The interdiffusion of Cu and Sn in internal SnNb3Sn superconductors

For Nb3Sn superconductors manufactured by the internal Sn process, the most appropriate heat treatment is still a matter of debate. The major complica tions are the low melting temperature of Sn and the 7 Cu-Sn phases. Each ph ase has its own diffusivity and atomic volume, which differs significantly from the pure Sn and Cn phases from which the diffusion starts. Void format ion near the Nb filaments, due to the presence of high-Sn phases, complicat es both the reaction kinetics and thermodynamics, often introducing local n on-uniformity. To try to understand these problems better, 24 hour heat tre atments approximately 10 degrees C above and below the Cu-Sn invariant temp eratures have been performed on a Nb3Sn ITER strand. The pure Sn core was r eplaced by eta-phase bronze after 24 hours at 237 degrees C. After 24 hours at 401 degrees C, the eta-phase itself was completely replaced by epsilon- phase. It appears that gamma-phase undergoes a eutectoid reaction, breaking down into alpha and delta phases. Mg initially in the Sn core was only fou nd in a Cu-Sn-Mg ternary at all heat treatment temperatures.