Microstructural evolution of nanodiamond-reinforced aluminum matrix composites during the process of hot extrusion

Nanodiamond (ND) has been recently considered to be an excellent reinforcement for strengthening metal matrix composites. However, most of the previous studies focused on manufacturing process, final structure, and mechanical properties. Systematic exploration of the microstructural evolution, which is critical to understand the effect mechanism of ND particles, still lacks. Herein, we seek to understand the effect of ND particles on microstructural evolution via comparative investigation of microstructures in different test points on extrusion remnant specimens of pure aluminum, 0.5 wt% and 1.0 wt% ND-reinforced aluminum composites prepared by powder metallurgy and hot extrusion. Finite element simulations were used to analyze the relative stress-strain states and deformation modes in different test points. Electron backscatter diffraction and transmission electron microscopy studies reveal that the mean grain size of ND-reinforced aluminum composite declined rapidly after entering the extrusion deformation zone. Microstructures in simple shear, tensile pure shear, and compressive pure shear deformation modes were quite different. The dispersed ND particles effectively promoted grain refinement and improved dislocation driving force with the increase in plastic strain. The large localized highcontent ND clusters were broken into small ones, and small ones acted as non-deformable particles to stimulate nucleation and recrystallization under high dislocation driving force. Test results show that the hardness of 0.5 wt% and 1.0 wt% ND-reinforced aluminum composites was 1.9 and 2.1 times that of pure aluminum, respectively. The 0.5 wt% ND-reinforced aluminum composites exhibited optimal comprehensive mechanical properties with a tensile strength of 205 MPa and elongation of 18%, which were 115% and 91% of pure aluminum, respectively.