Abstract
The effect of heat-treatment on the critical current density, Jc, was investigated in heavily cold-worked Nb-Ti alloys. The specimens containing 55 to 70 at%Ti and the balance Nb were reduced 75.00 to 99.99% in area by cold-working and then aged at temperatures from 350 to 550°C. The results are summarized as follows:
(1) When the specimens are aged, Jc increases initially, reaches a maximum value and then decreases. The maximum value of Jc becomes higher at a less cold-reduction prior to aging, at a lower aging temperature or with a higher Ti content.
(2) When the cold-drawn wires are heat-treated, the α phase precipitates preferentially at the fiber boundaries. The precipitates at the fiber boundaries grow coarser than those within the fibers and the amount of the former increases with increasing cold-reduction prior to the heat treatment.
(3) In precipitation of a α phase from a β supersaturated solid solution, precipitates become finer and more numerous at a lower precipitate temperature or with a higher solute Ti content in the β phase.
(4) The increase of Jc in specimens heat-treated after cold-working is mainly attributed to the precipitation of the α phase and Jc is found to be inversely proportional to a precipitate spacing. The highest Jc is obtained by the heat-treatment which produces a fine dispersion of precipitates.
(5) A Nb-65.0 at%Ti alloy is the most suitable composition for a superconducting magnet coil material. For this alloy cold-worked by 98.44% reduction and then heat-treated at 350°C for 340 h, the values of Jc at 4.2 K were 2×107 A/m2 at 3.2 MA/m, 1×107 A/m2 at 4.8 MA/m and 5×106 A/m2 at 6.4 MA/m, respectively.
