Cyclic deformation behavior of Cu-30% Zn single crystals oriented for single slip - II. Dislocation structures

The dislocation structures of Cu-30% Zn single crystals cyclically deformed at gamma(pl) = 3.8 x 10(-5)-6.4 x 10(-3) were studied through transmission electron microscopy in order to understand the cyclic deformation mechanisms of the material. It has been shown that the fatigue dislocation structures have two basic configurations depending on the strain amplitude applied. At low strain amplitudes (gamma(pl) <3 x 10(-4)), the dislocation structure is characterized by dislocation segments and multipoles, very similar to that formed in stage I of tensile deformation in the same material. At high strain amplitudes (gamma(pl) >3.0 x 10(-4)), however, the dislocation structure is dominated by planar dislocation loops and tangles; in this case a small amount of dislocation multipoles and a special zigzagged structure are also detected. Secondary slips were found to be activated from a very low strain amplitude (gamma(pl) = 3.8 x 10(-5)). This behavior is attributed to (i) the lower stress for dislocation generation compared with cyclic dow stresses and (ii) heterogeneous deformation at low strain amplitudes that results in a weak effect of local latent hardening. Cyclic deformation mechanisms at low and high strain amplitudes have been discussed in terms of the motion of dislocation multipoles and the dislocation reactions between the primary and secondary slip systems. The critical strain amplitude for the transition of the deformation mechanisms has been also theoretically determined to be 3 x 10(-4), in good agreement with results on the cyclic deformation response and the features of dislocation structures. The formation of specific dislocation structures such as the zigzagged structure has been discussed with detailed dislocation mechanisms. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.