Natural and artificial ageing in aluminium alloys – the role of excess vacancies

Non-equilibrium excess vacancies quenched-in after solutionising age-hardenable aluminium alloys at high temperature are known to play an important role in precipitation at lower temperatures. However, knowledge is still lacking on the extent of the contribution to precipitation at various temperatures. In this work, we revisit this classical problem and study the role of excess vacancies in an Al-Mg-Si alloy in natural and artificial ageing experimentally by hardness measurements, differential scanning calorimetry, and positron annihilation lifetime spectroscopy. We then apply a precipitation model involving a simulation of vacancy loss and solute diffusion based on parameters calculated by first principles. The experiments show that excess vacancies are largely removed in the initial seconds of artificial ageing with hardly any corresponding hardening, i.e., subsequent hardening is triggered primarily by equilibrium vacancies unlike natural ageing, where hardening is mainly driven by excess vacancies. We reproduce the anomaly that hardening for a given time can be faster at lower temperatures and explain this by the different activation energies of vacancy annihilation and solute diffusion. The role of excess vacancies demonstrated in Al-Mg-Si alloys could be similar in other age-hardenable alloys and hence be a universal phenomenon.