Nb<inf>3</inf>Al and its ternary A15 compound conductors prepared by a continuous liquid quenching technique

Abstract

The Nb 3 Al, Nb 3 (Al,Ge) and Nb 3 (Al,Si) tapes have been successfully fabricated by a continuous liquid quenching using a high-speed moving copper, Cu, substrate tape heated at a proper temperature. A small amount of silicon, Si, and/or germanium, Ge, substituted for alminum, Al, improves the fluidity of molten alloy and facilitates the formation of the quenched alloy tape. The Nb 3 Al alloy with small contents of Si(Ge) showed a supersaturated bcc phase as quenched state. This bcc alloy was then transformed into a metastable stoichiometric A15 phase, which had an extremely small grain size of a few hundred angstroms. The increase in Si(Ge) contents in the alloy led to the direct formation of A15 phase from the molten state, whose grain size was about 1 μm. J c of the sample transformed from the bcc phase is much larger than that of the A15 phase directly formed from the molten state because of the finer grains in the transformed A15 phase. The highest J c at 20 T and 4.2 K of about 105A/cm2was obtained for Nb-25 at % Al-2 at %Si alloy annealed at 820°C for 3 h.