Mechanism of cyclic softening and fracture of an Ni-Base γ′-Strengthened alloy under low-Cycle fatigue

Abstract

Evolution of the microstructure and its impact on the mechanical behavior of a γ′-strengthened Ni-base alloy under low-cycle fatigue (LCF) at 760 °C, 871 °C, and 982 °C were studied. A cyclic softening was observed, the extent of which increased with temperature. This was attributed to the coherency loss of the primary γ′ particles and partial shearing and dissolution of the secondary γ′ particles. The precipitates and carbides were resistant to coarsening during thermal exposure alone or LCF at the lower two temperatures; the primary γ′ particles, however, lost coherency and spheroidized during LCF at 982 °C. Fatigue cracking initiated at the surface was mainly responsible for failure, although cavities were found in localized areas of a few test specimens. The creep damage was attributed to an accumulation of plastic strains of high amplitude at inhomogeneities, either macroscopic or microscopic.