44 research outputs found

    Local deformations and chemical bonding in Fe-X (X = Si, Al, Ga, Ge) soft magnetic alloys

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    Dilute alloys based on ferromagnetic bcc iron modified by 3p (Al, Si) and 4p (Ga, Ge) elements are studied using the methods of the density-functional theory. It is shown that the local deformations and solution energies depend on the position of an alloying element in the periodic system. The nature of Fe-X chemical bonding varies from weak metallic in Fe-Ga to strong quasi-covalent in Fe-Si, which determines the values of local deformations in these alloys. The formation of pairs of impurity atoms in the position of the next-nearest neighbors leads to tetragonal lattice deformations, the value of which is highest for Si and Ge. The role of local deformations in the formation of unusual magnetic properties of Fe-X alloys is discussed. © 2013 Pleiades Publishing, Ltd

    Alloying element segregation and grain boundary reconstruction, atomistic modeling

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    Grain boundary (GB) segregation is an important phenomenon that affects many physical properties, as well as microstructure of polycrystals. The segregation of solute atoms on GBs and its effect on GB structure in Al were investigated using two approaches: First principles total energy calculations and the finite temperature large-scale atomistic modeling within hybrid MD/MC approach comprising molecular dynamics and Monte Carlo simulations. We show that the character of chemical bonding is essential in the solute–GB interaction, and that formation of directed quasi-covalent bonds between Si and Zn solutes and neighboring Al atoms causes a significant reconstruction of the GB structure involving a GB shear-migration coupling. For the solutes that are acceptors of electrons in the Al matrix and have a bigger atomic size (such as Mg), the preferred position is determined by the presence of extra volume at the GB and/or reduced number of the nearest neighbors; in this case, the symmetric GB keeps its structure. By using MD/MC approach, we found that GBs undergo significant structural reconstruction during segregation, which can involve the formation of single-or double-layer segregations, GB splitting, and coupled shear-migration, depending on the details of interatomic interactions. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.Ministry of Education and Science of the Russian Federation, MinobrnaukaFunding: The research has been performed in the framework of the state assignment of the Ministry of Education and Science of the Russian Federation (topic “Structure”, No. А18-118020190116-6 and “Pressure”, No. А18-118020190104-3)

    Origin of the vortex displacement field in twisted bilayer graphene

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    A model description of patterns of atomic displacements in twisted bilayer systems has been proposed. The model is based on the consideration of several dislocation ensembles, employing a language that is widely used for grain boundaries and film/substrate systems. We demonstrate explicitly how the dislocation language relates to vortex lattice motives found in experiment and simulations. We show that three ensembles of parallel screw dislocations are sufficient both to describe the rotation of the layers as a whole, and for the vortexlike displacements resulting from elastic relaxation. The results give a clear explanation of the observed features of the structural state such as vortices, accompanied by alternating stacking. © 2020 American Physical Society.This work of M.I.K. was supported by the JTC-FLAGERA Project GRANSPORT and the work of Y.N.G. was financed by the Russian Ministry of Education and Science (topic “Structure” A18-118020190116-6)

    Molecular dynamic simulation of a homogeneous bcc -> hcp transition

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    We have performed molecular dynamic simulations of a Martensitic bcc->hcp transformation in a homogeneous system. The system evolves into three Martensitic variants, sharing a common nearest neighbor vector along a bcc direction, plus an fcc region. Nucleation occurs locally, followed by subsequent growth. We monitor the time-dependent scattering S(q,t) during the transformation, and find anomalous, Brillouin zone-dependent scattering similar to that observed experimentally in a number of systems above the transformation temperature. This scattering is shown to be related to the elastic strain associated with the transformation, and is not directly related to the phonon response.Comment: 11 pages plus 8 figures (GIF format); to appear in Phys. Rev.

    Dislocation Structure and Mobility in the Layered Semiconductor InSe: a First-Principles Study

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    The structure and mobility of dislocations in the layered semiconductor InSe is studied within a multiscale approach based on generalized Peierls-Nabarro model with material-specific parametrization derived from first principles. The plasticity of InSe turns out to be attributed to peculiarities of the generalized stacking fault relief for the interlayer dislocation slips such as existence of the stacking fault with a very low energy and low energy barriers. Our results give a consistent microscopic explanation of recently observed (2020 Science 369, 542) exceptional plasticity of InSe. © 2021 The Author(s). Published by IOP Publishing Ltd

    Kinetics of Ordering and Decomposition in Ti-Al-X (X = Si, Zr) Alloys: Monte Carlo Modeling

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    To investigate the ordering and decomposition processes in Ti-Al-X [X = Si, Zr] alloys, the Monte Carlo simulations with first-principles parametrization of interatomic interactions were employed. It was shown that the processes of ordering and the precipitation in the Ti-Al system are closely related, and the stage of homogeneous ordering precedes the formation of ordered Ti3Al particles. It was found that the duration of homogeneous ordering is very sensitive to the annealing temperature and composition of alloy, and that precipitation becomes preferable as the temperature rises. In particular, uniform ordering of alloy Ti-12 at % Al was found during long-term holding at temperature below 850 K, while annealing at 1000 K resulted in formation of ordered Ti3Al particles. The obtained results agree well with the experimental data and allow explaining the features of the microstructure formed during annealing of the Ti-Al-X alloys. © 2022 by the authors.Ministry of Education and Science of the Russian Federation, Minobrnauka: AAA-A18-118020190116-6The research was carried out within the state assignment of the Ministry of Science and Higher Education of the Russian Federation (topic Structure, No. AAA-A18-118020190116-6)

    Dangling bonds and magnetism of grain boundaries in graphene

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    Grain boundaries with dangling bonds (DBGB) in graphene are studied by atomistic Monte Carlo and molecular dynamics simulations in combination with density functional (SIESTA) calculations. The most stable configurations are selected and their structure is analyzed in terms of grain boundary dislocations. It is shown that the grain boundary dislocation with the core consisting of pentagon, octagon and heptagon (5-8-7 defect) is a typical structural element of DBGB with relatively low energies. Electron energy spectrum and magnetic properties of the obtained DBGB are studied by density functional calculations. It is shown that the 5-8-7 defect is magnetic and that its magnetic moment survives after hydrogenation. The effects of hydrogenation and of out of plane deformations on the magnetic properties of DBGB are studied.Comment: 10 pages, 11 figures, 4 tables, the final version accepted in pr

    Mechanical Behavior of Polycrystalline Rhenium under 3-Points Bending at a Low Homological Temperature

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    Mechanical behaviour of polycrystalline rhenium under 3-points bending at such low homological temperature as room is discussed. Two metallurgical technologies (electron beam melting and powder metallurgy) were used for the samples preparation. Fine-grained samples (PM metal) exhibit some plasticity prior the failure, while coarse-grained ones (EBM metal) behave like a brittle solid. The intergranular fracture is the fracture mode of rhenium in both cases. Basal slip and prismatic slip of dislocations are not active in rhenium at low homological temperature, but the grain boundary sliding occurs under these conditions. Therefore, polycrystalline rhenium cannot be machined at room temperature despite the growth of grain boundary cracks are braked in the samples due to grain boundary sliding. © Published under licence by IOP Publishing Ltd.Russian Science Foundation, RSF: 18-19-00217This work is supported by the Russian Science Foundation (#18-19-00217)

    The frustration-based approach of supercooled liquids and the glass transition: a review and critical assessment

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    One of the most spectacular phenomena in physics in terms of dynamical range is the glass transition and the associated slowing down of flow and relaxation with decreasing temperature. That it occurs in many different liquids seems to call for a "universal" theory. In this article, we review one such theoretical approach which is based on the concept of "frustration". Frustration in this context describes an incompatibility between extension of the locally preferred order in a liquid and tiling of the whole space. We provide a critical assessment of what has been achieved within this approach and we discuss the relation with other theories of the glass transition.Comment: 48 pages, 13 figures, submitted to J. Phys : Cond. Matte
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