Composite Interfaces. Vol. 4. No. 5, pp. 287-298(1997) @vSP1997 Interfaces in metal matrix composites K. K. CHAWLA Department of Materials and Metallurgical Engineering, New Mexico institute of Mining and Technology, Socorro, New Mexico 87801, USA Received 23 June 1996: accepted 3 November 1996 Abstract-The interface region in a given composite has a great deal of importance in determining the ultimate properties of the composite. An interface is, by definition, a bidimensional region through which there occurs a discontinuity in one or more material parameters. In practice, there is always some volume associated with the interface region over which a gradual transition in material parameter(s)occurs. The importance of the interface region in composites stems from two main reasons: (i) the interface occupies a very large area in composites, and(ii) in general, the reinforcement and the metal matrix will form a system that is not in thermodynamic equilibrium. One can discuss the interface in a composite at various levels. An opti should be neither so simple that it covers only a few special cases nor so complex that it is not useful in designing composites from processing and applications points of view. In this paper. my objective is to give examples of interface microstructure in different metal matrix composite systems and suggest some ways of controlling the interface characteristics in order to control the properties of the composite. I shall give examples of the interface microstructure in different manx composites (particle and fiber reinforced as well as laminates)and discuss some of the ant implications various aspects of metal matrix composites, from the processing stage to ultimate perform Keywords: Metal matrix composite; interface: particle; fiber; reinforcement. 1 INTRODUCTION Metal matrix composites consist of a metal or an alloy as the continuous matrix and a reinforcement that can be particle, short fiber or whisker, ntinuous fib Table I provides examples of some important reinforcements used in metal matrix composites as well as their aspect(length/diameter) ratios and diameters. MMCs are really not new. Any heat treated steel or a two-phase metallic alloy is really a metal matrix composite. Hypereutectic Al-Si alloys represent a particulate metal matrix composite inasmuch as their microstructure consists of Si particles in an Al matrix. Metallurgists have controlled the shape and size of Si particles by means of alloy chemistry and solidification techniques. The new emphasis on MMCs involves mixing of reinforcement fiber or particles with a suitable metal matrix, generally larger volume fractions than ones found in steels and other alloys. In particular, it
Interfaces in metal matrix composites K. K. CHAWLA Department of Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA Received 23 June 1996; accepted 3 November 1996 Abstract_The interface region in a given composite has a great deal of importance in determining the ultimate properties of the composite. An interface is, by definition, a bidimensional region through which there occurs a discontinuity in one or more material parameters. In practice, there is always some volume associated with the interface region over which a gradual transition in material parameter(s) occurs. The importance of the interface region in composites stems from two main reasons: (i) the interface occupies a very large area in composites, and (ii) in general, the reinforcement and the metal matrix will form a system that is not in thermodynamic equilibrium. One can discuss the interface in a composite at various levels. An optimum one should be neither so simple that it covers only a few special cases nor so complex that it is not useful in designing composites from processing and applications points of view. In this paper, my objective is to give examples of interface microstructure in different metal matrix composite systems and suggest some ways of controlling the interface characteristics in order to control the properties of the composite. I shall give examples of the interface microstructure in different metal matrix composites (particle and fiber reinforced as well as laminates) and discuss some of the important implications on various aspects of metal matrix composites, from the processing stage to ultimate performance of the composite. Keywords: Metal matrix composite; interface; particle; fiber; reinforcement. 1. INTRODUCTION Metal matrix composites consist of a metal or an alloy as the continuous matrix and a reinforcement that can be particle, short fiber or whisker, or continuous fiber. Table 1 provides examples of some important reinforcements used in metal matrix composites as well as their aspect (length/diameter) ratios and diameters. MMCs are really not new. Any heat treated steel or a two-phase metallic alloy is really a metal matrix composite. Hypereutectic Al-Si alloys represent a particulate metal matrix composite inasmuch as their microstructure consists of Si particles in an Al matrix. Metallurgists have controlled the shape and size of Si particles by means of alloy chemistry and solidification techniques. The new emphasis on MMCs involves mixing of reinforcement fiber or particles with a suitable metal matrix, generally in larger volume fractions than ones found in steels and other alloys. In particular, it
KK Chawla Table 1 Typical reinforcements used in metal matrix composites Type Aspect ratio Diameter(um Examples Particle ~1-4 l-25 SiC, AlO3. BN. B4C Short fiber or whisker ~10-1000 SiC, AlzO3. Al2O3+ SiO, C Continuous fiber >1000 3-150 SiC, AlO3, C, B, w involves understanding and evaluating the characteristics of the interface region in order to obtain an optimum set of characteristics in the MMC The interface region in a composite is very important in determining the ultimate properties of the composite. An interface is the region through which there occurs a discontinu he or more material parameters such as elastic moduli, thermody namic parameters such as chemical potential, and the coefficient of thermal expansion The importance of the interface region in composites stems from two main reasons (i)the interface occupies a very large area in composites, and (ii)in general, the reinforcement and the matrix will form a system that is not in thermodynamic equi- There are many possible descriptions of the interface in a composite. An optimum one should be neither so simple that it covers only a few special cases nor so complex that it is not useful in designing composites from the points of view of processing and applications. In this paper, my objective is to give examples of interface microstructure in different metal matrix composite systems and suggest some means of controlling the interface characteristics in order to control the properties of the composite 2. WETTA BILITY An important parameter in regard to the interface is the wettability of reinforcement by the matrix. Wettability refers to the ability of a liquid to spread on a solid substrate Frequently, the contact angle between a liquid drop and a solid substrate is taken as a measure of wettability, a contact angle of 0 indicating perfect wetting while a contact angle of 180 indicating no wetting. Wettability is only a measure of the possibility of attaining an intimate contact between a liquid and a solid. good wetting is a necessary but not sufficient condition for strong bonding. One needs a good wetting even for purely mechanical bonding or weak van der Waals bonding, otherwise voids may form at the interface. Contact angle, 6, of the liquid metal matrix on the solid surface of the fiber is an important parameter to characterize the wettability. Commonly, the contact angle is measured by putting a sessile drop of the liquid on the flat surface of a solid substrate. When such a system is viewed from the side, the contact angle the angle between the tangents along three interfaces: solid/liquid, liquid/vapor, and solid/vapor. The angle 0 can be measured directly by a goniometer or calculated by using simple trigonometric relationships involving drop dimensions. In theory, one can use the following expression, called Youngs equation ysv= ysL nv cos 6
288 Table 1. Typical reinforcements used in metal matrix composites involves understanding and evaluating the characteristics of the interface region in order to obtain an optimum set of characteristics in the MMC. The interface region in a composite is very important in determining the ultimate properties of the composite. An interface is the region through which there occurs a discontinuity in one or more material parameters such as elastic moduli, thermodynamic parameters such as chemical potential, and the coefficient of thermal expansion. The importance of the interface region in composites stems from two main reasons: (i) the interface occupies a very large area in composites, and (ii) in general, the reinforcement and the matrix will form a system that is not in thermodynamic equilibrium. There are many possible descriptions of the interface in a composite. An optimum one should be neither so simple that it covers only a few special cases nor so complex that it is not useful in designing composites from the points of view of processing and applications. In this paper, my objective is to give examples of interface microstructure in different metal matrix composite systems and suggest some means of controlling the interface characteristics in order to control the properties of the composite. 2. WETTABILITY An important parameter in regard to the interface is the wettability of reinforcement by the matrix. Wettability refers to the ability of a liquid to spread on a solid substrate. Frequently, the contact angle between a liquid drop and a solid substrate is taken as a measure of wettability, a contact angle of 0° indicating perfect wetting while a contact angle of 180° indicating no wetting. Wettability is only a measure of the possibility of attaining an intimate contact between a liquid and a solid. Good wetting is a necessary but not sufficient condition for strong bonding. One needs a good wetting even for purely mechanical bonding or weak van der Waals bonding, otherwise voids may form at the interface. Contact angle, 0, of the liquid metal matrix on the solid surface of the fiber is an important parameter to characterize the wettability. Commonly, the contact angle is measured by putting a sessile drop of the liquid on the flat surface of a solid substrate. When such a system is viewed from the side, the contact angle is the angle between the tangents along three interfaces: solid/liquid, liquid/vapor, and solid/vapor. The angle 0 can be measured directly by a goniometer or calculated by using simple trigonometric relationships involving drop dimensions. In theory, one can use the following expression, called Young's equation
Interfaces in metal matrix composites where 8 is the contact angle and y is the surface energy per unit area. In very general terms, a small 6 indicates good wetting. In practice, it is rarely possible to obtain unique equilibrium value of 8. Most often, the ceramic reinforcement is rejected by the molten metal because of non-wettability or high contact angle. Sometimes the contact angle of a liquid drop on a solid substrate can be decreased by increasing the surface energy of the solid(ysv)or by decreasing the energy of the interface between the liquid and the solid(ysL). Thus, under certain circumstances, the wettability of a solid ceramic by a molten metal can be improved by making a small alloy addition to the matrix composition. An example of this is the addition of lithium to aluminum to improve the wettability in the alumina fiber/aluminum composite [1, 2]. However, besides wettability, there are other important factors such as chemical, mechanical thermal, and structural that affect the nature of bonding between reinforcement and matrix. As it happens, these factors frequently overlap and it may not always be possible to isolate these effects 3. MAJOR DISCONTINUITIES AT INTERFACES IN MMCS As we said above, at an interface there can occur a variety of discontinuities. The im- portant parameters that can show discontinuities in MMCs at a ceramic reinforcement/ metal matrix interface are as follows (i)Bonding. A ceramic reinforcement will have an ionic or a mixed ionic/covalent bonding while the metal matrix will have a metallic bonding (i)Crystallographic. The crystal structure and the lattice parameter of the matrix and the reinforcement will be different (iii) Moduli. Different elastic moduli of the matrix and the reinforcement. (iv) Chemical potential. The matrix and the reinforcement will not be in thermody namic equilibrium at the interface, i.e. there will be a driving force for a chemical reaction. Table 2 shows interfacial reaction products in some important MMC (v)Coefficient of thermal expansion(CTE). The matrix and the reinforcement will in general, have different CTE Interfacial reaction products in some important MMCs Reinforcement Matrix Reaction product(s) 二 TiC. Tis Si3 Al4C3 Alo MgO, MgAl2 O4(spinel) Al alloy Al alloy AlO3+ ZrO, Al alloy W None C Cu None None
289 where 9 is the contact angle and y is the surface energy per unit area. In very general terms, a small B indicates good wetting. In practice, it is rarely possible to obtain a unique equilibrium value of 9. Most often, the ceramic reinforcement is rejected by the molten metal because of non-wettability or high contact angle. Sometimes the contact angle of a liquid drop on a solid substrate can be decreased by increasing the surface energy of the solid (ysv) or by decreasing the energy of the interface between the liquid and the solid (ySL). Thus, under certain circumstances, the wettability of a solid ceramic by a molten metal can be improved by making a small alloy addition to the matrix composition. An example of this is the addition of lithium to aluminum to improve the wettability in the alumina fiber/aluminum composite [1, 2]. However, besides wettability, there are other important factors such as chemical, mechanical, thermal, and structural that affect the nature of bonding between reinforcement and matrix. As it happens, these factors frequently overlap and it may not always be possible to isolate these effects. 3. MAJOR DISCONTINUITIES AT INTERFACES IN MMCS As we said above, at an interface there can occur a variety of discontinuities. The important parameters that can show discontinuities in MMCs at a ceramic reinforcement/ metal matrix interface are as follows. (i) Bonding. A ceramic reinforcement will have an ionic or a mixed ionic/covalent bonding while the metal matrix will have a metallic bonding. (ii) Crystallographic. The crystal structure and the lattice parameter of the matrix and the reinforcement will be different. (iii) Moduli. Different elastic moduli of the matrix and the reinforcement. (iv) Chemical potential. The matrix and the reinforcement will not be in thermodynamic equilibrium at the interface, i.e. there will be a driving force for a chemical reaction. Table 2 shows interfacial reaction products in some important MMCs. (v) Coefficient of thermal expansion (CTE). The matrix and the reinforcement will, in general, have different CTE. Table 2. Interfacial reaction products in some important MMCs
K. K. Chawla 4. INTERFACIAL BONDING IN METAL MATRIX COMPOSITES We provide a summary of salient features of the interfacial region in some of the ost important metal matrix composites 4.1. Crystallographic nature In crystallographic terms, ceramic/metal interfaces in composites are, generally, inco- herent and high-energy interfaces. Accordingly, they can act as very efficient vacancy sinks, provide rapid diffusion paths, segregation sites, sites of heterogeneous precipita- tion, as well as sites for precipitate free zones. Among the possible exceptions to this are the eutectic composites [3] and the newer XDm type particulate composites [4] 4.2. Mechanical bonding Some bonding must exist between the ceramic reinforcement and the metal matrix for load transfer from matrix to fiber to occur. two main categories of bonding are mechanical and chemical. Mechanical keying effect between two surfaces can lead to bonding. Hill et al. [5] confirmed this experimentally for tungsten filaments in an aluminum matrix while Chawla and Metzger [6] observed mechanical gripping effects at AlO3/Al interfaces. The results of Chawla and Metzger [6] are shown in Fig. 1 in the form of the linear density of cracks in alumina as a function of strain in an alumina/aluminum composite for different degrees of interface roughness. The main message of this figure is that the crack density continues to increase to larger strain values in the case of a rough interface( deeply etched pits) vis-a-vis smooth or not very rough interface, i. e. the rougher the interface, the stronger the mechanical We can make an estimate of the radial stress at the fiber/ matrix interface due to roughness induced gripping [7] Emer E(1+m)+Em(1-) where E is Youngs modulus, v is Poisson's ratio, A is the amplitude of roughness, r is the radius of the fiber, and the subscripts m and f indicate matrix and fiber respectively. For a given composite, the compressive radial stress increases with the roughness amplitude and decreases with the fiber radius. Al uch an MMC, i.e. non-reacting components with a purely mechanical bond at the interface, is the filamentary superconducting composite consisting of niobium-titanium alloy filaments in a copper matrix 4.3. Chemical bon Ceramic/metal interfaces are generally formed at high temperatures. Diffusion and chemical reaction kinetics are faster at elevated temperatures. One needs to have knowledge of the chemical reaction products and, if possible, their properties. Molten
290 4. INTERFACIAL BONDING IN METAL MATRIX COMPOSITES We provide a summary of salient features of the interfacial region in some of the most important metal matrix composites. 4.1. Crystallographic nature In crystallographic terms, ceramic/metal interfaces in composites are, generally, incoherent and high-energy interfaces. Accordingly, they can act as very efficient vacancy sinks, provide rapid diffusion paths, segregation sites, sites of heterogeneous precipitation, as well as sites for precipitate free zones. Among the possible exceptions to this are the eutectic composites [3] and the newer XDTM type particulate composites [4]. 4.2. Mechanical bonding Some bonding must exist between the ceramic reinforcement and the metal matrix for load transfer from matrix to fiber to occur. Two main categories of bonding are mechanical and chemical. Mechanical keying effect between two surfaces can lead to bonding. Hill et al. [5] confirmed this experimentally for tungsten filaments in an aluminum matrix while Chawla and Metzger [6] observed mechanical gripping effects at A1203 /A1 interfaces. The results of Chawla and Metzger [6] are shown in Fig. 1 in the form of the linear density of cracks in alumina as a function of strain in an alumina/aluminum composite for different degrees of interface roughness. The main message of this figure is that the crack density continues to increase to larger strain values in the case of a rough interface (deeply etched pits) vis-a-vis smooth or not very rough interface, i.e. the rougher the interface, the stronger the mechanical bonding. We can make an estimate of the radial stress at the fiber/matrix interface due to roughness induced gripping [7] where E is Young's modulus, v is Poisson's ratio, A is the amplitude of roughness, r is the radius of the fiber, and the subscripts m and f indicate matrix and fiber, respectively. For a given composite, the compressive radial stress increases with the roughness amplitude and decreases with the fiber radius. An important example of such an MMC, i.e. non-reacting components with a purely mechanical bond at the interface, is the filamentary superconducting composite consisting of niobium-titanium alloy filaments in a copper matrix. 4.3. Chemical bonding Ceramic/metal interfaces are generally formed at high temperatures. Diffusion and chemical reaction kinetics are faster at elevated temperatures. One needs to have knowledge of the chemical reaction products and, if possible, their properties. Molten
Interfaces in metal matrix composites E ∝uo9omuo2 SMOOTH STEEP SDED PITS O PER cm GENTLY SLOPIN PERCENT ELONGATION Figure 1. Linear density of cracks in alumina as a function of strain in an alumina/aluminum composite for different degrees of interface roughness(from [4]) iron, nickel, titanium, low alloy steels, austenitic and ferritic stainless steels, nickel sed superalloys react with silicon containing ceramics to form eutectics, with the reaction products being mainly metal silicides and carbides. It is thus imperative to understand the thermodynamics and kinetics of reactions such that processing can be controlled and optimum properties can be obtained. We provide some examples Most metal matrix composite systems are nonequilibrium systems in the thermody- namic sense; that is, there exists a chemical potential gradient across the fiber/matrix interface. This means that given favorable kinetic conditions(which in practice means
291 Figure 1. Linear density of cracks in alumina as a function of strain in an alumina/aluminum composite for different degrees of interface roughness (from [4]J. iron, nickel, titanium, low alloy steels, austenitic and ferritic stainless steels, nickelbased superalloys react with silicon containing ceramics to form eutectics, with the reaction products being mainly metal silicides and carbides. It is thus imperative to understand the thermodynamics and kinetics of reactions such that processing can be controlled and optimum properties can be obtained. We provide some examples. Most metal matrix composite systems are nonequilibrium systems in the thermodynamic sense; that is, there exists a chemical potential gradient across the fiber/matrix interface. This means that given favorable kinetic conditions (which in practice means
K. Chawla a high enough temperature or long enough time), diffusion and/or chemical reactions will occur between the components. Two common morphologies of reaction products at an interface in common metal matrix composites are (a)a reaction layer that covers the ceramic reinforcement more or less uniformly, and (b)a discrete precipitate, particle or needle shaped, around the reinforcement. Type(a)reaction is controlled by diffusion of elements in the reaction layer and the square of the reaction zone thickness(r) varies linearly with time(t), i.e. x=Dr where D is the diffusion coefficient. Examples of such a reaction include b/Al and Sic/Ti [8]. An example of such a parabolic growth of the reaction zone in SCS-6 silicon carbide fiber/Ti-45Al-3 V-2Fe-2Mo(wt%)composites at three different temperatures is shown in Fig. 2 [9]. Figure 3 summarizes schematically the effect of time and temperature on the growth of the reaction layer. Silicon carbide fiber reinforced titanium matrix composites are attractive for some aerospace applications. In particular, titanium alloy matrix containing the SCS-6 silicon carbide fiber can have a very complex interfacial chemistry and microstructure Titanium and its alloy are very reactive in the liquid state; therefore, only solid state processing techniques such as diffusion bonding are used to make these composites A schematic of the interface region in these composites is shown in Fig. 4 [10] Type(b)reaction is controlled by nucleation process and discrete precipitation oc curs at the reinforcement/matrix interface. Examples include alumina/magnesium, carbon/aluminum, and alumina-zirconia/aluminum. Figure 5 shows an example,a dark field TEM micrograph, of the reaction zone between alumina fiber and magne sium matrix. Also to be seen in this figure are the deformation twins in the matrix, the result of thermal stresses on cooling during liquid metal infiltration. Aluminum o口△ F1.o[ N 200 Time"(s/2) growth of the reaction zone in SCS-6 silicon carbide fiber/Ti-45AI- 3V-2Fe-2Mo(wt%) composites nt temperatures(adapted from 161)
292 a high enough temperature or long enough time), diffusion and/or chemical reactions will occur between the components. Two common morphologies of reaction products at an interface in common metal matrix composites are: (a) a reaction layer that covers the ceramic reinforcement more or less uniformly, and (b) a discrete precipitate, particle or needle shaped, around the reinforcement. Type (a) reaction is controlled by diffusion of elements in the reaction layer and the square of the reaction zone thickness (x) varies linearly with time (t), i.e. x2 = Dt, where D is the diffusion coefficient. Examples of such a reaction include B/Al and SiC/Ti [8]. An example of such a parabolic growth of the reaction zone in SCS-6 silicon carbide fiber/Ti-4.SA1-3V 2Fe-2Mo (wt%) composites at three different temperatures is shown in Fig. 2 [9]. Figure 3 summarizes schematically the effect of time and temperature on the growth of the reaction layer. Silicon carbide fiber reinforced titanium matrix composites are attractive for some aerospace applications. In particular, titanium alloy matrix containing the SCS-6 silicon carbide fiber can have a very complex interfacial chemistry and microstructure. Titanium and its alloy are very reactive in the liquid state; therefore, only solid state processing techniques such as diffusion bonding are used to make these composites. A schematic of the interface region in these composites is shown in Fig. 4 [10]. Type (b) reaction is controlled by nucleation process and discrete precipitation occurs at the reinforcement/matrix interface. Examples include alumina/magnesium, carbon/aluminum, and alumina-zirconia/aluminum. Figure 5 shows an example, a dark field TEM micrograph, of the reaction zone between alumina fiber and magnesium matrix. Also to be seen in this figure are the deformation twins in the matrix, the result of thermal stresses on cooling during liquid metal infiltration. Aluminum Figure 2. An example of a parabolic growth of the reaction zone in SCS-6 silicon carbide fiber/ Ti-4.5AI- 3V-2Fe-2Mo (wt%) composites at three different temperatures (adapted from [6])
interfaces in metal matrix composites oxide can react with magnesium present in an aluminum alloy [ll] 3Mg+A2O3←→3MgO+2Al 3Mg+4Al2O3←→+3 MgOAl2O3+2Al At high levels of Mg and low temperatures, Mgo is expected to form while the spinel forms at low levels of magnesium [12] a good example of obtaining a processing window by exploiting the kinetics of interfacial reaction between the fiber and matrix can be had in the work of Isaacs et aL. 13]. These authors examined the interface structure in an aluminum matrix Exposure time→ Effects of Temperature 3>2>T1 Figure 3. A schematic of the effect of time and temperature on the growth of the reaction layer at an Second amorphous carbon layer First amorphous carbon layer Pyrocarbon layer Carbon core Carbon coating Mid-radial TiC Layer Figure 4. A schematic of the interface region in SCS-6 silicon carbide fiber/titanium alloy composite (after [lID)
293 oxide can react with magnesium present in an aluminum alloy [11]: At high levels of Mg and low temperatures, MgO is expected to form while the spinel forms at low levels of magnesium [12]. A good example of obtaining a processing window by exploiting the kinetics of interfacial reaction between the fiber and matrix can be had in the work of Isaacs et al. [13]. These authors examined the interface structure in an aluminum matrix Effects of . Time . Temperature T3 > T2 > T1 Figure 3. A schematic of the effect of time and temperature on the growth of the reaction layer at an interface. Figure 4. A schematic of the interface region in SCS-6 silicon carbide fiber/titanium alloy composite (after [11])
K. K. Chawla reinforced with an(alumina zirconia) fiber. The composite made by pressure filtration of the fibrous preform by liquid aluminum at 973 K(700C), with a dwell time of 13 min, showed faceted ZrAl3 platelets growing from the fiber into matrix However, they could suppress the kinetics of interfacial reaction by minimizing the high temperature exposure. No interfacial reaction product was observed when they processed the composite with the initial fibrous preform temperature below the melting point of aluminum and the solidification time less than I min Carbon fiber reacts with molten aluminum to form aluminum carbide which is a very brittle compound and highly susceptible to corrosion in humid environments hus, it becomes imperative to use a barrier coating on carbon fibers before bringing them in contact with the molten aluminum. The carbon fibers are coated with a co- deposition of Ti+ B(presumably giving TiB2). The starting materials for the coatin process are: TiCl4(g), BCl3(g), Zn(v)and the possible reaction products are: TiB TiCl3, TiCl3, ZnCl3. Any residual chloride in the coating is highly undesirable from a corrosion resistance point of view. The interface product(s) formed because of a reaction will generally have charac teristics different from those of either of the components. It should be pointed out, however. that at times some controlled amount of reaction at the interface. such as that shown in Fig. 5, may be desirable for obtaining strong bonding between the fiber FRZ M Figure 5. Microstructure of the reaction zone(Rz) between a continuous fiber a-Al2O3/Mg alloy(ZE41A) matrix,dark field (DF)TEM. M and F denote the matrix and the fiber, respectively. note also the defo mation twins in the matrix due to the thermal stresses during cooling
294 reinforced with an (alumina + zirconia) fiber. The composite made by pressure infiltration of the fibrous preform by liquid aluminum at 973 K (700°C), with a dwell time of 13 min, showed faceted ZrAl3 platelets growing from the fiber into matrix. However, they could suppress the kinetics of interfacial reaction by minimizing the high temperature exposure. No interfacial reaction product was observed when they processed the composite with the initial fibrous preform temperature below the melting point of aluminum and the solidification time less than 1 min. Carbon fiber reacts with molten aluminum to form aluminum carbide which is a very brittle compound and highly susceptible to corrosion in humid environments. Thus, it becomes imperative to use a barrier coating on carbon fibers before bringing them in contact with the molten aluminum. The carbon fibers are coated with a codeposition of Ti + B (presumably giving TiB2). The starting materials for the coating process are: TiCl4 (g), BC13 (g), Zn (v) and the possible reaction products are: TiB2, TiCl2, TiCl3, ZnCl3. Any residual chloride in the coating is highly undesirable from a corrosion resistance point of view. The interface product(s) formed because of a reaction will generally have characteristics different from those of either of the components. It should be pointed out, however, that at times, some controlled amount of reaction at the interface, such as that shown in Fig. 5, may be desirable for obtaining strong bonding between the fiber Figure 5. Microstructure of the reaction zone (RZ) between a continuous fiber ce-A]203/Mg alloy (ZE41A) matrix, dark field (DF) TEM. M and F denote the matrix and the fiber, respectively. Note also the deformation twins in the matrix due to the thermal stresses during cooling
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 the
295 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
PPER MATRIX DISLOCATION ETCH PITS SPECIMEN AXIS Figure 6. Schematic of the dislocation density on the primary slip plane of as-cast single crystal copper matrix containing tungsten filaments. There is a hard zone(high and a soft zone(low dislocation density away from the fiber A Sic 01m Figure 7. TEM micrograph showing dislocations in aluminum in the region near a silicon carbide parti cle(SiCp) omponents can lead to thermal residual stresses. Specifically, there will be introduced a radial thermal stress component which can be positive or negative. In order to btain the net radial stress at the fiber/ matrix interface, one should add the stresses algebraically from the two sources, viz., the contribution due to thermal mismatch between the reinforcement and the matrix and the stress arising due to the roughness induced gripping, as mentioned above. A combined expression for the radial stress
296 Figure 6. Schematic of the dislocation density on the primary slip plane of as-cast single crystal copper matrix containing tungsten filaments. There is a hard zone (high dislocation density) around each fiber and a soft zone (low dislocation density) away from the fiber. Figure 7. TEM micrograph showing dislocations in aluminum in the region near a silicon carbide particle (SiCP). components can lead to thermal residual stresses. Specifically, there will be introduced a radial thermal stress component which can be positive or negative. In order to obtain the net radial stress at the fiber/matrix interface, one should add the stresses algebraically from the two sources, viz., the contribution due to thermal mismatch between the reinforcement and the matrix and the stress arising due to the roughness induced gripping, as mentioned above. A combined expression for the radial stress
