Effect of Surface Mechanical Grinding Treatment on the Strain‐Amplitude‐Dependent Fatigue Response of CP‐Ti

The effect of surface mechanical grinding treatment (SMGT) on the low‐cycle fatigue (LCF) behavior of commercially pure titanium (CP‐Ti) is investigated under fully reversed strain‐controlled loading with amplitudes from ±0.25% to ±1.0%. Microstructural analysis reveals the formation of a ≈1000 μm gradient surface layer, characterized by a smooth transition from coarse grains in the bulk to a sub‐micron grain‐sized region near the surface. Compared to the as‐received (AR) material, SMGT specimens exhibit higher stress amplitudes, progressive cyclic softening, and a transition from non‐Masing to Masing behavior, attributed to the early stabilization of back stress within the gradient microstructure. At ±0.25% strain amplitude, the fatigue life improves by ≈93% due to delayed crack initiation and propagation facilitated by compressive residual stresses, grain refinement, and enhanced back‐stress generation. However, at higher strain amplitudes (>±0.25%), residual stress relaxation, excessive dislocation accumulation, and reduction in strain hardening ability induced by SMGT collectively undermine the fatigue improvement, leading to a shorter life compared with the AR condition. Postfatigue EBSD analysis reveals strain‐amplitude‐dependent grain growth in the gradient layer, where pronounced coarsening at low amplitudes contributes to microstructural stabilization by relieving local stresses, thereby enhancing fatigue performance.