Structure of Y and Zr segregated grain boundaries in alumina

Grain boundary segregation of Y and Zr in alpha-Al2O3 and the atomic structural environment around the Y and Zr atoms have been investigated using high resolution STEM and EXAFS. At dilute concentrations, the Y ions in alpha-Al2O3 grain boundaries, on average, are coordinated by 4 oxygens, at a distance of 2.30 Angstrom, which corresponds nearly to the Y-O bond length in cubic Y2O3, and Zr ions are coordinated by 5 oxygens at a distance of 2.14 Angstrom, which is approximately the same as the average Zr-O bond length in monoclinic ZrO2. However, in the EXAFS radial distribution function, the Y-cation and Zr-cation next nearest neighbor shell cannot be clearly identified. These results suggest that Y and Zr at dilute concentrations in alpha-Al2O3 occupy grain boundary sites with well defined nearest neighbor cation-oxygen bond lengths similar to those in their parent oxides, but with the next nearest neighbor cation-cation distances varying considerably from site to site. Grain growth can cause grain boundaries to become supersaturated by Y. In this case, both the Y-O nearest neighbor coordination number and the ordering of Y with respect to Al ions beyond nearest neighbor O are increased. This Y-Al distance is the same as that expected for the Y-Al distance when Y substitutes for Al while relaxing the Y-O distance to that in Y2O3. This may indicate that as the Y concentration increases, Y begins to occupy near-boundary sites in planes on each side of the geometrical boundary. In these near-boundary planes, the nearest neighbor ordering extends at least to nearest neighbor cations. Nucleation of the YAG phase leads to the depletion of these partially ordered layers.