Vol 118, No 7 (2017)

Using numerical computer simulation, the behavior of disordered dislocation systems under the action of monochromatic standing sound wave has been investigated in the grain of the model two-dimensional polycrystal containing nonequilibrium grain boundaries. It has been found that the presence of grain boundaries markedly affects the behavior of dislocations. The relaxation process and changes in the level of internal stresses caused by the rearrangement of the dislocation structure due to the ultrasonic action have been studied.

The methods of transmission and reflection electron diffraction have been used to investigate the structure of Fe₃O₄ films depending on the temperature of their synthesis on an Si substrate coated with an ultrathin layer of SiO₂. The thus-grown polycrystalline films of magnetite had a texture, the axis of which was perpendicular to the surface of the SiO₂ film. It has been revealed that, with an increase in the growth temperature, a structural rearrangement occurs which is characterized by an increase in the volume fraction of grains with the preferred (311) orientation. A study of the magnetotransport properties of the films has shown that the magnitude of their magnetoresistance increases with an increase in the temperature of their synthesis. It has been established that in the Fe₃O₄/SiO₂/Si system with a tunneling-thin layer of SiO₂ the magnetoresistance decreases as a result of the flow of an electric current through the silicon substrate.

Using numerical simulation in the ProCAST program complex, the conditions of the solidification of heat-resistant nickel alloy in curvilinear channels of a ceramic mold have been investigated. It has been shown that, in practically important cases, the vector of the temperature gradient is oriented along the axis of the curvilinear channel. In a spiral crystal selector, a cyclic change in the preferred direction of growth occurs because of the cyclic change in the direction of the vector of the temperature gradient. The fact that the vector of the temperature gradient is almost always directed along the axis of the curvilinear channel makes it possible to govern the orientation of the vector of the temperature gradient in space and, therefore, to obtain a grain with the preferred crystallographic orientation. Based on the results of this investigation, a method of the grain selection with a desired azimuthal orientation is proposed.

The effect of hydrogenation at 150 and 200°С on the structure and magnetic susceptibility of YBa₂Cu₃O_y (123) with different hydrogen contents has been studied. Upon the incorporation of hydrogen, the phase transition of the 123 phase into a defect tetragonal 124 phase occurs. In contrast to the transition upon hydration, the phase transition upon hydrogenation takes place only for compounds characterized by high oxygen contents (y > 6.5). Depending on the compound structure and oxygen content, hydrogen atoms can both occupy interstitials in Cu–O planes to form HYBa₂Cu₃O₆ and join with oxygen to form an oxide–hydroxide. In contrast to hydration, upon hydrogenation of YBa₂Cu₃O_y, the substitution of europium for yttrium and alloying with cerium and zirconium oxides do not block the intercalation of hydrogen into the structure of the 123 compound.

X-ray diffraction, Mössbauer spectroscopy, and magnetic measurements have been used to study the phase formation in (Fe_1–xNi_x) (with х = 0–0.20) nanocomposites upon severe plastic deformation in a planetary ball mill and subsequent annealings. It has been shown that the mechanical synthesis results in the formation of mainly a nickel-alloyed nanocrystalline (Fe, Ni)₃C cementite with a distorted crystal lattice and an amorphous Fe–Ni–C phase. During heating above 300°С, the amorphous phase crystallizes with the formation of cementite, which is characterized by a higher Ni content compared to that in mechanically synthesized cementite. The mechanically synthesized samples exhibit low coercive force (equal to several tens ampere per centimeter). In the course of annealing at temperatures of up to 500–550°С, crystal lattice distortions are removed; this results in reliving the magnetic anisotropy constant and high-coercivity state of cementite. At the same time, Ni-rich cementite areas decompose with the formation of γ-(Fe, Ni, C) phase (austenite); as a result, the average nickel content in the cementite substantially decreases. Annealings at higher temperatures cause the complete decomposition of cementite and lead to an abrupt decrease in the coercive force (Н_с) of samples. Alloying with nickel leads to an increase in the Curie temperature of cementite and a decrease in its specific saturation magnetization, coercive force, and thermal stability.

The analytical dependences of the stress-state characteristics of the single-component nanocrystalline-materials on the magnitude of the excess volume have been determined. It has been shown that local fluctuations in the internal stresses of these materials can significantly exceed the values of the macroscopic ultimate strength.

The scope of this paper is to evaluate and discuss the flow phenomena of Al–2Si and Al–12Si alloys, which were joined through friction stir welding processes. The feed rate of 50mm/min and tool rotation of 600, 900, and 1200 rpm were selected for the processes. There was much change in the surface morphology with smoothness for Al–2Si alloy and roughness for Al–12 Si alloys. The microstructure studies on the weld zone were performed. The mechanical properties like tensile strength and hardness are discussed in detail.