Scientific Output

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

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  • 2020 • 243 Post-synthetic Modification of DUT-5-based Metal Organic Frameworks for the Generation of Single-site Catalysts and their Application in Selective Epoxidation Reactions
    Yildiz, C. and Kutonova, K. and Oßwald, S. and Titze-Alonso, A. and Bitzer, J. and Bräse, S. and Kleist, W.
    ChemCatChem 12 1134-1142 (2020)
    New single-site catalysts based on mixed-linker metal-organic frameworks with DUT-5 structure, which contain immobilized Co2+, Mn2+ and Mn3+ complexes, have successfully been synthesized via post-synthetic modification. 2,2’-Bipyridine-5,5’-dicarboxylate linkers were directly metalated, while 2-amino-4,4’-biphenyldicarboxylate linkers were post-synthetically modified by their conversion to Schiff-base ligands and a subsequent immobilization of the metal complexes. The resulting materials were used as catalysts in the selective epoxidation of trans-stilbene and the activities and selectivities of the different catalysts were compared. The influence of various reaction parameters on conversion, yield and selectivity were investigated. Very low catalyst amounts of 0.02 mol % were sufficient to obtain a high conversion of trans-stilbene using molecular oxygen from air as the oxidant. For cobalt-containing MOF catalysts, conversions up to 90 % were observed and, thus, they were more active than their manganese-containing counterparts. Recycling experiments and hot filtration tests proved that the reactions were mainly catalyzed via heterogeneous pathways. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cctc.201901434
  • 2019 • 242 Concentration-dependent atomic mobilities in FCC CoCrFeMnNi high-entropy alloys
    Gaertner, D. and Abrahams, K. and Kottke, J. and Esin, V.A. and Steinbach, I. and Wilde, G. and Divinski, S.V.
    Acta Materialia 166 357-370 (2019)
    The diffusion kinetics in a CoCrFeMnNi high entropy alloy is investigated by a combined radiotracer–interdiffusion experiment applied to a pseudo-binary Co15Cr20Fe20Mn20Ni25/Co25Cr20Fe20Mn20Ni15 couple. As a result, the composition-dependent tracer diffusion coefficients of Co, Cr, Fe and Mn are determined. The elements are characterized by significantly different diffusion rates, with Mn being the fastest element and Co being the slowest one. The elements having originally equiatomic concentration through the diffusion couple are found to reveal up-hill diffusion, especially Cr and Mn. The atomic mobility of Co seems to follow a S-shaped concentration dependence along the diffusion path. The experimentally measured kinetic data are checked against the existing CALPHAD-type databases. In order to ensure a consistent treatment of tracer and chemical diffusion a generalized symmetrized continuum approach for multi-component interdiffusion is proposed. Both, tracer and chemical diffusion concentration profiles are simulated and compared to the measurements. By using the measured tracer diffusion coefficients the chemical profiles can be described, almost perfectly, including up-hill diffusion. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.033
  • 2019 • 241 Tuning the magnetic anisotropy of niptmnga by substitution and epitaxial strain
    Herper, H.C. and Grunebohm, A.
    IEEE Transactions on Magnetics 55 (2019)
    Large magnetocrystalline anisotropy (MCA) is of high technical relevance, in particular for magnetic actuators, permanent magnets, and memory devices with high density. Large MCAs have been reported for the low temperature L10 phase of Ni2MnGa. Both, Mn and Pt substitution can stabilize this phase at and above room temperature. Despite the larger spin-orbit coupling in the heavy 5d-element Pt, it has been reported that Pt substitution may result in degeneration of the MCA. In this paper, we study the MCA for a combination of epitaxial strain and Mn and Pt substitution based on density functional theory methods. We show that one can stabilize both large uniaxial and easy-plane anisotropies depending on the value of strain. In particular, small changes of the applied strain may allow to switch between low- and high-anisotropy states or even switch the direction of the easy-axis magnetization direction. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2018.2856461
  • 2019 • 240 Segregation-driven grain boundary spinodal decomposition as a pathway for phase nucleation in a high-entropy alloy
    Li, L. and Li, Z. and Kwiatkowski da Silva, A. and Peng, Z. and Zhao, H. and Gault, B. and Raabe, D.
    Acta Materialia 178 1-9 (2019)
    Elemental segregation to grain boundaries (GBs) can induce structural and chemical transitions at GBs along with significant changes in material properties. The presence of multiple principal elements interacting in high-entropy alloys (HEAs) makes the GB segregation and interfacial phase transformation a rather challenging subject to investigate. Here, we explored the temporal evolution of the chemistry for general high-angle GBs in a typical equiatomic FeMnNiCoCr HEA during aging heat treatment through detailed atom probe tomography (APT) analysis. We found that the five principal elements segregate heterogeneously at the GBs. More specifically, Ni and Mn co-segregate to some regions of the GBs along with the depletion of Fe, Co and Cr, while Cr is enriched in other regions of the GBs where Ni and Mn are depleted. The redistribution of these elements on the GBs follow a periodic characteristic, spinodal-like compositional modulation. The accumulation of elements at the GBs can create local compositions by shifting their state from a solid solution (like in the adjacent bulk region) into a spinodal regime to promote interfacial phase-like transitions as segregation proceeds. These results not only shed light on phase precursor states and the associated nucleation mechanism at GBs in alloy systems with multiple principal elements but also help to guide the microstructure design of advanced HEAs in which formation of embrittling phases at interfaces must be avoided. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.07.052
  • 2019 • 239 Revealing fracture mechanisms of medium manganese steels with and without delta-ferrite
    Sun, B. and Palanisamy, D. and Ponge, D. and Gault, B. and Fazeli, F. and Scott, C. and Yue, S. and Raabe, D.
    Acta Materialia 164 683-696 (2019)
    Medium Mn steels possess a composite like microstructure containing multiple phase constituents like metastable austenite, ferrite, δ-ferrite and α′-martensite with a wide range of fractions for each constituent. The high mechanical contrast among them and the deformation-driven evolution of the microstructure lead to complex fracture mechanisms. Here we investigate tensile fracture mechanisms of medium Mn steels with two typical types of microstructures. One group consists of ferrite (α) plus austenite (γ) and the other one of a layered structure with an austenite-ferrite constituent and δ-ferrite. Samples with the first type of microstructure show a dimple-type fracture due to void formation primarily at the ferrite/strain-induced α′-martensite (α′) interfaces. In contrast, the fracture surface of δ-ferrite containing steels shows a combination of cleavage in δ-ferrite and dimple/quasi-cleavage zones in the γ-α (or γ/α′-α) constituent. The embrittlement of δ-ferrite is due to the formation of B2 ordered phase. Failure of these samples is govern by crack initiation related to δ-ferrite and crack-arresting ability of the γ-α layers. Austenite stability is critical for the alloys' fracture resistance, in terms of influencing void growth and coalescence for the first type of microstructure and crack initiation and termination for the microstructure containing δ-ferrite. This effect is here utilized to increase ductility and toughness. By tailoring austenite stability towards higher fracture resistance, the total elongation of δ-ferrite containing steels increases from ∼13% to ∼33%. This approach opens a new pathway towards an austenite-stability-controlled microstructural design for substantially enhanced damage tolerance in steels containing metastable austenite and δ-ferrite. © 2018
    view abstractdoi: 10.1016/j.actamat.2018.11.029
  • 2019 • 238 Optical orientation of acceptor-bound hole magnetic polarons in bulk (Cd,Mn)Te
    Zhukov, E.A. and Kusrayev, Y.G. and Kirstein, E. and Thomann, A. and Salewski, M. and Kozyrev, N.V. and Yakovlev, D.R. and Bayer, M.
    Physical Review B 99 (2019)
    The optically induced long-lived spin polarization in the bulk diluted magnetic semiconductor (Cd,Mn)Te with small manganese concentration is studied by picosecond pump-probe Kerr rotation. At temperatures below 6 K and in transversal magnetic field the Kerr rotation signal contains three components: two oscillating components, corresponding to the Larmor precession of manganese spins and spins of photoexcited electrons, and a long-lived (up to 15 ns) nonoscillating component. The latter one is provided by optical orientation of equilibrium hole magnetic polarons involving holes bound to acceptors. The origin of the anisotropy controlling the orientation and the spin dynamics of the acceptor-bound hole magnetic polaron in bulk (Cd,Mn)Te is discussed. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.115204
  • 2018 • 237 Manganese(II) Molecular Sources for Plasma-Assisted CVD of Mn Oxides and Fluorides: From Precursors to Growth Process
    Barreca, D. and Carraro, G. and Fois, E. and Gasparotto, A. and Gri, F. and Seraglia, R. and Wilken, M. and Venzo, A. and Devi, A. and Tabacchi, G. and Maccato, C.
    Journal of Physical Chemistry C 122 1367-1375 (2018)
    A viable route to manganese-based materials of high technological interest is plasma-assisted chemical vapor deposition (PA-CVD), offering various degrees of freedom for the growth of high-purity nanostructures from suitable precursors. In this regard, fluorinated β-diketonate diamine Mn(II) complexes of general formula Mn(dik)2·TMEDA [TMEDA = N,N,N′,N′-tetramethylethylenediamine; Hdik = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (Hhfa), or 1,1,1-trifluoro-2,4-pentanedione (Htfa)] represent a valuable option in the quest of candidate molecular sources for PA-CVD environments. In this work, we investigate and highlight the chemico-physical properties of these compounds of importance for their use in PA-CVD processes, through the use of a comprehensive experimental-theoretical investigation. Preliminary PA-CVD validation shows the possibility of varying the Mn oxidation state, as well as the system chemical composition from MnF2 to MnO2, by simple modulations of the reaction atmosphere, paving the way to a successful utilization of the target compounds in the growth of manganese-containing nanomaterials for different technological applications. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.7b10277
  • 2018 • 236 Effects of strain rate on mechanical properties and deformation behavior of an austenitic Fe-25Mn-3Al-3Si TWIP-TRIP steel
    Benzing, J.T. and Poling, W.A. and Pierce, D.T. and Bentley, J. and Findley, K.O. and Raabe, D. and Wittig, J.E.
    Materials Science and Engineering A 711 78-92 (2018)
    The effects of quasi-static and low-dynamic strain rate (ε̇ = 10−4 /s to ε̇ = 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21 ± 3 mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.11.017
  • 2018 • 235 Annealing-time and annealing-temperature dependencies of the size of Ni-Mn-In shell-ferromagnetic nano-precipitates by Scherrer analysis
    Dincklage, L. and Scheibel, F. and Çaklr, A. and Farle, M. and Acet, M.
    AIP Advances 8 (2018)
    Shell-ferromagnetic effects are observed in Ni-Mn-based off-stoichiometric Heuslers decomposed into ferromagnetic precipitates embedded in an antiferromagnetic matrix when the surface-to-volume ratio of the precipitates are sufficiently large. However, since the size of the precipitates have until now not been determined, it is not known which ratios are involved. Here we carry out a Scherrer analysis on decomposed specimens to determine the precipitate-size as a function of decomposition temperature and time. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5018851
  • 2018 • 234 Competition between formation of carbides and reversed austenite during tempering of a medium-manganese steel studied by thermodynamic-kinetic simulations and atom probe tomography
    Kwiatkowski da Silva, A. and Inden, G. and Kumar, A. and Ponge, D. and Gault, B. and Raabe, D.
    Acta Materialia 147 165-175 (2018)
    We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-0.1C-0.5Si medium manganese steel during low temperature tempering. The material was annealed up to 24 h at 450 °C in order to follow the kinetics of precipitation. Using thermodynamics and kinetics simulations, we predicted the growth of M23C6 carbides according to the local-equilibrium negligible partition (LENP) mode where carbide growth is controlled by the diffusion of carbon, while maintaining local chemical equilibrium at the interface. Atom-probe tomography (APT) measurements performed on samples annealed for 1, 6 and 24 h at 450 °C confirmed that LENP is indeed the mode of carbide growth and that Mn segregation is necessary for the nucleation. Additionally, we observed the heterogeneous nucleation of transition carbides with a carbon content between 6 and 8 at% at segregated dislocations and grain boundaries. We describe these carbides as a complex face-centered cubic transition carbide type (CFCC-TmC phase) obtained by the supersaturation of the FCC structure by carbon that will act as precursor to the more stable γ-M23C6 carbide that forms at the dislocations and grain boundaries. The results suggest that the addition of carbon does not directly favor the formation of austenite, since Mn is consumed by the formation of the carbides and the nucleation of austenite is thus retarded to later stages of tempering as every FCC nucleus in the initial stages of tempering is readily converted into a carbide nucleus. We propose the following sequence of transformation: (i) carbon and Mn co-segregation to dislocations and grain boundaries; (ii) formation of FCC transition carbides; (iii) growth controlled according to the LENP mode and (iv) austenite nucleation and growth. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.01.022
  • 2018 • 233 Phase nucleation through confined spinodal fluctuations at crystal defects evidenced in Fe-Mn alloys
    Kwiatkowski Da Silva, A. and Ponge, D. and Peng, Z. and Inden, G. and Lu, Y. and Breen, A. and Gault, B. and Raabe, D.
    Nature Communications 9 (2018)
    Analysis and design of materials and fluids requires understanding of the fundamental relationships between structure, composition, and properties. Dislocations and grain boundaries influence microstructure evolution through the enhancement of diffusion and by facilitating heterogeneous nucleation, where atoms must overcome a potential barrier to enable the early stage of formation of a phase. Adsorption and spinodal decomposition are known precursor states to nucleation and phase transition; however, nucleation remains the less well-understood step in the complete thermodynamic sequence that shapes a microstructure. Here, we report near-atomic-scale observations of a phase transition mechanism that consists in solute adsorption to crystalline defects followed by linear and planar spinodal fluctuations in an Fe-Mn model alloy. These fluctuations provide a pathway for austenite nucleation due to the higher driving force for phase transition in the solute-rich regions. Our observations are supported by thermodynamic calculations, which predict the possibility of spinodal decomposition due to magnetic ordering. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03591-4
  • 2018 • 232 Element- and momentum-resolved electronic structure of the dilute magnetic semiconductor manganese doped gallium arsenide
    Nemšák, S. and Gehlmann, M. and Kuo, C.-T. and Lin, S.-C. and Schlueter, C. and Mlynczak, E. and Lee, T.-L. and Plucinski, L. and Ebert, H. and Di Marco, I. and Minár, J. and Schneider, C.M. and Fadley, C.S.
    Nature Communications 9 (2018)
    The dilute magnetic semiconductors have promise in spin-based electronics applications due to their potential for ferromagnetic order at room temperature, and various unique switching and spin-dependent conductivity properties. However, the precise mechanism by which the transition-metal doping produces ferromagnetism has been controversial. Here we have studied a dilute magnetic semiconductor (5% manganese-doped gallium arsenide) with Bragg-reflection standing-wave hard X-ray angle-resolved photoemission spectroscopy, and resolved its electronic structure into element- and momentum- resolved components. The measured valence band intensities have been projected into element-resolved components using analogous energy scans of Ga 3d, Mn 2p, and As 3d core levels, with results in excellent agreement with element-projected Bloch spectral functions and clarification of the electronic structure of this prototypical material. This technique should be broadly applicable to other multi-element materials. © 2018, The Author(s).
    view abstractdoi: 10.1038/s41467-018-05823-z
  • 2018 • 231 Elemental segregation to twin boundaries in a MnAl ferromagnetic Heusler alloy
    Palanisamy, D. and Raabe, D. and Gault, B.
    Scripta Materialia 155 144-148 (2018)
    Electron microscopy and atom probe tomography were combined to investigate the crystallography and chemistry of a single twin boundary (TB) in a rare-earth-free ferromagnetic MnAl Heusler alloy. The results establish a significant segregation of Mn along the twin boundaries. An enrichment of approx. ~8 at.% Mn was measured along the twin boundary with a confined depletion outside the twin boundary, suggesting short range solute diffusion occurring during massive transformation. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.06.037
  • 2018 • 230 In-situ SEM observation of phase transformation and twinning mechanisms in an interstitial high-entropy alloy
    Wang, M. and Li, Z. and Raabe, D.
    Acta Materialia 147 236-246 (2018)
    The recently developed interstitial high-entropy alloys (iHEAs) exhibit an enhanced combination of strength and ductility. These properties are attributed to dislocation hardening, deformation-driven athermal phase transformation from the face-centered cubic (FCC) γ matrix into the hexagonal close-packed (HCP) ε phase, stacking fault formation, mechanical twinning and precipitation hardening. For gaining a better understanding of these mechanisms as well as their interactions direct observation of the deformation process is required. For this purpose, an iHEA with nominal composition of Fe-30Mn-10Co-10Cr-0.5C (at. %) was produced and investigated via in-situ and interrupted in-situ tensile testing in a scanning electron microscope (SEM) combining electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) techniques. The results reveal that the iHEA is deformed by formation and multiplication of stacking faults along {111} microbands. Sufficient overlap of stacking faults within microbands leads to intrinsic nucleation of HCP ε phase and incoherent annealing twin boundaries act as preferential extrinsic nucleation sites for HCP ε formation. With further straining HCP ε nuclei grow into the adjacent deformed FCC γ matrix. γ regions with smaller grain size have higher mechanical stability against phase transformation. Twinning in FCC γ grains with a size of ∼10 μm can be activated at room temperature at a stress below ∼736 MPa. With increasing deformation, new twin lamellae continuously nucleate. The twin lamellae grow in preferred directions driven by the motion of the mobile partial dislocations. Owing to the individual grain size dependence of the activation of the dislocation-mediated plasticity, of the athermal phase transformation and of mechanical twinning at the different deformation stages, desired strain hardening profiles can be tuned and adjusted over the entire deformation regime by adequate microstructure design, providing excellent combinations of strength and ductility. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.01.036
  • 2018 • 229 Mn-Alloyed High-Strength Steels with a Reduced Austenitization Temperature: Thermodynamic Calculations and Experimental Investigations
    Windmann, M. and Opitz, T. and Klein, S. and Röttger, A. and Theisen, W.
    Steel Research International 89 (2018)
    High-strength steels (e.g., 1.5528–22MnB5), processed by direct press-hardening, are widely used for security-relevant structures in automotive bodyworks. In this study, the austenitization temperature AC3 of the steel 22MnB5 (approx. 840 °C) is decreased to enable a reduction in the heat-treatment temperature. Thermodynamic calculations using the CALPHAD method are used to assess the effect of alloying elements on the α–γ transformation temperatures. On this account, 22MnB5 steel is alloyed with 6 to 9.5 mass% manganese, which decreases the α–γ transformation temperature to 744 °C. Simultaneously, the martensite finish temperature decreases below room temperature, which is accompanied by the presence of retained austenite after hardening. Furthermore, ϵ-martensite is formed. High Mn-alloyed steel 22MnB5 (9.5 mass% Mn, AC3 = 744 °C) possesses a high strength of Rm = 1618 MPa, similar to the initial material 22MnB5. Elongation-to-fracture decreases to A5 = 3.5% due to the formation of ϵ-martensite. The material strength of the steel alloyed with 6 mass% manganese (AC3 = 808 °C) strongly increases to Rm = 1975 MPa as a result of α-martensite and solid-solution strengthening by the element manganese. This steel possesses a higher elongation-to-fracture of A5 = 7%. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/srin.201800166
  • 2017 • 228 Polybenzoxazine-Derived N-doped Carbon as Matrix for Powder-Based Electrocatalysts
    Barwe, S. and Andronescu, C. and Masa, J. and Ventosa, E. and Klink, S. and Genç, A. and Arbiol, J. and Schuhmann, W.
    ChemSusChem 10 2653-2659 (2017)
    In addition to catalytic activity, intrinsic stability, tight immobilization on a suitable electrode surface, and sufficient electronic conductivity are fundamental prerequisites for the long-term operation of particle- and especially powder-based electrocatalysts. We present a novel approach to concurrently address these challenges by using the unique properties of polybenzoxazine (pBO) polymers, namely near-zero shrinkage and high residual-char yield even after pyrolysis at high temperatures. Pyrolysis of a nanocubic prussian blue analogue precursor (KmMnx[Co(CN)6]y⋅n H2O) embedded in a bisphenol A and aniline-based pBO led to the formation of a N-doped carbon matrix modified with MnxCoyOz nanocubes. The obtained electrocatalyst exhibits high efficiency toward the oxygen evolution reaction (OER) and more importantly a stable performance for at least 65 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201700593
  • 2017 • 227 First principles modeling of 3d-metal doped three-layer fluorite-structured TiO2 (4,4) nanotube to be used for photocatalytic hydrogen production
    Bocharov, D. and Piskunov, S. and Zhukovskii, Y.F. and Spohr, E. and D'yachkov, P.N.
    Vacuum 146 562-569 (2017)
    We have estimated theoretically the photocatalytic suitability of thinnest single-wall fluorite-structured titania (4,4) nanotube (NT) possessing three layers each (O-Ti-O) and doped by Sc, V, Cr, Mn, Fe, Co, Ni, Cu and Zn atoms substituted for host Ti atoms. For this goal, we have performed large-scale ab initio calculations on TiO2 NTs with three-layer morphology doped by 3d transition metals, using (i) the method of linear combination of atom-centered Gaussian-type orbitals (LCAO) based on the hybrid density functional theory (DFT) incorporating the Hartree-Fock (HF) exchange contribution (DFT+HF) and (ii) the method of linearized augmented cylindrical waves (LACW) with the muffin-tin approximation based on the local density functional approach (LDA). We have compared the ground state electronic structure, particularly the one-electron densities of states (DOSs) from the LCAO and LACW calculations for periodic arrangements of the 3d-metal dopant atoms. The results show clear evidence for a potential photocatalytic application for water splitting in the case of the Sc-doped titania nanotubes only. These NTs show both a reduced band gap of 2.0 eV relative to the pristine NT and an absence of defect-induced levels between the redox potentials of hydrogen and oxygen, so that electron-hole recombination becomes unlikely. Other 3d dopants with higher atomic number, although their band gap also covers the favorable green to orange region of the solar spectrum, are unsuitable because their defect-induced levels are positioned between the redox potential of oxygen and hydrogen, which can be expected to lead to rapid electron-hole recombination. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.vacuum.2017.05.002
  • 2017 • 226 Shell-ferromagnetic precipitation in martensitic off-stoichiometric Ni-Mn-In Heusler alloys produced by temper-annealing under magnetic field
    Çakır, A. and Acet, M. and Wiedwald, U. and Krenke, T. and Farle, M.
    Acta Materialia 127 117-123 (2017)
    The variety of the multifunctional properties of martensitic Ni-Mn based Heusler alloys are related to the presence of a magnetostructural transition. We report here on a new functionality based on a newly observed property. The observed property is that all off-stoichiometric Ni-Mn-based Heuslers, here in the form Ni50Mn50−xInx with 0< x< 25, decompose into predominantly cubic ferromagnetic Ni50Mn25In25 and tetragonal antiferromagnetic NiMn components when temper-annealed. The new functionality is based on magnetic field assisted temper-annealing of a compound with stoichiometry x=5, whereby precipitates of Ni50Mn25In25 with a ferromagnetic shell are formed with spins in the field direction, strongly pinned by the surrounding antiferromagnetic anisotropy, even at temperatures as high as 500 K. The remanent pinning at high temperatures survives any thermal cycling between lowest temperatures and the annealing temperature and any magnetic field cycling between −9 and +9 T. The resulting product can serve as a thermally stable, magnetic-field-proof memory. © 2017
    view abstractdoi: 10.1016/j.actamat.2017.01.027
  • 2017 • 225 Dislocation interaction and twinning-induced plasticity in face-centered cubic Fe-Mn-C micro-pillars
    Choi, W.S. and Sandlöbes, S. and Malyar, N.V. and Kirchlechner, C. and Korte-Kerzel, S. and Dehm, G. and De Cooman, B.C. and Raabe, D.
    Acta Materialia 132 162-173 (2017)
    Deformation twinning contributes to a high work-hardening rate through modification of the dislocation structure and a dynamic Hall-Petch effect in polycrystalline steel. Due to the well-defined compression axis and limited deformation volume of micro-pillars, micro-compression testing is a suitable method to investigate the mechanisms of deformation twinning and the interactions of dislocations with twin boundaries. The material investigated is an austenitic Fe-22 wt%Mn-0.6 wt%C twining-induced plasticity steel. Micro-pillars oriented preferentially for deformation twinning and dislocation glide are compressed and the activated deformation systems are characterized. We observe that deformation twinning induces higher flow stresses and a more unstable work-hardening behavior than dislocation glide, while dislocation glide dominated deformation results in a stable work-hardening behavior. The higher flow stresses and unstable work-hardening behavior in micro-pillars oriented for deformation twinning are assumed to be caused by the activation of secondary slip systems and accumulated plastic deformation. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.04.043
  • 2017 • 224 Durable and cost-effective neutral density filters utilizing multiple reflections in glass slide stacks
    Czajkowski, K. and Schmid, M.
    IEEE Photonics Journal (2017)
    We propose an application of a stack of glass slides as a broadband neutral density (ND) filter with high (&#x003E;100 W/cm<formula><tex>$^2$</tex></formula>) damage threshold. The influence of multiple reflections on the filter transmittance is analyzed with transfer matrix method. The numerical study proves that the filter spectrum is determined by material dispersion and degree of wavelength averaging occurring due to light incoherence and limited measurement resolution. Varying the angle of light incidence can be used to extend the possible optical density range of the filters. The numerical results are verified by spectral measurements with setups containing coherent and incoherent light sources. The applicability of the filters is proven in a concentrated sun simulator. OAPA
    view abstractdoi: 10.1109/JPHOT.2017.2773500
  • 2017 • 223 The effects of prior austenite grain boundaries and microstructural morphology on the impact toughness of intercritically annealed medium Mn steel
    Han, J. and da Silva, A.K. and Ponge, D. and Raabe, D. and Lee, S.-M. and Lee, Y.-K. and Lee, S.-I. and Hwang, B.
    Acta Materialia 122 199-206 (2017)
    The effects of prior austenite (γ) grain boundaries and microstructural morphology on the impact toughness of an annealed Fe-7Mn-0.1C-0.5Si medium Mn steel were investigated for two different microstructure states, namely, hot-rolled and annealed (HRA) specimens and cold-rolled and annealed (CRA) specimens. Both types of specimens had a dual-phase microstructure consisting of retained austenite (γR) and ferrite (α) after intercritical annealing at 640 °C for 30 min. The phase fractions and the chemical composition of γR were almost identical in both types of specimens. However, their microstructural morphology was different. The HRA specimens had lath-shaped morphology and the CRA specimens had globular-shaped morphology. We find that both types of specimens showed transition in fracture mode from ductile and partly quasi-cleavage fracture to intergranular fracture with decreasing impact test temperature from room temperature to −196 °C. The HRA specimen had higher ductile to brittle transition temperature and lower low-temperature impact toughness compared to the CRA specimen. This was due to intergranular cracking in the HRA specimens along prior γ grain boundaries decorated by C, Mn and P. In the CRA specimen intergranular cracking occurred along the boundaries of the very fine α and α′ martensite grains. The results reveal that cold working prior to intercritical annealing promotes the elimination of the solute-decorated boundaries of coarse prior γ grains through the recrystallization of αʹ martensite prior to reverse transformation, hence improving the low-temperature impact toughness of medium Mn steel. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.09.048
  • 2017 • 222 Precipitation of T1 and θ' phase in Al-4Cu-1Li-0.25Mn during age hardening: Microstructural investigation and phase-field simulation
    Häusler, I. and Schwarze, C. and Bilal, M.U. and Ramirez, D.V. and Hetaba, W.d and Kamachali, R.D. and Skrotzki, B.
    Materials 10 (2017)
    Experimental and phase field studies of age hardening response of a high purity Al-4Cu-1Li-0.25Mn-alloy (mass %) during isothermal aging are conducted. In the experiments, two hardening phases are identified: the tetragonal θ' (Al2Cu) phase and the hexagonal T1 (Al2CuLi) phase. Both are plate shaped and of nm size. They are analyzed with respect to the development of their size, number density and volume fraction during aging by applying different analysis techniques in TEM in combination with quantitative microstructural analysis. 3D phase-field simulations of formation and growth of θ' phase are performed in which the full interfacial, chemical and elastic energy contributions are taken into account. 2D simulations of T1 phase are also investigated using multi-component diffusion without elasticity. This is a first step toward a complex phase-field study of T1 phase in the ternary alloy. The comparison between experimental and simulated data shows similar trends. The still unsaturated volume fraction indicates that the precipitates are in the growth stage and that the coarsening/ripening stage has not yet been reached. © 2017 by the authors.
    view abstractdoi: 10.3390/ma10020117
  • 2017 • 221 Overview of hydrogen embrittlement in high-Mn steels
    Koyama, M. and Akiyama, E. and Lee, Y.-K. and Raabe, D. and Tsuzaki, K.
    International Journal of Hydrogen Energy (2017)
    Hydrogen and fuels derived from it will serve as the energy carriers of the future. The associated rapidly growing demand for hydrogen energy-related infrastructure materials has stimulated multiple engineering and scientific studies on the hydrogen embrittlement resistance of various groups of high performance alloys. Among these, high-Mn steels have received special attention owing to their excellent strength - ductility - cost relationship. However, hydrogen-induced delayed fracture has been reported to occur in deep-drawn cup specimens of some of these alloys. Driven by this challenge we present here an overview of the hydrogen embrittlement research carried out on high-Mn steels. The hydrogen embrittlement susceptibility of high-Mn steels is particularly sensitive to their chemical composition since the various alloying elements simultaneously affect the material's stacking fault energy, phase stability, hydrogen uptake behavior, surface oxide scales and interstitial diffusivity, all of which affect the hydrogen embrittlement susceptibility. Here, we discuss the contribution of each of these factors to the hydrogen embrittlement susceptibility of these steels and discuss pathways how certain embrittlement mechanisms can be hampered or even inhibited. Examples of positive effects of hydrogen on the tensile ductility are also introduced. © 2017 Hydrogen Energy Publications LLC.
    view abstractdoi: 10.1016/j.ijhydene.2017.02.214
  • 2017 • 220 Confined chemical and structural states at dislocations in Fe-9wt%Mn steels: A correlative TEM-atom probe study combined with multiscale modelling
    Kwiatkowski da Silva, A. and Leyson, G. and Kuzmina, M. and Ponge, D. and Herbig, M. and Sandlöbes, S. and Gault, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 124 305-315 (2017)
    We investigated a high-purity cold-rolled martensitic Fe-9wt%Mn alloy. Tensile tests performed at room temperature after tempering for 6 h at 450 °C showed discontinuous yielding. Such static strain ageing phenomena in Fe are usually associated with the segregation of interstitial elements such as C or N to dislocations. Here we show by correlative transmission electron microscopy (TEM)/atom probe tomography (APT) experiments that in this case Mn segregation to edge dislocations associated with the formation of confined austenitic states causes similar effects. The local chemical composition at the dislocation cores was investigated for different tempering temperatures by APT relative to the adjacent bcc matrix. In all cases the Mn partitioning to the dislocation core regions matches to the one between ferrite and austenite in thermodynamic equilibrium at the corresponding tempering temperature. Although a stable structural and chemical confined austenitic state has formed at the dislocation cores these regions do not grow further even upon prolonged tempering. Simulation reveals that the high Mn enrichment along the edge dislocation lines (25 at.%Mn at 450 °C) cannot be described merely as a Cottrell atmosphere formed by segregation driven by size interaction. Thermodynamic calculations based on a multiscale model indicate that these austenite states at the dislocation cores are subcritical and defect-stabilized by the compression stress field of the edge dislocations. Phenomenologically, these states are the 1D equivalent to the so-called complexions which have been extensively reported to be present at 2D defects, hence have been named linear complexions. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.11.013
  • 2017 • 219 A TRIP-assisted dual-phase high-entropy alloy: Grain size and phase fraction effects on deformation behavior
    Li, Z. and Tasan, C.C. and Pradeep, K.G. and Raabe, D.
    Acta Materialia 131 323-335 (2017)
    We present a systematic microstructure oriented mechanical property investigation for a newly developed class of transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) with varying grain sizes and phase fractions. The DP-HEAs in both, as-homogenized and recrystallized states consist of a face-centered cubic (FCC) matrix containing a high-density of stacking faults and a laminate hexagonal close-packed (HCP) phase. No elemental segregation was observed in grain interiors or at interfaces even down to near-atomic resolution, as confirmed by energy-dispersive X-ray spectroscopy and atom probe tomography. The strength-ductility combinations of the recrystallized DP-HEAs (Fe50Mn30Co10Cr10) with varying FCC grain sizes and HCP phase fractions prior to deformation are superior to those of the recrystallized equiatomic single-phase Cantor reference HEA (Fe20Mn20Ni20Co20Cr20). The multiple deformation micro-mechanisms (including strain-induced transformation from FCC to HCP phase) and dynamic strain partitioning behavior among the two phases are revealed in detail. Both, strength and ductility of the DP-HEAs increase with decreasing the average FCC matrix grain size and increasing the HCP phase fraction prior to loading (in the range of 10–35%) due to the resulting enhanced stability of the FCC matrix. These insights are used to project some future directions for designing advanced TRIP-HEAs through the adjustment of the matrix phase's stability by alloy tuning and grain size effects. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.03.069
  • 2017 • 218 Influence of compositional inhomogeneity on mechanical behavior of an interstitial dual-phase high-entropy alloy
    Li, Z. and Raabe, D.
    Materials Chemistry and Physics (2017)
    In this study we present and discuss the influence of compositional inhomogeneity on the mechanical behavior of an interstitially alloyed dual-phase non-equiatomic high-entropy alloy (Fe49.5Mn30Co10Cr10C0.5). Various processing routes including hot-rolling, homogenization, cold-rolling and recrystallization annealing were performed on the cast alloys to obtain samples in different compositional homogeneity states. Grain sizes of the alloys were also considered. Tensile testing and microstructural investigations reveal that the deformation behavior of the interstitial dual-phase high-entropy alloy samples varied significantly depending on the compositional homogeneity of the specimens probed. In the case of coarse-grains (∼300 μm) obtained for cast alloys without homogenization treatment, ductility and strain-hardening of the material was significantly reduced due to its compositional inhomogeneity. This detrimental effect was attributed to preferred deformation-driven phase transformation occurring in the Fe enriched regions with lower stacking fault energy, promoting early stress-strain localization. The grain-refined alloy (∼4 μm) with compositional heterogeneity which was obtained for recrystallization annealed alloys without homogenization treatment was characterized by almost total loss in work-hardening. This effect was attributed to large local shear strains due to the inhomogeneous planar slip. These insights demonstrate the essential role of compositional homogeneity through applying corresponding processing steps for the development of advanced high-entropy alloys. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2017.04.050
  • 2017 • 217 Molecular Engineering of MnII Diamine Diketonate Precursors for the Vapor Deposition of Manganese Oxide Nanostructures
    Maccato, C. and Bigiani, L. and Carraro, G. and Gasparotto, A. and Seraglia, R. and Kim, J. and Devi, A. and Tabacchi, G. and Fois, E. and Pace, G. and Di Noto, V. and Barreca, D.
    Chemistry - A European Journal 23 17954-17963 (2017)
    Molecular engineering of manganese(II) diamine diketonate precursors is a key issue for their use in the vapor deposition of manganese oxide materials. Herein, two closely related β-diketonate diamine MnII adducts with different fluorine contents in the diketonate ligands are examined. The target compounds were synthesized by a simple procedure and, for the first time, thoroughly characterized by a joint experimental–theoretical approach, to understand the influence of the ligand on their structures, electronic properties, thermal behavior, and reactivity. The target compounds are monomeric and exhibit a pseudo-octahedral coordination of the MnII centers, with differences in their structure and fragmentation processes related to the ligand nature. Both complexes can be readily vaporized without premature side decompositions, a favorable feature for their use as precursors for chemical vapor deposition (CVD) or atomic layer deposition applications. Preliminary CVD experiments at moderate growth temperatures enabled the fabrication of high-purity, single-phase Mn3O4 nanosystems with tailored morphology, which hold great promise for various technological applications. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201703423
  • 2017 • 216 Diffusion-related SOFC stack degradation
    Menzler, N.H. and Beez, A. and Grünwald, N. and Sebold, D. and Fang, Q. and Vaßen, R.
    ECS Transactions 78 2223-2230 (2017)
    As part of two different stack tests with four-plane short stacks and their intensive post-test characterization, two varying diffusionrelated degradation mechanisms were investigated. The first was a short-term test (∼1250h) with two different chromium evaporation protection layers on the air-side metallic interconnect and frame and the second was a long-term endurance test (∼ 35,000h). For the first stack, two planes were coated with a manganese oxide layer applied by wet powder spraying (WPS), while the other two planes were coated with a manganese-cobalt-iron spinel layer by atmospheric plasma spraying (APS). The voltage loss in the planes with a WPS-coated interconnect was markedly higher than in those coated by means of APS. Finally, it was shown that the microstructure of the layers plays a key role in minimizing Cr evaporation. In this stack, gas-phase diffusion prevails over degradation. In the long-term stack, severe degradation due to solid-state manganese diffusion was observed. This paper draws an interaction hypothesis. © The Electrochemical Society.
    view abstractdoi: 10.1149/07801.2223ecst
  • 2017 • 215 Promotional Effect of Fe Impurities in Graphene Precursors on the Activity of MnOX/Graphene Electrocatalysts for the Oxygen Evolution and Oxygen Reduction Reactions
    Morales, D.M. and Masa, J. and Andronescu, C. and Schuhmann, W.
    ChemElectroChem 4 2835-2841 (2017)
    Bifunctional oxygen electrocatalysts were fabricated following a three-step synthesis method, which consisted of i) liquid-phase exfoliation of graphite in the presence of nitrogen-containing manganese macrocyclic complexes, using DMF as the dispersion medium under formation of few-layer graphene sheets. Subsequently, ii) solvent removal by vacuum filtration and drying, and iii) pyrolysis of the resulting composites under an inert gas atmosphere with subsequent mild calcination yielded manganese oxides embedded within a graphitic carbon matrix (MnOX/G). We further demonstrate that traces of Fe impurities in the used graphite result in enhanced electrocatalytic activity of the MnOX/G towards both the oxygen reduction and the oxygen evolution reactions, owing to synergistic interaction of the iron impurities with the species formed upon thermal decomposition of Mn macrocyclic complexes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201700496
  • 2017 • 214 Current-Induced Magnetic Polarons in a Colloidal Quantum-Dot Device
    Muckel, F. and Barrows, C.J. and Graf, A. and Schmitz, A. and Erickson, C.S. and Gamelin, D.R. and Bacher, G.
    Nano Letters 17 4768-4773 (2017)
    Electrical spin manipulation remains a central challenge for the realization of diverse spin-based information processing technologies. Motivated by the demonstration of confinement-enhanced sp-d exchange interactions in colloidal diluted magnetic semiconductor (DMS) quantum dots (QDs), such materials are considered promising candidates for future spintronic or spin-photonic applications. Despite intense research into DMS QDs, electrical control of their magnetic and magneto-optical properties remains a daunting goal. Here, we report the first demonstration of electrically induced magnetic polaron formation in any DMS, achieved by embedding Mn2+-doped CdSe/CdS core/shell QDs as the active layer in an electrical light-emitting device. Tracing the electroluminescence from cryogenic to room temperatures reveals an anomalous energy shift that reflects current-induced magnetization of the Mn2+ spin sublattice, that is, excitonic magnetic polaron formation. These electrically induced magnetic polarons exhibit an energy gain comparable to their optically excited counterparts, demonstrating that magnetic polaron formation is achievable by current injection in a solid-state device. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b01496
  • 2017 • 213 Low-Temperature Phase c-axis Oriented Manganese Bismuth Thin Films with High Anisotropy Grown from an Alloy Mn55Bi45 Target
    Sabet, S. and Hildebrandt, E. and Römer, F.M. and Radulov, I. and Zhang, H. and Farle, M. and Alff, L.
    IEEE Transactions on Magnetics 53 (2017)
    Manganese bismuth thin films were deposited from a Mn55Bi45 (at.%) alloy target onto glass substrates at room temperature using dc magnetron sputtering. The ferromagnetic low-temperature phase (LTP) of MnBi was formed through a subsequent vacuum annealing step. The resulting thin films were highly c-axis textured. Magnetic measurement shows a maximum saturation magnetization of 600 eμcm3 (0.60 MA/m). A magnetic uniaxial anisotropy energy density of \sim 1.86 {\cdot 10{7}} erg/cm3 (1.86 MJ/m3) was measured by torque magnetometry. The coercive field has a positive temperature coefficient and reaches 12 kOe (1.2 T) and 14 kOe (1.4 T) at 300 K for the out-of-plane and in-plane direction, respectively. Density functional theory calculations have confirmed that the magnetocrystalline anisotropy energy increases with increasing temperature as a result of a spin-reorientation occurring around 100 K. Growing LTP MnBi thin films directly from an alloy Mn55Bi45 target is an important step toward facilitating the synthesis of multilayers for spintronics or in an exchange spring magnet configuration. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2016.2636817
  • 2017 • 212 Magnetic properties of nanolaminated (Mo0.5Mn0.5)2GaC MAX phase
    Salikhov, R. and Meshkian, R. and Weller, D. and Zingsem, B. and Spoddig, D. and Lu, J. and Ingason, A.S. and Zhang, H. and Rosen, J. and Wiedwald, U. and Farle, M.
    Journal of Applied Physics 121 (2017)
    The magnetic properties of hexagonal (Mo0.5Mn0.5)2GaC MAX phase synthesized as epitaxial films on MgO (111) substrates with the c-axis perpendicular to the film plane are presented. The analysis of temperature-dependent ferromagnetic resonance (FMR) and magnetometry data reveals a ferro- to paramagnetic phase transition at 220 K. The electrical transport measurements at 5 K show a negative magnetoresistance of 6% in a magnetic field of 9 T. Further analysis confirms the spin-dependent scattering of charge carriers in this layered material. A small perpendicular (c-axis) magnetocrystalline anisotropy energy density (MAE) of 4.5 kJ/m3 at 100 K was found using FMR. Accordingly, (Mo0.5Mn0.5)2GaC behaves similar to the (Cr0.5Mn0.5)2GaC MAX phase as a soft magnetic material. The density functional theory calculations reveal that the sign and the amplitude of the MAE can be very sensitive to (Mo0.5Mn0.5)2GaC lattice parameters, which may explain the measured soft magnetic properties. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4982197
  • 2017 • 211 Shell-ferromagnetism in a Ni-Mn-In off-stoichiometric Heusler studied by ferromagnetic resonance
    Scheibel, F. and Spoddig, D. and Meckenstock, R. and Çakir, A. and Farle, M. and Acet, M.
    AIP Advances 7 (2017)
    Next to the multifunctional properties of Ni-Mn-based Heusler alloys new functionalities related to shell-ferromagnetism are emerging. To understand in more detail the properties of shell-ferromagnetism we examine a decomposed Ni50.0Mn45.1In4.9 off-stoichiometric compound using magnetic resonance techniques which provides details on magnetic interactions. We find that the ferromagnetic resonance profile of the shell-ferromagnetic state is symmetric for positive and negative fields and is independent of the direction of the field-sweep except for the hysteresis observed at small fields. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4976335
  • 2017 • 210 Room-temperature five-tesla coercivity of a rare-earth-free shell-ferromagnet
    Scheibel, F. and Spoddig, D. and Meckenstock, R. and Gottschall, T. and Çakır, A. and Krenke, T. and Farle, M. and Gutfleisch, O. and Acet, M.
    Applied Physics Letters 110 (2017)
    Ni2MnX-based Heusler (X: main group element), when enriched with Mn, will decompose into ferromagnetic Ni2MnX and antiferromagnetic NiMn when temper-annealed around 650 K. When the starting material is chosen such that the X-composition is about 5 at. % and the annealing takes place in the presence of a magnetic field of about 1 T, the resulting material is a composite of nanoprecipitate strongly pinned shell-ferromagnets with a soft ferromagnetic core embedded in the antiferromagnetic matrix. We show that the shells of the precipitates are so strongly pinned that the estimated field required to fully reorient the spins is in the order of 20 T. We examine in a Ni50.0Mn45.1In4.9 sample the pinning and the magnetic interactions of the precipitate and the matrix with magnetization and ferromagnetic resonance studies carried out in fields ranging up to 14 and 12 T, respectively. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4983199
  • 2017 • 209 Electronic structure and magnetism of epitaxial Ni-Mn-Ga(-Co) thin films with partial disorder: A view across the phase transition
    Schleicher, B. and Klar, D. and Ollefs, K. and Diestel, A. and Walecki, D. and Weschke, E. and Schultz, L. and Nielsch, K. and Fähler, S. and Wende, H. and Gruner, M.E.
    Journal of Physics D: Applied Physics 50 (2017)
    The influence of Co-doping in off-stoichiometric Ni-Mn-Ga and Ni-Mn-Ga-Co thin films on the magnetic coupling of the atoms is investigated with x-ray magnetic circular dichroism in both the martensitic as well as austenitic phase, respectively. Additionally, first principles calculations were performed to compare the experimentally obtained absorption spectra with theoretical predictions. Calculated exchange constants and density of states for the different atomic sites underline the large influence of chemical and magnetic order on the magnetocaloric properties of the material. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa8e7c
  • 2017 • 208 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 1-8 (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 abstractdoi: 10.1080/13621718.2017.1300744
  • 2017 • 207 Strengthening Fe – TiB2 based high modulus steels by precipitations
    Szczepaniak, A. and Springer, H. and Aparicio-Fernández, R. and Baron, C. and Raabe, D.
    Materials and Design 124 183-193 (2017)
    We systematically studied the microstructure, mechanical and physical properties of hyper-eutectic Fe – TiB2 high modulus steels (20 vol% TiB2) with (Si, Mn, Ni) and Cu additions for the formation of G-phase and Cu precipitates during ageing treatments. Alloying with Si, Mn and Ni led predominantly to pronounced solid solution strengthening, reaching tensile strength (UTS) values up to 1100 MPa after quenching. While G-phase formation could be observed in aged materials, its preferential formation on grain boundaries significantly deteriorated ductility. Its effects on strength were partially balanced by a decrease of grain boundary density. Additions of 1 and 2 wt% Cu, respectively, led to lower strength in the as quenched state, but also to significant strengthening via ageing. The peak ageing conditions as well as the Cu particle structure and size are comparable to values reported for Cu strengthened interstitial free steels and Fe-Cu alloys. Both alloying additions slightly lowered the specific elastic modulus of the HMS, most pronounced for Cu addition with a drop of about 3 GPa cm3 g− 1 per wt% and also promoted embrittlement. Microstructure-property relationships and consequences for future alloy design, especially towards more ductile hypo-eutectic HMS, are outlined and discussed. © 2017
    view abstractdoi: 10.1016/j.matdes.2017.03.042
  • 2017 • 206 Kinetic arrest of the ferrimagnetic state in indium-doped Mn1.82Co0.18Sb
    Tekgül, A. and Acet, M. and Farle, M. and Ünal, N.
    Journal of Alloys and Compounds 695 418-425 (2017)
    Substituting Mn with small amounts of Co in Mn2Sb leads to kinetic arrest and magnetic frustration effects. By adding small amounts of In in place of Sb, the tetragonality and thereby the kinetic arrest properties can be modified and controlled. We investigate the kinetic arrest effect in Mn1.82Co0.18Sb1−xInx with x = 0 and 0.5 at the first-order antiferromagnetic-ferrimagnetic magnetostructural transition. Kinetic arrest is observed in both systems, whereby the arrest is partial under 5 T field for Mn1.82Co0.18Sb and full for Mn1.82Co0.18Sb0.95In0.05. The FI state remains arrested as long as the cooling-field is maintained. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2016.11.093
  • 2017 • 205 The reversibility of the inverse magnetocaloric effect in Mn2−xCrxSb0.95Ga0.05
    Tekgül, A. and Acet, M. and Scheibel, F. and Farle, M. and Ünal, N.
    Acta Materialia 124 93-99 (2017)
    Mn2−xCrxSb exhibits an antiferromagnetic-ferrimagnetic transition of which the temperature can be adjusted by the Cr concentration. The transition temperature is at room temperature for x = 0.13, but an additional ferromagnetic MnSb impurity phase is usually inevitable at this concentration. To suppress the occurrence of this phase unfavorable for the magnetocaloric effect, we partially replace Sb by Ga. We show that Mn2−xCrxSb0.95Ga0.05 alloys have a narrow transitional hysteresis and exhibit the inverse magnetocaloric effect. We present here results on structural, magnetic, and magnetocaloric properties using isothermal magnetization and direct adiabatic temperature-change measurements. We find in particular for the sample with x = 0.13 a nearly hysteresis-free transition with a 2 K temperature-change around room temperature making it attractive for magnetic cooling technology. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.10.072
  • 2017 • 204 Strengthening and strain hardening mechanisms in a precipitation-hardened high-Mn lightweight steel
    Yao, M.J. and Welsch, E. and Ponge, D. and Haghighat, S.M.H. and Sandlöbes, S. and Choi, P. and Herbig, M. and Bleskov, I. and Hickel, T. and Lipinska-Chwalek, M. and Shanthraj, P. and Scheu, C. and Zaefferer, S. and Gault, B. an...
    Acta Materialia 140 258-273 (2017)
    We report on the strengthening and strain hardening mechanisms in an aged high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C, wt.%) studied by electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM), atom probe tomography (APT) and correlative TEM/APT. Upon isothermal annealing at 600 °C, nano-sized κ-carbides form, as characterized by TEM and APT. The resultant alloy exhibits high strength and excellent ductility accompanied by a high constant strain hardening rate. In comparison to the as-quenched κ-free state, the precipitation of κ-carbides leads to a significant increase in yield strength (∼480 MPa) without sacrificing much tensile elongation. To study the strengthening and strain hardening behavior of the precipitation-hardened material, deformation microstructures were analyzed at different strain levels. TEM and correlative TEM/APT results show that the κ-carbides are primarily sheared by lattice dislocations, gliding on the typical face-centered-cubic (fcc) slip system {111}<110>, leading to particle dissolution and solute segregation. Ordering strengthening is the predominant strengthening mechanism. As the deformation substructure is characterized by planar slip bands, we quantitatively studied the evolution of the slip band spacing during straining to understand the strain hardening behavior. A good agreement between the calculated flow stresses and the experimental data suggests that dynamic slip band refinement is the main strain hardening mechanism. The influence of κ-carbides on mechanical properties is discussed by comparing the results with that of the same alloy in the as-quenched, κ-free state. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.08.049
  • 2017 • 203 Formation Mechanism of Laser-Synthesized Iron-Manganese Alloy Nanoparticles, Manganese Oxide Nanosheets and Nanofibers
    Zhang, D. and Ma, Z. and Spasova, M. and Yelsukova, A.E. and Lu, S. and Farle, M. and Wiedwald, U. and Gökce, B.
    Particle and Particle Systems Characterization 34 1600225 (2017)
    Laser ablation in liquids (LAL) has emerged as a versatile approach for the synthesis of alloy particles and oxide nanomaterials. However, complex chemical reactions often take place during synthesis due to inevitable atomization and ionization of the target materials and decomposition/hydrolysis of solvent/solution molecules, making it difficult to understand the particle formation mechanisms. In this paper, a possible route for the formation of FeMn alloy nanoparticles as well as MnOx nanoparticles, -sheets, and -fibers by LAL is presented. The observed structural, compositional, and morphological variations are clarified by transmission electron microscopy (TEM). The studies suggest that a reaction between Mn atoms and Fe ions followed by surface oxidation result in nonstoichiometric synthesis of Fe-rich FeMn@FeMn2O4 core-shell alloy particles. Interestingly, a phase transformation from Mn3O4 to Mn2O3 and finally to Ramsdellite γ-MnO2 is accompanied by a morphology change from nanosheets to nanofibers in gradually increasing oxidizing environments. High-resolution TEM images reveal that the particle-attachment mechanism dominates the growth of different manganese oxides. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ppsc.201600225
  • 2017 • 202 Designing duplex, ultrafine-grained Fe-Mn-Al-C steels by tuning phase transformation and recrystallization kinetics
    Zhang, J. and Raabe, D. and Tasan, C.C.
    Acta Materialia 141 374-387 (2017)
    A novel, lightweight Fe-25.7Mn-10.6Al-1.2C (wt.%) steel is designed by exploiting the concurrent progress of primary recrystallization and phase transformation, in order to produce an ultrafine-grained, duplex microstructure. The microstructure consists of recrystallized austenite grains surrounded by submicron-sized ferrite grains, and recovered austenite regions with preferential nano-κ-carbide precipitation. This partially recrystallized duplex microstructure demonstrates excellent strength-ductility combinations, e.g. a yield strength of 1251 MPa, an ultimate tensile strength of 1387 MPa, and a total elongation of 43%, arising from the composite response by virtue of diverging constituent strength and strain hardening behaviors. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.09.026
  • 2016 • 201 Quantitative screening of an extended oxidative coupling of methane catalyst library
    Alexiadis, V.I. and Chaar, M. and van Veen, A. and Muhler, M. and Thybaut, J.W. and Marin, G.B.
    Applied Catalysis B: Environmental 199 252-259 (2016)
    A comprehensive microkinetic model, including catalyst descriptors, that accounts for the homogeneous as well as heterogeneously catalyzed reaction steps in Oxidative Coupling of Methane (OCM) was used in the assessment of large kinetic datasets acquired on five different catalytic materials. The applicability of the model was extended from alkali magnesia catalysts represented by Li/MgO and Sn-Li/MgO and alkaline earth lanthana catalysts represented by Sr/La2O3 to rare earth-promoted alkaline earth calcium oxide catalysts, represented by LaSr/CaO, and to a Na-Mn-W/SiO2 catalyst. The model succeeded in adequately simulating the performance of all five investigated catalysts in terms of reactant conversion and product selectivities in the entire range of experimental conditions. It was found that the activity of Sr/La2O3, in terms of methane conversion, is approximately 2, 5, 30 and 33 times higher than over the La-Sr/CaO, Sn-Li/MgO, Na-Mn-W/SiO2 and Li/MgO catalysts, respectively, under identical operating conditions. This was attributed mainly to the high stability of adsorbed hydroxyls, the high stability of adsorbed oxygen and the high concentration of active sites of Sr/La2O3. The selectivity towards C2 products was found to depend on the methyl radical sticking coefficient and the stability of the adsorbed oxygen and was the highest on the Na-W-Mn/SiO2 catalyst, that is 75% at about 1% methane conversion and 1023 K, 190 kPa and inlet molar CH4/O2 ratio of 4. © 2016 The Author(s)
    view abstractdoi: 10.1016/j.apcatb.2016.06.019
  • 2016 • 200 In-situ metal matrix composite steels: Effect of alloying and annealing on morphology, structure and mechanical properties of TiB2 particle containing high modulus steels
    Aparicio-Fernández, R. and Springer, H. and Szczepaniak, A. and Zhang, H. and Raabe, D.
    Acta Materialia 107 38-48 (2016)
    We systematically study the morphology, size and dispersion of TiB2 particles formed in-situ from Fe-Ti-B based melts, as well as their chemical composition, crystal structure and mechanical properties. The effects of 5 wt.% additions of Cr, Ni, Co, Mo, W, Mn, Al, Si, V, Ta, Nb and Zr, respectively, as well as additional annealing treatments, were investigated in order to derive guidelines for the knowledge based alloy design of steels with an increased stiffness/density ratio and sufficiently high ductility. All alloying elements were found to increase the size of the coarse primary TiB2 particles, while Co led to the most homogeneous size distribution. The size of the eutectic TiB2 constituents was decreased by all alloying additions except Ni, while their aspect ratio was little affected. No clear relation between chemical composition, crystal structure and mechanical properties of the particles could be observed. Annealing of the as-cast alloys slightly increased the size of the primary particles, but at the same time strongly spheroidised the eutectics. Additions of Co and Cr appear thus as the best starting point for designing novel in-situ high modulus metal matrix composite steels, while using Mn in concert with thermo-mechanical processing is most suited to adapt the matrix' microstructure and optimise the particle/matrix co-deformation processes. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.048
  • 2016 • 199 Effects of Mn additions on microstructure and properties of Fe–TiB2 based high modulus steels
    Baron, C. and Springer, H. and Raabe, D.
    Materials and Design 111 185-191 (2016)
    We studied the effects of Mn additions from 0 to 30 wt.% on microstructure, mechanical and physical properties of liquid metallurgy synthesised high modulus steels in hypo- and hyper-eutectic TiB2 concentrations. While Mn has little effect on density, both Young's modulus and mechanical properties were strongly affected by the achieved wide spectrum of matrix microstructures, ranging from ferrite to martensite, reverted austenite, ε-martensite and austenite. Mn additions of 20 and 30 wt.% did not translate into enhanced mechanical performance despite the higher inherent ductility of the predominantly austenitic matrix, and instead eliminate the intended weight saving potential by significantly reducing the Young's modulus. Martensitic matrices of Mn concentrations of 10 wt.%, on the other hand, are favourable for improved matrix/particle co-deformation without sacrificing too much of the composites' stiffness. In hypo-eutectic Fe – TiB2 based steels, mechanical properties on the level of high strength dual phase steels could be achieved (~ 900 MPa UTS and 20% tensile elongation) but with an enhanced Young's modulus of 217 GPa and reduced density of 7460 kg m− 3. These significantly improved physical and mechanical properties render Mn alloyed high modulus steels promising candidate materials for next generation lightweight structural applications. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2016.09.003
  • 2016 • 198 An experimental study of ultrafiltration for sub-10nm quantum dots and sub-150 nm nanoparticles through PTFE membrane and Nuclepore filters
    Chen, S.-C. and Segets, D. and Ling, T.-Y. and Peukert, W. and Pui, D.Y.H.
    Journal of Membrane Science 497 153-161 (2016)
    Ultrafiltration techniques (pore size of membrane below 100nm) are widely used in chemical engineering, semiconductor, pharmaceutical, food and beverage industries. However, for small particles, which are more and more attracting interests, the pore size often does not correlate well with sieving characteristics of the ultra-membranes. This may cause serious issues during modeling and prediction of retention efficiencies. Herein, a series of liquid filtration experiments with unfavorable conditions were performed. PTFE membranes (50, 100nm) and Nuclepore filters (50, 400nm) were challenged with 1.7nm manganese doped ZnS and 6.6 nm ZnO quantum dots (QDs), 12.4, 34.4 and 50 nm Au and 150 nm SiO2 nanoparticles. For larger and medium sized particles, sieving and eventually pore blockage phenomena were observed. In comparison, for small QDs, a high initial retention efficiency (&gt;0.4) in both filters was monitored, followed by a reduced efficiency with ongoing particle loading. This high initial retention of small nanoparticles was attributed to diffusion deposition rather than to sieving since the ratio of pore size to particle size was significantly high (up to 58). Our experimental results allow a basic understanding of the deposition mechanism of small nanoparticles (diffusion vs. sieving) in different filter structures. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2015.09.022
  • 2016 • 197 Structural and multifunctional properties of magnetron-sputtered Fe-P(-Mn) thin films
    Decker, P. and Stein, H.S. and Salomon, S. and Brüssing, F. and Savan, A. and Hamann, S. and Ludwig, Al.
    Thin Solid Films 603 262-267 (2016)
    Structural and magnetic properties of magnetron-sputtered Fe-P(-Mn) thin films with compositions around the Fe2P single phase region are reported, revealing the compositional range of the Fe2P-type structure and the change of the magnetic properties within this composition spread. The structural analysis shows that in order to obtain crystalline Fe-P phases the P content must be higher than (Fe0.97Mn0.03)2.33P. A maximum phase fraction of the Fe2P-type structure is obtained in the examined (Fe0.97Mn0.03)1.78P sample. The hysteresis loops for the Fe2P(-Mn) thin films show a two-step magnetic reversal with one part belonging to an amorphous phase fraction and the other to the Fe2P(-Mn) phase. A maximum coercivity of 0.36 T was measured for the Fe2P(-Mn) phase fraction also at the composition of (Fe0.97Mn0.03)1.78P. Furthermore, electrochemical properties of FeP2(-Mn) thin films as hydrogen evolution catalysts (HER) are studied. FeP2(-Mn) shows a HER onset potential about 200 mV lower than that of Pt. Chronoamperometric testing at - 11.5 mA/cm2 for over 3500 s revealed no obvious decay in current density, suggesting good stability under typical working conditions in a photoelectrochemical device. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2016.02.023
  • 2016 • 196 Spent Tea Leaf Templating of Cobalt-Based Mixed Oxide Nanocrystals for Water Oxidation
    Deng, X. and Chan, C.K. and Tüysüz, H.
    ACS Applied Materials and Interfaces 8 32488-32495 (2016)
    The facile synthesis of nanostructured cobalt oxides using spent tea leaves as a hard template is reported. Following an impregnation-calcination and template removal pathway, sheetlike structures containing nanosized crystallites of Co3O4 are obtained. Co3O4 incorporated with Cu, Ni, Fe, and Mn (M/Co = 1/8 atomic ratio) are also prepared, and the materials are thoroughly characterized using X-ray diffraction, electron microscopy, and N2 sorption. The method is applicable to several commercial tea leaves and is successfully scaled up to prepare over 7 g of Co3O4 with the same nanostructure. The oxides are then tested for electrochemical water oxidation, and Cu, Ni, and Fe incorporations show beneficial effect on the catalytic activity of Co3O4, achieving performance comparable to levels from benchmark electrocatalysts. These data suggest that tea leaf templating can be utilized as a facile and promising approach to prepare nanostructured functional catalyst. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsami.6b12005
  • 2016 • 195 Interface effects on the magnetic properties of layered Ni2MnGa/Ni2MnSn alloys: A first-principles investigation
    Dutta, B. and Opahle, I. and Hickel, T.
    Functional Materials Letters 9 (2016)
    The effect of interfaces on the magnetic properties of multilayers is analyzed forNi2MnGa/Ni2MnSn system using density functional theory. The Ni spin moments at the interface change by about 30% compared to the bulk value, whereas the effect on the Mn spin moments is much less pronounced. A similar strong effect is also observed for the Ni orbital moments at the interface. The magneto-crystalline anisotropy of the multilayer systems can be understood by the additive contribution of the respective values of strained bulk materials. © 2016 World Scientific Publishing Company.
    view abstractdoi: 10.1142/S1793604716420108
  • 2016 • 194 Ab initio Prediction of Martensitic and Intermartensitic Phase Boundaries in Ni-Mn-Ga
    Dutta, B. and Çaklr, A. and Giacobbe, C. and Al-Zubi, A. and Hickel, T. and Acet, M. and Neugebauer, J.
    Physical Review Letters 116 (2016)
    Despite the importance of martensitic transformations of Ni-Mn-Ga Heusler alloys for their magnetocaloric and shape-memory properties, the martensitic part of their phase diagrams is not well determined. Using an ab initio approach that includes the interplay of lattice and vibrational degrees of freedom we identify an intermartensitic transformation between a modulated and a nonmodulated phase as a function of excess Ni and Mn content. Based on an evaluation of the theoretical findings and experimental x-ray diffraction data for Mn-rich alloys, we are able to predict the phase diagram for Ni-rich alloys. In contrast to other mechanisms discussed for various material systems in the literature, we herewith show that the intermartensitic transformation can be understood solely using thermodynamic concepts. © 2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.116.025503
  • 2016 • 193 Giant Excitonic Exchange Splittings at Zero Field in Single Colloidal CdSe Quantum Dots Doped with Individual Mn2+ Impurities
    Fainblat, R. and Barrows, C.J. and Hopmann, E. and Siebeneicher, S. and Vlaskin, V.A. and Gamelin, D.R. and Bacher, G.
    Nano Letters 16 6371-6377 (2016)
    Replacing a single atom of a host semiconductor nanocrystal with a functional dopant can introduce completely new properties potentially valuable for "solotronic" information-processing applications. Here, we report successful doping of colloidal CdSe quantum dots with a very small number of manganese ions - down to the ultimate limit of one. Single-particle spectroscopy reveals spectral fingerprints of the spin-spin interactions between individual dopants and quantum-dot excitons. Spectrally well-resolved emission peaks are observed that can be related to the discrete spin projections of individual Mn2+ ions. In agreement with theoretical predictions, the exchange splittings are enhanced by more than an order of magnitude in these quantum dots compared to their epitaxial counterparts, opening a path for solotronic applications at elevated temperatures. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b02775
  • 2016 • 192 On the rich magnetic phase diagram of (Ni, Co)-Mn-Sn Heusler alloys
    Grünebohm, A. and Herper, H.C. and Entel, P.
    Journal of Physics D: Applied Physics 49 (2016)
    We put a spotlight on the exceptional magnetic properties of the metamagnetic Heusler alloy (Ni, Co)-Mn-Sn by means of first principles simulations. In the energy landscape we find a multitude of local minima, which belong to different ferrimagnetic states and are close in total magnetization and energy. All these magnetic states correspond to the local high spin state of the Mn atoms with different spin alignments and are related to the magnetic properties of Mn. Compared to pure Mn, the magneto-volume coupling is reduced by Ni, Co and Sn atoms in the lattice and no local low-spin Mn states appear. For the cubic phase we find a ferromagnetic ground state whereas the global energy minimum is a tetragonal state with a complicated spin structure and vanishing magnetization which so far has been overlooked in simulations. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/39/395001
  • 2016 • 191 Gold on Different Manganese Oxides: Ultra-Low-Temperature CO Oxidation over Colloidal Gold Supported on Bulk-MnO2 Nanomaterials
    Gu, D. and Tseng, J.-C. and Weidenthaler, C. and Bongard, H.-J. and Spliethoff, B. and Schmidt, W. and Soulimani, F. and Weckhuysen, B.M. and Schüth, F.
    Journal of the American Chemical Society 138 9572-9580 (2016)
    Nanoscopic gold particles have gained very high interest because of their promising catalytic activity for various chemicals reactions. Among these reactions, low-temperature CO oxidation is the most extensively studied one due to its practical relevance in environmental applications and the fundamental problems associated with its very high activity at low temperatures. Gold nanoparticles supported on manganese oxide belong to the most active gold catalysts for CO oxidation. Among a variety of manganese oxides, Mn2O3 is considered to be the most favorable support for gold nanoparticles with respect to catalytic activity. Gold on MnO2 has been shown to be significantly less active than gold on Mn2O3 in previous work. In contrast to these previous studies, in a comprehensive study of gold nanoparticles on different manganese oxides, we developed a gold catalyst on MnO2 nanostructures with extremely high activity. Nanosized gold particles (2-3 nm) were supported on α-MnO2 nanowires and mesoporous β-MnO2 nanowire arrays. The materials were extremely active at very low temperature (-80 °C) and also highly stable at 25 °C (70 h) under normal conditions for CO oxidation. The specific reaction rate of 2.8 molCO·h-1·gAu -1 at a temperature as low as -85 °C is almost 30 times higher than that of the most active Au/Mn2O3 catalyst. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/jacs.6b04251
  • 2016 • 190 Surface analysis of the Heusler Ni49.7Mn29.1Ga21.2 Alloy: The composition, phase transition, and twinned microstructure of martensite
    Horáková, K. and Cháb, V. and Heczko, O. and Drchal, V. and Fekete, L. and Honolka, J. and Kopeček, J. and Kudrnovský, J. and Polyak, Y. and Sajdl, P. and Vondráček, M. and Lančok, J. and Feyer, V. and Wiemann, C. and Schn...
    Journal of Applied Physics 120 (2016)
    Surface analysis was used to study the dynamics of the martensitic transformation on macro- and mesoscopic scales. The chemical state, morphology, and magnetic and surface structure were monitored at particular stages of the phase transition. At room temperature, the martensitic phase of the Ni49.7Mn29.1Ga21.2 (100) single crystal exhibited macroscopic a/c twinning and a corresponding magnetic domain structure characterized by magnetization vector in and out of the surface plane. Induced by radiation heating, the transformation from martensite to austenite takes place separately at the surface and in the bulk. Its dynamics depend on the history of the sample treatment which affects the crystallographic orientation of twins and minor changes of the surface stoichiometry. The interfaces (twin planes) between twin variants in the martensitic phase were noticeable also in the austenitic phase, thanks to the shape memory effect of this material. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4962648
  • 2016 • 189 Product distribution of CO2 hydrogenation by K- and Mn-promoted Fe catalysts supported on N-functionalized carbon nanotubes
    Kangvansura, P. and Chew, L.M. and Saengsui, W. and Santawaja, P. and Poo-arporn, Y. and Muhler, M. and Schulz, H. and Worayingyong, A.
    Catalysis Today 275 59-65 (2016)
    An iron based catalyst supported on an N-functionalized carbon nanotube (NCNT) was promoted with potassium and manganese as follows: Fe/NCNT, K/Fe/NCNT, Mn/Fe/NCNT, and K/Mn/Fe/NCNT for CO2 hydrogenation. Time-resolved reduction X-ray absorption near edge spectroscopy (XANES) showed mixed phases of Fe, FeO, Fe3O4, and Fe2O3 resulting from K/Fe/NCNT, and of FeO and Fe3O4 resulting from Mn/Fe/NCNT. The product distributions and growth probability of n-alkanes during CO2 hydrogenation indicated that the potassium-promoted iron catalysts performed Fischer-Tropsch (FT) synthesis under steady state at 60 h. 1-Alkenes desorbed from the FT sites with the potassium-promoted catalysts, (K/Fe/NCNT and K/Mn/Fe/NCNT), with low methane formation. Small amounts of 1-alkene, along with high methanation, were produced from the potassium-unpromoted catalysts, (Fe/NCNT and Mn/Fe/NCNT), indicating high local H2:CO ratios on the catalyst surfaces. K/Fe/NCNT and K/Mn/Fe/NCNT catalysts also produced ethanol. Thus, potassium is a key promoter providing active species of the catalysts for alkene and ethanol formation. Reduced surrounding of the NCNT support, potassium as an electronic promoter together with manganese as a structural promoter made the iron-active phase well suitable for CO2 hydrogenation producing mainly alkenes and ethanol. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2016.02.045
  • 2016 • 188 Magnetic proximity effect and shell-ferromagnetism in metastable Ni50Mn45Ga5
    Krenke, T. and Çakır, A. and Scheibel, F. and Acet, M. and Farle, M.
    Journal of Applied Physics 120 (2016)
    The present study on magnetic and structural properties of Ni50Mn45Ga5 confirms that structural metastability is an inherent property of Ni50Mn50- xXx Heusler alloys with X as In, Ga, and Sn. The ternary alloy transforms during temper-annealing into a dual-phase composite alloy. The two phases are identified to be cubic L21, Ni50Mn25Ga25, and tetragonal L10 Ni50Mn50. Depending on the annealing temperature, the magnetic-proximity effect giving rise to shell-ferromagnetism has been observed when annealing is carried out under an external magnetic field. The upper and lower remanence values MR+ and MR− have the same sign even at high temperatures. Such alloys can be promising candidates for heat- and magnetic-field-resistant magnetic recording media. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4972480
  • 2016 • 187 Interface-mediated ferroelectric patterning and Mn valency in nano-structured PbTiO3/La0.7Sr0.3MnO3
    Krug, I.P. and Doganay, H. and Nickel, F. and Gottlob, D.M. and Schneider, C.M. and Morelli, A. and Preziosi, D. and Lindfors-Vrejoiu, I. and Laskowski, R. and Barrett, N.
    Journal of Applied Physics 120 (2016)
    We employed a multitechnique approach using piezo-force response microscopy and photoemission microscopy to investigate a self-organizing polarization domain pattern in PbTiO3/La0.7Sr0.3MnO3 (PTO/LSMO) nanostructures. The polarization is correlated with the nanostructure morphology as well as with the thickness and Mn valence of the LSMO template layer. On the LSMO dots, the PTO is upwards polarized, whereas outside the nanodots, the polarization appears both strain and interface roughness dependent. The results suggest that the electronic structure and strain of the PTO/LSMO interface contribute to determining the internal bias of the ferroelectric layer. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4962007
  • 2016 • 186 Oxidation Behavior of the CrMnFeCoNi High-Entropy Alloy
    Laplanche, G. and Volkert, U.F. and Eggeler, G. and George, E.P.
    Oxidation of Metals 85 629-645 (2016)
    Oxidation of the Cr20Mn20Fe20Co20Ni20 (at%) high-entropy alloy (HEA) was investigated at 500–900 °C in laboratory air. At 600 °C the oxide was mainly Mn2O3 with a thin inner Cr2O3 layer; at 700 and 800 °C it was mainly Mn2O3 with some Cr enrichment; at 900 °C it was Mn3O4. The oxidation rate was initially linear but became parabolic at longer times with an activation energy of 130 kJ/mol, comparable to that of Mn diffusion in Mn oxides but much lower than that for sluggish diffusion of Mn in the HEA. The diffusion of Mn through the oxide is considered to be the rate-limiting process. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11085-016-9616-1
  • 2016 • 185 Dioxygen binding to Fe-MOF-74: Microscopic insights from periodic QM/MM calculations
    Moeljadi, A.M.P. and Schmid, R. and Hirao, H.
    Canadian Journal of Chemistry 94 1144-1150 (2016)
    Accurate MOF-FF parameter sets were determined for the ferrous and ferric forms of an iron-based metal-organic framework (MOF) called Fe-MOF-74. For this purpose, density functional theory (DFT) calculations were applied to truncated cluster models of Fe-MOF-74, and the DFT-calculated geometries and energy derivatives were used for the force-field parameterization. The resultant parameter sets performed remarkably well in reproducing the experimentally determined structure of the MOF. We also performed periodic quantum mechanics (QM) / molecular mechanics (MM) calculations employing a subtractive scheme called ONIOM, with the optimized MOF-FF parameters used for the MM calculations, in an attempt to evaluate the binding energies between O2 and several Fe-MOF-74 variants. The calculated binding energy for Fe-MOF-74 agreed very well with the experimental value, and QM/MM geometry optimization calculations confirmed that the O2-bound complex has a side-on geometry. Our calculations also predicted that, when the two neighboring iron ions around the O2-binding site are replaced with other metal ions (Mg2+, Ni2+, Zn2+, Co2+, or Mn2+), there are noticeable variations in the binding energy, indicating that these substituted metal ions affect the O2 binding indirectly. © 2016 Published by NRC Research Press.
    view abstractdoi: 10.1139/cjc-2016-0284
  • 2016 • 184 Digital Doping in Magic-Sized CdSe Clusters
    Muckel, F. and Yang, J. and Lorenz, S. and Baek, W. and Chang, H. and Hyeon, T. and Bacher, G. and Fainblat, R.
    ACS Nano 10 7135-7141 (2016)
    Magic-sized semiconductor clusters represent an exciting class of materials located at the boundary between quantum dots and molecules. It is expected that replacing single atoms of the host crystal with individual dopants in a one-by-one fashion can lead to unique modifications of the material properties. Here, we demonstrate the dependence of the magneto-optical response of (CdSe)13 clusters on the discrete number of Mn2+ ion dopants. Using time-of-flight mass spectrometry, we are able to distinguish undoped, monodoped, and bidoped cluster species, allowing for an extraction of the relative amount of each species for a specific average doping concentration. A giant magneto-optical response is observed up to room temperature with clear evidence that exclusively monodoped clusters are magneto-optically active, whereas the Mn2+ ions in bidoped clusters couple antiferromagnetically and are magneto-optically passive. Mn2+-doped clusters therefore represent a system where magneto-optical functionality is caused by solitary dopants, which might be beneficial for future solotronic applications. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.6b03348
  • 2016 • 183 Diffusion of solutes in fcc Cobalt investigated by diffusion couples and first principles kinetic Monte Carlo
    Neumeier, S. and Rehman, H.U. and Neuner, J. and Zenk, C.H. and Michel, S. and Schuwalow, S. and Rogal, J. and Drautz, R. and Göken, M.
    Acta Materialia 106 304-312 (2016)
    The interdiffusivity of Al and the transition metal solutes Ti, V, Cr, Mn, Fe, Nb, Mo, Ru, Ta, W, and Re in fcc Co is characterized at 1373 K, 1473 K and 1573 K by binary diffusion couples. The experimental results are complemented by first-principles calculations in combination with kinetic Monte Carlo simulations to investigate the diffusion of Re, W, Mo and Ta in fcc Co. The interdiffusion coefficients of alloying elements in fcc Co are generally smaller than in fcc Ni, but the correlation between interdiffusion coefficients and the atomic number of metal solutes is comparable in Co and Ni. With increasing atomic number and decreasing atomic radii the interdiffusion coefficients of the investigated elements, except for Mn and Fe, decrease strongly. The trends in the diffusivity determined by experiment and simulation are in excellent agreement. Re is the slowest diffusing element in fcc Co among the investigated elements. The electronic structure calculations indicate that this is caused by strong directional bonds between Re and neighboring Co atoms. The overall lower diffusivity of solute atoms in Co as compared to Ni suggests that diffusion controlled processes could be slower in Co-base superalloys. © 2016 Acta Materialia Inc. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.028
  • 2016 • 182 Ab initio-guided design of twinning-induced plasticity steels
    Raabe, D. and Roters, F. and Neugebauer, J. and Gutierrez-Urrutia, I. and Hickel, T. and Bleck, W. and Schneider, J.M. and Wittig, J.E. and Mayer, J.
    MRS Bulletin 41 320-325 (2016)
    The twinning-induced plasticity effect enables designing austenitic Fe-Mn-C-based steels with >70% elongation with an ultimate tensile strength >1 GPa. These steels are characterized by high strain hardening due to the formation of twins and complex dislocation substructures that dynamically reduce the dislocation mean free path. Both mechanisms are governed by the stacking-fault energy (SFE) that depends on composition. This connection between composition and substructure renders these steels ideal model materials for theory-based alloy design: Ab initio-guided composition adjustment is used to tune the SFE, and thus, the strain-hardening behavior for promoting the onset of twinning at intermediate deformation levels where the strain-hardening capacity provided by the dislocation substructure is exhausted. We present thermodynamic simulations and their use in constitutive models, as well as electron microscopy and combinatorial methods that enable validation of the strain-hardening mechanisms. Copyright © 2016 Materials Research Society.
    view abstractdoi: 10.1557/mrs.2016.63
  • 2016 • 181 Gold-manganese oxide core-shell nanoparticles produced by pulsed laser ablation in water
    Simao, T. and Chevrier, D.M. and Jakobi, J. and Korinek, A. and Goupil, G. and Lau, M. and Garbarino, S. and Zhang, P. and Barcikowski, S. and Fortin, M.-A. and Guay, D.
    Journal of Physical Chemistry C 120 22635-22645 (2016)
    A single-step procedure for the preparation of Au-MnOx NPs was achieved through pulsed laser ablation of a gold-manganese metal target made of a pressed metal powder mixture. First, a 248 nm nanosecond laser at 66.7 J cm-2 was used to synthesize Au-MnOx NPs from a gold-manganese metal target immersed in an aqueous solution at pH 11 (NaOH). It is demonstrated that the Au-MnOx NPs are made of a small Au core (around 5 nm in diameter) surrounded by a very thin manganese oxide layer (0.3-1.3 nm) as characterized by TEM, HAADF HR-STEM, and EELS. The superficial MnOx layer has a local structure that bears a close resemblance to that of Mn2O3 and MnO2 as revealed by EXAFS and XANES measurements. Comparative studies were also performed with a 1064 nm nanosecond laser at 1.4 J cm-2. In that case, the resulting colloids are mainly made of a mixture of Au NPs and MnOx NPs, with few Au-MnOx NPs, thereby suggesting the impact of the laser wavelength and fluence on the synthesis process. The mechanisms responsible for the production of Au-MnOx core-shell NPs are discussed. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.6b05838
  • 2016 • 180 Laser-synthesized ligand-free Au nanoparticles for contrast agent applications in computed tomography and magnetic resonance imaging
    Simão, T. and Chevallier, P. and Lagueux, J. and Côté, M.-F. and Rehbock, C. and Barcikowski, S. and Fortin, M.-A. and Guay, D.
    Journal of Materials Chemistry B 4 6413-6427 (2016)
    In recent years, pulsed laser ablation in liquids (PLAL) has emerged as a new green chemistry method to produce different types of nanoparticles (NPs). It does not require the use of reducing or stabilizing agents, therefore enabling the synthesis of NPs with highly-pure surfaces. In this study, pure Au NPs were produced by PLAL in aqueous solutions, sterically stabilized using minimal PEG excess, and functionalized with manganese chelates to produce a dual CT/MRI contrast agent. The small hydrodynamic size (36.5 nm), low polydispersity (0.2) and colloidal stability of Au NPs@PEG-Mn2+ were demonstrated by DLS. The particles were further characterized by TEM, XPS, FTIR and 1H NMR to confirm the purity of the Au surfaces (i.e. free from the common residual chemicals found after NP synthesis) and the presence of the different surface molecules. The potential of these particles as contrast agents for CT/MRI was assessed in vivo (e.g. chicken embryo). Au NPs@PEG-Mn2+ also demonstrated strong blood retention for at least 90 minutes following intravenous injection in mouse models. The promising performance of PEGylated PLAL-synthesized Au NPs containing manganese chelates could open new possibilities for the production of purer dual imaging contrast agents based on Au colloids. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6tb01162d
  • 2016 • 179 Combinatorial design of transitory constitution steels: Coupling high strength with inherent formability and weldability through sequenced austenite stability
    Springer, H. and Belde, M. and Raabe, D.
    Materials and Design 90 1100-1109 (2016)
    We introduce a novel alloying and processing scheme for high strength steels, which allows for precise and cost-effective cold forming due to high fractions of metastable austenite, and a subsequent low-distortion, coating-preserving strengthening through martensitic transformation induced by low temperature treatments. As the constitution is thus synchronised with the processing requirements, we refer to these materials as Transitory Constitution Steels. Suitable alloy compositions were identified by high throughput screenings through the exemplarily material systems Fe-5Ni-0.3C-(3-15)Mn and Fe-13.5Cr-6Mn-2Cu-0.2C-(0-2)Ni (wt.%) using combinatorial bulk metallurgical methods. The transformation behaviour, mechanical properties and underlying microstructural phenomena were studied in more detail after upscaling of selected compositions. The steel Fe-13.5Cr-6Mn-1.5Cu-0.2C (wt.%) exhibited an increase in yield strength from 300 to 1050. MPa after immersion into liquid nitrogen, as well as an ultimate tensile strength of more than 1700. MPa at a total elongation of more than 9%. Despite the ultra high strength, no embrittlement induced by Laser beam welding was observed, highlighting the inherent weldability of steels synthesised by the alloying and processing scheme presented here. Possibilities for flexible alloy design and processing variations are discussed. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2015.11.050
  • 2016 • 178 Microstructural analysis in the Fe-30.5Mn-8.0Al-1.2C and Fe-30.5Mn-2.1Al-1.2C steels upon cold rolling
    Souza, F.M. and Padilha, A.F. and Gutierrez-Urrutia, I. and Raabe, D.
    Revista Escola de Minas 69 167-173 (2016)
    Electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) were used to examine microstructural changes of the austenitic low-density Fe-30.5Mn-8.0Al-1.2C (8Al) and Fe-30.5Mn-2.1Al-1.2C (2Al) (wt.%) steels during cold rolling. As the strain increased, deformation mechanisms, such as stacking faults, slip, mechanical twinning, and shear banding were activated in both steels cold rolled up to strain of 0.69. Only slip was noted in these steels at low strain (ε=0.11) and slip dominance was detected in the 8Al steel at higher strains. Shear banding became active at higher strain (ε~0.7) in these materials. An inhomogeneous microstructure formed in both alloys at such strain level. More extensive mechanical twinning in the 2Al alloy than that in the 8Al alloy was observed. Fish bone-like structure patterns were revealed in the 8Al steel and a river-like structure in the 2Al steel. Detailed microstructure features as elongated and fragmented grains along the rolling direction (RD) were found for both steels, as already observed in other high-Mn steels. These deformed structures are composed by lamellar packets which can contain mechanical twins or slip lines and shear bands. © 2016, Escola de Minas. All rights reserved.
    view abstractdoi: 10.1590/0370-44672015690097
  • 2016 • 177 Thin film synthesis and characterization of a chemically ordered magnetic nanolaminate (V,Mn)3GaC2
    Tao, Q. and Salikhov, R. and Mockute, A. and Lu, J. and Farle, M. and Wiedwald, U. and Rosen, J.
    APL Materials 4 (2016)
    We report on synthesis and characterization of a new magnetic nanolaminate (V,Mn)3GaC2, which is the first magnetic MAX phase of a 312 stoichiometry. Atomically resolved energy dispersive X-ray mapping of epitaxial thin films reveals a tendency of alternate chemical ordering between V and Mn, with atomic layers composed of primarily one element only. Magnetometry measurements reveal a ferromagnetic response between 50 K and 300 K, with indication of a magnetic ordering temperature well above room temperature. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4961502
  • 2016 • 176 Investigation of the brazing characteristics of a new iron-based brazing filler metal
    Tillmann, W. and Wojarski, L. and Manka, M. and Trelenberg, A.
    Welding in the World 60 869-875 (2016)
    High temperature applications of new class of iron-based filler metals provide brazements with high corrosion resistance and mechanical properties. These brazements are cost-effective alternative to those made of the conventional brazing alloys. However, a wiser usage demands a deeper understanding of the wetting as well as gap filling behavior in conjunction with the resulting microstructure, which is mainly influenced by the applied brazing cycles. Therefore, this paper presents results of the investigation of specific brazing fundamentals for the new iron-based brazing alloy Fe-24Cr-20Ni-10Cu-7P-5Mn-5Si. Followed by DTA/DSC measurements, the spreading and gap filling behavior were examined by using stainless steel AISI 304 as base material. In wetting tests and wedge-gap experiments, the influence of the applied brazing temperature and the dwell time were investigated for vacuum brazing processes. The resulting microstructure was evaluated using a scanning electron microscope (SEM), equipped with an energy dispersive X-ray spectroscopy (EDS). Finally, strength tests were conducted in order to determine the influence of the brazing parameters on the mechanical properties of the joint. © 2016, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-016-0346-4
  • 2016 • 175 Effect of Si on the acceleration of bainite transformation by pre-existing martensite
    Toji, Y. and Matsuda, H. and Raabe, D.
    Acta Materialia 116 250-262 (2016)
    Bainite transformation was investigated focusing on the influence of pre-existing martensite on the transformation kinetics, morphology and crystallographic orientation of subsequently formed bainite using EBSD and atom probe tomography. Two 1.1 wt% C-3wt.%Mn steels with and without 2 wt% Si were used to clarify the effect of Si. Steels were rapidly cooled from 900 °C to 300 °C and held at this temperature, or quenched from 900 °C once in water to generate approximately 30 vol% martensite followed by holding at 300 °C. Bainite transformation was clearly accelerated by pre-existing martensite in both Si-containing and Si-free steels. Bainite surrounds the pre-existing martensite in the Si-free steel, whereas it grows to the interior of the austenite grains in the steel containing 2 wt% Si. The major orientation relationship between bainite and adjacent austenite was changed by the presence of martensite from Nishiyama-Wassermann (N-W) to Greninger-Troiano (G-T) regardless of Si content. Clear carbon partitioning from martensite into austenite was observed prior to the bainite transformation in the 2 wt% Si steel, which was not observed in the Si-free steel. We suggest that the dislocations introduced by the martensite transformation act as a primary factor accelerating the bainite transformation when martensite is introduced prior to the bainite transformation. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.06.044
  • 2016 • 174 Aging of atmospherically plasma sprayed chromium evaporation barriers
    Vaßen, R. and Grünwald, N. and Marcano, D. and Menzler, N.H. and Mücke, R. and Sebold, D. and Sohn, Y.J. and Guillon, O.
    Surface and Coatings Technology 291 115-122 (2016)
    Chromium evaporation barriers are frequently used in solid oxide fuel cells to protect the porous cathode from chromium poisoning. Volatile chromium species are generated at the operation temperature of about 600-900 °C in a humid atmosphere for chromia scale forming steels as interconnect materials. In order to reduce this effect, barrier coatings are applied, often by atmospheric plasma spraying. However, also in these coatings microstructural changes as densification and in parallel formation of large pores have been observed. In order to clarify these mechanisms plasma sprayed Mn1.0 Co1.9Fe0.1O4 ("MCF") are deposited on ferritic steels and furthermore coated with a perovskite based contact layer as used in stack build-up. These coatings are annealed in air up to 1000 h and the microstructural changes and bending of the samples are studied. The results show increasing bending with increasing aging time. High temperature curvature measurements indicate that the amount of bending is not significantly dependent on temperature. As an explanation the creep deformation of the substrate/coating system at high temperatures due to compressive stress levels in the coating is given. The origin of the stress is related to phase changes in combination with the oxidation of the coatings. In addition, interdiffusion and densification processes are discussed. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.02.005
  • 2016 • 173 Tuning the magnetism of ferrite nanoparticles
    Viñas, S.L. and Simeonidis, K. and Li, Z.-A. and Ma, Z. and Myrovali, E. and Makridis, A. and Sakellari, D. and Angelakeris, M. and Wiedwald, U. and Spasova, M. and Farle, M.
    Journal of Magnetism and Magnetic Materials 415 20-23 (2016)
    The importance of magnetic interactions within an individual nanoparticle or between adjacent ones is crucial not only for the macroscopic collective magnetic behavior but for the AC magnetic heating efficiency as well. On this concept, single-(MFe2O4 where M=Fe, Co, Mn) and core-shell ferrite nanoparticles consisting of a magnetically softer (MnFe2O4) or magnetically harder (CoFe2O4) core and a magnetite (Fe3O4) shell with an overall size in the 10 nm range were synthesized and studied for their magnetic particle hyperthermia efficiency. Magnetic measurements indicate that the coating of the hard magnetic phase (CoFe2O4) by Fe3O4 provides a significant enhancement of hysteresis losses over the corresponding single-phase counterpart response, and thus results in a multiplication of the magnetic hyperthermia efficiency opening a novel pathway for high-performance, magnetic hyperthermia agents. At the same time, the existence of a biocompatible Fe3O4 outer shell, toxicologically renders these systems similar to iron-oxide ones with significantly milder side-effects. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2016.02.098
  • 2016 • 172 Cyclic degradation in bamboo-like Fe-Mn-Al-Ni shape memory alloys - The role of grain orientation
    Vollmer, M. and Krooß, P. and Kriegel, M.J. and Klemm, V. and Somsen, C. and Ozcan, H. and Karaman, I. and Weidner, A. and Rafaja, D. and Biermann, H. and Niendorf, T.
    Scripta Materialia 114 156-160 (2016)
    In the present study the cyclic deformation behavior within differently oriented grains in Fe-34.8Mn-13.5Al-7.4Ni (at.%) shape memory polycrystals featuring a bamboo-like structure was investigated. In cyclic tensile tests up to 50 cycles, the degree of degradation in pseudoelasticity was evaluated and contributing elementary mechanisms are discussed. The results reveal rapid cyclic degradation in the bamboo-like samples. The unexpected stabilization of parent phase in reverse transformed areas and the proceeding activation of new martensite variants in subsequent cycles were found to be the prevailing degradation mechanisms. Dislocation activity is found to be the most detrimental factor. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2015.12.007
  • 2016 • 171 Spectral TRIP enables ductile 1.1 GPa martensite
    Wang, M.-M. and Tasan, C.C. and Ponge, D. and Raabe, D.
    Acta Materialia 111 262-272 (2016)
    Introduction of interlath reverted austenite is an effective method to design ductile lath martensitic steels. The challenge in this concept is that all reverted austenite films have similar mechanical stability, hence, they all undergo transformation-induced plasticity (TRIP) at the same strain level. Here we propose a new thermo-mechanical treatment route to activate the TRIP effect over a broad strain regime and refer to it as 'spectral TRIP effect'. It aims at spreading the micro-mechanical stability of reverted austenite grains by widening the austenite nucleation barrier in martensite. To validate the proposed thermo-mechanical treatment route, an as-quenched medium-Mn martensitic steel was cold rolled prior to the reversion treatment at 600 °C. Microstructure characterization was carried out by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). Mechanical tests show that the approach is effective. The spectral TRIP effect improves both, the strength and the ductility due to the well dispersed size distribution and the associated size-dependent deformation and phase transformation behavior of the reverted austenite grains, extending TRIP-related work hardening over a broad strain range. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.03.070
  • 2016 • 170 Strain hardening by dynamic slip band refinement in a high-Mn lightweight steel
    Welsch, E. and Ponge, D. and Hafez Haghighat, S.M. and Sandlöbes, S. and Choi, P. and Herbig, M. and Zaefferer, S. and Raabe, D.
    Acta Materialia 116 188-199 (2016)
    The strain hardening mechanism of a high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C (wt%)) is investigated by electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM). The alloy is characterized by a constant high strain hardening rate accompanied by high strength and high ductility (ultimate tensile strength: 900 MPa, elongation to fracture: 68%). Deformation microstructures at different strain levels are studied in order to reveal and quantify the governing structural parameters at micro- and nanometer scales. As the material deforms mainly by planar dislocation slip causing the formation of slip bands, we quantitatively study the evolution of the slip band spacing during straining. The flow stress is calculated from the slip band spacing on the basis of the passing stress. The good agreement between the calculated values and the tensile test data shows dynamic slip band refinement as the main strain hardening mechanism, enabling the excellent mechanical properties. This novel strain hardening mechanism is based on the passing stress acting between co-planar slip bands in contrast to earlier attempts to explain the strain hardening in high-Mn lightweight steels that are based on grain subdivision by microbands. We discuss in detail the formation of the finely distributed slip bands and the gradual reduction of the spacing between them, leading to constantly high strain hardening. TEM investigations of the precipitation state in the as-quenched state show finely dispersed atomically ordered clusters (size < 2 nm). The influence of these zones on planar slip is discussed. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.06.037
  • 2016 • 169 A crystal plasticity model for twinning- and transformation-induced plasticity
    Wong, S.L. and Madivala, M. and Prahl, U. and Roters, F. and Raabe, D.
    Acta Materialia 118 140-151 (2016)
    A dislocation density-based crystal plasticity model incorporating both transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) is presented. The approach is a physically-based model which reflects microstructure investigations of ε-martensite, twins and dislocation structures in high manganese steels. Validation of the model was conducted using experimental data for a TRIP/TWIP Fe-22Mn-0.6C steel. The model is able to predict, based on the difference in the stacking fault energies, the activation of TRIP and/or TWIP deformation mechanisms at different temperatures. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.07.032
  • 2016 • 168 Combined atom probe tomography and density functional theory investigation of the Al off-stoichiometry of κ-carbides in an austenitic Fe-Mn-Al-C low density steel
    Yao, M.J. and Dey, P. and Seol, J.-B. and Choi, P. and Herbig, M. and Marceau, R.K.W. and Hickel, T. and Neugebauer, J. and Raabe, D.
    Acta Materialia 106 229-238 (2016)
    We report on the investigation of the off-stoichiometry and site-occupancy of κ-carbide precipitates within an austenitic (γ), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the κ-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L′12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the κ/γ mismatch, which facilitates occupation of Al sites in κ-carbide by Mn atoms (Mnγ Al anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.01.007
  • 2016 • 167 Coherent spin dynamics of carriers in ferromagnetic semiconductor heterostructures with an Mn δ layer
    Zaitsev, S.V. and Akimov, I.A. and Langer, L. and Danilov, Y.A. and Dorokhin, M.V. and Zvonkov, B.N. and Yakovlev, D.R. and Bayer, M.
    Journal of Experimental and Theoretical Physics 123 420-428 (2016)
    The coherent spin dynamics of carriers in the heterostructures that contain an InGaAs/GaAs quantum well (QW) and an Mn δ layer, which are separated by a narrow GaAs spacer 2–10 nm thick, is comprehensively studied by the magnetooptical Kerr effect method at a picosecond time resolution. The exchange interaction of photoexcited electrons in QW with the ferromagnetic Mn δ layer manifests itself in magnetic-field and temperature dependences of the Larmor precession frequency of electron spins and is found to be very weak (several microelectron volts). Two nonoscillating components related to holes exist apart from an electron contribution to the Kerr signal of polarization plane rotation. At the initial stage, a fast relaxation process, which corresponds to the spin relaxation of free photoexcited holes, is detected in the structures with a wide spacer. The second component is caused by the further spin dephasing of energyrelaxed holes, which are localized at strong QW potential fluctuations in the structures under study. The decay of all contributions to the Kerr signal in time increases substantially when the spacer thickness decreases, which correlates with the enhancement of nonradiative recombination in QW. © 2016, Pleiades Publishing, Inc.
    view abstractdoi: 10.1134/S106377611607013X
  • 2015 • 166 Magnetic States of the Ni1.75Co0.25Mn1.25Cr0.25In0.5 Heusler Alloy
    Buchelnikov, V.D. and Sokolovskiy, V.V. and Gruner, M.E. and Entel, P.
    IEEE Transactions on Magnetics 51 (2015)
    The equilibrium magnetic and structural reference states of Co- and Cr-doped Ni2Mn1.5In0.5 Heusler alloy are investigated by means of the first-principles method using a supercell approach. Three different ferrimagnetic and one ferromagnetic (FM) spin configurations, as well as two supercells with different distributions of excess Mn and In atoms, were considered. It is found that for supercell #1, the FM spin state in austenite is stable, where the martensite with different spin configurations is unstable, while in the case of supercell #2, a ferrimagnetic configuration for both austenite and martensite is favorable. The different trends for martensitic transformation were studied by c/a calculations for the tetragonal and orthorombic distortions of supercells along the z -axis and the y -axis, showing martensitic variants for supercell #2 at a ratio c/a &gt; 1 and c/a < 1. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2015.2438953
  • 2015 • 165 Intermartensitic transitions and phase stability in Ni50Mn50-xSnx Heusler alloys
    Çakir, A. and Righi, L. and Albertini, F. and Acet, M. and Farle, M.
    Acta Materialia 99 140-149 (2015)
    Ni-Mn based Heusler alloys are of considerable interest due to their multifunctional properties such as magnetic shape memory, magnetocaloric effect and spintronics. The reason for these multifunctional properties is the presence of a first order martensitic transition and its strong coupling to the magnetization. In this work, one of the outstanding class of martensitic Heuslers, Ni-Mn-Sn, is investigated in relation to magneto-structural phase transitions and the stability of the various crystallographic structures under varying temperature. Temperature-dependent X-ray diffraction, resistance and magnetization measurements on Ni<inf>50</inf>Mn<inf>50-x</inf>Sn<inf>x</inf> alloys are performed in a broad valence electron concentration range 7.91 ≤ (e/a) ≤ 8.34 (5.1 ≤ x ≤ 20.3at.%). The results reveal that in addition to the austenite-martensite transition, further intermartensitic transitions take place with decreasing temperature. Depending on the composition, we observe that the parent martensite phase tends to transform to L1<inf>0</inf> martensite as the ground state phase when the temperature is lowered. A phase diagram of Ni<inf>50</inf>Mn<inf>50-x</inf>Sn<inf>x</inf> is constructed to include intermartensitic phase transition boundaries. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.07.072
  • 2015 • 164 Kinetic arrest in magnetically inhomogeneous C-deficient Mn3GaC
    Cakir, O. and Acet, M. and Farle, M. and Dias, E. and Priolkar, K.
    Journal of Magnetism and Magnetic Materials 390 96--99 (2015)
    Thermal broadening of the first order magnetostructural transition and enhancement in ferromagnetic exchange occurs in carbon deficient Mn3GaC0.9. We show from temperature and field-dependent magnetization measurements that this leads to inhomogeneous magnetism and causes frustration and kinetic arrest effects. The arrested state is deactivated and the system returns to its ground state when the cooling-field is removed. This causes open hysteresis loops with which we study the kinetic arrest effect in this system. (C) 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2015.04.084
  • 2015 • 163 Characteristics of 5M modulated martensite in Ni-Mn-Ga magnetic shape memory alloys
    Çaklr, A. and Acet, M. and Righi, L. and Albertini, F. and Farle, M.
    AIP Advances 5 (2015)
    The applicability of the magnetic shape memory effect in Ni-Mn-based martensitic Heusler alloys is closely related to the nature of the crystallographically modulated martensite phase in these materials. We study the properties of modulated phases as a function of temperature and composition in three magnetic shape memory alloys Ni49.8Mn25.0Ga25.2, Ni49.8Mn27.1Ga23.1 and Ni49.5Mn28.6Ga21.9. The effect of substituting Ga for Mn leads to an anisotropic expansion of the lattice, where the b-parameter of the 5M modulated structure increases and the a and c-parameters decrease with increasing Ga concentration. The modulation vector is found to be both temperature and composition dependent. The size of the modulation vector corresponds to an incommensurate structure for Ni49.8Mn25.0Ga25.2 at all temperatures. For the other samples the modulation is incommensurate at low temperatures but reaches a commensurate value of q 0.400 close to room temperature. The results show that commensurateness of the 5M modulated structure is a special case of incommensurate 5M at a particular temperature. © 2015 Author(s).
    view abstractdoi: 10.1063/1.4932233
  • 2015 • 162 Size and orientation effects in partial dislocation-mediated deformation of twinning-induced plasticity steel micro-pillars
    Choi, W.S. and De Cooman, B.C. and Sandlöbes, S. and Raabe, D.
    Acta Materialia 98 391-404 (2015)
    Abstract Bulk and micro-pillar single crystals were used to investigate the twinning-induced plasticity mechanism in austenitic Fe-22 wt%Mn-0.6 wt%C TWIP steel. Compression of micro-pillars oriented either for deformation-induced twinning or for perfect dislocation glide was carried out for pillars with diameters in the range of 600 nm to 4 μm. The same size dependence of the critical resolved shear stress was observed for both orientations. The critical micro-pillar diameter for size-independent plasticity was approximately 7.6 μm. Partial dislocation-mediated formation of twins and ε-martensite was observed in micro-pillars oriented for twinning by transmission electron microscopy. The elastic-plastic transition in micro-pillars oriented for deformation twinning did not involve twinning, and dislocation-dislocation interactions were a necessary precondition for twin formation. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.06.065
  • 2015 • 161 Design of a twinning-induced plasticity high entropy alloy
    Deng, Y. and Tasan, C.C. and Pradeep, K.G. and Springer, H. and Kostka, A. and Raabe, D.
    Acta Materialia 94 124-133 (2015)
    We introduce a liquid metallurgy synthesized, non-equiatomic Fe<inf>40</inf>Mn<inf>40</inf>Co<inf>10</inf>Cr<inf>10</inf> high entropy alloy that is designed to undergo mechanically-induced twinning upon deformation at room temperature. Microstructure characterization, carried out using SEM, TEM and APT shows a homogeneous fcc structured single phase solid solution in the as-cast, hot-rolled and homogenized states. Investigations of the deformation substructures at specific strain levels with electron channeling contrast imaging (ECCI) combined with EBSD reveal a clear change in the deformation mechanisms of the designed alloy starting from dislocation slip to twinning as a function of strain. Such twinning induced plasticity has only been observed under cryogenic conditions in the equiatomic FeMnNiCoCr high entropy alloy. Thus, despite the decreased contribution of solid solution strengthening, the tensile properties of the introduced lean alloy at room temperature are found to be comparable to that of the well-studied five component FeMnNiCoCr system. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.04.014
  • 2015 • 160 Interplay of strain and interdiffusion in Heusler alloy bilayers
    Dutta, B. and Hickel, T. and Neugebauer, J. and Behler, C. and Fähler, S. and Behler, A. and Waske, A. and Teichert, N. and Schmalhorst, J.-M. and Hütten, A.
    Physica Status Solidi - Rapid Research Letters 9 321-325 (2015)
    Combining conventional and inverse magnetocaloric materials promises to enhance solid state refrigeration. As a first step here we present epitaxial Ni-Mn-Ga/Ni-Mn-Sn bilayer films. We examine the dependence of the lateral and normal lattice constants on the deposition sequence by combining experimental and ab initio techniques. Structural properties are determined with X-ray diffraction as well as highresolution transmission electron microscopy, while ab initio calculations explain the interplay of strain, local relaxations and the interdiffusion of atoms. The latter is confirmed by Auger electron spectroscopy and is expected to have a noticeable impact on the functional properties of the Heusler materials. ( © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201510070
  • 2015 • 159 Large magnetocaloric effects in magnetic intermetallics: First-principles and Monte Carlo studies
    Entel, P. and Gruner, M.E. and Ogura, M. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Grünebohm, A. and Arróyave, R. and Uebayashi, K. and Singh, N. and Talapatra, A. and Duong, T. and Acet, M. and Çakir, A.
    MATEC Web of Conferences 33 (2015)
    We have performed ab initio electronic structure calculations and Monte Carlo simulations of frustrated ferroic materials where complex magnetic configurations and chemical disorder lead to rich phase diagrams. With lowering of temperature, we find a ferromagnetic phase which transforms to an antiferromagnetic phase at the magnetostructural (martensitic) phase transition and to a cluster spin glass at still lower temperatures. The Heusler alloys Ni-(Co)-Mn-(Cr)-(Ga, Al, In, Sn, Sb) are of particular interest because of their large inverse magnetocaloric effect associated with the magnetostructural transition and the influence of Co/Cr doping. Besides spin glass features, strain glass behavior has been observed in Ni-Co-Mn-In. The numerical simulations allow a complete characterization of the frustrated ferroic materials including the Fe-Rh-Pd alloys. © Owned by the authors, published by EDP Sciences, 2015.
    view abstractdoi: 10.1051/matecconf/20153302001
  • 2015 • 158 The metamagnetic behavior and giant inverse magnetocaloric effect in Ni-Co-Mn-(Ga, In, Sn) Heusler alloys
    Entel, P. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Ogura, M. and Gruner, M.E. and Grünebohm, A. and Comtesse, D. and Akai, H.
    Journal of Magnetism and Magnetic Materials 385 193-197 (2015)
    The magnetic and magnetocaloric properties of Ni-Co-Mn-(Ga, In, Sn) Heusler intermetallics are discussed on the basis of ab initio and Monte Carlo calculations. The main emphasis is on the different reference spin states and magnetic exchange coupling constants of high-temperature austenite and low-temperature martensite which are very important for the calculation of magnetocaloric effect. The origin of metamagnetic behavior is considered in the framework of orbital resolved magnetic exchange parameters of austenite and martensite. The decomposition of exchange constants on orbital contributions has shown that a strong ferromagnetic interaction of magnetic moments in austenite is caused by the more itinerant d-electrons with t2g states while a strong antiferromagnetic interaction in martensite is associated with the more localized eg states. In addition, the appearance of a paramagnetic gap between magnetically weak martensite and ferromagnetically ordered austenite can be realized because of strong competition of magnetic exchange interactions. As a result, large magnetization drop and giant inverse magnetocaloric effect can be achieved across the magnetostructural phase transition. ©2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmmm.2015.03.003
  • 2015 • 157 Spin-resolved low-energy and hard x-ray photoelectron spectroscopy of off-stoichiometric Co2MnSi Heusler thin films exhibiting a record TMR
    Fetzer, R. and Ouardi, S. and Honda, Y. and Liu, H.-X. and Chadov, S. and Balke, B. and Ueda, S. and Suzuki, M. and Uemura, T. and Yamamoto, M. and Aeschlimann, M. and Cinchetti, M. and Fecher, G.H. and Felser, C.
    48 (2015)
    Half-metallic Co<inf>2</inf>MnSi-based Heusler compounds have attracted attention because they yield very high tunnelling magnetoresistance (TMR) ratios. Record TMR ratios of 1995% (at 4.2 K) are obtained from off-stoichiometric Co<inf>2</inf>MnSi-based magnetic tunnel junctions. This work reports on a combination of band structure calculations and spin-resolved and photon-polarisation-dependent photoelectron spectroscopy for off-stoichiometric Heusler thin films with the composition Co<inf>2</inf>Mn<inf>1.30</inf>Si<inf>0.84</inf>. Co and Mn are probed by magnetic dichroism in angle-resolved photoelectron spectroscopy at the 2p core level. In contrast to the delocalised Co 3d states, a pronounced localisation of the Mn 3d states is deduced from the corresponding 2p core level spectra. The valence states are investigated by linear dichroism using both hard x-ray and very-low-photon-energy excitation. When a very low photon energy is used for excitation, the valence bands exhibit a spin polarisation of about 30% at the Fermi energy. First principles calculations reveal that the low spin polarisation might be caused by a spin-flip process in the photoelectron final states. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/16/164002
  • 2015 • 156 The geometric and electronic structure of TCNQ and TCNQ+Mn on Ag(0 0 1) and Cu(0 0 1) surfaces
    Feyer, V. and Graus, M. and Nigge, P. and Zamborlini, G. and Acres, R.G. and Schöll, A. and Reinert, F. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 204 125-131 (2015)
    Copper and silver surfaces can be used as model systems to study structure formation and interfacial bonding upon adsorption of organic molecules. We have investigated the geometric and electronic structure of ordered monolayers of TCNQ on Cu(0 0 1) and Ag(0 0 1) and of TCNQ+Mn on Ag(0 0 1) surfaces by LEED and photoelectron momentum microscopy. While TCNQ forms an incommensurable superstructure on Cu(0 0 1), two coverage-dependant, commensurable superstructures are established on Ag(0 0 1). Subsequent adsorption of Mn on top of TCNQ/Ag(0 0 1) results in the formation of a long-range ordered mixed metal-organic superstructure, which is also commensurable with the Ag(0 0 1) substrate. The photoelectron spectroscopy (PES) data shows a filling of the TCNQ LUMO by charge transfer from the substrate for all investigated interfaces and the coadsorption of Mn leads to an energy shift of the TCNQ HOMO and LUMO of 230 meV with respect to TCNQ/Ag(0 0 1). The characteristic angle-dependent intensity pattern of the TCNQ LUMO in PES was utilized to investigate the azimuthal orientation of the molecules in the respective unit cells. The angle-resolved PES data was further analyzed to identify lateral band dispersion effects in the adsorbate layers, but no significant dispersion was observed. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elspec.2015.02.010
  • 2015 • 155 Influence of alkylphosphonic acid grafting on the electronic and magnetic properties of La2/3Sr1/3MnO3 surfaces
    Galbiati, M. and Tatay, S. and Delprat, S. and Barraud, C. and Cros, V. and Jacquet, E. and Coloma, F. and Choueikani, F. and Otero, E. and Ohresser, P. and Haag, N. and Cinchetti, M. and Aeschlimann, M. and Seneor, P. and Mattana...
    353 24-28 (2015)
    Self-Assembled monolayers (SAMs) are highly promising materials for molecular engineering of electronic and spintronics devices thanks to their surface functionalization properties. In this direction, alkylphosphonic acids have been used to functionalize the most common ferromagnetic electrode in organic spintronics: La<inf>2/3</inf>Sr<inf>1/3</inf>MnO<inf>3</inf> (LSMO). However, a study on the influence of SAMs grafting on LSMO electronic and magnetic properties is still missing. In this letter, we probe the influence of alkylphosphonic acids-based SAMs on the electronic and magnetic properties of the LSMO surface using different spectroscopies. We observe by X-ray photoemission and X-ray absorption that the grafting of the molecules on the LSMO surface induces a reduction of the Mn oxidation state. Ultraviolet photoelectron spectroscopy measurements also show that the LSMO work function can be modified by surface dipoles opening the door to both tune the charge and spin injection efficiencies in organic devices such as organic light-emitting diodes. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.06.051
  • 2015 • 154 ICME for Crashworthiness of TWIP Steels: From Ab Initio to the Crash Performance
    Güvenç, O. and Roters, F. and Hickel, T. and Bambach, M.
    JOM 67 120-128 (2015)
    During the last decade, integrated computational materials engineering (ICME) emerged as a field which aims to promote synergetic usage of formerly isolated simulation models, data and knowledge in materials science and engineering, in order to solve complex engineering problems. In our work, we applied the ICME approach to a crash box, a common automobile component crucial to passenger safety. A newly developed high manganese steel was selected as the material of the component and its crashworthiness was assessed by simulated and real drop tower tests. The crashworthiness of twinning-induced plasticity (TWIP) steel is intrinsically related to the strain hardening behavior caused by the combination of dislocation glide and deformation twinning. The relative contributions of those to the overall hardening behavior depend on the stacking fault energy (SFE) of the selected material. Both the deformation twinning mechanism and the stacking fault energy are individually well-researched topics, but especially for high-manganese steels, the determination of the stacking-fault energy and the occurrence of deformation twinning as a function of the SFE are crucial to understand the strain hardening behavior. We applied ab initio methods to calculate the stacking fault energy of the selected steel composition as an input to a recently developed strain hardening model which models deformation twinning based on the SFE-dependent dislocation mechanisms. This physically based material model is then applied to simulate a drop tower test in order to calculate the energy absorption capacity of the designed component. The results are in good agreement with experiments. The model chain links the crash performance to the SFE and hence to the chemical composition, which paves the way for computational materials design for crashworthiness. © 2014, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1192-8
  • 2015 • 153 Influence of carbon content, particle size, and partial manganese substitution on the electrochemical performance of LiFexMn1-xPO4/carbon composites
    Hamid, N.A. and Wennig, S. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Ionics 21 1857-1866 (2015)
    LiFePO<inf>4</inf>/C and LiFe<inf>x</inf>Mn<inf>1-x</inf>PO<inf>4</inf>/C (x = 0.7) nanocomposites were successfully synthesized via scalable spray-flame synthesis followed by solid-state reaction. A solution of iron (III) acetylacetonate and tributyl phosphate in toluene was used to produce amorphous, nanosized FePO<inf>4</inf>⋅H<inf>2</inf>O in a spray-flame reactor which was then milled with Li<inf>2</inf>CO<inf>3</inf> and glucose to produce a LiFePO<inf>4</inf>/C composite material in a solid-state reaction. The influence of calcination temperature and carbon content on the properties of the resulting material was investigated using specific surface area measurements (BET), X-ray diffraction (XRD), electron microscopy, and electrochemical characterization. The impact of manganese addition on the electrochemical behavior was analyzed using cyclic voltammetry (CV) and constant-current (CC) measurements. XRD shows that the combination of gas-phase synthesis and subsequent solid-state reaction yields highly pure LiFePO<inf>4</inf>/C. BET measurement revealed that the particle size of LiFePO<inf>4</inf> in the composite depends on the amount of glucose. A discharge capacity of more than 140 mAh/g at C/20 is achieved for LiFePO<inf>4</inf>/C with a carbon content of 6 wt%. This material supports high charge as well as discharge rates delivering more than 60 mAh/g at 16 C and sustains good cycle stability providing 115 mAh/g at 1 C. The energy density of the olivine increases about 10 % by substituting 30 mol% of iron by manganese while preserving the electrochemical performance of pure LiFePO<inf>4</inf>/C. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s11581-015-1366-6
  • 2015 • 152 Manganese tetraboride, MnB4: High-temperature crystal structure, p-n transition, 55Mn NMR spectroscopy, solid solutions, and mechanical properties
    Knappschneider, A. and Litterscheid, C. and Brgoch, J. and George, N.C. and Henke, S.c and Cheetham, A.K. and Hu, J.G. and Seshadri, R. and Albert, B.
    Chemistry - A European Journal 21 8177-8181 (2015)
    The structural and electronic properties of MnB<inf>4</inf> were studied by high-temperature powder X-ray diffraction and measurements of the conductivity and Seebeck coefficient on spark-plasma-sintered samples. A transition from the room-temperature monoclinic structure (space group P2<inf>1</inf>/c) to a high-temperature orthorhombic structure (space group Pnnm) was observed at about 650K. The material remained semiconducting after the transition, but its behavior changed from p-type to n-type. 55Mn NMR measurements revealed an isotropic chemical shift of -1315ppm, confirming an oxidation state of Mn close to I. Solid solutions of Cr<inf>1-x</inf>Mn<inf>x</inf>B<inf>4</inf> (two phases in space groups Pnnm and P2<inf>1</inf>/c) were synthesized for the first time. In addition, nanoindentation studies yielded values of (496±26) and (25.3±1.7)GPa for the Young's modulus and hardness, respectively, compared to values of 530 and 37GPa obtained by DFT calculations. Phase transition: Monoclinic manganese tetraboride can be transformed into an orthorhombic phase thermally or by forming solid solutions with chromium tetraboride. The structural phase transition of semiconducting MnB<inf>4</inf> is accompanied by a p-n transition. 55Mn NMR spectroscopy confirmed the oxidation state I of the metal atom, and nanoindentation experiments resulted in hardness values that are in accordance with DFT calculations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201406631
  • 2015 • 151 Spatially and kinetically resolved mapping of hydrogen in a twinning-induced plasticity steel by use of Scanning Kelvin Probe Force Microscopy
    Koyama, M. and Bashir, A. and Rohwerder, M. and Merzlikin, S.V. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Journal of the Electrochemical Society 162 C638-C647 (2015)
    The hydrogen distribution in a hydrogen-charged Fe-18Mn-1.2C (wt%) twinning-induced plasticity austenitic steel was studied by Scanning Kelvin Probe Force Microscopy (SKPFM). We observed that 1-2 days after the hydrogen-charging, hydrogen showed a higher activity at twin boundaries than inside the matrix. This result indicates that hydrogen at the twin boundaries is diffusible at room temperature, although the twin boundaries act as deeper trap sites compared to typical diffusible hydrogen trap sites such as dislocations. After about 2 weeks the hydrogen activity in the twin boundaries dropped and was indistinguishable from that in the matrix. These SKPFM results were supported by thermal desorption spectrometry and scanning electron microscopic observations of deformation-induced surface cracking parallel to deformation twin boundaries. With this joint approach, two main challenges in the field of hydrogen embrittlement research can be overcome, namely, the detection of hydrogen with high local and chemical sensitivity and the microstructure-dependent and spatially resolved observation of the kinetics of hydrogen desorption. © 2015 The Electrochemical Society.
    view abstractdoi: 10.1149/2.0131512jes
  • 2015 • 150 Grain boundary segregation engineering and austenite reversion turn embrittlement into toughness: Example of a 9 wt.% medium Mn steel
    Kuzmina, M. and Ponge, D. and Raabe, D.
    Acta Materialia 86 182-192 (2015)
    We study grain boundary embrittlement in a quenched and tempered Fe-Mn high-purity model martensite alloy using Charpy impact tests and grain boundary characterization by atom probe tomography. We observe that solute Mn directly embrittles martensite grain boundaries while reversion of martensite to austenite at high-angle grain boundaries cleans the interfaces from solute Mn by partitioning the Mn into the newly formed austenite, hence restoring impact toughness. Microalloying with B improves the impact toughness in the quenched state and delays temper embrittlement at 450 °C. Tempering at 600 °C for 1 min significantly improves the impact toughness and further tempering at lower temperature does not cause the embrittlement to return. At higher temperatures, regular austenite nucleation and growth takes place, whereas at lower temperature, Mn directly promotes its growth. ©2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.12.021
  • 2015 • 149 Linear complexions: Confined chemical and structural states at dislocations
    Kuzmina, M. and Herbig, M. and Ponge, D. and Sandlöbes, S. and Raabe, D.
    Science 349 1080-1083 (2015)
    For 5000 years, metals have been mankind's most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We report chemical and structural states confined at dislocations. In a body-centered cubic Fe-9 atomic percent Mn alloy, we found Mn segregation at dislocation cores during heating, followed by formation of face-centered cubic regions but no further growth. The regions are in equilibrium with the matrix and remain confined to the dislocation cores with coherent interfaces. The phenomenon resembles interface-stabilized structural states called complexions. A cubic meter of strained alloy contains up to a light year of dislocation length, suggesting that linear complexions could provide opportunities to nanostructure alloys via segregation and confined structural states.
    view abstractdoi: 10.1126/science.aab2633
  • 2015 • 148 Atomic scale investigation of non-equilibrium segregation of boron in a quenched Mo-free martensitic steel
    Li, Y.J. and Ponge, D. and Choi, P. and Raabe, D.
    Ultramicroscopy 159 240-247 (2015)
    B-added low carbon steels exhibit excellent hardenability. The reason has been frequently attributed to B segregation at prior austenite grain boundaries, which prevents the austenite to ferrite transformation and favors the formation of martensite. The segregation behavior of B at prior austenite grain boundaries is strongly influenced by processing conditions such as austenitization temperatures and cooling rates and by alloying elements such as Mo, Cr, and Nb. Here an local electrode atom probe was employed to investigate the segregation behavior of B and other alloying elements (C, Mn, Si, and Cr) in a Cr-added Mo-free martensitic steel. Similar to our previous results on a Mo-added steel, we found that in both steels B is segregated at prior austenite grain boundaries with similar excess values, whereas B is neither detected in the martensitic matrix nor at martensite-martensite boundaries at the given cooling rate of 30 K/s. These results are in agreement with the literature reporting that Cr has the same effect on hardenability of steels as Mo in the case of high cooling rates. The absence of B at martensite-martensite boundaries suggests that B segregates to prior austenite grain boundaries via a non-equilibrium mechanism. Segregation of C at all boundaries such as prior austenite grain boundaries and martensite-martensite boundaries may occur by an equilibrium mechanism. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.03.009
  • 2015 • 147 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 84 110-123 (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 abstractdoi: 10.1016/j.actamat.2014.10.027
  • 2015 • 146 Phase stability of non-equiatomic CoCrFeMnNi high entropy alloys
    Ma, D. and Yao, M. and Pradeep, K.G. and Tasan, C.C. and Springer, H. and Raabe, D.
    Acta Materialia 98 288-296 (2015)
    Abstract The objective of this study is to experimentally and theoretically investigate the phase stability of non-equiatomic Fe<inf>x</inf>Mn<inf>62-x</inf>Ni<inf>30</inf>Co<inf>6</inf>Cr<inf>2</inf> based high entropy alloys, where x ranges from 22 to 42 at.%. Another aim is to systematically and critically assess the predictive capability of the CALPHAD approach for such high entropy alloy systems. We find that the CALPHAD simulations provide a very consistent assessment of phase stability yielding good agreement with experimental observations. These include the equilibrium phase formation at high temperatures, the constituent phases after non-equilibrium solidification processes, unfavorable segregation profiles inherited from solidification together with the associated nucleation and growth of low temperature phases, and undesired martensitic transformation effects. Encouraged by these consistent theoretical and experimental results, we extend our simulations to other alloy systems with equiatomic compositions reported in the literature. Using these other equiatomic model systems we demonstrate how systematic CALPHAD simulations can improve and accelerate the design of multicomponent alloy systems. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.030
  • 2015 • 145 Local electronic and magnetic properties of pure and Mn-containing magnetocaloric LaFe13-xSix compounds inferred from Mössbauer spectroscopy and magnetometry
    Makarov, S.I. and Krautz, M. and Salamon, S. and Skokov, K. and Teixeira, C.S. and Gutfleisch, O. and Wende, H. and Keune, W.
    Journal of Physics D: Applied Physics 48 (2015)
    Manganese containing La-Fe-Si alloys are important magnetocaloric compounds, since Mn atoms prevent segregation of hydrogen in partially hydrogenated La-Fe-Mn-Si alloys when their Curie temperature is tuned to room temperature by hydrogen. The effect of Mn alloying on the Fe atomic magnetic moment μ<inf>Fe</inf> is still rather unexplored. Therefore, we investigated the (local) magnetic and electric hyperfine interactions in the strongly magnetocaloric compound LaFe<inf>11.3</inf>Mn<inf>0.3</inf>Si<inf>1.4</inf> and, for comparison, LaFe<inf>11.6</inf>Si<inf>1.4</inf> by 57Fe Mössbauer spectroscopy, and the global magnetic properties by vibrating sample magnetometry. The NaZn<inf>13</inf> structure was confirmed by x-ray diffraction. Two non-equivalent Fe lattice sites are known to exist in this material: the (96i) sites (Fe<inf>II</inf>) of low local symmetry, and the highly symmetrical (8b) sites (Fe<inf>I</inf>). At room temperature in the paramagnetic state, the electric hyperfine parameters of Fe atoms on both sites were obtained. At low temperatures (4.8 K), the observed magnetically split nuclear Zeeman sextets with broad apparent lines were analyzed in terms of a distribution P(B<inf>hf</inf>) of hyperfine magnetic fields B<inf>hf</inf>. The average hyperfine field 〈B<inf>hf</inf>〉, originating predominantly from Fe<inf>II</inf> sites, was found to be rather high (30.7(1) T at 4.8 K) for LaFe<inf>11.6</inf>Si<inf>1.4</inf>, and the approximate relation 〈B<inf>hf</inf>〉 = Aμ<inf>Fe</inf> is confirmed for Fe<inf>II</inf> sites, with A = 14.2 T/μ<inf>B</inf>. 〈B<inf>hf</inf>〉 is significantly reduced (to 27.7(1) T at 4.8 K) for the Mn-containing sample LaFe<inf>11.3</inf>Mn<inf>0.3</inf>Si<inf>1.4</inf>, providing evidence for a reduction by 9.7% of the average Fe atomic moment μ<inf>Fe</inf> from ∼2.16 μ<inf>B</inf> to a value of ∼1.95 μ<inf>B</inf> by Mn substitution of Fe. Our Mössbauer results are in good agreement with magnetometry, which reveals a reduction of the saturation magnetization of M<inf>s</inf> = 163.1(1) Am2 kg-1 of LaFe<inf>11.6</inf>Si<inf>1.4</inf> by 10.5% due to Mn substitution. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/30/305006
  • 2015 • 144 Effect of dopants on the electrocaloric effect of 0.92 Pb(Mg1/3Nb2/3)O3-0.08 PbTiO3 ceramics
    Molin, C. and Sanlialp, M. and Shvartsman, V.V. and Lupascu, D.C. and Neumeister, P. and Schönecker, A. and Gebhardt, S.
    Journal of the European Ceramic Society 35 2065-2071 (2015)
    This paper presents an extract of modifications of lead magnesium niobate-lead titanate in order to optimize its electrocaloric properties. Electrocaloric entropy (δS<inf>EC</inf>) and temperature changes (δT<inf>EC</inf>) are measured in a temperature range from -5°C up to 100°C using differential scanning calorimetry. The influence of dopants (Li+, Mn2+, Sr2+, Ta5+) on the electrocaloric effect as well as on microstructure, dielectric and ferroelectric behavior is investigated. The maximum of relative permittivity can be shifted to lower or higher temperatures depending on the added dopant and decreased ferroelectric hysteresis losses were observed for all altered compositions. For the undoped PMN-8PT ceramics maximum δT<inf>EC</inf> of 0.58K and δS<inf>EC</inf> of 0.51Jkg-1K-1 were measured, when applying an electric field of 2kVmm-1 at 30°C. The electrocaloric temperature change showed lower values for all doped PMN-8PT. Remarkably, this is accompanied with an increase of the entropy change for the Li-doped sample. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jeurceramsoc.2015.01.016
  • 2015 • 143 The influence of stacking fault energy on the microstructural and strain-hardening evolution of Fe-Mn-Al-Si steels during tensile deformation
    Pierce, D.T. and Jiménez, J.A. and Bentley, J. and Raabe, D. and Wittig, J.E.
    Acta Materialia 100 178-190 (2015)
    Understanding the relationship between the stacking-fault energy (SFE), deformation mechanisms, and strain-hardening behavior is important for alloying and design of high-Mn austenitic transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The present study investigates the influence of SFE on the microstructural and strain-hardening evolution of three TRIP/TWIP alloys (Fe-22/25/28Mn-3Al-3Si wt.%). The SFE is increased by systemically increasing the Mn content from 22 to 28 wt.%. The Fe-22Mn-3Al-3Si alloy, with a SFE of 15 mJ m-2, deforms by planar dislocation glide and strain-induced ε<inf>hcp</inf>-/α<inf>bcc</inf>-martensite formation which occurs from the onset of plastic deformation, resulting in improved work-hardening at low strains but lower total elongation. With an increased SFE of 21 mJ m-2 in the Fe-25Mn-3Al-3Si alloy, both mechanical twinning and ε<inf>hcp</inf>-martensite formation are activated during deformation, and result in the largest elongation of the three alloys. A SFE of 39 mJ m-2 enables significant dislocation cross slip and suppresses ε<inf>hcp</inf>-martensite formation, causing reduced work-hardening during the early stages of deformation in the Fe-28Mn-3Al-3Si alloy while mechanical twinning begins to enhance the strain-hardening after approximately 10% strain. The increase in SFE from 15 to 39 mJ m-2 results in significant changes in the deformation mechanisms and, at low strains, decreased work-hardening, but has a relatively small influence on strength and ductility. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.08.030
  • 2015 • 142 From high-entropy alloys to high-entropy steels
    Raabe, D. and Tasan, C.C. and Springer, H. and Bausch, M.
    Steel Research International 86 1127-1138 (2015)
    Inspired by high-entropy alloys, we study the design of steels that are based on high configurational entropy for stabilizing a single-phase solid solution matrix. The focus is placed on the system Fe-Mn-Al-Si-C but we also present trends in the alloy system Fe-Mn-Al-C. Unlike in conventional high-entropy alloys, where five or more equiatomically proportioned components are used, we exploit the flat configurational entropy plateau in transition metal mixtures, stabilizing solid solutions also for lean, non-equiatomic compositions. This renders the high-entropy alloying concept, where none of the elements prevails, into a class of Fe-based materials which we refer to as high-entropy steels. A point that has received little attention in high-entropy alloys is the use of interstitial elements. Here, we address the role of C in face-centered cubic solid solution phases. High-entropy steels reveal excellent mechanical properties, namely, very high ductility and toughness; excellent high rate and low-temperature ductility; high strength of up to 1 GPa; up to 17% reduced mass density; and very high strain hardening. The microstructure stability can be tuned by adjusting the stacking fault energy. This enables to exploit deformation effects such as the TRIP, TWIP, or precipitation determined mechanisms. We present a class of massive solid solution steels with high configurational entropy. Focus is placed on the system Fe-Mn-Al-Si-C, i.e., considering also C interstitials. By exploiting the flat configurational entropy plateau in metal mixtures, solid solutions of lean, non-equiatomic compositions can be stabilized. This renders the high-entropy alloying concept, where none of the elements prevails, into high-entropy steels. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201500133
  • 2015 • 141 Effect of substitution on elastic stability, electronic structure and magnetic property of Ni-Mn based Heusler alloys: An ab initio comparison
    Roy, T. and Gruner, M.E. and Entel, P. and Chakrabarti, A.
    Journal of Alloys and Compounds 632 822-829 (2015)
    First-principles density functional theory based calculations have been used to predict the bulk mechanical properties of magnetic shape memory Heusler alloy Ni2MnGa substituted by copper (Cu), platinum (Pt), palladium (Pd) and manganese (Mn) at the Ni site. The elastic constants of Ni2MnGa alloy with and without substitution are calculated. We analyze and compare in detail the bulk mechanical properties for these alloys, in particular, the ratio between the calculated bulk and shear modulii, as well as the Poisson's ratio and Young's modulii. This analysis further based on an empirical relation, indicates that Pt2MnGa may inherently be the least brittle material, among the above-mentioned alloys. Interesting difference has been observed between the shear modulii calculated from Voigt's and Reuss's method. This has been explained in terms of the values of the tetragonal shear constant C′ of the materials. Study of Heisenberg exchange coupling parameters and Curie temperature as well as density of states of the materials shows the effect of substitution at the Ni site on the magnetic and electronic properties, respectively. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.01.255
  • 2015 • 140 Combinatorial synthesis and high-throughput characterization of the thin film materials system Co-Mn-Ge: Composition, structure, and magnetic properties
    Salomon, S. and Hamann, S. and Decker, P. and Savan, A. and Meshi, L. and Ludwig, Al.
    Physica Status Solidi (A) Applications and Materials Science 212 1969-1974 (2015)
    Co-Mn-Ge is a system of interest for magnetocaloric applications as a solid state magnetic refrigerant. A thin film materials library covering a large fraction of the Co-Mn-Ge ternary composition space was fabricated by sputter deposition. After deposition, it was annealed at 600°C for 3 h and quenched subsequently. An energy-dispersive X-ray spectroscopy and X-ray diffraction-based cluster analysis revealed the regions of existence for the CoMnGe and the Co<inf>2</inf>MnGe single phase areas. Furthermore, high intensity diffraction peaks revealed the presence of the hexagonal (Co, Mn)<inf>7</inf>Ge<inf>6</inf> phase in a region that also featured the CoMnGe phase. In this region, a non-linear, symmetric, and hysteretic shift of the (200) diffraction peak of the (Co, Mn)<inf>7</inf>Ge<inf>6</inf> phase was observed by temperature-dependent X-ray diffraction for Co<inf>23</inf>Mn<inf>33</inf>Ge<inf>44</inf>, indicating a structural phase transition taking place between 350 and 375 K upon heating and 325 and 300 K upon cooling. This coincides with a magnetic transition near 325 K from the ferromagnetic to the paramagnetic state. However, no magnetostructural coupling was identified in the temperature range from 330 to 300 K upon cooling. Magnetostriction and an undetected structural transition of the CoMnGe phase were ruled out as probable causes for the non-linear shifts. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532075
  • 2015 • 139 Dependence of the inverse magnetocaloric effect on the field-change rate in Mn3GaC and its relationship to the kinetics of the phase transition
    Scheibel, F. and Gottschall, T. and Skokov, K. and Gutfleisch, O. and Ghorbani-Zavareh, M. and Skourski, Y. and Wosnitza, J. and Çakir, Ö. and Farle, M. and Acet, M.
    Journal of Applied Physics 117 (2015)
    We study the dependence of the magnetocaloric effect on the magnetic field-change-rate the first order magnetostructural transition in Mn<inf>3</inf>GaC by measuring the adiabatic temperature change ΔT at three different time scales: 11 mT s-1, 700 mT s-1, and ∼1000 T s-1. We find that the maximum adiabatic temperature-change of about 5K is reached in the 11 mT s-1 and 700 mT s-1 rates, whereas for the ∼1000 T s-1-rate the transition lags the change in the magnetic field so that the maximum adiabatic temperature-change is not attained. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4922722
  • 2015 • 138 Alloying effects on microstructure formation of dual phase steels
    Schemmann, L. and Zaefferer, S. and Raabe, D. and Friedel, F. and Mattissen, D.
    Acta Materialia 95 386-398 (2015)
    In dual-phase (DP) steels, inherited microstructures and elemental distributions affect the kinetics and morphology of phase transformation phenomena and the mechanical properties of the final material. In order to study the inheritance process, we selected two model materials with the same average DP steel composition but with different initial microstructures, created by coiling at different temperatures after hot rolling. These samples were submitted to a DP-steel heat treatment consisting of a short isothermal annealing in the pure austenite region and a quenching process. The evolution of microstructure, chemical composition and mechanical properties (hardness) during this treatment was investigated. The initial samples had a bainitic-martensitic (B + M) microstructure for the material coiled at lower temperature and a ferritic-pearlitic (P + F) microstructure for that coiled at higher temperature. The P + F microstructure had a much more inhomogeneous distribution of substitutional elements (in particular of Mn) and of carbon. After complete heat treatment, both materials showed a typical DP microstructure (martensite islands embedded in ferrite) but the P + F material showed lower hardness compared to the B + M material. It was found that the inhomogeneous elemental distribution prevailed in the P + F material. The inheritance process was studied by combining measurements of the elemental distribution by Wavelength-Dispersive X-ray spectroscopy (WDX), simulations of the evolution of the elemental composition via the DICTRA (diffusion-controlled reactions) computer programme, dilatometry to observe the kinetics of phase transformation, and observation and quantification of the microstructures by Electron Backscatter Diffraction (EBSD) measurements. For the P + F material it was found that the α-γ transformation during annealing is slowed down in regions of lower Mn content and is therefore not completed. During the subsequent cooling the incompletely autenitized material does not require ferrite nucleation and the γ-α transformation starts at relative high temperatures. For B + M, in contrast, nucleation of ferrite is needed and the transformation starts at lower temperatures. As a result the B + M material develops a higher martensite content as well as a higher density of geometrically necessary dislocations (GNDs). It is speculated that for the B + M material the γ-α transformation occurs through a bainitic (i.e. partly displacive) process while the transformation at higher temperatures in the P + F material proceeds exclusively in a diffusive way. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.05.005
  • 2015 • 137 Crack propagation behavior of solution annealed austenitic high interstitial steels
    Schymura, M. and Stegemann, R. and Fischer, A.
    International Journal of Fatigue 79 25-35 (2015)
    Austenitic stainless steels provide a beneficial combination of chemical and mechanical properties and have been used in a wide field of applications for over 100 years. Further improvement of the chemical and mechanical properties was achieved by alloying nitrogen. But the solubility of N within the melt is limited and can be increased in substituting Ni by Mn and melting under increased pressure. In order to avoid melting under pressure and decrease production costs, a part of N can also be substituted by C. This leads to austenitic high interstitial steels (AHIS). Within the solution annealed state strength and ductility of AHIS is comparable or even higher of those of AHNS and can be further improved by cold working. Unfortunately the endurance limit does not follow this trend as it is known from cold-worked austenitic CrNi steels. This is due to the differences of the slip behavior which is governed by the stacking fault energy as well as other near field effects. Construction components operating under cyclic loads over long periods of time cannot be considered being free of voids or even cracks. Thus the crack propagation behavior is of strong interest as well. This contribution presents the tensile, fatigue, crack propagation and fracture toughness properties of AHNS and AHIS in comparison to those of CrNi-steels. The differences are discussed in relation to microstructural characteristic as well as their alterations under cyclic loading. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2015.04.014
  • 2015 • 136 New Developments in Martensitic Stainless Steels Containing C + N
    Seifert, M. and Siebert, S. and Huth, S. and Theisen, W. and Berns, H.
    Steel Research International 86 1508-1516 (2015)
    The use of nitrogen in martensitic stainless steels is limited by its solubility. Nitrogen solubility can be increased by alloying with elements such as Cr, Mn, and Mo and the use of pressure, such as in Pressurized ElectroSlag Remelting (PESR). Furthermore, the joint addition of C + N increases their solubility. Solid-state nitriding can be used for case hardening or N-enrichment of steel powders before sintering. However, the resulting stabilization of austenite can be a drawback for martensitic steels. Besides cryogenic treatment below the martensite finish temperature, ausforming, that is, metal working above Ms, could be promising. This contribution gives an overview about latest developments in N-rich martensitic stainless steels. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201400503
  • 2015 • 135 Effects of retained austenite volume fraction, morphology, and carbon content on strength and ductility of nanostructured TRIP-assisted steels
    Shen, Y.F. and Qiu, L.N. and Sun, X. and Zuo, L. and Liaw, P.K. and Raabe, D.
    Materials Science and Engineering A 636 551-564 (2015)
    With a suite of multi-modal and multi-scale characterization techniques, the present study unambiguously proves that a substantially-improved combination of ultrahigh strength and good ductility can be achieved by tailoring the volume fraction, morphology, and carbon content of the retained austenite (RA) in a transformation-induced-plasticity (TRIP) steel with the nominal chemical composition of 0.19C-0.30Si-1.76Mn-1.52Al (weight percent, wt%). After intercritical annealing and bainitic holding, a combination of ultimate tensile strength (UTS) of 1100. MPa and true strain of 50% has been obtained, as a result of the ultrafine RA lamellae, which are alternately arranged in the bainitic ferrite around junction regions of ferrite grains. For reference, specimens with a blocky RA, prepared without the bainitic holding, yield a low ductility (35%) and a low UTS (800. MPa). The volume fraction, morphology, and carbon content of RA have been characterized using various techniques, including the magnetic probing, scanning electron microscopy (SEM), electron-backscatter-diffraction (EBSD), and transmission electron microscopy (TEM). Interrupted tensile tests, mapped using EBSD in conjunction with the kernel average misorientation (KAM) analysis, reveal that the lamellar RA is the governing microstructure component responsible for the higher mechanical stability, compared to the blocky one. By coupling these various techniques, we quantitatively demonstrate that in addition to the RA volume fraction, its morphology and carbon content are equally important in optimizing the strength and ductility of TRIP-assisted steels. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.04.030
  • 2015 • 134 Predictions of a Large Magnetocaloric Effect in Co- and Cr-Substituted Heusler Alloys Using First-Principles and Monte Carlo Approaches
    Sokolovskiy, V.V. and Buchelnikov, V.D. and Zagrebin, M.A. and Grünebohm, A. and Entel, P.
    Physics Procedia 75 1381-1388 (2015)
    The effect of Co- and Cr-doping on magnetic and magnetocaloric poperties of Ni-Mn-(In, Ga, Sn, and Al) Heusler alloys has been theoretically studied by combining first principles with Monte Carlo approaches. The magnetic and magnetocaloric properties are obtained as a function of temperature and magnetic field using a mixed type of Potts and Blume-Emery-Griffiths model where the model parameters are obtained from ab initio calculations. The Monte Carlo calculations allowed to make predictions of a giant inverse magnetocaloric effect in partially new hypothetical magnetic Heusler alloys across the martensitic transformation. © 2015 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.phpro.2015.12.155
  • 2015 • 133 First-Principles Calculations of Magnetic Properties of Cr-Doped Ni45Co5Mn37In13 Heusler Alloys
    Sokolovskiy, V.V. and Buchelnikov, V.D. and Gruner, M.E. and Entel, P.
    IEEE Transactions on Magnetics 51 (2015)
    The magnetic and electronic properties of Co- and Cr-doped Ni50Mn37In13 Heusler alloys with a substitution of 5 at.% Co for Ni and 5 at.% Cr for Ni, Mn, or In are investigated in the framework of the density functional theory method. The chemical disorder in the off-stoichiometric Ni-Co-Mn-Cr-In systems was treated in the coherent potential approximation. Three different ferrimagnetic and one ferromagnetic (FM) spin states for austenite and martensite were considered in ab initio calculations. It is found that for both structures, the intersublattice interactions (MnY(Z)-Co, MnY(Z)-Ni, MnY(Z)-MnZ(Y), MnY(Z)-Cr, and Cr-Co) provide the largest contribution to the exchange due to the shorter distance compared with the intrasublattice interactions (MnY(Z)-MnY(Z), Co-Co, Ni-Ni, and Cr-Cr). Besides, the MnY-MnZ and MnY(Z)-Cr exchanges in the first shell become five times larger in martensite compared with austenite. The largest anti-FM interaction is observed between MnY(Z)-Cr atoms in martensite. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2015.2439391
  • 2015 • 132 Influence of film thickness and composition on the martensitic transformation in epitaxial Ni-Mn-Sn thin films
    Teichert, N. and Auge, A. and Yüzüak, E. and Dincer, I. and Elerman, Y. and Krumme, B. and Wende, H. and Yildirim, O. and Potzger, K. and Hütten, A.
    Acta Materialia 86 279-285 (2015)
    Two series of epitaxial Ni-Mn-Sn thin films of different thickness are investigated for the thickness and composition dependence of the martensitic transformation. Thin films ranging in thickness from 20 to 200 nm (series A) and 10 to 100 nm (series B) were prepared by magnetron co-sputtering and deposited on heated MgO(0 0 1) substrates. The structural characterization was done by temperature-dependent X-ray diffraction measurements. Magnetization and resistivity measurements were performed to investigate the transformation characteristics. We find a strong influence of the film thickness on the relative amount of material undergoing the martensitic transformation, the temperature range of the transformation, and the transformation temperatures. The main contribution originates from the rigid substrate which delays the transformation of the Ni-Mn-Sn near the interface and even leads to a layer of residual austenite at low temperatures. Another issue are size effects which presumably broaden the martensitic transformation and decrease the transformation temperatures. By variation of the thin film composition we find changes of the substrate influence due to a different mismatch between the lattice of MgO and austenite. A better phase compatibility between martensite and austenite, denoted by λ2, not only results in a smaller hysteresis but is also beneficial for the transformation of material close to the substrate. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.12.019
  • 2015 • 131 The structural, magnetic, and magnetocaloric properties of In-doped Mn2- xCrxSb
    Tekgül, A. and Çaklr, Ö. and Acet, M. and Farle, M. and Ünal, N.
    Journal of Applied Physics 118 (2015)
    Mn2- xCrxSb exhibits an antiferromagnetic-ferrimagnetic transition of which the temperature can be changed by controlling the Cr concentration. Increasing the Cr content from x = 0.05 to 0.13 raises the transition temperature from about 180 K up to around room temperature. To suppress the inevitable ferromagnetic MnSb impurity phase, we partially replace Sb by In. Mn2- xCrxSb1- yIny alloys have an antiferromagnetic-ferrimagnetic transition and a narrow transition hysteresis and exhibit the inverse magnetocaloric effect. We examine the magnetocaloric effect from the magnetization and direct adiabatic temperature-change measurements. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4934253
  • 2015 • 130 Non-aqueous semi-solid flow battery based on Na-ion chemistry. P2-type NaxNi0.22Co0.11Mn0.66O2-NaTi2(PO4)3
    Ventosa, E. and Buchholz, D. and Klink, S. and Flox, C. and Chagas, L.G. and Vaalma, C. and Schuhmann, W. and Passerini, S. and Morante, J.R.
    Chemical Communications 51 7298-7301 (2015)
    We report the first proof of concept for a non-aqueous semi-solid flow battery (SSFB) based on Na-ion chemistry using P2-type Na<inf>x</inf>Ni<inf>0.22</inf>Co<inf>0.11</inf>Mn<inf>0.66</inf>O<inf>2</inf> and NaTi<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> as positive and negative electrodes, respectively. This concept opens the door for developing a new low-cost type of non-aqueous semi-solid flow batteries based on the rich chemistry of Na-ion intercalating compounds. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4cc09597a
  • 2015 • 129 Enhancing Hydrogen Embrittlement Resistance of Lath Martensite by Introducing Nano-Films of Interlath Austenite
    Wang, M. and Tasan, C.C. and Koyama, M. and Ponge, D. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 3797-3802 (2015)
    Partial reversion of interlath austenite nano-films is investigated as a potential remedy for hydrogen embrittlement susceptibility of martensitic steels. We conducted uniaxial tensile tests on hydrogen-free and pre-charged medium-Mn transformation-induced plasticity-maraging steels with different austenite film thicknesses. Mechanisms of crack propagation and microstructure interaction are quantitatively analyzed using electron channelling contrast imaging and electron backscatter diffraction, revealing a promising strategy to utilize austenite reversion for hydrogen-resistant martensitic steel design. © 2015, The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-015-3009-y
  • 2015 • 128 Nanolaminate transformation-induced plasticity-twinning-induced plasticity steel with dynamic strain partitioning and enhanced damage resistance
    Wang, M.-M. and Tasan, C.C. and Ponge, D. and Dippel, A.-Ch. and Raabe, D.
    Acta Materialia 85 216-228 (2015)
    Conventional martensitic steels have limited ductility due to insufficient microstructural strain-hardening and damage resistance mechanisms. It was recently demonstrated that the ductility and toughness of martensitic steels can be improved without sacrificing the strength, via partial reversion of the martensite back to austenite. These improvements were attributed to the presence of the transformation-induced plasticity (TRIP) effect of the austenite phase, and the precipitation hardening (maraging) effect in the martensitic matrix. However, a full micromechanical understanding of this ductilizing effect requires a systematic investigation of the interplay between the two phases, with regards to the underlying deformation and damage micromechanisms. For this purpose, in this work, a Fe-9Mn-3Ni-1.4Al-0.01C (mass%) medium-Mn TRIP maraging steel is produced and heat-treated under different reversion conditions to introduce well-controlled variations in the austenite-martensite nanolaminate microstructure. Uniaxial tension and impact tests are carried out and the microstructure is characterized using scanning and transmission electron microscopy based techniques and post mortem synchrotron X-ray diffraction analysis. The results reveal that (i) the strain partitioning between austenite and martensite is governed by a highly dynamical interplay of dislocation slip, deformation-induced phase transformation (i.e. causing the TRIP effect) and mechanical twinning (i.e. causing the twinning-induced plasticity effect); and (ii) the nanolaminate microstructure morphology leads to enhanced damage resistance. The presence of both effects results in enhanced strain-hardening capacity and damage resistance, and hence the enhanced ductility. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.11.010
  • 2015 • 127 Nitrogen uptake of nickel free austenitic stainless steel powder during heat treatment-an XPS study
    Weddeling, A. and Lefor, K. and Hryha, E. and Huth, S. and Nyborg, L. and Weber, S. and Theisen, W.
    Surface and Interface Analysis 47 413-422 (2015)
    In austenitic stainless steel nitrogen stabilizes the austenitic phase improves the mechanical properties and increases the corrosion resistance. Nitrogen alloying enables to produce austenitic steels without the element nickel which is high priced and classified as allergy inducing. A novel production route is nitrogen alloying of CrMn-prealloyed steel powder via the gas phase. This is beneficial as the nitrogen content can be adjusted above the amount that is reached during conventional casting. A problem which has to be overcome is the oxide layer present on the powder surface which impedes both the sintering process and the uptake of nitrogen. This study focuses on whether heat treatment under pure nitrogen is an appropriate procedure to enable sintering and nitrogen uptake by reduction of surface oxides. X-ray photoelectron spectroscopy (XPS) in combination with scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) are used to investigate the surface of powdered FeMn19Cr17C0.4N heat treated under nitrogen atmosphere. The analyses showed reduction of iron oxides already at 500 °C leading to oxide-free metallic surface zones. Mn and Cr oxides are reduced at higher temperatures. Distinct nitrogen uptake was registered, and successful subsequent sintering was reached. Copyright © 2014 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.5730
  • 2015 • 126 Relationship Between Damping Capacity and Variations of Vacancies Concentration and Segregation of Carbon Atom in an Fe-Mn Alloy
    Wen, Y. and Xiao, H. and Peng, H. and Li, N. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 4828-4833 (2015)
    We investigated effects of quenching temperature and ageing on variations of vacancies concentration and segregation of solute atoms and their relationship with damping capacity in an Fe-Mn alloy. The damping capacity can be remarkably improved by lowering vacancies concentration but deteriorated by segregation of carbon atoms. A higher damping capacity can be obtained by furnace cooling or quenching and then ageing in Fe-Mn alloy with lower carbon content or addition of Ti or Nb. © 2015, The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-015-3111-1
  • 2015 • 125 Co3O4-MnO2-CNT Hybrids Synthesized by HNO3 Vapor Oxidation of Catalytically Grown CNTs as OER Electrocatalysts
    Xie, K. and Masa, J. and Madej, E. and Yang, F. and Weide, P. and Dong, W. and Muhler, M. and Schuhmann, W. and Xia, W.
    ChemCatChem 7 3027-3035 (2015)
    An efficient two-step gas-phase method was developed for the synthesis of Co<inf>3</inf>O<inf>4</inf>-MnO<inf>2</inf>-CNT hybrids used as electrocatalysts in the oxygen evolution reaction (OER). Spinel Co-Mn oxide was used for the catalytic growth of multiwalled carbon nanotubes (CNTs) and the amount of metal species remaining in the CNTs was adjusted by varying the growth time. Gas-phase treatment in HNO<inf>3</inf> vapor at 200 °C was performed to 1)open the CNTs, 2)oxidize encapsulated Co nanoparticles to Co<inf>3</inf>O<inf>4</inf> as well as MnO nanoparticles to MnO<inf>2</inf>, and 3)to create oxygen functional groups on carbon. The hybrid demonstrated excellent OER activity and stability up to 37.5h under alkaline conditions, with longer exposure to HNO<inf>3</inf> vapor up to 72h beneficial for improved electrocatalytic properties. The excellent OER performance can be assigned to the high oxidation states of the oxide nanoparticles, the strong electrical coupling between these oxides and the CNTs as well as favorable surface properties rendering the hybrids a promising alternative to noble metal based OER catalysts. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201500469
  • 2015 • 124 Route to the Smallest Doped Semiconductor: Mn2+-Doped (CdSe)13 Clusters
    Yang, J. and Fainblat, R. and Kwon, S.G. and Muckel, F. and Yu, J.H. and Terlinden, H. and Kim, B.H. and Iavarone, D. and Choi, M.K. and Kim, I.Y. and Park, I. and Hong, H.-K. and Lee, J. and Son, J.S. and Lee, Z. and Kang, K. and...
    Journal of the American Chemical Society 137 12776-12779 (2015)
    Doping semiconductor nanocrystals with magnetic transition-metal ions has attracted fundamental interest to obtain a nanoscale dilute magnetic semiconductor, which has unique spin exchange interaction between magnetic spin and exciton. So far, the study on the doped semiconductor NCs has usually been conducted with NCs with larger than 2 nm because of synthetic challenges. Herein, we report the synthesis and characterization of Mn2+-doped (CdSe)13 clusters, the smallest doped semiconductors. In this study, single-sized doped clusters are produced in large scale. Despite their small size, these clusters have semiconductor band structure instead of that of molecules. Surprisingly, the clusters show multiple excitonic transitions with different magneto-optical activities, which can be attributed to the fine structure splitting. Magneto-optically active states exhibit giant Zeeman splittings up to elevated temperatures (128 K) with large g-factors of 81(±8) at 4 K. Our results present a new synthetic method for doped clusters and facilitate the understanding of doped semiconductor at the boundary of molecules and quantum nanostructure. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b07888
  • 2015 • 123 A study on the geometry of dislocation patterns in the surrounding of nanoindents in a TWIP steel using electron channeling contrast imaging and discrete dislocation dynamics simulations
    Zhang, J.-L. and Zaefferer, S. and Raabe, D.
    Materials Science and Engineering A 636 231-242 (2015)
    Electron channeling contrast imaging under controlled diffraction conditions (cECCI) enables observation of crystal defects, especially dislocations, stacking faults and nano-twins, close to the surface of bulk samples. In this work cECCI has been employed to observe defects around nanoindents into the surface of {100}-, {110}-, {111}-oriented grains in a Fe-22Mn-0.65C (wt%) TWIP steel sample (fcc crystal structure, stacking fault energy ~20. mJ/m) using a cone-spherical indenter. The dislocation patterns show four- and two-fold symmetries for the {100}- and {110}-orientation, and a three-fold symmetry for the {111}-orientation which is, however, difficult to observe. Discrete dislocation dynamics (DDD) simulations of the indentation were carried out to complement the static experimental investigations. The simulations were carried out with both, cross-slip disabled and enabled conditions, where the former were found to match to the experimental results better, as may be expected for an fcc material with low stacking fault energy. The 3-dimensional geometry of the dislocation patterns of the different indents was analysed and discussed with respect to pattern formation mechanisms. The force-displacement curves obtained during indentation showed a stronger strain hardening for the {111} oriented crystal than that for the other orientations. This is in contrast to the behaviour of, for example, copper and is interpreted to be due to planar slip. Irrespective of orientation and indentation depth the radius of the plastically deformed area was found to be approximately 4 times larger than that of the indenter contact area. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.03.078
  • 2015 • 122 Nanoscale origins of the damage tolerance of the high-entropy alloy CrMnFeCoNi
    Zhang, Z. and Mao, M.M. and Wang, J. and Gludovatz, B. and Zhang, Z. and Mao, S.X. and George, E.P. and Yu, Q. and Ritchie, R.O.
    Nature Communications 6 (2015)
    Damage tolerance can be an elusive characteristic of structural materials requiring both high strength and ductility, properties that are often mutually exclusive. High-entropy alloys are of interest in this regard. Specifically, the single-phase CrMnFeCoNi alloy displays tensile strength levels of ∼1 GPa, excellent ductility (∼60-70%) and exceptional fracture toughness (KJIc &gt;200 MPa √m). Here through the use of in situ straining in an aberration-corrected transmission electron microscope, we report on the salient atomistic to micro-scale mechanisms underlying the origin of these properties. We identify a synergy of multiple deformation mechanisms, rarely achieved in metallic alloys, which generates high strength, work hardening and ductility, including the easy motion of Shockley partials, their interactions to form stacking-fault parallelepipeds, and arrest at planar slip bands of undissociated dislocations. We further show that crack propagation is impeded by twinned, nanoscale bridges that form between the near-tip crack faces and delay fracture by shielding the crack tip.
    view abstractdoi: 10.1038/ncomms10143
  • 2014 • 121 Ab Initio Predicted Impact of Pt on Phase Stabilities in Ni-Mn-Ga Heusler Alloys
    Dutta, B. and Hickel, T. and Entel, P. and Neugebauer, J.
    Journal of Phase Equilibria and Diffusion 35 695-700 (2014)
    The paper discusses the stabilization of the martensite in Ni2MnGa at finite temperatures that is caused by the substitution of Ni by Pt. For this purpose a recently developed ab initio based formalism employing density functional theory is applied. The free energies of the relevant austenite and martensite phases of Ni1.75Pt0.25MnGa are determined incorporating quasiharmonic phonons and fixed-spin magnons. In addition the dependence of the transition temperatures on the Pt concentration is investigated. Though our results are in qualitative agreement with estimates based on ground-state energies, they clearly demonstrate that a proper treatment of finite temperature contributions is important to predict the martensitic transition quantitatively. © 2014, ASM International.
    view abstractdoi: 10.1007/s11669-014-0342-6
  • 2014 • 120 Valence-band mixing effects in the upper-excited-state magneto-optical responses of colloidal Mn2+-doped CdSe quantum dots
    Fainblat, R. and Muckel, F. and Barrows, C.J. and Vlaskin, V.A. and Gamelin, D.R. and Bacher, G.
    ACS Nano 8 12669-12675 (2014)
    We present an experimental study of the magneto-optical activity of multiple excited excitonic states of manganese-doped CdSe quantum dots chemically prepared by the diffusion doping method. Giant excitonic Zeeman splittings of each of these excited states can be extracted for a series of quantum dot sizes and are found to depend on the radial quantum number of the hole envelope function involved in each transition. As seven out of eight transitions involve the same electron energy state, 1Se, the dominant hole character of each excitonic transition can be identified, making use of the fact that the g-factor of the pure heavy-hole component has a different sign compared to pure light hole or split-off components. Because the magnetic exchange interactions are sensitive to hole state mixing, the giant Zeeman splittings reported here provide clear experimental evidence of quantum-size-induced mixing among valence-band states in nanocrystals. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn505610e
  • 2014 • 119 Optimizing the magnetocaloric effect in Ni-Mn-Sn by substitution: A first-principles study
    Grünebohm, A. and Comtesse, D. and Hucht, A. and Gruner, M.E. and Maslovskaya, A. and Entel, P.
    IEEE Transactions on Magnetics 50 (2014)
    We optimize the magnetic and structural properties of Ni(Co,Cu)MnSn Heusler alloys for the magnetocaloric effect (MCE) by means of density functional theory combined with Monte Carlo simulations of a classical Heisenberg model. NiMnSn alloys show a drop of magnetization at the martensitic phase transition, which leads to the inverse MCE. We find either disordered or frustrated magnetic configurations directly below the martensitic transition temperature. However, the jump of magnetization at the magnetostructural transition is small as the austenite is in a ferrimagnetic state and not fully magnetized. For Co and Cu substitution, the structural phase transition temperature shifts to lower temperatures. In particular, Co substitution is promising, as the magnetization of the austenite increases by additional ferromagnetic interactions, which enhances the jump of magnetization. © 2014 IEEE.
    view abstractdoi: 10.1109/TMAG.2014.2330845
  • 2014 • 118 High strength and ductile low density austenitic FeMnAlC steels: Simplex and alloys strengthened by nanoscale ordered carbides
    Gutierrez-Urrutia, I. and Raabe, D.
    Materials Science and Technology (United Kingdom) 30 1099-1104 (2014)
    We introduce the alloy design concepts of high performance austenitic FeMnAlC steels, namely, Simplex and alloys strengthened by nanoscale ordered k-carbides. Simplex steels are characterised by an outstanding strain hardening capacity at room temperature. This is attributed to the multiple stage strain hardening behaviour associated to dislocation substructure refinement and subsequent activation of deformation twinning, which leads to a steadily increase of the strain hardening. Al additions higher that 5 wt-% promote the precipitation of nanoscale L912 ordered precipitates (so called k-carbides) resulting in high strength (yield stress ∼ 1.0 GPa) and ductile (elongation to fracture 7sim; 30%) steels. Novel insights into dislocation-particle interactions in a Fe- 30.5Mn-8.0Al-1.2C (wt-%) steel strengthened by nanoscale k-carbides are discussed. © 2014 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1179/1743284714Y.0000000515
  • 2014 • 117 Atom probe tomography observation of hydrogen in high-Mn steel and silver charged via an electrolytic route
    Haley, D. and Merzlikin, S.V. and Choi, P. and Raabe, D.
    International Journal of Hydrogen Energy 39 12221-12229 (2014)
    We investigate an electrolytic route for hydrogen charging of metals and its detection in Atom Probe Tomography (APT) experiments. We charge an austenitic Fe-30Mn-8Al-1.2C (wt.%) weight reduced high-Mn steel and subsequently demonstrate the detectability of deuterium in an APT experiment. The experiment is repeated with a deposited Ag film upon an APT tip of a high-Mn steel. It is shown that a detectable deuterium signal can be seen in the high-Mn steel, and a D:H ratio of 0.84 can be reached in Ag films. Additionally, it was found that the predicted time constraint on detectability of D in APT was found to be lower than predicted by bulk diffusion for the high-Mn steel. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2014.05.169
  • 2014 • 116 Tunable emission properties by ferromagnetic coupling Mn(II) aggregates in Mn-doped CdS microbelts/nanowires
    Kamran, M.A. and Liu, R. and Shi, L.-J. and Li, Z.-A. and Marzi, T. and Schöppner, C. and Farle, M. and Zou, B.
    Nanotechnology 25 (2014)
    Tunable optical emission properties from ferromagnetic semiconductors have not been well identified yet. In this work, high-quality Mn(II)-doped CdS nanowires and micrometer belts were prepared using a controlled chemical vapor deposition technique. The Mn doping could be controlled with time, precursor concentration and temperature. These wires or belts can produce both tunable redshifted emissions and ferromagnetic responses simultaneously upon doping. The strong emission bands at 572, 651, 693, 712, 745, 768, 787 and 803 nm, due to the Mn(II) 4T1(4G) → 6A 1(6s) d-d transition, can be detected and accounted for by the aggregation of Mn ions at Cd sites in the CdS lattice at high temperature. These aggregates with ferromagnetism and shifted luminescence are related to the excitonic magnetic polaron (EMP) and localized EMP formations; this is verified by ab initio calculations. The correlation between aggregation-dependent optical emissions and ferromagnetic responses not only presents a new size effect for diluted magnetic semiconductors (DMSs), but also supplies a possible way to study or modulate the ferromagnetic properties of a DMS and to fabricate spin-related photonic devices in the future. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/25/38/385201
  • 2014 • 115 Hydrogen embrittlement associated with strain localization in a precipitation-hardened Fe-Mn-Al-C light weight austenitic steel
    Koyama, M. and Springer, H. and Merzlikin, S.V. and Tsuzaki, K. and Akiyama, E. and Raabe, D.
    International Journal of Hydrogen Energy 39 4634-4646 (2014)
    Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2013.12.171
  • 2014 • 114 Influence of the PM-Processing Route and Nitrogen Content on the Properties of Ni-Free Austenitic Stainless Steel
    Lefor, K. and Walter, M. and Weddeling, A. and Hryha, E. and Huth, S. and Weber, S. and Nyborg, L. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 1154-1167 (2014)
    Ni-free austenitic steels alloyed with Cr and Mn are an alternative to conventional Ni-containing steels. Nitrogen alloying of these steel grades is beneficial for several reasons such as increased strength and corrosion resistance. Low solubility in liquid and δ-ferrite restricts the maximal N-content that can be achieved via conventional metallurgy. Higher contents can be alloyed by powder-metallurgical (PM) production via gas–solid interaction. The performance of sintered parts is determined by appropriate sintering parameters. Three major PM-processing routes, hot isostatic pressing, supersolidus liquid phase sintering (SLPS), and solid-state sintering, were performed to study the influence of PM-processing route and N-content on densification, fracture, and mechanical properties. Sintering routes are designed with the assistance of thermodynamic calculations, differential thermal analysis, and residual gas analysis. Fracture surfaces were studied by X-ray photoelectron spectroscopy, secondary electron microscopy, and energy dispersive X-ray spectroscopy. Tensile tests and X-ray diffraction were performed to study mechanical properties and austenite stability. This study demonstrates that SLPS process reaches high densification of the high-Mn-containing powder material while the desired N-contents were successfully alloyed via gas–solid interaction. Produced specimens show tensile strengths >1000 MPa combined with strain to fracture of 60 pct and thus overcome the other tested production routes as well as conventional stainless austenitic or martensitic grades. © 2014, The Author(s).
    view abstractdoi: 10.1007/s11661-014-2701-7
  • 2014 • 113 Mechanical tunability via hydrogen bonding in metal-organic frameworks with the perovskite architecture
    Li, W. and Thirumurugan, A. and Barton, P.T. and Lin, Z. and Henke, S. and Yeung, H.H.-M. and Wharmby, M.T. and Bithell, E.G. and Howard, C.J. and Cheetham, A.K.
    Journal of the American Chemical Society 136 7801-7804 (2014)
    Two analogous metal-organic frameworks (MOFs) with the perovskite architecture, [C(NH2)3][Mn(HCOO)3] (1) and [(CH2)3NH2][Mn(HCOO)3] (2), exhibit significantly different mechanical properties. The marked difference is attributed to their distinct modes of hydrogen bonding between the A-site amine cation and the anionic framework. The stronger cross-linking hydrogen bonding in 1 gives rise to Young's moduli and hardnesses that are up to twice those in 2, while the thermal expansion is substantially smaller. This study presents clear evidence that the mechanical properties of MOF materials can be substantially tuned via hydrogen-bonding interactions. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja500618z
  • 2014 • 112 Mn2+ cation-directed ionothermal synthesis of an open-framework fluorinated aluminium phosphite-phosphate
    Liu, H. and Tian, Z.-J. and Gies, H. and Wei, Y. and Marler, B. and Wang, L. and Wang, Y.-S. and Li, D.-W.
    RSC Advances 4 29310-29313 (2014)
    An open-framework fluorinated aluminium phosphite-phosphate, H 3.2Mn3.4[C6N2H11] 2{Al12(HPO3)15.0(HPO 4)3.0F12}·14H2O (DNL-2), was ionothermally synthesized by employing the in situ released Mn2+ cations as structure-directing agent. © 2014 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4ra05350h
  • 2014 • 111 MnxOy/NC and CoxOy/NC nanoparticles embedded in a nitrogen-doped carbon matrix for high-performance bifunctional oxygen electrodes
    Masa, J. and Xia, W. and Sinev, I. and Zhao, A. and Sun, Z. and Grützke, S. and Weide, P. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 53 8508-8512 (2014)
    Reversible interconversion of water into H2 and O2, and the recombination of H2 and O2 to H2O thereby harnessing the energy of the reaction provides a completely green cycle for sustainable energy conversion and storage. The realization of this goal is however hampered by the lack of efficient catalysts for water splitting and oxygen reduction. We report exceptionally active bifunctional catalysts for oxygen electrodes comprising Mn3O4 and Co 3O4 nanoparticles embedded in nitrogen-doped carbon, obtained by selective pyrolysis and subsequent mild calcination of manganese and cobalt N4 macrocyclic complexes. Intimate interaction was observed between the metals and nitrogen suggesting residual M-Nx coordination in the catalysts. The catalysts afford remarkably lower reversible overpotentials in KOH (0.1M) than those for RuO2, IrO2, Pt, NiO, Mn3O4, and Co3O4, thus placing them among the best non-precious-metal catalysts for reversible oxygen electrodes reported to date. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201402710
  • 2014 • 110 Metal-free catalysts for oxygen reduction in alkaline electrolytes: Influence of the presence of Co, Fe, Mn and Ni inclusions
    Masa, J. and Zhao, A. and Wei, X. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 128 271-278 (2014)
    Metal-free nitrogen modified carbon catalysts (NC) are very closely related to MNC catalysts which contain a transition metal(s) (M), usually Fe or Co as an essential constituent. We investigated the influence of metal inclusions on the activity of nitrogen-doped carbon black in the electrocatalysis of the oxygen reduction reaction (ORR). A reference metal-free NC catalyst was prepared by pyrolysis of a polypyrrole/Vulcan XC72 composite at 800 °C for 2 h under helium. Controlled amounts of Co, Fe, Mn and Ni in low concentrations were then introduced into NC by impregnating it with the corresponding meso-tetra(4-pyridyl) porphyrin metal complex followed by further pyrolysis at 650 °C for 2 h under helium. The resulting catalysts were investigated for ORR using rotating disk electrode and rotating-ring disk electrode voltammetry in 0.1 M KOH. Additionally, the rate of decomposition of hydrogen peroxide by the different catalysts was determined in order to probe the influence of the metal inclusions on the mechanism and selectivity of the ORR. The results show that Fe, Co and Mn inclusions cause a substantial decrease of the overpotential of the reaction and enhance the catalytic current, whereas the presence of Ni has a poisoning effect on ORR. In the presence of Fe, the catalysts apparently reduce oxygen selectively to OH- in a direct four electron transfer process as opposed to the two-step, two electron pathway involving hydrogen peroxide as an intermediate for the case of the NC catalyst. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2013.11.026
  • 2014 • 109 Designing Heusler nanoprecipitates by elastic misfit stabilization in Fe-Mn maraging steels
    Millán, J. and Sandlöbes, S. and Al-Zubi, A. and Hickel, T. and Choi, P. and Neugebauer, J. and Ponge, D. and Raabe, D.
    Acta Materialia 76 94-105 (2014)
    B2 NiMn and Ni2MnAl Heusler nanoprecipitates are designed via elastic misfit stabilization in Fe-Mn maraging steels by combining transmission electron microscopy (TEM) correlated atom probe tomography (APT) with ab initio simulations. Guided by these predictions, the Al content of the alloys is systematically varied, and the influence of the Al concentration on structure stability, size and distribution of precipitates formed during ageing at 450 °C is studied using scanning electron microscopy-electron backscatter diffraction, TEM and APT. Specifically, the Ni2MnAl Heusler nanoprecipitates exhibit the finest sizes and highest dispersion and hence lead to significant strengthening. The formation of the different types of precipitates and their structure, size, dispersion and effect on the mechanical properties of the alloys are discussed. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.05.016
  • 2014 • 108 Alloy Design, Combinatorial Synthesis, and Microstructure–Property Relations for Low-Density Fe-Mn-Al-C Austenitic Steels
    Raabe, D. and Springer, H. and Gutierrez-Urrutia, I. and Roters, F. and Bausch, M. and Seol, J.-B. and Koyama, M. and Choi, P.-P. and Tsuzaki, K.
    JOM 66 1845-1856 (2014)
    We present recent developments in the field of austenitic steels with up to 18% reduced mass density. The alloys are based on the Fe-Mn-Al-C system. Here, two steel types are addressed. The first one is a class of low-density twinning-induced plasticity or single phase austenitic TWIP (SIMPLEX) steels with 25–30 wt.% Mn and <4–5 wt.% Al or even <8 wt.% Al when naturally aged. The second one is a class of κ-carbide strengthened austenitic steels with even higher Al content. Here, κ-carbides form either at 500–600°C or even during quenching for >10 wt.% Al. Three topics are addressed in more detail, namely, the combinatorial bulk high-throughput design of a wide range of corresponding alloy variants, the development of microstructure–property relations for such steels, and their susceptibility to hydrogen embrittlement. © 2014, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11837-014-1032-x
  • 2014 • 107 Metallurgical aspects on the fatigue of solution-annealed austenitic high interstitial steels
    Schymura, M. and Fischer, A.
    International Journal of Fatigue 61 1-9 (2014)
    Austenitic stainless steels have been used for over 100 years for their combination of strength and ductility. In order to further improve the mechanical and chemical properties of austenitic high nitrogen steels (AHNS) were developed. Ni reduces the solubility of N and, therefore, was substituted by Mn in order to allow for up to 1 weight-% N to be alloyed. AHNS show an even higher strength for the solution annealed state, which can be increased further by cold working. Unfortunately the endurance limit did not follow this trend as it is known to for cold-worked Ni-containing steels. The solution annealed Ni-containing austenites allow for wavy slip and the generation of dislocation cells while the Mn-alloyed AHNS only show planar slip with twins and stacking faults. While the stacking fault energy was thought to be the main reason for planar slip, early results showed that there must be other near-field effects. The density of free electrons, which is mainly influenced by the sum and the ratio of C and N, might be responsible. Strain-controlled fatigue tests were carried out in CrMn-alloyed austenitic steels with different sums (C + N: 0.65-1.2) and ratios (C/N: 0.13-).©2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijfatigue.2013.11.018
  • 2014 • 106 Impact of Mn on the solution enthalpy of hydrogen in austenitic Fe-Mn alloys: A first-principles study
    Von Appen, J. and Dronskowski, R. and Chakrabarty, A. and Hickel, T. and Spatschek, R. and Neugebauer, J.
    Journal of Computational Chemistry 35 2239-2244 (2014)
    Hydrogen interstitials in austenitic Fe-Mn alloys were studied using density-functional theory to gain insights into the mechanisms of hydrogen embrittlement in high-strength Mn steels. The investigations reveal that H atoms at octahedral interstitial sites prefer a local environment containing Mn atoms rather than Fe atoms. This phenomenon is closely examined combining total energy calculations and crystal orbital Hamilton population analysis. Contributions from various electronic phenomena such as elastic, chemical, and magnetic effects are characterized. The primary reason for the environmental preference is a volumetric effect, which causes a linear dependence on the number of nearest-neighbour Mn atoms. A secondary electronic/magnetic effect explains the deviations from this linearity. © 2014 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jcc.23742
  • 2014 • 105 Large recovery strain in Fe-Mn-Si-based shape memory steels obtained by engineering annealing twin boundaries
    Wen, Y.H. and Peng, H.B. and Raabe, D. and Gutierrez-Urrutia, I. and Chen, J. and Du, Y.Y.
    Nature Communications 5 (2014)
    Shape memory alloys are a unique class of materials that can recover their original shape upon heating after a large deformation. Ti-Ni alloys with a large recovery strain are expensive, while low-cost conventional processed Fe-Mn-Si-based steels suffer from a low recovery strain (<3%). Here we show that the low recovery strain results from interactions between stress-induced martensite and a high density of annealing twin boundaries. Reducing the density of twin boundaries is thus a critical factor for obtaining a large recovery strain in these steels. By significantly suppressing the formation of twin boundaries, we attain a tensile recovery strain of 7.6% in an annealed cast polycrystalline Fe-20.2Mn-5.6Si-8.9Cr-5.0Ni steel (weight%). Further attractiveness of this material lies in its low-cost alloying components and simple synthesis-processing cycle consisting only of casting plus annealing. This enables these steels to be used at a large scale as structural materials with advanced functional properties © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms5964
  • 2014 • 104 Superplastic Mn-Si-Cr-C duplex and triplex steels: Interaction of microstructure and void formation
    Zhang, H. and Ponge, D. and Raabe, D.
    Materials Science and Engineering A 610 355-369 (2014)
    Duplex and triplex microstructures consisting initially of ferrite plus carbide or of martensite, ferrite plus carbide, respectively, can undergo strain induced austenite formation during superplastic deformation at 30K below Ae1 (Ae1: equilibrium pearlite-austenite transformation temperature) and low strain rate (e.g. 2×10-3s-1). The effect leads to excellent superplasticity of the materials (elongation ~500%, flow stress < 50MPa) through fine austenite grains (~10μm). Using a deformation temperature just below Ae1 leads to a weak driving force for both, carbide dissolution and austenite formation. Thereby a sufficient volume fraction of carbides (1-2μm, 15vol%) is located at austenite grain boundaries suppressing austenite grain growth during superplastic deformation. Also, void nucleation and growth in the superplastic regime are slowed down within the newly transformed austenite plus carbide microstructure. In contrast, austenite grains and voids grow fast at a high deformation temperature (120K above Ae1). At a low deformation temperature (130K below Ae1), strain induced austenite formation does not occur and the nucleation of multiple voids at the ferrite-carbide interfaces becomes relevant. The fast growth of grains and voids as well as the formation of multiple voids can trigger premature failure during tensile testing in the superplastic regime. EBSD is used to analyze the microstructure evolution and void formation during superplastic deformation, revealing optimum microstructural and forming conditions for superplasticity of Mn-Si-Cr-C steels. The study reveals that excellent superplasticity can be maintained even at 120K above Ae1 by designing an appropriate initial duplex ferrite plus carbide microstructure. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.05.061
  • 2014 • 103 Enhanced superplasticity in an Al-alloyed multicomponent Mn-Si-Cr-C steel
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Choi, P. and Tasan, C.C. and Raabe, D.
    Acta Materialia 63 232-244 (2014)
    Excellent superplasticity (elongation ∼720%) is observed in a novel multi-component (Mn-S-Cr-Al alloyed) ultrahigh carbon steel during tensile testing at a strain rate of 2 × 10-3 s-1 and a temperature of 1053 K (just above the equilibrium austenite-pearlite transformation temperature). In order to understand superplasticity in this material and its strong Al dependence, the deformation-induced microstructure evolution is characterized at various length scales down to atomic resolution, using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, energy-dispersive X-ray spectroscopy and atom probe tomography. The results reveal that 1 wt.% Al addition influences various microprocesses during deformation, e.g. it impedes Ostwald ripening of carbides, carbide dissolution, austenite nucleation and growth and void growth. As a result, the size of the austenite grains and voids remains relatively fine (< 10 μm) during superplastic deformation, and fine-grained superplasticity is enabled without premature failure. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.10.034
  • 2014 • 102 Spinel Mn-Co oxide in N-doped carbon nanotubes as a bifunctional electrocatalyst synthesized by oxidative cutting
    Zhao, A. and Masa, J. and Xia, W. and Maljusch, A. and Willinger, M.-G. and Clavel, G. and Xie, K. and Schlögl, R. and Schuhmann, W. and Muhler, M.
    Journal of the American Chemical Society 136 7551-7554 (2014)
    The notorious instability of non-precious-metal catalysts for oxygen reduction and evolution is by far the single unresolved impediment for their practical applications. We have designed highly stable and active bifunctional catalysts for reversible oxygen electrodes by oxidative thermal scission, where we concurrently rupture nitrogen-doped carbon nanotubes and oxidize Co and Mn nanoparticles buried inside them to form spinel Mn-Co oxide nanoparticles partially embedded in the nanotubes. Impressively high dual activity for oxygen reduction and evolution is achieved using these catalysts, surpassing those of Pt/C, RuO2, and IrO2 and thus raising the prospect of functional low-cost, non-precious-metal bifunctional catalysts in metal-air batteries and reversible fuel cells, among others, for a sustainable and green energy future. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja502532y
  • 2013 • 101 Thickness dependent exchange bias in martensitic epitaxial Ni-Mn-Sn thin films
    Behler, A. and Teichert, N. and Dutta, B. and Waske, A. and Hickel, T. and Auge, A. and Hütten, A. and Eckert, J.
    AIP Advances 3 (2013)
    A thickness dependent exchange bias in the low temperature martensitic state of epitaxial Ni-Mn-Sn thin films is found. The effect can be retained down to very small thicknesses. For a Ni50Mn32Sn18 thin film, which does not undergo a martensitic transformation, no exchange bias is observed. Our results suggest that a significant interplay between ferromagnetic and antiferromagnetic regions, which is the origin for exchange bias, is only present in the martensite. The finding is supported by ab initio calculations showing that the antiferromagnetic order is stabilized in the phase. © 2013 Author(s).
    view abstractdoi: 10.1063/1.4849795
  • 2013 • 100 Salen-based coordination polymers of manganese and the rare-earth elements: Synthesis and catalytic aerobic epoxidation of olefins
    Bhunia, A. and Gotthardt, M.A. and Yadav, M. and Gamer, M.T. and Eichhöfer, A. and Kleist, W. and Roesky, P.W.
    Chemistry - A European Journal 19 1986-1995 (2013)
    Treatment of N,N'-bis(4carboxysalicylidene)ethylenediamine (H 4L), with MnCl2·(H2O)4, and Ln(NO3)3·(H2O)m (Ln=Nd, Eu, Gd, Dy, Tb), in the presence of N,N-dimethylformamide (DMF)/pyridine at elevated temperature resulted (after work up) in the formation of 1D coordination polymers {[Ln2(MnLCl)2(NO3)2(dmf) 5]·4 DMF}n (1-5). In these coordination polymers the rare earth ions are connected through carboxylate groups from Mn-salen units in a 1D chain structure. Thus, the Mn-salen complex acts as a "metalloligand" with open coordination sites. All compounds were used as catalysts in the liquid-phase epoxidation of trans-stilbene with molecular oxygen, which resulted in the formation of stilbene oxide. Since the choice of the lanthanide had virtually no influence on the performance of the catalyst, only the manganese-gadolinium was studied in detail. The influence of solvent, catalyst concentration, reaction temperature, oxidant, and oxidant flow rate on conversion, yield, and selectivity was analyzed. A conversion of up to 70 %, the formation of 61 % stilbene oxide (88 % selectivity), and a TON of 84 were observed after 24 h. A hot filtration test confirmed that the reaction is mainly catalyzed through a heterogeneous pathway, although a minor contribution of homogeneous species could not be completely excluded. The catalyst could be reused without significant loss of activity. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201203636
  • 2013 • 99 Extended investigation of intermartensitic transitions in Ni-Mn-Ga magnetic shape memory alloys: A detailed phase diagram determination
    Çakir, A. and Righi, L. and Albertini, F. and Acet, M. and Farle, M. and Aktürk, S.
    Journal of Applied Physics 114 (2013)
    Martensitic transitions in shape memory Ni-Mn-Ga Heusler alloys take place between a high temperature austenite and a low temperature martensite phase. However, intermartensitic transformations have also been encountered that occur from one martensite phase to another. To examine intermartensitic transitions in magnetic shape memory alloys in detail, we carried out temperature dependent magnetization, resistivity, and x-ray diffraction measurements to investigate the intermartensitic transition in Ni50Mn50- xGax in the composition range 12 ≤ x ≤ 25 at. %. Rietveld refined x-ray diffraction results are found to be consistent with magnetization and resistivity data. Depending on composition, we observe that intermartensitic transitions occur in the sequences 7 M → L 1 0, 5 M → 7 M, and 5 M → 7 M → L 1 0 with decreasing temperature. The L1 0 non-modulated structure is most stable at low temperature. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4831667
  • 2013 • 98 Optimization of smart Heusler alloys from first principles
    Entel, P. and Siewert, M. and Gruner, M.E. and Chakrabarti, A. and Barman, S.R. and Sokolovskiy, V.V. and Buchelnikov, V.D.
    Journal of Alloys and Compounds 577 S107-S112 (2013)
    The strong magnetoelastic interaction in ternary X2YZ Heusler alloys is reponsible for the appearance of magnetostructural phase transitions and related functional properties such as the magnetocaloric and magnetic shape-memory effects. Here, X and Y are transition metal elements and Z is usually an element from the III-V group. In order to discuss possibilities to optimize the multifunctional effects, we use density functional theory calculations from which the martensitic driving forces of the magnetic materials can be derived. We find that the electronic contribution arising from the band Jahn-Teller effect is one of the major driving forces. The ab initio calculations also give a hint of how to design new intermetallics with higher martensitic transformation temperatures compared to the prototype alloy system Ni-Mn-Ga. As an example, we discuss quarternary PtxNi 2-xMnGa alloys which have properties very similar to Ni-Mn-Ga but exhibit a higher maximal eigenstrain of 14%. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2012.03.005
  • 2013 • 97 Complex magnetic ordering as a driving mechanism of multifunctional properties of Heusler alloys from first principles
    Entel, P. and Siewert, M. and Gruner, M.E. and Herper, H.C. and Comtesse, D. and Arróyave, R. and Singh, N. and Talapatra, A. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Albertini, F. and Righi, L. and Chernenko, V.A.
    European Physical Journal B 86 (2013)
    First-principles calculations are used to study the structural, electronic and magnetic properties of (Pd, Pt)-Mn-Ni-(Ga, In, Sn, Sb) alloys, which display multifunctional properties like the magnetic shape-memory, magnetocaloric and exchange bias effect. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior arising from competing ferro- and antiferromagnetic interactions with increasing number of Mn excess atoms in the unit cell. This information allows to optimize, for example, the magnetocaloric effect by using the strong influence of compositional changes on the magnetic interactions. Thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. We present guidelines of how to improve the functional properties. For Pt-Ni-Mn-Ga alloys, a shape memory effect with 14% strain can be achieved in an external magnetic field. © 2013 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1140/epjb/e2012-30936-9
  • 2013 • 96 Compositional trends and magnetic excitations in binary and ternary Fe-Pd-X magnetic shape memory alloys
    Gruner, M.E. and Hamann, S. and Brunken, H. and Ludwig, Al. and Entel, P.
    Journal of Alloys and Compounds 577 S333-S337 (2013)
    High throughput thin film experiments and first-principles calculations are combined in order to get insight into the relation between finite temperature transformation behavior and structural ground state properties of ternary Fe-Pd-X alloys. In particular, we consider the binding surface, i.e., the energy of the disordered alloy calculated along the Bain path between bcc and fcc which we model by a 108 atom supercell. We compare stoichiometric Fe 75Pd25 with ternary systems, where 4.6% of the Fe atoms were substituted by Cu and Mn, respectively. The computational trends are related to combinatorial experiments on thin film libraries for the systems Fe-Pd-Mn and Fe-Pd-Cu which reveal a systematic evolution of the martensitic start temperature with composition within the relevant concentration range for magnetic shape memory (MSM) applications. Our calculations include atomic relaxations, which were shown to be relevant for a correct description of the structural properties. Furthermore, we find that magnetic excitations can substantially alter the binding surface. The comparison of experimental and theoretical trends indicates that, both, compositional changes and magnetic excitations contribute significantly to the structural stability which may thus be tailored by specifically adding antiferromagnetic components. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2012.02.033
  • 2013 • 95 Microbanding mechanism in an Fe-Mn-C high-Mn twinning-induced plasticity steel
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 69 53-56 (2013)
    We study the microbanding mechanism in an Fe-22Mn-0.6C (wt.%) twinning-induced plasticity steel. Dislocation substructures were examined by electron channeling contrast imaging and electron backscatter diffraction. We observe a pronounced effect of the strain path on microbanding, which is explained in terms of Schmid's law. Microbands created under shear loading have a non-crystallographic character. This is attributed to the microbanding mechanism and its relation with the dislocation substructure. Further insights into the dislocation configuration of microbands are provided.© 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2013.03.010
  • 2013 • 94 Influence of Al content and precipitation state on the mechanical behavior of austenitic high-Mn low-density steels
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 68 343-347 (2013)
    We investigate the strain hardening of two austenitic high-Mn low density steels, namely, Fe-30.5Mn-2.1Al-1.2C and Fe-30.5Mn-8.0Al-1.2C (wt.%), containing different precipitation states. The strain hardening of the alloy with low Al content is attributed to dislocation and twin substructures. The precipitation of intergranular M3C-type carbides strongly influences the fracture mode. We associate the strain hardening behavior of the alloy with high Al content to the precipitation of shearable nanosized κ-carbides and their role in the development of planar dislocation substructures.© 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2012.08.038
  • 2013 • 93 Hydrogen-assisted failure in a twinning-induced plasticity steel studied under in situ hydrogen charging by electron channeling contrast imaging
    Koyama, M. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Acta Materialia 61 4607-4618 (2013)
    We investigated the hydrogen embrittlement of a Fe-18Mn-1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7 × 10-6 s -1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe-Mn-C twining-induced plasticity steel. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.030
  • 2013 • 92 Atomic scale investigation of redistribution of alloying elements in pearlitic steel wires upon cold-drawing and annealing
    Li, Y.J. and Choi, P. and Goto, S. and Borchers, C. and Raabe, D. and Kirchheim, R.
    Ultramicroscopy 132 233-238 (2013)
    A local electrode atom probe has been employed to analyze the redistribution of alloying elements including Si, Mn, and Cr in pearlitic steel wires upon cold-drawing and subsequent annealing. It has been found that the three elements undergo mechanical mixing upon cold-drawing at large strains, where Mn and Cr exhibit a nearly homogeneous distribution throughout both ferrite and cementite, whereas Si only dissolves slightly in cementite. Annealing at elevated temperatures leads to a reversion of the mechanical alloying. Si atoms mainly segregate at well-defined ferrite (sub)grain boundaries formed during annealing. Cr and Mn are strongly concentrated in cementite adjacent to the ferrite/cementite interface due to their lower diffusivities in cementite than in ferrite. © 2012.
    view abstractdoi: 10.1016/j.ultramic.2012.10.010
  • 2013 • 91 Understanding the detection of carbon in austenitic high-Mn steel using atom probe tomography
    Marceau, R.K.W. and Choi, P. and Raabe, D.
    Ultramicroscopy 132 239-247 (2013)
    A high-Mn TWIP steel having composition Fe-22Mn-0.6C (wt%) is considered in this study, where the need for accurate and quantitative analysis of clustering and short-range ordering by atom probe analysis requires a better understanding of the detection of carbon in this system. Experimental measurements reveal that a high percentage of carbon atoms are detected as molecular ion species and on multiple hit events, which is discussed with respect to issues such as optimal experimental parameters, correlated field evaporation and directional walk/migration of carbon atoms at the surface of the specimen tip during analysis. These phenomena impact the compositional and spatial accuracy of the atom probe measurement and thus require careful consideration for further cluster-finding analysis. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2013.01.010
  • 2013 • 90 Systematic selection of metalloporphyrin-based catalysts for oxygen reduction by modulation of the donor-acceptor intermolecular hardness
    Masa, J. and Schuhmann, W.
    Chemistry - A European Journal 19 9644-9654 (2013)
    Incisive modulation of the intermolecular hardness between metalloporphyrins and O2 can lead to the identification of promising catalysts for oxygen reduction. The dependency of the electrocatalytic reduction of O2 by metalloporphyrins on the nature of the central metal yields a volcano-type curve, which is rationalized to be in accordance with the Sabatier principle by using an approximation of the electrophilicity of the complexes. By using electrochemical and UV/Vis data, the influence of a selection of meso-substituents on the change in the energy for the π→π* excitation of manganese porphyrins was evaluated allowing one to quantitatively correlate the influence of the various ligands on the electrocatalysis of O2 reduction by the complexes. A manganese porphyrin was identified that electrocatalyzes the reduction of oxygen at low overpotentials without generating hydrogen peroxide. The activity of the complex became remarkably enhanced upon its pyrolysis at 650 °C. Finding the strength: Incisive modulation of the intermolecular hardness between metalloporphyrins and O2 can lead to the identification of promising catalysts for the oxygen reduction reaction (see figure). The feasibility of this principle is demonstrated in the selection and design of a manganese metalloporphyrin with promising high activity for electrocatalytic oxygen reduction. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201203846
  • 2013 • 89 High-throughput study of the structural stability and thermoelectric properties of transition metal silicides
    Opahle, I. and Parma, A. and McEniry, E.J. and Drautz, R. and Madsen, G.K.H.
    New Journal of Physics 15 (2013)
    The phase stability, electronic structure and transport properties of binary 3d, 4d and 5d transition metal silicides are investigated using high-throughput density functional calculations. An overall good agreement is found between the calculated 0 K phase diagrams and experiment. We introduce descriptors for the phase-stability and thermoelectric properties and hereby identify several candidates with potential for thermoelectric applications. This includes known thermoelectrics like Mn4Si7, β-FeSi2, Ru2Si3 and CrSi2 as well as new potentially meta-stable materials like Rh3Si5, Fe2Si3 and an orthorhombic CrSi2 phase. Analysis of the electronic structure shows that the gap formation in most of the semiconducting transition metal silicides can be understood with simple hybridization models. The transport properties of the Mn4Si 7, Ru2Ge3 and Ir3Si5 structure types and the orthorhombic CrSi2 phase are discussed. The calculated transport properties are in good agreement with available experimental data. It is shown that a better thermoelectric performance may be achieved upon optimal doping. Finally, the high-throughput data are analysed and rationalized using a simple tight-binding model. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/15/10/105010
  • 2013 • 88 Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite
    Raabe, D. and Sandlöbes, S. and Millán, J. and Ponge, D. and Assadi, H. and Herbig, M. and Choi, P.-P.
    Acta Materialia 61 6132-6152 (2013)
    In an Fe-9 at.% Mn maraging alloy annealed at 450 C reversed allotriomorphic austenite nanolayers appear on former Mn decorated lath martensite boundaries. The austenite films are 5-15 nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation) in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.06.055
  • 2013 • 87 Oxidation of divalent rare earth phosphors for thermal history sensing
    Rabhiou, A. and Kempf, A. and Heyes, A.
    Sensors and Actuators, B: Chemical 177 124-130 (2013)
    Knowledge of component temperatures under the extreme conditions in industrial prime movers is of great practical importance, but very hard to obtain. Thermal indicating paints offer one possible and practical way, but they have many disadvantages. A novel concept for utilising phosphorescent coatings as thermal history sensors was proposed by Feist et al. [1] in 2007. These phosphor coatings undergo irreversible changes when exposed to high temperatures that affect their photoluminescent emission properties in such a way that off-line analysis of the emission at room temperature can reveal the temperature history of the coating. In this paper, an investigation of the thermally activated oxidation of 2+ ions in phosphors such as BaMgAl10O 17:Eu2+, BaMgAl10O17:Eu 2+, Mn2+ and SrAl14O25:Eu 2+ is reported and used to demonstrate the potential for a phosphorescent thermal history sensor based on a new physical process. Phosphor powders were annealed at temperatures up to 1400 °C, and characterised using photoluminescence spectroscopy. An intensity ratio temperature measurand was defined and it was shown that the dynamic range of a thermal history sensor based on SrAl14O25:Eu2+ could provide a dynamic range extending from 600 °C to 1300 °C. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.snb.2012.10.012
  • 2013 • 86 Quantitative evaluation of size selective precipitation of Mn-doped ZnS quantum dots by size distributions calculated from UV/Vis absorbance spectra
    Segets, D. and Komada, S. and Butz, B. and Spiecker, E. and Mori, Y. and Peukert, W.
    Journal of Nanoparticle Research 15 (2013)
    We demonstrate the quantitative evaluation of the sharp classification of manganese-doped zinc sulfide (ZnS:Mn) quantum dots by size selective precipitation. The particles were characterized by the direct conversion of absorbance spectra to particle size distributions (PSDs) and high-resolution transmission electron micrographs (HRTEM). Gradual addition of a poor solvent (2-propanol) to the aqueous colloid led to the flocculation of larger particles. Though the starting suspension after synthesis had an already narrow PSD between 1.5 and 3.2 nm, different particle size fractions were subsequently isolated by the careful adjustment of the good solvent/poor solvent ratio. Moreover, due to the fact that for the analysis of the classification results the size distributions were available, an in-depth understanding of the quality of the distinct classification steps could be achieved. From the PSDs of the feed, as well as the coarse and the fine fractions with their corresponding yields determined after each classification step, an optimum after the first addition of poor solvent was identified with a maximal separation sharpness κ as high as 0.75. Only by the quantitative evaluation of classification results leading to an in-depth understanding of the relevant driving forces, a future transfer of this lab scale post-processing to larger quantities will be possible. © 2013 Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-013-1486-8
  • 2013 • 85 Direct evidence for the formation of ordered carbides in a ferrite-based low-density Fe-Mn-Al-C alloy studied by transmission electron microscopy and atom probe tomography
    Seol, J.-B. and Raabe, D. and Choi, P. and Park, H.-S. and Kwak, J.-H. and Park, C.-G.
    Scripta Materialia 68 348-353 (2013)
    We study the structure and chemical composition of the κ-carbide formed as a result of isothermal transformation in an Fe-3.0Mn-5.5Al-0.3C alloy using transmission electron microscopy and atom probe tomography. Both methods reveal the evolution of κ-particle morphology as well as the partitioning of solutes. We propose that the κ-phase is formed by a eutectoid reaction associated with nucleation growth. The nucleation of κ-carbide is controlled by both the ordering of Al partitioned to austenite and the carbon diffusion at elevated temperatures.© 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2012.08.013
  • 2013 • 84 Bulk combinatorial design of ductile martensitic stainless steels through confined martensite-to-austenite reversion
    Springer, H. and Belde, M. and Raabe, D.
    Materials Science and Engineering A 582 235-244 (2013)
    The effect of local martensite-to-austenite reversion on microstructure and mechanical properties was studied with the aim of designing ductile martensitic steels. Following a combinatorial screening with tensile and hardness testing on a matrix of six alloys (0-5. wt% Mn, 0-2. wt% Si, constant 13.5. wt% Cr and 0.45. wt% C) and seven martensite tempering conditions (300-500. °C, 0-30. min), investigations were focussed on martensite-to-austenite reversion during tempering as function of chemical composition and its correlation with the mechanical properties. While Mn additions promoted austenite formation (up to 35. vol%) leading to a martensitic-austenitic TRIP steel with optimum mechanical properties (1.5. GPa ultimate tensile strength and 18% elongation), Si led to brittle behaviour despite even larger austenite contents. Combined additions of Mn and Si broadened the temperature range of austenite reversion, but also significantly lowered hardness and yield strength at limited ductility. These drastically diverging mechanical properties of the probed steels are discussed in light of microstructure morphology, dispersion and transformation kinetics of the austenite, as a result of the composition effects on austenite retention and reversion. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2013.06.036
  • 2013 • 83 Revealing the strain-hardening behavior of twinning-induced plasticity steels: Theory, simulations, experiments
    Steinmetz, D.R. and Jäpel, T. and Wietbrock, B. and Eisenlohr, P. and Gutierrez-Urrutia, I. and Saeed-Akbari, A. and Hickel, T. and Roters, F. and Raabe, D.
    Acta Materialia 61 494-510 (2013)
    We present a multiscale dislocation density-based constitutive model for the strain-hardening behavior in twinning-induced plasticity (TWIP) steels. The approach is a physics-based strain rate- and temperature-sensitive model which reflects microstructural investigations of twins and dislocation structures in TWIP steels. One distinct advantage of the approach is that the model parameters, some of which are derived by ab initio predictions, are physics-based and known within an order of magnitude. This allows more complex microstructural information to be included in the model without losing the ability to identify reasonable initial values and bounds for all parameters. Dislocation cells, grain size and twin volume fraction evolution are included. Particular attention is placed on the mechanism by which new deformation twins are nucleated, and a new formulation for the critical twinning stress is presented. Various temperatures were included in the parameter optimization process. Dissipative heating is also considered. The use of physically justified parameters enables the identification of a universal parameter set for the example of an Fe-22Mn-0.6C TWIP steel. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.09.064
  • 2013 • 82 Surface morphology of MnSi thin films grown on Si(111)
    Suzuki, T. and Lutz, T. and Geisler, B. and Kratzer, P. and Kern, K. and Costantini, G.
    Surface Science 617 106-112 (2013)
    The surface morphology of MnSi thin films grown on Si(111)-7 × 7 substrates was investigated by systematically changing the amount of deposited Mn. A new 3 × 3 surface reconstruction was found at the very initial growth stages, whose atomic configuration was analyzed both experimentally and theoretically. At a coverage of 0.1 monolayers, the formation of nanometer-sized MnSi islands was observed in coexistence with Mn nanoclusters that fit within the 7 × 7 half unit cell. With increasing Mn deposition, the MnSi islands grow, develop extended flat tops and eventually coalesce into an atomically flat film with a high corrugated 3×3 reconstruction punctuated by several holes. The successive film growth mode is characterized by the formation of MnSi quadlayers with a low corrugated 3×3 reconstruction. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2013.08.005
  • 2013 • 81 Composition-dependent crystal structure and martensitic transformation in Heusler Ni-Mn-Sn alloys
    Zheng, H. and Wang, W. and Xue, S. and Zhai, Q. and Frenzel, J. and Luo, Z.
    Acta Materialia 61 4648-4656 (2013)
    In the present work, modulated four- and five-layered orthorhombic, seven-layered monoclinic (4O, 10M and 14M) and unmodulated double tetragonal (L10) martensites are characterized in Heusler Ni-Mn-Sn alloys using X-ray diffraction, high-resolution transmission electron microscopy, electron diffraction techniques and thermal analysis. All modulated layered martensites exhibit twins and stacking faults, while the L10 martensite shows fewer structural defects. The substitution of Sn with Mn in Ni 50Mn37+xSn13-x (x = 0, 2, 4) enhances the martensitic transition temperatures, while the transition temperatures decrease with increasing Mn content for constant Sn levels in Ni50-yMn37+ySn13 (y = 0, 2, 4). The compositional dependence of the martensitic transition temperatures is mainly attributed to the valence electron concentration (e/a) and the unit-cell volume of the high-temperature phase. With increasing transition temperatures (or e/a), the resultant martensitic crystal structure evolves in a sequence of 4O → 10M → 14M → L10 in bulk Ni-Mn-Sn alloys. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.035
  • 2013 • 80 Athermal nature of the martensitic transformation in Heusler alloy Ni-Mn-Sn
    Zheng, H. and Wang, W. and Wu, D. and Xue, S. and Zhai, Q. and Frenzel, J. and Luo, Z.
    Intermetallics 36 90-95 (2013)
    Martensitic transformations are generally classified into two groups, namely athermal and isothermal, according to their kinetics. In case of athermal transformations, the amount of the product phase only depends on temperature, and not on time. However, much debate rises about this issue due to unexpected experimental observations of isothermal effects in typically athermal transformations. Considering that the wide applications of Heusler Ni-Mn based materials are based on martensitic transformations, it is of importance to clarify the nature of their martensitic transformation. In this paper, we made an effort to study isothermal effects in a Ni-Mn-Sn alloy using differential scanning calorimetry (DSC). It is proposed that the martensitic transformation of Ni-Mn based materials is athermal in nature although a time-depending effect is observed through DSC interrupted measurements. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2013.01.012
  • 2012 • 79 Magnetization enhancement and cation valences in nonstoichiometric (Mn,Fe) 3-δO 4 nanoparticles
    Antic, B. and Kremenovic, A. and Jovic, N. and Pavlovic, M.B. and Jovalekic, C. and Nikolic, A.S. and Goya, G.F. and Weidenthaler, C.
    Journal of Applied Physics 111 (2012)
    We present a study of the structural and magnetic properties of (Mn,Fe) 3-δO 4 nanoparticles synthesized by soft mechanochemistry using Mn(OH) 2 × 2 H 2O and Fe(OH) 3 powders as starting compounds. The resulting nanoparticles with a composition of the (Mn,Fe) 3-δO 4 type are found to have a core/shell structure with different Mn/Fe ratios in the core and at the surface. XPS analysis points to valences of 2, 3, and 4 for Mn and 3 for Fe at the particle surface. Combined results of XRPD, Mössbauer spectroscopy, and EDX analysis suggest that there is a deviation from stoichiometry in the nanoparticle core compared to the shell, accompanied by creation of cation polyvalence and vacancies. The value of saturation magnetization, M S, of 73.5 emu/g at room temperature, is among the highest reported so far among nanocrystalline ferrite systems of similar composition. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.3700228
  • 2012 • 78 Single crystal growth and characterization of mullite-type Bi 2Mn 4O 10
    Burianek, M. and Krenzel, T.F. and Schmittner, M. and Schreuer, J. and Fischer, R.X. and Mühlberg, M. and Nénertd, G. and Schneider, H. and Gesing, T.M.
    International Journal of Materials Research 103 449-455 (2012)
    A part of the pseudo-binary join Bi 2O 3-Bi 2Mn 4O 10 of the ternary system Bi 2O 3 -MnO-MnO 2 was examined using thermo-analytical methods. Because Bi 2Mn 4O 10 melts incongruently single crystals of up to 20 mm in diameter were grown by the top seeded solution growth method in the temperature range from about 1223 K to 1173 K. Single crystal neutron diffraction confirmed the principles of the crystal structure of Bi 2Mn 4O 10 but revealed much smaller distortions of the cation coordination polyhedra. In contrast to the anisotropy observed in other mullite-type Bi containing compounds, the linear thermal expansion of Bi 2Mn 4O 10, as studied by means of dilatometry and X-ray powder diffraction techniques, is characterized by α 11 &gt; α;33 &gt; α22 at room temperature. The relatively large expansion along the a-axis can be attributed to the two oxygen atoms bridging two corner shared MnO 5 tetrahedral pyramids which alternate with the structural void between two adjacent Bi 3+ cations. © 2012 Carl Hanser Verlag.
    view abstractdoi: 10.3139/146.110714
  • 2012 • 77 On the effect of manganese on grain size stability and hardenability in ultrafine-grained ferrite/martensite dual-phase steels
    Calcagnotto, M. and Ponge, D. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 37-46 (2012)
    Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). The Mn distribution in ferrite and martensite is analyzed by high-resolution electron backscatter diffraction (EBSD) combined with energy dispersive X-ray spectroscopy (EDX). The experimental findings are supported by the calculated phase diagrams, equilibrium phase compositions, and the estimated diffusion distances using Thermo-Calc (Thermo-Calc Software, McMurray, PA) and Dictra (Thermo-Calc Software). Mn substantially enhances the grain size stability during intercritical annealing and the ability of austenite to undergo martensitic phase transformation. The first observation is explained in terms of the alteration of the phase transformation temperatures and the grain boundary mobility, while the second is a result of the Mn enrichment in cementite during large strain warm deformation, which is inherited by the newly formed austenite and increases its hardenability. The latter is the main reason why the ultrafine-grained material exhibits a hardenability that is comparable with the hardenability of the coarse-grained reference material. © 2011 The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-011-0828-3
  • 2012 • 76 Evaluation of corrosion performance of titanium/steel joint brazed by Cu-based filler metal
    Elrefaey, A. and Wojarski, L. and Tillmann, W.
    Journal of Materials Engineering and Performance 21 707-713 (2012)
    Furnace vacuum brazing has been employed to join commercially pure titanium and low carbon steel using copper-based filler metal with the composition of Cu-10.6Mn-1.9Ni, at.%. Three different brazing temperatures 930, 970, and 1000 °C and a holding time of 15 min were studied and evaluated. The corrosion behavior of the joint in 0.1 M sulfuric acid was investigated using immersion and electrochemical tests. Measurements of corrosion potential, corrosion current density, corrosion rate, polarization resistance, weight loss, and morphology of corrosion attack were used in this study. Experimental results showed that severe corrosion attack of the steel side at the interfacial area is clearly observed. Despite the difference in corrosion rate values obtained by electrochemical and weight loss measurements, the trend of results was identical to a large extent. Corrosion resistance of the joint showed a general tendency to increase with rising brazing temperature. The lowest corrosion rate was obtained for the couple bonded at 1000 °C. Meanwhile, at the lowest joining temperature of 930 °C, corrosion rate showed a higher value. The results of joints corrosion resistance were attributed to the difference in microstructure features and chemical analysis. © ASM International.
    view abstractdoi: 10.1007/s11665-012-0149-8
  • 2012 • 75 Basic properties of magnetic shape-memory materials from first-principles calculations
    Entel, P. and Dannenberg, A. and Siewert, M. and Herper, H.C. and Gruner, M.E. and Comtesse, D. and Elmers, H.-J. and Kallmayer, M.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 2891-2900 (2012)
    The mutual influence of phase transformations, magnetism, and electronic properties of magnetic-shape memory Heusler materials is a basic issue of electronic structure calculations based on density functional theory. In this article, we show that these calculations can be pursued to finite temperatures, which allows to derive on a first-principles basis the temperature versus composition phase diagram of the pseudo-binary Ni-Mn-(Ga, In, Sn, Sb) system. The free energy calculations show that the phonon contribution stabilizes the body-centered-cubic (bcc)-like austenite structure at elevated temperatures, whereas magnetism favors the lowtemperature martensite phase with body-centered-tetragonal (bct) or rather face-centeredtetragonal (fct) structure. The calculations also allow to make predictions of magnetostructural and magnetic field induced properties of other (new) magnetic Heusler alloys not based on NiMn such as Co-Ni-(Ga-Zn) and Fe-Co-Ni-(Ga-Zn) intermetallic compounds. © The Minerals, Metals & Materials Society and ASM International 2011.
    view abstractdoi: 10.1007/s11661-011-0832-7
  • 2012 • 74 Quantum confinement-controlled exchange coupling in manganese(II)-doped CdSe two-dimensional quantum well nanoribbons
    Fainblat, R. and Frohleiks, J. and Muckel, F. and Yu, J.H. and Yang, J. and Hyeon, T. and Bacher, G.
    Nano Letters 12 5311-5317 (2012)
    The impact of quantum confinement on the exchange interaction between charge carriers and magnetic dopants in semiconductor nanomaterials has been controversially discussed for more than a decade. We developed manganese-doped CdSe quantum well nanoribbons with a strong quantum confinement perpendicular to the c-axis, showing distinct heavy hole and light hole resonances up to 300 K. This allows a separate study of the s-d and the p-d exchange interactions all the way up to room temperature. Taking into account the optical selection rules and the statistical distribution of the nanoribbons orientation on the substrate, a remarkable change in particular of the s-d exchange constant with respect to bulk is indicated. Room-temperature studies revealed an unusually high effective g-factor up to ∼13 encouraging the implementation of the DMS quantum well nanoribbons for (room temperature) spintronic applications. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nl302639k
  • 2012 • 73 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 1543-1551 (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 abstractdoi: 10.1002/pssa.201228074
  • 2012 • 72 Bulk electronic structure of the dilute magnetic semiconductor Ga 1-x Mn x As through hard X-ray angle-resolved photoemission
    Gray, A.X. and Minár, J. and Ueda, S. and Stone, P.R. and Yamashita, Y. and Fujii, J. and Braun, J. and Plucinski, L. and Schneider, C.M. and Panaccione, G. and Ebert, H. and Dubon, O.D. and Kobayashi, K. and Fadley, C.S.
    Nature Materials 11 957-962 (2012)
    A detailed understanding of the origin of the magnetism in dilute magnetic semiconductors is crucial to their development for applications. Using hard X-ray angle-resolved photoemission (HARPES) at 3.2 keV, we investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor Ga 0.97 Mn 0.03 As, and the reference undoped GaAs. The data are compared to theory based on the coherent potential approximation and fully relativistic one-step-model photoemission calculations including matrix-element effects. Distinct differences are found between angle-resolved, as well as angle-integrated, valence spectra of Ga 0.97 Mn 0.03 As and GaAs, and these are in good agreement with theory. Direct observation of Mn-induced states between the GaAs valence-band maximum and the Fermi level, centred about 400 meV below this level, as well as changes throughout the full valence-level energy range, indicates that ferromagnetism in Ga 1-x Mn x As must be considered to arise from both p-d exchange and double exchange, thus providing a more unifying picture of this controversial material. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/nmat3450
  • 2012 • 71 Multistage strain hardening through dislocation substructure and twinning in a high strength and ductile weight-reduced Fe-Mn-Al-C steel
    Gutierrez-Urrutia, I. and Raabe, D.
    Acta Materialia 60 5791-5802 (2012)
    We investigate the kinetics of the deformation structure evolution and its contribution to the strain hardening of a Fe-30.5Mn-2.1Al-1.2C (wt.%) steel during tensile deformation by means of transmission electron microscopy and electron channeling contrast imaging combined with electron backscatter diffraction. The alloy exhibits a superior combination of strength and ductility (ultimate tensile strength of 1.6 GPa and elongation to failure of 55%) due to the multiple-stage strain hardening. We explain this behavior in terms of dislocation substructure refinement and subsequent activation of deformation twinning. The early hardening stage is fully determined by the size of the dislocation substructure, namely, Taylor lattices, cell blocks and dislocation cells. The high carbon content in solid solution has a pronounced effect on the evolving dislocation substructure. We attribute this effect to the reduction of the dislocation cross-slip frequency by solute carbon. With increasing applied stress, the cross-slip frequency increases. This results in a gradual transition from planar (Taylor lattices) to wavy (cells, cell blocks) dislocation configurations. The size of such dislocation substructures scales inversely with the applied resolved stress. We do not observe the so-called microband-induced plasticity effect. In the present case, due to texture effects, microbanding is not favored during tensile deformation and, hence, has no effect on strain hardening. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.018
  • 2012 • 70 Grain size effect on strain hardening in twinning-induced plasticity steels
    Gutierrez-Urrutia, I. and Raabe, D.
    Scripta Materialia 66 992-996 (2012)
    We investigate the influence of grain size on the strain hardening of two Fe-22Mn-0.6C (wt.%) twinning-induced plasticity steels with average grain sizes of 3 and 50 μm, respectively. The grain size has a significant influence on the strain hardening through the underlying microstructure. The dislocation substructure formed in the early deformation stages determines the density of nucleation sites for twins per unit grain boundary area which controls the developing twin substructure. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2012.01.037
  • 2012 • 69 Study of deformation twinning and planar slip in a TWIP steel by electron channeling contrast imaging in a SEM
    Gutierrez-Urrutia, I. and Raabe, D.
    Materials Science Forum 702-703 523-529 (2012)
    We study the dislocation and twin substructures in a high manganese twinning-induced-plasticity steel (TWIP) by means of electron channeling contrast imaging. At low strain (true strain below 0.1) the dislocation substructure shows strong orientation dependence. It consists of dislocation cells and planar dislocation arrangements. This dislocation substructure is replaced by a complex dislocation/twin substructure at high strain (true strain of 0.3-0.4). The twin substructure also shows strong orientation dependence. We identify three types of dislocation/twin substructures. Two of these substructures, those which are highly favorable or unfavorable oriented for twinning, exhibit a Schmid behavior. The other twin substructure does not fulfill Schmid's law. © (2012) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2012 • 68 Ab initio-based prediction of phase diagrams: Application to magnetic shape memory alloys
    Hickel, T. and Uijttewaal, M. and Al-Zubi, A. and Dutta, B. and Grabowski, B. and Neugebauer, J.
    Advanced Engineering Materials 14 547-561 (2012)
    An ultimate goal of material scientists is the prediction of the thermodynamics of tailored materials solely based on first principles methods. The present work reviews recent methodological developments and advancements providing thereby an up-to-date basis for such an approach. Key ideas and the performance of these methods are discussed with respect to the Heusler alloy Ni-Mn-Ga - a prototype magnetic shape-memory alloy of great technological interest for various applications. Ni-Mn-Ga shows an interesting and complex sequence of phase transitions, rendering it a significant theoretical challenge for any first principles approach. The primary goal of this investigation is to determine the composition dependence of the martensitic transition temperature in these alloys. Quasiharmonic phonons and the magnetic exchange interactions as well as the delicate interplay of vibrational and magnetic excitations are taken into account employing density functional theory. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201200092
  • 2012 • 67 Enhancing magnetocrystalline anisotropy of the Fe 70Pd 30 magnetic shape memory alloy by adding Cu
    Kauffmann-Weiss, S. and Hamann, S. and Gruner, M.E. and Schultz, L. and Ludwig, Al. and Fähler, S.
    Acta Materialia 60 6920-6930 (2012)
    Fe-Pd-Cu thin films are of great interest for applications in magnetic shape memory microsystems due to their increased martensitic transformation temperature. Here we analyse the consequences of Cu addition to Fe-Pd on the binding energy and magnetic properties by a combination of thin film experiments and first-principles calculations. Strained epitaxial growth of Fe 70Pd 30-xCu x with x = 0, 3, 7 is used to freeze intermediate stages during the martensitic transformation. This makes a large range of tetragonal distortion susceptible for analysis, ranging from body-centred cubic to beyond face-centred cubic (1.07 < c/a bct < 1.57). We find that Cu enhances the quality of epitaxial growth, while spontaneous polarization and Curie temperature are reduced only moderately, in agreement with our calculations. Beyond c/a bct &gt; 1.41 the samples undergo structural relaxations through adaptive nanotwinning. Cu enhances the magnetocrystalline anisotropy constant K 1 at room temperature, which reaches a maximum of -2.4 × 10 5 J m -3 around c/a bct = 1.33. This value exceeds those of binary Fe 70Pd 30 and the prototype Ni-Mn-Ga magnetic shape memory system. Since K 1 represents the maximum driving energy for variant reorientation in magnetic shape memory systems, we conclude that Fe-Pd-Cu alloys offer a promising route towards microactuator applications with significantly improved work output. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.08.001
  • 2012 • 66 Hydrogen-induced cracking at grain and twin boundaries in an Fe-Mn-C austenitic steel
    Koyama, M. and Akiyama, E. and Sawaguchi, T. and Raabe, D. and Tsuzaki, K.
    Scripta Materialia 66 459-462 (2012)
    Hydrogen embrittlement was observed in an Fe-18Mn-1.2C (wt.%) steel. The tensile ductility was drastically reduced by hydrogen charging during tensile testing. The fracture mode was mainly intergranular fracture, though transgranular fracture was also partially observed. The transgranular fracture occurred parallel to the primary and secondary deformation twin boundaries, as confirmed by electron backscattering diffraction analysis and orientation-optimized electron channeling contrast imaging. The microstructural observations indicate that cracks are initiated at grain boundaries and twin boundaries. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2011.12.015
  • 2012 • 65 Gas-solid interactions during nonisothermal heat treatment of a high-strength CrMnCN austenitic steel powder: Influence of atmospheric conditions and heating rate on the densification behavior
    Krasokha, N. and Weber, S. and Huth, S. and Zumsande, K. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 4237-4246 (2012)
    This work deals with gas-solid interactions between a high-alloyed steel powder and the surrounding atmosphere during continuous heating. It is motivated by the recently developed corrosion-resistant CrMnCN austenitic cast steels. Here, powder metallurgical processing would be desirable to manufacture highly homogeneous parts and/or novel corrosion-resistant metal-matrix composites. However, the successful use of this new production route calls for a comprehensive investigation of interactions between the sintering atmosphere and the metallic powder to prevent undesirable changes to the chemical composition, e.g., degassing of nitrogen or evaporation of manganese. In this study, dilatometric measurements combined with residual gas analysis, high-temperature X-ray diffraction (XRD) measurements, and thermodynamic equilibrium calculations provided detailed information about the influence of different atmospheric conditions on the microstructure, constitution, and densification behavior of a gasatomized CrMnCN steel powder during continuous heating. Intensive desorption of nitrogen led to the conclusion that a vacuum atmosphere is not suitable for powder metallurgical (PM) processing. Exposure to an N2-containing atmosphere resulted in the formation of nitrides and lattice expansion. Experimental findings have shown that the N content can be controlled by the nitrogen partial pressure. Furthermore, the reduction of surface oxides because of a carbothermal reaction at elevated temperatures and the resulting enhancement of the powder's densification behavior are discussed in this work. © The Minerals, Metals & Materials Society and ASM International 2012.
    view abstractdoi: 10.1007/s11661-012-1234-1
  • 2012 • 64 Depth-selective electronic and magnetic properties of a Co2MnSi tunnel magneto-resistance electrode at a MgO tunnel barrier
    Krumme, B. and Ebke, D. and Weis, C. and Makarov, S.I. and Warland, A. and Hütten, A. and Wende, H.
    Applied Physics Letters 101 (2012)
    We investigated the electronic structure as well as the magnetic properties of a Co2MnSi film on MgO(100) element-specifically at the interface to a MgO tunnel barrier by means of X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The electronic structure of the Co atoms as a function of the capping layer thickness remained unchanged, whereas the XA spectra of Mn indicate an increase of the unoccupied d states. The experimental findings are consistent with the interfacial structure proposed in the work by B. Hülsen [Phys. Rev. Lett. 103, 046802 (2009)], where a MnSi layer is present at the interface to the MgO with oxygen atoms at top positions in the first MgO layer. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4769180
  • 2012 • 63 Evolution of strength and microstructure during annealing of heavily cold-drawn 6.3 GPa hypereutectoid pearlitic steel wire
    Li, Y.J. and Choi, P. and Goto, S. and Borchers, C. and Raabe, D. and Kirchheim, R.
    Acta Materialia 60 4005-4016 (2012)
    Hypereutectoid steel wires with 6.35 GPa tensile strength after a cold-drawing true strain of 6.02 were annealed between 300 and 723 K. The ultrahigh strength remained upon annealing for 30 min up to a temperature of 423 K but dramatically decreased with further increasing temperature. The reduction of tensile strength mainly occurred within the first 2-3 min of annealing. Atom probe tomography and transmission electron microscopy reveal that the lamellar structure remains up to 523 K. After annealing at 673 K for 30 min, coarse hexagonal ferrite (sub)grains with spheroidized cementite, preferentially located at triple junctions, were observed in transverse cross-sections. C and Si segregated at the (sub)grain boundaries, while Mn and Cr enriched at the ferrite/cementite phase boundaries due to their low mobility in cementite. No evidence of recrystallization was found even after annealing at 723 K for 30 min. The stability of the tensile strength for low-temperature annealing (<473 K) and its dramatic drop upon high-temperature annealing (>473 K) are discussed based on the nanostructural observations. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.03.006
  • 2012 • 62 Component interactions after long-term operation of an SOFC stack with LSM cathode
    Malzbender, J. and Batfalsky, P. and Vaßen, R. and Shemet, V. and Tietz, F.
    Journal of Power Sources 201 196-203 (2012)
    The reliable long-term operation of stacks with a low degradation rate is a prerequisite for the commercialization of solid oxide fuel cell (SOFC) technology. A detailed post-test analysis of stacks is of major importance in understanding degradation mechanisms. Here the results are reported of a post-test analysis of an SOFC stack with anode supported cells with Ni/YSZ anode, 8YSZ electrolyte, and a lanthanum strontium manganite (LSM) cathode operated under steady-state conditions for 19,000 h. In particular, the microstructural and chemical analyses of the relevant metallic and ceramic components are reported. The interconnects were coated with a (Mn,Co,Fe) 3O 4 spinel by atmospheric plasma spraying, which prevented Cr evaporating into the cathode compartment. The diffusion of Mn from the (La,Sr)MnO 3 cathode into the 8YSZ electrolyte led to local enrichment at grain boundaries, which might have been responsible for the degradation via electronic pathways leading to partial short-circuiting across the electrolyte. However, the ultimate failure of the stack was the result of a weakening and fracture of the 8YSZ electrolyte along grain boundaries due to the local Mn enrichment. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2011.10.117
  • 2012 • 61 Electrochemical synthesis of metal-polypyrrole composites and their activation for electrocatalytic reduction of oxygen by thermal treatment
    Masa, J. and Schilling, T. and Bron, M. and Schuhmann, W.
    Electrochimica Acta 60 410-418 (2012)
    This work presents a new approach for synthesis of oxygen reduction catalysts constituted of a transition metal, nitrogen and carbon, by thermal treatment of electrochemically synthesized metal-polypyrrole (M-PPy) composites on glassy carbon electrodes. The synthesis procedure involves immobilization of PPy on glassy carbon followed by dosing of metal (M = Mn, Fe and Co) particles, alternately, by electropolymerization and electrochemical reduction respectively. Electrochemical characterization by cyclic voltammetry (CV) and hydrodynamic rotating disk electrode (RDE) measurements show that the M-PPy composites inherently catalyse the electroreduction of oxygen under acidic conditions. The activity of the composites is significantly augmented when they are heat treated at high temperatures (450-850 °C) under a continuous flow of nitrogen. The presence of metallic entities within the M-PPy composite structures and in the structures ensuing after heat treatment was confirmed by energy dispersive X-ray (EDX) analysis. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2011.11.076
  • 2012 • 60 Nanostructured Manganese Oxide Supported on Carbon Nanotubes for Electrocatalytic Water Splitting
    Mette, K. and Bergmann, A. and Tessonnier, J.-P. and Hävecker, M. and Yao, L. and Ressler, T. and Schlögl, R. and Strasser, P. and Behrens, M.
    ChemCatChem 4 851-862 (2012)
    Incipient wetness impregnation and a novel deposition symproportionation precipitation were used for the preparation of MnO x/CNT electrocatalysts for efficient water splitting. Nanostructured manganese oxides have been dispersed on commercial carbon nanotubes as a result of both preparation methods. A strong influence of the preparation history on the electrocatalytic performance was observed. The as-prepared state of a 6.5wt.% MnO x/CNT sample could be comprehensively characterized by comparison to an unsupported MnO x reference sample. Various characterization techniques revealed distinct differences in the oxidation state of the Mn centers in the as-prepared samples as a result of the two different preparation methods. As expected, the oxidation state is higher and near +4 for the symproportionated MnO x compared to the impregnated sample, where +2 was found. In both cases an easy adjustability of the oxidation state of Mn by post-treatment of the catalysts was observed as a function of oxygen partial pressure and temperature. Similar adjustments of the oxidation state are also expected to happen under water splitting conditions. In particular, the 5wt.% MnO/CNT sample obtained by conventional impregnation was identified as a promising catalytic anode material for water electrolysis at neutral pH showing high activity and stability. Importantly, this catalytic material is comparable to state-of-art MnO x catalyst operating in strongly alkaline solutions and, therefore, offers advantages for hydrogen production from waste and sea water under neutral, hence, environmentally benign conditions. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201100434
  • 2012 • 59 Covalent functionalization of carbon nanotubes with tetramanganese complexes
    Meyer, C. and Besson, C. and Frielinghaus, R. and Saelhoff, A.-K. and Flötotto, H. and Houben, L. and Kögerler, P. and Schneider, C.M.
    Physica Status Solidi (B) Basic Research 249 2412-2415 (2012)
    We present first results on the covalent chemical functionalization of single-walled carbon nanotubes with polynuclear {Mn4} coordination complexes. Raman spectra prove that the reaction can only be achieved for tubes which have been oxidized to create carboxylic groups. HRTEM is used to show that the reaction can be carried out directly on a substrate as well. Analysis of the D/G intensity ratio for different oxidation times shows that it is possible to reduce the amount of defects created. This is important for the future application of this material in transport devices. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201200135
  • 2012 • 58 The stacking fault energy and its dependence on the interstitial content in various austenitic steels
    Mujica, L. and Weber, S. and Theisen, W.
    Materials Science Forum 706-709 2193-2198 (2012)
    The stacking fault energy (SFE) is an intrinsic property of metals and is involved in the deformation mechanism of different kind of steels, such as TWIP (twinning induced plasticity), TRIP (transformation induced plasticity), HNS (high nitrogen), and high strength steels. The dependence of the SFE on the content of interstitial elements (C, N) is not yet fully understood, and different tendencies have been found by different authors. In order to study the influence of the interstitial elements on the SFE, experimental measurements extracted from literature were collected and analyzed to predict the individual and combined effect of carbon and nitrogen in different systems. The referenced austenitic steels are Fe-22Mn-C, Fe-30Ni-C, Fe-15Cr-17Mn-N, Fe-18Cr-16Ni-10Mn-N, Fe-18Cr-9Mn-C-N, Fe-18Mn-18Cr-C-N and Fe-(20-30)Mn-12Cr-C-N. The calculation of the SFE is based on the Gibbs free energy of the austenite to ε-martensite transformation (ΔG γ→ε), which is calculated by means of the Calphad method. The revision of the measured values reveals that on different ranges of interstitial contents the SFE behaves differently. At lower values (C, N or C+N up to 0.4%), a local minimum or maximum is found in most of the systems. At higher concentration levels, a proportional dependence seems to occur. These observations agree with the theory of the dependence of SFE on the free electron concentration. Alloying with Mn or Ni has a strong influence on the electronic configuration and magnetic properties of the austenite and therefore on the SFE. The results of this study provide valuable information for materials design, especially in the context of alloying with C, N or C+N. © 2012 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2012 • 57 Mechanical properties of (20-30)Mn12Cr(0.56-0.7)CN corrosion resistant austenitic TWIP steels
    Mújica Roncery, L. and Weber, S. and Theisen, W.
    Steel Research International 83 307-314 (2012)
    New developed (20-30)Mn12Cr(0.56-0.7)CN TWIP steels developed from thermodynamic calculations exhibit great mechanical properties, such as high strength (1800MPa UTS), deformability (80-100% elongation), toughness (300 J ISO-V), and impact wear resistance equivalent to that of Hadfield steel. In addition, they exhibit corrosion resistance by passivation in aqueous acidic media. Microstructure examination by SEM and EBSD at different degrees of deformation reveals that twinning takes place and is responsible for the high cold-work hardening of the steels. Stacking fault energy measurement of three different developed steels locates them in the range of 30-40mJm -2, being highly dependent on the N and Mn contents. Measurements carried out with digital image correlation indicate that at room temperature dynamic strain aging or Portevin-LeChatelier effect takes place. Measurements of impact toughness indicate that the steels have ductile to brittle transition at cryogenic temperatures as a consequence of the effect of nitrogen on the deformation mechanisms, resulting in a quasi-cleavage fracture along the {111} planes at -196°C. New Fe-Cr-Mn-C-N TWIP steels developed from thermodynamic calculations exhibit great mechanical properties, such as high strength (1800MPa UTS), deformability (80-100% elongation), toughness (300J ISO-V), high impact wear resistance, and corrosion resistance by passivation in aqueous acidic media. This work examines the microstructure, stacking fault energy, and dynamic strain aging to understand the tensile behavior and toughness of these materials. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201100316
  • 2012 • 56 The role of adaptive martensite in magnetic shape memory alloys
    Niemann, R. and Röáßler, U.K. and Gruner, M.E. and Heczko, O. and Schultz, L. and Fähler, S.
    Advanced Engineering Materials 14 562-581 (2012)
    Magnetic shape memory materials require a high twin boundary mobility and low hysteresis for applications mainly as actuators, sensors, and magnetocaloric cooling elements. Usually, outstanding properties are found only in samples with a modulated martensitic structure. Here, we analyze the question why a modulated structure is beneficial and show evidence that the modulated martensite is not an equilibrium phase but a nanoscale microstructure of non-modulated (NM) martensite. In this review, we combine results from continuum and atomistic theory, as well as local and integral measurements on the model system Ni-Mn-Ga. Following the concept of adaptive martensite the modulated phase forms to minimize elastic energy near the phase boundary by introducing low-energy twin boundaries between lamellae of the NM martensite that have widths of a few unit cells. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201200058
  • 2012 • 55 Microstructure evolution during recrystallization in dual-phase steels
    Peranio, N. and Roters, F. and Raabe, D.
    Materials Science Forum 715-716 13-22 (2012)
    The microstructure and texture of rolled and annealed dual-phase steels with 0.147 wt. % C, 1.868 wt. % Mn, and 0.403 wt. % Si were analyzed using SEM, EDX, and EBSD. Hot rolled sheets showed a ferritic-pearlitic microstructure with a pearlite volume fraction of about 40 % and ferrite grain size of about 6 μm. Ferrite and pearlite were heterogeneously distributed at the surface and distributed in bands at the center of the sheets. The hot rolled sheets revealed a throughthickness texture inhomogeneity with a plane-strain texture with strong α-fiber and γ-fiber at the center and a shear texture at the surface. After cold rolling, the ferrite grains showed elongated morphology and larger orientation gradients, the period of the ferrite-pearlite band structure at the center of the sheets was decreased, and the plane-strain texture components were strengthened in the entire sheet. Recrystallization, phase transformation, and the competition of both processes were of particular interest with respect to the annealing experiments. For this purpose, various annealing techniques were applied, i.e., annealing in salt bath, conductive annealing, and industrial hot-dip coating. The sheets were annealed in the ferritic, intercritical, and austenitic temperature regimes in a wide annealing time range including variation of heating and cooling rates. © (2012) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2012 • 54 Elastic properties of face-centred cubic Fe-Mn-C studied by nanoindentation and ab initio calculations
    Reeh, S. and Music, D. and Gebhardt, T. and Kasprzak, M. and Jäpel, T. and Zaefferer, S. and Raabe, D. and Richter, S. and Schwedt, A. and Mayer, J. and Wietbrock, B. and Hirt, G. and Schneider, J.M.
    Acta Materialia 60 6025-6032 (2012)
    We have studied experimentally and theoretically the influence of C and Mn content on the Young's modulus of Fe-Mn-C alloys. Combinatorial thin film and bulk samples were characterized regarding their structure, texture and Young's modulus. The following chemical composition range was investigated: 1.5-3.0 at.% C, 28.0-37.5 at.% Mn and 60.6-69.8 at.% Fe. The experimental lattice parameters change marginally within 3.597-3.614 Å with the addition of C and are consistent with ab initio calculations. The Young's modulus data are in the range of 185 ± 12-251 ± 59 GPa for the bulk samples and the thin film, respectively. C has no significant effect on the Young's modulus of these alloys within the composition range studied here. The ab initio calculations are 15-22% larger than the average Young's modulus values of the as-deposited and polished thin film at 3 at.% C. The comparison of thin film and bulk samples results reveals similar elastic properties for equivalent compositions, indicating that the applied research strategy consisting of the combinatorial thin film approach in conjunction with ab initio calculations is useful to study the composition dependence of the structure and elastic properties of Fe-Mn-C alloys. The very good agreement between the presented calculations and the experimentally determined lattice parameters and Young's modulus values implies that the here-adopted simulation strategy yields a reliable description of carbon in Fe-Mn alloys, important for future alloy design. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.038
  • 2012 • 53 Crystal chemistry and properties of mullite-type Bi 2M 4O 9: An overview
    Schneider, H. and Fischer, R.X. and Gesing, T.M. and Schreuer, J. and Mühlberg, M.
    International Journal of Materials Research 103 422-429 (2012)
    Bi 2M 4O 9 (M = Al 3+, Ga 3+, Fe 3+) belongs to the family of mullite-type crystal structures. The phases are orthorhombic with the space group Pbam. The backbones of the isostructural phases are edge-connected, mullite-type octahedral chains. The octahedral chains are linked by dimers of M 2O 7 tetrahedral groups and by BiO polyhedra. The Bi 3+ cations in Bi 2M 4O 9 contain stereo-chemically active 6s 2 lone electron pairs (LEPs) which are essential for the stabilization of the structure. Although the octahedral chains of the closely related Bi 2Mn 4O 10 are similar to those of Bi 2M 4O 9, Bi 2 Mn 4O 10 contains dimers of edge-connected, five-fold coordinated pyramids instead of four-fold coordinated tetrahedra. Also the 6s 2 LEPs of Bi 3+ in Bi 2Mn 4O 10 are not stereo-chemically active. Complete and continuous solid solutions exist for Bi 2(Al 1-xFe x) 4O 9 and Bi 2(Ga 1-x Fe x) 4O 9 (x = 0 - 1). Things are more complex in the case of the Bi 2(Fe 1-xMn x) 4O 9+y mixed crystals, where a miscibility gap occurs between x = 0.25 - 0.75. In the Fe-rich mixed crystals most Mn atoms enter the octahedra as Mn 4+, with part of the tetrahedral dimers being replaced by fivefold coordinated polyhedra, whereas in the Mn-rich compound Fe 3+ favorably replaces Mn 3+ in the pyramids. The crystal structure of Bi 2M 4O 9 directly controls its mechanical properties. The stiffnesses of phases are highest parallel to the strongly bonded octahedral chains running parallel to the crystallographic c-axis. Perpendicular to the octahedral chains little anisotropy is observed. The temperature- induced expansion perpendicular to the octahedral chains is probably superimposed by contractions. As a result the c-axis expansion appears as relatively high and does not display its lowest value parallel to c, as could be inferred. Maximally 6% of Bi 3+ is substituted by Sr 2+ in Bi 2Al 4O 9 corresponding to a composition of (Bi 0.94Sr 0.06) 2Al 4O 8.94. Sr 2+ for Bi 3+ substitution is probably associated with formation of vacancies of oxygen atoms bridging the tetrahedral dimers. Hopping of oxygen atoms towards the vacancies should strongly enhance the oxygen conductivity. Actually the conductivity is rather low (σ = 7 . 10 -2 S m -1 at 1073 K, 800 °C). An explanation could be the low thermal stability of Sr-doped Bi 2Al 4O 9, especially in coexistence with liquid Bi 2O 3. Therefore, Bi 2Al 4O 9 single crystals and polycrystalline ceramics both with significant amounts of M2+ doping (M = Ca 2+, Sr 2+) have not been produced yet. Thus the question whether or not M 2+-doped Bi 2M 4O 9 is an oxygen conducting material is still open. © 2012 Carl Hanser Verlag.
    view abstractdoi: 10.3139/146.110716
  • 2012 • 52 Atomic scale effects of alloying, partitioning, solute drag and austempering on the mechanical properties of high-carbon bainitic-austenitic TRIP steels
    Seol, J.-B. and Raabe, D. and Choi, P.-P. and Im, Y.-R. and Park, C.-G.
    Acta Materialia 60 6183-6199 (2012)
    Understanding alloying and thermal processing at an atomic scale is essential for the optimal design of high-carbon (0.71 wt.%) bainitic-austenitic transformation-induced plasticity (TRIP) steels. We investigate the influence of the austempering temperature, chemical composition (especially the Si:Al ratio) and partitioning on the nanostructure and mechanical behavior of these steels by atom probe tomography. The effects of the austempering temperature and of Si and Al on the compositional gradients across the phase boundaries between retained austenite and bainitic ferrite are studied. We observe that controlling these parameters (i.e. Si, Al content and austempering temperature) can be used to tune the stability of the retained austenite and hence the mechanical behavior of these steels. We also study the atomic scale redistribution of Mn and Si at the bainitic ferrite/austenite interface. The observations suggest that either para-equilibrium or local equilibrium-negligible partitioning conditions prevail depending on the Si:Al ratio during bainite transformation. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.064
  • 2012 • 51 A first-principles investigation of the compositional dependent properties of magnetic shape memory heusler alloys
    Siewert, M. and Gruner, M.E. and Hucht, A. and Herper, H.C. and Dannenberg, A. and Chakrabarti, A. and Singh, N. and Arróyave, R. and Entel, P.
    Advanced Engineering Materials 14 530-546 (2012)
    The interplay of structural and magnetic properties of magnetic shape memory alloys is closely related to their composition. In this study the influence of the valence electron concentration on the tetragonal transformation in Ni 2Mn 1 + xZ 1 - x (Z = Ga, In, Sn, Sb) and Co 2Ni 1 + xGa 1 - x is investigated by means of ab initio calculations. While the type of magnetic interaction is different for the two series, the trends of the total energy changes under a tetragonal transformation are very similar. We find that tetragonal structures become energetically preferred with respect to the cubic one as the valence electron concentration e/a is increased regardless of the system under consideration. In particular, the energy difference between the austenite and martensite structures increases linearly with e/a, which is in part responsible for the linear increase of the matensite transformation temperature. The substitution of nickel by platinum increases even further the transformation temperature. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201200063
  • 2012 • 50 Rapid alloy prototyping: Compositional and thermo-mechanical high throughput bulk combinatorial design of structural materials based on the example of 30Mn-1.2C-xAl triplex steels
    Springer, H. and Raabe, D.
    Acta Materialia 60 4950-4959 (2012)
    We introduce a new experimental approach to the compositional and thermo-mechanical design and rapid maturation of bulk structural materials. This method, termed rapid alloy prototyping (RAP), is based on semi-continuous high throughput bulk casting, rolling, heat treatment and sample preparation techniques. 45 Material conditions, i.e. 5 alloys with systematically varied compositions, each modified by 9 different ageing treatments, were produced and investigated within 35 h. This accelerated screening of the tensile, hardness and microstructural properties as a function of chemical and thermo-mechanical parameters allows the highly efficient and knowledge-based design of bulk structural alloys. The efficiency of the approach was demonstrated on a group of Fe-30Mn-1.2C-xAl steels which exhibit a wide spectrum of structural and mechanical characteristics, depending on the respective Al concentration. High amounts of Al addition (>8 wt.%) resulted in pronounced strengthening, while low concentrations (<2 wt.%) led to embrittlement of the material during ageing. © 2012 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2012.05.017
  • 2012 • 49 Hysteresis effects in the inverse magnetocaloric effect in martensitic Ni-Mn-In and Ni-Mn-Sn
    Titov, I. and Acet, M. and Farle, M. and González-Alonso, D. and Mañosa, L. and Planes, A. and Krenke, T.
    Journal of Applied Physics 112 (2012)
    The presence of a large inverse magnetocaloric effect around the martensitic transformation in Ni-Mn-Sn and Ni-Mn-In alloys is expected to lead to substantial cooling on applying a magnetic field. However, the occurrence of hysteresis around the transition causes limitations on adiabatic temperature-changes. We study the adiabatic temperature-change in both systems in relation to the hysteresis effects. Ni-Mn-In, having a relatively narrower hysteresis and a greater shift of the characteristic transition temperatures with applied field with respect to Ni-Mn-Sn, shows reversibility in the adiabatic-temperature change related to the inverse magnetocaloric effect when the state of the system is cycled within a minor transitional hysteresis loop. Ni-Mn-Sn does not show reversibility in the inverse magnetocaloric effect under cycling-fields up to 5 T. The reversibility in the adiabatic temperature-change is directly related to the reversibility in the relative amount of austenite and martensite in the sample when the field is cycled. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4757425
  • 2012 • 48 Magnetic anomaly and dielectric tunability of (Sr,Mn)TiO 3 thin films
    Tkach, A. and Okhay, O. and Wu, A. and Vilarinho, P.M. and Bedanta, S. and Shvartsman, V.V. and Borisov, P.
    Ferroelectrics 426 274-281 (2012)
    Polycrystalline Sr 0.98Mn 0.02TiO 3 thin films are prepared by sol-gel spin-coating method on Si/SiO 2/TiO 2/Pt substrates. Their dielectric permittivity, polarization, and magnetization are investigated as a function of temperature (from 10 to 300 K), electric and magnetic fields. Temperature dependences of the magnetization M ZFC-FH(T), real part of the dielectric permittivity (T), and dissipation factor tan(T) of Sr 0.98Mn 0.02TiO 3 films show anomalies around 41-45 K, implying an interrelation between polar and magnetic order. Butterfly-like dc bias dependences of the real part of the dielectric permittivity (E dc), as well as slim polarization and magnetization hysteresis loops are observed, suggesting that Sr 0.98Mn 0.02TiO 3 thin films belong to multiglass systems. © Taylor & Francis Group, LLC.
    view abstractdoi: 10.1080/00150193.2012.672038
  • 2012 • 47 Development of lean alloyed austenitic stainless steels with reduced tendency to hydrogen environment embrittlement
    Weber, S. and Martin, M. and Theisen, W.
    Materials Science Forum 706-709 1041-1046 (2012)
    Hydrogen gas is believed to play a more important role for energy supply in future instationary and mobile applications. In most cases, metallic materials are embrittled when hydrogen atoms are dissolved interstitially into their lattice. Concerning steels, in particular the ductility of ferritic grades is degraded in the presence of hydrogen. In contrast, austenitic steels usually show a lower tendency to hydrogen embrittlement. However, the so-called "metastable" austenitic steels are prone to hydrogen environmental embrittlement (HEE), too. Here, AISI 304 type austenitic steel was tensile tested in air at ambient pressure and in a 400 bar hydrogen gas atmosphere at room temperature. The screening of different alloys in the compositional range of the AISI 304 standard was performed with the ambition to optimize alloying for hydrogen applications. The results of the mechanical tests reveal the influence of the alloying elements Cr, Ni, Mn and Si on HEE. Besides nickel, a positive influence of silicon and chromium was found. Experimental results are supported by thermodynamic equilibrium calculations concerning austenite stability and stacking fault energy. All in all, the results of this work are useful for alloy design for hydrogen applications. A concept for a lean alloyed austenitic stainless steel is finally presented. © 2012 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2012 • 46 Atomic ordering effect in Ni 50Mn 37Sn 13 magnetocaloric ribbons
    Wu, D. and Xue, S. and Frenzel, J. and Eggeler, G. and Zhai, Q. and Zheng, H.
    Materials Science and Engineering A 534 568-572 (2012)
    High-performance Ni 50Mn 37Sn 13 magnetocaloric materials are produced using melt spinning technique in the present work and the atomic order dependence of phase transition behaviors and magnetic properties is established. The effective refrigeration capacity of the melt-spun ribbon annealed at 1273K for 15min reaches 95.27J/kg for a magnetic field change of 18kOe, demonstrating great potential for magnetic refrigeration applications near ambient temperature. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.12.009
  • 2012 • 45 Enrichment of paramagnetic ions from homogeneous solutions in inhomogeneous magnetic fields
    Yang, X. and Tschulik, K. and Uhlemann, M. and Odenbach, S. and Eckert, K.
    Journal of Physical Chemistry Letters 3 3559-3564 (2012)
    Applying interferometry to an aqueous solution of paramagnetic manganese ions, subjected to an inhomogeneous magnetic field, we observe an unexpected but highly reproducible change in the refractive index. This change occurs in the top layer of the solution, closest to the magnet. The shape of the layer is in accord with the spatial distribution of the largest component of the magnetic field gradient force. It turns out that this layer is heavier than the underlying solution because it undergoes a Rayleigh-Taylor instability upon removal of the magnet. The very good agreement between the magnitudes of buoyancy, associated with this layer, and the field gradient force at steady state provides conclusive evidence that the layer formation results from an enrichment of paramagnetic manganese ions in regions of high magnetic field gradient. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jz301561q
  • 2012 • 44 Characterization of the surface of Fe-19Mn-18Cr-C-N during heat treatment in a high vacuum - An XPS study
    Zumsande, K. and Weddeling, A. and Hryha, E. and Huth, S. and Nyborg, L. and Weber, S. and Krasokha, N. and Theisen, W.
    Materials Characterization 71 66-76 (2012)
    Nitrogen-containing CrMn austenitic stainless steels offer evident benefits compared to CrNi-based grades. The production of high-quality parts by means of powder metallurgy could be an appropriate alternative to the standard molding process leading to improved properties. The powder metallurgical production of CrMn austenitic steel is challenging on account of the high oxygen affinity of Mn and Cr. Oxides hinder the densification processes and may lower the performance of the sintered part if they remain in the steel after sintering. Thus, in evaluating the sinterability of the steel Fe-19Mn-18Cr-C-N, characterization of the surface is of great interest. In this study, comprehensive investigations by means of X-ray photoelectron spectroscopy and scanning electron microscopy combined with energy dispersive X-ray spectroscopy were performed to characterize the surface during heat treatment in a high vacuum. The results show a shift of oxidation up to 600 °C, meaning transfer of oxygen from the iron oxide layer to Mn-based particulate oxides, followed by progressive reduction and transformation of the Mn oxides into stable Si-containing oxides at elevated temperatures. Mass loss caused by Mn evaporation was observed accompanied by Mn oxide decomposition starting at 700 °C. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.matchar.2012.06.002
  • 2011 • 43 First-principles investigation of the effect of carbon on the stacking fault energy of Fe-C alloys
    Abbasi, A. and Dick, A. and Hickel, T. and Neugebauer, J.
    Acta Materialia 59 3041-3048 (2011)
    The intrinsic stacking fault energy (SFE) is a critical parameter that defines the type of plasticity mechanisms in austenitic high-Mn steels. We have performed ab initio investigations to study the effect of interstitial carbon atoms on the SFE of face-centred cubic (fcc) Fe-C alloys. Simulating the stacking fault explicitly, we observe a strong dependence of the SFE on the position of carbon atoms with respect to the stacking-fault layer and the carbon concentration. To determine the SFE for realistic carbon distributions we consider two scenarios, assuming (i) an equilibration of the carbon atoms in response to the stacking fault formation and (ii) a homogeneous distribution of the C atoms when creating the stacking fault (i.e. diffusion is suppressed). This distinction allows us to interpret two sets of apparently contradicting experimental data sets, where some find an almost negligible dependence on the carbon concentration while others report a large carbon dependence. In particular, our results for the second scenario show a significant increase in the SFE as a function of carbon concentration. These trends are consistently found for the explicit calculations as well as for the computationally much more efficient axial next-nearest-neighbour Ising approach. They will be decisive for the selection of specific plasticity mechanisms in steels (such as twin formation, martensitic transformations and dislocation gliding). © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.01.044
  • 2011 • 42 Optical orientation of Mn2+ ions in GaAs in weak longitudinal magnetic fields
    Akimov, I.A. and Dzhioev, R.I. and Korenev, V.L. and Kusrayev, Yu.G. and Sapega, V.F. and Yakovlev, D.R. and Bayer, M.
    Physical Review Letters 106 (2011)
    We report on optical orientation of Mn2+ ions in bulk GaAs subject to weak longitudinal magnetic fields (B≤100mT). A manganese spin polarization of 25% is directly evaluated by using spin-flip Raman scattering. The dynamical Mn2+ polarization occurs due to the s-d exchange interaction with optically oriented conduction band electrons. Time-resolved photoluminescence reveals a nontrivial electron spin dynamics, where the oriented Mn2+ ions tend to stabilize the electron spins. © 2011 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.106.147402
  • 2011 • 41 Optimizing the synthesis of cobalt-based catalysts for the selective growth of multiwalled carbon nanotubes under industrially relevant conditions
    Becker, M.J. and Xia, W. and Tessonnier, J.-P. and Blume, R. and Yao, L. and Schlögl, R. and Muhler, M.
    Carbon 49 5253-5264 (2011)
    An industrially applicable cobalt-based catalyst was optimized for the production of multiwalled carbon nanotubes (CNTs) from ethene in a hot-wall reactor. A series of highly active Co-Mn-Al-Mg spinel-type oxides with systematically varied Co: Mn ratios was synthesized by precipitation and calcined at different temperatures. The addition of Mn drastically enhanced the catalytic activity of the Co nanoparticles resulting in an extraordinarily high CNTyield of up to 249 g CNT/gcat. All quaternary catalysts possessed an excellent selectivity towards the growth of CNTs. The detailed characterization of the obtained CNTs by electron microscopy, Raman spectroscopy and thermogravimetry demonstrated that a higher Mn content results in a narrower CNT diameter distribution, while the morphology of the CNTs and their oxidation resistance remains rather similar. The temperature- programmed reduction of the calcined precursors as well as in situ X-ray absorption spectroscopy investigations during the growth revealed that the remarkable promoting effect of the Mn is due to the presence of monovalent Mn (II) oxide in the working catalyst, which enhances the catalytic activity of the metallic Co nanoparticles by strong metal-oxide interactions. The observed correlations between the added Mn promoter and the catalytic performance are of high relevance for the production of CNTs on an industrial scale. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2011.07.043
  • 2011 • 40 Indirect magnetic coupling of manganese porphyrin to a ferromagnetic cobalt substrate
    Chylarecka, D. and Kim, T.K. and Tarafder, K. and Müller, K. and Gödel, K. and Czekaj, I. and Wäckerlin, C. and Cinchetti, M. and Ali, Md.E. and Piamonteze, C. and Schmitt, F. and Wüstenberg, J.-P. and Ziegler, C. and Nolting,...
    115 1295-1301 (2011)
    The coupling mechanism of magnetic molecules to ferromagnetic surfaces is of scientific interest to design and tune molecular spintronic interfaces utilizing their molecular and surface architecture. Indirect magnetic coupling has been proposed earlier on the basis of density functional theory +U (DFT+U) calculations, for the magnetic coupling of manganese(II) porphyrin (MnP) molecules to thin Co films. Here we provide an experimental X-ray magnetic circular dichroism (XMCD) spectroscopy and scanning tunneling microscopy (STM) study of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) on rough (exhibiting a high density of monatomic steps) and smooth (exhibiting a low density of monatomic steps) thin Co films grown on a Cu(001) single crystal toward the assessment of the magnetic coupling mechanism. After deposition onto the surface, MnTPPCl molecules were found to couple ferromagnetically to both rough and smooth Co substrates. For high molecular coverage, we observed higher XMCD signals at the Mn L-edges on the smooth Co substrate than on the rough Co substrate, as expected for the proposed indirect magnetic coupling mechanism on the basis of its predominance on the flat surface areas. In particular, DFT+U calculations predict a weak ferromagnetic molecule-substrate coupling only if the chloride ion of the MnTPPCl molecule orients away (Co-Mn-Cl) from the Co surface. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp106822s
  • 2011 • 39 Chemical gradients across phase boundaries between martensite and austenite in steel studied by atom probe tomography and simulation
    Dmitrieva, O. and Ponge, D. and Inden, G. and Millán, J. and Choi, P. and Sietsma, J. and Raabe, D.
    Acta Materialia 59 364-374 (2011)
    Partitioning at phase boundaries of complex steels is important for their properties. We present atom probe tomography results across martensite/austenite interfaces in a precipitation-hardened maraging-TRIP steel (12.2 Mn, 1.9 Ni, 0.6 Mo, 1.2 Ti, 0.3 Al; at.%). The system reveals compositional changes at the phase boundaries: Mn and Ni are enriched while Ti, Al, Mo and Fe are depleted. More specific, we observe up to 27 at.% Mn in a 20 nm layer at the phase boundary. This is explained by the large difference in diffusivity between martensite and austenite. The high diffusivity in martensite leads to a Mn flux towards the retained austenite. The low diffusivity in the austenite does not allow accommodation of this flux. Consequently, the austenite grows with a Mn composition given by local equilibrium. The interpretation is based on DICTRA and mixed-mode diffusion calculations (using a finite interface mobility). © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.09.042
  • 2011 • 38 Ab-initio modeling of Fe-Mn based alloys and nanoclusters
    Entel, P. and Comtesse, D. and Herper, H.C. and Gruner, M.E. and Siewert, M. and Sahoo, S. and Hucht, A.
    Materials Research Society Symposium Proceedings 1296 80-91 (2011)
    New methods in steel design and basic understanding of the novel materials require large scale ab initio calculations of ground state and finite temperature properties of transition metal alloys. In this contribution we present ab initio modeling of the structural and magnetic properties of XYZ compounds and alloys where X, Y = Mn, Fe, Co Ni and Z = C, Si with emphasis on the Fe-Mn steels. The optimization of structural and magnetic properties is performed by using different simulation tools. In particular, the finite-temperature magnetic properties are simulated using a Heisenberg model with magnetic exchange interactions from first-principles calculations. Part of the calculations are extended to the nanoparticle range showing how ferromagnetic and antiferromagnetic trends influence the nucleation, morphologies and growth of Fe-Mn-based nanoparticles. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/opl.2011.1449
  • 2011 • 37 The influence of additions of Al and Si on the lattice stability of fcc and hcp Fe-Mn random alloys
    Gebhardt, T. and Music, D. and Ekholm, M. and Abrikosov, I.A. and Vitos, L. and Dick, A. and Hickel, T. and Neugebauer, J. and Schneider, J.M.
    Journal of Physics Condensed Matter 23 (2011)
    We have studied the influence of additions of Al and Si on the lattice stability of face-centred-cubic (fcc) versus hexagonal-closed-packed (hcp) Fe-Mn random alloys, considering the influence of magnetism below and above the fcc Néel temperature. Employing two different ab initio approaches with respect to basis sets and treatment of magnetic and chemical disorder, we are able to quantify the predictive power of the ab initio methods. We find that the addition of Al strongly stabilizes the fcc lattice independent of the regarded magnetic states. For Si a much stronger dependence on magnetism is observed. Compared to Al, almost no volume change is observed as Si is added to Fe-Mn, indicating that the electronic contributions are responsible for stabilization/destabilization of the fcc phase. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/24/246003
  • 2011 • 36 Structural and magnetic properties of ternary Fe1-xmnxpt nanoalloys from first principles
    Gruner, M.E. and Entel, P.
    Beilstein Journal of Nanotechnology 2 162-172 (2011)
    Background: Structural and magnetic properties of binary Mn-Pt and ternary Fe1-xMnxPt nanoparticles in the size range of up to 2.5 nm (561 atoms) have been explored systematically by means of large scale first principles calculations in the framework of density functional theory. For each composition several magnetic and structural configurations have been compared. Results: The concentration dependence of magnetization and structural properties of the ternary systems are in good agreement with previous bulk and thin film measurements. At an intermediate Mn-content around x = 0.25 a crossover between several phases with magnetic and structural properties is encountered, which may be interesting for exploitation in functional devices. Conclusion: Addition of Mn effectively increases the stability of single crystalline L10 particles over multiply twinned morphologies. This, however, compromises the stability of the ferromagnetic phase due to an increased number of antiferromagnetic interactions. The consequence is that only small additions of Mn can be tolerated for data recording applications. © 2011 Gruner and Entel.
    view abstractdoi: 10.3762/bjnano.2.20
  • 2011 • 35 Dislocation and twin substructure evolution during strain hardening of an Fe-22 wt.% Mn-0.6 wt.% C TWIP steel observed by electron channeling contrast imaging
    Gutierrez-Urrutia, I. and Raabe, D.
    Acta Materialia 59 6449-6462 (2011)
    We study the kinetics of the substructure evolution and its correspondence to the strain hardening evolution of an Fe-22 wt.% Mn-0.6 wt.% C TWIP steel during tensile deformation by means of electron channeling contrast imaging (ECCI) combined with electron backscatter diffraction (EBSD). The contribution of twin and dislocation substructures to strain hardening is evaluated in terms of a dislocation mean free path approach involving several microstructure parameters, such as the characteristic average twin spacing and the dislocation substructure size. The analysis reveals that at the early stages of deformation (strain below 0.1 true strain) the dislocation substructure provides a high strain hardening rate with hardening coefficients of about G/40 (G is the shear modulus). At intermediate strains (below 0.3 true strain), the dislocation mean free path refinement due to deformation twinning results in a high strain rate with a hardening coefficient of about G/30. Finally, at high strains (above 0.4 true strain), the limited further refinement of the dislocation and twin substructures reduces the capability for trapping more dislocations inside the microstructure and, hence, the strain hardening decreases. Grains forming dislocation cells develop a self-organized and dynamically refined dislocation cell structure which follows the similitude principle but with a smaller similitude constant than that found in medium to high stacking fault energy alloys. We attribute this difference to the influence of the stacking fault energy on the mechanism of cell formation. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.07.009
  • 2011 • 34 Design of a novel Mn-based 1 GPa duplex stainless TRIP steel with 60% ductility by a reduction of austenite stability
    Herrera, C. and Ponge, D. and Raabe, D.
    Acta Materialia 59 4653-4664 (2011)
    We report on the microstructure, texture and deformation mechanisms of a novel ductile lean duplex stainless steel (Fe-19.9Cr-0.42Ni-0.16N-4.79Mn-0.11C- 0.46Cu-0.35Si, wt.%). The austenite is stabilized by Mn, C, and N (instead of Ni). The microstructure is characterized by electron channeling contrast imaging (ECCI) for dislocation mapping and electron backscattering diffraction (EBSD) for texture and phase mapping. The material has 1 GPa ultimate tensile strength and an elongation to fracture of above 60%. The mechanical behavior is interpreted in terms of the strength of both the starting phases, austenite and ferrite, and the amount, dispersion, and transformation kinetics of the mechanically induced martensite (TRIP effect). Transformation proceeds from austenite to hexagonal martensite to near cubic martensite (γ → → α′). The -martensite forms in the austenite with an orientation relationship close to Shoji-Nishiyama. The α′-martensite nucleates at the intersections of deformation bands, especially -bands, with Kurdjumov-Sachs and Nishiyama-Wassermann relationships. The ferrite deforms by dislocation slip and contains cell substructures. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.04.011
  • 2011 • 33 Electrical properties and structure of grain boundaries in n-conducting BaTiO3 ceramics
    Hou, J. and Zhang, Z. and Preis, W. and Sitte, W. and Dehm, G.
    Journal of the European Ceramic Society 31 763-771 (2011)
    The electrical properties of positive temperature coefficient (PTC) ceramics are expected to strongly correlate with the potential barrier height at grain boundaries, which in turn may be influenced by the grain boundary structure and chemistry. In this study, n-conducting BaTiO3 ceramics co-doped by La and Mn were prepared, and the electrical properties were determined by impedance spectroscopy and dc four-point van der Pauw measurements. Detailed analysis of the grain boundary structure was performed by electron microscopy techniques across different length scales. The study revealed that the randomly oriented polycrystalline microstructure was dominated by large angle grain boundaries, which in the present case were dry although a secondary crystalline and glass phase formed at triple junctions. The relationship between the observed grain boundary atomic structures and electrical properties is briefly discussed. © 2010 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jeurceramsoc.2010.11.016
  • 2011 • 32 Induced magnetic Cu moments and magnetic ordering in Cu2MnAl thin films on MgO(0 0 1) observed by XMCD
    Krumme, B. and Herper, H.C. and Erb, D. and Weis, C. and Antoniak, C. and Warland, A. and Westerholt, K. and Entel, P. and Wende, H.
    Journal of Physics D: Applied Physics 44 (2011)
    The disorder-order transition of a highly defective A2-ordered Cu 2MnAl film on MgO(0 0 1) upon annealing at 600 K was monitored by means of x-ray absorption spectroscopy (XAS) at the Cu and Mn L2,3 edges. Additionally, x-ray magnetic circular dichroism (XMCD) was employed to determine element-specific orbital and spin resolved magnetic moments of the Cu and Mn atoms. A small induced total magnetic moment of ≈0.04 0.01μB per atom was detected at the Cu sites, whereas a total magnetic moment of 3.57 0.52μB is carried by the Mn atoms. The experimental XAS and XMCD spectra of Cu agree reasonably with the results from ab initio calculations, magnetic moments derived by the sum rules are in accordance with the calculations. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/41/415004
  • 2011 • 31 Precise chemical, electronic, and magnetic structure of binuclear complexes studied by means of X-ray spectroscopies and theoretical methods
    Kuepper, K. and Benoit, D.M. and Wiedwald, U. and Mögele, F. and Meyering, A. and Neumann, M. and Kappler, J.-P. and Joly, L. and Weidle, S. and Rieger, B. and Ziemann, P.
    Journal of Physical Chemistry C 115 25030-25039 (2011)
    We investigate two planar complexes MnNi and CoNi (see Scheme 1) by X-ray photoelec-tron spectroscopy (XPS) and ultralow-temperature X-ray magnetic circular dichroism (XMCD). In this way the valence states as well as the presence of uncompensated magnetic moments are obtained. The magnetism has been probed at a temperature of 0.6 K in order to reveal the magnetic ground state properties. We find that divalent Ni ions are in a diamagnetic low spin ground state in both complexes; however, in MnNi a small fraction of divalent nickel high-spin ions leads to a residual XMCD signal, indicating parallel spin alignment with the Mn spins. Mn and Co are found to be in a divalent high-spin configuration in both compounds. Theoretically, we address the energetic ordering of the different possible spin states of the binuclear complexes using (zeroth-order) relativistic approximation density functional calculations and a triple-ζ quality basis set. These results show that intermediate-spin states are often favored over low-spin states for most both metal combinations, in qualitative agreement with our experimental observations. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp2069804
  • 2011 • 30 Effect of alloying elements on hydrogen environment embrittlement of AISI type 304 austenitic stainless steel
    Martin, M. and Weber, S. and Theisen, W. and Michler, T. and Naumann, J.
    International Journal of Hydrogen Energy 36 15888-15898 (2011)
    The chemical composition of an AISI type 304 austenitic stainless was systematically modified in order to evaluate the influence of the elements Mo, Ni, Si, S, Cr and Mn on the material's susceptibility to hydrogen environment embrittlement (HEE). Mechanical properties were evaluated by tensile testing at room temperature in air at ambient pressure and in a 40 MPa hydrogen gas atmosphere. For every chemical composition, the corresponding austenite stability was evaluated by magnetic response measurements and thermodynamic calculations based on the Calphad method. Tensile test results show that yield and tensile strength are negligibly affected by the presence of hydrogen, whereas measurements of elongation to rupture and reduction of area indicate an increasing ductility loss with decreasing austenite stability. Concerning modifications of alloy composition, an increase in Si, Mn and Cr content showed a significant improvement of material's ductility compared to other alloying elements. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2011.09.013
  • 2011 • 29 Tight-binding simulation of transition-metal alloys
    McEniry, E.J. and Madsen, G.K.H. and Drain, J.F. and Drautz, R.
    Journal of Physics Condensed Matter 23 (2011)
    In order to perform atomistic simulations of steel, it is necessary to have a detailed understanding of the complex interatomic interactions in transition metals and their alloys. The tight-binding approximation provides a computationally efficient, yet accurate, method to investigate such interactions. In the present work, an orthogonal tight-binding model for Fe, Mn and Cr, with the explicit inclusion of magnetism, has been parameterized from ab initio density-functional calculations. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/27/276004
  • 2011 • 28 Characterization of nano-sized precipitates in a Mn-based lean maraging steel by atom probe tomography
    Millán, J. and Ponge, D. and Raabe, D. and Choi, P. and Dmitrieva, O.
    Steel Research International 82 137-145 (2011)
    We present atom probe tomography results of a precipitation-hardened Mn-based maraging steel (9 Mn, 1.9 Ni, 0.6 Mo, 1.1 Ti, 0.33 Al; in at.%). The alloy is characterized by the surprising effect that both, strength and total elongation increase upon aging. The material reveals a high ultimate tensile strength (UTS) up to 1GPa and good ductility (total elongation (TE) of up to 15% in a tensile test) depending on aging conditions. We map the evolution of the precipitates after 450°C aging treatment using atom probe tomography in terms of chemical composition and size distribution. Copyright © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201000274
  • 2011 • 27 Development and characterization of novel corrosion-resistant TWIP steels
    Mujica, L. and Weber, S. and Hunold, G. and Theisen, W.
    Steel Research International 82 26-31 (2011)
    Austenitic steels exhibiting twinning induced plasticity (TWIP) are materials with exceptional mechanical properties. In this work, the development of new grades of TWIP steels exhibiting corrosion resistance is presented. The alloy development was supported by thermodynamic and diffusion calculations within the (Fe-Mn-Cr)-(C-N) alloy system. For the calculations ambient pressure and primary austenitic solidification were considered as necessary to avoid nitrogen degassing in all processing steps. Manganese is used as an austenite stabilizer, chromium to increase nitrogen solubility and provide corrosion resistance, while carbon and nitrogen provide mechanical strength. Diffusion calculations were used in order to predict the extent of micro segregations and additionally to evaluate the effect of diffusion annealing treatments. The material was cast in a laboratory scale with a nominal composition of Fe-20Mn-12Cr-0.25C-0.3N. Diffusion annealing was followed by hot rolling and solution annealing resulting in a fully austenitic microstructure. Tensile tests at room temperature were performed, exhibiting yield strengths of 430 MPa and elongation to fracture of 93%. In addition, not only the mechanical properties but also the weldability was studied, focussing on the characterization of the microstructure of bead on plate welds obtained by laser and TIG welding. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201000219
  • 2011 • 26 Nucleation and precipitation kinetics of M23C6 and M2N in an Fe-Mn-Cr-C-N austenitic matrix and their relationship with the sensitization phenomenon
    Mújica Roncery, L. and Weber, S. and Theisen, W.
    Acta Materialia 59 6275-6286 (2011)
    This work addresses the austenite decomposition in Fe-20Mn-12Cr-0.24C-0.32N steel. Decomposition of austenite in C + N steels leads to the formation of M23C6 and M2N precipitates. Owing to the fact that decomposition is a temperature- and time-dependent phenomenon, thermodynamic and diffusion simulations of nucleation and growth of precipitates were performed based on the CALPHAD method. A well-known consequence of decomposition is sensitization of the areas surrounding the carbides and nitrides, which is normally associated with Cr depletion. In this study, the influence of element redistribution on the Gibbs free energy of the austenitic phase was analyzed. From the electrochemical equivalence of the difference in the Gibbs energy and the difference in the potential, it was found that the sensitized areas exhibit a less noble potential. A mechanism based on a sensitized anode and a matrix cathode is proposed and is correlated with experimental measures of intergranular corrosion. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.06.038
  • 2011 • 25 Designing shape-memory Heusler alloys from first-principles
    Siewert, M. and Gruner, M.E. and Dannenberg, A. and Chakrabarti, A. and Herper, H.C. and Wuttig, M. and Barman, S.R. and Singh, S. and Al-Zubi, A. and Hickel, T. and Neugebauer, J. and Gillessen, M. and Dronskowski, R. and Entel, P.
    Applied Physics Letters 99 (2011)
    The phase diagrams of magnetic shape-memory Heusler alloys, in particular, ternary Ni-Mn-Z and quarternary (Pt, Ni)-Mn-Z alloys with Z = Ga, Sn, have been addressed by density functional theory and Monte Carlo simulations. Finite temperature free energy calculations show that the phonon contribution stabilizes the high-temperature austenite structure while at low temperatures magnetism and the band Jahn-Teller effect favor the modulated monoclinic 14M or the nonmodulated tetragonal structure. The substitution of Ni by Pt leads to a series of magnetic shape-memory alloys with very similar properties to Ni-Mn-Ga but with a maximal eigenstrain of 14. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3655905
  • 2011 • 24 How to obtain structured metal deposits from diamagnetic ions in magnetic gradient fields?
    Tschulik, K. and Yang, X. and Mutschke, G. and Uhlemann, M. and Eckert, K. and Sueptitz, R. and Schultz, L. and Gebert, A.
    Electrochemistry Communications 13 946-950 (2011)
    Electrodeposition of Bi in magnetic gradient fields was performed from two different electrolytes. The first electrolyte contained only diamagnetic Bi 3+-ions; the second one additionally contained electrochemically inert paramagnetic Mn2+-ions. While homogeneous Bi deposits were obtained from the former, structured Bi layers were derived from the latter. The structured deposits show an inverse correlation between deposit thickness and superimposed magnetic field gradient. Minima of film thickness are observed in regions of maximum magnetic gradients. This demonstration of magneto-electrochemical structuring by deposition of diamagnetic ions is discussed considering the acting magnetic forces. Several possibilities explaining the structuring mechanism are presented. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2011.06.007
  • 2011 • 23 Lean-alloyed austenitic stainless steel with high resistance against hydrogen environment embrittlement
    Weber, S. and Martin, M. and Theisen, W.
    Materials Science and Engineering A 528 7688-7695 (2011)
    To address the upcoming austenitic stainless steel market for automotive applications involving hydrogen technology, a novel lean - alloyed material was developed and characterized. It comprises lower contents of nickel and molybdenum compared to existing steels for high - pressure hydrogen uses, for instance 1.4435 (AISI 316L). Alloying with manganese and carbon ensures a sufficient stability of the austenite at 8. wt.% of nickel while silicon is added to improve resistance against embrittlement by dissolved hydrogen. Investigations were performed by tensile testing in air and 400. bar hydrogen at 25 °C, respectively. In comparison to a standard 1.4307 (AISI 304L) material, a significant improvement of ductility was found. The materials concept is presented in general and discussed with regard to austenite stability and microstructure. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.06.049
  • 2011 • 22 Superplastic martensitic Mn-Si-Cr-C steel with 900% elongation
    Zhang, H. and Bai, B. and Raabe, D.
    Acta Materialia 59 5787-5802 (2011)
    High-strength (1.2-1.5)C-(2-2.5)Mn-(1.5-2)Si-(0.8-1.5)Cr steels (mass%) consisting of martensite and carbides exhibit excellent superplastic properties (e.g. strain rate sensitivity m ≈ 0.5, elongation ≈900% at 1023 K). A homogeneous martensitic starting microstructure is obtained through thermomechanical processing (austenitization plus 1.2 true strain, followed by quenching). Superplastic forming leads to a duplex structure consisting of ferrite and spherical micro-carbides. Through 1.5-2% Si alloying, carbides precipitate at hetero-phase interfaces and martensite blocks at the beginning of superplastic forming. Via Ostwald ripening, these interface carbides grow at the expense of carbides precipitating at martensite laths, thereby promoting ferrite dynamic recrystallization. Simultaneously, carbides at ferrite grain boundaries retard the growth of recrystallized ferrite grains. Due to 2-2.5% Mn and 0.8-1.5% Cr alloying, carbide coarsening is suppressed owing to the slow diffusion of these elements. As a result, fine and homogeneous ferrite plus spherical carbide duplex microstructures with a ferrite grain size of ∼1.5 μm are obtained after superplastic forming. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.05.055
  • 2011 • 21 Martensitic transformation in rapidly solidified Heusler Ni 49Mn39Sn12 ribbons
    Zheng, H. and Wu, D. and Xue, S. and Frenzel, J. and Eggeler, G. and Zhai, Q.
    Acta Materialia 59 5692-5699 (2011)
    In the present work, the microstructure evolution and kinetics of the martensitic transformation are investigated in as-spun and annealed ribbons of Heusler Ni49Mn39Sn12 using electron microscopy, X-ray diffraction and differential scanning calorimetry. Both ribbons undergo a reversible martensitic transformation during thermal cycling and the low-temperature martensite is confirmed to be a modulated four-layered orthorhombic (4O) structure through in situ cooling transmission electronic microscopy investigation. The annealing effect on the martensitic transformation behavior is discussed from the viewpoints of electron concentration, Mn-Mn interatomic distance, atomic order degree and grain size. A strong cooling-rate dependence of phase transition kinetics is found and the mechanism is analyzed. The satisfactory reproducibility obtained during thermal cycling test of this alloy ribbons offers great potential for practical applications. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.05.044
  • 2010 • 20 Ferromagnetic resonance in Ni-Mn based ferromagnetic Heusler alloys
    Aksoy, S. and Posth, O. and Acet, M. and Meckenstock, R. and Lindner, J. and Farle, M. and Wassermann, E.F.
    Journal of Physics: Conference Series 200 (2010)
    Ferromagnetic Ni-Mn based Heusler alloys undergo martensitic transformations leading to properties such as magnetic shape memory, magnetic field induced strain and magneto-caloric effects. The occurrence of such effects are closely related to the nature of magnetic interactions around the transition. These interactions can be closely examined by the ferromagnetic resonance (FMR) technique. Here, we report on the results of FMR studies performed at various temperatures in the martensite and austenite states of powder samples and discuss the mixed nature of the magnetic interactions in the martensitic state. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/200/9/092001
  • 2010 • 19 Local definition of spin polarization in a semiconductor by micro-scale current loops
    Chen, Y.S. and Halm, S. and Kümmell, T. and Bacher, G. and Wiater, M. and Wojtowicz, T. and Karczewski, G.
    Journal of Superconductivity and Novel Magnetism 23 111-114 (2010)
    We present an approach to electrical control of the spin polarization in a diluted magnetic semiconductor (DMS) structure. A variable magnetic field induced by a micro-scale current loop magnetizes the Mn 2+ ions in a CdMnTe/CdMgTe DMS quantum well, which via the sp-d exchange interaction polarizes photo-generated electron-hole pairs confined in the well. A maximum spin polarization degree of ±8.5% is obtained at 4.2 K without external magnetic field. The current-induced magnetic field and the current-generated heating of the spin system are quantitatively extracted by micro magneto-luminescence measurements. © 2009 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10948-009-0559-5
  • 2010 • 18 Using Ab initio calculations in designing bcc MgLi-X alloys for ultra-lightweight applications
    Counts, W.A. and Friák, M. and Raabe, D. and Neugebauer, J.
    Advanced Engineering Materials 12 1198-1205 (2010)
    Body centered cubic (bcc) Mg-Li-based alloys are a promising light-weight structural material. In order to tailor the Mg-Li composition with respect to specific industrial requirements, systematic materials-design concepts need to be developed and applied. Quantum-mechanical calculations are increasingly employed when designing new alloys as they accurately predict basic thermodynamic, structural, and functional properties using only the atomic composition as input. We have therefore performed a quantum-mechanical study using density functional theory (DFT) to systematically explore fundamental physical properties of a broad set of bcc MgLi-based compounds. These DFT-determined properties are used to calculate engineering parameters such as (i) the specific Young's modulus (Y/ρ) or (ii) the bulk over shear modulus ratio (B/G) which allow differentiating between brittle and ductile behavior. As we have recently shown, it is not possible to increase both specific Young's modulus, as a measure of strength, and B/G ratio, as a proxy for ductility, by changing only the composition in the binary bcc Mg-Li system. In an attempt to bypass such fundamental materials-design limitations, a large set of MgLi-X substitutional ternaries derived from stoichiometric MgLi with CsCl structure are studied. Motivated by the fact that for Mg-Li alloys (i) 3rd row Si and Al and (ii) 4th row Zn are industrially used as alloying elements, we probe the alloying performance of the 3rd (Na, Al, Si, P, S, Cl) and 4th row transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) elements. The studied solutes offer a variety of properties but none is able to simultaneously improve both specific Young's modulus and ductility. Therefore, in order to explore the alloying performance of yet a broader set of solutes, we predict the bulk modulus of MgX and LiX B2-compounds running over 40 different elements. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201000225
  • 2010 • 17 First-principles study of the structural stability of L11 order in Pt-based alloys
    Dannenberg, A. and Gruner, M.E. and Entel, P.
    Journal of Physics: Conference Series 200 (2010)
    We investigate in the framework of density functional theory the structural and electronic properties of stoichiometric L11 ordered transition metal alloys of Pt and the 3d transition metals Mn, Fe, Co, Ni and Cu. A marked dependence of the energy difference between L11 and L10 structure on the valence electron concentration is encountered, with the L1 1 order being the preferred structure for CuPt, whereas the other alloys favor the L10 arrangement. The changes of the electronic density of states on composition are well represented within a rigid-band-picture, while the transition from L10 to L11 order is accompanied by a characteristic redistribution of the minority spin states around the Fermi level. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/200/7/072021
  • 2010 • 16 Brazing of titanium to steel with different filler metals: Analysis and comparison
    Elrefaey, A. and Tillmann, W.
    Journal of Materials Science 45 4332-4338 (2010)
    Evaluations of vacuum brazed commercially pure titanium and low-carbon steel joints using one copper-based alloy (Cu-12Mn-2Ni) and two silver-based braze alloys (Ag-34Cu-2Ti, Ag-27.25Cu-12.5In-1.25Ti) have been studied. Both the interfacial microstructures and mechanical properties of brazed joints were investigated to evaluate the joint quality. The optical and scanning electron microscopic results showed that all the filler metals interact metallurgically with steel and titanium, forming different kinds of intermetallic compounds (IMC) such as CuTi, Cu2Ti, Cu4Ti3, and FeTi. The presence of IMC (interfacial reaction layers) at the interfacial regions strongly affects the shear strength of the joints. Furthermore, it was found that the shear strength of brazed joints and the fracture path strongly depend on the thickness of the IMC. The maximum shear strength of the joints was 113 MPa for the specimen brazed at 750 °C using an Ag-27.25Cu-12.5In-1.25Ti filler alloy. © 2010 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-010-4357-z
  • 2010 • 15 Fundamental aspects of magnetic shape memory alloys: Insights from ab initio and Monte Carlo studies
    Entel, P. and Gruner, M.E. and Dannenberg, A. and Siewert, M. and Nayak, S.K. and Herper, H.C. and Buchelnikov, V.D.
    Materials Science Forum 635 3-12 (2010)
    Ferromagnetic Heusler alloys like Ni-Mn-Z (Z = Al, Ga, In, Sn, Sb), which undergo a martensitic phase transformation, are on the edge of being used in technological applications involving actuator and magnetocaloric devices. The other class of ferromagnetic full Heusler alloys like Co-Mn-Z (Z = Al, Si, Ga, Ge, Sn) not undergoing a structural phase transition, are half-metals (in contrast to the Ni-based systems) with high spin polarization at the Fermi level and are of potential importance for future spintronics devices. On the basis of recent ab initio calculations, we highlight the main differences between the two classes of Heusler based materials. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2010 • 14 Study of Ni2 -Mn-Ga phase formation by magnetron sputtering film deposition at low temperature onto Si substrates and LaNi O3 /Pb (Ti,Zr) O3 buffer
    Figueiras, F. and Rauwel, E. and Amaral, V.S. and Vyshatko, N. and Kholkin, A.L. and Soyer, C. and Remiens, D. and Shvartsman, V.V. and Borisov, P. and Kleemann, W.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 28 6-10 (2010)
    Film deposition of Ni2 MnGa phaselike alloy by radio frequency (rf) magnetron sputtering was performed onto bare Si(100) substrates and LaNi O3 /Pb (Ti,Zr) O3 (LNO/PZT) ferroelectric buffer layer near room temperature. The prepared samples were characterized using conventional x-ray diffraction (XRD), superconducting quantum interference device, and electron dispersive x-ray spectroscopy from scanning electron microscope observations. The optimized films deposited under high rf power and low argon pressure present good surface quality and highly textured phase crystallization. The positioning distance between the substrate and the target-holder axis has some limited effect on the film's composition due to the specific diffusion behavior of each element in the sputtering plasma. Extended four pole high resolution XRD analysis allowed one to discriminate the intended Ni-Mn-Ga tetragonal martensitic phase induced by the (100) LNO/PZT oriented buffer. This low temperature process appears to be very promising, allowing separate control of the functional layer's properties, while trying to achieve high electromagnetoelastic coupling. © 2010 American Vacuum Society.
    view abstractdoi: 10.1116/1.3256200
  • 2010 • 13 Chemical trends in structure and magnetism of bimetallic nanoparticles from atomistic calculations
    Gruner, M.E.
    Journal of Physics D: Applied Physics 43 (2010)
    By means of large scale first-principles calculations in the framework of density functional theory, structure and magnetism of 561 atom nanoparticles are compared in order to obtain a systematic picture of the evolution with respect to a change in the constitutional elements. The investigation comprises ordered and disordered, cuboctahedral, icosahedral and decahedral morphologies of composition A265B296, where A is one of Mn, Fe and Co and B is Pt and, additionally, with A = Fe and B = Ni, Pd, Pt, Ir and Au. Fe-Ir and Fe-Pd and Co-Pt exhibit in comparison with Fe-Pt an increased tendency to form multiply-twinned structures and prefer segregation of the heavier element to the surface. The latter trend also applies to Fe-Au, where, on the other hand, icosahedral and crystalline motifs are very close in energy. Only in Mn-Pt the formation of multiply-twinned structures is effectively suppressed. The combinations with reduced valence electron concentration, Mn-Pt and Fe-Ir, exhibit a strong preference for antiferromagnetic spin order. The structural and magnetic trends are tentatively related to the change in features in the element and site-resolved electronic density of states. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/43/47/474008
  • 2010 • 12 Study of internal stresses in a TWIP steel analyzing transient and permanent softening during reverse shear tests
    Gutierrez-Urrutia, I. and Del Valle, J.A. and Zaefferer, S. and Raabe, D.
    Journal of Materials Science 45 6604-6610 (2010)
    Recent Bauschinger-type tests conducted on a twinning-induced plasticity (TWIP) steel highlights the important contribution of internal stresses to work hardening [1]. Along this line we present Bauschinger experiments in a Fe-22Mn wt%-0.6C wt% TWIP steel. The mechanical behavior upon load reversal shows transient and permanent softening effects. Determination of the internal stress from the magnitude of the permanent softening yields a contribution to work hardening of the order of 20%. Analysis of the transient softening, during strain reversal, indicates that internal stress is consistent with reported data on high carbon spheroidized steels. © 2010 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-010-4750-7
  • 2010 • 11 The effect of grain size and grain orientation on deformation twinning in a Fe-22wt.% Mn-0.6wt.% C TWIP steel
    Gutierrez-Urrutia, I. and Zaefferer, S. and Raabe, D.
    Materials Science and Engineering A 527 3552-3560 (2010)
    We investigate the effect of grain size and grain orientation on deformation twinning in a Fe-22wt.% Mn-0.6wt.% C TWIP steel using microstructure observations by electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Samples with average grain sizes of 3μm and 50μm were deformed in tension at room temperature to different strains. The onset of twinning concurs in both materials with yielding which leads us to propose a Hall-Petch-type relation for the twinning stress using the same Hall-Petch constant for twinning as that for glide. The influence of grain orientation on the twinning stress is more complicated. At low strain, a strong influence of grain orientation on deformation twinning is observed which fully complies with Schmid's law under the assumption that slip and twinning have equal critical resolved shear stresses. Deformation twinning occurs in grains oriented close to 〈1. 1. 1〉//tensile axis directions where the twinning stress is larger than the slip stress. At high strains (0.3 logarithmic strain), a strong deviation from Schmid's law is observed. Deformation twins are now also observed in grains unfavourably oriented for twinning according to Schmid's law. We explain this deviation in terms of local grain-scale stress variations. The local stress state controlling deformation twinning is modified by local stress concentrations at grain boundaries originating, for instance, from incoming bundles of deformation twins in neighboring grains. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.02.041
  • 2010 • 10 Coherent dynamics of localized spins in an inhomogeneous magnetic field
    Hohage, P.E. and Nannen, J. and Halm, S. and Puls, J. and Henneberger, F. and Bacher, G.
    Journal of Superconductivity and Novel Magnetism 23 135-137 (2010)
    We studied the influence of the inhomogeneity of fringe fields originating from nanostructured ferromagnets on the coherent dynamics of localized Mn 2+ spins in a (Zn,Cd,Mn)Se/ZnSe quantum well by time-resolved Kerr rotation. By studying hybrid systems with different geometry, we demonstrate that the spatially inhomogeneous fringe field results in two main consequences for the localized spin system: First, a temporal variation of the ensemble precession frequency is obtained, and second, a reduced ensemble spin dephasing time T 2 * is observed in comparison to an unstructured reference area of the sample. © 2009 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10948-009-0572-8
  • 2010 • 9 Multiferroic and magnetoelectric materials-novel developments and perspectives
    Kleemann, W. and Borisov, P. and Bedanta, S. and Shvartsman, V.V.
    IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 57 2228-2232 (2010)
    Magnetoelectric (ME) materials are of utmost interest in view of both fundamental understanding and novel desirable applications. Despite its smallness, the linear ME effect has been shown to control spintronic devices very efficiently, e.g., by using the classic ME antiferromagnet Cr 2O3. Similar nano-engineering concepts exist also for type-I multiferroic single phase materials like BiFeO3 and BiMnO 3. Record high ME response has been realized in stress-strain coupled multiphase magnetoelectrics like PZT/FeBSiC composites, enabling applications in sensors. In type-II multiferroics, whose ferroelectricity is due to modulated magnetic ordering, the ME coupling is of fundamental interest. Higher-order ME response characterizes disordered systems, which extend the conventional multiferroic scenario toward ME multiglass (e.g., Sr1-xMn xTiO3). © 2010 IEEE.
    view abstractdoi: 10.1109/TUFFC.2010.1682
  • 2010 • 8 Development of Mn-Cr-(C-N) corrosion resistant twinning induced plasticity steels: Thermodynamic and diffusion calculations, production, and characterization
    Roncery, L.M. and Weber, S. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 41 2471-2479 (2010)
    In this work, the development of corrosion-resistant twinning induced plasticity steels is presented, supported by thermodynamic and diffusion calculations within the (Fe-Mn-Cr)-(C-N) alloy system. For the calculations, ambient pressure and primary austenitic solidification were considered as necessary to avoid nitrogen degassing in all processing steps. Manganese is used as an austenite stabilizer, chromium is used to increase nitrogen solubility and provide corrosion resistance, and carbon and nitrogen are used as interstitial elements to provide mechanical strength. Isopleths of the different elements vs temperature as well as isothermal sections were calculated to determine the proper amount of Mn, Cr, total interstitial content, and the C/N ratio. Scheil and diffusion calculations were used to predict the extent of microsegregations and additionally to evaluate the effect of diffusion annealing treatments. The materials were produced in laboratory scale, being followed by thermomechanical processing and the characterization of the microstructure. Tensile tests were performed with three different alloys, exhibiting yield strengths of 460 Mpa to 480 MPa and elongations to fracture between 85 pct and 100 pct. © The Minerals, Metals & Materials Society and ASM International 2010.
    view abstractdoi: 10.1007/s11661-010-0334-z
  • 2010 • 7 Coherent magnetization precession in ferromagnetic (Ga,Mn)As induced by picosecond acoustic pulses
    Scherbakov, A.V. and Salasyuk, A.S. and Akimov, A.V. and Liu, X. and Bombeck, M. and Brüggemann, C. and Yakovlev, D.R. and Sapega, V.F. and Furdyna, J.K. and Bayer, M.
    Physical Review Letters 105 (2010)
    We show that the magnetization of a thin ferromagnetic (Ga,Mn)As layer can be modulated by picosecond acoustic pulses. In this approach a picosecond strain pulse injected into the structure induces a tilt of the magnetization vector M, followed by the precession of M around its equilibrium orientation. This effect can be understood in terms of changes in magnetocrystalline anisotropy induced by the pulse. A model where only one anisotropy constant is affected by the strain pulse provides a good description of the observed time-dependent response. © 2010 The American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.105.117204
  • 2010 • 6 Spin cluster glass and magnetoelectricity in Mn-doped KTaO3
    Shvartsman, V.V. and Bedanta, S. and Borisov, P. and Kleemann, W. and Tkach, A. and Vilarinho, P.M.
    Journal of Applied Physics 107 (2010)
    In ceramics of KTaO3 doped with 3 at. % of Mn the dielectric response is dominated by the polydispersive behavior of Mn2+ centered polar regions, whereas the magnetic and magnetoelectric (ME) behaviors reflect an intimate coupling between A -site substituted Mn2+ ions and minute amounts of Mn3 O4 precipitates mediated by the polar host material. This becomes apparent by the common onset at Tc ≈42 K of the ordering of ferrimagnetic Mn3 O4 and of a spin cluster glass, which is characterized by memory and rejuvenation effects. The composite magnetic system exposed to external magnetic and electric dc fields shows large third order ME susceptibility with a sharp anomaly at Tc and 1/ T2 dependence as T→0. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3430990
  • 2010 • 5 Thermal stability, vapor pressures, and diffusion coefficients of some metal 2,2,6,6-tetramethyl-3,5-heptandionate [M(tmhd)n] compounds
    Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B.
    Journal of Chemical and Engineering Data 55 2149-2154 (2010)
    Many metal 2,2,6,6-tetramethyl-3,5-heptandionate [M(tmhd)n] compounds are volatile enough to be useful as precursors of the metals in vapor-phase deposition processes, for example, metal organic chemical vapor deposition (MOCVD). The thermal stability, vapor pressures, and gaseous diffusion coefficients of these compounds are, therefore, of fundamental importance for achieving reproducible and effective depositions. The present communication reports the thermal stability, vapor pressures, enthalpies of sublimation, and diffusion coefficients (in nitrogen and/or helium) for some metal 2,2,6,6-tetramethyl-3,5-heptandionate compounds [M(tmhd)n], namely, [Al(tmhd)3], [Cr(tmhd)3], [Cu(tmhd)2], [Fe(tmhd)3], [Mn(tmhd)3], and [Ni(tmhd)2] at temperatures between (341 and 412) K at ambient pressure. All of these are found to be stable under the investigated experimental conditions and thus are suitable precursors for CVD. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/je9006822
  • 2010 • 4 Structure, lattice dynamics and Fermi surface of the magnetic shape memory system Co-Ni-Ga from first principles calculations
    Siewert, M. and Gruner, M.E. and Dannenberg, A. and Entel, P.
    Physics Procedia 10 138-143 (2010)
    Advanced magnetic shape memory materials like the prototypical Ni-Mn-Ga alloy system are limited to operating temperatures that are too low for many practical applications. To overcome this problem, an intensive search for new magnetic shape memory compounds has been started. One interesting system, showing magnetic as well as conventional shape memory behavior, is Co- Ni-Ga. In this work we report systematic studies of stoichiometric Co-Ni-Ga based alloys in the full and inverse Heusler structure by means of density functional theory. A prediction of the martensitic transition temperatures can be obtained by the structural energy differences calculated for different crystal structures. In prototype, near-stoichiometric Ni-Mn-Ga, the (pre-)martensitic transformation is accompanied by an anomalous softening of one transversal acoustic phonon branch along the [110] direction which has been frequently linked to nesting features of the Fermi surface in the past. In order to clarify this aspect for the Co-Ni- Ga system, we will discuss the influence of structure on the phonon dispersions determined from first principles and investigate whether the Fermi surface of the Co-Ni-Ga compound reveals nesting features as well.
    view abstractdoi: 10.1016/j.phpro.2010.11.089
  • 2010 • 3 Band-structure-dependent demagnetization in the Heusler alloy Co 2Mn1-xFexSi
    Steil, D. and Alebrand, S. and Roth, T. and Krauß, M. and Kubota, T. and Oogane, M. and Ando, Y. and Schneider, H.C. and Aeschlimann, M. and Cinchetti, M.
    105 (2010)
    We investigate the ultrafast demagnetization for two Heusler alloys (Co2Mn1-xFexSi) with a different lineup of the minority band gap and the Fermi level. Even though electronic spin-flip transitions are partially blocked by the band gap in one compound, the respective magnetization dynamics, as measured by the time-resolved Kerr effect, are remarkably similar. Based on a dynamical model that includes momentum and spin-dependent carrier scattering, we show that the magnetization dynamics are dominated by hole spin-flip processes, which are not influenced by the gap.
    view abstractdoi: 10.1103/PhysRevLett.105.217202
  • 2010 • 2 Spinel-Type Cobalt-Manganese-Based Mixed Oxide as Sacrificial Catalyst for the High-Yield Production of Homogeneous Carbon Nanotubes
    Tessonnier, J.-P. and Becker, M. and Xia, W. and Girgsdies, F. and Blume, R. and Yao, L. and Su, D.S. and Muhler, M. and Schlögl, R.
    ChemCatChem 2 1559-1561 (2010)
    doi: 10.1002/cctc.201000278
  • 2010 • 1 Flame-made MgAl2-xMxO4 (M = Mn, Fe, Co) mixed oxides: Structural properties and catalytic behavior in methane combustion
    van Vegten, N. and Baidya, T. and Krumeich, F. and Kleist, W. and Baiker, A.
    Applied Catalysis B: Environmental 97 398-406 (2010)
    Spinel-like oxides with the general formula MgAl2-xMxO4 (M = Mn, Fe, Co) were synthesized in a single step by flame-spray pyrolysis. High surface area materials were obtained, which showed improved resistance to sintering upon increasing the Al-content. XPS showed surface enrichment of the transition metal constituent, which favorably increased the fraction of transition metal exposed to the environment. By tuning the fraction of transition metal in the mixed oxide, materials were obtained with a good balance between thermal stability and activity in the catalytic combustion of methane. The activity of the materials increased in the order Fe &lt; Co &lt; Mn. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcatb.2010.04.026