Hierarchical nanostructures strengthen Al-Mg-Si alloys processed by deformation and aging

Processing Al alloys by combining aging and deformation have attracted considerable interests recently. However, contrary to the well defined precipitation-hardening in Al alloys obtained by conventional artificial aging, the strengthening mechanism in these alloys is still poorly understood. In this report, a thermo-mechanical route proceeding by solution treatment, water quenching, natural aging, cold-rolling and post-annealing was implemented to produce an Al-Mg-Si alloy sheet with significant improvement in strength and a comparable ductility to conventional T6 treated alloy. A relatively complete physical metallurgical picture of this Al-Mg-Si sheet alloy processed by deformation and aging was established based on electron microscopy. During post-annealing, the deformed structures underwent a considerable decrease in dislocation density and extensive sub-grains as well as nano-grains formed. At the same time, nano-sized wall-like precipitates that were rather different with those in traditionally aged Al-Mg-Si alloys formed with the mediation of lattice defects. Observing in the longitudinal plane, the sheet possessed a banded microstructure consisted of sub-lamellas with a thickness of about 100-200 nm. The hierarchical nanostructures achieved by deformation and aging due to concurrent precipitation and defects annihilation can enhance the capacity of the alloy to accommodate dislocations under tension deformation and thus led to a ductile failure. The combination of strength and ductility in our hierarchically nanostructured Al-Mg-Si alloy was discussed.