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Interactions of alloying elements (Si, Mg, Mn, Zr, Zn) and vacancies with coherent interfaces of theta' phase in Al-based alloys have been systematically studied by means of ab initio supercell calculations. The interface structure with a half-filled interfacial Cu layer is calculated to be lower in energy (by 0.1 eV per structural vacancy) than the structure with a filled Cu layer; the degree of interface reconstruction depends on the availability of vacancies. The presence of vacancies in the interfacial Cu layer plays a crucial role in the interaction of solutes with coherent theta' phase interfaces. The solute-interface interaction energies are calculated to be much weaker for elements having closed (Cu, Zn) or empty (Mg, Si) d-electron shells than for d-transition metals (Mn, Zr). To clarify the roles of alloying elements and interface structure in the stability of theta' phase precipitates, we analyze the solute-interface interactions in terms of electronic-structure and atomic-size contributions to interatomic bonding., QC 20210720

To understand the effects of doping elements on the formation and structure of Cu-enriched precipitates, different stages of Fe-Cu-Ni-Al alloy decomposition are studied using the combined MC + MD approach, which includes the Monte Carlo (MC) and molecular dynamics (MD) simulations. It is shown that the surface of the precipitates enriches in the doping elements at early stages of the decomposition, which is significant for the structure stability against the bcc -> 9R transition of the copper enriched precipitates., QC 20210719

The strengthening by coherent, nano-sized particles of metastable phases (pre-precipitates) continues to be the main design principle for new high-performance aluminium alloys. To describe the formation of such pre-precipitates in Al-Cu, Al-Mg, Al-Zn, and Al-Si alloys, we carry out cluster expansions of ab initio calculated energies for supercell models of the dilute binary Al-rich solid solutions. Effective cluster interactions, including many-body terms and strain-induced contributions due to the lattice relaxations around solute atoms, are thus systematically derived. Monte Carlo and statistical kinetic theory simulations, parameterized with the obtained effective cluster interactions, are then performed to study the early stages of decomposition in the binary Al-based solid solutions. We show that this systematic approach to multi-scale modelling is capable of incorporating the essential physical contributions (usually referred to as atomic size and electronic structure factors) to the free energy, and is therefore able to correctly describe the ordering temperatures, atomic structures, and morphologies of pre-precipitates in the four studied alloy systems. reserved., QC 20191022

A combined approach including the Monte Carlo and molecular-dynamics simulation, decomposition kinetics and segregation formation in the multicomponent low-alloy Fe-1.5Cu-2Ni-1.5Al (at %) alloy is studied. It is shown that the formation of Cu nanoparticles surface-enriched with Ni and Al (coprecipitation mode) includes several stages: (i) the formation of clusters consisting of several Cu atoms, (ii) their enrichment with Ni and Al atoms, and (iii) redistribution of Ni and Al atoms with the formation of a surface layer providing stabilization of Cu nanoparticles. Observed structural features of segregations and their stability in Fe-Cu-Ni-Al alloys is discussed., QC 20190610

We present a systematic analysis, based on ab initio calculations, of concentrated solute arrangements and precipitate phases in Fe-based alloys. The input data for our analysis are the calculated formation and interaction energies of point defects in the iron matrix, as well as the energies of ordered compounds that represent end-members in the 4-sublattice compound energy model of a multicomponent solid solution of Mg, Al, Si, P, S, Mn, Ni, and Cu elements and also vacancies in bcc Fe. The list of compounds also includes crystal structures obtained by geometric relaxation of the end-member compounds that in the cubic structure show weak mechanical instabilities (negative elastic constants) and also the G-phase Mn-6(Ni,Fe)(16)(Si,P)(7) having a complex cubic structure. A database of calculated thermodynamic properties (crystal structure, molar volume, enthalpy of formation, and elastic constants) of the most stable late-blooming-phase candidates is thus obtained. The results of this ab initio based theoretical analysis compare well with the recent experimental observations and predictions of thermodynamic calculations employing Calphad methodology., QC 20180919

The kinetics of decomposition of a polycrystalline Fe-Cu alloy and the formation of precipitates at the grain boundaries of the material have been investigated theoretically using the atomistic simulation on different time scales by (i) the Monte Carlo method implementing the diffusion redistribution of Cu atoms and (ii) the molecular dynamics method providing the atomic relaxation of the crystal lattice. It has been shown that, for a small grain size (D similar to 10 nm), the decomposition in the bulk of the grain is suppressed, whereas the copper-enriched precipitates coherently bound to the matrix are predominantly formed at the grain boundaries of the material. The size and composition of the precipitates depend significantly on the type of grain boundaries: small precipitates (1.2-1.4 nm) have the average composition of Fe-40 at % Cu and arise in the vicinity of low-angle grain boundaries, while larger precipitates that have sizes of up to 4 nm and the average composition of Fe-60 at % Cu are formed near grain boundaries of the general type and triple junctions., QC 20170412

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

This paper reviews recent progress in the field of ab initio based simulations of structure and properties of Fe-based alloys. We focus on thermodynamics of these alloys, their decomposition kinetics, and microstructure formation taking into account disorder of magnetic moments with temperature. We review modern theoretical tools which allow a consistent description of the electronic structure and energetics of random alloys with local magnetic moments that become totally or partially disordered when temperature increases. This approach gives a basis for an accurate finite-temperature description of alloys by calculating all the relevant contributions to the Gibbs energy from first-principles, including a configurational part as well as terms due to electronic, vibrational, and magnetic excitations. Applications of these theoretical approaches to the calculations of thermodynamics parameters at elevated temperatures (solution energies and effective interatomic interactions) are discussed including atomistic modeling of decomposition/clustering in Fe-based alloys. It provides a solid basis for understanding experimental data and for developing new steels for modern applications. The precipitation in Fe-Cu based alloys, the decomposition in Fe-Cr, and the short-range order formation in iron alloys with s-p elements are considered as examples., QC 20170320

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 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

QC 20170428