Non-destructive evaluation of the degradation of NI-base superalloy in the air by reflectance spectrum analysis

Thermal power generation is required to be highly efficient due to concerns such as environment and energy problems. In order to improve its efficiency, it is thermodynamically essential to increase operating temperature. In addition, since thermal power generation is expected to control its output to be coexistent with renewable energies of which output varies frequently depending on weather, not only simple fatigue or creep load but also creep-fatigue load is applied to its component because it is required to assure the safe and stable energy supply under random output of the renewable energies. Since the effective lifetime of heat-resistant alloys decreases drastically under creep-fatigue load, however, it is very important to develop a non-destructive inspection method which can detect the degradation of the crystallinity of the alloys such as local plastic deformation, local oxidation, and local change of micro texture (segregation/precipitation). In this research, the reflectance spectrum analysis of the component elements was applied to the observation of the change in the local crystallinity of Ni-base superalloy (Alloy 617). A creep-fatigue test was applied to a small specimen, and the change of the local reflectance spectrum was measured under the irradiation of a white light. It was confirmed that the change of the surface roughness in the damaged area caused by plastic deformation and the growth of the surface oxide were successfully observed by the spectrum analysis. In addition, the distribution of fine carbides and nitrides was visualized by the spectrum analysis. It was also confirmed that a thick Cr-rich oxide layer grew at the grain boundaries only in the heavily damaged area. Finally, it was concluded that the creep-fatigue damage was clearly visualized by the spectrum analysis.