Dynamics of Eutectic Microstructure during Unsteady and Oscillatory Solidification: Phase-Field Approach

A two-dimensional phase-field method was developed that allowed us to observe the evolution of the composition field and the microstructure of lamellar eutectics caused by variations in the freezing rate. Solid-phase nucleation, lamella termination, and migration of the three-phase junction were studied to elucidate the role of each in microstructure changes. This evolution is often complex due to the interplay between interface kinetics, the composition field, and the widths of the solid lamellae. When the freezing rate is increased, the supersaturation in front of the lamellae increases, causing formation of deep depressions, followed by nucleation of the other solid phase, instability, and movement of the three-phase junction. If the freezing rate is subsequently held constant, the microstructure eventually 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. With freezing rate oscillations, backmelting becomes important, especially for large lamellar spacing, large amplitudes, and low frequencies. Three ranges of oscillation amplitude and oscillation frequency were identified that result in different microstructures.