Classical estimates of the effective thermoelastic properties of copper–graphene composites

Significant research effort is concentrated worldwide on development of graphene-based metal-matrix composites with enhanced thermomechanical properties. In this work, we apply two classical micromechanical mean-field theories to estimate the effective thermoelastic properties that can be achieved in practice for a copper–graphene composite. In the modelling, graphene is treated as an anisotropic material, and the effect of its out-of-plane properties, which are less recognized than the in-plane properties, is studied in detail. To address the severe difficulties in processing of graphene-based metal-matrix composites, the copper–graphene composite is here assumed to additionally contain, due to imperfect processing, particles of graphite and voids. It is shown quantitatively that the related imperfections may significantly reduce the expected enhancement of the effective properties. The present predictions are also compared to the experimental data available in the literature.     

Architecting graphene nanowalls on diamond powder surface

Graphene nanowalls (GNWs) have been synthesized on diamond particle surface by a simple heat-treatment process in vacuum induction furnace. The thickness of the as-grown GNWs is in the range of 20–40 nm, and most of the nanowalls are smooth and flat. The growth of GNWs is affected by the type of catalyst and treatment temperature, and the mixed catalytic effect of Fe and Ni is better than Fe or Ni respectively. High-density GNWs was grown on the diamond particle entire surface, when the heat treatment process is performed at 1473 K. Al/diamond composites with high thermal conductivity of 423 W/(m K) was obtained, which was achieved by the formation of large number of GNWs on the diamond particle surface. Systematic analyses reveal that the growth models such as classic precipitation upon cooling (PUC) model are not applicable to explain this kind of GNW’s growth mechanism. Hereby, an extended PUC model is proposed to interpret the GNW’s growth process on diamond surface.     

Potential and challenges of metal-matrix-composites reinforced with carbon nanofibers and carbon nanotubes

With a continuous improvement of the production techniques for carbon nanofibers and carbon nanotubes along with an improvement of the available qualities of the materials, these reinforcements have been introduced into polymers, ceramics and metals. While in the field of polymers first success stories have been published on carbon nanofiller reinforcements, up to now metals containing these types of nanofillers are still a topic of intensive research. Basically a similar situation were found in those days, when micron sized carbon fibers came on the market. Today many applications of carbon fiber reinforced composites are existing, while metals reinforced with conventional carbon fibers are still only found in niche applications.     

A magnesium alloy matrix composite reinforced with metallic glass

Novel light-weight materials of advanced performance are now experiencing global interest due to the strong need to reduce energy consumption in land and air transportation sectors. Here we report on a novel magnesium alloy matrix composite material. The reinforcing phase in the magnesium alloy is a fine dispersion of metallic glass particles. The composite is sintered from the powder mixture of the alloy and metallic glass at a temperature slightly above the glass transition T_g of the metallic glass particles that is close to the Mg alloy’s solidus temperature. At the compaction temperature, the metallic glass acts as a soft liquid-like binder but upon cooling it becomes the hard reinforcement component of the composite. Processing, microstructure and mechanical properties of the composite are discussed.     

Thermal Expansion of Al Matrix Composites Reinforced with Hybrid Micro-/nano-sized Al_2O_3 Particles

The thermal expansion behavior of aluminum matrix composites reinforced with hybrid(nanometer and micrometer) Al_2O_3 particles was measured between 100 and 600℃ and compared to theoretical models.The results revealed that the nanoparticle concentration had significant effect on the thermal expansion behavior of the composites.For the composites with lower nanoparticle concentration,their coefficient of thermal expansion(CTE) is determined by a stress relaxation process.While for the composites with higher nanoparticle concentration,their CTE is determined by a percolation process.     

Compressive characteristics of metal matrix syntactic foams

The compressive behaviour of eight different metal matrix syntactic foams (MMSFs) are investigated and presented. The results showed that the engineering factors as chemical compositions of the matrix material, the size of the microballoons, the previously applied heat treatment and the temperature of the compression tests have significant effects on the compressive properties. The smaller microballoons with thinner wall ensured higher compressive strength due to their more flawless microstructure and better mechanical stability. According to the heat treatments, the T6 treatments were less effective than expected; the parameters of the treatment should be further optimised. The elevated temperature tests revealed ∼30% drop in the compressive strength. However, the strength remained high enough for structural applications; therefore MMSFs are good choices for light structural parts working at elevated or room temperature. The chemical composition - microballoon type - heat treatment combinations give good potential for tailoring the compressive characteristics of MMSFs.     

Micro-mechanical modeling the densification behavior of titanium metal matrix composites

Vacuum hot pressing has been used for the development of Ti-MMCs using foil–fiber–foil method, and a unified micro-mechanical model has been presented to determine the densification behavior. The effects of processing conditions on the consolidation, together with microstructural evolutions of the materials have been investigated. The explicit representation of fiber array, which is coupled with deformation behavior of matrix materials, is modeled in finite element simulation to determine the effect of geometrical arrangements on densification process. The approach is then used to model the densification behavior of porous plastic materials using the parameters obtained, and comparisons are made with experimental data. As shown by the results, either increasing temperature or pressure leads to increasing densification rate but the conditions should be determined by the precisely controlled geometrical arrangements with processing conditions. Further experimental investigation of the densification behavior of SiC/Ti–6Al–4V composites using thermo-acoustic emission analysis has been performed, and the results obtained are compared with the model predictions. Good comparisons are achieved.     

An experimental study on the in situ strength of SiC fibre in unidirectional SiC/Al composites

In order to investigate the effect of strain rate and high temperature exposure on the mechanical properties of the fibre in the unidirectional fibre reinforced metal-matrix composite, in situ SiC fibre bundles are extracted from two kinds of SiC/Al composite wires, which are heat-treated at two different temperatures (exposed in the air at 400 and 600 °C for 40 min after composition). Tensile tests for these two fibre bundles are performed at different strain rates (quasi-static test: 0.001 s⁻¹, dynamic test: 200, 700, and 1200 s⁻¹) and the stress–strain curves are obtained. The experimental results show that their mechanical properties are rate-dependent, the modulus E, strength σ_b and unstable strain _b (the strain corresponding to σ_b) all increase with increasing strain rate. Compared with the mechanical properties of the original SiC fibre, those of the two in situ fibres degrade to some extent, the degradation of the in situ fibre extracted from the composite wire exposed at 600 °C (hereafter referred to as in situ fibre 2) is more serious than that of the in situ fibre extracted from the composite wire exposed at 400 °C (hereafter referred to as in situ fibre 1). The mechanism of the degradation is investigated. A bi-modal Weibull statistical constitutive equation is established to describe the stress–strain relationship of the two in situ fibre bundles. The simulated stress–strain curves agree well with the experimental results.     

Tensile strength of axially loaded unidirectional Nextel 610™ reinforced aluminium: A case study in local load sharing between randomly distributed fibres

The tensile failure of unidirectional alumina fibre reinforced aluminium is studied in uniaxial loading along the fibre axis. The tensile strength is measured as a function of matrix yield strength, which is varied by varying the testing temperature, from RT to 600 °C. Over the range of matrix yield strength (i.e., of temperature) examined, the fracture mode remains brittle. Batdorf’s (J Reinforced Plastics Compos 1982;1:153-164) simple ideal local load-sharing model describes well the observed behaviour, under the condition that it be adapted to account for the actual number of nearest neighbours characteristic of the fibre distribution in the composite. This is shown to be close to three, i.e., at variance with the usually assumed idealized hexagonal or square fibre arrangement patterns.     

Novel wear-resistant materials - Carbon fiber reinforced low-carbon Stellite alloy composites

This paper reports the design and development of a class of new composite materials, which are low-carbon Stellite alloy matrices reinforced with carbon fibers. The focus of the research is to compare the different effects of carbon fibers versus carbides on Stellite alloys. Stellite 25 was selected as the matrix because of its very low carbon content (0.1 wt.%), thus minimal carbide volume fraction. The composite specimens are fabricated using the hot isostatic pressing and sintering techniques. The microstructures of the specimens are examined with optical microscopy in order to identify the possible carbide formation from the carbon fibers. The material characterization of the specimens is achieved through hardness test, sliding wear test and corrosion test. These novel materials exhibit superior properties compared to existing Stellite alloys and are expected to spawn a new generation of materials used for high temperature, severe corrosion, and wear resistant applications in various industries.     

Fabrication of aluminum matrix composites with enhanced mechanical properties reinforced by in situ generated MgAl2O4 whiskers

Aluminum matrix composite reinforced by in situ generated single crystalline MgAl₂O₄ whiskers was fabricated by chemical synthesis method in an Al-Mg-H₃BO₃ system. A large number of MgAl₂O₄ whiskers were generated during the sintering process and distributed homogeneously in the Al matrix. The whiskers penetrate into the matrix grains to form the framework of the materials, leading to an incredible increase in mechanical properties of the composites. The generation mechanism of the MgAl₂O₄ whiskers was also discussed.     

Thermal mismatch induced disorder of beta-eucryptite and its effect on thermal expansion of beta-eucryptite/Al composites

In this paper, beta-eucryptite/Al composites with different volume fraction of beta-eucryptite particles (Euc/Al) were prepared by spark plasma sintering process. The microstructures of the composites were studied by transmission electron microscope. The phase compositions and thermal physical properties of the composites were analyzed by X-ray diffraction and thermal dilatometer. The change of peaks intensity ratio of beta-eucryptite (2 0 0) to beta-eucryptite (2 0 2) plane after 12 months of aging in room condition and at elevated temperature (40–300 °C) was used to characterize the disorder of beta-eucryptite, which was due to the high thermal mismatch stress in the composite. The results indicate that the disorder of beta-eucryptite can recover when the thermal mismatch stress in beta-eucryptite is released after 12 months of aging or at elevated temperature. Both compressive stress and tensile stress could induce the disorder of eucryptite (2 0 0) plane. The relationships among CTE behavior of the composite, the phase transformation of beta-eucryptite and the thermal mismatch stress were discussed.     

Using integrated hybrid (Al + CNT) reinforcement to simultaneously enhance strength and ductility of magnesium

Magnesium nano-composites containing hybrid aluminium-carbon nanotube (Al-CNT) reinforcement were synthesized through powder metallurgy route using microwave assisted rapid sintering technique followed by hot extrusion. Compared to monolithic Mg, microstructural characterization revealed reasonably uniform distribution of Al-CNT particles in the matrix and reduction in average matrix grain size in the case of hybrid nano-composites. Compared to monolithic Mg, the Mg/Al-CNT nano-composites exhibited higher elastic modulus, strength and failure strain up to 1.00 wt.% Al content. The CNT content was kept constant at 0.18 wt.%. Among the different nano-composite formulations, the Mg/1.00Al-0.18CNT nano-composite exhibited the best improvement in elastic modulus (E), tensile yield strength (0.2%YS), ultimate tensile strength (UTS) and failure strain (up to +3.6%, +38%, +36% and +42%, respectively) compared to pure Mg. The effect of hybrid Al-CNT reinforcement integration on the enhancement of mechanical properties of Mg is investigated in this paper.