High-temperature low-cycle fatigue and fatigue–creep behaviour of Inconel 718 superalloy: Damage and deformation mechanisms

In this article, strain-controlled Low-Cycle Fatigue (LCF) and fatigue-creep tests were performed on Inconel 718 nickel-based superalloy at temperatures of 650 °C and 730 °C. LCF tests at elevated temperatures were performed with a mechanical strain rate of 1×10−3/s, while fatigue-creep tests involved either tensile or compressive strain dwell. Both the LCF and fatigue-creep tests revealed cyclic softening, with the mean stress evolving oppositely to the applied strain dwell in the fatigue-creep tests. Investigations into the damage mechanisms identified intergranular cracking as the predominant failure mode. Fatigue-creep loading with a compressive dwell resulted in multiple crack initiations from transgranular oxide intrusions, along with multiple creep cavities during loading at 730 °C. Deformation features such as persistent slip bands and deformation nanotwins were observed during cycling at 650 °C. In addition, fatigue-creep tests at 730 °C exhibited δ phase precipitation and a coarsening of strengthening precipitates, contributing to additional softening that increased over prolonged test durations. Finally, the observed lifetime during LCF tests decreased with increasing temperatures, and fatigue-creep loading was observed to be more damaging than LCF. On the other hand, fatigue-creep loading with a tensile strain dwell demonstrated a higher lifetime compared to LCF at 730 °C.