Controlling factors for the formation of Guinier-Preston zones in Al-Cu alloys: An atomistic study

Abstract

Nanosized precipitates of solute atoms, which are often called "Guinier-Preston (GP) zones," are known to play a significant role in the precipitation-hardening effect on the Al alloys. In this study, the controlling factors for the formation of solute nanoclusters as in GP1 and GP2 zones, which are observed in the early and middle stages, respectively, of aging in the Al-Cu system, were investigated by extracting and examining the intra-and intercluster interaction energies of Cu in Al using density functional theory (DFT). DFT-calculated two-and three-body binding energies for Cu indicated that a Cu-Cu pair tends to bind at the first nearest-neighbor (1NN) positions, and a Cu-Cu-Cu triplet energetically prefers the arrangement of the triangular atomic cluster that contains two Cu-Cu pairs at the 1NN positions on the same {100} plane. Such a characteristic short-range ordering was suggested to be dominated by attractive three-body interactions due to the chemical (charge localization) effect, leading to planar clustering as found in the GP1 zones along the {100} planes. Intercluster interaction energies between two Cu planar clusters in Al were also calculated based on DFT. The results indicated that the energetically preferable configuration was the one in which two planar clusters are aligned at an intercluster distance of 2a (where a is the lattice constant of Al); it is noteworthy that this distance was the same to that in the basic structures of the GP2 zones observed in the experiments. A potential model for a dilute Al-Cu system was constructed on the basis of the extracted intra-and intercluster interactions, and then applied to atomistic Monte Carlo modeling for predicting the planar segregation of Cu atoms at finite temperatures. As a result, the formation of planar Cu clusters and the alignment of two planer clusters separated by 2a were successfully reproduced within a specific temperature range. This demonstrated that these interactions were important controlling factors for the formation of a characteristic pattern of solute clusters in the Al-Cu system.