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

Abstract

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 αGB (grain boundary α) is preferentially precipitated, followed by αI (intragranular α) during the continuous cooling process. Orientation of αGB 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 αGB. Orientation of the flat αGB is characterized by a single orientation or two similar orientations when the θ＜30°. The irregular αGB contains a greater number of α variants, and there is no obvious dominant variant when the θ＞30°. The αI precipitated at different positions can contain the theoretical 12 α variants. However, αI nucleated at αGB is easily affected by the orientation of αGB. In this case, the strong variant selection occurs, which leads to the formation of colony structure. By contrast, the variant selection of αI, which is precipitated spontaneously in β phase, is not obvious. Under this condition, the basket-weave structure is formed. In addition, the existence of αGB with single orientation provides a convenient condition for the formation of colony αI. The basket-weave structure has better comprehensive properties, however, the opposite is true for colony structure. Therefore, strong variant selection of αI needs to be inhibited in order to effectively prevent the formation of colony structure, which requires that the orientation characteristics of αGB must be controlled.