High temperature low cycle fatigue behaviour of UDIMET 720 Li superalloy

The low cycle fatigue behaviour of UDIMET 720 Li has been investigated at 700°C in vacuum and air environments under strain control tests. Hold times (HTs) have been introduced at peak tensile strain in order to determine the effects of creep and fatigue interactions. The fatigue life of the alloy is practically independent of HT in both environments at high strain ranges while HT has beneficial effect at low strain ranges. The main factor that adversely affects the life of the alloy is oxidation, as the fatigue life is reduced in air compared to vacuum especially at high strain ranges. The increase in fatigue endurance due to HT at lower strain ranges is attributed to stress relaxation. A life prediction model has shown that nucleation occurs after few cycles at high strain ranges whereas a large proportion of the fatigue life is consumed in the nucleation process at low strain ranges.