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Al composite at elevated temperatures by change of interfacial state
Materials Science and Engineering A 486 (2008) 409–412
Improvement of the tensile strength of Al18B4O33w/Al composite at elevated temperatures by change of interfacial state H.Y. Yue, L.D. Wang, W.D. Fei ∗ School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Received 18 June 2007; received in revised form 7 September 2007; accepted 10 September 2007
Abstract In general, it is very difficult to obtain obviously reinforced effect in discontinuously reinforced aluminum matrix composites at the temperature above 400 ◦ C. In the present study, we report an effective method to improve the high-temperature tensile strength of Al18 B4 O33 w/Al composite by change of interfacial state. The pure aluminum matrix composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents were fabricated by squeeze casting. The results indicate that ZnAl2 O4 coating of the whiskers can effectively improve the high-temperature tensile strength of Al18 B4 O33 w/Al composite, although the tensile strength of the composite decreases with increasing the tensile temperature. On the basis of fractograph analysis, the fracture mechanism of the composites at elevated temperatures was investigated. © 2007 Elsevier B.V. All rights reserved. Keywords: Composites; Interface; High-temperature properties
1. Introduction Discontinuously reinforced aluminum matrix composites (DRAMCs) have a number of advantages over monolithic aluminum alloys, such as high-specific strength, stiffness and excellent high-temperature properties [1–3]. Researches on the properties of DRAMCs at elevated temperatures are not only to investigate the high-temperature deformation behaviors and fracture mechanisms but to promote the wider applications in many fields, such as pistons for internal combustion engines and automobile brake shaft, some parts in aeronautic fields and astronautic fields and so on [4,5]. However, extensive researches have indicated that for various DRAMCs, whatever the strengths of composites at room temperature are, they begin to converge at the temperature above 300 ◦ C and they are all nearly identical at above 400 ◦ C [6–8]. Even in some composites, the strengths of the composites are lower than those of the matrix alloys. The reason is generally believed that the shear strength of the matrix rapidly declines
and almost no load transfer from the matrix to the reinforcement exists at higher temperatures. Matrix alloy and reinforcement kinds, orientation and volume fraction of the reinforcement in composites, preparation and heat-treatment processes do not have too much influence on the tensile strength of DRAMCs at temperatures above 400 ◦ C [7,9–11]. Even for SiCw/Al composites with high-interfacial bonding strength, the similar results can be obtained [11]. Therefore, it is difficult to improve high-temperature tensile properties of DRAMCs according to the routine methods, although it is very important for the wider applications of DRAMCs. Up to now, little information can be available to improve the high-temperature strength of DRAMCs. In the present study, discontinuous ZnAl2 O4 particles were coated on the surface of aluminum borate whisker (Al18 B4 O33 w) by a sol–gel route. The effect of ZnAl2 O4 coating of the whiskers on the tensile strength and fracture mechanism of the composites were investigated at elevated temperatures. 2. Experimental
∗
Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 405, Harbin 150001, People’s Republic of China. E-mail address: [email protected] (W.D. Fei). 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.09.018
The reinforcement used was Al18 B4 O33 w with a diameter of 0.5–1.5 m and a length of 10–30 m. In order to obtain ZnAl2 O4 coating of the whiskers, ZnO was firstly coated on the surface of Al18 B4 O33 w by a sol–gel route, and then the ZnO-
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H.Y. Yue et al. / Materials Science and Engineering A 486 (2008) 409–412
are given in Fig. 1. Tensile fracture surfaces of the composites were examined by an S-3000 scanning electron microscope (SEM). 3. Results
Fig. 1. Dimensions of tensile specimens.
coated whiskers were sintered at 1000 ◦ C for 1 h. During the sintering process, the reaction between the whiskers and ZnO coating took place, so ZnAl2 O4 coating was introduced. The detailed coating process and reaction mechanism can be found in our previous study [12]. The pure aluminum matrix composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents were fabricated by squeeze casting. The volume fraction of Al18 B4 O33 w in the composite was 20%. In order to investigate the effect of coating content on tensile properties of the composites at elevated temperatures, the initial massive ratio of ZnO to ABOw selected was 0, 1:30 and 1:10, that is, the volume fraction of ZnAl2 O4 coating in the composites was 0%, 0.96% and 2.88%, respectively. The corresponding abbreviation separately is Al18 B4 O33 w/Al, Al18 B4 O33 w/ZnAl2 O4 /Al and Al18 B4 O33 w/3ZnAl2 O4 /Al. Interfacial microstructures of the composites were observed using a Philips CM-12 transmission electron microscope (TEM) with an operating voltage of 120 kV. Specimens used for TEM observations were abraded to a thickness of 30 m and finally thinned by ion-milling. Tensile experiments were carried out using an Instron-1186 testing machine at a constant strain rate of 1 mm/min in the temperature range from 200 to 500 ◦ C, and specimens were heated in an Instron-SF375D resistance furnace. A thermocouple was attached to the specimen and the specimen was held at the desired temperatures for 15 min to ensure a uniform temperature distribution before testing. The dimensions of tensile specimens used
Fig. 2 shows TEM morphologies of the interface in as-cast composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents. It can be seen that, in Al18 B4 O33 w/Al composite, the interface is smooth and no interphase exists. However, interphase particles can be clearly found in ZnAl2 O4 -coated Al18 B4 O33 w reinforced aluminum composites, as shown in Fig. 2b and c, which are verified as ZnAl2 O4 according to our previous study [12]. With the increase of the coating content, the sizes of interphase particles become larger and they discontinuously distribute on the whisker surface with an average size of about 30–100 nm. Because ZnAl2 O4 particles are the reaction products between the whiskers and ZnO coating, and ZnAl2 O4 do not react with aluminum during squeeze casting, thus, ZnAl2 O4 particles at the interface embed in both the whisker and aluminum matrix and form many steps on the surface of Al18 B4 O33 w at the interface. Tensile strengths of the composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents at elevated temperatures are shown in Fig. 3. It can be seen that the ultimate tensile strength (UTS) of the composites decreases with increasing the tensile temperature and increases with increasing ZnAl2 O4 coating content. At the tensile temperature of 200 ◦ C, the UTS of Al18 B4 O33 w/Al and Al18 B4 O33 w/3ZnAl2 O4 /Al composite is 164 MPa and 261 MPa, respectively. However, at the tensile temperature of 500 ◦ C, the UTS of Al18 B4 O33 w/Al and Al18 B4 O33 w/3ZnAl2 O4 /Al composite rapidly reduces to 41 MPa and 73 MPa, respectively. The UTS of ZnAl2 O4 -coated Al18 B4 O33 w reinforced aluminum composite is obviously higher than that of uncoated Al18 B4 O33 w reinforced aluminum composite. Even at the tensile temperature of 500 ◦ C, the increase ratio of the UTS of Al18 B4 O33 w/3ZnAl2 O4 /Al composite can reach 78% compared with that of Al18 B4 O33 w/Al composite. So, ZnAl2 O4 coating of Al18 B4 O33 w provides a very effective method to
Fig. 2. TEM micrographs of the interface in as-cast composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents: (a) 0%, (b) 0.96%, (c) 2.88%.
H.Y. Yue et al. / Materials Science and Engineering A 486 (2008) 409–412
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Fig. 3. Tensile strengths of the composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents at elevated temperatures.
improve the tensile strength of the composites at elevated temperatures. Fig. 4 is tensile fractographs of the composites at the tensile temperature of 500 ◦ C. The fractographs of the composites are quite different. In the fractograph of Al18 B4 O33 w/Al composite, as shown in Fig. 4a, debonding of many whiskers can be clearly seen and pull-out length of the whiskers is long, and almost no dimples exist. Obvious debonding of the whiskers from the matrix at the fracture surfaces indicates that Al18 B4 O33 w cannot transfer load due to the interface fracture. So the interface of Al18 B4 O33 w and aluminum dominates the fracture process at higher temperature. In Al18 B4 O33 w/ZnAl2 O4 /Al composite, some tear ridges on the surface of the whiskers can be observed and the pull-out amount of the whiskers becomes less. However, in Al18 B4 O33 w/3ZnAl2 O4 /Al composite (Fig. 4c), a small amount of pull-out of the whiskers, and more and larger tear ridges can be found on the surface of ZnAl2 O4 coated Al18 B4 O33 w, which indicates that ZnAl2 O4 coating of Al18 B4 O33 w can effectively prevent interfacial sliding and debonding. 4. Discussion According to the above results, one can conclude that ZnAl2 O4 coating of Al18 B4 O33 w can effectively enhance the high-temperature UTS of Al18 B4 O33 w reinforced aluminum composites. The variation of UTS of the composites reinforced by ZnAl2 O4 -coated Al18 B4 O33 w at higher temperature is very different from the previous researches [6–11]. In the previous researches, the UTSs of almost all DRAMCs are close to one value, that is to say, no reinforced effect above 400 ◦ C. As is well known, interfacial state plays an important role in the mechanical properties of composites [13,14]. Our previous research indicates that there is no interfacial reaction for the composites exposed at higher temperature (i.e. 520 ◦ C) for 1 h [15]. So we can exclude the effect of interfacial reaction on the UTS of the composites at elevated temperatures in the present study. According to the above TEM observation and fractograph
Fig. 4. Fractographs of the composites at the tensile temperature of 500 ◦ C: (a) Al18 B4 O33 w/Al, (b) Al18 B4 O33 w/ZnAl2 O4 /Al, (c) Al18 B4 O33 w/3ZnAl2 O4 /Al.
analysis, the effect of ZnAl2 O4 coating of the whiskers on the UTS of the composites is discussed as follows: On the one hand, in Al18 B4 O33 w/Al composite, there are microvoids and cracks formed at the interface due to poor wettability of Al18 B4 O33 w by molten aluminum during squeeze casting [16]. So the interfacial bonding strength is weak in Al18 B4 O33 w/Al composite. However, nano-ZnAl2 O4 particles with large specific areas on the surface of Al18 B4 O33 w can
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H.Y. Yue et al. / Materials Science and Engineering A 486 (2008) 409–412
improve the wettability of the whiskers by molten aluminum during squeeze casting [12], which causes microvoids and cracks at the interface to decrease greatly and enhance the interfacial bonding strength, so the load transfer capacity of the whisker/matrix interface increases. On the other hand, the interface of Al18 B4 O33 w/Al is smooth and the matrix is sheared away from the whiskers during the tensile process, so the contribution of the whiskers can be negligible to the tensile strength due to interfacial fracture at higher temperatures. Thus, it is easy to result in interfacial debonding at higher temperatures, as shown in Fig. 4a. However, nanoZnAl2 O4 particles embedded both in the matrix and whiskers can form many steps at the interface because of discontinuous distribution of ZnAl2 O4 particles. The steps can act as barriers to interface crack propagation, and make a large contribution to load transfer from the matrix to the whiskers during the tensile process of the composites, which also enhances the UTS of the composites at elevated temperatures. 5. Conclusions (1) The coating of nano-ZnAl2 O4 particles on the surface of Al18 B4 O33 w provides an effective method to increase the UTS of Al18 B4 O33 w/Al composites at elevated temperatures. (2) The UTS of the composites significantly increases with increasing ZnAl2 O4 coating content at the same temperature. At the tensile temperature of 500 ◦ C, the increase ratio of the UTS of Al18 B4 O33 w/3ZnAl2 O4 /Al composite can reach 78% compared with that of Al18 B4 O33 w/Al composite. (3) The steps at the interface caused by discontinuous distribution of nano-ZnAl2 O4 particles make the load transfer from the matrix to the whiskers more effective.
Acknowledgements The work is supported by the Reward of the Outstanding Young Teacher’s Teaching Scientific Research of Institution of Higher Education and National High Technology Research and Development Program of China. References [1] G.Q. Tong, K.C. Chan, Mater. Lett. 38 (1999) 326–330. [2] A.M. Davidson, D. Regener, Compos. Sci. Technol. 60 (2006) 865– 869. [3] D.J. Lloyd, Compos. Sci. Technol. 35 (1989) 159–179. [4] M. Taya, R.J. Arsenault, Metal Matrix Composite. Thermomechanical Behavior, Pergamon Press, Oxford, 1989. [5] J.P. Tu, Y.Z. Yang, Compos. Sci. Technol. 60 (2000) 1801–1809. [6] J. Dinwoodie, E. Moore, C.A.J. Langman, W.R. Symes, in: W.C. Harrigan, J. Strife, A.K. Dhingra (Eds.), Proceedings of the 5th International Conference on Composite Materials, San Diego, CA, TMS-AIME, Warrendale, PA, 1985, pp. 671–685. [7] M. Vedani, F. D’Errico, E. Gariboldi, Compos. Sci. Technol. 66 (2006) 343–349. [8] A. Sakamoto, H. Hasegawa, Y. Minoda, in: W.C. Harrigan, J. Strife, A.K. Dhingra (Eds.), Proceedings of the 5th International Conference on Composite Materials, San Diego, CA, TMS-AIME, Warrendale, PA, 1985, pp. 699–705. [9] A. Razaghian, D. Yu, T. Chandra, Compos. Sci. Technol. 58 (1998) 293–298. [10] K.M. Lee, I.H. Moon, Mater. Sci. Eng. A185 (1994) 165–170. [11] R.D. Schueller, F.E. Wawner, Compos. Sci. Technol. 40 (1991) 213– 223. [12] H.Y. Yue, W.D. Fei, L.D. Wang, Z.J. Li, Mater. Sci. Eng. A430 (2006) 260–265. [13] Y.Q. Wang, B.L. Zhou, Composites A27 (1996) 1139–1145. [14] M.Y. Zheng, K. Wu, M. Ling, S. Kamado, Y. Kojima, Mater. Sci. Eng. A372 (2004) 66–74. [15] H.Y. Yue, W.D. Fei, L.D. Wang, Mater. Sci. Eng. A (2007), doi:10.1016/j.msea.2007.03.066. [16] L.J. Yao, H. Fukunaga, Scripta Mater. 36 (1997) 1267–1271.
Improvement of the tensile strength of Al18B4O33w/Al composite at elevated temperatures by change of interfacial state H.Y. Yue, L.D. Wang, W.D. Fei ∗ School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Received 18 June 2007; received in revised form 7 September 2007; accepted 10 September 2007
Abstract In general, it is very difficult to obtain obviously reinforced effect in discontinuously reinforced aluminum matrix composites at the temperature above 400 ◦ C. In the present study, we report an effective method to improve the high-temperature tensile strength of Al18 B4 O33 w/Al composite by change of interfacial state. The pure aluminum matrix composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents were fabricated by squeeze casting. The results indicate that ZnAl2 O4 coating of the whiskers can effectively improve the high-temperature tensile strength of Al18 B4 O33 w/Al composite, although the tensile strength of the composite decreases with increasing the tensile temperature. On the basis of fractograph analysis, the fracture mechanism of the composites at elevated temperatures was investigated. © 2007 Elsevier B.V. All rights reserved. Keywords: Composites; Interface; High-temperature properties
1. Introduction Discontinuously reinforced aluminum matrix composites (DRAMCs) have a number of advantages over monolithic aluminum alloys, such as high-specific strength, stiffness and excellent high-temperature properties [1–3]. Researches on the properties of DRAMCs at elevated temperatures are not only to investigate the high-temperature deformation behaviors and fracture mechanisms but to promote the wider applications in many fields, such as pistons for internal combustion engines and automobile brake shaft, some parts in aeronautic fields and astronautic fields and so on [4,5]. However, extensive researches have indicated that for various DRAMCs, whatever the strengths of composites at room temperature are, they begin to converge at the temperature above 300 ◦ C and they are all nearly identical at above 400 ◦ C [6–8]. Even in some composites, the strengths of the composites are lower than those of the matrix alloys. The reason is generally believed that the shear strength of the matrix rapidly declines
and almost no load transfer from the matrix to the reinforcement exists at higher temperatures. Matrix alloy and reinforcement kinds, orientation and volume fraction of the reinforcement in composites, preparation and heat-treatment processes do not have too much influence on the tensile strength of DRAMCs at temperatures above 400 ◦ C [7,9–11]. Even for SiCw/Al composites with high-interfacial bonding strength, the similar results can be obtained [11]. Therefore, it is difficult to improve high-temperature tensile properties of DRAMCs according to the routine methods, although it is very important for the wider applications of DRAMCs. Up to now, little information can be available to improve the high-temperature strength of DRAMCs. In the present study, discontinuous ZnAl2 O4 particles were coated on the surface of aluminum borate whisker (Al18 B4 O33 w) by a sol–gel route. The effect of ZnAl2 O4 coating of the whiskers on the tensile strength and fracture mechanism of the composites were investigated at elevated temperatures. 2. Experimental
∗
Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 405, Harbin 150001, People’s Republic of China. E-mail address: [email protected] (W.D. Fei). 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.09.018
The reinforcement used was Al18 B4 O33 w with a diameter of 0.5–1.5 m and a length of 10–30 m. In order to obtain ZnAl2 O4 coating of the whiskers, ZnO was firstly coated on the surface of Al18 B4 O33 w by a sol–gel route, and then the ZnO-
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H.Y. Yue et al. / Materials Science and Engineering A 486 (2008) 409–412
are given in Fig. 1. Tensile fracture surfaces of the composites were examined by an S-3000 scanning electron microscope (SEM). 3. Results
Fig. 1. Dimensions of tensile specimens.
coated whiskers were sintered at 1000 ◦ C for 1 h. During the sintering process, the reaction between the whiskers and ZnO coating took place, so ZnAl2 O4 coating was introduced. The detailed coating process and reaction mechanism can be found in our previous study [12]. The pure aluminum matrix composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents were fabricated by squeeze casting. The volume fraction of Al18 B4 O33 w in the composite was 20%. In order to investigate the effect of coating content on tensile properties of the composites at elevated temperatures, the initial massive ratio of ZnO to ABOw selected was 0, 1:30 and 1:10, that is, the volume fraction of ZnAl2 O4 coating in the composites was 0%, 0.96% and 2.88%, respectively. The corresponding abbreviation separately is Al18 B4 O33 w/Al, Al18 B4 O33 w/ZnAl2 O4 /Al and Al18 B4 O33 w/3ZnAl2 O4 /Al. Interfacial microstructures of the composites were observed using a Philips CM-12 transmission electron microscope (TEM) with an operating voltage of 120 kV. Specimens used for TEM observations were abraded to a thickness of 30 m and finally thinned by ion-milling. Tensile experiments were carried out using an Instron-1186 testing machine at a constant strain rate of 1 mm/min in the temperature range from 200 to 500 ◦ C, and specimens were heated in an Instron-SF375D resistance furnace. A thermocouple was attached to the specimen and the specimen was held at the desired temperatures for 15 min to ensure a uniform temperature distribution before testing. The dimensions of tensile specimens used
Fig. 2 shows TEM morphologies of the interface in as-cast composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents. It can be seen that, in Al18 B4 O33 w/Al composite, the interface is smooth and no interphase exists. However, interphase particles can be clearly found in ZnAl2 O4 -coated Al18 B4 O33 w reinforced aluminum composites, as shown in Fig. 2b and c, which are verified as ZnAl2 O4 according to our previous study [12]. With the increase of the coating content, the sizes of interphase particles become larger and they discontinuously distribute on the whisker surface with an average size of about 30–100 nm. Because ZnAl2 O4 particles are the reaction products between the whiskers and ZnO coating, and ZnAl2 O4 do not react with aluminum during squeeze casting, thus, ZnAl2 O4 particles at the interface embed in both the whisker and aluminum matrix and form many steps on the surface of Al18 B4 O33 w at the interface. Tensile strengths of the composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents at elevated temperatures are shown in Fig. 3. It can be seen that the ultimate tensile strength (UTS) of the composites decreases with increasing the tensile temperature and increases with increasing ZnAl2 O4 coating content. At the tensile temperature of 200 ◦ C, the UTS of Al18 B4 O33 w/Al and Al18 B4 O33 w/3ZnAl2 O4 /Al composite is 164 MPa and 261 MPa, respectively. However, at the tensile temperature of 500 ◦ C, the UTS of Al18 B4 O33 w/Al and Al18 B4 O33 w/3ZnAl2 O4 /Al composite rapidly reduces to 41 MPa and 73 MPa, respectively. The UTS of ZnAl2 O4 -coated Al18 B4 O33 w reinforced aluminum composite is obviously higher than that of uncoated Al18 B4 O33 w reinforced aluminum composite. Even at the tensile temperature of 500 ◦ C, the increase ratio of the UTS of Al18 B4 O33 w/3ZnAl2 O4 /Al composite can reach 78% compared with that of Al18 B4 O33 w/Al composite. So, ZnAl2 O4 coating of Al18 B4 O33 w provides a very effective method to
Fig. 2. TEM micrographs of the interface in as-cast composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents: (a) 0%, (b) 0.96%, (c) 2.88%.
H.Y. Yue et al. / Materials Science and Engineering A 486 (2008) 409–412
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Fig. 3. Tensile strengths of the composites reinforced by Al18 B4 O33 w with different ZnAl2 O4 coating contents at elevated temperatures.
improve the tensile strength of the composites at elevated temperatures. Fig. 4 is tensile fractographs of the composites at the tensile temperature of 500 ◦ C. The fractographs of the composites are quite different. In the fractograph of Al18 B4 O33 w/Al composite, as shown in Fig. 4a, debonding of many whiskers can be clearly seen and pull-out length of the whiskers is long, and almost no dimples exist. Obvious debonding of the whiskers from the matrix at the fracture surfaces indicates that Al18 B4 O33 w cannot transfer load due to the interface fracture. So the interface of Al18 B4 O33 w and aluminum dominates the fracture process at higher temperature. In Al18 B4 O33 w/ZnAl2 O4 /Al composite, some tear ridges on the surface of the whiskers can be observed and the pull-out amount of the whiskers becomes less. However, in Al18 B4 O33 w/3ZnAl2 O4 /Al composite (Fig. 4c), a small amount of pull-out of the whiskers, and more and larger tear ridges can be found on the surface of ZnAl2 O4 coated Al18 B4 O33 w, which indicates that ZnAl2 O4 coating of Al18 B4 O33 w can effectively prevent interfacial sliding and debonding. 4. Discussion According to the above results, one can conclude that ZnAl2 O4 coating of Al18 B4 O33 w can effectively enhance the high-temperature UTS of Al18 B4 O33 w reinforced aluminum composites. The variation of UTS of the composites reinforced by ZnAl2 O4 -coated Al18 B4 O33 w at higher temperature is very different from the previous researches [6–11]. In the previous researches, the UTSs of almost all DRAMCs are close to one value, that is to say, no reinforced effect above 400 ◦ C. As is well known, interfacial state plays an important role in the mechanical properties of composites [13,14]. Our previous research indicates that there is no interfacial reaction for the composites exposed at higher temperature (i.e. 520 ◦ C) for 1 h [15]. So we can exclude the effect of interfacial reaction on the UTS of the composites at elevated temperatures in the present study. According to the above TEM observation and fractograph
Fig. 4. Fractographs of the composites at the tensile temperature of 500 ◦ C: (a) Al18 B4 O33 w/Al, (b) Al18 B4 O33 w/ZnAl2 O4 /Al, (c) Al18 B4 O33 w/3ZnAl2 O4 /Al.
analysis, the effect of ZnAl2 O4 coating of the whiskers on the UTS of the composites is discussed as follows: On the one hand, in Al18 B4 O33 w/Al composite, there are microvoids and cracks formed at the interface due to poor wettability of Al18 B4 O33 w by molten aluminum during squeeze casting [16]. So the interfacial bonding strength is weak in Al18 B4 O33 w/Al composite. However, nano-ZnAl2 O4 particles with large specific areas on the surface of Al18 B4 O33 w can
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H.Y. Yue et al. / Materials Science and Engineering A 486 (2008) 409–412
improve the wettability of the whiskers by molten aluminum during squeeze casting [12], which causes microvoids and cracks at the interface to decrease greatly and enhance the interfacial bonding strength, so the load transfer capacity of the whisker/matrix interface increases. On the other hand, the interface of Al18 B4 O33 w/Al is smooth and the matrix is sheared away from the whiskers during the tensile process, so the contribution of the whiskers can be negligible to the tensile strength due to interfacial fracture at higher temperatures. Thus, it is easy to result in interfacial debonding at higher temperatures, as shown in Fig. 4a. However, nanoZnAl2 O4 particles embedded both in the matrix and whiskers can form many steps at the interface because of discontinuous distribution of ZnAl2 O4 particles. The steps can act as barriers to interface crack propagation, and make a large contribution to load transfer from the matrix to the whiskers during the tensile process of the composites, which also enhances the UTS of the composites at elevated temperatures. 5. Conclusions (1) The coating of nano-ZnAl2 O4 particles on the surface of Al18 B4 O33 w provides an effective method to increase the UTS of Al18 B4 O33 w/Al composites at elevated temperatures. (2) The UTS of the composites significantly increases with increasing ZnAl2 O4 coating content at the same temperature. At the tensile temperature of 500 ◦ C, the increase ratio of the UTS of Al18 B4 O33 w/3ZnAl2 O4 /Al composite can reach 78% compared with that of Al18 B4 O33 w/Al composite. (3) The steps at the interface caused by discontinuous distribution of nano-ZnAl2 O4 particles make the load transfer from the matrix to the whiskers more effective.
Acknowledgements The work is supported by the Reward of the Outstanding Young Teacher’s Teaching Scientific Research of Institution of Higher Education and National High Technology Research and Development Program of China. References [1] G.Q. Tong, K.C. Chan, Mater. Lett. 38 (1999) 326–330. [2] A.M. Davidson, D. Regener, Compos. Sci. Technol. 60 (2006) 865– 869. [3] D.J. Lloyd, Compos. Sci. Technol. 35 (1989) 159–179. [4] M. Taya, R.J. Arsenault, Metal Matrix Composite. Thermomechanical Behavior, Pergamon Press, Oxford, 1989. [5] J.P. Tu, Y.Z. Yang, Compos. Sci. Technol. 60 (2000) 1801–1809. [6] J. Dinwoodie, E. Moore, C.A.J. Langman, W.R. Symes, in: W.C. Harrigan, J. Strife, A.K. Dhingra (Eds.), Proceedings of the 5th International Conference on Composite Materials, San Diego, CA, TMS-AIME, Warrendale, PA, 1985, pp. 671–685. [7] M. Vedani, F. D’Errico, E. Gariboldi, Compos. Sci. Technol. 66 (2006) 343–349. [8] A. Sakamoto, H. Hasegawa, Y. Minoda, in: W.C. Harrigan, J. Strife, A.K. Dhingra (Eds.), Proceedings of the 5th International Conference on Composite Materials, San Diego, CA, TMS-AIME, Warrendale, PA, 1985, pp. 699–705. [9] A. Razaghian, D. Yu, T. Chandra, Compos. Sci. Technol. 58 (1998) 293–298. [10] K.M. Lee, I.H. Moon, Mater. Sci. Eng. A185 (1994) 165–170. [11] R.D. Schueller, F.E. Wawner, Compos. Sci. Technol. 40 (1991) 213– 223. [12] H.Y. Yue, W.D. Fei, L.D. Wang, Z.J. Li, Mater. Sci. Eng. A430 (2006) 260–265. [13] Y.Q. Wang, B.L. Zhou, Composites A27 (1996) 1139–1145. [14] M.Y. Zheng, K. Wu, M. Ling, S. Kamado, Y. Kojima, Mater. Sci. Eng. A372 (2004) 66–74. [15] H.Y. Yue, W.D. Fei, L.D. Wang, Mater. Sci. Eng. A (2007), doi:10.1016/j.msea.2007.03.066. [16] L.J. Yao, H. Fukunaga, Scripta Mater. 36 (1997) 1267–1271.