Oct. 2008, Volume 2, No.10 (Serial No.11)
Journal of Materials Science and Engineering, ISSN1934-8959, USA
The compressive property investigation of SiCp/Al
functionally gradient materials
LI Jin1, ZHOU Chun-ling2, YANG Zhi-chun1
(1. School of Aeronauties of Northwestern Polytechnical University, Shanxi 710072, China;
2. Department of Physics and Electrical Information, Ningxia University, Ningxia 750021, China)
Abstract: SiCp/Al functionally gradient materials are fabricated
by powder metallurgy. The microstructure of FGM specimens
was observed by electron microscope(SEM), then the intension
of compression and compressive properties were investigated.
The result showed that when pressed at 85KN, the specimens of
SiCp/Al functionally gradient material appeared macroscopical
shear crack, which had the 45° with vertical direction. The
SiCp/Al FGM had good compress properties.
Key words: functionally gradient materials; microstructure;
property of compression
1. Introduction
Functionally gradient materials(FGMs) are
attracting considerable attention due to increasing
performance demands in modern engineering
applications. In particulate composites, a graded
structure can be obtained by either changing the
particle volume fraction or the particle size along the
thickness of the composite. Because SiC particulate
reinforcing Al matrix functionally gradient composite
materials have some excellent properties such as high
contrast intension, high contrast modulus, high
temperature tolerance, heat exchange, electric
conduction, low thermal expansion modulus and size
stability, it plays very important action in aviation,
spaceflight, energy sources, ocean engineering,
Acknowledgment:
The authors wish to thank the State
National Affairs Commission of PRC, the Second North
University for Minorities Key Laborating for Powder Materials
and Advanced Ceramics (0502), and the Nature Science Fund
Subsidize Project (NO. NZ0818).
Corresponding author: ZHOU Chun-ling (1983- ), female,
postgraduates; research fields: the fabrication of functionally
gradient materials and mechanics property. E-mail:
zhouchunl_333@163.com.
biomedical, electromagnetism, nuclear engineering,
etc. Presently, the studies of FGM at fabrication are in
the ‘small samples stage[1-4].To make the FGM widely
use in practice, and breakthrough design anamnesis in
some aspect, the study field should be widened. Now,
particulate composites have been extensively studied
for fabrication[5-10], crack analysis[11-15], interface
reactions[16-17],
thermal
properties[18-20],
and
[21-23]
mechanical properties
.
At present, some fabricate methods are in
common used in the composite study field, such as
Wang et al. have studied the fabrication of Mo/Cu
functionally gradient material by self-propagating
combustion and bidirectional explosive consolidation
with buffer action of water. The microstructure of
Mo/Cu FGM and the consolidation process has also
been investigated[8]. ZHAO, et al have investigated the
fabrication of gradient coatings by plasma spraying and
analyzed the microstructure and properties[21]. LIN, et
al have fabricated SiC/Al laminated graded composite
by vacuum hot-pressure method. The effective joining
among the composites of different volume fraction of
SiC particulates by heat diffusion pressure has come
true. And the bullet proof properties of the SiC/Al
laminated graded composites have been studied
through the armor piercing experiments[22].
Also the mechanical properties of FGM have been
widely investigated, the literatures of compressive
properties of SiCp/Al FGM are not many, and there are
a few of reports of testing the yield intension and
compressive intension. XU, et al. have fabricated SiC
43
The compressive property investigation of SiCp/Al functionally gradient materials
particulates reinforcing aluminum matrix functionally
gradient composite materials and analyzed the fatigue
crack growth behavior. It is observed that there is no
obvious interface among different layers, since Al
matrix is fusion together. However, the retardation of
fatigue crack growth occurs from the region with high
volume fraction of SiC to the low SiC region,
meanwhile the fatigue crack occurs deflection and
divarication[12].
R. Rodriguez, et al. have fabricated aluminum
matrix composite as SiC volume fraction (20%-44%)
of gradient variation by centrifugal casting method,
then tested the tension and fracture toughness
properties[7]. CHENG, et al have studied that specific
density of the materials can be increased and gradient
boundary layers can be eliminated by means of the
second compressing and resintering power metallurgy
technique. The true stress-strain curves of the materials
at the high temperature can be divided into two parts:
strain hardening and strain weakening. The matrix has
influence on strength at the high temperature but not
the particles, relatively sliding motion is not found in
the gradient boundary layers of the materials, when the
stress comes to the given degree. The flowing stress of
high volume fraction of SiC part is slow, and the
deformation changes much. The deformation of
functionally gradient materials accords with the
deformation standard of metal. When the functionally
gradient materials, which are symmetry and high
content of SiC in the middle of the materials, the
deformation of middle part is very much, and the drum
protruding is relatively serious.
In this paper, the SiCp/Al functionally gradient
materials are fabricated by power metallurgy
technique. We view and analyze the microstructure of
FGM specimens by SEM, and test the intension of
compression and study the compressive properties.
2. Experiment and analysis of properties
2.1 Experimental method
44
The green SiC particulates are the reinforcement
particulates, size is 7  m , aluminum powder size is
43  m , the chemical components are Fe  0.6%,
Si  0.3%, Cu  0.05%, N  0.01%, and aluminum
pure is 99%.
In the first step reinforcement particulates and
matrix powder are mixed by effective double-butterfly
mixture machine. In the first mixture powder the SiC
particulates volume fraction is 10%; in the second
mixture powder the SiC particulates volume fraction is
15%; in the third mixture powder the SiC particulates
volume fraction is 20%; in the forth mixture powder
the SiC particulates volume fraction is 25%. The
mixing time is 30min. In the second step the mixture
powders were paved serially as SiC volume fractions
are 0%, 10%, 15%, 20%, 25% in the carbon
model(  110mm). There are five layers in the model
and then use the vacuum hot-pressure method to sinter.
In the sintering process: heightening temperature and
giving pressure should be adagio. When the
temperature is 500℃, keep it for 1h, and keep the
pressure at 10t for 1h, too; when the temperature
heights to 620℃, the pressure comes to 12t; then
decreased the temperature naturally, but keep the
pressure for 1h then unload the pressure. The thickness
of the green FGM slab is 20.5mm and the diameter is
110mm on average. The surface is smooth and no
macroscopical porosities and cracks to be found.
2.2 Microstructure and density investigation
The FGM specimens which were 8mm×10mm×
20mm length, width and thickness, respectively, are cut
from the fabricated green FGM slab. The
microstructure of FGM is observed by SEM after the
microstructure specimens are polished. Fig. 1 shows
some fuscous particulates, which are surrounded by
grey object named matrix aluminum. The fuscous
particulates are SiC particulates. Each interface of the
layers is slur. There are no obviously cracks, and the
layers felt together well.
Fig. 2 shows that the SiC particulates disperse
uniformly, and there is no concentration in the whole
The compressive property investigation of SiCp/Al functionally gradient materials
specimen’s micro structural appearance. In the whole
FGM slab, SiC particulates disperse with no regulation
and the specimens with no obvious porosities, except
the layer of SiC volume fraction 10% being found
some porosity. SiC particulates and Al interfaces bond
well, and there are no obvious holes and slack, also Al
matrix have no melting phenomenon. The reasons of
SiC volume fraction 10% found porosities may be as
follows: the hot-press condition is not appropriate, or a
little of Al powder is melted or SiC particulates are
pulled when polish the specimens.
GB/T5163-1985 penetration metal materials
density standard is adopted for testing the density of
50m
AI-10%×200
FGM materials. The density of FGM materials is 2.695
g/cm3 by using the method of drainage (the instrument
is the analytical balance and precision is 0.1mg). The
relative density of the FGM materials is 97%, and the
whole materials are compact. Compared with
references [3] and [23], the experiment with no use of
cold press and second compressing and resintering
power metallurgy technique, but the FGM materials are
compact well, and the gradient layers interfaces are
continuous. It shows that the experiment keeps the
interfaces integrity, and Al matrix melt in integrity well
in the hot-press condition.
200m
10-15%×80
(a)
(b)
200m
200m
15-20%×80
20-25%×80
(c)
(d)
Fig. 1 Interface microstructure of each layer with different SiC contents
Notes: The SiC contents are (a) 0%-10%, (b) 10% -15%, (c) 15%-20%, and (d) 20%-25%, respectively;
The middle portrait of each figure is the interface of two layers, no obvious delamination.
45
The compressive property investigation of SiCp/Al functionally gradient materials
50m
15%×450
10m
25%×1500
(a)
Fig. 2
(b)
The microstructures of different content SiC Gradient layer microstructure
Notes: (a) Containing 15% SiC; (b) Containing 25% SiC.
2.3 Analysis of compressive property
The compressive specimens are cut from the
green FGM in height h=20.5 mm, diameter d=13 mm,
respectively. Fig. 3 shows that there are obvious
gradient layers on the macrostructure appearance.
From left to right the SiC volume fraction is 0%, 10%,
15%, 20% and 25%, serially.
vertical direction is 45° and it shows that it is the shear
deformation, shown in Fig. 5.
Fig. 4
Fig. 3 The appearance of compressive
specimen before compressing
The GB/T6525-1986 sintering power metallurgy
room-temperature standard is adopted and used the
JYE-2000 numerical control compress instrumental
for testing the property compression. The cross
section proportion of the specimens becomes larger
and larger and the middle section protrudes as the
pressure increases. Shown in Fig. 4 the macroscopical
cracks appear on the side surface of the specimens
when the press is 85 KN, and the cracks with the
46
The appearance of compressive
specimen after compressing
The result of the compressive experiment is
shown in Fig. 6. The initial stress-strain curve presents
linear relationship. The intensity of compression of
FGM specimens are 150 MPa when the strain is about
0.4; then the FGM specimens present plastic
deformation; and when the strain was 0.76, the FGM
specimens appear macroscopical crack and the
intensity of compression is 640.71 MPa. So the
materials are invalidated.
The elastic modulus of the SiCp/Al functionally
gradient materials are calculated about 375 MPa. Fig. 6
shows that the stress-strain curve presents linear
relationship in the elastic stage, and then the FGM
materials come into the plastic deformation stage. But
The compressive property investigation of SiCp/Al functionally gradient materials
Load(KN)
the stress increases as the strain increasing. It shows that
the plastic deformation is not symmetrical. The reasons
are SiC particulates prevent the plastic flow of Al
matrix, and the differences of SiC content in every
layer. Because the variation of the volume fraction of
SiC is not continuous, the deformation of each layer is
not the same. In addition, because the cross section
proportion of the FGM specimens becomes larger and
larger until the materials are invalidate, however the
stress keeps increasing. Therefore the compress
intension of FGM materials cannot be tested. Compared
with the reference [3], the present experiment is simpler
and the relative density of the fabricated FGM materials
is higher, and the intension of compress is favorable,
although the once sintering vacuum hot-press technique
was used. The compressive deformation of the FGM
materials is same with the continuous gradient
materials.
Displacement(mm)
Relation of compressed load and displacement
Stress(MPa)
Fig. 5
Fig. 6
Relation of compress stress and strain
3. Conclusions
(1) The experiment of powder metallurgy, which
is used to fabricate the SiCp/Al functionally gradient
materials, is simpler and easier to put into practice than
other techniques of experiment.
(2) The SiCp/Al functionally gradient material,
which is fabricated in the present experiment, has the
smooth appearance with no macroscopical cracks, and
the microstructure is favorable. The reinforcement
particles disperse uniformly and felt with matrix Al
well. The layers of the FGM bond compactly and the
materials have no obvious delaminations or cracks.
(3) The density of the SiCp/Al functionally
gradient material, which is fabricated in the present
experiment, is 2.695 g/cm3. In addition, the relative
density comes to 97%. The FGM material is compact
very well. Although the techniques of the experiment is
simple, the intension of compress is favorable and the
compress modulus come to be about 375 MPa.When
the pressure is 85 KN, the specimens of SiCp/Al FGM
appear macroscopical crack, which has the 45° with
vertical direction. It is asserted that the specimens of
SiCp/Al FGM occurs shear deformation.
(4) The experiment is adapted into manufacturing
in factory, and it has the meaning of practicality.
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(Edited by Tsyung and Taylor)