Microstructural evolution and tensile properties of the in situ Al-15%Mg₂Si composite with extra Si contents

In the present work, the effect of extra Si addition on the microstructure and tensile properties of Al-15%Mg₂Si composite has been investigated. The Al-15%Mg₂Si composite ingot was made by in situ process and different amounts of extra Si (0.5, 1, 1.5, 2, 5 and 7wt.% Si) were added to the remelted composite. Optical microscopy and scanning electron microscopy (SEM) indicated that the addition of extra Si up to 2wt.%, reduces the average Mg₂Si particle size from 39μm to 26μm and increases the volume fraction of α-Al phase from 6% to 22%. Addition of extra Si content up to 7wt.% leads to the formation of primary Mg₂Si particles with larger size (38μm). The results of tensile test revealed that the addition of extra Si improves ultimate tensile strength (UTS) and elongation values of the composite from 176MPa and 1.7% to 222MPa and 3.0% respectively. Fractographic analysis of specimens exposed a cellular nature for the fracture surface. On the cellular fracture surface, the features of both brittle and ductile fracture were present simultaneously. Raising the amount of extra Si up to 7% has increased the number and decreased the size of dimples. These microstructural findings led to a change in the mode of fracture from brittle to ductile and increased elongation values.