Precipitates and fluxoid pinning in a superconducting Nb-Hf alloy

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 from the quenched solution behaviour for short ageing times. This was attributed to the precipitate size being appreciably less than the calculated fluxoid core diameter. At longer times a broad peak in dB/dr developed at a particular value of external field H. Following its initial appearance the peak moved to lower values of H with further particle growth. Comparison of calculated fluxoid spacing at peak field with average particle spacing calculated from measurements on the micrographs indicated a one-to-one correspondence within the experimental limits. An approximate calculation of the point fluxoid pinning force was made from the bulk pinning, force and the results are compared to theoretical predictions.