Zan Li

Enhanced Mechanical Properties of Graphene (Reduced Graphene Oxide)/Aluminum Composites with a Bioinspired Nanolaminated Structure

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.

Uniform dispersion of graphene oxide in aluminum powder by direct electrostatic adsorption for fabrication of graphene/aluminum composites.

The excellent properties of graphene promote it as an ideal reinforcement in composites. However, dispersing graphene homogenously into metals is a key challenge that limits the development of high-performance graphene-reinforced metal matrix composites. Here, via simple electrostatic interaction between graphene oxide (GO) and Al flakes, uniform distribution of reduced graphene oxide (RGO) in an Al matrix is achieved. The adsorption process of GO on Al flakes is efficient, as it can be completed in minutes and proceeds spontaneously without any chemical agents. GO can be partially reduced by the electron interchange during the adsorption process and could be thoroughly reduced after subsequent thermal annealing. A densified RGO/Al composite can be obtained by hot pressing the RGO/Al composite powders. By employing the preceding fabrication process, a composite reinforced with only 0.3 wt.% of RGO shows an 18 and 17% increase in elastic modulus and hardness, respectively, over unreinforced Al, demonstrating RGO is a better reinforcement than most other reinforcements.

A Versatile Method for Uniform Dispersion of Nanocarbons in Metal Matrix Based on Electrostatic Interactions

Realizing the uniform dispersion of nanocarbons such as carbon nanotube and graphene in metals, is an essential prerequisite to fully exhibit their enhancement effect in mechanical, thermal, and electrical properties of metal matrix composites (MMCs). In this work, we propose an effective method to achieve uniform distribution of nanocarbons in various metal flakes through a slurry-based method. It relies on the electrostatic interactions between the negatively charged nanocarbons and the positively charged metal flakes when mixed in slurry. For case study, flake metal powders (Al, Mg, Ti, Fe, and Cu) were positively charged in aqueous suspension by spontaneous ionization or cationic surface modification. While nanocarbons, given examples as carboxylic multi-walled carbon nanotubes, pristine single-walled carbon nanotube, and carbon nanotube–graphene oxide hybrid were negatively charged by the ionization of oxygen-containing functional groups or anionic surfactant. It was found that through the electrostatic interaction mechanism, all kinds of nanocarbons can be spontaneously and efficiently adsorbed onto the surface of various metal flakes. The development of such a versatile method would provide us great opportunities to fabricate advanced MMCs with appealing properties.

Composite structural modeling and tensile mechanical behavior of graphene reinforced metal matrix composites

Owing to its distinguished mechanical stiffness and strength, graphene has become an ideal reinforcing material in kinds of composite materials. In this work, the graphene (reduced graphene oxide) reinforced aluminum (Al) matrix composites were fabricated by flaky powder metallurgy. Tensile tests of pure Al matrix and graphene/Al composites with bioinspired layered structures are conducted. By means of an independently developed Python-based structural modeling program, three-dimensional microscopic structural models of graphene/Al composites can be established, in which the size, shape, orientation, location and content of graphene can be reconstructed in line with the actual graphene/Al composite structures. Elastoplastic mechanical properties, damaged materials behaviors, graphene-Al interfacial behaviors and reasonable boundary conditions are introduced and applied to perform the simulations. Based on the experimental and numerical tensile behaviors of graphene/ Al composites, the effects of graphene morphology, graphene-Al interface, composite configuration and failure behavior within the tensile mechanical deformations of graphene/ Al composites can be revealed and indicated, respectively. From the analysis above, a good understanding can be brought to light for the deformation mechanism of graphene/Al composites.摘要石墨烯具有优异的机械性能, 已成为众多复合材料中的理想增强体材料. 本研究采用片状粉末冶金方法制备了具有仿生叠层结构的石墨烯/铝基复合材料, 同时对纯铝基体与石墨烯/铝基复合材料进行了拉伸试验. 通过基于Python语言自主研发的复合材料结构建模程序, 可以有效建立石墨烯/铝基复合材料的三维复合结构模型, 并实现石墨烯尺寸、形貌、取向、位置与含量等可控重构分布. 通过引入组分材料力学性能、损伤行为、界面行为及边界条件等实现了石墨烯/铝基复合材料的拉伸行为模拟, 并揭示了石墨烯形貌、石墨烯/铝界面、复合构型与失效行为等复合因素的影响规律, 对理解石墨烯/铝基复合材料的变形机理提供了有力依据.

Regain strain-hardening in high-strength metals by nanofiller incorporation at grain boundaries

Grain refinement to the nano/ultrafine-grained regime can make metals several times stronger, but this process is usually accompanied by a dramatic loss of ductility. Such strength-ductility trade-off originates from a lack of strain-hardening capacity in tiny grains. Here, we present a strategy to regain the strain-hardening ability of high-strength metals by incorporation of extrinsic nanofillers at grain boundaries. We demonstrate that the dislocation storage ability in Cu grains can be considerably improved through this novel grain-boundary engineering approach, leading to a remarkably enhanced strain-hardening capacity and tensile ductility (uniform elongation). Experiments and large-scale atomistic simulations reveal that a key benefit of incorporated nanofillers is a reduction in the grain-boundary energy, enabling concurrent dislocation storage near the boundaries and in the Cu grain interior during straining. The strategy of grain-boundary engineering through nanofillers is easily controllable, generally applicable, and may open new avenues for producing nanostructured metals with extraordinary mechanical properties.

Orientation-Dependent Tensile Behavior of Nanolaminated Graphene-Al Composites: An In Situ Study

We conducted in situ microtension experiments in a scanning electron microscope (SEM) to study the orientation-dependent mechanical behavior of nanolaminated graphene-Al composite. We found a transition from a weak-and-brittle behavior in the isostress composite configuration to a strong-yet-ductile tensile response in the composite under isostrain condition. This is explained by the excellent load-bearing capacity of the graphene nanosheets and a crack deflection mechanism rendered by the laminate structure. These in situ measurements enabled direct observation of the deformation procedure and the exact failure mode, which highlight the importance of microstructural control in tailoring the mechanical properties of advanced metal matrix composites (MMCs).

Grain boundary-assisted deformation in graphene–Al nanolaminated composite micro-pillars

ABSTRACT Micro-pillars with diameters varying from 0.5 to 3.5 µm were fabricated from bulk nanolaminated graphene (in the form of reduced grapheme oxide, RGO)–Al composite. Upon uniaxial compression, the pillar strengths exhibited no obvious size effect, and the pillars of larger diameters possessed smoother stress–strain response, as opposed to the jerky deformation of their smaller counterparts. A corresponding transition in the deformation mode from Al layer extrusion to shear fracture over decreasing pillar diameter was observed. These observations were explained by the competing effect of dislocation accumulation and annihilation, and a grain boundary-assisted deformation mechanism. GRAPHICAL ABSTRACT IMPACT STATEMENT The transition in the deformation mode of graphene–Al composite micro-pillars from localized shear facture to Al layer extrusion over increasing pillar diameter is attributed to a grain boundary-assisted deformation mechanism.