Precipitation Characteristics of Aluminium-Lithium Alloys

The structural ageing characteristics of aluminium alloys containing 2 and 4 wt.-% lithium are reported. One transition phase (AI3Li, δ′) occurs in the alloy system and time-temperature-transformation curves have been determined for this phase and the equilibrium precipitate. Strengthening in these alloys is associated with δ′ precipitation, the peak hardness increment occurring in the 2% alloy after ageing for 24 h at 200°C (473K). Thin-film electron microscopy was used to confirm the ordered L12-typestructure of the δ′ and to study the precipitate size and distribution over a range of ageing temperatures. At low temperatures the δ′, which has a spherical morphology, is very fine (∼ 0.01 μm at 100°C, 373K) and resists coarsening. Above 200°C (473 K) coarsening is rapid and the process has been analysed using the Lifshits-Wagner theory. The δ′ interfacial energy was estimated to be ∼ 240 erg/cm2 (0.24 J/m2) and the lattice mismatch 0.18%. At and below 60°C (333K), ageing is accompanied by an increase in electrical resistivity; this has been interpreted as a result of electron scattering from very small ordered particles of δ′.