Phase Evolution During Heat Treatment of Nb₃Sn 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.