Mechanical properties of squeeze-cast zinc alloy matrix composites containing α-alumina fibres

α-Alumina fibre-reinforced ZA12 alloy matrix composites, with fibre volume fractions ranging from 7.5 to 30%, were manufactured by squeeze casting. The alumina fibres were homogeneously distributed in the matrix and had a planar-random orientation. Mechanical properties of the composites such as hardness, tensile strength, Young's modulus, elongation and wear resistance were measured and the effect of fibre volume fraction on these properties was investigated. At room temperature the hardness, Young's modulus and wear resistance increased with increasing volume fraction of alumina fibres, but the other properties were inferior. At elevated temperature (above 80°C) the tensile strengths of the composites were higher than that of the matrix alloy.     

Prediction of Young's modulus of particulate two phase composites

Abstract

A new approach for predicting the Young's modulus of two phase composites has been proposed based on a topological transformation and the mean field theory. The new approach has been applied to Co/WC_p,Al/SiC_p, and glass filled epoxy composites. It is shown that the new theoretical predictions are well within the Hashin and Shtrikman lower and upper bounds (the HS bounds) and are in closer agreement with the experimental results for the corresponding composite systems than both the HS bounds and the predictions of the mean field theory. An advantage of the present approach over other continuum approaches is that it can predict not only the effect of volume fraction of the reinforcing phase, but also the effects of microstructural parameters such as grain shape and phase distribution on the stiffness of composites. It is also shown that the classical linear law of mixtures is a specific case (where the reinforcing phase is continuous and perfectly aligned) of the present approach. In contrast to the classical linear law of mixtures, the present approach can be applied to a two phase composite having any volume fraction, grain shape, and phase distribution. It is shown that in a particulate composite having a given volume fraction of reinforcement, the Young's modulus of the composite increases with increasing contiguity of the constituent phases and this increment is dependent on the stiffness ratio of the constituent phases. Furthermore, the present approach can provide a simple and effective solution to the problem of interaction between particles of the same phase.

MST/1587     

Alternating-current electrical properties of random metal-insulator composites

The complex a.c. impedance of three different random metal-insulator composites near their percolation threshold has been studied. These three metal-insulator systems include different shapes of nickel particles (filamentary and nodular shapes) in a matrix of polypropylene and silver particles in the matrix of potassium chloride. By using different metal-insulator structures and phases it is possible to elucidate the effect of different metal particle shapes and types of insulator phase on the electrical properties of these composites near their percolation threshold. Electrical properties, including d.c. conductivity, a.c. conductance, capacitance and dielectric loss tangent, of these metal-insulator composites as a function of metal volume fraction and frequency (5 Hz to 13 MHz) are presented. The results are correlated with structural characterization of these composites and are used to examine the applicability of different electrical transport models on these composite materials. The effect of different metal particle shapes on the percolation threshold and the power-law dependent percolation phenomenon is also discussed.     

Mechanical threshold of cementitious materials at early age

At early age, the mechanical characteristics of concrete, such as Young's modulus, follow a rapid rate of change. If strains are restricted or in the event of strain gradients, tensile stresses are generated and there is a risk of cracks occurring. Besides relaxation, change in Young's modulus as a function of the degree of hydration is a major parameter for the modeling of this phenomenon. In this evolution, a threshold of the degree of hydration has to be taken into account, below which concrete displays negligible stiffness. For cement pastes, a simplified hydration model shows that percolation of the solid phases depends on the w/c ratio, which is in accordance with experimental results. On the other hand, for mortar or concrete, the presence of aggregates means that the solid volumetric fraction is such that percolation is observed before hydration occurs. Therefore another parameter is introduced: cohesion due to hydration products. By coupling our model with a finite-element code (CAST3M), it is shown that the threshold for Young's modulus in mortar is almost independent of the w/c ratio, which is in accordance with experimental results.     

Distributions of elastic moduli in mechanically percolating composites

Power-law percolation models contain very little mechanics other than the theoretical or simulated value of a percolation threshold, the volume fraction where a connected microstructure forms. For mechanical percolation these theoretical values do not correspond well to experimental results and so the models are commonly used empirically; results are correlative rather than predictive. In recent work, the effective elastic properties of a model polymer nanocomposite were approximated using a computational micromechanics model within a Monte Carlo framework. Significantly, the statistical averages resulting from these simulations displayed distinct percolation-like behavior. Of equal interest is the distribution of properties that resulted from the randomly simulated microstructures. This strongly suggests that mechanical percolation in nanocomposites is the result of a combination of microstructural mechanisms. Analysis aimed at determining which microstructure produces what response is a challenging task if microstructure is the random variable. In this work, the effective composite properties are considered as the random variable; probability distribution functions (PDFs) of the properties at discrete volume fractions are developed using the Principle of Maximum Informational Entropy. The evolution of these PDFs with increasing volume fraction helps visualize and track the significant property changes that result from microstructural randomness.     

Effect of fibre volume fraction on fatigue behaviour of glass fibre reinforced composite

The aim of this paper is to study the fatigue behavior of GFRP composites manufactured by vacuum bagging process by varying the volume fraction. Constant‐amplitude flexural fatigue tests were performed at zero mean stress, i.e. a cyclic stress ratio R=?1 by varying the frequency of the testing machine. The relationship between stiffness degradation rate and fibre volume fraction, was observed, and the influence of volume fraction on the tensile strength was also investigated. The results show that, as the volume fraction increases the stiffness degradation rate initially decreases and then increases after reaching a certain limit for the volume fraction. Graph between volume fraction and Young's modulus shows that as the volume fraction increases Young's modulus also increases and reaches a limit and then it decreases with further increase in volume fraction, due to the increase in fibre content which changes the material properties of the composite material. The obtained results are in agreement with the available results.     

Electrical conductivity of random silver-potassium chloride composites

The complex alternating current (a.c.) impedance of random silver-potassium chloride (Ag-KCl) composite specimens near the percolation threshold has been measured in the frequency range 5 Hz to 13 MHz. The impedance of these composites as a function of metal volume fraction and direct current (d.c.) potential field are presented. This a.c. response, which is correlated with structural and compositional characterization of these composites, is used to examine the applicability of several different theoretical models as well as to determine the important factors in the electrical conductivity of such materials.     

Pressure infiltration casting process and thermophysical properties of high volume fraction SiCp/Al metal matrix composites

Abstract

SiC_p/Al composites containing high volume fraction SiC particles were fabricated using a pressure infiltration casting process, and their thermophysical properties, such as thermal conductivity and coefficient of thermal expansion (CTE), were characterised. High volume fraction SiC particulate preforms containing 50–70 vol.-%SiC particles were fabricated by ball milling and a pressing process, controlling the size of SiC particles and contents of an inorganic binder. 50–70 vol.-%SiC_p/Al composites were fabricated by high pressure infiltration casting an Al melt into the SiC particulate preforms. Complete infiltration of the Al melt into SiC preform was successfully achieved through the optimisation of process parameters, such as temperature of Al melt, preheat temperature of preform, and infiltration pressure and infiltration time after pouring. Microstructures of 50–70 vol.-%SiC_p/Al composites showed that pores resided preferentially at interfaces between the SiC particles and Al matrix with increasing volume fraction of SiC particles. The measured coefficients of thermal expansion of SiC_p/Al composites were in good agreement with the estimated values based on Turner's model. The measured thermal conductivity of SiC_p/Al composites agreed well with estimated values based on the 'rule of mixture' up to 70 vol.-% of SiC particles, while they were lower than the estimated values above 70 vol.-% of SiC particles, mainly due to the residual pores at SiC/Al interfaces. The high volume fraction SiC_p/Al composite is a good candidate material to substitute for conventional thermal management materials in advanced electronic packages due to their tailorable thermophysical properties.     

Elevated temperature X-ray measurement of residual stresses in a fibre reinforced Al alloy

Thermal residual stresses have been measured using X-ray diffraction in an Al-2% Mg matrix with 10, 20 or 26 vol % Al₂O₃ short fibres. Stress measurements were made at room temperature as well asin situ at elevated temperatures up to 250?C. The thermal stresses arise due to the difference in coefficient of thermal expansion (CTE) between the matrix and the reinforcement. The largest CTE is found in the matrix, resulting in tensile residual stresses after a temperature drop, e.g. after processing or annealing. A high fraction of reinforcement implies higher matrix stresses than a low fibre content. The stresses decrease with increasing temperature for all fibre volume fractions. Measurements are compared with calculations using a modified Eshelby model for equivalent inclusions. Parameters taken into account in the model are coefficient of thermal expansion, Young's modulus, and volume fraction and geometric shape of the reinforcing phase. A good correlation between calculations and experimental results has been found, bearing in mind that no plasticity is taken into account in the Eshelby model. The plastic behaviour of the composites has been described using a model based on a rigid spherical cavity in an elastic-plastic matrix.     

Electrical conductivity and mechanical properties of multiwalled carbon nanotube-reinforced polypropylene nanocomposites

In this paper, the electrical conductivity and mechanical properties such as elastic modulus of multiwalled carbon nanotubes (MWCNTs) reinforced polypropylene (PP) nanocomposites were investigated both experimentally and theoretically. MWCNT-PP nanocomposites samples were produced using injection mold at different injection velocities. The range of the CNT fillers is from 0 up to 12?wt%. The influence of the injection velocity and the volume fraction of CNTs on both electrical conductivity and mechanical properties of the nanocomposites were studied. The injection speed showed some effect on the electrical conductivity, but no significant influence on the mechanical properties such as elastic modulus and stress-strain relations of the composites under tensile loading. Parallel to the experimental investigation, for electrical conductivity, a percolation theory was applied to study the electrical conductivity of the nanocomposite system in terms of content of nanotubes. Both Kirkpatrick (Rev Mod Phys 45:574?C588, 1973) and McLachlan et?al. (J Polym Sci B 43:3273?C3287, 2005) models were used to determine the transition from low conductivity to high conductivity in which designates as percolation threshold. It was found that the percolation threshold of CNT/PP composites is close to 3.8?wt%. For mechanical properties of the system, several micromechanical models were applied to elucidate the elastic properties of the nanocomposites. The results indicate that the interphase between the CNT and the polymers plays an important role in determining the elastic modulus of the system.     

Graphene/poly(vinylidene fluoride) composites with high dielectric constant and low percolation threshold

In aiming to obtain highly flexible polymer composites with high dielectric performance, graphene/poly(vinylidene fluoride) (PVDF) composites with a multi-layered structure were proposed and prepared. Graphene sheets were prepared by reducing graphene oxide using phenylhydrazine, which could effectively alleviate aggregation of the graphene sheets. A two-step method, including solution casting and compression molding, was employed to fabricate the graphene/PVDF composites. The composites showed an alternative multi-layered structure of graphene sheets and PVDF. Due to their unique structure, the composites had an extremely low percolation threshold (0.0018 volume fraction of graphene), which was the lowest percolation threshold ever reported among PVDF-based polymer composites. A high dielectric constant of more than 340 at 100?Hz could be obtained within the vicinity of the percolation threshold when the graphene volume fraction was 0.00177. Above the percolation threshold, the dielectric constant continued to increase and a maximum value of as high as 7940 at 100?Hz was observed when the graphene volume fraction was 0.0177.     

ABS/石墨复合材料的交直流导电特性及流变性能

研究了石墨填充丙烯腈-丁二烯-苯乙烯共聚物(ABS)复合材料的直流(DC)和交流(AC)导电特性和线性粘弹行为。电性能测试结果表明,石墨体积分数为13.21%~16.36%时,ABS/石墨复合材料的DC电阻率突降6个数量级,说明发生电学逾渗;同时,AC电阻率在低频区不随频率而变化,且AC阻抗复平面图中阻抗实部与阻抗虚部呈现半圆弧,进一步证明导电网络的形成。流变性能测试结果表明石墨体积分数为10.24%~13.21%时复合体系的储能模量和复数黏度出现跳跃,损耗因子(tanδ)的峰值减小且逐渐向高频移动,说明复合体系从"类液态"转变为"类固态",发生流变逾渗现象。流变逾渗阈值小于导电逾渗阈值是因为传递电子时石墨之间的距离比阻碍聚合物分子链运动时石墨之间的距离小。     

High conductivity and low percolation threshold in polyaniline/graphite nanosheets composites

An easy process for the synthesis of polyaniline/graphite nanosheets (PANI/NanoG) composites was developed. NanoG were prepared by treating the expanded graphite with sonication in aqueous alcohol solution. Scanning electron microscopy (SEM), X-ray diffraction techniques (XRD), Fourier transform infrared (FT-IR), and transmission electron microscopy (TEM) were used to characterize the structures of NanoG and PANI/NanoG conducting composites. Electrical conductivity measurements indicated that the percolation threshold of PANI/NanoG composites at room temperature was as low as 0.32 vol.% and the conductivity of PANI/NanoG composites was 420 S/cm. The percolation theory, mean-field theory, and excluded volume theory were applied to interpret the conducting properties. Results showed that the low value of percolation threshold may be mainly attributed to nanoscale structure of NanoG forming conducting bridge in PANI matrix and there exists contact resistance in the percolation network formed within PANI/NanoG composites.     

Effect of metal volume fraction on the mechanical properties of alumina/aluminum composites

The influence of metal volume fraction on the mechanical properties of Al₂O₃/Al composites with constant diameter of metal ligaments was studied. Alumina/aluminum composites with interpenetrating networks and metal content between 12 and 34 vol.% were fabricated by gas-pressure infiltration technique. The fabricated composites exhibited good mechanical properties, e.g. the bending strength of 740 MPa for samples containing 12 vol.% of Al. The bending strength of the composites decreased with increasing volume fraction of metal phase. High strength of the fabricated composites was explained by strong interfacial bonding between alumina and aluminum. The fracture toughness of the composites increased, however, with increasing volume fraction of aluminum. The highest fracture toughness values of 6 MPa m were measured for the composites containing 25 vol.% of Al. Fractographic analysis of fractured surfaces showed deformed metal ligaments which demonstrated that crack bridging by plastic deformation of the metal phase is the main toughening mechanism in Al₂O₃/Al composites.     

Fabrication of aluminium composites reinforced with carbon fibres by a centrifugal infiltration process

Centrifugal-force infiltration was used for obtaining aluminium alloy composites reinforced with carbon fibre by the infiltration of preforms. The lost-wax-casting technique was used during the manufacturing process. Preforms fabricated with different percentages of reinforcement were heated to facilitate their filling with aluminium. Some samples were coated with nickel to favor the reinforcement wetting by the molten aluminium alloy. Composites with volume fraction of reinforcements above 7 vol.% and porosity values lower than 0.5 vol.% were obtained with this technique. The hardness of the composites increased with the volume fraction of reinforcement and the solution and the later precipitation of nickel coating caused an additional hardening effect.     

Mechanical properties of a diamond–copper composite with high thermal conductivity

Using pressureless infiltration of copper into a bed of coarse (180 μm) diamond particles pre-coated with tungsten, a composite with a thermal conductivity of 720 W/(m K) was prepared. The bending strength and compression strength of the composite were measured as 380 MPa. As measured by sound velocity, the Young's modulus of the composite was 310 GPa. Model calculations of the thermal conductivity, the strength and elastic constants of the copper–diamond composite were carried out, depending on the size and volume fraction of filler particles. The coincidence of the values of bending strength and compressive strength and the relatively high deformation at failure (a few percent) characterize the fabricated diamond–copper composite as ductile. The properties of the composite are compared to the known analogues — metal matrix composites with a high thermal conductivity having a high content of filler particles (~ 60 vol.%). In strength and ductility our composite is superior to diamond–metal composites with a coarse filler; in thermal conductivity it surpasses composites of SiC–Al, W–Cu and WC–Cu, and dispersion-strengthened copper.     

Effect of alumina particle size and thermal condition of casting on microstructure and mechanical properties of stir cast Al–Al2O3 composites

Abstract

Metal matrix composites are considered as a distinct category of the advanced materials, which have low weight, high strength, high modulus of elasticity, low thermal expansion coefficient and high wear resistance. Among them, Al–Al₂O₃ composites have achieved significant attention due to their desired properties. In the present research, Al–Al₂O₃ composites with 5 vol.-% alumina were produced by stir casting at a temperature of 800°C. Two different particle sizes of alumina were used as 53–63 and 90–105 μm. The microstructure of the samples was evaluated by SEM. In addition, the mechanical properties of the samples were measured, and hence, the optimum temperature and particle size of alumina to be added to the Al matrix were determined. The results demonstrated the positive effect of alumina on improving the properties of Al–Al₂O₃ composites.     

Nextel™ 610 alumina fibre reinforced aluminium: influence of matrix and process on flow stress

Continuous alumina fibre reinforced aluminium matrix composites are produced using two different liquid metal infiltration methods, namely direct squeeze casting and gas pressure infiltration. Net-shape fibre performs for longitudinal parallel tensile bars are prepared by winding the Nextel™ 610 alumina fibre (3M, St Paul, MN) into graphite moulds. High purity aluminium, two binary (Al–6% Zn and Al–1% Mg) and one ternary (Al–6% Zn–0.5% Mg) aluminium alloys are used as matrix materials. The composite is tested in uniaxial tension–compression, using unload–reload loops to monitor the evolution of Young's modulus. A linear dependence between Young's modulus and strain is observed; this is attributed, by deduction, to intrinsic elastic non-linearity of the alumina fibre. This conclusion is then used to compare on the basis of the in situ matrix flow curve the influence of matrix composition and infiltration process on the composite stress–strain behaviour.     

Mechanical properties of nanocomposite organosilicate films

Nanocomposite coatings have been deposited on plastic substrates by the dipping–drawing technique. The coatings were constituted by a matrice of a hybrid organomineral gel and a reinforcement made of amorphous silica. Two methods by which increase the silica content were investigated: silica was added via a silicon alkoxide compound or via dense silica particles of 10 nm size. Young's modulus and the hardness of the coating were measured using home-built equipment, and results compared to literature models. It is shown that the agreement between models and experimental values depends on the method of preparation of the nanocomposite coating. On the other hand, deviations appear when the volume fraction of reinforcement surpasses the three-dimensional percolation threshold.     

填充型聚合物基复合材料的导电和导热性能

研究了高密度聚乙烯为基体、炭黑和炭纤维为填料复合体系的导电和导热性能。发现当导电填料的含量达到渗流阈值时,复合材料的电导率急剧升高;而在渗流阈值附近,其热导率未出现突变。这表明电导渗流现象不完全是由导电粒子通过物理接触生成导电链所致。其导电机制是相当数量的导电粒子相互发生隧道效应。