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Which correctly describes the properties of a metal matrix composite (MMC) of an aluminum alloy reinforced with SiC particles?

  1. ACompared with the matrix alloy, it has a higher Young's modulus and wear resistance and lower thermal expansion, but lower ductility and toughness
  2. BCompared with the aluminum alloy matrix, its coefficient of thermal expansion is higher and its ductility is greatly improved
  3. CSiC particles do not react with molten aluminum at all, so no reaction products form at the interface whatever the processing temperature
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Answer: A. Compared with the matrix alloy, it has a higher Young's modulus and wear resistance and lower thermal expansion, but lower ductility and toughness

Adding SiC particles, which have a high Young's modulus of about 400 GPa and a low coefficient of thermal expansion of about 4×10⁻⁶ K⁻¹, makes the Young's modulus and wear resistance of the composite higher than those of the aluminum alloy matrix and its thermal expansion coefficient lower. Because the thermal expansion can be brought close to that of semiconductors and ceramic substrates, these composites are used for heat spreaders for power semiconductors and similar parts. On the other hand, cracks form easily around the hard particles, so ductility and toughness decrease. The statement that thermal expansion increases and ductility improves is backward on both counts. Also, SiC can react with hot molten aluminum to form brittle Al₄C₃, and the reaction is suppressed by controlling the processing temperature and the Si content of the matrix.

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