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A novel approach to synthesize SiC particles by in situ reaction in Al–Si–C alloys
Journal of Alloys and Compounds 588 (2014) 374–377
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
A novel approach to synthesize SiC particles by in situ reaction in Al–Si–C alloys Xiaofan Du, Tong Gao, DaKui Li, Yuying Wu, Xiangfa Liu ⇑ Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, 17923 Jingshi Road, Jinan 250061, PR China
a r t i c l e
i n f o
Article history: Received 22 September 2013 Received in revised form 14 November 2013 Accepted 15 November 2013 Available online 25 November 2013 Keywords: SiC particles Liquid–solid reaction Microstructure Al–Si–C alloy
a b s t r a c t In this article, a novel approach to synthesize SiC particles utilizing in situ liquid–solid reaction in Al–Si–C alloys was put forward. Unlike the traditional methods, the liquid–solid reaction method for in situ synthesis of SiC is direct, easy and can be used for mass production. The experimental results show that the synthesis temperature for SiC particles can be as low as 750 °C. The size of in situ synthesized SiC particles increases from 0.2 lm to 10 lm as the reaction temperature increases. Furthermore, it was found that SiC particles are in situ synthesized through gradual-reaction mechanism, and Al4C3 is the intermediate phase which is much easier to be synthesized in Al–Si–C system. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction SiC is an attractive reinforcing phase in several alloys due to its high melt point, high stiffness, good thermal stability, high hardness, good resistance to chemical attack at room temperature, and low thermal coefficient of expansion [1–4]. SiC reinforced aluminum matrix composites have been widely applied in aerospace, automotive and electronic productions since they exhibit some excellent properties, e.g. light weight, high specific modulus and excellent wear resistance [5,6]. Traditionally, SiC particulates reinforced aluminum matrix composites have been fabricated by several approaches such as melt-stirring method [7], the rheological casting technique [8–9], and infiltration method [10–11]. For the metal matrix composites, the melt-stirring method mixed of the reinforcements with the molten metal is cost effective. Seo and Sahin et al. fabricated the SiC particulates reinforced aluminum alloy composites by meltstirring [7,12,13]. The preheated SiC particles were injected into the molten aluminum, and the high-speed rotation of impeller is the driving force for melt mixing. However, it is extremely challenging for the conventional stirring casting to distribute and disperse nanoparticles uniformly in metal melts due to their large surface-to-volume ratio and their low wettability in metal melts. Secondly, due to the porosity and segregation at the interface between the matrix and reinforcing particles, the interface bonding strength may be lowered.
⇑ Corresponding author. Tel.: +86 531 88392006; fax: +86 531 88395414. E-mail address: xfl[email protected] (X. Liu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.099
Pressure infiltration method is mainly used for the preparation of high volume fractions of SiC particulates reinforced aluminum composites [10,14]. Nevertheless, the infiltration of liquid aluminum is associated with the complex physicochemical and hydrodynamic theory, it is difficult to control the reaction process. What is more, the fabrication of composites is complicated, the size of product is limited and the equipment is expensive, which has limited its application in industries. In addition, the interface reaction between SiC particles and aluminum matrix could not be avoided, thus resulting in the formation of brittle Al4C3, which is harmful for the mechanical properties of the composites. To overcome the limitations of conventional processes, new in situ processing techniques have been developed, in which the reinforcements are synthesized in matrix by chemical reactions. Compared to the conventional aluminum matrix composites produced by ex situ methods, the in situ composites exhibit the following advantages: (a) the in situ formed reinforcements are thermodynamically stable at the matrix, leading to less degradation under the elevated temperature conditions; (b) the reinforcement– matrix interfaces are clean, resulting in a strong interfacial bonding; (c) the in situ formed reinforcing particles are finer in size and their distribution in the matrix is more uniform [15,16]. However, the solubility of carbon in Al melts is quite limited [17], making it difficult to produce SiC phase with in situ synthesis method in Al melts. Therefore, rare work has been carried out on the in situ synthesis reaction method for SiC in Al–Si melts. In the present study, the feasibility of the synthesis of in situ formed SiCp/Al–Si composites utilizing liquid–solid reaction in Al–Si–C melts was investigated. The effect of sintering temperature on the microstructure of the Al–30Si–5C alloy was discussed.
X. Du et al. / Journal of Alloys and Compounds 588 (2014) 374–377
Meanwhile, the reaction mechanism of in situ synthesis of SiC particles was revealed. 2. Experimental In this work, the commercial aluminum powders (99.7%, all compositions quoted in this work are in wt.% unless otherwise stated, 100–200 mesh), commercial silicon powders (99.5%, 200 mesh) and the prepared carbon plastid (10 lm) were used for making the powder blends. The starting composition of the green samples was Al–30Si–5C (wt.%). After being sufficiently mixed, the required powders blends were pressed into cylindrical samples, 60 mm in diameter and 15 mm in length. The samples thus prepared were heated in a vacuum electric resistance furnace at 650, 700,750 and 950 °C, respectively, holding for 45 min. The vacuum degree of electric resistance furnace of 30KW was less than 103Pa. To study the in situ liquid–solid reaction behavior in the Al–Si–C system, the green sample of Al–5C (wt.%) was prepared by the same method without Si powders. The two kinds of green samples weighing about 20 mg were heated at 10 K/ min to about 1273 K under argon protective atmosphere in differential scanning calorimeter (DSC) (Netzsch STA 404C TG-DSC) apparatus. Metallographic specimens were taken from the center of each sample in the transverse section, then were mechanically ground and polished through standard routines. The microstructures of the investigated Al–30Si–5C alloys and the morphology of SiC particles were characterized using field emission scanning electron microscopy (FESEM, model SU-70, Japan), equipped with an energy dispersion spectrum (EDS) detector. X-ray diffraction (XRD, Rigaku D/max-rB) was carried out to identify the phase constitution.
3. Results and discussion 3.1. Morphology of SiC phase in Al–30Si–5C alloy Fig. 1 shows the XRD patterns of the Al–30Si–5C alloys prepared at 650, 700, 750 and 950 °C, respectively. It was not found the diffraction peak corresponding to SiC phase at 650 and 700 °C. Meanwhile, it is interesting to note that the main phase is Al4C3 ex-
375
cept for aluminum and silicon phases at 700 °C, as shown in Fig. 1b. However, when the temperature increases to 750 and 950 °C, it can be observed that the main phase changes to SiC. Since the carbon content is 5 wt.% in the alloy, it will consume about 12 wt.% of Si when totally form as SiC. As a result, there still remains 18 wt.% Si in the melt, leading to the formation of primary Si. Therefore, it can be found that there is Si phase in the alloy according to the XRD pattern. Fig. 2 presents the FESEM images of the Al–30Si–5C alloys fabricated by the liquid–solid reaction at 750 and 950 °C. The grey regions represent the Al matrix and the bright particles are SiC particles. When the reaction temperature is 750 °C, it can been seen that SiC particles can be in situ synthesized in large quantity. Fig. 2a shows the synthesized SiC particles exhibit spherical or nearly spherical. The SiC particles are nearly homogeneously distributed in aluminum matrix. The identification of the bright particles was further confirmed by EDS analysis, as shown in Fig. 3. Compositional image in Fig. 3b indicates that proper proportion of Si and C. Combining with the diffraction peaks of SiC as shown in Fig. 1, it is reasonable to consider that the SiC particles can be formed in Al–30Si–5C alloys. Fig. 2b shows microstructure of the sample prepared at 950 °C. Comparing with Fig. 2a, it is obviously found that the size of SiC particles increases. It is indicated that the size of SiC particles will vary with synthesis temperature. In order to clearly observe the distribution of SiC particle size, one hundred of particle diameters were measured for the samples. Fig. 4 shows the curves of the size distribution of SiC particles in the samples prepared at 750 and 950 °C, respectively. It is clear from the figure that the size of SiC particles concentrates at about 0.4 lm when the temperature is 750 °C. It should note that the size of most of SiC particles is less than 2 lm. There are two main peaks in the curve of 950 °C, which are located at 1.4 and 2.2 lm, respectively. While, some larger ones can be as large as 5 lm.The results demonstrate that the size of SiC particle increases with the increasing temperature.
3.2. Mechanism of reaction
Fig. 1. XRD patterns of Al–30Si–5C alloys prepared with different temperature: (a) 650 °C; (b) 700 °C; (c) 750 °C; (d) 950 °C.
Fig. 5 shows the DSC heating curves of Al–5C and Al–30Si–5C alloys, respectively. Form Fig. 5a, it can be seen that there are two main peaks, the endothermic peak at 668.3 °C corresponds to the melting aluminum and the exothermic reaction at about 679.2 °C indicates that Al4C3 phase is formed. By observing the DSC curve of Al–30Si–5C alloy, it can be observed that there are one exothermic peak and two endothermic peaks. The melt of aluminum– silicon eutectic corresponds to the endothermic peak at 586.2 °C. The exothermic peak at 679.7 °C is close to the temperature of the formation of Al4C3, as shown in Fig. 5a. According to the XRD pattern, the reaction product is SiC, as illustrated in Fig. 1. There-
Fig. 2. Microstructure analysis of Al–30Si–5C alloys: (a) 750 °C; (b) 950 °C.
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Fig. 3. EDS analysis of SiC particulates in Al–30Si–5C alloy fabricated at 750 °C.
It indicates that Al4C3 is the intermediate phase for in situ formed SiC particles. Therefore, it is reasonable to believe that SiC can be synthesized by the gradual reaction mechanism. A model is proposed to illustrate the formation of spherical SiC as shown in Fig. 6, and the reaction process is readily explained. Form Fig. 6a, it can be clearly seen that the carbon plastids uniformly distributes in Al–Si melts. The carbon released by carbon plastids reacts with aluminum forming Al4C3 phase as illustrated in Fig. 6b. The occurrence of Eq. (2) mainly depends on the diffusion of Si atoms. The system energy increases by both the heat released from the precipitation of Al4C3 and the increasing temperature of the melts. In addition, the Si content of the melts is enough at the initial stage. These two factors are conductive to the diffusion of Si atoms in the aluminum melt. According to the unidirectional diffusion mechanism, Si atoms diffuse to the periphery of Al4C3 phase. When the temperature and the Si content of melts are matching to the reaction conditions, Eq. (2) will occur. In this case, the SiC particles can be in situ formed in the alloy as shown in Fig. 6c.
Fig. 4. SiC particle size distributions at different temperature.
fore, it is reasonable to accept that endothermic peak at 757.0 °C corresponds to the formation of SiC. The chemical reactions between the reactants, which are keys to produce the in situ metal matrix composites, are extremely important. Based on the experimental results, we summarized that in situ formation of SiC particles is correlated with the below equation:
4Alð1Þ þ 3CðsÞ ! Al4 C3 ðsÞ
ð1Þ
Al4 C3 ðsÞ þ 3Sið1Þ ! 3SiCðsÞ þ 4Alð1Þ
ð2Þ
4. Discussion It is widely recognized that superior properties can be obtained by an in situ formed composites with thermodynamically more stable reinforcements by nucleation and growth form the parent matrix phase [18]. At the same time, the composites possess contaminated–free interfaces [19]. Thus, in situ processes are always promising techniques for composite fabrication. Some theoretical and experimental research on the SiC-reinforced aluminum alloy composites with in situ methods has been reported [20]. But, these effects mainly focus on the gas–liquid reaction method. Wu et al.
Fig. 5. The DSC heating carves of the mixtures of (a) Al–5C alloy and (b) Al–30Si–5C alloy heated at 10 K/min under argon atmosphere.
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Fig. 6. Schematic illustration of the reaction processes of formation of the nearly spherical SiC.
used the methane gas mixture to produce SiC-reinforced Al–Si alloy composites [21]. The bubbling of the gas mixture in the melt resulted in the formation of a layer of foam. Formed SiC particles were enriched in the foam and carried out of the crucible by the overflow foam to a composite collector located under the crucible. However, it requires the higher reaction temperature and the longer holding time, which greatly increases the cost of production. Besides, the amount of SiC particles is difficult to control and the size of SiC particles is larger. In our work, SiC particles are successfully in situ formed through liquid–solid reaction method in Al–Si melts. It overcomes the limitation of carbon solubility in aluminum melts. The spherical SiC particles can be synthesized in large quantities in Al– 30Si–5C alloy, and uniformly dispersed in aluminum matrix, as illustrated in Fig. 2a and b. When the temperature is 750 °C, the main size of SiC particles can be less than 1 lm, which is almost impossible to obtain using ex situ methods. Secondly, the reaction process is controllable, and the process parameters (temperature and holding time) play an important role for in situ synthesis of SiC particles. The reaction temperature can reduced to 750 °C and the holding time is shorter (45 min). The method contributes to saving the energy and laying the foundation for further industrial applications. Besides, it is interesting to note that the interfaces between SiC particles and matrix are clean since SiC particles are formed in situ in molten aluminum alloys. It is known that the reaction mainly depends on the thermodynamic and kinetic conditions. According to the DSC analysis, it is reasonable to accept that the formation of Al4C3 is favorable in Al–Si–C system. The main reason is that the formation energy of Al4C3 (168 kJ mol1) is more negative than that of SiC (84 kJ mol1) [22–24]. What’s more, it has been shown that Al4C3 could form as a transitory species under non-equilibrium conditions [25]. In addition, when the temperature is 700 °C, Al4C3 can be synthesized in Al–30Si–5C alloy, as shown in Fig. 2b. Therefore, it can be concluded that Al4C3 can be firstly synthesized in Al–30Si–5C alloy. Following the formation of Al4C3 as shown Eq. (1), the SiC can be synthesized through Eq. (2). Supposed that the Gibbs free energies of Al (l) and Si (l) are zero, the change of Gibbs free energy of Eq. (2) can be expressed as
DG0 ¼ 3DG0SiC D0Al4 C4 In this case, the change of Gibbs free energy of Eq. (2) (DG0 = 84 kJ mol1 < 0) is calculated. It is indicated that Eq. (2) meets the thermodynamic conditions. From Eq. (2), it can be found that SiC can be formed by replacement reaction of Si and Al4C3. Thus, the synthesized SiC particles in this way may contain a certain amount of Al element, which is conformity with the EDS analysis results as illustrated in Fig. 3. The gradual reaction mechanism of liquid–solid reaction method has a great superiority for synthesizing SiC particles in aluminum alloy.
Therefore, the liquid–solid reaction method for in situ synthesis of SiC is direct, easy and can be regulated, and it is a promising technology to produce SiC particles reinforced aluminum composites.
5. Conclusion In this study, a novel approach to synthesize SiC particles utilizing in situ liquid–solid reaction in Al–Si–C alloys was investigated. The experimental results show that the synthesis temperature of SiC particles can be reduced to 750 °C. The size of in situ synthesized SiC particles increases from 200 nm to 10 lm as the reaction temperature increases. The further study on DSC analysis presents that the SiC particles are in situ synthesized through gradual-reaction mechanism. Acknowledgment This research was financially supported by the National Natural Science Foundation of China (No.51271101) and the National Basic Research Program of China (973 Program, No. 2012CB825702). References [1] S. Ray, J. Mater. Sci. 28 (1993) 5397–5413. [2] N. Chawla, K. Chawla, USA: Springer Science–Business Media, Metal Matrix Composites 2005. [3] B.S. Unlu, Mater. Des. 29 (2008) 2002–2008. [4] C.S. Ramesh, R. Noor Ahmed, M.A. Mujeebu, M.Z. Abdullah, Mater. Des. 30 (2010) 1957–1965. [5] D.B. Miracle, Compos. Sci. Technol. 65 (2005) 2526–2540. [6] C. Srinivasa Rao, G.S. Upadhyaya, Mater. Des. 16 (1995) 359–366. [7] Y.H. Seoa, C.G. Kangb, Compos. Sci. Technol. 59 (1999) 643–654. [8] S.C. Lima, M. Guptaa, L. Rena, J.K.M. Kwok, J. Mater. Process. Technol. 89–90 (1999) 591–596. [9] U.A. Curle, L. Ivanchev, Trans. Nonferrous. Met. Soc. 20 (2010) 852–856. [10] H. Ahlatci, E. Candan, H. Çimenoglu, Wear 257 (2004) 625–632. [11] X.Y. Shen, S.B. Ren, X.B. He, M.L. Qin, X.H. Qu, J Alloys Comp. 468 (2009) 158– 163. [12] Y. Sahin, Mater. Des. 24 (2003) 671–679. [13] X.Y. Meng, H. Ding, Y.B. Chen, J.L. Wen, Rare. Met. 21 (3) (2002) 203–206. [14] E. Candan, H.V. Atkinson, H. Jones, J. Mater. Sci. 35 (2000) 4955–4960. [15] S.C. Tjong, Z.Y. Ma, Mater. Sci. Eng. 29 (2000) 49–113. [16] B.S.S. Daniel, V.S.R. Murthy, G.S. Murty, J. Mater. Process. Technol. 68 (1997) 132–155. [17] L.L. Oden, R.A. McCUNE, Metall. Trans A. 18 (1987) 2005–2014. [18] G. Chen, G.X. Sun, Z.G. Zhu, Mater. Sci. Eng A. 251 (1998) 226–231. [19] P. Yu, C.K. Kwok, C.Y. To, T.K. Li, Dickon H.L. Ng, Composites Part B 39 (2008) 327–331. [20] J.U. Ejiofor, R.G. Reddy, Jom. 49 (1997) 31–37. [21] B.Q. Wu, R.G. Reddy, Metall. Mater. Trans. B 33 (2002) 543–550. [22] L. Lu, A.K. Dahle, D.H. Stjohn, Scripta Mater. 54 (2006) 2197–2201. [23] L.Z. Cheng, Y.H. Zhang, S.Z. Ren, X.Z. Wang, W. Shi, Physical Chemistry, second ed., Shanghai Science and Technology Press, Shanghai, 2005. [24] J.L. Murray, A.C. McAlister, Bull. Alloy. Phase. Diag. 5 (1984) 74–84. [25] F. Barbeau, M. Peronnet, F. Bosselet, J.C. Viala, J. Mater. Sci. Lett. 19 (2000) 2039–2041.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
A novel approach to synthesize SiC particles by in situ reaction in Al–Si–C alloys Xiaofan Du, Tong Gao, DaKui Li, Yuying Wu, Xiangfa Liu ⇑ Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, 17923 Jingshi Road, Jinan 250061, PR China
a r t i c l e
i n f o
Article history: Received 22 September 2013 Received in revised form 14 November 2013 Accepted 15 November 2013 Available online 25 November 2013 Keywords: SiC particles Liquid–solid reaction Microstructure Al–Si–C alloy
a b s t r a c t In this article, a novel approach to synthesize SiC particles utilizing in situ liquid–solid reaction in Al–Si–C alloys was put forward. Unlike the traditional methods, the liquid–solid reaction method for in situ synthesis of SiC is direct, easy and can be used for mass production. The experimental results show that the synthesis temperature for SiC particles can be as low as 750 °C. The size of in situ synthesized SiC particles increases from 0.2 lm to 10 lm as the reaction temperature increases. Furthermore, it was found that SiC particles are in situ synthesized through gradual-reaction mechanism, and Al4C3 is the intermediate phase which is much easier to be synthesized in Al–Si–C system. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction SiC is an attractive reinforcing phase in several alloys due to its high melt point, high stiffness, good thermal stability, high hardness, good resistance to chemical attack at room temperature, and low thermal coefficient of expansion [1–4]. SiC reinforced aluminum matrix composites have been widely applied in aerospace, automotive and electronic productions since they exhibit some excellent properties, e.g. light weight, high specific modulus and excellent wear resistance [5,6]. Traditionally, SiC particulates reinforced aluminum matrix composites have been fabricated by several approaches such as melt-stirring method [7], the rheological casting technique [8–9], and infiltration method [10–11]. For the metal matrix composites, the melt-stirring method mixed of the reinforcements with the molten metal is cost effective. Seo and Sahin et al. fabricated the SiC particulates reinforced aluminum alloy composites by meltstirring [7,12,13]. The preheated SiC particles were injected into the molten aluminum, and the high-speed rotation of impeller is the driving force for melt mixing. However, it is extremely challenging for the conventional stirring casting to distribute and disperse nanoparticles uniformly in metal melts due to their large surface-to-volume ratio and their low wettability in metal melts. Secondly, due to the porosity and segregation at the interface between the matrix and reinforcing particles, the interface bonding strength may be lowered.
⇑ Corresponding author. Tel.: +86 531 88392006; fax: +86 531 88395414. E-mail address: xfl[email protected] (X. Liu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.099
Pressure infiltration method is mainly used for the preparation of high volume fractions of SiC particulates reinforced aluminum composites [10,14]. Nevertheless, the infiltration of liquid aluminum is associated with the complex physicochemical and hydrodynamic theory, it is difficult to control the reaction process. What is more, the fabrication of composites is complicated, the size of product is limited and the equipment is expensive, which has limited its application in industries. In addition, the interface reaction between SiC particles and aluminum matrix could not be avoided, thus resulting in the formation of brittle Al4C3, which is harmful for the mechanical properties of the composites. To overcome the limitations of conventional processes, new in situ processing techniques have been developed, in which the reinforcements are synthesized in matrix by chemical reactions. Compared to the conventional aluminum matrix composites produced by ex situ methods, the in situ composites exhibit the following advantages: (a) the in situ formed reinforcements are thermodynamically stable at the matrix, leading to less degradation under the elevated temperature conditions; (b) the reinforcement– matrix interfaces are clean, resulting in a strong interfacial bonding; (c) the in situ formed reinforcing particles are finer in size and their distribution in the matrix is more uniform [15,16]. However, the solubility of carbon in Al melts is quite limited [17], making it difficult to produce SiC phase with in situ synthesis method in Al melts. Therefore, rare work has been carried out on the in situ synthesis reaction method for SiC in Al–Si melts. In the present study, the feasibility of the synthesis of in situ formed SiCp/Al–Si composites utilizing liquid–solid reaction in Al–Si–C melts was investigated. The effect of sintering temperature on the microstructure of the Al–30Si–5C alloy was discussed.
X. Du et al. / Journal of Alloys and Compounds 588 (2014) 374–377
Meanwhile, the reaction mechanism of in situ synthesis of SiC particles was revealed. 2. Experimental In this work, the commercial aluminum powders (99.7%, all compositions quoted in this work are in wt.% unless otherwise stated, 100–200 mesh), commercial silicon powders (99.5%, 200 mesh) and the prepared carbon plastid (10 lm) were used for making the powder blends. The starting composition of the green samples was Al–30Si–5C (wt.%). After being sufficiently mixed, the required powders blends were pressed into cylindrical samples, 60 mm in diameter and 15 mm in length. The samples thus prepared were heated in a vacuum electric resistance furnace at 650, 700,750 and 950 °C, respectively, holding for 45 min. The vacuum degree of electric resistance furnace of 30KW was less than 103Pa. To study the in situ liquid–solid reaction behavior in the Al–Si–C system, the green sample of Al–5C (wt.%) was prepared by the same method without Si powders. The two kinds of green samples weighing about 20 mg were heated at 10 K/ min to about 1273 K under argon protective atmosphere in differential scanning calorimeter (DSC) (Netzsch STA 404C TG-DSC) apparatus. Metallographic specimens were taken from the center of each sample in the transverse section, then were mechanically ground and polished through standard routines. The microstructures of the investigated Al–30Si–5C alloys and the morphology of SiC particles were characterized using field emission scanning electron microscopy (FESEM, model SU-70, Japan), equipped with an energy dispersion spectrum (EDS) detector. X-ray diffraction (XRD, Rigaku D/max-rB) was carried out to identify the phase constitution.
3. Results and discussion 3.1. Morphology of SiC phase in Al–30Si–5C alloy Fig. 1 shows the XRD patterns of the Al–30Si–5C alloys prepared at 650, 700, 750 and 950 °C, respectively. It was not found the diffraction peak corresponding to SiC phase at 650 and 700 °C. Meanwhile, it is interesting to note that the main phase is Al4C3 ex-
375
cept for aluminum and silicon phases at 700 °C, as shown in Fig. 1b. However, when the temperature increases to 750 and 950 °C, it can be observed that the main phase changes to SiC. Since the carbon content is 5 wt.% in the alloy, it will consume about 12 wt.% of Si when totally form as SiC. As a result, there still remains 18 wt.% Si in the melt, leading to the formation of primary Si. Therefore, it can be found that there is Si phase in the alloy according to the XRD pattern. Fig. 2 presents the FESEM images of the Al–30Si–5C alloys fabricated by the liquid–solid reaction at 750 and 950 °C. The grey regions represent the Al matrix and the bright particles are SiC particles. When the reaction temperature is 750 °C, it can been seen that SiC particles can be in situ synthesized in large quantity. Fig. 2a shows the synthesized SiC particles exhibit spherical or nearly spherical. The SiC particles are nearly homogeneously distributed in aluminum matrix. The identification of the bright particles was further confirmed by EDS analysis, as shown in Fig. 3. Compositional image in Fig. 3b indicates that proper proportion of Si and C. Combining with the diffraction peaks of SiC as shown in Fig. 1, it is reasonable to consider that the SiC particles can be formed in Al–30Si–5C alloys. Fig. 2b shows microstructure of the sample prepared at 950 °C. Comparing with Fig. 2a, it is obviously found that the size of SiC particles increases. It is indicated that the size of SiC particles will vary with synthesis temperature. In order to clearly observe the distribution of SiC particle size, one hundred of particle diameters were measured for the samples. Fig. 4 shows the curves of the size distribution of SiC particles in the samples prepared at 750 and 950 °C, respectively. It is clear from the figure that the size of SiC particles concentrates at about 0.4 lm when the temperature is 750 °C. It should note that the size of most of SiC particles is less than 2 lm. There are two main peaks in the curve of 950 °C, which are located at 1.4 and 2.2 lm, respectively. While, some larger ones can be as large as 5 lm.The results demonstrate that the size of SiC particle increases with the increasing temperature.
3.2. Mechanism of reaction
Fig. 1. XRD patterns of Al–30Si–5C alloys prepared with different temperature: (a) 650 °C; (b) 700 °C; (c) 750 °C; (d) 950 °C.
Fig. 5 shows the DSC heating curves of Al–5C and Al–30Si–5C alloys, respectively. Form Fig. 5a, it can be seen that there are two main peaks, the endothermic peak at 668.3 °C corresponds to the melting aluminum and the exothermic reaction at about 679.2 °C indicates that Al4C3 phase is formed. By observing the DSC curve of Al–30Si–5C alloy, it can be observed that there are one exothermic peak and two endothermic peaks. The melt of aluminum– silicon eutectic corresponds to the endothermic peak at 586.2 °C. The exothermic peak at 679.7 °C is close to the temperature of the formation of Al4C3, as shown in Fig. 5a. According to the XRD pattern, the reaction product is SiC, as illustrated in Fig. 1. There-
Fig. 2. Microstructure analysis of Al–30Si–5C alloys: (a) 750 °C; (b) 950 °C.
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Fig. 3. EDS analysis of SiC particulates in Al–30Si–5C alloy fabricated at 750 °C.
It indicates that Al4C3 is the intermediate phase for in situ formed SiC particles. Therefore, it is reasonable to believe that SiC can be synthesized by the gradual reaction mechanism. A model is proposed to illustrate the formation of spherical SiC as shown in Fig. 6, and the reaction process is readily explained. Form Fig. 6a, it can be clearly seen that the carbon plastids uniformly distributes in Al–Si melts. The carbon released by carbon plastids reacts with aluminum forming Al4C3 phase as illustrated in Fig. 6b. The occurrence of Eq. (2) mainly depends on the diffusion of Si atoms. The system energy increases by both the heat released from the precipitation of Al4C3 and the increasing temperature of the melts. In addition, the Si content of the melts is enough at the initial stage. These two factors are conductive to the diffusion of Si atoms in the aluminum melt. According to the unidirectional diffusion mechanism, Si atoms diffuse to the periphery of Al4C3 phase. When the temperature and the Si content of melts are matching to the reaction conditions, Eq. (2) will occur. In this case, the SiC particles can be in situ formed in the alloy as shown in Fig. 6c.
Fig. 4. SiC particle size distributions at different temperature.
fore, it is reasonable to accept that endothermic peak at 757.0 °C corresponds to the formation of SiC. The chemical reactions between the reactants, which are keys to produce the in situ metal matrix composites, are extremely important. Based on the experimental results, we summarized that in situ formation of SiC particles is correlated with the below equation:
4Alð1Þ þ 3CðsÞ ! Al4 C3 ðsÞ
ð1Þ
Al4 C3 ðsÞ þ 3Sið1Þ ! 3SiCðsÞ þ 4Alð1Þ
ð2Þ
4. Discussion It is widely recognized that superior properties can be obtained by an in situ formed composites with thermodynamically more stable reinforcements by nucleation and growth form the parent matrix phase [18]. At the same time, the composites possess contaminated–free interfaces [19]. Thus, in situ processes are always promising techniques for composite fabrication. Some theoretical and experimental research on the SiC-reinforced aluminum alloy composites with in situ methods has been reported [20]. But, these effects mainly focus on the gas–liquid reaction method. Wu et al.
Fig. 5. The DSC heating carves of the mixtures of (a) Al–5C alloy and (b) Al–30Si–5C alloy heated at 10 K/min under argon atmosphere.
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377
Fig. 6. Schematic illustration of the reaction processes of formation of the nearly spherical SiC.
used the methane gas mixture to produce SiC-reinforced Al–Si alloy composites [21]. The bubbling of the gas mixture in the melt resulted in the formation of a layer of foam. Formed SiC particles were enriched in the foam and carried out of the crucible by the overflow foam to a composite collector located under the crucible. However, it requires the higher reaction temperature and the longer holding time, which greatly increases the cost of production. Besides, the amount of SiC particles is difficult to control and the size of SiC particles is larger. In our work, SiC particles are successfully in situ formed through liquid–solid reaction method in Al–Si melts. It overcomes the limitation of carbon solubility in aluminum melts. The spherical SiC particles can be synthesized in large quantities in Al– 30Si–5C alloy, and uniformly dispersed in aluminum matrix, as illustrated in Fig. 2a and b. When the temperature is 750 °C, the main size of SiC particles can be less than 1 lm, which is almost impossible to obtain using ex situ methods. Secondly, the reaction process is controllable, and the process parameters (temperature and holding time) play an important role for in situ synthesis of SiC particles. The reaction temperature can reduced to 750 °C and the holding time is shorter (45 min). The method contributes to saving the energy and laying the foundation for further industrial applications. Besides, it is interesting to note that the interfaces between SiC particles and matrix are clean since SiC particles are formed in situ in molten aluminum alloys. It is known that the reaction mainly depends on the thermodynamic and kinetic conditions. According to the DSC analysis, it is reasonable to accept that the formation of Al4C3 is favorable in Al–Si–C system. The main reason is that the formation energy of Al4C3 (168 kJ mol1) is more negative than that of SiC (84 kJ mol1) [22–24]. What’s more, it has been shown that Al4C3 could form as a transitory species under non-equilibrium conditions [25]. In addition, when the temperature is 700 °C, Al4C3 can be synthesized in Al–30Si–5C alloy, as shown in Fig. 2b. Therefore, it can be concluded that Al4C3 can be firstly synthesized in Al–30Si–5C alloy. Following the formation of Al4C3 as shown Eq. (1), the SiC can be synthesized through Eq. (2). Supposed that the Gibbs free energies of Al (l) and Si (l) are zero, the change of Gibbs free energy of Eq. (2) can be expressed as
DG0 ¼ 3DG0SiC D0Al4 C4 In this case, the change of Gibbs free energy of Eq. (2) (DG0 = 84 kJ mol1 < 0) is calculated. It is indicated that Eq. (2) meets the thermodynamic conditions. From Eq. (2), it can be found that SiC can be formed by replacement reaction of Si and Al4C3. Thus, the synthesized SiC particles in this way may contain a certain amount of Al element, which is conformity with the EDS analysis results as illustrated in Fig. 3. The gradual reaction mechanism of liquid–solid reaction method has a great superiority for synthesizing SiC particles in aluminum alloy.
Therefore, the liquid–solid reaction method for in situ synthesis of SiC is direct, easy and can be regulated, and it is a promising technology to produce SiC particles reinforced aluminum composites.
5. Conclusion In this study, a novel approach to synthesize SiC particles utilizing in situ liquid–solid reaction in Al–Si–C alloys was investigated. The experimental results show that the synthesis temperature of SiC particles can be reduced to 750 °C. The size of in situ synthesized SiC particles increases from 200 nm to 10 lm as the reaction temperature increases. The further study on DSC analysis presents that the SiC particles are in situ synthesized through gradual-reaction mechanism. Acknowledgment This research was financially supported by the National Natural Science Foundation of China (No.51271101) and the National Basic Research Program of China (973 Program, No. 2012CB825702). References [1] S. Ray, J. Mater. Sci. 28 (1993) 5397–5413. [2] N. Chawla, K. Chawla, USA: Springer Science–Business Media, Metal Matrix Composites 2005. [3] B.S. Unlu, Mater. Des. 29 (2008) 2002–2008. [4] C.S. Ramesh, R. Noor Ahmed, M.A. Mujeebu, M.Z. Abdullah, Mater. Des. 30 (2010) 1957–1965. [5] D.B. Miracle, Compos. Sci. Technol. 65 (2005) 2526–2540. [6] C. Srinivasa Rao, G.S. Upadhyaya, Mater. Des. 16 (1995) 359–366. [7] Y.H. Seoa, C.G. Kangb, Compos. Sci. Technol. 59 (1999) 643–654. [8] S.C. Lima, M. Guptaa, L. Rena, J.K.M. Kwok, J. Mater. Process. Technol. 89–90 (1999) 591–596. [9] U.A. Curle, L. Ivanchev, Trans. Nonferrous. Met. Soc. 20 (2010) 852–856. [10] H. Ahlatci, E. Candan, H. Çimenoglu, Wear 257 (2004) 625–632. [11] X.Y. Shen, S.B. Ren, X.B. He, M.L. Qin, X.H. Qu, J Alloys Comp. 468 (2009) 158– 163. [12] Y. Sahin, Mater. Des. 24 (2003) 671–679. [13] X.Y. Meng, H. Ding, Y.B. Chen, J.L. Wen, Rare. Met. 21 (3) (2002) 203–206. [14] E. Candan, H.V. Atkinson, H. Jones, J. Mater. Sci. 35 (2000) 4955–4960. [15] S.C. Tjong, Z.Y. Ma, Mater. Sci. Eng. 29 (2000) 49–113. [16] B.S.S. Daniel, V.S.R. Murthy, G.S. Murty, J. Mater. Process. Technol. 68 (1997) 132–155. [17] L.L. Oden, R.A. McCUNE, Metall. Trans A. 18 (1987) 2005–2014. [18] G. Chen, G.X. Sun, Z.G. Zhu, Mater. Sci. Eng A. 251 (1998) 226–231. [19] P. Yu, C.K. Kwok, C.Y. To, T.K. Li, Dickon H.L. Ng, Composites Part B 39 (2008) 327–331. [20] J.U. Ejiofor, R.G. Reddy, Jom. 49 (1997) 31–37. [21] B.Q. Wu, R.G. Reddy, Metall. Mater. Trans. B 33 (2002) 543–550. [22] L. Lu, A.K. Dahle, D.H. Stjohn, Scripta Mater. 54 (2006) 2197–2201. [23] L.Z. Cheng, Y.H. Zhang, S.Z. Ren, X.Z. Wang, W. Shi, Physical Chemistry, second ed., Shanghai Science and Technology Press, Shanghai, 2005. [24] J.L. Murray, A.C. McAlister, Bull. Alloy. Phase. Diag. 5 (1984) 74–84. [25] F. Barbeau, M. Peronnet, F. Bosselet, J.C. Viala, J. Mater. Sci. Lett. 19 (2000) 2039–2041.