  We report on atomic-scale analyses of grain boundary (GB) structures and
segregation in Nb3Sn coatings on Nb, prepared by the vapor-diffusion process,
for superconducting radiofrequency (SRF) cavity applications, utilizing
atom-probe tomography, high-resolution scanning transmission
electron-microscopy and first-principles calculations. We demonstrate that the
chemical composition of Nb3Sn GBs is correlated strongly with the diffusion of
Sn and Nb at GBs during the coating process. In a sample coated with a
relatively large Sn flux, we observe an interfacial width of Sn segregation at
a GB of ~3 nm, with a maximum concentration of ~35 at.%. After post-annealing
at 1100 oC for 3 h, the Sn segregated at GBs disappears and Nb segregation is
observed subsequently at GBs, indicating that Nb diffused into the Nb3Sn GBs
from the Nb substrate. It is also demonstrated that the amount of Sn
segregation in a Nb3Sn coating can be controlled by: (i) Sn flux; and (ii) the
temperatures of the Nb substrates and Sn source, which may affect the overall
kinetics including GB diffusion of Sn and Nb. An investigation of the
correlation between the chemical compositions of GBs and Nb3Sn SRF cavity
performance reveals that the Nb3Sn SRF cavities with the best performance
(high-quality factors at high accelerating electric-field gradients) do not
exhibit Sn segregation at GBs. Our results suggest that the chemical
compositions of GBs in Nb3Sn coatings for SRF cavities can be controlled by GB
engineering and can be utilized to optimize fabrication of high-quality Nb3Sn
coatings for SRF cavities.
