Cast metal matrix composites

Abstract

Metal matrix composites have been available in certain forms for at least two decades, e.g. boron fibre reinforced aluminium and various dispersed phase alloys and cermets. Recently, a range of alumina and silicon carbide fibres, whiskers, and particles with diameters <20 μm have become available. The possibilities of incorporating these materials into metals to improve stiffness, wear resistance, and elevated temperature strength without incurring weight penalties have attracted the attention of design engineers in the aerospace and automobile industries. The aim of the present paper is to outline the manufacturing processes for such composites, in particular those based upon liquid metal technology, e.g. squeeze casting and spray forming. Some of the mechanical and physical properties which have been determined for these materials are described. An analysis of how matrix alloy selection may influence tensile and fracture behaviour of short fibre and particle reinforced composites is attempted.

MST/770     

Fatigue of metal matrix composites

One unidirectional and two laminated 6061-0 A-B composite plates were tested under various cyclic loading conditions. Three types of material response to cyclic loading were identified; No evidence of damage at relatively low cyclic loads, damage accumulation caused primarily by growth of long matrix cracks parallel to the fibers in off-axis layers at higher loads, and sudden localized failure of the fibers. Quantitative analysis of the results shows that the extent of internal damage, demonstrated by a reduction in axial elastic modulus, depends on the applied stress range and is independent of mean stress. The stress range at which damage first starts to appear coincides with the shakedown range of the laminate.

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Résumé On a testé sous des conditions de contrainte cyclique variable des plaques constituées d'une part par des alliages 6061-0 A-B unidirectionnels et d'autre part du même alliage composite bilaminé. On a identifié trois types de réponse du matériau aux contraintes cycliques, à savoir la non-évidence d'un dommage pour des cycles de charge à faible amplitude relative, une accumulation du dommage causé principalement par la croissance de longues fissures matricielles parallèles aux fibres dans les couches éloignées du plan médian à des contraintes plus élevées, et une fissure soudainement localisée des fibres. L'analyse quantitative des résultats montre que 1'extension du dommage interne telle qu'elle est dóntree par une réduction du module axial d'élasticité, dépend de l'amplitude des contraintes appliquées et est indéndante de la contrainte moyenne. L'amplitude de contrainte auquelle un dommage commence à apparaître coíncide avec l'amplitude de l'adaptation plastique du laminé telle qu'établie par le théorème de Melan.

     

Design with metal matrix composites

Abstract

Applications for metal matrix composites (MMCs) have not emerged at the rate needed to justify the development costs. A reason for this may be that material developments have not been adequately linked to identified commercial needs. It is certainly true that some of the expectations raised about the potential offered by MMCs have been misguided. As the MMC business contracts, there is an ever greater need for a systematic method of linking material properties to the needs of engineering designers. This paper presents a methodology for evaluating materials in design, with the aim of linking MMCs to applications. The methodology has two main components: first, the use of performance indices and materials selection charts for specific design goals, to compare existing MMCs with competing materials; and secondly, the conceptual design of new MMC systems guided by those design goals. A selection of case studies illustrates that in mechanical applications the gains in using MMCs are frequently marginal, whereas in design for thermal management and vibration control, the materials can show very substantial improvements in performance. The methodology is general, and could be applied to other material systems.

MST/3094     

Processing techniques for particulate-reinforced metal aluminium matrix composites

The critical need for high strength, lightweight and high stiffness materials has, in recent years, resurrected much interest in discontinuously reinforced powder metallurgy metal matrix composites. These hybrid materials have combined both standard wrought alloys of aluminium and a wide variety of discontinuous reinforcements such as particulates and whiskers of ceramic materials. Renewed interest in these materials as attractive candidates for use in the aerospace and transportation industry has resulted from an attractive and unique combination of physical and mechanical properties, and an ability to offer near isotropic properties coupled with the low cost of these materials when compared with existing monolithic materials. In this paper, the primary processing categories for discontinuously-reinforced metal-matrix composites are highlighted and the salient features of the various techniques in each category are discussed. The variables involved in each processing technique are examined, and the influence of alloy chemistry highlighted. Novel processing techniques for these materials such as the variable co-deposition method is presented as a means to process these novel engineering materials in order to improve their overall mechanical performance.     

Closed form constitutive equations for metal matrix composites

Explicit constitutive equations are given for the prediction of the overall behavior of unidirectional fiber-reinforced composites whose constituents are elastoplastic materials. The closed form expressions in these constitutive relations solely involve the elastic and inelastic properties of the phases as well as the fibers volume ratio. In the elastic region the average stress-strain relations are expressed in terms of the effective elastic moduli of the composite, all of which are given by closed form expressions. The derived constitutive relations can be readily implemented for the analysis of metal matrix composites and inelastic composite structures.     

金属基复合材料的高应变速率超塑性

综述并评论了金属基复合材料的高应变速率超塑变形机制，描述了金属基复合材料在高应变速率超塑变形中的一些理化现象，说明了变形过程中的各种影响因素，总结了具有高应变速率超塑性能的金属基复合材料及其性能，并指出了在金属基复合材料的高应变速率超塑性研究方面的不足。     

Dislocation-induced damping in metal matrix composites

The damping response of crystalline metals and alloys is generally associated with the presence of defects in the crystal lattice. The disturbance of these defects, usually in response to an applied cyclic load, dissipates energy, a mechanism known as internal friction. The various defects commonly found in crystalline materials include point defects (e.g. vacancies), line defects (e.g. dislocations), surface defects (e.g. grain boundaries) and volume defects (e.g. inclusions). Among these, dislocations are noteworthy because they play a critical role, not only in the damping response of crystalline materials, but also in the overall mechanical behaviour of the materials. Among the various structural materials actively being developed, metal matrix composites (MMCs) have received considerable attention as a result of their potential to combine reinforcement properties of strength and environmental resistance, with matrix properties of ductility and toughness. Of interest is the generally observed phenomenon that MMCs exhibit unusually high concentrations of dislocations, an observation typically attributed to the difference in coefficient of thermal expansion between matrix and reinforcement. The objectives of the present paper are to provide an overview of the sources of dislocation generation in MMCs, and to provide insight into the effects that dislocations have on the damping response of MMCs. The presence of dislocations in MMCs is highlighted on the basis of transmission electron microscopy studies, and the dislocation damping mechanisms are discussed in light of the Granato-Lücke theory.     

Fabrication of CuSiC metal matrix composites

A CuSiC MMC heatspreader will offer high thermal conductivity between 250 and 325 W/mK and corresponding adjustable thermal expansion coefficient between 8.0 and 12.5 ppm/°C. The primary challenge of CuSiC manufacture was to prevent reaction between copper and silicon carbide during high temperature densification, which dramatically degraded the thermal conductivity. In this study, the key issue addressed was the Si attack of Cu at the temperatures necessary for CuSiC fabrication (850 to 1200°C). Decomposition of SiC in contact with copper will dissolve Si in Cu causing a dramatic decrease of Cu thermal conductivity. This diffusion of Si into Cu can be prevented by the application of reliable barrier layers to diminish mass transport through the diffusion path and thereby minimizing the chemical interaction. A reliable barrier coating was identified and used to fabricate the CuSiC composites. The CuSiC composites were then characterized by SEM, TEM, XRD and XPS. Chemical analysis and thermal conductivity by laser flash diffusivity measurement illustrated the effectiveness of the barriers. A CuSiC composite having thermal conductivity of 322.9 W/m-K was successfully fabricated.     

Beryllium metal matrix composites for aerospace and commercial applications

Abstract

There is a growing need in both aerospace and commercial markets for lighter weight, higher stiffness, higher thermal stability materials to solve the design engineers’ problems of reduced mass, higher access speeds, improved mechanical and thermal stability for today's advanced technology. To address those needs, Brush Wellman Inc. has developed, characterised, and put into high volume production a family of beryllium metal matrix composites. There are two classes of materials that have been developed to provide these engineering benefits to the designer in both the commercial and aerospace markets. The first family of materials is aluminium beryllium (AlBeMet, which is a registered tradename of Brush Wellman). This material is a metal matrix composite consisting of pure beryllium and aluminium, with 20–62 vol.-%Be and the remainder aluminium. The material is produced by both powder metallurgy and net shape technologies such as investment casting and semi-solid forming. The materials properties that make it attractive for the design engineer are a density that is 25% less than that of aluminium, a specific stiffness four times those of aluminium, titanium, steel, and magnesium, a higher dampening capacity than aluminium, and a coefficient of thermal expansion almost 50%lower than aluminium. The second family of materials is a beryllium–beryllium oxide metal matrix composite, which are called E materials. This material was developed to address the thermal management needs of the electronic packaging design engineer. The material properties that make this material attractive to the electronic packaging engineer are: a density 20–25% that of Kovar, Invar, and CuMoCu, and 30% less than Al–SiC; a thermal conductivity ranging from 210 to 240 W m^-1 K^-1and a tailorable coefficient of thermal expansion, ranging from 6×10^-6 to 8.7×10^-6 K^-1.     

Interference films for microstructural characterization of metal matrix composites

Metallographic preparation and phase identification of metal matrix composites poses a challenge which is difficult to meet by conventional techniques; especially when features other than the reinforcement and matrix, such as inclusions, must be identified and interfacial reaction zones characterized. The use of thin oxide films to produce interference film colour contrast between different phases can make a significant contribution to solving these problems. The technique should be particularly suitable to automated image analysis.     

Hot isostatic pressing of metal reinforced metal matrix composites

Metal reinforced Metal Matrix Composites (MMMCs) made by combining an aluminium alloy matrix with stainless steel reinforcing wires are potentially cheaper and tougher than continuous fibre ceramic reinforced Metal Matrix Composites (MMCs). Although they do not give as great enhancements in stiffness and strength, worthwhile gains are achieved. Such MMMCs can be produced by Hot Isostatic Pressing (HIPping), which reduces interfacial reactions in comparison with liquid metal routes. Here, stainless steel (316L) and commercial purity aluminium wires were used to make bundles which were inserted into mild steel cans for HIPping at 525 °C/120 min/100 MPa. Some stainless steel wires were pre-coated with A17Si, to examine the effect of coatings on mechanical properties. Specimens were evaluated in terms of their tensile and fatigue properties. During HIPping, cans collapsed anisotropically to give different cross-section shapes, and for larger diameter cans, there was also some longitudinal twisting. Wires tended to be better aligned after HIPping in the smaller diameter cans, which produced material having higher modulus and UTS. Higher volume fractions of reinforcement tend to give better fatigue properties. Composites with coated stainless steel wires gave higher composite elongation to failure than uncoated wires. Both uncoated and coated wires failed by fatigue during fatigue testing of the composite. This contrasts with ceramic reinforced MMCs where the fibres fracture at weak points and then pull out of the matrix.     

High-speed turning experiments on metal matrix composites

The hard abrasive ceramic component which increases the mechanical characteristics of metal matrix composites (MMC) causes quick wear and premature tool failure in the machining operations. The aim of the paper is to compare the behaviour of high rake angle carbide tools with their diamond coated versions in high-speed machining of an Al₂O₃Al 6061 MMC. The influence of the cutting parameters, in particular cutting feed and speed, on tool wear and surface finish has been investigated. The higher abrasion resistance of the coatings results in increased tool life performances and different chip formation mechanisms.     

Machining of aluminium based metal matrix composites

Although metal matrix composites (MMCs) are generally regarded as extremely difficult to machine, it is also acknowledged that their machining behaviour is not fully understood. The work reviewed here confirms this widely held view but also suggests that the machinability of these materials can be improved by appropriate selection of the reinforcing phase, its volume fraction, size, and morphology as well as the composition and hardness of the matrix material. Cemented carbide tools can be used to machine some of the less abrasive materials at slow speeds but if higher production rates are required or the more abrasive materials are to be machined, polycrystalline diamond tooling is required.     

Thermo-viscoplastic residual stresses in metal matrix composites

A three-dimensional thermo-viscoplastic ideal method is presented to determine the interfacial and cellular stresses which arise during and from manufacturing of an ideal periodic continuous unidirectional graphite/aluminum metal matrix composite (MMC) lamina. The particular manufacturing process examined is a liquid-infiltrated cast MMC with temperature excursions from the matrix melting temperature of 933 K to room temperature. The final stress state of the aluminum matrix is found to be in the vicinity of its room temperature yield strength and essentially independent of fiber volume fraction. The interface has compressive normal tractions with an insignificant shear traction component present for fiber volume fractions less than 0·70, while for higher volume fractions, approximately one-half the interface experiences tensile normal tractions. Increased fiber volume fraction lowers fiber axial stresses and decreases the uniformity of the interfacial tractions. The magnitude of the residual stress state can be reduced from the value obtained from a constant cooling rate history by using an alternative cooling profile which has a rapid initial cooling rate of 0·75 K/s until 400 K, and a subsequent slower cooling rate of 0·2 K/min.     

Asymmetric behaviour of fibrous metal matrix composites

Abstract

Experimental investigations have illustrated that unidirectional metal matrix composites (MMCs) show asymmetric behaviour under uniaxial tensile and compressive loading. This asymmetry occurs when the material is loaded along the fibre direction and also when loaded in the transverse direction. In this paper, results from finite element micromechanical models are presented. The models were used to study the asymmetric behaviour of unidirectional fibre reinforced MMCs subjected to longitudinal and transverse loading. The effects of the thermal residual stresses arising from the manufacturing process were included in the study. Also, the influence of the degree of bonding of fibre to matrix was examined, from perfectly bonded to completely debonded. Results reveal that thermal residual stresses are responsible for the asymmetric behaviour of the MMCs in the longitudinal direction. In transverse loading, both the degree of interface bonding and residual stresses account for the asymmetric behaviour. The predicted stress–strain response of the MMC shows good agreement with the available experimental data for both tènsile and compressive loading. Results also suggest that in order to predict accurately the yielding behaviour of MMCs, the current symmetric yield criteria require modification.     

Structural performance of discontinuous metal matrix composites

Abstract

Discontinuous metal matrix composites (i.e. short fibre and particle reinforced materials) have attained a significant degree of scientific and technological maturity as advanced structural materials. Initial commercialisation has been achieved, with the unique combinations of mechanical and physical properties afforded by metal-ceramic systems proving appropriate for a variety of structural and semistructural applications. In recent years there has been important consolidation in the understanding of basic structural properties in such composites, which are addressed in the present review. The outstanding requirement for an improved understanding of damage tolerance characteristics in these materials is particularly noted. ‘Mesoscopic’ materials architectures (e.g. laminated and functionally graded materials) are also discussed, and the associated potential for development offracture resistant discontinuous metal composite materials highlighted.     

Corrosion behaviour of aluminium-based metal matrix composites

The corrosion characteristics, in 3.5 wt% NaCl solution, of aluminium alloy composites containing a range of reinforcements have been investigated using potentiostatic measurements and simple immersion tests. Complementary microstructural studies carried out on corroded surfaces and sections through corroded material have identified a number of preferential corrosion sites; these include the fiber/matrix interface, especially where it contains chemical reaction products resulting from composite fabrication, as well as second phases and pores in the metal matrix. The effect on corrosion behaviour of the different reinforcements, with particular reference to their chemistry and geometry, is discussed, as is the influence of composite manufacturing route.     

Synthesis of cast metal matrix particulate composites

The present review begins by briefly tracing history in the early days of development of cast metal-matrix composites and also outlines different casting routes for their synthesis. The problems faced by the quality of cast products and their relation to the process variables and characteristics of a given process, constitute the main theme of the review. The development of microstructure has been discussed in view of nucleation behaviour anticipated on the basis of estimated interface energies. The solidification around dispersoids and in regions away from it has been highlighted. Porosity in cast composites (its origin and control in cast components by suitable mould design) has engaged attention because of damage to mechanical properties due to porosity. The chemical reactions at the interface between dispersoid and matrix during processing of certain important systems of composites, have been described and the means of controlling these reactions have been indicated. The review concludes by drawing attention to the potential for application of cast composites in different industrial components and underlines the necessity of research in certain related fields so that industrial application of cast metal-matrix composites will soon become a reality.