Interfaces and fracture surfaces in Saffil/Al-Mg-Cu metal-matrix composites

The interfacial phases were studied in pressure-cast Saffil/Al-4.5Cu-3Mg composite material using a variety of characterization techniques. The magnesium- and copper-rich phases were found to segregate near the fibre-matrix interfaces. The major phases were identified as MgAl₂O₄, Al₂CuMg and CuAl₂. Significant diffusion of silicon from the fibres into the matrix took place during the pressure-casting. A conclusion was drawn that solidification started with the less alloyed aluminium in the bulk of the matrix and proceeded towards the fibres. Auger and XPS analyses of composite materials fracturedin situ showed the fracture surface to lie within magnesium-rich (and not copper-rich) phases, most likely within the MgAl₂O₄ spinel phase. The fibre surface treatments which are being developed to improve interfacial wetting may also reverse the direction of crystallization and prevent formation of brittle phases in the vicinity of fibres, thereby improving the toughness of the composite materials.     

In situ scanning electron microscope studies of fracture in particulate-reinforced metal-matrix composites

In situ observations of specimen surfaces have been used to characterize the fracture behaviour of particulate-reinforced metal-matrix composites. Composites of silicon carbide particle sizes 3, 10 and 30 m with volume fractions of 5, 10 and 20% in commercial-purity aluminium and aluminium-1% magnesium matrices were studied. The results of this surface study are compared with complementary metallographic studies of sectioned specimens illustrating behaviour from the bulk. Significant differences between the results of these two studies are outlined. Previous work using these techniques is critically examined and recommendations made for the appropriate interpretation of in situ straining experiments.     

Interfacial phenomena in metal-matrix composites

The role of the reinforcement/matrix interface in processing and the control of properties is reviewed with special emphasis on recent developments in structural metal-matrix composites (mmcs). Basic considerations such as the physical nature of the interface, thermodynamic and kinetic factors, and models relating interfacial parameters to composite performance are introduced. Examples of the application of available experimental methods of study are used to illustrate the scope of these techniques. The role of the interface in mmc processing is summarized with particular emphasis on liquid-state processing. The role of the interface in property control is reviewed in terms of the influence of fibre damage, reaction products, load transfer and perturbations in matrix metallurgy. The complex interaction of these interfacial effects is illustrated by the case of property enhancement via hybridization of multifilament reinforcements with fine particles.     

Elastic nonlinearity in metal-matrix composites

Parameters characterizing the elastic nonlinearity in metal-matrix composites are studied. The composites consist of the aluminum alloys 8091 or 7064 containing silicon carbide particles up to 20% volume fraction. Two different ultrasonic measurements, namely the acoustoelastic effect and the harmonic generation, are used for the determination of acoustic nonlinearity parameters. Their dependence on the content of SiC in the composite is investigated. The values of the nonlinearity parameter are found to decrease with increasing volume fraction of SiC-particles. The changes are explained in terms of the effects of SiC-particles on the second and third order elastic constants of the composites.     

Ultrasonic anisotropy of metal-matrix composites

In recent work, Spies and Salama have attributed the ultrasonic velocity anisotropy observed in A1/SiC metal-matrix composites to the preferential alignment (texture) of crystallites in the aluminum matrix. In this paper it is shown that a simple relation exists between the compressional and shear wave anisotropy for an orthotropic orientation distribution of crystallites. It is shown that the data of Spies and Salama violates this relation and therefore that the ultrasonic anisotropy observed by these authors cannot be explained in terms of texture. It is proposed instead that the observed anisotropy is due to an anisotropic microstructure which may develop in the composites as a result of thermomechanical processing (extrusion), and a theory for this is presented.     

Size effects in the particle-reinforced metal-matrix composites

Summary Many experimental observations have shown the influences of particle size on the mechanical propertics of the particle-reinforced metal-matrix composite. However, the conventional theory cannot explain the phenomena because no length scale parameters are included in the conventional theory. In the present paper, the strain gradient theory proposed by Chen and Wang [32] is used, and a systematic research of the particle size effect in the particle-reinforced metal-matrix composite is carried out. Many composite factors, such as the particle size, the particle aspect ratio, the Young's modulus ratio of the particle to the matrix material, particle volume fraction and the strain hardening exponent of the matrix material, are investigated in detail. Two kinds of particle shapes, spheroidal particle and cylindrical particle, are considered to check the strength dependence of the particle shapes. Calculation to the special materials used by Ling [9] has been done, and the calculation results are consistent with the experimental results in Ling [9]. The material length scale parameter is predicted.     

Fatigue properties of particle-reinforced metal-matrix composites

Fatigue-lifetime behaviour has been examined for extruded 6061 aluminium alloy composites reinforced with 15 vol% SiC and 10 vol% Al₂O₃ particles. The peak particle sizes are at about 4.5 and 6 μm. Within measured S- N curves the fatigue lifetime at given stress amplitudes of SiC_p/AA6061 is superior to that of Al₂O_3p/AA6061 in the low-cycle fatigue region as well as in the high-cycle fatigue region. These results are discussed by consideration of theoretically evaluated crack propagation curves.     

Wear of ceramic particle-reinforced metal-matrix composites

Pin-on-disc dry sliding tests were carried out to study the wear mechanisms in a range of metal-matrix composites. 6061-aluminium alloys reinforced with 10 and 20 vol% SiC and Al₂O₃ particles were used as pin materials, and a mild steel disc was used as a counterface. A transition from mild wear to severe wear was found for the present composites; the wear rate increased by a factor of 10². The effects of the ceramic particles on the transition load and wear with varying normal pressure were thoroughly investigated. Three wear mechanisms were identified: abrasion in the running-in period, oxidation during steady wear at low load levels, and adhesion at high loads. A higher particle volume fraction raised the transition load but increased the wear rate in the abrasion and adhesion regimes. Increase of particle size was more effective than increase of volume fraction to prolong the transition from mild wear to adhesive wear. The reasons for different wear mechanisms were determined by analyses of the worn surfaces and wear debris.     

Particle fracture in metal-matrix composite friction joints

The influence of welding parameters, reinforcing particle chemistry and shape, matrix condition and silver interlayers on particle fracture during similar and dissimilar friction welding of aluminium-based metal-matrix composite (MMC) base material was investigated. Two composite base materials were examined, one containing Al₂O₃ particles and the other containing 72 wt% Al₂O₃–7 wt % Fe₂O₃–17 wt % SiO₂–3 wt % TiO₂ particles. The different material combinations comprised MMC/MMC, MMC/alloy 6061, MMC/AISI 304 stainless steel and MMC/1020 mild steel joints. Particle fracture was confined to a narrow region immediately adjacent to the dissimilar joint interface. The calculated normal pressure for fracture of Al₂O₃ particles ranges from 0.56–17.58 MPa and is in agreement with an experimentally measured pressure of 1.06 MPa found during sliding wear testing of aluminium-based composite base material. Because the lowest normal pressure applied during friction joining was 30 MPa, particle fracture occurs very early in the joining operation (immediately following contact between the two substrates). The application of a silver interlayer during dissimilar MMC/AISI 304 stainless steel joining decreased the particle fracture tendency. It is suggested that the presence of a silver interlayer decreased the coefficient of friction and lowered the stresses applied at the contact region. The particle fracture tendency was markedly increased when the MMC material contained blocky alumina particles. However, there was negligible particle fracture when the MMC base material contained spherical 72 wt % Al₂O₃–7 wt % Fe₂O₃–17 wt % SiO₂–3 wt % TiO₂ particles. This revised version was published online in November 2006 with corrections to the Cover Date.     

Workability of aluminium-based metal-matrix composites in cold compression

Cold compression tests have been carried out on aluminium-based metal-matrix composite (MMC) materials. Metallographic examinations have revealed very large local plastic flow in the matrix material subjected to compressive loading. It is found that localized shear flow of the matrix may control the failure of the composite under compression. A new processing technique has been developed and the cold workability of the MMCs has been greatly improved. The new processing technique is of significance for industrial application in that it shows the possibility of an alternative route for the forming of metal-matrix composites: hot forming to near net-shape, proper heat treatment and precision cold forging to final dimensions.     

The over-all instantaneous properties of metal-matrix composites

A micromechanical analysis is presented for the determination of the instantaneous effective properties of metal-matrix composites, which consist of elastic fibers reinforcing elastic-viscoplastic matrices. At any stage of loading, the current behavior of the initially isotropic matrix exhibits anisotropy that is load-level- and history-dependent. It is shown that the knowledge of the local instantaneous properties of the inelastic-matrix and elastic-fiber constants provides, in conjunction with the micromechanical analysis, the over-all instantaneous stiffnesses of the metal-matrix composite. The method is illustrated for the prediction of the instantaneous properties of unidirectional and laminated composite materials.     

Mechanical properties of aluminium-based particulate metal-matrix composites

A study has been made on a range of particulate-reinforced metal-matrix composites, all based on the aluminium alloy 7075, in underaged, peak aged and overaged conditions. Heat treatments were designed such that equivalent underaged and overaged matrix strengths were achieved. The reinforcement used was silicon carbide, in a range of particle sizes. All composites were produced by the co-spray deposition process. The tensile properties and fracture toughness of these materials were investigated at room temperature. Material reinforced with coarse particulate was observed to have poor yield strength, poor fracture stress and poor ductility compared with materials reinforced with finer particulate. Materials reinforced with the finest particulate had the greatest ductility. However, the coarse particulate-reinforced material exhibited reasonable toughness, similar to that of material containing the finest particles. A model to predict fracture toughness from tensile ductility and nominal interparticle spacing is proposed which offers an explanation for the observed experimental results.     

The temperature dependence of elastic nonlinearity in metal-matrix composites

Thermal stresses are very important in determining the strength of composites. In metal-matrix composites, these stresses are generated at the matrix-reinforcement interface as a result of the difference in thermal expansion coefficients of matrix and reinforcement during solidification. In order to evaluate these stresses, we studied the effect of temperature on the second- and third-order elastic constants in two metalmatrix composites consisting of the aluminum alloys 8091 and 7064 and silicon carbide particles up to 20% volume fraction. The elastic constants were determined at the temperatures 0, 25 and 55°C using measurements of absolute as well as changes of ultrasonic velocities as a function of applied stress. The values of these constants are used to calculate the acoustic nonlinearity parameters. In both composites, the acoustic nonlinearity parameters increase with the amount of reinforcement, which is opposite to that previously observed in aluminum alloys containing second-phase precipitates. Also, the temperature behavior of the nonlinearity parameters in the composites are opposite to those in the aluminum matrices. These differences in behavior are interpreted as due to the presence of thermal stresses at the matrix-reinforcement interface, and give promise to the possibility of using these parameters in the nondestructive evaluation of these stresses in metal-matrix composites.     

Fatigue crack growth with fiber failure in metal-matrix composites

Crack growth during the fatigue of fiber-reinforced metal-matrix composites can be predicted analytically by determining the reduction in the crack tip stress intensity range resulting from fiber bridging. Various canonical functions exist that relate the crack tip stress intensity range to bridged crack geometries and loading for both infinite and finite width specimens; however, comprehensive crack growth predictions incorporating fiber failure require knowledge of the maximum fiber stress in the bridged zone for all notch sizes and crack lengths. Previous modeling efforts have been extended to predict complete growth curves with fiber failure for specimens of finite width. Functions for maximum fiber stresses in the bridged zone are presented here for a center crack in tension and edge cracks in tension and bending. The rapid increase in crack growth when fibers fail emphasizes the importance of determining the loads and notch sizes that mark the beginning of fiber failure. Critical loads for given notch sizes and fiber strengths are easily determined for finite width specimens using the functions presented in this work.     

Thermal expansion and stress relaxation of metal-matrix composites

The coefficient of thermal expansion (CTE) of a series of Al-6%Si matrix samples, with reinforcements of carbon, SiC, Al₂O₃, or boron fibres, cloths, or ceramic particles was measured in the range 60°–220°C with a dilatometer. The anisotropy of the CTE was measured and found to be very large for specimens unidirectionally reinforced with carbon fibre. Relaxation of stresses due to the different thermal expansion of matrix and reinforcement was studied by using the bending of asymmetrically reinforced samples and the magnitude of the stresses evaluated using bending beam theory.     

Mechanical and tribological properties of zinc-aluminium metal-matrix composites

Recently, commercial Zn-Al foundry alloys such as ZA-27 have found increasing use for many applications and have competed effectively against copper, aluminium and iron-based foundry alloys. However, the elevated temperature (> 100°C) properties of zinc-aluminium alloys are unsatisfactory and restrict their use in some applications. One viable approach to improving the elevated temperature properties is to reinforce the zinc-aluminium alloys with alumina fibres. In this investigation, the mechanical properties of a Zn-Al alloy reinforced with alumina fibres were evaluated. Tensile, compression and impact properties were determined at 25, 100 and 150°C. Lubricated wear tests were also performed on the unreinforced alloy and composites. It was found that although fibre reinforcement did result in some improvement of tensile and compression properties at elevated temperatures, the composites had poor toughness and ductility. The presence of a brittle SiO₂ layer at the fibre/matrix interfaces resulted in fibre/matrix decohesion under tensile loading, impairing the performance of the reinforced materials. Some improvement in wear resistance was noted for the composite materials but fibre reinforcement did not yield significant improvement in fatigue resistance.     

A fibre coating process for advanced metal-matrix composites

A fibre coating process has been used to produce continuously reinforced advanced metal-matrix composites with up to 8% volume fraction of SiC fibre. Matrix materials were an / titanium alloy (Ti-Al-V), a dispersion-strengthened titanium alloy (Ti-Al-V-Y), a rapid solidification processed aluminium alloy (Al-4.3Cr-0.3Fe), and intermetallic compounds Ti₃Al and TiAl. Thick metal coatings are shown to adhere well to the fibres, no evidence is found for chemical reaction between the coating and the fibre during the coating process, and the coated fibres can be handled and bent without damage. Tensile test data for Ti-Al-V alloy reinforced with 21% SiC fibre show a modulus near to a theoretical prediction, but tensile strength significantly below prediction. Loss of strength is attributed to the formation of a brittle reaction product during hot consolidation. The advantages and potential of the coated-fibre route for MMC production are discussed.     

Texture of metal-matrix composites by ultrasonic velocity measurements

An ultrasonic method is developed for the nondestructive characterization of texture in metal-matrix composites. In this approach, it is assumed that the presence of reinforcement particles changes the elastic properties of the composite but only the texture of the matrix. The method utilizes the measurements of the six independent ultrasonic velocitiesV _ij and the formulation given by Bunge. The examined composites are the silicon carbide (SiC)-particle-reinforced aluminum 8091, 7064, and 6061 metal-matrix composites. The fourth-order expansion coefficients of the orientation distribution function are determined as a function of the SiC content in these composites. The results show that the expansion coefficients change with the presence of SiC where the coefficients C ₄ ¹¹ and C ₄ ¹³ increase as the volume fraction of SiC is increased and the coefficient C ₄ ¹² is zero in all composites examined. The analysis of these results indicates that ultrasonics can provide a promising technique for the texture characterization of metal-matrix composites.