THE BRAGG-WILLIAMS-GORSKY (BWG) ORDERING TREATMENT IN THE COMPOUND ENERGY FORMALISM (CEF)

The BWG type of approximation describing chemical ordering has been av ailable in thermodynamic calculation-optimisation software. Developmen ts were done for special cases of symmetry, e.g, the gamma-gamma' orde ring, based on the fee lattice, in Ni-based superalloys or the beta-be ta' ordering in the Cu-Zn system, based on the bce lattice. The techno logical importance of materials presenting order-disorder transformati ons, demands a versatile Gibbs energy modelling, able to describe the changes taking place during ordering as functions of composition and t emperature. Recently practical use of Gibbs energy descriptions of ord ered phases are becoming a daily procedure. In this communication we p resent a general method for deriving the constraints in the compound e nergy formalism (CEF) for the model parameters describing an order-dis order transformation. The cases discussed include, for example, the ox ygen ordering in its interstitial solution in alpha-Zr (hcp).