Dynamics of eutectic microstructure during phase nucleation and phase termination: phase-field model computer simulations

Computer simulation using two-dimensional phase-field method was performed in order to observe the evolution of the composition field and the microstructure of lamellar eutectics caused by change in freezing rate. Phase nucleation, phase termination, and migration of the 3-phase junction were studied to elucidate the role of each in microstructure changes. These phenomena are complex due to the interplay between interface kinetics, the composition field, and the phase fraction. When the freezing rate is increased, the supersaturation in front of lamellae increases, causing formation of depressions, followed by nucleation, instability, and motion of the 3-phase junction. Eventually the microstructure becomes stable. With a decrease in freezing rate, instability develops that changes the local growth direction and provokes changes in lamellar width followed by elimination of some lamellae. The time scales for phase nucleation and phase termination were also estimated assuming the overwhelming importance of lateral diffusion. The model explains the time scale - lamellar microstructure parameter dependence provided that the growth rate increases with a constant acceleration, and the volume fraction does not change during an increase in the freezing rate until nucleation occurs.