Al-7%Si-0.3%Mg alloys is evaluated as a function of cooling rate. The toughness of the Al-7%Si alloy is slightly improved when the primary and secondary arm spacings and the particle size of silicon are reduced with making fast the cooling rate. Cracks are initiated at the matrix-silicon particle interface by cleavage of silicon particles. The initial cracks at coarse silicon particles are interconnected and propagated. If the silicon particles are refined at faster cooling rate, the plane

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... n fracture toughness is raised. Coarse silicon particles are easily fractured. Fracture toughness of the Al-7%Si-0.3 Mg alloy is not so susceptible to the cooling rate because the dissolution of magnesium into the matrix and solution hardening oecur.