## 3 Results and Discussion  
Figure 1 shows the TEM results of the nanocrystalline surface layer on 304 stainless steel. As shown in Figure 1A, the grain size clearly changed in a gradient manner, becoming finer closer to the surface. Furthermore, selected area electron diffraction (SAED) was performed on the region near the surface in Figure 1A, and the diffraction pattern is shown in Figure 1B. It can be clearly seen that the pattern was mainly composed of \(\alpha^{\prime}\)-martensite, and the pattern of austenite was not obvious, indicating that its content is low and the main phase of the grains has transformed from austenite to martensite through strain-induced martensitic transformation. The TEM result indicates that a nanocrystalline layer consisting mainly of martensite was constructed on the surface of 304 austenite stainless steel through strain-induced martensitic transformation. Due to the small size of the surface grains, it is difficult to accurately identify their grain size. Therefore, an area 1-3 \(\upmu\)m away from the surface was selected to statistically analyze the grain size of the nanocrystalline layer, as shown in Figure 2. It can be seen that the grain size distribution of the nanocrystalline layer was between 40 and 340 nm, and the percentage of grains smaller than 100 nm was about 74%. The average grain size in the statistical area was also only around 89 nm. The result of the analysis revealed that the grain size of the nanocrystalline layer on the surface of 304 austenite stainless steel was within the expected range. Furthermore, the grain size was also found to be uniform across the layer. This indicates that the nanocrystalline layer on the surface of 304 austenite stainless steel was well-formed and homogeneous. Since the strain-hardening ability of martensite is weaker than that of austenite during cavitation erosion [19; 20], the martensite phase in the 304-Nano was partially conversely transformed into austenite by heat treatment, and two heat treatment temperatures (500 \({}^{\circ}\)C and 550 \({}^{\circ}\)C) were selected to adjust the phase content of the nanocrystalline layer. Figure 3 shows the XRD patterns of 304, 304-Nano, N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples. The 304 sample exhibited typical austenite peaks, which was consistent with previous studies [23; 24]. After deformation processing, additional peaks appeared in the XRD pattern, identified as martensite peaks [25; 26], which was consistent with the TEM result (Figure 1). Heat treatment at 500 \({}^{\circ}\)C and 550 \({}^{\circ}\)C did not trigger new peaks, and the peaks remained as austenite and martensite phases. It should be noted that the integrated intensity of the (111) and (220) planes of austenite and the (110) plane of martensite changed significantly at a heat treatment temperature of 500 \({}^{\circ}\)C, and this change was more noticeable when the heat treatment temperature raised from 500 to 550 \({}^{\circ}\)C. Furthermore, when the heat treatment  
Figure 1: TEM characterization of the 304-Nano sample. (**A**) TEM images of specimens cut by FIB, (**B**) the corresponding selected area electron diffraction (SAED) patterns. temperature was 500 \({}^{\circ}\)C, the diffraction peaks (200)\(\gamma\), (220)\(\gamma\), (110)\(\alpha^{\prime}\), (200)\(\alpha^{\prime}\), and (211)\(\alpha^{\prime}\) had a significant shift to the right compared to those of the 304-Nano. However, these five peaks did not shift further when the temperature increased to 550 \({}^{\circ}\)C. In addition, except for a few martensite peaks, the three austenite peaks (111), (200), and (220) at N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C did not shift relative to the 304. According to [PERSON]'s equation [27], residual tensile stress existed in the 304 formed by cutting processing of the 304-Nano, which was eliminated after heat treatment (N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C), causing the diffraction peaks to return to their corresponding angles.