On the effect of precipitates on the cyclic deformation behavior of an Al-Mg-Si alloy

Fatigue is one of the major failure modes of structural materials. While the effects of strengthening precipitates on the mechanical properties of heat treatable aluminum alloys during forming operations are well-studied, only little is known about the related mechanisms during fatigue. We study the influence of precipitates during low cycle fatigue of an Al-Si-Mg alloy by mechanical testing and microstructure characterisation using (scanning) transmission electron microscopy. Specifically, we have investigated under-aged, peak-aged, and over-aged precipitation states. The experiments reveal considerable influence of the precipitate state on the mechanical properties and the formed dislocation structures. Under-aged AA6016 experiences cyclic hardening accompanied by dynamic precipitation and precipitate growth during cyclic deformation, whereas peak-aged AA6016 shows a saturated cyclic stress behavior and the formation of a ‘prevein’-like dislocation structure aligned along [001] Al directions. Over-aged AA6016 exhibits cyclic softening, which is assumed to be due to frequent Orowan-looping of dislocations around incoherent precipitates.