On the role of nanocrystallization and cavitation to early damage of VHCF in selective laser melting Ti-6Al-4 V alloy

•In order to research the mechanism of VHCF internal initiation and early propagation, SLM-TC4 alloy containing LoF defects was selected as the model material and a secondary microcrack was observed by longitudinal cutting at the crack initiating-defect.•Cavitation was found in the process of severe plastic deformation caused by early damage of VHCF, and the formation mechanism of the cavity was proposed.•Through the multi-scale characterization of cracks and microstructures, the question of whether crack or fine grains exist first was solved. It is suggested that fine grains and cavities are the main forms of early damage.•Clear characterization and quantitative analysis of nanograins were carried out, and the evidence that nanograins came from recrystallization (MDRX and CDRX) during fatigue was found for the first time. Due to internal initiation and formation of FGA, it is difficult to observe the early damage of VHCF in situ. The inverse method based on the observation of cross-section in the fracture source region also fails to obtain the critical event of damage evolution. These make the early damage mechanism of long-life fatigue puzzling. Therefore, we proposed an idea of whether it is possible to prepare an observable microcrack initiating inside the material to observe the microstructure around the crack. Here, we selected selective laser melting Ti-6Al-4 V alloy with inherent defects as the model material for ultrasonic fatigue test, and successfully found a secondary micro-crack on the cross-section after dissecting numerous fractures along the crack initiating-defects, and then carried out multi-scale characterization and quantitative analysis. We found that both nanograins and microcavities were formed prior to cracking. Nano-crystallization results from the coupling of continuous recrystallization and migration recrystallization, and cavitation mainly results from grain boundary slip of hard oriented grains. This work elucidates the relationship and formation mechanism of nanograins, microcavities and cracks at nanometer scale. It provides a powerful support for decoding the severe plastic deformation damage in the early stage of VHCF.