Atomistic details of precipitates in lean Al–Mg–Si alloys with trace additions of Ag and Ge studied by HAADF-STEM and DFT

Abstract

Bonding energies and volume misfits for alloying elements and vacancies in multicomponent Al–Mg–Si alloys have been calculated using density functional theory (DFT). A detailed atomic scale analysis has been done for characteristic precipitate structures, using high-angle annular dark-field scanning transmission electron microscopy. Two new stacking configurations of the important strengthening phase β′′ were discovered in the Ge-added alloy. All three stacking variations were found to be energetically favourable to form from DFT calculations. The second stacking configuration, β₂′′, contains vacated columns in its unit cell, consequently requiring less solute to create the same volume fraction of precipitate needles. DFT suggests a lower formation enthalpy per atom for β₂′′ when Si is exchanged with Ge. In the alloy containing Ag additions, a new Q’/C-like local configuration containing Ag instead of Cu was discovered, also this phase was deemed energetically favourable from DFT.

Keywords:

• Aluminium alloys
• HAADF-STEM
• density-functional theory
• Si/Ge
• nanoscale precipitates
• crystal structure

Acknowledgements

Hydro Aluminium and the Research Council of Norway are greatly acknowledged for their support through the BIA RolEx project, No. 219371. The TEM and STEM work was carried out on the NORTEM JEOL ARM200F at the TEM Gemini Centre, NTNU, Norway. This research was also supported in part with computational resources provided by NOTUR, http://www.sigma2.no.