Revealing modification mechanism of Mg2Si in Sb modified Mg2Si/ AZ91 composites and its effect on mechanical properties

Abstract In this study, in-situ Mg2Si reinforced AZ91 composites unmodified and modified with 2.0 wt %Sb addition were fabricated. Investigation was done on the modification mechanism of Mg2Si phase morphology, intrinsic mechanical properties of Mg2Si phase and mechanical properties of Mg2Si/AZ91 composites containing unmodified and modified Mg2Si phase. The results showed that the morphology of primary Mg2Si phases was significantly modified from coarse dendritic shape to blocky polygonal shape with addition of Sb in Mg2Si/AZ91 composite. The 3-D morphology of modified Mg2Si phases was seen as perfect octahedron, and the average size was also refined into approximately 18 μm. Microstructure analysis revealed that Sb-rich layer with the thickness of 265 nm–744 nm formed on the surface of primary Mg2Si particles and suppressed the preferred growth along direction due to Sb atoms preferentially absorbed on the {100} surfaces. Meanwhile, approximately 4.18 at. % Sb was doped into Mg2Si crystals by substituting Si sites, which reduced the surface energy and restricted the growth of Mg2Si. Additionally, the most thermodynamically stable Mg3Sb2 phase formed during the solidification process acted as effective heterogeneous nucleus thus refining the size of primary Mg2Si. The intrinsic hardness of modified Mg2Si phase was greatly improved by 36.6% due to the lattice distortion induced by Sb doping. Moreover, Sb modified Mg2Si/AZ91 composite exhibited simultaneously enhanced strength and ductility when compared to unmodified Mg2Si/AZ91 composite, which were attributed to the improved CTE strengthening, increase in hardness of Mg2Si and reduction of stress concentration resulted from modification of morphology and size.