Low cycle fatigue and stress relaxation behaviours of powder metallurgy Ni-based superalloy FGH4098

Highlights: • Micro-mechanisms of cyclic deformation and stress relaxation behaviours in FGH4098 are investigated by SEM and TEM. • FGH4098 predominantly shows cyclic hardening behaviour due to dislocation interactions. • Cyclic hardening diminishes as temperature and dwell time increases due to the increasing shearing of secondary γ′. • Shearing of γ′ and micro-twinning contributes to stress relaxation at high temperature and long dwell period.. In this study, the effects of microstructure, temperature and dwell time in a loading cycle on low cycle fatigue (LCF) and stress relaxation behaviours of a powder metallurgy Ni-based superalloy (i.e. FGH4098) for aeroengine turbine disc application were studied along with detailed microscopic characterisation of fracture features and deformation substructures. The results indicate that the LCF failure mode of FGH4098 transits from transgranular to intergranular with the increase of temperature and dwell time, and the propensity of intergranular fatigue cracking is higher in fine-grained FGH4098. FGH4098 predominantly shows cyclic hardening behaviour which is mainly related to dislocation interactions. With the increase of temperature and/or dwell time, the cyclic hardening behaviour diminishes to some extent due to the increasing shearing of secondary γ′ precipitates by stacking fault and partial dislocation pairs. Similarly, stress relaxation in FGH4098 is also associated with the shearing of γ′ precipitates and becomes more prominent at high temperature with long dwell period. Micro-twinning may also contribute to stress relaxation at 750 °C.