Crystallography of α precipitates nucleated at β grain boundaries in metastable β titanium alloys

The crystallography of grain boundary alpha(HCP) precipitates nucleated at beta(BCC) matrix grain boundaries have been studied with TEM and SEM in a Ti-22 mass%V-4 mass%Al and a Ti-15 mass%V-3 mass%Cr-3 mass%Sn-3 mass%Al alloys. The relative orientation relationship between an alpha precipitate and adjacent beta grains was determined by the Kikuchi pattern analysis. The alpha precipitate shows a globular allotriomorph or an elongated film morphology. Their shape is an elongated lath with a growth direction lying close to the grain boundary plane. Although intragranular alpha precipitates have a near-Burgers orientation relationship (OR), the grain boundary alpha precipitates have a near-Potter or near-{1 (1) over bar 01}(alpha)/{110}(beta) OR with respect to at least one of the adjacent beta grains. According to the crystallographical feature, they are classified in two types; Type A: a near-(1 (1) over bar 01)alpha/{110}(beta) OR to one of the beta grains and no specific OR to the other one. Type B: a near-{1 (1) over bar 01}(alpha)/{110}(beta) OR to both beta grains. This {1 (1) over bar 01}(alpha) plane is confirmed to be parallel to one of the twelve {110}(beta) planes in the adjacent beta grains, which has smaller angle with the grain boundary plane. Therefore, the variants of an alpha precipitate at a beta grain boundary are selected from 48 (24 possible variants in each beta grain for the Potter OR) to 4 at the most, and it reduces in case of good coherency to the opposite beta grain.