Quantified effect of quench rate on the microstructures and mechanical properties of an Al–Mg–Si alloy

Abstract

It is well known that lower quench rate leads to the formation of precipitate free zones (PFZs) adjacent to grain boundaries in aging hardening materials such as Al–Mg–Si alloys. However, the combined effect of PFZs and intragranular precipitates on the yield strength of Al–Mg–Si alloys is not systematically understood. To clarify this, an Al–Mg–Si alloy was water/oil/air quenched after solid solution treatment, then aged at 180 °C for 6 h. It was found that a lower quench rate led to coarser precipitates and wider PFZs. The effects of PFZs and precipitates on the yield strength of the Al–Mg–Si alloy were separated by quantitative characterization of multi-scale microstructures and mechanical simulations. Wide PFZs were found to have a notable effect on yield strength, amounting over half the strengthening effect of intragranular precipitates. This effect increases as the width of PFZ increases, due to more serious strain localization caused by PFZs. A corresponding equation has thus been proposed to describe the deleterious effects of PFZs on yield strength of Al–Mg–Si alloys.