Vol 118, No 8 (2017)

The first-principles calculations of thermodynamic characteristics of bcc and sc structures of Ca_1–xSr_x solid solutions have been carried out. Taking into account insufficient accuracy of such calculations, for the description of phase transitions, the known experimental data on bcc and sc structures of calcium and strontium have been used to determine parameters for the calculation of Ca_1–xSr_x properties using linear interpolation. The possibility of the occurrence of bcc–sc structural phase transitions in Ca_1–xSr_x (x = 0.0625, 0.125, 0.25, 0.5, 0.75) solid solutions and their characteristics under different pressures have been investigated.

Specific features of magnetooptical images (MOIs) of stray fields near the faces of prismatic hard magnetic elements have been studied. Attention has primarily been focused on MOIs of fields near faces oriented perpendicular to the magnetic moment of hard magnetic elements. With regard to the polar sensitivity, MOIs have practically uniform brightness and geometrically they coincide with the figures of the bases of the elements. With regard to longitudinal sensitivity, MOIs consist of several sectors, the number of which is determined by the number of angles of the image. Each angle is divided by the bisectrix into two sectors of different brightnesses; therefore, the MOI of a triangular magnet consists of three sectors. A rectangle consists of four sectors separated by the bisectrices of the interior angles. In all types of figures, these lines converge at the center of the figure and form a singular point of the source or sink type.

Due to the difficulty of preparation and beneficial properties achievable, copper and iron matrix nanocomposites are materials for which fabrication via the powder metallurgy route is attracting increasing research interest. The presence of ceramic nanoparticles in their matrix can lead to considerable changes in the microstructure and morphology. The effects of the type of metallic matrix and ceramic nanoparticle on the distribution of nano reinforcements and the morphology of ball-milled composite powders were evaluated in this study. For this purpose, 25 wt % of Al₂O₃ and SiC nanoparticles were separately ball-milled in the presence of iron and copper metals. The SEM, FESEM, and XRD results indicated that as-received nanoparticles, which were agglomerated before milling, were partially separated and embedded in the matrix of both the metals after the initial stages of ball milling, while prolonged milling was not found to further affect the distribution of nanoparticles. It was also observed that the Al₂O₃ phase was not thermodynamically stable during ball milling with copper powders. Finally, it was found that the presence of nanoparticles considerably reduce the average size of metallic powder particles.

Using the results of differential scanning calorimetry and X-ray diffractometry, an analysis has been carried out of the initial stages of the eutectic and primary mechanisms of crystallization of a series of metallic glasses based on Fe and Al with the established temperature dependences of the effective diffusion coefficients. Analytical relationships, which relate the volume density of crystallites formed in the glasses at the temperatures of the onset of crystallization with the values of the effective diffusion coefficients at these temperatures have been proposed. It has been established that, in the glasses, the crystallization of which begins at the lower boundary of the threshold values of the effective diffusion coefficients (~10^–20 m²/s), structures are formed with the volume density of crystallites on the order of 10²³–10²⁴ m^–3 and, at the upper boundary (10^–18 m²/s), of the order of 10¹⁸ and 10²⁰ m^–3 in the glasses that are crystallized via the eutectic and primary mechanisms, respectively. Good agreement between the calculated and experimental estimates indicates that the threshold values of the effective diffusion coefficients are the main factors that determine the structure of glasses at the initial stages of crystallization.

Methods of the X-ray diffraction analysis and electron microscopy were used to study changes in the structural phase state and mechanical properties of bulk-nitrided 08Kh17T steel (0.08 wt % C, 17 wt % Cr, 0.8 wt % Ti, 0.5 wt % Si, 0.8 wt % Mn, 0.025 wt % S, and 0.035 wt % P) upon tempering in the temperature range of 400–700°C. The changes in the mechanical properties of the nitrided steel upon tempering are associated with the predominance of either the solid-solution or precipitation strengthening, i.e., with the presence of martensite in the steel structure at low temperatures of tempering and the precipitation of particles of Cr₂N nitrides of different dispersity upon increasing the tempering temperature. The greatest increase in the ultimate tensile strength and yield stress (1.8–2.5 times) at a satisfactory plasticity (no less than 10%) of the bulk-nitrided steel is achieved by tempering bulk-nitrided steel in a temperature range of 600–700°C.

A thermodynamic simulation of phase equilibria in the Cu–Ni–Si–Cr system has been carried out using a FactSage program package (version 7.0). The structure and phase composition of the Cu–2.6Ni–0.6Si–0.6Cr bronze have been studied in the quenched and additionally aged (under various conditions) states. Precipitates of the chromium silicide Cr₃Si have been revealed in the alloy both in the quenched and aged states. Particles of nickel silicide Ni₂Si 0.05–0.5 μm in size appear upon the precipitation hardening both homogeneously and heterogeneously (on particles of previously formed chromium silicides). The microhardness (HV) and the electrical resistivity of the experimental samples have been determined after conducting processes of precipitation strengthening.

Conditions have been considered for the formation of autowaves of localized plastic flow in the deformed metals upon the propagation of Lüders bands in the case of the Portevin–Le Chatelier effect, and upon the formation of a neck with taking into account the differences in the microscopic mechanisms of plastic deformation in the case of these phenomena. The laws that govern the development of the localized plastic flow of metals and the role of these laws in the development of the above effects have been investigated. It has been established that the main features of the deformation characteristic of these phenomena are determined by the differences in the properties of the active media that are formed in the material upon plastic deformation. The mechanisms of the generation of different autowave modes of the localized plastic flow upon the Lüders deformation, Portevin–Le Chatelier effect, and the formation of a neck in active media of different nature during deformation have been considered.