Experimental investigation on microcrack initiation process in nickel-based superalloy DAGH4169

An experimental investigation of the microcrack initiation process at 400 °C in nickel-based superalloy with direct aging, DAGH4169, is described. The fatigue tests were carried out using a mini servo-hydraulic fatigue machine in a scanning electronic microscope (SEM) chamber. The entire process of crack initiation (including microcrack nucleation, propagation and coalescence) and macrocrack formation was observed. It is found that the propagation behaviors of cracks nucleated at inclusions (Type I cracks) and in slip bands (Type II cracks) are quite different. According to observations from field emission scanning electron microscopy, crack closure caused by Ti(C, N) debris in cracking gaps is believed to be responsible for the low propagation rate of Type I cracks. A three-stage simulation model for the crack initiation process is proposed.