Microstructure and Superconducting Properties of Hf-Ta Addition Bronze-Route Nb3Sn Wire

The Nb3Sn superconducting wire is a promising candidate for cutting-edge high-field magnets such as the next generation particle accelerator and fusion reactor. Recently, Hf-Ta addition to Nb3Sn wires has attracted our great interest, because it results in significant grain refinement and J(c) enhancement, maintaining high B-c2. In this work, we studied how much effect the Hf-Ta addition has on the layer formation and superconducting characteristics, when it is applied to bronze-route Nb3Sn wires, through microstructural and microchemical analysis by using EBSD and EDS. For simplicity, single-core composite wires were fabricated and tested. The tested wires included conventional Ti-doped Nb3Sn and Hf-Ta-doped Nb3Sn wires. As a result, grain refinement in Hf-Ta addition was not significant in bronze process, compared with internal-tin process. Apparently, the thickness of Nb3Sn layer was thick especially when Ti was doped into the Cu-Sn bronze matrix. Sn content seems to be also high in Ti-doping to Cu-Sn bronze matrix. In the bronze-route process, impact of Ti addition on the promotion of Sn diffusion seems to be relatively large, compared with Hf-Ta addition.