The morphology and grain size of Nb₃Sn filaments in in situ prepared multifilamentary Nb₃Sn-Cu composite wire

Abstract

Multifilamentary Nb₃Sn-Cu wire was prepared from chill-cast Cu-20 wt% Nb ingots by drawing 12.5 mm ingots to 0.15 mm diameter wire, plating with 9 vol% Sn and reacting at temperatures ranging from 450 to 725° C. The morphology and grain size of the as-cast Nb filaments and the Nb₃Sn filaments formed on heat treatment were characterized by TEM and SEM studies, and the Sn diffusion process was evaluated using electron microprobe analysis. The as-drawn Nb filaments had a cross-section typically 50 Å by 2500 Å and were generally convoluted in the wide direction. Tin diffusion produced Nb₃Sn filaments with an essentially equi-axed cross-section having diameters of around 800 Å for the 550° reaction temperature and grains extending across the diameter. It was shown that the change in cross-sectional shape is produced largely by coarsening of the thin Nb filaments prior to diffusion of Sn to form the Nb₃Sn. Optimum J _c values at both 1 and 8 T were found to occur for diffusion temperatures of 550° C and times of 3 to 6 days where the grain size was 750 to 800 Å.