Coarse Nb₃ Sn Grain Formation and Phase Evolution During the Reaction of a High Sn Content Internal Tin Strand

The formation of coarse Nb3Sn grains in Internal Tin (IT) strands has been studied at the example of a prototype strand with high Sn content. Metallographic examination revealed that the comparatively low critical current density (Jc) of this strand is partly due to the formation of a significant fraction of coarse grained Nb3Sn at the periphery of the individual filaments within the subelements. The phase evolution during the reaction heat treatment has been determined in situ by high energy synchrotron X-ray diffraction as well as ex situ by Energy Dispersive X-ray Spectroscopy in a Scanning Electron Microscope (SEM) in order to identify the conditions under which the coarse grains form. Similar to what is observed in the tubular type strands, Nb3Sn coarse grain formation occurs in the filament areas that had first been transformed into NbSn2and Nb6Sn5, prior to Nb3Sn formation, and it accounts for an estimatedJcreduction of roughly 20%. The amount of Cu-Nb-Sn and NbSn2that is formed during the heat treatment can be reduced by increasing the temperature ramp rate, while the amount of Nb6Sn5formed appears to be hardly influenced by the different heat treatments that have been tested.