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γ-TiAl alloys at high temperatures
Journal of Alloys and Compounds 691 (2017) 489e497
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Role of alloying elements during thermocyclic oxidation of b/g-TiAl alloys at high temperatures Muhammad Naveed*, A. Flores Renteria, Sabine Weiß Physical Metallurgy and Materials Technology, Brandenburg Technical University, Cottbus-Senftenberg Konrad Wachsmann Allee 17, 03046, Cottbus, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 January 2016 Received in revised form 22 August 2016 Accepted 25 August 2016 Available online 28 August 2016
Gamma titanium aluminides are promising alloys known for their good mechanical properties and low densities, but their low oxidation resistance at high temperatures limits their application. This work discusses the thermocyclic oxidation behavior of newly developed b/g-TiAl alloys at temperatures between 600 C and 900 C. An influence of b-stabilizing alloying elements like Nb and V on the oxidation of these alloys has been investigated here. The selected alloys are tested in an in-house developed thermocyclic furnace. The oxidation study is supported by gravimetric measurements along with Scanning Electron Microscopy (SEM) and Electron Diffraction Spectroscopy (EDS) mapping of the oxide layers. Additionally, phase formation after oxidation has been determined using X-Ray Diffraction (XRD). Results show that the Nb containing alloys are more oxidation resistant as compared to V containing alloys. The formation of a mixture of Al2O3 and TiO2 layers was found for all the alloys. Additionally, the oxide kinetics controlled the oxide growth and formation of various phases at different testing temperatures. © 2016 Elsevier B.V. All rights reserved.
Keywords: g-TiAl Thermocyclic oxidation b-stabilizing elements Aluminum oxide Titanium oxide
1. Introduction For the last three decades, tremendous resources and efforts have been spent on research and development of g-TiAl alloys as a replacement of heavy nickel super alloys in aerospace applications [1]. Reports on high potential of these alloys in turbocharger wheels and exhaust valves in automobiles has also been discussed [2]. Even due to its low strength-to-weight ratio, only limited application is found due to insufficient ductility at ambient temperatures and oxidation resistance at high temperatures (above 800 C) [3,4]. Literature proposes various methods to improve the oxidation resistance of these alloys at high temperature which include preoxidation [5], ion implantation [6], overlay coatings [7] etc. Addition of alloying elements is one of the most discussed method for the improvement in oxidation resistance of g-TiAl [5,8e13]. Shida and Anada [14] classified these alloying elements as beneficial, neutral and detrimental, depending upon their effect during the oxidation of g-TiAl. Alloying of these elements influences the oxidation kinetics and scale growth on the alloy surface. This work also discusses the effect of some of the b-stabilizing
* Corresponding author. E-mail address: [email protected] (M. Naveed). http://dx.doi.org/10.1016/j.jallcom.2016.08.259 0925-8388/© 2016 Elsevier B.V. All rights reserved.
alloying elements added to improve ductility and oxidation resistance. Addition of Nb, which is classified as the most effective alloying element to improve the oxidation resistance of the alloy [15], has been presented here. In contrast, addition of V known for its detrimental effect on the oxidation behavior is also discussed in this work [16,17]. Moreover, effect of Mo, which is present in low amount, is also discussed in the present study. The present focuses on the formation of various oxide layers grown during thermocyclic oxidation of these alloys. A thermally stable protective oxide with slow growth rate and good adhesion to the metallic substrate can resist diffusion of O into the alloy surface [3]. In case of g-TiAl alloys, a dense Al2O3 is preferred over a porous TiO2 layer during the oxidation process. Since the equilibrium pressures of O for Al2O3 and TiO2 are similar, an activity between Ti and Al would control the formation of the preferential oxide layer [18]. Hence, this work focusses on understanding the growth kinetics of the oxide layers influenced by the addition of alloying elements. 2. Materials and methods Three different b/g-TiAl alloys containing V, Nb and a combination of both were compared in this work. The microstructure of
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Ti-42Al-8V-(2e4)Mo (Fig. 1a) is composed of grains containing (g þ a2)-lamellas encapsulated within a g-phase, which is embedded in a b-phase matrix. In case of Ti-45Al-8Nb-0.2C (Fig. 1b), the sample consists of large grains containing (gþa2)-lamellas with grain boundaries covered with bright b- and dark
Table 1 Volume fraction of various phases in g -TiAl alloys.
Ti-42Al-8V-(2e4)Mo Ti-45Al-8Nb-0.2C Ti-45Al-8Nb-(2e4)V-(2e4)Mo
a2 þ g
g
b
38 55.4 85.5
24.5 23.1 9.4
37.5 21.5 5.1
g-phase. Moreover, Ti-45Al-8Nb-(2e4)V-(2e4)Mo alloy (Fig. 1c) shows dark-grey grains composed of (gþa2)-lamellas embedded in a b-matrix with presence of few dark g-grains within the micro-
Fig. 1. (a) BSE Micrograph of the Ti-42Al-8V-(2e4)Mo alloy 1) BSE Micrograph of the Ti-45Al-8Nb-0.2C alloy (c) BSE Micrograph of the Ti-45Al-8Nb-(2e4)V-(2e4)Mo alloy.
structure. An evaluation of the volume fraction of different phases is given in Table 1, which would be discussed in detail in the next sections. Disc-shaped samples with a diameter of 20 mm and 1 mm thickness are used for the oxidation test. The samples are first polished to 2500-grit followed by ultrasonic (ethanol) cleaning and later dried in air. A Pt-wire is used to position the specimen on a ceramic specimen holder as shown in Fig. 2. Thermocyclic experiments are performed in an in-house developed tube furnace which can be programmed by a controller. Each cycle consists of an oxidation time of 60 min and a cooling time of 10 min, which is sufficient to cool the specimen below 50 C. Mass changes are documented by gravimetric measurements at different intervals during the oxidation process. Gravimetric measurements are made with a resolution of 105 g using a MC 21S microbalance from Sartorius (Spain). The surface morphologies and oxide interfaces are characterized by means of a SEM Mira (TESCAN, Czech Republic) integrated with an EDS-System (OXFORD, Wiesbaden Germany). Phase identification has been performed using a D8 Discovery diffractometer (Bruker, Karlsruhe, Germany) equipped with a LynxEye detector with Bragg Brentano technique. Phase detection software EVA (Bruker) with ICDD (PDF4) phase identification database has been used for the diffractogram analysis.
3. Results 3.1. Gravimetric analysis
Fig. 1. (continued).
Fig. 1. (continued).
Thermocyclic oxidation behavior is described by mass gain of a specimen as a function of oxidation cycles. Gravimetric analysis of Ti-42Al-8V-(2e4)Mo alloy (Fig. 3a) shows a very low mass gain after oxidation at 600 C whereas a significantly higher mass gain can be seen after oxidation at 700 C. Linear oxidation behavior is observed for both the cases (marked with red line). Rapid oxidation occurred during the first 150 with no further oxidation during the next 350 cycles at 800 C. A rapid oxidation ending in spallation of the oxide scale during the first 10 cycles took place after oxidation at 900 C. In case of Ti-45Al-8Nb-0.2C (Fig. 3b), no mass gain is observed after oxidation at 600 C, whereas a slow and linear oxidation (marked with red) occured at 700 C. A rapid linear mass increase (marked with red) from the initial stages of oxidation process is noticed at 800 C. Parabolic oxidation behavior is observed during thermocyclic oxidation resulting in spallation after 450 cycles at 900 C. Similar to previously discussed alloys, no mass gain was observed for Ti-45Al-8Nb-(2e4) V-(2e4)Mo after oxidation at 600 C whereas a slight linear increase (marked with red) has been noticed after oxidation at 700 C. Parabolic oxidation behavior is noticed at the initial stage, leading to a breakaway oxidation after 300 cycles at 800 C whereas a parabolic oxidation behavior is depicted for oxidation at 900 C.
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Fig. 2. Thermocyclic oxidation oven equipped with a controller and a specimen holder.
Fig. 3. (continued).
3.2. Surface morphology and elemental mapping
Fig. 3. (a) Thermogravimetric analysis of Ti-42Al-8V- (2e4)Mo at various temperatures (b) Thermogravimetric analysis of Ti-45Al-8Nb-0.2C at various temperatures (c) Thermogravimetric analysis of Ti-45Al-8Nb-(2e4)V-(2e4)Mo at various temperatures.
A study of the oxide growth during the thermocyclic process is discussed by analyzing surface morphologies and elemental mappings of the oxide layers (Figs. 4e6). Surface analysis of Ti-42Al-8V(2e4)Mo after oxidation at a temperature of 900 C is shown in Fig. 4a. The specimen surface is completely covered with a mixture of randomly oriented needle shaped grains with an approximate size of 2 mm. An oxide layer with an approximate thickness of 42 mm is found on the alloy surface (Fig. 4b). EDS analysis (Fig. 4cef) shows a thin and uneven top layer consisting of Ti, O and V. This top layer is followed by a thin discontinuous layer consisting of Al and O mixture. A richly doped Ti and V zone with traces of Al and O is situated directly below these surface layers and expands itself till the base material. The oxide morphology of Ti-45Al-8Nb-0.2C formed at 900 C is depicted in Fig. 5a. The complete surface is covered with homogeneously distributed partially equiaxed oxide grains with a grain size of approx. 1 mm. An oxide layer of approx. 25 mm is visible from
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Fig. 4. SEM image of Ti-42Al-8V-(2e4)Mo after 10-cycles(a) oxide layer morphology (b) cross-section of the oxide layer (cef) EDS elemental analysis of the oxide layer at 900 C.
Fig. 4. (continued).
the cross-section (Fig. 5b). Elemental mapping (Fig. 5cef) shows an oxide scale composed of a dense and uneven top layer containing a mixture of Al and O with a thickness of 2e3 mm. This top layer is followed by a mixture of Ti, O and Nb solid solution. Grains with a
mixture of Al and O are found below this layer expanding itself into the Ti-rich layer. The presence of Nb seems to be evenly distributed within the oxide layer with the exception to the top Al and O mixture.
Fig. 5. SEM image of Ti-45Al-8Nb-0.2C after 500 cycles (a) oxide layer morphology (b) cross-section of the oxide layer (cef) EDS elemental analysis of the oxide layer at 900 C.
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Fig. 5. (continued).
The surface morphology of the Ti-45Al-8Nb-(2e4)V-(2e4)Mo alloy after oxidation at 900 C is depicted in Fig. 6a. Densely populated, sharp-edged oxide grains of approx. 1 mm are visible on the alloy surface. Cross-sectional analysis (Fig. 6b) shows an oxide layer with a thickness of approximately 24 mm. An uneven and dense layer containing a mixture of Al and O is present at the top of the scale (Fig. 6cef). A solid solution of Ti, O and Nb including some traces of Al and O mixture, is aligned between the scale-substrate interface and this layer. As in the previous case, a thin, uneven layer containing a solid solution of Al and O can be seen at the bottom of oxide layer. Similarly, Nb present within the oxide layer with the exception of the top most Al and O mixture.
3.3. XRD analysis XRD analysis of the b/g-TiAl alloys as received conditions and after oxidation at 900 C is given in Fig. 7aec. Diffractograms in the non-oxidized state shows availability of a2-Ti3Al, g-TiAl and bTi2Al(Nb,Mo) for all alloys. Al2O3 in combination with TiO2 is present for 42Al-8V-(2e4)Mo after 10 oxidation cycles at 900 C (Fig. 7a). Moreover, the presence of alloying element oxides; VO2 and V2O5 along with intermetallic Al2Ti phase is are detected on the alloy surface. In case of Ti-45Al-8Nb-0.2C, formation TiO2 and Al2O3 with traces of Nb2O5 takes place at 900 C. The diffractograms of Ti45Al-8Nb-(2e4)V-(2e4)Mo alloy prove the presence of only Al2O3 and TiO2/TiO phases. No Nb or V based oxides can be seen in this case. The diffractogram of the tested alloys can be differentiated from number and intensity of peaks of their respective oxides. A quantitative analysis of the amount of the respective oxides (Rietveld Analysis), orientation analysis of the phases as well as grain size analysis of the present oxides can provide deep insight into the oxidation behavior of these alloys and are topics of future investigations.
4. Discussion 4.1. Ti-42Al-8V-(2e4)Mo Gravimetric analysis of Ti-42Al-8V-(2e4)Mo reveals that activation temperature for oxidation lies between 600 C and 700 C, as high oxidation rate is interpreted at 700 C. It is believed that formation of TiO2 took place during the initial oxidation stages which lead to the development of an Al2O3 and TiO2 mixture after 100 cycles, depicted through a parabolic behavior (Fig. 2a). SchmitzNiderau and Schütze [19] postulated that TiO2 cannot be considered as a protective due to its porous structure and high growing rate whereas a mixture of Al2O3 and TiO2 can result in controlled oxidation as Al2O3 offers a dense structure preventing further diffusion of O, leading to low oxide growth rates. EDS mapping shows formation of a top TiO2VX layer followed by a thin discontinuous Al2O3 layer. The formation of such a discontinuous und unstable layer leads to spallation of the oxide due to high stresses during the oxidation process [20e22]. This behavior was interpreted for the respective alloy, as a spallation of oxide layer can be seen after 10 cycles along with rapid mass loss. Underneath this discontinuous Al2O3 layer, Al2O3 and TiO2 particles embedded in a V matrix can be seen. The amount of secondary alloying element (V in this case) plays a vital role in defining the oxidation resistance of g -TiAl alloys. Herold-Schmidt et al. [23] reported no significant improvement in oxidation behavior after addition of 1% V to Ti-(40,48)Al at 900 C whereas Kekare et al. [24] found an improvement in Ti-48Al due to the addition of 2.2%V at 982 C. In contrast, Shida and Anada et al. [14] reported a deterioration in the oxidation behavior of Ti48Al when alloyed with 0.36e3.63%V. Similarly, Lee [13] reported on the deterioration for Ti-39.4Al when alloyed with 10%V. Cracking of and spalling of the oxide layer during oxidation was also reported for TiAl alloy with 5%V [11]. This deleterious effect is attributed to the formation of
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Fig. 6. SEM image of Ti-45Al-8Nb-(2e4)V-(2e4)Mo after 500 cycles (a) oxide layer morphology (b) cross-section of the oxide layer (cef) EDS elemental analysis of the oxide layer at 900 C.
polymorphic, volatile V-oxides (e.g. V2O5) which melt at 674 C [16,25]. Still presence of traces of V-oxide phases like VO2 along with TiO2 and Al2O3 can also be seen from the XRD diffractograms.
Fig. 7. (a) XRD analysis of Ti-42Al-8V-(2e4)Mo before and after oxidation (b)XRD analysis of Ti-45Al-8Nb-0.2C before and after oxidation (c) XRD analysis of Ti-45Al8Nb-(2e4)V-(2e4)Mo before and after oxidation.
Similar to our results, a depleted uneven top Al2O3 layer with presence of TiO2 phase has been reported by Becker et al. [26] for TiAl containing V, where the formation of volatile oxides took place and hindered the formation of a protective oxide layer. This leads to
Fig. 7. (continued).
M. Naveed et al. / Journal of Alloys and Compounds 691 (2017) 489e497
Fig. 7. (continued).
higher oxidation of the alloy with weak scale adherence with the alloy. 4.2. Ti-45Al-8Nb-0.2C In contrast to the previously discussed alloy, Ti-45Al-8Nb-0.2C (also known as TNB-V2) consists of Nb as secondary alloying element with traces of carbon. A rapid linear oxidation takes place at 800 C, whereas a constant oxidation rate was revealed after 50 cycles at 900 C. As obvious from the rapid oxidation, the oxidation kinetics support the formation of porous TiO2, unprotective Al2O3 phase or a mixture of TiO2 and Al2O3 phase at 800 C. EDS analysis shows the presence of a thin and uneven Al2O3 layer (Fig. 4d) which is followed by a second mixed layer of (TiO2)Nbx with Al2O3 grains in the bottom region. Finally, a thin (TiO2) Nbx layer is attached to the alloy surface. This high fraction of TiO2 can be a reason for the instability of the Al2O3 activating spalling after 450 cycles at 900 C. Such instability of the oxide layer result due to competitive growth of the two or more oxide forms grown during different cycle phases [3]. Moon and Lee postulated variation in compressive and tensile stresses during heating and cooling segments [22]. The amount of a particular alloying element determines the oxidation kinetics and the scale formation on the g-TiAl surface. An optimum 4e8% Nb content has been suggested for the formation of a protective stable Al2O3 layer during the oxidation process [27]. Literature [28] proves that even a 2% of Nb content can provide good oxidation resistance as in the case of Ti-48Al-2Cr-2Nb. Similar to our results, Sun€tter et al. [29]. found no oxidation of Ti-48Al-2Cr-2Nb even derko after 1200 cycles whereas an oxide layer mix of Al2O3 and TiO2 can be seen after 3000 cycles at 700 C and 800 C. 4.3. Ti-45Al-8Nb-(2e4)V-(2e4)Mo Gravimetric measurements (Fig. 2c) show lower oxidation rates at 600 C as well as at 700 C than at 800 C and 900 C. Hence, this alloy seems to be favorable for applications up to 700 C. Along with high oxidation rates, a breakaway oxidation can be seen at 800 C whereas a parabolic oxidation behavior is observed at 900 C. XRD analysis indicates the presence of a protective Al2O3 phase at 900 C with traces of TiO2. Elemental mapping shows a thick, dense Al2O3 layer followed by a (TiO2)Nbx layer too, which is characteristic for Nb containing g-TiAl alloys. This (TiO2)Nbx layer occupies almost
495
80% of the oxide volume and expands itself till the metal substrate. Such a combination of layers is due to the interchanging of Ti4þ radicals with Nb5þ in the TiO2 lattice. This doping mechanism of TiO2 with Nb, reduces the concentration of oxygen vacancies and titanium interstitials and thus the transport of oxygen inwards and titanium outwards is decelerated [15,27,30,31]. Along with Nb, Mo is also added as an alloying element, which is beneficial for oxidation applications. Inclusion of Mo leads to reduction of solubility of TiO2 leading to outwards growth of the oxide layer. Moreover, protective Al2O3 formed during oxidation of Mo is defect free and promotes resistance against further oxidation [32]. Shida and Anada [33] reported a 0.9% solubility of oxygen in Ti-34.5Al in comparison to 0.5% for Ti-34.5Al-4Mo which describes the beneficial effect of Mo as an alloying element. As a result of this addition, the internal oxide scale is suppressed leading to no further oxidation of the metal substrate. No detailed information on the effect of Mo as an alloying element is provided here. Additionally, a Nb enriched layer along with Al and Ti can be seen at the metal-oxide interface. This enrichment of Nb stabilizes the g-phase and promotes formation of protective Al2O3 at the oxide-metal interface [26] and would resist transport of O on the alloy surface and eventually leading to external oxidation [34]. 4.4. Comparison between the alloys A comparison of the oxidation behaviors of the three alloys is discussed in this section. Two alloys with different Al contents were chosen: 42 at.% and 45 at.%. Therefore, it is important to discuss the influence of Al content for the formation of a protective oxide layer during oxidation. Former studies show the formation of a stable TiO2 instead of Al2O3 for TiAl alloys containing Al 50 at.% [35]. In the current study, formation of TiO2 took place along with nonprotective Al2O3 layer too. Other studies indicate a requirement of 60e70 at. % Al to form a continuous/protective Al2O3 scale for ageing in air [14,18,36]. However, Al concentration is technologically restricted in the discussed alloys due to loss of ductility as formation of TiAl3 takes place [37]. Additionally, a clear change in oxidation behavior due to alloying of V and Nb can be estimated from the oxidation rates (Fig. 2aec). V containing alloys show higher mass gain in comparison to Nb containing alloys during the oxidation process. Oxide morphologies of the V containing alloys were found to be needle like structure whereas Nb containing alloys showed a uniform fine grained oxide structure. Cross-sectional analysis depicted a preferential development of a top Al2O3 layer for Nb containing alloys whereas the formation of a top TiO2 layer is seen for V containing alloy. The comparison between Nb containing alloys shows slightly lower oxidation rates for Ti-45Al-8Nb-0.2C than for Ti-45Al-8Nb(2e4)V-(2e4)Mo. Since Nb is present in similar amounts in both the alloys, the presence of low amount of ternary and quaternary elements (V and Mo) in Ti-45Al-8Nb-(2e4)V-(2e4)Mo respectively is beneficial for the oxidation behavior. Moreover, higher oxidation rates are observed at 800 C than at 900 C for both alloys. One of the reasons for such a behavior could be the formation of a mixed oxide scale (Al2O3 and TiO2) at 800 C whereas formation of Al2O3 is supported at 900 C. Linear oxidation occurs for Ti-45Al-8Nb-(2e4) V-(2e4)Mo followed by a breakaway oxidation in the later stages, which was not the case in Ti-42Al-8Nb-0.2C at 800 C. This breakaway oxidation generally follows a linear oxidation of the alloy. During the linear oxidation, a development of preferential TiO2 oxide takes place succeeded by an Al2O3 layer. With the increase in oxidation duration, the vacancies in TiO2 allow further oxidation of Al2O3 leading to a mixture of coarse grained Al2O3 and TiO2 top layer in form of breakaway oxidation [26].
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A direct comparison of the oxidation behavior of different g-TiAl can also be discussed by studying the microstructure of these alloys. Haanappel et al. [38] discussed the oxidation behavior of gTiAl alloys with various microstructures. He found low oxidation rates of fully lamellar structure in comparison to duplex and near gamma structures of Ti-48Al-2Cr alloy. Zhan et al. [39] postulated that addition of large amount of Nb (b-stabilizing element) can change the microstructure of the alloy to a large extent eventually leading to a variation in oxide morphology. Moreover, Perez et al. [40] postulated higher oxidation rates of coarse a2 structure in comparison to fine a2 structure. In our study, Ti-42A-8V-(2e4)Mo having a duplex structure showed higher affinity to oxidation than Ti-45Al-8Nb-0.2C and Ti-45Al-8Nb-(2e4)V-(2e4)Mo alloy having lamellar and near gamma structure respectively. A need to understand the oxygen diffusion in individual phases is important to explain these results. Sun et al. [41] reported that addition of b-stabilizing elements influences the microstructure and the volume fractions of phases within a TiAl alloy. V is considered as an intermediate b-phase stabilizer whereas addition of Nb weakly stabilizes the b-phase but is highly soluble in the gphase. Additionally, Nb leads to refinement and stabilization of the microstructure along with high temperature strength of gþa2 phase [42]. High temperature studies show that the g-phase supports the formation of stable Al2O3 whereas the a2 phase tends to form a TiO2 layer. In case, that the a2-phase is finely dispersed in gphase, formation of Al2O3 will over- or undergrow the initially formed TiO2, eventually covering the whole surface with a protective oxide layer [26,43]. The, b-phase shows a much faster diffusion of N and O in comparison to other phases [44,45]. These facts can be correlated with the presence of different volume fractions in the discussed alloys (Table 1). Alloying of Mo and V in case of Ti-42Al-8V-(2e4)Mo leads to b-matrix formation with g and a2 phases (Fig. 2a). This dominant presence of the b-phase (approx. 37.5% vol.) within the microstructure leads to high oxidation rates. In contrast, Ti-45Al-8Nb-(2e4)V-(2e4)Mo show low b-volume fraction even due to the presence of Nb and Mo shows only low oxidation rates. Additionally, a high volume fraction of g þ a2 in Ti45Al-8Nb-0.2C as well as in Ti-45Al-8Nb-(2e4)V-(2e4)Mo support a top Al2O3 layer formation while a TiO2 top layer is formed for Ti42Al-8V-(2e4)Mo. Hence, it can be concluded that an improvement in oxidation behavior can be achieved by an optimum design of the microstructure, for instance by addition of alloying elements or a heat treatment process.
5. Conclusions In this study, the oxidation behavior of three different g-TiAl alloys under thermocyclic ageing conditions at 600 C, 700 C, 800 C and 900 C was investigated. These TiAl alloys can be grouped in Nb and V containing alloys. The TiAl alloys containing Nb exhibited low oxidation even if V is also added, whereas the alloy containing only V showed an enhanced oxidation rate. Nb as ternary alloying element is accommodated in a solid solution with TiO2, improving the oxidation resistance by constraining oxygen mass transfer within TiO2. On the other hand, the V containing alloy also forms a solid solution with TiO2 but in this case no diffusion of oxygen can be prevented. According to the results, the oxidation behavior of g-TiAl could be improved by stabilizing the aluminum oxide at the metal/oxide interface either by preventing the aluminum depletion of the metal subsurface zone or by reducing the Al2O3 dissolution in TiO2. The effect of the alloying elements, and the ability of their oxides to form solid solutions with the growing oxide, especially TiO2 have a
decisive influence on the oxidation behavior. Moreover, the quantity of alloying elements, microstructural design and the oxidation conditions (temperature, partial pressure) significantly influence the formation of a protective oxide layer. Acknowledgements €nder and Mrs. Birgit The authors are thankful to Mr. Frank Holla Kunze for their technical assistance during the experiments. Many thanks to Mr. Sebastian Bolz, Mr. Julai Zhang, Mr. Aleksei Obrosov (BTU-Cottbus-Senftenberg) and Mr. Alexander Donchev (DECHEMA) for their scientific support during the research work. Very special thanks to GKSS for supplying the newly developed alloys for testing purpose. The authors would also like to thank Mr. Christoph Leyens (Technical University Dresden) for his support during the project. References [1] C. Leyens, Oxidation and protection of titanium alloys and titanium Aluminides, in: C. Leyens, M. Peters (Eds.), Titanium and Titanium Alloys, Wiley, Cologne, 2003, pp. 187e223. [2] C. Leyens, M. Peters, Gamma titanium aluminium alloys, in: C. Leyens, M. Peters (Eds.), Titanium and Titanium Alloys, WILEY-VCH, 2002, pp. 89e147. [3] I.C.I. Okafor, R.G. Reddy, The oxidation behaviour of high-temperature aluminides, J. Miner. Metals Mater. Soc. 51 (2007) 35e40. [4] J.W. Fergus, Review of the effect of alloy composition on the growth rates of scales formed during oxidation of gamma titanium alloys, Mater. Sci. Eng. A338 (2002) 108e125. € hlich, A. Ebach-Stahl, R. Braun, C. Leyens, Oxidation protective coatings [5] M. Fro for Gamma TiAl- recent trends, Mater. Wiss. Werst. 9 (2007). [6] H.J. Schmutzler, N. Zheng, W.J. Quadakkers, M.F. Strrnsnijder, The influence of niobium ion implantation on the high temperature oxidation behaviour of Ti48Al-2Cr, Surf. Coatings Technol. 83 (1999). [7] R. Braun, M. Froehlich, C. Leyens, D. Renusch, Oxidation behaviour of TBC systems on Gamma TiAl based alloy Ti-45Al-8Nb, Oxid. Metals 71 (2009). [8] L. Junpin, N. Zhang, Y.L. Wang, Y. Zhang, G.L. Chen, Influence of Y addition on the high temperature long-term oxidation resistance of high Nb coating TiAl based alloys, in: 3rd International Workshop on Gamma TiAl Technologies, Bamberg, Germany, 2006. [9] E. Godlewska, M. Mitoraj, J. Morgiel, Reaction and diffusion phenomena upon oxidation of a (gþa) TiAlNb alloy in air, Mater. High. Temp. 26 (2009) 101. [10] V.A.C. Haanappel, H. Clemens, M.F. Stroosnijder, The high temperature oxidation behaviour of high and low alloyed TiAl based intermetallics, Intermetallics (2002) 293e305. [11] B.G. Kim, G.M. Kim, C.J. Kim, Oxidation behavior of TiAl-X (X ¼ Cr, V, Si, Mo or Nb) intermetallics at elevated temperature, Scripta Metall. Mater. 33 (1995) 1117e1125. [12] C.H. Koo, J.W. Evans, K.Y. Song, T.H. Yu, High temperature oxidation of Ti3AlNb alloys, Oxid. Metals 42 (1994) 542e543. [13] D.-B. Lee, Effect of Cr, Nb, Mn, V, W and Si on high temperature oxidation of TiAl alloys, Metals Mater. Int. 11 (2005) 141e147. [14] Y. Shida, H. Anada, The effect of various ternary additives on the oxidation behavior of TiAl in high-temperature air, Oxid. Metals 45 (1996) 197e219. [15] H. Jiang, M. Hirohasi, Y. Lu, H. Imanari, Effect of Nb on the high temperature oxidation of Ti-(0-50 at.%)Al, Scr. Mater. 46 (2002) 639e643. [16] J.G. Keller, D.L. Douglas, The high temperature oxidation behavior of Vanadium-Aluminium alloys, Oxid. Metals 36 (1991). [17] H. Hindam, D.P. Whittle, Microstructure, adhesion and growth kinetics of protective scales on metals and alloys, Oxid. Metals 18 (1982) 245e283. [18] A. Rahmel, W.J. Quadakkers, M. Schütze, Fundamentals of TiAl oxidation - a critical review, Mater. Corros. (1995) 271e285. [19] M. Schmitz-Niderau, M. Schütze, Cracking and healing of oxide scales on Ti-Al alloys at 900 C, Oxid. Metals 52 (1999) 241e276. [20] K.S. Chan, A mechanics-based approach to cyclic oxidation, Metall. Mater. Trans. 28A (1997) 11. [21] D.P. Whittle, Spalling of protective oxide scales, Oxid. Metals 4 (1972) 9. [22] C.-O. Moon, S.-B. Lee, Analysis on failures of protective-oxide layers and cyclic oxidation, Oxid. Metals 39 (1992) 13. [23] U. Herold-Schmidt, B. Opolka, S. Schwanted, Prakt. Metallogr. 30 (1993) 7. [24] S.A. Kekare, P.B. Aswath, Oxidation of TiAl based intermetallics, J. Mater. Sci. 32 (1997) 2485. [25] B. Champin, L. Graff, M. Armand, Beranger, C. Coddet, Oxidation of titanium alloys at temperatures used in turbine engines, J. Less Common Met. 69 (1980) 20. [26] S. Becker, M. Schorr, M. Schutze, Mechanism of isothermal oxidation of the intermetallic TiAl and TiAl alloys, Oxid. Metals 38 (1992) 425e464. [27] S.K. Varma, A. Chan, R.N. Mahapatra, Static and cyclic oxidation of Ti-44Al and Ti-44Al-xNb alloys, Oxid. Metals 55 (2001) 423e435.
M. Naveed et al. / Journal of Alloys and Compounds 691 (2017) 489e497 €tter, M.F. Stroosnijder, The isothermal and [28] V.A.C. Haanappel, J.D. Sunderko cyclic high temperature oxidation behaviour of Tie48Ale2Mne2Nb compared with Tie48Ale2Cre2Nb and Tie48Ale2Cr, Intermetallics 7 (1999) 529e541. € tter, H.J. Schmutzler, V.A.C. Haanappel, R. Hofman, W. Glatz, [29] J.D. Sunderko H. Clemens, M.F. Stroosnijder, The high-temperature oxidation behaviour of Ti-47Al-2Cr-0.2Si and Ti-48Al-2Cr-2Nb compared with Ti-48Al-2Cr, Intermetallics 5 (1997) 525e534. [30] M.J. Weimer, T.J. Kelly, TiAl alloy 48Al-2Nb-2Cr, material database and application status, in: 3rd International Workshop on Gamma TiAl Alloys, Bamberg, Germany, 2006. [31] Y. Shen, X. Ding, F. Wang, Y. Tan, J.M. Yang, High temperature oxidation behaviour of Ti-Al-Nb ternary alloys, J. Mater. Sci. 39 (2004) 65e88. [32] Y. Shida, H. Anada, Role of W,Mo,Nb and Si on TiAl in air at high temperatures, Mater. Trans. 35 (1994) 626e630. [33] Y. Shida, H. Anada, Role of W, Mo, Nb and Si on oxidation of TiAl in air at high temperatures, materials transactions, JIM 35 (1994) 623e631. [34] C. Lang, M. Schütze, The initial stages in the oxidation of TiAl, Mater. Corros. (1997) 13e22. [35] M. Yamaguchi, H. Inui, TiAl compounds for structural applications, in: R. Darolia, J.J. Lewandowski, C.T. Liu, P.L. Martin, D.B. Miracle, M.V. Nathal (Eds.), TMS, Structural Intermetallics, Warendale PA, 1993, pp. 127e142. [36] G.H. Meier, Fundamental of the oxidation of high-temperature intermetallics, in: T. Grobstein, J. Doychak (Eds.), TMS, Oxidation of High-temperature Intermetallics, Wallendale, PA, 1988, pp. 1e16.
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[37] M.-R. Yang, S.-K. Wu, Oxidation resistance improvement of TiAl intermetallics using surface modification, Bulletin of the College of Engineering, N.T.U (2003) 3e19. € tter, W. Glatz, H. Clemens, [38] V.A.C. Haanappel, R. Hofmann, J.D. Sunderko M.F. Stroonijder, The influence of microstructure on the Isothermal and cyclic oxidation behavior of Ti-48Al-2Cr at 800 C, Oxid. Metals 48 (1997) 263. [39] C.-J. Zhan, T.-H. Yu, C.-H. Koo, Effects of high niobium addition on the microstructure and high temperature properties of Ti-40Al-xNb, Mater. Trans. 47 (2006) 2589e2593. [40] P. Perez, J.J.A, G. Frommeyer, P. Adeva, The influence of the alloy microstructure on the oxidation behavior of Ti-46Al-1Cr-0.2Si, Oxid. Metals 53 (2000) 99e124. [41] F.-S. Sun, C.-X. Cao, S.-E. Kim, Y.-T. Lee, M.-G. Yan, Alloying mechanism of beta stabilizers in a TiAl alloy, Metall. Mater. Trans. A 23A (2001) 1573e1589. [42] J.D.H. Paul, F. Appel, R. Wagner, The compression behaviour of niobium alloyed g-titanium alumindies, Acta Mater. 46 (1998) 1075e1085. [43] A. Gil, J. Hoven, E. Wallura, W.J. Quadakkers, The effect of microstructure on the oxidation behaviour of TiAl-based intermetallics, Corros. Sci. 3 (1993) 615e630. [44] H.F. Chladil, H. Clemens, V.G.A. Otto, S. Kremmer, A. Bartels, R. Gerling, Charakterisierung einer beta-erstarrenden gamma-TiAl-Basislegierung, BHM 151 (9) (2006) 356e361. [45] F. Dettenwanger, M. Shütze, Isothermal oxidation of a2-Ti3Al, Oxid. Metals 54 (1999) 121e138.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Role of alloying elements during thermocyclic oxidation of b/g-TiAl alloys at high temperatures Muhammad Naveed*, A. Flores Renteria, Sabine Weiß Physical Metallurgy and Materials Technology, Brandenburg Technical University, Cottbus-Senftenberg Konrad Wachsmann Allee 17, 03046, Cottbus, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 January 2016 Received in revised form 22 August 2016 Accepted 25 August 2016 Available online 28 August 2016
Gamma titanium aluminides are promising alloys known for their good mechanical properties and low densities, but their low oxidation resistance at high temperatures limits their application. This work discusses the thermocyclic oxidation behavior of newly developed b/g-TiAl alloys at temperatures between 600 C and 900 C. An influence of b-stabilizing alloying elements like Nb and V on the oxidation of these alloys has been investigated here. The selected alloys are tested in an in-house developed thermocyclic furnace. The oxidation study is supported by gravimetric measurements along with Scanning Electron Microscopy (SEM) and Electron Diffraction Spectroscopy (EDS) mapping of the oxide layers. Additionally, phase formation after oxidation has been determined using X-Ray Diffraction (XRD). Results show that the Nb containing alloys are more oxidation resistant as compared to V containing alloys. The formation of a mixture of Al2O3 and TiO2 layers was found for all the alloys. Additionally, the oxide kinetics controlled the oxide growth and formation of various phases at different testing temperatures. © 2016 Elsevier B.V. All rights reserved.
Keywords: g-TiAl Thermocyclic oxidation b-stabilizing elements Aluminum oxide Titanium oxide
1. Introduction For the last three decades, tremendous resources and efforts have been spent on research and development of g-TiAl alloys as a replacement of heavy nickel super alloys in aerospace applications [1]. Reports on high potential of these alloys in turbocharger wheels and exhaust valves in automobiles has also been discussed [2]. Even due to its low strength-to-weight ratio, only limited application is found due to insufficient ductility at ambient temperatures and oxidation resistance at high temperatures (above 800 C) [3,4]. Literature proposes various methods to improve the oxidation resistance of these alloys at high temperature which include preoxidation [5], ion implantation [6], overlay coatings [7] etc. Addition of alloying elements is one of the most discussed method for the improvement in oxidation resistance of g-TiAl [5,8e13]. Shida and Anada [14] classified these alloying elements as beneficial, neutral and detrimental, depending upon their effect during the oxidation of g-TiAl. Alloying of these elements influences the oxidation kinetics and scale growth on the alloy surface. This work also discusses the effect of some of the b-stabilizing
* Corresponding author. E-mail address: [email protected] (M. Naveed). http://dx.doi.org/10.1016/j.jallcom.2016.08.259 0925-8388/© 2016 Elsevier B.V. All rights reserved.
alloying elements added to improve ductility and oxidation resistance. Addition of Nb, which is classified as the most effective alloying element to improve the oxidation resistance of the alloy [15], has been presented here. In contrast, addition of V known for its detrimental effect on the oxidation behavior is also discussed in this work [16,17]. Moreover, effect of Mo, which is present in low amount, is also discussed in the present study. The present focuses on the formation of various oxide layers grown during thermocyclic oxidation of these alloys. A thermally stable protective oxide with slow growth rate and good adhesion to the metallic substrate can resist diffusion of O into the alloy surface [3]. In case of g-TiAl alloys, a dense Al2O3 is preferred over a porous TiO2 layer during the oxidation process. Since the equilibrium pressures of O for Al2O3 and TiO2 are similar, an activity between Ti and Al would control the formation of the preferential oxide layer [18]. Hence, this work focusses on understanding the growth kinetics of the oxide layers influenced by the addition of alloying elements. 2. Materials and methods Three different b/g-TiAl alloys containing V, Nb and a combination of both were compared in this work. The microstructure of
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Ti-42Al-8V-(2e4)Mo (Fig. 1a) is composed of grains containing (g þ a2)-lamellas encapsulated within a g-phase, which is embedded in a b-phase matrix. In case of Ti-45Al-8Nb-0.2C (Fig. 1b), the sample consists of large grains containing (gþa2)-lamellas with grain boundaries covered with bright b- and dark
Table 1 Volume fraction of various phases in g -TiAl alloys.
Ti-42Al-8V-(2e4)Mo Ti-45Al-8Nb-0.2C Ti-45Al-8Nb-(2e4)V-(2e4)Mo
a2 þ g
g
b
38 55.4 85.5
24.5 23.1 9.4
37.5 21.5 5.1
g-phase. Moreover, Ti-45Al-8Nb-(2e4)V-(2e4)Mo alloy (Fig. 1c) shows dark-grey grains composed of (gþa2)-lamellas embedded in a b-matrix with presence of few dark g-grains within the micro-
Fig. 1. (a) BSE Micrograph of the Ti-42Al-8V-(2e4)Mo alloy 1) BSE Micrograph of the Ti-45Al-8Nb-0.2C alloy (c) BSE Micrograph of the Ti-45Al-8Nb-(2e4)V-(2e4)Mo alloy.
structure. An evaluation of the volume fraction of different phases is given in Table 1, which would be discussed in detail in the next sections. Disc-shaped samples with a diameter of 20 mm and 1 mm thickness are used for the oxidation test. The samples are first polished to 2500-grit followed by ultrasonic (ethanol) cleaning and later dried in air. A Pt-wire is used to position the specimen on a ceramic specimen holder as shown in Fig. 2. Thermocyclic experiments are performed in an in-house developed tube furnace which can be programmed by a controller. Each cycle consists of an oxidation time of 60 min and a cooling time of 10 min, which is sufficient to cool the specimen below 50 C. Mass changes are documented by gravimetric measurements at different intervals during the oxidation process. Gravimetric measurements are made with a resolution of 105 g using a MC 21S microbalance from Sartorius (Spain). The surface morphologies and oxide interfaces are characterized by means of a SEM Mira (TESCAN, Czech Republic) integrated with an EDS-System (OXFORD, Wiesbaden Germany). Phase identification has been performed using a D8 Discovery diffractometer (Bruker, Karlsruhe, Germany) equipped with a LynxEye detector with Bragg Brentano technique. Phase detection software EVA (Bruker) with ICDD (PDF4) phase identification database has been used for the diffractogram analysis.
3. Results 3.1. Gravimetric analysis
Fig. 1. (continued).
Fig. 1. (continued).
Thermocyclic oxidation behavior is described by mass gain of a specimen as a function of oxidation cycles. Gravimetric analysis of Ti-42Al-8V-(2e4)Mo alloy (Fig. 3a) shows a very low mass gain after oxidation at 600 C whereas a significantly higher mass gain can be seen after oxidation at 700 C. Linear oxidation behavior is observed for both the cases (marked with red line). Rapid oxidation occurred during the first 150 with no further oxidation during the next 350 cycles at 800 C. A rapid oxidation ending in spallation of the oxide scale during the first 10 cycles took place after oxidation at 900 C. In case of Ti-45Al-8Nb-0.2C (Fig. 3b), no mass gain is observed after oxidation at 600 C, whereas a slow and linear oxidation (marked with red) occured at 700 C. A rapid linear mass increase (marked with red) from the initial stages of oxidation process is noticed at 800 C. Parabolic oxidation behavior is observed during thermocyclic oxidation resulting in spallation after 450 cycles at 900 C. Similar to previously discussed alloys, no mass gain was observed for Ti-45Al-8Nb-(2e4) V-(2e4)Mo after oxidation at 600 C whereas a slight linear increase (marked with red) has been noticed after oxidation at 700 C. Parabolic oxidation behavior is noticed at the initial stage, leading to a breakaway oxidation after 300 cycles at 800 C whereas a parabolic oxidation behavior is depicted for oxidation at 900 C.
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Fig. 2. Thermocyclic oxidation oven equipped with a controller and a specimen holder.
Fig. 3. (continued).
3.2. Surface morphology and elemental mapping
Fig. 3. (a) Thermogravimetric analysis of Ti-42Al-8V- (2e4)Mo at various temperatures (b) Thermogravimetric analysis of Ti-45Al-8Nb-0.2C at various temperatures (c) Thermogravimetric analysis of Ti-45Al-8Nb-(2e4)V-(2e4)Mo at various temperatures.
A study of the oxide growth during the thermocyclic process is discussed by analyzing surface morphologies and elemental mappings of the oxide layers (Figs. 4e6). Surface analysis of Ti-42Al-8V(2e4)Mo after oxidation at a temperature of 900 C is shown in Fig. 4a. The specimen surface is completely covered with a mixture of randomly oriented needle shaped grains with an approximate size of 2 mm. An oxide layer with an approximate thickness of 42 mm is found on the alloy surface (Fig. 4b). EDS analysis (Fig. 4cef) shows a thin and uneven top layer consisting of Ti, O and V. This top layer is followed by a thin discontinuous layer consisting of Al and O mixture. A richly doped Ti and V zone with traces of Al and O is situated directly below these surface layers and expands itself till the base material. The oxide morphology of Ti-45Al-8Nb-0.2C formed at 900 C is depicted in Fig. 5a. The complete surface is covered with homogeneously distributed partially equiaxed oxide grains with a grain size of approx. 1 mm. An oxide layer of approx. 25 mm is visible from
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Fig. 4. SEM image of Ti-42Al-8V-(2e4)Mo after 10-cycles(a) oxide layer morphology (b) cross-section of the oxide layer (cef) EDS elemental analysis of the oxide layer at 900 C.
Fig. 4. (continued).
the cross-section (Fig. 5b). Elemental mapping (Fig. 5cef) shows an oxide scale composed of a dense and uneven top layer containing a mixture of Al and O with a thickness of 2e3 mm. This top layer is followed by a mixture of Ti, O and Nb solid solution. Grains with a
mixture of Al and O are found below this layer expanding itself into the Ti-rich layer. The presence of Nb seems to be evenly distributed within the oxide layer with the exception to the top Al and O mixture.
Fig. 5. SEM image of Ti-45Al-8Nb-0.2C after 500 cycles (a) oxide layer morphology (b) cross-section of the oxide layer (cef) EDS elemental analysis of the oxide layer at 900 C.
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Fig. 5. (continued).
The surface morphology of the Ti-45Al-8Nb-(2e4)V-(2e4)Mo alloy after oxidation at 900 C is depicted in Fig. 6a. Densely populated, sharp-edged oxide grains of approx. 1 mm are visible on the alloy surface. Cross-sectional analysis (Fig. 6b) shows an oxide layer with a thickness of approximately 24 mm. An uneven and dense layer containing a mixture of Al and O is present at the top of the scale (Fig. 6cef). A solid solution of Ti, O and Nb including some traces of Al and O mixture, is aligned between the scale-substrate interface and this layer. As in the previous case, a thin, uneven layer containing a solid solution of Al and O can be seen at the bottom of oxide layer. Similarly, Nb present within the oxide layer with the exception of the top most Al and O mixture.
3.3. XRD analysis XRD analysis of the b/g-TiAl alloys as received conditions and after oxidation at 900 C is given in Fig. 7aec. Diffractograms in the non-oxidized state shows availability of a2-Ti3Al, g-TiAl and bTi2Al(Nb,Mo) for all alloys. Al2O3 in combination with TiO2 is present for 42Al-8V-(2e4)Mo after 10 oxidation cycles at 900 C (Fig. 7a). Moreover, the presence of alloying element oxides; VO2 and V2O5 along with intermetallic Al2Ti phase is are detected on the alloy surface. In case of Ti-45Al-8Nb-0.2C, formation TiO2 and Al2O3 with traces of Nb2O5 takes place at 900 C. The diffractograms of Ti45Al-8Nb-(2e4)V-(2e4)Mo alloy prove the presence of only Al2O3 and TiO2/TiO phases. No Nb or V based oxides can be seen in this case. The diffractogram of the tested alloys can be differentiated from number and intensity of peaks of their respective oxides. A quantitative analysis of the amount of the respective oxides (Rietveld Analysis), orientation analysis of the phases as well as grain size analysis of the present oxides can provide deep insight into the oxidation behavior of these alloys and are topics of future investigations.
4. Discussion 4.1. Ti-42Al-8V-(2e4)Mo Gravimetric analysis of Ti-42Al-8V-(2e4)Mo reveals that activation temperature for oxidation lies between 600 C and 700 C, as high oxidation rate is interpreted at 700 C. It is believed that formation of TiO2 took place during the initial oxidation stages which lead to the development of an Al2O3 and TiO2 mixture after 100 cycles, depicted through a parabolic behavior (Fig. 2a). SchmitzNiderau and Schütze [19] postulated that TiO2 cannot be considered as a protective due to its porous structure and high growing rate whereas a mixture of Al2O3 and TiO2 can result in controlled oxidation as Al2O3 offers a dense structure preventing further diffusion of O, leading to low oxide growth rates. EDS mapping shows formation of a top TiO2VX layer followed by a thin discontinuous Al2O3 layer. The formation of such a discontinuous und unstable layer leads to spallation of the oxide due to high stresses during the oxidation process [20e22]. This behavior was interpreted for the respective alloy, as a spallation of oxide layer can be seen after 10 cycles along with rapid mass loss. Underneath this discontinuous Al2O3 layer, Al2O3 and TiO2 particles embedded in a V matrix can be seen. The amount of secondary alloying element (V in this case) plays a vital role in defining the oxidation resistance of g -TiAl alloys. Herold-Schmidt et al. [23] reported no significant improvement in oxidation behavior after addition of 1% V to Ti-(40,48)Al at 900 C whereas Kekare et al. [24] found an improvement in Ti-48Al due to the addition of 2.2%V at 982 C. In contrast, Shida and Anada et al. [14] reported a deterioration in the oxidation behavior of Ti48Al when alloyed with 0.36e3.63%V. Similarly, Lee [13] reported on the deterioration for Ti-39.4Al when alloyed with 10%V. Cracking of and spalling of the oxide layer during oxidation was also reported for TiAl alloy with 5%V [11]. This deleterious effect is attributed to the formation of
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Fig. 6. SEM image of Ti-45Al-8Nb-(2e4)V-(2e4)Mo after 500 cycles (a) oxide layer morphology (b) cross-section of the oxide layer (cef) EDS elemental analysis of the oxide layer at 900 C.
polymorphic, volatile V-oxides (e.g. V2O5) which melt at 674 C [16,25]. Still presence of traces of V-oxide phases like VO2 along with TiO2 and Al2O3 can also be seen from the XRD diffractograms.
Fig. 7. (a) XRD analysis of Ti-42Al-8V-(2e4)Mo before and after oxidation (b)XRD analysis of Ti-45Al-8Nb-0.2C before and after oxidation (c) XRD analysis of Ti-45Al8Nb-(2e4)V-(2e4)Mo before and after oxidation.
Similar to our results, a depleted uneven top Al2O3 layer with presence of TiO2 phase has been reported by Becker et al. [26] for TiAl containing V, where the formation of volatile oxides took place and hindered the formation of a protective oxide layer. This leads to
Fig. 7. (continued).
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Fig. 7. (continued).
higher oxidation of the alloy with weak scale adherence with the alloy. 4.2. Ti-45Al-8Nb-0.2C In contrast to the previously discussed alloy, Ti-45Al-8Nb-0.2C (also known as TNB-V2) consists of Nb as secondary alloying element with traces of carbon. A rapid linear oxidation takes place at 800 C, whereas a constant oxidation rate was revealed after 50 cycles at 900 C. As obvious from the rapid oxidation, the oxidation kinetics support the formation of porous TiO2, unprotective Al2O3 phase or a mixture of TiO2 and Al2O3 phase at 800 C. EDS analysis shows the presence of a thin and uneven Al2O3 layer (Fig. 4d) which is followed by a second mixed layer of (TiO2)Nbx with Al2O3 grains in the bottom region. Finally, a thin (TiO2) Nbx layer is attached to the alloy surface. This high fraction of TiO2 can be a reason for the instability of the Al2O3 activating spalling after 450 cycles at 900 C. Such instability of the oxide layer result due to competitive growth of the two or more oxide forms grown during different cycle phases [3]. Moon and Lee postulated variation in compressive and tensile stresses during heating and cooling segments [22]. The amount of a particular alloying element determines the oxidation kinetics and the scale formation on the g-TiAl surface. An optimum 4e8% Nb content has been suggested for the formation of a protective stable Al2O3 layer during the oxidation process [27]. Literature [28] proves that even a 2% of Nb content can provide good oxidation resistance as in the case of Ti-48Al-2Cr-2Nb. Similar to our results, Sun€tter et al. [29]. found no oxidation of Ti-48Al-2Cr-2Nb even derko after 1200 cycles whereas an oxide layer mix of Al2O3 and TiO2 can be seen after 3000 cycles at 700 C and 800 C. 4.3. Ti-45Al-8Nb-(2e4)V-(2e4)Mo Gravimetric measurements (Fig. 2c) show lower oxidation rates at 600 C as well as at 700 C than at 800 C and 900 C. Hence, this alloy seems to be favorable for applications up to 700 C. Along with high oxidation rates, a breakaway oxidation can be seen at 800 C whereas a parabolic oxidation behavior is observed at 900 C. XRD analysis indicates the presence of a protective Al2O3 phase at 900 C with traces of TiO2. Elemental mapping shows a thick, dense Al2O3 layer followed by a (TiO2)Nbx layer too, which is characteristic for Nb containing g-TiAl alloys. This (TiO2)Nbx layer occupies almost
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80% of the oxide volume and expands itself till the metal substrate. Such a combination of layers is due to the interchanging of Ti4þ radicals with Nb5þ in the TiO2 lattice. This doping mechanism of TiO2 with Nb, reduces the concentration of oxygen vacancies and titanium interstitials and thus the transport of oxygen inwards and titanium outwards is decelerated [15,27,30,31]. Along with Nb, Mo is also added as an alloying element, which is beneficial for oxidation applications. Inclusion of Mo leads to reduction of solubility of TiO2 leading to outwards growth of the oxide layer. Moreover, protective Al2O3 formed during oxidation of Mo is defect free and promotes resistance against further oxidation [32]. Shida and Anada [33] reported a 0.9% solubility of oxygen in Ti-34.5Al in comparison to 0.5% for Ti-34.5Al-4Mo which describes the beneficial effect of Mo as an alloying element. As a result of this addition, the internal oxide scale is suppressed leading to no further oxidation of the metal substrate. No detailed information on the effect of Mo as an alloying element is provided here. Additionally, a Nb enriched layer along with Al and Ti can be seen at the metal-oxide interface. This enrichment of Nb stabilizes the g-phase and promotes formation of protective Al2O3 at the oxide-metal interface [26] and would resist transport of O on the alloy surface and eventually leading to external oxidation [34]. 4.4. Comparison between the alloys A comparison of the oxidation behaviors of the three alloys is discussed in this section. Two alloys with different Al contents were chosen: 42 at.% and 45 at.%. Therefore, it is important to discuss the influence of Al content for the formation of a protective oxide layer during oxidation. Former studies show the formation of a stable TiO2 instead of Al2O3 for TiAl alloys containing Al 50 at.% [35]. In the current study, formation of TiO2 took place along with nonprotective Al2O3 layer too. Other studies indicate a requirement of 60e70 at. % Al to form a continuous/protective Al2O3 scale for ageing in air [14,18,36]. However, Al concentration is technologically restricted in the discussed alloys due to loss of ductility as formation of TiAl3 takes place [37]. Additionally, a clear change in oxidation behavior due to alloying of V and Nb can be estimated from the oxidation rates (Fig. 2aec). V containing alloys show higher mass gain in comparison to Nb containing alloys during the oxidation process. Oxide morphologies of the V containing alloys were found to be needle like structure whereas Nb containing alloys showed a uniform fine grained oxide structure. Cross-sectional analysis depicted a preferential development of a top Al2O3 layer for Nb containing alloys whereas the formation of a top TiO2 layer is seen for V containing alloy. The comparison between Nb containing alloys shows slightly lower oxidation rates for Ti-45Al-8Nb-0.2C than for Ti-45Al-8Nb(2e4)V-(2e4)Mo. Since Nb is present in similar amounts in both the alloys, the presence of low amount of ternary and quaternary elements (V and Mo) in Ti-45Al-8Nb-(2e4)V-(2e4)Mo respectively is beneficial for the oxidation behavior. Moreover, higher oxidation rates are observed at 800 C than at 900 C for both alloys. One of the reasons for such a behavior could be the formation of a mixed oxide scale (Al2O3 and TiO2) at 800 C whereas formation of Al2O3 is supported at 900 C. Linear oxidation occurs for Ti-45Al-8Nb-(2e4) V-(2e4)Mo followed by a breakaway oxidation in the later stages, which was not the case in Ti-42Al-8Nb-0.2C at 800 C. This breakaway oxidation generally follows a linear oxidation of the alloy. During the linear oxidation, a development of preferential TiO2 oxide takes place succeeded by an Al2O3 layer. With the increase in oxidation duration, the vacancies in TiO2 allow further oxidation of Al2O3 leading to a mixture of coarse grained Al2O3 and TiO2 top layer in form of breakaway oxidation [26].
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A direct comparison of the oxidation behavior of different g-TiAl can also be discussed by studying the microstructure of these alloys. Haanappel et al. [38] discussed the oxidation behavior of gTiAl alloys with various microstructures. He found low oxidation rates of fully lamellar structure in comparison to duplex and near gamma structures of Ti-48Al-2Cr alloy. Zhan et al. [39] postulated that addition of large amount of Nb (b-stabilizing element) can change the microstructure of the alloy to a large extent eventually leading to a variation in oxide morphology. Moreover, Perez et al. [40] postulated higher oxidation rates of coarse a2 structure in comparison to fine a2 structure. In our study, Ti-42A-8V-(2e4)Mo having a duplex structure showed higher affinity to oxidation than Ti-45Al-8Nb-0.2C and Ti-45Al-8Nb-(2e4)V-(2e4)Mo alloy having lamellar and near gamma structure respectively. A need to understand the oxygen diffusion in individual phases is important to explain these results. Sun et al. [41] reported that addition of b-stabilizing elements influences the microstructure and the volume fractions of phases within a TiAl alloy. V is considered as an intermediate b-phase stabilizer whereas addition of Nb weakly stabilizes the b-phase but is highly soluble in the gphase. Additionally, Nb leads to refinement and stabilization of the microstructure along with high temperature strength of gþa2 phase [42]. High temperature studies show that the g-phase supports the formation of stable Al2O3 whereas the a2 phase tends to form a TiO2 layer. In case, that the a2-phase is finely dispersed in gphase, formation of Al2O3 will over- or undergrow the initially formed TiO2, eventually covering the whole surface with a protective oxide layer [26,43]. The, b-phase shows a much faster diffusion of N and O in comparison to other phases [44,45]. These facts can be correlated with the presence of different volume fractions in the discussed alloys (Table 1). Alloying of Mo and V in case of Ti-42Al-8V-(2e4)Mo leads to b-matrix formation with g and a2 phases (Fig. 2a). This dominant presence of the b-phase (approx. 37.5% vol.) within the microstructure leads to high oxidation rates. In contrast, Ti-45Al-8Nb-(2e4)V-(2e4)Mo show low b-volume fraction even due to the presence of Nb and Mo shows only low oxidation rates. Additionally, a high volume fraction of g þ a2 in Ti45Al-8Nb-0.2C as well as in Ti-45Al-8Nb-(2e4)V-(2e4)Mo support a top Al2O3 layer formation while a TiO2 top layer is formed for Ti42Al-8V-(2e4)Mo. Hence, it can be concluded that an improvement in oxidation behavior can be achieved by an optimum design of the microstructure, for instance by addition of alloying elements or a heat treatment process.
5. Conclusions In this study, the oxidation behavior of three different g-TiAl alloys under thermocyclic ageing conditions at 600 C, 700 C, 800 C and 900 C was investigated. These TiAl alloys can be grouped in Nb and V containing alloys. The TiAl alloys containing Nb exhibited low oxidation even if V is also added, whereas the alloy containing only V showed an enhanced oxidation rate. Nb as ternary alloying element is accommodated in a solid solution with TiO2, improving the oxidation resistance by constraining oxygen mass transfer within TiO2. On the other hand, the V containing alloy also forms a solid solution with TiO2 but in this case no diffusion of oxygen can be prevented. According to the results, the oxidation behavior of g-TiAl could be improved by stabilizing the aluminum oxide at the metal/oxide interface either by preventing the aluminum depletion of the metal subsurface zone or by reducing the Al2O3 dissolution in TiO2. The effect of the alloying elements, and the ability of their oxides to form solid solutions with the growing oxide, especially TiO2 have a
decisive influence on the oxidation behavior. Moreover, the quantity of alloying elements, microstructural design and the oxidation conditions (temperature, partial pressure) significantly influence the formation of a protective oxide layer. Acknowledgements €nder and Mrs. Birgit The authors are thankful to Mr. Frank Holla Kunze for their technical assistance during the experiments. Many thanks to Mr. Sebastian Bolz, Mr. Julai Zhang, Mr. Aleksei Obrosov (BTU-Cottbus-Senftenberg) and Mr. Alexander Donchev (DECHEMA) for their scientific support during the research work. Very special thanks to GKSS for supplying the newly developed alloys for testing purpose. The authors would also like to thank Mr. Christoph Leyens (Technical University Dresden) for his support during the project. References [1] C. Leyens, Oxidation and protection of titanium alloys and titanium Aluminides, in: C. Leyens, M. Peters (Eds.), Titanium and Titanium Alloys, Wiley, Cologne, 2003, pp. 187e223. [2] C. Leyens, M. Peters, Gamma titanium aluminium alloys, in: C. Leyens, M. Peters (Eds.), Titanium and Titanium Alloys, WILEY-VCH, 2002, pp. 89e147. [3] I.C.I. Okafor, R.G. Reddy, The oxidation behaviour of high-temperature aluminides, J. Miner. Metals Mater. Soc. 51 (2007) 35e40. [4] J.W. Fergus, Review of the effect of alloy composition on the growth rates of scales formed during oxidation of gamma titanium alloys, Mater. Sci. Eng. A338 (2002) 108e125. € hlich, A. Ebach-Stahl, R. Braun, C. Leyens, Oxidation protective coatings [5] M. Fro for Gamma TiAl- recent trends, Mater. Wiss. Werst. 9 (2007). [6] H.J. Schmutzler, N. Zheng, W.J. Quadakkers, M.F. Strrnsnijder, The influence of niobium ion implantation on the high temperature oxidation behaviour of Ti48Al-2Cr, Surf. Coatings Technol. 83 (1999). [7] R. Braun, M. Froehlich, C. Leyens, D. Renusch, Oxidation behaviour of TBC systems on Gamma TiAl based alloy Ti-45Al-8Nb, Oxid. Metals 71 (2009). [8] L. Junpin, N. Zhang, Y.L. Wang, Y. Zhang, G.L. Chen, Influence of Y addition on the high temperature long-term oxidation resistance of high Nb coating TiAl based alloys, in: 3rd International Workshop on Gamma TiAl Technologies, Bamberg, Germany, 2006. [9] E. Godlewska, M. Mitoraj, J. Morgiel, Reaction and diffusion phenomena upon oxidation of a (gþa) TiAlNb alloy in air, Mater. High. Temp. 26 (2009) 101. [10] V.A.C. Haanappel, H. Clemens, M.F. Stroosnijder, The high temperature oxidation behaviour of high and low alloyed TiAl based intermetallics, Intermetallics (2002) 293e305. [11] B.G. Kim, G.M. Kim, C.J. Kim, Oxidation behavior of TiAl-X (X ¼ Cr, V, Si, Mo or Nb) intermetallics at elevated temperature, Scripta Metall. Mater. 33 (1995) 1117e1125. [12] C.H. Koo, J.W. Evans, K.Y. Song, T.H. Yu, High temperature oxidation of Ti3AlNb alloys, Oxid. Metals 42 (1994) 542e543. [13] D.-B. Lee, Effect of Cr, Nb, Mn, V, W and Si on high temperature oxidation of TiAl alloys, Metals Mater. Int. 11 (2005) 141e147. [14] Y. Shida, H. Anada, The effect of various ternary additives on the oxidation behavior of TiAl in high-temperature air, Oxid. Metals 45 (1996) 197e219. [15] H. Jiang, M. Hirohasi, Y. Lu, H. Imanari, Effect of Nb on the high temperature oxidation of Ti-(0-50 at.%)Al, Scr. Mater. 46 (2002) 639e643. [16] J.G. Keller, D.L. Douglas, The high temperature oxidation behavior of Vanadium-Aluminium alloys, Oxid. Metals 36 (1991). [17] H. Hindam, D.P. Whittle, Microstructure, adhesion and growth kinetics of protective scales on metals and alloys, Oxid. Metals 18 (1982) 245e283. [18] A. Rahmel, W.J. Quadakkers, M. Schütze, Fundamentals of TiAl oxidation - a critical review, Mater. Corros. (1995) 271e285. [19] M. Schmitz-Niderau, M. Schütze, Cracking and healing of oxide scales on Ti-Al alloys at 900 C, Oxid. Metals 52 (1999) 241e276. [20] K.S. Chan, A mechanics-based approach to cyclic oxidation, Metall. Mater. Trans. 28A (1997) 11. [21] D.P. Whittle, Spalling of protective oxide scales, Oxid. Metals 4 (1972) 9. [22] C.-O. Moon, S.-B. Lee, Analysis on failures of protective-oxide layers and cyclic oxidation, Oxid. Metals 39 (1992) 13. [23] U. Herold-Schmidt, B. Opolka, S. Schwanted, Prakt. Metallogr. 30 (1993) 7. [24] S.A. Kekare, P.B. Aswath, Oxidation of TiAl based intermetallics, J. Mater. Sci. 32 (1997) 2485. [25] B. Champin, L. Graff, M. Armand, Beranger, C. Coddet, Oxidation of titanium alloys at temperatures used in turbine engines, J. Less Common Met. 69 (1980) 20. [26] S. Becker, M. Schorr, M. Schutze, Mechanism of isothermal oxidation of the intermetallic TiAl and TiAl alloys, Oxid. Metals 38 (1992) 425e464. [27] S.K. Varma, A. Chan, R.N. Mahapatra, Static and cyclic oxidation of Ti-44Al and Ti-44Al-xNb alloys, Oxid. Metals 55 (2001) 423e435.
M. Naveed et al. / Journal of Alloys and Compounds 691 (2017) 489e497 €tter, M.F. Stroosnijder, The isothermal and [28] V.A.C. Haanappel, J.D. Sunderko cyclic high temperature oxidation behaviour of Tie48Ale2Mne2Nb compared with Tie48Ale2Cre2Nb and Tie48Ale2Cr, Intermetallics 7 (1999) 529e541. € tter, H.J. Schmutzler, V.A.C. Haanappel, R. Hofman, W. Glatz, [29] J.D. Sunderko H. Clemens, M.F. Stroosnijder, The high-temperature oxidation behaviour of Ti-47Al-2Cr-0.2Si and Ti-48Al-2Cr-2Nb compared with Ti-48Al-2Cr, Intermetallics 5 (1997) 525e534. [30] M.J. Weimer, T.J. Kelly, TiAl alloy 48Al-2Nb-2Cr, material database and application status, in: 3rd International Workshop on Gamma TiAl Alloys, Bamberg, Germany, 2006. [31] Y. Shen, X. Ding, F. Wang, Y. Tan, J.M. Yang, High temperature oxidation behaviour of Ti-Al-Nb ternary alloys, J. Mater. Sci. 39 (2004) 65e88. [32] Y. Shida, H. Anada, Role of W,Mo,Nb and Si on TiAl in air at high temperatures, Mater. Trans. 35 (1994) 626e630. [33] Y. Shida, H. Anada, Role of W, Mo, Nb and Si on oxidation of TiAl in air at high temperatures, materials transactions, JIM 35 (1994) 623e631. [34] C. Lang, M. Schütze, The initial stages in the oxidation of TiAl, Mater. Corros. (1997) 13e22. [35] M. Yamaguchi, H. Inui, TiAl compounds for structural applications, in: R. Darolia, J.J. Lewandowski, C.T. Liu, P.L. Martin, D.B. Miracle, M.V. Nathal (Eds.), TMS, Structural Intermetallics, Warendale PA, 1993, pp. 127e142. [36] G.H. Meier, Fundamental of the oxidation of high-temperature intermetallics, in: T. Grobstein, J. Doychak (Eds.), TMS, Oxidation of High-temperature Intermetallics, Wallendale, PA, 1988, pp. 1e16.
497
[37] M.-R. Yang, S.-K. Wu, Oxidation resistance improvement of TiAl intermetallics using surface modification, Bulletin of the College of Engineering, N.T.U (2003) 3e19. € tter, W. Glatz, H. Clemens, [38] V.A.C. Haanappel, R. Hofmann, J.D. Sunderko M.F. Stroonijder, The influence of microstructure on the Isothermal and cyclic oxidation behavior of Ti-48Al-2Cr at 800 C, Oxid. Metals 48 (1997) 263. [39] C.-J. Zhan, T.-H. Yu, C.-H. Koo, Effects of high niobium addition on the microstructure and high temperature properties of Ti-40Al-xNb, Mater. Trans. 47 (2006) 2589e2593. [40] P. Perez, J.J.A, G. Frommeyer, P. Adeva, The influence of the alloy microstructure on the oxidation behavior of Ti-46Al-1Cr-0.2Si, Oxid. Metals 53 (2000) 99e124. [41] F.-S. Sun, C.-X. Cao, S.-E. Kim, Y.-T. Lee, M.-G. Yan, Alloying mechanism of beta stabilizers in a TiAl alloy, Metall. Mater. Trans. A 23A (2001) 1573e1589. [42] J.D.H. Paul, F. Appel, R. Wagner, The compression behaviour of niobium alloyed g-titanium alumindies, Acta Mater. 46 (1998) 1075e1085. [43] A. Gil, J. Hoven, E. Wallura, W.J. Quadakkers, The effect of microstructure on the oxidation behaviour of TiAl-based intermetallics, Corros. Sci. 3 (1993) 615e630. [44] H.F. Chladil, H. Clemens, V.G.A. Otto, S. Kremmer, A. Bartels, R. Gerling, Charakterisierung einer beta-erstarrenden gamma-TiAl-Basislegierung, BHM 151 (9) (2006) 356e361. [45] F. Dettenwanger, M. Shütze, Isothermal oxidation of a2-Ti3Al, Oxid. Metals 54 (1999) 121e138.