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Texture and superelastic behavior of cold-rolled TiNbTaZr alloy
Materials Science and Engineering A 491 (2008) 372–377
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Texture and superelastic behavior of cold-rolled TiNbTaZr alloy Liqiang Wang, Weijie Lu ∗ , Jining Qin, Fan Zhang, Di Zhang State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 2 January 2008 Received in revised form 6 February 2008 Accepted 7 April 2008 Keywords: Alloys X-ray diffraction Microstructure Elastic properties
a b s t r a c t This work investigates the deformation texture and strain-induced ␣ martensite texture of TiNbTaZr alloy during cold rolling. The alloy is rolled by 20% and 90% reductions without changing rolling direction. Textures of cold-rolled specimens are investigated by X-ray diffraction measurements. Besides {2 2 1} 1 1 4 twinning texture, {1 0 0} 0 0 1 texture is developed in the specimen with 20% reduction. In the 90% coldrolled specimen, {1 0 0} 0 1 1 texture appears along rolling direction and strain-induced ␣ martensite texture tends to [0 1 0] and [0 0 1] directions along rolling direction (RD) and transverse direction (TD), respectively. Superelastic strain (εSE ) exhibits higher value along RD and TD. Pure elastic strain (εE ) shows higher value along RD and 45◦ from RD. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As functional materials, shape memory alloys (SMAs) are attracting much attention in recent years [1–3]. Shape memory effect (SME) and superelasticity (SE) can be observed in shape memory alloys. SME is a phenomenon such that an apparent plastic strain given at a temperature below As recovers by heating to a temperature above Af , by virtue of the (crystallographically) reversible reverse transformation. SE, which is a pseudoelasticity occurring at a temperature above Af , is caused by a strain-induced martensitic transformation upon loading and by the subsequent reverse transformation upon unloading [4]. Ti–Ni alloys are the shape memory alloys which are presently used in practical biomedical applications due to their superior superelasticity and shape memory effect. Because of Ni-hypersensitivity and cytotoxicity of Ni, development of new Ni-free biomedical superelastic alloys is of vital importance. Nowadays, new kinds of Ni-free titanium alloys have been developed [5–8]. It has been reported by Baker that the shape memory effect can be observed in Ti–35Nb (wt.%) alloy [9]. By now, shape memory and superelasticity during newly developed  titanium alloys have also been studied [7,10]. In most  titanium alloys, the transformation from  to ␣ contributes much to the shape memory effect and superelasticity. In addition, strain-induced ␣ phase and deformation textures during cold deformation also play an important role in superelasticity of deformed alloys. Owing to the excellent cold workability of newly developed  titanium alloys, the study of
∗ Corresponding author. Tel.: +86 21 34202641; fax: +86 21 34202749. E-mail address: [email protected] (W. Lu). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.04.018
texture and superelasticity is very important in biomedical applications. This paper aims at studying the textures of  phase and straininduced ␣ martensite phase during cold rolling with different reductions. The effect of reduction and textures to superelasticity was investigated. The orientation dependence of superelastic strain and pure elastic strain was also discussed. Deformation characteristic of the studied alloy was investigated by X-ray diffraction (XRD) measurements. 2. Experimental procedure The TiNbTaZr alloy with a nominal composition of Ti–35Nb– 2Ta–3Zr (wt%) was prepared adopting arc-melting method. The ingot was re-melted three times to ensure compositional homogeneity. The alloy ingot was homogenized at 1223 K for 3.6 ks in vacuum condition and then forged into a quadrate casting. Followed by solution heat treatment at 1053 K for 0.5 h, the alloy was rolled with the reductions of 20% and 90% in thickness. X-ray diffraction measurements were carried out under the conditions of Cu K␣, 35 kV and 100 mA at room temperature. The distribution of diffraction intensities from three crystal planes {2 0 0}, {2 1 1} and {1 1 0} of the  phase and two crystal planes {2 0 0} and {0 2 1} of the ␣ martensite phase were measured. Orientation distribution functions (ODFs) were derived using the obtained pole figures. Microstructure observations were carried out by JEM-2000 EX transmission electron microscopy (TEM). Superelastic properties were measured through cyclic loading–unloading tensile test. Tensile tests were carried out on Zwick T1-Fr020TN materials testing machine at a strain rate of 1.5 × 10−4 s at room temperature.
L. Wang et al. / Materials Science and Engineering A 491 (2008) 372–377
Fig. 1. XRD patterns of 20% and 90% cold-rolled specimens.
3. Results and discussions 3.1. Textures and microstructure Fig. 1 shows the XRD profiles of cold-rolled specimens after solution heat treatment at 1053 K followed by air-cooling. As it is shown,  (bcc) phase and strain-induced ␣ (orthorhombic) martensite phase appeared when the reductions were 20% and 90%. With the increase of cold reduction, the peak intensity of (2 0 0)␣ increased. Besides the three major peaks of  phase, the intensity of peaks of ␣ martensite phase such as (0 2 1)␣ , (1 3 0)␣ and (2 2 0)␣ nearly kept the same regardless of the cold reduction. This result indicated that ␣ rolling textures along (2 0 0)␣ plane became predominant when the cold reduction was around 90%.
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Pole figure measurements were carried out using the three major pole planes (2 0 0), (2 1 1) and (1 1 0). The orientation functions (ODFs) were calculated based on the measured pole figures. Fig. 2 shows {2 0 0}, {2 1 1} and {1 1 0}  pole figures obtained from 20% and 90% cold-rolled specimens in the rolling planes. Fig. 3 shows ˚2 = 45◦ sections of ODFs obtained from the calculation of pole figures above. As the ODFs shown in Fig. 3a, when the cold reduction was 20%, preferred {1 0 0} 0 0 1 and {1 1 1} 1 1 0 textures were obtained along rolling direction (RD). In addition, weaker textures such as {0 1 1} 1 0 0 and {2 2 1} 1 1 4 appeared. When the cold reduction added up to 90%, stronger {1 0 0} 0 1 1 texture and weaker {1 1 1} 1 1 0 texture can be observed (Fig. 3b). It can be estimated that grains with 1 1 0 direction parallel to the rolling plane. Fig. 4 shows ODFs at around 45◦ from RD for  phase of the cold-rolled specimens. Besides {1 1 1} 1 1 2 and {2 2 1} 1 1 4 textures, strong {1 0 0} 0 1 1 texture appeared in the 20% rolled specimen (Fig. 4a). Preferred {1 0 0} 0 0 1 texture was also observed in the 90% rolled specimen (Fig. 4b). Plastic deformation in metal contained dislocation slip, deformation twinning and strain-induced phase transformation. Generally, as for BCC structure, shear deformation occurred on {1 1 0}, {1 1 2} and {1 2 3} planes in slip processes [11,12]. During plastic deformation, the orientation of grains tended to stable directions. The slip planes of B2 ordered structure alloys focused on {1 1 0} and {1 1 2} planes and the grains tended to form {1 1 1}1 1 2 stable texture [13]. As the researchers studied [14,15], during deformation, accompanying with deformation twinning, grains of  -CuZn alloy tended to 1 1 4 direction. As twinning texture, {2 2 1}1 1 4 texture was obtained. The orientation of grains changed from {0 0 1}1 1 0 to {2 2 1}1 1 4 when a large
Fig. 2. {2 0 0}, {2 1 1} and {1 1 0} pole figures in the rolling plane for  of cold-rolled specimens: (a) 20%; (b) 90%.
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Fig. 3. Sections (˚2 = 45◦ ) of the orientation distribution functions in the rolling plane for  of the specimens with cold rolling reductions of 20% (a) and 90% (b).
Fig. 4. Sections (˚2 = 45◦ ) of the orientation distribution functions at around 45◦ from RD for  of the specimens with cold rolling reductions of 20% (a) and 90% (b).
amount of twins appeared. The alloy of Ti35Nb3Zr2Ta has a similar structure with B2 ordered structure [16]. On the other hand, Ti35Nb3Zr2Ta can be considered as DO3 ordered structure. Fig. 5 shows the bright field TEM micrograph of the 20% rolled specimen. Plate-shaped deformation twinning was observed clearly. As mentioned above, {2 2 1} 1 1 4 texture which was the characteristic of deformation twinning appeared in the 20% rolled specimen. Strong {1 0 0} 0 1 1 texture was distinguished as the main texture in the specimen rolled by 90% reduction. As seen in the bright
field TEM micrograph of the 90% rolled specimen (Fig. 6), the shearing bands (SBs) appearing in slip processes were also observed. The result shows that the preferred slip direction rotated toward 0 1 1 direction which was parallel to the rolling direction. Fig. 7 shows (2 0 0)␣ and (0 2 1)␣ pole figures in the rolling plane as a function of rolling reduction. It was observed that (2 0 0)␣ texture became stronger when the reduction increased from 20% to 90%. In addition, as for (0 2 1)␣ texture appearing in 20% and 90% cold-rolled specimens, little difference can be
L. Wang et al. / Materials Science and Engineering A 491 (2008) 372–377
Fig. 6. Bright field TEM micrograph of the of the 90% rolled specimen. Fig. 5. Bright field TEM micrograph of the 20% rolled specimen.
Fig. 7. Pole figures for ␣ of (2 0 0)␣ and (0 2 1)␣ in rolling plane of cold-rolled specimens: (a) 20%; (b) 90%.
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Fig. 8. Stress–strain curves obtained by cyclic loading–unloading tensile tests for the specimens with cold reduction of 20% (a) and (b) and 90% (c) and (d).
found. With the increase of reduction, strain-induced ␣ martensite phase tended to form (2 0 0)␣ 0 1 0␣ texture in the RD and (2 0 0)␣ 0 0 1␣ texture appeared along transverse direction (TD), which was in good agreement with a result reported before [17].
Fig. 9. ˚ dependence of εSE and εE extracted from the stress–strain curves at strain of 1.5% and 2.5%.
3.2. Superelastic behavior Fig. 8 shows stress–strain curves obtained by cyclic loading–unloading tensile tests for the specimens with cold reduction of 20% and 90%. Tensile axes were along RD, 45◦ from RD and TD, respectively. In the first cycle, the tensile stress was applied when strain reached about 1.5%, and then the stress was removed. The measurement was repeated by increasing the maximum strain to 2.5%. The superelastic characteristic of cold-rolled specimens were described as two types of strains such as εSE and εE . εSE and εE were defined as superelastic strain and pure elastic strain upon unloading, respectively. εSE was considered as transformation strain recovered superelastically upon unloading and εE was considered as the elastic strain recovered elastically upon unloading. It had been studied [18,19] that the movement of twin-boundaries in martensites which were induced by the deformation could contribute much to superelasticity. As shown in Fig. 8a and b, superelasticity was observed in the 20% rolled specimen. Owing to the deformation twinning and strain-induced ␣ martensitic, the movement of twin-boundaries and slip of ␣ martensitic contributed much to superelasticity. No clear difference of εSE and εE was seen along the directions of ˚ = 0◦ (RD), ˚ = 45◦ and ˚ = 90◦ (TD). When the alloy was deformed by 90% reduction (Fig. 8c and d), εSE along the directions of ˚ = 0◦ (RD) and ˚ = 90◦ (TD) were larger than that at ˚ = 45◦ . On the other hand, εE kept higher value along the directions of ˚ = 0◦
L. Wang et al. / Materials Science and Engineering A 491 (2008) 372–377
(RD) and ˚ = 45◦ , as shown in Fig. 9. As research reported [20], transformation strain decreased with changing direction from [0 1 1] to [0 0 1]. εSE , as a part of transformation strain of  → ␣ in TiNbZrTa alloy, exhibited higher value along the direction of [0 1 1], which was parallel to RD or TD. Recently, Inamura et al. showed the anisotropic Young’s modulus in phase of TiNbAl alloy [21]. According to their result, the value of Young’s modulus was in the order of [0 0 1] < [1 1 0] < [1 1 1] . The texture along the direction of ˚ = 45◦ in the specimen rolled by 90% was mainly {1 0 0} 0 0 1 , which made εE higher in ˚ = 45◦ . In addition, strain-induced ␣ martensite phase tended to form (2 0 0)␣ 0 1 0␣ texture in RD. Among the three lattice constants a, b and c of ␣ martensite phase, b was the largest. The largest value of b decreased the Young’s modulus in RD. Therefore, εE in RD also exhibited higher value than that in TD. 4. Conclusions (1) Preferred {1 0 0} 0 0 1 and {1 1 1} 1 1 0 textures are obtained along rolling direction in the specimen with 20% reduction. {2 2 1} 1 1 4 twinning texture is also developed along rolling direction and 45◦ from rolling direction. As for the 90% rolled specimen, a well developed {1 0 0} 0 1 1 texture appears along the rolling direction and transverse direction. Strong {1 0 0} 0 0 1 texture is also observed along 45◦ from rolling direction. (2) Strain-induce ␣ martensite texture exhibits as preferred (2 0 0)␣ 0 1 0␣ texture in the rolling direction. Meanwhile, (2 0 0)␣ 0 0 1␣ texture appears along transverse direction in the 90% rolled specimen. (3) Clear anisotropy in superelastic strain and pure elastic strain in the 90% rolled specimen is observed. Owing to the anisotropy of  texture and ␣ martensite texture, εSE along RD and TD are larger than that at ˚ = 45◦ . On the other hand, εE keeps higher value along RD and ˚ = 45◦ .
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Acknowledgements We would like to acknowledge a financial support provided by High Technology Research and Development Program of China under Grant No.: 2006AA03Z559, 973 Program under Grant No.: 2007CB613806, A Foundation for the Author of National Excellent Doctoral Dissertation of PR China under Grant No.: 200332. References [1] H.Y. Kim, Y. Ikehara, J.I. Kim, H. Hosoda, S. Miyazaki, Acta Mater. 54 (2006) 2419–2429. [2] D.H. Ping, Y. Mitarai, F.X. Yin, Scripta Mater. 52 (2005) 1287–1291. [3] P. Laheurte, A. Eberhardt, M.J. Philippe, Mater. Sci. Eng. A 396 (2005) 223–230. [4] K. Otsuka, X.B. Ren, Intermetallics 7 (1999) 511–528. [5] M.J. Long, H.J. Rack, Biomaterials 19 (1998) 1621–1639. [6] E. Eisenbarth, D. Velten, M. Muller, R. Thull, J. BremeE, Biomaterials 25 (2004) 5705–5713. [7] J.I. Kim, H.Y. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Mater. Sci. Eng. A 403 (2005) 334–339. [8] W.F. Ho, C.P. Ju, J.H. Chern Lin, Biomaterials 20 (1999) 2115–2122. [9] C. Baker, Metall. Sci. J. 5 (1971) 92–100. [10] H. Hosoda, Y. Fukui, T. Inamura, K. Wakashima, S. Miyazaki, K. Inoue, Mater. Sci. Forum 426–432 (2003) 3121–3215. [11] D. Pionnier, M. Humbert, M.J. Philippe, Y. Combres, Acta Mater. 46 (1998) 5891–5898. [12] D. Roundy, C.R. Krenn, M.L. Cohen, J.W. Morris, Phil. Mag. A81 (2001) 1725–1747. [13] S. Takeuchi, Phil. Mag. A41 (1980) 541–553. [14] G.H. Zhu, W. Mao, Y. Yu, in: M.A. Imam (Ed.), PRICM3 Conference Proceedings, vol. 2, TMS, Hawaii, 1982, pp. 1983–1988. [15] G.H. Zhu, W. Mao, Y. Yu, in: J.A. Szpunar (Ed.), Proceedings of the 12th International Conference on Textures of Material, vol. 2, NRC Research Press, Montreal, 1999, pp. 629–634. [16] H. Ikehata, N. Nagasako, T. Furuta, A. Fukumoto, K. Miwa, T. Saito, Phys. Rev. B 70 (2004) 174113. [17] H. Matsumoto, S. Watanabe, S. Hanada, Mater. Trans. 46 (2005) 1070–1078. [18] K. Otsuka, C.M. Wayman, Shape Memory Materials, Cambridge University Press, 1998, p. 44. [19] T. Inamuraa, Y. Fukui, H. Hosoda, K. Wakashima, S. Miyazaki, Mater. Sci. Eng. C 25 (2005) 426–432. [20] H.Y. Kim, T. Sasaki, K. Okutsu, J.I. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Acta Mater. 54 (2006) 423–433. [21] T. Inamura, H. Hosoda, K. Wakashima, S. Miyazaki, Mater. Trans. 46 (2005) 1597–1603.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Texture and superelastic behavior of cold-rolled TiNbTaZr alloy Liqiang Wang, Weijie Lu ∗ , Jining Qin, Fan Zhang, Di Zhang State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 2 January 2008 Received in revised form 6 February 2008 Accepted 7 April 2008 Keywords: Alloys X-ray diffraction Microstructure Elastic properties
a b s t r a c t This work investigates the deformation texture and strain-induced ␣ martensite texture of TiNbTaZr alloy during cold rolling. The alloy is rolled by 20% and 90% reductions without changing rolling direction. Textures of cold-rolled specimens are investigated by X-ray diffraction measurements. Besides {2 2 1} 1 1 4 twinning texture, {1 0 0} 0 0 1 texture is developed in the specimen with 20% reduction. In the 90% coldrolled specimen, {1 0 0} 0 1 1 texture appears along rolling direction and strain-induced ␣ martensite texture tends to [0 1 0] and [0 0 1] directions along rolling direction (RD) and transverse direction (TD), respectively. Superelastic strain (εSE ) exhibits higher value along RD and TD. Pure elastic strain (εE ) shows higher value along RD and 45◦ from RD. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As functional materials, shape memory alloys (SMAs) are attracting much attention in recent years [1–3]. Shape memory effect (SME) and superelasticity (SE) can be observed in shape memory alloys. SME is a phenomenon such that an apparent plastic strain given at a temperature below As recovers by heating to a temperature above Af , by virtue of the (crystallographically) reversible reverse transformation. SE, which is a pseudoelasticity occurring at a temperature above Af , is caused by a strain-induced martensitic transformation upon loading and by the subsequent reverse transformation upon unloading [4]. Ti–Ni alloys are the shape memory alloys which are presently used in practical biomedical applications due to their superior superelasticity and shape memory effect. Because of Ni-hypersensitivity and cytotoxicity of Ni, development of new Ni-free biomedical superelastic alloys is of vital importance. Nowadays, new kinds of Ni-free titanium alloys have been developed [5–8]. It has been reported by Baker that the shape memory effect can be observed in Ti–35Nb (wt.%) alloy [9]. By now, shape memory and superelasticity during newly developed  titanium alloys have also been studied [7,10]. In most  titanium alloys, the transformation from  to ␣ contributes much to the shape memory effect and superelasticity. In addition, strain-induced ␣ phase and deformation textures during cold deformation also play an important role in superelasticity of deformed alloys. Owing to the excellent cold workability of newly developed  titanium alloys, the study of
∗ Corresponding author. Tel.: +86 21 34202641; fax: +86 21 34202749. E-mail address: [email protected] (W. Lu). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.04.018
texture and superelasticity is very important in biomedical applications. This paper aims at studying the textures of  phase and straininduced ␣ martensite phase during cold rolling with different reductions. The effect of reduction and textures to superelasticity was investigated. The orientation dependence of superelastic strain and pure elastic strain was also discussed. Deformation characteristic of the studied alloy was investigated by X-ray diffraction (XRD) measurements. 2. Experimental procedure The TiNbTaZr alloy with a nominal composition of Ti–35Nb– 2Ta–3Zr (wt%) was prepared adopting arc-melting method. The ingot was re-melted three times to ensure compositional homogeneity. The alloy ingot was homogenized at 1223 K for 3.6 ks in vacuum condition and then forged into a quadrate casting. Followed by solution heat treatment at 1053 K for 0.5 h, the alloy was rolled with the reductions of 20% and 90% in thickness. X-ray diffraction measurements were carried out under the conditions of Cu K␣, 35 kV and 100 mA at room temperature. The distribution of diffraction intensities from three crystal planes {2 0 0}, {2 1 1} and {1 1 0} of the  phase and two crystal planes {2 0 0} and {0 2 1} of the ␣ martensite phase were measured. Orientation distribution functions (ODFs) were derived using the obtained pole figures. Microstructure observations were carried out by JEM-2000 EX transmission electron microscopy (TEM). Superelastic properties were measured through cyclic loading–unloading tensile test. Tensile tests were carried out on Zwick T1-Fr020TN materials testing machine at a strain rate of 1.5 × 10−4 s at room temperature.
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Fig. 1. XRD patterns of 20% and 90% cold-rolled specimens.
3. Results and discussions 3.1. Textures and microstructure Fig. 1 shows the XRD profiles of cold-rolled specimens after solution heat treatment at 1053 K followed by air-cooling. As it is shown,  (bcc) phase and strain-induced ␣ (orthorhombic) martensite phase appeared when the reductions were 20% and 90%. With the increase of cold reduction, the peak intensity of (2 0 0)␣ increased. Besides the three major peaks of  phase, the intensity of peaks of ␣ martensite phase such as (0 2 1)␣ , (1 3 0)␣ and (2 2 0)␣ nearly kept the same regardless of the cold reduction. This result indicated that ␣ rolling textures along (2 0 0)␣ plane became predominant when the cold reduction was around 90%.
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Pole figure measurements were carried out using the three major pole planes (2 0 0), (2 1 1) and (1 1 0). The orientation functions (ODFs) were calculated based on the measured pole figures. Fig. 2 shows {2 0 0}, {2 1 1} and {1 1 0}  pole figures obtained from 20% and 90% cold-rolled specimens in the rolling planes. Fig. 3 shows ˚2 = 45◦ sections of ODFs obtained from the calculation of pole figures above. As the ODFs shown in Fig. 3a, when the cold reduction was 20%, preferred {1 0 0} 0 0 1 and {1 1 1} 1 1 0 textures were obtained along rolling direction (RD). In addition, weaker textures such as {0 1 1} 1 0 0 and {2 2 1} 1 1 4 appeared. When the cold reduction added up to 90%, stronger {1 0 0} 0 1 1 texture and weaker {1 1 1} 1 1 0 texture can be observed (Fig. 3b). It can be estimated that grains with 1 1 0 direction parallel to the rolling plane. Fig. 4 shows ODFs at around 45◦ from RD for  phase of the cold-rolled specimens. Besides {1 1 1} 1 1 2 and {2 2 1} 1 1 4 textures, strong {1 0 0} 0 1 1 texture appeared in the 20% rolled specimen (Fig. 4a). Preferred {1 0 0} 0 0 1 texture was also observed in the 90% rolled specimen (Fig. 4b). Plastic deformation in metal contained dislocation slip, deformation twinning and strain-induced phase transformation. Generally, as for BCC structure, shear deformation occurred on {1 1 0}, {1 1 2} and {1 2 3} planes in slip processes [11,12]. During plastic deformation, the orientation of grains tended to stable directions. The slip planes of B2 ordered structure alloys focused on {1 1 0} and {1 1 2} planes and the grains tended to form {1 1 1}1 1 2 stable texture [13]. As the researchers studied [14,15], during deformation, accompanying with deformation twinning, grains of  -CuZn alloy tended to 1 1 4 direction. As twinning texture, {2 2 1}1 1 4 texture was obtained. The orientation of grains changed from {0 0 1}1 1 0 to {2 2 1}1 1 4 when a large
Fig. 2. {2 0 0}, {2 1 1} and {1 1 0} pole figures in the rolling plane for  of cold-rolled specimens: (a) 20%; (b) 90%.
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Fig. 3. Sections (˚2 = 45◦ ) of the orientation distribution functions in the rolling plane for  of the specimens with cold rolling reductions of 20% (a) and 90% (b).
Fig. 4. Sections (˚2 = 45◦ ) of the orientation distribution functions at around 45◦ from RD for  of the specimens with cold rolling reductions of 20% (a) and 90% (b).
amount of twins appeared. The alloy of Ti35Nb3Zr2Ta has a similar structure with B2 ordered structure [16]. On the other hand, Ti35Nb3Zr2Ta can be considered as DO3 ordered structure. Fig. 5 shows the bright field TEM micrograph of the 20% rolled specimen. Plate-shaped deformation twinning was observed clearly. As mentioned above, {2 2 1} 1 1 4 texture which was the characteristic of deformation twinning appeared in the 20% rolled specimen. Strong {1 0 0} 0 1 1 texture was distinguished as the main texture in the specimen rolled by 90% reduction. As seen in the bright
field TEM micrograph of the 90% rolled specimen (Fig. 6), the shearing bands (SBs) appearing in slip processes were also observed. The result shows that the preferred slip direction rotated toward 0 1 1 direction which was parallel to the rolling direction. Fig. 7 shows (2 0 0)␣ and (0 2 1)␣ pole figures in the rolling plane as a function of rolling reduction. It was observed that (2 0 0)␣ texture became stronger when the reduction increased from 20% to 90%. In addition, as for (0 2 1)␣ texture appearing in 20% and 90% cold-rolled specimens, little difference can be
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Fig. 6. Bright field TEM micrograph of the of the 90% rolled specimen. Fig. 5. Bright field TEM micrograph of the 20% rolled specimen.
Fig. 7. Pole figures for ␣ of (2 0 0)␣ and (0 2 1)␣ in rolling plane of cold-rolled specimens: (a) 20%; (b) 90%.
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Fig. 8. Stress–strain curves obtained by cyclic loading–unloading tensile tests for the specimens with cold reduction of 20% (a) and (b) and 90% (c) and (d).
found. With the increase of reduction, strain-induced ␣ martensite phase tended to form (2 0 0)␣ 0 1 0␣ texture in the RD and (2 0 0)␣ 0 0 1␣ texture appeared along transverse direction (TD), which was in good agreement with a result reported before [17].
Fig. 9. ˚ dependence of εSE and εE extracted from the stress–strain curves at strain of 1.5% and 2.5%.
3.2. Superelastic behavior Fig. 8 shows stress–strain curves obtained by cyclic loading–unloading tensile tests for the specimens with cold reduction of 20% and 90%. Tensile axes were along RD, 45◦ from RD and TD, respectively. In the first cycle, the tensile stress was applied when strain reached about 1.5%, and then the stress was removed. The measurement was repeated by increasing the maximum strain to 2.5%. The superelastic characteristic of cold-rolled specimens were described as two types of strains such as εSE and εE . εSE and εE were defined as superelastic strain and pure elastic strain upon unloading, respectively. εSE was considered as transformation strain recovered superelastically upon unloading and εE was considered as the elastic strain recovered elastically upon unloading. It had been studied [18,19] that the movement of twin-boundaries in martensites which were induced by the deformation could contribute much to superelasticity. As shown in Fig. 8a and b, superelasticity was observed in the 20% rolled specimen. Owing to the deformation twinning and strain-induced ␣ martensitic, the movement of twin-boundaries and slip of ␣ martensitic contributed much to superelasticity. No clear difference of εSE and εE was seen along the directions of ˚ = 0◦ (RD), ˚ = 45◦ and ˚ = 90◦ (TD). When the alloy was deformed by 90% reduction (Fig. 8c and d), εSE along the directions of ˚ = 0◦ (RD) and ˚ = 90◦ (TD) were larger than that at ˚ = 45◦ . On the other hand, εE kept higher value along the directions of ˚ = 0◦
L. Wang et al. / Materials Science and Engineering A 491 (2008) 372–377
(RD) and ˚ = 45◦ , as shown in Fig. 9. As research reported [20], transformation strain decreased with changing direction from [0 1 1] to [0 0 1]. εSE , as a part of transformation strain of  → ␣ in TiNbZrTa alloy, exhibited higher value along the direction of [0 1 1], which was parallel to RD or TD. Recently, Inamura et al. showed the anisotropic Young’s modulus in phase of TiNbAl alloy [21]. According to their result, the value of Young’s modulus was in the order of [0 0 1] < [1 1 0] < [1 1 1] . The texture along the direction of ˚ = 45◦ in the specimen rolled by 90% was mainly {1 0 0} 0 0 1 , which made εE higher in ˚ = 45◦ . In addition, strain-induced ␣ martensite phase tended to form (2 0 0)␣ 0 1 0␣ texture in RD. Among the three lattice constants a, b and c of ␣ martensite phase, b was the largest. The largest value of b decreased the Young’s modulus in RD. Therefore, εE in RD also exhibited higher value than that in TD. 4. Conclusions (1) Preferred {1 0 0} 0 0 1 and {1 1 1} 1 1 0 textures are obtained along rolling direction in the specimen with 20% reduction. {2 2 1} 1 1 4 twinning texture is also developed along rolling direction and 45◦ from rolling direction. As for the 90% rolled specimen, a well developed {1 0 0} 0 1 1 texture appears along the rolling direction and transverse direction. Strong {1 0 0} 0 0 1 texture is also observed along 45◦ from rolling direction. (2) Strain-induce ␣ martensite texture exhibits as preferred (2 0 0)␣ 0 1 0␣ texture in the rolling direction. Meanwhile, (2 0 0)␣ 0 0 1␣ texture appears along transverse direction in the 90% rolled specimen. (3) Clear anisotropy in superelastic strain and pure elastic strain in the 90% rolled specimen is observed. Owing to the anisotropy of  texture and ␣ martensite texture, εSE along RD and TD are larger than that at ˚ = 45◦ . On the other hand, εE keeps higher value along RD and ˚ = 45◦ .
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