Fully-reversed strain-controlled fatigue tests were performed on polycrystalline specimens of 2N-purity commercial material. The specimens were cylindrical, with an effective gauge length section which was 6.5mm in diameter and 25mm long. Fatigue tests were carried out using symmetrical tension–compression loading under constant strain amplitude in laboratory air at room temperature. The longitudinal strain amplitude ranged from 10−3 to 1.1 x 10−2; with a constant strain rate of 0.0001/s. The cyclic deformation behavior was characterized by analyzing the cyclic hardening response, and by transmission electron microscopy. The cyclic stress–strain curve was characterized by the occurrence of a cyclic strain hardening, in which the saturation stress increased with plastic strain at all plastic strain amplitudes. The cyclic stress–strain behavior exhibited a grain-size dependence. This was in agreement with an equivalent Hall–Petch effect of the grain size upon the cyclic deformation behavior. An investigation of the effect of changes in strain amplitude, upon cyclic hardening, supported the suggestion that dislocation cell structures controlled the fatigue properties. For all strain ranges, the microstructures were formed mainly of dislocation cells, due to a high stacking-fault energy. This favored the activation of multiple glide systems and the formation of 3-dimensional dislocation structures. Persistent slip bands and labyrinth structures were not observed. The dislocation cellular structures were low-energy ones, which governed the plastic hardening (saturated stress) behavior of commercial purity polycrystalline Al.

Cyclic Stress–Strain Response and Dislocation Structures in Polycrystalline Aluminium. Y.El-Madhoun, A.Mohamed, M.N.Bassim: Materials Science and Engineering A, 2003, 359[1-2], 220-7