Mechanical and Thermal Evaluation of Aluminum Hybrid Nanocomposite Reinforced with Alumina and Graphene Oxide

Aluminum matrix composites are among the most widely used metal matrix composites in several industries, such as aircraft, electronics, automobile, and aerospace, due to their high specific strength, durability, structural rigidity and high corrosion resistance. However, owing to their low hardness and wear resistance, their usage is limited in demanding applications, especially in harsh environments. In the present work, aluminum hybrid nanocomposite reinforced with alumina (Al2O3) and graphene oxide (GO) possessing enhanced mechanical and thermal properties was developed using spark plasma sintering (SPS) technique. The focus of the study was to optimize the concentration of Al2O3 and GO content in the composite to improve the mechanical and thermal properties such as hardness, compressive strength, heat flow, and thermal expansion. The nanocomposites were characterized by FESEM, EDS, XRD and Raman spectroscopy to investigate their morphology and structural properties. In the first phase, different volume percent of alumina (10%, 20%, 30%) were used as reinforcement in the aluminum matrix to obtain (Al+X% Al2O3) composite with the best mechanical/thermal properties which was found to be 10 V% of Al2O3. In the second phase, a hybrid nanocomposite was developed by reinforcing the (Al + 10 V% Al2O3) with different weight percent (0.25%, 0.5%, 1%) of GO to obtain the optimum composition with improved mechanical/thermal properties. Results revealed that the Al\10 V% Al2O3\0.25 wt.% GO hybrid nanocomposite showed the highest improvement of about 13% in hardness and 34% in compressive strength as compared to the Al\10V% Al2O3 composite. Moreover, the hybrid nanocomposite Al\10 V% Al2O3\0.25 wt.% GO also displayed the lowest thermal expansion.


Introduction
Aluminum (Al) has traditionally been used for a variety of applications because of its lightweight, high corrosion resistance, high electrical, thermal conductivity and better formability compared to ferrous and other non-ferrous metals. However, the use of plain Al is generally not suitable for engineering applications because of its high ductility and low strength. This has led to the development of a variety of aluminum-based alloys with enhanced mechanical and tribological properties for a wide range of engineering applications. These alloys exhibit high strength to weight ratio, good machinability and lower cost of fabrication [1][2][3][4]. Further enhancement in the properties has been achieved by the development of aluminum metal matrix composites (MMCs) by adding different reinforcements [5]. The advantage of Al-MMCs is that they can be sintered with tailored

SEM and XRD Analysis of the as Received Powders
The morphology of the as received powders was analyzed by scanning electron microscopy (SEM). X-ray diffraction (XRD) was also conducted to determine the phases of the as received powders. XRD was carried out on a Rigaku Miniflex X-ray diffractometer, using Cu Kα radiation (λ = 0.15416 nm) in the 2θ range 5 • -120 • at a scanning speed of 2 • /min. Figure 1a,c displays the high magnification SEM images of as received Al powder, Al 2 O 3 and GO, respectively. It can be observed from Figure 1a that Al particles are spherically shaped with an average diameter of 30 µm. Figure 1b shows the Al 2 O 3 particles that are acicular in shape and which gather to form agglomerates in some areas, whereas Figure 1c shows small sheets of GO. The XRD spectra of the as received powders are displayed in Figure 1d-f, indicating that they exhibit the signature peaks of the as received Al, Al 2 O 3 and GO powders.

Experimental Procedure
The steps for fabricating Al\X% Al 2 O 3 nanocomposite and Al\X% Al 2 O 3 \Y% GO hybrid nanocomposite samples included ultrasonication, ball milling and spark plasma sintering. The different parameters used during each step are indicated in this section.

Ultrasonication of Al 2 O 3 and GO Powders
Prior to mixing the reinforcements with the matrix Al powder, each of the reinforcements, namely, Al 2 O 3 and GO were sonicated individually in ethanol for 10 min and 1 h, respectively using a probe sonicator (Sonics VCX 750, Newtown, CT, USA) at room Nanomaterials 2021, 11, 1225 5 of 19 temperature with an On\Off cycle of 20\5 s and an amplitude of 45%. Different volume percent (10%, 20%, 30%) of Al 2 O 3 and different weight percent (0.25, 0.5 and 1 wt.%) of GO were sonicated under the same conditions to prepare different compositions.

Ball-Milling Procedure
Pure Al with different Al 2 O 3 volume percent (10%, 20%, 30%) was loaded in zirconia vials and mixed for 24 h in a ball mill attritor (HD/HDDM/01, Union process, Inc. Akron, OH, USA) to produce a homogeneous mixture. The process was carried out under the flow of Argon (Ar) gas atmosphere to avoid oxidation. A total of 50 mL of ethanol was used as a process control agent (PCA) to avoid excessive cold welding and agglomeration. Zirconium oxide (ZrO 2 ) balls with a diameter of 5 mm were used with a ball-to-powder weight ratio (BPR) of 10:1. Mixing was performed at a speed of 200 rpm. The ball milling experiment was halted after the first hour of the process to remove any powder from the walls of the vial to eliminate its accumulation on the walls. The vials were purged with Ar gas during the whole mixing process. Subsequently, the powder mixture was dried in an oven at a temperature of 80 • C for 12 h. The same procedure was used for 48 h to mix the Al\X% Al 2 O 3 \Y% GO hybrid powders to obtain a homogeneous mixture. Table 2 summarizes the mixing parameters used for the nanocomposites and the hybrid nanocomposite powders.

Spark Plasma Sintering Procedure
As-received Al powder was used to fabricate a reference sample. Al powder was charged in a 20 mm graphite die. A graphite sheet approximately 0.35 mm thick was placed in between the die and the powder as well as between the powder and the punch to easily remove the sample and avoid the wear of the punch. Spark plasma sintering machine from FCT group, System GMBH, (Rauenstein, Germany) was used to sinter the Al, the Al-X% Al 2 O 3 nanocomposite and Al\X% Al 2 O 3 \Y% GO hybrid nanocomposite samples. In addition to the mentioned parameters in Table 3, the other SPS sintering parameters included, cooling rate = 100 • C/min to room temperature (20-35 • C), pulse = 1 ms, pause = 0, and number of pulse = 1. Circular samples of 20 mm diameter with a thickness of 6 mm were obtained after sintering. The sintered samples were mounted by using hot mounted and grounded using different grit papers starting from rougher to the finer grit (240, 320, 400, 600, 800, 1200) followed by polishing with 0.3 µm alumina paste to obtain a polished surface. The samples were subjected to ultrasonic cleaning for 10 min to remove any debris being subjected to further characterizations.

Densification, Mechanical, Thermal and Thermomechanical Analyses
Various characterization techniques were used to evaluate the mechanical and thermal properties of the nanocomposites and the hybrid nanocomposites. Density measurements were carried out in line with the Archimedes principle (Kern ABT weighing scale, 320 g capacity, Balingen, Germany). Hardness measurements were carried out using a Zwick Roell Vickers hardness testing machine (Ulm, Germany) at a load of 500 gf. An average of 10 readings was taken for each sample. Scanning electron microscopy (SEM) fitted with an electronic dispersive x-ray (EDX) (Quanta FEG 250, Thermo Fisher company, Waltham, MA USA) was used to evaluate the morphology and the chemical composition of the samples. A compression test was carried out on an Instron testing machine to determine the behavior or response of the nanocomposites when exposed to compressive loads. Al is a soft material, and prone to dimensional instability due to its expansion when exposed to high temperature. A higher coefficient of thermal expansion indicates a more expansion tendency of the material. Hence, to evaluate the thermal expansion of the developed nanocomposites, Mettler Toledo instrument (TMA/SDTA LF/100, Columbus, OH, USA) was used for thermal expansion measurement.

Results and Discussion
The results are presented in three sub-sections. Firstly, the morphology and mechanical characterization results for the Al-X vol% Al 2 O 3 composites are presented (Phase I) followed by the characterization of the hybrid nanocomposite (Phase II) and concluding with the mechanical and thermal characterization of the optimum hybrid nanocomposite (Phase III). and Al-30% Al 2 O 3 it can be observed the Al 2 O 3 particles are non-uniformly distributed with a significant amount of agglomeration. The agglomeration tends to increase as the volume content of Al 2 O 3 is increased from 20% to 30% Al 2 O 3 . Therefore, reducing the agglomeration would be a key element of improving the mechanical properties of Al-(20% and 30% Al 2 O 3 ) nanocomposites due to the restriction of the interfacial area between the matrix and the reinforcement.  Figure 3b. A few cracks are also observed around the grain boundaries of the sample containing 20 vol% Al 2 O 3 . This can be attributed to a higher volume percent of Al 2 O 3 content which makes the material brittle. Therefore, due to the brittleness of the sample which is associated with an increase in the volume percent of Al 2 O 3 , the fracture rate of the sample increased as was observed with Al-30 vol% Al 2 O 3 which fractured during the grinding and polishing of the sample. Hence, the SEM could not be taken owing to the difficulty faced during grinding and polishing. , shows the SEM images for the SPS sample of Al-10 vol% Al2O3 nanocomposite, where little porosity can be observed with fine grain size. The samples were etched by buffered hydrofluoric acid (HF) for 10 s (1 mL HF and 49 mL water) for imaging. Adding more amount of Al2O3, as in the Al-20 vol% Al2O3 sample, Al2O3 is mainly observed along the grain boundaries of Al as illustrated in Figure 3b. A few cracks are also observed around the grain boundaries of the sample containing 20 vol% Al2O3. This can be attributed to a higher volume percent of Al2O3 content which makes the material brittle. Therefore, due to the brittleness of the sample which is associated with an increase in the volume percent of Al2O3, the fracture rate of the sample increased as was observed with Al-30 vol% Al2O3 which fractured during the grinding and polishing of the sample. Hence, the SEM could not be taken owing to the difficulty faced during grinding and polishing.

Density of Al-X vol% Al2O3 Nanocomposites
After sintering and grinding, Al-X vol%Al2O3 nanocomposite samples, the density was measured based on the Archimedes method and the results are shown in Figure 4. It is observed that Al displays a higher density of 99.7% as compared to the nanocomposite samples, with a density of 97.5% and 93.7% for Al-10vol%Al2O3 and Al-20vol%Al2O3, respectively. This density decrease can be attributed to the reduction in the wettability between Al2O3 and Al matrix due to the agglomeration of Al2O3 particles, particularly at high volume content. However, for the sample containing 30 vol% Al2O3, the density could not be measured because of the challenges mentioned above. The theoretical densities were measured using the rule of mixtures and are shown in Figure 4 (Inset).

Density of Al-X vol% Al 2 O 3 Nanocomposites
After sintering and grinding, Al-X vol%Al 2 O 3 nanocomposite samples, the density was measured based on the Archimedes method and the results are shown in Figure 4. It is observed that Al displays a higher density of 99.7% as compared to the nanocomposite samples, with a density of 97.5% and 93.7% for Al-10 vol% Al 2 O 3 and Al-20 vol% Al 2 O 3 , respectively. This density decrease can be attributed to the reduction in the wettability between Al 2 O 3 and Al matrix due to the agglomeration of Al 2 O 3 particles, particularly at high volume content. However, for the sample containing 30 vol% Al 2 O 3 , the density could not be measured because of the challenges mentioned above. The theoretical densities were measured using the rule of mixtures and are shown in Figure 4 (Inset).  However, the addition of 10 vol% of Al 2 O 3 resulted in a significant increase in the hardness from 32 to 55.8 HV. This tremendous increase in the hardness can be attributed to the presence of the uniformly distributed hard and non-deformable nanoparticles of Al 2 O 3 particles within the Al matrix, as can be seen in Figure 3a. The presence of these particles thereby hinders the movement of dislocations, resulting in an increase in the hardness. Further increasing the amount of Al 2 O 3 to 20 vol% resulted in a reduction in the hardness from 55.8 to 47.2 HV. This reduction can be attributed to the lower densification triggered by the agglomeration of Al 2 O 3 particles, as clearly observed in the SEM image Figure 3b owing to high volume percent and non-uniform distribution of Al 2 O 3 . As mentioned earlier, with a further increase in the volume percent of Al 2 O 3 to 30%, the nanocomposite sample fractured due to an increased brittleness resulting from the agglomeration and cracks during sintering, due to which, the hardness measurements were not acquired.
by the agglomeration of Al2O3 particles, as clearly observed in the SEM image F owing to high volume percent and non-uniform distribution of Al2O3. As mentio lier, with a further increase in the volume percent of Al2O3 to 30%, the nanoco sample fractured due to an increased brittleness resulting from the agglomera cracks during sintering, due to which, the hardness measurements were not acqu    surface. The GO was embedded between the aluminium particles, as shown in the fractured surfaces, which promote the improvement in the toughness of the hybrid. SEM/EDS mapping micrographs, Figure 7f-h revealed the presence of both intergranular and transgranular fracture morphologies in the hybrid sample. Moreover, there are few regions where agglomerates were found which deteriorate and have an adverse effect on the properties of the hybrid sample.

Density Measurement of Al-10%Al2O3-Y%GO Hybrid Nanocomposites
After sintering and grinding/polishing of Al-10%Al2O3-Y wt.% GO hybrid nanocomposite samples, the experimental density was measured based on the Archimedes method and the results are shown in Figure 8. The results indicate that the relative density is reduced with increasing GO content, whereby, adding 0.25 wt.% of GO to the Al-10% Al2O3 nanocomposite decreases the relative density from 97.5% to 96.8 %. Further addition of GO to Al-10% Al2O3 nanocomposite gradually reduces the relative density to 95.4% and 94.6% corresponding to 0.5wt.% and 1wt.% GO content, respectively. This reduction in the density is attributed to the tendency of GO to distribute itself along the grain boundaries which impedes the densification process, consequently resulting in higher porosity with a higher content of GO as observed in SEM images in Figure 7.

Density Measurement of Al-10% Al 2 O 3 -Y% GO Hybrid Nanocomposites
After sintering and grinding/polishing of Al-10% Al 2 O 3 -Y wt.% GO hybrid nanocomposite samples, the experimental density was measured based on the Archimedes method and the results are shown in Figure 8. The results indicate that the relative density is reduced with increasing GO content, whereby, adding 0.25 wt.% of GO to the Al-10% Al 2 O 3 nanocomposite decreases the relative density from 97.5% to 96.8%. Further addition of GO to Al-10% Al 2 O 3 nanocomposite gradually reduces the relative density to 95.4% and 94.6% corresponding to 0.5 wt.% and 1 wt.% GO content, respectively. This reduction in the density is attributed to the tendency of GO to distribute itself along the grain boundaries which impedes the densification process, consequently resulting in higher porosity with a higher content of GO as observed in SEM images in Figure 7.

Hardness Measurement of Al-10%Al2O3-Y%GO Hybrid Nanocomposites
The hardness results of Al-10% Al2O3-Y% GO are presented in Figure 9. The highest hardness of 63 HV was observed for the hybrid sample containing 0.25 wt.% of GO among all the developed samples. The hardness reduced to 57 HV with an increase in the GO content to 0.5 wt.%. However, not much difference was observed in the hardness of the hybrid sample with a further increase in the GO content to 1 wt.%. The increase in the hardness of the hybrid nanocomposite with a low content of GO (0.25 wt.%) is attributed to the uniform distribution of both fillers, Al2O3 and GO, in the matrix. The homogeneous distribution of these fillers helps in the load transfer from the matrix leading to a higher hardness of the hybrid nanocomposite. Furthermore, these fillers as discussed above and shown by SEM, influence the microstructure of the hybrid nanocomposites resulting in finer grain size. The reason for observing relatively lower hardness in the samples containing 0.5 wt.% and 1 wt.% as compared to 0.25 wt.% GO sample is attributed to the low densification associated with the Al2O3 agglomeration and porosity.

Hardness Measurement of Al-10% Al 2 O 3 -Y% GO Hybrid Nanocomposites
The hardness results of Al-10% Al 2 O 3 -Y% GO are presented in Figure 9. The highest hardness of 63 HV was observed for the hybrid sample containing 0.25 wt.% of GO among all the developed samples. The hardness reduced to 57 HV with an increase in the GO content to 0.5 wt.%. However, not much difference was observed in the hardness of the hybrid sample with a further increase in the GO content to 1 wt.%. The increase in the hardness of the hybrid nanocomposite with a low content of GO (0.25 wt.%) is attributed to the uniform distribution of both fillers, Al 2 O 3 and GO, in the matrix. The homogeneous distribution of these fillers helps in the load transfer from the matrix leading to a higher hardness of the hybrid nanocomposite. Furthermore, these fillers as discussed above and shown by SEM, influence the microstructure of the hybrid nanocomposites resulting in finer grain size. The reason for observing relatively lower hardness in the samples containing 0.5 wt.% and 1 wt.% as compared to 0.25 wt.% GO sample is attributed to the low densification associated with the Al 2 O 3 agglomeration and porosity.   Figure 10 shows Raman spectra of GO powder and the hybrid nanocomposite powders after mixing. It can be observed that GO shows two signature peaks/bands. One at approximately 1580 cm −1 corresponding to the G band resulting from the stretching of the C-C bond in GO, and another one at approximately 1350 cm −1 , corresponding to the D band which is associated with the disorders or defects that occur from the resonance Raman spectra of Sp2 hybridized carbon. Both peaks can be observed in the Raman spectra for all the hybrid nanocomposite powders. However, their intensity increases with an increase in the GO content.  Figure 11 shows the X-ray diffraction pattern obtained for the SPS sin for Al-10%Al2O3 nanocomposites and all the developed hybrid nanocomp The XRD pattern of Al-10% Al2O3 nanocomposite shows slightly less broad both Al2O3 and Al, as compared to the hybrid nanocomposite samples. tributed to the effect of ball milling time where the nanocomposite was while the hybrid nanocomposite powders were milled for 48 h resulting in geneous and uniform distribution of the fillers in the Al matrix. However, not observed in the hybrid nanocomposite due to its very small amount. M observed that no chemical reaction occurred between GO and Al-Al2O3 n as there was no new phase such as intermetallic phase(s) nor the formatio carbide (Al4C3) was seen in any of the XRD patterns for the hybrid nanoco  Figure 11 shows the X-ray diffraction pattern obtained for the SPS sintered samples for Al-10% Al 2 O 3 nanocomposites and all the developed hybrid nanocomposite samples. The XRD pattern of Al-10% Al 2 O 3 nanocomposite shows slightly less broadened peaks for both Al 2 O 3 and Al, as compared to the hybrid nanocomposite samples. This can be attributed to the effect of ball milling time where the nanocomposite was milled for 24 h while the hybrid nanocomposite powders were milled for 48 h resulting in a more homogeneous and uniform distribution of the fillers in the Al matrix. However, GO phase was not observed in the hybrid nanocomposite due to its very small amount. Moreover, it was observed that no chemical reaction occurred between GO and Al-Al 2 O 3 nanocomposite as there was no new phase such as intermetallic phase(s) nor the formation of aluminum carbide (Al 4 C 3 ) was seen in any of the XRD patterns for the hybrid nanocomposites. The XRD patterns were normalised and the crystallite size and microstrain were also estimated by using the following equations: crystallite size (average in Å) = Kλ/(Bcosθ) and BT = CεTanθ (ca. C=4 for spherical particles), respectively. The average crystallite size was found to be slightly decreasing, with an increased ball milling duration and the average microstrain is found to slightly increase for all compositions. The slight increase in microstrain value can be attributed to the presence of hard alumina particles and the cold welding of soft aluminum.

Summary of Phase II
From the above results, it can be concluded that the hybrid nanocomposite sample containing 0.25%GO showed the highest hardness, density and uniform distribution of the fillers in the Al matrix. Hence, based upon the above results Al-10%Al2O3-0.25%GO was selected for further processing.

Results of Phase III-Mechanical and Thermal Characterization of Al-10 vol% Al2O3-0.25 wt.% GO Hybrid Nanocomposites
Mechanical and thermal characterization of the optimum hybrid nanocomposite which was obtained from Phase II to be Al-10 vol% Al2O3-0.25 wt.% GO were evaluated. Compressive strength, differential scanning calorimetry and thermal expansions for the hybrid nanocomposite were investigated and presented below. The results of compressive strength for Al as a reference matrix, Al-10% Al2O3 and the Al-10% Al2O3-0.25%GO hybrid nanocomposite are presented in Figure 12. The compressive strength of Al sample measured to be 75MPa significantly increased to 130 MPa for the Al-10% Al2O3 nanocomposite, whereas the compressive strain reduced to 0.4% as The XRD patterns were normalised and the crystallite size and microstrain were also estimated by using the following equations: crystallite size (average in Å) = Kλ/(Bcosθ) and BT = CεTanθ (ca. C = 4 for spherical particles), respectively. The average crystallite size was found to be slightly decreasing, with an increased ball milling duration and the average microstrain is found to slightly increase for all compositions. The slight increase in microstrain value can be attributed to the presence of hard alumina particles and the cold welding of soft aluminum.

Summary of Phase II
From the above results, it can be concluded that the hybrid nanocomposite sample containing 0.25%GO showed the highest hardness, density and uniform distribution of the fillers in the Al matrix. Hence, based upon the above results Al-10% Al 2 O 3 -0.25%GO was selected for further processing. The results of compressive strength for Al as a reference matrix, Al-10% Al 2 O 3 and the Al-10% Al 2 O 3 -0.25%GO hybrid nanocomposite are presented in Figure 12. The compressive strength of Al sample measured to be 75MPa significantly increased to 130 MPa for the Al-10% Al 2 O 3 nanocomposite, whereas the compressive strain reduced to 0.4% as compared to the Al sample. This effect could be attributed to the presence of the reinforcing Al 2 O 3 hard nanoparticles. Moreover, the compressive strength further increased in the Al-10% Al 2 O 3 -0.25%GO hybrid nanocomposite to values of 180 MPa, about 30% higher than that of Al-10%Al 2 O 3 nanocomposite. This improvement can be attributed to the presence of the uniformly distributed GO filler in the hybrid nanocomposite, leading to an improvement in the interfacial adhesion between Al 2 O 3 and Al without overlapping or agglomeration. However, an increase in the compressive strain for the hybrid composite sample was observed to a value of about 0.52% as compared to the 0.4% compressive strain for Al-10% Al 2 O 3 nanocomposite. This can be attributed to the structure of GO, which contains hydroxide (OH -) and (O -) chains, which in turn, leads to an increase in the length of the C-C bond at each hexagonal lattice as reported by Pop et al. [40]. These bonds will be dominant over Vander Waals attractions in (Al-10% Al 2 O 3 -0.25%GO) hybrid nanocomposites, thus results in strain improvement caused by the efficient load transfer from soft Al matrix to hard GO [41]. compared to the Al sample. This effect could be attributed to the presence of the reinforcing Al2O3 hard nanoparticles. Moreover, the compressive strength further increased in the Al-10% Al2O3-0.25%GO hybrid nanocomposite to values of 180 MPa, about 30% higher than that of Al-10%Al2O3 nanocomposite. This improvement can be attributed to the presence of the uniformly distributed GO filler in the hybrid nanocomposite, leading to an improvement in the interfacial adhesion between Al2O3 and Al without overlapping or agglomeration. However, an increase in the compressive strain for the hybrid composite sample was observed to a value of about 0.52% as compared to the 0.4% compressive strain for Al-10%Al2O3 nanocomposite. This can be attributed to the structure of GO, which contains hydroxide (OH -) and (O -) chains, which in turn, leads to an increase in the length of the C-C bond at each hexagonal lattice as reported by Pop et al. [40]. These bonds will be dominant over Vander Waals attractions in (Al-10%Al2O3-0.25%GO) hybrid nanocomposites, thus results in strain improvement caused by the efficient load transfer from soft Al matrix to hard GO [41]. Thermal expansion was carried out for the developed SPS samples, and the results obtained are presented in Figure 13. It is observed that the coefficient of thermal expansion linearly increased with increasing temperature for all the developed samples. The thermal expansion of Al which is the reference matrix was found to be 18.89 ppm °C −1 , whereas for Al-10%Al2O3 nancomposite it reduced to 15.51 ppm °C −l , leading to a reduction of 17%. Likewise, the thermal expansion further decreased for the Al-10% Al2O3-0.25%GO hybrid Thermal expansion was carried out for the developed SPS samples, and the results obtained are presented in Figure 13. It is observed that the coefficient of thermal expansion linearly increased with increasing temperature for all the developed samples. The thermal expansion of Al which is the reference matrix was found to be 18.89 ppm • C −1 , whereas for Al-10% Al 2 O 3 nancomposite it reduced to 15.51 ppm • C −l , leading to a reduction of 17%. Likewise, the thermal expansion further decreased for the Al-10% Al 2 O 3 -0.25%GO hybrid nanocomposite to a value of 14.82 ppm • C −1 , leading to a reduction of 4.4% in the coefficient of thermal expansion as compared to Al-10% Al 2 O 3 nanocomposite and reduction of 21% as compared to Al. mal expansion of the composite [42]. The thermal expansion of a composite is dominated by the component that has the most outstanding bulk modulus value. The bulk modulus values of aluminium and sintered alumina have been reported to be 62 GPa and 257 GPa, respectively. Additionally, indeed, we observed the lowest thermal expansion values for compositions having the highest tested alumina and GO content and this relationship was likely due to alumina and GO inclusions having a relatively much lower thermal expansion value and high bulk modulus as compared to the aluminium matrix. Generally, the thermal expansion decreased as alumina and GO was incorporated into the matrix. Thermal expansion of metal matrix composites is strongly influenced by voids and the breaking of bonds between constituents of the composite. Increasing the alumina and GO content of the composite coincided with the appearance of voids and porosity resulting in reduced effective thermal strain and lowering the coefficient of thermal expansion of the composite [42]. The thermal expansion of a composite is dominated by the component that has the most outstanding bulk modulus value. The bulk modulus values of aluminium and sintered alumina have been reported to be 62 GPa and 257 GPa, respectively. Additionally, indeed, we observed the lowest thermal expansion values for compositions having the highest tested alumina and GO content and this relationship was likely due to alumina and GO inclusions having a relatively much lower thermal expansion value and high bulk modulus as compared to the aluminium matrix. nanocomposite. On the other hand, the coefficient of thermal expansion was the lowest for the hybrid nanocomposite sample. Table 4 presents a summary and comparison of thermal and mechanical properties obtained in the present research for the hybrid sample to the mechanical and thermal properties of other developed aluminum composites in the literature.

Conclusions
Hybrid aluminum nanocomposites reinforced with alumina and graphene oxide were successfully produced by powder metallurgy technique and spark plasma sintering. The study was conducted in three phases whereby, in phase1, the optimum volume percent of alumina content out of 10%, 20%, 30% was determined. It was found that Al reinforced with 10 vol% Al 2 O 3 resulted in the best mechanical properties due to the uniform dispersion of Al 2 O 3 particles throughout the Al matrix as observed in SEM micrographs. In phase 2, different weight percent of GO (0.25%, 0.5% and 1%) were added to Al-10% Al 2 O 3 to form a hybrid nanocomposite. It was found that Al-10vol% Al 2 O 3 -0.25 wt.% GO resulted in the best mechanical properties in terms of hardness. The distribution of GO was identified to be along the Al grain boundaries while the Al 2 O 3 particle were distributed between the grain boundaries of Al grains. XRD results confirmed that no chemical reaction or intermetallic phase was formed in Al-10% Al 2 O 3 and Al-10% Al 2 O 3 -0.25%GO. The optimized hybrid nanocomposite was further characterized in phase 3 by measuring its compressive strength and its thermal expansion. It was found that adding 0.25 wt.% of GO into Al-10vol% Al 2 O 3 nanocomposite increased the compressive strength by 30%. Moreover, the Al-10% Al 2 O 3 -0.25%GO hybrid nanocomposite showed the lowest coefficient of thermal expansion.