Abstract
Solid-state precipitation is a key heat-treatment strategy for strengthening engineering alloys. Therefore, predicting the precipitation process of localized solute-rich clusters, such as Guinier–Preston (GP) zones, is necessary. We quantitatively evaluated the critical nucleus size and nucleation barrier of GP zones in Al–Cu alloys, illustrating the precipitation preferences of single-layer (GP1) and double-layer (GP2) GP zones. Based on classical nucleation theory using an effective multi-body potential for dilute Al–Cu systems, our model predicted GP1 and GP2 precipitation sequences at various temperatures and Cu concentrations in a manner consistent with experimental observations. The crossover between formation enthalpy curves of GP1 and GP2 with increasing cluster size determines the critical conditions under which GP2 zones can nucleate without prior formation of GP1 zones. This relationship reflects competing interactions within and between clusters. The results illustrate the underlying mechanisms of competing nucleation between zones, and provide guidance for tailoring aging conditions to achieve desired mechanical properties for specific applications.
Highlights
Precipitation hardening is one of the most common approaches used to increase the yield strength and hardness of Al alloys
We investigated the underlying mechanism of competing nucleation of the GP1 and GP2 zones in Al–Cu alloys
By exploring the free energy of formation for GP1 and GP2 zones as a function of temperature and solute concentration, we address important questions related to the critical conditions for the zone nucleation observed during the aging process, such as: (i) What mechanism could change the precipitation sequence of the two GP zones? (ii) Under what conditions can GP2 zones nucleate directly without the induction of GP1 zones? (iii) Why is the growth process of GP zones markedly discontinuous? (iv) What determines the upper limit of the size of the GP1 zones? To address these questions, we outline the computational approach and describe our findings from a combination of atomistic and classical nucleation theory (CNT) perspectives
Summary
Precipitation (or age) hardening is one of the most common approaches used to increase the yield strength and hardness of Al alloys. Hardness changes as a function of aging time were observed in the stages associated with the GP1 and GP2 zones. The two stages in the aging curves are separated by a plateau and can be clearly observed. This plateau becomes less evident as the aging temperature increases and/or the Cu content is reduced. During the growth of GP zones, the diameter of the zone with the GP1 structure remains quite constant across the plateau of the two-stage hardness curves, and increases rapidly at the end of the plateau as the GP1 structure changes to the GP2 structure. To the best of our knowledge, no conclusive explanation exists for the saturation of the GP1 zone size and its abrupt growth after transformation to the GP2 structure
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