Abstract

The impact of different industrial heat treatments on the precipitation kinetics and phase transformation of an Al-Li alloy is studied. AA2195 (Al -1.69 at.% Cu-3.87 at.% Li-0.39 at.% Mg-0.06 at.% Ag-0.04 at.% Zr), a commonly used aluminum alloy, was selected as a model system in this study. Evaluation of the microstructure is performed using differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and field ion microscopy (FIM). Three-dimensional atom probe tomography (APT) was utilized to investigate the intermetallic precipitates in the microstructure after applying T4, T8 and T6 heat treatment tempers. After conducting the T4 heat treatment, β' dispersoids were found to potentially act as nucleation sites for the T₁ phase. The segregation of Li and Cu to the β'/ matrix interface is reported. The microstructure of the samples subjected to the T8 heat treatment exhibit the intersections of different T₁ platelets, which favor a mechanism to hinder the motion of dislocations, resulting in a high hardness value. Growth of the T₁ platelet which proceeds via a ledge mechanism and the dissolution of the θ' phase were observed with increasing aging temperature. After conducting the T6 heat treatment, the microstructure predominantly consists of the T₁ phase. The nucleation of the T₁ platelets on Mg enriched regions that contain localized Ag atoms was observed, suggesting a potential role of Mg to enhance the nucleation of the T₁ phase.