Transmission electron microscopy study of precipitates in an artificially aged Al-12.7Si-0.7Mg alloy

An investigation of Al-12.7Si-0.7Mg alloy aged at 160 °C, 180 °C and 200 °C for 3 h was carried out in order to identify the precipitating phases. Regular transmission and high resolution electron microscopy (TEM and HREM) were employed for this purpose. The studies were focused on the dark spots and needle-shaped precipitates lying in (001)Al plane. Based on the HREM observations, dark spots and needle-shaped precipitates have different characteristics. The results revealed that the ellipsoidal and needle-shaped precipitates along <100> direction of the matrix coexist in the alloy by tilting experiments at given aging condition. The ellipsoidal dark spot precipitates viewing along [001]Al is not cross-sectional image of needle-shaped precipitates along <001>Al. Needle-shaped precipitate is coherent with the matrix. The diffraction pattern associated with the ellipsoidal precipitates is consistent with β″ reported in literature.