## 3 Results and Discussion
### Microstructural Analysis of the Composite Interface  
Figure 11 shows TEM images of the microstructure of the composite interface. In Figure 11a, the composite interface boundary is wavy, which is consistent with the results obtained by metallographic electron microscopy. Figure 11b shows a bright-field TEM image of the composite interface, where a certain amount of lath-like tissue appears in the near-composite interface area of the stainless steel (i.e., the composite zone), which is not austenite according to the microscopic morphological analysis. Figure 11c shows the microscopic morphology of the stainless steel grain boundary precipitates; the length of the precipitates is \(\approx\)100 nm. With the help of diffraction-spot calibration, crystal plane data calculation, and database matching, the precipitates were finally determined to be CrC (shown in Figure 11d). The causes of the formation of the carbon-chromium compounds which have been described in detail above.  
Figure 10: Composite interface electron-probe results: (**a**) EPMA line-sweep position; (**b**) C; (**c**) Fe; (**d**) Ni; (**e**) Cr.  
Stainless steel mainly consists of the following two types of alloying elements: the first type are austenite-stable elements, including C, Ni, and Mn, which promote the formation and stabilisation of austenite to room temperature, and the second type are austenite-unstable elements (e.g., Cr, Si, Mo, and Nb), which promote the transformation of austenite to ferrite [30]. When elements of both types are present, the type of stainless steel tissue can be determined by calculating the Cr and Ni equivalents of each element, combined with Schaeffler phase diagram (shown in Figure 12) [31]. The formulae for the calculation of the Cr and Ni equivalents are shown in Equations (1) and (2), as follows:  
\[\mathrm{Creq}=100\ \times\ \mathrm{(Cr+Mo+1.5\ Si+0.5\ Nb)} \tag{1}\]  
\[\mathrm{Ni_{eq}=100\ \times\ \mathrm{(Ni+30\ C+0.5\ Mn)}} \tag{2}\]  
The slatted tissue was spot-swept by the energy spectrometer, and Table 4 shows the elemental composition of the three test points and the corresponding Cr and Ni equivalents.  
Figure 11: TEM images showing the microstructure of the composite interface: (**a**) transmission sampling position; (**b**) bright-field TEM image of the composite interface; (**c**) microscopic morphology of the stainless steel grain-boundary precipitates; (**d**) electron-diffraction image of the precipitates.  
The Cr and Ni equivalents of the three test points are marked in the Schaeffler diagram. All three points lie in the martensitic region, so the slatted tissue can be identified as martensitic. Martensite forms mainly because Ni, Mn, and other alloying elements on the carbon steel side diffuse during the rolling process, making the C curve of the composite zone move to the right, thus, reducing the critical transformation rate of martensite, which can be induced by air cooling [19]. In addition, several dislocations were found within the martensite. This is due to the aggregation of dislocations at the composite interface owing to high temperature, high pressure, high speed, and large deformation; these dislocations are not effectively released during the transformation of austenite into martensite.