## 1 Introduction  
Aluminum-based metal matrix composites (MMCs) generally show low density, low coefficient of thermal expansion, and high specific strength and modulus, etc. ; therefore, they have been widely used in aerospace, military weapons, structural applications, and automobile products [1; 2; 3; 4; 5; 6; 7]. Boron carbide (B4C) has a low density (2.52 g/cm\({}^{3}\)), extremely high hardness, excellent chemical resistance, and a high neutron absorption cross-section [1; 2; 8; 9; 10; 11; 12; 13]. B4C-Al composites have the potential to combine the high toughness and hardness of B4C with the good ductility and lightweight structure of the Al matrix; thus, they have been recently investigated for various applications in high-strength structural materials, armor plate materials, substrate materials for computer hard disks, and neutron absorber materials in nuclear industries [7; 11; 14; 15]. Generally, the distribution of the reinforcing particles significantly affects the performance of particulate reinforced MMCs [3; 8]. B4C, Al2O3, and SiC-reinforced Al matrix composites were previously fabricated using a liquid melt-stirring process in air [5]. Below 1100 \({}^{\circ}\)C the wetting between Al and B4C is bad, so it is hard to prepare Al-B4C composites by dispersing particles into the liquid phase. In order to improve the wettability of reinforcing particles and enhance their incorporation into molten aluminium, reinforcing particles are often coated or heat treated in advance [2]. The B4C reacts drastically with molten Al, producing various binary and ternary compounds, such as Al3 BC, AlB2, Al4C3, etc., which are detrimental to the mechanical properties of the composites. On the other hand, the clustering of the reinforced particles tends to takes place when the particle size of the matrix exceeds that of the reinforcement significantly, which will worsen the mechanical properties of the composites [3]. With the aim of controlling the precise distribution of the super-hard B4C particles into the soft Al matrix, the powder metallurgy (P/M) process is focused in this work. The P/M route has potential advantages in (1) easing the harmful interfacial reaction between the matrix and reinforcement; (2) improving the possibility of adding higher amounts of reinforcement; (3) controlling the microstructure of the phases. Mechanical alloying (MA) is one of the P/M processes that produce uniform dispersion of the reinforcement particles in the matrix, which help to avoid the reinforcement clustering of agglomerates on the matrix, especially when the reinforcement particles size is small. By using the high-energy ball milling technique during the mechanical alloying process, a severe plastic deformation was imparted on the powder, leading to structural refinement by shear and the fracture of the powder mixtures and the re-crystallization process. The MA process is based on repeating welding-fracturing-welding of powder mixtures in a high-energy ball mill [3]. In the present work, the powder mixtures of Al and B4C were ball-milled at different energy levels followed by pressing, sintering, hot-extrusion, and hot-rolling to improve the mechanical and neutron absorption properties of the B4C-Al composite. It is well known that the nuclide boron-10 (B-10) has a large thermal neutron absorption cross section above 3800 barns [11; 12; 13; 14]. To apply the B4C-Al composites in nuclear industry, the urgent issue is the neutron absorption ability which is closely related to the safety of nuclear plants. For the B4C-Al composites prepared in this study, the distribution of the B4C particle could affect the properties of the thermal neutron absorption of the composite materials, which is mainly due to the differences of B-10 density in the matrix.