The microstructural state of Nb3Sn in a multifilamentary titanium doped bronze-process wire

The addition of titanium to bronze-processed Nb3Sn wires is known to increase the upper critical field and thus improve the high field performance. High resolution studies were done to determine the effect, if any, on the microstructure caused by Ti addition. This work used a Hitachi wire with a Nb filament diameter of 3.5 μm and a bronze to niobium ratio of 2.5. The bronze matrix had 7.5 at.% Sn and 0.4 at.% Ti. The microstructure of the Nb3Sn layer was studied in TEM on axial and longitudinal wire sections and in SEM on fractured wire surfaces. The results were compared with a previous study on a similiarly processed Ti free wire. It was found that the Ti addition caused no qualitative change in the microstructure. This result is contrasted with previous work on the beneficial effect of Mg additions which have been shown to be predominantly microstructural.