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This paper provides a critical review on the current status of graphene-reinforced metal matrix composites (GRMMCs) in an effort to guide future work on this topic. Metal matrix composites are preferred over other types of composites for their ability to meet engineering and structural demands. Graphene is considered an ideal reinforcement material for composites due to its unique structure and extraordinary physical, thermal, and electrical properties. Incorporating graphene as a reinforcement in metals is a way of harnessing its extraordinary properties, resulting in an enhanced metallic behavior for a wide variety of applications. Combining graphene with bulk metal matrices is a recent endeavor that has proven to have merit. A systematic study is needed to critically examine the efforts applied in this field, the successes achieved, and the challenges faced. This review highlights the three main pillars of GRMMCs: synthesis, structure, and properties. First, it discusses the synthesis techniques utilized for the fabrication of GRMMCs. Then, it highlights the resulting microstructures of the composites, including graphene dispersion and interfacial interactions. Finally, it summarizes the enhancements in the mechanical, electrical, thermal, and tribological properties of GRMMCs, while highlighting the effects of graphene type and content on those enhancements.

• Different milling times affected the structural defects in graphene and its distribution in the n-type Bi 2 Te 2.7 Se 0.3 matrix. • Long milling time induced higher levels of structural defects in graphene and reduced the composite's electrical conductivity • Ten minutes of mechanical milling was the optimum time to enhance the thermoelectric properties of the composite. • The improved ZT is attributed to the low level of structural defects and reduced agglomeration of graphene in the composite. This study examines the effects of the structural integrity of graphene on the thermoelectric properties of n-type bismuth telluride alloy. Graphene/Bi 2 Te 2.7 Se 0.3 composites were prepared via mechanical alloying and spark plasma sintering techniques. Different graphene concentrations (0.05 and 0.5 wt%) and addition times (20 hrs, 10 mins, and 1 min) were considered. The thermoelectric properties were measured, and the results showed that the milling time affects graphene structure as well as its agglomeration. It is revealed that the optimum time to add the two-dimensional filler is during the last phase of mechanical milling as it will preserve graphene's structure and boost the electrical conductivity. It is also shown that as the milling time of graphene increases, the Seebeck coefficient improves. Even though an increase in the thermal conductivity is expected due to the high electrical conductivity, a clear reduction in the lattice thermal conductivity part was obtained due to the increased scattering at the new interfaces. The figure-of-merit for the optimum sample with 0.05 wt% graphene added in the last 10 mins of milling had an improvement of 19% at room temperature reaching a value of 0.5, and 25% at 160 °C achieving a final value of 0.81. [ABSTRACT FROM AUTHOR]

• Tin Selenide/Graphene nanoplatelets (SnSe/GNPs) nanocomposite is synthesized by ball milling and hot pressing. • Milling time has a significant effect on enhancing the distribution of graphene within the matrix. • Homogeneous distribution of graphene enable decoupling the Seebeck coefficient and the electrical conductivity. • Homogeneously distributed graphene provides a continuous path for carriers to move, hence enhancing the carrier mobility. • Optimum milling time has a significant effect on enhancing the power factor of SnSe/GNPs nanocomposite. [Display omitted] Tin selenide graphene nanocomposites (SnSe/GNPs) were fabricated with high-energy ball milling and hot pressing by varying the milling time of graphene. The effect of ball milling time on the graphene integrity and the dispersion homogeneity was investigated and the consequential variation in electrical properties of SnSe/GNPs were analyzed. The evolution of graphene sheets during milling as well as the crystal structure of SnSe/GNPs nanocomposites were systematically studied by X-ray diffraction, Raman analysis, scanning electron microscopy, and transmission electron microscopy. It has been proven that graphene was able to keep its crystallinity at short milling times, but it exhibits agglomeration and poor dispersion within the matrix. However, long milling time has a significant effect on increasing the disorders on graphene structure while it provides well dispersion of graphene. The calculated power factor increases with the addition of graphene and with increasing graphene milling time. The increased power factor is attributed to the homogeneous distribution of graphene, which results in a significant increase in electrical conductivity. At 773 K, the lowest power factor value was reported for the 1-min graphene-milled sample, whereas a 40% enhancement was reported for the 2-h graphene-milled sample. Across a wide temperature range (298–720 K), the 12-h graphene-milled sample shows the best performance owing to the simultaneous increase of electrical conductivity and Seebeck coefficient. These findings indicate the positive effect of milling time on the distribution of graphene, which in turn enables graphene to form a continuous net for carriers to move. This study could provide a greater understanding of the control factors of the mechanical milling process for preparing SnSe/GNPs nanocomposites in order to take full advantage of graphene's extraordinary properties by improving its distribution within the tin-selenide- based composite. [ABSTRACT FROM AUTHOR]

Metal-graphene nanocomposites and plasmonic metal nanoparticles are two nanoscience fields of a rapidly growing interest due to their potential in advanced applications. In this study, we combine both fields by synthesizing plasmonic aluminum-lithium-graphene nanosheets (Al–Li-GNSs) with anisotropic morphologies using a simple ball-milling technique. Structural analysis using SEM and TEM revealed that the Al–Li-GNSs nanoparticles are single-crystals with a hexagonal platelet-like morphology of a ∼300–500 nm diagonal and a ∼60 nm thickness. Electron diffraction analysis indicated that the as-milled platelets have an FCC structure with (111) top and bottom facets and revealed the presence of 1/3(422) and 1/3(220) forbidden reflections. UV–Vis spectroscopy of the hexagonal Al-based nanoplatelets was found to exhibit plasmonic resonance absorption bands in the UV region at a wavelength of 214 nm and 345 nm. In this report, we confirm the feasibility of building epitaxial plasmonic metal-graphene systems inside bulk metal-graphene composites using a simple milling process. [Display omitted] [ABSTRACT FROM AUTHOR]

Interest in nanocrystalline (nc) aluminum-lithium (Al-Li) alloys is motivated by the demand for lightweight and high-performance materials for light-weighting applications and superior fuel consumption. Nonetheless, nc metals, including Al are thermally unstable, which hinders their technological applications. In this study, we explore the effect of combining dilute amounts of strontium (1.0 at% Sr) and graphene nanoplatelets (1.0 wt% GNPs) to investigate the thermal stability of a nc Al-Li alloy. Ball milling was used to prepare four samples: Al-Li, Al-Li-Sr, Al-Li-GNPs, and Al-Li-Sr-GNPs, to systematically investigate the role of each added element. Isothermal annealing was conducted at different temperatures to investigate the thermal stability. Despite maintaining a nanometric grain size and high hardness of 70 nm and 1.1 GPa, respectively, after annealing at 773 K for 1 h, the Al-Li-Sr-GNPs sample suffered the most significant grain growth and the highest drop in hardness when compared to the Al-Li-Sr and Al-Li-GNPs samples. Microstructural investigations suggested that competing effects resulting from the spontaneous reaction of both Sr and GNPs with Al at higher temperatures resulted in a declining thermal stability efficiency. The formation and distribution of the rod-like Al 4 C 3 phase at the grain boundaries stood in the way of proper Sr diffusion after annealing and caused the agglomeration of the Al 4 Sr phase. [Display omitted] • The thermal stability of nc Al-Li alloy was studied through Sr and GNPs additions. • The Al 4 Sr and Al 4 C 3 formed at the GBs contributed to the improved thermal stability. • Rod-like Al 4 C 3 formed at the grain boundaries prevented the proper Sr segregation. • Agglomeration of the Al 4 Sr resulted in a declined thermal stability efficiency. [ABSTRACT FROM AUTHOR]