Influence of high B4C contents on structural evolution of Al-B4C nanocomposite powders produced by high energy ball milling

The influence of high B4C contents on the structural evolution of Al-B4C nanocomposite powders during high energy ball milling was investigated. Al-B4C nanocomposite powders with various weight percentages of B4C particles (10, 30 and 40 wt%) were prepared. The scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis were carried out for the evaluation of structural evolution in the matrix. Nanocrystalline Al-B4C composite powders were successfully synthesized after milling. The morphology of matrix transformed from flat elongated to more fragmented with partly equiaxed, and the size of matrix powders decreased as the B4C content increased. The XRD analysis shows that the average grain sizes of 48 nm, 46 nm, and 45 nm corresponded to B4C contents of 10, 30 and 40 wt% respectively after 60 h of milling, TEM observation (51, 41 and 42 nm, respectively) shows good conformance with XRD analytical results. It illustrated that the grain size of Al was decreased with the increase of B4C. However an inverse variation relationship of lattice strain, dislocation density is evident with the increase of B4C content. It indicated that Al-30% B4C and 40% B4C nanocomposite powders induced the finer grain size even with low lattice strain and dislocation density. It could be explained as the enhanced and prolonged effect of the grain size decreased rapidly in the early stage of ball milling, but the lattice strain and dislocation density are still lower than that in the later stage. The results revealed that grain refinement can enhance with high reinforcement content even at low lattice strain and dislocation density. Furthermore, the increase in the B4C content can also lead to the transformation from irregular or flat elongated grains to equiaxed nanograin morphology.