Superlattice dislocations in the L1₂ ordered structure of Cu₂NiZn

Abstract

Dissociation criteria for superlattice dislocations on {111} planes have been applied to the ordering alloy Cu₂NiZn. The calculations showed that the splitting of a superlattice dislocation into two superlattice Shockley partials, with Burgers vector of type 1/3a ₀⟨211⟩, bounding a superlattice intrinsic stacking fault. (SISF), is energetically less favourable than the splitting inta two unit dislocations, with Burgers vector of type 1/2a ₀⟨110⟩, separated by an antiphase boundary (APB). This theoretical prediction is in agreement with the type of superlattice dislocation observed in the electron microscope. It is rather difficult to predict a priori whether each unit dislocation is split into two Shockley partials separated by a complex stacking fault (CRF) because the cut-off parameters for the dislocation cores are not known with sufficient accuracy. The splitting into two Shockley partials could not be resolved in the electron microscope, which means that either the separation between the Shockley partials is less than about 0.7 nm, or the twofold dissociation is energetically more favourable than the fourfold one.

Values for the APB energy have been calculated from the observed separations of the unit dislocations, constituting superlattice dislocations, in Cu₂NiZn single crystals quenched from several temperatures. The results are in agreement with theoretical predictions.