Crack path and oxidation behavior during crack growth of alloy 617 in steam environment at 750 °C

A high-efficiency coal-fired power plant, which is called an advanced ultra-supercritical (A-USC) plant, is being developed worldwide. For the high-temperature sections of A-USC plants, Ni-based superalloys are considered as candidate materials. Nowadays, particularly in Japan, renewable energy sources are widely promoted. Therefore, coal-fired power plants, including the A-USC plant, are expected to cover the need for load following operation to satisfy the electricity demand. However, it is expected that the output adjustment would induce fatigue damage in which the oxidation of material interacts. In this study, the crack growth behavior of alloy 617 in a steam environment at 750 °C was evaluated under cyclic loading. Under faster cyclic loading frequencies, the crack propagated perpendicularly to the loading direction. Under lower cyclic loading frequencies, the crack growth rates (CGRs) were accelerated and the crack paths were intergranular. Especially, in the intergranular crack, localized Al oxides were formed. It was thought that such intergranular oxides and precipitates affect the change in crack path depending on the applied loading frequency and result in differences between the CGRs.