Strain Dependence of Critical Current Associated with Constitution of Practical Nb₃Sn Superconducting Wires

Abstract

An investigation was made on the strain dependence of critical cuurent I_c for three kinds of practical Nb₃Sn superconducting wires which are in the course of development, with an emphasis on the correlation with wire constitution.
In the internal tin diffusion processed wires with a single tin core at the center, the reversible strain limit, ε_irrev was small and the strain sensitivity of I_c was high. This is ascribed to the strain concentration induced by Kirkendall voids observed in the center of fracture surface.
Internal reinforcement using SUS 405 steel increased both ε_m, the strain corresponding to peak I_c, and the strain sensitivity, and decreased the ε_irrev in addition to a marked increase of flow stress. In the light of both the serration in the stress-strain relation and fracture surface morphology, the degradation of strain characteristics are attributable to the strain concentration in Nb₃Sn layer associated with localized deformation of the steel at low temperatures.
In the external tin diffusion in situ processed wires, both Nb₃Sn layer formed in the periphery and the unreacted core in the center of the wire controlled the strain characteristics. The smaller the core size was, the larger the critical current density and the smaller the ε_m or ε_irrev. Inhomogeneity of tin diffusion in the cross section as well as in the longitudinal direction resulted in deviation of the core and tin balling, respectively. They deteriorated the strain characteristics of the wire significantly.
Improvements of the strain characteristics in these wires through proper constitution and heat treatment were briefly discussed.