Vacancy–Mg complexes and their evolution in early stages of aging of Al–Mg based alloys

Highlights: • Evolution of vacancy–solute complex in Al–Mg based alloys is studied during thermal treatment. • Vacancy–Mg complexes are observed in Al–Mg based alloys in the as-quenched state. • After aging at 180 °C, vacancy–Mg complexes decompose and evolve to Mg clusters. • Increase in hardness after aging is possibly due to formation of Mg clusters. - Abstract: The evolution of vacancy–solute complex in Al–Mg based alloys with different Mg contents during aging process was studied by positron annihilation spectroscopy together with Vickers micro-hardness and transmission electron microscopy measurements. For quenched Al–Mg based alloys, no obvious change in positron lifetime is observed during natural aging process. While during artificial aging at 180 °C after subsequent quenching, the positron lifetime and Doppler broadening S parameter show a fast decrease after the initial 1 min aging. Further aging for more than 10 min causes increase of these parameters. Coincidence Doppler broadening measurement indicates formation of vacancy–Mg complexes even in the as-quenched samples. Natural aging at room temperature has no effect on these vacancy–solute complexes. It is believed that with artificial aging the microstructure of Mg transforms from uniformly scattered vacancy–Mg complexes to Mg clusters. The Vickers micro-hardness changes slightly during the artificial aging, which is supposed to be related with the formation of Mg clusters