Metal matrix composites consist of a metal or an alloy as the continuous matrix and a reinforcement that can be particle, short fiber or whisker, or continuous fiber. In this chapter, we first describe important techniques to process metal matrix composites, then we describe the interface region and its characteristics, properties of different metal matrix composites, and finally, we summarize different applications of metal matrix composites.

Metal Matrix Composites

The most successful techniques for producing graphite-aluminum com posites have been those that utilize liquid-metal infiltration of graphite yarn. Almost all grades and types of graphite yarn can be infiltrated; how ever, a fiber coating is required, which results in wetting and provides a diffusion barrier to inhibit aluminum carbide formation. Two fiber coating methods are discussed in detail, including the sodium pretreatment method that results in intermetallic compound formation on the fiber and a chem ical vapor deposition method that applies a titanium-boron codeposit.

Progress in the Development of Graphite- Aluminum Composites Using Liquid Infiltration Technology:

This bibliography is comprised of approximately 1,600 reference citations related to four problem areas in the mechanics of failure in aerospace structures. The bibliography represents a search of the literature published in the period 1962-1976, the effort being largely limited to documents published in the United States. Listings are subdivided into the four problem areas: Hydrogen Embrittlement; Protective Coatings; Composite Materials; and Nondestructive Evaluation. An author index is included.

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Bibliography of information on mechanics of structural failure (hydrogen embrittlement, protective coatings, composite materials, NDE)

The mechanical properties of aluminum-graphite composites were measured at room temperature in the as-received condition, after elevated temperature exposure and after thermal cycling. The composites were fabricated by solid-state diffusion bonding of liquid-phase Al-infiltrated Thornel 50 fibers. The results showed that the maximum longitudinal tensile strength of the as-received material was 80,000 psi, which corresponds well with the rule of mixture value. The composite strength was observed to vary widely, depending on the extent of wetting of the fibers by the aluminum. The strength of the composites in the transverse direction was generally very low, due to poor interfacial bonding. Aluminum carbide (Al4C3) formed at the surface of the fibers at temperatures greater than 500 C. Development of the carbide was shown to be diffusion-controlled and was dependent on the time and temperature used. It was shown that the tensile strength was virtually unaffected by heat-treatment up to 500 C; beyond that temperature a drastic degradation of tensile strength occurred. Thermal cycling of the composites below 500 C resulted in an observable degradation of the composite strength.

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The effect of thermal exposure on the mechanical properties of aluminum-graphite composites

Compatibility between carbon fibre and various binary aluminium alloys is investigated. Series of aluminium alloys are coated onto the fibre surface and the wetting behaviour of these alloys are observed by scanning electron microscopy after heating the coated fibres at 1073 K, well above the melting point of the alloys. It is found that the aluminium alloys containing such elements as indium, lead and thallium showed excellent wetting behaviour. These alloying elements have significantly smaller surface energies as compared to aluminium and they are practically insoluble with aluminium, even in the liquid states. Tensile test shows that the strength of carbon fibres is not degraded after heating at 1073 K when fibres are coated with an Al-1 at % Tl alloy. The reasons for these elements in aluminium to substantially improve the compatibility against carbon fibres are discussed from the thermodynamical point of view.

Compatibility between carbon fibre and binary aluminium alloys

Room temperature uniaxial tensile tests have shown that composites of an Al-13 wt pct Si alloy and Thornel 50 graphite fibers have strengths greater than a theoretical value that was calculated on the basis of the law of mixtures. At 28 vol pct fibers, the average uniaxial tensile strength was found to be 106,000 psi, and several values between 130,000 and 144,000 psi were obtained. The modes of deformation and failure in the composites have been studied by the microexamination of polished surfaces and fractures of tested specimens. The reasons for the high strengths and the unusual modes of fracture that were observed cannot be explained on the basis of the presented data. Specimens of the composite have been thermally cycled between −193° and +20°C twenty times and others between −193° and +500°C twenty times. Tensile tests and microexamination of these thermally cycled specimens show that thermal cycling does not degrade the tensile properties of the composites or change their microstructure.

The tensile properties of a graphite-fiber-reinforced Al-Si alloy

: A composite consisting of an aluminum alloy matrix and a graphite fiber reinforcement was developed with strengths exceeding the 'rule of mixture' strength of 101 ksi. The composite is not degraded by temperature to 550C or by thermal cycling from -193 to +500C. The importance of various processing techniques to the successful development of this composite and the effects of certain process variables on properties are discussed. (Author)

Development of an Aluminum - Graphite Composite.

: High temperature uniaxial tensile tests have been performed on an aluminum matrix - graphite fiber composite material produced by liquid metal infiltration. A slight decrease in the composite fracture stress occurs at temperatures near the melting point of the matrix. Scanning electron microscope investigations of the fracture surfaces reveal an anomalously brittle high temperature fracture occurring in the matrix. Suggestions are made for possible fracture mechanisms at room and elevated temperatures. (Author)

High-Temperature Properties of Aluminum-Graphite Composites,

: Room temperature uniaxial tensile tests have shown that composites of eutectic aluminum - silicon alloy and Thornel 50 graphite fibers have strengths greater than a theoretical value that was calculated on the basis of the law of mixtures At 28 vol % fibers, the average uniaxial tensile strength was found to be 106,000 psi, and several values between 130,000 and 144,000 psi were obtained The modes of fracture in the composites have been studied by the microexamination of polished surfaces and fractures of tested specimens Specimens of the composite have been thermally cycled between -193C and +20C and -193C and +500C 20 times Tensile tests and microexamination of the thermally cycled specimens show that thermal cycling does not degrade the tensile properties of the composites or change their microstructure (Author)

John W. Upp

Papers

The tensile properties of a graphite-fiber-reinforced Al-Si alloy

Development of an Aluminum - Graphite Composite.

High-Temperature Properties of Aluminum-Graphite Composites,

Aluminum - graphite composites.