Influence of low additions of copper on properties of Al–Li–Mg–Zr alloy

Abstract

Low additions of Cu have been made to an Al–2·5Li–2Mg–0·14Zr (wt-%) alloy in order to improve its mechanical properties. The material was cast, homogenised, extruded as rod, solution treated, and aged at 170°C. Mechanical properties were tested using tensile and short rod fracture toughness specimens. Transmission electron microscopy was carried out to observe the microstructure and scanning electron microscopy permitted a study of the fracture surfaces of the tensile specimens. Field ion microscopy combined with atom probe analysis was used to locate the Cu atoms in the aged materials. It was found that the Cu additions increased the strength and decreased the fracture toughness. The optimum combination of mechanical properties was obtained in the underaged condition with an addition of 0·l%Cu. It is suggested that low additions of Cu influence the mechanical properties by affecting the nucleation of the strengthening δ′ precipitates and ultimately determine the size distribution and/or volume fraction of these precipitates. A mechanism based on Li–Cu–vacancy complexes is proposed to explain this effect.MST/1826