θ' structure in relation to nucleation sites for θ' phase in Al-Cu alloys

Abstract

Transmission electron microscopy was carried out on the Al-4%Cu alloy solution-treated at 793K for 7.2ks, quenched into iced water or directly quenched to an aging bath and subsequently aged for different times. Homogeneous precipitation of a θ' phase never occurs in any cases. The θ' structure actually observed is controlled by the θ' precipitates formed on dislocation loops, dislocation lines and/or θ". The θ' precipitates form on dislocation loops if specimens were water quenched or direct-quenched to comparatively low temperatures and aged; on considerably sparse dislocation lines if direct-quenched where neither loops nor θ" appear; and form on θ" if direct-quenched where loops never appear but θ" precipitates develope.