Understanding the formation of equiaxed α during dynamic precipitation in titanium alloys by elastoplastic phase field simulation

Dynamic α precipitation with hot deformation is commonly adopted to obtain equiaxed α for tailoring the mechanical properties of Ti alloys. The formation of an equiaxed shape rather than the typically observed α plate is closely correlated with the variations in nucleation rate and elastic strain energy induced by the external stress. However, it is still unclear how these changes result in the morphology transition of α precipitate. In this study, a multiphase-field and crystal plasticity integrated model was proposed to describe the coevolution of dynamic α precipitation and plastic deformation. It was found that the plasticity relaxes the elastic strain energy, retards lengthening but promotes the thickening of α precipitate. The elastic interaction between the applied stress and eigenstrain increases the elastic strain energy, promotes lengthening but has a weak effect on the thickening of α precipitate. The competition of these two factors determines the shape of α precipitates. In the case of a small nucleation rate, the promotion effect of the elastic interaction on lengthening of α precipitate always exceeds the retardation effect of plasticity, resulting in the formation of plate-like α. Increasing the nucleation rate under the same magnitude of external stress can induce a larger plastic strain within the soft β matrix in the initial growth stage, which results in the competitive advantage of the retardation effect of plasticity. Therefore, the lengthening of α precipitate decreases to a similar extent as its thickening, and the precipitate morphology changes into an equiaxed shape.