Browsing by Author "Zhang, Xi"

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    Ab initio simulations of the surface free energy of TiN(001)
    (2021) Forslund, Axel; Zhang, Xi; Grabowski, Blazej; Shapeev, Alexander V.; Ruban, Andrei V.
    The temperature dependence of the surface free energy of the industrially important TiN(001) system has been investigated by means of an extended two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD) methodology, to include the fully anharmonic vibrational contribution, as obtained from ab initio molecular dynamics (AIMD). Inclusion of the fully anharmonic behavior is crucial, since the standard low-temperature quasiharmonic approximation exhibits a severe divergence in the surface free energy due to a high-temperature dynamical instability. The anharmonic vibrations compensate for the quasiharmonic divergence and lead to a modest overall temperature effect on the TiN(001) surface free energy, changing it from around 78 meV Å-2 at 0 K to 73 meV Å-2 at 3000 K. The statistical convergence of the molecular dynamics is facilitated by the use of machine-learning potentials, specifically moment tensor potentials, fitted for TiN(001) at finite temperature. The surface free energy obtained directly from the fitted machine-learning potentials is close to that obtained from the full AIMD simulations.
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    Accurate electronic free energies of the 3d, 4d, and 5d transition metals at high temperatures
    (2017) Zhang, Xi; Grabowski, Blazej; Körmann, Fritz; Freysoldt, Christoph; Neugebauer, Jörg
    Free energies of bulk materials are nowadays routinely computed by density functional theory. In particular for metals, electronic excitations can significantly contribute to the free energy. For an ideal static lattice, this contribution can be obtained at low computational cost, e.g., from the electronic density of states derived at T = 0 K or by utilizing the Sommerfeld approximation. The error introduced by these approximations at elevated temperatures is rarely known. The error arising from the ideal lattice approximation is likewise unexplored but computationally much more challenging to overcome. In order to shed light on these issues we have computed the electronic free energies for all 3d, 4d, and 5d transition elements on the ideal lattices of the bcc, fcc, and hcp structures using finite-temperature density-functional theory. For a subset of elements we have explored the impact of explicit thermal vibrations on the electronic free energies by using ab initio molecular dynamics simulations. We provide an analysis of the observed chemical trends in terms of the electronic density of states and the canonical d band model and quantify the errors in the approximate methods. The electronic contribution to the heat capacities and the corresponding errors due to the different approximations are studied as well.
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    Calculating free energies of point defects from ab initio
    (2018) Zhang, Xi; Grabowski, Blazej; Hickel, Tilmann; Neugebauer, Jörg
    The formation and lifetime of point defects is governed by an interplay of kinetics and thermodynamic stability. To evaluate the stability under process conditions, empirical potentials and ab initio calculations at T 1⁄4 0 K are often not sufficient. Therefore, various concepts to determine the full temperature dependence of the free energy of point defects with ab initio accuracy are reviewed. Examples for the importance of accurately describing defect properties include the stabilization of vacancies by impurities and the non-Arrhenius behaviour of vacancy formation energies due to anharmonic lattice vibrations.
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    Interstitials in compositionally complex alloys
    (2023) Baker, Ian; Grabowski, Blazej; Divinski, Sergiy V.; Zhang, Xi; Ikeda, Yuji
    The effects of interstitial alloying on the mechanical and diffusive properties of compositionally complex alloys (CCAs), including high-entropy alloys (HEAs), are reviewed. The solubility of interstitial elements in CCAs can be extraordinarily high, a feature corroborated by ab initio density functional theory simulations. The yield stresses, work-hardening rates, and Hall-Petch slopes of CCAs are normally reported to increase due to interstitial alloying. In some CCAs, interstitial alloying has been found to enhance both strength and ductility, thus circumventing the traditional tradeoff between these properties. Self-diffusivities of the HEA CoCrFeMnNi are found to show complex dependences on interstitial C concentration as well as on temperature. Some CCAs with Laves phase or body-centered cubic crystal structures show potential as hydrogen-storage materials, with both experimental and computational research in this area steadily increasing. Based on the insights obtained, possible directions for further studies on the impacts of interstitial alloying in CCAs are suggested.
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    Recent advances in understanding diffusion in muti-principal element systems
    (2022) Dash, Anuj; Paul, Aloke; Sen, Sandipan; Divinski, Sergiy; Kundin, Julia; Steinbach, Ingo; Grabowski, Blazej; Zhang, Xi
    Recent advances in the field of diffusion in multiprincipal element systems are critically reviewed, with an emphasis on experimental as well as theo- retical approaches to determining atomic mobilities (tracer diffusion coef- ficients) in chemically complex multicomponent systems. The newly elaborated and augmented pseudobinary and pseudoternary methods provide a rigorous framework to access tracer, intrinsic, and interdiffusion coefficients in alloys with an arbitrary number of components. Utilization of the novel tracer-interdiffusion couple method allows for a high-throughput determination of composition-dependent tracer diffusion coefficients. A combination of these approaches provides a unique experimental toolbox to access diffusivities of elements that do not have suitable tracers. The pair-exchange diffusion model, which gives a consistent definition of diffusion matrices without specifying a reference element, is highlighted. Density-functional theory-informed calculations of basic diffusion properties - as required for the generation of extensive mobility databases for technological applications - are also discussed.
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    Strong impact of spin fluctuations on the antiphase boundaries of weak itinerant ferromagnetic Ni3Al
    (2023) Xu, Xiang; Zhang, Xi; Ruban, Andrei; Schmauder, Siegfried; Grabowski, Blazej
    Antiphase boundaries (APBs) are crucial to understand the anomalous temperature dependence of the yield stress of Ni3Al. However, the required, accurate prediction of temperature-dependent APB energies has been missing. In particular, the impact of magnetism at elevated temperatures has been mostly neglected, based on the argument that Ni3Al is a weak ferromagnet. Here, we show that this is an inappropriate assumption and that - in addition to anharmonic and electronic excitations - thermally-induced magnetic spin fluctuations strongly affect the APB energies, especially for the (100)APB with an increase of nearly up to 40% over the nonmagnetic data. We utilize an ab initio framework that incorporates explicit lattice vibrations, electronic excitations, and the impact of magnetic excitations up to the melting temperature. Our results prompt to take full account of thermally-induced spin fluctuations even for weak itinerant ferromagnetic materials. Consequences for large-scale modeling in Ni-based superalloys, e.g., of dislocations or the elastic-plastic behavior, can be expected.
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    Temperature dependence of the stacking-fault Gibbs energy for Al, Cu, and Ni
    (2018) Zhang, Xi; Grabowski, Blazej; Körmann, Fritz; Ruban, Andrei V.; Gong, Yilun; Reed, Roger C.; Hickel, Tilmann; Neugebauer, Jörg
    The temperature-dependent intrinsic stacking fault Gibbs energy is computed based on highly converged density-functional-theory (DFT) calculations for the three prototype face-centered cubic metals Al, Cu, and Ni. All relevant temperature-dependent contributions are considered including electronic, vibrational, magnetic, and explicit anharmonic Gibbs energy contributions as well as coupling terms employing state-of-the-art statistical sampling techniques. Particular emphasis is put on a careful comparison of different theoretical concepts to derive the stacking fault energy such as the axial-next-nearest-neighbor-Ising (ANNNI) model or the vacuum-slab approach. Our theoretical results are compared with an extensive set of previous theoretical and experimental data. Large uncertainties in the experimental data highlight the necessity of complementary parameter-free calculations. Specifically, the temperature dependence is experimentally unknown and poorly described by thermodynamic databases. Whereas calphad derived data shows an increase of the stacking fault energy with temperature for two of the systems (Cu and Ni), our results predict a decrease for all studied systems. For Ni, the temperature induced change is in fact so strong that in the temperature interval relevant for super-alloy applications the stacking fault energy falls below one third of the low temperature value. Such large changes clearly call for a revision of the stacking fault energy when modeling or designing alloys based on such elements.