Microstructural study on Sn–Zn/Cu–Ti diffusion reaction for internal tin Nb3Sn conductor development

Abstract

To take advantage of Zn and Ti doping effects on improvement of diffusion behavior in the Sn–Cu mixing step and the Nb3Sn layer formation for the internal tin process, a diffusion couple configuration of Nb/Cu–Ti/Sn–Zn has been proposed. In this work, a fundamental study on the diffusion behavior of Sn–Zn/Cu–Ti, with different heat treatments, was conducted to aid in a better understanding of Nb3Sn phase formation, via internal tin diffusion. The first feature is that at 210∘C, the γ-CuZn phase forms at the reaction interface of Sn-20 wt%Zn and Cu, following which the β-CuZn and η-CuSn phases form at 400∘C. Compared with the Sn/Cu-12 wt%Zn system, Kirkendall voids appear to be suppressed, as few ε-CuSn phases, in which many voids grow, form in the Sn–Zn/Cu system. At 550∘C, a dendritic mixed phase of α-CuZn and δ-CuSn phases form. At this temperature, Sn appears to diffuse faster than in the case of the Sn/Cu–Zn system. It is observed that, after heat treatment at 550∘C, very fine compound particles of Sn–Ti are homogeneously distributed in the outer regions originally known to be the Cu–Ti sheath area. In the multifilamentary wires consisting of Nb/Cu–Ti/Sn–Zn diffusion couple configuration, no Ti rich compound layer was formed at the boundary of the Nb sub-elements, which overcomes the problem to suppress a smooth Sn and Ti diffusion into the Nb sub-element, when Ti is doped to Sn cores. Thus, no Ti-rich compound layer segregation, few void formations in the Sn–Cu mixing step and distributed fine Sn–Ti particles in the matrix accounts for the improved Jc characteristics in Nb/Cu–Ti/Sn–Zn wires. With regards to the mechanical properties, alloying of 20 wt% Zn to Sn cores increases the Vickers hardness of the Sn core to approximately 20kgf/mm2, almost double that of pure Sn or Sn-alloys with a small amount of Ti.