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The interaction between solute atoms (Mg, Si, Ti) and grain boundaries (GBs) of different types in Al are investigated using two approaches: first principles total energy calculations and large scale atomistic simulations. We have found that both deformation (size effect) and electronic (charge transfer) mechanisms play an important role in solute-GB interaction. The deformation and electronic contributions to GB segregation energy for the considered solutes have been analyzed in dependence on the impurity and the GB type. Mg and Si atoms are calculated to segregate to GBs, while Ti atoms to repel from, GBs in Al. For the case of a symmetric special-type GB the interaction is found to be short-ranged. For a general-type GB the range of GB-solute interaction is found to be considerably longer. A method to estimate the segregation capacity of a GB has been proposed, which takes into account the solute-solute interactions, and shown to be able to correctly describe the GB enrichment in alloying elements. The features of the segregation formation in fine-grained materials produced by severe plastic deformation are discussed., QC 20160118

The effect of composition on the antiphase boundary (APB) energy of Ni-based L1(2)-ordered alloys is investigated by ab initio calculations employing the coherent potential approximation. The calculated APB energies for the {111} and {001} planes reproduce experimental values of the APB energy. The APB energies for the nonstoichiometric gamma' phase increase with Al concentration and are in line with the experiment. The magnitude of the alloying effect on the APB energy correlates with the variation of the ordering energy of the alloy according to the alloying element's position in the 3d row. The elements from the left side of the 3d row increase the APB energy of the Ni-based L1(2)-ordered alloys, while the elements from the right side slightly affect it except Ni. The way to predict the effect of an addition on the {111} APB energy in a multicomponent alloy is discussed., QC 20160720

Decomposition in bcc Fe-Cu-Ni and Fe-Cu-Mn alloys is studied using statistical thermodynamics simulations with ab initio effective interactions. It is demonstrated that magnetic state strongly affects the effective interactions in these systems, substantially increasing phase separation tendency with magnetization. Simulations show that Ni is promoting precipitation of Cu by segregating to the precipitate matrix interface, while Mn produces almost no effect distributing more homogeneously in the system. The obtained distributions of Ni and Mn are in good agreement with experimental data., QC 20150813

The energies of interaction between carbon impurity atoms in paramagnetic fcc iron (austenite) are calculated using electron density functional theory. Point defects in the paramagnetic matrix are described using a statistical approach that takes into account local magnetic fluctuations and atomic relaxation in the environment of impurity atoms. It is shown that, in addition to local deformations, magnetism significantly contributes to the energies of dissolution and interaction of carbon atoms. The values of the carbon-carbon interaction energy are indicative of a significant repulsion between these atoms in the first and second coordination spheres. The results of calculations are consistent with estimates obtained from experimental data on the activity of carbon impurity atoms in iron., Funding Agencies|Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Swedish Foundation for Strategic Research (SSF) [10-0026]; Russian Foundation for Basic Research [13-02-00-606a]

In the framework of disordered local moment approach by using magnetic sampling method, we suggested a model that takes into account the magnetic disorder in paramagnetic Fe with point defects. We calculate solution enthalpies of substitutional (Nb, V) and interstitial (C, N) impurities in paramagnetic face-centered cubic Fe and obtain results that are in agreement with available experimental data. It is found that both interstitial and substitutional atoms may favor the local magnetic polarization of the Fe host around the impurities by decreasing the potential energy of the system. The possibility of a formation of predominantly ferromagnetic Fe clusters around carbon in the temperature range of overcooled austenite is discussed., Funding Agencies|Russian Federation Ministry for Science and Education [14.Y26.31.0005]; Swedish Foundation for Strategic Research, program Succesful Research Leader [744 10-0026]; Russian Foundation for Basic Researches [13-02-00-606a]

When a crystal is subjected to a periodic potential, under certain circumstances it can adjust itself to follow the periodicity of the potential, resulting in a commensurate state. Of particular interest are topological defects between the two commensurate phases, such as solitons and domain walls. Here we report a commensurate-incommensurate transition for graphene on top of hexagonal boron nitride (hBN). Depending on the rotation angle between the lattices of the two crystals, graphene can either stretch to adapt to a slightly different hBN periodicity (for small angles, resulting in a commensurate state) or exhibit little adjustment (the incommensurate state). In the commensurate state, areas with matching lattice constants are separated by domain walls that accumulate the generated strain. Such soliton-like objects are not only of significant fundamental interest, but their presence could also explain recent experiments where electronic and optical properties of graphene-hBN heterostructures were observed to be considerably altered.

Investigation of Ga influence on the structure of Fe-Cr and Fe-Co alloys was performed with the use of Mossbauer spectroscopy and X-ray diffraction methods. The experimental results are compared with results of first-principles calculations of the mixing and solubility energies for Ga in an Fe-X (X = Co, Cr) alloy both in ferromagnetic and paramagnetic states. It is shown that Ga mainly goes into the solid solutions of the base alloys. In the alloys of the Fe-Cr system, doping with Ga handicaps the decomposition of solid solutions, observed in the binary alloys, and increases its stability. In the alloys with Co, Ga also favors the uniformity of solid solutions. The results of the first-principles calculations testify in favor of a preferable dissolution of Ga in the FeCo regions of a multicomponent structure rather than FeCr regions, both types of regions being in the ferromagnetic state at the temperature of annealing. The analysis of Mossbauer experiments gives some grounds to conclude that if, owing to liquation, clusterization, or initial stages of phase separation, there exist regions enriched in iron, some amount of Ga atoms prefer to enter the nearest surroundings of iron atoms, thus forming binary Fe-Ga regions (or phases)., QC 20141016

The temperature-dependent solubility of Cu in alpha-Fe and initial stages of Cu precipitation are investigated in first-principles calculations and statistical thermodynamic and kinetic modeling based on ab initio effective interactions. We demonstrate that the weakening of the phase separation tendency with increasing temperature, especially close to the magnetic phase transition, is related to the strong dependence of the "chemical" interactions on the global magnetic state. At the same time, our calculations demonstrate that the vibrational contribution obtained in the quasiharmonic approximation is relatively small for temperatures near the Curie point. The results of Monte Carlo simulations of Cu solubility and clustering are in good agreement with experimental data., QC 20131217

The methods of quantum-mechanical simulation have been used to study alloys of bcc iron with 3d transition metals in the ferromagnetic and paramagnetic states. It has been shown that the main factor that determines the solubility of the 3d elements is their electronic structure. The energy of the solution, mixing, and effective interatomic interactions vary regularly depending on the position of the element in the Periodic Table and on the magnetic state of the matrix. In some cases, depending on the magnetic state, changes in these quantities lead to the violation of the Hume-Rothery rules that determine the solubility of substitutional elements in alloys. The results obtained help us to understand the microscopic mechanisms that determine the solubility of alloying elements and their effect on the phase stability and structural state of steels., QC 20130913

Vacancy-solute interactions play a crucial role in diffusion-controlled processes, such as ordering or decomposition, which occur in alloys under heat treatment or under irradiation. Detailed knowledge of these interactions is important for predicting long-term behavior of nuclear materials (such as reactor steels and nuclear-waste containers) as well as for advancing our general understanding of kinetic processes in alloys. Using first-principles calculations based on the density functional theory and employing the locally self-consistent Green's function technique, we develop a database of vacancy-solute interactions in dilute alloys of bcc Fe with 3p (Al, Si, P, S), 3d (Sc-Cu), and 4d (Y-Ag) elements. Unrelaxed interactions within the first three coordination shells have been computed in the ferromagnetic state as well as in the paramagnetic (disordered local moment) state of the iron matrix. Magnetism is found to have a strong effect on the vacancy-solute interactions. Implications of the obtained results for interpreting the effects of vacancy trapping and enhanced impurity diffusion are discussed., QC 20120206

Vacancy-solute interactions play a crucial role in diffusion-controlled phase transformations, such as ordering or decomposition, which occur in alloys under heat treatment or under irradiation. The knowledge of these interactions is important for predicting long-term behavior of nuclear materials (such as reactor steels and nuclear-waste containers) under irradiation, as well as for advancing our general understanding of kinetic processes in alloys. Using first-principles calculations based on density functional theory and employing the locally self-consistent Green’s function technique, we develop a database of vacancy-solute interactions in dilute alloys of bcc Fe with 3p (Al, Si, P, S), 3d (Ti – Cu), and 4d (Nb – Ag) elements. Interactions within the first two coordination shells have been computed in the ferromagnetic state as well as in the paramagnetic (disordered local moment) state of the iron matrix. Magnetism is found to have a very strong effect on the vacancy-solute interactions., QC 20120217

In the frameworks of the model of regular solid solutions taking into account the different diffusion mobility of the components, the effect of fluxes of nonequilibrium vacancies on the kinetics of decomposition of the alloy is studied. It is shown that the joint action of thermodynamic stimuli of decomposition and the inverse Kirkendall effect gives rise to the precipitation of phases near sources and sinks for vacancies even if the alloy parameters fall into the region of mutual solubility of the components. It has been established that under specific conditions the effect of vacancy fluxes can lead to the formation of special space-time structures (traveling concentration waves). Features of the alloy decomposition upon severe plastic deformation, when grain boundaries serve as sources of nonequilibrium vacancies, are discussed. Copyright © 2005 by Pleiades Publishing, Inc.

To fulfill the great potential of intermetallic alloys for use in high temperature structural applications, it is necessary to understand on the microscopic level the mechanisms controlling their mechanical behavior, including such key phenomena as dislocation structure and mobility, crack blunting and propagation, the role and the effect of alloying additions: while they have been characterized by mesoscopic length and energy scales, they are determined on the microscopic level by the electronic structure.

A new fundamental phenomenon of phase instability (decomposition) of nanocrystalline alloys and appearance of metastable phases under intensive plastic deformation is considered. Various experimental data illustrating phase transformations of nanocrystalline alloys in the process of deformation treatment are analyzed. The fundamental physical criteria of instability of a solid solution and diffusion equations based on the idea of existence of nonequilibrium vacancy fluxes are formulated. The vacancy fluxes are generated on stress concentrators at grain boundaries in the process of their relaxation. The decomposition appears as a result of the self-consistent interaction of non-equilibrium vacancies with the alloy components having the different diffusion mobility. Within the framework of diffusion model the stability of a regular solid solution in relation to phase decomposition is theoretically investigated. On the base of qualitative and numerical analysis of the diffusion equations the phase instability of the solid solution is demonstrated.

A theoretical treatment of the fundamental phenomenon of phase instability of nanocrystalline alloys under intensive plastic deformation are performed. It is shown within the framework of a diffusion model that decomposition of nanocrystalline alloys is related to the existence of powerful sources of nonequilibrium vacancies that acting during deformation. Irreversibility of decomposition is defined by conservation a uni-directional vacancies flux between, for instance, triple junctions of grains in nanocrystals. The effect is observed when certain relationships are fulfilled between the characteristic times of vacancy flux existence, the homogenizing time and the time of life of a stable nanocrystalline structure. The evolution of substructure in the course of deformation may change the nature of transformation: back from decomposition to formation of a solid solution.