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

Highlights: • The morphology and size of Mg₂Si are significantly modified by Sb. • Modification mechanisms of Sb on Mg₂Si are clarified. • The intrinsic hardness of modified Mg₂Si phase is greatly improved. • Sb modified composite exhibits simultaneously enhanced strength and ductility. -- 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 < 100> 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.