Effect of cold compression on precipitation and conductivity of an Al-Li-Cu alloy

Transmission electron microscopy has been used to investigate the effect of increasing the degree of deformation applied by cold compression on the ageing kinetics and electrical conductivity response of an Al-Li-Cu alloy containing Mg and Ag. When cold compressed greater than 3%, the increased dislocation density accelerates the widespread precipitation of the T₁ phase resulting in an enhanced age hardening response. The lengthening rate of T₁ precipitates is also reduced in this cold compressed condition owing to the reduced local solute supersaturation, a result of the widespread precipitation of T₁ plates. Cold compression by less than 3% does not increase the age hardening response, and the precipitation of GP zones/θ″ appears to be suppressed. Precipitation of the T₁ phase is also not significantly enhanced compared with that of the more than 3% cold compressed conditions. The anomalous decrease in electrical conductivity is associated with the nucleation and growth of the T₁ phase. Strain fields around T₁ precipitates combined with the increased volume fraction of T ₁ are thought to be the cause of the anomalous conductivity behaviour.