Effect of Gd addition on the superconducting properties of Ti-based V, Nb, Ta alloys

The critical current density ($J_c$) of the body centered cubic (bcc) V_0.6Ti_0.4 alloy enhances significantly after the addition of rare earth Gd as the latter is immiscible in the matrix [S. Paul, et.al, IEEE Trans. Appl. Supercond. 31, 5 (2021)]. Very low solubility of Gd in other bcc elements like Ta and Nb is also well known [Jr. KA Gschneidner in Prog Sci Technol Rare Earths, vol. 1, pp. 222–258, 1964 $\&$ M Neuberger, et.al in Handbook of Electronic Materials, Vol 4, 1972]. We use these facts to find the effect of adding 1 at.% Gd into the Nb_0.6Ti_0.4 and Ta_0.4Ti_0.6 alloys on the superconducting properties e.g., the transition temperature ($T_c$), $J_c$, flux pinning force density ($F_p$) and the microstructure. In spite of Gd being ferromagnetic, the $T_c$ in these alloys change only marginally (increase by 0.3 K in Ta_0.4Ti_0.6 and decrease by 0.15 K in Nb_0.6Ti_0.4 after Gd addition. The $J_c$ ($H=1$ T, T = 4 K) increases by 5 and 1.5 times respectively in the Gd containing Nb_0.6Ti_0.4 and Ta_0.4Ti_0.6 alloys, which is quite small as compared to the increase observed in the V_0.6Ti_0.4 (20 times) system. With Gd addition, the grain size reduces approximately by 65% and 10% respectively in Nb_0.6Ti_0.4 and Ta_0.4Ti_0.6. Our analysis indicates that grain boundaries are the major flux line pinning centres in these alloys and the role of Gd in increasing the $J_c$ depends on the effectiveness of Gd in reducing the grain size. The grain boundary density depends strongly on the distribution of Gd precipitates, which is quite different from each other for two alloy systems under study. Moreover, our results suggest that the addition of Gd to commercial Nb-Ti (Nb_0.37Ti_0.63) alloy is a new promising route for achieving higher $J_c$ values.