Fig. 1. (a) Schematic illustration of PISG set-up, (b) an optical image showing deformation induced deflection of a twin boundary in PISGed sample, (c) measurement and the least square fitting of depth dependence of displacement of the twin boundary and (d) estimated shear strain, strain gradient as a function of depth.

Fig. 2. XRD profiles for different depths of PISG samples.

Fig. 3. Cross-sectional ECC image of PISG sample.

Fig. 4. TEM image of area in Z1.

Fig. 5. ECC (a) and EBSD (b) images of areas in Z2, <001> ploe figure of austenite (c) and martensite (d). Dot lines in (a) delineate one austenite grain containing multiple twins (T1, T2 and T3) and deformation induced martensites (M). In (b), the green color represents austenite, the red color refers to martensite and the black lines indicate twins. The calculated martensite variants (red dots) according to the austenite orientation in (c) following K-S relationship were overlapped in (d).

Fig. 6. ECC (a) and TEM (b) images of areas in Z3. Dot lines in (b) delineate the wavy twin boundaries, whereas the empty circles mark the sheared region. Shear direction is indicated by arrows.

Fig. 7. TEM image of topmost surface (a) and 15 μm deep (b) of PISG sample.

Fig. 8. Variation of hardness and boundary spacing as a function of depth of PISG sample.

Fig. 9. Schematic illustration of structural evolution of AISI 304 stainless steel toward nanometer regime.