Modification of Mg2Si in Mg–Si alloys with gadolinium

The modification effect of gadolinium (Gd) on Mg2Si in the hypereutectic Mg–3 wt.% Si alloy has been investigated using optical microscope, scanning electron microscope, X-ray diffraction and hardness measurements. The results indicate that the morphology of the primary Mg2Si is changed from coarse dendrite into fine polygon with the increasing Gd content. The average size of the primary Mg2Si significantly decreases with increasing Gd content up to 1.0 wt.%, and then slowly increases. Interestingly, when the Gd content is increased to 4.0 and 8.0 wt.%, the primary and eutectic Mg2Si evidently decrease and even disappear. The modification and refinement of the primary Mg2Si is mainly attributed to the poisoning effect. The GdMg2 phase in the primary Mg2Si is obviously coarsened as the Gd content exceeds 2.0 wt.%. While the decrease and disappearance of the primary and eutectic Mg2Si are ascribed to the formation of vast GdSi compound. Therefore, it is reasonable to conclude that proper Gd (1.0 wt.%) addition can effectively modify and refine the primary Mg2Si.