Observations of the effect of Zn on precipitation processes in an Al-Cu-Mg-Ag base alloy

This work examines the role of Zn additions to an Al-Cu-Mg-Ag alloy, focussing on the effects on hardening and microstructural evolution at 150 and 200°C in the T6 and T7 tempers. A detailed characterisation using analytical transmission electron microscopy in conjunction with hardness measurements showed that θ″ and θ′ predominate at temperatures below the GP zone solvus, whereas the Ω phase forms abundantly during ageing at 200°C. The effect of pre-ageing at room temperature prior to artificial ageing caused a slight increase in the level of hardening of both the Zn free and Zn containing alloys. Energy dispersive X-ray microanalysis of the Ω phase revealed an association of Zn and an increased stability of this phase was linked to this chemical difference.