Ultrasonically Stir Cast SiO 2 /A356 Metal Matrix Nanocomposites

: Metal matrix nanocomposites are a newly developed materials with promising applications in a wide variety of areas, ranging from medical to aerospace structures, owing to their lightweight high-strength properties. A light metal like aluminum is usually strengthened by a reinforcing agent of carbides, nitrides, oxides, carbon-based materials, or even elementals to boost the mechanical performance without sacriﬁcing lightweight; however, almost all reinforcing nanomaterials are commonly poorly wetted by metals leading to agglomerations, clusterings, among other problems, with diminished ductility and overall mechanical performance. To tackle the mentioned problems, a number of strategies including coatings, thermal, mechanical, or chemical treatments may be followed. In the present study, a particular focus is paid on the mechanical dispersion of nano-silica particles in a molten A356 alloy through applying high-intensity ultrasonic agitations in order to improve dispersibility, wettability, and interfacial afﬁnity. Nano-silica being an inexpensive high-strength nanomaterial is added to an A356 aluminum alloy melt and then dispersed and distributed by a 2-kW power ultrasonic system. Experimental results including microscopic observations and those mechanical experimentations revealed that the ultrasonication of the aforesaid solid–liquid system may greatly improve the afﬁnity between the de-agglomerated nano-silica particles and the host aluminum matrix with enhanced ductility.


Introduction
Metal matrix composites (MMCs) and nanocomposites (MMNCs), especially those produced by casting, are a class of advanced materials and structures with great potentials to be used in critical applications. A reinforcing agent is deliberately added to a host metal matrix to improve targeted properties. Size, shape, weight fraction, and dispersion/distribution quality as well as the chemical and physical properties are important parameters in any reinforcement-matrix system [1][2][3]. Stir-cast MMCs or MMNCs are of those lightweight, high-strength composite materials with common applications in automotive and aerospace structures; they basically are referred to a type of composite materials with at least two constituents: (i) a light metal such as aluminum or magnesium and (ii) a reinforcing agent like carbon-based nanomaterials, ceramics, or carbides, to name a few [3]. Owing to the enhanced properties of nano-sized materials, such as extremely high aspect ratio, the use of nano-reinforcement (instead of micron-size reinforcement) seems to be highly promising in composite manufacturing and technology. Many disadvantages with conventional composites, i.e., MMCs, including inferior mechanical performance, poor machinability, insignificant wear resistance, and microstructural inhomogeneity can be circumvented if nano-sized reinforcing material is properly employed [4,5]. It was The third strategy to enhance dispersion quality and improve the wetting behavior is thermal treatments wherein the reinforcing particles are first heated at elaborated temperature to change their surface chemistry [52][53][54][55][56]. For instance, the research reports of Tekmen et al. and Hashim et al. indicate that the heat treatment of SiC nano-particles may produce a layer of SiO 2 on the surface which has a lower contact angel with aluminum melt [52,54]. Unlike those reactive wetting processes, like coating, those processes based thermal treatment do not considerably change the composition as no external element is added to the melt; they may also modify particle surfaces [57], decrease contaminants or surface impurities, and improve particles retention [58]. It is repeatedly reported that the contact angle is greatly dependent on melt treating temperature and time of contact [27]. However, excessive temperature or overheat treatment may result in unwanted intermetallics/compounds, e.g., brittle carbides like Al 4 C 3 in aluminum matrix composites, being detrimental for mechanical and microstructural properties [27,59]. Finally, one of the most promising categories of improving dispersion quality and reinforcement-melt affinity is based on mechanical strategies such as the application of high-power ultrasonication or mechanical stirring [2]. Using a mechanical impeller, Naher et al. [60] and Moon [61] observed that stirring with higher amounts of shear rates could result in finer microstructural grains while manufacturing MMCs. Impeller position, size, geometry, stirring time, and speed are all the most important parameters in the stir casting of MMCs/MMNCs [62][63][64][65][66][67][68][69]. When a high-intensity ultrasonic irradiation is applied to a given melt composite slurry, a cloud of cavitation bubbles is formed in a solid-liquid system owing to the pressure differences occurred in the entire of melt volume [7,70,71]. Upon the collapse of each ultrasonic cavitation bubble, a high-speed microjet is developed through which the particles are deagglomerated, and an effective heterogenous nucleation is provided in the melt, all leading to severe grain refinement with homogenously dispersed reinforcing particles [72]. According to the literature, such a mechanical agitation promotes degassing [73] and acoustic streaming [74] with significant improvement in mass transport and diffusion phenomena [75,76], de-agglomerates particles [72], cleans the surfaces from contaminants [54,77], enhances the surface energy of solid reinforcement [7], distributes the solid phase evenly throughout the composite bulk [78], prevents segregation [74], and lowers those thermal residual stresses [2], among others. Unlike some other techniques, mechanical agitation is relatively inexpensive, simple, easy to use with high performance especially in MMNCs manufacturing wherein the de-agglomeration of nano-scale particles are problematic by other methods [79].
The present study aimed to fabricate a novel ultrasonically stir-cast MMNCs using inexpensive raw materials, namely A356 aluminum alloy as matrix as well as nano-silica powder as reinforcement. To this end, a high-power piezoelectric ultrasonic system is designed, simulated, and then fabricated to de-agglomerate silica nanomaterial in the metal matrix. Upon the fabrication process, a series of experimental tests has been carried out in order to microstructurally and mechanically characterize the properties. It was seen that ultrasonication could significantly improve the overall mechanical features by refining the grains and de-agglomerating the particles throughout the matrix materials.

Ultrasonic Dispersion System
A 2-kW high-power piezoelectric ultrasonic system vibrating at 20 kHz frequency was designed, simulated, and then fabricated to use for composite manufacturing. Shown in Figure 1, the transducer consists of six piezoelectric rings tighten between a steel backing and titanium matching by a steel bolt. This type of transducer can be used in a variety of manufacturing techniques ranging from machining to welding processes [80][81][82]. The whole transducer was kept in a cylindrical casing with air cooling system purging fresh air onto the piezoelectric rings to avoid thermal damages. Made of a titanium alloy Ti6Al4V, 2.0 cm of the transducer tip was placed into the melt for 5 min to mix the composite slurries under argon protection atmosphere. Figure 2a demonstrates the whole composite manufacturing setup including transducer, power supply, electrical furnace and its controller, crucible, and steel mold. Figure 2b,c schematically depicts how the cavitation bubbles may initiate, grow and collapse to develop high-speed micro-jets in the melts, de-agglomerating solid particles in composite melt. Moreover, this technology has a great capacity to produce an acoustic streaming with the flow patterns shown in Figure 2d mixing the composite slurry effectively and efficiently.
Ti6Al4V, 2.0 Cm of the transducer tip was placed into the melt for 5 min to m posite slurries under argon protection atmosphere. Figure 2a demonstrates composite manufacturing setup including transducer, power supply, electri and its controller, crucible, and steel mold. Figure 2b,c schematically depicts h itation bubbles may initiate, grow and collapse to develop high-speed micro melts, de-agglomerating solid particles in composite melt. Moreover, this tech a great capacity to produce an acoustic streaming with the flow patterns show 2d mixing the composite slurry effectively and efficiently.   According to our observations, the affinity of reinforcement-matrix increased because of a few important results. Silica particles usually agglomerate in A356 metal matrix and micro/macro agglomerates are usually appeared in the resultant composite when no treatment is applied on the melt slurries. Owing to differences in inherent properties, such as density values between the silica particles and the molten A356, the particles greatly tend to be floated on the top of melt, right after the addition of reinforcing particles, indicating very poor wettability in the mentioned pairs. However, we did not see obvious macro or even micro agglomerates in the microstructure of the ultrasonicated composites. No floatation was observed since the composite slurries were agitated by high-intensity ultrasonic vibrations, activating a cloud of high-pressure high-temperature cavitation bubbles in the melt volume, together with an effective melt streaming for mixing the particles and the liquid melts. It should be noted that without applying ultrasonication, nearly nothing could be incorporated in the host matrix. All these notes indicate the affinity in interfacial region may improve when treated by ultrasonics.

Materials and Methods
Commercial silica nanomaterials were provided by NanoMat Co., Isfahan, Iran, with a mean particle diameter of 30 nm and the following scanning electron microscopy image (see Figure 3). The amounts of 2.0, 4.0, and 6.0 gr silica nano-powder were added to a 1600 gr A356 aluminum alloy with the chemical composition given in Table 1 being supplied by the Materials Engineering Department, Isfahan University of Technology, Iran. The values of 2.0, 4.0, and 6.0 gr nano-silica are respectively equivalent to 0.125, 0.25, and 0.375 wt.%.
Metals 2021, 11, x FOR PEER REVIEW treatment is applied on the melt slurries. Owing to differences in inherent properties, as density values between the silica particles and the molten A356, the particles gr tend to be floated on the top of melt, right after the addition of reinforcing particles, cating very poor wettability in the mentioned pairs. However, we did not see obv macro or even micro agglomerates in the microstructure of the ultrasonicated compo No floatation was observed since the composite slurries were agitated by high-inte ultrasonic vibrations, activating a cloud of high-pressure high-temperature cavit bubbles in the melt volume, together with an effective melt streaming for mixing the ticles and the liquid melts. It should be noted that without applying ultrasonication, n nothing could be incorporated in the host matrix. All these notes indicate the affin interfacial region may improve when treated by ultrasonics.

Materials and Methods
Commercial silica nanomaterials were provided by NanoMat Co., Isfahan, Iran, a mean particle diameter of 30 nm and the following scanning electron microscopy i (see Figure 3). The amounts of 2.0, 4.0, and 6.0 gr silica nano-powder were added to a gr A356 aluminum alloy with the chemical composition given in Table 1    First of all, A356 raw material was weighted and then melted in a crucible under argon gas protection. The reinforcing powder was heated up to remove moisture and taminant and then gradually added to the melt matrix. Figure 3 shows the SEM ima the silica particles. The mixture of Silica/A356 was then processed by ultrasonic treat at 750 °C for 5 min; longer treatment may result in the dissolution of Ti tip with subseq contamination as well as the thermal damages to the transducer while shorter treatm may not provide effective de-agglomeration and homogenous dispersion. Upon ultr  First of all, A356 raw material was weighted and then melted in a crucible under inert argon gas protection. The reinforcing powder was heated up to remove moisture and contaminant and then gradually added to the melt matrix. Figure 3 shows the SEM image of the silica particles. The mixture of Silica/A356 was then processed by ultrasonic treatment at 750 • C for 5 min; longer treatment may result in the dissolution of Ti tip with subsequent contamination as well as the thermal damages to the transducer while shorter treatments may not provide effective de-agglomeration and homogenous dispersion. Upon ultrasonication, the melt slurry was immediately poured in a steal mold with the wall thickness of 10 mm and the cavity volume of 60 mm × 60 mm × 200 mm to avoid reagglomeration. After casting, a series of standard samples were cut from the cast composites using wire electro discharge machining (Wire-EDM, Tabriz Machines Co., Tabriz, Iran). From now on, COMP0 is referred to the monolithic alloy where no reinforcement has been added. COMP2, COMP4, and COMP6 are also respectively named as the composites with 2.0, 4.0, and 6.0 gr nano-reinforcement.
For microstructural observations, the cut specimens were mounted in a plastic body such that the cut face was polished by SiC emery clothes meshed 120, 220, 400, 600, 800, 1000, 1200, 1500, 2000, 2500, and 3000 on an inclined surface with flowing water. Afterward, Al 2 O 3 suspension (provided by Bohler Co., Seal Beach, CA, USA), was utilized on a rotary disk part with 250 rpm rotating speed. According to ASM handbook, vol. 9, the samples were first submerged in a 40% HF solution for 30 s and then washed by deionized water and alcohol. The samples were also gold plated by sputter coater to reach a 10 nm gold thickness under 10 −5 mbar atmosphere. The optical microscope as well as scanning electrone microscope were respectively Olympus PME-3 (LECO Co., Michigan, MI, USA) and Philips XL30 (FEI Co., Hillsboro, OR, USA).
The tensile stress tests were done based on the instructions given by ASTM E8. Based on the obtained stress-strain curves, the value of yield stress, tensile strength, elongation, and fracture toughness were determined. Further, each test was repeated three times to obtain reliability. Figure 4 shows the geometry of tensile test specimens. electro discharge machining (Wire-EDM, Tabriz Machines Co., Tabriz, Iran). From now on, COMP0 is referred to the monolithic alloy where no reinforcement has been added. COMP2, COMP4, and COMP6 are also respectively named as the composites with 2.0, 4.0, and 6.0 gr nano-reinforcement. For microstructural observations, the cut specimens were mounted in a plastic body such that the cut face was polished by SiC emery clothes meshed 120, 220, 400, 600, 800, 1000, 1200, 1500, 2000, 2500, and 3000 on an inclined surface with flowing water. Afterward, Al2O3 suspension (provided by Bohler Co., California, CA, USA), was utilized on a rotary disk part with 250 rpm rotating speed. According to ASM handbook, vol. 9, the samples were first submerged in a 40% HF solution for 30 s and then washed by deionized water and alcohol. The samples were also gold plated by sputter coater to reach a 10 nm gold thickness under 10 −5 mbar atmosphere. The optical microscope as well as scanning electrone microscope were respectively Olympus PME-3 (LECO Co., Michigan, MI, USA) and Philips XL30 (FEI Co., Hillsboro, Oregon, USA).
The tensile stress tests were done based on the instructions given by ASTM E8. Based on the obtained stress-strain curves, the value of yield stress, tensile strength, elongation, and fracture toughness were determined. Further, each test was repeated three times to obtain reliability. Figure 4 shows the geometry of tensile test specimens.

Microstructural Studies
Agglomeration has almost always been a great challenge in MMCs/MMNCs manufacturing as most of reinforcements, such as carbide/oxides/nitrides/carbon-based nanomaterials, have poor wetting with metals. As a result, inadequate dispersion in metals leads to the agglomerated phases and poor mechanical performance. Microstructural observation indicates that ultrasonic melt treatments not only de-agglomerate the silica particles but also refine the microstructure of A356 nanocomposites to a great extent. As discussed later, COMP2 exhibited the best mechanical performance. Figure 5 shows the optical microscopy (OM) results with severely refined grains of the composite materials.

Microstructural Studies
Agglomeration has almost always been a great challenge in MMCs/MMNCs manufacturing as most of reinforcements, such as carbide/oxides/nitrides/carbon-based nanomaterials, have poor wetting with metals. As a result, inadequate dispersion in metals leads to the agglomerated phases and poor mechanical performance. Microstructural observation indicates that ultrasonic melt treatments not only de-agglomerate the silica particles but also refine the microstructure of A356 nanocomposites to a great extent. As discussed later, COMP2 exhibited the best mechanical performance. Figure 5 shows the optical microscopy (OM) results with severely refined grains of the composite materials.
The results of scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) results are presented in the following figure. Fortunately, no trace of agglomeration, neither in OM nor in SEM results, has been detected, indicating the efficiency of ultrasonic dispersion of nano-silica particles within the A356 host aluminum matrix.
According to Figure 6, the possible phases in COMP0 are α-aluminum, Al-Si-Fe-Mg intermetallics, and those eutectic phases consisting O, Mg, Al, Si, and Fe [83]; the phase of Al 5 FeSi is also found in the microstructure of the composite samples being previously reported by others in the literature [83][84][85]. The results of scanning electron microscopy (SEM) and energy dispersive X-ray s troscopy (EDS) results are presented in the following figure. Fortunately, no trace o glomeration, neither in OM nor in SEM results, has been detected, indicating the effici of ultrasonic dispersion of nano-silica particles within the A356 host aluminum matr According to Figure 6, the possible phases in COMP0 are α-aluminum, Al-Si-Fe intermetallics, and those eutectic phases consisting O, Mg, Al, Si, and Fe [83]; the pha Al5FeSi is also found in the microstructure of the composite samples being previousl ported by others in the literature [83][84][85]. As shown in the OM results, the sample having 2.0 gr nano reinforcing agent (COMP2) has the finest grains compared to the other composite materials with different weight fractions of nano-silica particles. As discussed later, this sample has also the highest ductility and fracture toughness values being significant for those applications wherein both high static strength and enhanced fatigue performance are simultaneously demanded.

Mechanical Strength
The main goal of composite manufacturing is enhancing mechanical performance. While higher static strength values are demanded in some applications, the superior fatigue behavior is needed in some other special structures. In the case of automotive, aerospace or any other structures wherein the material is subjected to fatigue loading conditions, ductility is of crucial importance as it directly dictates the dynamic strength response. To best of our knowledge, the addition of reinforcing particles, e.g., SiC, to aluminum or magnesium melts usually resulted in lower ductility and fracture toughness, regardless of their effects on the static strength values; it is reportedly published in the literature that the addition of reinforcing agent might enhance static strength but greatly As shown in the OM results, the sample having 2.0 gr nano reinforcing agent (COMP2) has the finest grains compared to the other composite materials with different weight fractions of nano-silica particles. As discussed later, this sample has also the highest ductility and fracture toughness values being significant for those applications wherein both high static strength and enhanced fatigue performance are simultaneously demanded.

Mechanical Strength
The main goal of composite manufacturing is enhancing mechanical performance. While higher static strength values are demanded in some applications, the superior fatigue behavior is needed in some other special structures. In the case of automotive, aerospace or any other structures wherein the material is subjected to fatigue loading conditions, ductility is of crucial importance as it directly dictates the dynamic strength response. To best of our knowledge, the addition of reinforcing particles, e.g., SiC, to aluminum or magnesium melts usually resulted in lower ductility and fracture toughness, regardless of their effects on the static strength values; it is reportedly published in the literature that the addition of reinforcing agent might enhance static strength but greatly diminish the ductility and hence reduced fatigue behavior is expected [8,[86][87][88]. To find a composite with simultaneously enhanced static and dynamic strengths, both tensile and compression tests were conducted based on which the values of fracture toughness and ductility together with yield strength (YS), compression strength (US), and ultimate tensile strength (UTS) were measured.

Tensile Strength
The tensile strength tests were done based on the instructions presented by ASTM E8, at the room temperature of 20 ± 1.0 • C. The samples were cut by Wire-EDM from the cast aluminum sample. Figure 7 demonstrates the stress-strain behavior of the fabricated samples.
diminish the ductility and hence reduced fatigue behavior is expected [8,[86][87][88]. To find a composite with simultaneously enhanced static and dynamic strengths, both tensile and compression tests were conducted based on which the values of fracture toughness and ductility together with yield strength (YS), compression strength (US), and ultimate tensile strength (UTS) were measured.

Tensile strength
The tensile strength tests were done based on the instructions presented by ASTM E8, at the room temperature of 20 ± 1.0 °C. The samples were cut by Wire-EDM from the cast aluminum sample. Figure 7 demonstrates the stress-strain behavior of the fabricated samples.  As seen, although the wt.% of reinforcing particles goes up from the monolithic alloy to COMP6, the mechanical performance of the COMP2 is better than COMP4 and that COMP4 is greater than COMP6, owing probably to a degree of agglomerations appeared in the COMP4 and 6. In fact, owing to nanometric dimension of the reinforcing silica, it is believed the excessive addition of nanoparticles to the melt matrix may greatly affect the viscosity, surface tension as well as other fluid flow characteristics. Therefore, the same ultrasonication efficiency in COMP2 may not be obtained in a composite with higher filler loadings, such as COMP4 and COMP6 wherein the double and triple amount of filler materials are respectively incorporated. As a consequence, those composites with lower ultrasonic efficiency may have lower mechanical strength as well as coarse microstructural grains. Analyzing the fracture surface of the mechanical tests given in Figures 8-11, it is seen that the sample with 2.0 gr reinforcement (COMP2) has the cup and cone fracture surface with many dimples on the fracture surface being the indicator of ductile fracture behavior [83]. However, brittle cleavage fracture surface with no apparent plastic deformation is detected in COMP4 and COMP6 based on the SEM results. The presence of micro-voids and cracks in the fracture surface of COMP4 and 6 may result in a reduced mechanical strength. Finally, those agglomerated phases observed in composites (e.g., COMP6) with the higher amounts of silica additives may greatly contribute in lower mechanical strength.     Since no visible agglomerates is detected by OM and SEM on the fractured surfaces, it is believed the main influencing strengthening mechanisms are: (i) as a direct strengthening effect, load transfer from the aluminum matrix to high modulus silica nano-reinforcements can be a strengthening mechanism, (ii) as indirect mechanisms, Orowan strengthening and Hall-Petch effects are also thought to be important strengthening mechanisms. In other words, hard and non-shearable silica nanoparticles may pin and block the dislocations and therefore the composite can benefit from the Orowan effect; further, the presence of nano-reinforcing particles could greatly refine the microstructure and therefore improve the mechanical strength.
Later in the next section, we will compare the results of compression strength values to obtain which weight fraction is better for both static and dynamic loading conditions. We will see the higher amounts of ductility and fracture toughness of COMP2 can also be attributed to the ductile fracture behavior.

Compression Strength
Compression test samples were wire-EDM cut from the cast materials according to the instruction of ASTM-E8. The cylindrical samples with the diameter of 4 mm and the height of 6 mm were tested in room temperature. Figure 12 presents the results taken from the compression experiments with 60% strain.
According to the above stress-strain curves, it was found that the compression strength (at 60% strain) are 411.7, 460.6, 426.4, and 417.2, respectively for COMP0, COMP2, COMP4, and COMP6 wherein the composite material with 2.0 gr nano-silica reinforcement exhibited the highest compression strength of 460.6 MPa, indicating the superiority of COMP2 compared to the other weight fractions.

Compression Strength
Compression test samples were wire-EDM cut from the cast materials according to the instruction of ASTM-E8. The cylindrical samples with the diameter of 4 mm and the height of 6 mm were tested in room temperature. Figure 12 presents the results taken from the compression experiments with 60% strain. Taking into account all the experimental results obtained from different mechanical characterizations, Figure 13 shows that the yield strength value gradually increases from the monolithic sample to the composite sample with the highest reinforcement additives (COMP6); however, it is clear that COMP2 has the optimum value of reinforcing particles as all tensile strength, elongation at break, fracture toughness as well as compression strength values are the highest with this sample. It is believed that ultrasonication could de-agglomerate the strengthening particles in COMP2, increasing the aspect ratio and hence providing higher surface contacts between the solid materials and the melt aluminum alloy; further, ultrasonic cleaning of the particles' surface could facilitate strong bonding with the lowest interfacial imperfections and detachments. Ultrasonic might also greatly help with well-dispersion and distribution of nano-silica particles within the matrix alloy promoting heterogeneous nucleation throughout the composite volume. Therefore, an enhanced mechanical response is expected in COMP2. However, as the results of mechanical experiments show, the UTS, elongation and fracture toughness start to decrease when the amount of silica nano-reinforcements is higher than 2 gr, most probably owing to increasing the degree of agglomerations. In COMP4 and 6, we see lower ductility and fracture toughness as compared to COMP2, being probably due to agglomerates even if it is hard to be detected in SEM images. In the other words, COMP4 and COMP6 are not free of agglomerates being the responsible for diminishing mechanical properties, namely Taking into account all the experimental results obtained from different mechanical characterizations, Figure 13 shows that the yield strength value gradually increases from the monolithic sample to the composite sample with the highest reinforcement additives (COMP6); however, it is clear that COMP2 has the optimum value of reinforcing particles as all tensile strength, elongation at break, fracture toughness as well as compression strength values are the highest with this sample. It is believed that ultrasonication could deagglomerate the strengthening particles in COMP2, increasing the aspect ratio and hence providing higher surface contacts between the solid materials and the melt aluminum alloy; further, ultrasonic cleaning of the particles' surface could facilitate strong bonding with the lowest interfacial imperfections and detachments. Ultrasonic might also greatly help with well-dispersion and distribution of nano-silica particles within the matrix alloy promoting heterogeneous nucleation throughout the composite volume. Therefore, an enhanced mechanical response is expected in COMP2. However, as the results of mechanical experiments show, the UTS, elongation and fracture toughness start to decrease when the amount of silica nano-reinforcements is higher than 2 gr, most probably owing to increasing the degree of agglomerations. In COMP4 and 6, we see lower ductility and fracture toughness as compared to COMP2, being probably due to agglomerates even if it is hard to be detected in SEM images. In the other words, COMP4 and COMP6 are not free of agglomerates being the responsible for diminishing mechanical properties, namely ductility and toughness values. Although ultrasonication has a great potential to deagglomerate solid particles, it seems the efficiency has not been good enough to disperse excessive amount of solid loadings in COMP2 and COMP6 samples. Therefore, it is observed that such agglomerates could sharply decrease ductility in COMP6.

Conclusions
One of the greatest challenges in fabricating the cast metal matrix com (MMCs) or nanocomposites (MMNCs) is poor affinity/wetting conditions. Due to interparticle forces such as van der Waals as well as a very high aspect ratios of the sized particles, the reinforcing materials usually tend to significantly agglomerat cially when the fabrication route is based on a liquid state method like casting. F more, the addition of solid reinforcements to the melt usually degrades ductility an ture toughness, restricting the cast MMCs/MMNCs in some critical applications l tomotive or aerospace applications.
In the present paper, we aimed to produce inexpensive lightweight high-s aluminum matrix nanocomposite through ultrasonically stir-casting of A356 alu alloys filled by different weight fractions of nano-silica powders. It is found Figure 13. The values of (a) yield strength, (b) tensile strength, (c) elongation at break, (d) fracture toughness, as well as (e) compression strength for all monolithic and composite samples.

Conclusions
One of the greatest challenges in fabricating the cast metal matrix composites (MMCs) or nanocomposites (MMNCs) is poor affinity/wetting conditions. Due to strong interparticle forces such as van der Waals as well as a very high aspect ratios of the nano-sized particles, the reinforcing materials usually tend to significantly agglomerate especially when the fabrication route is based on a liquid state method like casting. Furthermore, the addition of solid reinforcements to the melt usually degrades ductility and fracture toughness, restricting the cast MMCs/MMNCs in some critical applications like automotive or aerospace applications.
In the present paper, we aimed to produce inexpensive lightweight high-strength aluminum matrix nanocomposite through ultrasonically stir-casting of A356 aluminum alloys filled by different weight fractions of nano-silica powders. It is found that a properly used ultrasonication could effectively de-agglomerate the strengthening particles and homogenously distribute them throughout the matrix alloy which subsequently leads to a significantly refined grain microstructures, improved tensile strength and compression behavior. The fabricated composite material exhibited a higher ductility and fracture toughness values as compared to monolithic alloy; such finding has rarely been reported in the literature. Based on the results obtained, it is concluded that ultrasonication is able to considerably affect the mechanical and microstructural properties of the cast aluminum matrix nanocomposites. In addition, still further characterization techniques, such as TEM, Atom probe, XAS, and neutron diffraction, can be used to unequivocally identify the ultrafine nanometric dispersion of the strengthening phase.