**2. Types of metal matrix composites and their methods of production**

### **2.1 An overview of metal matrix composites**

A composite is a mixture of two or more constituents or phases which are chemically distinct on a microscopic scale, separated by a distinct interface, and can easily be specified. In addition, other criteria are normally satisfied before a material can be called a composite. The constituents have to be present in reasonable proportions, and the constituent phases should have distinctly different properties, such that the properties of the composite are noticeably different from the properties of the constituents [4]. The constituent which is continuous and in most cases available in larger quantities is termed the *matrix*. It is commonly viewed that it is the properties of the matrix that are improved upon in the process of producing a composite. The second constituent is known as the reinforcing phase, or *reinforcement*, as it enhances or reinforces the mechanical properties of the matrix. In most cases the reinforcement is harder, stronger and stiffer than the matrix, although there are some exceptions. The matrix may be in form of a ceramic material, metallic or polymeric, with each of these three classes of materials having considerably different /unique mechanical properties. Generally, polymers have low Young's moduli and strengths; ceramics are strong, stiff and brittle; and metals have intermediate moduli, strengths and good ductility [6].

Composite materials are usually classified according to the physical or chemical nature of the matrix, e.g. metal matrix, polymer matrix and ceramic composites. Additionally, the emergence of the intermetallic matrix and carbon matrix composites as reported by [7] has broadened the scope of composites. Intermetallic compounds are metal-based systems centred on the fixed atomic compositions occurring in metallic systems of aluminium with nickel (Ni), titanium (Ti) and niobium (Nb), such as Ni3Al, Ti3Al, TiAl and Nb3Al. Intermetallic compounds are of interest because they often exhibit higher melting points and less ease of deformation due to the lattice arrangement of their atoms [8].

In certain applications, metal matrix composite materials, formed by combining two or more materials—one of which is a metal—exhibit a primary advantage over their counterpart organic matrix composites in regard to the maximum operating temperature. To support this point, [9] reports that the boron/aluminium composite offers useful mechanical properties up to a temperature of 510°C, whereas an equivalent boron/epoxy composite is limited to about 190°C. Furthermore, composites of graphite/aluminium, graphite/copper and graphite/magnesium exhibit higher thermal conductivity due to the significant contribution from the metallic matrix. A metal matrix composite retains the desirable properties of both the matrix and the reinforcement by combining the strength of its reinforcement with the ductility of its matrix [10]. The reinforcing constituent may be a particle, platelet, short fibre or continuous fibre and may range from sub-micrometre to millimetre in size. There is a difference between metal matrix composites and multiphase metallic alloys as the concept of MMCs introduces additional degrees of

freedom into designing the microstructure. Materials with desirable properties not obtainable by conventional alloying and heat treatment can be created compositing. This can be achieved by altering the reinforcement type (metallic, ceramic or polymeric), content (volume fraction), size, shape, distribution and orientation [11].

In the early development of MMCs, continuous ceramic fibres and single-crystal ceramic whiskers were the preferred reinforcements as they provided the most remarkable increase in strength and stiffness. Later, particulate and discontinuously reinforced MMCs then followed, registering substantial progress on many fronts especially in composites with aluminium as the metal matrix. In aluminium metal matrix composites (AlMMCs), aluminium or its alloy forms a percolating network and is the matrix phase, while the other constituent, which is embedded in this matrix, is the reinforcement. The reinforcement is usually ceramic such as silicon carbide (SiC) or aluminium oxide (Al2O3). The properties of AlMMCs can be varied by varying the nature of the constituent phases and their volume fractions [4].

Although the MMCs have been in existence since the 1960s, they have not been put to full commercial use due to their higher production costs and lack of proper understanding of their high-temperature behaviour [12]. The higher costs are mainly attributed to the machining processes requiring tool materials to have very high wear resistance because of the reinforcement component being extremely abrasive [13]. However, with the invention of functionally graded materials (FGMs), it is now possible to reduce the cost of secondary processing. FGMs are an emerging category of advanced materials that exhibit gradual microstructural transitions and/or the composition in a specific direction and hence different functional performances within a part [14, 15].

The rapid growth and development of AlMMCs happened in the years after the launch of the Aluminium Metal Matrix Composites Roadmap 2002, a policy document produced by the Aluminium Metal Matrix Composites Consortium with support from the Technology Research Corporation (TRC) of the United States and other stakeholders. The document spelt out a pathway for the AlMMCs' growth in 20 years from 2002 and asserted the industry's vision to position AlMMCs as the material of choice in a broad range of structural and nonstructural applications. This vision was to be achieved by addressing three strategic goals, namely:


By that time, AlMMCs had proved their potential in such applications as aerospace, automotive, electronic packaging, commercial and industrial markets. The market was projected to grow at a 14% overall rate to \$173 million by 2004. The industry believed then that there was much greater unrealised potential for growth [16].

#### **2.2 Classification of metal matrix composites**

Metal matrix composites can be classified into several distinct classes, generally defined with reference to the type, shape and method of their reinforcements. The following classification is relevant to MMCs with aluminium as the matrix metal as explained in [4] and [11]. Typical microstructures are shown in **Figures 1** and **2**.

*Particle-reinforced MMCs*: Invariably known as particulate-reinforced MMCs, these composites generally contain equi-axed ceramic reinforcements, mainly

**75**

**Figure 1.**

*Novel Applications of Aluminium Metal Matrix Composites*

oxides (e.g. alumina, Al2O3), carbides (e.g. silicon carbide, SiC) or borides (e.g. titanium bromide, TiB2), with an aspect ratio less than 5 and present in volume fraction less than 30%. They can be produced by blending metal and the ceramic powders, followed by solid-state sintering or by liquid-metal techniques such as stir casting,

*Continuous fibre-reinforced MMCs*: These contain either relatively fine continuous fibres, usually of Al2O3, SiC or carbon, with a diameter below 20 μm, or coarser fibres or monofilaments. The former can be either parallel or pre-woven prior to infiltration to form a composite, while the bending flexibility of the latter limits the range of shapes that can be produced. Monofilaments are large diameter (100–150 μm) fibres, usually produced by chemical vapour deposition (CVD) of

*Whisker- and short-fibre-reinforced MMCs*: These contain reinforcements with an aspect ratio of greater than 5 but are not continuous. Short Al2O3 fibre-reinforced MMCs have been dominantly used in pistons. Whisker-reinforced composites, produced by either powder metallurgy or squeeze infiltration into a fibre preform, are generally produced to net/near-net shape. However, usage of whiskers as reinforce-

*Hybrid MMCs*: Hybrid MMCs essentially contain more than one type of reinforcement, for example, a mixture of particle and whisker, a mixture of fibre and particle or a mixture of hard and soft reinforcements. With the discovery of carbon nanotubes (CNT), composites with superior mechanical properties over those of

Other MMCs with variety of matrices other than aluminium include: *Cemented carbides* (*cermets*)—which are made by powder blending of a high proportion (60–75%) of ceramic or titanium carbide (TiC) with a metal such as cobalt, followed by holding for a short period at a temperature sufficient to melt the metallic constituent (liquid-phase sintering). In situ composites—in which directional solidification is used to form relatively fine aligned two-phase fibre or lamellar structures, resulting in an intermetallic reinforcement with high stiffness and strength. *Co-deformed composites* - in which immiscible metals are co-deformed such that filaments of the second phase with very large aspect ratio are formed within the matrix material. Typical examples include Cu-Cr and Cu-Nb systems. Cermets have outstanding high-temperature strength and are

Primary compositing processes for manufacturing of AlMMCs at industrial scale can be classified into two main groups, namely, (1) liquid-state processes and (2) solid-state processes [4]. The liquid-state processes are further classified into liquid-metal-mixing processes and liquid-metal-infiltration processes. Specifically, liquid-metal mixing is the primary compositing route for producing materials

*Typical microstructures of AlMMCs. (a) Al/Al2O3 platelets. (b) Al/Al2O3 continuous fibres. (c) Al/SiCp.* 

*(d) Al/graphite with 20 vol.% graphite flakes taken along the basal plane (source: [17, 18]).*

either SiC or boron (B) into a core of carbon fibre or tungsten (W).

ments is being restricted due to perceived health hazards.

*DOI: http://dx.doi.org/10.5772/intechopen.86225*

squeeze infiltration and in situ processes.

carbon have been produced [19].

widely used for tool bits [11].

**2.3 Methods of production of AlMMCs**

freedom into designing the microstructure. Materials with desirable properties not obtainable by conventional alloying and heat treatment can be created compositing. This can be achieved by altering the reinforcement type (metallic, ceramic or polymeric), content (volume fraction), size, shape, distribution and orientation [11]. In the early development of MMCs, continuous ceramic fibres and single-crystal

ceramic whiskers were the preferred reinforcements as they provided the most remarkable increase in strength and stiffness. Later, particulate and discontinuously reinforced MMCs then followed, registering substantial progress on many fronts especially in composites with aluminium as the metal matrix. In aluminium metal matrix composites (AlMMCs), aluminium or its alloy forms a percolating network and is the matrix phase, while the other constituent, which is embedded in this matrix, is the reinforcement. The reinforcement is usually ceramic such as silicon carbide (SiC) or aluminium oxide (Al2O3). The properties of AlMMCs can be varied by varying the nature of the constituent phases and their volume fractions [4].

Although the MMCs have been in existence since the 1960s, they have not been put to full commercial use due to their higher production costs and lack of proper understanding of their high-temperature behaviour [12]. The higher costs are mainly attributed to the machining processes requiring tool materials to have very high wear resistance because of the reinforcement component being extremely abrasive [13]. However, with the invention of functionally graded materials (FGMs), it is now possible to reduce the cost of secondary processing. FGMs are an emerging category of advanced materials that exhibit gradual microstructural transitions and/or the composition in a specific direction

The rapid growth and development of AlMMCs happened in the years after the launch of the Aluminium Metal Matrix Composites Roadmap 2002, a policy document produced by the Aluminium Metal Matrix Composites Consortium with support from the Technology Research Corporation (TRC) of the United States and other stakeholders. The document spelt out a pathway for the AlMMCs' growth in 20 years from 2002 and asserted the industry's vision to position AlMMCs as the material of choice in a broad range of structural and nonstructural applications. This vision was to be achieved by addressing three strategic goals, namely:

i.To reduce the cost of discontinuously reinforced AlMMCs to be comparable to

ii.To develop the necessary infrastructure to provide design confidence for

By that time, AlMMCs had proved their potential in such applications as aerospace, automotive, electronic packaging, commercial and industrial markets. The market was projected to grow at a 14% overall rate to \$173 million by 2004. The industry believed then that there was much greater unrealised potential for growth [16].

Metal matrix composites can be classified into several distinct classes, generally defined with reference to the type, shape and method of their reinforcements. The following classification is relevant to MMCs with aluminium as the matrix metal as explained in [4] and [11]. Typical microstructures are shown in **Figures 1** and **2**. *Particle-reinforced MMCs*: Invariably known as particulate-reinforced MMCs, these composites generally contain equi-axed ceramic reinforcements, mainly

and hence different functional performances within a part [14, 15].

existing alternatives by 2010

iii.To increase the market size for AlMMCs

**2.2 Classification of metal matrix composites**

AlMMCs

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oxides (e.g. alumina, Al2O3), carbides (e.g. silicon carbide, SiC) or borides (e.g. titanium bromide, TiB2), with an aspect ratio less than 5 and present in volume fraction less than 30%. They can be produced by blending metal and the ceramic powders, followed by solid-state sintering or by liquid-metal techniques such as stir casting, squeeze infiltration and in situ processes.

*Continuous fibre-reinforced MMCs*: These contain either relatively fine continuous fibres, usually of Al2O3, SiC or carbon, with a diameter below 20 μm, or coarser fibres or monofilaments. The former can be either parallel or pre-woven prior to infiltration to form a composite, while the bending flexibility of the latter limits the range of shapes that can be produced. Monofilaments are large diameter (100–150 μm) fibres, usually produced by chemical vapour deposition (CVD) of either SiC or boron (B) into a core of carbon fibre or tungsten (W).

*Whisker- and short-fibre-reinforced MMCs*: These contain reinforcements with an aspect ratio of greater than 5 but are not continuous. Short Al2O3 fibre-reinforced MMCs have been dominantly used in pistons. Whisker-reinforced composites, produced by either powder metallurgy or squeeze infiltration into a fibre preform, are generally produced to net/near-net shape. However, usage of whiskers as reinforcements is being restricted due to perceived health hazards.

*Hybrid MMCs*: Hybrid MMCs essentially contain more than one type of reinforcement, for example, a mixture of particle and whisker, a mixture of fibre and particle or a mixture of hard and soft reinforcements. With the discovery of carbon nanotubes (CNT), composites with superior mechanical properties over those of carbon have been produced [19].

Other MMCs with variety of matrices other than aluminium include: *Cemented carbides* (*cermets*)—which are made by powder blending of a high proportion (60–75%) of ceramic or titanium carbide (TiC) with a metal such as cobalt, followed by holding for a short period at a temperature sufficient to melt the metallic constituent (liquid-phase sintering). In situ composites—in which directional solidification is used to form relatively fine aligned two-phase fibre or lamellar structures, resulting in an intermetallic reinforcement with high stiffness and strength. *Co-deformed composites* - in which immiscible metals are co-deformed such that filaments of the second phase with very large aspect ratio are formed within the matrix material. Typical examples include Cu-Cr and Cu-Nb systems. Cermets have outstanding high-temperature strength and are widely used for tool bits [11].
