GROUND-STATE STRUCTURES IN ORDERED BINARY HCP ALLOYS WITH PAIRWISE 3RD NEIGHBOR INTERACTIONS

Ground state structures in binary h.c.p. (non-ideal axial ratio) alloys have been studied using the cluster method proposed by Allen and Cahn [Acta metall. 20, 423 (1972)]. The method has been suitably modified and pairwise interactions up to third neighbour have been considered. Nine ground state structures have been identified. The structural details such as-lattice, number of atoms per unit cell, equivalent positions of atoms and their respective symmetries along with energy have been worked out. For off-stoichiometric compositions, the relative stabilities of solid solutions and two phase mixtures are also discussed. In the latter case, miscibility gaps arise. Three types of degeneracy have been found to occur. Two of these are associated with the possibility of arranging the same set of different types of atomic clusters in distinct ordered patterns. The third arises for h.c.p. alloys having ideal axial ratio.