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International Journal of Minerals, Metallurgy and Materials Volume 21, Number 3, March 2014, Page 295 DOI: 10.1007/s12613-014-0908-7

Mechanical and microstructural characterization of Al7075/SiC nanocomposites fabricated by dynamic compaction A. Atrian1), G.H. Majzoobi1), M.H. Enayati2), and H. Bakhtiari1) 1) Mechanical Engineering Department, Bu-Ali Sina University, Hamedan 65174, Iran 2) Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran (Received: 19 June 2013; revised: 17 November 2013; accepted: 19 November 2013)

Abstract: This paper describes the synthesis of Al7075 metal matrix composites reinforced with SiC, and the characterization of their microstructure and mechanical behavior. The mechanically milled Al7075 micron-sized powder and SiC nanoparticles are dynamically compacted using a drop hammer device. This compaction is performed at different temperatures and for various volume fractions of SiC nanoparticles. The relative density is directly related to the compaction temperature rise and indirectly related to the content of SiC nanoparticle reinforcement, respectively. Furthermore, increasing the amount of SiC nanoparticles improves the strength, stiffness, and hardness of the compacted specimens. The increase in hardness and strength may be attributed to the inherent hardness of the nanoparticles, and other phenomena such as thermal mismatch and crack shielding. Nevertheless, clustering of the nanoparticles at aluminum particle boundaries make these regions become a source of concentrated stress, which reduces the load carrying capacity of the compacted nanocomposite. Keywords: nanocomposites; metallic matrix composites; silicon carbide; nanoparticles; compaction

1. Introduction Due to its high strength-to-weight ratio and mechanical properties comparable with those of some steel materials, aluminum 7075 is widely used in aerospace, automotive, military industries, etc. The properties of this relatively ductile alloy can be improved by reinforcing it with hard nanoor micro-sized ceramic particles. The reinforced aluminum alloy, which is known as aluminum composite, enjoys the properties of a metal such as strength and high toughness, and the properties of a ceramic such as high temperature stability and wear resistance [1−2]. It must be mentioned that the main challenge in adding nanoparticles to reinforce a material is their natural tendency to cluster due to strong Van der Waals forces. The fine dimensions of such particles give rise to non-homogenous dispersion and agglomeration of the particles in a metal matrix [3]. Detrimental phenomena such as agglomeration can be prevented by controlling the temperature and using ultrasonic vibration during the addition of nanoparticles. Nanoceramics such as SiC, Al2O3, Corresponding author: G.H. Majzoobi

B4C, and TiB2 have received more attention due to their superiority to the conventional micron-sized particles over the past recent years. Among the nanoceramics, SiC has been widely used in Al-based composites due to its high resistance against temperature and wear [4]. Many fabrication techniques are available to produce nanoparticle-reinforced Al-based composites. Typical fabrication techniques for composite production are hot pressing [5−6], hot isostatic pressing (HIP) [3], hot extrusion [7−8] and cold pressing followed by conventional sintering [9−10]. All of these techniques are performed under quasi-static loading conditions. The techniques usually need hot sintering of the composite after or during its production. The composite can also be produced by dynamic or shock wave consolidation, and by high velocity compaction (HVC). The main advantage of these production techniques is that hot sintering is usually (but not always) eliminated from the production cycle. High velocity compaction techniques usually use explosives or compressed gas as propellants to accelerate a projectile for compaction of the powder, or use the impact of a dropping hammer for this purpose.

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The mass of the hammer varies from 5 to 1200 kg and the impact velocity ranges from 2 to 30 m/s [11]. Unlike the conventional quasi-static techniques, dynamic processes offer the possibility of producing the temperatures necessary for adequate local metallurgical bonding at the powder-particle interface, precisely where it is required, while the powder remains relatively cool elsewhere. Therefore, dynamic compaction techniques minimize microstructural changes, such as particle agglomeration and grain growth, which may otherwise occur due to the increases in temperature of the sample during compaction [12−13]. Wang et al. [14] employed dynamic consolidation to fabricate a W−25% Cu nanocomposite, where a shock wave induced by explosion was used to compact the powders. Anthony Fredenburg et al. [15] used a three-capsule gas-gun to consolidate nanocrystalline 6061−T6 aluminum powders, producing samples with 98%−99% of the theoretical mass density. Examples of dynamic compaction have been reported by Wang et al. [11], Yi et al. [16], and Yan et al. [17]. In this work, an Al7075/SiC nano-composite is produced under cold and hot dynamic compaction. The specimens are analyzed to determine the effects of temperature level and the volume fraction of nanoparticle reinforcement on their mechanical and microstructural behavior. Most prior investigations [3] on processing and characterization of Al7075/SiC nanocomposites have been performed mainly under quasi-static loading conditions. The main objective of this work is to explore the aspects of nanoparticle-reinforced Al7075 under dynamic loading conditions.

99.0%, specific surface area > 90 m2/g, nearly spherical morphology). The two material powders were mixed manually to produce two different mixtures with SiC content levels of 5vol% and 10vol%. The mixture was then suspended in ethanol and subjected to ultrasonic vibration for 20 min in order to prevent agglomeration of the particles and to obtain uniform dispersion of the nanoparticles. After drying, the mixture was milled in a planetary ball mill at room temperature in an inert argon atmosphere. The milling medium consisted of twenty-two hard chromium steel balls of 10 mm in diameter, encased in a 125 mL volume steel vial. About 30 g of the powder mixture, which has a ball-to-powder mass ratio of 3:1, was milled at a rotation speed of 300 r/min for 2 h. 0.5wt% of stearic acid was also added as a process control agent. Fig. 1 shows the morphology of as-received Al7075 and the milled mixture. The figure obviously shows that SiC nanoparticles have fully covered the surface of the Al7075 micron-sized particles after mechanical milling (Fig. 1(b)). Fig. 2 illustrates the XRD patterns of mixtures that were ball milled for 2 h and that differed in their content of SiC nanoparticles. The XRD patterns were recorded using a PHILIPS X’PERT PW3040 diffractometer (40 kV/30 mA) with Cu Kα radiation (λ = 0.154059 nm). No new phases are produced after ball milling of the mixture, due to the short duration of the milling. Fig. 2 shows the SiC diffraction peaks of specimens whose volume fraction of nanoparticle reinforcement was increased from 0 (Fig. 2(a)) to 10% (Fig. 2(b)).

2. Experimental

A general view of the die and the punch used for compaction of the composite is shown in Fig. 3(a). The die is made of 1.2344 heat-treated hot-work steel and the 15 mm diameter punch is made of 1.2542 shock-resisting steel. In order to pull the compacted specimen out of the die more easily, two 5-mm thick disk-shaped tablets were placed

2.1. Materials The as-received materials were Al7075−T6 (gas atomized, 100 μm, irregular morphology, Khorasan Powder Metallurgy Co., Iran) and SiC (average 50 nm, purity >

2.2. Compaction die and testing devices

Fig. 1. SEM micrographs of as-received Al7075 (a) and Al7075−5vol% SiC (b) after 2 h of ball milling.

A. Atrian et al., Mechanical and microstructural characterization of Al7075/SiC nanocomposites fabricated by…

Fig. 2. XRD patterns of specimens after 2 h of milling: (a) Al7075; (b) Al7075−5vol% SiC; (c) Al7075−10vol% SiC.

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sites were estimated using the rule of mixture to determine the corresponding densities ρA17075−5% SiC = 2832 kg/m3 and ρA17075−10% SiC = 2852 kg/m3. In order to study the effect of nanoparticle volume fraction and process temperature, cylindrical specimens of 8−9 mm in length and 15 mm in diameter of composites containing 0, 5vol%, and 10vol% of SiC were prepared at three process temperatures: room temperature, 225°C, and 425°C. The required temperature was supplied using a 1000 W ceramic cylindrical heating element. The temperature inside the die was measured using a wire thermocouple connected to an analog thermometer with a precision of ±10°C. It must be mentioned that the applied maximum temperature, 425°C, is the sintering temperature of Al7075, noting that the sintering temperature of a metal powder is about 75% of the melting point of the corresponding solid metal.

3. Results and discussion 3.1. XRD analysis Fig. 4 shows the XRD patterns of dynamically compacted Al7075−10vol% SiC nanocomposite specimens at different process temperatures. The XRD pattern of this nanocomposite powder before compaction is also depicted in Fig. 4(a). The figure suggests that no phase transformation occurs after dynamic compaction even at high temperatures.

Fig. 3. Schematic of the punch and die used for dynamic compaction (a) and a typical compacted specimen (b).

beneath and on the top of the powder. The tablets and the punch had the same diameter and were made of the same material. This was to reduce the spring-back effect of the specimen and also to improve the compaction [18]. A 2 kJ mechanical drop hammer supplied the impact loading required to compact the nanocomposite powders. The energy of the hammer is calculated for an impact velocity of 8 m/s and a hammer weight of m = 60 kg, using E = mV2/2 ( V = 2 gh ) for h = 3.5 m. Finite element simulation of the process reveals that this impact velocity produces a peak pressure of 1.2 GPa which can sinter the powder particles under optimized conditions. 5 g of the powder, with a tap density of about 55% of the theoretical density (ρA17075 = 2812 kg/m3 and ρSiC = 3220 kg/m3) was dynamically compacted using MoS2 as a lubricant for high-temperature tests. The theoretical densities of Al7075−5vol% SiC and Al7075−10vol% SiC nanocompo-

Fig. 4. XRD patterns of the dynamically compacted Al7075− 10vol% SiC nanocomposites at different temperatures: (a) milled powder; (b) compaction at 25°C; (c) compaction at 225°C; (d) compaction at 425°C.

Since strain in the crystal lattice also contributes to broadening of the XRD peaks, the Williamson–Hall approach was used to distinguish between the effects of crystallite size and strain [5]. The approach is described in the following equation: B cos θ =

0.9λ + 2ε sin θ D

(1)

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where B is the diffraction peak width at half maximum intensity, θ is the Bragg diffraction angle, λ is the wavelength of the radiation used, D is the average crystallite size, and ε is the average lattice strain. The average lattice strain can be estimated from the linear slope of Bcosθ versus sinθ, and the average crystallite size can be estimated from the intercept of the line at sinθ = 0. Williamson-Hall plots of dynamically compacted Al707510vol% SiC nanocomposites at different temperatures are depicted in Fig. 5. The figure shows that the Al crystallite size decreases from 138 nm to 27 nm when the processing temperature increases from 25°C to 425°C. The lattice strain also decreases from 0.35% to 0.05% with this increase in temperature. These severe changes may be attributed to dynamic recrystallization of the samples. The applied impact stress causes the dislocations to annihilate, and therefore facilitates recrystallization [19]. In contrast to the results of this investigation, the results reported by Ahmed et al. [3] show an increase in grain size of Al7075/SiC nanocomposites fabricated using hot isostatic pressing. These researchers believe that the coarser grain size of the composites may result from a lower extent of precipitation, which is required for microstructure refinement.


silicon carbide nanoparticles were compacted under dynamic loading. Fig. 6 shows how the relative density varies with the volume fraction of SiC. This figure suggests that the relative density of compacted specimens decreases by about 2% as the SiC content level increases from 0 to 10vol%. The figure also suggests that the relative density of Al7075−5vol% SiC is nearly 96%, which is lower than the relative density of 99% obtained by Ahmed et al. [3] for the same nanocomposite. This may be due to the difference in processing techniques. The present study uses dynamic compacting, but Ahmed et al. [3] used cold pressing followed by hot isostatic pressing and hot extrusion to fabricate the samples. Their three successive compacting steps, accompanied by very high pressures, could yield a higher density compared with our method of single-step dynamic compaction, where the powder is subjected to instantaneous peak pressure. In order to increase the accuracy of density measurement, the Archimedes principle was used to measure the density of the consolidated samples. Generally, the compaction and sintering of composite materials are more complex than those of monophase materials. The presence of hard and non-deformable particles in a ductile matrix reduces their compatibility. The reduction in compatibility increases for a higher volume fraction of reinforcement phase [9, 13]. Fig. 7 shows SEM micrographs of dynamically compacted specimens at 425°C for different content levels of SiC nanoparticles. As shown in this figure, the degree of porosity and the number and size of voids increase as the content level of SiC nanoparticles in the matrix increases.

Fig. 5. Williamson-Hall plots of dynamically compacted Al7075− 10vol% SiC nanocomposites at different temperatures (the equation corresponding to the trend line for each temperature is shown in the graph).

3.2. Effects of SiC volume fraction The properties of a composite depend not only on the properties of the matrix itself, but also on the reinforcing material and its interaction with the matrix. The volume fraction of hard phase particles in a multiphase microstructured composite is the most important factor affecting the composite’s mechanical properties such as strength and fracture toughness [13]. In order to study the effects of this parameter, specimens containing 0, 5vol%, and 10vol% of

Fig. 6. Effect of nano-SiC reinforcement on relative density at different process temperatures.

It should be noted that during dynamic compaction, the kinetic energy delivered to the specimen increases as the impact velocity increases according to the equation E = mV2/2. Therefore, the fine particles are easily pushed into the spaces between the coarse particles, reducing the porosity and increasing the green density of the sample. Furthermore, local-


ized heat generated by friction and displacement of powder particles gives rise to plastic deformation, which yields a sof-

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tened or molten layer which in turn enhances the bonding between particles and again increases the green density [11].

Fig. 7. SEM micrographs of dynamically compacted specimens at 425°C: (a) Al7075, (b) Al7075−5vol% SiC, and (c) Al7075− 10vol% SiC.

Fig. 8 is a SEM micrograph that shows nano-SiC particles settled at the boundary of two adjacent Al particles. The EDS point analysis shown in this figure also confirms that the dispersed particles at the boundary are SiC. These hard and stiff nanoparticles interfere with bonding between micron-sized Al particles, creating specimens that are more porous. This decrease in relative density upon increasing the content of the reinforcement phase has been frequently ob-

served in the literature. For example, Rahimnejad Yazdi et al. [20] reported that the relative density of alumina/SiC nanocomposites decreases with a higher SiC content. They attributed this behavior to the poor sintering of SiC at the temperatures that were used to sinter their samples. Dong et al. [21] stated that the decrease in relative density with particle content may arise because the reinforcing particles block grain boundary movement.

Fig. 8. (a) SEM micrograph showing interparticle settlement of nano-SiC and (b) EDS point analysis of the region around the SiC particle shown by the arrow in (a).

The variation of microhardness of compacted specimens versus SiC content is shown in Fig. 9. The figure suggests that the hardness increases with the increase of volume fraction of SiC. The Vickers microhardness was measured by applying a 0.98 N force for 15 s to the specimen using a tetragonal indenter. At least three measurements were made for each specimen. Improvement of hardness can be regarded as a benchmark for enhancement of material strength

as well. Yi et al. [16] have reported that dynamic compacted samples are harder than those compacted by conventional compaction methods and quasi-static compaction for the materials having the same density. This is thought to be probably due to stronger inter-particle combination and higher work hardening. As shown in Fig. 9, by adding the ceramic hard phase of SiC, the hardness of the composite increases by around 20%.

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In fact, despite of the reduction of density with the increase of SiC content, the nano reinforcement could enhance the hardness. The latter is speculated to be due to second-phase hardening effects of added nanophase and its intrinsic hardness. Similar observations have already been reported by Canakci [22] and Alizadeh et al. [7].

damage during synthesis of the composite [3]. Other factors which can significantly influence the material properties of the composite are interaction between the cracks and the residual stress field around the inclusions, and crack bridging (also known as crack shielding) by the intact SiC particles [25]. Fig. 11 shows that the uniform dispersion of nanoparticles is another important parameter, which can affect the mechanical behavior of the composite material. It is interesting to note that unlike our results, those reported by Ahmed et al. [3] show a significant drop in hardness and tensile properties of Al7075 reinforced by SiC nanoparticles. They attributed this behavior to Mg segregation at the oxidized Al−SiCnp interfaces and the grain boundaries, which does not occur in dynamic compaction. This can explain why the hardness and tensile properties obtained in this work differ from those reported by Ahmad et al. [3].

Fig. 9. Variation of microhardness with SiC content.

Fig. 10 shows the engineering stress-strain curves of Al7075/SiC with different content levels of SiC nanoparticles. The specimens for compressive tests were fabricated using a 60 kg drop hammer impacting at a velocity of 8 m/s and the testing temperature of 425°C. The compressive tests were also conducted at a velocity of 5 mm/min using a 60 t Instron universal testing machine. As observed in Fig. 10, SiC particles significantly improve the strength of the compacted composite specimens. The increase in ultimate strength is around 50%−60% for both 5vol% and 10vol% of SiC. The reason for the tremendous influence of nanoparticles on the strength of the Al7075 composite has been the subject of a number of investigations over the past several years [23−26]. Typical explanations include Orowan strengthening [27], thermal mismatch [27−29], shear lag and load-bearing effects. For instance, the Orowan mechanism considers both the interparticle distance and the particle size of the reinforcement material. According to this mechanism, the strengthening will depend on the uniformity of dispersion, and on the particle size distribution (the fraction of large micron-sized particles among the nanoparticles). The reduction of flaw size, which directly results from the inclusion of the nanophase in the composite, increases the threshold stress for crack initiation. This can explain the improvement in strength of the material. Basically, nanosized particles are more preferable than micron-sized particles in the sense that nano-sized particles are more effective in blocking the dislocation motions. Moreover, because of their small size, nanoparticles are less prone to crack or

Fig. 10. Engineering stress-strain curves of Al7075/SiC nanocomposites dynamically compacted at 425°C.

By comparing Figs. 11(a) and 11(b), which are SEM images of pure aluminum specimens and those reinforced with 10vol% SiC, respectively, one can observe that the nanoparticles are homogeneously dispersed around the surface of the Al micron-sized particles. The bright points in these figures correspond to the buildup of nano SiC around the Al particle surface. Nevertheless, agglomeration of the nanoparticles between the Al particles is also evident in Figs. 11(c) and 11(d). This is presumably due to the large difference between the sizes of aluminum and the SiC particles, which causes the SiC nanoparticles to intercalate into pores and vacancies between the aluminum particles [3]. Agglomeration is known to be the main obstacle for improving the strength and elongation of composite materials containing higher contents of SiC. Since SiC and its clusters are stiffer than the Al matrix, they boost the load bearing of the composite material by limiting its plastic deformations.


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Fig. 11. SEM micrographs of the etched surface of dynamically compacted Al7075 (a), Al7075−10vol% SiC (b), (c) partial enlargement of c region in (b) (uniform dispersion of SiC nanoparticles as bright points indicated in (c) and (b)), and (d) EDS point analysis of the region containing SiC agglomeration.

3.3. Effects of temperature Temperature plays an important role in dynamic compaction of powders. Fig. 12 shows how the relative density varies with temperature and SiC content for dynamically compacted samples. As can be observed in the figure, by increasing the process temperature from 25°C to 425°C the relative density increases by 7%. Typical optical micrographs of specimens, shown in Fig. 13, also reveal fewer voids and porosities at higher temperatures. This is a direct consequence of the higher relative density. It should be noted that the samples that were compacted at lower temperatures do exhibit voids with sharp corners, indicating that the specimens are not fully sintered. However, as shown in Fig. 13(c), at elevated temperatures the particle boundaries are welded and bonded together. This is because when a powder is heated up to a certain temperature, its yield stress and strain hardening reduce and the material becomes more

ductile. During dynamic compaction, the transmitted energy subjects the powder to large strain that changes instantaneously at high rates. Therefore, the process is adiabatic, i.e., the energy converted to heat does not have enough time to dissipate and as a result, the generated internal heat brings

Fig. 12. Relative density of dynamically compacted samples as a function of temperature and SiC content.

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Fig. 13. Temperature effect on the microstructure of dynamically compacted specimens: (a) T = 25°C (ρRel = 0.91); (b) T = 225°C (ρRel = 0.94); (c) T = 425°C (ρRel = 0.98).

about a local temperature rise in the powder and softens the particles surfaces [16]. The internal heat causes further plastic deformation of the particles and consequently, the material flows more easily to fill the pores and vacancies. Temperature can also help to strengthen a nanocomposite due to thermal mismatch between the nanoparticles and the matrix, as pointed out in the previous section. Thermal mismatch between the reinforcing phase and the matrix induces thermal stresses in the interface regions as the sample cools down from the compaction temperature. These stresses at the boundaries are large enough to cause plastic deformation, but reduce sharply as the distance from the boundary increases. This high stress gradient can generate small defects such as dislocation buildup in the vicinity of nano-sized particles. Dislocation buildup is thus contributes significantly to strengthen the material [27−30].

cles and increase the possibility that they will weld to each other during the hammer impact. The high processing temperature used in this study led finally to a 7% increase in the relative density of the composite.

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4. Conclusions [4]

(1) Reinforcement of Al7075 with SiC nanoparticles reduces the relative density about 2%. The hardness and stiffness of these nanoparticles, compared to the properties of the matrix itself, reduce the compressibility of the powder by being deposited at the boundaries between Al particles. (2) Because of the work-hardening effects of the added nanophase, and its intrinsic hardness, the hardness of the reinforced Al7075/SiC nanocomposite was enhanced about 20% with respect to the monolithic material. (3) Although a homogeneous dispersion of SiC nanoparticles can improve the strength and hardness of a composite material, a detrimental phenomenon such as agglomeration of SiC nanoparticles may also occur. The regions of agglomeration act as stress concentration sources, eventually reducing the strength of the compacted samples. Clustering of nanoparticles also prevents perfect densification, leading to lower densities. (4) Temperature also has a significant effect on relative density. A high processing temperature can soften the parti-

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