Abstract
The crystallography of grain boundary α(HCP) precipitates nucleated at β(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 α precipitate and adjacent β grains was determined by the Kikuchi pattern analysis. The α 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 α precipitates have a near-Burgers orientation relationship (OR), the grain boundary α precipitates have a near-Potter or near-{1\bar101}α⁄{110}β OR with respect to at least one of the adjacent β grains. According to the crystallographical feature, they are classified in two types; Type A: a near-{1\bar101}α⁄{110}β OR to one of the β grains and no specific OR to the other one. Type B: a near-{1\bar101}α⁄{110}β OR to both β grains. This {1\bar101}α plane is confirmed to be parallel to one of the twelve {110}β planes in the adjacent β grains, which has smaller angle with the grain boundary plane. Therefore, the variants of an α precipitate at a β grain boundary are selected from 48 (24 possible variants in each β grain for the Potter OR) to 4 at the most, and it reduces in case of good coherency to the opposite β grain.
