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 intensities, 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 intensities can be understood in terms of the higher grain boundary strength of the fine-grained structure.
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