Significant roles of minor Mg addition in microstructural evolution and mechanical properties of cast Al–3Li–2Cu-0.15Zr alloy

Abstract

The impact of minor Mg addition on microstructure and mechanical properties of cast Al–Li–Cu–Zr alloy with high lithium content (3 wt % Li) were studied to understand a qualitative assessment of sequence and kinetics of multiple precipitations and its effect on mechanical properties. Scanning Electron Microscope and X-ray diffraction results suggested that raising the content of Mg gradually altered the grain structure of the as-cast alloys and increased the types and volume fraction of the grain boundary intermetallic phase. Unusually, solvus temperature is higher than solidus temperature in 0.5 Mg alloy (Al–3Li–2Cu-0.5Mg-0.15Zr, wt. %), resulting in the incipient melting of the matrix before the complete dissolution of the intermetallic phases. Prominent enhancements in both age-hardening and precipitation kinetics of the base alloy with the addition of Mg were also easily identified, indicating the important role of Mg during isothermal aging. Transmission Electron Microscope and atom probe tomography results showed that minor Mg addition (0.2 wt %, denoted by 0.2 Mg alloy) could encourage the diffusion of Cu atoms by the aggregation of Mg–Cu-vacancy co-clusters, thereby promoting the nucleation of the T1-Al2CuLi and θ′-Al2Cu precipitates. In addition, S′-Al2CuMg precipitate was therefore introduced as a result of Mg addition. With further increasing the content of Mg to 0.5 wt %, the nucleation and growth of S′ precipitate would be promoted at the expense of T1 and θ′ precipitates, especially the latter. Based on the theoretical models and regression analysis, the formation mechanism of δ′-Al3Li precipitate-free-zones gradually changed from the initial vacancy-depletion mechanism to the solute-depletion mechanism, and Mg element could significantly inhibit the coarsening of δ′ precipitate-free-zones. The best balance between ductility and strength was obtained by 0.2 Mg alloy aged for 32 h, and the alloys have significant advantages in terms of density (2.437 g/cm3) and elastic modulus (82.65 GPa) over conventional cast aluminum alloys (density: 2.7–2.8 g/cm3; elastic modulus: 72 GPa).