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 highJcstrands 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 duringin-situreaction HT of a state-of-the-art highJcNb3Sn internal tin strand. In this strand, Cu3Sn is formed upon Cu6Sn5decomposition at 415degC, a Sn-rich ternary Cu-Nb-Sn phase is detected in the approximate temperature interval 345degC-575degC, and NbSn2is present in the temperature interval 545degC-630degC. The formation of voids in the strand subelements has been monitored by synchrotron microtomography duringin-situreaction HT.