Indian Journal of Engineering & Materials Sciences
Vol. 21, February 2014, pp. 111-115
Effects of silicon and chromium additions on glass forming ability and
microhardness of Co-based bulk metallic glasses
Aytekin Hitita*, Şükrü Talaşb & Rıza Karab
a
Department of Materials Science and Engineering, bDepartment of Metallurgical and Materials Engineering,
Afyon Kocatepe University, 03200 Afyonkarahisar, Turkey
Received 19 December 2011; accepted 5 September 2013
Effects of silicon and chromium additions on glass forming ability (GFA) and microhardness of a Co-Fe-Ta-B bulk
metallic glass are investigated by using differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and scanning
electron microscopy (SEM). It is found that partial substitution of boron by silicon promotes the GFA of the alloy. Fully
amorphous rod of 4 mm is fabricated by suction casting Co43Fe20Ta5.5B26.5Si5 alloy. However, partial replacement of cobalt
by chromium decreased the GFA significantly. In fact, critical casting thickness of Co39Cr4Fe20Ta5.5B26.5Si5 alloy is
determined as 2 mm. It is also determined that microhardness values of the amorphous alloys are about 1200HV300. This
value is lower than the hardness of the base alloy,1455HV300, and it is believed that decrease in hardness results from the
reduction in boron contents of the alloys.
Keywords: Bulk metallic glasses, Glass forming ability, Thermal analysis, Microhardness
For the last two decades, multicomponent bulk
metallic glasses (BMGs) have attracted great attention
because of their unusual physical, chemical and
mechanical properties. A large number of glassforming alloys with critical cooling rates less than
1000 K/s have been successfully developed in Zr1-3-,
La4,5-, Pd6,7-, Mg6-8-, Ti9,10-, Ni-11-13, Cu14,15-, Fe16-18and Co18-20- based systems, which have significantly
broadened the expectation of amorphous alloys for
both scientific and engineering applications. When
mechanical properties considered, the fracture
strength is in the range of 1500-1800 MPa for Zrbased alloys1-3, 1700-2000 MPa for Ti-based
alloys9,10, 1900-2500 MPa for Cu-based alloys11,14,
2700-3200 MPa for Ni-based alloys11,12, 3900-4500
MPa for Fe-based alloys17,18 and 4500-5500 MPa for
Co-based alloys18,19. The cobalt based alloy
Co43Fe20Ta5.5B31.5 is one of the alloys having the
highest fracture strength, but its critical casting
thickness is only 2 mm. It is believed that such a low
critical casting thickness limits utilization of this alloy
as a structural material. Therefore, critical casting
thickness of this alloy must be improved.
Unfortunately, there is no universal model to
predict the alloy compositions which has good glass
forming ability (GFA). Based on extensive
——————
*Corresponding author (E-mail: [email protected])
experimental results, three empirical rules have been
established to favor the formation of bulk metallic
glass21: (i) multi-component system with more than
three components, (ii) significant difference in atomic
size ratios above about 12% among the three main
constituent elements and (iii) large negative heats of
mixing among the three main constituent elements.
Although these rules can be useful guidelines for
alloy design, development of new alloys with high
GFA mainly depends on carrying out a series of
experiments where compositions are changed step by
step. Improvement of glass forming ability (GFA) is
often achieved by partial replacement of a constitute
element by another element, selected on the basis of
the empirical rules for bulk metallic glasses.
The Co-Fe-Ta-B alloy under investigation already
satisfies the three empirical rules described above.
In order to improve GFA of the Co-Fe-Ta-B alloy,
silicon and chromium were selected as candidate
elements. Examination of binary phase diagrams of
silicon with each of the constitutent elements of the
alloy reveals that for all cases, minor silicon
additions decrease liquidus temperatures of binary
alloys22. Similarly, binary phase diagrams of
chromium and the constitutent elements of the alloy
indicate that minor chromium additions also
decrease liquidus temperatures of binary alloys.
Therefore, proper utilization of these elements as
112
INDIAN J ENG. MATER. SCI., FEBRUARY 2014
substitutions for the alloying elements can improve
GFA of the alloy by lowering liquidus temperature.
Since silicon is a metalloid element, it was
subsituted for the metalloid element of the alloy,
boron, to keep the total fraction of metalloid
elements of the alloy constant. Chromium can
be used as substitution for both cobalt and iron
for the alloy under investigation due to the fact
that chromium, cobalt and iron have very similar
atomic radii, which are 0.1249 nm,0.1251 nm
and 0.1241 nm, respectively23. However, atomic
radius of tantalum is 0.143 nm and replacement
of tantalum by chromium violates the second
emprical rule described above. In this study,
chromium was substituted for cobalt not for iron.
There is no particular reason for not choosing
iron.
In this paper, we report the GFA and microhardness
of alloys designed by partial replacements of
alloying elements by silicon and chromium in
Co43Fe20Ta5.5B31.5 alloy.
Experimental Procedure
Multi-component Co-based alloy ingots with
composition of Co43-xCrxFe20Ta5.5B26.5Si5 (where
x=0,2,4) were prepared by arc melting the mixtures
of pure Co (99.8 wt%), Fe (99.9 wt%) and Ta
(99.9 wt%) and Cr (99.7 wt%) metals and pure
crystalline B (98 mass%) in a Ti-gettered high purity
argon atmosphere. In order to ensure homogenity,
master alloys were melted three times. The alloy
compositions represent nominal atomic percentages.
Bulk glassy alloys in a rod form with diameters up to
5 mm and a length of 50 mm were produced by
suction casting method in an arc furnace. The as-cast
structures were examined by X-ray diffraction (XRD)
(Shimadzu XRD-6000) with Cu-Kα radiation and
scanning electron microscope (SEM) (Leo 1430 VP ).
The glass transition temperature (Tg), crystallization
temperature (Tx), solidus temperature (Tm) and
liquidus temperatures (TL) of the alloys were
determined by differential scanning calorimetry
(DSC) (Netzsch STA 409 PC/PG ) at a heating rate of
0.33 K/s. Microhardness measurements were carried
out with a Vickers microhardness tester (Shimadzu
HMV 2L ) under a load of 2.94 N. For each alloy,
microhardnesses of as-cast samples having casting
thickness of 2 mm were measured. Twenty
measurements were carried out for each sample and
arithmetic mean of measurements were taken as
microhardness of the alloy.
Results and Discussion
XRD patterns of samples are shown in Fig. 1.
Co43Fe20Ta5.5B26.5Si5 alloy has critical casting
thickness of 4 mm. For the casting thickness of 5 mm,
precipitation of (Co,Fe)2B phase was observed for this
alloy. Critical casting thicknesses of chromium
containing alloys Co41Cr2Fe20Ta5.5B26.5Si5 and
Co39Cr4Fe20Ta5.5B26.5Si5 are found to be 3 mm and 2
mm, respectively. For both of these alloys, it was
determined that body-centered tetragonal (Co,Fe)2B
and face-centered cubic (Co,Fe)23B6 type phases
precipitate in the samples having diameters larger
than critical casting thicknesses.
Thermal stability of the alloys were investigated by
DSC (Fig. 2). During heating, all the DSC traces
showed an endothermic event, characteristics of glass
transition and followed by exothermic reactions
corresponding to crystallization of the undercooled
liquid. Tg and Tx of the base alloy, Co43Fe20Ta5.5B31.5,
alloy are 910 and 982 K, respectively24. Tg of
Co43Fe20Ta5.5B26.5Si5 alloy is 889K, which is about 20
K lower than that of Co43Fe20Ta5.5B31.5 alloy. Also Tx
of Co43Fe20Ta5.5B26.5Si5 alloy is 937 K, which is lower
than Tx of Co43Fe20Ta5.5B31.5 alloy. It was also
determined that TL of Co43Fe20Ta5.5B26.5Si5 alloy is
1450 K, which is about 65 K lower than the base
alloy.
Tg of the chromium containing alloys,
Co41Cr2Fe20Ta5.5B26.5Si5 and Co39Cr4Fe20Ta5.5B26.5Si5,
are 903 and 908 K, respectively. These values are
very close to Tg of the base alloy. Also, Tx of
Co41Cr2Fe20Ta5.5B26.5Si5 and Co39Cr4Fe20Ta5.5B26.5Si5
are determined to be 961 K and 979 K, respectively
and these values are very close to Tx of the base alloy.
Fig. 1—XRD patterns of the as-cast Co43-xCrxFe20Ta5.5B26.5Si5
(x=0,2 and 4) alloys
HITIT et al.: Co-BASED BULK METALLIC GLASSES
In addition, TL of Co41Cr2Fe20Ta5.5B26.5Si5 and
Co39Cr4Fe20Ta5.5B26.5Si5 alloys are determined to be
1487 and 1500 K, respectively. These values are also
very similar to the TL of the base alloy. Thermal
properties of the alloys are summarized in Table 1.
SEM image obtained from the center region of 4
mm sample of Co41Cr2Fe20Ta5.5B26.5Si5 alloy is shown
in Fig. 3a. Two types of particles are observed in the
SEM image. Because boron content of (Co,Fe)23B6
phase, which is 20 at.%, is lower than that of
(Co,Fe)2B phase, which is 33 at.%, (Co,Fe)23B6 phase
has a higher average atomic number. For this reason,
it is concluded that the brighter particles observed in
the SEM image are particles of (Co,Fe)23B6 phase and
the darker particles are particles of (Co,Fe)2B phase.
In addition, EDS results show that (Co,Fe)23B6 phase
contains some amount of tantalum (Fig. 3b). Also,
cubical morphology is observed for particles of
(Co,Fe)23B6 phase, which is not unexpected since the
particles have face-centered cubic structure.
Microhardnesses of the alloys are found to be
around 1200 HV300 (Table 1). During the
measurements, it was also observed that for each
alloy, measurements were quite consistent and
deviations from the average microhardness was less
113
than 5%. Also, microhardness values of the alloys are
determined to be quite close to each other and lower
than the microhardness of the base alloy.
It is believed that significant drop in liquidus
temperature of Co43Fe20Ta5.5B26.5Si5 alloy is the
reason for higher GFA of this alloy. Lowering TL for a
constant Tg of the alloy results in a decrease in the
temperature difference between TL and Tg ; for this
reason, a higher cooling rate can be achieved for the
same casting diameter. As chromium content is
increased there is no significant change for Tg of the
alloys. However, liquidus temperatures of the alloys
increase with chromium content and this must be the
reason for having lower GFA for these alloys. Also,
having shorter critical nucleation time for an alloy can
be another reason for the rapid crystallization of the
chromium containing alloys. Nevertheless, for
chromium containing alloys, it does not seem to be
possible that 2-4 at.% chromium additions cause such
a reduction in critical nucleation time of the phases
due to the fact that the precipitating phases do not
contain noticable amount of chromium.
Reduced glass transition temperatures (Tg/Tl)
of the alloys show very close correlation with
the critical casting thickness values (Fig. 4). Indeed,
Table 1—Thermal properties (Tg,Tx,Tl,Tm), parameters for GFA, critical casting thickness and microhardnesses
of Co-Fe-Ta-B-Cr-Si alloys
Alloy
Tg
(K)
Tx
(K)
Tm
(K)
TL
(K)
∆Tx
(K)
Tg/TL
Co43Fe20Ta5.5B31.518
Co43Fe20Ta5.5B26.5Si5
Co41Cr2Fe20Ta5.5B26.5Si5
Co39Cr4Fe20Ta5.5B26.5Si5
910
889
903
908
982
937
961
979
1295
1299
1304
1516
1450
1487
1500
72
48
58
71
0.599
0.613
0.607
0.605
γ(Tx/
(Tg+TL))
0.405
0.401
0.402
0.407
HV300
Dmax
(mm)
1455
1195
1219
1206
2
4
3
2
Fig. 2—DSC curves of the Co43-xCrxFe20Ta5.5B26.5Si5 (x=0,2 and 4) alloys: (a) low temperature measurements and (b) melting behaviour
114
INDIAN J ENG. MATER. SCI., FEBRUARY 2014
Fig. 4—Relationship between the critical casting thickness (Dmax)
for the formation of a glassy phase and reduced glass transition
temperature (Tg/TL).
Fig. 3—Microstructure of the Co41Cr2Fe20Ta5.5B26.5Si5
alloy (d=4mm): (a) SEM electron backscattered image,
(b) EDX result obtained from particles having lighter contrast
and (c) EDX result obtained from particles having darker
contrast
Co43Fe20Ta5.5B26.5Si5 alloy has the highest Tg/TL and
critical casting thickness values. Also, DSC
measurements show that eutectic temperatures of the
alloys are almost the same, which is about 1300 K,
and all the alloys have off eutectic compositions.
These result implies that if eutectic or near eutectic
compositions for these alloys are found, TL of the
alloys will be much lower. For this reason, the critical
casting thicknesses of these eutectic or near-eutectic
alloys are expected to be much higher. In addition
to reduced glass transition temperature, some other
well-known GFA parameters are also considered. It is
found that γ parameter does not show any agreement
with critical casting values of the alloys. Also, there is
no correlation with ∆Tx values and critical casting
thicknesses of the alloys either.
Microhardnesses of the alloys are determined to
be about 1200 HV300 (~11.7 GPa). Tensile yield
strength of the alloys can be estimated by using the
equation σy=Hv/3 24 . Based on the microhardness
values, the tensile yield strengths of the alloys
are determined to be about 3.9 GPa. Microhardness
values of the alloys are lower than the microhardness
value of the base alloy, which is 1455Hv.
This decrease in the hardness is believed to be due to
the reduction in number of Co-B, Fe-B and Ta-B
pairs in the alloys studied, which resulted from
substitution of silicon for boron. Similar results
indicating the effect of boron content on hardness
were observed in other Co- and Fe-based
bulk metallic glasses18. Based on these results, in
Co- and Fe-based bulk metallic glass systems, it is
HITIT et al.: Co-BASED BULK METALLIC GLASSES
obvious that replacement of boron by other elements
for the improvement GFA results in reduction in
hardness. For this reason, if the yield strength levels
are desired to be higher than 5 GPa, replacement of
boron by another element should not be a choice for
the improvement of the GFA of Co-based bulk
metallic glasses. In other words, partial replacement
of cobalt, iron and tantalum by suitable elements
should be the strategy for improvement of the
GFA of these alloys. It was also observed that,
chromium additions do not have any effect on the
microhardness values of the silicon modified alloy,
which suggests that there is no significant difference
in terms of bond strength between Co-B and
Cr-B pairs
Acknowledgements
This study was supported by grant no.104M124
from the The Support Programme for Scientific and
Technological Research Projects of the Scientific and
Technological Research Council of Turkey.
References
1
2
3
4
5
6
7
Conclusions
The following conclusions can be drawn from this
study:
(i)
Partial replacement of boron by silicon in
Co43Fe20Ta5.5B26.5 enhances the GFA by
lowering the liquidus temperature of the
alloy.
(ii)
Partial replacement of cobalt by chromium
reduces the GFA because of the fact that
chromium substitution increases the liquidus
temperatures of the alloys.
(iii)
Critical casting thicknesses of the alloys show
very good correlation with reduced glass
transition temperature, Tg/TL.
(iv)
Alloys having eutectic or near eutectic
compositions are expected to have much
higher critical casting thickness.
(v)
Replacement of boron by silicon caused
reduction in microhardness because of
the decrease in the number of (Co,Fe,Ta)B pairs.
115
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Peker A & Johnson W L, Appl Phys Lett, 63 (1993) 2342.
Inoue A & Zhang T, Mater Trans JIM, 36 (1995)1184
Fan J T, Wu F F , Zhang Z F , Jiang F & Sun J & Mao S X, J
Non-Cryst Solids, 353 (2007) 4707.
Jiang Q K, Zhang G Q, Chen L Y, Zeng Q S & Jiang J Z, J
Alloys Compounds, 424 (2006) 179.
Liu W Y , Zhang H F , Wang A M , Lia H & Hua Z Q, Mater
Sci Eng A, 459 (2007) 196.
Liu L, Zhao X, Ma C, Pang S & Zhang T, J Non-Cryst
Solids, 352 (2006) 5487.
Liu W Y, Zhang H F, Wang A M , Lia H & Hua Z Q ,
Mater Sci Eng A, 459 (2007) 196.
Zheng Q, Cheng J H, Strader E, Ma J & Xu J, Scripta Mater,
56 (2007) 161.
Inoue A, Mater Sci Forum, 307 (1999) 312.
Kim Y C, Yi S, Kim W T & Kim D H, Mater Res Soc Symp
Proc, 644 (2001) L4.9.1.
Inoue A, Zhang W, Zhang T & Kurosaka K, Acta Mater,
49 (2001) 2645.
Zhang T & Inoue A, Mater Trans, 43 (2002)708.
Liang W Z , Shen J & Sun J F, J Alloys Compounds,
420 (2006) 94.
Kim Y C, Lee J C, Cha P R, Ahn J P & Fleury E, Mater Sci
Eng A, 437 (2006) 248.
Fan J T, Zhang Z F, Jiang F, Sun J & Mao S X, Mater Sci
Eng A, 487 (2008) 144.
Li H X & Yi S, Mater Sci Eng A 449-451 (2007) 189.
Inoue A, Shen B L & Chang C T, Acta Mater, 52 (2004) 4093.
Inoue A, Shen B L, Chang C T, Intermetallics, 14 (2006)
936.
Inoue A., Shen B L, Koshiba H, Kato H & Yavari A R,
Acta Mater, 52 (2004) 1631.
Men H, Pan S J & Zhang T, J Mater Res, 21 (2006) 958.
Inoue A , Acta Mater, 48 (2000) 279.
Baker H (ed), ASM Handbook: Alloy Phase Diagrams, (1992).
International Tables for X-ray Crystallography, (1968).
Zhang P, Li S X & Zhang Z F, Mater Sci Eng A, 529 (2011) 62.