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 Al-Zn-Mg-Cu systems. These alloys commonly contain trace amounts of zirconium for grain refining purposes. In this paper, the influence of zirconium on the early stages of aging of an Al-Zn-Mg alloy is considered. It is shown that two different kinds of microstructures are developed for materials with and without zirconium when heat treated slightly above the GP (Guinier Preston) zone solvus. For the ternary alloy, the microstructure consists of both metastable MgZn, (eta') phase and ''stable GP zones(/clusters)''; the stable GP zones act as the nucleation sites for the eta' phase. Whilst, in the case of the zirconium bearing alloy, the microstructure consists only of a heterogeneous distribution of under developed eta' precipitates. These observations are discussed in relation to the critical role of vacancies in the formation and stability of GP zones and precipitate nuclei, and of zirconium in reducing the total numbers of available excess vacancies in these materials.