## 2 Materials and Methods  
Commercial ADC12 alloy was selected as the matrix. An electrical resistance furnace and a graphite crucible were chosen for preparation of the alloys. The ADC12 alloy was melted at 750 \({}^{\circ}\)C and held for 10 min. Then, the Al-10Y master alloy was added into the melt at 720 \({}^{\circ}\)C to prepare the composites with various contents of Y (0 wt%, 0.1 wt%, 0.2 wt%, and 0.3 wt%). The fabricated alloy was then deslagged and poured into a preheated mold (400 \({}^{\circ}\)C). An illustration of the mold used is shown in Figure 1. To evaluate the tensile properties, the test samples were machined into tensile test bars with a diameter of 6 mm and a gauge length of 30 mm, as seen in Figure 1, in accordance with China Standard GB/T228-2002. In order to ensure the uniformity of the various phases in the alloys, the amount of ADC12 alloy per casting was controlled at approximately 300 g. The tests were carried out at room temperature using a SUNC UMS105 electro-servo testing machine. A controlled tensile speed of 1 mm/min and a 100 KN load cell were used. In addition, the compositions of the fabricated alloys in this study were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES, as shown in Table 1.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Alloy** & **Si** & **Cu** & **Mg** & **Fe** & **Zn** & **Y** & **Al** \\ \hline ADC12 & 10.86 & 2.95 & 0.23 & 0.57 & 0.68 & - & Bal \\ ADC12+0.1 wt\% Y & 10.53 & 3.36 & 0.29 & 0.66 & 0.54 & 0.12 & Bal \\ ADC12+0.2 wt\% Y & 10.28 & 3.03 & 0.35 & 0.54 & 0.56 & 0.23 & Bal \\ ADC12+0.3 wt\% Y & 10.59 & 3.29 & 0.30 & 0.66 & 0.63 & 0.34 & Bal \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of the alloys (wt.%).  
Figure 1: Illustration of the mold and tensile test sample.  
The microstructure and mechanical properties of the composites were then investigated, and the optimum amount of added Y was selected. The composite samples with the optimal Y addition amounts were placed into an SX24-13 type resistance furnace. The holding time was 8 h and the solution temperature was set to 500, 520 and 540 \({}^{\circ}\)C. Immediately, the samples were taken out and placed in water to cool.  
The microstructures of all samples selected were observed with an optical microscope (OM, OLYMPUS BX51 microscope, Olympus Metrology, Inc., Tokyo, Japan) and a scanning electron microscope (SEM, Quanta 200 FEI Metrology, Inc., Hillsboro, OR, US) equipped with energy disperse spectroscopy (EDS, FEI Metrology, Inc., Hillsboro, OR, USA) after being etched by 0.5 vol pct hydrofluoric acid in water. The software IPP6.0 image (Media Cybernetics Co., Rockville, MD, USA) and OriginPro 8.5 (OriginLab Co., Northampton, MA, USA) were used for quantitative and data analysis, respectively. The calculation methods [10] of these parameters were as follows:  
\[AverageLength=\frac{1}{m}\sum_{j=1}^{m}\left(\frac{1}{n}\sum_{i=1}^{n}L\right) \tag{1}\]  
\[Meanarea=\frac{1}{m}\sum_{j=1}^{m}\left(\frac{1}{n}\sum_{i=1}^{n}A_{i}\right) \tag{2}\]  
\[Spectratio=\frac{1}{m}\sum_{j=1}^{m}\left(\frac{1}{n}\sum_{i=1}^{n}\left(\frac {L_{1}}{L_{s}}\right)\right) \tag{3}\]  
where \(A_{i}\) is the area of a single silicon particle, \(L_{1}/L_{S}\) is the ratio of the longest to shortest dimensions of a single silicon particle of a single field, \(L\) is the length of a single \(\beta\)-Fe phase or \(\alpha\)-Al phase secondary dendrite arm spacing (SDAS), \(n\) is the number of a single field and \(m\) is the number of the fields.