Impact of Ti-doping position on Nb3Sn layer formation in internal Sn-processed Nb3Sn superconducting wires

Further enhancement of critical current density of Nb3Sn superconductors is still of great interest and challenging towards realization of leading-edge magnets such as the future circular collider (FCC) and demonstration Power Station (DEMO) fusion reactors. In this study, the influence of the Ti-doping position on the Nb3Sn layer formation in internal Sn-processed Nb3Sn superconducting wires, which has not been sufficiently clarified yet, was investigated using the simplest diffusion pair structure that consists of a single Nb/Cu/Sn layer. Three types of specimens were prepared, in which Ti was doped into the Sn core, Nb, and Cu matrix (denoted as ST, NT, and CT, respectively). The NbCuSnTi compound phase appeared as a diffusion barrier at the boundary between the Nb3Sn and Cu-Sn phases in the ST and CT samples and remained during the Nb3Sn layer formation, although it was absent in NT. The electron backscatter diffraction analysis of the formed Nb3Sn grain morphologies showed no significant difference in terms of the average grain size and area fraction of the equiaxed grains regardless of the doping position. However, the average Nb3Sn layer thickness of NT was at least 1.6 times larger than that of ST and CT. The absence of the NbCuSnTi phase promotes Sn diffusion into Nb, which accounts for the thicker Nb3Sn layer and much higher I-c-B characteristics in NT.