## 3 Results and Discussion  
### Microstructure and Microhardness  
Optical micrographs of AlSi5 Cu1 Mg alloy samples with different contents of La are shown in Figure 2. It can be concluded that, with the adding of the content of La, there is a significant difference in the grain refinement effect. The microstructure of as-cast AlSi5 Cu1 Mg alloy consists of a wide range of intermetallic phases such as \(\alpha\)-Al, Al\({}_{4}\)Fe, \(\gamma\)-Al\({}_{3}\)FeSi, \(\alpha\)-Al\({}_{5}\)Fe\({}_{2}\)Si, \(\beta\)-Al\({}_{5}\)FeSi and Al\({}_{12}\)FeMnSi phase [25]. The matrix \(\alpha\)-Al of as-cast AlSi5 Cu1 Mg alloy exhibits coarse dendrites and irregular shapes of clutter (Figure 2a). With the addition of La, the contour of the matrix \(\alpha\)-Al becomes clearer and the shape becomes more regular. When the addition amount of La was 0.6 wt %, the dendrites disappeared, and the grains became small and round. In addition, the matrix \(\alpha\)-Al distribution is more uniform (Figure 2c). In contrast to Figure 2c,d, it can be seen that, if the amount of La increased to 0.9 wt %, the matrix \(\alpha\)-Al began to coarsen and be converted into slats. Additionally, with the increase in the amount of La, elongated eutectic silicon gradually becomes shorter and smoother without edges and corners. Furthermore, when the addition amount of La was 0.6 wt %, eutectic silicon was converted into granular form. Figure 1: (**a**) the loading positions (black prism) of wear test; (**b**) the wear test in pin-on-disc apparatus. With La was added to AlSi5 Cu1 Mg alloy, the size of the AlFeMnSi type phases in the coarse skeleton of the alloy were reduced, and the angular and needle-like AlFeSi type phases were refined into short rods or granules. This metamorphic effect can be attributed to the surface activity of rare earth elements. After the RE was added as the form of La rich master metal, a rod like Al\({}_{11}\)RE\({}_{3}\)(Al\({}_{4}\)RE) intermetallic phase [26] was observed in the microstructure. The SEM micrograph of AlSi5 Cu1 Mg + 0.6 wt % La alloy is shown in Figure 3a. In addition, the EDS spectrum of the precipitates (region A in Figure 3a) is shown in Figure 3b. The analysis revealed that the precipitates could be Al\({}_{11}\)La\({}_{3}\) phase. The Al\({}_{11}\)La\({}_{3}\) phase has a high thermal stability and can improve the mechanical properties of AlSi5 Cu1 Mg alloy [27]. It also can promote wear resistance. [PERSON] et al. [28] have reported about the positive role played by Al\({}_{11}\)RE\({}_{3}\) particles in the wear resistance of AZ91 alloy at high loads. Figure 3: Scanning electron microscope (SEM) micrograph of **(a)** AlSi5 Cu1 Mg + 0.6 wt % La alloy and **(b)** energy dispersive spectrum (EDS) analysis of region A in **(a)**. Figure 2: Microstructures of AlSi5 Cu1 Mg alloy samples with different contents of La: (**a**) 0 wt %, (**b**) 0.3 wt %, (**c**) 0.6 wt % and (**d**) 0.9 wt %. The microhardness and the grain size of the alloys are shown in Figure 4. It indicated that the microhardness of the AlSi5 Cu1 Mg + 0.6 wt % La was 34% higher than that of the matrix alloy. However, further addition of modifier resulted in decrease microhardness values. According to the Hall-Petch Equation (1) [29; 30]:  
\[\sigma_{y}=\sigma_{0}+\frac{k_{y}}{\sqrt{d}}, \tag{1}\]  
where \(\sigma_{y}\) is the yield load (It can be expressed by microscopic Vickers hardness [31]. ), \(\sigma_{0}\) is a materials constant for the starting load for dislocation movement (or the resistance of the lattice to dislocation motion), \(k_{y}\) is the strengthening coefficient (a constant specific to each material), and \(d\) is the average grain diameter. The finer grain of the alloy, the higher strength and the greater hardness of the alloy. The alloy with 0.6 wt % La had the highest microhardness due to its smallest and regular grains. However, increasing the content of La to 0.9 wt %, the microhardness of alloy decreased. This could be due to the fact that AlSi5 Cu1 Mg alloy contains a variety of alloying elements.