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Interface investigations of alumina and aluminosilicate short-fiber-reinforced aluminum-alloy composites
Composites Science and Technology 61 (2001) 545±550
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Interface investigations of alumina and aluminosilicate short-®berreinforced aluminum-alloy composites G.H. Cao a,*, Z.G. Liu a, J.M. Liu a, G.J. Shen b, S.Q. Wu c a
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR China b Analysis and Testing Center, Southeast University, Nanjing 210096, PR China c Department of Materials Science and Engineering, Southeast University, Nanjing 210096, PR China Received 24 January 2000; received in revised form 14 June 2000; accepted 19 October 2000
Abstract The interfacial microstructures of alumina (Al2O3) and aluminosilicate (Al2O3.SiO2) short-®ber-reinforced aluminum-alloy composites were studied by means of transmission electron microscopy (TEM). The experimental results showed that the SiO2 content in the ®ber has a marked eect on the interface microstructures of the composites even though a silica binder is not used. The reaction product MgAl2O4 spinel oxide was formed at the interface of the aluminosilicate-®ber-reinforce aluminum-alloy composite, however, no spinel oxide was observed at the interface of alumina-®ber-reinforced composite. The alumina ®ber could act as a heterogeneous nucleation substrate for the primary silicon phase as a result of the low disregistry (5.62%) between the (0001) of d-Al2O3 and the (110) of silicon. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Fibers; A. Metal-matrix composites (MMCs); B. Interface; D. TEM; F. Nucleation
1. Introduction Since the interface between the matrix and the reinforcement plays an important role in the properties of metal-matrix composites (MMCs), it is necessary to characterize the interfacial microstructures of a composite. Interfacial studies usually include identi®cation of the interfacial structures and/or reaction products and determination of crystallographic relationships [1,2]. It is obvious that scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis cannot determine the phase formed at the reinforcement and matrix interface. So further studies about the reinforcement and matrix interface must be done by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) [2] because of its good advantages such as high resolution , high magni®cation, ability to do selected area electron diraction (SAED), etc., which could characterize the atomic structures of interfacial products formed in metal-matrix composites. Interface microstructures of alumina short-®ber-reinforced aluminum-alloy composites have been widely studied, but there has no been a consensus. Dudek et al. * Corresponding author. Fax: +86-25-3595-535. E-mail address: [email protected] (G.H. Cao).
[3] have reported that MgAl2O4 spinel oxide was formed at the interface of d-Al2O3-®ber-reinforced aluminumpiston-alloy composite by means of HREM. On the basis of TEM analysis spinel is observed at the interface of alumina-®ber-reinforced aluminum-alloy composites [4±7]. However, smooth interfaces with no reaction products formation are also reported [8±10]. In general, most of the short reinforced metal-matrix composites are fabricated by the squeeze casting, which requires the use of a porous ®ber preform. In the process, silica is often added as a preform binder to resist the compressive stresses developed at the ®ber preform during squeeze casting. In practice, silica is also present as an additive (about 3±4 wt.%) in d-Al2O3 ®ber [10]. The purpose is to stabilize the d-Al2O3. From the thermodynamic considerations, the magnesium alloying element from the aluminum alloys tends to react with silica rather than with the Al2O3 ®ber [11,12], thereby producing a reaction product such as MgAl2O4 at the interface. Such an interface does not ensure an eective transfer of the applied load from the matrix to the reinforcement, leading to poor mechanical properties of the composites. Therefore, selection of an alternative binder for the ®ber preform is of particular interest in order to prevent the ®ber/binder/matrix interactions.
0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(00)00225-6
546
G.H. Cao et al. / Composites Science and Technology 61 (2001) 545±550
In terms of cost saving, the aluminosilicate ®ber is more attractive than pure alumina ®ber as the reinforcement material for the composites. Recently, Wu et al. [13,14] have used phosphoric acid instead of silica as a binder for the ®ber preform to fabricate the aluminosilicate short-®ber-reinforced Al±12Si-alloy composites. They have also studied the mechanical and wear properties of such composites. However, limited information is available on the interfacial microstructure of this type of composites. The purpose of this work is to study the interfacial microstructures of alumina and aluminosilicate ®ber reinforced aluminum alloy composites using phosphoric acid as a binder for the ®ber preform. In addition, the eects of SiO2 content in ®ber on the interface microstructures of the composites are also examined. 2. Experimental procedure The aluminum-alloy matrix approximates to an Al± 12Si alloy similar to SAE321. The Al±12Si alloy has a composition (in wt.%): Si:12.0, Cu:0.8, Mg:1.2, Ni:1.0, Fe:0.2, Mn:0.04, Zn:0.01 and the balance aluminum. The reinforcements, alumina and aluminosilicate ®ber, were obtained from the Luoyang Refractory Materials Company (PR China). The speci®cation of the ®bers is shown in Table 1. The diameter of the ®bers is about 10±12 mm. The short ®bers were chopped into 0.4±0.6 mm length. They were then dispersed in water in the presence of phosphoric acid colloid which acts as a binder (the weight ratio: H3PO4/H2O is about 2/100). The dispersions were ®ltered under an atmospheric pressure to produce a porous preform. It was air dried at 200 C, and ®nally sintered at 800 C. X-ray diraction analysis for the ®ber preform revealed that an eective bonding phase, Al(PO3)3 [15], was formed at the ®ber intersections. The short-®ber-reinforced aluminum-alloy composites containing 12.0 vol.% alumina and aluminosilicate, respectively, were fabricated by squeeze casting technique. In the process, the ®ber preform was held in a die heated at about 750 C for 15 min. Molten aluminum alloy was then poured into the die. The applied pressure was 20 MPa, and this pressure was maintained for 20 s during solidi®cation. In order to identify the mechanism controlling the formation of interfacial reaction product, the composites were examined without any subsequent heat treatment.
TEM samples were initially ground and polished, followed by argon-ion beam thinning. They were then examined in a JEOL 2000EX TEM, operating at 160 kV. 3. Results Fig. 1(a) is a TEM micrograph showing the interfacial microstructure of alumina-®ber-reinforced aluminumalloy composite. Electron diraction pattern [Fig. 1(b)] indicates that the lower right region in Fig. 1(a) is dAl2O3. Fig. 1(c) indicates that the upper region in Fig. 1(a) is aluminum matrix. It can also be seen from Fig. 1(a) that quadrangular particle at the alumina ®ber/matrix is interface precipitate. On the basis of the electron diraction pattern as shown in Fig. 1(d). Furthermore, this particle exhibits an A3 type structure with a lattice parameter (a) of 0.543 nm. Accordingly, this particle is identi®ed as silicon [16]. From Fig. 1(d), the presence of double diraction spots indicates that the silicon particle exhibits twinning. The thin foils used in TEM were examined under dierent diraction conditions by means of a double-tilt holder, but no reaction products such as MgAl2O4 spinel oxide were observed at the ®ber/matrix interface. Fig. 2(a) is a TEM micrograph of the interface of aluminosilicate-®ber-reinforced aluminum-alloy composite. It can be seen from Fig. 2(a) that a rectangular particle at a ®ber/matrix interface grows from the matrix into the ®ber. The aluminosilicate ®ber exhibits an amorphous structure because the electron diraction pattern, as shown in Fig. 2(b), shows the presence of a diuse halo ring. Fig. 2(c) and (d) are the [100] and [110] electron diraction patterns of the rectangular particle, which is H11 type structure and a=0.80 nm. This rectangular particle can be identi®ed as MgAl2O4 (spinel) [16]. 4. Discussion From Fig. 1(a), silicon particle is present at the alumina ®ber/matrix interface, which means that the alumina ®ber can act as a heterogeneous nucleation site for primary silicon phase during composite solidi®cation. Fig. 2(a) indicates that MgAl2O4 spinel oxide is formed at the interface of aluminosilicate short-®ber-reinforced aluminum-alloy composite. However, no reaction product such as spinel is formed at the interface of
Table 1 Speci®cation of alumina and aluminosilicate ®ber Materials
Compositions (wt.%)
Density (g/cm3)
UTS (MPa)
Young's modulus (GPa)
Crystal structure
Alumina
96Al2O3, 4SiO2 52Al2O3, 48SiO2
3.9
2100
285
Hexagonal, a=0.564 nm, b=2.265 nm
2.8
1300
210
Amorphous
Alumino-silicate
G.H. Cao et al. / Composites Science and Technology 61 (2001) 545±550
alumina-®ber-reinforced aluminum-alloy composite. Therefore, the interface in (1 x)Al2O3.x SiO2 ®ber system (x represents the weight percentage of the SiO2, the ®ber could be alumina, aluminosilicate, mullite, etc., depend on the x value) reinforced aluminum-alloy (containing magnesium) composites, is greatly dependent on the SiO2 content in the ®ber even if a silica binder is not used. The heterogeneous nucleation of silicon on the alumina ®ber, and the mechanisms
547
responsible for spinel oxide formation at the aluminosilicate ®ber/matrix interface, are discussed in detail. 4.1. Heterogeneous nucleation behavior of silicon on alumina ®ber The interfacial free energy at the nucleating interface is the controlling factor in heterogeneous nucleation behavior. However, a simple description of the interface
Fig. 1. (a) Interface TEM micrograph of Al2O3/aluminum-alloy composite; (b) electron diraction patterns of alumina ®ber; (c) aluminum matrix: [011]; (d) silicon: [1 10] (T refers to twinning).
548
G.H. Cao et al. / Composites Science and Technology 61 (2001) 545±550
energy is not realistic since the total interfacial energy of the system is composed of several contributory factors. Some of these factors are the chemical nature of the substrate, the topographic features of the substrate surface, the electrostatic potential between the substrate and the nucleated solid, and the lattice strain or disregistry between the two phases at the interface.
Generally, the lattice mismatch between the substrate and nucleated solid can be used as a criterion to judge heterogeneous nucleation behavior. The disregistry can be written as:
a0 a0
1
Fig. 2. (a) Interface TEM micrograph of Al2O3.SiO2/aluminum-alloy composite; (b) electron diraction patterns of aluminosilicate ®ber; (c) MgAl2O4 phase: [100]; (d) MgAl2O4 phase: [110].
G.H. Cao et al. / Composites Science and Technology 61 (2001) 545±550
where a0 =the dierence between the lattice parameter of the substrate and the nucleated solid a0 =the lattice parameter for the nucleated phase. This linear disregistry equation imposes a strict limitation for nucleating phase to select a proper crystallographic relationship, since only planes of similar atomic arrangement are considered. Bram®tt [17] modi®ed this equation from linear to planar in terms of angular dierence between the crystallographic directions within the planes, so that it could be applicable to crystallographic combinations of two phases with planes of diering atomic arrangements. The modi®ed equation is as follows: hkl
hklsn
3 X duvwi cos i1
s
duvwin =duvwin =3 100% 2
where hkls hkln uvws uvwn duvws duvwn
549
agreement with the observed experimental results. For aluminosilicate ®ber, because it exhibits an amorphous structure, it cannot act as a nucleation agent for primary silicon phase. 4.2. Mechanisms of spinel oxide formation Fig. 2(a) shows spinel growing from the matrix into the ®ber. Thus, we deduce that magnesium in matrix alloy may diuse into aluminosilicate ®ber. It is well known that the diusion process in amorphous is quicker. This process may be accelerated because of the interaction of high pressure (20 MPa) and high temperature (above 1000 K) during composite solidi®cation. From thermodynamic considerations [11], magnesium can react easily with SiO2 rather than with 3 4 Al2O3 as a result of G 1000 K > G1000 K . The reactions are given as follows: Mg 1=3 Al2 O3 ! MgO 2=3Al 3 G 1000
K
3
39 kJ
Mg 1=2SiO2 ! MgO 1=2Si = = = = = = =
a low-index plane of the substrate a low-index plane in the nucleated solid a low-index direction in hkls a low-index direction in hkln the interatomic spacing along uvws the interatomic spacing along uvws the angle between the uvws and uvwn
4 G 1000
K
4
128 kJ
The MgO generated from reaction (4) then reacts further with Al2O3 and SiO2 in aluminosilicate ®ber, the reactions are MgO Al2 O3 ! MgAl2 O4
5 G 1000
K
47 kJ 5
Eq. (2) was applied to the determination of the planar disregistry between the (0001) of the alumina ®ber and three low-index planes of silicon, namely, the (100), (110) and (111) planes. The corresponding parameters for Eq. (2) and the calculated planar disregistry are listed in Table 2. From the calculations, it can be seen that the most probable plane for silicon nucleation on the (0001) of d-Al2O3 is (110) since it has the lowest planar disregistry. The theoretical calculation is in good
MgO Al2 O3 ! MgSiO3
6 G 1000
K
34 kJ 6
5 G 1000 K
6 G 1000 K,
As < the reaction (5) is more thermodynamically stable. It is unlikely for SiO2 to react with Mg during composite fabrication as the result of a small SiO2 content (<4%) present in alumina ®ber. Thus, it cannot provide sucient oxygen for spinel
Table 2 Parameter for the planar disregistry equation (0001)Al2O3//(110)Si uvwAl2 O3 uvwSi duvwAl2 O3 duvwSi ( ) (%)
[0110] [111] 0.977 0.941 5.26
[1100] [111] 0.977 0.941 5.26 5.62
(0001)Al2O3//(100)Si [2110] [001] 0.564 0.543 30
[1210] [010] 0.564 0.543 0
[1010] [001] 0.977 0.543 0 41.9
(0001)Al2O3//(111)Si [2110] [011] 0.564 0.384 15
[2110] [101] 0.564 0.384 0
[1100] [211] 0.977 0.665 0 46.89
[1210] [110] 0.564 0.384 0
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G.H. Cao et al. / Composites Science and Technology 61 (2001) 545±550
formation [18]. In order to prevent spinel formation at the ®ber/matrix interface, alumina ®ber should be selected as the reinforcement of the aluminum-alloy composites, even though aluminosilicate ®ber is inexpensive. 5. Conclusion Alumina and aluminosilicate short-®ber-reinforced aluminum-alloy composites were fabricated by means of squeeze casting using phosphoric acid instead of silica as a binder for ®ber preform. The SiO2 content in ®ber has a dramatic eect on the interface microstructures. MgAl2O4 spinel oxide is formed at the interface of aluminosilicate short-®ber-reinforced aluminum-alloy composite, but no reaction product such as spinel is formed at the interface of alumina-®ber-reinforced aluminum-alloy composite. The alumina ®ber could act as a heterogeneous nucleation substrate for the primary silicon phase.
References [1] Shen GJ, Cai Y, Song JZ. Transmission electron microscopy study of SiC/Mg alloy interface in cast SiC particulate-reinforced Mg metal matrix composite. J Mater Sci Lett 1996;15:2058±60. [2] Cai Y, Shen GJ, Su HQ. The interface characteristics of as-cast SiCp/Mg (AZ80) composite. Scripta Materialia 1997;37:737±42. [3] Dudek HJ, Borath Rand, Kleine A. Spinel growth in the interface of d-Al2O3 ®ber reinforced aluminum piston alloys. J Mater Sci 1996;31:795±805. [4] Papworth A, Gree A, Fox P. Interface reactions in squeeze cast Sal ®ber/piston alloy composites. Mater Sci Technol 1998;14:47±53.
[5] Dudek HJ, Kleine A, Borath R, Neite G. Interfaces in alumina ®ber-reinforced piston alloys. Mater Sci Engng 1993;A167:129±37. [6] Wang SC, Dudek HJ. Fiber-matrix interaction in the d-Al2O3®ber reinforced aluminum piston alloy. Mater Sci Engng 1996;A205:180±6. [7] Molins R, Bartont JD, Bienvenu Y. Microstructural and analytical characterization of d-Al2O3±(Al±Mg) composite interfaces. Mater Sci Engng 1991;A135:111±7. [8] O'Rourke JM, Bushby RS, Scott VD. Evaluation of Al±Cu±Mg± Ag alloys containing discontinuous alumina ®bers: I. microstructural evolution. Comp Sci Technol 1996;56:957±65. [9] Cappleman GR, Watts JF, Clyne TW. The interface region in squeeze-in®ltrated composites containing a-alumina ®ber in an aluminum matrix. J Mater Sci 1985;20:2159±68. [10] Clyne TW, Bader MG, Cappleman GR, Hubert PA. The use of a d-alumina ®ber for metal-matrix composites. J Mater Sci 1985;20:85±96. [11] Hallstedt B, Liu ZK, Agren J. Fiber-matrix interactions during fabrication of Al2O3±Mg metal matrix composites. Mater Sci Engng 1990;A129:135±44. [12] Clyne TW, Withers J. An introduction to metal matrix composites. 1993, p.194. :Cambridge Cambridge University Press. [13] Wu SQ, Wang HZ, Tjong SC. Mechanical and wear properties of an Al/Si alloy metal-matrix composites reinforced with aluminosilicate ®ber. Comp Sci Technol 1996;56:1261±70. [14] Tjong SC, Wang HZ, Wu SQ. Wear behavior of aluminum-based metal matrix composites reinforced with a preform of aluminosilicate ®ber. Metall Trans 1996;A27:2385±9. [15] Chiou JM, Chung DDL. Improvement of the temperature resistance of aluminum-matrix composites using an acid phosphate binder. J Mater Sci 1993;28:1447±70. [16] Brandes EA. Smithell's metals reference book. 6th ed. London: Butterworth, 1983. [17] Bram®tt BL. The eects of carbide and nitride additions on the heterogeneous nucleation behavior of liquid iron. Metall Trans 1970;1:1987±95. [18] Wang N, Wang ZR, Georage C, Weatherly GC. Formation of magnesium aluminate (spinel) in cast SiC particulate-reinforced Al (A356) metal-matrix composites. Metall Trans 1992;A23:1423±30.
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Interface investigations of alumina and aluminosilicate short-®berreinforced aluminum-alloy composites G.H. Cao a,*, Z.G. Liu a, J.M. Liu a, G.J. Shen b, S.Q. Wu c a
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR China b Analysis and Testing Center, Southeast University, Nanjing 210096, PR China c Department of Materials Science and Engineering, Southeast University, Nanjing 210096, PR China Received 24 January 2000; received in revised form 14 June 2000; accepted 19 October 2000
Abstract The interfacial microstructures of alumina (Al2O3) and aluminosilicate (Al2O3.SiO2) short-®ber-reinforced aluminum-alloy composites were studied by means of transmission electron microscopy (TEM). The experimental results showed that the SiO2 content in the ®ber has a marked eect on the interface microstructures of the composites even though a silica binder is not used. The reaction product MgAl2O4 spinel oxide was formed at the interface of the aluminosilicate-®ber-reinforce aluminum-alloy composite, however, no spinel oxide was observed at the interface of alumina-®ber-reinforced composite. The alumina ®ber could act as a heterogeneous nucleation substrate for the primary silicon phase as a result of the low disregistry (5.62%) between the (0001) of d-Al2O3 and the (110) of silicon. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Fibers; A. Metal-matrix composites (MMCs); B. Interface; D. TEM; F. Nucleation
1. Introduction Since the interface between the matrix and the reinforcement plays an important role in the properties of metal-matrix composites (MMCs), it is necessary to characterize the interfacial microstructures of a composite. Interfacial studies usually include identi®cation of the interfacial structures and/or reaction products and determination of crystallographic relationships [1,2]. It is obvious that scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis cannot determine the phase formed at the reinforcement and matrix interface. So further studies about the reinforcement and matrix interface must be done by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) [2] because of its good advantages such as high resolution , high magni®cation, ability to do selected area electron diraction (SAED), etc., which could characterize the atomic structures of interfacial products formed in metal-matrix composites. Interface microstructures of alumina short-®ber-reinforced aluminum-alloy composites have been widely studied, but there has no been a consensus. Dudek et al. * Corresponding author. Fax: +86-25-3595-535. E-mail address: [email protected] (G.H. Cao).
[3] have reported that MgAl2O4 spinel oxide was formed at the interface of d-Al2O3-®ber-reinforced aluminumpiston-alloy composite by means of HREM. On the basis of TEM analysis spinel is observed at the interface of alumina-®ber-reinforced aluminum-alloy composites [4±7]. However, smooth interfaces with no reaction products formation are also reported [8±10]. In general, most of the short reinforced metal-matrix composites are fabricated by the squeeze casting, which requires the use of a porous ®ber preform. In the process, silica is often added as a preform binder to resist the compressive stresses developed at the ®ber preform during squeeze casting. In practice, silica is also present as an additive (about 3±4 wt.%) in d-Al2O3 ®ber [10]. The purpose is to stabilize the d-Al2O3. From the thermodynamic considerations, the magnesium alloying element from the aluminum alloys tends to react with silica rather than with the Al2O3 ®ber [11,12], thereby producing a reaction product such as MgAl2O4 at the interface. Such an interface does not ensure an eective transfer of the applied load from the matrix to the reinforcement, leading to poor mechanical properties of the composites. Therefore, selection of an alternative binder for the ®ber preform is of particular interest in order to prevent the ®ber/binder/matrix interactions.
0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(00)00225-6
546
G.H. Cao et al. / Composites Science and Technology 61 (2001) 545±550
In terms of cost saving, the aluminosilicate ®ber is more attractive than pure alumina ®ber as the reinforcement material for the composites. Recently, Wu et al. [13,14] have used phosphoric acid instead of silica as a binder for the ®ber preform to fabricate the aluminosilicate short-®ber-reinforced Al±12Si-alloy composites. They have also studied the mechanical and wear properties of such composites. However, limited information is available on the interfacial microstructure of this type of composites. The purpose of this work is to study the interfacial microstructures of alumina and aluminosilicate ®ber reinforced aluminum alloy composites using phosphoric acid as a binder for the ®ber preform. In addition, the eects of SiO2 content in ®ber on the interface microstructures of the composites are also examined. 2. Experimental procedure The aluminum-alloy matrix approximates to an Al± 12Si alloy similar to SAE321. The Al±12Si alloy has a composition (in wt.%): Si:12.0, Cu:0.8, Mg:1.2, Ni:1.0, Fe:0.2, Mn:0.04, Zn:0.01 and the balance aluminum. The reinforcements, alumina and aluminosilicate ®ber, were obtained from the Luoyang Refractory Materials Company (PR China). The speci®cation of the ®bers is shown in Table 1. The diameter of the ®bers is about 10±12 mm. The short ®bers were chopped into 0.4±0.6 mm length. They were then dispersed in water in the presence of phosphoric acid colloid which acts as a binder (the weight ratio: H3PO4/H2O is about 2/100). The dispersions were ®ltered under an atmospheric pressure to produce a porous preform. It was air dried at 200 C, and ®nally sintered at 800 C. X-ray diraction analysis for the ®ber preform revealed that an eective bonding phase, Al(PO3)3 [15], was formed at the ®ber intersections. The short-®ber-reinforced aluminum-alloy composites containing 12.0 vol.% alumina and aluminosilicate, respectively, were fabricated by squeeze casting technique. In the process, the ®ber preform was held in a die heated at about 750 C for 15 min. Molten aluminum alloy was then poured into the die. The applied pressure was 20 MPa, and this pressure was maintained for 20 s during solidi®cation. In order to identify the mechanism controlling the formation of interfacial reaction product, the composites were examined without any subsequent heat treatment.
TEM samples were initially ground and polished, followed by argon-ion beam thinning. They were then examined in a JEOL 2000EX TEM, operating at 160 kV. 3. Results Fig. 1(a) is a TEM micrograph showing the interfacial microstructure of alumina-®ber-reinforced aluminumalloy composite. Electron diraction pattern [Fig. 1(b)] indicates that the lower right region in Fig. 1(a) is dAl2O3. Fig. 1(c) indicates that the upper region in Fig. 1(a) is aluminum matrix. It can also be seen from Fig. 1(a) that quadrangular particle at the alumina ®ber/matrix is interface precipitate. On the basis of the electron diraction pattern as shown in Fig. 1(d). Furthermore, this particle exhibits an A3 type structure with a lattice parameter (a) of 0.543 nm. Accordingly, this particle is identi®ed as silicon [16]. From Fig. 1(d), the presence of double diraction spots indicates that the silicon particle exhibits twinning. The thin foils used in TEM were examined under dierent diraction conditions by means of a double-tilt holder, but no reaction products such as MgAl2O4 spinel oxide were observed at the ®ber/matrix interface. Fig. 2(a) is a TEM micrograph of the interface of aluminosilicate-®ber-reinforced aluminum-alloy composite. It can be seen from Fig. 2(a) that a rectangular particle at a ®ber/matrix interface grows from the matrix into the ®ber. The aluminosilicate ®ber exhibits an amorphous structure because the electron diraction pattern, as shown in Fig. 2(b), shows the presence of a diuse halo ring. Fig. 2(c) and (d) are the [100] and [110] electron diraction patterns of the rectangular particle, which is H11 type structure and a=0.80 nm. This rectangular particle can be identi®ed as MgAl2O4 (spinel) [16]. 4. Discussion From Fig. 1(a), silicon particle is present at the alumina ®ber/matrix interface, which means that the alumina ®ber can act as a heterogeneous nucleation site for primary silicon phase during composite solidi®cation. Fig. 2(a) indicates that MgAl2O4 spinel oxide is formed at the interface of aluminosilicate short-®ber-reinforced aluminum-alloy composite. However, no reaction product such as spinel is formed at the interface of
Table 1 Speci®cation of alumina and aluminosilicate ®ber Materials
Compositions (wt.%)
Density (g/cm3)
UTS (MPa)
Young's modulus (GPa)
Crystal structure
Alumina
96Al2O3, 4SiO2 52Al2O3, 48SiO2
3.9
2100
285
Hexagonal, a=0.564 nm, b=2.265 nm
2.8
1300
210
Amorphous
Alumino-silicate
G.H. Cao et al. / Composites Science and Technology 61 (2001) 545±550
alumina-®ber-reinforced aluminum-alloy composite. Therefore, the interface in (1 x)Al2O3.x SiO2 ®ber system (x represents the weight percentage of the SiO2, the ®ber could be alumina, aluminosilicate, mullite, etc., depend on the x value) reinforced aluminum-alloy (containing magnesium) composites, is greatly dependent on the SiO2 content in the ®ber even if a silica binder is not used. The heterogeneous nucleation of silicon on the alumina ®ber, and the mechanisms
547
responsible for spinel oxide formation at the aluminosilicate ®ber/matrix interface, are discussed in detail. 4.1. Heterogeneous nucleation behavior of silicon on alumina ®ber The interfacial free energy at the nucleating interface is the controlling factor in heterogeneous nucleation behavior. However, a simple description of the interface
Fig. 1. (a) Interface TEM micrograph of Al2O3/aluminum-alloy composite; (b) electron diraction patterns of alumina ®ber; (c) aluminum matrix: [011]; (d) silicon: [1 10] (T refers to twinning).
548
G.H. Cao et al. / Composites Science and Technology 61 (2001) 545±550
energy is not realistic since the total interfacial energy of the system is composed of several contributory factors. Some of these factors are the chemical nature of the substrate, the topographic features of the substrate surface, the electrostatic potential between the substrate and the nucleated solid, and the lattice strain or disregistry between the two phases at the interface.
Generally, the lattice mismatch between the substrate and nucleated solid can be used as a criterion to judge heterogeneous nucleation behavior. The disregistry can be written as:
a0 a0
1
Fig. 2. (a) Interface TEM micrograph of Al2O3.SiO2/aluminum-alloy composite; (b) electron diraction patterns of aluminosilicate ®ber; (c) MgAl2O4 phase: [100]; (d) MgAl2O4 phase: [110].
G.H. Cao et al. / Composites Science and Technology 61 (2001) 545±550
where a0 =the dierence between the lattice parameter of the substrate and the nucleated solid a0 =the lattice parameter for the nucleated phase. This linear disregistry equation imposes a strict limitation for nucleating phase to select a proper crystallographic relationship, since only planes of similar atomic arrangement are considered. Bram®tt [17] modi®ed this equation from linear to planar in terms of angular dierence between the crystallographic directions within the planes, so that it could be applicable to crystallographic combinations of two phases with planes of diering atomic arrangements. The modi®ed equation is as follows: hkl
hklsn
3 X duvwi cos i1
s
duvwin =duvwin =3 100% 2
where hkls hkln uvws uvwn duvws duvwn
549
agreement with the observed experimental results. For aluminosilicate ®ber, because it exhibits an amorphous structure, it cannot act as a nucleation agent for primary silicon phase. 4.2. Mechanisms of spinel oxide formation Fig. 2(a) shows spinel growing from the matrix into the ®ber. Thus, we deduce that magnesium in matrix alloy may diuse into aluminosilicate ®ber. It is well known that the diusion process in amorphous is quicker. This process may be accelerated because of the interaction of high pressure (20 MPa) and high temperature (above 1000 K) during composite solidi®cation. From thermodynamic considerations [11], magnesium can react easily with SiO2 rather than with 3 4 Al2O3 as a result of G 1000 K > G1000 K . The reactions are given as follows: Mg 1=3 Al2 O3 ! MgO 2=3Al 3 G 1000
K
3
39 kJ
Mg 1=2SiO2 ! MgO 1=2Si = = = = = = =
a low-index plane of the substrate a low-index plane in the nucleated solid a low-index direction in hkls a low-index direction in hkln the interatomic spacing along uvws the interatomic spacing along uvws the angle between the uvws and uvwn
4 G 1000
K
4
128 kJ
The MgO generated from reaction (4) then reacts further with Al2O3 and SiO2 in aluminosilicate ®ber, the reactions are MgO Al2 O3 ! MgAl2 O4
5 G 1000
K
47 kJ 5
Eq. (2) was applied to the determination of the planar disregistry between the (0001) of the alumina ®ber and three low-index planes of silicon, namely, the (100), (110) and (111) planes. The corresponding parameters for Eq. (2) and the calculated planar disregistry are listed in Table 2. From the calculations, it can be seen that the most probable plane for silicon nucleation on the (0001) of d-Al2O3 is (110) since it has the lowest planar disregistry. The theoretical calculation is in good
MgO Al2 O3 ! MgSiO3
6 G 1000
K
34 kJ 6
5 G 1000 K
6 G 1000 K,
As < the reaction (5) is more thermodynamically stable. It is unlikely for SiO2 to react with Mg during composite fabrication as the result of a small SiO2 content (<4%) present in alumina ®ber. Thus, it cannot provide sucient oxygen for spinel
Table 2 Parameter for the planar disregistry equation (0001)Al2O3//(110)Si uvwAl2 O3 uvwSi duvwAl2 O3 duvwSi ( ) (%)
[0110] [111] 0.977 0.941 5.26
[1100] [111] 0.977 0.941 5.26 5.62
(0001)Al2O3//(100)Si [2110] [001] 0.564 0.543 30
[1210] [010] 0.564 0.543 0
[1010] [001] 0.977 0.543 0 41.9
(0001)Al2O3//(111)Si [2110] [011] 0.564 0.384 15
[2110] [101] 0.564 0.384 0
[1100] [211] 0.977 0.665 0 46.89
[1210] [110] 0.564 0.384 0
550
G.H. Cao et al. / Composites Science and Technology 61 (2001) 545±550
formation [18]. In order to prevent spinel formation at the ®ber/matrix interface, alumina ®ber should be selected as the reinforcement of the aluminum-alloy composites, even though aluminosilicate ®ber is inexpensive. 5. Conclusion Alumina and aluminosilicate short-®ber-reinforced aluminum-alloy composites were fabricated by means of squeeze casting using phosphoric acid instead of silica as a binder for ®ber preform. The SiO2 content in ®ber has a dramatic eect on the interface microstructures. MgAl2O4 spinel oxide is formed at the interface of aluminosilicate short-®ber-reinforced aluminum-alloy composite, but no reaction product such as spinel is formed at the interface of alumina-®ber-reinforced aluminum-alloy composite. The alumina ®ber could act as a heterogeneous nucleation substrate for the primary silicon phase.
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