Effect of ECAP on Change in Microstructure and Critical Current Density of Low Temperature Super-Conducting Monowire

The critical current density (Jc), which is a major performance parameter of low temperature superconducting Nb-46.5 wt%Ti wire, can be improved by uniformly dispersed fine α-Ti precipitates since they serve as a flux pinning center. The high volume fraction of α-Ti precipitates at grain boundary intersections can be achieved by repetitive large deformation and intermediate heat treatments. For this aim, we investigated effect of severe plastic deformation of Nb-46.5 wt%Ti billet on the changes in microstructure and Jc by assigning equal channel angular process (ECAP) followed by groove rolling and drawing. In addition, intermediate heat treatments with different heating condition and holding time after each process were assigned to precipitate the fine α-Ti. It was observed that the curled flow lines like shear bands with the highly dense dislocations were formed after ECAP leading to recrystallization and the high volume fraction of α-Ti after subsequent heat treatment. The additional plastic deformation by groove rolling and drawing increased the volume fraction of the α-Ti and the aspect ratio of its shape. As a result, the critical current density at the lower range of magnetic field was enhanced with increasing the number of ECAP pass.