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Effect of an Al2O3 coating on the oxidation process of a γ-TiAl phase based alloy
Corrosion Science 63 (2012) 287–292
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Effect of an Al2O3 coating on the oxidation process of a c-TiAl phase based alloy J. Małecka Opole University of Technology, P.O. Box 321, 45-271 Opole, Poland
a r t i c l e
i n f o
Article history: Received 21 December 2011 Accepted 13 June 2012 Available online 23 June 2012 Keywords: A. Intermetallics C. High temperature corrosion C. Oxidation
a b s t r a c t The effect of Al2O3 coating on oxidation was investigated at 900 and 950 °C. Isothermal oxidation testing indicated that coating was very effective in reducing the oxidation rate of c-TiAl. The technology of magnetron sputtering used in this study allowed the deposition of a coating that ensured the improvement of high temperature oxidation resistance. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Intermetallic phase based Ti–Al alloys represent very attractive structural materials designed to operate at elevated temperatures and in aggressive chemical environments. The fields of application of these materials are space industry, aviation and automotive industry (parts of gas turbines and compressors) [1–3]. Ti–Al alloys based on intermetallic phases can yield a series of valuable properties i.e. low density, good strength at high temperatures or creep resistance, however, a method of increasing the resistance to high temperature oxidation [4–7] needs to be found. The limitation for wider application of c-TiAl phase based alloys lies in the inability to produce the protective layer of Al2O3, whereas the combination of aluminium oxide and rutile formed during oxidation shows too weak protective properties at high temperature [2,8]. The technological development, however, necessitates stricter requirements for materials in terms of their mechanical properties, corrosion effects or resistance to high temperatures. One of the methods of shaping the properties of these alloys is modifying their chemical composition accomplished by introducing certain alloying elements to the mass of material. As presented in papers [9–15] the addition of Nb [9,10], Zr [10], Cr [11,12], Si [12], Mo [12], W [13], B [13], Y [14] and Ta [15] and may improve the oxidation resistance of Ti–Al intermetallic phase based alloys. Nevertheless, the addition of such elements may result in increasing the density of the initial material and deterioration of mechanical properties of the alloys. Another way of improving Ti–Al alloy’s oxidation resistance is the application of appropriate protective coatings. Meeting these requirements is possible through the application of surface engineering methods, which allow shaping the structure, phase and chemical composition of the surface layers E-mail address: [email protected] 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.06.009
of given materials [16]. Therefore, one of the methods leading to improving high-temperature corrosion resistance is designing appropriate surface layers. Numerous attempts have been made to increase the heat resistance of these alloys [17–21]. This paper describes using the technique of magnetron deposition [22,23] of Al2O3 coating. The research studies presented in this paper use an innovative method of applying the layers: dielectric layers were obtained in metallic mode of the magnetron, which resulted in producing layers of Al2O3 with increased efficiency. 2. Experimental procedures The tests were performed on the multi-component alloy Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni with the content of 46% at Al 7% at Nb, 0.7% at Cr, 0.1% at Si and 0.2% at Ni purchased from Flowserve Corporation Titanium and Reactive Metals Foundry (USA). The structure of the test alloy can be specified as duplex [24]. The samples of alloy with dimensions of 20 15 2 mm were polished with 800 grade abrasive paper and subsequently degreased in acetone. The Al2O3 layer was obtained by means of magnetron method [22]. The deposition processes were conducted in a standard vacuum station type NP500 equipped with a circular magnetron type WMK-50. The WMK magnetron was switchedmode powered by DPS [25,26] power supply unit. The sprayed target was made of N5 grade aluminium (purity 99.97%), with a diameter of £50 mm and thickness of 7 mm directly cooled with water. The samples with Al2O3 coating of 3 lm obtained in this manner underwent tests of isothermal oxidation at the temperature of 900 and 950 °C during 250 h in static air atmosphere. Mass changes due to oxidation processes were controlled by precision scale with accuracy of 10 4 g. Trials were repeated three times
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and the presented test results are averaged. For comparison, the reference alloy were samples of the alloy without the coating. 3. Test result and analysis Previous research studies concerned determining the area of resistance to high temperature oxidation of Ti–46Al–7Nb–0.7Cr– 0.1Si–0.2Ni alloy without protective coatings [9,24]. It was determined that during oxidation a multi-phase scale is formed characterised by identical sequence of sublayers: the outer sublayer is made of rutile, the middle sublayer is rich in Al2O3 and contains a lesser amount of TiO2 and the sublayer containing Al2O3, TiO2 and oxides of Nb, Cr, Ni (Fig. 1). During the oxidation of the alloy in its initial state, columnar crystallites growing in various directions can be observed on the oxidised surface (Fig. 2). The qualitative analysis of the structure and composition of the isothermal air oxidation products i.e. quantitative dominance of Ti over Al, leads to the conclusion that the product covering the surface is rutile crystallites (Fig. 3). Such an outer layer formed during oxidation of uncoated Ti–46Al–7Nb– 0.7Cr–0.1Si–0.2Ni is created as a result of out-core diffusion of titanium. Therefore, the formation of Al2O3 layer on the surface could add to improving the oxidation resistance of the analysed alloy. For this purpose, the samples were subjected to similar tests following magnetron depositing a layer of Al2O3. The course of oxidation of the oxidised alloy was presented in Fig. 4. Isothermal oxidation at 900 and 950 °C during 250 h causes mass gain, which intensifies at 950 °C both in the case of alloy in initial state as well as for the samples with the coating of Al2O3. However, smaller mass gains were observed for the alloy with Al2O3 coating in the function of time. For the purpose of verification, each oxidation test was replicated three times using the same research methodology, which enabled a more precise analysis of their cycle. The conclusion was that the tests are characterised by a high repeatability of results. In three consecutive trials the mass gain recorded for samples coated with Al2O3 were as follows: (i) At the temperature of 900 °C: 3.02 mg/cm2, 3.03 and 2.98 mg/ cm2. The average mass gain: 3.01 mg/cm2. Standard deviation: r = 0.0216. (ii) At the temperature of 950 °C: 3.10 mg/cm2, 2.81 and 3.40 mg/ cm2. The average mass gain: 3.10 mg/cm2. Standard deviation: r = 0.2410. For the uncoated alloy:
Fig. 1. Cross-section of products and metallic sublayer of the oxidised Ti–46Al– 7Nb–0.7Cr–0.1Si–0.2Ni alloy.
Fig. 2. The surface Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni after 60 h of isothermal oxidation in air at 950 °C.
Fig. 3. EDX analysis results area inside the rectangle number 1 according to Fig. 2.
(i) At the temperature of 900 °C: 4.01 mg/cm2, 3.68 and 3.78 mg/cm2. The average mass gain: 3.82 mg/cm2. Standard deviation: r = 0.1382. (ii) At the temperature of 950 °C: 4.93 mg/cm2, 4.78 and 5.15 mg/cm2. The average mass gain: 4.95 mg/cm2. Standard deviation: r = 0.1520. As shown, the performed tests confirm the repeatability of the results. The differences in the results of the changes in the mass gain of the tested samples are minimal, and therefore in Fig. 4 averaged values of the obtained measurements were presented. The outcomes quoted above clearly prove the repeatability of the results. The demonstration of the mentioned repeatability is visualised by one of the lines in Fig. 4 (for the alloy coated with Al2O3 oxidised at 950 °C). Although (as shown in Fig. 4) at the initial stage of the oxidation slightly higher mass gains were observed for the coated alloy (up to about 60 h), yet along with the extension of the oxidation time, the mass gain of the coated alloy decreases, whereas the oxidation rate increases for the uncoated alloy (both at 900 and 950 °C). It can be concluded that the difference in mass gain between the uncoated alloy and the alloy coated with Al2O3 is negligible at 900 °C but it becomes considerable at 950 °C. Trials were repeated three times and the presented test results are averaged, which rules out any random changes. The sheer observation of the outer surface of oxidised sample allows the conclusion that the oxidation products are different for coated and uncoated alloys. The structure of the scale surface formed upon oxidation of the Al2O3 coated alloy is completely different (Fig. 5). The oxidation of the coated alloy results in the formation of singular efflorescences on the surface (Fig. 5a and b), which resemble rutile crystallites forming on the surface of the oxidised uncoated alloy (Fig. 2) [24]. In order to confirm it,
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Fig. 4. The mass change of Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni oxidised isothermally at 900 and 950 °C (continuous curves for initial state alloy and dashed curves for the alloy coated with Al2O3).
Fig. 5. The surface Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni with Al2O3 layer applied after 60 h of isothermal oxidation in air at 950 °C.
chemical composition analyses were carried out in these areas. The analyses showed the presence of Al and Ti (Fig. 6a). However, the distribution of elements shown in Fig. 7 precisely shows that only Ti is present whereas Al does not occur. Where does this discrepancy come from? In EDS analysis, the registered signals are generated not only on the sample’s surface. The electrons in the beam penetrate the sample up to a certain depth and may repeatedly interact with the sample’s atoms. Thence, this situation may result from the fact that the beam of electrons returned the information not only about the composition of the efflorescences but also about the adjacent or interlaying areas. On top of that, with the efflorescences being quite thin, the beam permeated them, penetrated into the substrate and therefore the registered information may be treated as collective. As can be seen these efflorescences are not formed on the whole surface of the alloy, but erupt between the cracks of the deposited layer of Al2O3 (the analysis presented in Fig. 6b shows the occurrence of aluminium which allows concluding that it is a deposited layer of Al2O3). The applied layer of Al2O3 definitely hinders the process out-core diffusion of titanium in this case adding to reduction in efflorescence growth with Ti content on the surface of the
oxidised content. Chemical composition microanalyses of the surface of the oxidised alloy show traces of Au (visible reflections in Fig. 6). The presence of Au reflections on the radiation spectrum is a consequence of the methodology of the preparation of samples for microscopic examination. The surface distribution of elements on the surface of the Ti– 46Al–7Nb–0.7Cr–0.1Si–0.2Ni alloy coated with Al2O3 after oxidation at 900 °C (Fig. 7) confirms previous observations. The areas rich in Al and O and eruptions of Ti are visible. The cross-section of the scale formed on Ti–46Al–7Nb–0.7Cr– 0.1Si–0.2Ni coated with Al2O3 is shown in Fig. 8. The produced scale was characterised by a two-layer structure. Two layers were distinguished, an inner and an outer one, which is particularly well pronounced in Fig. 8a. The chemical composition of the marked spots was shown in Fig. 9a–c. Directly above the substrate of c-TiAl alloy (spot x1 according to Fig. 8a), the layer of TiO2 (spot x2 according to Fig. 8a) can be seen. X-ray microanalysis performed in the spot x3 according to Fig. 8a (Fig. 9c) however, shows a clear dominance of aluminium in this layer. The presence of Ca, C and Fe in the results of X-ray microanalysis is caused by organic pollutions occurring on the surface of the sample and is taken into consider-
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Fig. 6. EDX analysis results: (a) in place 1 according to Fig. 5b and (b) in place 2 according to Fig. 5b.
Fig. 7. The surface distribution of elements on the surface of the Ti–46Al–7Nb alloy coated with Al2O3.
ation in quantitative microanalysis. Micro-voids were also noticed, which can be seen in BSE as the black fields between these two sublayers. It seems that due to oxidation, it is the original layer of Al2O3 that remains on the outside, whereas the alloy underneath it undergoes oxidation, which is proven by the results of analyses presented in Fig. 8a. The oxidation of Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni alloy not only includes scale formation, but also alterations in the substrate caused by out-core diffusion of alloying elements and by forming phases and solid solutions due to the in-core diffusion of oxygen
and nitrogen. In the case of the alloy with a deposited coating the oxidation mechanism occurred as a result of in-core diffusion of oxygen and out-core diffusion of alloying elements. On the metallic substrates side a light band occurs (Fig. 8a). It is a region rich in Nb. The deposited coating turned out to be an effective barrier against the in-core diffusion of oxygen, which secured the substrate material from oxidation effects as well as slowed and prevented further oxidation of the coating, which was confirmed in the research results presented. The scale formed on the surface of the samples was bonded with the substrate and did not chip off. Despite the fact that imme-
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Fig. 8. Cross-section of the scale formed on Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni coated with Al2O3 (a); products chipped observed (b).
Fig. 9. EDX analysis results: (a) in point # 1 marked in Fig. 8; (b) in point # 2 marked in Fig. 8; (c) in point # 3 marked in Fig. 8.
diately after the oxidation the products stuck to the surface of the samples, within the period of 24 h, a large amount of the oxidation products chipped away (Fig. 8b). It can be explained by the accumulation of compressive stresses within the products resulting from so-called ‘‘weekend effect’’ [27].
4. Conclusions The technology of magnetron sputtering used in this study allowed the deposition of a coating that ensured the improvement of high temperature oxidation. The heat resistance of the alloy
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coated with a protective film of Al2O3 is higher than in the initial state alloy. This coating adds to reducing the oxidation rate and causes the mass growth to be smaller compared to the uncoated layer. The dominating component of the formed scale is Al2O3 as expected, which has a positive impact on increasing heat resistance of the alloy. The presence of Al2O3 caused the increase of titanium activity and thus enhanced the tendency for selective oxidation of aluminium and improved the heat resistance of the substrate. As a consequence of the rutile eruptions are formed induced by out-core diffusion. Taking into consideration the presented results it can be concluded that magnetron generated coating of Al2O3 caused the heat resistance of the substrate alloy to increase.
Acknowledgement The research study was financed from the funds for science in 2010–2011 as research Project No. IP 2010 023870.
References [1] W. Szkliniarz, The alloys from the binary system of Ti–Al, in: Z. Bojar, W. Przetakiewicz (Eds.), Metallic Materials with the Participation of Intermetallic Phases, Technical Military Academy, Warsaw, 2006, pp. 66–88 (Chapter 2.2, in Polish). [2] M. Yoshihara, Y.W. Kim, Oxidation behaviour of gamma alloys designed for high temperature oxidation, Intermetallics 13 (2005) 952–958. [3] M. Yamaguchi, H. Inui, K. Ito, High-temperature structural intermetallic, Acta Materialia 48 (2000) 307–322. [4] S.A. Kakare, J.B. Toney, P.B. Aswath, Oxidation of ductile particle reinforced Ti– 48Al composite, Metallurgical and Materials Transactions 26A (1995) 1835– 1845. [5] K.S. Chan, Developing hydrogen-tolerant microstructures for an alpha-2 Titanium Aluminide Alloy, Metallurgical and Materials Transactions 23A (1992) 497–507. [6] A. Takasaki, Y. Furuya, Y. Taneda, Hydrogen uptake in titanium aluminides covered with oxide layers, Metallurgical and Materials Transactions 29A (1998) 307–314. [7] T.T. Cheng, M.R. Willis, I.P. Jones, Effects of major alloying addition on the microstructure and mechanical properties of c-TiAl, Intermetallics 7 (1999) 89–99. [8] S. Krol, M. Prazmowski, High-temperature oxidation of gamma-TiAl based alloys: generation of oxidized layer, Materials Engineering 3 (2006) 456–459 (in Polish).
[9] S. Krol, J. Małecka, L. Zemcik, The effect of niobium on the kinetics oxidation behaviour of c -TiAl, Protection against Corrosion 11 (2007) 124–128 (in Polish). [10] V. Shmet, M. Yurechko, A.K. Tyagi, W.J. Quadakkers, L. Singheiser, The influence of Nb and Zr additions on the high temperature oxidation mechanism of c-TiAl alloys in Ar/O2, gamma titanium aluminides 1999, in: Y.-W. Kim, D.M. Dimiduk, M.H. Loretto (Eds.), The Minerals, Metals & Materials Society, 1999, pp. 783–790. [11] N. Toshio, I. Takeshi, M. Yatagai, T. Yoshioka, Sulfidation processing and Cr addition to improve oxidation resistance of TiAl intermetallics in air at 1173 K, Intermetallics 8 (2000) 371–379. [12] Bong Goo Kim, Gil Moo Kim, Chong Jip Kim, Oxidation behaviour of TiAl– X(X = Cr, V, Si, Mo or Nb) intermetallics at elevated temperature, Scripta Metallurgica et Materialia 33 (1995) 1117–1125. [13] L. Huang, P.K. Liaw, C.T. Liu, Microstructural Evolution of TiAl-intermetallic Alloys Containing Tungsten and Boron, Oak Ridge National Laboratory, Managed by UT Battelle for the Department of Energy, Proceedings Paper, Session II, 13 June 2005. [14] Y. Wu, K. Hagihara, Y. Umakoshi, Improvement of cyclic oxidation resistance of Y-containing TiAl-based alloys with equiaxial gamma microstructures, Intermetallics 13 (2005) 879–884. [15] S. Krol, Cyclic oxidation of c-TiAl based multicomponent alloys with addition of Ta, Protection against Corrosion 11 (special issue-A) (2005) 194–198 [in Polish]. [16] T. Burakowski, T. Wierzchon, Surface Engineering of Metals: Principles, Equipment, Technologies, Publisher CRC, 1999. [17] L. Swadz´ba, G. Moskal, M. Hetman´czyk, B. Mendala, G. Jarczyk, Long-term cyclic oxidation of Al–Si diffusion coatings deposited by Arc-PVD on TiAlCrNb alloy, Surface and Coatings Technology 184 (2004) 93–101. [18] Z. Liu, G. Wang, Improvement of oxidation resistance of c-TiAl at 800 and 900 °C in air by TiAl2 coatings, Materials Science and Engineering A 397 (2005) 50–57. [19] G.S. Fox-Rabinovich, D.S. Wilkinson, S.C. Veldhuis, G.K. Dosbaeva, G.C. Weatherly, Oxidation resistance Ti–Al–Cr alloy for protective coating applications, Intermetallics 14 (2006) 189–197. [20] T. Izumi, T. Nishimoto, T. Narita, Superior long-term oxidation resistance of Ni–Al coated TiAl alloys, Intermetallics 13 (2005) 727–732. [21] S. Mridha, H.S. Ong, L.S. Poh, P. Cheang, Intermetallic coatings produced by TIG surface melting, Journal of Materials Processing Technology 113 (2001) 516– 520. [22] W. M. Posadowski, Unconventional magnetron sputtering systems for vacuum deposition of thin coatings, OWPW Wroclaw 2001 (in Polish). [23] S. Berg, T. Nyberg, Fundamental understanding and modelling of reactive sputtering processes, Thin Solid Films 476 (2005) 215–230. [24] J. Małecka, W. Grzesik, A. Hernas, An investigation on oxidation wear mechanisms of Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni intermetallic-based alloys, Corrosion Science 52 (2010) 263–272. [25] J. Dora, Resonant Power Supply Unit, Polish Patent Office, Patent Nr. 178285, Application 1996.06. [26] W.M. Posadowski, A. Wiatrowski, J. Dora, Z.J. Radzimski, Magnetron sputtering process control by medium-frequency power supply parameter, Thin Solid Films 516 (2008) 4478–4482. [27] J.L. Smialek, J.A. Nesbitt, C.A. Barrett, C.E. Lowell, Raport NASA, ITM-1999209769, 2000.
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Effect of an Al2O3 coating on the oxidation process of a c-TiAl phase based alloy J. Małecka Opole University of Technology, P.O. Box 321, 45-271 Opole, Poland
a r t i c l e
i n f o
Article history: Received 21 December 2011 Accepted 13 June 2012 Available online 23 June 2012 Keywords: A. Intermetallics C. High temperature corrosion C. Oxidation
a b s t r a c t The effect of Al2O3 coating on oxidation was investigated at 900 and 950 °C. Isothermal oxidation testing indicated that coating was very effective in reducing the oxidation rate of c-TiAl. The technology of magnetron sputtering used in this study allowed the deposition of a coating that ensured the improvement of high temperature oxidation resistance. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Intermetallic phase based Ti–Al alloys represent very attractive structural materials designed to operate at elevated temperatures and in aggressive chemical environments. The fields of application of these materials are space industry, aviation and automotive industry (parts of gas turbines and compressors) [1–3]. Ti–Al alloys based on intermetallic phases can yield a series of valuable properties i.e. low density, good strength at high temperatures or creep resistance, however, a method of increasing the resistance to high temperature oxidation [4–7] needs to be found. The limitation for wider application of c-TiAl phase based alloys lies in the inability to produce the protective layer of Al2O3, whereas the combination of aluminium oxide and rutile formed during oxidation shows too weak protective properties at high temperature [2,8]. The technological development, however, necessitates stricter requirements for materials in terms of their mechanical properties, corrosion effects or resistance to high temperatures. One of the methods of shaping the properties of these alloys is modifying their chemical composition accomplished by introducing certain alloying elements to the mass of material. As presented in papers [9–15] the addition of Nb [9,10], Zr [10], Cr [11,12], Si [12], Mo [12], W [13], B [13], Y [14] and Ta [15] and may improve the oxidation resistance of Ti–Al intermetallic phase based alloys. Nevertheless, the addition of such elements may result in increasing the density of the initial material and deterioration of mechanical properties of the alloys. Another way of improving Ti–Al alloy’s oxidation resistance is the application of appropriate protective coatings. Meeting these requirements is possible through the application of surface engineering methods, which allow shaping the structure, phase and chemical composition of the surface layers E-mail address: [email protected] 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.06.009
of given materials [16]. Therefore, one of the methods leading to improving high-temperature corrosion resistance is designing appropriate surface layers. Numerous attempts have been made to increase the heat resistance of these alloys [17–21]. This paper describes using the technique of magnetron deposition [22,23] of Al2O3 coating. The research studies presented in this paper use an innovative method of applying the layers: dielectric layers were obtained in metallic mode of the magnetron, which resulted in producing layers of Al2O3 with increased efficiency. 2. Experimental procedures The tests were performed on the multi-component alloy Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni with the content of 46% at Al 7% at Nb, 0.7% at Cr, 0.1% at Si and 0.2% at Ni purchased from Flowserve Corporation Titanium and Reactive Metals Foundry (USA). The structure of the test alloy can be specified as duplex [24]. The samples of alloy with dimensions of 20 15 2 mm were polished with 800 grade abrasive paper and subsequently degreased in acetone. The Al2O3 layer was obtained by means of magnetron method [22]. The deposition processes were conducted in a standard vacuum station type NP500 equipped with a circular magnetron type WMK-50. The WMK magnetron was switchedmode powered by DPS [25,26] power supply unit. The sprayed target was made of N5 grade aluminium (purity 99.97%), with a diameter of £50 mm and thickness of 7 mm directly cooled with water. The samples with Al2O3 coating of 3 lm obtained in this manner underwent tests of isothermal oxidation at the temperature of 900 and 950 °C during 250 h in static air atmosphere. Mass changes due to oxidation processes were controlled by precision scale with accuracy of 10 4 g. Trials were repeated three times
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and the presented test results are averaged. For comparison, the reference alloy were samples of the alloy without the coating. 3. Test result and analysis Previous research studies concerned determining the area of resistance to high temperature oxidation of Ti–46Al–7Nb–0.7Cr– 0.1Si–0.2Ni alloy without protective coatings [9,24]. It was determined that during oxidation a multi-phase scale is formed characterised by identical sequence of sublayers: the outer sublayer is made of rutile, the middle sublayer is rich in Al2O3 and contains a lesser amount of TiO2 and the sublayer containing Al2O3, TiO2 and oxides of Nb, Cr, Ni (Fig. 1). During the oxidation of the alloy in its initial state, columnar crystallites growing in various directions can be observed on the oxidised surface (Fig. 2). The qualitative analysis of the structure and composition of the isothermal air oxidation products i.e. quantitative dominance of Ti over Al, leads to the conclusion that the product covering the surface is rutile crystallites (Fig. 3). Such an outer layer formed during oxidation of uncoated Ti–46Al–7Nb– 0.7Cr–0.1Si–0.2Ni is created as a result of out-core diffusion of titanium. Therefore, the formation of Al2O3 layer on the surface could add to improving the oxidation resistance of the analysed alloy. For this purpose, the samples were subjected to similar tests following magnetron depositing a layer of Al2O3. The course of oxidation of the oxidised alloy was presented in Fig. 4. Isothermal oxidation at 900 and 950 °C during 250 h causes mass gain, which intensifies at 950 °C both in the case of alloy in initial state as well as for the samples with the coating of Al2O3. However, smaller mass gains were observed for the alloy with Al2O3 coating in the function of time. For the purpose of verification, each oxidation test was replicated three times using the same research methodology, which enabled a more precise analysis of their cycle. The conclusion was that the tests are characterised by a high repeatability of results. In three consecutive trials the mass gain recorded for samples coated with Al2O3 were as follows: (i) At the temperature of 900 °C: 3.02 mg/cm2, 3.03 and 2.98 mg/ cm2. The average mass gain: 3.01 mg/cm2. Standard deviation: r = 0.0216. (ii) At the temperature of 950 °C: 3.10 mg/cm2, 2.81 and 3.40 mg/ cm2. The average mass gain: 3.10 mg/cm2. Standard deviation: r = 0.2410. For the uncoated alloy:
Fig. 1. Cross-section of products and metallic sublayer of the oxidised Ti–46Al– 7Nb–0.7Cr–0.1Si–0.2Ni alloy.
Fig. 2. The surface Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni after 60 h of isothermal oxidation in air at 950 °C.
Fig. 3. EDX analysis results area inside the rectangle number 1 according to Fig. 2.
(i) At the temperature of 900 °C: 4.01 mg/cm2, 3.68 and 3.78 mg/cm2. The average mass gain: 3.82 mg/cm2. Standard deviation: r = 0.1382. (ii) At the temperature of 950 °C: 4.93 mg/cm2, 4.78 and 5.15 mg/cm2. The average mass gain: 4.95 mg/cm2. Standard deviation: r = 0.1520. As shown, the performed tests confirm the repeatability of the results. The differences in the results of the changes in the mass gain of the tested samples are minimal, and therefore in Fig. 4 averaged values of the obtained measurements were presented. The outcomes quoted above clearly prove the repeatability of the results. The demonstration of the mentioned repeatability is visualised by one of the lines in Fig. 4 (for the alloy coated with Al2O3 oxidised at 950 °C). Although (as shown in Fig. 4) at the initial stage of the oxidation slightly higher mass gains were observed for the coated alloy (up to about 60 h), yet along with the extension of the oxidation time, the mass gain of the coated alloy decreases, whereas the oxidation rate increases for the uncoated alloy (both at 900 and 950 °C). It can be concluded that the difference in mass gain between the uncoated alloy and the alloy coated with Al2O3 is negligible at 900 °C but it becomes considerable at 950 °C. Trials were repeated three times and the presented test results are averaged, which rules out any random changes. The sheer observation of the outer surface of oxidised sample allows the conclusion that the oxidation products are different for coated and uncoated alloys. The structure of the scale surface formed upon oxidation of the Al2O3 coated alloy is completely different (Fig. 5). The oxidation of the coated alloy results in the formation of singular efflorescences on the surface (Fig. 5a and b), which resemble rutile crystallites forming on the surface of the oxidised uncoated alloy (Fig. 2) [24]. In order to confirm it,
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Fig. 4. The mass change of Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni oxidised isothermally at 900 and 950 °C (continuous curves for initial state alloy and dashed curves for the alloy coated with Al2O3).
Fig. 5. The surface Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni with Al2O3 layer applied after 60 h of isothermal oxidation in air at 950 °C.
chemical composition analyses were carried out in these areas. The analyses showed the presence of Al and Ti (Fig. 6a). However, the distribution of elements shown in Fig. 7 precisely shows that only Ti is present whereas Al does not occur. Where does this discrepancy come from? In EDS analysis, the registered signals are generated not only on the sample’s surface. The electrons in the beam penetrate the sample up to a certain depth and may repeatedly interact with the sample’s atoms. Thence, this situation may result from the fact that the beam of electrons returned the information not only about the composition of the efflorescences but also about the adjacent or interlaying areas. On top of that, with the efflorescences being quite thin, the beam permeated them, penetrated into the substrate and therefore the registered information may be treated as collective. As can be seen these efflorescences are not formed on the whole surface of the alloy, but erupt between the cracks of the deposited layer of Al2O3 (the analysis presented in Fig. 6b shows the occurrence of aluminium which allows concluding that it is a deposited layer of Al2O3). The applied layer of Al2O3 definitely hinders the process out-core diffusion of titanium in this case adding to reduction in efflorescence growth with Ti content on the surface of the
oxidised content. Chemical composition microanalyses of the surface of the oxidised alloy show traces of Au (visible reflections in Fig. 6). The presence of Au reflections on the radiation spectrum is a consequence of the methodology of the preparation of samples for microscopic examination. The surface distribution of elements on the surface of the Ti– 46Al–7Nb–0.7Cr–0.1Si–0.2Ni alloy coated with Al2O3 after oxidation at 900 °C (Fig. 7) confirms previous observations. The areas rich in Al and O and eruptions of Ti are visible. The cross-section of the scale formed on Ti–46Al–7Nb–0.7Cr– 0.1Si–0.2Ni coated with Al2O3 is shown in Fig. 8. The produced scale was characterised by a two-layer structure. Two layers were distinguished, an inner and an outer one, which is particularly well pronounced in Fig. 8a. The chemical composition of the marked spots was shown in Fig. 9a–c. Directly above the substrate of c-TiAl alloy (spot x1 according to Fig. 8a), the layer of TiO2 (spot x2 according to Fig. 8a) can be seen. X-ray microanalysis performed in the spot x3 according to Fig. 8a (Fig. 9c) however, shows a clear dominance of aluminium in this layer. The presence of Ca, C and Fe in the results of X-ray microanalysis is caused by organic pollutions occurring on the surface of the sample and is taken into consider-
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Fig. 6. EDX analysis results: (a) in place 1 according to Fig. 5b and (b) in place 2 according to Fig. 5b.
Fig. 7. The surface distribution of elements on the surface of the Ti–46Al–7Nb alloy coated with Al2O3.
ation in quantitative microanalysis. Micro-voids were also noticed, which can be seen in BSE as the black fields between these two sublayers. It seems that due to oxidation, it is the original layer of Al2O3 that remains on the outside, whereas the alloy underneath it undergoes oxidation, which is proven by the results of analyses presented in Fig. 8a. The oxidation of Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni alloy not only includes scale formation, but also alterations in the substrate caused by out-core diffusion of alloying elements and by forming phases and solid solutions due to the in-core diffusion of oxygen
and nitrogen. In the case of the alloy with a deposited coating the oxidation mechanism occurred as a result of in-core diffusion of oxygen and out-core diffusion of alloying elements. On the metallic substrates side a light band occurs (Fig. 8a). It is a region rich in Nb. The deposited coating turned out to be an effective barrier against the in-core diffusion of oxygen, which secured the substrate material from oxidation effects as well as slowed and prevented further oxidation of the coating, which was confirmed in the research results presented. The scale formed on the surface of the samples was bonded with the substrate and did not chip off. Despite the fact that imme-
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Fig. 8. Cross-section of the scale formed on Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni coated with Al2O3 (a); products chipped observed (b).
Fig. 9. EDX analysis results: (a) in point # 1 marked in Fig. 8; (b) in point # 2 marked in Fig. 8; (c) in point # 3 marked in Fig. 8.
diately after the oxidation the products stuck to the surface of the samples, within the period of 24 h, a large amount of the oxidation products chipped away (Fig. 8b). It can be explained by the accumulation of compressive stresses within the products resulting from so-called ‘‘weekend effect’’ [27].
4. Conclusions The technology of magnetron sputtering used in this study allowed the deposition of a coating that ensured the improvement of high temperature oxidation. The heat resistance of the alloy
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coated with a protective film of Al2O3 is higher than in the initial state alloy. This coating adds to reducing the oxidation rate and causes the mass growth to be smaller compared to the uncoated layer. The dominating component of the formed scale is Al2O3 as expected, which has a positive impact on increasing heat resistance of the alloy. The presence of Al2O3 caused the increase of titanium activity and thus enhanced the tendency for selective oxidation of aluminium and improved the heat resistance of the substrate. As a consequence of the rutile eruptions are formed induced by out-core diffusion. Taking into consideration the presented results it can be concluded that magnetron generated coating of Al2O3 caused the heat resistance of the substrate alloy to increase.
Acknowledgement The research study was financed from the funds for science in 2010–2011 as research Project No. IP 2010 023870.
References [1] W. Szkliniarz, The alloys from the binary system of Ti–Al, in: Z. Bojar, W. Przetakiewicz (Eds.), Metallic Materials with the Participation of Intermetallic Phases, Technical Military Academy, Warsaw, 2006, pp. 66–88 (Chapter 2.2, in Polish). [2] M. Yoshihara, Y.W. Kim, Oxidation behaviour of gamma alloys designed for high temperature oxidation, Intermetallics 13 (2005) 952–958. [3] M. Yamaguchi, H. Inui, K. Ito, High-temperature structural intermetallic, Acta Materialia 48 (2000) 307–322. [4] S.A. Kakare, J.B. Toney, P.B. Aswath, Oxidation of ductile particle reinforced Ti– 48Al composite, Metallurgical and Materials Transactions 26A (1995) 1835– 1845. [5] K.S. Chan, Developing hydrogen-tolerant microstructures for an alpha-2 Titanium Aluminide Alloy, Metallurgical and Materials Transactions 23A (1992) 497–507. [6] A. Takasaki, Y. Furuya, Y. Taneda, Hydrogen uptake in titanium aluminides covered with oxide layers, Metallurgical and Materials Transactions 29A (1998) 307–314. [7] T.T. Cheng, M.R. Willis, I.P. Jones, Effects of major alloying addition on the microstructure and mechanical properties of c-TiAl, Intermetallics 7 (1999) 89–99. [8] S. Krol, M. Prazmowski, High-temperature oxidation of gamma-TiAl based alloys: generation of oxidized layer, Materials Engineering 3 (2006) 456–459 (in Polish).
[9] S. Krol, J. Małecka, L. Zemcik, The effect of niobium on the kinetics oxidation behaviour of c -TiAl, Protection against Corrosion 11 (2007) 124–128 (in Polish). [10] V. Shmet, M. Yurechko, A.K. Tyagi, W.J. Quadakkers, L. Singheiser, The influence of Nb and Zr additions on the high temperature oxidation mechanism of c-TiAl alloys in Ar/O2, gamma titanium aluminides 1999, in: Y.-W. Kim, D.M. Dimiduk, M.H. Loretto (Eds.), The Minerals, Metals & Materials Society, 1999, pp. 783–790. [11] N. Toshio, I. Takeshi, M. Yatagai, T. Yoshioka, Sulfidation processing and Cr addition to improve oxidation resistance of TiAl intermetallics in air at 1173 K, Intermetallics 8 (2000) 371–379. [12] Bong Goo Kim, Gil Moo Kim, Chong Jip Kim, Oxidation behaviour of TiAl– X(X = Cr, V, Si, Mo or Nb) intermetallics at elevated temperature, Scripta Metallurgica et Materialia 33 (1995) 1117–1125. [13] L. Huang, P.K. Liaw, C.T. Liu, Microstructural Evolution of TiAl-intermetallic Alloys Containing Tungsten and Boron, Oak Ridge National Laboratory, Managed by UT Battelle for the Department of Energy, Proceedings Paper, Session II, 13 June 2005. [14] Y. Wu, K. Hagihara, Y. Umakoshi, Improvement of cyclic oxidation resistance of Y-containing TiAl-based alloys with equiaxial gamma microstructures, Intermetallics 13 (2005) 879–884. [15] S. Krol, Cyclic oxidation of c-TiAl based multicomponent alloys with addition of Ta, Protection against Corrosion 11 (special issue-A) (2005) 194–198 [in Polish]. [16] T. Burakowski, T. Wierzchon, Surface Engineering of Metals: Principles, Equipment, Technologies, Publisher CRC, 1999. [17] L. Swadz´ba, G. Moskal, M. Hetman´czyk, B. Mendala, G. Jarczyk, Long-term cyclic oxidation of Al–Si diffusion coatings deposited by Arc-PVD on TiAlCrNb alloy, Surface and Coatings Technology 184 (2004) 93–101. [18] Z. Liu, G. Wang, Improvement of oxidation resistance of c-TiAl at 800 and 900 °C in air by TiAl2 coatings, Materials Science and Engineering A 397 (2005) 50–57. [19] G.S. Fox-Rabinovich, D.S. Wilkinson, S.C. Veldhuis, G.K. Dosbaeva, G.C. Weatherly, Oxidation resistance Ti–Al–Cr alloy for protective coating applications, Intermetallics 14 (2006) 189–197. [20] T. Izumi, T. Nishimoto, T. Narita, Superior long-term oxidation resistance of Ni–Al coated TiAl alloys, Intermetallics 13 (2005) 727–732. [21] S. Mridha, H.S. Ong, L.S. Poh, P. Cheang, Intermetallic coatings produced by TIG surface melting, Journal of Materials Processing Technology 113 (2001) 516– 520. [22] W. M. Posadowski, Unconventional magnetron sputtering systems for vacuum deposition of thin coatings, OWPW Wroclaw 2001 (in Polish). [23] S. Berg, T. Nyberg, Fundamental understanding and modelling of reactive sputtering processes, Thin Solid Films 476 (2005) 215–230. [24] J. Małecka, W. Grzesik, A. Hernas, An investigation on oxidation wear mechanisms of Ti–46Al–7Nb–0.7Cr–0.1Si–0.2Ni intermetallic-based alloys, Corrosion Science 52 (2010) 263–272. [25] J. Dora, Resonant Power Supply Unit, Polish Patent Office, Patent Nr. 178285, Application 1996.06. [26] W.M. Posadowski, A. Wiatrowski, J. Dora, Z.J. Radzimski, Magnetron sputtering process control by medium-frequency power supply parameter, Thin Solid Films 516 (2008) 4478–4482. [27] J.L. Smialek, J.A. Nesbitt, C.A. Barrett, C.E. Lowell, Raport NASA, ITM-1999209769, 2000.