aluminum composites

Accepted Manuscript Effects of graphene content on microstructures and tensile property of graphenenanosheets / aluminum composites Gang Li, Bowen Xiong PII:

S0925-8388(16)34065-8

DOI:

10.1016/j.jallcom.2016.12.147

Reference:

JALCOM 40069

To appear in:

Journal of Alloys and Compounds

Received Date: 9 November 2016 Revised Date:

8 December 2016

Accepted Date: 12 December 2016

Please cite this article as: G. Li, B. Xiong, Effects of graphene content on microstructures and tensile property of graphene-nanosheets / aluminum composites, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2016.12.147. 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 Effects of graphene content on microstructures and tensile property of graphene-nanosheets / aluminum composites Gang Li1, Bowen Xiong2,*

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1 School of Mechanical engineering, North China University of Water Resources and Electric Power, Zhengzhou 450045, China

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2 National Defence Key Discipline Laboratory of Light Alloy Processing Science and Technology, Nanchang Hangkong University, Nanchang 330063, China

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Abstract:

The graphene nanosheets (GNSs) reinforced Al matrix composites were successfully fabricated by high-energy ball milling and vacuum hot pressing. Effects of graphene content (0.25 wt. %, 0.5 wt. % and 1.0 wt. %) on microstructures and tensile properties

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of GNSs/Al composites were investigated. Microstructures and the distribution of GNSs were analyzed with scanning electron microscope (SEM), X-ray diffraction (XRD) and

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transmission electron microscope (TEM). Tensile properties and hardness were studied at room temperature. The results show that the GNSs/Al composites with the high

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relative density and dispersed distribution of GNSs were fabricated. A good interfacial bonding was obtaned in GNSs/Al composites. The aluminum carbide Al4C3 phases with granular and short rod-like morphology were found at interface. With the increase of GNSs content, the content of Al4C3 phases increases. The dislocations were also found near the interface, and the density of dislocation reduced with the increase of Al4C3 content. The Vickers hardness of GNSs/Al composites increases obviously with the Corresponding author Tel.: +86 791 86453167; fax: +86 791 86453167. E-mail address: [email protected] (B. Xiong). 1

ACCEPTED MANUSCRIPT increase of GNSs content. The yield strength and ultimate tensile strength of GNSs/Al composites have an obvious improvement compared to that of pure Al, but the elongation show a slight decrease. The yield strength and ultimate tensile strength of

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Al-0.25GNSs composites increase by 38.27% and 56.19% compare with that of pure Al. The GNSs pull-out is detected at the edges of dimples, and the number of dimples slight

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decreases with the increase of GNSs content.

Key words: Al matrix composite; Microstructure; Mechanical properties; Graphene

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nanosheets; Strengthening effect

1. Introduction

Graphene, as a newly emerged carbon material, has been considered as a popular

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nano-reinforcement for composites owing to its extraordinary mechanical, thermal and electrical properties [1-4]. Compared with carbon nanotubes, graphene has 2D

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sheet-like structure with larger surface area and much lower production cost, which makes it a good alternative strengthening material for composites [5]. Graphene

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nanosheets (GNSs) composed of a few graphene layers possess properties similar to that of single-layer graphene but are much easier to produce and handle [6]. Therefore, the GNSs have been employed as the reinforcement in the practical composites [7]. The aluminum matrix composites have been applied widely in the automotive and aerospace industry [8-10]. The nano-scale reinforcement materials have been used for the development of aluminum matrix composites, which have demonstrated a positive

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ACCEPTED MANUSCRIPT effect on the mechanical properties of the final composites. Recently, the GNSs have been used to reinforce aluminum matrix composites [6, 8, 11-14], however, from the available literatures, the investigations on GNSs/Al composites are still limited to

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compared to GNSs reinforced polymer matrix[15-17], especially, about the interface

structure is need to be in-depth studied. In the present study, the effect of GNSs content

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on microstructures and tensile properties of GNSs reinforced aluminum matrix

composites were investigated. In addition, the interfac structure was studied to control

2. Experimental procedure

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the mechanical properties.

The GNSs display the stack-like morphology with irregular, wrinkled and curled

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characteristics. The mean diameter of GNSs is less than 5µm and the thickness is several nanometers (5-8 nm). The matrix material is pure aluminum powder (99.5 wt.%)

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with an average diameter of about 40µm.

In order to disperse uniformly the GNSs in Al matrix, the ultrasonic processing was

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employed, which vibrates at a frequency of 35 kHz for 40 min. The GNSs were added in ethanol, and ultrasonic processing was carried out for about 45 min. Subsequently, the ultrasonicated GNSs solution was dropped into Al powder according to the content of 0.25 wt. %, 0.50 wt. % and 1.0 wt. %, respectively. The GNSs-Al mixtures were mixed by high-energy ball milling for 4 h in a high purity (> 99.99%) argon atmosphere to avoid element oxidation. The weight ratio of ball to mixture was 10:1. The rotational

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C for 24 h. The above prepared GNSs-Al mixtures were pressed into a graphite die with the

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dimensions of Ø60×20 mm. For easy taking out the samples, the graphitic paper was placed between the punch and sample as well as between the die and sample. The

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samples were sintered at 610 oC for 4 h under a pressure of 30 MPa in vacuum (20 Pa). For comparison, the pure Al sample was also prepared under the same conditions.

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The microstructural characterization samples were prepared by grinding paper from 320 to 1200 grit and metallographically polished with 1 µm alumina, and subsequently ultrasonically cleaned and etched using a reagent comprising 5 ml HF and 95 ml distilled water. The microstructures were examined using a JSMT-200 scanning electron

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microscope. D/MAX-IIIB X-ray diffraction was performed on the bulk specimens for identification of phases. The interface between the Al matrix and GNS was investigated

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using JEM2100 transmission electron microscope. Mechanical properties of sample were surveyed according to tensile properties and

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hardness at room temperature. The dimensions of tensile samples was shown in Fig.1. Tensile tests were carried out using INSTRON testing machine at a strain rate of 3×10-3 s-1. The hardness was measured by Vickers hardness machine at 100 g load. The densities of fabricated pure Al and composites were measured by Archimedes’ method. 3. Results and discussion 3.1 Microstructure

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GNSs content on its distribution is not obvious in this study. This phenomenon may be attribution to two aspects. First, before adding into Al powder, the GNSs were

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distributed uniformly in ethanol solution by ultrasonic processing. Second, the

ultrasonicated GNSs-Al powders were dispersed furtherly by high-energy ball milling.

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The well distribution of GNSs indicates that the used technology is fit to disperse the GNSs in present study. But, there are few micropores at boundary of Al grain that were found in Fig.2. The similar micropores in GNSs/Al composites were also detected by Muhammad Rashad [12]. This may be due to the existence of inevitable gas in the

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powders. Moreover, the fabrication pressure (30Mpa) is probably not enough to make it completely densification. In order to understand the densification degree of GNSs/Al

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composite, the densities of fabricated pure Al and composites were measured by Archimedes’ method. The experimental density was obtained according to the average

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value of five measurements, as listed in Table 1. The GNSs/Al composites with more than 99% of theoretical density were successful fabricated. The relative density of GNSs/Al composites is lower than that of pure Al. This is because the nano-GNSs can absorb the gas elements, such as O, N and CO et al, so resulting in the increase of air gap, which can lead to the decrease in relative density. Therefore, with the increase of GNSs content, the relative densities of GNSs/Al composites slight decrease observed

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ACCEPTED MANUSCRIPT from Table 1. The well-dispersed GNSs and desired relative density of composite indicate that this technology can effective fabricate GNSs/Al composite. In order to determine the phase composition of the composite, the X-ray diffraction

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was performed. The XRD patterns of the GNSs/Al composites were presented in Fig. 3. Individual phases were identified by matching the characteristic XRD peaks against

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JCPDS data. It is obvious from Fig. 3 that the peaks of Al and Al4C3 were detected in Al-0.50GNSs and Al-1.0GNSs composite, but the peaks of Al4C3was not detected in

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Al-0.25GNSs composites. Moreover, it was detected from XRD patterns that the intensity of Al4C3 phase peaks increases with the increase of GNSs content by comparing the XRD pattern of Al-0.50GNSs with Al-1.0GNSs composite. Therefore, the GNSs content has a significant effect on the content of Al4C3 phase. The aluminum

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carbide Al4C3 phase was found in GNSs/Al composites, which indicates the presence of chemical reaction between GNSs and Al matrix. The Al4C3 phase was also observed in

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GNSs/Al composite by Muhammad Rashad [12] and Bartolucci [18] et al. The interfacial phase and interfacial bonding between the GNSs and Al matrix are a very

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crucial issue for the mechanical properties. A good interface is favorable to transfer load between GNSs and the Al matrix and inhibit the movement of dislocations. Therefore, the interface between GNSs and Al matrix need to be inveatigated by TEM. On the other hand, no peak of Al4C3 phase was detected in Al-0.25GNSs composite, although the same fabrication conditions were employed with the Al-0.50GNSs and Al-1.0GNSs composites. The Al4C3 phase was not detected in Al-0.25GNSs composite may be

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ACCEPTED MANUSCRIPT because of detection limit of X-ray diffraction equipment for second phases [19]. Whether the formation of Al4C3 phase in Al-0.25GNSs composite is need to be further analyzed by TEM.

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Fig.4 present the TEM images of GNSs/Al composites. There are some obvious features of GNSs in Fig.4. The GNSs show a stack-like in Fig.4 (b) and curled or

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wrinkled edges in Fig.4 (c), which indicate that the GNSs were not destroyed, still maintain the original morphology. No unbonded interface was observed, but some

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interfacial products were detected. In order to further analyze the interfacial products, the phase regions were determined by the selected area diffraction (SAD) analysis, and the SAD pattern was given in Fig.4 (d). Combining with the results of XRD, the interfacial products were determined to be Al4C3 phases. The Al4C3 phases with the

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granular and short rod-like morphology were embedded at interface. Most Al4C3 phases are granular with 3-10 nm, and the rod-like Al4C3 phases are about 15 nm. Although the

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Al4C3 phases were not detected in Al-0.25GNSs composite by XRD, the results of TEM and SAD reveal that the Al4C3 phases are generated. The Al4C3 phase was not detected

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by XRD in Al-0.25GNSs composite is due to the detection limit of X-ray diffraction equipment for second phases. The formation of Al4C3 phase is known to occur between the Al matrix and graphene sheets, and this reaction is given by following reaction: 4Al(s) 3C(s)→Al4C3 In addition, the dislocations were also found in GNSs/Al composites from Fig.4. Because of the difference in coefficient of thermal expansions between GNSs and Al

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ACCEPTED MANUSCRIPT matrix, the dislocations are generated during cooling. An interesting phenomenon was found in Fig.4 that density of dislocation reduces with the increase in content of Al4C3 products. This phenomenon needs to be further confirmed and deep studied in the future

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work. 3.2 Mechanical properties

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The Vickers hardness results of pure Al and composites were given in table 2. It can be summarized that the Vickers hardness increases with the increase of GNSs content

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from Table 2. The improved hardness of GNSs/Al composites is ascribe to the presence of GNSs having extraordinary mechanical properties, and providing high restraining force for deforming during indentations. Furthermore, the densification of material also significant affects its hardness. With the increase of GNSs content, the relative densities

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of GNSs/Al composites slight decrease from Table 1, but the Vickers hardnesses of GNSs/Al composites still increase from Table 2. This phenomenon may be a result of

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reinforcement GNSs playing more vital role in improving hardness than the slight decrease in relative density, because of the high relative density of composites.

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The average values of yield strength, ultimate tensile strength and elongation obtained from five experimental results are present in Table 2. The variation tendencies of yield strength and ultimate tensile strength with GNSs content were described in Fig.5. The yield strength and ultimate tensile strength of GNSs/Al composites have an obvious improvement compared to that of pure Al. More importantly, it is only a slight decrease in elongation. The strength mechanism of GNSs in GNSs/Al composites can

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ACCEPTED MANUSCRIPT be explained from the following three aspects: stress transfer, dislocation strengthening and grain refinement. In present study, because the solid state hot-pressed sintering was employed, the effect of grain refinement is negligible. Therefore, the stress transfer and

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dislocation strengthening play the vital role in strengthening. The stress transfer

between Al matrix and GNSs primary depends on interfacial bonding. The good

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interfacial bonding was found from Fig.4, in spite of the existence of Al4C3 phase. The good interfacial bonding is conducive to the stress transfer. In addition, the GNSs

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possess the curled or wrinkled feature, which is favorable for forming the mechanical bonding between Al matrix and GNSs. This mechanical bonding can efficient transfer stress from ductility Al matrix to hard GNSs, resulting in the increases of yield strength and ultimate tensile strength. On the other hand, the large mismatch in thermal

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expansion coefficient between Al and GNSs will form the dislocations, and the hard GNSs will impede the movement of dislocation, leading to the increase of dislocation

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density and forming the dislocation strengthening. Therefore, the GNSs as reinforcement can valid improve the yield strength and ultimate tensile strength of Al

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matrix composite.

The maximum improvement of yield strength and ultimate tensile strength both appear in Al-0.25GNSs, and compare with that of pure Al increased by 38.27% and 56.19%, respectively. With the GNSs content increasing over 0.5 wt. %, the tensile properties of composites decrease in Fig.5. With the increase of GNSs content, the content of Al4C3 phases increase concluded from XRD results. It is well known that the

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ACCEPTED MANUSCRIPT brittle Al4C3 phase is harmful for the tensile properties [20]. The interfacial reaction also damages the GNSs, resulting in the decrease of tensile properties of GNSs. Furthermore, the introduced GNSs are few-layered graphenes, and the bonding force is much weak

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between layer and layer. When the GNSs are not to align along the tensile direction, the crack may originate at the interlayer of GNSs. Thus, with the increase of GNSs content,

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the crack origin also increases, resulting in the decrease of mechanical properties.

The tensile fracture surfaces of pure Al and GNSs/Al composites are given in Fig.6.

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All fracture surfaces display the ductile break including ductile fracture dimples and transgranular fracture surface. The phenomenon of GNSs pull-out was detected at the edges of dimples. Fig.6 indicates that the dimples slight decrease with the increases of GNSs content. This change in fracture surface was accompanied by the decrease in

4. Conclusions

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elongation, as given in Table 2.

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The GNSs reinforced Al matrix composites were successfully fabricated by high-energy ball milling and vacuum hot pressing. Effects of GNSs content (0.25 wt. %,

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0.5 wt. % and 1.0 wt. %) on microstructures and tensile properties of GNSs/Al composites were investigated. The results obtained are summarized as follows: (1) The GNSs/Al composites with the relative density over 99.0% were fabricated by high-energy ball milling and vacuum hot pressing. of composites. The GNSs were distributed uniformly throughout Al matrix. The well-dispersed GNSs and desired relative density of composites indicate that this technology can effective fabricate

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ACCEPTED MANUSCRIPT GNSs/Al composite. (2) A good interfacial bonding was obtaned in GNSs/Al composites. The aluminum carbide Al4C3 phases with granular and short rod-like morphology were found at

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interface. With the increase of GNSs content, the content of Al4C3 phases increases. The dislocations were also found near the interface, and the density of dislocation reduces

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with the increase of Al4C3 content.

(3) The Vickers hardness of GNSs/Al composites increases obviously with the increase

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of GNSs content. The yield strength and ultimate tensile strength of GNSs/Al composites have obvious improvement compared to that of pure Al, but the elongation show a slight decrease. The increased strength of GNSs/Al compostes is owe to the stress transfer and dislocation strengthening. The yield strength and ultimate tensile

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strength of Al-0.25GNSs composites increase by 38.27% and 56.19% compare with that of pure Al, respectively. With the GNSs content increasing over 0.5 wt. %, the tensile

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properties of composites decrease. The GNSs pull-out is detected at the edges of dimples, and the number of dimples slight decreases with the increase of GNSs content.

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Acknowledgements

This research was financially supported by Natural Science Foundation of Jiangxi Province (20151BAB206005) Reference [1] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, Nature 442 (2006) 282-286.

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ACCEPTED MANUSCRIPT [2] C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385-388. [3] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Nano Lett. 8 (2008) 902-907.

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Martinez-Sanchez, J. Alloys Compd. 615 (2014) S578-S582. [9] J.H. Li, S.L. Wang, Z.G. Wang, K.H. Xu, Q.F. Li, Y. Xiong, J. Chen, Failure

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Analysis and Prevention 2 (2016) 82-87.

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Prevention 3 (2007) 7-11.

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[12] M. Rashad, F.S. Pan, Z.W. Yu, M. Asif, H. Lin, R.J. Pan, Progress in Natural Science: Materials International 25 (2015) 460-470. [13] L.A. Yolshina, R.V. Muradymov, I.V. Korsun, G.A. Yakovlev, S.V. Smirnov, J.

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ACCEPTED MANUSCRIPT Alloys Compd. 663 (2016) 449-459. [14] X. Gao, H.Y Yue, E.J. Guo, H. Zhang, X.Y. Lin, L.H. Yao, B. Wang, Mater. Design 94 (2016) 54-60.

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[15] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D.Piner, S.T. Nguyen, R.S. Ruoff, Nature 442 (2006) 282–286.

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[17] S.C. Tjong, Mat. Sci. Eng. R 74 (2013) 281-350.

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[16] L.S. Walker, V.R. Marotto, M.A. Rafiee, N. Koratkar, E.L. Corral, ACS Nano 5

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[19] C. Suryanarayana, E. Ivanov, V. V. Boldyrev, Mater. Sci. Eng. A 151 (2001)

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497 (2010) L1-L4.

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ACCEPTED MANUSCRIPT Caption of figures: Fig.1 Dimensions of tensile sample (mm)

GNSs, (b) 0.50wt.% GNSs and (c) 1.0wt.% GNSs. Fig.3 XRD patterns of the GNSs/Al composites.

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Fig.2 The representative images of distribution of GNSs in composites (a) 0.25wt.%

Al-1.0GNSs and (d) Selected area diffraction patter.

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Fig.4 TEM images of GNSs/Al composites (a) Al-0.25GNSs; (b) Al-0.50GNSs; (c)

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Fig.5 The variation tendencies of yield strength and ultimate tensile strength with GNSs content.

Fig.6 The tensile fracture surfaces of pure Al and GNSs/Al composites (a) Pure Al; (b)

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Al-0.25GNSs; (c) Al-0.50GNSs and (d) Al-1.0GNSs.

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Experimental density Theoretical density Relative density ( g·cm -3 ) ( g·cm -3 ) (%)

Pure Al

2.6884

2.7000

Al-0.25GNSs 2.6756

2.6911

Al-0.50GNSs 2.6728

2.6976

Al-1.0GNSs

2.6943

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99.08

99.05

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2.6687

99.57

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Materials

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76

81

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Vickers hardness(Hv) 62

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Yield strength Ultimate tensile Elongation (Mpa) Strength ( MPa ) ( % )

Pure Al

81

Al-0.25GNSs 112

164

Al-0.50GNSs 104

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Al-1.0GNSs

138

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92

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105

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ACCEPTED MANUSCRIPT Highlights GNSs/Al composites with the high relative density and well distribution of GNSs were fabricated The Al4C3 phases with granular and short rod-like morphology were found

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The yield strength and ultimate tensile strength of composites have an obvious improvement