## 3 Results and Discussion
### Mechanical Properties
#### 3.5.2 Tensile Properties  
The engineering stress-strain curves of Al-Mg2 Si composite added with various contents of B4C ceramic particles is illustrated in Figure 12a. The ultimate tensile strength (UTS) and elongation percentage to failure (EI%) values of the composites are demonstrated in Figure 12b,c respectively. As seen, the addition of 2.5 and 5 wt% B4C to Al-20%Mg2 Si composite increased the tensile properties, in which the addition of 5 wt% B4C results is the highest value as the ultimate tensile strength (UTS) and percentage elongation (EI%) increased from 177 MPa and 4.6% in the base Al-20%Mg2 Si composite to 217 MPa and 7% in Al-20%Mg2 Si-5%B4C hybrid composite. It can be observed that there is a consistency between these results and the microstructural observations in Figure 3 as Al-20%Mg2 Si-5%B4C hybrid composite exhibits a uniform distribution of B4C reinforcements as well as refined primary Mg2 Si particles. The high strength values of Al-20%Mg2 Si-5%B4C hybrid composite is due to the existence of B4C particles in the composite matrix with uniform distribution, which creates dislocation pile-up in their neighborhood. Thus, the strength of the composite enhanced as a result of increasing dislocation interactions and dislocation density in the matrix near interfaces of the matrix and reinforcement. Figure 11: Vickers hardness of Al-20%Mg2 Si composite treated with various amount of B4C particles. In addition, partial refinement of primary Mg\({}_{2}\)Si particles has a significant impact on strength enhancement of the composite. As depicted in Figure 3a, in base Al-20%Mg\({}_{2}\)Si, the primary Mg\({}_{2}\)Si particles exist with polyhedral shape and sharp tips, in which these sharp areas are potential sites for stress concentration which cause a drop in tensile properties. Nevertheless, in Al-20%Mg\({}_{2}\)Si-5%B\({}_{4}\)C hybrid composite, the primary Mg\({}_{2}\)Si particles are finer and their morphology is regular (Figure 3c) compared to the base material, which results in tensile improvement. The most probable explanation of this behavior could be a deformation strengthening of hybrid composites which are brittle in nature but demonstrate higher strength and elongation when added with B\({}_{4}\)C particles due to Hall-Petch dependence on the Mg\({}_{2}\)Si particles size. The effect of particles size on tensile strength can be discribed through dislocation theory. The formation of plastic deformation needs to overcome the ultimate stress, which can attribute to a mass of moving dislocation. However, grain boundary will hinder the dislocation glide, and even leads to the dislocation pile-up. When the content of dislocation is pile-up increases a certain value, it will become the driving force of dislocation glide [37]. Furthermore, the particle size becomes larger, the content of dislocation pile-up is more; and the driving force of dislocation glide is bigger. That is why alloy with small particle size exhibits high strength. At the same time, due to small particle size increasing the number of particle boundary, the ability of hindering the dislocation motion is high. Therefore, the UTS for the plastic deformation is strongly influenced by Mg\({}_{2}\)Si particles size. Figure 12: (**a**) Engineering stress-strain curve of Al-20%Mg\({}_{2}\)Si with various content of B\({}_{4}\)C (**b**) the ultimate tensile strength (UTS) and (**c**) percentage elongation (El%) values of Al-Mg\({}_{2}\)Si composite with different B\({}_{4}\)C contents. However, as shown in Figure 12b,c, when the concentration of B4C increased to 10 wt%, the UTS and El% of the composite reduced to 150 MPa and 2.7%, which is lower than other fabricated composites. The reduction in the tensile properties of the composite from the addition of higher B4C concentration (10 wt%) is due to the agglomeration of B4C particles as well as their non-uniform distribution in the matrix (Figure 3d).