Physics of Metals and Metallography (v.118, #8)

The first-principles calculations of thermodynamic characteristics of bcc and sc structures of Ca1–x Sr x solid solutions have been carried out. Taking into account insufficient accuracy of such calculations, for the description of phase transitions, the known experimental data on bcc and sc structures of calcium and strontium have been used to determine parameters for the calculation of Ca1–x Sr x properties using linear interpolation. The possibility of the occurrence of bcc–sc structural phase transitions in Ca1–x Sr x (x = 0.0625, 0.125, 0.25, 0.5, 0.75) solid solutions and their characteristics under different pressures have been investigated.
Keywords: structural phase transitions; Ca–Sr alloys

Specific features of magnetooptical images (MOIs) of stray fields near the faces of prismatic hard magnetic elements have been studied. Attention has primarily been focused on MOIs of fields near faces oriented perpendicular to the magnetic moment of hard magnetic elements. With regard to the polar sensitivity, MOIs have practically uniform brightness and geometrically they coincide with the figures of the bases of the elements. With regard to longitudinal sensitivity, MOIs consist of several sectors, the number of which is determined by the number of angles of the image. Each angle is divided by the bisectrix into two sectors of different brightnesses; therefore, the MOI of a triangular magnet consists of three sectors. A rectangle consists of four sectors separated by the bisectrices of the interior angles. In all types of figures, these lines converge at the center of the figure and form a singular point of the source or sink type.
Keywords: inhomogeneous magnetic field; magnetooptical images; intensity; singular points; magnetically hard elements

The influence of the double heat treatment (T = 300 and 930°С) on the critical parameters of highly textured YBa2Cu3O6.96 and YBa2Cu3O6.8 ceramics has been investigated. It has been shown that, upon low-temperature annealing in humid air, planar stacking faults are formed in these ceramics. These defects are partly retained after reduction annealing (at T = 930°С) and are efficient pinning centers in magnetic fields applied parallel and perpendicular to the c axis. Due to the absorption of water, the oxygen content is increased in the ceramics, which is accompanied by an increase in the critical temperature of superconducting transition up to 94 K for YBa2Cu3O6.96 and up to 90 K for YBa2Cu3O6.8. Optimal conditions of the double annealing have been established, after which the critical-current density increased to j c ≥ 104 А/сm2 in an external magnetic field of up to 6 T. The low-temperature treatment in the neutral atmosphere saturated by water vapors deteriorates the current-carrying capacity of the highly textured ceramics, which is connected with the disappearance of texture due to the copper reduction and the precipitation of impurity phases.
Keywords: HTSC; electrophysical properties

Microstructure and morphological study of ball-milled metal matrix nanocomposites by Y. Afkham; R. Azari Khosroshahi; R. Kheirifard; R. Taherzadeh Mousavian; D. Brabazon (749-758).
Due to the difficulty of preparation and beneficial properties achievable, copper and iron matrix nanocomposites are materials for which fabrication via the powder metallurgy route is attracting increasing research interest. The presence of ceramic nanoparticles in their matrix can lead to considerable changes in the microstructure and morphology. The effects of the type of metallic matrix and ceramic nanoparticle on the distribution of nano reinforcements and the morphology of ball-milled composite powders were evaluated in this study. For this purpose, 25 wt % of Al2O3 and SiC nanoparticles were separately ball-milled in the presence of iron and copper metals. The SEM, FESEM, and XRD results indicated that as-received nanoparticles, which were agglomerated before milling, were partially separated and embedded in the matrix of both the metals after the initial stages of ball milling, while prolonged milling was not found to further affect the distribution of nanoparticles. It was also observed that the Al2O3 phase was not thermodynamically stable during ball milling with copper powders. Finally, it was found that the presence of nanoparticles considerably reduce the average size of metallic powder particles.
Keywords: ball milling; metal matrix nanocomposite; Al2O3 ; SiC

Structural transformations and properties of titanium–aluminum composite during heat treatment by L. B. Pervukhin; D. B. Kryukov; A. O. Krivenkov; S. N. Chugunov (759-763).
The link between the parameters of heat treatment of a layered titanium–aluminum composite material obtained by explosive welding with the formation of intermetallic compounds in it has been analyzed. The results of measurements of the microhardness of the composite and the thickness of the interlayer of the intermetallic phase obtained using different regimes of heat treatment have been discussed. Special attention has been paid to estimating the composition of the intermetallic phase in the composite prepared by explosive welding.
Keywords: composite; heat treatment; intermetallic compound; strength; phase composition; microhardness

Using the results of differential scanning calorimetry and X-ray diffractometry, an analysis has been carried out of the initial stages of the eutectic and primary mechanisms of crystallization of a series of metallic glasses based on Fe and Al with the established temperature dependences of the effective diffusion coefficients. Analytical relationships, which relate the volume density of crystallites formed in the glasses at the temperatures of the onset of crystallization with the values of the effective diffusion coefficients at these temperatures have been proposed. It has been established that, in the glasses, the crystallization of which begins at the lower boundary of the threshold values of the effective diffusion coefficients (~10–20 m2/s), structures are formed with the volume density of crystallites on the order of 1023–1024 m–3 and, at the upper boundary (10–18 m2/s), of the order of 1018 and 1020 m–3 in the glasses that are crystallized via the eutectic and primary mechanisms, respectively. Good agreement between the calculated and experimental estimates indicates that the threshold values of the effective diffusion coefficients are the main factors that determine the structure of glasses at the initial stages of crystallization.
Keywords: metallic glasses; onset crystallization temperature; threshold values of the diffusion coefficients; volume density of crystallites

Microstructure of YBa2Cu3O y subjected to severe plastic deformation by high pressure torsion by E. I. Kuznetsova; M. V. Degtyarev; N. A. Zyuzeva; I. B. Bobylev; V. P. Pilyugin (773-781).
The influence of plastic deformation carried out by high pressure torsion at room temperature on the microstructure of the YBa2Cu3O y (123) compound prepared by standard ceramic technology and annealed at low temperature (200°C) in a water-saturated atmosphere has been studied. It has been shown that the directional growth of recrystallized lamellar-shaped grains initiated by the 124–123 phase transformation takes place upon recovery (after deformation) annealing at 930°C in ceramics subjected to additional low-temperature annealing, which leads to the formation of the texture. A rodlike structure has been observed in samples prepared by standard technology, after deformation and recovery annealing (930°C).
Keywords: high-temperature superconductor; high pressure torsion; recrystallization

Methods of the X-ray diffraction analysis and electron microscopy were used to study changes in the structural phase state and mechanical properties of bulk-nitrided 08Kh17T steel (0.08 wt % C, 17 wt % Cr, 0.8 wt % Ti, 0.5 wt % Si, 0.8 wt % Mn, 0.025 wt % S, and 0.035 wt % P) upon tempering in the temperature range of 400–700°C. The changes in the mechanical properties of the nitrided steel upon tempering are associated with the predominance of either the solid-solution or precipitation strengthening, i.e., with the presence of martensite in the steel structure at low temperatures of tempering and the precipitation of particles of Cr2N nitrides of different dispersity upon increasing the tempering temperature. The greatest increase in the ultimate tensile strength and yield stress (1.8–2.5 times) at a satisfactory plasticity (no less than 10%) of the bulk-nitrided steel is achieved by tempering bulk-nitrided steel in a temperature range of 600–700°C.
Keywords: ferritic steel; nitriding; mechanical properties

Nitrogen alloying of the 12% Cr martensitic-ferritic steel by A. S. Kudryavtsev; D. A. Artem’eva; M. S. Mikhailov (788-794).
The influence of the nitrogen content on the structure and mechanical properties of heat and corrosion resistant 12% Cr martensitic-ferritic steel developed at the Central Research Institute of Structural Materials Prometey has been studied. Steel containing 0.061 wt % nitrogen possesses a high level of mechanical properties. The decrease in the nitrogen content to 0.017 wt % leads to an increase of structurally free ferrite fraction in the steel, a decrease in the density of dislocations, a decrease of structural dispersity and the absence of finely dispersed precipitates of niobium and vanadium nitrides and carbides. As a result, there is a decrease in the strength properties, especially in the heat resistance.
Keywords: heat-resistant steel; 12% chromium steel; long-term strength; nitrogen; ferrite; carbides; nitrides; carbonitrides

On the precipitation strengthening of Cu–2.6Ni–0.6Si–0.6Cr bronzes by N. T. Kareva; I. L. Yakovleva; O. V. Samoilova (795-801).
A thermodynamic simulation of phase equilibria in the Cu–Ni–Si–Cr system has been carried out using a FactSage program package (version 7.0). The structure and phase composition of the Cu–2.6Ni–0.6Si–0.6Cr bronze have been studied in the quenched and additionally aged (under various conditions) states. Precipitates of the chromium silicide Cr3Si have been revealed in the alloy both in the quenched and aged states. Particles of nickel silicide Ni2Si 0.05–0.5 μm in size appear upon the precipitation hardening both homogeneously and heterogeneously (on particles of previously formed chromium silicides). The microhardness (HV) and the electrical resistivity of the experimental samples have been determined after conducting processes of precipitation strengthening.
Keywords: strengthening; Cu–Ni–Si–Cr system; aging; microhardness

Is it possible for dislocations to self-lock after high-pressure torsion? by A. V. Plotnikov; B. A. Greenberg; M. A. Ivanov; V. P. Pilyugin; T. P. Tolmachev; O. V. Antonova; A. M. Patselov (802-809).
The evolution of the dislocation structure of the intermetallic compound Ni3Ge upon high-pressure torsion has been investigated. It has been determined that, under certain conditions, subsequent heating without loading leads to the self-locking of dislocations, while in other cases, this does not take place. The role played by the lattice distortions that arise under torsion is emphasized.
Keywords: torsion under pressure; dislocations; superdislocations; blocking; self-locking; yield stress; cross slip

Conditions have been considered for the formation of autowaves of localized plastic flow in the deformed metals upon the propagation of Lüders bands in the case of the Portevin–Le Chatelier effect, and upon the formation of a neck with taking into account the differences in the microscopic mechanisms of plastic deformation in the case of these phenomena. The laws that govern the development of the localized plastic flow of metals and the role of these laws in the development of the above effects have been investigated. It has been established that the main features of the deformation characteristic of these phenomena are determined by the differences in the properties of the active media that are formed in the material upon plastic deformation. The mechanisms of the generation of different autowave modes of the localized plastic flow upon the Lüders deformation, Portevin–Le Chatelier effect, and the formation of a neck in active media of different nature during deformation have been considered.
Keywords: plasticity; strength; dislocations; strengthening; autowaves; localization of deformation

The effect of ultrasound-assisted explosive welding on the structure formation and the properties of copper–aluminum joints is studied. Ultrasound-assisted explosive welding improves the quality of formed copper–aluminum joints, i.e., enhances their strength and significantly reduces the amount of fused metal over the entire weldability range. It is shown that ultrasound-assisted explosive welding can noticeably extend the weldability range of the copper–aluminum pair to obtain equal-in-strength joints with minimum structural heterogeneity in the wide welding range.
Keywords: explosive welding; composition material; weldability range