Fig. 1. (a) Schematic illustration of the influence of surface curvature on the calculation of deformation parameters according to the deflection of inner marker for pipe inner surface and rod outer surface, (b) Fitting the displacement-depth using Eqs. (2) and (3) of 8-pass SMGT Ni [6], 1-pass PISG 304 stainless steel [13] and the present 4-pass PISG 304 stainless steel; (c-f) Evaluation of the depth-dependent strain (e), strain gradient and strain rate (f) according to the displacement of twin boundary (d) measured in the SEM image (c).

Fig. 2. Cross-sectional SEM-ECC images of the 4-pass PISG sample, showing the gradient deformation microstructures along depth (a) and the structural detail in the rectangle marked areas (b, c). Three distinct deformation regions (Z1, Z2 and Z3) were observed. The dotted line in (b) guides the tilted twin boundaries and the white dashed lines indicate the SB.

Fig. 3. Cross-sectional TEM observations of the dislocation structure (a) and deformation twins (b) in Z1 of the 4-pass PISG sample. TEM sample was tilted with [011] parallel with the electron beam, where diffraction spots from twin and matrix are mirror symmetric along the twin plane normal indicated by the dashed line in the SAED pattern in (b).

Fig. 4. Cross-sectional SEM-ECC image (a) and dark filed TEM image (b) of the twin-twin intersection in Z1 of the 4-pass PISG sample. Note that deformation induced martensite (α) in the intersection sites, giving rise to an overlapped SAED pattern by α[111] and γ [110].

Fig. 5. (a) Dark field cross-sectional TEM image and (b) the enlargement of the rectangle marked area (a) of the local shearing of deformation twins in Z2 of the 4-pass PISG sample. Dotted lines in (b) indicate the sheared region and an arrow indicates the bended twin boundaries.

Fig. 6. (a) Bright and (b) dark field TEM images of the microstructure in Z2 of the 4-pass PISG sample, showing the SB formed in twin-matrix lamellae. (c) The local enlargement of (a) at a position where [011] of the SB is parallel with the electron beam. In (a), shear strain (ε) was calculated according to the angle between shearing direction and the original (θ0) and sheared (θ) twin boundaries. Note that SB contains twin substructure that is irrelevant to the original twin-matrix lamellae.

Fig. 7. (a) Cross-sectional TEM observation of SB formed in multiple twins (T1, T2) in Z2 of the 4-pass PISG sample. (b1) and (b2) the [011] SAED patterns from circled areas b1 and b2, respectively. Dashed lines in Fig. b1 indicate the twinning plane normal of T1 and T2, respectively.

Fig. 8. (a) Cross-sectional SEM-ECC image and (b) TEM observation of extensive local shearing in Z2 of the 4-pass PISG sample. (b1) and (b2) the SAED patterns from circled areas in (b), respectively. Dashed lines in (b) indicate the boundaries between SB and the remaining twinned area.

Fig. 9. (a) TEM observation and (b) the distribution of boundary spacing of extended microstructure in Z1 of the 4-pass PISG sample.

Fig. 10. (a) Cross-sectional TEM observation of fragment of extended structure in Z1 of the 4-pass PISG sample. (b) The HRTEM image and the inserted IFFT image of the low angle boundary in the extended structure.

Fig. 11. (a) TEM observation and (b) the boundary spacing distribution of the nanostructure in Z1 of the 4-pass PISG sample.

Fig. 12. Schematic illustration of the microstructural evolution of AISI 304 stainless steel during plastic deformation.

Fig. 13. Disappearance of twinning inside SBs of the 4-pass PISG sample. (a) TEM observation of the extended structure and twinned structure in SB; (b) HRTEM image of the propagation of extended structure toward twinned area in area “b” in a; (c) FFT and (d) IFFT image of the squared area “c” in b, indicating the disappearance of twinning by crystal rotation via dislocation slip.