Nanoscale patterning of chemical order induced by displacement cascades in irradiated alloys. I. A kinetic Monte Carlo study

Dense displacement cascades produced by irradiation with energetic particles lead to the formation of disordered zones in chemically ordered alloys. At temperatures below the order-disorder transition, these disordered zones, whose sizes range from a few to several nanometers, are annealed out by thermally activated atomic migration. Under sustained irradiation, the competition between these two dynamics may drive the system into various steady states of order. Kinetic Monte Carlo simulations are employed to identify these steady states in a model binary alloy that forms an L1₂ ordered phase at equilibrium. Besides the expected long-range ordered and disordered steady states, a new state is observed, where the microstructure is comprised of well-ordered domains of finite size. This steady-state patterning of order is identified by direct visualizations of the configurations, and by using an effective fluctuation-dissipation formula to analyze the behavior of the fluctuations of order upon approaching the long-range ordered steady state. It is shown that the patterning state becomes stable only when the disordered zones exceed a threshold size. Above this threshold size, reordering of cascade-induced disordered zones proceeds in two stages: new antiphase domains form first, and then shrink to the benefit of the matrix. This two-stage reordering is at the origin of the dynamical stabilization of patterns of order. The present results, which indicate that ion-beam processing could be used to synthesize ordered nanocomposites with tunable sizes, call for specific experimental tests.