We report on atomic-scale analyses of the microstructure of an Nb 3 Sn coating on Nb, prepared by a vapor diffusion process for superconducting radiofrequency (SRF) cavity applications using transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and first-principles calculations.Epitaxial growth of Nb 3 Sn on a Nb substrate is found and four types of orientation relationships (ORs) at the Nb 3 Sn/Nb interface are identified by electron diffraction or high-resolution scanning transmission electron microscopy (HR-STEM) analyses. Thin Nb 3 Sn grains are observed in regions with a low Sn flux and they have a specific OR: Nb 3 Sn (12 ̅ 0)//Nb (1 ̅ 11) and Nb 3 Sn (002)//Nb (01 ̅ 1). The Nb 3 Sn/Nb interface of thin grains has a large lattice mismatch, 12.3%, between Nb (01 ̅ 1) and Nb 3 Sn (002) and a high density of misfit dislocations as observed by HR-STEM. Based on our microstructural analyses of the thin grains, we conclude that the thin regions are probably a result of a slow interfacial migration with this particular OR. The Sn-deficient regions are seen to form initially at the Nb 3 Sn/Nb interface and remain in the grains due to the slow diffusion of Sn in bulk Nb 3 Sn. The formation of Sn-deficient regions and the effects of interfacial energies on the formation of Sn-deficient regions at different interfaces are estimated by first-principles calculations. The finding of ORs at the Nb 3 Sn/Nb interface provides important information about the formation of imperfections in Nb 3 Sn coatings, such as large thin-regions and Sn-deficient regions, which are critical to the performance of Nb 3 Sn SRF cavities for accelerators.

We report on atomic-scale analyses using high-resolution scanning transmission electron microscopy (HR-STEM), atom-probe tomography (APT), and first-principles calculations to study grain-boundary (GB) segregation behavior of Nb 3 Sn coatings on Nb, prepared by a vapor-diffusion process for superconducting radiofrequency (SRF) cavity applications. The results reveal Sn segregation at GBs of some Nb 3 Sn coatings, with a Gibbsian interfacial excess of ~10-20 Sn atoms/nm 2 . The interfacial width of Sn segregation at a GB is ~3 nm, with a maximum concentration of ~35 at.%. HR-STEM imaging of a selected [12 ̅ 0] tilt GB displays a periodic array of the structural unit at the core of the GB, and firstprinciple calculations for the GB implies that excess Sn in bulk Nb 3 Sn may segregate preferentially at GBs to reduce total internal energy. The amount of Sn segregation is correlated with two factors: (i) Sn supply; and (ii) the temperatures of the Nb substrate and Sn source, which may affect the overall kinetics including GB diffusion of Sn and Nb, and the interfacial reaction at Nb 3 Sn/Nb interfaces. An investigation of the correlation between the chemistry of GBs and Nb 3 Sn SRF cavity performance reveals no significant Sn segregation at GBs of high-performance Nb 3 Sn SRF cavities, indicating possible effects of GB segregation on the quality (Q 0 ) factor of Nb 3 Sn SRF cavities. Our results suggest that the chemistry of GBs of Nb 3 Sn coatings for SRF cavities can be controlled by grain-boundary engineering, and can be used to direct fabrication of high-quality Nb 3 Sn coatings for SRF cavities.

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