Large improvement in high-field critical current densities of Nb3Al conductors by the transformation-heat-based up-quenching method

The bcc supersaturated solid solution Nb(Al)(ss) obtained by rapid heating and quenching of a multifilamentary Nb/Al composite wire has shown a crystal structure change from a disordered to an ordered structure before transforming to the A15 Nb3Al phase. Such ordering of the bcc phase seems to be responsible for the A15 phase stacking faults that depress the critical temperature (T-c), the upper critical magnetic field (B-c2) and, hence, the critical current density (J(c)) of Nb3Al in high fields. A heat treatment around 1000 degreesC, higher than conventional transformation temperatures by about 200 degreesC, suppresses the ordering and yields a new phenomenon termed the 'transformation-heat-based up-quenching' (TRUQ). TRUQ is characterized by the self-heating of the bcc phase by the transformation heat, which propagates through the whole length of a composite wire and transforms it to Nb3Al. A subsequent annealing at 800 degreesC enhances the long range ordering of the Nb3Al phase and drastically improves the high-field critical current densities of the Nb3Al conductors.