EFFECTS OF DELETERIOUS IMPURITIES AND CERIUM MODIFICATION ON INTRINSIC AND EXTRINSIC TOUGHENING LEVELS OF AL-LI BASED ALLOYS

The toughness of Al-Li-Cu-Zr and Al-Li-Cu-Mg-Zr sheets containing vari ous impurities and cerium concentrations, and Al-Li-Mg-Zr sheets conta ining various cerium concentrations under two aged conditions has been investigated by establishing the variabilities of intrinsic and extri nsic toughening levels and by determining the fracture toughness and t he tensile ductility. The relationships of intrinsic and extrinsic tou ghening levels with fracture morphologies and microstructural paramete rs have been discussed. Iron and silicon impurities detract from the i ntrinsic toughening level although they do not have any important infl uence on the extrinsic toughening level. Sodium and potassium impuriti es 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 Al-Li-Cu-Zr alloy containing higher concentrations of iron and sili con impurities, resulting in an improvement of the fracture toughness. A small amount of cerium addition in Al-Li-Cu-Mg-Zr alloy rich in eit her Fe + Si or Na + K increases the intrinsic toughening level but dec reases the extrinsic toughening level so that, overall, the fracture t oughness is hardly affected. A certain amount of cerium addition in Al -Li-Cu-Mg-Zr alloy rich in iron and silicon improves the fracture toug hness because both the intrinsic toughening level and the extrinsic to ughening level are enhanced. However, for the Al-Li-Mg-Zr alloy, ceriu m modification does not bring about any improvement in the fracture to ughness as the extrinsic toughening level is generally damaged. When i mpurities result in increased crack branching or when cerium addition promotes more mal ked lamellar type splitting, the extrinsic toughenin g level can usually be increased.