Atomistic modeling of the Al-Mg-Si alloy system
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- Institutt for fysikk 
The atomic and electronic structure of bulk crystalline and solid state phases related to Al-Mg-Si (6xxx) alloys have been studied using Density Functional Theory (DFT) and total energy Tight Binding (TB). We present a procedure for fitting a non-orthogonal total energy tight binding model to full potential Linearized Augmented Plane Wave (LAPW) DFT calculations using a stochastic global optimization algorithm and increased emphasis on the angular symmetries of the bandstructure eigenvalues. We use this method to produce a tight binding parameterization for Aluminium which gives improved predictions for the generalized stacking fault energies. We present a method for applying ab initio calculations in the process of determining the atomic structure of the precipitate phases in Al-Mg-Si alloys. We propose atomic models for the recently identified U1 and U2 precipitate phases. We use DFT modelling to study the electronic structure of the β, β11, U1, U2, and β precipitate phases in Al-Mg-Si alloys. We show that the β11 phase is dominated by a network of covalently bonded Si and that the U1 and U2 phases contain tetragonally coordinated Al-Si bonding networks made possible by the donation of charge from the Mg atom, while the β phase is dominated by partially ionic Mg-Si bonds.