Cyclic response and fatigue behavior are sensitive to the microstructure of material induced by heat treatment. In this paper, three sets of high-temperature superalloy Inconel 718 with different heat treatment, namely annealed, aged, and directly aged high quality (DAHQ), are compared. Difference in grain size distribution, phase, and precipitate, etc., were investigated using an optical camera and scanning electron microscopy. Yield and ultimate strength were found to increase obviously after aging heat treatment. Self-heating phenomenon at 20 kHz was attenuated as grain size decreased. There was a transition from cyclic hardening to softening. Very-high cycle fatigue (VHCF) behavior of Inconel 718 was tested using an ultrasonic fatigue device. Crack initiation duration occupied greater than 99% of the total fatigue life. It concluded that average grain size influences VHCF strength and crack initiation mechanism, and that self-heating phenomenon is not a decisive factor on VHCF strength for Inconel 718.

Ultrasonic resonance fatigue test method at 20 kHz related to the very high cycle fatigue (VHCF) aims to accelerate a time-consuming experiment. In this paper, an ultrasonic fatigue device with a data acquisition system was improved for monitoring and recording the data from fatigue tests in which self-heating phenomenon exists. Symmetric tension-compression sinusoidal vibrating mode (R = −1) was observed in this study. VHCF behavior and mechanism of Inconel 718 were carried out using this device. It was concluded that more than 99% of fatigue life is consumed in initiation duration. Specimen temperature increase was not a decisive factor in VHCF strength for Inconel 718, as long as it was far less than the design temperature limitation. A single initiation site existed at the subsurface facet or grain cluster, observed from scanning electron microscope (SEM) micrographs. Quasi-cleavage fracture in transgranular ductile mode emerged and then tended to trace grain boundaries in an intergranular manner by cleavage-dominated mixed mode.