BULK PROPERTIES OF THE Cu50Zr50 ALLOY
V. Bykov, D. Yagodin, T. Kulikova, I. Sipatov, K. Shunyaev
The Institute of Metallurgy of Ural Division of Russian Academy of
Science, Russia
k_shun@mail.ru
One of the most extensive and thriving research lines of modern
material science is investigation of bulk amorphous metal alloys. Among
ensemble of such alloys the special attention is given to the systems
based on Cu-Zr. This system has a unique glass forming ability: the CuZr alloy is one of the few binary metal systems, which is succeeded to
form the bulk-amorphous samples. Furthermore, these samples are
possessed excellent mechanical properties, corrosion-resistance and high
values of thermal stability. Small quantity addition of other metals (Al,
Ni, Ti, rare earth metals, etc.) results in improvement of glass-forming
ability as well as application of additives enables to obtain bulkamorphous alloys with critical thickness, which is sufficient for
industrial application. Since obtaining the first bulk amorphous samples
in 2004 [1], research boom of the Cu-Zr system has been started.
The general mystery is the nature of glass-forming ability of the
Cu-Zr system as well as relations between the ability and system
composition and elements additive concentrations. It was experimentally
determined the bulk amorphization regions are concentrated in a very
narrow composition ranges (less than 1 at %), which are not correlated
with eutectic or other characteristic points at phase diagrams. All efforts
to explain such performance has been failed. However it is necessary to
note the range of experimental techniques and theoretical approaches,
which are applied as a rule for the task, is rather limited.
With reference to what has been noted above about the Cu-Zr
alloys, the main reason of interest is due to their glass-forming ability.
Therefore a number of physicochemical properties of alloys and
intermetallics based on Cu-Zr are little-studied in crystal state at high
temperatures. There are no detailed data about thermal properties
behaviour (thermal conductivity, heat diffusivity and capacity, density,
etc.) for the Cu-Zr alloys when rising temperature.
In the present work the density of the equiatomic Cu-Zr alloy
were studied by absolute gamma-radiation technique using the unit
described in the literature [2] and NETZSCH 402CD dilatometer using
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highly sensitive sensor-transducer of linear displacement. Dilatometry
tests were performed at constant rates of heating and cooling 2 K/min in
an high purity helium atmosphere. Investigated sample was synthesised
in an induction furnace
Density determination tests were performed by gamma-radiation
densimeter in a protective pure helium atmosphere at constant rates of
heating and cooling 2 K/min in 23 mm diameter graphite crucibles.
After fusion the liquid sample was thoroughly blended by a special
mechanical stirrer. The stirrer is tube of beryllium oxide which was
inserted into protective graphite cover withtungsten-rhenium
thermocouple. To ensure compositional uniformity and absence of
gaseous pores a gamma-quantum intensity was measured along vertical
axis starting from the melt surface (~8 mm) and down to the crucible
bottom. The beam of gamma-quantum were passed through crucible at
the 2 mm distance from its bottom. The thermocouple in protective
cover was inserted into the melt. The cover was placed slightly higher
the examined zone. At the exposure time of 1000 s the counting rate of
gamma-quanta passed through the sample was in the range of 1700 1800 s-1 that corresponds to the random error of 0.15 % at the accuracy
0.95.
The existence of the two phase’s composite-mechanical mixture
obtained by inductive remelting has been determined on evidence
derived from X-ray diffraction and microstructure analysis. The phases
are amorphous and crystalline.
Measurement results obtained by dilatometer in several heating cooling cycles of temperature dependence of density for the Cu 50Zr50 in
solid state are shown in Fig. 1. As may be inferred from Fig 1, the
sample was not fully crystalline even after 3 times heating up to 850oС
and followed by cooling.
The observed crystallization and amorphization processes cycling
for low heating and cooling rates is characteristic feature of bulkamorphous composites based on the Cu-Zr system. The problem was
scrutinized in the studies [3, 4].
The temperatures T1-3 corresponds to the following
transformations: T1 = 265oС – the initiation of martensite transformation
during first heating (As). The determined temperature value is in
agreement with experimental results [5], where As equals 255oС for the
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Cu50Zr50 alloy, T2 = 540oС – the amorphous phase crystallization;
T3 = 726oС – the eutectoid transformation CuZr ↔ Cu10Zr7 +CuZr2.
Fig. 1 The relation between density and temperature for the Cu50Zr50 alloy
obtained by dilatometer.
Measurement results for temperature dependence of density in
Cu50Zr50 sample for solid and liquid states obtained on gammadensimeter are shown in Fig. 2.
For crystal state (5th heating in Fig.1) the density dependence of
temperature is described by the equation (in the range of T = 25726 oС):
dS(T) = 7.34 – 2.27·10-4∙T –2.90·10-4∙T2
(1)
In liquid state density tends to decrease linearly with the increase
of temperature, this dependence is described by the equation
dL(t) = 7.49 – 5.43·10-4∙T
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(2)
Fig. 2 Temperature dependence of density in the Cu50Zr50 sample obtained by
gamma-densimeter
The difference between density values of composite sample (data
of the 1st heating) and crystalline one (data of the 5th heating) was
determined as 0.4% at room temperature. The density-temperature
dependence (ρcr – ρcom)/ρcom is shown in Fig. 3, which demonstrates the
density distinctions of composite and crystalline samples.
The gamma-radiation technique and dilatometry were applied to
density measurement of the equiatomic CuZr alloy in the temperature
range from room one and up to 1200°С, which correspond to solid and
liquid states. Parameters for linear approximation of density-temperature
dependences were calculated from the density values which were found
experimentally. Furthermore, the density differences between initial
composite sample and completely crystalline were determined.
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Fig. 3 Density differences between composite and crystalline samples
The cycling of crystallization and amorphization processes was
found out during heating and cooling of the sample obtained by
induction technique.
Acknowledgements
This work was supported by Russian Scientific Foundation (grant
RNF 14-13-00676).
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5.
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