- Email: [email protected]
Effect of thermal and thermo-mechanical cycling on the microstructure of Ni-rich NiTi shape memory alloys
Materials Letters 99 (2013) 150–153
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of thermal and thermo-mechanical cycling on the microstructure of Ni-rich NiTi shape memory alloys K.S. Suresh a, Subir K. Bhaumik b, Satyam Suwas a,n a b
Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India Materials Science Division, Council of Scientific and Industrial Research (CSIR), National Aerospace Laboratories, Bangalore 560017, India
art ic l e i nf o
a b s t r a c t
Article history: Received 19 November 2012 Accepted 6 March 2013 Available online 15 March 2013
Microstructural changes of Ni-rich NiTi shape memory alloy during thermal and thermo-mechanical cycling have been investigated using Electron Back Scattered Diffraction. A strong dependence of the orientation of the prior austenite grain on the misorientation development has been observed during thermal cycling and thermo-mechanical cycling. This effect is more pronounced at the grain boundaries compared to grain interior. At a larger applied strain, the volume fraction of stabilized martensite phase increases with increase in the number of cycling. Deformation within the martensite leads to stabilization of martensitic phase even at temperatures slightly above the austenite finish temperature. Modulus variation with respect to temperature has been explained on the basis of martensitic transformation. & 2013 Elsevier B.V. All rights reserved.
Keywords: Shape memory alloys Microstructure Electron back scattered diffraction Thermo-mechanical cycling Grain boundaries
1. Introduction Application of NiTi based shape memory alloys (SMA) involves repeated thermal cycling through the transformation range, commonly referred to as thermo-mechanical cycling (TMC) [1]. It is well known [2] that repetitive TMC of SMAs results in irreversible plastic deformation in the austenite phase and accumulation of stabilized martensite phase in the material. The stabilization of martensite phase is because of the generation of defects/dislocations in the microstructure [3,4]. Therefore, the present study was undertaken to analyze the distribution of misorientation in the microstructure of NiTi based SMAs during thermal cycling (TC) and thermo-mechanical cycling (TMC).
frequency of 10 Hz were applied on the sample. The temperature was varied between −75 1C and 100 1C and 10 such cycles were performed. The complex modulus was measured as a function of temperature. Microstructures of the selected samples were examined using a TSL electron back scattered diffraction (EBSD) detector attached to a FEI SIRION field emission gun scanning electron microscope (FEG-SEM). Using TSL-OIM analysis software, inverse pole figure (IPF) map and Kernal average misorientation (KAM) map were plotted. The KAM map describes the sub-structural aspects, for example, strain distribution that can be correlated to the presence of dislocation in the microstructure [5].
3. Results and discussion 2. Materials and methods
3.1. Effect of thermal cycling
A hot rolled alloy with nominal composition Ni50.6Ti49.4 was used as the starting material. The material was hot rolled at 700 1C to a true strain of 2.0 followed by annealing at 600 1C for 30 min. The martensite and austenite finish temperatures (denoted as Mf and Af, respectively) of the samples were determined to be −30 1C and 15 1C, respectively. TC was carried out by heating the sample to 40 1C (4Af) and then quenching it to −40 1C (oMf), and the process was repeated 1000 times. TMC was carried out in a GABO Eplexor Dynamical mechanical analyzer (DMA) using a three point bending set up. A static strain of 0.025 and a dynamic strain of 0.005 at a constant
Fig. 1 shows the EBSD generated microstructures of the as-rolled sample and after subjecting to TC for 1000 cycles, respectively in terms of IPF and KAM maps. The IPF map shows similar orientation for both the samples without any noticeable deviation. Nevertheless, a weak dependence of misorientation development is noticed in terms of higher KAM values at the grain boundaries. The continuous increase in the intra-grain misorientation could be attributed to the coherency strain at the austenite–martensite interface boundary [3,6]. The increase in the KAM value could be attributed to the increase in the geometrically necessary dislocations that give rise to the misorientation [5]. The generation of such dislocations resulting from transformation aspects can be understood by analyzing the transformation matrix. The lattice correspondence between the
n
Corresponding author. Tel.: þ91 80 22933245; fax: þ 91 80 23600472. E-mail address: [email protected] (S. Suwas).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.014
K.S. Suresh et al. / Materials Letters 99 (2013) 150–153
The middle Eigen value of the transformation matrix directly relates to the coherency level at the austenite and martensite interface. The calculated Eigen values λ1, λ2, and λ3 are 0.9456, 1.0253, and 0.9234, respectively. It is evident from these values that the interface is not completely coherent. Therefore, every thermal cycle involving martensitic transformation has to be necessarily accompanied by the misfit dislocations at the interface. With increase in the number of thermal cycles involving forward and reverse phase transformation, the number of generated dislocations increases leading to the development of misorientation in the microstructure. 3.2. Effect of thermo-mechanical cycling The variation of modulus with temperature for thermo-mechanically cycled samples is shown in Fig. 2. During the martensitic transformation, the decrease in modulus is very sharp reaching the minimum value upon completion of the transformation. A ‘V’ shape dependence of modulus on temperature was observed, the minimum being at the martensite finish (Mf) temperature. The changes in complex modulus values with temperature can be explained based on the elastic constants of the two phases and the fraction of phases at a given temperature. During the martensitic transformation, both austenite and martensite phases co-exist. As a result, macro modulus in this temperature range is the average modulus of these two phases based on their volume fraction. Previous studies [7–9] have shown that the elastic constants of martensite are higher than that of the austenite phase. Therefore, the modulus of martensite phase should be higher than that of the austenite phase. The results of the present investigation on modulus variation, however, shows a reverse trend which can be explained based on the stress induced martensitic
transformation (SIM). The additional displacement arising from the movement of austenite–martensite phase boundary and from detwinning leads to a lower modulus during transformation. With further decrease in temperature, the stress required for detwinning and martensite re-orientation increases. As a result, the contribution from detwinning and variant re-orientation decreases with decrease in temperature. This leads to an increase in the modulus of the martensite phase at T4Mf. Because of the large imposed strain in the present set of experiments, certain volume fraction of martensite phase gets stabilized in the material during each cycle. This martensite does not undergo reverse transformation upon heating. The volume fraction of stabilized martensite increases with the number of TMC (XRD patterns of the tested sample given in Supplementary figure confirms of presence of residual martensite). Fig. 3 shows the IPF and KAM maps for the starting material as well as after 5 and 10 TMC. The microstructure dependent strain is evident from the KAM maps. After 5 cycles, large misorientation is mostly visible at the grain boundaries. After 10 cycles, even the grain interior starts developing higher misorientation. The rate of generation of misorientation gradient differs based on the grain orientation. The IPF plotted in Fig. 3 for selected grains displays the differences in terms of the development of orientation gradient. Grain 1 develops higher misorientation after 10 cycles, whereas the orientation deviation in
Complex Modulus (GPa)
austenite phase B2 (space group: Pm3m) and martensite phase B19′ (space group: P21 =m) is as follows: [100]B2||[100]B19′, [0 1 1]B2|| [0 1 0]B19′ and [0 1 1]B2||[0 0 1]B19′. The calculated transformation matrix that relates the lattices of austenite and martensite phase crystal structures is given by: 0 1 0:9563 −0:0427 −0:0427 B −0:0427 1:0243 0:058 C @ A −0:0427 0:058 1:0243
151
50
ε =3*10-2 f= 10Hz
40
1st Cycle 5th Cycle
30
10th Cycle
20
10 -80
-40
0
40
80
120
T (°C) Fig. 2. Effect of thermo-mechanical cycling on complex modulus.
Fig. 1. Inverse pole figure (IPF) map and Kernel average misorientation (KAM) map for the (a) initial sample and (b) sample subjected to 1000 number of thermal cycling.
152
K.S. Suresh et al. / Materials Letters 99 (2013) 150–153
Fig. 3. (a) Inverse pole figure of the initial sample, (b) KAM map after 5th cycle and (c) KAM map after 10th cycle. The inverse pole figure shown in the bottom display the development of orientation gradient for selected grains (marked as 1 and 2).
grain 2 is not significant. Increase in intra-grain misorientation during TMC is high compared to TC. The higher misorientation value could be attributed to the additional constraints imposed by the stresses on the selection of certain variants, which are likely to generate dislocations at grain boundaries so as to maintain the compatibility.
Acknowledgement Authors acknowledge Mr. Nikhil Chawan-Dafle for his help during Dynamical Mechanical Analysis. The use of microscopes at the Advanced Facility for Microscopy and Microanalysis (AFMM) is also acknowledged.
4. Conclusion The study on the effect of thermal and thermo-mechanical cycling on the microstructure of a Ni50.6Ti49.4 has led to the following conclusions: (i) Misorientation development in NiTi SMA subjected to pure TC is purely attributed to the coherency level of the phase boundary. (ii) Misorientation development in NiTi SMA subjected to TMC is attributed to the constraints produced by the applied stress in addition to the coherency level of phase boundary. (iii) In both the TC and TMS, misorientation development is strongly influenced by the orientation of the parent austenite grains. (iv) Modulus variation with respect to temperature is basically related to the stress induced martensitic transformation and twin re-orientation.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.03.014.
References [1] Van Humbeeck J. Non-medical applications of shape memory alloys. Mater Sci Eng A 1999;273-275:134–48. [2] Jones NG, Dye D. Martensite evolution in a NiTi shape memory alloy when thermal cycling under an applied load. Intermetallics 2011;19:1348–58. [3] Simon T, Kroger A, Somsen C, Dlouhy A, Eggeler G. On the multiplication of dislocations during martensitic transformations in NiTi Shape memory alloys. Acta Mater 2010;58:1850–60.
K.S. Suresh et al. / Materials Letters 99 (2013) 150–153
[4] Basu R, Jain L, Maji BC, Krishnan M, Mani Krishna KV, Samajdar I, et al. Origin of Microstructural Irreversibility in Ni–Ti based shape memory alloys during thermal cycling. Metall Mater Trans A 2012;43:1277–87. [5] Zhong Y, Yin F, Sakaguchi T, Nagai K, Yang K. Dislocation structure evolution and characterization in the compression deformed Mn–Cu alloy. Acta Mater 2007;55:2747–56. [6] Miyazaki S, Igo Y, Otsuka K. Effect of thermal cycling on the transformation temperatures of TiNi alloys. Acta Metall 1986;34:2045–51.
153
[7] Hatcher N, Kontsevoi YO, Freeman AJ. Martensitic transformation path of NiTi. Phys Rev B Condens Matter 2009;79:020202 R. [8] Wagner MF-X Windl W. Lattice stability, elastic constants and macroscopic moduli of NiTi martensites form first principles. Acta Mater 2008;56:6232–45. [9] Rajagopalan S, Little AL, Bourke MAM, Vaidyanathan R. Elastic modulus of shape memory NiTi from in situ neutron diffraction during macroscopic loading instrumented indentation and extensometry. App Phys Lett 2005;86:081901.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of thermal and thermo-mechanical cycling on the microstructure of Ni-rich NiTi shape memory alloys K.S. Suresh a, Subir K. Bhaumik b, Satyam Suwas a,n a b
Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India Materials Science Division, Council of Scientific and Industrial Research (CSIR), National Aerospace Laboratories, Bangalore 560017, India
art ic l e i nf o
a b s t r a c t
Article history: Received 19 November 2012 Accepted 6 March 2013 Available online 15 March 2013
Microstructural changes of Ni-rich NiTi shape memory alloy during thermal and thermo-mechanical cycling have been investigated using Electron Back Scattered Diffraction. A strong dependence of the orientation of the prior austenite grain on the misorientation development has been observed during thermal cycling and thermo-mechanical cycling. This effect is more pronounced at the grain boundaries compared to grain interior. At a larger applied strain, the volume fraction of stabilized martensite phase increases with increase in the number of cycling. Deformation within the martensite leads to stabilization of martensitic phase even at temperatures slightly above the austenite finish temperature. Modulus variation with respect to temperature has been explained on the basis of martensitic transformation. & 2013 Elsevier B.V. All rights reserved.
Keywords: Shape memory alloys Microstructure Electron back scattered diffraction Thermo-mechanical cycling Grain boundaries
1. Introduction Application of NiTi based shape memory alloys (SMA) involves repeated thermal cycling through the transformation range, commonly referred to as thermo-mechanical cycling (TMC) [1]. It is well known [2] that repetitive TMC of SMAs results in irreversible plastic deformation in the austenite phase and accumulation of stabilized martensite phase in the material. The stabilization of martensite phase is because of the generation of defects/dislocations in the microstructure [3,4]. Therefore, the present study was undertaken to analyze the distribution of misorientation in the microstructure of NiTi based SMAs during thermal cycling (TC) and thermo-mechanical cycling (TMC).
frequency of 10 Hz were applied on the sample. The temperature was varied between −75 1C and 100 1C and 10 such cycles were performed. The complex modulus was measured as a function of temperature. Microstructures of the selected samples were examined using a TSL electron back scattered diffraction (EBSD) detector attached to a FEI SIRION field emission gun scanning electron microscope (FEG-SEM). Using TSL-OIM analysis software, inverse pole figure (IPF) map and Kernal average misorientation (KAM) map were plotted. The KAM map describes the sub-structural aspects, for example, strain distribution that can be correlated to the presence of dislocation in the microstructure [5].
3. Results and discussion 2. Materials and methods
3.1. Effect of thermal cycling
A hot rolled alloy with nominal composition Ni50.6Ti49.4 was used as the starting material. The material was hot rolled at 700 1C to a true strain of 2.0 followed by annealing at 600 1C for 30 min. The martensite and austenite finish temperatures (denoted as Mf and Af, respectively) of the samples were determined to be −30 1C and 15 1C, respectively. TC was carried out by heating the sample to 40 1C (4Af) and then quenching it to −40 1C (oMf), and the process was repeated 1000 times. TMC was carried out in a GABO Eplexor Dynamical mechanical analyzer (DMA) using a three point bending set up. A static strain of 0.025 and a dynamic strain of 0.005 at a constant
Fig. 1 shows the EBSD generated microstructures of the as-rolled sample and after subjecting to TC for 1000 cycles, respectively in terms of IPF and KAM maps. The IPF map shows similar orientation for both the samples without any noticeable deviation. Nevertheless, a weak dependence of misorientation development is noticed in terms of higher KAM values at the grain boundaries. The continuous increase in the intra-grain misorientation could be attributed to the coherency strain at the austenite–martensite interface boundary [3,6]. The increase in the KAM value could be attributed to the increase in the geometrically necessary dislocations that give rise to the misorientation [5]. The generation of such dislocations resulting from transformation aspects can be understood by analyzing the transformation matrix. The lattice correspondence between the
n
Corresponding author. Tel.: þ91 80 22933245; fax: þ 91 80 23600472. E-mail address: [email protected] (S. Suwas).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.014
K.S. Suresh et al. / Materials Letters 99 (2013) 150–153
The middle Eigen value of the transformation matrix directly relates to the coherency level at the austenite and martensite interface. The calculated Eigen values λ1, λ2, and λ3 are 0.9456, 1.0253, and 0.9234, respectively. It is evident from these values that the interface is not completely coherent. Therefore, every thermal cycle involving martensitic transformation has to be necessarily accompanied by the misfit dislocations at the interface. With increase in the number of thermal cycles involving forward and reverse phase transformation, the number of generated dislocations increases leading to the development of misorientation in the microstructure. 3.2. Effect of thermo-mechanical cycling The variation of modulus with temperature for thermo-mechanically cycled samples is shown in Fig. 2. During the martensitic transformation, the decrease in modulus is very sharp reaching the minimum value upon completion of the transformation. A ‘V’ shape dependence of modulus on temperature was observed, the minimum being at the martensite finish (Mf) temperature. The changes in complex modulus values with temperature can be explained based on the elastic constants of the two phases and the fraction of phases at a given temperature. During the martensitic transformation, both austenite and martensite phases co-exist. As a result, macro modulus in this temperature range is the average modulus of these two phases based on their volume fraction. Previous studies [7–9] have shown that the elastic constants of martensite are higher than that of the austenite phase. Therefore, the modulus of martensite phase should be higher than that of the austenite phase. The results of the present investigation on modulus variation, however, shows a reverse trend which can be explained based on the stress induced martensitic
transformation (SIM). The additional displacement arising from the movement of austenite–martensite phase boundary and from detwinning leads to a lower modulus during transformation. With further decrease in temperature, the stress required for detwinning and martensite re-orientation increases. As a result, the contribution from detwinning and variant re-orientation decreases with decrease in temperature. This leads to an increase in the modulus of the martensite phase at T4Mf. Because of the large imposed strain in the present set of experiments, certain volume fraction of martensite phase gets stabilized in the material during each cycle. This martensite does not undergo reverse transformation upon heating. The volume fraction of stabilized martensite increases with the number of TMC (XRD patterns of the tested sample given in Supplementary figure confirms of presence of residual martensite). Fig. 3 shows the IPF and KAM maps for the starting material as well as after 5 and 10 TMC. The microstructure dependent strain is evident from the KAM maps. After 5 cycles, large misorientation is mostly visible at the grain boundaries. After 10 cycles, even the grain interior starts developing higher misorientation. The rate of generation of misorientation gradient differs based on the grain orientation. The IPF plotted in Fig. 3 for selected grains displays the differences in terms of the development of orientation gradient. Grain 1 develops higher misorientation after 10 cycles, whereas the orientation deviation in
Complex Modulus (GPa)
austenite phase B2 (space group: Pm3m) and martensite phase B19′ (space group: P21 =m) is as follows: [100]B2||[100]B19′, [0 1 1]B2|| [0 1 0]B19′ and [0 1 1]B2||[0 0 1]B19′. The calculated transformation matrix that relates the lattices of austenite and martensite phase crystal structures is given by: 0 1 0:9563 −0:0427 −0:0427 B −0:0427 1:0243 0:058 C @ A −0:0427 0:058 1:0243
151
50
ε =3*10-2 f= 10Hz
40
1st Cycle 5th Cycle
30
10th Cycle
20
10 -80
-40
0
40
80
120
T (°C) Fig. 2. Effect of thermo-mechanical cycling on complex modulus.
Fig. 1. Inverse pole figure (IPF) map and Kernel average misorientation (KAM) map for the (a) initial sample and (b) sample subjected to 1000 number of thermal cycling.
152
K.S. Suresh et al. / Materials Letters 99 (2013) 150–153
Fig. 3. (a) Inverse pole figure of the initial sample, (b) KAM map after 5th cycle and (c) KAM map after 10th cycle. The inverse pole figure shown in the bottom display the development of orientation gradient for selected grains (marked as 1 and 2).
grain 2 is not significant. Increase in intra-grain misorientation during TMC is high compared to TC. The higher misorientation value could be attributed to the additional constraints imposed by the stresses on the selection of certain variants, which are likely to generate dislocations at grain boundaries so as to maintain the compatibility.
Acknowledgement Authors acknowledge Mr. Nikhil Chawan-Dafle for his help during Dynamical Mechanical Analysis. The use of microscopes at the Advanced Facility for Microscopy and Microanalysis (AFMM) is also acknowledged.
4. Conclusion The study on the effect of thermal and thermo-mechanical cycling on the microstructure of a Ni50.6Ti49.4 has led to the following conclusions: (i) Misorientation development in NiTi SMA subjected to pure TC is purely attributed to the coherency level of the phase boundary. (ii) Misorientation development in NiTi SMA subjected to TMC is attributed to the constraints produced by the applied stress in addition to the coherency level of phase boundary. (iii) In both the TC and TMS, misorientation development is strongly influenced by the orientation of the parent austenite grains. (iv) Modulus variation with respect to temperature is basically related to the stress induced martensitic transformation and twin re-orientation.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.03.014.
References [1] Van Humbeeck J. Non-medical applications of shape memory alloys. Mater Sci Eng A 1999;273-275:134–48. [2] Jones NG, Dye D. Martensite evolution in a NiTi shape memory alloy when thermal cycling under an applied load. Intermetallics 2011;19:1348–58. [3] Simon T, Kroger A, Somsen C, Dlouhy A, Eggeler G. On the multiplication of dislocations during martensitic transformations in NiTi Shape memory alloys. Acta Mater 2010;58:1850–60.
K.S. Suresh et al. / Materials Letters 99 (2013) 150–153
[4] Basu R, Jain L, Maji BC, Krishnan M, Mani Krishna KV, Samajdar I, et al. Origin of Microstructural Irreversibility in Ni–Ti based shape memory alloys during thermal cycling. Metall Mater Trans A 2012;43:1277–87. [5] Zhong Y, Yin F, Sakaguchi T, Nagai K, Yang K. Dislocation structure evolution and characterization in the compression deformed Mn–Cu alloy. Acta Mater 2007;55:2747–56. [6] Miyazaki S, Igo Y, Otsuka K. Effect of thermal cycling on the transformation temperatures of TiNi alloys. Acta Metall 1986;34:2045–51.
153
[7] Hatcher N, Kontsevoi YO, Freeman AJ. Martensitic transformation path of NiTi. Phys Rev B Condens Matter 2009;79:020202 R. [8] Wagner MF-X Windl W. Lattice stability, elastic constants and macroscopic moduli of NiTi martensites form first principles. Acta Mater 2008;56:6232–45. [9] Rajagopalan S, Little AL, Bourke MAM, Vaidyanathan R. Elastic modulus of shape memory NiTi from in situ neutron diffraction during macroscopic loading instrumented indentation and extensometry. App Phys Lett 2005;86:081901.