Synergistic effects of Mg۲Si formation and mold section thickness on the as-cast properties of the Mg-Si system

Magnesium alloys have attracted increasing attention due to their low density and high strength-to-weight ratio, which makes them attractive for lightweight engineering applications [۱-۳]. Despite these advantages, the as-cast mechanical performance of Mg alloys is commonly restricted by insufficient strength and ductility [۴]. Alloying is a widely used strategy to improve their behavior, and the Mg-Si system is recognized as an effective in-situ composite route because the addition of Si leads to the formation of Mg۲Si particles. Since Si has extremely low solubility in Mg, its addition produces eutectic Mg۲Si and primary Mg۲Si particles at higher Si levels [۵]. Although Mg۲Si is a hard and thermally stable phase, the as-cast morphology of primary particles is typically coarse and polygonal, which can reduce strength and promote early crack initiation [۶]. In this study, the effects of cooling rate and Mg₂Si particles evolution were systematically examined in eutectic Mg-۱.۳۴Si and hypereutectic Mg-۳Si alloys. The solidification rate was controlled by casting the alloys into a metallic mold with different section thicknesses, enabling the variation of geometrical modulus (M) and thus cooling rate. Thermal analysis confirmed that decreasing M significantly increased the cooling rate. This approach allowed direct investigation of how cooling rate influences grain size, dendritic features, eutectic morphology, and primary Mg۲Si particle size. The resulting microstructures were characterized using optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD), while shear punch testing (SPT) and hardness measurements were used to assess mechanical behavior. For both alloys, decreasing M (increasing cooling rate) produced pronounced grain refinement. With increasing cooling rate, the eutectic regions became finer and more uniformly distributed. In contrast, the Mg۳Si alloy showed a large number of primary Mg۲Si particles in addition to the eutectic constituent. These particles became significantly finer with increasing cooling rate. Quantitative analysis demonstrated that the size of primary Mg۲Si particles decreased substantially at higher cooling rates, confirming the strong influence of solidification conditions on particle evolution. Additionally, the coarsening of grain size in the Mg-۳Si alloy was attributed to the exothermic formation of primary Mg۲Si particles, which increases the latent heat released during solidification and effectively reduces the cooling rate, leading to grain growth. Mechanical evaluation revealed clear distinctions between the two alloys. For Mg-۱.۳۴Si, the ultimate shear strength (USS) consistently increased with cooling rate, mainly due to grain refinement and a higher contribution of fine eutectic phases. In contrast, the Mg-۳Si alloy exhibited the highest hardness values among the studied compositions, driven by the large volume fraction of hard Mg۲Si particles. However, its shear strength was significantly lower than that of the eutectic alloy. SPT results indicated that the coarse and polygonal primary Mg۲Si particles served as stress concentration sites, facilitating crack initiation and propagation. This effect reduced the shear load-bearing capacity. Observations of fractured regions confirmed that cracks preferentially initiated and propagated along Mg۲Si particles, illustrating the detrimental effect of their morphology on strength. Overall, the combined microstructural and mechanical findings demonstrated that increasing the cooling rate refined grains and secondary features in both alloys, with the Mg-۱.۳۴Si composition showing the highest shear strength due to the synergistic effects of grain refinement and the strengthening contribution of the eutectic constituent. In contrast, the Mg-۳Si alloy, despite attaining the greatest hardness because of its high fraction of Mg۲Si, exhibited the lowest shear strength owing to the presence of coarse primary Mg۲Si particles that strongly promoted crack initiation.

کلیدواژه ها:

Mg-Si alloys ، Solidification ، Grain refinement ، Cooling rate ، Mechanical properties