Characteristic compressive properties of hybrid metal matrix syntactic foams

TL;DR: In this paper, an AlSi12 matrix hybrid MMSF with monomodal Globocer (Al 2 O 3 and SiO 2 based ceramic) and pure Fe reinforcements were produced by pressure infiltration.

Abstract: Hybrid metal matrix syntactic foams (hybrid MMSFs) are particle reinforced composites in which the reinforcement is the combination of more than one grade of hollow spheres. The difference between the spheres can be in their chemical composition, dimension, physical properties etc. In this study AlSi12 matrix hybrid MMSFs with monomodal Globocer (Al 2 O 3 and SiO 2 based ceramic) and Globomet (pure Fe) reinforcements were produced by pressure infiltration. The investigation parameters were the ratio of the hollow sphere grades and the aspect ratio of the specimens. Microstructural investigations showed almost perfect infiltration and favourable interface layer, while quasi-static compression tests showed that the composition of the reinforcement and the aspect ratio of the specimens have determinative effect on the characteristic properties (compressive and flow strength, fracture strain, stiffness and absorbed energy). This nature of the MMSFs ensures the possibility to tailor their properties in order to optimise them for a given application.

Summary

1. Introduction

• As it is presented above, the mechanical properties of MMSFs have been more or less widely measured, but data about hybrid MMSFs is lacking.
• Therefore the aim of this paper is to give detailed introduction to the mechanical and microstructural properties of hybrid MMSFs.

2.1. Investigated materials and production method

• The vacuum pump was switched off and Ar gas was let into the chamber with a previously set 400 kPa pressure.
• The pressure difference (400 kPa in the chamber and vacuum under the liquid) forced the molten metal to infiltrate into the space between the hollow spheres.
• For further details about the production process please, refer to [29, 68] .
• The blocks were designated after their constituents: e. g. 20GC+80GM stands for an ASF block with AlSi12 matrix and with ~64 vol% of hollow spheres that is mixed from 20 vol% GC and 80 vol% GM grade hollow spheres respectively.
• The measured densities (ρm) of the blocks, determined by Archimedes' method, are listed in Table 1 .

2.2. Experimental

• The compression tests were performed on a MTS 810 type universal testing machine in a four column equipment at room temperature.
• The acting surface of the dies was ground and polished.
• The specimens and the dies were lubricated with Locktite antiseize material.
• Five specimens were compressed from each specimen group up to 25% engineering strain to get representative results and to verify repeatability.
• The results were evaluated according to the standard concerning the compression tests of cellular materials [41] and the characteristic properties (compressive and flow strength, fracture strain, structural stiffness and absorbed energies) were determined.

3.2. Compressive properties

• Based on the above mentioned descriptions it is worth mentioning that by the application of different composite layers with different GC and GM ratio, gradient materials can be easily built according to the requirements of given applications.
• By proper mixing, build-up and/or by planned distribution (either one-by-one placement or altering ratio) of the different grade spheres, gradient behaviour in different direction(s) can be ensured for given parts.
• This property of hybrid ASFs allows application as energy absorbers, hulls, collision dampers or vibration dampers.
• The altering ratio of the reinforcing grades can also ensure unique failure modes built-up from the basic failure mechanisms described in the next section.

Figures

Fig. 1. Schematic sketch of the infiltration chamber

Fig. 2. Micrographs about typical areas of 20GC+80GM ASFs

Fig. 6. The fracture strain (a) and the structural stiffness (b) of ASFs as the function of hollow sphere grade and aspect ratio

Fig. 5. The average compressive (a) yield (b) and plateau (c) stress of ASFs in the function of hollow sphere grade and aspect ratio

Table 1. The measured densities of the investigated MMSFs

Fig. 9. Macrographs about the failure mechanisms of AFs (a) brittle fracture (80GC+20GM), (b) transient failure mode (40GC+60GM) and (c) diffuse plastic deformation (20GC+80GM)

Fig. 4. Typical engineering stress – engineering strain curves of ASFs (H/D=1) (a) and the monitored properties (b)

Fig. 8. Representation of the rule of mixtures in the case of (a) yield and plateau strength and (b) structural stiffness and fracture strain

Fig. 3. BSE image (a) of a GM (left) and a GC (right) grade hollow sphere and EDS line-scan profile (b) of 60GC+40GM ASF

Table 3. Structural stiffness and absorbed energies of the investigated MMSFs

Fig. 7. The fracture energy (a) and the absorbed energy (b) of ASFs as the function of hollow sphere grade and aspect ratio

Full text

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This accepted author manuscript is copyrighted and published by
Elsevier. It is posted here by agreement between Elsevier and
MTA. The definitive version of the text was subsequently
published in Materials Science & Engineering A, 606, 248-256,
April 1, 2014, DOI: 10.1016/j.msea.2014.03.100. Available under
license CC-BY-NC-ND.
Characteristic compressive properties of hybrid metal matrix syntactic foams
Kornél MÁJLINGER
a,c
, Imre Norbert ORBULOV
a,b,d,
*
Department of Materials Science and Engineering, Faculty of Mechanical
Engineering, Budapest University of Technology and Economics, Bertalan Lajos utca
7., Budapest, Hungary, 1111
b
MTA–BME Research Group for Composite Science and Technology, Műegyetem
rakpart 3., Budapest, Hungary, 1111
c
vmkornel@eik.bme.hu
d
orbulov@eik.bme.hu
*Corresponding author
Address: Department of Materials Science and Engineering, Faculty of
Mechanical Engineering, Budapest University of Technology and Economics,
Bertalan Lajos utca 7., Budapest, Hungary, 1111
Tel: +36 1 463 2386
Fax: +36 1 463 1366

2
E-mail: orbulov@eik.bme.hu, orbulov@gmail.com

3
Abstract Hybrid metal matrix syntactic foams (hybrid MMSFs) are particle reinforced
composites in which the reinforcement is the combination of more than one grade of
hollow spheres. The difference between the spheres can be in their chemical
composition, dimension, physical properties etc. In this study AlSi12 matrix hybrid
MMSFs with monomodal Globocer (Al
2
O
3
and SiO
2
based ceramic) and Globomet
(pure Fe) reinforcements were produced by pressure infiltration. The investigation
parameters were the ratio of the hollow sphere grades and the aspect ratio of the
specimens. Microstructural investigations showed almost perfect infiltration and
favourable interface layer, while quasi-static compression tests showed that the
composition of the reinforcement and the aspect ratio of the specimens have
determinative effect on the characteristic properties (compressive and flow strength,
fracture strain, stiffness and absorbed energy). This nature of the MMSFs ensures
the possibility to tailor their properties in order to optimise them for a given
application.
Keywords: Mechanical characterisation; Cellular materials; Metal matrix composites;
Fracture; Syntactic foam
1. Introduction
Metal matrix syntactic foams (MMSFs) are special particle reinforced composites that
consist of a metal matrix (usually some kind of Al or Mg alloy, to get maximal weight
reduction) and a set of hollow, spherical particles. Most commonly the hollow spheres
are built up from some sort of ceramic or metallic material and they are commercially
available [1-5]. MMSFs have outstanding mechanical properties, like higher strength,
stiffness and energy absorption capacity compared to other metallic foams, while
their fracture strain is usually lower. Due to this, the MMSFs have promising

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application possibilities as lightweight parts or as hulls of public and/or military
vehicles [6, 7], as well as collision or vibration dampers.
In most cases MMSFs are made by stir casting or infiltration. Stir casting is cheaper
and faster, but it can only produce lower reinforcement volume fractions due to
hollow sphere breakage caused by mechanical stirring [8-16]. In the case of
infiltration two basic methods can be separated: gravity-fed infiltration (only in the
case of wetting matrix – reinforcement systems [17-20]) and pressure-assisted
infiltration (for non-wetting systems). In the latter case a threshold pressure must be
overcome in order to get acceptable workpieces. The threshold pressure can be
calculated (estimated) [21-28] or measured [29-31]. Pressure infiltration is capable of
producing MMSFs with maximal (~64 vol%) hollow sphere volume fraction and better
matrix dispersion, but it requires more investment and more sophisticated equipment
[29, 30, 32-38]. In practice, usually one kind of hollow sphere set with monomodal
diameter distribution is applied as reinforcement. Only a few efforts have been
published about MMSFs with bimodal hollow sphere diameter distribution [39]. Daoud
[10] produced closed cell foams by gas releasing method and added hollow spheres
into the ZnAl12 base metal. This hybrid foam showed ductile compressive
deformation and exhibited higher mechanical strength than pure ZnAl12 foams. Xia
et al. [40] produced and investigated Al99.5 based closed cell foams with different
kinds and contents of ceramic microspheres in the cell walls by melt-foaming method.
They showed that the microspheres have a significant effect on the strength, the
deformation capabilities and the energy absorption of the foams. However –
according to the best knowledge of the authors – no research results have been
published about other hybrid MMSFs containing at least two different reinforcement
grades. The difference between the reinforcements can be in the mean dimension,

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chemical composition, physical properties etc. For example, in the case of MMSFs
the material of the spheres can be different: metal and ceramic hollow spheres can
be combined.
The most common loading mode of the foams is compression. Therefore the
compressive properties of MMSFs have been widely studied and the quasi-static
testing method has been standardised in DIN50134 [41]. Balch et al. [42, 43]
examined the compressive properties and the load partitioning mechanisms in
aluminium based syntactic foams (ASFs). According to their results optimised
properties can be reached when the matrix and hollow spheres strengths are
properly matched. Dou et al. [44] performed quasi-static and high strain rate
compression tests on ASFs. The results showed distinct strain rate sensitivity. Goel
et al. [45] investigated the dynamic compressive properties of ASFs. They showed
that the compressive strength and energy absorption attained an optimum at a given
strain rate. Kiser et al. [46] investigated ceramic hollow sphere reinforced ASFs under
compressive loads. Uniaxial compressive failure has been initiated at small strains
through the collapse of the material within a localized deformation band. Under
constrained conditions, localization was suppressed and the flow stress increased
monotonically. Different matrix (Mg and Zn alloys) syntactic foams were studied by
Rohatgi et al., Daoud [8, 9, 47, 48] and Huang et al. [49, 50]. The hollow spheres
decreased the density and the foams became stiffer and stronger, than the
conventional ones. Castro and Nutt [20, 51] investigated the synthesis of steel matrix
syntactic foams with Al
2
O
3
hollow spheres. The MMSFs exhibited higher strength and
energy absorption capacity than the steel foams reported previously. The
compression and low-velocity impact behaviour of ASFs were also studied by the
same research group [52]. Luong et al. [53-55] investigated the strain rate sensitivity

Frequently asked questions

Q1. What is the failure mode of the hollow spheres?

The failure mode of the ASFs has changed between brittle shearing and plasticcollapse according to the actual fraction of the hollow sphere grades. 

Q2. What was the failure mode of the GC grade hollow spheres?

The compressive and yield strength as well as the structural stiffness increased,while the fracture strain decreased as the GC grade hollow sphere fraction increased, respectively. 

Q3. What was the common failure mode of the MMSFs?

In the case of mode C failure, a cone like volume near to either end of the specimen started to deform and densified intensively. 

Q4. What is the simplest explanation for the failure of a MMSF?

The failure started between the cones: first a few hollow spheres were broken, then the compressed zone expanded to a lens-like volume where the material was compressed and compacted; the specimen may also show some barrelling. 

Q5. What is the effect of higher GC content on the absorbed energies?

In the case of higher GC content the compressive and the plateau strengths were higher and therefore the absorbed energies became higher. 

Q6. How can the stress-strain curves be calculated?

The fracture energy and the total absorbed energy may be determined by the numerical integration of the stress-strain curves up to the fracture strain, or up to 25% strain respectively. 

Q7. What is the important stress property of the MMSFs?

Another important stress property is the plateau strength (σp) that can be defined as the average stress between 5% and 25% deformation. 

Q8. What is the strength of a MMSF?

The strength of MMSFs can be characterised by the first stress peak (compressive strength, σc) and by the strength at a given plastic deformation (similar to yield strength, σy). 

Q9. What are the properties of the MMSFs?

According to the standard [41] the compressive properties of the MMSFs can be sorted into strength, deformation and absorbed energy groups. 

Q10. What is the failure mechanism of a GC grade sphere?

As the fraction of GC grade hollow spheres increased the failure mechanism turned from brittle shearing (mode B, Fig. 9a) through mixed mode (Fig. 9b) to diffuse plastic collapse (mode A, Fig. 9c), as presented for 80GC+20GM, 60GC+40GM and 20GC+80GM ASFs respectively. 

Q11. What is the effect of the GC grade spheres on the wall?

The SEM and line EDS analysis highlighted (i) solution of Fe from GM gradespheres into the AlSi12 matrix, that can cause damage to the wall and leads to infiltrated hollow spheres and (ii) an exchange reaction between the AlSi12 matrix and the GC grade spheres, that was suppressed by the high Si content of the AlSi12 matrix. 

Q12. What is the effect of a higher GC content on the spheres?

In the case of lower GC content the strengths became lower, but the ductility of GM grade hollow spheres could balance and overcome this effect. 

Q13. What is the failure mode of a GC grade MMSF?

The failure modes of MMSFs containing steel hollow spheres were investigated by Rabiei and Vendra: plastic deformation of the MMSFs was reported that corresponds to mode A failure [17, 58]. 

Q14. What is the way to produce a hybrid ASF?

4. Conclusions From the above detailed investigations, the following conclusions can be drawn: Pressure infiltration is a convenient method to produce hybrid ASFs with highhollow sphere content and low uninfiltrated porosity. 

Q15. What is the failure mode of a GC grade sphere?

This failure mode occurred frequently in the case of higher aspect ratios, typically H/D=1.5 for GC grade reinforcement – as in their case as well. 

Q16. What was the failure mode of the GC grade spheres?

This failure mode occurred only in the case of the largest aspect ratio (H/D=2) and in the case of small (Ø<500 µm) ceramic hollow sphere reinforcement, as it was detailed in [69].