Study of Microstructural and Mechanical Properties of Al/SiC/TiO2 Hybrid Nanocomposites Developed by Microwave Sintering

Aluminum hybrid metal matrix nanocomposites (Al/SiC/TiO2) were synthesized through a microwave-assisted powder metallurgy process, and their evolved microstructure and mechanical properties were investigated. The Al/SiC/TiO2 hybrid nanocomposites were prepared by reinforcing aluminum (Al) matrix with a fixed amount of silicon carbide (SiC) nanoparticles (5 wt.%) and varying concentrations of titanium dioxide (TiO2) nanoparticles (3, 6, and 9 wt.%). The morphology results revealeda uniform distribution of SiC and TiO2 reinforcements in the aluminum matrix. An increase in the hardness and compressive strength of the Al/SiC/TiO2 hybrid nanocomposites was noticed with the increasein TiO2 nanoparticles. The Al/SiC/TiO2 hybrid nanocomposites that had an optimum amount of TiO2 nanoparticles (9 wt.%) showcased the best mechanical properties, with maximum increments of approximately 124%, 90%, and 23% of microhardness (83 ± 3 HV), respectively, yield strength (139 ± 8 MPa), and ultimate compression strength (375 ± 6 MPa) as compared to that of pure Al matrix. The Al/SiC/TiO2 hybrid nanocomposites exhibited the shear mode of fracture during their deformation process.


Introduction
In recent years, hybrid composites have been attractive because of their multifunctional characteristics and specific properties, such as low cost, lightweight, low density, high strength to weight ratio, and ease of fabrication. Hybrid matrix composites (HMCs) are developed by the combination of two or more reinforcements incorporated into a suitable matrix to allow for a superposition of their properties. HMCs provide a combination of unique properties that cannot be realized in conventional composite materials. HMCs are highly efficient and demonstrate promising properties, and thus, their rapid utility in various applications [1,2]. The important criteria for the use of hybrid composites are based on high strength, crack propagation resistance, and plasticity exhibited at high loads. The selection of materials is of key importance in designing and producing the components that conform to the end-user requirements. The possibility of mixing different reinforcements into the hybrid composite usually leads to enhanced structural, mechanical, electrical, and thermal characteristics, making them attractive for many applications, such as automobile, aerospace, aeronautical, marine, and other engineering applications [3,4].
Aluminum hybrid matrix composites (AHMCs) show better properties than conventional alloys [5]. AHMCs are the new HMC combination of at least two different types of reinforcements incorporated into an aluminum matrix. Different reinforcements are

Preparations of Hybrid Nanocomposite Samples
To synthesize the Al/SiC/TiO 2 hybrid nanocomposites, the stoichiometric amounts of precursor materials were weighed carefully by an analytical balance. The weighed powders were blended through a planetary ball mill PM 200 at a speed of 200 rpm for 2 h. The blended powder of approximately 1 gm was taken and compacted into cylindrical pellets with an applied pressure of 50 MPa for 60 s. The compacted green pallets were then sintered using a microwave sintering furnace (VB ceramic furnace, VBCC/MF/1600 • C/14/15, Chennai, India) at a temperature of 550 • C for 30 min, with a heating rate of approximately 10 • C/min. The sintered samples were further used for different characterizations. The powders were blended through a planetary ball mill PM 200 at a speed of 200 rpm for 2 h. The blended powder of approximately 1 gm was taken and compacted into cylindrical pellets with an applied pressure of 50 MPa for 60 s. The compacted green pallets were then sintered using a microwave sintering furnace (VB ceramic furnace, VBCC/MF/1600 °C/14/15, Chennai, India) at a temperature of 550 °C for 30 min, with a heating rate of approximately 10 °C/min. The sintered samples were further used for different characterizations. The samples were polished and ultrasonically cleaned in an alcoholic solution. A schematic representation of the Al/SiC/TiO2 hybrid nanocomposites is shown in Figure 1.

Characterization
The density and porosity of the Al/SiC/TiO2 hybrid nanocomposites were measured using Archimedes' principle. XRD analysis was performed on the polished microwave sintered samples using PANalyticalX'pert pro.
The SEM micrographs of the Al/SiC/TiO2 hybrid nanocomposites were carried out using a field emission scanning electron microscope (SEM-FEI Nova NanoSEM 450 FE-SEM, Hillsboro, OR, USA) with energy-dispersive X-ray spectroscopy (Bruker SDD-EDS, Coventry, UK). The surface roughness of the Al/SiC/TiO2 hybrid nanocomposites was carried out using an atomic force microscope (MFP-3D Nanoindenter, South Carolina, USA).
The Vickers microhardness of the developed Al/SiC/TiO2 hybrid nanocomposites was measured on a Vickers microhardness tester (MKV-h21) at a load of 50 gf for 10 s. The nanohardness of the hybrid nanocomposites was examined using the MFP-3D Nanoindenter system at a load of 100 mN. For microhardness testing, 4-sided pyramid tips were used, while for nanoindentation, 3-sided pyramid tips were employed. Both micro-and nanohardness experiments were conducted at room temperature (RT), and the reported results are an average of five indentations that were taken for each sample.
The compression strength and yield strength of the developed Al/SiC/TiO2 hybrid nanocomposites were measured on a universal testing machine (Lloyd-Ametek LR50K Plus) at room temperature under compression loading according to ASTM E9-89a. The average of the 3 samples was taken for each test. The fractured surfaces of the Al/SiC/TiO2 hybrid nanocomposites were studied using an SEM-FEI Nova NanoSEM 450 FE-SEM scanning electron microscope.

Characterization
The density and porosity of the Al/SiC/TiO 2 hybrid nanocomposites were measured using Archimedes' principle. XRD analysis was performed on the polished microwave sintered samples using PANalyticalX'pert pro.
The SEM micrographs of the Al/SiC/TiO 2 hybrid nanocomposites were carried out using a field emission scanning electron microscope (SEM-FEI Nova NanoSEM 450 FE-SEM, Hillsboro, OR, USA) with energy-dispersive X-ray spectroscopy (Bruker SDD-EDS, Coventry, UK). The surface roughness of the Al/SiC/TiO 2 hybrid nanocomposites was carried out using an atomic force microscope (MFP-3D Nanoindenter, Columbia, SC, USA).
The Vickers microhardness of the developed Al/SiC/TiO 2 hybrid nanocomposites was measured on a Vickers microhardness tester (MKV-h21) at a load of 50 gf for 10 s. The nanohardness of the hybrid nanocomposites was examined using the MFP-3D Nanoindenter system at a load of 100 mN. For microhardness testing, 4-sided pyramid tips were used, while for nanoindentation, 3-sided pyramid tips were employed. Both micro-and nanohardness experiments were conducted at room temperature (RT), and the reported results are an average of five indentations that were taken for each sample.
The compression strength and yield strength of the developed Al/SiC/TiO 2 hybrid nanocomposites were measured on a universal testing machine (Lloyd-Ametek LR50K Plus) at room temperature under compression loading according to ASTM E9-89a. The average of the 3 samples was taken for each test. The fractured surfaces of the Al/SiC/TiO 2 hybrid nanocomposites were studied using an SEM-FEI Nova NanoSEM 450 FE-SEM scanning electron microscope. Figure 2 illustrates the measured density and porosity results of the pure aluminum and its hybrid nanocomposites. It can be seen that the density of the hybrid nanocomposites increased with the increase in the amount of TiO 2 . However, the density of the hybrid nanocomposites was higher than that of the base aluminum matrix due to the presence of higher density reinforcements SiC (3.21 g/cc) and TiO 2 (4.23 g/cc) in the Al Crystals 2021, 11, 1078 4 of 13 matrix (2.7 g/cc). The higher density of the microwave sintered samples influenced the structural, hardness, and strength of the hybrid nanocomposites [14]. It can also be noticed from Figure 2 that the porosity increased with an increase in the contents of TiO 2 nanoparticles in the aluminum matrix. This can be ascribed to the effect of agglomeration at high reinforcement content and pore nucleation formation at the particle reinforcement surface [37][38][39]. Figure 2 illustrates the measured density and porosity results of the pure aluminum and its hybrid nanocomposites. It can be seen that the density of the hybrid nanocomposites increased with the increase in the amount of TiO2. However, the density of the hybrid nanocomposites washigher than that of the base aluminum matrix due to the presence of higher density reinforcements SiC (3.21 g/cc) and TiO2 (4.23 g/cc) in the Al matrix (2.7 g/cc). The higher density of the microwave sintered samples influenced the structural, hardness, and strength of the hybrid nanocomposites [14]. It can also be noticed from Figure 2 that the porosity increased with an increase in the contents of TiO2 nanoparticles in the aluminum matrix. This can be ascribed to the effect of agglomeration at high reinforcement content and pore nucleation formation at the particle reinforcement surface [37][38][39].  Figure 3 shows the XRD (X-ray diffraction) results of the Al/SiC/TiO2 hybrid nanocomposites. The XRD analysis results confirm the presence of SiC [40] and TiO2 [41] nanoparticles in the Al matrix [42].   Figure 3 shows the XRD (X-ray diffraction) results of the Al/SiC/TiO 2 hybrid nanocomposites. The XRD analysis results confirm the presence of SiC [40] and TiO 2 [41] nanoparticles in the Al matrix [42]. The sharp diffraction peaks of Al were at the 2θangles of 38. The XRD results confirm that there were no other peaks or impurities present. The presence of a small portion of reinforcements was hard to detect in the XRD spectra. However, the intensity of these peaks increased with an increasing number of reinforcements.

Microstructure Behavior
The SEM-EDS analysis was performed on the pure Al and its hybrid nanocomposites. Figure 4 shows the SEM micrographs of the pure Al and Al/SiC/TiO 2 hybrid nanocomposites. The SEM micrographs of the specimens clearly show that the SiC and TiO 2 reinforcements were uniformly distributed throughout the aluminum matrix. The light grey surface can beobserved, representing the aluminum matrix, while the dark grey and white spots confirm the presence of SiC and TiO 2 particles, respectively. The selection of suitable ball milling parameters, such as milling speed, time, powder to weight ratio, and so forth, and appropriate microwave sintering parameters, such as sintering temperature, time, power, holding time, and so forth, contributed to producing the compacts with uniform distribution reinforcements. Usually, an increasing percentage in reinforcement leads to an increase in the porosity of the composites, as reflected in Figure 2 [43,44].

Microstructure Behavior
The SEM-EDS analysis was performed on the pure Al and its hybrid nanocomposites. Figure 4 shows the SEM micrographs of the pure Al and Al/SiC/TiO2 hybrid nanocomposites. The SEM micrographs of the specimens clearly show that the SiC and TiO2 reinforcements wereuniformly distributed throughoutthe aluminum matrix. The light grey surface can beobserved, representing the aluminum matrix, while the dark grey and white spots confirm the presence of SiC and TiO2 particles, respectively. The selection of suitable ball milling parameters, such as milling speed, time, powder to weight ratio, and so forth, and appropriate microwave sintering parameters, such as sintering temperature, time, power, holding time, and so forth, contributed to producing the compacts with uniform distribution reinforcements. Usually, an increasing percentage in reinforcement leads to an increase in the porosity of the composites, as reflected in Figure 2 [43,44].
The EDS (energy-dispersive X-ray spectroscopy) analysis of the Al/SiC/TiO2 hybrid nanocomposites is depicted in Figure 5. The EDS spectrum in Figure 5A3-A5 confirms the presence of Al, Si, C, Ti, and O within the Al/SiC/TiO2 hybrid nanocomposites. Furthermore, Figure 5A5 presents the elemental mapping, which shows that Si (purple), C (blue), Ti (red), and O (yellow) elements were uniformly distributed throughoutthe Al matrix (green). Consequently, theuniform distribution of Si, C, Ti, and O elements within the Al matrix was successfully achieved.   The EDS (energy-dispersive X-ray spectroscopy) analysis of the Al/SiC/TiO 2 hybrid nanocomposites is depicted in Figure 5. The EDS spectrum in Figure 5A3-A5 confirms the presence of Al, Si, C, Ti, and O within the Al/SiC/TiO 2 hybrid nanocomposites. Furthermore, Figure 5A5 presents the elemental mapping, which shows that Si (purple), C (blue), Ti (red), and O (yellow) elements were uniformly distributed throughout the Al matrix (green). Consequently, the uniform distribution of Si, C, Ti, and O elements within the Al matrix was successfully achieved.   Figure 5. EDX spectra images of (A3) Al/5SiC/3TiO 2 , (A4) Al/5SiC/6TiO 2 , and (A5) EDX spectra, and elemental mapping of Al/5SiC/9TiO 2 hybrid nanocomposites. Figure 6 shows the 2D and 3D AFM images of the Al/SiC/TiO 2 hybrid nanocomposites, together with an average surface roughness value. From the AFM analysis, the average surface roughness (R a ) of pure aluminum was 21.793 nm. The average roughness (R a ) of the hybrid nanocomposites increased from 21.793 nm to 34.640 nm with an increase in TiO 2 . The average roughness (R a ) of the hybrid nanocomposites is shown in Figure 7. The increase in average roughness (R a )of Al/SiC/TiO 2 hybrid nanocomposites can be regarded as the effect of insoluble and hard ceramic particles and is consistent with the already reported studies [45,46]. The Al/SiC/TiO 2 hybrid nanocomposites show the varying number of protrusions (hills) and valleys along with the topographic surface, providing an idea about the surface morphology. Figure 7. The increase in average roughness (Ra)of Al/SiC/TiO2 hybrid nanocomposites can be regarded as the effect of insoluble and hard ceramic particles and is consistent with the already reported studies [45,46]. The Al/SiC/TiO2 hybrid nanocomposites show the varyingnumber of protrusions (hills) and valleys along with the topographic surface, providing an idea about the surface morphology.    crease in TiO2. The average roughness (Ra) of the hybrid nanocomposites is shown in Figure 7. The increase in average roughness (Ra)of Al/SiC/TiO2 hybrid nanocomposites can be regarded as the effect of insoluble and hard ceramic particles and is consistent with the already reported studies [45,46]. The Al/SiC/TiO2 hybrid nanocomposites show the varyingnumber of protrusions (hills) and valleys along with the topographic surface, providing an idea about the surface morphology.     Figure 8 shows the Vickers microhardness results and micrographs of the microhardness indentation of the Al/SiC/TiO 2 hybrid nanocomposites. The corresponding data are summarized in Table 2. The microhardness results show that the Al/SiC/TiO 2 hybrid nanocomposites exhibited higher hardness results than the pure aluminum. Moreover, the hardness of the Al/SiC/TiO 2 hybrid nanocomposites increased with the increasing amount of TiO 2 nanoparticles into the aluminum matrix [47]. The maximum hardness (83 ± 3 HV) of the hybrid nanocomposites was achieved at 9 wt.% of TiO 2 . The improvement in the hardness of the Al/SiC/TiO 2 hybrid nanocomposites can be attributed to the load transfer mechanism. The presence of the hard reinforcements (SiC and TiO 2 ) in the aluminum matrix offers more resistance to plastic deformation, leading to increased strength and hardness of the hybrid nanocomposite and an increase in the load-bearing capacity of the hybrid nanocomposites [48]. strength and hardness of the hybrid nanocomposite and an increase in the load-beari capacity of the hybrid nanocomposites [48].

Vickers Microhardness
The Vickers microhardness can be determined by the following equation

Nanohardness
The load-displacement curves and nanohardness results in the microwave sinter Al/SiC/TiO2 hybrid nanocomposites containing varying amounts of TiO2 nanopartic are shown in Figure 9.  The Vickers microhardness can be determined by the following equation [49]: where F is the load in Kgf and d is the athematic mean of d 1 and d 2 diagonals in mm.

Nanohardness
The load-displacement curves and nanohardness results in the microwave sintered Al/SiC/TiO 2 hybrid nanocomposites containing varying amounts of TiO 2 nanoparticles are shown in Figure 9. It can be noticed from the displacement curves in Figure 9A that the penetration depth of the hybrid nanocomposites decreased with the increasing amount of TiO2 nanoparticles. The decrease in penetration depth indicates the increase in the nanohardness of the hybrid nanocomposites, which agrees with our microhardness results (Figure 8). The lower penetration depth rate is attributed to the higher resistance offered by the aluminum matrix incorporated with hard ceramic nanoparticles. The Al/SiC/TiO2 hybrid nanocomposites containing 9 wt.% TiO2 exhibited the best performance, reaching ahardness value of 7.91 GPa. Figure 10 shows the compressive stress/strain curves and results (yield and compression strength) of the microwave-sintered pure Al and Al/SiC/TiO2 hybrid nanocomposites. For a clear comparison, the corresponding mechanical data is also shownin Table  2. It can be noticed that Al/SiC/TiO2 hybrid nanocomposites demonstrated higher strength when compared to the aluminum matrix. Furthermore, the strength of the Al/SiC/TiO2 hybrid nanocomposites increased with the increasing amount of TiO2 nanoparticles in the aluminum matrix. The Al/SiC/TiO2 hybrid nanocomposite containing a 9 wt.% of TiO2 nanoparticles showed a significant improvement in compressive strength (UCS_375 ± 6 MPa) and a 0.2% offset compressive yield strength (CYS_139 ± 8 MPa) as compared to the aluminum matrix. It can be noticed from the displacement curves in Figure 9A that the penetration depth of the hybrid nanocomposites decreased with the increasing amount of TiO 2 nanoparticles. The decrease in penetration depth indicates the increase in the nanohardness of the hybrid nanocomposites, which agrees with our microhardness results ( Figure 8). The lower penetration depth rate is attributed to the higher resistance offered by the aluminum matrix incorporated with hard ceramic nanoparticles. The Al/SiC/TiO 2 hybrid nanocomposites containing 9 wt.% TiO 2 exhibited the best performance, reaching ahardness value of 7.91 GPa. Figure 10 shows the compressive stress/strain curves and results (yield and compression strength) of the microwave-sintered pure Al and Al/SiC/TiO 2 hybrid nanocomposites. For a clear comparison, the corresponding mechanical data is also shownin Table 2. It can be noticed that Al/SiC/TiO 2 hybrid nanocomposites demonstrated higher strength when compared to the aluminum matrix. Furthermore, the strength of the Al/SiC/TiO 2 hybrid nanocomposites increased with the increasing amount of TiO 2 nanoparticles in the aluminum matrix. The Al/SiC/TiO 2 hybrid nanocomposite containing a 9 wt.% of TiO 2 nanoparticles showed a significant improvement in compressive strength (UCS_375 ± 6 MPa) and a 0.2% offset compressive yield strength (CYS_139 ± 8 MPa) as compared to the aluminum matrix.

Compression and Yield Strength
The improvements in strength and hardness of the hybrid nanocomposites can be attributed to many strengthening mechanisms, as reported in the literature [50][51][52][53]. In this present study, the improvements in strength and hardness of the Al/SiC/TiO 2 hybrid nanocomposites were mainly due to the load transfer mechanism, dispersion hardening effect, and Orowan strengthening mechanisms [54,55]. The load transfer mechanism is the most accepted strengthening mechanism that takes place due to the transfer of load (Equation (3)) from weaker matrix material to the harder reinforcements through their interface [56].
∆σ load = 0.5 fσ m where f is the volume fraction of reinforcements and σ m is the YS of the matrix (MPa). The improvements in strength and hardness of the hybrid nanocomposites can be attributed to many strengthening mechanisms, as reported in the literature [50][51][52][53]. In this present study, the improvements in strength and hardness of the Al/SiC/TiO2 hybrid nanocomposites were mainly due to the load transfer mechanism, dispersion hardening effect, and Orowan strengthening mechanisms [54,55]. The load transfer mechanism is the most accepted strengthening mechanism that takes place due to the transfer of load (Equation (3)) from weaker matrix material to the harder reinforcements through their interface [56].
where f is the volume fraction of reinforcements and σm is the YS of the matrix (MPa).  Figure 11 shows the fractography images of the pure aluminum and its hybrid nanocomposites fractured under compression loadings at room temperature. A close comparison suggests a shear mode fracture in the Al/SiC/TiO 2 hybrid nanocomposites. The compressive deformations obtained in the base matrix and hybrid nanocomposites weredifferent due to the dispersion hardening effect. The aluminum matrix's shear fracture was less than the hybrid nanocomposites because the reinforcements in the aluminum matrix acted as secondary phases that restricted the plastic deformation in the aluminum matrix.

Fractography
Crystals 2021, 11, x FOR PEER REVIEW 11  3.8. Fractography Figure 11 shows the fractography images of the pure aluminum and its hy nanocomposites fractured under compression loadings at room temperature. A comparison suggests a shear mode fracture in the Al/SiC/TiO2 hybrid nanocompo The compressive deformations obtained in the base matrix and hybrid nanocompo weredifferent due to the dispersion hardening effect. The aluminum matrix's s fracture was less than the hybrid nanocomposites because the reinforcements in aluminum matrix acted as secondary phases that restricted the plastic deformation i aluminum matrix. Figure 11. Fracture surface images of the (A1) pure Al, (A2) Al/5SiC, (A3) Al/5SiC/3TiO2, and Al/5SiC/9TiO2 hybrid nanocomposites.

Conclusions
Al/SiC/TiO 2 hybrid nanocomposites have been successfully synthesized using a combination of powder metallurgy (blending and compaction) and microwave sintering (MWS) techniques. The effects of the concentration of TiO 2 nanoparticles on microstructural and mechanical properties of the developed Al/SiC/TiO 2 hybrid nanocomposites wereinvestigated. The SEM results indicate a uniform distribution of SiC and TiO 2 reinforcements in the aluminum matrix. The nanohardness of the nanocomposites increased from 3.91 ± 0.3 GPa for pure Al to 7.91 ± 0.7 GPa for the Al/5SiC/9TiO 2 nanocomposite. Al/SiC/TiO 2 nanocomposites containing 9 wt.% of TiO 2 nanoparticles demonstrated superior hardness (83 ± 3 Hv), yield strength (139 ± 8 MPa), and compressive strength (375 ± 6 MPa) when compared to pure Al. Finally, the Al/SiC/TiO 2 hybrid nanocomposites underwent a shear mode of fracture during their plastic deformation instead of a brittle fracture mode.