Grain Growth, Morphology, and Composition Profiles in Industrial Nb3Sn Wires

The A15 formation process in Nb₃Sn wires has been revisited. The progress of the reaction was studied by a careful FE-SEM study after a series of heat treatments at various temperatures and times. It was found that in the very early stage of the reaction, the Sn flow is high and a very thin layer of equiaxed A15 grains is formed at the interface between Nb and the Cu-Sn bronze. Beyond the very first layer with equiaxed grains, the grains form with a columnar shape. With further progress of the A15 layer and the continuous flow of Sn through the already formed layers, a decomposition of the columnar grains to equiaxed grains was observed. By TEM/EDX, we have confirmed our earlier observation of a radial Sn gradient in the filaments. The equiaxed regions have a sufficiently high critical field to carry supercurrents at high magnetic fields, in contrast to the columnar regions, where the B values are lower, thus their contribution to the current transport at the higher fields is limited. We have also studied internal Sn wires and found equiaxed grains only, with a Sn content through the A15 layer reaching from 22 at.% to values close to 25 at.%.