Effect of Minor Elements Content on the Phase Formation and Mechanical Properties of 6xxx Series Aluminum Alloy Sheets

This study investigates the role of content of alloying element on the precipitate formation and the resulting mechanical properties of Si-rich (Si/Mg weight fraction > 1.5) 6xxx series aluminum alloy sheets. Compared to Si, Mg content dominantly controls the amount of Mg-Si precipitates (GP, β″, β′, and β). For alloys with less than 0.4 wt.% Mg, increasing the total amount of solute atoms rather than the amount of Si increases yield strength while decreasing ductility, which is a typical trade-off characteristic in solid solution hardening. Interestingly, increasing Zn content simultaneously improves strength and ductility, presumably by reducing stacking fault energy and enhancing plastic deformation capability. For alloys with more than 0.45 wt.% Mg, increasing the total amount of solute atoms also leads to a simultaneous increase in yield strength and ductility. High-resolution transmission electron microscopy and energy-dispersive spectroscopy (HR-TEM/EDS) analysis indicate that the solute atoms are consumed to form nanoscale α-AlMnSi (containing Fe and Cr) precipitates. With increasing Mg content, the needle-shaped β′-phase may act as a nucleation site for α-AlMnSi nano-precipitates, significantly improving yield strength and ductility. In particular, these mechanical properties are significantly improved as the Fe content increases and the nano-precipitates become Fe-rich.