Fatigue strength and formation behavior of surface damage in ultrafine grained copper with different non-equilibrium microstructures

Fatigue tests of ultrafine grained (UFG) copper were performed for four samples: (1) OFC4: oxygen-free copper (99.99 wt% Cu) pressed by ECAP using four passes, (2) OFC8: OFC pressed by ECAP using eight passes, (3) DLP4: deoxidized low-phosphorous copper (99.95 wt% Cu) pressed by ECAP using four passes and (4) OFC4A: OFC4 with post-ECAP short annealing. Fatigue strength at N = 3 × 107 cycles, σw, was enhanced by either an increase in the number of pressing cycles or a decrease in purity. σw for OFC8 and DLP4 showed 10% and 18% increases with respect to that for OFC4, respectively. Fatigue life in short- and medium-life fields was drastically decreased by post-ECAP annealing, whereas fatigue strength in long-life fields in excess of N = 107 cycles was slightly recovered. The formation process of surface damage was successively monitored to investigate the formation mechanism of the shear bands. A close relationship between the change in hardness and formation behavior of surface damage was observed. The physical background of fatigue damage was discussed from the viewpoints of shear band formation, non-equilibrium grain boundaries, grain coarsening, etc.