Fig. 1. Representative OM images ((a) and (b)) and corresponding statistical grain size distribution ((c) and (d)) of the #1-I ((a) and (c)) and #2-I ((b) and (d)) alloys, respectively.

Fig. 2. Representative TEM images showing intragranular β″ precipitates in the #1-I (a) and #2-I (b) alloys aged for 6 h, respectively. (c) and (d) are representative high resolution TEM image and corresponding Fourier Transformation image of a β″ precipitate, respectively. Insert in (c) is a sketch to show the atomic structure of the β″ precipitate along <010> direction. The half needle length and number density of the β″ precipitates are depicted in (e) and (f), respectively, as a function of aging time.

Fig. 3. Representative TEM images ((a) and (b)) and corresponding statistical cross-sectional grain size distribution ((c) and (d)) of the as-extruded #1-II ((a) and (c)) and #2-II ((b) and (d)) alloys, respectively. (e) is a typical TEM image showing some Al3Sc dispersoids located at grain boundaries in the as-extruded #1-II alloy (marked by solid arrows), and (f) showing some fine intragranular Al3Sc dispersoids in the #2-II alloy (marked by open arrows).

Fig. 4. Representative TEM images showing precipitates in the #1-II ((a), (c), and (e)) and #2-II ((b), (d), and (f)) alloys aged for 6 h, respectively. (a) and (b) are in low magnification, while (c) and (d) in high magnification. The β″ precipitates and Al3Sc precipitates in the aged #1-II alloy are marked by solid and open arrows, respectively, as shown in (c). A β' precipitate in the aged #2-II alloy is typically shown in (d), with corresponding selected-area diffraction pattern inserted in the upper-right corner. (e) shows a representative TEM image of an Al3Sc precipitate in the aged #1-II alloy, and (f) shows a representative TEM image of two Mg2Si particles formed at grain boundary in the aged #2-II alloy.

Fig. 5. Sketches to illustrate the microstructural evolution in the coarse-grained and ultrafine-grained #1 and #2 alloys before and after aging treatment.

Fig. 6. Hardness evolution in the coarse-grained (a) and ultrafine-grained (b) #1 and #2 alloys as a function of aging time.

Fig. 7. Calculated hardness in comparison with corresponding experimental data. Note that only the coarse-grained alloys are selected for calculations, and four data (both #1 and #2 alloys: before aging and aged for 6 h) are used for comparison.

Fig. 8. Electrical conductivity evolution in the coarse-grained (a) and ultrafine-grained (b) #1 and #2 alloys as a function of aging time.

Fig. 9. Hardness-electrical conductivity correlation of present Al-Mg-Si-Sc alloys with different grain size and Mg/Si ratio. The four data are selected from the best hardness/electrical conductivity combination among the four sets of alloys, i.e., aged #1-I, #2-I, #1-II, and #2-II alloys. Two regimes in (a) demonstrate the effect of grain size (I-type vs II-type), while those in (b) manifest the effect of Mg/Si ratio (#1 vs #2). The line is a guide for the eyes.