Fig. 1. (a) Representative HAADF image of the Al-Sc alloys aged at 250 °C for 14 h showing the bright contrast associated with the Sc-enriched, each bright dot in the atomic-resolution HAADF-STEM images is a Sc-rich atomic column. (b) Sc clusters enlarged from (a). HAADF analysis of Al3Sc precipitates in Al-Sc alloys aged at 300 °C for 14 h is shown in (c) and (d) respectively, and the corresponding selected-area electron diffraction pattern of Al3Sc particles (indicated by red arrows) is inserted in (c).

Fig. 2. Representative APT images to show the atom distribution of Al and Sc in the homogenized Al-Sc alloy (a, b) and aged at 250 °C for 2 h (c, d), respectively. Sc clusters are evident in (d). (Green = Al atoms and blue = Sc atoms, dimensions: 60 nm × 60 nm × 260 nm).

Fig. 3. Representative APT images typically showing the Sc cluster evolution in the Al-Sc alloys aged at 250 °C for 2 h (a), 8 h (b), and 14 h (c). (Blue = Sc atoms, dimensions: 30 nm × 30 nm × 60 nm). (d) The corresponding statistical results on the number density of Sc clusters, where the clusters are sorted into different sizes.

Fig. 4. (a) Variation of average atom number and number density of the Sc clusters with aging time (t) in the Al-Sc alloys aged at 250 °C. (b) Variation of the average radius and number density of Al3Sc precipitates with aging time in the 300 °C-aged Al-Sc alloy.

Fig. 5. (a) APT image showing the regions with the Sc cluster in Al-Sc alloys aged at 250 °C for 14 h (dimensions: 15 nm × 15 nm × 10 nm), (b) the corresponding proxigram displaying the respective element distributions across the Sc cluster/matrix interface.

Fig. 6. (a) Representative APT images typically showing the Sc cluster in the Al-Sc alloys aged at 300 °C for 14 h. (Blue = Sc atoms, dimensions: 60 nm × 60 nm × 160 nm), (b) the corresponding statistical results on the number density of Sc clusters, where the clusters are sorted into different sizes.

Fig. 7. Representative engineering stress-strain curves of the Al-Sc alloys aged at 250 °C and 300 °C for 14 h tensile tested at room temperature.

Fig. 8. Dependence of room-temperature yield strength (a) and the strain to fracture (b) on aging temperature as a function of aging time.

Fig. 9. Scaling relationship between the strain to fracture and the combined parameters of Sc clusters or Al3Sc precipitates.