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The longitudinal and transverse cross-sectional microstructures of several internal tin Nb3Sn strands have been systematically investigated. The critical current densities of these strands were then correlated with their design parameters. It is observed that the occurrence of certain coarse grain structures is related to the location of the filaments with respect to the subelements as well as to the strand. Experimental evidence suggests that the existence of these coarse grains is related to Sn distribution during the early stages of the heat treatment. It is also noticed that some coarse grains have high aspect ratio features, confirming the need to study the longitudinal fracture surface. We report in this paper the observation of some unusual grain sizes and morphologies. It appears that, of the high-Sn content Nb3Sn conductors investigated in this paper, a strand's global composition has a weaker influence on the critical current density (Jc) than local and structural factors, such as the local Cu:Nb area ratio (LAR) and filament layout.
Date of Publication: June 2011