Fabrication of Tantalum Sheet for Superconductor Diffusion Barriers

Poor deformation behavior of commercial Ta sheet used for Sn diffusion barriers in multifilamentary Nb₃Sn superconductors leads to excessive Ta-Cu interface roughening and the use of a thick Ta layer to overcome layer fracture. The problem stems from three factors: Ta strain hardens faster and to a much higher flow stress than Cu, pure Cu is adjacent to Ta, and the Ta sheet microstructure is non-uniform and non-optimum. The objective of our work is to fabricate Ta sheet with improved deformation behavior over present day commercial Ta sheet. The hypothesis is that a uniform, fine-grained, and well-textured sheet with a preferred orientation will have better codeformation characteristics. To test this thinking, 25 mm square cross-section bars of Ta were deformed by equal channel angular extrusion (ECAE) to strains of nine, sliced in half, annealed to various levels, and rolled to 0.5 mm thick sheet, and Cu-Ta monofilaments fabricated with the processed sheet. Experimental results and roughness measurements on the Ta layers indicate that severe plastic deformation (SPD) processed sheet shows better characteristics than commercial Ta layers. This results from the uniform microstructure and fine grains in precursor Ta sheet processed by SPD.