Phase Evolution During Heat Treatment of Nb$_3$Sn Wires Under Development for the FCC Study

In recent years, the phase formation sequence during heat treatment of Nb$_3$Sn wires, and the influence of the microstructure and compositional homogeneity of Nb$_3$Sn on in-field critical current ($I_c$), have received increasing attention. For RRP wires, the importance of understanding and managing the formation of the ternary phase nausite has been demonstrated. However, a published Cu–Nb–Sn phase diagram including this phase is still not available; and conductor development for the Future Circular Collider (FCC) study has introduced a variety of less-studied internal tin wire layouts. In this article, a study of phase transformations in the ternary Cu–Nb–Sn system is summarized, and selected isothermal sections of the re-evaluated phase diagram are presented. The phase transformations during low-temperature heat treatment steps of wires developed for the FCC study are also presented and analyzed in comparison to established RRP conductors.