Effect of Ca addition on modification of primary Mg2Si, hardness and wear behavior in Mg–Si hypereutectic alloys

The effect of Ca addition on modification of primary Mg2Si, hardness and wear behavior in Mg–5 wt.%Si hypereutectic alloy has been investigated. The results showed clearly that without Ca addition, most of primary Mg2Si appeared as coarse dendritic morphology with average size of about 215 μm. With the addition of 0.1 wt.%Ca, the average size of primary Mg2Si decreased to about 98 μm, but their morphologies did not significantly changed. As the addition level of Ca increased to 0.3 wt.%, the average size of primary Mg2Si decreased significantly to about 50 μm and their morphologies changed to polyhedral shape. However, with further increasing Ca addition to 0.6 wt.% and 1 wt.%, some needle-like and blocky CaMgSi particles formed and the average size of primary Mg2Si 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 Mg2Si and the adsorption effect of some Ca atoms in the Mg2Si 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.