MECHANISM OF NUCLEATION AND PRECIPITATION IN 6061-LI ALLOYS

Investigations into the hardening response of 6061 and 6061-Li alloys have been carried out using analytical electron microscopy. A nucleati on and precipitation mechanism has been suggested to explain the modif ications of the aging behaviour of 6061 caused by the Li additions. Ac cording to this mechanism, the preferential clustering of Li and vacan cies inhibits the formation of dislocation loops and diffusion of Si a nd Mg atoms into zones so that the precipitation of needle shaped Guin ier-Preston (GP) zones is limited and delayed. As a partial replacemen t of the needles, AlLiSi with a small lattice strain is formed through the preferential association of Li-nu (where nu denotes vacancies), S i-nu, and Al. Consequently, a dual precipitation behaviour of GP zones and AlLiSi occurs in 6061-0.7 Li, and an extremely restrained dual pr ecipitation of this type is also found in 6061-2.0 Li, in which delta' is dominant. In contrast to the initial inhibition, a promotion of ne edle shaped GP zones is observed on prolonged aging, these being forme d 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.