## 2 Materials and Methods  
Commercial powders of boron carbide (B4C, \(\geq\)95.0% purity, Jingangzuan Boron Carbide company Ltd., Mudanjiang, China) and aluminum (99.5% purity, Northeast Light Alloy Company Ltd., Haerbin, China) were used in this study. The microstructural morphology of the powders was investigated using a scanning electron microscope (SEM, Camscan Apollo 300). As shown in Figure 1, the B4C powder has a particle size of less than 10 \(\upmu\)m with a polygonal morphology. The aluminum powder has a particle size of less than 100 \(\upmu\)m with an ellipsoidal shape (Figure 2). From the inset picture in Figure 2, Al powders are actually made up of many particles with diameters less than 5 \(\upmu\)m. Energy dispersive X-ray analysis (EDX) showed that there is a thin oxide layer covering the Al particle surface.  
The as-received powder of 15 wt % B4C-Al and 20 wt % B4C-Al were ball-milled (Simolayer-CM01, Zoz Gmbh, Wenden, Germany) for three hours with rotation speeds of 800 rpm (high-energy ball milling process) and 100 rpm (low-energy ball milling process). The stainless steel balls with diameters of 5 mm were used and the ball-to-mass ratio was 10:1. All the powders were milled under Ar atmosphere without any process control agent. The ball-milled mixtures were cold-pressed (DY-20,KQGX Company Ltd., Tianjin, China) uniaxially in a cylindrical holder (\(\Phi\)50 mm \(\times\) 50 mm) with an external pressure of 25 MPa. The pressed samples with a theoretical density of 80% were heated in a vacuum furnace (VSF-335, Szvac Company Ltd., Shenyang, China) at 530 \({}^{\circ}\)C for three hours. The sintering procedures are illustrated in Figure 3. The sintered samples were hot extruded at 480 \({}^{\circ}\)C (with an extrusion rate of 9:1 and a extrusion speed of 1 cm\(\cdot\)s\({}^{-1}\), YL32-1000, Yili Company Ltd., Hanyang, China) followed by a hot rolling (with a speed of 0.3 m\(\cdot\)s\({}^{-1}\), LG-300, Yunnan Machine Tool Factory, Kunming, China) at 460 \({}^{\circ}\)C. The final products are plates with a thickness of 3 mm [18; 19; 20; 21; 22]. Sample plates prepared using the pure Al powder without B\({}_{4}\)C particles were used as a reference for comparison.  
Figure 1: Scanning electron microscope (SEM) micrograph of B\({}_{4}\)C powders.  
Figure 2: SEM micrograph of Al powders; the inset picture shows the magnified image of a single Al particle.  
Crystalline structures of the composites were examined by X-ray diffraction (XRD, Panalytical X'pert PRO MPD). Microstructural and fracture morphology was investigated using a scanning electron microscope (SEM, Camscan Apollo 300). Mechanical properties of the composites were studied at room temperature using a tensile test facility (MTS 810). Rectangular tensile samples were prepared according to the standard of GB/T 228-2002. The size of the tensile samples was 104 mm (L) \(\times\) 22.5 mm (W) \(\times\) 3 mm (T). Neutron radiography of the composites was taken along the thickness direction (normal direction, ND) in order to check the homogeneity of these composites. The neutron fluent rate was 7.8 \(\times\) 10\({}^{7}\) n\(\cdot\)cm\({}^{-2}\)\(\cdot\)s\({}^{-1}\), and the neutron energy was 0.0253 eV. The size of rectangle samples for neutron radiography was 40 mm (L) \(\times\) 10 mm (W) \(\times\) 3 mm (T). The center line of the all the studied samples parallels the hot-rolling direction (rolling direction, RD).