Fig. 1. Different morphologies of η-MgZn2 precipitates growing along the same orientation (η2). (a) Low magnification bright field images of nano-sized η precipitates in an overaged Al-Zn-Mg alloy. (b) Frequency count histogram of precipitates with different aspect ratios, taken from Fig. S2 like (a). (c) HRTEM image of one thin η2 precipitate in a white circle in (a). (d) HRTEM image of one thick η2 precipitate in a red circle in (a). All view direction are along [1$\overline{1}$2]Al.

Fig. 2. Defects in MgZn2 precipitates. (a, b) HAADF-STEM image of η2 precipitates containing 2R stacking faults on the edges. One precipitate is thin in (a) and another one is a bit thicker in (b). (c) HAADF-STEM image of an η2 precipitate containing 2R + 3R stacking faults and an irregular planar defect intersecting them. (d) Structural model for the area in the blue dashed rectangle in (c). All images are viewed along [1$\overline{1}$2]Al // [$\overline{1}$2$\overline{1}$0]η.

Fig. 3. Comparison among various defect structures in η2 precipitates. (a) HAADF-STEM image of a thick η2 precipitate with parallel 2R + 3R stacking faults and planar defects (in orange), the precipitate nucleates on an Al3Zr particle. (b) HAADF-STEM image of a thick η2 precipitate with reversed 2R + -2R / 2R + $\overline{3}$R stacking faults. (c) HAADF-STEM image of a standard thin C14 η2-MgZn2 precipitate. (d-f) Enlargements areas in blue dashed rectangles in (a-c), respectively. C14 crystal model and QSTEM simulated image are overlaid on (f). All precipitates with different morphologies have the same axis.

Fig. 4. Strain analysis of various η2 precipitates. (a-c) GPA images of a standard C14 η-MgZn2 precipitate (as in Fig. 3(c)), exx and eyy in (b) and (c), respectively. (d-f) GPA images of an η2 precipitate with one 2R (in Fig. 2(a)), exx and eyy in (e) and (f), respectively. (g-i) GPA images of an η2 precipitate (as in Fig. 3(b)) with reversed stacking faults, exx and eyy in (i) and (j), respectively. (j-l) GPA images of an η2 precipitate (as in Fig. 2(c)) with parallel stacking faults and planar defect, exx and eyy in (m) and (n), respectively. The reference region in each HAADF-STEM image is the farthest matrix (in the same image) away from the precipitate. All GPA in this work use the same parameters (shown in Supplementary Fig. S1).

Fig. 5. Five-fold symmetry structure. (a) HAADF-STEM images of round η precipitates with complicated defects including five-fold symmetry structures. (b) Atomic structure model for part of defects in (a). (c) Atomic structure model of the 72° rhombus unit. (d) Atomic structure model of the 72° flattened hexagon unit.

Fig. 6. Schematic diagram for the formation of self-accommodated planar-defect.

Fig. 7. Schematic diagram for the formation of 2R + -2R stacking faults couple.

Fig. 8. Schematic diagram for different growth behaviors of Laves phase precipitates reported in this work. A precipitate at the nucleating stage occurring in the alloy aged for 568 h was observed as inserted in the left part, in the same growth direction as η2.