Interface roughness in copper-tantalum wire and Nb3Sn superconductor composites

Poor deformation behavior of tantalum (Ta) sheet used for tin diffusion barriers in Nb3Sn composite superconductors can lead to Ta layer rupture and even strand fracture during wire drawing. These problems arise because the Ta layer deforms nonuniformly as it is reduced in thickness. The origin of the problem resides in the microstructure of the Ta and the co-deformation mechanics of relatively strong body centered cubic Ta with surrounding weaker and more ductile face centered cubic Cu. In an attempt to remedy this problem, 25 mm square bars of Ta were processed by multi-axis severe plastic deformation (SPD) via equal channel angular extrusion (ECAE), then rolled to sheet and recrystallized. The SPD processing was done to refine the microstructure and reduce nonuniformities in grain size and texture. Measurements of the Cu-Ta interface roughness in experimental Cu-Ta composite wires were made and compared with the interface roughness seen in commercial Ta diffusion barrier layers. Results show that Ta sheet made from SPD processed bulk Ta co-deforms well with Cu and leads to less interface roughening than is developed in commercial Ta sheet material fabricated into superconductor wire. © 2008 American Institute of Physics