On the low cycle fatigue behavior of a Ni-base superalloy containing high Co and Ti contents

Low cycle fatigue behavior was studied at 400, 650 and 725 °C in the total strain ranges of 0.79–1.22% for a recently developed Ni-base superalloy containing high Co and Ti contents. Detailed examinations were conducted on cyclic hardening/softening behavior, deformation substructure, fatigue life, as well as crack initiation and subsequent propagation. Continuous cyclic hardening at 400 °C was observed whereas cyclic softening at 650 and 725 °C was examined, except at 650 °C and low strain range (0.8%) where alloy exhibited relative stable stress response until crack initiation. Transmission electron microscopy analysis suggests that cyclic hardening is caused by the inhomogeneous dislocation activity and interactions of dislocations, and cyclic softening is related to shearing of gamma prime precipitates by stacking faults and coupled dislocation pairs combining with thermal activation process. The relation between fatigue life and plastic strain followed the Coffin–Manson law. Fatigue cracks often initiated on or near the surfaces of specimen during cycling, while subsurface carbide clusters initiation was also observed. Oxidation accelerated crack propagation at 650 and 725 °C.

► Cyclic hardening at 400 °C is mainly due to the interaction of dislocations.
► Cyclic softening at 650 and 725 °C is related to shearing of γ′ precipitates.
► The relation between fatigue life and plastic strain followed the Coffin–Manson law.
► Cracks often initiated on or near the surfaces of specimen.