Microstructural Changes During Heat Treatment of Sintered Al2O3

Samples of both high-purity and Mg-doped Al₂O₃ sintered in H₂, N₂, O₂, or vacuum were annealed at 1650° to 1850°C for times up to 64 h. In pore-free systems, grain growth is limited by the mobility of Mg-rich second-phase inclusions; in samples annealed in H₂, grain growth is limited by pore dragging with a transition toward limitation by solid-inclusion dragging at high dopant levels; in samples annealed in N₂ and O₂, grain growth is characterized by a transition from an "anchoring effect" of the pores toward a combination of pore dragging by and unpinning from the grain boundaries. Time-dependence of grain growth is insufficient to determine the mechanisms and provide an adequate foundation for model-based calculations. Observations of microstructure and its change with time, together with the rate of grain growth as a function of composition, allow elimination of alternate hypotheses and determination of the process which controls the rate of grain growth and change in pore size.