In order to reduce CO~2~ emission as a part of global warming countermeasures, thermal power plants are required to further improve their thermal efficiency. The operating conditions such as temperature and gas pressure, therefore, have become harsher and the maximum steam temperature in advanced thermal power plants has reached 750^o^C. In addition, due to the large-scale introduction of renewable energies of which the output strongly depends on the natural environment, the expected role of thermal power plants will be not only the conventional baseload power supply but also the variable correspondence of output to compensate the irregular change of the output of the renewable energies. Therefore, not only simple creep but also creep-fatigue load is applied to the structural materials used in those plants. Under the creep-fatigue load at elevated temperatures, the effective lifetime of the material was found to drastically decrease, and the main reason for this drastic decrease was attributed to the degradation of the crystallinity of grain boundaries and thus, their effective strength. Since brittle intergranular cracking accelerates the crack growth rate, it is important to detect the initial intergranular cracking during the operation of thermal power plants. Therefore, it is essential to establish a new non-destructive inspection technology to detect the degradation of the local crystallinity of grain boundaries in the air. The purpose of this study is to develop a new technology for monitoring the change of the crystallinity of grain boundaries during operation.

Since each element has a unique reflectance spectrum in the visible-light wavelength region, it is possible to measure the local composition of an alloy by quantitatively analyzing the local reflectance spectrum under the irradiation of a white light. The reflectance spectrum was observed by using a hyperspectral camera NH-8 MS (Eva Japan) at room temperature in the atmosphere. The measured wavelength range was from 450 nm to 750 nm. The spectrum was analyzed with resolution of 5 nm. Before the observation, the surface of each sample was slightly polished by using chemical mechanical polishing technique. A white light (halogen lamp) was used for the light source. Using this method, we succeeded in visualizing the rafting damage of the precipitation-strengthening structure of Ni-base superalloys. Both gamma (Ni-base matrix) and gamma’ (Ni~3~Al) phases in CM247LC were clearly observed by using the spectrum range from 450 nm to 460 nm. This is because the reflectance of Ni decreases drastically in the wavelength shorter than 500 nm. Since the composition of Ni in gamma’ phase is larger than that in gamma phase, the gamma’ phase shows lower intensity in this wavelength range. Clear periodic distribution of both phases was observed by this method. The change of the micro texture under creep loading was monitored successfully. In addition, the change of surface roughness due to plastic deformation was detected by the change of the intensity of the spectrum. It was confirmed that there was no change in the wavelength dependency of the spectrum during plastic deformation. Only the intensity of the spectrum was found to monotonically decrease with the increase of the surface roughness. The change of the local composition was also observed. The grain boundary oxidation was found to be accelerated in the region where necking occurred. The clear spectrum of Cr oxide was observed and this result was validated by EDX analysis using scanning electron microscope. Precipitation of Nb and NbO at grain boundaries in Alloy 625 was also detected during the creep test. Therefore, the micro texture of Ni-base superalloys was successfully observed in the atmosphere using a visible light. The difference in the reflectance spectrum of each phase and each precipitate under the irradiation of a white light was analyzed in detail by using a hyperspectral camera. These results clearly indicate the possibility of the non-destructive inspection of the degradation of the micro texture of these alloys in the atmosphere.


Non-Destructive Evaluation of the Degradation of Ni-Base Superalloy in the Air by Reflectance Spectrum Analysis

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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