Nanoscale patterning of chemical order induced by displacement cascades in irradiated L 10 alloys: Scaling analysis of the fluctuations of order

Atomistic kinetic Monte Carlo simulations are employed to analyze the dynamical stabilization of nanoscale patterning of L 10 chemical order in a model binary alloy subjected to sustained irradiation. The effect of irradiation-induced displacement cascades on the chemical order is modeled by the introduction at a controlled rate of nearly fully disordered spherical zones, which compete with the reordering promoted by the thermally activated migration of vacancies. When the size of the disordered zones is small, the alloy reaches a steady state that is either long-range ordered at low irradiation-induced ballistic jump frequency, Γb, or disordered at high Γb, with a first-order dynamical transition between these two steady states at Γb = Γbc. Furthermore, in the disordered steady state, the intensity of order fluctuations scales with the reduced variable Γb Γbc, a scaling that is consistent with an effective temperature approach. For larger cascade sizes, however, an additional steady state is stabilized at intermediate ballistic jump frequency, with a microstructure comprised of well-ordered nanoscale domains. In this patterning-of-order steady state, the above rescaling breaks down but we show that, after deconvolution of the structure factor into Gaussian and Lorentzian components, scaling of the Gaussian component is recovered by introducing a new reduced variable, Γb Γbp, where 1 Γbp is interpreted as the characteristic time for new domains to form in a disordered zone. This new scaling relationship provides a rigorous definition of the regime of patterning of order. This regime corresponds to the steady states stabilized by cascade sizes and ballistic jump frequencies satisfying Γbc ≤ Γb ≤ Γbp. A dynamical phase diagram based on this new criterion is constructed and it agrees well with direct visualization of atomic configurations. Extensions to nonstoichiometric compositions are investigated. Consequences for the direct synthesis of functional nanocomposite structures comprised of chemically ordered phases are discussed.