Interfaces in metal matrix composites and the matrix, but too thick an interaction zone will adversely affect the composite Silicon carbide particle reinforced aluminum composites have been investigated extensively. An important processing technique for these MMCs involves liquid metal infiltration of a particulate preform. In a silicon-free aluminum alloy matrix, silicon carbide and molten aluminum can react as follows: 4Al()+3SiC(s)+ Al4 C3(s)+ 3Si(s) The forward reaction will add silicon to the matrix. as the silicon level increases in the molten matrix, the melting point of alloy decreases with time. The reaction can be made to go to the left by using high silicon alloys. This of course restricts the hoice of Al alloys for liquid route processing 4. 4. Thermal stresses In general, ceramic reinforcements( fibers, whiskers, or particles) have a coefficient of thermal expansion greater than that of most metallic matrices. This means that when the composite is subjected to a temperature change, thermal stresses will be generated in both the components. This observation is true for all composites--polymer, metal and ceramic-matrix composites. What is unique of metal matrix composites is the ability of a metal matrix to undergo plastic deformation in response to the thermal stresses generated and thus alleviate them. Chawla and Metzger [14], working wi a single crystal copper matrix containing large diameter tungsten fibers, showed the importance of thermal stresses in MMCs. Specifically, they employed a dislocation etch-pitting technique to delineate dislocations in single crystal copper matrix and showed that near the fiber the dislocation density was much higher in the matrix than the dislocation density far away from the fiber. The situation in the as-cast composite can be depicted as shown schematically in Fig. 6, where a primary plane section of the composite is shown having a hard zone(high dislocation density) around each fiber and a soft zone(low dislocation density) away from the fiber [15]. The enhanced dislocation density in the copper matrix near the fiber arises because of the plastic deformation in response to the thermal stresses generated by the thermal mismatch between the fiber and the matrix. It should be mentioned that the intensity of the gradient in dislocation density will depend on the interfiber spacing. The dislocation density gradient will decrease with a decrease in the interfiber spacing. The existence of a plastically deformed zone containing high dislocation density in the metallic matrix in the vicinity of the reinforcement has since been confirmed by transmission electron microscopy by a number of researchers, both in fibrous and particulate metal matrix composites [16-18]. Figure 7 shows an example of dislocation generation in the matrix near the silicon carbide particle/aluminum interface due to the temperature excursion during the processing of the composite. Such high dislocation density in the matrix can alter the precipitation behavior, and, consequently, the aging behavior in MMCs in those composites that have a precipitation hardenable alloy matrix [19] We mentioned the roughness induced radial compression stress at the fiber/matrix nterface in Section 4.2, as discussed above, the thermal mismatch between the295 and the matrix, but too thick an interaction zone will adversely affect the composite properties. Silicon carbide particle reinforced aluminum composites have been investigated extensively. An important processing technique for these MMCs involves liquid metal infiltration of a particulate preform. In a silicon-free aluminum alloy matrix, silicon carbide and molten aluminum can react as follows: The forward reaction will add silicon to the matrix. As the silicon level increases in the molten matrix, the melting point of alloy decreases with time. The reaction can be made to go to the left by using high silicon alloys. This of course restricts the choice of Al alloys for liquid route processing. 4.4. Thermal stresses In general, ceramic reinforcements (fibers, whiskers, or particles) have a coefficient of thermal expansion greater than that of most metallic matrices. This means that when the composite is subjected to a temperature change, thermal stresses will be generated in both the components. This observation is true for all composites-polymer-, metal-, and ceramic-matrix composites. What is unique of metal matrix composites is the ability of a metal matrix to undergo plastic deformation in response to the thermal stresses generated and thus alleviate them. Chawla and Metzger [14], working with a single crystal copper matrix containing large diameter tungsten fibers, showed the importance of thermal stresses in MMCs. Specifically, they employed a dislocation etch-pitting technique to delineate dislocations in single crystal copper matrix and showed that near the fiber the dislocation density was much higher in the matrix than the dislocation density far away from the fiber. The situation in the as-cast composite can be depicted as shown schematically in Fig. 6, where a primary plane section of the composite is shown having a hard zone (high dislocation density) around each fiber and a soft zone (low dislocation density) away from the fiber [15]. The enhanced dislocation density in the copper matrix near the fiber arises because of the plastic deformation in response to the thermal stresses generated by the thermal mismatch between the fiber and the matrix. It should be mentioned that the intensity of the gradient in dislocation density will depend on the interfiber spacing. The dislocation density gradient will decrease with a decrease in the interfiber spacing. The existence of a plastically deformed zone containing high dislocation density in the metallic matrix in the vicinity of the reinforcement has since been confirmed by transmission electron microscopy by a number of researchers, both in fibrous and particulate metal matrix composites [16-18]. Figure 7 shows an example of dislocation generation in the matrix near the silicon carbide particle/aluminum interface due to the temperature excursion during the processing of the composite. Such high dislocation density in the matrix can alter the precipitation behavior, and, consequently, the aging behavior in MMCs in those composites that have a precipitation hardenable alloy matrix [19]. We mentioned the roughness induced radial compression stress at the fiber/matrix interface in Section 4.2. As discussed above, the thermal mismatch between the