TEM observation of extracted precipitates in T5 heat treated Al–Si–Mg alloy

The precipitation behavior of an Al–6.8 %Si–0.38 %Mg cast alloy subjected to T5 heat treatment was investigated using transmission electron microscopy (TEM). At the peak aging condition of 200 °C, granular and rod-shaped precipitates were observed at the center of the dendrite. Rod-shaped precipitates were successfully extracted after aging at 250 °C for 3.6 ks. Energy-dispersive X-ray spectroscopy (EDS) confirmed that the extracted precipitates had a Mg:Al:Si atomic ratio of approximately 1:4:5, consistent with previously reported Type-A precipitates. Selected area electron diffraction (SAED) analysis further confirmed their hexagonal crystal structure, with lattice parameters a = 0.405  nm and c = 0.67  nm. These findings indicate that Type-A precipitates, which are typically observed in excess-Si Al–Mg–Si alloys, also form in T5-treated Al–Si–Mg cast alloys.