A model for co-clusters and their strengthening in Al–Cu–Mg based alloys: a comparison with experimental data

A model for the thermodynamics of and strengthening due to Cu–Mg co-clusters in Al–Cu– Mg based alloys is analysed and tested. The formulation uses a single interaction enthalpy between dissimilar alloying elements (e.g. Cu and Mg atoms in an Al–Cu–Mg based alloy) combined with the configurational entropy. The metastable solvus in Al–Cu–Mg based alloys is calculated. Recently published small angle X-ray scattering experiments, 3 dimensional atom probe and yield strength data on these type of alloys support the model. The small angle X-ray scattering and hardness experiments, as well as calorimetry experiments, are sensitive to the main free energy (or enthalpy) changes, which are dominated by Cu–Mg bonds formed by the dimers and the local electron densities related to these bonds. 3 dimensional atom probe is less sensitive to dimers, and will detect agglomeration of dimers to form larger clusters.