Precipitation in Al–Zn–Mg–(Cu, Zr) Alloys

The effect of Cu and Zr on the precipitation and reversion processes in high strength Al–Zn–Mg alloys was investigated by compression and microhardness tests and by electron microscopy. The addition of Cu increases the supersaturation in the quenched alloy strongly changing thereby the upper temperature limit of Guinier-Preston (GP) zone formation, the reversion of GP zones and the transition from GP zone to intermediate phase. The grain size of the Zr containing alloy is by more than an order of magnitude smaller than that of the Al–Zn–Mg and the Al–Zn–Mg–Cu alloys. Because of the large fraction of material near grain boundaries, the concentration changes caused by segregation and grain boundary precipitation have a pronounced effect on the precipitation process in the Zr containing alloy.