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AC8A composite by nitrided nano-coating
Materials Letters 58 (2004) 833 – 836 www.elsevier.com/locate/matlet
Controlling the interface reaction in Al18B4O33w/AC8A composite by nitrided nano-coating Gang Bi a, Qunli Rao b,*, Fen Xi c a
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China b Instrumental Analysis Center, Shanghai Jiao Tong University, Shanghai 200030, PR China c Shanghai Baojiu Technology Co. Ltd., Shanghai 201900, PR China Received 25 March 2003; received in revised form 27 June 2003; accepted 2 July 2003
Abstract Controlling the interface reaction in aluminum borate whisker (Al18B4O33w)/AC8A composite by nitrided nano-coating was studied. The results showed that the nitrided nano-layer is an effectively protective barrier for Al18B4O33w/Al – Si – Mg – Cu interfaces and can impede the formation of spinel MgAl2O4. So the serious interfacial reaction is reduced, and it will certainly help to modify the interface properties of Al18B4O33w/AC8A. D 2003 Elsevier B.V. All rights reserved. Keywords: Al18B4O33w/AC8A composite; Interfacial reaction; Nitrided nano-coating
1. Introduction
2. Experimental
Aluminum borate whisker (Al18B4O33w)-reinforced aluminum-based composites, fabricated by squeeze casting, exhibit strength and modulus comparable to those of SiC or Si3N4 whisker-reinforced aluminum composites. However, their thermal expansion is lower and abrasion resistance is better. Moreover, the superior feature of this whisker is that its price is far below that of the SiC whisker (almost one twentieth). Therefore, Al18B4O33 whisker will give a good possibility for expanding the applications of the aluminum matrix composites in the future [1– 3]. However, on the basis of experimental and theoretical studies [2– 5], it is established that Al18B4O33 whisker is unstable in aluminum alloy and chemical reaction occurs at the interface. It has been suggested that the serious interfacial reaction is a direct reason for the decrement of the composite strength. In order to control the interfacial reaction, the nitridation process of Al18B4O33 whisker is proposed based on a thermodynamic calculation [6]. The objective of the present work is to produce nitrided protective nano-coating on the surface of the whisker. The influence of the nitrided nano-coating on interfacial reaction in the Al18B4O33w/AC8A composite will be reported.
Al18B4O33 whiskers, supplied from NICHIAS in Japan, were used either untreated or after surface nitridation. For surface nitridation, the whiskers were nitrided at the first step (held at 1300 jC for a period of 9 h in ammonia) [6]. An Al alloy-AC8A was selected as a matrix in this study. The chemical composition of the AC8A alloy was: Si 11– 13 wt.%, Mg 0.8– 1.3 wt.%, Cu 0.5 – 1.5 wt.%, Ni 0.8 –1.5 wt.%, Al balance. The composites were manufactured by a squeeze casting method using a molten aluminum temperature of 750 jC, a preform temperature of 750 jC, a mould temperature of 350 jC and a pressure of about 100 MPa. Some of the composite specimens were heat-treated under the conditions (T6): 520 jC, 3 h for solid solution, and quenching at 75 jC, then 160 jC, 24 h for aging treatment. Both SEM and TEM were used to characterize the nature of the interfacial reaction.
* Corresponding author. 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.07.046
3. Results and discussion Fig. 1a shows that the surface of untreated whisker is very smooth. After the artificial nitridation, a nitrided nano-coating, which is very continuous and homoge-
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Fig. 1. HREM morphology of aluminum borate whiskers: (a) untreated whisker; and (b) the surface-nitrided whisker.
neous, is found to cover the Al18B4O33 whisker (see Fig. 1b). Phase analysis indicates that the nitrided coating consists of boron nitride and alumina. The nitridation reaction is: 9Al2 O3 2B2 O3 þ 4NH3 ¼ 9Al2 O3 þ 4BN þ 6H2 Oz
ð1Þ
Detailed information about the nitrided nano-coating can be found elsewhere [6]. Observation of the surface of whiskers in deep etched composites were presented in Fig. 2. Fig. 2a shows little reaction products on whisker surface in an as cast composite, whereas in T6 composite, some reaction products appear on the whisker surface (Fig. 2b). A further TEM examination of thin foils from two composites confirms the above results. For as cast composite, the nitrided nano-coating with a thickness of 40– 60 nm isolates the whisker from the surrounding matrix, and the Al/coating interface is free of the reaction products (Fig. 3a). But for T6 composite, Fig. 3b and c shows that the
interfacial reaction occurred between the nitrided nanocoating and the matrix alloy. By indexing the diffraction pattern (see Fig. 3d), it can be determined that the interfacial reaction product is MgAl2O4 with spinel structure. In principle, boron nitride in the nitrided nano-coating has good chemical inertness to many molten metals, including Al, Mg and their alloys [7,8]. It will not take part in the interfacial reaction. However, alumina in the nitrided nanocoating is unstable in aluminum alloys containing magnesium. Thermodynamically, the MgAl2O4 reaction is favored at low Mg content ( < 4 wt.%) [9– 11], and the interfacial reaction [2] can occur: 3Mg þ 4Al2 O3 ¼ 3MgAl2 O4 þ 2Al Moreover, nitrided with forced AC8A the interfacial
ð2Þ
comparing the TEM micrographs of the the corresponding untreated whisker-reincomposites (see Fig. 4), it demonstrates that reaction can be greatly restrained due to the
Fig. 2. SEM morphology of Al18B4O33 whiskers: (a) extracted whisker from surface nitride nano-coating Al18B4O33-reinforced AC8A composite (as cast); and (b) extracted whisker from surface nitride nano-coating Al18B4O33-reinforced AC8A composite (T6).
G. Bi et al. / Materials Letters 58 (2004) 833–836
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Fig. 3. TEM morphology of the interface in the nano-coating Al18B4O33w/AC8A composites: (a) as cast composite; (b) and (c) transverse and longitudinal section of T6 composite; and (d) SADP of reaction production corresponding to (c).
existence of the nitrided nano-coating. Moreover, the interfacial reaction in the nano-coating Al18B4O 33w/ AC8A composite (T6) is even weaker than that of the untreated Al18B4O33w/AC8A composite (as cast). Finally, it is worth noting from Fig. 3 that the interfacial reaction only consumes the nitrided nano-coating instead of the reinforcement whisker, and the structure integrity of the whisker is preserved.
4. Conclusions The nitrided nano-layer, which separates the whisker from the surrounding Al matrix, is an effectively protective barrier that can impede the formation of spinel MgAl2O4. It will certainly help to modify the interface properties of Al18B4O33w/Al and reduce the degradation of aluminum borate whiskers. Additionally, Mg depletion from the matrix
Fig. 4. TEM morphology of the interface in untreated Al18B4O33-reinforced AC8A composites: (a) as-cast composite; and (b) T6 composite.
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due to spinel formation at the interface can decrease the age harding responsibility of the matrix.
References [1] K. Suganuma, T. Fujita, N. Suzuki, K. Niihara, Journal of Materials Science Letters 9 (1990) 633 – 635. [2] K. Suganuma, T. Fujita, G. Sasaki, N. Suzuki, Journal of Japan Institute of Light Metal 41 (1991) 297 – 303. [3] L.J. Yao, H. Fukunnaga, Scripta Materialia 36 (1997) 1267 – 1271. [4] X.G. Ning, J. Pan, Y.K. Hu, H.Q. Ye, Materials Letters 13 (1992) 377 – 381.
[5] G. Bi, H.W. Wang, R.J. Wu, D. Zhang, Material Engineering Sinica 200 (2000) 27 – 30. [6] G. Bi, H.W. Wang, R.J. Wu, D. Zhang, Journal of Materials Science Letters 20 (2001) 799 – 801. [7] S. Rudolph, Interceram 42 (1993) 302 – 309. [8] M.G. Nicholas, D.A. Mortimer, Journal of Materials Science 25 (1991) 2679 – 2687. [9] D.J. Lloyd, I. Jin, G.C. Weatherly, Scripta Metallurgica et Materialia 31 (1994) 393 – 396. [10] A.D. Mcleod, C.M. Gabryel, Metallurgical Transactions 23A (1992) 1279 – 1283. [11] M. Pfeifer, J.M. Rigsbee, K.K. Chawla, Journal of Materials Science 25 (1990) 1563 – 1569.
Controlling the interface reaction in Al18B4O33w/AC8A composite by nitrided nano-coating Gang Bi a, Qunli Rao b,*, Fen Xi c a
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China b Instrumental Analysis Center, Shanghai Jiao Tong University, Shanghai 200030, PR China c Shanghai Baojiu Technology Co. Ltd., Shanghai 201900, PR China Received 25 March 2003; received in revised form 27 June 2003; accepted 2 July 2003
Abstract Controlling the interface reaction in aluminum borate whisker (Al18B4O33w)/AC8A composite by nitrided nano-coating was studied. The results showed that the nitrided nano-layer is an effectively protective barrier for Al18B4O33w/Al – Si – Mg – Cu interfaces and can impede the formation of spinel MgAl2O4. So the serious interfacial reaction is reduced, and it will certainly help to modify the interface properties of Al18B4O33w/AC8A. D 2003 Elsevier B.V. All rights reserved. Keywords: Al18B4O33w/AC8A composite; Interfacial reaction; Nitrided nano-coating
1. Introduction
2. Experimental
Aluminum borate whisker (Al18B4O33w)-reinforced aluminum-based composites, fabricated by squeeze casting, exhibit strength and modulus comparable to those of SiC or Si3N4 whisker-reinforced aluminum composites. However, their thermal expansion is lower and abrasion resistance is better. Moreover, the superior feature of this whisker is that its price is far below that of the SiC whisker (almost one twentieth). Therefore, Al18B4O33 whisker will give a good possibility for expanding the applications of the aluminum matrix composites in the future [1– 3]. However, on the basis of experimental and theoretical studies [2– 5], it is established that Al18B4O33 whisker is unstable in aluminum alloy and chemical reaction occurs at the interface. It has been suggested that the serious interfacial reaction is a direct reason for the decrement of the composite strength. In order to control the interfacial reaction, the nitridation process of Al18B4O33 whisker is proposed based on a thermodynamic calculation [6]. The objective of the present work is to produce nitrided protective nano-coating on the surface of the whisker. The influence of the nitrided nano-coating on interfacial reaction in the Al18B4O33w/AC8A composite will be reported.
Al18B4O33 whiskers, supplied from NICHIAS in Japan, were used either untreated or after surface nitridation. For surface nitridation, the whiskers were nitrided at the first step (held at 1300 jC for a period of 9 h in ammonia) [6]. An Al alloy-AC8A was selected as a matrix in this study. The chemical composition of the AC8A alloy was: Si 11– 13 wt.%, Mg 0.8– 1.3 wt.%, Cu 0.5 – 1.5 wt.%, Ni 0.8 –1.5 wt.%, Al balance. The composites were manufactured by a squeeze casting method using a molten aluminum temperature of 750 jC, a preform temperature of 750 jC, a mould temperature of 350 jC and a pressure of about 100 MPa. Some of the composite specimens were heat-treated under the conditions (T6): 520 jC, 3 h for solid solution, and quenching at 75 jC, then 160 jC, 24 h for aging treatment. Both SEM and TEM were used to characterize the nature of the interfacial reaction.
* Corresponding author. 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.07.046
3. Results and discussion Fig. 1a shows that the surface of untreated whisker is very smooth. After the artificial nitridation, a nitrided nano-coating, which is very continuous and homoge-
834
G. Bi et al. / Materials Letters 58 (2004) 833–836
Fig. 1. HREM morphology of aluminum borate whiskers: (a) untreated whisker; and (b) the surface-nitrided whisker.
neous, is found to cover the Al18B4O33 whisker (see Fig. 1b). Phase analysis indicates that the nitrided coating consists of boron nitride and alumina. The nitridation reaction is: 9Al2 O3 2B2 O3 þ 4NH3 ¼ 9Al2 O3 þ 4BN þ 6H2 Oz
ð1Þ
Detailed information about the nitrided nano-coating can be found elsewhere [6]. Observation of the surface of whiskers in deep etched composites were presented in Fig. 2. Fig. 2a shows little reaction products on whisker surface in an as cast composite, whereas in T6 composite, some reaction products appear on the whisker surface (Fig. 2b). A further TEM examination of thin foils from two composites confirms the above results. For as cast composite, the nitrided nano-coating with a thickness of 40– 60 nm isolates the whisker from the surrounding matrix, and the Al/coating interface is free of the reaction products (Fig. 3a). But for T6 composite, Fig. 3b and c shows that the
interfacial reaction occurred between the nitrided nanocoating and the matrix alloy. By indexing the diffraction pattern (see Fig. 3d), it can be determined that the interfacial reaction product is MgAl2O4 with spinel structure. In principle, boron nitride in the nitrided nano-coating has good chemical inertness to many molten metals, including Al, Mg and their alloys [7,8]. It will not take part in the interfacial reaction. However, alumina in the nitrided nanocoating is unstable in aluminum alloys containing magnesium. Thermodynamically, the MgAl2O4 reaction is favored at low Mg content ( < 4 wt.%) [9– 11], and the interfacial reaction [2] can occur: 3Mg þ 4Al2 O3 ¼ 3MgAl2 O4 þ 2Al Moreover, nitrided with forced AC8A the interfacial
ð2Þ
comparing the TEM micrographs of the the corresponding untreated whisker-reincomposites (see Fig. 4), it demonstrates that reaction can be greatly restrained due to the
Fig. 2. SEM morphology of Al18B4O33 whiskers: (a) extracted whisker from surface nitride nano-coating Al18B4O33-reinforced AC8A composite (as cast); and (b) extracted whisker from surface nitride nano-coating Al18B4O33-reinforced AC8A composite (T6).
G. Bi et al. / Materials Letters 58 (2004) 833–836
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Fig. 3. TEM morphology of the interface in the nano-coating Al18B4O33w/AC8A composites: (a) as cast composite; (b) and (c) transverse and longitudinal section of T6 composite; and (d) SADP of reaction production corresponding to (c).
existence of the nitrided nano-coating. Moreover, the interfacial reaction in the nano-coating Al18B4O 33w/ AC8A composite (T6) is even weaker than that of the untreated Al18B4O33w/AC8A composite (as cast). Finally, it is worth noting from Fig. 3 that the interfacial reaction only consumes the nitrided nano-coating instead of the reinforcement whisker, and the structure integrity of the whisker is preserved.
4. Conclusions The nitrided nano-layer, which separates the whisker from the surrounding Al matrix, is an effectively protective barrier that can impede the formation of spinel MgAl2O4. It will certainly help to modify the interface properties of Al18B4O33w/Al and reduce the degradation of aluminum borate whiskers. Additionally, Mg depletion from the matrix
Fig. 4. TEM morphology of the interface in untreated Al18B4O33-reinforced AC8A composites: (a) as-cast composite; and (b) T6 composite.
836
G. Bi et al. / Materials Letters 58 (2004) 833–836
due to spinel formation at the interface can decrease the age harding responsibility of the matrix.
References [1] K. Suganuma, T. Fujita, N. Suzuki, K. Niihara, Journal of Materials Science Letters 9 (1990) 633 – 635. [2] K. Suganuma, T. Fujita, G. Sasaki, N. Suzuki, Journal of Japan Institute of Light Metal 41 (1991) 297 – 303. [3] L.J. Yao, H. Fukunnaga, Scripta Materialia 36 (1997) 1267 – 1271. [4] X.G. Ning, J. Pan, Y.K. Hu, H.Q. Ye, Materials Letters 13 (1992) 377 – 381.
[5] G. Bi, H.W. Wang, R.J. Wu, D. Zhang, Material Engineering Sinica 200 (2000) 27 – 30. [6] G. Bi, H.W. Wang, R.J. Wu, D. Zhang, Journal of Materials Science Letters 20 (2001) 799 – 801. [7] S. Rudolph, Interceram 42 (1993) 302 – 309. [8] M.G. Nicholas, D.A. Mortimer, Journal of Materials Science 25 (1991) 2679 – 2687. [9] D.J. Lloyd, I. Jin, G.C. Weatherly, Scripta Metallurgica et Materialia 31 (1994) 393 – 396. [10] A.D. Mcleod, C.M. Gabryel, Metallurgical Transactions 23A (1992) 1279 – 1283. [11] M. Pfeifer, J.M. Rigsbee, K.K. Chawla, Journal of Materials Science 25 (1990) 1563 – 1569.