Carbon nanotube induced microstructural characteristics in powder metallurgy Al matrix composites and their effects on mechanical and conductive properties

In this study, Al matrix composites (AMCs) containing different contents (0-1 vol.%) of carbon nanotubes (CNTs) were fabricated by powder metallurgy (PM). A mechanical mixing process was used to disperse CNTs in Al powders. The powder mixtures were consolidated by spark plasma sintering and subsequent hot-extrusion. The microstructures, tensile properties, electrical conductivity (E.C.) and thermal conductivity (T.C.) of CNT/Al composites were examined. Within 0.75 vol.%, CNTs could be well dispersed and aligned in Al matrix. However, 1 vol.% CNTs led to agglomerations and resultant degraded tensile properties. With CNT additions, Al grains were refined under the pinning effect of CNTs at grain boundaries. An oxygen-rich layer was observed at PM CNT-Al interfaces by combination of transmission electronic microscopy and energy dispersive spectrometer analysis. The oxygen-rich layer was favorable to improve the bonding conditions between CNTs and Al matrix, leading to CNT fracture during composite failure. The refined grains and oxygen-rich interfaces were responsible for the improvement of tensile strength, but decreases of E.C. and T.C. of CNT/Al composites. Considering the CNT induced microstructure characteristics, grain & interface modified models were further set up. They showed good predictions on both the tensile strength and conductivity of CNT/Al composites. The results might provide new insights for designing simultaneously strong and high-conductivity metal matrix composites.