Vol 118, No 3 (2017)

Results of investigations of the stability of a structural Nb–Cr alloy subjected to cyclic mechanical loading have been presented. It has been established that, upon deformation, the alloy shows a structural instability that manifests in the form of a local cyclic microinhomogeneous redistribution of elements depending on the applied external loads and, as a process for the refinement and growth of crystallites. Based on these phenomena, the oscillating character of significant internal structural stresses has been explained, which are changed in the region that is limited by the strengthening–softening curves and characterizes the cyclic strength of the alloy.

The effect of the types and conditions of heat treatment on the element composition, microstructure, phase composition, kinetics and temperatures of phase transformations, crystal lattice and substructure parameters, and mechanical characteristics of the shape-memory 45Ti–45Ni–10Nb (at %) alloy in the ascast and extruded states was studied. The interrelation between the structure features of the alloy and its mechanical characteristics was found. The conditions of heat treatment and the state of the alloy, which assure its high mechanical characteristics, were determined.

In this work, the magnetic contribution to the isothermal entropy change ΔS upon switching on a magnetic field has been investigated in correlated metallic ferromagnets within the Hubbard nondegenerate model. The analytical expression ΔS for obtained in the mean-field approximation depends substantially on the electronic structure (density of electron states), which presents new ways to increase the absolute value of ΔS relative to the known result obtained within the Heisenberg model. The temperature dependence of ΔS has been calculated at different values of the Coulomb interaction U and the number of electrons n for the Bethe infinite-dimensional lattice and square lattice with allowance for transfer integrals in the first (t) and the second (t') coordination shells. It has been found that the presence of Van Hove singularities in the electronic spectrum near the Fermi level makes it possible to considerably increase |ΔS| at a fixed magnetic field. The possibility of first-order magnetic phase transitions depending on the model parameters has been analyzed.

The microstructure and the phase composition of a heat-resistant Fe–Cr–Ni alloy (0. 45C–25Cr–35Ni) has been investigated in the cast state and after annealing at 1150°C for 2–100 h. After a 2-h high-temperature annealing, the fragmentation of the crystal structure of the eutectic M₇C₃ carbides into domains of ~500 nm in size with a partial transition into M₂₃C₆ carbides is observed. After a 100-h holding, the complete transition of the hexagonal M₇C₃ carbides into M₂₃C₆ with a face-centered cubic structure occurs. The carbide transition M₇C₃ →M₂₃ can be considered to be an in situ transformation.

Scanning tunnel microscopy is used to directly estimate the relative energy of internal interfaces in ultrafine-grained copper after equal-channel angular pressing, followed by rolling. Estimates of the boundary energy for as-prepared samples and samples annealed at different temperatures indicate that ultrafinegrained copper has nonequilibrium boundaries with energy higher than that in coarse-grained copper. The cumulative distribution functions of the relative boundary energy in the grain–subgrain structure allow the qualitative estimation of the redistribution of excess energy between the boundaries of various types during structure evolution upon annealing. Differences between the behaviors of the cumulative distribution functions of relative boundary energy in ultrafine-grained copper and nickel produced by the same technology are revealed. These differences are related to the characteristics of the structure formation of two metals during severe plastic deformation and its subsequent evolution during annealing, which depend on the stacking fault energy and the melting point of copper and nickel.

Initial single-crystal nickel deformed by shear under pressure at the temperature of 150°C has been studied. It has been found that, under these conditions, dynamic recrystallization develops in nickel. As a result, after true strain in the range of 4 < е < 9, a heterogeneous structure that consist of recrystallized grains of different defectiveness and microcrystallites is formed. Calorimetric studies have shown that the stored energy varies nonmonotonically with increasing true strain, which is associated with the cyclic character of dynamic recrystallization. In the calorimetric dependence, several peaks of heat release have been revealed that are connected with the nonsimultaneous occurrence of static recrystallization upon heating in nickel with a heterogeneous structure formed upon dynamic recrystallization.

The changes in the structure, phase composition, and in the microhardness after heating in the temperature range of 600–1000°C and subsequent quenching in a Ti–6Al–4V–ELI alloy preliminarily subjected to equal-channel angular pressing at 600°C have been studied by methods of structural analysis, X-ray diffraction analysis, and microdurometry. The presence of the α₂ phase in the deformed alloy has been detected. The diagram of the change in the phase composition of the alloy upon quenching was constructed. The interrelation between the character of structural and phase transformations, and the level of the microhardness of the alloy after quenching from different temperatures has been found.