Ab initio modeling of clean and Y-doped grain boundaries in alumina and intergranular glassy films (IGF) in β-Si3N4

Based on large and relaxed grain boundary (GB) models in alumina and intergranular glassy film (IGF) models in polycrystalline beta-Si3N4, ab initio modeling and theoretical tensile experiments were carried out for both clean and Y-doped models. It is shown that the increased covalent bonding between Y and O or N through the participation of the Y-4d and Y-3p orbitals is the mechanism by which Y ions enhance the mechanical and elastic properties of the Y-doped GB and IGF models. In alumina, this explains the improved creep behavior in the presence of Y doping. Preliminary results on the electronic structure and bonding of a specific GB model (Sigma 37) in alpha-Al2O3 is presented. For the IGF models, the distribution patterns of Y ions in the glassy region were investigated by total energy calculations. Y ions prefer to be at the interfacial region between the IGF and bulk crystal. Defect-like states of different origin can be identified near the valence band and the conduction band edges. These theoretical predictions obtained from the calculation of the fundamental electronic structure of the materials can be used to derive local strain fields of dissimilar "particles" that may be linked to continuum level theories via finite element methods.