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| METAL-MATRIX COMPOSITES (MMC) |
METAL-MATRIX COMPOSITES
A composite material (1) is a material consisting of two or more physically and/or chemically distinct phases. The composite generally has superior characteristics than those of each of the individual components. Usually the reinforcing component is distributed in the continuous or matrix component. When the matrix is a metal, the composite is termed a metal-matrix composite (MMC). In MMCs, the reinforcement usually takes the form of particles, whiskers, short fibers, or continuous fibers (see COMPOSITE MATERIALS).
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2. Types of Metal-Matrix Composites
Metal-matrix composites are generally distinguished by characteristics of the reinforcement: particle- reinforced MMCs, short fiber- or whisker-reinforced MMCs, and continuous fiber- or layered MMCs.
Table 1 shows examples of some important reinforcements used in metal-matrix composites. These are categorized by the diameter and aspect ratio (length/diameter) of the reinforcement.
The aspect ratio of the reinforcement is an important quantity, because the degree of load transfer from the matrix to the reinforcement is directly proportional to the reinforcement aspect ratio. Thus, continuous fibers typically provide the highest degree of load transfer, because of their very high aspect ratio, which results in a significant amount of strengthening along the fiber direction.
Particle or short fiber reinforced metals have a much lower aspect ratio, so they exhibit lower strengths than their continuous fiber counterparts, although the properties of these composites are much more isotropic. Figures 1 a and b show typical microstructures of a continuous alumina fiber/Mg composite and silicon carbide particle/ Al composite, respectively.
3. Processing
Metal-matrix composites can be processed by several techniques. Some of these important techniques are described below.
3.1 Liquid-State Processes.
Casting or liquid infiltration involves infiltration of a fibrous or particulate reinforcement preform by a liquid metal. Liquid-phase infiltration of MMCs is not straightforward,mainly because of difficulties with wetting the ceramic reinforcement by the molten metal.
When the infiltration of a fiber preform occurs readily, reactions between the fiber and the molten metal may take place which significantly degrade the properties of the fiber. Fiber coatings (qv) applied prior to infiltration, which improve wetting and allow control of interfacial reactions, have been developed and are producing some encouraging results.
In this case, however, the disadvantage is that the fiber coatings must not be exposed to air prior to infiltration because surface oxidation of the coating takes place (2). One liquid infiltration process involving particulate reinforcement, called the Duralcan process, has been quite successful (Fig. 2). Ceramic particles and ingot-grade aluminum are mixed and melted. The melt is stirred slightly above the liquidus temperature (600−700°C). The solidified ingot may also undergo secondary processing by extrusion orrolling.
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The Duralcan process of making particulate composites by a liquid metal casting route involves the use of 8−12 μm particles. For particles that are much smaller (2−3 μm), the result is a very large interface region and, thus, a very viscous melt. In foundry-grade MMCs, high Si aluminum alloys (eg, A356) are used to prevent the formation of the brittle compound Al4C3, which is formed from the interfacial reaction between Al and SiC. Al4C3 is extremely detrimental to mechanical properties, particularly toughness and corrosion resistance.
Alternatively, tows of fibers can be passed through a liquid metal bath, where the individual fibers are wet by the molten metal, wiped of excess metal, and a composite wire is produced. A bundle of such wires can be consolidated by extrusion to make a composite.
Another pressureless liquid metal infiltration process of making MMCs is the Primex process (Lanxide), which can be used with certain reactive metal alloys such as Al− Mg to infiltrate ceramic preforms, Fig. 3. For an Al− Mg alloy, the process takes place between 750−1000°C in a nitrogen-rich atmosphere, and typical infiltration rates are less than 25 cm/h.
Squeeze casting or pressure infiltration involves forcing a liquid metal into a fibrous or particulate preform (2-4) (Fig. 4). Pressure is applied until solidification is complete. By forcing the molten metal through small pores of the fibrous preform, this method obviates the requirement of good wettability of the reinforcement by the molten metal. Composites fabricated with this method have the advantage of minimal reaction between the reinforcement and molten metal because of the short processing time involved. Such composites are also typically free from common casting defects such as porosity and shrinkage cavities.
Infiltration of a fibrous preform by means of a pressurized inert gas is another variant of the liquid metal infiltration technique. The process is conducted in the controlled environment of a pressure vessel and rather high fiber volume fractions; complex shaped structures are obtainable (3, 4). Alumina fiber- reinforced intermetallic matrix composites, e.g., TiAl, Ni3Al, and Fe3Al matrix materials, have also been prepared by pressure casting (5). The technique involves melting of the matrix alloy in a crucible in vacuum, while the fibrous preform is heated separately. The molten matrix material (at ∼100°C above Tm) is poured onto the fibers and argon gas is introduced simultaneously. Argon gas pressure forces the melt, which contains additives to aid wetting of the fibers, to infiltrate the preform.
3.2 Solid-State Processes
Diffusion bonding is a common solid-state processing technique for joining similar or dissimilar metals.
Interdiffusion of atoms between clean metallic surfaces, in contact at an elevated temperature, leads to bonding. The principal advantages of this technique are the ability to process a wide variety of metal matrices and control of fiber orientation and volume fraction. Among the disadvantages are long processing times, high processing temperatures and pressures (which makes the process expensive), and a limitation on the complexity of shapes that can be produced. There are many variants of the basic diffusion bonding process, although all of them involve simultaneous application of pressure and high temperature. Matrix alloy foil and fiber arrays (composite wire) or monolayer laminae are stacked in a predetermined order (Fig. 5).
Vacuum hot pressing is an important step in the diffusion bonding processes for metal- matrix composites. Hot isostatic pressing (HIP), instead of uniaxial pressing, can also be used. In HIP, gas pressure against a can consolidates the composite inside the can. With HIP it is relatively easy to apply high pressures at elevated temperatures with variable geometries.
Deformation processing can also be used to deform and/or densify the composite material. In metal−metal composites mechanical processing (swaging, extrusion, drawing, or rolling) of a ductile two-phase material causes the two phases to co-deform, causing one of the phases to elongate and become fibrous in nature within the other phase. These materials are sometimes referred to as in situ composites.
The properties of a deformation processed composite depend largely on the characteristics of the starting material, which is usually a billet of a two-phase alloy that has been prepared by casting or powder metallurgy methods (see METALLURGY, POWDER). Roll bonding is a common technique used to produce a laminated composite consisting of different metals in layered form (6). Such composites are called sheet laminated metal-matrix composites. Roll bonding and hot pressing have also been used to make laminates of Al sheets and discontinuously reinforced MMCs (7,8). Figure 6 shows the roll bonding process for making a laminated MMC. Other examples of deformation processed metal-matrix composites are niobium-based conventional filamentary superconductors with a copper matrix and the high-TC superconductors with a silver matrix. There are two main types of the conventional niobium-based superconductors: Nb−Ti⁄Cu and Nb3Sn⁄Cu.
Niobium−titanium ( ∼50−50) form a ductile system. Rods of Nb−Ti are inserted in holes drilled in a block of copper, evacuated, sealed, and subjected to a series of drawing operations interspersed with appropriate annealing treatments to obtain the final composite superconducting wire. In the case of Nb3Sn⁄Cu, a process called the bronze route is used to make this composite. Nb3Sn, an A-15-type intermetallic, cannot be processed like Nb−Ti because of its extreme brittleness. Instead, the process starts with a bronze (Cu−13% Sn) matrix; pure niobium rods are inserted in holes drilled in bronze, evacuated, sealed, and subjected to wire drawing operations as in the case of Nb−Ti⁄Cu . The critical step is the final heat treatment ( ∼700°C) that drives out the tin from the bronze matrix to combine with niobium to form stoichiometric, superconducting Nb3Sn , leaving behind copper matrix. A similar process, called the oxide- powder-in-tube (OPIT) method (9), is used to fabricate silver matrix high TC superconducting composites .
In this process, the oxide powder of appropriate composition (stoichiometry, phase content,,purity, etc.) is packed inside a metal tube (generally silver), sealed, and degassed. Commonly, swaging and drawing are used to make wires and rolling is used for tapes. Heat treatments, intermediate and/or subsequent to deformation, are conducted to form the desired phase, promote grain interconnectivity and crystallographic alignment of the oxide, and obtain proper oxygenation (9).
Powder processing methods in conjunction with deformation processing are used to fabricate particulate or short fiber reinforced composites. This typically involves cold pressing and sintering, or hot pressing to fabricate primarily particle- or whisker-reinforced MMCs (10). The matrix and the reinforcement powders are blended to produce a homogeneous distribution, Figure 7. The blending stage is followed by cold pressing to produce what is called a green body, which is about 80% dense and can be easily handled.
The cold pressed green body is canned in a sealed container and degassed to remove any absorbed moisture from the particle surfaces. The material is hot pressed, uniaxially or isostatically, to produce a fully dense composite and extruded. The rigid particles or fibers cause the matrix to be deformed significantly. In addition, during hot extrusion, dynamic recrystallization takes place at the particle/matrix interface, yielding randomly oriented grains near the interface, and relatively textured grains far from the interface, Figure 8 (11).
Sinter-forging is a novel and low cost deformation processing technique (12). In sinter-forging a powder mixture of reinforcement and matrix is cold compacted, sintered, and forged to nearly full density, Figure 9. The main advantage of this technique is that forging is conducted to produce a near-net shape material, and machining operations and material waste are minimized. The low cost, sinter-forged composites have tensile and fatigue properties that are comparable to those of materials produced by extrusion.
Deposition techniques for metal-matrix composite fabrication involve coating individual fibers in a tow with the matrix material needed to form the composite followed by diffusion bonding to form a consolidated composite plate or structural shape. The main disadvantage of using deposition techniques is that they are time consuming. However, there are several advantages. (1)
The degree of interfacial bonding is easily controllable; interfacial diffusion barriers and compliant coatings can be formed on the fiber prior to matrix deposition or graded interfaces can be formed. (2) Filament-wound thin monolayer tapes can be produced that are easier to handle and easier to mold into structural shapes than other precursor forms; unidirectional or angle-plied composites can be easily fabricated in this way.
Several deposition techniques are available: immersion plating, electroplating, spray deposition, chemical vapor deposition (CVD), and physical vapor deposition (PVD) (see THIN FILMS ). Dipping or immersion plating is similar to infiltration casting except that fiber tows are continuously passed through baths of molten metal, slurry, sol, or organometallic precursors. Electroplating (qv) produces a coating from a solution containing the ion of the desired material in the presence of an electric current. Fibers are wound on a mandrel, which serves as the cathode, and placed into the plating bath with an anode of the desired matrix material.
The advantage of this method is that the temperatures involved are moderate and no damage is done to the fibers. Problems with electroplating involve void formation between fibers and between fiber layers, possible poor adhesion of the deposit to the fibers, and limited numbers of alloy matrices available for this type of processing. A spray deposition operation may also be used.
This technique typically consists of winding fibers onto a foil-coated drum and spraying molten metal onto them to form a monotape. The source of molten metal may be powder or wire feedstock which is melted in a flame, arc, or plasma torch. The advantages of spray deposition are the easier control of fiber alignment and rapid solidification of the molten matrix. In the CVD process, a vaporized component decomposes or reacts with another vaporized chemical on the substrate to form a coating on that substrate. The processing is generally carried out at elevated temperatures.
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