GROUND-STATE STRUCTURES IN ORDERED BINARY HCP ALLOYS

Ground-state ordered structures in binary h.c.p. alloys have been studied. Non-ideal axial ratios and pairwise interactions up to third-nearest neighbours have been considered. Ordered structures have been constructed by dividing the atomic sites into several sublattices ranging from a minimum of 6 to a maximum of 28. Ordered structures have been generated by identifying distinct layers arising for each sublattice division by populating them with two types of atoms, A and B, and then stacking such layers. Twelve energetically distinct structures with lowest energies for certain values of the interchange energy parameters have been identified. Three other structures have been found which probably belong to the ground-state. Complete structural details have been worked out and energy expressions found for these. Degeneracy occurs in several ways. The first is associated with the occurrence of several stoichiometric structures with the same energy. A second type of degeneracy is exhibited by stoichiometric structures with compositions intermediate to two terminal ground-state structures and energies equal to those of a mixture of the latter. For off-stoichiometric situations, composition regions corresponding to the presence of miscibility gaps or solid solutions are discussed. The latter show a third type of degeneracy where the solid solution, which is partially ordered, has the same energy as that of an ordered structure.