Cyclic deformation behavior and dislocation structures of [001] copper single crystals--I cyclic stress-strain response and surface feature
详细信息 查看全文
文摘
Cyclic deformation behavior of [001] copper single crystals was investigated in symmetrical pull-push fatigue tests at the ambient temperature and with constant plastic shear strain amplitudes (γpl) in the range of 1.0 × 10−4 to 3.0 × 10−3. The formation and development of surface slip bands (SBs) were examined in situ by a light microscope. A rapid initial hardening stage followed by a pronounced overshooting or significant softening was found with the crystals tested at γpl ≥ 4.8 × 10−4. The cyclic stress-strain curve of [001] crystals does not show any plateau behavior, instead it follows, if corrected with the Taylor factor, the power law function for copper polycrystals. The fatigue limit, defined as the critical strain amplitude below which SBs do not form, was found to be approximately 1.7 × 10−4, a value much higher than that for single slip oriented crystals, but very close to that for polycrystals modified by the Taylor factor. In situ observation revealed that the primary SBs appeared at the very beginning of cyclic deformation and frequently they were not persistent. The secondary (critical) SBs usually formed at the overshooting stage after rapid hardening. Deformation bands with the characteristics of kink bands were detected in the final stage of the cyclic deformation. Analysis indicated that the rapid initial cyclic hardening is caused by the formation of Lomer-Cottrell locks, while the overshooting/softening behavior is related to the operation of secondary slip or the formation of dislocation labyrinth structure. The preferred combination of primary and critical slip during cyclic deformation is also discussed in terms of dislocation reactions.