Influence of Nb alloying on Nb recrystallization and the upper critical field of Nb3Sn

Abstract

${\mathrm{Nb}}_3\mathrm{Sn}$ conductors are important candidates for high-field magnets for particle accelerators, and they continue to be widely used for many laboratory and nuclear magnetic resonance magnets. However, the critical current density, $J_c$, of present ${\mathrm{Nb}}_3\mathrm{Sn}$ conductors declines swiftly above 12–15 T. State-of-the-art Ta- and Ti-doped strands exhibit upper critical field, $H_{c2}$, values of ∼24–26.5 T (4.2 K) and do not reach the Future Circular Collider target $J_c$, which serves as the present stretch target for ${\mathrm{Nb}}_3\mathrm{Sn}$ development. As recently demonstrated, to meet this goal requires enhanced vortex pinning but an independent and supplementary approach is to significantly enhance $H_{c2}$. In this study, we have arc melted multiple Nb alloys with added Hf, Zr, Ta, and Ti and drawn them successfully into monofilament wires to investigate the possibilities of $H_{c2}$ enhancement through alloying. $H_{c2}\left(T\right)$ was measured for all samples in fields up to 16 T and some up to 31 T. We have found that all alloys show good agreement with the standard Werthamer, Helfand, and Hohenberg fitting procedure without the need to adjust the paramagnetic limitation parameter (α) and spin-orbit scattering parameter $\left({\lambda }_{\mathrm{so}}\right)$. The evaluation of $d{H}_{c2}/dT$ near $T_c$, which is proportional to the electronic specific heat coefficient γ and the normal state resistivity ${\rho }_{\mathrm{n}}$, allows a better understanding of the induced disorder introduced by alloying in the A15 phase. So far, we have observed that Hf alloying of pure Nb can enhance $H_{c2}\left(0\right)$ by 3–4 T to ∼28 T, while adding just $1\text{at.}\%$ Hf or Zr into a Nb-4 at. % Ta base alloy can raise $H_{c2}\left(0\right)$ to ∼31 T. Very importantly we find that Hf and Zr raise the alloy recrystallization temperature above the usual A15 reaction temperature range of $650{}^{\circ }\mathrm{C}-750{}^{\circ }\mathrm{C}$, thus ensuring denser A15 phase nucleation in the Nb alloy grain boundaries, possibly leading to a more homogeneous A15 phase Sn content and refined A15 grain size. The potential for further advancements in ${\mathrm{Nb}}_3\mathrm{Sn}$ properties is explored in relation to the recrystallization of the Nb alloy and the factors controlling the upper critical field.