Fig. 1. (a) A cross-sectional SEM observation of the GNG surface layer with different microstructures (nanograins, submicron-grains and deformation twins) in the as-SMGT Co sample. (b) Average grain size and average Vickers hardness along the depth from the treated surface.

Fig. 2. Structural characteristics of submicron-grains at depth of 40--60 μm: (a) Bright-field TEM image and corresponding selected area electron diffraction pattern showing $\{10\bar{1}2\}$ twins; (b, c) g = $[01\bar{1}0]$ and g = [0002] two-beam bright field images showing dislocation array in matrix and twin (The blue arrows represent the <a> dislocation array); (d) Formation of low-angle GBs perpendicular to the basal plane (the dash line represents basal plane trace); $[\bar{1}2\bar{1}0]$zone axis HRTEM image (e) and inversed FFT image (f) using a pair of $\{1\bar{1}00\}$reflections, showing the presence of extra half plans for <a> dislocations.

Fig. 3. Microstructural characteristics of nanograins at depth of 0～10 μm: (a) Bright field image showing the approximately equiaxed nanograins (corresponding selected area electron diffraction pattern is inset); (b) Dislocation density distribution calculated by HAADF-STEM images (The inset presents significant slip planes and Burger vector of dislocations); (c) $[\bar{1}2\bar{1}0]$ zone axis HAADF-STEM image showing various dislocations and SFs in nanograin; (d) Formation of the low-angle GB by <c+a> dislocations from the nanograin presented in the inset image.

Fig. 4. Cross-sectional orientation mapping and typical GB characterizations of the Co sample subject to SMGT: (a) and (b) 40 μm depth, (c) and (d) 10 μm depth. (e) Grain size effect on the fraction of twinned grains by SEM-TKD. (f) Misorientation angle distributions of the two deformation microstructures shown in a and c (various twins angles are identified by dash lines). (g, h) Shear direction (SD) inverse pole figures of submicron-grains and nanograins in the same position. Correspondingly, grain orientations containing $\{10\bar{1}2\}$ $<\bar{1}011>$ twins are shown in (i, j).

Fig. 5. Hardness variation against inverse square root of the average grain size of the Co sample treated by SMGT.