Fig. 1. Micrographs taken after forging and heat-treatments of TNM alloy: (a, f) SEM images taken in BSE mode of as-forged and two-step treatments at 1230 °C /1 h /AC + 950 °C /1 h /AC (sample B). β0, (α2 /γ) lamellar colonies, globular γ phase and irregular γ phase are indicted by white arrows, and special platelets are indicted by yellow rectangles. (b, c), TEM micrograph and SAED patterns of β0 phase for as-forged sample. (d, e) TEM micrograph and SAED patterns of β0 phase for solution heat treatment sample at 1230 °C /1 h /AC (sample A).

Fig. 2. TEM images of the γ twins and α2-platelet in β0 phase. (a) The morphology of γ perpendicular precipitation. (b) SAED patterns of α2-platelet and γ twins along [$11\bar{2}0$] α2. (c) SAED patterns of γ and β0 phases along [111] β0. (d) Dark-field micrograph of α2-platelet showing a plate-like morphology. The dark field image is obtained using the (0001) α2 reflection. (e) The OR between α2 and β0: [111] β0 //[$11\bar{2}0$] α2, (0-11) β0 //(0001) α2. (f) The interface structure of α2 and β0. Inverse Fourier-transformed (IFFT) images show the interface dislocation between α2 and β0 phases.

Fig. 3. TEM images of the γ twins in β0 phase. (a) A magnified TEM image of the framed area of Fig. 2(a). The stacking faults (SFs) are indicated by the dashed blue line. (b) HRTEM image of γ twins along [$0\bar{1}1$] γ matrix. (c) A magnified TEM image of γ twins in β0 phase obtained at region c in (a). (d) SAED patterns of γ phase and β0 recorded from the [111] β0 direction.

Fig. 4. SFs inside the α2-platelet. (a) TEM micrograph of SFs inside α2-platelet, viewed from the [$11\bar{2}0$] α2 direction. The FFT image is inserted into the right bottom part of (a). (b) Atomic resolution HAADF image showing the stacking sequence of I1-type SF (ABA $\dot{B}$CBCB) in α2-platelet.

Fig. 5. HADDF-STEM images of SFs in α2 phase. (a-j) HADDF-STEM images. (a1-j1) The magnification of the areas in (a-j).

Fig. 6. The atomic stacking sequence adjacent to the γ twins, viewed from the [$11\bar{2}0$] α2 direction. (a) Two kinds of stacking sequence γ platelets (denoted as γ and γT) (b-e) The FFT patterns taken at sections Ⅰ →Ⅲ and the interface between section Ⅰ and section Ⅱ. (f) A higher magnify of the section Ⅲ.

Fig. 7. HADDF-STEM micrographs obtained with B=[$11\bar{2}0$] α2, showing the perfect disconnections with two-layer (a) and four-layer (b) steps.

Fig. 8. Schematic diagram showing the formation mechanism of γ twins in β0 phase. α2-platelets with I1- type SFs → γ and γT→ γ twins. b1= 1/3[$\bar{1}010$], b2= 1/3 [$10\bar{1}0$]. The red atoms indicate the present positions, while the white atoms indicate the initial positions. (a) The formation of I1- type SFs in α2 phase, yields two γ embryos, which has been indicated with blue lines. (b) γ and γT with different stacking sequences are formed by the slipping of partial dislocations b1 and b2, respectively. (c) γ twins formed by the encountering of γ and γT.