Effect of milling time on morphology and microstructure of ... - CiteSeerX

International Journal of Advances in Engineering Sciences Vol.4, Issue 2, April, 2014

Effect of milling time on morphology and microstructure of Al-Mg/Al2O3 nanocomposite powder produced by mechanical alloying A.Wagih Department of Mechanical Design and Production Engineering, Faculty of Engineering, Zagazig University, P.O. Box 44519, Egypt AMADE, Polytechnic School, Universitat de Girona, Campus Montilivi s/n, 17071 Girona, Spain Email: Ahmedwagih@ zu.edu.eg, [email protected]. Tel: +2-0106-1474023; Fax: +2-055-2304987

fracturing of the welded particles impedes the clustering of the particles promoting the transfer of the high ball collision energy to all particles and produces new, clean surfaces without oxide layers accelerating the diffusion [4, 5]. The most important advantage of this method with respect to other alloying methods is feasibility of addition of alloying elements for improvement of mechanical and physical properties of alloys. Ceramic nano-particles have received great attention owing to their property advantages over conventional coarse grained counterparts [6-10]. Among various reinforcements, Al2O3is one of the most widely used dispersoids in Al-based composites. There are some publications about the production of this type of composite using the mechanical milling method they are successfully incorporated microscale Al2O3 particulates in pure Al matrix using the mechanical milling process [11-15]. However, there are only few works on the effect of the addition of nanometric particulates on the structural and morphological behavior of Al powders. Al–Mg alloys as matrix are interesting because of their high specific strengths and good corrosion resistances [16, 17]. Mechanical alloying has been used previously for the preparation of a range of Al– Mg alloys with bulk Mg concentrations varying from 3 to 70 at% [16–18]. The equilibrium solid solubility of Mg in Al at room temperature is only about 1 at% [19], indeed, the solubility of Mg in aluminum has been increased substantially by the mechanical alloying process [20] Therefore, the objective of present work is the investigation of effect of milling time on morphological, and microstructural changes of Al/10 wt.% Al2O3 and Al–10 wt.% Mg/10 wt.% Al2O3 powder mixtures during the mechanical alloying process.

Abstract The effect of milling time on the microstructure and morphology of Al and Al–10 wt.% Mg matrix nanocomposites reinforced with 10 wt.% Al2O3 during mechanical alloying was investigated. Steadystate situation was occurred in Al–10Mg/10Al2O3 nanocomposite after 20 h, due to solution of Mg into Al matrix, while the situation was not observed in Al/10Al2O3 nanocomposite at the same time. For the binary Al–Mg matrix, after 20 h, the average crystallite size was 25 nm. Up to 20 h, the lattice strain increased to about 0.67% for Al–Mg matrix.

Key words Al–Mg matrix nanocomposite, Mechanical alloying, Scanning electron microscopy, X-ray diffraction, Crystallite size, lattice strain

1. Introduction Interest on powder metallurgy (P/M) aluminum metal matrix composites (MMCs) is increasing, since there is a potential field of applications in aerospace, chemical, transportation, structural and automotive industries. P/M aluminum MMCs have improved strength, high elastic modulus, increased wear resistance, low density, and high stiffness over conventional base alloys [1, 2]. High energy ball milling is a simple and useful technique for attaining a homogeneous distribution of the inert fine particles within a fine grained matrix [3]. During ball milling two essential processes occur, cold welding between the different particles and fracturing of the cold welded particles due to high energy collision [4]. The cold welding minimizes the diffusion distance between the atoms of the different components. The

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2. Materials and experimental procedure

mass ratio was about 20:1. Stearic acid (3 wt. %) was used as the process control agent to prevent excessive cold welding of powder particles.

2.1 materials Commercial pure aluminum powder with an average particle size of 80 µm and 99.5% purity (Figure 1), Al2O3 (99.5% purity 80 µm size) (Figure 2) and Mg powder of 99.98% purity with average particle size of (100μm) were used as raw materials for composite fabrication.

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a

b

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Figure 2 (a) SEM of Al2O3 pure powder (b) average particle size distribution.

2.3 Morphological analysis The powders produced after milling were investigated by using SEM Model Quanta 250 FEG (Field Emission Gun) attached with EDX unit (Energy Dispersive X-ray Analyses), with accelerating voltage 30 kV. Morphology, size and particle distribution were examined of the milled powders were quantified by visual basic software using several SEM images and their morphology was characterized by scanning electron microscopy.

Figure 1 (a) SEM of Al pure powder (b) average particle size distribution.

2.2 ball milling Pure powder were milled to form Al-xAl2O3, (x=5, 10 wt. %) composites in a Fritsch planetary ball mill, while confined in sealed 250 ml steel containers rotated at 250 rpm for 20 hr. in order to study the effect of Mg add, pure Al was milled with Mg and Al2O3 for 20 h to form Al-10Mg/10wt.% AL2O3 nanocomposite The container was loaded with a blend of balls (φ = 10, 20 mm). The total weight of the powder was about 25 g and the ball to powder

2.4 Structural evolutions X-ray diffraction (XRD) patterns were carried in a Rigaku-DXR 3000 X-ray diffractometer using Cu

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Kα radiation (λ = 0.15406 nm) at 30 kV and 30 mA settings. The XRD patterns were recorded in the 2θ range of 20–70° with a step size of 0.02° and a scanning rate of 1.5 degs./min.

have become more rounded in shapes and finer in average size.

The crystallite size and lattice strain of milled aluminum powders were estimated by XRD peak broadening using William–Hall method as follows [19]: (1) (1) Where B, λ, θ, D and ε are full width at half maximum (FWHM), the wave length, peak position crystallite size and lattice strain, respectively.

3. Results and discussion 3.1 morphology In Figure 3 one possible scheme of the mechanical milling process of this system is proposed. In the first stage of milling, the ductile particles undergo deformation while brittle particles undergo fragmentation. Then, when ductile particles start to weld, the brittle particles come between two or more ductile particles at the instant of the ball collision [21]. As a result, fragmented reinforcement particles will be placed in the interfacial boundaries of the welded metal particles, and the result is the formation of a real composite particle. As welding is the predominant mechanism in the process, the particles change their morphology by piling up the laminar particles. These phenomena, deformation, welding and solid dispersion harden the material and increase the fracture process, which also contributes to the equiaxed morphology. Welding and fracture mechanisms then reach equilibrium, promoting the formation of composite particles with randomly orientation interfacial boundaries. At the steady state, the microstructure undergoes a great refinement, and the interfacial boundaries are no longer visible by optical microscopy. Figure 4 shows the SEM micrograph for Al-10Mg/10Al2O3 nanocomposite powders mechanically alloyed for different milling Times. At the beginning of milling (from 2 to 5 h), the powders are still soft and cold welding predominates. Consequently, the particles size increases Figure 4(a,b) Particles shape has become flattened due to cold working effects during milling. Figure 4.c shows that after 10 h of milling, particles

Figure 3 the various stages of a ductile-brittle system during mechanical alloying and experimental SEM images for Al-Al2O3 nanocomposite powder.

That’s may be because of the predominant processes at this stage are cold working and fracturing with probably some recovery. Powder particles after 15 h seem equiaxed with almost the same size, but some coarse particles still remain as shown Figure 4.d, after 20 h of milling a balance is established between the cold welding and fracturing events and a steady-

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state situation is obtained as shown in Figure 4.e. It means that the distribution of Al2O3 particles in the Al–Mg matrix is very uniform at this stage [15]. And Figure 4.f shows the average particle size of Al2O3 particles after 20 h milling. The cold worked structure of Al matrix with high dislocation density provides suitable conditions for diffusion of bigger Mg atoms (Mg atoms have a larger atomic radius than Al atoms) along dislocation lines into Al lattice [5]. Additionally, the decreased particle size during milling reduces the diffusion distances between particles and facilitates diffusion. Diffusion is further aided by the increased defect density and a local rise in temperature (at Al and Mg interfaces). The combination of these effects would permit sufficient diffusion to occur in the interfacial regions of the nanocrystalline grains to form solid solutions [23].

and/or the low volume fraction of the Al2O3 phase [24].

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3.2 particle size

Figure 4 SEM micrographs of Al–10Mg/10Al2O3 powders milled for different times: (a) 2 h, (b) 5 h, (c) 10 h, (d) 15 h, (e) 20 h and (f) high magnification of 20 h.

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Al-10%Al2O3 Al-10%Mg/10%Al2O3

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Particle size (μm)

Figure 5 shows the variation of average particles size versus milling time for Al/10wt.% Al2O3 and Al–10% Mg/10wt.% Al2O3 powder mixture. It is observed that the trend shows a decrease in particles size as milling time increased. The effect of milling time on the particles size of ductile powders has been studied on pure and composite powders [3]. In all cases a similar trend in powder particle size was observed, i.e. an initial increase is followed by a decrease and then steady state in powder size was observed. This behavior can be attributed to the cold welding of initial ductile particles followed by work hardening and thus the fracturing of powder particles. When the rate of cold welding and fracturing processes equals, the steady state is achieved. The presence of reinforcement particles mixed with aluminum changes the mechanical milling classification to a ductile-brittle component system. It is obviously seen that at the same milling time the particle size of Al–10% Mg/10wt.% Al2O3 powder mixture is smaller than the particle size of Al / 10wt.% Al2O3 powder mixture that is can be explained as The cold worked structure of Al matrix with high dislocation density provides suitable conditions for diffusion of bigger Mg atoms (Mg atoms have a larger atomic radius than Al atoms) along dislocation lines into Al lattice [5].

100 80 60 40 20 0 0

3.3 Structural Analysis

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Millingtime(h)

X-ray diffraction patterns of Al–10Mg/10Al2O3 powder mixtures for different milling times are shown in Figure 6. With increasing the milling time to 20 h the Al2O3 lines reduced in height due to the reduction of alumina particles to submicron sizes

Figure 5 Variation of particles size as a function of milling time of Al/10%Al2O3 and Al-10%Mg/10%Al2O3 powder mixture.

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Also Mg peaks disappeared completely, which indicates the formation of the FCC supersaturated solid solution of Al–Mg (Al(Mg)ss)due to the diffusionn of Mg atoms into the Al lattice. This was

∆ □

(a) Intensity(A.U)

inferred by the clear shift of the Al peaks toward lower angles as shown in Figure 6.b, due to the solution of the larger Mg atoms into the Al matrix.

20 h

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∆ Al ● Al2O3 □ Al (Mg)ss □ ∆

15 h 10 h 5h 2h

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25

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Two-theta (deg)

Intesity(A.U)

(b) 20 h 15 h 10 h 5h 2h 37,8

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Two-theta (deg) Figure 6 X-ray diffraction patterns of Al-10Mg/10Al2O3 powder mixture for different milling time.

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Moreover, neither the presence of Al–Mg intermetallics nor unalloyed Mg was revealed from the diffraction pattern, which suggests completed solid solution formation. The results were in good agreement with previous findings [18, 20, 25, 26], in which only Al–Mg solid solution was present in alloys up to 30 at.% Mg. Furthermore milling to 20 h leads to broadening of the Al(Mg)ss peaks and decreasing of their intensities, which indicate reduction in crystallite size and accumulation of heterogeneous strain in the materials.

lattice strain (%)

0,7 0,6 0,5 0,4 0,3

Al/10Al2O3 Al-10Mg/10Al2O3

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120

Crystallite size (nm)

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Al/10Al2O3 Al-10Mg/10Al2O3

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Milling Time (h)

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25

Figure 8 Evolution of lattice strain for Al/10wt.% Al2O3 and

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Al–10Mg/10wt.% Al2O3 powder mixtures on several milling

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times.

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Figure 8 showing the effect of milling time on the lattice strain of composite powders. up to 10 h the lattice strains increase rapidly to about 0.4 and 0.66% for Al and Al–Mg matrix, respectively .It may be caused by the introduction of dislocations, vacancies, impurities and other lattice defects during milling [23]. Further milling to 20 h had no observable effect on micro strain level. Also for each milling time the micro-strain of alloyed matrix was higher than pure Al due to the presence of Mg. with diffusion of Mg atoms in Al lattice, interaction of these atoms with dislocations during milling leads to increase in dislocation density and subsequently strain accumulation in particles.

20 0

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Milling time (h)

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Figure 7 Evolution of crystallite size for Al/10wt.% Al2O3 and Al–10Mg/10wt. % Al2O3 powder mixtures on several milling times.

Figure 7 shows the effect of milling time on the crystallite size of composite powders. Up to 10 h, the crystallite size decreases rapidly to about 70 and 43 nm for Al/10Al2O3 and Al–10Mg/10Al2O3 composites respectively and from 10 to 20 h slow grain refinement occurs. Increase of crystal defects such as point defects and dislocations was caused by Plastic deformation of powder particles during milling [22, 27]. Moreover for each milling time, the crystallite size for Al–Mg matrix was lower than for Al. It can be due to the formation of Al–Mg solid solution with higher work hardening effect than pure Al which leads to the crystallite size reduction during milling and facilitation of nanostructure formation. The decrease in the crystallite size with milling time is attributed to dislocation generation caused by severe plastic deformation [23, 28]. Also in Al–Mg systems, the Mg concentration has a strong effect on the defect structure. With increasing nominal Mg content of the powder mixture the dislocation density increases [5].

4. Conclusion The results of the study of microstructure and microhardness of Al/10wt.% Al2O3and Al– 10Mg/10wt.% Al2O3 nanocomposite produced by mechanical alloying are remarked as belows: 1- For the binary Al–Mg matrix, alloying process completed after 10 h milling and the predominant phase was an Al–Mg solid solution. 2- Steady-state situation occurred in Al– 10Mg/Al2O3 nanocomposite after 20 h milling, due to solution of Mg in the Al matrix, which accelerates the mechanical milling stages. 3- By milling for 20 h, decreasing in the crystallite size (to 52 nm for Al and 25 nm

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for Al–Mg) and increasing in the lattice strain (to 0.45% for Al and 0.67% for Al– Mg) due to the dislocation generation are observed. On the other hand, profound effect of Mg on these variations is because of diffusion of Mg atoms in Al lattice and formation of Al-Mg solid solution which causes generation of more dislocations.

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