- Email: [email protected]
Effect of coatings on oxidation resistance and mechanical performance of Ti60 alloy
Materials Science and Engineering A255 (1998) 133 – 138
Effect of coatings on oxidation resistance and mechanical performance of Ti60 alloy Zhaolin Tang a,*, Fuhui Wang a, Weitao Wu a, Qingjiang Wang b, Dong Li b a
State key Laboratory for Corrosion and Protection, Institute of Corrosion and Protection of Metals, The Chinese Academy of Sciences, Wencui Road 62, Shenyang 110015, People’s Republic of China b Institute of Metals Research, The Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China Received 26 May 1998
Abstract The effect of several sputtered coatings on oxidation resistance and mechanical performance of Ti60 alloy was investigated. Sputtered pure Al, Ti–36Al, Ni–16Cr–2.5Al(wt%) coatings and reactively-sputtered Al2O3 film were effective in inhibiting the oxidation and oxygen-embrittlement of Ti60 alloy at 600 – 700°C, while Al2O3 and TiAl coatings exhibited a better effect than Al and NiCrAl coatings from the point of view of coating-substrate compatibility. The tensile test after 100h exposure at 600°C in air also showed that Ti60 alloy with Al2O3 and TiAl coatings exhibited a higher ductility than that with Al and NiCrAl coatings, which exhibited a good correlation with coating-substrate compatibility. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Ti60 alloy; Oxidation; Oxygen-barrier coating; Mechanical properties
1. Introduction High temperature titanium alloys are considered as good candidate materials for many aerospace applications due to their high specific strengths [1,2]. However severe oxidation and oxygen embrittlement problems limit the technical application of titanium alloys to temperatures much lower than that permitted by their mechanical properties. When titanium alloys are exposed to oxidising environments, the oxygen reacts with the alloys to form non-protective oxides (generally the rutile TiO2). Moreover, oxygen dissolves into the alloys to form a hardened, oxygen rich layer or ‘alpha-case’ (oxygen embrittlement), which may degrade the mechanical properties of titanium alloys [3 – 5]. Because of the technological importance of extending the operational temperatures of titanium alloys for aerospace applications, there is an ongoing interest in the development of the anti-oxidation alloys and oxygen-barrier coatings. In general, it met with only limited success to improve the oxidation resistance of titanium alloy with alloying addition and microstructure control. * Corresponding author. Tel.: + 86-24-23915911; fax: + 86 24 23894149; e-mail: [email protected].
Therefore, an oxygen barrier coatings may be needed, especially for the application at temperatures above 600°C. Recently more and more attentions have been paid to the protective coatings which prevent the titanium alloys from oxidation and oxygen embrittlement [6–15]. The results generally showed that, many kinds of oxygen-barrier coatings are very effective in reducing the oxidation rate of titanium alloys. However, the coating-substrate inter-diffusion may be harmful to the mechanical properties of Ti alloy. In the present study, aiming to search oxygen barrier coatings with good protectiveness and coating-substrate compatibility, the effect of several sputtered coatings on oxidation resistance and mechanical properties of Ti60 alloy will be investigated.
2. Experimental Ti60 alloy is a near-alpha alloy which is designed to be used at 600°C [16]. The nominal chemical composition (wt%) of Ti60 alloy is Ti–5.6Al–4.8Sn–2Zr– 1Mo–0.32Si–1Nd. Two types of specimens were prepared: tensile specimens, a cylindrical specimens with a gauge length of 25 mm and diameter of 5 mm;
0921-5093/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00789-8
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thermogravimetrical specimens (10×10 × 3 mm rectangular coupons). Three kinds of metallic coatings with the thickness of 10 mm were prepared with magnetron-sputtering technique. Their nominal compositions (wt%) were as follows; Al coatings-99.99% Al; NiCrAl coatings-Ni– 16Cr–2.5Al; TiAl coatings-Ti – 36Al. An Al2O3 film with thickness about 1 mm was produced by reactively sputtering technique. Oxidation tests of thermogravimetrical specimens tests were conducted at 600 and 700°C in static air. The specimens were placed in alumina crucibles and then oxidised at desired temperatures and cooled down to room temperature at regular intervals of 20 h for mass measurement. The sensitivity of the balance used in the study was 10 − 4 g. Subsequent to oxidation, the specimens were analysed using optical metallography (OM), X-ray diffractometer (XRD), scanning electron microscope (SEM) with dispersive X-ray analysis (EDAX). The tensile specimens were thermally exposed to 600°C for 100 h in air and then the tensile tests were conducted at room temperature.
reduced the mass gain of Ti60 alloy as a result of the formation of more protective scales on these coatings. From their corresponding cross-sectional microstructure, it is seen that a thick oxide scale formed on bare Ti60 alloy, while the oxide scale on all coatings were very thin. Fig. 3 shows the cross sections of bare Ti60 alloy with and without Al and NiCrAl coatings after 100 h oxidation at 700°C. It should be noted that severe inter-diffusion existed between the NiCrAl coating-Ti60 alloy interface and the depth of diffusion zone is about 15 mm, EDAX analysis indicated that it is mainly due to the diffusion of Ni into substrate. Ni is a b-phase stabilising elements, so diffusion of Ni into Ti60 alloy resulted in a b-stabilised zone [15] at the alloy surface. After high temperature exposure, X-ray results indicated that the Al coating transferred to TiAl3 phase and there are some cracks across the coating. These kinds of cracks also appeared in the pack cementation TiAl3 coating on Ti alloys due to the brittleness of TiAl3 phase [7–9]. However there is not an obvious reaction
3. Results and discussion
3.1. Oxidation resistance Fig. 1 shows the oxidation kinetics of Ti60 alloy with and without coatings. It is seen that all coatings can reduce the weight gains of Ti60 alloy at 600 – 700°C. At 600°C, the positive effect of coatings is not very obvious because the weight gains of Ti60 alloy are very low (0.15 mg/cm2 for 100 h oxidation). At 700°C, however, several coatings are very effective to reduce the weight gains of Ti60 alloys. Among these coatings, the Al and Al2O3 coatings showed the best effect, they reduce the weight gains of Ti60 alloy by a factor of three. All kinetics curves exhibited the parabolic rule, which suggested that the rate-limiting step should be diffusionrelated. Fig. 2 shows the surface morphologies of Ti60 alloy with and without coatings after 100 h oxidation at 700°C. It can be seen that the oxide scale formed on all specimens are very adherent. X-ray diffraction results indicated that the oxide is mainly TiO2 on Ti60 alloy; Mainly Al2O3 on Al and NiCrAl coating; Al2O3 plus TiO2 on TiAl coatings. Due to the rapid diffusion of O2 − and Ti4 + in TiO2, the TiO2 scale is much less protective than Al2O3 scale, so Al and NiCrAl coated Ti60 alloy exhibited much less weight gains that bare Ti60 alloy, while TiAl coating on Ti60 alloy is not so effective. At this temperature, the Al2O3 coating is very adherent, which act as a very effective oxygen-diffusion barrier. Therefore Al2O3 coating reduce the weight gains of Ti60 alloy the most. It is clear that the coatings
Fig. 1. Oxidation kinetics of Ti60 alloy with and without coatings.
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Fig. 2. Surface morphologies of Ti60 alloy (a) and Ti60 alloy with Al (b), Al2O3 (c), TiAl (d) and NiCrAl (e) coatings after 100 h oxidation at 700°C.
between TiAl, Al2O3 coating and their substrates which indicated a good coating-substrate compatibility. TiAl coatings have a close composition to Ti60 alloy, so the coating-substrate reaction would be limited; Al2O3 is very stable under this condition. The a-case etching of the metallographic cross-sections can visualise the a-phase enrichment in the nearsurface regions of Ti alloy [14]. Fig. 4 shows the alpha case (the bright zone near the alloy surface) of Ti60 alloy with and without Al2O3 coatings. For bare Ti60 alloy after 100 h oxidation at 700°C, the depth of a-case zone is about 30 mm. However the depth of a-case zone on Al2O3 coated Ti60 alloy is only about 10 mm. The a-case zone on Al, TiAl and NiCrAl coated Ti60 alloy is not obvious. Due to the high diffusion
coefficient and solid solution (34 at%) of oxygen in a-Ti alloy, oxygen reacts easily with the alloy to form oxygen rich ‘alpha-case’ (oxygen embrittlement) [3]. Obviously, these coatings are very effective in inhibiting the oxygen-embrittlement of Ti60 alloy. Figs. 5–8 exhibit the effect of coatings on the mechanical properties of Ti60 alloy after 100 h exposure at 600°C (specimens labelled as ‘Ti60’ were free of the surface oxidation and oxygen-embrittlement zone because the surface zone was removed after exposure; on the contrary, specimen labelled ‘None’ kept the surface for tensile testing.). It is seen that the surface oxidation and oxygen-embrittlement zone exhibited significant harmful effect on the post-exposure ductility (d and c) of Ti60 alloy, and all coatings is effective to improve
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Fig. 3. Cross sections of Ti60 alloy (a) and Ti60 alloy with Al (b) and NiCrAl (c) coatings after 100 h oxidation at 700°C.
Fig. 4. The a-case zone on Ti60 alloy (a) and Ti60 alloy with Al2O3 (b) coatings after 100 h oxidation at 700°C.
the post-exposure ductility (d and c), while the ultimate tensile strength (sb) and yield tensile strength (s0.2) display just a little change. The tensile elongation of specimen labelled as None is much lower than that of specimens labelled as Ti60, which is caused by the surface oxidation and oxygen embrittlement of Ti60 alloy. During exposure at elevated temperature, non-protective TiO2 scale formed on the Ti60 alloy, and in the meantime, oxygen penetrated through the oxide scale and dissolved into the alloys to form a solid solution with the base metal. The solubility of oxygen in a-Ti is up to 34 at%, while the alloy experiences a reduction of ductility at oxygen concentration more that 1.5 at% [11].
Oxygen-barrier coatings improve the post-exposure tensile elongation of Ti60 alloy in different degree. Among them, Al2O3 and TiAl coatings exhibited the best effect, they increase the post-exposure tensile elongation by a factor of 1. It is seen that a correlation between weight gain and tensile elongation seems poor. As mentioned above, four coatings is very effective to inhibit the oxidation and oxygen embrittlement of Ti60 alloy, but the coating-substrate inter-diffusion may be another factor to degrade the post-exposure tensile elongation. According to Groves [17], the presence of diffusion coatings containing b-phase stabilising elements such as Fe, Ni and Cr may cause loss of ductility in alloys such as Ti-6242 after exposure at elevated
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137
Fig. 5. Effect of coatings on the sb of Ti60 alloy after 100 h exposure in air at 600°C.
Fig. 7. Effect of coatings on the d of Ti60 alloy after 100 h exposure in air at 600°C.
temperatures. Therefore, the diffusional b-stabilised zone under NiCrAl coating would cause loss of ductility of Ti60 alloys. The formation of brittle TiAl3 phase on Ti60 alloy may also lead to the low ductility of Al coated Ti60 alloy. Therefore, a better correlation might be made between the tensile elongation and the coatingsubstrate compatibility. The oxygen rich a-case or diffusional b embrittled zone is at the surface, the surface cracks may act as stress risers that cause failure of the specimens at low strain levels and cause the loss of ductility; however the strength of Ti60 alloy was not affected obviously because the brittle zone is very thin.
The effect of sputtered pure Al, Ti – 36Al, Ni–16Cr– 2.5Al(wt%) coatings and reactively-sputtered Al2O3 coatings on oxidation resistance and mechanical prop-
erties of Ti60 alloy was investigated. The following results will be obtained: 1. All coatings are very effective in reducing the oxidation rate of Ti60 alloy. Among them Al and Al2O3 coatings exhibited the best effect due to the protectiveness of Al2O3 scales. These coatings can remarkably reduce the depth of a-case zone. 2. Severe inter-diffusion existed between the NiCrAl coating and Ti60 alloy interface and the depth of diffusion zone is about 15 mm. After exposure, the Al coating transferred to brittle TiAl3 phase and there are some cracks across the coating. However there is not obvious reaction between TiAl, Al2O3 coating and Ti60 alloy. 3. All coatings are effective in improving the post-exposure tensile elongation of Ti60 alloy, while other mechanical properties (sb, s0.2) display just a little change. Among them, Al2O3 and TiAl coatings exhibited the best effect, and a good correlation can be made between the tensile elongation and the coating-substrate compatibility.
Fig. 6. Effect of coatings on the s0.2 of Ti60 alloy after 100 h exposure in air at 600°C.
Fig. 8. Effect of coatings on the c of Ti60 alloy after 100 h exposure in air at 600°C.
4. Conclusions
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Acknowledgements This project was supported by the National Natural Sciences Foundation of China (NSFC) for Outstanding Young Scientists and by the National Advanced Material Committee of China.
References [1] R.R. Boyer, Adv. Perform. Mater. 2 (1995) 349. [2] P.J. Bania, in: P. Lacombe, R. Tricot, G. Beanger (Eds.), Les Editions De Physique, France, 1988, p. 825. [3] J. Unnam, R.N. Shenoy, R.K. Clark, Oxid met. 26 (1986) 231. [4] R.K. Clark, J. Unnam, TMS-AIME Paper No. A84-57, TMSAIME Annual Meeting Los Angeles, (1984). [5] G. Welsch, A.I. Kahveci, in: T. Grobstein, J. Doy (Eds.), Oxidation of High Temperature Intermetallics, TMS, Warrendale, PA, 1988, p. 207.
.
[6] N.W. Kearns, J.E. Restall, in: P. Lacombe, R. Tricot, G. Beanger (Eds.), Les Editions De Physique, France, 1988, p. 1753. [7] Y. Zhang, Y. Ten, L Zhou, T. Li, Rare Met. Sci. Eng. 22 (1993) 27. [8] J. Subrahanyan, J. Annapura, Oxid Met. 26 (1986) 275. [9] H. Mabuchi, T. Asaiand, Y. Hakayama, Scripta Metal. 23 (1989) 685. [10] R. Steriff and S. Poize, in: High Temperature Corrosion, NACE-6, R.A. Rapp (Ed.), Houston, 1983, p 591. [11] R.K. Clark, J. Unnam, K.E. Widemann, Oxid. Met. 28 (1987) 391. [12] C. Leyens, M. Peters, W.A. Kaysser, Surf. Coat. Technol. 34 (1997) 94 – 95. [13] D.W. Mckee, K.L. Luthra, Surf. Coat. Technol. 56 (1993) 109. [14] H. Benien, J. Maushart, M. Meyer, R. Suchentruck, Mater. Sci. Eng. A139 (1991) 126. [15] F. Wang, D. Li, W. Wu and Z. Hu, Acta Metall. Sinica, S31 (1995) 602. [16] G. Li, D. Li, Y. Liu, Q. Wang, S. Guan, Q. Li, J. Aeronautical Mater. 17 (4) (1997) 21. [17] M.T. Groves, En6ironmental Protection to 922K for Titanium Alloys, NASA Report. CR-134537, Washington, DC: NASA, 1973.
Effect of coatings on oxidation resistance and mechanical performance of Ti60 alloy Zhaolin Tang a,*, Fuhui Wang a, Weitao Wu a, Qingjiang Wang b, Dong Li b a
State key Laboratory for Corrosion and Protection, Institute of Corrosion and Protection of Metals, The Chinese Academy of Sciences, Wencui Road 62, Shenyang 110015, People’s Republic of China b Institute of Metals Research, The Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China Received 26 May 1998
Abstract The effect of several sputtered coatings on oxidation resistance and mechanical performance of Ti60 alloy was investigated. Sputtered pure Al, Ti–36Al, Ni–16Cr–2.5Al(wt%) coatings and reactively-sputtered Al2O3 film were effective in inhibiting the oxidation and oxygen-embrittlement of Ti60 alloy at 600 – 700°C, while Al2O3 and TiAl coatings exhibited a better effect than Al and NiCrAl coatings from the point of view of coating-substrate compatibility. The tensile test after 100h exposure at 600°C in air also showed that Ti60 alloy with Al2O3 and TiAl coatings exhibited a higher ductility than that with Al and NiCrAl coatings, which exhibited a good correlation with coating-substrate compatibility. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Ti60 alloy; Oxidation; Oxygen-barrier coating; Mechanical properties
1. Introduction High temperature titanium alloys are considered as good candidate materials for many aerospace applications due to their high specific strengths [1,2]. However severe oxidation and oxygen embrittlement problems limit the technical application of titanium alloys to temperatures much lower than that permitted by their mechanical properties. When titanium alloys are exposed to oxidising environments, the oxygen reacts with the alloys to form non-protective oxides (generally the rutile TiO2). Moreover, oxygen dissolves into the alloys to form a hardened, oxygen rich layer or ‘alpha-case’ (oxygen embrittlement), which may degrade the mechanical properties of titanium alloys [3 – 5]. Because of the technological importance of extending the operational temperatures of titanium alloys for aerospace applications, there is an ongoing interest in the development of the anti-oxidation alloys and oxygen-barrier coatings. In general, it met with only limited success to improve the oxidation resistance of titanium alloy with alloying addition and microstructure control. * Corresponding author. Tel.: + 86-24-23915911; fax: + 86 24 23894149; e-mail: [email protected].
Therefore, an oxygen barrier coatings may be needed, especially for the application at temperatures above 600°C. Recently more and more attentions have been paid to the protective coatings which prevent the titanium alloys from oxidation and oxygen embrittlement [6–15]. The results generally showed that, many kinds of oxygen-barrier coatings are very effective in reducing the oxidation rate of titanium alloys. However, the coating-substrate inter-diffusion may be harmful to the mechanical properties of Ti alloy. In the present study, aiming to search oxygen barrier coatings with good protectiveness and coating-substrate compatibility, the effect of several sputtered coatings on oxidation resistance and mechanical properties of Ti60 alloy will be investigated.
2. Experimental Ti60 alloy is a near-alpha alloy which is designed to be used at 600°C [16]. The nominal chemical composition (wt%) of Ti60 alloy is Ti–5.6Al–4.8Sn–2Zr– 1Mo–0.32Si–1Nd. Two types of specimens were prepared: tensile specimens, a cylindrical specimens with a gauge length of 25 mm and diameter of 5 mm;
0921-5093/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00789-8
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thermogravimetrical specimens (10×10 × 3 mm rectangular coupons). Three kinds of metallic coatings with the thickness of 10 mm were prepared with magnetron-sputtering technique. Their nominal compositions (wt%) were as follows; Al coatings-99.99% Al; NiCrAl coatings-Ni– 16Cr–2.5Al; TiAl coatings-Ti – 36Al. An Al2O3 film with thickness about 1 mm was produced by reactively sputtering technique. Oxidation tests of thermogravimetrical specimens tests were conducted at 600 and 700°C in static air. The specimens were placed in alumina crucibles and then oxidised at desired temperatures and cooled down to room temperature at regular intervals of 20 h for mass measurement. The sensitivity of the balance used in the study was 10 − 4 g. Subsequent to oxidation, the specimens were analysed using optical metallography (OM), X-ray diffractometer (XRD), scanning electron microscope (SEM) with dispersive X-ray analysis (EDAX). The tensile specimens were thermally exposed to 600°C for 100 h in air and then the tensile tests were conducted at room temperature.
reduced the mass gain of Ti60 alloy as a result of the formation of more protective scales on these coatings. From their corresponding cross-sectional microstructure, it is seen that a thick oxide scale formed on bare Ti60 alloy, while the oxide scale on all coatings were very thin. Fig. 3 shows the cross sections of bare Ti60 alloy with and without Al and NiCrAl coatings after 100 h oxidation at 700°C. It should be noted that severe inter-diffusion existed between the NiCrAl coating-Ti60 alloy interface and the depth of diffusion zone is about 15 mm, EDAX analysis indicated that it is mainly due to the diffusion of Ni into substrate. Ni is a b-phase stabilising elements, so diffusion of Ni into Ti60 alloy resulted in a b-stabilised zone [15] at the alloy surface. After high temperature exposure, X-ray results indicated that the Al coating transferred to TiAl3 phase and there are some cracks across the coating. These kinds of cracks also appeared in the pack cementation TiAl3 coating on Ti alloys due to the brittleness of TiAl3 phase [7–9]. However there is not an obvious reaction
3. Results and discussion
3.1. Oxidation resistance Fig. 1 shows the oxidation kinetics of Ti60 alloy with and without coatings. It is seen that all coatings can reduce the weight gains of Ti60 alloy at 600 – 700°C. At 600°C, the positive effect of coatings is not very obvious because the weight gains of Ti60 alloy are very low (0.15 mg/cm2 for 100 h oxidation). At 700°C, however, several coatings are very effective to reduce the weight gains of Ti60 alloys. Among these coatings, the Al and Al2O3 coatings showed the best effect, they reduce the weight gains of Ti60 alloy by a factor of three. All kinetics curves exhibited the parabolic rule, which suggested that the rate-limiting step should be diffusionrelated. Fig. 2 shows the surface morphologies of Ti60 alloy with and without coatings after 100 h oxidation at 700°C. It can be seen that the oxide scale formed on all specimens are very adherent. X-ray diffraction results indicated that the oxide is mainly TiO2 on Ti60 alloy; Mainly Al2O3 on Al and NiCrAl coating; Al2O3 plus TiO2 on TiAl coatings. Due to the rapid diffusion of O2 − and Ti4 + in TiO2, the TiO2 scale is much less protective than Al2O3 scale, so Al and NiCrAl coated Ti60 alloy exhibited much less weight gains that bare Ti60 alloy, while TiAl coating on Ti60 alloy is not so effective. At this temperature, the Al2O3 coating is very adherent, which act as a very effective oxygen-diffusion barrier. Therefore Al2O3 coating reduce the weight gains of Ti60 alloy the most. It is clear that the coatings
Fig. 1. Oxidation kinetics of Ti60 alloy with and without coatings.
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Fig. 2. Surface morphologies of Ti60 alloy (a) and Ti60 alloy with Al (b), Al2O3 (c), TiAl (d) and NiCrAl (e) coatings after 100 h oxidation at 700°C.
between TiAl, Al2O3 coating and their substrates which indicated a good coating-substrate compatibility. TiAl coatings have a close composition to Ti60 alloy, so the coating-substrate reaction would be limited; Al2O3 is very stable under this condition. The a-case etching of the metallographic cross-sections can visualise the a-phase enrichment in the nearsurface regions of Ti alloy [14]. Fig. 4 shows the alpha case (the bright zone near the alloy surface) of Ti60 alloy with and without Al2O3 coatings. For bare Ti60 alloy after 100 h oxidation at 700°C, the depth of a-case zone is about 30 mm. However the depth of a-case zone on Al2O3 coated Ti60 alloy is only about 10 mm. The a-case zone on Al, TiAl and NiCrAl coated Ti60 alloy is not obvious. Due to the high diffusion
coefficient and solid solution (34 at%) of oxygen in a-Ti alloy, oxygen reacts easily with the alloy to form oxygen rich ‘alpha-case’ (oxygen embrittlement) [3]. Obviously, these coatings are very effective in inhibiting the oxygen-embrittlement of Ti60 alloy. Figs. 5–8 exhibit the effect of coatings on the mechanical properties of Ti60 alloy after 100 h exposure at 600°C (specimens labelled as ‘Ti60’ were free of the surface oxidation and oxygen-embrittlement zone because the surface zone was removed after exposure; on the contrary, specimen labelled ‘None’ kept the surface for tensile testing.). It is seen that the surface oxidation and oxygen-embrittlement zone exhibited significant harmful effect on the post-exposure ductility (d and c) of Ti60 alloy, and all coatings is effective to improve
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Fig. 3. Cross sections of Ti60 alloy (a) and Ti60 alloy with Al (b) and NiCrAl (c) coatings after 100 h oxidation at 700°C.
Fig. 4. The a-case zone on Ti60 alloy (a) and Ti60 alloy with Al2O3 (b) coatings after 100 h oxidation at 700°C.
the post-exposure ductility (d and c), while the ultimate tensile strength (sb) and yield tensile strength (s0.2) display just a little change. The tensile elongation of specimen labelled as None is much lower than that of specimens labelled as Ti60, which is caused by the surface oxidation and oxygen embrittlement of Ti60 alloy. During exposure at elevated temperature, non-protective TiO2 scale formed on the Ti60 alloy, and in the meantime, oxygen penetrated through the oxide scale and dissolved into the alloys to form a solid solution with the base metal. The solubility of oxygen in a-Ti is up to 34 at%, while the alloy experiences a reduction of ductility at oxygen concentration more that 1.5 at% [11].
Oxygen-barrier coatings improve the post-exposure tensile elongation of Ti60 alloy in different degree. Among them, Al2O3 and TiAl coatings exhibited the best effect, they increase the post-exposure tensile elongation by a factor of 1. It is seen that a correlation between weight gain and tensile elongation seems poor. As mentioned above, four coatings is very effective to inhibit the oxidation and oxygen embrittlement of Ti60 alloy, but the coating-substrate inter-diffusion may be another factor to degrade the post-exposure tensile elongation. According to Groves [17], the presence of diffusion coatings containing b-phase stabilising elements such as Fe, Ni and Cr may cause loss of ductility in alloys such as Ti-6242 after exposure at elevated
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Fig. 5. Effect of coatings on the sb of Ti60 alloy after 100 h exposure in air at 600°C.
Fig. 7. Effect of coatings on the d of Ti60 alloy after 100 h exposure in air at 600°C.
temperatures. Therefore, the diffusional b-stabilised zone under NiCrAl coating would cause loss of ductility of Ti60 alloys. The formation of brittle TiAl3 phase on Ti60 alloy may also lead to the low ductility of Al coated Ti60 alloy. Therefore, a better correlation might be made between the tensile elongation and the coatingsubstrate compatibility. The oxygen rich a-case or diffusional b embrittled zone is at the surface, the surface cracks may act as stress risers that cause failure of the specimens at low strain levels and cause the loss of ductility; however the strength of Ti60 alloy was not affected obviously because the brittle zone is very thin.
The effect of sputtered pure Al, Ti – 36Al, Ni–16Cr– 2.5Al(wt%) coatings and reactively-sputtered Al2O3 coatings on oxidation resistance and mechanical prop-
erties of Ti60 alloy was investigated. The following results will be obtained: 1. All coatings are very effective in reducing the oxidation rate of Ti60 alloy. Among them Al and Al2O3 coatings exhibited the best effect due to the protectiveness of Al2O3 scales. These coatings can remarkably reduce the depth of a-case zone. 2. Severe inter-diffusion existed between the NiCrAl coating and Ti60 alloy interface and the depth of diffusion zone is about 15 mm. After exposure, the Al coating transferred to brittle TiAl3 phase and there are some cracks across the coating. However there is not obvious reaction between TiAl, Al2O3 coating and Ti60 alloy. 3. All coatings are effective in improving the post-exposure tensile elongation of Ti60 alloy, while other mechanical properties (sb, s0.2) display just a little change. Among them, Al2O3 and TiAl coatings exhibited the best effect, and a good correlation can be made between the tensile elongation and the coating-substrate compatibility.
Fig. 6. Effect of coatings on the s0.2 of Ti60 alloy after 100 h exposure in air at 600°C.
Fig. 8. Effect of coatings on the c of Ti60 alloy after 100 h exposure in air at 600°C.
4. Conclusions
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Acknowledgements This project was supported by the National Natural Sciences Foundation of China (NSFC) for Outstanding Young Scientists and by the National Advanced Material Committee of China.
References [1] R.R. Boyer, Adv. Perform. Mater. 2 (1995) 349. [2] P.J. Bania, in: P. Lacombe, R. Tricot, G. Beanger (Eds.), Les Editions De Physique, France, 1988, p. 825. [3] J. Unnam, R.N. Shenoy, R.K. Clark, Oxid met. 26 (1986) 231. [4] R.K. Clark, J. Unnam, TMS-AIME Paper No. A84-57, TMSAIME Annual Meeting Los Angeles, (1984). [5] G. Welsch, A.I. Kahveci, in: T. Grobstein, J. Doy (Eds.), Oxidation of High Temperature Intermetallics, TMS, Warrendale, PA, 1988, p. 207.
.
[6] N.W. Kearns, J.E. Restall, in: P. Lacombe, R. Tricot, G. Beanger (Eds.), Les Editions De Physique, France, 1988, p. 1753. [7] Y. Zhang, Y. Ten, L Zhou, T. Li, Rare Met. Sci. Eng. 22 (1993) 27. [8] J. Subrahanyan, J. Annapura, Oxid Met. 26 (1986) 275. [9] H. Mabuchi, T. Asaiand, Y. Hakayama, Scripta Metal. 23 (1989) 685. [10] R. Steriff and S. Poize, in: High Temperature Corrosion, NACE-6, R.A. Rapp (Ed.), Houston, 1983, p 591. [11] R.K. Clark, J. Unnam, K.E. Widemann, Oxid. Met. 28 (1987) 391. [12] C. Leyens, M. Peters, W.A. Kaysser, Surf. Coat. Technol. 34 (1997) 94 – 95. [13] D.W. Mckee, K.L. Luthra, Surf. Coat. Technol. 56 (1993) 109. [14] H. Benien, J. Maushart, M. Meyer, R. Suchentruck, Mater. Sci. Eng. A139 (1991) 126. [15] F. Wang, D. Li, W. Wu and Z. Hu, Acta Metall. Sinica, S31 (1995) 602. [16] G. Li, D. Li, Y. Liu, Q. Wang, S. Guan, Q. Li, J. Aeronautical Mater. 17 (4) (1997) 21. [17] M.T. Groves, En6ironmental Protection to 922K for Titanium Alloys, NASA Report. CR-134537, Washington, DC: NASA, 1973.