多結晶Ni基耐熱合金の高温クリープ疲労損傷に及ぼす負荷・除荷ひずみ速度の影響

抄録

Ni-based superalloys have been subjected to random creep-fatigue load in high-temperature environment to 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 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 IQ (Image Quality) values obtained by EBSD (Electron Back-Scatter Diffraction) method. In particular, the effect of strain rate during loading and unloading processes were evaluated in detail. 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.