Fig. 1. (a) Optical microstructure of the as-solidified ingot. (b) BF-STEM image of the as-solidified ingot. The lamellae were indicated by green arrows. (c) Magnified BF-STEM image of the region c in b. (d) BF-TEM image corresponding to region d in c. (e) SAED patterns of the region containing α matrix, lamellae 1 and 2 showing twinning and B-type OR. The FFT patterns of lamellae 1-6 were shown at the bottom of this figure.

Fig. 2. (a) HRTEM image of an area with lamellae 3 and 4 viewed along [2$\bar{1}$$\bar{1}$0]HCP and[$\bar{1}$10]FCC direction. (b-e) Enlarged HRTEM images of the region b-e in b, respectively, showing corresponding stacking sequences.

Fig. 3. (a) Standard simulation diffraction patterns viewed along [2$\bar{1}$$\bar{1}$0] direction of HCP structure. (b) FFT patterns corresponding to lamella 4. (c) Atomic model of HCP and S1 structures viewed along [2$\bar{1}$$\bar{1}$0] direction. The left side for the HCP and the right side for the S1.

Fig. 4. (a), (b) and (c) Stacking sequences of HCP, S1 and FCC structures viewed along [2$\bar{1}$$\bar{1}$0]HCP direction, correspondingly. (d) Schematic illustration of HCP→ S1 →FCC phase transformation. Dashed lines represent slip planes; Blue arrows indicate slip direction; HCP, S1 and FCC are marked by blue, red and black boxes, respectively.

Fig. 5. Schematic illustration for the formation of S1.