THE INFLUENCE OF ZIRCONIUM ON THE EARLY STAGES OF AGING OF A TERNARY AL-ZN-MG ALLOY

The commercial 7000 series aluminum alloys are based on Al-Zn-Mg and A l-Zn-Mg-Cu systems. These alloys commonly contain trace amounts of zir conium for grain refining purposes. In this paper, the influence of zi rconium on the early stages of aging of an Al-Zn-Mg alloy is considere d. It is shown that two different kinds of microstructures are develop ed for materials with and without zirconium when heat treated slightly above the GP (Guinier Preston) zone solvus. For the ternary alloy, th e microstructure consists of both metastable MgZn, (eta') phase and '' stable GP zones(/clusters)''; the stable GP zones act as the nucleatio n sites for the eta' phase. Whilst, in the case of the zirconium beari ng alloy, the microstructure consists only of a heterogeneous distribu tion of under developed eta' precipitates. These observations are disc ussed in relation to the critical role of vacancies in the formation a nd stability of GP zones and precipitate nuclei, and of zirconium in r educing the total numbers of available excess vacancies in these mater ials.