Biomedical Engineering Reference
In-Depth Information
Zhang and Chen model
This is a modified version of the shear-lag model and is mostly commonly
used in recent works (Zhang and Chen, 2008). However, in this model, only
dislocation density, the Orowan strengthening effect, and the load-bearing
improvement factor are considered:
s
yc
¼ s
ym
ð
1
þ
f
l
Þð
1
þ
f
d
Þð
1
þ
f
Or
Þ
½
6
:
5
where
σ
ym
is the yield strength of the
monolithic matrix with the same process, and f
1
, f
d
, f
Or
are improvement
factors from the load-bearing effect, enhanced dislocation density by
thermal mismatch, and the Orowan strengthening effect.
σ
yc
is the yield strength of the MMNC,
Sanaty-Zadeh model
Sanaty-Zadeh (2011) proposed a modified Clyne method as follows:
2
2
2
2
2
s
yc
¼ s
ym
þ½ðDs
l
Þ
þðDs
Or
Þ
þðDs
H
P
Þ
þðDs
EM
Þ
þðDs
CTE
Þ
2
1
=
2
þðDs
WH
Þ
½
6
:
6
The Sanaty-Zadeh model takes into account the Hall-Petch strengthening
mechanism, which was not considered before. The Sanaty-Zadeh model is in
good agreement with experimental values.
6.4
Different manufacturing methods for MMNCs
Based on the sources of nanoreinforcement, MMNC fabrication methods
can be categorized as ex-situ and in-situ.
For the ex-situ route, nanoreinforcements are externally added to the
matrix. Due to the existence of electrostatic and van der Waals forces
between nanoreinforcements, especially nanoparticles, the added nano-
reinforcements tend to agglomerate. Uniform dispersal of nanoreinforce-
ment into a metal matrix is the major concern for ex-situ methods. Different
processes can be used to homogeneously disperse the reinforcement. These
ex-situ processes can be classified, based on the process temperature, as
liquid-phase processes, solid-state processes, and two-phase processes.
For in-situ routes, nanoreinforcements are produced by chemical
reactions within the materials during the fabrication process. In-situ routes
are used to produce both MMNC powders, which are consolidated to form
MMNC bulk material, and MMNCs bulk material directly. Reinforcements
created in-situ are usually fine and well distributed. However, in-situ
reinforcements have fewer options in terms of material selection than ex-situ
reinforcements because of the complex reactions involved in the in-situ
fabrication process.
