Prolonged Ageing of an Aluminium-Zinc-Magnesium Alloy

Abstract

Prolonged ageing behaviors of an Al-5.06 wt% Zn-0.94 wt% Mg alloy over 10000 hr at 100°C have been investigated by means of the specific heat measurements and the tensile test. It has been shown that the G. P. zones and the intermediate and the equilibrium phases are formed on ageing at 100°C. The intermediate phase precipitates only in the early stages of ageing, while the G. P. zones continue to exist over a long ageing period. Changes in the internal energy of the specimen during ageing and respective contributions of the G. P. zones and the precipitates to it have been determined as functions of the ageing time, from which the ageing characteristics of them have been deduced. It has been suggested that the precipitates grow at the sacrifice of the G. P. zones in the later stages of ageing.Remarkable age-hardening has been observed. The maximum strength (0.2% proof stress) of about 24 kg/mm2 has been obtained after ageing for about 500 hr. In the whole ageing intervals studied, the strength changes almost parallel to the variations in the internal energy due to the G. P. zones, suggesting that the zones make a predominant contribution to age-hardening of this alloy at 100°C. The strength decreases to the as-quenched level when the alloy is reversion heat-treated at 250°C for 30 sec even after ageing to the maximum of the strength.The serrated stress-strain curve has been obtained on the alloy aged for a short period. Proposed mechanisms of the serrations have been discussed and it has been shown that the serrations should be explained by a mechanism based on the change in dislocation mobility due to the formation of the G. P. zones.