Novel aluminum-graphene and aluminum-graphite metallic composite materials: Synthesis and properties

Article abstract of DOI:10.1016/j.jallcom.2015.12.084

A novel method of creating new lightweight, aluminum-metallic, composite materials under halides melt at temperatures 973-1073 K under air atmosphere is proposed. The method for synthesizing aluminum-based metallic composite materials, containing up to 2 wt. % graphene sheets uniformly distributed in a metal matrix, is entirely new, having no analogies in current science and practice. The synthesis of graphene nanosheets in a metal matrix is one-step, simultaneous process, taking place directly in molten aluminum under alkali halides melt without the necessity of a separate stage of synthesis and introduction of graphene. This has the potential to facilitate the inexpensive synthesis of aluminum-graphene composites with a high concentration of graphene. The aluminum-graphene composites formed according to this method are characterized by a high uniformity of graphene films with linear dimensions from 100 nm to 50 μm and a thickness from one to three nm in the metal bulk. No aluminum carbide forms under synthesis; the aluminum-graphene and aluminum-graphite composites are resistant to corrosion in NaCl solution. The hardness, strength and ductility of aluminum-graphene composites are at least 2-3 times higher than the initial aluminum material, proportional to the concentration of graphene.

Full text of DOI:10.1016/j.jallcom.2015.12.084

Accepted Manuscript
Novel Aluminum-Graphene And Aluminum-Graphite Metallic Composite Materials:
Synthesis And Properties
L.A. Yolshina, R.V. Muradymov, I.V. Korsun, G.A. Yakovlev, S.V. Smirnov
PII:
S0925-8388(15)31873-9
10.1016/j.jallcom.2015.12.084
JALCOM 36173
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 30 August 2015
Revised Date: 3 November 2015
Accepted Date: 10 December 2015
Please cite this article as: L.A. Yolshina, R.V. Muradymov, I.V. Korsun, G.A. Yakovlev, S.V. Smirnov,
Novel Aluminum-Graphene And Aluminum-Graphite Metallic Composite Materials: Synthesis And
Properties, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2015.12.084.
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ACCEPTED MANUSCRIPT

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NOVEL ALUMINUM-GRAPHENE AND ALUMINUM-GRAPHITE METALLIC
COMPOSITE MATERIALS: SYNTHESIS AND PROPERTIES
L.A.Yolshina1,2, R.V.Muradymov1, I.V.Korsun1, G.A.Yakovlev2, S.V.Smirnov3
1- Institute of High-Temperature Electrochemistry, UB RAS, Ekaterinburg, Russia
2- Ural Federal University named after B.N. Yeltsin, Ekaterinburg, Russia
3 - Institute of Engineering Science, UB RAS, Ekaterinburg, Russia
ABSTRACT
A novel method of creating new lightweight, aluminum-metallic, composite materials
under halides melt at temperatures 973-1073 К under air atmosphere is proposed. The method
for synthesizing aluminum-based metallic composite materials, containing up to 2 wt. %
graphene sheets uniformly distributed in a metal matrix, is entirely new, having no analogies in
current science and practice. The synthesis of graphene nanosheets in a metal matrix is one-step,
simultaneous process, taking place directly in molten aluminum under alkali halides melt without
the necessity of a separate stage of synthesis and introduction of graphene. This has the potential
to facilitate the inexpensive synthesis of aluminum-graphene composites with a high
concentration of graphene.
The aluminum-graphene composites formed according to this method are characterized by
a high uniformity of graphene films with linear dimensions from 100 nm to 50 µm and a
thickness from one to three nm in the metal bulk.
No aluminum carbide forms under synthesis; the aluminum-graphene and aluminum-
graphite composites are resistant to corrosion in NaCl solution.
The hardness, strength and ductility of aluminum-graphene composites are at least 2-3
times higher than the initial aluminum material, proportional to the concentration of graphene.
Keywords: graphene; aluminum; metal-matrix composites; mechanical properties; corrosion.
__________________________________
*Corresponding author. E-mail: yolshina@ihte.uran.ru; yolshina06@rambler.ru. Tel:
+73433745044. Present address: Institute of High-Temperature Electrochemistry UB RAS,
Akademicheskaya str., 20, Ekaterinburg, 620990, Russian Federation.

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1. INTRODUCTION
The production of novel aluminum alloys and composites with a decreased specific density
and improved mechanical properties is one of the most significant goals of contemporary
material science investigations. The creation of new aluminum metallic composites with a high
content of nano- and microparticles of carbon in different allotrope modifications, or with
ceramic nano- and microparticles such as aluminum oxide and silicon carbide, presents both a
scientific challenge and an industrial necessity for numerous applications in automotive, aircraft
or battery technologies.
Studies of interactions between aluminum and carbon have been carried out both by
experimental method and computer simulation [1, 2]. According to Schrunk’s data [3], the
solubility of carbon in liquid aluminum is about 0.03 at.%. The carbon (graphite) does not allow
wetting by liquid aluminum at temperatures below 1373 К; this makes graphite the most usable
materials for anodes in aluminum production. Consequently, the synthesis of aluminum
composite materials with carbon concentrations above the equilibrium value presents a
particularly difficult challenge.
Currently, the most commonly utilized processes of production of aluminum-carbon
metallic composites are the sintering of aluminum into carbon fiber matrices [4], or the
impregnation of carbon fibers with liquid aluminum [5]. The carbon fibers are typically covered
by thin layers of silver, copper, nickel 0.4 to 1.0 µm thick for the enhancing of carbon fiber
wetting [6]. Other existing methods include “mixing in the melt” of particles, wicks and short
fibers of carbon (up to 50 wt. %) with aluminum melt under standard casting technology [7].
Due to a significant difference in the densities of carbon and aluminum, and the weak
wettability of carbon by molten aluminum, it is difficult to obtain a homogeneous material;
consequently, aluminum-carbon composite materials produced by traditional methods tend to
have poor operational and technological characteristics. The fibrous, uniaxial composite
materials are characterized by anisotropy of mechanical properties along and across the fibers,
consequently exhibiting a low-resistance interlayer shift and tendency to transversal breakage.
In addition, an increase of the process temperature up to 1373 К for wetting by aluminum
of carbon particles and fibers improvement leads to a dissolution of the carbon with the
formation of significant quantities of aluminum carbide. The aluminum carbide fully hydrolyzes
in aqueous media with aluminum hydroxide and gaseous hydrocarbons such as methane or
acetylene. This leads to a deterioration in the corrosion resistance of the composite material and
is a common contributory factor in mechanical failure.
Metallic materials with degrees of reinforcement up to 50% can also be produced by means
of powder metallurgy [8, 9]. Typically, the degree of reinforcement does not exceed 25%

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because fracture toughness is reduced at further increases of carbon content. The aluminum-
carbon composites produced by powder metallurgy with carbon content up to 25 wt. % are
characterized by high porosity and poor sintering due to the aluminum oxide layer on the surface
of the aluminum powder.
Some alternative, high-energy methods for the creation of aluminum-carbon metallic
composites have been proposed in recent years. Thus, in the course of the implantation of carbon
ions into an aluminum target [10-12], carbon embeds to a depth of 150 nm with formation of
aluminum carbide. The accurate, prolonged ball milling of carbon nanoparticles or carbon
nanotubes (CNT) with aluminum nanoparticles [13, 14] – or the combination of different above-
mentioned synthesis methods [15] – allows various aluminum-carbon composite materials to be
obtained.
A method for synthesizing aluminum-carbon composite materials with weak mechanical
anisotropy, low porosity and good sintering is described [16]. This method is based on the
interaction of molten aluminum with graphite in K2TiF6 and/or cryolite melt media. Reinforcing
titanium / aluminum carbide and titanium aluminide particles are formed at high-temperature
interaction. The fact that the dispersion of carbon and titanium carbide particles is non-uniform
greatly limits the applicability of this method.
A significant number of papers on synthesis methods and mechanical properties of
aluminum-graphene composites have been published in recent years [17-23]. Typical synthesis
processes as prolonged milling and annealing in an inert atmosphere have preliminary
synthesized graphene or graphene oxide with powdered aluminum; this is also the case with
application of CNT as a reinforcing additive.
Nearly all researchers have noted the graphene hardening effect on an aluminum matrix
with aluminum carbide, which formation was confirmed by X-Ray investigations. The formation
of a thin layer of aluminum carbide at the ND–Al interface may be beneficial to the overall
performance of the metal-matrix composites as it improves the bonding strength between the
reinforcement agent and the metal matrix [24].
In contrast, even small aluminum carbide additions are damaging to aluminum-graphene
composite materials in aggressive environments – and even in conditions of atmospheric
corrosion – because of non-uniform, pitting corrosion. This limits the practical use of thus
obtained aluminum-graphene composite materials.
Thus, we can conclude that there are currently no reliable and productive methods of
aluminum-carbon synthesis with high concentration and uniform distribution of carbon as
graphite or graphene in aluminum matrix without the unwanted production of aluminum carbide.

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Even in impurity amounts, the presence of aluminum carbide negatively impacts on the
corrosion-resistant properties of metallic composite materials.
The subject of this study is to obtain aluminum-graphene and aluminum-graphite metallic
composites with carbon concentration up to 2 wt. % having an improved structure and physical /
chemical properties without aluminum carbide formation at temperatures under 1073 K by direct
chemical interaction of carbon-containing substances “in situ” with molten aluminum under a
layer of molten alkali halides in air.
2. EXPERIMENTAL
The interaction between the molten aluminum and the carbon-containing component was
performed in molten alkali halide media using an alumina crucible. The aluminum sample with
0.65 wt. % impurities (Si – up to 0.62 wt. %; Fe, Cu, Mn, Mg – up to 0.05 wt. % in total; Zn – up
to 0.1 wt. %) comprises a disc 3 cm in diameter and 1 cm in height. The aluminum sample size
was equal to the inner crucible diameter.
Salt mixtures of alkali chlorides such as LiCl, NaCl, KCl, CsCl and CaCl2 with melting
points of less than 700ºС were used as base electrolyte. The small fluoride additions of NaF or
AlF3 (less than 2 wt. %) formed part of the salt flux electrolyte. Metal or non-metal carbide
powders such as WC, TiC, ZrC, Mo2C, SiC or B4C were used as the carbon-containing additive.
A pre-melted salt electrolyte was thoroughly mixed with a specific amount (not more than
0.2 to 6.5 wt. %. of the total salt weight) of carbide powder that had an average grain size of 100
to 200 ꢀm. The mixture of alkali-halide metals with the carbide addition was placed on the
bottom of the crucible.
The aluminum was loaded into part of the salt mixture on the bottom of the crucible. The
mixture of alkali-halide metals with the carbide addition was also placed on the top of the
aluminum in order to eliminate the oxidation of molten aluminum through contact with air
oxygen. The alumina crucible was covered with a cap and then placed into the vertical heating
furnace. The temperature interval for the experiments was 973-1073 К. The interaction
temperature was chosen under the melting point of both the aluminum (935 K) and the salt
mixture. These temperatures are lower than that of aluminum carbide synthesis. The exposure
time varied from 0.5 to 5 hours. After the high temperature exposure, the liquid aluminum
globule was poured into the cold alumina crucible with a controlled cooling period. After the salt
mixture had fully solidified, it was dissolved and the salts were washed off the surface of the
aluminum globule. The metal drop was weighed and its specific density defined according to its
chemical composition and structure.

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The chemical composition of aluminum carbon-containing composites was determined by
chemical X-Ray fluorescent analysis using an XRF-1800 X-Ray fluorescent spectrometer by
Shimadzu, Japan and CS-230 infrared analyzer of carbon and sulfur absorption by LECO (USA).
The aluminum-carbon composite was then divided into two halves. The first half was
poured by current-carrying gum and then polished by six different silicon solutions using a
Struers disc-finishing machine (Austria). The specimen was then examined as follows: scanning
electron microscopy (SEM) using JEOL 5900LV and Zeiss Σigma VP microscopes; Raman
spectroscopy, using a Renishaw-1000 Raman microscope-spectrometer with a green 514 nm
argon laser; servo-hydraulic testing machine INSTRON 8801 and Neophot 32 optical
microscope, micro hardness testing, using the system for micro hardness testing WIN-HCU
FISCHERSCOPE HM 2000 XYm which designed to measure Martens test under a load in
accordance with standard ISO 14577. Elastic modulus was determined by unloading curves by
standard Oliver - Pharr method. The Oliver–Pharr method [25] is the most frequently adopted
method in instrumented indentation testing to probe the elastic modulus of materials. Electron
backscatter diffraction (EBSD) patterns were obtained using HKL NORDLYSS console
software Channel 5.
Another part of the aluminum was dissolved in a 20 wt. % HCl solution; fine transparent
films were then deposited on the titanium foil substrate for further analysis. The transparent films
deposited on the titanium foil were photographed by SEM microscope and then analyzed by
grazing-incidence X-ray diffraction (Rigaku DMAX 2200PC) and by Raman spectroscopy
(Renishaw).
3. RESULTS AND DISCUSSION
It has been shown that the mixing of carbon micro- and even nanopowders into molten
aluminum is impractical because of the extremely poor carbon wettability by liquid aluminum,
resulting in composites that are very porous and non-homogeneous. The problem of wettability
of carbon particles by molten aluminum can be easily solved by application of molten alkali
chloride and sometimes fluoride as a reaction media. The ion diffusion coefficients in molten
halides are compatible with diffusion coefficients in molten aluminum [26]. This permits the
chemical interaction of carbon-containing substances with liquid aluminum to be carried out
with a large yield of high carbon content aluminum composites at relatively low temperatures
under air.
The synthesis of carbon particles inside an aluminum matrix proceeds by one-step reaction
of carbon-containing components of molten halide mixture with the molten aluminum resulting
graphene sheets and/or graphite crystals obtained in molten aluminum, which are well wetted by

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aluminum. The total reaction of carbide ion oxidation by air oxygen within molten aluminum
under halide flux can be described by the following equations, depending on the type of carbon
precursors (Table 1).
It is obvious that all the reactions from Table 1 are thermodynamically favorable at
relatively low temperatures of 973-1073 К. The main interaction products – aluminum oxide and
carbon – were determined by X-Ray, EDS spectra and chemical analysis at interaction of all the
above-mentioned metal or non-metal carbides with liquid aluminum under molten salts.
Aluminum oxide nanopowders form in the volume of molten salt electrolyte; while the large
amounts of reduced carbon are embedded into aluminum, partially as a graphene on the surface
of the salt melt. The other interaction products – TiO2 or tungsten powders – are also generally
deposited as solids in the salt melt; boron oxide fully dissolves in the melt.
The two main types of aluminum-carbon metallic composites – aluminum-graphene and
aluminum-graphite – have been synthesized. Aluminum-carbon composites produced by the
proposed method are compact and non-porous, with a characteristic metallic shine (Fig.1) and
good electrical conductivity. The carbon content of the composites is consistent from the surface
of the aluminum drop to its center.
The lower specific densities of the aluminum-carbon composites compared to the initial
aluminum precursor are strongly dependent on the carbon content: 2.5 g/cm-3 for the composite
with 1 wt. % of carbon and 2.4 g/cm-3 for the composite with 2 wt. % of carbon.
The optical and electron microscopy study of the surfaces (Fig. 2a, b) and polished cross-
section of aluminum following the interaction with the carbide-containing halide melt
demonstrated that there are multiple, well-formed crystals (Fig. 3a, b) or transparent single- or
multilayer films (Fig. 4a, b). According to the chemical analysis, these crystals and films consist
of pure carbon (Fig. 3c and 4c). The thin graphene films remain on the surface of the polished
cross-section of the specimen after mechanical cutting and 6-times finishing in different silicon
solutions, as well as after rolling into a foil with a thickness of 100 µm (Fig. 5 a-c). It is evident
that, following rolling of the aluminum-graphene composite, the concentration of graphene
sheets on the metallic surface increases as if the pressure in the rolling extrudes graphene from
deeper layers to the surface. Graphene does not form a continuous net through all aluminum
surfaces but represents many separate films.
The phase purity and crystallinity of carbon inclusions was investigated by Raman micro
spectroscopy because of the sensitivity of the method to the full range of structural states of
carbon (diamond, graphite, graphene and others). Distinctive Raman bands are observed for each
of these individual forms of carbon, making it possible to distinguish between sp2 and sp3
hybridization.

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The Raman spectra were recorded on a Renishaw U1000 Raman microscope-spectrometer
equipped with a confocal Leica DML microscope, 50×, 100× objectives, a notch filter, and
cooled charge-coupled device (CCD) detector. An Ar+ laser (model 1160 spectra Physics)
operating at 514.5 nm and 20 mW was employed as an excitation source. The spectral resolution
was 2 cm-1, the wave number accuracy was about ± 1cm-1 and the laser beam had a focal-spot ~
1ꢀm in diameter; typical spectra acquisition time was 20 to 30s.
Fig. 6a illustrates a typical Raman spectrum of transparent carbon film in the aluminum
matrix. The dimensions of single-layer and multilayer graphene sheets are quite significant: 25 to
35 ꢀm. In addition, there are a large number of very small inclusions – about 100 to 300 nm. The
Raman spectrum represents the typical graphene one. It contains all characteristic graphene
peaks: G - at 1585 cm-1; D – 1353 cm-1; 2D – 2700 cm-1; D+G- at 2938 cm-1. All peaks are very
well defined and symmetrical. The intensity ratio I2D/IG is equal to 0.31. This implies the
formation of a three-layered graphene [27, 28].
Fig. 6b presents a typical Raman spectrum of carbon crystal in the aluminum matrix. There
is only one extremely intensive line at 1585 cm-1, which corresponds to the G-bond and
asymmetric sharp peak at 2717 cm-1. This Raman spectrum confirms the presence of perfect
pyrolitic quality graphite. The well-edged graphite crystals were formed from atoms in molten
metallic aluminum; therefore, they are well wetted and do not crumble when rolling and
polishing.
The presence of aluminum carbide in the spectra is typically defined by the appearance of
the Raman peak around 850 cm-1 that is assigned to aluminum carbide (Al4C3) [29]. No traces of
aluminum carbide have been determined in numerous Raman spectra, which are very sensitive to
any micro impurities. Therefore, we can conclude the absence of aluminum carbide formation
inside the aluminum-carbon composites.
The halves of aluminum-carbon samples were dissolved in 20 wt. % HCl solution. The thin
films floating on the solution surface were deposited onto titanium foil (Fig. 7a). The Raman
spectra of the thin film on the titanium substrate corresponded to that of graphene, showing all
the characteristic peaks (Fig. 7b). The X-Ray analysis of graphene film on titanium shows broad
diffusion peaks despite the formation conditions (different carbides and temperatures) (Fig. 7c).
The XRD pattern of aluminum-graphene composites containing carbon in concentrations
from 0.2 to 2 wt. % does not include any carbon peaks such as those peaks characteristic of
aluminum carbide (Fig. 8a). The crystal lattice of the synthesized aluminum-graphene samples
strongly corresponds to the face-centered cubic lattice with symmetry of Fm-3m. Despite the fact
that the carbon content is significant enough for X-Rays to define it, it is likely that the reason

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for the lack of carbon peaks in the X-Ray diffraction pattern is due to the sizes or thickness of
carbon inclusions being less than 20 nm: X-Ray definition of such small particles is not possible.
At the formation of rather large, well-grained graphite crystals in the aluminum matrix
there were four characteristic graphite peaks. The 100% graphite peak at 26.46º is well defined
and high in the X-Ray diagram (Fig.8b). However, in paper [30] no carbon peaks were registered
in X-Ray diagrams even at higher graphite concentrations in an aluminum matrix.
Both large crystals and thin films were studied by EBSD analysis. Electron backscatter
diffraction (EBSD) is a microanalysis technique used by scanning electron microscopes to
determine the crystal micro- and nanostructure of the sample. The comparison of obtained
diffraction lines was performed with the carbon of different allotropic modifications, i.e. graphite
and cubic diamond with different lattice parameters and others.
Kikuchi lines pairing up to form bands in electron diffraction from single crystal
specimens were registered by EBSD. Because each Kikuchi line is associated with Bragg
diffraction from one side of a single set of lattice planes, these lines can be labeled with the same
Miller or reciprocal-lattice indices that are used to identify individual diffraction spots.
Representations of Kikuchi lines of large carbon crystals are presented in Figure 9. The
large crystal consists of 100% carbon, representing graphite with the characteristic angles of 90º
× 90º × 120º and lattice parameters of 0.246 × 0.246 × 0.671 nm.
When trying to obtain EBDS spectra of graphene inclusions, Kikuchi lines were not
obtained because of their insufficient thickness (less than 10 atomic layers).
Thermal analysis of aluminum-carbon composites was carried out by means of an STA
449C Jupiter® thermal analyzer by NETZSCH (Germany), allowing measurements to be
performed by differential scanning calorimetry (DSC) and thermogravimetry (TG) methods
across the wide temperature range. The measurement conditions were qualified by a heating rate
– 10K/min under argon atmosphere in a stationary regime.
The different scanning calorimetric curves of aluminum-graphite and aluminum-graphene
alloy indicate that they have almost the same onset point of pure aluminum but different
enthalpies (Fig. 10 a and b). The melting point of the aluminum-graphene composite material
decreases with an increase of the graphene content in aluminum, equal to 660ºС for the
composite with 1 wt. % of graphene and 658ºС for the composite with 2 wt. % of graphene.
However, the melting point of the aluminum-graphite composite at 666ºС is higher than
aluminum. Thus, there is congruent melting. This means that the melting- and crystallization
peaks are reproduced by reheating; the peak of crystallization is expressed by the single DSC
anomaly. The melting and crystallization peak areas are equal because the melt composition

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coincides with the solid-state composition. There are no additional peaks characteristic of carbon
in the DSC curves.
The complex of the main properties of aluminum-graphite and aluminum-graphene
composites with some graphene concentrations is presented in Table 2.
The mechanical properties (Vickers hardness and elastic modulus) of so-formed Al-
graphene and Al-graphite metallic composites were studied. The optical photographs of cross-
sections without etching of Al-carbon composites before (Fig. 11a) and after hardness testing
represent a typical aluminum-graphene face with uniformly distributed large graphene films in it
(Fig. 11b) and aluminum-graphite with small well-grained graphite crystals (Fig. 11c). As can be
seen from these photographs, the graphene films are predominantly self-produced on the grain
boundaries of aluminum; however, a part of them is formed directly inside the grain. Despite the
great difference in the surface areas occupied by graphene and graphite, the total concentration
of carbon in aluminum is the same because of differences in the thickness of carbon inclusions.
As Table 2 shows, the hardness increases in the row aluminum – aluminum-graphite
composite – aluminum-graphene composite; additionally, in the aluminum-graphene composite,
the hardness is directly proportional to the graphene concentration in the composite up to 2 wt.
%. Similarly, the elastic modulus increases in this row. The simultaneous increase of hardness
and elastic modulus of aluminum-graphene composites is very unusual and can be explained
only by the influence of a high concentration of graphene, which simultaneously exhibits high
elasticity and hardness [31]. The hardness of aluminum-graphene composites is at least 2-3 times
higher than that of initial aluminum.
Typical stress–strain curves of the aluminum-graphene composites reinforced with varying
graphene content are shown in Fig. 12. It is obvious that the tensile strength increases with the
increase of graphene content up to 2 wt. % in the composite; this is because graphene
concentration in aluminum strongly effects the enhancement of the mechanical properties of the
aluminum-graphene composites. As is shown in Fig. 12, the composites with an addition of 1 or
2 wt. % graphene possess ultimate tensile strengths of 80.4 and 93.78 MPa, which are 1.3 and
1.52 times greater than that of the aluminum matrix sample (61.9 MPa). The same strong
improvement of tensile strength is seen in the aluminum-graphene composite [18, 32] but it is
accompanied by a reduction of elongation. While significant hardening is accompanied by
elongation of the samples with the addition of 1 and 2 wt. % graphene, which increases with the
greater addition of graphene and reaches values of 20.4 % and 24.2 % at 1 and 2 wt. % graphene
composite, compared with 13 % for pure aluminum. This means that the hardness and the
ductility of the aluminum-graphene composite simultaneously improve with graphene
concentration in the aluminum matrix.

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It is well known that improving a material’s hardness usually leads to brittle deformation.
However, the introduction of graphene into an aluminum matrix in concentrations up to 2 wt. %
allows a simultaneous increase in the strength, hardness, ductility and elasticity of the resultant
aluminum-graphene metal composite. The possible explanation of such unique mechanical
properties of aluminum-graphene composites is the fact that numerous graphene flakes in the
aluminum have a significantly higher specific area per unit mass in comparison with MWCNTs
(at least 2πr times more), for example. The strong interface interaction between the graphene and
aluminum matrix can effectively transfer the load from the aluminum to the graphene. Because
of graphene’s high tensile strength, the application of graphene directly formed and uniformly
distributed in an aluminum matrix may be the best strengthening carbon additive for improving
the mechanical properties of aluminum.
The carbon concentrations in aluminum matrix obtained by the proposed method are
unprecedentedly high. However, the data on the solubility of carbon in aluminum (0.03 at.%)
contained in most databases refer to the solubility of macroscopic carbon in aluminum. The most
recent computer simulations [1] show that when considering metastable body-centered
aluminum crystal cells, the interstitial sites occupied by carbon may be the same value as the
maximum carbon solubility in bcc at 1473 K in Al-C-Fe. In the above-referenced paper, the
formation of aluminum carbide Al4C3 is considered.
It is well known that significant amounts of aluminum carbide form only at temperatures
above 1373 K. The proposed method of aluminum-graphene and aluminum-graphite composites
precedes at temperatures of 350-400 K less. Therefore, there is the only phase – aluminum with
some increase in the lattice parameters according to the X-Ray analysis of aluminum-graphene
metallic composites (Fig. 8a). There are peaks of aluminum and carbon conforming to X-Ray
analysis of aluminum composites with large graphite crystals (Fig. 8b). There is no transition
zone between carbon inclusions and aluminum by microprobe analysis. Accordingly, no traces
of aluminum carbide presence in aluminum-carbon composites were established by any of the
methods of investigation.
The full hydrolysis of aluminum carbide in aqueous solutions with the formation of
aluminum hydroxide and methane is highly detrimental to the corrosion resistance of aluminum-
carbon metallic composites if aluminum carbide exists in composites. We carried out prolonged
corrosion tests of the samples of aluminum-graphite and aluminum-graphene composites that
were cut from different parts of the obtained metallic globule in 3.5 wt. % NaCl solution under
air at room temperature. No gaseous products or aluminum hydroxide formation were observed
during 4 months corrosion exposure of aluminum-carbon composites in 3.5 wt. % NaCl solution.
The corrosion rate of aluminum-carbon composites had higher values than the initial aluminum

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ones (Table 2). Corrosion of aluminum-graphene composite as with aluminum is uniform,
without any pitting; some graphene films are visually evident on the surface following the
corrosion test (Fig. 13 b). The corrosion of the aluminum-graphite composite is non-uniform
with numerous pitting formations in place of graphite crystals egress as shown in Fig 13 c and d.
Despite some increase in the corrosion rate when modifying by graphene flakes and
graphite inclusions, the extent of corrosion characteristics is class 4, corresponding to corrosion
resistant materials. This compares to pure aluminum, which is quite resistant with class 3.
If any aluminum carbide forms in the composite pitting of alloys or composites, this leads
to subsequent crumbling of the carbon-constituents of the alloy and very rapid dissolution.
Therefore, we can conclude that there is no formation of any significant quantity of aluminum
carbide at electroless interaction of molten aluminum with the carbide-containing salt melt
within the investigated temperature interval.
The introduction of carbon into aluminum in quantities exceeding its solubility is possible
only at the atomic level. Carbon atoms extracted at the surface of the aluminum and diffused
inside aluminum and salt melts take place at quantities greatly exceeding its solubility limit.
After aluminum melt cooling, all extra-carbon forms graphene sheets or graphite crystals inside
aluminum drop depending on cooling regimes, i.e. cooling rate or subsequent thermal treatment.
Graphene films are formed parallel to the whole plane of solidification; their concentrations do
not change from layer to layer of the composite. Such an arrangement of graphene films
enhances the strength, hardness, ductility and elasticity of the composite as well as providing
high electrical conductivity.
Carbon as graphene or graphite inclusion is uniformly distributed in the aluminum bulk.
The carbon concentration is the same in the surface layer of the aluminum drop as in its center.
Since the synthesis of carbon inclusions as graphene and graphite is carried out from carbon
atoms directly embedded in the aluminum matrix, the composites so produced are distinguished
by their unique wettability. There are no transition zones between aluminum and carbon in cross-
sectional samples.
The method of production of aluminum-graphene metallic composites is based on direct
chemical interaction of carbides with molten aluminum under the layer of chloride and/or
halogenide melt at temperatures 973-1073 K in air without any special inert atmosphere. The
one-step synthesis of nano- and microparticles of carbon as graphene films or graphite
microcrystals takes place directly in molten aluminum without a separate stage of synthesis and
allocation of carbon nanomaterials as is the case with all existing methods of metal-graphene
composite synthesis. This significantly reduces difficulties in producing of aluminum composites
with a high graphene or graphite content. The high uniformity of graphene sheets with sizes from

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50 nm to 100 µm and thickness from 1 to 3 nm inside the metallic bulk leads to high uniformity
of properties of the thus obtained aluminum-graphene composites.
The non-porous compact aluminum-graphene and aluminum-graphite metallic composites
obtained by chemical interaction of carbides with molten aluminum possess increased hardness,
elasticity and sufficient resistance to corrosion. The melting point of aluminum-graphene
composites strongly corresponds to those of aluminum.
The presence of separate, uniformly distributed graphene or graphite inclusions in
aluminum is confirmed by several independent investigation methods. It was shown that
aluminum carbide, which could negatively affect the corrosion resistance of metallic composite,
does not form under the investigated experimental conditions.
Samples of the aluminum-graphene metallic composites retain a shiny, bright surface when
stored in air for long periods of time (1.5 to 2 years) and are not covered by oxide films.
Aluminum-graphene composites can be easily re-melted for casting using traditional
technologies and rolled into thin foil without the loss of their original properties.

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Conclusions
The aluminum-graphene and aluminum-graphite metallic composite materials, with
uniformly distributed carbon inclusions inside an aluminum matrix, were synthesized for the first
time by chemical interaction of metal or non-metal carbides with molten aluminum in a
temperature range of 973-1073 К under air. The metallic composites thus obtained have specific
densities from 2.4 to 2.5 and possess a characteristic metallic shine and electrical conductivity.
The synthesis of aluminum-carbon metallic composites with improved structure and
unique mechanical properties proceeds at relatively low temperatures without aluminum carbide
formation; as a consequence, these composites have sufficiently high corrosion resistance. The
hardness, strength, ductility and elasticity of aluminum-graphene composites are higher than the
initial aluminum materials, corresponding to the concentration of graphene.
Aluminum-graphene composites can be rolled into thin foil or re-melted without loss of
mechanical properties.
Acknowledgments
The authors gratefully acknowledge the “Composition of Compounds” Shared Access
Centre for analytical support and “Plastometry” Shared Access Centre for mechanical testing.
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Figure captions
Fig.1. Photo of aluminum-graphene composite samples.
Fig.2. SEM-images of the surfaces of a- aluminum-graphite composite, b – aluminum-graphene
composite.
Fig.3. SEM-image (a, b) and EDS spectrum (c) of thick carbon inclusion in the cross section of
aluminum matrix.
Fig.4. SEM-image (a, b) and EDS spectrum (c) of thin carbon film in the cross section of
aluminum matrix.
Fig.5. SEM images of graphene films on the surface of aluminum-graphene foil after rolling; b, c
– at higher magnifications..
Fig.6. Raman spectra of thin carbon film (a) and carbon crystal (b) in aluminum.
Fig.7. SEM-image (a), Raman spectrum (b) and X-Rays diagrams (c) of thin films obtained with
WC (top) and TiC (bottom) additives after dissolution of aluminum-graphene composite in 20
wt.% HCl solution and graphene deposition on titanium foil.
Fig.8. X-Rays diagrams of: a – aluminum-graphene composite; b – aluminum-graphite
composite.
Fig.9. Kikuchi lines of large graphite crystal.
Fig.10. DSC and TG curves of: a – Al composite with 2 wt.% graphene; b – Al-composite with 2
wt.% graphite.
Fig.11. Optic image of cross-section of Al-graphehe: a – before; b – after hardness test; c –
image of Al-graphite after hardness test.
Fig.12. Stress-strain curves of initial aluminum, aluminum -1wt.% graphene (ACT1) and
aluminum-2 wt.% graphene (ACT2).
Fig.13. SEM-image of the samples after 4 months corrosion test in 3.5 wt.% NaCl solution: a –
initial aluminum; b – aluminum-graphene composite; c, d – aluminum-graphite composite.

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Table 1
Gibbs energies (kJ/mol) of possible reactions in the system molten Al-carbide
Reaction
973 К
1023 К
1073 К
-4372.670 -4677.916
-3333.713 -3292.155
-2703.960 -2670.820
-4623.364
-3250.668
-2637.724
4Al+B4C+6O2=2Al2O3+C+2B2O3 (1)
4Al+TiC+4O2=2Al2O3+C+TiO2 (2)
4Al+WC+3O2=2Al2O3+C+W (3)
Table2
Properties of aluminum-carbon composites
Al-2%
graphite
Al
Al-1% graphene Al-2% graphene
6620C
6600C
6580C
2.4
6660C
Melting point, K
Specific density, g·cm-3
HV 2000 mH
2.7
2.5
2.4
16.72
60.61
41.46
61.87
13.04
0.0016
0.005
45.32
82.48
47.61
80.4
25.52
58.54
43.92
75.5
57.19
87.93
Elastic modulus, GPa
Tensile Strength, σ0.2,
MPa
48.1
93.78
Yield Strength, σВ, MPa
20.6
19.04
0.0130
0.045
24.2
Related length, δ5, %
Corrosion rate, g/m2·h
0.0035
0.012
0.0048
Depth of corrosion,
mm/y
0.017
3
4
4
4
Grade of corrosion
resistance
Quite durable
Durable
Durable
Durable
Uniform
Uniform
Uniform
Pitting
Corrosion character

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Fig.1.
Fig2a
Fig.2b

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Fig.3a
Fig.3b
Fig.3c

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Fig.4a
Fig.4b.

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Fig.4c
Fig.5a
Fig.5b

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Fig.5c
Fig.6a

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20000
15000
10000
5000
0
500
1000
1500
2000
2500
3000
3500
Raman shift, cm-1
Fig.6b
Fig.7a

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7000
6000
5000
4000
3000
2000
1000
0
500
1000
1500
2000
2500
3000
Raman shift, cm-1
Fig.7b
Fig.7c

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Fig.8a
Fig.8b

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Fig.9.
Fig.10a

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Fig.10b
Fig.11a