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 °C, higher than conventional transformation temperatures by about 200 °C, 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 °C enhances the long-range ordering of the Nb₃Al phase and drastically improves the high-field critical current densities of the Nb₃Al conductors.