## 3 Results and Discussion
### Compositional Analyses  
Table 2 shows the results obtained from the XRF analysis of the samples. The composition of the samples is in good agreement with the nominal compositions, with the Zn being around 2 and 3 wt.% for the two alloys and the Si being in the 0.58-0.65 wt.% range. Additionally, Figure 4 shows results for the thermodynamic simulations of the solidification processes in the equilibrium state of the two alloy compositions. The simulation results suggest the formation of three distinct phases. As the alloy cools from the liquid state, the primary solidification of Mg is anticipated, which forms the matrix of the alloy. This is followed by the solidification of the eutectic microconstituent Mg + Mg\({}_{2}\)Si. In the final stage, the MgZn phase forms in the solid state. Furthermore, note that while three phases (Mg, Mg\({}_{2}\)Si, and MgZn) are expected in the microstructure, we can observe the Mg\({}_{2}\)Si phase only in the SEM/EDS data, and due to limitations on the contrast between Mg and Si phases at the available X-ray photon energy, the Mg\({}_{2}\)Si phase is not clearly visible in the XCT data. In addition to the thermodynamic simulations, SEM and EDS analyses of the alloys were also carried out. Figure 5a shows the microstructure obtained from the BSE imaging mode in the SEM with the bright regions representing the high density (Zn rich) phases (green arrow) of the microstructure and the light and dark grey regions representing the Mg\({}_{2}\)Si (blue arrow) and Mg phases, respectively. The elemental dot maps for Si, Mg, and Zn obtained from the EDS analyses are shown in Figure 5b-d, respectively, and clearly highlight the compositions of the three different phases (as predicted in the thermodynamic simulations). Note that due to the high solubility of Zn in Mg and the low solidification temperature of the MgZn phase, under equilibrium conditions, the formation of MgZn is expected via precipitation in the solid-state Mg matrix, rather than the formation of a eutectic structure [27]; however, under non-equilibrium conditions, the MgZn phase tends to \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Alloy** & **Mg** & **Si** & **Zn** & **Al** & **Ca** & **Fe** & **Cu** \\ \hline Mg-0.6 Si-2 Zn & 97.413 & 0.580 & 1.953 & 0.041 & 0.001 & 0.007 & 0.002 \\ Mg-0.6 Si-3 Zn & 96.304 & 0.654 & 2.976 & 0.053 & 0.001 & 0.008 & 0.001 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: X-ray fluorescence-based compositional analyses of the two ally compositions used in this study to assess the impacts of change in composition and change in cooling rate on the morphology of intermetallic particles within the alloys. Figure 4: Thermodynamic calculations of solidification processes of (**a**) Mg-0.6 Si-2 Zn and (**b**) Mg-0.6 Si-3 Zn alloys considering the equilibrium state. appear in its eutectic form (as seen in Figure 6). In the binary Mg-Zn system, under non-equilibrium conditions, we can expect to observe a partition coefficient (\(K\)) (ratio of concentration of Zn in the solid state to the concentration of Zn in liquid state) smaller than 1, indicating that a significantly higher concentration of Zn exists in the liquid phase compared to the solidified Mg phase, ahead of the solid/liquid interface [45; 46; 47]. In the final stages of solidification, the concentration of Zn can increase to near eutectic composition (51.3 wt.% Zn), leading to the formation of a eutectic microconstituent in the last regions of liquid remaining within the material. This can explain why we see the eutectic form of MgZn in these Zn-rich phases of the microstructure. For simplicity of nomenclature, we refer to these Zn-rich eutectic particles as MgZn particles throughout the manuscript. Higher-magnification images, shown in Figure 6, also clearly reveal the eutectic nature of the MgZn particles. Furthermore, point-EDS analyses clearly show that the MgZn particles have a MgZn phase with islands of Mg in between.