Effect of Li content on precipitation behavior and mechanical properties of Al–Cu–Li alloys under aging treatment

The influence of Li content (0.25, 0.85 and 1.20 wt.%) on the precipitation behavior and mechanical properties of Al-Cu-Li alloys was systematically investigated under two aging conditions (direct aging at 175 °C and 3.5% pre-stretching followed by aging at 155 °C). The alloys were studied through hardness testing, tensile property evaluation, transmission electron microscopy (TEM), three-dimensional atom probe (3DAP) analysis and density functional theory (DFT) calculations to establish correlations between microstructural evolution and mechanical performance. The results reveal that increasing Li content significantly enhances peak strength, with the most pronounced improvement under 155 °C aging after pre-deformation. While all alloys predominantly formed T 1 and θ′ phases, the low-Li alloy (0.25 wt.%) additionally exhibited Li-containing Ω phases precipitation. For the 0.25 wt.% Li alloy, full strengthening potential from T 1 and Ω phases was achieved during 175 °C/36 h aging, whereas 155 °C/36 h aging after pre-deformation increased θ′ phase contributions but reduced overall precipitation strengthening efficiency. At Li contents ≥0.85 wt.%, higher Li concentration preferentially enhanced T 1 nucleation while suppressing growth during 175 °C/36 h aging. Notably, pre-deformation combined with 155 °C/36 h aging in high-Li alloys (≥0.85 wt.%) significantly increased T 1 phase density with limited growth, triggering a transition from bypass to shearing-dominated strengthening mechanisms. Further increasing Li content to 1.20 wt.% promoted T 1 phase growth, resulting in substantial strength enhancement for the high-Li alloy. This work establishes critical relationships between Li content, aging processing and precipitation control strategies for optimizing Al-Cu-Li alloy performance.