Structural changes in the lamellar eutectic Al-Al2Cu during heat treatment

X-ray diffraction techniques are used to study the structural changes in the lamellar eutectic Al-Al 2 Cu under various heat treatments. Only α-phase lamellae show changes in the structure during ageing after quenching from 520°C. The observed phase changes follow practically the same sequence as in Al-rich Al-Cu metastable solid solution, namely: GP zones→ θ′ → θ″ → θ. However, in completely aged-as well as in slow cooled samples-θ-phase particles do not precipitate in α-phase lamellae but coherently with θ-phase lamellae. Comparison of hardness variations and X-ray results during ageing at 165°C shows that the maximum hardness corresponds to maximum development of θ″-phase, in agreement with previous observations made by other authors in Al-4-4, 5% Cu metastable solid solution.