Fundamental Study on the Effect of Zn Addition Into Cu Matrix in DT Method Nb3Sn Conductors

Abstract

Kobe Steel, Ltd. has developed brass matrix distributed-tin (DT) method Nb3Sn wires, aiming to achieve both high J c performance and high robustness. In this work, microstructural study on the diffusion reaction behavior of those wires was conducted, to understand in detail the characteristic differences in use of Cu and brass matrix, and the effect of Zn addition into Cu matrix was discussed towards further J c improvement. There were clear differences in the reaction behavior of Sn/Cu interdiffusion between the pure Cu and the brass matrix wires before Nb3Sn formation: for instance, after a heat treatment at 400 °C/200 h, δ and ϵ phases form at the reaction layer in the Cu matrix wire, while β-CuZn and ϵ phases form in the brass matrix wire. The β phase is solid and appears to be effective to suppress void growth: it is preferable to avoid void formation, as void physically suppresses the Sn diffusion. Brass matrix promoted Sn penetration into Nb subelement modules. Those characteristics in use of brass matrix are advantageous to improve Sn distribution across the cross-section prior to Nb3Sn formation step, which would contribute to more homogeneous Nb3Sn layer formation across the cross-section. The non-Cu J c of 2nd trial brass DT Nb3Sn wire at 16 T and 4.2 K is 1,100 A/mm2.