Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

Figure 4.4 SEM micrographs showing tensile fracture surfaces of
(a) 2024 Al/1 wt% MWNT and (b) 2024 Al/2 wt% MWNT
nanocomposites. Reproduced with permission from [Chap. 2,
Ref. 35]. Copyright Ó (2007) Elsevier.
elongation of nanocomposites remains nearly unchanged by adding nanotubes up
to 1 wt%; this is due to the superior flexibility of CNTs. As a result, CNTs can bridge
advancing crack during tensile deformation, thereby improving the tensile ductility
of nanocomposites having nanotube content 1 wt% (Figure 4.4(a)). On the other
hand, pull-out of nanotubes can be readily seen in the fracture surface of 2024 Al/2 wt
% MWNT nanocomposite, indicating poor interfacial nanotube–matrix bonding
strength (Figure 4.4(b)).
Nanocrystalline metals generally exhibit limited tensile plasticity due to a lack
of work hardening capability. Therefore, cracks are readily initiated in nanocrystalline
metals during the earlier stage of tensile deformation. Figure 4.5 shows typical tensile
true stress–strain curve of nanocrystalline Al prepared by extruding ball-milled
powders [18]. Apparently, nanocrystalline Al exhibits a high mechanical strength of
Figure 4.5 Tensile true stress–strain curves of nanocrystalline
(nc) Al and its composite reinforced with 0.7 vol% SWNT. These
materials were prepared by hot extrusion of ball-milled powders.
Reproduced with permission from [18]. Copyright Ó (2007)
Elsevier.
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