Enhanced tensile properties of aluminium matrix composites reinforced with graphene encapsulated SiC nanoparticles

Due to a high propensity of nano-particles to agglomerate, making aluminium matrix composites with a uniform dispersion of the nano-particles using liquid routes is an exceptionally difficult task. In this study, an innovative approach was utilised to prevent agglomeration of nano-particle by encapsulating SiC nano-particles using graphene sheets during ball milling. Subsequently, the milled mixture was incorporated into A356 molten alloy using non-contact ultrasonic vibration method. Two different shapes for graphene sheets were characterised using HRTEM, including onion-like shells encapsulating SiC particles and disk-shaped graphene nanosheets. This resulted in 45% and 84% improvement in yield strength and tensile ductility, respectively. The former was ascribed to the Orowan strengthening mechanism, while the latter is due primarily to the fiber pull-out mechanism, brought about by the alteration of the solidification mechanism from particle pushing to particle engulfment during solidification as a consequence of high thermal conductive graphene sheets encapsulating SiC particles.     

Effect of tungsten addition on thermal conductivity of graphite/copper composites

Graphite/copper composites with high thermal conductivity were fabricated by tungsten addition, which formed a thin tungsten carbide layer at the interface. The microstructure and thermal conductivity of the composite material were studied. The results indicated that the insertion of tungsten carbide layer obviously suppressed spheroidization of copper coating on the graphite particles during the sintering process, and decreased the interfacial thermal resistance of the composites. Compared with the graphite/copper composites without tungsten, the thermal conductivity of the obtained composites was increased by 43.6%.     

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.     

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.     

Size dependent strengthening mechanisms in carbon nanotube reinforced metal matrix composites

The matrix grain size plays a dual role in metal matrix composites (MMCs). Contrary to enhance the strength of matrix, grain refinement can weaken the thermal expansion mismatch strengthening induced by the reinforcement. In this article, a dislocation density based model is developed to describe the factors affecting the strengthening mechanisms in Carbon nanotube (CNT)-reinforced MMCs with different matrix grain sizes. Two kinds of thermal expansion mismatch strengthening mechanisms are considered, i.e., geometrically necessary dislocations (GNDs) are distributed in entire matrix and GNDs are limited in dislocation punched zones (DPZs). In addition, comparisons between the predictions and some available experimental results are also performed.     

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.     

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.     

Interfacial analysis between Mg matrix and carbon nanotubes in Mg-6 wt.% Al alloy matrix composites reinforced with carbon nanotubes

Magnesium containing 6 wt.% aluminum alloy composites reinforced with carbon nanotubes were fabricated with powder metallurgy based wet-processing. Yield stress and tensile strength were successfully improved by the addition of carbon nanotubes. Field emission-transmission electron microscopy microstructural analysis clarified that needle-like ternary carbides of Al₂MgC₂ were synthesized at some interfaces between magnesium matrix and carbon nanotubes, and the other interfaces were clean without any other materials or defects. Tensile loading transfer from magnesium matrix to carbon nanotubes was effectively strengthened by both the production of Al₂MgC₂ compounds and the clean interface between magnesium matrix and carbon nanotubes.     

Interfacial design of Cu/SiC composites prepared by powder metallurgy for heat sink applications

In order to dissipate the heat generated in electronic packages, suitable materials must be developed as heat spreaders or heat sinks. Metal matrix composites (MMCs) offer the possibility to tailor the properties of a metal (Cu) by adding an appropriate reinforcement phase (SiC) to meet the demands for high thermal conductivities in thermal management applications. Copper/SiC composites have been produced by powder metallurgy. Silicon carbide is not stable in copper at the temperature needed for the fabrication of Cu/SiC. The major challenge in development of Cu/SiC is the suppression of this reaction between copper and SiC. Improvements in bonding strength and thermo-physical properties of the composites have been achieved by a vapour deposited molybdenum coating on SiC powders to control the detrimental interfacial reactions.     

Investigation of mechanical and tribological behaviors of multilayer graphene reinforced Ni3Al matrix composites

Multilayer graphene (MLG) shows an attractive prospect for the demanding engineering applications. This paper reports the mechanical and tribological properties of MLG reinforced Ni₃Al matrix composites (NMCs) under dry sliding at varying sliding speed. The hardness and elastic modulus of the NMCs are significantly influenced with MLG content. It is found that the hardness and elastic modulus of the NMCs are found to be increased by increasing MLG content up to 1.0 wt.%, while decreased when MLG content is above 1.0 wt.%. Tribological experiments suggest that MLG can dramatically improve the wear resistance and decrease the friction coefficient of the NMCs. Such marked improvement of wear resistance is attributed to the reinforcing mechanisms of MLG, such as crack deflection and pull-out, and reduction of friction coefficient is related to the formation of a tribofilm on the sliding contact surface.     

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.     

The influence of thermal residual stresses and thermal generated dislocation on the mechanical response of particulate-reinforced metal matrix nanocomposites

Due to thermal expansion mismatch between reinforcing particles and matrix, thermal induced dislocations are generated in metal matrix nanocomposites (MMNCs) during cooling down from the processing temperature. These dislocations have been identified as an important strengthening mechanism in particulate-reinforced MMNCs. In this study, the development of thermal residual stresses and thermal induced dislocations in MMNCs are predicted using discrete dislocation simulation, assuming the whole material is under uniform temperature change. Shear deformation is applied after the composites are cooled down to room temperature and the influence of thermal residual stresses and thermal generated dislocation on the overall response of particulate-reinforced MMNCs are investigated. The results show that the thermal residual stresses are high enough to generate dislocations and the dislocation density is higher in the interfacial region than the rest of the matrix. The predicted mechanical behavior of the MMNCs matches the experimental results better when thermal residual stresses are included in the simulations.     

Coefficients of thermal expansion for single crystal piezoelectric fiber composites

Piezoelectric fiber composites were developed to overcome drawbacks of typical monolithic piezoceramic (PZT) actuators. Although piezoelectric fiber composites had many improvements over the monolithic PZT, there are still improvements. Thus, the single crystal piezoelectric fiber composite actuator is proposed. Single crystal piezoelectric materials such as PMN-PT have larger piezoelectric strain constants, higher bandwidth and higher energy density than polycrystalline counterparts. Piezoelectric fiber composites can improve the performance of various structures, and can be subject to wide temperature range where the thermoelastic behavior is important. Therefore, this paper studies the coefficients of thermal expansion (CTE) for single crystal piezoelectric fiber composites. The Macro Fiber Composite (MFC) as the piezoelectric fiber composite is considered. To calculate the effective properties of two orthotropic layers of the MFC, PMN-PT(or PZT)/epoxy and copper/epoxy layers, the rule of mixture is adopted. With the effective properties known for each layers, the two CTE of the MFC actuator are obtained from the classical lamination theory considering thermal effects. The difference of the CTE between the single crystal MFC and the standard MFC is studied.     

Evaluation of thermal stability of ultrafine grained aluminium matrix composites reinforced with carbon nanotubes

The effect of carbon nanotubes on the thermal stability of ultrafine grained aluminium alloy processed by the consolidation of nano-powders obtained by mechanical alloying was evaluated via measurements of grain size and mechanical property changes upon annealing at various temperatures. It was found that the grain size of the samples containing carbon nanotubes is stable up to high temperatures and even after annealing at 450 °C (0.7T_m) no evident grain growth was observed. The limited grain boundary migration was attributed to the presence of entangled networks of carbon nanotubes located at grain boundaries and to the formation of nanoscale particles of aluminium carbide Al₄C₃. It was also revealed that carbon nanotubes decompose at a relatively low temperature of 450 °C and form fine Al₄C₃ precipitates. This transformation does not significantly affect the mechanical properties due to the nanoscale size of the carbides.     

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.     

Wear characteristics of hybrid aluminium matrix composites reinforced with graphite and silicon carbide particulates

This investigation studies the dry sliding wear behaviour of Al matrix composites reinforced with Gr and SiC particulate up to 10%, to study the effect of % reinforcement, load, sliding speed and sliding distance on stir cast Al–SiC–Gr hybrid composites, Al–Gr and Al–SiC composites. Parametric studies indicate that the wear of hybrid composites has a tendency to increase beyond% reinforcement of 7.5% as its values are 0.0242 g, 0.0228 g and 0.0234 g respectively at 3%, 7.5% and 10% reinforcement. The corresponding values are 0.0254 g, 0.0240 g and 0.0242 g in Al–Gr composites and 0.0307 g, 0.0254 g and 0.0221 g in Al–SiC composites, clearly indicating that hybrid composites exhibit better wear characteristics. Increase of speed reduces wear and increase of either load or sliding distance or both increases wear. Statistical analysis has revealed interactions among load, sliding speed and sliding distance in composites with Gr particulates.     

Enhanced strength and excellent transport properties of a superaligned carbon nanotubes reinforced copper matrix laminar composite

Superaligned carbon nanotube (SACNT) reinforced copper matrix laminar composites have been fabricated by means of the traditional copper sulfate electroplating process. The mechanical properties and transport properties of the Cu/SACNT composites with different SACNT content have been studied systematically, and the experimental results show that the as-prepared composites possess a better comprehensive performance than pure copper. The simple rule of mixtures (ROM) has been used to estimate the potential maximum properties of the Cu/SACNT composites. The Cu/SACNT composite is considered to be a promising material for electronics and communications applications.     

Estimation of thermal conductivity for polypropylene/hollow glass bead composites

The thermal conductivity of hollow glass bead (HGB)-filled polypropylene (PP) composites was estimated using the thermal conductivity equation of inorganic hollow microsphere-filled polymer composites published in the previous paper. The estimations were compared with the measured data of the PP composites filled with two kinds of HGB with different size (the mean diameter was respectively 35 μm and 70 μm). The results showed that the predictions of the thermal conductivity were in good agreement with the measured data except to individual data points. Furthermore, both the estimated and measured thermal conductivity decreased roughly linearly with increasing the HGB volume fraction when the HGB volume fraction was less than 20%; the influence of the particle diameter on the thermal conductivity was insignificant.     

Modeling of the thermal conductivity and thermomechanical behavior of diamond reinforced composites

Semi-analytical Mori-Tanaka methods and numerical models for studying the overall thermal conduction behavior of metal matrix composites reinforced by diamond particles are presented, special emphasis being put on the effects of finite interfacial conductances. Good agreement between the simulation approaches is obtained and the influence of particle shapes and homogeneous vs. inhomogeneous interfacial conductances on local and global responses is studied. Analogous methods are applied to modeling the elastic and thermoelastoplastic behavior of diamond reinforced metals.