CALCULATION OF GAMMA-GAMMA EQUILIBRIUM PHASE COMPOSITIONS IN NICKEL-BASE SUPERALLOYS BY CLUSTER VARIATION METHOD

The gamma'/gamma equilibrium phase boundaries in ternary, quaternary and even higher order nickel-base superalloys are calculated by the cluster variation method (CVM) which utilizes the tetrahedron approximation and the phenomenological Lennard-Jones pair interaction potential. The potential parameters are first estimated from thermochemical data and /or semi-empirical formulae on cohesive energies and lattice parameters of binary alloys or compounds. Some of these values are adjusted by a small amount to obtain better agreement with the phase bpundaries in lower order alloys reported in the literature. These paramter values are shown to reproduce gamma'/gamma equilibrium phase compositions, gamma' volume fraction and misfit between the two lattices etc. in higher order alloys close to experimental observation in the temperature range from 750 to 1250-degrees-C. Hence, this method may be applicable to the designing of engineering nickel base superalloys in which the improvement of high temperature performance is effected by multiple alloying additions.