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Effect of hot compressive deformation on the martensite transformation of Ti–10V–2Fe–3Al titanium alloy
Materials Science and Engineering A 530 (2011) 591–601
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of hot compressive deformation on the martensite transformation of Ti–10V–2Fe–3Al titanium alloy Liming Lei a , Xu Huang a , Minmin Wang b , Liqiang Wang b,∗ , Jining Qin b , Hongping Li c , ShiQiang Lu d a
Titanium Alloys Laboratory, Beijing Institute of Aeronautical Materials, Beijing 100095, China State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200240, China c Institute of Commercial Aircraft Company, Shanghai 200120, China d Department of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China b
a r t i c l e
i n f o
Article history: Received 10 April 2011 Received in revised form 20 August 2011 Accepted 6 October 2011 Available online 18 October 2011 Keywords: Microanalysis Titanium alloys Martensitic transformations
a b s t r a c t The hot deformation behavior of as-cast TB6 alloy was investigated by isothermal hot compressive deformation from 800 ◦ C to 1150 ◦ C at the strain rates between 10−3 s−1 and 10 s−1 . The microstructural evolution of martensite transformation of the deformed TB6 was observed, and the volume fraction of martensite was also studied. The results show that the deformation temperature plays an important role on the stress-induced martensite transformation for TB6 titanium alloy. When the strain rate is 0.001 s−1 , 0.01 s−1 , 0.1 s−1 , 1 s−1 , and 10 s−1 , the deformation temperature with more martensite content is 900–975 ◦ C, 925–1050 ◦ C, 825–1100 ◦ C, 910–1000 ◦ C, and 925–1100 ◦ C, respectively. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental processing
Owing to their high specific strength, excellent fracture toughness, good creep and corrosion resistance, titanium alloys have been used widely in many fields such as aerospace, biomedical and sporting industries [1–5]. TB6 (Ti–10V–2Fe–3Alwt%), known as Ti-1023 titanium alloy, is a metastable beta titanium alloy with two very important features: (1) the comprehensive performance is better than that of any other known titanium alloy; (2) malleability is better than that of any other known titanium alloys, especially for the isothermal forging and hot forging. Because of its excellent forging performance, TB6 is very favorable for a variety of near net shape forging with the isothermal forging and hot forging process. Near net shape deformation process can ensure low-cost processing. In addition, it can produce very uniform cross-section of the forgings with different thickness. By now, few investigations about the effect of hot compressive deformation on the martensite transformation of TB6 titanium alloy has been reported [6–9]. The object of this paper is to investigate the microstructure evolution, especially martensite transformation during hot compressive deformation for Ti–10V–2Fe–3Al alloy. Effect of deformation temperature on flow stress of as-cast TB6 titanium alloy and effect of strain rate on flow stress of the alloy were also discussed.
The detailed chemical composition of Ti-1023 alloy used in this paper is 10.2%V, 1.8%Fe, 3.1%Al (wt%), and balance Ti. The beta transformation temperature was determined to be (802 + 3)◦ C and the microstructure of as-cast material was shown in Fig. 1. The as-cast Ti–10V–2Fe–3Al alloy with diameter of 8 mm and height of 15 mm was selected. The hot deformation behavior of as-cast TB6 alloy was investigated by isothermal constant strain rate compression test at temperatures between 800 ◦ C and 1150 ◦ C and strain rates from 10−3 s−1 to 10−1 s−1 in Thermecmaster-Z thermal simulation machine with a true strain of 0.92. Fig. 2 shows the schematic diagram of hot compression test. Deformed specimens were sectioned paralleled to the compression axis and the surface was prepared for metallographic examination using standard techniques. The specimens were etched with 2%HF + 4%HNO3 and microstructures were observed using conventional optical microscopy (OM). The content of martensite was also calculated by the optical microscopy.
∗ Corresponding author. Tel.: +86 21 34202641; fax: +86 21 34202749. E-mail address: wang [email protected] (L. Wang). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.10.028
3. Results and discussions 3.1. Martensite transformation at different temperature The stress-induced martensite of as-cast TB6 titanium alloy was caused mainly by the uniformity and the size of internal stress. The ˇ stable coefficient of TB6 alloy is about 1.1. The start transformation temperature of martensite (Ms) is below
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less content of martensite appears. As for the as-cast TB6 titanium alloy, martensite transformation can be induced in the region with lower ˇ stable coefficient, while no martensite can be induced even with larger stress in the region with larger ˇ stable coefficient. The effect of deformation temperature on the content of straininduced martensite of as-cast TB6 titanium alloy can be concluded as follows: (1) Under certain conditions, the uniformity of alloy increased with the increase of deformation temperature. (2) The deformation temperature also effects the dynamic recrystallization of alloy and volume fraction of martensite indirectly. In generally, other conditions being equal, raising the deformation temperature, the volume fraction of the dynamic recrystallization is increased. 3.2. The amount of martensite under different hot compressive deformation
Fig. 1. Microstructures of as-cast TB6 titanium alloy: (a) low magnification (b) high magnification.
room temperature, which means that no martensite appears in the process of solution treatment. However, when internal stress exists within certain region of the alloy, the start transformation temperature of martensite can be increased in this region. Generally, the content of martensite increases with the increase of stress applied. In addition, when the stress keeps in a constant value, the higher content of the ˇ stable element is, the
Fig. 2. Schematic diagram of hot compression test.
3.2.1. Martensite transformation under the strain rate of 0.001 s−1 Fig. 3 shows the optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.001 s−1 and the strain of 0.92. As shown in Fig. 3, deformation temperature played an important role on martensite transformation. With the increase of deformation temperature, much more martensite appeared. When the deformation temperature increased to 950 ◦ C, the amount of martensite reached the maximum value. It can be observed that 15% martensite transformation was obtained at 950 ◦ C. However, fewer martensite was obtained when the deformation temperature was higher than 950 ◦ C. No martensite transformation can be observed at 1150 ◦ C. The relationship between the amount of martensite and deformation temperature can be clearly shown in Fig. 4. As seen in Fig. 4, for the as-cast TB6 alloy, when the deformation temperature varied from 800 ◦ C to 1150 ◦ C, more martensite appeared between 900 ◦ C and 975 ◦ C. When the deformation temperature was lower (T < 900 ◦ C), less dynamic recrystallization appeared. In addition, the stress was larger with relatively uniform composition. Therefore, in the region with more ˇ stable elements, little martensite transformation appeared even if there was larger internal stress. With the increase of deformation temperature (900 ◦ C < T < 975 ◦ C), an optimal combination of internal stress and composition uniformity was obtained and maximum content of martensite about 15% was obtained. When the deformation temperature was above 975 ◦ C, although the composition was uniform and the ˇ stable factor of the alloy tended to 1.1, much more recrystallization appeared. Less content of martensite was obtained because of the lower internal stress. 3.2.2. Martensite transformation under the strain rate of 0.01 s−1 Fig. 5 shows the optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.01 s−1 and the strain of 0.92. Fig. 6 shows the relationship between the content of martensite transformation and deformation temperature. As shown in Fig. 5, maximum content of martensite about 39% was obtained at 1000 ◦ C. However, less martensite can be observed when the deformation temperature was higher than 1000 ◦ C (Fig. 5(e)). Compared with the martensite transformation under the strain rate of 0.001 s−1 , similar characteristic of martensite transformation was found under the strain rate of 0.01 s−1 . As shown in Fig. 6, more martensite transformation appeared between 925 ◦ C and 1050 ◦ C. No significant differences of the needle-like martensite observed in the ˇ
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Fig. 3. Optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.001 s−1 : (a) 800 ◦ C, (b) 850 ◦ C, (c) 900 ◦ C, (d) 950 ◦ C, (e) 1000 ◦ C, (f) 1050 ◦ C, (g) 1100 ◦ C.
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Fig. 4. The relationship between the amount of martensite and deformation temperature under the strain rate of 0.001 s−1 .
value and then decreased to a lower value. However, the maximum content of martensite was different with the strain rate. Under the low strain rate (0.001 s−1 ), the maximum amount of martensite was about 15%. Under the medium strain rate (0.01 s−1 , 0.1 s−1 , 1 s−1 ), the maximum amount of martensite has greatly been improved. The maximum value of martensite was 39%, 40%, and 50%, respectively under the strain rate of 0.01 s−1 , 0.1 s−1 and 1 s−1 . While under the high strain rate (10 s−1 ), the content of martensite decreased to 25%. As mentioned in the optical microscopies shown above, twins were the secondary microstructure of martensite, As the deformation defect, the appear of martensite accelerated the process of grain refinement and dynamic recrystallization during hot deformation. However, a large amount of martensite contributed much to the strength of the alloy, which was not so good for the further deformation. Therefore, appropriate content of martensite was needed with regards to applications, i.e. forging. 3.3. Effect of deformation temperature on flow stress of as-cast TB6 titanium alloy
grain and grain boundaries can be found. Acicular martensite is formed by the criss-cross martensite group. Martensite appearing at the grain boundaries was mostly distributed in the two grains (Fig. 5(a) and (c)). In addition, there were also martensite groups appearing through several grains (Fig. 5(e)). As seen in Fig. 5, many dendritic martensite stacking together was observed, clearly. 3.2.3. Martensite transformation under the strain rate of 0.1 s−1 Fig. 7 shows the optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.1 s−1 and the strain of 0.92. Fig. 8 shows the relationship between the content of martensite transformation and deformation temperature. As shown in Fig. 8, maximum content of martensite was obtained when the deformation temperature were 825 ◦ C and 1100 ◦ C, which were 40% and 33%, respectively. 3.2.4. Martensite transformation under the strain rate of 1.0 s−1 Fig. 9 shows the optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 1.0 s−1 and the strain of 0.92. As shown in Fig. 9(c), when the deformation temperature was 925 ◦ C, maximum content of martensite was obtained. Dendritic martensite appeared between grain boundaries and the content was about 50%. Fig. 10 shows the relationship between the martensite transformation and the deformation temperature of as-cast TB6 titanium alloy. It can be found that between 910 ◦ C and 1000 ◦ C, much more martensite was obtained, which was similar with the martensite transformation under the strain rate of 0.001 s−1 . 3.2.5. Martensite transformation under the strain rate of 10.0 s−1 The microstructures of as-cast TB6 titanium alloy under the strain rate of 10 s−1 with the strain of 0.92 were shown in Fig. 11. The deformation temperature was from 800 ◦ C to 1150 ◦ C. As shown in Fig. 11, with the increase of deformation temperature, more martensite transformation appeared. However, when the temperature increased over 1100 ◦ C, less martensite transformation was obtained. Maximum value of martensite was observed between 925 ◦ C and 1100 ◦ C. As indicated in Fig. 12, 25% martensite transformation appeared at 1100 ◦ C. As mentioned above, under the five different strain rates, the characteristic of the influence of deformation temperature on the martensite transformation was nearly the same. With the increase of deformation, the amount of martensite increased to a peak
Fig. 13 shows the stress–strain curve of as-cast TB6 titanium alloy at different strain rate. In the initial stage of deformation, flow stress increased rapidly with the increasing of strain. While arriving at the maximum value (ε <0.1), flow stress decreased with the increasing of strain. The lower the temperature was, the more obvious softening was obtained. At certain strain, such as 0.001 s−1 , little continuous reduction of flow stress can be observed (Fig. 13(a)), which was dominated by the interaction effect of hardening and softening. Strain hardening was obtained at the strain rate of 0.01 s−1 , 0.1 s−1 and 1.0 s−1 (Fig. 13(b)–(d)). However, no strain hardening appeared at the strain rate of 10.0 s−1 and 0.001 s−1 , as shown in Fig. 13(e). At higher strain rate such as 10 s−1 , the strain hardening was caused by the accumulation of dislocations during deformation. Much more hardening occurred because of deformation defects at higher strain rate. However, accompanying with a large amount of dislocations, dynamic softening took place easily at high temperature, which decreased the flow stress. In addition, at lower strain rate such as 0.001 s−1 , there was sufficient time for dynamic recovery and dynamic recrystallization. Therefore, no strain hardening was observed at 10 s−1 and 0.001 s−1 rates. Except at the strain rate of 0.001 s−1 , the effect of temperature on flow stress was similar at both higher temperature and low temperature. When the temperature increased by 25 ◦ C, flow stress is decreased by about 4–9 MPa. It can be observed that deformation temperature played an important role in the flow stress. At certain strain rate, the higher the temperature was, the lower the flow stress was obtained. This was mainly because with the increase of temperature, the enhanced role of thermal activation and average kinetic energy of atoms increased, the critical slip shear stress decreased which reduced the obstacles of dislocation movement and the slip between crystal planes. Furthermore, along with the increasing temperature, dynamic recovery and dynamic recrystallization took place more easily, and the dislocation density was reduced, which offset the hardening caused by plastic deformation, thus reducing the flow stress of the material. At the beginning of deformation, the flow stress increased rapidly with increasing strain rate, and showed flow softening characteristics after reaching a maximum value. This is mainly because in the initial stage of deformation, the appearing of hardening and a large number of proliferation and tangles of dislocations contributed most to the hardening of the material. Therefore, in this condition, the flow stress increased to a larger value significantly. With the further increase of deformation,
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Fig. 5. Optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.01 s−1 : (a) 800 ◦ C, (b) 850 ◦ C, (c) 900 ◦ C, (d) 950 ◦ C, (e) 1000 ◦ C, (f) 1050 ◦ C, (g) 1100 ◦ C.
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Fig. 6. The relationship between the amount of martensite and deformation temperature under the strain rate of 0.01 s−1 .
softening effects, such as dynamic recovery and dynamic recrystallization appeared gradually. At the same time, the flow stress began to decline as the dominant softening effects. In addition, at certain temperature, with the increase of strain rate, the flow stress increased gradually. Take the stress–strain curve of as-cast TB6 titanium alloy at 800 ◦ C for example, when the strain rate was 0.001 s−1 , 0.01 s−1 , 0.1 s−1 , 1.0 s−1 , and 10 s−1 , the largest flow stress was 51 MPa, 86 MPa, 108 MPa, 160 MPa, and 225 MPa, respectively, which were shown in Fig. 13(a)–(e). The trend can
Fig. 8. The relationship between the amount of martensite and deformation temperature under the strain rate of 0.1 s−1 .
also be found when the temperature ranged from 825 ◦ C to 1150 ◦ C. 3.4. Effect of strain rate on flow stress of as-cast TB6 titanium alloy Fig. 14 shows the stress–strain curve of as-cast TB6 titanium alloys with different strain rate at various temperatures. As shown
Fig. 7. Optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.1 s−1 : (a) 800 ◦ C, (b) 900 ◦ C, (c) 1100 ◦ C.
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Fig. 9. Optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 1.0 s−1 : (a) 800 ◦ C, (b) 925 ◦ C, (c) 1100 ◦ C.
in Fig. 14, flow stress increased rapidly with the increasing of strain rate at low deformation temperature. However, when the deformation temperature was higher, flow stress increased slowly with the increasing of strain rate. At 800 ◦ C, flow stress increased by 45 MPa when the strain rate increased by an order of magnitude. In addition, at 1150 ◦ C, flow stress increased by 25 MPa when the strain rate increased by an order of magnitude. When the strain rate was over 0.1 s−1 , flow stress increased quickly with the increase of strain
rate. While when the strain rate was less than 0.1 s−1 , flow stress was not so sensitive to strain rate. As shown in Fig. 14, the elastic modulus of the TB6 titanium alloy seemed similar for strain rates of 0.01 s−1 and 0.1 s−1 under the temperature of 950 ◦ C. However, they separated at higher temperatures above 950 ◦ C. It can be concluded that under the strain rates of 0.01 s−1 and 0.1 s−1 , the compressive elastic modulus of the TB6 alloy was related higher with the temperature when the temperature was above 950 ◦ C, which meant
Fig. 10. The relationship between the amount of martensite and deformation temperature under the strain rate of 1.0 s−1 .
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Fig. 11. Optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 10.0 s−1 : (a) 800 ◦ C, (b) 925 ◦ C, (c) 1100 ◦ C, (d) 1150 ◦ C.
Fig. 12. The relationship between the amount of martensite and deformation temperature under the strain rate of 10.0 s−1 .
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Fig. 13. Stress–strain curve of as-cast TB6 titanium alloy at different strain rate: (a) 0.001 s−1 ; (b) 0.01 s−1 ; (c) 0.1 s−1 ; (d) 1.0 s−1 ; (e) 10 s−1 .
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Fig. 14. Stress–strain curve of TB6 titanium alloys with different strain rate at various temperatures: (a) 800 ◦ C; (b) 875 ◦ C; (c) 950 ◦ C; (d) 1050 ◦ C; (e) 1150 ◦ C.
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that at higher deformation temperature, at certain deformation strain, a larger compressive stress was applied at the strain rate of 0.1 s−1 . 4. Conclusions The microstructure of as-cast TB6 titanium alloy was studied. The deformation temperature varied from 800 ◦ C to 1150 ◦ C and the strain rate was between 0.001 s−1 and 10 s−1 under the total strain of 0.92. The main conclusions are as follows: (1) As for the mechanical instability of ˇ phase in TB6 titanium alloy, stress-induced martensite transformation occurs in the cooling process after hot deformation. The dendritic morphology martensite appears in the ˇ grain and grain boundaries. (2) Deformation temperature plays an important role on the stressinduced martensite transformation for TB6 titanium alloy. Only at moderate deformation temperature, when a best combination of the alloy composition uniformity and internal stress is achieved, maximum content of martensite is obtained. (3) The content of martensite phase changed with the different strain rate at certain deformation temperature. When the strain rate is 0.001 s−1 , 0.01 s−1 , 0.1 s−1 , 1 s−1 , and 10 s−1 , respectively, the deformation temperature with more martensite content is 900–975 ◦ C, 925–1050 ◦ C, 825–1100 ◦ C, 910–1000 ◦ C, and 925–1100 ◦ C, respectively. (4) In the initial stages of deformation, flow stress increased rapidly with increasing strain rate. Under small strain rate (ε < 0.1),
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after reaching its peak, flow stress showed a flow softening, and the lower the temperature was, the more evident the softening phenomenon was. Under certain strain rate, such as 0.001 s−1 , the reduction of flow stress was not continuous. In addition to strain rate of 0.001 s−1 , compared with high temperature, the effect of the temperature on the flow stress was more significant than at low temperature. Little difference about effect of the temperature on the flow stress can be obtained at other strain rates. Acknowledgement We would like to acknowledge financial support provided by the National Basic Research Program of China under grant no. 2007CB613803. References [1] N. Poondla, T.S. Srivatsan, A. Patnaik, M. Petraroli, Journal of Alloys and Compounds 486 (2009) 162–167. [2] M. Niinomi, Science and Technology of Advanced Materials 4 (2003) 445–454. [3] S. Hardt, H.J. Maier, J. Christ, International Journal of Fatigue 21 (1999) 779–789. [4] I.V. Gorynin, Materials Science and Engineering A 263 (1999) 112–116. [5] J.P. Immarigeon, R.T. Holt, A.K. Koul, L. Zhao, W. Wallace, J.C. Beddoes, Materials Characterization 35 (1995) 41–67. [6] W. Chen, Q.Y. Sun, L. Xiao, J. Sun, Materials Science and Engineering A527 (2010) 225–7234. [7] R.Q. Bao, X. Huang, C.X. Cao, Transactions of Nonferrous Metals Society of China 16 (2006) 274–280. [8] I. Weiss, S.L. Semiatin, Materials Science and Engineering A 243 (1998) 46–65. [9] G.W. Kuhlman, A.K. Chakrabarti, R. Pishko, J.W. Nelson, G. Terlinde, in: P. Lacombe, R. Tricot, G. Beranger (Eds.), Sixth World Conference on Titanium, Societe Francaise de Metallurgie, Les Ulis Cedex, France, 1988, pp. 1269–1275.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of hot compressive deformation on the martensite transformation of Ti–10V–2Fe–3Al titanium alloy Liming Lei a , Xu Huang a , Minmin Wang b , Liqiang Wang b,∗ , Jining Qin b , Hongping Li c , ShiQiang Lu d a
Titanium Alloys Laboratory, Beijing Institute of Aeronautical Materials, Beijing 100095, China State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200240, China c Institute of Commercial Aircraft Company, Shanghai 200120, China d Department of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China b
a r t i c l e
i n f o
Article history: Received 10 April 2011 Received in revised form 20 August 2011 Accepted 6 October 2011 Available online 18 October 2011 Keywords: Microanalysis Titanium alloys Martensitic transformations
a b s t r a c t The hot deformation behavior of as-cast TB6 alloy was investigated by isothermal hot compressive deformation from 800 ◦ C to 1150 ◦ C at the strain rates between 10−3 s−1 and 10 s−1 . The microstructural evolution of martensite transformation of the deformed TB6 was observed, and the volume fraction of martensite was also studied. The results show that the deformation temperature plays an important role on the stress-induced martensite transformation for TB6 titanium alloy. When the strain rate is 0.001 s−1 , 0.01 s−1 , 0.1 s−1 , 1 s−1 , and 10 s−1 , the deformation temperature with more martensite content is 900–975 ◦ C, 925–1050 ◦ C, 825–1100 ◦ C, 910–1000 ◦ C, and 925–1100 ◦ C, respectively. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental processing
Owing to their high specific strength, excellent fracture toughness, good creep and corrosion resistance, titanium alloys have been used widely in many fields such as aerospace, biomedical and sporting industries [1–5]. TB6 (Ti–10V–2Fe–3Alwt%), known as Ti-1023 titanium alloy, is a metastable beta titanium alloy with two very important features: (1) the comprehensive performance is better than that of any other known titanium alloy; (2) malleability is better than that of any other known titanium alloys, especially for the isothermal forging and hot forging. Because of its excellent forging performance, TB6 is very favorable for a variety of near net shape forging with the isothermal forging and hot forging process. Near net shape deformation process can ensure low-cost processing. In addition, it can produce very uniform cross-section of the forgings with different thickness. By now, few investigations about the effect of hot compressive deformation on the martensite transformation of TB6 titanium alloy has been reported [6–9]. The object of this paper is to investigate the microstructure evolution, especially martensite transformation during hot compressive deformation for Ti–10V–2Fe–3Al alloy. Effect of deformation temperature on flow stress of as-cast TB6 titanium alloy and effect of strain rate on flow stress of the alloy were also discussed.
The detailed chemical composition of Ti-1023 alloy used in this paper is 10.2%V, 1.8%Fe, 3.1%Al (wt%), and balance Ti. The beta transformation temperature was determined to be (802 + 3)◦ C and the microstructure of as-cast material was shown in Fig. 1. The as-cast Ti–10V–2Fe–3Al alloy with diameter of 8 mm and height of 15 mm was selected. The hot deformation behavior of as-cast TB6 alloy was investigated by isothermal constant strain rate compression test at temperatures between 800 ◦ C and 1150 ◦ C and strain rates from 10−3 s−1 to 10−1 s−1 in Thermecmaster-Z thermal simulation machine with a true strain of 0.92. Fig. 2 shows the schematic diagram of hot compression test. Deformed specimens were sectioned paralleled to the compression axis and the surface was prepared for metallographic examination using standard techniques. The specimens were etched with 2%HF + 4%HNO3 and microstructures were observed using conventional optical microscopy (OM). The content of martensite was also calculated by the optical microscopy.
∗ Corresponding author. Tel.: +86 21 34202641; fax: +86 21 34202749. E-mail address: wang [email protected] (L. Wang). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.10.028
3. Results and discussions 3.1. Martensite transformation at different temperature The stress-induced martensite of as-cast TB6 titanium alloy was caused mainly by the uniformity and the size of internal stress. The ˇ stable coefficient of TB6 alloy is about 1.1. The start transformation temperature of martensite (Ms) is below
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less content of martensite appears. As for the as-cast TB6 titanium alloy, martensite transformation can be induced in the region with lower ˇ stable coefficient, while no martensite can be induced even with larger stress in the region with larger ˇ stable coefficient. The effect of deformation temperature on the content of straininduced martensite of as-cast TB6 titanium alloy can be concluded as follows: (1) Under certain conditions, the uniformity of alloy increased with the increase of deformation temperature. (2) The deformation temperature also effects the dynamic recrystallization of alloy and volume fraction of martensite indirectly. In generally, other conditions being equal, raising the deformation temperature, the volume fraction of the dynamic recrystallization is increased. 3.2. The amount of martensite under different hot compressive deformation
Fig. 1. Microstructures of as-cast TB6 titanium alloy: (a) low magnification (b) high magnification.
room temperature, which means that no martensite appears in the process of solution treatment. However, when internal stress exists within certain region of the alloy, the start transformation temperature of martensite can be increased in this region. Generally, the content of martensite increases with the increase of stress applied. In addition, when the stress keeps in a constant value, the higher content of the ˇ stable element is, the
Fig. 2. Schematic diagram of hot compression test.
3.2.1. Martensite transformation under the strain rate of 0.001 s−1 Fig. 3 shows the optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.001 s−1 and the strain of 0.92. As shown in Fig. 3, deformation temperature played an important role on martensite transformation. With the increase of deformation temperature, much more martensite appeared. When the deformation temperature increased to 950 ◦ C, the amount of martensite reached the maximum value. It can be observed that 15% martensite transformation was obtained at 950 ◦ C. However, fewer martensite was obtained when the deformation temperature was higher than 950 ◦ C. No martensite transformation can be observed at 1150 ◦ C. The relationship between the amount of martensite and deformation temperature can be clearly shown in Fig. 4. As seen in Fig. 4, for the as-cast TB6 alloy, when the deformation temperature varied from 800 ◦ C to 1150 ◦ C, more martensite appeared between 900 ◦ C and 975 ◦ C. When the deformation temperature was lower (T < 900 ◦ C), less dynamic recrystallization appeared. In addition, the stress was larger with relatively uniform composition. Therefore, in the region with more ˇ stable elements, little martensite transformation appeared even if there was larger internal stress. With the increase of deformation temperature (900 ◦ C < T < 975 ◦ C), an optimal combination of internal stress and composition uniformity was obtained and maximum content of martensite about 15% was obtained. When the deformation temperature was above 975 ◦ C, although the composition was uniform and the ˇ stable factor of the alloy tended to 1.1, much more recrystallization appeared. Less content of martensite was obtained because of the lower internal stress. 3.2.2. Martensite transformation under the strain rate of 0.01 s−1 Fig. 5 shows the optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.01 s−1 and the strain of 0.92. Fig. 6 shows the relationship between the content of martensite transformation and deformation temperature. As shown in Fig. 5, maximum content of martensite about 39% was obtained at 1000 ◦ C. However, less martensite can be observed when the deformation temperature was higher than 1000 ◦ C (Fig. 5(e)). Compared with the martensite transformation under the strain rate of 0.001 s−1 , similar characteristic of martensite transformation was found under the strain rate of 0.01 s−1 . As shown in Fig. 6, more martensite transformation appeared between 925 ◦ C and 1050 ◦ C. No significant differences of the needle-like martensite observed in the ˇ
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Fig. 3. Optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.001 s−1 : (a) 800 ◦ C, (b) 850 ◦ C, (c) 900 ◦ C, (d) 950 ◦ C, (e) 1000 ◦ C, (f) 1050 ◦ C, (g) 1100 ◦ C.
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Fig. 4. The relationship between the amount of martensite and deformation temperature under the strain rate of 0.001 s−1 .
value and then decreased to a lower value. However, the maximum content of martensite was different with the strain rate. Under the low strain rate (0.001 s−1 ), the maximum amount of martensite was about 15%. Under the medium strain rate (0.01 s−1 , 0.1 s−1 , 1 s−1 ), the maximum amount of martensite has greatly been improved. The maximum value of martensite was 39%, 40%, and 50%, respectively under the strain rate of 0.01 s−1 , 0.1 s−1 and 1 s−1 . While under the high strain rate (10 s−1 ), the content of martensite decreased to 25%. As mentioned in the optical microscopies shown above, twins were the secondary microstructure of martensite, As the deformation defect, the appear of martensite accelerated the process of grain refinement and dynamic recrystallization during hot deformation. However, a large amount of martensite contributed much to the strength of the alloy, which was not so good for the further deformation. Therefore, appropriate content of martensite was needed with regards to applications, i.e. forging. 3.3. Effect of deformation temperature on flow stress of as-cast TB6 titanium alloy
grain and grain boundaries can be found. Acicular martensite is formed by the criss-cross martensite group. Martensite appearing at the grain boundaries was mostly distributed in the two grains (Fig. 5(a) and (c)). In addition, there were also martensite groups appearing through several grains (Fig. 5(e)). As seen in Fig. 5, many dendritic martensite stacking together was observed, clearly. 3.2.3. Martensite transformation under the strain rate of 0.1 s−1 Fig. 7 shows the optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.1 s−1 and the strain of 0.92. Fig. 8 shows the relationship between the content of martensite transformation and deformation temperature. As shown in Fig. 8, maximum content of martensite was obtained when the deformation temperature were 825 ◦ C and 1100 ◦ C, which were 40% and 33%, respectively. 3.2.4. Martensite transformation under the strain rate of 1.0 s−1 Fig. 9 shows the optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 1.0 s−1 and the strain of 0.92. As shown in Fig. 9(c), when the deformation temperature was 925 ◦ C, maximum content of martensite was obtained. Dendritic martensite appeared between grain boundaries and the content was about 50%. Fig. 10 shows the relationship between the martensite transformation and the deformation temperature of as-cast TB6 titanium alloy. It can be found that between 910 ◦ C and 1000 ◦ C, much more martensite was obtained, which was similar with the martensite transformation under the strain rate of 0.001 s−1 . 3.2.5. Martensite transformation under the strain rate of 10.0 s−1 The microstructures of as-cast TB6 titanium alloy under the strain rate of 10 s−1 with the strain of 0.92 were shown in Fig. 11. The deformation temperature was from 800 ◦ C to 1150 ◦ C. As shown in Fig. 11, with the increase of deformation temperature, more martensite transformation appeared. However, when the temperature increased over 1100 ◦ C, less martensite transformation was obtained. Maximum value of martensite was observed between 925 ◦ C and 1100 ◦ C. As indicated in Fig. 12, 25% martensite transformation appeared at 1100 ◦ C. As mentioned above, under the five different strain rates, the characteristic of the influence of deformation temperature on the martensite transformation was nearly the same. With the increase of deformation, the amount of martensite increased to a peak
Fig. 13 shows the stress–strain curve of as-cast TB6 titanium alloy at different strain rate. In the initial stage of deformation, flow stress increased rapidly with the increasing of strain. While arriving at the maximum value (ε <0.1), flow stress decreased with the increasing of strain. The lower the temperature was, the more obvious softening was obtained. At certain strain, such as 0.001 s−1 , little continuous reduction of flow stress can be observed (Fig. 13(a)), which was dominated by the interaction effect of hardening and softening. Strain hardening was obtained at the strain rate of 0.01 s−1 , 0.1 s−1 and 1.0 s−1 (Fig. 13(b)–(d)). However, no strain hardening appeared at the strain rate of 10.0 s−1 and 0.001 s−1 , as shown in Fig. 13(e). At higher strain rate such as 10 s−1 , the strain hardening was caused by the accumulation of dislocations during deformation. Much more hardening occurred because of deformation defects at higher strain rate. However, accompanying with a large amount of dislocations, dynamic softening took place easily at high temperature, which decreased the flow stress. In addition, at lower strain rate such as 0.001 s−1 , there was sufficient time for dynamic recovery and dynamic recrystallization. Therefore, no strain hardening was observed at 10 s−1 and 0.001 s−1 rates. Except at the strain rate of 0.001 s−1 , the effect of temperature on flow stress was similar at both higher temperature and low temperature. When the temperature increased by 25 ◦ C, flow stress is decreased by about 4–9 MPa. It can be observed that deformation temperature played an important role in the flow stress. At certain strain rate, the higher the temperature was, the lower the flow stress was obtained. This was mainly because with the increase of temperature, the enhanced role of thermal activation and average kinetic energy of atoms increased, the critical slip shear stress decreased which reduced the obstacles of dislocation movement and the slip between crystal planes. Furthermore, along with the increasing temperature, dynamic recovery and dynamic recrystallization took place more easily, and the dislocation density was reduced, which offset the hardening caused by plastic deformation, thus reducing the flow stress of the material. At the beginning of deformation, the flow stress increased rapidly with increasing strain rate, and showed flow softening characteristics after reaching a maximum value. This is mainly because in the initial stage of deformation, the appearing of hardening and a large number of proliferation and tangles of dislocations contributed most to the hardening of the material. Therefore, in this condition, the flow stress increased to a larger value significantly. With the further increase of deformation,
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Fig. 5. Optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.01 s−1 : (a) 800 ◦ C, (b) 850 ◦ C, (c) 900 ◦ C, (d) 950 ◦ C, (e) 1000 ◦ C, (f) 1050 ◦ C, (g) 1100 ◦ C.
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Fig. 6. The relationship between the amount of martensite and deformation temperature under the strain rate of 0.01 s−1 .
softening effects, such as dynamic recovery and dynamic recrystallization appeared gradually. At the same time, the flow stress began to decline as the dominant softening effects. In addition, at certain temperature, with the increase of strain rate, the flow stress increased gradually. Take the stress–strain curve of as-cast TB6 titanium alloy at 800 ◦ C for example, when the strain rate was 0.001 s−1 , 0.01 s−1 , 0.1 s−1 , 1.0 s−1 , and 10 s−1 , the largest flow stress was 51 MPa, 86 MPa, 108 MPa, 160 MPa, and 225 MPa, respectively, which were shown in Fig. 13(a)–(e). The trend can
Fig. 8. The relationship between the amount of martensite and deformation temperature under the strain rate of 0.1 s−1 .
also be found when the temperature ranged from 825 ◦ C to 1150 ◦ C. 3.4. Effect of strain rate on flow stress of as-cast TB6 titanium alloy Fig. 14 shows the stress–strain curve of as-cast TB6 titanium alloys with different strain rate at various temperatures. As shown
Fig. 7. Optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 0.1 s−1 : (a) 800 ◦ C, (b) 900 ◦ C, (c) 1100 ◦ C.
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Fig. 9. Optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 1.0 s−1 : (a) 800 ◦ C, (b) 925 ◦ C, (c) 1100 ◦ C.
in Fig. 14, flow stress increased rapidly with the increasing of strain rate at low deformation temperature. However, when the deformation temperature was higher, flow stress increased slowly with the increasing of strain rate. At 800 ◦ C, flow stress increased by 45 MPa when the strain rate increased by an order of magnitude. In addition, at 1150 ◦ C, flow stress increased by 25 MPa when the strain rate increased by an order of magnitude. When the strain rate was over 0.1 s−1 , flow stress increased quickly with the increase of strain
rate. While when the strain rate was less than 0.1 s−1 , flow stress was not so sensitive to strain rate. As shown in Fig. 14, the elastic modulus of the TB6 titanium alloy seemed similar for strain rates of 0.01 s−1 and 0.1 s−1 under the temperature of 950 ◦ C. However, they separated at higher temperatures above 950 ◦ C. It can be concluded that under the strain rates of 0.01 s−1 and 0.1 s−1 , the compressive elastic modulus of the TB6 alloy was related higher with the temperature when the temperature was above 950 ◦ C, which meant
Fig. 10. The relationship between the amount of martensite and deformation temperature under the strain rate of 1.0 s−1 .
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Fig. 11. Optical microscopy of as-cast TB6 titanium alloy at different deformation temperature under the strain rate of 10.0 s−1 : (a) 800 ◦ C, (b) 925 ◦ C, (c) 1100 ◦ C, (d) 1150 ◦ C.
Fig. 12. The relationship between the amount of martensite and deformation temperature under the strain rate of 10.0 s−1 .
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Fig. 13. Stress–strain curve of as-cast TB6 titanium alloy at different strain rate: (a) 0.001 s−1 ; (b) 0.01 s−1 ; (c) 0.1 s−1 ; (d) 1.0 s−1 ; (e) 10 s−1 .
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Fig. 14. Stress–strain curve of TB6 titanium alloys with different strain rate at various temperatures: (a) 800 ◦ C; (b) 875 ◦ C; (c) 950 ◦ C; (d) 1050 ◦ C; (e) 1150 ◦ C.
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that at higher deformation temperature, at certain deformation strain, a larger compressive stress was applied at the strain rate of 0.1 s−1 . 4. Conclusions The microstructure of as-cast TB6 titanium alloy was studied. The deformation temperature varied from 800 ◦ C to 1150 ◦ C and the strain rate was between 0.001 s−1 and 10 s−1 under the total strain of 0.92. The main conclusions are as follows: (1) As for the mechanical instability of ˇ phase in TB6 titanium alloy, stress-induced martensite transformation occurs in the cooling process after hot deformation. The dendritic morphology martensite appears in the ˇ grain and grain boundaries. (2) Deformation temperature plays an important role on the stressinduced martensite transformation for TB6 titanium alloy. Only at moderate deformation temperature, when a best combination of the alloy composition uniformity and internal stress is achieved, maximum content of martensite is obtained. (3) The content of martensite phase changed with the different strain rate at certain deformation temperature. When the strain rate is 0.001 s−1 , 0.01 s−1 , 0.1 s−1 , 1 s−1 , and 10 s−1 , respectively, the deformation temperature with more martensite content is 900–975 ◦ C, 925–1050 ◦ C, 825–1100 ◦ C, 910–1000 ◦ C, and 925–1100 ◦ C, respectively. (4) In the initial stages of deformation, flow stress increased rapidly with increasing strain rate. Under small strain rate (ε < 0.1),
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after reaching its peak, flow stress showed a flow softening, and the lower the temperature was, the more evident the softening phenomenon was. Under certain strain rate, such as 0.001 s−1 , the reduction of flow stress was not continuous. In addition to strain rate of 0.001 s−1 , compared with high temperature, the effect of the temperature on the flow stress was more significant than at low temperature. Little difference about effect of the temperature on the flow stress can be obtained at other strain rates. Acknowledgement We would like to acknowledge financial support provided by the National Basic Research Program of China under grant no. 2007CB613803. References [1] N. Poondla, T.S. Srivatsan, A. Patnaik, M. Petraroli, Journal of Alloys and Compounds 486 (2009) 162–167. [2] M. Niinomi, Science and Technology of Advanced Materials 4 (2003) 445–454. [3] S. Hardt, H.J. Maier, J. Christ, International Journal of Fatigue 21 (1999) 779–789. [4] I.V. Gorynin, Materials Science and Engineering A 263 (1999) 112–116. [5] J.P. Immarigeon, R.T. Holt, A.K. Koul, L. Zhao, W. Wallace, J.C. Beddoes, Materials Characterization 35 (1995) 41–67. [6] W. Chen, Q.Y. Sun, L. Xiao, J. Sun, Materials Science and Engineering A527 (2010) 225–7234. [7] R.Q. Bao, X. Huang, C.X. Cao, Transactions of Nonferrous Metals Society of China 16 (2006) 274–280. [8] I. Weiss, S.L. Semiatin, Materials Science and Engineering A 243 (1998) 46–65. [9] G.W. Kuhlman, A.K. Chakrabarti, R. Pishko, J.W. Nelson, G. Terlinde, in: P. Lacombe, R. Tricot, G. Beranger (Eds.), Sixth World Conference on Titanium, Societe Francaise de Metallurgie, Les Ulis Cedex, France, 1988, pp. 1269–1275.