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Journal ArticleDOI

On the microstructure of ceramic hollow microspheres

01 Jan 2010-Periodica Polytechnica Mechanical Engineering-Vol. 54, Iss: 2, pp 89-94

AbstractMetal matrix syntactic foams (MMSFs) are relatively new materials which have increasing interest in the field of aviation and packaging industry. They are metal matrix composites, and their porosity is ensured by the incorporation of ceramic hollow microspheres. In this paper the microstructure of the ceramic hollow microspheres as reinforcing element was investigated. SL150, SLG and SL300 type ceramic microspheres were investigated. They contain various oxides, mainly Al2O2 and SiO2. Energy dispersive X-ray spectroscopy (EDS) maps were recorded from the sections of the microspheres’ wall. The results showed that the Al2O3 and SiO2 distribution was not equal; the Al2O3 phase was embedded in the surrounding mullite and SiO2 phase in the form of needles. EDS along a line in aluminium matrix syntactic foams was carried out in order to investigate the possible reaction between the aluminium matrix and the ceramic microspheres. Due to the uneven distribution of Al2O3 rich particles, the molten aluminium can reduce the SiO2 rich parts of the microspheres and the wall of the microspheres become damaged and degraded. This chemical reaction between the microspheres and the walls can make the infiltration easier, but the resulting mechanical properties will be lower due to the damaged microsphere walls.

Topics: Syntactic foam (58%), Ceramic (55%), Aluminium (51%)

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Journal ArticleDOI
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.

51 citations


Journal ArticleDOI
Abstract: Metal matrix syntactic foams (MMSFs) were produced by pressure infiltration. Two parameters of the infiltration process (pressure and time) were varied and the infiltrated length was measured as the function of infiltration parameters in order to get data for the implementation of pressure infiltration as mass-production of MMSFs similar to injection mould casting, especially in the short infiltration time range (

51 citations


Journal ArticleDOI
Abstract: Aluminum alloy matrix syntactic foams were produced by inert gas pressure infiltration. Four different alloys and ceramic hollow spheres were applied as matrix and filler material, respectively. The effects of the chemical composition of the matrix and the different heat-treatments are reported at different strain-rates and in compressive loadings. The higher strain rates were performed in a Split-Hopkinson pressure bar system. The results show that, the characteristic properties of the materials strongly depends on the chemical composition of the matrix and its heat-treatment condition. The compressive strength of the investigated foams showed a limited sensitivity to the strain rate, its effect was more pronounced in the case of the structural stiffness and fracture strain. The failure modes of the foams have explicit differences showing barreling and shearing in the case of quasi-static and high strain rate compression respectively.

50 citations


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Journal ArticleDOI
Abstract: Metal matrix syntactic foams of ceramic (mullite) cenospheres (10–40% v/v) embedded in pure aluminium matrix, were fabricated by powder metallurgy technique at different sintering temperatures (610–710 °C). Density, vis-a-vis porosity and microstructural characteristics were examined by using scanning electron microcopy and energy dispersive X-ray spectroscopy. A first systematic description of the sintering mechanism of the composites is presented. The in situ reaction between silica (phase of mullite) and aluminium particles, apart from any degradation of the mullite cell wall and the precipitation of silicon in the matrix, changes the sintering mechanism of syntactic foams due to the formation of eutectic aluminium–silica liquid quantities. Transient liquid phase sintering, with surface material diffusion, and liquid phase sintering, with bulk diffusion, selected to be the main sintering mechanisms, are described in detail for sintering temperatures below and above the melting point of aluminium matrix, respectively.

31 citations


Journal ArticleDOI
Abstract: Closed-cell AZ31 Mg alloy foams were successfully prepared by melt-foaming method. The effects of specimen aspect ratio (the thickness/width ratio, AR) on the compressive properties of closed-cell Mg alloy foams were investigated systematically. The results showed that the length of stress–strain plateau stage extended and ideality energy absorption efficiency improved with the specimen AR increasing and the yield strength decreased. Specimens with the AR = 1.00 possess good combination of yield strength, plateau stage length and compressive stability when compressed under the experiment conditions.

22 citations


References
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Journal ArticleDOI
Abstract: Loose beds of hollow fly ash particles (cenospheres) were pressure infiltrated with A356 alloy melt to fabricate metal-matrix syntactic foam, using applied pressure up to 275 kPa. The volume fractions of cenospheres in the composites were in the range of 20–65%. The processing variables included melt temperature, gas pressure and particles size of fly ash. The effect of these processing variables on the microstructure and compressive properties of the synthesized composites is characterized. Compressive tests performed on these metal-matrix composites containing different volume fractions of hollow fly ash particles showed that their yield stress, Young's modulus, and plateau stress increase with an increase in the density. Variations in the compressive properties of the composites in the present study were compared with other foam materials.

243 citations


Journal ArticleDOI
Abstract: Syntactic foams were fabricated by liquid metal infiltration of commercially pure and 7075 aluminum into preforms of hollow ceramic microspheres. The foams exhibited peak strengths during quasi-static compression ranging from −100 to −230 MPa, while dynamic compression loading showed a 10–30% increase in peak strength magnitude, with strain rate sensitivities similar to those of aluminum–matrix composite materials. X-ray tomographic investigation of the post-compression loaded foam microstructures revealed sharp differences in deformation modes, with the unalloyed-Al foam failing initially by matrix deformation, while the alloy–matrix foams failed more abruptly through the formation of sharp crush bands oriented at about 45° to the compression axis. These foams displayed pronounced energy-absorbing capabilities, suggesting their potential use in packaging applications or for impact protection; proper tailoring of matrix and microsphere strengths would result in optimized syntactic foam properties.

202 citations


Journal ArticleDOI
10 May 2007-Wear
Abstract: In the present experimental investigation, Al (12 wt% Si) as matrix material and up to 15 wt% of flyash particulate composite was fabricated using the liquid metallurgy route. The wear and friction characteristics of the composite in the as-cast conditions were studied by conducting sliding wear test, slurry erosive wear test and fog corrosion test. The sliding wear behavior of the MMCs were investigated by varying parameters like normal load, percentage flyash, and track velocity. Pin-on-disc wear testing machine was used for investigating sliding wear behavior. In slurry erosive wear studies, percentage flyash and pH value of the slurry were used as variables. Corrosion studies were carried out using fog corrosion test. The specimens were exposed to a fog of NaCl. The worn surfaces were analyzed using optical microscope and scanning electron microscope. The results indicate that the wear resistance of the flyash reinforced material increased with increase in flyash content, but decreases with increase in normal load, and track velocity. The microscopic examination of the worn surfaces, wear debris and subsurface shows that the base alloy wears primarily because of micro cutting. But the MMCs wear because of delamination, micro cutting, oxidation and thermal softening. Corrosion has increased with increase in flyash content.

185 citations


Journal ArticleDOI
Abstract: Syntactic foams were fabricated by pressure-infiltrating liquid aluminum (commercial purity and 7075-Al) into a packed preform of silica–mullite hollow microspheres. These foams were subjected to a series of uniaxial compression stresses while neutron or synchrotron X-ray diffraction measurements of elastic strains in the matrix and the microspheres were obtained. As for metal matrix composites with monolithic ceramic reinforcement, load transfer in the pure aluminum foams is apparent between the two phases during elastic deformation, and is affected at higher stresses by matrix plasticity. Calculating effective stresses from the lattice strains shows that the microspheres unload the pure aluminum matrix by a factor of about 2. In the aluminum alloy foams, an in situ reaction between silica and the melt leads to the conversion of silica to alumina in the microsphere walls and the precipitation of silicon particles in the matrix. This affects the load transfer between the matrix and the reinforcement (microspheres and particles), and increases the macroscopic foam stiffness by over 40%, as compared to the pure aluminum foams. Composite micromechanical modeling provides good predictions of the elastic moduli of the syntactic foams, capturing the effects of load transfer and suggesting that significant stiffness improvements can be achieved in syntactic foams by the use of microspheres with stiff walls and/or by the incorporation of a stiff reinforcing phase within the metallic matrix.

135 citations


Journal ArticleDOI
Gaohui Wu1, Zuoyong Dou1, D.L. Sun1, Longtao Jiang1, B.S. Ding1, B.F. He1 
Abstract: A new method has been established to predict the compressive strength of cenosphere–aluminum syntactic foams, showing the relation between the relative wall thickness of the cenopshere and the compressive strength of such foams. Quasi-static compression tests indicated that annealed cenosphere–aluminum syntactic foams can deform plastically at a relatively higher stress (∼45–75 MPa) and their energy-absorbing capacity can reach ∼20–35 MJ m−3. The deformation mechanisms of syntactic foams have also been discussed.

129 citations