Transformation–dislocation dipoles in Laves phases: A high-resolution transmission electron microscopy analysis

Images of synchro-Shockley partial dislocation arrangements in the Laves phases NbCr₂ and HfCr₂ have been obtained by high-resolution transmission electron microscopy. The analysis of the stacking sequences around these arrangements revealed the role of the constituting partial dislocations in enabling the polytypic C14 → C36/6H phase transformation in both alloys. The synchro-Shockley partial dislocations occur in and move through the crystals mainly as dipoles of partials of opposite signs, leading to—as compared to isolated partials—strain fields of lesser extent. In NbCr₂ a single, probably quenched synchro-Shockley partial dislocation dipole, consisting of two partials with Burgers vectors of opposite sign, was identified. The ordered passage of a series of this type of line defects brings about the C14 → C36 transformation. In HfCr₂ a complex synchro-Shockley partial dislocation configuration was revealed. It can be regarded as an “antiphase boundary” between two C36 domains. It is likely that this defect structure had formed by impingement of two domains of C36 growing perpendicular to the stacking direction by glide of synchro-Shockley partial dislocation dipoles.