  We report an atomic-scale analysis of the microstructure of Nb3Sn coating on
Nb prepared by vapor diffusion process for superconducting radiofrequency (SRF)
cavity application using transmission electron microscopy (TEM). Epitaxial
growth of Nb3Sn on the Nb substrate is found and four types of orientation
relationships at the Nb3Sn/Nb interface are identified by electron diffraction
or high-resolution scanning transmission electron microscopy (STEM) analysis.
Thin Nb3Sn grains are observed in regions with low Sn flux and they have the
specific orientation relationship, Nb3Sn (1-20)//Nb (-111) and Nb3Sn (002)//Nb
(0-11). The Nb3Sn/Nb interface of thin grains had a large lattice mismatch,
12.3 at.%, and a high density of misfit dislocations was observed by HR-STEM.
Based on our microstructural analysis of the thin grains, we conclude that the
thin regions are probably a result of a slow interfacial reaction with this
particular orientation relationship at the interface. The Sn-deficient regions
are seen to form initially at the Nb3Sn/Nb interface and remain in the grains
due to the slow diffusion of Sn in bulk Nb3Sn. The formation of Sn-deficient
regions and the effects of strain and interfacial energies on the formation of
Sn-deficient regions at various interfaces were also estimated by
first-principle calculation. The finding of orientation relationships at the
Nb3Sn/Nb interface provides important information about the formation of
defects in Nb3Sn coatings such as large thin regions, Sn-deficient regions,
which are critical to the performance of Nb3Sn superconducting radiofrequency
cavities for accelerators.
