Cyclic deformation behavior of Cu–30 wt% Zn single crystals oriented for single slip—I. Cyclic deformation response and slip band behavior

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Cyclic deformation behavior of Cu–30% Zn single crystals oriented for single slip was studied at constant plastic shear strain amplitudes (γpl) in the range of 3.8×10−5–6.4×10−3 in order to understand systematically the fundamental fatigue behavior of low stacking fault energy materials. Results indicate that the cyclic hardening behavior strongly depends on the strain amplitude applied. For low strain amplitudes (γpl<3×10−4), cyclic saturation occurred after an initial cyclic hardening stage, but for high strain amplitudes (γpl≥6.0×10−4) saturation could not be reached until fatigue failure. The initial cyclic hardening rate (θ0.2) was found to decrease with increase in the applied strain amplitude. Slip bands were found to behave very similarly to Lüders band appearance at the beginning as well as in the middle stage of cyclic deformation. Particularly, the similarity of the cyclic hardening behavior at low and high strain amplitudes to the work hardening response in stages I and II of tensile deformation of the same alloy has been pointed out. Cyclic stress was further decomposed into two terms, the effective stress and the internal stress, and both were found to increase continuously with cyclic deformation. It has been demonstrated that the activities of secondary slips played an important role in the continuous cyclic hardening at high γpl. A comparison of the present result with previous relevant work on both wavy and planar slip materials has been attempted. The transition of wavy slip mode to planar slip mode of Cu–Al and Cu–Zn alloys has been discussed in terms of the electron–atom ratio and the critical value of the ratio for such a transition is found to be 1.18–1.19 for both alloys.