**4. Discussion**

### *4.1. E*ff*ect of RE on Carbon Di*ff*usion*

As shown in Figures 4 and 5, after ion implantation, the 20Cr2Ni4A matrix formed an injection layer containing the RE, in which the RE atoms existed in the form of oxides and solid solution atoms. The results of the TEM analysis in Figure 7 showed that RE ion implantation caused substantial damage to the matrix lattice.

Due to the characteristics of the electron cloud of RE element shells and its atomic size effect, a series of cascade collisions occurred between ion-implanted RE atoms and substrate atoms, resulting in a large number of dislocations and damage in the matrix lattice [34,35]. The estimation of mean ion energy transferred to the target were reasonably evaluated by SRIM. The average primary knock-on atom energy of the La and Y ions were 94.6 and 96.8 Kev/ion during the ion implantation, respectively. As shown in Figure 14, the maximum electronic stopping powers of La and Y ions along the depth direction were 537.8 and 370.3 Kev/Å, respectively (electron stopping power is the blocking effect of nucleus and electrons in the target matrix on the implanted ions during ion implantation [34]). A schematic diagram is shown in Figure 15. Subsequently, the disorder degree of the matrix atoms, the crystal interface area and the crystal structure defects increased, which became the channels for carbon diffusion during carburizing and played an extremely important role in improving the carbon diffusion coefficient [11].

**Figure 14.** SRIM simulation of mean ion energy during ion implantation of (**a**) La and **(b**) Y into steel.

**Figure 15.** Physical model of an RE solid solution atom in a crystal face in an Fe lattice.

When solid solution RE atoms expand the lattice defects in the matrix, the distorted lattice region of the surrounding iron atoms becomes a trap for carbon atoms during carburization. Carbon atoms segregate into the voids of the distortion region [36], resulting in the formation of a nanoscale Cottrell atmosphere with the RE as the core carbon atom [2], as shown in Figure 16. Moreover, as shown in Figure 16a, carbon uniformly diffuses in the volume from the outside to the inside, in accordance with the concentration gradient in the austenite during conventional carburizing. The grain boundary diffusion was faster than that in the grains, and the finer the grain was, the faster was the diffusion rate. However, once the Cottrell atmosphere was formed during carburizing with RE ions, the diffusion mode changed from uniform diffusion to non-uniform diffusion [37]. As shown in Figure 16b, the Cottrell atmosphere also acted as an accelerator for diffusion [38]. First, the concentration of carbon atoms in the air mass was much higher than that in the matrix, resulting in a high concentration difference. Then, the larger the density of the air mass and the higher the average carbon concentration, the larger was the diffusion flux, such that the diffusion velocity and diffusion flux were significantly increased. Finally, the mechanism of carbon diffusion during RE carburization could be considered to be a result of jumping the short-circuit diffusion when the carbon atoms at the top of the last gas mass jumped to the next one. It could be concluded that the lattice damage and associated Cottrell atmosphere produced by the RE ion implantation changed the diffusion mode of the carbon atoms during the conventional carburizing processes and increased the diffusion coefficient of the carbon atoms.

**Figure 16.** Diffusion models of (**a**) conventional carburizing and the (**b**) RE carburizing processes.

### *4.2. E*ff*ect of REs on the Microstructure and Hardness of the Carburized Layer*

According to the differences in the microstructure, phase, and hardness of the carburized layers between the non-implanted and implanted samples, as determined by the OM, SEM, and the XRD, it could be concluded that RE ion implantation can improve the carburized layer [10]. From Figure 8, the diffraction peaks of the ion-implanted lanthanum and yttrium specimens shift to the left, and the

intensity of the peaks increased after vacuum carburizing. From the Bragg equation, the smaller the di ffraction angle, the larger the lattice plane distance. Due to the large radius of RE atoms, the iron lattice in steel is distorted [39]. Moreover, the (110) α di ffraction peak showed and obvious increase, which indicated that the preferred orientation of cryptocrystalline martensite was (110) α after RE ion implantation, and its content also increased. From the results in Figures 9 and 10, it could be concluded that the structure of the carburized layer after ion implantation was obviously refined, the content of the cryptocrystalline martensite was increased, and the minimum size of the surface carbide was 0.17 μm. Figure 11 shows that the surface hardness increased after RE ion implantation.

These results could be explained according to the following two aspects. First, the RE ion implantation pretreatment could change the martensite transformation mode. Since martensite has an explosive shear growth, the first martensite could not pass through the austenite large-angle grain boundary, and it was impossible to cut through the carbides. During the conventional carburization process, substantial amounts of carbides do not exist in the austenite crystal, so the martensite tends to be coarse. Carbides precipitated from austenite crystals are often required during RE carburizing. In the presence of these carbides, the martensite shear is blocked and the martensite is forced to become superfine [10]. Moreover, the quenching performance was also improved. Figure 17 shows the martensite transformation patterns of the two carburizing methods. Figure 17a shows the transformation mode of austenite to martensite during conventional carburizing and quenching, and it can be seen that plate martensite needles often appeared. Figure 17b shows that during the carburizing and quenching processes with REs, the Cottrell atmosphere with RE ions as the core becomes the nucleation core for a carbide, which results in the precipitation of fine dispersed carbides on the surface, during carburization and makes the martensite superfine, as shown in Figure 10c,e. The fine, dispersed, spherical carbides were embedded in the martensite matrix, which inhibited the growth of the austenite grains and improved the hardness and wear resistance of the steel.

**Figure 17.** Transformation models of martensite in the grain interior for (**a**) conventional carburization and the (**b**) RE carburization processes.

In addition, RE ion implantation pretreatment could dramatically refine the microstructure of the carburized layer. The undercooled austenite before quenching was uniform and stable during conventional carburization. RE carburization results in a typical non-uniform structure, as its stability is very poor due to the carbide nucleus [40]. Without undercooling, the carbon concentration around the carbide increased to the highest level and decreased far away from the carbide. The carbon concentration periodically changed according to the distance between the carbides. With an increase in the undercooling before quenching, the carbon saturation in austenite around the carbide increased sharply, causing the carbon atoms in the austenite to remain around the carbide. With the uphill di ffusion process from the austenite to the carbide, that is, from a low concentration to a high concentration of carbon, the carbon in the austenite became depleted. After this transformation, the superfine martensite was formed. The resistance of the superfine martensite to fatigue crack initiation and propagation was greatly improved, so the microstructure contributed to increasing the service life of the different parts.
