Cyclic response and dislocation evolution of a nickel-based superalloy under low cycle fatigue deformation

Abstract

The low cycle fatigue behavior of Inconel 750H superalloy, a newly designed Nickel-based superalloy, was studied in this work. The superalloy exhibited an initial cyclic hardening and a following continuous cyclic softening behavior at the strain amplitudes from 0.35% to 0.6%, while a stress saturation at the strain amplitude of 0.3%. Cyclic response considering cyclic stress, plastic strain, and hysteresis loop area was studied. Four interrupted cycle tests were conducted at the strain amplitude of 0.35% to investigate the microstructure evolution during fatigue deformation. As fatigue cycles increased, the fraction of γ′ phase decreased while the diameter of γ’ phase increased. The planar slip deformation with dislocation pile-ups at half cycles was observed, and finally evolved into simple dislocation lines when the specimen ruptured. The coarsening γʹ precipitation degraded the strengthening effect, resulting in more uneven slip deformation. Compared with chain-shaped M23C6 carbides, a larger dimension of the block like MC carbides (i.e. TiC and NbC) led to higher stress at the interface between MC and matrix, where cavities preferred to nucleate and propagate to a micro-crack.