CLUSTER EXPANSIONS OF ALLOY ENERGETICS IN TERNARY INTERMETALLICS

Ternary alloy energetics are examined for substitutional systems by a formal cluster expansion. In contrast to the binary-alloy problem, sev eral distinct basis sets are possible for the ternary cluster expansio n. Several of these sets of ternary basis functions are examined and c ompared, and relationships are derived between the expansion coefficie nts, or effective cluster interactions, expressed in various bases. Th e method of direct configurational averaging (DCA) (based on a tight-b inding, linearized muffin-tin-orbital Hamiltonian) is extended to trea t ternary alloy systems. Using the DCA, ternary, fcc-based effective p air and triplet interactions are computed for the Rh-V-Ti, Pd-Rh-V, an d Ag-Pd-Rh systems, and convergence of the expansion is examined. By c ombining the cluster expansion with the results of the DCA computation s, formation energies are obtained for the completely disordered state as a function of alloy composition. Both pair and triplet interaction s are seen to be crucial towards obtaining quantitatively converged en ergetics.