Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

Figure 2.13 TEM image of bulk Al/5 wt% CNT bulk
nanocomposite showing homogeneous dispersion of CNTs
within the metal matrix. Reproduced with permission from [46].
Copyright Ó (2007) Wiley-VCH Verlag GmbH.
2.4 Aluminum-Based Nanocompositesj59
turns applied to the disk and the magnitude of applied pressure control the final grain
sizes of deformed materials. In ECAP, an intense plastic strain is imposed by
pressing a sample in a die consisting of two channels equal in cross section,
intersecting at an angle ranging from 90 to 157 . The billet retains the same
cross-sectional area so that repetitive pressings can be applied to accumulate large
plastic strains [47]. In practice, ECAP is more attractive because it can refine the grain
sizes of bulk MMCs billets to the submicrometer region [48–50]. ECAP treatment can
also improve the homogeneity of reinforcement distribution in the matrix of Al-based
MMCs [50]. Apart from grain refinement of metallic materials, HPT has been used to
consolidate powder materials to form titanium matrix nanocomposites [51]. An
in-depth understanding on the microstructure and deformation behavior of the HPT
and ECAP prepared metal-matrix nanocomposites is still lacking.
More recently, Tokunaga et al. attempted to use HPT to produce Al-based nanocomposites
reinforced with 5 wt% SWNT and 5 wt% fullerene, respectively [52, 53].
In the process, Al powder (75 mm) was mixed with either SWNTor fullerene in ethanol
ultrasonically followed by evaporating the solvent. The powder mixture was then
placed in a central hole located at the lower anvil. The two anvils were brought into
contact to apply a pressure on the disk. The lower anvil was rotated with respect to
the upper anvil at a rotation speed of 1 rpm. A pressure of 2.5 GPa was applied during
the operation at room temperature. Figure 2.14(a) and (b) show bright field and
dark field TEM images for pure Al and Al/5 wt% SWNTnanocomposite, respectively.
Large shear straining generates many dislocations that subsequently rearrange into
subgrain boundaries. From dark field TEM images, HPTproduces finer grain size of
Al matrix ( 100 nm) of nanocomposite compared with that of pure aluminum
( 500 nm). The grain size of Al matrix of Al/5 wt% SWNT is even smaller than the
grain size reported on the bulk sample deformed by severe plastic deformation. The
presence of rings in the selected area diffraction patterns indicates the formation of