High-cycle fatigue strength and small-crack growth behavior of ultrafine-grained copper with post-ECAP annealing

The effects of post equal channel angular pressing annealing on microstructure and high-cycle fatigue behavior of ultrafine-grained copper were examined. Grains with sizes up to a few tens of micrometers surrounded by fine grains were formed after the annealing as a result of discontinuous recrystallization. The fatigue life of annealed samples in the short- and medium-life fields was found to decrease, while in the long-life field, the fatigue strength at 3 × 107 cycles was about 9% greater than that of ultrafine-grained copper without annealing. Long-term stressing for as-received ultrafine-grained samples produced large grains over 100 μm as a result of dynamic recovery and recrystallization. For annealed samples, there were no significant differences in grain sizes between pre- and post-stressing, showing the higher stability of bimodal microstructure. This stability was related to the slightly increased fatigue strength in long life fields. The effect of bimodal microstructure on the growth behavior of small surface-cracks was discussed.