Reversion of natural ageing in Al-Mg-Si alloys.

Abstract Al-0.59Mg-0.79Si (wt.%) alloy was naturally aged for 2 weeks after solutionising and quenching to produce a high number density of atomic clusters. This caused an increase of both hardness and electrical resistivity. Samples were then subjected to reversion ageing (RA) by applying temperature treatments at 250 °C of 1 s–5 min duration. The associated changes of hardness, resistivity, positron lifetime and the DSC dissolution signal were measured. Subsequent secondary ageing at 20 °C was studied in an analogous way. Ideas are developed how solute supersaturation and vacancy fraction vary during RA. It is found that the most reverted state still contains enough clusters to trap positrons efficiently and to contribute to hardness. Longer ageing at 250 °C leads to the formation of coherent and later incoherent precipitates. The differences between the evolution of resistivity and hardness indicate that besides the level of solute supersaturation the Mg:Si ratio in the matrix changes during reversion. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]