954Aluminum Metal Matrix Composites

A relatively new product, aluminum metal matrix composites (MMCs) consist of an aluminum alloy matrix with carbon, metallic, or, most commonly, ceramic reinforcement. Of all metals, aluminum is the most commonly used matrix material in MMCs. MMCs combine the low density of aluminum with the benefits of ceramics such as strength, stiffness (by increasing the modulus of elasticity), wear resistance, and high-temperature properties. They can be formed from both solid and molten states into forgings, extrusions, sheet and plate, and castings. Disadvantages include decreased ductility and higher cost; MMCs cost about three times more than conventional aluminum alloys. Yet even though they're still being developed, MMCs have been applied in automotive parts such as diesel engine pistons, cylinder liners, drive shafts, and brake components such as rotors.

Reinforcements are characterized as continuous or discontinuous depending on their shape and make up 10-70% of the composite by volume. Continuous fiber or filament reinforcements (designated f) include graphite, silicon carbide (SiC), boron, and aluminum oxide (Al2O3). Discontinuous reinforcements include SiC whiskers (designated w), SiC or Al2O3 particles (designated p), or short or chopped (designated c) Al2O3 or graphite fibers. The Aluminum Association standard designation system for aluminum MMCs identifies each as:

matrix material / reinforcement material / reinforcement volume %, form

For example, 2124/SiC/25 w is aluminum alloy 2124 reinforced with 25% by volume of silicon carbide whiskers; 6061/Al2O3/10p is aluminum alloy 6061 reinforced with 10% by volume of aluminum oxide particles. Chapter 5 has additional information on metal matrix composites.

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