Creep of polycrystalline Y ₃ Al ₅ O ₁₂ and Lu ₃ Al ₅ O ₁₂ garnet in air and steam

Abstract Creep rates of polycrystalline yttrium–aluminum garnet (Y 3 Al 5 O 12 , YAG) and lutetium–aluminum garnet (Lu 3 Al 5 O 12 , LuAG) under 50 to 200 MPa compressive stress were measured at 1300°C in air and in steam. Measurements for 2% Er and 2% Yb doped YAG were also done at 1300°C, and for YAG at 1400°C. The effect of 0.3 to 8 µm grain size variation on creep rate was determined. Flow stress exponents for all materials were determined, and the creep rate activation energy for YAG was determined. Creep rates in steam were slightly faster than those in air. Er and Yb dopants in YAG had little discernible effect on creep rates. LuAG creep rates, adjusted for grain size, were consistently higher than those for YAG. The Nabarro–Herring mechanism best describes the creep of polycrystalline doped and undoped YAG in air and steam for all samples except those with a grain size less than 0.5 µm. The creep mechanism for polycrystalline LuAG was not determined and may be a complex combination of mechanisms. Creep mechanisms for polycrystalline garnets are compared and discussed.