Microstructure and Mechanical Properties of Carbon Nanotubes-Reinforced 7055Al Composites Fabricated by High-Energy Ball Milling and Powder Metallurgy Processing

In the recent years, lightweight and high-strength structural materials have gained much attention in engineering applications. Carbon nanotube (CNT)-reinforced Al (CNT/Al) composites are promising structural materials owing to the good mechanical properties and high reinforcing efficiency of CNTs. Previous studies on these composites mainly focused on fabricating CNT-reinforced low-strength or medium-high-strength Al alloys (such as pure Al, or 2xxx series or 6xxx series Al alloys) composites via various dispersion methods. However, only few studies investigated composites with super-high-strength Al alloys as the matrices. In the present work, CNT/7055Al composites with CNT volume fractions of 0%, 1%, and 3% were prepared by high-energy ball milling combined with powder metallurgy. The CNT distribution, grain structure, interface, and mechanical properties of the CNT/7055Al composite were investigated using OM, SEM, TEM, and tensile tests. The strengthening mechanism and anisotropy of the composite were analyzed. The results indicated that the composite had a bimodal grain structure consisting of CNT-free coarse grain zones and CNT-enriched ultrafine grain zones. CNTs were well dispersed in the ultrafine grain zones of the Al matrix, and the CNT/Al interface was clean. There were only few reaction products at the interface. The tensile strength of the 3%CNT/7055Al composite reached 816 MPa, but the elongation was only 0.5%. Grain refinement and Orowan strengthening were the main strengthening mechanisms of the CNT/7055Al composite. Because of the load transfer efficiency of CNTs and a coarse grain band structure, the composite exhibited stronger anisotropy than the matrix alloy. The tensile properties of the CNT/7055Al composite normal to the extrusion direction were weaker than those in the extrusion direction.