Universität Stuttgart

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Subject: search.filters.subject.530Institute: search.filters.UBSInstitut.Institut für MaterialwissenschaftAuthor: search.filters.author.Körmann, FritzStart date: 2019

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Now showing 1 - 8 of 8
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    Finite-temperature interplay of structural stability, chemical complexity, and elastic properties of bcc multicomponent alloys from ab initio trained machine-learning potentials
    (2021) Gubaev, Konstantin; Ikeda, Yuji; Tasnádi, Ferenc; Neugebauer, Jörg; Shapeev, Alexander V.; Grabowski, Blazej; Körmann, Fritz
    An active learning approach to train machine-learning interatomic potentials (moment tensor potentials) for multicomponent alloys to ab initio data is presented. Employing this approach, the disordered body-centered cubic (bcc) TiZrHfTax system with varying Ta concentration is investigated via molecular dynamics simulations. Our results show a strong interplay between elastic properties and the structural ω phase stability, strongly affecting the mechanical properties. Based on these insights we systematically screen composition space for regimes where elastic constants show little or no temperature dependence (elinvar effect).
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    Impact of thermal excitations on the stabilization of the disordered VCoNi alloy
    (2026) Körmann, Fritz; Forslund, Axel; Ikeda, Yuji; Tirunilai, Aditya Srinivasan; Laplanche, Guillaume; Münchhalfen, Marie; Schreuer, Jürgen; Neugebauer, Jörg; Grabowski, Blazej
    The VCoNi alloy is a face-centered cubic medium-entropy alloy with exceptionally high yield strength, serving as a prototypical system for investigating short-range order and phase stability in compositionally complex alloys. However, density functional theory calculations underestimate the stability of the random solid solution by several hundred Kelvin. To resolve this discrepancy, we present accurate Gibbs energy calculations for both the random solid solution and a prototypical L12 ordered phase. Our findings reveal that vibrational and electronic excitations account for nearly half of the entropy difference between the ordered phase and disordered solid solution. These factors reduce the energy difference between the two phases by about one-third and help stabilize the solid solution. Our thermodynamic analysis is validated through direct comparison with experimental thermodynamic data.
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    Correlation analysis of strongly fluctuating atomic volumes, charges, and stresses in body-centered cubic refractory high-entropy alloys
    (2020) Ishibashi, Shoji; Ikeda, Yuji; Körmann, Fritz; Grabowski, Blazej; Neugebauer, Jörg
    Local lattice distortions in a series of body-centered cubic alloys, including refractory high-entropy alloys, are investigated by means of atomic volumes, atomic charges, and atomic stresses defined by the Bader charge analysis based on first-principles calculations. Analyzing the extensive data sets, we find large distributions of these atomic properties for each element in each alloy, indicating a large impact of the varying local chemical environments. We show that these local-environment effects can be well understood and captured already by the first and the second nearest neighbor shells. Based on this insight, we employ linear regression models up to the second nearest neighbor shell to accurately predict these atomic properties. Finally, we find that the elementwise-averaged values of the atomic properties correlate linearly with the averaged valence-electron concentration of the considered alloys.
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    Performance of the standard exchange-correlation functionals in predicting melting properties fully from first principles: application to Al and magnetic Ni
    (2020) Zhu, Li-Fang; Körmann, Fritz; Ruban, Andrei; Neugebauer, Jörg; Grabowski, Blazej
    We apply the efficient two-optimized references thermodynamic integration using Langevin dynamics method [Phys. Rev. B 96, 224202 (2017)] to calculate highly accurate melting properties of Al and magnetic Ni from first principles. For Ni we carefully investigate the impact of magnetism on the liquid and solid free energies including longitudinal spin fluctuations and the reverse influence of atomic vibrations on magnetic properties. We show that magnetic fluctuations are effectively canceling out for both phases and are thus not altering the predicted melting temperature. For both elements, the generalized gradient approximation (GGA) and the local-density approximation (LDA) are used for the exchange-correlation functional revealing a reliable ab initio confidence interval capturing the respective experimental melting point, enthalpy of fusion, and entropy of fusion.
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    Anharmonicity in bcc refractory elements : a detailed ab initio analysis
    (2023) Srinivasan, Prashanth; Shapeev, Alexander; Neugebauer, Jörg; Körmann, Fritz; Grabowski, Blazej
    Explicit anharmonicity, defined as the vibrational contribution beyond the quasiharmonic approximation, is qualitatively different between the group V and group VI bcc refractory elements. Group V elements show a small and mostly negative anharmonic entropy, whereas group VI elements have a large positive anharmonic entropy, strongly increasing with temperature. Here, we explain this difference utilizing highly accurate anharmonic free energies and entropies from ab initio calculations for Nb and Ta (group V), and Mo and W (group VI). The numerically calculated entropies are in agreement with prior experimental data. The difference in behavior between the two sets of elements arises not from their high-temperature behavior but rather from the 0K　quasiharmonic reference state. We understand this by analyzing the 0K and the high-temperature phonon density of states and the electronic density of states. The qualitative difference disappears when the anharmonicity is instead referenced with a high-temperature effective harmonic potential. However, even for an optimized effective harmonic reference, the remaining effective anharmonicity is significant. The reason is that the anharmonicity in the bcc systems - carried by asymmetric distributions in the nearest neighbors - can never be accounted for by a harmonically restricted potential.
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    Melting properties of the refractory metals V and W and the binary VW alloy fully from first principles
    (2024) Zhu, Li-Fang; Srinivasan, Prashanth; Gong, Yilun; Hickel, Tilmann; Grabowski, Blazej; Körmann, Fritz; Neugebauer, Jörg
    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 capacity difference between solid and liquid. Our results thus prompt us to scrutinize this approximation which is often used in phase diagram parametrizations in the CALPHAD approach, as well as for materials, such as W, that have a large electronic DOS difference between solid and liquid at the melting temperature.
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    Thermodynamics up to the melting point in a TaVCrW high entropy alloy : systematic ab initio study aided by machine learning potentials
    (2022) Zhou, Ying; Srinivasan, Prashanth; Körmann, Fritz; Grabowski, Blazej; Smith, Roger; Goddard, Pooja; Duff, Andrew Ian
    Multi-principal-component alloys have attracted great interest as a novel paradigm in alloy design, with often unique properties and a vast compositional space auspicious for materials discovery. High entropy alloys (HEAs) belong to this class and are being investigated for prospective nuclear applications with reported superior mechanical properties including high-temperature strength and stability compared to conventional alloys. Computational materials design has the potential to play a key role in screening such alloys, yet for high-temperature properties, challenges remain in finding an appropriate balance between accuracy and computational cost. Here we develop an approach based on density-functional theory (DFT) and thermodynamic integration aided by machine learning based interatomic potential models to address this challenge. We systematically evaluate and compare the efficiency of computing the full free energy surface and thermodynamic properties up to the melting point at different stages of the thermodynamic integration scheme. Our new approach provides a ×4 speed-up with respect to comparable free energy approaches at the level of DFT, with errors on high-temperature free energy predictions less than 1 meV/atom. Calculations are performed on an equiatomic HEA, TaVCrW - a low-activation composition and therefore of potential interest for next generation fission and fusion reactors.
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    Ab initio phase stabilities and mechanical properties of multicomponent alloys: a comprehensive review for high entropy alloys and compositionally complex alloys
    (2019) Ikeda, Yuji; Grabowski, Blazej; Körmann, Fritz
    Multicomponent alloys with multiple principal elements including high entropy alloys (HEAs) and compositionally complex alloys (CCAs) are attracting rapidly growing attention. The endless possibilities to explore new alloys and the hope for better combinations of materials properties have stimulated a remarkable number of research works in the last years. Most of these works have been based on experimental approaches, but ab initio calculations have emerged as a powerful approach that complements experiment and serves as a predictive tool for the identification and characterization of promising alloys. The theoretical ab initio modeling of phase stabilities and mechanical properties of multi-principal element alloys by means of density functional theory (DFT) is reviewed. A general thermodynamic framework is laid down that provides a bridge between the quantities accessible with DFT and the targeted thermodynamic and mechanical properties. It is shown how chemical disorder and various finite-temperature excitations can be modeled with DFT. Different concepts to study crystal and alloy phase stabilities, the impact of lattice distortions (a core effect of HEAs), magnetic transitions, and chemical short-range order are discussed along with specific examples. Strategies to study elastic properties, stacking fault energies, and their dependence on, e.g., temperature or alloy composition are illustrated. Finally, we provide an extensive compilation of multi-principal element alloys and various material properties studied with DFT so far (a set of over 500 alloy-property combinations).