Mechanism of Decomposition of the Cubic Solid Solutions in the System ZrO2—MgO

The phase diagram for the system ZrO2-MgO was reinvestigated by using coprecipitated gels. The thermal stability of the defective cubic solid solutions was investigated by a study of kinetics. The decomposition curves were sigmoidal, indicating that the formation of the tetragonal solid solutions proceeded by a process of nucleation and growth. Decomposition comprised: (1) formation of the nuclei of the tetragonal solid solution on the grain surface, (2) growth of the nuclei leading to the formation of an interface, and (3) the irregular growth of that interface into the interior of the grain. During the decomposition magnesium ions diffused out of the regions of the growth of the tetragonal phase and partly precipitated as magnesium oxide at the grain boundaries. The decomposition rate increased with the degree of ordering or clustering of defects and impurities and with the number of the anion defects; it showed a maximum at about 1200°C.