High-cycle and very-high-cycle fatigue behaviors of additively manufactured vs. conventional titanium alloys: Microstructural and mechanical insights

This study presents a comparative investigation on the high-cycle and very-high-cycle fatigue (HCF–VHCF) behaviors of additively manufactured (AM) and conventionally processed (CP) titanium alloys under fully reversed loading (R = –1). Emphasis is placed on clarifying how microstructure and defect characteristics jointly govern the transition between microstructure-sensitive and defect-dominated fatigue mechanisms. The AM Ti-6Al-4V alloy, possessing a lamellar martensitic structure and void-type defects such as lack-of-fusion (LoF) pores and gas porosity, exhibits a duplex S–N curve and internal-origin fractures with smooth fine granular areas (FGAs) and fish-eye (FiE) morphologies. In contrast, the CP alloy with an equiaxed α+β microstructure shows a nearly linear S–N relation, surface or α/β interface crack initiation, and irregular FGAs, indicating a microstructure-controlled fatigue process. Micro- and nano-structural observations indicate the formation of nanoscale grains within the crack-initiation zone, suggesting a universal mechanism of cyclic plastic refinement in the VHCF regime. The results provide mechanistic insights into the distinct fatigue responses of AM and CP titanium alloys and underscore the importance of defect morphology control and microstructural design in improving the long-life reliability of AM components.