CALCULATION OF PHASE-DIAGRAMS OF BINARY FCC AND IDEAL HCP ALLOYS UNDERGOING ORDERING TRANSITIONS

The tetrahedron approximation of the cluster variation method (CVM) ha s been employed to investigate phase diagrams having fee-based ordered and disordered phases. This approximation is also applicable to the b inary hcp ordered structures with ideal axial ratio. The CVM developed by Kikuchi consists of calculating approximate expressions for the nu mber of configurations and hence entropy of a crystal lattice having d efinite distribution of clusters (points, pairs, triangles, tetrahedra , etc.) of lattice points which in general may be occupied by one of a given set of atomic species. Tetrahedral multi-atom interactions deno ted by alpha and beta are utilized for expressing the configurational energy. The equilibrium cluster distribution is then found by minimizi ng the free energy by utilizing the natural iteration method developed by Kikuchi. The effect of alpha and beta parameters on the topology o f the resulting phase diagrams is observed by assigning several negati ve and positive values to them. The invariant reactions were also dete rmined in each case. Finally a study was made on the Cd-Mg diagram.