Two-phase dynamical equilibria driven by irradiation in ordered alloys

We study here two-phase equilibria in driven compounds, where two dynamics are acting in parallel: thermally activated atomic jumps and forced jumps; such is the case for an alloy under irradiation where nuclear collisions induce ballistic jumps. We propose a deterministic treatment of the concentration and degree of order fields (one or two dimensional) and identify two-phase locally stable steady states: Dynamical equilibrium phase diagrams are thus computed. It is shown that in a body-centered-cubic alloy an A2-B2 order-disorder transition of the second kind at thermal equilibrium becomes of the first kind beyond a temperature-dependent critical forcing intensity. As a result, two-phase steady states can be stabilized by irradiation. Interface properties are then studied: Surface-tension-like effects are observed; introduction of antiphase boundaries destabilizes ordered precipitates, leading to their dissolution and redistribution. In order to compare the relative stability of the different steady states, a stochastic description is then proposed: We build a mean-field grand-canonical potential which governs the steady-state probability distribution of the concentration and long-range order parameter. It shows that the most stable steady state is indeed two-phase under suitable irradiation conditions.