Role of Cu on the mechanical properties and microstructures evolution of Al-xCu-1Li-0.4Mg-1Zn-0.1Zr alloys

In this paper, mechanical properties and microstructures of the cast and extruded Al-xCu-1Li-0.4Mg-1Zn-0.1Zr alloys (x = 2.0, 3.0, 4.0, 4.5, 5.5, 6.5 wt%) were investigated. The results showed that the as-cast micrographs of alloys with different Cu content primarily consist of dendritic alpha-Al grains, and the intermetallic phases at the grain boundaries were Cu and Zr rich phases. After solution treatment, no recrystallization took place in extruded alloys, and the alloys mainly represented mixed textures composed of <111> and <100> fibers. Cu significantly affected the precipitation behavior of the extruded alloys. Only T-1 (Al2CuLi) phases were observed in T6 (aging at 175 degrees C for 32 h) and T8 (3% pre-strain followed by aging at 150 degrees C for 32 h) treated alloy with 2 wt% Cu. Further increasing Cu contents (4.5 and 6.5 wt%) enabled the co-precipitation of T-1 and theta' (Al2Cu) phases, as well as increased their density. In addition, the microstructures of grain boundaries of alloys with different Cu content showed dramatic differences. T-1 phases precipitated at the grain boundaries of alloy containing 2 wt% Cu, while rare precipitation of T-1 phases was observed at the grain boundaries of alloy with 6.5 wt% Cu. The yield strength (YS) of the alloys increased with the Cu content and the elongation (EL) showed an opposite trend. A good combination of ultra-high strength (YS = 684 MPa, ultimate tensile strength = 726 MPa) and ductility (EL = 6.3%) was obtained in T8 treated alloy containing 4.5 wt% Cu. The presence of the undesired coarse T-2 (Al6CuLi3) phases at the grain boundaries resulted in the premature failure of the high Cu content alloys.