- Email: [email protected]
Mechanical response of proton beam irradiated nitinol
Physica B 406 (2011) 8–11
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Mechanical response of proton beam irradiated nitinol Naveed Afzal, I.M. Ghauri n, F.E. Mubarik, F. Amin Centre for Advanced Studies in Physics, GC University, Lahore, Pakistan
a r t i c l e in f o
a b s t r a c t
Article history: Received 11 February 2010 Received in revised form 30 July 2010 Accepted 28 September 2010
The present investigation deals with the study of mechanical behavior of proton beam irradiated nitinol at room temperature. The specimens in austenitic phase were irradiated over periods of 15, 30, 45 and 60 min at room temperature using 2 MeV proton beam obtained from Pelletron accelerator. The stress–strain curves of both unirradiated and irradiated specimens were obtained using a universal testing machine at room temperature. The results of the experiment show that an intermediate rhombohedral (R) phase has been introduced between austenite and martensite phase, which resulted in the suppression of direct transformation from austenite to martensite (A–M). Stresses required to start R-phase (sRS) and martensitic phase (sMS) were observed to decrease with increase in exposure time. The hardness tests of samples before and after irradiation were also carried out using Vickers hardness tester. The comparison reveals that the hardness is higher in irradiated specimens than that of the unirradiated one. The increase in hardness is quite sharp in specimens irradiated for 15 min, which then increases linearly as the exposure time is increased up to 60 min. The generation of R-phase, variations in the transformation stresses sRS and sMS and increase in hardness of irradiated nitinol may be attributed to lattice disorder and associated changes in crystal structure induced by proton beam irradiation. & 2010 Elsevier B.V. All rights reserved.
Keywords: Shape memory alloy Irradiation Deformation Mechanical properties
1. Introduction Shape memory alloys are intelligent materials that have gained worldwide recognition because of their unique properties, such as superelasticity and shape memory effect [1,2]. These alloys undergo a reversible martensitic phase transformation under the action of either temperature or applied stress, where austenite phase is transformed into a martensite phase. The transformation process is accompanied by shape memory effect and superelasticity [1]. Nitinol is a well known shape memory alloy which is commonly used in biomedical, aerospace and nuclear engineering applications [3–8]. Therefore its thermo-mechanical response under irradiation environment is of principal importance. In this regard various attempts in the past have been made to investigate the irradiation effects of electrons, protons, neutrons and heavy ions on the physical and metallurgical behavior of nitinol [9–20]. Occurrence of transition from crystalline to amorphous (C–A) structure has been observed in nitinol after irradiation [9–15]. Cheng and Ardell [14] reported that C–A transition in nitinol is a function of both irradiation dose and temperature. Mori et al. [15] studied the electron irradiation induced C-A transition in nitinol. It was found that electron irradiation induces a localized amorphization which mainly occurs along dislocation lines and grain boundaries. Effects of irradiation on transformation characteristics of nitinol were studied by Hoshiya et al. [16] and later on by Konopleva
n
Corresponding author. E-mail address: [email protected] (I.M. Ghauri).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.09.040
et al. [17]. They observed that neutron irradiation of nitinol decreased its transformation. Similar studies were also carried out by Al-Aql et al. [18] using proton beam irradiation. Wang et al. [19] studied the existence of R-phase in proton beam irradiated TiNi alloy. The alloy specimens were exposed to 18 MeV proton beam and the irradiation resulted in a decrease in R-phase transformation start temperature and the reverse martensitic transformation finish temperature with increase in irradiation dose. Investigating the effects of proton irradiation energy on the martensitic transformation temperature of nitinol, Dughaish [20] reported a decrease in transformation temperature with increase in irradiation energy. The foregoing analysis of previous work reveals that irradiation induced C–A transitions and irradiation effects on the transformation temperatures in nitinol has been the main focus of research. The aim of the present study is to investigate transformation characteristics of nitinol (NiTi) alloy through its mechanical response after irradiation for different exposure times with 2 MeV proton beam.
2. Experimental work NiTi alloy in the form of wire with 50.5 at% Ni and 49.5 at% Ti was obtained from Alfa Aeisar (USA). The specimens, each of length 60 mm and diameter 1 mm, were cut from the as-received wire and then mechanically polished using diamond paste of grades 6, 3 and 0.1 mm to remove any distortion from the surface.
N. Afzal et al. / Physica B 406 (2011) 8–11
1400 1. Unirradiated Nitinol 2. 15 min Irradiated Nitinol 3. 30 min Irradiated Nitinol 4. 45 min Irradiated Nitinol 5. 60 min Irradiated Nitinol
1200 1000 Stress (MPa)
The polished specimens were annealed at 703 K for 1 h in a high temperature vacuum furnace. The annealed samples were then subjected to 2 MeV proton beam irradiation for 15, 30, 45 and 60 min under vacuum at 298 K to a fluence of 1015/cm2. The irradiations were carried out using Pelletron accelerator installed at accelerator lab of Government College University, Lahore. The temperature of samples during irradiation was kept constant by continuous circulation of liquid nitrogen in the irradiation chamber. A digital temperature controller attached to the chamber indicated variations of a maximum of 71 K during the entire duration of irradiation. The XRD analysis of unirradiated and irradiated specimens was carried out using Phillips Panalytical XRD system. Both unirradiated and irradiated specimens were deformed at room temperature using Universal Testing Machine with a cross-head speed of 0.5 mm/min. The deformation of specimens was recorded in the form of stress–strain curves, which were used for further analysis. Micro-hardness of specimens was also measured using Vickers hardness tester at a maximum load of 200 g. The data thus obtained, associated with micrographs, was utilized to compare the mechanical response of both unirradiated and proton beam irradiated specimens.
9
D
E
E
800
D
600 / MS B
400 B
RS
C
2 3
4
5 1
200
C
B
0A
0
2
4
6
8
10
12
14
16
18
20
22
Strain (%) Fig. 2. Comparison between stress–strain curves of unirradiated and 15–60 min proton beam irradiated nitinol.
3. Results and discussion
650
The stress–strain curve of unirradiated nitinol deformed at room temperature is shown in Fig. 1. Nitinol is in austenitic phase before deformation (point A). The austenite phase continues to deform with elastic modulus EA, as the elastic deformation proceeds. At point B, a transformation of phase from austenitic to martensitic begins to take place and the corresponding stress is called martensite transformation start stress (sMS). As deformation continues, a stress plateau BC appears in the stress–strain curve indicating that a large strain in the material has been produced on the application of small stress beyond sMS. The deformation in region BC is mainly carried out by transformation from austenitic to martensitic crystal structure through detwinning. As stress is increased further beyond point C, the curve again shows a linear stress–strain relationship indicating that the martensite is deformed elastically with elastic modulus EM. Beyond point D, the material deforms plastically, leading to its failure. Fig. 2 shows the comparison of stress–strain curves of unirradiated and 15–60 min proton beam irradiated nitinol. It can be seen that as a result of irradiation, an additional intermediate region BB/ of rhombohedral (R) phase has appeared between austenitic (A) and martensitic (M) phase. The comparison
600
Hardness (HV)
550 500 450 400 350 300 250 0
10
20
30
40
50
60
Time (minutes) Fig. 3. Variations of micro-hardness of nitinol with irradiation exposure time.
1000 E
Stress (MPa)
800
D
600 EM
400
200
MS
C
B
EA
0
A
0
2
4
6
8
10
12
14
16
18
20
22
Strain (%) Fig. 1. Stress–strain behavior of unirradiated nitinol.
Fig. 4. Microstructure of unirradiated nitinol (magnification 1000 ).
10
N. Afzal et al. / Physica B 406 (2011) 8–11
Fig. 5. Microstructure of 30 min proton beam irradiated nitinol (magnification 1000 ).
Fig. 6. Microstructure of 60 min proton beam irradiated nitinol (magnification 1000 ).
between unirradiated and 15 min proton beam irradiated nitinol clearly shows that irradiation has resulted in an increase in the transformation stress sMS. However the R-phase start stress sRS and M-phase start stress sMS decrease with increase in exposure time. The fracture stress for irradiated specimens is much higher than that of the unirradiated one. The results of micro-hardness tests of samples reveal that hardness is higher in irradiated specimens than that of unirradiated one. The increase in hardness is quite sharp after 15 min of irradiation, which then increases linearly with further increase in exposure time up to 60 min (Fig. 3). The comparison between the microstructures of unirradiated specimen (Fig. 4) with that of 30 min proton beam irradiated specimen (Fig. 5) shows that irradiation produces wear and tear on the surface and it also causes the formation of precipitates, which are non-uniformly distributed over the surface. The defects/precipitates seem to agglomerate with increase in irradiation time up to 60 min (Fig. 6). XRD spectra of unirradiated and irradiated specimens are shown in Fig. 7. The spectrum of unirradiated nitinol shows a prominent peak of cubic NiTi2 at 2y value of 381 corresponding to (4 2 2) plane. The spectrum also indicate a NiTi peak at 431 corresponding to (0 0 2) plane and two monoclinic peaks at 551, and 691 corresponding to (1 2 0) and (2 1 0) planes respectively. The monoclinic peaks however disappear and a new peak at 421 corresponding to (1 1 0) plane of cubic NiTi appears when the specimens are irradiated with 2 MeV proton beam for 15–60 min. Similarly two more peaks of Ni3Ti also appear at 2y value of 641 and 821 corresponding to planes (3 0 1) and (2 1 5), respectively. The generation of Ni3Ti phases and the re-crystallization of monoclinic NiTi into cubic NiTi are clear indications of structural evolution induced by irradiation. The presence of Ni3Ti peaks shows the formation of precipitates responsible for the internal stresses within the material. It has been established in the literature that proton irradiation of nitinol produces small displacement cascades and thus generates vacancies, interstitials and/or precipitates [19]. The lattice disorder and stress fields associated with the defects/ precipitates produced during irradiation are responsible for the generation of R-phase that nucleates around the local stress fields and grows through the matrix [19,21]. With increase in irradiation exposure time, the density of defects also increases, resulting in an increase in local stress concentration, which then decreases the sRS and sMS. Radiation induced defects in the
(002) ∗
NiTi2 ∗ NiTi o Ni3Ti
(422) ∗
(120) ∗
210 ∗
Untreated
(110) ∗
1000 Intensity
∗∗
(215) o
(301) o
∗ ∗
500
o
o
o
o
o
o
0 30
40
30 min
∗
∗
20
15 min
50 2 Theta
60
70
80
Fig. 7. XRD spectra of unirradiated and 15–60 min proton beam irradiated nitinol.
45 min
60 min
90
N. Afzal et al. / Physica B 406 (2011) 8–11
material enhance the shear movement of atoms at the beginning of martensitic transformation and cause an increase in anti-phase boundaries [20,22], thus resulting in a decrease in sMS. Once the martensitic phase begins to form, the deformation in the material is carried out by detwinning under the action of internal stresses. As deformation increases beyond point C, the detwinning process continues. The planes that are not favorably oriented with respect to applied stress require higher stresses for their re-orientation. As the stress reaches a critical level, the detwinning process is completed and upon any further increase in the stress levels, plastic deformation takes place through the generation of dislocations, which ultimately leads to the fracture of the specimens. The observed higher values of fracture stress in irradiated specimens as compared to unirradiated ones may be attributed to the interaction of mobile dislocations with radiation induced defects.
4. Conclusions The following conclusions can be drawn from the foregoing analysis: 1) Irradiation of nitinol suppresses the direct austenite to martensite (A–M) phase transformation by introducing an intermediate R-phase between A and M phases. 2) The transformation stresses sRS and sMS decrease with increase in exposure time. 3) The micro-hardness of nitinol increases as the exposure time increases. 4) The generation of R-phase and decrease of sRS and sMS can be attributed to the lattice disorder and stress fields associated with the defects produced during irradiation.
11
Acknowledgement The authors would like to thank the accelerator group of CASP for their help during irradiation of the samples.
References [1] W.J. Buehler, F.E. Wang, Ocean Engineering 1 (1968) 105. [2] J. Perkins, Mater. Sci. Eng. 51 (1981) 181. [3] S. Kujala, A. Pajala, M. Kallioinen, A. Pramila, J. Tuukkanen, J. Ryhanen, Biomaterials 25 (2) (2004) 353. [4] F.E. Wang, W.J. Buehler, S. Pickart, J. Appl. Phys 36 (1965) 3232. [5] Z.G. Wang, X.T. Zu, X.D. Feng, J.Y. Dai, Mater. Lett. 54 (1) (2002) 55. [6] T.W. Duerig, K.N. Melton,D. Stockel, C.M. Wayman(Eds.), Engineering Aspects of Shape Memory Alloys, Butterworth–Heinemann, London, 1990, pp. 181–194. ISBN 0-750-61009-3. [7] M. Nishikawa, T. Narikawa, M. Iwamoto, K. Watanabe, Fusion Technol. 9 (1986) 101. [8] T. Hoshiya, M. Ohmi, Y. Matsui, M. Nishikawa, J. Nucl. Mater. 258–263 (1998) 2036. [9] G. Thomas, H. Mori, H. Fujita, R. Sinclair, Scr. Metall. 16 (1982) 589. [10] H. Mori, H. Fujita, Jpn. J. Appl. Phys. 21 (1982) L494. [11] J.L. Brimhall, H.E. Kissinger, A.R. Pelton, Radiat. Eff. 90 (1985) 241. [12] D.F. Pedraza, J. Less-Common Methods 140 (1988) 219. [13] D.E. Luzzi, H. Mori, H. Fujita, M. Meshii, Acta Metall. 34 (1986) 629. [14] J. Cheng, A.J. Ardell, Nucl. Instrum. Methods B 44 (1990) 336. [15] H. Mori, H. Fujita, M. Fujita, Jpn. J. Appl. Phys. 22 (1983) L94. [16] T. Hoshiya, S. Den, H. Ito, S. Takamura, Y. Ichihashi, J. Jpn. Instrum. Methods 55 (1991) 1054. [17] R.F. Konopleva, I.V. Nazarkin, V.L. Solovei, V.A. Chekanov, S.P. Belyaev, A.E. Volkov, A.I. Razov, Phys. Solid State 40 (1998) 1550. [18] A.A. Al-Aql, Physica B 239 (1997) 345. [19] Z.G. Wang, X.T. Zu, L.J. Liu, S. Zhu, Y. Huo, L.B. Lin b, X.D. Feng, L.M. Wang, Nucl. Instrum. Methods Phys. Res. B 211 (2003) 239. [20] Z.H. Dughaish, Mater. Lett. 32 (1997) 29. [21] L. Bataillard, J.E. Biadux, R. Gotthardt, Philos. Mag. A 78 (2) (1998) 327. [22] F.E. Wang, B.F. Desavage, W.J. Buehler, W.R. Hosler, J. Appl. Phys. 39 (1968) 2166.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Mechanical response of proton beam irradiated nitinol Naveed Afzal, I.M. Ghauri n, F.E. Mubarik, F. Amin Centre for Advanced Studies in Physics, GC University, Lahore, Pakistan
a r t i c l e in f o
a b s t r a c t
Article history: Received 11 February 2010 Received in revised form 30 July 2010 Accepted 28 September 2010
The present investigation deals with the study of mechanical behavior of proton beam irradiated nitinol at room temperature. The specimens in austenitic phase were irradiated over periods of 15, 30, 45 and 60 min at room temperature using 2 MeV proton beam obtained from Pelletron accelerator. The stress–strain curves of both unirradiated and irradiated specimens were obtained using a universal testing machine at room temperature. The results of the experiment show that an intermediate rhombohedral (R) phase has been introduced between austenite and martensite phase, which resulted in the suppression of direct transformation from austenite to martensite (A–M). Stresses required to start R-phase (sRS) and martensitic phase (sMS) were observed to decrease with increase in exposure time. The hardness tests of samples before and after irradiation were also carried out using Vickers hardness tester. The comparison reveals that the hardness is higher in irradiated specimens than that of the unirradiated one. The increase in hardness is quite sharp in specimens irradiated for 15 min, which then increases linearly as the exposure time is increased up to 60 min. The generation of R-phase, variations in the transformation stresses sRS and sMS and increase in hardness of irradiated nitinol may be attributed to lattice disorder and associated changes in crystal structure induced by proton beam irradiation. & 2010 Elsevier B.V. All rights reserved.
Keywords: Shape memory alloy Irradiation Deformation Mechanical properties
1. Introduction Shape memory alloys are intelligent materials that have gained worldwide recognition because of their unique properties, such as superelasticity and shape memory effect [1,2]. These alloys undergo a reversible martensitic phase transformation under the action of either temperature or applied stress, where austenite phase is transformed into a martensite phase. The transformation process is accompanied by shape memory effect and superelasticity [1]. Nitinol is a well known shape memory alloy which is commonly used in biomedical, aerospace and nuclear engineering applications [3–8]. Therefore its thermo-mechanical response under irradiation environment is of principal importance. In this regard various attempts in the past have been made to investigate the irradiation effects of electrons, protons, neutrons and heavy ions on the physical and metallurgical behavior of nitinol [9–20]. Occurrence of transition from crystalline to amorphous (C–A) structure has been observed in nitinol after irradiation [9–15]. Cheng and Ardell [14] reported that C–A transition in nitinol is a function of both irradiation dose and temperature. Mori et al. [15] studied the electron irradiation induced C-A transition in nitinol. It was found that electron irradiation induces a localized amorphization which mainly occurs along dislocation lines and grain boundaries. Effects of irradiation on transformation characteristics of nitinol were studied by Hoshiya et al. [16] and later on by Konopleva
n
Corresponding author. E-mail address: [email protected] (I.M. Ghauri).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.09.040
et al. [17]. They observed that neutron irradiation of nitinol decreased its transformation. Similar studies were also carried out by Al-Aql et al. [18] using proton beam irradiation. Wang et al. [19] studied the existence of R-phase in proton beam irradiated TiNi alloy. The alloy specimens were exposed to 18 MeV proton beam and the irradiation resulted in a decrease in R-phase transformation start temperature and the reverse martensitic transformation finish temperature with increase in irradiation dose. Investigating the effects of proton irradiation energy on the martensitic transformation temperature of nitinol, Dughaish [20] reported a decrease in transformation temperature with increase in irradiation energy. The foregoing analysis of previous work reveals that irradiation induced C–A transitions and irradiation effects on the transformation temperatures in nitinol has been the main focus of research. The aim of the present study is to investigate transformation characteristics of nitinol (NiTi) alloy through its mechanical response after irradiation for different exposure times with 2 MeV proton beam.
2. Experimental work NiTi alloy in the form of wire with 50.5 at% Ni and 49.5 at% Ti was obtained from Alfa Aeisar (USA). The specimens, each of length 60 mm and diameter 1 mm, were cut from the as-received wire and then mechanically polished using diamond paste of grades 6, 3 and 0.1 mm to remove any distortion from the surface.
N. Afzal et al. / Physica B 406 (2011) 8–11
1400 1. Unirradiated Nitinol 2. 15 min Irradiated Nitinol 3. 30 min Irradiated Nitinol 4. 45 min Irradiated Nitinol 5. 60 min Irradiated Nitinol
1200 1000 Stress (MPa)
The polished specimens were annealed at 703 K for 1 h in a high temperature vacuum furnace. The annealed samples were then subjected to 2 MeV proton beam irradiation for 15, 30, 45 and 60 min under vacuum at 298 K to a fluence of 1015/cm2. The irradiations were carried out using Pelletron accelerator installed at accelerator lab of Government College University, Lahore. The temperature of samples during irradiation was kept constant by continuous circulation of liquid nitrogen in the irradiation chamber. A digital temperature controller attached to the chamber indicated variations of a maximum of 71 K during the entire duration of irradiation. The XRD analysis of unirradiated and irradiated specimens was carried out using Phillips Panalytical XRD system. Both unirradiated and irradiated specimens were deformed at room temperature using Universal Testing Machine with a cross-head speed of 0.5 mm/min. The deformation of specimens was recorded in the form of stress–strain curves, which were used for further analysis. Micro-hardness of specimens was also measured using Vickers hardness tester at a maximum load of 200 g. The data thus obtained, associated with micrographs, was utilized to compare the mechanical response of both unirradiated and proton beam irradiated specimens.
9
D
E
E
800
D
600 / MS B
400 B
RS
C
2 3
4
5 1
200
C
B
0A
0
2
4
6
8
10
12
14
16
18
20
22
Strain (%) Fig. 2. Comparison between stress–strain curves of unirradiated and 15–60 min proton beam irradiated nitinol.
3. Results and discussion
650
The stress–strain curve of unirradiated nitinol deformed at room temperature is shown in Fig. 1. Nitinol is in austenitic phase before deformation (point A). The austenite phase continues to deform with elastic modulus EA, as the elastic deformation proceeds. At point B, a transformation of phase from austenitic to martensitic begins to take place and the corresponding stress is called martensite transformation start stress (sMS). As deformation continues, a stress plateau BC appears in the stress–strain curve indicating that a large strain in the material has been produced on the application of small stress beyond sMS. The deformation in region BC is mainly carried out by transformation from austenitic to martensitic crystal structure through detwinning. As stress is increased further beyond point C, the curve again shows a linear stress–strain relationship indicating that the martensite is deformed elastically with elastic modulus EM. Beyond point D, the material deforms plastically, leading to its failure. Fig. 2 shows the comparison of stress–strain curves of unirradiated and 15–60 min proton beam irradiated nitinol. It can be seen that as a result of irradiation, an additional intermediate region BB/ of rhombohedral (R) phase has appeared between austenitic (A) and martensitic (M) phase. The comparison
600
Hardness (HV)
550 500 450 400 350 300 250 0
10
20
30
40
50
60
Time (minutes) Fig. 3. Variations of micro-hardness of nitinol with irradiation exposure time.
1000 E
Stress (MPa)
800
D
600 EM
400
200
MS
C
B
EA
0
A
0
2
4
6
8
10
12
14
16
18
20
22
Strain (%) Fig. 1. Stress–strain behavior of unirradiated nitinol.
Fig. 4. Microstructure of unirradiated nitinol (magnification 1000 ).
10
N. Afzal et al. / Physica B 406 (2011) 8–11
Fig. 5. Microstructure of 30 min proton beam irradiated nitinol (magnification 1000 ).
Fig. 6. Microstructure of 60 min proton beam irradiated nitinol (magnification 1000 ).
between unirradiated and 15 min proton beam irradiated nitinol clearly shows that irradiation has resulted in an increase in the transformation stress sMS. However the R-phase start stress sRS and M-phase start stress sMS decrease with increase in exposure time. The fracture stress for irradiated specimens is much higher than that of the unirradiated one. The results of micro-hardness tests of samples reveal that hardness is higher in irradiated specimens than that of unirradiated one. The increase in hardness is quite sharp after 15 min of irradiation, which then increases linearly with further increase in exposure time up to 60 min (Fig. 3). The comparison between the microstructures of unirradiated specimen (Fig. 4) with that of 30 min proton beam irradiated specimen (Fig. 5) shows that irradiation produces wear and tear on the surface and it also causes the formation of precipitates, which are non-uniformly distributed over the surface. The defects/precipitates seem to agglomerate with increase in irradiation time up to 60 min (Fig. 6). XRD spectra of unirradiated and irradiated specimens are shown in Fig. 7. The spectrum of unirradiated nitinol shows a prominent peak of cubic NiTi2 at 2y value of 381 corresponding to (4 2 2) plane. The spectrum also indicate a NiTi peak at 431 corresponding to (0 0 2) plane and two monoclinic peaks at 551, and 691 corresponding to (1 2 0) and (2 1 0) planes respectively. The monoclinic peaks however disappear and a new peak at 421 corresponding to (1 1 0) plane of cubic NiTi appears when the specimens are irradiated with 2 MeV proton beam for 15–60 min. Similarly two more peaks of Ni3Ti also appear at 2y value of 641 and 821 corresponding to planes (3 0 1) and (2 1 5), respectively. The generation of Ni3Ti phases and the re-crystallization of monoclinic NiTi into cubic NiTi are clear indications of structural evolution induced by irradiation. The presence of Ni3Ti peaks shows the formation of precipitates responsible for the internal stresses within the material. It has been established in the literature that proton irradiation of nitinol produces small displacement cascades and thus generates vacancies, interstitials and/or precipitates [19]. The lattice disorder and stress fields associated with the defects/ precipitates produced during irradiation are responsible for the generation of R-phase that nucleates around the local stress fields and grows through the matrix [19,21]. With increase in irradiation exposure time, the density of defects also increases, resulting in an increase in local stress concentration, which then decreases the sRS and sMS. Radiation induced defects in the
(002) ∗
NiTi2 ∗ NiTi o Ni3Ti
(422) ∗
(120) ∗
210 ∗
Untreated
(110) ∗
1000 Intensity
∗∗
(215) o
(301) o
∗ ∗
500
o
o
o
o
o
o
0 30
40
30 min
∗
∗
20
15 min
50 2 Theta
60
70
80
Fig. 7. XRD spectra of unirradiated and 15–60 min proton beam irradiated nitinol.
45 min
60 min
90
N. Afzal et al. / Physica B 406 (2011) 8–11
material enhance the shear movement of atoms at the beginning of martensitic transformation and cause an increase in anti-phase boundaries [20,22], thus resulting in a decrease in sMS. Once the martensitic phase begins to form, the deformation in the material is carried out by detwinning under the action of internal stresses. As deformation increases beyond point C, the detwinning process continues. The planes that are not favorably oriented with respect to applied stress require higher stresses for their re-orientation. As the stress reaches a critical level, the detwinning process is completed and upon any further increase in the stress levels, plastic deformation takes place through the generation of dislocations, which ultimately leads to the fracture of the specimens. The observed higher values of fracture stress in irradiated specimens as compared to unirradiated ones may be attributed to the interaction of mobile dislocations with radiation induced defects.
4. Conclusions The following conclusions can be drawn from the foregoing analysis: 1) Irradiation of nitinol suppresses the direct austenite to martensite (A–M) phase transformation by introducing an intermediate R-phase between A and M phases. 2) The transformation stresses sRS and sMS decrease with increase in exposure time. 3) The micro-hardness of nitinol increases as the exposure time increases. 4) The generation of R-phase and decrease of sRS and sMS can be attributed to the lattice disorder and stress fields associated with the defects produced during irradiation.
11
Acknowledgement The authors would like to thank the accelerator group of CASP for their help during irradiation of the samples.
References [1] W.J. Buehler, F.E. Wang, Ocean Engineering 1 (1968) 105. [2] J. Perkins, Mater. Sci. Eng. 51 (1981) 181. [3] S. Kujala, A. Pajala, M. Kallioinen, A. Pramila, J. Tuukkanen, J. Ryhanen, Biomaterials 25 (2) (2004) 353. [4] F.E. Wang, W.J. Buehler, S. Pickart, J. Appl. Phys 36 (1965) 3232. [5] Z.G. Wang, X.T. Zu, X.D. Feng, J.Y. Dai, Mater. Lett. 54 (1) (2002) 55. [6] T.W. Duerig, K.N. Melton,D. Stockel, C.M. Wayman(Eds.), Engineering Aspects of Shape Memory Alloys, Butterworth–Heinemann, London, 1990, pp. 181–194. ISBN 0-750-61009-3. [7] M. Nishikawa, T. Narikawa, M. Iwamoto, K. Watanabe, Fusion Technol. 9 (1986) 101. [8] T. Hoshiya, M. Ohmi, Y. Matsui, M. Nishikawa, J. Nucl. Mater. 258–263 (1998) 2036. [9] G. Thomas, H. Mori, H. Fujita, R. Sinclair, Scr. Metall. 16 (1982) 589. [10] H. Mori, H. Fujita, Jpn. J. Appl. Phys. 21 (1982) L494. [11] J.L. Brimhall, H.E. Kissinger, A.R. Pelton, Radiat. Eff. 90 (1985) 241. [12] D.F. Pedraza, J. Less-Common Methods 140 (1988) 219. [13] D.E. Luzzi, H. Mori, H. Fujita, M. Meshii, Acta Metall. 34 (1986) 629. [14] J. Cheng, A.J. Ardell, Nucl. Instrum. Methods B 44 (1990) 336. [15] H. Mori, H. Fujita, M. Fujita, Jpn. J. Appl. Phys. 22 (1983) L94. [16] T. Hoshiya, S. Den, H. Ito, S. Takamura, Y. Ichihashi, J. Jpn. Instrum. Methods 55 (1991) 1054. [17] R.F. Konopleva, I.V. Nazarkin, V.L. Solovei, V.A. Chekanov, S.P. Belyaev, A.E. Volkov, A.I. Razov, Phys. Solid State 40 (1998) 1550. [18] A.A. Al-Aql, Physica B 239 (1997) 345. [19] Z.G. Wang, X.T. Zu, L.J. Liu, S. Zhu, Y. Huo, L.B. Lin b, X.D. Feng, L.M. Wang, Nucl. Instrum. Methods Phys. Res. B 211 (2003) 239. [20] Z.H. Dughaish, Mater. Lett. 32 (1997) 29. [21] L. Bataillard, J.E. Biadux, R. Gotthardt, Philos. Mag. A 78 (2) (1998) 327. [22] F.E. Wang, B.F. Desavage, W.J. Buehler, W.R. Hosler, J. Appl. Phys. 39 (1968) 2166.