Vacancies and Copper Ordering in Al–Cu alloys

A vacancy-induced mechanism of copper aggregate formation in Al–Cu alloy is proposed. For this purpose, various Cu cluster configurations in bulk Al lattice were explored in the framework of the density functional theory within the projector augmented plane-wave method and generalized gradient approximation. Investigation of different models of small copper clusters in 4 × 4 × 4 Al supercells containing totally 256 atoms reveals that the most energy-favorable configurations correspond to flat copper aggregates aligned along the 100-degree plane. Vacancies play a critical role in copper ordering in the Al–Cu alloy. It is found that vacancies preferably localize in the vicinity of Cu clusters and make them less soluble in the aluminum lattice. During the process of Cu aggregation, there are two competing effects. On one hand, raising the Cu atom number in the cluster decreases the mixing enthalpy, stabilizes ordered Cu structures in the Al lattice, and impedes Cu atom diffusion from the aggregate. On the other hand, vacancies near small Cu clusters result in the increment of the Al–Cu mixing enthalpy to positive values and make copper clusters insoluble in aluminum lattice. This effect makes small Cu clusters diffuse through solute to aggregate with other small clusters until large enough stable Cu aggregate will be formed.