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1 Aluminum matrix composites: Most of the industrial work on MMCs focuses on aluminum as the matrix metal. Aluminum alloys are very common due to the combination of light weight, environmental adaptability, and good mechanical properties; these same properties also make aluminum an excellent choice as a base metal. Aluminum has a sufficiently high melting point to meet many application requirements, yet a sufficiently low melting point to make the manufacturing of composite materials reasonably convenient. Aluminum can also accommodate various types of reinforcements. Although much of the early work on aluminum MMCs focused on continuous fiber types, many current studies are centered on discontinuously reinforced (particle or whisker) aluminum MMCs, due to their easier fabrication, lower production costs, and relatively isotropic properties. 01. Coding system for aluminum MMCs: Since the production of aluminum MMCs far exceeds that of MMCs made from all other matrix alloys combined, the Aluminum Association (AA) developed a standard coding system for MMCs, which has now been adopted by the American National Standards Institute. ANSI 35.5‑1992 specifies the identification of aluminum MMCs as follows: For example, 2124/SiC/25w denotes alloy 2124 reinforced with 25% by volume of whisker-shaped silicon carbide as registered with AA; 7075/Al2O3/10p denotes alloy 7075 reinforced with 10% by volume of particulate alumina as registered with AA; 6061/SiC/47f denotes alloy 6061 reinforced with 47% by volume of continuous silicon carbide fibers as registered with AA; and A356/C/05C is a cast alloy reinforced with 5% by volume of chopped graphite fibers as registered with AA. 02. Discontinuous aluminum MMCs: In discontinuously reinforced aluminum composites, the most commonly used reinforcing materials are SiC and Al2O3; although silicon nitride (Si3N4), TiB2, and graphite are also used in some specialized applications. For example, the aluminum–graphite alloy has been developed for use in tribology due to its excellent anti-friction, wear-resistant, and anti-seize properties. The processing methods for discontinuous aluminum MMC include various casting techniques: liquid metal infiltration, spray deposition, and powder metallurgy. The effect of enhancers on performance. The mechanical properties of MMC depend not only on those of the matrix material, but also on the quantity, size, shape, and distribution of the dispersed phase (reinforcer), as well as the properties of the interfaces. By definition, a composite material generally requires a certain number of dispersed phases of a certain size (>1μm), with a volume fraction of >1%; unlike in dispersion-strengthened materials, where these phases merely serve to control dislocation movement, they are intended to bear loads. The shape of the dispersed phase is so important in determining its load-bearing capacity that composite materials are classified accordingly: ① Fiber-reinforced composites with continuous and discontinuous fibers ； ②Particulate or whisker-reinforced composite materials. Its shape is characterized by the aspect ratio. In continuous fiber composites, the load acts directly on the matrix and fibers. In discontinuous fiber composites or particle-reinforced composites, the load is transmitted from the matrix to the dispersoids. Mathematical models can be used to predict the performance of MMC, which requires inputting parameters related to the properties and geometry of its components. However, in general, the effects of hard particle reinforcements (such as SiC) on the mechanical and physical properties of discontinuous aluminum MMCs are summarized as follows: ● Both tensile strength and yield strength increase as the proportion of the reinforcement increases (it should be noted that these properties decrease with the addition of soft particles such as graphite). ● Fracture toughness and plasticity (elongation and strain to failure) decrease as the volume fraction of the reinforcement increases. ● The elastic modulus increases as the volume fraction of the reinforcement increases ； ● The thermal conductivity, electrical conductivity, and thermal expansion coefficient all decrease as the volume fraction of the reinforcement increases. 03. Cast aluminum MMC: During the stir casting (or mixing/turbulation) process, the pre-treated and pre-fabricated reinforcing phase is poured into a continuously stirred molten matrix, and then cast into sand molds, metal molds, or pressure die molds. Melting is carried out under the shielding of an inert gas mixed with an Ar³SF6 mixture, which is used for fluxing and degassing; these steps are essential to prevent the absorption of gases. It can also be mixed using ultrasonic waves, or with the help of back-and-forth stirring rods, centrifugation, or weightless processes. The reinforcing agents include SiC or Al2O3 particles sized at 10–20 μm, with a volume content ranging from 10% to 30%. The development of cast aluminum MMC has promoted the automotive industry. Its current and potential applications include brake rotors and drums, brake calipers, brake pad backs, and cylinder liners. 04. Die-cast aluminum MMC Die-casting is a casting process in which molten metal is solidified under pressure applied to a closed mold, which is placed between the platens of a hydraulic press. The pressure exerted by the molten metal, along with its instantaneous contact with the mold surface, creates conditions for rapid heat transfer, thereby producing castings without porosity and with fine grain structure; moreover, their mechanical properties are similar to those of forged products. The die-casting of aluminum MMC involves first placing a porous ceramic preform inside a preheated mold, followed by pouring in liquid metal and applying pressure. At this point, the liquid metal is forced under pressure to penetrate into the porous ceramic preform, thereby forming a dense cerametallic composite material. Die casting is particularly valued because this process minimizes the use of materials and energy, produces parts in their final shape, and offers the ability to incorporate reinforcing agents as a choice. Two types of reinforcements, discontinuous and continuous, have been produced: aluminum–copper, aluminum–silicon, and aluminum–manganese reinforced alloys with a content of up to 45% (by volume). 05. Rheo-cast aluminum MMCs. Rheo-casting is also known as composite casting; it is similar to the metal stirring method, but instead of mixing particles into fully liquid metal, mixing takes place in a semi-solid (thixotropic) state, after which casting is carried out under pressure. Particles and discontinuous fibers of SiC, Al2O3, TiC, Si3N4, graphite, mica, glass, slag, magnesium oxide, and boron carbide can all be added to a strongly stirred and partially solidified aluminum alloy melt using this technique. 06.P/M Aluminum MMC: The powder metallurgy process for aluminum MMC involves granular and whisker-shaped SiC; however, Al2O3 particles and Si3N4 whiskers can also be used. Its processing procedure includes: ① Mixing the gas-atomized matrix alloy with powdered reinforcing agents ； ②The homogeneous mixture is pressed into a green body with a density of about 80% (cold pressing) ； ③Degas the preform (a structure with interconnected pores) to remove volatile impurities (lubricants, mixtures, and additive compounds), water vapor, and gases ； ④Compaction is carried out using vacuum hot pressing or hot isostatic pressing. The hot-pressed cylindrical billet can then be extruded, rolled, or forged. Whisker-reinforced aluminum MMCs can allow the whiskers to be arranged to a certain extent during extrusion or rolling. Controlling the alignment of whiskers enables the properties of the produced aluminum MMC products to be directional, which is essential for certain high-performance applications. Transverse rolling of thin plates can produce a more planar arrangement of whiskers, thereby achieving isotropy in two dimensions. 07. Aluminum/Graphite MMC: The development of this material began in the early 1960s, driven solely by the attractive commercial prospects of carbon fibers. The various properties of carbon fiber include an elastic modulus of up to 966 GPa (140 psi × 106), and a negative CTE (coefficient of thermal expansion) of as low as -1.62×10-6/°C (-0.9×10-6/°F). However, it is difficult to process and combine carbon and aluminum into a composite material. The difficulty in reacting between carbon and aluminum, the inability of carbon to be wetted by liquid aluminum, and the oxidizability of carbon all constitute major technical obstacles in producing such composite materials. Currently, there are two processes for producing industrial aluminum MMC: impregnation of liquid metal as the matrix within a fiber mat, and hot-press bonding of the fiber mat between aluminum foils. By utilizing two materials, namely precursor fibers and metal-coated fibers, secondary processes such as diffusion bonding or pultrusion are required to manufacture its structural components. Die casting can also be used to manufacture this composite material. Precision aerospace structural components that require high accuracy due to dimensional stability need materials with high rigidity, low weight, and low thermal deformation. Aluminum/graphite MMC has the potential to meet the above requirements. Unidirectional P100Gr/6061 aluminum extruded tubes exhibit a higher elastic modulus in the direction of the fibers compared to steel, while their density is only about 1/3 that of steel. 08. Aluminum/Al2O3 MMC: This material can be manufactured using various methods, but liquid or semi-solid processing techniques are generally employed. Alumina fibers, including Fiber FP (99.5% Al2O3) and Saffil (96% Al2O3 – 4% SiO2), are cost-effective composite materials whose properties have been improved compared to unreinforced aluminum alloys. For example, composite materials with improved wear resistance and resistance to thermal fatigue deformation, along with a reduced coefficient of thermal expansion. Continuous fiber Al/Al2O3 MMC is created by forming Al2O3 ribbon fibers into preforms in the desired direction, then inserting them into a mold and impregnating them with molten aluminum using vacuum. The adhesion of the reinforcement to the matrix is achieved by adding a small amount of lithium to the liquid metal. 2 Titanium matrix composites: Titanium is chosen as the matrix material due to its good specific strength at room temperature and at moderately high temperatures, as well as its excellent corrosion resistance. Since titanium can maintain its strength at higher temperatures better than aluminum, it is increasingly being used in place of aluminum for various aircraft and missile structural components where flight speeds increase from subsonic to supersonic. 01. Continuous fiber titanium MMC: Silicon carbide fibers are used as reinforcements for titanium MMC. SCS⁃6 fiber is a carbon-core monofilament with a diameter of 142 micrometers. A single tungsten-core fiber with a carbon coating and a diameter of 127 μm was also developed. Tungsten core filaments with a diameter of 102 μm and coated with carbon and titanium diboride are also used. A fiber content of (volume fraction) 30%–40% is quite common. Traditional matrix alloys include Ti³₆Al₃₄V for low-temperature applications, and Ti³₆Al₃₂Sn₃₄Zr₂Mo (Ti-6242) when higher creep resistance is required and the temperature exceeds the maximum tolerance for Ti³₆Al₃₄V. The Ti₃₆₂₄₂ alloy is used for the pistons of built-in exhaust bypass valves and reinforced fan frames in turbine engines. Recently, ordered intermetallic compounds of alumina, such as Ti³²Al²³Nb and Ti³²Al²⁶Nb, have been used as matrix materials. These materials are being developed for rotary blades and propellers. The processing techniques used for titanium MMC in aerospace applications include the fiber-film-fiber process, as well as cast strips or wound wires combined with thermal isostatic pressing. The titanium matrix can also be deposited on the fibers using plasma spraying technology. Similarly, physical vapor deposition using an electron beam to coat metals on fibers has also proven to be feasible. 02. Particle-reinforced titanium MMC is made using P/M. Different materials have been studied, and the most common combination is Ti³⁶Al³⁴V reinforced with 10%–20% (by volume) TiC. These composite materials possess increasingly high hardness and wear resistance superior to that of traditional titanium alloys. 3 Magnesium matrix composites: These composites possess properties that are essentially identical to those of aluminum MMCs – high stiffness, low weight, and a low coefficient of thermal expansion. In fact, as a matrix, a choice is usually made between aluminum and magnesium based on their weight and corrosion resistance. The density of magnesium is about 2/3 that of aluminum, but it is more reactive in corrosive environments. Magnesium also has a lower thermal conductivity, which is sometimes a factor in choosing it. Magnesium MMCs include continuous fiber Gr/Mg for aerospace structural components, short U-shaped fiber Al2O3/Mg for automotive engine parts, and discontinuous SiC or B4C/Mg for engine parts and low-expansion electronic packaging materials. The matrix alloys include AZ31, AZ91, ZE41, QE22, and EZ33. The process method used is similar to that of the corresponding aluminum MMC. 4 Copper-based composites: These materials are produced using continuous fiber reinforcements of tungsten, silicon carbide, and graphite. Among them, the continuous graphite/copper MMC has been the most extensively studied. The successful development of advanced graphite fibers has sparked interest in continuous graphite/copper MMCs. Copper has good thermal conductivity, but it is relatively heavy, and its mechanical properties at high temperatures are poor. Asphalt-based graphite fibers with better axial thermal conductivity than copper at room temperature have now been developed. In addition to reducing density, increasing stiffness, and raising the operating temperature compared to copper, this fiber also features a mechanism for customizing its coefficient of thermal expansion. One method for manufacturing graphite/copper MMCs is to use the electroplating process, that is, coating each graphite fiber with pure copper, thereby giving the MMC fibers sufficient flexibility to be woven into a fabric. 5 Composite materials based on superalloy matrices: The use of fusible metal (tungsten, molybdenum, and niobium) wires as fiber reinforcements in high-temperature superalloy MMCs continues to receive significant attention, despite their poor oxidation resistance and high density. Although the theoretical specific strength potential of fiber-reinforced composites made from refractory alloys is lower than that of those made from ceramic fibers, the higher plasticity of metal fiber systems allows for greater fiber-matrix interaction and thermal expansion mismatch. If refractory metal fibers are used to reinforce a plastic and oxidation-resistant matrix, they can be protected from oxidation, and the specific strength of the resulting composite materials is **higher than that of superalloys** at high temperatures. The production of superalloy MMC is accomplished through solid-state, liquid-state, or deposition processes. Its methods include investment casting, the use of matrix metal in the form of very thin sheets, the use of matrix metal rolled into powder sheets from powder and organic binders, powder metallurgy techniques, slurry casting of metal alloy powders, and arc spraying. 6 Composites based on intermetallic compound matrices. One drawback of superalloy MMCs is their high density, which limits the potential minimum weight of parts manufactured from these materials. The high melting point and relatively low density of intermetallic compound matrix composites (IMCs) make them an important material choice for lighter turbine engine components. Aluminides of nickel, titanium, and iron have attracted attention as matrix materials for IMC.