Grain-size effect on high temperature fatigue crack growth in polycrystalline Al₂O₃

High temperature cyclic crack growth experiments were conducted on polycrystalline Al₂O₃ of different grain sizes. The effect of grain size depended on the level of applied stress intensity. At high stress intensifies, a larger grain size provided a superior crack growth resistance, similar to the effect of grain size on crack growth at room temperature. However, at low stress intensities, the grain-size dependence was reversed. While the effect of grain size at high stress intensities resulted from enhanced grain bridging in the coarse-grained microstructure, the behavior at low stress intensifies can be understood in terms of the higher grain boundary strength of the fine-grained structure.