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

The phase transformation of theta ''->theta' in an Al-5.7Cu alloy was investigated by aberration-corrected scanning transmission electron microscopy, and the tranformation mode of theta->theta ' during aging treatment was clarified. In the presence of the theta '' phases, theta' was found to be formed by in-situ transformation from theta '' with the same plate shape, size and broad faces. The transformation starts from multiple sites within the theta' precipitate and the whole theta' phase finally forms as the preferential theta sections grow and connect with each other. Antiphase domain boundaries are also found in some theta '' precipitates when the disregistry exists between different theta' sections. (C) 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.