## 3 Results
### Effects of Y Addition on the Si and Fe-Rich Phases  
Figure 4 exhibits the morphology of the eutectic Si and Fe-rich phases in ADC12 alloy with different amounts of Y addition. The aspect ratio and mean area of the eutectic Si phases are illustrated in Figure 4e,f. In Figure 4a we can see that the eutectic Si phases of unmodified alloy presented a coarse acicular or long axis morphology. The mean area and aspect ratio were 44 \(\upmu\)m\({}^{2}\) and 15.2 \(\upmu\)m,  
Figure 3: Optical micrographs of as-cast ADC12 alloys with different contents of Y addition: (**a**) unmodified; (**b**) 0.1 wt% Y; (**c**) 0.2 wt% Y; (**d**) 0.3 wt% Y; and, (**e**) SDAS of \(\alpha\)-Al phases with different additions Y.  
Figure 2: SEM images and EDS of ADC12 alloys. (**a**) SEM image; (**b**) Corresponding EDS results. respectively. In contrast, the microstructures of the alloy were significantly optimized after the addition of Y. As shown in Figure 4b, the eutectic Si particles exhibited a short rod structure when Y addition was 0.1 wt%. Correspondingly, the mean area and aspect ratio decreased to 11.9 \(\upmu\)m\({}^{2}\) and 3.8 \(\upmu\)m, respectively. As seen in Figure 4c, the eutectic Si particles were clearly spheroidized with a small fibrous or granular morphology. The mean area and aspect ratio further declined to 3.2 \(\upmu\)m\({}^{2}\) and 2.2 \(\upmu\)m, respectively, which was 92% and 75% lower than the matrix, respectively. However, as the Y addition amount increased to 0.3 wt%, the mean area and length-width ratio of the eutectic Si phase exhibited a slight increase. According to the impurity-induced twinning mechanism (IIT) [13; 14; 15; 16] and the restricted twin plane reentrant edge growth mechanism (TPRE) [17], the modified Y atoms easily concentrate near the Si solid-liquid interface or Si grooves. This phenomenon may change the growth mode and eventually lead to the isotropic growth of grain. Thereby, the Y atoms of the modification agents will refine the eutectic Si and change its morphology from the coarse acicular or long axis shape to a small fibrous or granular shape. Moreover, the previous works conducted by [PERSON] et al [18] and [PERSON] et al. [19] revealed that the large degree of negative mixing between Y and Si led to a decrease in the eutectic temperature and an improvement of the microstructure. Additionally, Figure 5 reveals typical SEM images of ADC12 alloys with different amounts of Y addition. The results indicate that the Y-rich phases mainly contain three elements (Al, Si and Y). This phase is Al\({}_{2}\)Si\({}_{2}\)Y, according to [PERSON] et al. [7]. As can be seen from Figure 5a,b, the Al\({}_{2}\)Si\({}_{2}\)Y phase displayed a coarser microstructure when the rear earth Y addition reached 0.8 wt.%. This phenomenon indicates that an excessive amount of rare earth Y may cause the phase to grow sharply. When the amount of Y reached 0.8 wt%, a large amount of Y and Si atoms were consumed and coarse Y-rich phases were formed, as shown in Figure 5b. This greatly reduced the modification effect. Therefore, the metamorphic effect would get worse when too much rare earth Y was added to the alloy. Similar results after adding Zr to A356 casting alloy have been reported by [PERSON] et al. [20]. Figure 4: High magnification morphology of primary Si crystals and Fe-rich phases in ADC12 alloy: (**a**) 0 wt%; (**b**) 0.1 wt%; (**c**) 0.2 wt%; (**d**) 0.3 wt%; and, (**e**, **f**) the mean area and aspect ratio of the eutectic Si phases. Figure 6 represents the average length of the Fe-rich phases with different Y additions. The average length of the Fe-rich phases gradually decreased while the Y content increased from 0 to 0.2 wt%. The average length of the Fe-rich phases in the unmodified alloy was approximately 22.3 \(\upmu\)m and the phases were characterized by a coarse needle-like morphology, as shown in Figure 4a. When the amount of Y was 0.1%, the average length decreased to 7.9 \(\upmu\)m and the morphology of the Fe-rich phases changed from needle-like to small flake, as shown in Figure 4b.