Scanning transmission electron microscopy analysis of grain boundaries in creep-resistant yttrium- and lanthanum-doped alumina microstructures

High-spatial-resolution analytical electron microscopy using energy-dispersive X-ray (EDX) and electron energy-loss spectrometry (EELS) of yttrium- and lanthanum-doped Al₂O₃ has been conducted to ascertain the level of segregation of these impurities to grain boundaries. Line profile analyses indicate that the segregation is confined to a layer thickness of <3 nm. Similar amounts of excess solute have been observed in both dopant systems: 4.4±1.5 and 4.5±0.9 at./nm² for yttrium and lanthanum, respectively. Assuming all the segregant is uniformly distributed within ±0.5 nm of the boundary, this excess corresponds to 9±3 at.% for yttrium-doped Al₂O₃ and 10±2 at.% for lanthanum-doped Al₂O₃. For both dopant systems, examination of the spatially resolved electron energy-loss near-edge structures (ELNES) on the Al-L_2,3 edge suggests a loss in octahedral symmetry and a slight Al-O bond-length expansion. No significant change is noted in the O-K edge.