Severe shot peening (SSP) as a surface severe plastic deformation (S2PD) process over a wide range of coverages was applied to generate gradient microstructures with the grain size increasing from nanometer- and micron-scale to initial grain size on the surface layers of 321 stainless steel. The microstructural evolutions including the grain size distribution and phase transformation were systematically investigated in-depth for different surface coverages. GI-XRD, FE-SEM and TEM techniques were used to reveal the microstructure modification mechanisms as a function of the surface coverage. Experimental results show that dislocation slip plays a key role in the grain refinement of this alloy so that with increasing the surface coverage, different structures including dislocation walls, dislocation tangles, and lamella-shaped cells sequentially appear in the initial coarse grains. The results confirmed that these dense dislocation structures during ultrahigh plastic deformation produce ultrafine- (115-192 nm) and nano-grains (68-82 nm) to minimize the total energy of the system. In line with the grain refinement, the γ (austenite) → α̍ (strain-induced martensite) phase transformation is more affected as the plastic strain increases. So that the volume fraction of α̍ phase increases to 58.4 % for ultrahigh strains. Gradient variation of microhardness with the depth were also obtained for various samples.

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