Influence of Cu on Strength and Ductility of Al-Mg-Si Alloys with Excess Fe and Si Processed by High-Pressure Torsion

This study presents tensile properties of Al-Mg-Si alloys after processing by severe plastic deformation through high-pressure torsion (HPT). The alloys were fabricated so as to include excess Fe and Si with different additions of Cu as model alloys for recycling purpose. The tensile strength well exceeded 500 MPa with the total elongation more than 15% after HPT-processing under 2 GPa for 1 turn in all the model alloys. The strength further increases to more than 700 MPa with increasing addition of Cu while maintaining reasonable ductility (~8%). Transmission electron microscopy confirmed that the grain size was reduced to 180 nm and further reduced to 160 nm with increasing Cu addition. Microstructural analyses using scanning transmission electron microscopy and atom probe tomography revealed that Cu was segregated at grain boundaries, contributing to the increase in the tensile strength. The high strength with enhanced ductility is discussed in terms of strain rate sensitivities measured from strain rate change tests.