Modelling of the transformation kinetics of the beta -> alpha + beta transformation in Ti-4.5Fe-6.8Mo-1.5Al based on dilatometric results and the morphology of the alpha plates

摘要

The kinetics of the isothermal beta -> alpha + beta transformation in a metastable beta titanium alloy was studied by dilatometry in the temperature range 500-770 deg C. Metallographic analysis shows that normal alpha plates form predominantly on beta grain boundaries above 700 deg C, whereas below 630 deg C so-called black plates are distributed uniformly in the beta matrix. For intermediate temperatures (630-700 deg C) both morphologies of alpha phase form. This morphological transition with decreasing temperature is associated with a change in transformation mechanism. Black plates form partition-less and therefore the growth is much faster than the diffusion-controlled growth of normal alpha plates. Based on thermodynamic calculations, the volume diffusion of Mo in the titanium matrix ahead of the interface, and the morphology of the normal alpha plates, a Zener-like model was developed. Subsequently, fraction curves were calculated using a three-dimensional model of the beta grain in which a number of plates are simulated to grow. The simulated growth rate of the alpha phase shows good agreement with dilatometric results in the temperature range 700-750 deg C. A discrepancy between the simulated and experimental data is seen below 700 deg C when black plates start to form, which is obviously due to the different mechanism.