Temperature-dependent nucleation kinetics of Guinier-Preston zones in Al-Cu alloys: An atomistic kinetic Monte Carlo and classical nucleation theory approach

Guinier-Preston (GP) zones, which form in Al-Cu alloys, exhibit characteristic patterns of Cu-rich, disk shaped atomic clusters along the {000} planes. GP zones have received much attention not only for their precipitation-hardening effect on the Al matrix, but also from fundamental interest in the physics and chemistry of the nanoscale organization of solute atoms. In this study, we established an atomistic kinetic Monte Carlo (kMC) modeling technique for exploring the nucleation and formation processes of GP zones in Al-Cu alloys by constructing an effective on-lattice multibody potential for a dilute Al-Cu-vacancy system by density functional theory calculations. Our model can describe the clustering of Cu atoms via successive exchanges with vacancies and reproduce the characteristic planar nanoprecipitates along the {100} planes in a manner consistent with the crystallographic nature of the GP zones in the early-stage aging of Al-Cu alloys. The time evolution and critical nucleus size of Cu clusters were characterized at various temperatures based on the kMC results. Consequently, we predicted the existence of an optimum temperature (i.e., nose temperature) for the formation of Cu clusters at which the cluster growth was maximized, which was attributable to the interplay between the critical nucleation barrier and the diffusion rate. In addition, the critical nucleus size and temperature for cluster formation were examined based on classical nucleation theory along with the developed multibody potential. These provided an insight into the competition between the enthalpic and entropic effects on the formation of GP zones in the Al-Cu system. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.