Phase fraction evolution in hot working of a two-phase titanium alloy: experiment and modeling

In the present work, the coupled effects of initial structure and processing parameters on microstructure of a two-phase titanium alloy were investigated to predict the microstructural evolution in multiple hot working. It is found that microstructure with different constituent phases can be obtained by regulating the initial structure and hot working conditions. The variation of deformation degree and cooling rate can change the morphology of the constituent phases, but do not alter the phase fraction. The phase transformation during heating and holding determines the phase fraction for a certain initial structure. β–α–β transformation occurs during heating and holding. β to α transformation leads to a significant increase in content and size of lamellar α. The α to β transformation occurs simultaneously in equiaxed α and lamellar α. The thickness of lamellar α increases with temperature, which is caused by the vanishing of fine α lamellae due to phase transformation and coarsening by termination migration. By assuming a quasi-equilibrium phase transformation in heating and holding, a modeling approach is proposed for predicting microstructural evolution. The three stages of phase transformation are modeled separately and combined to predict the variation of phase fraction with temperature. Model predictions agree well with the experimental results.