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Influence of yttrium on microstructure, mechanical properties and deformability of Ti–43Al–9V alloy
Intermetallics 13 (2005) 263–266 www.elsevier.com/locate/intermet
Influence of yttrium on microstructure, mechanical properties and deformability of Ti–43Al–9V alloy Yuyong Chen*, Fantao Kong, Jiecai Han, Ziyong Chen, Jing Tian School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Available online 22 October 2004
Abstract The influences of rare earth metal (Y) on microstructure, mechanical properties and deformability of Ti–43A1–9V alloy are reported. The results show that Ti–43Al–9V–0.3Y alloy is composed of g, a2, B2 and YAl2 phases. Adding 0.3at.% Y refines the grain size, decreases a2/g/B2 lamellar thickness of Ti–43Al–9V alloy, and promotes the formation of fine a2/g lamellae. Mechanical property tests show that adding 0.3at.% Y can apparently enhance strength and plastic property of Ti–43Al–9V alloy. The hot deformation experiment at 1200 8C indicates that adding 0.3at.% Y can improve deformability of Ti–43Al–9V alloy, because Ti–43Al–9V–0.3Y alloy has smaller grain size, lower resistance to deformation, and fine and uniform recrystallized grains. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl
1. Introduction Because TiAl intermetallic materials have shown promising combinations of low density, good specific strength and modulus ratio, excellent creep resistance and anti-oxidation properties, they have received increasing attention, particularly for aerospace vehicle, energy and automotive industry [1–4]. However, the disadvantages of TiAl alloys are low ductility at room temperature and poor workability. It is difficult for them to be deformed plastically even at elevated temperature by conventional methods. Depending on the alloy composition and processing parameters, various microstructures can be obtained in this alloy system [5–9]. Tensile and other mechanical properties of TiAl alloys are sensitively influenced by the microstructure. Particularly, the room temperature tensile properties and plastic deformability are improved by microstructural refinement [10–12]. Recent research shows that rare earth element Y can refine microstructure and purify melt of TiAl alloys [13–15]. In this paper, the effects of yttrium additions on * Corresponding author. Tel.: C86-451-86418734; fax: C86-45186415776. E-mail address: [email protected] (Y. Chen). 0966-9795/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2004.07.014
the microstructure, mechanical properties and plastic deformability of Ti–43Al–9V alloy have been examined, and results of the observations are discussed. The purpose of this investigation is to explore methods of enhancing properties of TiAl alloys.
2. Experimental procedures Two alloy ingots with composition of Ti–43Al–9V and Ti–43Al–9V–0.3Y (all in atomic percent) were prepared by induction skull melting (ISM). The ingots were homogenized by hot isostatic pressing (HIP) at 1250 8C for 4 h under an argon pressure of 170 MPa and then heat treated (HT) at 900 8C for 48 h followed by furnace cooling to room temperature. The microstructures of Ti–43Al–9V and Ti–43Al–9V– 0.3Y alloy were studied by optical microscopy (OM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). Specimens for OM were prepared by etching in Kroll solution (H2O 100 ml, HF 3 ml and HNO3 6 ml). Thin foils for TEM were prepared by twin-jet polishing. The electrolyte used for twin-jet polishing was a solution of 60% methanol, 35% n-butyl alcohol and 5% perchloric acid. Phases of the alloy were determined
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Table 1 . Chemical composition of experimental alloys after ISM (at.%) Alloy
Al
V
Y
Ti
Ti–43Al–9V Ti–43Al–9V–0.3Y
42.58 42.64
8.98 9.03
0.22
Bal. Bal.
by X-ray diffraction (XRD) analysis, and the measurement parameters were as follows: Cu Ka radiation, accelerating voltage 40 kV, tube current 40 mA. Chemical composition analyses of specimens were performed by energy dispersive X-ray (EDX) method in SEM. Table 1 shows chemical composition of the present alloys. Tensile testing on plate specimens with a gage section of 15!6.0!2.0 mm was conducted on an Instron testing machine at room temperature, and strain rate of 10K3 sK1. At least five samples were tested under identical conditions to obtain an average. Cylindrical specimens 8.0 mm in diameter by 12 mm height were then taken from the HIP/HT ingots and coated with a graphite lubricant for the isothermal hotcompression tests. The deformation behaviors were examined at the temperature of 1200 8C, average nominal strain rates of 0.1 sK1, and amount of deformation of 60%.
Fig. 2. BSE images of Ti–43Al–9V–0.3Y alloy.
(in Fig. 2, white regions contain rare earth-rich phase). The reason of grain refinement appears to be rare earth-rich phase segregation at grain boundaries preventing grain growth. The chemical composition of the rare earth-rich phase was analyzed using EPMA. EPMA revealed that the rare
3. Results and discussion 3.1. Effects of yttrium on microstructure The microstructure of the Y free alloy was coarsegrained with grain sizes of 100–200 mm (average size is 170 mm). Fig. 1(a) shows one such example, which is a representative OM image obtained from the Ti–43Al–9V alloy. Adding yttrium leads to microstructure change for Ti– 43Al–9V alloy. The microstructure of Ti–43Al–9V–0.3Y alloy is shown in Fig. 1(b), and this alloy has a grain size of 50–100 mm (average size is 80 mm). It can be seen that a coarse-grained structure could be refined to a fine one with an average size of 80 mm merely through the addition of rare earth element. In Ti–43Al–9V–0.3Y alloy, most of the rare earth-rich phase is distributed at grain boundaries
Fig. 1. Microstructure of (a) Ti–43Al–9V and (b) Ti–43Al–9V–0.3Y alloy.
Fig. 3. X-ray diffraction spectrum of (a) Ti–43Al–9V and (b) Ti–43Al–9V– 0.3Y alloy.
Y. Chen et al. / Intermetallics 13 (2005) 263–266
265
Fig. 4. Lamellar microstructure of (a) Ti–43Al–9V and (b) Ti–43Al–9V– 0.3Y alloy.
earth-rich phase shown in Fig. 2 has an average composition of 66.17Al, 32.49Y, and 1.34Ti (at.%). The XRD patterns of Ti–43Al–9V and Ti–43Al–9V– 0.3Y alloy are shown in Fig. 3. The result confirms that Ti– 43Al–9V alloy is composed of three phases. In addition to the a2 and g phases, the alloy contained a bcc phase with an ordered B2 (CsCl) structure. The formation of B2 phase is due to the V addition, which is known to be a bcc stabilizing element. B2 phase may be formed either from retained high temperature b phase via concurrent b/B2 and b/a/ a2Cg transformations, or from the decomposition of the high temperature a phase via (b/) a/a2CgCB2 transformations. Comparing with the RE-free alloy, Ti– 43Al–9V–0.3Y alloy is composed of g, a2, B2 and YAl2 intermetallic phases. EPMA indicates that the rare earthrich phase is YAl2. Wu et al. reported that adding Y to TiAl alloys could result in the formation of oxide of Y(Y2O3) [14], but diffraction peaks of Y2O3 phase was not found in the present XRD experiment. The reason for the absence of Y2O3 phase may be that the melt of Ti–43Al–9V–0.3Y has low oxygen content during ISM melting (oxygen content! 300 ppm) and the diffraction peaks of Y2O3 can not be easily discerned by XRD. TEM images of Ti–43Al–9V and Ti–43Al–9V–0.3Y alloy are presented in Fig. 4. From Fig. 4, it can be seen that both alloys have lamellar microstructure. The average interlamellar spacing of a2/g/B2 plates decreases from 1.9 mm of Ti–43Al–9V alloy to 1.1 mm after adding 0.3Y to the base composition (see the coarse lamellar part in Fig. 4(a) and (b)), and there are many fine a2/g lamellae in Ti–43Al–9V–0.3Y alloy (see the thin lamellar part in Fig. 4(b)). Therefore, the results show that adding yttrium can also refine the a2/g/B2 lamellar thickness of Ti–43Al–9V alloy, and promote the formation of fine a2/g lamellae.
Fig. 5. True stress–strain curves of (a) Ti–43Al–9V and (b) Ti–43Al–9V– 0.3Y alloy.
3.2. Mechanical property The results of the tensile tests are summarized in Table 2. Mechanical property tests show that, compared with REfree alloy, 0.3at.% Y addition can apparently enhance RT strength and elongation. The reason of this improvement may be as follows: (1) RE refines grain sizes and lamellar thickness (so far, all studies indicate that TiAl alloys with refined colony size and lamellar thickness possess superior mechanical properties for structural applications at elevated temperatures [10]); (2) RE purifies TiAl alloys (such as decreasing oxygen content of the g phase due to the strong binding among Y and O atoms [14,15]).
3.3. Deformation behavior OM observations showed no crack in the Ti–43Al–9V– 0.3Y alloy specimens compressed to 60%, but a few small cracks were found on the surfaces of Ti–43Al–9V alloy at the same deformation conditions. The hot deformation
Table 2 Room temperature tensile properties of TiAl alloys Alloy
s0.2 (MPa)
sb (MPa)
d (%)
Ti–43Al–9V Ti–43Al–9V–0.3Y
402.8 453.5
461.5 516.4
1.1 1.6
Fig. 6. Deformed microstructure of (a) Ti–43Al–9V and (b) Ti–43Al–9V– 0.3Y alloy.
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also contribute to the deformability of Ti–43Al–9V–0.3Y alloy.
4. Conclusions
Fig. 7. Recrystallized grains of Ti–43Al–9V–0.3Y alloy.
experiment results show that adding 0.3at.%Y has great benefits to the hot-compression deformability of this alloy. The deformation behaviors of Ti–43Al–9V and Ti–43Al– 9V–0.3Y alloy were described by the true stress–true compressive strain curves at 1200 8C and strain rate of 0.1 sK1 (Fig. 5). Compared with Ti–43Al–9V alloy, Ti–43Al–9V–0.3Y alloy exhibited much lower peak stress and flow stress. Fig. 6 shows two typical optical microstructures of isothermal hot-compression samples. The deformed samples show a near-lamellar microstructure consisting of lamellar colonies, with a few equiaxed recrystallized g-grains at the lamellar-grain boundaries. From Fig. 6(a), most of the lamellae can be seen to have become bent as a result of heavy deformation and there are few recrystallized grains at the lamellar-grain boundaries. However, Figs. 6(b) and 7 show that there are plenty of smaller and uniform recrystallized grains formed in Ti–43Al–9V–0.3Y alloy at the same deformation conditions. This indicates that adding Y can increase apparently capability of recrystallization of Ti–43Al–9V alloy. Comparing with Ti–43Al–9V alloy, the lower resistance to deformation as well as the smaller grains make Ti–43Al–9V–0.3Y alloy easier to deform plastically. Besides, the fine recrystallized g-grains formed in the process of the isothermal hot-compression tests may
Ti–43Al–9V–0.3Y alloy is composed of g, a2, B2 and YAl2 phases. Adding 0.3at.% Y refines the grain size, decreases a2/g/B2 lamellar thickness of Ti–43Al–9V alloy, and promotes the formation of fine a 2/g lamellae. Mechanical property tests show that 0.3at.% Y added can apparently enhance RT strength and plastic property of Ti–43Al–9V alloy. The hot deformation experiment at 1200 8C indicates that adding 0.3at.%Y can improve deformability of Ti–43Al–9V alloy, because Ti–43Al–9V–0.3Y alloy has smaller grain size, lower resistance of deformation, and fine and uniform recrystallized grains.
Acknowledgements This research was funded by the National Natural Science Foundation of China under Grant 50274035.
References [1] Kim Y-W, Dimiduk DM. J Metals 1991;43(8):40. [2] Nazmy M, Staubli M. Scri Metall Mater 1994;31:829. [3] Kim Y-W. Trans of Nonferrous Metals Society of China 1999; 9(suppl.):298. [4] Kumpfert J, Kim Y-W, Dimiduk DM. Mater Sci Eng 1995;A192/193: 465. [5] Ahmed T, Flower HM. Mater Sci Technol 1994;10:272. [6] Prabir KC, Rack HJ. Scri Metall Mater 1992;26:691. [7] Diplas S, Tsakiropoulos P, Shao G, Watts JF, Matthew JAD. Acta Mater 2002;50:1951. [8] Shao G, Tsakiropoulos P, Miodownik AP. Mater Sci Eng 1996;A216:1. [9] Shao G, Tsakiropoulos P, Miodownik AP. Intermetallics 1995;3:315. [10] Liu CT, Maziasz PJ. Intermetallics 1998;6:653. [11] Chen YY, Kong FT, Tian J, Chen ZY, Xiao SL. Trans of Nonferrous Metals Society of China 2002;12(4):605. [12] Park HS, Nam SW, Kim NJ. Scripta Mater 1999;41(11):1197. [13] Kong FT, Dong MH, Chen YY. J Chin Rare Earth Soc 2002;20(1):72. [14] Wu Y, Hwang SK. Acta Mater 2002;50:1479. [15] Wu Y, Hwang SK, Nam SW, Kim NJ. Scripta Mater 2003;48:1655.
Influence of yttrium on microstructure, mechanical properties and deformability of Ti–43Al–9V alloy Yuyong Chen*, Fantao Kong, Jiecai Han, Ziyong Chen, Jing Tian School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Available online 22 October 2004
Abstract The influences of rare earth metal (Y) on microstructure, mechanical properties and deformability of Ti–43A1–9V alloy are reported. The results show that Ti–43Al–9V–0.3Y alloy is composed of g, a2, B2 and YAl2 phases. Adding 0.3at.% Y refines the grain size, decreases a2/g/B2 lamellar thickness of Ti–43Al–9V alloy, and promotes the formation of fine a2/g lamellae. Mechanical property tests show that adding 0.3at.% Y can apparently enhance strength and plastic property of Ti–43Al–9V alloy. The hot deformation experiment at 1200 8C indicates that adding 0.3at.% Y can improve deformability of Ti–43Al–9V alloy, because Ti–43Al–9V–0.3Y alloy has smaller grain size, lower resistance to deformation, and fine and uniform recrystallized grains. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl
1. Introduction Because TiAl intermetallic materials have shown promising combinations of low density, good specific strength and modulus ratio, excellent creep resistance and anti-oxidation properties, they have received increasing attention, particularly for aerospace vehicle, energy and automotive industry [1–4]. However, the disadvantages of TiAl alloys are low ductility at room temperature and poor workability. It is difficult for them to be deformed plastically even at elevated temperature by conventional methods. Depending on the alloy composition and processing parameters, various microstructures can be obtained in this alloy system [5–9]. Tensile and other mechanical properties of TiAl alloys are sensitively influenced by the microstructure. Particularly, the room temperature tensile properties and plastic deformability are improved by microstructural refinement [10–12]. Recent research shows that rare earth element Y can refine microstructure and purify melt of TiAl alloys [13–15]. In this paper, the effects of yttrium additions on * Corresponding author. Tel.: C86-451-86418734; fax: C86-45186415776. E-mail address: [email protected] (Y. Chen). 0966-9795/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2004.07.014
the microstructure, mechanical properties and plastic deformability of Ti–43Al–9V alloy have been examined, and results of the observations are discussed. The purpose of this investigation is to explore methods of enhancing properties of TiAl alloys.
2. Experimental procedures Two alloy ingots with composition of Ti–43Al–9V and Ti–43Al–9V–0.3Y (all in atomic percent) were prepared by induction skull melting (ISM). The ingots were homogenized by hot isostatic pressing (HIP) at 1250 8C for 4 h under an argon pressure of 170 MPa and then heat treated (HT) at 900 8C for 48 h followed by furnace cooling to room temperature. The microstructures of Ti–43Al–9V and Ti–43Al–9V– 0.3Y alloy were studied by optical microscopy (OM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). Specimens for OM were prepared by etching in Kroll solution (H2O 100 ml, HF 3 ml and HNO3 6 ml). Thin foils for TEM were prepared by twin-jet polishing. The electrolyte used for twin-jet polishing was a solution of 60% methanol, 35% n-butyl alcohol and 5% perchloric acid. Phases of the alloy were determined
264
Y. Chen et al. / Intermetallics 13 (2005) 263–266
Table 1 . Chemical composition of experimental alloys after ISM (at.%) Alloy
Al
V
Y
Ti
Ti–43Al–9V Ti–43Al–9V–0.3Y
42.58 42.64
8.98 9.03
0.22
Bal. Bal.
by X-ray diffraction (XRD) analysis, and the measurement parameters were as follows: Cu Ka radiation, accelerating voltage 40 kV, tube current 40 mA. Chemical composition analyses of specimens were performed by energy dispersive X-ray (EDX) method in SEM. Table 1 shows chemical composition of the present alloys. Tensile testing on plate specimens with a gage section of 15!6.0!2.0 mm was conducted on an Instron testing machine at room temperature, and strain rate of 10K3 sK1. At least five samples were tested under identical conditions to obtain an average. Cylindrical specimens 8.0 mm in diameter by 12 mm height were then taken from the HIP/HT ingots and coated with a graphite lubricant for the isothermal hotcompression tests. The deformation behaviors were examined at the temperature of 1200 8C, average nominal strain rates of 0.1 sK1, and amount of deformation of 60%.
Fig. 2. BSE images of Ti–43Al–9V–0.3Y alloy.
(in Fig. 2, white regions contain rare earth-rich phase). The reason of grain refinement appears to be rare earth-rich phase segregation at grain boundaries preventing grain growth. The chemical composition of the rare earth-rich phase was analyzed using EPMA. EPMA revealed that the rare
3. Results and discussion 3.1. Effects of yttrium on microstructure The microstructure of the Y free alloy was coarsegrained with grain sizes of 100–200 mm (average size is 170 mm). Fig. 1(a) shows one such example, which is a representative OM image obtained from the Ti–43Al–9V alloy. Adding yttrium leads to microstructure change for Ti– 43Al–9V alloy. The microstructure of Ti–43Al–9V–0.3Y alloy is shown in Fig. 1(b), and this alloy has a grain size of 50–100 mm (average size is 80 mm). It can be seen that a coarse-grained structure could be refined to a fine one with an average size of 80 mm merely through the addition of rare earth element. In Ti–43Al–9V–0.3Y alloy, most of the rare earth-rich phase is distributed at grain boundaries
Fig. 1. Microstructure of (a) Ti–43Al–9V and (b) Ti–43Al–9V–0.3Y alloy.
Fig. 3. X-ray diffraction spectrum of (a) Ti–43Al–9V and (b) Ti–43Al–9V– 0.3Y alloy.
Y. Chen et al. / Intermetallics 13 (2005) 263–266
265
Fig. 4. Lamellar microstructure of (a) Ti–43Al–9V and (b) Ti–43Al–9V– 0.3Y alloy.
earth-rich phase shown in Fig. 2 has an average composition of 66.17Al, 32.49Y, and 1.34Ti (at.%). The XRD patterns of Ti–43Al–9V and Ti–43Al–9V– 0.3Y alloy are shown in Fig. 3. The result confirms that Ti– 43Al–9V alloy is composed of three phases. In addition to the a2 and g phases, the alloy contained a bcc phase with an ordered B2 (CsCl) structure. The formation of B2 phase is due to the V addition, which is known to be a bcc stabilizing element. B2 phase may be formed either from retained high temperature b phase via concurrent b/B2 and b/a/ a2Cg transformations, or from the decomposition of the high temperature a phase via (b/) a/a2CgCB2 transformations. Comparing with the RE-free alloy, Ti– 43Al–9V–0.3Y alloy is composed of g, a2, B2 and YAl2 intermetallic phases. EPMA indicates that the rare earthrich phase is YAl2. Wu et al. reported that adding Y to TiAl alloys could result in the formation of oxide of Y(Y2O3) [14], but diffraction peaks of Y2O3 phase was not found in the present XRD experiment. The reason for the absence of Y2O3 phase may be that the melt of Ti–43Al–9V–0.3Y has low oxygen content during ISM melting (oxygen content! 300 ppm) and the diffraction peaks of Y2O3 can not be easily discerned by XRD. TEM images of Ti–43Al–9V and Ti–43Al–9V–0.3Y alloy are presented in Fig. 4. From Fig. 4, it can be seen that both alloys have lamellar microstructure. The average interlamellar spacing of a2/g/B2 plates decreases from 1.9 mm of Ti–43Al–9V alloy to 1.1 mm after adding 0.3Y to the base composition (see the coarse lamellar part in Fig. 4(a) and (b)), and there are many fine a2/g lamellae in Ti–43Al–9V–0.3Y alloy (see the thin lamellar part in Fig. 4(b)). Therefore, the results show that adding yttrium can also refine the a2/g/B2 lamellar thickness of Ti–43Al–9V alloy, and promote the formation of fine a2/g lamellae.
Fig. 5. True stress–strain curves of (a) Ti–43Al–9V and (b) Ti–43Al–9V– 0.3Y alloy.
3.2. Mechanical property The results of the tensile tests are summarized in Table 2. Mechanical property tests show that, compared with REfree alloy, 0.3at.% Y addition can apparently enhance RT strength and elongation. The reason of this improvement may be as follows: (1) RE refines grain sizes and lamellar thickness (so far, all studies indicate that TiAl alloys with refined colony size and lamellar thickness possess superior mechanical properties for structural applications at elevated temperatures [10]); (2) RE purifies TiAl alloys (such as decreasing oxygen content of the g phase due to the strong binding among Y and O atoms [14,15]).
3.3. Deformation behavior OM observations showed no crack in the Ti–43Al–9V– 0.3Y alloy specimens compressed to 60%, but a few small cracks were found on the surfaces of Ti–43Al–9V alloy at the same deformation conditions. The hot deformation
Table 2 Room temperature tensile properties of TiAl alloys Alloy
s0.2 (MPa)
sb (MPa)
d (%)
Ti–43Al–9V Ti–43Al–9V–0.3Y
402.8 453.5
461.5 516.4
1.1 1.6
Fig. 6. Deformed microstructure of (a) Ti–43Al–9V and (b) Ti–43Al–9V– 0.3Y alloy.
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Y. Chen et al. / Intermetallics 13 (2005) 263–266
also contribute to the deformability of Ti–43Al–9V–0.3Y alloy.
4. Conclusions
Fig. 7. Recrystallized grains of Ti–43Al–9V–0.3Y alloy.
experiment results show that adding 0.3at.%Y has great benefits to the hot-compression deformability of this alloy. The deformation behaviors of Ti–43Al–9V and Ti–43Al– 9V–0.3Y alloy were described by the true stress–true compressive strain curves at 1200 8C and strain rate of 0.1 sK1 (Fig. 5). Compared with Ti–43Al–9V alloy, Ti–43Al–9V–0.3Y alloy exhibited much lower peak stress and flow stress. Fig. 6 shows two typical optical microstructures of isothermal hot-compression samples. The deformed samples show a near-lamellar microstructure consisting of lamellar colonies, with a few equiaxed recrystallized g-grains at the lamellar-grain boundaries. From Fig. 6(a), most of the lamellae can be seen to have become bent as a result of heavy deformation and there are few recrystallized grains at the lamellar-grain boundaries. However, Figs. 6(b) and 7 show that there are plenty of smaller and uniform recrystallized grains formed in Ti–43Al–9V–0.3Y alloy at the same deformation conditions. This indicates that adding Y can increase apparently capability of recrystallization of Ti–43Al–9V alloy. Comparing with Ti–43Al–9V alloy, the lower resistance to deformation as well as the smaller grains make Ti–43Al–9V–0.3Y alloy easier to deform plastically. Besides, the fine recrystallized g-grains formed in the process of the isothermal hot-compression tests may
Ti–43Al–9V–0.3Y alloy is composed of g, a2, B2 and YAl2 phases. Adding 0.3at.% Y refines the grain size, decreases a2/g/B2 lamellar thickness of Ti–43Al–9V alloy, and promotes the formation of fine a 2/g lamellae. Mechanical property tests show that 0.3at.% Y added can apparently enhance RT strength and plastic property of Ti–43Al–9V alloy. The hot deformation experiment at 1200 8C indicates that adding 0.3at.%Y can improve deformability of Ti–43Al–9V alloy, because Ti–43Al–9V–0.3Y alloy has smaller grain size, lower resistance of deformation, and fine and uniform recrystallized grains.
Acknowledgements This research was funded by the National Natural Science Foundation of China under Grant 50274035.
References [1] Kim Y-W, Dimiduk DM. J Metals 1991;43(8):40. [2] Nazmy M, Staubli M. Scri Metall Mater 1994;31:829. [3] Kim Y-W. Trans of Nonferrous Metals Society of China 1999; 9(suppl.):298. [4] Kumpfert J, Kim Y-W, Dimiduk DM. Mater Sci Eng 1995;A192/193: 465. [5] Ahmed T, Flower HM. Mater Sci Technol 1994;10:272. [6] Prabir KC, Rack HJ. Scri Metall Mater 1992;26:691. [7] Diplas S, Tsakiropoulos P, Shao G, Watts JF, Matthew JAD. Acta Mater 2002;50:1951. [8] Shao G, Tsakiropoulos P, Miodownik AP. Mater Sci Eng 1996;A216:1. [9] Shao G, Tsakiropoulos P, Miodownik AP. Intermetallics 1995;3:315. [10] Liu CT, Maziasz PJ. Intermetallics 1998;6:653. [11] Chen YY, Kong FT, Tian J, Chen ZY, Xiao SL. Trans of Nonferrous Metals Society of China 2002;12(4):605. [12] Park HS, Nam SW, Kim NJ. Scripta Mater 1999;41(11):1197. [13] Kong FT, Dong MH, Chen YY. J Chin Rare Earth Soc 2002;20(1):72. [14] Wu Y, Hwang SK. Acta Mater 2002;50:1479. [15] Wu Y, Hwang SK, Nam SW, Kim NJ. Scripta Mater 2003;48:1655.