Carbon nanotubes as reinforcements and interface modifiers in metal matrix composites
Multi-wall carbon nanotubes (MWCNTs), catalytically grown by chemical vapour deposition (CVD), have been successfully employed in the development of bulk composites, where aluminium alloys (Al-4%Cu-1%Mg-0.5%Ag, called WFA) or commercial pure magnesium have been used as matrix, while MWCNTs served as interface modifiers and/or reinforcing phase. Four different composites, WFA or Mg reinforced with 25% volume fraction of Al2O3 SAFFIL fibres coated or not with MWCNTs were processed by low pressure infiltration. Two other composites, made of pure Mg reinforced with 1% and 2% weight fraction of MWCNTs, were fabricated by powder metallurgy. Scanning Electron Microscope (SEM) revealed fine microstructures of the produced specimens: locally homogeneous distribution of carbon nanotubes (CNTs) in 1% and 2% of CNTs reinforced Mg composites, and overall uniform dispersion of SAFFIL fibres in WFA-25%SAFFIL-CNTs and Mg-25%SAFFIL-CNTs. However, CNTs are not visible in WFA-25%SAFFIL-CNTs and Mg-25%SAFFIL-CNTs in this scale. It could be assumed that together with SAFFIL fibres, they are embedded completely in the matrix. A substantial increase in Young's modulus has been attributed to the addition of CNTs as well as the effective interface bonding between CNTs and matrix. Elastic and anelastic properties of the specimens of the six composites have been investigated by mechanical spectroscopy. Mechanical loss and shear modulus were characterized as a function of temperature. The obtained experimental results have demonstrated positive impacts of MWCNTs on the elastic and anelastic properties of these composites, by comparing to the corresponding MWCNTs-free materials, respectively, and the advantages of using CNTs as reinforcements in metal matrix composites (MMCs). A significant increase in transient damping due to CNTs has been measured in WFA-25%SAFFIL-CNTs and Mg-25%SAFFIL-CNTs. It indicates that a SAFFIL-matrix interface softening has been achieved. This has been interpreted by theoretical modelling as due to an additional density of mobile dislocations nucleated by CNT tips in the interface between SAFFIL fibres and matrix. On the other hand, the CNTs far away from the SAFFIL-matrix interface are assumed to pin dislocation loops, leading to a decrease in their mobility. Dislocation motion controlled by solid friction has been identified as the relaxation mechanism of thermal stresses. Damping in Mg-2%CNTs remains approximately as high as in pure Mg. Mechanical loss spectrum obtained in both Mg-1%CNTs and Mg-2%CNTs is composed of a relaxation peak superposed on an exponential background. The activation enthalpy of the peak is of 0.83 eV and the limit relaxation time of 2.3 × 10-11s. The peak is explained by a dislocation dragging model. An enhancement in the shear modulus has been measured in WFA-CNTs, Mg- 25%SAFFIL-CNTs and Mg-2%CNTs due to the addition of CNTs. Hot Isostatic Pressing (HIP), which was used in processing Mg-2%CNTs, was found to be advantageous in obtaining a CNTs - Mg composite with better interface bonding.