Tailored precipitation route for the effective utilisation of Sc and Zr in an Al-Mg-Si alloy

In this work, we develop and assess two potential heat treatment methods tailored for the effective utilization of Sc and Zr in an Al-Mg-Si model alloy. (Al,Si)3(Sc,Zr) dispersoids and MgSi precipitates were identified and characterised using a combination of SEM, STEM and APT. The dispersoid composition and structure changed significantly depending on the heat treatment route. The strength increment due to Sc and the effectiveness of the heat treatments after hot extrusion and artificial ageing was determined using tensile tests. A tensile yield strength increase of approximately 60 MPa could be attained through the addition of 0.05 wt.% Sc and 0.1 wt.% Zr, provided a suitable heat treatment methodology was utilised. An analytical strengthening model was utilised to confirm the individual strengthening contributions of the (Al,Si)3(Sc,Zr) dispersoids and the various other active strengthening mechanisms in the final product. The strength increase from Sc and Zr additions to Al-Mg-Si was found to come from precipitation hardening owing the presence of nano-sized dispersoids. The (Al,Si)3(Sc,Zr) dispersoids were also revealed to act as preferential nucleation sites for the β’’ precipitates. These results can be used as an outline for the design of a suitable heat treatment for Sc and Zr augmented Al-Mg-Si alloys.