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The influence of Nb ion implantation upon oxidation behavior and hardness of a Ti-48 at.% Al alloy
Surface and Coatings Technology 100-101 ( 1998) 214P218
The influence of Nb ion implantation upon oxidation behavior and hardness of a Ti-48 at.% Al alloy Y.G. Zhang a,*, X.Y. Li a, C.Q. Chen a, W. Wang a, Vincent Ji b a Department
qf Materials
Science and Engineering. Beijing Unirersity oj’Aeronautics and Astronautics. b LM3 ENSAM, 151 boulevard de I’Hopital75013 Paris. France
Beijing. 100083. P.R. China
Abstract The influence of Nb ion implantation with different doses upon microhardness and high-temperature oxidation resistance of a Ti-48 at.% Al alloy was investigated. The results showed that the surface microhardness of the alloy was significantly increased by Nb implantation with doses from 3 x 1015 ions cm-l to 3 x 1Or6 ions cm-‘, and reached a constant level at a dose of 3 x 10” ions cmm2. The isothermal oxidation resistance of the alloy at 900 ‘C and 950 “‘C in an atmospheric environment could be remarkably improved by Nb ion implantation with a dose of 3 x IO” ions cm-‘. which was attributed to the effect of the implanted Nb atoms on the nucleation and formation of the oxides and the oxidation scale microstructure. The influence of both the primary and secondary effects of Nb ion implantation upon the microhardness and oxidation behavior of Ti-48 at.% Al alloy is discussed. 0 1998ElsevierScienceS.A. Keywords:
Ion implantation;
Microhardness;
Oxidation
resistance; TiAl intermetallic
1. Introduction The TiAl intermetallic compound has received an increasing amount of interest as an attractive light, hightemperature structural material to be potentially used in the engines of the next generation of space planes and other advanced transport systemsbecauseof its low density and high strength-to-density ratio at elevated temperatures [I]. Various efforts have been made in understanding and improving its room-temperature ductility, which is considered to be a major obstacle in the application of the alloy, and tremendous progress has been achieved. However, the integration of TiAl into actual service has also been hindered by its poor oxidation resistance above 800 “C 121. In addition, it would be necessary to modify its surface properties such as hardness and wear resistance, etc. when it is used as a part in contact with other components. These aspects have attracted much attention most recently [3]. There are many surface modification techniques that may be used to improve the oxidation resistance of alloys and metals. Among them, ion implantation shows * Corresponding author. Fax: + 86 IO 62015347; e-mail: [email protected] 0257~8972/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved, PII
SO257-8972(97)00616-6
compound
unique modification advantages in comparison with others [4]. Because this process does not produce a coating, the dimension and bulk properties of the part remain unchanged, and implanted surfaces have no problems with adhesion and residual stress, etc. [5]. These features are particularly attractive when both oxidation and wear resistanceare considered to be issues in a component. In this work, the inlluence of Nb ion implantation with different doses upon microhardness and oxidation resistanceof a Ti-48 at.% Al alloy was investigated, and the possible mechanisms of the effects were proposed on the basis of microstructure analysis of the oxidation scales.
2. Experimental 2.1. Substrate alloy and specimens The alloy used in the present study was produced by a non-consumable argon arc furnace, using a titanium granular sponge (99.9%) and a pure aluminum plate (99.99%). The ingot was melted twice and then
Y. G. Zhang
et al. 1 Swfhce
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underwent a homogenizing treatment at 1000 “C for 7 days. The nominal composition of the ingot was Ti-48 at.% Al (hereafter referred to as 48Al). It was isothermally forged at 1150 “C with the compression ratio of 60% and subsequently heat-treated at 1330 C/2hAC +980 “C/8hAC, which led to a duplex microstructure. Specimens measuring about 2x 5 x 9mm3 were machined from the ingot, and polished with up to SOO# SIC paper and cleaned with acetone and ethanol before ion implantation, microhardness and isothermal oxidation tests. 2.2. Ion impluntation and tests Ion implantation was carried out in a MEVVA Source Ion Implanter, and the conditions are listed in Table 1. A microhardness test was performed using a Vickers profile indenter, a load of 50 g was applied for 10 s and then the length of indentation diagonal was measured through a microscope. Five indentations were made on each specimen, and both diagonals of each indentation were measured separately, the final microhardness data representing the mean of the five measurements. The isothermal oxidation was performed at 900 “C and 950 “C for different times up to 100 h under an atmospheric environment in a vertical furnace. The weight change of the specimens being oxidized was continuously recorded by a balance with a resolution of 0.01 mg. Two specimens were used for each isothermal oxidation test. 2.3. Metallographic
examinations
The distribution of ion-implanted elements in the specimens was examined by auger electron spectroscopy (AES). Outer surfaces and cross-sections of the oxidation scales were observed by scanning electron microscopy (SEM).
Sputter Time(min) Fig. 1. AES Nb-implanted ing time.
analysis of elemental concentration distributions in 48AI with a dose of 3 x lOI ions cm-’ versus sputter-
Nb appears beneath the surface of the specimen at a depth of about 16.5 nm. 3.2. Change in microhurdness The effects of Nb ion implantation upon the microhardness of 48Al alloy with different doses are presented in Fig. 2. It can be seen that the microhardness of implanted alloy with a dose 3 x 1015 ions cmm2 has a noticeable improvement compared to the unimplanted one, and it has increased rapidly when the implantation dose was increased from 3 x 1015 ions cm-2 to 3 x 1016 ions cm -‘; a further increase in dose (to 3 x 10” ions cmm2) appeared not to improve the hardness any further. This result implies that there may be a critical dose value between 3 x 1016 ions cm-2 and
3. Results 3.1. Distribution
of implunted Nb
Fig. 1 shows the element concentration distributions in Nb-implanted 48Al alloy analyzed by AES with a sputtering rate of about 5.5 nm mini. The element concentrations are a function of sputtering time and roughly follow a Gaussian curve, and a peak concentration of Table 1 Conditions
of Nb ion implantation Dcs0
Element
Dose (ions cm-‘)
Nb
3 x 10’5, 3 x IO’b.
Energy 3 x lOI
(keV
40 x 1. 1, 3
)
Current 1, 1, 3
(mA)
Fig. 2. Changes of microhardness of Nb-implanted 48Al with different doses: (A) unimplanted, (B) 3 x 10’5ionscm-z, (C) 3 x 1016 ions cm -‘, and (D) 3 x IO” ions cm-‘.
heavy dose is more effective for the Nb ion implantation to be used to modify the oxidation properties of the alloy. 3.4. Microstructure
0
looo2ooo3rm4ooo5oooeooo
Time. min Fig. 3. Oxidation kinetics of 48AI and Nb-implanted 48AI with a dose of 3 x 10”ions cm-’ oxidized at 900 and 950 ‘C for 100 h.
3 x 10” ions cm-’ that may correspond to the maximum microhardness value of Nb-implanted 48Al alloy. 3.3. Isothermul
oxidutiorz kinetics
The isothermal oxidation kinetics of unimplanted 48Al and Nb-implanted 48Al with a dose of 3 x 10” ions cmm2 oxidized at 900 and 950 “C for 100 h is shown in Fig. 3. It can been seen that the oxidation resistance of the alloy was remarkably promoted by Nb implantation in comparison with that of the unimplanted one, and the promotion effect was maintained considerably, even at 950 “C for 100 h. The oxidation kinetics curves are mainly near parabola. The effect of dose on the oxidation kinetics of Nb implanted alloy with an increase in the dose from 3 x 10” ions cmP2 to 3 x 10” ions cm-’ oxidized at 900 “C for 100 h is presented in Fig. 4. The result shows that the oxidation rate of Nb-implanted specimens decreased with the increase in dose, and therefore a
4.5 4 3.5 3 2.5 2 1.5 1 0.5 0
Fig. 4. Erect of dose on the oxidation kinetics with doses from 3 x lOI ions cm-’ to 3 x IO” 900 ‘C for 100 h.
of Nb-implanted 48AI ions cm-’ oxidized at
of oxidution scales
The morphology of outer surfaces of the Nb-implanted and unimplanted 48Al oxidized at 900 “C for different times is shown in Fig. 5. It should be noted that the shape and grain size of the oxides developed on the surfaces of Nb-implanted 48Al and unimplanted 48Al after oxidation at 900 ‘C for 24 h (Fig. 5a, b) are significantly different to each other, which clearly demonstrates that Nb ion implantation has a strong effect on the nucleation and formation of the oxidation products at the beginning of the oxidation reaction. A layer of dense TiO, grains with a regular shape completely covered the surface of the unimplanted 48Al (Fig. 5a), but only some oxide particles with an irregular shape were formed on the surface of Nb-implanted specimens. After a much longer oxidation period, e.g. undergoing oxidation at 900 ‘C for 100 h, the oxides on the outer surfaces of the oxidation scales for both the implanted and unimplanted specimens almost have the same shape with the grain size increasing with exposure time, but to different extents. The average grain size of the oxides developed on the Nb-implanted surface with a dose of 3 x 10” ions cm-’ is much smaller than that on the unimplanted 48Al alloy (Fig. 5c, d). Fig. 6 shows the morphology of the cross-sections of oxidation scales formed on the unimplanted and Nb-implanted specimens after oxidation at 950 “C for 100 h. The typical cross-section structure of the oxidation scale of unimplanted 48Al is multi-layer oxides, which is in agreement with other reports [6,7]. It was noted that the Al,O, formed in the outer part of the oxidation scale was discontinuously dispersed in the TiO, layer like isolated islands (Fig. 6a); a layer of almost pure TiO, appeared next to this part. The internal region is uniformly and loosely dispersed with mixture of TiO, and Al,O,. There is no layer of continuous and compact AlzO, that would act as a barrier inhibiting the outward and inward fluxes of elements and oxygen during the oxidation process. In contrast, it can be seen that a layer consisting of continuous Al,O, was formed in the oxidation scale of Nb-implanted alloy with a dose of 3 x 10” ions cm-l (Fig. 6b), and the oxidation scale is relatively thinner than that of the unimplanted one. In addition to the outer continuous Al,O, layer, the Al,O, particles in the internal region are almost completely linked with each other and appear as a network. This type of structure of Al,O, could retard and even block the diffusion process and effectively reduce the oxide growing rate. Hence, this result demonstrated that Nb implantation with a dose of 3 x 10” ions cmm2 could enhance the formation of Al,O, to act as a barrier
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Fig. 5. Secondary electron (SE) images of the outer surfaces of unimplanted and Nb-implanted 48Al oxidized at 900 “C for different unimplanted 48A1, 900 C/24h; (b) Nb-implanted 48Al (3 x IO” ions cm-‘), 900 C for 24 h; (c) unimplanted 48A1, 900 “C for Nb-implanted 48AI (3 x IO” ions cm-‘), 900 ‘C for 100 h.
against the diffusion process oxidation rate of 48Al alloy.
and thus slow down
the
4. Discussion It is well known that in addition to doping, a primary process of implantation, many secondary processes such as lattice damage and density increase of dislocations and other defects are introduced into the implanted region and its affected zone during the implantation process. The high density of point defects (vacancies and interstitials) and dislocations created in the affected zone would be expected to harden the surface layer. The
Fig. 6. Back-scattering electron (BSE) images of cross-sections of the oxidation scales formed on unimplanted and Nb-implanted 48AI alloy. (a) Unimplanted 48A1, 950 “C for 100 h and (b) Nb-implanted 48Al (3 x IO” ions cm-‘), 950 “C for 100 h.
217
times. (a) 100 h: (d)
doped Nb atoms might introduce solution hardening effect as well. These effects are expected to be very effective at the early implantation stage or with small doses, and with an increase in the dose, dynamic annihilation processes between point defects (vacancies and interstitial) and between point defects and dislocations may lead to a non-equilibrium steady-state concentration, or density of the defects at a critical dose; alternatively, more defects created with the increase of dose might mainly enlarge the affected zone rather than increase the defect density and, therefore, the hardness related to the defect density might not be expected to be increased further. This argument may account for the microhardness behavior of the Nb-implanted 48Al alloy, but further experimental evidence, e.g. changes in defect density and distribution with the implantation dose, is needed to give a more accurate explanation. The parabolic nature of the oxidation kinetics indicates that the initial oxidation is due to diffusion of the metals to the surface but, after formation of the oxide films, the oxidation proceeds by diffusion of oxygen through the initial layers towards the metal. The experimental results showed that the main oxidation products are TiO,, and Al,O, with TiOz made up the outermost oxide layer. This is because the kinetics of Ti oxide formation is typically greater than that of AllO,. The high-concentration doped Nb near the surface might reduce the diffusion of Ti outward and thus slow down the oxidation at an early stage. TiO, is an n-type semiconductor and contains the interstitials based on Ti3+and Ti4+ existing together with doubly ionized oxygen vacancies [ 81. Kekare et al. indicated that the
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doubly ionized oxygen vacancies are the rate-controlling species during the growth of TiO, over Ti and, therefore, any dopant in the TiO, that is able to reduce the concentration of the vacancies will retard the oxidation of TiAl alloys [9]. Additions of Nb, which belongs to the VB group, could form cations with a valency of more than +4, and may substitute the Ti site in the TiO, space lattice, thus reducing the number of interstitials and oxygen ion vacancies in the oxides. This effect can decrease the diffusion rate in the oxidation scale. In addition to the valent-control rule, the implanted element Nb may be incorporated with the oxidation reaction of the base alloy and modify the structure of the oxidation scale as shown above. The defects introduced during the implantation are expected to enhance diffusion of all elements considerably and, therefore, may play an opposite role in the improvement of the oxidation resistance of the alloy. Taking the role of doped elements into account, it may be assumed that the oxidation behavior of the implanted specimens may be considered as a result of competition and balance of the two processes, i.e. the primary and the secondary processes; if the doped element such as Nb in 48Al plays a positive role in the oxidation resistance, the implantation with a proper dose could enhance the oxidation resistance of the alloy. Thus, it may be understood that at a low dose, e.g. 3 x lOi ions cm-‘, the secondary process may dominate and, as a result, the oxidation resistance of the implanted specimens was even poorer than the unimplanted ones. but the hardness was increased remarkably. It may be expected that if the implanted elements do not play a positive role in the oxidation resistance, the implantation must be harmful to the oxidation properties of the alloy. This has been clearly demonstrated to be true in the 48Al implanted with vanadium [ lo]. However, it may be expected that if the secondary effects can be eliminated or suppressed by suitable post-implantation annealing, the oxidation resistance of Nb-implanted 48Al alloy would be expected to be improved further, although the microhardness also may be decreased, which has been demonstrated to be true in our primary work [lo].
Technology
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5. Conclusions Nb implantation with doses from 3 x 1015 ions cme2 to 3 x 1016 ions cm-2 significantly enhanced the surface microhardness of Ti-48 at.% Al alloy, which might be mainly due to the high density of the defects introduced during the implantation. The oxidation rate of Nb-implanted Ti-48 at.% Al at 900 and 950 “C decreases with increasing dose from 3 x 1015 ions cmm2 to 3 x 10” ions crne2 , and the oxidation resistance of Ti-48 at.% Al alloy may be remarkably improved by Nb implantation with doses of 3 x 1016 ions cm-’ and 3 x 10” ions cmp2. Niobium implantation with proper doses may enhance the formation of a layer consisting of continuous and compact Al,O, to act as a barrier of diffusion processes during long-term oxidation, and thus improve the oxidation resistance of Ti-48 at.% Al alloy.
Acknowledgement The authors are grateful to professor X.J. Zhang and professor T.H. Zhang of the Institute of Low Energy Nuclear Physics of Beijing Normal University for the provision of implantation facilities and useful discussions, and thanks are also to the financial support of Aeronautical Science Foundation of China.
References [l] [2] [3] [4] [5] [6] [7] [X] [9] [IO]
Y.-W. Kim, D.M. Dimiduk, JOM 43 (8) (1991) 40. G.H. Meier, F.S. Pettit, Mater. Sci. Eng. A I53 ( 1992) 548. T.C. Munro. B. Gleeson, Metall. Mater. Trans. 27A (1996) 3761. P.Y. Hou, J. Stringer. Oxid. Met. 13 (1988) 45. J.R. Treglio, A.J. Peny, R.J. Stinner, Adv. Mater. Pro. 5 (1995) 29. M. Yoshihara, R. Tanaka, Bull. Japan Inst. Met. 30 (1991) 61. Y. Shida, H. Anada, J. Japan Inst. Met. 55 (1991) 690. P. Kofstad, P.B. Anderson, O.J. Krudtaa, J. Less Common Metals 3 (1961) 89. S.A. Kekare. P.B. Aswath, J. Mater. Sci. 32 ( 1997) 2485. X.Y. Li, PhD thesis. Beijing University of Aronautics and Astronautics. 1997.
The influence of Nb ion implantation upon oxidation behavior and hardness of a Ti-48 at.% Al alloy Y.G. Zhang a,*, X.Y. Li a, C.Q. Chen a, W. Wang a, Vincent Ji b a Department
qf Materials
Science and Engineering. Beijing Unirersity oj’Aeronautics and Astronautics. b LM3 ENSAM, 151 boulevard de I’Hopital75013 Paris. France
Beijing. 100083. P.R. China
Abstract The influence of Nb ion implantation with different doses upon microhardness and high-temperature oxidation resistance of a Ti-48 at.% Al alloy was investigated. The results showed that the surface microhardness of the alloy was significantly increased by Nb implantation with doses from 3 x 1015 ions cm-l to 3 x 1Or6 ions cm-‘, and reached a constant level at a dose of 3 x 10” ions cmm2. The isothermal oxidation resistance of the alloy at 900 ‘C and 950 “‘C in an atmospheric environment could be remarkably improved by Nb ion implantation with a dose of 3 x IO” ions cm-‘. which was attributed to the effect of the implanted Nb atoms on the nucleation and formation of the oxides and the oxidation scale microstructure. The influence of both the primary and secondary effects of Nb ion implantation upon the microhardness and oxidation behavior of Ti-48 at.% Al alloy is discussed. 0 1998ElsevierScienceS.A. Keywords:
Ion implantation;
Microhardness;
Oxidation
resistance; TiAl intermetallic
1. Introduction The TiAl intermetallic compound has received an increasing amount of interest as an attractive light, hightemperature structural material to be potentially used in the engines of the next generation of space planes and other advanced transport systemsbecauseof its low density and high strength-to-density ratio at elevated temperatures [I]. Various efforts have been made in understanding and improving its room-temperature ductility, which is considered to be a major obstacle in the application of the alloy, and tremendous progress has been achieved. However, the integration of TiAl into actual service has also been hindered by its poor oxidation resistance above 800 “C 121. In addition, it would be necessary to modify its surface properties such as hardness and wear resistance, etc. when it is used as a part in contact with other components. These aspects have attracted much attention most recently [3]. There are many surface modification techniques that may be used to improve the oxidation resistance of alloys and metals. Among them, ion implantation shows * Corresponding author. Fax: + 86 IO 62015347; e-mail: [email protected] 0257~8972/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved, PII
SO257-8972(97)00616-6
compound
unique modification advantages in comparison with others [4]. Because this process does not produce a coating, the dimension and bulk properties of the part remain unchanged, and implanted surfaces have no problems with adhesion and residual stress, etc. [5]. These features are particularly attractive when both oxidation and wear resistanceare considered to be issues in a component. In this work, the inlluence of Nb ion implantation with different doses upon microhardness and oxidation resistanceof a Ti-48 at.% Al alloy was investigated, and the possible mechanisms of the effects were proposed on the basis of microstructure analysis of the oxidation scales.
2. Experimental 2.1. Substrate alloy and specimens The alloy used in the present study was produced by a non-consumable argon arc furnace, using a titanium granular sponge (99.9%) and a pure aluminum plate (99.99%). The ingot was melted twice and then
Y. G. Zhang
et al. 1 Swfhce
and Coatings
Technology
100~101
(l%%‘)
214-218
215
underwent a homogenizing treatment at 1000 “C for 7 days. The nominal composition of the ingot was Ti-48 at.% Al (hereafter referred to as 48Al). It was isothermally forged at 1150 “C with the compression ratio of 60% and subsequently heat-treated at 1330 C/2hAC +980 “C/8hAC, which led to a duplex microstructure. Specimens measuring about 2x 5 x 9mm3 were machined from the ingot, and polished with up to SOO# SIC paper and cleaned with acetone and ethanol before ion implantation, microhardness and isothermal oxidation tests. 2.2. Ion impluntation and tests Ion implantation was carried out in a MEVVA Source Ion Implanter, and the conditions are listed in Table 1. A microhardness test was performed using a Vickers profile indenter, a load of 50 g was applied for 10 s and then the length of indentation diagonal was measured through a microscope. Five indentations were made on each specimen, and both diagonals of each indentation were measured separately, the final microhardness data representing the mean of the five measurements. The isothermal oxidation was performed at 900 “C and 950 “C for different times up to 100 h under an atmospheric environment in a vertical furnace. The weight change of the specimens being oxidized was continuously recorded by a balance with a resolution of 0.01 mg. Two specimens were used for each isothermal oxidation test. 2.3. Metallographic
examinations
The distribution of ion-implanted elements in the specimens was examined by auger electron spectroscopy (AES). Outer surfaces and cross-sections of the oxidation scales were observed by scanning electron microscopy (SEM).
Sputter Time(min) Fig. 1. AES Nb-implanted ing time.
analysis of elemental concentration distributions in 48AI with a dose of 3 x lOI ions cm-’ versus sputter-
Nb appears beneath the surface of the specimen at a depth of about 16.5 nm. 3.2. Change in microhurdness The effects of Nb ion implantation upon the microhardness of 48Al alloy with different doses are presented in Fig. 2. It can be seen that the microhardness of implanted alloy with a dose 3 x 1015 ions cmm2 has a noticeable improvement compared to the unimplanted one, and it has increased rapidly when the implantation dose was increased from 3 x 1015 ions cm-2 to 3 x 1016 ions cm -‘; a further increase in dose (to 3 x 10” ions cmm2) appeared not to improve the hardness any further. This result implies that there may be a critical dose value between 3 x 1016 ions cm-2 and
3. Results 3.1. Distribution
of implunted Nb
Fig. 1 shows the element concentration distributions in Nb-implanted 48Al alloy analyzed by AES with a sputtering rate of about 5.5 nm mini. The element concentrations are a function of sputtering time and roughly follow a Gaussian curve, and a peak concentration of Table 1 Conditions
of Nb ion implantation Dcs0
Element
Dose (ions cm-‘)
Nb
3 x 10’5, 3 x IO’b.
Energy 3 x lOI
(keV
40 x 1. 1, 3
)
Current 1, 1, 3
(mA)
Fig. 2. Changes of microhardness of Nb-implanted 48Al with different doses: (A) unimplanted, (B) 3 x 10’5ionscm-z, (C) 3 x 1016 ions cm -‘, and (D) 3 x IO” ions cm-‘.
heavy dose is more effective for the Nb ion implantation to be used to modify the oxidation properties of the alloy. 3.4. Microstructure
0
looo2ooo3rm4ooo5oooeooo
Time. min Fig. 3. Oxidation kinetics of 48AI and Nb-implanted 48AI with a dose of 3 x 10”ions cm-’ oxidized at 900 and 950 ‘C for 100 h.
3 x 10” ions cm-’ that may correspond to the maximum microhardness value of Nb-implanted 48Al alloy. 3.3. Isothermul
oxidutiorz kinetics
The isothermal oxidation kinetics of unimplanted 48Al and Nb-implanted 48Al with a dose of 3 x 10” ions cmm2 oxidized at 900 and 950 “C for 100 h is shown in Fig. 3. It can been seen that the oxidation resistance of the alloy was remarkably promoted by Nb implantation in comparison with that of the unimplanted one, and the promotion effect was maintained considerably, even at 950 “C for 100 h. The oxidation kinetics curves are mainly near parabola. The effect of dose on the oxidation kinetics of Nb implanted alloy with an increase in the dose from 3 x 10” ions cmP2 to 3 x 10” ions cm-’ oxidized at 900 “C for 100 h is presented in Fig. 4. The result shows that the oxidation rate of Nb-implanted specimens decreased with the increase in dose, and therefore a
4.5 4 3.5 3 2.5 2 1.5 1 0.5 0
Fig. 4. Erect of dose on the oxidation kinetics with doses from 3 x lOI ions cm-’ to 3 x IO” 900 ‘C for 100 h.
of Nb-implanted 48AI ions cm-’ oxidized at
of oxidution scales
The morphology of outer surfaces of the Nb-implanted and unimplanted 48Al oxidized at 900 “C for different times is shown in Fig. 5. It should be noted that the shape and grain size of the oxides developed on the surfaces of Nb-implanted 48Al and unimplanted 48Al after oxidation at 900 ‘C for 24 h (Fig. 5a, b) are significantly different to each other, which clearly demonstrates that Nb ion implantation has a strong effect on the nucleation and formation of the oxidation products at the beginning of the oxidation reaction. A layer of dense TiO, grains with a regular shape completely covered the surface of the unimplanted 48Al (Fig. 5a), but only some oxide particles with an irregular shape were formed on the surface of Nb-implanted specimens. After a much longer oxidation period, e.g. undergoing oxidation at 900 ‘C for 100 h, the oxides on the outer surfaces of the oxidation scales for both the implanted and unimplanted specimens almost have the same shape with the grain size increasing with exposure time, but to different extents. The average grain size of the oxides developed on the Nb-implanted surface with a dose of 3 x 10” ions cm-’ is much smaller than that on the unimplanted 48Al alloy (Fig. 5c, d). Fig. 6 shows the morphology of the cross-sections of oxidation scales formed on the unimplanted and Nb-implanted specimens after oxidation at 950 “C for 100 h. The typical cross-section structure of the oxidation scale of unimplanted 48Al is multi-layer oxides, which is in agreement with other reports [6,7]. It was noted that the Al,O, formed in the outer part of the oxidation scale was discontinuously dispersed in the TiO, layer like isolated islands (Fig. 6a); a layer of almost pure TiO, appeared next to this part. The internal region is uniformly and loosely dispersed with mixture of TiO, and Al,O,. There is no layer of continuous and compact AlzO, that would act as a barrier inhibiting the outward and inward fluxes of elements and oxygen during the oxidation process. In contrast, it can be seen that a layer consisting of continuous Al,O, was formed in the oxidation scale of Nb-implanted alloy with a dose of 3 x 10” ions cm-l (Fig. 6b), and the oxidation scale is relatively thinner than that of the unimplanted one. In addition to the outer continuous Al,O, layer, the Al,O, particles in the internal region are almost completely linked with each other and appear as a network. This type of structure of Al,O, could retard and even block the diffusion process and effectively reduce the oxide growing rate. Hence, this result demonstrated that Nb implantation with a dose of 3 x 10” ions cmm2 could enhance the formation of Al,O, to act as a barrier
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et al. J Surjtice
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Fig. 5. Secondary electron (SE) images of the outer surfaces of unimplanted and Nb-implanted 48Al oxidized at 900 “C for different unimplanted 48A1, 900 C/24h; (b) Nb-implanted 48Al (3 x IO” ions cm-‘), 900 C for 24 h; (c) unimplanted 48A1, 900 “C for Nb-implanted 48AI (3 x IO” ions cm-‘), 900 ‘C for 100 h.
against the diffusion process oxidation rate of 48Al alloy.
and thus slow down
the
4. Discussion It is well known that in addition to doping, a primary process of implantation, many secondary processes such as lattice damage and density increase of dislocations and other defects are introduced into the implanted region and its affected zone during the implantation process. The high density of point defects (vacancies and interstitials) and dislocations created in the affected zone would be expected to harden the surface layer. The
Fig. 6. Back-scattering electron (BSE) images of cross-sections of the oxidation scales formed on unimplanted and Nb-implanted 48AI alloy. (a) Unimplanted 48A1, 950 “C for 100 h and (b) Nb-implanted 48Al (3 x IO” ions cm-‘), 950 “C for 100 h.
217
times. (a) 100 h: (d)
doped Nb atoms might introduce solution hardening effect as well. These effects are expected to be very effective at the early implantation stage or with small doses, and with an increase in the dose, dynamic annihilation processes between point defects (vacancies and interstitial) and between point defects and dislocations may lead to a non-equilibrium steady-state concentration, or density of the defects at a critical dose; alternatively, more defects created with the increase of dose might mainly enlarge the affected zone rather than increase the defect density and, therefore, the hardness related to the defect density might not be expected to be increased further. This argument may account for the microhardness behavior of the Nb-implanted 48Al alloy, but further experimental evidence, e.g. changes in defect density and distribution with the implantation dose, is needed to give a more accurate explanation. The parabolic nature of the oxidation kinetics indicates that the initial oxidation is due to diffusion of the metals to the surface but, after formation of the oxide films, the oxidation proceeds by diffusion of oxygen through the initial layers towards the metal. The experimental results showed that the main oxidation products are TiO,, and Al,O, with TiOz made up the outermost oxide layer. This is because the kinetics of Ti oxide formation is typically greater than that of AllO,. The high-concentration doped Nb near the surface might reduce the diffusion of Ti outward and thus slow down the oxidation at an early stage. TiO, is an n-type semiconductor and contains the interstitials based on Ti3+and Ti4+ existing together with doubly ionized oxygen vacancies [ 81. Kekare et al. indicated that the
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Y.G. Zhang
et al. / Surfh-ce and Coatings
doubly ionized oxygen vacancies are the rate-controlling species during the growth of TiO, over Ti and, therefore, any dopant in the TiO, that is able to reduce the concentration of the vacancies will retard the oxidation of TiAl alloys [9]. Additions of Nb, which belongs to the VB group, could form cations with a valency of more than +4, and may substitute the Ti site in the TiO, space lattice, thus reducing the number of interstitials and oxygen ion vacancies in the oxides. This effect can decrease the diffusion rate in the oxidation scale. In addition to the valent-control rule, the implanted element Nb may be incorporated with the oxidation reaction of the base alloy and modify the structure of the oxidation scale as shown above. The defects introduced during the implantation are expected to enhance diffusion of all elements considerably and, therefore, may play an opposite role in the improvement of the oxidation resistance of the alloy. Taking the role of doped elements into account, it may be assumed that the oxidation behavior of the implanted specimens may be considered as a result of competition and balance of the two processes, i.e. the primary and the secondary processes; if the doped element such as Nb in 48Al plays a positive role in the oxidation resistance, the implantation with a proper dose could enhance the oxidation resistance of the alloy. Thus, it may be understood that at a low dose, e.g. 3 x lOi ions cm-‘, the secondary process may dominate and, as a result, the oxidation resistance of the implanted specimens was even poorer than the unimplanted ones. but the hardness was increased remarkably. It may be expected that if the implanted elements do not play a positive role in the oxidation resistance, the implantation must be harmful to the oxidation properties of the alloy. This has been clearly demonstrated to be true in the 48Al implanted with vanadium [ lo]. However, it may be expected that if the secondary effects can be eliminated or suppressed by suitable post-implantation annealing, the oxidation resistance of Nb-implanted 48Al alloy would be expected to be improved further, although the microhardness also may be decreased, which has been demonstrated to be true in our primary work [lo].
Technology
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5. Conclusions Nb implantation with doses from 3 x 1015 ions cme2 to 3 x 1016 ions cm-2 significantly enhanced the surface microhardness of Ti-48 at.% Al alloy, which might be mainly due to the high density of the defects introduced during the implantation. The oxidation rate of Nb-implanted Ti-48 at.% Al at 900 and 950 “C decreases with increasing dose from 3 x 1015 ions cmm2 to 3 x 10” ions crne2 , and the oxidation resistance of Ti-48 at.% Al alloy may be remarkably improved by Nb implantation with doses of 3 x 1016 ions cm-’ and 3 x 10” ions cmp2. Niobium implantation with proper doses may enhance the formation of a layer consisting of continuous and compact Al,O, to act as a barrier of diffusion processes during long-term oxidation, and thus improve the oxidation resistance of Ti-48 at.% Al alloy.
Acknowledgement The authors are grateful to professor X.J. Zhang and professor T.H. Zhang of the Institute of Low Energy Nuclear Physics of Beijing Normal University for the provision of implantation facilities and useful discussions, and thanks are also to the financial support of Aeronautical Science Foundation of China.
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