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Correlation between ceramic additions and compressive properties of Zn–22Al matrix composite foams
Journal of Alloys and Compounds 476 (2009) 220–225
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Correlation between ceramic additions and compressive properties of Zn–22Al matrix composite foams Jiaan Liu a , Sirong Yu a,∗ , Xianyong Zhu b , Ming Wei a , Yanru Luo b , Yaohui Liu a a Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun 130025, China b Institute of Mechanical Science and Engineering, Jilin University, Changchun 130025, China
a r t i c l e
i n f o
Article history: Received 18 July 2008 Received in revised form 8 September 2008 Accepted 12 September 2008 Available online 1 November 2008 Keywords: Metal matrix composites Zn–22Al alloy Ceramics Microstructure Compressive behavior
a b s t r a c t The compressive behaviors of Zn–22Al composite foams using SiC particles as reinforcement and stabilizing agent (ZA22/SiCp composite foams) were investigated in this study. To observe the deformation mechanisms, the deformation processes of the composite foams were recorded, and the cracks under the compressive load were observed by scanning electron microscope (SEM). Owing to the presence of the particles, ZA22/SiCp composite foams show more brittle compressive behavior with a large stress fluctuation than Zn–22Al alloy foams (ZA22 foams). The compressive processes of composite foams are characterized by the formation and propagation of the localized deformation band. The photomicrographs of the cracks indicate that a complicated effect of SiC particles on the compressive properties of the foams result in a high compressive strength. As a consequence, ZA22/SiCp composite foams exhibit slightly high energy absorption capacities, although low energy absorption efficiencies, as compared with ZA22 foams. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Metallic foams have received extensive interest as new structural and functional materials because of their energy absorption, flame resistance, vibration reduction, and acoustic insulation [1]. In particular, they can absorb most of the plastic deformation energy at low constant stress during compression. They are adopted for protective aim in traffic and packing industry [2,3]. Therefore, the compressive property is an important characteristic of metallic foams. Generally, the compressive properties of metallic foams depend on the property of cell wall material [4–7], the relative density [1–3], the cell structure (e.g., shape, orientation and defect of cell) [8,9], and the type of load (e.g., static or dynamic, free or constrained compression) [10,11]. Secondary phase or component in cell wall materials is an important factor affecting the mechanical response of the foams [5,11–17]. SiC particles commonly utilized as the stabilizing agent for the fabrication of metallic foams can dramatically change the mechanical properties of metallic foams [5,14–17]. Parkash et al. [14] found that the localized deformation took place in Al/SiCp composite foams due to SiCp dispersed in cell structure. Yu et al. [5]
also denoted that the presences of SiC particles in Zn–22Al/10 vol.% SiCp composite foams resulted in a large compressive stress fluctuation. Moreover, the studies on the composite foams fabricated by powder metallurgy process also demonstrated that the composite foams showed a brittle deformation behavior and a slight degradation in the compressive properties as compared with Al alloy foams [15,16]. However, the studies on the correlation between ceramic particles and compressive property of metallic foams, especially on the nucleation and propagation of cracks, are limited. Therefore, in present study, the deformation process and the cracks of ZA22/SiCp composite foams were observed in order to understand the compressive deformation mechanisms of composite foams. 2. Experimental 2.1. Raw materials The raw materials for preparing composite foams included ZA22 alloy (22.0 wt.% Al, 1.0 wt.% Cu, 0.03 wt.% Mg, and Zn balance), SiC particles (98.0 wt.% in purity, about 28 m in size), and CaCO3 powders (99.5 wt.% in purity). SiC particles and CaCO3 powders were used as stabilizing agent and blowing agent, respectively. To improve the wettability between SiC particles and ZA22 melt, SiC particles were heat-treated at 930 ◦ C for 6 h and at 420 ◦ C for 2 h, respectively. 2.2. Fabrication of composite foams
∗ Corresponding author. Tel.: +86 431 85095862; fax: +86 431 85095876. E-mail addresses: [email protected], [email protected] (S. Yu). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.069
Zn–22Al composite slurry with 7 vol.% SiCp was prepared by conventional stir-casting technique [18], and then CaCO3 powders were added into the melt.
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221
Afterwards, the composite slurry was stirred for 2 min with a steel stirrer at a rate of 900 r/min and then held at 700–720 ◦ C to allow the blowing agent to release gas bubbles. 2.3. Compression test Size effect is an important issue in the mechanical testing of metallic foams [19–21]. That means the value of the specimen size relative to the mean cell size has great influence on the mechanical properties of the metallic foams. The specimen with insufficient number of cells (<6) can cause a significant loss of the mechanical properties. When a specimen contains more than 6 cells, the size effects can be ignored [19,20]. Therefore, the large specimens used for compression test are favoring for eliminating the size effects. But large specimens are discommodious for observation in SEM. A small specimen is required for the observations on the microstructure failure. Song et al. [21] found that the failure process of a specimen with insufficient cell is homologous to that of a specimen with many cells. Both of them are defect-directed or weakness-directed processes. That means that the failure mechanisms of small and large specimens are homologous. Therefore, in this work, we selected the large specimen used for compression test and the small specimen used for the observation on the microstructure failure. Specimens with the large dimensions of 15 mm × 15 mm × 30 mm were prepared for the compressive test in order to eliminate size effects [5]. The compressive tests were carried out on a universal mechanical testing machine at a nominal strain rate of 2.2 × 10−3 s−1 . The obtained data were used for drawing the compressive stress–strain curves. The deformation processes of the large foam samples were recorded by a numeral camera to study the deformation mechanism of the macrostructure in the composite foams. The rectangular sections of small foam samples (10 mm × 10 mm × 12 mm) were polished, and then the samples were compressed at certain strain. Afterwards, the polished sections of the samples were observed immediately without further metallographic treatment. 2.4. Characterization of composite foams The cracks of the deformed foams were observed by means of scanning electron microscopy (SEM) (Model JSM-5310, Japan). The porosities of the composite foams were calculated using the following formula [2]:
P=
1−
∗ s
× 100%
(1)
where P is the porosity of the composite foams, * and s are the densities of the composite foams and the cell wall material, respectively, and (* /s ), which is called the relative density of the composite foams, indicates the ratio of the density of the composite foams to the density of cell wall material.
Fig. 1. Compressive stress–strain curves of (a) ZA22 foams and (b) ZA22/SiCp composite foams.
3. Results and discussion 3.1. Compressive curves of the foams Fig. 1 shows the compressive stress–strain curves of ZA22 foams and ZA22/SiCp composite foams with various relative densities. It can be seen that all the stress–strain curves exhibit three distinct regions [7,10]: a linear-elastic region at very low strain, a collapse plateau region, and a densification region where the stress rises rapidly. Owing to the presence of ceramic particles, ZA22/SiCp composite foams have a complex mechanical response as compared with ZA22 foams. It can be found that the curves of composite foams continually fluctuated with increasing strain, and the stress peak appeared time and again (Fig. 1(b)). As is generally accepted, the brittle foams exhibit a serrate compressive stress–strain curve, while the plastic foams show a smooth stress–strain curve [2,3]. It can be found that from Fig. 1, the composite foams show more brittle compressive behavior than Zn–22Al alloy foams. 3.2. Deformation mechanisms of composite foams in macrostructure and microstructure To observe the deformation mechanisms, the deformation processes of the composite foams with different strains were recorded
(Fig. 2). The formation and propagation of the localized deformation band, which is approximately perpendicular to the compression direction, can be observed. This deformation process is in accordance with the universal investigations on the deformation of metallic foams with closed-cell structure [3,4,7,9,14,16]. The position of the initially local deformation and the propagation of the deformation band rely on the cell structure. It seems to be that the isolated large pore and deep pore are easy to deform because they are thought to be a weak portion in the foam structure. There are two different categories of alloy foams: One is ductile foams, and another is brittle foams. It is generally accepted that the compressive failure of ductile foams is controlled by the cell edge buckling and cell wall bending, whereas the compressive failure of brittle foams is governed by the cell edge fracture and cell wall tearing [3,4,16]. The ductile foams were characterized by a smooth curve in compressive response. On the contrary, the brittle foams were characterized by a large fluctuation in plateau region [2]. Figs. 1 and 2 further indicated that composite foams show a partly brittle deformation behavior because some pores were crushed. Therefore, the deformation mechanism of composite foams can be summarized as follows: the deformation starts from an elastic one of cell structure. Then the cell structure collapses, and a localized deformation band occurs per-
222
J. Liu et al. / Journal of Alloys and Compounds 476 (2009) 220–225
Fig. 2. Deformation processes of composite foams with different strains ε (a) ε = 0, (b) ε = 0.1 and (c) ε = 0.2.
pendicularly to the compression direction. This deformation band then propagates outwards from the initial location to the rest of the cell structure. The sequential formations of the deformation bands take place, resulting in the densification of the whole foams. The compressive response of foams with closed-cell structure is rather complex. When the compressive load is applied, cell edges bend and then fracture, and meanwhile, cell walls stretch perpendicularly to the compression direction [2]. In metallic foams, the cracks often start at the locations of the cell defects because of the shrinkages or thermal stresses induced during the preparation of the foams [3,9,16,22]. From Fig. 3(a), the nucleation and growth of the cracks can be found in the form of discontinuous ones in cell wall and in the form of continuous ones in cell edge. Note that the cracks grow along not only traverse direction but also longitudinal direction, which indicates the complex response under the compression. According to Fig. 3(b), it can be seen that some cracks propagate along the interfaces of matrix/particles, which may degrade the compressive strength of the composite foams. The debonding in the interfaces of matrix/particles often occurs when the particles are attached to the pore because the bonding strength of such interface is weak [14]. On the contrary, the fragmentized particles in alloy matrix are proven to have a reinforced effect on the compressive strength and an impeditive effect on the crack propagation, as is denoted by the arrowhead in Fig. 3(c). Therefore, a complicated effect of SiC particles on the compressive behavior of the foams can be drawn: when the composite foams are compressed, the stress concentration takes place at the interfaces of matrix/particles, which strengthen the compressive strength of the composite foams. But as soon as the cracks nucleate, i.e., collapse occurs, the crack bridges are prone to
propagate along the interfaces of matrix/particles till the cracks stop in alloy matrix, leading to a decrease of the compressive stress. Owing to the circular effect mentioned above, the compressive curves of the composite foams continually fluctuate with increasing strain, and the stress peaks appeared time and again.
3.3. Compressive strength The compressive strength of the foams here is defined as the first peak value in compressive curves. Fig. 4 shows the compressive strengths of ZA22 foams and ZA22/SiCp composite foams with different relative densities. ZA22/SiCp composite foams have a higher compressive strength than ZA22 foams, especially high density foams. Two factors are responsible for the strengthening effect. Direct strengthening effect can be observed in Fig. 3(c), although this effect is impaired by the debonding in the interfaces of matrix/particles. Indirect strengthening effect can be explained by the grains refinement effect of SiC particles on matrix alloy, as is generally accepted in SiC particles reinforced metal matrix composites [22].
3.4. Energy absorption characteristic Energy absorption characteristic is an important technological property of the foams [1–3]. Two parameters are utilized in this study, i.e., energy absorption capacity and energy absorption efficiency. The energy absorption capacity of metallic foams, W, can be obtained from the area under the stress–strain curve up to a certain
J. Liu et al. / Journal of Alloys and Compounds 476 (2009) 220–225
223
Fig. 3. Cracks of the composite foams at about 15% strain (a) cracks in cell wall and cell edge (b) crack bridges in the interface of matrix/particles and (c) cracks propagation in composite foams.
strain [1,2,23], namely:
W=
ε
dε
(2)
0
where ε is the compressive strain, and the compressive stress. Energy absorption efficiency, I, can be calculated using the following equation [1,2,19,20]: I=
1 max ε
ε
dε
(3)
0
where max is the maximal stress in plateau region. Fig. 5 shows the energy absorption capacities of ZA22 foams and ZA22/SiCp composite foams with various strains. It can be seen that the energy absorption capacities of both ZA22 foams and ZA22/SiCp composite foams exhibit the dependence on the relative density and the compressive strain. Fig. 6 shows the energy absorption efficiencies of ZA22 foams and ZA22/SiCp composite foams with various strains. Both two kinds of the foams exhibit an irrelevant tendency to relative density. It is widely received that the energy absorption efficiencies of metallic foams are found to be independent of the relative density, but they have strong correlation with the alloy component and heat treatment [24,25]. In addition, ZA22 foams, in
Fig. 4. Comparison of the compressive strength between ZA22 foams and ZA22/SiCp composite foams.
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Fig. 5. Energy absorption capacity of (a) ZA22 foams and (b) ZA22/SiCp composite foams.
general, show higher energy absorption efficiencies than ZA22/SiCp composite foams at the strain from 0.1 to 0.5. High energy absorption efficiency is obtained when the plateau region is horizontal and no peaks occur [1,23]. Therefore, the large stress fluctuation in composite foams results in the low energy absorption efficiency. However, the energy absorption efficiencies of ZA22/SiCp composite foams keep steadier than those of ZA22 foams at the strain from 0.1 to 0.6. This result demonstrates that the ceramic particles in the foams can widen the range of the applied strains. Fig. 7 shows the energy absorption capacities of ZA22 foams and ZA22/SiCp composite foams at the strain of 0.6. It can be seen that ZA22/SiCp composite foams exhibit slightly higher energy absorption capacities than ZA22 foams. The energy absorption capacities of metallic foams are mainly due to the yielding, buckling, and fracture of the cells and the friction between cell walls when they contact each other [2]. As mentioned above, ZA22/SiCp composite foams have higher compressive strength than ZA22 foams. In addition, the composite foams were crushed in course of collapse. The friction between fragmentized cell walls will dissipate much energy. Therefore, ZA22/SiCp composite foams show high energy absorption capacities, though this tendency is partly impaired by low energy absorption efficiencies.
Fig. 6. Energy absorption efficiencies of (a) ZA22 foams and (b) ZA22/SiCp composite foams.
Fig. 7. Comparison of energy absorption capacity between ZA22 foams and ZA22/SiCp composite foams.
J. Liu et al. / Journal of Alloys and Compounds 476 (2009) 220–225
4. Conclusion The present study demonstrates a strong correlation between SiC ceramic particles and compressive properties of Zn–22Al matrix composite foams. ZA22/SiCp composite foams exhibit a brittle compressive behavior with large stress fluctuation. The deformation processes of the composite foams are characterized by the formation and propagation of the localized deformation band. Both the reinforced effect and depressed effect of SiC ceramic particles on the compressive strength can be observed by the nucleation and propagation of the cracks. Moreover, ZA22/SiCp composite foams show slightly higher energy absorption capacity, although lower energy absorption efficiency, than ZA22 foams for a given relative density. Acknowledgement This work was supported by “Program for New Century Excellent Talents in University” and “985 project” of Jilin University of PR China. References [1] J. Banhart, Prog. Mater. Sci 46 (2001) 559–632. [2] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, second ed., Cambridge University Press, Oxford, 1997.
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[3] H.P. Degischer, Handbook of Cellular Metals. WILEY-VCH Gerlag GmbH, Foreword, 2002. [4] A.E. Simone, L.J. Gibson, Acta Mater. 46 (1998) 3109–3123. [5] S. Yu, J. Liu, Y. Luo, et al., Mater. Sci. Eng. A 457 (2007) 325–328. [6] J. Kovacik, F. Simancik, Kovove Mater. 42 (2004) 70–90. [7] J. Liu, S. Yu, X. Zhu, et al., Mater. Lett. 62 (2008) 683–685. [8] T.G. Nieh, K. Higashi, J. Wadsworth, Mater. Sci. Eng. A 283 (2000) 105–110. [9] I. Jeon, T. Asahina, Acta Mater. 53 (2005) 3415–3423. [10] S. Yu, Y. Luo, J. Liu, Mater. Sci. Eng. A 487 (2008) 394–399. [11] B. Vamsi Krishna, S. Bose, A. Bandyopadhyay, Mater. Sci. Eng. A 452–453 (2007) 178–188. [12] A.E. Markaki, T.W. Clyne, Acta Mater. 49 (2001) 1677–1686. [13] F. Campana, D. Pilone, Mater. Sci. Eng. A 479 (2008) 58–64. [14] O. Parkash, H. Sang, J.D. Embury, Mater. Sci. Eng. A 199 (1995) 195– 203. [15] S. Esmaeelzadeh, A. Simchi, D. Lehmhus, Mater. Sci. Eng. A 424 (2006) 290– 299. [16] M. Guden, S. Yuksel, J. Mater. Sci 41 (2006) 4075–4084. [17] Y. Luo, S. Yu, W. Li, J. Liu, M. Weu, J. Alloys Compd. 460 (2008) 294–298. [18] J. Hashim, L. Looney, M.S.J. Hashmi, J. Mater. Process. Technol. 92–93 (1999) 1–7. [19] E.W. Andrews, G. Gioux, P. Onck, L.J. Gibson, Int. J. Mech. Sci. 43 (2001) 701– 713. [20] H. Yu, Z. Guo, B. Li, et al., Mater. Sci. Eng. A 454–455 (2007) 542–546. [21] H.-W. Song, Q.-J. He, J.-J. Xie, A. Tobota, Compos. Sci. Technol. 68 (2008) 2441–2450. [22] W. Zhou, Z.M. Xu, J. Mater. Process. Technol. 63 (1997) 358–363. [23] D. Lehmhus, J. Banhart, Mater. Sci. Eng. A 349 (2003) 98–110. [24] P.H. Thornton, C.L. Magee, Metall. Trans. 6A (1975) 1253–1263. [25] S. Yu, J. Liu, X. Zhu, Y. Luo, M. Wei, Adv. Eng. Mater. 9 (2007) 679–683.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Correlation between ceramic additions and compressive properties of Zn–22Al matrix composite foams Jiaan Liu a , Sirong Yu a,∗ , Xianyong Zhu b , Ming Wei a , Yanru Luo b , Yaohui Liu a a Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun 130025, China b Institute of Mechanical Science and Engineering, Jilin University, Changchun 130025, China
a r t i c l e
i n f o
Article history: Received 18 July 2008 Received in revised form 8 September 2008 Accepted 12 September 2008 Available online 1 November 2008 Keywords: Metal matrix composites Zn–22Al alloy Ceramics Microstructure Compressive behavior
a b s t r a c t The compressive behaviors of Zn–22Al composite foams using SiC particles as reinforcement and stabilizing agent (ZA22/SiCp composite foams) were investigated in this study. To observe the deformation mechanisms, the deformation processes of the composite foams were recorded, and the cracks under the compressive load were observed by scanning electron microscope (SEM). Owing to the presence of the particles, ZA22/SiCp composite foams show more brittle compressive behavior with a large stress fluctuation than Zn–22Al alloy foams (ZA22 foams). The compressive processes of composite foams are characterized by the formation and propagation of the localized deformation band. The photomicrographs of the cracks indicate that a complicated effect of SiC particles on the compressive properties of the foams result in a high compressive strength. As a consequence, ZA22/SiCp composite foams exhibit slightly high energy absorption capacities, although low energy absorption efficiencies, as compared with ZA22 foams. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Metallic foams have received extensive interest as new structural and functional materials because of their energy absorption, flame resistance, vibration reduction, and acoustic insulation [1]. In particular, they can absorb most of the plastic deformation energy at low constant stress during compression. They are adopted for protective aim in traffic and packing industry [2,3]. Therefore, the compressive property is an important characteristic of metallic foams. Generally, the compressive properties of metallic foams depend on the property of cell wall material [4–7], the relative density [1–3], the cell structure (e.g., shape, orientation and defect of cell) [8,9], and the type of load (e.g., static or dynamic, free or constrained compression) [10,11]. Secondary phase or component in cell wall materials is an important factor affecting the mechanical response of the foams [5,11–17]. SiC particles commonly utilized as the stabilizing agent for the fabrication of metallic foams can dramatically change the mechanical properties of metallic foams [5,14–17]. Parkash et al. [14] found that the localized deformation took place in Al/SiCp composite foams due to SiCp dispersed in cell structure. Yu et al. [5]
also denoted that the presences of SiC particles in Zn–22Al/10 vol.% SiCp composite foams resulted in a large compressive stress fluctuation. Moreover, the studies on the composite foams fabricated by powder metallurgy process also demonstrated that the composite foams showed a brittle deformation behavior and a slight degradation in the compressive properties as compared with Al alloy foams [15,16]. However, the studies on the correlation between ceramic particles and compressive property of metallic foams, especially on the nucleation and propagation of cracks, are limited. Therefore, in present study, the deformation process and the cracks of ZA22/SiCp composite foams were observed in order to understand the compressive deformation mechanisms of composite foams. 2. Experimental 2.1. Raw materials The raw materials for preparing composite foams included ZA22 alloy (22.0 wt.% Al, 1.0 wt.% Cu, 0.03 wt.% Mg, and Zn balance), SiC particles (98.0 wt.% in purity, about 28 m in size), and CaCO3 powders (99.5 wt.% in purity). SiC particles and CaCO3 powders were used as stabilizing agent and blowing agent, respectively. To improve the wettability between SiC particles and ZA22 melt, SiC particles were heat-treated at 930 ◦ C for 6 h and at 420 ◦ C for 2 h, respectively. 2.2. Fabrication of composite foams
∗ Corresponding author. Tel.: +86 431 85095862; fax: +86 431 85095876. E-mail addresses: [email protected], [email protected] (S. Yu). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.069
Zn–22Al composite slurry with 7 vol.% SiCp was prepared by conventional stir-casting technique [18], and then CaCO3 powders were added into the melt.
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221
Afterwards, the composite slurry was stirred for 2 min with a steel stirrer at a rate of 900 r/min and then held at 700–720 ◦ C to allow the blowing agent to release gas bubbles. 2.3. Compression test Size effect is an important issue in the mechanical testing of metallic foams [19–21]. That means the value of the specimen size relative to the mean cell size has great influence on the mechanical properties of the metallic foams. The specimen with insufficient number of cells (<6) can cause a significant loss of the mechanical properties. When a specimen contains more than 6 cells, the size effects can be ignored [19,20]. Therefore, the large specimens used for compression test are favoring for eliminating the size effects. But large specimens are discommodious for observation in SEM. A small specimen is required for the observations on the microstructure failure. Song et al. [21] found that the failure process of a specimen with insufficient cell is homologous to that of a specimen with many cells. Both of them are defect-directed or weakness-directed processes. That means that the failure mechanisms of small and large specimens are homologous. Therefore, in this work, we selected the large specimen used for compression test and the small specimen used for the observation on the microstructure failure. Specimens with the large dimensions of 15 mm × 15 mm × 30 mm were prepared for the compressive test in order to eliminate size effects [5]. The compressive tests were carried out on a universal mechanical testing machine at a nominal strain rate of 2.2 × 10−3 s−1 . The obtained data were used for drawing the compressive stress–strain curves. The deformation processes of the large foam samples were recorded by a numeral camera to study the deformation mechanism of the macrostructure in the composite foams. The rectangular sections of small foam samples (10 mm × 10 mm × 12 mm) were polished, and then the samples were compressed at certain strain. Afterwards, the polished sections of the samples were observed immediately without further metallographic treatment. 2.4. Characterization of composite foams The cracks of the deformed foams were observed by means of scanning electron microscopy (SEM) (Model JSM-5310, Japan). The porosities of the composite foams were calculated using the following formula [2]:
P=
1−
∗ s
× 100%
(1)
where P is the porosity of the composite foams, * and s are the densities of the composite foams and the cell wall material, respectively, and (* /s ), which is called the relative density of the composite foams, indicates the ratio of the density of the composite foams to the density of cell wall material.
Fig. 1. Compressive stress–strain curves of (a) ZA22 foams and (b) ZA22/SiCp composite foams.
3. Results and discussion 3.1. Compressive curves of the foams Fig. 1 shows the compressive stress–strain curves of ZA22 foams and ZA22/SiCp composite foams with various relative densities. It can be seen that all the stress–strain curves exhibit three distinct regions [7,10]: a linear-elastic region at very low strain, a collapse plateau region, and a densification region where the stress rises rapidly. Owing to the presence of ceramic particles, ZA22/SiCp composite foams have a complex mechanical response as compared with ZA22 foams. It can be found that the curves of composite foams continually fluctuated with increasing strain, and the stress peak appeared time and again (Fig. 1(b)). As is generally accepted, the brittle foams exhibit a serrate compressive stress–strain curve, while the plastic foams show a smooth stress–strain curve [2,3]. It can be found that from Fig. 1, the composite foams show more brittle compressive behavior than Zn–22Al alloy foams. 3.2. Deformation mechanisms of composite foams in macrostructure and microstructure To observe the deformation mechanisms, the deformation processes of the composite foams with different strains were recorded
(Fig. 2). The formation and propagation of the localized deformation band, which is approximately perpendicular to the compression direction, can be observed. This deformation process is in accordance with the universal investigations on the deformation of metallic foams with closed-cell structure [3,4,7,9,14,16]. The position of the initially local deformation and the propagation of the deformation band rely on the cell structure. It seems to be that the isolated large pore and deep pore are easy to deform because they are thought to be a weak portion in the foam structure. There are two different categories of alloy foams: One is ductile foams, and another is brittle foams. It is generally accepted that the compressive failure of ductile foams is controlled by the cell edge buckling and cell wall bending, whereas the compressive failure of brittle foams is governed by the cell edge fracture and cell wall tearing [3,4,16]. The ductile foams were characterized by a smooth curve in compressive response. On the contrary, the brittle foams were characterized by a large fluctuation in plateau region [2]. Figs. 1 and 2 further indicated that composite foams show a partly brittle deformation behavior because some pores were crushed. Therefore, the deformation mechanism of composite foams can be summarized as follows: the deformation starts from an elastic one of cell structure. Then the cell structure collapses, and a localized deformation band occurs per-
222
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Fig. 2. Deformation processes of composite foams with different strains ε (a) ε = 0, (b) ε = 0.1 and (c) ε = 0.2.
pendicularly to the compression direction. This deformation band then propagates outwards from the initial location to the rest of the cell structure. The sequential formations of the deformation bands take place, resulting in the densification of the whole foams. The compressive response of foams with closed-cell structure is rather complex. When the compressive load is applied, cell edges bend and then fracture, and meanwhile, cell walls stretch perpendicularly to the compression direction [2]. In metallic foams, the cracks often start at the locations of the cell defects because of the shrinkages or thermal stresses induced during the preparation of the foams [3,9,16,22]. From Fig. 3(a), the nucleation and growth of the cracks can be found in the form of discontinuous ones in cell wall and in the form of continuous ones in cell edge. Note that the cracks grow along not only traverse direction but also longitudinal direction, which indicates the complex response under the compression. According to Fig. 3(b), it can be seen that some cracks propagate along the interfaces of matrix/particles, which may degrade the compressive strength of the composite foams. The debonding in the interfaces of matrix/particles often occurs when the particles are attached to the pore because the bonding strength of such interface is weak [14]. On the contrary, the fragmentized particles in alloy matrix are proven to have a reinforced effect on the compressive strength and an impeditive effect on the crack propagation, as is denoted by the arrowhead in Fig. 3(c). Therefore, a complicated effect of SiC particles on the compressive behavior of the foams can be drawn: when the composite foams are compressed, the stress concentration takes place at the interfaces of matrix/particles, which strengthen the compressive strength of the composite foams. But as soon as the cracks nucleate, i.e., collapse occurs, the crack bridges are prone to
propagate along the interfaces of matrix/particles till the cracks stop in alloy matrix, leading to a decrease of the compressive stress. Owing to the circular effect mentioned above, the compressive curves of the composite foams continually fluctuate with increasing strain, and the stress peaks appeared time and again.
3.3. Compressive strength The compressive strength of the foams here is defined as the first peak value in compressive curves. Fig. 4 shows the compressive strengths of ZA22 foams and ZA22/SiCp composite foams with different relative densities. ZA22/SiCp composite foams have a higher compressive strength than ZA22 foams, especially high density foams. Two factors are responsible for the strengthening effect. Direct strengthening effect can be observed in Fig. 3(c), although this effect is impaired by the debonding in the interfaces of matrix/particles. Indirect strengthening effect can be explained by the grains refinement effect of SiC particles on matrix alloy, as is generally accepted in SiC particles reinforced metal matrix composites [22].
3.4. Energy absorption characteristic Energy absorption characteristic is an important technological property of the foams [1–3]. Two parameters are utilized in this study, i.e., energy absorption capacity and energy absorption efficiency. The energy absorption capacity of metallic foams, W, can be obtained from the area under the stress–strain curve up to a certain
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223
Fig. 3. Cracks of the composite foams at about 15% strain (a) cracks in cell wall and cell edge (b) crack bridges in the interface of matrix/particles and (c) cracks propagation in composite foams.
strain [1,2,23], namely:
W=
ε
dε
(2)
0
where ε is the compressive strain, and the compressive stress. Energy absorption efficiency, I, can be calculated using the following equation [1,2,19,20]: I=
1 max ε
ε
dε
(3)
0
where max is the maximal stress in plateau region. Fig. 5 shows the energy absorption capacities of ZA22 foams and ZA22/SiCp composite foams with various strains. It can be seen that the energy absorption capacities of both ZA22 foams and ZA22/SiCp composite foams exhibit the dependence on the relative density and the compressive strain. Fig. 6 shows the energy absorption efficiencies of ZA22 foams and ZA22/SiCp composite foams with various strains. Both two kinds of the foams exhibit an irrelevant tendency to relative density. It is widely received that the energy absorption efficiencies of metallic foams are found to be independent of the relative density, but they have strong correlation with the alloy component and heat treatment [24,25]. In addition, ZA22 foams, in
Fig. 4. Comparison of the compressive strength between ZA22 foams and ZA22/SiCp composite foams.
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Fig. 5. Energy absorption capacity of (a) ZA22 foams and (b) ZA22/SiCp composite foams.
general, show higher energy absorption efficiencies than ZA22/SiCp composite foams at the strain from 0.1 to 0.5. High energy absorption efficiency is obtained when the plateau region is horizontal and no peaks occur [1,23]. Therefore, the large stress fluctuation in composite foams results in the low energy absorption efficiency. However, the energy absorption efficiencies of ZA22/SiCp composite foams keep steadier than those of ZA22 foams at the strain from 0.1 to 0.6. This result demonstrates that the ceramic particles in the foams can widen the range of the applied strains. Fig. 7 shows the energy absorption capacities of ZA22 foams and ZA22/SiCp composite foams at the strain of 0.6. It can be seen that ZA22/SiCp composite foams exhibit slightly higher energy absorption capacities than ZA22 foams. The energy absorption capacities of metallic foams are mainly due to the yielding, buckling, and fracture of the cells and the friction between cell walls when they contact each other [2]. As mentioned above, ZA22/SiCp composite foams have higher compressive strength than ZA22 foams. In addition, the composite foams were crushed in course of collapse. The friction between fragmentized cell walls will dissipate much energy. Therefore, ZA22/SiCp composite foams show high energy absorption capacities, though this tendency is partly impaired by low energy absorption efficiencies.
Fig. 6. Energy absorption efficiencies of (a) ZA22 foams and (b) ZA22/SiCp composite foams.
Fig. 7. Comparison of energy absorption capacity between ZA22 foams and ZA22/SiCp composite foams.
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4. Conclusion The present study demonstrates a strong correlation between SiC ceramic particles and compressive properties of Zn–22Al matrix composite foams. ZA22/SiCp composite foams exhibit a brittle compressive behavior with large stress fluctuation. The deformation processes of the composite foams are characterized by the formation and propagation of the localized deformation band. Both the reinforced effect and depressed effect of SiC ceramic particles on the compressive strength can be observed by the nucleation and propagation of the cracks. Moreover, ZA22/SiCp composite foams show slightly higher energy absorption capacity, although lower energy absorption efficiency, than ZA22 foams for a given relative density. Acknowledgement This work was supported by “Program for New Century Excellent Talents in University” and “985 project” of Jilin University of PR China. References [1] J. Banhart, Prog. Mater. Sci 46 (2001) 559–632. [2] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, second ed., Cambridge University Press, Oxford, 1997.
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