TY - JOUR A1 - Sözen, H.I. A1 - Ener, S. A1 - Maccari, F. A1 - Fayyazi, B. A1 - Gutfleisch, O. A1 - Neugebauer, J. A1 - Hickel, Tilmann T1 - Combined ab initio and experimental screening of phase stabilities in the Ce-Fe-Ti-X system (X = 3d and 4d metals) N2 - One of the main challenges for the synthesis and application of the promising hard-magnetic compound CeFe11Ti is the formation of Laves phases that are detrimental for their thermodynamic stability and magnetic properties. In this paper, we present an ab initio based approach to modify the stability of these phases in the Ce-Fe-Ti system by additions of 3d and 4d elements. We combine highly accurate free-energy calculations with an efficient screening technique to determine the critical annealing temperature for the formation of Ce(Fe,X)11Ti. The central findings are the dominant role of the formation enthalpy at T = 0 K on chemical trends and the major relevance of partial chemical decompositions. Based on these insights, promising transition metals to promote the stability of the hard-magnetic phase, such as Zn and Tc, were predicted. The comparison with suction casting and reactive crucible melting experiments for Ce-Fe-Ti-X (X = Cu, Ga, Co, and Cr) highlights the relevance of additional phases and quaternary elements. KW - Density functional theory KW - Phase stability KW - Energy materials PY - 2023 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-568850 DO - https://doi.org/10.1103/PhysRevMaterials.7.014410 SN - 2475-9953 VL - 7 SP - 1 EP - 15 AN - OPUS4-56885 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Zhu, L. A1 - Srinivasan, P. A1 - Gong, Y. A1 - Hickel, Tilmann A1 - Grabowski, B. A1 - Körmann, Fritz A1 - Neugebauer, J. T1 - Melting properties of the refractory metals V and W and the binary VW alloy fully from first principles N2 - We investigate the melting properties of the bcc refractory metals V and W, and the disordered equiatomic VW alloy from first principles. We show that thermal vibrations have a large impact on the electronic density of states (DOS) and thus considerably affect the electronic contribution to the free energy. For W, the impact of vibrations on the electronic free energy of solid and liquid is different. This difference substantially impacts the computed melting point and also triggers a large electronic heat capacity difference between solid and liquid. For V, although vibrations likewise affect the electronic free energy, the effect on the melting properties cancels out to a large degree. For the binary VW alloy we observe a similar impact as for W, but slightly weaker. The underlying physics is explained in terms of the electronic DOS of the solid and liquid phases. Based on our accurate first-principles results, we reveal critical limitations of the Sommerfeld approximation in predicting the electronic heat KW - Crystal melting KW - Thermodynamics KW - Metals KW - Ab initio molecular dynamics KW - Density functional theory PY - 2024 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-618100 DO - https://doi.org/10.1103/PhysRevB.109.094110 SN - 2469-9950 VL - 109 IS - 9 SP - 1 EP - 18 PB - American Physical Society (APS) AN - OPUS4-61810 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Zhou, X. A1 - Mathews, P. A1 - Berkels, B. A1 - Delis, W. A1 - Saood, S. A1 - Alhassan, A. A1 - Keuter, P. A1 - Schneider, J. A1 - Korte-Kerzel, S. A1 - Sandlöbes, S. A1 - Raabe, D. A1 - Neugebauer, j. A1 - Dehm, G. A1 - Hickel, Tilmann A1 - Scheu, C. A1 - Zhang, S. T1 - Materials Design by Constructing Phase Diagrams for Defects N2 - Phase transformations and crystallographic defects are two essential tools todrive innovations in materials. Bulk materials design via tuning chemicalcompositions is systematized using phase diagrams. It is shown here that thesame thermodynamic concept can be applied to manipulate the chemistry atdefects. Grain boundaries in Mg–Ga system are chosen as a model system,because Ga segregates to the boundaries, while simultaneously improving thestrength and ductility of Mg alloys. To reveal the role of grain boundaries,correlated atomic-scale characterization and simulation to scope and buildphase diagrams for defects are presented. The discovery is enabled bytriggering phase transformations of individual grain boundaries through localalloying, and sequentially imaging the structural and chemical changes usingatomic-resolution scanning transmission electron microscopy. Ab initiosimulations determined the thermodynamic stability of grain boundaryphases, and found out that increasing Ga content enhances grain boundarycohesion, relating to improved ductility. The methodology to trigger, trace,and simulate defect transformation at atomic resolution enables a systematicdevelopment of defect phase diagrams, providing a valuable tool to utilizechemical complexity and phase transformations at defects. KW - Automatic pattern recognition KW - Defect phase diagram KW - Density functional theory KW - Grain boundary complexion KW - Transmission electron microscopy PY - 2024 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-618063 DO - https://doi.org/10.1002/adma.202402191 SN - 1521-4095 SP - 1 EP - 9 PB - Wiley AN - OPUS4-61806 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Sestan, A. A1 - Sreekala, L. A1 - Markelj, S. A1 - Kelemen, M. A1 - Zavasnik, J. A1 - Liebscher, C. A1 - Dehm, G. A1 - Hickel, Tilmann A1 - Ceh, M. A1 - Novak, S. A1 - Jenus, P. T1 - Non-uniform He bubble formation in W/W2C composite: Experimental and ab-initio study N2 - Tungsten-tungsten carbide (W/W2C) composites are considered as possible structural materials for future nuclear fusion reactors. Here, we report on the effect of helium (He) implantation on microstructure evolution of polycrystalline W/W2C composite consolidated by field-assisted sintering technique (FAST), homogenously implanted at room temperature with 1 MeV 4He+ ions at the fluence of 8 × 1016 ions cm−2 and annealed at 1873 K for 20 minutes. Samples were analysed by scanning and transmission electron microscopy to study the presence and size of He bubbles. Monomodal He bubbles in W (30-80 nm) are limited to point defects and grain boundaries, with a considerable void denuded zone (150 nm). Bubbles do not form in W2C, but at the W|W2C interface and are considerably larger (200-400 nm). The experimental observations on He behaviour and migration in W and W2C were assessed by density functional theory (DFT) calculations, suggesting He migration and accumulation in the composite are determined by the effective He-He binding in clusters, which will give rise to decohesion. In the presence of He clusters, the decohesion of bulk W into free surfaces is energetically highly favourable but not sufficient in the W2C; hence bubbles are only observed in W grains and interfaces and not within bulk W2C. KW - Density functional theory KW - Tungsten KW - Ditungsten carbide KW - FAST KW - Helium implantation PY - 2022 DO - https://doi.org/10.1016/j.actamat.2021.117608 VL - 226 SP - 1 EP - 11 PB - Elsevier Ltd. AN - OPUS4-54364 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Hickel, Tilmann T1 - Ab initio Thermodynamics of defect phases in metals N2 - Thermodynamic bulk phase diagrams have become the roadmap used by researchers to identify alloy compositions and process conditions that result in novel materials with tailored microstructures. On the other hand, recent progress in experimental atomic-scale characterization techniques allows one to study the local chemical composition at individual defects such as interfaces, grain boundaries, dislocations and surfaces. They show that changes in the alloy composition can drive not only transitions in the bulk phases present in a material, but also in the concentration and type of defects they contain. Defect phase diagrams, using chemical potentials as thermodynamics variables, provide a natural route to study these chemically driven defects. Our results show, however, that the direct application of thermodynamic approaches can fail to reproduce the experimentally observed defect formation. Therefore, we extend the concept to metastable defect phase diagrams to account for kinetic limitations that prevent the system from reaching equilibrium. We use ab initio calculations based on density functional theory to quantify the extension of regions where defect formation is expected. We successfully applied this concept to explain the formation of large concentrations of planar defects in supersaturated Fe2Nb Laves phases. In addition, we identify in a joint study with experiments conditions and structures in Mg-Al-Ca alloys for defect phase occurrence. The concept offers new avenues for designing materials with tailored defect structures. T2 - 19th Int. Conference on Diffusion in Liquids and Solids CY - Heraklion, Greece DA - 26.06.2023 KW - Density functional theory KW - Defects KW - Phase diagrams KW - Mg alloys PY - 2023 AN - OPUS4-58845 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hickel, Tilmann A1 - Divinski, S. A1 - Starikov, S. A1 - Soisson, F. A1 - Mény, C. A1 - Hegde, O. A1 - Gerlitz, M. A1 - Magnifouet, G. A1 - Schneider, A. A1 - Barreteau, C. A1 - Mirebeau, I. A1 - Tran, V.T. A1 - Förster, G. A1 - Front, A. A1 - Egorov, A. A1 - Wilde, G. A1 - Amara, H. A1 - Hammerschmidt, T. A1 - Mrovec, M. A1 - Pierron-Bohnes, V. A1 - Drautz, R. A1 - Fu, C. T1 - Magnetism in iron alloys: methodological advances for thermodynamics, defects, and kinetics N2 - Steels are among the technologically and economically most relevant materials. Key innovations in important sectors of human society such as mobility, energy and safety, are currently based on alloying of Fe with other transition-metal elements such as Mn, Cr, or Co. Due to strong impacts and conceptual challenges related to magnetism, however, the fundamental understanding and the ability to computationally design these steels in high-throughput approaches lags behind other classes of alloys. In this article, we will provide a substantial review of the role of magnetism, magnetic excitations and transformations for alloy thermodynamics, point defects, interfaces and kinetics. This will be achieved by combining insights from different methods: Ab initio simulations have the advantage that the magnetic ground state is intrinsic part of the electronic minimization. Due to the coarsening of the many-electron structures and therewith magnetic interactions, tight-binding methods can handle larger system sizes. Effective interaction models provide the freedom to exploit more sophisticated magnetic interactions. The performance of these methods in terms of magnetic properties of Fe alloys will be evaluated by providing state-of-the-art results for their sensitivity to magnetism. Furthermore, dedicated experiments will be discussed to complete the understanding of magnetic effects in Fe alloys and to validate the modeling strategy. KW - Magnetic excitations KW - Defects KW - Diffusion KW - Density functional theory KW - Tight-binding KW - Effective interaction models PY - 2025 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-634530 DO - https://doi.org/10.1515/ijmr-2023-0225 SN - 1862-5282 VL - 60 IS - 99 SP - 1 EP - 14761 PB - Walter de Gruyter GmbH AN - OPUS4-63453 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -