Vol 125, No 6 (2024)

Based on the replica exchange algorithm of the Monte Carlo method, the influence of an external magnetic field on phase transitions and thermodynamic properties of the two-dimensional antiferromagnetic Potts model with the number of spin states q = 4 on a hexagonal lattice was studied. The studies were conducted in the range of external magnetic field values of 0.0 ≤ h ≤ 10.0. The magnetic field is measured in relative units of exchange interaction between nearest neighbors |J₁|. The magnetic structures of the ground state were obtained, and the nature of phase transitions in the considered field range was analyzed. It has been established that in the intervals 0.0 ≤ h ≤ 3.0 and 6.0 ≤ h ≤ 6.5 a first-order phase transition is observed. It is shown that at the external magnetic field values h = 3.5 and 5.5, the system is frustrated, and in the magnetic field intervals 4.0 ≤ h ≤ 5.0 and 7.0 ≤ h ≤ 8.5, the system approaches the frustration regime. It was found that strong magnetic fields (h ≥ 9.0) suppress the phase transition in the system.

The linear electro-optic effect is investigated in films of RFeO₃ compounds (R = Y, Lu, Yb) possessing hexagonal symmetry. A general equation for the optical indicatrix of the film at arbitrary orientation of an external electric field is derived. The equation of the indicatrix in the principal axes, as well as the principal refractive indices, are determined for the particular case of an external electric field orientation along the [010] axis. Two novel optical axes are identified in the presence of an electric field. It was determined that two isonormal modes propagate in the film when light propagates along the [100] direction. The refractive indices associated with these modes were also found. The phase shift associated with the propagation of these modes and caused by the presence of an electric field is calculated. The shift is linear along the electric field, thereby demonstrating the Pockels effect.

The effect of grain boundary retardation by annealing twins in pure copper has been experimentally and theoretically investigated. A model that describes the influence of annealing twins on the migration of grain boundaries in pure metals has been proposed. The retarding force from the twins has been demonstrated to be analogous to the Zener retarding force created by incoherent fine particles. An equation has been derived to calculate the retarding force induced by annealing twins. The force has been demonstrated to be inversely proportional to the size of the twins and directly proportional to their volume fraction. The simulation results have been compared with the experimental results. A satisfactory correlation between the theoretical and experimental results has been achieved.

The commercial Ti–6Al–4V alloy was obtained in an almost single-phase state, formed by finely dispersed globular α-grains with an average size of 12 μm, using thermomechanical processing, including hot rolling. The microtexture and structure of the alloy were studied using X-ray diffractometry and transmission and scanning electron microscopy, including orientation microscopy. It is found that for α-grains the Burgers orientation relationships are satisfied, and twin orientations are ensured in the rolling plane. A significant scattering of the crystallographic orientations of α-grains relative to each other (up to 10°–15°) is established for each group of close Burgers orientations as a result of plastic deformation by rolling at high temperatures. Clusters of microtexture regions in the layered microstructure of grains and the formation mechanisms and mutual crystallographic misorientations of microtexture regions and grains in the alloy have been identified.

The effect of nanosecond pulse laser processing of the Zr–1% Nb alloy surface of specimens in the annealed state and after their two-stage deformation treatment by abc-pressing and rolling has been investigated. The morphology of the modified surface of specimens is described using optical and scanning microscopy. Furthermore, the microrelief formed as a result of vaporization and melting of a thin layer of material subjected to laser processing is evaluated quantitatively. Durometric measurements were conducted to ascertain the hardness of the near-surface layer and the impact of laser-induced shock waves on its hardness. The electron backscattering diffraction (EBSD) analysis data were employed to describe the structure of the specimens in the near-surface layer. The influence of the initial grain size on the quality of the modified surface, as well as on the depth and hardening of the near-surface layers has been established.

The structure and mechanical properties of rolled sheets with a thickness of 40 mm made of lowcarbon low-alloyed steel after thermomechanical treatment of different modes are investigated. The structural factors that affect obtaining high mechanical properties are established. It is shown that a complex treatment including quenching from the rolling temperature and high-temperature tempering with a subsequent additional cycle of quenching and tempering leads to formation of rolled sheet uniform over the cross section with a fragmented bainite structure with disperse niobium carbides and carbides of cementite type. Such structure provides a yield strength of no less than 630 MPa in combination with high values of the impact energy at negative temperatures and a level of plasticity of δ ≈ 20%.

For a polycrystalline material with a hexagonal close-packed lattice, a model is proposed that allows estimating the yield strength at a given texture. The plasticity properties of an individual grain are described by the generalized von Mises criterion. The most widespread averaging approaches are considered to determine the yield strength of a polycrystal. An original averaging method for a heterogeneous medium under plastic deformation conditions is proposed that takes into account the presence of undeformed grains whose share is determined by means of the percolation theory. Using each approach, the problem about tension/compression of a homogeneous rod of square cross-section is solved for two limiting cases: no texture and rigid basis texture. The calculation results are juxtaposed with the available literature data. The effect of the texture on the yield strength is considered. A qualitative explanation of generating a texture is given.

The process of precipitation of Al₃Zr particles in cast Al–(0.25–0.4) wt % Zr alloys manufactured by the induction casting method is studied. The effect of zirconium concentration on the microstructural parameters, hardness, and specific electrical resistance (SER) of cast alloys is investigated. The dependences of hardness and SER on the time of cast alloys annealing at 500°C are plotted. The parameters of the Johnson–Mehl–Avrami–Kolmogorov equation for alloys with different Zr content are determined. The optimal regimes of cast ingot aging are found. Fine wires with ∅ 0.3 mm are manufactured by the drawing method; their strength, SER, and hardness are studied in the initial state and after heat treatment. The tests of thermal stability of wires are carried out according to the state standard GOST R MEK 62004–2014.