Yu. N. Akshentsev

Ordering kinetics in ternary Ni3Al-X alloys

Abstract Measurements of electrical resistivity were used to study the kinetics of ordering in stoichiometric Ni 3 Al alloyed with a third element (Nb, Ti, V, W, Cr, Co, Fe). The activation energies of ordering and disordering were found.

Choice of copper-based alloys for ribbon substrates with a sharp cube texture

It has been shown that, in some copper-based alloys subjected to cold deformation by rolling to 98.6–99% followed by recrystallization annealing, a sharp cube texture can be produced. Optimum conditions of annealing have been determined, which make it possible to produce a sharp biaxial texture in Cu-Ni, Cu-Fe, and Cu-Cr alloys with the fraction of cube grains of more than 95%; this opens a possibility of using thin ribbons made of these alloys as substrates for multilayer film compositions, in particular when developing second-generation high-temperature superconductors.

Investigation of the structure and magnetic and mechanical properties of textured substrates of an Ni-Cr-W alloy

Based on the data of X-ray diffraction, electron-backscatter diffraction, dilatometry, thermal analysis, and scanning electron microscopy, we determined the optimum regimes of heat treatment of cold-rolled tape substrates of an alloy Ni88.4Cr9.2W2.4 that permit obtaining a perfect cube texture. It has been shown that the use of two-stage annealings makes it possible to obtain in this ternary nickel alloy a cube texture in which the scatter about the rolling direction (RD) and the transverse direction (TD) of the substrates varied within 6.75°–7.20° and 4.8°–5.4°, respectively.

Mechanisms of the formation of a bulk superconducting phase MgB2 at high temperatures

The structure of MgB2 samples synthesized from magnesium flakes and a boron powder at temperatures of 900–1000°C has been investigated. It has been found that the samples have “dendrite-like” and layered structures formed as a result of the dissolution of solid boron in liquid magnesium followed by crystallization. The obtained structures have been analyzed within the theoretical concepts of crystallization from melt.

Mechanism of the formation and specific features of the structure of massive samples of compound MgB2

Using different methods, it has been revealed that two MgB2 phases with the same hexagonal lattice, which differ in the contents of Mg and B (in the limits of the homogeneity range), as well as in the concentration of impurity oxygen and in the microstructure, are formed. The regions that correspond to these two phases of MgB2 have relatively large sizes (100–500 μm) and, alternatingly, fill the entire volume of the sample. It is assumed that the two-phase state of MgB2 is due to the specific features of the mechanism of its formation (when synthesizing at 800–1000°C), which includes the stage of the melting of magnesium, the dissolution of solid boron in the melt to a concentration that corresponds to the composition of the MgB2 compound, and the subsequent crystallization of the MgB2 compound in the melt with the formation of a dendrite-like structure, which is accompanied by an appropriate redistribution of the main components and impurities.

Effect of deformation with Bridgman anvils on the structure, hardness, and critical current of a massive MgB2-based sample

The structure and properties of synthesized massive MgB2-based samples subjected to deformation with Bridgman anvils have been studied. Deformation results in the formation of fine-grained structure of the MgB2 phase, enhancement of interconnection of grains, complete disappearance of friable MgB2-phase areas, and abrupt increase in the microhardness.

Texture and mechanical properties of tape substrates from binary and ternary copper alloys for second-generation high-temperature superconductors

The process of texture formation in tapes made of a number of binary and ternary copper alloys upon cold rolling to degrees of deformation of 98.6–99% and subsequent recrystallization annealing has been studied. The possibility of designing multicomponent alloys based on the binary Cu-30% Ni alloy additionally alloyed with elements that strengthen the fcc matrix, such as iron or chromium, has been shown. The opportunity of obtaining a perfect cube texture in a thin tape made of binary and ternary copper alloys opens prospects for their use as substrates in the technology of second-generation HTSC cables. Optimum regimes of annealing have been determined, which make it possible to obtain in the Cu-M and Cu-(30–40)Ni-M (M = Fe, Cr, Mn) alloys a perfect biaxial texture with the fraction of cube grains {001}〈100〉 on the surface of the tape more than 94%. The estimation of the mechanical properties of the textured tapes of the investigated alloys demonstrates a yield strength that is 2.5–4.5 times greater than that in the textured tape of pure copper.

Suppression of superconductivity in YNi 2 B 2 C at the atomic disordering

The behavior of the electrical resistivity ρ(T), the superconducting transition temperature Tc, and the upper critical field Hc2(T) of a polycrystalline sample of YNi2B2C irradiated by thermal neutrons with the subsequent high-temperature isochronous annealing in the temperature interval Tann = 100–1000°C has been studied. It has been found that the irradiation of YNi2B2C with a fluence of 1019cm−2 leads to the suppression of the superconductivity. The final disordered state is reversible; i.e., the initial ρ(T), Tc, and Hc2(T) values are almost completely recovered upon annealing at up to Tann = 1000°C. The quadratic dependence ρ(T) = ρ0 + a2T2 is observed for the sample in the superconducting state (Tc = 5.5−14.5 K). The coefficient a2 (proportional to the square of the electron mass m*) hardly changes. The form of the dependence of Tc on ρ0 can be interpreted as the suppression of the two superconducting gaps, Δ1 and Δ2 (Δ1 ∼ 2Δ2). The degradation rate of Δ1 is about three times higher than that of Δ2. The dependences dHc2/dT on ρ0 and Tc may be described by the relations for a superconductor in the intermediate limit (the coherence length ζ0 is on the order of the electron mean free path ltr) under the assumption of a nearly constant electron density of states on the Fermi level N(EF). The observed behavior of Tc obviously does not agree with the widespread opinion about the purely electron-phonon mechanism of superconductivity in the compounds of this type supposing the anomalous type of superconducting pairing.

Structure and mechanical properties of an Ni3Al single crystal upon high-temperature deformation

Structure and strength properties of single-crystal 〈001〉 samples of Ni3Al have been studied in the as-grown and homogenized state during tensile tests in the temperature range of 1150–1250°C. At the strain rate of 1.32 mm/min (2 × 10−5 m/s), the samples are in the state of superplasticity. The basic mechanism of relaxation is dynamic recovery; in some regions of the sample, recrystallized grains are formed. At 1250°C, coarse twins are observed in the zone of fracture, which indicates the “switching on” of additional slip systems necessary to guarantee the relaxation process.

Structure of crystals of iodide titanium obtained by a solid-state reaction using α → β → α phase transitions

Results of metallographic and electron-microscopic studies of a structure observed on the surface of crystals of iodide α-Ti that are produced by thermal cycling in a textured matrix are presented. It is shown that the structure in the form of slip and twin traces, which is formed under the effect of a phase transition and thermal stresses, is oriented with respect to the rolling direction of the textured matrix. The results of electronmicroscopic analysis have demonstrated that the character of the dislocation structure of weakly deformed α-titanium crystals does not depend on the crystallographic orientation.