Effect of milling process on microstructural and properties of a ball-milled Al-based nanocomposite powder

1. Effect of milling process on microstructural and properties of
a ball-milled Al-based nanocomposite powder
Hossein Ramezanalizadeh
Department of Materials Engineering and Polymer, Faculty of Engineering, Hakim Sabzevari University,
Sabzevar, Iran
ABSTRACT
The goal of this study was investigation of milling process condi-
tions on microstructural and features of an aluminium matrix nano-
composite reinforced with 10 wt. % Al3Mg2. For this, the initial
powder mixtures were subjected to high-energy mechanical milling
in an attrition ball mill at different times (0, 2, 5, 7, 10, 15 and 20 h).
The phase composition, crystallite size and lattice strain of the
milled nanocomposite powders were characterised by X-ray diffrac-
tion analysis. Phase determination and the morphology of milled
powders were studied by using field emission scanning electron
microscope (FESEM) and transmission electron microscope (TEM) as
well as energy dispersive spectrometer (EDS). The results showed
that the crystallite size of nanocomposite powders decreases with
increasing milling time while saved lattice strain increases seriously.
In addition, according to the TEM results, a uniform distribution of
Al3Mg2 in the Al matrix was obtained after 15 h milling. The
agglomeration of nanocomposite reduced noticeably after milling
completion, too.
ARTICLE HISTORY
Accepted 25 February 2021
1. Introduction
Nowadays, composite materials have been synthesised in order to improve the properties
of metals that do not meet the requirements of specific applications. On the other hand,
metal matrix nanocomposites (MMNCs) are most promising in producing balanced
mechanical properties between nano- and micro-structured materials, i.e. enhanced
hardness, Young’s modulus, 0.2% yield strength, ultimate tensile strength and ductility,
due to the addition of nanosized reinforcing particles into the matrix [1].
One new type of candidates as reinforcing agents in MMCs is complex metallic alloy
(CMA), an intermetallic compound with giant unit cells, comprising up to more than
a thousand atoms per unit cell [2,3]. In particular, CMAs show many attractive properties
for reinforcing applications, such as high strength-to-weight ratio, good oxidation
resistance and high-temperature strength [4].
Powder metallurgy can produce metal matrix composites without suffering from
casting defects and segregation [4,5]. One type of powder metallurgy routes is mechanical
alloying/milling (MA/MM). The MA/MM process, using ball-milling technique, has
CONTACT Hossein Ramezanalizadeh h.ramezanalizadeh@hsu.ac.ir Department of Materials Engineering and
Polymer, Faculty of Engineering, Hakim Sabzevari University, P. O. Box 391, Sabzevar 9617976487, Iran
ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES
https://doi.org/10.1080/2374068X.2021.1896862
© 2021 Informa UK Limited, trading as Taylor & Francis Group

2. received much attention as a powerful tool for fabrication of several advanced materials
including equilibrium, nonequilibrium (e.g. amorphous, quasicrystals, nanocrystalline)
and composite materials [6–8].
In this study, Al matrix nanocomposite powders reinforced with intermetallic CMA-
type of Al3Mg2 particles as a new MMNC was successfully fabricated using mechanical
milling. Because the better understanding and final control of the next consolidation
process, as well as describing the mechanical behaviour of finish counterparts, could be
related to the powder properties, this study investigated the powder features of the
mentioned MMNC. In this regard, the effect of milling time on some powder character-
istics was mainly studied.
2. Experimental
High-purity aluminium powders and Al3Mg2 nanoparticles were used as starting
materials. The preparation of Al3Mg2 is described in [9]. The morphology of as-
received Al and Al3Mg2 powder particles is shown in Figures 1 and 2, respectively.
As can be seen, as-received Al powders had a random morphology, and their average
particle size was about 63 μm (Figure 1). The sizes of Al3Mg2 nanoparticles were
between 20 and 70 nm (Figure 2). 10 wt. % of Al3Mg2 nanoparticles were mixed with
aluminium powders and then mechanically milled up to 20 h to produce Al–Al3Mg2
nanocomposites. The AC10 code throughout the text refers to this composite. To
minimise the extreme cold welding of aluminium powders, 2 wt. % of stearic acid
was used as a process control agent (PCA) in all experiments. Ball milling was executed
using an attrition ball mill with the rotation speed of 400 rpm and ball-to-powder
weight ratio of 12:1. In this work, hardened chromium steel vial, containing steel balls
with a diameter of 10 and 6 mm, was utilised. To prevent oxidation during the milling
process, the vial was evacuated and then filled with pure argon gas. The Fe contamina-
tion of the powders during MM was traced by using inductively coupled plasma (ICP)
mass spectrometry.
Figure 1. FESEM micrograph of the as-received Al powder.
2 H. RAMEZANALIZADEH

3. Structural evolution of the as-milled powders was studied by X-ray diffraction (XRD,
Philips X’ Pert) using Cu Kα radiation (λ = 0.15406 nm) at 40 kV and step of 0.02°. The
crystallite size (D) and lattice strain (ε) of samples were estimated from the broadening of
XRD peaks using Scherrer method as follows [10,11]:
D ¼
0:9λ
B cos θ
(1)
ε ¼ B=4 tan θ (2)
where λ is wavelength = 1.54059 Å (CuKα radiation), B is the full width at half
maximum in radians and θ is the angle.
Transmission electron microscope (TEM) and field emission scanning electron micro-
scope (FESEM), equipped with an energy dispersive spectrometer (EDS), was employed
to investigate the morphology and particle size of the milled powders after different
milling times.
3. Results and discussion
3.1. Sem analysis
As mentioned in [12], the main process, which happens during milling, is the repeated
welding, fracturing and rewelding procedure of a powder mixture. The morphology of
the initial powders is expected to be changed when subjected to ball collisions. It is worth
Figure 2. Typical TEM micrograph of initial Al3Mg2 powder.
ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 3

4. noting that the effects of collisions on the milled powders depend on the type of the
constituent particles. It has been shown that the initial ball-powder-ball collision causes
the ductile metal powders to be flattened and then work hardened due to the cold-
welding and heavily mechanically deformation. They come in close contact with each
other and form a layered structure of composite particles consisting of various combina-
tions of the starting ingredients [12].
Of course, further milling results in cold welding and deformation of the layered
particles and a refined microstructure. Due to the initially low hardness of the
starting elemental powders, the lamellar spacing of the agglomerated particles is
quickly reduced upon more milling. As expected, increasing milling time increases
the hardness, and this leads to fracturing of the agglomerated powders into smaller
particles. In the next stage, the welding phenomenon predominates and causes the
formation of the equiaxed particle. Then, welding and fracture mechanisms reach
equilibrium, and this results in the formation of particles with randomly oriented
interfacial boundaries. The steady-state process, in which the microstructural refine-
ment can continue but the particle size and size distribution remain approximately
constant, means the final stage. The observations of high-energy mechanical milling
of Al powder in Al3Mg2 particles presence are in good agreement with this
description.
After short milling times (2 and 5 h) of Al-Al3Mg2 powder mixture, there is a pre-
dominance of deformed-flat particles (Figure 3(a,b)). It is important to notice that
intermetallic particles undergo plastic deformation with more frequency fragmentation
and a little agglomeration.
Then, before ductile particles are going to weld, the brittle particles locate between two
or more of them by the help of ball collisions. As a result, fragmented reinforcing
particles will be placed in the interfacial boundaries of the welded metal particles, and
this leads to the formation of a real composite particle [13]. After 7 and 10 h of milling
time, welded particles are observed clearly (Figure 3(c,d)), which indicates that welding is
the predominant phenomenon at this stage of milling. Therefore, the particle morphol-
ogies are changed by piling up the laminar particles. These phenomena of deformation,
welding and solid dispersion harden the material and so increase the fracture process,
contributing to form the equiaxed morphology [13].
Welding and fracture mechanisms then reach equilibrium, promoting the forma-
tion of composite particles with randomly orientated interfacial boundaries. Figure 3
(ef) depicts the microstructures of the composite powder particles at the steady state
after 15 and 20 h milling. As it can be seen, the morphology of powders changes
from flattened to the equiaxed type. From Figure 3(e,f), no change is observed in
particle morphologies and sizes with extending the milling time from 15 to 20 h. In
addition, Figure 4 shows the EDS analysis of nanocomposite after 15 h that confirms
the formation of Al/Al3Mg2 nanocomposite powders in this step of milling.
Therefore, the optimum value for milling time in this study could be 15 h. It should
be noted that the time of milling is a critical parameter to achieve optimum
properties of final products. Moreover, it should be considered that the level of
contamination increases and some undesirable phases form if the powder is milled
for times longer than required [12].
4 H. RAMEZANALIZADEH

5. 3.2. Tem analysis
Figure 5(a) shows the bright-field TEM image of AC10-2HM powder sample, in which
the Al3Mg2 particles have been dispersed in the nanocrystalline Al matrix. In addition,
the bright-field TEM image of AC10-15HM is shown in Figure 5(b), indicating the
homogeneous dispersion of the second phase in the matrix. Analysis of some parts
(point1) in Figure 5(b) is given in Figure 5(c), which corresponds to the composition of
Al3Mg2 nanoparticle. Also, the point analysis in other parts in the microstructure (point2
in Figure 5(b)), which contain Fe in the composition (Figure 5(d)), indicates the
contamination of the powder by Fe during the mechanical milling process. The con-
tamination amount for the AC10-15HM nanocomposite measured about 0.2 wt. % by
ICP mass spectrometry. It should be noted that in the case of aluminium and its alloys,
Figure 3. FESEM micrograph of the AC10 nanocomposite after (a) 2, (b) 5, (c) 7, (d) 10, (e) 15 and (f)
20 h of milling.
ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 5

6. there is a typical oxide layer that coats the powder surface [2]. Indeed, it is mentionable
that the amount of these Fe and O elements is not high enough to be detected by XRD
diffractometer and to affect the MMC final products seriously.
3.3. XRD analysis
Figure 6 shows the XRD patterns of AC10 samples after different milling times. Only two
phases, i.e. Al and Al3Mg2 were detected in the patterns of milled powders, following to
the card numbers (Al: 004–0787 and Al3Mg2: 029–0048) [14]. According to the card
number of 029–0048, the main peaks of Al3Mg2 with high intensity are at the 2θ of 37.52,
35.95 and 36.52, respectively. In the XRD pattern of Figure 6, all three mentioned 2θ are
available but due to the minor amount, small particle size and lower density of Al3Mg2 in
comparison with Al, the intensity of Al3Mg2 peaks is weaker than Al ones [4]. With
increasing milling time, the diffraction peaks of Al and Al3Mg2 become broader and their
intensities are weaker.
Crystallite size and lattice strain are important parameters for milled powders,
since they have significant effect on both compacting of the powders during sinter-
ing process and properties of the finally obtained aluminium matrix strengthened by
fine particles [4]. The average crystallite size of Al matrix in the composite was
estimated from broadening of XRD peaks. Figure 7 presents the effect of milling
time on the crystallite size and lattice strain of examined powder particles, indicat-
ing that the crystallite size decreases with increasing milling time.
In this work, the most intensive crystallite refinement occurs in the early stage of
milling, up to 10 h. With prolonged time, the crystallite size of the milled powders
decreases slowly. The lattice strain increases while crystallite size reduces with
Figure 4. The EDS analysis of AC10 nanocomposite after 15 h.
6 H. RAMEZANALIZADEH

7. increasing milling time due to the distortion effect caused by dislocation in the
lattice [4]. With increasing milling time, severe plastic deformation brings about
a deformed lattice with high density of dislocations [4].
4. Conclusion
Al reinforced with Al3Mg2 particles was prepared with the MM process. The evolution of
the microstructural during the MM process was investigated using XRD, SEM and TEM
analyses. Some of conclusions can be summarised as follows:
Ɣ The Al3Mg2 particles experienced a large reduction in size, <100 nm, and were
uniformly distributed in the Al matrix; no segregation or agglomeration was
observed.
Figure 5. Bright-field TEM micrographs of (a) AC10-2HM and (b) AC10-15HM powder samples, (c)
chemical analysis of the point1 in (b) corresponding to the Al3Mg2 composition and (d) chemical
analysis of the point2 in (b), indicating contamination of the powder by Fe during the mechanical
milling process.
ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 7

8. Figure 6. XRD patterns of AC10 powder composites produced after different milling times.
0
10
20
30
40
50
0
0.1
0.2
0.3
0.4
0.5
0 5 10 15 20 25
Crystallite&Strain vs Milling time
Crystallite size Lattice strain
Crystallite
size
(nm)
Lattice
strain
(%)
Milling time (h)
Figure 7. Crystallite size and lattice strain of AC10 samples at different milling times.
8 H. RAMEZANALIZADEH

9. Ɣ The crystallite size has been decreased while the lattice strain was found to be more
with increasing milling time due to distortion effect caused by dislocation in the
lattice.
Ɣ According to the TEM results, a uniform distribution of Al3Mg2 in the Al matrix
was obtained after 15 h milling
Disclosure statement
No potential conflict of interest was reported by the authors.
References
[1] Zhang Z, Chen DL. Consideration of Orowan strengthening effect in particulate-reinforced
metal matrix nanocomposites: a model for predicting their yield strength. Scr Mater.
2006;54(7):1321–1326.
[2] Ramezanalizadeh H, Emamy M, Shokouhimehr M. Wear Behavior of Al/CMA-Type Al3
Mg2 Nanocomposites Fabricated by Mechanical Milling and Hot Extrusion. Tribol Trans.
2016;59(2):219–228.
[3] Ramezanalizadeh H. Fabrication and characterization of an Al-based nanocomposite with
high specific strength and good elongation using large amount CMA nanoparticles. J Alloys
Compd. 2020;822:153667.
[4] Ramezanalizadeh H, Emamy M, Shokouhimehr M. A novel aluminum based nanocompo-
site with high strength and good ductility. J Alloys Compd. 2015;649:461–473.
[5] Kumar S, Singh R, Hashmi MSJ. Adv Mater Process Technol. 2020;6:13–24.
[6] Ramezanalizadeh H, Heshmati-Manesh S. Preparation of MoSi2–Al2O3 nano-composite via
MASHS route. Int J Ref Met Hard Mater. 2012;31:210–217.
[7] Ramezanalizadeh H, Heshmati-Manesh S. Mechanochemical Reduction of MoO3 Powder
by Silicone to Synthesize Nanocrystalline MoSi2. Adv Mater Res. 2011;264:1364–1369.
[8] Ramezanalizadeh H, Heshmati-Manesh S. Mechanochemical reduction of MoO3 powder by
Al and Si to synthesize nanocrystalline MoSi2. Int J Mod Phys Conf Ser. 2012;5:119–126.
[9] Ramezanalizadeh H. Adv Mater Process Technol. 2020. DOI:10.1080/
2374068X.2020.1794229
[10] Tang F, Anderson IE, Biner SB. Microstructures and mechanical properties of pure Al
matrix composites reinforced by Al;Cu;Fe alloy particles. Mater Sci Eng A. 2003;363:20–29.
[11] Smagorinski ME, Tzantrizos PG, Grenier S, et al. The properties and microstructure of
Al-based composites reinforced with ceramic particles. Mater Sci Eng A. 1998;244:86–90.
[12] Suryanarayana C. Prog Mater Sci. 2001;46:1–184.
[13] Fogagnolo JB, Robert MH, Torralba JM. Mechanically alloyed AlN particle-reinforced
Al-6061 matrix composites: powder processing, consolidation and mechanical strength
and hardness of the as-extruded materials. Mater Sci Eng A. 2006;426:85–94.
[14] Swanson HE, Tatge E. Natl. standard x-ray diffraction powder patterns. Bur Stand (US)
Circ. 1953;539:1.
ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 9