EFFECT OF DISLOCATION DISSOCIATION ON CRACK TIP PLASTICITY IN L12 INTERMETALLIC ALLOYS

A model describing the energy associated with emission of a dissociated a <110BAR> {111} dislocation from a crack tip in ordered intermetallic alloys with the L1(2) crystal structure has been formulated. The model incorporates five material parameters and two parameters defining the slip system orientation relative to the crack. Numerical calculations using the model allow determination of the range of slip system orientations for which there is no energy barrier for emission of a dissociated superlattice dislocation from a crack tip. Application of the model to a variety of intermetallic alloys with the L1(2) crystal structure has revealed a correlation between the observed fracture mode and the calculated range of slip system orientations for which there is no energy barrier for emission of a dissociated superlattice dislocation. The model has also been used to predict the effect of lowering the {111} APB or SISF energy on the fracture mode of Al67Ni8Ti25 and Al3Sc.