Fig. 1. The heat treatment and cold mechanical processing of Ti-11 V alloy used in this study.

Fig. 2. XRD patterns of Ti-11 V samples at various strains. The initial ST state was also included for comparison.

Fig. 3. Optical micrographs of Ti-11V samples cold forged to different strains: (a) the ST state. The inset shows the as-received hot-rolled morphology for comparison; (b) 0.05; (c) 0.1; (d) 0.4; (e) 0.7 and (f) 1.2. Kink bands and slip bands were marked by red arrows and blue arrows, respectively.

Fig. 4. EBSD characterization of Ti-11 V sample forged at a strain of 0.1: (a) grayscale image quality map; (b) misorientation profiles respectively along the ‘b1b1’ line crossing slip bands, and along the ‘b2b2’, ‘b3b3’, ‘b4b4’ lines crossing kink bands in (a); (c) corresponding IPF map of (a) showing several isolated pre-kinks. The inset is the legend; (d) IPF map showing the growth and coalescence of pre-kinks. The inset is corresponding grayscale image quality map; (e) IPF map showing the as-formed whole kink bands.

Fig. 5. EBSD characterization of Ti-11 V sample forged at a strain of 0.4: (a) grayscale image quality map showing the coalescence of kink bands; (b) corresponding IPF map of (a). Black color represents unindexed areas; (c) misorientation profile along the ‘1234’ line crossing the kink bands in (a); (d) the other IPF map showing the widened kink bands; (e) corresponding KAM map of (d); (f) misorientation profile along the ‘ff'’ line crossing the kink band in (d).

Fig. 6. EBSD characterization of Ti-11 V sample forged at a strain of 0.7: (a) grayscale image quality map showing the fragmentation of kink bands; (b) corresponding KAM map and (c) IPF map, respectively; (d) pole figure of (c) along the {110} pole.

Fig. 7. TEM images of Ti-V samples forged at a strain of 0.4: (a) kink bands showing the inner dense dislocation tangling; (b) kink bands showing the inner dislocation walls; (c) kink bands containing the bend boundaries; (a'), (b') and (c') corresponding SAED patterns of the circled areas in (a), (b) and (c) under zone axes of [$\bar{1}$11], respectively; (d) and (d') kink bands showing obvious distortion and fragmentation of the boundaries.

Fig. 8. TEM micrographs of Ti-V samples forged at a strain of 0.7: (a) kink band divided by dislocation cells along the longitudinal direction; (b) SAED patterns of the circle areas of ‘b1’, ‘b2’ and ‘b3’ in (a), respectively; (c) DF-image of the kink band corresponding to (a).

Fig. 9. TEM micrographs of Ti-V samples forged at a strain of 1.2: (a) overall BF-image showing the nanocrystallization within the kink band; (b) the magnification of the rectangular in (a); (c) SAED pattern of (b). The diffraction rings were indexed; (d) DF-image of (b); (e) and (f) SAED patterns taken from the surrounding β-matrix, and the boundaries of kink band/β-matrix, respectively.

Fig. 10. Nanoindentation inside both kink bands and the outer neighboring β-matrix at various strains: load-displacement curves at the strain of (a) 0.1, (b) 0.4, (c) 0.7 and (d) 1.2. The inset in (a) is typical AFM morphology of the indent; (b) variations of the average hardness (HN) as a function of the strain.

Fig. 11. Schematical showing the formation processes of kink bands. The crystal direction is indexed, and dislocations are marked as ‘⊥’. Gray arrows point in the direction of increasing strain.

Fig. 12. Schematical illustration of the proposed microstructural refinement mechanism within kink bands. Gray arrows point in the direction of increasing strain.

Fig. 13. Variations of dislocation density as a function of the applied strain: (a) the calculated total dislocation density ρtot on basis of nanoindentation testing in Fig. 10; (b) the measured ρGND according to EBSD maps.