Alumina is normally regarded as truly brittle below ∼ 1000°C. Contrary to this view, it is argued that plastic deformation at the crack tip plays a necessary part in the fracture. This is used to explain the minimum observed at 250°C in the fracture stress of polycrystalline plates containing drilled cracks and in the stress intensity required for crack-branching. Other details of the fracture of single crystal and polycrystalline alumina, including changes in the proportion of intergranular and transgranular fracture with temperature, crack length and crack-branching, are also explained by the involvement of plastic deformation.
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Congleton et al. (1969) studied this question.