Point defects and formation driving forces of complex metallic alloys: Atomic-scale study of Al4Cu9

A comprehensive atomic-scale investigation of γ1-Al4Cu9, based on density functional theory and the independent-point-defect approximation, is performed. It provides a detailed description of the point defect structure and metastable parts of free energy in this compound, revealing an unusual shallow shape around stoichiometry. Including the energetics of the fcc Al–Cu random solid solution on a wide composition range, our analysis allows to evaluate the chemical driving force for the formation of γ1, which is favourable in various Cu- but also Al-rich environments. Moreover, since the γ1 driving force is strongly enhanced by off-stoichiometry, Al excess may allow γ1-Al4+xCu9 to compete with the θ equilibrium phase in Al-rich solid solutions. These conclusions shed light on the persistence of γ1-Al4Cu9, instead of the expected θ phase, in various processes.