Letter pubs.acs.org/NanoLett
Enhanced Mechanical Properties of Graphene (Reduced Graphene Oxide)/Aluminum Composites with a Bioinspired Nanolaminated Structure Zan Li, Qiang Guo,* Zhiqiang Li, Genlian Fan, Ding-Bang Xiong, Yishi Su, Jie Zhang, and Di Zhang* State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China S Supporting Information *
ABSTRACT: Bulk graphene (reduced graphene oxide)-reinforced Al matrix composites with a bioinspired nanolaminated microstructure were fabricated via a composite powder assembly approach. Compared with the unreinforced Al matrix, these composites were shown to possess significantly improved stiffness and tensile strength, and a similar or even slightly higher total elongation. These observations were interpreted by the facilitated load transfer between graphene and the Al matrix, and the extrinsic toughening effect as a result of the nanolaminated microstructure.
KEYWORDS: Metal matrix composites, graphene, nanolaminated structure, ductility, toughening nder the principle of “survival of the fittest”, biological species evolve over time to acquire particular characteristics to adapt to the surrounding environment. Constituted by materials available in nature that generally exhibit poor macroscale mechanical properties, biological structural materials can achieve orders of magnitude increase in strength and toughness over their individual constituents.1−3 This remarkable property optimization in biological materials is mainly the result of their exquisite microstructures. Nanolaminated structure is widely adopted by hard biological materials, such as bone, nacre, and biosilica, which are composites basically composed of brittle minerals, and a small fraction of soft organic constituents.3−5 Deriving the advantages from the intricate design of the lamellar structure, these materials possess the combination of superior strength and toughness,5−8 which are mutually exclusive in most man-made structural materials.5 The strategies of biological materials in dealing with the strength-toughness conflict by nanolaminated structures provide a potential solution for advanced artificial materials used for structural applications. However, in materials such as metal matrix composites (MMCs), guided by the design principle of homogeneous dispersion of reinforcements, few attempts in microstructure design have been carried out, and the composites usually make a compromise between strength and toughness,9−11 which greatly hinders their broad engineering applications in the fields of transportation, aerospace, and military industries.11 The advantages of biological materials have inspired studies on developing MMCs with a nanolaminated structure. Recently, efforts have been made to fabricate carbon nanotube
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© 2015 American Chemical Society
(CNT)/metal composites with sandwiched structure, demonstrating the advantage of nanolaminated structure in strengthening12−14 and toughening14 the composites. While compared to the one-dimensional CNT, the two-dimensional geometry of graphene is intrinsically more compatible with the planar laminated structure and is thus considered an ideal reinforcement in nanolaminated MMCs.15−17 Until now, most of the studies on graphene/metal composite emphasize the homogeneous dispersion of graphene and the suppression of its agglomeration, and there are limited attempts in careful microstructure design for graphene/metal composites.18,19 Co-milling of metal powders (such as Al powders) with graphene is an effective and widely adopted way to disperse graphene uniformly in the metal matrix. However, the high energy milling process would seriously damage the integrity of graphene and promote interfacial reactions, causing a marginal or even a negative strengthening effect of graphene in the metal matrix.20,21 Moreover, the distribution of graphene in co-milled composites is usually random, which is unfavorable for fully realizing its strengthening capability due to its strong anisotropy, and also cause a poor match between strength and ductility in the as-fabricated composites.19 By employing a molecular-level mixing process, Hwang et al.22 fabricated millimeter-sized graphene/Cu composite samples and explained the good strengthening effect of graphene by the strong adhesion energy between graphene and Cu. However, Received: September 1, 2015 Revised: November 10, 2015 Published: November 17, 2015 8077
DOI: 10.1021/acs.nanolett.5b03492 Nano Lett. 2015, 15, 8077−8083
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Nano Letters
Figure 1. Microstructural characterization of RGO/Al nanolaminated composites with RGO concentration of 1.50 vol %. (a) Cross-sectional TEM image of the hot-rolled composites. (b) An HRTEM image of the interface of RGO and Al matrix. (c) An EBSD image of the ND-RD cross-section, from which the spacing of lamellar boundaries parallel to ND (dT) and the spacing of interconnecting boundaries parallel to RD (dL) were estimated. (d) Distribution of the boundary spacing of the elongated Al grains. At least 150 boundaries were measured and statistically averaged.
the distribution of graphene is random in the as-fabricated composites, and the fabrication process is relatively complicated and difficult to be scaled-up. By alternately evaporating metal thin films and transferring monolayer or bilayer graphene onto the metal-deposited substrate, Kim et al.16 fabricated graphene/ Cu and graphene/Ni nanolaminated composite films and demonstrated a significant strengthening effect of graphene in nanolayered composites by dislocation blockade mechanism. Nevertheless, the method is time-consuming and only applicable for thin film-typed samples. Therefore, the development of a strategy to synthesize bulk graphene/metal composites with nanolaminated structure is critical for further demonstrating its strengthening and toughening mechanisms as well as its potential for large scale applications. In this study, flake powder metallurgy, a bottom-up assembly process of composite flake powders, was used to prepare bulk graphene/Al composite with bioinspired nanolaminated structure. Tensile test reveals that graphene in the nanolaminated composites has remarkably higher strengthening and stiffening efficiencies than those of other reinforcements, and the composites maintain a similar or even slightly higher total elongation than the unreinforced Al matrix. The deformation behavior of the composites is interpreted in terms of a competition between dislocation accumulation and dynamic recovery at the graphene/Al interfaces, and the large elongation after peak stress is attributed to a toughening effect from the laminated structure, as revealed by in situ tensile tests in a transmission electron microscope (TEM). This work highlights the importance of structural control in the stiffening, strengthening, and toughening of the composites, and sheds new light on the development of graphene-reinforced MMCs with the potential for large-scale applications.
Different from the conventional powder metallurgy process, which pays little attention to the geometry of the powders, the composite powder assembly process adopted in this work underlines the morphology control of initial composite powders, which serve as building blocks and are assembled to form the final bulk composites. Graphene oxide (GO) monolayers (in-plane dimension of the monolayer ranged from about 200 nm to almost 1 μm) in the aqueous suspension have a tendency to get uniformly adsorbed on bare Al nanoflakes (lateral size ∼50 μm and thickness 250 ± 30 nm) through electrostatic interactions when they are mixed.23 After the adsorbed GO were converted to graphene (reduced graphene oxide, RGO) upon thermal reduction, the composite nanoflakes with large aspect ratio can then be stacked into a bulk specimen under the action of gravity during uniaxial compaction, resulting in a composite billet with nanolaminated microstructure. Further densification processes, including hot pressing and rolling, were employed to obtain fully densified bulk RGO/Al nanolaminated composite with a thickness of 10 mm. In this work, composite samples of 0.75 vol % and 1.50 vol % RGO concentrations were fabricated, and unreinforced Al matrix sample was prepared by nominally the same process for comparison. Details of the fabrication process and microstructure evolution can be found in Sample Preparation section and Supporting Information. As shown in Figure 1a, the microstructure of the composite is characterized by elongated Al grains along the rolling direction, forming a laminated structure where each lamella contains predominantly a single grain through its thickness in the normal direction (ND). High-resolution TEM (HRTEM) characterization performed at the RGO/Al interface with RGO concentration of 1.50 vol % (Figure 1b) shows that the interface comprises a few RGO layers (
