Crystallography of grain boundary α precipitates in a β titanium alloy

The crystallography of α(hcp) precipitates formed on the β(bcc) matrix grain boundaries has been studied with transmission electron microscopy (TEM) in a Ti-15V-3Cr-3Sn-3Al alloy. The a precipitates have a near-Burgers orientation relationship with respect to at least one of the adjacent β grains. Among the possible 12 variants in this orientation relationship, the variant that [112̄0]_α is parallel to the 〈111〉_β closest to the grain boundary plane tends to be preferred by the α precipitates. Additionally, further variant selections are made so as to minimize the deviation of orientation relationship with respect to the "opposite" β 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 β matrix changes the variant of α 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 a precipitates. Discussion of these results is based upon a classical nucleation theory.