Phase-field study of grain growth in porous polycrystals

Abstract

During sintering, a multitude of mechanisms act in different ways resulting in densification and coarsening. Since the material properties depend on the microstructure, the manufacturing of advanced ceramics requires a deep understanding of the sintering process. The present study focuses on the occurrence of grain growth during final stage sintering. In a porous microstructure, the pores exert a dragging force to grain boundary motion retarding the grain growth. However, as soon as the porosity becomes low enough, the grain growth exceeds the dragging force and the microstructure starts to coarsen. This interdependency of densification and grain growth is still not fully understood. The present study uses a 3D phase-field model to investigate grain growth in porous microstructures during final stage sintering. A model extension treats pore dynamics under consideration of pressure stability as well as pore coalescence. To account for the size effect of pores on its dragging force, a surface diffusion based mobility approache is incorporated. Large-scale 3D grain growth simulations are conducted to analyze the effect of different porosities and pore sizes with statistical relevance comparable to real microstructures. Depending on the porosity, two dominating effects on the reduction of grain growth rate are found. For low porosities, the growth rate depends on the number of pores whereas for higher porosities the pore size and their mobility dominate the process. The results show the need to consider the effects of the pore size and their distribution during final stage sintering.