Precipitates and tensile fracture mechanism in a sand cast A356 aluminum alloy

In this paper, precipitates and tensile fracture mechanism in a precision sand cast A356-T6 aluminum alloy were investigated. In the T6 heat treated condition, both needle-shaped Mg2Si precipitates with length of about 0.5 μm and width of less than 50 nm, and fine polygonal-shaped Si precipitates (<1 μm) were observed in Al matrix. The silicon precipitates are mainly distributed in α-Al dendritic phase. The precipitate-free-zone (PFZ) was observed along the α-Al dendrite cell and grain boundaries. In the studied alloy, the width of PFZ is over 0.5 μm. During tensile straining, multiple cracks were observed in the large eutectic silicon particles. In many cases, the fracture of silicon particles is normal to the macroscopic tension axis, even when this was the longest fracture path in the particles. The fracture mode in the studied alloy is transgranular.