Microstructural investigation of the bcc to A15 phase transition in sputter-deposited Nb3Al superconductors

The bcc to A15 phase transition in sputter-deposited Nb3Al (Cb3Al) superconductors was studied using transmission electron microscopy. Four distinct microstructures were observed for the sputter-deposited and isochronally heat treated samples. Heat treatments were for 24 h at temperatures between 500 and 7000C. Deposition at 100C produced the bcc phase with a small grain size of 1000A. The bcc grains contained a high density of dislocations and fine precipitates. Coarse precipitates were found in the bcc grains and at the grain boundaries after heat treatment between 500 and 6000C. At 6500C, the A15 phase was detected in the presence of the bcc phase; the nucleation of the A15 phase originated at the grain boundary. Finally, a homogeneous microstructure of A15 phase resulted from heat treatment at 7000C. The A15 phase had a small, but defect free grain of 2500A. It is believed that such microstructures of A15 phase could have enhanced superconducting critical current (J/sub c/) and critical temperature