ALUMINIUM-LITHIUM-COPPER-MAGNESIUM-ZIRCONIUM ALLOYS WITH HIGH STRENGTH AND HIGH TOUGHNESS - SOLVING THE PERCEIVED DICHOTOMY

Throughout the past decade extensive research and development has been carried out on aluminium-lithium base alloys because of the attractive combination of lower density and higher modulus that can be achieved in this system compared with "conventionalw aluminium alloys.Much of this effort has been directed at understanding and overcoming their "Achilles heel" of low ductility and poor fracture toughness (particularly for crack planes perpendicular to the short transverse direction).This study reviews the metallurgical features which affect ductility and damage tolerance in the 8090 and 8091 alloys developed at Alcan and the Royal Aircraft Establishment.The paper shows that control of these metallurgical features can be used to markedly improve these properties.Three examples will be given.In the first case the production of material with a high damage tolerance will be discussed.It will be shown that by control of grain structure and ageing practice, 8090 sheet with a plane stress fracture toughness (Kc) of >I30 MPa/iii can be achieved at similar strength levels to 2024-T3 sheet.There is also a decrease in fatigue crack growth rate.This results in sheet with an overall damage tolerance capability comparable with that of 2024-T3.The other examples concern the processing of 8090 and 8091 plate to meet medium and high strength airframe requirements respectively.8090 plate ( (100 mm thick) can achieve the property requirements of 7010-T7651, with short transverse ductilities in excess of 3% and short transverse fracture toughness of >18 MPa /iiT.This is accomplished by controlled thermomechanical treatment.Similar treatments applied to the 8091 alloy result in a material competitive with 7150-T651 for specific ap~lications (e.g.upper wing skins,where short transverse performance is sacrificed in favour of in-plane properties).