Precipitation structure of Al-10 mass%Si-0.3 mass%Mg alloy produced by high pressure die casting and permanent mold casting with T5 treatment

The precipitation structures in as-cast and T5-treated specimens of high pressure die casting and permanent mold casting in Al10-mass%Si-0.3 mass%Mg alloys were investigated using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM). No precipitates were observed in any of the as-cast materials. Fine needle-shaped precipitates, coarse rod-shaped precipitates, and granular precipitates were observed in all peak-aged specimens, which are thought to be the main cause of the increase in hardness during T5 treatment. These precipitates became finer near the eutectic phases than inside the α-Al phase cells, because the contact interface between the α-Al phase and the eutectic Si phase can be considered to become an area of vacancy sink and inhibit the growth of the precipitates. It became clear that almost all the precipitates in the die cast and the permanent mold cast specimens of this alloy were not due to the as-cast heat; rather, they were generated by the T5 treatment.