SITE SUBSTITUTION OF TERNARY ADDITIONS TO NI3AL (GAMMA') FROM ELECTRONIC-STRUCTURE CALCULATIONS

A real-space, Green-function method is proposed to examine the electro nic structure and site preference of substitutional ternary additions to Ni3Al (gamma') using the tight-binding method of linear muffin-tin orbitals (TB-LMTO). Pairs of atoms are embedded in a random Ni0.75Al0. 25 medium (of approximately 1000 atoms), one-electron energies are obt ained by performing recursion on the TB-LMTO Hamiltonian, and the effe ctive pair interactions (EPI's) determined by configurational averagin g. Depending on the relative values of the three EPI's, W(Ni-Al), W(Ni -X) and W(Al-X), the ternary addition may occupy either the Ni site or the Al site exclusively, or the site preference may be a function of alloy stoichiometry. The site-substitution behavior in both stoichiome tric and non-stoichiometric Ni3Al alloys is computed for six ternary a dditions, X (X = Co,Cu,Zn,Rh,Pd,Si). The theoretical predictions are i n excellent agreement with experimental data, where available.