Enhancement of irreversibility field and critical current density of rare earth containing V0.60Ti0.40 alloy superconductor by cold-working and annealing

Abstract

β-V1−xTix alloy superconductors are considered to be promising materials for high magnetic field applications. So far, attempts to improve the critical current density (JC) of β-V1−xTix alloys have shown limited success. Improving JC requires a controlled generation of defects. Similar to the V0.6Ti0.4-RE (RE = Gd, Y) alloys, RE = Ce, Dy and Nd are also immiscible in the V0.6Ti0.4 matrix. The superconducting transition temperature (TC), upper critical field (HC2), irreversibility field (HIrr), and JC increase with the RE addition. However, the tensile strength of V0.6Ti0.4-Gd alloy is observed to be significantly lower than that of V0.6Ti0.4 alloy. Cold-working is found to further improve the TC, HC2, HIrr, and JC of all the V0.6Ti0.4-RE alloys. Successive cold-working (with 50% reduction of thickness each time) and annealing (SCA) at 450∘C for 5 hrs is found to significantly improve the HIrr, and JC of V0.6Ti0.4-RE (RE = Gd, Ce) alloys. The tensile strength is also found to increase to about 60–70% of the V0.6Ti0.4 alloy after the third annealing. It is observed that α′ and ω phases form at the defect sites at various stages of cold-working and annealing. JC(H = 0) and HIrr are about 840 Amm−2 and 7 T respectively for the as-cast V0.6Ti0.4-RE alloys. Cold-working on the V0.6Ti0.4-RE alloys further improves the JC(H = 0) and HIrr to about 1250 Amm−2 and 8.45 T respectively. SCA increases the JC(H = 0) and HIrr to about 4000 Amm−2 (or more) and 9 T respectively, and the JC(7 T) to about 500 Amm−2 in V0.6Ti0.4-RE alloy at 4 K. We present a detailed description of the defect structure in these alloys and its role in pinning the magnetic flux lines, thereby improving the overall JC.