Microstructure evolution and mechanical properties of carbon nanotubes reinforced Al matrix composites

Carbon nanotubes reinforced pure aluminum (CNT/Al) composites were fabricated by combined ball milling and powder metallurgy (PM) techniques. The distribution of CNTs, the evolution of the average Al grain size of the powder mixtures and as-prepared composite bulks were investigated, and the mechanical properties of the composites were also tested. With increasing ball milling time, the entangled CNTs were broken, gradually achieving a uniform dispersion within the Al matrix. The microstructure became denser and the Al grains in the powder mixture and extruded composites got significantly refined. Some small-sized Al4C3 needles along the Al grain boundaries were observed using transmission electron microscopy (TEM). The in-situ formed Al4C3 rods have an orientation relation with the Al matrix as Al4C3 [110]//Al [(1) over bar 12], which strongly improved the Al-CNT interface bonding. The yield and the ultimate tensile strength of the composites significantly increased, when the ball milling time increased from 2 to 12 h, finally reaching about 210 +/- 4.2 MPa and 253 +/- 3.7 MPa, respectively, for the composite milled for 12 h. The enhancement of mechanical properties mainly stems from the uniform distribution of CNTs, the grain refinement of the Al matrix and the in-situ formed Al4C3.