The microstructure, hardness and tensile properties of Al–15%Mg2Si in situ composite with yttrium addition

Current study investigates the effect of different concentrations of yttrium (0.1–1.0 wt.%) on the microstructure, hardness and tensile properties of Al–15%Mg2Si cast composite. The microstructural study of the composite revealed the presence of both primary and secondary Mg2Si phases in all specimens and also Y-containing intermetallics at higher concentrations of the respected element. It was also found that Y addition does not change the size and morphology of primary Mg2Si particles considerably, but the pseudo-eutectic Mg2Si was changed from a flake-like morphology to fine fibrous or rod-like. The results obtained from mechanical testing demonstrated that the addition of Y increases both hardness and ultimate tensile strength (UTS) values. Further investigations on tensile test revealed optimum Y level (0.5 wt.%) for improving both UTS and elongation values. Fracture surfaces via scanning electron microscopy (SEM) revealed that all specimens with large facets of primary Mg2Si particles succumb to brittle fracture. The large and brittle Mg2Si phases may act as crack initiators, while fine rod-like morphology of eutectic Mg2Si in Al–Mg2Si–0.5Y can be a barrier to the propagation of cracks and thus enhance the elongation values. At higher Y contents, an intermetallic phase (Al2Y) introduced on eutectic cell boundaries, appears to be the favored path for crack propagation.