Phase Transformations During the Reaction Heat Treatment of Internal Tin Nb $_{3}$ Sn Strands With High Sn Content

The phase transformations that occur during the reaction heat treatment (HT) of Nb₃Sn superconductors depend on the overall elemental composition of the strand subelements. In the case of modern high Jc strands with a relatively low Cu content, liquid phases are present during large temperature intervals and phases that can be detrimental for the microstructural and microchemical homogeneity of the fully reacted strand are formed. We report synchrotron X-ray diffraction measurements during in-situ reaction HT of a state-of-the-art high Jc Nb₃ Sn internal tin strand. In this strand, Cu₃Sn is formed upon Cu ₆Sn ₅ decomposition at 415degC, a Sn-rich ternary Cu-Nb-Sn phase is detected in the approximate temperature interval 345degC-575degC, and NbSn₂ is present in the temperature interval 545degC-630degC. The formation of voids in the strand subelements has been monitored by synchrotron microtomography during in-situ reaction HT.