Crystallography of grain boundary alpha precipitates in a beta titanium alloy

The crystallography of alpha(hcp) precipitates formed on the beta(bcc) matrix grain boundaries has been studied with transmission electron microscopy (TEM) in a Ti-15V-3Cr-3Sn-3Al alloy. The alpha precipitates have a near-Burgers orientation relationship with respect to at least one of the adjacent beta grains. Among the possible 12 variants in this orientation relationship, the variant that [11 (2) over bar 0](alpha) is parallel to the [111](beta) closest to the grain boundary plane tends to be preferred by the alpha precipitates. Additionally, further variant selections are made so as to minimize the deviation of orientation relationship with respect to the ''opposite'' beta grain from the Burgers one. Such rules in variant selection often result in the formation of precipitates with a single variant at a planar grain boundary. Prior small deformation of beta matrix changes the variant of alpha precipitates at the deformed portion of grain boundary. It is considered that the stress field of dislocations in the slip bands intersecting with the boundary strongly affects the variants of alpha precipitates. Discussion of these results is based upon a classical nucleation theory.