Effects of Si content and Ca modification on microstructure and thermal expansion property of Mg-Si alloys

The effect of Si content on the coefficient of thermal expansion (CTE) of Mg-xSi (x = 1, 2, 3, 4 wt%) alloys and the influence of Ca addition of Mg-4Si-yCa (y = 0.2, 0.4, 0.6, 0.8, 1.0 wt%) alloys have been studied. The results show that with the increase of Si content, the CTE of Mg-Si alloys decrease sharply, which is decreased by 30.8%, from 26 x 10(-6)/K of pure Mg to 17.98 x 10(-6)/K of Mg-4Si alloy at room temperature. When appropriate amount of Ca is added, the morphology of Mg2Si in Mg-4Si alloy can be effectively modified, but excessive addition of Ca about 1 wt% can result in over-modification. When 0.2 wt% Ca is added, the CTE of Mg-4Si alloy is increased compared with no Ca addition. After this, along with the increase of Ca content, the CTE of the alloy is decreased.