Reinforcement with graphene nanoflakes in titanium matrix composites

Journal of Alloys and Compounds 696 (2017) 498e502

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Reinforcement with graphene nanoflakes in titanium matrix composites Zhen Cao a, b, *, Xudong Wang a, b, Jiongli Li a, b, Yue Wu a, b, Haiping Zhang a, b, Jianqiang Guo a, b, Shengqiang Wang a, b a b

Beijing Institute of Aeronautical Materials, Beijing 100095, China Beijing Engineering Research Center of Advanced Aluminum Alloys and Application, Beijing 100095, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 September 2016 Received in revised form 20 November 2016 Accepted 21 November 2016 Available online 22 November 2016

Graphene reinforced bulk titanium matrix composites (TMCs) were successfully fabricated via powder metallurgy approach. 0.5 wt% graphene nanoflakes (GNFs) and Ti6Al4V mixture powders were prepared by a wet process. The composites were then consolidated using hot isostatic pressing (HIP) with a pressure of 150 MPa at 700
C followed by isothermal forging with a forging ratio of 3 at 970
C. The microstructure and mechanical properties of TMCs had been investigated by optical microscopy, SEM, TEM and static tensile tests. Microstructure observation illustrated a uniform distribution of graphene in the composite and in-situ formed TiC particles at the metallurgical interface between titanium matrix and graphene. Compared with the unreinforced titanium matrix, the 0.5 wt% GNFs reinforced composite exhibits significantly improved strength without losing ductility, which demonstrates that GNFs could actually act as superb reinforcements in TMCs. © 2016 Elsevier B.V. All rights reserved.

Keywords: Graphene nanoflakes Titanium matrix composite Powder metallurgy Mechanical properties

1. Introduction Graphene is a single-atomic-layer material consisting of sp2hybridized carbon atoms. It has attracted great scholarly attention recently due to its superior properties, such as excellent mechanical properties, high electrical conductivity and good thermal conductivity [1e3]. Graphene nanoflakes (GNFs) composed of multiple layer graphene could be produced from chemical reduced graphene oxide. They possess properties similar to that of single-layer graphene but are much easier to prepare and handle. To utilize their superior properties, GNFs are generally dispersed into various matrices (e.g. polymers, metals and ceramics). GNFs have recently been reported as reinforcements in metal matrices (e.g. aluminum [4e6], magnesium [7,8] and copper [9,10]) to improve the properties of the matrices. The results indicated that graphene is sufficiently robust to improve mechanical properties of the metal matrix composites. For example, Li et al. [11] have synthesized the aluminum/graphene composites via powder metallurgy process and found that both tensile and yield strengths were remarkably

* Corresponding author. Beijing Institute of Aeronautical Materials, Huanshancun, Haidian District, Beijing 100095, China. E-mail address: [email protected] (Z. Cao). http://dx.doi.org/10.1016/j.jallcom.2016.11.302 0925-8388/© 2016 Elsevier B.V. All rights reserved.

increased by GNFs without loss of ductility performance. Titanium (Ti) and titanium alloys are extensively used in aeronautical, marine and chemical industries because of their high specific strength, high specific modulus, good oxidation and corrosion resistance [12e15]. Considering energy saving and fuel consumption, the further improvement of mechanical properties of Ti alloys is significantly important when they are applied to those industries. Dispersion strengthening is one of the effective methods to improve the mechanical response of titanium matrix composites (TMCs). In previous works, reinforcement materials such as ceramic particles (SiC [16], TiC [17], TiB [18]), carbon nanotubes [19,20], carbon fibers [21] and SiC fibers [22] were used to improve the strength of Ti and its alloys by powder metallurgy method. However, the above-mentioned reinforced TMCs showed a combination of high strength and low elongation-to-failure. Namely, the incorporation of hard reinforcements increases the tensile strength and stiffness yet decreases their tensile ductility and fracture toughness. It results in a poor reliability of the composite materials [23]. Compared with the above filling materials, GNFs have higher strength, elongation and larger specific surface area. GNFs could cause a good match between strength and ductility in the asfabricated composites so that they would be a favorable candidate to reinforce TMCs. However, few works present TMCs reinforced with GNFs in current science and practice.

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TMCs reinforced by GNFs have the potential to become a major structural material in the next-generation. In this study, they were prepared through powder metallurgy approach. It is strongly expected that the combination of titanium and GNFs will serve superior mechanical properties to the conventional TMCs. The microstructures and the tensile properties of the composites were investigated and compared with those of the matrix alloy. 2. Experimental

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phase composition was characterized by Rigaku D/Max 2500 v/pc X-ray diffraction (XRD) with Cu Ka radiation. The bulk density of the composite was measured by the Archimedes method. The tensile specimens with a gage length of 25 mm and a diameter of 5 mm were machined perpendicular to the forging direction and pulled to failure on a testing machine (Instron 5887) at ambient temperature. For minimizing the experimental error, three tensile specimens for each sample were tested with stretching rate of 1 mm/min. 3. Results and discussion

Gas-atomized Ti6Al4V powder with a mean particle diameter of ~35 mm was supplied as the starting matrix material. Table 1 shows chemical composition of the matrix alloy used in this study. Fig. 1a is the scanning electron microscopy (SEM) image of titanium powder which has a spherical morphology. GNFs applied in this investigation were chemically synthesized from graphene oxides which were prepared by Hummer's method [24]. Fig. 1b shows the transmission electron microscopy (TEM) image of GNFs which have a large specific surface area and two-dimensional high aspect ratio sheet geometry. The GNFs reinforced Ti6Al4V composites were prepared in a number of steps: blending, degassing, hot isostatic pressing (HIP) and isothermal forging. Samples of pure Ti6Al4V powder and Ti6Al4V powder reinforced with 0.5 wt% GNFs were prepared respectively. Dispersion of GNFs in metal matrix is more challenging than other reinforcement materials due to their greater interfacial contact area [3], hence a low weight fraction of graphene was chosen for this study. Take Ti/0.5 wt% GNFs composite for example, the detailed fabrication processes are as follows. At the first step, Ti6Al4V powder (2 kg) and GNFs (10 g) were mechanically mixed with stearic acid (4 g) using a modified V-blender for 24 h. The mixed powder was added into ethyl alcohol (1000 ml) and mechanically stirred at 70
C in a water bath until it turned into semi-dry state. Then it was fully dried in an oven at 60
C. Fig. 1c shows the morphology of the mixed powder after blending. It presents that the titanium particles are covered by several welldistributed GNFs. After that, the mixture was sealed in a cylindershaped 304 stainless steel can, and pumped to 5  102 Pa. In order to further remove the moisture, the remaining gas and stearic acid from the mixed powder, the steel can was heated to 480
C for 6 h. Subsequently, the can was hot isostatic pressed at 700
C and 150 MPa for 2 h. Then the canned composite powder was isothermal forged at 970
C with a forging ratio of 3. Finally, the specimen was subjected to an anneal heat treatment at 780
C for 2 h. Morphology and property characterization SEM images were obtained with a field emission scanning electron microscope (SEM; JEOL JSM-7001) equipped with an energy-dispersive X-ray spectrometer (EDS). The microstructures of the bulk Ti/GNFs composite were investigated by optical microscopy, SEM and transmission electron microscopy (TEM) which was performed with JEOL JEM2100 microscope operated at an accelerating voltage of 200 kV. The interfaces between titanium and graphene were also investigated by high resolution transmission electron microscopy (HRTEM) on JEOL JEM-2100 microscope. The TEM specimens were prepared from the forged material parallel to the forging direction. The structure integrity of GNF was examined by Raman spectroscopy (Renishaw inVia) with an Arþ laser wavelength of 633 nm. The

Table 1 Chemical composition (wt%) of the as-atomized titanium powders. Powder

Al

V

Fe

O

N

C

H

Ti

Ti6Al4V

6.24

3.98

0.20

0.087

0.01

<0.01

0.002

Bal

The mechanical properties of TMCs are not only determined by the volume fraction, morphology and type of reinforcements but also affected by the morphology of matrix and the distribution of reinforcements [2,25]. Fig. 2a shows the optical microscopy of the Ti/GNFs composite reinforced with 0.5 wt% GNFs after HIP. XRD scan for the Ti/GNFs composite (Fig. 2b) shows the peaks corresponding to a-Ti phase. It is found the sample consists of equiaxial and elongated a-Ti phase. XRD pattern does not show formation of any new phase (such as TiC), namely that GNFs did not react with titanium matrix at 700
C during HIP process. Furthermore, the GNFs dispersed in the matrix could not be recognized clearly by optical microscopy (Fig. 2a). Fig. 2c presents SEM image of the Ti/ GNFs composite after HIP. The GNFs that dispersed in titanium matrix are confirmed by the Raman spectra in Fig. 2d, which shows the characteristic peaks of D (1351 cm1), G (1592 cm1) and 2D (2660 cm1) band of the GNFs. In the process of HIP, the titanium powders were consolidated effectively. Consequently, the GNFs that covered on the titanium powders (Fig. 1c) appeared to be linelike in the SEM image (Fig. 2c). It can be seen that GNFs are homogeneously distributed in the titanium matrix. Fig. 3a shows the optical microscopy the Ti/GNFs composite reinforced with 0.5 wt% GNFs after isothermal forging and heat treatment at 780
C for 2 h. The microstructure of the composite consisted of equiaxed bright a-Ti phase grains and intergranular gray b-Ti phase, which is typical from aþb titanium-base alloys that have been underwent an annealing treatment. To investigate the microstructure characteristics more clearly, SEM observation of the as-forged composite at high magnification is shown in Fig. 3b. It can be seen that the stripped or granular b-Ti phase located at intragranular and grain boundary sites. Additionally, the density of the composite after isothermal forging is 4.40 g/cm3, which is 99.1% of the theoretical density (4.44 g/cm3). It can be concluded that the whole process including blending, degassing, HIP and isothermal forging is effective to fabricate compact Ti/GNFs composite. Fig. 4a and b presents bright-field TEM images of graphene in the fabricated Ti/GNFs composite. The results show that the graphene located around the boundary of titanium grains. According to the EDS spectrum, the ribbon-like region consisted of pure carbon (Fig. 4c) completely, indicating the graphene still remained in the composite. Besides, Fig. 4b exhibits that the interlayer space is about 0.34 nm, which confirms the structure of graphitic layers. In order to characterize the interface microstructure, the HRTEM image of the area “A” in Fig. 4a was carried out (Fig. 4b). It demonstrated that there is a strong interface bond between titanium grain and graphene with formed TiC particles in the Ti/GNFs composite. The existence of TiC layer surrounding the graphene indicates that the titanium matrix reacted with graphene to form titanium carbide during isothermal forging. The results also elucidate that when the temperature is relatively high (970
C), GNF is reactive to form titanium carbide with titanium matrix. Due to the high stability of its chemical properties, it could still maintain its layered microstructure to some extent. Table 2 shows the tensile properties of the Ti/GNFs composite containing 0.5 wt% GNFs together with the tensile response of pure

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Fig. 1. (a) SEM image of the Ti6Al4V powder; (b) TEM image of the GNFs; (c) SEM image depicting the morphology of the mixed powder after blending.

Fig. 2. Optical microscopy (a), X-ray diffraction (b) and SEM image (c) of the Ti/GNFs composite reinforced with 0.5 wt% GNFs after HIP; (d) Raman spectrum of GNF in Fig. 2c.

Fig. 3. Optical microscopy (a) and SEM image (b) of the Ti/GNFs composite reinforced with 0.5 wt% GNFs after isothermal forging.

titanium matrix fabricated using identical processing conditions.

The Ti/0.5 wt% GNFs composite has a tensile strength of 1058 MPa, a

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Fig. 4. TEM micrographs of Ti/GNFs composite reinforced with 0.5 wt% GNFs after isothermal forging: (a) Bright-field image showing the interface of graphene and titanium matrix; (b) HRTEM image of the area “A” in Fig. 4a. The insets show selection electron diffraction of TiC particle and graphitic layers with a layer interval of 0.34 nm; (c) EDS spectrum of the graphene in Fig. 4a.

Table 2 Tensile Properties of the Ti/GNFs composite and the Unreinforced Titanium Matrix. Samples

Elasticity modulus (GPa)

Ultimate tensile strength (MPa)

Yield strength (MPa)

Total elongation (%)

Ti Ti þ 0.5 wt% GNFs

109 ± 1 125 ± 2

942 ± 3 1058 ± 3

850 ± 5 1021 ± 2

9.4 ± 0.3 9.3 ± 0.3

yield strength of 1021 MPa and a Young's modulus of 125 GPa, which are 12.3%, 20.1% and 14.6% higher than that of the pure titanium matrix, respectively. The result clearly indicates that GNFs is a highly effective reinforcement in the titanium matrix. In the case of ductility, the total elongation of the Ti/GNFs is 9.3%, which is comparable to that of the titanium matrix. Though the same strong improvement of tensile strength was seen in the titanium and other reinforcement composites [16e19,26e28], it was always accompanied by an apparent reduction of elongation. Namely, the incorporation of those hard reinforcements to titanium increases the tensile strength and hardness with losing ductility, which is one of the main barriers that TMCs must confront with. However, in this study, the ductility of the Ti/GNFs composite remained virtually unchanged while a significant improvement of strength had been achieved. A possible explanation for this is that the well-distributed GNFs in titanium matrix have a super-high strength and significantly high specific area. In previous works, the strengthening mechanism of graphene reinforcement in aluminum matrix was discussed [4,11]. The precipitates strengthening of GNFs and load transfer effect between GNFs and aluminum matrix were regarded as the main strengthening mechanisms in aluminum/GNFs composites. In this work, the in-situ formed TiC around the interface between the graphene and titanium matrix could be a reinforcement and tend to drastically strengthen the interface. The strong interface interaction

could effectively transfer the load from the titanium to the graphene, which may be the main strengthening mechanism of the Ti/ GNFs composite. At the same time, the remaining of GNF with large specific area of 2D structure is expected to impede the coarsening of matrix grain during thermal processing, and restrain dislocation motion and crack propagation during tensile testing. Additionally, the remarkable ductility property of the composite could be attributed to the multiply wrinkled structure of the GNFs (Fig. 1b) and their good combination with the matrix (Fig. 4). During plastic deformation, GNFs are straightened and flattened, which give rise to ductility maintaining. Although some valuable results have been obtained, there is still much work to do to achieve the full potential of GNF as reinforcements in TMCs. Our further investigation will focus on the effect of GNFs content on TMCs, optimization of the processing parameters and interfacial reaction of Ti/graphene to further improve the mechanical properties of TMCs. 4. Conclusion Graphene reinforced bulk Ti6Al4V matrix composite was fabricated by powder metallurgy process. The results indicate that GNFs could be homogeneously introduced into titanium matrix. It is also found that the in-situ formed TiC particles during isothermal forging at 970
C could strengthen the interface bond between titanium and graphene. Tensile strength and yield strength were

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remarkably improved by the reinforcement of 0.5 wt% GNFs with nearly no loss of ductility. The preliminary results achieved here demonstrate for the first time that GNFs could actually act as effective reinforcements in TMCs. The GNFs and in-situ synthesized TiC particles presumably promoted the mechanical properties of the titanium composite.

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Acknowledgements

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We gratefully acknowledge the support by National Natural Science Foundation of China (Grant No.51374187 and 51404221).

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