Mechanical properties of modified and nonmodified eutectic Al-Si alloys

Abstract

Instrumented Charpy Impact Test has been used to evaluate the effect of strontium (Sr) level and solidification cooling rate on the impact toughness and its parameters (crack initiation and crack propagation energies) of high purity eutectic Al-Si alloy. In addition, tensile properties has been also investigated. It has been found that, both crack initiation and crack propagation energies are improved gradually with increasing Sr content with a consequent increase in the toughness. This confirms also the high strength and ductility displayed by modified alloy. The improvement amount not only depends on the Sr level but also on the cooling rate imposed during solidification. Fractographic study revealed that the crack is initiated in the eutectic phase by fracture of Si particles. This crack seems to propagate in a brittle fashion in a nonmodified alloy while it proceeds in a ductile manner in a modified one. Dimple pattern of fracture is found to be the characteristic of modified alloy. The higher ductility and toughness of modified alloy in addition to the change in the fracture characteristics can be attributed only to the fine fibrous morphology of Si particles achieved by modification and the stress-strain state in the matrix material associated with the fineness of Si particles.