Experimental study of the cyclic mechanical behaviour of two Ni-based superalloys at evaluated temperature

The fatigue tests of two Ni-base superalloys, powder metallurgy (PM) FGH95 and GH4169, were performed at 650 °C with four different dwells to study their cyclic mechanical behaviour, particularly the evaluation law of cyclic mean stress and cyclic hardening/softening. The cyclic mean stress of the two alloys decreases with the number of fatigue cycles under tensile dwell, while it increases with the number of fatigue cycles under compressive dwell. PM FGH95 is found to undergo cyclic hardening, and its stress range -logarithmic cycle curve can be described by a quadratic polynomial. GH4169 exhibits cyclic softening, and its stress range -logarithmic cycle curve in the stable softening phase can be described by a linear equation. Furthermore, the hardening amplitude factor (HAF), the hardening velocity factor (HVF), the softening amplitude factor (SAF) and the softening velocity factor (SVF) are defined, based on the features of cyclic stress range, to correlate the total strain range, which are used to determine a fatigue damage parameter that introduces the effect of loading history and consequently improve the accuracy of low cycle fatigue (LCF) life prediction.