Phase Evolution during Heat Treatment of Nb3Sn Wires under Development for the FCC Study

Abstract

In recent years, the phase formation sequence during heat treatment of Nb3Sn wires, and the influence of the microstructure and compositional homogeneity of Nb3Sn on in-field critical current (Ic), 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.