CYCLIC DEFORMATION BEHAVIOR OF Cu-30/100 Zn SINGLE CRYSTALS ORIENTED FOR SINGLE SLIP--II. DISLOCATION STRUCTURES

摘要

The dislocation structures of Cu-30/100 Zn single crystals cyclically deformed at γ_pl = 3.8 x l0~-5 -6.4 x l0~-s were studied through transmission electron microscopy in order to understand the cyclic defor- mation 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 (γ_pl < 3 x l0~-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 (γ_pl > 3.0 x l0~-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 (γ_pl= 3.8 x l0~-5). This behavior is attributed to (i) the lower stress for dislocation generation compared with cyclic flow stresses and (ii) heterogeneous deformation at low strain amp1itudes that results in a weak effect of local latent hardening. Cyclic defor- mation mechanisms at low and high strain amplitudes have been discussed in terms of the motion of dislo- cation 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 deter- mined to be 3 x l0~-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.