Nanoscale grain formation at crack initiation region under very-high-cycle fatigue for Ti-6Al-4 V manufactured by selective laser melting

The very-high-cycle fatigue (VHCF) behavior of Ti-6Al-4 V alloy produced by selective laser melting (SLM) was systematically investigated, with particular emphasis on the microstructural features at crack initiation sites. Fatigue cracks predominantly originated from internal pores, accompanied by the formation of a fine granular area (FGA) and characteristic fisheye (FiE) morphologies. For the first time in the selectively laser melted (SLMed) Ti-6Al-4 V, a nanocrystalline layer at the immediate crack surface within the FGA was characterized in detail, providing direct experimental evidence in support of the numerous cyclic pressing (NCP) model for crack initiation and early-stage propagation. The interrelation among pore size, applied stress amplitude, and the degree of microstructural refinement within the FGA was clarified. These findings offer new insights into the fatigue behavior of additively manufactured (AM) alloys and underscore the critical influence of process-induced defects and post-processing treatments on VHCF performance.