The energetics and thermodynamics of fcc-based Al-Ag alloys have important consequences for the formation of Guinier-Preston (GP) zones in Al-rich alloys. Bulk thermodynamic properties of stable and metastable fcc-based phases in the Ag-Al system are studied from first principles using quantum-mechanical, total-energy methods combined with the cluster-variation method. From the results of these calculations the metastable fcc-based Ag-Al phase diagram has been calculated. Our calculated phase diagram features a monotectoid reaction. A miscibility gap in the fcc solid solution phase is present above the monotectoid temperature and a two-phase field between the Al-rich solid solution and an ordered fcc-based Ag2Al phase is predicted for low temperatures. Therefore, our calculations provide clear evidence that the structure of ε (high temperature) and η (low temperature) GP-zone precipitates should be disordered and long-range ordered, respectively. Our preliminary findings using a full-potential electronic-structure technique indicate that lattice relaxations in the alloys can significantly alter the formation energies.
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