Using two different kinds of many-body potentials as well as the Lennard-Jones potential for hexagonal close packed metals, we have found that c + a edge dislocations with dislocation lines along 1 100 can split onto the basal plane, forming a non-planar sessile structure. The 'type I' undissociated dislocation core, observed in previous papers, is shown to be stable only for small simulations. The observed dissociated core structure has a large distorted region that we interpret as a (11 21) twin nucleus, which may help the formation of (11 21) tension twins. We also find that this core structure is lower in energy than the previously observed glissile 'type II' configuration, dissociated on the (11 22) plane. The sessile splitting of the core gives rise to non-Schmid behaviour, with the twin nucleus expanding under c-axis tension, and contracting under compression.

We present a detailed study of both the topological and the chemical short-range orders for liquid Na - Cd alloys using interatomic potentials derived from a non-local model pseudopotential theory. First, the mixing entropy is predicted using a hard-sphere Yukawa reference system, with the parameters being measured from the pseudopotential-calculated mean interatomic interactions and the ordering potentials. It is found that the ordering contribution to the entropy reaches its largest magnitude at around 30 to 50 at.% (atomic per cent) of Na, while above 70 at.% Na, the contribution is small. Then, the atomic structures are calculated using molecular dynamics simulations combined with an energy mapping technique. From the careful examination of the calculated structural distribution functions, the chemical short-range-order parameters and the pair analysis results, we found that with increasing Na concentration, the Na - Cd alloy system exhibits a chemical ordering tendency from compound forming to phase separation, and a topological ordering tendency from a Cd-like structure characterized by 1311- and 1422-type atomic bonded pairs to a Na-like one characterized by 1551-type atomic bonded pairs.