Observations on the Effects of Cu, Sc, Ti and Zr Addition on Precipitation in Al-Li-Based Alloys

The present study investigates the effect of the addition of 3% Cu, 0.3% Zr and 0.19% Sc on the precipitation behavior of Al-2%Li-0.3%Zr alloys solidified at 0.15 °C/s in the as-cast as well as T6 and T7 conditions. Microstructure was examined using optical, field emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy (STEM). The results reveal that the coherent δ′ (Al₃Li) phase particles are the principal hardening source in Al-Li alloy. Considering the other two alloys, δ′ (Al₃Li), the T1 (Al₂CuLi), and the θ′ (Al₂Cu) phases are the major hardening agents. The addition of Sc resulted in the precipitation of dispersoid particles in the liquid state in the form of star-like particles with composition close to Al₃(Sc, Zr, Ti). This type of dispersoids consists of several layers, with each layer having its chemical composition. During T6 treatment, fine Al₃Sc particle precipitates at the grain boundaries, restricting the grain growth leading to about 85% reduction in the original grain size. Spheroidization and coarsening of Al₂Cu precipitates occur at the cost of the dissolution of the small ones (Ostwald ripening mechanism). In the T6 condition, coherent Al₃Sc particles have complex crystallographic aspects. The average interplanar spacing is about 2.32 Å and 2.18 Å for the Al₃Sc particle and Al, respectively. The results were backed by measuring the microhardness of samples aged in the range of 160 °C to 200 °C for times up to 30 h. Under these conditions, the peak hardness of the aged Al-Li, Al-Li-Cu and Al-Li-Cu-Sc alloys are, respectively, 97.2 HV, 163.6 HV and 182.6 HV.