ABSTRACT
Lithium has always been of interest to research metallurgists as it is one of only eight elements whose solid solubilities in aluminium exceed 1 at% and can promote precipitation strengthening effects, thereby offering the possibility that useful engineering alloys could be developed. It was realized in the early 1900s that binary aluminium-lithium alloys were of little industrial use due to low strength, low ductility and high brittleness, but that lithium additions could enhance the properties of aluminium alloys containing other soluble elements such as copper and zinc. In particular, with lithium being the lightest metallic element, its incorporation at appropriate levels into aluminium based alloys could lead to useful density reductions. The first such alloy to be fully commercialized was the 2020 alloy registered in the late 1950s by Alcoa, of nominal composition Al-4.5Cu-1.1Li-0.5Mn0.2Cd. Due primarily to its high compressive strength, increased modulus (arising from lithium additions in excess of 1wt%) and enhanced elevated temperature strength and stability, with reduced density a secondary advantage, 2020 was widely incorporated in the wings and horizontal stabilizer of the RA-5C Vigilante Mach 2 attack and reconnaissance aircraft. A total of 117 Vigilantes was built from 1958 onwards incorporating 2020, giving over 25 years of active and operational service with the US Navy without major problems, but special design and manufacturing procedures had to be instigated to accommodate low toughness, low ductility and relatively high notch sensitivity. These deficiencies led to difficulties in meeting the increasing damage tolerance requirements of the mid to late 1960s and, consequently, there were no further applications for 2020 after the last Vigilante and spares were made.
