6.1.3 In Situ Fabrication Techniques -Controlled unidirectional solidification of a eutectic alloy can result in a two-phase microstructure with one of the phases, present in lamellar or fiber form, distributed in the matrix At low solidification rates -> The TaC fibers are square in cross section At higher solidification rates -> Blades of TaC form The number of fibers per square centimeter also increased with increasing solidification rate

6.1.2 Liquid State Fabrication Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

6.2 Interface in Metal Matrix Composites - In MMCs, as in other composites, we can have mechanical bonding as well as chemical bonding Mechanical Bonding Mechanical bonding restores the strength to levels Achieved in reaction bonding Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

6.2.2 Chemical Bonding - Some controlled amount of reaction at the interface may even be desirable for obtaining strong bonding between the fiber and the matrix. - Too thick an interaction zone, however, will adversely affect the composite properties. - Improvements in interfacial bonding in MMCs are frequently obtained by two methods : Fiber surface treatment, Matrix modification

6.2.2 Chemical Bonding Fiber Surface Treatment (a) To improve wettability and adhesion (b) To prevent any adverse chemical interaction between the fiber and the matrix at elevated temperature Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

6.2.2 Chemical Bonding In the case of carbon fibers in aluminum, poor wettability is a major problem. - The idea is to alter the matrix composition in such a way that dopants would react with the fiber. - In a controlled manner to give a thin fiber surface layer that will be wetted by the liquid matrix alloy.

6.2.3 Interfaces in In Situ Composites λ 2 R = const. λ : The spacing between rods or lamellae R : The solidification rate R Hall-Petch equation σ = σ 0 + kλ -1/2 σ 0 : intrinsic strength k : material constant

6.3 In Situ MMCs Van Suchtelen has classified eutectic or in situ composites into two broad categories from an electronic property viewpoint : 1. Combination type properties : subdivision into (a) sum type, (b) product type (a)Sum type : properties of the constituent phases contribute proportionally to their amount : ex. Heat conduction, density, elastic modulus (b) Product type : physical output of one phase serves as input for the other phase: ex. Conversion of a magnetic signal into an electrical signal in a eutectic composite with one phase magneto-strictive and the other piezoelectric

6.3 In Situ MMCs 2. Morphology- dependent properties : The properties depend on the periodicity and anisotropy of the microstructure, the shape and size of the phases, and the amount of interface area between the phases.

6.4 Discontinuous Reinforcement of MMCs -Silicon carbide whiskers(SiCw) and particles(SiCp) in an aluminum matrix

6.5 Properties -For a tough MMC, we need (a)Maximum inter-fiber spacing for a given fiber volume fraction. (b)Exploits the crack blunting characteristics of tough metals (c)Fibers with a high in situ strength and a low density of critical flaws.

6.5 Properties - The mechanical properties of MMCs are highly anisotropic Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

6.5 Properties - The strength properties vary linearly with fiber volume fraction Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

6.5 Properties - Thermal stress Chawla and Metzger : The importance of thermal stresses on microstructure, strength, cracking, general stress-strain behavior of tungsten/single- crystal copper matrix, tungsten/polycrystalline copper matrix, B/Al, B/Mg composites -> For a small change in temperature -> large enough to deform the matrix plastically

6.5 Properties Nieh and Karlak : The presence of B4C particles in 6061 Al matrix accelerated the aging response of the matrix. - high dislocation density, thermal mismatch between the reinforcement and the matrix, the presence of a highly diffusive interface - Porosity : major defects in cast MMCs owing to the shrinkage of the metallic matrix during solidification

6.5 Properties - Microstructure of the metallic matrix in a fiber composite can differ significantly from that of the unreinforced metal.

6.5 Properties Cross section of an SCS-2 silicon carbide fiber/Al-4.5%Cu matrix Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

6.5 Properties - in the reinforced region the dendritic morphology is controlled by the fiber distribution - the second phase appears preferentially at the fiber/matrix interface or in the narrow interfiber spaces It is possible to control the location of the second phase in the matrix, the amount of microsegregation, the grain size of the matrix, as well as characteristics of the fiber/metal interface.

6.5 Properties - Superior high-temperature properties of MMCs : For example. Silicon carbide whiskers(SiCw) Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

6.5 Properties - Very high stiffness : High-modulus carbon fiber/aluminum composites combine a very high stiffness with a very low thermal expansion due mainly to the almost zero longitudinal expansion coefficient of carbon fibers. - The Creep behavior of MMCs

6.5 Properties - Toughness and fatigue : the fatigue behavior of SiC whisker reinforced 2124 Al alloy composites. - Reducing the clustering of SiC and the number and size of the intermatallics resulted in increased fatigue life.

6.6 Applications - B/Al composites Boron fiber : high elastic modulus values, high tensile and compressive strength, low density → compression loading applications (Space Shuttle Orbiter) - SiC whiskers + Aluminum : reinforced pistons in diesel engines (Toyota Motor Co.) -Light, stiff, low expansion coefficient MMCs : space applications