Effects of ballistic atom movements on ordering transitions of binary alloys

We studied chemical order-disorder transitions in equiatomic alloys on a square lattice in the presence of both thermal and ballistic atom movements. Using Monte Carlo simulations with a vacancy mechanism, we determined the steady states of the alloys for various combinations of f (fraction of ballistic atom movements) and T (temperature), and located the order-disorder phase boundary on a diagram of T vs f. For symmetric interatomic potentials, the dynamical critical temperature decreased with f as (1-1.58f) when f≤0.36, and decreased rapidly with f when 0.36<f≤0.43. No ordered phase was stable at any temperature when f was greater than a critical value of fc0>0.43. An Onsager-type kinetic rate equation was modified to include ballistic atom movements, and was used to identify two reasons why the ballistic atom movements suppress the dynamical critical temperature: (1) the ballistic atom movements dilute the enthalpy driving force for ordering, and (2) at low temperatures the ratio of thermal mobility coefficient to ballistic mobility coefficient becomes small.