Degradation of the Strength of a Grain Boundary in Ni-Base Superalloy Under Creep-Fatigue Loading

Electron back-scatter diffraction method was applied to analyze the degradation process of Ni-base superalloy Alloy 617 under various loading conditions at elevated temperatures from the view point of the change of the order of atom arrangement in grains and grain boundaries. The local damage evolution around grain boundaries was evaluated by applying the intermittent creep and creep-fatigue tests. The creep-fatigue test was operated at 800 °C under the stress of 150 MPa with strain rate of 0.4%/s and hold time of 10 min in inert gas (99.9999% Ar). Each test was stopped at a certain strain and the micro texture was evaluated step by step at the same area in each test sample continuously. The crystallinity of grains and grain boundaries was evaluated by applying image quality (IQ) and confidence index (CI) values obtained from EBSD analyses. It was found that degradation of the crystallinity was accelerated at certain grain boundaries drastically and the first crack always initiated at a grain boundary when the IQ value decreased to a critical value. It was also confirmed that this degradation decreased the strength of the grain boundary. Finally, it was concluded that the strength of a grain boundary was dominated by its crystallinity and there was a critical IQ value at which a crack initiated at the grain boundary.