High temperature deformation characteristics of sintered alumina having amorphous grain boundaries

MgO-doped alumina was prepared by pressureless sintering. HIPed pure alumina was prepared by pressureless sintering followed by HIPing. High resolution electron micrographs of both samples showed the presence of an amorphous phase along grain boundaries. Calorimetric analysis during heating to 1773 K showed that the amorphous grain boundaries were melted at 1675 and 1638 K, respectively, in the HIPed and the MgO-doped alumina. The effects of melting of grain boundaries on the mechanical properties were studied by compressive tests at temperatures between 1573 and 1773 K. The MgO-doped alumina showed an excellent ductility at all examined temperatures. However, the HIPed alumina showed a ductile-to-brittle transition above 1673 K when the grain boundaries were melted. The differences in the deformation behavior between the HIPed and the MgO-doped alumina were discussed in terms of the degree of charge compensation and the nature of chemical bonding in the grain boundary phase.