On the effect of precipitates on the cyclic deformation behavior of an Al-Mg-Si alloy
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ue 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.
I. INTRODUCTION
Aluminum alloys are one of the most important lightweight structural materials. Particularly Al–Mg–Si (AA6xxx) alloys which have a desirable combination of good formability and high strengthening potential are widely used in the automotive industry due to increasing demands to improve the fuel efficiency and lower vehicle emissions.1–3 In most automotive applications, the materials are subjected to alternating loads, hence, their fatigue properties are critical. Generally, fatigue-related failure is one of the major reasons for the damage of metals and metallic alloys.4 Specifically, it has been reported that about 90% of mechanical service failures are caused by cyclic loading.5 Unfortunately, the fatigue performance of Al alloys is not very high and particularly that of precipitation hardened aluminum alloys. For example, the fatigue ratio (fatigue strength/ultimate tensile strength) of engineering steels amounts to ;0.56,7; while that of precipitation strengthened aluminum alloys is as low as ;0.3 under high cycle fatigue conditions.6,7 The high strength of Al–Mg–Si alloys is obtained through precipitation hardening by formation of nanosized coherent and/or semicoherent metastable precipitates.8,9 Contributing Editor: Mathias Göken Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2017.350
Numerous studies on the characterisation of the precipitate phases and their precipitation sequence have been published during the past decades.2,10–13 Generally, the precipitation sequence in these alloys follows2: SSSS ! atomic clusters=GPzones ! b00 ! b0 ! b ; where SSSS is a super-saturated solid solution and atomic clusters are clusters of solute atoms in the fcc Al matrix phase without a specific structure. The atomic clusters are usually considered as a prestage for the formation of Gui
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