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Physics-based computational thermokinetic modeling of initial precipitation stages in AA6xxx

Wednesday (06.11.2019)
11:25 - 11:45
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The modeling of cluster phases and GP-zones in the Al-Mg-Si system [1] has been shown to represent a feasible modeling base, which can predict nucleation, growth and transformations of early Mg-Si-type clusters close to a Mg:Si ratio (at.%) of 1. Recently, it has been proven by 3D-APT that the earliest precipitation stages in 6xxx can significantly deviate from this composition, approaching almost pure Si and Mg-rich clusters [2]. In order to predict the precipitation sequence in technological alloys based on the Al-Mg-Si system by means of predictive simulation in a wide range of nominal alloy compositions and thermo-mechanical treatments, initial precipitate compositions need to be reproduced by the thermodynamic models. In this paper, we demonstrate the successful computations of composition variations of initial precipitates in Al-Mg-Si alloys with varying nominal compositions, as well as their competitions during quench- and continuous heat treatments. In the proposed thermodynamic models, Calphad constraints are obeyed: The Compound Energy Model is used, allowing for physically proper extensions of precipitate-forming subsystems from unaries to binaries towards the ternary Al-Mg-Si system, and early precipitates remain metastable throughout the Al-Mg-Si composition range at all temperatures. Computed enthalpy and entropy trends, as well as precipitate fractions, sizes and compositions are discussed in comparison with experimental findings from recent high-resolution analyses. Whereas pure Mg nano-precipitates cannot be stabilised in line with evaluated thermodynamic data in the Al-Mg-Si system, cubic, ordered Al-Mg nano-particles of the L12-type are predicted to form at low temperature aging in Mg-rich specifications of AA6xxx. On the other hand, Si-rich clusters are formed in Si-rich specifications. From low to high Si-alloying, intermediate Mg-Si clusters and ordered GP-zones can form kinetically. Quenched vacancies play a key role for the stabilisation of all early precipitate stages.