## 3 Results and Discussion
### Mechanical Properties
#### 3.5.3 Fracture Surface  
The SEM images of tensile fracture surfaces of the Al-20%Mg2 Si composite added with different percentages of B4C are exhibited in Figure 13a-f. Primary Mg2 Si particle is intrinsically brittle and has more intrinsic defects, which cause cellular fracture mode in the fracture surface of base Al-20%Mg2 Si composite due to cleavage in fracture planes of coarse Mg2 Si particles that are indicated by arrows (Figure 3a). Furthermore, the sharp corners of the irregular primary Mg2 Si particles in the base Al-Mg2 Si composite serve as stress concentration sites for crack initiation and propagation which results in fracture of these coarse particles (Figure 3b) [39]. However, after adding B4C particles up to 5 wt% to the Al-20%Mg2 Si composite, the fracture mode changed from brittle in the base composite to ductile, as depicted in Figure 13c,d. An interesting result is achieved for Al-20%Mg2 Si-5%B4C hybrid composite, in which the fracture surface contains decohered primary Mg2 Si particles and a more ductile area. The obtained result is because of the presence of partially refined primary Mg2 Si particles and also B4C particles, which have uniform distribution and good bonding with the Al matrix. The matrix alloy can be reinforced with a uniform distribution of the particle due to the effective role of the particle-rich region in the prevention of crack propagation; however, a particle-poor region offers canals for the propagation of crack [40]. Due to the decreasing size and aspect ratio of Mg2 Si particles after the addition of 5 wt% B4C particles (Figure 4), the Mg2 Si provides less potential areas for crack nucleation compared to the base composite, and hence crack initiation is hindered. Furthermore, as the bonding between B4C particles with the matrix is good, fracturing almost occurs in the B4C particles rather than in the particle-matrix interface. This is in accordance with the model proposed by [PERSON] et al. [41] in simulation of the fracture behavior in MMCs. Based on their suggestion, regardless of particle density, the fracture of interface is strength controlled, and decohesion occurred once the tensile stress in the particles exceeded the interface strength as a critical value. Based on these results, it is concluded that the Al-B4C interface is steady and stable, hence, the initiation and propagation of the cracks is from Mg2 Si particles and not the Al-B4C interface. Moreover, as seen in Figure 13d, the fracture surface exhibits several dimples, which are indications of a ductile fracture. This result is aligned with high strength and elongation of composite, as shown in Figure 12.  
Figure 13e,f depicts the fracture surface of Al-20%Mg2 Si-10%B4C hybrid composite. As observed, B4C particles agglomerate in some areas on the fracture surface. The particle agglomeration increases the local stresses and proffers substrates for nucleation of crack and low energy propagation canals across the linked brittle particles. Failure can be portrayed by higher stresses generated in these areas. The interface of B4C/matrix where the B4C particles agglomerate shows interfacial decohesion, in which the failure took place along this interface. If the bonding strength in the interface between particle and matrix is weak, debonding occurs between matrix and particles before a particle fracture [39]. As the B4C particles debonded from the matrix in a brittle manner, this feature reveals a weak  
interface bonding which creates favorable substrates for the initiation of crack and drops the load bearing capacity of the composite. In this condition, the crack needs less energy to propagate through the interface which can be reconciled with low strength and elongation of composite, as depicted in Figure 12. The crack initiates at the interface between particle and matrix, then propagates through the matrix, and finally joins other cracks, resulting in composite failure. Porousity and bifilm on fracture surface are clearly observed in Figure 13e.