Abstract: The precipitation evolution and mechanical properties of a commercial Al-Cu-Li-Mg alloy in natural aging (NA) tempers (T3 and T4) were investigated in detail. It was demonstrated that GPI zones were the predominant precipitate throughout the NA process. In the T3-aged sample, the precipitation sequence was determined: supersaturated solid solution (SSSS) → atom clusters → GPI zones, whereas in the T4-aged sample, it was SSSS → atom clusters → GPI zones → GPI zones + δ′/GPI zones/δ′. For samples without NA, the strength of the T3 sample was 76.0 MPa higher than that in the T4 sample, which was attributed to the pre-deformation-introduced dislocations. During the early stage of NA, GPI zone nucleation was substantially restricted in the T3 sample, resulting in a much lower number density of GPI zones. It retarded the aging response and strength improvement, consequently narrowing the strength difference between T3 and T4 samples. As the NA process progressed, the average diameter of GPI zones in the T4 sample increased to approximately 4.5 nm, accompanied by the formation of δ′/GPI zones/δ′ composite precipitate. However, GPI zone growth and δ′/GPI zones/δ′ composite precipitate formation were significantly inhibited in the T3 sample, as evidenced by its much smaller average diameter of approximately 2.1 nm and the absence of composite precipitates. It inhibited the strength improvement of the T3 sample. Therefore, after a long period (5 months) of NA, the strength of the T4 sample was about 13.0 MPa higher than that of the T3 samples, instead.

Key words: Al-Cu-Li-Mg alloy, Precipitation evolution, Mechanical properties, Natrual aging, HAADF-STEM