|
Ti-6Al-4V alloy specimens cut form a forged plate with a duplex microstructure, similar to the microstructure used in fan blades were tested under conditions of high-cycle fretting fatigue. The contact geometry, the normal stress, as well as the cyclic stress were selectee such that the mixed, slip-stick regime prevails during the experiments. Following testing, the specimens as well as the fretting pads were characterized by a variety of techniques including white light interference profilometry, scanning electron microscopy, ultrasonic force microscopy, microhardness testing, 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 beta phase has been decreased substantially. The elastic property variation of this region was determined by ultrasonic force microscopy; the results revealed that in contrast to what found for the bulk of the material, there are significant local differences of the elastic properties inside the fretting-affected zone. In addition, the changes in the plastic behavior of the region below the slip zone, was determined using microhardness measurements. It was found that this transformed microstructure area, has also a higher hardness compared to the hardness of the bulk structure. Booth elastic and plastic property variations were attributed to the increased percent of alpha phase and the decreased amount of beta in the transformed zone, since the former phase exhibits higher elastic moduli as well as flow stresses.In addition, changes in the concentration of the oxygen at the specimen's surface as well as inside the transformed zone were examined by energy dispersive spectroscopy (EDS). The EDS analysis revealed a high concentration of oxygen on the specimen's surface only at the slip region of the two contacting materials. This finding indicates that elevated temperatures were developed during the fretting fatigue testing, which enable the diffusion of oxygen from the atmosphere to the alloy. However, within the transformed zone, no detectable differences in the oxygen concentration were revealed. This finding allowed us to assume that stress induced transformation is the most probable mechanism.
|
