Undesirable natural aging (NA) in Al-6xxx delays subsequent artificial aging
(AA) but the size, composition, and evolution of clustering are challenging to
measure. Here, atomistic details of early-stage clustering in Al-1\%Mg-0.6\%Si
during NA are studied computationally using a chemically-accurate
neural-network potential. Feasible growth paths for the preferred β′′
precipitates identify: dominant clusters differing from β′′ motifs;
spontaneous vacancy-interstitial formation creating 14-18 solute atom
β′′-like motifs; and lower-energy clusters requiring chemical
re-arrangement to form β′′ nuclei. Quasi-on-lattice kinetic Monte Carlo
simulations reveal that 8-14 solute atom clusters form within 1000 s but that
growth slows considerably due to vacancy trapping inside clusters, with
trapping energies of 0.3-0.5 eV. These findings rationalize why cluster growth
and alloy hardness saturate during NA, confirm the concept of ''vacancy
prisons", and suggest why clusters must be dissolved during AA before formation
of β′′. This atomistic understanding of NA may enable design of
strategies to mitigate negative effects of NA