## 3 Results and Discussion
### Mechanical Properties
Consequently, the poor interfacial bonding between the Al matrix and the B\({}_{4}\)C particles facilitates the easy crack propagation, which deteriorates the mechanical properties of the composites. For 15 wt % and 20 wt % B\({}_{4}\)C-Al composites prepared by the high-energy ball milling process, BSTL images reveal a homogeneous distribution of the B\({}_{4}\)C particles (with size of \(\sim\)10 \(\upmu\)m) inside the Al matrix and a tight bonding between the B\({}_{4}\)C particles and the Al matrix without apparent porosity (Figure 10d,e). Accordingly, the high-energy ball milling is an effective process to induce a uniform dispersion of the B\({}_{4}\)C particles inside the Al matrix. In such cases, the load transfer from the matrix to the reinforcement is upgraded, leading to the dramatic enhancement of the yield and tensile strength of B\({}_{4}\)C-Al composites (Figures 8 and 9). The de-cohesion of particle-matrix interfaces is facilitated and plasticity is improved (Figure 7). Both the 15 wt % and 20 wt % B\({}_{4}\)C-Al composites prepared by the high-energy ball milling process are stronger than those prepared by the low-energy ball milling process. XRD results show that the phases of all prepared composites are almost the same. Therefore, the differences in the mechanical properties of the composites could be attributed to the distribution of B\({}_{4}\)C particles inside the Al matrix and the bonding between the B\({}_{4}\)C particles and the Al matrix. After hot extrusion and hot rolling processing, the fracture surfaces of the composites prepared by low-energy ball milling process show the apparent B\({}_{4}\)C clusters or agglomerates inside the Al matrix, with a weak bonding between the reinforcement and the matrix (see Figures 5a,b and 10a,b). [PERSON] et al. [3; 26; 27; 28; 29] reported that when the particle size of the matrix materials was considerably larger than the reinforcement particle size, the agglomeration of the reinforcement could take place, which worsened the mechanical properties of the composites and decreased the material fabricability. When the reinforcement particles were homogeneously distributed in the matrix, the P/M-processed composites showed the best characteristics, especially when the ratio of the matrix particle size to the reinforcement particle size was close to or less than unity. In this study, even though the particles size of the matrix materials is about ten times of the size of the reinforcement particle, a homogeneous distribution of B\({}_{4}\)C particles and a good coherent bonding between the reinforcing particles and the matrix is still observed in the samples prepared by high-energy ball milling process (see Figures 5c,d and 10d,e), which contributes to the remarkable improvement of the mechanical properties of Al-B\({}_{4}\)C composites. These are coincident with our present tensile experiment. The deformation energy, obtained by calculating the areas below stress-strain curves, can be used to compare the tough or plastic deformation ability of the composites. As shown in Figure 7, the deformation energies of the composites prepared by high-energy ball milling process are \(\sim\)1851 J\(\cdot\)cm\({}^{-3}\) and \(\sim\)1524 J\(\cdot\)cm\({}^{-3}\) for 15 wt % and 20 wt % B\({}_{4}\)C-Al composites, respectively, which are approximately an order of magnitude higher than those prepared by low-energy ball milling process (\(\sim\)1035 J\(\cdot\)cm\({}^{-3}\) and \(\sim\)560 J\(\cdot\)cm\({}^{-3}\) for 15 wt % and 20 wt % B\({}_{4}\)C-Al composites, respectively). It can be deduced that a homogeneous distribution of B\({}_{4}\)C particles would increase the plastic deformation ability of B\({}_{4}\)C-Al composites. In brief, results indicated that the high-energy ball milling is an effective process to prevent the agglomeration of the reinforcement particles even if the matrix-to-reinforcement particle size ratio was nearly 10.