Effect of Hf Addition to Nb on Nb$_{\text{3}}$Sn Grain Morphology Under High Sn Diffusion Driving Force

Grain refinement and performance improvements for Nb
$_{\text{3}}$
Sn superconducting wires are highly desired for realizing next-generation high-field magnets for the Future Circular Collider (FCC) project. The target performance is a non-Cu critical current density of 1500 A/mm
²
at 4.2 K under a 16 T field. Hf addition to Nb, which leads to Nb
$_{\text{3}}$
Sn grain refinement, has attracted intensive interest from the viewpoint of applicability to conventional wire manufacturing. The grain-refinement effect of Hf addition is speculated to be due to promotion of Nb
$_{\text{3}}$
Sn phase nucleation, which is caused by an increase of nucleation sites such as dislocations or misorientations in the parent Hf-doped Nb; the deformation-induced fine microstructure remains even at the Nb
$_{\text{3}}$
Sn phase formation temperature. Nevertheless, our previous study on the addition of Hf–Ta to bronze-route Nb
$_{\text{3}}$
Sn wires did not reveal a substantial effect on grain refinement. This lack of effect might be attributed to a smaller Sn diffusion driving force in the case of the bronze route. Therefore, we here study the Nb
$_{\text{3}}$
Sn phase formation behavior in the case of a higher Sn diffusion driving force using tapes with a diffusion couple of Nb and high-Sn-content Sn–Cu alloy. We prepared samples with a diffusion couple of Nb–4at%Ta–1at%Hf / Sn–10at%Cu, Nb–2at%Ti–1at%Hf / Sn–10at%Cu and conducted microstructural and microchemical analyses to evaluate the formation of Nb
$_{\text{3}}$
Sn during the heat treatment. Grain refinement in Hf-doped Nb
$_{\text{3}}$
Sn wires was confirmed under a high Sn diffusion driving force. However, the grain refinement effect was weaker than the effect expected on the basis of previous works.