THE IRON - NIOBIUM PHASE DIAGRAM AND THE VISCOSITY OF LIQUID
ALLOYS IN THIS SYSTEM
K.Yu. Shunyaev, N.V. Korchemkina, V.L.Lisin, V.P.Chentsov, N.V.Pechischeva
Institute of Metallurgy, Ural Division of RAS, Ekaterinburg, Russia
The research results of kinematic viscosity of pure iron and Fe-Nb alloys containing up
to 60 at% Nb are discussed in this article. The method of damping rotating oscillations
was used in experiments. Measurements were made in temperature interval from liquidus
up to 1800oС. The liquidus and solidus temperatures of alloys are defined. Experimental
data, obtained for compositions 40 – 60 at.% Nb, have allowed to correct phase diagram
of the Fe-Nb system in this interval.
The iron - niobium alloys are widely used for microdoping of steel. The viscosity of
melts is important technological parameter, defining hydrodynamic and diffusion processes at
dissolution of ferroalloys in molten metal. Besides, research of viscosity gives the information on
character of interparticle interaction in melts, because the viscosity is a structure - sensitive
property. The melting point is the important technical characteristics of a ferroalloy.
Many publications are devoted to study the phase diagram of Fe-Nb system, for example
[1-4]. Different authors data are in good agreement only in the concentration interval 0 40 at.% Nb. As far as concentration interval from 40 up to 80 at.% Nb is concerned, the authors
obtain contradictory information about liquidus (tL) and solidus (tS) temperatures of alloys as
well as eutectics and intermetallic compounds on the Fe-Nb phase diagram. Until recently it was
stated, that there are four intermetallic compounds (Fe2Nb, Fe21Nb19, Fe2Nb3 and FexNby,
containing about 90 at.% Nb) in Fe-Nb system, and Fe2Nb and Fe2Nb3 compound is congruent
[1, 2].
The detailed experiments which have been carried out by the authors [3,4] with use of
methods of optical metallography, X-ray analysis, differential - thermal analysis (DTA) and
electronic microprobe, have shown, that the system has only one congruent intermetallic
compound Fe2Nb and two compounds of Fe21Nb19, Fe2Nb3. The Fe21Nb19 phase is formed as a
result of peritectic reaction, the Fe2Nb3 phase is metastable. The high-temperature phase at
Fe11Nb89 – stoichiometric composition was not found in this system according to [3, 4]
publications.
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The
kinematic
viscosity measuring
method
for
metal
melts
developed
by
E.G.Shvidkovsky [5], allows to determine liquidus (tL) and solidus (tS) temperatures of alloys by
measuring of rotating oscillations logarithmic decrement (δ) of a sample (fig.1).
Fig.1. Temperature dependence of δ for
Fe - 47.4at.% Nb alloy.
Fig.2. Fragment of the Fe-Nb system phase diagram [3, 4] and the results of measurings tS (▲) and tL
(▼) from temperature dependence of a rotating oscillations logarithmic decrement of a sample.
The melt transition from liquid state to two-phase area is accompanied by sharp
decreasing of δ value and clearly revealed on «δ – t» graph curve. At solidus temperature and
after cooling by 100-200oС δ value does not vary practically. The measurement accuracy of tL
and tS is comparable to the differential-thermal analysis results. Advantage of this method in
comparison with (DTA) is the independence of the data on specific heat phase transition value.
As well as in the (DTA) method, the two-phase area boundaries data obtained at heating from
solid state, are more reliable. The random error is possible during experiments in cooling mode
because of fluid overcooling. The measuring error of liquidus temperature tL is  5, and solidus
temperature tS is  100.
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The high-temperature vacuum viscosimeter was used in experiments. The measurements
of viscosity, tL and tS values for Fe-Nb alloys were carried out in purified helium atmosphere
using cooling sample mode from 1800 oС to complete crystallization. The isothermal stand-up
technique at every chosen temperature was applied. Then experiments were repeated in heating
mode. Beryllium oxide crucibles were used in experiments. The error of viscosity measuring was
 3 %. Obtained values of tL и tS are represented in the table.
Table. Kinematic viscosity, liquidus and solidus temperatures of Fe-Nb alloys
Niobium
concentration
at.% (mas.%)
tL ,
С
tS ,
С
Temperature
interval
Co
0 (0)
1535
-
2,5 (4,1 )
1490
1360
6,3 (10,0)
1445
1370
11,5 (17,8)
1360*
1360*
1475
1620
1630
1595
1560
1550
1530
1550
1370
1365
1620
1530
1530
1530
1500
1485
1535 – 1630
1630 - 1800
tL – 1575
1575 - 1800
tL – 1575
1575 - 1800
tL – 1575
1575 - 1800
tL - 1800
tL - 1800
tL - 1800
tL - 1800
tL - 1800
tL - 1800
tL - 1800
tL - 1800
16,7 (25,0)
28,6 (40,0)
37,5 (50,0)
47,5 (60,0)
50,0 (62,5)
53,9 (66,0)
59,5 (71,0)
60,7 (72,0)
* - from data [5].
t = Аexp(E/RT)
А107,
E,
m2/c
KJ/mol
0,264
0,522
1,126
0,579
0,408
0,317
1,283
0,448
0,295
0,294
0,206
0,184
0,310
0,304
0,083
0,080
51,5
40,9
31,9
41,9
47,4
51,5
31,0
46,7
55,4
58,5
65,6
69,2
60,1
61,5
83,2
84,9
The liquidus and solidus temperatures of alloys in the concentration interval 45 60 at.% Nb (fig.2) coincide with the phase diagram in error limits of the authors [3, 4].
According to our data, tS = 1530C for alloys containing 47.4; 50.0 и 53.9 at.% Nb. Authors
[3, 4] obtained this value as tS = 1520C in this area of peritectic transmutation L +   . The
authors [3, 4] have used (DTA) technique for study Fe-Nb system. The proximity of values tL and
tS, obtained by different methods, allows to recommend the phase diagram of Fe-Nb system,
developed by the authors [3, 4], as most reliable.
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The polythermal curves of a kinematic viscosity () of iron and Fe-Nb melts are treated
by a mean squares method and represented by exponential dependence:
  A   exp E
RT ,
where Еv is activation energy of viscous flow. The correlation coefficient is r  0.95.
Temperature dependence of viscosity for iron (fig. 3) reveals discontinuity at t  1630С,
which value is ~ 6,5 %. A polythermal curve of viscosity of iron is approximated by two
exponential curves. The measuring were performed in heating and cooling mode with a
temperature steps 5 – 15oC after isothermal stand-up (10 - 15 of minutes), during which the
value of a logarithmic decrement was stabilized. The composition of a gaseous atmosphere (pure
hydrogen or purified helium) practically does not effect on polythermal curves form Fe (fig.3).
The experimental results are in good quantitative agreement with data [6] for iron with oxygen
content of 0,002 % (see fig.3). This experimental data give real argument to suppose short order
structure changes in liquid iron at ~ 1630oС as it was stated earlier by certain authors at study of
viscosity, structure and other properties [6-8].
Fig.3. Temperature dependence of
iron viscosity (oxygen concentration
0,003 %), obtained in heating (▲,●)
and in cooling (Δ,○) mode of sample
in helium (▲,Δ) and in hydrogen
atmosphere (●,○);
+ - from data [6].
The break on polythermal curves of a kinematic viscosity in Fe-Nb alloys at 2.5; 6.3 and
11.5 at.% Nb concentrations is observed, but at lower temperature (~ 1575oС). The similar
results are obtained in work [9]. Polythermal curves of viscosity for melts with concentration of
niobium more than 15 at.% have no breaks, the viscosity monotonically decreases with
temperature grows.
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The viscosity isotherms and equal superheating curves (fig.4) for melts of Fe-Nb system
were drown using data of the table.
Fig. 4. Concentration dependence of Fe-Nb
melts viscosity:
a - viscosity isotherms at 1650С (1) and
1800С (2);
b - equal overheating curves at tL+10С (1)
and tL+100С (2).
c - Fe-Nb phase diagram fragment [3, 4];
▲ - tS, ▼ - tL (our data).
Niobium addition to iron induces viscosity increase of melt. The viscosity change in a
Fe-Nb system is nonmonotone. Alloy composition 2.5 at.% Nb is characterized by sharp growth
of viscosity (~ by 20 %). Viscosity level in before eutectic area of alloys varies insignificantly.
Viscosity isotherm at 11.5 at.% Nb (composition, close to eutectic) has break. Viscosity increases
in an interval 11.5 – 47.5 at.% Nb, however there is no extremum at concentration of Fe2Nb
intermetallic compound. At a content of niobium about 60 at.% viscosity of melt more than
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twice higher than viscosity of pure iron. An isotherm of viscosity in the concentration interval 40
– 60 at.% Nb have been smoothing with a raise of temperature.
The considerable influence of niobium on fluctuation of iron is explained by the more
size of niobium atom, than iron atom. Besides there are formed more strong bond between
heterogeneous atoms in comparison with homogenous atoms in melt. The results of viscosity
measuring give evidence of strong interparticle interaction in Fe-Nb melts. It is possible to
assume formation of stable complexes according to Fe2Nb compound concentration in melts. The
correlation of viscosity with the phase diagram of Fe-Nb system testifies to possibility of
presence in melts of bond, proper for alloys in a solid state.
The equal overheating curves of viscosity also correlate with the phase diagram for FeNb system (fig.4). However, viscosity level of melt on these curves for 11 – 40 at.% Nb
concentration is approximately constant. It is possible to assume, that at concentrations area of
Fe2Nb compound on the phase diagram the viscous flux units in melt are the associates based on
this compound. If independence of associates nature on niobium concentration is assumed, than
the stable level of viscosity is natural. However quantity of such associates increases with
niobium concentration increasing, therefore growth of  - value on a viscosity isotherm in this
concentration interval is observed.
The presence of strong interparticle interaction in Fe-Nb melts, which is revealed in
viscosity measurements result, is confirmed also by mixing heats of these melts [10].
Conclusion.
1. It’s found, that the niobium considerably increases viscosity of iron. Adding
2.5 at.% Nb in melt increases viscosity approximately by 20 %. The viscosity increases by 2 –
2.5 times at 50 - 60 at.% Nb.
2. Correlation of viscosity with the phase diagram for Fe-Nb system in the concentration
interval 0 – 60 at.% Nb proves presence of associates in a liquid, probably, close to composition
of stable Fe2Nb compound.
3. The results of tL and tS measuring from temperature dependence of rotating
oscillations logarithmic decrement have proved, that in disputable area of the Fe-Nb phase
diagram (40 – 60 at.% Nb) there is peritectic transmutation, as well as eutectic equilibrium, but
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stable Fe2Nb3 intermetallic compound is absent. Hence, the reliability of the Fe-Nb diagram,
published by authors [3, 4], proves to be true.
The work is made with financial support of Russian Foundation for Basic Research
(grant № 02-03-96453-Ural) and grant «Leading scientific schools» (№ 00-15-97420).
REFERANCES
1. Goldschmidt H.J. The constitution of the iron-niobium-silicon system. - J. of the Iron
and Steel Inst., 194 (1960) 2, p.169-180.
2. Kubashevsky O. The phase diagrams of binary systems based on iron. Translation from
English under edition of L.A.Petrova. - M.: Metallurgy (1985).
3. Bejarano J.M.Z., Gama S., Ribeiro C.A., Effenberg G., Santos C. On the Existence of
the Fe2Nb3 Phase in the Fe-Nb System. - Z. Metallkunde, 82 (1991) 8, p.615-620.
4. Bejarano J.M.Z., Gama S., Ribeiro C.A., Effenberg G. The Iron-Niobium Phase
Diagram. - Z. Metallkunde, 84 (1993) 3, p.160-164.
5. Shvidkovsky E.G. Some problems of molten metals viscosity. - M.: Gos. Izd. teh.-teor.
literature,1955 (in Russian).
6. Bazin J.A., Igoshin I.N., Baum B.A., Tretyakova E.E. Kinematic viscosity of liquid
alloys of iron with oxygen. - Izv. Vuzov. Black metallurgy (1985) 9, p. 16-20 (in Russian).
7. Vatolin N.A., Pastuchov E.A. Diffraction researches of high-temperature melts
structure. - М: Nauka, 1980 (in Russian).
8. Ostrovsky O.I., Grigoryan V.A., Vishkarev A.F. Properties of metal melts. - М:
Metallurgy. 1988. - 304 p. (in Russian).
9. Ershov G.C., Kasatkin A.A. Influence of alloying elements on viscosity of liquid iron
and steels. - Izv. Vuzov. Black metallurgy (1976) 4, p. 141-146 (in Russian).
10. Frohberg M.G., Schaefers K., Kuppermann G. Mixing enthalpies of liquid iron with
Vb and VIb elements. - Steel research, 66 (1995) 9, p.367-371.
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