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Development of TiAl alloys with excellent mechanical properties and oxidation resistance
Materials and Design 54 (2014) 814–819
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Development of TiAl alloys with excellent mechanical properties and oxidation resistance Seong-Woong Kim ⇑, Jae Keun Hong, Young-Sang Na, Jong-Taek Yeom, Seung Eon Kim Light Metal Division, Korea Institute of Materials Science (KIMS), Changwon 642-831, Republic of Korea
a r t i c l e
i n f o
Article history: Received 17 June 2013 Accepted 23 August 2013 Available online 30 August 2013 Keywords: Intermetallic compounds Structural materials Electron microscopy Mechanical properties Microstructure Turbine wheels
a b s t r a c t Mechanical and oxidation properties of newly-developed TiAl alloys were investigated. The TiAl alloys in this study were manufactured by casting and no further heat-treatment was conducted. In this study, Ti– (40–44)Al–(3,6)Nb–(W,Cr)–Si–C alloys were developed and the possibility of using them for turbine wheels in automobile engines was examined in comparison with commercial TiAl alloys. The new alloys developed in this study showed excellent tensile strength at room temperature and high temperature (900 °C) as well as good oxidation resistance at 900 °C compared to the commercial TiAl alloy. Moreover, the new alloys showed much better castability than the commercial TiAl alloy. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Gamma titanium aluminides (TiAl) have gained great interest for research on high-temperature applications due to their weight saving in combination with excellent high temperature properties such as creep and oxidation resistance [1–4]. Furthermore, fully lamellar TiAl alloys composed of TiAl (c) and a small amount of Ti3Al (a2) exhibit superior fracture toughness and creep resistance. However, their poor room temperature ductility and machinability have hindered their application in areas such as aerospace and automobile products. To overcome this barrier, enormous efforts have been devoted to preparing TiAl alloys with refined and homogeneous microstructure by alloying [5], heat-treatment [6], thermo-mechanical treatment (TMP) [7–9] including isothermal forging, pack forging and rolling. Powder metallurgy have been tried, however, their mechanical properties were not good enough for practical applications [10]. Other approach for achieving balanced mechanical properties is by directional solidification (DS) techniques [11,12]. The mechanical properties of two-phase lamellar TiAl alloys are known to be dependent on the lamellar orientation relative to the testing axis; thus it is essential to be able to control the orientation during directional solidification in order to obtain desirable properties [13]. However, DS process cannot be applied to large ingots, thus, its practical application is very difficult.
⇑ Corresponding author. Tel.: +82 55 280 3837; fax: +82 55 280 3255. E-mail address: [email protected] (S.-W. Kim). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.08.083
Recently, new TiAl alloys that contains less aluminum and some beta stabilizers such as Mo, Nb or W were reported to have relatively higher elongation at room temperature; thus, many efforts are now underway to develop beta phase containing TiAl alloys [14–16]. The beta phase containing TiAl alloys use beta solidification instead of peritectic reaction (L + b ? a), and the beta phase can be beneficial for higher ductility [14]. In this study, we developed new TiAl alloys for turbocharger wheels of automobile engines. The basic mechanical properties at room temperature (RT) and high temperature (900 °C) as well as the oxidation resistance at 900 °C were examined and compared with commercial TiAl alloys that have been used in automobile engines. No post processing or heat-treatment was conducted because the turbocharger wheels of automobile engines are manufactured by casting. Based on this study, we recommend new TiAl alloys for automobile and aerospace applications. 2. Experimental details The alloys used in this research were produced from 99.99 wt.% Ti, 99.99 wt.% Al, 99.9 wt.% Nb, 99.9 wt.% W, 99.99 wt.% Cr, 99 wt.% TiC powder and 99.99 wt.% Si. Button ingots of (Ti–(40–44) Al–(3,6)Nb–W–(Cr)–Si–C alloys and Ti–47.21Al–6.28Nb–0.49Cr– 0.28Si–0.15Ni–0.17V) were made by vacuum arc melting in an argon atmosphere and the resulting ingots were re-melted at least five times or more to obtain homogeneity. The composition of a commercial alloy (Ti–47.21Al–6.28Nb–0.49Cr–0.28Si–0.15Ni– 0.17V) was analyzed from the real turbocharger wheel component of a Porche car engine using EDS in a scanning electron microscope
S.-W. Kim et al. / Materials and Design 54 (2014) 814–819 Table 1 Alloy compositions of newly developed commercial alloys. Specimen number
Composition (at.%)
#1 #2
Ti–44Al–3Nb–2W–0.3Si Ti–44Al–6Nb–2Cr–0.3Si–0.1C
#3
Ti–47.21Al–6.28Nb–0.49Cr–0.28Si– 0.15Ni–0.17V
Remarks
KIMS recommended Commercial alloy
(SEM). Vickers hardness, tensile stress/strain and oxidation resistance tests of as-cast alloys were examined in this study. The microstructure of the alloys was examined by optical microscope, SEM as well as transmission electron microscope (TEM). The samples for TEM observation were prepared by twin-jet polishing
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using a solution of 925 ml methanol and 75 ml perchloric acid in a 20 °C to 40 °C range and a voltage of 12–15 V. Room temperature and high temperature (900 °C) tensile tests were performed at a nominal strain rate 2 10 4/s using a screw driven universal testing machine. Flat tensile specimens were electro-discharge machined, and each specimen had a 15 mm gauge length with a 2.5 mm 2 mm cross section for the RT specimen and 5 mm gauge length with a 5 mm 1 mm cross section for the 900 °C specimen. All the tensile specimens were polished with 2000 grit emery paper and electropolished in a solution of 5% HClO4-35% n-butanol–60% methanol by volume at 65 °C. Oxidation resistance was evaluated by measuring the mass gain of the 40 mm 8 mm (t = 4 mm) specimens while the specimens were exposed to air at 900 °C for 168 h using a thermal analysis system (TGA) from Setaram Instrumentation.
Fig. 1. Microstructure of as-cast alloys of (a) Ti–44Al–3Nb–2W–0.3Si (#1), (b) Ti–44Al–6Nb–2Cr–0.3Si–0.1C (#2) and (c) Ti–47.21Al–6.28Nb–0.49Cr–0.28Si–0.15Ni–0.17V (#3) by optical microscope.
Fig. 2. Bright field TEM images of (a and b) Ti–44Al–3Nb–2W–0.3Si (#1), (c and d) Ti–44Al–6Nb–2Cr–0.3Si–0.1C (#2) and (e and f) Ti–47.21Al–6.28Nb–0.49Cr–0.28Si– 0.15Ni–0.17V (#3) alloys. The inset of (f) indicates the selected area diffraction pattern of the circled area.
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S.-W. Kim et al. / Materials and Design 54 (2014) 814–819 Table 3 Tensile stress and strain to failure from a tensile test at 900 °C. Specimen number
Tensile stress (MPa)
Strain to failure (%)
#2 #3
560.3 505.5
27.5 50.7
Fig. 3. Room temperature stress–strain curves from tensile tests of #8, #11 and #13 alloys.
Table 2 Summary of fracture stresses from a tensile test and Vickers hardness at room temperature. Specimen number
Fracture stress (MPa)
Vickers hardness
#1 #2 #3
494.9 539.6 390.1
405.2 407.9 268.4
Casting defects were observed using a conventional X-ray scanning inspection method after the specimens were manufactured by a centrifugal casting method. Casting was not conducted at KIMS but ordered from a company, so we only used the casting samples to see the casting defects. Instead, we used arc melting buttons for the other mechanical and oxidation tests. To observe the voids or defects of the casting specimens, advanced high-resolution 3D Xray microscopy (Versa XRM-500) from Xradia was also used.
Fig. 5. Tensile stresses of #2 and #3 alloys from tensile tests at 900 °C together with the results from references. Tensile stresses for alloys 1 and 2 are from [2] and alloy 3 is from [22].
3. Results and discussion 3.1. Alloy design The composition range of the beta phase containing TiAl alloys that was proposed in previous studies [14–16] was Ti–(42–44) Al–Nb–(Mo, V)–(B,C) with some variations on the amount of beta stabilizers and minor elements. In this study, we developed new alloys under following design concepts: (i) change the Al contents in a range of 40–44 at.%, (ii) keep the Nb content for oxidation resistance [17,18], (iii) add small amount of W instead of Mo, V which has stronger beta stabilizing effect [11], (iv) add small amount of Cr to improve ductility [5] and (v) add small amount of Si and C to improve high temperature mechanical properties
Fig. 6. Mass gain after exposure in air at 900 °C for 168 h of #2 and #3 alloy.
Fig. 4. SEM images of the fracture surface after room temperature tensile tests of (a) Ti–44Al–3Nb–2W–0.3Si (#1), (b) Ti–44Al–6Nb–2Cr–0.3Si–0.1C (#2) and (c) Ti–47.21Al– 6.28Nb–0.49Cr–0.28Si–0.15Ni–0.17V (#3) alloys.
S.-W. Kim et al. / Materials and Design 54 (2014) 814–819 Table 4 Summary of mechanical properties at RT and 900 °C, the mass gain from an isothermal oxidation test at 900 °C and casting defects by X-ray inspection. Properties
KIMS alloy (#2)
Commercial alloy (#3)
Remarks Superior
Fracture stress at RT (MPa)
540
390
Tensile stress at 900 °C (MPa)
560.3
505.5
Superior
Oxidation resistance (mg/cm2) (900 °C 168 h) Casting defect
2.226155
2.268986
Similar
Few
Many
Superior
and creep resistance [12,19]. With these alloy design concepts, we designed several alloy compositions and selected two alloys for comparison with the commercial alloy that has been used in a turbocharger engine of Porsche cars. The compositions of our alloys (#1 and #2) and the commercial TiAl alloy (#3) are summarized in Table 1. Arc melting button ingots of the alloys were manufactured and used to examine the room temperature and high temperature properties.
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phases were found as shown in Fig. 1. However, the lamellar spacing and detailed microstructure needed to be confirmed to understand their mechanical properties, so TEM was used to observe the detailed microstructures. The microstructures of the alloys observed by TEM are shown in Fig. 2. Alloy #1 (Fig. 2a and b) mostly revealed a fully lamellar structure with lamellar spacing of 300 nm, and some beta phases were found in the gamma matrix as reported in previous studies [15,16]. Alloy #2 (Fig. 2c and d) also showed a fully lamellar structure with much finer lamellar spacing (100 nm), including some beta phases as in alloy #1. In alloy #3 (Fig. 2e and f), a stacking fault array was observed and it was possibly due to Ni addition for which the stacking fault energy is lower than other face centered cubic metals [20]. However, the effect of stacking faults on mechanical properties is not clear. The grain size of alloy #3 was small (Fig. 1), but the lamellar structure was quite coarse (300 nm) compared to alloy #2. The lamellar spacing of alloy #2 was fine, which was possibly due to Si and C additions [12,21], and the fine lamellar structure was expected to contribute to better mechanical properties such as higher tensile stress and fracture toughness.
3.2. Microstructures The as-cast microstructures of the alloys were observed by optical microscope (Fig. 1). The alloys had a fully lamellar microstructure, and the grain size of alloy #3 (200 lm) was smaller than that of alloys #1 and #2 (500 lm). No precipitates or second
3.3. Room temperature and high-temperature tensile tests Room temperature tensile test results are shown in Fig. 3, and the values of the fracture stress and Vickers hardness are
Fig. 7. Oxide layers after isothermal oxidation tests by SEM at 900 °C for 168 h of (a) the #2 alloy and (b) the #3 alloy.
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S.-W. Kim et al. / Materials and Design 54 (2014) 814–819
Fig. 8. Casting defects examination by (a and b) conventional X-ray scanning and (c and d) high-resolution 3D X-ray scanning of (a and c) the #2 and (b and d) #3 alloys.
summarized in Table 2. As the tensile tests were performed with the specimens from as-cast ingots without any post processing (e.g., heat-treatment and/or forging and HIP), no ductility was shown in the tensile tests results. Therefore, we compared the fracture stresses of the alloys. The alloys had a very similar Young’s modulus (180–220 GPa) (Fig. 3) and the fracture stress of alloy #2 was the highest among the alloys (Fig. 3 and Table 2). Moreover, the Vickers hardness of alloy #2 was also much higher than that of the other alloys (Table 2). The fracture surfaces after the tensile test were examined by SEM (Fig. 4). In alloy #1, the cleavage fracture surface was clearly revealed (Fig. 4a). However, in alloy #2, a streak morphology was observed due to the fine lamellar structure of the alloy which contributed to high fracture stress (Fig. 4b). No ductile evidence, such as dimples, was found, but the fracture surface of alloy #2 had many steps which indicated much more complex fracture behavior compared with alloy #1. The fracture surface of alloy #3 (Fig. 4c) was in between that of #1 and #2 alloys showing many steps mixed with some cleavage fracture surface. High temperature tensile tests on alloys #2 and #3 were performed at 900 °C and the summary of the tensile stress and strain to failure is shown in Table 3. Alloy #1 was not used for high-temperature tensile and oxidation tests since its room temperature properties were not as good as alloy #2. The tensile stress of alloys #3 and #2 are plotted in Fig. 5 together with the results from [2,22]. In [2,22], the alloys were hot extruded and heat-treated or a TMP process was performed, while the results in this study were obtained from as-cast ingots. However, the new alloy in this study (#2) showed the highest tensile stress values compared with commercial alloy (#3) and references (Fig. 5). The strain to failure of alloy #3 was 50.7% which was almost two times higher than that of alloy #2. However, too much strain during the high-temperature tensile test may indicate low creep resistance; thus, we expect that
the creep resistance of alloy #2 will be much better than alloy #3. Creep tests are now underway and will be reported later. Tensile tests at RT and 900 °C and Vickers hardness measurements at RT confirmed that alloy #2 had excellent mechanical properties. 3.4. Oxidation resistance Isothermal oxidation tests on alloys #2 and #3 were conducted at 900 °C for 168 h as shown in Fig. 6. The mass gain of the alloys after exposure for 168 h in air is summarized in Table 4. Table 4 and Fig. 6 revealed that alloys #2 and #3 had similar oxidation resistance. The alloy element effect on oxidation resistance is not fully understood, but a similar amount of Nb addition was believed to contribute to oxidation resistance [4,5]. Cross-section of the oxidation layers of each alloy, which exhibited a lower mass gain in Table 4, was observed by SEM (Fig. 7). Both alloys had very similar oxidation layers, and stable TiO2 layers were formed in the outer layer (note that the yellow line indicates oxygen) [23]. Thus, the alloys had quite excellent oxidation resistance. 3.5. Casting defects Casting defects of alloys #2 and #3 were examined using a conventional X-ray scanning inspection method (Fig. 8a and b) as well as advanced high-resolution 3D X-ray microscopy (Versa XRM-500 from Xradia) (Fig. 8c and d) after manufacturing cylindrical samples by a centrifugal casting method. From X-ray scanning examination (Fig. 8a and b), samples from alloy #3 exhibited casting defects such as shrinkages and pores, while samples from alloy #2 rarely showed casting defects. Less than 50% of the samples in alloy #3 were identified as good samples that contained a small amount of defects. One sample of each alloy (sample ‘A’ in Fig. 8a
S.-W. Kim et al. / Materials and Design 54 (2014) 814–819
and b) was picked and scanned using the Versa XRM-500 machine to observe the defects/voids in more detail (Fig. 8c and d). In alloy #3, very large shrinkages as well as small pores were clearly seen (Fig. 8d). However, alloy #2 rarely showed casting pores (Fig. 8c). In Fig. 8, yellow and blue indicate pores while red indicates particles. Castability is very important for industrial applications as many TiAl components are manufactured by casting. The cost of manufacturing is increased by a high number of casting defects, which makes application difficult. All the results from mechanical, oxidation tests and casting defect examination are summarized in Table 4. A newly developed alloy (#2, the KIMS alloy) in this study showed superior or similar properties compared with commercial TiAl alloy (#3) over all properties. Therefore, it is expected that the newly developed KIMS alloy can be a strong candidate for turbine wheels of automobile engines. 4. Conclusions New TiAl alloys (KIMS alloys) were developed and their mechanical and oxidation behaviors were examined. The KIMS alloy exhibited superior mechanical properties at RT and 900 °C and they also exhibited similar oxidation resistance at 900 °C compared with commercial alloys. Furthermore, the KIMS alloy showed superior castability compared with commercial alloys. This indicates that KIMS alloy can be a good candidate for the cast part applications of TiAl alloys. Acknowledgements This work was supported by the Fundamental R&D Program of the Korea Institute of Materials Science. The authors are grateful to Dr. Naomi Kotwal of Xradia Inc. for advanced X-ray inspection using the VersaXRM-500 machine. References [1] Kim Y-W. Intermetallic alloys based on gamma titanium aluminide. JOM 1989;41:24–30. [2] Liu CT, Schneibel JH, Maziasz PJ, Wright JL, Easton DS. Tensile properties and fracture toughness of TiAl alloys with controlled microstructures. Intermetallics 1996;4:429–40.
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[3] Liu CT, Maziasz PJ. Microstructural control and mechanical properties of dualphase TiAl alloys. Intermetallics 1998;6:653–61. [4] Wu X. Review of alloy and process development of TiAl alloys. Intermetallics 2006;14:1114–22. [5] Imayev RM, Imayev VM, Oehring M, Appel F. Alloy design concepts for refined gamma titanium aluminide based alloys. Intermetallics 2007;15:451–60. [6] Clemens H, Bartels A, Bystrzanowski S, Chladil H, Leitner H, Dehm G, et al. Grain refinement in c-TiAl-based alloys by solid state phase transformations. Intermetallics 2006;14:1380–5. [7] Tetsui T, Shindo K, Kobayashi S, Takeyama M. Strengthening a high-strength TiAl alloy by hot-forging. Intermetallics 2003;11:299–306. [8] Draper SL, Krause D, Lerch B, Locci IE, Doehnert B, Nigam R, et al. Development and evaluation of TiAl sheet structures for hypersonic applications. Mater Sci Eng: A 2007;464:330–42. [9] Carneiro T, Kim YW. Evaluation of ingots and alpha-extrusions of gamma alloys based on Ti–45Al–6Nb. Intermetallics 2005;13:1000–7. [10] Srivastava D, Chang ITH, Loretto MH. The optimisation of processing parameters and characterisation of microstructure of direct laser fabricated TiAl alloy components. Mater Des 2000;21:425–33. [11] Jung IS, Jang HS, Oh MH, Lee JH, Wee DM. Microstructure control of TiAl alloys containing b stabilizers by directional solidification. Mater Sci Eng: A 2002;329–331:13–8. [12] Kim SW, Wang P, Oh MH, Wee DM, Kumar KS. Mechanical properties of Si- and C-doped directionally solidified TiAl–Nb alloys. Intermetallics 2004;12:499–509. [13] Inui H, Oh MH, Nakamura A, Yamaguchi M. Room-temperature tensile deformation of polysynthetically twinned (PST) crystals of TiAl. Acta Metall Et Mater 1992;40:3095–104. [14] Shih DS, Kim Y-W. Sheet rolling and performance evaluation of beta gamma (b–g) alloys. In: Ninomi M, Akiyama S, Ikeda M, editors. Ti-2007 science and enginnering (C). Kyoto, Japan: The Japan Institute of Metals; 2007. p. 1021–4. [15] Chen Y, Niu H, Kong F, Xiao S. Microstructure and fracture toughness of a b phase containing TiAl alloy. Intermetallics 2011;19:1405–10. [16] Niu HZ, Kong FT, Xiao SL, Chen YY, Yang F. Effect of pack rolling on microstructures and tensile properties of as-forged Ti–44Al–6V–3Nb–0.3Y alloy. Intermetallics 2012;21:97–104. [17] Lee JK, Lee HN, Lee HK, Oh MH, Wee DM. Effects of Al–21Ti–23Cr coatings on oxidation and mechanical properties of TiAl alloy. Surf Coat Technol 2002;155:59–66. [18] Kim JS, Lee YH, Kim Y-W, Lee CS. High temperature deformation behavior of beta–gamma TiAl alloy. Mater Sci For 2007;539–543:1531. [19] Lapin J. Creep behaviour of a cast TiAl-based alloy for industrial applications. Intermetallics 2006;14:115–22. [20] Dillamore IL, Smallman RE. The stacking-fault energy of F.C.C. metals. Philos Magaz 1965;12:191–3. [21] Kim SW, Kumar KS, Oh MH, Wee DM. Crack propagation behavior in TiAl–Nb single and Bi-PST crystals. Intermetallics 2007;15:976–84. [22] Kim YW. Effects of microstructure on the deformation and fracture of c-TiAl alloys. Mater Sci Eng: A 1995;192-193(Part 2):519–33. [23] Kim BG, Kim GM, Kim CJ. Oxidation behavior of TiAl–X (X = Cr, V, Si, Mo or Nb) intermetallics at elevated temperature. Scr Metall Et Mater 1995;33:1117–25.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Development of TiAl alloys with excellent mechanical properties and oxidation resistance Seong-Woong Kim ⇑, Jae Keun Hong, Young-Sang Na, Jong-Taek Yeom, Seung Eon Kim Light Metal Division, Korea Institute of Materials Science (KIMS), Changwon 642-831, Republic of Korea
a r t i c l e
i n f o
Article history: Received 17 June 2013 Accepted 23 August 2013 Available online 30 August 2013 Keywords: Intermetallic compounds Structural materials Electron microscopy Mechanical properties Microstructure Turbine wheels
a b s t r a c t Mechanical and oxidation properties of newly-developed TiAl alloys were investigated. The TiAl alloys in this study were manufactured by casting and no further heat-treatment was conducted. In this study, Ti– (40–44)Al–(3,6)Nb–(W,Cr)–Si–C alloys were developed and the possibility of using them for turbine wheels in automobile engines was examined in comparison with commercial TiAl alloys. The new alloys developed in this study showed excellent tensile strength at room temperature and high temperature (900 °C) as well as good oxidation resistance at 900 °C compared to the commercial TiAl alloy. Moreover, the new alloys showed much better castability than the commercial TiAl alloy. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Gamma titanium aluminides (TiAl) have gained great interest for research on high-temperature applications due to their weight saving in combination with excellent high temperature properties such as creep and oxidation resistance [1–4]. Furthermore, fully lamellar TiAl alloys composed of TiAl (c) and a small amount of Ti3Al (a2) exhibit superior fracture toughness and creep resistance. However, their poor room temperature ductility and machinability have hindered their application in areas such as aerospace and automobile products. To overcome this barrier, enormous efforts have been devoted to preparing TiAl alloys with refined and homogeneous microstructure by alloying [5], heat-treatment [6], thermo-mechanical treatment (TMP) [7–9] including isothermal forging, pack forging and rolling. Powder metallurgy have been tried, however, their mechanical properties were not good enough for practical applications [10]. Other approach for achieving balanced mechanical properties is by directional solidification (DS) techniques [11,12]. The mechanical properties of two-phase lamellar TiAl alloys are known to be dependent on the lamellar orientation relative to the testing axis; thus it is essential to be able to control the orientation during directional solidification in order to obtain desirable properties [13]. However, DS process cannot be applied to large ingots, thus, its practical application is very difficult.
⇑ Corresponding author. Tel.: +82 55 280 3837; fax: +82 55 280 3255. E-mail address: [email protected] (S.-W. Kim). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.08.083
Recently, new TiAl alloys that contains less aluminum and some beta stabilizers such as Mo, Nb or W were reported to have relatively higher elongation at room temperature; thus, many efforts are now underway to develop beta phase containing TiAl alloys [14–16]. The beta phase containing TiAl alloys use beta solidification instead of peritectic reaction (L + b ? a), and the beta phase can be beneficial for higher ductility [14]. In this study, we developed new TiAl alloys for turbocharger wheels of automobile engines. The basic mechanical properties at room temperature (RT) and high temperature (900 °C) as well as the oxidation resistance at 900 °C were examined and compared with commercial TiAl alloys that have been used in automobile engines. No post processing or heat-treatment was conducted because the turbocharger wheels of automobile engines are manufactured by casting. Based on this study, we recommend new TiAl alloys for automobile and aerospace applications. 2. Experimental details The alloys used in this research were produced from 99.99 wt.% Ti, 99.99 wt.% Al, 99.9 wt.% Nb, 99.9 wt.% W, 99.99 wt.% Cr, 99 wt.% TiC powder and 99.99 wt.% Si. Button ingots of (Ti–(40–44) Al–(3,6)Nb–W–(Cr)–Si–C alloys and Ti–47.21Al–6.28Nb–0.49Cr– 0.28Si–0.15Ni–0.17V) were made by vacuum arc melting in an argon atmosphere and the resulting ingots were re-melted at least five times or more to obtain homogeneity. The composition of a commercial alloy (Ti–47.21Al–6.28Nb–0.49Cr–0.28Si–0.15Ni– 0.17V) was analyzed from the real turbocharger wheel component of a Porche car engine using EDS in a scanning electron microscope
S.-W. Kim et al. / Materials and Design 54 (2014) 814–819 Table 1 Alloy compositions of newly developed commercial alloys. Specimen number
Composition (at.%)
#1 #2
Ti–44Al–3Nb–2W–0.3Si Ti–44Al–6Nb–2Cr–0.3Si–0.1C
#3
Ti–47.21Al–6.28Nb–0.49Cr–0.28Si– 0.15Ni–0.17V
Remarks
KIMS recommended Commercial alloy
(SEM). Vickers hardness, tensile stress/strain and oxidation resistance tests of as-cast alloys were examined in this study. The microstructure of the alloys was examined by optical microscope, SEM as well as transmission electron microscope (TEM). The samples for TEM observation were prepared by twin-jet polishing
815
using a solution of 925 ml methanol and 75 ml perchloric acid in a 20 °C to 40 °C range and a voltage of 12–15 V. Room temperature and high temperature (900 °C) tensile tests were performed at a nominal strain rate 2 10 4/s using a screw driven universal testing machine. Flat tensile specimens were electro-discharge machined, and each specimen had a 15 mm gauge length with a 2.5 mm 2 mm cross section for the RT specimen and 5 mm gauge length with a 5 mm 1 mm cross section for the 900 °C specimen. All the tensile specimens were polished with 2000 grit emery paper and electropolished in a solution of 5% HClO4-35% n-butanol–60% methanol by volume at 65 °C. Oxidation resistance was evaluated by measuring the mass gain of the 40 mm 8 mm (t = 4 mm) specimens while the specimens were exposed to air at 900 °C for 168 h using a thermal analysis system (TGA) from Setaram Instrumentation.
Fig. 1. Microstructure of as-cast alloys of (a) Ti–44Al–3Nb–2W–0.3Si (#1), (b) Ti–44Al–6Nb–2Cr–0.3Si–0.1C (#2) and (c) Ti–47.21Al–6.28Nb–0.49Cr–0.28Si–0.15Ni–0.17V (#3) by optical microscope.
Fig. 2. Bright field TEM images of (a and b) Ti–44Al–3Nb–2W–0.3Si (#1), (c and d) Ti–44Al–6Nb–2Cr–0.3Si–0.1C (#2) and (e and f) Ti–47.21Al–6.28Nb–0.49Cr–0.28Si– 0.15Ni–0.17V (#3) alloys. The inset of (f) indicates the selected area diffraction pattern of the circled area.
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S.-W. Kim et al. / Materials and Design 54 (2014) 814–819 Table 3 Tensile stress and strain to failure from a tensile test at 900 °C. Specimen number
Tensile stress (MPa)
Strain to failure (%)
#2 #3
560.3 505.5
27.5 50.7
Fig. 3. Room temperature stress–strain curves from tensile tests of #8, #11 and #13 alloys.
Table 2 Summary of fracture stresses from a tensile test and Vickers hardness at room temperature. Specimen number
Fracture stress (MPa)
Vickers hardness
#1 #2 #3
494.9 539.6 390.1
405.2 407.9 268.4
Casting defects were observed using a conventional X-ray scanning inspection method after the specimens were manufactured by a centrifugal casting method. Casting was not conducted at KIMS but ordered from a company, so we only used the casting samples to see the casting defects. Instead, we used arc melting buttons for the other mechanical and oxidation tests. To observe the voids or defects of the casting specimens, advanced high-resolution 3D Xray microscopy (Versa XRM-500) from Xradia was also used.
Fig. 5. Tensile stresses of #2 and #3 alloys from tensile tests at 900 °C together with the results from references. Tensile stresses for alloys 1 and 2 are from [2] and alloy 3 is from [22].
3. Results and discussion 3.1. Alloy design The composition range of the beta phase containing TiAl alloys that was proposed in previous studies [14–16] was Ti–(42–44) Al–Nb–(Mo, V)–(B,C) with some variations on the amount of beta stabilizers and minor elements. In this study, we developed new alloys under following design concepts: (i) change the Al contents in a range of 40–44 at.%, (ii) keep the Nb content for oxidation resistance [17,18], (iii) add small amount of W instead of Mo, V which has stronger beta stabilizing effect [11], (iv) add small amount of Cr to improve ductility [5] and (v) add small amount of Si and C to improve high temperature mechanical properties
Fig. 6. Mass gain after exposure in air at 900 °C for 168 h of #2 and #3 alloy.
Fig. 4. SEM images of the fracture surface after room temperature tensile tests of (a) Ti–44Al–3Nb–2W–0.3Si (#1), (b) Ti–44Al–6Nb–2Cr–0.3Si–0.1C (#2) and (c) Ti–47.21Al– 6.28Nb–0.49Cr–0.28Si–0.15Ni–0.17V (#3) alloys.
S.-W. Kim et al. / Materials and Design 54 (2014) 814–819 Table 4 Summary of mechanical properties at RT and 900 °C, the mass gain from an isothermal oxidation test at 900 °C and casting defects by X-ray inspection. Properties
KIMS alloy (#2)
Commercial alloy (#3)
Remarks Superior
Fracture stress at RT (MPa)
540
390
Tensile stress at 900 °C (MPa)
560.3
505.5
Superior
Oxidation resistance (mg/cm2) (900 °C 168 h) Casting defect
2.226155
2.268986
Similar
Few
Many
Superior
and creep resistance [12,19]. With these alloy design concepts, we designed several alloy compositions and selected two alloys for comparison with the commercial alloy that has been used in a turbocharger engine of Porsche cars. The compositions of our alloys (#1 and #2) and the commercial TiAl alloy (#3) are summarized in Table 1. Arc melting button ingots of the alloys were manufactured and used to examine the room temperature and high temperature properties.
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phases were found as shown in Fig. 1. However, the lamellar spacing and detailed microstructure needed to be confirmed to understand their mechanical properties, so TEM was used to observe the detailed microstructures. The microstructures of the alloys observed by TEM are shown in Fig. 2. Alloy #1 (Fig. 2a and b) mostly revealed a fully lamellar structure with lamellar spacing of 300 nm, and some beta phases were found in the gamma matrix as reported in previous studies [15,16]. Alloy #2 (Fig. 2c and d) also showed a fully lamellar structure with much finer lamellar spacing (100 nm), including some beta phases as in alloy #1. In alloy #3 (Fig. 2e and f), a stacking fault array was observed and it was possibly due to Ni addition for which the stacking fault energy is lower than other face centered cubic metals [20]. However, the effect of stacking faults on mechanical properties is not clear. The grain size of alloy #3 was small (Fig. 1), but the lamellar structure was quite coarse (300 nm) compared to alloy #2. The lamellar spacing of alloy #2 was fine, which was possibly due to Si and C additions [12,21], and the fine lamellar structure was expected to contribute to better mechanical properties such as higher tensile stress and fracture toughness.
3.2. Microstructures The as-cast microstructures of the alloys were observed by optical microscope (Fig. 1). The alloys had a fully lamellar microstructure, and the grain size of alloy #3 (200 lm) was smaller than that of alloys #1 and #2 (500 lm). No precipitates or second
3.3. Room temperature and high-temperature tensile tests Room temperature tensile test results are shown in Fig. 3, and the values of the fracture stress and Vickers hardness are
Fig. 7. Oxide layers after isothermal oxidation tests by SEM at 900 °C for 168 h of (a) the #2 alloy and (b) the #3 alloy.
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Fig. 8. Casting defects examination by (a and b) conventional X-ray scanning and (c and d) high-resolution 3D X-ray scanning of (a and c) the #2 and (b and d) #3 alloys.
summarized in Table 2. As the tensile tests were performed with the specimens from as-cast ingots without any post processing (e.g., heat-treatment and/or forging and HIP), no ductility was shown in the tensile tests results. Therefore, we compared the fracture stresses of the alloys. The alloys had a very similar Young’s modulus (180–220 GPa) (Fig. 3) and the fracture stress of alloy #2 was the highest among the alloys (Fig. 3 and Table 2). Moreover, the Vickers hardness of alloy #2 was also much higher than that of the other alloys (Table 2). The fracture surfaces after the tensile test were examined by SEM (Fig. 4). In alloy #1, the cleavage fracture surface was clearly revealed (Fig. 4a). However, in alloy #2, a streak morphology was observed due to the fine lamellar structure of the alloy which contributed to high fracture stress (Fig. 4b). No ductile evidence, such as dimples, was found, but the fracture surface of alloy #2 had many steps which indicated much more complex fracture behavior compared with alloy #1. The fracture surface of alloy #3 (Fig. 4c) was in between that of #1 and #2 alloys showing many steps mixed with some cleavage fracture surface. High temperature tensile tests on alloys #2 and #3 were performed at 900 °C and the summary of the tensile stress and strain to failure is shown in Table 3. Alloy #1 was not used for high-temperature tensile and oxidation tests since its room temperature properties were not as good as alloy #2. The tensile stress of alloys #3 and #2 are plotted in Fig. 5 together with the results from [2,22]. In [2,22], the alloys were hot extruded and heat-treated or a TMP process was performed, while the results in this study were obtained from as-cast ingots. However, the new alloy in this study (#2) showed the highest tensile stress values compared with commercial alloy (#3) and references (Fig. 5). The strain to failure of alloy #3 was 50.7% which was almost two times higher than that of alloy #2. However, too much strain during the high-temperature tensile test may indicate low creep resistance; thus, we expect that
the creep resistance of alloy #2 will be much better than alloy #3. Creep tests are now underway and will be reported later. Tensile tests at RT and 900 °C and Vickers hardness measurements at RT confirmed that alloy #2 had excellent mechanical properties. 3.4. Oxidation resistance Isothermal oxidation tests on alloys #2 and #3 were conducted at 900 °C for 168 h as shown in Fig. 6. The mass gain of the alloys after exposure for 168 h in air is summarized in Table 4. Table 4 and Fig. 6 revealed that alloys #2 and #3 had similar oxidation resistance. The alloy element effect on oxidation resistance is not fully understood, but a similar amount of Nb addition was believed to contribute to oxidation resistance [4,5]. Cross-section of the oxidation layers of each alloy, which exhibited a lower mass gain in Table 4, was observed by SEM (Fig. 7). Both alloys had very similar oxidation layers, and stable TiO2 layers were formed in the outer layer (note that the yellow line indicates oxygen) [23]. Thus, the alloys had quite excellent oxidation resistance. 3.5. Casting defects Casting defects of alloys #2 and #3 were examined using a conventional X-ray scanning inspection method (Fig. 8a and b) as well as advanced high-resolution 3D X-ray microscopy (Versa XRM-500 from Xradia) (Fig. 8c and d) after manufacturing cylindrical samples by a centrifugal casting method. From X-ray scanning examination (Fig. 8a and b), samples from alloy #3 exhibited casting defects such as shrinkages and pores, while samples from alloy #2 rarely showed casting defects. Less than 50% of the samples in alloy #3 were identified as good samples that contained a small amount of defects. One sample of each alloy (sample ‘A’ in Fig. 8a
S.-W. Kim et al. / Materials and Design 54 (2014) 814–819
and b) was picked and scanned using the Versa XRM-500 machine to observe the defects/voids in more detail (Fig. 8c and d). In alloy #3, very large shrinkages as well as small pores were clearly seen (Fig. 8d). However, alloy #2 rarely showed casting pores (Fig. 8c). In Fig. 8, yellow and blue indicate pores while red indicates particles. Castability is very important for industrial applications as many TiAl components are manufactured by casting. The cost of manufacturing is increased by a high number of casting defects, which makes application difficult. All the results from mechanical, oxidation tests and casting defect examination are summarized in Table 4. A newly developed alloy (#2, the KIMS alloy) in this study showed superior or similar properties compared with commercial TiAl alloy (#3) over all properties. Therefore, it is expected that the newly developed KIMS alloy can be a strong candidate for turbine wheels of automobile engines. 4. Conclusions New TiAl alloys (KIMS alloys) were developed and their mechanical and oxidation behaviors were examined. The KIMS alloy exhibited superior mechanical properties at RT and 900 °C and they also exhibited similar oxidation resistance at 900 °C compared with commercial alloys. Furthermore, the KIMS alloy showed superior castability compared with commercial alloys. This indicates that KIMS alloy can be a good candidate for the cast part applications of TiAl alloys. Acknowledgements This work was supported by the Fundamental R&D Program of the Korea Institute of Materials Science. The authors are grateful to Dr. Naomi Kotwal of Xradia Inc. for advanced X-ray inspection using the VersaXRM-500 machine. References [1] Kim Y-W. Intermetallic alloys based on gamma titanium aluminide. JOM 1989;41:24–30. [2] Liu CT, Schneibel JH, Maziasz PJ, Wright JL, Easton DS. Tensile properties and fracture toughness of TiAl alloys with controlled microstructures. Intermetallics 1996;4:429–40.
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[3] Liu CT, Maziasz PJ. Microstructural control and mechanical properties of dualphase TiAl alloys. Intermetallics 1998;6:653–61. [4] Wu X. Review of alloy and process development of TiAl alloys. Intermetallics 2006;14:1114–22. [5] Imayev RM, Imayev VM, Oehring M, Appel F. Alloy design concepts for refined gamma titanium aluminide based alloys. Intermetallics 2007;15:451–60. [6] Clemens H, Bartels A, Bystrzanowski S, Chladil H, Leitner H, Dehm G, et al. Grain refinement in c-TiAl-based alloys by solid state phase transformations. Intermetallics 2006;14:1380–5. [7] Tetsui T, Shindo K, Kobayashi S, Takeyama M. Strengthening a high-strength TiAl alloy by hot-forging. Intermetallics 2003;11:299–306. [8] Draper SL, Krause D, Lerch B, Locci IE, Doehnert B, Nigam R, et al. Development and evaluation of TiAl sheet structures for hypersonic applications. Mater Sci Eng: A 2007;464:330–42. [9] Carneiro T, Kim YW. Evaluation of ingots and alpha-extrusions of gamma alloys based on Ti–45Al–6Nb. Intermetallics 2005;13:1000–7. [10] Srivastava D, Chang ITH, Loretto MH. The optimisation of processing parameters and characterisation of microstructure of direct laser fabricated TiAl alloy components. Mater Des 2000;21:425–33. [11] Jung IS, Jang HS, Oh MH, Lee JH, Wee DM. Microstructure control of TiAl alloys containing b stabilizers by directional solidification. Mater Sci Eng: A 2002;329–331:13–8. [12] Kim SW, Wang P, Oh MH, Wee DM, Kumar KS. Mechanical properties of Si- and C-doped directionally solidified TiAl–Nb alloys. Intermetallics 2004;12:499–509. [13] Inui H, Oh MH, Nakamura A, Yamaguchi M. Room-temperature tensile deformation of polysynthetically twinned (PST) crystals of TiAl. Acta Metall Et Mater 1992;40:3095–104. [14] Shih DS, Kim Y-W. Sheet rolling and performance evaluation of beta gamma (b–g) alloys. In: Ninomi M, Akiyama S, Ikeda M, editors. Ti-2007 science and enginnering (C). Kyoto, Japan: The Japan Institute of Metals; 2007. p. 1021–4. [15] Chen Y, Niu H, Kong F, Xiao S. Microstructure and fracture toughness of a b phase containing TiAl alloy. Intermetallics 2011;19:1405–10. [16] Niu HZ, Kong FT, Xiao SL, Chen YY, Yang F. Effect of pack rolling on microstructures and tensile properties of as-forged Ti–44Al–6V–3Nb–0.3Y alloy. Intermetallics 2012;21:97–104. [17] Lee JK, Lee HN, Lee HK, Oh MH, Wee DM. Effects of Al–21Ti–23Cr coatings on oxidation and mechanical properties of TiAl alloy. Surf Coat Technol 2002;155:59–66. [18] Kim JS, Lee YH, Kim Y-W, Lee CS. High temperature deformation behavior of beta–gamma TiAl alloy. Mater Sci For 2007;539–543:1531. [19] Lapin J. Creep behaviour of a cast TiAl-based alloy for industrial applications. Intermetallics 2006;14:115–22. [20] Dillamore IL, Smallman RE. The stacking-fault energy of F.C.C. metals. Philos Magaz 1965;12:191–3. [21] Kim SW, Kumar KS, Oh MH, Wee DM. Crack propagation behavior in TiAl–Nb single and Bi-PST crystals. Intermetallics 2007;15:976–84. [22] Kim YW. Effects of microstructure on the deformation and fracture of c-TiAl alloys. Mater Sci Eng: A 1995;192-193(Part 2):519–33. [23] Kim BG, Kim GM, Kim CJ. Oxidation behavior of TiAl–X (X = Cr, V, Si, Mo or Nb) intermetallics at elevated temperature. Scr Metall Et Mater 1995;33:1117–25.