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Microstructure evolution of alumina borate whisker reinforced AA2024 composite after T6 treatment
Materials Science and Engineering A333 (2002) 170– 175 www.elsevier.com/locate/msea
Microstructure evolution of alumina borate whisker reinforced AA2024 composite after T6 treatment J. Hu *, W.D. Fei, C.K. Yao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001 People’s Republic of China Received 23 April 2001; received in revised form 18 September 2001
Abstract The microstructure of alumina borate whisker reinforced AA2024 composite, subjected to different aging states, was studied by transmission electron microscopy. The characteristics of interfacial phases and the morphology of precipitates in the matrix were investigated. The morphology and distribution of precipitates near the interface between the whiskers and matrix in the composite were also studied. The results indicated that two typical interfacial phases existed at the interface between the whisker and the matrix. These two interfacial phases were different in composition and shape. The morphology and distribution of precipitates were also different near the interface with the different interfacial phases and the geometric features of whisker surface. The precipitates in the matrix were also investigated. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Microstructure; Aging; Composite
1. Introduction Metal matrix composites (MMCs) are attractive as structural materials because of their high specific strength and stiffness. They also offer improved wear resistance, optimized thermal expansion and are capable of maintaining their excellent mechanical properties up to much higher temperatures than the unreinforced metals. Because of the reduction in fabrication cost, the application of MMCs has been widely extended to lightweight automobile parts, aerospace components and other industrial applications [1 – 4]. As a new material, alumina borate whisker (Al18B4O33w) reinforced aluminum composites have attracted interest because of their good mechanical properties, thermal expansion coefficients and low cost [5 – 8]. Many researchers [5 – 9] indicate that Al18B4O33 whisker reinforced aluminum composites are the best type of whisker reinforced aluminum composite. Furthermore, the price of alumina borate whisker is far below that of silicon carbide whisker, which has also promoted this composite’s wide application.
* Corresponding author. E-mail address: [email protected] (J. Hu).
For Al18B4O33 whisker reinforced aluminum alloy composites, the interfacial reaction has been regarded as one of the main factors that significantly affect the properties of the composite [9]. It is widely accepted that interfaces play important roles in the properties of composites [10 –12]. Aging is one of the main strengthening methods for MMCs. Aging condition can change the microstructure of the matrix in composites. The microstructure of the matrix also has a great influence on the composite properties. As a result, the study of the microstructure is very important for optimization of the composite properties. Because interfacial reactions occur between Al18B4O33 whisker and aluminum alloy, the effect of T6 treatment on the mechanical properties of Al18B4O33w/ Al composites has not been understood adequately. Studies on the effect of heat treatment on the mechanical properties are conflicting. Some research [13] indicates that T6 treatment reduces the mechanical properties of composites that contain Mg in the matrix, while other research [14] indicates that T6 treatment enhances the mechanical properties. Many factors, such as the chemical composition of the matrix and heat treatment, can affect the microstructure of composites. Although there have been some investigations on the mechanisms and products of
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 8 4 2 - 1
J. Hu et al. / Materials Science and Engineering A333 (2002) 170–175 Table 1 Composition of the AA2024 Cu
Mg
Mn
Si
Fe
Al
4.4
1.5
0.6
0.5
B0.5
Balance
the interfacial reactions of Al18B4O33w/Al composites, little research has been carried out on the effects of aging on the microstructure of composites. In the present investigation, the Al18B4O33w/AA2024 composite was studied using transmission electron microscopy (TEM). Particular attention was paid to the interface between the whisker and the matrix. More specifically, the various morphologies of interfacial phases, the distribution and the morphology of precipitates near the interface and some of the characteristics of precipitation in the matrix at different aging conditions were investigated.
2. Experimental Alumina borate whisker reinforced AA2024 composites were fabricated using the squeeze casting method. The volume fraction of the whiskers was 20%. Table 1 gives the chemical composition of the AA2024. The size of the Al18B4O33w ranged from 0.5 to 1 mm in diameter and 10– 30 mm in length. Solution treatment was carried out for 30 min at 495 °C in a salt bath furnace followed by quenching in cold water. The samples were artificially aged at 190 °C with various holding times and Vicker’s hardness of aged specimens was measured. The measurement of hardness showed that the peak aging time was 5 h for the composite. The specimens selected for TEM analysis were underaged (UA: 190 °C– 3 h), peak-aged (PA: 195 °C –5 h)
171
and over-aged (OA: 190 °C–8 h) composites. TEM analysis of the microstructure of the composites was carried out on a Philips CM12 TEM with an operating voltage of 120 kV. Energy dispersive analysis of X-ray (EDAX) was employed during TEM observations to examine the phase composition in the composite. Specimens for TEM observation were mechanically abraded to a thickness of : 20 mm, then thinned by ion milling.
3. Results and discussion
3.1. Typical interfacial phase Fig. 1 shows the morphology and corresponding selected area diffraction pattern (SADP) of the interfacial phase (interphase) with irregular shape. Using the indexing of the SADP and previous studies [6,7,9], it was determined that the interphase was MgAl2O4, with a spinel structure, which resulted from an interfacial reaction [9]. The interfacial reaction was found in many of the Al18B4O33 whiskers reinforced aluminum alloys containing Mg. Previous research [6] has indicated that the interfacial reactions take place more easily at rough surfaces than at even surfaces of the Al18B4O33 whiskers. In general, interfacial reactions occur during squeeze casting and solution treatment [10]. Fig. 2 shows the morphologies and corresponding SADPs of the interphase with regular shape. The SADP (Fig. 2b,d) was indexed as the q-phase (Al2Cu), the EDAX analysis also indicated the atomic ratio of Al and Cu in the phase was 2:1. So the interphase can be determined as the q-phase (Al2Cu). Comparing Fig. 2(a) and Fig. 2(c), it was found that the size of the q-phase increased with increasing aging time. In addition, no q-phase was found at the interface in the as-quenched specimen, so the q-phase is precipitation at interface.
Fig. 1. TEM micrographs of the interface with interfacial reaction in the composite: (a) morphology and (b) SADP pattern.
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The micrograph of the transverse section of the whisker shows two types of surfaces (Fig. 2c): even whisker surface and rough whisker surface. From Fig. 2(c) it can be seen that the q-phase exists at even surfaces of the Al18B4O33 whisker and MgAl2O4 (interfacial reaction product) exists at rough surfaces of the Al18B4O33 whisker. This agrees well with previous studies, which state that the interfacial reactions take place preferentially at rough surfaces of the Al18B4O33 whisker [6]. As shown in Fig. 3, the q-phase can also precipitate at the interface between the interfacial reaction product MgAl2O4 and the matrix. However, this was seldomly observed in the present study. According to the SADP (shown in Fig. 3b), the orientation relationship between the q-phase and the MgAl2O4 phase can be found and is given in Eq. (1), which also suggests that the q-phase precipitates after the interfacial reaction. 131MgAl2O4//1( 1( 2( q
{311}MgAl2O4//{112}q
(1)
3.2. Precipitation in the matrix Fig. 4 shows the TEM image of needle-like precipitation which was observed both in the matrix of underaged and peak-aged composites, respectively. The precipitation of the q%-phase in AA2024 and other Cu containing aluminum alloys has been well studied [15]. Therefore, the q%-phase can be determined conveniently through its shape. Fig. 5 is the TEM morphology and SADP of the bulk-shape precipitation observed only in under-aged and peak-aged composites. The SADP analysis indicated that the precipitation is the S-phase (Al2CuMg), the lattice parameters were found to be a= 0.4 nm, b= 0.92 nm and c= 0.71 nm, which are in agreement with ASTM data of power diffraction of X-ray data and previous research [15].
Fig. 2. TEM micrographs of the interphase and corresponding SADPs: (a,b) aged at 190 °C for 5 h, (c,d) transverse section, aged at 190 °C for 8 h.
J. Hu et al. / Materials Science and Engineering A333 (2002) 170–175
173
Fig. 3. TEM micrographs of the interface with interphase q-phase and MgAl2O4: (a) morphology and (b) SADP pattern.
Fig. 6(a) is a TEM image of the precipitation observed mainly in peak-aged and over-aged composites. Using the SADP (Fig. 6b), the precipitation was determined to be the q-phase (Al2Cu).
3.3. Characterization of the precipitation near the interface It was noted that in the region near the interface with an interfacial reaction, only the q-phase precipitated, as shown in Fig. 7. No precipitation free zone (PFZ) was found in this region. However, a PFZ existed in the region surrounded by the whiskers. An interfacial reaction existed on one whisker surface and the precipitates existed on the other whisker surface (shown in Fig. 8). Fig. 9 shows a clear interface between the whisker and the matrix, both the q-phase and the S-phase can be seen in the region near the interface without the interfacial reaction and the interfacial precipitation.
energy of these interfaces, i.e. the weaker interface bonding between Al18B4O33 whisker and the matrix in composites. Suganuma et al. [9] pointed out that the interfacial reaction in the Al18B4O33w/AC8A composite may be as follows: Al18B4O33 + 4Al 11k − Al2O3 + 4B
(2)
k −Al2O3 + Mg + [O] MgAl2O4
(3)
It is clear that the Mg atoms in the matrix are consumed by the above interfacial reaction. The Mg content is lower near the interface due to the interfacial reaction. Therefore, the formation of the Al2CuMg phase is very difficult near the interface with an interfacial reaction. Thus, the precipitation is only the q-phase (Al2Cu) near the interface and no PFZ exists at the interface (Fig. 7). The presence of interfacial precipitation of Al2Cu leads to a Cu content decrease near the
4. Discussion Suganuma et al. [9] indicated that the interfacial reaction product can form in the Al18B4O33w/AC8A and Al18B4O33w/6061 composites. Hu et al. [6] found that no interfacial reaction could occur between Al18B4O33 whiskers and pure aluminum. Their study indicated that the presence of magnesium in the matrix alloy is one of the essential conditions under which the interfacial reaction can take place in Al18B4O33w/ AC8A. Because of the presence of Mg in the AA2024 matrix, the interfacial reaction in Al18B4O33w/AA2024 composites can occur easily. Although Al18B4O33 whisker does not react with Cu, the q%-phase or q-phase (Al2Cu) can precipitate at interfaces during aging, which is caused by the higher
Fig. 4. Typical TEM image of q%-phase in the composite.
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Fig. 5. TEM micrographs of the precipitation S-phase: (a) morphology and (b) SADP pattern.
Fig. 6. TEM micrographs of the precipitation q-phase: (a) morphology and (b) SADP pattern.
interface, so that q%-phase or q-phase (Al2Cu phase) cannot form near the interface. The PFZ of Al2Cu phase, close to the interface with interfacial precipitation, can be observed clearly (Fig. 2a). Only a small amount of the S-phase can be seen due to the presence of Mg atoms. Because the interfacial reaction product is MgAl2O4 and the interfacial precipitation is the Al2Cu phase, the reaction and the precipitation are controlled by diffusion of Mg and Cu atoms in the matrix. Therefore, it is easily understood that no precipitation is present in the region near the interfaces with both interfacial reaction and interfacial precipitation. This is due to the interfacial reaction and the interfacial precipitation consumption of the Mg and the Cu. However, two types of precipitates, CuAl2 and Al2CuMg, are present near the interface without the interfacial reaction and the interfacial precipitation, because Mg and Cu atoms are not consumed.
Fig. 7. Microstructure of the precipitation near the interface with interfacial reaction in the overaged composite.
J. Hu et al. / Materials Science and Engineering A333 (2002) 170–175
175
The difference of interfacial phases leads to a varying precipitation distribution near the interface between the whiskers and the matrix in Al18B4O33w/AA2024 composite. There is no PFZ (Al2Cu) near the interface with the interfacial reaction product MgAl2O4. No precipitation is present in the region near the interfaces with both the reaction and the precipitation products, due to the Mg and Cu being consumed in this region. Two types of precipitates exist near the interface without the interfacial reaction and the interfacial precipitation in Al18B4O33w/AA2024 composites, one is the S-phase (Al2CuMg), the other is the q-phase (Al2Cu). The phases or precipitates in the matrix are the S-phase, q%-phase and q-phase. Fig. 8. TEM morphology of the region surrounded by whiskers in the peak-aged composite.
Acknowledgements This investigation was supported by the National Science Foundation of China.
References
Fig. 9. TEM morphology of the region near the interface without interfacial reaction and interfacial precipitation.
5. Conclusions There are two interfacial phases at the interface between whisker and matrix for Al18B4O33w/AA2024 composite. The interfacial phases are the reaction product MgAl2O4 and the precipitation Al2Cu phase, respectively.
[1] F.M. Hosking, F.F. Partillo, R. Wunderlin, R. Mehrabian, J. Mater. Sci. 17 (1982) 477 – 498. [2] C.K. Fang, C.C. Huang, T.H. Chuang, Metal. Mater. Trans. A30 (1999) 643. [3] T. Zeuner, P. Stojanov, P.R. Sahm, H. Ruppert, A. Engels, Mater. Sci. Technol. 14 (1998) 857. [4] A.J. Shakesheff, G. Purdue, Mater. Sci. Technol. 14 (1998) 851. [5] K. Suganuma, T. Fujita, N. Suzuki, K. Niihara, Light Met. 40 (1991) 297 in Japanese. [6] J. Hu, W.D. Fei, C. Li, C.K. Yao, J. Mater. Sci. Lett. 13 (1994) 1794. [7] X.G. Ning, J. Pan, K. Hu, H. Ye, Mater. Lett. 13 (1992) 377. [8] J. Hu, Q.F. Xing, C.K. Yao, J. Mater. Sci. Lett. 9 (1997) 835. [9] K. Suganuma, T. Fujita, N. Suzuki, K. Niihara, J. Mater. Sci. Lett. 9 (1990) 633. [10] W.D. Fei, X.D. Jiang, C. Li, C.K. Yao, Mater. Sci. Technol. 13 (1997) 918. [11] W.D. Fei, X.D. Jiang, S. Wu, C. Li, C.K. Yao, J. Mater. Res. 1 (1993) 1 in Chinese. [12] J. Hu, W.D. Fei, C. Li, C.K. Yao, J. Mater. Sci. Technol. 1 (1991) 1 in Chinese. [13] P. Jin, Ph.D Thesis of Harbin Institute of Technology, Harbin Institute of Technology, 1995. [14] J. Hu, W.D. Fei, C.K. Yao, J. Mater. Sci. 13 (2001) 1. [15] L.F. Mondolfo, Aluminum Alloys: Structure and Properties, Butterworth, London, Boston, 1979, p. 502.
Microstructure evolution of alumina borate whisker reinforced AA2024 composite after T6 treatment J. Hu *, W.D. Fei, C.K. Yao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001 People’s Republic of China Received 23 April 2001; received in revised form 18 September 2001
Abstract The microstructure of alumina borate whisker reinforced AA2024 composite, subjected to different aging states, was studied by transmission electron microscopy. The characteristics of interfacial phases and the morphology of precipitates in the matrix were investigated. The morphology and distribution of precipitates near the interface between the whiskers and matrix in the composite were also studied. The results indicated that two typical interfacial phases existed at the interface between the whisker and the matrix. These two interfacial phases were different in composition and shape. The morphology and distribution of precipitates were also different near the interface with the different interfacial phases and the geometric features of whisker surface. The precipitates in the matrix were also investigated. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Microstructure; Aging; Composite
1. Introduction Metal matrix composites (MMCs) are attractive as structural materials because of their high specific strength and stiffness. They also offer improved wear resistance, optimized thermal expansion and are capable of maintaining their excellent mechanical properties up to much higher temperatures than the unreinforced metals. Because of the reduction in fabrication cost, the application of MMCs has been widely extended to lightweight automobile parts, aerospace components and other industrial applications [1 – 4]. As a new material, alumina borate whisker (Al18B4O33w) reinforced aluminum composites have attracted interest because of their good mechanical properties, thermal expansion coefficients and low cost [5 – 8]. Many researchers [5 – 9] indicate that Al18B4O33 whisker reinforced aluminum composites are the best type of whisker reinforced aluminum composite. Furthermore, the price of alumina borate whisker is far below that of silicon carbide whisker, which has also promoted this composite’s wide application.
* Corresponding author. E-mail address: [email protected] (J. Hu).
For Al18B4O33 whisker reinforced aluminum alloy composites, the interfacial reaction has been regarded as one of the main factors that significantly affect the properties of the composite [9]. It is widely accepted that interfaces play important roles in the properties of composites [10 –12]. Aging is one of the main strengthening methods for MMCs. Aging condition can change the microstructure of the matrix in composites. The microstructure of the matrix also has a great influence on the composite properties. As a result, the study of the microstructure is very important for optimization of the composite properties. Because interfacial reactions occur between Al18B4O33 whisker and aluminum alloy, the effect of T6 treatment on the mechanical properties of Al18B4O33w/ Al composites has not been understood adequately. Studies on the effect of heat treatment on the mechanical properties are conflicting. Some research [13] indicates that T6 treatment reduces the mechanical properties of composites that contain Mg in the matrix, while other research [14] indicates that T6 treatment enhances the mechanical properties. Many factors, such as the chemical composition of the matrix and heat treatment, can affect the microstructure of composites. Although there have been some investigations on the mechanisms and products of
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 8 4 2 - 1
J. Hu et al. / Materials Science and Engineering A333 (2002) 170–175 Table 1 Composition of the AA2024 Cu
Mg
Mn
Si
Fe
Al
4.4
1.5
0.6
0.5
B0.5
Balance
the interfacial reactions of Al18B4O33w/Al composites, little research has been carried out on the effects of aging on the microstructure of composites. In the present investigation, the Al18B4O33w/AA2024 composite was studied using transmission electron microscopy (TEM). Particular attention was paid to the interface between the whisker and the matrix. More specifically, the various morphologies of interfacial phases, the distribution and the morphology of precipitates near the interface and some of the characteristics of precipitation in the matrix at different aging conditions were investigated.
2. Experimental Alumina borate whisker reinforced AA2024 composites were fabricated using the squeeze casting method. The volume fraction of the whiskers was 20%. Table 1 gives the chemical composition of the AA2024. The size of the Al18B4O33w ranged from 0.5 to 1 mm in diameter and 10– 30 mm in length. Solution treatment was carried out for 30 min at 495 °C in a salt bath furnace followed by quenching in cold water. The samples were artificially aged at 190 °C with various holding times and Vicker’s hardness of aged specimens was measured. The measurement of hardness showed that the peak aging time was 5 h for the composite. The specimens selected for TEM analysis were underaged (UA: 190 °C– 3 h), peak-aged (PA: 195 °C –5 h)
171
and over-aged (OA: 190 °C–8 h) composites. TEM analysis of the microstructure of the composites was carried out on a Philips CM12 TEM with an operating voltage of 120 kV. Energy dispersive analysis of X-ray (EDAX) was employed during TEM observations to examine the phase composition in the composite. Specimens for TEM observation were mechanically abraded to a thickness of : 20 mm, then thinned by ion milling.
3. Results and discussion
3.1. Typical interfacial phase Fig. 1 shows the morphology and corresponding selected area diffraction pattern (SADP) of the interfacial phase (interphase) with irregular shape. Using the indexing of the SADP and previous studies [6,7,9], it was determined that the interphase was MgAl2O4, with a spinel structure, which resulted from an interfacial reaction [9]. The interfacial reaction was found in many of the Al18B4O33 whiskers reinforced aluminum alloys containing Mg. Previous research [6] has indicated that the interfacial reactions take place more easily at rough surfaces than at even surfaces of the Al18B4O33 whiskers. In general, interfacial reactions occur during squeeze casting and solution treatment [10]. Fig. 2 shows the morphologies and corresponding SADPs of the interphase with regular shape. The SADP (Fig. 2b,d) was indexed as the q-phase (Al2Cu), the EDAX analysis also indicated the atomic ratio of Al and Cu in the phase was 2:1. So the interphase can be determined as the q-phase (Al2Cu). Comparing Fig. 2(a) and Fig. 2(c), it was found that the size of the q-phase increased with increasing aging time. In addition, no q-phase was found at the interface in the as-quenched specimen, so the q-phase is precipitation at interface.
Fig. 1. TEM micrographs of the interface with interfacial reaction in the composite: (a) morphology and (b) SADP pattern.
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The micrograph of the transverse section of the whisker shows two types of surfaces (Fig. 2c): even whisker surface and rough whisker surface. From Fig. 2(c) it can be seen that the q-phase exists at even surfaces of the Al18B4O33 whisker and MgAl2O4 (interfacial reaction product) exists at rough surfaces of the Al18B4O33 whisker. This agrees well with previous studies, which state that the interfacial reactions take place preferentially at rough surfaces of the Al18B4O33 whisker [6]. As shown in Fig. 3, the q-phase can also precipitate at the interface between the interfacial reaction product MgAl2O4 and the matrix. However, this was seldomly observed in the present study. According to the SADP (shown in Fig. 3b), the orientation relationship between the q-phase and the MgAl2O4 phase can be found and is given in Eq. (1), which also suggests that the q-phase precipitates after the interfacial reaction. 131MgAl2O4//1( 1( 2( q
{311}MgAl2O4//{112}q
(1)
3.2. Precipitation in the matrix Fig. 4 shows the TEM image of needle-like precipitation which was observed both in the matrix of underaged and peak-aged composites, respectively. The precipitation of the q%-phase in AA2024 and other Cu containing aluminum alloys has been well studied [15]. Therefore, the q%-phase can be determined conveniently through its shape. Fig. 5 is the TEM morphology and SADP of the bulk-shape precipitation observed only in under-aged and peak-aged composites. The SADP analysis indicated that the precipitation is the S-phase (Al2CuMg), the lattice parameters were found to be a= 0.4 nm, b= 0.92 nm and c= 0.71 nm, which are in agreement with ASTM data of power diffraction of X-ray data and previous research [15].
Fig. 2. TEM micrographs of the interphase and corresponding SADPs: (a,b) aged at 190 °C for 5 h, (c,d) transverse section, aged at 190 °C for 8 h.
J. Hu et al. / Materials Science and Engineering A333 (2002) 170–175
173
Fig. 3. TEM micrographs of the interface with interphase q-phase and MgAl2O4: (a) morphology and (b) SADP pattern.
Fig. 6(a) is a TEM image of the precipitation observed mainly in peak-aged and over-aged composites. Using the SADP (Fig. 6b), the precipitation was determined to be the q-phase (Al2Cu).
3.3. Characterization of the precipitation near the interface It was noted that in the region near the interface with an interfacial reaction, only the q-phase precipitated, as shown in Fig. 7. No precipitation free zone (PFZ) was found in this region. However, a PFZ existed in the region surrounded by the whiskers. An interfacial reaction existed on one whisker surface and the precipitates existed on the other whisker surface (shown in Fig. 8). Fig. 9 shows a clear interface between the whisker and the matrix, both the q-phase and the S-phase can be seen in the region near the interface without the interfacial reaction and the interfacial precipitation.
energy of these interfaces, i.e. the weaker interface bonding between Al18B4O33 whisker and the matrix in composites. Suganuma et al. [9] pointed out that the interfacial reaction in the Al18B4O33w/AC8A composite may be as follows: Al18B4O33 + 4Al 11k − Al2O3 + 4B
(2)
k −Al2O3 + Mg + [O] MgAl2O4
(3)
It is clear that the Mg atoms in the matrix are consumed by the above interfacial reaction. The Mg content is lower near the interface due to the interfacial reaction. Therefore, the formation of the Al2CuMg phase is very difficult near the interface with an interfacial reaction. Thus, the precipitation is only the q-phase (Al2Cu) near the interface and no PFZ exists at the interface (Fig. 7). The presence of interfacial precipitation of Al2Cu leads to a Cu content decrease near the
4. Discussion Suganuma et al. [9] indicated that the interfacial reaction product can form in the Al18B4O33w/AC8A and Al18B4O33w/6061 composites. Hu et al. [6] found that no interfacial reaction could occur between Al18B4O33 whiskers and pure aluminum. Their study indicated that the presence of magnesium in the matrix alloy is one of the essential conditions under which the interfacial reaction can take place in Al18B4O33w/ AC8A. Because of the presence of Mg in the AA2024 matrix, the interfacial reaction in Al18B4O33w/AA2024 composites can occur easily. Although Al18B4O33 whisker does not react with Cu, the q%-phase or q-phase (Al2Cu) can precipitate at interfaces during aging, which is caused by the higher
Fig. 4. Typical TEM image of q%-phase in the composite.
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Fig. 5. TEM micrographs of the precipitation S-phase: (a) morphology and (b) SADP pattern.
Fig. 6. TEM micrographs of the precipitation q-phase: (a) morphology and (b) SADP pattern.
interface, so that q%-phase or q-phase (Al2Cu phase) cannot form near the interface. The PFZ of Al2Cu phase, close to the interface with interfacial precipitation, can be observed clearly (Fig. 2a). Only a small amount of the S-phase can be seen due to the presence of Mg atoms. Because the interfacial reaction product is MgAl2O4 and the interfacial precipitation is the Al2Cu phase, the reaction and the precipitation are controlled by diffusion of Mg and Cu atoms in the matrix. Therefore, it is easily understood that no precipitation is present in the region near the interfaces with both interfacial reaction and interfacial precipitation. This is due to the interfacial reaction and the interfacial precipitation consumption of the Mg and the Cu. However, two types of precipitates, CuAl2 and Al2CuMg, are present near the interface without the interfacial reaction and the interfacial precipitation, because Mg and Cu atoms are not consumed.
Fig. 7. Microstructure of the precipitation near the interface with interfacial reaction in the overaged composite.
J. Hu et al. / Materials Science and Engineering A333 (2002) 170–175
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The difference of interfacial phases leads to a varying precipitation distribution near the interface between the whiskers and the matrix in Al18B4O33w/AA2024 composite. There is no PFZ (Al2Cu) near the interface with the interfacial reaction product MgAl2O4. No precipitation is present in the region near the interfaces with both the reaction and the precipitation products, due to the Mg and Cu being consumed in this region. Two types of precipitates exist near the interface without the interfacial reaction and the interfacial precipitation in Al18B4O33w/AA2024 composites, one is the S-phase (Al2CuMg), the other is the q-phase (Al2Cu). The phases or precipitates in the matrix are the S-phase, q%-phase and q-phase. Fig. 8. TEM morphology of the region surrounded by whiskers in the peak-aged composite.
Acknowledgements This investigation was supported by the National Science Foundation of China.
References
Fig. 9. TEM morphology of the region near the interface without interfacial reaction and interfacial precipitation.
5. Conclusions There are two interfacial phases at the interface between whisker and matrix for Al18B4O33w/AA2024 composite. The interfacial phases are the reaction product MgAl2O4 and the precipitation Al2Cu phase, respectively.
[1] F.M. Hosking, F.F. Partillo, R. Wunderlin, R. Mehrabian, J. Mater. Sci. 17 (1982) 477 – 498. [2] C.K. Fang, C.C. Huang, T.H. Chuang, Metal. Mater. Trans. A30 (1999) 643. [3] T. Zeuner, P. Stojanov, P.R. Sahm, H. Ruppert, A. Engels, Mater. Sci. Technol. 14 (1998) 857. [4] A.J. Shakesheff, G. Purdue, Mater. Sci. Technol. 14 (1998) 851. [5] K. Suganuma, T. Fujita, N. Suzuki, K. Niihara, Light Met. 40 (1991) 297 in Japanese. [6] J. Hu, W.D. Fei, C. Li, C.K. Yao, J. Mater. Sci. Lett. 13 (1994) 1794. [7] X.G. Ning, J. Pan, K. Hu, H. Ye, Mater. Lett. 13 (1992) 377. [8] J. Hu, Q.F. Xing, C.K. Yao, J. Mater. Sci. Lett. 9 (1997) 835. [9] K. Suganuma, T. Fujita, N. Suzuki, K. Niihara, J. Mater. Sci. Lett. 9 (1990) 633. [10] W.D. Fei, X.D. Jiang, C. Li, C.K. Yao, Mater. Sci. Technol. 13 (1997) 918. [11] W.D. Fei, X.D. Jiang, S. Wu, C. Li, C.K. Yao, J. Mater. Res. 1 (1993) 1 in Chinese. [12] J. Hu, W.D. Fei, C. Li, C.K. Yao, J. Mater. Sci. Technol. 1 (1991) 1 in Chinese. [13] P. Jin, Ph.D Thesis of Harbin Institute of Technology, Harbin Institute of Technology, 1995. [14] J. Hu, W.D. Fei, C.K. Yao, J. Mater. Sci. 13 (2001) 1. [15] L.F. Mondolfo, Aluminum Alloys: Structure and Properties, Butterworth, London, Boston, 1979, p. 502.