Microstructural morphology in A120, containing trace amounts ( ~0 . 2 5 ~01%) of a silicate-based amorphous phase was studied, as afinction of MgO additions, using transmission electron microscopy. Incipient abnormal grains develop during hot-pressing of undoped Al,O, powder and are characterized by a large aspect ratio ( > 3 ) and long basal plane {OOOI} 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 MgOdoped 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 amorphousfilms. I t is believed the additive accomplishes this primarily by reducing the mobility of the clean grain boundaries via a solid-solution pinning mechanism. [
It is experimentally observed that the peritectic reaction, 211 + liquid -+ 123, can be driven essentially to completion in 1 h at an undercooling of only ~3 0°C . The kinetic data, together with the observed microstructures, are inconsistent with the normal mechanism of the peritectic reaction. It is proposed that the mechanism of the reaction involves dissolution of 211 particles into the liquid and precipitation of solid 123. The aligned grain structure is explained through sympathetic nucleation of new 123 grains on existing grains. [
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