Fig. 1. (a) True stress-strain curves of both FG and CG specimens deformed at temperature of 950 °C and strain rate of 0.01 s-1. Deformed samples at various strain levels for microstructural observation are marked with dotted lines; (b) relative softening-strain curves of both FG and CG specimens corresponding to the true strain-stress curves in (a).

Fig. 2. Typical SEM microstructures at strains of 0, 0.5 and 1.2, where dark areas indicate α phase: (a-c) FG alloys; (d-f) CG alloys.

Fig. 3. Quantified evolution of α phase fraction with increased strain during hot deformation.

Fig. 4. EBSD micrographs of the studied samples deformed at temperature of 950 °C and strain rate of 0.01 s-1: CG samples at strains of (a) 0, (b) 0.1, (c) 0.5, (d) 0.9 and (e) 1.2; FG alloys at strains of (f) 0.5 and (g) 1.2.

Fig. 5. TEM bright field images of CG specimens deformed at strains of (a) 0, (b) 0.5 and (c) 0.9, respectively; (d, f) observed lath α′ phase coupled with stacking faults and the corresponding (e) SAED pattern and (g, h) HRTEM images.

Fig. 6. (a) Evolution of critical stress of DT with increased strain levels in FG and CG alloys, (b) evolution of energy barriers opposing DT with increased strain levels in both alloys and (c) relationship of total energy barriers and actual driving force of DT at various strains.

Fig. 7. (a, b) Two typical EBSD maps of CG alloy deformed to the same strain of 0.5 with higher magnification. The hexagonal wireframes of marked grains are also given. The (c) hcp (0 0 0 1) and (d) bcc (1 0 0) pole figures show the original and reconstructed orientation relationships of marked points in (a) and (b), respectively.