Preparation of ductile NbAl powders for the fabrication of Nb 3Al superconductors

Abstract

The influence of rapid quenching and hydrogenation on the structure and phase composition of niobium-rich NbAl alloys was investigated. Starting ingots were prepared by conventional arc melting of compacted powder mixtures. The phase diagram for NbAl, presented by Jorda et al., was confirmed in the examined concentration range up to about 30 at.% aluminium. Specimens were quenched with cooling rates not less than 10 4 K s −1, both from the solid and liquid state. A metastable extension of the niobium solid solution up to 27 at.% aluminium was obtained after quenching small droplets from temperatures T ⩾ 2200 °C. Contrary to previous publications, it is shown that rapid quenching of solids (19–24 at.% aluminium) from 1750 °C in a eutectic GaIn bath does not lead to the formation of metastable states in the region of the Nb 3Al phase. Ductile Nb-Al powders of an average composition “Nb 3Al” were prepared with a combined hydridingdehydriding process. The retransformation of the metastable powders was determined to occur in the temperature range between 710 and 860 °C depending on the aluminium concentration. Based on these findings, a new perspective results for the powder metallurgical fabrication of Nb 3Al superconductors.