Phase equilibria of L1(0)-type order in Cu-Au-Pd and Cu-Au-Ni ternary systems

The IKL-ALCHEMI (intersecting Kikuchi-line-atom-location by electron channeling-enhanced X-ray microanalysis) method of electron diffraction was employed to study the atomic configuration in L1(0)-type ordered phases in Cu-Au-Pd and Cu-Au-Ni ternary alloys annealed at 573 K. In the L1(0) phase of Cu-Au-Pd, Au and Pd atoms tend to share the same sublattice, while Cu atoms prefer the other sublattice. If the sum of Au and Pd contents exceeds 50 at.%, most Pd atoms still remain on the favourite sites, while excess Au atoms are driven to the opposite sublattice. In the ordered Cu-Au-Ni alloys, Ni atoms weakly prefer the Cu sublattice rather than the other for Au, but the distribution of Ni atoms depends on the alloy composition. Most Ni atoms reside on the Cu sublattice when the Au content is 50 at.%, while about two thirds of Ni atoms are found on the Au sublattice in the alloy with 50 at.% Cu. The cluster variation method (CVM) in the tetrahedral approximation explains well not only the experimental results of atomic configuration but also the equilibrium phase diagrams of both ternary systems. The appropriate values of parameters for pairwise interactions as well as four-body interactions of atoms have been determined in the framework of the CVM tetrahedral approximation.