The interdiffusion of Cu and Sn in internal Sn Nb/sub 3/Sn superconductors

For Nb/sub 3/Sn superconductors manufactured by the internal Sn process, the most appropriate heat treatment is still a matter of debate. The major complications are the low melting temperature of Sn and the 7 Cu-Sn phases. Each phase has its own diffusivity and atomic volume, which differs significantly from the pure Sn and Cu phases from which the diffusion starts. Void formation near the Nb filaments, due to the presence of high-Sn phases, complicates both the reaction kinetics and thermodynamics, often introducing local non-uniformity. To try to understand these problems better, 24 hour heat treatments approximately 10/spl deg/C above and below the Cu-Sn invariant temperatures have been performed on a Nb/sub 3/Sn ITER strand. The pure Sn core was replaced by /spl eta/-phase bronze after 24 hours at 237/spl deg/C. After 24 hours at 401/spl deg/C, the /spl eta/-phase itself was completely replaced by /spl epsi/-phase. It appears that /spl gamma/-phase undergoes a eutectoid reaction, breaking down into /spl alpha/ and /spl delta/ phases. Mg initially in the Sn core was only found in a Cu-Sn-Mg ternary at all heat treatment temperatures.