## 3 Results and Discussion
### Refinement Mechanism of B\({}_{4}\)C Addition
Moreover, the average number of primary Mg\({}_{2}\)Si particles per unit (mm\({}^{2}\)) (density) decreased with the increasing B\({}_{4}\)C concentration up to 10 wt%, as illustrated in Figure 3. The drop in the density of primary Mg\({}_{2}\)Si particles as a result of B\({}_{4}\)C addition can be associated with the formation of some boron-containing compounds in the matrix of the composite. According to [PERSON] et al. [25], and [PERSON] et al. [33], some B-containing compounds are detected in the various B\({}_{4}\)C reinforced aluminium composites, in which some B elements are dissolved from B\({}_{4}\)C particles into the liquid-solid interface. As observed in Figure 10, the SEM micrograph with corresponding EDX results depicts that the presence of these boride compounds consist of Al and Mg. The formation of Al-Mg-B consumes the existed Mg in the melt, and hence, the amount of Mg atoms to react with Si to produce Mg\({}_{2}\)Si decreased. This findings are aligned with the results by [PERSON] et al. [34] where 1%B was added to Al-15%Mg\({}_{2}\)Si composite. In fact, partial dissolution of B\({}_{4}\)C in Al leads to the formation of B-containing compounds. [PERSON] et al. [35] have pointed out that as a result of the reaction between Al and B\({}_{4}\)C at temperatures above 700 \({}^{\circ}\)C, AlB\({}_{2}\), Al\({}_{3}\)BC, and Al\({}_{4}\)BC are produced. The dissolution of B\({}_{4}\)C immediately saturates the molten Al with carbon and boron since the solubility of these elements in Al is low [36]. However, as the amount of Al-Mg-B phase is small, it is difficult to detect them in the XRD patterns (Figure 2). The observation in the density of primary Mg\({}_{2}\)Si particles as a result of increasing B\({}_{4}\)C content is consistent with the decreasing trend of nucleation temperature (T\({}_{\rm N}\)) of primary Mg\({}_{2}\)Si phase in Figure 6. In fact, the reduction of Mg in the melt as a result of the formation of Al-Mg-B compound leads to a shift in the composition of the Al-Mg\({}_{2}\)Si composite to lower the Mg\({}_{2}\)Si content (eutectic point); hence, the nucleation temperature of primary Mg\({}_{2}\)Si phase decreased. According to [PERSON] et al. [34], in the Al-Mg\({}_{2}\)Si composite treated with B element, beside the segregation of boron on the solid-liquid interface, some B atoms also adsorbed on the growth front and changed the surface energy of the Mg\({}_{2}\)Si crystals. Nevertheless, in the present study, due to the interaction of B with the melt and formation of B-rich intermetallic compounds, more B atoms are consumed, in which the extra B atoms are inadequate in the melt to absorb the primary Mg\({}_{2}\)Si growth front and poison its anisotropic growth and alter its morphology significantly. Therefore, as shown in Figures 3 and 4, addition of B\({}_{4}\)C particles into Al-20%Mg\({}_{2}\)Si composite has a minor impact on the aspect ratio of primary Mg\({}_{2}\)Si particles, in which the maximum alteration of the aspect ratio occurred after the addition of 5 wt% B\({}_{4}\)C (5.22%) compared to the base composite. This result demonstrates that B\({}_{4}\)C addition has no obvious changes on the structure of primary Mg\({}_{2}\)Si particles.