Phase-Field Simulation of Grain Growth During Final Stage of Sintering in Two-Phase Composite Fuels

A numerical model based on phase-field method to simulate the grain growth during the final sintering stage of a two-phase UO2-UN composite fuel was established, systematically investigating the grain evolution within the composite fuel system. Firstly, a simplified model of a void held between two grains was established to elucidate the interaction mechanism between grain boundary (GB) and voids. The results show that voids near GBs exhibit shrinkage dynamics and evolve into a ellipsoidal shape. Additionally, voids in contact with grains of high interfacial energy show significantly accelerated shrinkage rate. The triple interface angle in the system is determined by the ratio of the two-phase GB energy to the interface energy. Furthermore, a quantitative analysis was conducted on the grain growth process within the two-phase polycrystalline system. The investigation into the effect of phase volume fraction reveals that grain migration is significantly constrained by phase interface. As the volume fraction of the secondary phase increases, the increased phase interface density reduces the grain growth rate. Finally, the grain growth model for the two-phase polycrystalline system containing voids was developed to investigate the pinning effect induced by voids and to elucidate the growth kinetics at the final sintering stage. The results show that voids induce GB pinning, with the pinning strength positively correlated with void density. Non-uniform local void distribution can trigger abnormal grain growth. A three-dimensional void pinning analysis further shows that complex grain topology enhances the void pinning effect, resulting in more distinctive morphological features of abnormal grain growth in three-dimensional systems.