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High temperature oxidation behaviour of a TiAl–Al2O3 intermetallic matrix composite
Corrosion Science 46 (2004) 1997–2007 www.elsevier.com/locate/corsci
High temperature oxidation behaviour of a TiAl–Al2O3 intermetallic matrix composite Z.W. Li a, W. Gao
a,*
, D.L. Zhang b, Z.H. Cai
b
a
Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand b Department of Materials and Processes Engineering, The University of Waikato, Private Bag 3105, Hamilton, New Zealand Received 4 March 2003; accepted 9 October 2003 Available online 10 January 2004
Abstract A TiAl-based intermetallic matrix composite has been produced through sintering of mechanically milled Al/TiO2 composite powder. The composite contains 42–50 vol.% of a-Al2 O3 as the particulate reinforcement phase. Oxidation experiments were carried out at 800–900 C in air up to 500 h to evaluate its oxidation and scale spallation resistance. A cast Ti–50at.%Al alloy was also tested for comparison. The composite samples showed much lower oxidation mass gain than the cast alloy under all testing conditions. Moreover, the composite samples exhibited extremely strong scale spallation resistance. Spallation could never be recorded and observed even under long-time intensive cyclic oxidation exposure. Based on the kinetic and microstructural studies, the mechanisms for the improved oxidation and spallation resistance are discussed. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Metal matrix composites; C. Oxidation; Scale spallation
1. Introduction Titanium aluminides (based on a2 -Ti3 Al, TiAl3 , and especially c-TiAl) are attracting more and more attention recently due to their merits of low density, high specific strength, and relatively good properties at elevated temperatures. Therefore
*
Corresponding author. Tel.: +64-9-373-7599x88175; fax: +64-9-373-7463. E-mail address: [email protected] (W. Gao).
0010-938X/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2003.10.026
1998
Z.W. Li et al. / Corrosion Science 46 (2004) 1997–2007
they are being considered as prospective structural materials for applications in aerospace and automobile industries [1–3]. However, their practical applications are still hindered by the relatively poor room temperature mechanical properties, machinability, and oxidation resistance at high temperatures. The development of TiAl matrix composites is a promising way to cope these problems. SiC and Al2 O3 fibre reinforced intermetallic matrix composites (IMCs) are therefore being developed and tested [4–7]. Particulates such as TiB, TiC, TiN and Si3 N4 reinforced composites are being studied recently due to their isotropic property, amenability of component forming and low cost [8]. It was also reported that the addition of particles could exert influences on the oxidation resistance of the composites. For example, Lee et al. [9,10] found that additions of SiC or TiB2 could decrease the oxidation rate of Ti3 Al and TiAl. Previous work has demonstrated that Ti3 Al-based in situ composites can be produced by sintering of mechanically milled Al/TiO2 powder [11–13]. It was observed that the composite had an oxidation mass gain close to Nb alloyed Ti3 Al, much lower than the cast Ti3 Al alloy [14]. Additionally, this composite has superior cyclic oxidation resistance similar to its isothermal oxidation; no scale spallation could be observed. In a similar way, TiAl–Al2 O3 intermetallic matrix composites have also been synthesized [15]. This paper studies the high temperature oxidation behaviour of one of these composites.
2. Experimental The TiAl–Al2 O3 composite was prepared by pressureless sintering of an Al/TiO2 composite powder, which was produced by high-energy ball milling of TiO2 and Al powders. It consists of 42–50 vol.% of a-Al2 O3 particles, which formed through the in situ reaction between Al and TiO2 . Details on the sintering process and the microstructure of this composite will be published elsewhere [16]. The sintered bar was cut into rectangular samples with a dimension of 8 · 8 · 2 mm. Before oxidation, all surfaces were ground successively to 1200-grit SiC paper, cleaned with alcohol and acetone, and followed by blow-drying in hot air. Slow cyclic-oxidation tests were carried out in a horizontal tube furnace in ambient air. After desired amount of time (20 h), the specimens were pulled out of the hot zone, cooled in a desiccator, and then weighed with an electronic balance with an accuracy of 0.01 mg. Rapid cyclic-oxidation tests were performed in a vertical furnace. Oxidation time in the furnace and cooling in ambient air were 60 and 10 min respectively for one cycle, and controlled automatically by a time-step unit. Under both testing conditions, the specimens were held in quartz crucibles separately (the crucible had been heated at a higher temperature to achieve a constant mass). Therefore, all the oxide scale spalled could be collected in the crucible, and the total oxidation mass gain and amount of scale spallation could be accurately measured with time. The surface morphologies and polished cross-sections of the oxidised samples were studied with a high-resolution field emission gun scanning electron microscope
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(FEG-SEM, Phillips XL-30S) equipped with an energy dispersive X-ray spectrometer (EDS). The phases in the oxide scales were also analyzed using an X-ray diffractometer (Bruker D8) with Cu-Ka radiation.
3. Results 3.1. Characterization of the composite Fig. 1 represents the polished surface morphology of the TiAl–Al2 O3 in situ composite. The bright phase has a size of 15–30 lm, and was detected to contain mainly Al and O. Based on this information and XRD analysis, it was confirmed that this phase was a-Al2 O3 . The dark phase contains an average 47.8 at.% Al balanced with Ti. Oxygen could be occasionally detected by EDS on this phase. However, the content appeared to be low, generally below 1 at.%. This oxygen content is much lower than that detected in the previous Ti3 Al(O)–Al2 O3 composite, which had an average oxygen content of 10 at.% in the Ti3 Al phase. Again this
Fig. 1. Polished surface morphology of TiAl–Al2 O3 in situ composite; arrows show the transition region and bond between alumina particle and TiAl substrate.
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information and XRD analysis confirmed that this phase was TiAl. Along the interface of bright and dark phases, a narrow transition region could be observed. With EDS line scanning, concentration gradients of Ti, Al, and O were found. Cracks could not be observed in these regions, indicating good interface bonding probably formed by the in situ reaction of the composite powder. 3.2. Oxidation kinetics Slow cyclic-oxidation kinetics at 800 and 900 C are presented in Fig. 2. In the first 40 h of exposure at 800 C, the mass gain of the composite increased fast, then leveled off in the following exposure with a very low oxidation rate. In contrast, the mass gain of the cast Ti–50Al alloy increased dramatically after 25 h oxidation, which appeared to be related with the scale spallation as seen in the spallation result. After 280 h of exposure, the oxidation kinetics of the cast alloy changed into a linear behaviour, corresponding to the continuous scale spallation. At 900 C, the cast Ti–50Al alloy showed non-protective scaling behaviour. The oxidation kinetics basically followed a linear rate law, therefore leading to extremely large and irregular mass gains and scale spallation after 40 h oxidation. Oxidation behaviour of the composite sample appeared to obey a parabolic rate law; the mass gain was more than one order of magnitude lower than that of the cast alloy, with no scale spallation being detected at all.
Ti-50at.%Al
3.2 2.4 1.6 TiAl-Al2O3 Composite
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Mass Gain (mg/cm2)
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(d)
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Fig. 2. Slow cyclic-oxidation mass gain and amount of scale spalled of Ti–50at.%Al alloy and TiAl–Al2 O3 in situ composite at 800–900 C in air, (a)–(b) 800 C and (c)–(d) 900 C.
Z.W. Li et al. / Corrosion Science 46 (2004) 1997–2007
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Fig. 3. Rapid cyclic-oxidation mass gain and amount of scale spalled of Ti–50at.%Al alloy and TiAl– Al2 O3 in situ composite at 800–900 C in air, (a)–(b) 800 C and (c)–(d) 900 C.
Mass gain vs. time curves measured during the rapid cyclic-oxidation at 800 and 900 C are shown in Fig. 3. While the mass gains for the cast alloy and the composite were not too different at 800 C, the scale spallation was very different as shown in Fig. 3b. The composite specimen did not show any scale spallation up to 500 cycles. It was observed that the scale spallation of the cast alloy was in the form of small flakes, resulting in a relatively smooth spallation curve with no sudden increases. Rapid cyclic-oxidation behaviour at 900 C for both Ti–50Al and composite are similar to the results obtained during slow cyclic-oxidation test: huge mass gain and extensive spallation for the cast alloy, while significantly lower mass gain and no spallation for the composite. 3.3. Surface and cross-section morphologies After 500 h of exposure in air at 800 C, the outer surface of the cast Ti–50Al alloy was covered with thick rutile crystals. The polished cross-section shows a multilayered scale composed of alternative rutile and alumina sub-layers with a total thickness of about 120 lm (Fig. 4b). Micro-pores were present in the oxide scale with observable extensive cracks and spallation. The oxide scale formed on the composite did not cover the whole surface since the oxide only formed on the TiAl grains (Fig. 4a). The top scale consisted mainly of rutile, while the inner part showed smaller grains with high Al content as found by EDS analysis. Cross-section micrograph showed a relatively dense but not uniform layer (Fig. 4c).
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Fig. 4. Surface and cross-section morphologies of Ti–50at.%Al alloy and TiAl–Al2 O3 in situ composite after slow cyclic-oxidation at 800–900 C in air; (a) composite, 800 C, 500 h, (b) TiAl, 800 C, 500 h, (c) composite, 800 C, 500 h, (d) TiAl, 900 C, 400 h, showing the formation of thick and non-protective rutile scale on the surface, and (e) composite, 900 C, 400 h.
Severe scale cracking and spallation were observed on the cast Ti–50Al sample after 400 h oxidation at 900 C (Fig. 4d). On some locations, mixed layers with micro- and macro-pores still remained on the substrate. At the interface, internal oxidation zone (IOZ) with long needle-like Al2 O3 grains and oxygen-dissolved region could be clearly seen. This region has an average composition closed to the phase of Ti5 Al3 O2 (Z-phase) according to EDS analysis. The surface morphology of the oxidised composite appears similar to that at 800 C. The oxide scale covered the majority of the outer surface area. The scale also
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Fig. 5. Surface and cross-section morphologies of Ti–50at.%Al alloy and TiAl–Al2 O3 in situ composite after rapid cyclic-oxidation at 800–900 C in air; (a) TiAl, 800 C, 500 cycles, (b) composite, 800 C, 500 cycles, (c) TiAl, 900 C, 400 cycles, showing severe scale cracking and spallation, and (d) composite, 900 C, 400 cycles.
consisted mainly of rutile crystals but with increased sizes (Fig. 4e). Cross-section showed that the oxidation front penetrated into the matrix with a depth of 20 lm. The surface and cross-section morphologies of composite samples after rapid cyclic-oxidation at 800 and 900 C were basically the same as those after slow cyclicoxidation, so did the cast Ti–50Al at 900 C (Fig. 5). As indicated from the kinetic results obtained during 400 cycles at 800 C for the Ti–50Al alloy (Fig. 3a and b), the oxide scale on the cast alloy underwent severe spallation. The remained scale was thin after spallation; and long, horizontal cracks could be easily seen in the scale (Fig. 5a). 3.4. Summary of results In general, the TiAl–Al2 O3 in situ composite showed much lower oxidation mass gain than the cast Ti–50Al alloy under both oxidation conditions. The mass gain of the composite is 2–4 and more than 10 times lower than the cast TiAl for 800 and 900 C, respectively. The most significant result, however, is that all composite samples showed superior scale spallation resistance even under rapid cyclic-oxidation conditions up to 500 cycles, which is very important for practical engineering applications. The following section discusses the mechanisms by which the Al2 O3 particles
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in the composite reduce the oxidation rate and improve the scale integrity and spallation resistance.
4. Discussion Poor oxidation resistance of TiAl-based alloys above 800 C is a major obstacle for their practical applications as lightweight structural materials in aerospace and energy-conversion systems. Research has been very active in an effort to improve their oxidation resistance, including ternary- and/or quaternary-alloying additions, and application of various surface-coating systems [17]. The main approach is to promote the formation of a-Al2 O3 , therefore changing the property of the scale (usually a mixture of TiO2 + Al2 O3 ) and decreasing its growth rate. It was reported that additions of Mo, Nb, Si, and W were beneficial to the microstructural modification of the mixed oxide layer, while protective alumina layers could be developed on Cr- or Ag-containing alloy systems [18–20]. 4.1. Decreasing oxidation rate In the present study, formation of an external a-Al2 O3 layer was not achieved on the TiAl–Al2 O3 in situ composite under both oxidation conditions. However the composite exhibited clearly lower mass gains than that of the cast alloy. It is obvious that incorporation of the stable a-Al2 O3 particles contributes to the mass gain reduction partially; since this oxide will not oxidise further. 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 the alumina particles and TiAl grains could provide additional heterogeneous nucleation sites for the formation of Al and Ti oxides, resulting in very fine oxide grains in the initial stage [21,22] (the fine TiAl grains in the matrix will also increase the nucleation rate). This can be verified by Fig. 4a in which the underlying grains are seen extremely small. This will promote the formation of a protective oxide layer, sometimes even rich in Al2 O3 , on the surface between alumina particles (Fig. 5d). Our previous result on the oxidation of TiAl under low oxygen partial pressures indicated that this type of oxide scale could provide better oxidation protection in a certain time period [23]. In addition, compared with the oxide scale formed on the cast TiAl sample, it can be seen that fewer micro-defects, such as pores and cracks, were present in the scale on the composite. An oxide scale with dense, strong adherence and less cracking feature can provide better protection to the underlying substrate against the aggressive environment since the diffusion of reactants through this scale could be retarded. 4.2. Improving scale spallation resistance Significant improvement on the cracking/spallation resistance is the most encouraging result obtained with this composite. Spallation could never be observed
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under both oxidation conditions; and its kinetic and scaling behaviour are quite similar, implying that some effective mechanisms have played an active role. Large temperature changes exist during the heating and cooling stages in the cyclic oxidation, resulting in thermal stresses in both oxide 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 [24]. It is generally accepted that these thermal stresses could be relieved through creep of the oxide and the substrate, and cracking/detachment of the oxide scale. The thermal stresses generated in the oxide scale could be calculated with [25]: roxide ¼
Eoxide DT ðasub aoxide Þ Eoxide doxide ð1 mÞ 1 þ Esub dsub
ð4:1Þ
where E, a, m, d and DT are the Young’s modulus, CTE, Poisson’s ratio, thickness, and temperature change, respectively. The subscripts ‘‘oxide’’ and ‘‘sub’’ refer to the oxide scale formed and the unoxidised substrate. It can be seen that the smaller the difference between the CTEs, the smaller the thermal stress generated during temperature change. The actual CTE of the present TiAl–Al2 O3 in situ composite was not measured, but it might be calculated using the Turner equation, acomposite ¼
ðar Br Fr =qr Þ þ ðam Bm Fm =qm Þ ðBr Fr =qr Þ þ ðBm Fm =qm Þ
ð4:2Þ
where B, F , and q represent bulk modulus, mass fraction and density, respectively, while ‘‘r’’ and ‘‘m’’ refer to the reinforcement and matrix phases respectively. According to the data from literature [26–30], the CTE of the composite is calculated to be 9.9 · 106 K1 . This value is much lower than that of TiAl, 12.6 · 106 K1 , and close to that of a-Al2 O3 and TiO2 (8.3 · 106 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 TiAl– Al2 O3 in situ composite is significantly smaller than that on the cast TiAl alloy due to the special material fabrication process, which combines high-energy mechanical milling and powder compact sintering. It is believed that plastic deformation of oxide scale through high temperature creep could relieve the growth/thermal stresses partially, and the diffusion creep rate, e0 , is favored by small grain size as shown in the following equation [31]: rX B2 dDB 0 e ¼ 2 B1 D v þ ð4:3Þ d kT d where r is the stress, X is the atomic volume, d is the width of grain boundary, d is the average grain size, k is the Boltzmann constant, Dv and DB are the volume and grain boundary diffusion coefficients respectively, and B1 and B2 are constants. Thus, finer grains would promote higher creep rate, relieving the stresses more effectively,
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decreasing the possibility of scale cracking/detachment, and then improving the scale spallation resistance. The incorporated a-Al2 O3 can also contribute to the improvement of scale spallation resistance. Firstly, these a-Al2 O3 particles formed originally through in situ reaction in the fine Al–TiO2 composite powder, seem to develop a local threedimensional network in the matrix [14]. It was 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 (Figs. 4c, 5b and d) (this might be highly related with the gradient compositional and structural changes in the original transition region). Therefore, the alumina 3-D network can hold the matrix and the oxide together strongly. Secondly, it can be seen that micro-pores and micro-cracks play an important role in the mechanical failure of the oxide scale, especially under rapid thermal cycling. The cracks tend to propagate laterally (sometimes also vertically), resulting in detachment of the scale from the inner part, and exhibiting scale spallation, sometimes with large pieces. It appears that the alumina particles in the scale can change the layered structure and stop the propagation of cracks, therefore effectively inhibiting the development of micro-cracks in the scale. 5. Conclusions A TiAl-based intermetallic matrix composite is fabricated through the in situ reaction of mechanically milled Al/TiO2 composite powder. This composite composed of TiAl and 42–50 vol.% of a-Al2 O3 particles. High temperature oxidation tests have been carried out at 800 and 900 C in air to evaluate its oxidation/scale spallation resistance in comparison with a cast Ti–50at.%Al alloy. The results showed that the composite had much lower oxidation rates than the cast alloy, especially at 900 C. The composite also exhibited superior scale spallation resistance even under intensive thermal cycling conditions. It is believed that the incorporation of alumina particulates into the metal matrix decreases the CTE of the substrate (thereby reducing the difference of the CTE between the oxide and substrate), forms a local 3-D network structure that can hold the oxide scale, and inhibit the propagation of micro-cracks in the scale. Formation of oxide scale with finer particle size, stronger adherence, less micro-defects, slower growth rate, and higher plastic deformation ability could contribute to the improvement of oxidation and scale spallation resistance.
Acknowledgements This research is partially supported by the Foundation for Research, Science and Technology (FRST), New Zealand through a New Economy Research Fund (NERF) grant. ZWL would like to thank the University of Auckland and FRST Postdoctoral Fellowships. The authors would also like to express their thanks to the
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technical staff members at the Department and the Research Center for Surfaces and Materials Science (RCSMS) at the University of Auckland for various help during this study. Titanox Development Limited is the licenser of the composite materials production technology.
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F.H. Froes, C. Suryanarayana, D. Eliezer, J. Mater. Sci. 27 (1992) 5113. E.A. Loria, Intermetallics 8 (2000) 1339. H. Clemens, H. Kestler, Adv. Eng. Mater. 2 (2000) 551. C.C. Koch, Mater. Sci. Eng. A 244 (1998) 39. C.M. Ward-Close, R. Minor, P.J. Doorbar, Intermetallics 4 (1996) 217. C.M. Ward-Close, L. Chandrasekaran, J.G. Robertson, S.P. Godfrey, D.P. Murgatroyde, Mater. Sci. Eng. A 263 (1999) 314. S. Ranganath, J. Mater. Sci. 32 (1997) 1. T.M.T. Godfrey, P.S. Goodwin, C.M. Ward-Close, Adv. Eng. Mater. 2 (2000) 85. D.B. Lee, J.H. Park, Y.H. Park, Y.J. Kim, Mater. Trans. JIM 38 (1997) 306. D.B. Lee, M.H. Kim, C.W. Yang, S.H. Lee, M.H. Yang, Y.J. Kim, Oxid. Met. 56 (2001) 215. S.P. Gaus, M.P. Haemer, H.M. Chan, H.S. Caram, J. Bruhn, N. Claussen, J. Am. Ceram. Soc. 83 (2000) 1606. D.L. Zhang, Z.H. Cai, M. Newby, Mater. Technol. Adv. Mater. 18 (2003) 94. Z.H. Cai, PhD Thesis, Waikato University, New Zealand, 2002. W. Gao, Z.W. Li, D.L. Zhang, Oxid. Met. 57 (2002) 99. D.L. Zhang, Z.H. Cai, Mat. Sci. Forum. 437–438 (2003) 297. Z.H. Cai, D.L. Zhang, Mat. Sci. Forum. 437–438 (2003) 451. G. Welsch, P.D. Desal, Oxidation and Corrosion of Intermetallic Alloys, Purdue University, West Lafayette, Indiana, 1996. M.P. Brady, J.L. Smialek, J. Smith, D.L. Humphrey, Acta Mater. 45 (1997) 2357. M.P. Brady, J.L. Smialek, J. Smith, D.L. Humphrey, Acta Mater. 45 (1997) 2371. V. Shemet, A.K. Tyagi, J.S. Becker, P. Lersch, L. Singheiser, W.J. Quadakkers, Oxid. Met. 54 (2000) 211. Y.D. He, Z.W. Li, H.B. Qi, W. Gao, Mater. Res. Innov. 1 (1997) 157. Y.D. He, F.H. Stott, Corros. Sci. 36 (1994) 1869. Z.W. Li, W. Gao, Y.D. He, High Temp. Mater. Process. 21 (2002) 35. P. Kofstad, Oxid. Met. 24 (1985) 265. H.E. Evans, Int. Mater. Rev. 40 (1995) 1. J.H. Westbrook, R.L. Fleischer, Intermetallic Compounds, Principles and Practice, vol. 1, John Wiley & Sons, Chichester, 1995 (Chapter 37). G. Sauthoff, Intermetallics, VCH, Weinheim, 1995. G.L. Chen, J.P. Lin, Ordered Intermetallic Compounds for Structural Application Basis of Physical Metallurgy (in Chinese), Metallurgical Press, Beijing, 1999. R.G. Munro, J. Am. Ceram. Soc. 80 (8) (1997) 1919. G. Frommeyer, C. Derder, J. Phys. III France 7 (1997) 2393. P. Kofstad, High Temperature Corrosion, Elsevier Applied Science, London and New York, 1988.
High temperature oxidation behaviour of a TiAl–Al2O3 intermetallic matrix composite Z.W. Li a, W. Gao
a,*
, D.L. Zhang b, Z.H. Cai
b
a
Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand b Department of Materials and Processes Engineering, The University of Waikato, Private Bag 3105, Hamilton, New Zealand Received 4 March 2003; accepted 9 October 2003 Available online 10 January 2004
Abstract A TiAl-based intermetallic matrix composite has been produced through sintering of mechanically milled Al/TiO2 composite powder. The composite contains 42–50 vol.% of a-Al2 O3 as the particulate reinforcement phase. Oxidation experiments were carried out at 800–900 C in air up to 500 h to evaluate its oxidation and scale spallation resistance. A cast Ti–50at.%Al alloy was also tested for comparison. The composite samples showed much lower oxidation mass gain than the cast alloy under all testing conditions. Moreover, the composite samples exhibited extremely strong scale spallation resistance. Spallation could never be recorded and observed even under long-time intensive cyclic oxidation exposure. Based on the kinetic and microstructural studies, the mechanisms for the improved oxidation and spallation resistance are discussed. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Metal matrix composites; C. Oxidation; Scale spallation
1. Introduction Titanium aluminides (based on a2 -Ti3 Al, TiAl3 , and especially c-TiAl) are attracting more and more attention recently due to their merits of low density, high specific strength, and relatively good properties at elevated temperatures. Therefore
*
Corresponding author. Tel.: +64-9-373-7599x88175; fax: +64-9-373-7463. E-mail address: [email protected] (W. Gao).
0010-938X/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2003.10.026
1998
Z.W. Li et al. / Corrosion Science 46 (2004) 1997–2007
they are being considered as prospective structural materials for applications in aerospace and automobile industries [1–3]. However, their practical applications are still hindered by the relatively poor room temperature mechanical properties, machinability, and oxidation resistance at high temperatures. The development of TiAl matrix composites is a promising way to cope these problems. SiC and Al2 O3 fibre reinforced intermetallic matrix composites (IMCs) are therefore being developed and tested [4–7]. Particulates such as TiB, TiC, TiN and Si3 N4 reinforced composites are being studied recently due to their isotropic property, amenability of component forming and low cost [8]. It was also reported that the addition of particles could exert influences on the oxidation resistance of the composites. For example, Lee et al. [9,10] found that additions of SiC or TiB2 could decrease the oxidation rate of Ti3 Al and TiAl. Previous work has demonstrated that Ti3 Al-based in situ composites can be produced by sintering of mechanically milled Al/TiO2 powder [11–13]. It was observed that the composite had an oxidation mass gain close to Nb alloyed Ti3 Al, much lower than the cast Ti3 Al alloy [14]. Additionally, this composite has superior cyclic oxidation resistance similar to its isothermal oxidation; no scale spallation could be observed. In a similar way, TiAl–Al2 O3 intermetallic matrix composites have also been synthesized [15]. This paper studies the high temperature oxidation behaviour of one of these composites.
2. Experimental The TiAl–Al2 O3 composite was prepared by pressureless sintering of an Al/TiO2 composite powder, which was produced by high-energy ball milling of TiO2 and Al powders. It consists of 42–50 vol.% of a-Al2 O3 particles, which formed through the in situ reaction between Al and TiO2 . Details on the sintering process and the microstructure of this composite will be published elsewhere [16]. The sintered bar was cut into rectangular samples with a dimension of 8 · 8 · 2 mm. Before oxidation, all surfaces were ground successively to 1200-grit SiC paper, cleaned with alcohol and acetone, and followed by blow-drying in hot air. Slow cyclic-oxidation tests were carried out in a horizontal tube furnace in ambient air. After desired amount of time (20 h), the specimens were pulled out of the hot zone, cooled in a desiccator, and then weighed with an electronic balance with an accuracy of 0.01 mg. Rapid cyclic-oxidation tests were performed in a vertical furnace. Oxidation time in the furnace and cooling in ambient air were 60 and 10 min respectively for one cycle, and controlled automatically by a time-step unit. Under both testing conditions, the specimens were held in quartz crucibles separately (the crucible had been heated at a higher temperature to achieve a constant mass). Therefore, all the oxide scale spalled could be collected in the crucible, and the total oxidation mass gain and amount of scale spallation could be accurately measured with time. The surface morphologies and polished cross-sections of the oxidised samples were studied with a high-resolution field emission gun scanning electron microscope
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(FEG-SEM, Phillips XL-30S) equipped with an energy dispersive X-ray spectrometer (EDS). The phases in the oxide scales were also analyzed using an X-ray diffractometer (Bruker D8) with Cu-Ka radiation.
3. Results 3.1. Characterization of the composite Fig. 1 represents the polished surface morphology of the TiAl–Al2 O3 in situ composite. The bright phase has a size of 15–30 lm, and was detected to contain mainly Al and O. Based on this information and XRD analysis, it was confirmed that this phase was a-Al2 O3 . The dark phase contains an average 47.8 at.% Al balanced with Ti. Oxygen could be occasionally detected by EDS on this phase. However, the content appeared to be low, generally below 1 at.%. This oxygen content is much lower than that detected in the previous Ti3 Al(O)–Al2 O3 composite, which had an average oxygen content of 10 at.% in the Ti3 Al phase. Again this
Fig. 1. Polished surface morphology of TiAl–Al2 O3 in situ composite; arrows show the transition region and bond between alumina particle and TiAl substrate.
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information and XRD analysis confirmed that this phase was TiAl. Along the interface of bright and dark phases, a narrow transition region could be observed. With EDS line scanning, concentration gradients of Ti, Al, and O were found. Cracks could not be observed in these regions, indicating good interface bonding probably formed by the in situ reaction of the composite powder. 3.2. Oxidation kinetics Slow cyclic-oxidation kinetics at 800 and 900 C are presented in Fig. 2. In the first 40 h of exposure at 800 C, the mass gain of the composite increased fast, then leveled off in the following exposure with a very low oxidation rate. In contrast, the mass gain of the cast Ti–50Al alloy increased dramatically after 25 h oxidation, which appeared to be related with the scale spallation as seen in the spallation result. After 280 h of exposure, the oxidation kinetics of the cast alloy changed into a linear behaviour, corresponding to the continuous scale spallation. At 900 C, the cast Ti–50Al alloy showed non-protective scaling behaviour. The oxidation kinetics basically followed a linear rate law, therefore leading to extremely large and irregular mass gains and scale spallation after 40 h oxidation. Oxidation behaviour of the composite sample appeared to obey a parabolic rate law; the mass gain was more than one order of magnitude lower than that of the cast alloy, with no scale spallation being detected at all.
Ti-50at.%Al
3.2 2.4 1.6 TiAl-Al2O3 Composite
0.8
2.5 2.0 1.5 1.0 0.5
0
80
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240 320 Time (hrs)
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36 24 TiAl-Al2O 3 Composite
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12
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TiAl-Al2O3 Composite
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80 60 40 20 TiAl-Al2O3 Composite
0
0
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(c)
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Mass Gain (mg/cm2)
4.0
80
160 240 Time (hrs)
320
400
0
(d)
80
160 240 Time (hrs)
320
400
Fig. 2. Slow cyclic-oxidation mass gain and amount of scale spalled of Ti–50at.%Al alloy and TiAl–Al2 O3 in situ composite at 800–900 C in air, (a)–(b) 800 C and (c)–(d) 900 C.
Z.W. Li et al. / Corrosion Science 46 (2004) 1997–2007
Ti-50at.%Al
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TiAl-Al2O3 Composite
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0 0
(c)
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160 240 No. of 1hr-cycle
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0
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80
160 240 No. of 1hr-cycle
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Fig. 3. Rapid cyclic-oxidation mass gain and amount of scale spalled of Ti–50at.%Al alloy and TiAl– Al2 O3 in situ composite at 800–900 C in air, (a)–(b) 800 C and (c)–(d) 900 C.
Mass gain vs. time curves measured during the rapid cyclic-oxidation at 800 and 900 C are shown in Fig. 3. While the mass gains for the cast alloy and the composite were not too different at 800 C, the scale spallation was very different as shown in Fig. 3b. The composite specimen did not show any scale spallation up to 500 cycles. It was observed that the scale spallation of the cast alloy was in the form of small flakes, resulting in a relatively smooth spallation curve with no sudden increases. Rapid cyclic-oxidation behaviour at 900 C for both Ti–50Al and composite are similar to the results obtained during slow cyclic-oxidation test: huge mass gain and extensive spallation for the cast alloy, while significantly lower mass gain and no spallation for the composite. 3.3. Surface and cross-section morphologies After 500 h of exposure in air at 800 C, the outer surface of the cast Ti–50Al alloy was covered with thick rutile crystals. The polished cross-section shows a multilayered scale composed of alternative rutile and alumina sub-layers with a total thickness of about 120 lm (Fig. 4b). Micro-pores were present in the oxide scale with observable extensive cracks and spallation. The oxide scale formed on the composite did not cover the whole surface since the oxide only formed on the TiAl grains (Fig. 4a). The top scale consisted mainly of rutile, while the inner part showed smaller grains with high Al content as found by EDS analysis. Cross-section micrograph showed a relatively dense but not uniform layer (Fig. 4c).
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Fig. 4. Surface and cross-section morphologies of Ti–50at.%Al alloy and TiAl–Al2 O3 in situ composite after slow cyclic-oxidation at 800–900 C in air; (a) composite, 800 C, 500 h, (b) TiAl, 800 C, 500 h, (c) composite, 800 C, 500 h, (d) TiAl, 900 C, 400 h, showing the formation of thick and non-protective rutile scale on the surface, and (e) composite, 900 C, 400 h.
Severe scale cracking and spallation were observed on the cast Ti–50Al sample after 400 h oxidation at 900 C (Fig. 4d). On some locations, mixed layers with micro- and macro-pores still remained on the substrate. At the interface, internal oxidation zone (IOZ) with long needle-like Al2 O3 grains and oxygen-dissolved region could be clearly seen. This region has an average composition closed to the phase of Ti5 Al3 O2 (Z-phase) according to EDS analysis. The surface morphology of the oxidised composite appears similar to that at 800 C. The oxide scale covered the majority of the outer surface area. The scale also
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Fig. 5. Surface and cross-section morphologies of Ti–50at.%Al alloy and TiAl–Al2 O3 in situ composite after rapid cyclic-oxidation at 800–900 C in air; (a) TiAl, 800 C, 500 cycles, (b) composite, 800 C, 500 cycles, (c) TiAl, 900 C, 400 cycles, showing severe scale cracking and spallation, and (d) composite, 900 C, 400 cycles.
consisted mainly of rutile crystals but with increased sizes (Fig. 4e). Cross-section showed that the oxidation front penetrated into the matrix with a depth of 20 lm. The surface and cross-section morphologies of composite samples after rapid cyclic-oxidation at 800 and 900 C were basically the same as those after slow cyclicoxidation, so did the cast Ti–50Al at 900 C (Fig. 5). As indicated from the kinetic results obtained during 400 cycles at 800 C for the Ti–50Al alloy (Fig. 3a and b), the oxide scale on the cast alloy underwent severe spallation. The remained scale was thin after spallation; and long, horizontal cracks could be easily seen in the scale (Fig. 5a). 3.4. Summary of results In general, the TiAl–Al2 O3 in situ composite showed much lower oxidation mass gain than the cast Ti–50Al alloy under both oxidation conditions. The mass gain of the composite is 2–4 and more than 10 times lower than the cast TiAl for 800 and 900 C, respectively. The most significant result, however, is that all composite samples showed superior scale spallation resistance even under rapid cyclic-oxidation conditions up to 500 cycles, which is very important for practical engineering applications. The following section discusses the mechanisms by which the Al2 O3 particles
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in the composite reduce the oxidation rate and improve the scale integrity and spallation resistance.
4. Discussion Poor oxidation resistance of TiAl-based alloys above 800 C is a major obstacle for their practical applications as lightweight structural materials in aerospace and energy-conversion systems. Research has been very active in an effort to improve their oxidation resistance, including ternary- and/or quaternary-alloying additions, and application of various surface-coating systems [17]. The main approach is to promote the formation of a-Al2 O3 , therefore changing the property of the scale (usually a mixture of TiO2 + Al2 O3 ) and decreasing its growth rate. It was reported that additions of Mo, Nb, Si, and W were beneficial to the microstructural modification of the mixed oxide layer, while protective alumina layers could be developed on Cr- or Ag-containing alloy systems [18–20]. 4.1. Decreasing oxidation rate In the present study, formation of an external a-Al2 O3 layer was not achieved on the TiAl–Al2 O3 in situ composite under both oxidation conditions. However the composite exhibited clearly lower mass gains than that of the cast alloy. It is obvious that incorporation of the stable a-Al2 O3 particles contributes to the mass gain reduction partially; since this oxide will not oxidise further. 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 the alumina particles and TiAl grains could provide additional heterogeneous nucleation sites for the formation of Al and Ti oxides, resulting in very fine oxide grains in the initial stage [21,22] (the fine TiAl grains in the matrix will also increase the nucleation rate). This can be verified by Fig. 4a in which the underlying grains are seen extremely small. This will promote the formation of a protective oxide layer, sometimes even rich in Al2 O3 , on the surface between alumina particles (Fig. 5d). Our previous result on the oxidation of TiAl under low oxygen partial pressures indicated that this type of oxide scale could provide better oxidation protection in a certain time period [23]. In addition, compared with the oxide scale formed on the cast TiAl sample, it can be seen that fewer micro-defects, such as pores and cracks, were present in the scale on the composite. An oxide scale with dense, strong adherence and less cracking feature can provide better protection to the underlying substrate against the aggressive environment since the diffusion of reactants through this scale could be retarded. 4.2. Improving scale spallation resistance Significant improvement on the cracking/spallation resistance is the most encouraging result obtained with this composite. Spallation could never be observed
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under both oxidation conditions; and its kinetic and scaling behaviour are quite similar, implying that some effective mechanisms have played an active role. Large temperature changes exist during the heating and cooling stages in the cyclic oxidation, resulting in thermal stresses in both oxide 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 [24]. It is generally accepted that these thermal stresses could be relieved through creep of the oxide and the substrate, and cracking/detachment of the oxide scale. The thermal stresses generated in the oxide scale could be calculated with [25]: roxide ¼
Eoxide DT ðasub aoxide Þ Eoxide doxide ð1 mÞ 1 þ Esub dsub
ð4:1Þ
where E, a, m, d and DT are the Young’s modulus, CTE, Poisson’s ratio, thickness, and temperature change, respectively. The subscripts ‘‘oxide’’ and ‘‘sub’’ refer to the oxide scale formed and the unoxidised substrate. It can be seen that the smaller the difference between the CTEs, the smaller the thermal stress generated during temperature change. The actual CTE of the present TiAl–Al2 O3 in situ composite was not measured, but it might be calculated using the Turner equation, acomposite ¼
ðar Br Fr =qr Þ þ ðam Bm Fm =qm Þ ðBr Fr =qr Þ þ ðBm Fm =qm Þ
ð4:2Þ
where B, F , and q represent bulk modulus, mass fraction and density, respectively, while ‘‘r’’ and ‘‘m’’ refer to the reinforcement and matrix phases respectively. According to the data from literature [26–30], the CTE of the composite is calculated to be 9.9 · 106 K1 . This value is much lower than that of TiAl, 12.6 · 106 K1 , and close to that of a-Al2 O3 and TiO2 (8.3 · 106 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 TiAl– Al2 O3 in situ composite is significantly smaller than that on the cast TiAl alloy due to the special material fabrication process, which combines high-energy mechanical milling and powder compact sintering. It is believed that plastic deformation of oxide scale through high temperature creep could relieve the growth/thermal stresses partially, and the diffusion creep rate, e0 , is favored by small grain size as shown in the following equation [31]: rX B2 dDB 0 e ¼ 2 B1 D v þ ð4:3Þ d kT d where r is the stress, X is the atomic volume, d is the width of grain boundary, d is the average grain size, k is the Boltzmann constant, Dv and DB are the volume and grain boundary diffusion coefficients respectively, and B1 and B2 are constants. Thus, finer grains would promote higher creep rate, relieving the stresses more effectively,
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decreasing the possibility of scale cracking/detachment, and then improving the scale spallation resistance. The incorporated a-Al2 O3 can also contribute to the improvement of scale spallation resistance. Firstly, these a-Al2 O3 particles formed originally through in situ reaction in the fine Al–TiO2 composite powder, seem to develop a local threedimensional network in the matrix [14]. It was 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 (Figs. 4c, 5b and d) (this might be highly related with the gradient compositional and structural changes in the original transition region). Therefore, the alumina 3-D network can hold the matrix and the oxide together strongly. Secondly, it can be seen that micro-pores and micro-cracks play an important role in the mechanical failure of the oxide scale, especially under rapid thermal cycling. The cracks tend to propagate laterally (sometimes also vertically), resulting in detachment of the scale from the inner part, and exhibiting scale spallation, sometimes with large pieces. It appears that the alumina particles in the scale can change the layered structure and stop the propagation of cracks, therefore effectively inhibiting the development of micro-cracks in the scale. 5. Conclusions A TiAl-based intermetallic matrix composite is fabricated through the in situ reaction of mechanically milled Al/TiO2 composite powder. This composite composed of TiAl and 42–50 vol.% of a-Al2 O3 particles. High temperature oxidation tests have been carried out at 800 and 900 C in air to evaluate its oxidation/scale spallation resistance in comparison with a cast Ti–50at.%Al alloy. The results showed that the composite had much lower oxidation rates than the cast alloy, especially at 900 C. The composite also exhibited superior scale spallation resistance even under intensive thermal cycling conditions. It is believed that the incorporation of alumina particulates into the metal matrix decreases the CTE of the substrate (thereby reducing the difference of the CTE between the oxide and substrate), forms a local 3-D network structure that can hold the oxide scale, and inhibit the propagation of micro-cracks in the scale. Formation of oxide scale with finer particle size, stronger adherence, less micro-defects, slower growth rate, and higher plastic deformation ability could contribute to the improvement of oxidation and scale spallation resistance.
Acknowledgements This research is partially supported by the Foundation for Research, Science and Technology (FRST), New Zealand through a New Economy Research Fund (NERF) grant. ZWL would like to thank the University of Auckland and FRST Postdoctoral Fellowships. The authors would also like to express their thanks to the
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technical staff members at the Department and the Research Center for Surfaces and Materials Science (RCSMS) at the University of Auckland for various help during this study. Titanox Development Limited is the licenser of the composite materials production technology.
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