In order to reduce CO~2~ emission and improve the efficiency of power generation, the steam temperature in A-USC (Advanced-Ultra Super Critical) is going to be elevated to about 750^o^C. Therefore, the heat-resistant materials used in A-USC should be replaced from conventional ferritic steels to new materials such as nickel-base super-alloys (Alloy 617, Alloy 625, Alloy 718 and so on). However, it was reported that the effective lifetime of Ni-base alloy decreased drastically under the creep-fatigue loading, which corresponds to the random change of output of the power plants for assuring the stable power supply with various renewable energies. Therefore, it is very important to evaluate the total damage of Ni-base superalloy under complicated creep-fatigue loadings at elevated temperature. It has been found that the effective lifetime dropped to about one tenth of the lifetime expected based on the conventional linear damage rule. The degradation speed of the Alloy 617 was markedly accelerated by the combination of fatigue and creep loads. It was clarified that the main reason for this drastic decrease in the lifetime of the alloy was the change in the fracture mode from ductile transgranular fracture to brittle intergranular fracture. Intergranular cracking accelerated the crack growth rate drastically. Therefore, it is indispensable for establishing a quantitative theory for evaluating the damage of Ni-base superalloy under complicated creep-fatigue loading at elevated temperatures.

In this study, the degradation process of two different Ni-base alloys (Alloy 625 and Alloy 617) were investigated in detail. In order to establish a quantitative theory for evaluating the initial damage under creep-fatigue loadings, intermittent creep test was firstly applied to Alloy 617 specimens. According to the previous research, it was clarified that the dominant factors of the initial intergranular cracks were the defect density around grain boundaries and the combination of crystallographic orientation of nearby grains. In particular, the accumulation of dislocations was found to accelerate the intergranular cracking. This degradation of the local crystallinity around grain boundaries was quantitatively characterized by EBSD (Electron Back-Scatter Diffraction) analysis. The IQ (Image Quality) value which indicates the clarity of the obtained Kikuchi pattern was applied to the quantitative analysis of the total density of defects such as vacancies and dislocations. It was found that there was a critical IQ value at which the fracture mode changed from transgranular cracking to intergranular cracking. Since the effective strength of a grain boundary decreased and that of a grain increased monotonically with the decrease of the IQ value, intergranular cracking started to occur when the strength of a grain boundary became lower than that of nearby grains. The activation energy of the stress-induced acceleration of the generation and diffusion of dislocations was obtained from the analyses obtained from the data at various temperatures. In addition, it was also found that the accumulation of dislocations around grain boundaries was accelerated under the application of creep-fatigue loading. The local heavy plastic deformation occurred around grain boundaries during loading and unloading processes. On the other hand, the effective lifetime of Alloy 625 was dominated by not only the accumulation of dislocations around grain boundaries but also precipitation of Nb and NbO around grain boundaries. The accumulation of fine voids around the precipitates was drastically accelerated during the creep-fatigue loading. The effective strength of this alloy varied a lot depending on the distribution and density of the precipitates. The growth of the precipitates was accelerated by the magnitude of the applied load. Since these changes of the local crystallinity of Ni-base alloys were dominated by the diffusion of component elements and dislocations, a mathematical model based on the atomic diffusion theory was established to evaluate the critical value of the appearance of the initial damage under different creep loading conditions. Finally, the quantitative theories of the stress-induced acceleration of the diffusion and the degradation of the crystallinity of Ni-base alloys based on the diffusion were proposed.


Quantitative Evaluation of the Dominant Factors of Intergranular Cracking of Ni-Base Superalloys Under Creep-Fatigue Loadings at Elevated Temperature

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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