Ni-based superalloys used for thermal power plants have been subjected to random creep-fatigue load in high-temperature environment to counter global warming and compensate the fluctuation of the output of renewable energies. However, under the creep-fatigue condition, intergranular cracking was found to be accelerated, and the fracture life of the alloy was significantly decreased. Therefore, it is essential to elucidate the mechanism of this acceleration of crack propagation rate and to develop a quantitative evaluation method of the creep-fatigue damage. In this study, intermittent creep-fatigue tests were applied to Alloy 617, one of the Ni-based superalloys, and the degradation process of its crystallinity was monitored by EBSD (Electron Back-Scatter Diffraction) method. In particular, the effect of strain rate during loading and unloading processes were evaluated in detail. IQ (Image Quality) value which indicates the quality of crystallinity, in other words, the concentration of various defects and Schmid factor were applied to the crystallinity analysis, and the degradation process of the microstructure was continuously monitored. The results showed that under creep-fatigue loading, the faster the strain rate during unloading, the earlier the IQ value decreased and the shorter the time for the initiation of intergranular cracking. It was also found that intergranular cracking appeared when the IQ value reached a critical value regardless of the strain rate. The creep-fatigue damage was mainly accumulated grain boundaries with large difference in Schmid factor between nearby grains, indicating large local strain due to the large lattice mismatch. The unloading rate was one of the dominant factors of the degradation of the crystallinity around grain boundaries.
Initiation mechanism of intergranular cracking in SUS316L under creep loading at elevated temperatures was investigated quantitatively by applying electron back-scatter diffraction (EBSD) analysis. It was found that intergranular cracking started to appear by 40% of the normalized lifetime of the steel and the cracked grain boundaries were close to perpendicular to the direction of the applied load and the Schmid factor of one of the grains which consisted of the cracked grain boundaries was larger than 0.43. In addition, the difference of the Schmid factor between the nearby grains was large. Not only dislocations but also vacancies were found to accumulate faster around the cracked grain boundaries. The reason for the accelerated accumulation was attributed to the local mechanical stress concentration around the grain boundaries, which was caused by the large lattice mismatch between the nearby grains. The degradation process of the crystallinity of the grain boundaries was continuously monitored by using the image quality (IQ) value obtained from the EBSD analysis. The degradation process was analyzed by using the modified Arrhenius equation, in which the effective activation energy of the damage accumulation decreased locally with the large strain around the cracked grain boundaries. The local activation energy was confirmed to be more than 30% lower than that obtained under the thermodynamically stable condition.
In this study, intermittent creep-fatigue tests were applied on Ni-based Alloy 617 and Alloy 625. Scanning electron microscope (SEM) observation and electron back-scatter diffraction (EBSD) analysis were employed to the evaluation of the degradation of the crystallinity under the creep-fatigue loads. It was found that the initial damage under creep-fatigue load basically appeared as intergranular cracks. In addition, the lifetimes of Alloy 625 samples fluctuated significantly due to the growth of NbC precipitates. The initial damage of these two alloys was dominated by the growth and accumulation of dislocations and vacancies around the interface that consisted of large lattice mismatch. Local atomic diffusion was activated when the summation of the nominal stress and local stress caused by the large lattice mismatch exceeded a critical value. The stress-induced acceleration of the degradation of the crystallinity of the alloys was analyzed by applying the modified Arrhenius equation. Instead of the most popular inversion methods according to the final failure mode, the quantitative description in life prediction was developed based on the dynamic progress for acceleration mechanism of the degradation. It is of importance to perfecting the frontiers of damage mechanics approach.
Degradation mechanism of the strength of a grain boundary in Ni-base superalloy under creep-fatigue loading at elevated temperature was investigated by using the modified Arrhenius equation, which explained the stress-induced acceleration of the local generation and diffusion of dislocations and vacancies. EBSD analysis confirmed that dislocations and vacancies started to generate and accumulate around grain boundaries and the interface between precipitates and matrix in grains. The generation and accumulation were accelerated around the interfaces with large difference in the lattice constant between the nearby crystallographic phases and grains. The activation energies of the diffusion of dislocations and vacancies measured under the harsh condition was much lower than those measured under the thermodynamically stable conditions. It was confirmed that there are two main acceleration mechanisms of the degradation of the crystallinity and strength of grain boundaries under a tensile stress at elevated temperatures: the acceleration of the generation and diffusion of dislocations and the acceleration of accumulation of voids due to the outward diffusion of component atoms from the grain boundaries. These phenomena were explained by the modified Arrhenius equations in which the effective activation energies were changed by the summation of the applied nominal stress and the localized internal stress around various interfaces quantitatively.
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