Precipitation Process in an Al–Zn–Mg Alloy

Ageing characteristics in a supersaturated Al–6 wt% MgZn2 quasi-binary alloy have been investigated principally by a calorimetric method with particular emphasis on the thermal stability of Guinier-Preston zones. Ageing sequence in the alloy is as follows; supersaturated solid solution—G. P. zones—intermediate M′ phase—stable M (MgZn2) phase. It has been found that G. P. zones can be formed on ageing below 340°C which corresponds to the solvus temperature of the M phase. The intermediate precipitate phase M′ is formed preferentially on ageing below 70°C and the equilibrium M phase precipitates predominantly above 100°C.Reversion temperature of the zones increases with ageing time for a certain period, after which it has a tendency to saturate to a definite temperature depending on the ageing temperature. The reversion temperature varies from 170°C for the alloy aged at room temperature to 300°C for the alloy aged at 190°C.It has been found that the activation energy for the reversion of G. P. zones determined from the specific heat versus temperature curve depends upon the ageing temperature.