Solubility of Magnesia in Polycrystalline Alumina at High Temperatures

High‐purity Al 2 O 3 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 Al 2 O 3 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 × 10 4 / 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 Al 2 O 3 were probably not done in single‐phase Al 2 O 3 solid solutions. However, the conclusions on sintering and grain‐growth mechanisms in prior research work in MgO‐doped A 2 O 3 may be correct. The effects of SiO 2 impurity and grain size on MgO solubility are discussed. Previous grain‐growth experiments in MgO‐doped Al 2 O 3 are described that demonstrate the clearest evidence for grain‐boundary mobility controlled by a solid‐solution mechanism.