Cross-scale early damage mechanism of VHCF based on the initiation and evolution of fine granular area in selective laser melting Ti-6Al-4 V alloy

Due to the initiation of fine granular area inside the material and the formation of nanoscale grains, it is difficult to conduct in-situ observation and high-scale characterization. This is the main reason why the formation mechanism of fine granular area in very high cycle fatigue has been unknown and controversial. Therefore, on the basis of fracture analysis method to invert the fine granular area formation, we further put forward an experimental proposal whether a microcrack in the fine granular area formation stage can be prepared to observe the critical event of early damage evolution. Here, we selected selective laser melting Ti-6Al-4 V alloy with inherent defects as the model material to obtain significant defect-initiating fine granular areas, found two secondary microcracks after dissecting along the defects, and then carried out multiscale characterization and quantitative analysis of main cracks and secondary cracks. We found that the fine grains originate from the severe plastic deformation in local of crack tip plastic zone and lowered the cracking threshold by the grain boundary sliding between the hard-oriented grains, which results in grain refinement, cavitation and cracking. This work systematically describes damage evolution mechanism, which has guiding significance for the reliability evaluation and fatigue resistance design of materials.