Phase stabilization principle and precipitate-host lattice influences for Al–Mg–Si–Cu alloy precipitates

• DOI: 10.1007/s10853-014-8371-4
• Publication Date: 2014-06-19T10:54:28Z
• Authors: F. J. H. Ehlers; S. Wenner; S. J. Andersen; C. D. Marioara; W. Lefebvre; C. B. Boothroyd; R. Holmestad
• Keywords: Metastability; Substructure

In this work, we seek to elucidate a common stabilization principle for the metastable and equilibrium phases of the Al–Mg–Si–Cu alloy system, through combined experimental and theoretical studies. We examine the structurally known well-ordered Al–Mg–Si–Cu alloy metastable precipitates along with experimentally observed disordered phases, using high angle annular dark field scanning transmission electron microscopy. A small set of local geometries is found to fully explain all structures. Density functional theory based calculations have been carried out on a larger set of structures, all fully constructed by the same local geometries. The results reveal that experimentally reported and hypothetical Cu-free phases from the set are practically indistinguishable with regard to formation enthalpy and composition. This strongly supports a connection of the geometries with a bulk phase stabilization principle. We relate our findings to the Si network substructure commonly observed in all Mg–Al–Si(–Cu) metastable precipitates, showing how this structure can be regarded as a direct consequence of the local geometries. Further, our proposed phase stabilization principle clearly rests on the importance of metal-Si interactions. Close links to the Al–Mg–Si precipitation sequence are proposed.