Bulk amorphous and nanocrystalline Al83Fe17 alloys prepared by consolidation of mechanically alloyed amorphous powder

Article abstract of DOI:10.1016/j.jallcom.2009.10.177

In the present work, bulk amorphous and nanocrystalline Al83Fe17 alloys were obtained by consolidation of mechanically alloyed powders. Mechanical alloying of Al-17% Fe powder mixture yielded powder with an amorphous structure. Thermal behaviour of the milling product was examined using differential scanning calorimetry. This investigation revealed that the amorphous phase crystallised above 380 °C. The amorphous powder was compacted under a pressure of 7.7 GPa in different conditions: at 380 °C for 600 s and at 1000 °C for 180 s. Structural investigations of the bulk material revealed that the amorphous structure was retained after consolidation process applied at 380 °C. Compaction under high pressure at 1000 °C caused crystallisation of the amorphous phase and appearance of metastable nanocrystalline phases, different to those which crystallised during heating in the calorimeter under atmospheric pressure. The microhardness of the bulk amorphous sample is 760 HV0.2, whereas that of bulk nanocrystalline one is 630 HV0.2. The density of the bulk amorphous material is 3.505 g/cm³ and is ～3% lower than that of the nanocrystalline one. The open porosity of both consolidated materials is 0.4%. The results obtained show that the quality of compaction preserving amorphous structure as well as of the one yielding nanocrystalline structure is satisfactory and the products' microhardness is relatively high. The results also indicate that application of high pressure affects crystallisation of amorphous alloy influencing the phase composition of the products of this process.

Full text of DOI:10.1016/j.jallcom.2009.10.177

Journal of Alloys and Compounds 495 (2010) 382–385
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Bulk amorphous and nanocrystalline Al83Fe17 alloys prepared by consolidation
of mechanically alloyed amorphous powder
M. Krasnowski∗, T. Kulik
Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2008
Received in revised form 21 October 2009
Accepted 22 October 2009
Available online 31 October 2009
In the present work, bulk amorphous and nanocrystalline Al83Fe17 alloys were obtained by consolida-
tion of mechanically alloyed powders. Mechanical alloying of Al–17% Fe powder mixture yielded powder
with an amorphous structure. Thermal behaviour of the milling product was examined using differential
scanning calorimetry. This investigation revealed that the amorphous phase crystallised above 380 ◦C.
The amorphous powder was compacted under a pressure of 7.7 GPa in different conditions: at 380 ◦C for
600 s and at 1000 ◦C for 180 s. Structural investigations of the bulk material revealed that the amorphous
structure was retained after consolidation process applied at 380 ◦C. Compaction under high pressure
at 1000 ◦C caused crystallisation of the amorphous phase and appearance of metastable nanocrystalline
phases, different to those which crystallised during heating in the calorimeter under atmospheric pres-
sure. The microhardness of the bulk amorphous sample is 760 HV0.2, whereas that of bulk nanocrystalline
one is 630 HV0.2. The density of the bulk amorphous material is 3.505 g/cm3 and is ∼3% lower than that
of the nanocrystalline one. The open porosity of both consolidated materials is 0.4%. The results obtained
show that the quality of compaction preserving amorphous structure as well as of the one yielding
nanocrystalline structure is satisfactory and the products’ microhardness is relatively high. The results
also indicate that application of high pressure affects crystallisation of amorphous alloy influencing the
phase composition of the products of this process.
Keywords:
Amorphous materials
Nanostructures
Intermetallics
Mechanical alloying
Powder metallurgy
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
the form of a powder, thus consolidation is a necessary step for
milled powders for possible practical applications. Consolidation
The main advantages of aluminium based alloys are low den-
sity and a good corrosion resistance. Among the aluminium alloys,
iron aluminide intermetallics seem to be of technological inter-
est [1]. This is due to their advantageous properties, in particular
a high specific strength (strength-to-density ratio), high specific
stiffness, good strength at intermediate temperatures and excellent
corrosion resistance at elevated temperatures under oxidizing, car-
burizing and sulfidizing atmospheres [1]. A new class of Al-based
alloys, which has received a great deal of attention over the last
few years, are amorphous as well as nanocrystalline ones. Amor-
phous alloys can have unusual combination of properties such as
high strength, good ductility, high fracture toughness and good cor-
rosion resistance [2]. Nanocrystalline materials exhibit enhanced
properties, such as high strength and hardness [3,4], compared
to the materials with conventional grain size. Mechanical alloying
(MA) is a widely used processing route for synthesis of amorphous
and nanocrystalline materials [5]. However, MA products are in
of amorphous powders into bulk, full-density material providing
amorphous structure’s maintenance or creating nanocrystalline
one is not easy to achieve. Crystallisation of amorphous phase and
extensive grain growth occur during conventional consolidation at
elevated temperature. High temperature is required for good con-
solidation of powders into bulk materials, i.e. to remove all porosity
and to obtain good interparticle bonding, this is why the serious
challengeofcompaction processes is toretain theamorphous struc-
ture or to preserve nanocrystalline structure if such was formed
during consolidation. To fulfil this, the applying of a high pressure
during consolidation as well as limiting of the high temperature
exposure time should be taken into account. High pressure hot-
pressing method has been successfully used for producing bulk
nanocrystalline samples [6–11]. Recently, we demonstrated that
application of a high pressure influences grain growth at elevated
temperature by hindering it [8–10].
Amorphous aluminium-rich Al–Fe powder alloys have been pre-
pared using the MA process [12,13]. However, works devoted to
consolidation of these powders are not plentiful [14].
In the present work, we obtained bulk amorphous as well as bulk
nanocrystalline Al–17% Fe alloys by consolidation of amorphous
powders. The structural and phase transformations taking place
∗
Corresponding author. Tel.: +48 22 6608716; fax: +48 22 6608514.
E-mail addresses: makr@inmat.pw.edu.pl, mkrasnow@inmat.pw.edu.pl (M.
Krasnowski).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.10.177

M. Krasnowski, T. Kulik / Journal of Alloys and Compounds 495 (2010) 382–385
383
during the mechanical alloying, during subsequent heating of the
final milling product and during consolidation were also studied.
2. Experimental
Pure Al and Fe elemental powders of composition Al–17% Fe (all compositions
are given in at.% throughout this paper) were mechanically alloyed in a SPEX 8000
D high-energy shaker ball mill. The ball-to-powder weight ratio was about 10:1. In
order to minimize oxygen contamination the milling process was performed under
argon atmosphere.
The thermal behaviour of the milling product was examined by differential
scanning calorimetry (DSC) method using a PerkinElmer DSC7 calorimeter in a
temperature range from 50 to 720 ◦C at a constant rate of 40 ◦C/min.
A press equipped with a toroid-type high pressure cell was used for consoli-
dation of the milled powder. The shape of the cell and the material of the gasket
ensure that the compacting conditions were close to isostatic ones. The compaction
process was performed under a pressure of 7.7 GPa at a temperature of 380 ◦C for
10 min and at a temperature of 1000 ◦C for 3 min. The heating and cooling rate was
1000 ◦C/min.
The phase changes that occurred in the powder during milling as well as the
structure of the materials after heating in the calorimeter and after consolida-
tion were investigated by X-ray diffraction (XRD) method using a Philips 1830
diffractometer using CuK␣ radiation. The lattice parameter, the mean crystallite
size and the mean lattice strain, the latter two determined by the Williamson–Hall
method, were calculated from the XRD data taking into account CuK␣1 radiation,
after K␣2 stripping using the Rachinger method. The instrumental broadening was
determined using an Si standard and subtracted from the experimental breadth to
obtain a “physical” broadening of each diffraction line, which was then used for the
Williamson–Hall calculations.
A Hitachi S-3500N scanning electron microscope (SEM) as well as a Reichert
MeF2 light microscope was used for observation of the surface of the bulk sam-
ples. Samples for SEM and light microscopy were prepared using standard polishing
techniques.
The hardness of the compacts was measured using a ZWICK microhardness
tester under a load of 200 g imposed for 15 s. Vickers microhardness value was the
average of at least 25 indentations. The density of bulk samples was determined
using a Gibertini E154 balance equipped with a device for measuring the density of
solids (Archimedes method). The error in density determination was 0.5%. Basing
on mass measurements performed during density determination, open porosity of
bulk samples was calculated.
Fig. 1. XRD patterns of the Al83Fe17 powder mixture after various milling times
and after DSC examination of the final MA product.
was found in compositions with Al concentration ranging from 67%
to 83% during mechanical alloying of Al–Fe alloys in low-energy
conventional ball mill [12].
3. Results and discussion
Fig. 1 shows the XRD patterns of the powder mixture of Al–17%
Fe in the initial state, after various milling times and after DSC
examination (up to 720 ◦C) of the final MA product. On the basis
of these patterns, phase transformations occurring in the powder
samples during the MA process and during heating in the calorime-
ter can be analysed. The XRD results demonstrate that with the
increase of milling time, the intensity of the fcc Al diffraction peaks
decreases progressively with respect to that of the bcc Fe ones.
Simultaneously, the Fe diffraction lines become slightly shifted
towards lower angles, which indicates that the lattice parameter
of Fe has increased. These changes suggest that a bcc Fe(Al) solid
solution forms during this stage of the process. Another feature
which can be seen in this pattern is a broadening of the diffrac-
tion lines due to the reduction in grain size and the increase in
lattice strain. In the XRD pattern obtained after 25-h milling, one
can see broad Fe-based solid solution peaks, the residual Al peaks
and a small broad halo due to an amorphous component seeming
to overlap the most intense peak. After 35 h of MA, the Al peaks
vanish completely and only an amorphous halo and very weak
Fe(Al) peaks are visible. The next XRD pattern reveals amorphous
halo only, which indicates that further milling up to 50 h induces
complete amorphisation of the milled material. Thus, the product
of performed mechanical alloying of Al–17% Fe powder mixture
is an amorphous alloy. According to the Fe–Al phase equilibrium
diagram, for the concentration of 83% of Al there is a two-phase
FeAl3 + Al field. However, mechanical alloying is a non-equilibrium
processing technique, hence the phase composition of its products
can differ from that expected taking into account phase equilib-
rium diagrams. For example, an almost complete amorphisation
In order to study the thermal behaviour of the milling product,
the powder sample was examined in the calorimeter. Fig. 2 shows
the DSC trace for the powders after 50 h of the MA process. The
DSC curve reveals three exothermic peaks at temperature between
about 380 and 650 ◦C and endothermic one at 670 ◦C. The exother-
mic effects, or at least one of them, can be related to crystallisation
of amorphous phase, while the endothermic one to Al melting.
The powder sample after DSC examination was investigated
using the XRD method in order to define the phase changes that
occurred during heating. The diffraction spectrum of the final
milling product after heating in the calorimeter is shown at the top
of Fig. 1. Comparing this spectrum with the one obtained before
heating, one can see that diffraction lines appear, which confirm
the origin of the exothermic effects. These lines are attributed to
the Al13Fe4 phase (50-0797 ICDD card) and to fcc Al.
Fig. 2. DSC curve of the final milling product after 50 h of MA.

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M. Krasnowski, T. Kulik / Journal of Alloys and Compounds 495 (2010) 382–385
was retained during compaction. In Fig. 3b, for the sample pro-
cessed at 1000 ◦C, diffraction peaks are present, which indicates
that crystallisation of amorphous phase occurred during consolida-
tion. However, the diffraction lines in Fig. 3b cannot be assigned to
any phase in Al–Fe system [15] nor to any Al–Fe phase in the ICDD
PDF4 database, hence products of the crystallisation under high
pressure of the amorphous Al83Fe17 alloy are metastable phases.
Some of the peaks of the pattern in Fig. 3b can be indexed in cubic
system and attributed to a phase with bcc structure and unit cell
parameter of 2.975 Å. The estimated crystallite size and the mean
lattice strain of this phase are 36 nm and 0.1%, respectively. It is
worthwhile to comment on the pressure’s effect on crystallisation
ofthe amorphous Al83Fe17 alloy. Duringheating in thecalorimeter,
when material was under atmospheric pressure, the crystallisation
products were the Al13Fe4 phase and fcc Al, while during heat-
ing throughout the consolidation process, when high pressure was
applied, crystallisation yielded metastable phases different from
the ones mentioned above. Hence, in our case, we can infer that
application of high pressure affects crystallisation of amorphous
alloy influencing the phase composition of the products of this
process.
Fig. 3. XRD patterns of the bulk samples after consolidation of the final milling
product at (a) 380 ◦C and (b) 1000 ◦C.
Compacted samples were investigated using light microscopy
and SEM in order to verify the quality of consolidation. Fig. 4 shows
the images of the polished samples’ surface. One can see that the
surface is smooth and free of pores between bound powder parti-
cles, which evidences good quality of consolidation.
The consolidated material was also characterised using micro-
hardness, density and open porosity measurements. The average
Vickers microhardness value of the consolidated samples is equal
to 760 HV0.2 with a standard deviation of 23 (3.0%) and 630 HV0.2
The final MA product was subjected to high pressure consolida-
tion. Two temperatures were chosen for this process: 380 ◦C for the
purpose of providing amorphous structure’s maintenance during
compaction, and 1000 ◦C, which was usually used in our previous
works for consolidation of nanocrystalline powders [8–11,14]. The
XRD patterns of the consolidated materials are shown in Fig. 3. In
Fig. 3a, for the sample processed at 380 ◦C, only an amorphous halo
is visible. This indicates that amorphous structure of the material
Fig. 4. Micrographs of the polished surface of the consolidated samples: (a, b) amorphous material; (c, d) nanocrystalline material; (a, c) light microscopy images; (b, d) SEM
images.

M. Krasnowski, T. Kulik / Journal of Alloys and Compounds 495 (2010) 382–385
385
with a standard deviation of 37 (6.0%) for bulk amorphous and
nanocrystalline sample, respectively. No information about hard-
ness of a bulk amorphous or nanocrystalline Al–Fe alloys with
similar content of Al to that in this study has been found in
literature. However, data for some Al-based alloys are available
and can be quoted for comparison. For example, a microhardness
540 HV0.1 for the Al88Mm5Ni5Fe2 alloy with mixed amorphous-
nanocrystalline structure, consolidated using the same equipment
as in our work, has been reported [16]. For the hot-pressed
Al85Ni5Y8Co2 alloy with mixed amorphous-nanocrystalline struc-
ture, a microhardness value around 500 HV was obtained [17],
while for the hot-extruded Al88.7Ni7.9Mm3.4 alloy microhardness
253 HV was achieved [18]. On the basis of the quoted results, we
can assume that the microhardness of the bulk samples obtained
in this work is relatively high.
HV0.2, whereas that of bulk nanocrystalline one is 630 HV0.2. The
density of the bulk amorphous material is 3.505 g/cm3 and is ∼3%
lower than that of the nanocrystalline one. The open porosity of
both consolidated materials is 0.4%. The results obtained show
that the quality of compaction preserving amorphous structure as
well as of the one yielding nanocrystalline structure is satisfac-
tory and the products’ microhardness is relatively high. The results
also indicate that application of high pressure affects crystallisa-
tion of amorphous alloy, influencing the phase composition of the
products of this process.
Acknowledgements
The authors would like to thank Dr. S. Gierlotka (Institute of
High Pressure Physics of the Polish Academy of Sciences, Warsaw,
Poland) for assistance in performing the hot-pressing consolida-
tion, and Mrs A. Antolak-Dudka (Faculty of Materials Science and
Engineering, WUT) for assistance in SEM observations and in hard-
ness and density measurements.
The density values of the bulk samples are 3.505 and 3.604 g/cm3
for amorphous and nanocrystalline material, respectively. It is
known that the density of an amorphous phase is often a few per-
cent lower than its corresponding crystalline phase. This is because
the transformation from an amorphous to a crystalline phase
involves a negative total volume change due to excess volume in an
amorphous phase. In our case, the density of the amorphous mate-
rial is ∼3% lower than that of the nanocrystalline one. The open
porosity of both consolidated samples is 0.4%.
This work was supported by the Ministry of Science and Higher
Education (grant no. N507 057 31/1290).
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