## 3 Results and Discussion
### Microstructural Analysis of the Heat-Treated Specimens
In the literature, to enhance the study of strengthening particles, some authors have proposed performing aging treatments for times ranging from 2 to 200 h and up to 5000 h [36; 51]. In these works, the use of transmission electron microscopy has been effective in detecting and identifying nanometric precipitates. Hence, two specimens subjected to long-time aging treatments, i.e., the HT4 and HT5 ones, were analyzed in depth by using TEM. A TEM analysis was conducted on specimens that were HT4- and HT5-treated, focusing on the detection and identification of strengthening precipitates, specifically CRPs and Nb carbides, which can be expected after a long-time aging treatment. In Figure 6, two representative TEM micrographs of HT4 and HT5 specimens acquired in the BF imaging mode exhibit a polycrystalline microstructure. Both HT4 and HT5 feature a lathy martensite matrix. In the micrographs, the different contrast is mainly ascribed to the orientation of the crystal domains according to the direction of the electron beam of the TEM, while small-sized contrast details are an indication of defects within the matrix. The analysis of the electron diffraction pattern from selected areas (SAED) of the specimens confirmed the \(\alpha^{\prime}\)-martensite phase for both the HT4 and HT5 specimens. As previously mentioned, the SHT carried out at 1040 \({}^{\circ}\)C for 30 min promoted the dissolution of \(\delta\)-ferrite and most of the alloying elements within the matrix. The subsequent aging treatment performed at 480 \({}^{\circ}\)C after rapid quenching was expected to promote the precipitation of nanometric Cu-rich particles and at least Nb carbides [48]. It is worth highlighting that, during the first 30-60 min of the aging treatment at 480 \({}^{\circ}\)C, the steel was expected to achieve its maximum hardness due to the precipitation of Cu clusters, which nucleate and grow coherently with the matrix until reaching the optimized size and density [48,52]. The TEM image shown in Figure 7a, obtained in the STEM-HAADF imaging mode to promote the compositional contrast, shows an example of a Nb carbide  
Figure 5: X-ray diffraction spectra of the single tracks heat-treated according to the (**a**) HT1, (**b**) HT2, and (**c**) HT3 routes. Figure 6: BF TEM micrographs of the microstructures of (**a**) HT4 and (**b**) HT5 specimens. particle. The white arrow in Figure 7a points to a low-contrast Nb carbide precipitate. Indeed, the decrease in contrast with respect to the surrounding iron matrix was ascribed to a lower value of both the average atomic number and density for the NbC precipitate [53]. In addition, the shape and size of the mentioned detail correspond to the NbC precipitates described in [36]. These carbides were dispersed in the thin lamella with an average distance of about 3.5 \(\upmu\)m, and they measured about 92 nm in equivalent diameter. It is well-known that this type of carbide precipitates at 480 \({}^{\circ}\)C after 2 h of aging adjacent to Cu-rich nanometric particles [21]. In the HT4 specimen, an investigation performed on the grains at a high magnification and in the high-resolution imaging mode (Figure 7b) revealed the presence of small particles of a few nanometers in size. These features were identified as Cu-rich nanoprecipitates by the contrast fringes arising from the overlap between the particles and the martensitic lattice, as well as the analysis of the spatial frequencies in the high-resolution images. Figure 7c shows the Fast Fourier Transform (FFT) of the image reported in Figure 7b and features a regular lattice of spatial frequencies corresponding to the [0 0 1] zone-axis projection of the martensitic Fe; in addition, two extra spots marked by the white triangles highlight the presence of spatial frequencies not pertaining to the lattice of the Fe phase, and the lattice distance corresponding to these extra frequencies fits the [2 2 4] reflection for CuNi\({}_{3}\) at 0.101 nm [54].