Precipitates and fluxoid pinning in a superconducting Nb-Hf alloy

Abstract

Abstract The relationship between superconducting fluxoid pinning and precipitate dispersion was investigated in a high-field type II superconducting alloy. The alloy, Nb-38 at. % Hf, is a b.c.c. solid solution above 1200°C; at lower temperatures the equilibrium structure is two-phase: b.c.c. β and Hf-rich h.c.p. a. The quenched alloy was aged at 600°C. The changes in microstructure during ageing were examined by transmission electron microscopy. The attendant changes in superconducting properties—transition temperature (T c) and critical flux gradient (dB/dr)—were measured by an a.c. susceptibility technique. It was found that precipitation at 600°C proceeds by the formation and growth of strained, partially coherent particles in the b.c.c. matrix termed transition alpha, αt. Particle size measurements indicated that growth of at follows a t 1/3 coarsening law after times not longer than ∼ 0·5 hr. Transition temperature measurements substantiate this finding. Measurements of dB/dr showed little change fr...