Fig. 1. Schematic diagram of the heat treatment process.

Fig. 2. Original microstructure of the Ti-22Al-25Nb alloy. (a) Distribution and content of the B2 and O phase; (b) O phase morphology; (c) B2 grain orientation.

Fig. 3. Microstructure obtained after holding for 15?min at 930?°C. (a) EBSD image of the phase distribution and local B2 phase orientation distribution; (b) spheroidized O phase and nucleation of the α2 phase; (c) strip O phase; (d)-(f) TEM images of the composition distribution.

Fig. 4. Microstructure obtained after holding for 15?min at 960?°C. (a) O phase distribution inside the B2 grains; (b) O phase orientation distribution inside the B2 grains; (c) α2 phase precipitated inside the spheroidized O phase at the B2 grain boundaries; (d) orientation relationship between the O and α2 phases.

Fig. 5. Composition analysis at the grain boundaries at 930?°C and 960?°C. (a) Scanning electron microscopy (SEM) image of the microstructure at 930?°C; (b) line scan of elements at the grain boundaries of (a); (c) SEM image of the microstructure at 960?°C; (d) line scan of elements at the grain boundaries of (c).

Fig. 6. EBSD image of the microstructure obtained after holding for 15?min at 990?°C during the heating process. (a) Phase distribution; (b) Z-axis inverse pole figure (IPF) map.

Fig. 7. Schematic diagram of the O→α2 phase transformation mechanism.

Fig. 8. Free energies of formation for the α2 phase and O phase. (a) Free energies of formation for the α2 phase and O phase as a function of the Nb content; (b) difference in the free energies of formation for the α2 phase and O phase as a function of the Nb content.

Fig. 9. Variation in lattice parameters of the α2 phase with increasing Nb content. (a) γ angle; (b) a, b, and c values; (c) crystal cell of the α2 phase when the Nb content is 0%; (d) crystal cell of the α2 phase when the Nb content is 25 %.

Fig. 10. Backscattered SEM image of the microstructure at 1200?°C.

Fig. 11. Microstructures in the samples cooled from 1200?°C to different temperatures in the furnace. (a) (d) 1020?°C; (b) (e) 930?°C; (c) (f) room temperature (25?°C).

Fig. 12. Microstructure in the sample cooled from 1200?°C to 930?°C in the furnace. (a) (b) EBSD image of the phase distributed in the blue box in Fig. 11(e); (c) standard Kikuchi patterns of the B2 phase and O phase; (d) Kikuchi patterns at points ①-⑤.

Fig. 13. Microstructures in the samples cooled from 1200?°C to 1020?°C in the furnace, quenched, and held at 960?°C for different time. (a) 15?min; (b) 60?min; (c) partial enlargement of (b).