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Effect of short-time annealing treatment on the superelastic behavior of cold drawn Ni-rich NiTi shape memory wires
Journal of Alloys and Compounds 554 (2013) 32–38
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of short-time annealing treatment on the superelastic behavior of cold drawn Ni-rich NiTi shape memory wires Fatemeh Khaleghi a, Jafar Khalil-Allafi a,⇑, Vahid Abbasi-Chianeh a,b, Soheil Noori a a b
Research Center for Advanced Materials and Mineral Processing, Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran Faculty of Mechanical Engineering, Urmia University of Technology, Urmia, Iran
a r t i c l e
i n f o
Article history: Received 24 July 2012 Received in revised form 26 November 2012 Accepted 28 November 2012 Available online 7 December 2012 Keywords: Cold drawing Short-time annealing Super elastic behavior Ni-rich NiTi wires
a b s t r a c t The effect of short-time annealing treatment for 3, 9 and 18 s at 700 °C on the superelastic behavior of cold drawn Ni-rich NiTi wires was studied. Superelastic properties of cold drawn and annealed samples, including upper and lower plateau stresses, residual strain and mechanical hysteresis, were specified comparatively. The results show that for all cold drawn wires, annealing at 700 °C for only 3 s leads to the best superelastic behavior. Microstructure investigations were performed using optical microscopy and X-ray diffractometry. It was demonstrated that annealing at 700 °C for 3 s leads to production of an ultra fine grained structure. Also increasing the annealing time, only about a few seconds, results in occurrence of grain growth phenomenon which causes a sharp drop in superelastic behavior. Transformation behavior of the samples was specified by differential scanning calorimetry and R-phase was detected in ultra fine grained materials. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Shape memory alloys (SMAs) are the materials that possess two unique properties named superelasticity (SE) and shape memory effect (SME) [1]. They both are based on a reversible thermo-elastic phase transformation from a high temperature B2 austenite phase to a low temperature B190 martensite phase on cooling and reverse transformation on heating [2]. In addition to mentioned characteristics, SMAs have high corrosion resistance, favorable biocompatibility and high intrinsic damping capacity [3–6]. Therefore, they have been employed in many applications, especially medical usages including orthodontic wires and drills, cardiovascular guidewires, implants and self-expanding stents [7–10]. Additional improvements of the properties of SMAs can be accomplished by specific thermomechanical treatments like cold working and subsequent annealing [11]. Cold working improves the superelastic behavior with increasing the critical stress for dislocations slip relative to critical stress for twinning mechanism [12]. Besides, NiTi shape memory alloys are prone to be amorphous by severe plastic deformation [13]. Koike et al. [14] reported that amorphization phenomenon will take place in NiTi alloys, if the density of dislocations induced by plastic deformation exceeds an estimated amount of 1013–1014 cm2. Post deformation annealing will result in the
⇑ Corresponding author. Address: Faculty of Materials Engineering, Sahand University of Technology, P.O. Box 51335-11996, Tabriz, Iran. Tel.: +98 412 3459454; fax: +98 412 3444333. E-mail address: allafi@sut.ac.ir (J. Khalil-Allafi). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.183
formation of an ultra fine grained structure (UFG) if the optimum thermomechanical course is selected [15]. UFG shape memory alloys have superior properties over their coarse grained counterparts involving favorable ductility, high strength and upper level of recovery strains and stresses up to 10% and 1.5 GPa respectively [16,17]. Numerous investigations have been devoted to various SPD methods, most remarkably to high pressure torsion (HPT), equal-channel angular pressing (ECAP) and repeated cold rolling [18–20]. However the aforesaid techniques are not feasible for the wires. Thus, cold drawing is an attractive method to obtain thin wires with improved shape memory and superelastic properties depending on the selection of an appropriate thermomechanical regime. In the present work the influence of a novel annealing treatment i.e. a short-time annealing process at 700 °C on the superelastic response of cold drawn NiTi wires was studied. 2. Experimental works NiTi wires with nominal composition of Ti-50.9 at.% Ni (0.34 mm), provided by Memry corporation of the United States, was used in the present study. They were annealed at 850 °C for 1 h and quenched into room temperature water. Regarding the severe influence of oxygen on this solution annealing treatment, the wires were placed in a quartz tube filled with titanium powders and a high purity argon gas. After annealing, no remarked variation was observed in the diameter and appearance of the wires. However, the samples were chemically etched out to reduce the oxygen-affected layer after annealing. The solution annealed wires were cold drawn to a reduction area of 46–71% corresponding to a true strain of 0.6–1.2 using poly crystalline diamond dies. The number of reduction passes ranged from three for e = 0.6 to six for e = 1.2 with approximately 0.2 reduction per each step. Drawing process was performed at a speed of 0.6 m/min, without intermediate annealing. Cold drawn wires were annealed at 700 °C in an air furnace. For each reduction,
F. Khaleghi et al. / Journal of Alloys and Compounds 554 (2013) 32–38
33
3. Results and discussions
Fig. 1. Optical micrographs of the (a) solution annealed wire and as-drawn wires with logarithmic strains of (b) e = 0.6, (c) e = 0.8, (d) e = 1.0 and (e) e = 1.2.
one sample was maintained for 3 s, one for 9 s and one for 18 s in a vertical furnace with a small hole in its port. After annealing the samples appeared in an azure blue color while they had a bright gray surface before that. In order to minimize the effect of oxidation, the wires were chemically etched until their diameters were reduced up to 50 lm. Microstructure studies of the wires were carried out using optical microscopy (OM, Olympus PMG3). Transformation behavior of the samples was investigated by differential scanning calorimetry (DSC, Netzsch DSC 404C). The samples were subjected to a cooling/heating cycle in the temperature range of 100 to +100 °C with a rate of 10 °C/min after heating up to +100 °C. DSC measurements for determining the microstructure evolutions of as-drawn wires after cold working were performed between 25 and 650 °C with a rate of 10 °C. Crystallite size and crystal structure of the samples were determined by X-ray diffractometry (XRD, Siemens D 5000) using CuKa radiation (k = 0.1541 nm) operated at 40 kV/30 mA. The superelastic behavior of the wires was studied using an Adamel Lohmargy DY26 tensile test instrument equipped with an electrical oven with accuracy of ±0.5 °C and a 100-kg load cell. The tests were conducted at 37 °C and a cross-head speed of 0.1 mm/min in a loading and unloading course up to 6.2% strain. Overall length of the wires between the grasps was selected to be 40 ± 0.02 mm. Diameters of the samples were measured using a digital micrometer with accuracy of 1 lm.
Fig. 1 shows the optical micrographs of the wires, before and after cold drawing. As can be seen, the microstructure of solution annealed sample (Fig. 1a) contains equiaxed grains while after cold working (Fig. 1b–e) the grains are drawn in the drawing direction. Stress–strain curves of solution annealed and as-drawn samples are presented in Fig. 2. It is observed that the as-drawn sample with logarithmic strain of e = 0.6 displays a linear superelastic behavior which is specified by the absence of stress plateau in stress–strain curve [21]. The recovery strain of the sample is equal to about 4.8% which is a remarkable value and much larger than that obtained for solution annealed sample (1.2%). The cause of strain recovery, of the as-cold worked samples, has been attributed to reversible movement of (0 0 1) microtwins inside h0 1 1i type II twin lamella of B190 by Zheng et al. [22]. Further increase in the level of cold work from e = 0.8 up to e = 1.2 causes the failure of the wires in the strains less than 6.2%. Fig. 3 illustrates DSC profiles of as-drawn wires with logarithmic strains of e = 0.6 and e = 1.2. For the sample with the plastic deformation of e = 0.6, no B2 ? B190 peak is visible in the cooling cycle which is related to the peak widening due to introduction of lattice defects as a result of cold working. However, during the heating cycle one peak could be detected that would be due to reverse transformation of B190 ? B2. Peak occurrence during the heating cycle in the temperature range of 25 °C up to 0 °C indicates that the cold drawn sample with logarithmic strain of e = 0.6 is not fully martensitic at room temperature. Increasing the level of cold work up to e = 1.2 leads to annihilation of detected peak in sample with e = 0.6 that is due to peak broadening during both cooling and heating regimes or martensite stabilization because of severe plastic deformation. The presence of stabilized martensite in a Ti-50.8Ni wire, cold drawn to an area reduction of 45 ± 5%, has been evidenced by X-ray synchrotron measurements and TEM observations in [23,24]. Fig. 4 demonstrates X-ray diffraction patterns of NiTi wires in the solution annealed state and after cold drawing with logarithmic strains of e = 0.6 and e = 1.2. It can be seen that after cold drawing, some peaks are revealed that are not related to B2 structure. Newly revealed peaks are related to the B190 phase. However they are very broad and have very low intensities compared to (1 1 0) B2 peak of solution annealed sample. Because of low intensity, broad peak area and the probability of peak shift due to cold work, the newly revealed peaks are not indexed strictly. Since peak broadening takes place due to high density of dislocations [25], cold drawing has been caused to introduction of a dislocation stabilized martensitic structure. Peak broadening would also be related to the occurrence of amorphizaion as a result of subjecting the alloy to severe plastic deformation. The amorphization phenomenon
Fig. 2. Stress–strain curves of (a) solution annealed and (b) as-drawn specimens.
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F. Khaleghi et al. / Journal of Alloys and Compounds 554 (2013) 32–38
Fig. 3. DSC profiles of as-drawn wires with logarithmic strains of (a) e = 0.6 and (b) e = 1.2.
Fig. 4. X-ray diffraction patterns of NiTi wires in the solution annealed state and after cold drawing with logarithmic strains of e = 0.6 and e = 1.2.
Fig. 5. DSC profiles for as-drawn wires with logarithmic strains of e = 0.6 up to e = 1.2.
has been reported by Tsuchiya et al. [26] in the Ti-50.9Ni wires, cold drawn up to e = 1.2. Therefore it can be proposed that in the cold worked samples of this study, the microstructure contains both deformation stabilized B190 phase and partially amorphous areas. DSC profiles recorded for investigation of microstructure evolutions after cold working is demonstrated in Fig. 5. For all specimens, there is an exothermic peak observed in the temperature range of 300–400 °C. It seems that the peak correlates with the devitrification of the amorphous phase which has been formed during cold-drawing. It also should be noted that in the samples with logarithmic strains of e = 0.6 up to e = 1.0, another exothermic peak is appeared in the temperature range of 500–600 °C which are marked with arrows in Fig. 5. The appearance of the second peak can be attributed to the nucleation of new recrystallized grains in the areas with high dislocation densities. Increasing the driving force for the recrytallization phenomenon with increasing the level of plastic deformation leads to the decrease in the onset temperature of the peak. Surprisingly, the crystallization peak is not observed in the DSC profile of the sample deformed up to e = 1.2. Therefore it can be proposed that the recrystallization has been completed dynamically on cold drawing process due to applying high level of strains. Another prospect is the merging of the devitrification and recrystallization peaks due to the decrease in the onset temperature of the latter one. However, the microstructures of the as-drawn samples are possibly a mixture of dislocation substructure, amorphous phase and nanocrystalline B190 phase. Brailovski et al. [27] has observed the above mentioned structure in NiTi alloy subjected to cold rolling up to e = 1.0 by TEM. Fig. 6 shows the optical micrographs of the samples with logarithmic strains of e = 0.6 and e = 1.2, annealed at 700 °C for 3 and 18 s. It is observed that the microstructure of the samples, annealed for 3 s is analogous to those before annealing (Fig. 1b and e). The grains are as fine that they are not observable by optical microscopy. However, increasing the annealing time up to 18 s, results in emerging very fine grains in both samples. Thus, it is found out that the rate of the microstructure evolutions is very high and the new recrystallized grains appear only in a few seconds. Very fast nature of microstructure evolutions which transform the highly cold-worked microstructure into an annealed microstructure during annealing treatment for very short times has also been shown by Delville et al. [24]. Fig. 7 exhibits X-ray diffraction patterns of the samples with logarithmic strains of e = 0.6 and e = 1.2, annealed at 700 °C for 3 and 18 s. It can be seen that the characteristic peaks of the B2 phase exist in the X-ray diffractograms. Crystallite size of the samples was estimated by Williamson–Hall equation. They were 110 nm and 981 nm for the samples deformed up to e = 0.6, annealed for 3 and 18 s
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Fig. 6. Optical micrographs of the samples with logarithmic strains of e = 0.6 annealed at 700 °C for (a) 3 s, (b) 9 s and (c) 18 s and e = 1.2 annealed at 700 °C for (a) 3 s, (b) 9 s and (c) 18 s.
Fig. 7. X-ray diffraction patterns of the samples with logarithmic strains of e = 0.6 annealed at 700 °C for (a) 3 s and (b) 18 s and e = 1.2 annealed at 700 °C for (c) 3 s and (d) 18 s.
subsequently and 70 nm and 1387 nm for the samples deformed up to e = 1.2, annealed for 3 and 18 s subsequently. The determined values for samples annealed for 18 s are in good agreement with the optical microscopy observations. DSC profiles of the samples with logarithmic strains of e = 0.6 and e = 1.2 which have been annealed at 700 °C for 3, 9 and 18 s, are presented in Fig. 8. For both e = 0.6 and e = 1.2, the samples annealed for 3 s yield a B2 ? R transformation peak in the cooling cycle. It is known that transformation behavior of nanocrytalline NiTi alloys intensively depends on the grain size. The nanograins with the size of less than 150 nm yield two-step martensitic transformation (B2 ? R ? B190 ) [28]. So, it seems that the appearance of the R-phase peak could be related to the presence of the nanograins with the size
of less than 150 nm or high density of dislocations. Grain growth progress and decreasing the dislocation density with increasing the annealing time up to 18 s, result in the elimination of R-phase peak in the DSC curves. Increasing the annealing time also leads to peak sharpening. It indicates that the major part of the cold work has been annealed. Fig. 9 demonstrates stress–strain curves of all cold drawn wires that have been annealed at 700 °C for 3, 9 and 18 s. Upper plateau stress, lower plateau stress, mechanical hysteresis and residual strain alterations with annealing time, are given in Fig. 10. It is observed that for all cold drawn wires, annealing for 3 s leads to the very low residual strains with high stress value of plateaues. Increasing the annealing time reduces the amount of recovered strain. It is known that post deformation annealing
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F. Khaleghi et al. / Journal of Alloys and Compounds 554 (2013) 32–38
Fig. 8. DSC profiles of the samples with logarithmic strains of e = 0.6 annealed at 700 °C for (a) 3 s,(b) 9 s, (c) 18 s and e = 1.2 annealed at 700 °C for (d) 3 s, (e) 9 s and (f) 18 s.
Fig. 9. Stress–strain curves of all cold drawn wires which have been annealed at 700 °C for 3, 9 and 18 s.
F. Khaleghi et al. / Journal of Alloys and Compounds 554 (2013) 32–38
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Fig. 10. (a) Upper plateau stress, (b) lower plateau stress, (c) mechanical hysteresis and (d) residual strain alterations with annealing time for all cold drawn samples.
results in the occurrence of following processes: (i) recovery, polygonization and recrystallization in dislocation substructure, (ii) crystallization of the amorphous structure and (iii) the coarsening of the nanocrystalline structure, induced either directly from cold work or from crystallized amorphous structure [29]. Interaction of the above mentioned phenomena affects the superelastic behavior of the specimens, annealed at various conditions. For the samples deformed up to e = 0.6 and e = 0.8, the superelasticity parameters vary in a similar manner. Increasing the annealing time from 3 to 9 s causes a sharp increase in residual strain and mechanical hysteresis and a sharp decrease in upper and lower plateau stresses which are related to grain growth and recovery processes. Mechanical hysteresis is resulted from a friction energy which originates from the austenite/martensite interface motion [28]. It has been evidenced that dislocations assist in the formation of martensite from austenite [30]. Thus decreasing the density of dislocations with the continuance of the recovery process increases the mechanical hysteresis. Residual strain and mechanical hysteresis do not differ considerably with increasing the annealing time from 9 to 18 s because of the completeness of the recovery process within 9 s. In addition, smooth reduction of upper plateau stress from 9 to 18 s introduces the possibility of initiating the recrystallization process in the dislocation substructure. Changes of superelasticity parameters with annealing time for the sample, deformed up to e = 1.0 is similar to the samples e = 0.6 and e = 0.8 except the mechanical hysteresis which does not vary tangibly with annealing time. It might be considered that the recovery process has been finished within 3 s and continuation of annealing procedure causes the occurrence of recrystallization and grain growth phenomena. The superelastic behavior of the e = 1.2 samples, annealed at 700 °C for 3, 9 and 18 s is some deal different from the previous samples. Contrary to the samples with e = 0.6 up to e = 1.0, variation of upper plateau stress and residual strain with increasing the annealing time from 9 to 18 s is remarkable. This behavior proposes that the recrystallization process has been completed within 3 s. Thus, elevating the annealing time leads to the grain growth phenomenon.
One can observe that increasing the level of cold work will be effective on improving the superelastic behavior if the annealing time is selected not more than 3 s. For the samples which have been annealed for 3 s, increasing the level of plastic deformation leads to a steady increase in upper plateau stress and mechanical hysteresis. Raising the upper plateau stress with increasing the strain hardening is because of the grain refinement which increases the yield stress of the austenite. The reason for monotonous intensifying the mechanical hysteresis can be related to increasing the ratio of volume fraction of recrystallized grains to dislocations network with increasing the level of cold work.
4. Conclusion In the present assay, we studied the effect of short-time annealing treatment at 700 °C on the superelastic behavior of cold drawn wires and obtained the following conclusions: (i) Annealing of the cold drawn Ni-rich NiTi wires with logarithmic strains of e = 0.6–1.2 at 700 °C for 3 s results in formation of an ultra fine grained structure. Increasing the annealing time to 18 s leads to the occurrence of grain growth to the extent that the grains get observable with optical microscopy. (ii) Comparative study between the superelastic properties of the samples which have been subjected to different thermomechanical courses shows that for all cold drawn samples annealing for 3 s results in the best superelastic behavior. Furthermore, increasing the level of cold work improves the superelastic properties of the wires, annealed for 3 s. (iii) All the treated samples of the present study displays much larger recovered strains compared to the solution annealed wire. It shows that despite the high rate of microstructure evolutions at 700 °C, elimination of the entire cold work needs more annealing times.
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of short-time annealing treatment on the superelastic behavior of cold drawn Ni-rich NiTi shape memory wires Fatemeh Khaleghi a, Jafar Khalil-Allafi a,⇑, Vahid Abbasi-Chianeh a,b, Soheil Noori a a b
Research Center for Advanced Materials and Mineral Processing, Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran Faculty of Mechanical Engineering, Urmia University of Technology, Urmia, Iran
a r t i c l e
i n f o
Article history: Received 24 July 2012 Received in revised form 26 November 2012 Accepted 28 November 2012 Available online 7 December 2012 Keywords: Cold drawing Short-time annealing Super elastic behavior Ni-rich NiTi wires
a b s t r a c t The effect of short-time annealing treatment for 3, 9 and 18 s at 700 °C on the superelastic behavior of cold drawn Ni-rich NiTi wires was studied. Superelastic properties of cold drawn and annealed samples, including upper and lower plateau stresses, residual strain and mechanical hysteresis, were specified comparatively. The results show that for all cold drawn wires, annealing at 700 °C for only 3 s leads to the best superelastic behavior. Microstructure investigations were performed using optical microscopy and X-ray diffractometry. It was demonstrated that annealing at 700 °C for 3 s leads to production of an ultra fine grained structure. Also increasing the annealing time, only about a few seconds, results in occurrence of grain growth phenomenon which causes a sharp drop in superelastic behavior. Transformation behavior of the samples was specified by differential scanning calorimetry and R-phase was detected in ultra fine grained materials. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Shape memory alloys (SMAs) are the materials that possess two unique properties named superelasticity (SE) and shape memory effect (SME) [1]. They both are based on a reversible thermo-elastic phase transformation from a high temperature B2 austenite phase to a low temperature B190 martensite phase on cooling and reverse transformation on heating [2]. In addition to mentioned characteristics, SMAs have high corrosion resistance, favorable biocompatibility and high intrinsic damping capacity [3–6]. Therefore, they have been employed in many applications, especially medical usages including orthodontic wires and drills, cardiovascular guidewires, implants and self-expanding stents [7–10]. Additional improvements of the properties of SMAs can be accomplished by specific thermomechanical treatments like cold working and subsequent annealing [11]. Cold working improves the superelastic behavior with increasing the critical stress for dislocations slip relative to critical stress for twinning mechanism [12]. Besides, NiTi shape memory alloys are prone to be amorphous by severe plastic deformation [13]. Koike et al. [14] reported that amorphization phenomenon will take place in NiTi alloys, if the density of dislocations induced by plastic deformation exceeds an estimated amount of 1013–1014 cm2. Post deformation annealing will result in the
⇑ Corresponding author. Address: Faculty of Materials Engineering, Sahand University of Technology, P.O. Box 51335-11996, Tabriz, Iran. Tel.: +98 412 3459454; fax: +98 412 3444333. E-mail address: allafi@sut.ac.ir (J. Khalil-Allafi). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.183
formation of an ultra fine grained structure (UFG) if the optimum thermomechanical course is selected [15]. UFG shape memory alloys have superior properties over their coarse grained counterparts involving favorable ductility, high strength and upper level of recovery strains and stresses up to 10% and 1.5 GPa respectively [16,17]. Numerous investigations have been devoted to various SPD methods, most remarkably to high pressure torsion (HPT), equal-channel angular pressing (ECAP) and repeated cold rolling [18–20]. However the aforesaid techniques are not feasible for the wires. Thus, cold drawing is an attractive method to obtain thin wires with improved shape memory and superelastic properties depending on the selection of an appropriate thermomechanical regime. In the present work the influence of a novel annealing treatment i.e. a short-time annealing process at 700 °C on the superelastic response of cold drawn NiTi wires was studied. 2. Experimental works NiTi wires with nominal composition of Ti-50.9 at.% Ni (0.34 mm), provided by Memry corporation of the United States, was used in the present study. They were annealed at 850 °C for 1 h and quenched into room temperature water. Regarding the severe influence of oxygen on this solution annealing treatment, the wires were placed in a quartz tube filled with titanium powders and a high purity argon gas. After annealing, no remarked variation was observed in the diameter and appearance of the wires. However, the samples were chemically etched out to reduce the oxygen-affected layer after annealing. The solution annealed wires were cold drawn to a reduction area of 46–71% corresponding to a true strain of 0.6–1.2 using poly crystalline diamond dies. The number of reduction passes ranged from three for e = 0.6 to six for e = 1.2 with approximately 0.2 reduction per each step. Drawing process was performed at a speed of 0.6 m/min, without intermediate annealing. Cold drawn wires were annealed at 700 °C in an air furnace. For each reduction,
F. Khaleghi et al. / Journal of Alloys and Compounds 554 (2013) 32–38
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3. Results and discussions
Fig. 1. Optical micrographs of the (a) solution annealed wire and as-drawn wires with logarithmic strains of (b) e = 0.6, (c) e = 0.8, (d) e = 1.0 and (e) e = 1.2.
one sample was maintained for 3 s, one for 9 s and one for 18 s in a vertical furnace with a small hole in its port. After annealing the samples appeared in an azure blue color while they had a bright gray surface before that. In order to minimize the effect of oxidation, the wires were chemically etched until their diameters were reduced up to 50 lm. Microstructure studies of the wires were carried out using optical microscopy (OM, Olympus PMG3). Transformation behavior of the samples was investigated by differential scanning calorimetry (DSC, Netzsch DSC 404C). The samples were subjected to a cooling/heating cycle in the temperature range of 100 to +100 °C with a rate of 10 °C/min after heating up to +100 °C. DSC measurements for determining the microstructure evolutions of as-drawn wires after cold working were performed between 25 and 650 °C with a rate of 10 °C. Crystallite size and crystal structure of the samples were determined by X-ray diffractometry (XRD, Siemens D 5000) using CuKa radiation (k = 0.1541 nm) operated at 40 kV/30 mA. The superelastic behavior of the wires was studied using an Adamel Lohmargy DY26 tensile test instrument equipped with an electrical oven with accuracy of ±0.5 °C and a 100-kg load cell. The tests were conducted at 37 °C and a cross-head speed of 0.1 mm/min in a loading and unloading course up to 6.2% strain. Overall length of the wires between the grasps was selected to be 40 ± 0.02 mm. Diameters of the samples were measured using a digital micrometer with accuracy of 1 lm.
Fig. 1 shows the optical micrographs of the wires, before and after cold drawing. As can be seen, the microstructure of solution annealed sample (Fig. 1a) contains equiaxed grains while after cold working (Fig. 1b–e) the grains are drawn in the drawing direction. Stress–strain curves of solution annealed and as-drawn samples are presented in Fig. 2. It is observed that the as-drawn sample with logarithmic strain of e = 0.6 displays a linear superelastic behavior which is specified by the absence of stress plateau in stress–strain curve [21]. The recovery strain of the sample is equal to about 4.8% which is a remarkable value and much larger than that obtained for solution annealed sample (1.2%). The cause of strain recovery, of the as-cold worked samples, has been attributed to reversible movement of (0 0 1) microtwins inside h0 1 1i type II twin lamella of B190 by Zheng et al. [22]. Further increase in the level of cold work from e = 0.8 up to e = 1.2 causes the failure of the wires in the strains less than 6.2%. Fig. 3 illustrates DSC profiles of as-drawn wires with logarithmic strains of e = 0.6 and e = 1.2. For the sample with the plastic deformation of e = 0.6, no B2 ? B190 peak is visible in the cooling cycle which is related to the peak widening due to introduction of lattice defects as a result of cold working. However, during the heating cycle one peak could be detected that would be due to reverse transformation of B190 ? B2. Peak occurrence during the heating cycle in the temperature range of 25 °C up to 0 °C indicates that the cold drawn sample with logarithmic strain of e = 0.6 is not fully martensitic at room temperature. Increasing the level of cold work up to e = 1.2 leads to annihilation of detected peak in sample with e = 0.6 that is due to peak broadening during both cooling and heating regimes or martensite stabilization because of severe plastic deformation. The presence of stabilized martensite in a Ti-50.8Ni wire, cold drawn to an area reduction of 45 ± 5%, has been evidenced by X-ray synchrotron measurements and TEM observations in [23,24]. Fig. 4 demonstrates X-ray diffraction patterns of NiTi wires in the solution annealed state and after cold drawing with logarithmic strains of e = 0.6 and e = 1.2. It can be seen that after cold drawing, some peaks are revealed that are not related to B2 structure. Newly revealed peaks are related to the B190 phase. However they are very broad and have very low intensities compared to (1 1 0) B2 peak of solution annealed sample. Because of low intensity, broad peak area and the probability of peak shift due to cold work, the newly revealed peaks are not indexed strictly. Since peak broadening takes place due to high density of dislocations [25], cold drawing has been caused to introduction of a dislocation stabilized martensitic structure. Peak broadening would also be related to the occurrence of amorphizaion as a result of subjecting the alloy to severe plastic deformation. The amorphization phenomenon
Fig. 2. Stress–strain curves of (a) solution annealed and (b) as-drawn specimens.
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Fig. 3. DSC profiles of as-drawn wires with logarithmic strains of (a) e = 0.6 and (b) e = 1.2.
Fig. 4. X-ray diffraction patterns of NiTi wires in the solution annealed state and after cold drawing with logarithmic strains of e = 0.6 and e = 1.2.
Fig. 5. DSC profiles for as-drawn wires with logarithmic strains of e = 0.6 up to e = 1.2.
has been reported by Tsuchiya et al. [26] in the Ti-50.9Ni wires, cold drawn up to e = 1.2. Therefore it can be proposed that in the cold worked samples of this study, the microstructure contains both deformation stabilized B190 phase and partially amorphous areas. DSC profiles recorded for investigation of microstructure evolutions after cold working is demonstrated in Fig. 5. For all specimens, there is an exothermic peak observed in the temperature range of 300–400 °C. It seems that the peak correlates with the devitrification of the amorphous phase which has been formed during cold-drawing. It also should be noted that in the samples with logarithmic strains of e = 0.6 up to e = 1.0, another exothermic peak is appeared in the temperature range of 500–600 °C which are marked with arrows in Fig. 5. The appearance of the second peak can be attributed to the nucleation of new recrystallized grains in the areas with high dislocation densities. Increasing the driving force for the recrytallization phenomenon with increasing the level of plastic deformation leads to the decrease in the onset temperature of the peak. Surprisingly, the crystallization peak is not observed in the DSC profile of the sample deformed up to e = 1.2. Therefore it can be proposed that the recrystallization has been completed dynamically on cold drawing process due to applying high level of strains. Another prospect is the merging of the devitrification and recrystallization peaks due to the decrease in the onset temperature of the latter one. However, the microstructures of the as-drawn samples are possibly a mixture of dislocation substructure, amorphous phase and nanocrystalline B190 phase. Brailovski et al. [27] has observed the above mentioned structure in NiTi alloy subjected to cold rolling up to e = 1.0 by TEM. Fig. 6 shows the optical micrographs of the samples with logarithmic strains of e = 0.6 and e = 1.2, annealed at 700 °C for 3 and 18 s. It is observed that the microstructure of the samples, annealed for 3 s is analogous to those before annealing (Fig. 1b and e). The grains are as fine that they are not observable by optical microscopy. However, increasing the annealing time up to 18 s, results in emerging very fine grains in both samples. Thus, it is found out that the rate of the microstructure evolutions is very high and the new recrystallized grains appear only in a few seconds. Very fast nature of microstructure evolutions which transform the highly cold-worked microstructure into an annealed microstructure during annealing treatment for very short times has also been shown by Delville et al. [24]. Fig. 7 exhibits X-ray diffraction patterns of the samples with logarithmic strains of e = 0.6 and e = 1.2, annealed at 700 °C for 3 and 18 s. It can be seen that the characteristic peaks of the B2 phase exist in the X-ray diffractograms. Crystallite size of the samples was estimated by Williamson–Hall equation. They were 110 nm and 981 nm for the samples deformed up to e = 0.6, annealed for 3 and 18 s
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Fig. 6. Optical micrographs of the samples with logarithmic strains of e = 0.6 annealed at 700 °C for (a) 3 s, (b) 9 s and (c) 18 s and e = 1.2 annealed at 700 °C for (a) 3 s, (b) 9 s and (c) 18 s.
Fig. 7. X-ray diffraction patterns of the samples with logarithmic strains of e = 0.6 annealed at 700 °C for (a) 3 s and (b) 18 s and e = 1.2 annealed at 700 °C for (c) 3 s and (d) 18 s.
subsequently and 70 nm and 1387 nm for the samples deformed up to e = 1.2, annealed for 3 and 18 s subsequently. The determined values for samples annealed for 18 s are in good agreement with the optical microscopy observations. DSC profiles of the samples with logarithmic strains of e = 0.6 and e = 1.2 which have been annealed at 700 °C for 3, 9 and 18 s, are presented in Fig. 8. For both e = 0.6 and e = 1.2, the samples annealed for 3 s yield a B2 ? R transformation peak in the cooling cycle. It is known that transformation behavior of nanocrytalline NiTi alloys intensively depends on the grain size. The nanograins with the size of less than 150 nm yield two-step martensitic transformation (B2 ? R ? B190 ) [28]. So, it seems that the appearance of the R-phase peak could be related to the presence of the nanograins with the size
of less than 150 nm or high density of dislocations. Grain growth progress and decreasing the dislocation density with increasing the annealing time up to 18 s, result in the elimination of R-phase peak in the DSC curves. Increasing the annealing time also leads to peak sharpening. It indicates that the major part of the cold work has been annealed. Fig. 9 demonstrates stress–strain curves of all cold drawn wires that have been annealed at 700 °C for 3, 9 and 18 s. Upper plateau stress, lower plateau stress, mechanical hysteresis and residual strain alterations with annealing time, are given in Fig. 10. It is observed that for all cold drawn wires, annealing for 3 s leads to the very low residual strains with high stress value of plateaues. Increasing the annealing time reduces the amount of recovered strain. It is known that post deformation annealing
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Fig. 8. DSC profiles of the samples with logarithmic strains of e = 0.6 annealed at 700 °C for (a) 3 s,(b) 9 s, (c) 18 s and e = 1.2 annealed at 700 °C for (d) 3 s, (e) 9 s and (f) 18 s.
Fig. 9. Stress–strain curves of all cold drawn wires which have been annealed at 700 °C for 3, 9 and 18 s.
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Fig. 10. (a) Upper plateau stress, (b) lower plateau stress, (c) mechanical hysteresis and (d) residual strain alterations with annealing time for all cold drawn samples.
results in the occurrence of following processes: (i) recovery, polygonization and recrystallization in dislocation substructure, (ii) crystallization of the amorphous structure and (iii) the coarsening of the nanocrystalline structure, induced either directly from cold work or from crystallized amorphous structure [29]. Interaction of the above mentioned phenomena affects the superelastic behavior of the specimens, annealed at various conditions. For the samples deformed up to e = 0.6 and e = 0.8, the superelasticity parameters vary in a similar manner. Increasing the annealing time from 3 to 9 s causes a sharp increase in residual strain and mechanical hysteresis and a sharp decrease in upper and lower plateau stresses which are related to grain growth and recovery processes. Mechanical hysteresis is resulted from a friction energy which originates from the austenite/martensite interface motion [28]. It has been evidenced that dislocations assist in the formation of martensite from austenite [30]. Thus decreasing the density of dislocations with the continuance of the recovery process increases the mechanical hysteresis. Residual strain and mechanical hysteresis do not differ considerably with increasing the annealing time from 9 to 18 s because of the completeness of the recovery process within 9 s. In addition, smooth reduction of upper plateau stress from 9 to 18 s introduces the possibility of initiating the recrystallization process in the dislocation substructure. Changes of superelasticity parameters with annealing time for the sample, deformed up to e = 1.0 is similar to the samples e = 0.6 and e = 0.8 except the mechanical hysteresis which does not vary tangibly with annealing time. It might be considered that the recovery process has been finished within 3 s and continuation of annealing procedure causes the occurrence of recrystallization and grain growth phenomena. The superelastic behavior of the e = 1.2 samples, annealed at 700 °C for 3, 9 and 18 s is some deal different from the previous samples. Contrary to the samples with e = 0.6 up to e = 1.0, variation of upper plateau stress and residual strain with increasing the annealing time from 9 to 18 s is remarkable. This behavior proposes that the recrystallization process has been completed within 3 s. Thus, elevating the annealing time leads to the grain growth phenomenon.
One can observe that increasing the level of cold work will be effective on improving the superelastic behavior if the annealing time is selected not more than 3 s. For the samples which have been annealed for 3 s, increasing the level of plastic deformation leads to a steady increase in upper plateau stress and mechanical hysteresis. Raising the upper plateau stress with increasing the strain hardening is because of the grain refinement which increases the yield stress of the austenite. The reason for monotonous intensifying the mechanical hysteresis can be related to increasing the ratio of volume fraction of recrystallized grains to dislocations network with increasing the level of cold work.
4. Conclusion In the present assay, we studied the effect of short-time annealing treatment at 700 °C on the superelastic behavior of cold drawn wires and obtained the following conclusions: (i) Annealing of the cold drawn Ni-rich NiTi wires with logarithmic strains of e = 0.6–1.2 at 700 °C for 3 s results in formation of an ultra fine grained structure. Increasing the annealing time to 18 s leads to the occurrence of grain growth to the extent that the grains get observable with optical microscopy. (ii) Comparative study between the superelastic properties of the samples which have been subjected to different thermomechanical courses shows that for all cold drawn samples annealing for 3 s results in the best superelastic behavior. Furthermore, increasing the level of cold work improves the superelastic properties of the wires, annealed for 3 s. (iii) All the treated samples of the present study displays much larger recovered strains compared to the solution annealed wire. It shows that despite the high rate of microstructure evolutions at 700 °C, elimination of the entire cold work needs more annealing times.
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