Nanostructured B2-CoAl has been successfully synthesized directly by MA of nominal composition of
Co50Al50. The CoAl phase was found to be formed in an ordered structure via a gradual reaction which
completed in less than 10 h. However, further milling up to 45 h led to disordering of the CoAl with a relatively
constant long-range order parameter of 0.82. Lattice parameter measurements showed that disordering
of CoAl was caused by triple defect mechanism. The results of isothermal annealing showed that
the grains grew after isothermal annealing up to 0.7 Tm but still remained below 100 nm. The grain
growth behavior was well described by the parabolic kinetics equation. The grain growth exponent remained
constant above 873 K offering that grain growth mechanism does not change at high temperatures.
The grain growth exponents and activation energies offered the mechanism of diffusing Co and Al atoms in
the two separate sublattices at high temperatures. The equation 4 4 13
D D0 6 10 exp 175 RT t was
suggested to describe the grain-growth kinetics of nano-crystalline CoAl during isothermal annealing at
temperatures above 0.5Tm (873 K).
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3511

Enayati, M.H.

Hosseini, S.N.

Karimzadeh, F.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 1 No 3, 03CNN15(5pp) (2012) 
 
 
2304-1862/2012/1(3)03CNN15(5) 03CNN15-1  2012 Sumy State University 
Structural Evolutions of Nanostructured CoAl Intermetallic Compound during Mechanical 
Alloying and Subsequent Heat Treatment  
 
F. Karimzadeh,*, M.H. Enayati,†, S.N. Hosseini,‡ 
 
Department of Materials Engineering, Isfahan University of Technology, 84156-83111, Isfahan, Iran 
 
(Received 19 June 2012; published online 23 August 2012) 
 
Nanostructured B2-CoAl has been successfully synthesized directly by MA of nominal composition of 
Co50Al50. The CoAl phase was found to be formed in an ordered structure via a gradual reaction which 
completed in less than 10 h. However, further milling up to 45 h led to disordering of the CoAl with a rela-
tively constant long-range order parameter of 0.82. Lattice parameter measurements showed that disor-
dering of CoAl was caused by triple defect mechanism. The results of isothermal annealing showed that 
the grains grew after isothermal annealing up to 0.7 Tm but still remained below 100 nm. The grain 
growth behavior was well described by the parabolic kinetics equation. The grain growth exponent re-
mained constant above 873 K offering that grain growth mechanism does not change at high temperatures. 
The grain growth exponents and activation energies offered the mechanism of diffusing Co and Al atoms in 
the two separate sublattices at high temperatures. The equation 4 4 13
0 6 10 exp 175D D RT t  was 
suggested to describe the grain-growth kinetics of nano-crystalline CoAl during isothermal annealing at 
temperatures above 0.5Tm (873 K). 
 
Keywords: Intermetallic Compounds, CoAl, Nanostructures, Grain Growth, Mechanical Alloying. 
 
  PACS numbers: 71.20.Lp, 81.20.Ev 
 
 
                                                          
* Karimzadeh_f@cc.iut.ac.ir 
† ena78@cc.iut.ac.ir 
‡ golenarges63@yahoo.com  
1. INTRODUCTION 
 
The intermetallic compound CoAl with B2 structure 
is a promising material for many high-temperature 
applications due to its melting temperature (1648°C), 
density (6 086 Mg m3), moderate oxidation resistance 
and thermal conductivity [1]. A common feature of the 
B2 type CoAl phase is that it has a broad composition 
range and the highly ordered bcc structure of the CoAl 
phase is maintained up to its melting temperature [1-
2]. It is known that the physical and mechanical prop-
erties of brittle intermetallics can significantly alter by 
controlling the microstructure in the nanoscale. Nano-
crystalline materials can exhibit improved ductility and 
fracture toughness. Non-equilibrium processing tech-
niques such as mechanical alloying milling can produce 
metastable microstructure with the grain size reduced 
to nanoscale and therefore, this processing technique
has been extensively employed to synthesize interme-
tallics. These materials can be used as, for example, 
dispersoids, coatings, high specific surface area pow-
ders, functional nanodevices and engine parts [3-4].  
In general, MA of elemental metal blend may lead 
to the formation of the intermetallic, solid solution or 
amorphous phases [5]. In Co-Al binary system, it is 
reported that MA of Co50-Al50 led to the formation of 
CoAl intermetallic compound [6]. Although it has been 
reported that prolonged mechanical milling (MM) of 
fully ordered CoAl produced by other techniques than 
MA leads to partly disordering [7], no report exists on 
ordering-disordering behavior of CoAl during MA.  
Grain growth in polycrystalline materials, in gen-
eral, occurs to decrease the free energy of the system by 
decreasing the total grain boundary energy. In nano-
crystalline (nc) materials, high amount of grain bound-
ary leads to a significant fraction of high energy and 
provides a strong driving force for grain growth. Since 
the unique properties of these materials are closely 
related to the fine grain size and the large volume frac-
tion of grain boundaries, it is of vital importance to 
maintain the microstructure at a nanometeric scale 
during consolidation or service as structural compo-
nents at elevated temperatures [8-11]. With this re-
gard, many studies have been focused on the grain 
growth behavior of nc materials (e.g., see References 
[12-14]).  
Although different models were proposed grain 
growth models for nc materials taking into accounts of 
the impurities and solute drags, it was reported that 
some nc materials still obeyed the parabolic kinetic 
equation of grain growth for isothermal annealing in 
the form [15-17] 
 
 
0
n nD D Kt  (1) 
 
where D , 0D  and n  are the mean grain size at time t
, mean initial grain size, and the grain-growth expo-
nent, respectively. K is a constant that satisfies the 
following equation [15-17] 
 
 0 exp( )K K Q RT  (2) 
 
Where Q  is the activation energy required for grain 
growth and 0K , R  and T  are constant, gas constant 
and temperature, respectively. Eqs. (1) and (2) can be 
rewritten as [17] 
 0 0 exp
n nD D K Q RT t  (3) 
 F. KARIMZADEH, S.N. HOSSEINI, M.H. ENAYATI PROC. NAP 1, 03CNN15 (2012) 
 
 
03CNN15-2 
 
In addition, the rate of grain growth, dD dt  , can 
be derived as 
 
 log ( ) ( 1) log ( ) log logdD t dt n D t K n  (4) 
 
The relationship between log ( )dD t dt  and 
log ( )D t  gives the values of n  and K . Also, the activa-
tion energy Q  can be obtained from Equation (3) by 
plotting 
0log /
n nD D t  as a function of 1 T . A 
variety of n  values has been reported for various sys-
tems and the n  value can be a useful factor to describe 
the difference in growth mechanism between the differ-
ent systems [16]. So, the aim of the present work is 
mainly to investigate MA of the Co50-Al50 mixture and 
to investigate thermal stability and grain growth kinet-
ics of the nanostructured CoAl synthesized by MA. 
 
2. EXPERIMENTAL PROCEDURE 
 
MA of the Co-50at.%Al powder blend was carried 
out at ambient temperature in a planetary high energy 
ball mill, using hardened chromium steel vial and balls. 
No process control agent (PCA) was used. The related 
parameters for the MA are presented in Table 1. Iso-
thermal annealing was performed to investigate the 
grain growth kinetics of milled powders. Powder sam-
ples were sealed and then annealed in a conventional 
tube furnace. The annealing process parameters are 
displayed in Table 1. X-ray diffraction (XRD) technique 
by a Philips X’PERT MPD diffractometer (Cu Kα radia-
tion: λ  0 154 nm at 20 kV and 30 mA) was used to 
characterize the phase composition and structural 
changes of the powders. The Williamson–Hall formula 
was used to estimate the grain size and internal strain 
using broadening of XRD peaks [18]. 
 
Table 1 – Parameters for MA and heat treatment. 
 
Parameters Value 
Rotation speed (r min)  600 
Ball to powder weight ratio 15 1 
Ball milling time (h)  1, 3, 6, 10, 25 and 45 
Protective atmosphere Ar 
Annealing temperature (K)  873, 973, 1073 and 1173 
Holding time (h) 0.5, 1, 1.5 
Cooling mode Cooling in air 
 
3. RESULTS AND DISCUSSION  
 
3.1 MA of Co-50 at Al 
 
Fig 1 shows XRD traces for Co50Al50 powders as-
received and after different MA times. During MA the 
sharp crystalline diffraction peaks of the as-received 
powder broadened progressively with increasing pro-
cessing time associated with accumulated internal 
strain and refinement of grain size. The Co and Al grain 
size and lattice strain after 1 and 3 h MA, estimated 
from broadening of XRD peaks using Williamson–Hall 
formula [18], were presented in Table 2.  
 
 
Fig. 1 – The XRD patterns of the Co50-Al50 powder mixture 
at different MA time. 
 
It can be seen that after 3 h MA, the final grain size 
of Co is smaller and its lattice strain is greater than 
that for Al. Although the initial grain size was smaller 
for Co, this might be due to their different crystal struc-
tures. Cobalt (with hcp structure) is a material with low 
stacking fault energy in which dislocation slip is the 
dominant deformation mechanism. But it is reported 
[19] that the nanocrystalline cobalt deforms predomi-
nantly by mechanical twinning. Dislocation slip in the 
nanocrystalline cobalt is hindered by the small grain 
size and twinning (in despite of higher activation energy 
requirement) proceeds. Although the classical twinning 
process based on the pole mechanism would be difficult 
to apply for very small grain sizes, a recent molecular 
dynamics simulation study [20] showed that, at high 
stress levels, both heterogeneous (starting at grain 
boundaries) and homogenous nucleation of twins can 
occur for materials with grain sizes less than 50 nm. 
Homogeneous nucleation took place in the grain interi-
ors and involved the overlap of stacking faults of disso-
ciated dislocations. So, the material exhibits an unusu-
ally faulted microstructure [19]. However, in the case of 
nanocrystalline Al, the higher stacking fault energy for 
Al (200 mJ/m2) [21] in comparison with cobalt 
(31 mJ/m2)[19] and the tendency of Al (with fcc struc-
ture) to slip will make deformation based on twinning 
more difficult. 
 
Table 2 – Grain size and lattice strain of Co and Al at differ-
ent MA times. 
 
MA time  Co Al 
(h) d (nm) ε  d (nm) ε  
1 23 0.46 55 0 35 
3 9 0 63 20 0 53 
 
From 1 to 3 h MA, the powder particles were ag-
glomerated and large amount of powders were stuck to 
the surfaces of the container and balls. Further milling 
resulted in the release of coated Co and Al and appear-
ance of CoAl peaks. Transformation of elemental Co and 
Al powder mixture to the CoAl intermetallic phase was 
completed after 10h MA. The detection of superlattice 
 STRUCTURAL EVOLUTIONS OF NANOSTRUCTURED COAL… PROC. NAP 1, 03CNN15 (2012) 
 
 
03CNN15-3 
reflection (100) indicates that the B2-CoAl phase has an 
ordered structure. The long range order parameter (S) 
was calculated considering the (100) superlattice reflec-
tion and (110) fundamental reflection of cubic B2 struc-
ture formed by MA using the following formula [22]: 
 
 
100 110
100 110
obs
std
I I
S
I I
 (5) 
 
Where subscripts ‘‘obs’’ and ‘‘std’’ stand for observed 
and Standard ratios of the intensities of the mentioned 
diffracting atomic planes. Here standard ratio refers to 
that for a completely ordered structure.  
The S parameter, grain size and lattice strain of the 
B2-CoAl calculated for different MA times, are present-
ed in Table 3.  
 
Table 3 – Structural characteristics of the CoAl phase at dif-
ferent MA times  
 
MA 
time (h) 
d 
(nm) 
ε 
 
Lattice  
parameter (A○) 
S 
6 6 0 75 2 862 1 
10 7 0 36 2 862 1 
25 7 1 41 2 857 0 82 
45 6 1 63 2 856 0 82 
 
As can be seen the S value is equal to 1 during 6 to 
10 h milling suggests that formed B2-CoAl is fully or-
dered. The S value decreases and reaches a constant 
value of about 0 82 after long milling periods. The grain 
size of the formed CoAl is about 6 nm that remains ap-
proximately constant with further milling. These results 
are in agreement with the general observation that up-
on ball milling, for most intermetallic compounds a re-
finement of the internal grain size is observed to typical-
ly 5 to 20 nm together with an increase of atomic level 
strains typically between 0.7 and 2.5%[23] (see Table 3). 
The minimum grain size achievable by milling is deter-
mined by the competition between the plastic defor-
mation via dislocation motion and the recovery and re-
crystallization behavior of the material. This balance 
gives a lower bound for the grain size of the material. 
Besides, a number of process variables (such as method 
to estimate the grain size, milling intensity, milling 
temperature, alloying effects, contamination, etc.) can 
influence the minimum achievable grain size [7].  
In some instances, ordered intermetallics have been 
found to be formed directly on MA, although it is partic-
ularly true in Al-transition metal systems [7]. At the 
early stage of milling, nanometric grain sizes and high 
defect densities produced by MA accelerate the Co–Al 
interdiffusion process at room temperature. Therefore, 
CoAl phase starts to form after 6 h MA. The heat re-
leased from this reaction is consumed to release stored 
energy, which was incorporated during ball milling. So, 
the lattice strain of the B2-CoAl decreases from 0 75 to 
0 36%. After completion of CoAl formation, the lattice 
strain increases again and S value decreases on further 
milling due to severe plastic deformation. It is reported 
that disordering in B2-CoAl compound occurs with tri-
ple defect mechanism that results in reducing the lattice 
parameter [3,7,24-25]. So, triple defects formation can 
be responsible for decrease of lattice parameter from 
2 862 to 2 856 Aο. 
 
3.2 Heat Treatment 
 
In order to investigate grain growth, annealing of the 
as-milled powder was performed at different temperatures 
and durations. The corresponding XRD patterns are 
shown in Fig. 2. The grain size of nano-crystalline CoAl, 
calculated from XRD patterns, are plotted as a function of 
annealing time (see Fig. 3). 
 
 
 
 
Fig. 2 – XRD patterns of the isothermally annealed powders at a)873, b)973, c)1073, d)1173 and e)1273 K for 0.5, 1 and 1.5 h 
 
 F. KARIMZADEH, S.N. HOSSEINI, M.H. ENAYATI PROC. NAP 1, 03CNN15 (2012) 
 
 
03CNN15-4 
 
 
Fig. 3 – Grain size of the CoAl as a function of annealing time 
for different temperatures  
 
As it is expected, the grain size increases with increas-
ing the temperature. The grain size does not hugely vary 
with annealing time especially at T≥973 K. The grain size 
increases rapidly first and then almost evenly at the long-
er annealing times. This can be interpreted as a result of 
the decrease in the interfacial energy during grain growth 
[7]. The plots of log ( )dD t dt  versus log( )D  (Fig. 4) were 
obtained by fitting the data to Eq. (4) using a non-linear 
fitting route. The value of n  is about 3 for 873 K while 
reaches to a constant value of 4 at higher temperatures 
(T/Tm≥0.5). It is reported that n  value depends on grain 
growth mechanism [15-16]. So, it is appeared that grain 
growth mechanism of the nano-crystalline CoAl compound 
is the same at the temperature range of 973-1273 K. 
Normal grain growth of pure materials (n   2) is con-
trolled by curvature-driven  migration of grain bounda-
ries. When n   3, grain growth process is governed by 
volume diffusion. If n   4, the stochastic jumping of at-
oms across the grain boundaries is predominant. As ob-
served experimentally, the n  values usually ranges from 
2 to >5 for conventional pure metals as well as some of the 
MAed intermetallic compounds e.g. nano-crystalline Fe 
with the n  value in the range of 3-11 [9-10, 15, 17]. In 
conventional materials, the value of n  is dependent on 
both mobility and energy of the grain boundaries. Wang et 
al. [15, 17] reported that for nano-Fe, similar to coarse-
grained Fe, n  decreases toward 2 as T/Tm increases to 
≈0.6. The growth exponent n  obtained in this work indi-
cates that nano-CoAl shows high resistance against grain 
growth compared to nano-Fe or conventional coarse-
grained Fe. In addition, this value offers that the grain 
growth mechanism of the nanostructured CoAl at temper-
atures above 873K (T/Tm≈0.5) can be the random jump of 
atoms across the grain boundaries. Considering the fact 
that nanostructured CoAl consists of nanometer-sized 
grains and has a large volume fraction of grain bounda-
ries, grain growth through atom diffusion in these bound-
aries is expected much easier than that through conven-
tional lattice diffusion. 
By plotting 
0log /
n nD D t  versus 1 T  and fit-
ting a straight line to the data points (Fig. 5), the activa-
tion energy for grain growth of nano-crystalline CoAl 
compound was calculated to be 175±20 kJ/mol. This value 
of Q  is quite smaller than the activation energy of both 
interdiffusion in conventional CoAl (≈432 kJ/mol) [2] and 
self-diffusion of Co in conventional CoAl (≈416 kJ/mol) 
[26]. But, it is approximately close to the activation energy 
for diffusion of Co in Al (83 Kj/mol) [27] and also the acti-
vation energy of Al volume self-diffusion (144 kJ/mol) [10]. 
As mentioned before, the CoAl phase has a disordered 
structure with the triple defects formation mechanism in 
which CoAl structure is maintained by remaining Al at-
oms on their own sublattices. So, it can be said that the 
grain growth in disordered CoAl phase is accomplished by 
diffusing Co and Al atoms in the two separate Al and Co 
sublattices. 
 
 
 
Fig. 4 – Plots of log ( )dD t dt versus log( )D for CoAl phase 
at different annealing temperatures  
 
 
 
Fig. 5 – Plot of 
0log /
n nD D t  against 1000 T  for 
CoAl grains 
 
Consequently, the grain-growth kinetics of nano-
crystalline CoAl under isothermal annealing at tempera-
tures above 873 K can be well described using 
 
 4 4 130 6 10 exp 175 /D D RT t  (6) 
 
Eq. (6) provides the means by which the grain size of 
nano-crystalline CoAl can be controlled by simply govern-
ing the annealing temperature and time 
 STRUCTURAL EVOLUTIONS OF NANOSTRUCTURED COAL… PROC. NAP 1, 03CNN15 (2012) 
 
 
03CNN15-5 
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Structural Evolutions of Nanostructured CoAl Intermetallic Compound during Mechanical Alloying and Subsequent Heat Treatment

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 1 No 3, 03CNN15(5pp) (2012)   2304-1862/2012/1(3)03CNN15(5) 03CNN15-1  2012 Sumy State University Structural Evolutions of Nanostructured CoAl Intermetallic Compound during Mechanical Alloying and Subsequent Heat Treatment   F. Karimzadeh,*, M.H. Enayati,†, S.N. Hosseini,‡  Department of Materials Engineering, Isfahan University of Technology, 84156-83111, Isfahan, Iran  (Received 19 June 2012; published online 23 August 2012)  Nanostructured B2-CoAl has been successfully synthesized directly by MA of nominal composition of Co50Al50. The CoAl phase was found to be formed in an ordered structure via a gradual reaction which completed in less than 10 h. However, further milling up to 45 h led to disordering of the CoAl with a rela-tively constant long-range order parameter of 0.82. Lattice parameter measurements showed that disor-dering of CoAl was caused by triple defect mechanism. The results of isothermal annealing showed that the grains grew after isothermal annealing up to 0.7 Tm but still remained below 100 nm. The grain growth behavior was well described by the parabolic kinetics equation. The grain growth exponent re-mained constant above 873 K offering that grain growth mechanism does not change at high temperatures. The grain growth exponents and activation energies offered the mechanism of diffusing Co and Al atoms in the two separate sublattices at high temperatures. The equation 4 4 130 6 10 exp 175D D RT t  was suggested to describe the grain-growth kinetics of nano-crystalline CoAl during isothermal annealing at temperatures above 0.5Tm (873 K).  Keywords: Intermetallic Compounds, CoAl, Nanostructures, Grain Growth, Mechanical Alloying.    PACS numbers: 71.20.Lp, 81.20.Ev                                                             * Karimzadeh_f@cc.iut.ac.ir † ena78@cc.iut.ac.ir ‡ golenarges63@yahoo.com  1. INTRODUCTION  The intermetallic compound CoAl with B2 structure is a promising material for many high-temperature applications due to its melting temperature (1648°C), density (6 086 Mg m3), moderate oxidation resistance and thermal conductivity [1]. A common feature of the B2 type CoAl phase is that it has a broad composition range and the highly ordered bcc structure of the CoAl phase is maintained up to its melting temperature [1-2]. It is known that the physical and mechanical prop-erties of brittle intermetallics can significantly alter by controlling the microstructure in the nanoscale. Nano-crystalline materials can exhibit improved ductility and fracture toughness. Non-equilibrium processing tech-niques such as mechanical alloying milling can produce metastable microstructure with the grain size reduced to nanoscale and therefore, this processing techniquehas been extensively employed to synthesize interme-tallics. These materials can be used as, for example, dispersoids, coatings, high specific surface area pow-ders, functional nanodevices and engine parts [3-4].  In general, MA of elemental metal blend may lead to the formation of the intermetallic, solid solution or amorphous phases [5]. In Co-Al binary system, it is reported that MA of Co50-Al50 led to the formation of CoAl intermetallic compound [6]. Although it has been reported that prolonged mechanical milling (MM) of fully ordered CoAl produced by other techniques than MA leads to partly disordering [7], no report exists on ordering-disordering behavior of CoAl during MA.  Grain growth in polycrystalline materials, in gen-eral, occurs to decrease the free energy of the system by decreasing the total grain boundary energy. In nano-crystalline (nc) materials, high amount of grain bound-ary leads to a significant fraction of high energy and provides a strong driving force for grain growth. Since the unique properties of these materials are closely related to the fine grain size and the large volume frac-tion of grain boundaries, it is of vital importance to maintain the microstructure at a nanometeric scale during consolidation or service as structural compo-nents at elevated temperatures [8-11]. With this re-gard, many studies have been focused on the grain growth behavior of nc materials (e.g., see References [12-14]).  Although different models were proposed grain growth models for nc materials taking into accounts of the impurities and solute drags, it was reported that some nc materials still obeyed the parabolic kinetic equation of grain growth for isothermal annealing in the form [15-17]   0n nD D Kt  (1)  where D , 0D  and n  are the mean grain size at time t, mean initial grain size, and the grain-growth expo-nent, respectively. K is a constant that satisfies the following equation [15-17]   0 exp( )K K Q RT  (2)  Where Q  is the activation energy required for grain growth and 0K , R  and T  are constant, gas constant and temperature, respectively. Eqs. (1) and (2) can be rewritten as [17]  0 0 expn nD D K Q RT t  (3)  F. KARIMZADEH, S.N. HOSSEINI, M.H. ENAYATI PROC. NAP 1, 03CNN15 (2012)   03CNN15-2  In addition, the rate of grain growth, dD dt  , can be derived as   log ( ) ( 1) log ( ) log logdD t dt n D t K n  (4)  The relationship between log ( )dD t dt  and log ( )D t  gives the values of n  and K . Also, the activa-tion energy Q  can be obtained from Equation (3) by plotting 0log /n nD D t  as a function of 1 T . A variety of n  values has been reported for various sys-tems and the n  value can be a useful factor to describe the difference in growth mechanism between the differ-ent systems [16]. So, the aim of the present work is mainly to investigate MA of the Co50-Al50 mixture and to investigate thermal stability and grain growth kinet-ics of the nanostructured CoAl synthesized by MA.  2. EXPERIMENTAL PROCEDURE  MA of the Co-50at.%Al powder blend was carried out at ambient temperature in a planetary high energy ball mill, using hardened chromium steel vial and balls. No process control agent (PCA) was used. The related parameters for the MA are presented in Table 1. Iso-thermal annealing was performed to investigate the grain growth kinetics of milled powders. Powder sam-ples were sealed and then annealed in a conventional tube furnace. The annealing process parameters are displayed in Table 1. X-ray diffraction (XRD) technique by a Philips X’PERT MPD diffractometer (Cu Kα radia-tion: λ  0 154 nm at 20 kV and 30 mA) was used to characterize the phase composition and structural changes of the powders. The Williamson–Hall formula was used to estimate the grain size and internal strain using broadening of XRD peaks [18].  Table 1 – Parameters for MA and heat treatment.  Parameters Value Rotation speed (r min)  600 Ball to powder weight ratio 15 1 Ball milling time (h)  1, 3, 6, 10, 25 and 45 Protective atmosphere Ar Annealing temperature (K)  873, 973, 1073 and 1173 Holding time (h) 0.5, 1, 1.5 Cooling mode Cooling in air  3. RESULTS AND DISCUSSION   3.1 MA of Co-50 at Al  Fig 1 shows XRD traces for Co50Al50 powders as-received and after different MA times. During MA the sharp crystalline diffraction peaks of the as-received powder broadened progressively with increasing pro-cessing time associated with accumulated internal strain and refinement of grain size. The Co and Al grain size and lattice strain after 1 and 3 h MA, estimated from broadening of XRD peaks using Williamson–Hall formula [18], were presented in Table 2.    Fig. 1 – The XRD patterns of the Co50-Al50 powder mixture at different MA time.  It can be seen that after 3 h MA, the final grain size of Co is smaller and its lattice strain is greater than that for Al. Although the initial grain size was smaller for Co, this might be due to their different crystal struc-tures. Cobalt (with hcp structure) is a material with low stacking fault energy in which dislocation slip is the dominant deformation mechanism. But it is reported [19] that the nanocrystalline cobalt deforms predomi-nantly by mechanical twinning. Dislocation slip in the nanocrystalline cobalt is hindered by the small grain size and twinning (in despite of higher activation energy requirement) proceeds. Although the classical twinning process based on the pole mechanism would be difficult to apply for very small grain sizes, a recent molecular dynamics simulation study [20] showed that, at high stress levels, both heterogeneous (starting at grain boundaries) and homogenous nucleation of twins can occur for materials with grain sizes less than 50 nm. Homogeneous nucleation took place in the grain interi-ors and involved the overlap of stacking faults of disso-ciated dislocations. So, the material exhibits an unusu-ally faulted microstructure [19]. However, in the case of nanocrystalline Al, the higher stacking fault energy for Al (200 mJ/m2) [21] in comparison with cobalt (31 mJ/m2)[19] and the tendency of Al (with fcc struc-ture) to slip will make deformation based on twinning more difficult.  Table 2 – Grain size and lattice strain of Co and Al at differ-ent MA times.  MA time  Co Al (h) d (nm) ε  d (nm) ε  1 23 0.46 55 0 35 3 9 0 63 20 0 53  From 1 to 3 h MA, the powder particles were ag-glomerated and large amount of powders were stuck to the surfaces of the container and balls. Further milling resulted in the release of coated Co and Al and appear-ance of CoAl peaks. Transformation of elemental Co and Al powder mixture to the CoAl intermetallic phase was completed after 10h MA. The detection of superlattice  STRUCTURAL EVOLUTIONS OF NANOSTRUCTURED COAL… PROC. NAP 1, 03CNN15 (2012)   03CNN15-3 reflection (100) indicates that the B2-CoAl phase has an ordered structure. The long range order parameter (S) was calculated considering the (100) superlattice reflec-tion and (110) fundamental reflection of cubic B2 struc-ture formed by MA using the following formula [22]:   100 110100 110obsstdI ISI I (5)  Where subscripts ‘‘obs’’ and ‘‘std’’ stand for observed and Standard ratios of the intensities of the mentioned diffracting atomic planes. Here standard ratio refers to that for a completely ordered structure.  The S parameter, grain size and lattice strain of the B2-CoAl calculated for different MA times, are present-ed in Table 3.   Table 3 – Structural characteristics of the CoAl phase at dif-ferent MA times   MA time (h) d (nm) ε  Lattice  parameter (A○) S 6 6 0 75 2 862 1 10 7 0 36 2 862 1 25 7 1 41 2 857 0 82 45 6 1 63 2 856 0 82  As can be seen the S value is equal to 1 during 6 to 10 h milling suggests that formed B2-CoAl is fully or-dered. The S value decreases and reaches a constant value of about 0 82 after long milling periods. The grain size of the formed CoAl is about 6 nm that remains ap-proximately constant with further milling. These results are in agreement with the general observation that up-on ball milling, for most intermetallic compounds a re-finement of the internal grain size is observed to typical-ly 5 to 20 nm together with an increase of atomic level strains typically between 0.7 and 2.5%[23] (see Table 3). The minimum grain size achievable by milling is deter-mined by the competition between the plastic defor-mation via dislocation motion and the recovery and re-crystallization behavior of the material. This balance gives a lower bound for the grain size of the material. Besides, a number of process variables (such as method to estimate the grain size, milling intensity, milling temperature, alloying effects, contamination, etc.) can influence the minimum achievable grain size [7].  In some instances, ordered intermetallics have been found to be formed directly on MA, although it is partic-ularly true in Al-transition metal systems [7]. At the early stage of milling, nanometric grain sizes and high defect densities produced by MA accelerate the Co–Al interdiffusion process at room temperature. Therefore, CoAl phase starts to form after 6 h MA. The heat re-leased from this reaction is consumed to release stored energy, which was incorporated during ball milling. So, the lattice strain of the B2-CoAl decreases from 0 75 to 0 36%. After completion of CoAl formation, the lattice strain increases again and S value decreases on further milling due to severe plastic deformation. It is reported that disordering in B2-CoAl compound occurs with tri-ple defect mechanism that results in reducing the lattice parameter [3,7,24-25]. So, triple defects formation can be responsible for decrease of lattice parameter from 2 862 to 2 856 Aο.  3.2 Heat Treatment  In order to investigate grain growth, annealing of the as-milled powder was performed at different temperatures and durations. The corresponding XRD patterns are shown in Fig. 2. The grain size of nano-crystalline CoAl, calculated from XRD patterns, are plotted as a function of annealing time (see Fig. 3).     Fig. 2 – XRD patterns of the isothermally annealed powders at a)873, b)973, c)1073, d)1173 and e)1273 K for 0.5, 1 and 1.5 h   F. KARIMZADEH, S.N. HOSSEINI, M.H. ENAYATI PROC. NAP 1, 03CNN15 (2012)   03CNN15-4   Fig. 3 – Grain size of the CoAl as a function of annealing time for different temperatures   As it is expected, the grain size increases with increas-ing the temperature. The grain size does not hugely vary with annealing time especially at T≥973 K. The grain size increases rapidly first and then almost evenly at the long-er annealing times. This can be interpreted as a result of the decrease in the interfacial energy during grain growth [7]. The plots of log ( )dD t dt  versus log( )D  (Fig. 4) were obtained by fitting the data to Eq. (4) using a non-linear fitting route. The value of n  is about 3 for 873 K while reaches to a constant value of 4 at higher temperatures (T/Tm≥0.5). It is reported that n  value depends on grain growth mechanism [15-16]. So, it is appeared that grain growth mechanism of the nano-crystalline CoAl compound is the same at the temperature range of 973-1273 K. Normal grain growth of pure materials (n   2) is con-trolled by curvature-driven  migration of grain bounda-ries. When n   3, grain growth process is governed by volume diffusion. If n   4, the stochastic jumping of at-oms across the grain boundaries is predominant. As ob-served experimentally, the n  values usually ranges from 2 to >5 for conventional pure metals as well as some of the MAed intermetallic compounds e.g. nano-crystalline Fe with the n  value in the range of 3-11 [9-10, 15, 17]. In conventional materials, the value of n  is dependent on both mobility and energy of the grain boundaries. Wang et al. [15, 17] reported that for nano-Fe, similar to coarse-grained Fe, n  decreases toward 2 as T/Tm increases to ≈0.6. The growth exponent n  obtained in this work indi-cates that nano-CoAl shows high resistance against grain growth compared to nano-Fe or conventional coarse-grained Fe. In addition, this value offers that the grain growth mechanism of the nanostructured CoAl at temper-atures above 873K (T/Tm≈0.5) can be the random jump of atoms across the grain boundaries. Considering the fact that nanostructured CoAl consists of nanometer-sized grains and has a large volume fraction of grain bounda-ries, grain growth through atom diffusion in these bound-aries is expected much easier than that through conven-tional lattice diffusion. By plotting 0log /n nD D t  versus 1 T  and fit-ting a straight line to the data points (Fig. 5), the activa-tion energy for grain growth of nano-crystalline CoAl compound was calculated to be 175±20 kJ/mol. This value of Q  is quite smaller than the activation energy of both interdiffusion in conventional CoAl (≈432 kJ/mol) [2] and self-diffusion of Co in conventional CoAl (≈416 kJ/mol) [26]. But, it is approximately close to the activation energy for diffusion of Co in Al (83 Kj/mol) [27] and also the acti-vation energy of Al volume self-diffusion (144 kJ/mol) [10]. As mentioned before, the CoAl phase has a disordered structure with the triple defects formation mechanism in which CoAl structure is maintained by remaining Al at-oms on their own sublattices. So, it can be said that the grain growth in disordered CoAl phase is accomplished by diffusing Co and Al atoms in the two separate Al and Co sublattices.    Fig. 4 – Plots of log ( )dD t dt versus log( )D for CoAl phase at different annealing temperatures     Fig. 5 – Plot of 0log /n nD D t  against 1000 T  for CoAl grains  Consequently, the grain-growth kinetics of nano-crystalline CoAl under isothermal annealing at tempera-tures above 873 K can be well described using   4 4 130 6 10 exp 175 /D D RT t  (6)  Eq. (6) provides the means by which the grain size of nano-crystalline CoAl can be controlled by simply govern-ing the annealing temperature and time  STRUCTURAL EVOLUTIONS OF NANOSTRUCTURED COAL… PROC. NAP 1, 03CNN15 (2012)   03CNN15-5 REFERENCES  1. Y. Takano, J. Am. Ceram. Soc. 84, 2445 (2001). 2. R. Nakamura, K. Takasawa, Y. Yamazaki, Y. Iijima, In-termetallics 10, 195 (2002). 3. H. Bakker, G.F. Zhou and H. Yang, Progr. Mater. Sci. 39, 159 (1995).   4. N.K. Mukhopadhyay, D. Mukherjee, S. Dutta, R. Manna, D.H. Kim, I. Manna, J. Alloys Compd. 457, 177 (2008).  5. P. Nandi, P.M.G. Nambissan and I. Manna, J. 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Structural Evolutions of Nanostructured CoAl Intermetallic Compound during Mechanical Alloying and Subsequent Heat Treatment

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