THETA-PRIME PRECIPITATION IN ALUMINIUM-COPPER-CADMIUM ALLOYS

Dilatometer, resistivity and electron microscope measurements have been used to study the nucleation and growth of the θ′ phase in an Al-5.12% Cu-0.14% Cd alloy. The results suggested that nucleation of θ′ took place by the coalescence of mobile copper-cadmium-vacancy clusters, the activation energy for the process being 0.5 eV. The growth characteristics of the θ′ precipitate could all be satisfactorily explained by assuming the growth of the precipitate was controlled by the movement of clusters to it. During the early stages of ageing the clusters were rich in vacancies so that precipitate growth was rapid. In the later stages of ageing the clusters contained less vacancies per solute atom and in consequence their movement became more sluggish which resulted in very slow precipitate growth. When extensive Cd′ precipitation started to occur the cadmium atoms left the clusters, which therefore dispersed and thus allowed more rapid growth of the θ′ precipitates. The activation energy for growth of the θ′ precipitate varied from 0.7 eV for the early stages of ageing, to 1.5 eV for the later stages. © 1968.