Atomic-scale mechanism of the θ″ → θ′ phase transformation in Al-Cu alloys

The phase transformation of θ″ → θ′ in an Al-5.7Cu alloy was investigated by aberration-corrected scanning transmission electron microscopy, and the tranformation mode of θ″ → θ′ during aging treatment was clarified. In the presence of the θ″ phases, θ′ was found to be formed by in-situ transformation from θ″ with the same plate shape, size and broad faces. The transformation starts from multiple sites within the θ″ precipitate and the whole θ′ phase finally forms as the preferential θ′ sections grow and connect with each other. Antiphase domain boundaries are also found in some θ′ precipitates when the disregistry exists between different θ′ sections.