Novel mechanism for order patterning in alloys driven by irradiation

Kinetic Monte Carlo simulations have been performed to investigate the evolution of ordered domains in model alloys under irradiation. The alloys investigated were equiatomic binary alloys on a simple square lattice with first and second nearest-neighbor interactions, chosen so that a 2×2 ordered structure is the equilibrium phase below a critical order-disorder transition temperature Tc. The ratio of second to first nearest-neighbor interactions R was varied from 0 to 0.45 to explore the effect of the thermodynamic frustrations induced by the proximity of the 2×1 phase boundary, which occurs at R=0.5 for T=0. The atomic mixing produced by nuclear collisions was modeled by forcing the ballistic exchange of pairs of atoms at a controlled rate Γb. This disordering process competed with thermodynamic reordering, resulting in nonequilibrium steady states. Two trivial steady states were found, a disordered state at high Γb and low T, and a long-range ordered state at low Γb and low T. In the R=0.45 alloy, however, a third steady state was identified at intermediate Γb and T values, where multiple long-range ordered domains coexisted dynamically. It is shown that this state of patterning of order resulted from the coupling of the thermodynamic frustrations present in that alloy with the disorder introduced by irradiation. The practical relevance of this novel mechanism for patterning of order under irradiation is discussed in the context of recent observations of domain coexistence in irradiated Cu3Au.