Modeling over-ageing in Al-Mg-Si alloys by a multi-phase CALPHAD-coupled Kampmann-Wagner Numerical model Qiang Du*, Kai Tang, Calin D. Marioara, Sigmund J. Andersen, SINTEF Materials and Chemistry, Trondheim, Norway Bjørn Holmedal, Randi Holmestad, Norwegian University of Science and Technology, Trondheim, Norway *corresponding author: qiang.du@sintef.no Abstract The formation of the equilibrium precipitation phase during ageing treatment of Al-Mg-Si alloys is preceded by a series of metastable phases. Given inappropriate ageing time, higher ageing temperature or elevated temperature service condition, β'', the main hardening phase, would be replaced by the more stable metastable phases such as β', B', U1 and U2. The post-β'' microstructure evolution, called "over-ageing", leads to a steep drop in the hardness evolution curve. This paper aims to predict directly over-ageing in Al-Mg-Si alloys by extending a CALPHAD-coupled Kampmann-Wagner Numerical framework towards handling the coexistence of several types of stoichiometric particles of different phases. We demonstrate how the proposed modeling framework, calibrated with a limited amount of experimental measurement data, can aid in understanding the precipitation kinetics of a mix of different types particles. Simulation results will be presented for some earlier reported transmission electron microscopy measurements [1] to shed light on how the alloy composition and ageing treatment influence the post- β'' phase selection. 1. Introduction The precipitation kinetics in heat-treatable aluminum alloys is quite complex. The formation of the equilibrium precipitate phase is preceded by a series of metastable ones due to their ease of nucleation. Examples include the needle β'' precipitate in Al-Mg-Si [2], the plate θ' precipitate in Al-Cu [3], the platelet η' precipitate in Al-Zn-Mg [4], and the lath S' precipitate in Al-Cu-Mg alloys [5]. It is these metastable precipitates rather than their stable counterparts that are contributing to peak hardening. However, given longer ageing time, higher ageing temperature or extended service time at an elevated temperature the precipitates responsible for the peak hardness will be replaced by other more stable precipitates, and eventually the equilibrium phases will form. This phenomenon is termed "over-ageing". During over-aging the precipitate density will decrease and the precipitate size will increase. Since a majority of the precipitates are non-