Journal of Science: Advanced Materials and Devices (2019) 568e576 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article The effects of shape-setting on transformation temperatures of pseudoelastic shape memory alloy springs Farideh Jahanbazi Asl a, Mahmoud Kadkhodaei a, *, Fathallah Karimzadeh b a b Department of Mechanical Engineering, Isfahan University of Technology, Isfahan, 84156-83111, Iran Department of Materials Engineering, Isfahan University of Technology, Isfahan, 84156-83111, Iran a r t i c l e i n f o a b s t r a c t Article history: Received 13 April 2019 Received in revised form 21 October 2019 Accepted 24 October 2019 Available online November 2019 Since shape memory alloy (SMA) wires can hardly ever be reliably employed under compressive loadings, SMA springs are developed as axial actuators with the ability of withstanding both tension and compression In this paper, shape memory alloy helical springs are produced by shape-setting two types of wires: one with shape memory effect (SME) and the other with pseudoelasticity (PE) at the ambient temperature Phase transformation temperatures of the produced springs are measured by differential scanning calorimetry (DSC), and the influences of effective parameters including cold work, heat treatment temperature and duration, and cooling rate are investigated on transformation temperatures of the products The results show that phase transition temperatures of the fabricated springs can be tuned by performing cold work and by adjusting temperature and duration of the conducted heat treatment as well as the subsequent cooling rate It is found that transformation temperatures of the springs fabricated using the SME wire increase as the heat treatment temperature increases However, for samples manufactured using PE wire, transformation temperatures first increase and then decrease with the increase in the heat treatment temperature An increase in the cooling rate leads to a decrease in the austenite final temperature (Af), and an increase in the extent of cold work leads to the increase in transformation temperatures especially Af © 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Shape memory alloy SMA Transformation temperature Heat treatment Shape-setting Introduction Shape memory alloys (SMAs) are a class of smart materials which exhibit two extraordinary behaviors of shape memory effect (SME) and pseudoelasticity (PE) (or superelasticity) owing to thermoelastic martensitic phase transformation between the two phases of martensite and austenite The so-called shape memory effect is the thermally-driven recovery of the initial configuration of an SMA when residual strains appear after an inelastic loading/ unloading cycle In PE, recovery occurs spontaneously upon unloading once a large deformation is induced during inelastic loadings [1] Shape memory alloys with several shapes have been widely employed in various fields, such as robotics, biomechanics and microelectromechanical systems [2] In particular, beside SMA wires, spring actuators are vastly used owing to their simplicity of * Corresponding author E-mail addresses: fj.eng88@yahoo.com (F Jahanbazi Asl), kadkhodaei@cc.iut.ac ir (M Kadkhodaei), karimzadeh_f@cc.iut.ac.ir (F Karimzadeh) Peer review under responsibility of Vietnam National University, Hanoi fabrication [3] compared to other shapes such as tubes Moreover, in contrary to wires, helical springs can be subjected to both tension and compression Many studies have been so far accomplished to investigate the behaviors of SMA springs both theatrically and experimentally In theoretical modeling of SMA springs, the main goal is to predict force-displacement response of a helical spring Toi et al [4] presented a finite element formulation for analysis of superelasticity in SMA springs using linear Timoshenko beam elements Aguiar et al [5] proposed a numerical method based on the operator split technique in this regard Mirzaeifar et al [6,7] studied pure torsion of SMA bars with circular cross sections to investigate the pseudoelastic response of SMA helical springs under an axial force Heidari et al [8] proposed an enhanced one-dimensional constitutive model to describe the shear stressestrain response within the coils of an SMA spring Their model was based on the von-Mises effective stress and strain, and it was further extended to take large deformations into account [9] To fabricate SMA elements with a desirable shape, specific thermomechanical treatments called “shape-setting” are required https://doi.org/10.1016/j.jsamd.2019.10.005 2468-2179/© 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/) F Jahanbazi Asl et al / Journal of Science: Advanced Materials and Devices (2019) 568e576 [10] Shape-setting mainly includes annealing but may be accompanied by cold work and quenching on the products Although several microstructural analyses have been carried out [11e16] to investigate the detailed mechanisms of the various stages in shapesetting, macroscopic studies have gained a great attention in order to directly observe the influences of each stage on the phase transition and other features of an SMA specimen after shape-setting Wang et al [17] investigated the effect of annealing on the transformation characteristics of TiNi shape memory alloys by differential scanning calorimetry (DSC) Their results showed that the Rphase transformation appeared at low annealing temperatures The R-phase disappeared and austenite directly transformed to martensite as the annealing temperature exceeded 550 C When the R-phase appends to austenite and martensite, several thermomechanical behaviors of an SMA may be affected [18e21,22,23] Yeung et al [24] found that phase transition temperatures can be manipulated by adjusting the heat treatment parameters including duration, temperature and cooling rate Liu et al [25] showed that the austenite final temperature (Af) mostly increases after ageing Eggeler et al [26] realized that, in general, the transformation temperatures increase as the aging time increases Sadiq et al [27] investigated the effect of annealing temperature on the transformation temperatures They found that, after annealing the NiTi alloy above the recrystallization temperature, the R-phase disappears so that a direct transformation from austenite to martensite takes place The transformation temperatures increase as the annealing temperature increases; however, the R-phase transforms at lower temperatures Wang et al [28] investigated the microstructure, martensitic transformation, shape memory effect and superelastic property of the Ti49.6Ni45.1Cu5Cr0.3 alloy They concluded that, in the course of elevating the annealing temperature, the transformation temperatures increased for annealing temperatures in the range of 623 Ke873 K Then, they decreased for the temperature range of 873 Ke1023 K and did not considerably vary after 1023 K until 1273 K Heidari et al [8] fabricated SMA helical springs by shape-setting the NiTi wires and evaluated the thermomechanical characteristics of their products They found that transformation temperatures of the fabricated springs increase as the annealing temperature increases; however, the start and final temperatures of the R-phase transition decrease so that the Rphase eventually disappears at the annealing temperature of 600 C Motemani et al [29] studied the effect of cooling rate on the phase transformation and mechanical properties of Ni-rich NiTi shape memory alloy They realized that the austenite final temperature Af increases as the cooling rate decreases while the martensite final temperature Mf is reduced Consequently, a furnace-cooled sample has the highest phase transformation hysteresis (AfeMf) compared to samples with lower cooling rates Mitwally and Farag [30] studied the effect of cold work on the superelasticity, the shape memory effect and the hardness of NiTi alloy They found that the shape memory recovery is diminished by cold rolling as well as bending Lin and Wu [31] observed that, by augmenting the extent of applied cold work, the products would show lower residual strains upon unloading Grossmann et al [32] proposed a procedure to fabricate an SMA spring and to characterize its microstructural evolution during the production Costanza et al [33] investigated the shape memory effect in NieTi springs and presented a technique to design a linear actuator made of SMAs Follador et al [3] fabricated an SMA spring actuator They further described the mechanical characteristics of the SMA spring by a simple linear-elastic model whose parameters depend on the crystalline characteristics In most of the available works on fabrication of SMA springs, the products are martensitic at the ambient temperature Moreover, 569 the influences of all shape-setting parameters on transformation temperatures of the fabricated springs are not thoroughly investigated Since pseudoelastic SMA springs at the ambient temperatures are vastly required, in this paper, the main purpose is to fabricate austenitic springs using shape memory alloy wires Two types of NiTi (Ti-55.87 at % Ni) wires (one of which is martensitic at ambient temperature and the other is austenitic) are utilized The wires are wound and locked on a screw and are then heat treated at temperatures within the range of 300e1000 C for 5e1260 The effects of various production parameters such as cold work, heat treatment temperature and duration, and the subsequent cooling rate on transformation temperatures of the fabricated springs are further investigated Materials and methods The present investigation is carried out on commercial SMA wires provided by Memry Co with the nominal composition of Ti55.87% Ni and the diameter of 1.5 mm Two types of wires, one with shape memory effect (SME) and the other with pseudoelasticity (PE) at the ambient temperature, were utilized Fig and Fig illustrate results of differential scanning calorimetry (DSC), and Table shows the transformation temperatures of these wires To fabricate springs using SME and PE wires, according to Fig 3, each wire was first wound and locked on a screw Once the temperature of the furnace has reached a desired number, the specimen was placed in the furnace for the required time interval Then, the specimen was cooled down To prevent oxidation of the specimens, the whole wound wire together with the fastening components were covered by a stainless steel foil Heat treatments were performed on the fabricated springs at different conditions so that various combinations of the influential parameters were examined For instance, the springs S-11, S-12 and S-13 were made using SME wire by heat treating at 750 C for 10 min; the samples S-11 and S13 were quenched in a water-ice bath, but S-12 was quenched in a dry-ice bath Cold work was performed on S-13, but specimens S-11 and S-12 were fabricated with no cold work Sample S-14 indicates the SME wire which was heat treated at 1000 C for 10 and finally was quenched in the water-ice bath For fabricating the spring S-15, sample S-14 was first wound and locked on the screw; then, it was heat treated at 1000 C for 10 Eventually, quenching in the water-ice bath was performed This procedure of heat treatment followed by quenching was repeated to fabricate spring S-16 from S-15 A summary of heat treatment temperatures, duration, and cooling method is given in Appendix DSC tests were used to determine the transformation temperatures of the products DSC samples were cut into 30e50 mg in mass, and the tests were performed at the heating/cooling rate of C/min between À50 C and ỵ150 C To induce a thermal shock in order to reduce the transformation temperatures of the samples S-13 and S-16, heat treatment without using stainless steel foil was carried out in three stages as follows: Heat treatment for the initial wire at the required temperature followed by quenching in the water-ice bath; Wounding the heat-treated wire on the screw followed by quenching in the water-ice bath; Heat treatment of the fabricated spring followed by quenching in the water-ice bath Additionally, cold work was carried out on some of the samples in order to obtain desirable transformation temperatures To this end, cold rolling was used to reduce the thickness by the amounts of 3.5% and 32.1% A wide range of temperatures from 300 C to 1000 C was chosen for the shape-setting; however, in order to 570 F Jahanbazi Asl et al / Journal of Science: Advanced Materials and Devices (2019) 568e576 Fig Transformation temperatures for the SME wire Fig Transformation temperatures for the PE wire Table Transformation temperatures of the utilized wires Initial wire Af ( C) As ( C) Rs ( C) Rf ( C) Ms ( C) Mf ( C) SME wire PE wire 64.00 15.00 43.63 À5.00 50.00 17.30 28.00 À10.00 8.00 À45.00 À26.00 À F Jahanbazi Asl et al / Journal of Science: Advanced Materials and Devices (2019) 568e576 Fig Constrained SMA wire on a screw prevent oxidation of the samples, heat treatment at lower temperatures is more preferred Results and discussion In this section, the effects of temperature and duration of the heat treatments, cooling rate, and cold work on the transformation temperatures of the products are investigated Transformation temperatures of the products are given in Appendix Since the utilized instrument was limited to the minimum temperature of À50 C, determination of Mf and Ms for some specimens was not possible Fig shows one of these cases for the sample S-09 R-phase transition occurs for most of the specimens (for instance, in Fig where the DSC curve of sample S-13 is shown) while it is not seen for some specimens (for instance, in sample S-24 whose DSC curve is shown in Fig 6) The samples S-18, S-19, S-20, S-21, S-22 and 23 were pseudoelastic springs at the room temperature while the other ones showed shape memory effect Fig illustrates one of the fabricated pseudoelastic springs from two different views As shown in Appendix 2, the austenitic transformation temperatures of the specimens S-18, S-19, S-20 and S-22 are lower than the ambient temperature It should be noted that sample S21 is similar to S-22 except the difference that sample S-21 was placed into a quartz capsule under vacuum to prevent from oxidation as much 571 as possible The specimen S-23 was heat treated to find the lowest applicable temperature for achieving a pseudoelastic spring Accordingly, 630 C was the minimum temperature to optimize the process of fabricating a pseudoelastic spring with the PE wire Samples S-21 and S-23 were mechanically shown to be pseudoelastic: they were examined under inelastic stretch followed by inelastic compression and could return to their original configuration after unloading The samples S-24, S-25 and S-26 are 32.1% cold-worked SME wires As formerly indicated, in some specimens, the reverse transformation (during the heating cycle) occurs in two stages: from martensite to an intermediate R-phase and then to austenite Similarly, for some samples, the forward transformation (during the cooling cycle) occurs in two stages: from austenite (parent phase) to R-phase and then to martensite Table shows transformation temperatures of R-phase during the heating cycle for such specimens Since pseudoelastic springs are planned to be manufactured in the present work, Af of the products should be lower than the ambient temperature Therefore, the effects of the various parameters on the austenite transformation temperatures are investigated in the following subsections 3.1 Influence of heat treatment on the transformation temperatures Heat treatment parameters including temperature, cooling rate, and the duration affect transformation temperatures of the fabricated springs Here, the findings of this work are presented 3.1.1 Effect of the heat treatment temperature Fig shows the influence of the heat treatment temperature on the austenite transformation temperatures of the springs fabricated by using the SME wire It is seen that transformation temperatures increase with the increment in the heat treatment temperature Moreover, for the manufactured spring at the Fig Transformation temperatures for sample S-09 572 F Jahanbazi Asl et al / Journal of Science: Advanced Materials and Devices (2019) 568e576 Fig Transformation temperatures for sample S-13 Fig Transformation temperatures for sample S-24 minimum temperature of 750 C, As and Af are higher than the ambient temperature The rapid growth of Af with the increase in the heat treatment temperature indicates that the fabrication of a pseudoelastic spring with an SME wire is impossible at these conditions Variationsof the transformation temperatures for thesprings made from PE wire with the heat treatment temperature are shown in Fig It is observed that the transformation temperatures strongly depend on the heat treatment temperature According to this figure, there are three trends for the variations in the transformation temperatures: Transformation temperatures are enhanced by elevating the heat treatment temperature until 500 C Transformation temperatures, except Ms, are reduced by increasing the heat treatment temperature between 550 C to 650 C F Jahanbazi Asl et al / Journal of Science: Advanced Materials and Devices (2019) 568e576 Fig A fabricated pseudoelastic helical spring Table Transformation temperatures of Rhombohedral phase during heating cycle Specimen Code Rs ( C) Rf ( C) S-18 S-19 S-20 S-24 S-25 S-26 À7 À7 À10 55 53 52 66 68 66 Transformation temperatures except Rs and Rf are nearly constant with the increase in the heat treatment temperature beyond 650 C until 750 C In fact, with further increasing the heat treatment temperature, transformation temperatures remain fairly constant This is why heat treatment at temperatures above 650 C results in pseudoelastic products Fig Effect of the heat treatment temperature on the transformation temperatures for the spring made from SME wire 573 3.1.2 Effect of cooling rate According to the data provided in Appendix 2, the comparison between the transformation temperatures of specimens S-19 and S-22 and between those of S-10 to S-12 indicates that transformation temperatures of the products mostly decrease when quenching in the water-ice bath is done instead of cooling in the furnace for both series of springs made using PE and SME wires However, by comparing the samples S-11 and S-12, it is seen that Mf increases by increasing the cooling rate This trend coincides with the findings of investigations carried out by Motemani et al [29] on Ni-rich NieTi shape memory alloys In other words, the furnacecooled specimen has the highest phase transformation hysteresis (AfeMf) compared to the other samples The increase in transformation temperatures for the samples quenched in dry ice, compared to the samples quenched in water-ice, is due to the application of the steel foil to prevent oxidation of the parts When the sample is cooled in the water-ice bath, since the steel foil is not completely sealed, water may leak during the cooling time Consequently, cooling using dry ice is performed more gradually 3.1.3 Effect of the heat treatment duration Variations of the austenite transformation temperatures with the duration of the heat treatment for springs made from the SME wire are shown in Fig 10 As it is seen, As and Af first increase slightly with the increase in the heat treatment duration Then, by further prolonging the heat treatment, their increase is suppressed Fig 10 Effect of the heat treatment duration on the transformation temperatures of sample springs made from SME wire Fig Effect of the heat treatment temperature on the transformation temperatures of samples manufactured using PE wires 574 F Jahanbazi Asl et al / Journal of Science: Advanced Materials and Devices (2019) 568e576 Fig 11 Effect of heat treatment duration on the transformation temperatures of springs made from PE wire so that As is reduced a little Fig 11 shows the effect of the heat treatment time on the transformation temperatures for springs made from the PE wire It is seen that the increase in duration of the heat treatment leads to a very negligible rise in As and Af compared to the observed increase for the springs made of the SME wire Moreover, the effect of the heat treatment duration on transformation temperatures is less than that of the other production parameters 3.2 Effect of cold work Using a flat rolling machine, the SME wire with the initial diameter of 1.5 mm was cold-rolled to achieve a 0.75 mm-thick strip The cold-rolled specimen was then further wound and locked on a screw Then, the prepared spring was heat treated at 750 C for 10 followed by quenching in the water-ice bath Specimen S-13 was manufactured with the use of a 3.5% cold-worked SME wire According to Appendix 2, comparison of transformation temperatures for samples S-11 and S-13 indicates that the SME wire becomes austenitic at the room temperature when the cold work percentage increases to 32.1% Moreover, transformation temperatures of 32.1% cold-worked wires S-24 to S-26 are generally higher than those of the as-received wire This is in agreement with the findings reported by Mitwally and Farag [30] According to the findings of this work, the effects of the various shape-setting stages on the transformation temperatures for the fabricated samples using PE wire are schematically illustrated in Fig 12 By elevating the heat treatment temperature, the transformation temperatures increase and then decrease Consequently, in early stages of annealing when the temperature is not high enough, the possibility of achieving pseudoelastic parts may be lessen even with the use of initially austenitic wires In other words, if one needs to reduce the transformation temperatures after annealing, the set temperature is recommended to be chosen as high as possible A more pronounced cooling rate gives rise to the decrease in transformation temperatures; thus, severe cooling such as quenching is beneficial when pseudoelastic products are desirable Cold work on the specimens leads to the rise in transformation temperatures; therefore, such processes may be useful in achieving martensitic parts and are not recommended when austenitic products are desired Conclusion In this paper, pseudoelastic helical springs were manufactured by shape-setting two types of NiTi wires: one was martensitic at the ambient temperature and the other was austenitic The effects of the various stages such as cold work, heat treatment temperature and duration, and the subsequent cooling rate on the transformation temperatures of the products were investigated The main results obtained in this research can be summarized as follows: Heat treatment duration has the less effect on the transformation temperatures than the other adjustments of the shapesetting process Transformation temperatures of the springs fabricated using the SME wire increase as the heat treatment temperature increases However, for samples manufactured using PE wire, transformation temperatures first increase and then decrease with the increase in the heat treatment temperature The increase in cooling rate results in a decrease in Af which is beneficial to fabricate PE springs at the ambient temperature Cold work leads to an increase in transformation temperatures, especially Af Depending on the desired type of products with either the pseudoelasticity or the shape memory effect, the presented findings regarding the effects of the various shape-setting stages on transformation temperatures of an SMA spring can be considered as guidelines for fabricating SMA springs with a predefined phase (austenite or martensite) at the ambient temperature Metallurgical investigations will improve the present understanding, and such studies are under way In addition to the procedures used in the current study, the various thermomechanical treatments cause a well-developed dislocation sub-structure or nanocrystalline structure and may be used for improving the properties of SMAs in future works Declaration of interests Fig 12 Schematic illustration of the effect of various parameters on transformation temperatures for springs sample made of PE wire The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper F Jahanbazi Asl et al / Journal of Science: Advanced Materials and Devices (2019) 568e576 575 Appendix Summary of Heat Transfer Conditions Specimen Code Material Code Heat Treatment Temperature ( C) Heat Treatment Duration (min) Cooling Method S-01 S-02 S-03 S-04 S-05 S-06 S-07 S-08 S-09 S-10 S-11 S-12 S-13 S-14 S-15 S-16 S-17 S-18 S-19 S-20 S-21 SME SME SME SME SME SME SME SME SME SME SME SME SME SME SME SME PE PE PE PE PE 300 300 300 300 300 300 400 868 868 750 750 750 750 1000 1000 1000 600 750 750 650 750 25 180 300 480 780 1260 780 10 10 10 10 10 10 10 10 10 1260 1260 10 10 10 S-22 S-23 S-24 S-25 S-26 PE PE SME SME SME 750 630 750 750 1000 10 10 10 10 10 Ambient Temperature Ambient Temperature Ambient Temperature Ambient Temperature Ambient Temperature Ambient Temperature Ambient Temperature Cooled in Furnace Quenched in water-ice bath Cooled in Furnace Quenched in water-ice bath Quenched in dry-ice bath Quenched in water-ice bath Quenched in water-ice bath Quenched in water-ice bath Quenched in water-ice bath Ambient Temperature Cooled in Furnace Cooled in Furnace Cooled in Furnace Quenched in water-ice bath (Quartz Capsule) Quenched in water-ice bath Quenched in water-ice bath Quenched in water-ice bath Quenched in liquid N2 bath Quenched in liquid N2 bath Appendix Transformation temperatures of selected specimens Specimen code Af ( C) As ( C) Rs ( C) Rf ( C) Ms ( C) Mf ( C) S-09 S-10 S-11 S-12 S-13 S-14 S-15 S-16 S-17 S-18 S-19 S-20 S-22 S-24 S-25 S-26 64 71 57.18 58 58 69.51 70 72.68 41 20 16 14 À7.5 77.5 77.5 77.5 44 47.5 32.50 37.5 28 45 45.38 32.50 17.5 10 7 À25 74 75 74 34 34 45 48 45 33 27.50 36 24 39 22 7.5 À28 À À À 10 20 22.5 20 À4 7.50 16 À7.5 À6 À1.5 À À À À À À 14 17.5 12.5 À À À À1 À31 À39 À36 À 42.5 42.5 42 À À À20 À17 À25 À À À À27.5 À À À À 28 27.5 22.5 References [1] S.H Youn, Y.S Jang, J.H Han, Development of a three-axis hybrid mesh isolator using the pseudoelasticity of a shape memory alloy, Smart Mater Struct 20 (7) (2011) 075017, https://doi.org/10.1088/0964-1726/20/7/ 075017 [2] K.K Alaneme, S Umar, Mechanical behaviour and damping properties of Ni modified CueZneAl shape memory 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Depending on the desired type of products with either the pseudoelasticity or the shape memory effect, the presented findings regarding the effects of the various shape- setting stages on transformation. .. compared to the observed increase for the springs made of the SME wire Moreover, the effect of the heat treatment duration on transformation temperatures is less than that of the other production parameters