Composite precipitates in a commercial Al–Li–Cu–Mg–Zr alloy

Composite precipitates with duplex and triplex association of single precipitates commonly found in commercial Al–Li alloys were observed by transmission electron microscopy (TEM) in a re-solution and quenched AA2090 alloy after aging. These novel precipitates were the duplex association of δ′(Al3Li) and T1 (Al2CuLi) as well as the triplex δ′/β′(Al3Zr)/T1 and δ′/β′/θ′(Al2Cu). The reduction of elastic energy associated with composite precipitate formation is suggested as a basic mechanism with plate-like T1 and θ′ particles serving as preferential sites for nucleation of either δ′ or δ′/β′ spherical particles, which although in much smaller numbers, represent a volume fraction of 10 to 20% of all precipitates.