Microstructural and surface characterization of Ti–6Al–4V alloys after fretting fatigue

Ti–6Al–4V alloy specimens were tested under conditions of fretting fatigue, with the contact geometry, the normal stress, as well as the cyclic stress selected such that the mixed, slip-stick regime prevails during the experiments. Following testing, the specimens were characterized using white light interference profilometry, scanning electron microscopy, microhardness, and electron dispersive spectroscopy (EDS). The results revealed that the surface roughness of the slip region increases compared to the roughness of the stick, and non-contact ones. In addition, at the higher spatial frequencies, the power spectral density (PSD) of the slip region increases compared to the PSD of the stick and non-contact regions, thus revealing that an increase of the population of the smaller size asperities occurs. The microstructure of the material below the slip zone was found to be transformed to a finer one; and the percentage of the transformed β phase has been decreased substantially. This area of the transformed microstructure, has also a higher hardness compared to the hardness of the bulk structure. EDS analysis revealed a high concentration of oxygen on the specimen’s surface at the slip region of the two contacting bodies. This finding indicates that elevated temperatures are developed during fretting fatigue and enable the diffusion of oxygen from the atmosphere to the alloy.