Marina B. Lazareva

Nb-Ti superconductors of a high current-carrying capacity

The effect of multiple prolonged heat treatment on the structure and critical current density in Nb-50 wt.% Ti composites was studied in the case of monofilamentary superconductors. The treatment temperature range, in which high Jc values can be obtained, is determined. The influence of intermediate treatments on the increase in the quantity of precipitated alpha -phase is studied. The exponent specifying the kinetics of particle growth from the supersaturated solid solution is estimated. The interrelation between the critical current density and the quantity of precipitated phase is established. The maximum critical current density is 4*105 A cm-1 (B=5 T).

Structure and properties of differently directed deformed niobium-titanium alloy

Cylindrical blanks of Nb-48.5 wt.%Ti alloy were prepared with a variable degree of plastic strain at different temperatures of the treatment by using the thermomechanical treatment consisting of deformation of the ingot via upsetting in a closed container and its subsequent extrusion from this container to the initial size. The true strain value over one cycle of such a treatment is approximately 1.6, with the number of cycles varying from 1 to 5. Measurements were taken of strength, plastic and elastic characteristics of the differently directed deformed alloy over different stages of superconductor fabrication. TEM was used to study the microstructure of the alloy in the course of plastic deformation. It is established that such combined treatment promotes formation of a better homogeneous dispersed structure. With that the current characteristics of the niobium-titanium superconductor increase. The highest critical current density after such a treatment was 4.1 k/mm/sup 2/ under the applied magnetic field 5 T at 4.2 K.

Structure parameters and degradation of the critical current of the alloy NT-50 after cryogenic drawing in an ultrasonic field

The parameters of the defect and phase structure of the alloy NT-50 are investigated after low-temperature (77 K) deformation by drawing in an ultrasonic field, and the influence of these parameters on the amount of degradation of the critical current under tensile loading at 4.2 K is studied. It is found that cryogenic ultrasonic deformation leads to intensified decomposition of the β-solid solution with precipitation of Ti-rich phases according to the kinetics of the spontaneous martensitic transformation and results in a reduction of the internal stress level in the alloys. The structural features found promote stabilization of the β-solid solution during a subsequent deep cooling, as is manifested in a lowering of the degree of completion of the low-temperature deformation-induced martensitic transformation, which in turn raises the threshold for degradation of the critical current and reduces the degree of degradation in a wide range of external mechanical loads.

Current characteristics and microstructure in a multifilamentary Nb-37Ti-22Ta superconductor

The 210-filamentary superconductor based on Nb-37Ti-22Ta, wt.% alloy was obtained by using the electric arc melting and deformation processing. During alloy drawing we used four isochronal intermediate heat treatments at the temperature 425/spl deg/C. The total heat treatment duration was varied from 50 to 1000 hours. The true plastic strain value between heat treatments was 0.4 and after the final treatment was 3-5. The kinetics of phase transformation in the ternary alloy was investigated by the method of transmission electron microscopy. The J/sub c/ value increases with an increasing quantity of the precipitated alpha-phase in medium magnetic fields. The J/sub c/ (5 T, 4.2 K) value reaches 3000 A/mm/sup 2/ or more. In high fields, J/sub c/(10.4 T, 2.05 K) for the given superconductor was equal to 1600 A/mm/sup 2/. The parameters responsible for the shape of pinning force curve are determined for different treatment conditions.

About the nature of inelastic deformation of HTSC ceramics in region temperature below Tc

Abstract For Y- and Bi-based ceramics in superconducting state the results of the experimental studying the deformation under the loading are presented. The acoustic emission signals, the peculiarities of the intrinsic friction spectrum and ones of the planar contact resistance are studied. These results confirm the full analogy of the inelastic deformation processes at T c for HTSC ceramics and for metals.

Investigation of flux-trapping effect in Bi-based ceramics at 65–300 K

The influence of low magnetic fields on the temperature dependence of attenuation decrement is studied. The conservation of this effect after field switching off is discovered.

On the softening effect in superconducting state

It is shown that in superconducting state the increase of creep rate and peculiariries of attenuation decrement have dislocation origin.

The effects of low temperature deformation on the microstructure and Jc of Nb-49Ti superconductor

It is shown that cryogenic deformation (77 K) speeds up the kinetics of particle precipitation in the second phase, using the subsequent heat treatments in order to form the structure with increased volume contents of α-phase. Studied is the microstructural parametric influence upon Jc.

The effect of low-temperature strain on the structure and the critical-current degradation of the superconducting alloy Nb-Ti

The structural-phase aspects of the behavior of the critical current under load in the superconductive alloy Nb–48.5 wt% Ti are studied. It is shown that the optimum preliminary strain at T=77 K with subsequent annealing suppresses the tendency of the β matrix of the alloy to undergo phase transformations according to martensitic kinetics under uniaxial tension at T=4.2 K, and this is manifested in the reduction of the critical-current degradation effect when loaded to σ=0.9σfr in a transverse magnetic field of H=5 T. The paper discusses possible critical-current degradation mechanisms under mechanical loading in the niobium-titanium alloy, based on models of critical current flow in the β matrix reinforced with martensitic interlayers.

Dependence of the shear modulus of a Bi ceramic on external influences in the temperature interval 65–130 K

The dynamic shear modulus of a two-phase bismuth ceramic (Bi-2212 and Bi-2223) is measured as a function of temperature, magnetic field, and deformation. The results attest to the multistage penetration of magnetic field into the sample. It is found that the level of the shear modulus is sensitive to the magnetic and defect structure of the ceramic. The conjecture that the observed defects of the shear modulus are due to phase separation in the CuO2 planes of the 2212 phase is discussed.