Yagai Tsuyoshi

Further performance improvement in Nb3Sn conductors is strongly demanded towards realization of upcoming high-field magnet applications such as the Future Circular Collider (FCC) and the DEMOnstration power plant (DEMO). However, we are facing the problem that the J(c) performance of the Nb3Sn strands are almost fully optimized in terms of Nb/Cu/Sn area ratio, cross-sectional design, Nb filament diameter, and so on. We thus need some breakthrough to overcome the problem. In this context, we have been studying the new microstructure control by the element addition into the Cu matrix of the internal tin processed Nb3Sn conductors. In this paper, some results of the more fundamental study on the effect of the Zn addition are reported. in addition, results of the simultaneous addition of Zn and a small amount of Mg, and the influence of Ti-doping position on the microstructure and J(c )performance are reported. It was found that significantly different diffusion behaviors happen in the Cu-Zn/Sn diffusion reaction. For instance, in the Cu-Zn/Sn diffusion, a solid ternary Cu-Sn- Zn phase widely forms at the outermost reaction front at 400 degrees C heat treatment, whereas the porous epsilon phase widely forms in the Cu/Sn diffusion. A small addition of Mg into the brass matrix resulted in finer grain size and better performance. Ti doping to the Nb filaments but not Sn cores leads to elimination of Ti rich layer at the boundary that forms in Ti doping to Sn cores. The absence of Ti-rich layer contributes significantly to improvement of Sn and Ti distribution across the cross-section.