Effect of chemical bonding state on high-temperature plastic flow behavior in fine-grained, polycrystalline cation-doped Al₂O₃

High-temperature plastic deformation characteristics such as flow stress and elongation to failure in polycrystalline Al₂O₃ with an average grain size of about 1 μm is remarkably changed by the doping of 0.1 mol% YO_1.5, SiO₂, TiO₂ or ZrO₂ at 1400°C under an initial strain rate of 1.2 × 10^-4 s^-1. The difference in the flow stress and the tensile ductility is considered to be originated from change in the grain boundary chemistry in Al₂O₃ due to segregation of the dopant cation in the vicinity of the grain boundary. A change in the chemical bonding state in the cations-doped Al₂O₃ is examined by first-principle molecular orbital calculations using DV-Xα method based on [Al₅O₂₁]^27- cluster model. Chemical shift observed in electron energy-loss spectra (EELS) at the grain boundary due to the dopant segregation can be roughly reproduced by the molecular orbital calculations. A correlation is found between the flow stress and product of net charges of aluminum and oxygen ions. Moreover, the tensile elongation to failure seems to correlate with bond overlap population between Al and O. The change in the chemical bonding strength at the grain boundary must dominantly affect to the grain boundary diffusion and bond strength at the grain boundary, and thus seems to be an important factor to determine the plastic flow behavior in polycrystalline Al₂O₃.