## 3 Results and Discussion  
### Effects of (Pr+Ce) Addition on Microstructure and Morphology  
The microstructures of the AlSi5 Cu1 Mg alloys with different contents of the (Pr+Ce) are shown in Figure 1. The Fe-containing phases (in Figures 1 and 2) of the AlSi5 Cu1 Mg alloy consisted of the Y-Al3 FeSi, Al4 Fe, \(\beta\)-Al5 FeSi, \(\alpha\)-Al8 Fe2 Si, and Al12 FeMnSi phases [16]. As shown in Figure 3a, the eutectic Si phase in the matrix was a long-needle-like morphology. When 0.3 wt.% (Pr+Ce) was added, the eutectic Si phase was refined to a certain extent, the dendrites slowly disappeared, as shown in Figure 3b. The content of the mixed rare earth elements was insufficient; thus, the modification was incomplete and the eutectic Si phase required further refinement. When the addition of (Pr+Ce) increased to 0.6 wt.% (as shown in Figure 3c), The short rod-like of the eutectic Si phase was replaced by the fine granule of the eutectic Si phase structure. When the addition of (Pr+Ce) increased to 0.9 wt.%, the eutectic Si phase became coarser on some level (in Figure 3d). The size of Fe-containing phases were slightly larger than the phases in Figure 3b. The additional (Pr+Ce) had a significant modification effect on the AlSi5 Cu1 Mg alloy. Figure 1: _Cont._  
Figure 1: Backscattered SEM images of AlSi5 Cu1 Mg (**a**) at low; (**b**) high magnification and area scanning of AlFeMnSi phases of elements; (**c**) Al; (**d**) Si; (**e**) Fe; (**f**) Mn. Figure 2: _Cont._  
Figure 4 shows the change that various additions of the (Pr+Ce) caused on the dimension of eutectic Si. The mean length and aspect ratio were 15.1 \(\upmu\)m and 10.6 for the matrix. When the additional (Pr+Ce) increased to 0.6 wt.%, the mean length and the aspect ratio were 3.2 \(\upmu\)m and 3.5, which decreased by 78.8% and 67.4%. When the additional (Pr+Ce) increased 0.9 wt.%, the mean length and aspect ratio increased to 8.2 \(\upmu\)m and 6.4. Figure 3: Microstructures of AlSi5 Cu1 Mg alloy with different (Pr+Ce) content: (**a**) 0; (**b**) 0.3 wt.%; (**c**) 0.6 wt.%; (**d**) 0.9 wt.%. Figure 2: Backscattered SEM images of AlSi5 Cu1 Mg (**a**) high magnification and area scanning of AlFeSi phases of elements; (**b**) Al; (**c**) Fe; (**d**) Si. The SDAS (Secondary dendrite arm spacing) of the AlSi5 Cu1 Mg-x(Pr+Ce) (x = 0, 0.3 wt.%, 0.6 wt.% and 0.9 wt.% increasing amount) alloys are shown in Figure 5. The SDAS decreased initially and then increased with additional of (Pr+Ce). When the additional (Pr+Ce) increased to 0.6 wt.%, the SDAS decreased by 50.2% compared to the value of the matrix alloy. For the Al-Si alloys, a smaller SDAS itself reduced the elemental segregation and improved the microporosity [17]. A density between the surfactant film [8] between the particles and the solution formed, due to the addition of the rare earth elements. The grain growth was largely inhibited and the grain was refined. The eutectic reaction in the alloy occurred with the additional (Pr+Ce) [18]: L \(\rightarrow\)\(\alpha\)-Al + Al\({}_{11}\)RE\({}_{3}\). A eutectic product (Al\({}_{11}\)RE\({}_{3}\)) was used as a nucleation core for \(\alpha\)-Al, which allowed grain refinement. Some studies [19] showed that many Al-rich phases (mainly including Al\({}_{11}\)RE\({}_{3}\), Al\({}_{3}\)RE, and AlRE\({}_{3}\)) existed at the grain boundaries, which played an important role in inhibiting the grain boundary sliding and the matrix deformation, further improving the mechanical properties [20]. The results of XRD (X-ray diffraction) phase analysis in Figure 6 show that the addition of (Pr+Ce) can react with Fe-rich intermetallics compounds to form ternary Fe-rich intermetallics compounds of Al\({}_{5}\)CeFe\({}_{4}\), which can improve the mechanical properties of large-size Fe-rich intermetallics relative alloys.