Analysis of Microstructure and Mechanical Properties of Bi-Modal Nanoparticle-Reinforced Cu-Matrix

: Bi-modal particles are used as reinforcements for Cu-matrix. Nano TiC and/or Al 2 O 3 were mechanically mixed with Cu particles for 24 h. The Cu-TiC/Al 2 O 3 composites were successfully produced using spark plasma sintering (SPS). To investigate the effect of TiC and Al 2 O 3 nanoparticles on the microstructure and mechanical properties of Cu-TiC/Al 2 O 3 nanocomposites, they were added, whether individually or combined, to the copper (Cu) matrix at 3, 6, and 9 wt.%. The results showed that titanium carbide was homogeneously distributed in the copper matrix, whereas alumina nanoparticles showed some agglomeration at Cu grain boundaries. The crystallite size exhibited a clear reduction as a reaction to the increase of the reinforcement ratio. Furthermore, increasing the TiC and Al 2 O 3 nanoparticle content in the Cu-TiC/Al 2 O 3 composites reduced the relative density from 95% for Cu-1.5 wt.% TiC and 1.5 wt.% Al 2 O 3 to 89% for Cu-4.5 wt.% TiC and 4.5 wt.% Al 2 O 3 . Cu-9 wt.% TiC achieved a maximum compressive strength of 851.99 N/mm 2 . Hardness values increased with increasing ceramic content.


Introduction
Copper strengthening is a current priority due to the pressing need to use it in various applications requiring a balance of properties [1][2][3]. Metal-matrix composites are most promising in achieving balanced mechanical properties between nano and microstructure materials [4][5][6][7][8]. Copper is used in many industries owing to its low cost, ease of manufacturing, and good corrosion resistance [9]. The main drawbacks of pure copper are its substantial low strength, high coefficient of thermal expansion (CTE), and generally poor mechanical properties [10]. One effective way to overcome these limitations is to reinforce copper with ceramic particles to obtain composites with superior properties. The effectiveness of dispersed particles in matrix strengthening depends primarily on particle characteristics: size, distribution, spacing, thermodynamic stability, and low solubility and diffusivity of its constituent elements in the matrix. Among ceramic particles, alumina nanoparticles have shown outstanding mechanical properties even at high temperatures, as well as low production costs [11,12]. In addition, TiC is an attractive candidate for metallic matrices such as copper (Cu), iron (Fe), aluminum (Al), titanium (Ti), and nickel (Ni) because of its high hardness, high melting point, and abrasion resistance with good

Spark Plasma Sintering (SPS)
The sintering process was performed using a spark plasma sintering technique (DR. SINTER LAB Model: SPS-1030, Syntex, Osaka, Japan). In all experiments, the powder was loaded into a graphite die with an inner diameter of 15 mm with graphite foil and enclitic by 0.5 mm thick graphite cover to prevent the friction of the sample with the die during the compaction process and to minimize heat loss. Before sintering, the SPS chamber was evacuated to a pressure below 5 Pa. The samples were heated from room temperature up to 950 • C by pulsed D.C. current using the heating rate of 20 • C/min. The samples were then held at the maximum temperature for 45 min under a uniaxial pressure of 30 MPa applied since the first minute of heating. This processing route was used to fabricate the Cu-TiC/Al 2 O 3 nanocomposites.

Mechanical Properties
The hardness was measured along the polished surface of the specimen using a Vickers hardness tester (HMV-2T Model SHIMADZU, Kyoto, Japan). The test was carried out under 100 g load for 15 s dwell time.
The microhardness values were evaluated for an average of twelve readings on the surface of each sample. The compression test for the investigated specimens was carried out using a universal testing machine. In the compression test, three samples were investigated, and average results were obtained. The dimensions of the specimens for compression tests were 6 mm in diameter and 15 mm in length. The applied crosshead speed was 0.05 mm/s, and the test was performed at room temperature. Figure 2 shows the XRD patterns of the prepared ten samples, pure Cu and 3, 6, 9 wt.% TiC/Al 2 O 3 (individual and hybrid) nanocomposites. Only peaks corresponding to Cu, TiC, and Al 2 O 3 appeared, whereas pattern-like Cu was observed in the case of 3 wt.% Cu/TiC/Al 2 O 3 samples; this may be due to the lower percentage of both TiC and Al 2 O 3 that are below the limits of the XRD device. This may be attributed to the controlled milling and sintering process in an argon atmosphere which shows that no other peaks for any new phases or intermetallic compounds were formed due to the rapid consolidation process (45 min) during the SPS technique.

XRD Analysis
The crystallite size was assessed by the classical Williamson-Hall method (FWHM) from the broadening of XRD peaks and using the following formula [37,38]: where β is the full width at half maximum height (FWHM), θ is the Bragg's angle of the peak, λ is the wavelength of X-ray (0.15406 nm), K is a dimensionless shape factor (0.9), which depends on the material, d is the crystallite size, and ε is the microstrain.  Figure 3 shows the effect of ceramic ratio on the crystallite size. A clear reduction of the crystallite size with increasing wt.% of ceramic additives was observed. Al2O3 and TiC are ceramic materials that act as internal balls that reduce the particle size [39,40]. In addition, the SPS technique achieved the consolidation process, which is a rapid method for the sintering in which no chance for the grain growth of the particles occurs [41,42]. The crystallite size of pure copper was ~105 nm, whereas the crystallite size for the produced composites was in the range of 5-25 nm. The crystallite size was assessed by the classical Williamson-Hall method (FWHM) from the broadening of XRD peaks and using the following formula [37,38]: where β is the full width at half maximum height (FWHM), θ is the Bragg's angle of the peak, λ is the wavelength of X-ray (0.15406 nm), K is a dimensionless shape factor (0.9), which depends on the material, d is the crystallite size, and ε is the microstrain. Figure 3 shows the effect of ceramic ratio on the crystallite size. A clear reduction of the crystallite size with increasing wt.% of ceramic additives was observed. Al 2 O 3 and TiC are ceramic materials that act as internal balls that reduce the particle size [39,40]. In addition, the SPS technique achieved the consolidation process, which is a rapid method for the sintering in which no chance for the grain growth of the particles occurs [41,42]. The crystallite size of pure copper was~105 nm, whereas the crystallite size for the produced composites was in the range of 5-25 nm.

Densification
The density of composite material is the most important parameter, which significantly affects both physical and mechanical properties. The relative density is calculated and plotted in Figure 4. Relative density is the ratio of the measured and theoretical density of the sample. Measured density was determined by the Archimedes method, and the theoretical density was calculated from the simple rule of mixtures. Each percent of pure Cu and 3, 6, 9 wt.% TiC/Al2O3 (individual and hybrid) nanocomposite were tested by three samples, and the average results were obtained. It was observed that the relative density decreased with increasing reinforcement content for all composites, as shown in Figure 4.

Densification
The density of composite material is the most important parameter, which significantly affects both physical and mechanical properties. The relative density is calculated and plotted in Figure 4. Relative density is the ratio of the measured and theoretical density of the sample. Measured density was determined by the Archimedes method, and the theoretical density was calculated from the simple rule of mixtures. Each percent of pure Cu and 3, 6, 9 wt.% TiC/Al 2 O 3 (individual and hybrid) nanocomposite were tested by three samples, and the average results were obtained. It was observed that the relative density decreased with increasing reinforcement content for all composites, as shown in Figure 4. The maximum relative density (~96%) was achieved by adding 3 wt.% TiC to copper, whereas the minimum relative density (89%) was obtained for 9 wt.% Al 2 O 3 /copper composite. TiC (4.91 g/cm 3 ) and Al 2 O 3 (3.987 g/cm 3 ) also have lower densities than Cu [7,35]. So, the addition of a light material to a denser one decreased the overall density of the prepared composites. This may be attributed to the presence of hard ceramic material with a high melting point into a ductile metal such as Cu that may hinder the high densification and increase the porosity content accompanied by the high fraction of reinforcement [7,43].

Densification
The density of composite material is the most important parameter, which significantly affects both physical and mechanical properties. The relative density is calculated and plotted in Figure 4. Relative density is the ratio of the measured and theoretical density of the sample. Measured density was determined by the Archimedes method, and the theoretical density was calculated from the simple rule of mixtures. Each percent of pure Cu and 3, 6, 9 wt.% TiC/Al2O3 (individual and hybrid) nanocomposite were tested by three samples, and the average results were obtained. It was observed that the relative density decreased with increasing reinforcement content for all composites, as shown in Figure 4. The maximum relative density (~96%) was achieved by adding 3 wt.% TiC to copper, whereas the minimum relative density (89%) was obtained for 9 wt.% Al2O3/copper composite. TiC (4.91 g/cm 3 ) and Al2O3 (3.987 g/cm 3 ) also have lower densities than Cu [7,35]. So, the addition of a light material to a denser one decreased the overall density of the prepared composites. This may be attributed to the presence of hard ceramic material with a high melting point into a ductile metal such as Cu that may hinder the high densification and increase the porosity content accompanied by the high fraction of reinforcement [7,43]. Ayman Elsayed et al. [11] studied experimental investigations for the synthesis of W-Cu nanocomposite through spark plasma sintering, and they concluded that using the SPS technique led to reaching a maximum of 90% relative density. On the other hand, a relative density of 98.1% was reached for Cu-Fe-Al 2 O 3 -MoS 2 composite sintered using the SPS route [33]. Moreover, Babapoor et al. [44] investigated the effects of spark plasma sintering temperature on the densification of TiC. They reached a relative density of 99.4% at 1900 • C for 7 min under 40 MPa using the TiC powder with a mean particle size of 7 µm. They also suggested that there is an optimum temperature for reaching the maximum density. Figure 5 shows the FE-SEM micrograph of TiC-reinforced copper composite using 3%, 6%, and 9% TiC addition to Cu. Two phases are observed; the dark-gray phase represents the Cu matrix, and the black phase is the TiC particles. For 3 wt.% samples, TiC and Al 2 O 3 particles are concentrated along the grain boundaries in a chain form, while 6 and 9 wt.% samples were homogeneously distributed all over the Cu matrix. This may be attributed to the suitable mechanical milling parameters and good SPS technique applied. The SPS technique leads to a finer structure compared with traditional routes [18,44]. density. Figure 5 shows the FE-SEM micrograph of TiC-reinforced copper composite using 3%, 6%, and 9% TiC addition to Cu. Two phases are observed; the dark-gray phase represents the Cu matrix, and the black phase is the TiC particles. For 3 wt.% samples, TiC and Al2O3 particles are concentrated along the grain boundaries in a chain form, while 6 and 9 wt.% samples were homogeneously distributed all over the Cu matrix. This may be attributed to the suitable mechanical milling parameters and good SPS technique applied. The SPS technique leads to a finer structure compared with traditional routes [18,44]. The SEM investigation of Cu/Al2O3 microstructure is shown in Figure 6. Two phases are observed; the dark-gray phase represents the Cu matrix while the white phase represents the Al2O3 particles. The dispersion of Al2O3 inside the copper matrix is observed for Cu/3% Al2O3 with a little agglomeration of Al2O3 reinforcement. On the other hand, white areas of agglomerated alumina reinforcement are revealed within the Cu/6% Al2O3 matrix grain boundaries, whereas very fine particles are dispersed within the grain interior. Moreover, the SEM of Cu/9% Al2O3 composite shows that most of the Al2O3 nanoparticles are agglomerated along grain boundaries, and a small percentage are dispersed with the grains. Some authors have also concluded that increasing agglomeration steadily occurs, along with increasing the weight percentage of reinforcement [9,15]. The SEM investigation of Cu/Al 2 O 3 microstructure is shown in Figure 6. Two phases are observed; the dark-gray phase represents the Cu matrix while the white phase represents the Al 2 O 3 particles. The dispersion of Al 2 O 3 inside the copper matrix is observed for Cu/3% Al 2 O 3 with a little agglomeration of Al 2 O 3 reinforcement. On the other hand, white areas of agglomerated alumina reinforcement are revealed within the Cu/6% Al 2 O 3 matrix grain boundaries, whereas very fine particles are dispersed within the grain interior. Moreover, the SEM of Cu/9% Al 2 O 3 composite shows that most of the Al 2 O 3 nanoparticles are agglomerated along grain boundaries, and a small percentage are dispersed with the grains. Some authors have also concluded that increasing agglomeration steadily occurs, along with increasing the weight percentage of reinforcement [9,15]. The combination of both TiC and Al2O3 for reinforcing copper (Cu/hybrid nanocomposite) with point analysis EDS is shown in Figure 7a,b. The homogeneous distribution of TiC nanoparticles is predominant, while the agglomeration of some Al2O3 is observed along grain boundaries. More agglomeration of alumina particles along grain boundaries is observed with increasing hybrid percentage (TiC and Al2O3).

Microstructure Analysis
(a)  Figure 8 represents the stress-strain curves for pure copper and TiC/Al 2 O 3 -reinforced copper matrix composites, while the key mechanical properties obtained from the compression test are plotted in Figure 9. The compressed samples are photographed in Figure 10. The addition of TiC enhanced the compression strength of copper and reached its maximum compression strength of~852 N/mm 2 at 9 wt.% TiC, whereas Al 2 O 3 additions exhibited a dramatic effect on the Cu strength. Increasing Al 2 O 3 from 3 to 6 wt.% increased the Cu strength, but a clear failure of strength is noticed at 9 wt.% ratios at which a minimum value  Figure 10). clearpage Figure 6. SEM of Cu/Al2O3 nanocomposite with Al2O3 percentage of 3, 6, and 9%.

Mechanical Strength
The combination of both TiC and Al2O3 for reinforcing copper (Cu/hybrid nanocomposite) with point analysis EDS is shown in Figure 7a,b. The homogeneous distribution of TiC nanoparticles is predominant, while the agglomeration of some Al2O3 is observed along grain boundaries. More agglomeration of alumina particles along grain boundaries is observed with increasing hybrid percentage (TiC and Al2O3).  Moreover, the strength of hybrid composites increased firstly with increasing the percentage of reinforcement up to 6% and then slightly decreased. The extreme drop in the compression strength of the Cu/9% Al 2 O 3 composite may be attributed to particleto-particle contact resulting from ceramic particle agglomeration (see Figure 6). The high compression strength of TiC-strengthened Cu prepared by the SPS route compared with Cu/Al 2 O 3 composite with the same wt.% is attributed to a combined effect of ultrafine grain (UFG) structure by the Hall-Petch mechanism and the obstruction of dislocation movement by nanoscale ceramic particles in the grain interior by the Orowan mechanism [8,43].
The effect of ceramic additions on the hardness of copper is illustrated in Figure 11. The hardness steadily increases with increasing the wt.% of reinforcement for synthesized composites. Cu/9% Al 2 O 3 obtained the maximum hardness (211 HV), whereas two composites that obtained the minimum hardness value of 112 HV are Cu/3% TiC and Cu/3% hybrid. Many reasons could explain this. The first is that adding high-hardness and high-strength ceramic materials such as TiC and Al 2 O 3 on the ductile Cu matrix increases the overall hardness. The second is that the addition of nanomaterials with the incorporation of nanoparticles between the Cu particles improves the hardness as a grain reinforcement takes place accordingly.
composites. Cu/9% Al2O3 obtained the maximum hardness (211 HV), whereas two composites that obtained the minimum hardness value of 112 HV are Cu/3% TiC and Cu/3% hybrid. Many reasons could explain this. The first is that adding high-hardness and highstrength ceramic materials such as TiC and Al2O3 on the ductile Cu matrix increases the overall hardness. The second is that the addition of nanomaterials with the incorporation of nanoparticles between the Cu particles improves the hardness as a grain reinforcement takes place accordingly.

Strengthening Criteria
Increasing the strength of metallic materials is based on two competing factors. The first is work hardening, and the second is dynamic softening. Work hardening is caused by dislocations, multiplication, pileup, and tangle. Dynamic softening is caused by dislocations, rearrangement, and interactions. In the present work, the compression test and hardness measurement are carried out at room temperature, which is why the dynamic softening factor would not be probable, and the work hardening mechanism would affect the enhancement of strength and hardness. This is true for pure metal and alloys, unlike the composite materials where the contribution of ceramic additions to the matrix to enhance the properties should be considered.
Moreover, it is worth mentioning that some authors have considered the strengthening mechanisms in ceramic-reinforced composites [6,7,15,40,[42][43][44]. The addition of TiC nanoparticles to the Cu matrix retained grain growth during sintering due to the peening effect of TiC for grain boundary movement and the strengthening effect of dispersed TiC in the Cu matrix grains where a mismatch of coefficient of thermal expansion is present [45] (see Figure 12). Furthermore, increasing the TiC fraction increased the strength and hardness of Cu-TiC composites in the present work. Another strengthening mechanism of TiC dispersion is the Orowan mechanism, especially at low fractions of TiC [6]. Compared to the other two cases, the composites with 3% and 6% Al2O3 gave the highest yield strength. This is a signal of increasing material strength with decreasing ductility. This may be attributed to the good adhesion between Cu matrix and TiC nanoparticles than between Cu and Al2O3 [46]. An extreme drop in the Cu-Al2O3 composite strength is noticed at 9 wt.% of Al2O3. This unexpected behavior may be attributed to the agglomeration of some alumina particles in the Cu matrix, increasing the chance for particle-to-particle Figure 11. A diagram of the measured hardness for copper-based nanoceramics.

Strengthening Criteria
Increasing the strength of metallic materials is based on two competing factors. The first is work hardening, and the second is dynamic softening. Work hardening is caused by dislocations, multiplication, pileup, and tangle. Dynamic softening is caused by dislocations, rearrangement, and interactions. In the present work, the compression test and hardness measurement are carried out at room temperature, which is why the dynamic softening factor would not be probable, and the work hardening mechanism would affect the enhancement of strength and hardness. This is true for pure metal and alloys, unlike the composite materials where the contribution of ceramic additions to the matrix to enhance the properties should be considered.
Moreover, it is worth mentioning that some authors have considered the strengthening mechanisms in ceramic-reinforced composites [6,7,15,40,[42][43][44]. The addition of TiC nanoparticles to the Cu matrix retained grain growth during sintering due to the peening effect of TiC for grain boundary movement and the strengthening effect of dispersed TiC in the Cu matrix grains where a mismatch of coefficient of thermal expansion is present [45] (see Figure 12). Furthermore, increasing the TiC fraction increased the strength and hardness of Cu-TiC composites in the present work. Another strengthening mechanism of TiC dispersion is the Orowan mechanism, especially at low fractions of TiC [6]. Compared to the other two cases, the composites with 3% and 6% Al 2 O 3 gave the highest yield strength. This is a signal of increasing material strength with decreasing ductility. This may be attributed to the good adhesion between Cu matrix and TiC nanoparticles than between Cu and Al 2 O 3 [46]. An extreme drop in the Cu-Al 2 O 3 composite strength is noticed at 9 wt.% of Al 2 O 3 . This unexpected behavior may be attributed to the agglomeration of some alumina particles in the Cu matrix, increasing the chance for particle-to-particle contact (see Figure 13). A balanced behavior was observed with the hybrid (TiC/Al 2 O 3 ) additions to the Cu matrix in which the combined effects of both ceramics are clear.

Conclusions
Cu-(TiC and/or Al2O3) nanocomposites were synthesized successfully using mechanical milling followed by the spark plasma sintering (SPS) technique.
The density decreased with increasing percentages of the nano reinforcements (TiC and/or Al2O3). A maximum relative density of ~96% was achieved with the addition of 3 wt.% TiC to copper, whereas the minimum relative density (89%) was obtained by adding

Conclusions
Cu-(TiC and/or Al2O3) nanocomposites were synthesized successfully using mechanical milling followed by the spark plasma sintering (SPS) technique.
The density decreased with increasing percentages of the nano reinforcements (TiC and/or Al2O3). A maximum relative density of ~96% was achieved with the addition of 3 Reinforcing the Cu matrix, which is ductile in nature with two types of ceramic materials-ceramic carbide (TiC) and ceramic oxide (Al 2 O 3 )-helps to improve the mechanical properties of the Cu matrix. Both TiC and Al 2 O 3 are at the nanoscale; therefore, by high ball milling, they filled the interstitial voids between Cu particles. As a consequence, the strengthening effect of both of them is distributed all over the Cu matrix. The hardness estimation test increased as the nano-ceramic hard particles were increased. This can also be explained by the resistance of the hard ceramic particles to the indenter from greater depth in the Cu-composite surface.
Consequently, the hardness is enhanced [46,47]. For the compression test, the presence of the nano-ceramic particles dispersed formally in the Cu matrix prevents the dislocation of the particles. In addition, as these hybrid reinforcements are at the nanoscale, they fill the voids; consequently, the strength of samples is increased [47][48][49]. Table 2 shows a comparison between the present study and previous work used to fabricate copper composites reinforced with alumina and titanium carbide nanoparticles. In this work, different concentrations of nano alumina and/or nano titanium carbide particles were used as a reinforcement material to the Cu matrix manufactured by the SPS technique. This work is compared with the same composites prepared by traditional sintering, vacuum sintering, hot pressing, and hot extrusion. The table shows that the composites produced by the SPS technique have the best mechanical properties compared with the other consolidation techniques.

Conclusions
Cu-(TiC and/or Al 2 O 3 ) nanocomposites were synthesized successfully using mechanical milling followed by the spark plasma sintering (SPS) technique.
The density decreased with increasing percentages of the nano reinforcements (TiC and/or Al 2 O 3 ). A maximum relative density of~96% was achieved with the addition of 3 wt.% TiC to copper, whereas the minimum relative density (89%) was obtained by adding 9 wt.% Al 2 O 3 to copper. Agglomerated areas of Al 2 O 3 nanoparticles around grain boundaries were observed, and increased with increasing Al 2 O 3 fractions that, in turn, adversely affect the mechanical properties.
The compression strength of Cu/TiC increased with increasing the TiC fraction, and a maximum value of 851.99 N/mm 2 was obtained by Cu/9% TiC. A dramatic behavior was observed for Cu/Al 2 O 3 composites that gave the minimum compressive strength of 367.8 N/mm 2 resulted at 9% Al 2 O 3 .
The maximum hardness of 211 HV was obtained by Cu/9% Al 2 O 3 , whereas two types of composites obtained the minimum hardness value of 112 HV: the Cu/3% TiC and Cu/3% hybrid.

Data Availability Statement:
The data that support the findings of this study are included in the article. Any further requested information can be addressed to the corresponding author.