The effect of low Cu additions on precipitate crystal structures in overaged Al-Mg-Si(-Cu) alloys

This study concerns the effect of low Cu additions (≲0.1 at.%) on the precipitate crystal structure evolution in three Al-Mg-Si(-Cu) alloys during overageing. The evolution was assessed through a combination of atomic resolution scanning transmission electron microscopy, scanning electron diffraction, and differential scanning calorimetry. It was found that relatively small changes in the Cu level and the Si:Mg ratio had significant effects on the resulting distribution of precipitate phases, their structural evolution, and their thermal stability. Two Si-rich alloys formed hybrid β′ phase and Q′ phase precipitates on overageing. A third Mg-rich alloy primarily formed L phase precipitates, which exhibited superior thermal stability. Three distinct Cu-containing sub-units that form the basis for all Al-Mg-Si-Cu precipitate phases were identified: the three-fold symmetric Q′/C and the βCu′ sub-units, in addition to a newly discovered C sub-unit. The formation of each sub-unit was discussed, and the atomic structures and connections to other precipitate phases in the Al-Mg-Si(-Cu) system were elaborated. The work presented provides new insights into the complex precipitation of Cu-added Al-Mg-Si alloys, with implications for material properties. The results obtained will be of importance in future alloy and process development, and are thought to be of high value in modelling work on the quaternary Al-Mg-Si-Cu system. [Display omitted] •Three Cu-containing sub-units form in the quaternary Al-Mg-Si-Cu system.•The analysis of 1000s of precipitate phases is enabled by a scanning electron diffraction approach.•The Si-rich alloys form hybrid β′ and Q′ phase precipitates with overageing.•The Mg-rich alloy forms L phase precipitates exhibiting superior thermal stability.•Minor changes to the Cu level in the Si-rich alloys determine the stability of hexagonal Q phase relative to cubic β phase.