Influence of heat treatment on the cyclic deformation properties of aluminium alloy AA7030

An extruded AA7030 alloy (Al-5.4Zn-1.2Mg) was tested in low-cycle fatigue at different total strain amplitudes to determine the cyclic deformation and fracture behaviour and to investigate the effect of heat treatment-natural ageing (NA) or peak ageing (PA). The results show that the fatigue life is longer in the NA temper at a given plastic strain amplitude. The fatigue life can be described by a total strain amplitude approach, where both NA and PA fall on the same straight line. The NA temper shows continuous cyclic hardening until the onset of fatal crack growth while the PA temper hardens during the first 20-30% of the fatigue life and then softens due to crack growth. The crack propagation mechanisms are different between the tempers, where NA favours transgranular cracking while intergranular propagation prevails in PA. Finally, a tension/compression stress asymmetry was observed under symmetric straining.