Genetic structural phase evolution from Li-containing S-like phase precipitates towards S-phase in AlCuLiMg alloys

The S-phase is one of the most effective strengthening precipitates in the aluminum alloys for aerospace applications. Its precipitation mechanism upon thermal aging seems to have been well revealed and understood in the studies of AlCuMg alloys. Here we report an entirely different scenario of S-phase formation in an AlCuLiMg alloy. Using atomic-resolution imaging in transmission electron microscopy, it is shown that upon aging this alloy can form a precipitate microstructure dominated by Li-containing Guinier-Preston-Bagaryatsky zones and so-called S-like phase precipitates that are previously unknown. Upon formation a S-like precipitate initiates with a monoclinic structure of Al ₂CuMg _2/3(MgLi) _1/3, and may then genetically evolve with a dynamic composition of Al ₂CuMgLi _x (1/3 ≥ x > 0) towards the orthorhombic S-phase (Al ₂CuMg). This phase change is more a genetic structural phase evolution rather than a conventional phase transformation. Our study shows that it is possible for AlCuLiMg alloys to form a featured microstructure overwhelmed by S-like/S-phase and T ₁-phase precipitates, without the commonly observed δ′/θ′/δ′ composite precipitates.