Composite Materials Research Progress

Recent Advances in Discontinuously Reinforced Aluminum… 285
Figure 10. Comparison of creep behavior between Al/1vol.%Si-N-C (25 nm) nanocomposite (open
symbol) and Al/15vol% SiC (3.5μm) composite (solid symbol) at 573 and 623 K [41].
The creep behavior of the nanoparticle reinforced composites is mainly depended on the
matrix materials selected, i.e pure aluminum and aluminum solid solution alloy. The high
temperature creep strength of micrograined aluminum is also greatly improved by the
addition of low volume content of ceramic nanoparticles. Tjong et al. demonstrated that the
creep resistance of the Al/1vol.%Si3N4 (15nm) and Al/1vol.%Si-N-C (25 nm)
nanocomposites is about two orders of magnitude higher than that of the [40, 41]. Fig. 8
shows the variation of steady creep rate vs applied stress for the Al/1vol.%Si-N-C (25 nm)
nanocomposite. The Arrhenius plot of creep rate against 10 3 /T for this nanocomposite is
shown in Fig. 9. The nanocomposite exhibits an apparent stress exponent (n) varying from
15.7 to 23.0 and an apparent creep activation energy (Q) of 248 kJ/mol. The apparent
activation energy of the Al/1vol.%Si-N-C nanocomposite is much higher than that for lattice
diffusion of aluminum (142 kJ/mol). Similar high apparent values of n and Q values are also
observed for the Al/1vol.% Si3N4 nanocomposite. For the purposes of comparison, the creep
rates of the Al/15vol.% SiCp (3.5 µm) microcomposite and the Al/1vol.%Si-N-C (25 nm)
nanocomposite at 573 and 623 K are presented in Fig. 10. It is evident that the creep rate of
the Al/1vol.%Si-N-C nanocomposite is about two orders of magnitude lower comparing to
the Al/15vol.% SiCp (3.5 µm) microcomposite. To rationalize the high apparent values of n
and Q of the Al/1vol.%Si-N-C nanocomposite, a slip creep mechanism of constant
substructure as given in Eq (2) is applied to the Al/1vol.%Si-N-C nanocomposite (Fig. 11). It
appears that the datum points at three temperatures can be fitted linearly. It is considered that
the nanoparticles of very small volume content (1 vol.%) pin the subgrain boundaries
effectively. Consequently, the microstructure of nanocomposite remains unchanged during
creep deformation. The threshold stress can be determined from Fig. 12 by extrapolating the
linear regression line to zero strain rates. The values of threshold stress are determined to be
36.3, 25.7 and 17.3 MPa at 573, 623 and 673 K, respectively. It is obvious that the threshold