Variant selection of grain boundary α by special prior β grain boundaries in titanium alloys

Abstract

During β-processing of α/β and β titanium alloys, variant selection (VS) of grain boundary α (GBα) is one of the key factors in determining the final transformation texture and mechanical properties. It has been observed frequently that GBα prefers its 〈0001〉α pole to be parallel to a common 〈110〉β pole of the two adjacent β grains and results in a micro-textured region across the grain boundary (GB) and, as a consequence, slip transmission may take place more easily across that GB. In order to investigate how such a special prior β GB contributes to VS of GBα, we develop a crystallographic model based on the Burgers orientation relationship (BOR) between GBα and one of the two β grains. The model predicts all possible special β grain boundaries at which GBα is able to maintain BOR with both β grains. A new measure for VS of GBα, (βΔJβBOR), i.e., a measure of the deviation of the actual OR between the GBα and the non-Burgers grain from the BOR, is proposed. For the particular alloy chosen for experimental observations, Ti-5553, it is found that when the misorientation angle θm of (βΔJβBOR) (instead of the closeness between two closet {110}β poles between two β grains widely used in literature) is less than 15°, misorientation between the two β grains dominates the VS of GBα and, in particular, the variant with minimum θm is always selected for GBα. A possible effect due to grain boundary plane inclination on VS is also discussed.