The Effect of Dislocation Dissociation on Crack Tip Plasticity in L1₂ and B2 Intermetallic Alloys

Abstract

Low toughness at ambient temperatures is an inherent problem in many ordered intermetallic alloys that otherwise have attractive properties for high temperature applications. One potential explanation of low toughness is the existence of an energy barrier for crack tip dislocation emission. A model describing the energy associated with emission of a dissociated superlattice dislocation from a crack tip in ordered intermetallic alloys has been formulated. Application of the model to a wide variety of intermetallic alloys with the L1₂ and B2 crystal structures has revealed a correlation between the observed macroscopic fracture mode and the calculated range of slip system orientations for which emission of a dissociated superlattice dislocation at ambient temperatures is possible. Additionally, the model has been used to predict the effects of lowering the stacking fault energy, increasing the thermal energy available for thermally activated dislocation emission, and changing the active slip system. Preliminary fractographic analyses have been conducted with a chromium-modified Al₃Ti alloy to verify the model predictions.