03 Fakultät Chemie

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Author: search.filters.author.Grabowski, BlazejStart date: 2020

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Now showing 1 - 10 of 24
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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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    Molecular dynamics simulations of screw dislocation mobility in bcc Nb
    (2021) Zotov, Nikolay; Grabowski, Blazej
    The screw dislocation mobility in bcc Nb has been studied by molecular dynamics (MD) simulations at different strain rates and temperatures using an embedded-atom method (EAM) potential. Static properties of the screw dislocation, as determined with the EAM potential, are in agreement with previous density-functional-theory calculations. The elementary slip plane of the screw dislocation remains (110) for all studied strain rates (in the range 6.3 × 107-6.3 × 109 s-1) and temperatures (5 to 550 K). However, the consecutive cross-slip on different symmetry-equivalent (110) planes leads to an effective glide on (112) planes. It is demonstrated that the screw dislocation trajectories, velocities and waviness of the screw dislocation depend on the crystallographic indices, (110) or (112), of the maximum resolved shear stress plane. The waiting time for the start of the screw dislocation motion increases exponentially with decreasing strain rate, substantiating the necessity to apply in future accelerated MD techniques in order to compare with macroscopic stress-strain experiments.
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    Magnetic Moment Tensor Potentials for collinear spin-polarized materials reproduce different magnetic states of bcc Fe
    (2022) Novikov, Ivan; Grabowski, Blazej; Körmann, Fritz; Shapeev, Alexander
    We present the magnetic Moment Tensor Potentials (mMTPs), a class of machine-learning interatomic potentials, accurately reproducing both vibrational and magnetic degrees of freedom as provided, e.g., from first-principles calculations. The accuracy is achieved by a two-step minimization scheme that coarse-grains the atomic and the spin space. The performance of the mMTPs is demonstrated for the prototype magnetic system bcc iron, with applications to phonon calculations for different magnetic states, and molecular-dynamics simulations with fluctuating magnetic moments.
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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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    Chemically induced local lattice distortions versus structural phase transformations in compositionally complex alloys
    (2021) Ikeda, Yuji; Gubaev, Konstantin; Neugebauer, Jörg; Grabowski, Blazej; Körmann, Fritz
    Recent experiments show that the chemical composition of body-centered cubic (bcc) refractory high entropy alloys (HEAs) can be tuned to enable transformation-induced plasticity (TRIP), which significantly improves the ductility of these alloys. This calls for an accurate and efficient method to map the structural stability as a function of composition. A key challenge for atomistic simulations is to separate the structural transformation between the bcc and the ω phases from the intrinsic local lattice distortions in such chemically disordered alloys. To solve this issue, we develop a method that utilizes a symmetry analysis to detect differences in the crystal structures. Utilizing this method in combination with ab initio calculations, we demonstrate that local lattice distortions largely affect the phase stability of Ti-Zr-Hf-Ta and Ti-Zr-Nb-Hf-Ta HEAs. If relaxation effects are properly taken into account, the predicted compositions near the bcc–hcp energetic equilibrium are close to the experimental compositions, for which good strength and ductility due to the TRIP effect are observed.
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    High-accuracy thermodynamic properties to the melting point from ab initio calculations aided by machine-learning potentials
    (2023) Jung, Jong Hyun; Srinivasan, Prashanth; Forslund, Axel; Grabowski, Blazej
    Accurate prediction of thermodynamic properties requires an extremely accurate representation of the free-energy surface. Requirements are twofold - first, the inclusion of the relevant finite-temperature mechanisms, and second, a dense volume–temperature grid on which the calculations are performed. A systematic workflow for such calculations requires computational efficiency and reliability, and has not been available within an ab initio framework so far. Here, we elucidate such a framework involving direct upsampling, thermodynamic integration and machine-learning potentials, allowing us to incorporate, in particular, the full effect of anharmonic vibrations. The improved methodology has a five-times speed-up compared to state-of-the-art methods. We calculate equilibrium thermodynamic properties up to the melting point for bcc Nb, magnetic fcc Ni, fcc Al, and hcp Mg, and find remarkable agreement with experimental data. A strong impact of anharmonicity is observed specifically for Nb. The introduced procedure paves the way for the development of ab initio thermodynamic databases.
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    Thermally-activated dislocation mobility in bcc metals : an accelerated molecular dynamics study
    (2021) Grabowski, Blazej; Zotov, Nikolay
    Plastic deformation in metals is controlled by dislocation density and mobility. In bcc metals the mobility of screw dislocations, which takes place by temperature- and stress-driven nucleation of critical kink-pairs, is most essential for deformation. However, the critical resolved shear stresses at low temperatures, as determined from molecular dynamics (MD) simulations performed at constant strain rate, are typically 2–3 times larger than the yield stresses measured experimentally. Here, an accelerated MD procedure is developed and employed to investigate the onset of dislocation mobility in the prototypical system bcc Nb. The method combines constant strain and temperature MD with hyperdynamics, using a bond-boost potential. We demonstrate, with a careful statistical analysis, that the method enables nucleation of critical kink-pairs and the determination of the Gibbs energy of kink-pair formation from accelerated MD simulations at experimentally-measured shear stresses.
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    Li5Sn, the most lithium-rich binary stannide : a combined experimental and computational study
    (2022) Stelzer, Robert U.; Ikeda, Yuji; Srinivasan, Prashanth; Lehmann, Tanja S.; Grabowski, Blazej; Niewa, Rainer
    From reaction of excess lithium with tin, we isolate well-crystallized Li5Sn and solve the crystal structure from single-crystal X-ray diffraction data. The orthorhombic structure (space group Cmcm) features the same coordination polyhedra around tin and lithium as previously predicted by electronic structure calculations for this composition, however differently arranged. An extensive ab initio analysis, including thermodynamic integration using Langevin dynamics in combination with a machine-learning potential (moment tensor potential), is conducted to understand the thermodynamic stability of this Cmcm Li5Sn structure observed in our experiments. Among the 108 Li5Sn structures systematically derived using the structure enumeration algorithm, including the experimental Cmcm structure and those obtained in previous ab initio studies, another new structure with the space group Immm is found to be energetically most stable at 0 K. This computationally discovered Immm structure is also found to be thermodynamically more stable than the Cmcm structure at finite temperatures, indicating that the Cmcm Li5Sn structure observed in our experiments is favored likely due to kinetic reasons rather than thermodynamics.
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    Structural and magnetic properties of newly found BaFeO2.667 synthesized by oxidizing BaFeO2.5 obtained via nebulized spray pyrolysis
    (2021) Wollstadt, Stephan; Ikeda, Yuji; Sarkar, Abhishek; Vasala, Sami; Fasel, Claudia; Alff, Lambert; Kruk, Robert; Grabowski, Blazej; Clemens, Oliver
    A new vacancy-ordered perovskite-type compound Ba3Fe3O8 (BaFeO2.667) was prepared by oxidizing BaFeO2.5 (P21/c) with the latter compound obtained by a spray-pyrolysis technique. The structure of Ba3Fe3O8 was found to be isotypic to Ba3Fe3O7F (P21/m) and can be written as Ba3Fe3+2Fe4+1O8. Mössbauer spectroscopy and ab initio calculations were used to confirm mixed iron oxidation states, showing allocation of the tetravalent iron species on the tetrahedral site and octahedral as well as square pyramidal coordination for the trivalent species within a G-type antiferromagnetic ordering. The uptake and release of oxygen was investigated over a broad temperature range from RT to 1100 °C under pure oxygen and ambient atmosphere via a combination of DTA/TG and variable temperature diffraction measurements. The compound exhibits a strong lattice enthalpy driven reduction to monoclinic and cubic BaFeO2.5 at elevated temperatures.
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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.