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     

Effect of boron carbide particle size and volume fraction of TiB–TiC reinforcement on steady state creep behaviour of PM processed titanium matrix composites

Abstract

TiB–TiC reinforced titanium matrix composites (TMCs) were synthesised through pressureless sintering of titanium and boron carbide (B₄C) powder compacts. Effect of boron carbide (B₄C) particle size and volume fraction of TiB–TiC reinforcement on steady state compression creep behaviour of composites was investigated in the temperature range of 773–873 K. The creep rates of composites are found to be about two orders of magnitude lower than those of unreinforced titanium. The creep rates further lowered with decrease in size of B₄C particles (from 16 to 3 μm) used in preparation of composites as well as with increase in volume fraction of the TiB–TiC reinforcement from 10 to 30 vol.%. By using the concept of effective stress as well as incorporation of load transfer and substructural strengthening effect produced by the reinforcement into analysis, the entire creep data of Ti and the composites can be made to merge on to a single line within a scatter band of factor of 2–3 and can be represented by a unified power-law equation.     

Discontinuously reinforced intermetallic matrix composites via XD synthesis

Abstract

A review is given of recent results obtained for discontinuously reinforced intermetallic matrix composites produced using the XD process. Intermetallic matrixes investigated include NiAl, multiphase NiAl+Ni₂AlTi, CoAl, near γ titanium aluminides (i.e. TiAl+Ti₃Al), and L1₂ trialuminides containing minor amounts of second phase. A judicious match of the matrix and reinforcement can enhance both physical and mechanical properties. Particulate reinforcements can affect properties directly by enhancing modulus and strength or indirectly by refining grain size, enhancing microstructural homogeneity in a casting, or providing additional nucleation sites which improve phase transformation kinetics. Such mechanical properties as low and high temperature strength, compressive and tensile creep, elastic modulus, ambient ductility, and fracture toughness are discussed as functions of reinforcement size, shape, and volume fraction. Microstructures before and after deformation are examined and correlated with measured properties. An observation of interest in many of the systems examined is ‘dispersion weakening’ at high temperatures and high strain rates. This behaviour is not specific to the XD process; rather similar observations have been reported in other discontinuous composites. Proposed mechanisms for this behaviour are presented.

MST/1561     

Reference stress approach for predicting creep deformation of metal matrix composites

Abstract

A sound, mechanics based approach, using the reference stress concept, has been provided to allow the effects of volume ratio, fibre aspect ratio, and fibre spacing on the creep behaviour of uniaxial metal matrix composites to be quickly assessed. It is shown that fibres are much more effective than particles in reducing creep deformations. In addition, volume ratio and fibre aspect ratio have a large effect on creep properties, while fibre spacing has a relatively small effect. The existence of cracks at the ends of fibres is shown to reduce seriously the effectiveness of the reinforcement. The creep properties for loading in transverse directions are much lower than for loading in longitudinal directions.

MST/2059     

A new approach to the effective viscosity of suspensions

Study of the effective viscosity of suspensions is not only of interest in science, but also of great practical relevance to industries, such as the petrochemical industry, food and nutrition, materials processing and so on. In this paper, an attempt is made to establish theoretically the correlation between the effective viscosity of suspensions and their microstructural features. Firstly, the method for microstructural characterization developed by Fanet al. will be introduced to describe effectively the particle distribution in a suspension, and then the analogy between viscosity and field properties will be used to develop a new approach for the effective viscosity of suspensions. The new approach considers implicitly the effects of size, shape, orientation and distribution of the solid particles within the suspension through the topological parameters. Therefore, it can be applied to a suspension containing solid particles with any size, shape, orientation and distribution. Compared with other models available in the literature, the present approach is more realistic and more versatile. It can be applied to both liquids containing solid particles with a very high viscosity, and porous suspensions where the second phase has a vanishing viscosity. Perhaps more importantly, the present approach can predict the well-known S-shaped log-volume fraction curve in the whole range of microstructures (from completely continuous to completely discontinuous) and is in better agreement with experimental results.     

Effect of fiber arrangement on mechanical properties of short fiber reinforced composites

The present paper developed a three-dimensional (3D) “tension–shear chain” theoretical model to predict the mechanical properties of unidirectional short fiber reinforced composites, and especially to investigate the distribution effect of short fibers. The accuracy of its predictions on effective modulus, strength, failure strain and energy storage capacity of composites with different distributions of fibers are validated by simulations of finite element method (FEM). It is found that besides the volume fraction, shape, and orientation of the reinforcements, the distribution of fibers also plays a significant role in the mechanical properties of unidirectional composites. Two stiffness distribution factors and two strength distribution factors are identified to completely characterize this influence. It is also noted that stairwise staggering (including regular staggering), which is adopted by the nature, could achieve overall excellent performance. The proposed 3D tension–shear chain model may provide guidance to the design of short fiber reinforced composites.     

Processing and Creep Resistance of Nickel/Yttria Composites

In this study, pure nickel and yttria (Y2O3) were selected as a model system to investigate the feasibility of processing metal matrix composites (MMCs) through a powder metallurgy approach for the in-situ formation of a continuous three-dimensional reinforcement network or the in-situ formation of discrete reinforcements with certain degrees of interconnected clusters. Composites with a volume fraction of Y2O3 ranging from 20 to 50 % were prepared through hot pressing. The density, microstructure and creep resistance of these composites were evaluated as a function of the yttria volume fraction. It was found that a continuous Y2O3 network was formed in composites with 40 and 50 vol % Y2O3, while yttria was discrete with some degrees of interconnected clusters in composites with 20 and 30 vol % Y2O3. The creep rate was reduced by two to three orders of magnitude with the addition of 20 to 30 vol % Y2O3, and it continued to decrease with increasing the volume fraction of yttria to 50%. The analysis indicated that the load transfer to isolated yttria particles could not account for the improved creep resistance of composites with 20 and 30 vol % Y2O3, while the load transfer to a continuous yttria network in composites with 40 and 50 vol % Y2O3 could not be approximated by the model of the load transfer to continuous fibres. The discrepancies are believed to be related to the presence of interconnected yttria clusters, the low relative density of the yttria phase in the composite, and the low load-carrying capability through a three-dimensional network in comparison with the load-carrying capability through continuous fibres. It is suggested that the density of the yttria phase and hence the creep resistance of the composite can be further improved over what have been obtained in this study by densifying the composite at high temperatures and pressures.     

界面对硅酸铝短纤维增强AZ91D镁基复合材料蠕变性能的影响

界面对复合材料蠕变性能的影响很大。在试验分析的基础上建立了硅酸铝短纤维增强AZ91D镁基复合材料理论分析模型,利用三维有限元分析方法,系统研究了界面特性、界面上应力应变分布和短纤维位向变化对硅酸铝短纤维增强AZ91D镁基复合材料蠕变性能的影响。研究表明：界面特性,如厚度、模量,均对纤维最大轴应力和稳态蠕变速率有影响,当界面厚度增加,纤维最大轴应力减小而稳态蠕变速率增大;当界面模量增大,纤维最大轴应力增大而稳态蠕变速率减小,但当界面模量高于基体模量时,纤维最大轴应力和稳态蠕变速率均保持不变;纤维位向也影响轴应力分布和稳态蠕变速率,纤维在其末端界面上存在较大的应力和应变,此处容易产生微裂纹而使材料抗蠕变能力下降;界面对硅酸铝短纤维增强AZ91D镁基复合材料的蠕变曲线和蠕变断裂机制也有影响,其影响程度还与纤维位向有关。     

超声化学原位合成纳米Al_2O_3/6063Al复合材料组织及高温蠕变性能

为研究纳米颗粒增强铝基复合材料的高温蠕变特性,基于6063Al-Al2(SO4)3体系,采用超声化学原位合成技术,制备出不同Al2O3体积分数(5%、7%)的纳米Al2O3/6063Al复合材料,通过高温蠕变拉伸试验测试其高温蠕变性能,利用XRD、OM、SEM及TEM分析其微观形貌。结果表明:施加高能超声可显著细化增强体颗粒并提高其分布的均匀性,所生成的Al2O3增强颗粒以圆形或近六边形为主,尺寸为20~100nm;纳米Al2O3/6063Al复合材料的名义应力指数、表观激活能和门槛应力值与基体相比大幅提高,均随着增强体体积分数的增加而提高,表明纳米Al2O3/6063Al复合材料的抗蠕变性能提高;纳米Al2O3/6063Al复合材料的真应力指数为8,说明复合材料蠕变机制符合微结构不变模型,即受基体晶格扩散的控制;纳米Al2O3/6063Al复合材料的高温蠕变断口特征以脆性断裂为主,高应力下形成穿晶断裂,低应力下形成沿晶断裂和晶界孔洞;纳米Al2O3/6063Al复合材料的主要强化机制为位错强化与弥散强化。     

Computer Simulation of the Indentation Creep Tests on Particle-Reinforced Composites

A systematical simulation has been carried out on the indentation creep test on particle-reinforced composites.The deformation ,failure mechanisms and life are analyzed by three reasonable models.The following five factors have been considered simultaneously:creep property of the particle,creep property of the matrix,the shape of the particle, the volume faction of the particle and the size(relative size to the particle )of the indentation indenter.For all the cases,the power law respecting to the applied stress can be used to model the steady indentation creep depth rate of the indenter,and the detail expressions have been presented.The computer simulation is analyzed by the two-phase model and the three-phase model.Two places of the stress concentration are found in the composites.One is ahead of the indentation indenter, where the high stress state is deduced by the edge of theindenter and will decrease rapidly near to a steady value with the creep time The other one is at the interface,where the high stress state is deduced by the misfit of material properties between the particles and matrix.It has been found that the creep dissipation energy density other than a stress parameter can be used to be the criterion to model the debonding of the interfaces.With the criterion of the critical creep dissipation energy density, a power law to the applied stress with negative exponent can be used to model the failure life deduced by the debonding of interfaces.The influences of the shape of the particles and the matching of creep properties of particle and matrix can be discussed for the failure.With a crack model,the further growthe of interface crack is analyzed, and some important experimental phenomena can be predicted.The failure mechanism which the particle will be punched into matrix has been also discussed.The critical differences between the creep properties of the particles and matrix have been calculated, after a parameter has been defined.In the view of competition of failure mechanisms, the best matching of the creep properties of the two phases and the best shape of the particles are discussed for the composite design.     

搅拌摩擦加工制备颗粒增强铝基复合材料的研究现状及展望

铝合金作为现代工程和高新技术领域发展的关键材料之一,具有密度小、比强度和比刚度高、耐蚀性好等特点。通过在铝基体中添加增强相颗粒,制备得到的颗粒增强铝基复合材料既有铝合金良好的强度、韧性、易成形性等特点,又有颗粒的高强、高模等优点,是近年来应用最广的一类金属基复合材料。 目前,制备铝基复合材料的方法主要有粉末冶金法、铸造以及超声波法等,但这些方法在制备过程中需要较高的温度,颗粒与金属基体容易发生不良的界面反应,从而影响界面结合效果,降低复合材料的性能。搅拌摩擦加工(FSP)作为一种新型的固相加工技术,可同时实现材料微观组织的细化、致密化和均匀化。目前,FSP直接法已在铝基复合材料制备方面取得应用,主要是将增强相颗粒通过打盲孔或开槽的方式预置在金属基体内再进行FSP,进而制备出高致密度的颗粒增强铝基复合材料。因为FSP过程的温度低,颗粒与铝基体不会发生界面反应,所以该方法也被用于制备具有形状记忆效应(SME)的铝基功能复合材料。 近年研究结果表明,颗粒相对FSP制备的铝基复合材料晶粒细化起到显著作用,这有助于提高复合材料的拉伸强度、显微硬度及疲劳强度等力学性能。随着颗粒含量的增加和颗粒尺寸的减小,复合材料的力学性能得以增强。再者,减小颗粒尺寸有利于改善颗粒与基体之间的结合。另外,通过优化搅拌头的结构、形状和尺寸,以及FSP工艺参数,已经可以实现加工后颗粒相在基体中的均匀分布。 鉴于搅拌摩擦加工(FSP)直接法在制备颗粒增强铝基复合材料方面所具备的短流程、高效能以及基体与增强相颗粒界面无杂质等优势,本文对目前FSP直接法制备颗粒增强铝基复合材料的最新研究现状进行了总结。主要综述了FSP制备颗粒增强铝基复合材料过程中颗粒的含量、类型及尺寸对复合材料组织与力学性能的影响,并对颗粒分布均匀性以及颗粒与铝基体的界面问题做了阐述。文章最后深入分析了当前研究中的不足之处并展望了未来的研究方向。     

Modeling the influence of particulate geometry on the thermal conductivity of composites

The influence of particle shape and orientation on the thermal conductivity of low volume, particulate composites was examined through two-dimensional numerical simulations using the finite element method, FEM. The simulations demonstrate that the conductivity of such composites is influenced by not only the relative volume and conductivity of the embedded particles, but also their general shape, elongation, and orientation relative to the direction of global heat flow. The functional form of the Halpin–Tsai equation was utilized to characterize the composite thermal conductivity through a derived expression of the geometric distribution factor, ζ. The proposed expression of ζ differs from commonly assumed values and is shown to be highly dependent on the shape of the embedded particles. This approach was further extended to examine and explicitly characterize the anisotropic behavior of composites containing multiple, randomly distributed particles of a uniform size and shape based on their relative orientation. This work demonstrates the viability of using numerical tools to examine composites with complex geometries.     

Thermal properties of diamond/SiC/Al composites with high volume fractions

The diamond/SiC/Al composites with high volume fractions and a large ratio of diamond to SiC particle size (7.8:1) were fabricated by gas pressure infiltration. The results show that the fine SiC particles occupy efficiently the interstitial positions around coarse diamond particles; the main fracture mechanism of the composite is matrix ductile fracture, and diamond brittle fracture was observed which confirms a high interfacial bonding strength; the diamond/SiC/Al composites with 80% and 66.7% volume fraction of diamond in the reinforcement have the higher volume fraction in the reinforcement and lower coefficient of thermal expansion compared to the diamond/Al composite. Turner and Kerner models are not in good agreement with the experimental data for the composites based on reinforcement with two phases different in shape and component. When the effect of the coating layer considered, differential effective medium (DEM) model is confirmed a reliable model in designing a composite with a given thermal conductivity based on reinforcement with two phases different in size.     

Creep behaviour of unreinforced and short fibre reinforced AlSi12CuMgNi piston alloy

The isothermal creep resistance of AlSi12CuMgNi alloys produced by squeeze casting, unreinforced and reinforced with 10, 15 and 20 vol% of short alumina fibres is investigated by means of long-term tensile creep tests at 300 °C. Dislocation creep-mechanisms associated with a Norton exponent n = 3 are attributed to both the unreinforced and the short fibre reinforced materials (SFRMs) in the stress range between 10 and 50% of the yield strength at 300 °C. The embedding of short fibres reduces the creep rate of the alloy by more than one order of magnitude. The SFRM with 15 vol% of reinforcement is the most creep resistant, while the 20 vol% SFRM is less creep resistant than the 10 and 15 vol% SFRMs due to its higher defect density and larger interface area.

Load changes are imposed during creep tests to study the influence of periods of overloading. Returning to the initial load causes the stationary creep rates of the SFRMs to be further reduced down to one third of their initial values. Furthermore, a small increase in the creep exponent n of the SFRMs is observed after an overloading cycle. The increase of the connectivity of the Si/Al₂O₃-short fibre network in the direction of loading enhances the load transfer effect from the matrix to the reinforcement decreasing thus the external stress acting on the matrix. The yielding of highly stressed zones in the matrix around the ceramic reinforcement during the overload is able to produce a more uniform distribution of internal stresses when returned to a lower stress level. Both effects are considered to be responsible for the observed reduction of the stationary creep rate.

The unexpected low creep resistance of the 20 vol% SFRM in comparison with the two other composites can be explained by the larger number of micro-structural defects (≤1 vol% porosity) found in this material after the infiltration process. Although the small portion of these defects does not affect the high temperature tensile properties of the composite, it accelerates the diffusion controlled creep mechanisms. The larger interface area present in the materials with higher reinforcement volume fraction increases the diffusivity as well. The connectivity of the Si/Al₂O₃-short fibre (SF) network is higher for the 20 vol% SFRM than for the other SFRMs already before creep exposure. Thus the training effect by overloading periods is less efficient in the 20 vol% SFRM than in SFRMs with less fibre content. Those effects are counteracted by the fact that the higher the reinforcement volume fraction the smaller the load acting on the matrix. Yet for the investigated 20 vol% SFRM samples, the negative contributions prevail resulting in a lower creep resistance than for the other two composites.     

Modeling elastic properties and volume change in dental composites

A modeling approach was applied to study elastic properties and volume change in dental composites. Mechanics modeling results were compared with experimental data in model materials of known composition where the filler content was varied. Composite behavior was predicted based on polymer and filler properties in order to improve basic understanding. Model predictions agree well with data. The models were used to discuss effects of resin properties, filler volume fraction and microstructure (particle shape and filler size distribution).     

Deformation and failure of PP composites reinforced with lignocellulosic fibers: Effect of inherent strength of the particles

PP/wood composites were prepared from two lignocellulosic fibers with different particle size and aspect ratio in order to determine the effect of these factors on the deformation and failure mechanism as well as on the properties of the composites. Wood content was changed from 0 to 80 wt%. Maleinated polypropylene (MAPP) was added to improve interfacial adhesion. The MAPP/wood ratio was kept constant at 0.1. Mechanical properties were determined by tensile testing. Micromechanical deformation processes were followed by acoustic emission (AE) and volume strain (VOLS) measurements, and by the study of fracture surfaces. The results proved that micromechanical deformations change drastically both with decreasing particle size and changing interfacial adhesion. Less debonding, fiber pull out and fiber fracture occur in composites containing small particles. Hardly any change was observed in the mechanical properties of the composites with decreasing particle size, in spite of the drastic modification of the deformation mechanism. The apparently slight influence of particle size on composite strength results from the smaller aspect ratio of the small particles, which indicates that orientation and orientation distribution must have a strong effect on reinforcement. Further improvement in composite strength is possible only through the optimization of particle size, aspect ratio and the inherent strength of wood.     

Finite element analysis of observed high strengthening in composites with regularly segregated microstructures

Abstract

A distinct dual phase composite has been developed, comprising spherical reinforcement clusters and an unreinforced matrix, according to numerical simulation of crack initiation and propagation in discontinuously reinforced MMCs. The present work is aimed at interpretation of the high strengthening ratios which were actually measured in such dual phase composites. Elastic-plastic finite element modelling is utilised to analyse the strengthening ratio in a two-dimensional idealised microstructure with periodic clustering. As the degree of clustering increases, the strengthening ratio is predicted to increase. In composites with a networking cluster, much more strengthening is exhibited together with relatively uniform strain distribution. The primary mechanism leading to additional strengthening due to clustering derives from an optimum ratio in deformation resistance between a matrix and a reinforcing phase. In the proposed dual phase composites, each cluster can behave as a single reinforcement which can deform plastically and there is no distinct interface between the cluster and the softer phase.     

Main and interaction effects of matrix particle size,reinforcement particle size and volume fraction on wear characteristics of Al–SiCp composites using central composite design

The aim of this study was to investigate the effects of matrix particle size, reinforcement particle size, volume fraction, and their interactions on the wear characteristics of Al–SiC_p composites. Central composite design method was used to perform a series of experiments. The statistical analysis of experimental results showed that both main effect and interaction effect of factors investigated were effective on the wear behavior of Al–SiC_p composites. Wear loss decreased as volume fraction increased; however, beyond volume fraction of 17.5%, it increased due to reinforcement particle clustering depending on volume fraction and matrix particle size to reinforcement particle size ratio. With decreasing of matrix particle size and increasing of reinforcement particle size, wear loss also decreased. However, after a certain volume fraction, large sized reinforcement particles had a negative effect on the wear resistance.     

析出相对Ti60钛合金蠕变和持久性能的影响

研究了固溶态的Si、硅化物以及α2相对Ti60高温钛合金蠕变和持久性能的影响.结果表明,α片层之间析出的硅化物能提高Ti60钛合金的600℃蠕变抗力,且当α片层内部有α2相析出时蠕变抗力提高更明显,但是硅化物的大量析出和大颗粒硅化物的存在却降低了Ti60钛合金的600℃持久性能;α2相的析出同时提高材料的蠕变抗力和持久性能;减少硅化物的析出以提高固溶态的Si对低应力下蠕变抗力的作用不显著,但是能改善高应力下的持久性能.在蠕变和持久实验条件下固溶态的硅和硅化物的不同作用,可通过不同外加应力水平下材料变形机制的差异加以解释.     

石墨热压还原Cu/Cu_(2)O金属陶瓷电导逾渗行为与微观结构分形表征EI北大核心CSCD

采用石墨热压还原法制备Cu/Cu_(2)O金属陶瓷复合材料,并测试不同导通相(Cu)体积含量Cu/Cu_(2)O金属陶瓷复合材料的直流电导率。为了实现对导通相形状、大小和分布状态的定量表征,通过对复合材料微观结构图像的二值化处理进行导通相分形维数计算,结合Cu/Cu_(2)O金属陶瓷复合材料的电逾渗行为,分析复合材料微观结构与电性能之间的对应关系。结果表明:随着导通相体积分数的增加,逾渗无限团簇和逾渗骨架的总量随之增大,但逾渗骨架密度在逾渗阈值附近波动。此外,Cu/Cu_(2)O金属陶瓷复合材料垂直热压方向与平行热压方向的分形维数相差约0.1。分形计算为定量表征导体/绝缘体双相复合材料中导通相的微观结构提供了一种计算方法,有助于对第二相随机分布的复合材料实现微观结构定量表征。