The effect of thermo-mechanical treatment on the formation of T1 phase and δ′/θ′/δ′ composite precipitate in an Al-Cu-Li-Mg alloy

Abstract

The T1-precipitates and δ′/θ′/δ′ composite precipitates are the two primary strengthening precipitates and co-exist in the age-hardened 3rd generation AlLi alloys. The relative proportion of these two types of precipitates formed in the alloys is strongly dependent of the thermo-mechanical-treatment (TMT) processes applied. In the present study, using electron microscopy and properties characterization, the competitive precipitation behaviors of these two types of precipitates in an Al-4.0Cu-0.9Li-0.2Mg (wt.%) alloy, and the property of the alloy were investigated systematically in relation to TMT processes employed. It is shown that a 5% pre-deformation can promote the formation of T1 precipitates but depress the δ′/θ′/δ′ composite precipitates, which improves the strength but decreases the plasticity of the alloy significantly. A pre-natural-aging will promote the fine δ′/θ′/δ′ composite precipitates and T1 precipitates to form uniformly during subsequent artificial aging, improving the strength while maintaining good plasticity. A pre-artificial-aging at 180 °C will depress the formation of T1 precipitates, and greatly reduce the plasticity of the alloy. Our study indicates that when the T1 and the δ′/θ′/δ′ precipitates are equally developed, the alloy may achieve optimized properties.