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Forming and two-way shape memory effect of NiTi alloy induced by laser shock imprinting
Optics and Laser Technology 120 (2019) 105762
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Full length article
Forming and two-way shape memory effect of NiTi alloy induced by laser shock imprinting
T
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Xiong Feia, Yang Haifenga,b, , Liu Kuna, Man Jiaxianga, Chen Haoxuea a b
School of Mechatronic Engineering, China University of Mining and Technology, XuZhou 221116, China Jiangsu Key Laboratory of Mine Mechanical and Electrical Equipment, China University of Mining and Technology, XuZhou 221116, China
H I GH L IG H T S
shape memory effect (TWSME) of NiTi alloy was induced successfully. • AThetwo-way forming and TWSME were realized simultaneously at a micron scale. • Laserprecise shock imprinting can improve the mechanical properties of the formed part. • The TWSME by laser shock imprinting has a good thermal cycling stability. • Strengtheninginduced mechanism and thermally controllable microstructures were discussed. •
A R T I C LE I N FO
A B S T R A C T
Keywords: NiTi shape memory alloy Two-way shape memory effect Laser shock imprinting Mechanical properties Thermal cycling stability
Nickel–Titanium shape memory alloy (NiTi SMA) is an intelligent material that is a widely applied and studied. The two-way shape memory effect (TWSME) of the NiTi SMA has garnered particular attraction in the further miniaturisation and integration of micromechanical systems. To realise the micro-forming and TWSME of the NiTi SMAs at a micron scale, this study reports a method that involves laser shock imprinting (LSI) technology to induce the TWSME. The production efficiency of this method was significantly higher than that of the thermo–mechanical treatment; the proposed method integrated the induction and forming processes. In this study, the precise and rapid forming of microstructures was realised; meanwhile, the TWSME was induced successfully at the micron scale. Moreover, the experimental results indicated that this method can improve the mechanical properties of the formed part. The LSI-induced TWSME exhibited good thermal cycling stability during the thermal cycling attenuation test, the maximum phase transition angle of bending workpiece began to remain stable after the fourth thermal cycle, and the stability value was approximately 19.0% of the initial bending angle. Furthermore, the feasibility of the LSI-induced TWSME was proved by theoretical analysis, and its strengthening mechanism, phase transition process, and thermal cycling stability were discussed.
1. Introduction A Nickel–Titanium shape memory alloy (NiTi SMA) exhibits a phase transition between austenite and martensite at certain stresses or temperatures, and it has a shape memory effect (SME) with the ability of remembering the original shape of the SMA materials [1,2]. Currently, the NiTi SMA has become a popular intelligent materials for research and application [3,4]. This material has great development potential in thermally controllable surface morphologies, microstructures, and microactuators [5,6]. Further, it is widely used in the fields of microelectro-mechanical system (MEMS), microsensors, biomedicine, aviation, and civil engineering [4,7–9].
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It is well known that the spontaneous shape change of an SMA occurs only at heating, but thermo–mechanical treatment can be used to train the two-way shape memory effect (TWSME) and induce the strain variation, that is, the TWSME refers to the ability of SMA can reversibly restore the two shapes of high-temperature austenite (A) phase and low-temperature martensite (M) phase with the change in temperature (heating or cooling) [10,11]. The thermo–mechanical treatment has been widely investigated; previous studies have shown that pre-strain, training methods, training times, training temperatures, and alloy composition all affect the TWSME of the NiTi SMA during the thermo–mechanical treatment [12–14]. Further, the two significant parameters of the strain amplitude and stability were used to characterize
Corresponding author at: School of Mechatronic Engineering, China University of Mining and Technology, XuZhou 221116, China. E-mail address: [email protected] (H. Yang).
https://doi.org/10.1016/j.optlastec.2019.105762 Received 25 April 2019; Received in revised form 19 June 2019; Accepted 2 August 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
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the TWSME, which had been extensively studied by researchers. For instance, Hornbogen [15] mentioned that the TWSME strain would decrease during the thermal cycle, and the decrease continued until the TWSME disappeared completely. Wang [16] and Falvo [12] found that the strain amplitude of the TWSME for the NiTi SMA increased with the increase in the number of the thermal cycling training. Moreover, Falvo [12] also found that the strain amplitude decreased dramatically during the first 20 cycles of the subsequent thermal cycling attenuation test, and then stabilized gradually. So far, this method has become mature; however, this method still encounters the problems of a process complexity, long production cycle, and high cost. Recently, the quasistatic indentation method has been used widely in TWSME training. Although this method is simple and can induce a relatively stable TWSME, it still presents significant limitations on the micron and nano scales or thin film samples, and accurate formation of complex shapes is difficult [1,17]. The Cheng team of Purdue University used the methods of laser shock peening (LSP) [18] and laser shock indentation [6] to produce indentations on NiTi SMA sheets, thus inducing the TWSME at a smaller scale. These studies also proved that the TWSME induced by these methods has good strain amplitude and stability. However, these methods belong to the dynamic loading, which is different from the quasistatic indentation method, and a super-high strain rate could affect the phase transformation behavior of the NiTi SMA [19,20]. This conclusion had been confirmed by the research team of Wang [21,22], and they also found that the stress-induced martensite transformation takes place in the NiTi SMA processed by the laser shock processing. Laser shock imprinting (LSI), as the extensive technology of LSP and laser shock forming, belongs to a non-contact process and exhibits the advantages of simple processing, fast forming speed, and short production cycle [23,24]. Owing to the better controllability and smaller beam diameter of laser, LSI can localise the processing area at a micron scale, and realise the precise forming of complex shapes at the micron or even nano scales. Furthermore, LSI inherits some advantages of LSP, such that it can improve the mechanical properties of the target material effectively, such as corrosion resistance, hardness, and fatigue life [25,26]. Meanwhile, the improvement of mechanical properties of SMA is of great value in engineering applications, and Wang [21] and Chang [27] found that LSP also can improve the mechanical properties of SMA. Therefore, according to the LSI processing characteristics, this technique can be used to realise the TWSME training of the NiTi SMA and the precise processing of the thermally controllable surface morphologies, microstructures, and microactuators. In this study, in order to induce the TWSME of the NiTi SMA at the micron or even nano scales, achieve the synchronization of the microforming and TWSME, as well as obtain the TWSME with a good stability, we report a method that microstructures are fabricated on the surface of the NiTi SMA by the LSI technique. Firstly, the changes in the profile curve and the forming height were observed in the thermal cycle, the mechanical properties of the formed part were evaluated, and the TWSME stability of the bending workpiece was evaluated by a thermal cycling attenuation test. Further, the strengthening mechanism of the LSI, phase transition process, and thermal cycling stability of the TWSME induced by LSI were analysed. In summary, it is expected that this method would promote the development and application of thermally controllable surface morphologies.
Table 1 Composition of the NiTi SMA plate. Compositions
Ni
C
O+N
H
Ti
(wt.%)
56.85
0.007
0.041 + 0.0014 ≦ 0.045
0.0009
balance
Fig. 1. DSC curve of the NiTi SMA initial material.
ultrasonic cleaner in anhydrous alcohol. Before cleaning, the sample surface was ground using a 2000-grid sandpaper followed by final polishing with 0.05-μm diamond paste. The phase transition process and phase transition temperatures of the NiTi SMA initial material were measured using a differential scanning calorimeter (DSC), as shown in Fig. 1. The austenite start temperature (As) and austenite finish temperature (Af) are 296 K and 305 K, respectively; the martensite start temperature (Ms) and martensite finish temperature (Mf) are 278 K and 260 K, respectively. Before LSI, the samples were cooled to approximately 240 K and subsequently heated to 293 K to ensure these samples were in a fully martensitic condition. 2.2. Experiment methods Fig. 2 shows a schematic diagram of the LSI process, in which the pulsed laser, confinement layer, ablative layer, NiTi SMA sample, and mould are indispensable in these experiments. The focused pulse laser beam with a short pulse duration (tp) had a high power density (I0) in the LSI process. The laser beam passed through the transparent confinement layer and irradiated the ablation layer surface. Then, the ablation layer absorbed laser energy and rapidly gasified, and a large number of dense high-temperature (> 104 K) and high-pressure (> 1 GPa) plasmas formed almost simultaneously. Subsequently, the plasma continued to absorb laser energy such that it heated up and expanded quickly; a high-intensity shock wave was subsequently produced by the plasma explosion, and finally acted on the NiTi SMA surface. Additionally, owing to the confinement effect of the confinement layer for plasmas, the peak pressure and action time of the shock wave were increased. When the peak pressure of the shock wave was greater than the Hugoniot elastic limit (HEL) of plastic deformation for the NiTi SMA, the NiTi SMA sample formed according to the mould's shape, thereby resulting in residual stress in the formed parts after imprinting [28,29]. In this work, the LSI experiments were carried out at 293 K; a pulse laser (Hercules-1000-TH, wavelength 1064 nm, pulse duration 7 ns) was selected; a K9 glass of 3-mm thickness and black aluminium foil with a 45-μm thickness were used as the confinement layer and ablative layer, respectively. During this LSI process, the peak pressure of the shock wave could be adjusted by changing the spot diameter at the
2. Experiments and characterisation 2.1. Materials In this research, a 0.1-mm-thick 56.85 wt% Ni of the NiTi SMA plate was chosen, and its composition is listed in Table 1. The experimental samples with the separate dimensions of 4 mm × 4 mm and 4 mm × 0.2 mm were cut by pulsed laser cutting from the NiTi SMA plate, and the samples were subsequently cleaned and degreased by an 2
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Fig. 2. LSI process: (a) schematic diagram of the LSI process, (b) mould, (c) the formed parts and bending workpiece.
3. Results and discussions
Table 2 Significant experimental parameters of the laser. Dimension (mm)
Frequency (HZ)
Pulse energy (J)
Spot diameter (mm)
Power density (GW/cm2)
4 × 4 × 0.1 4 × 0.2 × 0.1
1 1
0.3 0.3
1 2.5
5.46 0.87
3.1. Results After LSI, regular circular microstructures (diameter is approximately 126 μm) were successfully copied on the NiTi SMA plate surface. These microstructures of the formed part were convex, and their shapes and sizes coincide with the corresponding holes on the surface of mould 1 (diameter 126.8 μm, depth 11.7 μm), as shown in Fig. 3. In addition, the forming accuracy and forming quality of the formed part were excellent, and these microstructures were uniform. The profile curve in Fig. 3(d) shows that the initial forming height of the circular convex microstructures was approximately 11.5 μm. Fig. 4 shows the DSC curve of the formed part fabricated by the LSI technique. After LSI, the phase transition process in the formed part had not changed during the thermal cycle, the phase transition in heating stage was still M → A, and that in cooling stage was still A → M. Further, the corresponding phase transition temperatures did not change significantly during the phase transition process, as listed in Table 3. It could be seen that the LSI has little effect on the overall phase transition process and phase transition temperatures of the NiTi SMA. Fig. 5 shows the variation in the section profile curve and the forming height for the circular convex microstructures on the formed part surface during a thermal cycle. As shown in Fig. 5(a), the microstructures’ shape remains unchanged throughout the thermal cycle, and the microstructures shrinks and extends separately at temperatures higher than Af or lower than Mf. Meanwhile, the height change of the convex microstructures was reversible, and this thermal cycle could be repeated many times. Therefore, we found that the microstructures imprinted by LSI exhibits a TWSME, that is, the height of the convex microstructures could be regulated by the change in temperature. The initial forming height of the circular convex microstructures was the largest after imprinting, i.e. approximately 11.5 μm, as illustrated in Fig. 5(b). During the thermal cycle, when the formed part was heated above Af, the section profile curve shrank in both the longitudinal and transverse directions, and the height of the circular convex microstructures decreased to 9.6 μm. The height change of 1.9 μm implied that the initial deformation recovered by 16.5% in the height (longitudinal) direction, as illustrated in Fig. 5(c). This height ratio represented the proportion of the non-plastic deformation (deformation caused by phase transition) of the circular convex microstructures in the height direction. Subsequently, the section profile curve extended in both the longitudinal and transverse directions at a temperature below Mf, and the forming height increased to 11.0 μm, as shown in Fig. 5(d). Consequently, the forming height change for the circular convex microstructures was 1.4 μm during the first thermal cycle, at approximately 12.2% of the initial forming height. This height ratio represented the proportion of the maximum phase transition height of the circular convex microstructures in the initial forming height during the
same single pulse energy to control the material deformation degree. Therefore, the formed part and the bending workpiece were manufactured by changing the mould and the spot diameter, respectively (see Fig. 2(b) and (c)), and the corresponding significant parameters are listed in Table 2. For guaranteeing the quality of the formed parts, improving its uniformity, and realising the repeatable operation of this experiment, the K9 glass, black aluminium foil, and NiTi SMA samples must be pressed firmly and adhered closely on the mould surface [30]. In the thermal cycling attenuation test, a complete thermal cycling process involves the bending workpiece imprinted by LSI to first increase from 293 K to 360 K, subsequently decrease to 240 K, and finally increase to 293 K again. To measure the stability of the TWSME induced by LSI, 10 thermal cycles were performed in this work. 2.3. Measurements and characterisation After the LSI process or the thermal cycle, we used a scanning electron microscope (SEM) (Quanta 250) to observe the microstructures’ morphologies on the formed part surface and measured their profile curves and forming heights via an optical profiler (DVM5000, Leica, Germany). The microhardness of the formed parts were determined by a microhardness tester (HVSA-1000A) with a 50-g load and a 15-s duration time. The deformation process and bending angle of the bending workpiece were determined by a universal material microscope (LEICA DM4 M) during the thermal cycle. For the measurements above, the average of three measurements was used as the final measured result to characterise the formed part and reduce measurement errors. In this Paper, the DSC curves of the initial material and formed parts were measured using a differential scanning calorimeter (DSC) (America TA DSC250) with a heating or cooling rate of 10 K/min and a sample mass of 30 mg. Furthermore, to evaluate the mechanical properties of the formed part, the load–displacement curves, nanohardness, and elastic modulus of the initial materials and the formed parts were measured by nanoindentation (Agilent U9820A, Nano Indenter G200) with a standard Berlcovich diamond indenter, and the corresponding contact stiffness between the samples and indenter were calculated from the load–displacement curve. Last but not least, the microhardness test and nanohardness test were carried out at 293 K. In addition, the formed part was firstly cooled to 240 K and subsequently heated to 293 K to ensure these were in a fully martensitic condition before the tests of the microhardness or nanohardness. 3
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Fig. 3. (a) the surface morphology, profile curve, and depth of the mould 1, (b) the SEM images (500×) of the formed part, (c) the SEM images (1500×) of the formed part, (d) the morphology, profile curve, and forming height of the formed part.
is 60 μm. As shown, the microhardness has improved after LSI, indicating that LSI has a certain strengthening effect on the NiTi SMA material. The microhardness was discrepant at different positions, its curve exhibited an upward trend as the measurement position gradually approached the convex microstructures, and the top of the convex microstructures was the highest microhardness values. The initial microhardness of the NiTi SMA material was 210 HV, while the average value of the microhardness for the convex microstructures was approximately 352 HV, which increased by 67.6%. After LSI, the minimum microhardness of the formed parts was 260 HV, implying that the monolithic microhardness of the formed parts was increased by approximately 23.8%. Table 4 shows the test results of the nanoindentation. The nanohardness of the convex microstructures was found to be the highest, followed by the flat region of the formed parts, and subsequently the initial material without imprinting. The test results were consistent with the microhardness, thus indicating that LSI technology can improve the deformation resistance of the formed parts. Compared with the nanohardness (2.7 GPa) of the initial material, the nanohardness (3.9 GPa) of the convex microstructures increased by 44.4%. In addition, the elastic modulus and contact stiffness demonstrated the same results, that is, the elastic modulus and contact stiffness of the convex microstructures were the largest and those of the initial material was the smallest. Compared with the elastic modulus (17.8 GPa) and contact stiffness (29.5 μN/nm) of the initial material, the elastic modulus (46.3 GPa) and contact stiffness (212.0 μN/nm) of the convex microstructures increased by 160.1% and 618.6%, respectively. This indicated that LSI technology can improve the elastic deformation resistance of the formed parts.
Fig. 4. DSC curve of the formed part. Table 3 Phase transition temperatures of the initial material and formed part. Sample
Ms (K)
Mf (K)
As (K)
Af (K)
Initial material Formed part
278 278
260 261
296 296
305 304
first thermal cycle. Fig. 6 shows the microhardness of the formed part and the initial material of the NiTi SMA. Further, the distance of each measuring point 4
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Fig. 5. Variation in the profile curve and the forming height for the circular convex microstructures during the thermal cycle.
To investigate the influence of the LSI on the mechanical properties of the formed part, the load–displacement curves were monitored during the whole nanoindentation process, as shown in Fig. 7. In the loading curve, the indentation load required by the convex microstructures was the largest at the same indentation depth, followed by the flat, and subsequently the initial material, thus proving that the deformation resistance of the formed parts is strengthened. According to Table 4, the maximum indentation load (25.1 mN) of the initial material was significantly lower than that of the flat (41.7 mN) and convex microstructures (85.4 mN) at a fixed indentation depth (1500 nm), which also showed that LSI technique can improve the alloy properties of the NiTi SMA. The NiTi SMA material was processed into a bending workpiece by LSI technology to obtain a larger deformation, as shown in Fig. 8(a) and (b). The bending workpiece also exhibited the TWSME and the same deformation effect as the formed parts during the thermal cycles, and the initial bending angle of the bending workpiece was 48.5°. When the temperature was heated to 360 K, the bending workpiece recovered, the bending angle decreased to 9.0°, and the recovery rate was approximately 81.4%, as illustrated in Fig. 8(c). The bending angle subsequently increased to 21.5° when the temperature was cooled to 240 K, and the change in the bending angle is 12.5°, at approximately 25.8% of the initial bending angle, as illustrated in Fig. 8(d). In this experiment, the thermal cycling attenuation test of the TWSME included the heating and cooling for the bending workpiece; subsequently, changes in the plastic bending angle (θpn) and the
Table 4 Nanoindentation test results and calculated values for the different positions. Positions
hmax (nm)
Fmax (mN)
H (GPa)
E (GPa)
K (μN/nm)
Initial Material Convex Flat
1500 1500 1500
25.1 85.4 41.7
2.7 3.9 3.7
17.8 46.3 26.9
29.5 212.0 56.8
maximum phase transition angle (θmaxn) were measured to characterise its thermal cycling stability performance, as shown in Fig. 9. As shown in Fig. 9(a), θ0 (θ0 = 48.5°) represents the initial bending angle; θ1n and θ2n represent the bending angle after heating and cooling during the nth thermal cycle, respectively. Fig. 9(b) shows a schematic diagram for the thermal cycling attenuation test of the TWSME, in which ABCDE is the first thermal cycle, and EFGHI is the second thermal cycle. As shown, the bending workpiece could not return to its initial position after the thermal cycle. Among them, the θpn and θmaxn at the nth thermal cycle could be calculated as follows:
θpn = θ0 − θ2n
(n ≧ 1)
θmaxn = θ2n − θ1n
ΔnTWSME =
(n ≧ 1)
θmax1 − θmaxn × 100% (n ≧ 2) θmax1
(1) (2)
(3)
where n is a positive integer, and it represents the nth thermal cycle;
Fig. 6. Microhardness at the different positions of the formed part. 5
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phase M (including the twin martensite and untwinned martensite), and R-phase (R) at different stresses and temperatures, and the SME of an SMA originates from the reversible strain variation induced by microcosmic phase transformation in SMA crystals, as shown in Fig. 11 [31,32]. Among them, phase A only exists stably in the high-temperature region; phase M only exists stably in the low-temperature region; R-phase often appears during the phase transformation process from the phase A to Phase M; the σs and σf are the start stress and finish stress of the martensite detwinning, respectively [32]. In this work, it can be seen that the phases A and phase M exist in the phase transformation process according to the DSC curves; however, the R-phase does not appear. During LSI, when the pressure generated by the shock wave exceeds the Hugoniot elastic limit for martensite detwinning (HELM) and plastic slip-deformation (HELP), the NiTi SMA creates the martensite detwinning (as shown in Fig. 12) and an obvious plastic deformation, respectively. In additions, the samples in a fully phase M state first creates an elastic deformation when the shock wave acts on the surface of NiTi SMA. Subsequently, when the shock wave pressure is greater than HELM, the deformation, reversed by heating, can be accommodated by the martensite detwinning. The plastic deformation is sequentially accommodated through dislocation slip after the martensite detwinning, which is irreversible [23]. Further, the transformation from the phase M to phase A doesn’t occur during this imprinting process, and this conclusion had been confirmed by Liu [33]. Martensite detwinning and slip-deformation are the necessary conditions for inducing the TWSME, and the TWSME of the NiTi SMA sample demands that the martensite is mainly detwinned before the dislocations induced by slip-deformation are generated in the phase M, this reason for which is that only these dislocations can stabilize the variant distribution of the deformed martensite induced by the LSI technique [6]. We can verify that this technique can sequentially achieve the martensite detwinning and slip-deformation by the followed calculation. The calculation method of the HEL is as follows:
Fig. 7. Load–displacement curves for the formed part and initial material and the nanoindentation morphology.
ΔnTWSME is the attenuation rate of the TWSME after the nth thermal cycle. The relationship curves between the change in bending angle and thermal cycling number is shown in Fig. 10. With increasing thermal cycling number, the θ1n (heating) and θpn (plastic bending angle) exhibited an upward trend, while the θ2n (cooling) and θmaxn (maximum phase transition angle) exhibited a downward trend. Further, they tended to be stable at the end. From this figure, we also found that the θ1n and θ2n began to remain stable after the fourth thermal cycle, and that the bending angles hovered at 10.1° and 21.5°. The θpn and θmaxn were also unchanged after the fourth thermal cycle, and their stability values were the θsp (the stability value of the plastic bending angle was 29.2°) and θsmax (the stability value of the maximum phase transition angle was 9.2°). Additionally, the specific changes and trend of each parameter are listed in Table 5. It can be concluded that the TWSME induced by LSI demonstrates a good stability; the θpn and θmaxn remain unchanged after the initial four thermal cycles, the final attenuation rate of the TWSME is only 26.4%, and the θsmax is approximately 19.0% of the initial bending angle.
HEL (MPa) =
1−v D σY (MPa) 1 − 2v
(4) 6 −1
(strain rate ε = 10 s ) is the where v is the Poisson ratio, v = 0.33; dynamic yield strength. Additionally, the σYD can be estimated by the method (as shown the formula (5)) from the research of Fei [6], D D and σY2 correspond to the HELM and HELP, respectively. whereas σY1
σYD
σYD (MPa) = KσYS (MPa)
3.2. Discussions
(5) −3 −1
(strain rate ε = 3 × 10 s ) is the where K is a ratio, K = 1.387; static yield strength. In this research, the strain rate of LSI is up to
σYS
Typically, the NiTi SMA can exhibit three different phases: phase A,
Fig. 8. Forming and thermal cycling process of the bending workpiece. 6
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Fig. 9. (a) Definition of the bending angle and (b) schematic diagram for the thermal cycling attenuation test of the TWSME.
by formula (7) was 1.71 × 106 g cm−2 s−1, and the calculated value of the peak pressures for the formed part and bending workpiece were 5402 MPa and 2156 MPa, respectively. The peak pressures of both experiments were greater than the HELM and HELP of the NiTi SMA, and the HELP was approximately three times that of the HELM, such that the martensite detwinning of NiTi SMA first occurred (as shown in Fig. 12), and subsequently the plastic slip-deformation could be observed. After imprinting, the residual stress field with anisotropy, created by the plastic slip-deformation, distributed around the dislocations in the phase M. This resulted in the preferential selection of martensite variants during the cooling process from the phase A, which enabled the NiTi SMA to possess the shape memory ability of the low-temperature phase M [23]. Therefore, the two shapes of high-temperature phase A and low-temperature phase M can be reversibly restored by changing the temperature, and the NiTi SMA material possesses the TWSME after imprinting. The LSI process is similar to that of LSP, and they have the similar strengthening mechanism. During the LSI, the NiTi SMA was subjected to dynamic loading that leads to the plastic deformation that confirms by a partial strain recovery occurs on heating. The dislocation source and proliferation mechanism in the NiTi SMA were activated at the shock wave pressure, which would proliferate the dislocation and increase the dislocation density [18,21,27], as illustrated in Fig. 12. The increase of the dislocation density in the dynamic loading led to the alloy hardening and improved the alloy properties. After imprinting, a grain refinement and nanocrystallines layers were created in the NiTi SMA surface. The reason for this result was that the nonequilibrium defects caused by the ultra-high strain rate would induce grain differentiation, and the grain refinement and nanocrystallines layers also could improve the alloy mechanical properties [34]. Thus, the plastic deformation in the LSI process improves the alloy properties, including the microhardness, nanohardness, elastic modulus, and contact stiffness. NiTi SMA was pressed into the mould cavity by the shock wave produced by LSI, and more severe plastic deformation and nanocrystallisation occurred at the mould’s edge, thus resulting in the strengthening degree of the convex microstructures being higher than that of the flat. The primary reason for the difference in hardness at different positions may be that the material thickness, local temperature, local strain, and internal stress had changed in different degrees during the imprinting process. The TWSME of the NiTi SMA was created by the dislocation and residual stress in the phase M. The dislocations induced by LSI in phase M were aligned on the specific crystal planes (as shown in Fig. 13), and the stress fields around the dislocations decided the selection of the
Fig. 10. Relationship curves between change in bending angle and thermal cycling number at the thermal cycling attenuation test. Table 5 Specific changes and trends of each parameter during multiple thermal cycles. Parameters
Initial
Stable
Changes
Ratio
Trend
θ1n θ2n θpn θmaxn
θ11 = 9° θ21 = 21.5° θp1 = 27° θmax1 = 12.5°
10.1° 19.3° θsp = 29.2° θsmax = 9.2°
1.1° 2.2° 2.2° 3.3°
12.2% 10.2% 8.1% 26.4%
Increased Reduced Increased Reduced
S 106 s−1, the measured σY1 is the static detwinning-yield strength, S S σY1 = 149 MPa; the measured σY2 is the static slip-yield strength, D D S σY2 = 512 MPa. Consequently, σY1, σY2 , HELM, and HELP can be calculated, and these calculated values are listed Table 6. During LSI, the shock pressure reached a peak at 0.6tp (4.2 ns) [34,35].The peak pressure of the shock wave can be calculated according to [36] as follows:
Pmax (GPa) = 0.01 2/ Z = 1/ Z1 + 1/ Z2
a 2a + 3
Z (g cm - 2 s - 1) I0 (GW cm - 2)
(6) (7)
where a is the efficiency of the absorbed energy converted to the thermal energy, a = 0.1; Z is the reduced acoustic impedance between the confinement layer (K9 glass, Z1 = 1.14 × 106 g cm−2 s−1) [37] and the target (NiTi SMA, Z2 = 3.44 × 106 g cm−2 s−1) [22]; I0 is the laser power density. Therefore, the reduced acoustic impedance calculated 7
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Fig. 11. Schematic diagram of the NiTi SMA phase transformation.
Fig. 12. Martensite detwinning process and strengthening mechanism for the LSI technique. Table 6 Calculation results in the LSI process. Parameters
Size
I0
Pmax
S σY2 (MPa)
D σY1 (MPa)
D σY2 (MPa)
HELP
(MPa)
S σY1 (MPa)
HELM
(GW/cm2)
(MPa)
(MPa)
5.46
5402
149
502
206.7
696.3
407.2
1372.1
the untwinned martensite, as shown in Fig. 13(a). When the temperature was above Af, the phase transition of M → A occurred, and phase M completely transformed to phase A, as shown in Fig. 13(b). At the moment, the circular convex microstructures shrank as a whole, and the recovery of the bending workpiece returned to the state before imprinting. When the temperature was below Mf, the phase transition of
martensitic transition at the cooling process [11]. Furthermore, lattice distortion led to the residual stress, while the lattice distortion is primarily caused by the dislocation [34,38]. Therefore, the stability of phase Md was determined by the dislocation induced by the LSI process [18]. After the martensite detwinning, the twin martensite transformed to
Fig. 13. Stability mechanism and phase transition process of TWSME during thermal cycling attenuation test. 8
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A → M occurred, and the phase A completely transformed to phase M (as shown in Fig. 13(c)), which resulted in that the circular convex microstructures extended and the bending angle of the bending workpiece increased in the reverse direction. During the thermal cycling attenuation test, the residual stresses in the formed parts and bending workpiece were released slowly, and the dislocation density was decreased. Furthermore, the residual dislocations in the crystal supported the preferred orientation of the martensite variants during the cooling process, and a change of the macroscopic shape was caused by the bias in the variant selection mechanism of habit plane during martensitization [39,40]. Therefore, the TWSME strain couldn’t be completely restored, and the plastic strain of the formed part and bending workpiece increased. In other words, after a complete thermal cycle, the formed parts or bending workpiece could not restore to the original state before the thermal cycle. With the increase in the thermal cycle, the unstable dislocations disappeared gradually, and the residual stress tended to be stable. The θpn and θmaxn of the bending workpiece tended to be stable quickly in the thermal cycling attenuation test, and this result could be attributed to that the TWSME stability included by LSI was affected by the stability of the dislocation and residual stress in the NiTi SMA.
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4. Conclusions In this work, the regular circular convex microstructures were successfully transferred on the surface of the NiTi SMA plate by the LSI technique. By monitoring the strain recovery of the formed part as well as bending workpiece during the thermal cycle and the subsequent theoretical calculations, it was proven that the LSI technique can induce the TWSME at the micron scale. From the DSC curves, it was believed that the LSI does not change the phase transition process and phase transition temperatures of the NiTi SMA. In the subsequent mechanical properties tests, the formed part and initial material were measured; the microhardness, nanohardness, elastic modulus, and contact stiffness of the formed part were improved to varying degrees, and the improvement degree was also different in different positions, which indicated that the LSI technique can improve the mechanical properties of the NiTi SMA material. Ultimately, the thermal cycling attenuation test was performed for the bending workpiece; it was found that the TWSME exhibited good thermal cycling stability; the θmaxn of the bending workpiece tended to be stable only after four thermal cycles, and the stability value of the θmaxn was approximately 19.0% of the initial bending angle. This method enabled the SMA to induce a TWSME with high-thermal cycling stability at the micron or even nano scales; it would be crucial in promoting the development and application of thermally controllable surface morphologies, microstructures, and microactuators. Acknowledgements This work was supported by the Discipline Research Initiative of CUMT [2015XKQY12], and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions [PAPD]. References [1] Y.J. Zhang, Y.T. Cheng, D.S. Grummon, Shape memory surfaces, Appl. Phys. Lett. 89 (2006) 1–3, https://doi.org/10.1063/1.2222173. [2] C. Urbina, S. De La Flor, F. Gispert-Guirado, F. Ferrando, New understanding of the influence of the pre-training phase transformation behaviour on the TWSME in NiTi SMA wires, Exp. Mech. 53 (2013) 1415–1436, https://doi.org/10.1007/s11340013-9756-z. [3] T.Y. Lai, K.P. Lin, S.W. Liang, T.H. Fang, Mechanical properties and mechanism of NiTi pillars using in-situ compression and indentation, Mater. Res. Express. 6 (111322) (2019) 1–23, https://doi.org/10.1088/2053-1591/aafb17. [4] Y. Fu, W. Huang, H. Du, X. Huang, J.P. Tan, X.Y. Gao, Characterization of TiNi shape-memory alloy thin films for MEMS applications, Surf. Coat. Tech. 145 (2001)
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Full length article
Forming and two-way shape memory effect of NiTi alloy induced by laser shock imprinting
T
⁎
Xiong Feia, Yang Haifenga,b, , Liu Kuna, Man Jiaxianga, Chen Haoxuea a b
School of Mechatronic Engineering, China University of Mining and Technology, XuZhou 221116, China Jiangsu Key Laboratory of Mine Mechanical and Electrical Equipment, China University of Mining and Technology, XuZhou 221116, China
H I GH L IG H T S
shape memory effect (TWSME) of NiTi alloy was induced successfully. • AThetwo-way forming and TWSME were realized simultaneously at a micron scale. • Laserprecise shock imprinting can improve the mechanical properties of the formed part. • The TWSME by laser shock imprinting has a good thermal cycling stability. • Strengtheninginduced mechanism and thermally controllable microstructures were discussed. •
A R T I C LE I N FO
A B S T R A C T
Keywords: NiTi shape memory alloy Two-way shape memory effect Laser shock imprinting Mechanical properties Thermal cycling stability
Nickel–Titanium shape memory alloy (NiTi SMA) is an intelligent material that is a widely applied and studied. The two-way shape memory effect (TWSME) of the NiTi SMA has garnered particular attraction in the further miniaturisation and integration of micromechanical systems. To realise the micro-forming and TWSME of the NiTi SMAs at a micron scale, this study reports a method that involves laser shock imprinting (LSI) technology to induce the TWSME. The production efficiency of this method was significantly higher than that of the thermo–mechanical treatment; the proposed method integrated the induction and forming processes. In this study, the precise and rapid forming of microstructures was realised; meanwhile, the TWSME was induced successfully at the micron scale. Moreover, the experimental results indicated that this method can improve the mechanical properties of the formed part. The LSI-induced TWSME exhibited good thermal cycling stability during the thermal cycling attenuation test, the maximum phase transition angle of bending workpiece began to remain stable after the fourth thermal cycle, and the stability value was approximately 19.0% of the initial bending angle. Furthermore, the feasibility of the LSI-induced TWSME was proved by theoretical analysis, and its strengthening mechanism, phase transition process, and thermal cycling stability were discussed.
1. Introduction A Nickel–Titanium shape memory alloy (NiTi SMA) exhibits a phase transition between austenite and martensite at certain stresses or temperatures, and it has a shape memory effect (SME) with the ability of remembering the original shape of the SMA materials [1,2]. Currently, the NiTi SMA has become a popular intelligent materials for research and application [3,4]. This material has great development potential in thermally controllable surface morphologies, microstructures, and microactuators [5,6]. Further, it is widely used in the fields of microelectro-mechanical system (MEMS), microsensors, biomedicine, aviation, and civil engineering [4,7–9].
⁎
It is well known that the spontaneous shape change of an SMA occurs only at heating, but thermo–mechanical treatment can be used to train the two-way shape memory effect (TWSME) and induce the strain variation, that is, the TWSME refers to the ability of SMA can reversibly restore the two shapes of high-temperature austenite (A) phase and low-temperature martensite (M) phase with the change in temperature (heating or cooling) [10,11]. The thermo–mechanical treatment has been widely investigated; previous studies have shown that pre-strain, training methods, training times, training temperatures, and alloy composition all affect the TWSME of the NiTi SMA during the thermo–mechanical treatment [12–14]. Further, the two significant parameters of the strain amplitude and stability were used to characterize
Corresponding author at: School of Mechatronic Engineering, China University of Mining and Technology, XuZhou 221116, China. E-mail address: [email protected] (H. Yang).
https://doi.org/10.1016/j.optlastec.2019.105762 Received 25 April 2019; Received in revised form 19 June 2019; Accepted 2 August 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
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the TWSME, which had been extensively studied by researchers. For instance, Hornbogen [15] mentioned that the TWSME strain would decrease during the thermal cycle, and the decrease continued until the TWSME disappeared completely. Wang [16] and Falvo [12] found that the strain amplitude of the TWSME for the NiTi SMA increased with the increase in the number of the thermal cycling training. Moreover, Falvo [12] also found that the strain amplitude decreased dramatically during the first 20 cycles of the subsequent thermal cycling attenuation test, and then stabilized gradually. So far, this method has become mature; however, this method still encounters the problems of a process complexity, long production cycle, and high cost. Recently, the quasistatic indentation method has been used widely in TWSME training. Although this method is simple and can induce a relatively stable TWSME, it still presents significant limitations on the micron and nano scales or thin film samples, and accurate formation of complex shapes is difficult [1,17]. The Cheng team of Purdue University used the methods of laser shock peening (LSP) [18] and laser shock indentation [6] to produce indentations on NiTi SMA sheets, thus inducing the TWSME at a smaller scale. These studies also proved that the TWSME induced by these methods has good strain amplitude and stability. However, these methods belong to the dynamic loading, which is different from the quasistatic indentation method, and a super-high strain rate could affect the phase transformation behavior of the NiTi SMA [19,20]. This conclusion had been confirmed by the research team of Wang [21,22], and they also found that the stress-induced martensite transformation takes place in the NiTi SMA processed by the laser shock processing. Laser shock imprinting (LSI), as the extensive technology of LSP and laser shock forming, belongs to a non-contact process and exhibits the advantages of simple processing, fast forming speed, and short production cycle [23,24]. Owing to the better controllability and smaller beam diameter of laser, LSI can localise the processing area at a micron scale, and realise the precise forming of complex shapes at the micron or even nano scales. Furthermore, LSI inherits some advantages of LSP, such that it can improve the mechanical properties of the target material effectively, such as corrosion resistance, hardness, and fatigue life [25,26]. Meanwhile, the improvement of mechanical properties of SMA is of great value in engineering applications, and Wang [21] and Chang [27] found that LSP also can improve the mechanical properties of SMA. Therefore, according to the LSI processing characteristics, this technique can be used to realise the TWSME training of the NiTi SMA and the precise processing of the thermally controllable surface morphologies, microstructures, and microactuators. In this study, in order to induce the TWSME of the NiTi SMA at the micron or even nano scales, achieve the synchronization of the microforming and TWSME, as well as obtain the TWSME with a good stability, we report a method that microstructures are fabricated on the surface of the NiTi SMA by the LSI technique. Firstly, the changes in the profile curve and the forming height were observed in the thermal cycle, the mechanical properties of the formed part were evaluated, and the TWSME stability of the bending workpiece was evaluated by a thermal cycling attenuation test. Further, the strengthening mechanism of the LSI, phase transition process, and thermal cycling stability of the TWSME induced by LSI were analysed. In summary, it is expected that this method would promote the development and application of thermally controllable surface morphologies.
Table 1 Composition of the NiTi SMA plate. Compositions
Ni
C
O+N
H
Ti
(wt.%)
56.85
0.007
0.041 + 0.0014 ≦ 0.045
0.0009
balance
Fig. 1. DSC curve of the NiTi SMA initial material.
ultrasonic cleaner in anhydrous alcohol. Before cleaning, the sample surface was ground using a 2000-grid sandpaper followed by final polishing with 0.05-μm diamond paste. The phase transition process and phase transition temperatures of the NiTi SMA initial material were measured using a differential scanning calorimeter (DSC), as shown in Fig. 1. The austenite start temperature (As) and austenite finish temperature (Af) are 296 K and 305 K, respectively; the martensite start temperature (Ms) and martensite finish temperature (Mf) are 278 K and 260 K, respectively. Before LSI, the samples were cooled to approximately 240 K and subsequently heated to 293 K to ensure these samples were in a fully martensitic condition. 2.2. Experiment methods Fig. 2 shows a schematic diagram of the LSI process, in which the pulsed laser, confinement layer, ablative layer, NiTi SMA sample, and mould are indispensable in these experiments. The focused pulse laser beam with a short pulse duration (tp) had a high power density (I0) in the LSI process. The laser beam passed through the transparent confinement layer and irradiated the ablation layer surface. Then, the ablation layer absorbed laser energy and rapidly gasified, and a large number of dense high-temperature (> 104 K) and high-pressure (> 1 GPa) plasmas formed almost simultaneously. Subsequently, the plasma continued to absorb laser energy such that it heated up and expanded quickly; a high-intensity shock wave was subsequently produced by the plasma explosion, and finally acted on the NiTi SMA surface. Additionally, owing to the confinement effect of the confinement layer for plasmas, the peak pressure and action time of the shock wave were increased. When the peak pressure of the shock wave was greater than the Hugoniot elastic limit (HEL) of plastic deformation for the NiTi SMA, the NiTi SMA sample formed according to the mould's shape, thereby resulting in residual stress in the formed parts after imprinting [28,29]. In this work, the LSI experiments were carried out at 293 K; a pulse laser (Hercules-1000-TH, wavelength 1064 nm, pulse duration 7 ns) was selected; a K9 glass of 3-mm thickness and black aluminium foil with a 45-μm thickness were used as the confinement layer and ablative layer, respectively. During this LSI process, the peak pressure of the shock wave could be adjusted by changing the spot diameter at the
2. Experiments and characterisation 2.1. Materials In this research, a 0.1-mm-thick 56.85 wt% Ni of the NiTi SMA plate was chosen, and its composition is listed in Table 1. The experimental samples with the separate dimensions of 4 mm × 4 mm and 4 mm × 0.2 mm were cut by pulsed laser cutting from the NiTi SMA plate, and the samples were subsequently cleaned and degreased by an 2
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Fig. 2. LSI process: (a) schematic diagram of the LSI process, (b) mould, (c) the formed parts and bending workpiece.
3. Results and discussions
Table 2 Significant experimental parameters of the laser. Dimension (mm)
Frequency (HZ)
Pulse energy (J)
Spot diameter (mm)
Power density (GW/cm2)
4 × 4 × 0.1 4 × 0.2 × 0.1
1 1
0.3 0.3
1 2.5
5.46 0.87
3.1. Results After LSI, regular circular microstructures (diameter is approximately 126 μm) were successfully copied on the NiTi SMA plate surface. These microstructures of the formed part were convex, and their shapes and sizes coincide with the corresponding holes on the surface of mould 1 (diameter 126.8 μm, depth 11.7 μm), as shown in Fig. 3. In addition, the forming accuracy and forming quality of the formed part were excellent, and these microstructures were uniform. The profile curve in Fig. 3(d) shows that the initial forming height of the circular convex microstructures was approximately 11.5 μm. Fig. 4 shows the DSC curve of the formed part fabricated by the LSI technique. After LSI, the phase transition process in the formed part had not changed during the thermal cycle, the phase transition in heating stage was still M → A, and that in cooling stage was still A → M. Further, the corresponding phase transition temperatures did not change significantly during the phase transition process, as listed in Table 3. It could be seen that the LSI has little effect on the overall phase transition process and phase transition temperatures of the NiTi SMA. Fig. 5 shows the variation in the section profile curve and the forming height for the circular convex microstructures on the formed part surface during a thermal cycle. As shown in Fig. 5(a), the microstructures’ shape remains unchanged throughout the thermal cycle, and the microstructures shrinks and extends separately at temperatures higher than Af or lower than Mf. Meanwhile, the height change of the convex microstructures was reversible, and this thermal cycle could be repeated many times. Therefore, we found that the microstructures imprinted by LSI exhibits a TWSME, that is, the height of the convex microstructures could be regulated by the change in temperature. The initial forming height of the circular convex microstructures was the largest after imprinting, i.e. approximately 11.5 μm, as illustrated in Fig. 5(b). During the thermal cycle, when the formed part was heated above Af, the section profile curve shrank in both the longitudinal and transverse directions, and the height of the circular convex microstructures decreased to 9.6 μm. The height change of 1.9 μm implied that the initial deformation recovered by 16.5% in the height (longitudinal) direction, as illustrated in Fig. 5(c). This height ratio represented the proportion of the non-plastic deformation (deformation caused by phase transition) of the circular convex microstructures in the height direction. Subsequently, the section profile curve extended in both the longitudinal and transverse directions at a temperature below Mf, and the forming height increased to 11.0 μm, as shown in Fig. 5(d). Consequently, the forming height change for the circular convex microstructures was 1.4 μm during the first thermal cycle, at approximately 12.2% of the initial forming height. This height ratio represented the proportion of the maximum phase transition height of the circular convex microstructures in the initial forming height during the
same single pulse energy to control the material deformation degree. Therefore, the formed part and the bending workpiece were manufactured by changing the mould and the spot diameter, respectively (see Fig. 2(b) and (c)), and the corresponding significant parameters are listed in Table 2. For guaranteeing the quality of the formed parts, improving its uniformity, and realising the repeatable operation of this experiment, the K9 glass, black aluminium foil, and NiTi SMA samples must be pressed firmly and adhered closely on the mould surface [30]. In the thermal cycling attenuation test, a complete thermal cycling process involves the bending workpiece imprinted by LSI to first increase from 293 K to 360 K, subsequently decrease to 240 K, and finally increase to 293 K again. To measure the stability of the TWSME induced by LSI, 10 thermal cycles were performed in this work. 2.3. Measurements and characterisation After the LSI process or the thermal cycle, we used a scanning electron microscope (SEM) (Quanta 250) to observe the microstructures’ morphologies on the formed part surface and measured their profile curves and forming heights via an optical profiler (DVM5000, Leica, Germany). The microhardness of the formed parts were determined by a microhardness tester (HVSA-1000A) with a 50-g load and a 15-s duration time. The deformation process and bending angle of the bending workpiece were determined by a universal material microscope (LEICA DM4 M) during the thermal cycle. For the measurements above, the average of three measurements was used as the final measured result to characterise the formed part and reduce measurement errors. In this Paper, the DSC curves of the initial material and formed parts were measured using a differential scanning calorimeter (DSC) (America TA DSC250) with a heating or cooling rate of 10 K/min and a sample mass of 30 mg. Furthermore, to evaluate the mechanical properties of the formed part, the load–displacement curves, nanohardness, and elastic modulus of the initial materials and the formed parts were measured by nanoindentation (Agilent U9820A, Nano Indenter G200) with a standard Berlcovich diamond indenter, and the corresponding contact stiffness between the samples and indenter were calculated from the load–displacement curve. Last but not least, the microhardness test and nanohardness test were carried out at 293 K. In addition, the formed part was firstly cooled to 240 K and subsequently heated to 293 K to ensure these were in a fully martensitic condition before the tests of the microhardness or nanohardness. 3
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Fig. 3. (a) the surface morphology, profile curve, and depth of the mould 1, (b) the SEM images (500×) of the formed part, (c) the SEM images (1500×) of the formed part, (d) the morphology, profile curve, and forming height of the formed part.
is 60 μm. As shown, the microhardness has improved after LSI, indicating that LSI has a certain strengthening effect on the NiTi SMA material. The microhardness was discrepant at different positions, its curve exhibited an upward trend as the measurement position gradually approached the convex microstructures, and the top of the convex microstructures was the highest microhardness values. The initial microhardness of the NiTi SMA material was 210 HV, while the average value of the microhardness for the convex microstructures was approximately 352 HV, which increased by 67.6%. After LSI, the minimum microhardness of the formed parts was 260 HV, implying that the monolithic microhardness of the formed parts was increased by approximately 23.8%. Table 4 shows the test results of the nanoindentation. The nanohardness of the convex microstructures was found to be the highest, followed by the flat region of the formed parts, and subsequently the initial material without imprinting. The test results were consistent with the microhardness, thus indicating that LSI technology can improve the deformation resistance of the formed parts. Compared with the nanohardness (2.7 GPa) of the initial material, the nanohardness (3.9 GPa) of the convex microstructures increased by 44.4%. In addition, the elastic modulus and contact stiffness demonstrated the same results, that is, the elastic modulus and contact stiffness of the convex microstructures were the largest and those of the initial material was the smallest. Compared with the elastic modulus (17.8 GPa) and contact stiffness (29.5 μN/nm) of the initial material, the elastic modulus (46.3 GPa) and contact stiffness (212.0 μN/nm) of the convex microstructures increased by 160.1% and 618.6%, respectively. This indicated that LSI technology can improve the elastic deformation resistance of the formed parts.
Fig. 4. DSC curve of the formed part. Table 3 Phase transition temperatures of the initial material and formed part. Sample
Ms (K)
Mf (K)
As (K)
Af (K)
Initial material Formed part
278 278
260 261
296 296
305 304
first thermal cycle. Fig. 6 shows the microhardness of the formed part and the initial material of the NiTi SMA. Further, the distance of each measuring point 4
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Fig. 5. Variation in the profile curve and the forming height for the circular convex microstructures during the thermal cycle.
To investigate the influence of the LSI on the mechanical properties of the formed part, the load–displacement curves were monitored during the whole nanoindentation process, as shown in Fig. 7. In the loading curve, the indentation load required by the convex microstructures was the largest at the same indentation depth, followed by the flat, and subsequently the initial material, thus proving that the deformation resistance of the formed parts is strengthened. According to Table 4, the maximum indentation load (25.1 mN) of the initial material was significantly lower than that of the flat (41.7 mN) and convex microstructures (85.4 mN) at a fixed indentation depth (1500 nm), which also showed that LSI technique can improve the alloy properties of the NiTi SMA. The NiTi SMA material was processed into a bending workpiece by LSI technology to obtain a larger deformation, as shown in Fig. 8(a) and (b). The bending workpiece also exhibited the TWSME and the same deformation effect as the formed parts during the thermal cycles, and the initial bending angle of the bending workpiece was 48.5°. When the temperature was heated to 360 K, the bending workpiece recovered, the bending angle decreased to 9.0°, and the recovery rate was approximately 81.4%, as illustrated in Fig. 8(c). The bending angle subsequently increased to 21.5° when the temperature was cooled to 240 K, and the change in the bending angle is 12.5°, at approximately 25.8% of the initial bending angle, as illustrated in Fig. 8(d). In this experiment, the thermal cycling attenuation test of the TWSME included the heating and cooling for the bending workpiece; subsequently, changes in the plastic bending angle (θpn) and the
Table 4 Nanoindentation test results and calculated values for the different positions. Positions
hmax (nm)
Fmax (mN)
H (GPa)
E (GPa)
K (μN/nm)
Initial Material Convex Flat
1500 1500 1500
25.1 85.4 41.7
2.7 3.9 3.7
17.8 46.3 26.9
29.5 212.0 56.8
maximum phase transition angle (θmaxn) were measured to characterise its thermal cycling stability performance, as shown in Fig. 9. As shown in Fig. 9(a), θ0 (θ0 = 48.5°) represents the initial bending angle; θ1n and θ2n represent the bending angle after heating and cooling during the nth thermal cycle, respectively. Fig. 9(b) shows a schematic diagram for the thermal cycling attenuation test of the TWSME, in which ABCDE is the first thermal cycle, and EFGHI is the second thermal cycle. As shown, the bending workpiece could not return to its initial position after the thermal cycle. Among them, the θpn and θmaxn at the nth thermal cycle could be calculated as follows:
θpn = θ0 − θ2n
(n ≧ 1)
θmaxn = θ2n − θ1n
ΔnTWSME =
(n ≧ 1)
θmax1 − θmaxn × 100% (n ≧ 2) θmax1
(1) (2)
(3)
where n is a positive integer, and it represents the nth thermal cycle;
Fig. 6. Microhardness at the different positions of the formed part. 5
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phase M (including the twin martensite and untwinned martensite), and R-phase (R) at different stresses and temperatures, and the SME of an SMA originates from the reversible strain variation induced by microcosmic phase transformation in SMA crystals, as shown in Fig. 11 [31,32]. Among them, phase A only exists stably in the high-temperature region; phase M only exists stably in the low-temperature region; R-phase often appears during the phase transformation process from the phase A to Phase M; the σs and σf are the start stress and finish stress of the martensite detwinning, respectively [32]. In this work, it can be seen that the phases A and phase M exist in the phase transformation process according to the DSC curves; however, the R-phase does not appear. During LSI, when the pressure generated by the shock wave exceeds the Hugoniot elastic limit for martensite detwinning (HELM) and plastic slip-deformation (HELP), the NiTi SMA creates the martensite detwinning (as shown in Fig. 12) and an obvious plastic deformation, respectively. In additions, the samples in a fully phase M state first creates an elastic deformation when the shock wave acts on the surface of NiTi SMA. Subsequently, when the shock wave pressure is greater than HELM, the deformation, reversed by heating, can be accommodated by the martensite detwinning. The plastic deformation is sequentially accommodated through dislocation slip after the martensite detwinning, which is irreversible [23]. Further, the transformation from the phase M to phase A doesn’t occur during this imprinting process, and this conclusion had been confirmed by Liu [33]. Martensite detwinning and slip-deformation are the necessary conditions for inducing the TWSME, and the TWSME of the NiTi SMA sample demands that the martensite is mainly detwinned before the dislocations induced by slip-deformation are generated in the phase M, this reason for which is that only these dislocations can stabilize the variant distribution of the deformed martensite induced by the LSI technique [6]. We can verify that this technique can sequentially achieve the martensite detwinning and slip-deformation by the followed calculation. The calculation method of the HEL is as follows:
Fig. 7. Load–displacement curves for the formed part and initial material and the nanoindentation morphology.
ΔnTWSME is the attenuation rate of the TWSME after the nth thermal cycle. The relationship curves between the change in bending angle and thermal cycling number is shown in Fig. 10. With increasing thermal cycling number, the θ1n (heating) and θpn (plastic bending angle) exhibited an upward trend, while the θ2n (cooling) and θmaxn (maximum phase transition angle) exhibited a downward trend. Further, they tended to be stable at the end. From this figure, we also found that the θ1n and θ2n began to remain stable after the fourth thermal cycle, and that the bending angles hovered at 10.1° and 21.5°. The θpn and θmaxn were also unchanged after the fourth thermal cycle, and their stability values were the θsp (the stability value of the plastic bending angle was 29.2°) and θsmax (the stability value of the maximum phase transition angle was 9.2°). Additionally, the specific changes and trend of each parameter are listed in Table 5. It can be concluded that the TWSME induced by LSI demonstrates a good stability; the θpn and θmaxn remain unchanged after the initial four thermal cycles, the final attenuation rate of the TWSME is only 26.4%, and the θsmax is approximately 19.0% of the initial bending angle.
HEL (MPa) =
1−v D σY (MPa) 1 − 2v
(4) 6 −1
(strain rate ε = 10 s ) is the where v is the Poisson ratio, v = 0.33; dynamic yield strength. Additionally, the σYD can be estimated by the method (as shown the formula (5)) from the research of Fei [6], D D and σY2 correspond to the HELM and HELP, respectively. whereas σY1
σYD
σYD (MPa) = KσYS (MPa)
3.2. Discussions
(5) −3 −1
(strain rate ε = 3 × 10 s ) is the where K is a ratio, K = 1.387; static yield strength. In this research, the strain rate of LSI is up to
σYS
Typically, the NiTi SMA can exhibit three different phases: phase A,
Fig. 8. Forming and thermal cycling process of the bending workpiece. 6
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Fig. 9. (a) Definition of the bending angle and (b) schematic diagram for the thermal cycling attenuation test of the TWSME.
by formula (7) was 1.71 × 106 g cm−2 s−1, and the calculated value of the peak pressures for the formed part and bending workpiece were 5402 MPa and 2156 MPa, respectively. The peak pressures of both experiments were greater than the HELM and HELP of the NiTi SMA, and the HELP was approximately three times that of the HELM, such that the martensite detwinning of NiTi SMA first occurred (as shown in Fig. 12), and subsequently the plastic slip-deformation could be observed. After imprinting, the residual stress field with anisotropy, created by the plastic slip-deformation, distributed around the dislocations in the phase M. This resulted in the preferential selection of martensite variants during the cooling process from the phase A, which enabled the NiTi SMA to possess the shape memory ability of the low-temperature phase M [23]. Therefore, the two shapes of high-temperature phase A and low-temperature phase M can be reversibly restored by changing the temperature, and the NiTi SMA material possesses the TWSME after imprinting. The LSI process is similar to that of LSP, and they have the similar strengthening mechanism. During the LSI, the NiTi SMA was subjected to dynamic loading that leads to the plastic deformation that confirms by a partial strain recovery occurs on heating. The dislocation source and proliferation mechanism in the NiTi SMA were activated at the shock wave pressure, which would proliferate the dislocation and increase the dislocation density [18,21,27], as illustrated in Fig. 12. The increase of the dislocation density in the dynamic loading led to the alloy hardening and improved the alloy properties. After imprinting, a grain refinement and nanocrystallines layers were created in the NiTi SMA surface. The reason for this result was that the nonequilibrium defects caused by the ultra-high strain rate would induce grain differentiation, and the grain refinement and nanocrystallines layers also could improve the alloy mechanical properties [34]. Thus, the plastic deformation in the LSI process improves the alloy properties, including the microhardness, nanohardness, elastic modulus, and contact stiffness. NiTi SMA was pressed into the mould cavity by the shock wave produced by LSI, and more severe plastic deformation and nanocrystallisation occurred at the mould’s edge, thus resulting in the strengthening degree of the convex microstructures being higher than that of the flat. The primary reason for the difference in hardness at different positions may be that the material thickness, local temperature, local strain, and internal stress had changed in different degrees during the imprinting process. The TWSME of the NiTi SMA was created by the dislocation and residual stress in the phase M. The dislocations induced by LSI in phase M were aligned on the specific crystal planes (as shown in Fig. 13), and the stress fields around the dislocations decided the selection of the
Fig. 10. Relationship curves between change in bending angle and thermal cycling number at the thermal cycling attenuation test. Table 5 Specific changes and trends of each parameter during multiple thermal cycles. Parameters
Initial
Stable
Changes
Ratio
Trend
θ1n θ2n θpn θmaxn
θ11 = 9° θ21 = 21.5° θp1 = 27° θmax1 = 12.5°
10.1° 19.3° θsp = 29.2° θsmax = 9.2°
1.1° 2.2° 2.2° 3.3°
12.2% 10.2% 8.1% 26.4%
Increased Reduced Increased Reduced
S 106 s−1, the measured σY1 is the static detwinning-yield strength, S S σY1 = 149 MPa; the measured σY2 is the static slip-yield strength, D D S σY2 = 512 MPa. Consequently, σY1, σY2 , HELM, and HELP can be calculated, and these calculated values are listed Table 6. During LSI, the shock pressure reached a peak at 0.6tp (4.2 ns) [34,35].The peak pressure of the shock wave can be calculated according to [36] as follows:
Pmax (GPa) = 0.01 2/ Z = 1/ Z1 + 1/ Z2
a 2a + 3
Z (g cm - 2 s - 1) I0 (GW cm - 2)
(6) (7)
where a is the efficiency of the absorbed energy converted to the thermal energy, a = 0.1; Z is the reduced acoustic impedance between the confinement layer (K9 glass, Z1 = 1.14 × 106 g cm−2 s−1) [37] and the target (NiTi SMA, Z2 = 3.44 × 106 g cm−2 s−1) [22]; I0 is the laser power density. Therefore, the reduced acoustic impedance calculated 7
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Fig. 11. Schematic diagram of the NiTi SMA phase transformation.
Fig. 12. Martensite detwinning process and strengthening mechanism for the LSI technique. Table 6 Calculation results in the LSI process. Parameters
Size
I0
Pmax
S σY2 (MPa)
D σY1 (MPa)
D σY2 (MPa)
HELP
(MPa)
S σY1 (MPa)
HELM
(GW/cm2)
(MPa)
(MPa)
5.46
5402
149
502
206.7
696.3
407.2
1372.1
the untwinned martensite, as shown in Fig. 13(a). When the temperature was above Af, the phase transition of M → A occurred, and phase M completely transformed to phase A, as shown in Fig. 13(b). At the moment, the circular convex microstructures shrank as a whole, and the recovery of the bending workpiece returned to the state before imprinting. When the temperature was below Mf, the phase transition of
martensitic transition at the cooling process [11]. Furthermore, lattice distortion led to the residual stress, while the lattice distortion is primarily caused by the dislocation [34,38]. Therefore, the stability of phase Md was determined by the dislocation induced by the LSI process [18]. After the martensite detwinning, the twin martensite transformed to
Fig. 13. Stability mechanism and phase transition process of TWSME during thermal cycling attenuation test. 8
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A → M occurred, and the phase A completely transformed to phase M (as shown in Fig. 13(c)), which resulted in that the circular convex microstructures extended and the bending angle of the bending workpiece increased in the reverse direction. During the thermal cycling attenuation test, the residual stresses in the formed parts and bending workpiece were released slowly, and the dislocation density was decreased. Furthermore, the residual dislocations in the crystal supported the preferred orientation of the martensite variants during the cooling process, and a change of the macroscopic shape was caused by the bias in the variant selection mechanism of habit plane during martensitization [39,40]. Therefore, the TWSME strain couldn’t be completely restored, and the plastic strain of the formed part and bending workpiece increased. In other words, after a complete thermal cycle, the formed parts or bending workpiece could not restore to the original state before the thermal cycle. With the increase in the thermal cycle, the unstable dislocations disappeared gradually, and the residual stress tended to be stable. The θpn and θmaxn of the bending workpiece tended to be stable quickly in the thermal cycling attenuation test, and this result could be attributed to that the TWSME stability included by LSI was affected by the stability of the dislocation and residual stress in the NiTi SMA.
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4. Conclusions In this work, the regular circular convex microstructures were successfully transferred on the surface of the NiTi SMA plate by the LSI technique. By monitoring the strain recovery of the formed part as well as bending workpiece during the thermal cycle and the subsequent theoretical calculations, it was proven that the LSI technique can induce the TWSME at the micron scale. From the DSC curves, it was believed that the LSI does not change the phase transition process and phase transition temperatures of the NiTi SMA. In the subsequent mechanical properties tests, the formed part and initial material were measured; the microhardness, nanohardness, elastic modulus, and contact stiffness of the formed part were improved to varying degrees, and the improvement degree was also different in different positions, which indicated that the LSI technique can improve the mechanical properties of the NiTi SMA material. Ultimately, the thermal cycling attenuation test was performed for the bending workpiece; it was found that the TWSME exhibited good thermal cycling stability; the θmaxn of the bending workpiece tended to be stable only after four thermal cycles, and the stability value of the θmaxn was approximately 19.0% of the initial bending angle. This method enabled the SMA to induce a TWSME with high-thermal cycling stability at the micron or even nano scales; it would be crucial in promoting the development and application of thermally controllable surface morphologies, microstructures, and microactuators. Acknowledgements This work was supported by the Discipline Research Initiative of CUMT [2015XKQY12], and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions [PAPD]. References [1] Y.J. Zhang, Y.T. Cheng, D.S. Grummon, Shape memory surfaces, Appl. Phys. Lett. 89 (2006) 1–3, https://doi.org/10.1063/1.2222173. [2] C. Urbina, S. De La Flor, F. Gispert-Guirado, F. Ferrando, New understanding of the influence of the pre-training phase transformation behaviour on the TWSME in NiTi SMA wires, Exp. Mech. 53 (2013) 1415–1436, https://doi.org/10.1007/s11340013-9756-z. [3] T.Y. Lai, K.P. Lin, S.W. Liang, T.H. Fang, Mechanical properties and mechanism of NiTi pillars using in-situ compression and indentation, Mater. Res. Express. 6 (111322) (2019) 1–23, https://doi.org/10.1088/2053-1591/aafb17. [4] Y. Fu, W. Huang, H. Du, X. Huang, J.P. Tan, X.Y. Gao, Characterization of TiNi shape-memory alloy thin films for MEMS applications, Surf. Coat. Tech. 145 (2001)
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