Oxidation of niobium aluminide NbAl3

Materials Science and Engineering, A 120 (1989) 55 - 59

55

Oxidation of Niobium Aluminide NbAI3* M. STEINHORST and H. J. GRABKE Max-Planck-lnstitut f~r Eisenforschung GmbH, Max-Planck-Strasse 1, D-4000 Diisseldorf (F.R.G.) (Received February 23, 1989)

Abstract The oxidation of the intermetallic compound NbAI3 and of materials with some excess aluminium (NbAl3 + AI) was investigated at temperatures between 650 and 1200 °C in He-133.3mbar 02. At temperatures between 1000 and 1200 °C, the NbAI3 single phase shows a quasi-linear oxidation rate caused by cracking and spallation of the protective oxide scale. The Nb2Al phase is formed at the oxide-scale interface on which the fast-growing oxides AINb04 and Nb205 are formed upon cracking of the oxide scale. The oxidation kinetics of (Nb.413 + AI) can be described by ~t-A1203 growth according to the parabolic rate law between 1000 and 1200 °C. Between 650 and I000 °C, NbAI3 shows the phenomenon of disintegration with no appreciable bulk oxidation, known as "'pest". A maximum in the oxidation rate of the NbAI3 single phase is observed at 750 °C. An excess of aluminium decreases the oxidation rate and no disintegration occurs.

1. Introduction NbAI3 is an interesting aluminide for high temperature use because of its high melting point of about 1600 °C, low density (p = 4.54 g cm -3) and high aluminium concentration for a promising oxidation resistance. The oxidation behaviour has been investigated at high temperatures [1-6] but most probably not for the pure NbA13 phase. NbAI3 is a line compound with aluminium and Nb2AI as neighbouring phases and in most cases reported in the literature, it was probably not the NbAI 3 single phase that was investigated but NbA13 with an excess of aluminium. This can be deduced from the method of preparation of the investigated materials [ 1, 3, 6].

*Invited paper. 0921-5093/89/$3.50

At intermediate temperatures in oxygen-conmining atmospheres NbAI3 shows the phenomenon of grain boundary disintegration, known as "pest" [7]. The mechanism of grain boundary oxidation has not been explained satisfactorily up to now [8, 9]. The aim of this study was to elucidate the oxidation mechanism of NbAI3 in :the temperature range 650-1200 °C. It will be shown that an excess of aluminium drastically changes the oxidation kinetics between 650 and 1200 °C compared with the NbA13 single phase. 2. Preparation of the intermetallie phases The production of the NbAI 3 single phase is very difficult because this aluminide only exists in one definite composition and the difference in the melting points of niobium and aluminium is approximately 1800 °C. Therefore in melting these elements aluminium evaporates and pores can appear in the cast product. The single phase shows a distinct sensitivity to thermal shock which decreases at very high aluminium excess. Therefore a combination of melting and a powder-metallurgy processing technique was used to obtain materials near the stoichiometric composition and materials with small amounts of aluminium at the grain boundaries. The chemical analyses (in weight per cent) of the samples investigated are listed in Table 1. The sample NbA13(I) would correspond to the singlephase material; NbAI3(4) has a high aluminium excess so that it could be prepared by arc-melting. TABLE 1 Chemical analysis (wt.%) of the intermetalhc

compounds Aluminide

A1

Nb

0

S

N

C

NbA13(1) NbAI3(2 ) NbA]3(3 ) NbAI3(4)

45.7 46.5 46.2 53.3

54.1 53.5 53.9 46.9

0.06 0.09 0.08 0.02

0.0009 0.0014 0.0013 0.0055

0.0005 0.0017 0.0013 0.0028

0.023 0.019 0.030 0.046

© Elsevier Sequoia/Printed in The Netherlands

56 3. Experimental details

The oxidation kinetics of NbAI3 was investigated by thermogravimetric measurements at 1000-1200 °C. All investigations were performed in He-133.3 m b a r O 2. The samples were cut by spark erosion, ground and polished. The measurements were started by heating the sample to reaction temperature in purified He. At this temperature, the oxygen was added to the reaction gas flow. Failure of the oxide scale could be detected by acoustic emission analysis. The disintegration process at intermediate temperatures was examined by gravimetric oxidation experiments in the temperature range 6501000 °C, in which the temperature was raised in steps. The temperature in these step-oxidation experiments was increased by 50 °C every 24 h and, finally, the specimens were oxidized at 1000 °C for 70 h. In addition, fracture samples were oxidized at the most critical temperature and broken in UHV. The transient state of the oxidation was analyzed by short-time experiments at 1000 °C. The corrosion products were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and by taking Auger electron spectroscopy (AES) sputter profiles.

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.| 4. Oxidation behaviour at I000, II00 and 1200 °C

o

17500-

4.1. Isothermal gravimetric rexperiments The oxidation ~ kinetics of the NbAI3 intermetallic phase was investigated by thermogravimetric measurements at I000-1200 °C in He-O2. At I000-1200°C for the NbAI3(I ) single phase a transition from a parabolic to a quasilinear oxidation kinetics occurs, which was Caused by cracking and spallation of the oxide scale, as seen from acoustic emission analysis (Figs. l(a) and l(b)). Investigations by SEM and X R D showed that the oxide scale consists of several layers: a sequence of an outer AINbO4 layer, a layer of ~-A1203, a layer of AINbO4 with small amounts of Nb2Os, followed by acAI203 on the metal phase. The aluminiumdepleted Nb2AI phase is formed in the metallic matrix at the oxide-metal interface. From these results, it follows that the oxidation mechanism of NbAI3 is as follows. At first an ~-A1203 oxide scale grows on the NbAI3 single phase so

SSO

412,5

i

0,~

(b)

137,5

275 Time in h

~12,5

550

Fig. I. (a) Thermogravimetric measurement and (b) acoustic emission analysis of the NbAI3(1 ) intermetallic phase at I000 °C in He-133.3 mbar 02.

that the aluminium-depleted Nb2AI phase is formed. In the continued oxidation process the g-A1203 scale cracks and the atmosphere gains access to the metallic matrix, which is now the aluminium-depleted Nb2AI phase. Accelerated oxidation follows because the faster-growing oxides AINbO4 and Nb205 are formed on Nb2A1. In the growth of AINbO4 and Nb205, niobium ions enter the oxide phase so that now the

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He-O2, P0f 133,3mbar

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u

.~ 10"~

10-B

I0 -~' 0,0

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75

10,0

Excess of aluminium in w~

Fig. 2. Influence of the excess aluminium on the parabolic rate constant in the temperature range 1000-1200°C in H e 133.3 mbar 02.

niobium activity at the oxide-metal interface decreases with increasing scale thickness. Finally, the growth of an alumina scale beneath the niobate starts again and the described process is repeated. With increasing aluminium content of the aluminide leading to aluminium precipitation at grain boundaries, the oxidation resistance improves. In the temperature range 1000-1200°C the alloys NbAI3(2)-(4) showed an oxidation rate which was close to parabolic behaviour, indicating it to be controlled by diffusion processes. Nevertheless, the oxidation behaviour deteriorated with higher aluminium concentrations on the grain boundaries. The influence of the aluminium at the grain boundaries on the parabolic rate constant is shown in Fig. 2. At low aluminium concentrations the oxidation rate increases according to the presence of Nb 5+ ions in the ~-A1203 lattice, as indicated by X R D analysis. If the aluminium content at the grain boundaries increases, the oxidation behaviour deteriorates because of bursting of grains out of the metallic matrix. In this case the aluminium-depleted Nb2A1 phase has not been observed at the oxide-metal interface.

4.2. Short-time experiments The niobium oxides NbO, NbO2 and Nb205 are thermodynamically stable at 1000 °C at an oxygen pressure of 133.3 mbar, as is A1203, and can be formed in the transient state of oxidation, i.e. the initial period when the metal phase is exposed to the atmosphere or is still in equilibrium With the scale. This was to be proved by short-time experiments at 1000 °C. Auger depth profiles and XPS investigations after oxidation for 5 s showed, especially on the NbAI3(1 ) single phase, that AI203 and Nb205 grow simultaneously and that aluminium depletion develops in the metallic matrix (Fig. 3). Later, Nb205 is undergrown by AI203 and A1NbO4 is formed by the reaction of these oxides. The apparent increase in the niobium concentration in the depth profiles is caused by reduction of the Nb2Os by the sputtering process. (AES investigations with ionic sputtering of pure Nb205 showed that the oxygen in the Nb2Os was preferentially sputtered leaving niobium and lower oxides were formed.) For samples with aluminium at the grain boundaries (aAi = 1), an increasing attack of the grain boundaries was observed so that metallic grains burst out of the intermetallic compound during the initial oxidation and holes could be detected. AES investigations demonstrated that the grains were metallic. In these investigations, AI203 was found, as was also the intermetallic compound covered by adsorbed oxygen.

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0.0

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10.0

20.0

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40.0

Sputter time

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60.0

70.0

inrain

Fig. 3. Auger depth profile of the oxide scale formed on NbAl3(l) after 5 s oxidation at 1000 °C in He-133.3 mbar 02 .

58 Aluminium melts at 660°C, thus the aluminium at the grain boundaries of the NbA13(2)(4) would be liquid at the oxidation temperature and would evaporate during the heating phase in the He flow. This was demonstrated in oxidation experiments with samples of NbAI3(2), which were encapsuled in quartz ampoules, after 24 h in pure He or 02 at 1000 °C for either metallic or preoxidized samples. In the experiments with the metallic NbAI3(2), a strong evaporation of aluminium was observed in pure He and only a slight aluminium deposit in pure 02. No evaporation could be detected during the experiment with the pre-oxidized sample. In accordance with the evaporation of aluminium, pores and channels were formed at the grain boundaries into the alloy and when oxygen was added, A1203 could be formed at the grain boundaries. A1203 has a greater volume than the metallic aluminium at the grain boundaries and its formation generates compressive strains. Finally, the contact of the grain with the metallic matrix breaks and the grain bursts out. Simultaneously, an oxide scale grows and stops the evaporation of aluminium. Reducing the aluminium concentration decreases the bursting out of grains. Investigations by XRD, XPS and AES showed that less Nb205 was formed at the beginning of the oxidation on NbA13(2) than on the NbAI3(1) single phase and that no aluminium depletion occurred. The alumina identified in the shorttime experiments was formed in the corundum modification.

Temperature in 650 0,7 1

700 t

750 t

800 850 t t

900 t

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NbAL3 111 N'oAI~ (21 POf 133,3tabar

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5. Oxidation behaviour between 650 and 1000 °C

5. I. Gravimetric step-oxidation experiments Investigations of the grain boundary disintegration ("pest") were performed by gravimetric step-oxidation experiments in the temperature range 650-1000 °C to determine the most critical temperature at which "pest" occurs. These measurements give a qualitative overview of the oxidation kinetics in dependence of the temperature. Figures 4(a) and 4(b) represent the results of NbAI3 of two different compositions. The NbAI3(1) single phase disintegrated at 750 °C in a continuous mass loss. Correspondingly, a stepoxidation with acoustic emission analysis showed a maximum in the acoustic emission at the same temperature. Investigations of the sample by SEM indicated that there was almost no external oxidation but a strong internal oxidation and the

0,0

l e

650 (b)

700

750

800 850 Temperature in *l:

9DO

950

1000

Fig. 4. (a) Mass increase measured in gravimetric step oxidation experiments and (b) acoustic emission analysis of NbAI3(1) and (2) in the temperature range 650-I000°C in He-133.3 mbar 02.

formation of cracks deep into the intermetallic phase. Figure 5 represents, for example, the AES results of intergranular fracture surfaces ot NbAI3(1) which was oxidized for 24 h at 750 °C. At this temperature the outer intergranular surfaces were covered by A1203 and the inner faces by absorbed oxygen; only in the middle of the fracture surface was there no oxygen, as proved

59

6. Conclusions

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T= I50*C t.: 21,h

g_ 0,2"

0,0 Centre

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Distance from the outer-surface

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Fig. 5. Auger analysis of the intergranutar fracture surface of NbA13(1 ) after 24 h oxidation at 750 °C in He-133.3 mbar 02.

by AES. The sample oxidized at 1000 °C and the unoxidized sample showed no oxygen on the intergranular fracture surfaces. It can therefore be concluded that the oxygen diffuses on grain boundaries in the intermetallic compound at 750 °C and A1203 precipitates at the grain boundariers. The A1203 produces compressive stresses and causes cracks into the intermetallic phase. Through these cracks the oxygen penetrates deeper into the metallic matrix and further oxide formation at least causes the disintegration of the NbAI3(1) single phase at 750 °C. In the gravimetric step-oxidation experiments and in the acoustic emission the NbAI3(2) sample showed a slight maximum of the oxidation rate only at 800-850 °C (Figs. 4(a) and 4(b)). According to X R D the oxide scale consisted of ~c-A1203 and small amounts of A1NbO4. When aluminium was present at the grain boundaries, very little oxygen diffusion into the sample was measured by AES. The alumina "clogs" the grain boundaries so that on the one hand the oxygen cannot diffuse at the grain boundaries into the intermetaUic phase and on the other the evaporation of aluminium is stopped. The result is a drastically reduced oxidation rate in the temperature range 650-1000 °C. The temperature of the maximum in the oxidation rate is increased from 750 °C up to 800-850 °C, and no grain boundary disintegration occurs.

The NbAI3 single phase shows quasi-linear oxidation kinetics above 1000°C because the parabolic growth is repeatedly interrupted by cracking and spallation of the scale. The oxidation of NbAI 3 is a repeating process of (i) ~-A1203 growth, aluminium depletion and Nb2AI formation below the scale, (ii) cracking of the scale and AINbO4 and Nb205 growth on the Nb2AI and (iii) niobium depletion and A1203 growth again. An excess of aluminium located at the grain boundaries markedly improves the oxidation properties. In this case, the oxidation kinetics obey the parabolic rate law. (NbAI3 + AI) forms protective ~-A1203 oxide scales and the aluminium-depleted Nb2AI phase has not been observed. In the transient state of NbAI3 and (NbAI 3 + AI) oxidation a simultaneous growth of A1203 and Nb205 is observed. The alumina undergrows the Nb205 in the subsequent oxidation period. The oxidation of aluminium at the grain boundaries of (NbAI 3 + A1) causes burst out of metallic grains of the intermetaUic phase. The NbAI 3 single phase disintegrates at 750 °C by grain boundary disintegration ("pest"). The "pest" is initiated by oxygen diffusion at grain boundaries into the intermetallic compound at intermediate temperatures and by the formation of alumina precipitates at the grain boundaries. An excess of aluminium decreases the oxidation rate at intermediate temperatures and increases the critical temperature of maximum oxidation from 750 to 800-850 °C, and no "pest" takes place. References 1 R. M. Paine, A. J. Stonehouse and W. W. Beaver, WADC TR 59-29, Parts I and II, 1960 (U.S. Dept. Comm. OTS). 2 C. S. Wukusick, XDC 61-4-54 and GEMP-(3A, 5/1, 7A), 1961 (General Electric Co., Cincinnati, OH, NSA 16-2177

OTS). 3 R. C. Svedberg, in Proc. Symp. on Properties of High Temperature Alloys, Vol. 77-1, pp. 331-362. The Electrochemical Society, Pennington, NJ, 1976. 4 R. A. Perkins, K. T. Chiang and G. H. Meier, Scr. Metall., 22 (1988) 419-424. 5 M. G. Hebsur, J. R. Stephens, J. L. Smialek, C. A. Barrett and D. S. Fox, Workshop on the Oxidation of High Temperature Intermetallics, Cleveland, OH, 1988. 6 G. Raisson and A. Vignes, Rev. Phys. Appl., 5(1970) 535. 7 E. A. Aitken, in J. H. Westbrook (ed.), Intermetallic Compounds, Wiley, New York, 1967. 8 J. H. Westbrook, J. Inst. Met., 91 (1962-1963) 174. 9 P. A. Turner, R. T. Pasco¢ and C. W. A. Newley, J. Mater. Sci., 2 (1967) 197.