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 of Nb$_{3}$Sn superconductors depend on the overall elemental composition of the strand sub-elements. 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 heat treatment of a state-of-the-art high Jc Nb$_{3}$Sn Internal Tin (IT) strand. In this strand Cu3Sn is formed upon Cu6Sn5 decomposition at 415 °C, a Sn-rich ternary Cu-Nb-Sn phase is detected in the approximate temperature interval 345-575°C and NbSn2 is present in the temperature interval 545-630°C. The formation of voids in the strand sub-elements has been monitored by synchrotron micro-tomography during in-situ reaction heat treatment.