Morphology evolution and orientation characteristics of precipitate of ultra-high strength titanium alloy

Morphology evolution and orientation characteristics of precipitate of ultra-high strength titanium alloy was investigated in the present work. The continuous cooling tests were performed to control the precipitation of α phase after preservation in single-phase zone. The α_GB (grain boundary α) is preferentially precipitated, followed by α_I (intragranular α) during the continuous cooling process. Orientation of α_GB is related to orientation of β grains on both sides of grain boundary and habit plane. The θ is proposed to represent the angle between the axis of misorientation and normal of the habit plane, and it can be used as an important index to judge orientation characteristics of α_GB. Orientation of the flat α_GB is characterized by a single orientation or two similar orientations when the θ＜30°. The irregular α_GB contains a greater number of α variants, and there is no obvious dominant variant when the θ＞30°. The α_I precipitated at different positions can contain the theoretical 12 α variants. However, α_I nucleated at α_GB is easily affected by the orientation of α_GB. In this case, the strong variant selection occurs, which leads to the formation of colony structure. By contrast, the variant selection of α_I, which is precipitated spontaneously in β phase, is not obvious. Under this condition, the basket-weave structure is formed. In addition, the existence of α_GB with single orientation provides a convenient condition for the formation of colony α_I. The basket-weave structure has better comprehensive properties, however, the opposite is true for colony structure. Therefore, strong variant selection of α_I needs to be inhibited in order to effectively prevent the formation of colony structure, which requires that the orientation characteristics of α_GB must be controlled.