Microstructures of rapidly-heated/quenched and transformed Nb3 Al multifilamentary superconducting wires

Microstructures of rapidly-heated/quenched and transformed Nb₃Al multifilamentary wires were studied by using transmission electron microscopy. Nb/Al composite wires are fabricated by a jelly-roll process. The Nb/Al composite filaments changed into an Nb-Al supersaturated bcc solid solution with rapid-heating/quenching. The Nb-Al bcc phase consists of many crystal grains with diameters of 2-4 μm, surrounded with large-angle grain boundaries. Some spherical voids, about 0.1 micron in diameter, were also observed at the intra- and intergrains. All grain boundaries of the Nb-Al bcc phases are simple flat planes. Then, the Nb-Al bcc phases were transformed into A15 phases (grain size: 0.5-2.0 μm in diameter) with additional annealing. Secondary phases were not observed in the A15 filaments. Grain boundaries of the A15 phases show zig-zag shape unlike those of the Nb-Al bcc phases, and every grain of A15 phases is an aggregation of sub-grains of 80-150 nm in diameter. Sub-grain boundaries are small-angle ones. Moreover we found that many stacking faults formed in the A15 sub-grains in parallel with spaces of 10-20 nm. These numerous plane defects seem to be the main pinning centers in the rapidly-heated/quenched and transformed Nb₃Al wires