Trans. Nonferrous Met. Soc. China 31(2021) 878-886

Preparation and properties of graphene nanoplatelets reinforced aluminum composites

Zhong ZHENG¹, Xiao-xia YANG², Jian-chao LI¹, Xue-xi ZHANG ¹, Imran MUHAMMAD^1,3, Lin GENG¹

1. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;

2. College of Materials Science and Engineering, Shandong University, Jinan 250014, China;

3. Mechanical Engineering Department, University of Engineering & Technology Taxila, Taxila 47050, Pakistan

Received 23 May 2020; accepted 28 December 2020

Abstract: 5.0 vol.% graphene nanoplatelets (GNPs) and aluminum powders were mixed to prepare GNPs/Al composites via high-energy ball milling (HEBM). The mixed powders were subjected to spark plasma sintering (SPS) and subsequent hot extrusion. The microstructure and mechanical properties of extruded composites were investigated by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and tensile tests. In the extruded composites, 5.0 vol.% GNPs were dispersed homogeneously and no serious GNP-Al interfacial reaction occurred. As a result, the yield strength and ultimate tensile strength of the extruded GNPs/Al composites reached 462 and 479 MPa, which were 62% and 60% higher than those of the extruded Al matrix, respectively. The enhanced mechanical properties were attributed to the effective load transfer capacity of dispersed GNPs. This demonstrated that it may be promising to introduce dispersed high-content GNPs via HEBM, SPS and hot extrusion techniques and GNP-Al interfacial reaction can be controlled.

Key words: aluminum matrix composites; graphene nano-platelets; powder metallurgy; interface; microstructure; mechanical properties

1 Introduction

As a kind of high-performance reinforcement, graphene has been applied successfully to aluminum matrix composites (AMCs) [1-4]. From Refs. [5-31], it can be seen that the hardness, yield strength (YS) and ultimate tensile strength (UTS) of AMCs with 0.06-6.7 vol.% graphene exhibited an increment of 13%-71% [5,6,9,16,18], 14%-85% [8,11-13,15,17,19-21,25,27-31] and 9%-114% [7,8,10-15,17,20-31], respectively, compared with those of samples without graphene reinforcement. Since graphene-reinforced AMCs were reported in 2011, the interfacial reaction between graphene and Al has been found to be an important factor  that hindered the improvement of graphene/Al composites’ properties [5,32].

Up to now, a majority of graphene/Al composites have been produced by powder metallurgy (PM). LI et al [15] developed oxide graphene (GO)/Al composites with nano-laminate architecture via a flake assemble technique. They found that the elastic modulus and UTS of GO/Al composites were 21% and 50% higher than those of monolithic Al matrix, respectively. Although PM technique was proved to be effective in eliminating graphene aggregates in Al matrix, it may lead to unfavorable damage of graphene and formation of Al₄C₃ phase [33,34], which has detrimental effects on the mechanical properties of graphene/Al composites due to its brittle nature. Al₄C₃ was prone to be formed at high temperature for a prolonged time (e.g., casting [35] and 3D printing [36]). So, the graphene/Al interfacial reaction should be well controlled by low temperature casting [26] or cryo- milling followed by hot extrusion [13]. Thus, low-temperature, short-time hot working like spark plasma sintering (SPS) may hinder the formation of Al₄C₃ [30]. For example, no Al₄C₃ was formed in graphene/Al composites prepared by sintering at temperatures of 823-873 K [6,23]. In the graphene/ Al composites prepared by wet mixing and sintering [17], the UTS reached the peak value at a graphene content of 0.4 vol.%. For composites prepared by wet mixing [6] and flake assembly [37] techniques, such critical graphene content may be 2.0 vol.%.

Flake powder metallurgy via ball milling is a feasible strategy to improve the strength of graphene/Al composites [38]. In this process, a high specific surface area of Al flakes is necessary to accommodate the graphene. Our recent work has demonstrated that 1.0 vol.% GNPs may be effectively attached on Al flake surface induced by the shear stress during ball milling (200 r/min for 5 h) [27]. However, even higher content of GNPs, e.g. 5.0 vol.%, cannot be homogeneously dispersed with Al flakes under low-energy ball milling (LEBM) conditions. Here, 5.0 vol.% GNPs were dispersed with Al powders via high-energy ball milling (HEBM, 300 r/min for 10 h). The mixed powders were then subjected to sintering by SPS and extrusion with a high extrusion ratio 25:1.  The produced composites exhibited enhanced mechanical properties due to the homogeneous distribution of GNPs, demonstrating a promising method for fabricating high-content GNPs/Al composites.

2 Experimental

Al powder (purity ≥99.99%) with an average diameter of ~10 μm was supplied by Tianjiu Changsha Technology Company, China. The as- received GNPs (purity ≥99.9%) with an average thickness of 5-10 nm were supplied by Jicang Nano Technology Company, China. The Al powders were mixed with as-received GNPs in a planetary ball mill for 10 h at 300 r/min with ball to material mass ratio of 16:1. In our works, ball milling speeds of 100 r/min [33] and 200 r/min [27] were defined as LEBM or low-speed ball milling (LSBM). Here, a higher ball milling speed (300 r/min) is considered HEBM. The HEBM process was performed with 0.5 wt.% stearic acid as process control agent in argon atmosphere. The milled powders were placed into a graphite mold and subjected to sintering at a temperature of 723 K for 5 min to remove the stearic acid and then the green ingots were fabricated under a uniaxial pressure of 50 MPa in an HP-D250-1 SPS machine. The sintered ingot was heated to 723 K, kept at this temperature for 30 min and extruded into a rod with an extrusion ratio of 25:1. Graphite was used as the lubricant to reduce friction between sintered ingot and extrusion mold. For comparison, Al matrix was prepared at the same parameters.

Determined by Archimedes method, the relative densities of sintered Al matrix and composites are >99%. The morphology of mixed powders and microstructures of composites were observed by Zeiss field emission SEM Merlin Compact. Raman spectra tracking the structural integrity of GNPs in blended powders were acquired using a Renishaw inVia Reflex Raman confocal microscope (Gloucestershire, UK). X-ray photoelectron spectroscopy (XPS) measurement was performed using ESCLAB 250Xi equipped with a monochromator aluminum source. A Talos F200X TEM was used to investigate the morphology of GNPs and GNPs/Al composites. The samples for TEM analysis were prepared by ion thinning. Dog-bone shaped samples cut along the extrusion direction were used for tensile tests on a universal testing machine (Instron-5569).

3 Results and discussion

3.1 Graphene/Al interfacial reaction

For graphene/Al interfacial reaction, the relationship between Gibbs free energy change (△G_T) and temperature (T) can be described by △G_T=A+BT, where A and B are temperature independent constants [39]. The obtained △G_T is far less than zero at temperatures of 700-900 K (Fig. 1), implying that Al may react with graphene to form Al₄C₃. The thickness of reaction layer Z and time t can be given by , where the theoretical kinetic diffusion coefficient (k) can be expressed by Arrhenius formula [40,41]. The empirical expression of k and T may be expressed by k=k₀exp[-Q/(RT)] (Fig. 1). Apparently, k is almost zero at below 823 K and increases exponentially with the increase of T.

Fig. 1 Thermodynamic and kinetic curves for reaction between graphene and Al

3.2 Microstructure of GNPs/Al composites

Figure 2(a) shows the morphology of 5.0 vol.% GNPs/Al composites produced by HEBM. Distributions of the diameters of Al particles and lateral sizes of GNPs were determined using Image Pro Plus software, as plotted in Figs. 2(b) and (c), respectively. It can be seen that the average diameter of Al particles produced by HEBM is 110 μm, which is larger than that of the as-received Al powders, implying the occurrence of cold- welding during ball milling. On the other hand, GNP slices (inset in Fig. 2(a)) attached on the surface of Al particles were observed. The average lateral size of GNP slice is 0.20 μm, which is one eighth of as-received GNPs (~1.5 μm in lateral size). The reduction in GNP lateral size may be attributed to the shear stress induced by HEBM.

The Raman spectra of GNPs/Al composites and as-received GNPs are shown in Fig. 3. One strong broad peak centered at 1343 cm^-1 corresponds to D band (disordered defect structure) and that centered at 1575 cm^-1 corresponding to G band (ordered graphene structure) [42]. Generally speaking, the intensity ratio of D-band to G-band (I_D/I_G) represents defect density or disorder degree in graphene. The I_D/I_G value increased from 0.1:1 in as-received GNPs to 1.4:1 in GNPs/Al composites, implying the increase of defect density in GNPs induced by HEBM. At the same time, the I_D/I_G value of GNPs/Al composites by HEBM (300 r/min for 10 h) is similar to that of LEBM (150 r/min for 1.5 h [43]), indicating that embedding of GNPs in Al particles may protect GNPs from serious damage.

Fig. 2 Morphology and size of 5.0 vol.% GNPs/Al composites produced by HEBM

While SPS at higher temperature is helpful for the densification of composites, graphene and Al are prone to react at such high temperatures [22,36]. As the value of k is very small at 723 K (Fig. 1), interfacial reaction between graphene and Al is possibly not serious. In order to reveal the interfacial reaction state, XPS spectra were obtained, as shown in Fig. 4. The peaks centered at 743 and 284.5 eV correspond to Al and C, respectively. High resolution Al 2p and C 1s XPS spectra of sintered GNPs/Al composites are shown in Figs. 4(a) and (b), respectively. No peaks at 72.7 and 282.2 eV, corresponding to Al₄C₃ phase [36], were observed, implying that the interfacial reaction between GNPs and Al may be too weak to be detectable via XPS.

Fig. 3 Raman spectra of 5.0 vol.% GNPs/Al composites and as-received GNPs

Fig. 4 XPS spectra of sintered 5.0 vol.% GNPs/Al composites

Figure 5(a) shows the GNPs distribution state in the extruded 5.0 vol.% GNPs/Al composites. HRTEM image (inset in Fig. 5(a)) shows that the lattice fringe has a spacing of ~0.34 nm, corresponding to the interplanar spacing of graphite (0002) [44]. GNPs are dispersed homogeneously in extruded composites (Fig. 5(c)), which is favorable for improving mechanical properties of the composites [45]. Nano-sized grains are formed in extruded composites and Al matrix (Figs. 5(b) and (d)). Compared with the average grain size of Al matrix (279 nm), the grain size of composite is reduced to 196 nm, indicating that addition of GNPs is favorable for grain refinement [46]. This may be related to the presence of GNPs at Al grain boundaries, which reduced the mobility of Al grain boundaries during high temperature dwelling and deformation processes [33].

3.3 Mechanical properties and fracture behaviors

The YS, UTS and fracture elongation are summarized in Table 1. The YS of composites (~462 MPa) is 1.6 times as high as that of Al matrix (~286 MPa), which is the outcome of well- dispersed GNPs and fine Al grains in the composites (Fig. 5). Furthermore, the UTS of composites reaches ~479 MPa, which is comparable or even higher than that of many graphene/Al composites with lower or similar  GNP contents [8,10,11,13,17,18,20-24,26-28,30,47]. However, the fracture elongation of the composite is ~2.7%, which is much smaller than that of Al matrix (~10.9%). This may be related to the damage of GNPs after HEBM (I_D/I_G value increased from 0.1:1 for the as-received GNPs to 1.4:1 for GNPs/Al composites, see Fig. 3). This shows that, in order to enhance the strength and plasticity of GNPs/Al composites, it is necessary to achieve homogeneous distribution of GNPs without damage GNPs structure.

The fracture surface morphology of 5.0 vol.% GNPs/Al composites is displayed in Fig. 6. It can be seen that dimples and GNP slices exist on the fracture surfaces of composites. It is also noted that GNPs (marked circles in Fig. 6(b)) may bridge cracks, leading to the crack propagation resistance for composite. Therefore, the dispersed GNPs exhibit significant load transfer strengthening effect; thus, mechanical properties of the composites were improved.

Fig. 5 TEM images and Al grain size of extruded 5.0 vol.% GNPs/Al composite and Al matrix

Table 1 Yield strength (YS), ultimate tensile strength (UTS) and fracture elongation (δ) of extruded Al matrix and 5.0 vol.% GNPs/Al composites

In order to reveal the reinforcing efficiency of GNPs, a normalized parameter Δσ=σ_c-σ_m, where σ_c and σ_m are UTS values of composites and Al matrix, respectively, is proposed. The UTS increment Δσ of graphene/Al composites and their fracture elongations in open literatures are summarized in Fig. 7. The fabrication techniques and graphene contents in Refs. [7,8,10-13,15,20,22-28,37] are given in Table 2. Here, different Δσ may be attributed to graphene dispersion state and graphene/Al interfacial reaction state by various fabrication techniques. Usually, graphene/Al composites possess significantly enhanced UTS owning to the uniform distribution of graphene in Al matrix by employing ball milling [12,24] or flake assemble [15,20] techniques. In addition, improving the distribution of graphene in Al matrix via secondary-processing techniques, such as rolling [12] and extrusion [13,24], exhibited higher UTS. From Fig. 7 and Table 1, it is also noticed that the UTS increment of the composite is linked inversely to its fracture elongation. Generally, graphene/Al composites show low fracture elongation owing to damage graphene structure [6] or serious interfacial reaction [11].

In this study, GNPs were embedded in cold- welded Al particles effectively, which protected GNPs from the serious damage during HEBM.

Fig. 6 SEM images showing fracture surfaces of extruded 5.0 vol.% GNPs/Al composites

Fig. 7 Increment of ultimate tensile strength (UTS) in graphene/Al composites prepared by various fabricating techniques including wet mixing + spark plasma sintering [22,23]/hot extrusion [8,11]/high pressure torsion [10], flake assembly + hot rolling [15,20]/hot extrusion [7,37], ball milling + spark plasma sintering[28]/cold drawing [27]/hot rolling [26]/friction stir processing [25]/hot extrusion [13,24]/hot rolling [12]

Table 2 Al composites with different contents of graphene prepared by various fabricating techniques

Subsequently, the mixed powders were sintered by SPS at a low temperature of 723 K in order to avoid unfavorable interfacial reaction. Meanwhile, the high shear strain produced during hot extrusion help for eliminating GNP dense zones and realizing homogeneous distribution of GNPs (Fig. 5). As a result, the UTS and YS of GNPs/Al composites reached as high as about 479 and 462 MPa, respectively (Table 1) because of high load transfer strengthening efficiency of GNPs.

4 Conclusions

(1) Homogeneous dispersion of high-content GNPs in Al matrix was achieved via cold-welding during HEBM. This was favorable for avoiding serious damage of GNP structure during HEBM and preventing unfavorable interfacial reaction during SPS.

(2) The SPS and hot extrusion temperatures were determined to be 723 K. The hot extrusion induced homogeneous distribution of GNPs in composites, while the low sintering temperatures prevented interfacial reaction.

(3) The extruded 5.0 vol.% GNPs/Al composites had YS of ~462 MPa and UTS of ~479 MPa, which were 62% and 60% higher than those of Al matrix, respectively. The enhanced YS and UTS were attributed to well-dispersed GNPs and nano-grains in the extruded composites.

Acknowledgments

The authors are grateful for the financial supports from National Key R&D Program of China (2017YFB0703103), and Key Area R&D Program of Guangdong Province, China (2019B010942001)

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石墨烯纳米片增强铝基复合材料的制备与性能

郑 忠¹，杨晓霞²，李建超¹，张学习¹，Imran MUHAMMAD^1,3，耿 林¹

1. 哈尔滨工业大学 材料科学与工程学院，哈尔滨 150001；

2. 山东大学 材料科学与工程学院，济南 250014；

3. Mechanical Engineering Department, University of Engineering & Technology Taxila, Taxila 47050, Pakistan

摘  要：采用高能球磨、放电等离子烧结以及热挤压工艺制备含量为5.0%(体积分数)的石墨烯增强铝基复合材料。分别采用X射线光电子能谱、透射电镜及拉伸试验研究挤压态复合材料的显微组织与力学性能，发现5.0%(体积分数)的石墨烯分散在铝晶界上，并且未与铝基体发生界面反应。最终，挤压态复合材料的屈服强度和抗拉强度高达462 MPa和479 MPa，分别比挤压态铝基体提高62%和60%。断口分析表明，在断裂过程中复合材料中分散的石墨烯起到明显的载荷传递的作用。上述结果表明，采用高能球磨、放电等离子烧结以及热挤压制备工艺可将高含量石墨烯分散于铝合金中，且能控制石墨烯和铝基体之间的界面反应。

关键词：铝基复合材料；石墨烯纳米片；粉末冶金；界面；显微组织；力学性能

(Edited by Wei-ping CHEN)

Corresponding author: Xue-xi ZHANG; Tel: +86-451-86415894; Fax: 86-451-86413921; E-mail: xxzhang@hit.edu.cn

DOI: 10.1016/S1003-6326(21)65546-2

1003-6326/ 2021 The Nonferrous Metals Society of China. Published by Elsevier Ltd & Science Press