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Al composites fabricated by squeeze casting technology
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Scripta Materialia 59 (2008) 619–622 www.elsevier.com/locate/scriptamat
Mechanical properties of SiC/Gr/Al composites fabricated by squeeze casting technology Jinfeng Leng,a,* Gaohui Wu,a Qingbo Zhou,b Zuoyong Doua and XiaoLi Huanga a
Center for Metal Matrix Composites Engineering Technology, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China b Northeast Light Alloys Company Ltd., Harbin, Heilongjiang 150060, China Received 11 January 2008; revised 24 April 2008; accepted 14 May 2008 Available online 27 May 2008
SiC/Gr/Al composites were fabricated by squeeze casting with graphite volume fractions of 3–7% and particles size of 1, 6, 10, 20 and 70 lm. No Al4C3 brittle interfacial product could be detected by transmission electron microscopy. With increasing volume fraction and particle size of graphite, the tensile strength (rb) decrease from 420 to 235 MPa and the elastic modulus (E) decrease from 166 to 116 GPa. These changes were in close accordance with the linear function: E = 224rb + 61,695. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Casting; Metal–matrix composites (MMC); Mechanical properties; Graphite
Silicon carbide particle-reinforced aluminum metal– matrix composites (MMC) are a unique class of advanced engineered materials that have been developed and qualified for use in aerospace structures, lightweight optical assemblies and inertial guidance systems over the past 20 years [1,2]. Such materials are as light as aluminum but exhibit significantly greater specific strengths and stiffness. Moreover, these materials are isotropic and more resistant to compressive microcreep than beryllium. They also can be tailored to match materials, including stainless steel, beryllium and nickel. The presence of hard, brittle and abrasive SiC reinforcement makes the material difficult to form or machine using traditional manufacturing processes [3–6]. In order to improve machinability of the SiCp/Al composites, graphite has been added to the composites [7,8]. However, with the addition of graphite particles, the mechanical properties of composites decrease, and this limits their large-scale industrial applications as structure materials. Therefore, how to maintain the higher mechanical properties of SiC/Gr/Al composites has become the focus of much research. To date, most of the studies have been concerned with the fabrication technique of the SiC/Gr/Al composites [9–15], such as stir-casting and spray co-deposition. For SiC/Gr/Al composites fabricated by stir-casting,
* Corresponding author. Tel.: +86 451 86402373; fax: +86 451 86412164; e-mail: jfl[email protected]
the mechanical properties of the composites are low due to the presence of coarse graphite particles (the size of graphite must above 20 lm), the segregation of particles and the presence of Al4C3 intermetallic compound. The spray co-deposition technique also has some drawbacks, such as the inhomogeneous distribution of particles and the relative low density, leading to poor mechanical properties of the composites. To the best to our knowledge, the composite fabricated by squeeze casting exhibits better mechanical properties due to the presence of fewer common defects such as porosity and shrinking cavities, and the elimination of segregation of the reinforcement. However, SiC/Gr/Al composites fabricated by this technique have seldom been reported on, and details of their mechanical properties are still lacking. Therefore, the present study concentrated on the fabrication of SiC/Gr/Al composites, and the microstructure and mechanical properties of the composites are also reported. The matrix alloy was 2024Al, with the chemical composition (wt.%): 4.79% Cu, 1.49% Mg, 0.611% Mn, 0.245% Fe, 0.168% Si, 0.068% Zn, 0.046% Ti, 0.013% Ni, 0.049% Cr and the balance Al. SiC particles with a volume fraction of 40% and an average size of 3 lm and flaky graphite particles with volume fractions of 3%, 5% and 7% and average sizes of 1, 6, 10, 20 and 70 lm were used as the reinforcements. The composite was fabricated by squeeze casting technology. First, the SiC and graphite particles were mixed by mechanical balling for 30 min at a rotational speed of 350 rpm.
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.05.018
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J. Leng et al. / Scripta Materialia 59 (2008) 619–622
Then they were filled and pressed into a mold to produce a SiC/Gr preform and preheated. At the same time, the aluminum alloy was melt, degassed and cleaned in a graphite crucible and heated to 800 °C. Subsequently, the molten aluminum was poured into the tool steel die and a vertical pressure of up to 100 MPa was applied to force the molten aluminum to completely infiltrate the SiC/Gr preform. The pressure was maintained for 180 s until the solidification was complete. The samples were all treated according to the following protocol: solution treating at 495 °C for 1 h, quenching into water, then aging at 160 °C for 10 h. For comparison, the SiC/ Al composite specimens were prepared by the same method. XRD analyses were performed on an X’Pert X-ray ˚ ), operated at diffractometer with Cu Ka (k = 1.5406 A 40 kV and 40 mA. The microstructures of the composites and fracture surfaces were examined using a S-570 scanning electron microscope. Interfacial morphologies between the reinforcement and alloy matrix were analysed using a CM12 transmission electron microscope. Tensile tests were conducted on an Instron5569 testing machine at ambient temperature with the crosshead moving rate of 0.5 mm min1. Each tensile strength value as well as the elastic modulus value was the average of at least six measurements. The details of the tensile tests, such as the shape and dimensions of the tensile specimen, are given elsewhere [16]. Figure 1 shows the microstructure of 40%SiC/ 5%Gr(70 lm)/Al composite as portrayed by scanning electron microscopy (SEM). The SiC particles and flaky graphite particles are distributed homogeneously in the Al matrix. The composites were free from common defects, such as porosity and shrinking cavities. Dense microstructures were obtained by the high pressure during the solidification process, which contributed to the improvement in the mechanical properties. Al4C3 is often found in SiC/Gr/Al composites made by stir-casting. Al4C3 is a brittle phase and results from the interfacial reactions between SiC and Al and between graphite and Al. It deteriorates the mechanical properties of the composite. Thus avoiding the formation of Al4C3 is a primary concern for the successful fabrication of SiC/Gr/Al composite. In order to reveal the interfacial details of reinforcement and Al matrix, transmission electron microscopy (TEM) was used to study the interfacial microstructure of the composites. Figure 2a and b shows the morphologies of the interface between the reinforcements and the
Figure 2. TEM micrographs of 40%SiC/5%Gr(70 lm)/Al composites.
Al matrix. As shown in the figure, the interface between SiC or graphite and Al matrix is free from any interfacial reaction products. This indicates that squeeze casting is a viable technique for the fabrication of SiC/Gr/ Al composite that avoids the shortcomings of the composite fabricated by stir-casting technology, such as introducing the brittle intermetallic compound Al4C3. The addition of graphite and SiC particles to molten aluminum alloy often leads to severe reactivity between graphite or SiC and Al under the thermodynamic conditions that are normally present in common casting fabrication techniques. These reactions include 4Al þ 3C ! Al4 C3
ð1Þ
4Al þ 3SiC ! Al4 C3 þ 3Si
ð2Þ
In this work, no interfacial reactivity was observed on the interfaces between Al and SiC or graphite. This is for two reasons. First, reactions (1) and (2) are mainly controlled by the kinetics of Al4C3 formation. Molten aluminum alloy was filtered into the SiC/Gr preform, such that SiC or graphite particles were in contact with molten aluminum. Due to the cooling effect of the preform and the mold, the contact time between the reinforced particles and the molten aluminum was shortened, and this decreased the possibility of interfacial reactions. Secondly, the SiC/Gr perform had been preheated at 600 °C, which induces the formation of SiO2 oxidation layers on the SiC particles. SiO2 layers prevent any direct contact between the SiC and the molten Al, and this inhibits the formation of Al4C3, as reported in the literature [17]. Graphite particles with a high degree of graphitization do not readily react with molten aluminum because their chemical properties are relatively stable compared with carbon. According to the Bragg equation, interlayer spacing can be obtained, and, on basis of the model given by Maire and Mering [18,19], the value of the degree of graphitization can be calculated from the following equation: G¼
Figure 1. SEM composites.
microstructure
of
40%SiC/5%Gr(70 lm)/Al
0:3440 d ð002Þ 0:3440 0:3354
ð3Þ
where G is the degree of graphitization (%), 0.3440 is the interlayer spacing of the fully nongraphitized carbon (nm), 0.3354 is the interlayer spacing of the ideal graphite crystallite and the d(002) is the interlayer spacing (nm) derived from X-ray diffraction (XRD). Figure 3 shows the XRD pattern of the graphite particles. It can be seen that the value of 2h for graphite particles is 26.57°.
J. Leng et al. / Scripta Materialia 59 (2008) 619–622
621
where Vp is the volume fraction of graphite particles and d is the particle size. Based on formula (4), we can see that the particle spacing decreased exponentially with increasing volume content of graphite particles, which was due to the matrix being dramatically divided by graphite particles. This could explain why the mechanical properties of composites decreased with increasing volume fraction of graphite particles. In addition, with increasing particle size of graphite, the tensile strength of SiC/Gr/Al showed a significant reduction, i.e. the tensile strength of 40%SiC/5%Gr/Al composites are 420, 282 and 235 MPa with graphite particle sizes of 1, 20 and 70 lm, respectively. This indicates that the tensile strength of the SiC/Gr/Al composite is affected remark-
ably by the volume fraction and particle size of the graphite particles. The hybrid-reinforced composites may be divided into two parts – SiC + Al and graphite – since there is a strong bond between SiC particles [21] and Al but a weak bond between graphite and Al. In other words, SiC + Al can be considered as a matrix, while graphite is the reinforcement embedded in the SiC + Al matrix. The good interfacial bonding between SiC particles and Al might be due to the fact that there is no brittle phase in the interfaces, as shown in Figure 3. Also, loads can be effectively transferred from the Al to SiC, thus SiC is the main load-bearing body. Hence, the overall mechanical properties of the composite are improved. Graphite is a soft phase, which deteriorates the mechanical properties significantly. The fracture surfaces of the composite were examined by SEM, and the fractography is shown in Figure 5. It can be seen that SiC + Al matrix failed in a ductile manner while graphite particles fail in a brittle manner. Cleavage fracture along the graphite basal plane of the graphite particles is found, as shown by A and B in Figure 5. Cracks propagate between flakes and/or along the Al/graphite interface. Graphite particles in composites tend to fracture in a brittle fashion, which could be ascribed to two factors. On the one hand, graphite particle layers parallel to the basal plane are held together by weak van der Waals forces. At the same time, weakbonding interfaces exist between the graphite particles and Al. These become a crack source under applied stress and the crack then propagates rapidly between the flakes and/or along the Al/graphite interface. The crack propagation path in the composite with fine graphite particles is shorter than that with coarse graphite particles because crack propagation is inhibited by the plastic deformation of the SiC + Al matrix. Thus the composite with fine graphite particles has a higher tensile strength than that with coarse ones. In addition, with an increasing volume fraction of graphite particles, the crack sources increase correspondingly, hence the tensile strength of composite are reduced. Further, the elastic modulus also depends on graphite. According to the Hashin–Shtrikman model and the rule of mixture, the elastic modulus of the composite is affected mainly by the elastic modulus of the reinforcement and that of the matrix alloy. Compared to the SiC + Al matrix, the graphite particles have an extremely low elastic modulus. Therefore, the elastic modulus of the composites must come mainly from the SiC + Al matrix. The significant decrease in the elastic modulus
Figure 4. Effect of the volume fraction and size of graphite particles on mechanical properties of SiC/Gr/Al composites at room temperature.
Figure 5. SEM tensiling fractographs of 40%SiC/5%Gr(70 lm)/Al.
(002)
100
80
Intensity
60
40
20
26.57 0 25.6 25.8 26.0
26.2
26.4 26.6 26.8 27.0 27.2 Deg, 2θ
27.4
Figure 3. XRD pattern of graphite particles.
According to above calculation, the value of the degree of graphitization is 98.64%. This indicates that the structure of graphite particles chosen is close to that of ideal graphite crystallite. Thus, the reinforced particles with a high degree of graphitization inhibit the interfacial reaction of Al/graphite. The effects of the volume fraction and size on mechanical properties of SiC/Gr/Al composites at room temperature are showed in Figure 4a and b. It can be clearly seen that the tensile strength of SiC/Al composites decreases with the addition of graphite particles, which is mainly attributed to the significantly lower strength of graphite (about 20–30 MPa) compared with the matrix alloy and SiC. With the addition of 3%, 5% and 7% graphite particles (particle size: 6 lm) in SiC/ Al, the tensile strength is 412, 405 and 365 MPa respectively. Compared to 510 MPa for the SiC/Al composite, the tensile strength of the SiC/Gr/Al composites are decreased by 19%, 21% and 28%, respectively. Assuming the particles to be equiaxed, particles spacing (k) is calculated using the following relationship [20]: k ¼ 0:77dV p1=2
ð4Þ
622
J. Leng et al. / Scripta Materialia 59 (2008) 619–622 180
Elastic modulus(GPa)
170 160 150 140 130 120 110 100 200
250
300 350 400 Tensile strengh(MPa)
450
Figure 6. Relationship between tensile strength and elastic modulus.
is obtained with increasing graphite particles, as shown in Figure 4a and b. When the volume fraction of graphite particles is increased from 0% to 3%, 5% and 7%, the value of elastic modulus decreased from 172 to 152, 150 and 144 GPa, and the extent of reduction is 12%, 13% and 16%, respectively. In addition, with the increasing of particle size of graphite, the tensile strength of SiC/ Gr/Al shows a significant reduction, i.e. the elastic modulus of 40%SiC/5%Gr/Al composites are 166, 125 and 116 GPa with the graphite particle sizes of 1, 20 and 70 lm, respectively. From above discussion we know that the tensile strength of the SiC/Gr/Al composites clearly depends on the volume fraction and particles size of graphite, and the elastic modulus is also affected remarkably by the volume fraction and particles size of graphite particles. Figure 6 shows the relationship between the tensile strength and elastic modulus of SiC/Gr/Al composites. As can be seen, the elastic modulus decreases with a decrease in tensile strength. Moreover, this phenomenon can be described by the following linear function: E ¼ Krb þ C
ð5Þ
where E is the elastic modulus, rb is the tensile strength, and K and C are material constants. The result of calculation is K = 224, C = 61,695. For SiC/Gr/Al composites, cracks initiate on the interfaces between the graphite particles and the matrix under applied stress, and then propagate rapidly between flakes and/or along the Al/graphite interface until finally the samples fracture. Thus, the tensile strength of SiC/Gr/Al composites depends greatly on the size and distribution of the graphite particles, namely the continuity of matrix. The decrease in elastic modulus is also induced by the graphite particles. Cracks are initiated under applied stress, leading to the failure of the composite’s continuity, and as a result the elastic deformation cannot be transferred effectively. Therefore, decreases in both tensile strength and elastic modulus have same physical essence and both mechanical properties are closely interrelated. By the addition of graphite with different volume fractions and particle sizes, SiC/Gr/Al composites were
fabricated by squeeze casting. These composites are macroscopically dense and homogeneous, with no distinct presence of Al4C3 in the composites. The tensile strengths range from 235 to 420 MPa, depending on the volume fraction and the size of the graphite particles. With the addition of 3%, 5% and 7% graphite particles to SiC/Al composites, the tensile strengths are 412, 405 and 365 MPa, which are decreased by 19%, 21% and 28%, respectively. The tensile strength of 40%SiC/ 5%Gr/Al composites decreased with the particle size of the graphite increasing from 1 to 70 lm, with the highest value of 420 MPa for the composite with 1 lm graphite particles and the lowest value of 235 MPa for that with 70 lm particles. The elastic modulus of the composites tends to decrease with increasing volume fraction and particle size of graphite. Both the tensile strength and the elastic modulus depended on the volume fraction and the size of graphite particles. Moreover, this phenomenon could be described by the linear function: E = 224rb + 61,695. [1] W.R. Mohn, D. Vukobratorich, J. Mater. Eng. 10 (1988) 225. [2] W.R. Mohn, SAMPE (January/February 26) (1988). [3] M. Ei-Gallab, M. Sklad, J. Mater. Process. Technol. 83 (1998) 151. [4] R.L. Deuis, C. Subramanian, J.M. Yellup, Wear 201 (1996) 132. [5] F. Bergman, S. Jacobason, Wear 179 (1994) 89. [6] N.P. Hung, K.A. Boey, C.A. Khor, J. Mater. Process. Technol. 48 (1995) 292. [7] J.F. Leng, G.H. Wu, Chin. J. Rare Metals 30 (2006) 20, (in Chinese). [8] V. Songmene, M. Balazinski, CIRP Ann. – Manuf. Technol. 48 (1999) 77. [9] W. Ames, A.T. Alpas, Metall. Mater. Trans. A 26A (1995) 85. [10] A.R. Riahi, A.T. Alpas, Wear 251 (2001) 1396. [11] P.K. Rohatgi, D. Nath, S.S. Singh, J. Mater. Sci. 29 (1994) 5975. [12] M.C. Gui, S.B. Kang, Mater. Lett. 51 (2001) 396. [13] R.J. Perze, J. Zhang, E.J. Lavernia, Metal. Trans. A 24 (1993) 701. [14] J. Zhang, R.J. Perze, E.J. Lavernia, Acta Metal. Mater. 42 (1994) 395l. [15] E.J. Lavernia, R.J. Perze, J. Zhang, Metal. Mater. Trans. A 26 (1995) 2803. [16] M. Zhao, G.H. Wu, D.Z. Zhu, L.T. Jiang, Zuoyong Dou, Mater. Lett. 58 (2004) 1899. [17] Z.P. Luo, Y.G. Song, S.Q. Zhang, Scripta Mater. 45 (2001) 1183. [18] J. Mering, J. Maire, J. Chem. Phys. 57 (1960) 803. [19] J. Maire, J. Mering, Chem. Phys. Carbon 6 (1970) 125. [20] G. Leroy, J.D. Embury, G. Edward, M.F. Ashby, Acta Metall. 29 (1981) 1509. [21] Y. Flom, R.J. Arsenault, Mater. Sci. Eng. 77 (1986) 191.
Scripta Materialia 59 (2008) 619–622 www.elsevier.com/locate/scriptamat
Mechanical properties of SiC/Gr/Al composites fabricated by squeeze casting technology Jinfeng Leng,a,* Gaohui Wu,a Qingbo Zhou,b Zuoyong Doua and XiaoLi Huanga a
Center for Metal Matrix Composites Engineering Technology, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China b Northeast Light Alloys Company Ltd., Harbin, Heilongjiang 150060, China Received 11 January 2008; revised 24 April 2008; accepted 14 May 2008 Available online 27 May 2008
SiC/Gr/Al composites were fabricated by squeeze casting with graphite volume fractions of 3–7% and particles size of 1, 6, 10, 20 and 70 lm. No Al4C3 brittle interfacial product could be detected by transmission electron microscopy. With increasing volume fraction and particle size of graphite, the tensile strength (rb) decrease from 420 to 235 MPa and the elastic modulus (E) decrease from 166 to 116 GPa. These changes were in close accordance with the linear function: E = 224rb + 61,695. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Casting; Metal–matrix composites (MMC); Mechanical properties; Graphite
Silicon carbide particle-reinforced aluminum metal– matrix composites (MMC) are a unique class of advanced engineered materials that have been developed and qualified for use in aerospace structures, lightweight optical assemblies and inertial guidance systems over the past 20 years [1,2]. Such materials are as light as aluminum but exhibit significantly greater specific strengths and stiffness. Moreover, these materials are isotropic and more resistant to compressive microcreep than beryllium. They also can be tailored to match materials, including stainless steel, beryllium and nickel. The presence of hard, brittle and abrasive SiC reinforcement makes the material difficult to form or machine using traditional manufacturing processes [3–6]. In order to improve machinability of the SiCp/Al composites, graphite has been added to the composites [7,8]. However, with the addition of graphite particles, the mechanical properties of composites decrease, and this limits their large-scale industrial applications as structure materials. Therefore, how to maintain the higher mechanical properties of SiC/Gr/Al composites has become the focus of much research. To date, most of the studies have been concerned with the fabrication technique of the SiC/Gr/Al composites [9–15], such as stir-casting and spray co-deposition. For SiC/Gr/Al composites fabricated by stir-casting,
* Corresponding author. Tel.: +86 451 86402373; fax: +86 451 86412164; e-mail: jfl[email protected]
the mechanical properties of the composites are low due to the presence of coarse graphite particles (the size of graphite must above 20 lm), the segregation of particles and the presence of Al4C3 intermetallic compound. The spray co-deposition technique also has some drawbacks, such as the inhomogeneous distribution of particles and the relative low density, leading to poor mechanical properties of the composites. To the best to our knowledge, the composite fabricated by squeeze casting exhibits better mechanical properties due to the presence of fewer common defects such as porosity and shrinking cavities, and the elimination of segregation of the reinforcement. However, SiC/Gr/Al composites fabricated by this technique have seldom been reported on, and details of their mechanical properties are still lacking. Therefore, the present study concentrated on the fabrication of SiC/Gr/Al composites, and the microstructure and mechanical properties of the composites are also reported. The matrix alloy was 2024Al, with the chemical composition (wt.%): 4.79% Cu, 1.49% Mg, 0.611% Mn, 0.245% Fe, 0.168% Si, 0.068% Zn, 0.046% Ti, 0.013% Ni, 0.049% Cr and the balance Al. SiC particles with a volume fraction of 40% and an average size of 3 lm and flaky graphite particles with volume fractions of 3%, 5% and 7% and average sizes of 1, 6, 10, 20 and 70 lm were used as the reinforcements. The composite was fabricated by squeeze casting technology. First, the SiC and graphite particles were mixed by mechanical balling for 30 min at a rotational speed of 350 rpm.
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.05.018
620
J. Leng et al. / Scripta Materialia 59 (2008) 619–622
Then they were filled and pressed into a mold to produce a SiC/Gr preform and preheated. At the same time, the aluminum alloy was melt, degassed and cleaned in a graphite crucible and heated to 800 °C. Subsequently, the molten aluminum was poured into the tool steel die and a vertical pressure of up to 100 MPa was applied to force the molten aluminum to completely infiltrate the SiC/Gr preform. The pressure was maintained for 180 s until the solidification was complete. The samples were all treated according to the following protocol: solution treating at 495 °C for 1 h, quenching into water, then aging at 160 °C for 10 h. For comparison, the SiC/ Al composite specimens were prepared by the same method. XRD analyses were performed on an X’Pert X-ray ˚ ), operated at diffractometer with Cu Ka (k = 1.5406 A 40 kV and 40 mA. The microstructures of the composites and fracture surfaces were examined using a S-570 scanning electron microscope. Interfacial morphologies between the reinforcement and alloy matrix were analysed using a CM12 transmission electron microscope. Tensile tests were conducted on an Instron5569 testing machine at ambient temperature with the crosshead moving rate of 0.5 mm min1. Each tensile strength value as well as the elastic modulus value was the average of at least six measurements. The details of the tensile tests, such as the shape and dimensions of the tensile specimen, are given elsewhere [16]. Figure 1 shows the microstructure of 40%SiC/ 5%Gr(70 lm)/Al composite as portrayed by scanning electron microscopy (SEM). The SiC particles and flaky graphite particles are distributed homogeneously in the Al matrix. The composites were free from common defects, such as porosity and shrinking cavities. Dense microstructures were obtained by the high pressure during the solidification process, which contributed to the improvement in the mechanical properties. Al4C3 is often found in SiC/Gr/Al composites made by stir-casting. Al4C3 is a brittle phase and results from the interfacial reactions between SiC and Al and between graphite and Al. It deteriorates the mechanical properties of the composite. Thus avoiding the formation of Al4C3 is a primary concern for the successful fabrication of SiC/Gr/Al composite. In order to reveal the interfacial details of reinforcement and Al matrix, transmission electron microscopy (TEM) was used to study the interfacial microstructure of the composites. Figure 2a and b shows the morphologies of the interface between the reinforcements and the
Figure 2. TEM micrographs of 40%SiC/5%Gr(70 lm)/Al composites.
Al matrix. As shown in the figure, the interface between SiC or graphite and Al matrix is free from any interfacial reaction products. This indicates that squeeze casting is a viable technique for the fabrication of SiC/Gr/ Al composite that avoids the shortcomings of the composite fabricated by stir-casting technology, such as introducing the brittle intermetallic compound Al4C3. The addition of graphite and SiC particles to molten aluminum alloy often leads to severe reactivity between graphite or SiC and Al under the thermodynamic conditions that are normally present in common casting fabrication techniques. These reactions include 4Al þ 3C ! Al4 C3
ð1Þ
4Al þ 3SiC ! Al4 C3 þ 3Si
ð2Þ
In this work, no interfacial reactivity was observed on the interfaces between Al and SiC or graphite. This is for two reasons. First, reactions (1) and (2) are mainly controlled by the kinetics of Al4C3 formation. Molten aluminum alloy was filtered into the SiC/Gr preform, such that SiC or graphite particles were in contact with molten aluminum. Due to the cooling effect of the preform and the mold, the contact time between the reinforced particles and the molten aluminum was shortened, and this decreased the possibility of interfacial reactions. Secondly, the SiC/Gr perform had been preheated at 600 °C, which induces the formation of SiO2 oxidation layers on the SiC particles. SiO2 layers prevent any direct contact between the SiC and the molten Al, and this inhibits the formation of Al4C3, as reported in the literature [17]. Graphite particles with a high degree of graphitization do not readily react with molten aluminum because their chemical properties are relatively stable compared with carbon. According to the Bragg equation, interlayer spacing can be obtained, and, on basis of the model given by Maire and Mering [18,19], the value of the degree of graphitization can be calculated from the following equation: G¼
Figure 1. SEM composites.
microstructure
of
40%SiC/5%Gr(70 lm)/Al
0:3440 d ð002Þ 0:3440 0:3354
ð3Þ
where G is the degree of graphitization (%), 0.3440 is the interlayer spacing of the fully nongraphitized carbon (nm), 0.3354 is the interlayer spacing of the ideal graphite crystallite and the d(002) is the interlayer spacing (nm) derived from X-ray diffraction (XRD). Figure 3 shows the XRD pattern of the graphite particles. It can be seen that the value of 2h for graphite particles is 26.57°.
J. Leng et al. / Scripta Materialia 59 (2008) 619–622
621
where Vp is the volume fraction of graphite particles and d is the particle size. Based on formula (4), we can see that the particle spacing decreased exponentially with increasing volume content of graphite particles, which was due to the matrix being dramatically divided by graphite particles. This could explain why the mechanical properties of composites decreased with increasing volume fraction of graphite particles. In addition, with increasing particle size of graphite, the tensile strength of SiC/Gr/Al showed a significant reduction, i.e. the tensile strength of 40%SiC/5%Gr/Al composites are 420, 282 and 235 MPa with graphite particle sizes of 1, 20 and 70 lm, respectively. This indicates that the tensile strength of the SiC/Gr/Al composite is affected remark-
ably by the volume fraction and particle size of the graphite particles. The hybrid-reinforced composites may be divided into two parts – SiC + Al and graphite – since there is a strong bond between SiC particles [21] and Al but a weak bond between graphite and Al. In other words, SiC + Al can be considered as a matrix, while graphite is the reinforcement embedded in the SiC + Al matrix. The good interfacial bonding between SiC particles and Al might be due to the fact that there is no brittle phase in the interfaces, as shown in Figure 3. Also, loads can be effectively transferred from the Al to SiC, thus SiC is the main load-bearing body. Hence, the overall mechanical properties of the composite are improved. Graphite is a soft phase, which deteriorates the mechanical properties significantly. The fracture surfaces of the composite were examined by SEM, and the fractography is shown in Figure 5. It can be seen that SiC + Al matrix failed in a ductile manner while graphite particles fail in a brittle manner. Cleavage fracture along the graphite basal plane of the graphite particles is found, as shown by A and B in Figure 5. Cracks propagate between flakes and/or along the Al/graphite interface. Graphite particles in composites tend to fracture in a brittle fashion, which could be ascribed to two factors. On the one hand, graphite particle layers parallel to the basal plane are held together by weak van der Waals forces. At the same time, weakbonding interfaces exist between the graphite particles and Al. These become a crack source under applied stress and the crack then propagates rapidly between the flakes and/or along the Al/graphite interface. The crack propagation path in the composite with fine graphite particles is shorter than that with coarse graphite particles because crack propagation is inhibited by the plastic deformation of the SiC + Al matrix. Thus the composite with fine graphite particles has a higher tensile strength than that with coarse ones. In addition, with an increasing volume fraction of graphite particles, the crack sources increase correspondingly, hence the tensile strength of composite are reduced. Further, the elastic modulus also depends on graphite. According to the Hashin–Shtrikman model and the rule of mixture, the elastic modulus of the composite is affected mainly by the elastic modulus of the reinforcement and that of the matrix alloy. Compared to the SiC + Al matrix, the graphite particles have an extremely low elastic modulus. Therefore, the elastic modulus of the composites must come mainly from the SiC + Al matrix. The significant decrease in the elastic modulus
Figure 4. Effect of the volume fraction and size of graphite particles on mechanical properties of SiC/Gr/Al composites at room temperature.
Figure 5. SEM tensiling fractographs of 40%SiC/5%Gr(70 lm)/Al.
(002)
100
80
Intensity
60
40
20
26.57 0 25.6 25.8 26.0
26.2
26.4 26.6 26.8 27.0 27.2 Deg, 2θ
27.4
Figure 3. XRD pattern of graphite particles.
According to above calculation, the value of the degree of graphitization is 98.64%. This indicates that the structure of graphite particles chosen is close to that of ideal graphite crystallite. Thus, the reinforced particles with a high degree of graphitization inhibit the interfacial reaction of Al/graphite. The effects of the volume fraction and size on mechanical properties of SiC/Gr/Al composites at room temperature are showed in Figure 4a and b. It can be clearly seen that the tensile strength of SiC/Al composites decreases with the addition of graphite particles, which is mainly attributed to the significantly lower strength of graphite (about 20–30 MPa) compared with the matrix alloy and SiC. With the addition of 3%, 5% and 7% graphite particles (particle size: 6 lm) in SiC/ Al, the tensile strength is 412, 405 and 365 MPa respectively. Compared to 510 MPa for the SiC/Al composite, the tensile strength of the SiC/Gr/Al composites are decreased by 19%, 21% and 28%, respectively. Assuming the particles to be equiaxed, particles spacing (k) is calculated using the following relationship [20]: k ¼ 0:77dV p1=2
ð4Þ
622
J. Leng et al. / Scripta Materialia 59 (2008) 619–622 180
Elastic modulus(GPa)
170 160 150 140 130 120 110 100 200
250
300 350 400 Tensile strengh(MPa)
450
Figure 6. Relationship between tensile strength and elastic modulus.
is obtained with increasing graphite particles, as shown in Figure 4a and b. When the volume fraction of graphite particles is increased from 0% to 3%, 5% and 7%, the value of elastic modulus decreased from 172 to 152, 150 and 144 GPa, and the extent of reduction is 12%, 13% and 16%, respectively. In addition, with the increasing of particle size of graphite, the tensile strength of SiC/ Gr/Al shows a significant reduction, i.e. the elastic modulus of 40%SiC/5%Gr/Al composites are 166, 125 and 116 GPa with the graphite particle sizes of 1, 20 and 70 lm, respectively. From above discussion we know that the tensile strength of the SiC/Gr/Al composites clearly depends on the volume fraction and particles size of graphite, and the elastic modulus is also affected remarkably by the volume fraction and particles size of graphite particles. Figure 6 shows the relationship between the tensile strength and elastic modulus of SiC/Gr/Al composites. As can be seen, the elastic modulus decreases with a decrease in tensile strength. Moreover, this phenomenon can be described by the following linear function: E ¼ Krb þ C
ð5Þ
where E is the elastic modulus, rb is the tensile strength, and K and C are material constants. The result of calculation is K = 224, C = 61,695. For SiC/Gr/Al composites, cracks initiate on the interfaces between the graphite particles and the matrix under applied stress, and then propagate rapidly between flakes and/or along the Al/graphite interface until finally the samples fracture. Thus, the tensile strength of SiC/Gr/Al composites depends greatly on the size and distribution of the graphite particles, namely the continuity of matrix. The decrease in elastic modulus is also induced by the graphite particles. Cracks are initiated under applied stress, leading to the failure of the composite’s continuity, and as a result the elastic deformation cannot be transferred effectively. Therefore, decreases in both tensile strength and elastic modulus have same physical essence and both mechanical properties are closely interrelated. By the addition of graphite with different volume fractions and particle sizes, SiC/Gr/Al composites were
fabricated by squeeze casting. These composites are macroscopically dense and homogeneous, with no distinct presence of Al4C3 in the composites. The tensile strengths range from 235 to 420 MPa, depending on the volume fraction and the size of the graphite particles. With the addition of 3%, 5% and 7% graphite particles to SiC/Al composites, the tensile strengths are 412, 405 and 365 MPa, which are decreased by 19%, 21% and 28%, respectively. The tensile strength of 40%SiC/ 5%Gr/Al composites decreased with the particle size of the graphite increasing from 1 to 70 lm, with the highest value of 420 MPa for the composite with 1 lm graphite particles and the lowest value of 235 MPa for that with 70 lm particles. The elastic modulus of the composites tends to decrease with increasing volume fraction and particle size of graphite. Both the tensile strength and the elastic modulus depended on the volume fraction and the size of graphite particles. Moreover, this phenomenon could be described by the linear function: E = 224rb + 61,695. [1] W.R. Mohn, D. Vukobratorich, J. Mater. Eng. 10 (1988) 225. [2] W.R. Mohn, SAMPE (January/February 26) (1988). [3] M. Ei-Gallab, M. Sklad, J. Mater. Process. Technol. 83 (1998) 151. [4] R.L. Deuis, C. Subramanian, J.M. Yellup, Wear 201 (1996) 132. [5] F. Bergman, S. Jacobason, Wear 179 (1994) 89. [6] N.P. Hung, K.A. Boey, C.A. Khor, J. Mater. Process. Technol. 48 (1995) 292. [7] J.F. Leng, G.H. Wu, Chin. J. Rare Metals 30 (2006) 20, (in Chinese). [8] V. Songmene, M. Balazinski, CIRP Ann. – Manuf. Technol. 48 (1999) 77. [9] W. Ames, A.T. Alpas, Metall. Mater. Trans. A 26A (1995) 85. [10] A.R. Riahi, A.T. Alpas, Wear 251 (2001) 1396. [11] P.K. Rohatgi, D. Nath, S.S. Singh, J. Mater. Sci. 29 (1994) 5975. [12] M.C. Gui, S.B. Kang, Mater. Lett. 51 (2001) 396. [13] R.J. Perze, J. Zhang, E.J. Lavernia, Metal. Trans. A 24 (1993) 701. [14] J. Zhang, R.J. Perze, E.J. Lavernia, Acta Metal. Mater. 42 (1994) 395l. [15] E.J. Lavernia, R.J. Perze, J. Zhang, Metal. Mater. Trans. A 26 (1995) 2803. [16] M. Zhao, G.H. Wu, D.Z. Zhu, L.T. Jiang, Zuoyong Dou, Mater. Lett. 58 (2004) 1899. [17] Z.P. Luo, Y.G. Song, S.Q. Zhang, Scripta Mater. 45 (2001) 1183. [18] J. Mering, J. Maire, J. Chem. Phys. 57 (1960) 803. [19] J. Maire, J. Mering, Chem. Phys. Carbon 6 (1970) 125. [20] G. Leroy, J.D. Embury, G. Edward, M.F. Ashby, Acta Metall. 29 (1981) 1509. [21] Y. Flom, R.J. Arsenault, Mater. Sci. Eng. 77 (1986) 191.