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 longer 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 (KWN) framework towards handling the coexistence of several different types of stoichiometric particles. 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 are presented with some earlier reported transmission electron microscopy measurements [1,2] to shed light on how the alloy composition and ageing treatment influence the post- β″ phase selection.