The effect of major constituents on microstructure and mechanical properties of cast Al-Li-Cu-Zr alloy

Abstract

The effect of major constituents (the Li content increases from 0.5 to 3 wt% and the Cu content decreases from 4.5 to 2 wt%) on microstructure and mechanical properties of cast Al-Li-Cu-Zr alloy were studied to understand a qualitative assessment of sequence and kinetics of multiple precipitations. Increasing the Li content progressively raises the melting temperature from 520 °C in 1# alloy (Al-0.5Li-4.5Cu, wt%) and 2# alloy (Al-1Li-4.5Cu) to 560 °C in 6# alloy (Al-3Li-2Cu). In addition, the variations of the major constituents result in the changes of the type and volume fraction of the inter- and intra-granular intermetallic phases. TEM results indicate that minor modifications to alloy composition greatly alter precipitation behavior during isothermal aging. The Li-poor alloy (2# alloy) presents a much shorter incubation time of T1-Al2CuLi precipitate than the Li-rich alloy (Al-1.5Li-4.5Cu, 3# alloy), while the nucleation of θ'-Al2Cu precipitate is promoted in Li-rich alloy. No typical spherical δ'-Al3Li precipitate is observed in 2# and 3# alloys during isothermal aging, except for the as-quenched 3# alloy. The preferential nucleation of δ' precipitate on the coherent and broad face of θ' precipitate is observed in 2# and 3# alloys, leading to the formation of “sandwich-like” δ'/θ'/δ' phases. Further decreasing Cu content to 2 wt% and increasing Li content to 3 wt% (6# alloy), the predominant phase in the matrix is δ' precipitate, accompanied by a small amount of unevenly distributed θ' and T1 precipitates. The best balance between ductility and strength is obtained by 2# alloy aged for 32 h, but 6# alloy has significant advantages in terms of density (2.437 g/cm3) and elastic modulus (82.65 GPa) over 2# alloy (density: 2.675 g/cm3; elastic modulus: 75.45 GPa).