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Microstructure and interfacial reactions of β-eucryptite particles in aluminum matrix composites
Materials Science and Engineering A 433 (2006) 291–297
Microstructure and interfacial reactions of -eucryptite particles in aluminum matrix composites L.D. Wang, W.D. Fei ∗ School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 405, Harbin 150001, PR China Received 14 September 2005; received in revised form 22 June 2006; accepted 22 June 2006
Abstract It has been found that aluminum matrix composites reinforced by -eucryptite particles and aluminum borate whiskers exhibit much lower coefficient of thermal expansion and higher strength. In the present study, the interfacial reactions of -eucryptite particles in aluminum matrix composites with different matrices were investigated. The interfacial reaction products on the surface of -eucryptite particles and the temperature of the reaction were identified; the chemical reaction formulas were put forward to interpret the reactions between eucryptite and Al and the alloying element Mg. © 2006 Elsevier B.V. All rights reserved. Keywords: -Eucryptite; Aluminum matrix composite; Interfacial reaction; DSC
1. Introduction Materials with a low coefficient of thermal expansion (CTE) and high strength have important applications as structural parts in aerospace system, measuring instruments, electric packaging, general optics, antennas, etc. It was reported [1,2] that low CTE and high tensile strength were attained simultaneously in an aluminum matrix composite reinforced by -eucryptite particles (LiAlSiO4 , denoted by Euc) which have a negative CTE [3] and aluminum borate whiskers (Al18 B4 O33 , denoted by ABO) which have high strength [4] (the composite is denoted by (Euc+ABO)/Al in this paper). It was also mentioned [5] that interfacial reactions can occur on the surfaces of both -eucryptite particles and aluminum borate whiskers. The interfacial reaction affects the CTE of the composite greatly. Previous researches [4,6,7] on aluminum matrix composites reinforced by ABO whiskers have indicated that an interfacial reaction occurs between the whiskers and the matrix alloys. The degree of the interfacial reaction can be changed by casting technique and heat treatment [8]. In addition, some studies have been carried out on the effect of interfacial reaction on the mechani-
∗
Corresponding author. Tel.: +86 45186418647; fax: +86 45186418647. E-mail address: [email protected] (W.D. Fei).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.06.092
cal properties of ABO whiskers reinforced aluminum composite [9]. It is well known that the interface plays an important role in the properties of composites. It is necessary to study the interfacial reaction in composites to improve their properties, control their interface morphology and broaden their applications. But, to our knowledge, the interfacial reactions between -eucryptite and different aluminum alloys have not been reported in detail before. In this study, the interfacial reactions of -eucryptite in (Euc+ABO)/Al composites with different aluminum matrices such as pure aluminum, AA6061 and AA2024 were studied. 2. Experimental The reinforcements were -eucryptite particles with a diameter of 5–10 m, which were synthesized in our laboratory by the method of a Chinese patent [10], and aluminum borate whiskers with a diameter of 0.5–1 m and a length of 10–30 m. Pure aluminum (AA1050), AA6061 and AA2024 were used as matrices. The chemical compositions of the alloys are listed in Table 1. The aluminum matrix composite reinforced by both -eucryptite particles and aluminum borate whiskers was fabricated by squeeze casting. The total volume fraction of eucryptite and aluminum borate is 40%, and the volume ratio between -eucryptite and aluminum borate is 2:1. The pres-
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Table 1 Chemical compositions of aluminum alloys used (mass%) Alloy
Mg
Si
Cu
Mn
Fe
Cr
Al
AA1050 AA6061 AA2024
0.05 0.8–1.2 1.2–1.8
0.25 0.4–0.8 <0.5
0.05 0.15–0.4 3.8–4.9
0.05 <0.15 0.3–0.9
0.4 <0.7 <0.5
– 0.04–0.35 <0.1
Balance Balance Balance
sure for squeeze casting was 30 MPa. The casting temperature of pure aluminum and aluminum alloys was 800 ◦ C, and the die temperature was about 520 ◦ C. An ABO reinforced pure Al matrix composite was also fabricated using the above conditions for the purpose of investigating the interfacial reactions in the composites. In order to investigate the effect of heat treatment on the interfacial reactions of the composites, specimens of the composites were heated to 490 or 520 ◦ C for different lengths of time and cooled to room temperature in air. Microstructures were investigated with a Philips CM-12 type transmission electron microscope. Specimens for TEM observation were thinned by ion milling. The diffractional scanning calorimeter (DSC) analysis was carried out on a Netzsch STA 449 DSC with a heating rate of 10 ◦ C/min under the protection of Ar gas.
3. Results and discussion 3.1. Interfacial reaction in (ABO+Euc)/pure-Al composite The interfacial reactions in ABO whiskers reinforced aluminum composite and their effect on the mechanical properties of the composite have been intensely studied in many investigations [4,6–9,11]. So in this paper, we do not pay attention to the interfacial reactions of ABO whiskers but focus on the interfacial reactions of -eucryptite. Fig. 1 shows the micrographs of the interface between eucryptite particles and the matrix in the (Euc+ABO)/pure-Al composite annealed at 490 ◦ C for 3, 7 and 9 h. The interfacial reactions between -eucryptite particles and the pure Al matrix were found to be sensitive to the solution time. For specimens of the composite treated for 3 h (Fig. 1a), the interfacial reaction
Fig. 1. TEM micrographs of eucryptite particles in (Euc+ABO)/p-Al composite treated at 490 ◦ C for (a) 3 h, (b) 7 h and (c) 9 h.
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products between -eucryptite particles and pure Al are fine. The specimens treated for 7 and 9 h (Fig. 1b and c) display more interfacial reaction products and -eucryptite particles are almost wholly covered by the interfacial reaction products. Therefore, it can be concluded that the amount of the interfacial reaction on the interface between -eucryptite and pure Al matrix increases obviously with the increasing of the heat treatment time. Fig. 1b and c and Fig. 2a show interfacial reaction products with different morphologies: one near the interface and inside the particle (area A in Fig. 2a) looks strip-like; the other outside of the interface (area B in Fig. 2a) looks particle-like. The selected area electron diffraction patterns (SADPs) corresponding to the interfacial reaction products are given in Fig. 2b and c, respectively. The SADP in Fig. 2b can be indexed as two components: one is tetragonal LiAlO2 (a = 0.28003 nm, c = 1.4216 nm), the
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other is cubic Si0.7276 Al0.2724 O2 (a = 2.4358 nm). The SADP in Fig. 2c can be indexed as ␥-Al2 O3 (a = 0.7924 nm). 3.2. Interfacial reaction in (ABO+Euc)/AA6061 composite The interfacial reaction product between -eucryptite and AA6061 has been studied before [5], it was found that the degree of interfacial reaction on the surface of -eucryptite particles increases with the increasing of solution time, and that the interfacial reaction products were initially identified as orthorhombic Mg2 SiO4 (forsterite). The present study shows that more than one kind of interfacial product forms on the interface of -eucryptite and AA6061. Fig. 3 shows the micrograph of (Euc+ABO)/AA6061 composite treated at 520 ◦ C for 5 h. It is obvious that the interfacial reaction of -eucryptite was extensive. The SADP
Fig. 2. TEM micrographs and SADP spectrum of the interfacial reaction products of eucryptite in (Euc+ABO)/p-Al composite treated at 490 ◦ C for 7 h (a) TEM micrographs of interfacial reaction products, (b) SADP of interfacial products adjacent to A and (c) SADP of interfacial product adjacent to B.
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Fig. 3. TEM micrographs and SADP of (Euc+ABO)/AA6061 composite treated at 520 ◦ C for 5 h (a) micrograph and (b) SADP of interfacial reaction products on the surface of eucrptite particle.
of the interface of -eucryptite shown in Fig. 3b can be indexed as Mg2 SiO4 (forsterite) with the lattice parameters of a = 0.47 6 nm, b = 1.020 nm and c = 0.600 nm, cubic LiAl5 O8 (a = 0.7908 nm), and -eucryptite. So the interfacial reaction products of -eucryptite and AA6061 include both Mg2 SiO4 and LiAl5 O8 . 3.3. Interfacial reaction in (ABO+Euc)/AA2024 composite Fig. 4 shows the micrographs of -eucryptite particles in the (Euc+ABO)/AA2024 composite. It can be seen that there are few interfacial products on the surface of -eucryptite in the ascast composite. However, the -eucryptite particles are covered by a heavy interfacial product layer in the composites treated for 5 and 9 h. So the degree of interfacial reaction increases greatly with the increasing of the heat treatment time. Fig. 4c shows the appearance of the interfacial reaction product of AA2024 matrix composite which is similar to that of pure Al matrix and AA6061 matrix composites. The indexing of the SADP in Fig. 3c indicates that orthorhombic Mg2 SiO4 , cubic LiAl5 O8 , and ␥-Al2 O3 are formed on the interface. 3.4. DSC analysis of the interfacial reactions in the composites The DSC curves of the three composites are shown in Fig. 5. For the three curves in the temperature range of 100–620 ◦ C, there is no peak for ABO/pure-Al composite, and there are weak and broad exothermal peaks for (Euc+ABO)/AA6061 and (Euc+ABO)/AA2024 composite. Each curve has an endothermal peak during 600–680 ◦ C corresponding to the melting of the matrix. Between 680 and 850 ◦ C only one exothermal peak with the peak temperature of 766 ◦ C can be found for ABO/pure-Al composite, while two exothermal peaks with the peak tempera-
tures of 734 and 766 ◦ C can be found for (Euc+ABO)/AA6061 and (Euc+ABO)/AA2024 composite. 3.5. Discussion It is surprising to see that there is a broad peak with a peak temperature of 344 ◦ C in Fig. 5b and c. This peak may be caused by a reaction between -eucryptite and Mg. TEM observation (Fig. 3a) shows that there are more interfacial products on the surface of -eucryptite than on the surface of ABO in (Euc+ABO)/AA6061 composite treated at 520 ◦ C for 5 h. This suggests that the degree of interfacial reaction for -eucryptite is higher than that of ABO. Thus Mg and -eucryptite appears to react more readily than Mg and ABO. Since Mg can react with ABO at the temperature of 480–600 ◦ C [11], it is reasonable to deduce that the reaction corresponding to -eucryptite and Mg may occur at a lower temperature. Thus, we deduce that the broad exothermal peak in the temperature range 100–480 ◦ C in Fig. 5b and c is probably related to the reaction of active Mg and -eucryptite. When the temperature ranges from 449 to 690 ◦ C, only Mg reacts with ABO whiskers; when the temperature is higher than 690 ◦ C, both Mg and Al react with ABO whiskers [11]. So for the Mg-containing composites, an exothermal peak must exist between 480 and 600 ◦ C. The exothermal peak between 480 and 600 ◦ C is clear in Fig. 5c, but it is less distinct in Fig. 5b. This presumably occurs because there is less Mg in the AA6061 matrix than in the AA2024 matrix. Most of the Mg in AA6061 matrix may be consumed by the reaction corresponding to Mg and -eucryptite, which leave less Mg for the reaction of Mg and ABO. Thus, the exothermal peak between 480 and 600 ◦ C in Fig. 5b is too weak to see. The exothermal peak on the DSC curve of ABO/pure-Al composite between 690 and 830 ◦ C is caused by the reaction between
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Fig. 4. TEM micrographs of the eucryptite in (Euc+ABO)/2024Al composite treated at 490 ◦ C: (a) as-cast, (b) 5 h, (c) 9 h and (d) SADP on the interface in (c).
ABO and Al [11]. In Fig. 5b, there are two exothermal peaks between 690 and 833 ◦ C, one is strong and the other is weak. By comparing Fig. 5b and c with Fig. 5a, it is clear that the weak exothermal peak near 766 ◦ C is also caused by the reaction between ABO and Al. In our opinion, the stronger peak at 734 ◦ C in Fig. 5b and c is caused by the reaction between -eucryptite and Al. The reasons are as follows: Firstly, the exothermal peak between 690 and 750 ◦ C is very strong, which means it is not possible for it to result from the reaction between additive such as Mg and the reinforcements. It can only be caused by the reaction between Al matrix and the reinforcements. Secondly, according to the preceding discussion, the reaction between Mg and -eucryptite may be much easier than that between Mg and ABO, and it occurs at a lower temperature. It is logical to deduce that -eucryptite can react more easily with Al compared with ABO when Al is melting. Comparing Fig. 5b with Fig. 5c, it is interesting to see that the peaks between 100 and 620 ◦ C in Fig. 5b are weaker than those in Fig. 5c, and the peaks between 690 and 833 ◦ C in Fig. 5c are weaker than those in Fig. 5b. The difference in the DSC
curves is caused by the interfacial reactions on the reinforcements. Because the Mg content in AA2024 is greater than in AA6061, more Mg was involved in the reactions in AA2024 matrix composite, which made the exothermal peaks in the temperature range 100–620 ◦ C in Fig. 5c more obvious than those in Fig. 5b. At the same time, interfacial reaction products can prevent the reactions between reinforcements and Al when Al is melting. The more interfacial reaction products forming in the temperature range 100–620 ◦ C, the less degree of reaction between Al and eucryptite or ABO in the temperature range 690–833 ◦ C. That is why the peaks between 690 and 833 ◦ C in Fig. 5c are not as strong as those in Fig. 5b. The interfacial reactions between Al and -eucryptite can be given by the following formulas: (4/3)Al + LiAlSiO4 → LiAlO2 + (2/3)Al2 O3 + Si
(1)
(4/3)Al + 4.5614LiAlSiO4 → 4.5614LiAlO2 + 4.8948Al0.2724 Si0.7276 O2 + Si
(2)
The reaction between Al and -eucryptite occurs when liquid Al exists during squeeze casting or heating. In the beginning,
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Fig. 5. DSC curves of ABO/pure-Al (a), (Euc+ABO)/AA6061 (b) and (Euc+ABO)/AA2024 (c).
molten Al and -eucryptite can contact directly. The reaction is controlled by formula (1). Once the interfacial reactions begin, accumulating interfacial reaction products can prevent the Al from contacting the -eucryptite. The relative content of Al to -eucryptite is less, and the reaction in formula (2) may happen. That is why cubic Si0.7276 Al02724 O2 was just found near the eucryptite particle (Fig. 2, area A) and Al2 O3 was found farther from the -eucryptite particle (Fig. 2, area B). For Mg-containing aluminum alloy matrix composites, Mg is an important reactant which can react with -eucryptite easily in the temperature range 100–480 ◦ C as described previously. But we found if the interfacial reaction products are Al2 O3 , LiAl5 O8 and Mg2 SiO4 , the reactants must include not only Mg and eucryptite but also Al. The interfacial reaction between Mg, Al and -eucryptite can proceed in terms of the following formula: 8Al + 2Mg + 8LiAlSiO4 → 3Li2 O + 2LiAl5 O8 + Mg2 SiO4 + 3Al2 O3 + 7Si
(3)
So we found Mg2 SiO4 , LiAl5 O8 and Al2 O3 simultaneously on the surface of -eucryptite in (Euc+ABO)/AA6061 and (Euc+ABO)/AA2024. 4. Conclusions (1) For pure Al matrix composite, the interfacial reaction products on the surface of -eucryptite are tetragonal LiAlO2 , cubic Al0.2724 Si0.7276 O2 and ␥-Al2 O3 . For Mg-containing
aluminum alloy matrix composite, the interfacial reaction products on the surface of -eucryptite include Mg2 SiO4 , cubic LiAl5 O8 and ␥-Al2 O3 . (2) -Eucryptite can react with Mg and Al in the temperature range 100–480 ◦ C and -eucryptite can react with Al in the temperature range 700–750 ◦ C. (3) The interfacial reactions between -eucryptite, Al and Mg can be given by the formulas as follows: (4/3)Al + LiAlSiO4 → LiAlO2 + (2/3)Al2 O3 + Si
(4)
(4/3)Al + 4.5614LiAlSiO4 → 4.5614LiAlO2 + 4.8948Al0.2724 Si0.7276 O2 + Si
(5)
8Al + 2Mg + 8LiAlSiO4 = 3Li2 O + 3Al2 O3 + 2LiAl5 O8 + Mg2 SiO4 + 7Si
(6)
References [1] L.D. Wang, W.D. Fei, M. Hu, L.S. Jiang, C.K. Yao, Mater. Lett. 53 (2002) 20–24. [2] L.D. Wang, W.D. Fei, L.S. Jiang, C.K. Yao, J. Mater. Sci. Lett. 21 (2002) 737–738. [3] H. Xu, P.J. Heaney, D.M. Yates, J. Mater. Res. 14 (1999) 3138–3158. [4] K. Suganuma, T. Fujita, N. Suzukl, K. Niihara, Light Met. 40 (1991) 297–303 (in Japanese). [5] L.D. Wang, W.D. Fei, C.K. Yao, Mater. Sci. Eng. A336 (2002) 110–116.
L.D. Wang, W.D. Fei / Materials Science and Engineering A 433 (2006) 291–297 [6] J. Hu, W.D. Fei, C. Li, C.K. Yao, J. Mater. Sci. Lett. 13 (1994) 1797–1799. [7] L.J. Yao, Gen. Sasaki, Hideharu Fukunaga, Mater. Sci. Eng. A225 (1997) 59–68. [8] F.J. Guide, D. Walton, R.D. Adams, D. Short, Composites 7 (1976) 173–179.
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[9] W.D. Fei, X.D. Jiang, C.K. Yao, Mater. Sci. Technol. 13 (1997) 918. [10] Chinese Patent, Patent Number ZL03 1 32477.0. [11] Gang Bi, Haowei Wang, Renjie Wu, Di Zhang, Zhongliang Shi, Transactions of Nonferrous Metals Society of China (English Edition) 9 (1999) 785–790.
Microstructure and interfacial reactions of -eucryptite particles in aluminum matrix composites L.D. Wang, W.D. Fei ∗ School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 405, Harbin 150001, PR China Received 14 September 2005; received in revised form 22 June 2006; accepted 22 June 2006
Abstract It has been found that aluminum matrix composites reinforced by -eucryptite particles and aluminum borate whiskers exhibit much lower coefficient of thermal expansion and higher strength. In the present study, the interfacial reactions of -eucryptite particles in aluminum matrix composites with different matrices were investigated. The interfacial reaction products on the surface of -eucryptite particles and the temperature of the reaction were identified; the chemical reaction formulas were put forward to interpret the reactions between eucryptite and Al and the alloying element Mg. © 2006 Elsevier B.V. All rights reserved. Keywords: -Eucryptite; Aluminum matrix composite; Interfacial reaction; DSC
1. Introduction Materials with a low coefficient of thermal expansion (CTE) and high strength have important applications as structural parts in aerospace system, measuring instruments, electric packaging, general optics, antennas, etc. It was reported [1,2] that low CTE and high tensile strength were attained simultaneously in an aluminum matrix composite reinforced by -eucryptite particles (LiAlSiO4 , denoted by Euc) which have a negative CTE [3] and aluminum borate whiskers (Al18 B4 O33 , denoted by ABO) which have high strength [4] (the composite is denoted by (Euc+ABO)/Al in this paper). It was also mentioned [5] that interfacial reactions can occur on the surfaces of both -eucryptite particles and aluminum borate whiskers. The interfacial reaction affects the CTE of the composite greatly. Previous researches [4,6,7] on aluminum matrix composites reinforced by ABO whiskers have indicated that an interfacial reaction occurs between the whiskers and the matrix alloys. The degree of the interfacial reaction can be changed by casting technique and heat treatment [8]. In addition, some studies have been carried out on the effect of interfacial reaction on the mechani-
∗
Corresponding author. Tel.: +86 45186418647; fax: +86 45186418647. E-mail address: [email protected] (W.D. Fei).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.06.092
cal properties of ABO whiskers reinforced aluminum composite [9]. It is well known that the interface plays an important role in the properties of composites. It is necessary to study the interfacial reaction in composites to improve their properties, control their interface morphology and broaden their applications. But, to our knowledge, the interfacial reactions between -eucryptite and different aluminum alloys have not been reported in detail before. In this study, the interfacial reactions of -eucryptite in (Euc+ABO)/Al composites with different aluminum matrices such as pure aluminum, AA6061 and AA2024 were studied. 2. Experimental The reinforcements were -eucryptite particles with a diameter of 5–10 m, which were synthesized in our laboratory by the method of a Chinese patent [10], and aluminum borate whiskers with a diameter of 0.5–1 m and a length of 10–30 m. Pure aluminum (AA1050), AA6061 and AA2024 were used as matrices. The chemical compositions of the alloys are listed in Table 1. The aluminum matrix composite reinforced by both -eucryptite particles and aluminum borate whiskers was fabricated by squeeze casting. The total volume fraction of eucryptite and aluminum borate is 40%, and the volume ratio between -eucryptite and aluminum borate is 2:1. The pres-
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Table 1 Chemical compositions of aluminum alloys used (mass%) Alloy
Mg
Si
Cu
Mn
Fe
Cr
Al
AA1050 AA6061 AA2024
0.05 0.8–1.2 1.2–1.8
0.25 0.4–0.8 <0.5
0.05 0.15–0.4 3.8–4.9
0.05 <0.15 0.3–0.9
0.4 <0.7 <0.5
– 0.04–0.35 <0.1
Balance Balance Balance
sure for squeeze casting was 30 MPa. The casting temperature of pure aluminum and aluminum alloys was 800 ◦ C, and the die temperature was about 520 ◦ C. An ABO reinforced pure Al matrix composite was also fabricated using the above conditions for the purpose of investigating the interfacial reactions in the composites. In order to investigate the effect of heat treatment on the interfacial reactions of the composites, specimens of the composites were heated to 490 or 520 ◦ C for different lengths of time and cooled to room temperature in air. Microstructures were investigated with a Philips CM-12 type transmission electron microscope. Specimens for TEM observation were thinned by ion milling. The diffractional scanning calorimeter (DSC) analysis was carried out on a Netzsch STA 449 DSC with a heating rate of 10 ◦ C/min under the protection of Ar gas.
3. Results and discussion 3.1. Interfacial reaction in (ABO+Euc)/pure-Al composite The interfacial reactions in ABO whiskers reinforced aluminum composite and their effect on the mechanical properties of the composite have been intensely studied in many investigations [4,6–9,11]. So in this paper, we do not pay attention to the interfacial reactions of ABO whiskers but focus on the interfacial reactions of -eucryptite. Fig. 1 shows the micrographs of the interface between eucryptite particles and the matrix in the (Euc+ABO)/pure-Al composite annealed at 490 ◦ C for 3, 7 and 9 h. The interfacial reactions between -eucryptite particles and the pure Al matrix were found to be sensitive to the solution time. For specimens of the composite treated for 3 h (Fig. 1a), the interfacial reaction
Fig. 1. TEM micrographs of eucryptite particles in (Euc+ABO)/p-Al composite treated at 490 ◦ C for (a) 3 h, (b) 7 h and (c) 9 h.
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products between -eucryptite particles and pure Al are fine. The specimens treated for 7 and 9 h (Fig. 1b and c) display more interfacial reaction products and -eucryptite particles are almost wholly covered by the interfacial reaction products. Therefore, it can be concluded that the amount of the interfacial reaction on the interface between -eucryptite and pure Al matrix increases obviously with the increasing of the heat treatment time. Fig. 1b and c and Fig. 2a show interfacial reaction products with different morphologies: one near the interface and inside the particle (area A in Fig. 2a) looks strip-like; the other outside of the interface (area B in Fig. 2a) looks particle-like. The selected area electron diffraction patterns (SADPs) corresponding to the interfacial reaction products are given in Fig. 2b and c, respectively. The SADP in Fig. 2b can be indexed as two components: one is tetragonal LiAlO2 (a = 0.28003 nm, c = 1.4216 nm), the
293
other is cubic Si0.7276 Al0.2724 O2 (a = 2.4358 nm). The SADP in Fig. 2c can be indexed as ␥-Al2 O3 (a = 0.7924 nm). 3.2. Interfacial reaction in (ABO+Euc)/AA6061 composite The interfacial reaction product between -eucryptite and AA6061 has been studied before [5], it was found that the degree of interfacial reaction on the surface of -eucryptite particles increases with the increasing of solution time, and that the interfacial reaction products were initially identified as orthorhombic Mg2 SiO4 (forsterite). The present study shows that more than one kind of interfacial product forms on the interface of -eucryptite and AA6061. Fig. 3 shows the micrograph of (Euc+ABO)/AA6061 composite treated at 520 ◦ C for 5 h. It is obvious that the interfacial reaction of -eucryptite was extensive. The SADP
Fig. 2. TEM micrographs and SADP spectrum of the interfacial reaction products of eucryptite in (Euc+ABO)/p-Al composite treated at 490 ◦ C for 7 h (a) TEM micrographs of interfacial reaction products, (b) SADP of interfacial products adjacent to A and (c) SADP of interfacial product adjacent to B.
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Fig. 3. TEM micrographs and SADP of (Euc+ABO)/AA6061 composite treated at 520 ◦ C for 5 h (a) micrograph and (b) SADP of interfacial reaction products on the surface of eucrptite particle.
of the interface of -eucryptite shown in Fig. 3b can be indexed as Mg2 SiO4 (forsterite) with the lattice parameters of a = 0.47 6 nm, b = 1.020 nm and c = 0.600 nm, cubic LiAl5 O8 (a = 0.7908 nm), and -eucryptite. So the interfacial reaction products of -eucryptite and AA6061 include both Mg2 SiO4 and LiAl5 O8 . 3.3. Interfacial reaction in (ABO+Euc)/AA2024 composite Fig. 4 shows the micrographs of -eucryptite particles in the (Euc+ABO)/AA2024 composite. It can be seen that there are few interfacial products on the surface of -eucryptite in the ascast composite. However, the -eucryptite particles are covered by a heavy interfacial product layer in the composites treated for 5 and 9 h. So the degree of interfacial reaction increases greatly with the increasing of the heat treatment time. Fig. 4c shows the appearance of the interfacial reaction product of AA2024 matrix composite which is similar to that of pure Al matrix and AA6061 matrix composites. The indexing of the SADP in Fig. 3c indicates that orthorhombic Mg2 SiO4 , cubic LiAl5 O8 , and ␥-Al2 O3 are formed on the interface. 3.4. DSC analysis of the interfacial reactions in the composites The DSC curves of the three composites are shown in Fig. 5. For the three curves in the temperature range of 100–620 ◦ C, there is no peak for ABO/pure-Al composite, and there are weak and broad exothermal peaks for (Euc+ABO)/AA6061 and (Euc+ABO)/AA2024 composite. Each curve has an endothermal peak during 600–680 ◦ C corresponding to the melting of the matrix. Between 680 and 850 ◦ C only one exothermal peak with the peak temperature of 766 ◦ C can be found for ABO/pure-Al composite, while two exothermal peaks with the peak tempera-
tures of 734 and 766 ◦ C can be found for (Euc+ABO)/AA6061 and (Euc+ABO)/AA2024 composite. 3.5. Discussion It is surprising to see that there is a broad peak with a peak temperature of 344 ◦ C in Fig. 5b and c. This peak may be caused by a reaction between -eucryptite and Mg. TEM observation (Fig. 3a) shows that there are more interfacial products on the surface of -eucryptite than on the surface of ABO in (Euc+ABO)/AA6061 composite treated at 520 ◦ C for 5 h. This suggests that the degree of interfacial reaction for -eucryptite is higher than that of ABO. Thus Mg and -eucryptite appears to react more readily than Mg and ABO. Since Mg can react with ABO at the temperature of 480–600 ◦ C [11], it is reasonable to deduce that the reaction corresponding to -eucryptite and Mg may occur at a lower temperature. Thus, we deduce that the broad exothermal peak in the temperature range 100–480 ◦ C in Fig. 5b and c is probably related to the reaction of active Mg and -eucryptite. When the temperature ranges from 449 to 690 ◦ C, only Mg reacts with ABO whiskers; when the temperature is higher than 690 ◦ C, both Mg and Al react with ABO whiskers [11]. So for the Mg-containing composites, an exothermal peak must exist between 480 and 600 ◦ C. The exothermal peak between 480 and 600 ◦ C is clear in Fig. 5c, but it is less distinct in Fig. 5b. This presumably occurs because there is less Mg in the AA6061 matrix than in the AA2024 matrix. Most of the Mg in AA6061 matrix may be consumed by the reaction corresponding to Mg and -eucryptite, which leave less Mg for the reaction of Mg and ABO. Thus, the exothermal peak between 480 and 600 ◦ C in Fig. 5b is too weak to see. The exothermal peak on the DSC curve of ABO/pure-Al composite between 690 and 830 ◦ C is caused by the reaction between
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Fig. 4. TEM micrographs of the eucryptite in (Euc+ABO)/2024Al composite treated at 490 ◦ C: (a) as-cast, (b) 5 h, (c) 9 h and (d) SADP on the interface in (c).
ABO and Al [11]. In Fig. 5b, there are two exothermal peaks between 690 and 833 ◦ C, one is strong and the other is weak. By comparing Fig. 5b and c with Fig. 5a, it is clear that the weak exothermal peak near 766 ◦ C is also caused by the reaction between ABO and Al. In our opinion, the stronger peak at 734 ◦ C in Fig. 5b and c is caused by the reaction between -eucryptite and Al. The reasons are as follows: Firstly, the exothermal peak between 690 and 750 ◦ C is very strong, which means it is not possible for it to result from the reaction between additive such as Mg and the reinforcements. It can only be caused by the reaction between Al matrix and the reinforcements. Secondly, according to the preceding discussion, the reaction between Mg and -eucryptite may be much easier than that between Mg and ABO, and it occurs at a lower temperature. It is logical to deduce that -eucryptite can react more easily with Al compared with ABO when Al is melting. Comparing Fig. 5b with Fig. 5c, it is interesting to see that the peaks between 100 and 620 ◦ C in Fig. 5b are weaker than those in Fig. 5c, and the peaks between 690 and 833 ◦ C in Fig. 5c are weaker than those in Fig. 5b. The difference in the DSC
curves is caused by the interfacial reactions on the reinforcements. Because the Mg content in AA2024 is greater than in AA6061, more Mg was involved in the reactions in AA2024 matrix composite, which made the exothermal peaks in the temperature range 100–620 ◦ C in Fig. 5c more obvious than those in Fig. 5b. At the same time, interfacial reaction products can prevent the reactions between reinforcements and Al when Al is melting. The more interfacial reaction products forming in the temperature range 100–620 ◦ C, the less degree of reaction between Al and eucryptite or ABO in the temperature range 690–833 ◦ C. That is why the peaks between 690 and 833 ◦ C in Fig. 5c are not as strong as those in Fig. 5b. The interfacial reactions between Al and -eucryptite can be given by the following formulas: (4/3)Al + LiAlSiO4 → LiAlO2 + (2/3)Al2 O3 + Si
(1)
(4/3)Al + 4.5614LiAlSiO4 → 4.5614LiAlO2 + 4.8948Al0.2724 Si0.7276 O2 + Si
(2)
The reaction between Al and -eucryptite occurs when liquid Al exists during squeeze casting or heating. In the beginning,
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Fig. 5. DSC curves of ABO/pure-Al (a), (Euc+ABO)/AA6061 (b) and (Euc+ABO)/AA2024 (c).
molten Al and -eucryptite can contact directly. The reaction is controlled by formula (1). Once the interfacial reactions begin, accumulating interfacial reaction products can prevent the Al from contacting the -eucryptite. The relative content of Al to -eucryptite is less, and the reaction in formula (2) may happen. That is why cubic Si0.7276 Al02724 O2 was just found near the eucryptite particle (Fig. 2, area A) and Al2 O3 was found farther from the -eucryptite particle (Fig. 2, area B). For Mg-containing aluminum alloy matrix composites, Mg is an important reactant which can react with -eucryptite easily in the temperature range 100–480 ◦ C as described previously. But we found if the interfacial reaction products are Al2 O3 , LiAl5 O8 and Mg2 SiO4 , the reactants must include not only Mg and eucryptite but also Al. The interfacial reaction between Mg, Al and -eucryptite can proceed in terms of the following formula: 8Al + 2Mg + 8LiAlSiO4 → 3Li2 O + 2LiAl5 O8 + Mg2 SiO4 + 3Al2 O3 + 7Si
(3)
So we found Mg2 SiO4 , LiAl5 O8 and Al2 O3 simultaneously on the surface of -eucryptite in (Euc+ABO)/AA6061 and (Euc+ABO)/AA2024. 4. Conclusions (1) For pure Al matrix composite, the interfacial reaction products on the surface of -eucryptite are tetragonal LiAlO2 , cubic Al0.2724 Si0.7276 O2 and ␥-Al2 O3 . For Mg-containing
aluminum alloy matrix composite, the interfacial reaction products on the surface of -eucryptite include Mg2 SiO4 , cubic LiAl5 O8 and ␥-Al2 O3 . (2) -Eucryptite can react with Mg and Al in the temperature range 100–480 ◦ C and -eucryptite can react with Al in the temperature range 700–750 ◦ C. (3) The interfacial reactions between -eucryptite, Al and Mg can be given by the formulas as follows: (4/3)Al + LiAlSiO4 → LiAlO2 + (2/3)Al2 O3 + Si
(4)
(4/3)Al + 4.5614LiAlSiO4 → 4.5614LiAlO2 + 4.8948Al0.2724 Si0.7276 O2 + Si
(5)
8Al + 2Mg + 8LiAlSiO4 = 3Li2 O + 3Al2 O3 + 2LiAl5 O8 + Mg2 SiO4 + 7Si
(6)
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