Molecular dynamics simulations of basal and pyramidal system edge dislocations in sapphire

Directionally solidified ceramic eutectics containing alumina as a topologically continuous majority component with, e.g., yttria-stabilized cubic-zirconia or with YAG as minority components have become of considerable interest as members of a new family of potential high temperature structural materials. The attractive creep resistance of such eutectics in fiber-form is based on their remarkably tight [0001] texture of the alumina component in which neither the basal plane nor the prism plane can be activated to result in glide that can extend the eutectic by creep in tension. Under such conditions creep in such eutectics must be governed by climb of the (1/3)((1) over tilde 101) edge dislocations out of either the (11 (2) over tilde0) prism plane or the (1 (1) over tilde 02) pyramidal plane. To develop better understanding of the core structure of these dislocations and their potential role in the creep resistance of the alumina component in eutectics with tight [0001] textures, a molecular dynamics (MD) simulation was carried out of the comparative core structures of both the (1/3)(2 (1) over tilde(1) over tilde0) basal edge dislocations and the (1/3)((1) over tilde 101) pyramidal edge dislocations on the (1 (1) over tilde 02) plane in sapphire. The MD simulation revealed that the equilibrium structure of the core of the pyramidal edge dislocations undergo a dissociation into two half strength partial edge dislocations displaced vertically out of the best glide plane of cation holes with weak covalent bonding and possible fair glide resistance into two adjacent pyramidal planes of very strong covalent bonding, and consequent, very high glide resistance. While this explains the immobility in glide of such dislocations on the pyramidal system, no important structural impediment was found for their climb motion out of the pyramidal planes. (c) 2005 Elsevier Ltd. All rights reserved.