The fracture mode in Al–Si–Mg casting alloys

Samples of commercial Al–7Si–0.4/0.7Mg–T6 casting alloys deformed to fracture were anodised to reveal the grain boundaries and the proportion of transgranular/intergranular fracture was determined. The number and area fraction of Si particles cracked on cell and grain boundaries was also determined. Samples of unmodified and Sr-modified alloy, with magnesium contents of 0.4 and 0.7% and dendrite arm spacings (DAS) between 17 and 65 μm were used for the study. Cracking of the Si particles in samples with small DAS occurs preferentially on grain boundaries and the fracture path is predominantly intergranular, while in samples with large DAS, particle cracking concentrates on the cell boundaries and the fracture mode is transgranular, along the cell boundaries. The relation between the fracture mode and the tensile ductility of the alloys is discussed.