The evolution of precipitate crystal structures in an Al-Mg-Si(-Cu) alloy studied by a combined HAADF-STEM and SPED approach

Highlights: • Scanning electron diffraction and machine learning complement lattice imaging • Developed approach gives increased objectivity in precipitate phase analysis • Precipitate phase fractions and solute content obtained with superior statistics • 0.01 at.% Cu changes the Al-Mg-Si system precipitation during overageing • Q'-phase eventually dominates precipitate structures and shows varying Cu-occupancy - Abstract: This work presents a detailed investigation into the effect of a low Cu addition (0.01 at.%) on precipitation in an Al-0.80Mg-0.85Si alloy during ageing. The precipitate crystal structures were assessed by scanning transmission electron microscopy combined with a novel scanning precession electron diffraction approach, which includes machine learning. The combination of techniques enabled evaluation of the atomic arrangement within individual precipitates, as well as an improved estimate of precipitate phase fractions at each ageing condition, through analysis of a statistically significant number of precipitates. Based on the obtained results, the total amount of solute atoms locked inside precipitates could be approximated. It was shown that even with a Cu content close to impurity levels, the Al-Mg-Si system precipitation was significantly affected with overageing. The principal change was due to a gradually increasing phase fraction of the Cu-containing Q′-phase, which eventually was seen to dominate the precipitate structures. The structural overtake could be explained based on a continuous formation of the thermally stable Q′-phase, with Cu atomic columns incorporating less Cu than what could potentially be accommodated.