The detailed study of the temperature dependence of the viscosity of (Co,Fe)-(Si,B) amorphous metallic alloys up to crystallization temperatures was carried out. The kinetics of the non-equilibrium viscosity was described using Vogel-Fulcher-Tammann law below the glass transition temperature

A Juríková

J Miškuf

K Csach

P Vojtaník

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Vol. 113 (2008) ACTA PHYSICA POLONICA A No. 1
Proceedings of the CSMAG’07 Conference, Košice, July 9–12, 2007
Creep Behaviour
of Co-Based Amorphous Alloys
K. Csach, J. Mǐskuf, A. Juŕıková
Institute of Experimental Physics, Slovak Academy of Sciences
Watsonova 47, 040 01 Košice, Slovakia
and P. Vojtańık
Institute of Physics, Faculty of Science, P.J. Šafárik University
Park Angelinum 9, 040 01 Košice, Slovakia
The detailed study of the temperature dependence of the viscosity of
(Co,Fe)–(Si,B) amorphous metallic alloys up to crystallization temperatures
was carried out. The kinetics of the non-equilibrium viscosity was described
using Vogel–Fulcher–Tammann law below the glass transition temperature.
PACS numbers: 61.43.Dq, 62.20.Hg
1. Introduction
Amorphous metallic glasses based on transition metals are in the center of the
interest due to their useful mainly magnetic properties. Especially Co-based alloys
are interesting for their magnetoelastic behaviour [1, 2]. The plastic deformation
ability is very important property for the practical exploitation of these materials.
The magnetic properties are sensitive to mechanical stresses and structural defects.
Stress sensitive defects play the important role in the forming ability of these
alloys. The homogeneous (i.e. creep) deformation is controlled by the creation
and anihilation of defects and their mobility. For liquids as well as for undercooled
liquids the model for the temperature dependence of a viscosity η(T ) in equilibrium
condition is usually used [3, 4]. The empirical Vogel–Fulcher–Tammann (VFT)
equation is the Arrhenius law modified by terms of some constants B and T0 that
reflect the temperature dependence of a deformation defect concentration
η(T ) = η0 exp
B
T − T0 . (1)
Amorphous metals below the glass transition temperature are in a metastable state
and the non-equilibrium viscosity is observed [4, 5]. Structural relaxation effects
influence the structure of amorphous alloys towards more stable state [6]. The
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92 K. Csach, J. Mǐskuf, A. Juŕıková, P. Vojtańık
aim of this work is to examine the temperature dependence of the non-equilibrium
viscosity of some metal–metalloid glasses and to check the validity of VFT formula
for these alloys at temperatures below the glass transition.
2. Experimental details
Amorphous metallic ribbons Co82B18, Co75Si15B10, Co37.5Fe37.5Si15B10 and
Fe75Si15B10 in as-quenched state were used in experiments. Uniaxial tensions
of 15 MPa and 150 MPa were applied during the linear heating with the rate of
10 K/min and the elongation of samples was measured using the thermomechanical
analyser Setaram TMA92 with the resolution of 0.01 µm. A flow of pure argon
atmosphere was used to protect samples. The viscosity was determined using
the method described in [7]. This method is based on measuring of the sample
elongation ∆l(t, T, σ) during the linear heating under different stresses σ:
∆l(t, T, σ) = l0
∫ T
T0
α(T )dT + l0σ
∫ T
T0
α(T )
E(T )
dT + l(T )Kσ
∫ t(T )
0
1
η(T )
dt. (2)
The first term corresponds to the thermal dilatation of the sample with the length
l0 and the coefficient of thermal expansion α(T ). The second term is given by
the elasticity under an applied stress σ. The last term reflects the flow of the
sample under an applied stress, K is a numerical constant. Considering the small
temperature dependence of elastic properties it is possible to eliminate the first and
the second term in Eq. (2) by repeating the identical experiment in tension under
low and high stresses. The creep deformation ε was obtained by the subtraction
of length changes ∆l obtained under the high stress from the ones obtained under
the low stress. Finally, the shear viscosity was calculated using the deformation
rate ε̇ as η = ∆σ/3ε̇.
3. Results and discussion
The thermal elongation of samples measured under the low applied stress is
shown in Fig. 1. During the linear heating the structural relaxation occurs and a
deviation from the simple thermal dilatation is visible. For all samples besides CoB
the crystallization is observed as a contraction. Only the temperature interval up
to the crystallization temperature onset was taken into account for further analysis.
The measured temperature dependences of η as well as the corresponding
fits according to VFT equation can be seen in Fig. 2. It is shown that the viscosity
of CoB alloy in the whole examined temperature interval is higher than for other
samples. Increasing the content of both metalloids (Si and B) from 18% to 25%
leads to the decrease in the viscosity η due to the increased amount of more mobile
defects. Higher stability of CoB alloy is argued by the fact that during the heating
up to the temperature of 830 K, the amorphous structure is saved whereas the
amorphous alloys with other compositions undergo the crystallization.
Creep Behaviour of Co-Based Amorphous Alloys 93
Fig. 1. Dilatation curves measured under the low stress of 15 MPa.
Fig. 2. The measured viscosity η (symbols) and VFT fits (lines).
For amorphous alloys FeSiB the lowest viscosity is observed. The partial
substitution of Fe atoms by Co increases the viscosity η at all temperatures. Ad-
ditional substitution of all atoms Fe by Co does not influence the creep behaviour
significantly. The viscosity of CoSiB alloy is lower than for CoB alloy. This phe-
nomenon can be caused by the different content of metalloids and their nature.
It should be noted that the direct conclusion about the influence of the metal–
metalloid type bonds on the creep behaviour cannot be made due to the fact that
the structure of the used as-quenched samples is metastable up to the temperatures
below the glass transition temperature.
4. Summary
The detailed study of the temperature dependence of the viscosity of
(Co,Fe)–(Si,B) amorphous alloys at temperatures up to crystallization temper-
atures has been carried out. It was shown that the non-equilibrium viscosity can
94 K. Csach, J. Mǐskuf, A. Juŕıková, P. Vojtańık
be described by the Vogel–Fulcher–Tammann law. The increase in the metalloid
content in amorphous alloys causes the decrease in their viscosity. The FeSiB alloy
has lower viscosity than CoSiB with the identical content of metalloids.
Acknowledgments
The work was supported by the Slovak Grant Agency for Science — VEGA.
References
[1] D.C. Jiles, Acta Mater. 51, 5907 (2003).
[2] P. Vojtańık, J. Magn. Magn. Mater. 304, 159 (2006).
[3] I. Avramov, J. Non-Cryst. Solids 351, 3163 (2005).
[4] K. Russew, Mater. Sci. Eng. A 226-228, 779 (1997).
[5] V.A. Khonik, J. Non-Cryst. Solids 296, 147 (2001).
[6] K. Csach, V. Oceĺık, IEEE Trans. Magn. 30, 496 (1994).
[7] G. Vlasák, P. Švec, P. Duhaj, Mater. Sci. Eng. A 304-306, 472 (2001).


Creep Behaviour of Co-Based Amorphous Alloys

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Creep Behaviour of Co-Based Amorphous Alloys

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