Initial Grain-Boundary Cracking of Ni-Base Superalloys under Creep-Fatigue Loadings at 750°C

摘要

The acceleration of the crack growth of Ni-base superalloys under creep-fatigue loadings at elevated temperatures was attributed to the drastic degradation of the strength of grain boundaries due to the accumulation of dislocations and vacancies around the grain boundaries. The accumulation process was clearly visualized by the intermittent EBSD analysis under the loadings. It was quantitatively analysed by using the image quality value obtained from the analysis, which indicates the total density of various defects such as vacancies, dislocations, and local strain. Initial cracks always appeared at random grain boundaries perpendicular to the direction of the applied uniaxial load. There was the critical value of the IQ value at which the initial crack started to open. The abnormal accumulation was explained by the stress-induced acceleration of self-diffusion of dislocations. The micro tensile test system using a bicrystal specimen was applied to the strength measurement of grain boundaries with different IQ value. It was confirmed that the strength of a grain boundary decreased monotonically with the decrease in the IQ value. There was the critical IQ value at which the strength of a grain boundaries became lower than that of a grain. Therefore, two fracture modes appeared, brittle fracture at a grain boundary and ductile fracture in a grain, as a strong function of the IQ value. It is possible to predict the lifetime of the crack initiation of the Ni-base superalloys at elevated temperatures from the viewpoint of the degradation of their crystallinity under creep-fatigue loadings.