Mechanism for the Role of Magnesia in the Sintering of Alumina Containing Small Amounts of a liquid Phase

Microstructural morphology in A1₂0₃ containing trace amounts (～0.25 vol%) of a silicate-based amorphous phase was studied, as a finction of MgO additions, using transmission electron microscopy. Incipient abnormal grains develop during hot-pressing of undoped Al₂O₃ powder and are characterized by a large aspect ratio (>3) and long basal plane {0001} facets. The facets are completely wet by a thin (～3 nm) amorphous phase in contrast with grain ends, which appear to be devoid of an amorphous phase. The shape of the grains is believed to result from differences in mobility between clean (i.e., unwet) grain boundaries and intergranular films which are liquid at the firing temperature. Doping with trace additions of MgO (Mg/A1=250 ppm) results in smaller, more uniform grain structures. Many grains in the MgO doped material exhibit all the features of the incipient abnormal grains in the undoped material, with the exception of the large aspect ratio. It is concluded that the role of MgO in such a system is a grain-growth inhibitor and a microstructural stabilizer. It is proposed that the additive operates by reducing the mobility difference between clean (unwet) boundaries and grain boundaries wetted by thin amorphous films. It is believed the additive accomplishes this primarily by reducing the mobility of the clean grain boundaries via a solid-solution pinning mechanism.