Ab initio theoretical tensile test on Y-doped Σ=3 grain boundary in α-Al2O3

It has been known for a long time that Y ions segregate to the grain boundaries (GBs) in polycrystalline alumina with the beneficial effects of enhanced mechanical properties and increased creep resistance. No detailed microscopic theory exists to explain this so called "Y-effect". We provide a quantum mechanical explanation for this effect through a series of carefully designed large-scale computations. The results of our theoretical tensile experiments show that the maximum stresses in pure crystalline alpha-Al2O3, undoped Sigma = 3 GB, and Y-doped Sigma = 3 GB models are, respectively 55, 31 and 39 GPa at the uni-axial strains of 17%, 14% and 16% in the direction perpendicular to the GB. The participation of the Y-4d and Y-4p orbitals enhances the covalent character of the Y-O bond, making it stronger than the Al-O bond it replaces. This could be the major reason for the improved mechanical properties of Y-doped alpha-Al2O3. (C) 2004 Published by Elsevier Ltd on behalf of Acta Materialia Inc.