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-dispers ive X-ray (EDX) and electron energy-loss spectrometry (EELS) of yttrium- an d lanthanum-doped Al2O3 has been conducted to ascertain the level of segreg ation of these impurities to grain boundaries. Line profile analyses indica te 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,/mm(2) for yttrium and lanthanum, respectively, Ass uming all the segregant is uniformly distributed within +/-0.5 nm of the bo undary, this excess corresponds to 9 rt 3 at.% for yttrium-doped Al2O3 and 10 +/- 2 at.% for lanthanum-doped Al2O3. For both dopant systems, examinati on of the spatially resolved electron energy-loss near-edge structures (ELN ES) on the AI-L,, edge suggests a loss in octahedral symmetry and a slight Al-L-2,L-3 bond-length expansion. No significant change is noted in the O-K edge.