EAM analysis of the lattice parameter effect in order-disorder transformation

The embedded atom method (EAM) was used to theoretically analyze the effect of the lattice parameter variation on the order-disorder transformation in binary alloys. The simple model was considered in which the configuration energy E as the functions of the lattice parameter a and the long-range order parameter σ was given by Taylor expansion at σ=0. The following results were found for AB alloy. At T=0 K, the stable state existed in the completely ordered phase which was also found in the case of A₃B compound or AB₃ compound. The order-disorder was found to be a second order transition. Only one kind of order-disorder was found for AB alloy. Three groups of order-disorder transformation can be observed for A₃B compound or AB₃ compound. For group I, when the temperature is below the critical temperature, the order-disorder is a first order transformation. For group II, the order-disorder transformation is a first order transition. For group III, the order-disorder transformation is found to be a second order transition. The lattice parameter variations have a significant effect on E₂ coefficient, which is related to the ordering energy. These results are in good agreement with those obtained by using EAM and CVM (cluster variation method) calculations.