## 3 Results and Discussion
### Effect of (Pr+Ce) Addition on Mechanical Properties  
The UTS (ultimate tensile strength) and the breaking elongation of the AlSi5 Cu1 Mg with different (Pr+Ce) additions are shown in Figure 8. The UTS and the breaking elongation were remarkable elevated by the (Pr+Ce) addition, reaching 0-0.6 wt.%. The matrix alloy demonstrated the lowest tensile strength (230.5 MPa) and breaking elongation (5.4%). Among with the various additions of (Pr+Ce), AlSi5 Cu1 Mg-0.6 wt.% (Pr+Ce) exhibited a better UTS (280.4 MPa) and breaking elongation (5.8%). Compared to the matrix alloy, an increase of the UTS and the breaking elongation of 21.7% and 8.0% is achieved. When the addition of (Pr+Ce) increased to 0.9 wt.%, the mechanical properties of alloy declined gradually. Initially, (Pr+Ce) was separated at the grain boundary and strengthened the grain boundary. As additional (Pr+Ce) was added, the precipitated RE-rich phases acted as a dispersion strengthener. When the addition of (Pr+Ce) exceed 0.6 wt.%, the RE-enriched intermetallic compound accumulated in the front of the interface, and the grains became coarse by encroaching and overlapping, causing the mechanical degradation [18]. The mechanical properties of the cast Al-Si alloy were related to the morphology of the eutectic Si and Fe-containing phases [28]. The matrix alloy had many long-needle -like morphologies of the eutectic Si phases, bone-like or rod-like morphologies of the Fe-containing phases (in Figures 1 and 2), and the AlSi5 Cu1 Mg alloy exhibited lower mechanical properties. Figure 8: Tensile properties of AlSi5 Cu1 Mg alloy with different (Pr+Ce) additions (**a**) strain and stress; (**b**) breaking elongation and UTS. Figure 7: Backscattered SEM images of the alloy with AlSi5 Cu1 Mg-0.9 wt.% (Pr+Ce) (**a**) high magnification and area scanning of RE- rich (Rare earth-rich) intermetallic compounds of elements; (**b**) Al; (**c**) Si; (**d**) Fe; (**e**) Mn; \(f\). Ce; (**g**) Pr; (**h**) Cu. Figure 9 shows a partial fracture surface of the AlSi5 Cu1 Mg alloy under various (Pr+Ce) additions. The fracture of the AlSi5 Cu1 Mg alloy was a typical cleavage fracture. Because the Si phases were long and needle-like, and it could easily generate internal stress concentration when it was subjected to an external force, causing cracking of the Si phase. The cracks at the adjacent Si phase were connected and gradually extended throughout the entire fracture surface, resulting in the alloy fracturing. Compared with the fractures of the matrix, the fracture of the 0.6 wt% (Pr+Ce) metamorphic alloy was mainly composed of dimples and a certain number of cleavage surfaces. The fracture mode was a quasi-cleavage fracture between the cleavage fracture and the dimple fracture. The internal grain boundary density of the cleavage fracture was relatively large, the intergranular interfacial bonding force was poor, and the grain boundary bonding force of the quasi-cleavage fracture was higher than the intergranular interfacial bonding force, resulting in the elongation of quasi-cleavage fracture being superior than the cleavage of the fracture. The grain size can have an important effect on the fracture mechanism of the alloy. As the addition of (Pr+Ce) increased further, most of the Si phase and Fe-containing phase become coarse, leading to the toughness and the breaking elongation of the alloy decreasing. The grain size also has an important effect on the mechanical mechanism of the alloy, the smaller the size of the grain, the more grain boundaries. The dislocation motion between the grain boundaries was hindered. The alloy was strengthened and the tensile strength improved [29].