Cyclic stress–strain response and dislocation structures in polycrystalline aluminum

Fully reversed strain-controlled fatigue tests were performed on polycrystalline specimens of commercially 99.0% purity aluminum. The objective was to reveal the influence of plastic strain amplitude and fatigue cycles on dislocation arrangements and investigate the role of dislocation structures on cyclic deformation behavior of aluminum. The test specimens were cylindrical in shape having an effective gauge length section 6.5 mm in diameter and 25 mm long. The fatigue tests were run using symmetrical tension–compression loading under constant strain amplitude in laboratory air environment and room temperature. The longitudinal strain amplitudes used for the testing were in the range of 1.0×10−3–1.1×10−2 with a constant strain rate of 0.0001 s−1. Cyclic deformation behavior was characterized by analyzing the cyclic hardening response, and microstructural observations by means of transmission electron microscopy. The cyclic stress–strain curve of polycrystalline aluminum is characterized with the occurrence of cyclic strain hardening at which the saturation stress increases with plastic strain at all plastic strain amplitudes tested. In addition, the cyclic stress–strain behavior obtained in this study showed grain size dependence, which is in agreement with an equivalent Hall–Petch effect of grain size on cyclic deformation behavior. An investigation on the effect of changing strain amplitude on cyclic hardening reinforces the analysis that dislocation cell structures control fatigue properties. In all strain ranges investigated, microstructures are mainly formed by dislocation cells due to high stacking fault energy, which favors an activation of multiple glide systems and formation of three-dimensional dislocation structures. Persistent slip bands and labyrinth structures were not observed. The observed dislocation cellular structures are low energy structures, which govern plastic hardening (saturated stress) behavior of commercial purity polycrystalline aluminum.