Abstract
The formation mechanism of the granular regions often observed in subsurface fractures of Ti6Al4V alloy was experimentally investigated. To simulate and simplify the phenomenon of crack opening and closure, which is strongly related to fatigue crack propagation and strength properties, uniaxial cyclic compression tests were conducted in air and vacuum using test pieces with rounded surfaces under contact conditions. The results showed ring-like patterns on the parts of the rounded surfaces subjected to cyclic contact regardless of the ambient environmental conditions. However, in the case of vacuum only, fine surface asperities were observed, which were quite similar to those in the subsurface fractures. Microstructural refinement was confirmed by cross-sectional observation of the concave and convex parts. In the refined microstructure, voids and cracks were observed, which were formed during the refinement and adhesion processes. Microstructural refinement and metallic adhesion under compression loading in vacuum are key phenomena for the formation of fine surface asperities. Based on the experimental results, a new model for the formation mechanism of the granular region was proposed. This model can explain the characteristics of the granular region and the properties of subsurface fractures, which occur at lower stress and have longer lifetimes than surface fractures.
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