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We report on atomic-scale analyses of grain boundary (GB) structures andsegregation in Nb3Sn coatings on Nb, prepared by the vapor-diffusion process,for superconducting radiofrequency (SRF) cavity applications, utilizingatom-probe tomography, high-resolution scanning transmissionelectron-microscopy and first-principles calculations. We demonstrate that thechemical composition of Nb3Sn GBs is correlated strongly with the diffusion ofSn and Nb at GBs during the coating process. In a sample coated with arelatively large Sn flux, we observe an interfacial width of Sn segregation ata GB of ~3 nm, with a maximum concentration of ~35 at.%. After post-annealingat 1100 oC for 3 h, the Sn segregated at GBs disappears and Nb segregation isobserved subsequently at GBs, indicating that Nb diffused into the Nb3Sn GBsfrom the Nb substrate. It is also demonstrated that the amount of Snsegregation in a Nb3Sn coating can be controlled by: (i) Sn flux; and (ii) thetemperatures of the Nb substrates and Sn source, which may affect the overallkinetics including GB diffusion of Sn and Nb. An investigation of thecorrelation between the chemical compositions of GBs and Nb3Sn SRF cavityperformance reveals that the Nb3Sn SRF cavities with the best performance(high-quality factors at high accelerating electric-field gradients) do notexhibit Sn segregation at GBs. Our results suggest that the chemicalcompositions of GBs in Nb3Sn coatings for SRF cavities can be controlled by GBengineering and can be utilized to optimize fabrication of high-quality Nb3Sncoatings for SRF cavities.

Grain-boundary structure and segregation in Nb3Sn coatings on Nb for high-performance superconducting radiofrequency cavity applications