Ab initio interface configuration determination for β″ in Al–Mg–Si: Beyond the constraint of a preserved precipitate stoichiometry

• Interface configurations examined without the limitations of a preserved precipitate stoichiometry.
• β″ precipitate composition Mg5Si6 rejected.
• New locally stable interface configurations and increased β″ compositional disorder proposed.
• Importance of examining the solute atom diffusivity within β″ stressed.

The precipitate-host lattice interface configuration stabilities for the β″ phase in the Al–Mg–Si alloy system have been examined using density functional theory. Usually, the supercell based calculations underlying such studies assume a preserved precipitate stoichiometry. Relaxing this assumption in the present work, we highlight significant results that may easily be overlooked when the stoichiometry constraint is operative. Our main findings are the following: (i) we reject the often proposed Mg5Si6 composition for β″. (ii) We stress the possibility of significant Mg–Al compositional disorder for the more likely β″-Mg5Al2Si4. Finally, (iii) we propose the existence of more than one locally stable interface configuration for the coherent β″/Al interfaces. It is argued that quantifying the solute atom diffusivity near the interface as well as within β″ is fundamental to addressing the importance of points (ii and iii).