Phase-field study for the growth kinetic and topology transformation mechanism of grains during solidification process

Based on the continuum phase-field dynamic model, the microstructure evolution, dynamics change and topology transformation during the solidification process of nucleation, growth and coarsening were studied, and the relationship between nucleation and growth was analyzed. Results show that volume fraction gradually increases to the equilibrium value, while the total surface area of grains increases first and then decreases during the process of nucleation and growth. For nucleation, volume free energy is driving force and surface energy is resistance force. Nucleation process is accompanied by growth process; these two processes are overlapped and competed. During the growth process, grains with more than six sides continue to grow, and that with less than six sides gradually reduces. The mechanisms of small grain disappearing are neighbor switching mechanism, direct annihilation mechanism of three, four and five sided grains, the intersection of four sides separated and resulting in the disappearance of small grain, and direct disappearance mechanism of grain boundary. The simulation results are in good accordance with experiment results.