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Two-stage superelasticity of a Ce-added laser-welded TiNi alloy
Materials Letters 62 (2008) 3539–3541
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Two-stage superelasticity of a Ce-added laser-welded TiNi alloy Xingke Zhao a,⁎, Wei Wang a, Li Chen b, Fangjun Liu b, Geng Chen a, Jihua Huang a, Hua Zhang a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China National Key Laboratory for High Energy Density Beam Processing Technology, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, China
A R T I C L E
I N F O
Article history: Received 8 October 2007 Accepted 17 March 2008 Available online 25 March 2008 Keywords: Shape memory materials Laser processing Mechanical properties TiNi alloy Cerium
A B S T R A C T Microstructure, phase transformation and stress–strain behaviors of a Ce-added laser welded TiNi alloy have been studied. An Nd:YAG laser was used for butt fusion of two Ti–50.9Ni alloy plates with a (Ti–50.9Ni)–5Ce stripe pre-assembled between their joint. Microstructure, phase constitution, phase transformation and stress–strain behavior of several TiNi alloy specimens were researched with SEM, XRD, DSC and tensile test, respectively. Unlike the typical monotonous superelasticity of base metal and Ce-free welded TiNi, the Ceadded welded TiNi specimen shows a peculiar two-stage superelasticity. Based on results of SEM, XRD and DSC tests, a mechanism for the two-stage superelasticity has been discussed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction TiNi alloys have been finding more and more applications in numerous fields due to its excellent shape memory effect, superelasticity and good mechanical properties [1]. However, the low formability of these intermetallic alloys requires a suitable joining technique to obtain devices and components with complex geometries. Laser welding is one of the most important joining techniques for this class of materials [2–4]. The mechanical and the functional behavior of TiNi alloys are strongly influenced by the thermal effects and chemical contamination associated with the welding process. An accurate knowledge of the modifications induced by the laser welding process is essential to design complex shaped miniaturized parts in many branches of engineering. Furthermore, new types of smart sensors and actuators could be obtained by joining TiNi components with different functional properties. In present work, two-stage superelasticity in a Ceadded laser-weld TiNi alloy is reported. 2. Experiments Ti50.9Ni plate, hot-rolled at 900 °C with a thickness of 1.6 mm, was used in this study. Weld specimens were cut into the size of 1.6 mm × 25 mm × 50 mm. Two specimens were assembled together to a butt joint with a thin strip of filling alloy between them. The composition of filling alloy was (Ti–50.9Ni)–5Ce. The specimens and
⁎ Corresponding author. Tel.: +86 10 62334859. E-mail address: [email protected] (X. Zhao). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.03.042
fillings were heat-treated at 723 K for 0.5 h, and then cleaned with acetone before welding. An Nd:YAG laser source (AM356, Gsi Lumonics) was used for this experiment. Phase transformation measurements were carried out using differential scanning calorimetry (DSC) under a controlled cooling/ heating rate of 10 K/min. The weld joint was cross-sectioned using a low speed diamond saw. The polished cross-section was etched with an acidic etchant (HF-2HNO3–3H2O) for analysis by using scanning scanning-electron microscopy (SEM). Phase identification was carried out with an X-ray diffractometer (XRD) using Cu-Kα radiation. Superelasticity behaviors were investigated with a tensile test at room temperature. A tensile specimen with size of 0.5 mm×1.6 mm × 50 mm was loaded under a displacement rate of 0.02 mm/s to a maximum strain value of 4% and then unloaded back to zero stress. The stress–strain curve was recorded automatically during loading and unloading cycle. 3. Results and discussions 3.1. DSC results Fig. 1 shows DSC curves of several TiNi alloy specimens, marked as base metal, Cefree weld and Ce-added weld, respectively. All these three curves in Fig. 1 are very alike. Their transformation temperatures are almost the same. All three curves do not show the presence of the R-phase transformation during either cooling or heating. Cai has reported that the Ms of TiNi–Ce alloy increases rapidly with the increase of Ce content when the Ce content is less than 2%, and then keeps constant with further increasing the Ce content [5]. This conclusion were drawn from the results of adding Ce into a Ti50Ni50 alloy, the increase of Ms were thought to be attributed to that some Nirich particles exist in the TiNiCE alloys, which decrease the Ni/Ti ratio of the matrix. In the present study, the experimental material is Ti49.1–Ni50.9, and Ce content in weld is less than 1% considering the dilution effect. Ni-rich particles forming should not affect the Ni/Ti ratio of the matrix. So the Ce-added weld has the similar phase transformation behavior.
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X. Zhao et al. / Materials Letters 62 (2008) 3539–3541
Fig. 1. DSC curves of several TiNi alloy specimens.
3.2. Microstructure and phase constitution Fig. 2(a) and (b) shows the typical microstructures of center of a Ce-added weld and a Ce-free weld of TiNi alloy. Both pictures show a primarily single-phase matrix. However, there are obvious differences between the two pictures. Ce-free weld has a typical solidification microstructure with coarse branch-grains and wide grain boundaries. While in the Ce-added weld, grain boundaries are obscure and the matrix is more compact and homogeneous. In addition, there are lots of white fine granular particles as well as gray granular particles dispersing uniformly in the matrix. In another word, Ce-added weld resembles a heat-treatment microstructure rather than a casting one. XRD patterns of weld show that Ce-free weld is mainly TiNi austenite phase. Some new diffraction peaks and the peaks broadening indicate the formation of some second phases. These second phases are identified to be TiO2 anatase (JCPDS 83-2242) and Ni3Ti (JCPDS 05-0723). Ce-added weld displays much more diffraction peaks than Cefree weld. The main diffraction peaks can be indexed to TiNi austenite, TiO2 anatase and Ni3Ti phase. In addition, CeO2 (JCPDS 78-0694) has been detected in the Ce-added weld. Based on results of XRD, phases in Fig. 2 can be identified as follows: the dark gray
Fig. 2. SEM images of (a) Ce-added and (b) Ce-free laser weld of TiNi alloy.
Fig. 3. Stress vs. Strain behaviors of TiNi alloy specimens.
particle being Ni3Ti phase, white particle being CeO2 phase. TiO2 anatase in weld may exist in the form of thin layer at the grain boundaries. 3.3. Stress–strain behavior Fig. 3 shows tensile stress–strain curves of several TiNi specimens. From left to right the curves are of base metal, Ce-added weld and Ce-free weld, respectively. The curve of the Ce-free weld is similar to that of base metal, showing a typical monotonous superelasticity, this is in accord with the literatures [6]. But the curve of Ce-added weld specimen shows a peculiar two-stage superelasticity behavior, having two stress plateaus with the value of 80 MPa and 450 MPa during loading half-cycle and one stress plateau during unloading half-cycle. An obvious stress hysteresis appears in a loading and unloading cycle, and forming two rectangle-like regions below and above the loading lower stress plateau. The total tensile strain is 4%, about 1.5% come from the low stress plateau and 0.8% from upper stress plateau during loading. During unloading, about 1.5% recovery strains come from unloading stress plateau. As unloading to zero stress, about 0.4% strain remain. To further confirm the two-stage superelasticity of the Ce-added welding sample, a tensile curve till fracture is carried out for the understanding whether the second plateau is true, since if the second plateau is close to the fracture strength, it does mean the superelastic plateau. The presence of CeO2 and TiO2 at the joint position with a high content might also imply a deterioration of the mechanical property. The tensile cures till fracture is shown in Fig. 4. The fracture tensile stress is about 770 MPa, and close to the second plateau stress, which means the second superelastic plateau does exist. This kind of two-stage superelasticity is common for a Cu–Al–Ni single crystal [7] but uncommon for a TiNi alloy. In near-equiatomic TiNi alloys, superelasticity occurs in association with the thermo-elastic martensitic transformation. A familiar stress–strain is characterized by a monotonous increase of stress with strain increasing during loading and a monotonous decrease of stress with strain decreasing during unloading. This attributes to the one stage phase transformation (M-B2). After certain thermomechanical treatments or adding a third element, the transformation sequence can be changed from one-stage (M-B2) to two-stage (such as M-R-B2) transformation accordingly a two-stage superelasticity appeared [8,9]. The two-stage superelasticity
Fig. 4. Stress vs. Strain behaviors of TiNi alloy specimens.
X. Zhao et al. / Materials Letters 62 (2008) 3539–3541 in present work is different from that usually caused by R-phase transformation. Rphase transformation generally can provide a recovery strain less than 1%. And R phase has not been detected in our DSC measurement. Besides R-phase transformation, the coexistence of different types of martensite in a TiNi specimen will also induce multistage strain recovery behavior [10]. The two-stage superelasticity in present work can be explained by martensite coexistence. Comparing with base metal and Ce-free weld, Ce-added weld resembles a heat-treatment microstructure with lots of fine granular particles of CeO2 as well as Ti3Ni4 dispersing uniformly in the matrix. During laser welding process the weld joint undergoes a special thermal cycle, which makes a thermo-mechanical effects and produces residual stress and residual strain in a narrow weld region. The value of residual stress or residual strain relates to joint rigidity. The higher strength a weld is, the larger rigidity a weld joint. Ce-added weld has higher strength than that of Ce-free weld according to their microstructure characteristics in Fig. 2, and accordingly inducing a two-stage martensitic transformation.
4. Summary In this paper microstructure and two-stage superelasticity of a Ce-added laser welded TiNi alloy have been presented. Tensile tests show a peculiar stress–strain curve with two rectangle-like regions in a loading and unloading cycle. SEM, DSC and XRD results of a Ceadded weld show no evidence of R-phase transformation. The multistage superelasticity is referred to the corporate effect of the special
3541
microstructure of Ce-added laser weld and the thermo-mechanical history of the TiNi specimen. Acknowledgement The authors would like to thank the Chinese Military Research Foundation for supporting the presented investigations in the “Fundamental research on laser weldability of Shape Memory alloys”. References [1] K. Otsuka, X. Ren, Prog. Mater. Sci. 50 (2005) 511–678. [2] Y.T. Hsu, Y.R. Wang, S.K. Wu, C. Chen, Metall. Mater. Trans., A Phys. Metall. Mater. Sci. 32 (2001) 569–576. [3] P. Schloßmacher, T. Hass, A. Schüßler, Proceedings SMST conference, 1994, pp. 7–10. [4] H. Hosoda, S. Hanada, K. Inoue, T. Fukui, Y. Mishima, T. Suzuki, Intermetallics 6 (1998) 291–301. [5] W. Cai, X. Meng, L. Zhao, Curr. Opin. Solid State Mater. Sci. 9 (2005) 296–302. [6] A. Falvo, F.M. Furgiuele, C. Maletta, Sci. Eng. Abroad 412 (2005) 235–240. [7] K. Otsuka, H. Sakamoto, K. Shimizu, Acta Metall. 270 (1976) 585–601. [8] S. Miyazaki, K. Otsuka, Metall. Trans., A, Phys. Metall. Mater. Sci. 17 (1986) 53–63. [9] J.I. Kim, Y. Liu, S. Miyazaki, Acta Mater. 52 (2004) 487–499. [10] L. Cui, Y. Li, Y. Zheng, D. Yang, Mater. Lett. 47 (2001) 286–289.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Two-stage superelasticity of a Ce-added laser-welded TiNi alloy Xingke Zhao a,⁎, Wei Wang a, Li Chen b, Fangjun Liu b, Geng Chen a, Jihua Huang a, Hua Zhang a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China National Key Laboratory for High Energy Density Beam Processing Technology, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, China
A R T I C L E
I N F O
Article history: Received 8 October 2007 Accepted 17 March 2008 Available online 25 March 2008 Keywords: Shape memory materials Laser processing Mechanical properties TiNi alloy Cerium
A B S T R A C T Microstructure, phase transformation and stress–strain behaviors of a Ce-added laser welded TiNi alloy have been studied. An Nd:YAG laser was used for butt fusion of two Ti–50.9Ni alloy plates with a (Ti–50.9Ni)–5Ce stripe pre-assembled between their joint. Microstructure, phase constitution, phase transformation and stress–strain behavior of several TiNi alloy specimens were researched with SEM, XRD, DSC and tensile test, respectively. Unlike the typical monotonous superelasticity of base metal and Ce-free welded TiNi, the Ceadded welded TiNi specimen shows a peculiar two-stage superelasticity. Based on results of SEM, XRD and DSC tests, a mechanism for the two-stage superelasticity has been discussed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction TiNi alloys have been finding more and more applications in numerous fields due to its excellent shape memory effect, superelasticity and good mechanical properties [1]. However, the low formability of these intermetallic alloys requires a suitable joining technique to obtain devices and components with complex geometries. Laser welding is one of the most important joining techniques for this class of materials [2–4]. The mechanical and the functional behavior of TiNi alloys are strongly influenced by the thermal effects and chemical contamination associated with the welding process. An accurate knowledge of the modifications induced by the laser welding process is essential to design complex shaped miniaturized parts in many branches of engineering. Furthermore, new types of smart sensors and actuators could be obtained by joining TiNi components with different functional properties. In present work, two-stage superelasticity in a Ceadded laser-weld TiNi alloy is reported. 2. Experiments Ti50.9Ni plate, hot-rolled at 900 °C with a thickness of 1.6 mm, was used in this study. Weld specimens were cut into the size of 1.6 mm × 25 mm × 50 mm. Two specimens were assembled together to a butt joint with a thin strip of filling alloy between them. The composition of filling alloy was (Ti–50.9Ni)–5Ce. The specimens and
⁎ Corresponding author. Tel.: +86 10 62334859. E-mail address: [email protected] (X. Zhao). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.03.042
fillings were heat-treated at 723 K for 0.5 h, and then cleaned with acetone before welding. An Nd:YAG laser source (AM356, Gsi Lumonics) was used for this experiment. Phase transformation measurements were carried out using differential scanning calorimetry (DSC) under a controlled cooling/ heating rate of 10 K/min. The weld joint was cross-sectioned using a low speed diamond saw. The polished cross-section was etched with an acidic etchant (HF-2HNO3–3H2O) for analysis by using scanning scanning-electron microscopy (SEM). Phase identification was carried out with an X-ray diffractometer (XRD) using Cu-Kα radiation. Superelasticity behaviors were investigated with a tensile test at room temperature. A tensile specimen with size of 0.5 mm×1.6 mm × 50 mm was loaded under a displacement rate of 0.02 mm/s to a maximum strain value of 4% and then unloaded back to zero stress. The stress–strain curve was recorded automatically during loading and unloading cycle. 3. Results and discussions 3.1. DSC results Fig. 1 shows DSC curves of several TiNi alloy specimens, marked as base metal, Cefree weld and Ce-added weld, respectively. All these three curves in Fig. 1 are very alike. Their transformation temperatures are almost the same. All three curves do not show the presence of the R-phase transformation during either cooling or heating. Cai has reported that the Ms of TiNi–Ce alloy increases rapidly with the increase of Ce content when the Ce content is less than 2%, and then keeps constant with further increasing the Ce content [5]. This conclusion were drawn from the results of adding Ce into a Ti50Ni50 alloy, the increase of Ms were thought to be attributed to that some Nirich particles exist in the TiNiCE alloys, which decrease the Ni/Ti ratio of the matrix. In the present study, the experimental material is Ti49.1–Ni50.9, and Ce content in weld is less than 1% considering the dilution effect. Ni-rich particles forming should not affect the Ni/Ti ratio of the matrix. So the Ce-added weld has the similar phase transformation behavior.
3540
X. Zhao et al. / Materials Letters 62 (2008) 3539–3541
Fig. 1. DSC curves of several TiNi alloy specimens.
3.2. Microstructure and phase constitution Fig. 2(a) and (b) shows the typical microstructures of center of a Ce-added weld and a Ce-free weld of TiNi alloy. Both pictures show a primarily single-phase matrix. However, there are obvious differences between the two pictures. Ce-free weld has a typical solidification microstructure with coarse branch-grains and wide grain boundaries. While in the Ce-added weld, grain boundaries are obscure and the matrix is more compact and homogeneous. In addition, there are lots of white fine granular particles as well as gray granular particles dispersing uniformly in the matrix. In another word, Ce-added weld resembles a heat-treatment microstructure rather than a casting one. XRD patterns of weld show that Ce-free weld is mainly TiNi austenite phase. Some new diffraction peaks and the peaks broadening indicate the formation of some second phases. These second phases are identified to be TiO2 anatase (JCPDS 83-2242) and Ni3Ti (JCPDS 05-0723). Ce-added weld displays much more diffraction peaks than Cefree weld. The main diffraction peaks can be indexed to TiNi austenite, TiO2 anatase and Ni3Ti phase. In addition, CeO2 (JCPDS 78-0694) has been detected in the Ce-added weld. Based on results of XRD, phases in Fig. 2 can be identified as follows: the dark gray
Fig. 2. SEM images of (a) Ce-added and (b) Ce-free laser weld of TiNi alloy.
Fig. 3. Stress vs. Strain behaviors of TiNi alloy specimens.
particle being Ni3Ti phase, white particle being CeO2 phase. TiO2 anatase in weld may exist in the form of thin layer at the grain boundaries. 3.3. Stress–strain behavior Fig. 3 shows tensile stress–strain curves of several TiNi specimens. From left to right the curves are of base metal, Ce-added weld and Ce-free weld, respectively. The curve of the Ce-free weld is similar to that of base metal, showing a typical monotonous superelasticity, this is in accord with the literatures [6]. But the curve of Ce-added weld specimen shows a peculiar two-stage superelasticity behavior, having two stress plateaus with the value of 80 MPa and 450 MPa during loading half-cycle and one stress plateau during unloading half-cycle. An obvious stress hysteresis appears in a loading and unloading cycle, and forming two rectangle-like regions below and above the loading lower stress plateau. The total tensile strain is 4%, about 1.5% come from the low stress plateau and 0.8% from upper stress plateau during loading. During unloading, about 1.5% recovery strains come from unloading stress plateau. As unloading to zero stress, about 0.4% strain remain. To further confirm the two-stage superelasticity of the Ce-added welding sample, a tensile curve till fracture is carried out for the understanding whether the second plateau is true, since if the second plateau is close to the fracture strength, it does mean the superelastic plateau. The presence of CeO2 and TiO2 at the joint position with a high content might also imply a deterioration of the mechanical property. The tensile cures till fracture is shown in Fig. 4. The fracture tensile stress is about 770 MPa, and close to the second plateau stress, which means the second superelastic plateau does exist. This kind of two-stage superelasticity is common for a Cu–Al–Ni single crystal [7] but uncommon for a TiNi alloy. In near-equiatomic TiNi alloys, superelasticity occurs in association with the thermo-elastic martensitic transformation. A familiar stress–strain is characterized by a monotonous increase of stress with strain increasing during loading and a monotonous decrease of stress with strain decreasing during unloading. This attributes to the one stage phase transformation (M-B2). After certain thermomechanical treatments or adding a third element, the transformation sequence can be changed from one-stage (M-B2) to two-stage (such as M-R-B2) transformation accordingly a two-stage superelasticity appeared [8,9]. The two-stage superelasticity
Fig. 4. Stress vs. Strain behaviors of TiNi alloy specimens.
X. Zhao et al. / Materials Letters 62 (2008) 3539–3541 in present work is different from that usually caused by R-phase transformation. Rphase transformation generally can provide a recovery strain less than 1%. And R phase has not been detected in our DSC measurement. Besides R-phase transformation, the coexistence of different types of martensite in a TiNi specimen will also induce multistage strain recovery behavior [10]. The two-stage superelasticity in present work can be explained by martensite coexistence. Comparing with base metal and Ce-free weld, Ce-added weld resembles a heat-treatment microstructure with lots of fine granular particles of CeO2 as well as Ti3Ni4 dispersing uniformly in the matrix. During laser welding process the weld joint undergoes a special thermal cycle, which makes a thermo-mechanical effects and produces residual stress and residual strain in a narrow weld region. The value of residual stress or residual strain relates to joint rigidity. The higher strength a weld is, the larger rigidity a weld joint. Ce-added weld has higher strength than that of Ce-free weld according to their microstructure characteristics in Fig. 2, and accordingly inducing a two-stage martensitic transformation.
4. Summary In this paper microstructure and two-stage superelasticity of a Ce-added laser welded TiNi alloy have been presented. Tensile tests show a peculiar stress–strain curve with two rectangle-like regions in a loading and unloading cycle. SEM, DSC and XRD results of a Ceadded weld show no evidence of R-phase transformation. The multistage superelasticity is referred to the corporate effect of the special
3541
microstructure of Ce-added laser weld and the thermo-mechanical history of the TiNi specimen. Acknowledgement The authors would like to thank the Chinese Military Research Foundation for supporting the presented investigations in the “Fundamental research on laser weldability of Shape Memory alloys”. References [1] K. Otsuka, X. Ren, Prog. Mater. Sci. 50 (2005) 511–678. [2] Y.T. Hsu, Y.R. Wang, S.K. Wu, C. Chen, Metall. Mater. Trans., A Phys. Metall. Mater. Sci. 32 (2001) 569–576. [3] P. Schloßmacher, T. Hass, A. Schüßler, Proceedings SMST conference, 1994, pp. 7–10. [4] H. Hosoda, S. Hanada, K. Inoue, T. Fukui, Y. Mishima, T. Suzuki, Intermetallics 6 (1998) 291–301. [5] W. Cai, X. Meng, L. Zhao, Curr. Opin. Solid State Mater. Sci. 9 (2005) 296–302. [6] A. Falvo, F.M. Furgiuele, C. Maletta, Sci. Eng. Abroad 412 (2005) 235–240. [7] K. Otsuka, H. Sakamoto, K. Shimizu, Acta Metall. 270 (1976) 585–601. [8] S. Miyazaki, K. Otsuka, Metall. Trans., A, Phys. Metall. Mater. Sci. 17 (1986) 53–63. [9] J.I. Kim, Y. Liu, S. Miyazaki, Acta Mater. 52 (2004) 487–499. [10] L. Cui, Y. Li, Y. Zheng, D. Yang, Mater. Lett. 47 (2001) 286–289.