Disappearance and reappearance of serrated plastic flow under cyclic loading: A study of dislocation substructures

Serrated plastic flow was observed in cyclic stress–strain hysteresis loops both in tensile and in compressive components during strain controlled low cycle fatigue of annealed Zircaloy-2 at a strain rate of 10−4 s−1 and different strain amplitudes, at room temperature. Although there were only D-type serrations at higher strain amplitudes (>±0.75%) there were mixed serrations of D, A, B and C type at lower strain amplitudes (≤±0.75%). Depending on the cyclic stress response, transitions in dislocation structure from dislocation pile-ups to cell structure and cells to Corduroy structure were observed in the region of softening and secondary hardening. Disappearance and reappearance of serrated flow in the hysteresis loops were observed with number of cycles and it was proposed to be associated with transition in dislocation structure from planar to cell walls and finally from cell wall to Corduroy structure.