Optimization studies for processing Nb3Al by a rapid ohmic-heating and quenching method

Rapid heating to different temperatures in the vicinity of 2000°C followed by quenching to room temperature was used to prepare short samples of jelly-roll-type Nb₃Al superconductor. The as quenched material has either a bcc or A15 structure depending on the presence or absence of a specific endothermic reaction during the rapid heating, but after a secondary heat treatment at 800°C an A15 structure is obtained for both cases. Significantly higher critical current densities at magnetic fields up to 18 T (4.2 K) are obtained for samples quenched as bcc, while the A15-quenched material is superior at higher fields due to a larger upper critical field. For the bcc-quenched samples the critical current density is optimized (16 T, 4.2 K) when they are quenched from temperatures just above or 400°C higher than the temperature of the endothermic reaction, with a noticeable drop in the middle range. A 10 hour secondary heat treatment gives rise to better superconducting properties than a 3 hour heat treatment (both at 800°C). Some of the observed dependencies are explained using SEM microstructure analysis