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Interfacial microstructure and compressive properties of Al–Mg syntactic foam reinforced with glass cenospheres
Journal of Alloys and Compounds 655 (2016) 301e308
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Interfacial microstructure and compressive properties of AleMg syntactic foam reinforced with glass cenospheres Yingfei Lin, Qiang Zhang*, Gaohui Wu School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 August 2015 Received in revised form 14 September 2015 Accepted 18 September 2015 Available online 25 September 2015
Glass cenospheres/Al syntactic foams are successfully produced with about 50% volume fraction using pressure infiltration process. The interfacial reactions between glass cenospheres and AleMg alloys (5A03 with 3wt% Mg and 5A06 with 6wt% Mg) have been deeply studied. The MgAl2O4 crystals produced by the reaction of 2Al(l)þMg(l)þ2SiO2(s) / 2MgAl2O4(S) þ 2Si(S) are the main reaction products in the size of ~100 nm and the reaction layer is about 800 nm thick. The interfacial reaction region is composed of a large amount of MgAl2O4 uniformly coating on the glass cenospheres due to the spherical structure of cenosphere. Si is believed to diffuse through the grain boundaries between the MgAl2O4 crystals and aggregate on the surface of the reaction zone. With the increase of Mg content in matrix such as 5A06 alloy, Si formed above transforms into Mg2Si by the reaction of 2 Mg(l) þ Si(s) / Mg2Si(s). The compressive strength of composites increases with the increase of Mg content which is not only contributed to the improved strength of matrix but the suitable interfacial reaction coating and the forming of Mg2Si in matrix. © 2015 Elsevier B.V. All rights reserved.
Keywords: Glass cenospheres AleMg alloys Interfacial reactions Compressive properties Metal matrix composites
1. Introduction Aluminum alloy syntactic foams are a class of new structural and functional composite materials synthesized by aluminum alloy filling with the hollow particles known as ceramic hollow spheres such as alumina cenospheres [1e3] and fly ash [4e7]. It has been paid great attention on aluminum alloy syntactic foams because of their excellent combination of physical and mechanical properties such as high energy absorbing capabilities, high stiffness, improved strength and damping capacities in addition to their light weight characteristics [8e11]. These properties make syntactic foams possible to use in a various applications such as, core in sandwich structures, crash safety, and damping panels [12,13]. For cellular materials, the function is usually affected by the characteristic of porous structure, and mechanical properties are dependent on the interface structure [14e16]. Under the impact loadings, the ceramic hollow spheres embedded in foams can be thought as a supporting role in syntactic foams due to the high strengths of the ceramic spherical shell [17,18]. The syntactic foams reinforced with ceramic hollow spheres
* Corresponding author. P.O. Box 433, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. E-mail address: [email protected] (Q. Zhang). http://dx.doi.org/10.1016/j.jallcom.2015.09.175 0925-8388/© 2015 Elsevier B.V. All rights reserved.
exhibits the advantages of isotropic mechanical properties, higher strengths with extensive strain accumulation resulting in excellent energy-absorption capacities [19]. In addition, hollow spheres can be classified into different densities using flotation methods. On the other hand, the undamaged spheres can be divided into different sizes by sieves with corresponding mesh sizes. The variety of cenospheres provides a possibility for designing excellent syntactic foams meeting application demands [20,21]. Glass cenospheres, a kind of ceramic hollow sphere, are used to be fillers in polymeric foams [22,23]. Similarly, these syntactic foams generally have a good energy absorption capacity but are easy to be sensitive to strain rates due to the characteristic of polymer [24]. That is to say, glass cenospheres have a great perspective application in aluminum syntactic foams. AleMg alloys belong to the group of aluminum alloys which cannot be strengthened by heat treatment. These alloys possess a series of great properties, such as corrosion resistance, good weldability and cutting performance [25e27]. Meanwhile, AleMg alloys have the advantage of good shock resistance and ductility in aluminum alloys. Magnesium can decrease the surface tension of molten aluminum which can promote the infiltration. It is generally stated that magnesium commonly plays an important role in increasing the strength and improving the wetting properties with SiO2 in light weight Al alloys [7,28]. It is generally stated that Mg in
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the matrix will react with glass phase such as Al2O3 and SiO2 [29,30]. In the present work, we investigate the feasibility of synthesis the AleMg syntactic foam filled with glass cenospheres by pressure infiltration process. In view of the availability of syntactic foams and the active magnesium in matrix, the microstructural features, interfacial reactions between glass cenospheres and AleMg matrix, and compressive properties of the syntactic foams have been mainly discussed in detail in order to lay a foundation for further research and application. 2. Materials and experimental 2.1. Fabrication of the AleMg syntactic foam The compositions of matrices, commercial pure aluminum (1100Al), 5A03, and 5A06, were listed in Table 1. The main difference in these alloys is the content of Magnesium which is about 0%, 3%, and 6% by weight, respectively. The glass cenospheres, named S38 considering its density supplied by 3M Company, were to serve as fillers. The morphology of glass cenospheres is shown in Fig. 1. It shows spherical shape for most of cenospheres with generally smooth exterior surfaces. The main elements detecting by energy dispersive X-ray analysis are 53.99wt% O, 34.25wt% Si, 8.26wt% Ca, and 3.51wt% Na. Their size ranges from 20 to 90 mm and the average size is 40 mm. The chemical composition of glass cenospheres was listed in Table 2 (3M glass bubbles product information from 3M Company). The cenosphere is mainly composed of SiO2 and CaO, which can act as indicators of cenosphere. The main physical properties of the ceramic cenospheres were listed in Table 3 (3M glass bubbles product information from 3M Company). It can be seen that the glass cenospheres in this study process a certain of strength with high porosity. The syntactic foams were fabricated by pressure infiltration technique. Glass cenospheres, filling in a steel mold, were pressed with a vertical pressure of ~0.5 MPa as a cylindrical preform with a 40 mm height. The perform was then preheated to 600 C. Concurrently, matrix alloy was heated in a graphite crucible to a temperature of 850 C at which point the molten metal was poured into the steel mold. Then a mechanical pressure of ~0.8 MPa was applied to press the molten matrix vertically, causing downwards infiltration of matrix into cenospheres perform. The applied pressure was maintained for 5 min until the foam solidified completely. All processing was in air. 2.2. Density The density of the composites was determined by means of Archimedes' principle, using a microbalance (BP211D-0CE from Satorius AG) with ethanol as suspending medium. The porosity of the composite was calculated using rules of mixture as:
rc ¼ rm 1 Vf þ rf Vf
(1)
VP ¼ P0 Vf
(2)
where rc is the density of composite determined by Archimedes' Table 1 Typical chemical composition of matrix in weight procent. Alloy
Cu
Mg
Mn
Fe
Si
Zn
Ti
Al
1100 5A03 5A06
0.05e0.2 0.1 0.1
e 3.2e3.8 5.8e6.8
0.05 0.3e0.6 0.5e0.8
0.5 0.5 0.4
0.4 0.5e0.8 0.4
0.1 0.2 0.2
e 0.15 0.02e0.1
Bal. Bal. Bal.
Fig. 1. The SEM image of raw cenospheres.
principle, rf (shown in Table 3) is the density of fillers (glass cenospheres), Vf is the volume fraction of fillers in composite, P0 (shown in Table 3) is the porosity in cenosphere, and VP is the porosity of composite. 2.3. Analysis of microstructure Specimens for microstructure investigations were cut from composites castings and annealed for 30 min at 325 C for the purpose of eliminating stress. The specimens were polished using standard metallographic technique with ethanol which can prevent the oxidation of matrix in composites. In consideration of the interfacial reactions between matrix and cenospheres, composites were corroded by acid solution for extracting the products of interface. The microstructural features and phases of composites, raw cenospheres, and cenospheres extracted from composites were revealed by scanning electron microscope (SEM, Helios Nanolab600i) and X-ray diffraction analysis (XRD, Rigaku D/max2200), respectively. The backscattered electron (BSE) image was employed to characterize the precipitated-phase or impurity phase in composites. SEM with energy dispersive X-ray analysis (EDXA) is used for the elemental analysis of certain phases observed in the composites. Transmission electron microscopic (TEM, TecnaiF2F30) is performed on confirming the phases in interfacial reaction zone and discussing the process of interfacial reactions. However, the TEM sample of composite was difficult to prepare in a normal way due to its porous nature. There is no enough thin area for observing the interfacial zone in TEM sample. So, cenospheres were compressed into pieces for preparing bulk composite with the same process above. The TEM samples in this paper were obtained from the bulk composites for further study. 2.4. Compression test To evaluate the influence of Mg content on the compressive properties of the composites, quasi-static compression tests were performed on a Zwick universal testing machine (Instron 5569) according to the GB/T 7314-2005 standard for metallic materials compression testing at ambient temperature. Cylindrical
Table 2 Typical chemical composition of glass microspheres in weight procent. SiO2
CaO
Na2O
MgO
Al2O3
P2O5
SO3
LOIa
77.85
14.18
6.41
0.18
0.15
0.76
0.31
0.16
a
Loss on ignition.
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Table 3 Physical properties of the glass cenospheres. Density rf (g/cm3)
Bulk density rb (g/cm3)
Porosity P0 (%)
Wall thickness t (mm)
Strength sc (MPa)
0.38
2.54
85.04
1.05
27.56
Fig. 2. The SEM images of composites cenosphere/1100 (a); cenospheres/5A03 (b); cenospheres/5A06 (c).
specimens with height/diameter ratio of 1.5 and diameter 8 mm were used in uniaxial compression tests. The testing was carried out at constant crosshead speed with an initial strain rate of 103 s1. 3. Results and discussion 3.1. Density In this paper, the composites have been successtully fabricated. The average densities measured for cenospheres/1100, cenospheres/5A03, and cenosphere/5A06 with Eq. (1) were 1.17 g/cm3, 1.22 g/cm3 and 1.31 g/cm3, respectively. The corresponding porosities of composites referring to Eq. (2) were 56.0%, 53.8% and 49.5%. Addition of glass cenospheres to the Al alloy resulted in an obviously decrease in the density of the resulting composite in comparison to the base alloy. The composites in this work are generally lighter than other aluminum matrix syntactic foams under the same porosity [8,31]. It can be attributed to the lightweight cenospheres and AleMg alloy. In addition, the high composites' porosity implies the great compressive densification strain and efficient energy absorption.
3.2. Microstructure and phases in composites The microstructures of Al syntactic foams in annealed condition are shown in Fig. 2. Visual inspection of polishing surface of Al syntactic foams shows uniform distribution of cenospheres throughout the composite. There are two positions that cenospheres perform in composites, such as the A and B area display in Fig. 2aec. A majority of cenospheres in composites stay as integrated as the cenosphere which shown in area A. A few of cenospheres are too weak to bear the pressure during infiltration, they are broken and perforated, molten matrix alloy flows into, completely fills in the core of cenosheres, which presented in area B. However, the amount of perforated cenospheres is too few to affect the porosity of composites. X-ray diffraction analysis is required to determine the phases in composites. The XRD patterns of the prepared composites and the corresponding matrix alloys are presented in Fig. 3. In matrix alloy's XRD patterns, Al is the only phase can be observed. As compared to the standard pattern, the diffraction peaks of aluminum in AleMg alloy have moved to left about 0.3 . The deviation of peaks in 5A06 is more than that in 5A03. It indicates that the major solute element Mg in AleMg alloy play a significant role in this kind diffraction peaks moving. The lattice constant of a-Al will increase with the increasing of Mg content. According to the Bragg equation, 2dsinq¼nl (where d means interplanar distance, q means diffraction angle, l means x-ray wavelength, and n means diffraction series), diffraction angle q will decrease with lattice constant increasing. Different from the patterns of AleMg alloy, the diffraction peaks of Al in syntactic foams are no deviation from standard pattern. There is no other phase except Al can be seen in the pattern of cenosphere/1100. On the other hand, there are trace amount of Si and MgAl2O4 being found in conosphere/AleMg composites. What's interesting is that some weak peaks of Mg2Si can be found in cenospheres/5A06 XRD pattern. Considering the finding of Si and MgAl2O4 in AleMg syntactic foams, it implies that the Mg element in prepared composite precipitate out from a-Al by reacting with cenospheres (SiO2) resulting the diffraction peaks of Al back to the certified values. 3.3. Interfacial reaction
Fig. 3. The XRD of matrix alloys and composites.
Mg element is an active in aluminum alloys which is prone to reacting with reinforcements in composite [29,30]. Fig. 4 shows the
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Fig. 4. The SEM images of extracted cenosphere from cenosphere/1100 (a); cenospheres/5A03 (b); cenospheres/5A06 (c); the cross-section of cenospheres/5A03 (d); and surface morphology cenospheres/5A03 (e).
Table 4 The DEXA analysis of Fig. 4. Area
Element
OK
Na K
Mg K
Al K
Si K
Ca K
Total
A
wt% at% wt% at% wt% at%
61.42 73.82 47.31 60.12 33.91 47.36
3.69 3.08 e e e e
e e 10.20 8.45 e e
5.28 3.77 24.26 18.29 1.91 1.58
25.03 17.14 17.68 12.85 64.18 54.07
4.58 2.20 0.54 0.29 e e
100
B C
Fig. 5. The XRD of raw cenospheres and extracted cenosheres.
100 100
microstructure of cenospheres extracted from cenosphere/1100, cenospheres/5A03, and cenospheres/5A06 syntactic foams. The corresponding energy dispersive X-ray analysis of part A, B, and C are listed in Table 4. The exterior surface of cenosphere extracted from cenosphere/1100 stay as smooth as the raw cenospheres, which can be seen in Fig. 4a. The composition in part A of cenosphere extracted from cenosphere/1100 is similar to raw cenospheres. Compared to the smooth surface of raw cenospheres, the cenospheres extracted from AleMg composite are covered by a coating, shown as Fig. 4b and c. From the section of extracted cenospheres in Fig. 4d, it is found that the coating is composed of micro-particles layer by layer. Different from the EDXA result of raw cenospheres, a number of Mg and Al elements are observed in part B which refers to the surface of extracted conosphere. The atomic ratio of Mg, Al and O in part B almost meets the ratio of MgAl2O4. It suggests that the dense micro-particles on extracted cenosphere could be MgAl2O4 particles. Besides the micro-particles, there is overlapped dendritic substance attaching on the extracted cenosphere, shown in Fig. 4e. As compared to the EDXA result of raw cenospheres, the substance contains much more Si and no Ca or Na. Fig. 5 is the XRD patterns of cenospheres which can clarify the interfacial phase forming on the cenospheres. Glass cenospheres consist of different inorganic peroxides with a complex microstructure, such as a mixture of amorphous. The XRD pattern of the extracted cenospheres from cenosphere/1100 is the same as raw cenospheres. Differing from the raw cenospheres' XRD patterns, there are some diffraction peaks referring to spinel MgAl2O4 and Si in the XRD patterns of the cenospheres extracted from AleMg syntactic foams. As a matter of fact, spinel MgAl2O4 and Si are also found in composites, but much less than that in extracts. On the other hand, MgAl2O4 and Si are not the phases in the system of
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Fig. 6. (a) The TEM images of cenospheres/5A06 composites; the diffraction pattern of part I (b); II (c); III (d).
Fig. 7. Microstructure (a) and corresponding area distribution analysis of Al (b); Mg (c); O (d); Si (e) and Ca (f) in part IV of Fig. 6a.
AleMg alloy [32]. To some extent, the coating layer forming on cenospheres results from the interfacial reaction between cenospheres and Al and Mg elements in the matrix and then the products are spinel MgAl2O4 and Si. In order to indentify the interfacial reaction path of cenospheres and matrix, TEM observations were carried out. Figs. 6e8 show transmission electron micrographs of interfacial region in the case of the prepared AleMg syntactic foams. An interfacial reaction region of approximate 800 nm thickness is clearly seen in Fig. 6a. The main phases of cenosphere's wall (area I), interfacial region (area II) and matrix (area III) were characterized by the diffraction patterns of Fig. 6bed. It proves again that cenosphere is composed of amorphous crystals confirming by the amorphous ring diffraction pattern. The matrix consists of coarse Al grains. The interfacial reaction region consists of a large number of fine spinel MgAl2O4 crystals (~100 nm) as identified from their diffraction pattern in area II. The area distribution of element Al, Mg, O, Si, and Ca in area IV of Fig. 6a was analyzed and shown in Fig. 7. As can be seen, a high concentration of Mg uniformly distributes in the interfacial region and degrades in the cenospheres' wall. The trend of O-concentration distribution is similar to element Mg but the degradation to cenospheres' wall is smaller than that of Mg. Different from the element Mg, the concentration of element Si increases to the matrix while element Ca is little in the matrix and no segregation in cenosphere region or interfacial region. It reveals that element Si gradually diffuses to the matrix when the interfacial reaction is in process and segregates in the matrix close to the interfacial region. There is no reaction between Ca and matrix. Fig. 8a shows the TEM image of interfacial reaction zone for cenosphere/5A03 composite, Fig. 8c shows for cenosphere/5A06 composite. There is a large piece of Si (confirmed by the diffraction pattern in Fig. 8b) precipitating
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Fig. 8. The TEM images of (a) cenospheres/5A03; (c) cenospheres/5A06; the diffraction patterns of part A (b); B (d).
on the surface of reaction region in cenospheres/5A03 foams. That is to say, Si phase would precipitate while the concentration of element Si reaches a certain amount in matrix. In the case of cenospheres/5A06 foam whose Mg content is more than that in cenospheres/5A03 foam, the enrichment of Mg still exist in the matrix near the interfacial region, then the Mg will continue to react with Si and Mg2Si (identified by diffraction pattern in Fig. 8d) forms on the surface of the reaction region. The driving force for interface products forming is the Gibbs free energy associated with the reaction of cenospheres and matrix. From the thermodynamic point of view, the most likely interfacial reactions are listed as following [29]:
2AlðlÞ þMgðlÞ þ2SiO2ðsÞ /2MgAl2 O4ðsÞ þ2SiðsÞ DG850 C ¼2962kJ (3) 2MgðlÞ þ SiðsÞ /Mg2 SiðsÞ
DG850 C ¼ 59kJ
(4)
It infers that the dominant content (SiO2) in cenospheres prefers to react with Mg and Al under the fabrication temperature of syntactic foams. The interfacial reaction leads to producing spinel MgAl2O4 micro-particles on the surface of cenospheres and Si beside the interfacial region. There is a possibility that the Si in cenospheres would be driven away from cenospheres when SiO2
reacts with Al and Mg. In the view of Fig. 8a, rod-shaped Si grows away from interfacial reaction region and precipitates in matrix. It can be speculated that Si produced by the reaction would diffuse through the grain boundaries between MgAl2O4 crystals then precipitate in the matrix beside the interfacial reaction zone. With the increase of Mg content in matrix, as the 5A06 alloy in this paper, the redundant Mg would continue to react with Si forming from reaction (3), producing Mg2Si after no MgAl2O4 forming further. 3.4. Compressive properties Compressive properties of the syntactic foams can be understood by studying the stressestrain curves. Fig. 9 shows the quasistatic compressive stressestrain curves for the cenosphere/Al syntactic foams in this work. It is shown that as the Mg content in matrix increases from 0wt% to 6wt%, the corresponding compressive peak strength increases from 77.8 to 137.2 MPa. Generally, the strength of AleMg alloys will increase with the Mg content increasing, therefore, an increase in compressive strength of the composite is observed with the increase of Mg content in matrix. Ferguson et al. [31] modified a model that could predict the compressive strength (peak strength, sp) of Al matrix syntactic foams.
iA h w sp ¼ 2A%m sym þ A%s hsfw As
(5)
where sym is the yield strength of the matrix, A%m and A%s indicate the area fraction of the matrix and the cenospheres, respectively.sfw is the fracture strength of the wall material. h is the parameter considering the defeats of monolithic wall material. The parameter h in the model assumed as 0.357. The ratio of Aw/As is the smallest ratio of wall area to cross-sectional area. This model assumed that the cenospheres are considered as solid spheres having a density equal to the bulk density of the cenospheres. So, the volume percentage of the cenospheres (Vf) is thought to be equivalent to the area fraction of the cenospheres (A%s). That is to say the area fraction of the matrix A%m equal to the volume percentage of the matrix (1 Vf). The fracture strength of the wall material sfw in this work can be predicted by using the following equation [33]: 3
sc ¼ sfw Cð1 P0 Þ2
Fig. 9. The compressive stressestrain curves of composites.
(6)
where C is assumed to be 0.3, sc and P0 is the strength and the void volume fraction in cenospheres (see Table 3). The sfw is calculated to be 1587.7 MPa for cenosphere wall material. There are three
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Table 5 Compressive properties and calculated peak strength of cenospheres/Al syntactic foams. Composite
Experimental peak strength spe (MPa)
Densification strain εD (%)
Energy absorption E (MJ/m3)
Calculated peak strength spc (MPa)
Variance (%)
Cenospheres/1100 Cenospheres/ 5A03 Cenospheres/ 5A06
77.8 109.0
60.1 57.6
36.4 52.4
79.7 82.2
2 32
137.2
48.8
68.2
82.4
66
main assumptions inherent the predict peak strength of the syntactic foams: (1) the infiltration of broken or perforated spheres is thought to have no effect; (2) the matrix is considered to have no shrinkage or gas porosity; and (3) the properties of the spheres are assumed to relate to the average sphere size. Table 5 shows the compressive properties data and the variance between the experimental peak strength and those predicted by the model of the syntactic foams in this work. The yield strength of matrix alloys 1100Al, 5A03, and 5A06 for model are 50 MPa, 118 MPa, and 167 MPa, respectively. The difference of densification strain εD can be contributed to the difference of porosity and matrix's ductility among the syntactic foams. The One of the important technological properties to estimate the application of metal matrix syntactic foams is energy absorption characteristic, the energy can be evaluated by integrating the area under the compressive stressestrain curve up to the onset of densification strain εD as follow:
ZεD E¼
sðεÞdε
(7)
0
As the small difference of the densification strain εD among the syntactic foams, the energy absorption increases with the increasing of compressive stress. The peak strength of cenospheres/ 1100 syntactic foam which has no interfacial region shows excellent agreement to the model, the variance is just 2%. For cenospheres/ 5A03 and cenospheres/5A06 syntactic foams, the peak strength is higher than the predicted one. Meanwhile, the variance value between experimental peak stress and calculated peak stress of cenospheres/5A06 is 66% which much higher than 32% of cenospheres/5A03. It suggests that a certain interfacial reaction between cenospheres and matrix can improve the strength of syntactic foams. Firstly, the forming of MgAl2O4 coating is beneficial to the interface bonding between matrix and cenospheres. Secondly, the fracture strength of the MgAl2O4 coating may be stronger than that of cenospheres' wall material. The MgAl2O4 crystals in this paper uniformly precipitate on the glass cenosphere, while in the case of glass particles filled in AleMg alloy, the MgAl2O4 crystals precipitate locally on the glass particles. The glass cenospheres in this paper have smooth exterior surfaces and homogeneous wall. For the reason of the same curvature in a cenosphere, the probability of MgAl2O4 crystals nucleation on the each position of cenosphere is same which leads to uniformly distribution on cenospheres. Thus, most of cenospheres in composite still keep pore structure and spherical shape after interfacial reaction. In principle, when there is a few conospheres in composite, an over-reaction would happen and consume lots of SiO2 causing perforation on the cenospheres, which makes the cracking or collapse of cenospheres. But, if there is a high volume fraction of cenospheres in composite (just as the composite in this paper), the reactants and the reaction degree would decrease in each position with the increase of interface. A suitable interfacial reaction will not only keep pore structure but strengthen the interface bonding in syntactic foams. Moreover, the
spe spc spc
Mg2Si is a reinforcing phase in Al alloy which has high melting point, low density and good mechanical properties. The precipitation of Mg2Si in the matrix of cenosphere/5A06 syntactic foam is contributed to improving the mechanical property of composite by second phase strengthening mechanism. This is why the variance value in cenosphere/5A06 syntactic foam is larger than cenosphere/ 5A03 syntactic foam. There are three main strengthening effects correlated with element Mg on composite. On the one hand, the interfacial reaction layer MgAl2O4 because of the addition of Mg in matrix alloy were uniformly coating on the cenospheres, which can improve the compressive properties of cenospheres. On the other hand, with the increase of Mg content in matrix, there is Mg2Si phase precipitating in matrix produced by the reaction between Mg and Si. The Mg2Si phase also can enhance the mechanical properties of matrix, finally improve the compressive properties of the composites. In generally, the strength of AleMg alloys would increase with the increasing addition of element Mg because of the solution strengthening of Mg element in Al alloy. Although the solution strength effect of Mg in Al alloys would weaken by the interfacial reaction, it still cannot be neglected in term of the high Mg concentration in matrix. The strengthening effects of Mg in cenospheres/AleMg syntactic foams are diverse and complex. The intercoupling of these strengthening effects finally improves the mechanical properties and energy absorption of these syntactic foams. 4. Conclusion In this work, the Al matrix syntactic foams reinforced with cenopsheres were successfully fabricated by pressure infiltration process. The average densities of prepared foams were comprehensively lower than most of other aluminum syntactic foams in present study, ranged from 1.17 g/cm3 to 1.30 g/cm3 with 50e56% porosity. The interfacial reactions between cenospheres and AleMg alloy were detailed investigated. During the process of fabrication, element Al and Mg in matrix first diffused to the surface of cenospheres, reacted with SiO2 in cenospheres and formed spinel MgAl2O4 coating. The thickness of MgAl2O4 coating reached 800 nm. In the meantime, element Si was forced to move away from cenospheres and segregated outside the interfacial region, subsequently, rod-shaped Si came into being and then grew towards matrix. With increasing of Mg content in matrix, the Si produced above would continue to react with Mg to form Mg2Si. The precipitation of Mg2Si can improve the mechanical property of composite. The spinel MgAl2O4 particles uniformly precipitate on glass cenospheres because of the spherical structure of cenosphere rather than glass particles. A suitable interfacial reaction will obtain with the tailor of the content of cenospheres and matrix which can not only keep pore structure but strengthen the interface bonding in syntactic foams. The strength of composites increases with the increase of Mg content. It is not only contributed to the improved strength of matrix but also the interfacial bonding which is related to element Mg. The strengthening effects of interfacial MgAl2O4 coating, precipitating Mg2Si, and matrix with increasing Mg
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content are complex and finally improve the mechanical properties and energy absorption of syntactic foams. Acknowledgments The authors would like to thank the Chinese government for the support rendered to this work through the National Natural Science Foundation of China (Grant No. 51001037). References [1] M.Y. He, F.W. Zok, M. Kiser, The mechanical response of ceramic microballoon reinforced aluminum matrix composites under compressive loading, Acta Mater. 47 (1999) 2685e2694, http://dx.doi.org/10.1016/S1359-6454(99) 00129-9. [2] M. Alizadeh, M. Mirzaei-Aliabadi, Compressive properties and energy absorption behavior of AleAl2O3 composite foam synthesized by space-holder technique, Mater. Des. 35 (2012) 419e424, http://dx.doi.org/10.1016/ j.matdes.2011.09.059. [3] Daoud, Compressive response and energy absorption of foamed A359eAl2O3 particle composites, J. Alloys Compd. 486 (2009) 597e605, http://dx.doi.org/ 10.1016/j.allcom.2009.07.013. [4] P.K. Rohatgi, J.K. Kim, R.Q. Guo, D.P. Robertson, M. Gajdardziska-josifovska, Age-hardening characteristics of aluminum alloy-hollow fly ash composites, Metall. Mater. Trans. A 33A (2002) 1541e1547, http://dx.doi.org/10.1007/ s11661-002-0076-7. [5] P.K. Rohatgi, Thermal expansion of aluminum-fly ash cenosphere composites synthesized by pressure infiltration technique, J. Compos. Mater 40 (2005) 1163e1174, http://dx.doi.org/10.1177/0021998305057379. [6] P.K. Rohatgi, A. Daoud, B.F. Schultz, T. Puri, Microstructure and mechanical behavior of die casting AZ91D-fly ash cenosphere composites, Compos. Part A. 40 (2009) 883e896, http://dx.doi.org/10.1016/j.compositesa.2009.04.014. [7] Daoud, Effect of fly ash addition on the structure and compressive properties of 4032efly ash particle composite foams, J. Alloys Compd. 487 (2009) 618e625, http://dx.doi.org/10.1016/j.allcom.2009.08.026. [8] I.N. Orbulov, Compressive properties of aluminium matrix syntactic foams, Mater. Sci. Eng. A 555 (2012) 52e56, http://dx.doi.org/10.1016/ j.msea.2012.06.032. [9] R.A. Palmer, K. Gao, T.M. Doan, L. Green, G. Cavallaro, Pressure infiltrated syntactic foamsdprocess development and mechanical properties, Mater. Sci. Eng. A 464 (2007) 85e92, http://dx.doi.org/10.1016/j.msea.2007.01.116. [10] W.S. Sanders, L.J. Gibson, Mechanics of hollow sphere foams, Mater. Sci. Eng. A 347 (2003) 70e85, http://dx.doi.org/10.1016/S0921-5093(02)00583-X. [11] D.D. Luong, O.M. Strbik, V.H. Hammond, N. Gupta, K. Cho, Development of high performance lightweight aluminum alloy/SiC hollow sphere syntactic foams and compressive characterization at quasi-static and high strain rates, J. Alloys Compd. 550 (2013) 412e422, http://dx.doi.org/10.1016/ j.allcom.2012.10.171. [12] J. Banhart, Manufacture, characterisation and application of cellular metals and metal foams, Prog. Mater. Sci. 46 (2001) 559e632, http://dx.doi.org/ 10.1016/S0079-6425(00)00002-5. [13] M.D. Goel, V.A. Matsagar, A.K. Gupta, Blast resistance of stiffened sandwich panels with aluminum cenosphere syntactic foam, Int. J. Impact Eng. 77 (2015) 134e146. [14] M.F. Ashby, A.G. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley, Metal Foams: a Design Guide, Butterworth-Heinemann, Warrendale, 2000, http://dx.doi.org/10.1016/j.ijimpeng.2014.11.017. [15] R. Li, G. Wu, L. Jiang, D. Sun, Characterization of multi-scale porous structure of fly ash/phosphate geopolymer hollow sphere structures: from submillimeter to nano-scale, Micron 68 (2015) 54e58, http://dx.doi.org/10.1016/
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Interfacial microstructure and compressive properties of AleMg syntactic foam reinforced with glass cenospheres Yingfei Lin, Qiang Zhang*, Gaohui Wu School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 August 2015 Received in revised form 14 September 2015 Accepted 18 September 2015 Available online 25 September 2015
Glass cenospheres/Al syntactic foams are successfully produced with about 50% volume fraction using pressure infiltration process. The interfacial reactions between glass cenospheres and AleMg alloys (5A03 with 3wt% Mg and 5A06 with 6wt% Mg) have been deeply studied. The MgAl2O4 crystals produced by the reaction of 2Al(l)þMg(l)þ2SiO2(s) / 2MgAl2O4(S) þ 2Si(S) are the main reaction products in the size of ~100 nm and the reaction layer is about 800 nm thick. The interfacial reaction region is composed of a large amount of MgAl2O4 uniformly coating on the glass cenospheres due to the spherical structure of cenosphere. Si is believed to diffuse through the grain boundaries between the MgAl2O4 crystals and aggregate on the surface of the reaction zone. With the increase of Mg content in matrix such as 5A06 alloy, Si formed above transforms into Mg2Si by the reaction of 2 Mg(l) þ Si(s) / Mg2Si(s). The compressive strength of composites increases with the increase of Mg content which is not only contributed to the improved strength of matrix but the suitable interfacial reaction coating and the forming of Mg2Si in matrix. © 2015 Elsevier B.V. All rights reserved.
Keywords: Glass cenospheres AleMg alloys Interfacial reactions Compressive properties Metal matrix composites
1. Introduction Aluminum alloy syntactic foams are a class of new structural and functional composite materials synthesized by aluminum alloy filling with the hollow particles known as ceramic hollow spheres such as alumina cenospheres [1e3] and fly ash [4e7]. It has been paid great attention on aluminum alloy syntactic foams because of their excellent combination of physical and mechanical properties such as high energy absorbing capabilities, high stiffness, improved strength and damping capacities in addition to their light weight characteristics [8e11]. These properties make syntactic foams possible to use in a various applications such as, core in sandwich structures, crash safety, and damping panels [12,13]. For cellular materials, the function is usually affected by the characteristic of porous structure, and mechanical properties are dependent on the interface structure [14e16]. Under the impact loadings, the ceramic hollow spheres embedded in foams can be thought as a supporting role in syntactic foams due to the high strengths of the ceramic spherical shell [17,18]. The syntactic foams reinforced with ceramic hollow spheres
* Corresponding author. P.O. Box 433, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. E-mail address: [email protected] (Q. Zhang). http://dx.doi.org/10.1016/j.jallcom.2015.09.175 0925-8388/© 2015 Elsevier B.V. All rights reserved.
exhibits the advantages of isotropic mechanical properties, higher strengths with extensive strain accumulation resulting in excellent energy-absorption capacities [19]. In addition, hollow spheres can be classified into different densities using flotation methods. On the other hand, the undamaged spheres can be divided into different sizes by sieves with corresponding mesh sizes. The variety of cenospheres provides a possibility for designing excellent syntactic foams meeting application demands [20,21]. Glass cenospheres, a kind of ceramic hollow sphere, are used to be fillers in polymeric foams [22,23]. Similarly, these syntactic foams generally have a good energy absorption capacity but are easy to be sensitive to strain rates due to the characteristic of polymer [24]. That is to say, glass cenospheres have a great perspective application in aluminum syntactic foams. AleMg alloys belong to the group of aluminum alloys which cannot be strengthened by heat treatment. These alloys possess a series of great properties, such as corrosion resistance, good weldability and cutting performance [25e27]. Meanwhile, AleMg alloys have the advantage of good shock resistance and ductility in aluminum alloys. Magnesium can decrease the surface tension of molten aluminum which can promote the infiltration. It is generally stated that magnesium commonly plays an important role in increasing the strength and improving the wetting properties with SiO2 in light weight Al alloys [7,28]. It is generally stated that Mg in
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the matrix will react with glass phase such as Al2O3 and SiO2 [29,30]. In the present work, we investigate the feasibility of synthesis the AleMg syntactic foam filled with glass cenospheres by pressure infiltration process. In view of the availability of syntactic foams and the active magnesium in matrix, the microstructural features, interfacial reactions between glass cenospheres and AleMg matrix, and compressive properties of the syntactic foams have been mainly discussed in detail in order to lay a foundation for further research and application. 2. Materials and experimental 2.1. Fabrication of the AleMg syntactic foam The compositions of matrices, commercial pure aluminum (1100Al), 5A03, and 5A06, were listed in Table 1. The main difference in these alloys is the content of Magnesium which is about 0%, 3%, and 6% by weight, respectively. The glass cenospheres, named S38 considering its density supplied by 3M Company, were to serve as fillers. The morphology of glass cenospheres is shown in Fig. 1. It shows spherical shape for most of cenospheres with generally smooth exterior surfaces. The main elements detecting by energy dispersive X-ray analysis are 53.99wt% O, 34.25wt% Si, 8.26wt% Ca, and 3.51wt% Na. Their size ranges from 20 to 90 mm and the average size is 40 mm. The chemical composition of glass cenospheres was listed in Table 2 (3M glass bubbles product information from 3M Company). The cenosphere is mainly composed of SiO2 and CaO, which can act as indicators of cenosphere. The main physical properties of the ceramic cenospheres were listed in Table 3 (3M glass bubbles product information from 3M Company). It can be seen that the glass cenospheres in this study process a certain of strength with high porosity. The syntactic foams were fabricated by pressure infiltration technique. Glass cenospheres, filling in a steel mold, were pressed with a vertical pressure of ~0.5 MPa as a cylindrical preform with a 40 mm height. The perform was then preheated to 600 C. Concurrently, matrix alloy was heated in a graphite crucible to a temperature of 850 C at which point the molten metal was poured into the steel mold. Then a mechanical pressure of ~0.8 MPa was applied to press the molten matrix vertically, causing downwards infiltration of matrix into cenospheres perform. The applied pressure was maintained for 5 min until the foam solidified completely. All processing was in air. 2.2. Density The density of the composites was determined by means of Archimedes' principle, using a microbalance (BP211D-0CE from Satorius AG) with ethanol as suspending medium. The porosity of the composite was calculated using rules of mixture as:
rc ¼ rm 1 Vf þ rf Vf
(1)
VP ¼ P0 Vf
(2)
where rc is the density of composite determined by Archimedes' Table 1 Typical chemical composition of matrix in weight procent. Alloy
Cu
Mg
Mn
Fe
Si
Zn
Ti
Al
1100 5A03 5A06
0.05e0.2 0.1 0.1
e 3.2e3.8 5.8e6.8
0.05 0.3e0.6 0.5e0.8
0.5 0.5 0.4
0.4 0.5e0.8 0.4
0.1 0.2 0.2
e 0.15 0.02e0.1
Bal. Bal. Bal.
Fig. 1. The SEM image of raw cenospheres.
principle, rf (shown in Table 3) is the density of fillers (glass cenospheres), Vf is the volume fraction of fillers in composite, P0 (shown in Table 3) is the porosity in cenosphere, and VP is the porosity of composite. 2.3. Analysis of microstructure Specimens for microstructure investigations were cut from composites castings and annealed for 30 min at 325 C for the purpose of eliminating stress. The specimens were polished using standard metallographic technique with ethanol which can prevent the oxidation of matrix in composites. In consideration of the interfacial reactions between matrix and cenospheres, composites were corroded by acid solution for extracting the products of interface. The microstructural features and phases of composites, raw cenospheres, and cenospheres extracted from composites were revealed by scanning electron microscope (SEM, Helios Nanolab600i) and X-ray diffraction analysis (XRD, Rigaku D/max2200), respectively. The backscattered electron (BSE) image was employed to characterize the precipitated-phase or impurity phase in composites. SEM with energy dispersive X-ray analysis (EDXA) is used for the elemental analysis of certain phases observed in the composites. Transmission electron microscopic (TEM, TecnaiF2F30) is performed on confirming the phases in interfacial reaction zone and discussing the process of interfacial reactions. However, the TEM sample of composite was difficult to prepare in a normal way due to its porous nature. There is no enough thin area for observing the interfacial zone in TEM sample. So, cenospheres were compressed into pieces for preparing bulk composite with the same process above. The TEM samples in this paper were obtained from the bulk composites for further study. 2.4. Compression test To evaluate the influence of Mg content on the compressive properties of the composites, quasi-static compression tests were performed on a Zwick universal testing machine (Instron 5569) according to the GB/T 7314-2005 standard for metallic materials compression testing at ambient temperature. Cylindrical
Table 2 Typical chemical composition of glass microspheres in weight procent. SiO2
CaO
Na2O
MgO
Al2O3
P2O5
SO3
LOIa
77.85
14.18
6.41
0.18
0.15
0.76
0.31
0.16
a
Loss on ignition.
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Table 3 Physical properties of the glass cenospheres. Density rf (g/cm3)
Bulk density rb (g/cm3)
Porosity P0 (%)
Wall thickness t (mm)
Strength sc (MPa)
0.38
2.54
85.04
1.05
27.56
Fig. 2. The SEM images of composites cenosphere/1100 (a); cenospheres/5A03 (b); cenospheres/5A06 (c).
specimens with height/diameter ratio of 1.5 and diameter 8 mm were used in uniaxial compression tests. The testing was carried out at constant crosshead speed with an initial strain rate of 103 s1. 3. Results and discussion 3.1. Density In this paper, the composites have been successtully fabricated. The average densities measured for cenospheres/1100, cenospheres/5A03, and cenosphere/5A06 with Eq. (1) were 1.17 g/cm3, 1.22 g/cm3 and 1.31 g/cm3, respectively. The corresponding porosities of composites referring to Eq. (2) were 56.0%, 53.8% and 49.5%. Addition of glass cenospheres to the Al alloy resulted in an obviously decrease in the density of the resulting composite in comparison to the base alloy. The composites in this work are generally lighter than other aluminum matrix syntactic foams under the same porosity [8,31]. It can be attributed to the lightweight cenospheres and AleMg alloy. In addition, the high composites' porosity implies the great compressive densification strain and efficient energy absorption.
3.2. Microstructure and phases in composites The microstructures of Al syntactic foams in annealed condition are shown in Fig. 2. Visual inspection of polishing surface of Al syntactic foams shows uniform distribution of cenospheres throughout the composite. There are two positions that cenospheres perform in composites, such as the A and B area display in Fig. 2aec. A majority of cenospheres in composites stay as integrated as the cenosphere which shown in area A. A few of cenospheres are too weak to bear the pressure during infiltration, they are broken and perforated, molten matrix alloy flows into, completely fills in the core of cenosheres, which presented in area B. However, the amount of perforated cenospheres is too few to affect the porosity of composites. X-ray diffraction analysis is required to determine the phases in composites. The XRD patterns of the prepared composites and the corresponding matrix alloys are presented in Fig. 3. In matrix alloy's XRD patterns, Al is the only phase can be observed. As compared to the standard pattern, the diffraction peaks of aluminum in AleMg alloy have moved to left about 0.3 . The deviation of peaks in 5A06 is more than that in 5A03. It indicates that the major solute element Mg in AleMg alloy play a significant role in this kind diffraction peaks moving. The lattice constant of a-Al will increase with the increasing of Mg content. According to the Bragg equation, 2dsinq¼nl (where d means interplanar distance, q means diffraction angle, l means x-ray wavelength, and n means diffraction series), diffraction angle q will decrease with lattice constant increasing. Different from the patterns of AleMg alloy, the diffraction peaks of Al in syntactic foams are no deviation from standard pattern. There is no other phase except Al can be seen in the pattern of cenosphere/1100. On the other hand, there are trace amount of Si and MgAl2O4 being found in conosphere/AleMg composites. What's interesting is that some weak peaks of Mg2Si can be found in cenospheres/5A06 XRD pattern. Considering the finding of Si and MgAl2O4 in AleMg syntactic foams, it implies that the Mg element in prepared composite precipitate out from a-Al by reacting with cenospheres (SiO2) resulting the diffraction peaks of Al back to the certified values. 3.3. Interfacial reaction
Fig. 3. The XRD of matrix alloys and composites.
Mg element is an active in aluminum alloys which is prone to reacting with reinforcements in composite [29,30]. Fig. 4 shows the
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Fig. 4. The SEM images of extracted cenosphere from cenosphere/1100 (a); cenospheres/5A03 (b); cenospheres/5A06 (c); the cross-section of cenospheres/5A03 (d); and surface morphology cenospheres/5A03 (e).
Table 4 The DEXA analysis of Fig. 4. Area
Element
OK
Na K
Mg K
Al K
Si K
Ca K
Total
A
wt% at% wt% at% wt% at%
61.42 73.82 47.31 60.12 33.91 47.36
3.69 3.08 e e e e
e e 10.20 8.45 e e
5.28 3.77 24.26 18.29 1.91 1.58
25.03 17.14 17.68 12.85 64.18 54.07
4.58 2.20 0.54 0.29 e e
100
B C
Fig. 5. The XRD of raw cenospheres and extracted cenosheres.
100 100
microstructure of cenospheres extracted from cenosphere/1100, cenospheres/5A03, and cenospheres/5A06 syntactic foams. The corresponding energy dispersive X-ray analysis of part A, B, and C are listed in Table 4. The exterior surface of cenosphere extracted from cenosphere/1100 stay as smooth as the raw cenospheres, which can be seen in Fig. 4a. The composition in part A of cenosphere extracted from cenosphere/1100 is similar to raw cenospheres. Compared to the smooth surface of raw cenospheres, the cenospheres extracted from AleMg composite are covered by a coating, shown as Fig. 4b and c. From the section of extracted cenospheres in Fig. 4d, it is found that the coating is composed of micro-particles layer by layer. Different from the EDXA result of raw cenospheres, a number of Mg and Al elements are observed in part B which refers to the surface of extracted conosphere. The atomic ratio of Mg, Al and O in part B almost meets the ratio of MgAl2O4. It suggests that the dense micro-particles on extracted cenosphere could be MgAl2O4 particles. Besides the micro-particles, there is overlapped dendritic substance attaching on the extracted cenosphere, shown in Fig. 4e. As compared to the EDXA result of raw cenospheres, the substance contains much more Si and no Ca or Na. Fig. 5 is the XRD patterns of cenospheres which can clarify the interfacial phase forming on the cenospheres. Glass cenospheres consist of different inorganic peroxides with a complex microstructure, such as a mixture of amorphous. The XRD pattern of the extracted cenospheres from cenosphere/1100 is the same as raw cenospheres. Differing from the raw cenospheres' XRD patterns, there are some diffraction peaks referring to spinel MgAl2O4 and Si in the XRD patterns of the cenospheres extracted from AleMg syntactic foams. As a matter of fact, spinel MgAl2O4 and Si are also found in composites, but much less than that in extracts. On the other hand, MgAl2O4 and Si are not the phases in the system of
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Fig. 6. (a) The TEM images of cenospheres/5A06 composites; the diffraction pattern of part I (b); II (c); III (d).
Fig. 7. Microstructure (a) and corresponding area distribution analysis of Al (b); Mg (c); O (d); Si (e) and Ca (f) in part IV of Fig. 6a.
AleMg alloy [32]. To some extent, the coating layer forming on cenospheres results from the interfacial reaction between cenospheres and Al and Mg elements in the matrix and then the products are spinel MgAl2O4 and Si. In order to indentify the interfacial reaction path of cenospheres and matrix, TEM observations were carried out. Figs. 6e8 show transmission electron micrographs of interfacial region in the case of the prepared AleMg syntactic foams. An interfacial reaction region of approximate 800 nm thickness is clearly seen in Fig. 6a. The main phases of cenosphere's wall (area I), interfacial region (area II) and matrix (area III) were characterized by the diffraction patterns of Fig. 6bed. It proves again that cenosphere is composed of amorphous crystals confirming by the amorphous ring diffraction pattern. The matrix consists of coarse Al grains. The interfacial reaction region consists of a large number of fine spinel MgAl2O4 crystals (~100 nm) as identified from their diffraction pattern in area II. The area distribution of element Al, Mg, O, Si, and Ca in area IV of Fig. 6a was analyzed and shown in Fig. 7. As can be seen, a high concentration of Mg uniformly distributes in the interfacial region and degrades in the cenospheres' wall. The trend of O-concentration distribution is similar to element Mg but the degradation to cenospheres' wall is smaller than that of Mg. Different from the element Mg, the concentration of element Si increases to the matrix while element Ca is little in the matrix and no segregation in cenosphere region or interfacial region. It reveals that element Si gradually diffuses to the matrix when the interfacial reaction is in process and segregates in the matrix close to the interfacial region. There is no reaction between Ca and matrix. Fig. 8a shows the TEM image of interfacial reaction zone for cenosphere/5A03 composite, Fig. 8c shows for cenosphere/5A06 composite. There is a large piece of Si (confirmed by the diffraction pattern in Fig. 8b) precipitating
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Fig. 8. The TEM images of (a) cenospheres/5A03; (c) cenospheres/5A06; the diffraction patterns of part A (b); B (d).
on the surface of reaction region in cenospheres/5A03 foams. That is to say, Si phase would precipitate while the concentration of element Si reaches a certain amount in matrix. In the case of cenospheres/5A06 foam whose Mg content is more than that in cenospheres/5A03 foam, the enrichment of Mg still exist in the matrix near the interfacial region, then the Mg will continue to react with Si and Mg2Si (identified by diffraction pattern in Fig. 8d) forms on the surface of the reaction region. The driving force for interface products forming is the Gibbs free energy associated with the reaction of cenospheres and matrix. From the thermodynamic point of view, the most likely interfacial reactions are listed as following [29]:
2AlðlÞ þMgðlÞ þ2SiO2ðsÞ /2MgAl2 O4ðsÞ þ2SiðsÞ DG850 C ¼2962kJ (3) 2MgðlÞ þ SiðsÞ /Mg2 SiðsÞ
DG850 C ¼ 59kJ
(4)
It infers that the dominant content (SiO2) in cenospheres prefers to react with Mg and Al under the fabrication temperature of syntactic foams. The interfacial reaction leads to producing spinel MgAl2O4 micro-particles on the surface of cenospheres and Si beside the interfacial region. There is a possibility that the Si in cenospheres would be driven away from cenospheres when SiO2
reacts with Al and Mg. In the view of Fig. 8a, rod-shaped Si grows away from interfacial reaction region and precipitates in matrix. It can be speculated that Si produced by the reaction would diffuse through the grain boundaries between MgAl2O4 crystals then precipitate in the matrix beside the interfacial reaction zone. With the increase of Mg content in matrix, as the 5A06 alloy in this paper, the redundant Mg would continue to react with Si forming from reaction (3), producing Mg2Si after no MgAl2O4 forming further. 3.4. Compressive properties Compressive properties of the syntactic foams can be understood by studying the stressestrain curves. Fig. 9 shows the quasistatic compressive stressestrain curves for the cenosphere/Al syntactic foams in this work. It is shown that as the Mg content in matrix increases from 0wt% to 6wt%, the corresponding compressive peak strength increases from 77.8 to 137.2 MPa. Generally, the strength of AleMg alloys will increase with the Mg content increasing, therefore, an increase in compressive strength of the composite is observed with the increase of Mg content in matrix. Ferguson et al. [31] modified a model that could predict the compressive strength (peak strength, sp) of Al matrix syntactic foams.
iA h w sp ¼ 2A%m sym þ A%s hsfw As
(5)
where sym is the yield strength of the matrix, A%m and A%s indicate the area fraction of the matrix and the cenospheres, respectively.sfw is the fracture strength of the wall material. h is the parameter considering the defeats of monolithic wall material. The parameter h in the model assumed as 0.357. The ratio of Aw/As is the smallest ratio of wall area to cross-sectional area. This model assumed that the cenospheres are considered as solid spheres having a density equal to the bulk density of the cenospheres. So, the volume percentage of the cenospheres (Vf) is thought to be equivalent to the area fraction of the cenospheres (A%s). That is to say the area fraction of the matrix A%m equal to the volume percentage of the matrix (1 Vf). The fracture strength of the wall material sfw in this work can be predicted by using the following equation [33]: 3
sc ¼ sfw Cð1 P0 Þ2
Fig. 9. The compressive stressestrain curves of composites.
(6)
where C is assumed to be 0.3, sc and P0 is the strength and the void volume fraction in cenospheres (see Table 3). The sfw is calculated to be 1587.7 MPa for cenosphere wall material. There are three
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307
Table 5 Compressive properties and calculated peak strength of cenospheres/Al syntactic foams. Composite
Experimental peak strength spe (MPa)
Densification strain εD (%)
Energy absorption E (MJ/m3)
Calculated peak strength spc (MPa)
Variance (%)
Cenospheres/1100 Cenospheres/ 5A03 Cenospheres/ 5A06
77.8 109.0
60.1 57.6
36.4 52.4
79.7 82.2
2 32
137.2
48.8
68.2
82.4
66
main assumptions inherent the predict peak strength of the syntactic foams: (1) the infiltration of broken or perforated spheres is thought to have no effect; (2) the matrix is considered to have no shrinkage or gas porosity; and (3) the properties of the spheres are assumed to relate to the average sphere size. Table 5 shows the compressive properties data and the variance between the experimental peak strength and those predicted by the model of the syntactic foams in this work. The yield strength of matrix alloys 1100Al, 5A03, and 5A06 for model are 50 MPa, 118 MPa, and 167 MPa, respectively. The difference of densification strain εD can be contributed to the difference of porosity and matrix's ductility among the syntactic foams. The One of the important technological properties to estimate the application of metal matrix syntactic foams is energy absorption characteristic, the energy can be evaluated by integrating the area under the compressive stressestrain curve up to the onset of densification strain εD as follow:
ZεD E¼
sðεÞdε
(7)
0
As the small difference of the densification strain εD among the syntactic foams, the energy absorption increases with the increasing of compressive stress. The peak strength of cenospheres/ 1100 syntactic foam which has no interfacial region shows excellent agreement to the model, the variance is just 2%. For cenospheres/ 5A03 and cenospheres/5A06 syntactic foams, the peak strength is higher than the predicted one. Meanwhile, the variance value between experimental peak stress and calculated peak stress of cenospheres/5A06 is 66% which much higher than 32% of cenospheres/5A03. It suggests that a certain interfacial reaction between cenospheres and matrix can improve the strength of syntactic foams. Firstly, the forming of MgAl2O4 coating is beneficial to the interface bonding between matrix and cenospheres. Secondly, the fracture strength of the MgAl2O4 coating may be stronger than that of cenospheres' wall material. The MgAl2O4 crystals in this paper uniformly precipitate on the glass cenosphere, while in the case of glass particles filled in AleMg alloy, the MgAl2O4 crystals precipitate locally on the glass particles. The glass cenospheres in this paper have smooth exterior surfaces and homogeneous wall. For the reason of the same curvature in a cenosphere, the probability of MgAl2O4 crystals nucleation on the each position of cenosphere is same which leads to uniformly distribution on cenospheres. Thus, most of cenospheres in composite still keep pore structure and spherical shape after interfacial reaction. In principle, when there is a few conospheres in composite, an over-reaction would happen and consume lots of SiO2 causing perforation on the cenospheres, which makes the cracking or collapse of cenospheres. But, if there is a high volume fraction of cenospheres in composite (just as the composite in this paper), the reactants and the reaction degree would decrease in each position with the increase of interface. A suitable interfacial reaction will not only keep pore structure but strengthen the interface bonding in syntactic foams. Moreover, the
spe spc spc
Mg2Si is a reinforcing phase in Al alloy which has high melting point, low density and good mechanical properties. The precipitation of Mg2Si in the matrix of cenosphere/5A06 syntactic foam is contributed to improving the mechanical property of composite by second phase strengthening mechanism. This is why the variance value in cenosphere/5A06 syntactic foam is larger than cenosphere/ 5A03 syntactic foam. There are three main strengthening effects correlated with element Mg on composite. On the one hand, the interfacial reaction layer MgAl2O4 because of the addition of Mg in matrix alloy were uniformly coating on the cenospheres, which can improve the compressive properties of cenospheres. On the other hand, with the increase of Mg content in matrix, there is Mg2Si phase precipitating in matrix produced by the reaction between Mg and Si. The Mg2Si phase also can enhance the mechanical properties of matrix, finally improve the compressive properties of the composites. In generally, the strength of AleMg alloys would increase with the increasing addition of element Mg because of the solution strengthening of Mg element in Al alloy. Although the solution strength effect of Mg in Al alloys would weaken by the interfacial reaction, it still cannot be neglected in term of the high Mg concentration in matrix. The strengthening effects of Mg in cenospheres/AleMg syntactic foams are diverse and complex. The intercoupling of these strengthening effects finally improves the mechanical properties and energy absorption of these syntactic foams. 4. Conclusion In this work, the Al matrix syntactic foams reinforced with cenopsheres were successfully fabricated by pressure infiltration process. The average densities of prepared foams were comprehensively lower than most of other aluminum syntactic foams in present study, ranged from 1.17 g/cm3 to 1.30 g/cm3 with 50e56% porosity. The interfacial reactions between cenospheres and AleMg alloy were detailed investigated. During the process of fabrication, element Al and Mg in matrix first diffused to the surface of cenospheres, reacted with SiO2 in cenospheres and formed spinel MgAl2O4 coating. The thickness of MgAl2O4 coating reached 800 nm. In the meantime, element Si was forced to move away from cenospheres and segregated outside the interfacial region, subsequently, rod-shaped Si came into being and then grew towards matrix. With increasing of Mg content in matrix, the Si produced above would continue to react with Mg to form Mg2Si. The precipitation of Mg2Si can improve the mechanical property of composite. The spinel MgAl2O4 particles uniformly precipitate on glass cenospheres because of the spherical structure of cenosphere rather than glass particles. A suitable interfacial reaction will obtain with the tailor of the content of cenospheres and matrix which can not only keep pore structure but strengthen the interface bonding in syntactic foams. The strength of composites increases with the increase of Mg content. It is not only contributed to the improved strength of matrix but also the interfacial bonding which is related to element Mg. The strengthening effects of interfacial MgAl2O4 coating, precipitating Mg2Si, and matrix with increasing Mg
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