Relationship Between BCC-Deformation, Transformation Temperature and Microstructure in Nb3A1 Wires.

The authors have been developing the Nb3Al conductors through transformation process, where the Nb3AL phase forms from the BCC (body-centered cubic) Nb(Al) supersaturated solid solution phase; the BCC phase is quenched by rapid-cooling from a temperature around 2000°C. It was reported several times that deformation to the quenched BCC Nb(AI) phase influences the superconducting properties, usually improving Jc. But, the results have not been discussed in detail so far from a metallurgical viewpoint. This work investigates the fundamental relationship between the BCC-deformation, transformation temperature, microstructure and superconducting properties. The transformation temperature is identified by DTA measurement, detecting the release of the free energy between the BCC and the Nb3Al phases. It was found that the transformation temperature has a tendency of monotonic decrease with the deformation. But, the transformation temperature does not seem to go down below a temperature of about 720°C even by strong deformation. The microstructures observed by FESEM suggests that the grain size is reduced almost proportionally with the deformation. In case of reduction in area of 99%, the grain size is reduced down to about 250 nm from 1.2 μm. The Jc increases with deformation, not showing a peak. The Jc of the wire (ME365) with reduction in area of 86% shows the best performance of the Nb3A1 wire ever reported. But, at least high-field Jc seems to saturate at a certain value. [ABSTRACT FROM AUTHOR]