Magnesium (Mg) contributes ~2.7% by weight to the earth’s crust, making it the sixth most abundant element [1
]. Mg-based materials show enormous potential in weight-sensitive applications, as they are the lightest structural material available. Mg has a density of only 1.74 g/cc which is ~33%, ~61% and ~77% lower than that of aluminium (Al), titanium (Ti), and iron (Fe), respectively [2
]. Increased demand for lightweighting drives the interest in Mg-based materials to be used in a variety of structural components in sectors striving for weight reduction, higher fuel efficiency, and payload capacity, such as automotive, aerospace, and defense. In addition to its light weight, Mg-based materials also exhibit better specific mechanical properties, damping characteristics, and impact resistance [3
]. However, Mg exhibits low corrosion resistance, room temperature ductility, and high temperature strength, which limits its wide scale adaptability [4
]. These facts necessitate increasing thrust in developing Mg-based syntactic foams with superior specific mechanical properties.
Metal matrix syntactic foams (MMSFs) are produced by dispersing hollow particulate fillers in the metal matrix. Incorporating porosity through hollow particles instead of embedding air/gas voids provides a reinforcing effect to each pore, and imparts properties similar or superior to what would be found in monolithic cellular materials, in particular metallic foams [6
]. The incorporation of porosity in a material makes the material lightweight and enhances the compressibility. Energy absorption and ductility can be further enhanced by increasing the extent of porosity. Open cell foams lack in strength and modulus, which limits their application. MMSFs exhibit superior mechanical properties for the same amount of porosity due to their closed cell structure [7
]. Further, syntactic foams offer an interesting and wide spectrum of properties, such as lower densities, low moisture uptake, high specific stiffness, superior blast and energy absorption, low thermal conductivity, and better dimensional stability, which would make them ideal for many applications in ground vehicle transportation, aerospace, defense, electronic packaging, biomedical, and marine applications [8
]. Recent review articles have detailed the synthesis methods, microstructures, mechanical properties and applications of Mg, Al, and Ti matrix syntactic foams [7
]. Engineered hollow spheres of SiC and Al2
], inexpensive fly ash cenospheres, and hollow glass microballoons (GMB) [11
] have been used as fillers in developing these syntactic foams.
Though Mg is expanding into more promising lightweight regime and medical applications, very little attention has been given to studies on Mg matrix syntactic foams (MgMSFs) compared to Al and Ti syntactic foams. This is likely due to the high reactivity of Mg and processing challenges that can lead to the microballoon breakage and related infiltration issues. From the open literature, a limited number of Mg alloy systems (AZ31B, AM 20, AM 50, AZ91, AZ91D and ZC63) were used for the synthesis of MgMSFs [7
], and only few publications on monolithic Mg-based syntactic foams are available [16
]. Fly ash cenospheres are widely used as fillers in these studies to synthesize syntactic foams because of their low cost. Cenospheres are recovered from coal combustion ash and are available in large quantities in thermal power plants. However, being waste by-products, they contain numerous defects in their structures, which is also reflected in the properties of developed foams. As a result, studies have experimented with the engineered hollow particles of ceramics such as SiC and Al2
for high-performance applications. Recently, a Mg-AZ91 matrix reinforced with SiC particles has been developed by Anantharaman et al. [9
] and Rivero et al. [18
] with a density as low as 0.92 g/cc and were found to have better performance than Al matrix syntactic foams (AlMSFs) on unit weight basis, wherein MgMSFs showed 44% higher specific compressive strength as compared to AlMSFs.
Syntactic foams made of Mg and hollow GMB have not been explored so far. In recent studies, the compressive and corrosion properties of AZ91D/sodalime–borosilicate GMB syntactic foams [19
] were characterized. The compressive properties of the AZ91D matrix showed an increase in compressive yield strength (CYS) from 112 to 143, 161 and 168 MPa with 15, 20 and 23 wt %, GMB additions respectively. A similar trend was observed in ultimate compressive strength (UCS) with a maximum of 243 MPa (~52% rise). Also, the addition of GMB particles caused a decrease in the α-Mg phase, which resulted in improved corrosion resistance for the syntactic foams. Nevertheless, this study dealt increasing the processing complexities of an Mg alloy and quantifying additional phase formations. Also, it has been demonstrated that the sodalime-borosilicate hollow glass microspheres (GMB) used in most applications are susceptible to significant degradation under high temperature and long-term exposures [20
]. In response to concerns over the long-term stability of sodalime-borosilicate filled syntactic foams, GMB comprised of borosilicate glass or silica can be used in developing MgMSFs. Particles with this glass chemistry have been demonstrated to be inert to degradation in wet environments and also presents a number of other potential benefits for use in syntactic foams, as the glass chemistry has lower thermal expansion and a higher softening temperature as compared with sodalime-borosilicate glass [20
]. The present work is aimed at exploring the possibility of using silica GMB for the development of high performance Mg-based syntactic foams using a disintegrated melt deposition (DMD) approach.
For fabricating MgMSFs, pressure infiltration, stir casting, and conventional powder metallurgy techniques are currently used as the main processing techniques [8
]. Each technique has its own advantages and limitations for the development of MgMSFs. The limitations of infiltration technique include the fracturing of hollow particles, matrix metal filling inside the spheres due to high infiltration pressure, and high residual porosity due to low infiltration pressure. Wettability and undesired phase formation between the matrix/sphere interface and the permeability of the reinforcement bed are also challenging issues. The major drawback of the stir-casting technique is flotation of low-density hollow particles and particle fracture. This technique is also particularly sensitive to segregation and agglomeration of the hollow reinforcement. The major issue associated with the powder metallurgy technique is the breakage of hollow particles at high volume fractions of the reinforcement. Therefore, to successfully develop MgMSFs, the use of different or new processing techniques and/or optimization of processing parameters, theoretically and experimentally, are still needed. DMD is a unique cost-effective technique that brings together the advantages of conventional casting and spray processing, utilizing lower impinging gas jet velocities, and higher superheat temperatures to produce bulk composite material. The advantage of DMD is the ability to obtain a uniform distribution of secondary reinforcements and equiaxed grains, resulting in a highly dense material with enhanced properties [22
Reinforcing pure Mg with glass microballoons using a DMD route of processing is quite a challenging and interesting task, hence adopted in this work. Mg/GMB foams are investigated for coefficient of thermal expansion (CTE), hardness, ignition, elastic modulus, and compressive properties. The GMB amount is varied in the Mg matrix by 5, 15 and 25 wt % which corresponds to ~8, 22.6 and 35.5 vol %. Pure Mg samples are also casted for comparison. Structure–property correlations are elaborately discussed with micrography.