Atomic structural environment of grain boundary segregated Y and Zr in creep resistant alumina from EXAFS

Dopants Y and Zr at 100 p.p.m. level (Y or Zr to Al atomic ratio) in ultra- high purity polycrystalline alumina have been found to be mainly segregated to the alumina grain boundaries. The atomic structural environments around the Y and Zr segregants have been investigated by Extended X-ray Absorptio n Fine Structure (EXAFS), On average, Y ions in alpha-Al2O3 grain boundarie s are coordinated by four oxygens, at a distance of 2.30 Angstrom, which co rresponds nearly to the Y-O bond length in cubic Y2O3, and Zr ions are coor dinated by five oxygens at a distance of 2.14 Angstrom, which is approximat ely the same as the average Zr-O bond length in monoclinic ZrO2. However, i n the EXAFS radial distribution function, the Y-cation and Zr-cation next n earest neighbor shell cannot be clearly identified. These results suggest t hat Y and Zr at 100 p.p.m. concentrations in alpha-Al2O3 occupy grain bound ary sites with well defined nearest neighbor cation-oxygen bond lengths sim ilar to those in their parent oxides but with the next nearest neighbor cat ion-cation distances varying considerably from site to site. From EXAFS, th e Y grain boundary saturation concentration is estimated to be 6.0 atoms/nm (2), which is consistent with the estimate from STEM of 4.4 atoms/nm(2) The differences in the Zr-O and Y-O nearest neighbor distances and coordinatio n numbers in Al2O3 grain boundaries are related to the Y-Al and Zr-Al size mismatches. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science L td. All rights reserved.