficiency than the nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffered one). To explain the better efficiency of the sample with nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffer layer, combined transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies were performed. In the pure In<inf>2</inf>S<inf>3</inf> buffered sample, a CuIn<inf>3</inf>Se<inf>5</inf> ordered-defect compound is observed at the CIGSSe surface, whereas in the nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffered sample no such compound is detected. The absence of an ordered-defect compound in the latter sample is explained either by the presence of the ZnS nanodots, which may act as a barrier layer against Cu diffusion in CIGSSe hindering the formation of CuIn<inf>3</inf>Se<inf>5</inf>, or by the presence of Zn at the CIGSSe surface, which may disturb the formation of this ordered-defect compound. In the nano-ZnS/In<inf>2</inf>S<inf>3</inf> sample, Zn was found in the first monolayers of the absorber layer, which may lead to a downward band bending at the surface. This configuration is very stable (Fermi level pinning at the conduction band, as observed for Cd in Cu(In,Ga)Se<inf>2</inf>) and reduces the recombination rate at the interface. This effect may explain why the sample with ZnS nanodots possesses a higher efficiency. This work demonstrates the capability of correlative transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies in investigating buried interfaces. The study provides essential information for understanding and modeling the p-n junction at the nanoscale in CIGSSe solar cells. Copyright © 2014 John Wiley & Sons, Ltd.
    view abstract10.1002/pip.2484
  • 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 (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 abstract10.1016/j.actamat.2015.02.005
  • Mechanisms of subgrain coarsening and its effect on the mechanical properties of carbon-supersaturated nanocrystalline hypereutectoid steel
    Li, Y.J. and Kostka, A. and Choi, P. and Goto, S. and Ponge, D. and Kirchheim, R. and Raabe, D.
    Acta Materialia 84 (2015)
    Carbon-supersaturated nanocrystalline hypereutectoid steels with a tensile strength of 6.35 GPa were produced from severely cold-drawn pearlite. The nanocrystalline material undergoes softening upon annealing at temperatures between 200 and 450°C. The ductility in terms of elongation to failure exhibits a non-monotonic dependence on temperature. Here, the microstructural mechanisms responsible for changes in the mechanical properties were studied using transmission electron microscopy (TEM), TEM-based automated scanning nanobeam diffraction and atom probe tomography (APT). TEM and APT investigations of the nanocrystalline hypereutectoid steel show subgrain coarsening upon annealing, which leads to strength reduction following a Hall-Petch law. APT analyzes of the Mn distribution near subgrain boundaries and in the cementite give strong evidence of capillary-driven subgrain coarsening occurring through subgrain boundary migration. The pronounced deterioration of ductility after annealing at temperatures above 350°C is attributed to the formation of cementite at subgrain boundaries. The overall segregation of carbon atoms at ferrite subgrain boundaries gives the nanocrystalline material excellent thermal stability upon annealing. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.10.027
  • 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 (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 abstract10.1007/s10853-015-9177-8
  • Microstructure refinement for high modulus in-situ metal matrix composite steels via controlled solidification of the system Fe-TiB2
    Springer, H. and Aparicio Fernandez, R. and Duarte, M.J. and Kostka, A. and Raabe, D.
    Acta Materialia 96 (2015)
    Microstructures of Fe-TiB<inf>2</inf> metal-matrix-composites formed in-situ from Fe-Ti-B melts were investigated for hypo- and hyper-eutectic concentrations down to atomic-scale resolution. Special emphasis is laid on the influence of the solidification rate on particle size, morphology and distribution as well as their relation to mechanical properties. Innovative routes for the cost-effective production of stiff and ductile high modulus steels for lightweight structural applications are discussed, focusing on hyper-eutectic compositions due to their high stiffness/density ratio: firstly, very slow cooling allows the primary particles floating to the top of the cast, from which they can either be easily removed for retaining bulk material containing only fine-dispersed eutectic particles, or be kept and utilised as a wear resistant surface. Secondly, annealing of amorphous matrix material obtained from very fast solidification leads to fine dispersed nano-scaled precipitation of TiB<inf>2</inf> particles. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2015.06.017
  • 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 (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 abstract10.1016/j.msea.2015.03.045
  • Stability of Dealloyed Porous Pt/Ni Nanoparticles
    Baldizzone, C. and Gan, L. and Hodnik, N. and Keeley, G.P. and Kostka, A. and Heggen, M. and Strasser, P. and Mayrhofer, K.J.J.
    ACS Catalysis 5 (2015)
    We provide a comprehensive durability assessment dedicated to a promising class of electrocatalysts for the oxygen reduction reaction (i.e., porous platinum nanoparticles). The stability of these nanoengineered open structures is tested under two accelerated degradation test conditions (ADT), particularly selected to mimic the potential regimes experienced by the catalyst during the operative life of a fuel cell (i.e., load cycles (up to 1.0 V<inf>RHE</inf>) and start-up cycles (up to 1.4 V<inf>RHE</inf>)). To understand the evolution of the electrochemical performance, the catalyst properties are investigated by means of fundamental rotating disc electrode studies, identical location-transmission electron microscopy (IL-TEM) coupled with electron energy loss spectroscopy chemical mapping (IL-EELS), and post-use chemical analysis and online highly sensitive potential resolved dissolution concentration monitoring by scanning flow cell inductively coupled plasma-mass spectrometry (SFC-ICP-MS). The experimental results on the nanoporous Pt revealed distinctive degradation mechanisms that could potentially affect a wide range of other nanoengineered open structures. The study concludes that, although providing promising activity performance, under the relevant operational conditions of fuel cells, the nanoporosity is only metastable and subjected to a progressive reorganization toward the minimization of the nanoscale curvature. The rate and pathways of this specific degradation mechanism together with other well-known degradation mechanisms like carbon corrosion and platinum dissolution are strongly dependent on the selected upper limit potential, leading to distinctly different durability performance. © 2015 American Chemical Society.
    view abstract10.1021/acscatal.5b01151
  • The crystallographic template effect assisting the formation of stable α-Al2O3 during low temperature oxidation of Fe-Al alloys
    Brito, P. and Pinto, H. and Kostka, A.
    Corrosion Science (2015)
    The role of thermally grown α-Fe2O3 on the nucleation of α-Al2O3 during oxidation of binary Fe-Al alloys with 15 and 26 at.%Al at 700°C was investigated. Surface morphology of the oxide scales indicated direct nucleation of α-Al2O3 preferentially instead of conversion from metastable Al2O3 polymorphs. Oxide scale development over time was also monitored by use of synchrotron X-ray diffraction and Raman spectroscopy. The results showed that the α-Fe2O3 crystal lattice decreases in volume as oxidation progresses, which was found to be consistent with an Al3+ enrichment of α-Fe2O3 as confirmed by the change in relative intensity of α-Fe2O3 Raman peaks. © 2016 Elsevier Ltd.
    view abstract10.1016/j.corsci.2016.01.007
  • The nucleation of Mo-rich Laves phase particles adjacent to M23C6 micrograin boundary carbides in 12% Cr tempered martensite ferritic steels
    Isik, M.I. and Kostka, A. and Yardley, V.A. and Pradeep, K.G. and Duarte, M.J. and Choi, P.P. and Raabe, D. and Eggeler, G.
    Acta Materialia 90 (2015)
    We study the nucleation of Mo-rich Laves phase particles during aging and creep of 12 wt.% Cr tempered martensite ferritic steels (TMFS). Recently, in Isik et al. (2014) we reported that Laves phase particles tend to form at micrograin boundaries of TMFSs after Mo and Si had segregated from the ferritic matrix to these internal interfaces. In the present work, we employ transmission electron microscopy (TEM) and atom probe tomography (APT) to study the formation of Laves phase particles. We investigate the preference of Laves phase particles to nucleate next to M23C6 micrograin boundary carbides. Our results suggest that this joint precipitation effect is due to the combined segregation of Mo and Si from the matrix to the micrograin boundaries and Si and P enrichment around the growing carbides.
    view abstract10.1016/j.actamat.2015.01.027
  • Crystallization, phase evolution and corrosion of Fe-based metallic glasses: An atomic-scale structural and chemical characterization study
    Duarte, M.J. and Kostka, A. and Jimenez, J.A. and Choi, P. and Klemm, J. and Crespo, D. and Raabe, D. and Renner, F.U.
    Acta Materialia 71 (2014)
    Understanding phase changes, including their formation and evolution, is critical for the performance of functional as well as structural materials. We analyze in detail microstructural and chemical transformations of the amorphous steel Fe50Cr15Mo14C15B6 during isothermal treatments at temperatures ranging from 550 to 800 °C. By combining high-resolution transmission electron microscopy and Rietveld analyses of X-ray diffraction patterns together with the local chemical data obtained by atom probe tomography, this research provides relevant information at the atomic scale about the mechanisms of crystallization and the subsequent phases evolution. During the initial stages of crystallization a stable (Fe,Cr) 23(C,B)6 precipitates as well as two metastable intermediates of M3(C,B) and the intermetallic χ-phase. When full crystallization is reached, only a percolated nano-scale Cr-rich (Fe,Cr) 23(C,B)6 and Mo-rich η-Fe3Mo3C structure is detected, with no evidence to suggest that other phases appear at any subsequent time. Finally, the corrosion behavior of the developed phases is discussed from considerations of the obtained atomic information. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.02.027
  • Enhanced Quantum Confined Stark Effect in a mesoporous hybrid multifunctional system
    Gogoi, M. and Deb, P. and Sen, D. and Mazumder, S. and Kostka, A.
    Solid State Communications 187 (2014)
    Quantum Confined Stark Effect in hybrid of CdTe quantum dot with superparamagnetic iron oxide nanoparticles in both nonporous and mesoporous silica matrix has been realized. The observed QCSE is due to the local electric field induced by charge dispersion at SiO2/polar solvent interface. Enhanced Stark shift of 89.5 meV is observed in case of mesoporous hybrid structure and the corresponding local electric field has been evaluated as 4.38×104 V/cm. The enhancement is assumed to be caused by greater density of charge in the mesoporous hybrid. The conjugation of superparamagnetic nanoparticles in this tailored hybrid microstructure has not imparted any alteration to the Stark shift, but has added multifunctional attribute. The present study on the local electric field induced enhanced QCSE with wavelength modulation towards red end paves the way of developing magneto-fluorescent hybrid systems for biomedical imaging application. © 2014 Elsevier Ltd.
    view abstract10.1016/j.ssc.2014.02.011
  • Erratum: Shear-induced mixing governs codeformation of crystalline- amorphous nanolaminates (Physical Review Letters (2014) 113 (035501))
    Guo, W. and Jägle, E.A. and Choi, P.-P. and Yao, J. and Kostka, A. and Schneider, J.M. and Raabe, D.
    Physical Review Letters 113 (2014)
    view abstract10.1103/PhysRevLett.113.069903
  • Interaction of cobalt nanoparticles with oxygen- and nitrogen- functionalized carbon nanotubes and impact on nitrobenzene hydrogenation catalysis
    Chen, P. and Yang, F. and Kostka, A. and Xia, W.
    ACS Catalysis 4 (2014)
    The type and the amount of functional groups on the surface of carbon nanotubes (CNTs) were tuned to improve the activity of supported Co nanoparticles in hydrogenation catalysis. Surface nitrogen species on CNTs significantly promoted the decomposition of the cobalt precursor and the reduction of cobalt oxide, and improved the resistance of metallic Co against oxidation in ambient atmosphere. In the selective hydrogenation of nitrobenzene in the gas phase, Co supported on CNTs with the highest surface nitrogen content showed the highest activity, which is ascribed to the higher reducibility and the lower oxidation state of the Co nanoparticles under reaction conditions. For Co nanoparticles supported on CNTs with a smaller amount of surface nitrogen groups, a repeated reduction at 350 °C was essential to achieve a comparable high catalytic activity reaching 90% conversion at 250 °C, pointing to the importance of nitrogen species for the supported Co nanoparticles in nitrobenzene hydrogenation. © 2014 American Chemical Society.
    view abstract10.1021/cs500173t
  • Long-term microstructural stability of oxide-dispersion strengthened Eurofer steel annealed at 800 °c
    Zilnyk, K.D. and Sandim, H.R.Z. and Bolmaro, R.E. and Lindau, R. and Möslang, A. and Kostka, A. and Raabe, D.
    Journal of Nuclear Materials 448 (2014)
    Oxide-dispersion strengthened ferritic martensitic steels such as ODS-Eurofer grade are good candidates for structural applications in future fusion power reactors. Long-term annealing treatments in vacuum were carried out in cold-rolled samples (80% reduction in thickness) from 1 h up to 4320 h (6 months) at 800 °C, i.e. the maximum temperature in the ferritic phase field, to follow its softening behavior. The microstructural stability of this steel was mapped using several characterization techniques including scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, Vickers microhardness testing, X-ray diffraction texture measurements, low-temperature electrical resistivity, and magnetic coercive field measurements. ODS-Eurofer steel displays good microstructural stability. Discontinuous recrystallization occurs at the early stages of annealing resulting in a low volume fraction of recrystallized grains. Extended recovery is the predominant softening mechanism at this temperature for longer times. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jnucmat.2014.01.032
  • Nanoscale understanding of bond formation during cold welding of aluminum and steel
    Altin, A. and Wohletz, S. and Krieger, W. and Kostka, A. and Groche, P. and Erbe, A.
    Advanced Materials Research 966-967 (2014)
    Cold welding, e.g. by cold forging, is a smart manufacturing technology, enabling novel multi material designs. A material combination, which is particularly attractive for manufacturing, though challenging to handle in a cold welding process, is steel and aluminum. We investigate the bond formation between cold forged C 15 (mainly primary heat treated) and AW 6082. Analysis starts with numerical simulations using the finite element analysis (FEA) to identify optimum conditions for bond formation. The bond strength was determined by tensile tests from samples eroded from the cold-welded specimen. Best performing samples showed a maximum tensile strength of ~200 MPa with ductile failure in the AW 6082. Transmission electron microscopy (TEM) inspection of the bonded area between aluminum and steel show a reaction layer consisting of iron and aluminum of few nm thickness throughout the sample. The formation of such a reaction layer is hypothesized to be crucial for bond formation. © (2014) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/AMR.966-967.445
  • On the nucleation of Laves phase particles during high-temperature exposure and creep of tempered martensite ferritic steels
    Isik, M.I. and Kostka, A. and Eggeler, G.
    Acta Materialia 81 (2014)
    This paper reports on the formation of an Mo-rich Laves phase during high-temperature exposure and creep of a tempered martensite ferritic steel with 12 wt.% Cr and 1 wt.% Mo. The material was exposed to 550 °C for time intervals between 864 and 81,984 h. For comparison, a few creep tests were carried out at 550 °C and 120 MPa (duration between 864 and 12,456 h). All tests were interrupted after specific time periods and microstructures were investigated using transmission electron microscopy and atom probe tomography. Laves phase formation occurs during both heat treatment and creep. Creep stress and strain have no significant effect on the early stages of Laves phase formation. In the present work we show that prior to Laves phase nucleation Si and Mo segregate to micrograin boundaries, where subsequently Laves phase particles appear next to M23C6carbides. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.08.008
  • Segregation stabilizes nanocrystalline bulk steel with near theoretical strength
    Li, Y. and Raabe, D. and Herbig, M. and Choi, P.-P. and Goto, S. and Kostka, A. and Yarita, H. and Borchers, C. and Kirchheim, R.
    Physical Review Letters 113 (2014)
    Grain refinement through severe plastic deformation enables synthesis of ultrahigh-strength nanostructured materials. Two challenges exist in that context: First, deformation-driven grain refinement is limited by dynamic dislocation recovery and crystal coarsening due to capillary driving forces; second, grain boundary sliding and hence softening occur when the grain size approaches several nanometers. Here, both challenges have been overcome by severe drawing of a pearlitic steel wire (pearlite: lamellar structure of alternating iron and iron carbide layers). First, at large strains the carbide phase dissolves via mechanical alloying, rendering the initially two-phase pearlite structure into a carbon-supersaturated iron phase. This carbon-rich iron phase evolves into a columnar nanoscaled subgrain structure which topologically prevents grain boundary sliding. Second, Gibbs segregation of the supersaturated carbon to the iron subgrain boundaries reduces their interface energy, hence reducing the driving force for dynamic recovery and crystal coarsening. Thus, a stable cross-sectional subgrain size <10nm is achieved. These two effects lead to a stable columnar nanosized grain structure that impedes dislocation motion and enables an extreme tensile strength of 7 GPa, making this alloy the strongest ductile bulk material known. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.113.106104
  • Sequential growth of zinc oxide nanorod arrays at room temperature via a corrosion process: Application in visible light photocatalysis
    Iqbal, D. and Kostka, A. and Bashir, A. and Sarfraz, A. and Chen, Y. and Wieck, A.D. and Erbe, A.
    ACS Applied Materials and Interfaces 6 (2014)
    Many photocatalyst systems catalyze chemical reactions under ultraviolet (UV) illumination, because of its high photon energies. Activating inexpensive, widely available materials as photocatalyst using the intense visible part of the solar spectrum is more challenging. Here, nanorod arrays of the wide-band-gap semiconductor zinc oxide have been shown to act as photocatalysts for the aerobic photo-oxidation of organic dye Methyl Orange under illumination with red light, which is normally accessible only to narrow-band semiconductors. The homogeneous, 800-1000-nm-thick ZnO nanorod arrays show substantial light absorption (absorbances >1) throughout the visible spectral range. This absorption is caused by defect levels inside the band gap. Multiple scattering processes by the rods make the nanorods appear black. The dominantly crystalline ZnO nanorod structures grow in the (0001) direction, i.e., with the c-axis perpendicular to the surface of polycrystalline zinc. The room-temperature preparation route relies on controlled cathodic delamination of a weakly bound polymer coating from metallic zinc, an industrially produced and cheaply available substrate. Cathodic delamination is a sequential synthesis process, because it involves the propagation of a delamination front over the base material. Consequently, arbitrarily large sample surfaces can be nanostructured using this approach. © 2014 American Chemical Society.
    view abstract10.1021/am504299v
  • Shear-induced mixing governs codeformation of crystalline-amorphous nanolaminates
    Guo, W. and Jägle, E.A. and Choi, P.-P. and Yao, J. and Kostka, A. and Schneider, J.M. and Raabe, D.
    Physical Review Letters 113 (2014)
    Deformation of ductile crystalline-amorphous nanolaminates is not well understood due to the complex interplay of interface mechanics, shear banding, and deformation-driven chemical mixing. Here we present indentation experiments on 10 nm nanocrystalline Cu-100 nm amorphous CuZr model multilayers to study these mechanisms down to the atomic scale. By using correlative atom probe tomography and transmission electron microscopy we find that crystallographic slip bands in the Cu layers coincide with noncrystallographic shear bands in the amorphous CuZr layers. Dislocations from the crystalline layers drag Cu atoms across the interface into the CuZr layers. Also, crystalline Cu blocks are sheared into the CuZr layers. In these sheared and thus Cu enriched zones the initially amorphous CuZr layer is rendered into an amorphous plus crystalline nanocomposite. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.113.035501
  • Smaller is less stable: Size effects on twinning vs. transformation of reverted austenite in TRIP-maraging steels
    Wang, M.-M. and Tasan, C.C. and Ponge, D. and Kostka, A. and Raabe, D.
    Acta Materialia 79 (2014)
    Steels containing reverted nanoscale austenite (γRN) islands or films dispersed in a martensitic matrix show excellent strength, ductility and toughness. The underlying microstructural mechanisms responsible for these improvements are not yet understood, but are observed to be strongly connected to the γRN island or film size. Two main micromechanical effects are conceivable in this context, namely: (i) interaction of γRN with microcracks from the matrix (crack blunting or arresting); and (ii) deformation-induced phase transformation of γRN to martensite (TRIP effect). The focus here is on the latter phenomenon. To investigate size effects on γRN transformation independent of other factors that can influence austenite stability (composition, crystallographic orientation, defect density, surrounding phase, etc.), a model (TRIP-maraging steel) microstructure is designed with support from diffusion simulations (using DICTRA software) to have the same, homogeneous chemical composition in all γRN grains. Characterization is conducted by in-situ tension and bending experiments in conjunction with high-resolution electron backscatter diffraction mapping and scanning electron microscopy imaging, as well as post-mortem transmission electron microscopy and synchrotron X-ray diffraction analysis. Results reveal an unexpected "smaller is less stable" effect due to the size-dependent competition between mechanical twinning and deformation-induced phase transformation. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2014.07.020
  • Structural and functional characterization of enamel pigmentation in shrews
    Dumont, M. and Tütken, T. and Kostka, A. and Duarte, M.J. and Borodin, S.
    Journal of Structural Biology 186 (2014)
    Pigmented tooth enamel occurs in several vertebrate clades, ranging from mammals to fish. Although an iron compound is associated with this orange to red colored pigmentation, its chemical and structural organization within the enamel is unknown. To determine the nature of the iron compound, we investigated heavily pigmented teeth of the northern short-tailed shrew Blarina brevicauda using combined characterization techniques such as scanning and transmission electron microscopy and synchrotron X-ray diffraction. We found that the pigmentation of the enamel with an iron content of around 8. wt% results from a close to amorphous magnetite phase deposited around the nm-sized enamel crystals. Furthermore, the influence of the pigmentation on the enamel hardness was determined by nanoindentation measurements. Finally, the biomechanical function and biological context are discussed in light of the obtained results. © 2014 Elsevier Inc.
    view abstract10.1016/j.jsb.2014.02.006
  • Control of phase coexistence in calcium tantalate composite photocatalysts for highly efficient hydrogen production
    Wang, P. and Chen, P. and Kostka, A. and Marschall, R. and Wark, M.
    Chemistry of Materials 25 (2013)
    Design and fabrication of semiconductor based composite photocatalysts with matching band structure is an important strategy to improve charge separation of photogenerated electron-hole pairs for photocatalytic hydrogen production. In our study, by aid of the simple and cost-effective molten salts method, a series of phase-controlled and composition-tuned calcium tantalate composite photocatalysts has been prepared by adjusting the initial atomic ratio of Ta/Ca precursors. We demonstrate the strong correlation between the photocatalytic activities of calcium tantalate composite photocatalysts for hydrogen evolution and respective phase compositions. Without any cocatalysts, these composites with the optimized phase composition of cubic α-CaTa2O 6/hexagonal Ca2Ta2O7, cubic CaTa2O6/hexagonal Ca2Ta2O 7/orthorhombic β-CaTa2O6, or cubic α-CaTa2O6/orthorhombic β-CaTa2O 6 showed very high photocatalytic H2 production activities in the presence of methanol. It is attributed mainly to a significantly improved photoexcited charge carrier separation via the junctions and interfaces in the composites. Further by in situ photodeposition of noble metal nanoparticles (Pt or Rh) as cocatalysts the photocatalytic activity of these composites was greatly promoted for H2 production. The study on convenient fabrication of phase-coexisting composite photocatalysts with matching band structure for improving the photocatalytic hydrogen production sheds light on developing efficient composite photocatalyst as a means for conversion of solar energy to chemical energy. © 2013 American Chemical Society.
    view abstract10.1021/cm402708h
  • Grain boundary segregation in a bronze-route Nb3Sn superconducting wire studied by atom probe tomography
    Sandim, M.J.R. and Tytko, D. and Kostka, A. and Choi, P. and Awaji, S. and Watanabe, K. and Raabe, D.
    Superconductor Science and Technology 26 (2013)
    Atom probe tomography was used to characterize the A15 phase in a bronze-route Nb3Sn superconducting wire with a bronze matrix composition of Cu-8Sn-0.3Ti (in at.%). We observed depletion of niobium and segregation of Cu and Ti atoms at Nb3Sn grain boundaries. While the Nb depletion is about 15% relative to the grain interior, the average ratio between Cu and Ti excess values is 9 to 2. Segregation extends to a distance d ∼ 9 Å from the point of maximum Cu and Ti concentrations. Such local variation in the stoichiometry at the grain boundary region can be an additional source of flux-pinning in the Nb3Sn phase. Other microstructural parameters, such as the grain size and chemical composition of the Nb 3Sn layer, were investigated by electron backscatter diffraction and transmission electron microscopy. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0953-2048/26/5/055008
  • Interface Segregation in Advanced Steels Studied at the Atomic Scale
    Raabe, D. and Ponge, D. and Kirchheim, R. and Assadi, H. and Li, Y. and Goto, S. and Kostka, A. and Herbig, M. and Sandl, S. and Kuzmina, M. and Millán, J. and Yuan, L. and Choi, P.-P.
    Microstructural Design of Advanced Engineering Materials (2013)
    view abstract10.1002/9783527652815.ch11
  • Purified oxygen- and nitrogen-modified multi-walled carbon nanotubes as metal-free catalysts for selective olefin hydrogenation
    Chen, P. and Chew, L.M. and Kostka, A. and Xie, K. and Muhler, M. and Xia, W.
    Journal of Energy Chemistry 22 (2013)
    Oxygen- and nitrogen-functionalized carbon nanotubes (OCNTs and NCNTs) were applied as metal-free catalysts in selective olefin hydrogenation. A series of NCNTs was synthesized by NH3 post-treatment of OCNTs. Temperature-programmed desorption, N2 physisorption, Raman spectroscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were employed to characterize the surface properties of OCNTs and NCNTs, aiming at a detailed analysis of the type and amount of oxygen- and nitrogen-containing groups as well as surface defects. The gas-phase treatments applied for oxygen and nitrogen functionalization at elevated temperatures up to 600 °C led to the increase of surface defects, but did not cause structural damages in the bulk. NCNTs showed a clearly higher activity than the pristine CNTs and OCNTs in the hydrogenation of 1,5-cyclooctadiene, and also the selectivity to cyclooctene was higher. The favorable catalytic properties are ascribed to the nitrogen-containing surface functional groups as well as surface defects related to nitrogen species. In contrast, oxygen-containing surface groups and the surface defects caused by oxygen species did not show clear contribution to the hydrogenation catalysis. Copyright © 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
    view abstract10.1016/S2095-4956(13)60038-8
  • Simple synthesis of superparamagnetic magnetite nanoparticles as highly efficient contrast agent
    Jha, D.K. and Shameem, M. and Patel, A.B. and Kostka, A. and Schneider, P. and Erbe, A. and Deb, P.
    Materials Letters 95 (2013)
    Magnetite nanoparticles have been prepared by one-pot thermal decomposition process using iron (III) acetylacetonate in stearic acid in ambient environment. In this process, stearic acid acts as solvent as well as capping agent for the particles. These as-prepared hydrophobic magnetite nanoparticles have been converted into a hydrophilic form using tetramethylammonium hydroxide. This controlled surface functionalization approach limits microstructural and phase alteration due to the ligand exchange. A detailed investigation was carried out on the microstructural characteristics of these nanoparticles with the aid of X-ray diffraction, infrared spectroscopy, XPS and transmission electron microscopy. The hydrophilic superparamagnetic magnetite particles posses extraordinary transverse relaxivity and contrast property, making them potential T2 contrast agent in clinical magnetic resonance imaging. © 2013 Elsevier B.V.
    view abstract10.1016/j.matlet.2012.12.096
  • The structural and electronic promoting effect of nitrogen-doped carbon nanotubes on supported Pd nanoparticles for selective olefin hydrogenation
    Chen, P. and Chew, L.M. and Kostka, A. and Muhler, M. and Xia, W.
    Catalysis Science and Technology 3 (2013)
    A high-performance Pd catalyst for selective olefin hydrogenation was synthesized by supporting Pd nanoparticles on nitrogen-doped carbon nanotubes (NCNTs). X-ray diffraction, hydrogen chemisorption, transmission electron microscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize Pd supported on NCNTs and nitrogen-free oxygen-functionalized CNTs (OCNTs). The Pd nanoparticles were stabilized on NCNTs with narrower size distribution compared with OCNTs. The XPS analysis revealed that the nitrogen functional groups favor the reduction of Pd on CNTs suggesting an electronic promoter effect. The Pd/NCNT catalyst showed extraordinary catalytic performance in terms of activity, selectivity and stability in the selective hydrogenation of cyclooctadiene, which is related to the structural and electronic promoting effect of the NCNT support. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c3cy00097d
  • Degradation mechanisms of Pt/C fuel cell catalysts under simulated start-stop conditions
    Meier, J.C. and Galeano, C. and Katsounaros, I. and Topalov, A.A. and Kostka, A. and Schüth, F. and Mayrhofer, K.J.J.
    ACS Catalysis 2 (2012)
    This manuscript investigates the degradation of a Pt/Vulcan fuel cell catalyst under simulated start-stop conditions in an electrochemical half-cell. Identical location transmission electron microscopy (IL-TEM) is used to visualize the several different degradation pathways occurring on the same catalyst material under potential cycling conditions. The complexity of degradation on the nanoscale leading to macroscopic active surface area lossis demonstrated and discussed. Namely, four different degradation pathways at one single Pt/Vulcan aggregate are clearly observed. Furthermore, inhomogeneous degradation behavior for different catalyst locations is shown, and trends in degradation mechanisms related to the platinum particle size are discussed in brief. Attention is drawn to the vast field of parameters influencing catalyst stability. We also present the development of a new technique to study changes of the catalyst not only with 2D projections of standard TEM images but also in 3D. For this purpose, identical location tomography (IL-tomography) is introduced, which visualizes the 3D structure of an identical catalyst location before and after degradation. © 2012 American Chemical Society.
    view abstract10.1021/cs300024h
  • Differential tunability effect on the optical properties of doped and undoped quantum dots
    Gogoi, M. and Deb, P. and Kostka, A.
    Physica Status Solidi (A) Applications and Materials Science 209 (2012)
    We have studied the optical properties of size-tunable undoped and Mn 2+-doped cadmium telluride (CdTe) quantum dots (QDs) of monodisperse suspensions with emission wavelength varying between 500 and 680 nm. The role of the surface is investigated for obtaining size-tunable, bright, and stable CdTe QDs. Differentially tunable optical properties of doped and undoped CdTe QDs has been reported. Unlike the continuous redshifting in the absorption spectra of pristine CdTe system, an unusual blueshift is observed for doped CdTe system after a certain time period of refluxion, followed by again a redshift and the disappearance of earlier peak with further refluxing. The available size range and optical properties are significantly controlled through Mn 2+ doping. The surface adsorbed Mn promotes ripening of the doped system as well as disintegration into smaller fraction after saturation in growth occurs. An optimum size fraction is identified for both the systems based on their photoluminescence (PL) quantum yield. The photophysics of Mn 2+-doped nanocrystals has been proposed. Copyright © 2012 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201228074
  • Direct monophasic replacement of fatty acid by DMSA on SPION surface
    Gogoi, M. and Deb, P. and Vasan, G. and Keil, P. and Kostka, A. and Erbe, A.
    Applied Surface Science 258 (2012)
    Tailoring the surface and understanding the surface characteristics is necessary for biomedical applications of superparamagnetic nanoparticles. In this paper, superparamagnetic iron oxide nanoparticles (SPIONs) were prepared by thermal decomposition of iron nitrate in presence of stearic acid as surfactant. Due to the multilayer organization of surfactant molecules over the nanoparticle surface, the surface potential can be tuned by pH changes and hence the nanoparticles can be made dispersible in nonpolar as well as in polar solvents. We have presented a simple, facile procedure for controlled replacement of stearic acid from maghemite surface and subsequent derivatization by biocompatible dimercaptosuccinic acid (DMSA) to obtain ultrastable hydrophilic nanoparticles with unaltered morphology, phase and properties. The surface chemistry of the functionalized SPIONs was analyzed by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) revealing the presence of bound and unbound thiol groups and disulfides, leading to its prolonged stability in aqueous medium. The consequence of spatially selective functionalization on the stability and solubility of surface hydrophilic SPION has also been realized. © 2012 Elsevier B.V.
    view abstract10.1016/j.apsusc.2012.06.011
  • Dislocation engineering and its effect on the oxidation behaviour
    Naraparaju, R. and Christ, H.-J. and Renner, F.U. and Kostka, A.
    Materials at High Temperatures 29 (2012)
    Shot-peening of the surface of steel prior to oxidation can have a beneficial effect. Shot-peening can improve the oxidation resistance by introducing a localised plastic deformation in the near surface region resulting in an increase of the dislocation density. These dislocations can act in Cr-containing steels as fast diffusion paths for Cr promoting the formation of protective Cr-oxides. However, the effect of shot-peening has some limitations such as working temperature and microstructure. It has different effects on austenitic steels and ferritic martensitic steels. The effect of shot-peening can become futile due to recovery and recrystallisation of the alloy when subjected to higher temperatures for longer periods. In the present work, the main emphasis is put on the type of dislocation arrangement promoting the positive effect on the oxidation behaviour. Dislocation engineering was applied on shot-peened samples by means of some pre-annealing procedures resulting in a recovery process. During the process, dislocations were assumed to rearrange and form certain combinations nearer to the alloy grain boundaries. These arrays of dislocations can result in different oxidation behaviour. In the present study, 18 wt% Cr and 12 wt% Cr steels were shot-peened and vacuum annealed at 750°C for 1 h, 2 h, 3 h, 5 h and 15 h. Subsequently these steels were oxidised at 750°C. The mass gain in all cases is different for both steels, and in the case of both 12 wt% Cr and 18 wt% Cr steels the best oxidation resistance was achieved for the shot-peened 1 h pre-annealed sample.
    view abstract10.3184/096034012X13322687148749
  • Erratum: Direct monophasic replacement of fatty acid by DMSA on SPION surface (Applied Surface Science (2012) 258 (9685))
    Gogoi, M. and Deb, P. and Vasan, G. and Keil, P. and Kostka, A. and Erbe, A.
    Applied Surface Science 263 (2012)
    view abstract10.1016/j.apsusc.2012.09.085
  • Friction-stir dissimilar welding of aluminium alloy to high strength steels: Mechanical properties and their relation to microstructure
    Coelho, R.S. and Kostka, A. and dos Santos, J.F. and Kaysser-Pyzalla, A.
    Materials Science and Engineering A 556 (2012)
    The use of light-weight materials for industrial applications is a driving force for the development of joining techniques. Friction stir welding (FSW) inspired joints of dissimilar materials because it does not involve bulk melting of the basic components. Here, two different grades of high strength steel (HSS), with different microstructures and strengths, were joined to AA6181-T4 Al alloy by FSW. The purpose of this study is to clarify the influence of the distinct HSS base material on the joint efficiency. The joints were produced using the same welding parameter/setup and characterised regarding microstructure and mechanical properties. Both joints could be produced without any defects. Microstructure investigations reveal similar microstructure developments in both joints, although there are differences e.g. in the size and amount of detached steel particles in the aluminium alloy (heat and thermomechanical affected zone). The weld strengths are similar, showing that the joint efficiency depends foremost on the mechanical properties of the heat and the thermomechanical affected zone of the aluminium alloy. © 2012 Elsevier B.V.
    view abstract10.1016/j.msea.2012.06.076
  • Microstructural evolution of a Ni-based superalloy (617B) at 700 °c studied by electron microscopy and atom probe tomography
    Tytko, D. and Choi, P.-P. and Klöwer, J. and Kostka, A. and Inden, G. and Raabe, D.
    Acta Materialia 60 (2012)
    We report on the microstructural evolution of a polycrystalline Ni-based superalloy (Alloy 617B) for power plant applications at a service temperature of 700 °C. The formation of secondary M 23C 6-carbides close to grain boundaries (GBs) and around primary Ti(C,N) particles is observed upon annealing at 700 °C, where γ′ is found to nucleate heterogeneously at M 23C 6 carbides. Using atom probe tomography, elemental partitioning to the phases and composition profiles across phase and grain boundaries are determined. Enrichments of B at γ/M 23C 6 and γ′/M 23C 6 interfaces as well as at grain boundaries are detected, while no B enrichment is found at γ/γ′ interfaces. It is suggested that segregation of B in conjunction with γ′ formation stabilizes a network of secondary M 23C 6 precipitates near GBs and thus increases the creep rupture life of Alloy 617B. Calculations of the equilibrium phase compositions by Thermo-Calc confirm the chemical compositions measured by atom probe tomography. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.11.020
  • On the correlation between thermal cycle and formation of intermetallic phases at the interface of laser-welded aluminum-steel overlap joints
    Szczepaniak, A. and Fan, J. and Kostka, A. and Raabe, D.
    Advanced Engineering Materials 14 (2012)
    A laser beam welding process via heat conduction was applied to join DC01 steel with aluminum (Al) in overlap configuration without filler wire. The effect of the applied laser power (1.7, 1.8, 2.1, and 2.4 kW) on the formation and evolution of the interfaces between steel and Al was analyzed. Two intermetallic compounds were found at the interface, namely, one adjacent to the steel layer (Al 5Fe 2) and one close to the solidified Al (Al 13Fe 4). The thickness of the intermetallic reaction layer increases with laser power, while the morphology of its individual components evolves due to differences in accumulated thermal cycles. Correlations between simulations and measurements show that the peak temperature has significantly stronger influence on the thickness of the intermetallic reaction layer than cooling time and the integral of temperature over the time. Shear/tensile strength tests reveal that all the specimens fail in the Al heat affected zone. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201200075
  • Stability investigations of electrocatalysts on the nanoscale
    Meier, J.C. and Katsounaros, I. and Galeano, C. and Bongard, H.J. and Topalov, A.A. and Kostka, A. and Karschin, A. and Schüth, F. and Mayrhofer, K.J.J.
    Energy and Environmental Science 5 (2012)
    The search for more stable electrocatalyst materials for electrochemical energy conversion requires a fundamental understanding of the underlying degradation processes. Advanced characterization techniques like identical location transmission electron microscopy (IL-TEM) can provide invaluable insight into the stability of electrode materials on the nanoscale. In this review, the basic principles and the methodology of IL-TEM are described, and its capabilities are revealed by demonstrating the recent progress that has been achieved in research on the stability of fuel cell catalysts. Moreover, we provide future perspectives of the identical location approach towards implementing other electron microscopic and tomographic applications, which will help us to gain an even broader view of the degradation of electrocatalysts. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2ee22550f
  • Synthesis of titanium carbonitride coating layers with star-shaped crystallite morphology
    Garcia, J. and Pitonak, R. and Agudo, L. and Kostka, A.
    Materials Letters 68 (2012)
    Titanium carbonitride coating layers with star-shaped crystallite morphology were produced by chemical vapor deposition. Crystallites presenting a pentagonal symmetry nucleate at 880 °C and grow perpendicular to the surface by controlling the heating rate of the deposition process. Detailed transmission electron microscopy analyses of star-shaped crystallites along the [110] zone axis showed that each crystallite consists of five tetrahedra separated by (111) twins. A small-angle boundary consisting of edge dislocations forms as a result of elastic stress relaxation in the crystallites. The coatings presented a preferential texture in the direction (110) and an overall composition of Ti(C 0.15 N 0.85). © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.matlet.2011.10.008
  • Effect of shot-peening on the oxidation behaviour of boiler steels
    Naraparaju, R. and Christ, H.-J. and Renner, F.U. and Kostka, A.
    Oxidation of Metals 76 (2011)
    The presence of short diffusion paths is very important for rapid diffusion processes which are involved in forming protective oxide layers against high temperature corrosion, e.g. on boiler steels. Rapid diffusion paths can be produced by applying cold work such as shot-peening to the surface of the boiler steels prior to oxidation. The effect of shot-peening on oxidation behaviour was tested experimentally on 12 wt% Cr martensitic steel and 18 wt% Cr austenitic steel. Isothermal oxidation tests were performed at 700 and 750 °C. The surface treatment proved to be very effective in improving oxidation protection at 700 °C. Shot-peening the surface prior to the oxidation has an influential effect in changing the diffusion mechanisms of the elements involved in oxidation and changes the oxidation kinetics substantially at the applied conditions in this study. © 2011 Springer Science+Business Media, LLC.
    view abstract10.1007/s11085-011-9250-x
  • Hierarchical microstructure of explosive joints: Example of titanium to steel cladding
    Song, J. and Kostka, A. and Veehmayer, M. and Raabe, D.
    Materials Science and Engineering A 528 (2011)
    The microstructure of explosive cladding joints formed among parallel Ti and steel plates was examined by electron microscopy. The bonding interface and the bulk materials around it form pronounced hierarchical microstructures. This hierarchy is characterized by the following features: at the mesoscopic scale of the hierarchy a wavy course of the interface characterizes the interface zone. This microstructure level is formed by heavy plastic shear waves (wavelength≈0.5mm) which expand within the two metal plates during the explosion parallel to the bonding interface. At the micro-scale range, intermetallic inclusions (size≈100-200μm) are formed just behind the wave crests on the steel side as a result of partial melting. Electron diffraction revealed FeTi and metastable Fe9.64Ti0.36. Most of the observed phases do not appear in the equilibrium Fe-Ti phase diagram. These intermetallic inclusions are often accompanied by micro-cracks of similar dimension. At the smallest hierarchy level we observe a reaction layer of about 100-300nm thickness consisting of nano-sized grains formed along the entire bonding interface. Within that complex hierarchical micro- and nanostructure, the mesoscopic regime, more precisely the type and brittleness of the intermetallic zones, seems to play the dominant role for the mechanical behavior of the entire compound. © 2010 Elsevier B.V.
    view abstract10.1016/j.msea.2010.11.092
  • Influence of intermetallic phases and Kirkendall-porosity on the mechanical properties of joints between steel and aluminium alloys
    Springer, H. and Kostka, A. and dos Santos, J.F. and Raabe, D.
    Materials Science and Engineering A 528 (2011)
    The formation of intermetallic reaction layers and their influence on mechanical properties was investigated in friction stir welded joints between a low C steel and both pure Al (99.5wt.%) and Al-5wt.% Si. Characterisation of the steel/Al interface, tensile tests and fractography analysis were performed on samples in the as-welded state and after annealing in the range of 200-600°C for 9-64min. Annealing was performed to obtain reaction layers of distinct thickness and composition. For both Al alloys, the reaction layers grew with parabolic kinetics with the η phase (Al5Fe2) as the dominant component after annealing at 450°C and above. In joints with pure Al, the tensile strength is governed by the formation of Kirkendall-porosity at the reaction layer/Al interface. The tensile strength of joints with Al-5wt.% Si is controlled by the thickness of the η phase (Al5Fe2) layer. The pre-deformation of the base materials, induced by the friction stir welding procedure, was found to have a pronounced effect on the composition and growth kinetics of the reaction layers. © 2011 Elsevier B.V.
    view abstract10.1016/j.msea.2011.02.057
  • Investigations of Wear Mechanisms in Diamond Tools with Fe-Based Matrices Reinforced with WC-Co Particles
    Garcia, J. and Weber, S. and Kostka, A. and Pyzalla, A. and Garcia, L.F. and Lammer, A.
    Friction, Wear and Wear Protection: International Symposium on Friction, Wear and Wear Protection 2008 Aachen, Germany (2011)
    view abstract10.1002/9783527628513.ch92
  • On the formation and growth of intermetallic phases during interdiffusion between low-carbon steel and aluminum alloys
    Springer, H. and Kostka, A. and Payton, E.J. and Raabe, D. and Kaysser-Pyzalla, A. and Eggeler, G.
    Acta Materialia 59 (2011)
    The formation of intermetallic reaction layers was investigated for interdiffusion between a low-carbon steel and commercially pure aluminum (99.99%) and between a low-carbon steel and an aluminum-silicon alloy (Al-5 wt.% Si). Solid/solid, solid/semi-solid and solid/liquid diffusion couples were produced at both 600 and 675 °C. The total width of the reaction layer is governed mainly by the parabolic diffusion-controlled growth of the η phase (Al5Fe2), which exhibits orientation-dependent growth kinetics. The addition of Si to Al, which is known to decelerate reaction layer growth in interdiffusion experiments with Al melts, was found to accelerate the reaction layer growth in solid/semi-solid interdiffusion experiments. This phenomenon is discussed in light of previous atomistic explanations and the apparent activation energy calculated for the growth of the η phase (Al 5Fe2). © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2010.11.023
  • Size and size distribution of apatite crystals in sauropod fossil bones
    Dumont, M. and Kostka, A. and Sander, P.M. and Borbely, A. and Kaysser-Pyzalla, A.
    Palaeogeography, Palaeoclimatology, Palaeoecology 310 (2011)
    Two complementary techniques, X-ray diffraction and transmission electron microscopy, were used to analyze the size and size distribution of apatite crystallites in subfossil mammal and fossil sauropod dinosaur bones. The transmission electron microscope (TEM) samples were prepared by Focused Ion Beam (FIB). X-ray diffraction indicates that crystals in sauropod bones are larger than in subfossil mammal bones. For sauropods the average crystallite size (both in length and width) does not seem to increase with bone length. A similar result was obtained from TEM investigations regarding crystallites aspect ratio (length/width). The distribution of crystallite sizes in a juvenile sauropod was found to be much wider compared to an adult bone originating from the same bed and locality, consistent with the hypothesis that young animal bone contains a mixture of crystallite sizes, with a beneficial effect on mechanical properties. Diagenesis effects on crystal size dimensions have been considered. © 2011 Elsevier B.V.
    view abstract10.1016/j.palaeo.2011.06.021
  • Thermal stability of TiAIN/CrN multilayer coatings studied by atom probe tomography
    Choi, P.-P. and Povstugar, I. and Ahn, J.-P. and Kostka, A. and Raabe, D.
    Ultramicroscopy 111 (2011)
    This study is about the microstructural evolution of TiAlN/CrN multilayers (with a Ti:Al ratio of 0.75:0.25 and average bilayer period of 9 nm) upon thermal treatment. Pulsed laser atom probe analyses were performed in conjunction with transmission electron microscopy and X-ray diffraction. The layers are found to be thermally stable up to 600 °C. At 700 °C TiAlN layers begin to decompose into Ti- and Al-rich nitride layers in the out-of-plane direction. Further increase in temperature to 1000 °C leads to a strong decomposition of the multilayer structure as well as grain coarsening. Layer dissolution and grain coarsening appear to begin at the surface. Domains of AlN and TiCrN larger than 100 nm are found, together with smaller nano-sized AlN precipitates within the TiCrN matrix. Fe and V impurities are detected in the multilayers as well, which diffuse from the steel substrate into the coating along columnar grain boundaries. © 2010 Elsevier B.V.
    view abstract10.1016/j.ultramic.2010.11.012
  • Design and characterization of novel wear resistant multilayer CVD coatings with improved adhesion between Al2O3 and Ti(C,N)
    Garcia, J. and Pitonak, R. and Weißenbacher, R. and Köpf, A. and Soldera, F. and Suarez, S. and Miguel, F. and Pinto, H. and Kostka, A. and Mücklich, F.
    Advanced Engineering Materials 12 (2010)
    Multilayer CVD coatings for high speed cutting applications were designed to achieve high wear and heat resistance during machining of steel alloys. In this work the microstructure and cutting performance of these novel multilayer CVD coatings are investigated and compared with standard CVD multilayer coatings. 3D-FIB tomography is used to characterize the microstructure of the layers, especially the transition between the Ti(C,N) and the Al 2O3 layer. The 3D reconstruction of the surface of the Ti(C,N) layer shows the formation of protruded Ti(C,N) grains with a very particular architecture, which penetrate into the Al2O3 top-layer, providing a mechanical anchoring between both layers. Cemented carbides coated with the novel CVD multilayer present reduced crater and flank wear as well as improved adherence between the Al2O3 top-layer and the Ti(C,N) layer leading to a dramatic improvement of cutting performance. The microstructure and cutting performance of multilayer CVD coatings with a novel transition between the Ti(C,N) and the Al 2O3 layer are investigated. 3D-FIB tomography shows the formation of protruded Ti(C,N) grains with a particular architecture, which penetrate into the Al2O3 top-layer, providing a mechanical anchoring between both layers. Cutting tools coated with the novel CVD multilayer show dramatic improvement of cutting performance, due to reduced crater and flank wear and improved adherence between the Ti(C,N) and the Al 2O3 top-layer. © 2010 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/adem.201000130
  • Where Does the Lithium Go? - A Study of the Precipitates in the Stir Zone of a Friction Stir Weld in a Li-containing 2xxx Series Al Alloy
    Rao, J.C. and Payton, E.J. and Somsen, C. and Neuking, K. and Eggeler, G. and Kostka, A. and Dos Santos, J.F.
    Advanced Engineering Materials 12 (2010)
    The main strengthening precipitates of aluminum alloy 2198-T8, which are of the T1 phase, dissolve during friction stir welding, sending many Li atoms into solid solution. The stir zone precipitates are characterized using high-resolution transmission electron microscopy, energy dispersive spectroscopy, and selected area diffraction techniques to begin answering questions about the microstructural evolution and the relationship between microstructure and mechanical properties in friction stir welding of the next generation of lightweight Li-containing Al alloys. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.200900284
  • mechanical properties

  • microstructure

  • scanning electron microscopy

  • superalloys

  • transmission electron microscopy

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