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The BWG type of approximation describing chemical ordering has been available in thermodynamic calculation-optimisation software. Developments were done for special cases of symmetry, e.g. the γ-γ′ ordering, based on the fcc lattice, in Ni-based superalloys or the β-β′ ordering in the Cu-Zn system, based on the bcc lattice.

The technological importance of materials presenting order-disorder transformations, demands a versatile Gibbs energy modelling, able to describe the changes taking place during ordering as functions of composition and temperature. Recently practical use of Gibbs energy descriptions of ordered phases are becoming a daily procedure.

In this communication we present a general method for deriving the constraints in the compound energy formalism (CEF) for the model parameters describing an order-disorder transformation. The cases discussed include, for example, the oxygen ordering in its interstitial solution in α-Zr (hep).