Distribution and stability of carbon nanotubes during multi-pass friction stir processing of carbon nanotube/aluminum composites

Multi-pass friction stir processing was used to fabricate multi-walled carbon nanotube/aluminum composites with a high volume fraction (>50%) of reinforcing material homogeneously dispersed in the matrix. The microhardness of the produced composites were two times higher than the original aluminum alloy. SEM and TEM analyses of the microstructures indicated that the reinforcing phase was uniformly distributed after three friction stir passes but the thermo-mechanical cycles destroyed the tubular structure of the carbon nanotubes. TEM micrographs revealed that carbon nanotubes were mostly transformed to polyaromatic and turbostratic carbon structures along with Al4C3 after 3 passes.