Development, heat treatment optimization and microstructural characterization of Nb3Sn superconductor wire

The optimization of the heat treatment profile for Nb₃Sn superconductor wires is one of the most difficult steps to process this material. The present work had the objective to optimize the heat treatment profile to form the Nb₃Sn phase. It was developed an internal-tin Nb₃Sn strand with 1.0 mm in diameter, 12 065 Nb filaments with average diameter of 3.4 μm, 26 cores of Sn and internal stabilization of Cu surrounded by Ta diffusion barrier. Samples were removed for heat treatment optimization using different profiles to analyze the Sn diffusion and CuSn formation at temperatures up to 575 °C. A final treatment at 700 °C was used to form the Nb₃Sn phase. After the heat treatments, samples were removed for microstructural characterization. Measurements of T_c and J_c were performed to analyze the influence of the heat treatments on the superconducting phase and on the transport properties. It was concluded that the use of a two step heat treatment of 575 °C/100 h + 700 °C/100 h is as efficient as the use of a heat treatment with more low temperature steps.