The basic processes affecting two-step ageing in an Al-Mg-Si alloy

Abstract

A study has been made, by thin-film electron microscopy, of the structures resulting from two-step ageing treatments of Al-1·2% Mg₂Si. It is shown that the number, and hence the size, of the needle precipitates in the fully aged condition can be influenced appreciably by the ageing which occurs during the first of the two ageing treatments. This pre-treatment can lead to either a refinement or a coarsening of the final aged structure, in a controlled manner, depending upon conditions. A basis for a theoretical model to explain the detailed behaviour resulting from two-step ageing has already been suggested (Pashley et al. 1966), and this kinetic model has been developed further in order to explain the extensive microstructural observations. There is excellent qualitative agreement between the predictions of the model and the experimental results. It is suggested that the model should apply to other alloy systems.