Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

Table 4.2 Density and mechanical properties of AZ91D/MWNT nanocomposites.
Materials Density (g cm 3 )
Elastic
modulus
(GPa)
Tensile
strength
(MPa)
Yield stress
(MPa) Ductility (%)
AZ91D 1.80 0.007 40 2 315 5 232 6 14 3
AZ91D/0.5%CNT 1.82 0.008 43 3 383 7 281 6 6 2
AZ91D/1%CNT 1.83 0.006 49 3 388 11 295 5 5 2
AZ91D/3%CNT 1.84 0.005 51 3 361 9 284 6 3 2
AZ91D/5%CNT 1.86 0.003 51 4 307 10 277 4 1 0.5
Reproduced with permission from [Chap. 2, Ref. 63]. Copyright Ó (2008) Elsevier.
4.2 Tensile Deformation Behaviorj111
increase slowly with increasing CNF content from 1.5 to 7.5 wt%. The yield stress and
tensile strength of the AZ91D/1.5% CNF nanocomposite are 342 and 400 MPa,
respectively. The yield stress and tensile strength of the AZ91D/7.5% CNF nanocomposite
further increase to 416 and 470 MPa, respectively. The strengthening
effect of nanofibers is pronounced and can be attributed to better dispersion of
nanofibers in the magnesium alloy matrix. This arises from the employment of
multiple casting processes and extrusion as well as the improvement in the
wettability of nanofibers through the use of silicon coating. The mechanisms
responsible for the strengthening of AZ91D/CNF nanocomposites include load
transfer effect (Equation 4.4) and grain refinement strengthening (Hall–Petch
relationship). Furthermore, the contribution of load transfer to the yield stress
enhancement is twice of that of grain refinement.
In addition to squeeze casting, disintegrated melt deposition (DMD) seems to be
an effective technique for depositing near net shape metal-matrix composites [Chap.
2, Ref. 61]. Gupta and workers used the DMD technique to fabricate the Mg/MWNT
nanocomposites containing fillers from 0.3 up to 2 wt%. The resulting nanocomposites
were then extruded [Chap. 2, Ref. 5, 19]. Figure 4.7 shows the stress–strain
curves of pure Mg and Mg/MWNT nanocomposites. The density and mechanical
properties of these specimens are listed in Table 4.3. Apparently, the density of
nanocomposites remains almost unchanged by adding nanotubes up to 1.6 wt%.
At 2 wt% MWNT, the density begins to decrease due to the formation of micropores.
Tensile test data show that the yield and tensile strengths as well as tensile ductility of
Mg-based nanocomposites increase with increasing nanotube content up to 1.3 wt%.
At 1.6 and 2 wt% MWNT, the yield stress, tensile strength and ductility reduce
considerably due to the agglomeration of nanotubes and micropore formation. The
improvement in tensile ductility of nanocomposites with MWNTcontent 1.3 wt% is
attributed to the high activity of the basal slip system and the initiation of prismatic
hai slip [Chap. 2, Ref. 5, 19]. Magnesium generally exhibits low tensile ductility due to
its hexagonal close-packed (HCP) structure having only three independent slip
system. MWNTs assist the activation of prismatic and cross-slip in the matrix during
extrusion. Texture analysis reveals that the basal planes of magnesium matrix tend to
align with the extrusion direction (Figure 4.8). Such dislocation slip behavior has
been confirmed by transmission electron microscopyTEM observations.