Stress-oriented nucleation of ω-phase plates in an Al-Cu-Mg-Ag alloy

Abstract

The effect of an external stress applied during formation of ω-phase plates in an Al-Cu-Mg-Ag alloy is examined by two-step ageing: stress ageing followed by stress-free ageing or vice versa. A compressive stress applied at the beginning of ageing is shown to promote the formation of the ω-phase plates lying on the {111} planes perpendicular to the stress axis. The stress effect is found to be dominant only in the early stage of ageing, indicating that it affects nucleation. Quantitative data have been obtained on stress-assisted nucleation by analysing correlation between the number of the ω-phase plates and the stress acting on the {111} habit planes in different grains in a polycrystal. From the stress-orienting effect described above and the transformation strain (eigenstrain) measured by high-resolution electron microscopy, a critical size of the ω plate nucleus is estimated to be about 0.3 nm³, consisting of approximately ten atoms.