The chemistry of the iron-niobium intermetallics

Infermetallics 4 (1996) 211-216 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0966-9795/96/$15.00

0966-9795(95)00035-6 ELSEVIER

The chemistry of the iron-niobium intermetallics Clludio Gerald0 Schiin” & Jorge Albert0 Soares Ten&io”* “Max-Planck-Institut fir Eisenforschung GmbH, Postfach 140444, D-40074 DiisseldorJ; Germany bDepartamento de Engenharia Metahirgica e de Materiais, Escola PoIitPcnica da Universidade de SZo Paulo, 05508-900 &To Paula-SP, Brazil (Received

4 May 1995; revised version

received

12 July 1995; accepted

12 July 1995)

The chemistry of the CL-FeNb and r]-Fe,Nb, phases was studied in ferroniobium samples of two different impurity contents. Silicon and aluminium partition to the p-phase, presenting a negative concentration gradient at the primary plates of this phase. Titanium, phosphorus and sulphur, on the contrary, partition to the n-Fe,Nb, particles in the eutectic. Evidence was found for the decisive role of phosphorus and sulphur in the stabilisation of n-Fe,Nb,. Considerations are made about the equilibrium relations in the binary Fe-Nb system. Keywords: A. niobium aluminides, diagram, impurities effects.

B. crystal

INTRODUCTION

should

of intermetallics,

phase

binary phases are more precise, but the present set of data can be used as a starting point for such investigations in the future.

Even though a huge set of data is already available on binary intermetallic compounds (see Villars and Calvert’ for a compilation), it is far from complete. The experimental difficulties involved in preparing a sample free from impurities and in a true equilibrium state are still large - this was even more so in earlier times, when the control of the impurity content was more difficult and the accuracy of the experimental techniques was lower. The present work attempts to give a contribution to the determination of the true equilibrium phases in the Nb-rich part of the Fe-Nb system. In particular, we try to clarify the question of the existence of the controversial q-Fe,Nb, phase as a stable or metastable true binary phase. Our approach is unusual: we do not try to prepare a very pure binary alloy and study the equilibrium state, since earlier attempts to do that provided inconclusive results.2 We study a commercial alloy (a ferroalloy) and try to infer information on the binary system by extrapolation. Much information is obtained in this way, which helps to determine the chemistry of the studied phases. Traditional equilibrium methods, such as diffusion couple experiments and arc melt synthesis of the pure *To whom correspondence

chemistry

THE Fe-Nb SYSTEM

Most of the work on the Fe-Nb system is concentrated on the technologically important iron-rich side, where the equilibria between ct/& and y-iron with the e-Fe,Nb Laves phase and the corresponding niobium solubility in the terminal solid solutions were early stablished (see Paul and Schwartzendruber3). The medium and high niobium concentrations were first studied in the work of Eggers and Peter,4 who found two intermetallics: one with Fe,Nb, stoichiometry (later identified to be the e-Fe,Nb Laves phase) and one with higher niobium content (unidentified Fe,Nb, stoichiometry, but more than 70 wt.% Nb in the composition). The first complete Fe-Nb phase diagram was proposed by Goldschmidt in 1957, who found three intermediate phases: the e-Fe,Nb Laves phase; a a-Fe2,Nbi9 phase (Strukturbericht notation: D8,, prototype a-CrFe) and an q-Fe,Nb, phase, isostructural with Ti,Ni (equivalent to the 7)-M,C carbide) and possibly associated to Eggers’ Fe,Nb, compound. This author also suggested the existence of a fourth high temperature phase with

be addressed. 211

212

C. G. Schtin, J. A. S. Tenbrio

unidentified crystal structure and containing about 94 wt.% Fe. This work was the basis for the binary Fe-Nb phase diagram until 1982 (see, e.g. 0. Kubaschewsk?$ and this diagram is still used in recent work.7 On the other hand, Raman* in 1967 could only find two intermediate phases, the e-Fe,Nb Laves phase and a new phase, p-Fe2’Nb19, which is different from the a-phase reported by Goldschmidt for the same stoichiometry. Raman determined the structure of this phase as D85 (prototype p-Fe7W6), with space group R3m and 13 atoms in the rhomboedric unit ce11.9 The equilibrium between P-Nb and p-Fe2’Nb19 was also confirmed by other researchers. “3” Based on these results the most recent phase diagrams3,12 only display the e-Laves phase and the p-phase as stable phases. Goldschmidt’s interpretation conflicts with that of Raman,* who found only two intermediate phases in the Fe-Nb system: the l-Fe,Nb Laves phase and a F-FezlNb19 phase. According to Raman,’ the p-phase has a homogeneity range of 47-49 at.% Nb, but Kripyakevitch et ~1.‘~ found this range to be situated in the 50-52 at.% Nb region. Wetzig14 studied the compositions between E- and p-phases and found a lamellar eutectic at 48 at.% Nb, suggesting a (lOi1, IllOil,} orientation relationship between the phases for the growth of the eutectic. Kripyakevitch et al.13 found, however, an q-Fe2Nb3 phase (Fd5m Space Group, prototype Ti,Ni) in alloys containing 20-30 at.% Nb, annealed at 900°C under argon, and attributed it to a reaction of the sample with residual oxygen in the atmosphere. Lii and Jack” claim the v-Fe,Nb, phase to be, in reality, an oxide (Fe,Nb@) or a nitride (Fe,Nb,N), both precipitating at the lowest partial pressures of the gaseous component and presenting an 77-M,C carbide-type structure. Attempts to stabilise an equivalent carbide did not succeed. Raman’ had already alloyed Fe-Nb alloys with carbon, but did not observe the formation of the q-Fe,Nb, compound. The hypothesis of the stabilisation of q-Fe2Nb3 by interstitials originates from similar observations in other Ti,Ni-type compounds, the crystal structure of which presents some interstitial sites with large dimensions and is capable to dissolve large amounts of these elements.16 Tenorio and Takano17 reported the observation of a high volume fraction (-10%) of a phase with approximate q-Fe2Nb, stoichiometry in standard ferroniobium alloy. The iron and niobium compositions of this phase clearly follow a 2/3 relationship

and thus are incompatible with Lti’s metal concentrations in the nitride/oxide (a 1:l relationship”). This phase was later proposed to be the n-Fe,Nb, already observed by Goldschmidt since some of the lines attributed to it in an X-ray powder diffraction experiment were indeed found at the predicted Bragg angles. l8 Its structure, however, does not correspond to a Ti,Ni-like crystallographic model, principally due to a set of lines overlapping with the stronger p-phase reflections. This suggests that the attribution of the Ti2Ni carbide structure to this phase could be an artefact due to the lower resolution of the first film diffractograms.18 Bejarano et al2 observed the q-Fe,Nb, phase in DTA samples solidified at low cooling rate (5 Wmin). These authors attributed two thermal events at 1490-1460°C respectively, to the precipitation and dissolution of this phase. They concluded that the q-Fe,Nb, should be metastable.2 The most recent evaluation of the Fe-Nb phase diagram by 0kamoto12 rectified the diagram of Paul and Schwartzendruber,3 based on Raman’s interpretation. This diagram is presented as the most acceptable phase diagram at the time and does not include the v-Fe,Nb, compound as an equilibrium phase. Table 1 summarises the lattice parameter data of the p- and q-Fe,Nb, compounds as available in the literature.

EXPERIMENTAL

Table 2 presents the nominal compositions of the ferroniobium samples used in this work.22 The main impurities are silicon, aluminium, titanium and carbon, but the sulphur and phosphorus residuals play a significant role in the results, as will be shown later. The samples were metallographically prepared by the standard metallographic techniques (grinding on SIC paper and polishing with 6 and lprn diamond paste). Particular care was taken for eliminating the pores created during cutting and grinding of the sample, but due to the inherent brittleness some residual density of pores is unavoidable. The energy-dispersive X-ray spectroscopy (EDS) and the wave-length-dispersive X-ray spectroscopy (WDS) measurements were always performed far from the curved surfaces near the pores to warrant the precision of the correction algorithms. The EDS measurements were performed in a Cambridge S200 Scanning Electron Microscope,

Chemistry of Fe-Nb intermetallics

213

Table 1. Lattice parameter data for the CL-FeNb and vFe,Nba Lattice parameters, Phase

Homogeneity

p (FeNb) CL(FeNb) p (FeNb) p (FeNb) /1 (FeNb) CL(FeNb)

a

C

Fe saturated Nb saturated

0.4926 0.4952 0.4932 0.4928 0.4929 0.4947

2.663 2.667 2.681 2,683 2,880 2.691

At At At At At At

RT RT RT RT RT RT

8 8 9 13 15 18

0.493

2.682

At RT

18

0.492

2.679

At RT

19

At At At At

20 21 15 15

51.2-51.7

at.% Nb; 0.060.4 at.% Si; 0.2-0.7 at.% Al 50.3-51.2 at.% Nb; 3.5-5.1 at.% Si; 1.7 at.% Al; 0.3 at.% Ti 49.5550.5

EL(FeNb) Fe,Nb, Fe*Nb, Fe,Nb,O FejNb,N

nm

range, at.% Nb

? 50 ‘)

/I (FeNb)

compounds

Line Line Line Line

1.1239 1.1262 1.1347 1.1251

compound compound compound compound

Reference

Comment

-

RT RT RT RT

Table 2. Nominal composition of the ferroniobium samples (wt%)**

Standard Vacuum grade

Nb

Fe

Si

Al

C

Ti

P

S

67.3 63 Min.

29.4 Bal.

1.20 1.5 Max.

0.46 0.25 Max.

0.15 0.1 Max.

0.32 0.1 Max.

0.05 0.01 Max.

0.08 0.01 Max.

other phases present in the microstructure in addition: the q-Fe,Nb, compound and the niobium carbide (NbC) particles. The long plates of p-phase are denominated PE- (proeutectic) ,u, to differentiate it from the p-phase present in the matrix (E-, or eutectic, p). Table 3 presents the results of the EDS measurements in the standard ferroniobium samples. The results are an average of 10 measurements (except for the E-p case, in which only 4 measurements were used to calculate the average) in various particles of the phases in a single sample. The difference of silicon concentration at the center and the edge of the PE-E.Lplates indicates that the silicon partition coefficient for the solidifi71. The cation of this phase24 is Ksi = x$$:’ behaviour of aluminium seems to be similar to that of silicon, but due to the smaller variation of the content of aluminium no clear conclusion can be drawn. These two components are well-known

equipped with Lynk AN10000 EDS detector. The correction algorithm ZAF-4 was used. The WDS measurements were performed in a Cameca SX50 Microprobe, equipped with LIF, PET, TAP and spacings: crystals (interplanar PC 1 analyser 4.0267, 8.75, 25745 and 60.2 A respectively). The correction algorithm ZAF-PAP23 was used. All the samples were measured in the ‘as-cast’ condition, so that rigorously the measurements do not give equilibrium compositions.

RESULTS

AND DISCUSSION

The microstructure of the ferroniobium samples was studied by Tenorio and Takano.” It consists of coarse plates of the p-phase in a multiphase matrix, based on the p/P-Nb eutectic of the binary Raman-type diagrams. This eutectic corresponds to the final liquid to solidify and contains the

Table 3. EDS analysis of the phases in the standard ferroniobium samples (at.%), n is the number of measurements used to calculate the averages Phase

n

@Nb PE-p middle PE-p border E-IL Fe,Nb,

10 10 10 4 10

Nb 93.5 50.3 51.2 50.9 57.6

* + f f f

Fe 0.4 0.5 0.5 0.5 0.5

6.4 42.6 43.0 43.6 39.6

f 0.2 +- 0.4 f 0.4 f 0.4 f 0.6

Al

Si co.2 5.1 + 0.3 3.8 f 0.3 3.5 * 0.3 1.2 zt 0.3

0.5 1.7 1.6 1.6

f + f f

0.2 0.2 0.2 0.2 1.o f 0.2

Ti 0.3 0.3 0.5 0.5 1.3

I!I0.1 * 0.1 + 0.2 -+ 0.1 f 0.2

C. G. Sch&,

214

sigma-like electron compound stabilisers, and so this behaviour is not surprising since both (T- and p-phases are closely related and considered as electron compounds rather than size factor compounds. l6 The titanium present in the alloy partitions mainly to the r]-Fe2Nb3 phase, suggesting that t.his element could contribute to its stabilisation. It will be shown later that other elements (P and S) have a much more important stabilizing effect. The spatial distribution of Nb in the p-phase is approximately constant, indicating that silicon and aluminium dissolve preferentially by substituting for iron, i.e. in the B-atom sublattices of this phase.16 Table 4 presents the results of the WDS measurements in the ‘vacuum grade’ ferroniobium sample. The results are in fact the average of ten points per phase inside a single sample. Apart from the lower content of impurities, characteristic of the alloy, no difference is observed between the measurements performed in the standard and ‘vacuum grade’ ferroniobium. The partition of the elements between the phases in particular remains the same, with silicon and aluminium concentrating in the p-phase, preferentially in the pro-eutectic plates. The very low titanium content measured in all phases in this alloy indicates that the presence of this element is not a necessary condition for the stabilisation of the r)-Fe,Nb, phase. The WDS measurements in the p-phase confirm the tendency of silicon and aluminium to substitute for iron in its sublattices. The systematical deficit in the sum of weight fractions of the components in the q-Fe,Nb, phase, observed either in EDS or in WDS measurements, suggests that some component was missing in the evaluation of the X-ray spectrum. To find this component the complete WDS spectrum of the r]-Fe2Nb3 and E-p phases was determined and analysed in detail.

J. A. S. Tendrio

Figure 1 presents the spectra of the /..Land r]-Fe,Nb3 phases obtained by the PC1 crystal. The greater interplanar spacing of this analyser makes it particularly, adequate to study the WDS spectrum in the region of very large wavelengths for the determination of the contents of light elements such as carbon, oxygen and nitrogen. The results show that the CKa and OKa lines are clearly discernible in the two spectra, apparently with the same intensity in both phases (no partitioning). The precise determination of the contents of these elements is very difficult due to the systematic errors originating from the contamination or oxidation of the sample surface, but the absence of partitioning is sufficient to show that these elements play no main role in the determination of the equilibrium between both intermetallics. A significant partitioning of phosphorus and sulphur between the two intermetallics was counts 400

350 300 250

-I 20

400 350 300

Table 4. WDS measurements of the compositions in wt.% of the phases in the ‘vacuum grade’ ferroniobium sample (values in bold are the corresponding atomic fractions in at.%). The titanium content was lower than O-01 wt.% in all phases

250

Phases

100

/3-Nb PE-/L E-CL Fe,Nb,

Fe 2.85 463 35.4 47.3 36.1 48.5 28.0 40.0

+ f f f + + + +

Nb 0.2 0.3 0.5 0.5 0.4 0.3 0.3 0.3

97.6 95.3 64.3 51.7 63.4 51.2 69.7 59.9

f + f f f f + f

Si 0.3 0.3 0.5 0.5 0.6 0.3 0.5 0.3

Traces COGI 0.15 f 0.02 0.40 f 0.06 0.02 f 0.01 0.06 zk~04 Traces co.02

Al Traces CO.06 0.23 ?I 10.01 0.65 + 0.02 0.08 f 0.03 0.22 + 0.07 0.014 + 0.01 0.04 f 0.03

200 150

25

30

40

45

50

55

l-i--IO,

-

E-p (FeNb)

50 0

Fig. 1. Details of the WDS spectra of the q-Fe,Nb, as obtained by the PC1 analyser.

and E-p

Chemistry

of Fe-Nb

observed, as can be seen in the PET spectra in Fig. 2. These elements concentrate in the q-Fe,Nb, phase, indicating very low solid solubility at the p-phase. The inclusion of phosphorus in the quantitative analysis of the spectra confirms this partitioning (Table 5). The phosphorus content of phase v-Fe,Nb, varied from particle to particle in the sample, always in the range between 0.9 and 2.5 at.%. The sulphur analysis was not performed since the great overlap with the Nb,, lines would lead to poor precision in the determination, but the SKa line is clearly discernible near the Nb,, line in the PET spectrum of the v-Fe,Nb, phase. The results suggest that phosphorus and sulphur are responsible for the presence of the v-Fe,Nb, phase in the ferroniobium samples. The possibility that in reality this phase is a sulphide or a phosphide should not be discarded, but, at least in the

215

intermetallics

Table 5. Composition in wt.% (with bold numbers for at.%) of the phases including phosphorus in the analysis of the WDS spectra. Individual measurements are presented Phase

Fe

Nb

Si

FezNb,

28.3 39.4 28.6 39.6 28.2 39.2 28.6 39.6 28.6 40-l 28.3 39.5 28.0 39.1 28.5 39.9 36.7 48.7 3.09 5.04

69.9 58.5 IO.2 58.5 69.9 58.3 69.7 58.0 69.8 58.8 69.8 58.5 69.7 58.5 68.9 58.0 63 9 51.0 96.7 94.8

0.02 0.05 0.04

Fe*Nb, FezNb, FezNb, FezNb, FezNb, FezNbj Fe,Nb, E-CL P-Nb

O-11

0.03 0.08 Traces < 0.03 Traces < o-01 0.02 0.06 0.02 0% Traces C 0.02 0.03 0.08 Traces < 0.02

Al

P

Traces CO.04 Traces < 6.06 0.02 0.06 Traces CO.05 0.02 0.07 Traces < 0.05 0.02 0% 0.04 O-10 0.05 o-14 Traces < 0.04

0.80 2.03 0.67 1.68 0.95 2.36 0.91 2.27 0.38 0.96 0.79 1.98 0.88 2.22 0.77 l-93 0.06 o-13 Traces < 0.03

counts 400 350 300 250 200 kl

150 100

NbL”

50 0

* 5,2

5,3

5,4

6,0

6,l

6,2

6,3

h (4

counts 400

I

350 300

Fe-Nb-P system, no known phosphide would correspond to the measured stoichiometry.25 An analysis of the ternary diagram Fe-Nb-P25 shows that the phosphide in equilibrium with the /3-Nb and p-phases is the ternary FeNb,P compound, which contains 16.7 at.% P. All other ternary phosphides in this system form on a line joining FeP to P-Nb and have stoichiometries of the type Fe,XNbYP. 25 Following this rule, a hypothetical Fe,Nb,P would also have 16.7 at.% P. The lower content of phosphorus in the q-Fe,Nb, phase suggests that this element, possibly together with sulphur, stabilises the metastable binary phase rather than forming a new ternary (or quaternary) compound. The observed volume fractions and compositions of q-Fe2Nb3 are compatible with the total phosphorus and sulphur content of the alloy and the greater purity of the ‘vacuum grade’ ferroniobium is reflected in a lower volume fraction of the q-Fe,Nb, phase.

250 200

SUMMARY

5,2

5,3

5,4

6,0

6,l

6,2

6,3

Fig. 2. Details of the WDS spectra of the q-Fe,Nb, phases as obtained by the PET analyser.

and E-p

Titanium, phosphorus and sulphur are presented as possible stabilisers of the q-Fe,Nb, phase in the ferroniobium alloy. The low content of titanium in the ‘vacuum grade’ ferroniobium shows that this element is not necessary to the stabilisation of this phase. The more pronounced partitioning of phosphorus and sulphur to the r]-FezNb, phase suggests that their role is decisive, but new experiments are

216

C. G. Schiin, J. A. S. Tendrio

necessary to clarify this question. This fact could explain the apparent conflict between the various studies of the Fe-Nb system, since very low contents of P and S seem sufficient to stabilise the qFe2Nb, phase. The control of these impurities is a well-known problem in metallurgy, principally in the earlier times, when high purity raw materials were not easily available. The results show that silicon and aluminium show high solid solubilities in the p-phase, having partition coefficients larger than one for the solidification of this phase. This confirms the stabilising character of these elements for the pphase and clarifies the reason why the earlier attempts to stabilise the q-Fe2Nb3 phase with these two elements did not succeed. The addition of silicon and aluminium to the p-phase at constant niobium content indicates that these elements are dissolved in substitution for iron in the corresponding sublattices.

ACKNOWLEDGEMENT

The authors would like to thank Mr U. Wellms at the Max-Planck-Institut fur Eisenforschung in Dusseldorf for the help in obtaining the WDS measurements. REFERENCES 1. Villars, P. & Calvert, L. D., Pearson’s Handbook, of Crystalloqraphic Data for Intermetallic Phases, ASM International, Metals Park, OH, USA, 199 1. 2. Bejarano, J. M. Z., Gama, S., Ribeiro, C. A., Effenberg, G. & Santos, C., Z. Metallk., 82 (1991) 615.

L. J., Bull. Alloy Phase 3. Paul, E. & Schwartzendruber, Diagrams, 7 (1986) 248. 4. Eggers, H. & Peter, W., Mitt. Kaiser-Wilhelm Inst. Eisenforsch., 20 (1938) 199. H. J., Research, 10 (1957) 289. 5. Goldschmidt, Iron Binary Phase Diagrams, O., 6. Kubaschewski, Springer-Verlag, Berlin, 1982. 7. Ichise, E. & Horikawa, K., ZSIJ International, 29 (1989) 843. 8. Raman, A., Proc. Indian Acad. Sci., 65A (1967) 256. 9. Raman, A., Z Metallk., 57 (1966) 301. W., Arch. Eisenhiittenwes., 33 10. Vogel, R. & Bleichroth, (1962) 195. 11. Denham, A. W., J. Iron Steel Inst., 205 (1967) 435. 12. Okamoto, H., J. Phase Equilibria, 14 (1993) 650. P. I., Gladyshevskii, E. I. & Skolozdra, 13. Kripyakevitch, R. V., Sov. Phys. Crystallogr., 12 (1968) 525. 14. Wetzig, K., Phys. Status Solidi, 34 (1969) K79. 15. Lii, F. X. & Jack, K. H., J. Less Common Metals, 14 (1985) 123. J. H., Intermetallic Compounds, John Willey 16. Westbrook, & Sons Inc., New York, USA, 1966, p. 220. J. A. S. & Takano, C., Ferroaleaciones ‘88, 17. Tenorio, Instituto Latinoamericano de1 Fierro y Acero/ILAFA, Salvador-BA, Brazil, 1988, p. kl. 18. Schbn, C. G. & Tenorio J. A. S., Anals of the IOlhBrazilian Congress on Materials Science and Engineering (CBECZMAT ‘92), Vol 1, eds E. C. da Silva ,& L. P. Cardoso, Universidade Estadual de Campinas, Aguas de Lindoia-SP, Brazil, 1992, p. 507. 19. Asrar, N., Meshkov, L. L. & Sokolovskaya, E. M., J. Less Common Metals, 114 (1988) 41. 20. Goldschmidt, H. J., J. Iron Steel Inst., 194 (1960) 169. 21. Drobyshev, V. N. & Rezukhina, T. N., Russian Metall., (Feb. 1966) 85. 22. Paraiso Filho, 0. S. & de Fuccio Jr., R., Niobium: Proceedings of the International Symposium, ed. H. Stuart, The Metallurgical Society of the AIME, San Francisco, CA, USA, 1981, p. 113. 23. Ponchon, J. L. & Pichoir, F., La Recherche Aerospatiale, 1984-3 (1984) 43. 24. Kurz, W. & Fisher, D. J., Fundamentals of solidtj?cation, 31d. Edition, Trans Tech Publications, Aedermannsdorf, Switzerland, 1989. V., Phase Diagrams of Ternary Iron Alloys, 25. Raghavan, Part III, The Indian Institute of Metals, Calcutta, India, 1989, p. 111.