Characterizing and modeling the precipitation of Mg-rich phases in Al 5xxx alloys aged at low temperatures

Al 5083 alloys (5.25 at.% Mg) of different tempers (H131 and H116) were aged at low temperatures (50 and 70 °C) for 41 months. Scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS), and atom probe tomography (APT) were applied to characterize precipitates formed in the sensitized samples. Experimental results revealed that the size of Mg-rich precipitates increased with aging time at 70 °C for both alloys. APT results showed that Mg-rich precipitates of different Mg concentrations and morphologies formed in Al matrix and on the interface of Al matrix/pre-existing particles. In addition, a model based on local equilibrium of chemical potential and multi-class precipitates number evolution was adopted to predict the multiphase precipitation process in the Al-Mg binary system. The overall trend of precipitate radius and number density predicted by the model matched well with the experimental results. Moreover, modeling results revealed that nucleation and coarsening occurred faster in Al 5083 H131 than in Al 5083 H116 when aged at same temperature. The high density of dislocations and the pipe diffusion mechanism of dislocations can be used to explain such behavior.