Abstract
It is demonstrated that good thermal stability in Al-Mg-Si-Cu aluminum alloys correlates with a high density of fine lath-shaped, Cu-containing, disordered L-precipitates. Alloys optimized for L retained hardness above 90 HV after 3 weeks over-aging at 473 K (200 °C). Further improvement was achieved by substituting Si by Ge in one alloy. High-angle annular dark-field scanning transmission electron microscopy showed that at peak-hardness conditions, L coexists with more common needle-shaped precipitates, often with Cu-enriched interfaces.