Influence of Ti-Hf Doping on the Nb$_{3}$Sn Layer Formation in Bronze-Processed Nb$_{3}$Sn Wire Structure

Hf-Ta doping of Nb
$_{3}$
Sn superconductors is of considerable interest, as it can lead to a significant refinement of the grain morphology of the Nb
$_{3}$
Sn layer, maintaining the critical magnetic field. In this study, we investigated the microstructure and superconducting properties of bronze-processed Nb
$_{3}$
Sn wires using an Nb-Ti-Hf alloy core / Cu-Sn matrix. The wires had a single-core structure for simplicity. Conventional structure wires with Nb-Ti/Cu-Sn and Nb/Cu-Sn-Ti diffusion couple combinations were also prepared as references. The direct Ti-Hf doping of the Nb core resulted in a better
$J_{\mathrm{c}}$
property than direct Ti doping. However, the thickness of the Nb
$_{3}$
Sn layer of the conventional external Ti-doped (Ti-bronze) wire sample was large. A high Ti composition in the Nb
$_{3}$
Sn layer was also observed in this sample. Electron backscatter diffraction analysis suggested that there was no significant difference in the morphological features, although the addition of Ti-Hf, as well as Hf-Ta, to the Nb core resulted in an increase in the internal strain energy. Consequently, the layer
$J_{\mathrm{c}}$
—
$B$
characteristic of the conventional Ti-bronze sample was the highest, and the impact of Ti-Hf doping was not significant in the bronze process compared with conventional external Ti doping. Thus, the Ti doping of the bronze matrix enhanced the Sn diffusion driving force, which had a relatively large impact on improving the Nb
$_{3}$
Sn layer growth kinetics and grain refinement during the bronze process.