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

Highlights: ► Effect of extra Si contents on the microstructure of Al–15%Mg2Si composite. ► The optimum concentration of extra Si was found to be 2 wt.%. ► UTS values changed from 176 MPa to 222 MPa by adding extra Si. ► The addition of 2 wt.% extra Si increased the elongation values from 1.7% to 2.9%. ► Extra Si addition conducted the fracture mode of the MMC from brittle to ductile. -- Abstract: 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 7 wt.% Si) were added to the remelted composite. Optical microscopy and scanning electron microscopy (SEM) indicated that the addition of extra Si up to 2 wt.%, reduces the average Mg2Si particle size from 39 μm to 26 μm and increases the volume fraction of α-Al phase from 6% to 22%. Addition of extra Si content up to 7 wt.% leads to the formation of primary Mg2Si particles with larger size (38 μm). The results of tensile test revealed that the addition of extra Si improves ultimate tensile strength (UTS) and elongation values of the composite from 176 MPa and 1.7% to 222 MPa 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.