Limitation of Critical Current Density by Intermetallic Formation in Fine Filament Nb-Ti Superconductors

Two experiments have been performed to investigate the role that the intermetallic reaction between the copper matrix and the Nb-Ti filaments plays in limiting the critical current density (Jc) of Nb 46.5 wt% Ti composites. The first experiment involved composites which were industrially extruded. It was found that as the number of heat treatments increased, the Jc declined, the resistive transition broadened and the filaments sausaged. The filament sausaging was initiated by intermetallic particles at the filament matrix interface. A series of many heat treatment procedures were then applied to composites fabricated in our own laboratories without extrusion. Very high Jc values were obtained at filament sizes of 20 ¿m. When the same heat treatment procedures were applied to 4 - 5 ¿m conductors, extensive sausaging and degraded Jc values resulted. This degradation was also found to be due to the formation of Cu-Nb-Ti intermetallic compounds. It is concluded that a reliable filament diffusion barrier technology is necessary to permit full flexibility in the heat treatment of 2 - 5 ¿m filament Nb-Ti composites.