Surface & Coatings Technology 197 (2005) 322 – 326 www.elsevier.com/locate/surfcoat

The oxidation behavior of TiAlNb intermetallics with coatings at 800 8C Yuming Xiong*, Shenglong Zhu, Fuhui Wang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Received 5 April 2004; accepted in revised form 12 November 2004 Available online 9 April 2005

Abstract The discontinuous and cyclic oxidation behavior of TiAlNb intermetallics with coatings such as NiCrAlY, TiAlCr and ultrafine enamel coatings at 800 8C was studied. The results indicated that the three coatings could decrease the mass gains of TiAlNb alloys during discontinuous and cyclic oxidation at 800 8C. However, heavy interdiffusion occurred at the interface of NiCrAlY/TiAlNb during oxidation. Some discontinuous Al2O3 scales formed at interface of NiCrAlY/TiAlNb, where interdiffusion might be depressed. A protective Al2O3 scale could form on TiAlCr coating during oxidation at 800 8C; however, the outward diffusion of Nb and inward diffusion of Cr through the interface and the formation of Al-depletion zone at the subsurface might be detrimental to the long-term oxidation resistance of coating. Enamel coating kept intact during oxidation at 800 8C. But the thermodynamic instability of enamel in contact with TiAl alloys resulted in the formation of an Al-depletion zone and TiSiO3 layer at the interface of enamel/TiAlNb, which would be harmful to the adherence of the enamel coating to the substrate. D 2004 Published by Elsevier B.V. Keywords: TiAlNb intermetallics; Discontinuous and cyclic oxidation; Coatings

1. Introduction g-TiAl-based intermetallics are the most attractive heatresisting light materials for some industries such as automobile, aircraft and power generation [1]. The lack of oxidation resistance at temperatures above 800 8C, however, has become an important obstacle for the application of these materials [2,3]. Therefore, research work undertaken in the past decade or so has been concentrated on the improvement of both mechanical properties and oxidation resistance. However, a simultaneous improvement of the above two methods (though alloying) is hardly reached. For example, Cr addition with high concentrations promotes the formation of a continuous Al2O3 scale on TiAlCr, while the alloy becomes brittle due to Cr addition [4,5]. Furthermore, Ag addition could improve the oxidation resistance of TiAl alloys due to the formation of Z-phase [6,7]. And Nb is one

* Corresponding author. Tel.: +86 24 23904856; fax: +86 24 23893624. E-mail address: [email protected] (Y. Xiong). 0257-8972/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2004.11.019

of the most attractive elements since it improves not only the oxidation resistance of TiAl-based intermetallics, but also their creep resistance and room temperature toughness [8–10]. But Nb addition will become detrimental to the oxidation resistance of TiAl with the increase of its concentration due to the formation of an additional oxide phase (TiNb2O7 or AlNbO4) [11]. Over the past 20 years at least, numerous reports showed that it was a more effective method by protective coatings to provide improved oxidation resistance to TiAl-based alloys than by alloying. Several types of coatings including enamel, MCrAlY, TiAl3 and TiAl–Ag coatings, thereby, have been used for the oxidation protection of TiAl intermetallic compounds [12–14]. However, it should be a concern for many practical uses due to the inherent brittleness of TiAl3, interdiffusion between MCrAlY and TiAl-based substrate, and high cost of TiAlAg preparation. The enamel coating may be a novel promising choice for TiAl due to its good thermal and chemical stability, and the good matching of its coefficient of thermal expansion (CTE) with the substrate [15].

Y. Xiong et al. / Surface & Coatings Technology 197 (2005) 322–326 Table 1 The nominal composition (wt.%) of enamel frit SiO2

Al2O3

ZrO2

ZnO

B2O3

CaO

Na2O

Balance

60

8.1

6.2

9.00

4.8

4.6

3.40

3.9

In the present paper, the effectiveness of NiCrAlY, TiAlCr and enamel coating for protection of TiAl–5Nb intermetallic compounds from discontinuous and cyclic oxidation at 800 8C in air was studied.

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were weighed after oxidation for some cycles. One cycle was named that the specimens was oxidized at 800 8C for 1 h every cooling for 15 min in air. Samples after oxidation were examined by means of Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-Ray Diffraction (XRD) to characterize the microstructure and identify the oxide phases. The oxidized specimens were mounted in epoxy resins, cross-sectioned, and then polished to reveal their microstructures.

3. Results 2. Experimental procedure The test alloy Ti–46.5Al–5Nb (at.%) was prepared by melting the high-purity metals in an induction furnace under an argon atmosphere, followed by pouring the melt into a cylindrical mould. The ingots were cut into 15103.0 mm specimens and ground down to 600#-SiC paper, and then cleaned in acetone by ultrasonic cleaner before use. The composition of enamel glaze was desired by empirical calculations [16–19] and shown in Table 1. The mineral materials were mixed by milling and melting for about 8 h at 1450 8C, and then the melt was poured into water. The clinker became the traditional enamel frit (50 Am) by milling for 20 h. In order to lower the firing temperature, it was further milled to ultrafine enamel frit (300 nm) [20]. The specimens were firstly sand blasted. Enamel frit mixed with ethanol was air-sprayed on the coarse surface of these alloys, dried at 1008C and then fired in air at 900 8C for 45 min. The Ti–35.45Al–20.05Cr and Ni–30Cr–6Al–0.5Y (at.%) coatings were prepared by magnetron-sputtering technique with direct current power supplies using argon plasma [21]. The specimens were oxidized at 800 8C in air for times up to 100 h. They were cooled in air to room temperature every 10 h or 20 h oxidation, and weighed by using a balance with an accuracy of 10 4 g. Cyclic oxidation test was carried out in a cyclic oxidation furnace with an auto elevator. The specimens (the spalled oxides were collected)

Mass change (mg/cm2)

0.5

Discontinuous oxidation

0.4 0.3 0.2 0.1

TiAlNb TiAlNb+Enamel TiAlNb+NiCrAlY TiAlNb+TiAlCr

1.2

Mass change (mg/cm2)

TiAlNb TiAlNb+TiAlCr TiAlNb+Enamel TiAlNb+NiCrAlY

0.6

Fig. 1 shows the discontinuous and cyclic oxidation kinetics of TiAlNb alloy without and with coatings at 800 8C in air. Coatings could decrease the mass gains of TiAlNb alloys during both discontinuous and cyclic oxidation at 800 8C. And both TiAlCr and enamel coating are more effective among the three coatings. Fig. 2 shows the cross-sectional microstructures of TiAlNb alloy without and with coatings after discontinuous oxidation for 100 h at 800 8C in air. XRD analysis results show that Al2O3+TiO2 mixed oxides scale forms on bare TiAlNb alloy after discontinuous oxidation for 100 h at 800 8C. A little amount of Nb is doped in the oxide scale according to EDS results. And a white thin layer rich in Nb forms beneath the external oxide scale (Fig. 2a). XRD results show that a discontinuous Al2O3+Cr2O3 scale forms on NiCrAlY coating during oxidation at 800 8C. I, II, III and IV interaction zones could be observed at the interface of NiCrAlY/TiAlNb after oxidation (Fig. 2b). According to EDS results, I is Al2O3 scale, and the contents (at.%) of (Ni, Al and Ti) at the other three zones were shown in Table 2. A continuous protective Al2O3 scale forms on TiAlCr coating during oxidation. However, a thin Nb and Cr interdiffusion layer appears at the interface of coating/alloy. After oxidation, the surface of enamel coating keeps uniform except that reaction layer I and II forms at the interface of coating/alloy. According to EDS results, I is rich

1.0

Cyclic oxidation

0.8 0.6 0.4 0.2

0.0 0

20

40

60

Oxidation time (h)

80

100

0.0

0

20

40

60

Cycles

Fig. 1. Discontinuous and cyclic oxidation kinetics of TiAlNb intermetallics at 800 8C.

80

100

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Fig. 2. Cross-sectional microstructures of TiAlNb alloy without (a) and with NiCrAlY (b), TiAlCr (c) and enamel (d) coatings after discontinuous oxidation for 100 h at 800 8C.

in Al2O3 with little silicates and rutile, and II is Al depletion. Fig. 3 shows the cross-sectional microstructures of TiAlNb alloy without and with coatings after cyclic oxidation for 100 cycles at 800 8C in air. Comparing to the discontinuous oxidation results, a thicker layered mixed oxides scale forms on bare TiAlNb alloy after cyclic oxidation for 100 cycles. From the cross-sectional microstructures, there hardly exists change at the interface of coatings/TiAlNb after cyclic oxidation comparing with the corresponding discontinuous oxidation results.

4. Discussion The concentration of Al at the surface of TiAlNb alloy is not enough to form a continuous protective Al2O3 scale during oxidation at elevated temperature although Nb could promote the formation of Al2O3 [22,23]. At the initial oxidation stage, TiO2 scale doped with Nb forms on TiAlNb alloy. The inward-diffusion of oxygen leads to the formation

Table 2 The contents (at.%) of Ni, Al, Ti and Nb at sections II, III and IV in Fig. 2b Zones

II III IV

Contents Ni

Al

Ti

Nb

74.4 49.5 9.2

14.7 28.7 35.6

10.9 21.8 49.5

/ / 5.7

of TiO2+Al2O3 mixed scales beneath the external TiO2 layer. There exist a lot of cracks in NiCrAlY coating due to its mismatched thermal expansion coefficient (1310 6/8C) with TiAlNb intermetallics (9.510 6/8C). Severe internal oxidation occurs in coating during oxidation. And a discontinuous NiO+Cr2O3+Al2O3 mixed scale form on coating according to X-ray results. Besides, serious interdiffusion occurs at the interface of NiCrAlY/TiAlNb due to the difference of composition between coating and substrate (Figs. 2b and 3b). However, some discontinuous alumina scales were formed due to the selective oxidation of Al at part of low oxygen partial pressure interface of NiCrAlY/ TiAlNb, then may induce to the spallation of the coating from the substrate (I in Fig. 2b). A continuous protective Al2O3 scale could quickly form on TiAlCr coating during oxidation to improve the oxidation resistance of the substrate significantly [24]. However, the formation of Al depletion at the subsurface might influence the long-term oxidation resistance of coating. The interdiffusion of Nb and Cr at the interface of coating/substrate may also shorten the life of the coating. The recent researches showed that the enamel coating could improve the oxidation and corrosion resistance of titanium alloys due to its good thermal chemistry stability and matched expansion coefficient (10.810 6/8C) with the substrate [20,25]. Furthermore, it may be a candidate for the industrial application due to its low cost and simple preparation process.

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Fig. 3. Cross-sectional microstructures of TiAlNb alloy without (a) and with NiCrAlY (b), TiAlCr (c) and enamel (d) coatings after cyclic oxidation for 100 cycles at 800 8C.

The enamel coating keeps intact during discontinuous and cyclic oxidation at 800 8C although the reaction layer appears at the interface of enamel/TiAlNb. At the low oxygen partial pressure interface of enamel/TiAlNb, alumina forms due to the selective oxidation of Al and replacement reaction of Al to SiO2. And then a layer rich in Nb and Ti appears due to the consumption of Al at the interface. And according to the Ellingham/Richardson diagram and the thermodynamic calculation, Ti might react with SiO2 which is the main component of enamel coating (the interface of coating/TiAlNb shown in Figs. 2d and 3d). The formation of reaction layer at the interface of enamel/ TiAlNb may be detrimental to the adherence of coating to the substrate.

5. Conclusions TiAlNb alloy shows good discontinuous and cyclic oxidation resistance at 800 8C in air due to the formation of adhesive TiO2+Al2O3 scale doped with Nb. NiCrAlY and enamel coating is thermodynamically not stable in contact with TiAl intermetallics, then heavy interdiffusion and reaction between coatings and substrate might cause the spallation of coatings from the substrate. Comparing to NiCrAlY and enamel coatings, TiAlCr showed an excellent protection for TiAlNb alloy from oxidation at experimental temperature. However, the interdiffusion of Nb and Cr at the interface of TiAlCr/TiAlNb may be detrimental to the mechanical performance and long-term oxidation resistance.

Acknowledgement This project was supported by the NSFC of China (No. 59625103) and also by the Chinese Academy of Sciences. The authors wish to thank Professor Weitao Wu for many helpful discussions.

References [1] M. Yamaguchi, H. Inui, K. Ito, Acta Mater. 48 (2000) 307. [2] G.H. Meier, D. Appalonia, R.A. Perkins, K.T. Chiang, in: T. Grobstein, J. Doychak (Eds.), Oxidation of high temperature intermetallics, TMS, Warrendale, PA, USA, 1988, p. 185. [3] S. Becker, A. Rahmel, M. Schorr, M. Schutze, Oxid. Met. 38 (1992) 425. [4] M.P. Brady, W.J. Brindley, J.L. Smialek, I.E. Locci, JOM 48 (1996) 46. [5] Z. Tang, F. Wang, W. Wu, Oxid. Met. 48 (1997) 511. [6] V. Shemet, A.K. Tyagi, J.Sl. Becker, P. Lersch, L. Singheiser, W.J. Quadakkers, Oxid. Met. 54 (2000) 211. [7] L. Niewolak, V. Shemet, C. Thomas, P. Lersch, L. Singheiser, W.J. Quadakkers, Intermetallics 12 (2004) 1387. [8] P.L. Martin, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 243 (1998) 25. [9] T. Nakano, A. Yokoyama, Y. Umakoshi, Scr. Metall. Mater. 27 (1992) 1253. [10] G. Venketaraman, K.R. Teal, F.H. Froes, Mater. Sci. Eng. 98 (1988) 257. [11] H. Jiang, M. Hirohasi, Y. Lu, H. Imanari, Scr. Mater. 46 (2002) 639. [12] S. Taniguchi, Mater. Corros. 48 (1997) 1. [13] Z. Tang, F. Wang, W. Wu, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 276 (2000) 70. [14] L. Niewolak, V. Shemet, A. Gil, L. Singheiser, W.J. Quadakkers, Adv. Eng. Mater. 3 (2001) 496.

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[15] F. Wang, Z. Tang, C. Guan, Y. Xiong, W. Wu, Chinese Patent, No. 98114349.0. [16] U. Winkelman Schott, Ann. Phys. 49 (1893) S401. [17] W. Gou, Calculations of enamel glaze (in Chinese), 1964, Report of Chin. Chongqing Ceram. Soc. [18] A. Fetterolf, C.W. Parmelee, J. Am. Ceram. Soc. 12 (1927) 193. [19] S. English, W.E.S. Turner, J. Am. Ceram. Soc. 12 (1929) 760.

[20] [21] [22] [23] [24] [25]

Y. Xiong, S. Zhu, F. Wang, Surf. Coat. Technol. 190 (2005) 195. Z. Tang, F. Wang, W. Wu, Intermetallics 7 (1999) 1271. Y. Shida, H. Anada, Oxid. Met. 45 (1996) 197. W.J. Quadakkers, N. Zheng, E. Wallura, Werkst. Korros. 48 (1997) 28. Z. Tang, F. Wang, W. Wu, Surf. Coat. Technol. 110 (1998) 57. Y. Xiong, F. Wang, W. Wu, Y. Niu, Mat. Sci. Forum 369–372 (2001) 743.