TiAl composites at 900°C in static air

International Journal of Minerals, Metallurgy and Materials Volume 16, Number 3, June 2009, Page 339

Materials

Oxidation behavior of in-situ Al2O3/TiAl composites at 900°C in static air Tao-tao Ai1), Fen Wang2), and Xiao-ming Feng1) 1) Department of Materials Science and Engineering, Shaanxi University of Technology, Hanzhong 723003, China 2) School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China (Received 2008-06-17)

Abstract: In-situ Al2O3/TiAl composites were successfully synthesized from the starting powders of Ti, Al, TiO2 and Nb2O5. The oxidation behavior of the composites at 900°C in static air was investigated. The results indicate that the composite samples present a much lower oxidation mass gain. Under long-time intensive oxidation exposure, the formed oxide scale is multi-layer. The formation of the outer TiO2 layer is fine and dense, the internal Al2O3 scale has good adhesiveness with the outer TiO2 scale, and the TiO2+Al2O3 mixed layer forming the protective oxide scale is favorable for the improvement of oxidation resistance. It is believed that the incorporation of Al2O3 particulates into the metal matrix decreases the coefficient of thermal expansion of the substrate, and forms a local three-dimensional network structure that can hold the oxide scale. The formation of the oxide scale with finer particle size, stronger adherence, less micro-defects and slower growth rate can contribute to the improvement of oxidation resistance. Nb element plays an important role in reducing the internal oxidation action of the materials, restraining the growth of TiO2 crystals and promoting the stable formation of the Al2O3-riched layer, which is beneficial to improve the oxidation properties. Key words: intermetallic matrix composites; alumina; oxidation resistance; scale spallation; high temperature

[This work was financially supported by the Special Program of the Education Bureau of Shaanxi Province of China (No.08JK240) and the Breeding Program for Provincial Level Key Research Base of Shaanxi University of Technology (No.SLGJD0806).]

1. Introduction Intermetallic compounds have become a subject of intensive research with the aim of developing new materials that can be used in a wide range of high temperature applications [1-2]. Particular attention has been paid to aluminides, especially -TiAl, which is one of the most promising candidates owing to its low density, high melting point, excellent corrosion and oxidation resistance compared with Ti3Al ( 2) and orthorhombic (Ti2AlNb) classes of aluminides. However, the major problems limiting the practical use of this compound are its low ductility and fracture toughness at ambient temperature and poor oxidation resistance above 800°C [3-5]. To improve these properties, Ti2AlC, SiC and Al2O3 fiber reinforced intermetallic matrix composites were investigated [6-9]. Particulates such as TiB, TiC, TiN and Si3N4 reinforced composites were studied recently due to their isotropic property, amenability of component forming Corresponding author: Tao-tao Ai, E-mail: [email protected] © 2009 University of Science and Technology Beijing. All rights reserved.

and low cost [10]. It was also reported that the addition of particles could exert influences on the oxidation resistance of the composites. Such a composite with good oxidation resistance would extend its potential for engine component applications, such as high pressure compressors, turbines and some combustors. For example, Lee et al. [11-12] found that the addition of SiC or TiB2 could decrease the oxidation rate of Ti3Al and TiAl. Ti3Al-based in-situ composites produced by sintering of mechanically milled Al/TiO2 powders had an oxidation mass gain close to Nb-alloyed Ti3Al, and much lower than the cast Ti3Al alloy [13-15]. In the present work, in-situ Al2O3/TiAl intermetallic matrix composites have been synthesized [16-17]. The high temperature oxidation behavior of these composites in static air will be discussed in this article.

2. Experiment Al2O3/TiAl composites were prepared by sintering Also available online at www.sciencedirect.com

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of Ti/Al/TiO2/Nb2O5 composite powders. The basic composition (wt%) was corresponding to 43.9Ti38.6Al-17.5TiO2 with 2wt%, 6wt% and 10wt% Nb2O5 content, respectively. The specific sintering process and the morphology of the composites have been published elsewhere [16-17]. The as-sintered samples were cut into rectangular samples with a dimension of 7 mm×8 mm×3 mm before oxidation, all surfaces were ground successively by 700-grit SiC paper, cleaned with alcohol, and followed by blow-drying in hot air. The specimens were held in alumina crucibles which had been heated at a higher temperature to achieve a constant mass. Cyclic-oxidation tests were carried out in a furnace in ambient air. After desired amount of time (10 h) at 900°C, the specimens were pulled out from the crucibles, cooled in air, and then weighed with an electronic balance with an accuracy of 0.1 mg. Oxidating in the furnace and cooling in ambient air as one cycle proceeded for 12 times. The total oxidation mass gain could be accurately measured with time. The surface morphologies and polished cross-sections of the oxidized samples were studied with a scanning electron microscope (JSM-6700F,

JEOL) equipped with a dispersive X-ray spectrometer (EDS). The phases in the oxide scales were also analyzed using an X-ray diffractometer (D/max 2000PC) with Cu K radiation.

3. Results 3.1. Characterization of the composites In our previous work [16-17], the as-sintered composites consisted of -TiAl, 2-Ti3Al and Al2O3 as three major phases, as well as a small amount of NbAl3 phase. Fig. 1 represents the polished surface morphologies of the Al2O3/TiAl in-situ composites. The dark phase detected to mainly contain Al and O is corresponding to Al2O3. The bright phase is corresponding to the intermetallic phase. Oxygen can be occasionally detected by EDS on the thicker lamellar phase. However, the content appears to be low. According to EDS analysis, this phase is -TiAl. The darker lamellar is too thin to quantitative EDS analyze. Qualitatively, this phase is more Ti-riched and contains some oxygen, which apparently corresponds to the 2-Ti3Al phase. With increasing Nb2O5 content, the composites are remarkably grain-refined, as shown in Fig. 1(c), particularly Al2O3 particulates, which dispersed and uniformly distributed in the whole area.

Fig. 1. Polished surface morphologies of Al2O3/TiAl in-situ composites with different Nb2O5 contents: (a) 2wt%; (b) 6wt%; (c) 10wt%.

3.2. Oxidation kinetics Cyclic-oxidation kinetics at 900°C is presented in Fig. 2. At the beginning of 40 h exposure in air, the mass gain of the composites with 6wt% and 10wt% Nb2O5 increases fast, and then levels off in the following exposure with a very low oxidation rate; the oxidation behavior of the composite samples appear to obey parabolic rate laws. In contrast, the mass gain of the composite with 2wt% Nb2O5 increases dramatically after 40 h oxidation, and the oxidation kinetics of the composite changes into a linear behavior. In a word, the mass gain of the composite with 10wt% Nb2O5 is lower than other composites. The observations above indicate that the increase in Nb2O5 content is favorable for the improvement of

oxidation resistance of Al2O3/TiAl composites at high temperature. The effects of Nb2O5 will be discussed later. 3.3. Phase constituents X-ray diffraction patterns of the oxide scales formed on the samples after exposed in air at 900°C for 120 h are shown in Fig. 3. As can be seen clearly in Fig. 3, the oxide scales formed on the samples are mainly composed of TiO2 and Al2O3 phases. But it is also found that -TiAl and layered ternary nitride phases (Ti2AlN) only exist in the sample with 10wt% Nb2O5. 3.4. Surface and cross-section morphologies After exposed at 900°C for 120 h in air, the outer

T.T. Ai et al., Oxidation behavior of in-situ Al2O3/TiAl composites at 900°C in static air

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surface of the composites is covered with thick rutile crystals, as illustrated in Fig. 4. With increasing Nb2O5 content, the grain size decreases significantly with an average size of 3 μm, as shown in Fig. 4(b). It seems that the Nb2O5 content in the present composites may play a more important role in improving the oxidation resistance.

Fig. 2. Cyclic-oxidation mass gain of Al2O3/TiAl in-situ composites at 900°C in air with different Nb2O5 contents.

Fig. 4. Surface morphologies of Al2O3/TiAl in-situ composites after oxidation at 900°C in air for 120 h: (a) 2wt% Nb2O5; (b) 10wt% Nb2O5.

Fig. 3. X-ray diffraction patterns of the oxide scales formed on the samples after exposed at 900°C for 120 h.

The cross-section morphologies of Al2O3/TiAl in-situ composites with 10wt% Nb2O5 content after oxidizing at 900°C in air for 120 h are presented in Fig. 5. The cross-section shows a multi-layered scale composed of alternative rutile and alumina sub-layers with a total thickness of about 20 μm. The top scale is mainly consisted of rutile with a thickness of 5 μm, while the inner part shows smaller grains with high Al content as found by EDS analysis corresponding to an Al2O3 scale with a thickness of about 3 μm. The secondary inner is corresponding to a mixed Al2O3/TiO2 scale with a thickness of about 5 μm. Near the substrate, cross-section micrograph shows a relatively loose layer, and micro- and macro-pores remain on this layer, which is a transition layer and transfers from the Al2O3/TiO2 scale to substrate.

Fig. 5. Cross-section morphologies of Al2O3/TiAl in-situ composites with 10wt% Nb2O5 after oxidation at 900°C in air for 120 h

4. Discussion 4.1. Effect of Nb2O5 As shown in Fig. 2, the composite sample with 10wt% Nb2O5 has the optimal oxidation resistance, it is severely related with the existence of 2 phase. As indicated earlier [16-17], with increasing Nb2O5 content, the 2 content of the composites increased gradually, and formed + 2 lamellar structure, as shown in

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Fig. 1. The existence of proper 2 phase can increase the stability of Al2O3, accordingly improve the oxidation resistance of the composites, which is in good agreement with the results in Ref. [18]. Moreover, the -TiAl and layered ternary nitride phases (Ti2AlN) only exist in the sample with 10wt% Nb2O5 content. These phenomena indicate that the oxide layer of the sample is thinner. Due to a unique combination of properties [19], including their ability to be readily machined and high activity of Al in Ti2AlN [20], some of these layered ternary nitrides accelerate forming Al2O3 scales in air despite their low Al content (comparable to or less than that of Ti3Al, which does not form Al2O3). Reduced oxygen permeability in these structures compared with the corresponding binary Ti3Al phases of equivalent Al content is likely a key contributor to their ability to form Al2O3. Hence, the existence of Ti2AlN phase is beneficial to reduce the outdiffusion of metal atoms and the indiffusion of oxygen atoms, and improve the oxidation resistance of the composites. In other words, Nb element plays an important role in reducing the internal oxidation action of the materials [21-22], restraining the growth of TiO2 crystals and promoting the stable formation of Al2O3-riched layer [23], which is beneficial to improve the oxidation properties. The results are in accordance with the cross-section morphologies in Fig. 5. EDS analysis indicates that from outer layer to inner, TiO2+Al2O3 mixed layer presents the transit of Al-riched oxide to Ti-riched oxide mixed layer. 4.2. Decrease of oxidation rate There are two key factors for protective scale formation: thermodynamic stability and protective scale continuity. During the oxidation stage, all thermodynamically stable oxides transformed by the alloy components will form nuclei, including rapidly oxidizing elements. In order to achieve optimal protective oxidation behavior, the most thermodynamically stable oxide of all the major base alloy components must be Al2O3. Under this constraint, if a continuous layer of the protective oxide can be established, the oxygen potential is reduced below the alloy/scale interface and at which the less-protective oxides are stable. Steady-state selective oxidation governed by Al2O3 formation then ensures. In practice, kinetic factors, not thermodynamic stability, nearly always determine whether or not a continuous scale of the protective oxide-forming element can be established. As can be seen clearly in Figs. 4 and 5, the formation of an external Al2O3 layer is not

achieved on the Al2O3/TiAl in-situ composite, but an inner continuous Al2O3 scale and a correspondingly relatively low oxidation rate are observed. To establish a continuous Al2O3 scale, the selection of alloying additions and the proposed mechanisms for their beneficial or detrimental effects on the oxidation behavior of intermetallics have often erroneously overemphasized the importance of thermodynamic stability. In other words, it is obviously that the in-situ synthesis of stable Al2O3 particles contributes to mass gain reduction partially since this oxide will not oxidize further. On the other hand, Al2O3 oxides offer potential protection if they are formed as a continuous layer in which the rates of metal and oxygen diffusion are sufficiently low, so that the composites grow at an acceptably slow rate. The oxidation process can only proceed at a reduced surface area, resulting in a lower apparent mass gain. Furthermore, it appears that the transition region between alumina particles and TiAl grains can provide additional heterogeneous nucleation sites for the formation of Al and Ti oxides, resulting in very fine oxide grains in the initial stage [24-25] (fine TiAl grains in the matrix will also increase the nucleation rate). This can be verified by Fig. 5 in which the underlying grains are extremely small. This will promote the formation of a protective oxide layer, sometimes even form Al2O3-riched layer. In addition, it can be seen that fewer micro-defects, such as pores and cracks, are present in the scale on the composite. An oxide scale with dense and strong adherence and less cracking feature can provide a better protection to the underlying substrate against the aggressive environment since the diffusion of reactants through this scale can be retarded. 4.3. Improvement of scale spallation resistance Significant improvement on the cracking/spallation resistance is the most encouraging result obtained with this composite. Spallation cannot be observed, implying that some effective mechanisms have played an active role. Large temperature change exists during the heating and cooling stages in the cyclic oxidation, resulting in thermal stresses in both the scale and metal substrate due to the mismatch of their coefficients of thermal expansion (CTE). Generally, the oxide scale experiences a compressive stress during cooling, resulting in scale fracture and spallation; whereas the substrate has a tensile stress [26]. It is generally accepted that these thermal stresses can be relieved through creep of the

T.T. Ai et al., Oxidation behavior of in-situ Al2O3/TiAl composites at 900°C in static air

oxide and the substrate, and cracking/detachment of the oxide scale. In Ref. [27], it can be seen that the smaller the difference between the CTEs, the smaller the thermal stress generated during temperature change. According to the data from Ref. [28-29], the CTE of TiAl-Al2O3 is calculated to be about 9.9×106 K1. This value is much lower than that of TiAl (12.6×106 K1), and close to that of Al2O3 and TiO2 (8.3×106 K-1 and 7.3×106 K1). This means that the thermal stress in the oxide scale formed on the composite should be significantly lower than that on the TiAl intermetallic matrix. On the other hand, it is observable that the oxide grain size formed on the Al2O3-TiAl in-situ composite is significantly smaller than that on the cast TiAl alloy due to the special material fabrication process, which combines in-situ reaction and hot-pressing sintering. It is believed that plastic deformation of oxide scales through high temperature creep can relieve the growth/thermal stresses partially, and the diffusion creep rate (İ0) is favored by small grain size according to the equation [30]: İ0 =

ıȍ B įD ˜ ( B1 D V  2 B ) d 2 kT d

(1)

where ı is the stress, ȍ the atomic volume, į the width of grain boundary, k the Boltzmann constant, d the average grain size, DV and DB the volume and grain boundary diffusion coefficients respectively, and B1 and B2 constants. It can be found that finer grains would promote a higher creep rate, relieve the stresses more effectively, decrease the possibility of scale cracking/detachment, and then improve the scale spallation resistance. The incorporated Al2O3 can also contribute to the improvement of scale spallation resistance. These Al2O3 particles which formed originally through in-situ reaction in the fine Al-TiO2 and Al-Nb2O5 composite powders seem to develop a local three-dimensional network in the matrix. It is observable that these particles exhibit good adhesion with the TiAl matrix before and after oxidation, and they also have strong connection with the thermally grown oxides (Fig. 5). Therefore, the alumina 3-D network can hold the matrix and the oxide together strongly.

5. Conclusion Al2O3/TiAl in-situ composites have been produced through sintering of Ti/Al/TiO2/Nb2O5 composite powders. Oxidation experiments were carried out at 900°C in air for 120 h to evaluate the oxidation resistance of these composites. The results show that the

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composite samples show a much lower oxidation mass gain. Under long-time intensive oxidation exposure, the formation of the outer TiO2 layer is fine and dense, the internal Al2O3 scale has good adhesiveness with the outer TiO2 scale, and the TiO2+Al2O3 multi-layer forming the protective oxide scale is favorable for the improvement of oxidation resistance. It is believed that the incorporation of alumina particulates into the metal matrix decreases the CTE of the substrate, and forms a local 3-D network structure that can hold the oxide scale. The formation of the oxide scale with finer particle size, stronger adherence, less micro-defects and slower growth rate can contribute to the improvement of oxidation resistance.

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