Composite Materials Research Progress

282
S.C. Tjong
nanoparticles were prepared by means of the laser induced gas-phase reactions [39-41]. They
reported that the mechanical strength of nanoparticle strengthened composites is far superior
to that of microparticle reinforce composite with a similar volume content of particulate. In
other words, the tensile strength of Al/1vol% Si3N4 (15 nm) and Al/1vol.% Si-N-C (25
nm)nanocomposites is comparable to that of Al/15vol% SiC (3.5μm) composite, but the yield
stress of such nanocomposites is significantly higher than that of the microcomposite. The
tensile ductility of nanocomposites is also higher than that of microcomposite (Table 1).
However, increasing the Si-N-C nanoparticle content to 5 vol.% leads to deterioration of
mechanical properties as a result of the particle agglomeration. The strengthening mechanism
of nanocomposites is derived from the Orowan stress. It is well known that the Orowan
strengthening results from interaction between dislocation and the dispersed particles during
mechanical loading. Recently, Kang and Chan [36] also reported that the tensile strength of
the Al/1vol.% Al2O3 nanocomposite is similar to that of the Al/10 vol.%SiCp (13 μm)
composite, and the yield strength of the former is higher than that of the latter (Fig. 7). This
figure reveals that the yield and tensile strengths of Al reinforced with Al2O3 nanoparticles
increase with increasing filler content up to 4 vol.% Al2O3 at the expense of tensile ductility.
Above 4 vol.%, the strengthening effects level off owing to the agglomeration of alumina
nanoparticles as shown in Fig. 5. The main strengthening effect in such nanocomposites also
arises from the Orowan stress. It is worth-noting that both the tensile strength and tensile
ductility of cast Al-based composites are improved considerably as a result of better
dispersion of nanoparticles in the alloy matrix via laser assisted melting [10,11].
Figure 7. Tensile properties of Al/Al 2O 3 nanocomposites prepared by conventional powder metallurgy
method. The tensile properties of Al/10 vol.% SiC (10 μm) microcomposites are also show for the
purposes of comparison [36].