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Investigations of Impurities in TiC-C Eutectic System as a Fixed Point
A. Bourdakin, M. Sakharov, B. Khlevnoy, S. Ogarev, V. Sapritsky 1
A. Elyutin 2
1 All-Russian Research Institute for Optical and Physical Measurements (VNIIOFI), Moscow, Russia
2 Academician of Russian Academy of Science
Abstract. Data on elimination of impurities from TiC-C
are presented and discussed. The idea of less sensitivity of
eutectics as the fixed points to impurity, than that in case
of pure materials, is suggested. It is concluded that criteria
of both initial metals purity and metal-carbon eutectics
purity are not as tough as for fixed points on pure metals.
Statement of the problem
In paper [1] there were presented results conducted at
VNIIOFI investigations of melting/freezing phase
transition in eutectic system TiC-C as a prospective fixed
point for radiometry, photometry and radiation
thermometry. The biggest requirement for a fixed point is
its temperature reproducibility. Further investigations are
to be directed at lowering the non-reproducibility and
discovering its fundamental roots.
Deviations from equilibrium transition temperature
could be classified in the following way. 1) Caused by
physico-chemical properties of the real materials. 2)
Caused by thermal non-equilibrium in the sample volume
in conditions of real experiment. The present work focuses
on deviations originated from intrinsic material properties.
It is commonly accepted that the main factor of
non-equilibrium of phase transition in mono-component
(uniform) metals and eutectic alloys is impurity [2-4].
In uniform metal short-range order of atoms disposition
remains in molten state and thus melting involves just
minimal rearrangement of atoms [5]. Melting mechanism
keeps identical in the whole sample volume if structural
defects are neglected. In case of high-purity metal melting
kinetic factor of impurity distribution between solid and
liquid phases does not essentially change the mechanism.
On the contrary, even in totally pure eutectic system
mechanism of melting is not identical in the sample
volume. Structural factor can not be neglected now.
Melting transition of eutectic alloy, occurring in
accordance with equilibrium phase diagram, can be
preceded by premelting on interphase surfaces and grain
boundaries [5].
Kinetics of eutectic transformation leads to additional
deviation from equilibrium, because melting of extremely
non-uniform eutectic system requires mass transfer of
eutectic components through liquid phase. Atoms of
eutectic alloy are transferred for distances comparable to
structural parameters of solid eutectic. That is many times
as much as for melting transition in pure metal where only
minimal displacements of atoms from crystal lattice nodes
are required.
Influence of structural factor on melting point and
melting plateau shape was discussed by example of
eutectic Fe-C [6]. Slight decrease of melting temperature,
which was preceded by “fast” solidification from melt, the
authors explained by two main grounds. 1) After “fast”
solidification eutectic comes to metastable state. Having
more refined phase structure than eutectic in stable
equilibrium state it possesses enhanced interphase surface
Gibbs energy. 2) Premelting on more developed grain
boundaries provides for more substantial contribution to
entire melting process than in stable state.
The problem is that eutectic alloys intrinsically are
systems with highly refined phase structure and developed
interphase surfaces. It is unclear so far what eutectic
structure should be attributed to stable state, and what
structure – to metastable. Consequently, it remains unclear
what degree of fine-dyspersation of eutectic phases forces
us to take into account excessive surface energy.
We suppose that there is a certain “threshold purity
level” for different substances above which other causes of
non-equilibrium become more important than content of
impurities. In our opinion, for eutectic systems this level
should be lower than that of relatively simple uniform
metals, for the reason of complexity of the mechanism and
kinetics of eutectic transformation.
While reaching “the threshold purity level” structural
and kinetic factors should play the key role in deviation
from equilibrium temperature of eutectic melting/freezing.
Proceeding from this assumption we set the task to
evaluate the level of impurities content in metal-carbon
eutectics, above which further purification brings no result
in terms of melting plateau quality and reproducibility.
Experimental
At the first stage we investigated influence of initial
titanium purity on final composition of eutectic TiC-C.
Earlier Yamada et al. observed elimination of impurities in
initial rhenium after smelting Re and C.
Two samples of initial titanium and four eutectic alloys
TiC-C were analyzed by SSMS method with susceptibility
at 0.001 ppm. For very low impurity content uncertainty
can exceed 100%, mostly due to strong non-uniformity of
impurity distribution and extreme locality of this method.
As will be seen later, it is quite sufficient for targeted goal.
The first and the second samples TiC-C were prepared
out of titanium hydride 0.9999 purity (Ti#1) and 0.999995
extra purity carbon powder by smelting in argon flux and
under vacuum correspondingly. The third and the fourth
TiC-C samples were prepared in the same way, but out of
titanium powder 0.9985 purity (Ti#2). Ti#2 was
preliminary eliminated of hydrogen.
Whatever initial titanium had been taken, the final
purity of solid ingots was 0.99995 after smelting in argon
and 0.99997 after smelting under vacuum. Results obtained
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 279