Microstructure Evolution and Damage Mechanisms of Inconel 718 Superalloy After Overloading Under Hybrid Stress–Strain Controlled Creep‐Fatigue Loading

ABSTRACT This study investigates constant stress overloading effects of Inconel 718 at various lifespan stages on microstructure evolution and damage in hybrid stress–strain controlled creep–fatigue interaction (HCFI) tests at 650°C. The interrupting tests were conducted at the 10%, 25%, 50%, and 65% lifespan of HCFI tests. Based on the interrupting tests, overload interruption experiments were conducted at 10%, 25%, 50%, and 65% lifespan with a constant stress 800 MPa and a dwell time 180 min, respectively. The EBSD and SEM characterization techniques were employed to reveal microstructure evolution and damage mechanisms after overload at different service stages. Results show that implied overload at different stages caused varied damage accumulation rates, dislocation pile‐up, and grain boundary stress concentration. The changes in the δ phase coupled with the stress concentration at the grain boundaries promote the formation of cavities and the crack propagation, ultimately accelerating the accumulation of damage within the material.