X. DU et al.: MICROSTRUCTURE AND MECHANICAL PROPERTIES OF GRAPHENE-REINFORCED ...
763–768
MICROSTRUCTURE AND MECHANICAL PROPERTIES OF
GRAPHENE-REINFORCED ALUMINUM-MATRIX COMPOSITES
MIKROSTRUKTURA IN MEHANSKE LASTNOSTI Z GRAFENOM
OJA^ANIH KOMPOZITOV Z ALUMINIJEVO MATRICO
Xiaoming Du, Kaifeng Zheng, Fengguo Liu
Shenyang Ligong University, School of Materials Science and Engineering, Nanping Middle-Road 6, Hunnan Street, Shenyang 110159, China
du511@163.com
Prejem rokopisa – received: 2018-02-13; sprejem za objavo – accepted for publication: 2018-06-28
doi:10.17222/mit.2018.021
Graphene has a high fracture strength, good ductility and low coefficient of thermal expansion, making it an ideal reinforcement
for composite materials. This paper reports on an aluminum-matrix composite reinforced with graphene (graphene/Al) using
ball milling and hot-press sintering. The effect of the graphene content on the microstructures, mechanical properties and
fracture mechanisms of the composites were investigated. It is shown that 0.25 w/% and 0.5 w/% graphene contents modified
with copper are homogeneously dispersed into the aluminum matrix. The composite with the 0.5 w/% graphene content exhibits
the highest tensile strength and elongation; its improvement over the unreinforced aluminum matrix is 31.0 % and 30.6 %,
respectively. With the increasing graphene content up to 1.0 w/%, the aggregation of graphene in the composite is apparent,
seriously reducing the tensile properties. The fracture mechanism of the composites changes from ductile to brittle.
Keywords: aluminum-matrix composites, graphene, microstructure, mechanical properties
Grafen ima visoko lomno trdnost, dobro duktilnost in nizek koeficient termi~nega raztezka, zato je idealen za oja~itev kompozit-
nih materialov. V ~lanku avtorji poro~ajo o kompozitu na osnovi aluminija, oja~anem z grafenom (grafen/Al), izdelanem s
krogli~nim mletjem in sintranjem pod tlakom. Raziskovali so vpliv dodatka grafena na mikrostrukturo, mehanske lastnosti in
mehanizem loma izdelanih kompozitov. V prispevku so pokazali, da je dodatek 0,25 w/% in 0,5 w/% grafena, modificiranega z
bakrom, homogeno porazdeljen v matrici Al kompozita. Kompozit z 0,5 w/% grafena ima najvi{jo natezno trdnost in raztezek,
kar je 31,0 % oz. 30,6 % bolje v primerjavi z neoja~anim kompozitom. Z nara{~ajo~o vsebnostjo vsebnosti grafena do 1,0 w/%
se pojavlja kopi~enje grafena v matrici kompozita, kar mo~no zmanj{a njegove natezne lastnosti. Mehanizem loma kompozita
se tako spreminja iz duktilnega v krhkega.
Klju~ne besede: kompoziti na osnovi aluminija, grafen, mikrostruktura, mehanske lastnosti
1 INTRODUCTION
Aluminium-matrix composites have a high strength-
to-weight ratio, high modulus-to-weight ratio, good
damage tolerance, good wear resistance, excellent ther-
mal conductivity and corrosion resistance, which make
them highly competitive against the conventional alumi-
nium alloys or even titanium alloys and steels for many
structural applications, such as the construction, aero-
space, transportation, automotive and military indus-
tries.
1–3
At present, particulate (such as graphite, Si
3
N
4
,
SiC, TiN, BN, MgO and Al
2
O
3
, etc. ) reinforced alumi-
num-matrix composites are widely studied because of
their advantages such as low cost, isotropic properties
and the possibility of secondary processing, facilitating
the fabrication of secondary components.
4–7
The main research trend of aluminum-matrix com-
posites is to continue to improve their comprehensive
properties (such as tensile and fracture strength, thermal
conductivity, friction and wear properties); meanwhile,
they have good ductility and fatigue resistance. In order
to get high-ductility, high-strength aluminum-matrix
composites, high-performance reinforcement materials
are necessary. Among them, carbon materials are out-
standing, such as graphite whiskers, carbon fibers,
carbon nanotubes (CNTs) and graphene (GNP).
In recent years, carbonaceous nanomaterials includ-
ing graphene and carbon nanotubes (CNTs) have
emerged as an important class of new materials for
structural engineering and functional device applications
due to their extraordinary high elastic modulus and me-
chanical strength as well as excellent electrical and
thermal conductivities. In addition, most studies show
that graphene is considered a most effective reinforcing
filler for fabricating composite materials.
8–11
It is because
graphene has some exceptional properties: excellent
electric properties,
12
high thermal conductivity,
13
high
Young’s modulus
14
and high tensile stress.
15
Addition-
ally, graphene is extremely light. The density of gra-
phene was estimated to be as low as 1.06 g·cm.
3,16
These
exceptional properties make it a perfect reinforcement
for metal-matrix composites. A lot of research work has
been carried out for the fabrication of aluminium compo-
sites reinforced with graphene nanosheets,
17–19
graphene
nanoflakes,
18–21
few-layered graphene,
22
or its derivatives
such as graphene oxide and reduced graphene oxide.
23,24
Cold compaction followed by hot isostatic pressing,
18
as
well as hot extrusion,
17,24,20,21
vacuum hot press and/or
Materiali in tehnologije / Materials and technology 52 (2018) 6, 763–768 763
UDK 67.017:620.168:546.262.5 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(6)763(2018)

hot rolling
22
are commonly used, according to the litera-
ture, to manufacture solid pure aluminium and graphe-
ne-reinforced aluminium-matrix composite specimens.
Wang et al. reported that the tensile strength of Al
composites reinforced with only 0.3 w/% graphene nano-
sheets using powder metallurgy was 249 MPa.
24
It was a
62-% enhancement over the unreinforced Al matrix. Li
et al. reported that an Al composite reinforced with only
0.3 w/% graphene oxide showed an 18-% and 17-% in-
crease in the elastic modulus and hardness, respectively,
over the unreinforced Al.
23
Yan Shao-jiu et al. reported
that the tested tensile strength and yield strength of
0.3 w/% graphene-reinforced Al-matrix composites
fabricated with powder metallurgy, showed an increase
of 25 % and 58 %, respectively, compared to the pristine
Al alloy.
18
The ductility of the composite was almost as
good as or even a little better than the pristine alloy.
Although the above studies showed that the mechanical
properties of aluminum alloys can be improved by
adding graphene, it is still a great challenge to produce
aluminum-matrix composites reinforced by well-dis-
persed graphene via a conventional metallurgical process
due to a high-temperature environment and a huge den-
sity difference between graphene and aluminum matrix.
Great difficulties are encountered during the synthesis
and unknown interfacial chemical reactions between
graphene and metal matrices.
In this work, Al composites reinforced with graphene
were fabricated by ball milling and hot-press sintering.
The distribution and morphology of graphene in the Al
matrix were analyzed. The mechanical properties of Al
composites reinforced with graphene were tested and the
relevant strengthening mechanisms of graphene were
discussed based on the primary experimental evidences.
2 EXPERIMENTAL PART
2.1 Materials
The graphene sheets used in our experiments were
prepared with modified Hummers method as reported
previously.
25
The experimental material was natural gra-
phite with a particle size of about 45 um. Natural
graphite was reduced at 95 °C for 24 h using hydrazine
hydrate. The preparation method was similar to that from
Reference
25
.
To improve the interface wetting between graphene
and aluminum matrix, graphene was coated with copper
using the chemical plating method. Graphene was added
into a plating solution including 10 mL/L CuSO
4
,
1 mL/L NiSO
4
, 30 mL/L NaH
2
PO
2
, 30 mL/L H
3
BO
3
and
15 mL/L Na
3
C
6
H
5
O
7
. The solution was stirred with a
magnetic stirrer, then heated to 65 °C in a water bath for
2 h in order to keep a homogeneous growth of copper on
the surface of graphene.
Atomized pure-aluminum powder (Anshan Steel
Industrial Fine Aluminum, Inc., China) with an average
size of 20 um and chemical composition of (Fe 0.071
w/%, Si 0.067 w/%, Cu 0.002 w/%) was used as the
matrix material.
2.2 Composite preparation
A planetary machine was used for ball milling
without an interruption under high-purity (99.999 %)
argon gas. The stainless-steel vial was sealed with an
elastomeric O ring. The stainless-steel balls to powder
weight ratio was 7:1, and the rotation rate of the vial was
40 min
–1
under the total milling time of 72 h.
A variety of ball-milled powders were fabricated by
varying the mass fraction of graphene sheets (i.e., 0.25,
0.5, and 1.0 w/%). The ball-milled powder was contain-
erized in a heat-resisting steel die (50 mm in diameter,
200 mm in height, and 10 mm in thickness) and was
compacted. The compacted composite billets were
sintered at 610 °C for 1.5 h under a pressure of 25 MPa
with hot-press sintering. For comparison, a pure Al
sample was also fabricated under the same conditions.
2.3 Characterization
An X-ray diffraction analysis of polished sintered
sample bars was carried out with an X-ray diffractometer
(Rigaku Ultima IV), using Cu K
 radiation for 20–90
degrees. Raw XRD data were refined and analyzed via
the MDI Jade 6.0 program (Materials Data Incorporated:
Livermore, CA, USA). The surface morphology was
observed with a scanning electron microscope (SEM)
S-3400N equipped with an energy-dispersive spectro-
meter (EDS).
The tensile properties of the sintered samples with
the standard specimen sizes of (20×4×2)mmwere de-
termined with a testing machine (UTM4304, Shenzhen
Suns Technology Stock Co., Ltd, China) at ambient tem-
perature using a displacement rate of 0.5 mm/min.
3 RESULTS AND DISCUSSION
3.1. Microstructure
Figure 1 shows the representative morphology of
graphene and graphene-coated Cu. Graphene has a two-
dimensional high-aspect-ratio sheet geometry, and the
wrinkles and folds are also shown on the exfoliated
graphene sheet, as depicted in Figure 1a. The graphene
consists of platelets with a morphology of irregularly
shaped flakes, whose mean diameters are in a range of
0.5–15 um. Agglomerated powders have an opaque
structure. The thickness of the graphene sheets is several
nanometers (3–20 nm), corresponding to approximately
10–50 sheets of graphene (assuming that the thickness of
monolayer graphene is 0.35 nm
26
). In Figure 1b, the gra-
phene is covered with fine crystal particles. These
particles are distributed uniformly on the surface of the
graphene slice. It is concluded that these particles con-
tain a large amount of copper particles.
X. DU et al.: MICROSTRUCTURE AND MECHANICAL PROPERTIES OF GRAPHENE-REINFORCED ...
764 Materiali in tehnologije / Materials and technology 52 (2018) 6, 763–768

Figure 2 shows the distribution of graphene on the
aluminum surfaces of the samples with different gra-
phene contents. As shown in Figure 2, there are many
fine platelets on the surfaces of the graphene/Al compo-
site powders (Figures 2b to2d ), while the surface of the
pure Al flakes is very clear. As the graphene contents
increase, the fine platelets show a dense distribution. It is
indicated that the fine platelets are signs of a homo-
geneous dispersion of the graphene nanosheets. It is
believed that, in this study, the shape compatibility of the
Al flakes and graphene nanosheets is crucial to achieve a
uniform dispersion.
Figure 3 shows X-ray diffraction patterns from pure
Al and its composites in the sintered condition. As
shown in Figure 3, a small amount of magnesium (2 =
35.2°) is included in pure Al. No diffraction peaks of the
carbon element are present in the graphene/Al compo-
sites. This is attributed to the nanometric size and the
low content of the reinforcement phase, which cannot be
detected due to the detection limit presented by XRD for
the second phases.
27
However, no aluminium carbide
(Al
4
C
3
) peak is recorded for any of the samples. It is
believed that there is no chemical reaction between
graphene and the Al matrix under this condition.
Bartolucci et al. reported a formation of aluminum
carbide in graphene/aluminum composites processed
with hot extrusion.
17
Bustamante et al. thought that the
formation of aluminum carbide is strongly dependent on
the processing temperature of the production of the
composites.
28
Our results calculated in the previous work
showed that the interaction between C and Al atoms is in
the Van Der Waals range, indicating that there is no
chemical bonding between graphene and Al.
29
As also
shown in Figure 3, an amount of the Al
2
CuMg (S) phase
is also detected in the graphene/Al composites, which is
the main strengthening phase of Al-Cu-Mg alloys. How-
ever, in the current study, Al
2
CuMg phases are generated
from the copper-coated graphene and Al matrix during
the sintering process.
3.2 Mechanical properties
The tensile properties of the graphene/Al composites
with different graphene contents are shown in Figure 4.
X. DU et al.: MICROSTRUCTURE AND MECHANICAL PROPERTIES OF GRAPHENE-REINFORCED ...
Materiali in tehnologije / Materials and technology 52 (2018) 6, 763–768 765
Figure 1: SEM and TEM images depicting the morphology of gra-
phene, a) SEM morphology of graphene, b) TEM image of the
copper-coated graphene
Figure 3: XRD patterns of pure Al and various graphene/Al compo-
sites
Figure 2: SEM micrographs of the mixture of aluminum powder
particles and different graphene amounts after 72 h, a) pure Al, b) 0.25
w/%, c) 0.5 w/%, d) 1.0 w/%

Figure 4a shows the representative engineering stress-
strain curves of the graphene/Al composites with diffe-
rent graphene contents. Figure 4b shows the relationship
of the ultimate tensile strength (UTS) and elongation to
fracture ( ) of the graphene/Al composites. The
strengthening effects are first enhanced and then they
deteriorate with the increasing graphene contents. When
the graphene content is 0.5 w/%, the UTS and  of the
composites reach the maximum value. As shown in
Figure 4b, the composites with additions of 0.25 or 0.5
w/% graphene exhibit UTS values of 174.4 and 178.3
MPa, which indicate 28.1-% and 31-% enhancements
over the aluminum-matrix sample (136.1 MPa). The
same strong improvement of the tensile strength is seen
in the aluminum-graphene composite.
24
Generally, the
strengthening mechanisms include: a) Orowan
strengthening, b) grain refinement, c) dislocation genera-
tion due to a different coefficient of thermal expansion
(CTE) of reinforcements and matrix, and d) load transfer
of graphene.
30
In the present work, the contribution of
Orowan strengthening can be ignored considering the
size and volume fraction of the graphene.
31
The strengthening mechanism can be explained as
follows: Firstly, the graphene with a super-high strength,
a nanometric size and a homogeneous dispersion in the
aluminum matrix (Figures 2a and 2b) acts as the rein-
forcement contributing to the mechanical strengthening
of bulk nanostructured graphene/Al composites. Second-
ly, the Cu on the graphene surfaces improves the inter-
facial bonding between the aluminum matrix and
graphene (Figures 2d and 3), effectively loading the
transfer from the aluminum matrix to graphene, reported
as the strongest material ever measured (ultimate
strength of 130 GPa )
14
. Thirdly, graphene is uniformly
dispersed at the grain boundaries of the aluminum
matrix, as shown in Figures 2a and 2b, thus hampering
the migration of the grain boundaries and effectively
hindering the grain growth during the hot-press sintering
at elevated temperatures. Finally, the thermal-expans-
ion-coefficient mismatch between aluminum and
graphene (–0.8 × 10
–6
/K for graphene
32
and 23.6 × 10
–6
/K
for Al
30
) may result in a multidirectional thermal stress at
the graphene-Al interface and an improvement in the
dislocation densities, which will increase the strength of
the composites.
Meanwhile, a significant ductility is accompanied by
elongation of the samples with the additions of 0.25 w/%
and 0.5 w/% graphene, which increases with a greater
addition of graphene and reaches the values of 9.2 % and
11.1 % for the 0.25 w/% and 0.5 w/% graphene contents,
compared with 8.5 % for pure aluminum. This means
that the strength and the ductility of the graphene/Al
composites simultaneously increase. A possible reason
for such unique mechanical properties of graphene/Al
composites is the strong interface interaction between
the graphene and aluminum matrix, leading to an effect-
ive transfer of the load from aluminum to graphene.
In contrast, the effect of a larger content (1.0 w/%) of
graphene on the tensile properties is adverse. Both the
tensile strength and the elongation decrease with an
increase in the graphene additions. In particular, the
elongation of the graphene/Al composite dropped
sharply from 8.5 % of pure aluminum to 3.2 % of the
graphene/Al composite with a 1.0 w/% graphene content.
This is attributed to the fact that it would be more and
more difficult for nanostructured graphene with a large
aspect ratio and great specific surface area to be dis-
persed homogeneously into the aluminum matrix. Thus,
a poor dispersion of graphene (>1.0 w/%) resulted in
increased amounts of agglomeration and clusters of
graphene, which in turn degraded both the strength and
ductility of the graphene/Al composite.
20
The tensile-fracture behavior of pure Al and gra-
phene/Al composites with various contents of graphene
is presented in Figure 5. It can be seen that dimples and
tear ridges are tiny and uniform with distinguishing
features of an obvious trend along the loading direction
of the tensile stress, indicating a ductile fracture of pure
Al. Compared with the pure Al (Figure 5a), the fracture
surfaces of the graphene/Al composites are flat. The
number of the dimples and tear ridges decreases. A few
X. DU et al.: MICROSTRUCTURE AND MECHANICAL PROPERTIES OF GRAPHENE-REINFORCED ...
766 Materiali in tehnologije / Materials and technology 52 (2018) 6, 763–768
Figure 4: Tensile properties of graphene/Al composites with different
graphene contents, a) stress-strain curves, b) relationship of UTS and
 with the graphene content

graphene sheets are observed on the fracture surface. For
the 0.25 w/% and 0.5 w/% graphene/Al composites
(shown in Figures 5b and 5c), the fracture surfaces show
graininess-distribution characteristics, indicating a brittle
fracture. This is because graphene is mainly distributed
around the grain boundaries in composite materials.
When subjected to an external stress, the cracks will pre-
ferentially nucleate at the grain boundaries and expand
continuously, which will eventually lead to a fracture of
the composite. However, a plastic deformation of com-
posite materials during a tension process is depressed by
graphene. With an increase in the external force, the
stress concentration occurs in the aluminum matrix
sandwiched in graphene, in which holes appear and
gradually grow up. These would cause a formation and
continuous expansion of the cracks, indicating an
obviously ductile fracture. Thus, there is a combination
of fracture mechanisms, including ductile fracture and
brittle fracture, affecting the graphene/Al composites
with a lower graphene content.
With the increasing graphene contents up to 1.0 w/%,
it is found that some graphene sheets are pulled out at
the edges of the tear ridges, and the graphene/Al com-
posite exhibits many flat fracture surfaces of brittle
fracture, characterized by flat dimples of different sizes
or almost no dimples. Many graphene sheets are ob-
served on the fracture surface (Figure 5d). The aggre-
gation of the graphene is apparent as it is seriously
separated from the matrix, so the cracks preferentially
form in the graphene and extend to the aluminum matrix
during the tensile process, decreasing the tensile strength
and elongation of the graphene/Al composite. The frac-
ture mechanism of the graphene/Al composite changes
from ductile fracture to brittle fracture with the in-
creasing graphene content.
4 CONCLUSIONS
Graphene-reinforced aluminum-matrix composites
were successfully fabricated with ball milling and hot-
press sintering. Ball milling can cause a uniform disper-
sion of graphene into the aluminum matrix. A modi-
fication of the graphene surface with a copper coating
can improve the interfacial bonding between the gra-
phene and aluminum matrix. When the graphene content
was small, the interfacial bonding between the graphene
and aluminum matrix was good. Both the UTS and  of
the graphene/Al composites increased with the in-
creasing graphene content. The composite with a 0.5
w/% graphene content exhibited a tensile strength and
elongation improvement of 31.0 % and 30.6 %, respect-
ively, over the unreinforced aluminum matrix. This im-
provement of the strength and ductility can be attributed
to the homogeneous distribution of graphene in the alu-
minum matrix. However, as the content of graphene
increased (especially up to 1.0 w/%), the extent of the
agglomeration of graphene within the aluminum matrix
increased, lowering both the UTS and  of the gra-
phene/Al composites. The fracture mechanism changed
from ductile to brittle with the increasing graphene
content.
Acknowledgment
This work was supported by the Natural Science
Foundation of Liaoning (2015602642) and Shenyang
Science and Technology Project (no. 18-013-0-33) in
Liaoning Province, China.
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