Том 117, № 8 (2016)

The generalized Thomson formula T_m = T_m^(∞)(1-δ)R for the melting point of small objects T_m has been analyzed from the viewpoint of the thermodynamic theory of similarity, where R is the radius of the particle and T_m^(∞) is the melting point of the corresponding large crystal. According to this formula, the parameter δ corresponds to the value of the radius of the T_m(R^-1) particle obtained by the linear extrapolation of the dependence to the melting point of the particle equal to 0 K. It has been shown that δ = αδ₀, where α is the factor of the asphericity of the particle (shape factor). In turn, the redefined characteristic length δ₀ is expressed through the interphase tension σ_sl at the boundary of the crystal with its own melt, the specific volume of the solid phase v_s and the macroscopic value of the heat of fusion λ_∞:δ₀ = 2σ_slv_s/λ_∞. If we go from the reduced radius of the particle R/δ to the redefined reduced radius R/r₁ or R/d, where r₁ is the radius of the first coordination shell and d ≈r₁ is the effective atomic diameter, then the simplex δ/r₁ or δ/d will play the role of the characteristic criterion of thermodynamic similarity. At a given value of α, this role will be played by the simplex Estimates of the parameters δ₀ and δ₀/d have been carried out for ten metals with different lattice types. It has been shown that the values of the characteristic length δ₀ are close to 1 nm and that the simplex δ₀/d is close to unity. In turn, the calculated values of the parameter δ agree on the order of magnitude with existing experimental data.

A modified Davidenkov hysteresis equation of the state has been proposed for describing the saturation of the effects of amplitude-dependent internal friction in polycrystalline metals and other solids, which possess imperfect elasticity. Using this equation, an exact analytical solution of the problem of the propagation of a periodic sawtooth wave in media characterized by quadratic hysteresis with nonlinear loss saturation has been obtained. Regularities of variations in the characteristics of a sawtooth wave, such as nonlinear loss, the change in the velocity of the propagation of the wave, and the amplitudes of the higher harmonics of the wave, have been determined. A graphical analysis of the evolution of the shape and the spectral components of the wave has been carried out.

A synthesized MgB₂ superconductor has been investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and by the measurements of the superconducting characteristics and microhardness after cold high-pressure deformation in a Toroid chamber and in Bridgman anvils and subsequent high-temperature annealing. A nanocrystalline structure is formed in the superconductor after high-pressure treatment, but internal cracks appear, and the critical current density decreases strongly. The annealing leads to a coarsening of the structure and to an increase in the critical current density up to 5.8–6.7 × 10⁴ А/сm², which is more than three times greater than that in the initial state.

The Al–5% Cu alloy-based metal-matrix composite materials reinforced with 5-μm B₄C particles have been produced using mechanical mixing-in method. A process of addition of the B₄C particles into the melt has been developed. A homogeneous distribution of the B₄C reinforcing particles in the metal-matrix composite matrix was obtained. Using X-ray diffraction analysis, the formation of Al₃BC and AlB₂ phases has been revealed at the interphase matrix/particle boundary, which indicates a good interaction in the phases. With increasing B₄C content in the matrix alloy, an insignificant increase in the porosity (from 1 to 3.1%) occurs. The average linear thermal-expansion coefficient is reduced from 24.5 to 22.6 × 10^–6 K^–1 in the temperature range of 20–100°C.

Mechanical alloying (MA) was employed to synthesize Al–Zn–Mg–Cu alloys of high weight percentage of the nickel component from the elemental powders of constituents via high-energy ball milling. The mixed powders underwent 15 h of milling time at 350 rpm speed and 10: 1 balls/powder weight ratio. The samples were cold-compacted and sintered thereafter. The sintered compacts underwent homogenization treatments at various temperatures conditions and were aged at 120°C for 24 h (T6). The milled powders and heat-treated Al alloy products were characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The crystallite sizes and microstrains of the alloyed powder were estimated via measuring the broadening of XRD peaks using the Williamson–Hall equation. The results have revealed that optimum MA time of 15 h has led to the formation of Al-based solid solutions of Zn, Mg, Cu, and Ni. The outcomes showed that the Vickers hardness of the sintered Al–Zn–Mg–Cu compacts of Ni alloys was enhanced following aging at T6 tempering treatments. Higher compression strength of Al-alloys with the addition of 15% nickel was obtained next to the aging treatment.

Effect of the size of the elements of the mixed structure of B2 austenite, which consists of nanosized grains and subgrains of a polygonized substructure, on the functional properties of the Ti–50.7 at % Ni alloy preliminarily subjected to a low-temperature thermomechanical treatment (LTMT) and post-deformation annealing (PDA), has been investigated. The generation of the shape-memory effect (SME) and reversible two-way SME (TWSME) was performed using bending deformation. A maximum (for the Ti–Ni alloys) value of the recovery strain ε_r = 15.5 ± 0.5% has been obtained after annealing at 600°C for 1 h (recrystallized structure) and after LTMT + PDA at 430°C for 10 h (mixed nanocrystalline and nanosubgrain structure). The behavior of the parameters of the SME and TWSME in different structural states has been considered. A comparative study of the effect of the temperature and time of holding at a temperature upon the PDA on the formation of the microstructure and submicrostructure of the B2 austenite has been performed.

Temper-resistant low-carbon Cr–Mn–Ni–Мо–V–Nb steels with concentrations of carbon of 0.15 and 0.27 wt % have been studied. It has been shown that, upon quenching, various morphological types of the α phase can be formed. The structure of the steels is stable in the course of heating below critical temperatures and remains a lath-type structure in the intercritical temperature range. Specific features of structural and phase transformations, as well as the dependence of the mechanical characteristics of the steels, on the tempering temperature have been determined.

It has been shown that, in metastable austenitic Fe–18Cr–10Ni–Ti steel, under conditions of torsion under pressure, local reversible (forward plus reverse) (γ → α′ → γ) martensitic transformations can occur, which are one of the mechanisms of the formation of nanostructured states. An increase in the rotation rate, which leads to an increase in the deformation temperature, stimulates the reverse (α′ → γ) transformation. The evolution of the structural and phase states is represented as the following sequence: (1) mechanical twinning; (2) nucleation of martensitic plates in the microtwinned structure of the austenite with the formation of two-phase (γ + α′) structures, packet α′ martensite, and structural states with a high curvature of the crystal lattice; (3) reverse (α′ → γ)-transformations; and (4) the fragmentation of nanosized crystals via the formation of a nanotwinned structure in the austenite and of a nanoscale banded structure of the ε martensite in the α′ martensite.