The Role of Vacancies in the Aging of Al-Mg-Si Alloys

In this paper the role of vacancies in the aging of Al-Mg-Si alloys is examined and novel concepts to improve their aging behavior are presented. It has been proposed that the technologically favored fast nucleation of the major hardening phase during artificial aging requires quenched-in vacancy assisted diffusion. The well-known interdependence of natural aging and subsequent artificial in Al-Mg-Si alloys can be understood in terms of quenched-in vacancy trapping in Mg/Si-clusters formed during natural aging. Diffusion during artificial aging is then determined by the dissolution of these vacancy-containing Mg/Si-clusters. This simple concept can guide the development of strategies to avoid the negative effect of natural aging. It is shown that the aging behavior of Al-Mg-Si alloys can be improved not only by processing related measures, but also by compositional interventions, which apply the following recipe: (i) avoid the trapping of vacancies in Mg/Si-clusters, (ii) prevent the vacancy annihilation during RT, and (iii) make them available for diffusion during artificial aging. It is shown that this strategy can be executed in Al-Mg-Si alloys by adding defined trace amounts of elements with an attractive binding energy to vacancies and sufficient solubility in the aluminum matrix.