Investigation of microstructural and mechanical properties of cell walls of closed-cell aluminium alloy foams

Author’s Accepted Manuscript Investigation of microstructural and mechanical properties of cell walls of closed-cell aluminium alloy foams M.A. Islam, M.A. Kader, P.J. Hazell, A.D. Brown, M. Saadatfar, M.Z Quadir, J.P. Escobedo www.elsevier.com/locate/msea

PII: DOI: Reference:

S0921-5093(16)30430-0 http://dx.doi.org/10.1016/j.msea.2016.04.046 MSA33575

To appear in: Materials Science & Engineering A Received date: 3 March 2016 Accepted date: 15 April 2016 Cite this article as: M.A. Islam, M.A. Kader, P.J. Hazell, A.D. Brown, M. Saadatfar, M.Z Quadir and J.P. Escobedo, Investigation of microstructural and mechanical properties of cell walls of closed-cell aluminium alloy foams, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.04.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Investigation of microstructural and mechanical properties of cell walls of closed-cell aluminium alloy foams M.A. Islam1, M.A. Kader1, P.J. Hazell1, A.D. Brown1, M. Saadatfar2, M.Z. Quadir3 and J.P. Escobedo1*, 1

School of Engineering and Information Technology, UNSW Canberra, ACT 2610, Australia

2

Department of Applied Mathematics, Australian National University, Canberra, ACT 0200, Australia 3

UNSW Australia, Sydney, NSW 2502, Australia

Corresponding author: [email protected]

Abstract This study investigates the influence of microstructure on the strength properties of individual cell walls of closed-cell stabilized aluminium foams (SAFs). Optical microscopy (OM), micro-computed X-ray tomography (µ-CT), electron backscattering diffraction (EBSD), and energy dispersive X-ray spectroscopy (EDS) analyses were conducted to examine the microstructural properties of SAF cell walls. Novel micro-tensile tests were performed to investigate the strength properties of individual cell walls. Microstructural analysis of the SAF cell walls revealed that the material consists of eutectic Al-Si and dendritic a-Al with an inhomogeneous distribution of intermetallic particles and micro-pores (void defects). These microstructural features affected the micro-mechanism fracture behaviour and tensile strength of the specimens. Laser-based extensometer and digital image correlation (DIC) analyses were employed to observe the strain fields of individual tensile specimens. The tensile failure mode of these materials has been evaluated using microstructural analysis of post-mortem specimens, revealing a brittle cleavage fracture of the cell wall materials. The microporosities and intermetallic particles reduced the strength under tensile loading, limiting the elongation to fracture on average to ~3.2% and an average ultimate tensile strength to ~192

MPa. Finally, interactions between crack propagation and obstructing intermetallic compounds during the tensile deformation have been elucidated. Keywords: Closed-cell foams, microstructure, micro-tensile, cell wall properties.

1. Introduction Closed-cell aluminium foams are well known for their unique physical and mechanical properties, including, but not limited to: ultra-lightweight, high specific strength, high stiffness and extraordinary energy absorption capabilities during compressive loading. These characteristics appeal to numerous applications in aerospace, civil and defence engineering as well as protective equipment [1-4]. It is well known that mechanical performance of closedcell foams macroscopically depends on relative density and geometric features such as cell size and cell wall thickness [5-7]. Numerous experimental [8-12] and numerical [13-15] investigations into the structural behaviour of metallic foams have been extensively carried out over the past two decades; primarily focusing on geometric parameters relating the cell structure to mechanical performance. One the other hand, the effect of microstructural features of metallic cellular materials on their mechanical properties has not been investigated except in a few recent studies [16-19]. The strength of individual foam cells is governed by microstructural features such as grain size, grain boundaries, and precipitation of foaming agents (stabilized particles) in the microstructure [20-21]. Closed-cell foams of identical density demonstrated different compressive plateau strengths due to variations in the alloying composition [21]. Closed-cell foams usually produced in the melt route contain approximately 10-30% ceramic particles by volume, which are added during foaming [22]. These particles enhance the stability of the foaming process; however, melting of the base alloy and the addition of ceramic particles changes the microstructural and mechanical properties of the final material [23-24]. The role of these stabilized particles on mechanical properties is unclear to date. Moreover, mechanical property changes such as elastic-plastic transition and ductile-brittle formation during the foaming are a general phenomenon of cast alloys [24]. Approximately 8-10% of the SAF alloy is silicon, which increases the fluidity significantly [24], but at the expense of

ductility and an increase in brittleness [25-27]. Thus, it is expected that the resultant cast material is likely to exhibit different mechanical behaviours than the base alloy material.

Several studies have aimed to mechanically test individual struts from open cell foams [19, 28, 29]. However, the cell size and shape distribution in closed-cell foams is not homogeneous, which leads to challenges when fabricating specimens for tensile tests from the cell walls. To the authors’ knowledge, comprehensive studies of individual cell-wall strength properties of closed-cell metallic foams are currently unavailable in the open literature. This is important for simulating the behaviour of these materials under structural loads, quasi-static and dynamic [30]. Hence, it is of interest to investigate strength properties of individual cell walls of closed-cell aluminium foams, which are the building blocks of the entire cellular structure. In depth microstructural analyses of SAF cell walls and its influence on foamed materials are essential since the alloying compositions may alter the crystallography and microstructural features. In this research, systematic microstructural characterization of cell wall materials is performed using optical microscopy, EBSD and SEM/EDS analysis. Furthermore, tensile strength and failure behaviour of individual cell walls have been provided via micro-tensile testing.

2. Materials and Methods 2.1 Materials The SAFs used in this study are manufactured through melting route and the base material consists of an aluminium alloy [31]; the composition of the base alloy material investigated in this study is given in Table 1. During processing the bulk alloy is melted and then stabilized particles consisting of ceramic compounds (Al2O3) are added to the molten materials [31], which is then poured into a foaming box. Bubbles are created by injecting gas in a controlled

fashion through a rotating impeller. The bubbles then rise through the liquid metal matrix, resulting in a foam structure. The cell size is controlled by varying the operating conditions, such as: gas pressure and temperature [32]. In this experiment, CYMAT™ SAFs with density of 0.17 g/cc (relative density 6.18 %) were investigated. The physical properties of the SAFs are illustrated in Table 2. During the removal of cell walls from the foam structure, several visible defects were documented, such as: a) rupturing of cell walls visible from the surface, b) coagulation of adjacent cell walls and c) non-flat “wrinkled” cell wall regions. The effects of these morphological inhomogeneities have only been reported in a few previous studies [33, 34]. Cell walls with pre-existing defects were avoided to study approximately homogenous cell walls. Since the cell shape and size is irregular, only relatively larger cells have been chosen for the ease of producing quality tensile samples. Further descriptions of the tensile samples are given in the tensile test section (3.2).

Table 1. The weight percent composition of the base SAF alloy Name

Al

Si

Fe

Cu

Mg

Mn

Ti

Zn

Percentage

>89

8-11

0-1.0

0-1.00

0-0.5

0.1-0.8

0-0.2

0-1.0

(weight)

Table 2. Physical properties of closed-cell aluminium foams Density

Relative Density

Porosity

Cell size

Cell-wall

(g/cc)

(%)

(%)

(mm)

thickness(mm)

0.17±0.1

6.30±0.25

93.70±0.50

9.00±1.25

0.10±0.02

2.2 Microstructural Examination Cell wall specimens were sectioned using a high precision cutting blade to avoid damaging the specimen. The fine cut cell-wall parts were embedded in an acrylic resin in both the through-thickness and in-plane orientations.

Microstructural observations of cell-wall

materials of the SAFs were carried out using OM, µ-CT, scanning electron microscopy, EDS and EBSD analysis. All metallographic procedures for the sample preparation for the aforementioned analyses have been carried out using a Struers’ automated polisher for mechanical grinding with silica-carbide paper through final polishing steps using colloidal silica suspension in a vibratory polisher. The samples used for EBSD analysis were oriented to analyze the through thickness and the in-plane surface of the material. The samples were then mounted in epoxy, mechanically polished with SiC papers, 1µm diamond suspension, and finished with colloidal silica in the vibratory polisher for 2.5 hours.

2.3 Tensile testing Micro-scale tensile tests were conducted on cell wall specimens with an average thickness of 100 µm, 3 mm in width and 4.5 mm in length. The sample dimensions have been chosen to maximize the size of the specimens while minimizing geometric inhomogeneity. The samples were prepared (Fig 1.a) using stainless steel high precision scissors. The samples were then

connected with a mechanical grip buffer layer (1mm thick Al. sheet of size 30mm x 30mm) using very high strength metal to metal adhesive (Master Bond MB303). A total of 35 samples were manufactured using this procedure. The experimental set-up was equivalent to ASTM D4018, for which the major components of the test set up are: loading actuator, gripper system, strain measuring systems and a load measuring system. Experiments have been carried out using a modified Shimadzu tensile testing machine outfitted with an additional laser extensometer and a DIC setup (section 3.4) mounted for accurate in-situ strain measurement. A small load cell (maximum 350 N) and laser based strain measuring system were used to measure the load and strain. A pneumatically controlled gripper system was implemented to provide sufficient gripping pressure without any slip during the tensile loading, which was performed with a constant cross-head velocity of 0.1 mm/min (strain rate ~10-4 s-1). The uniaxial tensile test setup is shown as a schematic in Fig.1 (b). Care was taken during the tests due to two aspects; a) the sample can be damaged during mounting due to comparatively heavier grips attached to the specimen and b) alignment of the specimen.

Fig.1. Experimental overview: a) example of an exhumed tensile specimen from a cell wall; b) schematic of experimental setup

3. Experimental Observations 3.1 Cell-wall microstructure A typical optical micrograph of cell-wall surfaces is given in Fig. 2. It was found that the microstructure consists of dendritic α-Al (15-25 µm) with a eutectic Al-Si domain approximately 10-20 µm in size. The two primary microstructural features examined in the cell wall materials are ceramic particles and eutectic silicon segregated into inter-dendritic regions, which is similar to other cast Al-Si alloys of similar base compositions [35]. The eutectic silicon in the form of globular and semi-fibrous crystal of a modified structure is predominant and the presence of several intermetallic compounds is also documented.

The distribution of particles visible from the cell wall cross-sections are shown in Fig.2. It is observed that the particles’ coverage in regions of the cell wall cross-section varied from 0 to 20% (Fig.2.a to 2.d), with a global area fraction of 7.50 %. The distributions of these intermetallic particles are heterogeneous over the surface of the cell wall and are predominantly found near the wall boundaries and plateau borders, or, cell wall joints. This mosaic distribution of the particles along the cross-section and surface of the cell wall can be further organized with size and shape. It is shown that larger particles were mainly concentrated directly on cell wall surfaces whereas smaller particles are heavily concentrated around the micro-pores and near the free surface edges.

In addition to dispersive

intermetallic compounds, an irregular distribution of micro-pores and micro-cracks were observed. The size of the micro-pores varied from 5µm to 80 µm and micro-crack lengths were observed exceeding 80 µm in some locations. It is proposed that these micro-pores are a

result from the micro-bubbles developing inside the cell wall during isothermal holding of molten alloy after the gas injection. Fig.2. Microstructure of cell-wall materials of SAFs. The cell wall has been magnified into four subsets: (a) eutectic aluminium particle coverage of 0%, (b) particles dispersed around a micro-pore and overall particle coverage of ~10%, (c) ~12 % and (d) 15%.

It was found that all micro-pores are surrounded by particles (Fig. 2.b to 2.d); however, all particles are not accumulated around micro-pores and are dispersed throughout the cell walls. High particle densities were observed near the plateau border of the cell walls compared to central regions of the cell walls. Figure 2(a) to 2(d) show regions containing approximately 0%, 5%, 10% and 20% localized area fraction of particles to metal matrix. In short, the particles agglomeration on the cell wall of the foam can be random, surface segregation and

cluster type. Closed cell metal foams without particles are not possible since the stabilization of foaming is inherently relying on the particles’ addition [32]. The role of these particles on the bulk strength properties has been further elucidated in section (4.2). Porosity defects in the microstructure of cell walls have been further analysed using µ-CT. Figure 3 shows µ-CT images of a cell wall surface, the plateau border and a cell wall crosssection. A dispersion of both micro and macro-cracks are observed in µ-CT image. Furthermore, crack in the cell wall also appears which have the maximum crack length exceeding 100µm as is in Fig. 3.c). In comparison to the optical microstructure, a higher porosity fraction was found from the µ-CT data. This is to be expected since a single 2-D slice cannot yield volumetric information about the porosity and serial sectioning was not performed via mechanical polishing. The defect fraction of microstructure has been measured using the open sourced ImageJ software. It has been found that the void fraction of the porosity encompasses up to 3.20% of total area of the microstructure of a typical cell-wall surface. However the area fraction of defect (void) does not provide the overall of void in a single cell wall specimen. To measure the exact volume fraction, we developed the x-ray data into 3D reconstructed cell wall using 3D image rendering software ‘Drishti’. Fig.3. also shows the 3D reconstructed (Fig. 3.d) cell wall and void distributions (Fig. 3.e) in a cell wall of SAFs. The reconstructed cell wall reveals that various shaped and sized bubbles presence in the cell wall. A void distribution in the cell wall has been obtained by eliminating material using above mentioned software. The gas bubbles entrapped in the cell wall during the foaming process were size from ~200 nm to 80 µm, while the crack length found are exceeding the 100 µm in some cases. The volumetric defect fraction of the porosity was 3.34 % which is slightly higher than area fraction of defects. Thus, evidence of massive porosity within the cell walls of SAF is well established from our OM and µ-CT analyses. A single macro pore in the cross-section of a specimen greatly increases the vulnerability of the

specimen to failure since thickness of the specimen approaches the same length scale (~100 µm) of some identified macro pores. It is hypothesized that the material’s response to tensile loading will be affected by the void fraction and distribution of these defects and is further discussed in section 4.2..

Fig.3. Micro-CT images of a cell wall. Black regions within the cell wall material are from porosity and micro-cracks, while the lighter grey regions represent the ceramic particles distribution. The orientations shown are: (a) the cell wall surface, (b) plateau border (cell wall joint), (c) cross-section of the cell wall, (d) 3D reconstruction of cell wall and (e) total void distribution in a cell wall material.

EBSD analysis was carried out to investigate the microstructural and crystallographic features of the SAFs. Figure 4 shows the orientation mapping of an SAF cell wall (in-plane). The color level of each pixel in the inverse pole figure (IPF) is defined according to the deviation of the measured orientation. Black regions indicate the presence of second phase intermetallic particles. Grain boundary mapping shows that very few sub-grain structures are present. It is shown that the grain sizes within the cell wall are up to several hundred micrometers. Notably, the thickness of the cell wall is less than the average grain size and the effects of grain size on the thickness of FCC polycrystalline tensile specimens have been discussed in [36]. It has been found by [36] that limited numbers of grains across one of the dimensions of the specimen (first order size effect) weaken the polycrystalline structure. Hence, the presence of first order size effects [36] is expected to affect the tensile properties in this experiment due to limited thickness.

Fig.4. Grain map and orientation of cell-wall materials of SAFs

The intermetallic phases of the alloy have been further studied using scanning electron microscopy coupled with EDS to identify the point wise elemental compositions. The base alloy has been found enriched with Al enriched phase whereas intermetallic phase was enriched with Si and Fe. It is noted that Fe is not present in the base alloy and that the presence of Fe may come from the addition of particles [31].

3.2 Characteristics of tensile specimen Cell wall thicknesses have been measured using sharp angle high precision conical micrometres and OM image analyses. The tolerance for the measurements using the conical micrometre is ±5 µm. The average thickness of the cell walls was found to be between 70 µm to 140 µm. Measurements were taken at 250 points from 50 cell walls on the same foam to capture the variance in thickness of cell walls. It has been observed that the cell wall thickness is not uniform throughout the specimen length; thicknesses of cell walls were varied between the plateau borders. Figure 5(a) shows the typical thickness distribution of cell walls between two plateau borders (sample 12), which exhibited the approximate average thickness values across all specimens. As observed in Fig.5, the thickness remains 100 ±10 µm within the constant wall thickness section, with few local inhomogeneities.

Fig.5. Cell wall geometry: a) thickness distribution between two wall joint (plateau border) and b) Cell-wall thickness distribution of the specimen 12.

Figure 6 shows optical micrographs typical cell wall cross-section. It was observed that the microstructure along the cross-section is almost identical to the microstructure along longitudinal surface. The particle distribution is again random with micro-pores and microcracks present along the cross-section of cell wall.

Fig.6. Cross-section of cell walls (left) and microstructure through thickness of cell wall

3.4 Digital Image Correlation Digital image correlation (DIC) was implemented to measure the deformation and strain fields in situ. Strain measurements from the DIC technique were compared to an

extensometer integrated with the testing machine in addition to a laser extensometer. Prior to testing, the specimens were speckled on both sides with white and black spray paint to create a surface with random gray level sufficient for software recognition. A digital camera (Canon 70D) was placed 0.5m away from the specimen and was positioned perpendicular to the specimen surface to avoid artifacts from false out of plane distortion. A continuous video recording of 2D deformation was taken for each sample. A LED light source was used to illuminate the specimen for optimal image quality. The recordings were segmented into 100 images (1280x1024 pixels) with an initial undeformed image used as a reference image and a time interval between images was set at 15 seconds to capture the entire strain to failure process. The measurement of displacements on the surface of the sample and calculation of strains were performed using open image processing software Ncorr and were later verified using commercial image analysis software Vic-2D. The DIC analysis process was followed similar to [36].

Y

X

Fig.7. Experimental full longitudinal strain field (εyy ) of the specimen resulting from the tensile test.

The DIC analyses show that the strain measured using the laser extensometer and normal extensometer were equivalent to within ±5%. Aside from an initial strain reading, the extensometer reading was slightly larger than the DIC measurements. Figure 7 shows the strain field and measured strain of three representative specimens at various stages of deformation. The strain and displacement fields obtained by DIC software reveal a wide variation in the material responses between specimens. This is likely due to casting defects varying between samples: porosity size differences and inhomogeneity in the porosity and ceramic particle distributions. Consequently, the tensile behavior of cell wall samples of SAF demonstrated a range of tensile strengths, as discussed in the next section.

4. Results and Discussions 4.1 Tensile Properties A test summary has been presented in Table 3, Fig. 8 and Fig. 9 provides visual aide for the distribution of ultimate tensile strength (UTS), percentage of elongation at UTS, elastic modulus and yield strength at 0.2% offset. The average UTS, yield strength, modulus of elasticity and elongation at UTS of 25 specimens with standard deviation have been estimated as: 192± 14.69 MPa, 134± 8.05 MPa, 9.7±1.54 GPa and 3.2±0.38 % respectively. Representative uniaxial tensile stress versus strain curves of the cell wall material of SAF are shown in the Fig.8. Each curve of different cell walls shows approximately linear elastic behavior and tensile strength reaches the ultimate value when the elastic limit is about to end followed by sudden drop. The stress-strain curves demonstrate the linear elongation behavior and the slope of the curves represents the elastic modulus ~ 9.7 GPa. The specimens fully fractured just after the ultimate tensile load was attained. All 33 specimens were tested under the same conditions. However, a few of the samples were discarded during the test due to

some local bending at the time of insertion of the specimen in the pneumatic gripper and are omitted from Table 3 and subsequent figures. Table 3: Test summary Specimen No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 18 19 20 21 22 23 24 26 29 31

Yield stress(MPa) 0.2% offset 141 127 137 121 128 148 146 135 128 133 148 135 143 124 138 132 128 131 121 131 132 148 127 141 138

Ultimate tensile stress (MPa) 211 191 175 186 172 210 191 187 198 208 188 196 182 164 217 206 191 195 193 172 184 207 237 168 207

Ultimate tensile strain 2.79 2.69 3.21 2.46 2.63 3.42 3.24 3.36 3.81 3.41 3.39 3.25 2.81 3.45 3.38 2.8 3.56 3.91 3.12 3.81 3.7 2.75 3.41 3.51 2.87

Modulus of Elasticity(GPa) 11.21 9.20 8.78 11.31 7.84 7.71 9.57 8.89 9.6 9.88 12.70 13.35 9.78 9.50 11.25 8.5 9.85 8.54 9.41 10.58 9.21 11.41 12.44 8.72 10.22

Specimens were analyzed post mortem; Figure 10 shows the representative fracture profiles of tensile specimens and DIC images of the strain fields just before fracture for five specimens that are representative of the full range of strain to failure. The DIC analyses show that specimens exhibiting larger total strains to failure contain a larger area of high strain than specimens exhibiting lower strains to failure. The fracture surfaces of the specimens reveal two types of failure: a cleavage of the particles surrounded by the micro-pores and a “dimple and tear” fracture. The cleavage fracture mode occurs due to the high amount of silicon content between the particles. The “dimple and tear” fracture mode is due to the presence of particle and void defects acting as concentrators for

crack nucleation and propagation. It is reasonable to assume that under the applied stress conditions the elastic and plastic deformation properties of intermetallic particles are different from that of the metal matrix. Moreover, micro-cracks and micro pores reduced the elastic energy absorption in the cell wall materials of the SAF. According to Griffith crack theory and energy balance theory, the existence of cracks decreases the elastic energy in the system [37, 38]. Consequently, the decrease of elastic energy due to the micro-cracks and voids results in spontaneous core brittle cracks. The intermetallic particles are also considered to act as stress raiser and also serve as weak links in the microstructure. These intermetallic particles in the materials assisted in favour of severing the metallic matrix decreasing the ductility and eventually resulted cleavage fracture.

Fig.8 Stress-strain curves of tensile tests

(a)

(b)

Fig.9 Ultimate tensile stress and strain measured in the experiment.

(c)

Fig.10 DIC failure analysis of tensile specimens.

(d)

The elastic limit of the base metal matrix undergoing quasi-static tension is considered to be lower than the material containing second phase particles. Deformation proceeds through the metal matrix results a cleavage surface and a separation between the metal matrix and particles takes place. It is considered that the cleavage occurred firstly in the more brittle βAlFeSi second phase particles, which are present throughout the entire cell wall. Only a small portion of area was fractured in ductile manner, evident from the minimal necking of α-Al as the cell wall contains a large amount of the relatively hard and brittle β-AlFeSi particles. As a result, cracks propagate through the randomly oriented network of intermetallic phase and fracture path is trans-granular. Although the cell walls are considered to be multicrystalline due to the large grain size in relation to the cell wall thickness, the grain boundaries do not play a significant role in the fracture behavior in this uniaxial tensile experiment.

Fig.11 Average stress-strain curve of cell-wall materials of SAF and base alloy.

The average tensile response was compared to the tensile response of the nearest base alloy available found in the open literature [40, 41]. It is found that the tensile strength for the individual cell-wall of SAFs is lower than its nearest base alloy properties (Fig. 11). Moreover, strain to failure of cell wall materials is approximately half of the base alloy

materials. Accountability for this low strain to failure again can be ascribed to porosities in cell wall materials and mismatching of elastic energy of intermetallic particles and metal matrix. 4.2 Microstructure Variation in the final elongation and UTS of the specimens are considerable, 31% and 34%, respectively, therefore; two specimens have been chosen that are characteristic of the lowest and highest strain to failure for post mortem analysis of the fracture surface to elucidate the roll of microstructure on crack propagation.

Fig.12 Crack propagation during tensile failure for specimen. 12(a) to 12(f) represents different location of crack propagation. The strain field of specimen 6 (lowest elongation, 2.6%) shows that the deformation remains limited in a narrow lateral strip of the specimen (Fig.10). Conversely, specimen 27 exhibits larger deformation (4.2%) to failure. Tensile specimens 6 and 27 are further analyzed by means of OM to investigate the variation of elongation as is shown by Fig. 12 and Fig.13. From comparing the microstructure of longitudinal section of the fractured surface of these

two specimens, it was found that along the crack path of specimen 6, there were a number of large voids, reducing the cross sectional area, thusly increasing the stress during the tensile testing The width of the displacement fields obtained from DIC (strain variation) are influenced by the particles and micro-pore distribution in the matrix. The microstructure of longitudinal sections near the fracture surfaces of both specimens reveal that the crack propagates in the eutectic Al-Si, circumventing clusters of Al-dendrites. Microstructures of both specimens show that ceramic particles along the crack path were broken and disordered. The crack profile also indicates that the crack path followed the weakest path along the crosssection of the specimen, following regions with a high void fraction with respect to the thickness and weak regions rich with b-AlFeSi. It is likely that the fracture path initially started from a single macro-void surrounded by a high concentration of particle and bAlFeSi.

Fig.13 Crack propagation during tensile failure; 12(a) to 12(f) represent different location of crack propagation

Finally, the cell wall materials have demonstrated brittle behaviour due to the b-AlFeSi phase as shown in the EDS analysis. Ductility of the cell wall is expected to increase if the Fe and Si content in the alloy are reduced. According to Miserez et al. and Dutta et al. [42, 43], heat treatment of the material may increase the ductility since it transform from b-AlFeSi phase to a-AlFeSi phase, however; heat treatment of the closed cell foam is a complex process and the internal gas pressure inside the cell may cause further reduction of strength.

5. Conclusions The microstructure along with the uniaxial tensile and deformation behaviours of individual cell wall elements of closed-cell SAF was studied. From this study, it is clear that the nature of the microstructure (void defects and particles) of the cell walls has a significant effect on the mechanical behaviour of closed-cell SAF. The following specific conclusions can be drawn from the investigation of microstructural and strength properties of SAFs: 1. The microstructure of the cell wall materials of closed cell aluminium alloy foams reveal that the cell wall materials consist of eutectic Al-Si and dendritic α-Al structure with an irregular distribution of intermetallic particles, micro-pores and micro-cracks. The grains were, on average, on a similar length scale as the thickness of the cell walls; varying in size between 50 -700µm. 2. DIC analysis showed the strain fields of individual cell wall specimens during the tensile loading. Narrow DIC displacement and strain fields with respect to the loading direction were prevalent in specimens that underwent lower strains to failure. Post mortem OM analysis along the crack path showed such specimens contained larger voids and irregular particle depositions across the areas of localized strain shown in the DIC analysis. 3. Tensile behaviour of cell wall elements of SAFs demonstrated a range of (UTS: 164 211MPa) tensile strengths. Post-mortem analysis of fractured surface provided insight

into the micromechanics of cell wall failure. The failure has been observed as combination of little ductile tearing and largely cleavage fracture. The presence of micropores and micro-cracks provided a natural path of crack propagation. Specifically, the combination of high void fraction regions and b-AlFeSi was the preferred pathway of crack propagation. Moreover, the mismatch of elastic and plastic properties between particles and metal matrix resulted in brittle fracture of the specimens. 4. Finally, future

optimization of the closed cell metallic foam clearly require the

understanding of the interplay between processing condition, cell wall microstructure and mechanical characterization under various types of applied load.

Acknowledgements The authors gratefully acknowledge the UNSW Canberra Defence Related Research Scheme that part-funded this work. The authors would like to thank Pat Nolan for the modification of tensile setup. M.A.I also thanks David Sharp for his insightful suggestions on sample preparation for micro analysis. Finally, authors thankfully acknowledge the laboratory facility provided by Department of Applied Mathematics, Australian National University for the µCT and image analysis.

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