On the core structure of ⟨112] edge dislocations in γ-TiAl

Weak-beam image simulations have been carried out using the CUFOUR programme of a ] edge dislocation in γ-Ti–50 wt% Al, dissociated into three ⟨112] partial dislocations bounding superlattice intrinsic stacking-fault (SISF) and superlattice extrinsic stacking-fault (SESF) ribbons of equal width. The simulations included one of the two core configurations of the composite Shockley partial dislocation found by Kumar et al. to explain their observed image contrast, and two others that involve dissociation into stair-rods. These latter simulated images cannot be distinguished from those for the model proposed by Kumar et al. for the composite Shockley dislocation and therefore account for the observed image contrast equally well. The claim of these workers that configurations consisting of stair-rod dislocations can be ruled out is not substantiated. The core structures involving sessile stair-rod dislocations explain the locked nature of the edge dislocation. The values found by Kumar et al. for the true sep...