  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.
