Metal Matrix Composites

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Metal Matrix Composites and Nanocomposites

Metal Matrix Composites and Nanocomposites (MMCs and MMNCs) are composite materials with at least two phases, one being the metallic matrix and the other being the reinforcements. The difference between MMCs and MMNCs is the size of the reinforcements: reinforcements for MMNCs have at least one dimension less than 100 nm. The reinforcements can take the shape of particles, short fibers, or continuous fibers[1]. The motivation for the development of MMCs for commercial applications is the enhanced mechanical properties of the MMCs in comparison to the corresponding metals/alloys. There has been an explosive growth in research on MMNCs in the last decade to reveal the unusual enhancement effects by the nanoscale reinforcements. As compared with conventional MMCs that are reinforced, this new class of material can overcome many disadvantages of MMCs such as poor ductility, low fracture toughness and poor machinability[2]. Zhang et al. [3] manufactured TiB2 nanoparticles reinforced Inconel 625 alloy through the laser-aided additive manufacturing technique, resulting in a 40% increase in the hardness, 21.5% increase in the ultimate tensile strength and 35% increase in the yield strength of Inconel 625 due to the presence of the reinforcement nanoparticles. There is also scope for additional enhancement of properties like electrical conductivity, corrosion resistance, wear rate and coefficient of friction. MMC/MMNC manufacturing processes can be divided into conventional and additive manufacturing (AM) techniques.

Types of Metal Matrix Composites and Nanocomposites

The MMCs/MMNCs that are currently being produced and used in the industry can be broadly divided based on what type of matrix/reinforcement materials are used and whether the reinforcement materials are introduced ex-situ or formed in-situ.

  • Reinforcement Types: The selection criteria for the reinforcements within the MMCs/MMNCs depend on the desired mechanical properties and the intended area of application. The Reinforcements can be divided into five major classes based on size and shape considerations: Continuous and discontinuous fibers, whiskers, wires and particulates [4]. All the above-mentioned reinforcements are generally ceramics. Other reinforcements include carbon nanotubes (CNTs) and graphene. In recent times, nanoparticles have systematically replaced other mentioned reinforcements due to their ability to improve fracture toughness, creep resistance, surface roughness, wear resistance along with the enhancements to strength and hardness of the matrix material. Examples of ceramic nanoparticles include borides, carbides, nitrides, oxides, or their mixtures, such as TiC, TiB2, WC, Al2O3, SiC, B4C and ZrB2.
  • Ex-situ MMCs/MMNCs. In the ex-situ composites, the reinforcement materials are added into the metallic matrix directly. Ex-situ reinforcement processes have some drawbacks. Particle agglomeration and clustering resulting from improper dispersion may lead to inhomogeneous microstructures. In high-temperature processing techniques, irregular reinforcement presence may lead to interfacial reactions resulting in the formation of unwanted phases[5]. Storjohann et al. studied the feasibility of Al-based MMCs from fusion welding processes and observed the formation of the unwanted needle-like Al4C3 phase and gray faceted silicon phase in the fusion zone (FZ) of the 20 vol. % SiC reinforced 2124 alloy matrix [6]. However, ex-situ incorporation of stable reinforcements can offer enhancements for different materials.
  • In-situ MMCs/MMNCs. In the in-situ composites, the reinforcement in the matrix is formed because of a chemical reaction between different components. The chemical reactions occurring for the formation of reinforcements offer thermodynamic stability at high temperatures along with a clean interface with strong bonding quality [7]. However, the in-situ generation of the reinforcements only applies to specific materials. The probability of reinforcement formation via in-situ processes can be known by observing the reactive processes associated with the process. If an Al-Ti-C reactive system was considered, it is observed that there is scope for the formation of different phases like TiC, Al4C3, and Al3T; since this is a multicomponent system. However, since the type of phase formed is also related to the free energy change ∆G due to the reaction, it is important to select the starting raw materials and process parameters such that the intended reinforcement phase has the highest probability for formation. Sahoo et al. reported that TiC formation within the material results in the greatest negative free energy changes out of all the constituent phases, provided the process parameters are favorable [8].


  1. Clyne, T., and Withers, P., 1995, An introduction to metal matrix composites, Cambridge University Press.
  2. Everett, R., and Arsenault, R. J., 1991, "Metal matrix composites: mechanisms and properties."
  3. Zhang, B., Bi, G., Wang, P., Bai, J., Chew, Y., and Nai, M. S., 2016, "Microstructure and mechanical properties of Inconel 625/nano-TiB 2 composite fabricated by LAAM," Materials & Design, 111, pp. 70-79
  4. Gilliland, R. G., 1988, "Metal Matrix Composites, their structure, design, processing and applications," Process Symposium on future industrial technologyKobe.
  5. Borgonovo, C., and Apelian, D., "Manufacture of aluminum nanocomposites: a critical review," Proc. Materials Science Forum, Trans Tech Publ, pp. 1-22.
  6. Storjohann, D., Barabash, O., David, S., Sklad, P., Bloom, E., and Babu, S., 2005, "Fusion and friction stir welding of aluminum-metal-matrix composites," Metallurgical and Materials Transactions A, 36(11), pp. 3237-3247.
  7. Tjong, S. C., and Ma, Z., 2000, "Microstructural and mechanical characteristics of in situ metal matrix composites," Materials Science and Engineering: R: Reports, 29(3), pp. 49-113.
  8. Sahoo, P., and Koczak, M. J., 1991, "Analysis of in situ formation of titanium carbide in aluminum alloys," Materials Science and Engineering: A, 144(1-2), pp. 37-44.
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