Abstract: Imparting a gradient microstructure combined with residual compressive stress in the surface layer is an attractive strategy for enhancing the fatigue resistance of alloys. However, a counterintuitive reduction in the fatigue strength (σ_âˆ'1) in such alloys is often reported, and its underlying mechanism remains unclear. In this study, the Inconel 718 nickel-based alloy was subjected to an ultrasonic surface rolling process, and the effects of microstructural, residual stress, and hardness gradients on the fully reversed tension-compression σ_âˆ'1 are studied. Experimental results show a peak in the data of σ_âˆ'1 versus normalized fatigue crack initiation radius ratio (A), a normalized parameter that captures geometric effects on crack initiation sites and is associated with stress state (stress triaxiality (Tr)). Detailed analysis and microstructural characterization show that the observed peak is a result of the competition between the Tr—governed dislocation driving force (Ï„d) and the microstructure-dictated resistance (Ï„c) at fatigue small crack tip, which was substantiated by recourse to molecular dynamics simulations. Based on these findings, a fracture mechanics-based model that considers the residual stress fields, A, Tr, Ï„d, and Ï„cfor predicting the σ_âˆ'1 variations is developed and validated. This work establishes a theoretical framework and design methodology for enhancing the σ_âˆ'1 of structural components subjected to cyclic loads through surface modification.

Key words: Inconel 718 alloy, Fatigue strength (Ơ-1), Stress triaxiality (Tr), Fatigue crack initiation radius ratio (A), Fatigue small crack (FSC) growth