Indexed on: 01 Nov '88Published on: 01 Nov '88Published in: Journal of Materials Science
Transmission electron microscopy has been used to investigate the effect of varying the heat treatment temperature, the duration and the frequency on the microstructure of high current density Nb-Ti superconducting composites. It was found that multiple heat treatment and drawing cycles produced between 16 and 25 vol % ofα-Ti precipitate. The longer and more frequent were the heat treatments, the greater was the volume fraction of precipitate. An increase in the number of heat treatment and drawing cycles significantly reduced the size of the precipitates formed at heat treatment size. If the composites were drawn to final wire size following final heat treatment, theα-Ti precipitates were distorted into folded sheets which appeared as ribbons in transverse cross-section, with average thickness varying from 1.6 nm to 2.6 nm. The specific flux pinning force of the ribbons varied from 360 to 260 N m−2 and was found to decrease with ribbon thickness. Some evidence of an extrinsic limit on theJc was provided by a study of the filament cross-section variability and the resistive transition index.