Analysis of the beta'Ni2TiAl/beta NiTi equilibrium in Ni-Ti-Al alloys by the cluster variation method

The cluster variation method with the Lennard-Jones potential, previously used to analyze the gamma'(Ll(2) Ni3Al)/gamma(fcc Ni) equilibrium in NI-AI base multicomponent alloys, was used to calculate the beta'(L2(1) Ni2TiAl)/beta (B2 NiTi) equilibrium phase boundaries in the Ni-Ti-Al system. Results are compared with the isothermal and vertical sections of the ternary phase diagram in the literature. The lattice misfit between beta' and beta compound lattices, which is considered to play an important role for the formation of a rafted structure, passed through a minimum at 50%Ni, but was considerably larger compared to gamma'/gamma alloys. It is thus necessary to consider alloying addition for the optimization of high temperature properties of beta'/beta alloys by controlling the size and distribution of the beta' phase and the lattice misfit. (C) 1997 Elsevier Science Limited.