Effect of Processing on Mechanically Alloyed and Spark Plasma Sintered Al-Al<sub>2</sub>O<sub>3</sub> Nanocomposites

Saheb, Nouari; Shahzeb Khan, Muhammad; Hakeem, Abbas Saeed

doi:https://doi.org/10.1155/2015/609824

Journal of Nanomaterials

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Abstract Introduction Materials Results and Discussion Conclusion Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2015 | Article ID 609824 | https://doi.org/10.1155/2015/609824

Effect of Processing on Mechanically Alloyed and Spark Plasma Sintered Al-Al₂O₃ Nanocomposites

Nouari Saheb,^1,2Muhammad Shahzeb Khan,¹and Abbas Saeed Hakeem²

Academic Editor: Wei Chen

Received28 Jul 2015

Revised15 Oct 2015

Accepted18 Oct 2015

Published05 Nov 2015

Abstract

Metal matrix nanocomposites are advanced materials developed using ceramic nanoreinforcements and nanocrystalline metal matrices. These composites have outstanding properties and high potential for large number of functional and structural applications. In this work, nanocrystalline aluminium and Al-Al₂O₃ nanocomposites were synthesised using mechanical alloying and consolidated through spark plasma sintering technique. Scanning electron microscopy, X-ray diffraction, and mapping were used to characterize the powders and sintered samples. Density and hardness of sintered samples were measured using densimeter and hardness tester, respectively. It was found that milling of pure aluminium for 24 h reduced its crystallite size to less than 100 nm. For Al-Al₂O₃ nanocomposites, milling for 24 h decreased the crystallite size of the aluminium phase and resulted in uniform dispersion of the reinforcement. Sintering of the synthesised powders led to grain growth. Al₂O₃ contributed to growth inhibition when samples were sintered for 20 minutes and improved the hardness but reduced densification. The Al-10 vol.% Al₂O₃ nanocomposite had the highest Vickers hardness value of 1460 MPa.

1. Introduction

The growing demand for light-weight and high-performance structural materials, in the aerospace, military, and automotive industries, to reduce energy consumption and air pollution [1, 2], is gradually leading to a shift from conventional metallic alloys to metal matrix composites [3] and nanocomposites [4]. Among these composites, particle reinforced aluminium matrix composites received the serious attention of researchers and engineers. This is due to the fact that reinforcing a metal with ceramic particles improves its mechanical and physical properties. However, although the addition of the reinforcement improves hardness, stiffness, and strength of the matrix, it decreases the ductility and toughness of the composite. This is due to cracking of ceramic particles during loading which leads to early failure of the composites. Fortunately, reducing the size of the reinforcement and/or matrix grains from micro- to nanodimensions to produce nanocomposites [4] led to significant improvement in ductility, toughness, and strength [5, 6]. Grain size is a key microstructural factor that significantly affects the properties and mechanical behaviour of polycrystalline materials; and one way to design materials with desired properties is controlling their grain size [7]. Moreover, it is known that grain refinement is the only method that improves the strength of the material without compromising the ductility and flow properties [8]. Severe plastic deformation processes can be used to reduce matrix grain size to submicrometer or nanometer dimension. These processes include equal channel angular extrusion/pressing (ECAE/ECAP) [9], multiaxial forging [10], cyclic extrusion and compression (CEC) [11], high pressure torsion (HPT) [12], accumulative roll bonding (ARB) [13, 14], and mechanical alloying (MA) [5]. However, all these processes, except ARB and MA, suffer from two main drawbacks. The first is the fact that they require large load capacity machines and expensive tools. The second is the limited productivity and amount of produced material. In addition to grain size refinement, addition of nanoparticles to metals leads to the enhancement of toughness because of the reduced particle volume fraction needed to achieve high strength. However, the development of nanoparticle reinforced metal matrix nanocomposites for commercial applications is still facing some challenges and the race for better materials performance is never ending [7]. These challenges include inhomogeneous microstructures because of nanoparticles’ agglomeration, grain growth if sintering is required, and limitations on the amount of the produced material. Fortunately, overcoming the above challenges is possible through proper selection and combination of processing techniques. In this regard, mechanical alloying is used to process homogenous nanocomposite powders [5]. It allows not only the design of specific microstructures with uniform dispersion of the reinforcement and small grain size of the matrix, but also the elevation of the limitations on the amount of the produced material. On the other side, spark plasma sintering (SPS) also named field assisted sintering (FAST) is a binderless and a direct process to consolidate powdered materials in short times at relatively low temperatures. It involves the application of uniaxial force and high intensity current at low voltage [16]. The advantages of the process include the ability to sinter the material at low sintering temperature and in short sintering time which allows the retention of the initial fine structure of the material that leads to superior properties. In addition, SPS is a cost-effective process due to the fact that it is binderless process and direct, does not require a precompaction step, and can be performed at low temperatures for short times compared to other powder metallurgy processes. The MA and/or SPS techniques were used to process nanostructured aluminium [17–20], aluminium alloys [21–23], and Al-Al₂O₃ nanocomposites [24, 25]. Despite the importance of Al-Al₂O₃ nanocomposites, work dedicated to their processing using MA and SPS is very scarce in the literature. In a previous review paper [26], the authors concluded that reaching uniform distribution of the reinforcement in the matrix, which could be achieved through optimization of processing parameters, is vital for the development of nanocomposites with the anticipated properties. In another work [15], the authors successfully synthesised and consolidated Al-SiC nanocomposites using MA and SPS techniques. The objective of the present work is to synthesize nanocrystalline aluminium and homogenous Al-Al₂O₃ nanocomposite powders using mechanical alloying and consolidate them through spark plasma sintering technique. The second objective is to investigate the influence of milling and sintering conditions on the microstructure, densification, and hardness of the developed materials.

2. Materials and Experimental Procedures

Aluminium powder, 99.88% purity supplied by Aluminium Powder Co. Ltd., and α-Al₂O₃ (with an average particle size of 150 nm), 99.85% purity, supplied by ChemPUR Germany, were used in this investigation. The chemical composition and particle size distribution of the aluminium powder are presented in Tables 1 and 2, respectively.

Mechanical alloying was used to synthesize Al-Al₂O₃ composite powders reinforced with 2, 10, and 15 vol.% Al₂O₃ nanoparticles. In addition, pure aluminium, as a reference material, was milled for 24 h. A planetary ball mill (Fritsch Pulverisette, P5, Idar-Oberstein, Germany) was used to perform the milling experiments. The powders were milled in argon inert gas to avoid oxidation. Milling conditions of ball to powder weight ratio of 10 : 1 and speed of 200 rpm were maintained constant in all experiments. Stainless steel vials (250 mL in volume) and balls (10 mm in diameter) were used. Each powder mixture was milled for 0.5, 1, 2, 3, 4, 5, 6, 8, 10, 15, 20, and 24 h. The sticking of the powder to milling balls and vials was minimised using stearic acid (1.5 wt.%). Spark plasma sintering machine (FCT Systeme, Germany), model HP D 5, was used to consolidate the mechanically alloyed powders. More details on SPS process were reported in [16]. Disc shaped specimens were prepared using a graphite die of 20 mm diameter. A compaction pressure of 50 MPa and heating rate of 200°C/min were used in all sintering experiments. Aluminium unmilled and milled for 24 h and Al-Al₂O₃ nanocomposite powders containing 2, 10, and 15 vol.% Al₂O₃ milled for 24 h were sintered. The samples were sintered at a temperature of 550°C for holding times of 5 and 20 minutes. FE-SEM equipped with EDS was used to characterize both mechanically alloyed powders and sintered samples. X-ray elemental mapping, using 20 frames, was performed to examine the distribution of the reinforcement in the matrix. X-ray diffraction experiments were carried out using a diffractometer (Bruker D8, USA, with a wavelength nm) to characterize the crystallite size of the aluminium matrix. The bulk density of the consolidated samples was measured according to the Archimedes principle using Mettler Toledo balance density determination KIT model AG285. Digital microhardness tester (Buehler, USA) was used to measure the microhardness of the developed materials. The obtained hardness values were the average of 12 readings. Conditions of a load of 100 gf and a time of 12 s were maintained in all measurements.

3. Results and Discussion

FE-SEM micrograph of aluminium powder is shown in Figure 1(a). Its particles have various sizes and shapes. A TEM micrograph of the as-received Al₂O₃ nanopowder is presented in Figure 1(b). The Al₂O₃ nanopowder has particles with different shapes and average particle size of 150 nm. It can be noticed that Al₂O₃ nanoparticles are agglomerated because of their small particle size.

(a)

(b)

The Al-Al₂O₃ powders were milled for different milling times to achieve uniform dispersion of the reinforcement in the matrix. Figure 2 shows typical FE-SEM micrographs of Al-10 vol.% Al₂O₃ powder milled for (a) 3 h; (b) 6 h; (c) 20 h; and (d) 24 h. The morphology and size of aluminium particles changed with milling time. Initially, milling led to flattening of particles and increase in their size. However, at long milling time of 24 h, Figure 2(d), milling led to particle size decrease and particles had smaller size compared to the size of unmilled particles as presented in Figure 1(a). In addition, particles became more equiaxed.

(a)

(b)

(c)

(d)

Figures 3(a) and 3(b) show FE-SEM micrographs of powders containing 2 and 15 vol.% of Al₂O₃, respectively, milled for 24 h. The nanocomposite powders reinforced with 2 and 15 vol.% of Al₂O₃ nanoparticles showed similar behaviour compared to the Al-10 vol.% Al₂O₃ composite. Analysis of the size and morphology of aluminium particles in powders containing 2, 10, and 15 vol.% Al₂O₃ and milled for 24 hours, presented in Figures 3(a), 2(d), and 3(b), respectively, shows that the higher the Al₂O₃ content the smaller the particle size; and more equiaxed particles were formed. Therefore, it can be concluded that, under the same milling conditions, the increase in Al₂O₃ content enhanced the milling effect. The same effect was observed in Al-SiC nanocomposites where SiC was reported to enhance the milling effect in mechanically alloyed Al-SiC nanocomposites prepared under milling speed of 200 rpm, milling time up to 20 h, and BPR of 10 : 1 [15]. This is due to the fact that the increase in reinforcement volume fraction leads to the increase in milling rate and reduces the time to reach a balance between fracturing and rewelding of the milled particles [15].

(a)

(b)

Elemental mapping of aluminium and oxygen in Al-10 vol.% Al₂O₃ composite mechanically milled for 2 h; 8 h; 15 h; and 24 h and corresponding FE-SEM micrographs are shown in Figure 4. The micrographs confirm the decrease of particle size and formation of more equiaxed particles with the increase in milling time from 2 to 24 h as discussed above. It can be noticed that after milling for 15 hours, from the one hand, the size of aluminium particles remained relatively large, and from the other hand, Al₂O₃ nanoparticles remained relatively agglomerated as it is evident from the mapping of Al and O, respectively. However, a homogenous composite powder with small aluminium particles and uniform distribution of Al₂O₃ nanoparticles was obtained when the milling time was increased to 24 h. The uniform dispersion of Al₂O₃ particles is clear from the mapping of aluminium and oxygen at 24 h. This time is the same time used to obtain a uniform distribution of SiC nanoparticles in mechanically alloyed Al-SiC nanocomposite powders prepared under similar conditions [15]. It is worth mentioning here that the time to reach uniform dispersion of the reinforcement in mechanically alloyed nanocomposites strongly depends on the nature of the matrix, amount and type of the reinforcement, and milling conditions. Yadav [27] obtained uniform dispersion of SiC particles in Al-SiC composites (containing 5, 10, and 20 wt.% SiC) at a milling time of 30 minutes. In his work, acetone and polyacrylic were used as process control and dispersive agents, respectively; and a ball to powder weight ratio (BPR) of 5 : 1 and a speed of milling of 500 rpm were employed. Homogenous distribution of SiC nanoparticles was achieved in Al-SiC composites containing 2.5, 7.5, and 12.5 vol.% SiC after 10 h of milling [28] at milling conditions of a speed of 320 rpm and BPR of 20 : 1 and 10 : 1.

(a)

(b)

(c)

(d)

The composite powders reinforced with 2 and 15 vol.% of Al₂O₃ displayed similar behaviour. Uniform distribution of Al₂O₃ particles was reached after milling for 24 h as can be seen from mapping of oxygen presented in Figure 5. The Al₂O₃ particles remained fairly distributed despite the increase in their volume fraction to 15%.

(a)

(b)

Analysis of Figures 4(d), 5(a), and 5(b), showing elemental mapping in composite powders reinforced with 10, 2, and 15 vol.% of Al₂O₃, respectively, indicates that under the employed milling conditions, a milling time of 24 h was suitable to process homogenous composites with uniform dispersion of the reinforcement. On the other hand, nanostructured aluminium matrix was obtained at this milling time as will be discussed below. Thus, only powders milled for 24 h were consolidated using SPS for subsequent characterization.

Figure 6 shows XRD spectra of aluminium, unmilled, Figure 6(a), and milled for 24 h, Figure 6(b). Only reflections from α-Al solid solution are present. The α-Al phase has FCC crystal structure. Mechanical alloying of pure aluminium for 24 h decreased the intensity of the α-Al peaks. This decrease was accompanied with broadening of the peaks, as can be seen in Figure 6(b). This is due to the fact that mechanical milling of metallic powders is usually associated with a decrease in crystallite size and increase in lattice strain [29]. The decrease in crystallite size is believed to take place in three stages, “The first stage is characterized by the formation of shear bands with high density of dislocations. In the second stage, annihilation and recombination of these dislocations give rise to small angle grain boundaries separating the individual grains. In the last stage, the orientation of the single crystalline grains become random with respect to their adjacent grains” [29]. It should be understood that “in XRD analysis, when the size of a crystal is used, it usually refers to the size of crystallites concerning a factor, which makes a diffraction peak broad” [30].

Figure 7 shows XRD spectra of nanocomposites reinforced with 2, 10, and 15 vol.% of Al₂O₃ nanoparticles and mechanically alloyed for 24 h. It is worth mentioning here that for Figure 6 XRD patterns were recorded for the same powder, that is, aluminium unmilled and milled for 24 h; but, for Figure 7, XRD patterns were recorded for three powders containing different volume fractions of the Al and Al₂O₃ phases and milled for the same time of 24 h. Furthermore, particle size and shape for the three powders are noted to be the same as it is clearly seen in Figures 3(a), 2(d), and 3(b), for composites containing 2, 10, and 15 vol.% Al₂O₃, respectively. This is due to the fact that Al₂O₃ acts as a grinding medium and the increase in its volume fraction contributes to more refinement of the particles as explained above. Therefore, the volume fraction of the two phases present and the size and shape (flattened of equiaxed) of the particles may affect the intensity of peaks in the powders. Figure 7 shows that, for the composite powders, milling reduced the intensity of peaks and led to their broadening, compared with the unmilled aluminium powder presented in Figure 6(a). The crystallite size of the α-Al phase decreased because of milling and addition of Al₂O₃ as explained below.

The formation of secondary phases in the mechanically alloyed nanocomposite powders was not revealed by XRD. This could be attributed to the fact that the composites are processed through a powder metallurgy route and not casting. Moreover, peaks of these phases may not be easily seen on XRD spectra because of the reduced intensity as a result of milling [31] and their small amount which may be below the detectability of the equipment [32]. Peaks of Al₂O₃ phase were not observed in the composite containing 2% alumina and milled for 24 h, Figure 7(a), because of the low volume fraction of alumina. However, the peaks were detected in XRD spectra of composites containing 10 and 15% alumina nanoparticles and milled for 24 h, Figure 7(a and b), respectively. Overall, Al₂O₃ peaks had low intensity which could be attributed to the fact that Al₂O₃ nanoparticles have average particle size of 150 nm and their crystallite size is very small which contributes to peak broadening in addition to broadening that results from plastic deformation and increase in strain as a result of milling [30, 33, 34].

Crystallite size of the α-aluminium phase in pure aluminium and composites milled for 24 h was calculated [15, 30] using where is the full width at half-maximum (FWHM) of the diffraction peak after instrument correction; is constant (with a value of 0.9); is wavelength of the X-ray radiation ( nm); and are crystallite size and lattice strain, respectively; and is the Bragg angle. is related to the measured width of the peak () and peak broadening caused by factors except the particle size effect (), frequently called instrumental broadening factor and calculated using a fully annealed sample, through the following formula [15, 30]:Crystallite size of the α-Al phase is presented in Figure 8. The α-Al phase of the as-received pure aluminium had a crystallite size of 298 nm and decreased to 62.29 nm after milling for 24 h. This size is close to the crystallite size of the aluminium phase reported by other researchers who investigated the effect of milling on monolithic Al powder [22, 23]. It was reported that dry milling of monolithic Al decreased the α-Al crystallite size to 38 nm in 8 h [22] and 75 nm in 20 h [23]. As for the composite containing 2 vol.% of Al₂O₃, milling for 24 h decreased the crystallite size of the α-Al phase to 39.82 nm. Further increase of Al₂O₃ to 10 and 15 vol.% led to further decrease in the crystallite phase to 32.69 and 27.24 nm, respectively. This clearly shows that Al₂O₃ nanoparticles acted as grinding medium and enhanced the milling effect which contributed to further decrease of the crystallite size of the α-aluminium phase. A summary of the crystallite size of the aluminium phase in mechanically alloyed Al-based nanocomposites is presented in Table 3.

Mechanical alloying of Al-based nanocomposites was reported to decrease the crystallite size of the aluminium matrix to nanodimensions [15, 28, 39]. α-Al crystallite sizes of 175.6 and 97.9 nm were reported for Al-SiC composites reinforced with 12.5 and 2.5 vol.% SiC, respectively, and mechanically alloyed for 10 h [28]. In another work, Al5083-10 wt.% SiC composites were mechanically alloyed for 15 h at a speed of 400 rpm using a BPR of 20 : 1; and a crystallite size of 25 nm was reported [39]. In Al-SiC composites reinforced with 1, 5, and 10 wt.% SiC nanoparticles, mechanical alloying for 24 h was reported to decrease the crystallite size of the α-Al phase to 140, 50, and 32 nm, respectively [15].

The microstructure of unmilled aluminium sintered for 5 and 20 min is presented in Figures 9(a) and 9(b), respectively. The effect of milling on the microstructure of aluminium sintered for 5 and 20 min can be clearly seen in Figures 9(c) and 9(d), respectively. The microstructure of unmilled and sintered aluminium for 5 minutes is characterized by relatively large grains. The increase in sintering time from 5 to 20 minutes resulted in grain growth. The effect of milling for 24 h and sintering for 5 minutes can be clearly seen in Figure 9(c) where a very fine microstructure characterized by elongated grains was obtained. The increase in sintering time from 5 to 20 minutes led to significant grain growth as seen in Figure 9(d). It is well known that sintering time plays an important role in isothermal grain growth, which can be expressed with the formula , where and are the grain sizes at initial time and isothermal holding time , respectively. is a temperature-dependent parameter.

(a)

(b)

(c)

(d)

It can be concluded that grain growth in the milled nanocrystalline aluminium was much less compared to the unmilled aluminium. This is may be due to recrystallization of the heavily milled and deformed structure. Moreover, it is claimed that nanocrystalline materials often exhibit a remarkable resistance to grain growth [40].

Typical microstructures of Al-15 vol.% Al₂O₃ sintered for 5 and 20 min are presented in Figures 10(a) and 10(b), respectively. The increase in sintering time from 5 to 20 minutes led to marginal grain growth. Analysis of the microstructure of aluminium, Figure 9(c), and Al-15 vol.% Al₂O₃, Figure 10(a), milled for 24 h and sintered for 5 minutes shows that both materials have microstructures with similar features and very elongated grains. The grains in the composite material have relatively large size compared to the monolithic material. However, for the same samples sintered for 20 minutes, the monolithic material, Figure 9(d), had a microstructure with relatively large and equiaxed grains while the composite material, Figure 10(b), had a microstructure with relatively small and elongated grains. It can be concluded that Al₂O₃ significantly contributed to grain growth inhibition when samples were sintered for 20 minutes.

(a)

(b)

Relative density of the consolidated samples is presented in Figure 11. The as-received pure aluminium consolidated at 550°C for 5 minutes had a relative density of 98.88%. The increase in sintering time to 20 minutes increased its relative density to 100%. The pure aluminium milled for 24 h and consolidated at 550°C for 5 minutes had a relative density of 95.92%. The increase in sintering time to 20 minutes resulted in a minor change of density to 95.18%.

The Al-2 vol.% Al₂O₃ composite consolidated at 550°C for 5 minutes had a relative density of 98.16%. The increase in sintering time to 20 minutes decreased its relative density to 95.50%. The Al-10 vol.% Al₂O₃ composite consolidated at 550°C for 5 minutes had a relative density of 100%. The increase in sintering time to 20 minutes did not change its density. The Al-15 vol.% Al₂O₃ composite consolidated at 550°C for 5 minutes had a relative density of 91.69%. The increase in sintering time to 20 minutes did not affect its density.

Analysis of density results presented above shows that relative density of 100% was achieved for pure unmilled aluminium sintered for 20 min; that is, it was fully densified. However, milling for 24 h reduced its densification to 95%. The addition of 2 vol.% of Al₂O₃ increased the densification compared to pure aluminium milled for 24 h. Further increase of Al₂O₃ content to 10 vol.% led to full densification of the sample. However, the increase in Al₂O₃ content to 15 vol.% reduced the densification to 91%.

Generally, it is believed that “compaction of powder containing grains of nanometer dimensions, especially when ceramic or carbide nanoparticles are incorporated, is more difficult than the compaction of micron-sized powder of the same metal or alloy. This is because plasticity in nanocrystalline samples requires order of magnitude larger stresses and the spring back is more pronounced. Therefore, nanostructured samples are likely to be more porous” [35, 41, 42] and may not sinter to full density easily. On the other hand, conventional sintering involves compaction followed by sintering, but consolidation using SPS is done in one single step which makes sintering and densification behaviours more complex. Overall, sintering time had marginal effect on the densification of samples except the composite reinforced with 2 vol.% of Al₂O₃ which showed a small decrease in its density with the increase in sintering time from 5 to 20 minutes. This small decrease in density could be attributed to the effect of sintering time on the formation and elimination of open and closed pores, which may have been affected by the large size and flattened shape of particles shown in Figure 3(a). It is worth mentioning here that the density of samples was measured according to Archimedes method with the possibility of water intrusion into any remaining open pores [43]; this will slightly affect density values if open pores are present. The increase in Al₂O₃ content to 10 vol.% contributed to the decrease of particle size and led to the formation of more equiaxed particles as seen in Figure 2(d), which may have improved the densification. However, further increase in Al₂O₃ content to 15 vol.% reduced the densification. This could be due to the fact that consolidation of nanocomposite powders becomes more difficult at higher fractions of ceramic nanoparticles.

The hardness of the developed materials is presented in Figure 12. The as-received pure aluminium consolidated at 550°C for 5 minutes had a Vickers hardness of 369.3 MPa. The increase in sintering time to 20 minutes decreased its hardness to 326.3 MPa. This can be attributed to grain growth observed in Figures 9(a) and 9(b). It is understood that the yield strength of polycrystalline materials depends on the grain size according to the expression (Hall-Petch relationship), where is the lattice friction stress and is a Hall-Petch slope. On the other hand, Vickers hardness and yield strength could be related through a simple formula . As a result, hardness can be related to the grain size through , where and are constants. Therefore, the hardness of a material decreases with the increase in grain size.

Pure aluminium ball milled for 24 h and consolidated at 550°C for 5 minutes had a Vickers hardness of 837.8 MPa. The increase in sintering time to 20 minutes decreased its hardness to 421 MPa. The milled aluminium had similar behaviour in terms of sintering time effect. However, milled aluminium had higher hardness compared to unmilled aluminium because of smaller grain size.

It is well understood that reducing the crystallite size in metals results in significant enhancement in their mechanical properties. The reduction in crystallite size corresponds to increased dislocation density which causes higher resistance to the sliding of atomic planes and, as a consequence, the metal becomes stronger. Whereas bulk metals can be strengthened through several cold working methods, metals in the form of powders can be effectively strengthened by reducing their crystallite size using mechanical alloying (MA) process. The crystallite size of ball milled metallic powders can be effectively reduced to nanometer range. In the process, “the powder particles undergo excessive plastic deformation producing complex dense networks of dislocations. Initially, the internal strain increases with increasing dislocation density. As the dislocation density reaches a threshold value in the heavily strained region, the grains break up into much smaller grains with low angle boundaries. These smaller grains undergo further deformation and disintegration as the milling time is increased. Ultimately, the orientation of the grains with respect to each other becomes completely random and the final grain size approaches nanometer scale” [44]. It is worth mentioning here that once the nanostructure is fully developed, additional decrease of the grain size will be more difficult because large stresses are usually required for the deformation of nanograins. The creation and movement of dislocations under these circumstances are difficult and hence grain boundary sliding becomes the dominant deformation mechanism. The rate of recovery is another parameter which limits the grain size reduction during milling. Partial recovery may occur during mechanical milling, especially for metals with low melting point. For such metals, the formation of high dislocation density regions is hindered by the increased recovery [45]. Once nanocrystalline powders obtained by milling are consolidated, grain growth may take place; however, grain size will remain relatively small compared to unmilled powders, as seen in Figures 9(c) and 9(d), which leads to improved hardness.

The Al-2 vol.% Al₂O₃ composite consolidated at 550°C for 5 minutes had a Vickers hardness of 1113.6 MPa. The increase in sintering time to 20 minutes decreased its hardness to 1098 MPa. The Al-10 vol.% Al₂O₃ composite consolidated at 550°C for 5 minutes had a Vickers hardness of 1463.3 MPa. The increase in sintering time to 20 minutes decreased its hardness to 1309.7 MPa. The Al-15 vol.% Al₂O₃ composite consolidated at 550°C for 5 minutes had a Vickers hardness of 923.9 MPa. The increase in sintering time to 20 minutes decreased its hardness to 821.9. Hardness followed the same trend as densification. Overall, the increase in Al₂O₃ content up to 10 vol.% increased hardness; a further increase to 15 vol.% decreased it as a result of poor densification and probably less uniform distribution of the reinforcement. The composite reinforced with 10% vol. of Al₂O₃ nanoparticles displaced higher hardness than other aluminium based nanocomposites reinforced with CNTs [46–48] or SiC nanoparticles [33]. However, its hardness was lower than the hardness of Al-10 wt.% SiC nanocomposite [15] and Al5083-10 wt.% SiC [39]. Table 4 shows hardness values of aluminium based nanocomposites consolidated by spark plasma sintering. Data presented in Table 4 clearly shows that the hardness of aluminium based nanocomposites strongly depends on the nature of the matrix, whether pure aluminium or alloy, sintering parameters, and the amount of the reinforcing phase.

As for the composites, the increase in hardness can be attributed to the same factors which lead to the increase in the strength of particle reinforced metal matrix composites. This includes small grain size of the matrix (Hall-Petch theory), presence of nanoparticles (Orowan strengthening), increase in dislocations’ density, load transfer from the matrix to the reinforcement, and strain gradient [49–52].

4. Conclusion

Nanocrystalline aluminium and homogenous Al-Al₂O₃ nanocomposites were developed using MA and SPS techniques. The influence of reinforcement content, milling, and sintering conditions on the microstructure, densification, and hardness of the developed materials was investigated. It was found that milling of pure aluminium for 24 h decreased its crystallite size less than 100 nm. For Al-Al₂O₃ nanocomposites, milling for 24 h decreased the crystallite size of the aluminium phase and resulted in uniform dispersion of the reinforcement. Sintering of the synthesised powders led to grain growth. Al₂O₃ contributed to grain growth inhibition when samples were sintered for 20 minutes. Milling improved hardness of aluminium but reduced its densification. Addition of Al₂O₃ nanoparticles resulted in further improvement of hardness. The Al-10 vol.% Al₂O₃ nanocomposite had the highest Vickers hardness value of 1460 MPa.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science and Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through Project 12-NAN2374-04 as part of the National Science, Technology and Innovation Plan.

References

T. Dursun and C. Soutis, “Recent developments in advanced aircraft aluminium alloys,” Materials & Design, vol. 56, pp. 862–871, 2014.
View at: Publisher Site | Google Scholar
J. Hirsch, “Recent development in aluminium for automotive applications,” Transactions of Nonferrous Metals Society of China, vol. 24, no. 7, pp. 1995–2002, 2014.
View at: Publisher Site | Google Scholar
D. B. Miracle, “Metal matrix composites—from science to technological significance,” Composites Science and Technology, vol. 65, no. 15-16, pp. 2526–2540, 2005.
View at: Publisher Site | Google Scholar
R. Casati and M. Vedani, “Metal matrix composites reinforced by nano-particles: a review,” Metals, vol. 4, no. 1, pp. 65–83, 2014.
View at: Publisher Site | Google Scholar
C. Suryanarayana and N. Al-Aqeeli, “Mechanically alloyed nanocomposites,” Progress in Materials Science, vol. 58, no. 4, pp. 383–502, 2013.
View at: Publisher Site | Google Scholar
S. C. Tjong, “Novel nanoparticle-reinforced metal matrix composites with enhanced mechanical properties,” Advanced Engineering Materials, vol. 9, no. 8, pp. 639–652, 2007.
View at: Publisher Site | Google Scholar
Y. Estrin and A. Vinogradov, “Extreme grain refinement by severe plastic deformation: a wealth of challenging science,” Acta Materialia, vol. 61, no. 3, pp. 782–817, 2013.
View at: Publisher Site | Google Scholar
A. Bhowmik, S. Biswas, S. Suwas, R. K. Ray, and D. Bhattacharjee, “Evolution of grain-boundary microstructure and texture in interstitial-free steel processed by equal-channel angular extrusion,” Metallurgical and Materials Transactions A, vol. 40, no. 11, pp. 2729–2742, 2009.
View at: Publisher Site | Google Scholar
E. A. Prokofiev, A. M. Jorge Jr., W. J. Botta, and R. Z. Valiev, Comprehensive Materials Processing, Volume 3: Advanced Forming Technologies, Elsevier, 2014.
R. Kapoor, A. Sarkar, R. Yogi, S. K. Shekhawat, I. Samajdar, and J. K. Chakravartty, “Softening of Al during multi-axial forging in a channel die,” Materials Science and Engineering: A, vol. 560, pp. 404–412, 2013.
View at: Publisher Site | Google Scholar
N. Pardis, C. Chen, M. Shahbaz, R. Ebrahimi, and L. S. Toth, “Development of new routes of severe plastic deformation through cyclic expansion-extrusion process,” Materials Science and Engineering A, vol. 613, pp. 357–364, 2014.
View at: Publisher Site | Google Scholar
M. Kawasaki and T. G. Langdon, “Review: achieving superplasticity in metals processed by high-pressure torsion,” Journal of Materials Science, vol. 49, no. 19, pp. 6487–6496, 2014.
View at: Publisher Site | Google Scholar
Y. Saito, N. Tsuji, H. Utsunomiya, T. Sakai, and R. G. Hong, “Ultra-fine grained bulk aluminum produced by accumulative roll-bonding (ARB) process,” Scripta Materialia, vol. 39, no. 9, pp. 1221–1227, 1998.
View at: Publisher Site | Google Scholar
M. Göken and H. W. Höppel, “Tailoring nanostructured, graded, and particle-reinforced Al laminates by accumulative roll bonding,” Advanced Materials, vol. 23, no. 22-23, pp. 2663–2668, 2011.
View at: Publisher Site | Google Scholar
N. Saheb, I. K. Aliyu, S. F. Hassan, and N. Al-Aqeeli, “Matrix structure evolution and nanoreinforcement distribution in mechanically milled and spark plasma sintered Al-SiC nanocomposites,” Materials, vol. 6, no. 9, pp. 6748–6767, 2014.
View at: Publisher Site | Google Scholar
N. Saheb, Z. Iqbal, A. Khalil et al., “Spark plasma sintering of metals and metal matrix nanocomposites: a review,” Journal of Nanomaterials, vol. 2012, Article ID 983470, 13 pages, 2012.
View at: Publisher Site | Google Scholar
G. M. Le, A. Godfrey, and N. Hansen, “Structure and strength of aluminum with sub-micrometer/micrometer grain size prepared by spark plasma sintering,” Materials and Design, vol. 49, pp. 360–367, 2013.
View at: Publisher Site | Google Scholar
G. M. Le, A. Godfrey, N. Hansen, W. Liu, G. Winther, and X. Huang, “Influence of grain size in the near-micrometre regime on the deformation microstructure in aluminium,” Acta Materialia, vol. 61, no. 19, pp. 7072–7086, 2013.
View at: Publisher Site | Google Scholar
G. Le, G. Andrew, and W. Liu, “Microstructures and mechanical properties of sintered fine-grained Al,” Acta Metallurgica Sinica, vol. 49, no. 8, pp. 939–945, 2013.
View at: Publisher Site | Google Scholar
G. A. Sweet, M. Brochu, R. L. Hexemer, I. W. Donaldson, and D. P. Bishop, “Microstructure and mechanical properties of air atomized aluminum powder consolidated via spark plasma sintering,” Materials Science and Engineering A, vol. 608, pp. 273–282, 2014.
View at: Publisher Site | Google Scholar
Y. Xiong, D. Liu, Y. Li et al., “Spark plasma sintering of cryomilled nanocrystalline Al alloy—part I: microstructure evolution,” Metallurgical and Materials Transactions A, vol. 43, no. 1, pp. 327–339, 2012.
View at: Publisher Site | Google Scholar
D. Liu, Y. Xiong, T. D. Topping et al., “Spark plasma sintering of cryomilled nanocrystalline Al alloy. Part II. Influence of processing conditions on densification and properties,” Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, vol. 43, no. 1, pp. 340–350, 2012.
View at: Publisher Site | Google Scholar
A. Eldesouky, M. Johnsson, H. Svengren, M. M. Attallah, and H. G. Salem, “Effect of grain size reduction of AA2124 aluminum alloy powder compacted by spark plasma sintering,” Journal of Alloys and Compounds, vol. 609, pp. 215–221, 2014.
View at: Publisher Site | Google Scholar
K. Edalati, M. Ashida, Z. Horita, T. Matsui, and H. Kato, “Wear resistance and tribological features of pure aluminum and Al-Al₂O₃ composites consolidated by high-pressure torsion,” Wear, vol. 310, no. 1-2, pp. 83–89, 2014.
View at: Publisher Site | Google Scholar
N. Guo, B. Luan, F. He, Z. Li, and Q. Liu, “Influence of flake thickness on the shape and distribution of Al₂O₃ particles in Al matrix composites fabricated by flake powder metallurgy,” Scripta Materialia, vol. 78-79, pp. 1–4, 2014.
View at: Publisher Site | Google Scholar
N. Saheb, N. U. Qadir, M. U. Siddiqui, A. F. M. Arif, S. S. Akhtar, and N. Al-Aqeeli, “Characterization of nanoreinforcement dispersion in inorganic nanocomposites: a review,” Materials, vol. 7, no. 6, pp. 4148–4181, 2014.
View at: Publisher Site | Google Scholar
V. Yadav, Spark plasma sintering of aluminum matrix composites [M.S. thesis], Oklahoma State University, 2011.
A. A. El-Daly, M. Abdelhameed, M. Hashish, and W. M. Daoush, “Fabrication of silicon carbide reinforced aluminum matrix nanocomposites and characterization of its mechanical properties using non-destructive technique,” Materials Science and Engineering A, vol. 559, pp. 384–393, 2013.
View at: Publisher Site | Google Scholar
A. S. Edelstein and R. C. Cammarata, Nanomaterials: Synthesis, Properties and Application, Institute of Physics, Philadelphia, Pa, USA, 2002.
Y. Waseda, K. Shinoda, and E. Matsubara, X-Ray Diffraction Crystallography, Springer, Berlin, Germany, 2011.
L. Lu and M. O. Lai, Mechanical Alloying, Kluwer Academic, Boston, Mass, USA, 1998.
N. Saheb, A. S. Hakeem, A. Khalil, N. Al-Aqeeli, and T. Laoui, “Synthesis and spark plasma sintering of Al-Mg-Zr alloys,” Journal of Central South University, vol. 20, no. 1, pp. 7–14, 2013.
View at: Publisher Site | Google Scholar
N. Al-Aqeeli, K. Abdullahi, A. S. Hakeem, C. Suryanarayana, T. Laoui, and S. Nouari, “Synthesis, characterisation and mechanical properties of SiC reinforced Al based nanocomposites processed by MA and SPS,” Powder Metallurgy, vol. 56, no. 2, pp. 149–157, 2013.
View at: Publisher Site | Google Scholar
N. Al-Aqeeli, K. Abdullahi, C. Suryanarayana, T. Laoui, and S. Nouari, “Structure of mechanically milled CNT-reinforced Al-alloy nanocomposites,” Materials and Manufacturing Processes, vol. 28, no. 9, pp. 984–990, 2013.
View at: Publisher Site | Google Scholar
J. J. Grácio, C. R. Picu, G. Vincze et al., “Mechanical behavior of Al-SiC nanocomposites produced by ball milling and spark plasma sintering,” Metallurgical and Materials Transactions A, vol. 44, no. 11, pp. 5259–5269, 2013.
View at: Publisher Site | Google Scholar
S. Kamrani, R. Riedel, S. M. Seyed Reihani, and H. J. Kleebe, “Effect of reinforcement volume fraction on the mechanical properties of Alg–SiC nanocomposites produced by mechanical alloying and consolidation,” Journal of Composite Materials, vol. 44, no. 3, pp. 313–326, 2010.
View at: Publisher Site | Google Scholar
H. Kwon, S. Cho, M. Leparoux, and A. Kawasaki, “Dual-nanoparticulate-reinforced aluminum matrix composite materials,” Nanotechnology, vol. 23, no. 22, Article ID 225704, 2012.
View at: Publisher Site | Google Scholar
R. Vintila, A. Charest, R. A. L. Drew, and M. Brochu, “Synthesis and consolidation via spark plasma sintering of nanostructured Al-5356/B4C composite,” Materials Science and Engineering A, vol. 528, no. 13-14, pp. 4395–4407, 2011.
View at: Publisher Site | Google Scholar
S. Bathula, R. C. Anandani, A. Dhar, and A. K. Srivastava, “Microstructural features and mechanical properties of Al 5083/ ${SiC}_{p}$ metal matrix nanocomposites produced by high energy ball milling and spark plasma sintering,” Materials Science and Engineering A, vol. 545, pp. 97–102, 2012.
View at: Publisher Site | Google Scholar
Y. Saberi, S. M. Zebarjad, and G. H. Akbari, “On the role of nano-size SiC on lattice strain and grain size of Al/SiC nanocomposite,” Journal of Alloys and Compounds, vol. 484, no. 1-2, pp. 637–640, 2009.
View at: Publisher Site | Google Scholar
S. Kamrani, Z. Razavi Hesabi, R. Riedel, and S. M. Seyed Reihani, “Synthesis and characterization of Al-SiC nanocomposites produced by mechanical milling and sintering,” Advanced Composite Materials, vol. 20, no. 1, pp. 13–27, 2011.
View at: Publisher Site | Google Scholar
G. M. Candido, V. Guido, G. Silva, and K. R. Cardoso, “Effect of the reinforcement volume fraction on mechanical alloying of AA2124-SiC composite,” Materials Science Forum, vol. 660-661, pp. 317–324, 2010.
View at: Publisher Site | Google Scholar
N. S. Weston, F. Derguti, A. Tudball, and M. Jackson, “Spark plasma sintering of commercial and development titanium alloy powders,” Journal of Materials Science, vol. 50, no. 14, pp. 4860–4878, 2015.
View at: Publisher Site | Google Scholar
H. J. Fecht, E. Hellstern, Z. Fu, and W. L. Johnson, “Nanocrystalline metals prepared by high-energy ball milling,” Metallurgical Transactions A, vol. 21, no. 9, pp. 2333–2337, 1990.
View at: Publisher Site | Google Scholar
S. Hwang, C. Nishimura, and P. G. McCormick, “Mechanical milling of magnesium powder,” Materials Science and Engineering A, vol. 318, no. 1-2, pp. 22–33, 2001.
View at: Publisher Site | Google Scholar
N. Saheb, “Characterization of mechanically milled and spark plasma sintered Al2124-CNTs Nanocomposite,” Science of Sintering, vol. 47, pp. 119–129, 2015.
View at: Google Scholar
A. M. Al-Qutub, A. Khalil, N. Saheb, and A. S. Hakeem, “Wear and friction behavior of Al6061 alloy reinforced with carbon nanotubes,” Wear, vol. 297, no. 1-2, pp. 752–761, 2013.
View at: Publisher Site | Google Scholar
N. Al-Aqeeli, “Processing of CNTs reinforced al-based nanocomposites using different consolidation techniques,” Journal of Nanomaterials, vol. 2013, Article ID 370785, 10 pages, 2013.
View at: Publisher Site | Google Scholar
M. A. Muñoz-Morris, C. G. Oca, and D. G. Morris, “An analysis of strengthening mechanisms in a mechanically alloyed, oxide dispersion strengthened iron aluminide intermetallic,” Acta Materialia, vol. 50, no. 11, pp. 2825–2836, 2002.
View at: Publisher Site | Google Scholar
L. H. Dai, Z. Ling, and Y. L. Bai, “A strain gradient-strengthening law for particle reinforced metal matrix composites,” Scripta Materialia, vol. 41, no. 3, pp. 245–251, 1999.
View at: Publisher Site | Google Scholar
N. Chawla and Y.-L. Shen, “Mechanical behavior of particle reinforced metal matrix composites,” Advanced Engineering Materials, vol. 3, no. 6, pp. 357–370, 2001.
View at: Publisher Site | Google Scholar
R. Casati and M. Vedani, “Metal matrix composites reinforced by nano-particles—a review,” Metals, vol. 4, pp. 65–83, 2014.
View at: Google Scholar

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Copyright © 2015 Nouari Saheb et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Journal of Nanomaterials

Effect of Processing on Mechanically Alloyed and Spark Plasma Sintered Al-Al2O3 Nanocomposites

Abstract

1. Introduction

2. Materials and Experimental Procedures

3. Results and Discussion

4. Conclusion

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Effect of Processing on Mechanically Alloyed and Spark Plasma Sintered Al-Al₂O₃ Nanocomposites