Thermal fatigue behavior of K4169 Ni-based superalloy for thin-walled investment castings in the process of aero-engines service was analyzed by the fluorescent microscopic examination method

optical microscopy（OM）and scanning electron microscope（SEM）

and the fatigue crack initiation and propagation mechanism were also described.The results indicate that almost all the primary fatigue cracks are initiated from the carbides at the notch sensitivity of specimen formed during the machining process

Subsequently

only one or two small thermal fatigue cracks can continue to grow and propagate along apreferential direction with the dendrite growth direction or along the intergranular region which carbides can be easy cracked due to different thermal expanding coefficients of the matrix and the carbides

and the crack of MC carbides either at grain boundaries or within the grain interior as well as the high-temperature oxidation are played an important role on crack propagation.The rapid fatigue fracture morphology exhibits quasi-cleavage and dimples feature.