- Email: [email protected]
Analysis of the mechanical and shape memory behaviour of nitrogen ion-implanted NiTi alloy
ARTICLE IN PRESS
Vacuum 81 (2007) 1283–1287 www.elsevier.com/locate/vacuum
Analysis of the mechanical and shape memory behaviour of nitrogen ion-implanted NiTi alloy N. Levintant Institute of Fundamental Technological Research PASc., 21 Swietokrzyska Street, 00-049 Warsaw, Poland
Abstract Experimental results of an accumulation and return strain behaviour of the modified surface of NiTi alloy, as well as mechanical and shape memory behaviour, are shown in this paper. Surface of equiatomic NiTi shape memory alloy (in martensitic form) has been modified by high-dose ion-implantation technique using nitrogen ion beam. The low-energy (65 keV) and following high doses have been used: 1 1017, 5 1017 and 1 1018 J/cm2. Correlation between subsurface layers elemental composition of NiTi alloy, microstructure and shape memory properties is shown. r 2007 Elsevier Ltd. All rights reserved. Keywords: Ion implantation; Shape memory effect; Differential scanning calorimetry (DSC); NiTi alloy; Indentation test
1. Introduction An increasing interest in the properties of a nearequiatomic nickel–titanium (NiTi) alloy is recently observed. Shape memory alloys (SMAs) are characterised by such interesting properties like pseudoelasticity and shape memory effect (SME), which increase their possibilities of industrial applications [1]. Equiatomic superelastic NiTi alloys are a unique class of materials having properties that are very difficult to describe using an engineering approach typical for traditional materials. Different approach has to be specified in order to describe adequately the NiTi materials, for example, such features like transformation temperatures, surface properties, mechanical and thermomechanical conditions. Indeed, to understand the behaviour of shape memory alloys from scientific as well as application points of view, one needs to have a very good knowledge of basic physical, mechanical and chemical properties [2,3]. Nickel–titanium alloy having near equiatomic concentration is practically the most important shape memory alloy, mainly due to the unique functional properties such as shape memory effect Tel.: +48 22 826 98 06; fax: +48 22 826 98 15.
E-mail address: [email protected]. 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.01.058
and superelasticity. The reasons for that phenomeon are the thermoelastic martensite phase transformations of the high-temperature B2 phase into the B190 martensite phase [4]. It is known that the problem of creating a protective surface coating for the shape memory alloy is the most acute for potential applications of this material: such as orthopaedic devices, surgical tools, medical implants, endodontic instruments and orthodontic wires in dentistry [5,6]. Thus, the problem of increasing of surface protective properties and, at the same time, simultaneous preservation of functional properties of shape memory materials is a subject of research and development. To solve that problem, plasma and laser treatment, ion implantation, CVD of hydroxylapatite, TiN, and Ti (C, N) layers have been used for surface modification of NiTi-based alloys. Apparently, all these protective coatings can influence functional properties of shape memory or superelasticity. In general, implantation process may improve the mechanical properties (wear and fatigue resistance of surface) [7]. The ion implantation method possesses precision capabilities for modification of thin surface layers of metals and alloys including their chemical composition, crystal structure, physical and mechanical properties. However, a successful solution of the problem requires a more comprehensive approach, including the study of the
ARTICLE IN PRESS 1284
N. Levintant / Vacuum 81 (2007) 1283–1287
Fig. 1. Results of DSC (Pyris-1). Transformation temperatures NiTi alloy before and after nitrogen ion implantation (65 keV, 1 1017 J/cm2): (1) before implantation and (2) after ion implantation.
physical properties of surface layer interfaces, as well as the plastic behaviour and functional mechanical properties (shape memory effect, superelasticity) of NiTi alloys after surface treatment. A previous study [6] on B+, Mo+ and Ar+-implanted NiTi alloys has shown that argon implantation may be beneficial for wear, corrosion and fatigue resistance of NiTi. Gas nitriding, ion nitriding, B+, Ar+ implantation [6,7] have been already used to improve the fatigue, wear and corrosion resistance of NiTi alloys, too. Moreover, the mechanical properties and shape memory behaviour of these alloys are dependent upon element composition and surface properties, as has been discussed by numerous authors in the literature [6–9]. The objective of the present work is to study, analyse and interpret of mechanical and shape memory behaviour of nitrogen ion-implanted equiatomic NiTi alloy. This paper contains discussion of experimental results of an accumulation and return strain behaviour of the NiTi alloy with modified surface by high-dose ion-implantation
technique as well as mechanical and shape memory behaviour of this alloy. Correlation between subsurface layers elemental composition of NiTi alloy, microstructure and shape memory properties are shown as well.
2. Experimental Martensitic form of equiatomic NiTi (51.5 at% Ni) alloy was used in this study. Specimens were cut out from commercial NiTi type. The samples of dimensions 22 5.4 0.25 mm were prepared. Before the measurements, the samples were annealed at 803 K for 30 min in vacuum and then cooled in the furnace. The surface of NiTi was cleaned using a solution containing 10% HClO4 and 90% acetic acid. Ion implantation was carried out using typical semiindustrial implanter IMJON (Surface Layer Division, PASc., Warsaw). Samples were implanted at low energy with 65 keV N+ and at several doses: 1 1017, 5 1017 and
ARTICLE IN PRESS N. Levintant / Vacuum 81 (2007) 1283–1287
1285
a C
RAW INTENSITY
2000 Ni
1000
Ti
O 0
0
20
40 50 SPUTTERING TIME (min)
60
b
RAW INTENSITY
2000
Ti+N
Ti 1000
C
Fig. 3. Optical photomicrographs of cross-sectional showing martensitic microstructure of NiTi shape memory alloy: (a) before ion implantation and (b) microstructure induced by ion implantation.
Ni O
0
0
5 10 SPUTTERING TIME (min)
15
Fig. 2. Auger electron spectroscopy (AES) analysis: (a) non-implanted samples NiTi and (b) ion-implanted samples NiTi with 65 keV energy and dose 5 1017 J/cm2.
1 1018 J/cm2. The beam current density varies between 0.8 and 1 mA. Temperatures of phase transformation were determined using differential scanning calorimetry (DSC; Pyris-1). In this presentation, the elemental depth profiles of the near-surface region of NiTi before and after nitrogen ion implantation were examined by Auger electron spectroscopy (AES; PHI-600 Perkin, Elmer). The changes of NiTi microstructure induced by ion implantation were investigated using optical (Neophot-2) and scanning electron microscopy (SEM; Perkin, Elmer). Mechanical properties and shape memory behaviour were investigated by the depth sensing micro-indentation test conducted using device with diamond (type Brinell) indentor (diameter: 400 mm; range of force: 4, 7, 10, 13, 16, 20 N) and scanning profilometry measurements of durable imprints induced by indentation procedure (Talysurf-5120). Indentation and profilometry experiments were
carried out on the unimplanted and ion-implanted areas of NiTi samples. 2.1. Experimental procedure The procedure of experiment is given below showing gradual step-by-step approach: 1. Indentation test. NiTi samples were loaded by force (from 4 to 20 N)—measurement of accumulation strain. 2. Profilometry test. Measurement of accumulation strain and shape of durable imprints. 3. Carry out of gradually shape memory effect. NiTi samples were heated up to the temperature Tj (AspTjpAf). 4. Profilometry test. Measure of return strain—durable imprint evolution.
3. Results and discussion DSC using Pyris-1 was used to monitor the transformation characteristics of the samples. Fig. 1 shows the change
ARTICLE IN PRESS N. Levintant / Vacuum 81 (2007) 1283–1287
1286
depth, µm
of transformation temperatures induced by ion implantation (1, curve before implantation; 2, curve after implantation). It is known that the parent austenite B2-phase is found to undergo the B22B190 martensitic transformation (B2-phase has a CsCl-type ordered structure, B190 phase has a rhombohedral martensite structure with monoclinic distortion). It can be seen that the temperature ranges of B22B190 martensitic transformation for nonimplanted and ion-implanted NiTi samples, respectively, are as follows: Ms ¼ 301, 285 K; Mf ¼ 275, 270 K; As ¼ 318, 328 K; Af ¼ 338, 353 K (Ms, Mf: forward start and finish temperatures of transformation; As, Af: the B190 -B2 backward start and finish temperatures of transformation). We found that the start temperatures of B22B190 are lower for implanted alloy than those for non-implanted one. However, in this study, NiTi alloy containing R-phase was investigated as well. It means that R-phase at temperatures above Ms is forming easier in the ion-implanted alloy than in the non-implanted one. AES analysis of NiTi samples before and after ion implantation shows that the coincidence of their elemental compositions is achieved at different depth from the surface. Fig. 2(a, b) shows distribution of the chemical elements composition depending on sputtering time of the
11 10 9 8 7 6 5 4 3 2 1
implanted non-implanted
0
1
2
3
4
5
6
7
load, N
Fig. 4. Indentation curves for several forces of non-implanted and ionimplanted NiTi shape memory alloy surface.
NiTi shape memory alloy with (a) non-implanted and (b) ion-implanted surface. It is seen that the thickness of modified layer is around 280–300 nm. Moreover, the absence of Ni-content in the 30–40 nm thickness and a noticeable depletion of this element in the layer down to the depth of 100 nm is found. Fig. 3(a, b) is an optical photomicrograph showing the typical (a) and changed martensite microstructure (b) induced by ion implantation. Figs. 4 and 5 show experimental results of an accumulation and return strain behaviour of the NiTi alloy with modified surface by high-dose ion implantation. NiTi samples with modified surface have more stiff mechanical properties at 4 and 7 N forces. The changes of shape memory properties (Fig. 5(a, b)) are in correlation with subsurface layers elemental composition and with the changes of transformation temperatures.
4. Conclusions On the basis of experimental results obtained in this work, it is shown that the ion-modified layer on the NiTi alloy surface plays a complex multifunctional role and changes of the mechanical behaviour of these materials; the effect of the surface implantation treatment on mechanical and shape memory properties of NiTi are correlated with the evolution of thermomechanical properties. Also the shape memory properties are changed by: (1) the formation of nitrogen and carbon ion layer near the surface and formation residual stress due to ion implantation; (2) the change of the ratio of titanium to nickel atoms due to preferential sputtering of nickel atoms and subsequently the change in transformation temperature. Results presented have also demonstrated that the indentation behaviour of NiTi is sensitive to the martensite transformation temperatures. Further work may prove an application of indentation test as a tool to extract the local transformation temperatures of NiTi.
Fig. 5. Scanning profilometry measure of durable imprint evolution (return strain) after heated up to 75 1C of implanted NiTi alloy.
ARTICLE IN PRESS N. Levintant / Vacuum 81 (2007) 1283–1287
Acknowledgements The author is grateful to Prof. A.D. Pogrebnjak, Director of Institute for Surface Modification, Sumy, Ukraine, and Ph.D.S. Kucharski, Division of Surface Layers, Institute of Fundamental Technological Research PASc., Warsaw, for their support for this work. References [1] Lehnert T, Tixier S, Boni P, Gotthardt R. Mater Sci Eng 1999;A273–275:713–6.
1287
[2] Humbeeck J. Scripta Mater 2004;50:179–80. [3] Raniecki B, Lexcelent C. Eur J Mech A. Solids 1998;17–20: 185–205. [4] Madangopal K. Scripta Mater 2005;53:875–9. [5] Chrobak D, Morawiec H. Scripta Mater 2001;44:725–30. [6] Meisner LL, Sivokha VP, Lotkov AI, Derevyagina LA. Physica B 2001;307:251–7. [7] Pelletier H, Muller D, Mille P, Grob JJ. Surf Coat Technol 2002;158–159:309–17. [8] Carroll MC, Somsen Ch, Eggler G. Scripta Mater 2004;50:187–92. [9] Mandl S, Gerlach JW, Rauschenbach B. Surf Coat Technol 2005;196:293–7.
Vacuum 81 (2007) 1283–1287 www.elsevier.com/locate/vacuum
Analysis of the mechanical and shape memory behaviour of nitrogen ion-implanted NiTi alloy N. Levintant Institute of Fundamental Technological Research PASc., 21 Swietokrzyska Street, 00-049 Warsaw, Poland
Abstract Experimental results of an accumulation and return strain behaviour of the modified surface of NiTi alloy, as well as mechanical and shape memory behaviour, are shown in this paper. Surface of equiatomic NiTi shape memory alloy (in martensitic form) has been modified by high-dose ion-implantation technique using nitrogen ion beam. The low-energy (65 keV) and following high doses have been used: 1 1017, 5 1017 and 1 1018 J/cm2. Correlation between subsurface layers elemental composition of NiTi alloy, microstructure and shape memory properties is shown. r 2007 Elsevier Ltd. All rights reserved. Keywords: Ion implantation; Shape memory effect; Differential scanning calorimetry (DSC); NiTi alloy; Indentation test
1. Introduction An increasing interest in the properties of a nearequiatomic nickel–titanium (NiTi) alloy is recently observed. Shape memory alloys (SMAs) are characterised by such interesting properties like pseudoelasticity and shape memory effect (SME), which increase their possibilities of industrial applications [1]. Equiatomic superelastic NiTi alloys are a unique class of materials having properties that are very difficult to describe using an engineering approach typical for traditional materials. Different approach has to be specified in order to describe adequately the NiTi materials, for example, such features like transformation temperatures, surface properties, mechanical and thermomechanical conditions. Indeed, to understand the behaviour of shape memory alloys from scientific as well as application points of view, one needs to have a very good knowledge of basic physical, mechanical and chemical properties [2,3]. Nickel–titanium alloy having near equiatomic concentration is practically the most important shape memory alloy, mainly due to the unique functional properties such as shape memory effect Tel.: +48 22 826 98 06; fax: +48 22 826 98 15.
E-mail address: [email protected]. 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.01.058
and superelasticity. The reasons for that phenomeon are the thermoelastic martensite phase transformations of the high-temperature B2 phase into the B190 martensite phase [4]. It is known that the problem of creating a protective surface coating for the shape memory alloy is the most acute for potential applications of this material: such as orthopaedic devices, surgical tools, medical implants, endodontic instruments and orthodontic wires in dentistry [5,6]. Thus, the problem of increasing of surface protective properties and, at the same time, simultaneous preservation of functional properties of shape memory materials is a subject of research and development. To solve that problem, plasma and laser treatment, ion implantation, CVD of hydroxylapatite, TiN, and Ti (C, N) layers have been used for surface modification of NiTi-based alloys. Apparently, all these protective coatings can influence functional properties of shape memory or superelasticity. In general, implantation process may improve the mechanical properties (wear and fatigue resistance of surface) [7]. The ion implantation method possesses precision capabilities for modification of thin surface layers of metals and alloys including their chemical composition, crystal structure, physical and mechanical properties. However, a successful solution of the problem requires a more comprehensive approach, including the study of the
ARTICLE IN PRESS 1284
N. Levintant / Vacuum 81 (2007) 1283–1287
Fig. 1. Results of DSC (Pyris-1). Transformation temperatures NiTi alloy before and after nitrogen ion implantation (65 keV, 1 1017 J/cm2): (1) before implantation and (2) after ion implantation.
physical properties of surface layer interfaces, as well as the plastic behaviour and functional mechanical properties (shape memory effect, superelasticity) of NiTi alloys after surface treatment. A previous study [6] on B+, Mo+ and Ar+-implanted NiTi alloys has shown that argon implantation may be beneficial for wear, corrosion and fatigue resistance of NiTi. Gas nitriding, ion nitriding, B+, Ar+ implantation [6,7] have been already used to improve the fatigue, wear and corrosion resistance of NiTi alloys, too. Moreover, the mechanical properties and shape memory behaviour of these alloys are dependent upon element composition and surface properties, as has been discussed by numerous authors in the literature [6–9]. The objective of the present work is to study, analyse and interpret of mechanical and shape memory behaviour of nitrogen ion-implanted equiatomic NiTi alloy. This paper contains discussion of experimental results of an accumulation and return strain behaviour of the NiTi alloy with modified surface by high-dose ion-implantation
technique as well as mechanical and shape memory behaviour of this alloy. Correlation between subsurface layers elemental composition of NiTi alloy, microstructure and shape memory properties are shown as well.
2. Experimental Martensitic form of equiatomic NiTi (51.5 at% Ni) alloy was used in this study. Specimens were cut out from commercial NiTi type. The samples of dimensions 22 5.4 0.25 mm were prepared. Before the measurements, the samples were annealed at 803 K for 30 min in vacuum and then cooled in the furnace. The surface of NiTi was cleaned using a solution containing 10% HClO4 and 90% acetic acid. Ion implantation was carried out using typical semiindustrial implanter IMJON (Surface Layer Division, PASc., Warsaw). Samples were implanted at low energy with 65 keV N+ and at several doses: 1 1017, 5 1017 and
ARTICLE IN PRESS N. Levintant / Vacuum 81 (2007) 1283–1287
1285
a C
RAW INTENSITY
2000 Ni
1000
Ti
O 0
0
20
40 50 SPUTTERING TIME (min)
60
b
RAW INTENSITY
2000
Ti+N
Ti 1000
C
Fig. 3. Optical photomicrographs of cross-sectional showing martensitic microstructure of NiTi shape memory alloy: (a) before ion implantation and (b) microstructure induced by ion implantation.
Ni O
0
0
5 10 SPUTTERING TIME (min)
15
Fig. 2. Auger electron spectroscopy (AES) analysis: (a) non-implanted samples NiTi and (b) ion-implanted samples NiTi with 65 keV energy and dose 5 1017 J/cm2.
1 1018 J/cm2. The beam current density varies between 0.8 and 1 mA. Temperatures of phase transformation were determined using differential scanning calorimetry (DSC; Pyris-1). In this presentation, the elemental depth profiles of the near-surface region of NiTi before and after nitrogen ion implantation were examined by Auger electron spectroscopy (AES; PHI-600 Perkin, Elmer). The changes of NiTi microstructure induced by ion implantation were investigated using optical (Neophot-2) and scanning electron microscopy (SEM; Perkin, Elmer). Mechanical properties and shape memory behaviour were investigated by the depth sensing micro-indentation test conducted using device with diamond (type Brinell) indentor (diameter: 400 mm; range of force: 4, 7, 10, 13, 16, 20 N) and scanning profilometry measurements of durable imprints induced by indentation procedure (Talysurf-5120). Indentation and profilometry experiments were
carried out on the unimplanted and ion-implanted areas of NiTi samples. 2.1. Experimental procedure The procedure of experiment is given below showing gradual step-by-step approach: 1. Indentation test. NiTi samples were loaded by force (from 4 to 20 N)—measurement of accumulation strain. 2. Profilometry test. Measurement of accumulation strain and shape of durable imprints. 3. Carry out of gradually shape memory effect. NiTi samples were heated up to the temperature Tj (AspTjpAf). 4. Profilometry test. Measure of return strain—durable imprint evolution.
3. Results and discussion DSC using Pyris-1 was used to monitor the transformation characteristics of the samples. Fig. 1 shows the change
ARTICLE IN PRESS N. Levintant / Vacuum 81 (2007) 1283–1287
1286
depth, µm
of transformation temperatures induced by ion implantation (1, curve before implantation; 2, curve after implantation). It is known that the parent austenite B2-phase is found to undergo the B22B190 martensitic transformation (B2-phase has a CsCl-type ordered structure, B190 phase has a rhombohedral martensite structure with monoclinic distortion). It can be seen that the temperature ranges of B22B190 martensitic transformation for nonimplanted and ion-implanted NiTi samples, respectively, are as follows: Ms ¼ 301, 285 K; Mf ¼ 275, 270 K; As ¼ 318, 328 K; Af ¼ 338, 353 K (Ms, Mf: forward start and finish temperatures of transformation; As, Af: the B190 -B2 backward start and finish temperatures of transformation). We found that the start temperatures of B22B190 are lower for implanted alloy than those for non-implanted one. However, in this study, NiTi alloy containing R-phase was investigated as well. It means that R-phase at temperatures above Ms is forming easier in the ion-implanted alloy than in the non-implanted one. AES analysis of NiTi samples before and after ion implantation shows that the coincidence of their elemental compositions is achieved at different depth from the surface. Fig. 2(a, b) shows distribution of the chemical elements composition depending on sputtering time of the
11 10 9 8 7 6 5 4 3 2 1
implanted non-implanted
0
1
2
3
4
5
6
7
load, N
Fig. 4. Indentation curves for several forces of non-implanted and ionimplanted NiTi shape memory alloy surface.
NiTi shape memory alloy with (a) non-implanted and (b) ion-implanted surface. It is seen that the thickness of modified layer is around 280–300 nm. Moreover, the absence of Ni-content in the 30–40 nm thickness and a noticeable depletion of this element in the layer down to the depth of 100 nm is found. Fig. 3(a, b) is an optical photomicrograph showing the typical (a) and changed martensite microstructure (b) induced by ion implantation. Figs. 4 and 5 show experimental results of an accumulation and return strain behaviour of the NiTi alloy with modified surface by high-dose ion implantation. NiTi samples with modified surface have more stiff mechanical properties at 4 and 7 N forces. The changes of shape memory properties (Fig. 5(a, b)) are in correlation with subsurface layers elemental composition and with the changes of transformation temperatures.
4. Conclusions On the basis of experimental results obtained in this work, it is shown that the ion-modified layer on the NiTi alloy surface plays a complex multifunctional role and changes of the mechanical behaviour of these materials; the effect of the surface implantation treatment on mechanical and shape memory properties of NiTi are correlated with the evolution of thermomechanical properties. Also the shape memory properties are changed by: (1) the formation of nitrogen and carbon ion layer near the surface and formation residual stress due to ion implantation; (2) the change of the ratio of titanium to nickel atoms due to preferential sputtering of nickel atoms and subsequently the change in transformation temperature. Results presented have also demonstrated that the indentation behaviour of NiTi is sensitive to the martensite transformation temperatures. Further work may prove an application of indentation test as a tool to extract the local transformation temperatures of NiTi.
Fig. 5. Scanning profilometry measure of durable imprint evolution (return strain) after heated up to 75 1C of implanted NiTi alloy.
ARTICLE IN PRESS N. Levintant / Vacuum 81 (2007) 1283–1287
Acknowledgements The author is grateful to Prof. A.D. Pogrebnjak, Director of Institute for Surface Modification, Sumy, Ukraine, and Ph.D.S. Kucharski, Division of Surface Layers, Institute of Fundamental Technological Research PASc., Warsaw, for their support for this work. References [1] Lehnert T, Tixier S, Boni P, Gotthardt R. Mater Sci Eng 1999;A273–275:713–6.
1287
[2] Humbeeck J. Scripta Mater 2004;50:179–80. [3] Raniecki B, Lexcelent C. Eur J Mech A. Solids 1998;17–20: 185–205. [4] Madangopal K. Scripta Mater 2005;53:875–9. [5] Chrobak D, Morawiec H. Scripta Mater 2001;44:725–30. [6] Meisner LL, Sivokha VP, Lotkov AI, Derevyagina LA. Physica B 2001;307:251–7. [7] Pelletier H, Muller D, Mille P, Grob JJ. Surf Coat Technol 2002;158–159:309–17. [8] Carroll MC, Somsen Ch, Eggler G. Scripta Mater 2004;50:187–92. [9] Mandl S, Gerlach JW, Rauschenbach B. Surf Coat Technol 2005;196:293–7.