Dislocation structures of fatigued polycrystalline copper under variable amplitudes

Cyclic behaviors of polycrystalline copper are studied under variable strain amplitudes. It is shown that the specimens tested at incremental strain amplitudes have the same stress as at conventional constant strain amplitudes. However, the specimens tested at decremental strain amplitudes have a history effect on the stress as compared with those at constant strain amplitudes. TEM observations show that the incremental loading produces a dislocation wall structure similar to that at constant amplitudes. Under the decremental loading, the cellular dislocation structure formed at higher strain amplitude recovers to labyrinth structure. This degeneration process is found to be incomplete and the dislocation structure remains as dense dipolar walls.