Historical Progress in the Fabrication Process for a Nb₃Al Conductor

Abstract

Nb₃Al with stoichiometric composition shows T_c above 19K and H_c2 (4.2K) above 30T, much higher than in commercially available Nb-Ti wire and Nb₃Sn wire. Therefore the developments of fabrication processes for Nb₃Al conductors were performed early on with zeal, but ended without success because stoichiometric Nb₃Al is unstable at any temperatures except about 2, 000°C. Moreover, the fabrication speed of Nb₃Al through solid-state diffusion reaction is very slow. To overcome these difficulties, a recent new process, RHQT (rapid-heat, quench, and transformation), was developed for fabricating Nb₃Al multifilamentary conductors with near-stoichiometric composition and fine-grained crystals. Through this process we have succeeded in the fabrication of Nb₃Al multifilamentary wires showing 2 to 5 times larger J_c at 4.2K than those of commercialized superconductors in any field, and their T_c of about 17.6K and H_c2 (4.2K) of about 26T are similar to those of the (Nb, Ti)₃Sn wire. Furthermore, excellent tolerance to mechanical strain is also obtained for the Nb₃Al conductor. I believe the new Nb₃Al conductor should soon be commercialized and take the place of Nb₃Sn conductor. In this paper I would like to describe the historical progress in the Nb₃Al conductor fabrication process up to the birth of the RHQT process.