Precipitation sequence in Al–Mg–Si–Sc–Zr alloys during isochronal aging

Abstract

6xxx-series Al alloys are the most commonly used alloys in the automotive industry as they have an appropriate balance of strength, corrosion resistance and formability. These alloys experience significant strengthening from the precipitation of coherent GP-zones and β'' precipitates. One opportunity to achieve stronger 6xxx-series Al alloys, without affecting corrosion and formability, is the use of Sc. The strengthening benefits from Sc are obtained by the formation of the Al3Sc dispersoids. The addition of Zr, together with Sc, results in the formation of hybrid Al3(Sc,Zr) dispersoids with a core–shell morphology. The microstructural development in 6xxx-series alloys containing Sc is still largely unknown. Here we show, that combining conductivity, hardness and modelling allows for the prediction of the precipitation sequence in an Al–Sc–Zr, an Al–Mg–Si and an Al–Mg–Si–Sc–Zr alloy. The evolution of conductivity and hardness during isochronal aging is discussed in terms of solute depletion and precipitate formation. APT, TEM and SEM are also conducted in key conditions to help interpret the resistivity and hardness fluctuations. Modelling allows for estimation of the precipitation kinetics of the Mg-Si precipitates and Sc–Zr dispersoids during isochronal aging. The precipitation kinetics of Al3(Sc,Zr) is found to be accelerated in the presence of Mg and Si and APT reveals that this is due to the preferred nucleation of the Al3(Sc,Zr) dispersoids on the MgSi precipitates. These results pave the way for the design of suitable heat treatments for 6xxx-series alloys containing Sc that would allow for the formation of both Al3(Sc,Zr) dispersoids and fine MgSi precipitates.