Investigation on formation mechanism of T1 precipitate in an Al-Cu-Li alloy

The formation mechanism of T1 (Al2CuLi) precipitate in an Al-Cu-Li alloy was investigated by using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging, atom probe tomography (APT) and first-principles energy calculations. The HAADF-STEM images reveal the existence of the T1 precursor phase, which is termed as T1p in this study. The T1p phase has a lattice structure identical to the Al matrix, and consists of a stacking of Li-rich and Cu-rich layers along the {111}Al planes. APT analysis further confirms the existence of T1p phase, in which the concentrations of Li and Cu are significantly lower than those in T1 plate. Based on the first-principles energy calculations, the structural evolution from T1p phase to T1 phase is proved to be energetically favorable. Correspondingly, one possible evolution process from T1p phase to T1 phase involving the change of stacking sequence is proposed and discussed.