Fatigue mechanisms in ultrafine-grained copper

Fatigue behaviour of ultrafine-grained (UFG) copper produced by equal-channel angular pressing (ECAP) was studied at room temperature and at depressed temperatures of 173 K and 103 K. Effect of material purity was investigated at room temperature on UFG coppers of three substantially different purities. Temperature sensitivity of fatigue strength and cyclic flow stress of UFG copper is lower than that of copper with conventional grain size. Fatigue strength strongly depends on purity at low stress amplitudes and is independent of it at high stress amplitudes. No substantial changes of microstructure were detected after cyclic loading. Surface fatigue slip markings along the traces of the shear planes of the ECAP process were observed. Two concurrent mechanisms of cyclic plastic deformation in UFG copper axe proposed: (i) bulk mechanism consisting of irreversible movement of dislocations and (ii) surface mechanism operating in the surface layer consisting of grain boundary sliding along shear planes of ECAP process.