The Influence of a Subgrain Structure on Precipitation-Hardening and Nucleation of Precipitates in an Al-Zn-Mg Alloy

Abstract

The influence of a dislocation subgrain structure on the precipitation behaviour of a super-purity-based aluminium-zinc-magnesium alloy (Al-5½% Zn-l¾% Mg) has been examined through a study of its age-hardening behaviour and by a transmission electron microscope investigation of the distribution of precipitates developed. The presence of the subgrain structure inhibits hardening at 120° C and above, but has no effect at lower temperatures. After an induction period of ∼ 30 min, room-temperature ageing before elevated-temperature ageing progressively reduces the inhibiting effect of the subgrain structure on hardening behaviour. The results are interpreted in terms of a mechanism for the nucleation of hardening precipitates which envisages that nuclei develop either from simple solute atom clusters produced during room-temperature ageing or from solute atom-vacancy aggregates formed during rapid quenching. The implications of these modes of nucleation are briefly discussed in relation to the development of ageing treatments.