- Email: [email protected]
6061Al composites
Journal of Alloys and Compounds 697 (2017) 11e18
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of whisker surface treatment on microstructures, tensile properties and aging behaviors of Al18B4O33w/6061Al composites H.Y. Yue a, *, S.S. Song a, B. Wang a, X. Gao a, S.L. Zhang b, L.H. Yao a, X.Y. Lin a, E.H. Guan a, H.J. Zhang a, E.J. Guo a a b
School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, People's Republic of China Department of Physics, Donguk University, Seoul 100715, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 September 2016 Received in revised form 24 November 2016 Accepted 11 December 2016 Available online 13 December 2016
CuO was coated on the surface of Al18B4O33 whiskers by hydrothermal synthesis and CuO coated Al18B4O33 whiskers reinforced 6061Al composites with different coating contents were fabricated by squeeze casting. The preparation process and characterizations of the CuO coated Al18B4O33 whiskers were studied. The effects of CuO coating of whiskers on the microstructures, tensile properties and aging behaviors of the composite were investigated. The results showed that a uniform CuO coating of Al18B4O33 whiskers could be prepared by hydrothermal synthesis. CuO reacted with molten 6061Al during squeeze casting and meanwhile a layer of continuous MgAl2O4 formed at the interface. With increasing the contents of CuO coating on whiskers, the ultimate tensile strength of the composite gradually increased. The elongation to fracture of the composite initially increased and later decreased. Impressively, the ultimate tensile strength of the composite could be further improved after T6 heattreatment. CuO coating of the whiskers could also obviously change the aging behaviors of the composite. © 2016 Elsevier B.V. All rights reserved.
Keywords: Metal matrix composites Microstructures Mechanical properties Surfaces and interfaces
1. Introduction Aluminum borate whiskers reinforced aluminum matrix composites (Al18B4O33w/Al) have been extensively investigated on account of their low density, improved mechanical properties and fairly low cost, which make them very attractive in the fields of aerospace, aircraft, automobile and sports industry [1e4]. However, poor interfacial wettability between Al18B4O33w and matrix, and serious interfacial reactions between Al18B4O33w and Mg in the alloy matrix are regarded as the major obstacles to synthesize high-performance Al18B4O33w/Al composites [5,6]. Unfortunately, there are few effective methods to improve the wettability and hinder the reactions simultaneously. It is widely accepted that the interface between the matrix and reinforcement plays a critical role in determining the mechanical properties of composites [7]. Reinforcement coating is one of the most effective techniques to improve the interfacial properties of composites [8], which is expected to simultaneously solve above
* Corresponding author. E-mail address: [email protected] (H.Y. Yue). http://dx.doi.org/10.1016/j.jallcom.2016.12.141 0925-8388/© 2016 Elsevier B.V. All rights reserved.
two issues. The coating of whiskers is commonly prepared by precipitation, electroless plating, sol-gel and hydrothermal process [9e15]. Hu et al. [9] prepared SnO2 coated Al18B4O33w by precipitation, however, the SnO2 coating on the surface of Al18B4O33w was not uniform. Gao et al. [10] prepared uniform Cu coated Al18B4O33w by electroless plating. However, their electroless plating process was very complex involving activation, sensitization and pre-treatment of Al18B4O33w. Sol-gel is an advanced process to prepare highquality oxide film on the surface of Al18B4O33w [11e13]. However, before the preform was prepared, the coated Al18B4O33w must be sintered at high temperature to transfer the xerogel to metal oxide, which can result in the aggregation of the coated whiskers, and thus additional ball-milling treatment or proper dispersion process was required. On the other hand, hydrothermal synthesis is a facile and universal chemical synthesis method which is processed under hightemperature and high-pressure, offering some significant advantages over above techniques [14,15]. Therefore, it can be used as a promising technique to fabricate uniform metal oxide coating on the surface of whiskers. If proper metal oxide is selected, the improved interfacial wettability and suppressed interfacial
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H.Y. Yue et al. / Journal of Alloys and Compounds 697 (2017) 11e18 Table 1 Specification for the coated whiskers and composites with different CuO coating contents. Mass ratio between coating and whiskers
Coated whiskers
Composites
0 1:40 1:20 1:10
Al18B4O33w 40Al18B4O33w/CuO 20Al18B4O33w/CuO 10Al18B4O33w/CuO
Al18B4O33w/6061Al 40Al18B4O33w/CuO/6061Al 20Al18B4O33w/CuO/6061Al 10Al18B4O33w/CuO/6061Al
reactions can be simultaneously achieved. In the present study, CuO was selected and coated on the surface of Al18B4O33w by hydrothermal synthesis. CuO coated Al18B4O33w reinforced 6061Al composites were prepared by squeeze casting. The effects of coating contents of CuO on the microstructures, tensile properties and aging behaviors of the composites were investigated in detail.
2. Experimental The reinforcement was Al18B4O33w with a diameter of 0.5e1 mm and a length of 10e30 mm (Shikoku Chemical Company of Japan). 6061Al was selected as the matrix. The chemical compositions of 6061Al are 0.34 wt% Cu, 0.75 wt% Mg, 1.26 wt% Si, 0.22 wt% Mn and balanced Al. CuO was coated on the surface of Al18B4O33w by a hydrothermal synthesis depicted as follows. Copper nitrate was dissolved in distilled water to form a transparent solution and the Al18B4O33w was added into the solution under magnetic stirring. Then, NH3$H2O solution was dropped. The mixture was transferred into a Teflon-lined autoclave and maintained at 200 C for 8 h. After that, it was cooled down to room temperature naturally in air. The whiskers was washed and dispersed in water to prepare a preform by mould pressing. The preform was dried at room temperature and sintered at 900 C for 1 h, thus CuO coating was introduced on the surface of Al18B4O33w. CuO coated Al18B4O33w reinforced 6061Al composite was
fabricated by squeeze casting. The whisker preform and the mould were preheated at 500 C, and then molten 6061Al at 780 C was poured into the mould with a pressure of 100 MPa. The volume fraction of whiskers in the composite was ~20%. In order to investigate the effects of CuO coating contents on tensile properties of as-cast composites, the mass ratio of CuO to Al18B4O33w was selected as 1:40, 1:20 and 1:10, respectively. The corresponding abbreviations for the coated whiskers and composites are listed in Table 1. For comparison, pure Al18B4O33w reinforced 6061Al composite was also prepared under the same condition. Tensile tests of the composites were carried out on an Instron 1186 test machine at the tensile rate of 0.5 mm/min at room temperature and three specimens were tested for each composite. The morphologies of the coated whiskers and tensile fractographs of the composites were observed by an FEI siron-200 scanning electron microscope (SEM) with an operating voltage of 20 kV. The phase compositions of the coated whiskers with different coating contents were carried out on a Philips X'pert X-ray diffractometer (XRD) with a radiation of Cu Ka, a tube voltage of 40 kV and a tube current of 40 mA. The interfacial microstructures of the composites were investigated using a JEM-2100 transmission electron microscope (TEM) with an operating voltage of 200 kV. The thinned foils of the composites were prepared by mechanical and ion milling for TEM observation. T6 heat-treatment of the composites was performed by a solid-
Fig. 1. SEM morphologies of the coated Al18B4O33w with different coating contents. (a) Al18B4O33w, (b) 40Al18B4O33w/CuO, (c) 20Al18B4O33w/CuO and (d) 10Al18B4O33w/CuO.
H.Y. Yue et al. / Journal of Alloys and Compounds 697 (2017) 11e18
13
which indicates the coating is comprised of CuO. With increasing the contents of CuO coating on whiskers, the thickness of CuO coating also increases, which is evidenced by the increased diffraction peaks of CuO. 3.2. Microstructures of as-cast composites The interfacial microstructures of as-cast composites are shown in Fig. 3. In as-cast Al18B4O33w/6061Al composite, almost no interfacial products exist at the interface. Since the contact time between Al18B4O33w and molten 6061Al at high temperatures is short during squeeze casting, it is not evident of the interfacial reactions between Al18B4O33w and Mg in molten 6061Al. However, in as-cast 10Al18B4O33w/CuO/6061Al composite, a continuous product with large square grain distributes at the interface. It is determined that the product is MgAl2O4 by the indexing of selected area diffraction pattern. The interfacial reactions during squeeze casting may be expressed as follows: Fig. 2. XRD patterns of the coated Al18B4O33w with different coating contents.
solution treatment at 520 C for 4 h and aging treatment at various temperatures (150, 170, 190 C) for different time. In order to characterize the aging kinetics of the composites, Vickers hardness tests were conducted on a HXD-1000 microhardness machine with a 500 g loading for 30 s. The peak-aged time was carefully determined by the time of the highest hardness value of the composites. 3. Results 3.1. Characterizations of CuO coated Al18B4O33w SEM morphologies of the coated Al18B4O33w with different coating contents calcined at 900 C for 1 h are shown in Fig. 1. The surface of pure Al18B4O33w is smooth and clear. After the coating, the surface of Al18B4O33w becomes rough. With increasing the contents of coating on whiskers, some nanoparticles appear on the surface of Al18B4O33w. XRD patterns of the coated Al18B4O33w with different coating contents calcined at 900 C for 1 h are shown in Fig. 2. The diffraction peaks of CuO and Al18B4O33w can be clearly observed,
2Al þ 3CuO / Al2O3 þ 3Cu
(1)
Mg þ CuO / MgO þ Cu
(2)
MgO þ Al2O3 / MgAl2O4
(3)
3Mg þ 4Al2O3 / 3MgAl2O4 þ 2Al
(4)
The studies on thermodynamics stability of Al-Mg oxides in AlMg alloys showed that the formation of MgAl2O4 and MgO primarily depended on the Mg contents [16,17]. In this study, the formation of MgAl2O4 at the interface is favorable due to low contents of Mg in the 6061Al. 3.3. Tensile properties of as-cast composites Fig. 4 shows the variations of ultimate tensile strength (UTS) and elongation to fracture (d) of as-cast composites with different contents of CuO coating on whiskers. The UTS of the composites increases gradually with increasing the contents of CuO coating on whiskers. The UTS of 10Al18B4O33w/CuO/6061Al composite reaches the largest value, which increases about 30% more compared with
Fig. 3. TEM micrographs of as-cast composites. (a) Al18B4O33w/6061Al and (b) 10Al18B4O33w/CuO/6061Al.
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Fig. 4. Tensile properties of as-cast composites with different contents of CuO coating on whiskers. (a) UTS and (b) d.
that of Al18B4O33w/6061Al composite. However, the d of the composites initially increases and later declines with increasing the contents of CuO coating on whiskers, which indicates an optimal coating content of whiskers exists in the composites. In 40Al18B4O33w/CuO/6061Al composite, the d of the composite reaches the highest value. The tensile fractographs of as-cast composites with different contents of CuO coating on whiskers are shown in Fig. 5. In Al18B4O33w/6061Al composite, many microcracks and holes can be found, which results from the unfilled regions of molten 6061Al due to poor wettability during squeeze casting or the interfacial debonding of whiskers and matrix during the tensile deformation. With increasing the contents of CuO coating on whiskers, the amount of pull-out of whiskers decreases, however, fracture of whiskers increases. In 40Al18B4O33w/CuO/6061Al composite, pullout of whiskers (the amount is ~0.04/mm2) takes the dominant position. However, in 10Al18B4O33w/CuO/6061Al composite, many fractures of whiskers instead of pull-out of whiskers (the amount is ~0.02/mm2) are observed.
3.4. Microstructures of the composites after T6 heat-treatment Fig. 6 is the TEM micrographs of composites after solid-solution treatment at 520 C for 4 h and aging treatment at 170 C for 4 h. It can be seen that the amount of precipitation phase is less in Al18B4O33w/6061Al composite (~25/mm2) because the content of Mg in the 6061Al matrix is consumed by the reaction of Al18B4O33w and Mg at high temperatures. MgAl2O4 forms at the interface [16], meanwhile, the integrity of the Al18B4O33w is seriously destroyed. However, in 10Al18B4O33w/CuO/6061Al composite, the amount of precipitation phase is more (~30/mm2). This indicates that the existence of continuous MgAl2O4 formed at the interface during squeeze casting can effectively hinder the interfacial reactions of Al18B4O33w and Mg in the 6061Al matrix at high temperatures. 3.5. Tensile properties of the composite after T6 heat-treatment Fig. 7 presents the tensile properties of 10Al18B4O33w/CuO/ 6061Al composite after solid-solution treatment at 520 C for 4 h
Fig. 5. Fractographs of as-cast composites with different contents of CuO coating on whiskers. (a) Al18B4O33w/6061Al, (b) 40Al18B4O33w/CuO/6061Al, (c) 20Al18B4O33w/CuO/6061Al and (d) 10Al18B4O33w/CuO/6061Al.
H.Y. Yue et al. / Journal of Alloys and Compounds 697 (2017) 11e18
15
Fig. 6. TEM micrographs of composites after solid-solution treatment at 520 C for 4 h and aging treatment at 170 C for 4 h (a) Al18B4O33w/6061Al and (b) 10Al18B4O33w/CuO/ 6061Al.
Fig. 7. Tensile properties of 10Al18B4O33w/CuO/6061Al composite after solid-solution treatment at 520 C for 4 h and aging treatment at 170 C for different time. (a) UTS and (b) d.
and aging treatment at 170 C for different time. After T6 heattreatment, the UTS of the composite can be significantly improved due to the effects of solid-solution and aging strengthening effects compared with that of the as-cast composite (360 MPa). Moreover, the UTS of the composite can be affected by the aging time and it is the largest when the aging time is 4 h. The d of the composite shows a similar variation tendency with the UTS of the composite, however, it declines to some extent compared
with that of as-cast composite. The tensile fractographs of 10Al18B4O33w/CuO/6061Al composite after the solid-solution treatment at 520 C for 4 h and aging treatment at 170 C for different time are shown in Fig. 8. It can be observed that all the fracture surfaces are made up of dimples, pullout and fracture of whiskers. After aging at 170 C for 4 h, the amount of matrix dimples is more than that of sample after aging at 170 C for 1 h and 7 h, respectively.
Fig. 8. Tensile fractographs of 10Al18B4O33w/CuO/6061Al composite after solid-solution treatment at 520 C for 4 h and aging treatment at 170 C for different time. (a) 1 h, (b) 4 h and (c) 7 h.
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H.Y. Yue et al. / Journal of Alloys and Compounds 697 (2017) 11e18
Fig. 9. Aging characteristics of Al18B4O33w/6061Al at different temperatures. (a) 150 C, (b) 170 C and (c) 190 C.
Fig. 10. Aging characteristics of 10Al18B4O33w/CuO/6061Al at different temperatures. (a) 150 C, (b) 170 C and (c) 190 C.
3.6. Aging behaviors of the composites Aging characteristics of Al18B4O33w/6061Al and 10Al18B4O33w/ CuO/6061Al composites at different temperatures are separately shown in Figs. 9 and 10, which are obtained by the hardness test at different time points. For comparison, the aging characteristics of 6061Al are also shown in Fig. 11. The composites undergo the processes of under-aged, peak-aged and over-aged, which correspond to a phase transformation from solid-solution to b'', b0 and bMg2Si in the matrix alloys according to previous research on the aging behavior of 6061Al composites [18]. The peak-aged time is
shorten with increasing the aging temperature and the hardness level is enhanced by the CuO coating on whiskers at the same aging temperature. In 6061Al, the peak-aged time is 12, 8 and 4 h at the aging temperature of 150, 170 and 190 C, respectively. In Al18B4O33w/6061Al and 10Al18B4O33w/CuO/6061Al composites, the hardness level increases and the peak-aged time decreases compared with those of 6061Al due to the addition of the whiskers. In Al18B4O33w/6061Al composite, the peak-aged time is 2, 1.5 and 0.5 h at the aging temperature of 150, 170 and 190 C, respectively. In 10Al18B4O33w/CuO/6061Al composite, the peak-aged time is 6, 4
H.Y. Yue et al. / Journal of Alloys and Compounds 697 (2017) 11e18
17
Al18B4O33w prepared by a novel hydrothermal synthesis process, the reactions of CuO and molten 6061Al can improve the interfacial wettability during squeeze casting. Thus, the UTS of the composites increases gradually with increasing the contents of CuO coating on whiskers. However, the d of the composites initially increases and later decreases. In Al18B4O33w/6061Al composite, the d of the composite is low due to the existence of microcracks and holes in the composite. When the CuO coating contents is large, the d of the composite is also low due to the fracture of brittle phase MgAl2O4 with large sizes. Only when the CuO coating contents is suitable, whiskers can slide in the matrix by consuming grinding force during the tensile deformation. After T6 heat-treatment, the tensile properties of the composite are significantly increased due to the effects of solid-solution and aging strengthening. It is the important factors of the Mg content in the matrix alloy to determine the aging kinetics of composites. In Al18B4O33w/ 6061Al composite, the interfacial reaction between Al18B4O33w and Mg at high temperatures consumes Mg content of 6061Al matrix. Therefore, the amount of secondary phases needed to precipitate from the matrix is rather small, accelerating the peak-aged time. However, in 10Al18B4O33w/CuO/6061Al composite, continuous MgAl2O4 forms at the interface during squeeze casting and meanwhile it acts as an effective barrier layer to hinder the reaction of Al18B4O33w and Mg in the 6061Al matrix at high temperatures. Therefore, the Mg content in the 6061Al matrix is reserved and Cu is also introduced into the 6061Al matrix, which results in a longer time for the secondary phases to precipitate from the matrix and reach the peak-aged stage. 5. Conclusions Uniform CuO coating can be successfully prepared on the surface of Al18B4O33w by hydrothermal synthesis. CuO reacts with molten 6061Al during squeeze casting, which can improve the interfacial wettability and tensile properties of as-cast composites. Meanwhile, MgAl2O4 formed at the interface can effectively hinder further serious interfacial reactions between Al18B4O33w and Mg of the 6061Al matrix. With increasing the contents of CuO coating on whiskers, the UTS of the composites increases gradually. The d of the composites initially increases and later decreases. The UTS of the composite can be significantly improved after T6 heattreatment. The CuO coating of whiskers prolongs the peak-aged time of the composite. Acknowledgements
Fig. 11. Aging characteristics of 6061Al at different temperatures. (a) 150 C, (b) 170 C and (c) 190 C.
and 2.5 h at the aging temperature of 150, 170 and 190 C, respectively. This indicates after CuO coating of Al18B4O33w, the peak-aged time is significantly prolonged. 4. Discussion It is well known that the interface plays a significant role in determining mechanical properties of metal matrix composites. In as-cast Al18B4O33w/6061Al composite, the poor wettability between Al18B4O33w and 6061Al results in microholes in the composite during squeeze casting and interfacial debonding during tensile deformation. However, after uniform CuO coating of
This work is supported by the Natural Science Foundation of Heilongjiang Province (LC2015020), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (2015192), the Innovative Talent Fund of Harbin city (2016RAQXJ185) and Science Fund for the Young Innovative Talents of HUST (201306). References [1] G. Liu, W.C. Ren, Y.L. Sun, J. Hu, Damping behavior of Bi2O3-coated Al18B4O33 whisker-reinforced pure Al composite, Mater. Sci. Eng. A 527 (2010) 5136e5142. [2] P.T. Zhao, L.D. Wang, Z.M. Du, S.C. Xu, P.P. Jin, W.D. Fei, Low temperature extrusion of 6061 aluminum matrix composite reinforced with SnO2-coated Al18B4O33 whisker, Compos. A 43 (2012) 183e188. [3] L. Wang, L.D. Wang, W.D. Fei, Fractal analysis of fracture surfaces in aluminum borate whiskers-reinforced aluminum alloy 6061 composite, Trans. Nonferrous Metals Soc. China 21 (2011) 461e466. [4] H.T. Zhang, J. Cao, H. Lu, Reactive brazing of aluminum to aluminum-based composite reinforced with alumina borate whiskers with Cu interlayer, Vacuum 84 (2010) 474e477. [5] W.G. Wang, K. Matsugi, H. Fukushima, G. Sasaki, Interfacial reaction in AZ91D
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of whisker surface treatment on microstructures, tensile properties and aging behaviors of Al18B4O33w/6061Al composites H.Y. Yue a, *, S.S. Song a, B. Wang a, X. Gao a, S.L. Zhang b, L.H. Yao a, X.Y. Lin a, E.H. Guan a, H.J. Zhang a, E.J. Guo a a b
School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, People's Republic of China Department of Physics, Donguk University, Seoul 100715, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 September 2016 Received in revised form 24 November 2016 Accepted 11 December 2016 Available online 13 December 2016
CuO was coated on the surface of Al18B4O33 whiskers by hydrothermal synthesis and CuO coated Al18B4O33 whiskers reinforced 6061Al composites with different coating contents were fabricated by squeeze casting. The preparation process and characterizations of the CuO coated Al18B4O33 whiskers were studied. The effects of CuO coating of whiskers on the microstructures, tensile properties and aging behaviors of the composite were investigated. The results showed that a uniform CuO coating of Al18B4O33 whiskers could be prepared by hydrothermal synthesis. CuO reacted with molten 6061Al during squeeze casting and meanwhile a layer of continuous MgAl2O4 formed at the interface. With increasing the contents of CuO coating on whiskers, the ultimate tensile strength of the composite gradually increased. The elongation to fracture of the composite initially increased and later decreased. Impressively, the ultimate tensile strength of the composite could be further improved after T6 heattreatment. CuO coating of the whiskers could also obviously change the aging behaviors of the composite. © 2016 Elsevier B.V. All rights reserved.
Keywords: Metal matrix composites Microstructures Mechanical properties Surfaces and interfaces
1. Introduction Aluminum borate whiskers reinforced aluminum matrix composites (Al18B4O33w/Al) have been extensively investigated on account of their low density, improved mechanical properties and fairly low cost, which make them very attractive in the fields of aerospace, aircraft, automobile and sports industry [1e4]. However, poor interfacial wettability between Al18B4O33w and matrix, and serious interfacial reactions between Al18B4O33w and Mg in the alloy matrix are regarded as the major obstacles to synthesize high-performance Al18B4O33w/Al composites [5,6]. Unfortunately, there are few effective methods to improve the wettability and hinder the reactions simultaneously. It is widely accepted that the interface between the matrix and reinforcement plays a critical role in determining the mechanical properties of composites [7]. Reinforcement coating is one of the most effective techniques to improve the interfacial properties of composites [8], which is expected to simultaneously solve above
* Corresponding author. E-mail address: [email protected] (H.Y. Yue). http://dx.doi.org/10.1016/j.jallcom.2016.12.141 0925-8388/© 2016 Elsevier B.V. All rights reserved.
two issues. The coating of whiskers is commonly prepared by precipitation, electroless plating, sol-gel and hydrothermal process [9e15]. Hu et al. [9] prepared SnO2 coated Al18B4O33w by precipitation, however, the SnO2 coating on the surface of Al18B4O33w was not uniform. Gao et al. [10] prepared uniform Cu coated Al18B4O33w by electroless plating. However, their electroless plating process was very complex involving activation, sensitization and pre-treatment of Al18B4O33w. Sol-gel is an advanced process to prepare highquality oxide film on the surface of Al18B4O33w [11e13]. However, before the preform was prepared, the coated Al18B4O33w must be sintered at high temperature to transfer the xerogel to metal oxide, which can result in the aggregation of the coated whiskers, and thus additional ball-milling treatment or proper dispersion process was required. On the other hand, hydrothermal synthesis is a facile and universal chemical synthesis method which is processed under hightemperature and high-pressure, offering some significant advantages over above techniques [14,15]. Therefore, it can be used as a promising technique to fabricate uniform metal oxide coating on the surface of whiskers. If proper metal oxide is selected, the improved interfacial wettability and suppressed interfacial
12
H.Y. Yue et al. / Journal of Alloys and Compounds 697 (2017) 11e18 Table 1 Specification for the coated whiskers and composites with different CuO coating contents. Mass ratio between coating and whiskers
Coated whiskers
Composites
0 1:40 1:20 1:10
Al18B4O33w 40Al18B4O33w/CuO 20Al18B4O33w/CuO 10Al18B4O33w/CuO
Al18B4O33w/6061Al 40Al18B4O33w/CuO/6061Al 20Al18B4O33w/CuO/6061Al 10Al18B4O33w/CuO/6061Al
reactions can be simultaneously achieved. In the present study, CuO was selected and coated on the surface of Al18B4O33w by hydrothermal synthesis. CuO coated Al18B4O33w reinforced 6061Al composites were prepared by squeeze casting. The effects of coating contents of CuO on the microstructures, tensile properties and aging behaviors of the composites were investigated in detail.
2. Experimental The reinforcement was Al18B4O33w with a diameter of 0.5e1 mm and a length of 10e30 mm (Shikoku Chemical Company of Japan). 6061Al was selected as the matrix. The chemical compositions of 6061Al are 0.34 wt% Cu, 0.75 wt% Mg, 1.26 wt% Si, 0.22 wt% Mn and balanced Al. CuO was coated on the surface of Al18B4O33w by a hydrothermal synthesis depicted as follows. Copper nitrate was dissolved in distilled water to form a transparent solution and the Al18B4O33w was added into the solution under magnetic stirring. Then, NH3$H2O solution was dropped. The mixture was transferred into a Teflon-lined autoclave and maintained at 200 C for 8 h. After that, it was cooled down to room temperature naturally in air. The whiskers was washed and dispersed in water to prepare a preform by mould pressing. The preform was dried at room temperature and sintered at 900 C for 1 h, thus CuO coating was introduced on the surface of Al18B4O33w. CuO coated Al18B4O33w reinforced 6061Al composite was
fabricated by squeeze casting. The whisker preform and the mould were preheated at 500 C, and then molten 6061Al at 780 C was poured into the mould with a pressure of 100 MPa. The volume fraction of whiskers in the composite was ~20%. In order to investigate the effects of CuO coating contents on tensile properties of as-cast composites, the mass ratio of CuO to Al18B4O33w was selected as 1:40, 1:20 and 1:10, respectively. The corresponding abbreviations for the coated whiskers and composites are listed in Table 1. For comparison, pure Al18B4O33w reinforced 6061Al composite was also prepared under the same condition. Tensile tests of the composites were carried out on an Instron 1186 test machine at the tensile rate of 0.5 mm/min at room temperature and three specimens were tested for each composite. The morphologies of the coated whiskers and tensile fractographs of the composites were observed by an FEI siron-200 scanning electron microscope (SEM) with an operating voltage of 20 kV. The phase compositions of the coated whiskers with different coating contents were carried out on a Philips X'pert X-ray diffractometer (XRD) with a radiation of Cu Ka, a tube voltage of 40 kV and a tube current of 40 mA. The interfacial microstructures of the composites were investigated using a JEM-2100 transmission electron microscope (TEM) with an operating voltage of 200 kV. The thinned foils of the composites were prepared by mechanical and ion milling for TEM observation. T6 heat-treatment of the composites was performed by a solid-
Fig. 1. SEM morphologies of the coated Al18B4O33w with different coating contents. (a) Al18B4O33w, (b) 40Al18B4O33w/CuO, (c) 20Al18B4O33w/CuO and (d) 10Al18B4O33w/CuO.
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which indicates the coating is comprised of CuO. With increasing the contents of CuO coating on whiskers, the thickness of CuO coating also increases, which is evidenced by the increased diffraction peaks of CuO. 3.2. Microstructures of as-cast composites The interfacial microstructures of as-cast composites are shown in Fig. 3. In as-cast Al18B4O33w/6061Al composite, almost no interfacial products exist at the interface. Since the contact time between Al18B4O33w and molten 6061Al at high temperatures is short during squeeze casting, it is not evident of the interfacial reactions between Al18B4O33w and Mg in molten 6061Al. However, in as-cast 10Al18B4O33w/CuO/6061Al composite, a continuous product with large square grain distributes at the interface. It is determined that the product is MgAl2O4 by the indexing of selected area diffraction pattern. The interfacial reactions during squeeze casting may be expressed as follows: Fig. 2. XRD patterns of the coated Al18B4O33w with different coating contents.
solution treatment at 520 C for 4 h and aging treatment at various temperatures (150, 170, 190 C) for different time. In order to characterize the aging kinetics of the composites, Vickers hardness tests were conducted on a HXD-1000 microhardness machine with a 500 g loading for 30 s. The peak-aged time was carefully determined by the time of the highest hardness value of the composites. 3. Results 3.1. Characterizations of CuO coated Al18B4O33w SEM morphologies of the coated Al18B4O33w with different coating contents calcined at 900 C for 1 h are shown in Fig. 1. The surface of pure Al18B4O33w is smooth and clear. After the coating, the surface of Al18B4O33w becomes rough. With increasing the contents of coating on whiskers, some nanoparticles appear on the surface of Al18B4O33w. XRD patterns of the coated Al18B4O33w with different coating contents calcined at 900 C for 1 h are shown in Fig. 2. The diffraction peaks of CuO and Al18B4O33w can be clearly observed,
2Al þ 3CuO / Al2O3 þ 3Cu
(1)
Mg þ CuO / MgO þ Cu
(2)
MgO þ Al2O3 / MgAl2O4
(3)
3Mg þ 4Al2O3 / 3MgAl2O4 þ 2Al
(4)
The studies on thermodynamics stability of Al-Mg oxides in AlMg alloys showed that the formation of MgAl2O4 and MgO primarily depended on the Mg contents [16,17]. In this study, the formation of MgAl2O4 at the interface is favorable due to low contents of Mg in the 6061Al. 3.3. Tensile properties of as-cast composites Fig. 4 shows the variations of ultimate tensile strength (UTS) and elongation to fracture (d) of as-cast composites with different contents of CuO coating on whiskers. The UTS of the composites increases gradually with increasing the contents of CuO coating on whiskers. The UTS of 10Al18B4O33w/CuO/6061Al composite reaches the largest value, which increases about 30% more compared with
Fig. 3. TEM micrographs of as-cast composites. (a) Al18B4O33w/6061Al and (b) 10Al18B4O33w/CuO/6061Al.
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Fig. 4. Tensile properties of as-cast composites with different contents of CuO coating on whiskers. (a) UTS and (b) d.
that of Al18B4O33w/6061Al composite. However, the d of the composites initially increases and later declines with increasing the contents of CuO coating on whiskers, which indicates an optimal coating content of whiskers exists in the composites. In 40Al18B4O33w/CuO/6061Al composite, the d of the composite reaches the highest value. The tensile fractographs of as-cast composites with different contents of CuO coating on whiskers are shown in Fig. 5. In Al18B4O33w/6061Al composite, many microcracks and holes can be found, which results from the unfilled regions of molten 6061Al due to poor wettability during squeeze casting or the interfacial debonding of whiskers and matrix during the tensile deformation. With increasing the contents of CuO coating on whiskers, the amount of pull-out of whiskers decreases, however, fracture of whiskers increases. In 40Al18B4O33w/CuO/6061Al composite, pullout of whiskers (the amount is ~0.04/mm2) takes the dominant position. However, in 10Al18B4O33w/CuO/6061Al composite, many fractures of whiskers instead of pull-out of whiskers (the amount is ~0.02/mm2) are observed.
3.4. Microstructures of the composites after T6 heat-treatment Fig. 6 is the TEM micrographs of composites after solid-solution treatment at 520 C for 4 h and aging treatment at 170 C for 4 h. It can be seen that the amount of precipitation phase is less in Al18B4O33w/6061Al composite (~25/mm2) because the content of Mg in the 6061Al matrix is consumed by the reaction of Al18B4O33w and Mg at high temperatures. MgAl2O4 forms at the interface [16], meanwhile, the integrity of the Al18B4O33w is seriously destroyed. However, in 10Al18B4O33w/CuO/6061Al composite, the amount of precipitation phase is more (~30/mm2). This indicates that the existence of continuous MgAl2O4 formed at the interface during squeeze casting can effectively hinder the interfacial reactions of Al18B4O33w and Mg in the 6061Al matrix at high temperatures. 3.5. Tensile properties of the composite after T6 heat-treatment Fig. 7 presents the tensile properties of 10Al18B4O33w/CuO/ 6061Al composite after solid-solution treatment at 520 C for 4 h
Fig. 5. Fractographs of as-cast composites with different contents of CuO coating on whiskers. (a) Al18B4O33w/6061Al, (b) 40Al18B4O33w/CuO/6061Al, (c) 20Al18B4O33w/CuO/6061Al and (d) 10Al18B4O33w/CuO/6061Al.
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Fig. 6. TEM micrographs of composites after solid-solution treatment at 520 C for 4 h and aging treatment at 170 C for 4 h (a) Al18B4O33w/6061Al and (b) 10Al18B4O33w/CuO/ 6061Al.
Fig. 7. Tensile properties of 10Al18B4O33w/CuO/6061Al composite after solid-solution treatment at 520 C for 4 h and aging treatment at 170 C for different time. (a) UTS and (b) d.
and aging treatment at 170 C for different time. After T6 heattreatment, the UTS of the composite can be significantly improved due to the effects of solid-solution and aging strengthening effects compared with that of the as-cast composite (360 MPa). Moreover, the UTS of the composite can be affected by the aging time and it is the largest when the aging time is 4 h. The d of the composite shows a similar variation tendency with the UTS of the composite, however, it declines to some extent compared
with that of as-cast composite. The tensile fractographs of 10Al18B4O33w/CuO/6061Al composite after the solid-solution treatment at 520 C for 4 h and aging treatment at 170 C for different time are shown in Fig. 8. It can be observed that all the fracture surfaces are made up of dimples, pullout and fracture of whiskers. After aging at 170 C for 4 h, the amount of matrix dimples is more than that of sample after aging at 170 C for 1 h and 7 h, respectively.
Fig. 8. Tensile fractographs of 10Al18B4O33w/CuO/6061Al composite after solid-solution treatment at 520 C for 4 h and aging treatment at 170 C for different time. (a) 1 h, (b) 4 h and (c) 7 h.
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Fig. 9. Aging characteristics of Al18B4O33w/6061Al at different temperatures. (a) 150 C, (b) 170 C and (c) 190 C.
Fig. 10. Aging characteristics of 10Al18B4O33w/CuO/6061Al at different temperatures. (a) 150 C, (b) 170 C and (c) 190 C.
3.6. Aging behaviors of the composites Aging characteristics of Al18B4O33w/6061Al and 10Al18B4O33w/ CuO/6061Al composites at different temperatures are separately shown in Figs. 9 and 10, which are obtained by the hardness test at different time points. For comparison, the aging characteristics of 6061Al are also shown in Fig. 11. The composites undergo the processes of under-aged, peak-aged and over-aged, which correspond to a phase transformation from solid-solution to b'', b0 and bMg2Si in the matrix alloys according to previous research on the aging behavior of 6061Al composites [18]. The peak-aged time is
shorten with increasing the aging temperature and the hardness level is enhanced by the CuO coating on whiskers at the same aging temperature. In 6061Al, the peak-aged time is 12, 8 and 4 h at the aging temperature of 150, 170 and 190 C, respectively. In Al18B4O33w/6061Al and 10Al18B4O33w/CuO/6061Al composites, the hardness level increases and the peak-aged time decreases compared with those of 6061Al due to the addition of the whiskers. In Al18B4O33w/6061Al composite, the peak-aged time is 2, 1.5 and 0.5 h at the aging temperature of 150, 170 and 190 C, respectively. In 10Al18B4O33w/CuO/6061Al composite, the peak-aged time is 6, 4
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Al18B4O33w prepared by a novel hydrothermal synthesis process, the reactions of CuO and molten 6061Al can improve the interfacial wettability during squeeze casting. Thus, the UTS of the composites increases gradually with increasing the contents of CuO coating on whiskers. However, the d of the composites initially increases and later decreases. In Al18B4O33w/6061Al composite, the d of the composite is low due to the existence of microcracks and holes in the composite. When the CuO coating contents is large, the d of the composite is also low due to the fracture of brittle phase MgAl2O4 with large sizes. Only when the CuO coating contents is suitable, whiskers can slide in the matrix by consuming grinding force during the tensile deformation. After T6 heat-treatment, the tensile properties of the composite are significantly increased due to the effects of solid-solution and aging strengthening. It is the important factors of the Mg content in the matrix alloy to determine the aging kinetics of composites. In Al18B4O33w/ 6061Al composite, the interfacial reaction between Al18B4O33w and Mg at high temperatures consumes Mg content of 6061Al matrix. Therefore, the amount of secondary phases needed to precipitate from the matrix is rather small, accelerating the peak-aged time. However, in 10Al18B4O33w/CuO/6061Al composite, continuous MgAl2O4 forms at the interface during squeeze casting and meanwhile it acts as an effective barrier layer to hinder the reaction of Al18B4O33w and Mg in the 6061Al matrix at high temperatures. Therefore, the Mg content in the 6061Al matrix is reserved and Cu is also introduced into the 6061Al matrix, which results in a longer time for the secondary phases to precipitate from the matrix and reach the peak-aged stage. 5. Conclusions Uniform CuO coating can be successfully prepared on the surface of Al18B4O33w by hydrothermal synthesis. CuO reacts with molten 6061Al during squeeze casting, which can improve the interfacial wettability and tensile properties of as-cast composites. Meanwhile, MgAl2O4 formed at the interface can effectively hinder further serious interfacial reactions between Al18B4O33w and Mg of the 6061Al matrix. With increasing the contents of CuO coating on whiskers, the UTS of the composites increases gradually. The d of the composites initially increases and later decreases. The UTS of the composite can be significantly improved after T6 heattreatment. The CuO coating of whiskers prolongs the peak-aged time of the composite. Acknowledgements
Fig. 11. Aging characteristics of 6061Al at different temperatures. (a) 150 C, (b) 170 C and (c) 190 C.
and 2.5 h at the aging temperature of 150, 170 and 190 C, respectively. This indicates after CuO coating of Al18B4O33w, the peak-aged time is significantly prolonged. 4. Discussion It is well known that the interface plays a significant role in determining mechanical properties of metal matrix composites. In as-cast Al18B4O33w/6061Al composite, the poor wettability between Al18B4O33w and 6061Al results in microholes in the composite during squeeze casting and interfacial debonding during tensile deformation. However, after uniform CuO coating of
This work is supported by the Natural Science Foundation of Heilongjiang Province (LC2015020), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (2015192), the Innovative Talent Fund of Harbin city (2016RAQXJ185) and Science Fund for the Young Innovative Talents of HUST (201306). References [1] G. Liu, W.C. Ren, Y.L. Sun, J. Hu, Damping behavior of Bi2O3-coated Al18B4O33 whisker-reinforced pure Al composite, Mater. Sci. Eng. A 527 (2010) 5136e5142. [2] P.T. Zhao, L.D. Wang, Z.M. Du, S.C. Xu, P.P. Jin, W.D. Fei, Low temperature extrusion of 6061 aluminum matrix composite reinforced with SnO2-coated Al18B4O33 whisker, Compos. A 43 (2012) 183e188. [3] L. Wang, L.D. Wang, W.D. Fei, Fractal analysis of fracture surfaces in aluminum borate whiskers-reinforced aluminum alloy 6061 composite, Trans. Nonferrous Metals Soc. China 21 (2011) 461e466. [4] H.T. Zhang, J. Cao, H. Lu, Reactive brazing of aluminum to aluminum-based composite reinforced with alumina borate whiskers with Cu interlayer, Vacuum 84 (2010) 474e477. [5] W.G. Wang, K. Matsugi, H. Fukushima, G. Sasaki, Interfacial reaction in AZ91D
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