## 3 Results and Discussion
### Microstructure Observation after RHT  
RHT was carried out on the precept specimens with different levels of strain. To clarify the microstructural evolution during RHT, samples were taken and inspected by SEM and TEM, and a comparison was made among samples at different stages. First of all, the grain size of different samples after RHT was examined by optical microscope, and no substantial difference was detected compared with the initial microstructure. Figure 6 shows the morphology of \(\gamma^{\prime}\) precipitates in the DC with different creep stain. It can be seen that rafted \(\gamma^{\prime}\) was absent and cuboidal \(\gamma^{\prime}\) precipitates were uniformly dispersed in the \(\gamma\) matrix in the sample with accumulated strain of 0.03, which is almost similar to those of original microstructure. With the increase of accumulated strain to 0.05 or above, some \(\gamma^{\prime}\) precipitates of a smaller size were found in some localized regions, as indicated by  
Figure 5: TEM micrographs showing the dislocation configuration in the samples with creep strain of (**a**) 0.03, (**b**) 0.05, and (**c**) 0.07. The arrows indicate the dislocations cutting into \(\gamma^{\prime}\) particles. the arrows shown in Figure 6b and c. Moreover, the morphology of these precipitates appears to be irregular. A similar phenomenon was also reported previously [30]. It was found that the \(\gamma^{\prime}\) particles in the sample prepared by epitaxial laser metal forming also had an irregular morphology after heat treatment, which was associated with the internal stress induced by rapid cooling. In the present study, creep deformation was induced, and internal stress may still remain after RHT, especially in the sample with high creep strain. The morphology and distribution of \(\gamma^{\prime}\) particles in the ID region after RHT is shown in Figure 7. Two types of particles of different size coexisted, which was similar to that in the initial microstructure. However, the shape of precipitates became more irregular, especially in the sample with large precret strain as can be seen in Figure 7c. Moreover, the size of \(\gamma^{\prime}\) particles in this sample seems to be still larger than that of the initial microstructure. The carbide transformations before and after RHT were also examined in different samples. The SEM micrographs shown in Figure 8 provide one typical example for revealing the carbide transformation in the sample with precret strain of 0.07. In Figure 8a, the blocky MC carbides can be easily found in the ID region, which is indicated by the arrows. However, the MC type carbides were almost absent after RHT in the same sample, as can be seen in Figure 8b. This implies that the MC type carbides transformed into M\({}_{6}\)C carbides during RHT. The dissolution temperature for MC carbides is above 128 \({}^{\circ}\)C according to the differential thermal analysis (DTA) results. Therefore, it is unlikely that RHT in the present study induced the dissolution of MC carbides but rather enhanced their decomposition. To quantitatively evaluate the transformation of carbides, the ratio of area fraction of M\({}_{6}\)C carbides to the total area fraction of carbide colonies within five inspected areas was measured by an image analysis approach. The result is summarized in Figure 9. It can be seen that the ratio of M\({}_{6}\)C carbides was increased gradually during the creep test. With the subsequent RHT, the ratio of M\({}_{6}\)C carbides significantly increases. This result demonstrates that MC type carbides will continuously transform into M\({}_{6}\)C type carbides with the thermally-aided diffusional process. RHT cannot reverse the carbide transformation. Figure 6: SEM micrographs showing the morphology of the \(\gamma^{\prime}\) phase in the dendritic core with creep strain of (**a**) 0.03, (**b**) 0.05, and (**c**) 0.07 after rejuvenation heat treatment (RHT). Arrows indicate the \(\gamma^{\prime}\) particles with small size and irregular morphology.