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The effect of Ca addition on modification of primary Mg 2 Si, hardness and wear behavior in Mge5 wt.%Si hypereutectic alloy has been investigated. The results showed clearly that without Ca addition, most of primary Mg 2 Si appeared as coarse dendritic morphology with average size of about 215 mm. With the addition of 0.1 wt.%Ca, the average size of primary Mg 2 Si decreased to about 98 mm, but their morphologies did not significantly changed. As the addition level of Ca increased to 0.3 wt.%, the average size of primary Mg 2 Si decreased significantly to about 50 mm and their morphologies changed to polyhedral shape. However, with further increasing Ca addition to 0.6 wt.% and 1 wt.%, some needlelike and blocky CaMgSi particles formed and the average size of primary Mg 2 Si increased slightly, which could described as over-modification. The present work showed that the optimal modification effect could be obtained when the Ca content in the investigated alloy reached 0.3 wt.%. The modification mechanism may be referred mainly due to poisoning effect resulting from the segregation of Ca atoms at the growth front of the Mg 2 Si and the adsorption effect of some Ca atoms in the Mg 2 Si crystal growth plane. The 0.3 wt.%Ca-added alloy has the highest hardness value and the best wear resistance among all other alloys. An excessive Ca addition resulted in the formation of some needle-like and blocky CaMgSi particles, which was detrimental to hardness and wear behavior of the 0.6 wt.% and 1 wt.%Ca-added alloys. The wear mechanism of investigated alloys is a mild abrasive oxidative wear with little adhesion.

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