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 (Tc), the upper critical magnetic field (Bc2) and, hence, the critical current density (Jc) 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.

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The innermost coil of a new high-field NMR magnet such as a 1-GHz NMR magnet is designed by using a new superconductor. It is therefore important that a superconducting joint between the new superconductor and the NbTi conductor was developed. We have developed a new superconducting joint method for the Nb 3 Al conductor, which is expected to be used for the innermost coil of the 1-GHz NMR magnet. This joint method uses the Nb 3 Sn layer that is formed on the Nb matrix surface of the Nb 3 Al conductor. This Nb 3 Sn layer enables the soldering of the Nb 3 Al conductor; thus, it is possible to solder the Nb 3 Al conductor and NbTi conductor. The superconducting property of the Nb 3 Al NbTi joint that is joined by the superconducting solder (for example, SnBiPb solder) showed good performance under a magnetic field of 0.1 T.