Abstract
Since a starting point of fatigue failure is usually in surfaces, clarifying the mechanism of surface damage formation due to cyclic stresses should lead to a better understanding of the fatigue characteristics of ultrafine grained (UFG) metals. With regard to surface damage, shear bands (SBs) in cyclically deformed UFG copper have been reported. The morphology of the SBs was found to be similar to the extrusions formed by persistent slip bands (PSBs) in large grain metals. An attempted to explain the SB formation with relation to the oriented distribution of defects along the shear direction of the last pressing has been performed. In many studies, observations of SBs have been performed on surfaces of post-fatigued specimens or surfaces after SB formation. Therefore, in order to study the formation mechanism of surface damage, the formation process of surface morphologies should be clarified. However, no such experiment has yet to be conducted. In the present study, fatigue tests of ECAP processed copper with grain size of about 250 nm were made. The successive observation of surface was made by optical microscopy and scanning electron microscopy. The formation mechanism of the surface damage of UFG copper was discussed through the observation of changes in surface morphological features.
