Temperature dependence of cyclic saturation in low amplitude fatigue of copper single crystals

Single slip oriented copper single crystals were cyclically saturated in tension-compression at constant amplitudes of plastic strain at a temperature of 77 K. The cyclic stress-strain curve for the crystals displays a plateau associated with persistent slip bands (PSBs), whose volume fraction increases when the plastic strain amplitude is increased. By comparison with the room-temperature behaviour it appears that although the plateau stress and the length of the plateau are highly sensitive to the temperature, their ratio is temperature-independent, unless secondary cyclic hardening intervenes to shorten the plateau. Existing models based on cross-slip of screw dislocations are consistent with this result. Electron microscopy revealed significant deviations from the regularity usually assumed for the structure of PSBs. The deviations allow the Eshelby model to account for the plateau stress as the sum of long-range internal stress carried by tilted PSB walls and the stresses required for dislocation bowing or passing.