Compositional and microstructural profiles across Nb/sub 3/Sn filaments produced by different fabrication methods

The next generation of high field magnets for fusion and accelerator applications requires the development of Nb/sub 3/Sn strands 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 bronze-processed Nb/sub 3/Sn filaments with sharp interfaces for both Cu(Sn)-Nb/sub 3/Sn and Nb/sub 3/Sn-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.