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Tensile properties and fracture mechanisms of ZnO and ZnAl2O4-coated Al18B4O33 whisker reinforced aluminum composites
MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 5 4 2–5 4 7
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Tensile properties and fracture mechanisms of ZnO and ZnAl2O4-coated Al18B4O33 whisker reinforced aluminum composites H.Y. Yue⁎, Z.M. Yu, E.J. Guo, L.P. Wang, F.L. Sun School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, People's Republic of China
AR TIC LE D ATA
ABSTR ACT
Article history:
ZnO and ZnAl2O4-coated Al18B4O33 whisker reinforced pure aluminum composites were
Received 15 September 2009
fabricated by squeeze casting. The effects of the coating contents on the ultimate tensile
Received in revised form
strength (UTS), 0.2% proof stress (σ0.2) and elongation to fracture (δ) of the composites were
23 December 2009
investigated. The results show that the UTS of ZnO-coated Al18B4O33 whisker reinforced
Accepted 16 February 2010
aluminum composites increases almost linearly as the coating contents increase. However,
Keywords:
initially and increases later. The ZnAl2O4 coating of the whiskers is more favorable for an
the UTS of ZnAl2O4-coated Al18B4O33 whisker reinforced aluminum composites declines Whisker reinforced composites
increase in the σ0.2 of the composites. The δ of the composites obtains its maximum value
Coating
with an appropriate coating content. Fracture mechanisms of the composites were also
Tensile properties
investigated.
ZnO
© 2010 Elsevier Inc. All rights reserved.
ZnAl2O4
1.
Introduction
Al18B4O33 whisker reinforced aluminum composites (ABOw/Al) have attracted interest on account of their excellent mechanical properties and low costs [1,2]. Therefore, ABOw/Al composites will possess an optimistic commercial prospect in many fields, such as automobile and aerospace industries. The interface between the matrix and the reinforcement plays an important role in the mechanical properties of the composites [3,4]. In general, the interfacial wettability is poor in ABOw/Al composites [5]. Therefore, one of the most important issues is to improve the interfacial wettability of the composites. Reinforcement coating is one of the most effective techniques to improve interfacial wettability of composites [6]. Unfortunately, there are little researches on the coating of ABOw. Moreover, there are no adequate methods to deal with the dispersion of the coated whiskers. Although Ding et al. [7]
prepared Al2O3-coated-ABOw by a sol–gel process, ball-milling treatment was used to disperse the coated whiskers, resulting in a large amount of whisker fracture and severely affecting the mechanical properties of the composites. Therefore, it is necessary to adopt a better process for the dispersion of the coated whiskers. In our previous study [8], ZnO and ZnAl2O4 coatings of the ABOw were prepared by a sol–gel process and dispersed by an ultrasonic vibration technique. In this paper, the tensile properties and fracture mechanisms of ZnO and ZnAl2O4coated-ABOw reinforced aluminum composites are investigated and discussed.
2.
Experimental
The reinforcement was ABOw with a diameter of 0.5–1 μm and a length of 10–30 μm. ZnO and ZnAl2O4 coating of the ABOw
⁎ Corresponding author. Tel.: +86 451 86392556. E-mail address: [email protected] (H.Y. Yue). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.02.010
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 5 4 2 –5 4 7
Table 1 – Abbreviations of the composites. Initial mass ZnO-coated-ABOw/Al ZnAl2O4-coatedABOw/Al composites ratio of ZnO composites and ABOw 0 1:40 1:30 1:20 1:10
ABOw/Al 40ABOw/ZnO/Al 30ABOw/ZnO/Al 20ABOw/ZnO/Al 10ABOw/ZnO/Al
ABOw/Al 40ABOw/ZnAl2O4/Al 30ABOw/ZnAl2O4/Al 20ABOw/ZnAl2O4/Al 10ABOw/ZnAl2O4/Al
were prepared by a sol–gel method and dispersed by an ultrasonic dispersion technique. The detailed coating process can be found in our previous study [8]. The coated whiskers were dispersed in water by an ultrasonic vibration technique and then poured into a mould to obtain the preform. Then, the pure aluminum composites reinforced by ABOw with different coating contents were fabricated by squeeze casting. The mould-preheating temperature and molten aluminum-pouring temperature were 500 °C and 800 °C, respectively. The volume fraction of ABOw in the composites was about 20%. In order to investigate the effects of the coating contents on the mechanical properties and fracture mechanisms of the composites, the initial mass ratios of ZnO and ABOw were selected as 0, 1:40, 1:30, 1:20 and 1:10. The corresponding abbreviations of the composites are listed in Table 1. Interfacial microstructures of the composites were observed using a Philips CM-12 Transmission Electron Microscope (TEM) with an accelerating voltage of 120 kV. Specimens used for TEM observations were abraded to a thickness of about 30 μm and finally thinned by an ion-milling method. Phase compositions of the composites were characterized on a Philips X'Pert X-ray diffractometer employing a radiation of Cu Kα, a tube voltage of 40 kV and a tube current of 35 mA. Tensile tests were carried out on an Instron-1186 tension machine at room temperature with a tensile speed of 0.5 mm/min. Out of consideration for the scattering effect
543
that occurred in the composite, five specimens were tested. The tensile properties of each composite were the average values and the error bar represented the main variation of 5 tests for each point. Fractographs of the composites were examined by an S-4300 Scanning Electron Microscope (SEM) operating at 20 kV and the elemental compositions of the fractured particles on the fractographs of the composites were obtained using an Energy Dispersive X-ray Spectroscopy (EDS) analyzer linked to this instrument.
3.
Results
3.1.
Interfacial Microstructures of the Composites
TEM photographs of 10ABOw/ZnO/Al and 10ABOw/ZnAl2O4/Al composites are given in Fig. 1. It can be seen that the interface between the whisker and Al matrix is made up of about 30–50 and 50–100 nm discontinuous particles for 10ABOw/ZnO/Al and 10ABOw/ZnAl2O4/Al composites, respectively. The insert in Fig. 1a indicates that the interfacial phase is γ-Al2O3, which suggests that ZnO reacts with the molten aluminum during squeeze casting. The interfacial reaction equation can be written as follows [9]: 3ZnO þ 2Al → γ Al2 O3 þ 3Zn þ Q
ð1Þ
where, + Q represents an exothermic reaction (QT = − 637,820 + 13.4T [10], Q1000K = 624.4 KJ/mol). The interfacial reaction can release a large amount of heat during squeeze casting, resulting in a significant increase in the local temperature near the interface. So the weaker interfacial bonding strength between ABOw and Al can be improved by the heating effect [11–13]. In the present study, the amount of Zn produced by the reaction is very little and Zn can easily dissolve into the Al matrix at high temperature according to the Al–Zn phase diagram [14]. Hence, when the composites are cooled down from high temperature, it is very difficult to find Zn. The insert in Fig. 1b shows that the interfacial phase is ZnAl2O4, which
Fig. 1 – TEM photographs of the composites: (a) 10ABOw/ZnO/Al; (b) 10ABOw/ZnAl2O4/Al.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 5 4 2–5 4 7
suggests that no interfacial reaction between ZnAl2O4 and Al takes place during squeeze casting.
3.2.
X-ray Diffraction (XRD) Analysis of the Composites
Fig. 2 shows the XRD patterns of ABOw/Al, 10ABOw/ZnO/Al and 10ABOw/ZnAl2O4/Al composites. It can be seen that only the diffraction peaks of ABOw and Al can be found in both ABOw/Al and 10ABOw/ZnO/Al composites. Moreover, the peak position of Al with the strongest diffraction (at about 38.5°) in Fig. 2b has shifted slightly to a higher angle compared with that in Fig. 2a, as shown in the insert of Fig. 2. This may be the reason that Zn dissolves into Al matrix during squeeze casting. It is commonly known that the atomic radius of Zn is smaller than that of Al (rZn = 0.1332 nm, rAl = 0.1432 nm) [15]. When some of Al atoms in the lattice are replaced by Zn, the resultant Al(Zn) solid solution will have a smaller lattice parameter, so the diffraction peak position will shift to a higher angle [9]. Meanwhile, the amount of γ-Al2O3 produced is little, so it cannot be detected by the method of XRD. However, in the XRD patterns of 10ABOw/ZnAl2O4/Al composite, the diffraction peaks of ZnAl2O4 are also found besides those of ABOw and Al. This suggests that ZnAl2O4 cannot react with molten Al during squeeze casting. These results are in accordance with the results of TEM analysis.
Fig. 3 – Effects of the coating contents on the σ0.2 of ZnO and ZnAl2O4-coated-ABOw/Al composites.
Fig. 3 shows the effects of the coating contents on the σ0.2 of ZnO and ZnAl2O4-coated-ABOw reinforced aluminum composites (ZnO and ZnAl2O4-coated-ABOw/Al). It can be seen that the σ0.2 of the composites increases as the coating contents increase. When the coating content is small, the increase ratio of σ0.2 is basically the same for both composites. However, when the coating content is large, it can be seen that the ZnAl2O4 coating of ABOw is more effective for the increase of σ0.2. Fig. 4 shows the effects of the coating contents on the UTS of ZnO and ZnAl2O4-coated-ABOw/Al composites. For ZnOcoated-ABOw/Al composites, the UTS of the composites in-
creases almost linearly as the ZnO coating contents increase. The UTS of ABOw/Al composite is 260 MPa, however, the UTS of 10ABOw/ZnO/Al composite reaches 326 MPa. For ZnAl2O4coated-ABOw/Al composites, the UTS of the composites declines initially and then increases as the ZnAl2O4 coating content increases. The UTS of 40ABOw/ZnAl2O4/Al composite is the lowest (250 MPa) and the UTS of 10ABOw/ZnAl2O4/Al composite is the highest (292 MPa). Therefore, the ZnO coating of ABOw is more effective for the increase of UTS. Fig. 5 shows the effects of the coating contents on the δ of ZnO and ZnAl2O4-coated-ABOw/Al composites. It can be seen that the δ of the composites increases initially and declines later as the coating contents increase. This indicates that the coating contents significantly affect the δ of the composites. For ZnO-coated-ABOw/Al composites, the δ of 40ABOw/ZnO/Al composite is the highest (8.1%); for ZnAl2O4-coated-ABOw/Al composites, the δ of 30ABOw/ZnAl2O4/Al composite is the highest (7.9%). Fractographs of the composites and EDS results of the fractured particles are shown in Fig. 6. As shown by the arrows in Fig. 6a, microcracks and microholes can be found on the surface of the fractograph in ABOw/Al composite. This may be
Fig. 2 – XRD patterns of the composites: (a) ABOw/Al; (b) 10ABOw/ZnO/Al; (c) 10ABOw/ZnAl2O4/Al.
Fig. 4 – Effects of the coating contents on the UTS of ZnO and ZnAl2O4-coated-ABOw/Al composites.
3.3. Effects of the Coating Contents on the Tensile Properties and Fractographs of the Composites
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 5 4 2 –5 4 7
545
resulted from the unfilled regions of molten aluminum caused by poor wettability during squeeze casting or interfacial debonding between the whiskers and matrix during the tensile deformation. In 40ABOw/ZnO/Al and 30ABOw/ ZnAl2O4/Al composites, aluminum matrix dimples with large sizes and amount can be clearly seen, and the holes due to the pull-out of the whiskers are relatively rough. In 10ABOw/ZnO/ Al and 10ABOw/ZnAl2O4/Al composites, many small fractured particles can be observed at the interface. The EDS results of the fractured particles in Fig. 6d and e are shown in Fig. 6f. It can be deduced that they are separately γ-Al2O3 (in Fig. 6d) and ZnAl2O4 (in Fig. 6e) according to the EDS results and the above TEM analysis. Also note that the amount of fractured whiskers in 10ABOw/ZnAl2O4/Al composite is larger than that in 10ABOw/ZnO/Al composite. Fig. 5 – Effects of the coating contents on the δ of ZnO and ZnAl2O4-coated-ABOw/Al composites.
Fig. 6 – Fractographs of the composites and EDS results of the fractured particles: (a) ABOw/Al; (b) 40ABOw/ZnO/Al; (c) 30ABOw/ZnAl2O4/Al; (d) 10ABOw/ZnO/Al; (e) 10ABOw/ZnAl2O4/Al; (f) EDS of the fractured particles in d and e.
546
4.
MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 5 4 2–5 4 7
Discussion
The tensile properties and fracture mechanisms of the composites can be analyzed according to the above experimental results. It is known that there are two kinds of wettability mechanisms: physical wettability and chemical wettability. And the latter is more effective than the former for the increase of wettability [16–20]. The interfacial bonding strength is weak in ABOw/Al composites [5]. According to the wettability mechanisms, ZnAl2O4 nano-particles with large specific areas can improve the interfacial wettability, which belongs to physical wettability. ZnO can react with molten aluminum during squeeze casting to improve the wettablity, which belongs to chemical wettability. The increase of wettability is favorable for the increase of interfacial bonding strength [21,22]. Thus, the σ0.2 and UTS of the composites should increase as the coating contents increase. The σ0.2 of the composites increases as the coating contents increase due to the improvement of interfacial wettability. Although the contribution of ZnAl2O4 nano-particles to interfacial wettability is worse than that of ZnO, the σ0.2 value of ZnAl2O4-coated-ABOw/Al composites is higher than that of ZnO-coated-ABOw/Al composites. This may relate to the sizes of interfacial phases besides the interfacial wettability. Interfacial phases can form many steps at the interface according to Fig. 1. ABOw and interfacial phases do not fracture under a small tensile deformation (0.2%) of the composites, so the steps can provide an additional load transfer from the matrix to the whiskers. The size of ZnAl2O4 is obviously larger than that of γ-Al2O3, which can also be viewed as a contributing factor to its higher σ0.2 value. For ZnO-coated-ABOw/Al composites, the UTS of the composites increases as the coating contents increase. This attributes to three reasons: the improvement of interfacial wettablity, the solid solution strengthening effect of the aluminum matrix dissolved by Zn and the preservation of ABOw integrity. ZnO do not react with ABOw during the preparation of ZnO-coated-ABOw preform, so the reaction between ZnO and molten aluminum do not destroy the integrity of ABOw during squeeze casting. However, for ZnAl2O4-coated-ABOw/Al composites, the UTS shows a drop before increasing as the coating contents increase. This may relate to the integrity of ABOw. The integrity of ABOw is destroyed due to the reaction between ABOw and ZnO during the preparation of ZnAl2O4 coated-ABOw preform [8]. When the ZnAl2O4 coating content is small, the contribution of ZnAl2O4 nano-particles to the wettability may be smaller than the destruction to the ABOw integrity, so the UTS of the composites decreases initially. Only when the coating content is large enough, the contribution of ZnAl2O4 nano-particles to the wettability is larger than the destruction to the integrity of ABOw, thus, the UTS of the composites can be enhanced effectively. Moreover, the sizes of ZnAl2O4 nano-particles at the interface are larger than those of γ-Al2O3, causing more fracture of ABOw during the tensile deformation of the composites. So, the UTS of ZnAl2O4-coated-ABOw/Al composites is lower than that of ZnO-coated-ABOw/Al composites. The δ of the composites greatly relies on the coating contents. In ABOw/Al composite, the interfacial strength is
weak, so ABOw can be easily pulled out from aluminum matrix and form many microcracks and microholes, resulting in a low δ value of the composite. When the coating content is large enough, interphases with large sizes produce the stress concentration and fracture of ABOw as well as brittle particles at the interface during the tensile deformation, as shown in Fig. 6b and c, hence the δ value of the composite decreases. Only when the coating content is appropriate, the coated whiskers can slide in the matrix and consume the grinding force during the tensile deformation of the composites so that the δ of the composites reaches the maximum value.
5.
Conclusions
During squeeze casting, ZnO reacts with molten aluminum and ZnAl2O4 nano-particles have large specific areas, which both can improve the wettability of the whiskers by molten aluminum. The σ0.2 of the composites increases as the coating contents increase. And the ZnAl2O4 coating of ABOw is more favorable for an increase in the σ0.2 of the composites, especially when its coating content is large. As the coating contents increase, the UTS of ZnO-coated-ABOw/Al composites shows a nearly linear increase. However, the UTS of ZnAl2O4-coated-ABOw/Al composites shows a drop before increasing gradually. The δ of the composites increases initially and declines later as the coating contents increase. For both analyzed composites, the δ value reaches the maximum in 40ABOw/ZnO/Al and 30ABOw/ZnAl2O4/Al composites, respectively.
Acknowledgement This work was supported by the National High Technology Research and Development Program of China.
REFERENCES [1] Tu JP, Matsumura M. Interfacial reaction effects on erosion of aluminum matrix composites. Scr Mater 1999;40:645–50. [2] Zhu SJ, Iizuka T. Fatigue behavior of Al18B4O33 whisker-framework reinforced Al matrix composites at high temperatures. Comp Sci Tech 2003;63:265–71. [3] Wang YQ, Zhou BL. Behaviour of coatings on reinforcements in some metal matrix composites. Composites 1996;A27: 1139–45. [4] Zheng MY, Wu K, Ling M, Kamado S, Kojima Y. The effect of thermal exposure on the interface and mechanical properties of Al18B4O33w/AZ91 magnesium matrix composite. Mater Sci Eng 2004;A372:66–74. [5] Fei WD, Jiang XD, Li C, Yao CK. Effect of interfacial reaction on fracture behaviour aluminium borate whisker reinforced aluminium composite. Mater Sci Technol 1997;13:918–22. [6] Narciso J, Alonso A, Pamies A, Garcia-Cordovilla C, Louis E. Wettability of binary and ternary alloys of the system Al–Si–Mg with SiC particulates. Scr Metall Mater 1994;31: 1495–500. [7] Ding DY, Wang JN, Ning CQ, Dai KR. On the thermal stability of coating in an Al18B4O33w/α–Al2O3/6061Al composite. Mater Sci Eng 2003;A358:159–63.
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[8] Yue HY, Fei WD, Li ZJ, Wang LD. Sol–gel process of ZnO and ZnAl2O4 coated aluminum borate whiskers. J Sol-Gel Sci Technol 2007;44:259–62. [9] Yu P, Deng CJ, Ma NG, Ng Dickon HL. A new method of producing uniformly distributed alumina particles in Al-based metal matrix composite. Mater Lett 2004;58:679–82. [10] Rao YK. Stoichiometry and thermodynamics of metallurgical processes. Cambridge University Press; 1985. [11] Zhou XB, De Hosson JThM. Microstructure and interfaces of a reaction coating on aluminium alloys by laser processing. J Phys IV 1993;3:1007–11. [12] Fei WD, Li YB. Effect of NiO coating of whisker on tensile strength of aluminum borate whisker-reinforced aluminum composite. Mater Sci Eng 2004;A379:27–32. [13] Mortensen A, Hodaj F, Eustathopoulos N. On thermal effects in reactive wetting. Scr Mater 1998;38:1411–7. [14] Mondolfo LF. Aluminum alloys: structure and properties. Butterworths London-Boston Press; 1976. p. 400. [15] Askeland DR. The science and engineering of materials. Boston: PWS; 1994. p. 796.
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[16] Eustathopoulos N, Nicholas M, Drevet B. Wettability at high temperatures Pergamon Materials Series 3; 1999. [17] Zhou XB, De Hosson JThM. Reactive wetting of liquid metals on ceramic substrates. Acta Mater 1996;44:421–6. [18] Shen P, Fujii H, Matsumoto T, Nogi K. Critical factors affecting the wettability of α-alumina by molten aluminum. J Am Ceram Soc 2004;87:1265–73. [19] OH SY, Cornie JA, Russel KC. Wetting of ceramic particulates with liquid aluminum alloys: Part II. Study of wettability. Metall Trans 1989;20A:533–41. [20] Landry K, Eustathopoulos N. Dynamics of wetting in reactive metal/ceramic systems: linear spreading. Acta Mater 1996;44: 3923–32. [21] Shorowordi KM, Laoui T, Haseeb ASMA, Celis JP, Froyen L. Microstructure and interface characteristics of B4C, SiC and Al2O3 reinforced Al matrix composites: a comparative study. J Mater Process Technol 2003;142:738–43. [22] Peteves SD, Paulasto M, Ceccone G, Stamos V. The reactive route to ceramic joining: fabrication, interfacial chemistry and joint properties. Acta Mater 1998;46:2407–14.
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Tensile properties and fracture mechanisms of ZnO and ZnAl2O4-coated Al18B4O33 whisker reinforced aluminum composites H.Y. Yue⁎, Z.M. Yu, E.J. Guo, L.P. Wang, F.L. Sun School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, People's Republic of China
AR TIC LE D ATA
ABSTR ACT
Article history:
ZnO and ZnAl2O4-coated Al18B4O33 whisker reinforced pure aluminum composites were
Received 15 September 2009
fabricated by squeeze casting. The effects of the coating contents on the ultimate tensile
Received in revised form
strength (UTS), 0.2% proof stress (σ0.2) and elongation to fracture (δ) of the composites were
23 December 2009
investigated. The results show that the UTS of ZnO-coated Al18B4O33 whisker reinforced
Accepted 16 February 2010
aluminum composites increases almost linearly as the coating contents increase. However,
Keywords:
initially and increases later. The ZnAl2O4 coating of the whiskers is more favorable for an
the UTS of ZnAl2O4-coated Al18B4O33 whisker reinforced aluminum composites declines Whisker reinforced composites
increase in the σ0.2 of the composites. The δ of the composites obtains its maximum value
Coating
with an appropriate coating content. Fracture mechanisms of the composites were also
Tensile properties
investigated.
ZnO
© 2010 Elsevier Inc. All rights reserved.
ZnAl2O4
1.
Introduction
Al18B4O33 whisker reinforced aluminum composites (ABOw/Al) have attracted interest on account of their excellent mechanical properties and low costs [1,2]. Therefore, ABOw/Al composites will possess an optimistic commercial prospect in many fields, such as automobile and aerospace industries. The interface between the matrix and the reinforcement plays an important role in the mechanical properties of the composites [3,4]. In general, the interfacial wettability is poor in ABOw/Al composites [5]. Therefore, one of the most important issues is to improve the interfacial wettability of the composites. Reinforcement coating is one of the most effective techniques to improve interfacial wettability of composites [6]. Unfortunately, there are little researches on the coating of ABOw. Moreover, there are no adequate methods to deal with the dispersion of the coated whiskers. Although Ding et al. [7]
prepared Al2O3-coated-ABOw by a sol–gel process, ball-milling treatment was used to disperse the coated whiskers, resulting in a large amount of whisker fracture and severely affecting the mechanical properties of the composites. Therefore, it is necessary to adopt a better process for the dispersion of the coated whiskers. In our previous study [8], ZnO and ZnAl2O4 coatings of the ABOw were prepared by a sol–gel process and dispersed by an ultrasonic vibration technique. In this paper, the tensile properties and fracture mechanisms of ZnO and ZnAl2O4coated-ABOw reinforced aluminum composites are investigated and discussed.
2.
Experimental
The reinforcement was ABOw with a diameter of 0.5–1 μm and a length of 10–30 μm. ZnO and ZnAl2O4 coating of the ABOw
⁎ Corresponding author. Tel.: +86 451 86392556. E-mail address: [email protected] (H.Y. Yue). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.02.010
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 5 4 2 –5 4 7
Table 1 – Abbreviations of the composites. Initial mass ZnO-coated-ABOw/Al ZnAl2O4-coatedABOw/Al composites ratio of ZnO composites and ABOw 0 1:40 1:30 1:20 1:10
ABOw/Al 40ABOw/ZnO/Al 30ABOw/ZnO/Al 20ABOw/ZnO/Al 10ABOw/ZnO/Al
ABOw/Al 40ABOw/ZnAl2O4/Al 30ABOw/ZnAl2O4/Al 20ABOw/ZnAl2O4/Al 10ABOw/ZnAl2O4/Al
were prepared by a sol–gel method and dispersed by an ultrasonic dispersion technique. The detailed coating process can be found in our previous study [8]. The coated whiskers were dispersed in water by an ultrasonic vibration technique and then poured into a mould to obtain the preform. Then, the pure aluminum composites reinforced by ABOw with different coating contents were fabricated by squeeze casting. The mould-preheating temperature and molten aluminum-pouring temperature were 500 °C and 800 °C, respectively. The volume fraction of ABOw in the composites was about 20%. In order to investigate the effects of the coating contents on the mechanical properties and fracture mechanisms of the composites, the initial mass ratios of ZnO and ABOw were selected as 0, 1:40, 1:30, 1:20 and 1:10. The corresponding abbreviations of the composites are listed in Table 1. Interfacial microstructures of the composites were observed using a Philips CM-12 Transmission Electron Microscope (TEM) with an accelerating voltage of 120 kV. Specimens used for TEM observations were abraded to a thickness of about 30 μm and finally thinned by an ion-milling method. Phase compositions of the composites were characterized on a Philips X'Pert X-ray diffractometer employing a radiation of Cu Kα, a tube voltage of 40 kV and a tube current of 35 mA. Tensile tests were carried out on an Instron-1186 tension machine at room temperature with a tensile speed of 0.5 mm/min. Out of consideration for the scattering effect
543
that occurred in the composite, five specimens were tested. The tensile properties of each composite were the average values and the error bar represented the main variation of 5 tests for each point. Fractographs of the composites were examined by an S-4300 Scanning Electron Microscope (SEM) operating at 20 kV and the elemental compositions of the fractured particles on the fractographs of the composites were obtained using an Energy Dispersive X-ray Spectroscopy (EDS) analyzer linked to this instrument.
3.
Results
3.1.
Interfacial Microstructures of the Composites
TEM photographs of 10ABOw/ZnO/Al and 10ABOw/ZnAl2O4/Al composites are given in Fig. 1. It can be seen that the interface between the whisker and Al matrix is made up of about 30–50 and 50–100 nm discontinuous particles for 10ABOw/ZnO/Al and 10ABOw/ZnAl2O4/Al composites, respectively. The insert in Fig. 1a indicates that the interfacial phase is γ-Al2O3, which suggests that ZnO reacts with the molten aluminum during squeeze casting. The interfacial reaction equation can be written as follows [9]: 3ZnO þ 2Al → γ Al2 O3 þ 3Zn þ Q
ð1Þ
where, + Q represents an exothermic reaction (QT = − 637,820 + 13.4T [10], Q1000K = 624.4 KJ/mol). The interfacial reaction can release a large amount of heat during squeeze casting, resulting in a significant increase in the local temperature near the interface. So the weaker interfacial bonding strength between ABOw and Al can be improved by the heating effect [11–13]. In the present study, the amount of Zn produced by the reaction is very little and Zn can easily dissolve into the Al matrix at high temperature according to the Al–Zn phase diagram [14]. Hence, when the composites are cooled down from high temperature, it is very difficult to find Zn. The insert in Fig. 1b shows that the interfacial phase is ZnAl2O4, which
Fig. 1 – TEM photographs of the composites: (a) 10ABOw/ZnO/Al; (b) 10ABOw/ZnAl2O4/Al.
544
MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 5 4 2–5 4 7
suggests that no interfacial reaction between ZnAl2O4 and Al takes place during squeeze casting.
3.2.
X-ray Diffraction (XRD) Analysis of the Composites
Fig. 2 shows the XRD patterns of ABOw/Al, 10ABOw/ZnO/Al and 10ABOw/ZnAl2O4/Al composites. It can be seen that only the diffraction peaks of ABOw and Al can be found in both ABOw/Al and 10ABOw/ZnO/Al composites. Moreover, the peak position of Al with the strongest diffraction (at about 38.5°) in Fig. 2b has shifted slightly to a higher angle compared with that in Fig. 2a, as shown in the insert of Fig. 2. This may be the reason that Zn dissolves into Al matrix during squeeze casting. It is commonly known that the atomic radius of Zn is smaller than that of Al (rZn = 0.1332 nm, rAl = 0.1432 nm) [15]. When some of Al atoms in the lattice are replaced by Zn, the resultant Al(Zn) solid solution will have a smaller lattice parameter, so the diffraction peak position will shift to a higher angle [9]. Meanwhile, the amount of γ-Al2O3 produced is little, so it cannot be detected by the method of XRD. However, in the XRD patterns of 10ABOw/ZnAl2O4/Al composite, the diffraction peaks of ZnAl2O4 are also found besides those of ABOw and Al. This suggests that ZnAl2O4 cannot react with molten Al during squeeze casting. These results are in accordance with the results of TEM analysis.
Fig. 3 – Effects of the coating contents on the σ0.2 of ZnO and ZnAl2O4-coated-ABOw/Al composites.
Fig. 3 shows the effects of the coating contents on the σ0.2 of ZnO and ZnAl2O4-coated-ABOw reinforced aluminum composites (ZnO and ZnAl2O4-coated-ABOw/Al). It can be seen that the σ0.2 of the composites increases as the coating contents increase. When the coating content is small, the increase ratio of σ0.2 is basically the same for both composites. However, when the coating content is large, it can be seen that the ZnAl2O4 coating of ABOw is more effective for the increase of σ0.2. Fig. 4 shows the effects of the coating contents on the UTS of ZnO and ZnAl2O4-coated-ABOw/Al composites. For ZnOcoated-ABOw/Al composites, the UTS of the composites in-
creases almost linearly as the ZnO coating contents increase. The UTS of ABOw/Al composite is 260 MPa, however, the UTS of 10ABOw/ZnO/Al composite reaches 326 MPa. For ZnAl2O4coated-ABOw/Al composites, the UTS of the composites declines initially and then increases as the ZnAl2O4 coating content increases. The UTS of 40ABOw/ZnAl2O4/Al composite is the lowest (250 MPa) and the UTS of 10ABOw/ZnAl2O4/Al composite is the highest (292 MPa). Therefore, the ZnO coating of ABOw is more effective for the increase of UTS. Fig. 5 shows the effects of the coating contents on the δ of ZnO and ZnAl2O4-coated-ABOw/Al composites. It can be seen that the δ of the composites increases initially and declines later as the coating contents increase. This indicates that the coating contents significantly affect the δ of the composites. For ZnO-coated-ABOw/Al composites, the δ of 40ABOw/ZnO/Al composite is the highest (8.1%); for ZnAl2O4-coated-ABOw/Al composites, the δ of 30ABOw/ZnAl2O4/Al composite is the highest (7.9%). Fractographs of the composites and EDS results of the fractured particles are shown in Fig. 6. As shown by the arrows in Fig. 6a, microcracks and microholes can be found on the surface of the fractograph in ABOw/Al composite. This may be
Fig. 2 – XRD patterns of the composites: (a) ABOw/Al; (b) 10ABOw/ZnO/Al; (c) 10ABOw/ZnAl2O4/Al.
Fig. 4 – Effects of the coating contents on the UTS of ZnO and ZnAl2O4-coated-ABOw/Al composites.
3.3. Effects of the Coating Contents on the Tensile Properties and Fractographs of the Composites
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resulted from the unfilled regions of molten aluminum caused by poor wettability during squeeze casting or interfacial debonding between the whiskers and matrix during the tensile deformation. In 40ABOw/ZnO/Al and 30ABOw/ ZnAl2O4/Al composites, aluminum matrix dimples with large sizes and amount can be clearly seen, and the holes due to the pull-out of the whiskers are relatively rough. In 10ABOw/ZnO/ Al and 10ABOw/ZnAl2O4/Al composites, many small fractured particles can be observed at the interface. The EDS results of the fractured particles in Fig. 6d and e are shown in Fig. 6f. It can be deduced that they are separately γ-Al2O3 (in Fig. 6d) and ZnAl2O4 (in Fig. 6e) according to the EDS results and the above TEM analysis. Also note that the amount of fractured whiskers in 10ABOw/ZnAl2O4/Al composite is larger than that in 10ABOw/ZnO/Al composite. Fig. 5 – Effects of the coating contents on the δ of ZnO and ZnAl2O4-coated-ABOw/Al composites.
Fig. 6 – Fractographs of the composites and EDS results of the fractured particles: (a) ABOw/Al; (b) 40ABOw/ZnO/Al; (c) 30ABOw/ZnAl2O4/Al; (d) 10ABOw/ZnO/Al; (e) 10ABOw/ZnAl2O4/Al; (f) EDS of the fractured particles in d and e.
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4.
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Discussion
The tensile properties and fracture mechanisms of the composites can be analyzed according to the above experimental results. It is known that there are two kinds of wettability mechanisms: physical wettability and chemical wettability. And the latter is more effective than the former for the increase of wettability [16–20]. The interfacial bonding strength is weak in ABOw/Al composites [5]. According to the wettability mechanisms, ZnAl2O4 nano-particles with large specific areas can improve the interfacial wettability, which belongs to physical wettability. ZnO can react with molten aluminum during squeeze casting to improve the wettablity, which belongs to chemical wettability. The increase of wettability is favorable for the increase of interfacial bonding strength [21,22]. Thus, the σ0.2 and UTS of the composites should increase as the coating contents increase. The σ0.2 of the composites increases as the coating contents increase due to the improvement of interfacial wettability. Although the contribution of ZnAl2O4 nano-particles to interfacial wettability is worse than that of ZnO, the σ0.2 value of ZnAl2O4-coated-ABOw/Al composites is higher than that of ZnO-coated-ABOw/Al composites. This may relate to the sizes of interfacial phases besides the interfacial wettability. Interfacial phases can form many steps at the interface according to Fig. 1. ABOw and interfacial phases do not fracture under a small tensile deformation (0.2%) of the composites, so the steps can provide an additional load transfer from the matrix to the whiskers. The size of ZnAl2O4 is obviously larger than that of γ-Al2O3, which can also be viewed as a contributing factor to its higher σ0.2 value. For ZnO-coated-ABOw/Al composites, the UTS of the composites increases as the coating contents increase. This attributes to three reasons: the improvement of interfacial wettablity, the solid solution strengthening effect of the aluminum matrix dissolved by Zn and the preservation of ABOw integrity. ZnO do not react with ABOw during the preparation of ZnO-coated-ABOw preform, so the reaction between ZnO and molten aluminum do not destroy the integrity of ABOw during squeeze casting. However, for ZnAl2O4-coated-ABOw/Al composites, the UTS shows a drop before increasing as the coating contents increase. This may relate to the integrity of ABOw. The integrity of ABOw is destroyed due to the reaction between ABOw and ZnO during the preparation of ZnAl2O4 coated-ABOw preform [8]. When the ZnAl2O4 coating content is small, the contribution of ZnAl2O4 nano-particles to the wettability may be smaller than the destruction to the ABOw integrity, so the UTS of the composites decreases initially. Only when the coating content is large enough, the contribution of ZnAl2O4 nano-particles to the wettability is larger than the destruction to the integrity of ABOw, thus, the UTS of the composites can be enhanced effectively. Moreover, the sizes of ZnAl2O4 nano-particles at the interface are larger than those of γ-Al2O3, causing more fracture of ABOw during the tensile deformation of the composites. So, the UTS of ZnAl2O4-coated-ABOw/Al composites is lower than that of ZnO-coated-ABOw/Al composites. The δ of the composites greatly relies on the coating contents. In ABOw/Al composite, the interfacial strength is
weak, so ABOw can be easily pulled out from aluminum matrix and form many microcracks and microholes, resulting in a low δ value of the composite. When the coating content is large enough, interphases with large sizes produce the stress concentration and fracture of ABOw as well as brittle particles at the interface during the tensile deformation, as shown in Fig. 6b and c, hence the δ value of the composite decreases. Only when the coating content is appropriate, the coated whiskers can slide in the matrix and consume the grinding force during the tensile deformation of the composites so that the δ of the composites reaches the maximum value.
5.
Conclusions
During squeeze casting, ZnO reacts with molten aluminum and ZnAl2O4 nano-particles have large specific areas, which both can improve the wettability of the whiskers by molten aluminum. The σ0.2 of the composites increases as the coating contents increase. And the ZnAl2O4 coating of ABOw is more favorable for an increase in the σ0.2 of the composites, especially when its coating content is large. As the coating contents increase, the UTS of ZnO-coated-ABOw/Al composites shows a nearly linear increase. However, the UTS of ZnAl2O4-coated-ABOw/Al composites shows a drop before increasing gradually. The δ of the composites increases initially and declines later as the coating contents increase. For both analyzed composites, the δ value reaches the maximum in 40ABOw/ZnO/Al and 30ABOw/ZnAl2O4/Al composites, respectively.
Acknowledgement This work was supported by the National High Technology Research and Development Program of China.
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