Materials Transactions, Vol. 44, No. 4 (2003) pp. 601 to 610
Special Issue on Platform Science and Technology for Advanced Magnesium Alloys, II
#2003 The Japan Institute of Metals
Solid State Synthesis of Non-Equilibrium Phase in Mg–Co and Mg–Fe Systems
via Bulk Mechanical Alloying
Tatsuhiko Aizawa1 , Ken-Ichi Hasehira1; * and Chikashi Nishimura2
1
2
Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo 153-8904, Japan
National Institute of Materials Science, Tsukuba 305-0047, Japan
Mg–Co and Mg–Fe systems were employed as a candidate hydrogen storage alloy. Different from Mg–Ni system, there exist no line
compounds of Mg2 Co and Mg2 Fe. Non-equilibration of these compounds is indispensable to make solid state synthesis. Bulk mechanical
alloying was applied to this non-equilibration of Mg2 Co with success. Planetary ball milling was also utilized to discuss the process efficiency of
bulk mechanical alloying. In particular, the on-line monitored energy density was used to describe the homogeneous refining and solid-state
reaction with increasing the number of cycles. Through SEM observation of intermediate phase change, the solid-state reaction commences
when the total energy density exceeds the critical limit. SEM/EDX and XRD analyses assured that the synthesized non-equilibrium phase should
be Mg2 Co. The Goldschmidt-factor analysis was used to determine that the synthesized Mg2 Co has mainly fcc-structure. No significant change
of XRD profiles was observed even when increasing the holding temperature. This Mg2 Co is quasi-stable, non-equilibrium phase even at the
elevated temperature. In case of Mg–Fe system, the initial elemental particle mixture was homogeneously refined. Under the similar condition to
the solid-state synthesis of Mg2 Co, however, Mg2 Fe was not synthesized even via bulk mechanical alloying. Through precise analysis, nonequilibrium phase with high iron content was recognized, so that non-equilibration via the bulk mechanical alloying might well be effective to
investigate the solid state synthesis of binary compounds even in Mg–Fe system.
(Received November 13, 2002; Accepted February 5, 2003)
Keywords: non-equilibration, bulk mechanical alloying, solid-state synthesis, Mg–Co, Mg–Fe, Mg2 Co, hydrogen absorption
1.
Introduction
As illustrated in the periodic table, nickel, cobalt and iron
have common physical and chemical properties. Table 1
listed the typical features common to these three elements.
Since they are typical, transient metallic elements, they are
widely used in various applications especially as ferromagnetic materials.1) Since magnesium is a para-magnetic
element, the magnetic behavior for elemental mixture or
alloy in Mg–X (X = Ni, Co and Fe) system becomes an
indicator of refinement or reaction.
Pure magnesium with the purity of five to six nines, has
good corrosion resistance as reported in Ref. 2).
Reference 3) told that magnesium alloys indicate severe
corrosion when the content of nickel, cobalt or iron exceeds a
limit. Typical example is seen in Mg–Fe system: small
addition of iron into magnesium exaggerated the corrosion
rate. Considering that Mg + 2(H2 O) ! Mg(OH)2 + H2 takes
place in this corrosion reaction, this significant sensitivity to
amount of additive elements must be attributed to the
Table 1
Physical properties of nickel, cobalt and iron.
Nickel
Cobalt
Iron
(Ni)
(Co)
(Fe)
28
27
26
Atomic mass
58.7
58.9
55.8
Atomic radius/nm
0.124
0.125
0.126
Melting point/K
1726
1768
1808
Crystalline structure
fcc
hcp
bcc
Saturated magnetization/G
484
1422
1714
Item
Atomic number
*Graduate
Student. The University of Tokyo.
catalytic role of nickel, cobalt and iron in the magnesium.
The nickel, cobalt or iron segregates form a local electrochemical cell to enhance the formation reaction of hydroxide.
In other words, the affinity of Mg–H can be controlled by the
affinities between Ni/Co/Fe–H.
Significant difference among these three elements can be
seen in their binary phase diagram against magnesium.3) As
compared in Fig. 1, the solid solubility of nickel, cobalt and
iron into magnesium is very low: e.g., <0:01 at% for nickel,
<0:001 at% for cobalt, and 0.00043 at% for iron. There are
two line compounds in Mg–Ni system: hydrogen absorbing
compound, Mg2 Ni and Laves phase, MgNi2 . One line
compound in the Laves phase is reported in Mg–Co system:
MgCo2 . No compounds exist in Mg–Fe system. Especially
both in Mg–Co and Mg–Fe systems, there are no equilibrium
phases other than original elements for the magnesium
content less than 50 at%.
To be interested, their hydrides are reported as a stable
phase: Mg2 NiH4 , Mg2 CoH5 and Mg2 FeH6 . As listed in
Table 2, the number of hydrogen atoms increases one-by-one
in the order of nickel, cobalt to iron. In Mg–Ni system,
Mg2 Ni corresponds as a metallic phase (or -phase) to
Mg2 NiH4 . While no metallic phases exist in the corresponding manner to Mg2 CoH5 and Mg2 FeH6 . This difference
suggests that the affinity of Co–H or Fe–H is sufficiently large
to control the affinity of Mg–H due to the weak bondage
between Mg and Co/Fe. This situation might well be
attractive to reduce the enthalpy barrier during hydrogenation
and de-hydrogenation and to reduce the operating temperature for hydrogen absorption and desorption. Hence, nonequilibrium, metallic phase in Mg–Co and Mg–Fe systems
must be attractive, not only to search for new types of
material system with hydrogen absorption and desorption,
working at the relatively low temperature, but also to
investigate how to modify the original hydrogenation–de-
602
T. Aizawa, K.-I. Hasehira and C. Nishimura
Table 2 Properties and performance of hydrides Mg2 XHy for X = Ni, Co
and Fe and for y ¼ 4; 5 and 6.
Mg2 CoH5
Mg2 FeH6
—Þ
—Þ
1.33
1.67
2.00
Dissociation heat
64 kJ/H2
86 kJ/H2
98 kJ/H2
X–H interaction
Small
Medium
Large
Item
Starting metallic
phase
H/M ratio
Þ
Mg2 NiH4
Mg2 Ni
(with C16 structure)
No metallic compounds were reported in literature.
hydrogenation processes of magnesium.4–6) In fact, the ballmilling type mechanical alloying was applied to synthesize
the non-equilibrium phases in Ref. 6).
In this paper, Mg–Co and Mg–Fe systems are employed to
investigate the possibility of solid state synthesis of metallic
compounds to be utilized as a starting phase in the
hydrogenation process. Bulk mechanical alloying (BMA) is
mainly used to synthesize the non-equilibrium phase in both
systems. Different from the synthesis of compounds in Mg–
Ni via BMA,7–10) the adequate chemical composition as well
as BMA-process conditions must be surveyed in the study. In
addition, the solid state reaction to non-equilibrium phase, or,
non-equilibration process, is also described objectively by
using the time history of applied energy density and the
change of magnetic properties during BMA. The planetary
ball milling is also utilized to evaluate the processing
efficiency via BMA. XRD (X-ray diffraction), SEM (Scanning Electron Microscope) and EDX (Energy dispersive Xray spectroscopy) are used to make structural analysis of the
synthesized phase. Hydrogen absorbing capacity is evaluated
from the pressure composition isotherm measurement as a
preliminary experiment.
2.
Fig. 1 Characteristic features of binary phase diagram for Mg–X
(X: Ni, Co and Fe).
Design of Non-equilibrium Phases for Hydrogen
Storage Alloys
In the material search for a candidate hydrogen storage
alloy, there are two ways to be selected. In the first approach,
the targeting hydride is once synthesized to investigate its
desorption process. As seen in Ref. 11), most of the
synthesized hydrides sometimes become too stable to release
the hydrogen even in partial. In the second way, the metallic
phase is first formed to investigate the hydrogen absorption
behavior with or without activation treatment. In Mg–Ni
system, Mg2 Ni was selected for solid-state synthesis to have
investigated the absorption behavior8) and to have discussed
the nickel enrichment effect on the modification of hydrogenation process.12) As had been stressed in Refs. 12) and
13), if the non-equilibrium phase materials or the amorphous
materials are used as a starting metallic phase, then, the
hydrogen absorption process is often enhanced to commence
the hydrogenation even at the room temperature, or, to have
low-temperature hydride phase. Furthermore, as had been
reported in Ref. 14), the starting materials might well be
designed to have polymorphous structure.
In the non-equilibrium phase, hydrogen absorbing material
design, there are another two ways. The first way is small
addition of second constituent element to hydrogen absorb-
Solid State Synthesis of Non-Equilibrium Phase in Mg–Co and Mg–Fe Systems via Bulk Mechanical Alloying
603
ing, pure magnesium. Just as seen in the results, where small
addition of nickel significantly modified the hydrogen
absorbing behavior of magnesium without reduction of
hydrogen storage capacity,15) small addition of cobalt or
iron leads to significant modification of original properties to
pure magnesium. In particular, remembering that iron has
very little solubility to magnesium and that corrosion of
magnesium is enhanced with iron segregates in magnesium,
super-saturated solid solution of iron into magnesium must
be a new way to change the original reactivity of magnesium
to hydrogen.
The second way is non-equilibration of compounds or
alloys having the pseudo-stoichiometric composition. As
pointed out in the non-equilibration of Mg2 Ni, it has a
relatively wide miscibility gap as the crystalline structure of
Mg2 Ni.12) Hence, since no compounds in the equilibrium
phase are present, Mg2 Co or Mg2 Fe have the possibility to
have wider miscibility gap as a compound. To be noticed in
the following experiment, nearly the same XRD profiles were
obtained for the synthesized product having higher cobalt
contents; various phases can be available to investigate the
Co-enrichment effect on the hydrogen-absorbing behavior of
Mg2 Co.
3.
Experimental Procedure
In the present experimental approach for solid-state
synthesis, two types of mechanical alloying were used to
make non-equilibration for Mg–Co and Mg–Fe systems. The
bulk mechanical alloying (BMA), was mainly utilized to
synthesize the non-equilibrium phase, magnesium base
cobalt and iron alloys. As had been discussed in Ref. 5),
the applied mechanical energy was on-line monitored to
describe the transient behavior of processed materials with
increasing the number of cycles. The planetary ball milling
was used here to discuss the possibility of solid-state
synthesis during the ball milling and to compare the
processing time between two different approaches.
Figure 2(a) showed the planetary milling apparatus. A vial,
housing the mixture of milling balls and initial powders with
the specified ratio, was placed in this apparatus. The
controlling parameters are: rotational and revolving speeds
to accelerate the refining and solid-state reaction during
milling. On the other hand, the laboratory-scaled BMAsystem was depicted in Fig. 2(b). As had been insisted in
Refs. 16–18), the BMA-base powder metallurgy process is
composed of the above laboratory-scaled BMA for materials
search via the solid state synthesis and the productionoriented BMA for fabrication of targeting materials with high
efficiency.
As had been stated elsewhere,19–21) the repeated plastic
work is applied to the starting materials by controlling the
movement of lower/upper punches and dies. Table 3 showed
that 40 to 45% of total input energy is used for plastic
working. The rest was wasted for rigid motion of punches,
friction and heat losses. The loading pass schedule in BMA is
designed to have the materials inside the die cavity undergo
severe shear stress and pressure. In the laboratory-scaled
BMA, one forward extrusion mode and two compression
modes are scheduled into one cyclic loading; eighty to ninety
Fig. 2 (a) Outlook of the planetary milling apparatus. (b) Outlook of the
laboratory-scaled bulk mechanical alloying.
Table 3 Contribution of mechanical work and loss items to the total energy
input through the bulk mechanical alloying.
Contribution to total
applied energy
Specific energy per cycle
For AZ 91
Plastic work
40–45%
3.5 kJ/cycle
Rigid motion
55–50%
4.5 kJ/cycle
5%
0.6 kJ/cycle
Item
Heat loss
Frictional work
Total applied energy
minimum
—
100%
8.6 kJ/cycle
percentage of the plastic work (Wp ) is supplied in the shear
flow of materials.22) This Wp is given in general by
integration of (flow stress) (equivalent strain rate) over
the plastically deforming portion. Although the strain rate
distributes in the processed sample, the average strain rate is
prescribed by (threading speed in punching)/(stroke). Hence,
the average strain rate becomes constant when using the same
loading pass schedule in BMA. In the rough sense, Wp is
proportional to the flow stress. In fact, as depicted in Fig. 3,
T. Aizawa, K.-I. Hasehira and C. Nishimura
W(plastic) / MJ kg-1
604
Table 4 Estimate of the plastic work per a cyclic loading for refining and
mixing in BMA.
80
AZ91
60
Al-12%Si
Mg
40
Item
Al
20
BMA-Sample (N=200)
0
0
50
100
150
200
Yield Stress of Alloys, σ/MPa
Fig. 3 Dependence of the plastic work on the flow stress for various sample
materials.
Energy Consumption per Cycle, E / kJ
experimentally measured Wp increases linearly with increase
of the flow stress for various sample materials. Hence, if little
change takes place in the flow stress, the plastic work must be
nearly constant.
In BMA, total applied energy per a cycle is on-line
monitored all through the process and stored into disc as its
time history. Figure 4 depicted a typical time history of online monitored mechanical energy per a cycle in Mg–Co. In
the normal operation during BMA, mixing, refining and
homogenizing processes take place in materials under the
constant supply of energy density, so that the monitored
energy per a cycle should be nearly constant irrespectively of
the number of cycles. If these normal processes are followed
by solid solution formation, or, solid-state synthesis, the
increase of applied energy per a cycle is detected in its time
history.
Assuming that the whole sample could be in elasto-plastic
state, the plastic work can be roughly evaluated by integration of the plastic energy density during forward extrusion.
Ref. 21) suggested that the average shear strain rate might be
1 s1 ; necessary data were listed in Table 4. The flow stress
of cobalt and magnesium powder mixture with the molar
ratio of Mg70C030, was provided by using the mixture rule
and porous-media approximation:22) ¼ ð1 Þð0:7 Mg þ 0:3 Co Þ. denotes the porosity, and, Mg and Co ,
the flow stress of pure magnesium and cobalt, respectively.
Since the processing time for forward extrusion is two
seconds in BMA, the specific energy per a cycle, was
estimated by 780 J/cycle. Remembering that the plastic work
per a cycle (Wp ) is 40% of total specific applied energy (W),
W can be provided by 2 kJ/cycle. This assures, that
W ¼ 2{3 kJ/cycle in Fig. 4 is just corresponding to the
20
18
16
14
12
10
8
6
4
2
0
0
200
400
600
800
1000
1200
1400
Number of Cycles
Fig. 4
Typical time history of the monitored energy in Mg–X system.
Flow stress
Flow stress of powder mixture
Pure magnesium: 40 MPa
Pure cobalt: 80 MPa
Mg70Co30 mixture: 52 MPa
Average strain rate
1/s
Mass of processed sample
30 g
Volume of processed sample
7.5 cm3
Specific plastic power
390 J/s
Specific plastic work per a cycle
780 J/cycle
energy wasted for plastic work. In other words, the applied
energy of W ¼ 2{3 kJ cycle for N < 1750 is only responsible
for plastic work to make homogeneous refining and mixing.
Hence, no change could occur if W were kept constant in the
level of 2–3 kJ/cycle althrough the processes in BMA;
acceleration of mechanical energy supply must be indispensable to ignite the solid state synthesis.
As had been discussed in Refs. 4) and 5), the processing
condition must be optimized to make solid state synthesis
even via the bulk mechanical alloying. Both the initial molar
ratio and the relative density during processing were varied in
Mg–Co system: the cobalt atomic percentage from 10 to 40%
and the relative density from 80% to 65% T.D. In general,
when the initial cobalt atomic percentage is less than 20%, no
increase of Wp was detected in the online-monitoring during
BMA. In BMA with the constant relative density, 80% T.D.,
the friction loss is sometimes exaggerated too much to make
continuation of BMA. In the following experiments,
Mg70Co30 and Mg70Fe30 were employed as an initial
molar ratio of magnesium versus cobalt or iron. The relative
density was constant, 70% T.D. The blended of pure
magnesium and cobalt with the weight of about 30 g, was
poured into a die cavity and subjected to repetitive plastic
loading with the prescribed pass schedule.
4.
Experimental Results
Mg–Co and Mg–Fe binary systems were employed to
investigate whether non-equilibrium phase can be synthesized in the solid state.
4.1 Mg–Fe system
Magnesium granules with the purity of 99.99% and the
size of 2–3 mm, and iron powders with the purity of 99.9%
and particle size under #300 mesh, were blended and mixed
with the molar ratio of Mg70Fe30. This starting elemental
mixture was subjected to the cyclic loading during BMA with
the constant relative density of 70% T.D. Figure 5 depicted
the variation of XRD profiles with increasing the number of
cycles. With increasing the number of cycles up to
N ¼ 2000, the peak intensities both for Mg and Fe were
monotonically reduced; no new peaks were seen in their
XRD profiles. When N ¼ 5000, the peaks for iron can be
seen while the typical triplet peaks for magnesium were
invisible. To be noticed is that new peaks can be seen in XRD
profiles.
Fe(44.67)
Fe(65.02)
N=5000
Fe
(82.33)
Energy Consumption per Cycle, E / kJ
Solid State Synthesis of Non-Equilibrium Phase in Mg–Co and Mg–Fe Systems via Bulk Mechanical Alloying
605
20
18
16
14
12
10
8
6
4
2
0
N=2000
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Number of Cycles, N
Fig. 6 Variation of monitored energy per cycle with increasing the number
of cycles for Mg–Fe system.
N=1500
Mg
Mg
Mg
Mg
Mg
Mg
Mg Mg
Mg
Mg
70°
80°
N=1000
20°
30°
40°
50°
60°
2θ
Fig. 5 Variation of XRD profiles with increasing the number of cycles for
Mg–Fe system.
No significant increase of input mechanical energy was
detected in Fig. 6. This suggested that most of plastic work
during BMA might be wasted for refining and homogeneous
mixing of iron particles in the magnesium matrix. Figure 7(a)
depicted a typical microstructure of BMA sample processed
to N ¼ 5000. Fine particles with the diameter of 2–3 mm,
distribute in the matrix. As proven by EDX analysis in Fig.
7(b), main composition of these particles is iron while 6 at%
iron was detected in the magnesium matrix. This noticeable
solid solution of iron into magnesium corresponds to the new
phase in Fig. 5. Remembering that iron has very little
solubility into magnesium, this enhancement of solid
solubility might be intrinsic to microstructure control via
the bulk mechanical alloying.
4.2 Mg–Co system
The same pure magnesium granules as used in the above
were premixed with the cobalt powders with the purity of
99.9% and the particle size under #300 mesh. This powder
mixture is blended with the molar ratio of Mg70Co30 and
poured into a die cavity. In the similar processing conditions,
the above elemental mixture was BMAed to the specified
number of cycles. Figure 8 showed the variation of XRD
profiles with increasing the number of cycles (N). Up to
N ¼ 1750, the intensities of characteristic triplet peaks to
magnesium and cobalt, were reduced with N. When
N > 1750, the XRD profile changes itself significantly from
that for the refined powder mixture of magnesium and cobalt.
This implies that non-equilibration reaction commences for
N > 1750.
Figure 9 depicted the time history of on-line monitored
mechanical energy per cycle with N. Although the same pass
schedule was utilized all through the process, the energy per
cycle with 3–4 kJ/cycle in the initial stage began to increase
with N for N > 800 and became constant, about 9 kJ/cycle
for N > 1400. Considering that significant broadening of
peaks took place in the XRD profiles for N > 1000, this
increase of the applied energy must be caused by the increase
of flow stress during homogeneous refining of cobalt into
magnesium matrix. For N > 1200 in Fig. 9, higher specific
energy was applied to continue the bulk mechanical alloying
process. Hence, the solid state reaction takes place in this
high energy transfer condition.
Significant XRD profile change between N ¼ 1750 and
N ¼ 2000, implies that the solid state reaction should be
enhanced at this stage. Microstructure of BMA sample at
N ¼ 1750 was shown in Fig. 10. Local chemical compositions were analyzed by EDX with the spatial resolution of
about 1 mm. This microstructure was divided into three
regions: gray matrix, light gray zone and white area. Owing
to EDX, the gray zone was a magnesium matrix, the white
area was a residual cobalt particle and the light gray zone was
Mg2 Co. Hence, when N ¼ 1750, the solid state reaction to
Mg2 Co commences together with homogeneous refining of
cobalt particles in the magnesium matrix. On the contrary,
when N ¼ 2000, most of starting materials were synthesized
to Mg2 Co together with small amount of residual cobalt
dotted in the reacted phase matrix, as shown in Fig. 11. This
assures that non-equilibration with solid state synthesis
commences in the early stage of high-energy transfer region
and the yield of synthesized Mg2 Co is significantly promoted
with increasing the energy density.
5.
Discussion
Mixture of magnesium with nickel, cobalt or iron is a
composite of para-magnetic and ferro-magnetic materials.
Hence, variation of magnetic properties with increasing the
number of cycles during BMA must be an indicator of
commencement of solid state synthesis. As had been
discussed in Refs. 23–25), Co–Cu system was employed to
describe the solid solution formation with increasing the
number of cycles in BMA by monotonic decreasing of
606
T. Aizawa, K.-I. Hasehira and C. Nishimura
Energy per Cycle, E / kJ
Fig. 7 Microstructure of BMA sample at N ¼ 5000: a) SEM micrograph and b) typical EDX analysis.
N=2000
N=1750
N=1250
N=1000
N= 750
40°
50°
16
14
12
10
8
6
4
2
0
200
400
600
800
1000
1200
1400
1600
Number of Cycles
Fig. 9 Variation of monitored energy per cycle with increasing the number
of cycles for Mg–Co system.
N= 250
30°
18
0
N= 500
Co
20
Mg
Mg
60°
Mg
70°
2θ
Fig. 8 Variation of XRD profiles with increasing the number of cycles for
Mg–Co system.
saturated magnetization. In that case, ferro-magnetic cobalt
forms the solid solution with non-magnetic copper, and, the
formed solid solution, Cu (Co), is also non-magnetic. Hence,
monotonic reduction of saturated magnetization with increasing the number of cycles, is just corresponding to
gradual solid solution formation in Co–Cu system.
Solid State Synthesis of Non-Equilibrium Phase in Mg–Co and Mg–Fe Systems via Bulk Mechanical Alloying
607
100
Magnetization /emu/g
500 < N <1750
80
N=500
60
N=2000
40
N=250
20
20
-40
-20
0
External filed
40
/kOe
-20
-40
Fig. 10 Microstructure of BMA sample at N ¼ 1750.
-60
-80
Fig. 12 Variation of magnetization curves with increasing the number of
cycles in BMA for Mg–Co system.
Fig. 11 Microstructure of BMA sample at N ¼ 2000.
Figure 12 depicted the variation of magnetization curves
against the applied magnetic field with increasing the number
of cycles. Except for the initial stage with N < 500, nearly
the same magnetization response was obtained for N < 1750.
The measured sample includes 30 at% cobalt, or, about
70 mass% cobalt. The saturated magnetization at the room
temperature (Ms) is 160 emu/g for cobalt. Since cobalt has
little solubility into magnesium, the theoretical bound of Ms
for the mixture sample is given by about 110 emu/g. The
magnetization curves with Ms = 80–90 emu/g for N < 1750
implies that the processed materials are homogeneously
refined but are still a mixture of magnesium and cobalt. Ms
commenced to decrease at N ¼ 1750 and it was significantly
reduced at N ¼ 2000 in Fig. 12. This reduction of Ms
indicates the onset of solid state reaction from refined
elemental mixture in Mg–Co to Mg2 Co, which might be in
the non-magnetic phase.
In previous studies,23–25) the coercive force, Hc was found
to be sensitive to the refined cobalt particles size (RCO ). In
particular, when RCO is reduced below the critical size, Hc
becomes nearly zero, and, the materials are in the super-para
magnetic state. Hc decreased slightly with increasing the
number of cycles but Hc ¼ 280 Oe even at N ¼ 2000. This
suggests that solid-state synthesis might take place even
when the refined cobalt size is relatively large enough to have
ferro-magnetization. The detail studies are necessary to
investigate the effect of cobalt particle size in refined
elemental mixture on the commencement of solid-state
synthesis.
Both the planetary milling and the bulk mechanical
alloying were utilized for particle size refinement and
solid-state synthesis. The processing time is often an issue
of discussion for industrial application of mechanical alloying process. Variation of XRD profiles with the elapsed time
was first compared between two processes. Figure 13
depicted a typical variation of XRD profiles obtained by
the planetary ball milling. It was found from comparison to
Fig. 8, that nearly the same intensities of magnesium as
observed at N ¼ 1750 in Fig. 8 were detected when
t ¼ 540 ks (150 h). Since one cyclic loading requires eight
seconds in BMA, the total time up to N ¼ 1750 was 14 ks or
3.9 h. Hence, the processing time can be reduced into about
1=40 of the milling time by using BMA. As before
mentioned, a production-oriented BMA system can be
utilized to further improve the production efficiency.
The point to be noted here is that no commencement of
solid-state synthesis was recognized in the planetary ball
milling even by increasing the milling time up to 900 ks
(250 h). This might be because the mechanical energy by
milling is insufficient to refinement of cobalt in Mg–Co
mixture and to ignite the reaction to Mg2 Co. In Ref. 6), solidstate synthesis of Mg2 Co via ball milling was detected.
Hence, applied energy control must be essential when using
the ball milling type of mechanical alloying. In case of the
bulk mechanical alloying, since the applied energy per a
cycle is constant, the totally applied energy (W) to the
processed samples monotonically increases with the number
of cycles. As shown in Fig. 9, the applied energy per a cycle
increases to enhance the solid reaction: W ¼ 4420 kJ at N ¼
608
T. Aizawa, K.-I. Hasehira and C. Nishimura
Mg
Co
tH = 36 ks
Mg
Mg
tH = 108 ks
tH = 180 ks
tH = 252 ks
tH = 324 ks
tH = 396 ks
tH = 468 ks
20°
40°
60°
2θ
Fig. 13 Variation of XRD profiles with increasing the milling time in the
planetary ball milling.
1750 and W ¼ 5674 kJ at N ¼ 2000. Remembering that 40%
of this energy is transformed to materials as a plastic work,
the total energy density (U) is given by 64.3 kJ/g at N ¼ 1750
and U ¼ 82:6 kJ/g at N ¼ 2000. Synthesis of Mg2 Co over
N ¼ 2000 implies that mechanical energy exceeding the
critical limit must be applied to materials to ignite the solidstate reaction. U ¼ 80 kJ/g might be an effective limit to
commence the solid-state reaction from the refined Mg–Co
mixture to Mg2 Co via the bulk mechanical alloying.
Table 5
Comparison of Bragg diffraction angles between the calculated by the Goldschmidt-factor analysis and the experimental results.
Model
bcc-random
distribution
fcc-random
distribution
Model/Experiment
Bcc-random
distribution
Fcc-random
distribution
Experimental
observation
In order to identify the crystalline structure of Mg2 Co, the
Goldschmidt factor analysis was used. In this analysis,
random distribution of constituent atoms in the specified
crystalline structure is assumed to estimate the lattice
constant, the lattice distance and the diffraction angles.
Direct comparison of the diffraction angles between estimation and experimental analysis, determines the most appropriate crystalline structure. In this simulation, the
Goldschmidt atomic radius for magnesium and cobalt, or,
R(Mg) and R(Co) were prescribed by R(Mg) = 0.16 nm and
R(Co) = 0.125 nm, respectively. Table 5 summarized typical
two simulations assuming bcc-random and fcc-random
distributions for Mg2 Co. The calculated diffraction angles
of 2 by assuming the fcc-random distribution, were in fairly
good agreement with the experimental data, so that the
synthesized Mg2 Co has mainly fcc-structure.
In general, the non-equilibrium phase material often
becomes unstable at the elevated temperature. Heating is an
important tool to accelerate hydrogenation process, so that
the candidate hydrogen storage alloys must have sufficient
stability in the operating temperature range. Figure 14
depicted the variation of XRD profiles with increasing and
decreasing the holding temperature. Little or no change of
XRD profiles was seen even at 773 K. Although MgCo2
could be synthesized, no MgCo2 peaks were detected even in
trace level; with increasing the holding temperature, peak
intensities of fcc-structured Mg2 Co increased. This implies
that the synthesized Mg2 Co must be stable up to 773 K and
full reaction to Mg2 Co takes place during heat treatment. To
be noticed, both bcc-structured and cubic-structured Mg2 Co
peaks were also detected with main peaks of fcc-structured
Mg2 Co. This suggests that Mg2 Co is possible to have
polymorphic crystalline structure.
As the preliminary study on the hydrogen absorption and
desorption behavior of the synthesized Mg2 Co, the pressurecomposition isothermal curves were measured at the room
temperature (293 K) without activation operation. Although
the hydrogen absorption rate was very slow without activation process, the hydrogen can be absorbed up to H/M =
Lattice
(100)-lattice
(111)-lattice
(110)-lattice
constant
distance
distance
distance
(nm)
(nm)
(nm)
(nm)
0.3325
0.1662
0.1439
0.2350
0.4199
0.2699
0.2424
0.1984
2-diffraction angle
2-diffraction angle
2-diffraction angle
of (100)
of (111)
of (110)
55.2
38.2
64.7
43.04
37.04
62.51
43.0
37.0
62.5
Solid State Synthesis of Non-Equilibrium Phase in Mg–Co and Mg–Fe Systems via Bulk Mechanical Alloying
Fig. 14 Variation of XRD profiles with increasing the holding temperature.
609
temperature. Different from the previous report in Ref. 6),
no new phases of Mg2 Co can be synthesized via the ballmilling type mechanical alloying. The energy criterion must
be effective as a necessary condition for full-reaction into
Mg2 Co through the solid-state synthesis via the bulk
mechanical alloying, or, the ball-milling.
No new phases like Mg2 Fe can be synthesized in Mg–Fe
system even by using the bulk mechanical alloying. Although
very little solid solubility of iron is allowed to magnesium,
significant solid solubility more than five atomic percent can
be seen in the synthesized magnesium matrix. The rest iron
particle size in this matrix was finely reduced to sub-micron
order. This suggests that chemical modification by solid
solution of iron into magnesium should be successfully
performed to control the hydrogenation and de-hydrogenation processes at the presence of iron in the magnesium.
Magnetization is thought to be a indicator to describe the
homogeneous mixing, refining and alloying processes in Mg–
X systems for X = Ni, Co and Fe. In the present paper,
different from Cu–Co system, fine correlation between
variations in the measured magnetization via VSM and the
XRD profiles with increasing the number of cycles in BMA,
cannot be seen. Further study is also necessary to consider the
solid-state reaction process of X-element into magnesium.
Acknowledgements
This study was financially supported in part by the Grandin-Aid from the MEST (Ministry of Education, Culture,
Sports, Science and Technology) on the priority group of
platform science and technology on the advanced magnesium
alloys with the contract of #11225205.
REFERENCES
Fig. 15 Comparison of XRD profiles before and after PCT measurements.
0.84–0.95, or, 2.7–3.0 mass%. Figure 15 compared the XRD
profiles before and after this PCT measurement. No change
observed in this comparison, assured that de-hydrogenation
process from Mg2 CoHx to Mg2 Co takes place normally.
Mg2 Co has mainly fcc structure and H/M is nearly unity; then
x ¼ 3 for Mg2 CoHx . This implies that only octahedral
interstices are occupied by hydrogen while tetrahedral
interstices are vacant. This characteristic feature was also
observed in the PCT measurement even when increasing the
holding temperature. Further precise study is necessary to
investigate the relationship between the interstitial occupation of hydrogen in Mg2 Co and its crystalline structure.
6.
Conclusion
Mg–Co and Mg–Fe binary systems were employed to
investigate the possibility of solid state synthesis of nonequilibrium, metallic phase material as a starting compound
for hydrogenation. Mg2 Co can be successfully synthesized
via the bulk mechanical alloying. The synthesized compound
might have a capacity of hydrogenation at the room
1) S. Iida, K. Ohno, K. Kamimae, H. Kumagai and S. Sawada, Eds.:
Handbook on Physical Properties (Asakura-Shoten, 1994) 124–128.
2) H. Tsubakino, A. Watanabe, A. Yamamoto and S. Fukumoto: Mater.
Sci. Forum 351 (2000) 235–240.
3) J. D. Hanawalt and C. E. Nelson: Trans. AIME 147 (1942) 273–277.
4) T. Aizawa and K.-I. Hasehira: Mater. Sci. Forum 419–422 (2002) 989–
994.
5) T. Aizawa, K.-I. Hasehira and C. Nishimura: Proc. 11th Int.
Symposium on Processing and Fabrication of Advanced Materials, X
(2002, Oct. Ohio) (in press).
6) J.-L. Bobet, E. Akiba and B. Darrier: J. Alloys Compd. 297 (2000) 192–
198.
7) T. Aizawa, T. Kuji and H. Nakano: J. Alloys Compd. 291 (1999) 248–
253.
8) T. Aizawa: Mater. Sci. Forum 351 (2000) 299–310.
9) T. Aizawa, T. Kuji and H. Nakano: Mater. Trans. 42 (2001) 1284–
1292.
10) T. Aizawa, T. Kuji and H. Nakano: Proc. 10th Int. Symposium on
Processing and Fabrication of Advanced Materials, ASM International
(2001) 164–183.
11) H. Takamura, T. Amemiya, A. Kamegawa and M. Okada: Mater. Sci.
Forum 351 (1999) 315–320.
12) T. Kuji, N. Nakano and T. Aizawa: J. Alloys Compd. 330–332 (2002)
590–596.
13) S. Orimo and H. Fujii: J. Appl. Phys. A 72 (2001) 167–186.
14) E. Akiba, H. Enoki and Y. Nakamura: Pro. 10th Int. Symp. Processing
and Fabrication of Advanced Materials, X, ASM International (2001)
199–213.
15) J.-I. Yagi, T. Akiyama and L. Li: Pro. 10th Int. Symp. Processing and
Fabrication of Advanced Materials, X, ASM International (2001) 223–
610
T. Aizawa, K.-I. Hasehira and C. Nishimura
237.
16) M. Mabuchi, K. Halada and T. Aizawa: Mater. Trans. 42 (2001) 285–
291.
17) K. Kondoh and T. Aizawa: Proc. Powder Metallurgy World Congress—2002, 1 (2002, June, Orlado) 115–128.
18) T. Aizawa, R. Tsuzuki and K. Kondoh: Proc. Powder Metallurgy World
Congress—2002, 1 (2002, June, Orlando) 137–148.
19) T. Aizawa, K. Tatsuzawa and J. Kihara: J. Faculty of Engineering,
University of Tokyo. XLII (1993) 164–183.
20) T. Aizawa, T. Luangvaranunt and K. Kodoh: J. Japan Inst. Metals 65
(2001) 581–588.
21) T. Luangvaranunt, K. Kondoh and T. Aizawa: Mater. Trans. 43 (2002)
1178–1182.
22) T. Aizawa, J. Kihara and D. J. Benson: Mater. Trans., JIM 36 (1995)
138–145.
23) C. Zhou, T. Aizawa and K. Tatsuzawa: J. Jpn. Powder & Powder
Metallurgy 46 (1998) 1202–1206.
24) T. Aizawa and C. Zhou: Mater. Sci. Eng. A 285 (2000) 1–7.
25) T. Aizawa and K. Kondoh: Scr. Mater. 44 (2001) 1751–1755.