Aging phenomena in Al-Zn-Mg alloys ranging from (α+T) to (α+η) phase region

Abstract

Aging phenomena in Al-Zn-Mg alloys having 5.5wt% of (Zn+Mg) and ranging from the (α+T) to the (α+η) phase region were investigated by means of hardness, electrical resistivity and specific heat measurements and transmission electron microscopy. Six alloys were solution treated at 470°C for 1hr, quenched into iced water and aged at various temperatures between 50 and 150°C. The results obtained were as follows: The tendency that the initial aging rate increased with the increasing ratio of Zn/Mg was clearly observed in the (α+T) type specimens. However, the initial rate in the (α+T+η) and the (α+η) type specimens was nearly the same as that in the (α+T) type specimen with high Zn contents. As the ratio of Zn/Mg increased, all the specimen tended to be overaging. Therefore, the (α+T) type specimen developed the largest hardness when it was aged between 125 and 150°C. Electron micrographs as well as selected area diffraction patterns showed that nearly the same intermediate phase, η', appeared in all the specimens.