Continuous precipitate modes of the δ′-Al₃Li phase in Al-Li alloys

Owing to the fast kinetics of the precipitation, the formation mechanism of the δ′ phase in Al-Li alloys is still open to question. Using ab initio calculations, here, we reported two continuous precipitate modes (Bulk and Planar modes based on Li atoms arrangements) to complete the ordered δ′ phase through progressive Al-Li solid solutions. Under the vacancy-dependent solid-state precipitation mechanism, we found that these intermediate solid solutions were dominated by sequent Al-Li-vacancy complex structures that exhibit strong binding effects from Li atoms to vacancies. In turn, vacancies located in different sub-lattices exert short- and long-range effects to tailor the bonding strength of the constituted atoms. Based on the diffusion activation energy calculations, we suggested that the ordering transition of the δ′ phase prefers the Planar mode. In later stages of precipitation, the kinetics of precipitation accelerates, resulting in great difficulties in capturing this structural performance in experiments. Our findings are informative for understanding the energetic and phase transformation mechanism behind continuous precipitate for the precipitated phases with fast kinetics.