Fatigue damage of ultrafine-grain copper in very-high cycle fatigue region

Ultrafine-grained copper produced by ECAP technique was tested in ultrasonic fatigue in the region of lives from 108 to 2 × 1010 cycles. The fatigue strength of ultrafine-grained copper is by a factor of 2 higher than that of conventional-grain copper. The occurrence of surface fatigue slip markings is rare. No grain coarsening was observed either in bulk or in areas underneath the surface slip markings. Localized collective grain-boundary slip of near-by oriented grains is considered to be the main damage mechanism leading to fatigue microcrack initiation either directly in the roots of surface intrusions or underneath the surface slip markings.