Coarse ${\rm Nb}_{3}{\rm Sn}$ 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 (J_c) 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 NbSn₂ and Nb₆Sn₅, prior to Nb₃Sn formation, and it accounts for an estimated Jc reduction of roughly 20%. The amount of Cu-Nb-Sn and NbSn₂ that is formed during the heat treatment can be reduced by increasing the temperature ramp rate, while the amount of Nb₆Sn₅ formed appears to be hardly influenced by the different heat treatments that have been tested.