Vol 126, No 6 (2025)

The ⁵⁷Fe NMR spectra and relaxation times in θ-Fe₃C cementite in the region of long-range magnetic order were obtained and analyzed. The values of hyperfine magnetic fields for two nonequivalent positions of iron atoms in the structure (Pnma) of this compound and their evolution in the temperature range from 4.2 to 350 K were determined. From the analysis of the ⁵⁷Fe Mossbauer spectra obtained at T = 295 K, the components of the electric field gradient tensor and the values of hyperfine fields were determined.

Currently, projects of large-scale magnetic systems based on second-generation high-temperature superconductors (HTS) are actively being developed all over the world. Significant mechanical stresses occur during the production and operation of conductive elements, which can adversely affect the conductivity. In this study, an analysis was conducted on the changes in conductivity of HTS tapes with various architectures after applying compressive mechanical loads at room temperature. The HTS tapes produced at S-Innovations and those manufactured on the Kurchatov Institute's production line were used for investigation. It was shown that additional coating of HTS tape with POS-61 solder (Sn61/Pb39) lead to a decrease in conductivity at lower compressive mechanical loads compared to non-coated samples. It was also found that compressive loads of 50 MPa led to a significant increase in longitudinal inhomogeneity and local increase in critical current above values in original tape. Comparative analysis of critical currents obtained using transport method and calculated from Hall magnetometry data was carried out. Results obtained by these complementary methods were consistent for all samples if exponential form of local volt-ampere characteristic was assumed.

Computer simulation of phase transitions in a three-state antiferromagnetic Potts model on a simple cubic lattice is carried out. Using the Monte Carlo method, the temperature dependences of the order parameter m_AF, the susceptibility χ, and the heat capacity C. Systems with linear dimensions L×L×L=N, L=10–50 are considered. Using histogram data analysis and the fourth-order Binder cumulant method it is shown that a second-order phase transition occurs in the Potts model under consideration. The critical temperature T_c has been determined with good accuracy.

Experiments were performed on the convergence of cylindrical shells made of austenitic steel 12H18N10T under the action of an explosion. The initiation of the explosive layer surrounding the shell was carried out from eight points evenly distributed on the cylindrical surface. It was established that the convergence process consists of the division of the shells into similar fragments and their movement toward the shell axis. The study of the microstructure showed that shear bands are formed, which create teeth when they reach the surface. Comparison of the processes of convergence of steel shells and previously studied copper ones showed that there are two different mechanisms of convergence: solid-state and hydrodynamic. The deformation criterion for the onset of instability was determined and the size of the instability region was estimated

The structure of Cu–Ni alloys processed by high-pressure torsion (HPT) by 5 revolutions at room temperature and its thermal stability under subsequent annealing have been studied using transmission electron microscopy and microhardness measurements. The influence of three main factors on the structure and microhardness under deformation and annealing depending on the composition has been revealed: the changes in homologous temperature and stacking fault energies (SFE) and the effect of solid solution formation. The combination of these factors results in the non-monotonic change in the structure obtained by HPT (crystallite sizes and microhardness) and its thermal stability depending on the alloy composition. In copper-rich alloys, a decrease in the homologous temperature with an increase in Ni content plays a decisive role in the refinement of the structure, strengthening and thermal stability. When Ni is alloyed with Cu, the structure and its thermal stability do not change as significantly, since the increase in homologous temperature and the decrease in SFE compensate each other, but due to the effect of solid solution formation in alloys with a nearly equiatomic composition, the most dispersed and thermally stable structure is formed.

Using the example of metallic nickel, the possibility of synthesizing periodic carbon structures by chemical vapor deposition (CVD) without applying an additional catalyst to the surface of metals that are catalytically active in hydrocarbon pyrolysis processes has been experimentally demonstrated. Planar and non-planar structures of sub-millimeter (~500×500 μm²) scale have been obtained on polycrystalline nickel substrates. The surface structuring of nickel substrates was performed by local modification of the surface catalytic properties, as well as by forming the surface relief by the method of plastic surface deformation using a template. For planar structures, the modification of the surface properties was carried out by ion bombardment through a mask. For non-planar structures, the local activation of the passivated surface was carried out by mechanical grinding-polishing. The obtained structures contain two forms of carbon: turbo-substrate graphite and carbon nanotubes. It has been established that both forms of carbon can be produced in a single CVD process.

A model of voids formation during plastic deformation is proposed, based on the concept of crack formation in the field of elastic stresses of a negative disclination and its subsequent blunting due to accommodative plastic deformation. A detailed analysis of the main characteristics of a disclination crack is carried out. Analytical expressions are obtained for the stress intensity factors at its tips, the distribution of the Burgers vector density and the profile of its opening depending on the material parameters and disclination characteristics. The stability conditions of a symmetric microcrack with a disclination located in its center are investigated. For this configuration of defects, the dependence of the value of plastic crack opening on the disclination strength, the elastic field screening radius and the mechanical constants of the material is obtained.