Significance of non-equilibrium grain boundaries in surface damage formation of ultrafine-grained copper in high-cycle fatigue

High-cycle fatigue tests were carried out on specimens of ultrafine-grained copper both after equal channel angular pressing and after subsequent annealing at 160 °C for 3 min. The formation behavior of surface damage and the change in surface hardness during stressing was monitored. A close relationship was observed between the change in hardness and the formation behavior of surface damage. The physical background for the formation of surface fatigue damage is discussed from the viewpoint of the non-equilibrium states of the microstructure.