Institute of Quantum Materials Science

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.

The electronic structure of the Cr(001) surface with its sharp resonance at the Fermi level is a subject of controversial debate of many experimental and theoretical works. To date, it is unclear whether the origin of this resonance is an orbital Kondo or an electron-phonon coupling effect. We have combined ab initio density functional calculations with dynamical mean-field simulations to calculate the orbitally resolved spectral function of the Cr(001) surface. The calculated orbital character and shape of the spectrum is in agreement with data from (inverse) photoemission experiments. We find that dynamic electron correlations crucially influence the surface electronic structure and lead to a low energy resonance in the ${d}_{{z}^{2}}$ and ${d}_{xz/yz}$ orbitals. Our results help to reconvene controversial experimental results from (I)PES and STM measurements.

Dilute alloys based on ferromagnetic bcc iron modified by 3p (Al, Si) and 4p (Ga, Ge) elements are studied using the methods of the densityfunctional 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 quasicovalent 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 nextnearest 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 dis� cussed.

Phase diagram and pattern formation in a two-dimensional Ising model with coupling between order parameter and lattice vibrations are investigated by Monte Carlo simulations. It is shown that if the coupling is strong enough (or phonons are soft enough) a short-range order exists in the disordered phase for a broader temperature interval. Different types of this short-range order (stripe-like, chessboard-like, etc.) depending on the temperature and model parameters are investigated. With further increase of the coupling, a reconstruction of the ground state happens and new ordered phases appear at low enough temperatures.

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.