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Al–Mg–Si–(Cu) alloys form the basis of a wide variety of commercial precipitation-hardened alloys, and the observed precipitation sequence in these alloys is complex and involves a wide variety of metastable phases (e.g. GP zones, β″, U1, U2, B^′, β^′). Calculations of metastable phase equilibria in these alloys are virtually nonexistent due to the lack of quantitative information on the thermodynamics of the precipitate phases. We have undertaken an extensive, systematic first-principles study of energetics of all the reported precipitate phases of Al–Mg–Si–(Cu) alloys, using density functional-based calculations in both the local density and generalized gradient approximations. Our calculations help clarify the energetics of the metastable precipitate phases, and in certain cases, provide insight into the compositional changes of precipitates during aging. In addition to energetics, we also examine the relative …