Fine Grained Tantalum for Composite Nb₃Sn Superconductor Diffusion Barrier Sheet

Poor deformation behavior of commercial polycrystalline Ta sheet used for Sn diffusion barriers in Nb₃Sn composite superconductors leads to the use of more Ta than may be necessary in these conductors, and to strand fracture during wire drawing. These problems arise because of nonuniform deformation of the Ta layer when co-drawn with Cu. The origin of the problem resides in the microstructure of the Ta and the code formation mechanics of relatively strong BCC Ta with weaker and more ductile adjacent FCC Cu. In an attempt to remedy this problem, we have processed 25 mm square bars of Ta by multi-axis severe plastic deformation (SPD) via equal channel angular extrusion, then rolled the bars to sheet and annealed the as-worked sheet. The processing is done to refine the microstructure and reduce nonuniformities in grain size and texture. Measurements of hardness, microstructure and mechanical properties are reported for bulk and sheet Ta. The results of SPD processing are encouraging.