Microstructure and properties characterization of tungsten–copper composite materials doped with graphene

Accepted Manuscript Microstructure and properties characterization of tungsten–copper composite materials doped with graphene Longlong Dong, Wenge Chen, Chenghao Zheng, Nan Deng PII:

S0925-8388(16)33444-2

DOI:

10.1016/j.jallcom.2016.10.310

Reference:

JALCOM 39479

To appear in:

Journal of Alloys and Compounds

Received Date: 5 August 2016 Revised Date:

26 October 2016

Accepted Date: 30 October 2016

Please cite this article as: L. Dong, W. Chen, C. Zheng, N. Deng, Microstructure and properties characterization of tungsten–copper composite materials doped with graphene, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.310. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Microstructure and properties characterization of tungsten–copper

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composite materials doped with graphene

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Longlong Dong*, Wenge Chen*, Chenghao Zheng, Nan Deng

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School of Materials Science and Engineering, Xi’an University of Technology,

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Shaanxi, Xi’an, 710048, China

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Abstract: Graphene is considered as an excellent reinforcement in composite

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materials because of its unique physical and mechanical properties. In this study,

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graphene was successfully prepared at the reducing temperature of ambient

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temperature by oxidation reduction process using a green reducing agent and an

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attempt was made to fabricate graphene/W70Cu30 composites by mechanical alloy

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and pressureless infiltration sintering technology. Effect of graphene addition on

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W70Cu30 powders and W70Cu30 composites were characterized by using

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transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman

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spectroscopy, Optical microscopy (OM) and HB-3000 hardness tester. The results

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show that the graphene could remain after ball milling and sintering process. But, the

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carbides (like WC and W2C) were also formed in bulk composites, which revealed

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that some of graphene would react with W during the sintering process, but the

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graphene coated with Cu phase in W70Cu30 alloy still keep intrinsic structures. And

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W grain size is efficiently refined with addition of graphene. The relative density of

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graphene/W70Cu30 composites enhance with the content of graphene increasing.

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When the graphene content is 1.0wt %, the relative density of composites reach up to

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*

Corresponding author: Professor Wenge Chen E – mail addresses: [email protected] (Wenge Chen), [email protected] (Longlong Dong)

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ACCEPTED MANUSCRIPT 98.4%, which is considered that graphene could improve the wettability of W and Cu.

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The incorporation of graphene into W70Cu30 alloy gradually increases the hardness

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of composites, which is enhanced about 21% compared with the pure W70Cu30 when

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the graphene content is 1.0wt%. The electrical conductivity of composites increase

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gradually firstly and decrease sharply with the increase of graphene content. When the

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0.5wt % graphene was added to the W70Cu30 alloys, the maximum conductivity

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reaches ~46% IACS.

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Key words: Graphene, WCu composites, Mechanical properties, Pressureless

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infiltration sintering 1.

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Introduction

W-Cu composite materials are a typical pseudo-alloy consisting of BCC structure

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W and FCC structure Cu, W and Cu neither dissolve each other nor form intermetallic

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compound in W-Cu system pseudo-alloys [1]. Fortunately, these materials combine

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the high melting point, low thermal coefficient of expansion and high strength of W

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with the outstanding thermal and electrical conductivities of Cu, so it is widely used

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in various industrial fields, such as high voltage electrical contacts, heat sinks,

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welding electrodes, etc. due to its high hardness, high arc erosion resistance and

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fusion welding performance and high temperature oxidation resistant properties [2].

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Graphene, a single atomic thick layer of sp2 hybridized carbon, is known as the “star

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material in the 21st century” owing to its fascinating electrical (1.5×104 cm2/V·s) [3],

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thermal (5×103 W/m·K) [4] and mechanical (1TPa of Yong’s modulus and 130GPa of

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tensile strength) [5] properties, etc.. Therefore, those outstanding performances make

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ACCEPTED MANUSCRIPT graphene to be considered as an excellent reinforcement in composite materials. For

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example, graphene has a significant effect on the hardness, relative density and the

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grain size of matrix materials. It is well known that high relative density is the

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prerequisite of materials with high performance. In the past few years, several

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researchers have attempted to synthesize graphene reinforced Cu matrix, Al matrix,

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Ni, Mg matrix, etc. [6-11]. S. J. Yan et al. [12] investigated the graphene reinforced

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Al-matrix nanocomposites, and experimental results revealed that the tensile strength

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and yield strength of graphene-Al nanocomposites are 25% and 58%, respectively,

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higher than the pristine Al alloy at a nanofiller mass fraction of 0.3%. Also, M.

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Rashad. et al. prepared aluminum matrix composites strengthened by graphene

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nanoplatelets via a semi-powder process followed by a hot extrusion technique. They

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found that synthesized composite exhibited higher hardness and tensile strength which

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might be attributed to the geometry necessary dislocation generation, Orowan looping

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and efficient load transfer from soft matrix to the strong reinforcement [13]. At the

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same time, 14.3% increase of tensile strength was achieved in Mg-Al-Sn alloys by

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filling 0.18 wt% graphene nanoplatelets [14]. Moreover, the high-temperature

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hardness of Cu-graphene composite was enhanced while the CTE of it was also

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reduced because of the addition of the 0.5 wt% graphene according to Ref. [15]. In

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addition, micro-hardness, tensile and compression strengths were increased 43.3%,

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34.5% and 23.5%, respectively, with 1.5 wt% graphene addition in the Mg-Zn alloys

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[16]. However, to the best of our knowledge, open literature reports so far revealed

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that no attempt is made to investigate the microstructure, mechanical and physical

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properties of WCu composite materials reinforced with graphene. In present work, the mechanical alloy and pressureless infiltration sintering

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technology were used to fabricate W70Cu30-X graphene (X=0, 0.1, 0.5, 1.0 wt %)

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composite materials. Firstly, the effect of graphene addition on microstructures,

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mechanical and physical properties of W70Cu30 composites was examined. Secondly,

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it would also expand the graphene application in metal or alloy matrix.

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2.

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2.1 Raw materials

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Materials and Experimental details

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The commercially available tungsten powder (Fig. 1(a), average particle size

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5~7µm, oxygen content ＜600 ppm, purity ≥ 99.9%) and electrolytic copper powder

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(Fig. 1(b), particle size 48µm, purity ≥ 99.9%) were all purchased from Zhuzhou

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Cemented Carbide group Co., Ltd, Hunan, China. (Chemical components are listed in

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table 1). The raw natural flake graphite (Fig. 1(c), 200 mesh, average flake diameter

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75µm, purity ≥ 98.5%) was purchased from Nanjing Xian Feng Nano Materials

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Technology Co. Ltd. Jiangsu, China. All other chemicals were obtained from

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Sinopharm Chemical Reagent Co. Ltd., Shanghai, China.

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2.2 Preparation of graphene

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To prepare graphene, graphene oxide (GO) was synthesized by hummers method

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[17] firstly. A typical experiment is carried out as follows: (a) Take 5g of graphite,

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150mL of H2SO4, 2.5g of NaNO3 and 30g of KMnO4 according to proportion of

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graphite: concentrated H2SO4: KMnO4: NaNO3= 1g: (1~60)ml: (1~6)g: 0.5g. (b)

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Graphite and NaNO3 were put into H2SO4 solution at an ice-bath environment with 4

ACCEPTED MANUSCRIPT constant stirring for 60min. (c) Raised to 30 oC and hold it for 2.5h after KMnO4 was

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gradually added into mixture. (d) Raised to 90 oC and then 200 mL of purified water

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was added gradually into the system and stirred for additional 30 min. The mixture

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was filtered vacuum and washed with hydrochloric acid (HCl) and deionized water

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for many times. (e) A certain amount GO solution was dispersed for 10min, after that

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excess reductive agent (thiourea dioxide) was slowly added into GO solution with

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agitation at room temperature for 30min. (f) The obtained mixture solution was

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separated by high speed centrifugation and washed with ultrapure water for many

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times, then the mixture was vacuum freeze dried at 50 oC for 24h. (g) Lastly, the as

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prepared graphene was successfully obtained. The analysis of the diffraction peak of

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graphene was examined by X-ray diffraction meter (XRD-7000). The microstructure

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and structure of prepared graphene were observed by transmission electron

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microscopy (TEM, Tecnai G2 F20 S-TWIN).

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2.3 Preparation of graphene / W70Cu30 composites

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As shown in Fig. 2, the powder metallurgy route was used to fabricate the

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graphene/W70Cu30 composite materials. As-prepared graphene was dispersed in

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C2H5OH solution for 5 min by ultrasonic cleaner with power of 120W (Fig. 2(a)). At

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the same time, W and Cu powders were mixed in ethanol solvent with continuous

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stirring for 30min by mechanic agitator (Fig. 2(b)). Then, the graphene dispersion

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solution was slowly added into WCu mixture powders solution and the mixture was

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agitated for 20min (Fig. 2(c)). Finally, the graphene and WCu mixture powders

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solution were put inside a stainless steel ball mill jar under a high purity Ar

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ACCEPTED MANUSCRIPT atmosphere for 8h at 800 rmp in a high energy planetary (QM-BP, Nanjing NanDa

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Instrument Plant) ball mill using stainless steel balls of 0.5 mm, 2 mm and 5 mm in

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diameter as milling media, respectively (Mass Ratio, 5:3:1). The ball-to-powder

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weight ratio was 5:1 (Fig. 2(d)). And the ball mill mixture was dried at 50 oC for 24h

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in a vacuum of -0.08MPa in vacuum drying oven (ZA-1A) (Fig. 2(e)). After these

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steps, the WCu-X graphene (X=0.1 wt%, 0.5 wt%, 1.0 wt%) powders were obtained

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successfully. In order to obtain designed green compact density (85% of the

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theoretical density), the composite powders were compacted in the form of cylindrical

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bars with dimensions of Φ 11 mm×5 mm by equal height control under a universal

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testing machine (Type WE-100) (Fig. 2(f)). Then, the green compacts were put onto

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quartz ark and sintered at a temperature of 1350 oC, dwell time of 90min via

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melt-infiltration under tube furnace (GSL-1700X) (Fig. 2(g)). To avoid the samples

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oxidation and improve the wettability of Cu phase to W skeleton, it should be noted

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that the whole process of sintering was performed under H2 protective atmosphere.

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For comparison, W70Cu30 composites with any graphene addition specimens were

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also prepared in the same process.

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2.4 Characterization

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The X-ray investigations were carried out to identify phases in the composites

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powders and bulk materials using X-ray Diffractometer (XRD-3700) operating at 40

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kV and 40 mA with Cu Kα radiation and a scan rate of 0.02o s-1 in a 2θ range of 20o -

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80o. Transmission electron microcopy (TEM) observations were carried out using a

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JEOL 2010F field emission TEM. Raman spectra with 533 nm laser wavelength were 6

ACCEPTED MANUSCRIPT collected to examine the existence of graphene in composites at room temperature

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using LabRAM XploRA Plus (HORIBA Scientific) equipped with a SWIFT detector

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over a range of 500 ~ 3000 cm-1. About two hundred grains within the same size

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scope of metallographs were randomly selected to statistically analyze the average

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grain size by Nano Measurer software. Hardness test was conducted on the polished

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surface of the samples by HB-3000 hardness tester with a load of 750 kg and dwell

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time of 30s, and every sample was performed on three measurements to obtain

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average value. Also, the electrical conductivity of polished samples was evaluated by

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a digital metal conductivity measuring instrument (D60K, Xiamen xin bo te

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technology co., Ltd) at room temperature and at least 5 measurements were performed

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for each specimen. Additionally, the densities of the all bulk samples were determined

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by using the Archimedes principle with deionized water. The measured density ( ρ ),

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theoretical density ( ρ 0 ) and relative density ( φ ) of the graphene / W70Cu30 materials

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were calculated using Equation (1), (2) and (3), respectively:

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m1 = ρ H 2O V

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m1 m1－m 2

•

(1)

Where ρ is the measured density of the samples, ρ H 2O is the density of purified water

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under the temperature of 25 oC ( ρ H 2O =0.9960g/cm3), m1 and m2 are the measured

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weight of the samples in air and water, respectively.

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ρ

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Where

ρ

WCu

0

=

m V

=

graphene

+ m WCu

WCu

m

m m

ρ

WCu

+

ρ

graphene

(2)

graphene

is the theoretical density of W70Cu30 alloys, and

ρ

Graphene

is the 7

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theoretical density of graphene,

ϕ=

2

ρ

WCu

=14.337 g/cm3 [18],

ρ

Graphene

ρ ρ0

=2.2 g/cm3 [19]. (3)

Where ρ, ρ 0 and Φ are the measured density, theoretical density and relative density

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of samples, respectively.

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3.

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3.1 Graphene characterization

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3.1.1

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Results and discussion

The TEM, XRD, XPS and Raman spectra of graphene

In Fig. 3, the prepared graphene was characterized by using XRD, XPS, Raman

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spectroscopy and TEM, respectively. The XRD shows a broad diffraction peak about

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at 22.5o was detected (Fig. 3(a)), which suggested that oxygen-containing functional

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groups were partially removed and a few layers structure of graphene were produced

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[20]. In Fig. 3(b), the Raman spectra exhibited two peaks around 1329 cm-1 and 1581

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cm-1 corresponding to the D band and G band in carbon materials, respectively, which

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was in accordance with the Raman spectra of GO reduced by vitamin C [21]. The

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XPS analysis of graphene was shown in Fig. 3 (c) and (d). As seen from 3(c), there

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are four kinds of functional groups obtaining carbon: C=C (~284.5 eV)，C-O (~286.4

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eV)，C=O (~287.8 eV)，broad peak of COOH (~289.0 eV) [22]. As depicted in Fig. 3

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(d), the atomic % of C in graphene is 95.89% and that of O as 4.11%, which confirm

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that most of oxygen functional groups have been successfully removed during the

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process of reduction reaction. Fig. 3 (e) shows the TEM image of graphene, the large

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and thin graphene with typical wrinkled structures, which is caused by the

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overlapping of graphene edges, were observed. The HRTEM and SADE of graphene

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ACCEPTED MANUSCRIPT were shown in Fig. 3(f), it can be observed that graphene presents about 4~5 layers.

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Also, the reflections arranged in hexagonal pattern in the inset of Fig. 3(f) revealed

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that graphene has a good crystal structure. These results suggested that the graphene

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was successfully obtained in this work.

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3.2 Characterizations of bulk graphene / W70Cu30 composites

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The existence of graphene in composites

The Raman spectra of the prepared graphene, ball-milled 1.0 wt% graphene /

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W70Cu30 composite powders and sintered 1.0 wt% graphene / W70Cu30 bulk

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composites were shown in Fig. 4. It can be seen that the characteristic peaks of

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graphene, i. e., D band (~1329cm-1) and G band (~1581cm-1), were all observed in all

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samples. G and D band reflected the ordered state of sp2 hybridized carbon lattice. In

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composites powders, the intensity of D peak increased dramatically, which suggested

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that defects or disorder structure of graphene increased in during ball-milling process.

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Additionally, C. He et al. [23] had reported that the relative intensity ratio of D band

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and G band (IG/ID) was considered to be a signal of the existence of graphene or CNTs

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in composites. It is reported that the ratio is 0.7 after ball-milling for 5h [24].

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Interestingly, in this paper, the ratio increased to 0.8964 after ball-milling for 8h, it

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could be caused that the breakage of graphene was reduced by balls of small diameter.

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Unfortunately, compared with composites powders, the ratio of bulk composite

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decreased to 0.5397 after infiltration sintering in 1350 oC, which indicated that the

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crystal structure of graphene was destroyed a lot after sintered. It was also

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demonstrated in the XRD and microstructure analysis.

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3.2.2

XRD analysis Fig. 5 presents the XRD pattern of the various samples after ball-milled and

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sintered with different contents of graphene. In Fig. 5(a), it is clearly seen from the

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data that all samples have major W and Cu peaks at 2θ equal to ~ 40.264 (110),

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~58.274 (200), ~73.195 (211), ~43.297 (111), ~50.433 (200), respectively. Except that,

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there is no detectable second phase indicating that all powders were not oxidized in

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the process of ball milling. In addition, the intensity of W70Cu30 powders with

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different content of graphene is higher than that of pure W70Cu30 powders which

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may be attributed to the presence of graphene in composites. However, it is notable

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that the new peaks were observed after sintering in Fig. 5(b). Addition of 1.0 wt%

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graphene, the tungsten carbides (WC and W2C phases) were identified from the peaks

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emerged at 2θ equal to 31.511 (001), 35.641 (100), 48.296 (101) and 39.563 (102),

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respectively. The similar results were all observed in Al alloys reinforced with CNTs

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and graphene by Stephen F. Bartolucci and co-workers [25]. In general, tungsten

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carbide is beneficial to the mechanical properties of metal-matrix composites [26],

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which is described in Fig. 9 in this work. Thermodynamics calculation indicates that

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the W and graphite can react, and then form WC and W2C below 2000K. Also,

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previous works have found the W2C formed in the CNTs/WCu composites after

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sintering at 1400 oC [27]. Because the sintering temperature is below W melting point

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(3410 oC), graphene and W could occur reaction diffusion, whose reaction rate is

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related to the size and homogeneity of raw materials apart from the reaction

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temperature and time. The Raman spectra results show that some of the graphitic

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structure is broken with formation of defects during the ball milling process (Fig. 4),

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which will promote the reaction between W and graphene below W melting

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temperature.

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3.2.3

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Microstructures The microstructures of W70Cu30 composites with different contents of graphene

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are shown in Fig. 6. It can be clearly observed that W grain size is efficiently refined

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with addition of graphene. The average value about grain size of W phase measure

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using linear intercept method was listed in table 2. As seen from table 2, the grain size

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of W reduced with increasing contents of graphene. The composites containing 1.0

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wt% graphene present finer microstructure with average grain size of ~14.67 µm. The

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similar results can also be observed in Mg alloys reinforced with graphene

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nanoplatelets [13]. The graphene may refine the grains of the W grain size in

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W70Cu30 composites in two ways in this work, on the one hand, the graphene will

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accelerate the milling process, obtaining much smaller particles compared with pure

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W70Cu30 powders. On the other hand, the graphene and tungsten carbide (W2C and

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WC) phases may hinder the grain growth during the infiltration sintering process.

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In order to further confirm the presence of the graphene in bulk W70Cu30

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composites, the TEM was conducted in Fig. 7. It was noted that there are only two

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phases in Fig. 7(a), namely, black area sets inside bright white area, and the

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corresponding selected electronic diffraction (SAED) pattern reveals that the black

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particle is made of tungsten with a single crystal structure of BCC (Fig. 7(b)) while

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the bright white area is made of copper with a single crystal structure of FCC (Fig. 11

ACCEPTED MANUSCRIPT 7(c)). The TEM images of marked A zone in Fig. 7 (d) confirmed the presence of

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graphene in WCu matrix. As seen from Fig. 7(e), it can be noted that the graphene

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have distinctly wrinkled and folding features the same as the prepared graphene (Fig.

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3(e)), suggesting that the sintering did not damage the graphene intrinsic structures in

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Cu phase which could be ascribed to be that graphene was coated by Cu powder in

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ball-milling process. And the structural integrity of graphene is also supported by the

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HRTEM (in the right top corner) and SAED (in the right bottom corner) of graphene.

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The HRTEM and SAED images of marked B zone in Fig. 7(d) are shown in Fig.

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7(f) and (g), respectively. The interplanar distances of WC and W2C in Fig. 7(f) are

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measured as 0.32nm and 0.188nm, respectively. These measurements are extremely

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close to the theoretical interplanar distance of W (111) and W2C (102), providing

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further evidence for the strong interfacial bonding between W and Cu. It was noted

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that B zone exhibited much complex electron diffraction patterns in Fig. 7(g). It is

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calculated that the diffraction pattern of the main phase is BCC structure of W phase

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according to crystallography. Additional, some diffraction rings and spots that are

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consistent with interplanar distance of graphene, WC and W2C are also observed. That

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may be attributed to the fact that W and graphite can react and form carbide below

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2000K, which corresponding to graphene/W70Cu30 composites of XRD spectrum

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(Fig. 5(b)).

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In general, the graphene would be broken into fragments and dispersed in the

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WCu matrix after the ball milling process. Some of the fragments, which keep the

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intrinsic structure, will survive and be embedded in the bulk graphene/W70Cu30 12

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composites after the infiltration sintering process. But some fragments, whose

2

structure are broken into defects or disorder structure, will react with W form WC and

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W2C phases in the composites. In our opinion, the formation of carbide would deteriorate the properties of WCu

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composites, especially high temperature performance. Hence, in our future work, the

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graphene will be coated by copper powders and then graphene coated with copper by

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electroless plating was added into WCu powders in order to separate graphene from

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W grains to a certain extent and the properties of the composites would be further

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improved.

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3.2.4

Mechanical and physical properties

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3.2.4.1 Density of graphene/W70Cu30 composites

Fig. 8 shows the variation of theoretical density, measured density and relative

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density of W70Cu30 composites containing different weight fractions of graphene: 0,

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0.1, 0.5 and 1.0 wt%, respectively. As depicted in Fig. 8, the density of W70Cu30

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alloys remarkably decreased with the increase in amount of the graphene added into

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W70Cu30 composites. When the amount of the graphene is 1.0 wt%, the density of

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W70Cu30 composites obtained the minimum lower value of 12.859 g/cm3, which is

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related to the theoretical density of graphene. In general, the theoretical density of

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graphene is ~ 2.2 g/cm3 [19], which is far less than that of W (19.35 g/cm3) and Cu

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(8.96 g/cm3). On the other hand, the gap between W-W skeleton was not completely

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filled with melting Cu liquid during the infiltration-sintering because the highest

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relative density is only 98.4% which do not reach fully densification. At the same time,

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ACCEPTED MANUSCRIPT the relative density of W70Cu30 has been improved with the additive amount of

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graphene. This result could be explained as the two main reasons. At first, the relative

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density of WCu composite was dominated by W particles rearrangement. Owing to

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the good wettability of graphene, W particles rearrangement could be promoted and

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accelerated to some extent, which improved the densification behavior of WCu

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composite materials. Secondly, graphene has a very good wettability on Cu at high

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temperature [28]. The wettability of graphene and Cu is better than that of W and Cu,

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which would greatly promote the ability of Cu liquid filling into W skeleton.

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Moreover, graphene reacts with W form the tungsten carbide which would improve

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the wettability of W and Cu phase by reaction wetting, also, small piece of layer

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structure of graphene (Fig. 3(e) and (f)) is beneficial to the wettability. Hence, the

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densification of WCu composites was increased a lot.

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3.2.4.2 Hardness of graphene/W70Cu30 composites

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Fig. 9 summarizes results of the hardness of graphene/W70Cu30 composites. As

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seen from Fig. 9, it confirms that the hardness values of W70Cu30 alloys with

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addition of graphene are greatly improved than that of no additive. Compared to the

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hardness value of the pure W70Cu30 of around 172HB, the hardness of

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graphene/W70Cu30 composites with 1.0wt % graphene can reach 208HB, which is

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up to 21% approximately. The ball mill can refine grain and increase the solid

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solubility between W and Cu, thus the W and Cu phase distribute more uniformly,

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which not only improve the relative density but also increase the hardness of

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W70Cu30 composites. Additionally, the ability of activated sintering of W-W

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ACCEPTED MANUSCRIPT particles was enhanced due to the addition of CNTs and graphene [29-30]. J.J. Sha et

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al. had reported that the activated sintering of W in their work is due to an enhanced

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diffusion of W atoms along the grain boundaries induced by CNTs [29]. Hence, the W

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particles can be promoted uniform distribution in copper in the process of

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melt-infiltration sintering at a temperature of 1350 oC. Moreover, the formation of the

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hard phase carbides WC and W2C has a significant contribution on the hardness of the

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graphene/W70Cu30 composites.

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3.2.4.3 Electrical conductivity of graphene/W70Cu30 composites

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According to the national standard value (GB/T 8320-2003), the electrical

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conductivity of the W70Cu30 composite material is IACS 42%. As shown in Fig. 9,

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the results of electrical conductivity of graphene/W70Cu30 composites were

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illustrated. It can be seen from the picture that the electrical conductivity tends to

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increase gradually firstly and decrease sharply then with increasing the graphene

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content. This is similar to those reported in WCu alloys reinforced with CNTs by J. H.

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Nie et al. [31]. The electrical conductivity of graphene/W70Cu30 was ~46% IACS

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when the doping amount of graphene was 0.5 wt %, which is attributed to fascinating

17

electrical of graphene. But the electrical conductivity of 1.0 wt % bulk composites

18

was only 38.3% IACS, which was reduced by 10% compared with W70Cu30 alloys

19

without any additive (IACS 42%). For W70Cu30 composites with 1.0 wt% graphene

20

addition, the WC and W2C phase was produced (Fig. 5(b)). Because graphene was

21

also carbon source, so the W grains were carbonized by graphene during the high

22

temperature infiltration sintering process. For the electrical conductivity of W70Cu30

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ACCEPTED MANUSCRIPT alloy, it is equivalent to add some impurities in tungsten copper alloys. Furthermore,

2

the electrical conductivity of tungsten carbide (WC and W2C) is much lower than that

3

of the pure metal W and Cu, and tungsten carbide can inevitably increase additional

4

interfaces and electron scattering. [32-33] As a result, W70Cu30 alloys with 1.0wt%

5

graphene addition have lower electrical conductivity in comparison with that of

6

W70Cu30 alloys without any addition.

7

4. Conclusions

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In the present work, tungsten-copper composite materials doped with graphene

9

were successfully fabricated by the mechanical alloy and pressureless infiltration

10

sintering technology. The microstructures and properties of tungsten–copper

11

composites doped with graphene were also investigated. The following conclusions

12

can be drawn from the present work:

13

(1) Graphene was successfully synthesized via oxidation reduction process using a

14

friendly reductant at room temperature.

15

(2) Graphene/W70Cu30 composites were also successfully fabricated by the

16

mechanical alloy and pressureless infiltration sintering technology at a temperature

17

1350 oC for 90min in pure H2 atmosphere.

18

(3) W grain size is efficiently refined with addition of graphene. At the same time,

19

the graphene coated with Cu phase in WCu alloy still keep intrinsic structures.

20

However, graphene reacts with W formed W2C and WC during the sintering process

21

when the graphene content is 1.0wt%.

22

(4) The relative density and hardness of composites both increase with the increase

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of graphene content, while electrical conductivity tends to first decrease and then

2

increase. When the graphene content is 1.0wt%, the relative density and hardness of

3

composites reach up to 98.4% and 208HB, respectively.

4

Acknowledgements The authors would like to acknowledge the support from Xi’an science and

6

technology plan Projects, Project No. CXY1342(2), and also express their gratitude to

7

the Institute of Materials Science and Engineering of Xi’an University of Technology

8

for providing help in testing relevant properties.

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ACCEPTED MANUSCRIPT List of figure captions: Fig. 1 The SEM of raw materials: (a) W powder, (b) Cu powder, (c) Graphite. Fig. 2 The illustration of the preparation process of Graphene/WCu composites.

(c) and (d) XPS; (e) TEM and (f) HRTEM image.

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Fig. 3 Prepared graphene of: (a) XRD pattern; (b) Raman spectra with typical D, G and 2D bands;

graphene/W70Cu30 composite materials.

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Fig. 4 Raman spectra of graphene, 1.0 wt% graphene/W70Cu30 powders and bulk

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Fig. 5 XRD patterns of W70Cu30 composites doped with different graphene amount of 0, 0.1, 0.5, 1.0wt %: (a) Graphene/W70Cu30 powders; (b) Bulk graphene/W70Cu30. Fig. 6 Optical microscopic images of graphene/W70Cu30 composites: (a) W70Cu30; (b) 0.1wt% graphene/W70Cu30; (c) 0.5wt% graphene/W70Cu30; (d) 1.0wt% graphene/W70Cu30.

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Fig. 7 TEM images of graphene/W70Cu30 composites: (a) Raw W70Cu30 composites; (b) Electronic diffraction pattern and calibration of black area of in Fig. 8(a); (c) Electronic diffraction pattern and calibration of white particle of in Fig. 8(a); (d) 1.0 wt% graphene/W70Cu30

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composites; (e) High-magnification images of marked A zone in Fig. 8(d) showing graphene in

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composites; (f) HRTEM of marked B zone in Fig. 8(d) showing little graphene embedded in the W matrix and some react with W; (g) Electronic diffraction pattern and calibration of marked B area in Fig. 8(d).

Fig. 8 The theoretical density, measured density and relative density of W70Cu30 composites containing 0 wt%, 0.1 wt%, 0.5 wt% and 1.0 wt% graphene, respectively. Fig. 9 Variation of Brinell hardness (black) and electrical conductivity (blue) of bulk graphene/W70Cu30 with varying graphene weight fraction. 1

ACCEPTED MANUSCRIPT List of figure captions: Table 1 Chemical compositions of W and Cu powders (mass fraction, %). Table 2 An average value about grain size of W phase measure based on linear intercept method (~

(a)

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(b)

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two hundred W grains).

Fig. 1 The SEM of raw materials: (a) W powder, (b) Cu powder, (c) Graphite.

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Cu powder W powder

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60r/min

(b) W and Cu powder stirring in C2H5OH

(c) WCu and graphene powder solution stirring in C2H5OH

(d) Put the composites in the ball mill jar

(g) Infiltration sintering under H2 protective atmosphere

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(a) Graphene sonication in C2H5OH

60r/min

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C2H5OH Graphene

(f) Compaction by controlling green billet height

(e) Put mixture powders in vacuum drying

Fig. 2 The illustration of the preparation process of Graphene/W70Cu30 composite.

3

ACCEPTED MANUSCRIPT (b)

D band ~1329cm-1 G band ~1581cm-1

10

20

30

40

2θ θ/(°)

The ratio ID/IG =1.07 2D band ~2700cm-1

1000

50

2000

C 1S

Intensity(cps)

C-O (~286.4 eV)

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C=O (~287.8eV)

3000

Atomic % C1S O1S

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(d)

C=C (~284.5eV)

2500

3500

Raman shift/cm-1

(c)

Intensity(cps)

1500

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Intensity/(a.u.)

Intensity (a.u.)

(a)

Graphene 95.89 4.11

O 1S Graphene

COOH (~289eV)

275

280

285

290

295

Binding Energy(eV)

300

0

200

400

600

800

Binding Energy(eV)

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4~5 layers

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Wrinkled structures

Fig. 3 Prepared graphene of: (a) XRD pattern; (b) Raman spectra with typical D, G and 2D bands; (c) and (d) XPS; (e) TEM and (f) HRTEM image.

4

ACCEPTED MANUSCRIPT D Materials

Powder

Bulk

IG/ID

0.8964

0.5397

Intensity(a.u.)

G

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Bulk graphene/WCu composite

Graphene/WCu powder Graphene 1000

1500

2000

2500

3000

3500

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Raman Shift(cm-1)

Fig. 4 Raman spectra of graphene, 1.0 wt% graphene/WCu powder and bulk graphene/W70Cu30

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♠

20

30

40

2θ(°)

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WCu

50

Graphene

♦

♦

WCu+0.5 WCu+0.1

WC • Cu ♦W ♥ W2C

▲

(b)

Intensity(a.u.)

Intensity(a.u.)

♠

WCu+1.0

♦

♦W ♠ Cu

Graphene

(a)

60

70

80

WCu+1.0

▲ ▲

♥

•

♦ ▲

♦ •

WCu+0.5 WCu+0.1 WCu

20

30

40

50

60

70

80

2θ(°)

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Fig. 5 XRD patterns of W70Cu30 composites doped with different graphene amount of 0.1, 0.5,

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1.0wt%: (a) Graphene/W70Cu30 powders; (b) Bulk graphene/W70Cu30 composites. (a)

(b)

W W

50µm

50µm

5

ACCEPTED MANUSCRIPT (c)

(d)

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W

W

50µm

50µm

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Fig. 6 Optical microscopic images of graphene/W70Cu30 composites: (a) W70Cu30; (b) 0.1 wt%

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(a)

Bright white area Black area

Cu

(c)

131

111

220

042 000

220

131 111

220

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(b)

110

020

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110

110 110 200

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000

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ACCEPTED MANUSCRIPT (d)

(c) (e)

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Graphene A

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B

wrinkled and folding structure

(f)

d=0.188nm

Graphene

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W2C(102)

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20nm

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W(111)

d=0.32nm

(g)

WC(1010) (0000) W(000)

(111)

(200) (111) Graphene (001) WC(0111) W2C(1102) 7 WC(1101)

ACCEPTED MANUSCRIPT Fig. 7 TEM images of graphene/W70Cu30 composites: (a) Raw WCu composites; (b) Electronic diffraction pattern and calibration of black area of in Fig. 8(a); (c) Electronic diffraction pattern and calibration of white particle of in Fig. 8(a); (d) 1.0 wt% graphene/W70Cu30 composites; (e)

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High-magnification images of marked A zone in Fig. 8(d) showing graphene in composites; (f) HRTEM of marked B zone in Fig. 8(d) showing little graphene embedded in the W matrix and some react with W; (g) Electronic diffraction pattern and calibration of marked B area in Fig. 8(d).

14.2

97.7

97.2 96.7

98.4

98

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Density/g⋅⋅cm-3

14.0

100

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Relative density Theoretical density Measured density

13.8

13.6

13.4

0.2

0.4

0.6

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0.0

0.8

96

94

Relative density/%

14.4

92

90 1.0

Graphene Contents/wt%

Fig. 8 The theoretical density, measured density and relative density of W70Cu30 composites

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containing 0wt%, 0.1wt%, 0.5wt% and 1.0wt% graphene, respectively.

200

44

195

43

190

42

185

41

180

Brinell hardness/HB

205

45

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Electrical conductivity / % IACS

210

Electrical conductivity Brinell hardness

46

40 175 39 170 38

0.0

0.2

0.4

0.6

0.8

1.0

Graphene Contents /wt%

Fig. 9 Variation of Brinell hardness (black) and electrical conductivity (blue) of bulk 8

ACCEPTED MANUSCRIPT graphene/W70Cu30 with varying graphene weight fraction. Table 1 Chemical compositions of W and Cu powders (mass fraction, %). W

Cu

Fe

Cr

Ni

Mn

W (wt%)

Bal.

0.05

0.002

＜0.002

-

＜0.005

Cu (wt%)

-

Bal.

0.008

-

＜0.1

＜0.005

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Powders

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Table 2 An average value about grain size of W phase measure based on linear intercept method (~ two hundred W grains).

Grain size of W/ µm

0

45.50

0.1 0.5

32.35 28.24 14.67

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Graphene Contents /wt%

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ACCEPTED MANUSCRIPT Highlights Graphene was prepared via the reduction of GO used a green reductant.




Graphene can effectively refine W grain size.




WCu-doping 0.5wt% graphene results high electrical conductivity.




Graphene can activate sintering WCu alloys.




The appropriate range of graphene doping content is obtained.

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