Fig. 1. Schematic of the dimensions of the tensile test samples used in this study.

Fig. 2. Microstructures of as-cast Al-2.5Li-1.5Cu-1Zn-0.5Mg-0.15 Zr-(0.2Sc) alloys: (a) OM image of as-cast Alloy A; (b) OM image of as-cast Alloy B; (c) SEM image of as-cast Alloy A surrounded by EDS mapping.

Fig. 3. Microstructures of Alloy A after solutionizing: (a) SEM graph accompanied by a spot scanning result of a residual phase, indicating Al3(Sc, Zr) particles hard to be dissolved; (b) Superlattice-centered dark field TEM graph surrounding the rod-shaped discontinuous precipitating Al3(Sc, Zr) particles in Alloy A (corresponding selected area diffraction pattern (B= [001]) and bright field image inset).

Fig. 4. OM graphs of the studied alloys suffered to different ageing schemes: (a) Alloy A aged at 150 °C for 64 h; (b) Alloy A aged at 150 °C for 128 h; (c) Alloy A aged at 175 °C for 64 h; (d) Alloy A aged at 175 °C for 128 h; (e) Alloy B aged at 150 °C for 64 h; (f) Alloy B aged at 175 °C for 64 h.

Fig. 5. TEM Micrographs of the Alloy A in different ageing states:(a)-(d) BF images of Alloy A aged at 150 °C (together with the SAED pattern, B=[$1\bar{1}0$] in image (a) and B= [$11\bar{2}$] in others); (e) Higher magnification BF image of Alloy A aged at 150 °C for 256 h, showing some fine T1 phases (B=[$11\bar{2}$]); (f) BF image together with SAED pattern(B = [$11\bar{2}$], inset) of Alloy A aged at 175 °C for 64 h.

Fig. 6. TEM Micrographs of the Alloy B in different ageing states: (a) BF images of Alloy B aged at 150 °C for 64 h (B= [$11\bar{2}$]); (b) BF image of Alloy B aged at 175 °C for 64 h (B= [$1\bar{1}0$]).

Fig. 7. Dark field TEM micrographs comparing the δ' in the studied alloys aged at different states: (a)-(d) DF images of Alloy A aged at 150 °C for 32 h, 64 h, 128 h and 256 h respectively; (e) DF images of Alloy A aged at 175 °C for 64 h, showing lager particle spacing and size compared to the samples aged at 150 °C; (f) DF image of Alloy B aged at 150 °C for 64 h.

Fig. 8. TEM Micrographs of the studied alloys aged at 150 °C for 512 h: (a) BF images of Alloy A (B=[$11\bar{2}$]); (b) δ'-centered dark field image of Alloy A; (c) BF image of Alloy B (B=[$11\bar{2}$]); (d) δ'-centered dark field image of Alloy B.

Fig. 9. High magnification TEM images (B =[$11\bar{2}$]) of Alloy A aged at 150 °C for 64 h: (a) the interaction state between lath-shaped S' and a spherical Al3(Zr, Sc) particle; (b) HRTEM image of the red box in Fig. 9(a); (c) the coexistence state of Al3(Zr, Sc), plate-like T1, and lath-shaped S' phases; (d) HRTEM image of the red box in Fig. 9(c).

Fig. 10. δ' CDF images of the Alloy A aged at 150 °C for: (a) 32 h; (b) 64 h; (c) 128 h; (d) 256 h observed near the grain boundaries (corresponding BF images inset), indicating the presence of δ'-PFZs.

Fig. 11. δ' CDF images of the Alloy A aged at 175 °C for: (a) 32 h; (b) 64 h; (c) 128 h; (d) 256 h observed near the grain boundaries (corresponding BF images inset).

Fig. 12. δ' CDF images of the Alloy B: (a) aged at 150 °C for 64 h; (b) at 175 °C for 64 h observed near the grain boundaries (corresponding BF images inset).

Fig. 13. δ' CDF images of the studied alloys aged at 150 °C for 512 h: (a) Alloy A; (b) Alloy B observed near the grain boundaries.

Fig. 14. Variation of Vickers hardness as a function of artificial ageing time for Alloy A and Alloy B aged at 150 °C and 175 °C.

Fig. 15. Typical engineering stress-strain curves and detailed values of the samples in this study: (a) Engineering stress-strain curves of the selected samples in this study; (b) digest of UTS-EL data of cast Al-Li-Cu alloys and high-performance 2XXX alloys [7,20,28,43] ; (c) detailed mechanical properties values of the studied alloys suffered to different ageing scheme.

Fig. 16. Cubic of the measured δ' average diameters in the Alloy A and Alloy B aged at 150 °C and 175 °C plotted as function of the ageing time.

Fig. 17. (a) Measured δ'-PFZs half widths in the studied alloys aged at 150 °C and 175 °C plotted as a function of square root of the ageing time; (b) the scatter plot and linear fitting of all measured half-width of δ'-PFZs and corresponding EL in alloy A.