Compositional and microstructural profiles across Nb3Sn filaments producedby different fabrication methods

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