Ca Addition Effects on the Microstructure, Tensile and Corrosion Properties of Mg Matrix Alloy Containing 8 wt.% Mg2Si

The microstructure, tensile properties and corrosion behavior of the Mg-8 wt.% Mg2Si-x%Ca alloy have been studied by the use of optical microscopy, scanning electron microscopy equipped with energy-dispersive spectroscopy, x-ray diffraction, standard tensile testing, polarization test and electrochemical impedance spectroscopy (EIS) measurements. Microstructural studies indicated that Ca modifies both primary and eutectic Mg2Si phase. It was found that the average size of primary Mg2Si particles is about 60 mu m, which is dropped by about 82% in the alloy containing 0.05 wt.% Ca. By the addition of different Ca contents, Ca-rich intermetallics (i.e., CaSi2 and CaMgSi) were formed. The modification mechanism of adding Ca during solidification was found to be due to the strong effect of CaMgSi phase as a heterogonous nucleation site, apart from CaSi2 which was reported before, for Mg2Si intermetallics. Tensile testing results ascertained that Ca addition enhances both ultimate tensile strength (UTS) and elongation values. The optimum amount of Ca was found to be 0.1 wt.%, which improved UTS and elongation values from about 130 MPa and 2% to 165 MPa and 5.5%, whereas more Ca addition (i.e., 3 wt.%) reduced the tensile properties of the alloy to about 105 MPa and 1.8%, which can be due to the formation of CaMgSi intermetallics with deteriorating needle-like morphology. Polarization and EIS tests also showed that the Mg-3%Si-0.5%Ca alloy pronounces as the best anti-corrosion alloy. Nevertheless, further added Ca (up to 3 wt.%) deteriorated the corrosion resistance due to predominance of worse galvanic coupling effect stemmed from the presence of stronger CaMgSi cathode in comparison with Mg2Si. With higher Ca additions, an adverse effect was seen on corrosion resistance of the Mg-3%Si alloy, as a result of forming a weak film on the alloy specimen surface.