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Strain induced martensite stabilization and shape memory effect of Ti–20Zr–10Nb–4Ta alloy
Author’s Accepted Manuscript Strain induced martensite stabilization and shape memory effect of ti-20Zr-10Nb-4Ta alloy Chengyang Xiong, Li Yao, Bifei Yuan, Wentao Qu, Yan Li www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(16)30106-X http://dx.doi.org/10.1016/j.msea.2016.01.104 MSA33291
To appear in: Materials Science & Engineering A Received date: 14 December 2015 Revised date: 27 January 2016 Accepted date: 28 January 2016 Cite this article as: Chengyang Xiong, Li Yao, Bifei Yuan, Wentao Qu and Yan Li, Strain induced martensite stabilization and shape memory effect of ti-20Zr10Nb-4Ta alloy, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.01.104 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Strain induced martensite stabilization and shape memory effect of Ti-20Zr-10Nb-4Ta alloy Chengyang Xionga, Li Yaoa, Bifei Yuanb, Wentao Qub, Yan Li a, c*, a
School of Materials Science and Engineering, Beihang University, Beijing 100191, China b
School of Mechanical Engineering, Xi'an Shiyou University, Xi'an 710065, China
c
Beijing Key Laboratory for Advanced Functional Materials and Thin Film Technology, Beihang University, Beijing 100191, China
Abstract: The phase transformation, the microstructure and the shape memory effect of the Ti-20Zr-10Nb-4Ta alloy are investigated. The X-ray diffraction measurements indicated that the alloy is composed of single orthorhombic α″-martensite. The alloy showed a two-stage yielding behavior upon tension at 0.5 % and 6 % strain with a yield stress of 215 MPa and 565 MPa, respectively. The strain induced martensite stabilization was identified because the reverse martensite transformation start temperature of the alloy increases from 348 to 405 K, with the pre-strain increasing from 0 to 8 %. This can be ascribed to the martensite reorientation that occurred at a low strain level and the dislocations formed at a large strain level. The maximum shape memory strain is 3.3 % in the Ti-20Zr-10Nb-4Ta alloy.
Key words: Ti-Zr; Shape memory alloy; Martensite transformation; Martensite Stabilization. * Corresponding author. Fax: +86 10 82315989 100, +86 10 82338200. Email: [email protected]
1
Shape memory alloys (SMAs) have been intensively investigated regarding their phase transformations and functional properties, shape memory effect, and pseudoelasticity [1-4]. In recent years, some Ti-based SMAs, e.g., Ti-Nb, Ti-Zr and Ti-Ta, have drawn much attention as new candidates for smart materials with high performance, especially in biomedical and high-temperature applications [5-8]. In 2011, binary Ti-Zr alloys were reported as shape memory alloys with a high phase transformation temperature of up to 900 K between the hexagonal α′-martensite and bcc β phases [6]. The phase stability and hardness of Ti-Zr-based alloys have been demonstrated to be sensitive to the addition of alloying elements; e.g., Nb, Cr, Al, Mo and Fe are all β-Stability elements that decrease the transformation temperature between the α and β phases [9-12]. Ti-20Zr-10Nb alloy [13] and Ti-30Zr-1Fe [14] alloys have demonstrated improved shape memory effect and excellent mechanical properties. Recently, a large shape memory recovery strain and a pseudoelasticity strain of 4.1 % and 4.7 %, respectively, were reported in Ti-19.5Zr-10Nb-0.5Fe [15] and Ti-19Zr-10Nb-1Fe [16] alloys. These results indicate that the addition of an alloying element is an effective approach to modify the phase stability and the corresponding shape memory effect or pseudoelasticity. Martensite stabilization in a shape memory alloy is the increase of the reverse martensitic transformation temperature via some thermal or mechanical features [17-22]. In the early studies, it was found that martensite stabilization occurred when martensite aging was performed in some shape memory alloys (SMAs), such as Au-Cd [17], Cu-Al-Ni [23], Ni-Ti-Hf [24] and Ni-Mn-Ga [19, 25]. The mechanism 2
can be ascribed to the symmetry-conforming property for short-range order of point defects [26, 27]. Moreover, it may trigger a new avenue to obtain new high temperature shape memory alloys [28]. Alternatively, martensite stabilization has usually been observed when the phase transformation temperatures of deformed martensite phase were measured in NiTi alloys [20-22, 29, 30] and some Heusler type SMAs [31, 32]. The strain induced martensite stabilization was attributed to the alteration of the internal elastic energy and the irreversible energy during the variants reorientation and plastic deformation [20, 33]. Although many investigations have focused on the thermo-mechanical and phase transformation behaviors, little research has considered the martensite stabilization of Ti-based SMAs. In the present work, the martensite stabilization and the shape memory effect in a new Ti-20Zr-10Nb-4Ta alloy induced by tensile strain associated with the structural analysis were investigated. 1. Experimental procedure The Ti-20Zr-10Nb-4Ta (at.%) alloy was prepared using high-purity (> 99.9 wt.%) raw materials. The ingots (approximately 1.5 kg in weight) were re-melted five times for homogeneity via the non-consumable arc-melting method under Ar atmosphere protection. The as-melted ingots were homogenized at 1273 K for 6 h in a vacuum of 10-3 Pa, cut into 4-mm thick pieces, and then cold rolled into sheets with 75 % reduction in thickness. Next, alloy sheets were annealed at 873 K in vacuum for 30 min and quenched in water. The tensile tests were performed at room temperature (298K) on a SANS CMT5504 universal test machine with the sample gauge length of 3
20 mm and strain rate of 1.67×10-4 s-1. Some of the alloy samples were deformed in tension to different pre-strains, from 2 % to 8 %, and then the unloaded alloy samples were cut into small samples for structural or phase transformation measurements. In order to determine the shape memory effect, tensile specimens with different pre-strains, 2 %, 4 %, 6 % and 8 %, were heated to 473 K for shape recovery. The microstructure was analyzed using optical microscopy, X-ray diffraction (XRD, Rigaku D/Max 2500 PC diffractometer with a Cu-Kα radiation (λ = 0.15406 nm)) and transmission electron microscopy (TEM, JEOL 2100F, operated at 200 kV). The phase transformation temperatures were measured by differential scanning calorimetry DSC (Q800) with a cooling/heating rate of 10 K min-1. 2. Results and discussion 2.1. Microstructures The optical micrograph and XRD patterns of the Ti-20Zr-10Nb-4Ta alloy are shown in Fig. 1. It can be seen that the alloy is composed of a polycrystalline structure with fine grains ranging from 5 to 15 μm in size, which is very similar to the structure of the cold-rolled and 873 K annealed Ti-20Zr-10Nb alloy [13]. The XRD result indicates that the only orthorhombic α″-martensite phase with lattice parameters of a=0.3170 nm, b=0.5004 nm and c=0.4755 nm exists in the alloy (Fig. 1b). Unlike the Ti-20Zr-10Nb alloy [13], no phase appears in the Ti-20Zr-10Nb-4Ta alloy. This observation suggests that the formation of the ω phase may be suppressed by the addition of Ta in Ti-Zr-Nb alloys. A similar effect of Ta has been found in Ti-Nb-based shape memory alloys [34]. 4
2.2. Mechanical properties Fig. 2 shows the tensile stress-strain curve of the Ti-20Zr-10Nb-4Ta alloy. It is seen that the stress-strain curve exhibits two yielding stages, with critical stress values of 215 MPa (σM) and 565 MPa (σS) at the strains of 0.5 % and 6 %, respectively. The tensile stress-strain curve obviously corresponds to the reorientation and plastic deformation of martensites, as commonly seen in the mechanical tests on the martensitic shape memory alloy, e.g., NiTi [33, 35], Ti-Zr-Nb [13, 15], Ni-Mn-Ga [36, 37] and Co-Ni-Ga [38] alloys, because the Ti-20Zr-10Nb-4Ta alloy is composed of single α″-martensite, as mentioned above. The tensile stress of the Ti-20Zr-10Nb-4Ta alloy is approximately 565 MPa, which is higher than that of the Ti-20Zr-10Nb alloy [13] due to the solid solution strengthening effect of the Ta element. 2.3. Effect of pre-strain on the reverse martensite transformation Fig. 3 displays the DSC curves of the Ti-20Zr-10Nb-4Ta alloy with different pre-strains (0, 2%, 4%, 6% and 8%). It is seen that there is one endothermic peak on the heating curve, which is confirmed to be the reverse transformation from the α″-martensite phase to the β phase with the start temperature (As) of 348 K compared with the phase transformations of some other Ti-Zr-Nb SMAs [13, 15]. It is seen that no obvious exothermic peak on cooling appears for the sample without deformation. This phenomenon has been found not only in the Ti-20Zr-10Nb-4Ta alloy but also in some other Ti-based shape memory alloys, e.g. Ti–30Ta [39] and Ti–30Nb–3Pd [40]. The reason may be due to a partial transformation of the β phase to the α″-martensite phase or the intrinsic low enthalpy of the phase transformation between the 5
α″-martensite and β parent [13, 15, 39, 40]. The other samples show the similar phase transformation behavior. Note that As temperature increases with increasing pre-strain. This phenomenon is the typical strain induced martensite stabilization. Fig. 4 shows the TEM images and SAED patterns of the Ti-20Zr-10Nb-4Ta alloy with different pre-strains. It is seen in Fig. 4a that the Ti-20Zr-10Nb-4Ta alloy without deformation is mainly composed of self-accommodation martensite variants exhibited as plates with many fine twins inside. The corresponding SAED pattern recorded along the α″ [101]/[-101] zone axis clearly indicates the twinned structure of martensites. Meanwhile, no ω and residual β phase was observed, as reported in Ti-20Zr-10Nb [13] and Ti-19.5Zr-10Nb-0.5Fe alloys [15], respectively. This observation is in good agreement with the XRD results in Fig. 1b. The TEM image of the sample with pre-strain of 2 % is very similar to Fig. 4a, so it is not presented here. For the sample with pre-strain of 4 %, as shown in Fig. 4b, the reorientation of the martensite twins occurred as characterized by the partial disappearance of the self-accommodation martensite morphology and the reduction in the fine inner twins. After being pre-strained to 6 %, most of the martensite plates are de-twinned, as shown in Fig. 4c. Some martensite plates are separated by the traces of the original plate boundaries, where fine inner twins disappear and dislocations appear. If the sample was deformed to a pre-strain of 8 %, more severe martensite reorientation occurs, as indicated by the rearrangement of martensites to nearly the same direction, and more dislocations appear, as shown in Fig. 4d. Therefore, it can be concluded that during the tensile loading on the Ti-20Zr-10Nb-4Ta alloy up to a pre-stain of 8 %, 6
martensite reorientation occurs at low strain level and dislocations forms at high strain level. To clearly explain the mechanism of martensite stabilization, the relationship between As and pre-strain of the Ti-20Zr-10Nb-4Ta alloy as well as the corresponding tensile stress-strain curve are shown in Fig. 5. Based on the TEM results in Fig. 4, the tensile stress-strain of a martensite shape memory alloy can be divided in to three stages. After the elastic deformation finished at the pre-strain of 0.5 %, a stress-plateau controlled by the reorientation of α"-martensite appears until the pre-strain of 3 % (stage I). Next, the tensile stress rapidly increases with increasing pre-strain to the second yielding at the pre-strain of approximately 6 % (stage II), which is dominated by further reorientation and elastic deformation of α"-martensite. Finally, the deformation continues at an almost constant stress level until fracture (stage III). This stage is companied by the generation of a high density of dislocations due to the slip deformation of martensite. According to some previous studies, the mechanism of the martensitic stabilization can be ascribed to the release of stored elastic energy in martensites [25, 26], the influence of dislocations in plastic deformation [38], or to their combined effects [41]. Based on the above analysis, therefore, it is believed that the martensite stabilization is influenced by the release of the stored elastic energy due to reorientation of martensite twins at the initial deformation, e.g., 4 % or 6 %. With the further deformation, e.g., 8 %, the effect of dislocations should be the predominant factor. 2.4. Shape memory effect of the Ti-20Zr-10Nb-4Ta alloy 7
Fig. 6 shows the tensile loading-unloading curves and the shape memory behavior of Ti-20Zr-10Nb-4Ta alloy. It can be seen that the shape memory recovery ratio decreases with the increasing pre-strain, and the entire recovery occurs in the sample with pre-strain of 2 %. The shape memory strain of Ti-20Zr-10Nb-4Ta alloy increases with the increasing pre-strain and reaches the maximum value of 3.3 % when the pre-strain reaches 6 % or 8%. Therefore, the shape memory effect of the Ti-20Zr-10Nb-4Ta alloy is superior to those of the Ti–22Nb–6Ta (~ 2.1 %) [42], Ti-20Zr-10Nb (~ 2.5 %) [13] and Ti-30Nb-3Pd (~ 2.7 %) [40] alloys. The critical stress for martensite reorientation (σM) of the Ti-20Zr-10Nb-4Ta alloy is 215 MPa, as shown in Fig. 2, which is much lower than that of the Ti-20Zr-10Nb alloy (~ 240 MPa) [13]. It is well known that a lower critical stress for martensite reorientation (σM) is beneficial to shape memory recovery. Moreover, the shape memory recovery during the reverse martensitic transformation of the Ti-20Zr-10Nb-4Ta alloy will not be influenced by the ω phase transformation as occurred in the Ti-20Zr-10Nb alloy [13]. Therefore, the shape memory effect of the Ti-20Zr-10Nb alloy can be improved by the addition of Ta. 3. Conclusions Microstructures, phase transformation, and shape memory effect of the Ti-20Zr-10Nb-4Ta alloy were investigated. The results show that the alloy is composed of single orthorhombic α″-martensite, without and β phase. The alloy exhibit a double yielding behavior upon tension at the strains of 0.5 % and 6 %, corresponding to the yield stresses of 215 MPa and 565 MPa, respectively. The 8
reverse martensite transformation start temperature of the alloy increases from 348 to 405 K with the pre-strain increasing from 0 to 8%. The strain induced martensite stabilization can be attributed to the martensite reorientation that occurred at the low strain level and the dislocations formed at the high strain level. The maximum shape memory strain is 3.3 % in the Ti-20Zr-10Nb-4Ta alloy. Acknowledgements This work is supported by the National Basic Research Program of China (No. 2012CB619400), the National Natural Science Foundation of China (NSFC, No. 51371016) and the Aeronautical Science Foundation of China (2014ZF51070). References: [1] S. Hao, L. Cui, D. Jiang, X. Han, Y. Ren, J. Jiang, Y. Liu, Z. Liu, S. Mao, Y. Wang, Y. Li, X. Ren, X. Ding, S. Wang, C. Yu, X. Shi, M. Du, F. Yang, Y. Zheng, Z. Zhang, X. Li, D.E. Brown, J. Li, Science 339 (2013) 1191-1194. [2] T. Omori, K. Ando, M. Okano, X. Xu, Y. Tanaka, I. Ohnuma, R. Kainuma, K. Ishida, Science 333 (2011) 68-71. [3] Y. Liu, S.P. Galvin, Acta Mater. 45 (1997) 4431-4439. [4] Y. Liu, Z. Xie, J. Van Humbeeck, L. Delaey, Acta Mater. 46 (1998) 4325-4338. [5] H.Y. Kim, Y. Ikehara, J.I. Kim, H. Hosoda, S. Miyazaki, Acta Mater. 54 (2006) 2419-2429. [6] Y. Li, Y. Cui, F. Zhang, H. Xu, Scr. Mater. 64 (2011) 584-587. [7] Y.L. Zhou, M. Niinomi, T. Akahori, Mater. Sci. Eng. A 371 (2004) 283-290. [8] D.C. Zhang, Y.F. Mao, Y.L. Li, J.J. Li, M. Yuan, J.G. Lin, Mater. Sci. Eng. A 559 (2013) 706-710. 9
[9] L. López Pavón, H.Y. Kim, H. Hosoda, S. Miyazaki, Scr. Mater. 95 (2015) 46-49. [10] P. Wang, Y. Feng, F. Liu, L. Wu, S. Guan, Mater. Sci. Eng. C 51 (2015) 148-152. [11] M.F. Ijaz, H.Y. Kim, H. Hosoda, S. Miyazaki, Mater. Sci. Eng. C 48 (2015) 11-20. [12] S.X. Liang, L.X. Yin, L.Y. Zheng, M.Z. Ma, R.P. Liu, Mater. Sci. Eng. A 639 (2015) 699-704. [13] Y. Cui, Y. Li, K. Luo, H. Xu, Mater. Sci. Eng. A 527 (2010) 652-656. [14] F. Zhang, Y. Cui, P.F. Xue, Y. Li, Rare Metal Mater. Eng. 10 (2013) 2131-2135. [15] P. Xue, Y. Li, F. Zhang, C. Zhou, Scr. Mater. 101 (2015) 99-102. [16] P. Xue, Y. Li, K. Li, D. Zhang, C. Zhou, Mater. Sci. Eng. C 50 (2015) 179-186. [17] N. Nakanishi, T. Mori, S. Miura, Y. Murakami, S. Kachi, Philos. Mag. 28 (1973) 277-280. [18] K. Otsuka, X. Ren, Mater. Sci. Eng. A 312 (2001) 207-218. [19] C. Seguí, E. Cesari, J. Font, J. Muntasell, V.A. Chernenko, Scr. Mater. 53 (2005) 315-318. [20] M. Piao, K. Otsuka, S. Miyazaki, H. Horikawa, Mater. Trans. JIM 34 (1993) 919-923. [21] Y. Liu, D. Favier, Acta Mater. 48 (2000) 3489-3499. [22] Y. Li, H.B. Xu, L.S. Cui, D.Z. Yang, Metall. Mater. Trans. A 34 (2003) 219-223. [23] H. Sakamoto, K. Otsuka, H. Shimizu, Scr. Mater. 11 (1977) 607-611. [24] R. Santamarta, C. Segu, J. Pons, E. Cesari, Scr. Mater. 41 (1999) 867-872. [25] Y. Xin, Y. Li, L. Chai, H. Xu, Scr. Mater. 54 (2006) 1139-1143. [26] S. Miura, T. Mori, N. Nakanishi, Scr. Mater. 7 (1973) 697-700. [27] P.M. Kadletz, P. Kroo, Y.I. Chumlyakov, M.J. Gutmann, W.W. Schmahl, H.J. Maier, T. Niendorf, Mater. Lett. 159 (2015) 16-19. [28] T. Niendorf, P. Krooß, C. Somsen, G. Eggeler, Y.I. Chumlyakov, H.J. Maier, Acta Mater. 89 10
(2015) 298-304. [29] L. Cui, Y. Li, Y. Zheng, D. Yang, Mater. Lett. 47 (2001) 286-289. [30] Y. Li, L. Cui, Y. Zheng, D. Yang, Mater. Lett. 51 (2001) 73-77. [31] V.A. Chernenko, J. Pons, E. Cesari, I.K. Zasimchuk, Scr. Mater. 50 (2004) 225-229. [32] G.T. Li, Z.H. Liu, X.Q. Ma, S.Y. Yu, Y. Liu, Mater. Lett. 107 (2013) 239-242. [33] Y. Liu, Z. Xie, J. Van Humbeeck, L. Delaey, Scr. Mater. 41 (1999) 1273-1281. [34] S. Dubinskiy, V. Brailovski, S. Prokoshkin, V. Pushin, K. Inaekyan, V. Sheremetyev, M. Petrzhik, M. Filonov, J. Mater. Eng. Perform 9 (2013) 2656-2664. [35] F. Liu, Z. Ding, Y. Li, H. Xu, Intermetallics 13 (2005) 357-360. [36] Y. Xin, Y. Li, L. Chai, H. Xu, Scr. Mater. 57 (2007) 599-601. [37] Y. Li, Y. Xin, L. Chai, Y. Ma, H. Xu, Acta Mater. 58 (2010) 3655-3663. [38] H.C. Lin, S.K. Wu, Metall. Trans. A 24 (1993) 293-299. [39] X.H. Zheng, J.H. Sui, X. Zhang, X.H. Tian, W. Cai, J. Alloys Compd. 539 (2012) 144-147. [40] D.H. Ping, Y. Mitarai, F.X. Yin, Scr. Mater. 52 (2005) 1287-1291. [41] H. Kato, Y. Yasuda, K. Sasaki, Acta Mater. 59 (2011) 3955-3964. [42] H.Y. Kim, T. Sasaki, K. Okutsu, J.I. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Acta Mater. 54 (2006) 423-433.
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Fig. 1. Optical micrograph (a) and XRD pattern (b) of Ti-20Zr-10Nb-4Ta alloy.
Fig. 2. Tensile stress-strain curve of the Ti-20Zr-10Nb-4Ta alloy.
Fig. 3. DSC curves of Ti-20Zr-10Nb-4Ta alloy with different pre-strains (0, 2%, 4%, 6% and 8%). 12
Fig. 4. TEM bright field images and the SAED patterns of the Ti-20Zr-10Nb-4Ta alloy with different pre-strains: (a) 0 % (martensite variant boundaries indicated by solid lines and fine inner twins in martensite variants indicated by white arrows), (b) 13
4 % (the disappearance of fine inner twins in martensite variants indicated by black arrows), (c) 6 % (the traces of the original plate boundaries are indicated by dotted lines and the dislocations are indicated by hollow arrows) and (d) 8 % (more dislocations indicated by hollow arrows).
Fig. 5. Tensile stress-strain curve and the relationship between As and the pre-strain of Ti-20Zr-10Nb-4Ta alloy.
Fig. 6. (a) The tensile loading-unloading curves of Ti-20Zr-10Nb-4Ta alloy (the dashed line with an arrow indicates the shape recovery after the reverse transformation upon heating) and (b) the shape recovery strain and shape memory recovery ratio versus pre-strain. 14
PII: DOI: Reference:
S0921-5093(16)30106-X http://dx.doi.org/10.1016/j.msea.2016.01.104 MSA33291
To appear in: Materials Science & Engineering A Received date: 14 December 2015 Revised date: 27 January 2016 Accepted date: 28 January 2016 Cite this article as: Chengyang Xiong, Li Yao, Bifei Yuan, Wentao Qu and Yan Li, Strain induced martensite stabilization and shape memory effect of ti-20Zr10Nb-4Ta alloy, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.01.104 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Strain induced martensite stabilization and shape memory effect of Ti-20Zr-10Nb-4Ta alloy Chengyang Xionga, Li Yaoa, Bifei Yuanb, Wentao Qub, Yan Li a, c*, a
School of Materials Science and Engineering, Beihang University, Beijing 100191, China b
School of Mechanical Engineering, Xi'an Shiyou University, Xi'an 710065, China
c
Beijing Key Laboratory for Advanced Functional Materials and Thin Film Technology, Beihang University, Beijing 100191, China
Abstract: The phase transformation, the microstructure and the shape memory effect of the Ti-20Zr-10Nb-4Ta alloy are investigated. The X-ray diffraction measurements indicated that the alloy is composed of single orthorhombic α″-martensite. The alloy showed a two-stage yielding behavior upon tension at 0.5 % and 6 % strain with a yield stress of 215 MPa and 565 MPa, respectively. The strain induced martensite stabilization was identified because the reverse martensite transformation start temperature of the alloy increases from 348 to 405 K, with the pre-strain increasing from 0 to 8 %. This can be ascribed to the martensite reorientation that occurred at a low strain level and the dislocations formed at a large strain level. The maximum shape memory strain is 3.3 % in the Ti-20Zr-10Nb-4Ta alloy.
Key words: Ti-Zr; Shape memory alloy; Martensite transformation; Martensite Stabilization. * Corresponding author. Fax: +86 10 82315989 100, +86 10 82338200. Email: [email protected]
1
Shape memory alloys (SMAs) have been intensively investigated regarding their phase transformations and functional properties, shape memory effect, and pseudoelasticity [1-4]. In recent years, some Ti-based SMAs, e.g., Ti-Nb, Ti-Zr and Ti-Ta, have drawn much attention as new candidates for smart materials with high performance, especially in biomedical and high-temperature applications [5-8]. In 2011, binary Ti-Zr alloys were reported as shape memory alloys with a high phase transformation temperature of up to 900 K between the hexagonal α′-martensite and bcc β phases [6]. The phase stability and hardness of Ti-Zr-based alloys have been demonstrated to be sensitive to the addition of alloying elements; e.g., Nb, Cr, Al, Mo and Fe are all β-Stability elements that decrease the transformation temperature between the α and β phases [9-12]. Ti-20Zr-10Nb alloy [13] and Ti-30Zr-1Fe [14] alloys have demonstrated improved shape memory effect and excellent mechanical properties. Recently, a large shape memory recovery strain and a pseudoelasticity strain of 4.1 % and 4.7 %, respectively, were reported in Ti-19.5Zr-10Nb-0.5Fe [15] and Ti-19Zr-10Nb-1Fe [16] alloys. These results indicate that the addition of an alloying element is an effective approach to modify the phase stability and the corresponding shape memory effect or pseudoelasticity. Martensite stabilization in a shape memory alloy is the increase of the reverse martensitic transformation temperature via some thermal or mechanical features [17-22]. In the early studies, it was found that martensite stabilization occurred when martensite aging was performed in some shape memory alloys (SMAs), such as Au-Cd [17], Cu-Al-Ni [23], Ni-Ti-Hf [24] and Ni-Mn-Ga [19, 25]. The mechanism 2
can be ascribed to the symmetry-conforming property for short-range order of point defects [26, 27]. Moreover, it may trigger a new avenue to obtain new high temperature shape memory alloys [28]. Alternatively, martensite stabilization has usually been observed when the phase transformation temperatures of deformed martensite phase were measured in NiTi alloys [20-22, 29, 30] and some Heusler type SMAs [31, 32]. The strain induced martensite stabilization was attributed to the alteration of the internal elastic energy and the irreversible energy during the variants reorientation and plastic deformation [20, 33]. Although many investigations have focused on the thermo-mechanical and phase transformation behaviors, little research has considered the martensite stabilization of Ti-based SMAs. In the present work, the martensite stabilization and the shape memory effect in a new Ti-20Zr-10Nb-4Ta alloy induced by tensile strain associated with the structural analysis were investigated. 1. Experimental procedure The Ti-20Zr-10Nb-4Ta (at.%) alloy was prepared using high-purity (> 99.9 wt.%) raw materials. The ingots (approximately 1.5 kg in weight) were re-melted five times for homogeneity via the non-consumable arc-melting method under Ar atmosphere protection. The as-melted ingots were homogenized at 1273 K for 6 h in a vacuum of 10-3 Pa, cut into 4-mm thick pieces, and then cold rolled into sheets with 75 % reduction in thickness. Next, alloy sheets were annealed at 873 K in vacuum for 30 min and quenched in water. The tensile tests were performed at room temperature (298K) on a SANS CMT5504 universal test machine with the sample gauge length of 3
20 mm and strain rate of 1.67×10-4 s-1. Some of the alloy samples were deformed in tension to different pre-strains, from 2 % to 8 %, and then the unloaded alloy samples were cut into small samples for structural or phase transformation measurements. In order to determine the shape memory effect, tensile specimens with different pre-strains, 2 %, 4 %, 6 % and 8 %, were heated to 473 K for shape recovery. The microstructure was analyzed using optical microscopy, X-ray diffraction (XRD, Rigaku D/Max 2500 PC diffractometer with a Cu-Kα radiation (λ = 0.15406 nm)) and transmission electron microscopy (TEM, JEOL 2100F, operated at 200 kV). The phase transformation temperatures were measured by differential scanning calorimetry DSC (Q800) with a cooling/heating rate of 10 K min-1. 2. Results and discussion 2.1. Microstructures The optical micrograph and XRD patterns of the Ti-20Zr-10Nb-4Ta alloy are shown in Fig. 1. It can be seen that the alloy is composed of a polycrystalline structure with fine grains ranging from 5 to 15 μm in size, which is very similar to the structure of the cold-rolled and 873 K annealed Ti-20Zr-10Nb alloy [13]. The XRD result indicates that the only orthorhombic α″-martensite phase with lattice parameters of a=0.3170 nm, b=0.5004 nm and c=0.4755 nm exists in the alloy (Fig. 1b). Unlike the Ti-20Zr-10Nb alloy [13], no phase appears in the Ti-20Zr-10Nb-4Ta alloy. This observation suggests that the formation of the ω phase may be suppressed by the addition of Ta in Ti-Zr-Nb alloys. A similar effect of Ta has been found in Ti-Nb-based shape memory alloys [34]. 4
2.2. Mechanical properties Fig. 2 shows the tensile stress-strain curve of the Ti-20Zr-10Nb-4Ta alloy. It is seen that the stress-strain curve exhibits two yielding stages, with critical stress values of 215 MPa (σM) and 565 MPa (σS) at the strains of 0.5 % and 6 %, respectively. The tensile stress-strain curve obviously corresponds to the reorientation and plastic deformation of martensites, as commonly seen in the mechanical tests on the martensitic shape memory alloy, e.g., NiTi [33, 35], Ti-Zr-Nb [13, 15], Ni-Mn-Ga [36, 37] and Co-Ni-Ga [38] alloys, because the Ti-20Zr-10Nb-4Ta alloy is composed of single α″-martensite, as mentioned above. The tensile stress of the Ti-20Zr-10Nb-4Ta alloy is approximately 565 MPa, which is higher than that of the Ti-20Zr-10Nb alloy [13] due to the solid solution strengthening effect of the Ta element. 2.3. Effect of pre-strain on the reverse martensite transformation Fig. 3 displays the DSC curves of the Ti-20Zr-10Nb-4Ta alloy with different pre-strains (0, 2%, 4%, 6% and 8%). It is seen that there is one endothermic peak on the heating curve, which is confirmed to be the reverse transformation from the α″-martensite phase to the β phase with the start temperature (As) of 348 K compared with the phase transformations of some other Ti-Zr-Nb SMAs [13, 15]. It is seen that no obvious exothermic peak on cooling appears for the sample without deformation. This phenomenon has been found not only in the Ti-20Zr-10Nb-4Ta alloy but also in some other Ti-based shape memory alloys, e.g. Ti–30Ta [39] and Ti–30Nb–3Pd [40]. The reason may be due to a partial transformation of the β phase to the α″-martensite phase or the intrinsic low enthalpy of the phase transformation between the 5
α″-martensite and β parent [13, 15, 39, 40]. The other samples show the similar phase transformation behavior. Note that As temperature increases with increasing pre-strain. This phenomenon is the typical strain induced martensite stabilization. Fig. 4 shows the TEM images and SAED patterns of the Ti-20Zr-10Nb-4Ta alloy with different pre-strains. It is seen in Fig. 4a that the Ti-20Zr-10Nb-4Ta alloy without deformation is mainly composed of self-accommodation martensite variants exhibited as plates with many fine twins inside. The corresponding SAED pattern recorded along the α″ [101]/[-101] zone axis clearly indicates the twinned structure of martensites. Meanwhile, no ω and residual β phase was observed, as reported in Ti-20Zr-10Nb [13] and Ti-19.5Zr-10Nb-0.5Fe alloys [15], respectively. This observation is in good agreement with the XRD results in Fig. 1b. The TEM image of the sample with pre-strain of 2 % is very similar to Fig. 4a, so it is not presented here. For the sample with pre-strain of 4 %, as shown in Fig. 4b, the reorientation of the martensite twins occurred as characterized by the partial disappearance of the self-accommodation martensite morphology and the reduction in the fine inner twins. After being pre-strained to 6 %, most of the martensite plates are de-twinned, as shown in Fig. 4c. Some martensite plates are separated by the traces of the original plate boundaries, where fine inner twins disappear and dislocations appear. If the sample was deformed to a pre-strain of 8 %, more severe martensite reorientation occurs, as indicated by the rearrangement of martensites to nearly the same direction, and more dislocations appear, as shown in Fig. 4d. Therefore, it can be concluded that during the tensile loading on the Ti-20Zr-10Nb-4Ta alloy up to a pre-stain of 8 %, 6
martensite reorientation occurs at low strain level and dislocations forms at high strain level. To clearly explain the mechanism of martensite stabilization, the relationship between As and pre-strain of the Ti-20Zr-10Nb-4Ta alloy as well as the corresponding tensile stress-strain curve are shown in Fig. 5. Based on the TEM results in Fig. 4, the tensile stress-strain of a martensite shape memory alloy can be divided in to three stages. After the elastic deformation finished at the pre-strain of 0.5 %, a stress-plateau controlled by the reorientation of α"-martensite appears until the pre-strain of 3 % (stage I). Next, the tensile stress rapidly increases with increasing pre-strain to the second yielding at the pre-strain of approximately 6 % (stage II), which is dominated by further reorientation and elastic deformation of α"-martensite. Finally, the deformation continues at an almost constant stress level until fracture (stage III). This stage is companied by the generation of a high density of dislocations due to the slip deformation of martensite. According to some previous studies, the mechanism of the martensitic stabilization can be ascribed to the release of stored elastic energy in martensites [25, 26], the influence of dislocations in plastic deformation [38], or to their combined effects [41]. Based on the above analysis, therefore, it is believed that the martensite stabilization is influenced by the release of the stored elastic energy due to reorientation of martensite twins at the initial deformation, e.g., 4 % or 6 %. With the further deformation, e.g., 8 %, the effect of dislocations should be the predominant factor. 2.4. Shape memory effect of the Ti-20Zr-10Nb-4Ta alloy 7
Fig. 6 shows the tensile loading-unloading curves and the shape memory behavior of Ti-20Zr-10Nb-4Ta alloy. It can be seen that the shape memory recovery ratio decreases with the increasing pre-strain, and the entire recovery occurs in the sample with pre-strain of 2 %. The shape memory strain of Ti-20Zr-10Nb-4Ta alloy increases with the increasing pre-strain and reaches the maximum value of 3.3 % when the pre-strain reaches 6 % or 8%. Therefore, the shape memory effect of the Ti-20Zr-10Nb-4Ta alloy is superior to those of the Ti–22Nb–6Ta (~ 2.1 %) [42], Ti-20Zr-10Nb (~ 2.5 %) [13] and Ti-30Nb-3Pd (~ 2.7 %) [40] alloys. The critical stress for martensite reorientation (σM) of the Ti-20Zr-10Nb-4Ta alloy is 215 MPa, as shown in Fig. 2, which is much lower than that of the Ti-20Zr-10Nb alloy (~ 240 MPa) [13]. It is well known that a lower critical stress for martensite reorientation (σM) is beneficial to shape memory recovery. Moreover, the shape memory recovery during the reverse martensitic transformation of the Ti-20Zr-10Nb-4Ta alloy will not be influenced by the ω phase transformation as occurred in the Ti-20Zr-10Nb alloy [13]. Therefore, the shape memory effect of the Ti-20Zr-10Nb alloy can be improved by the addition of Ta. 3. Conclusions Microstructures, phase transformation, and shape memory effect of the Ti-20Zr-10Nb-4Ta alloy were investigated. The results show that the alloy is composed of single orthorhombic α″-martensite, without and β phase. The alloy exhibit a double yielding behavior upon tension at the strains of 0.5 % and 6 %, corresponding to the yield stresses of 215 MPa and 565 MPa, respectively. The 8
reverse martensite transformation start temperature of the alloy increases from 348 to 405 K with the pre-strain increasing from 0 to 8%. The strain induced martensite stabilization can be attributed to the martensite reorientation that occurred at the low strain level and the dislocations formed at the high strain level. The maximum shape memory strain is 3.3 % in the Ti-20Zr-10Nb-4Ta alloy. Acknowledgements This work is supported by the National Basic Research Program of China (No. 2012CB619400), the National Natural Science Foundation of China (NSFC, No. 51371016) and the Aeronautical Science Foundation of China (2014ZF51070). References: [1] S. Hao, L. Cui, D. Jiang, X. Han, Y. Ren, J. Jiang, Y. Liu, Z. Liu, S. Mao, Y. Wang, Y. Li, X. Ren, X. Ding, S. Wang, C. Yu, X. Shi, M. Du, F. Yang, Y. Zheng, Z. Zhang, X. Li, D.E. Brown, J. Li, Science 339 (2013) 1191-1194. [2] T. Omori, K. Ando, M. Okano, X. Xu, Y. Tanaka, I. Ohnuma, R. Kainuma, K. Ishida, Science 333 (2011) 68-71. [3] Y. Liu, S.P. Galvin, Acta Mater. 45 (1997) 4431-4439. [4] Y. Liu, Z. Xie, J. Van Humbeeck, L. Delaey, Acta Mater. 46 (1998) 4325-4338. [5] H.Y. Kim, Y. Ikehara, J.I. Kim, H. Hosoda, S. Miyazaki, Acta Mater. 54 (2006) 2419-2429. [6] Y. Li, Y. Cui, F. Zhang, H. Xu, Scr. Mater. 64 (2011) 584-587. [7] Y.L. Zhou, M. Niinomi, T. Akahori, Mater. Sci. Eng. A 371 (2004) 283-290. [8] D.C. Zhang, Y.F. Mao, Y.L. Li, J.J. Li, M. Yuan, J.G. Lin, Mater. Sci. Eng. A 559 (2013) 706-710. 9
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Fig. 1. Optical micrograph (a) and XRD pattern (b) of Ti-20Zr-10Nb-4Ta alloy.
Fig. 2. Tensile stress-strain curve of the Ti-20Zr-10Nb-4Ta alloy.
Fig. 3. DSC curves of Ti-20Zr-10Nb-4Ta alloy with different pre-strains (0, 2%, 4%, 6% and 8%). 12
Fig. 4. TEM bright field images and the SAED patterns of the Ti-20Zr-10Nb-4Ta alloy with different pre-strains: (a) 0 % (martensite variant boundaries indicated by solid lines and fine inner twins in martensite variants indicated by white arrows), (b) 13
4 % (the disappearance of fine inner twins in martensite variants indicated by black arrows), (c) 6 % (the traces of the original plate boundaries are indicated by dotted lines and the dislocations are indicated by hollow arrows) and (d) 8 % (more dislocations indicated by hollow arrows).
Fig. 5. Tensile stress-strain curve and the relationship between As and the pre-strain of Ti-20Zr-10Nb-4Ta alloy.
Fig. 6. (a) The tensile loading-unloading curves of Ti-20Zr-10Nb-4Ta alloy (the dashed line with an arrow indicates the shape recovery after the reverse transformation upon heating) and (b) the shape recovery strain and shape memory recovery ratio versus pre-strain. 14