Effect of Defects and Oxidation on CNT-Copper Interface: First-Principles Calculation and Experiment.

In this paper, the effects of carbon nanotube defects and a copper surface oxide layer on a carbon nanotube-copper interface were studied via first-principles. A defect-free CNT-Cu interface, Stone-Wales defect CNT-Cu interface, single-hole and double-hole defect CNT-Cu interface, and Cu2O-Cu interface were simulated and calculated. By simulating the differential charge density, atomic population, bond population and density of states of the interface model, the effects of various defects on the interface bonding and electrical conductivity of the composites during the preparation of the CNT-reinforced copper matrix composites were analyzed, which provided theoretical guidance for the preparation of CNT/Cu composites. After that, copper matrix composites with different CNT defect contents were prepared via different rolling deformation processes. Their hardness and electrical conductivity were tested, and the results were consistent with the results obtained via the first-principles calculations.

Study on the Preparation of Network Ti-N/Ti Composites by Nitridation of Ti Powders.

Composite structure design is an important way to improve reinforcement strengthening efficiency. The dispersion of the external reinforcement is often not uniform enough, however, and it is agglomerated in the matrix, which cannot uniformly and effectively bear the load. The interconnected reinforcement network prepared by the in-situ self-growth method is expected to obtain higher material properties. In this paper, the TiN shell was formed on the surface of Ti powder by the in-situ nitriding method, and then the network TiN/Ti composites were prepared by sintering. In the control group, TiN was dispersed by mechanical ball milling, and it was found that TiN powder was coated on the surface of Ti particles, and the sintered TiN/Ti composites formed a discontinuous structure with a great deal of TiN agglomeration. A uniform TiN nitride layer of 5~7 µm was formed on the surface of Ti powder by the in-situ nitriding method, and a connected TiN network was formed in the sintered Ti-N/Ti composites. The composites prepared by nitriding have higher compressive strength, hardness, and plasticity. The hardness of the Ti-N/Ti composite is 685.7 HV and the compressive strength is 1468.5 MPa. On this basis, the influence of the connected TiN structure on the material properties was analyzed, which provided theoretical guidance for the structural design of the network structure-reinforced titanium matrix composites.

Microstructure and Mechanical Properties of Core-Shell B4C-Reinforced Ti Matrix Composites.

Composite material uses ceramic reinforcement to add to the metal matrix to obtain higher material properties. Structural design is an important direction of composite research. The reinforcement distribution of the core-shell structure has the unique advantages of strong continuity and uniform stress distribution. In this paper, a method of preparing boron carbide (B4C)-coated titanium (Ti) powder particles by ball milling and preparing core-shell B4C-reinforced Ti matrix composites by Spark Plasma Sintering was proposed. It can be seen that B4C coated on the surface of the spherical Ti powder to form a shell structure, and B4C had a certain continuity. Through X-ray diffraction characterization, it was found that B4C reacted with Ti to form layered phases of titanium boride (TiB) and titanium carbide (TiC). The compressive strength of the composite reached 1529.1 MPa, while maintaining a compressive strain rate of 5%. At the same time, conductivity and thermal conductivity were also characterized. The preparation process of the core-shell structure composites proposed in this paper has high feasibility and universality, and it is expected to be applied to other ceramic reinforcements. This result provides a reference for the design, preparation and performance research of core-shell composite materials.

Effect of Interfacial Strength on Mechanical Behavior of Be/2024Al Composites by Pressure Infiltration.

In this paper, two kinds of Be/2024Al composites were prepared by the pressure infiltration method using two different beryllium powders as reinforcements and 2024Al as a matrix. The effect of interfacial strength on the mechanical behavior of Be/2024Al composites was studied. Firstly, the microstructure and mechanical properties of the two composites were characterized, and then the finite element analysis (FEA) simulation was used to further illustrate the influence of interfacial strength on the mechanical properties of the two Be/2024Al composites. The mechanical tensile test results show that the tensile strength and elongation of the beryllium/2024Al composite prepared by the blocky impact grinding beryllium powder (blocky-Be/2024Al composite) are 405 MPa and 1.58%, respectively, which is superior to that of the beryllium/2024Al composite prepared by the spherical atomization beryllium powder (spherical-Be/2024Al composite), as its strength and elongation are 331 MPa and 0.38%, respectively. Meanwhile, the fracture of the former shows brittle fracture of beryllium particles and ductile fracture of aluminum, while the latter shows interface debonding. Further FEA simulation illustrates that the interfacial strength of the blocky-Be/2024Al composite is 600 MPa, which is higher than that of the spherical-Be/2024Al composite (330 MPa). Therefore, it can be concluded that the better mechanical properties of the blocky-Be/2024Al composite contribute to its stronger beryllium/aluminum interfacial strength, and the better interfacial strength might be due to the rough surface and microcrack morphology of blocky beryllium particles. These research results provide effective experimental and simulation support for the selection of beryllium powder and the design and preparation of high-performance beryllium/aluminum composites.

Effect of Volume Fraction of Reinforcement on Microstructure and Mechanical Properties of In Situ (Ti, Nb)B/Ti2AlNb Composites with Tailored Three-Dimensional Network Architecture.

The mechanical properties of (Ti, Nb)B/Ti2AlNb composites were expected to improve further by utilizing spark plasma sintering (SPS) and inducing the novel three-dimensional network architecture. In this study, (Ti, Nb)B/Ti2AlNb composites with the novel architecture were successfully fabricated by ball milling the LaB6 and Ti2AlNb mixed powders and subsequent SPS consolidation. The influence of the (Ti, Nb)B content on the microstructure and mechanical properties of the composites was revealed by using the scanning electron microscope (SEM), transmission electron microscopy (TEM) and electronic universal testing machine. The microstructural characterization demonstrated that the boride crystallized into a B27 structure and the α2-precipitated amount increased with the (Ti, Nb)B increasing. When the (Ti, Nb)B content reached 4.9 vol%, both the α2 and reinforcement exhibited a continuous distribution along the prior particle boundaries (PPBs). The tensile test displayed that the tensile strength of the composites presented an increasing trend with the increasing (Ti, Nb)B content followed by a decreasing trend. The composite with a 3.2 vol% reinforcement had the optimal mechanical properties; the yield strengths of the composite at 25 and 650 °C were 998.3 and 774.9 MPa, showing an 11.8% and 9.2% improvement when compared with the Ti2AlNb-based alloy. Overall, (Ti, Nb)B possessed an excellent strengthening effect and inhibited the strength weakening of the PPBs area at high temperatures; the reinforcement content mainly affected the mechanical properties of the (Ti, Nb)B/Ti2AlNb composites by altering the α2-precipitated amount and the morphology of (Ti, Nb)B in the PPBs area. Both the continuous precipitation of the brittle α2 phase and the agglomeration of the (Ti, Nb)B reinforcement dramatically deteriorated the mechanical properties.

Effect of Reinforcement Size on Mechanical Behavior of SiC-Nanowires-Reinforced 6061Al Composites.

In the present study, the effects of SiC nanowires (SiCnws) with diameters of 100 nm, 250 nm and 450 nm on the microstructure and mechanical behavior of 20 vol.% SiCnws/6061Al composites prepared by pressure infiltration were studied. It was found that the interface between SiCnws and Al matrix was well bonded, and no interface product was found. The thicker SiCnws are beneficial to improve the density. In addition, the bamboo-like and bone-like morphologies of SiCnws produce a strong interlocking effect between SiCnws and Al, which helps to improve the strength and plasticity of the material. The tensile strength of the composite prepared by SiCnws with a diameter of 450 nm reached 544 MPa. With a decrease in the diameter of SiCnws, the strengthening effect of SiCnws increases. The yield strength of SiCnws/6061Al composites prepared by 100 nm is 13.4% and 28.5% higher than that of 250 nm and 450 nm, respectively. This shows that, in nano-reinforced composites, the small-size reinforcement has an excellent improvement effect on the properties of the composites. This result has a guiding effect on the subsequent composite structure design.

Microstructure Evolution of Graphene and the Corresponding Effect on the Mechanical/Electrical Properties of Graphene/Cu Composite during Rolling Treatment.

Rolling enables the directional alignment of the reinforcements in graphene/Cu composites while achieving uniform graphene dispersion and matrix grain refinement. This is expected to achieve a breakthrough in composite performance. In this paper, the process parameters of rolling are investigated, and the defects, thickness variations of graphene and property changes of the composite under different parameters are analyzed. High-temperature rolling is beneficial to avoid the damage of graphene during rolling, and the prepared composites have higher electrical conductivity. The properties of graphene were investigated. Low-temperature rolling is more favorable to the thinning and dispersion of graphene; meanwhile, the relative density of the composites is higher in the low-temperature rolling process. With the increase of rolling deformation, the graphene defects slightly increased and the number of layers decreased. In this paper, the defect states of graphene and the electrical conductivity with different rolling parameters is comprehensively investigated to provide a reference for the rolling process of graphene/copper composites with different demands.

Evaluation of the Effects of Surface Treatment Methods on the Properties of Coral Aggregate and Concrete.

Coral concrete has low cost and convenient materials, making it an excellent raw material for processing. However, its lower strength limits the application of coral concrete. Surface modification is expected to increase the properties of porous coral concrete. In this study, single and compound modification treatments were applied to the surface of a coral aggregate to improve its properties for promoting the mechanical performance of coral concrete. The results showed that the micro-aggregate effect and pozzolanic activity of granulated blast furnace slag (GBFS) and the permeability and polycondensation of sodium silicate (SS) could be mutually promoted. The GBFS and SS could effectively fill the pores of the coral aggregate, enhancing the properties of the aggregate, such as density and load-bearing capacity, and reducing the water absorption and crushing index by more than 50%. GBFS and SS could intensify and accelerate the hydration of cement, and generate a large number of hard hydration products at the interfacial transition zone (ITZ), which could strengthen the bonding between the aggregate and mortar, improving the strength of the ITZ. The compressive strength of the coral concrete was significantly increased.

Properties of Concrete Prepared with Silane Coupling Agent-Impregnated Coral Aggregate and Coral Concrete.

Silane coupling agent (SCA), a kind of organic solvent, was introduced to improve the performance of coral coarse aggregates and enhance the interfacial adhesion between the inorganic coral aggregate and the cement paste of coral concrete. The crushing indicator and water absorption of the coral aggregates over various dipping times were measured, and the slump, interface microhardness, and compressive strength of coral concrete tested. The microscopic appearances of the coral concrete before and after modification were analyzed based on SEM images. The experimental results indicate that SCA can effectively reduce the crushing indicator and water absorption of coral coarse aggregates, and the modification performance becomes better over time. SCA facilitates the generation of chemical forces between the coral aggregates and cement mortars, improves adhesion between the aggregates and mortars, augments the microhardness of the interface, and increases the compressive strength. According to the microscopic appearance of the treated and untreated coral aggregate interfaces, the aggregates and the mortars are in closer combination after modification.

Design and Evaluation of an Ultrahigh-Strength Coral Aggregate Concrete for Maritime and Reef Engineering.

In this paper, an ultrahigh-strength marine concrete containing coral aggregates is developed. Concrete fabricated from marine sources is considered an effective and economical alternative for marine engineering and the construction of remote islands. To protect sea coral ecosystems, the coral aggregates used for construction are only efflorescent coral debris. To achieve the expected mechanical performance from the studied concrete, an optimal mixture design is conducted to determine the optimal proportions of components, in order to optimize the compressive strength. The mechanical properties and the autogenous shrinkage, as well as the heat flow of early hydration reactions, are measured. The hydration products fill up the pores of coral aggregates, endowing our concrete with flowability and self-compacting ability. The phases in the marine concrete are identified via X-ray diffraction analysis. The 28-day compressive and flexural strength of the developed marine concrete achieve 116.76 MPa and 18.24 MPa, respectively. On account of the lower cement content and the internal curing provided by coral aggregates, the volume change resulting from autogenous shrinkage is only 63.11% of that of ordinary reactive powder concrete.

Spark Plasma Sintering of AlN/Al Functionally Graded Materials.

In this paper, six-layer AlN/Al gradient composites were prepared by a spark plasma sintering process to study the influences of sintering temperature and holding time on the microstructure and mechanical properties. The well-bonded interface enables the composite to exhibit excellent thermal and mechanical properties. The hardness and thermal expansion properties of the composite exhibit a gradient property. The hardness increased with the volume fraction of AlN while the CTE decreased as the volume fraction of AlN. The thermal expansion reaches the lowest value of 13-14 ppm/K, and the hardness reaches the maximum value of 1.25 GPa, when the target volume fraction of AlN is 45%. The simulation results show that this gradient material can effectively reduce the thermal stress caused by the mismatch of the thermal expansion coefficient as a transmitter and receiver (T/R) module. This paper attempts to provide experimental support for the preparation of gradient Al matrix composites.

Study on the Evolution of Graphene Defects and the Mechanical and Thermal Properties of GNPs/Cu during CVD Repair Process.

Graphene has extremely high theoretical strength and electrothermal properties, and its application to Cu-based composites is expected to achieve a breakthrough in the performance of existing composites. As a nano-reinforced body, graphene often needs a long time of ball milling to make it uniformly dispersed, but the ball milling process inevitably brings damage to the graphene, causing the performance of the composite to deviate from expectations. Therefore, this paper uses CH4 as a carbon source to repair graphene through a CVD process to prepare low-damage graphene/Cu composites. The process of graphene defect generation was studied through the ball milling process. The effects of defect content and temperature on the graphene repair process were studied separately. The study found that the graphene defect repair process, the decomposition process of oxygen-containing functional groups, and the deposition process of active C atoms existed simultaneously in the CVD process. When the repair temperature was low, the C atom deposition process and the oxygen-containing functional group decomposition process dominated. In addition, when the repair temperature is high, the graphene defect repair process dominated. 3 wt% graphene/Cu composites were prepared by pressure infiltration, and it was found that the bending strength was increased by 48%, the plasticity was also slightly increased, and the thermal conductivity was increased by 10-40%. This research will help reduce graphene defects, improve the intrinsic properties of graphene, and provide theoretical guidance for the regulation of C defects in composites.