高温クリープ負荷環境におけるNi基超合金Alloy617の粒界割れ機構の解明

抄録

In order to reduce the emission of CO2 for solving the global warming issue, the operating temperature of thermal power plants is continuously increasing. Under the creep and creep-fatigue loading at elevated temperature, the effective lifetime of heat-resistant alloys such as Ni-base superalloy Alloy 617 which is expected to be used high temperature components of thermal power plants was found to decrease drastically. This is caused by the change in the crack propagation path from transgranular to intergranular, and thus, it is very important to quantitatively evaluate the acceleration mechanism of grain boundary cracking. In this study, an intermittent creep test was applied to Alloy 617 to continuously observe the change of microstructure by electron back-scatter diffraction (EBSD) analysis. It was found that vacancies accumulated around grain boundaries perpendicular to the applied uniaxial stress, and it was clearly seen that voids started to appear and accumulate around those grain boundaries in the early stage of the creep test. When the image quality (IQ) value of the damaged grain boundaries decreased to a critical value, grain boundary cracking started to occur at the grain boundaries.