Creep of polycrystalline yttrium aluminum garnet (YAG) at elevated temperature in air and in steam

Compressive creep of high-purity polycrystalline yttrium aluminum garnet (YAG, Y3Al5O12) was investigated at 1300 °C and 50–200 MPa in air and in steam. Compressive creep behavior of silica-doped polycrystalline YAG (Y3Al5O12–0.14 wt% SiO2) was also studied. Creep specimens were microstructurally characterized by optical microscopy and TEM before and after creep. Steam slightly increased creep rates of material with grain size less than 1 μm (the undoped YAG), but otherwise had little effect. The flow stress exponent was n≈1 for both SiO2-doped YAG and undoped YAG. Creep rates and microstructural observations are consistent with the Nabarro-Herring creep mechanism, with creep rate limited by lattice diffusion of yttrium cations (Y3+). Silica-doped YAG had a larger grain size of 2.41 μm and lower creep rates than undoped YAG with 0.92 μm grain size. However, creep rates normalized by grain size for Nabarro-Herring creep were higher in SiO2-doped YAG. Possible effects of SiO2 doping and steam on creep of YAG are discussed.