EFFECT OF STRAIN RATE ON THE CREEP-FATIGUE DAMAGE OF POLYCRYSTALLINE NI-BASE SUPERALLOY AT ELEVATED TEMPERATURE

Ni-based superalloys used for thermal power plants have been subjected to random creep-fatigue load in hightemperature 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.