Biomedical Engineering Reference
In-Depth Information
the degree of deformation that occurs during consolidation. The grain size
can also influence properties via the Hall-Petch relationship. The grain size
can be refined by ultrasonic vibration and large plastic deformation. The
role of clean interfaces, which will effectively permit load transfer, can also
greatly affect the properties.
6.3.2 Strengthening mechanisms for particulate strength
MMNCs
There are two strengthening mechanisms that are typically associated with
conventional MMCs - direct and indirect strengthening. Direct strengthen-
ing is the result of load transfer from the matrix to the reinforcement.
Indirect strengthening is caused by the effect of reinforcement on the matrix
microstructure or deformation mode, such as finer grain size and dislocation
strengthening induced by mismatch between the reinforcement and the
matrix (Sanaty-Zadeh, 2011). The strengthening mechanisms of MMNCs
are similar to traditional MMCs. However, due to the dimensional
difference, some mechanisms have a more significant effect than others.
The model proposed by Zhang and Chen (2006) was the first to predict the
yield strength of particulate reinforced MMNCs. Zhang and Chen's model
added the Orowan strengthening mechanism to the Ramakrishnan model, a
MMCs reinforcing model. The model agrees with some published data.
Sanaty-Zadeh (2011) quantitatively analyzed the Zhang and Chen model
and the Clyne model, added Hall-Petch, elastic modulus mismatch, and
work hardening into the model, and proposed a modified Clyne model.
Strengthening mechanisms
The different strengthening mechanisms of MMNCs analyzed in this section
are the Hall-Petch relationship, Orowan strengthening, the Taylor relation-
ship and the load-bearing effect.
Hall-Petch relationship
The Hall-Petch relationship is used to predict the effect of grain size on the
strength of materials.
k
y
d
1
=
2
s
y
¼ s
0
þ
½
6
:
1
where
σ
y
is the strength with grain refinement,
σ
0
is the initial strength, k
y
is a
constant, and d is the grain size.
For nanocomposite materials, the grain size could be affected by process
parameters, reinforcement size, and volume ratio. The most significant
