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Improvement of the high temperature oxidation resistance of Ti50Al via ion-implantation
Nuclear Instruments and Methods in Physics Research B 148 (1999) 858±862
Improvement of the high temperature oxidation resistance of Ti50Al via ion-implantation U. Hornauer
a,*
, E. Richter a, E. Wieser a, W. M oller a, G. Schumacher b, C. Lang c, M. Sch utze b
a
b
Forschungszentrum Rossendorf e.V., FWII, PO Box 510119, D-01314 Dresden, Germany Karl-Winnacker-Institut der DECHEMA e.V., HTW, Theodor Heuss Allee 25, D-60486 Frankfurt, Germany c Prinz-Konstantin Straûe 12, 81737 M unchen, Germany
Abstract The TiAl intermetallic compound is very promising for high temperature applications, because of its good high temperature strength and its low density. At temperatures exceeding 800°C, the low oxidation resistance is a limiting factor. It is known, that Cl doping reduces the oxidation strongly even in very low concentrations of about 500 ppm (`microalloy'). In the present investigation ion beam implantation is used to dope the material close to the surface quantitatively. The well-de®ned depth pro®le obtained after implantation provided a means to monitor the diusion of additives during oxidation. Implantation of Cl ions (1 MeV, 1015 ±1017 cmÿ2 ) results in a systematic reduction of the oxidation at 900°C in air for doses P 1016 cmÿ2 . AES measurements were performed to investigate the diusion process during oxidation. A microscopic model will be proposed for the enhanced oxidation resistance. For bene®cial eects of Silicon a higher concentration is required (`macroalloy'). Therefore high-dose implantations were carried out (upto 8 ´ 1017 cmÿ2 ). The change in phase composition, microstructure and the oxidation behaviour will be discussed. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 81.65M; 81.40; 61.10 Keywords: TiAl; Ion implantation; Oxidation resistance; Depth pro®ling
1. Introduction c-TiAl based intermetallic compounds are very promising for structural materials in high temperature application, because of the low density of 3.6 g/cm3 . The problems, which hinder the use of
* Corresponding author. Tel.: +49 351-260-3674; fax: +49 351-260-2703; e-mail: [email protected]
this material, are the low ductility at room temperature and the oxidation above 700°C. The basic eects of oxidation and alloying ternary elements are still not understood completely. Because the equilibrium oxygen partial pressure of Ti/TiO and Al/Al2 O3 is very similar [1], a mixed oxide layer is formed during oxidation. Whether a dense protective layer of Al2 O3 is formed or a fast growing scale, depends on the local activities of the metals and the oxides formed, which is in¯uenced by the
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 8 2 0 - 9
U. Hornauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 858±862
859
oxygen partial pressure, but also by the alloying elements [2,3] Unfortunately alloying elements which are usually used for good ductility (i.e. Cr, Mn) deteriorate the oxidation behaviour and those which inhibit oxidation deteriorate the mechanical properties (i.e. W, Nb, Mo, Si). In Ref. [4], it is found that very small amounts of halogenides improve the oxidation behaviour dramatically. A good oxidation resistance has been found for mechanically alloyed TiAl. This was explained by the remaining Cl content, since the Ti sponge was not molten during the powder metalurgical routine. The ÔCl-eectÕ protects TiAl even at very low concentration below 500 ppm. The underlying mechanism and the in¯uence of Cl concentration is discussed in [5]. In the present paper, a systematic variation of [Cl] is accomplished via ion implantation. In order to investigate the eects of doping a material, ion implantation is a very useful tool. First of all the dopant concentration and depth can be reproduced and controlled with high precision, especially in the low concentration region of doping. Secondly it gives a possibility to make a fast screening of doping elements. A well-de®ned variation of the implantation parameters for each doping element is needed to ®nd optimum oxidation protection [6]. For application, it can be imagined to optimise the mechanical and the oxidation properties separately, because only a layer close to the surface (<1 lm) is in¯uenced by implantation. A strong in¯uence of the composition of the material is found [7]. Therefore a c-Ti50Al alloy (GFE) is used in the present study to investigate the basic eects on oxidation.
order to ®nd the optimal parameters for the Cl concentration an extensive screening has been done (energy: 200 keV±2 MeV with constant ¯uence of 1016 Cl /cm2 and ¯uence: 1015 ±1017 cmÿ2 at 1 MeV). For bene®cial eects of Si, a higher concentration is required [3,6]. Therefore highdose implantations were carried out (2 ´ 1017 ± 8 ´ 1017 cmÿ2 ). An energy of 1 MeV Si was chosen, which leads to a range, Rp , of about 1lm. The local Si concentration varies from 8 at% to 35 at%. In all cases the implantation temperature was below 70°C. Both 100 mm2 sides of the samples were implanted in order to optimise the thermogravimetric tests. The outer edges remained unimplanted (relative area <20%). The change in phase composition is analysed via grazing angle X-ray diraction (GXRD) and the element distribution is measured by Auger depth pro®ling (AES). An approximated depth scale is achieved by determination of the ®nal sputter crater depths and the assumption of a constant sputter rate. The implanted pro®les closely follow the predicted pro®les using the TRIM95 code [8]. The thermal treatment has been done in a conventional furnace and in a rapid thermal annealing unit (RTA). Thermogravimetric oxidation tests (TGA) were performed at 900°C in air (rel. humidity of 25% at 295 K) for 100 h using a thermobalance to continuously record the mass gain.
2. Experimental
Fig. 1 shows the TGA results obtained with dierent ¯uences at 1 MeV [9]. The specimens implanted with 1015 and 5 ´ 1015 /cm2 did not show a change in oxidation compared with the unimplanted sample Ti50Al (not shown in Fig. 1). For a suciently high concentration (1016 ±1017 cmÿ2 ), the mass gain almost stops after an incubation period. The oxidation kinetics is similar to that of Al2 O3 forming materials. The oxidation rates are reduced by about 2 orders of magnitude. The duration of the incubation period increases with increasing concentration of Cl. By varying the
Specimens of the dimension 10 ´ 10 ´ 1 mm3 were prepared from the ingot. After grinding they were polished with 4000 grit SiC paper. Before implantation organic substances were removed in a 10 min ultrasonic acetone bath. The samples were mounted on a water cooled sample holder using a special carbon foil to improve the heat contact. The implantations were performed using a 500 kV implanter (HVEE) and a high current tandetron (HVEE) for energies upto 3 MeV. In
3. Results and discussion 3.1. Microalloying: implantation of Cl
860
U. Hornauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 858±862
Fig. 1. Thermogravimetric oxidation tests (TGA) at 900°C in air for 100 h [9]. Ti50Al (dotted line) represents an unimplanted sample. The oxidation kinetics for 1á1015 and 5á1015 /cm2 are unchanged (not shown). The Cl implanted samples (1 MeV, 1á1016 ±1á1017 /cm2 ) show a protective behaviour.
energy from 200 keV to 2 MeV, a comparable reduction of oxidation occurs for energies up to 1 MeV. The sample implanted with 2 MeV exhibits no protection. The results of all implantations are compiled in Fig. 2. The variation of energy and ¯uence results in a variation of the peak concentration Nmax and the projected range Rp . A region of a protective eect is visible towards low energy and above a threshold of 1016 /cm2 . Regarding the incubation time, there is a narrow region for optimum implantation parameters. Compared to homogeneously doped material the concentration Nmax needed for oxidation protection is about ten times higher (Fig. 2). This is probably due to the fast diusion of Cl in TiAl at 900°C. In order to monitor the redistribution process in the ®rst stage, a high dose implantation has been carried out (200 keV, 2 ´ 1017 /cm2 ). The resulting AES depth pro®le is shown in Fig. 3(a). Because in the beginning a very fast oxidation takes place [9] and the maximum sputter depth of AES is limited, the annealing was done in pure Ar (O2 , H2 O, N2 < 5 ppm) to avoid too fast oxidation. It can be seen in Fig. 3(b) that already after 1 min at 900°C almost 70% of the incorporated Cl is lost out of the sample. An enrichment of Al close to the Cl pro®le indicates an early stage of the formation of Al2 O3 . After 10 min at 900°C this is more pro-
Fig. 2. Compilation of the Cl implantation parameters described in this study. Nmax represents the local concentration in the maximum of the pro®le at a depth Rp . The errorbars mark the straggling DRp . Closed symbols stand for TGA measurements showing a protective eect, for open symbols the oxidation is not improved. The line at 0.05at% represents the order of magnitude needed for the `Cl-eect' in bulk samples.
nounced (Fig. 3(c)). At the approx. depth of 400 nm no titanium is detectable and a dense layer of Al2 O3 has formed. The Cl concentration decreases further. A local concentration of 1at% Cl remains close to the Al2 O3 -layer. The pronounced structure of the oxide scale and the shallow penetration depth of oxygen compared to the unimplanted sample (Fig. 3(d)) shows, that already after 10 min a certain ÔCl-eectÕ is detectable. In Ref. [5], Sch utze and Hald describe a model based on thermodynamic calculations, in which the oxidation behaviour is determined by a catalytic process of volatile chlorides. At certain conditions the vapour pressure for aluminium chloride is higher than that for titanium chloride. This leads to a signi®cant selective Al transport via pores and ®ssures. Therefore formation of Al2 O3 is favoured in a speci®c position in the scale. When the Cl concentration is too high both metals are transported, the positive Cl eect turns into a negative eect. Since the TGA proves, that the protection holds for up to at least 100 h and that in the early
U. Hornauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 858±862
stages of oxidation important processes take place [10], this might be a quick way to investigate the Nmax ±Rp plane (Fig. 2) of the Cl-eect.
Fig. 3. AES of the very ®rst stage of oxidation. (a) as implanted (200 keV Cl , 2á1017 /cm2 ) results to a depth of 200 nm and a peak concentration of 30 at%. (b) 1 min at 900°C. Almost 70% of the incorporated Cl is lost out of the sample. (c) 10 min at 900°C. At the approx. depth of 400 nm is no titanium detectable, a dense layer of Al2 O3 has formed. (d) reference sample.
861
3.2. Macroalloying: implantation of Si TGA oxidation tests of Si implanted samples (Fig. 4) show a positive eect in the beginning of oxidation for a ¯uence of 8 ´ 1017 cmÿ2 . The oxidation kinetics are similar to unimplanted TiAl, but the mass gain after 100 h is still 30% smaller. The lower ¯uence (2 ´ 1017 cmÿ2 ) shows linear oxidation kinetics after a positive eect in the ®rst hours. The oxidation rate becomes even higher than for unimplanted Ti50Al. Since Si and Nb show a comparable [3] eect on the oxidation behaviour, the eect of the variation of ¯uence of Si is dierent from that for Nb implantation [6] and still not understood completely. In order to investigate the phase formation of the implanted Si isochronal annealing in Ar (O2 ,N2 ,H2 O <5 ppm) for dierent temperatures was performed. Fig. 5(a) shows the AES results after implantation and two temperature steps. Surprisingly titanium diuses into the implanted region at 650°C, whereas the Si-pro®le remains unchanged. At the same time GXRD proves the formation of Ti5 Si3 , which is similar to mechanical
Fig. 4. TGA at 900°C in air for 100 h of Si implanted samples. In comparison to a reference sample, the oxidation is improved mainly in the ®rst hours for the implanted specimens. This improvement retains up to 100 h for the high ¯uence, but the kinetics are similar to unimplanted Ti50Al. The lower ¯uence (2 ´ 1017 cmÿ2 ) shows linear kinetics after a positive eect in the ®rst hours.
862
U. Hornauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 858±862
systematic screening of the implantation parameters shows that there is a narrow regime for the optimal ¯uence of Cl ion implantation. The implantation energy is not a sensitive parameter, because the implanted pro®le changes very quickly during high temperature oxidation. It has been shown that important processes take place in the ®rst minutes of oxidation. Further investigations have to be undertaken to understand the eects in detail and to optimise the implantation. After Si doping an improvement in the beginning of oxidation is observed. During oxidation this protection disappears. For a ¯uence of 8 ´ 1017 cmÿ2 the oxidation dynamics are similar to untreated Ti50Al.
Acknowledgements This work is ®nancially supported by the Volkswagen Stiftung. The authors wish to thank also Dr. Sch oneich, Dr. Friedrich (Implantation), Dr. Matz (XRD), Dr. K ogler (RTA) and Dr. Reuther (AES) for excellent work.
Fig. 5. AES of a Si implanted specimen. (a) as implanted; (b) isochronal annealing step at 650°C/1 h showing a change in the Ti distribution; (c) at 850°C step oxidation up to the Si pro®le, which acts as a diusion barrier.
alloyed Ti±Al±Si [11]. In spite of the low O2 partial pressure, after 750°C a pronounced oxidation starts. The depth pro®le for 850°C (Fig.5(c)) shows an almost constant oxygen distribution up to the Si pro®le, which acts as a diusion barrier. XRD shows the existence of a mixed TiO2 /Al2 O3 scale, the Ti5 Si3 phase is still detectable. A detailed description will be published elsewhere. 4. Conclusions It has been shown that it is possible to improve the oxidation resistance of Ti50Al using the microalloying eect. Cl implanted Ti50Al can be protected at 900°C in air for at least 100 h. A
References [1] A. Rahmel, W.J. Quadakkers, M. Sch utze, Mater. and Corrosion 46 (1995) 217. [2] K. Maki, M. Shioda, M. Sayashi, T. Shimizu, S. Isobe, Mat. Sci. Eng. A 153 (1992) 591. [3] Y. Shida, H. Anada, Mater. Trans. 35 (9) (1994) 623. [4] M. Kumagai, K. Shibue, K. Mok-Soon, Y. Makoto, Intermetallics 4 (1996) 557. [5] M. Sch utze, M. Hald, Mat. Sci. Eng. A 239-240 (1997) 847. [6] M.F. Stroosnijder, N. Zheng, W.J. Quadakkers, R. Hofman, A. Gil, F. Lanza, Oxidation of Metals 46 (1996) 19. [7] V.A. C Haanappel, M.F. Stroosnijder, Surf. and Coating Technol. 105 (1998) 147. [8] J. Ziegler, J. Biersack, U. Littmark, The Stopping of Ions in Solids, Program Version 1995, Pergamon Press, New York, 1995. [9] G. Schumacher, C. Lang, M. Sch utze, U. Hornauer, E. Richter, E. Wieser, W. M oller, Mater. and Corrosion, accepted. [10] C. Lang, M. Sch utze, Mater. and Corrosion 48 (1997) 13. [11] Z.Q. Guan, Th. Pfullmann, M. Oehring, R. Bormann, J. Alloys and Comp. 252 (1997) 245.
Improvement of the high temperature oxidation resistance of Ti50Al via ion-implantation U. Hornauer
a,*
, E. Richter a, E. Wieser a, W. M oller a, G. Schumacher b, C. Lang c, M. Sch utze b
a
b
Forschungszentrum Rossendorf e.V., FWII, PO Box 510119, D-01314 Dresden, Germany Karl-Winnacker-Institut der DECHEMA e.V., HTW, Theodor Heuss Allee 25, D-60486 Frankfurt, Germany c Prinz-Konstantin Straûe 12, 81737 M unchen, Germany
Abstract The TiAl intermetallic compound is very promising for high temperature applications, because of its good high temperature strength and its low density. At temperatures exceeding 800°C, the low oxidation resistance is a limiting factor. It is known, that Cl doping reduces the oxidation strongly even in very low concentrations of about 500 ppm (`microalloy'). In the present investigation ion beam implantation is used to dope the material close to the surface quantitatively. The well-de®ned depth pro®le obtained after implantation provided a means to monitor the diusion of additives during oxidation. Implantation of Cl ions (1 MeV, 1015 ±1017 cmÿ2 ) results in a systematic reduction of the oxidation at 900°C in air for doses P 1016 cmÿ2 . AES measurements were performed to investigate the diusion process during oxidation. A microscopic model will be proposed for the enhanced oxidation resistance. For bene®cial eects of Silicon a higher concentration is required (`macroalloy'). Therefore high-dose implantations were carried out (upto 8 ´ 1017 cmÿ2 ). The change in phase composition, microstructure and the oxidation behaviour will be discussed. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 81.65M; 81.40; 61.10 Keywords: TiAl; Ion implantation; Oxidation resistance; Depth pro®ling
1. Introduction c-TiAl based intermetallic compounds are very promising for structural materials in high temperature application, because of the low density of 3.6 g/cm3 . The problems, which hinder the use of
* Corresponding author. Tel.: +49 351-260-3674; fax: +49 351-260-2703; e-mail: [email protected]
this material, are the low ductility at room temperature and the oxidation above 700°C. The basic eects of oxidation and alloying ternary elements are still not understood completely. Because the equilibrium oxygen partial pressure of Ti/TiO and Al/Al2 O3 is very similar [1], a mixed oxide layer is formed during oxidation. Whether a dense protective layer of Al2 O3 is formed or a fast growing scale, depends on the local activities of the metals and the oxides formed, which is in¯uenced by the
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 8 2 0 - 9
U. Hornauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 858±862
859
oxygen partial pressure, but also by the alloying elements [2,3] Unfortunately alloying elements which are usually used for good ductility (i.e. Cr, Mn) deteriorate the oxidation behaviour and those which inhibit oxidation deteriorate the mechanical properties (i.e. W, Nb, Mo, Si). In Ref. [4], it is found that very small amounts of halogenides improve the oxidation behaviour dramatically. A good oxidation resistance has been found for mechanically alloyed TiAl. This was explained by the remaining Cl content, since the Ti sponge was not molten during the powder metalurgical routine. The ÔCl-eectÕ protects TiAl even at very low concentration below 500 ppm. The underlying mechanism and the in¯uence of Cl concentration is discussed in [5]. In the present paper, a systematic variation of [Cl] is accomplished via ion implantation. In order to investigate the eects of doping a material, ion implantation is a very useful tool. First of all the dopant concentration and depth can be reproduced and controlled with high precision, especially in the low concentration region of doping. Secondly it gives a possibility to make a fast screening of doping elements. A well-de®ned variation of the implantation parameters for each doping element is needed to ®nd optimum oxidation protection [6]. For application, it can be imagined to optimise the mechanical and the oxidation properties separately, because only a layer close to the surface (<1 lm) is in¯uenced by implantation. A strong in¯uence of the composition of the material is found [7]. Therefore a c-Ti50Al alloy (GFE) is used in the present study to investigate the basic eects on oxidation.
order to ®nd the optimal parameters for the Cl concentration an extensive screening has been done (energy: 200 keV±2 MeV with constant ¯uence of 1016 Cl /cm2 and ¯uence: 1015 ±1017 cmÿ2 at 1 MeV). For bene®cial eects of Si, a higher concentration is required [3,6]. Therefore highdose implantations were carried out (2 ´ 1017 ± 8 ´ 1017 cmÿ2 ). An energy of 1 MeV Si was chosen, which leads to a range, Rp , of about 1lm. The local Si concentration varies from 8 at% to 35 at%. In all cases the implantation temperature was below 70°C. Both 100 mm2 sides of the samples were implanted in order to optimise the thermogravimetric tests. The outer edges remained unimplanted (relative area <20%). The change in phase composition is analysed via grazing angle X-ray diraction (GXRD) and the element distribution is measured by Auger depth pro®ling (AES). An approximated depth scale is achieved by determination of the ®nal sputter crater depths and the assumption of a constant sputter rate. The implanted pro®les closely follow the predicted pro®les using the TRIM95 code [8]. The thermal treatment has been done in a conventional furnace and in a rapid thermal annealing unit (RTA). Thermogravimetric oxidation tests (TGA) were performed at 900°C in air (rel. humidity of 25% at 295 K) for 100 h using a thermobalance to continuously record the mass gain.
2. Experimental
Fig. 1 shows the TGA results obtained with dierent ¯uences at 1 MeV [9]. The specimens implanted with 1015 and 5 ´ 1015 /cm2 did not show a change in oxidation compared with the unimplanted sample Ti50Al (not shown in Fig. 1). For a suciently high concentration (1016 ±1017 cmÿ2 ), the mass gain almost stops after an incubation period. The oxidation kinetics is similar to that of Al2 O3 forming materials. The oxidation rates are reduced by about 2 orders of magnitude. The duration of the incubation period increases with increasing concentration of Cl. By varying the
Specimens of the dimension 10 ´ 10 ´ 1 mm3 were prepared from the ingot. After grinding they were polished with 4000 grit SiC paper. Before implantation organic substances were removed in a 10 min ultrasonic acetone bath. The samples were mounted on a water cooled sample holder using a special carbon foil to improve the heat contact. The implantations were performed using a 500 kV implanter (HVEE) and a high current tandetron (HVEE) for energies upto 3 MeV. In
3. Results and discussion 3.1. Microalloying: implantation of Cl
860
U. Hornauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 858±862
Fig. 1. Thermogravimetric oxidation tests (TGA) at 900°C in air for 100 h [9]. Ti50Al (dotted line) represents an unimplanted sample. The oxidation kinetics for 1á1015 and 5á1015 /cm2 are unchanged (not shown). The Cl implanted samples (1 MeV, 1á1016 ±1á1017 /cm2 ) show a protective behaviour.
energy from 200 keV to 2 MeV, a comparable reduction of oxidation occurs for energies up to 1 MeV. The sample implanted with 2 MeV exhibits no protection. The results of all implantations are compiled in Fig. 2. The variation of energy and ¯uence results in a variation of the peak concentration Nmax and the projected range Rp . A region of a protective eect is visible towards low energy and above a threshold of 1016 /cm2 . Regarding the incubation time, there is a narrow region for optimum implantation parameters. Compared to homogeneously doped material the concentration Nmax needed for oxidation protection is about ten times higher (Fig. 2). This is probably due to the fast diusion of Cl in TiAl at 900°C. In order to monitor the redistribution process in the ®rst stage, a high dose implantation has been carried out (200 keV, 2 ´ 1017 /cm2 ). The resulting AES depth pro®le is shown in Fig. 3(a). Because in the beginning a very fast oxidation takes place [9] and the maximum sputter depth of AES is limited, the annealing was done in pure Ar (O2 , H2 O, N2 < 5 ppm) to avoid too fast oxidation. It can be seen in Fig. 3(b) that already after 1 min at 900°C almost 70% of the incorporated Cl is lost out of the sample. An enrichment of Al close to the Cl pro®le indicates an early stage of the formation of Al2 O3 . After 10 min at 900°C this is more pro-
Fig. 2. Compilation of the Cl implantation parameters described in this study. Nmax represents the local concentration in the maximum of the pro®le at a depth Rp . The errorbars mark the straggling DRp . Closed symbols stand for TGA measurements showing a protective eect, for open symbols the oxidation is not improved. The line at 0.05at% represents the order of magnitude needed for the `Cl-eect' in bulk samples.
nounced (Fig. 3(c)). At the approx. depth of 400 nm no titanium is detectable and a dense layer of Al2 O3 has formed. The Cl concentration decreases further. A local concentration of 1at% Cl remains close to the Al2 O3 -layer. The pronounced structure of the oxide scale and the shallow penetration depth of oxygen compared to the unimplanted sample (Fig. 3(d)) shows, that already after 10 min a certain ÔCl-eectÕ is detectable. In Ref. [5], Sch utze and Hald describe a model based on thermodynamic calculations, in which the oxidation behaviour is determined by a catalytic process of volatile chlorides. At certain conditions the vapour pressure for aluminium chloride is higher than that for titanium chloride. This leads to a signi®cant selective Al transport via pores and ®ssures. Therefore formation of Al2 O3 is favoured in a speci®c position in the scale. When the Cl concentration is too high both metals are transported, the positive Cl eect turns into a negative eect. Since the TGA proves, that the protection holds for up to at least 100 h and that in the early
U. Hornauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 858±862
stages of oxidation important processes take place [10], this might be a quick way to investigate the Nmax ±Rp plane (Fig. 2) of the Cl-eect.
Fig. 3. AES of the very ®rst stage of oxidation. (a) as implanted (200 keV Cl , 2á1017 /cm2 ) results to a depth of 200 nm and a peak concentration of 30 at%. (b) 1 min at 900°C. Almost 70% of the incorporated Cl is lost out of the sample. (c) 10 min at 900°C. At the approx. depth of 400 nm is no titanium detectable, a dense layer of Al2 O3 has formed. (d) reference sample.
861
3.2. Macroalloying: implantation of Si TGA oxidation tests of Si implanted samples (Fig. 4) show a positive eect in the beginning of oxidation for a ¯uence of 8 ´ 1017 cmÿ2 . The oxidation kinetics are similar to unimplanted TiAl, but the mass gain after 100 h is still 30% smaller. The lower ¯uence (2 ´ 1017 cmÿ2 ) shows linear oxidation kinetics after a positive eect in the ®rst hours. The oxidation rate becomes even higher than for unimplanted Ti50Al. Since Si and Nb show a comparable [3] eect on the oxidation behaviour, the eect of the variation of ¯uence of Si is dierent from that for Nb implantation [6] and still not understood completely. In order to investigate the phase formation of the implanted Si isochronal annealing in Ar (O2 ,N2 ,H2 O <5 ppm) for dierent temperatures was performed. Fig. 5(a) shows the AES results after implantation and two temperature steps. Surprisingly titanium diuses into the implanted region at 650°C, whereas the Si-pro®le remains unchanged. At the same time GXRD proves the formation of Ti5 Si3 , which is similar to mechanical
Fig. 4. TGA at 900°C in air for 100 h of Si implanted samples. In comparison to a reference sample, the oxidation is improved mainly in the ®rst hours for the implanted specimens. This improvement retains up to 100 h for the high ¯uence, but the kinetics are similar to unimplanted Ti50Al. The lower ¯uence (2 ´ 1017 cmÿ2 ) shows linear kinetics after a positive eect in the ®rst hours.
862
U. Hornauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 858±862
systematic screening of the implantation parameters shows that there is a narrow regime for the optimal ¯uence of Cl ion implantation. The implantation energy is not a sensitive parameter, because the implanted pro®le changes very quickly during high temperature oxidation. It has been shown that important processes take place in the ®rst minutes of oxidation. Further investigations have to be undertaken to understand the eects in detail and to optimise the implantation. After Si doping an improvement in the beginning of oxidation is observed. During oxidation this protection disappears. For a ¯uence of 8 ´ 1017 cmÿ2 the oxidation dynamics are similar to untreated Ti50Al.
Acknowledgements This work is ®nancially supported by the Volkswagen Stiftung. The authors wish to thank also Dr. Sch oneich, Dr. Friedrich (Implantation), Dr. Matz (XRD), Dr. K ogler (RTA) and Dr. Reuther (AES) for excellent work.
Fig. 5. AES of a Si implanted specimen. (a) as implanted; (b) isochronal annealing step at 650°C/1 h showing a change in the Ti distribution; (c) at 850°C step oxidation up to the Si pro®le, which acts as a diusion barrier.
alloyed Ti±Al±Si [11]. In spite of the low O2 partial pressure, after 750°C a pronounced oxidation starts. The depth pro®le for 850°C (Fig.5(c)) shows an almost constant oxygen distribution up to the Si pro®le, which acts as a diusion barrier. XRD shows the existence of a mixed TiO2 /Al2 O3 scale, the Ti5 Si3 phase is still detectable. A detailed description will be published elsewhere. 4. Conclusions It has been shown that it is possible to improve the oxidation resistance of Ti50Al using the microalloying eect. Cl implanted Ti50Al can be protected at 900°C in air for at least 100 h. A
References [1] A. Rahmel, W.J. Quadakkers, M. Sch utze, Mater. and Corrosion 46 (1995) 217. [2] K. Maki, M. Shioda, M. Sayashi, T. Shimizu, S. Isobe, Mat. Sci. Eng. A 153 (1992) 591. [3] Y. Shida, H. Anada, Mater. Trans. 35 (9) (1994) 623. [4] M. Kumagai, K. Shibue, K. Mok-Soon, Y. Makoto, Intermetallics 4 (1996) 557. [5] M. Sch utze, M. Hald, Mat. Sci. Eng. A 239-240 (1997) 847. [6] M.F. Stroosnijder, N. Zheng, W.J. Quadakkers, R. Hofman, A. Gil, F. Lanza, Oxidation of Metals 46 (1996) 19. [7] V.A. C Haanappel, M.F. Stroosnijder, Surf. and Coating Technol. 105 (1998) 147. [8] J. Ziegler, J. Biersack, U. Littmark, The Stopping of Ions in Solids, Program Version 1995, Pergamon Press, New York, 1995. [9] G. Schumacher, C. Lang, M. Sch utze, U. Hornauer, E. Richter, E. Wieser, W. M oller, Mater. and Corrosion, accepted. [10] C. Lang, M. Sch utze, Mater. and Corrosion 48 (1997) 13. [11] Z.Q. Guan, Th. Pfullmann, M. Oehring, R. Bormann, J. Alloys and Comp. 252 (1997) 245.