Solubility of Magnesia in Polycrystalline Alumina at High Temperatures

High‐purity Al2O3 compacts were doped with 0–350 ppm (by weight) of MgO using a liquid immersion technique and equilibrated at temperatures between 1700° and 2000°C under hydrogen. The solubility limits of MgO in Al2O3 at temperatures of 1720° and 1880°C were very low, ∼75 and 175 ppm, respectively. Variation of MgO solubility with temperature could be represented by the equation, ln Mg/Al = 3.80–2.63 × 104/T. The small MgO solubilities were understood by the high enthalpy (326 kJ/mol) of solution. The results of this study suggested that previous investigations on sintering and grain‐growth mechanisms in MgO‐doped Al2O3 were probably not done in single‐phase Al2O3 solid solutions. However, the conclusions on sintering and grain‐growth mechanisms in prior research work in MgO‐doped A2O3 may be correct. The effects of SiO2 impurity and grain size on MgO solubility are discussed. Previous grain‐growth experiments in MgO‐doped Al2O3 are described that demonstrate the clearest evidence for grain‐boundary mobility controlled by a solid‐solution mechanism.