Microstructural evolution during low cycle fatigue of a 3003 aluminum alloy

A polycrystalline 3003 aluminum alloy was cyclically deformed at room temperature under constant stress amplitudes in the range of 40–68 MPa. The mechanical responses of the stress-controlled tests were compared with those of the strain-controlled tests conducted earlier. Similar to the strain-controlled tests, cyclic softening after initial hardening was found at medium strain amplitudes in the stress-controlled tests. Secondary hardening occurred in the later stage of fatigue after softening. Microstructural analysis using transmission electron microscopy (TEM), revealed evolutions of dislocation structures at each stress amplitude. A labyrinth structure, which is composed of over two sets of dislocation-dipole walls with spacing of about l μm, was typically formed during the softening stage. Veins, i.e. tangled edge dislocation dipoles, were formed during the monotonic hardening stage at both lower strain and stress amplitudes. On the other hand, cell structures were dominant at the final stage in the high stress amplitude tests. The evolutions of dislocation structures in stress-controlled tests are well correlatable with those in strain-controlled tests.