Improving the critical current density of the $\mathrm{V}_{0.59}\mathrm{Ti}_{0.40}\mathrm{Ce}_{0.01}$ alloy superconductor through successive cold-working and annealing at different temperatures

Abstract

The critical current density ($J_c$) of V-Ti alloy superconductors is strongly influenced by the size and distribution of microstructural defects that pin magnetic flux lines. In this work, the effect of successive cold working and annealing (SCA) at 550°C on the microstructure, superconducting properties, and flux-pinning behaviour of the V$_{0.59}$Ti$_{0.40}$Ce$_{0.01}$ alloy is investigated and compared with the previously reported SCA at 450 and 650°C. During SCA at 550°C, the superconducting transition temperature increases gradually with successive processing steps. The first annealing after cold rolling to 50% thickness produces a significant enhancement in $J_c$ over the measured field range, whereas subsequent SCA cycles result in only marginal changes. The $J_c$ remains relatively weakly dependent on magnetic field over wide magnetic-field range, and the final cold-worked sample exhibits a finite $J_c$ up to 8.5 T. Pinning force density analysis shows that grain boundaries dominate flux pinning in low magnetic fields, whereas dislocations and $β$-$α'$ interfaces become the dominant pinning centres in higher fields. Comparison of the different SCA temperatures shows that 650°C provides the highest low-field $J_c$, whereas 450°C gives the best high-field performance. In contrast, SCA at 550°C provides most balanced field dependence and the largest enhancement in $J_c$ relative to the corresponding as-cast alloy. Although the highest absolute $J_c$ in the high-field regime is achieved after the third SCA cycle at 450°C, a comparable value is obtained after only the first annealing at 550°C. These results demonstrate that the intermediate annealing temperature of 550°C provides an effective balance between defect generation, phase evolution, and recovery, resulting in enhanced flux pinning over a wide magnetic-field range.