The effect of high temperature plastic flow on the superconducting transition in A15 compounds

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I,

INTRODUCTION

A number of studies have now shown that the A15 compounds Nb3Sn, V3Si, and V3Ga can undergo extensive plastic deformation, either under conditions of high hydrostatic pressure z-5 or high homologous temperature. 6-" At ambient pressure, the ductile-to-brittle transition is at about two-thirds of the absolute melting range. Hot single crystal studies of V3Si have noted {100} (010) slip behavior, 8"9and dislocations with cube direction Burgers vectors have been observed.~2 There are only three independent {100} (010) slip systems, however, and polycrystalline ductility indicates additional modes of microplasticity. Hot deformed microstructures have displayed polygonized dislocation structures. ~3"~4 Stress-strain rate-temperature relationships are largely those of "power law creep", with activation energies for creep roughly in the 400 to 500 kJ/mol range. 6'I~ Grain size refinement strengthens polycrystals.~~ Much of this research has been motivated by interest in AI5 phase deformation processing. Such processing might be useful as a superconductor fabrication expedient or as a means of optimizing microstructure and related electrical performance. For example, plastic deformation has been shown to increase the critical current density of V~Si.7 In any case, it is important to consider the effect plastic deformation has on the critical superconducting transformation temperature, T,.. Lower temperature AI5 plastic deformation generally lowers To, broadens the transition, and/or creates two transition temperatures. 2'5"~5This degradation in Tc can be largely removed by annealing. A summary of published data is shown in Table I. In addition to the Nb3Sn results shown in Table I, Rohr and Mfiller ~5deformed powdered Nb3AI and Nb3A10.76Ga024. The T,- degradation was much the same as in Nb3Sn, but could not be removed by 750 ~ annealing. The effect of hot deformation on the T,. behavior of V3Si single crystals was evaluated by Mahajan, et al.9 Plastic ROGER N. WRIGHT is Professor and Acting Chairman, Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12181. JAMES C. HO is Professor, Physics Department, Wichita State University, Wichita, KS 67208. Manuscript submitted June 14, 1983.

METALLURGICALTRANSACTIONS A

strains of 3 to 7 pet, achieved at a strain rate of 1.2 x 10-4/ sec and in a temperature range of from 1200 to 1800 ~ resulted in essentially no change in T,: behavior. The superconducting transition was determined inductively. The present paper presents superconducting transition temperature characterizations for a series of variously fabricated and hot deformed Nb3Sn and V~Ga polycrystals. Unlike previous work, the 7",: characterization has been made through heat capacity measurements.

II.

EXPERIMENTAL

The materials tested consisted of polycrystalline cylinders of Nb3Sn and V3Ga. The preparation of these materials has been described in detail in separate publications. 1~.J4Briefly, two different Nb3Sn materials were prepared from Nb and Sn powder stock by hot isostati