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.