Flux Pinning in RHQT-Processed<tex>$rm Nb_3rm Al$</tex>After Various Transformation Heat Treatments

Abstract

Jelly-roll Nb-Al strands were rapidly heated and quenched (RHQ) to bcc phase from temperatures around 1900/spl deg/C. Transformation heat treatments were subsequently applied to convert the as-quenched bcc phase into superconducting A15 phase. These heat treatments consisted of a fast ramp (0.5 to 10 min) to 1000/spl deg/C followed by a long hold (10 h) at 800/spl deg/C. Magnetization measurements performed by vibrating sample magnetometry under fields up to 30 T were used to determine the irreversibility field, the upper critical field and the dependence of the critical current density on the field. These properties were all found to be higher for samples with high initial heating rate in the transformation heat treatment. Some variability of the critical current density was observed for samples prepared under apparently the same conditions. The scaling of the bulk pinning force density over temperatures ranging from 4.2 K to 8 K is analyzed.