Effect of microstructural characteristics on the low cycle fatigue behaviors of cast Ni-base superalloys

Low cycle fatigue behaviors of two cast Ni-base superalloys have been investigated. Effects of microstructure of a solution hardened alloy and a precipitation strengthened alloy on cyclic stress responses were studied at various temperatures and strain ranges. In the case of a solution hardened alloy, Hastelloy X, fatigue lives at lower temperatures are longer than those at higher temperatures regardless of total strain range. IN738LC which is strengthened by γ′ particles is founded to have longer fatigue life at low temperature than high temperature when total strain range is low. However, the alloy has longer fatigue life at high temperature than low temperature when the total strain range is high. Because γ′ particles are cut by coupled dislocation and slip band forms, deformation is inhomogeneous, and strain is concentrated to local region during low temperature fatigue test, while thermally assisted dislocation movements result in less stress concentration at high temperature. Therefore, fatigue life of the alloy at high temperature is longer than that of low temperature under high strain range condition. When strain range is low, oxidation plays important role in the fatigue behavior. Oxidation decreases fatigue life by means of affecting crack initiation and growth behavior at high temperature.