Fig. 1. Schematic diagrams (a) the SPDT process and (b) the SPDT-induced surface prow deformation for the formation of GNS.

Fig. 2. Cross-sectional OM images of the (a) original and (b) SPDT L4 alloy. (c) EBSD map of the cross-sectional surface of SPDT L4 alloy. (d) XRD patterns of original and SPDT alloy at different depths of 80?μm, 50?μm, 30?μm, and surface, respectively. (e) Variation of both hardness and grain size against the distance from the topmost surface to the CG matrix of the SPDT L4 alloy.

Fig. 3. Typical longitudinal section BF TEM images of the SPDT L4 alloy at different depths: (a) 65?μm, (b) 50?μm, (c) 30?μm, (d) 15?μm, (e) 5?μm. (f) Topmost surface. Insets are the corresponding HRTEM image, SAED patterns and grain size distribution.

Fig. 4. (a) TEM image of the TTW. (b) HRTEM of TB of the dotted square in (a). (c) Fourier-filtered image of SF.

Fig. 5. (a) TEM image of the CTW. (b) HRTEM of TB of the dotted square in (a). (c) Fourier-filtered image of step.

Fig. 6. (a) TEM image of UFG. (b) HRTEM image of UFG, inset is the FFT pattern. (c), (d) Fourier-filtered image of SF marked by the white dashed square in (b). (e) TEM image of NG. (f) HRTEM image of SF from the marked region in (e) and high density of dislocations existing in the GBs.

Fig. 7. (a-c) Microstructure evolution of the MD simulation model at three different stages during cutting, from (a) stage 1, (b) intermediate stage 2, to (c) stage 3. Inset is the enlarged image of the red dash region in (c). The atoms are colored using common neighbor analysis.

Fig. 8. (a) Contour map distributions of equivalent stress in SPDT sample. (b) The statistics of varied equivalent stress with depth calculated by FEA in SPDT processing.

Fig. 9. Atomic structure evolutions show the nucleation and growth processes of TTW and CTW, respectively. (a)-(c) Nucleation and growth process of TTW. (d)-(e) Nucleation and growth process of CTW.

Fig. 10. Schematic of twinning nucleation and growth of (a) $\left\{ 10\bar{1}1 \right\}$CTW. Summary of twinning nucleation and growth in GNS L4 alloy based on HRTEM and MD results: (b) partial dislocations pile up at GBs, (c) nucleation of twinning, (d) growth of twinning.

Fig. 11. (a) Effective stresses of various Mg alloys plotted against grain size showing Hall-Petch relationship and corresponding dominated plastic deformation mechanisms for the refinement process. (b) Schematic illustration of grain refinement process in the L4 GNS layer from the matrix to surface.