Relationship Between BCC-Deformation, Transformation Temperature and Microstructure in Nb3Al 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 2000degC. It was reported several times that deformation to the quenched BCC Nb(Al) phase influences the superconducting properties, usually improving . 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 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 degC 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 1.2mum . The increases with deformation, not showing a peak. The J_c of the wire (ME365) with reduction in area of 86% shows the best performance of the wire ever reported. But, at least high-field J_c seems to saturate at a certain value.