Fatigue Crack Initiation and Propagation in UNSM‐Treated Biomedical Ti–6Al–4V ELI Alloy: Role of Fisheye and Surface Crack Coalescence Under RFT and AFT Loading

ABSTRACT This study examines the fatigue crack initiation and propagation mechanisms in Ti–6Al–4V ELI alloy subjected to ultrasonic nanocrystal surface modification (UNSM) under rotating bending fatigue (RFT) and axial fatigue (AFT) conditions. Microscopic observations and EDS analysis revealed that fisheye cracks dominated fatigue life, consuming approximately 93%–96%. Multiple‐small‐fatigue‐crack formation was driven by micro‐slip accumulation, leading to crack coalescence and semicircular morphologies with micro‐ridges. Under AFT, fisheye initiated at depths of 0.45–0.6 mm and exhibited facet‐type fracture due to weakened grain boundaries between α‐phase and discontinuous β‐phase regions. In RFT, fisheye cracks formed beyond 1.7 × 10 6 cycles and were associated with localized carbon and oxide inclusions. The study demonstrates that internal crack growth can be inferred from surface crack behavior and highlights the role of microstructural and chemical heterogeneities in subsurface crack evolution. These findings provide insight into fatigue damage mechanisms in UNSM‐treated Ti–6Al–4V ELI alloys under multiaxial cyclic loading.