## 4 Results and Discussion
### Analysis of IMCs  
Figure 6 shows the quantitative analysis of section B. The A356 alloy contained eutectic Si phases and Fe-rich IMCs (\(\pi\)-AlMgFeSi, \(\beta\)-FeSiAl) around the Al matrix. The Fe-rich IMCs were identified by EDS and were divided into a gray \(\pi\)-AlMgFeSi and a needle-shaped light-gray \(\beta\)-FeSiAl phase (Figure 7c). After ST-WQ and ST-WQ-AG, \(\pi\)-AlMgFeSi phases were transformed into \(\beta\)-FeSiAl phases owing to the low melting temperature of the Mg solute atoms. Differences in the sizes and morphologies of the \(\beta\)-FeSiAl phases were not observed. The eutectic Si in the non-heated A356 alloys was present with a relatively uniform concentration. However, the concentrations of Si solute atoms constituting the \(\pi\)-AlMgFeSi and \(\beta\)-FeSiAl phases were slightly increased.  
Figure 6: Secondary electron images and chemical element quantitative mapping obtained by EPMA in section B: (**a**) non-heated, (**b**) ST-E, (**c**) ST-WQ, and (**d**) ST-WQ-AG samples, (**e**) magnifications of (**b**).  
After the ST-WQ, the microstructure was relatively finer than before heat treatment, and the solute micro-segregation of Si solute atoms was reduced. The Mg solute micro-segregation distributed in the Si and Fe are shown in Figure 6b. These corresponded to the concentrated Mg solute atoms, as shown in Figure 7 (Points 2 and 3). The microstructures of the ST-WQ and ST-WQ-AG A356 alloys were not significantly different, as shown in Figure 6c,d. However, the hardness in section A increased from 64.1 HV to 81.2 HV, and in section B from 63.7 HV to 82.5 HV (Compared to the ST-WQ and ST-WQ-AG A356 alloys).  
Figure 8 shows the DSC analytical results of the precipitation and dissolution of the phases. In general, DSC is used to observe the temperatures at which fine phases are precipitated. The convex upward peak is an exothermic peak when a phase is formed, and the concave downward peak is an endothermic peak when the phase is re-dissolved into the Al matrix. An exothermic peak around 400 K is known as the Guinier-Preston zone (GP zone). In this zone, a fine-scale metallurgical phenomenon of Mg-Si nanocluster formation occurs [31]. The exothermic peaks between 500 and 600 K were generated by the formation  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Point** & **Al** & **Si** & **Mg** & **Fe** \\ \hline
1 & 77.04 & 16.75 & - & 6.22 \\
2 & 84.71 & 14.26 & 1.02 & - \\
3 & 75.55 & 20.16 & 4.29 & - \\
4 & 49.06 & 26.78 & 14.34 & 9.83 \\
5 & 71.94 & 11.20 & - & 16.87 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Qualitative analysis (EDS/EDAX) results of points in Figure 7 (wt.%).  
Figure 7: Field-emission scanning microscopy images of samples after ST-E heat treatment: (**a,b**) located in section B, and (**c**) representative phases of \(\pi\)-AlMgFeSi and \(\beta\)-FeSiAl in these A356 alloys. Chemical compositions of these points are shown in Table 3.  
of \(\beta\)\" and \(\beta^{\prime}\) phases [32]. The values of the entropy/heat of reaction are calculated by this method [33]. A higher entropy value means that a lot of phases have been formed. When considering the temperature of artificial aging, the increase of average hardness in ST-WQ-AG A356 alloys by 18 HV compared to the ST-WQ A356 alloys was determined to be because of a precipitation-strengthening effect that occurred by the formation of the GP zone in the Al matrix.