Effect of Hf Addition to Nb on Nb3Sn Grain Morphology Under High Sn Diffusion Driving Force

Grain refinement and performance improvements for Nb3Sn 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 Nb3Sn 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 Nb3Sn 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 Nb3Sn phase formation temperature. Nevertheless, our previous study on the addition of Hf-Ta to bronze-route Nb3Sn 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 Nb3Sn 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 Nb3Sn during the heat treatment. Grain refinement in Hf-doped Nb3Sn 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.