Nonuniform Distribution of Elements in RRP® Nb3Sn Wires Doped Using Nb–Ti Rods

In the restacked rod process (RRP^®) Nb3Sn wires studied, Ti doping is achieved by replacing some Nb filaments with the standard Nb-47 wt% Ti alloy. We observed that, after full heat treatment, the amount of Ti in Nb₃Sn does not increase linearly with the nominal doping, suggesting segregation of Ti. We found that, locally, at the doping sites, the Ti content can be as much as an order of magnitude higher than nominal, but this is not always the case: the composition at the doping sites varies dramatically. Furthermore, in the highest nominal doping wire studied, Ti-containing compounds were also observed surrounding the Cu-Sn core. The relative composition trend of Cu and Ti appears to go in opposite directions. Using Auger electron spectroscopy (AES), significant spatial variations of Ti concentration were found, and Ti LMV peak broadening was observed at the original doping sites.