Effects of Mg and Sc additions on the microstructure, mechanical properties, and thermal stability of a cast Al-2Li-2Cu-0.2Zr alloy after thermal exposure

Abstract

This work was undertaken to investigate the influence of independent and combined additions of Mg and Sc on the microstructure, mechanical properties, and thermal stability of cast Al-2Li-2Cu-0.2Zr base alloy after thermal exposure. All studied alloys were exposed at 100 °C for 1000 h after a prior artificial ageing of 24 h at 150 °C to simulate the thermal exposure under service conditions. A significant increase in strength took place in all alloys, accompanied by different variations of toughness as a result of exposure. It is proposed that the toughness reduction in base Al-2Li-2Cu-0.2Zr alloy should be primarily attributed to the precipitation of θ′′/θ′ (Al3Cu/Al2Cu) plates together with complementary nucleation and coarsening of δ′ (Al3Li) particles. Addition of Mg has a pronounced effect on the competitive precipitation behavior, which will suppress the precipitation of θ′′/θ′ plates and encourage formation of S′ (Al2CuMg) phase. Significant grain refinement and increased Al3(Li, Sc, Zr) precipitation was obtained with minor Sc addition, contributing to the significant enhancement of ductility. Sc addition is revealed to promote the precipitation of refined θ′′/θ′ plates, primarily contributing to the significant strength enhancements. The additions of Mg and/or Sc could efficiently improve the strength and thereby reduce the susceptibility to exposure embrittlement of the base alloy during exposure. An excellent combination of strength (YS = 377 MPa, UTS = 478 MPa) and ductility (EL = 8.4%) after exposure could be obtained in the alloy with combined Mg and Sc addition.