Effects of temperature and strain rate on isothermal low-cycle fatigue behaviour of Inconel 718 superalloy: Damage mechanisms, microstructure evolution, and life prediction

In this article, strain-controlled Low-Cycle Fatigue (LCF) tests were performed on Inconel 718 nickel-based superalloy at temperatures of 300 °C, 650 °C, and 730 °C. The LCF tests were conducted at various mechanical strain amplitudes between 3.5×10−3 and 1×10−2, and three different mechanical strain rates: 1×10−4/s, 1×10−3/s, and 1×10−2/s. Cyclic straining resulted in cyclic softening under all investigated loading conditions, with the effect being more significant at higher temperatures. The cyclic softening was attributed to the formation of persistent slip bands and the shearing of coherent precipitates. At 730 °C, δ phase precipitation in LCF tests conducted at low strain rates contributed to additional softening. Investigations into the damage mechanisms revealed that the predominant failure mode shifted from transgranular at 300 °C to intergranular at 650 °C and 730 °C. In addition, fatigue crack initiation sites most frequently involved broken or oxidized carbides. The fatigue lifetime decreased with an increasing temperature and a decreasing strain rate, primarily due to oxidation-assisted intergranular cracking at high temperatures, involving the formation of brittle oxides at grain boundaries. Finally, a multi-mechanism-based damage model was proposed to predict fatigue lifetime, accounting for contributions from oxidation, creep, and fatigue damage. The model exhibited a good correlation between the predicted and the observed lifetimes.