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.