*3.1. Microstructure Characterizations*

Figure 3 shows the SEM image of the RASP-processed sample in cross-section. According to the grain deformation degree, we found that the depth of the deformation layer of the carbon steel processed 20 min by the RASP was about 120 μm. It was shown that the top surface of the sample produced a distinct plastic deformation and the grain within the layer was remarkably refined. Black and white arrows indicate the treated surface and deformed region of the sample, respectively. It shows that the RASP successfully produced a steel with a structural gradient. Some bending striations were found near the surface. Duan et al. [19,20] also found these deformation characteristics under high strain, and two competing factors were suggested. One was the work-hardening effect through the increase of strain. The other was the thermal-softening effect caused by adiabatic temperature rise. In this study, severe plastic deformation (SPD) was introduced into the surface, resulting in the enhancement of the lattice distortion and micro-strain. This is a reason why the bending striations were formed.

**Figure 3.** SEM image of the RASP-processed for 20 min sample.

Figure 4 shows the SEM images of the different RASP-processed samples in cross-sections. One can see that with the prolongation of the RASP time, the thickness of the deformed layer at the top surface increased. With a peening time of 5 min, the sample had a harder surface without defects. However, prominent micro-cracks and holes were produced easily in the samples when the RASP was processed for more than 5 min, as shown in Figure 4 by the red arrows.

**Figure 4.** SEM images of the RASP-processed samples: (**a**) 5 min, (**b**) 10 min, (**c**) 15 min, (**d**) 20 min.
