AbstractInvestigations into the hardening response of 6061 and 6061–Li alloys have been carried out using analytical electron microscopy. A nucleation and precipitation mechanism has been suggested to explain the modifications of the aging behaviour of 6061 caused by the Li additions. According to this mechanism, the preferential clustering of Li and vacancies inhibits the formation of dislocation loops and diffusion of Si and Mg atoms into zones so that the precipitation of needle shaped Guinier–Preston (GP) zones is limited and delayed. As a partial replacement of the needles, AILiSi with a small lattice strain is formed through the preferential association of Li–v (where v denotes vacancies), Si–v, and AI. Consequently, a dual precipitation behaviour ofGP zones and AILiSi occurs in 6061–0·7Li, and an extremely restrained dual precipitation of this type is alsofound in 6061–2·0Li, in whichδ′is dominant. In contrast to the initial inhibition, a promotion of needle shaped GP zones is observed on prolonged aging, these being formed with the aid of defects. The inhibited nucleation and precipitation and the reduced age hardening can be improved either by increasing the Si excess or by applying two step aging.MST/1972