Development of nanometer scale structures in composites of Nb-Ti and their effect on the superconducting critical current density

An investigation has been made of an optimized, high critical current density composite of Nb-46.5 wt% Ti. Details of the fabrication of the composite are given and an extensive metallurgical characterization of the microstructure at many stages in the fabrication and optimization process is presented. It is shown that up to 20% at α-Ti is produced by the heat treatment process. The mechanism by which the precipitate distribution is refined with increasing number of heat treatment and initially equaixed precipitates turned into very thin, high aspect ratio ribbons by large drawing strains is quantitatively described. Critical current density measurements show that the optimum pinning forces are obtained for precipitates of -- 1 nm in thickness, spaced -- 4nm apart. These dimensions are considerably smaller than those previously reported. There appears to be little or no proximity effect suppression of the pinning force even though the precipitate thickness is of order 0.2 coherence length. The specific pinning force of the precipitates ranges from 170 to 260 N/m/sup 2/, a value similar to that measured earlier for grain boundary film precipitation in this alloy