Fig. 1. TEM images of the sample deformed at 800 °C to strain ε=30%. (a) An overview image showing the numerous deformation twins in the γ laths, introducing a number of intersection areas of γT and the original α2 lath; (b) low-magnification HRTEM image of the area marked by a rectangle in (a), showing the distribution of twin-induced ωo phase formed in the original α2 lath; (c) reverse transformation of ωo phase into α2 phase: the variants α2V1 and α2V2 have the same orientation relationship to the ωo phase (sections I/III and I/IV, FFTs I/III and I/IV). The basal planes of these two α2 variants (the yellow and white dashed lines) include an angle of 60° corresponding to the angle between two ωo planes (regions III and IV, FFT IV). However, the angle between the {0001} basal planes of α2V1 (the white dashed line) and the α2 matrix (the green dashed line) is 68.5° (FFTs II and III). Therefore, it is assumed that the ωo phase is the parent phase for α2V1 and α2V2.

Fig. 2. Phase constitution and microstructure observed after deformation and subsequent 100 h annealing at 850 °C. (a) SEM micrograph showing ωo precipitates (appearing bright) at the lamellar boundaries of deformed colonies. (b) TEM micrograph showing mechanical twins within γ lamellae. (c) Mixed structure of α2 and ωo grains in a former α2 lath, sandwiched between twinned γ phase. The inserted diffraction pattern confirms the presence of the phases γ, α2 and ωo. (d) HEXRD pattern, the βo phase has largely transformed into the ωo phase, as indicated by the splitting of the (110)βo peak into the two ωo peaks (11$\bar{2}$0)ωo and (10$\bar{1}$2)ωo at about 3.12 °

Fig. 3. Structure of α2 and ωo grains in former α2 laths observed after deformation and subsequent 100 h annealing at 850 °C. All micrographs taken along zone axis [11$\bar{2}$0]α2/[0001]ωo. (a) Low-magnification HRTEM image showing an overview of the structure. Note the numerous stacking faults in the α2 phase whose habit planes run parallel to the basal plane. The inserts show the ωo and α2 phases imaged in thinner areas of the same grain. (b) Two α2 grains joined by a (2$\bar{2}$01)α2 symmetrical tilt boundary. Note the orientation relationships between adjacent α2 and ωo grains. (c) Grain boundaries between α2 grains twisted around [11$\bar{2}$0]α2 at different angles.

Fig. 4. Distribution of the angles formed between (0001) basal planes of neighboring α2 grains. (a) After compression and (b) after compression and subsequent 100 h annealing at 850 °C.

Fig. 5. HAADF-STEM image of an (α2+ωo) lath between twinned γ lamellae together with an EDX line scan recorded along the line indicated by the yellow arrow in the HAADF image, showing the composition redistribution between neighboring ωo and α2 phases in the sample compressed at 800 °C and subsequently annealed at 850 °C for 100 h.

Fig. 6. APT results of the lamellar structure after annealing at 850 °C for 100 h. (a)-(d) The distribution of Al, Ti, Nb and O in the α2 and γ phases, respectively; (e) the composition fluctuation of α2/γ interface in (d). Note the composition fluctuation at a planar defect marked by green arrows in the lower part of (d), shown in (f).