Phase-field simulation of structure evolution at high growth velocities during directional solidification of Ti55Al45 alloy

A phase-field model whose free energy of the solidification system is derived from Calphad thermodynamic modeling of phase diagram is used to simulate structure evolution of Ti55Al45 alloy during directional solidification at growth velocities sufficiently higher than the critical velocity of transition from cells to dendrites, but lower than the absolute stability. The liquid-solid phase transition of L→L+β(Ti) is chosen. Firstly, the dynamics of the breakdown of initially planar interfaces into cellular structures then cellular dendrites are shown. Then the transition from cellular dendrites to fine cellular structures are shown at higher growth velocities. The solute segregation patterns are investigated at different growth velocities. The appearance of solute trapping is also investigated by determining the solute partition coefficients as a function of growth velocities. Agreement is reached with the theory of rapid directional solidification.