Fig. 1. A set of bright-field TEM micrographs and the corresponding SAED patterns of Mg-In-Sm alloy aged at 200 °C for 24 h. The electron beam is parallel to (a, b) ${{\left[ 0001 \right]}_{\alpha }}$ and (c, d) ${{\left[ 1\bar{2}10 \right]}_{\alpha }}$.

Fig. 2. Low magnification HAADF-STEM images of the prismatic platelets in Mg-In-Sm alloys aged at 200 °C for (a, b) 24 h and (c, d) 96 h. The incident beam is parallel to (a, c) ${{\left[ 0001 \right]}_{\alpha }}$ and (b, d) ${{\left[ 11\bar{2}0 \right]}_{\alpha }}$. The inset in (d) is a distribution of the aspect ratio based on the statistical results of 30 prismatic plates.

Fig. 3. Series of STEM images of the prismatic platelets in Mg-In-Sm alloy aged at 200 °C for 24 h. (a, b) High magnification and atomic scale ${{\left[ 0001 \right]}_{\alpha }}$ HAADF-STEM images. (c, d) A pair of ${{\left[ 11\bar{2}0 \right]}_{\alpha }}$ HAADF and ABF-STEM images. The insets in (a–c) are the intensity profiles of the corresponding rectangle and the insets in (d) are the FFT and IFFT images of the green dotted frame.

Fig. 4. EDS elemental mappings of the prismatic platelets in Mg-In-Sm alloy aged for 24 h.

Fig. 5. A series of analysis results from the ${{\left\{ 10\bar{1}0 \right\}}_{\alpha }}$ prismatic platelet viewed along ${{\left[ 0001 \right]}_{\alpha }}$(upper) and ${{\left[ 11\bar{2}0 \right]}_{\alpha }}$ (below). (a, b) Atomic arrangements of the ideal and relaxed states. (c) Charge distribution. (d) Simulated HAADF-STEM images.

Fig. 6. Simulated composite diffraction patterns from the platelet (red) and matrix (black).

Fig. 7. Projections of the Mg3Sm-D019 (β″) and (Mg2In)Sm-D019 (generalized β″) supercells along (a) ${{\left[ 0001 \right]}_{\alpha }}$ and (b) $\langle 11\bar{2}0{{\rangle }_{\alpha }}$.

Fig. 8. Schematic diagram of the stability and formation ranges of β″ phase in α-Mg and Mg/In matrixes, and the left pictures give compared experimental images of β″ phases in Mg-La and Mg-In-Sm alloys. The gray background in table denotes the unstudied RE element in Mg alloys.

Fig. 9. A schematic diagram of the formation process of Mg2InSm prismatic platelet with an extended aging time. (a) Isolated Mg3In-type $\text{D}{{0}_{19}}$ structure, (b) The agglomeration of Mg3In-type $\text{D}{{0}_{19}}$ structures and formation of Mg3In-type $\text{D}{{0}_{19}}$ two-dimensional structure, (c) The nucleation of Mg2InSm-type $\text{D}{{0}_{19}}$ structures, (d) The growth and formation of Mg2InSm prismatic platelets.