Compositional and Microstructural Profiles across Nb3Sn Filaments Produced by Different Fabrication Methods

The next generation of high field magnets for fusion and accelerator applications requires the development of Nb3Sn strand with significantly higher critical current densities. Our recent work has shown that when the specific pinning force is normalized to the grain boundary density (the primary pinning site in this material), a strong increase in pinning force is observed for high Sn content strand, suggesting a strong compositional dependency for pinning in this material. In this study we use advanced FESEM techniques to quantify the compositional and microstructural variations to a sub-100 nm level. A stepped compositional and microstructural variation was observed across a bronze-processed Nb3Sn filaments with sharp interfaces for both Cu(Sn)-Nb3Sn and Nb3Sn-Nb. Microstructural variation was observed on the scale of the original filaments in high Sn, MJR but not across the coalesced filament mass. A very high microstructural uniformity was observed in a PIT monofilament. Thus a major influence of Sn appears to be exerted through its influence on the composition gradient.