An insight into the precipitate evolution and mechanical properties of a novel high-performance cast Al-Li-Cu-Mg-X alloy

Abstract

High-performance cast Al-Li alloys are highly desirable for a wide range of weight-critical and stiffness-critical applications. This work was undertaken to investigate the microstructural evolution, coarsening kinetics of main precipitates and related mechanical characteristics of a newly-developed cast Al-Li-Cu-Mg-X alloy during aging at 150, 175, 200 and 225 °C. The as-aged alloys studied primarily comprised of δ′ (Al3Li), T1 (Al2CuLi), S′ (Al2CuMg) and core-shell Al3(Li, Sc, Zr) phases. δ′ phases exhibited the relatively lowest thermal stability. The increasing aging temperature led to the greatly enhanced coarsening rate of δ′. It was found that the coarsening of metastable δ′ was a diffusion-controlled process and a linear relationship of cube average particle radius (R̅3) against aging time t was revealed. The activation energy for the δ′ coarsening was estimated to be ~98.7 kJ/mol, which was somewhat lower than those in wrought Al-Li alloys. Besides, the activation energy for the growth of T1 plate was measured to be ~95.6 kJ/mol, which was close to that of grain boundary diffusion of Cu in α-Al (~105 kJ/mol), indicating that the growth of T1 should be governed by the diffusion rate of Cu toward T1 plates. With the successively increased aging temperature and prolonged aging time, the number density and volume fraction of δ′ particles were significantly reduced, primarily contributing to the significant strength reduction. The relatively best combination of strength and ductility was achieved in the studied alloy aged at 175 °C for 32 h.