Modification of Cluster Variation Method Entropy Functional for Binary fcc Phases using Tetrahedron Approximation

The binary prototype phase diagrams for fcc structures calculated with the tetrahedron, tetrahedron-octahedron and quadruple tetrahedron approximations of the cluster variation method and Monte-Carlo simulations (MCS) for first neighbor interactions show significant differences in the computed triple points of the A1, L1(0) and L1(2) phases.The finite number of terms retained in the cluster variation method entropy functional in different approximations leads to differences in the accuracy of the approximation. In general, larger cluster approximations and MCS results are accepted to be more accurate, but are associated with large computational burden. The multiplicities of the basic clusters in the entropy functional for tetrahedron-octahedron approximation were earlier modified to reproduce many of the results of larger clusters with lower computational burden. In the present communication, a similar approach is adopted for the case of tetrahedron approximation for binary fcc phases. This approximation has a smaller number of variables, but yields the results which are in good agreement with the best known results.