Effect of thermal annealing on the superconductivity of Nb-Ta-Hf-based multicomponent alloys

We investigated the effects of thermal annealing on superconducting critical properties—critical temperature (T_c), critical current density (J_c), and upper critical field (H_c2)—of several multicomponent alloys of Nb_2/6Ta_1/6Hf_1/6Zr_1/6Ti_1/6, Nb_2/6Ta_2/6Hf_1/6Zr_1/6, Nb_2/6Ta_2/6Hf_1/6Ti_1/6, and Nb_2/6Ta_2/6Hf_2/6. Because the mixing entropy in multicomponent alloys strongly depends on the number of principal elements, the alloy Nb_2/6Ta_1/6Hf_1/6Zr_1/6Ti_1/6 is expected to exhibit the highest level of atomic disorder among the four. Interestingly, despite its high disorder, this high-entropy alloy (HEA) Nb_2/6Ta_1/6Hf_1/6Zr_1/6Ti_1/6 demonstrates superior superconducting performance compared to the other medium-entropy alloys (MEAs) Nb_2/6Ta_2/6Hf_1/6Zr_1/6, Nb_2/6Ta_2/6Hf_1/6Ti_1/6, and Nb_2/6Ta_2/6Hf_2/6. This suggests a synergistic interaction among its constituent elements that enhances superconductivity. Following thermal annealing, all alloys showed significant improvement in J_cs, which was primarily attributed to enhanced normal point pinning. These results offer valuable insights into the underlying mechanisms of thermal annealing and its potential for optimizing the performance of multicomponent superconducting alloys.