is the EPMA image of the as-synthesized YAG composite structure ceramics. The YAG fine grains grow with each other closely and no secondary phase was found. There are some pores in the grains. The sizes of the pores are less than 1 μm. The grain size was determined with the EPMA by freely drawing a line on a high resolution image of the sample. The average grain size was calculated by the formula below [21] and [22]: