Volume 118, Nº 4 (2017)

Despite of the appearance of numerous new materials, the iron based alloys and steels continue to play an essential role in modern technology. The properties of a steel are determined by its structural state (ferrite, cementite, pearlite, bainite, martensite, and their combination) that is formed under thermal treatment as a result of the shear lattice reconstruction γ (fcc) → α (bcc) and carbon diffusion redistribution. We present a review on a recent progress in the development of a quantitative theory of the phase transformations and microstructure formation in steel that is based on an ab initio parameterization of the Ginzburg–Landau free energy functional. The results of computer modeling describe the regular change of transformation scenario under cooling from ferritic (nucleation and diffusion-controlled growth of the α phase) to martensitic (the shear lattice instability γ → α). It has been shown that the increase in short-range magnetic order with decreasing the temperature plays a key role in the change of transformation scenarios. Phase-field modeling in the framework of a discussed approach demonstrates the typical transformation patterns.

In terms of the dynamic theory, one of the possible scenarios of the formation of transformation twins in crystals of α martensite of thin-plate morphology characteristic of Fe–Ni–C alloys with low temperatures of the onset of the γ–α martensitic transformation has been discussed. It has been shown that, in the case of matched velocities of the propagation of relatively short s waves and relatively long l waves, the attenuation of s waves leads to the fragmentation of the twin structure with a monotonic decrease in the fraction of the main component of the twin structure inside fragments.

A micromagnetic investigation of the dynamics of two dipole-coupled magnetic vortices in a magnetic tunnel nanocolumn under the action of an external magnetic field directed perpendicularly to the plane of the sample and of a spin-polarized electric current has been carried out. Three regimes of motion of the vortices have been shown to exist that differ in critical values of the current. The dependence of the magneticfield strength that separately switches the polarity of the cores of the vortices depending on the density of the spin-polarized current has been found. The possibility of controlling the frequency of the stationary motion of the vortices and of the fine adjustment of the amplitude of the controlling currents using an external magnetic field has been suggested.

Variations in magnetic properties of the heavy-fermion YbNi₂ alloy when milled in a high energy ball milling system have been investigated. The ferromagnetic transition (T_C = 10.4 K) in the initial sample almost vanishes after milling, which leads to the appearance of a magnetic transition at T* = 3.2 K in nanocrystallites. Before milling, processes of spin–lattice relaxation of the Orbach–Aminov type with the participation of the first excited Stark sublevel of the Yb³⁺ ion located at 75 K are dominating in the electron spin dynamics in the paramagnetic phase of the alloy. A comparative study of the temperature dependence of the magnetic properties and spectra of electron paramagnetic resonance in poly- and nanocrystalline samples indicates the existence of a magnetic inhomogeneity of the compound arising upon milling.

The composition of an Al–Cu–Mg ternary eutectic alloy was chosen to be Al–30 wt% Cu–6 wt % Mg to have the Al₂Cu and Al₂CuMg solid phases within an aluminum matrix (α-Al) after its solidification from the melt. The alloy Al–30 wt % Cu–6 wt % Mg was directionally solidified at a constant temperature gradient (G = 8.55 K/mm) with different growth rates V, from 9.43 to 173.3 μm/s, by using a Bridgman-type furnace. The lamellar eutectic spacings (λ_E) were measured from transverse sections of the samples. The functional dependencies of lamellar spacings λ_E (\({\lambda _{A{l_2}CuMg}}\) and \({\lambda _{A{l_2}Cu}}\) in μm), microhardness H_V (in kg/mm²), tensile strength σ_T (in MPa), and electrical resistivity ρ (in Ω m) on the growth rate V (in μm/s) were obtained as \({\lambda _{A{l_2}CuMg}} = 3.05{V^{ - 0.31}}\), \({\lambda _{A{l_2}Cu}} = 6.35{V^{ - 0.35}}\), \({H_V} = 308.3{\left( V \right)^{ - 0.33}}\); σ_T= 408.6(V)^0.14, and ρ = 28.82 × 10^–8(V)^0.11, respectively for the Al–Cu–Mg eutectic alloy. The bulk growth rates were determined as \(\lambda _{A{l_2}CuMg}^2V = 93.2\) and \(\lambda _{A{l_2}Cu}^2V = 195.76\) by using the measured values of \({\lambda _{A{l_2}CuMg}}\), \({\lambda _{A{l_2}Cu}}\) and V. A comparison of present results was also made with the previous similar experimental results.

Laws of the formation of substructure and of changes in the hardness and in the mechanical properties have been established for sheets of 1545K alloy obtained by tension according to different technologies at various accumulated strains. With an increase in cold deformation (e_cold) from 0 to 2.64, the yield stress of cold-worked sheets increases from 355 to 466 МPа and the relative elongation decreases insignificantly from 4 to 3.5%. The maximum strength with σ_0.2 = 410 МPа, σ_u = 460 МPа, and δ = 6.5% is provided by annealing at 150°C for 1 h of the sheets obtained via the technology with the maximum fraction of cold deformation (e_cold = 2.64). After annealing at 300°C for 30 min, a twofold increase in the plasticity is observed without a significant reduction in the strength characteristics a follows: σ_0.2 = 385 МPа, σ_u = 436 МPа and δ = 13%. It has been shown that the level of mechanical properties is determined by the substructure that is formed inside deformed grains during annealing.