Precipitate coarsening during overaging of Al–Zn–Mg–Cu alloy

Abstract

The overaging kinetics of Al–Zn–Mg–Cu alloy 7050 at 430–455 K (157–182°C) was investigated by electrical resistivity and tensile property measurements, which were supplemented by TEM and differential scanning calorimetry (DSC). Overaging in this alloy system was shown to follow classical coarsening behaviour as characterised by a time^-1/3 particle size dependence. Changes in solute supersaturation, as measured by electrical resistivity, could be directly related through the Gibbs–Thompson– Freundlich equation to particle size variations, which were inferred from the DSC results. Strength changes, when interpreted in terms of the dislocation looping mechanism, also fit the classical coarsening theory. The activation energy for the coarsening reaction was estimated to be about 160 kJ mol^-1, which is 30–40 kJ mol^-1 higher than that for solute (Mg,Zn) diffusion. This difference is ascribed to the solution enthalpy of the coarsening precipitate. Precipitate dissolution under non-isothermal conditions (DSC) had an activation energy (∼135 kJ mol^-1) closer to that for diffusion.