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Study of Al composites prepared by high-energy ball milling; Effect of processing conditions
Journal of Alloys and Compounds xxx (2015) xxx–xxx
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Study of Al composites prepared by high-energy ball milling; Effect of processing conditions J.M. Mendoza-Duarte, I. Estrada-Guel ⇑, C. Carreño-Gallardo, R. Martínez-Sánchez Centro de Investigación en Materiales Avanzados (CIMAV), Laboratorio Nacional de Nanotecnología, Miguel de Cervantes # 120, C.P. 31109 Chihuahua, Chih., Mexico
a r t i c l e
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Article history: Available online xxxx Keywords: Aluminum Composites High-energy milling
a b s t r a c t The present work deals with the synthesis of some Al-based composites prepared by mechanical milling and processing by powder metallurgy followed by the evaluation of process conditions as: type of additive, their concentration and milling intensity studying its effect on the characteristics of the powder composite and mechanical performance of the composite. Powder samples were microstructural characterized by electronic microscopy (SEM–TEM) and the mechanical response was followed by hardness and compressive tests. A pronounced effect on the mechanical response of the specimens was evident after the addition of reinforced particles and milling intensity. Microscopy studies showed a uniform dispersion of the reinforcing particles in the metallic matrix at nanometric scale and an important grain refinement of the Al matrix was confirmed. After processing, a 66% increase on the mechanical response was reached with 1% of additive complemented with short milling intensities. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Today, there is a rising need for advanced materials in order to achieve a better performance for engineering materials that cannot be easily reached using conventional alloys. Thus, production and application of composite materials have increased in recent years [1]. Metal matrix composites (MMC) are engineering materials that contain at least two insoluble components: a metallic matrix (high ductility) and reinforcement particles (high stiffness) in the form of a particulate, short or continuous fiber with a well-defined interface between the matrix and the reinforcement phases [2]. These novel materials have the potential to provide tailored properties that are an average of the matrix and reinforcement properties for specific applications by adjusting the ratio of matrix and reinforcement [3]. Among many metals, aluminum (Al) has been preferred in many cases due to its relatively low density, reasonable mechanical properties, and good workability [4]. Due to its universal applications, AlMMCs are valuable alternatives for demanding industries as: transportation, aerospace, defense, etc., [5] because of the fact, that addition of small amounts of reinforcements particles in the metallic matrix, offers an important increase of mechanical properties compared to their unreinforced counterparts [6,7]. Some processes have been used for composites manufacture, which can coarsely be divided into three types: (a) liquid-state methods (pressure infiltration, stir casting, spray deposition and ⇑ Corresponding author.
in-situ processing), (b) semi-solid state methods (compocasting) and (c) solid-state methods (powder metallurgy, mechanical alloying). The first method involves melting and casting processes, however, this method was unable to give good homogeneity of dispersions in metal matrix because of the high interfacial energy between the molten metal and dispersoid [8]. It is extremely difficult to obtain a uniform dispersion of components especially in viscose molten alloys in semi-solid techniques where particles have high clustering tendency [9]. It is possible to produce a fine and homogeneous distribution of hardening particles with a very fine particle size in the metallic matrix [2] using the solid-state or mechanical milling (MM) route (via high-energy ball milling), avoiding the reinforcement-particles clustering [10]. This is important due that fine dispersion of the strengthened phase in the Al structure has a preponderant effect on mechanical properties of prepared composites [11]. The amount that the dispersoids strengthen the final composite depends of their specific characteristics as: particle type, size, morphology, volume fraction and their physical distribution. Also, formation of good interface between matrix and reinforcement is very important [6] and plays a key role on the overall composite performance. The technological challenge is to increase the bonding strength of the interphase by controlling manufacture process based on some particle features as size, morphology and volume fraction. Oxides, carbures, nitrides as silicon carbide, boron carbide [12], titanium carbide [13] alumina [9,14] and hard metals such as titanium and tungsten have been used as reinforcement for aluminum composites [15] but for some
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applications, these materials are not appropriate due to their high cost, use of additives and complicated processing conditions. On the other hand, natural graphite (Gr) has received significant attention for its low chemical activity, low density, high melting temperature, excellent tribological properties, high availability and especially low cost. In order to improve the interfacial bonding between components, a metal coating with Cu, Ni, and Co, has been demonstrated to be a feasible method to solve this inconvenience [5], also can inhibit interfacial reaction under high temperatures during the fabrication process [16]. The present research deals with the synthesis of some Al–Gr composites, the effect of three different metals as Cu, Ni and Ag on the graphite and Al/MeGr interphase and the evaluation of process conditions as: type of additive, their concentration and milling intensity on the characteristics of the processed powders and their mechanical performance.
2. Experimental procedure The studied composites were formulated using as raw materials pure powders of aluminum (99.5% purity and 44 lm, in size) and metallized graphite (MeGr) as reinforcing phase. Pure elemental powders of natural graphite (99.9%, 85 + 180), copper (99.5%, 100), silver (99.95%, 150) or nickel (99.8%, 44) were used for the MeGr preparation. Mixtures of Metal–Graphite powder mixtures (where Metal = Cu, Ag or Ni) with a concentration of 10 at.% metal were prepared and processed using a high-energy SPEX 8000M mill under an inert Ar atmosphere, selected milling intensities was 0, 1, 2, 4 and 8 h. The structure of Ni, Cu and Ag coatings was evaluated by X-ray diffraction (XRD) analysis. Al–MeGr composites preparation was carried out milling mixtures of pure Al powder and MeGr particles with a concentration of 1.0 (wt.%) during five time intervals (0, 1, 2, 4 and 8 h) as shown in Table 1 (due to space restriction, only the better mechanical results of each family are showed). Methanol was used as a process control agent in order to avoid excessive Al agglomeration. Pure Al samples were used as a reference for comparison proposes. Cold-consolidated forms were obtained from the powders using a two-step pressing method: pressing the powders in a circular die (900 MPa) in one direction and rotating the die to press the form in the opposite direction in order to obtain an ideal compaction level (this condition was calculated using density–pressure curve using a universal machine and plotted the curve searching for an asymptotic zone [17]. Sintering process was carried out with a heating ramp of 10 K/min until 723 K for 3 h under an Ar atmosphere. The morphology, size and metal-particle distribution of milled powder was examined using a JEOL-JSM 7201F SEM/EDS. X-rays diffraction (XRD) analysis was carried out using a Pan Analytical X’pert PRO X-ray diffractometer with Cu Ka radiation. Rockwell F hardness was measured on polished samples using a 1/1600 ball at a load of 60 kg and converted to Vickers scale.
At least five hardness measurements were done and the average is reported. Compression tests were done at room temperature in an Instron Universal tester and 0.2% yield strength (ry) was measured.
3. Results and discussion 3.1. Microstructural characterization The Fig. 1 shows the effect of milling on the general morphology of pure Al powder. As-mixed sample (Fig. 1a) presents semi-spherical particles, typical of gas-atomized metals. Milled products exhibit particles with higher size compared with an un-processed sample (0 h) due to Al ductile characteristics. After 4 h of milling (Fig. 1b), the metal particles are plastically deformed by highenergy collisions between grinding media and container. Morphology changes from spherical to irregular with large aggregates formed at the expenses of trapped minuscule particles are evident. Further milling (8 h) hardened large particles lead to fracture process activation (Fig. 1c), forming flake-like sub-particles on the material surface. The Fig. 2(a–c) shows the morphology of pure Gr particles and the effect of milling on its physical characteristics. Contrarily to the ductile behavior of Al, Gr is fragmented into tiny pieces as a consequence of high impact forces and defoliation evidence is observed after 8 h of milling (Fig. 2c). The Fig. 3a shows the morphology of CuGr metalized graphite particles; it is evident that particles are micrometric in size exhibiting a laminar structure. Bright zones correspond to zones with high metal concentration (Cu) surrounded by defoliated graphite layers, as high-magnification micrograph shows (bottom square). EDS analyses reveal the existence of an interphase composed of a mixture of Cu–Gr with a heterogenic composition creating an interlapping of elements as Shehata et al. described in their work [8]. In Fig. 3b a STEM micrograph shows the distribution of Cu and C in an isolated particle, graphite signal shows a rich distribution of carbon on the particle surface from one side to the center; while Cu signal is low reaching a maximum in the central part of the particle, indicating the presence of a metal core inside. Similar behavior was found with AgGr and NiGr particles. A possible mechanism involves the homogenization of spatial distribution of reinforcement particles at initial stages of milling by the control
Table 1 Nomenclature of metalized graphite and prepared composites.
a b c
Description
Nomenclature
Composition
Milling time (h)
Example
Milled Gr Metallized Gr
Grxh MeGrxh
x = 0, 1, 2, 4 and 8
Gr2 ha CuGr4 hb
Composite Al–Gr
Al–1%MeGrxh–M4
Pure graphite Graphite + Me Me = Cu, Ni, Ag. Al + Metallized Gr Al + MeGrxh
M = 0, 1, 2, 4 and 8
Al–1%AgGr8 hM4c
Gr milled 2 h. Cu/Gr milled 4 h. Al + 1% (wt.%) Ag/Gr milled 8 h and the mixture milled 4 h.
Fig. 1. SEM micrograph (2.5 Kx) of pure Al after (a) 0, (b) 4 and (c) 8 h of milling.
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Fig. 2. SEM micrograph (2, 5 and 20 Kx) of pure graphite after (a) 0, (b) 4 and (c) 8 h of milling.
Fig. 3. (a) SEM and (b) STEM micrographs of CuGr2 h particles.
20
30
40
50
60
70
80
90
20
311
004
002
Intensity (a.u.) 0h
222
220
311
8h
Intensity (a.u.)
222
220
Al C (Gr)
111
(b)
200
111
Al 200
(a)
30
40
50
8h
0h
60
70
80
90
2θ (Degrees)
2θ (Degrees)
Fig. 4. (a) XRD patterns of Al and (b) Al–1%CuGr2 h powder composites milled at different milling times.
process agent (MeOH in our case) allowing a better collision rate between particles and balls within the milling containers as Goya explains [18]. 3.2. X-rays diffraction (XRD) The XRD patterns of pure Al and Al–1%CuGr2 h composites processed from 0 to 8 h are shown in Fig. 4a and b. The signal of graphite (26.6° and 54.6°) is weak due to low concentration of reinforcement in the un-milled composite; after milling, both peaks disappear and are no longer visible. Similarly, as effect of milling, all Al peaks substantially reduced their size and became wider because the grain refining and internal stress accumulation induced by cold deformation [12]. The XRD patterns of composites show the same tendency of broadening and intensity reduction of the peaks, it means that the refinement of the aluminum grain is not affected by the addition of reinforcing particles (at least with this concentration). No new peaks appear and Al peaks do not shift (contrary to Nemati evidence [13] in Al–4.5 wt.%CuTiC system). This indicates that reinforcing particles do not react or form other phases with aluminum matrix staying insoluble in order to strengthen a material with a second phase. This behavior is beneficial because the mechanical
Table 2 Calculated lattice parameter of studied composites [nm]. Milling (h)
Alp
Al– 1%Gr2 h
Al– 1%CuGr2 h
Al– 1%NiGr2 h
Al– 1%AgGr2 h
0 1 2 4 8
0.404 0.404 0.405 0.405 0.404
0.405 0.404 0.404 0.404 0.404
0.405 0.404 0.404 0.404 0.405
0.404 0.404 0.404 0.404 0.404
0.404 0.404 0.404 0.404 0.404
properties are increased. This behavior is confirmed in Table 2 (results of 5 samples), where Al parameter was calculated using the main Al diffraction lines, showing an irrelevant variation not far from its reported value (0.404 nm) in all samples. 3.3. Mechanical characterization The reason to use a low concentration of reinforcement in this work is related to avoid the pores formation provoked by clusters as was reported by Nie et al. [5]. Other authors emphasize that the mechanical properties of a composite are related to the amount
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500
(a)
(b)
Stress (MPa)
100
Al 4h
400
Al 0h
Al-1%NiGr2h-M0 Al-1%AgGr2h-M0
Al 8h
300 Al 2h
Al 1h
Al-1%CuGr2h-M0
50
200 100 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 0.00
0.01
0.02
0.03
0.04
0.05
Strain (mm/mm) Fig. 5. Compression strain–stress curves of composites (a) Al pure reference at different milling times and (b) magnificated square of un-milled Al–MeGr composites.
500
AlAgGr1-2h Alp 4h
Stress (MPa)
400
AlGr8-2h
300
200
AlNiGr4-2h AlCuGr2-2h Alp 0h
100
0 0.00
0.05
0.10
0.15
0.20
0.25
0.30
Strain (mm/mm) Fig. 6. Best response strain–stress curves of each family (Gr, CuGr, NiGr and AgGr).
[19] and size of its reinforcements [1] upgrading their properties with higher concentration of reinforcement phase and/or decreasing its size [2]. Sameezadeh et al. [20] comments if nanoparticle content in a composite exceeds a critical value, the improvement in mechanical properties diminishes by saturation of grain boundaries and agglomeration. Following these observations, the mechanical properties of the sintered composites (1 wt.%) were evaluated by compression tests along consolidation direction and their yield strength was measured. Some strain–stress curves are shown in Figs. 5 and 6. As-mixed samples present a low mechanical response, being practically similar in all samples despite of MeGr addition (Fig. 5b). On the other hand, a remarkable increase on mechanical response in pure milled aluminum is observed; this effect is particularly evident with Alp-2 h sample (Fig. 5a). This behavior agrees with Karbalaei et al. [9] research where the compressive strength of the composites was higher compared with a non-reinforced alloy due to strengthening effect of nanoparticles on aluminum matrix. Yang et al. [22] comments that the increase in toughness is believed to be associated with the homogenous
dispersion reinforcement particles within the matrix and its strong interface between them and the matrix. It is evident a reduction in the elongation in all samples when comparing the curves of samples after 1 h of milling (Fig. 5a). This effect is due to an increase in the required stress for dislocations movement between the nanoparticles or can be caused from problems with particle packing, which can produce ductility loss [12]. Therefore, mechanical properties of porous composites generally depend in a macro level on their density. Moreover, porosity has been considered to be a critical factor deteriorating mechanical properties of materials. Additionally, El-Kady et al. [1] mentions that the degradation of mechanical properties of the composites is caused by interphase problems in a micro level where particles cannot transfer loads from soft matrix to the hard reinforcements. From strain–stress curves (Fig. 6), the yield strength was calculated observing that Al–1%CuGr2 h–M2 composite presents the highest value (66% of increase compared with pure Al sample processed under same conditions), with un-milled samples negative values appeared even though with reinforcements addition, this is because reinforcement particles do not form a consistent structure with the matrix, thus the interface is weak and the distribution of loads is not efficient. According to Van Dick, this condition rather than improving the properties deteriorates them since this poor union may create pores or initiate fractures [21]. Yield strength values (Table 3) can be compared to two commercial alloys: the 6000 series with 280 MPa and 4000 series (295 MPa) after T5 thermic treatment. Hardness is a physical parameter that implies the material’s capability to resist local plastic deformation. In MMCs, dispersed nanoparticles act as reinforcing phases becoming the obstacles to the movement of dislocation when plastic deformation occurs [8]. Fig. 7 presents a plot with hardness values measured on the composites as a function of milling time. Milled samples (1 h) are harder than un-milled ones; this is a direct consequence of work hardening despite the addition of MeGr as described before. As it can be seen, milling process causes a raise in hardness of powders; MM leads a microstructure refinement resulting in a
Table 3 Calculated yield strength (MPa) of the best composites compared with pure aluminum sample. The numbers between parentheses indicate the property increment (%). Values in bold, italics and underline are the highest response reach of each composite family of samples. Milling (h)
Alp
Al–1%Gr8 h
Al–1%CuGr2 h
Al–1%NiGr4 h
Al–1%AgGr2 h
0 1 2
85.9 235.9 233.3
81.0 (5.8) 281.4 (19.3)
93.7 (9.0) 321.5 (36.3)
83.1 (3.3) 293.6 (24.5)
82.8 (3.6) 295.2 (25.2)
4 8
245.5 238.7
353.4 (51.4) 312.9 (27.5) 305.9 (28.1)
387.3 (66.0) 341.3 (39.0) 341.2 (42.9)
379.7 (62.7) 341.7 (39.2) 329.5 (38.0)
323.0 (38.4) 321.3 (30.9) 321.9 (34.9)
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120
Al–1%CuGr2 h–2 h sample. This behavior is in a good agreement with the evidence found in compression curves. As it was mentioned before, the particle size not only has an important effect on densification, porosity but also in the mechanical behavior of composites. Therefore, an optimal distribution of particle sizes gives an increase on the final mechanical properties of composites.
100
Hardness (HV)
5
80 60
Al-1%Gr2h Al-1%CuGr2h Al-1%NiGr4h Al-1%AgGr2h Alp
40 20
3.4. TEM characterization
0 0
1
2
4
6
8
Milling Time (h) Fig. 7. Hardness determination as a function of milling intensity and dispersoid type.
nanocrystalline structure with high lattice microstrain. Moreover, the presence of reinforcing particles enhances the work hardening of the matrix. The pure Al hardness before milling was undetermined (value below selected scale). After milling, the hardness of composite powder increased rapidly to over 87 HV. But the hardness values fall with further processing, it can be assumed that behavior can be attributed to high grain refinement, large strain [12] and increased dislocation density [20] introduced by MM. At macro level, higher milling time induces particle agglomeration (big sizes) meaning poor compaction and high porosity finding a loss of property after 4 h of milling (2 h for Alp sample) in the composites. It is evident that MeGr addition increases the hardness of all composites compared to the pure aluminum alloy reference, because the presence of ceramic reinforcement has a preponderant effect on mechanical properties of composites [11]. This increase is expected because particulate reinforcements act as a barrier to the dislocation movement [15] within the matrix and exhibit greater resistance to indentation of the hardness tester [13,20]. According to the Orowan mechanism, this enhancement is also attributed because nanoparticles can restrain the growth of crystal grains and refine the crystal grains of the matrix [8]. Li et al. mentions that there are two strengthening mechanisms that are typically associated with conventional MMCs: load transfer from the metal matrix to the reinforcing particle (direct) and the influence of reinforcement on matrix microstructure (indirect) induced by the deformation mismatch between the reinforcement and the matrix [3]. Hardness increases with the addition of reinforcement particles as a function of preparation conditions reaching a maximum with
Composites reinforced by nanoparticles have a larger reinforcement/matrix interfacial area and a reduced interparticle spacing, which can transfer more loads from the soft matrix to the hard reinforcements improving the strength of the composites. Thus, the formation of a clean [3] and cohesive [6] interface between matrix and reinforcement is important to permit an appropriate load transfer. Likewise, in carbon nanotubes reinforced composites, Ahmad et al. [22] mentions that strengthening/toughening phenomena strongly depend on the interfacial bonding. Fig. 8 shows STEM micrographs of an Al–1%CuGr2 h–M2 composite. We can notice that CuGr particles (bright dots) are dispersed into the metallic matrix (Fig. 8a), a good linking between the reinforcement particle and aluminum matrix is evident. The Fig. 8b shows the interaction between the reinforcement particle and a dislocation line; it is not clear if the particle stops or initiates the dislocation. Higher magnifications TEM image (Fig. 8c) exhibit a Cu particle of copper perfectly covered by graphite since are neither cracks nor gaps between these two elements. This layer is so effective, so Cu was not dissolved into Al matrix during the thermal treatment maintaining it as a second phase. Thus, the carbon layer acts as an efficient barrier avoiding reactions between nano reinforcement particles and the metallic matrix working as an efficient load transfer system as mentioned.
4. Conclusions The method improves the mechanical properties of Al-composites reinforced with MeGr as a direct consequence of the uniform distribution of ceramic nanoparticles. A significant increase on the mechanical response of the specimens was evident with addition of reinforcement particles. A 66% increase on the mechanical response was reached with small amounts of reinforcement particles (1 wt.%) complemented with low milling intensities (2 h). Electron microscopy studies showed a uniform dispersion of the particles and good interaction with the metallic matrix. This interaction occurred by the formation of a thin layer of graphite on the metallic particle. The graphite layer acts as a barrier, generating good adhesion between the interfaces and avoiding unwanted dissolution of metal in the metal matrix.
Fig. 8. STEM Micrographs of Al–1%CuGr2 h–M2 composite with z contrast to evidence the elemental distribution of components.
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Acknowledgements CONACYT Project 169262 supported this research. Thanks to Redes Temáticas de Nanotecnología y Nanociencias Reg. 0124886 and 0124623; respectively. The technical assistance of A. Hernández-Gutiérrez, D. Lardizabal and E. Torres Moye is gratefully acknowledged. References [1] Omyma El-Kady, A. Fathy, Effect of SiC particle size on the physical and mechanical properties of extruded Al matrix nanocomposites, Mater. Des. 54 (2014) 348–353. [2] C. Suryanarayana, Synthesis of nanocomposites by mechanical alloying, J. Alloys Comp. 509S (2011) S229–S234. [3] Y. Li, Y.H. Zhao, V. Ortalan, W. Liu, Z.H. Zhang, R.G. Vogt, N.D. Browning, E.J. Lavernia, J.M. Schoenung, Investigation of aluminum-based nanocomposites with ultra-high strength, Mater. Sci. Eng. A 527 (2009) 305–316. [4] Heekyu Choi, Lin Wang, Dongkeun Cheon, Woong Lee, Preparation by mechanical alloying of Al powders with single-, double-, and multi-walled carbon nanotubes for carbon/metal nanocomposites, Compos. Sci. Technol. 74 (2013) 91–98. [5] Jun-hui Nie, Cheng-chang Jia, Na Shi, Ya-feng Zhang, Yi Li, Xian Jia, Aluminum matrix composites reinforced by molybdenum-coated carbon nanotubes, Int. J. Miner. Metall. Mater. 18–6 (2011) 695–702. [6] D. Özyürek, S. Tekeli, A. Güral, A. Meyveci, M. Gürü, Effect of Al2O3 amount on microstructure and wear properties of Al–Al2O3 metal matrix composites prepared using mechanical alloying method, Powder Metall. Met. Ceram. 49 (5-6) (2010) 289–294. [7] Zuhair M. Gasem, Fatigue crack growth behavior in powder-metallurgy 6061 aluminum alloy reinforced with submicron Al2O3 particulates, Composites Part B 43 (2012) 3020–3025. [8] F. Shehata, A. Fathy, M. Abdelhameed, S.F. Moustafa, Fabrication of copper– alumina nanocomposites by mechano-chemical routes, J. Alloys Comp. 476 (2009) 300–305. [9] M. Karbalaei Akbari, H.R. Baharvandi, O. Mirzaee, Fabrication of nano-sized Al2O3 reinforced casting aluminum composite focusing on preparation process of reinforcement powders and evaluation of its properties, Composites Part B 55 (2013) 426–432.
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Study of Al composites prepared by high-energy ball milling; Effect of processing conditions J.M. Mendoza-Duarte, I. Estrada-Guel ⇑, C. Carreño-Gallardo, R. Martínez-Sánchez Centro de Investigación en Materiales Avanzados (CIMAV), Laboratorio Nacional de Nanotecnología, Miguel de Cervantes # 120, C.P. 31109 Chihuahua, Chih., Mexico
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Aluminum Composites High-energy milling
a b s t r a c t The present work deals with the synthesis of some Al-based composites prepared by mechanical milling and processing by powder metallurgy followed by the evaluation of process conditions as: type of additive, their concentration and milling intensity studying its effect on the characteristics of the powder composite and mechanical performance of the composite. Powder samples were microstructural characterized by electronic microscopy (SEM–TEM) and the mechanical response was followed by hardness and compressive tests. A pronounced effect on the mechanical response of the specimens was evident after the addition of reinforced particles and milling intensity. Microscopy studies showed a uniform dispersion of the reinforcing particles in the metallic matrix at nanometric scale and an important grain refinement of the Al matrix was confirmed. After processing, a 66% increase on the mechanical response was reached with 1% of additive complemented with short milling intensities. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Today, there is a rising need for advanced materials in order to achieve a better performance for engineering materials that cannot be easily reached using conventional alloys. Thus, production and application of composite materials have increased in recent years [1]. Metal matrix composites (MMC) are engineering materials that contain at least two insoluble components: a metallic matrix (high ductility) and reinforcement particles (high stiffness) in the form of a particulate, short or continuous fiber with a well-defined interface between the matrix and the reinforcement phases [2]. These novel materials have the potential to provide tailored properties that are an average of the matrix and reinforcement properties for specific applications by adjusting the ratio of matrix and reinforcement [3]. Among many metals, aluminum (Al) has been preferred in many cases due to its relatively low density, reasonable mechanical properties, and good workability [4]. Due to its universal applications, AlMMCs are valuable alternatives for demanding industries as: transportation, aerospace, defense, etc., [5] because of the fact, that addition of small amounts of reinforcements particles in the metallic matrix, offers an important increase of mechanical properties compared to their unreinforced counterparts [6,7]. Some processes have been used for composites manufacture, which can coarsely be divided into three types: (a) liquid-state methods (pressure infiltration, stir casting, spray deposition and ⇑ Corresponding author.
in-situ processing), (b) semi-solid state methods (compocasting) and (c) solid-state methods (powder metallurgy, mechanical alloying). The first method involves melting and casting processes, however, this method was unable to give good homogeneity of dispersions in metal matrix because of the high interfacial energy between the molten metal and dispersoid [8]. It is extremely difficult to obtain a uniform dispersion of components especially in viscose molten alloys in semi-solid techniques where particles have high clustering tendency [9]. It is possible to produce a fine and homogeneous distribution of hardening particles with a very fine particle size in the metallic matrix [2] using the solid-state or mechanical milling (MM) route (via high-energy ball milling), avoiding the reinforcement-particles clustering [10]. This is important due that fine dispersion of the strengthened phase in the Al structure has a preponderant effect on mechanical properties of prepared composites [11]. The amount that the dispersoids strengthen the final composite depends of their specific characteristics as: particle type, size, morphology, volume fraction and their physical distribution. Also, formation of good interface between matrix and reinforcement is very important [6] and plays a key role on the overall composite performance. The technological challenge is to increase the bonding strength of the interphase by controlling manufacture process based on some particle features as size, morphology and volume fraction. Oxides, carbures, nitrides as silicon carbide, boron carbide [12], titanium carbide [13] alumina [9,14] and hard metals such as titanium and tungsten have been used as reinforcement for aluminum composites [15] but for some
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applications, these materials are not appropriate due to their high cost, use of additives and complicated processing conditions. On the other hand, natural graphite (Gr) has received significant attention for its low chemical activity, low density, high melting temperature, excellent tribological properties, high availability and especially low cost. In order to improve the interfacial bonding between components, a metal coating with Cu, Ni, and Co, has been demonstrated to be a feasible method to solve this inconvenience [5], also can inhibit interfacial reaction under high temperatures during the fabrication process [16]. The present research deals with the synthesis of some Al–Gr composites, the effect of three different metals as Cu, Ni and Ag on the graphite and Al/MeGr interphase and the evaluation of process conditions as: type of additive, their concentration and milling intensity on the characteristics of the processed powders and their mechanical performance.
2. Experimental procedure The studied composites were formulated using as raw materials pure powders of aluminum (99.5% purity and 44 lm, in size) and metallized graphite (MeGr) as reinforcing phase. Pure elemental powders of natural graphite (99.9%, 85 + 180), copper (99.5%, 100), silver (99.95%, 150) or nickel (99.8%, 44) were used for the MeGr preparation. Mixtures of Metal–Graphite powder mixtures (where Metal = Cu, Ag or Ni) with a concentration of 10 at.% metal were prepared and processed using a high-energy SPEX 8000M mill under an inert Ar atmosphere, selected milling intensities was 0, 1, 2, 4 and 8 h. The structure of Ni, Cu and Ag coatings was evaluated by X-ray diffraction (XRD) analysis. Al–MeGr composites preparation was carried out milling mixtures of pure Al powder and MeGr particles with a concentration of 1.0 (wt.%) during five time intervals (0, 1, 2, 4 and 8 h) as shown in Table 1 (due to space restriction, only the better mechanical results of each family are showed). Methanol was used as a process control agent in order to avoid excessive Al agglomeration. Pure Al samples were used as a reference for comparison proposes. Cold-consolidated forms were obtained from the powders using a two-step pressing method: pressing the powders in a circular die (900 MPa) in one direction and rotating the die to press the form in the opposite direction in order to obtain an ideal compaction level (this condition was calculated using density–pressure curve using a universal machine and plotted the curve searching for an asymptotic zone [17]. Sintering process was carried out with a heating ramp of 10 K/min until 723 K for 3 h under an Ar atmosphere. The morphology, size and metal-particle distribution of milled powder was examined using a JEOL-JSM 7201F SEM/EDS. X-rays diffraction (XRD) analysis was carried out using a Pan Analytical X’pert PRO X-ray diffractometer with Cu Ka radiation. Rockwell F hardness was measured on polished samples using a 1/1600 ball at a load of 60 kg and converted to Vickers scale.
At least five hardness measurements were done and the average is reported. Compression tests were done at room temperature in an Instron Universal tester and 0.2% yield strength (ry) was measured.
3. Results and discussion 3.1. Microstructural characterization The Fig. 1 shows the effect of milling on the general morphology of pure Al powder. As-mixed sample (Fig. 1a) presents semi-spherical particles, typical of gas-atomized metals. Milled products exhibit particles with higher size compared with an un-processed sample (0 h) due to Al ductile characteristics. After 4 h of milling (Fig. 1b), the metal particles are plastically deformed by highenergy collisions between grinding media and container. Morphology changes from spherical to irregular with large aggregates formed at the expenses of trapped minuscule particles are evident. Further milling (8 h) hardened large particles lead to fracture process activation (Fig. 1c), forming flake-like sub-particles on the material surface. The Fig. 2(a–c) shows the morphology of pure Gr particles and the effect of milling on its physical characteristics. Contrarily to the ductile behavior of Al, Gr is fragmented into tiny pieces as a consequence of high impact forces and defoliation evidence is observed after 8 h of milling (Fig. 2c). The Fig. 3a shows the morphology of CuGr metalized graphite particles; it is evident that particles are micrometric in size exhibiting a laminar structure. Bright zones correspond to zones with high metal concentration (Cu) surrounded by defoliated graphite layers, as high-magnification micrograph shows (bottom square). EDS analyses reveal the existence of an interphase composed of a mixture of Cu–Gr with a heterogenic composition creating an interlapping of elements as Shehata et al. described in their work [8]. In Fig. 3b a STEM micrograph shows the distribution of Cu and C in an isolated particle, graphite signal shows a rich distribution of carbon on the particle surface from one side to the center; while Cu signal is low reaching a maximum in the central part of the particle, indicating the presence of a metal core inside. Similar behavior was found with AgGr and NiGr particles. A possible mechanism involves the homogenization of spatial distribution of reinforcement particles at initial stages of milling by the control
Table 1 Nomenclature of metalized graphite and prepared composites.
a b c
Description
Nomenclature
Composition
Milling time (h)
Example
Milled Gr Metallized Gr
Grxh MeGrxh
x = 0, 1, 2, 4 and 8
Gr2 ha CuGr4 hb
Composite Al–Gr
Al–1%MeGrxh–M4
Pure graphite Graphite + Me Me = Cu, Ni, Ag. Al + Metallized Gr Al + MeGrxh
M = 0, 1, 2, 4 and 8
Al–1%AgGr8 hM4c
Gr milled 2 h. Cu/Gr milled 4 h. Al + 1% (wt.%) Ag/Gr milled 8 h and the mixture milled 4 h.
Fig. 1. SEM micrograph (2.5 Kx) of pure Al after (a) 0, (b) 4 and (c) 8 h of milling.
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Fig. 2. SEM micrograph (2, 5 and 20 Kx) of pure graphite after (a) 0, (b) 4 and (c) 8 h of milling.
Fig. 3. (a) SEM and (b) STEM micrographs of CuGr2 h particles.
20
30
40
50
60
70
80
90
20
311
004
002
Intensity (a.u.) 0h
222
220
311
8h
Intensity (a.u.)
222
220
Al C (Gr)
111
(b)
200
111
Al 200
(a)
30
40
50
8h
0h
60
70
80
90
2θ (Degrees)
2θ (Degrees)
Fig. 4. (a) XRD patterns of Al and (b) Al–1%CuGr2 h powder composites milled at different milling times.
process agent (MeOH in our case) allowing a better collision rate between particles and balls within the milling containers as Goya explains [18]. 3.2. X-rays diffraction (XRD) The XRD patterns of pure Al and Al–1%CuGr2 h composites processed from 0 to 8 h are shown in Fig. 4a and b. The signal of graphite (26.6° and 54.6°) is weak due to low concentration of reinforcement in the un-milled composite; after milling, both peaks disappear and are no longer visible. Similarly, as effect of milling, all Al peaks substantially reduced their size and became wider because the grain refining and internal stress accumulation induced by cold deformation [12]. The XRD patterns of composites show the same tendency of broadening and intensity reduction of the peaks, it means that the refinement of the aluminum grain is not affected by the addition of reinforcing particles (at least with this concentration). No new peaks appear and Al peaks do not shift (contrary to Nemati evidence [13] in Al–4.5 wt.%CuTiC system). This indicates that reinforcing particles do not react or form other phases with aluminum matrix staying insoluble in order to strengthen a material with a second phase. This behavior is beneficial because the mechanical
Table 2 Calculated lattice parameter of studied composites [nm]. Milling (h)
Alp
Al– 1%Gr2 h
Al– 1%CuGr2 h
Al– 1%NiGr2 h
Al– 1%AgGr2 h
0 1 2 4 8
0.404 0.404 0.405 0.405 0.404
0.405 0.404 0.404 0.404 0.404
0.405 0.404 0.404 0.404 0.405
0.404 0.404 0.404 0.404 0.404
0.404 0.404 0.404 0.404 0.404
properties are increased. This behavior is confirmed in Table 2 (results of 5 samples), where Al parameter was calculated using the main Al diffraction lines, showing an irrelevant variation not far from its reported value (0.404 nm) in all samples. 3.3. Mechanical characterization The reason to use a low concentration of reinforcement in this work is related to avoid the pores formation provoked by clusters as was reported by Nie et al. [5]. Other authors emphasize that the mechanical properties of a composite are related to the amount
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500
(a)
(b)
Stress (MPa)
100
Al 4h
400
Al 0h
Al-1%NiGr2h-M0 Al-1%AgGr2h-M0
Al 8h
300 Al 2h
Al 1h
Al-1%CuGr2h-M0
50
200 100 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 0.00
0.01
0.02
0.03
0.04
0.05
Strain (mm/mm) Fig. 5. Compression strain–stress curves of composites (a) Al pure reference at different milling times and (b) magnificated square of un-milled Al–MeGr composites.
500
AlAgGr1-2h Alp 4h
Stress (MPa)
400
AlGr8-2h
300
200
AlNiGr4-2h AlCuGr2-2h Alp 0h
100
0 0.00
0.05
0.10
0.15
0.20
0.25
0.30
Strain (mm/mm) Fig. 6. Best response strain–stress curves of each family (Gr, CuGr, NiGr and AgGr).
[19] and size of its reinforcements [1] upgrading their properties with higher concentration of reinforcement phase and/or decreasing its size [2]. Sameezadeh et al. [20] comments if nanoparticle content in a composite exceeds a critical value, the improvement in mechanical properties diminishes by saturation of grain boundaries and agglomeration. Following these observations, the mechanical properties of the sintered composites (1 wt.%) were evaluated by compression tests along consolidation direction and their yield strength was measured. Some strain–stress curves are shown in Figs. 5 and 6. As-mixed samples present a low mechanical response, being practically similar in all samples despite of MeGr addition (Fig. 5b). On the other hand, a remarkable increase on mechanical response in pure milled aluminum is observed; this effect is particularly evident with Alp-2 h sample (Fig. 5a). This behavior agrees with Karbalaei et al. [9] research where the compressive strength of the composites was higher compared with a non-reinforced alloy due to strengthening effect of nanoparticles on aluminum matrix. Yang et al. [22] comments that the increase in toughness is believed to be associated with the homogenous
dispersion reinforcement particles within the matrix and its strong interface between them and the matrix. It is evident a reduction in the elongation in all samples when comparing the curves of samples after 1 h of milling (Fig. 5a). This effect is due to an increase in the required stress for dislocations movement between the nanoparticles or can be caused from problems with particle packing, which can produce ductility loss [12]. Therefore, mechanical properties of porous composites generally depend in a macro level on their density. Moreover, porosity has been considered to be a critical factor deteriorating mechanical properties of materials. Additionally, El-Kady et al. [1] mentions that the degradation of mechanical properties of the composites is caused by interphase problems in a micro level where particles cannot transfer loads from soft matrix to the hard reinforcements. From strain–stress curves (Fig. 6), the yield strength was calculated observing that Al–1%CuGr2 h–M2 composite presents the highest value (66% of increase compared with pure Al sample processed under same conditions), with un-milled samples negative values appeared even though with reinforcements addition, this is because reinforcement particles do not form a consistent structure with the matrix, thus the interface is weak and the distribution of loads is not efficient. According to Van Dick, this condition rather than improving the properties deteriorates them since this poor union may create pores or initiate fractures [21]. Yield strength values (Table 3) can be compared to two commercial alloys: the 6000 series with 280 MPa and 4000 series (295 MPa) after T5 thermic treatment. Hardness is a physical parameter that implies the material’s capability to resist local plastic deformation. In MMCs, dispersed nanoparticles act as reinforcing phases becoming the obstacles to the movement of dislocation when plastic deformation occurs [8]. Fig. 7 presents a plot with hardness values measured on the composites as a function of milling time. Milled samples (1 h) are harder than un-milled ones; this is a direct consequence of work hardening despite the addition of MeGr as described before. As it can be seen, milling process causes a raise in hardness of powders; MM leads a microstructure refinement resulting in a
Table 3 Calculated yield strength (MPa) of the best composites compared with pure aluminum sample. The numbers between parentheses indicate the property increment (%). Values in bold, italics and underline are the highest response reach of each composite family of samples. Milling (h)
Alp
Al–1%Gr8 h
Al–1%CuGr2 h
Al–1%NiGr4 h
Al–1%AgGr2 h
0 1 2
85.9 235.9 233.3
81.0 (5.8) 281.4 (19.3)
93.7 (9.0) 321.5 (36.3)
83.1 (3.3) 293.6 (24.5)
82.8 (3.6) 295.2 (25.2)
4 8
245.5 238.7
353.4 (51.4) 312.9 (27.5) 305.9 (28.1)
387.3 (66.0) 341.3 (39.0) 341.2 (42.9)
379.7 (62.7) 341.7 (39.2) 329.5 (38.0)
323.0 (38.4) 321.3 (30.9) 321.9 (34.9)
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Al–1%CuGr2 h–2 h sample. This behavior is in a good agreement with the evidence found in compression curves. As it was mentioned before, the particle size not only has an important effect on densification, porosity but also in the mechanical behavior of composites. Therefore, an optimal distribution of particle sizes gives an increase on the final mechanical properties of composites.
100
Hardness (HV)
5
80 60
Al-1%Gr2h Al-1%CuGr2h Al-1%NiGr4h Al-1%AgGr2h Alp
40 20
3.4. TEM characterization
0 0
1
2
4
6
8
Milling Time (h) Fig. 7. Hardness determination as a function of milling intensity and dispersoid type.
nanocrystalline structure with high lattice microstrain. Moreover, the presence of reinforcing particles enhances the work hardening of the matrix. The pure Al hardness before milling was undetermined (value below selected scale). After milling, the hardness of composite powder increased rapidly to over 87 HV. But the hardness values fall with further processing, it can be assumed that behavior can be attributed to high grain refinement, large strain [12] and increased dislocation density [20] introduced by MM. At macro level, higher milling time induces particle agglomeration (big sizes) meaning poor compaction and high porosity finding a loss of property after 4 h of milling (2 h for Alp sample) in the composites. It is evident that MeGr addition increases the hardness of all composites compared to the pure aluminum alloy reference, because the presence of ceramic reinforcement has a preponderant effect on mechanical properties of composites [11]. This increase is expected because particulate reinforcements act as a barrier to the dislocation movement [15] within the matrix and exhibit greater resistance to indentation of the hardness tester [13,20]. According to the Orowan mechanism, this enhancement is also attributed because nanoparticles can restrain the growth of crystal grains and refine the crystal grains of the matrix [8]. Li et al. mentions that there are two strengthening mechanisms that are typically associated with conventional MMCs: load transfer from the metal matrix to the reinforcing particle (direct) and the influence of reinforcement on matrix microstructure (indirect) induced by the deformation mismatch between the reinforcement and the matrix [3]. Hardness increases with the addition of reinforcement particles as a function of preparation conditions reaching a maximum with
Composites reinforced by nanoparticles have a larger reinforcement/matrix interfacial area and a reduced interparticle spacing, which can transfer more loads from the soft matrix to the hard reinforcements improving the strength of the composites. Thus, the formation of a clean [3] and cohesive [6] interface between matrix and reinforcement is important to permit an appropriate load transfer. Likewise, in carbon nanotubes reinforced composites, Ahmad et al. [22] mentions that strengthening/toughening phenomena strongly depend on the interfacial bonding. Fig. 8 shows STEM micrographs of an Al–1%CuGr2 h–M2 composite. We can notice that CuGr particles (bright dots) are dispersed into the metallic matrix (Fig. 8a), a good linking between the reinforcement particle and aluminum matrix is evident. The Fig. 8b shows the interaction between the reinforcement particle and a dislocation line; it is not clear if the particle stops or initiates the dislocation. Higher magnifications TEM image (Fig. 8c) exhibit a Cu particle of copper perfectly covered by graphite since are neither cracks nor gaps between these two elements. This layer is so effective, so Cu was not dissolved into Al matrix during the thermal treatment maintaining it as a second phase. Thus, the carbon layer acts as an efficient barrier avoiding reactions between nano reinforcement particles and the metallic matrix working as an efficient load transfer system as mentioned.
4. Conclusions The method improves the mechanical properties of Al-composites reinforced with MeGr as a direct consequence of the uniform distribution of ceramic nanoparticles. A significant increase on the mechanical response of the specimens was evident with addition of reinforcement particles. A 66% increase on the mechanical response was reached with small amounts of reinforcement particles (1 wt.%) complemented with low milling intensities (2 h). Electron microscopy studies showed a uniform dispersion of the particles and good interaction with the metallic matrix. This interaction occurred by the formation of a thin layer of graphite on the metallic particle. The graphite layer acts as a barrier, generating good adhesion between the interfaces and avoiding unwanted dissolution of metal in the metal matrix.
Fig. 8. STEM Micrographs of Al–1%CuGr2 h–M2 composite with z contrast to evidence the elemental distribution of components.
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Acknowledgements CONACYT Project 169262 supported this research. Thanks to Redes Temáticas de Nanotecnología y Nanociencias Reg. 0124886 and 0124623; respectively. The technical assistance of A. Hernández-Gutiérrez, D. Lardizabal and E. Torres Moye is gratefully acknowledged. References [1] Omyma El-Kady, A. Fathy, Effect of SiC particle size on the physical and mechanical properties of extruded Al matrix nanocomposites, Mater. Des. 54 (2014) 348–353. [2] C. Suryanarayana, Synthesis of nanocomposites by mechanical alloying, J. Alloys Comp. 509S (2011) S229–S234. [3] Y. Li, Y.H. Zhao, V. Ortalan, W. Liu, Z.H. Zhang, R.G. Vogt, N.D. Browning, E.J. Lavernia, J.M. Schoenung, Investigation of aluminum-based nanocomposites with ultra-high strength, Mater. Sci. Eng. A 527 (2009) 305–316. [4] Heekyu Choi, Lin Wang, Dongkeun Cheon, Woong Lee, Preparation by mechanical alloying of Al powders with single-, double-, and multi-walled carbon nanotubes for carbon/metal nanocomposites, Compos. Sci. Technol. 74 (2013) 91–98. [5] Jun-hui Nie, Cheng-chang Jia, Na Shi, Ya-feng Zhang, Yi Li, Xian Jia, Aluminum matrix composites reinforced by molybdenum-coated carbon nanotubes, Int. J. Miner. Metall. Mater. 18–6 (2011) 695–702. [6] D. Özyürek, S. Tekeli, A. Güral, A. Meyveci, M. Gürü, Effect of Al2O3 amount on microstructure and wear properties of Al–Al2O3 metal matrix composites prepared using mechanical alloying method, Powder Metall. Met. Ceram. 49 (5-6) (2010) 289–294. [7] Zuhair M. Gasem, Fatigue crack growth behavior in powder-metallurgy 6061 aluminum alloy reinforced with submicron Al2O3 particulates, Composites Part B 43 (2012) 3020–3025. [8] F. Shehata, A. Fathy, M. Abdelhameed, S.F. Moustafa, Fabrication of copper– alumina nanocomposites by mechano-chemical routes, J. Alloys Comp. 476 (2009) 300–305. [9] M. Karbalaei Akbari, H.R. Baharvandi, O. Mirzaee, Fabrication of nano-sized Al2O3 reinforced casting aluminum composite focusing on preparation process of reinforcement powders and evaluation of its properties, Composites Part B 55 (2013) 426–432.
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