Influence of Nb/Al Composition on the Microstructure and Superconducting Properties of RHQT-processed Nb3Al Wire.

Nb/Al precursor wire with a composition of Nb-25at%Al is converted into a bcc supersaturated solid solution (Nb(Al)SS) through RHQ treatment, of which the maximum heating temperature is around 2,000°C. However, the solubility limit of the bcc phase is 21.5at%Al. Therefore, the solid solution phase does not exist in a state of equilibrium at 25at%Al. Why 25at%Al bcc Nb(Al)SS forms is not clear. Accordingly, we studied the high-temperature state of the solid solution phase at around 2,000°C. We have made four kinds of samples with different Al compositions, ranging from 22.6at% to 30.0at%, and subjected them to RHQ treatment at temperatures from 1,900°C to 2,400°C. We measured variations in the microstructure and superconducting properties at the maximum heating temperature for each composition. We found that the temperature range where only the solid solution phase forms decreases as Al concentration increase. This clearly indicates that the solid solution phase extends to at least 27at%Al.