Precipitation and evolution behaviors of grain boundary α and α side plates for ultra-high strength titanium alloy during continuous cooling

In this paper, precipitation behavior and evolution mechanism of grain boundary α (α_GB) and α side plates (α_SP) of ultra-high strength titanium alloy are investigated. Precipitation of α_GB can change original morphology of β grain boundary into irregular shapes, and it is much more obvious at slower cooling rate. The λ, which denotes the minimum value of misorientations between 12 α variants of β_nBOR and α_GB, is introduced to quantitatively evaluate morphology of α_GB. The β phase that maintains Burgers orientation relationship (BOR) with α_GB is labeled as β_BOR, conversely, it is labeled as β_nBOR. The deformation degree of α_GB increases with the increasing λ. However, the λ is not the only factor that determines morphology of α_GB, and it is also related to the angle between habit plane and grain boundary plane. Orientation of α_GB is affected by β phase on both sides. The α_GB maintains BOR with β_BOR and the s-BOR with β_nBOR. The s-BOR is defined as (0 0 0 1) plane of α_GB is not parallel to (1 1 0) plane of β_nBOR, but one of 〈1 1 –2 0〉 directions of α_GB is parallel or close to one of β_nBOR’s 〈1 1 1〉 directions.