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Effect of Nb content on deformation behavior and shape memory properties of Ti–Nb alloys
Journal of Alloys and Compounds 577S (2013) S435–S438
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of Nb content on deformation behavior and shape memory properties of Ti–Nb alloys H. Tobe a , H.Y. Kim a,∗ , T. Inamura b , H. Hosoda b , T.H. Nam c , S. Miyazaki a,c,d,∗∗ a
Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan School of Materials Science and Engineering & ERI, Gyeongsang National University, 900 Gazwadong, Jinju, Gyeongnam 660-701, Republic of Korea d Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia b c
a r t i c l e
i n f o
Article history: Received 31 October 2011 Received in revised form 20 January 2012 Accepted 1 February 2012 Available online 15 February 2012 Keywords: Shape memory alloy Ti–Nb Martensite Deformation twin
a b s t r a c t Deformation behavior and shape memory properties of Ti–(20, 23) at.% Nb alloys in a single ␣ martensite state were investigated. The Ti–20Nb alloy exhibited a higher stress for the reorientation of martensite variants when compared with the Ti–23Nb alloy. The recovery strain due to the shape memory effect in the Ti–20Nb alloy was smaller than that in the Ti–23Nb alloy. Transmission electron microscope (TEM) observation revealed that the reorientation of martensite variants occurred by the deformation of {1 1 1} type I and 2 1 1 type II twins. The Nb content dependence of the deformation behavior and shape memory properties was discussed considering the magnitude of twinning shear of the twins. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, -type Ti–Nb based alloys consisting of non-toxic elements have attracted considerable interest as Ni-free biomedical shape memory alloys [1–10]. These alloys undergo a thermoelastic martensitic transformation from  phase to ␣ phase upon cooling. The crystal structure of the  phase is disordered bcc and that of the ␣ phase is disordered C-centered orthorhombic [11,12]. The martensitic transformation start temperature (Ms ) depends on the alloy composition. For the case of Ti–Nb binary alloys, the Ms increases with decreasing Nb content and becomes higher than room temperature (RT) when the Nb content is lower than 25.5 at.% [1]. Furthermore, it has been reported that the transformation strain calculated from the lattice parameters of the  and ␣ phases increases with decreasing Nb content [1]. The shape memory effect of the alloys in a single ␣ martensite state is due to the reorientation of martensite variants during loading and the reverse transformation upon heating after unloading. Thus the investigation of the deformation behavior, i.e., the reorientation mechanism of the ␣ martensite variants is important for the understanding of the shape memory properties of -type Ti-based alloys. However,
∗ Corresponding author. Tel.: +81 29 853 6942; fax: +81 29 853 6942. ∗∗ Corresponding author at: Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan. Tel.: +81 29 853 5283; fax: +81 29 853 5283. E-mail addresses: [email protected] (H.Y. Kim), [email protected] (S. Miyazaki). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.02.023
little has been reported on the effect of Nb content on the deformation behavior and shape memory properties of Ti–Nb alloys in the single ␣ martensite state. In this study, deformation behavior and shape memory properties of Ti–20 at.% Nb and Ti–23 at.% Nb alloys were investigated. The Nb content dependence of the deformation behavior and shape memory properties was discussed on the basis of the results obtained by tensile tests and transmission electron microscope (TEM) observation. 2. Experimental Ti–20 at.% Nb and Ti–23 at.% Nb alloy ingots were fabricated using an Ar arc melting method. The ingots were sealed in a vacuumed quartz tube and homogenized at 1273 K for 7.2 ks and then quenched into water. The homogenized ingots were cold-rolled at RT with a reduction of 98.5% in thickness. The final thickness of the sheets was about 150 m. Specimens for X-ray diffraction (XRD) measurements and tensile tests were cut by an electro-discharge machine from the sheets. The oxidized surface was removed by chemical etching in a solution of 10 vol.% HF, 40 vol.% HNO3 and 50 vol.% H2 O at RT. All the specimens were solution-treated at 1173 K for 1.8 ks in an Ar atmosphere, followed by quenching into water. XRD measurements were carried out at RT using Cu K␣ radiation. Tensile tests were conducted at RT along the rolling direction at a strain rate of 2.5 × 10−4 s−1 . The gage length of the specimens was 20 mm. Specimens for TEM observation were prepared by a twin-jet polishing technique. TEM observation was conducted at RT in a JEOL 2010F instrument operated at 200 kV.
3. Results and discussion Fig. 1 shows the XRD profiles obtained at RT for the Ti–20Nb and Ti–23Nb alloys subjected to the solution treatment. In both alloys
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H. Tobe et al. / Journal of Alloys and Compounds 577S (2013) S435–S438 Table 1 Lattice correspondence variants (CVs).
CV1 CV2 CV3 CV4 CV5 CV6
Fig. 1. XRD profiles obtained at room temperature for the Ti–20Nb and Ti–23Nb alloys.
all diffraction peaks are indexed as ␣ martensite phase with an orthorhombic structure, indicating that the martensitic transformation finish temperatures of the alloys are higher than RT. This result is consistent with previous reports [2,8]. The 2 positions of the peaks observed in the Ti–20Nb and Ti–23Nb alloys are slightly different, indicating that the lattice parameters of the ␣ martensite are different from each other. The lattice parameters of the Ti–20Nb alloy were determined to be a␣ = 0.31257 nm, b␣ = 0.48704 nm and c␣ = 0.46456 nm, while those of the Ti–23Nb alloy were a␣ = 0.31645 nm, b␣ = 0.48291 nm and c␣ = 0.46378 nm. Tensile tests were conducted in order to characterize deformation behavior and shape memory properties of the Ti–20Nb and Ti–23Nb alloys. Fig. 2 shows the stress-strain curves obtained at RT for the solution-treated alloys loaded along the rolling direction. The load was applied until 2.5% strain was reached, then the load was removed. The specimens were heated after unloading to measure shape recovery strain induced by heating: the heating temperature is about 600 K, which is higher than the reverse martensitic transformation finish temperature of the Ti–20Nb and Ti–23Nb alloys [8]. The dashed lines with an arrow in Fig. 2 represent the strain recovered by heating. Almost perfect shape recovery is observed in the Ti–23Nb alloy, while the Ti–20Nb alloy exhibits incomplete shape recovery. It is also seen that the yield stress of the Ti–20Nb alloy is higher than that of the Ti–23Nb alloy, where the yield stress corresponds to the stress for the reorientation of martensite variants. It should be noted here that the maximum recoverable strain, i.e., transformation strain, associated with the –␣ transformation of Ti–Nb alloys increases with decreasing Nb content [1]. As a result, it is reasonably considered that the smaller
Fig. 2. Stress-strain curves obtained at room temperature for the Ti–20Nb and Ti–23Nb alloys.
[1 0 0]␣
[0 1 0]␣
[0 0 1]␣
[1 0 0] [1 0 0] [0 1 0] [0 1 0] [0 0 1] [0 0 1]
[0 1 1] [0 1¯ 1] [1 0 1] ¯  [1 0 1]
[0 1¯ 1] ¯  [0 1¯ 1] ¯  [1 0 1] ¯  [1¯ 0 1] [1¯ 1 0] [1¯ 1¯ 0]
[1 1 0] [1¯ 1 0]
recovery strain in the Ti–20Nb alloy is due to the higher yield stress compared with the Ti–23Nb alloy, because plastic deformation occurs more easily in the alloy with a higher stress for the reorientation of martensite variants upon loading. The effect of texture should be considered on the deformation behavior since the crystal orientation affects the stress for the reorientation of martensite variants and transformation strain. XRD pole figure measurements were conducted at RT for the solution-treated alloys using diffraction intensities from planes (0 0 2)␣ , {1 1 1}␣ and (2 0 0)␣ . There are six lattice correspondence variants (CVs) in the ␣ martensite [13,14] as listed in Table 1. The (0 0 2)␣ and {1 1 1}␣ planes correspond to {1 1 0} planes, whereas the (2 0 0)␣ plane corresponds to {2 0 0} planes. The texture of the parent phase was determined from the pole figures of the martensite phase based on the lattice correspondence between the martensite phase and the parent phase. The texture formed in the Ti–20Nb alloy is {1 1 2} 1 1 0 . On the other hand, the texture formed in the Ti–23Nb alloy is composed of a mixture of {1 1 2} 1 1 0 and {0 01 } 1 1 0 . The transformation strain calculated from the lattice parameters of the  and ␣ phases varies according to crystal orientation [1]. The maximum transformation strain can be obtained when a sample is loaded along 1 1 0 directions. In both the Ti–20Nb and Ti–23Nb alloys, one of the 1 1 0 directions is parallel to the rolling direction, indicating that the effect of the textures on the transformation strain and stress for reorientation is similar. In order to investigate the reorientation mechanism of martensite variants, TEM observation was carried out for the specimen subjected to the solution treatment and for the specimen deformed at RT by a tensile test. Fig. 3(a) and (b) show the bright field images of the solution-treated Ti–20Nb alloy, where Fig. 3(b) is the magnified image of the framed area in Fig. 3(a). All the six CVs are confirmed in Fig. 3(a). V-shaped and triangular clusters formed by thick martensite plates were frequently observed. These morphologies have been reported as the self-accommodation morphologies in the ␣ martensite [13]. The thick martensite plates are mainly related to {1 1 1} type I twinning: the selected area diffraction (SAD) patterns obtained at interfaces between variants CV3 and CV6 and variants CV2 and CV5 are shown in Fig. 3(c) and (d), respectively. The martensite plates related to 2 1 1 type II twinning were also observed. Fig. 3(e) and (f) shows the SAD patterns exhibiting the 2 1 1 type II twinning taken from the interfaces between variants CV6 and CV1 and variants CV6 and CV3, respectively. The twinning elements of the {1 1 1} type I and 2 1 1 type II twins were calculated by the Bilby–Crocker theory [15] using the lattice parameters of the ␣ martensite in the Ti–20Nb alloy, and they are listed in Table 2. The K2 and 1 elements of the {1 1 1} type I twin and the K1 and 2 elements of the 2 1 1 type II twin have irrational Miller indices. It is noted that the 2 1 1 type II twin is the conjugate of the {1 1 1} type I twin. Fig. 4(a) shows the bright field image of the deformed Ti–20Nb alloy. The Ti–20Nb alloy was loaded until the strain leached 2.5% and unloaded, then used for the TEM observation. The morphology of the deformed martensite is clearly different from that of the self-accommodated martensite observed in the solution-treated Ti–20Nb alloy (Fig. 3(a)). A nearly single variant microstructure formed by the reorientation of martensite
H. Tobe et al. / Journal of Alloys and Compounds 577S (2013) S435–S438
S437
Fig. 3. Bright field images ((a) and (b)) and selected area diffraction patterns with key diagrams exhibiting {1 1 1} type I twinning ((c) and (d)) and 2 1 1 type II twinning ((e) and (f)) of the Ti–20Nb alloy.
Table 2 Twinning elements of {1 1 1} type I and 2 1 1 type II twins calculated from the lattice parameters of the Ti–20Nb alloy. K1 {1 1 1} Type I
2 1 1 Type II
1
K2
{1 1 1}
{3.201, 5.402, 1}
{3.201, 5.402, 1}
{1 1 1}
variants can be seen in the upper right region of Fig. 4(a). The single variant grown by the reorientation is identified as CV6. It was confirmed that the interfaces between the CV6 and the other CVs are {1 1 1} type I or 2 1 1 type II twin boundaries as shown in Fig. 4(b) and (c). Saburi et al. [16] showed in several -phase alloys such as Cu–Al–Ni, Cu–Zn–Ga etc. that the recoverable deformation of shape memory alloys is produced by mutual conversion among CVs, and the mechanism of the conversion is twinning deformation. For the case of the ␣ martensite in -type Ti alloys, deformation of the {1 1 1} type I and 2 1 1 type II twins is equivalent to the conversion of a CV to the other CVs, as reported by Inamura et al. [14] recently. Therefore, it is considered that the large single variant CV6 observed in Fig. 4(a) seems to be produced by conversion among
5.037, 4.037, 1 211
2
211
s
5.037, 4.037, 1
0.1613 0.1613
the CVs by the movement of the {1 1 1} type I and 2 1 1 type II twin boundaries between the CVs and/or the introduction of the {1 1 1} type I and 2 1 1 type II twins in each CV. It is noted that the magnitude of twinning shear of the {1 1 1} type I and 2 1 1 type II
Table 3 Magnitudes of twinning shear of {1 1 1} type I and 2 1 1 type II twins for the Ti–20Nb and Ti–23Nb alloys.
{1 1 1} Type I
2 1 1 Type II
Ti–20Nb
Ti–23Nb
0.1613
0.1256
0.1613
0.1256
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H. Tobe et al. / Journal of Alloys and Compounds 577S (2013) S435–S438
Fig. 4. Bright field image (a) and selected area diffraction patterns with key diagrams taken form the interfaces between variants CV6 and CV1 (b) and variants CV6 and CV3 (c) of the Ti–20Nb alloy loaded until the strain leached 2.5% and unloaded.
twins depends on lattice parameters of the ␣ martensite, i.e., Nb content of the Ti–Nb alloys. The calculated magnitudes of twinning shear for the Ti–20Nb and Ti–23Nb alloys are listed in Table 3. The twinning shears of the two types of twins in the Ti–20Nb alloy are larger than those in the Ti–23Nb alloy. This implies that a higher stress and a larger strain are required for deformation by the {1 1 1} type I and 2 1 1 type II twinning in the Ti–20Nb alloy when compared with the Ti–23Nb alloy. Therefore, it is suggested that the higher stress for the reorientation of martensite variants and the smaller recovery strain observed in the Ti–20Nb alloy are due to the larger shear of the {1 1 1} type I and 2 1 1 type II twins. 4. Conclusions Deformation behavior and shape memory properties of Ti–20Nb and Ti–23Nb alloys in a single ␣ martensite state were investigated by tensile tests and TEM observation. TEM observation revealed that the reorientation of martensite variants occurred by the deformation of the {1 1 1} type I and 2 1 1 type II twins. The magnitude of twinning shear in the Ti–20Nb alloy is larger than that in the Ti–23Nb alloy. The measured recovery strain due to the shape memory effect in the Ti–20Nb alloy was smaller than that in the Ti–23Nb alloy, although a larger transformation strain is expected considering the larger twinning shear strains of the former alloy. The Ti–20Nb alloy exhibited a higher stress for the reorientation of martensite variants when compared with the Ti–23Nb alloy, resulting in larger plastic strain in the former alloy. The higher stress for the reorientation of martensite variants observed in the Ti–20Nb alloy is due to the larger twinning shear. Acknowledgments This work was partially supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan and the Grant-in-Aid for JSPS Fellows from the Japan Society for the Promotion of Science. This work was also partially supported by the WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (Grant Number: R32-2008-000-20093-0). References [1] H.Y. Kim, Y. Ikehara, J.I. Kim, H. Hosoda, S. Miyazaki, Acta Mater. 54 (2006) 2419–2429. [2] H.Y. Kim, H. Satoru, J.I. Kim, H. Hosoda, S. Miyazaki, Mater. Trans. 45 (2004) 2443–2448. [3] E. Takahashi, T. Sakurai, S. Watanabe, N. Masahashi, S. Hanada, Mater. Trans. 43 (2002) 2978–2983. [4] Y. Fukui, T. Inamura, H. Hosoda, K. Wakashima, S. Miyazaki, Mater. Trans. 45 (2004) 1077–1082. [5] J.I. Kim, H.Y. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Mater. Sci. Eng. A 403 (2005) 334–339. [6] H.Y. Kim, S. Hashimoto, J.I. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Mater. Sci. Eng. A 417 (2006) 120–128. [7] H.Y. Kim, N. Oshika, J.I. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Mater. Trans. 48 (2007) 400–406. [8] Y. Al-Zain, H.Y. Kim, H. Hosoda, T.H. Nam, S. Miyazaki, Acta Mater. 58 (2010) 4212–4223. [9] J.I. Kim, H.Y. Kim, H. Hosoda, S. Miyazaki, Mater. Trans. 46 (2005) 852–857. [10] M. Tahara, H.Y. Kim, H. Hosoda, S. Miyazaki, Funct. Mater. Lett. 2 (2009) 79–82. [11] A.R.G. Brown, D. Clark, J. Eastabrook, K.S. Jepson, Nature 201 (1964) 914–915. [12] T. Inamura, H. Hosoda, H.Y. Kim, S. Miyazaki, Philos. Mag. 90 (2010) 3475–3498. [13] Y.W. Chai, H.Y. Kim, H. Hosoda, S. Miyazaki, Acta Mater. 57 (2009) 4054–4064. [14] T. Inamura, J.I. Kim, H.Y. Kim, H. Hosoda, K. Wakashima, S. Miyazaki, Philos. Mag. 87 (2007) 3325–3350. [15] B.A. Bilby, A.G. Crocker, Proc. R. Soc. London Ser. A 288 (1965) 240–255. [16] T. Saburi, S. Nenno, in: H.I. Aaronson, D.E. Laughlin, R.F. Sekerka, C.M. Wayman (Eds.), Proc. Int. Conf. on Solid–Solid Phase Transformations, Warrendale, PA, Metall. Soc. AIME (1982) 1455–1479.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of Nb content on deformation behavior and shape memory properties of Ti–Nb alloys H. Tobe a , H.Y. Kim a,∗ , T. Inamura b , H. Hosoda b , T.H. Nam c , S. Miyazaki a,c,d,∗∗ a
Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan School of Materials Science and Engineering & ERI, Gyeongsang National University, 900 Gazwadong, Jinju, Gyeongnam 660-701, Republic of Korea d Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia b c
a r t i c l e
i n f o
Article history: Received 31 October 2011 Received in revised form 20 January 2012 Accepted 1 February 2012 Available online 15 February 2012 Keywords: Shape memory alloy Ti–Nb Martensite Deformation twin
a b s t r a c t Deformation behavior and shape memory properties of Ti–(20, 23) at.% Nb alloys in a single ␣ martensite state were investigated. The Ti–20Nb alloy exhibited a higher stress for the reorientation of martensite variants when compared with the Ti–23Nb alloy. The recovery strain due to the shape memory effect in the Ti–20Nb alloy was smaller than that in the Ti–23Nb alloy. Transmission electron microscope (TEM) observation revealed that the reorientation of martensite variants occurred by the deformation of {1 1 1} type I and 2 1 1 type II twins. The Nb content dependence of the deformation behavior and shape memory properties was discussed considering the magnitude of twinning shear of the twins. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, -type Ti–Nb based alloys consisting of non-toxic elements have attracted considerable interest as Ni-free biomedical shape memory alloys [1–10]. These alloys undergo a thermoelastic martensitic transformation from  phase to ␣ phase upon cooling. The crystal structure of the  phase is disordered bcc and that of the ␣ phase is disordered C-centered orthorhombic [11,12]. The martensitic transformation start temperature (Ms ) depends on the alloy composition. For the case of Ti–Nb binary alloys, the Ms increases with decreasing Nb content and becomes higher than room temperature (RT) when the Nb content is lower than 25.5 at.% [1]. Furthermore, it has been reported that the transformation strain calculated from the lattice parameters of the  and ␣ phases increases with decreasing Nb content [1]. The shape memory effect of the alloys in a single ␣ martensite state is due to the reorientation of martensite variants during loading and the reverse transformation upon heating after unloading. Thus the investigation of the deformation behavior, i.e., the reorientation mechanism of the ␣ martensite variants is important for the understanding of the shape memory properties of -type Ti-based alloys. However,
∗ Corresponding author. Tel.: +81 29 853 6942; fax: +81 29 853 6942. ∗∗ Corresponding author at: Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan. Tel.: +81 29 853 5283; fax: +81 29 853 5283. E-mail addresses: [email protected] (H.Y. Kim), [email protected] (S. Miyazaki). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.02.023
little has been reported on the effect of Nb content on the deformation behavior and shape memory properties of Ti–Nb alloys in the single ␣ martensite state. In this study, deformation behavior and shape memory properties of Ti–20 at.% Nb and Ti–23 at.% Nb alloys were investigated. The Nb content dependence of the deformation behavior and shape memory properties was discussed on the basis of the results obtained by tensile tests and transmission electron microscope (TEM) observation. 2. Experimental Ti–20 at.% Nb and Ti–23 at.% Nb alloy ingots were fabricated using an Ar arc melting method. The ingots were sealed in a vacuumed quartz tube and homogenized at 1273 K for 7.2 ks and then quenched into water. The homogenized ingots were cold-rolled at RT with a reduction of 98.5% in thickness. The final thickness of the sheets was about 150 m. Specimens for X-ray diffraction (XRD) measurements and tensile tests were cut by an electro-discharge machine from the sheets. The oxidized surface was removed by chemical etching in a solution of 10 vol.% HF, 40 vol.% HNO3 and 50 vol.% H2 O at RT. All the specimens were solution-treated at 1173 K for 1.8 ks in an Ar atmosphere, followed by quenching into water. XRD measurements were carried out at RT using Cu K␣ radiation. Tensile tests were conducted at RT along the rolling direction at a strain rate of 2.5 × 10−4 s−1 . The gage length of the specimens was 20 mm. Specimens for TEM observation were prepared by a twin-jet polishing technique. TEM observation was conducted at RT in a JEOL 2010F instrument operated at 200 kV.
3. Results and discussion Fig. 1 shows the XRD profiles obtained at RT for the Ti–20Nb and Ti–23Nb alloys subjected to the solution treatment. In both alloys
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H. Tobe et al. / Journal of Alloys and Compounds 577S (2013) S435–S438 Table 1 Lattice correspondence variants (CVs).
CV1 CV2 CV3 CV4 CV5 CV6
Fig. 1. XRD profiles obtained at room temperature for the Ti–20Nb and Ti–23Nb alloys.
all diffraction peaks are indexed as ␣ martensite phase with an orthorhombic structure, indicating that the martensitic transformation finish temperatures of the alloys are higher than RT. This result is consistent with previous reports [2,8]. The 2 positions of the peaks observed in the Ti–20Nb and Ti–23Nb alloys are slightly different, indicating that the lattice parameters of the ␣ martensite are different from each other. The lattice parameters of the Ti–20Nb alloy were determined to be a␣ = 0.31257 nm, b␣ = 0.48704 nm and c␣ = 0.46456 nm, while those of the Ti–23Nb alloy were a␣ = 0.31645 nm, b␣ = 0.48291 nm and c␣ = 0.46378 nm. Tensile tests were conducted in order to characterize deformation behavior and shape memory properties of the Ti–20Nb and Ti–23Nb alloys. Fig. 2 shows the stress-strain curves obtained at RT for the solution-treated alloys loaded along the rolling direction. The load was applied until 2.5% strain was reached, then the load was removed. The specimens were heated after unloading to measure shape recovery strain induced by heating: the heating temperature is about 600 K, which is higher than the reverse martensitic transformation finish temperature of the Ti–20Nb and Ti–23Nb alloys [8]. The dashed lines with an arrow in Fig. 2 represent the strain recovered by heating. Almost perfect shape recovery is observed in the Ti–23Nb alloy, while the Ti–20Nb alloy exhibits incomplete shape recovery. It is also seen that the yield stress of the Ti–20Nb alloy is higher than that of the Ti–23Nb alloy, where the yield stress corresponds to the stress for the reorientation of martensite variants. It should be noted here that the maximum recoverable strain, i.e., transformation strain, associated with the –␣ transformation of Ti–Nb alloys increases with decreasing Nb content [1]. As a result, it is reasonably considered that the smaller
Fig. 2. Stress-strain curves obtained at room temperature for the Ti–20Nb and Ti–23Nb alloys.
[1 0 0]␣
[0 1 0]␣
[0 0 1]␣
[1 0 0] [1 0 0] [0 1 0] [0 1 0] [0 0 1] [0 0 1]
[0 1 1] [0 1¯ 1] [1 0 1] ¯  [1 0 1]
[0 1¯ 1] ¯  [0 1¯ 1] ¯  [1 0 1] ¯  [1¯ 0 1] [1¯ 1 0] [1¯ 1¯ 0]
[1 1 0] [1¯ 1 0]
recovery strain in the Ti–20Nb alloy is due to the higher yield stress compared with the Ti–23Nb alloy, because plastic deformation occurs more easily in the alloy with a higher stress for the reorientation of martensite variants upon loading. The effect of texture should be considered on the deformation behavior since the crystal orientation affects the stress for the reorientation of martensite variants and transformation strain. XRD pole figure measurements were conducted at RT for the solution-treated alloys using diffraction intensities from planes (0 0 2)␣ , {1 1 1}␣ and (2 0 0)␣ . There are six lattice correspondence variants (CVs) in the ␣ martensite [13,14] as listed in Table 1. The (0 0 2)␣ and {1 1 1}␣ planes correspond to {1 1 0} planes, whereas the (2 0 0)␣ plane corresponds to {2 0 0} planes. The texture of the parent phase was determined from the pole figures of the martensite phase based on the lattice correspondence between the martensite phase and the parent phase. The texture formed in the Ti–20Nb alloy is {1 1 2} 1 1 0 . On the other hand, the texture formed in the Ti–23Nb alloy is composed of a mixture of {1 1 2} 1 1 0 and {0 01 } 1 1 0 . The transformation strain calculated from the lattice parameters of the  and ␣ phases varies according to crystal orientation [1]. The maximum transformation strain can be obtained when a sample is loaded along 1 1 0 directions. In both the Ti–20Nb and Ti–23Nb alloys, one of the 1 1 0 directions is parallel to the rolling direction, indicating that the effect of the textures on the transformation strain and stress for reorientation is similar. In order to investigate the reorientation mechanism of martensite variants, TEM observation was carried out for the specimen subjected to the solution treatment and for the specimen deformed at RT by a tensile test. Fig. 3(a) and (b) show the bright field images of the solution-treated Ti–20Nb alloy, where Fig. 3(b) is the magnified image of the framed area in Fig. 3(a). All the six CVs are confirmed in Fig. 3(a). V-shaped and triangular clusters formed by thick martensite plates were frequently observed. These morphologies have been reported as the self-accommodation morphologies in the ␣ martensite [13]. The thick martensite plates are mainly related to {1 1 1} type I twinning: the selected area diffraction (SAD) patterns obtained at interfaces between variants CV3 and CV6 and variants CV2 and CV5 are shown in Fig. 3(c) and (d), respectively. The martensite plates related to 2 1 1 type II twinning were also observed. Fig. 3(e) and (f) shows the SAD patterns exhibiting the 2 1 1 type II twinning taken from the interfaces between variants CV6 and CV1 and variants CV6 and CV3, respectively. The twinning elements of the {1 1 1} type I and 2 1 1 type II twins were calculated by the Bilby–Crocker theory [15] using the lattice parameters of the ␣ martensite in the Ti–20Nb alloy, and they are listed in Table 2. The K2 and 1 elements of the {1 1 1} type I twin and the K1 and 2 elements of the 2 1 1 type II twin have irrational Miller indices. It is noted that the 2 1 1 type II twin is the conjugate of the {1 1 1} type I twin. Fig. 4(a) shows the bright field image of the deformed Ti–20Nb alloy. The Ti–20Nb alloy was loaded until the strain leached 2.5% and unloaded, then used for the TEM observation. The morphology of the deformed martensite is clearly different from that of the self-accommodated martensite observed in the solution-treated Ti–20Nb alloy (Fig. 3(a)). A nearly single variant microstructure formed by the reorientation of martensite
H. Tobe et al. / Journal of Alloys and Compounds 577S (2013) S435–S438
S437
Fig. 3. Bright field images ((a) and (b)) and selected area diffraction patterns with key diagrams exhibiting {1 1 1} type I twinning ((c) and (d)) and 2 1 1 type II twinning ((e) and (f)) of the Ti–20Nb alloy.
Table 2 Twinning elements of {1 1 1} type I and 2 1 1 type II twins calculated from the lattice parameters of the Ti–20Nb alloy. K1 {1 1 1} Type I
2 1 1 Type II
1
K2
{1 1 1}
{3.201, 5.402, 1}
{3.201, 5.402, 1}
{1 1 1}
variants can be seen in the upper right region of Fig. 4(a). The single variant grown by the reorientation is identified as CV6. It was confirmed that the interfaces between the CV6 and the other CVs are {1 1 1} type I or 2 1 1 type II twin boundaries as shown in Fig. 4(b) and (c). Saburi et al. [16] showed in several -phase alloys such as Cu–Al–Ni, Cu–Zn–Ga etc. that the recoverable deformation of shape memory alloys is produced by mutual conversion among CVs, and the mechanism of the conversion is twinning deformation. For the case of the ␣ martensite in -type Ti alloys, deformation of the {1 1 1} type I and 2 1 1 type II twins is equivalent to the conversion of a CV to the other CVs, as reported by Inamura et al. [14] recently. Therefore, it is considered that the large single variant CV6 observed in Fig. 4(a) seems to be produced by conversion among
5.037, 4.037, 1 211
2
211
s
5.037, 4.037, 1
0.1613 0.1613
the CVs by the movement of the {1 1 1} type I and 2 1 1 type II twin boundaries between the CVs and/or the introduction of the {1 1 1} type I and 2 1 1 type II twins in each CV. It is noted that the magnitude of twinning shear of the {1 1 1} type I and 2 1 1 type II
Table 3 Magnitudes of twinning shear of {1 1 1} type I and 2 1 1 type II twins for the Ti–20Nb and Ti–23Nb alloys.
{1 1 1} Type I
2 1 1 Type II
Ti–20Nb
Ti–23Nb
0.1613
0.1256
0.1613
0.1256
S438
H. Tobe et al. / Journal of Alloys and Compounds 577S (2013) S435–S438
Fig. 4. Bright field image (a) and selected area diffraction patterns with key diagrams taken form the interfaces between variants CV6 and CV1 (b) and variants CV6 and CV3 (c) of the Ti–20Nb alloy loaded until the strain leached 2.5% and unloaded.
twins depends on lattice parameters of the ␣ martensite, i.e., Nb content of the Ti–Nb alloys. The calculated magnitudes of twinning shear for the Ti–20Nb and Ti–23Nb alloys are listed in Table 3. The twinning shears of the two types of twins in the Ti–20Nb alloy are larger than those in the Ti–23Nb alloy. This implies that a higher stress and a larger strain are required for deformation by the {1 1 1} type I and 2 1 1 type II twinning in the Ti–20Nb alloy when compared with the Ti–23Nb alloy. Therefore, it is suggested that the higher stress for the reorientation of martensite variants and the smaller recovery strain observed in the Ti–20Nb alloy are due to the larger shear of the {1 1 1} type I and 2 1 1 type II twins. 4. Conclusions Deformation behavior and shape memory properties of Ti–20Nb and Ti–23Nb alloys in a single ␣ martensite state were investigated by tensile tests and TEM observation. TEM observation revealed that the reorientation of martensite variants occurred by the deformation of the {1 1 1} type I and 2 1 1 type II twins. The magnitude of twinning shear in the Ti–20Nb alloy is larger than that in the Ti–23Nb alloy. The measured recovery strain due to the shape memory effect in the Ti–20Nb alloy was smaller than that in the Ti–23Nb alloy, although a larger transformation strain is expected considering the larger twinning shear strains of the former alloy. The Ti–20Nb alloy exhibited a higher stress for the reorientation of martensite variants when compared with the Ti–23Nb alloy, resulting in larger plastic strain in the former alloy. The higher stress for the reorientation of martensite variants observed in the Ti–20Nb alloy is due to the larger twinning shear. Acknowledgments This work was partially supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan and the Grant-in-Aid for JSPS Fellows from the Japan Society for the Promotion of Science. This work was also partially supported by the WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (Grant Number: R32-2008-000-20093-0). References [1] H.Y. Kim, Y. Ikehara, J.I. Kim, H. Hosoda, S. Miyazaki, Acta Mater. 54 (2006) 2419–2429. [2] H.Y. Kim, H. Satoru, J.I. Kim, H. Hosoda, S. Miyazaki, Mater. Trans. 45 (2004) 2443–2448. [3] E. Takahashi, T. Sakurai, S. Watanabe, N. Masahashi, S. Hanada, Mater. Trans. 43 (2002) 2978–2983. [4] Y. Fukui, T. Inamura, H. Hosoda, K. Wakashima, S. Miyazaki, Mater. Trans. 45 (2004) 1077–1082. [5] J.I. Kim, H.Y. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Mater. Sci. Eng. A 403 (2005) 334–339. [6] H.Y. Kim, S. Hashimoto, J.I. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Mater. Sci. Eng. A 417 (2006) 120–128. [7] H.Y. Kim, N. Oshika, J.I. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Mater. Trans. 48 (2007) 400–406. [8] Y. Al-Zain, H.Y. Kim, H. Hosoda, T.H. Nam, S. Miyazaki, Acta Mater. 58 (2010) 4212–4223. [9] J.I. Kim, H.Y. Kim, H. Hosoda, S. Miyazaki, Mater. Trans. 46 (2005) 852–857. [10] M. Tahara, H.Y. Kim, H. Hosoda, S. Miyazaki, Funct. Mater. Lett. 2 (2009) 79–82. [11] A.R.G. Brown, D. Clark, J. Eastabrook, K.S. Jepson, Nature 201 (1964) 914–915. [12] T. Inamura, H. Hosoda, H.Y. Kim, S. Miyazaki, Philos. Mag. 90 (2010) 3475–3498. [13] Y.W. Chai, H.Y. Kim, H. Hosoda, S. Miyazaki, Acta Mater. 57 (2009) 4054–4064. [14] T. Inamura, J.I. Kim, H.Y. Kim, H. Hosoda, K. Wakashima, S. Miyazaki, Philos. Mag. 87 (2007) 3325–3350. [15] B.A. Bilby, A.G. Crocker, Proc. R. Soc. London Ser. A 288 (1965) 240–255. [16] T. Saburi, S. Nenno, in: H.I. Aaronson, D.E. Laughlin, R.F. Sekerka, C.M. Wayman (Eds.), Proc. Int. Conf. on Solid–Solid Phase Transformations, Warrendale, PA, Metall. Soc. AIME (1982) 1455–1479.