Effects of grain boundary amorphous phase on high-temperature ductility in alumina polycrystals

Two types of polycrystalline alumina were prepared by centrifugal compaction; MgO-doped alumina sintered in air and pure alumina HIPed in argon atmosphere with carbon susceptor. Both MgO-doped and HIP alumina had amorphous grain boundaries that were segregated, respectively by Mg and by C and S. The amorphous phase was found to melt near 1650 K, as evidenced by an endothermic peak in differential scanning calorimeter data. Melting of the grain boundary phase resulted in a ductile-to-brittle transition during the compressive deformation of the HIP alumina. In contrast, the MgO-doped alumina showed a ductile behavior up to the highest test temperature of 1773 K. The difference in ductility between two alumina specimens may be related to charge compensation, localization of bonding electrons in the grain boundary phase, and the spatial distribution of the grain boundary phase.