Comparative drawability and recrystallization evaluation of Nb4Ta and Nb4Ta1Hf alloys, and the beneficial influence of Hf on developing finer Nb3Sn grain size

Abstract

Nb3Sn superconducting wires with high critical current density (Jc, 4.2 K) at fields greater than 15 T are needed for next-generation superconducting magnets for high-energy physics and nuclear magnetic resonance applications. It has been demonstrated that the addition of one atomic % Hf can significantly increase the high field Jc of Ta-doped Nb3Sn and offer the potential to meet the Future Circular Collider (FCC) specification of 1500 A/mm2 at 16 T, 4.2 K if the pure Nb filaments in conventional Nb3Sn composites can be replaced by Nb4Ta1Hf alloy. In this paper we demonstrate that a commercially produced Nb4Ta1Hf alloy is ductile and drawable in a Cu matrix to a large true strain of 15 without intermediate annealing. The hardness and work-hardening rates of Nb4Ta1Hf are shown to be higher than Nb and commercial Nb4Ta, but are comparable to ductile Nb47Ti alloy at high strains, which is significant because more than 1000 tons of Nb-Ti rods are co-drawn in Cu each year. Very importantly for the Nb3Sn properties is that just 1 at%Hf shifts the recrystallization curve of Nb4Ta to temperatures above the normal 600 °C −700 °C A15 range of reaction temperatures used for Nb3Sn wires. For the Hf-based 19 filament rod in tube (RIT) conductors of this study, we observe a refined (<100 nm) Nb3Sn grain size, and an improved upper critical field of 24.6 T (4.2 K), ∼1 T higher than that in a corresponding Nb4Ta composite. The demonstration of improved performance and good drawability with commercially produced Nb4Ta1Hf alloy indicates a pathway for Nb3Sn strand development for next-generation Nb3Sn magnets.