Vol 120, No 4 (2019)

The crystalline and magnetic structure of the nonstoichiometric intermetallic compound Tm₂Fe₁₈ has been investigated in the temperature interval from 3 to 300 K using the methods of neutron diffraction, synchrotron radiation, and ultra-small-angle neutron scattering. A theoretical interpretation of the spin-reorientation transition has been given and the value and temperature dependence of the first constants of magnetic anisotropy for the sublattices of iron and thulium have been determined. Based on the results of experiments and theoretical interpretation of small-angle neutron scattering, a conclusion has been made on a reconstruction of the domain structure upon spin-reorientation phase transition.

The crystal structure and magnetotransport properties of stoichiometric and anion-deficient Ba²⁺‑alloyed cobaltites with a perovskite structure have been studied. It was shown that the development of ferromagnetic state is related to the presence of cobalt ions mainly with single e_g electron (intermediate spin IS state), whereas the existence of two e_g electrons (high-spin HS state) leads to the antiferromagnetic state. The covalent component of the chemical bond stabilizes an electron configuration that is close to the intermediate spin state. The colossal magnetoresistance appears at either concentration or temperature boundary corresponding to the coexistence of ferromagnetic and antiferromagnetic phases or clusters and results from the filed-induced spin transition from the HS/LS mixture to the IS state. Cobalt ions in the IS state are responsible for the ferromagnetism and high resistivity.

Mössbauer studies of the anisotropy of hyperfine interactions (HFIs) of the ⁵⁷Fe nuclei in the quasi-binary intermetallic system Zr_{1 –x}Sc_xFe₂ (0 ≤ x ≤ 1) with the structure of the cubic Laves phase of the C15 type have been performed. Within the framework of the tensor description of the anisotropy of magnetic HFIs, taking the crystal and magnetic structure into account, the hyperfine parameters of the partial spectra have been expressed through the constant of the quadrupole interaction, the isotropic and anisotropic magnetic fields, and also the azimuthal and polar angles, which specify the orientation of the direction of the easy axis of magnetization (EA). It has been established that at temperatures of 87 and 297 K the EA deviates in the plane \((1\bar {1}0)\) from the crystallographic direction [111] by ~15°, and with an increase in the concentration x, the orientation of the EA almost does not change. The changes in the isotropic field and in the shift of the Mössbauer line have an anomalous character. With an increase in x, an increase can first be observed, and then, at x > 0.4, a decrease of the magnitude of the isotropic field can be observed. The shift of the Mössbauer line decreases with an increase in x; at x ~ 0.4, a bent is observed. This anomalous behavior of the isotropic field and of the shift correlates with a change in the lattice parameter and in the magnetic moment of the Fe atoms. The anisotropic field decreases linearly with an increase in the concentration of Sc atoms, while the constant of the quadrupole interaction almost does not change, ~1 mm/s.

The precipitation kinetics of overlapping process from nucleation and growth to coarsening of the γ′(Ni₃Al) phase in Ni–Al alloys are investigated quantitatively by using the interface diffusion-controlled phase field model. It is found that the time exponent of average particles radius of the γ′ phase is about 1/3 at the growth and coarsening stage, while the exponent is smaller than 1/3 at the later steady-state coarsening stage. The decrease rate of the number density of the γ′ phase is larger at the steady-state coarsening than that of the growth and coarsening stage. The particle size distribution (PSD) is widened at the nucleation and growth stage, then is narrowed at the growth and coarsening stage, and becomes wide again at the steady-state coarsening stage; the width of PSDs obtained by the quantitative calculation is greater than 0.215 proposed by the LSW theory. Moreover, the position of the PSDs peak moves from 1.0 of the normalized radius at the nucleation and growth stage to less than 1.0 of the growth and coarsening stage, and then moves back to 1.0 at steady-state coarsening stage. The non-monotonically variation of kinetics of the γ′ phase during the precipitation process are reasonable and theoretically significant for the kinetics evolution.

Ta–Be solid-solution–alloys have been obtained for the first time by ion-plasma sputtering and co-deposition of ultradispersed Ta and Be particles in the range of concentrations of Be from 1.1 to 85.8 at % in the film, which is a confirmation of the thermofluctuation melting and coalescence of small particles. A regular character of changes of the unit-cell volume of the solid solution of Be in β-Ta has been established. Upon the vacuum annealing at 800°C of a sample with Be concentration at a level of 26.8 at %, there was a new phase, identified as Ta₃Be, with a tetragonal face-centered lattice with p arameters a = 0.7579 ± 0.0003 nm and c = 0.7370 ± 0.0005 nm. The X-ray diffraction data for the identification of this intermetallic compound are given. The performed electron-microscopic investigations have shown that the thermal expansion coefficient of the Ta₃Be phase is greater than that of the solid solution of Be in β-Ta. The study of the temperature dependence of the electrical resistance of the Ta₃Be phase upon cooling from room temperature to 10 K has shown that this phase has a metallic character of the R(T) dependence. The Ta₃Be phase does not possess superconducting properties down to 10 K.

The effect of the time and energy parameters of pulsed laser treatment on the structure formation and hardness of high-speed steel after annealing upon surface melting has been studied. The results of the metallographic analysis and energy-dispersive X-ray microanalysis of the steel, as well as of the microhardness measurements, have been discussed. It has been shown that the laser-induced remelting of the annealed high-speed steel causes a significant refinement of both the solid solution and of the carbide constituent. The structure of the laser-affected zone and the morphology of dendrites depend on the laser-irradiation regimes.

In this research, hot deformation behavior of NiTiNb alloy was explored. Then, the activation energy and the constitutive equation were evaluated and obtained. The results show that the compressive process of TiNiNb alloy is a typical rheological process. The thermal activation energy of TiNiNb alloy in the temperature range of 720–840°C is 198.004 kJ/mol. Both NiTi matrix phase and Nb solution have been refined. The amounts of (Ti, Nb)₂Ni hard brittle phase and Nb solution were gradually reduced with increasing the strain rate and deformation temperature.

The process of obtaining a gradient structure by thermomechanical treatment (TMT) has been demonstrated using the example of a model disk workpiece made of the powder-metallurgy (PM) EP741NP nickel-based superalloy. The TMT included the main deformation leading to the development of recrystallization and the formation of a fine-grained microstructure, heat treatment in a gradient temperature field, additional deformation, and final strengthening heat treatment. In the disk workpiece after TMT, a gradient structure was obtained: a “necklace” structure in the peripheral part and a fine-grained “microduplex” structure in the center. The estimation of mechanical properties of samples cut out at different distances from the axis of symmetry of the disk workpiece showed that the central part of the disk is characterized by enhanced strength and ductility, and the peripheral part possesses enhanced high-temperature strength and impact toughness. The obtained results indicate that the approach used is promising for achieving a gradient structure and gradient mechanical properties in disks made of PM nickel based superalloys.