A NEW SURFACE SEGREGATION ISOTHERM DESCRIPTION AND ITS APPLICATION TOBINARY-ALLOYS

In previous years a new theory, modern thermodynamic calculation of in terface properties (MTCIP), was developed. Its general equations on th e thermodynamic equilibrium were solved in a first approximation (MTCI P-1A) and applied to several dilute binary alloy systems in good agree ment with experimental results, even in the case of surface reactions with the environment. Here, first a more developed second approximatio n (MTCIP-2A) is shortly outlined, allowing, among other new results, f or any number of interface sublayers, and being valid in the total com positional range of the bulk. Then, by some simplifications, a new seg regational isotherm is developed from it. Using the two dilute limits as described by the MTCIP-2A, then even with a single surface composit ion (measured at any bulk composition and temperature) this segregatio nal isotherm permits to obtain surface compositions over the total bul k compositional range. Results are shown for the low-index surfaces of PtNi at temperatures of about 1100 K, mostly in better agreement with experimental values than other present theories.