Fabrication and Characterization of Aluminum-Graphene Nano-Platelets—Nano-Sized Al₄C₃ Composite

Abstract

Reinforcement of aluminum and aluminum alloys with graphene has been intensively practiced by researchers in the past dozen years. The role of Al₄C₃, which could be produced unintentionally or purposefully during the composite production, was controversial until it was found that nano-sized carbides were beneficial for strengthening the composites. aluminum-graphene-nano-sized Al₄C₃ composites were produced by us using the powder metallurgical method and subsequent annealing. The microstructure was investigated using light microscopy (LM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and high resolution transmission electron microscopy HRTEM. Nano-sized carbides were found at the interface aluminum-graphene. The formation of a chemical bond between aluminum and graphene during annealing was proved. Lower values of the microhardness and strength characteristics of the composites after extrusion and subsequent annealing during which nano-sized carbides are formed were found in comparison with those obtained after extrusion. It could be supposed that the annealing processes contribute more to the reduction in microhardness and strength characteristics than nano-sized carbides contribute to its increase. The presence of a strong chemical bond between the graphene and the aluminum is manifested in the failure pattern, which is characterized by graphene nano-platelets and nano-sized carbides fracture and semi-pulled out or semi-slipped Al₄C₃ from the matrix.

1. Introduction

Numerous articles whose object of research is aluminum-based composites reinforced by graphene nano-platelets (GNPs) in quantities from 0.1 to 2.0 wt. % have been published in the last ten years [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]. Most of these cited works have established the formation of a mechanical bond between the graphene and the aluminum matrix. However, when tensile testing the resulting composite, higher hardness and strength characteristics were obtained compared to pure aluminum. Relatively older documents testify that Al₄C₃ was used for strengthening composites and alloys [25]. Researchers claim that when micro-sized carbides of the Al₄C₃ type are formed during composite production, the mechanical properties of the composite deteriorate [12,26]. Specifically, the composite’s strength decreases because stress transfer occurs through the interface. Li et al. [13] and A. Saboori et al. [16] stated that Al₄C₃ is brittle and thermodynamically unstable, which may cause a decrease in the tensile strength of the composite. There are some studies claiming that when nano-sized Al₄C₃ carbides are produced at the aluminum matrix–graphene interface, they are locked in the aluminum matrix, creating an anchor effect between graphene nano-platelets and the matrix due to which the composite’s strength properties are improved [14,15,27]. The mechanical and chemical bonds between aluminum matrix and graphene are defined and illustrated in [17], as well as the relevant failure behaviors of the composites. The effect of the size of Al₄C₃ on the mechanical properties of aluminum composites reinforced with SiC and nanotubes is investigated in [26]. It was found that the nano-sized Al₄C₃ helps to increase the interface shear stress and the strength of composites, while micro-sized Al₄C₃ significantly deteriorates the mechanical properties.

Some reports specify the methods and conditions when the nano-sized carbides at the aluminum-graphene interface are formed [14,15,17,26,27,28,29]. It was established that the conditions for obtaining the nano-sized carbides are different for the different methods of producing the composite. Moreover, in some cases, micro-sized carbides are obtained, which is undesirable, considering the deterioration of the mechanical properties of the composite due to the brittleness of Al₄C₃ [13,16].

It should be mentioned that GNPs react with aluminum, and nano-sized Al₄C₃ are formed in an extruded and drawn specimen during annealing at 650 °C for 30 min [17]. As a result, the fracture mode of the composite changed from GNPs de-bonding to GNPs fracture because the interfacial bonding changed from mechanical bonding to chemical bonding. It is also found in [17] that the strengthening mechanism of load transfer plays a significant role in the enhancement of strength.

Our goal is to produce aluminum-graphene-nano-sized Al₄C₃ composite by powder metallurgical method and subsequent heat treatment and investigate the microstructure, microhardness, mechanical properties, and failure behavior of the composite during tensile testing. We would like to clarify whether the nano-sized Al₄C₃ will help to increase the strength properties of the composite obtained in this way.

We will strive in our future works to study the mechanisms of strengthening Al-GNPs-nano-sized Al₄C₃ composites.

2. Materials and Methods

2.1. Materials

The materials used to prepare the composites were:

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Aluminum powder with a chemical purity of 99.5%. The average particle size was measured by us to be 37 µm.

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Graphene nano-platelets produced by io-li-tech Company with thickness 6–8 nm and purity 99.5%.

2.2. Experimental Procedure

The two constituents (Al powder and graphene nano-platelets) were mixed in a planetary agate ball mill with 7 balls; the weight of each ball was 11.6 g; speed—700 revolutions per minute; time—30 min; room temperature; in an argon atmosphere. The powders were mixed in a way that the graphene content varied from 0.1 wt.% to 1.1 wt.% with an increment of 0.2 wt.%.

The weight of each mixture was 300 g. Extrusion was carried out on a hydraulic press, “RUE 250 SS”, after preliminary densification by cold isostatic pressing. For extrusion, a mold with its own heater and the possibility of changing the degree of reduction was used. This stage was carried out by straight pressing. Seven compacts, in the form of cylinders with a diameter of 40 mm, were produced by double-sided pressing for 60 s at 381 MPa. The compacts were heated at 370 °C for 20 min in order to temper the entire system and degas the pre-pressed sample body. The mold was preheated to 400 ± 10 °C, where the compacts stayed for 4.5 min and were extruded under a pressure of 457 MPa, extrusion lasting for 60 s/apiece. In order to reduce friction during pressing, a high-temperature lubricant, “Vapor”, was used.

The final product, a bar of aluminum-based nanocomposite reinforced with graphene nano-platelets—12 mm in diameter, was cooled in the air.

From the aluminum composite bar reinforced by 1.1 wt.% GNPs, pieces were cut off for heat treatments with the following parameters: heating temperature—610 °C; holding time—3 h; cooling ambiance—air out of the furnace in order to determine the heat treatment mode in which nano-sized carbides are obtained at the aluminum–GNPs interface. The used furnace is Labor Müszeripari Müvek Eszteercom Typ LR-202. This specimen was investigated using all listed methods, and then the rest of the specimens were heat treated at the same conditions.

2.3. Research Methodology

The LM and SEM samples were prepared from the rods according to standard procedures in the longitudinal direction with respect to the extrusion. The cross-sections of the samples were wet ground with sandpaper from 320# to 3000#, mechanically polished with diamond suspension 1/4 μm, and etched in 0.5% water solution of HF.

The LM and SEM samples from the extruded and subsequently annealed rods were prepared in the same way.

The LM observations were performed by a metallographic microscope MIT500 at magnifications up to 1000×.

The SEM analysis was carried out using a HIROX SH-5500P microscope with an integrated Energy-Dispersive X-ray Spectroscopy (EDS) system “QUANTAX 100 Advanced” by Bruker under the following conditions: (a) an accelerating voltage: 20 kV; (b) work distance: 3 to 7 mm; (c) secondary electrons detector. The HIROX SH-5500P microscope was delivered by HIROX-Europe, Jyfel, 300 RN 6—Le Bois des Côtes, Bât A, F-69760 Limonest, France.

The micro-hardness measurements were carried out using the device MicroDuromat 4000 (firm Reichert-Jung, Austria) with a load of 10 g, time for reaching a load of 10 s, and holding time of 10 s.

The XRD analysis was carried out on a powder diffractometer, Bruker D8 Advance equipped with LynxEye detector, (Bruker AXS, GmbH, Karlsruhe, Germany) 3 kW generator, and 2.4 kW X-ray tube with Cu-anode. The evaluation of the diffraction patterns was performed using the Bruker AXS. EVA 2, DIFFRACplus Evaluation Package. 2009, package in combination with the ICDD-PDF-2 database.

The TEM and HRTEM investigations were performed on a JEOL 2100 transmission electron microscope (XEDS: Oxford Instruments, X-MAX N 80 T, CCD Camera ORIUS 1000, 11 Mp, GATAN) at an accelerating voltage of 200 kV [30,31]. The specimens were prepared by Leica EM UC7 ultramicrotome, Danaher Corporation, to obtain ultrathin sections for electron microscopy. The measurements of lattice-fringe spacing recorded in HRTEM micrographs were made using digital image analysis of reciprocal space parameters. The analysis was carried out by the Digital Micrograph software.

The tensile tests were carried out on a standard testing machine, ISO 6892-1. Three samples with the same content of graphene in extruded state and three samples with the same content of graphene in extruded and annealed state were tested. The mean arithmetic value was determined for each group of samples.

3. Results

3.1. Light Microscopy

The light microscopy observations showed that there was no nano-sized formation at GNPs periphery in the samples after extrusion and before heat treatment (Figure 1a), while after annealing at 610 °C for 3 h, nano-sized spherical grains formed at the interface GNPs–aluminum (Figure 1b). It could be only supposed at this stage of the investigation that these small grains are nano-sized Al₄C₃.

3.2. Microhardness

After extrusion, the microhardness increases with increasing content of GNPs until it reaches 49.3 kgf/mm² at 0.5 wt.% GNPs (Figure 2). The maximum value of microhardness or any strength property is reached when the graphene content increases and before the formation of GNPs’ agglomerates begins. Obviously, the best mixing of the powders in the ball mill and the most homogeneous microstructure in the composite was obtained at a graphene content of up to 0.5 wt.% because the microhardness reaches there the maximum value. It increases from 0 wt.% GNPs to 0.5 wt.% GNPs with 6%. The measured microhardness values after annealing are generally lower, but the pattern of variation of this property is the same. Here the difference between the maximum and minimum values is 18%. The lower values of the microhardness in the annealing state compared with the values in the deformed state show that the annealing processes contribute more to the reduction of microhardness than the suspected nano-sized carbides contribute to its increase.

3.3. SEM-EDS

The carried out SEM observations and EDS analyses of the sample reinforced by 1.1 wt.% GNPs after heat treatment gives reasons to believe that the platelet shown in Figure 3 is a GNP and the spherical formations around it are nano-sized aluminum carbides (Al₄C₃). We hypothesize that the spheroids at the periphery of the lamellae and the lamellae themselves are imaged with different contrast when analyzed with an SE detector due to their different chemical composition, which should be C and Al₄C₃.

3.4. XRD

Figure 4 shows the XRD powder patterns obtained from the sample containing 1.1 [wt.%] graphene before and after heat treatment of the same sample. Both patterns contain very intensive peaks of aluminum, a small peak corresponding to graphene, and a hump related to the amorphous phase. On the heat-treated sample, a very small peak at about 54° 2θ may be due to the trace of Al₄C₃. Since the intensity of the latter peak is very low and lies within the uncertainty of the instrument, the XRD analysis could not present reliable evidence for the presence of the carbide phase, which is not a strong indication that a small quantity of nano-carbides is missing. The fact that aluminum carbides are not registered may be due to the integral nature of the method. For this reason, we studied this sample using TEM and HRTEM.

3.5. TEM and HRTEM

The most convincing evidence of obtaining nano-sized A₄C₃ after HT we received as a result of the study using TEM and HRTEM. Figure 5a shows the polycrystalline SAED pattern of Al PDF 89-4037 a = 4.0490 Å SG Fm-3m. The image in diffraction mode confirms the presence of submicron-sized particles (spots along the polycrystalline rings) (Figure 5a), and the HRTEM image Figure 5b shows two areas of Al₄C₃ crystallographic phase (Al₄C₃ PDF 79-1736 a = 3.3350 Å, c= 24.9670 Å SG R-3m) with d₀₀₁ = 2.50 nm and d₀₀₂ = 1.25 nm [32,33]. The size of the particles is about 20 nm. The interface between nano-sized Al₄C₃ and the aluminum matrix is given in Figure 5c. In Figure 5d, we present experimental and corresponding Fourier-filtered HRTEM images of crystallographic phase Al₄C₃ (left-hand and right-hand sides) in another orientation with inter-planar distance d₀₀₆ = 0.416 nm.

Figure 6a,b show areas of the longitudinal section of the neck of a tensile-tested specimen. Both broken GNPs and carbide rods (Figure 6a) and semi-pulled out or semi-slipped A₄C₃ rods from the matrix are observed (Figure 6b) in composites where GNPs were reacted with aluminum matrix and Al₄C₃ were obtained due to the heat treatment. The failure behavior could be defined as intermediate or mixed—the behavior of breaking the particles and “interface de-bonding and pull-outs”. The fracture behavior of the composites after extrusion is entirely “interface de-bonding and pull-outs” [16,23], which is determined by the presence of a mechanical bond between GNPs and aluminum matrix.

3.6. Tensile Testing

The results from tensile testing of the specimens after extrusion and subsequent heat treatment are shown graphically in Figure 7. It can be seen that the strength characteristics reach their maximal values at 0.7 wt.% GNPs—R_m = 123 MPa and R_0.2 = 79 MPa. The respective values in the extruded state, according to our previous research [23], are R_m = 134.5 MPa and R_0.2 = 97.5 MPa. From the comparison of these values, it follows that the studied composites have higher strength properties after extrusion than after extrusion and subsequent annealing. These results could be explained by assuming that the processes taking place during annealing, which are also strength decreasing, have a greater share than the strengthening effect of nano-sized aluminum carbides.

4. Discussion

Obtaining nano-sized carbides at the graphene–aluminum interface in graphene-reinforced aluminum-based composites is a challenge for modern researchers, as usually either no carbides are obtained, or microsized carbides are obtained. The implemented research has reached its goal of producing aluminum–GNPs composite with nano-sized Al₄C₃ at the interface aluminum-graphene, which assures a chemical bond between the two phases and investigates the microstructure, microhardness, strength properties, and fracture behavior. The specific conditions for composite fabrication by powder metallurgical method have been established. It is shown that nano-sized Al₄C₃ could be obtained by extrusion and subsequent annealing at 610 °C for 3 h. The carbides are formed, namely during annealing. This temperature and this holding time are sufficient for a reaction between aluminum and carbon from graphene to take place and to form nano-sized carbides. The microhardness of both extruded and extruded and subsequently annealed samples is higher than the microhardness of pure aluminum in the respective state. The microhardness scattering in the two cases has the same character, as the maximum values have been reached at 0.5 wt.% GNPs, i.e., after this graphene content, agglomerates of GNPs are formed during mixing, and the microhardness of the composite decreases. Using LM and SEM, it has been supposed, and using TEM and HRTEM, it has been conclusively proven the obtaining of nano-sized Al₄C_3. The size of the found nano-carbides is about 20 nm. The observation of the aluminum matrix-nano-sized Al₄C₃ interface testifies to the existence of a chemical bond between them which is stronger than the mechanical one. Regardless of the presence of nano-sized carbides and the formation of a chemical bond between them and the aluminum matrix, the microhardness of the composites in the extruded and annealed state is lower than the corresponding one in the extruded state. Comparing the maximum values of the strength characteristics in the two states shows an analogous result. This could be explained by the greater impact of the processes taking place in the microstructure during annealing, which weaken the matrix than the presence of nano-sized carbides and chemical bonding which strengthens the composite. This shows that not in all cases, the presence of nano-sized carbides Al₄C₃ and a chemical bond between them and the aluminum matrix in Al–GNPs composite leads to an increase in its strength properties. The presence of a stronger chemical bond between the graphene nano-platelets and the aluminum matrix is manifested in the failure pattern of the specimens during tensile testing. Therefore, in the extruded and subsequently annealed samples in which nano-sized carbides were formed, GNPs and A₄C₃ fracture and semi-pulled out or semi-slipped Al₄C₃ from the matrix are observed. Unlike composites with a mechanical bond between GNPs and the aluminum matrix where the failure mechanism is characterized by interface de-bonding and pull-outs.

5. Conclusions

(1)

Aluminum-based composites with graphene nano-platelets from 0.1 to 1.1 wt.% and nano-sized carbides at the aluminum-graphene interface can be successfully produced by powder metallurgy via extrusion and subsequent annealing at 610 °C and holding time of 3 h.

(2)

The presence of nano-sized carbides type Al₄C₃ can be most conclusively proved, and their dimensions determined by HRTEM. They are around 20 nm.

(3)

The microhardness and the strength properties of extruded samples, whose microstructure constituents are aluminum matrix and GNPs, are the highest. The microhardness and the strength properties of extruded and annealed samples, whose microstructure constituents are aluminum matrix, GNPs, and nano-sized carbides, are lower. This is due to the greater strength-reducing of the material, which occurs as a result of annealing, than the strengthening, which is due to the presence of graphene nano-platelets and nano-sized carbides. The microhardness and the strength properties of pure aluminum in the respective state are the lowest.

(4)

GNPs and A₄C₃ fracture and semi-pulled out or semi-slipped Al₄C₃ from the matrix are observed as a result of the chemical bond existing between GNPs and aluminum matrix.