High‐ and Very High‐Cycle Fatigue Behaviors and Crack Initiation Mechanism of Alloy 718 at 650°C

ABSTRACT The high‐cycle fatigue (HCF) and very high‐cycle fatigue (VHCF) behaviors of alloy 718 were investigated via ultrasonic axial cycling at room temperature (RT) and 650°C. The lower bound stress amplitude ( σ a ) for survival at 10 9 cycles was 618 MPa at RT and 475 MPa at 650°C. Interestingly, two distinct crack initiation modes were identified at 650°C, leading to significant life differences. When σ a is lower than 520 MPa, single‐crack initiation was observed, triggered by slip‐induced deformation localization. In contrast, when σ a ≥ 520 MPa, multi‐crack initiation was facilitated by deformation localization and stress‐assisted oxidation, where oxygen preferentially reacted with grain boundary δ phases, forming brittle oxides. In addition, fatigue data were incorporated into the modified Yang et al.'s model, showing life factors within ±2.5 for single‐crack initiation samples but ±3.5 for multi‐crack initiation samples. This study emphasized the critical role of stress‐assisted oxidation in high‐temperature life prediction.