Influence of water vapour on the isothermal oxidation behaviour of TiAl at high temperatures

Materials Science and Engineering A307 (2001) 107– 112 www.elsevier.com/locate/msea

Influence of water vapour on the isothermal oxidation behaviour of TiAl at high temperatures Shigeji Taniguchi *, Narihito Hongawara, Toshio Shibata Department of Materials Science and Processing, Graduate School of Engineering, Osaka Uni6ersity, 2 -1 Yamadaoka, Suita, Osaka 565 -0871, Japan Received 1 November 2000

Abstract The isothermal oxidation behaviour of TiAl coupons has been studied at 1100 and 1200 K in a flow of O2, mixtures of O2 and H2O (water vapour) or Ar–H2O under atmospheric pressure. Metallographic examinations were performed for the specimens oxidised under specified conditions of temperature, time and oxidant, using X-ray diffractometry, optical microscopy, scanning electron microscopy combined with energy dispersive spectrometry and microprobe analysis. The mass gain due to the oxidation increases as the H2O content increases at 1100 K. At 1200 K, the mass gain increases significantly as the H2O content increases for up to : 75% H2O over which it decreases to some degree. The oxidation follows approximately linear kinetic laws at 1200 K. The oxide scales formed in the H2O-containing gases are characterised by a two-layer structure: an outer layer consisting mainly of TiO2 (rutile) grains showing very directional growth and an inner layer which is a porous mixture of very fine TiO2 and a-Al2O3 grains. A thin Al2O3-rich layer, which is usually formed at the interface between the two layers in O2 or air, is not formed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: TiAl; Oxidation; High temperature; Water vapour; Scale; Linear kinetics

1. Introduction The improvement in the oxidation resistance of gTiAl based alloys at high temperatures is of considerable concern because of their potential application to automobile and aircraft engines. Therefore, several trials including alloying additions [1 – 4] and surface treatments [5–8] have been performed to improve their oxidation resistance. A few successful results have been reported. However, most of these studies were carried out in air or oxygen atmospheres. With a view to their potential application to engine parts, it is very necessary to understand the influence of CO2 and H2O (water vapour), in addition to N2. Their resistance to the attack by the environment should be well established before their practical use. There are several studies on the influence of N2 [9 –11]. The present authors [12] already reported that the addition of CO2 to O2 enhances the oxidation * Corresponding author. Tel./fax: +81-6-8797471. E-mail address: [email protected] (S. Taniguchi).

significantly and the scale becomes more porous and is associated with TiC. On the other hand, the study on the influence of H2O has been quite few; Kremer and Auer [13] studied it and found that no compact Al2O3 layer was formed in the scale resulting in enhanced oxidation. With the above background, the present study deals with the influence of H2O on the oxidation behaviour of TiAl for providing the relevant knowledge.

2. Experimental procedures

2.1. Specimen A TiAl ingot was produced by Ar-arc melting using a non-consumable electrode and annealed in a vacuum at 1473 K for 86.4 ks for compositional homogenisation. Its chemical composition is shown in Table 1. Coupon specimens used in this study were machined out of the ingot to have dimensions of : 15×10×1 in mm. Metallographic examinations and X-ray diffrac-

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tometry (XRD) of the specimen revealed that it consists mainly of equiaxial grains of g-TiAl phase with a few grains consisting of g-TiAl and a2-Ti3Al phases. The grain size was : 50 – 400 mm. The coupon specimens were polished with a series of SiC polishing paper of up to c 1000 and further polished with alumina powders of 0.3 mm in size to mirror finish. They were ultrasonically washed in acetone and methanol before oxidation.

2.2. Oxidation tests The oxidation runs were performed in a flow of purified O2, mixtures of O2 and H2O (water vapour) or H2O all under atmospheric pressure. The fraction of H2O was controlled by passing purified O2 through water bath heated to a specified temperature so that partial pressure of H2O can be adjusted. When H2O was used as oxidant, purified Ar gas was used as a carrier gas and it was passed through boiling distilled water with a flow rate of 50 cm3 min − 1. This gas is referred to as 100% H2O in this study. The gas flow rate was controlled with mass flow controllers. Oxygen gas and Ar gas were purified by passing it through towers of silica gel, phosphorous pentoxide and CO2 remover in this order. The oxidation runs were performed using a horizontal electric furnace. In order to include the mass of the spalled scales (if any) each specimen was placed in an alumina crucible and two sets were inserted in a quartz reaction tube. The scale spallation was very rare. When O2 was used, the reaction tube was flashed with it using a vacuum pump three times. Table 1 Chemical composition of TiAl specimen (mass %) Al

Fe

H

C

N

O

Ti

36.5

0.096

0.002

0.008

0.005

0.098

Balance

Fig. 1. Variation in the mass gain due to oxidation at 1100 and 1200 K for 100 ks with the H2O fraction.

When H2O-containing gases were used the reaction tube was flashed with the purified Ar and the furnace was switched on. When the specimen temperature reached 423 K, the specified gas was introduced. The specimen temperature was monitored by a thermocouple which was spot-welded to a dummy specimen. It took :3 and 3.5 ks to get to test temperatures of 1100 and 1200 K, respectively. After a specified period of oxidation, the furnace was switched off and the specimens were cooled in the furnace. The oxidant gases were switched to Ar gas when the specimen temperature reached :450 K to prevent the condensation of H2O except for the oxidation in O2.

2.3. Metallographic examinations The specimens oxidised under specified conditions of temperature, time and gas composition were examined with XRD using Cu-Ka radiation at 40 kV and 30 mA for identifying the phases present. Cross-sections of the specimens were observed with an optical microscope. Outer surfaces and fractured sections of the oxidised specimens were observed and examined by scanning electron microscopy (SEM) combined with an energy dispersive X-ray spectroscopy (EDS) unit. Polished sections were examined with electron probe X-ray microanalysis (EPMA).

3. Results

3.1. Kinetic tests The results of isothermal oxidation tests at 1100 and 1200 K are summarised in Fig. 1, where the mass gain due to oxidation per original specimen area for 100 ks is plotted against the H2O fraction. The mass gains are very small at 1100 and 1200 K for the oxidation in O2. The mass gain increases gradually with an increase in the H2O content for up to 100% at 1100 K. Contrary to this, an addition of 5% H2O significantly enhances the oxidation at 1200 K resulting in large mass gains. Further large mass gains follow with an increase in the H2O content of up to 75%, over which it decreases gradually. The mass gain in 100% H2O is, however, still much larger than in O2. In order to identify any clear kinetic law at 1200 K, logarithm of the mass gain is plotted against logarithm of the oxidation time in Fig. 2. The slopes in the figure are almost unity for three levels of the H2O content except for initial periods of :5 ks, indicating that the oxidation kinetics approximately follows linear rate laws in O2 –H2O and H2O. This is quite different from the oxidation in O2 where near parabolic kinetics were observed.

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Fig. 2. Logarithm of the mass gain plotted against the logarithm of time for the oxidation in 50, 90 and 100% H2O at 1200 K.

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richer in TiO2 and thicker as the H2O content increases. These tendencies are similar for the oxidation at 1200 K, except for the oxidation in 90 and 100% H2O. In order to understand the scale structure, stepwise XRD was performed. A part of the scale was removed by slight polishing parallel to the substrate surface and XRD was performed, and a set of these procedures was repeated until the high peaks for g-TiAl was obtained. In other words, the substrate surface was almost exposed. Fig. 4 shows an example of such measurement for the specimen oxidised in 50% H2O at 1200 K for 100 ks. The peaks for TiO2 become lower and those for Al2O3 and g-TiAl become higher towards the substrate. However, peaks for Al2O3 did not become major peaks, implying the formation of no continuous Al2O3 layer which is thick and dense enough to become an effective barrier layer. The XRD was also performed for the specimens oxidised for various periods to follow the sequence of development of scale structure. Fig. 5 is such an example. The chart for 1 ks oxidation at 1200 K shows peaks for TiO2, Al2O3 and g-TiAl. It is clear that during the initial oxidation period TiO2 and Al2O3 are formed, however TiO2 prevails as the oxidation proceeds. Fig. 6(a,b) are outer surfaces of the specimen oxidised at 1200 K for 100 ks in O2 and (c) is a fractured section of the same specimen. In Fig. 6(a) white particles are TiO2 grains growing on a thin Al2O3 layer (virtually Al2O3 scale). Thin parallel lines reflect the lamellar structure of two-phase grains of the substrate.

Fig. 3. XRD charts for the specimens oxidised in various O2 – H2O gases at 1100 K for 100 ks.

3.2. X-ray diffractometry and metallographic examinations Fig. 3 shows XRD charts for the specimens oxidised at 1100 K in various gases. The chart for 5% H2O is characterised by the peaks for g-TiAl, TiO2 (rutile) and a-Al2O3. In the following TiO2 and Al2O3 mean rutile and a-Al2O3, respectively unless otherwise stated. The highest peaks for g-TiAl indicate that the scale is thin and/or there are areas where the substrate is exposed. However, no scale spallation was observed with the specimen. The peaks for TiO2 and Al2O3 indicate that the scale consists of these oxides. In general, the peaks for TiO2 become higher, and those for g-TiAl and Al2O3 become lower, as the H2O content increases. This indicates that the scale becomes

Fig. 4. XRD charts for the specimen subjected to stepwise polishing from outer surface toward substrate, oxidised in O2 – 50% H2O at 1200 K for 100 ks.

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Fig. 5. XRD charts for the specimens oxidised in O2 –50% H2O at 1200 K for various periods.

Fig. 7(a –c) are outer surfaces of the specimens oxidised at 1100 K for 100 ks in 5, 50 and 100% H2O, respectively. The specimen surface for 5% H2O is covered with a very thin scale accompanied by small oxide nodules consisting mainly of TiO2 grains. This tendency varies from grain to grain of the substrate. When the H2O content was increased to 50 and 100%, most of the surfaces were covered with these nodules. The surface of TiO2 grains shows directional growth, or they have blade-like or whicker-like shapes. This is quite different from those formed by the oxidation in dry oxygen or in air, where well facetted TiO2 grains were observed [2,4]. When the oxidation temperature was raised to 1200 K, the above mentioned characteristics were enhanced as shown in Fig. 8(a –c), where surfaces of the specimens oxidised at 1200 K for 100 ks in 5, 50 and 100% H2O, respectively are shown. This tendency is weak in Fig. 8(c). The directional growth of TiO2 grains is clearly seen in Fig. 9 which is a higher magnification view of an area of Fig. 8 (b). Relatively large agglomerates of TiO2 grains consist of small platelets of varying directions.

Fig. 6. Outer surfaces (a,b) and a fractured section (c) of the specimen oxidised at 1200 K for 100 ks in dry O2. Fig. 8. Outer surfaces of the specimens oxidised at 1200 K for 100 ks in (a) 5%; (b) 50%; and (c) 100% H2O, showing directional growth of TiO2 crystals.

Fig. 7. Outer surfaces of the specimens oxidised at 1100 K for 100 ks in (a) 5%; (b) 50%; and (c) 100% H2O, showing directional growth of TiO2 crystals.

Fig. 6(b) is a higher magnification view, showing an area above grain boundary of the substrate. Here, relatively large angular grains are TiO2 and small round grains are Al2O3. The growth rates of TiO2 grains in neighboring TiAl grains are different. The scale is B1 mm thick as Fig. 6(c) shows and was found to consist mainly of Al2O3. There is a thin continuous layer beneath the scale where Al is depleted to some degree. Some details of this layer have been shown elsewhere [14 –16].

Fig. 9. A higher magnification view of an area in Fig. 8 (b), showing agglomerates of TiO2 platelets.

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4.1. Oxidation enhanced by the addition of H2O

Fig. 10. Fractured sections of the specimens oxidised at 1200 K for 100 ks in (a) 5%; (b) 50%; and (c) 100% H2O, showing structures of the scales and the substrates. The scales consist of porous outer layers and porous inner layers made of fine oxide grains.

Fig. 10(a –c) are fractured sections of the specimens oxidised at 1200 K for 100 ks in 5, 50 and 100% H2O, respectively. Examinations with XRD, SEM and EDS revealed that all the scales are characterised by porous two-layer structures. The outer layer consists of relatively large TiO2 grains and the inner layer is a porous mixture of fine TiO2 and Al2O3 grains. There is no Al2O3-rich layer at the interface between the two layers, though there was very slight enrichment of Al2O3 which cannot be a barrier layer.

4. Discussion The main points obtained in this study are as follows; (a) the oxidation of the TiAl specimen is much enhanced by the addition of H2O to O2 at 1100 and 1200 K; (b) TiO2 grains at the outer scale surface show directional growth; and (c) the scale consists of fine grains in comparison with the oxidation in O2. The following discussion will be extended mainly to these points. In general, scales formed on unalloyed TiAl in dry oxygen consist of two layers [2,4]; an outer layer consisting of TiO2 grains with a small fraction of isolated Al2O3 grains and a porous inner layer which is a mixture of TiO2 and Al2O3 grains. The outer layer seems almost dense, though there are relatively large pores near the interface of the two layers. In addition, there is a thin Al2O3-rich layer between the two layers. This layer is, however, not dense or thick enough, though it works as a barrier layer to some degree. The above knowledge is useful in the following discussion. In the present study, only TiO2 and Al2O3 were identified as oxides. It is, therefore, reasonable to conclude that the addition of H2O does not change the kind of oxide formed, but changes the nature of TiO2 and probably Al2O3 grains. In addition, no thin Al2O3rich layer was formed in the scale.

Fig. 1 clearly shows that the addition of H2O to O2 significantly enhances the oxidation. Similar results were reported by Kremer and Auer [13], who suggested a possibility of solution of H in TiO2 and also in Al2O3. They also found TiH2 with XRD, though they did not discuss this finding. Sarrazin et al. [17] studied the oxidation behaviour of pure Ti and Ti –6Al –4V (in mass %) alloy in O2 –H2O and concluded that their results can be explained by an assumption that H is incorporated in TiO2 lattice in a form of OH−. Furthermore, Kusabiraki et al. [18] found that the oxidation of pure Ti in Ar –1% and Ar –10% H2O follows parabolic kinetics and the oxidation rate is increased in comparison with the oxidation in O2. The scales were found to have a two-layer structure; an outer layer consisting of relatively large TiO2 grains and an inner layer consisting of fine TiO2 grains. The outer layer contains relatively large pores and the inner layer fine pores. They concluded that the outer layer grows by the diffusion of titanium ions and the inner layer by the diffusion of oxygen ions. Therefore, the solubility of H in TiO2 and Al2O3 and its influence on the diffusion process in TiO2 should be understood. The solubility of H or its isotopes in TiO2 [19,20] and in Al2O3 [21,22] was reported and the former has much larger solubility than the latter. Therefore, the oxidation rate enhanced by the addition of H2O is attributable to the enhanced diffusion through TiO2, though direct mechanism for this is not yet known. Basing on the above description, we can conclude that the influence of H2O in this study appears mainly through the diffusion of the titanium ions and oxygen ions in TiO2; namely the outer layer of the scale grows predominantly by the diffusion of titanium ions via interstitial sites and the inner layer by the diffusion of oxygen ions via vacancy sites. The relationship between the solution of H into TiO2 and the enhanced diffusion should be clarified in a future study. The diffusion of oxygen ions seems to be so fast in the H2O-containing atmospheres that aluminium in the substrate is oxidised without its significant movement and thus, the formation of a thin Al2O3-rich layer is prevented. The diffusion through Al2O3 seems very slow and therefore, does not seem to influence the oxidation kinetics. The nearly linear oxidation kinetics found in this study suggest chemical reaction control as Kremer and Auer [13] did, though a further study is needed for a better understanding.

4.2. Directional growth of TiO2 The detailed metallographic observations revealed that TiO2 grains show directional growth, cf. Fig.

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7(b,c), Fig. 9 and Fig. 10(b) and that the relatively large TiO2 grains are agglomerates of platelets as shown in Fig. 9. This makes good contrast with the TiO2 grains formed in dry oxygen or air which are usually surrounded by well-developed facets [2,4], indicating their recrystallisation and grain growth. A similar directional growth of TiO2 grains was reported by Sarrazin et al. [17] for the oxidation of Ti and Ti – 6Al – 4V alloy. The rutile structure possesses tetragonal symmetry with titanium ions arrayed on a body-centred tetragonal sublattice and octahedrally coordinated to the oxygen [23]. It has considerable anisotropy such that there are open channels in the lattice parallel to c-axis. Mass transport of interstitial impurities along c-axis is much faster than that along a-axis. For instance, it was reported [19,20] that the diffusion of isotopes of hydrogen in a direction parallel to c-axis is about two orders of magnitude faster than that in a direction parallel to a-axis. In addition, the diffusion of titanium ions shows similar anisotropy [23]. Therefore, we can conclude that the directional growth of TiO2 is attributable to the anisotropy which is enhanced by the presence of H in TiO2 lattice.

4.3. Fine grains in the scale During the oxidation of TiAl TiO2 grains grow by recrysallisation between the neighbouring grains. However, these formed in the H2O containing atmospheres are fine as shown in Fig. 10. Similar fine grains were observed when TiAl was oxidised in O2 – CO2 mixtures. In this case fine TiC grains were found in the scale and thought to have prevented the growth of TiO2 grains by disturbing their contact. In the present study, there was no such material which would prevent the grain growth. Adsorption of H2O molecules, therefore, seems to be responsible for this, since porous structure of the scale allows the transport of H2O molecules, for instance, by dissociation mechanism. However, there is a controversy view that the presence of H2O in the oxidant enhanced recrystallisation of TiO2 grains at the outer scale surface [17].

5. Conclusions The addition of water vapour (H2O) to O2 significantly enhances the oxidation of TiAl at 1100 and 1200 K. The scale consists of a porous outer TiO2 layer and an inner layer which is a porous mixture consisting of very fine Al2O3 and TiO2 grains. An Al2O3-rich layer,

.

which is generally formed in air or O2 at the interface between the two layers, was not formed. TiO2 grains at the outer scale surface show directional growth, implying that the anisotropic nature of TiO2 was intensified by the presence of H2O. Acknowledgements The authors are grateful to Mr. J. Nakata of Osaka University for his partial assistance in the experimental work. A part of the present work was supported by the Grant-in-Aid of Scientific Research of the Ministry of Education, Science, Sports and Culture of Japan (No. 12450295). References [1] D.W. McKee, S.C. Huang, Corros. Sci. 33 (1992) 1899. [2] S. Becker, A. Rahmel, M. Schorr, M. Schu¨tze, Oxid. Met. 38 (1992) 425. [3] Y. Shida, H. Anada, Corros. Sci. 35 (1993) 945. [4] S. Taniguchi, T. Shibata, Intermetallics 4 (1996) S85. [5] M. Yoshihara, T. Suzuki, R. Tanaka, ISIJ Int. 31 (1991) 1201. [6] S. Taniguchi, MRS Bull. 19 (1994) 31. [7] S. Taniguchi, A. Murakami, T. Shibata, Oxid. Met. 41 (1994) 103. [8] A. Gil, B. Rajchel, N. Zheng, W.J. Quadakkers, H. Nickel, J. Mater. Sci. 30 (1995) 5793. [9] U. Figge, A. Elschner, N. Zheng, H. Schuster, W.J. Quadakkers, Fres. J. Anal. Chem. 346 (1993) 75. [10] J. Rakowski, F.S. Pettit, G.H. Meier, F. Dettenwanger, E. Schumann, M. Ru¨hle, Scr. Metall. Mater. 33 (1995) 997. [11] F. Dettenwanger, E. Schumann, J. Rakowski, G.H. Meier, M. Ru¨hle, Mater. Sci. For. 251-254 (1997) 211. [12] S. Taniguchi, N. Hongawara, T. Shibata, J. Japan Inst. Metal. 62 (1998) 542. [13] R. Kremer, W. Auer, Mater. Corros. 48 (1997) 35. [14] N. Zheng, W. Fischer, H. Grubmeier, V. Schemet, W.J. Quadakkers, Scr. Metall. Mater. 33 (1995) 47. [15] Y.F. Cheng, F. Dettenwanger, J. Mayer, E. Schumann, M. Ru¨hle, Scr. Mater. 34 (1996) 707. [16] S. Taniguchi, S. Fujimoto, T. Kato, T. Shibata, Mater. High Temp. 17 (2000) 35. [17] P. Sarrazin, F. Motte, J. Besson, C. Coddet, J. Less-Common Met. 59 (1978) 111. [18] K. Kusabiraki, T. Sugihara, T. Ooka, Tetsu-to-Hagane 78 (1992) 829. [19] O.W. Johnson, S.H. Paek, J.W. DeFord, J. Appl. Phys. 46 (1975) 1026. [20] J.V. Cathcart, R.A. Perkins, J.B. Bates, L.C. Manley, J. Appl. Phys. 50 (1979) 4110. [21] S.K. Roy, R.L. Coble, J. Am. Ceram. Soc. 50 (1967) 435. [22] R.M. Roberts, T.S. Elleman, K. Verghese, J. Am. Ceram. Soc. 62 (1979) 495. [23] P. Kofstad, Nonstoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides, Robert E. Krieger, Malabar, FL, 1983.