Relationship of Mg$_{2}$Si phase-content and thermal expansion properties of Mg–Si and Mg–Si–Ca alloys

The thermal expansion properties of Mg–$x$Si ($x$ = 1 wt.%, 1.38 wt.%, 2 wt.%, 3 wt.%, and 4 wt.%) binary alloys and Mg–4Si–$y$Ca ($y$ = 0.2 wt.%, 0.4 wt.%, 0.6 wt.%, 0.8 wt.%, and 1.0 wt.%) alloys over a temperature range of 25–300$^\circ$C were systematically studied. The results show that the Mg–Si binary alloys consist of $\alpha$-Mg and Mg$_{2}$Si phases. The volume fraction of Mg$_{2}$Si phase increases with the increase of Si content in Mg–Si alloys, from 30.72% to 46.50% when Si content increases from 1 wt.% Si to 4 wt.% Si. The coefficient of thermal expansion (CTE) of Mg–$x$Si binary alloys decreases with the increase of Mg$_{2}$Si volume fraction. The addition of Ca element to Mg–4Si alloys has an obvious modification effect on the Mg$_{2}$Si phase. When the Ca content increases constantly, the CTE of the Mg–4Si–Ca alloys increases at first, then it continues to decline. The mechanism is mainly related to the formation of the CaMgSi phase.