Overview of the effects of impurities and rare earth elements in Al−Li alloys

Abstract

The current status of the effects of impurities and rare earth elements on the microstructure and the mechanical properties of Al−Li alloys has been surveyed in the present paper. Particular attention has been paid to the mechanical properties in respect of different conditions and the mechanisms responsible for both impurity-induced embrittlement and rare earth element microalloying. Al−Li alloys usually show a high sensitivity to impurity-induced embrittlement. High hydrogen concentration in Al−Li alloys results in severe hydrogen embrittlement. Impurities Fe and Si could form some brittle compound particles to produce the negative effects on the microstructure. Alkali metal impurities such as Na and K could result in the formation of grain boundary liquid, recrystallized grains, coarse particles distributed along grain boundaries and δ′ (Al 3Li) precipitation-free zones (δ′-PFZ). By reducing the tendency for brittle intergranular fracture, the effects of rare earth elements, such as cerium and scandium, as alloying additions, on the microstructure and the mechanical properties are usually beneficial. Rare earth modification for Al−Li alloys improves ductility, fracture toughness and possibly reduces their anisotropy to a certain degree. These microalloying effects of rare earth additions might suppress the impurity impairment. Adding an amount of rare earth elements somewhat reduces the stress concentration sensitivity in high strength Al−Li alloys while the notch strength is hardly affected. Though the microalloying of rare earth elements improves the mechanical properties, in particular, the ductility of smooth specimens, high strength Al−Li alloys still have high sensitivity to stress concentrations. This might be a possible obstruction to the widespread application of high strength Al−Li alloys in primary aerospace structures containing geometry notch design. In order to achieve an essential improvement in the ductility and fracture toughness of Al−Li alloys, it is necessary to find further efficient alloying approaches associated with rare earth constitutents.