The composition of Guinier–Preston zones in Al–Zn–Mg alloys

Abstract

The composition of GP zones in the metastable equilibrium state at room temperature was studied in a series of Al–Zn–Mg alloys with Zn concentrations from 0–72 to 2–5 at.% and Mg concentration!) from 0–61 to 3–33 at.%. It was found that the volume fraction of GP zones in the equilibrium stale is determined by the Zn content of the alloys. The Zn concentration inside the GP zones was determined by X–ray small–angle scattering and was found to be 42–2 at.% and that in the remaining matrix 0–74 at.%. Further analysis gives the approximate positions and 3lopes of tho tie–lines and the Mg concentrations in the metastable miseibility gap. With knowledge of the this gap, the contribution of GP zones to the electrical resistivity of the alloy in the state of metastable equilibrium at room temperature was determined. It was found that the electrical resistivity contribution per unit volume fraction of zones depends to a good approximation on the linear combination of the Zn and Mg concentrations in the zones. The contribution of Mg atoms to the electrical resistivity of GP zones was found to be 1–8 times larger than that of the Zn atoms. The heat of dissolution of the room-temperature GP zones was determined by differential scanning ealorimetry measurements and was found to increase linearly with increasing volume fraction of zones yielding 2–9 J g⁻¹ %⁻¹ for the heat of dissolution of one percent volume fraction of zones. This value appears to be independent of the composition of tho zones in the alloys investigated here.