Cyclic response and residual life prediction of Inconel 718 superalloy after overloading under hybrid stress-strain controlled creep-fatigue loading

Inconel 718 is a material for manufacturing turbine disks, which is subjected to complex loads of creep-fatigue during service. This paper examines the effect of constant stress overloading at various lifespan stages on subsequent cyclic deformation, fracture behavior, and residual lifetime in hybrid stress and strain-controlled creep-fatigue interaction (HCFI) experiments conducted at 650℃. Overloading of 800 MPa constant stress with 180 min dwell time is applied at the 10 %, 25 %, 50 %, 65 %, and 70 % lifespan of HCFI tests. Results show that overloading reduces the residual life of the material obviously and makes a significant impact on the subsequent creep-fatigue deformation behavior, and additional damage is induced when overloading is applied towards the end of the lifespan. Moreover, more severe grain boundary damage leading to the final intergranular fracture is identified through microstructural analysis following overloading. Finally, equivalent damage is proposed to quantify the creep-fatigue damage affected by overloading, resulting in accurate life prediction outcomes.