Materials Transactions, Vol. 49, No. 3 (2008) pp. 527 to 531
#2008 The Japan Institute of Metals
Thermal and Magnetic Properties of Mechanically Alloyed fcc Cu-Fe
Supersaturated Solid Solutions
Jean-Claude Crivello1 , Tohru Nobuki1 and Toshiro Kuji2; *
1
2
School of High Technology for Human Welfare, Tokai University, Numazu 410-0395, Japan
Courses of Materials Science and Chemistry Unified Graduate School, Tokai University, Numazu 410-0395, Japan
Copper and iron are immiscible elements according to the equilibrium phase diagram, but they can form metastable phases by mechanical
alloying process.
In this present work, mixtures of Cu-Fe powders in the range 0, 12, 25 and 40 atomic% of Fe have been prepared by ball milling. The
analysis of alloyed samples shows a single phase described in the fcc-Cu structure, except for the 40%-Fe compound, which presents the
additional bcc-Fe phase. The study of microstructure and magnetic properties under thermal treatments suggests a decomposition of the
metastable phase with increasing temperature.
In the Cu-richer domain, the fcc cell parameter increases with increasing Fe content. This effect is explained from the fact that the
ferromagnetic Fe phase is dispersed in the form of nanosized particles in the paramagnetic Cu matrix, in agreement with previous reports.
[doi:10.2320/matertrans.MRA2007189]
(Received August 2, 2007; Accepted December 25, 2007; Published February 20, 2008)
Keywords: copper iron alloys, mechanical alloying, non-equilibrium compound, microstructure, thermal and magnetic properties
1.
Introduction
Cu-Fe alloys are expected to be high strength and high
electric conductive materials.1) It is also indicated that these
alloys exhibit giant magneto-resistance and other outstanding
physical properties.2) But the applications associated to their
great potentials are limited since Cu-Fe alloys cannot form
stable phases: they are described in a peritectic system with a
miscibility gap.3) According to the Miedema model,4) the
enthalpy of mixing are largely positive (Hmix ’ þ13
kJ mol1 at 50–50 composition), in agreement with the
values calculated recently with an empirical potential description.5)
Mechanical alloying (MA), as for example the high energy
ball milling (BM), was proved to be useful to synthesize various phases even with positive mixing enthalpy.6–8) It has
been suggested that the energy stored in the large grain-boundary and interfacial surface areas serves as a driving force for
alloy formation. In the early 1990’s, several groups reported
the formation of metastable solid solutions in the Cu-Fe system by these techniques.9–12) It has been shown that the mechanical process reduces the crystalline size as the milling
time increases. More recently, the Cu-Fe alloys properties
were reviewed by several groups.13–15) Particularly, the magnetic properties of Cu-Fe have been studied extensively. It
should be noticed that fcc-Cu supersaturated solid solutions
with the intrusion of Fe atoms have a large magnetic moment
comparable to bcc -Fe.9,16,17) Ino et al.18,19) concluded that
the ferromagnetic state of the fcc Cu-Fe alloys is realized
by the expansion of the lattice in accordance with the
Bethe-Slater curve. In addition, Chien et al.20) have shown
that the Curie temperature is very sensitive to the initial composition. Furthermore, the annealing of the MA specimens
effects the decomposition of the solid solution into fcc-Cu
and Fe.21) However, all the thermal and magnetic experiments were performed at temperatures lower than 400 C.
*Corresponding
author, E-mail: tkuji@urchin.fc.u-tokai.ac.jp
The purpose of the present paper is to clarify the
dependence of the alloy stability towards temperature and
particularly according to its microstructure and magnetic
properties. Measurements of Curie temperature for two
samples Cu75 Fe25 and Cu60 Fe40 are presented with associated
magnetization measurements above 400 C.
2.
Experimental Details
The alloys were prepared from mixtures of Cu and Fe
powders (Kōjundo Chemical Lab., size < 150 mm, 99.9%
purity). The milling was conducted with 20 h of BM alloying
using a high energy Nisshin-Giken Super-Misuni NEV #8,
under argon atmosphere and a rotation speed of 720 rpm. The
vial and the balls were in zirconia. 14 balls (10 mm diameter)
were used with the ball to powder weight ratio (BPR) of
10 : 1.
After preparation of samples on the focused ions bean
apparatus (Hitachi FB-2000A), transmission electron microscopy (TEM) analysis was determined using high resolution TEM (Hitachi FE-TEM HF 2200TV), operating at
200 kV. The scanning electron microscopy (SEM) used is the
Hitachi S-4000 model (20 kV), equipped with the Horiba
EMAX-2770 EDX spectrometer.
The differential scanning calorimetry (DSC) measurements
were performed with a Rigaku DSC8230 instrument with a
heating rate of 10 K/min under flow of purified argon gas.
The powders were characterized by X-ray diffraction (XRD
type MAC science MXP3 and Shimazu XRD-6100) with
Cu K radiation at 40 kV–30 mA settings, 2 from 40 to
140 at room temperature, and 2 from 40 to 55 with
heating at 50, 150 and 300 C under vacuum. The resulting
profiles were refined with the Rietveld program Fullprof.22)
Magnetic hysteresis and Curie point measurements were
carried out using vibrating sample magnetometer (VSM)
apparatuses with a heating rate of 10 K/min.
528
J.-C. Crivello, T. Nobuki and T. Kuji
(a) 100% Cu
Intensity, I / arb. units
(b) 88% Cu + 12% Fe
Fig. 2 XRD powder diffraction patterns of Cu88 Fe12 after alloying, during
heating at 50-150-300 C, and after treatments at room temperature.
(c) 75% Cu + 25% Fe
(d) 60% Cu + 40% Fe (2 phases)
40
50
60
70
80
90
100
110
120
130
140
2 θ (°)
Fig. 1 Observed (points), calculated (line) and difference (bottom line) of
XRD powder diffraction patterns of Cu100X FeX milled for (a) X ¼ 0, (b)
X ¼ 12, (c) X ¼ 25, (d) X ¼ 40. The Bragg positions of fcc-Cu and bcc-Fe
are indicated by vertical bars.
3.
Results and Discussion
3.1 Structural and thermal analysis
The XRD patterns for the 4 samples after milling are
shown in Fig. 1. The single fcc-Cu phase remains for all the
compounds from 0% to 25% of Fe. On the pattern of the 40%
sample, bcc-Fe peaks appear and show a separation into two
phases. These results are not completely similar to previous
works,12,16,23) where it was found that the single fcc phase
extends up to 60% of Fe in fcc solution. By increasing the
atomic composition of Fe, the average intensity of peaks
decreases compared to the background line, which means that
crystallization process is poor. The second effect is the
decrease of fcc peaks position with smaller angles and
associated increase of fcc cell parameter. The fcc lattice
parameter expansion has already been reported by several
experiments.12,18) This effect cannot be explained by the
smaller atom size of Fe compared to Cu (radius 0.126 against
0.128 nm), but by the magnetovolume effect: when the
magnetic iron nanoparticules are alloyed into the sub-matrix
of copper, the interatomic potential for Cu has a stronger
core-core repulsion due to the filled d-band shell.20,24) This
effect leads to the extension of the fcc cell parameter.2,25)
The effect of the temperature on the lattice expansion was
also investigated. At low angles, the in situ X-ray diffraction
was made by gradual increase of temperature from 50, 150 to
300 C. As an example, XRD patterns of the 12% Fe
compound are presented in Fig. 2. Considering this sample,
the decomposition into two phases (fcc-Cu and bcc-Fe)
appears since annealing starts at 50 C. After all the heating
processes, irreversible decomposition remains at room
temperature. By the Rietveld method, fcc cell parameters
have been identified. Figure 3 sums up these results, obtained
with accuracies of 2 < 4 and refined agreement factors
Rwp < 9%. They show a linear increase of fcc lattice
parameter with increasing Fe content and with increasing
temperature. As a reference in Fig. 3, the value of nonalloyed Cu parameter is lower than all results found, except
for the ball-milled pure-Cu at room temperature.
With DSC analysis, the decomposition process was clearly
identified. Figure 4 shows DSC traces for 20 mg of the 25%
and 40%-Fe samples. Each presents an exothermic peak
respectively at TX ¼ 424 C and 392 C, corresponding to the
temperature of decomposition into two fcc and bcc phases
(results checked by XRD). The solution stability decreases
with increasing Fe-content. Thermal decomposition of the
supersaturated fcc solution has already been reported at
similar temperatures.10,12,16,21)
Thermal and Magnetic Properties of Mechanically Alloyed fcc Cu-Fe Supersaturated Solid Solutions
529
EXO
Fig. 3 fcc lattice parameter of Cu100X FeX according to the % Fe-content
at room temperature and during heating at 50-150-300 C.
Fig. 4 DSC traces from 20 mg Cu60 Fe40 (dashed line) and Cu75 Fe25 (line)
samples.
Fig. 5 SEM micrograph of the Cu75 Fe25 sample, after milling (a) and post-annealing (b).
3.2 Microscopy
Figure 5 shows an overview of Cu75 Fe25 powder on SEM
microscopy. The sample after milling (a) and after heat
treatment at 750 C for 1 hour (b) present particles with the
same size range (about 300 mm). Modifications on the
microstructure coming from heating can be identified by a
smaller scale: TEM bright-field micrographs and corresponding Z-contrast STEM images of the same sample present
significant differences (Fig. 6). Whereas the compound just
after milling (a,a0 ) has a homogeneous structure, the postannealing compound (b,b0 ) presents different regions of
contrast. The white frame in Fig. 6(b0 ) is analyzed in Fig. 7,
which contains elemental distribution maps. The corresponding EDX results are summarized on Table 1: the elements
composition analysis in Fig. 7 imply a composition very
close to the initial composition Cu:Fe (74.96:25.04). The
TEM micrograph shows a distribution of brighter regions
with a small size range (from 100 nm to 400 nm). EDX spot
analysis suggests that this brighter region corresponds to iron
particles. The darker region is richer in copper.
3.3 Magnetic properties
Shown in Fig. 8 are the hysteresis loops of magnetization
measured for 0.35 g of Cu75 Fe25 and Cu60 Fe40 powders. The
coercive field HC measured is respectively about 455 Oe and
365 Oe, which are larger than pure bcc-Fe under the same
conditions.
Figure 9 shows the magnetization at 5 kOe of the same two
25% and 40%-Fe samples according to temperature T, during
continuous heating and subsequent cooling. Magnetization
drops significantly near to 200 C, slightly lower than 2 other
measurements at the Cu50 Fe50 composition: 230 C for
Yavari et al.10) and 240 C for Jiang et al.26) By increasing
T higher than 400 C, a second magnetization drop shows a
typical signature of ferromagnetic iron at respectively
TC ’ 775 C and 780 C, close to the Curie temperature of
Fe (770 C). We concluded that this anomalous behavior is
justified by means of spinodal decomposition at inflection
point TX , which corresponds to the decomposition into bcc Fe
and fcc Cu predicted by DSC analysis at TX . This chemical
phase-separation was already found for richer Fe compositions.13,21,27)
4.
Conclusions
We have shown that the high-energy ball milling of
elemental powders can be used to alloy the immiscible
copper-iron system with formation of metastable solid
solutions. Regarding the solubility of Fe elements in
Cu100X FeX sub-matrix, we found that single fcc phase
occurs until X 25% of Fe. As reported previously, the
increase of fcc cell parameter by increasing Fe-content may
be attributed to magnetovolume effect. The effect of heating
causes iron particles to precipitate out of the copper matrix as
530
J.-C. Crivello, T. Nobuki and T. Kuji
Fig. 6 Bright-field TEM micrograph and corresponding Z-contrast STEM image of the Cu75 Fe25 sample, after milling (a,a0 ) and postannealing (b,b0 ).
Fig. 7 Bright-field TEM micrograph from Fig. 6(b0 ) with the corresponding distribution image of Cu and Fe.
Fig. 8 Hysteresis loops of magnetization measured at 5 kOe from 0.35 g Cu60 Fe40 (dashed line) and Cu75 Fe25 (line) samples.
Thermal and Magnetic Properties of Mechanically Alloyed fcc Cu-Fe Supersaturated Solid Solutions
Table 1 Chemical compositions measured in Fig. 6 and corresponding
spots EDX analysis of the Cu75 Fe25 sample.
area
spot 1
spot 2
% Cu-K
74.96
77.15
50.01
% Fe-K
25.04
22.85
49.99
Fig. 9 Magnetization according to the temperature at 5 kOe from 0.03 g
Cu60 Fe40 (dashed line) and Cu75 Fe25 (line) samples.
a decomposition process at temperature around 400 C. This
result was been shown in mutual agreements by several
analyses in the present work.
Acknowledgements
This work was partially supported financially by the
French Ministry for Foreign Affairs. The authors thank
Dr. Sumida for the help during the in-situ XRD analysis in
Tokyo University. Dr. Saito of Chiba Institute of Technology
and associate Professor Chiba of Tokai University are
acknowledged for their support in the instrumental VSM
analysis.
531
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