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Influence of carburization on oxidation behavior of High Nb contained TiAl alloy
Surface & Coatings Technology 277 (2015) 210–215
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of carburization on oxidation behavior of High Nb contained TiAl alloy Tianhang Yao, Yong Liu ⁎, Bin Liu, Min Song, Kun Zhao, Weidong Zhang, Yuehui He State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, China
a r t i c l e
i n f o
Article history: Received 13 December 2014 Revised 6 May 2015 Accepted in revised form 26 July 2015 Available online 29 July 2015 Keywords: TiAl alloy Carburization Oxidation
a b s t r a c t In this work, surface carburization has been performed on Ti–45Al–7Nb–0.3W alloy. The isothermal oxidation behavior of the alloy has been investigated in static air at 950 °C for 100 h. X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were applied to study the microstructural evolution. The results reveal that the carburized layer, with a thickness of 7.4 μm, is composed of nanostructured Ti2AlC and Ti3AlC phases. The oxidation weight gain of the carburized TiAl is half of that of the bare TiAl. The oxide scale of the carburized TiAl is compact without delamination and spallation. The inward diffusion of oxygen atom was hindered due to the existence of the carburized layer. Therefore, the carburization process is an effective and feasible treatment to improve the oxidation resistance of high Nb contained TiAl alloys. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent decades TiAl based alloys have attracted extensive attentions, and have been regarded as potential structural materials for high temperature applications, due to their high strength to weight ratio [1–3]. However, several shortcomings substantially limit their industrial applications, such as intrinsic brittleness and poor oxidation resistance at high temperatures. It has been shown that large amount Nb addition to TiAl alloys is an effective way to improve the high temperature tolerance and strength [4]. In addition, the addition of Nb also enhances the oxidation resistance of TiAl alloys by promoting the formation of Al-rich oxides scale, which acts as a barrier to the inward diffusion of oxygen [5]. However, it should be noted that even large amount Nb addition is introduced, severe oxidation and spallation of the oxide layers can still happen when the temperature increases to 900 °C. Thus, lots of efforts have been made to improve the oxidation resistance of TiAl alloys through coating techniques and the surface alloying [6,7]. In comparison with the surface alloying, coatings or hard films are not very useful due to the interfacial compatibility between the coating and the substrate. The carburization process has been widely applied on steel, Ni-based alloys and Ti-based alloys [8–10]. It aims to form a continuous C-containing ceramic layer to enhance the surface anti-wear and anti-oxidation properties. Previous investigations have applied the carburization on TiAl alloys to improve the surface hardness, mechanical property and anti-corrosion property [11,12]. However, little information is reported on the enhancement of the oxidation resistance of TiAl alloys by the carburization.
⁎ Corresponding author. E-mail address: [email protected] (Y. Liu).
http://dx.doi.org/10.1016/j.surfcoat.2015.07.058 0257-8972/© 2015 Elsevier B.V. All rights reserved.
In contrast to other technologies such as plasma carburization or surface ion implantation, solid carburization has advantages of low energy consumption, easy controlling and easy access to carbon source. The purpose of this work is to investigate the feasibility of the formation of C-containing ceramic layer on high Nb contained TiAl alloys by pack carburization, and effects of the layer on the oxidation behaviors at high temperatures has been systematically studied. 2. Experimental 2.1. Materials The pre-alloyed Ti–45Al–7Nb–0.3W (at.%) powders were prepared by plasma rotating electrode processing (PREP) under the protection of inert gas. The powders were filled into a steel can and degassed at 400 °C, then hot isostatic pressed (HIPed) at 1240 °C, and 140 MPa for 4 h. The specimens with the size of 10 mm × 10 mm × 1.5 mm were cut from as-HIPed billet by electric discharge machining (EDM). Before the carburization and oxidation, each specimen was polished using 1500 grit emery paper, and ultrasonically cleaned in acetone for 15 min. 2.2. Carburization Carburization treatments were performed in a corundum crucible which was filled with solid carburizer. The solid carburizer consists of 90% active charcoal, 8% barium carbonate and 2% sodium carbonate as the catalyzing reagent. Fresh carburizer was used in each carburization treatment, which was conducted in a muffle furnace at 1223 K for 4 h followed by furnace cooling. The heating and cooling rates were 5 K/min and 7–8 K/min, respectively. After the carburization, all the specimens were ultrasonically cleaned in acetone for 15 min.
T. Yao et al. / Surface & Coatings Technology 277 (2015) 210–215
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with a voltage of 20 V at − 30 °C. The TEM observation used a JEOL2100 F transmission electron microscope operated at 200 kV. 3. Results 3.1. Microstructures
Fig. 1. X-ray diffraction patterns of the carburized TiAl alloy.
2.3. Oxidation behaviors Isothermal oxidation tests were performed in a muffle furnace at 950 °C for 100 h. During the oxidation tests, each specimen was placed in corundum crucible. After being oxidized for a certain time, the crucible with the specimen were taken out from the furnace, cooled in air for 30 min and weighed using a balance with a resolution of 0.01 mg. Then the specimen was placed to the furnace again for further oxidation.
Fig. 1 shows the GAXRD pattern of the carburized surface. It can be seen from Fig. 1 that except for the substrate peaks of TiAl and Ti3Al, the peaks of carburized layer mainly consist of Ti2AlC (MAX-phase [13]), Ti3AlC and Al2Nb3C phases. Fig. 2 shows the surface and crosssectional microstructures of the alloy. In Fig. 2(a), a carburized layer with a thickness of 7.4 μm can be observed on the surface of the TiAl alloy. No cracks or porosities can be found in the layer and the substrate. The surface of the carburized layer shows the formation of carbide phases, which have an average size of about 1 μm, as shown in Fig. 2(b). It should be noted that the microstructures are very similar to the carburized layer fabricated by plasma carburization [11]. TEM images from the carburized layer of the TiAl alloy are shown in Fig. 3, indicating the formation of very fine grained Ti3AlC and Ti2AlC phases (Fig. 3a and b). Fig. 3(c) shows the microstructure of the carburized layer close to the substrate. The substrate mainly includes γ-TiAl phase, while the carburized layer is composed of very fine carbides. There are some interactions between the carbide layer and the substrate. Fig. 3(d) is the HRTEM image of the region from the carburized layer in Fig. 3(c) and corresponding FFT pattern. It shows the precipitation of Ti2AlC from the γ-TiAl phase. The Ti2AlC particle has a size of 16 nm. The orientation relationship of (0001) Ti2AlC // {111} γ can be determined from the FFT pattern, which in accordance with other study [14]. 3.2. Oxidation behaviors
2.4. Structural characterization The carburized layer was analyzed using gracing angle X-ray (GAXRD) diffraction in order to detect the phases at typical depth in a range of a few dozen to hundreds of nanometers. X-ray diffraction (XRD) was used to identify phases in the oxide scales. XRD and GAXRD spectra were performed on the same X-ray diffractometer (XRD: D/MAX-255) using CuKα1 = 1.5406 Å radiation. The crosssection and surface microstructures and compositions were analyzed using a scanning electron microscope (NOVA NANO SEM 230) equipped with energy dispersive spectrometer (EDS). To prepare the TEM foils, the carburized specimens were firstly mechanically thinned using 1500 grit emery paper to obtain slices with a thickness of about 70 μm. During the mechanically thinning, the carburized side was kept intact. Then the foils were thinned by twin-jet polishing method in a solution of 5% perchloric acid, 35% n-butyl alcohol and 60% methanol
3.2.1. Oxidation kinetics Fig. 4 shows the weight gain during the oxidation of TiAl alloy at 950 °C. It can be found that the carburized TiAl alloy shows much less mass gain than that of the untreated counterpart. For the untreated TiAl alloy, a rapid oxidation was observed during the initial exposure of 7 h, and the mass gain obeys a parabolic law until 70 h. After 70 h, a linear oxidation kinetic curve can be observed due to the oxide scale losing the protective function and beginning to spallation. For the carburized TiAl alloy, the oxidation kinetic shows a parabolic behavior during the whole testing process. The velocity of mass gain can be calculated by the following equation: ΔWn ¼ Kp t
ð1Þ
where ΔW, Kp, t and n are the mass gain of unit area (mg/cm2),
Fig. 2. SEM images of the (a) cross-section and (b) surface of the carburized TiAl alloy.
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Fig. 3. (a) TEM bright field image and (b) the corresponding SAED pattern of the carburized layer, (c) TEM bright field image shows the interaction between substrate and carburized layer and (d) HRTEM of carburized layer and FFT pattern of the corresponding region.
oxidation reaction rate constant (mg2/cm4h), oxidation time (h), and power exponent (n), respectively. From Eq. (1), the power exponent and the oxidation reaction rate constant Kp can be determined by fitted linear regression line between log (ΔW) and log (t). As shown in Table 1, the rate constants of the untreated and carburized TiAl alloys are 0.1579 mg2/cm4h and 0.0934 mg2/cm4h, respectively. The smaller the rate constant, the higher the oxidation resistance of the alloy [15]. Therefore, the carburized TiAl alloy exhibits a better oxidation resistance, which is nearly twice of the untreated one. 3.2.2. Phase structures Fig. 5 shows the X-ray diffraction patterns of both carburization treated and untreated TiAl alloys after oxidation at 950 °C for 100 h. For the untreated specimen, peaks of rutile and corundum can be identified. The strong peak intensity of rutile shows that it is the main oxidation product. Besides, a small amount of TiN can also be found. For the carburized TiAl alloy, the oxidation scale consists of the similar phases
to that of the untreated one, but it has a higher content of corundum. No nitride peaks are detected in the XRD pattern. 3.2.3. The oxide scales Fig. 6 shows the surface morphology of the oxide scales on TiAl alloys. For the untreated TiAl alloy, the oxide scale was peeled off at the early stage of the oxidation, and developed into two parts: the top layer and the interface. In the top layer, the oxides are rod-like with a length of several microns. In the interface, the oxides have a granule shape with very fine size. In Fig. 6(b), some insular clusters of large grains exist on the top layer. The insular cluster is made of
Table 1 Kinetic parameters for the oxidation of the TiAl alloys at 950 °C.
Bare TiAl Carburized TiAl
Fig. 4. Oxidation kinetics curves of the TiAl alloys at 950 °C for 100 h.
n
kp (mg2/cm4h)
2.3759 2.2333
0.1579 0.0934
Fig. 5. XRD patterns of oxide scales on TiAl alloys.
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Fig. 6. (a) (b) Surface morphology and (c) cross-sectional microstructures of oxide scale on the bare TiAl alloy.
coarse-grained TiO2 surrounded by fine-grained Al2O3 which agrees well with previous investigations [16]. Fig. 6(c) shows the crosssectional microstructures of TiAl alloys. The thickness of the oxide scale is about 13.6 μm, and it was regard as the thickness of the top layer. Due to the repeating spallation of the scale and many spalt flakes existed at the bottom of the crucible, the accumulated thickness of oxide scale was believed to be very high, and it was directly reflected on the oxidation mass gain. The top layer is composed of multi-layered Ti- and Al-rich oxides. The surface oxide scale is Al2O3-rich while the intermediate layer is TiO2-rich. This structure is in accordance with Nb-containing TiAl alloys in other study [17], but different from Nb free alloys [6]. After the oxidation at 950 °C for 100 h, an evident crack can be observed between the top layer and the interface. EDS analyses in Table 2 shows that there is a nitrogen-containing region near the crack. Fig. 7(a) shows the oxidation scale of the carburized TiAl. There is only one oxide layer with no spallation from the top view of the specimen. The oxides mainly consist of large polygon TiO2 grains and particular Al2O3 grains. Both TiO2 and Al2O3 grains are uniformly distributed on the surface of the oxide scale. Fig. 7(b) shows the cross-sectional morphology of the oxide scale on the carburized TiAl, the thickness of the oxide scale is 7.1 μm, about half of that of the untreated TiAl alloy.
Table 2 EDS analyses of oxide layer on the bare TiAl alloy. Sample
1 2 3 4 5
Element content (at.%) Ti
Al
Nb
O
N
W
11.76 30.16 38.79 45.93 43.57
44.75 3.85 15.09 32.98 40.51
1.32 4.37 4.98 5.92 8.00
42.17 50.57 35.75 14.62 7.16
/ 11.04 5.39 / /
/ / / 0.55 0.77
The scale is overall compact, and well bonded with the substrate. There are intersecting Ti-(marked as ‘1’ and ‘3’) and Al-rich (marked as ‘2’) layers. After the oxidation, the carburized layer remains contact. EDS analyses in Table 3 show that, compared to the oxide layer, the content of oxygen is significantly decreased in the carburized layer (points ‘4’ and ‘5’).
4. Discussions 4.1. Carburization mechanism It has been shown that the carburizing process of metals and alloys can be divided into three stages [12]: (1) the adsorption of the active carbon atoms, (2) the diffusion of C atom and (3) the rearrangement of the C atoms. The diffusion rate and the solubility of C atoms vary evidently with the microstructure of the substrate materials. Usually, γ phase in TiAl alloys dissolves less C than α2 phase. C atom preferentially localizes in “Ti6” type octahedral interstices, in α2 phase with a DO19 structure, instead of in “Ti4Al2” or “Ti2Al4” type in γ-phase with an L10 structure [12]. In the present study, the as-HIPed TiAl–7Nb alloy has a near γ microstructure, without enough “Ti6” type octahedral interstices for C atoms to locate. However, antisite of Al sublattice occupied by Ti atom occurs in high Nb contained TiAl alloys, especially in the alloys with lower Al concentration such as Ti–45Al–7Nb–0.3W [18]. It is more likely that Ti6-type sites are formed in TiAl–7Nb alloys, and the solubility of C in the structure increases. With the C atoms accumulating to a critical concentration, Ti2AlC phase preferentially precipitates along the heterogeneous sites, such as dislocations, grain boundaries, interfacial ledges and stacking faults [14]. Ti atoms diffuse outward to the surface to form a continuous C-containing and Ti-rich layer. The remnant Al and Nb enrich in the intermediate layer, resulting in the formation of Al2Nb3C. Consequently, a multi-phases carburized layer which shown in Fig. 2 forms on high Nb contained TiAl alloy.
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Fig. 7. (a) Surface morphology and (b) cross-sectional microstructures of oxide scale on the carburized TiAl alloy.
4.2. Oxidation mechanism According to the Ellingham–Richardson chart [19], Al2O3 preferentially forms on the surface than TiO2 during the oxidation of TiAl alloys. However, the activity of Al is inversely proportional to its concentration in the matrix [20]. According to Lin et al. [21], the formation of Al2O3 will dominate the oxidation process only when the Al content is higher than 59 at.% in binary TiAl alloy. Therefore, in the present TiAl alloy, the formation of TiO2 is more kinetically favorable than that of Al2O3. The addition of Nb is beneficial to the improvement of the oxidation resistance of the TiAl alloys by the “doping effect” [22]. Nb substitutes for Ti in rutile as a cation with a valence of +5, decreasing the concentration of oxygen vacancies in TiO2 and favoring the formation of Al2O3. However, when the oxidation temperature is up to 950 °C, the volume of corundum is too low to form a dense and continuous oxide scale, and serious oxidation occurs. The oxides formed on the alloy are only rutile and corundum for TiAl–7Nb. According to Yoshihara et al. [23], Nb oxide is formed only until Nb content is higher than 15 at.%. It should not be neglected that nitrogen in TiAl alloys remarkably affects the oxidation behavior, especially for TiAl–Nb alloys. Previous study has shown that the presence of nitrogen has a positive effect in the oxidation of Nb-containing TiAl by lowering pO2/pN2-ratio at the oxide/substrate interface, and increasing Al/Ti-activity ratio [24]. However, due to the large difference in the thermal coefficients of TiN and oxides, cracks may form during cooling, leading to the spallation of the oxide scale. For the carburized TiAl alloy, the carbide layer has a remarkably influence on the oxidation behavior. Previous study also indicated that the bonding between Ti and Al is much weaker than the covalent bonding between Ti and C in Ti2AlC [25]. When the oxidation occurs at elevated temperatures, the activity of Ti atom is significantly decreased by the interaction between Ti and C, resulting in a preferential oxidation of Al. The dense Al2O3 layer is helpful for hindering further diffusion of O. Ti atom will enrich underneath the Al2O3, and form a layer of rutile. According to Marie Sonestedt et al. [26], C in the
surface of carburized TiAl is oxidized to CO at the interface between the oxide and the substrate. Then CO gas is transported through the defects in the scale. In this study, the nano-crystallined Ti2AlC and Ti3AlC provide enough grain boundary to transport CO to prevent the formation of pores. In addition, other study has shown that the presence of CO favors the formation of Al2O3 because it facilitates the transportation of oxygen through the formation of oxygen vacancies [27]. This might be a possible explanation for the high content of Al2O3 in the oxide scale of the carburized TiAl. The lack of nitride in the oxide scale of the carburized alloy may be due to the formation of Ti carbides, which prevent the diffusion of nitrogen and the reaction between N and Ti. 5. Conclusions In this paper, carburization was performed on a high Nb contained TiAl alloy. By characterizing the carburized layer of the alloy and oxidation tests of both carburized and untreated samples in static air at 950 °C for 100 h, the following conclusion can be drawn: (1) A dense, uniform and pore-free carburized layer can be obtained on Ti–45Al–7Nb–0.3W substrate after the carburization process. The carburized layer is composed of nano-structured phases, including Ti2AlC, Ti3AlC and Al2Nb3C. (2) The oxidation resistance of TiAl is significantly improved after the carburization process. The constants of the oxidation reaction rate for the untreated and carburized TiAl alloy are 0.1579 mg2/cm4h and 0.0934 mg2/cm4h, respectively. (3) The introducing of C changes the formation of oxide scales, and therefore the oxidation mechanism. The carburized TiAl alloy has higher proportion of protective Al2O3 in the oxide scale than the bare TiAl alloy. The existence of carburized layer also effectively hinders the inward diffusion of O and N atoms.
Acknowledgment The authors would like to thank financial supports of National Key Fundamental Research and Development Project of China (2011CB605505).
Table 3 EDS analyses of oxide layer on the carburized TiAl alloy. Sample
1 2 3 4 5 6
Element content (at.%) Ti
Al
Nb
C
O
W
27.60 4.92 27.96 46.97 49.42 46.77
19.35 44.32 19.54 30.60 27.56 45.03
1.35 1.41 2.91 7.77 7.88 7.79
7.97 7.21 7.76 9.30 9.33 /
43.74 42.13 41.84 5.38 5.81 /
/ / / / / 0.42
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[16] J. Rakowski, F. Pettit, G. Meier, F. Dettenwanger, E. Schumann, M. Ruhle, The effect of nitrogen on the oxidation of γ-TiAl, Scr. Metall. Mater. 33 (1995) 997–1003. [17] M. Mitoraj, E. Godlewska, O. Heintz, N. Geoffroy, S. Fontana, S. Chevalier, Scale composition and oxidation mechanism of the Ti–46Al–8Nb alloy in air at 700 and 800 °C, Intermetallics 19 (2011) 39–47. [18] W. Zhang, S. Deevi, G. Chen, On the origin of superior high strength of Ti–45Al–10Nb alloys, Intermetallics 10 (2002) 403–406. [19] M. Li, High Temperature Corrosion of Metals, Metallurgical Press, Beijing, 2001. [20] F. Wang, Z. Tang, Oxidation and protection of TiAl. Intermetallics, Chin, J. Mater. Res. 12 (1998) 337–344. [21] J. Lin, L. Zhao, G. Li, L. Zhang, X. Song, F. Ye, G. Chen, Effect of Nb on oxidation behavior of high Nb containing TiAl alloys, Intermetallics 19 (2011) 131–136. [22] M. Stroosnijder, N. Zheng, W. Quadakkers, R. Hofman, A. Gil, F. Lanza, The effect of niobium ion implantation on the oxidation behavior of a γ-TiAl-based intermetallic, Oxid. Met. 46 (1996) 19–35. [23] M. Yoshihara, K. Miura, Effects of Nb addition on oxidation behavior of TiAl, Intermetallics 3 (1995) 357–363. [24] P. Schaaf, W. Quadakkers, N. Zheng, E. Wallura, A. Gil, Beneficial and detrimental effects of nitrogen on the oxidation behaviour of TiAl-based intermetallics, Mater. Corros. 48 (1997) 28–34. [25] X. Wang, Y. Zhou, High-temperature oxidation behavior of Ti2AlC in air, Oxid. Met. 59 (2003) 303–320. [26] M. Sonestedt, J. Frodelius, M. Sundberg, L. Hultman, K. Stiller, Oxidation of Ti2AlC bulk and spray deposited coatings, Corros. Sci. 52 (2010) 3955–3961. [27] X. Wang, Y. Zhou, Oxidation behavior of Ti3AlC2 at 1000–1400 °C in air, Corros. Sci. 45 (2003) 891–907.
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of carburization on oxidation behavior of High Nb contained TiAl alloy Tianhang Yao, Yong Liu ⁎, Bin Liu, Min Song, Kun Zhao, Weidong Zhang, Yuehui He State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, China
a r t i c l e
i n f o
Article history: Received 13 December 2014 Revised 6 May 2015 Accepted in revised form 26 July 2015 Available online 29 July 2015 Keywords: TiAl alloy Carburization Oxidation
a b s t r a c t In this work, surface carburization has been performed on Ti–45Al–7Nb–0.3W alloy. The isothermal oxidation behavior of the alloy has been investigated in static air at 950 °C for 100 h. X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were applied to study the microstructural evolution. The results reveal that the carburized layer, with a thickness of 7.4 μm, is composed of nanostructured Ti2AlC and Ti3AlC phases. The oxidation weight gain of the carburized TiAl is half of that of the bare TiAl. The oxide scale of the carburized TiAl is compact without delamination and spallation. The inward diffusion of oxygen atom was hindered due to the existence of the carburized layer. Therefore, the carburization process is an effective and feasible treatment to improve the oxidation resistance of high Nb contained TiAl alloys. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent decades TiAl based alloys have attracted extensive attentions, and have been regarded as potential structural materials for high temperature applications, due to their high strength to weight ratio [1–3]. However, several shortcomings substantially limit their industrial applications, such as intrinsic brittleness and poor oxidation resistance at high temperatures. It has been shown that large amount Nb addition to TiAl alloys is an effective way to improve the high temperature tolerance and strength [4]. In addition, the addition of Nb also enhances the oxidation resistance of TiAl alloys by promoting the formation of Al-rich oxides scale, which acts as a barrier to the inward diffusion of oxygen [5]. However, it should be noted that even large amount Nb addition is introduced, severe oxidation and spallation of the oxide layers can still happen when the temperature increases to 900 °C. Thus, lots of efforts have been made to improve the oxidation resistance of TiAl alloys through coating techniques and the surface alloying [6,7]. In comparison with the surface alloying, coatings or hard films are not very useful due to the interfacial compatibility between the coating and the substrate. The carburization process has been widely applied on steel, Ni-based alloys and Ti-based alloys [8–10]. It aims to form a continuous C-containing ceramic layer to enhance the surface anti-wear and anti-oxidation properties. Previous investigations have applied the carburization on TiAl alloys to improve the surface hardness, mechanical property and anti-corrosion property [11,12]. However, little information is reported on the enhancement of the oxidation resistance of TiAl alloys by the carburization.
⁎ Corresponding author. E-mail address: [email protected] (Y. Liu).
http://dx.doi.org/10.1016/j.surfcoat.2015.07.058 0257-8972/© 2015 Elsevier B.V. All rights reserved.
In contrast to other technologies such as plasma carburization or surface ion implantation, solid carburization has advantages of low energy consumption, easy controlling and easy access to carbon source. The purpose of this work is to investigate the feasibility of the formation of C-containing ceramic layer on high Nb contained TiAl alloys by pack carburization, and effects of the layer on the oxidation behaviors at high temperatures has been systematically studied. 2. Experimental 2.1. Materials The pre-alloyed Ti–45Al–7Nb–0.3W (at.%) powders were prepared by plasma rotating electrode processing (PREP) under the protection of inert gas. The powders were filled into a steel can and degassed at 400 °C, then hot isostatic pressed (HIPed) at 1240 °C, and 140 MPa for 4 h. The specimens with the size of 10 mm × 10 mm × 1.5 mm were cut from as-HIPed billet by electric discharge machining (EDM). Before the carburization and oxidation, each specimen was polished using 1500 grit emery paper, and ultrasonically cleaned in acetone for 15 min. 2.2. Carburization Carburization treatments were performed in a corundum crucible which was filled with solid carburizer. The solid carburizer consists of 90% active charcoal, 8% barium carbonate and 2% sodium carbonate as the catalyzing reagent. Fresh carburizer was used in each carburization treatment, which was conducted in a muffle furnace at 1223 K for 4 h followed by furnace cooling. The heating and cooling rates were 5 K/min and 7–8 K/min, respectively. After the carburization, all the specimens were ultrasonically cleaned in acetone for 15 min.
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with a voltage of 20 V at − 30 °C. The TEM observation used a JEOL2100 F transmission electron microscope operated at 200 kV. 3. Results 3.1. Microstructures
Fig. 1. X-ray diffraction patterns of the carburized TiAl alloy.
2.3. Oxidation behaviors Isothermal oxidation tests were performed in a muffle furnace at 950 °C for 100 h. During the oxidation tests, each specimen was placed in corundum crucible. After being oxidized for a certain time, the crucible with the specimen were taken out from the furnace, cooled in air for 30 min and weighed using a balance with a resolution of 0.01 mg. Then the specimen was placed to the furnace again for further oxidation.
Fig. 1 shows the GAXRD pattern of the carburized surface. It can be seen from Fig. 1 that except for the substrate peaks of TiAl and Ti3Al, the peaks of carburized layer mainly consist of Ti2AlC (MAX-phase [13]), Ti3AlC and Al2Nb3C phases. Fig. 2 shows the surface and crosssectional microstructures of the alloy. In Fig. 2(a), a carburized layer with a thickness of 7.4 μm can be observed on the surface of the TiAl alloy. No cracks or porosities can be found in the layer and the substrate. The surface of the carburized layer shows the formation of carbide phases, which have an average size of about 1 μm, as shown in Fig. 2(b). It should be noted that the microstructures are very similar to the carburized layer fabricated by plasma carburization [11]. TEM images from the carburized layer of the TiAl alloy are shown in Fig. 3, indicating the formation of very fine grained Ti3AlC and Ti2AlC phases (Fig. 3a and b). Fig. 3(c) shows the microstructure of the carburized layer close to the substrate. The substrate mainly includes γ-TiAl phase, while the carburized layer is composed of very fine carbides. There are some interactions between the carbide layer and the substrate. Fig. 3(d) is the HRTEM image of the region from the carburized layer in Fig. 3(c) and corresponding FFT pattern. It shows the precipitation of Ti2AlC from the γ-TiAl phase. The Ti2AlC particle has a size of 16 nm. The orientation relationship of (0001) Ti2AlC // {111} γ can be determined from the FFT pattern, which in accordance with other study [14]. 3.2. Oxidation behaviors
2.4. Structural characterization The carburized layer was analyzed using gracing angle X-ray (GAXRD) diffraction in order to detect the phases at typical depth in a range of a few dozen to hundreds of nanometers. X-ray diffraction (XRD) was used to identify phases in the oxide scales. XRD and GAXRD spectra were performed on the same X-ray diffractometer (XRD: D/MAX-255) using CuKα1 = 1.5406 Å radiation. The crosssection and surface microstructures and compositions were analyzed using a scanning electron microscope (NOVA NANO SEM 230) equipped with energy dispersive spectrometer (EDS). To prepare the TEM foils, the carburized specimens were firstly mechanically thinned using 1500 grit emery paper to obtain slices with a thickness of about 70 μm. During the mechanically thinning, the carburized side was kept intact. Then the foils were thinned by twin-jet polishing method in a solution of 5% perchloric acid, 35% n-butyl alcohol and 60% methanol
3.2.1. Oxidation kinetics Fig. 4 shows the weight gain during the oxidation of TiAl alloy at 950 °C. It can be found that the carburized TiAl alloy shows much less mass gain than that of the untreated counterpart. For the untreated TiAl alloy, a rapid oxidation was observed during the initial exposure of 7 h, and the mass gain obeys a parabolic law until 70 h. After 70 h, a linear oxidation kinetic curve can be observed due to the oxide scale losing the protective function and beginning to spallation. For the carburized TiAl alloy, the oxidation kinetic shows a parabolic behavior during the whole testing process. The velocity of mass gain can be calculated by the following equation: ΔWn ¼ Kp t
ð1Þ
where ΔW, Kp, t and n are the mass gain of unit area (mg/cm2),
Fig. 2. SEM images of the (a) cross-section and (b) surface of the carburized TiAl alloy.
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Fig. 3. (a) TEM bright field image and (b) the corresponding SAED pattern of the carburized layer, (c) TEM bright field image shows the interaction between substrate and carburized layer and (d) HRTEM of carburized layer and FFT pattern of the corresponding region.
oxidation reaction rate constant (mg2/cm4h), oxidation time (h), and power exponent (n), respectively. From Eq. (1), the power exponent and the oxidation reaction rate constant Kp can be determined by fitted linear regression line between log (ΔW) and log (t). As shown in Table 1, the rate constants of the untreated and carburized TiAl alloys are 0.1579 mg2/cm4h and 0.0934 mg2/cm4h, respectively. The smaller the rate constant, the higher the oxidation resistance of the alloy [15]. Therefore, the carburized TiAl alloy exhibits a better oxidation resistance, which is nearly twice of the untreated one. 3.2.2. Phase structures Fig. 5 shows the X-ray diffraction patterns of both carburization treated and untreated TiAl alloys after oxidation at 950 °C for 100 h. For the untreated specimen, peaks of rutile and corundum can be identified. The strong peak intensity of rutile shows that it is the main oxidation product. Besides, a small amount of TiN can also be found. For the carburized TiAl alloy, the oxidation scale consists of the similar phases
to that of the untreated one, but it has a higher content of corundum. No nitride peaks are detected in the XRD pattern. 3.2.3. The oxide scales Fig. 6 shows the surface morphology of the oxide scales on TiAl alloys. For the untreated TiAl alloy, the oxide scale was peeled off at the early stage of the oxidation, and developed into two parts: the top layer and the interface. In the top layer, the oxides are rod-like with a length of several microns. In the interface, the oxides have a granule shape with very fine size. In Fig. 6(b), some insular clusters of large grains exist on the top layer. The insular cluster is made of
Table 1 Kinetic parameters for the oxidation of the TiAl alloys at 950 °C.
Bare TiAl Carburized TiAl
Fig. 4. Oxidation kinetics curves of the TiAl alloys at 950 °C for 100 h.
n
kp (mg2/cm4h)
2.3759 2.2333
0.1579 0.0934
Fig. 5. XRD patterns of oxide scales on TiAl alloys.
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Fig. 6. (a) (b) Surface morphology and (c) cross-sectional microstructures of oxide scale on the bare TiAl alloy.
coarse-grained TiO2 surrounded by fine-grained Al2O3 which agrees well with previous investigations [16]. Fig. 6(c) shows the crosssectional microstructures of TiAl alloys. The thickness of the oxide scale is about 13.6 μm, and it was regard as the thickness of the top layer. Due to the repeating spallation of the scale and many spalt flakes existed at the bottom of the crucible, the accumulated thickness of oxide scale was believed to be very high, and it was directly reflected on the oxidation mass gain. The top layer is composed of multi-layered Ti- and Al-rich oxides. The surface oxide scale is Al2O3-rich while the intermediate layer is TiO2-rich. This structure is in accordance with Nb-containing TiAl alloys in other study [17], but different from Nb free alloys [6]. After the oxidation at 950 °C for 100 h, an evident crack can be observed between the top layer and the interface. EDS analyses in Table 2 shows that there is a nitrogen-containing region near the crack. Fig. 7(a) shows the oxidation scale of the carburized TiAl. There is only one oxide layer with no spallation from the top view of the specimen. The oxides mainly consist of large polygon TiO2 grains and particular Al2O3 grains. Both TiO2 and Al2O3 grains are uniformly distributed on the surface of the oxide scale. Fig. 7(b) shows the cross-sectional morphology of the oxide scale on the carburized TiAl, the thickness of the oxide scale is 7.1 μm, about half of that of the untreated TiAl alloy.
Table 2 EDS analyses of oxide layer on the bare TiAl alloy. Sample
1 2 3 4 5
Element content (at.%) Ti
Al
Nb
O
N
W
11.76 30.16 38.79 45.93 43.57
44.75 3.85 15.09 32.98 40.51
1.32 4.37 4.98 5.92 8.00
42.17 50.57 35.75 14.62 7.16
/ 11.04 5.39 / /
/ / / 0.55 0.77
The scale is overall compact, and well bonded with the substrate. There are intersecting Ti-(marked as ‘1’ and ‘3’) and Al-rich (marked as ‘2’) layers. After the oxidation, the carburized layer remains contact. EDS analyses in Table 3 show that, compared to the oxide layer, the content of oxygen is significantly decreased in the carburized layer (points ‘4’ and ‘5’).
4. Discussions 4.1. Carburization mechanism It has been shown that the carburizing process of metals and alloys can be divided into three stages [12]: (1) the adsorption of the active carbon atoms, (2) the diffusion of C atom and (3) the rearrangement of the C atoms. The diffusion rate and the solubility of C atoms vary evidently with the microstructure of the substrate materials. Usually, γ phase in TiAl alloys dissolves less C than α2 phase. C atom preferentially localizes in “Ti6” type octahedral interstices, in α2 phase with a DO19 structure, instead of in “Ti4Al2” or “Ti2Al4” type in γ-phase with an L10 structure [12]. In the present study, the as-HIPed TiAl–7Nb alloy has a near γ microstructure, without enough “Ti6” type octahedral interstices for C atoms to locate. However, antisite of Al sublattice occupied by Ti atom occurs in high Nb contained TiAl alloys, especially in the alloys with lower Al concentration such as Ti–45Al–7Nb–0.3W [18]. It is more likely that Ti6-type sites are formed in TiAl–7Nb alloys, and the solubility of C in the structure increases. With the C atoms accumulating to a critical concentration, Ti2AlC phase preferentially precipitates along the heterogeneous sites, such as dislocations, grain boundaries, interfacial ledges and stacking faults [14]. Ti atoms diffuse outward to the surface to form a continuous C-containing and Ti-rich layer. The remnant Al and Nb enrich in the intermediate layer, resulting in the formation of Al2Nb3C. Consequently, a multi-phases carburized layer which shown in Fig. 2 forms on high Nb contained TiAl alloy.
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Fig. 7. (a) Surface morphology and (b) cross-sectional microstructures of oxide scale on the carburized TiAl alloy.
4.2. Oxidation mechanism According to the Ellingham–Richardson chart [19], Al2O3 preferentially forms on the surface than TiO2 during the oxidation of TiAl alloys. However, the activity of Al is inversely proportional to its concentration in the matrix [20]. According to Lin et al. [21], the formation of Al2O3 will dominate the oxidation process only when the Al content is higher than 59 at.% in binary TiAl alloy. Therefore, in the present TiAl alloy, the formation of TiO2 is more kinetically favorable than that of Al2O3. The addition of Nb is beneficial to the improvement of the oxidation resistance of the TiAl alloys by the “doping effect” [22]. Nb substitutes for Ti in rutile as a cation with a valence of +5, decreasing the concentration of oxygen vacancies in TiO2 and favoring the formation of Al2O3. However, when the oxidation temperature is up to 950 °C, the volume of corundum is too low to form a dense and continuous oxide scale, and serious oxidation occurs. The oxides formed on the alloy are only rutile and corundum for TiAl–7Nb. According to Yoshihara et al. [23], Nb oxide is formed only until Nb content is higher than 15 at.%. It should not be neglected that nitrogen in TiAl alloys remarkably affects the oxidation behavior, especially for TiAl–Nb alloys. Previous study has shown that the presence of nitrogen has a positive effect in the oxidation of Nb-containing TiAl by lowering pO2/pN2-ratio at the oxide/substrate interface, and increasing Al/Ti-activity ratio [24]. However, due to the large difference in the thermal coefficients of TiN and oxides, cracks may form during cooling, leading to the spallation of the oxide scale. For the carburized TiAl alloy, the carbide layer has a remarkably influence on the oxidation behavior. Previous study also indicated that the bonding between Ti and Al is much weaker than the covalent bonding between Ti and C in Ti2AlC [25]. When the oxidation occurs at elevated temperatures, the activity of Ti atom is significantly decreased by the interaction between Ti and C, resulting in a preferential oxidation of Al. The dense Al2O3 layer is helpful for hindering further diffusion of O. Ti atom will enrich underneath the Al2O3, and form a layer of rutile. According to Marie Sonestedt et al. [26], C in the
surface of carburized TiAl is oxidized to CO at the interface between the oxide and the substrate. Then CO gas is transported through the defects in the scale. In this study, the nano-crystallined Ti2AlC and Ti3AlC provide enough grain boundary to transport CO to prevent the formation of pores. In addition, other study has shown that the presence of CO favors the formation of Al2O3 because it facilitates the transportation of oxygen through the formation of oxygen vacancies [27]. This might be a possible explanation for the high content of Al2O3 in the oxide scale of the carburized TiAl. The lack of nitride in the oxide scale of the carburized alloy may be due to the formation of Ti carbides, which prevent the diffusion of nitrogen and the reaction between N and Ti. 5. Conclusions In this paper, carburization was performed on a high Nb contained TiAl alloy. By characterizing the carburized layer of the alloy and oxidation tests of both carburized and untreated samples in static air at 950 °C for 100 h, the following conclusion can be drawn: (1) A dense, uniform and pore-free carburized layer can be obtained on Ti–45Al–7Nb–0.3W substrate after the carburization process. The carburized layer is composed of nano-structured phases, including Ti2AlC, Ti3AlC and Al2Nb3C. (2) The oxidation resistance of TiAl is significantly improved after the carburization process. The constants of the oxidation reaction rate for the untreated and carburized TiAl alloy are 0.1579 mg2/cm4h and 0.0934 mg2/cm4h, respectively. (3) The introducing of C changes the formation of oxide scales, and therefore the oxidation mechanism. The carburized TiAl alloy has higher proportion of protective Al2O3 in the oxide scale than the bare TiAl alloy. The existence of carburized layer also effectively hinders the inward diffusion of O and N atoms.
Acknowledgment The authors would like to thank financial supports of National Key Fundamental Research and Development Project of China (2011CB605505).
Table 3 EDS analyses of oxide layer on the carburized TiAl alloy. Sample
1 2 3 4 5 6
Element content (at.%) Ti
Al
Nb
C
O
W
27.60 4.92 27.96 46.97 49.42 46.77
19.35 44.32 19.54 30.60 27.56 45.03
1.35 1.41 2.91 7.77 7.88 7.79
7.97 7.21 7.76 9.30 9.33 /
43.74 42.13 41.84 5.38 5.81 /
/ / / / / 0.42
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