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Evolution of microstructure and property of NiTi alloy induced by cold rolling
Journal of Alloys and Compounds 653 (2015) 156e161
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Evolution of microstructure and property of NiTi alloy induced by cold rolling Y. Li a, J.Y. Li b, M. Liu a, Y.Y. Ren c, F. Chen a, G.C. Yao a, Q.S. Mei a, * a
School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China School of Mechanical Engineering, Wuhan Polytechnic University, Wuhan 430023, China c School of Physics and Technology, Wuhan University, Wuhan 430072, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 August 2015 Received in revised form 2 September 2015 Accepted 8 September 2015 Available online 10 September 2015
We investigated the combination effect of plastic deformation and phase transformation on the evolution of microstructure and property of NiTi alloy. Samples of Ni50.9Ti49.1 alloy were deformed by cold rolling to different strains/thickness reductions (4%e56%). X-ray diffraction, transmission electronic microscopy (TEM) and microhardness measurements were applied for characterization of the microstructure and property of the cold-rolled samples. Experimental results indicated the non-monotonic variations of microstructure parameters and mechanical property with strain, indicating the different processes in microstructure and property evolution of NiTi subjected to cold rolling. TEM observations further showed the dominating mechanisms of microstructure evolution at different strain levels, leading to the gradual reduction of grain size of NiTi to the nanoscale by cold rolling. The results were discussed and related to deformation of martensite, forward and reverse martensitic transformations and dynamic recrystallization. The present study provided experimental evidences for the enhanced formation of nanograins in NiTi by plastic deformation coupled with phase transformation. © 2015 Elsevier B.V. All rights reserved.
Keywords: NiTi alloy Nanostructure Cold rolling Martensitic transformation
1. Introduction The excellent shape memory effect and superelasticity of NiTi alloys are related to the reversible martensitic transformation from austenite phase B2 to martensite phase B190 [1,2], and have attracted numerous fundamental studies [3e6]. Nanostructured materials have attracted widespread interests because of their superior properties compared with traditional counterparts [7e9]. Preparation of nanostructured NiTi alloys is expected to further improve their performances by reducing the grain size to nanoscale [10e12]. Recent investigations showed that the heat accumulation in cyclic loading of superelastic NiTi shape memory alloys, as one of the sources of poor fatigue response, can be reduced by extreme grain refinement [13]. Severe plastic deformation (SPD) has been used as one of the most effective method for producing bulk nanostructured metals. By using SPD technologies such as high-pressure torsion (HPT) and equal-channel angular pressing (ECAP), large plastic strains are imposed on the sample to refine grains to the nanoscale [14,15]. The
main problems with the SPD methods are the rigorous requirements of the equipment, limitation of the size of the sample, and trade-off between strength and ductility [16,17], for which formation of nanograins with less lattice defects at reduced strain levels is highly needed. For NiTi alloy, plastic deformation can lead to both changes of lattice defects and phase transformations between martensite and austenite [18,19]. For the SPD method, formation of nanograins mainly depends on the accumulation and evolution of lattice defects, while the combination of phase transformation is expected to provide a mechanism for production of new refined grains. Therefore, it is interesting to find out how plastic deformation coupled with phase transformation can affect the microstructure evolution and refinement of NiTi alloy. In this study, we investigated the combination effect of plastic strain and phase transformation on the evolution of microstructure and property of NiTi alloy. Experimental results indicated the coupling effect of different processes on microstructure evolution of martensitic NiTi sample induced by cold rolling.
2. Experimental methods * Corresponding author. E-mail address: [email protected] (Q.S. Mei). http://dx.doi.org/10.1016/j.jallcom.2015.09.056 0925-8388/© 2015 Elsevier B.V. All rights reserved.
The original Ni50.9Ti49.1 alloys were solution-treated at 900 C
Y. Li et al. / Journal of Alloys and Compounds 653 (2015) 156e161
for 2 h in vacuum. The original sample has an average grain size about 70 mm with a martensite starting temperature (Ms) of 17 C and austenite starting temperature (As) of 7 C. Cold rolling of the sample with an initial thickness of ~1 mm was performed at the martensite state by immersing the samples into liquid nitrogen, to various thickness reductions (defined by (t0tf)/t0 100%, where t0 and tf are sample thickness before and after cold rolling respectively) from 4% to 56%. Temperatures of the sample before and after cold rolling, measured by an infrared thermometer, are about 30 C and 10 C, respectively. During cold rolling, the NiTi sample was sandwiched in a stainless steel plate to facilitate the uniform deformation. X-Ray diffraction (XRD) analysis was performed on a Brueker D8 advance diffractometer using Cu Ka radiation. A step size of 0.02 and a scanning rate of 0.5 /min were used in the 2q range from 35 to 50 . Transmission electron microscopy (TEM) observations were carried out on JEM-2010HT microscope with an accelerating voltage of 200 kV. Samples for TEM observations were prepared by mechanically thinning, punching 3 mm disks and then subjected to twin-jet method in an electrolyte of 30% nitric acid and 70% methanol by volume. Microhardness tests were performed on a HXS-1000A microhardness tester with a load of 300 g and a duration of 5 s. Each data was obtained from an average of 10 indentations. All experiments were performed on the rolling plane of the cold rolled samples. 3. Results and discussion 3.1. XRD analysis
(110)B2
Fig. 1 shows the XRD profiles of the original sample and samples cold rolled to different thickness reductions as indicated. As can be seen from Fig. 1, the XRD pattern of the original sample shows a main peak corresponding to the (110) diffraction of the B2 phase. After cold rolling at the martensite state, the (110)B2 diffraction
157
peak is observed in all samples, owing to the reverse transformation either because of the temperature increase of samples or plastic strain induced by cold rolling. For samples with thickness reductions of 4%e12%, small shoulder peaks corresponding to B190 phase can also be observed, indicating the existence of retained martensite due to martensite stabilization effect [5,6]. Moreover, significant peak broadening is seen in the cold-rolled samples, which is related to the reduction of grain size and the production of lattice strain [20]. Fig. 2 shows the variations of the full-width-at-half-maximum (FWHM) and peak intensity of (110)B2 with thickness reduction, respectively. Interestingly, instead of a successive variation trend as one may expect, these data show non-monotonic behaviors, suggesting that different processes were involved in microstructure evolution of NiTi induced by cold rolling. At small strain levels (thickness reduction <24%), the FWHM of (110)B2 peak increases and the peak intensity decreases with the increase of thickness reduction. At medium strain levels (thickness reduction 24%e32%), the FWHM of (110)B2 peak decreases and the peak intensity increases with the increase of thickness reduction, i.e., reverse trends compared with those at small strain levels were observed. At high strain levels (thickness reduction >32%), the FWHM of (110)B2 peak increases and the peak intensity decreases with the increase of thickness reduction. For the diffraction peak broadening, both the presence of grain refinement and the uneven microscopic strain can be responsible: the former is caused by the non-Bragg reflection and the latter is due to fluctuations of lattice spacing [21]. The high plastic strain leads to intense lattice distortion and microscopic strain in the sample, which can result in significant grain refinement [22,23]. The decrease of peak intensity can be due to the increased lattice defects with the increase of deformation, while a reverse transformation can also lead to an increase of peak intensity. Dynamic recrystallization is also supposed to be responsible for the increase in peak intensity that is associated with the increased degree of crystallinity. Meanwhile, microscopic strain was released due to dynamic recrystallization, leading to the decrease of FWHM. 3.2. TEM observations
56% 48%
40%
Intensity (a.u.)
32% 28%
The XRD results indicate the non-monotonic behaviors in the microstructural evolution of NiTi subjected to cold rolling, for which several mechanisms are considered to be responsible. To further clarify the different processes of microstructure evolution as indicated by the XRD results, TEM observations were performed on the cold-rolled samples. As shown in Fig. 3, the interface of 300
24% 16% 12% 8% 4%
0%
35
40
45
50
2θ (deg.) Fig. 1. XRD patterns of the NiTi samples cold rolled to different thickness reductions as indicated. The original sample corresponds to a thickness reduction of 0%.
Fig. 2. FWHM and intensity of (110)B2 as functions of thickness reduction for the cold rolled samples.
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Fig. 3. Bright field (a) and dark field (b) TEM micrographs of the NiTi sample cold rolled to a thickness reduction of 16%. Bright field (c) and dark filed (d) TEM micrographs of the NiTi sample cold rolled to a thickness reduction of 32%.
martensite reveals curved and distorted features and the martensite is more refined with the average width of ~54 nm in the 32%-deformed sample, while the average width of martensite in 16%-deformed sample is ~71 nm. Deformation and segmenting of the martensite variants occurred dominantly at this process, generating gradually flexuous and distorted boundaries inside the original martensite lath, resulting in the structure refinement. At the same time, the favorably oriented martensite variants accommodated the deformation by consuming the unfavorably oriented ones [24,25]. Here, the increase of FWHM and decrease of (110)B2 intensity at small strain level can be mainly attributed to the accumulation of lattice strain. Fig. 4 shows TEM images of the 32%-deformed sample, where one can see a martensite lath ~490 nm wide and refined isometric B2 crystals with an average size of ~27 nm inside. The selected area diffraction pattern (SAED) shows evidence for the existence of both martensite and austensite phases and the remarkable grain
refinement as indicated from the continuous rings. It is believed that the large size difference between the deformed martensite lath and the refined B2 crystals inside provides an evidence for dynamic recrystallization coupled with reverse martensitic transformation. Such process is believed to be responsible for the decrease of FWHM and increase of (110)B2 intensity as a result of the recovery of lattice strain and reverse transformation. Fig. 5 shows TEM micrographs of the samples with thickness reductions of 40%, 48% and 56%, respectively. As shown in Fig. 5, refined equiaxed B2 nanocrystals without noticeably distinct grain boundaries were observed inside the martensite laths. With the increase of deformation, the diffraction rings in SAED patterns become more continuous, indicating the increase of grain refinement and the decrease of texture. Fig. 6 depicts the measured sizes of martensite and austenite grains shown in Fig. 5. As shown in Fig. 6, different from the significant difference in the size of the two phases as shown in Fig. 4, the two phases have similar sizes and both become
Fig. 4. Bright field TEM micrograph (a) and dark field micrograph and corresponding SAED pattern (b) of the NiTi sample cold rolled to a thickness reduction of 32%.
Y. Li et al. / Journal of Alloys and Compounds 653 (2015) 156e161
159
Fig. 5. Bright field and dark field micrographs and corresponding SAED patterns of NiTi samples cold rolled to different thickness reductions (a) and (b) 40%, (c) and (d) 48%, (e) and (f) 56%.
120
B19' B2
Grain/lath size (nm)
100 80 60 40 20 40
44
48
52
56
Thickness reduction (%) Fig. 6. Grain/lath size of B2 and B190 as functions of thickness reduction for cold rolled NiTi samples.
smaller with the increase of deformation (thickness reduction 40%e56%). The above results indicate the strain-induced grain refinement [26e28] coupled with reverse transformation for the microstructure evolution at high strain levels, i.e., formation and continuous refinement of nanograined austenite phase from the refined martensite by reverse martensitic transformation. Such reverse martensitic transformation induced by high plastic strain can be understood considering the martensite destabilization due to the significant increase of free energy of martensite when the size is reduced to the nanoscale [4]. Obviously, the significant grain refinement and accumulation of lattice strain lead to the increase of FWHM and decrease of (110)B2 intensity at high strain level. In the cold rolled samples, existence of retained martensite is indicated from the SAED pattern, due to the deformation induced martensite stabilization effect [5,6]. Fig. 7 shows the TEM micro graphs of the 56%-deformed sample after annealing at 300 C for 1 h. It is seen from Fig. 7 that annealing can lead to formation of more refined B2 nanocrystals, as the SAED pattern of the annealed sample shows continuous rings corresponding to single B2 structure. Obviously, the reverse martensitic transformation induced by annealing further enhanced the production of B2 nanocrystals from the retained B190 .
160
Y. Li et al. / Journal of Alloys and Compounds 653 (2015) 156e161
Fig. 7. Bright field TEM micrograph and corresponding SAED pattern (a) and dark field micrograph (b) of the NiTi sample cold rolled to a thickness reduction of 56% and annealed at 300 C for 1 h.
3.3. Microhardness From the XRD and TEM analysis, it is identified that the microstructural evolution of NiTi subjected to cold rolling is mediated by several different processes, resulting in the non-monotonic variations of microstructure parameters. At small strain levels (thickness reduction <24%), deformation and refinement of martensite is dominant. At medium strain levels (thickness reduction 24%e32%), dynamic recrystallization coupled with reverse transformation is dominant. At high strain levels (thickness reduction >32%), strain induced continuous microstructure refinement coupled with reverse transformation is dominant. To further see how these microstructural evolutions are correlated with the property, the microhardness of the cold-rolled samples with different thickness reductions was measured, as shown in Fig. 8. It can be seen from Fig. 8 that there is an obvious increase in the hardness of cold rolled samples: the microhardness of the 56%-deformed sample is ~1.6 times that of the original sample. Moreover, the microhardness varies with thickness reduction in a fluctuation behavior: with increasing thickness reduction, it first increases for thickness reduction <16%, and decrease for thickness reduction from 16% to 24%, and then increases for thickness reduction >24%. Here the microhardness result can be understood considering the different microstructure evolution processes and effect of retained martensite. The increase of microhardness for thickness reduction <16% can be attributed to the increased lattice defects during
deformation of martensite which are retained after back transformation to B2 at room temperature (above As). For thickness reduction from 16% to 24%, more martensite phase can be retained in the sample even at room temperature due to enhanced martensite stabilization effect (increase of As), which can result in a decrease of microhardess. For thickness reduction >24%, reverse transformation is enhanced either by dynamic recrystallization or strain induced microstructure refinement. Hence the formation of more B2 nanocrystals results in the microhardness increment. 4. Conclusions In this study, Ni50.9Ti49.1 alloy was deformed at martensite state by cold rolling to thickness reductions from 4% to 56%. Experimental results indicated the different processes in microstructure and property evolution of NiTi samples subjected to cold rolling, which can be related to the deformation of martensite, forward and reverse martensitic transformations and dynamic recrystallization. Observations indicated that plastic deformation coupled with phase transformation can enhance the formation of nanograins in NiTi. The present study provides a better understanding of the microstructure refinement mechanism of NiTi alloys subjected to traditional cold rolling, and important clues for synthesis of bulk nanostructured NiTi alloy by plastic deformation coupled with phase transformation. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (grant 51371128) and the research foundation of Wuhan University (grant 410100018).
Microhardness (Hv)
600
References
500
400
300 0
8
16
24
32
40
48
56
Thickness reduction (%) Fig. 8. Microhardness of NiTi samples cold rolled to different thickness reductions. The original sample corresponds to a thickness reduction of 0%.
[1] K. Otsuka, C.M. Wayman, Shape Memory Materials, Cambridge University Press, Cambridge, UK, 1998. [2] K. Otsuka, X. Ren, Physical metallurgy of Ti-Ni-based shape memory alloys, Prog. Mater. Sci. 50 (2005) 511e678. [3] A.A. Klopotov, V.E. Gyunter, T.L. Chekalkin, E.V. Kozlov, Crystal chemistry and martensite phase diagrams of titanium-nickel based ternary alloys, Tech. Phys. Lett. 28 (2002) 803e805. [4] T. Waitz, V. Kazykhanov, H.P. Karnthaler, Martensitic phase transformations in nanocrystalline NiTi studied by TEM, Acta Mater. 52 (2004) 137e147. [5] Y.N. Lin, D. Favier, Stabilisation of martensite due to shear deformation via variant reorientation in polycrystalline NiTi, Acta Mater. 48 (2000) 3489e3499. [6] S. Wang, K. Tsuchiya, L. Wang, Martensitic stabilization and defects induced by deformation in TiNi shape memory alloys, Int. Metall. Min. Mater. 18 (2011) 66e69. [7] R. Valiev, Nanostructuring of metals by severe plastic deformation for advanced properties, Nat. Mater. 3 (2004) 511e516.
Y. Li et al. / Journal of Alloys and Compounds 653 (2015) 156e161 [8] P.V. Liddicoat, et al., Nanostructural hierarchy increases the strength of aluminium alloys, Nat. Commun. 1 (2010) 63. [9] W.P. Tong, N.R. Tao, Z.B. Wang, J. Lu, K. Lu, Nitriding iron at lower temperature, Science 299 (2003) 286e288. [10] R. Valiev, D. Gunderov, E. Prokofiev, V. Pushin, Y. Zhu, Nanostructuring of TiNi alloy by SPD processing for advanced properties, Mater. Trans. 49 (2008) 97e101. [11] V.G. Pushin, V.V. Stolyarov, R.Z. Valiev, T.C. Lowe, Y.T. Zhu, Nanostructured TiNi-based shape memory alloys processed by severe plastic deformation, Mater. Sci. Eng. A 410 (2005) 386e389. [12] K. Tsuchiya, et al., Production of TiNi amorphous/nanocrystalline wires with high strength and elastic modulus by severe cold drawing, Scr. Mater. 60 (2009) 749e752. [13] A. Aslan, Q.P. Sun, Effects of Grain Size on the rate-dependent thermomechanical responses of nanostructured superelastic NiTi, Acta Mater. 57 (2014) 671e679. [14] R.Z. Valiev, T.G. Langdon, Principles of equal-channel angular pressing as a processing tool for grain refinement, Prog. Mater. Sci. 51 (2006) 881e981. [15] P. Zhilyaev Alexander, T.G. Langdon, Using high-pressure torsion for metal processing: Fundamentals and applications, Prog. Mater. Sci. 53 (2008) 893e979. [16] Y. Wang, M. Chen, F. Zhou, E. Ma, High tensile ductility in a nanostructured metal, Nature 419 (2002) 912e915. [17] T.C. Lowe, Y.T. Zhu, Commercialization of nanostructured metals produced by severe plastic deformation processing, Adv. Eng. Mater. 5 (2003) 373e378. [18] A.V.K. Karaman, Z.P. Luo, Transformation behavior and unusual twinning in a NiTi shape memory alloy ausformed using equal channel angular extrusion,
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Philos. Mag. 85 (2005) 1729e1745. [19] K. Tsuchiya, M. Inuzuka, D. Tomus, et al., Martensitic transformation in nanostructured TiNi shape memory alloy formed via severe plastic deformation, Mater. Sci. Eng. A 438 (2006) 643e648. [20] A.K. Srivastava, D. Schryvers, V. Humbeeck, Grain growth and precipitation in an annealed cold-rolled Ni50.9Ti49.1 alloy, Intermetallics 15 (2007) 1538e1547. [21] X.C. Hu, X-ray diffraction analysis carbide coating application, Carbide 25 (2009) 236e241. [22] N. Tsuji, R. Ueji, Y. Minamino, et al., A new and simple process to obtain nanostructured bulk low-carbon steel with superior mechanical property, Scr. Mater. 46 (2002) 305e310. [23] R. Ueji, N. Tsuji, Y. Minamino, et al., Ultragrain refinement of plain low carbon steel by cold-rolling and annealing of martensite, Acta Mater. 50 (2002) 4177e4189. [24] T. Kurita, H. Matsumoto, H. Abe, Transformation behavior in rolled NiTi, J. Alloy. Comp. 381 (2004) 158e161. [25] Y. Liu, Z.L. Xie, H.J. Van, L. Delaey, Effect of texture orientation on the martensite deformation of NiTi shape memory alloy sheet, Acta Mater. 47 (1999) 645e660. [26] Y.F. Zheng, J.X. Zhang, L.C. Zhao, H.Q. Ye, HREM studies on the microstructure of severely cold-rolled TiNi alloy after reverse martensitic transformation, Mater. Lett. 41 (1999) 9e15. [27] R. Singh, et al., Microstructure evolution in nanocrystalline NiTi alloy produced by HPT, J. Alloy. Comp. 509 (2011) S290eS293. [28] K. Lu, N. Hansen, Structural refinement and deformation mechanisms in nanostructured metals, Scr. Mater. 60 (2009) 1033e1038.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Evolution of microstructure and property of NiTi alloy induced by cold rolling Y. Li a, J.Y. Li b, M. Liu a, Y.Y. Ren c, F. Chen a, G.C. Yao a, Q.S. Mei a, * a
School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China School of Mechanical Engineering, Wuhan Polytechnic University, Wuhan 430023, China c School of Physics and Technology, Wuhan University, Wuhan 430072, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 August 2015 Received in revised form 2 September 2015 Accepted 8 September 2015 Available online 10 September 2015
We investigated the combination effect of plastic deformation and phase transformation on the evolution of microstructure and property of NiTi alloy. Samples of Ni50.9Ti49.1 alloy were deformed by cold rolling to different strains/thickness reductions (4%e56%). X-ray diffraction, transmission electronic microscopy (TEM) and microhardness measurements were applied for characterization of the microstructure and property of the cold-rolled samples. Experimental results indicated the non-monotonic variations of microstructure parameters and mechanical property with strain, indicating the different processes in microstructure and property evolution of NiTi subjected to cold rolling. TEM observations further showed the dominating mechanisms of microstructure evolution at different strain levels, leading to the gradual reduction of grain size of NiTi to the nanoscale by cold rolling. The results were discussed and related to deformation of martensite, forward and reverse martensitic transformations and dynamic recrystallization. The present study provided experimental evidences for the enhanced formation of nanograins in NiTi by plastic deformation coupled with phase transformation. © 2015 Elsevier B.V. All rights reserved.
Keywords: NiTi alloy Nanostructure Cold rolling Martensitic transformation
1. Introduction The excellent shape memory effect and superelasticity of NiTi alloys are related to the reversible martensitic transformation from austenite phase B2 to martensite phase B190 [1,2], and have attracted numerous fundamental studies [3e6]. Nanostructured materials have attracted widespread interests because of their superior properties compared with traditional counterparts [7e9]. Preparation of nanostructured NiTi alloys is expected to further improve their performances by reducing the grain size to nanoscale [10e12]. Recent investigations showed that the heat accumulation in cyclic loading of superelastic NiTi shape memory alloys, as one of the sources of poor fatigue response, can be reduced by extreme grain refinement [13]. Severe plastic deformation (SPD) has been used as one of the most effective method for producing bulk nanostructured metals. By using SPD technologies such as high-pressure torsion (HPT) and equal-channel angular pressing (ECAP), large plastic strains are imposed on the sample to refine grains to the nanoscale [14,15]. The
main problems with the SPD methods are the rigorous requirements of the equipment, limitation of the size of the sample, and trade-off between strength and ductility [16,17], for which formation of nanograins with less lattice defects at reduced strain levels is highly needed. For NiTi alloy, plastic deformation can lead to both changes of lattice defects and phase transformations between martensite and austenite [18,19]. For the SPD method, formation of nanograins mainly depends on the accumulation and evolution of lattice defects, while the combination of phase transformation is expected to provide a mechanism for production of new refined grains. Therefore, it is interesting to find out how plastic deformation coupled with phase transformation can affect the microstructure evolution and refinement of NiTi alloy. In this study, we investigated the combination effect of plastic strain and phase transformation on the evolution of microstructure and property of NiTi alloy. Experimental results indicated the coupling effect of different processes on microstructure evolution of martensitic NiTi sample induced by cold rolling.
2. Experimental methods * Corresponding author. E-mail address: [email protected] (Q.S. Mei). http://dx.doi.org/10.1016/j.jallcom.2015.09.056 0925-8388/© 2015 Elsevier B.V. All rights reserved.
The original Ni50.9Ti49.1 alloys were solution-treated at 900 C
Y. Li et al. / Journal of Alloys and Compounds 653 (2015) 156e161
for 2 h in vacuum. The original sample has an average grain size about 70 mm with a martensite starting temperature (Ms) of 17 C and austenite starting temperature (As) of 7 C. Cold rolling of the sample with an initial thickness of ~1 mm was performed at the martensite state by immersing the samples into liquid nitrogen, to various thickness reductions (defined by (t0tf)/t0 100%, where t0 and tf are sample thickness before and after cold rolling respectively) from 4% to 56%. Temperatures of the sample before and after cold rolling, measured by an infrared thermometer, are about 30 C and 10 C, respectively. During cold rolling, the NiTi sample was sandwiched in a stainless steel plate to facilitate the uniform deformation. X-Ray diffraction (XRD) analysis was performed on a Brueker D8 advance diffractometer using Cu Ka radiation. A step size of 0.02 and a scanning rate of 0.5 /min were used in the 2q range from 35 to 50 . Transmission electron microscopy (TEM) observations were carried out on JEM-2010HT microscope with an accelerating voltage of 200 kV. Samples for TEM observations were prepared by mechanically thinning, punching 3 mm disks and then subjected to twin-jet method in an electrolyte of 30% nitric acid and 70% methanol by volume. Microhardness tests were performed on a HXS-1000A microhardness tester with a load of 300 g and a duration of 5 s. Each data was obtained from an average of 10 indentations. All experiments were performed on the rolling plane of the cold rolled samples. 3. Results and discussion 3.1. XRD analysis
(110)B2
Fig. 1 shows the XRD profiles of the original sample and samples cold rolled to different thickness reductions as indicated. As can be seen from Fig. 1, the XRD pattern of the original sample shows a main peak corresponding to the (110) diffraction of the B2 phase. After cold rolling at the martensite state, the (110)B2 diffraction
157
peak is observed in all samples, owing to the reverse transformation either because of the temperature increase of samples or plastic strain induced by cold rolling. For samples with thickness reductions of 4%e12%, small shoulder peaks corresponding to B190 phase can also be observed, indicating the existence of retained martensite due to martensite stabilization effect [5,6]. Moreover, significant peak broadening is seen in the cold-rolled samples, which is related to the reduction of grain size and the production of lattice strain [20]. Fig. 2 shows the variations of the full-width-at-half-maximum (FWHM) and peak intensity of (110)B2 with thickness reduction, respectively. Interestingly, instead of a successive variation trend as one may expect, these data show non-monotonic behaviors, suggesting that different processes were involved in microstructure evolution of NiTi induced by cold rolling. At small strain levels (thickness reduction <24%), the FWHM of (110)B2 peak increases and the peak intensity decreases with the increase of thickness reduction. At medium strain levels (thickness reduction 24%e32%), the FWHM of (110)B2 peak decreases and the peak intensity increases with the increase of thickness reduction, i.e., reverse trends compared with those at small strain levels were observed. At high strain levels (thickness reduction >32%), the FWHM of (110)B2 peak increases and the peak intensity decreases with the increase of thickness reduction. For the diffraction peak broadening, both the presence of grain refinement and the uneven microscopic strain can be responsible: the former is caused by the non-Bragg reflection and the latter is due to fluctuations of lattice spacing [21]. The high plastic strain leads to intense lattice distortion and microscopic strain in the sample, which can result in significant grain refinement [22,23]. The decrease of peak intensity can be due to the increased lattice defects with the increase of deformation, while a reverse transformation can also lead to an increase of peak intensity. Dynamic recrystallization is also supposed to be responsible for the increase in peak intensity that is associated with the increased degree of crystallinity. Meanwhile, microscopic strain was released due to dynamic recrystallization, leading to the decrease of FWHM. 3.2. TEM observations
56% 48%
40%
Intensity (a.u.)
32% 28%
The XRD results indicate the non-monotonic behaviors in the microstructural evolution of NiTi subjected to cold rolling, for which several mechanisms are considered to be responsible. To further clarify the different processes of microstructure evolution as indicated by the XRD results, TEM observations were performed on the cold-rolled samples. As shown in Fig. 3, the interface of 300
24% 16% 12% 8% 4%
0%
35
40
45
50
2θ (deg.) Fig. 1. XRD patterns of the NiTi samples cold rolled to different thickness reductions as indicated. The original sample corresponds to a thickness reduction of 0%.
Fig. 2. FWHM and intensity of (110)B2 as functions of thickness reduction for the cold rolled samples.
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Fig. 3. Bright field (a) and dark field (b) TEM micrographs of the NiTi sample cold rolled to a thickness reduction of 16%. Bright field (c) and dark filed (d) TEM micrographs of the NiTi sample cold rolled to a thickness reduction of 32%.
martensite reveals curved and distorted features and the martensite is more refined with the average width of ~54 nm in the 32%-deformed sample, while the average width of martensite in 16%-deformed sample is ~71 nm. Deformation and segmenting of the martensite variants occurred dominantly at this process, generating gradually flexuous and distorted boundaries inside the original martensite lath, resulting in the structure refinement. At the same time, the favorably oriented martensite variants accommodated the deformation by consuming the unfavorably oriented ones [24,25]. Here, the increase of FWHM and decrease of (110)B2 intensity at small strain level can be mainly attributed to the accumulation of lattice strain. Fig. 4 shows TEM images of the 32%-deformed sample, where one can see a martensite lath ~490 nm wide and refined isometric B2 crystals with an average size of ~27 nm inside. The selected area diffraction pattern (SAED) shows evidence for the existence of both martensite and austensite phases and the remarkable grain
refinement as indicated from the continuous rings. It is believed that the large size difference between the deformed martensite lath and the refined B2 crystals inside provides an evidence for dynamic recrystallization coupled with reverse martensitic transformation. Such process is believed to be responsible for the decrease of FWHM and increase of (110)B2 intensity as a result of the recovery of lattice strain and reverse transformation. Fig. 5 shows TEM micrographs of the samples with thickness reductions of 40%, 48% and 56%, respectively. As shown in Fig. 5, refined equiaxed B2 nanocrystals without noticeably distinct grain boundaries were observed inside the martensite laths. With the increase of deformation, the diffraction rings in SAED patterns become more continuous, indicating the increase of grain refinement and the decrease of texture. Fig. 6 depicts the measured sizes of martensite and austenite grains shown in Fig. 5. As shown in Fig. 6, different from the significant difference in the size of the two phases as shown in Fig. 4, the two phases have similar sizes and both become
Fig. 4. Bright field TEM micrograph (a) and dark field micrograph and corresponding SAED pattern (b) of the NiTi sample cold rolled to a thickness reduction of 32%.
Y. Li et al. / Journal of Alloys and Compounds 653 (2015) 156e161
159
Fig. 5. Bright field and dark field micrographs and corresponding SAED patterns of NiTi samples cold rolled to different thickness reductions (a) and (b) 40%, (c) and (d) 48%, (e) and (f) 56%.
120
B19' B2
Grain/lath size (nm)
100 80 60 40 20 40
44
48
52
56
Thickness reduction (%) Fig. 6. Grain/lath size of B2 and B190 as functions of thickness reduction for cold rolled NiTi samples.
smaller with the increase of deformation (thickness reduction 40%e56%). The above results indicate the strain-induced grain refinement [26e28] coupled with reverse transformation for the microstructure evolution at high strain levels, i.e., formation and continuous refinement of nanograined austenite phase from the refined martensite by reverse martensitic transformation. Such reverse martensitic transformation induced by high plastic strain can be understood considering the martensite destabilization due to the significant increase of free energy of martensite when the size is reduced to the nanoscale [4]. Obviously, the significant grain refinement and accumulation of lattice strain lead to the increase of FWHM and decrease of (110)B2 intensity at high strain level. In the cold rolled samples, existence of retained martensite is indicated from the SAED pattern, due to the deformation induced martensite stabilization effect [5,6]. Fig. 7 shows the TEM micro graphs of the 56%-deformed sample after annealing at 300 C for 1 h. It is seen from Fig. 7 that annealing can lead to formation of more refined B2 nanocrystals, as the SAED pattern of the annealed sample shows continuous rings corresponding to single B2 structure. Obviously, the reverse martensitic transformation induced by annealing further enhanced the production of B2 nanocrystals from the retained B190 .
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Fig. 7. Bright field TEM micrograph and corresponding SAED pattern (a) and dark field micrograph (b) of the NiTi sample cold rolled to a thickness reduction of 56% and annealed at 300 C for 1 h.
3.3. Microhardness From the XRD and TEM analysis, it is identified that the microstructural evolution of NiTi subjected to cold rolling is mediated by several different processes, resulting in the non-monotonic variations of microstructure parameters. At small strain levels (thickness reduction <24%), deformation and refinement of martensite is dominant. At medium strain levels (thickness reduction 24%e32%), dynamic recrystallization coupled with reverse transformation is dominant. At high strain levels (thickness reduction >32%), strain induced continuous microstructure refinement coupled with reverse transformation is dominant. To further see how these microstructural evolutions are correlated with the property, the microhardness of the cold-rolled samples with different thickness reductions was measured, as shown in Fig. 8. It can be seen from Fig. 8 that there is an obvious increase in the hardness of cold rolled samples: the microhardness of the 56%-deformed sample is ~1.6 times that of the original sample. Moreover, the microhardness varies with thickness reduction in a fluctuation behavior: with increasing thickness reduction, it first increases for thickness reduction <16%, and decrease for thickness reduction from 16% to 24%, and then increases for thickness reduction >24%. Here the microhardness result can be understood considering the different microstructure evolution processes and effect of retained martensite. The increase of microhardness for thickness reduction <16% can be attributed to the increased lattice defects during
deformation of martensite which are retained after back transformation to B2 at room temperature (above As). For thickness reduction from 16% to 24%, more martensite phase can be retained in the sample even at room temperature due to enhanced martensite stabilization effect (increase of As), which can result in a decrease of microhardess. For thickness reduction >24%, reverse transformation is enhanced either by dynamic recrystallization or strain induced microstructure refinement. Hence the formation of more B2 nanocrystals results in the microhardness increment. 4. Conclusions In this study, Ni50.9Ti49.1 alloy was deformed at martensite state by cold rolling to thickness reductions from 4% to 56%. Experimental results indicated the different processes in microstructure and property evolution of NiTi samples subjected to cold rolling, which can be related to the deformation of martensite, forward and reverse martensitic transformations and dynamic recrystallization. Observations indicated that plastic deformation coupled with phase transformation can enhance the formation of nanograins in NiTi. The present study provides a better understanding of the microstructure refinement mechanism of NiTi alloys subjected to traditional cold rolling, and important clues for synthesis of bulk nanostructured NiTi alloy by plastic deformation coupled with phase transformation. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (grant 51371128) and the research foundation of Wuhan University (grant 410100018).
Microhardness (Hv)
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300 0
8
16
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32
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48
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Thickness reduction (%) Fig. 8. Microhardness of NiTi samples cold rolled to different thickness reductions. The original sample corresponds to a thickness reduction of 0%.
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