Tantalum Tube for Diffusion Barriers

Diffusion barriers used in Nb3Sn wire are currently fabricated by wrapping Ta sheet into a tube with an overlap seam. The result of current practice is non- uniform deformation in the Ta sheet as it thins by wire drawing because of non-uniform grain size and texture. In addition, a significant portion of the diffusion barrier is twice as thick as need be. The overlap is wasteful of Ta and produces asymmetric internal deformation forces and consequent non-uniform deformation in the wire. Severe plastic deformation (SPD) processing will be used to refine and homogenize the microstructure of seam-welded Ta tube for subsequent fabrication into Nb3Sn diffusion barriers. Careful selection of deformation route and heat treatment conditions during the processing steps will produce a fine grain uniform microstructure with favorable texture in the tube, leading to improved co-deformability with other conductor components (Cu, Nb, and Sn) and lower manufacturing costs. Phase I work includes 1) developing equipment and procedures for SPD processing seam-welded Ta tube, 2 refining/homogenizing the grain structure in Ta tube, and 3) microstructure examination and mechanical testing of processed Ta tube to assess quality for diffusion barrier applications. Material property comparisons with commercial Ta sheet will be used as a baseline for evaluating project success. Phase II work includes construction of scaled-up equipment for SPD processing of Ta tube, SPD processing of appropriate outside diameter thin wall Ta tube, pilgering or drawing of the processed tube into prototype thin wall diffusion barrier tube, and fabricating/testing prototype Nb3Sn wire including the Ta tube. Wire manufacturing and performance experiments will assess the improvement in fabricability of the specially processed Ta tube compared with comparable wire made with commercial Ta sheet. A cost/benefit analysis will assess technical viability of the new technology for fabrication of Ta tubes. Commercial Applications and Other Benefits: The commercial application is to apply this technology to the production of Ta tube for lower cost and higher quality Nb3Sn conductors. Cabot and H.C. Starck, Inc. may be interested in using this technology for other tubular products. The approach used could be applicable to the fabrication of other tube materials including niobium, titanium and zirconium.