Nonequilibrium self-organization in alloys under irradiation leading to the formation of nanocomposites

Abstract

Alloys under irradiation are continuously driven away from equilibrium: Every time an external particle interacts with the atoms in the solid, a perturbation very localized in space and time is produced. Under this external forcing, phase and microstructural evolution depends ultimately on the dynamical interaction between the external perturbation and the internal recovery kinetics of the alloy. We consider the nonequilibrium steady state of an immiscible binary alloy subject to mixing by heavy-ion irradiation. It has been found that the range of the forced atomic relocations taking place during collision cascades plays an important role on the final microstructure: when this range is large enough, it can lead to the spontaneous formation of compositional patterns at the nanometer scale. These results were rationalized in the framework of a continuum model solved by deriving a nonequilibrium thermodynamic potential. Here we derive the nonequilibrium structure factor by including the role of fluctuations. In order to consider an experimentally relevant situation, we perform kinetic Monte Carlo simulations of temperature-controlled heavy-ion irradiation in the immiscible model system Ag–Cu, where we obtain specific irradiation data by molecular dynamics simulations. We find that irradiation with 1 MeV Kr ions can indeed induce the spontaneous formation of a nanocomposite, in agreement with recent experimental results. Our results suggest, on the other hand, that this type of microstructure could not be obtained by irradiation with lighter particles, such as He ions. The simulation results predict, as well, the dependency of the structure factor on the type of irradiation particle used and are in qualitative agreement with the formula derived analytically. We propose that these statements can be tested by small angle scattering experiments.