Nickel-based superalloy, Inconel 718, has high temperature strength effected by γ"(Ni3Nb) phase at approximately 973 K. It has been found that the creep damage of Inconel 718 was induced by γ"phase disappearance. In the study, we applied the nonlinear ultrasonic method, nonlinear three-wave interaction, for evaluation of the creep damages and the microstructural degradation in γ" phase for Inconel 718 at 973 K, 310MPa. In the nonlinear three-wave interaction method, two intersecting ultrasonic waves produced a scattered wave when the resonance condition was satisfied. The amplitude in resonant scattering wave was measured. We analyzed the relation between the microstructural change and nonlinear acoustic characterizations by the electromagnetic acoustic resonance (EMAR) throughout the creep life, and the nonlinearity corresponded to change of the dislocation density. This was supported with X-ray diffraction, EBSD and SEM observation.
