Strain‐Dependent Evolution of Fatigue and Oxidation Damage in a Nickel‐Based Single‐Crystal Superalloy: The Dominant Mechanism Under Low‐Cycle and Thermomechanical Fatigue

ABSTRACT Systematic studies on the fatigue deformation and damage mechanisms of nickel‐based single‐crystal superalloys (Ni‐SXs) under combined thermal and mechanical conditions remain limited, particularly regarding the influence of strain amplitude on the relative contributions of fatigue and oxidation damage. This study employs advanced characterization techniques to investigate the fatigue failure behavior of a second‐generation Ni‐SX (DD6) under strain‐controlled low‐cycle fatigue (LCF, 980°C) and out‐of‐phase thermomechanical fatigue (OP‐TMF, 600°C–980°C) at strain amplitudes of 0.6%, 0.8%, and 0.9%. The results indicate that fatigue life degradation under OP‐TMF increases more rapidly with strain amplitude than under LCF. Under LCF, micro‐deformation evolves from dislocation climb at 0.6% strain amplitudes to interfacial dislocation networks and γ′ phase shearing at higher strains, followed by significant dislocation recovery contributing to cyclic stress softening. In contrast, under OP‐TMF, the deformation under 0.6% is dominated by dislocation cross‐slip and Kear–Wilsdorf (K‐W) locks, whereas higher strains promote planar slip, Lomer–Cottrell (L‐C) locks and dislocation entanglement with cyclic softening in the high‐temperature half‐cycle and hardening in the low‐temperature half‐cycle. Additionally, by correlating dislocation configurations with fatigue damage and oxide‐layer morphology with oxidation damage, it is shown that at a strain amplitude of 0.6%, OP‐TMF exhibits lower fatigue damage than LCF, whereas the associated life reduction is primarily controlled by oxidation. With increasing strain, fatigue damage becomes dominant in OP‐TMF and exceeds that in LCF. Oxidative passivation suppresses oxidation damage in LCF, whereas the severe life reduction in OP‐TMF reflects the highly destructive combined effects of oxidation and fatigue damage. Overall, the fatigue behavior of Ni‐SXs is controlled by the combined contributions of strain‐dependent fatigue damage and oxidation damage under complex loading conditions.