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

The formation mechanism of T-1 (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 T-1 precursor phase, which is termed as T-1p in this study. The T-1p 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 T-1p phase, in which the concentrations of Li and Cu are significantly lower than those in T-1 plate. Based on the first-principles energy calculations, the structural evolution from T-1p phase to T-1 phase is proved to be energetically favorable. Correspondingly, one possible evolution process from T-1p phase to T-1 phase involving the change of stacking sequence is proposed and discussed. (C) 2017 Elsevier B.V. All rights reserved.