Composition of β′ precipitates in Al-Mg-Si alloys by atom probe tomography and first principles calculations

The composition of beta(') precipitates in an Al-Mg-Si alloy has been investigated by atom probe tomography, ab initio density functional calculations, and quantitative electron diffraction. Atom probe analysis of an Al-0.72% Si-0.58% Mg (at. %) alloy heat treated at 175 degrees C for 36 h shows that the beta(') phase contains similar to 20 at. % Al and has a Mg/Si-ratio of 1.1, after correcting for a local magnification effect and for the influence of uneven evaporation rates. The composition difference is explained by an exchange of some Si with Al relative to the published beta(')-Mg5Si6 structure. Ab initio calculations show that replacing the Si-3-site by aluminum leads to energetically favorable compositions consistent with the other phases in the precipitation sequence. Quantitative electron nanodiffraction is relatively insensitive to this substitution of Al by Si in the beta(')-phase.