Biomedical Engineering Reference
In-Depth Information
factor is the process used to produce the nanocomposite. For some
processes, the influence of nanosize reinforcement has proven to be able to
refine the grain size in some fabrication methods. The existence of
nanoparticles can also lead to a smaller grain size during the post-treatment
process. Li et al. (2009) pointed out that ceramic particles enclosed by a thin
nanocrystalline layer produced an average grain size of 35 nm and this was
estimated to be the dominant strengthening mechanism, followed by
dislocation strengthening and then secondary-phase strengthening.
Goussous et al. (2009) observed a nanoscaled grain size due to large
deformation during the back pressure equal-channel angular pressing (BP-
ECAP) process; nanograin was the main factor that contributed to the great
increase in hardness. Cao et al. (2008a) used ultrasonic vibration to disperse
nanoparticles into liquid metal and grain refining was observed. Choi et al.
(2011) produced nanocomposite powder of different grain sizes by changing
the milling time. It has also been shown that grain refinement occurs when a
large amount of Y 2 O 3 is added to the matrix. MMNCs should have a higher
strength due to the grain refining effect of nanoparticles during solidification
and also the heat treatment process.
Based on the analysis above, the Hall-Petch mechanism can be a major
strengthening mechanism in routes where the grain refining effect is
achieved either by adding nanoreinforcements or by altering the manu-
facturing process.
Orowan and dispersion strengthening
Orowan strengthening is used to express the strengthening effect of
dimension and interparticulate spacing of secondary-phase dispersoids.
Orowan strengthening caused by the resistance of closely spaced hard
particles to the passing of dislocations is important for MMNCs, but is not
a significant strengthening factor for MMCs. Zhang and Chen's model takes
it into consideration as a major strengthening factor. The general equation is
(Sanaty-Zadeh, 2011):
￿ ￿ ￿ ￿ ￿ ￿
ln ð d
M0
:
4Gb
=
b
Þ
s Or ¼
½
6
:
2
1 = 2
l
1
1 = 2 d, M is the mean orientation factor, G is the shear
modulus of the matrix (Pa), b is the Burgers vector (m),
where d
¼ð
2
=
3
Þ
n
is the Poisson
ratio, d is the particle diameter, and
is the interparticle spacing. For
nanosize reinforcement, Orowan bowing, the Orowan dislocation bowing
mechanism, and the Orowan loop mechanism are used to explain the effect
of Orowan strengthening for MMNCs.
λ
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