The Iron - Niobium Phase Diagram and the Viscosity of Liquid Alloys in This System

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 60at%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 – 60at.%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 - 40at.%Nb. As far as concentration interval from 40 up to 80at.%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 90at.%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.

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).


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.

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 tSare represented in the table.

Table. Kinematic viscosity, liquidus and solidus temperatures of Fe-Nb alloys

at.% (mas.%) / tL ,
С / tS ,
С / Temperature interval
Co / t = Аexp(E/RT)
m2/c / E,
0 (0) / 1535 / - / 1535 – 1630
1630 - 1800 / 0,264
0,522 / 51,5
2,5 (4,1 ) / 1490 / 1360 / tL – 1575
1575 - 1800 / 1,126
0,579 / 31,9
6,3 (10,0) / 1445 / 1370 / tL – 1575
1575 - 1800 / 0,408
0,317 / 47,4
11,5 (17,8) / 1360* / 1360* / tL – 1575
1575 - 1800 / 1,283
0,448 / 31,0
16,7 (25,0) / 1475 / 1370 / tL - 1800 / 0,295 / 55,4
28,6 (40,0) / 1620 / 1365 / tL - 1800 / 0,294 / 58,5
37,5 (50,0) / 1630 / 1620 / tL - 1800 / 0,206 / 65,6
47,5 (60,0) / 1595 / 1530 / tL - 1800 / 0,184 / 69,2
50,0 (62,5) / 1560 / 1530 / tL - 1800 / 0,310 / 60,1
53,9 (66,0) / 1550 / 1530 / tL - 1800 / 0,304 / 61,5
59,5 (71,0) / 1530 / 1500 / tL - 1800 / 0,083 / 83,2
60,7 (72,0) / 1550 / 1485 / tL - 1800 / 0,080 / 84,9

* - from data [5].

The liquidus and solidus temperatures of alloys in the concentration interval 45 - 60at.%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 53.9at.%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.

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:


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].


1 -


1 -

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 15at.% have no breaks, the viscosity monotonically decreases with temperature grows.

The viscosity isotherms and equal superheating curves (fig.4) for melts of Fe-Nb system were drown using data of the table.


1 -


1 -

Niobium addition to iron induces viscosity increase of melt. The viscosity change in a Fe-Nb system is nonmonotone. Alloy composition 2.5at.%Nb is characterized by sharp growth of viscosity (~ by 20%). Viscosity level in before eutectic area of alloys varies insignificantly. Viscosity isotherm at 11.5at.%Nb (composition, close to eutectic) has break. Viscosity increases in an interval 11.5 – 47.5at.%Nb, however there is no extremum at concentration of Fe2Nb intermetallic compound. At a content of niobium about 60at.% viscosity of melt more than twice higher than viscosity of pure iron. An isotherm of viscosity in the concentration interval 40 – 60at.%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 Fe-Nb system (fig.4). However, viscosity level of melt on these curves for 11 – 40at.%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].


1. It’s found, that the niobium considerably increases viscosity of iron. Adding 2.5at.%Nb in melt increases viscosity approximately by 20%. The viscosity increases by 2 – 2.5 times at 50 - 60at.%Nb.

2. Correlation of viscosity with the phase diagram for Fe-Nb system in the concentration interval 0 – 60at.%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 – 60at.%Nb) there is peritectic transmutation, as well as eutectic equilibrium, but 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).


1.GoldschmidtH.J. The constitution of the iron-niobium-silicon system. - J. of the Iron and Steel Inst., 194 (1960) 2, p.169-180.

2.KubashevskyO. 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.FrohbergM.G., SchaefersK., KuppermannG. Mixing enthalpies of liquid iron with Vb and VIb elements. - Steel research, 66 (1995) 9, p.367-371.


1 -