7 Metal,Ceramic,and Carbon Matrix Composites In the earlier chapters of this book,we considered the performance,manufac- turing,and design issues pertaining to polymer matrix composites.In this chapter,we review the thermomechanical properties of metal,ceramic,and carbon matrix composites and a few important manufacturing methods used in producing such composites. The history of development of metal,ceramic,and carbon matrix compos- ites is much more recent than that of the polymer matrix composites.Initial research on the metal and ceramic matrix composites was based on continuous carbon or boron fibers,but there were difficulties in producing good quality composites due to adverse chemical reaction between these fibers and the matrix.With the development of newer fibers,such as silicon carbide or aluminum oxide,in the early 1980s,there has been a renewed interest and an accelerated research activity in developing the technology of both metal and ceramic matrix composites.The initial impetus for this development has come from the military and aerospace industries,where there is a great need for materials with high strength-to-weight ratios or high modulus-to-weight ratios that can also withstand severe high temperature or corrosive environments. Presently,these materials are very expensive and their use is limited to appli- cations that can use their special characteristics,such as high temperature resistance or high wear resistance.With developments of lower cost fibers and more cost-effective manufacturing techniques,it is conceivable that both metal and ceramic matrix composites will find commercial applications in automobiles,electronic packages,sporting goods,and others. The carbon matrix composites are more commonly known as carbon- carbon composites,since they use carbon fibers as the reinforcement for carbon matrix.The resulting composite has a lower density,higher modulus and strength,lower coefficient of thermal expansion,and higher thermal shock resistance than conventional graphite.The carbon matrix composites have been used as thermal protection materials in the nose cap and the leading edges of the wing of space shuttles.They are also used in rocket nozzles,exit cones,and aircraft brakes,and their potential applications include pistons in 2007 by Taylor&Francis Group.LLC
7 Metal, Ceramic, and Carbon Matrix Composites In the earlier chapters of this book, we considered the performance, manufacturing, and design issues pertaining to polymer matrix composites. In this chapter, we review the thermomechanical properties of metal, ceramic, and carbon matrix composites and a few important manufacturing methods used in producing such composites. The history of development of metal, ceramic, and carbon matrix composites is much more recent than that of the polymer matrix composites. Initial research on the metal and ceramic matrix composites was based on continuous carbon or boron fibers, but there were difficulties in producing good quality composites due to adverse chemical reaction between these fibers and the matrix. With the development of newer fibers, such as silicon carbide or aluminum oxide, in the early 1980s, there has been a renewed interest and an accelerated research activity in developing the technology of both metal and ceramic matrix composites. The initial impetus for this development has come from the military and aerospace industries, where there is a great need for materials with high strength-to-weight ratios or high modulus-to-weight ratios that can also withstand severe high temperature or corrosive environments. Presently, these materials are very expensive and their use is limited to applications that can use their special characteristics, such as high temperature resistance or high wear resistance. With developments of lower cost fibers and more cost-effective manufacturing techniques, it is conceivable that both metal and ceramic matrix composites will find commercial applications in automobiles, electronic packages, sporting goods, and others. The carbon matrix composites are more commonly known as carbon– carbon composites, since they use carbon fibers as the reinforcement for carbon matrix. The resulting composite has a lower density, higher modulus and strength, lower coefficient of thermal expansion, and higher thermal shock resistance than conventional graphite. The carbon matrix composites have been used as thermal protection materials in the nose cap and the leading edges of the wing of space shuttles. They are also used in rocket nozzles, exit cones, and aircraft brakes, and their potential applications include pistons in 2007 by Taylor & Francis Group, LLC
internal combustion engines,gas turbine components,heat exchangers,and biomedical implants. 7.1 METAL MATRIX COMPOSITES The metal matrix composites(MMC)can be divided into four general categories: 1.Fiber-reinforced MMC containing either continuous or discontinuous fiber reinforcements;the latter are in the form of whiskers with approxi- mately 0.1-0.5 um in diameter and have a length-to-diameter ratio up to200. 2.Particulate-reinforced MMC containing either particles or platelets that range in size from 0.5 to 100 um.The particulates can be incorporated into the metal matrix to higher volume fractions than the whiskers. 3.Dispersion-strengthened MMC containing particles that are 20 years;however,they have found limited use due to problems in controlling the chemical reaction between the fibers and the molten metal at the high processing temperatures used for such composites.The result of this chemical reaction is a brittle interphase that reduces the mechanical properties of the composite.Fiber surface treatments developed to reduce this problem increase the cost of the fiber.Additionally,the manufacturing cost of continuous carbon or boron fiber-reinforced MMC is also high,which makes them less attractive for many applications.Much of the recent work on MMC is based on silicon carbide whiskers (SiCw)or silicon carbide particulates (SiCp).SiC is less prone to oxidative reactions at the processing temperatures used.Furthermore,not only they are less expensive than carbon or boron fibers,but also they can be incorporated into metal matrices using common manufacturing techniques,such as powder metallurgy and casting. 7.1.1 MECHANICAL PROPERTIES In Chapter 2,we discussed simple micromechanical models in relation to polymer matrix composites in which fibers carry the major portion of the composite load by virtue of their high modulus compared with the polymer matrix,such as epoxy.The same micromechanical models can be applied to MMC with some modifications.The modulus of metals is an order of magni- tude higher than that of polymers (Table 7.1).Many metals are capable of 2007 by Taylor Francis Group,LLC
internal combustion engines, gas turbine components, heat exchangers, and biomedical implants. 7.1 METAL MATRIX COMPOSITES The metal matrix composites(MMC) can be divided into four general categories: 1. Fiber-reinforced MMC containing either continuous or discontinuous fiber reinforcements; the latter are in the form of whiskers with approximately 0.10.5 mm in diameter and have a length-to-diameter ratio up to 200. 2. Particulate-reinforced MMC containing either particles or platelets that range in size from 0.5 to 100 mm. The particulates can be incorporated into the metal matrix to higher volume fractions than the whiskers. 3. Dispersion-strengthened MMC containing particles that are 20 years; however, they have found limited use due to problems in controlling the chemical reaction between the fibers and the molten metal at the high processing temperatures used for such composites. The result of this chemical reaction is a brittle interphase that reduces the mechanical properties of the composite. Fiber surface treatments developed to reduce this problem increase the cost of the fiber. Additionally, the manufacturing cost of continuous carbon or boron fiber-reinforced MMC is also high, which makes them less attractive for many applications. Much of the recent work on MMC is based on silicon carbide whiskers (SiCw) or silicon carbide particulates (SiCp). SiC is less prone to oxidative reactions at the processing temperatures used. Furthermore, not only they are less expensive than carbon or boron fibers, but also they can be incorporated into metal matrices using common manufacturing techniques, such as powder metallurgy and casting. 7.1.1 MECHANICAL PROPERTIES In Chapter 2, we discussed simple micromechanical models in relation to polymer matrix composites in which fibers carry the major portion of the composite load by virtue of their high modulus compared with the polymer matrix, such as epoxy. The same micromechanical models can be applied to MMC with some modifications. The modulus of metals is an order of magnitude higher than that of polymers (Table 7.1). Many metals are capable of 2007 by Taylor & Francis Group, LLC
Taykr Francis Group. TABLE 7.1 Properties of Some Metal Alloys Used in Metal Matrix Composites Tensile Density, Modulus, YS, UTS,MPa Failure CTE 10-6 Melting Material g/cm3 GPa(Gsi) MPa (ksi) (ksi) Strain,% per℃ Point,℃ Aluminum alloy 2024-T6 2.78 70(10.1) 468.9(68) 579.3(84) 11 23.2 6061-T6 2.70 70(10.1) 275.9(40) 310.3(45) 17 23.6 7075-T6 2.80 70(10.1) 503.5(73) 572.4(83) 23.6 8009 2.92 88(12.7 407(59) 448(64.9)】 17 23.5 380(As cast) 2.71 70(10.1) 165.5(24) 331(48) 540 Titanium alloy Ti-6A1-4V 4.43 110(16) 1068(155) 1171(170) 9.5 1650 (Solution-treated and aged) Magnesium alloy AZ91A 1.81 45(6.5) 158.623) 234.5(34) 26 650 Zinc-aluminum alloy ZA-27(Pressure die-cast) 5 78(11.3) 370(53.6) 425(61.6) 3 26 375
TABLE 7.1 Properties of Some Metal Alloys Used in Metal Matrix Composites Material Density, g=cm3 Tensile Modulus, GPa (Gsi) YS, MPa (ksi) UTS, MPa (ksi) Failure Strain, % CTE 106 per 8C Melting Point, 8C Aluminum alloy 2024-T6 2.78 70 (10.1) 468.9 (68) 579.3 (84) 11 23.2 6061-T6 2.70 70 (10.1) 275.9 (40) 310.3 (45) 17 23.6 7075-T6 2.80 70 (10.1) 503.5 (73) 572.4 (83) 11 23.6 8009 2.92 88 (12.7) 407 (59) 448 (64.9) 17 23.5 380 (As cast) 2.71 70 (10.1) 165.5 (24) 331 (48) 4 — 540 Titanium alloy Ti-6A1-4V (Solution-treated and aged) 4.43 110 (16) 1068 (155) 1171 (170) 8 9.5 1650 Magnesium alloy AZ91A 1.81 45 (6.5) 158.6 (23) 234.5 (34) 3 26 650 Zinc–aluminum alloy ZA-27 (Pressure die-cast) 5 78 (11.3) 370 (53.6) 425 (61.6) 3 26 375 2007 by Taylor & Francis Group, LLC
undergoing large plastic deformations and strain hardening after yielding.In general,they exhibit higher strain-to-failure and fracture toughness than poly- mers.Furthermore,since the processing temperature for MMCs is very high, the difference in thermal contraction between the fibers and the matrix during cooling can lead to relatively high residual stresses.In some cases,the matrix may yield under the influence of these residual stresses,which can affect the stress-strain characteristics as well as the strength of the composite. 7.1.1.1 Continuous-Fiber MMC Consider an MMC containing unidirectional continuous fibers subjected to a tensile load in the fiber direction.Assume that the matrix yield strain is lower than the fiber failure strain.Initially,both fibers and matrix deform elastically The longitudinal elastic modulus of the composite is given by the rule of mixtures: EL EfVf EmVm. (7.1) After the matrix reaches its yield strain,it begins to deform plastically,but the fiber remains elastic.At this point,the stress-strain diagram begins to deviate from its initial slope (Figure 7.1)and exhibits a new longitudinal modulus, which is given by: do EL=Evf十 Vm? (7.2) /m do where is slope of the stress-strain curve of the matrix at the composite strain se.The stress-strain diagram of the composite in this region is not elastic. In addition,it may not be linear if the matrix has a nonuniform strain- hardening rate. For brittle fiber MMCs,such as SiC fiber-reinforced aluminum alloys,the composite strength is limited by fiber fracture,and the MMCs fail as the com- posite strain becomes equal to the fiber failure strain.For ductile fiber MMCs, such as tungsten fiber-reinforced copper alloys [5]and beryllium fiber-reinforced aluminum alloys [6],the fiber also yields and plastically deforms along with the matrix.In addition,the composite strength is limited by the fiber failure strain, unless the fibers fail by necking.If the fibers exhibit necking before failure and its failure strain is lower than that of the matrix,the strain at the ultimate tensile stress of the composite will be greater than that at the ultimate tensile stress of the fiber alone. If the composite failure is controlled by the fiber failure strain,the longitudinal composite strength is given by OLm OfuVf +om(1 -Vf), (7.3) 2007 by Taylor Francis Group,LLC
undergoing large plastic deformations and strain hardening after yielding. In general, they exhibit higher strain-to-failure and fracture toughness than polymers. Furthermore, since the processing temperature for MMCs is very high, the difference in thermal contraction between the fibers and the matrix during cooling can lead to relatively high residual stresses. In some cases, the matrix may yield under the influence of these residual stresses, which can affect the stress–strain characteristics as well as the strength of the composite. 7.1.1.1 Continuous-Fiber MMC Consider an MMC containing unidirectional continuous fibers subjected to a tensile load in the fiber direction. Assume that the matrix yield strain is lower than the fiber failure strain. Initially, both fibers and matrix deform elastically. The longitudinal elastic modulus of the composite is given by the rule of mixtures: EL ¼ Efvf þ Emvm: (7:1) After the matrix reaches its yield strain, it begins to deform plastically, but the fiber remains elastic. At this point, the stress–strain diagram begins to deviate from its initial slope (Figure 7.1) and exhibits a new longitudinal modulus, which is given by: EL ¼ Efvf þ ds d« m vm, (7:2) where ds d« m is slope of the stress–strain curve of the matrix at the composite strain «c. The stress–strain diagram of the composite in this region is not elastic. In addition, it may not be linear if the matrix has a nonuniform strainhardening rate. For brittle fiber MMCs, such as SiC fiber-reinforced aluminum alloys, the composite strength is limited by fiber fracture, and the MMCs fail as the composite strain becomes equal to the fiber failure strain. For ductile fiber MMCs, such as tungsten fiber-reinforced copper alloys [5] and beryllium fiber-reinforced aluminum alloys [6], the fiber also yields and plastically deforms along with the matrix. In addition, the composite strength is limited by the fiber failure strain, unless the fibers fail by necking. If the fibers exhibit necking before failure and its failure strain is lower than that of the matrix, the strain at the ultimate tensile stress of the composite will be greater than that at the ultimate tensile stress of the fiber alone. If the composite failure is controlled by the fiber failure strain, the longitudinal composite strength is given by sLtu ¼ sfuvf þ sm 0 (1 vf), (7:3) 2007 by Taylor & Francis Group, LLC.
Brittle fiber Ductile fiber Ductile fiber ssens composite Brittle fiber composite Secondary modulus Primary modulus Matrix Strain FIGURE 7.1 Schematic representation of longitudinal tensile stress-strain diagram of a unidirectional continuous fiber-reinforced MMC. where om is the matrix flow stress at the ultimate fiber strain that is determined from the matrix stress-strain diagram. Equation 7.3 appears to fit the experimental strength values for a number of MMCs,such as copper matrix composites(Figure 7.2)containing either brittle or ductile tungsten fibers [5].In general,they are valid for MMCs in which (1) there is no adverse interfacial reaction between the fibers and the matrix that produces a brittle interphase,(2)there is a good bond between the fibers and the matrix,and(3)the thermal residual stresses at or near the interface are low. The longitudinal tensile strength predicted by Equation 7.3 is higher than the experimental values for carbon fiber-reinforced aluminum alloys.In these systems,unless the carbon fibers are coated with protective surface coating,a brittle Al4C3 interphase is formed.Cracks initiated in this interphase cause the fibers to fail at strains that are lower than their ultimate strains.In some cases, the interfacial reaction is so severe that it weakens the fibers,which fail at very low strains compared with the unreacted fibers [7].If the matrix continues to carry the load,the longitudinal tensile strength of the composite will be OLtu Omu(1-Vf), (7.4) 2007 by Taylor&Franeis Group.LLC
where sm 0 is the matrix flow stress at the ultimate fiber strain that is determined from the matrix stress–strain diagram. Equation 7.3 appears to fit the experimental strength values for a number of MMCs, such as copper matrix composites (Figure 7.2) containing either brittle or ductile tungsten fibers [5]. In general, they are valid for MMCs in which (1) there is no adverse interfacial reaction between the fibers and the matrix that produces a brittle interphase, (2) there is a good bond between the fibers and the matrix, and (3) the thermal residual stresses at or near the interface are low. The longitudinal tensile strength predicted by Equation 7.3 is higher than the experimental values for carbon fiber-reinforced aluminum alloys. In these systems, unless the carbon fibers are coated with protective surface coating, a brittle Al4C3 interphase is formed. Cracks initiated in this interphase cause the fibers to fail at strains that are lower than their ultimate strains. In some cases, the interfacial reaction is so severe that it weakens the fibers, which fail at very low strains compared with the unreacted fibers [7]. If the matrix continues to carry the load, the longitudinal tensile strength of the composite will be sLtu ¼ smu(1 vf), (7:4) Brittle fiber Ductile fiber Ductile fiber composite Brittle fiber composite Secondary modulus Primary modulus Matrix Strain Stress FIGURE 7.1 Schematic representation of longitudinal tensile stress–strain diagram of a unidirectional continuous fiber-reinforced MMC. 2007 by Taylor & Francis Group, LLC
1200 (edW) 1000 u6uens 800 O Equation 7.3 600 400 ● 200 Equation 7.4 0 0 0.5 1.0 Fiber volume fraction,Vi FIGURE 7.2 Longitudinal tensile strength variation of a unidirectional continuous tungsten fiber/copper matrix composite at various fiber volume fractions.(Adapted from Kelly,A.and Davies,G.J.,Metall.Rev.,10,1,1965.) which is less than the matrix tensile strength omu.Thus,in this case,the matrix is weakened in the presence of fibers instead of getting strengthened (Figure 7.3). 7.1.1.2 Discontinuously Reinforced MMC In recent years,the majority of the research effort has been on SiCw-and SiCp- reinforced aluminum alloys [8].Titanium,magnesium,and zinc alloys have also been used;however,they are not discussed in this chapter.Reinforcements other than SiC,such as Al2O3,have also been investigated.Tensile properties of some of these composites are given in Appendix A.9. McDanels [9]has reported the mechanical properties of both SiCw-and SiCp-reinforced aluminum alloys,such as 6061,2024/2124,7075,and 5083. Reinforcement content is in the range of 10-40 vol%.These composites were produced by powder metallurgy,followed by extrusion and hot rolling.His observations are summarized as follows: 2007 by Taylor Francis Group,LLC
which is less than the matrix tensile strength smu. Thus, in this case, the matrix is weakened in the presence of fibers instead of getting strengthened (Figure 7.3). 7.1.1.2 Discontinuously Reinforced MMC In recent years, the majority of the research effort has been on SiCw- and SiCpreinforced aluminum alloys [8]. Titanium, magnesium, and zinc alloys have also been used; however, they are not discussed in this chapter. Reinforcements other than SiC, such as Al2O3, have also been investigated. Tensile properties of some of these composites are given in Appendix A.9. McDanels [9] has reported the mechanical properties of both SiCw- and SiCp-reinforced aluminum alloys, such as 6061, 2024=2124, 7075, and 5083. Reinforcement content is in the range of 10–40 vol%. These composites were produced by powder metallurgy, followed by extrusion and hot rolling. His observations are summarized as follows: 1200 1000 800 600 Longitudinal tensile strength (MPa) 400 200 0 0 0.5 Fiber volume fraction, vf 1.0 Equation 7.4 Equation 7.3 FIGURE 7.2 Longitudinal tensile strength variation of a unidirectional continuous tungsten fiber=copper matrix composite at various fiber volume fractions. (Adapted from Kelly, A. and Davies, G.J., Metall. Rev., 10, 1, 1965.) 2007 by Taylor & Francis Group, LLC
2000 GLtu=V:Ofu +(1-Vi)Omu ofu (as received) ofu (as extracted) 1500 1000 O SiC/CP-AI ●SiC/A384 500 0o o00 Equation 7.4 ● 0 0.5 1.0 Fiber volume fraction,Vi FIGURE 7.3 Longitudinal tensile strength variation of a unidirectional continuous SiC fiber-reinforced high-purity aluminum and A384 aluminum alloy composites at various fiber volume fractions.(Adapted from Everett,R.K.and Arsenault,R.J.,eds.,Metal Matrix Composites:Processing and Interfaces,Academic Press,San Diego,1991.) 1.The tensile modulus of the composite increases with increasing reinforcement volume fraction;however,the increase is not linear.The modulus values are much lower than the longitudinal modulus predicted by Equation 7.1 for continuous-fiber composites.Furthermore,the reinforcement type has no influence on the modulus. 2.Both yield strength and tensile strength of the composite increase with increasing reinforcement volume fractions;however,the amount of increase depends more on the alloy type than on the reinforcement type.The higher the strength of the matrix alloy,the higher the strength of the composite. 3.The strain-to-failure decreases with increasing reinforcement volume fraction (Figure 7.4).The fracture mode changes from ductile at low volume fractions(below 15%)to brittle (flat and granular)at 30-40 vol%. 2007 by Taylor Francis Group.LLC
1. The tensile modulus of the composite increases with increasing reinforcement volume fraction; however, the increase is not linear. The modulus values are much lower than the longitudinal modulus predicted by Equation 7.1 for continuous-fiber composites. Furthermore, the reinforcement type has no influence on the modulus. 2. Both yield strength and tensile strength of the composite increase with increasing reinforcement volume fractions; however, the amount of increase depends more on the alloy type than on the reinforcement type. The higher the strength of the matrix alloy, the higher the strength of the composite. 3. The strain-to-failure decreases with increasing reinforcement volume fraction (Figure 7.4). The fracture mode changes from ductile at low volume fractions (below 15%) to brittle (flat and granular) at 30–40 vol%. 2000 1500 1000 Longitudinal tensile strength (MPa) 500 0 0 0.5 Fiber volume fraction, vf Equation 7.4 1.0 SiC/CP-Al SiC/A384 sLtu=vf sfu + (1−vf )smu sfu (as received) sfu (as extracted) FIGURE 7.3 Longitudinal tensile strength variation of a unidirectional continuous SiC fiber-reinforced high-purity aluminum and A384 aluminum alloy composites at various fiber volume fractions. (Adapted from Everett, R.K. and Arsenault, R.J., eds., Metal Matrix Composites: Processing and Interfaces, Academic Press, San Diego, 1991.) 2007 by Taylor & Francis Group, LLC
600 SiCp:40% SiCw 30% 500 SiCp.30% SiCp:20% SiC 20% 400 SiCp:15% SiCw 10% 300 200 100 SiC/6061 A1 Composites 0 0 8 10 Strain(percent) FIGURE 7.4 Tensile stress-strain diagrams of SiCp-and SiCw-reinforced 6061-T6 aluminum alloy composites.(Adapted from McDanels,D.L.,Metall.Trans.,16A, 1105,1985.) McDanels [9]did not observe much directionality in SiC-reinforced aluminum alloys.Since MMCs manufactured by powder metallurgy are transformed into bars and sheets by hot rolling,it is possible to introduce differences in orien- tation in SiCw-reinforced alloys with more whiskers oriented in the rolling direction.Repeated rolling through small roll gaps can break whiskers and particulates into smaller sizes,thereby reducing the average particle size or the average length-to-diameter ratio of the whiskers.Both whisker orientation and size reduction may affect the tensile properties of rolled MMCs. Johnson and Birt [10]found that the tensile modulus of both SiCp-and SiCw-reinforced MMCs can be predicted reasonably well using Halpin-Tsai equations(Equations 3.49 through 3.53).However,the strength and ductility of MMCs with discontinuous reinforcements are difficult to model in terms of reinforcement and matrix properties alone,since the matrix microstructure in the composite may be different from the reinforcement-free matrix due to complex interaction between the two.The particle size has a significant influ- ence on yield strength,tensile strength,and ductility of SiCp-reinforced MMCs [11].Both yield and tensile strengths increase with decreasing particle size.Such behavior is attributed to the generation of thermal residual stresses,increase in dislocation density,and constraints to dislocation motion,all due to the pres- ence of particles.The ductility of the composite also increases with decreasing 2007 by Taylor Francis Group,LLC
McDanels [9] did not observe much directionality in SiC-reinforced aluminum alloys. Since MMCs manufactured by powder metallurgy are transformed into bars and sheets by hot rolling, it is possible to introduce differences in orientation in SiCw-reinforced alloys with more whiskers oriented in the rolling direction. Repeated rolling through small roll gaps can break whiskers and particulates into smaller sizes, thereby reducing the average particle size or the average length-to-diameter ratio of the whiskers. Both whisker orientation and size reduction may affect the tensile properties of rolled MMCs. Johnson and Birt [10] found that the tensile modulus of both SiCp- and SiCw-reinforced MMCs can be predicted reasonably well using Halpin–Tsai equations (Equations 3.49 through 3.53). However, the strength and ductility of MMCs with discontinuous reinforcements are difficult to model in terms of reinforcement and matrix properties alone, since the matrix microstructure in the composite may be different from the reinforcement-free matrix due to complex interaction between the two. The particle size has a significant influence on yield strength, tensile strength, and ductility of SiCp-reinforced MMCs [11]. Both yield and tensile strengths increase with decreasing particle size. Such behavior is attributed to the generation of thermal residual stresses, increase in dislocation density, and constraints to dislocation motion, all due to the presence of particles. The ductility of the composite also increases with decreasing SiCp, 40% SiCw, 30% SiCp, 30% SiCp, 20% SiCw, 20% SiCw, 10% SiCp, 15% SiC/6061 A1 Composites 6 8 10 Strain (percent) 0 2 4 0 100 200 Stress (MPa) 300 400 500 600 FIGURE 7.4 Tensile stress–strain diagrams of SiCp- and SiCw-reinforced 6061-T6 aluminum alloy composites. (Adapted from McDanels, D.L., Metall. Trans., 16A, 1105, 1985.) 2007 by Taylor & Francis Group, LLC
particle size;however,after attaining a maximum value at particle diameters between 2 and 4 um,it decreases rapidly to low values.The failure of the composite is initiated by cavity formation at the interface or by particle fracture. Other observations on the thermomechanical properties of SiCp-or SiCw- reinforced aluminum alloys are 1.Both CTE and thermal conductivity of aluminum alloys are reduced by the addition of SiCp [11,12]. 2.The fracture toughness of aluminum alloys is reduced by the addition of SiCp.Investigation by Hunt and his coworkers [13]indicates that frac- ture toughness is also related to the particle size.They have also observed that overaging,a heat treatment process commonly used for 7000-series aluminum alloys to enhance their fracture toughness,may produce lower fracture toughness in particle-reinforced aluminum alloys. 3.The long-life fatigue strength of SiCw-reinforced aluminum alloys is higher than that of the unreinforced matrix,whereas that of SiCp- reinforced aluminum alloys is at least equal to that of the unreinforced matrix (Figure 7.5). 4.The high-temperature yield strength and ultimate tensile strength of SiC-reinforced aluminum alloys are higher than the corresponding values of unreinforced alloys.The composite strength values follow similar functional dependence on temperature as the matrix strength values (Figure 7.6) 42 35 □ SiCw/6061-Al 28- 6061 Al Matrix 21 on g 14- SiCp/6061-Al 7 103 10 105 105 107 109 Cycles to failure FIGURE 7.5 S-N curves for SiCw-and SiCp-reinforced 6061 aluminum alloy.(Adapted from Rack,H.J.and Ratnaparkhi,P.,J.Metals,40,55,1988.) 2007 by Taylor&Francis Group.LLC
particle size; however, after attaining a maximum value at particle diameters between 2 and 4 mm, it decreases rapidly to low values. The failure of the composite is initiated by cavity formation at the interface or by particle fracture. Other observations on the thermomechanical properties of SiCp- or SiCwreinforced aluminum alloys are 1. Both CTE and thermal conductivity of aluminum alloys are reduced by the addition of SiCp [11,12]. 2. The fracture toughness of aluminum alloys is reduced by the addition of SiCp. Investigation by Hunt and his coworkers [13] indicates that fracture toughness is also related to the particle size. They have also observed that overaging, a heat treatment process commonly used for 7000-series aluminum alloys to enhance their fracture toughness, may produce lower fracture toughness in particle-reinforced aluminum alloys. 3. The long-life fatigue strength of SiCw-reinforced aluminum alloys is higher than that of the unreinforced matrix, whereas that of SiCpreinforced aluminum alloys is at least equal to that of the unreinforced matrix (Figure 7.5). 4. The high-temperature yield strength and ultimate tensile strength of SiC-reinforced aluminum alloys are higher than the corresponding values of unreinforced alloys. The composite strength values follow similar functional dependence on temperature as the matrix strength values (Figure 7.6). 42 35 Alternating stress (MPa) 28 21 14 7 0 103 104 105 106 Cycles to failure SiCw/6061–Al 6061 Al Matrix SiCp /6061–Al 107 108 FIGURE 7.5 S–N curves for SiCw- and SiCp-reinforced 6061 aluminum alloy. (Adapted from Rack, H.J. and Ratnaparkhi, P., J. Metals, 40, 55, 1988.) 2007 by Taylor & Francis Group, LLC
500 400 21 vol%SiC/2024 Al 21vo%SiC/2024A1 8 400 50 ao edW 300 300 ● 200 200 2024AI Q 2024A1 100 ● 100 ● 0 0 100 200300 400 100 200300 400 Temperature,C Temperature,C FIGURE 7.6 Effects of increasing temperature on the ultimate tensile strength and yield strength of unreinforced and 21 vol%SiC-reinforced 2024 aluminum alloy.(Adapted from Nair,S.V.,Tien,J.K.,and Bates,R.C.,Int.Metals Rev.,30,275,1985.) 5.Karayaka and Sehitoglu [14]conducted strain-controlled fatigue tests on 20 vol%SiCp-reinforced 2xxx-T4 aluminum alloys at 200C and 300C.Based on stress range,the reinforced alloys have a superior fatigue performance than the unreinforced alloys. 6.Creep resistance of aluminum alloys is improved by the addition of either SiCw or SiCp.For example,Morimoto et al.[15]have shown that the second-stage creep rate of 15 vol%SiCw-reinforced 6061 alu- minum alloy is nearly two orders of magnitude lower than that of the unreinforced alloy (Figure 7.7). 6061AI o=26 MPa p 1 15 vol%SiC /6061 o=70 MPa T=300C 0 0.5 1.0 1.5 Time,105s FIGURE 7.7 Comparison of creep strains of unreinforced and 15 vol%SiCw-reinforced 6061 aluminum alloy.(Adapted from Morimoto,T.,Yamaoka,T.,Lilholt,H.,and Taya,M.,J.Eng.Mater.Technol.,110,70,1988.) 2007 by Taylor Francis Group,LLC
5. Karayaka and Sehitoglu [14] conducted strain-controlled fatigue tests on 20 vol% SiCp-reinforced 2xxx-T4 aluminum alloys at 2008C and 3008C. Based on stress range, the reinforced alloys have a superior fatigue performance than the unreinforced alloys. 6. Creep resistance of aluminum alloys is improved by the addition of either SiCw or SiCp. For example, Morimoto et al. [15] have shown that the second-stage creep rate of 15 vol% SiCw-reinforced 6061 aluminum alloy is nearly two orders of magnitude lower than that of the unreinforced alloy (Figure 7.7). 2024 Al 2024 Al 21 vol% SiC/2024 Al 21 vol% SiC/2024 Al 500 400 Ultimate tensile strength, MPa Yield strength, MPa 300 200 100 0 400 300 200 100 0 0 100 200 Temperature, C 300 400 0 100 200 Temperature, C 300 400 FIGURE 7.6 Effects of increasing temperature on the ultimate tensile strength and yield strength of unreinforced and 21 vol% SiC-reinforced 2024 aluminum alloy. (Adapted from Nair, S.V., Tien, J.K., and Bates, R.C., Int. Metals Rev., 30, 275, 1985.) 0 0 1 Strain, % 2 0.5 Time, 105 s 1.0 1.5 6061 Al s =26 MPa 15 vol% SiCw/6061 s =70 MPa T =300°C FIGURE 7.7 Comparison of creep strains of unreinforced and 15 vol% SiCw-reinforced 6061 aluminum alloy. (Adapted from Morimoto, T., Yamaoka, T., Lilholt, H., and Taya, M., J. Eng. Mater. Technol., 110, 70, 1988.) 2007 by Taylor & Francis Group, LLC.