099 Fatigue crack initiation and early crack propagation in ultrafine-grained copper in high-cycle fatigue region

Fatigue crack initiation and early crack propagation in ultrafine-grained copper prepared by equal channel angular pressing was experimentally investigated in high-cycle and giga-cycle region. The results of the observations are compared with those known for conventionally grained material. Dissimilarities and peculiarities of behaviour of ultrafine-grained structure are discussed. The cyclic slip localization in ultrafine-grained copper takes place in cyclic slip bands. The mechanism consists in highly localized slip within the individual ultrafine-grains resulting in development of cavities and voids arranged along the planes of highest cyclic shear stress and in formation of extrusions. The mechanism of the crack initiation in highand giga-cycle fatigue region does not require grain coarsening often discussed in literature. Fatigue cracks initiate in long slip bands, which grow predominantly in zones of near-by oriented grains by subsequent linking of individual slip bands. A process of growth and linking of cavities and voids produced by the irreversible cyclic slip by dislocation movement generating point defects governs the early stage of the development of fatigue cracks. Sufficiently long cracks created by this mechanism and lying in suitably oriented long slip bands finally transform to fatigue cracks propagating by common opening mode with a plastic zone generated at their tip.