Diffusion induced grain-boundary migration and mechanical property improvement in Fe-doped alumina

When 1 wt.%-Fe2O3-doped Al2O3 samples were sintered at 1600 °C in a reducing atmosphere, the additive remained as Fe precipitates at the triple grain-junctions. On annealing the sintered samples in an oxidizing atmosphere, however, the Fe precipitates dissolved into the Al2O3 grains at the surface region and induced grain-boundary migration. This diffusion induced grain-boundary migration (DIGM) resulted in the corrugation of grain boundaries and the formation of misfit dislocations in the migration region. The mechanical properties of the samples were evaluated by the Hertizian indentation technique under static and cyclic loading. The sample with DIGM showed better mechanical properties than that without DIGM: improvement in the critical load for cone crack initiation under static loading and in the number of cycles for crack initiation under cyclic loading. In terms of microstructure, the cone crack propagation was suppressed and a quasi-plastically deformed region appeared under static loading, and the surface chipping at the contact area was much reduced under cyclic loading. An insignificantly low value of residual stresses at the surface of the sample with DIGM suggested that the mechanical property improvement was due to the microstructural changes, the grain boundary corrugation and the misfit dislocations, rather than to any compressive stresses introduced in the migration layer. DIGM appears to be a possible means of improving the mechanical properties of Al2O3.