Assessment of alpha phase evolution in deformation of two-phase Ti-alloys under the off-equilibrium condition

In this paper, the microstructure development and flow behavior in the near isothermal forming of two-phase titanium alloys were studied through designing an off-equilibrium analog experiment. The results show that deformation decreases the volume fraction of primary alpha (α_p) phase significantly at high temperature. This can be ascribed to the occurrence of dynamic transformation of α_p→β by calculating the Gibbs energy barrier based on solution thermodynamics and the drive force from stress difference between α_p and β phases. With the decrease of temperature, α_p phase fraction varies little with deformation as a result of the counteraction between dynamic transformation of α_p→β and strain-induced phase transformation of β→α_p. However, the deformation under the off-equilibrium condition can accelerate the precipitation kinetics of secondary alpha (α_s) phase evidently. Moreover, a large amount of fine equiaxed α_s phase can be formed by intragranular nucleation. Finally, it is found that, with the deformation at high temperature, the loss of Hall–Petch strengthening is principal source of flow softening. Meanwhile, the quantity of flow softening is less than that of the loss of Hall–Petch strengthening, which is associated with pronounced precipitation of α_s phase. At the low temperature, flow softening by the loss of Hall–Petch strengthening is minor compared to that by α_s laths rotation.