Synthesis and characterization of bulk amorphous steels

Available online at www.sciencedirect.com
Journal of Non-Crystalline Solids 354 (2008) 3284–3290
www.elsevier.com/locate/jnoncrysol
Synthesis and characterization of bulk amorphous steels
M. Iqbal a,b,*, J.I. Akhter b,*, H.F. Zhang a, Z.Q. Hu a,1
a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,
72 Wenhua Road, Shenyang 110016, PR China
b
Physics Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan
Received 11 October 2007; received in revised form 28 January 2008
Available online 18 April 2008
Abstract
Bulk amorphous steels (BASs) are a novel class of advanced materials having very attractive physical, thermal and mechanical properties and have applications as structural materials. Two BASs Fe50Cr14Mo14C14B6M2 (M = Y and Dy) were designed following the
Greer’s confusion principle and cylinders of thickness 3–5 mm were synthesized by Cu mold casting technique. Characterization was
carried out by techniques of X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy
(SEM) with attachment of energy dispersive spectroscopy (EDS). The alloys show high glass-forming ability (GFA) as well as high thermal stability. Hardness and elastic moduli of the present steels were found to be about 3–4 times higher as compared to the conventional
steels. Steel containing Dy has superior mechanical and thermal properties as compared to the steel containing Y.
Ó 2008 Elsevier B.V. All rights reserved.
PACS: 61.05.cp; 61.10.Nz; 61.43.Dq; 62.20.x
Keywords: Amorphous metals; Metallic glasses; Alloys; X-ray diffraction; Hardness; Scanning electron microscopy
1. Introduction
Metallic glasses have been extensively studied during the
last two decades owing to their various potential technological applications. Fe-based bulk metallic glasses
(BMGs) are of special importance due to their relatively
low cost and unique mechanical properties such as ultrahigh strength and high corrosion resistance as compared
to conventional materials [1]. Among these the amorphous
steels have shown great potential for some structural applications [2]. In order to compete commercially with conventional materials, amorphous steels need to be made of
*
Corresponding authors. Address: Physics Division, Pakistan Institute
of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan.
Tel.: +92 51 2207224; fax: +92 51 9290275 (J.I. Akhter).
E-mail addresses: miqbalchishti@yahoo.com (M. Iqbal), jiakhter@
yahoo.com, akhterji@pinstech.org.pk (J.I. Akhter), zqhu@imr.ac.cn
(Z.Q. Hu).
1
Tel.: +86 24 23971827, +86 24 23992092; fax: +86 24 23992092.
0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jnoncrysol.2008.02.009
relatively inexpensive components, while maintaining a
large critical size for a given application. Bulk amorphous
steels (BASs) offer important advantages over their crystalline counterparts such as much lower material cost, higher
strength, better magnetic properties, better corrosion resistance and higher thermal stability as well as better glassforming ability (GFA) [3,4]. Structural amorphous steels
(SASs) having thickness up to 12–16 mm have been produced with glass transition temperature Tg above 900 K
by Cu mold casting [5–8]. However, most of these steels
have negligible ductility and they are brittle [9]. The limited
GFA of amorphous steels is another major concerning
point. Efforts have been devoted to improve the GFA of
Fe-based alloys in order to enhance their ability to form
bulk glassy steels under conventional industrial conditions,
using, for example, commercial-grade raw materials, low
vacuum, conventional casting techniques, etc. Recently, a
great progress has been made in fabricating bulk amorphous steels [5–10] and nonmagnetic bulk amorphous
steels have been developed which are three to four times
M. Iqbal et al. / Journal of Non-Crystalline Solids 354 (2008) 3284–3290
stronger as well as have superior corrosion resistance compared to the ordinary steels.
A number of compositions of BASs have been reported,
in the recent past [5,9,11], with varied amounts of Fe contents. However, the supercooled liquid region of these
steels has been less than 50 K in most cases indicating
low GFA of these materials. Therefore, improvement of
GFA and brittleness are still major tasks in this regard.
In the present study two FeCrMoCB steels have been synthesized by adding 2 at.% Y and Dy to investigate their
effect on thermal and mechanical properties.
2. Experimental
The steels of compositions Fe50Cr14Mo14C14B6M2,
(where M = Y and Dy), designated as S1 and S2 in Table
1, were designed according to the Greer’s confusion principle [12]. The alloy buttons were produced from 1-3N pure
constituent materials by arc melting at least four times to
get chemical homogeneity. The Fe–B master alloy was used
along with other alloy constituents having precise weights.
The final casting of bulk samples having thickness (3–
5 mm) and length 60 mm was done in an induction furnace
by Cu mold casting. In order to determine the thermal
parameters, low temperature DSC was performed at heating rate ‘r’ of 10, 20 and 40 K/min. High temperature DSC
was also performed at 20 and 40 K/min using NETZSCH
DSC 404C to determine the melting and liquid temperatures. Samples of both steels were annealed at various temperatures ranging from 873 to 1123 K for 20 minutes under
inert atmosphere. For structural characterization, XRD
Table 1
Steel composition (at.%) with their designations and density (g/cm3)
Steel
Composition
q1 Arc
melted
(±0.002)
q2 Induction
cast (±0.002)
Difference
q1 q2
S1
S2
Fe50Cr14Mo14C14B6Y2
Fe50Cr14Mo14C14B6Dy2
8.431
8.600
8.277
8.355
0.154
0.245
S1
Intensity (a.u.)
11 1
2
2
873 K/20 min
20
30
40
Fig. 1(a) and (b) shows the XRD patterns of as-cast
samples of the steels S1 and S2 of varying thickness. It is
clear that both the steels with thickness upto 4 mm show
broad bands in the XRD patterns, which indicate amorphous nature of the steels. XRD patterns of both the steels
having thickness 5 mm revealed that they are partially crystalline. Physical appearance of induction cast ingots shows
excellent metallic luster, indicating the amorphous nature
of the steels. XRD analysis of the annealed samples
revealed that both the steels contain crystalline phases,
namely, c-Fe and Cr23C6 as shown in Fig. 1(a) and (b).
SEM examination of as-cast polished samples of both
steels reveal featureless surface with out any second phase
particles or segregation which reconfirms the amorphous
nature of the steel samples. Fig. 2(a)–(c) shows the microstructure of the samples annealed at 1123 K for 20 min.
EDS analysis confirmed the presence of Fe and Cr rich precipitates. The magnified view of crystalline matrix is shown
in Fig. 2(b) indicating presence of precipitates in steel S1.
In addition to the above mentioned phases, the steel S1
contains dendritic structure as shown in Fig. 2(a).
γ -Fe 2
50
2
S2
1
1 1 11 1 1
1 1
Cr23C6 1
11 2
1123 K/20 min
11
11
11
1
953 K/20 min
As-cast dia. 5 mm
As-cast dia. 5 mm
As-cast dia. 4 mm
As-cast dia. 4 mm
As-cast dia. 3 mm
60
2 theta (deg)
2
1
11
1123 K/20 min
2
3. Results
Cr23C6 1
1
1 1
was conducted using D/Max-2500 Rigaku diffractometer
˚ ) radiation. Uniaxial compreswith Cu Ka1 (k = 1.54056 A
sion tests were applied on as-cast samples with aspect ratio
2 at a constant strain rate of 4.2 10–4/s. As-cast and
annealed samples were examined in scanning electron
microscope (SEM) to study the microstructure. Analysis
of the samples was carried out by electron probe microanalyzer (EPMA) attached with SEM. Vicker’s hardness ‘HV’
was measured by MVK-H3 Mitutoya hardness testing
machine taking average of at least eight to ten readings.
Nanohardness ‘H’ and elastic modulus ‘E’ was measured
employing Nanoindenter XP with Berkovich indenter
using Oliver and Pharr method [13] under a load of
10 mN and taking average of at least six measurements.
Density ‘q’ of arc-melted buttons and induction melted
ingots was measured by Archimedes principle.
Intensity (a.u.)
γ -Fe 2
3285
70
80
20
30
40
50
2 theta (deg)
Fig. 1. XRD patterns of two as-cast and annealed steels S1 (a) and S2 (b).
60
70
80
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M. Iqbal et al. / Journal of Non-Crystalline Solids 354 (2008) 3284–3290
Heat flow (a.u.) Exo.
r = 10 K/min
Tg Tx
S2
Tg
Tx
S1
Tp1
450
600
750
Tp2
900
1050
1200
Temperature (K)
Heat flow (a.u.) Exo.
r = 20K/min
Tm Tl
Tg Tx
S2
S1
Tp1
450
600
750
Tp2
900
1050
1200
1350
1500
Temperature (K)
Tm Tl
Heat flow (a.u.) Exo.
r = 40K/min
Tg
S2
Tx
Tp1 T
p2
S1
Tp1
Fig. 2. Microstructure of annealed samples at 1123 K/20 min. Steel S1 at
low magnification (a) magnified view of crystalline matrix (b) and steel S2
(c).
450
600
750
Tp2
900
1050
1200
1350
1500
Temperature (K)
Fig. 3. Low (a) and high temperature DSC scans (b,c) of two BASs S1 and
S2 at heating rates of 10, 20 and 40 K/min.
Low and high temperature DSC were conducted at the
heating rates of 10, 20 and 40 K/min and DSC scans are
shown in Fig. 3(a)–(c). The results reveal that multistage
crystallization occurs in both the steels. The thermal
parameters like glass transition temperature Tg, crystallization temperature Tx, peak temperature Tp, melting and
liquid temperatures Tm and Tl are taken from the DSC
scans. The maximum variation in these temperature measurements is less than ±0.5 K. Using these values the supercooled liquid region DTx (=Tx Tg), reduced glass
transition temperature Trg1 (=Tg/Tm) and Trg2 (=Tg/Tl),
gamma ‘c’ parameter (=Tx/(Tl + Tg)) and delta ‘d’ parameter (=Tx/(Tl Tg)) were calculated. In addition to these
parameters, Weinberg parameter KW [=(Tx Tg)/Tm],
Hruby parameter KH (=(Tx Tg)/(Tm Tx)), thermal
parameter KLL (=Tx/(Tg + Tm)), K1 (=Tm Tg),
Table 2a
Thermal parameters by low temperature DSC of two steels S1 and S2
Steel
r
Tg
Tx1
Tp1
DTx
Tx2
Tp2
S1
S2
10
10
823
835
88
899
893
908
65
64
913
923
938
943
All temperatures are in K.
K2 = DTx, K3 (=Tx/Tm), K4 (=(Tp Tx)(Tx Tg)/Tm)
and stability parameter KSP (=(Tp Tx)(Tx Tg)/Tg)
[14] were also evaluated. All the parameters are summarized in Tables 2a–2c. These parameters are generally used
to indicate the thermal stability and GFA [15,16]. The maximum supercooled liquid region was found to be 70 K for
S2. The values of DTx are better than those reported by
M. Iqbal et al. / Journal of Non-Crystalline Solids 354 (2008) 3284–3290
3287
Table 2b
Thermal parameters of two BASs at heating rates of 20 and 40 K/min from high temperature DSC
Steel
r
Tg
Tx1
DTx
Tm
Tl
Tg/Tm
Tg/Tl
c
d
b
S1
S1
S2
S2
20
40
20
40
825
840
840
859
892
908
905
929
67
68
65
70
1373
1385
1384
1375
1413
1435
1418
1415
0.601
0.606
0.607
0.625
0.584
0.585
0.592
0.607
0.399
0.399
0.401
0.409
1.516
1.526
1.566
1.671
2.71
2.75
2.89
3.38
All temperatures are in K.
Table 2c
Few more thermal parameters of two BASs at heating rates ‘r’
Steel
r
Tp1
Tx2
Tp2
K1
K2
K3
K4
KH
KW
KLL
S1
S1
S2
S2
20
40
20
40
909
919
919
958
929
938
934
1018
946
958
949
1028
548
545
544
516
67
68
65
70
0.6497
0.6556
0.6539
0.6756
0.8590
0.5401
0.6763
1.5069
0.1393
0.1426
0.1357
0.1570
0.049
0.041
0.047
0.051
0.0406
0.0408
0.0407
0.0416
All temperatures are in K.
Lu et al. [17] for Fe-based alloys containing 2 at.% Y. The
results on Trg, c parameter (=Tx/(Tl + Tg)) and d parameter (=Tx/(Tl Tg)) [18] indicate that the present values are
better than many Fe-based alloys. Steel S2 containing Dy
has the highest values of all the parameters. From the data
reported by Gu et al. [9] for more than 20 Fe-based BMGs,
DTx, Trg (Tg/Tl), c and d parameters were calculated and
the maximum values are 67 K, 0.584, 0.391 and 1.488,
respectively. The values of thermal parameters determined
in the present study were found to be 70 K, 0607, 0.409 and
1.671 respectively as given in Table 2b, which indicate good
GFA of the present steels. Thermal parameters for present
steels were also found to be better than Luo’s steels [19].
Thermal parameters like KH, KW, KLL, KSP, K1, K2, K3
and K4, etc. [15] are summarized in Table 2c. In order to
calculate the activation energy for crystallization, Kissinger
and Ozawa plots are drawn as shown in Fig. 4(a) and (b)
for both steels. The data was fitted to Kissinger equation
lnðr=T 2p Þ ¼ Eac =RT p + constant and Ozawa equation
ln(r) = Eac/Tp + constant, where ‘r’ is the heating rate,
Tp is the peak temperature in DSC curves, Eac is the activation energy, R is real gas constant 8.3145 J/mol K, with
slope E/R = B, where B is a constant. The activation
energy for first and second stage crystallization was deter-
mined from Kissinger and Ozawa plots using the values of
R and B. The results are summarized in Table 3, which
again indicates high thermal stability of both the steels.
The density of BASs was found to be in the range of 8.3–
8.4 g/cm3 as given in Table 1, which is higher than the density found by Lu and Liu for BASs (7.8–7.9 g/cm3) [5].
Density of arc-melted ingots was found to be higher than
the as-cast induction cast samples (3 mm thick) because
the arc-melted alloy buttons contain crystalline phases
while as-cast samples are fully amorphous. Results on
HV of as-cast and the annealed samples are given in Table
4. It shows that the hardness of as-cast steels S1 and S2 is
1219 ± 15 and 1272 ± 15 respectively. The HV exceeds
from 1500 at annealing temperature 1123 K for both the
steels. The maximum value of hardness was found to be
1550 for S2.
Nanohardness H (GPa) of as-cast steels was measured
and given in Table 4, which is found to be higher than
many BMGs, e.g. Zr-based BMGs. The as-cast steels S1
and S2 have H in the range of 16.7–17.2 GPa. With
annealing temperature, H also increases and maximum
value was found to be 22.3 GPa for S2 at 1123 K. The elastic modulus E (GPa) is one of the fundamental properties
of materials as it shows the bonding between the atoms
4.0
-9.5
-10.0
ln(r)
2
ln(r/Tp )
3.5
S1
S2
-10.5
3.0
2.5
-11.0
-11.5
1.04
S1
S2
1.06
1.08
1.10
-1
1000/Tp(K )
1.12
2.0
1.04
1.06
1.08
1.10
-1
1000/Tp(K )
Fig. 4. Kissinger (a) and Ozawa (b) plots for both steels.
1.12
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M. Iqbal et al. / Journal of Non-Crystalline Solids 354 (2008) 3284–3290
Table 3
Activation energies (kJ/mol) of two steels S1and S2
Steel
Eac1
Kissinger
Ozawa
S1
S2
348.4 ± 2.6
358.3 ± 2.8
363.6 ± 2.6
374.2 ± 2.8
Difference (%)
Eac2
Difference (%)
Kissinger
Ozawa
4.34
4.44
505.6 ± 9.5
436.8 ± 14.4
521.4 ± 9.5
472.7 ± 15.2
3.12
8.22
Table 4
Mechanical properties of the steels S1 and S2
Sample
HV (±15)
H (±0.2) GPa
E (±5) GPa
H/E
h (nm)
hc (nm)
hf (nm)
hmax (nm)
hf/hmax
%R
S1
S1
S1
S2
S2
S2
1219
1280
1515
1272
1420
1548
17.2
17.9
21.5
16.7
21.3
22.3
263
267
321
257
326
337
0.0652
0.0670
0.0668
0.0651
0.0655
0.0660
179.2
180.7
165.8
185.8
165.7
162.5
142.1
143.0
130.1
148.2
130.5
127.4
133.7
128.6
112.9
134.6
113
111.6
187.0
180.8
165.8
187.5
165.7
162.5
0.715
0.711
0.681
0.718
0.682
0.687
28.5
28.9
31.9
28.2
31.8
31.3
as-cast
873 K
1123 K
as-cast
953 K
1123 K
shown in Fig. 5(a) and (b). The maximum load (Pmax) used
is 10 mN. The development of pop-in marks (displacement discontinuities) in first loading of the indent and
pop-out marks in the unloading curves were observed.
The pop-in marks indicate a sudden penetration of the
tip of the indent into the sample. The non-uniform penetration is also due to sudden plastic deformation or formation
of cracks. Iqbal et al. and Wang et al. [20–24] have also
observed pop-in marks in loading curves of BMGs. The
and is dependent on interatomic distances. The elastic
moduli of the present steels (given in Table 4) were found
to be much higher than many BMGs [20–24]. The elastic
moduli E of as-cast steels S1and S2 were found to be 263
and 257 GPa respectively, which are higher than many
BMGs. Maximum elastic modulus was found to be 337
for S2 at 1123 K.
Loading and unloading curves (P–h curves) obtained by
nanoindentation of as-cast and heat-treated S1 and S2 are
S1
As-cast
873 K/20 min
1123 K/20 min
10
Loading curves
Pop-out
Pop-out
Pop-in marks
6
Loading curves
25
50
75
100
125
150
Penetration depth h (nm)
175
200
Pmax
4
Pop-out
Unloading curves
hf
0
0
Pop-in marks
2
Unloading curves
hf
hmax
0
Pmax
Load (mN)
Load(mN)
6
2
As-cast
953 K
1123 K
8
8
4
S2
10
225
Pop-in marks
0
25
50
hmax
75
100
125
150
175
200
225
Penetration depth h (nm)
Fig. 5. Loading and unloading curves (P-h curves) of two as-cast and annealed steels S1 (a) and S2 (b) showing pop-in and pop-out marks.
Fig. 6. Shear band formation in compression tested as-cast steels S1 (a) and S2 (b).
M. Iqbal et al. / Journal of Non-Crystalline Solids 354 (2008) 3284–3290
higher slope of the unloading curves indicates a higher stiffness or elastic modulus while higher penetration depth of
indent shows lower hardness. Fig. 6(a) and (b) shows
SEM micrographs of as-cast fractured samples of both
the steels, which reveal the formation of parallel and
curved shear bands without liquid droplets and veins
patterns.
4. Discussion
Two bulk amorphous steels were synthesized from low
purity materials to investigate the effect of Y and Dy, having large atomic sizes (0.18015 nm and 0.17740 nm respectively), on the thermal stability, GFA and mechanical
properties. Over all analysis of the results suggests that
both the steels have good GFA as well as thermal stability.
The GFA of BMGs has been described in terms of a number of thermal parameters like DTx, Trg, c, d and b
[Tg Tx/(Tl Tx)2] [25–27]. Chen et al. [18] reported these
thermal parameters of a number of Fe-based alloys. The
thermal parameters DTx, Trg, c, d and b for the Fe48Cr15Mo14C15B6Y2 alloy, having composition similar to the
present steels, were 47 K, 0.587, 0.385, 1.418 and 2.225,
respectively. It is clear from Table 2b that most of the
parameters of the present steels are higher than those
reported by Chen et al. [18]. Shen and Schwarz [28] have
reported DTx to be an important parameter regarding the
estimation of GFA of Fe-based alloys. The striking feature
of the present study was that supercooled liquid regions of
67 and 70 K were achieved for steels containing Y and Dy
respectively. The high values of Tg and DTx suggest high
thermal stability of the present steels. The enhanced thermal stability of the alloys is also evident from the high values of the activation energy for crystallization. The value is
much higher for the steel S2, for which even sample with
thickness 5 mm was fairly amorphous.
The other important feature of the present study was
that mechanical properties of the synthesized steels were
much higher compared to the conventional steels. The
microhardness values of 1219 and 1272 are comparable
to Lu’s [5] steels, while these values are much higher than
960–1150 reported by Hess et al. [3] for Fe48Cr15C15Mo14B6Er2 bulk amorphous steel. Like microhardness, the
nanohardness and elastic moduli of the present steels were
found to be higher than many BMGs [20–24]. It was
observed that the microhardness and elastic moduli
increase by more than 25% as the samples are annealed.
The enhancement in hardness and elastic moduli is due
to nucleation of crystalline phases, which act as obstacles
to the dislocation movement [29]. Nanohardness to elastic
modulus ratio, i.e. H/E of as-cast and annealed steel samples was calculated and found to be in the range of
0.0651–0.0670, which is comparable with H/E ratios
reported by Wang et al. [24]. H/E ratio 0.1 indicates
that bonding in these BMGs is most probably of covalent
nature. The elastic recovery hf /hmax and percentage elastic
recovery of displacement on unloading % R = [(hmax 3289
hf)/hmax) 100%] are two important parameters that were
calculated from the P–h curves, where hf is the final
indentation depth and hmax is the maximum penetration
depth of the indenter. This parameter is independent of
indentation depth due to self-similar geometry of the
indenter. The limits of this parameter are generally
0 6 hf/hmax 6 1, where the lower limit corresponds to fully
elastic deformation and the upper limit reflects characteristic of rigid plastic materials, for which there is no elastic
recovery [30]. The value of elastic recovery (hf/hmax) for
glass and Al are 0.687 and 0.951, respectively. The elastic
recovery limits in present case are found to be in the
range of 0.681–0.718, while percentage elastic recovery
% R ranges in between 28.2% and 31.9% for the steels
under study.
5. Conclusions
Two amorphous steels having diameter 4 mm and very
good thermal and mechanical properties were synthesized
by Cu mold casting technique. The maximum supercooled
liquid region of 70 K was achieved for steel S2. High values
of thermal parameters for the present steels indicate better
glass-forming ability. High temperature DSC of present
steels reveals multistage crystallization. Higher values of
activation energy for crystallization show higher thermal
stability of the present steels. Hardness and elastic moduli
of the present steels were found to be about 3–4 times
higher as compared to the conventional steels. Steel S2 containing Dy has superior mechanical/thermal properties
than steels containing Y.
Acknowledgements
M. Iqbal is grateful to IMR and Chinese Academy of
Sciences for offering PhD Scholarship. National Natural
Science Foundation of China (Grant No. 50731005) and
the Ministry of Science and Technology of China
(2006CB605201, 2005DFA50860) supported this work.
Thanks are also due to Profs. W.S. Sun, W. Wei, X.P.
Song, J.Z. Zhao, A.M. Wang and H. Li for good cooperation and extending help during the experimental part of the
work. Thanks to PAEC officials for providing the chance
to do this work at IMR, Shenyang, China.
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