High‐Temperature Failure of Polycrystalline Alumina: I, Crack Nucleation

A combination of techniques has been used to compare the compositional and microstructural characteristics of two commercially hot‐pressed aluminas. Second‐phase material containing Ni was observed in one material, mainly as small intergranular particles. Thermal treatment in air caused Ni to concentrate at the surfaces, in small precipitates. Large‐grained heterogeneities, with Ti at the core, were identified in the same material. Hot‐pressing flaws were observed in the second material. The large‐scale heterogeneities act as crack nucleation sites during creep. Stress concentrations associated with these heterogeneities are considered to contribute to the premature crack nucleation, in conjunction with the presence of localized regions of amorphous second phase.