The decomposition of the solid solution state in the temperature range 20 to 200° C in an Al-Zn-Mg alloy

Abstract

The decomposition of the supersaturated solid solution of an Al-3.2wt% Zn-2.2 wt% Mg alloy has been investigated in the temperature of 20 to 200° C, by small-angle X-ray scattering, electrical resistivity and mechanical measurements. On the basis of the results obtained, three subsequent stages of the decomposition process can be distinguished. Between 20 and 70° the basic process is the nucleation and growth of G.P. zones, the volume fraction of which increases logarithmically with time. A transition stage is observed between 80 and 100° in which the volume fraction increases linearly with time. Above 90° C, the growth kinetics of the volume fraction shows a definite incubation period at the beginning of ageing, while the yield stress increases monotonically. In the temperature range 100 to 160° C, the formation of theη′ phase takes place. Below 100° a linear connection between the yield stress and (fR)^1/2 is found from which the specific surface energy to cut a G.P. zone is calculated to be Γ₀= 0.21 Nm m⁻².