Atomic-scale investigation on the evolution of T1 precipitates in an aged Al-Cu-Li-Mg-Ag alloy

The T-1 (Al-2 CuLi) phase is one of the most effective strengthening nanoscale-precipitate in Al-Cu alloys with Li. However, its formation and evolution still need to be further clarified during aging due to the complex precipitation sequences. Here, a detailed investigation has been carried out on the atomic structural evolution of T-1 precipitate in an aged Al-Cu-Li-Mg-Ag alloy using state-of-the-art Cs-corrected high-angle annular dark field (HAADF)- coupled with integrated differential phase contrast (iDPC)-scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDXS) techniques. An intermediate T-1' phase between T-1p and T-1 phase, with a crystal structure and orientation relationship consistent with T-1, but exhibiting different atomic occupancy and chemical composition was found. We observed the atomic structural transformation from T-1p to T-1' phase (fcc -> hcp), involving only 1/12 < 112 >(Al )shear component. DFT calculation results validated our proposed structural models and the precipitation sequence. Besides, the distributions of minor solute elements (Ag, Mg, and Zn) in the precipitates exhibited significant differences. These findings may contribute to a further understanding of the nucleation mechanism of T-1 precipitate. (c) 2024 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.