First-principle investigation on the interfacial structure evolution of the δ'/θ'/δ' composite precipitates in Al-Cu-Li alloys

The precipitation sequence in Al-Cu-Li alloys is sensitively dependent on the Cu/Li ratio. In the low ratio Cu/Li alloys of 1–2.5, the δ' phases usually nucleate and grow from the θ' precipitates, forming δ'/θ'/δ' composite precipitates. In this work, we present a first-principle study on atomic structures and their relative stabilities of the growing δ'/θ'/δ' composite precipitates in Al-Cu-Li alloys. Based on the analysis of the interface formation energy, constituted interface and coherent strains energies, an “anti-phase 1/2[110]” relationship for the opposite δ' has been proposed when the inward θ' has an odd number of Cu-layers. It may be achieved by translating one side of the δ' by 2/2a along the [110] slip direction, which is an energetically most favorable path. By analyzing the bonding characteristics, both the “zigzag Al-Li combined with Cu” and the “zigzag Al-Al” interfacial terminals are found to control the interface structure of the growing δ'/θ'/δ'. According to the calculated ideal tensile strength, the “anti-phase 1/2 [110]” structure is most stable to some extent. When Li atoms at the interface enter decohesion mode along the applied strain, the stable δ'/θ'/δ' is prone to failure because of relatively weak Li-Al covalent bonds. Therefore, the really thin δ' in δ'/θ'/δ' composite precipitates may be explained by the continuous disassociation of Li atoms from the interface. In addition, a very weak Cu-Li covalent bond was suggested in the δ'/θ'/δ' composite precipitates. This is in sharp contrast to previous reports.