Fig. 1. (a) SEM image of the annealed TC17, showing the coarse-grained microstructure; (b) SEM image taken from the severe plastic deformation zone of the HESP processed TC17, showing the deformed microstructure; (c) bright-field (BF) TEM image taken from the topmost surface layer of the HESP processed TC17, with the inset being an SAED pattern; (d) dark-field (DF) TEM image showing the refined grains of β phase.

Fig. 2. (a) BF TEM image taken from the HESP processed TC17 at 0.25 MPa and 30 min, with a corresponding SAED pattern; (b) DF TEM image, showing the refined grains of β phase; (c) HRTEM image corresponding to region A marked by the red square in (a); (d) FFT image of (c); (e) IFFT image in the region C of (c), showing the hcp stacking sequence; (f) HRTEM image in the region B marked by the blue square in (a); (g) FFT image of (f); (h) IFFT image in the region D of (f), showing the fcc stacking sequence.

Fig. 3. (a) BF TEM image taken from the HESP processed TC17 at 0.35 MPa and 10 min, with the inset being a SAED pattern; (b) HRTEM image corresponding to region A marked by the red square in (a), with the inset being FFT image; (c) HRTEM and FFT image in the region C of (b); (d) HRTEM and FFT image in the region D of (b).

Fig. 4. (a) HRTEM image taken from the region B of Fig. 3(a), with the insets being FFT images corresponding to region I and II respectively; (b) IFFT image of region I in (a), with the inset showing the interplanar distance of (111)fcc crystal planes; (c) IFFT image of region II in (a), with the inset showing the interplanar distance of (0001)hcp crystal planes.

Fig. 5. (a) HRTEM image taken from an area containing different crystal structures in the HESP processed surface layer of TC17; (b) FFT image and IFFT image obtained from the region I in (a); (c) FFT image and IFFT image obtained from the region II in (a); (d) FFT image corresponding to region III in (a), showing the OR between hcp-Ti and fcc-Ti; (e) IFFT image obtained from the region III in (a), showing the atom stacking sequences on close-packed planes of hcp-Ti and fcc-Ti.

Fig. 6. (a) HRTEM image in the interface between hcp-Ti and fcc-Ti; (b) FFT image corresponding to upper part; (c) IFFT image in the region I of (a); (d) FFT image corresponding to lower part; (e) IFFT image in the region II of (a); (f) IFFT image in the region III of (a), showing the change of atomic stacking sequence from hcp to fcc; (g) IFFT image in the region IV of (a).

Fig. 7. (a) HAADF image of the ultrafine hcp-Ti containing several nano-sized fcc-Ti lamellas; (b) HRTEM image corresponding to region A in (a); (c) FFT image corresponding to region I in (b); (d) IFFT image corresponding to (c); (e) FFT image corresponding to region II in (b); (f) IFFT image corresponding to (e).

Fig. 8. (a) HRTEM image of the plate-like structures in Fig. 2(a), with the FFT images (a1) and (a2) corresponding to regions I and II respectively; (b) FFT image in the region III, showing the twinning relationship; (c) IFFT image in the region III of (a).

Fig. 9. (a) BF TEM image of the ultrafine grain taken from the HESP processed TC17 at 0.35 MPa and 10 min; (b) HRTEM image in the region A of (a), FFT images (b1) and (b2) corresponding to the region I and region II, respectively; (c) one-dimensional IFFT image in the region I of (b), showing the dislocations; (d) FFT image in the region III of (b), showing the twinning relationship; (e) IFFT image in the region III (b).

Fig. 10. (a) BF TEM image taken from the HESP processed TC17 at 0.25 MPa and 30 min; (b) HRTEM image in the region A of (a), with the inset being FFT image corresponding to the region I; (c) FFT image in the region II of (b), showing the twinning relationship; (d) FFT image in the region III, showing extra spots; (e) IFFT image in the region III of (b), showing the periodic structure.

Fig. 11. (a) HRTEM image taken from the HESP processed TC17 at 0.35 MPa and 10 min; (b) FFT image in the region I of (a), showing the twinning relationship; (c) FFT image in the region II, showing extra spots due to the presence of periodic structure; (d) IFFT image in the region II of (c), showing the periodic structure.

Fig. 12. (a, b) Schematic illustrations of the hcp-Ti and fcc-Ti respectively; (c) atomic schematic of the interface between hcp-Ti and fcc-Ti viewed along [2-1-10]hcp and [-110]fcc orientations; (d) schematic illustration of the formation for fcc structure by gliding of the Shockley partials with Burgers vectors of 1/3[01⇓⇓⇓⇓⇓⇓⇓⇓-10] on the (0001)hcp basal plane.

Fig 13. (a) Schematic of the typical crystal structure in fcc-Ti; (b) schematic of the Shockley partial b1 for fcc structure viewed from [111]fcc orientation; (c) process of forming the I-type twin by the glide of b1 Shockley partial on successive (111)fcc planes; (d) atomic schematic after the first glide of the b1 Shockley partial viewed from [-110]fcc orientation, producing an intrinsic stacking fault (ISF); (e) atomic schematic after fourth slip of the b1 Shockley partial viewed from [-110]fcc orientation, forming a four-layered deformation twin; (f) HRTEM image of a four-layered deformation twin.

Fig. 14. (a) Schematics of three Shockley partials b1, b2 and b3 for fcc structure viewed from [111]fcc orientation; (b) process of forming the II-type twin by the cooperative slip of three partials on successive (111)fcc planes and the 9R structure induced by slipping once every three (111)fcc planes of partial b1; (c) HRTEM image of 9R structure; (d) FFT image of the 9R structure in (c).