Characterization of Nb3Sn diffusion layer (A-15) material

Microstructural characterization of Nb
3
Sn diffusion layers grown by Sn-vapor reaction with single crystal Nb substrates has been carried out using Auger and X-ray spectroscopy, X-ray and reflection electron diffractometry, and scanning electron microscopy. The layers exhibit preferred crystallographic orientation traceable to the underlying Nb-substrate orientation. The
\langle 111 rangle
-Nb-substrate layers differ substantially from all other-Nb-direction layers. In particular, they exhibit surface-like composition profiles over several hundred angstroms into the bulk, a very rough micro-structure including numerous microcracks, and a surface superconducting transition temperature of < 1 K. Non-
\langle 111 rangle
-Nb-substrate layers with 1 μm columnar grain structure and surface T
c
's ∼ 18 K appear to have breaking stresses far in excess of bulk polycrystalline Nb
3
Sn.