First-principles study of stability of small vacancy clusters in aluminum

The vacancy cluster is one of the most common defects in aluminum alloys. Using density-functional theory-based first-principles calculations, the energetics of small vacancy clusters in aluminum, including voids, stacking fault tetrahedra (SFT), and vacancy platelets on the {111} plane, was investigated. It is found that the significant difference in monovacancy formation energies reported in literatures is mainly related to their used exchange-correlation functions. On average, LDA is the most reliable approximation for the monovacancy formation energies in Al, followed by PBE, PBEsol, PW91, and AM05. The results in this work confirm that the divacancy in Al is indeed energetically unfavorable. Moreover, vacancy clusters with a size smaller than five in any form are unstable against their corresponding number of isolated monovacancies. The SFT is the most stable form of most small vacancy clusters, followed by the void and vacancy platelet. These results are helpful to understand the experimentally observed size distributions of vacancy clusters in Al.