effect of cooling rate on plastic deformation of zr-based bulk metallic glasses
TRANSCRIPT
Chinese Materials Research Society
Progress in Natural Science: Materials International
Progress in Natural Science: Materials International 2012;22(1):21–25
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ORIGINAL RESEARCH
Effect of cooling rate on plastic deformation of Zr-based
bulk metallic glasses
Chunyan Lia,nn, Shengzhong Koua,b,n, Yanchun Zhaoa, Guangqiao Liub, Yutian Dinga
aState Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, ChinabKey Laboratory of Non-ferrous Metal Alloys and Processing of the Ministry of Education, Lanzhou University of Technology,
Lanzhou 730050, China
Received 14 November 2011; accepted 9 December 2011
Available online 14 February 2012
KEYWORDS
Bulk metallic glasses;
Cooling rate;
Serration;
Plastic deformation;
Free volume
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Abstract The effect of cooling rate on plastic deformation of Zr-based bulk metallic glasses has been
studied. The specimens with the diameters of 3 mm, 4 mm and 6 mm cut from the same ladder-shaped
Zr55Al10Ni5Cu30 BMG rod were used in the present investigation. The experimental results indicated
that the different plastic strains were induced for the three type compression specimens. The
compressive plastic strain increased with the decreasing diameter for the as-cast specimens, which
means that the compressive plastic strain increased with the increasing cooling rate. The reason of the
difference is attributed to the free volume existed in the BMGs during the rapid solidification, and the
larger free volume was contained in the specimen fabricated with the higher cooling rate. It has been
found that the prominent serrations and multiple shear bands appeared during the compressive
deformation tests of the specimen with smaller diameter of 3 mm, indicating that the specimen with
smaller diameter exhibits better plasticity due to more free volume content.
& 2012. Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
esearch Society. Production
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nese Materials Research
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aboratory of Gansu Advanced
u University of Technology,
76682; fax:þ86 931 2976578.2976646; fax:þ86 931 2976578.
C. Li), [email protected] (S. Kou).
1. Introduction
Bulk metallic glasses (BMGs), as the promising structural and
functional materials, have received considerable attention owing
to their unique physical and mechanical properties. Being in a
metastable state, the amorphous structure of BMGs is most
likely to be influenced by different variables in the process of
materials preparation, such as, cooling rate, overheat tempera-
ture and impurities, among which the cooling rate was thought
to play an important role in the forming of BMGs [1–5]. The
macroscopic plastic deformation of most BMGs is limited
because of the lack of the dislocation or other structural defects
[6–10]. In recent years, multicomponent BMGs with improved
Fig. 1 Morphology of ladder Zr55Al10Ni5Cu30 BMG rod:
(a) diameter: 3 mm specimen, (b) diameter: 4 mm specimen and
C. Li et al.22
deformation have been successfully designed and fabricated
[11–15]. It has also been found that for metallic glasses with
the same compositions, the specimens with smaller sizes are
much softer than those with larger dimensions [16–18]. There-
fore, the cooling rate which determines the glassy structure of the
BMGs is directly influenced by the sizes of the as-cast specimens,
on the other hand, it seems to strongly affect the mechanical
properties of BMGs [8]. Recently, the effect of cooling rate on
mechanical properties of BMGs has attracted increasing atten-
tions [5,19]. However, the cooling-rate effect on plastic deforma-
tion and the plastic deformation mechanism of BMGs are still
need further research.
In this paper, ladder Zr55Al10Ni5Cu30 BMG rods with different
diameters were fabricated by copper mold suction casting.
The compressive deformation behavior of Zr55Al10Ni5Cu30as-cast BMGs with different diameters from the same ladder rods
has been presented, the plastic deformation mechanism and the
relationships between compressive plastic strain and cooling rate
have been discussed.
(c) diameter: 6 mm specimen.20 40 60 80
6mm
4mm
Inte
nsity
(cou
nts/
s)
3mm
2. Experimental procedures
Zr55Al10Ni5Cu30 alloy ingots were prepared by levitation melting.
The mixture was composed of pure elements Zr (99.8 mass%),
Cu (99.99mass%), Ni (99.99 mass%) and Al (99.99 mass%).
Ladder-shaped BMG rods with diameters of 3 mm, 4 mm and
6 mm were fabricated by copper mold suction casting under an
argon atmosphere. The amorphous structure of specimens was
identified by Bruker Axs D8 Advance X-ray diffraction (XRD,
Cu Ka radiation). The glass transition and crystallization behavior
of the BMG was evaluated by STA409 thermal differential
calorimeter (DSC) at a heating rate of 0.33 K/s. Uniaxial
compression tests of the specimens were carried out by WDW-
100D universal testing machine at room temperature with the
strain rate of 2.7� 10�4 s�1, and the aspect ratio of all specimens
was about 2:1. The outer surfaces of specimens were carefully
polished in order to decrease the roughness effect on the plastic
deformation.
2θ (degrees)Fig. 2 XRD patterns of Zr55Al10Ni5Cu30 BMG specimens with
different diameters.
0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.0400.0
0.5
1.0
1.5
2.0
φ 4mm
φ 6mm
φ 3mm
Stre
ss �
(GP
a)
1%
Strain (%)
Fig. 3 Compressive stress–strain curves of Zr55Al10Ni5Cu30BMG specimens with different diameters.
3. Results and discussion
Fig. 1 shows the ladder-shaped Zr55Al10Ni5Cu30 BMG rod
with the diameters of 3 mm, 4 mm and 6 mm. Fig. 2 shows
XRD patterns of the Zr55Al10Ni5Cu30 rods with different
diameters. It can be seen that the XRD patterns of all the
three kinds specimens with the diameters of 3 mm, 4 mm and
6 mm consist of only one broad diffuse peak between diffrac-
tion angles 301 and 451. This indicates that these specimens
have a perfect glassy structure.
Fig. 3 shows compressive stress–strain curves of
Zr55Al10Ni5Cu30 BMG with different diameters. At the strain
rate of 2.7� 10�4 s�1, the specimens of diameter 3 mm and
diameter 4 mm exhibited elastic–plastic behavior with the yield
strengths of 1.83 GPa and 1.65 GPa, and maximum strengths
of 1.91 GPa and 1.84 GPa, while their plastic deformation
differs prominently from each other. The plastic strain of
2.2% was obtained for the diameter 3 mm specimen, and
prominent serrations appeared on the compressive stress–
strain curve. The plastic strain decreases to 1% for the
diameter 4 mm specimen, and to 0.1% for the diameter 6 mm
Table 1 Compressive mechanical property parameters of
Zr55Al10Ni5Cu30 BMG specimens with different diameters.
Diameters
of
specimens
(mm)
Yield
strength,
ss (GPa)
Maximum
strength,
smax (GPa)
Plastic
strain,
ep (%)
Elastic
modulus,
E (GPa)
3 1.83 1.91 2.20 115.20
4 1.65 1.84 1.00 110.11
6 1.685 1.690 0.10 100.02
3 4 5 6
1.7
1.8
1.9
Stre
ss
(GP
a)
Diameters of specimens (mm)
0
2P
lasi
c st
rain
(%)
Compressive fracture strengthPlastic strain
Fig. 4 Compressive fracture strength and plastic strain for
Zr55Al10Ni5Cu30 BMG specimens with different diameters.
10µm
Fig. 5 Compressive fracture morphologies for Zr55Al10Ni5Cu30 BMG
(b) diameter: 4 mm specimen and (c) diameter: 6 mm specimen.
Effect of cooling rate on plastic deformation of Zr-based metallic glasses 23
specimen. By comparing plastic strains and stress–strain curves
of three specimens, it has been found that more plastic
deformation was induced before fracture for the diameter
3 mm specimen, and only very limited plastic deformation
occurred (is produced) for the diameter 6 mm specimen with
the fracture strength of 1.69 GPa.
Compressive mechanical properties have been tested for three
times to ensure the data’s reliablility. The yield strength (ss),maximum strength (smax), plastic strain (ep) and elastic modulus
(E) of the specimens with different diameters are listed in Table 1.
It can be seen clearly that, with the increasing diameter of the
specimens, smax, ep and E decreased gradually. Fig. 4 shows the
rules of smax and ep for specimens with different diameters.
Fig. 5 shows compressive fracture morphologies for
Zr55Al10Ni5Cu30 BMG specimens with different diameters.
The typical fracture morphology of full bulk metallic glass is
shown in Fig. 5(a)–(c), which is a typical characteristic of
fracture feature with well-developed vein patterns. The local
melting and the softened alloy, which look like liquid droplets,
can be observed on the fracture surface as shown in Fig. 5(b)
and (c). However, there is no liquid droplets in Fig. 5(a), and
vein patterns are shallow, and this is consistent with the good
plastic deformation of diameter 3 mm specimen. The previous
study reported that the density of the vein patterns can reflect
the changes of the strength and the plasticity of the specimen,
i.e., the larger the vein patterns density, the higher the strength
and the plasticity of the specimens [20]. Fig. 5 shows that the
density of the vein patterns of diameter 3 mm specimen was the
largest among the three specimens and hence diameter 3 mm
specimen has the highest strength and plasticity.
It is generally known that the plastic behavior of amorphous
alloy is closely related to the free volume [21]. During plastic
deformation process, the difference of elastic modulus between the
10µm
10µm
specimens with different diameters: (a) diameter: 3 mm specimen,
Exo
ther
mic
hea
t flo
w(m
W/m
g)
Temperature (K)500 600 700
500 600 700 800 900 1000 1100 1200Tempreture(K)
Exo
ther
mic
hea
t flo
w(m
W/m
g)
Fig. 6 DSC curves of Zr55Al10Ni5Cu30 BMG specimens with
different diameters (b) is the magnified figure of (a) in the
temperature range of 500–710 K.
C. Li et al.24
region with a lot of free volume and the adjacent region induces
local stress concentration, which leads to nucleation of shear
bands. For amorphous alloys with same chemical compositions,
the smaller the specimen is, the larger the cooling rate is in
solidification process, thus more excess free volume is obtained.
Under compression, if the specimens contain more excess free
volume, more shear bands can be nucleated and propagated in the
deformation process, which contribute to more superior plasticity.
According to Cohen and Turnbull [22,23], the excess free
volume is arrested as a structural defect during the rapid
solidification. Different amorphous structures can be obtained
under different cooling rates, i.e., the content of excess free volume
is actually determined by the cooling rate. The higher the cooling
rate is, the more excess free volume will be quenched in. The
critical cooling rate T can be calculated by the following formula
T¼4K(Tm�Tg)/CR2 [24], where K¼0.01 Wmm�1 s�1 K�1 is
thermal coefficient, Tg (¼673 K) and Tm (1107 K) are glass
transition temperature and fusion temperature, respectively,
obtained from DSC curve shown in Fig. 4(a). And C is volume
heat capacity, C¼0.004 J mm�3 K�1, diameter of alloy is denoted
by R with unit mm. Based on the relationship between the critical
cooling rate and the specimen diameter, the cooling rates of
diameter 3 mm, diameter 4 mm and diameter 6 mm specimens are
482, 272 and 120 K s�1 respectively. The results indicate that, with
the increasing size of specimens, the cooling rate is almost multiply
decreasing, which suggests that the cooling rate is very sensitive to
the size changes of specimens. Accordingly, the amount of free
volume trapped in the amorphous alloys should be smaller in the
larger specimens than that in the specimen with a smaller size,
which explains the difference in plasticity of the Zr-based
BMGs with different diameters.
It is also known that, during heating amorphous alloys, the
heat release of the exothermic reaction occurred below glass
transition temperature Tg is attributed to structural relaxation
[25]. The heat release is sensitive to the cooling rate and
proportional to the free volume changes during the structural
relaxation [26,27]. For a better understanding of the differences in
plastic deformation of the Zr55Al10Ni5Cu30 BMGs with different
diameters, thermal properties related to the free volume were
measured. The DSC curves of Zr55Al10Ni5Cu30 BMG specimens
with different diameters are shown in Fig. 4. The specimens with
different diameters showed almost identical DSC curves with a
distinct glass transition temperature (Tg) at 673 K, crystallization
temperature (Tx) at 757 K and fusion temperature (Tm) at
1107 K, as shown in Fig. 6(a). Fig. 6(b) shows the magnified
DSC curves in the temperature range of 500–710 K, the heat
releases are 2.8� 104, 1.6� 104 and 1.2� 104 J kg�1 for the
diameter 3 mm, diameter 4 mm and diameter 6 mm specimens,
respectively. The results indicate that the specimens of larger size
possess a more relaxed structure, which means that the specimens
with larger size possess smaller free volume than that of the
specimens with smaller dimension. Therefore, when the diameter
3 mm specimen is compressed, the regions concentrating more
free volume facilitates the forming of multiple shear bands, and
then leads to a larger plastic strain, as shown in Fig. 3.
4. Conclusions
1)
The cooling rate affect the plastic deformation behavior ofZr-based BMGs significantly. The different compressive
plastic deformation of diameter 3 mm, diameter 4 mm and
diameter 6 mm Zr55Al10Ni5Cu30 BMG as-cast specimens
from the same ladder-shaped rod is attributed to the different
cooling rates during the preparation of the specimens. The
plastic strain of the specimens with the diameters of 3 mm,
4 mm and 6 mm were 2.2%, 1% and 0.1%, respectively.
2)
The better plastic deformation of the specimens with highcooling rate may be attributed to the generation of more
free volume in the specimens to facilitate the formation of
multiple shear bands.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (Grant no. 50961008, 51061008) and the
‘973’ Special Preliminary Program (Grant no. 2011CB612203).
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