Research progress on the characterization and repair of graphene defects

Graphene has excellent theoretical properties and a wide range of applications in metal-based composites. However, because of defects on the graphene surface, the actual performance of the material is far below theoretical expectations. In addition, graphene containing defects could easily react with a matrix alloy, such as Al, to generate brittle and hydrolyzed phases that could further reduce the performance of the resulting composite. Therefore, defect repair is an important area of graphene research. The repair methods reported in the present paper include chemical vapor deposition, doping, liquid-phase repair, external energy graphitization, and alloying. Detailed analyses and comparisons of these methods are carried out, and the characterization methods of graphene are introduced. The mechanism, research value, and future outlook of graphene repair are also discussed at length. Graphene defect repair mainly relies on the spontaneous movement of C atoms or heteroatoms to the pore defects under the condition of applied energy. The repair degree and mechanism of graphene repair are also different according to different preparations. The current research on graphene defect repair is still in its infancy, and it is believed that the problem of defect evolution will be explained in more depth in the future.

Microstructural evolution and thermal conductivity of diamond/Al composites during thermal cycling

The microstructural evolution and performance of diamond/Al composites during thermal cycling has rarely been investigated.In the present work,the thermal stability of diamond/Al composites during thermal cycling for up to 200 cycles was explored.Specifically,the thermal conductivity(λ)of the composites was measured and scanning electron microscopy of specific areas in the same samples was carried out to achieve quasi-in situ observations.The interface between the(100)plane of diamond and the Al matrix was well bonded with a zigzag morphology and abundant needle-like Al4C3 phases.By contrast,the interface between the(111)plane of diamond and the Al matrix showed weak bonding and debonded during thermal cycling.The debonding length increased rapidly over the first 100 thermal cycles and then increased slowly in the following 100 cycles.Theλof the diamond/Al composites decreased abruptly over the initial 20 cycles,increased afterward,and then decreased monotonously once more with increasing number of thermal cycles.Decreases in theλof the Al matrix and the corresponding stress concentration at the diamond/Al interface caused by thermal mismatch,rather than interfacial debonding,may be the main factors influencing the decrease inλof the diamond/Al composites,especially in the initial stages of thermal cycling.

Deformation treatment and microstructure of graphene-reinforced metal matrix nanocomposites: A review of graphene post-dispersion

Graphene/aluminum(Gr/Al)composites have attracted the attention of researchers all over the world due to their excellent properties.However,graphene agglomerates easily because of the van der Waals force between graphite sheets,thereby affecting the performance of the composites.Decreasing the agglomeration of graphene and dispersing it uniformly in the Al matrix is a key challenge.In the preparation process,predispersion treatment and deformation treatment can play important roles in graphene dispersion.Researchers have conducted a series of research and literature reviews of the graphene predispersion and consolidation of composites.However,they paid less attention to post-deformation processing.This review summarizes different deformation treatments involved in the preparation process of Gr/Al composites and the evolution of the microstructure during the process.Research on deformation parameters is expected to further improve the properties of Gr/Al composites and would provide a deep understanding of the strengthening effect of graphene.

Mechanical Property of M40J_f/5A06Al Composite at Elevated Temperatures

In this work, aluminum alloy with a high concentration of magnesium （5A06） was reinforced with 55 vol% unidirectional ultra-high modulus and highly graphitized carbon fiber （M40J） using pressure infiltration method. The effect of temperature on the bending strength of the Cf/A1 composites was investigated from room temperature to 500 ℃. The experimental results showed that the strength of M40Jf/5A06A1 composites was not affected by temperature from room temperature to 200 ℃. The bending strength of the composite at 300 ℃ was decreased by 30% compared with that at room temperature. In order to evaluate the extent of interface weakening, the length of fiber pullout was measured. The results showed that the pullout length reached the maximum at 300 and 500 ℃, which indicated weak interface at the corre- sponding temperature. The DSC curve presented obvious heat absorption peak at around 300 ℃, which may be attributed to the dissolution of the interfacial product （A13Mg2） phases at the C/A1 interface. The bending fracture surfaces of the composites after three-point bending tests were observed by SEM, plastic-viscous flow of the matrix were observed at the samples tested at 500 ℃. The predominant mechanisms for high-temperature damage of M40Jf/5A06A1 composites are matrix softening caused by dislocation recovery and interface weakening caused by the dissolution of interfacial products.

Mechanical behaviors and energy absorption properties of Y/Cr and Ce/Cr coated open-cell nickel-based alloy foams

In this study, Y-and Ce-modified Cr coatings applied by pack cementation method were prepared on the surface of open-cell nickel-based alloy foam. The morphologies and microstructures of Y- and Ce-modified Cr coatings with various Y and Ce contents were investigated in detail. Then, the effects of Y and Ce addition on the mechanical properties of open-cell nickel-based alloy foams were analyzed and compared. Simultaneously, the energy absorption capacity and energy absorption efficiency of the Y- and Ce-modified Cr coated alloy foams were discussed and compared at the room and high temperatures. The results show that Cr coatings containing minor amounts of rare earth element （Y and Ce） are well adhered to the nickel-based foam struts. Especially, the microstructure of the 2 wt% Ce-modified Cr coating is denser and uniform. In addition, the compressive strength and plateau stress of Y- and Ce-modified Cr coated alloy foams firstly increase and then decrease by increasing the Y and Ce contents at room and high temperatures. The energy absorption capacity of Y/Cr and Ce/Cr coated alloy foams increases linearly with the strains increasing. The Ce/Cr coated alloy foams can absorb more energy than Y/Cr coated alloy foams in the plateau and densification regions at room temperature. Compared to those at room temperature, the Y- and Ce-modified Cr coated alloy foams show higher energy absorption efficiency when deforma- tion within 10%-30% at high temperature.

Electrical Discharge Machining of Al2024-65 vol%SiC Composites

In the present work, the wire electrical discharge machining（WEDM） process of the 65 vol% SiCp/2024 Al composite prepared by pressure infiltration methods has been investigated. The microstructure of the machined composite was characterized by scanning electron microscope, the average surface roughness（Ra）, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy（TEM） techniques. Three zones from the surface to the interior（melting zone, heat affected zone and un-affected zone） were found in the machined composites, while the face of SiC particles on the surface toward the outside was ‘‘cut＇＇ to be flat. Increase in Al and Si but decrease in C and O were observed in the core areas of the removed particles. Si phase, which was generated due to the decomposition of SiC, was detected after the WEDM process. The irregular and spherical particles were further observed by TEM. Based on the microstructure observation, it is suggested that the machining mechanism of 65 vol% SiCp/2024 Al composite was the combination of the melting of Al matrix and the decomposition of SiC particles.