Effects of deleterious impurities and cerium modificationon intrinsic and extrinsic toughening levels of AI-Li based alloys

Abstract

The toughness of AI-Li-Cu-Zr and AI-Li-Cu-Mg-Zr sheets containing various impurities and cerium concentrations, and AI-Li-Mg-Zr sheets containing various cerium concentrations under two aged conditions has been investigated by establishing the variabilities of intrinsic and extrinsic toughening levels and by determining the fracture toughness and the tensile ductility. The relationships of intrinsic and extrinsic toughening levels with fracture morphologies and microstructural parameters have been discussed. Iron and silicon impurities detract from the intrinsic toughening level although they do not have any important influence on the extrinsic toughening level. Sodium and potassium impurities significantly degrade the intrinsic toughening level although they may benefit the extrinsic toughening level to a certain degree. Cerium modification increases the intrinsic and extrinsic toughening levels of AI-Li-Cu-Zr alloy containing higher concentrations of iron and silicon impurities, resulting in an improvement of the fracture toughness. A small amount of cerium addition in Al-Li-Cu-Mg-Zr alloy rich in either Fe + Si or Na + K increases the intrinsic toughening level but decreases the extrinsic toughening level so that, overall, the fracture toughness is hardly affected. A certain amount of cerium addition in AI-Li-Cu-Mg-Zr alloy rich in iron and silicon improves the fracture toughness because both the intrinsic toughening level and the extrinsic toughening level are enhanced. However, for the AI-Li-Mg-Zr alloy, cerium modification does not bring about any improvement in the fracture toughness as the extrinsic toughening level is generally damaged. When impurities result in increased crack branching or when cerium addition promotes more marked lamellar type splitting, the extrinsic toughening level can usually be increased.