Change of the effective strength of grain boundaries in Alloy 617 under creep-fatigue loadings at 800°C

In this study, both EBSD (Electron Back-Scatter Diffraction) analysis and a micro tensile test system with FIB (Focused Ion Beam) equipment were applied to bicrystal specimens of Alloy 617 in order to establish a quantitative theory of the lifetime evaluation method under creep-fatigue loadings. The IQ (Image Quality) value which is obtained from the diffraction pattern (Kikuchi pattern) indicates the total density of defects such as vacancies, dislocations, and local strain was used for quantitatively evaluating the crystallinity of the alloy. KAM (Kernel Average Misorientation) value was also used for density analysis of GN (Generally Necessary) dislocations. Continuous changes of microtexture of grains and grain boundaries were observed by applying an intermittent creep fatigue test. As a result, it was confirmed that the IQ value around specific grain boundaries in damaged specimen was drastically degraded compared to that of the initial specimen. Intergranular cracks always occurred when the IQ value decreased to a certain value. Therefore, there is a correlation between the IQ value and the strength of a grain boundary in this alloy. The decrease of the IQ value was attributed to the accumulation of both dislocations and vacancies around the specific grain boundaries under creep loading. The accumulation rate under the creep-fatigue loading was clearly higher than that under simple fatigue and creep loadings. Finally, it was clarified that the degradation of the crystallinity in the vicinity of a grain boundary was a criterion of an intergranular crack under creep-fatigue loadings.