- Email: [email protected]
Thermal stability and high-temperature shape memory effect of Ni55.2Mn24.7Ga19.9Gd0.2 thin film
Accepted Manuscript Thermal stability and high-temperature shape memory effect of Ni55.2Mn24.7Ga19.9Gd0.2 thin film Jian Yao, Bo Cui, Xiaohang Zheng, Ye Wu, Jiehe Sui, Wei Cai PII:
S0042-207X(17)31273-3
DOI:
10.1016/j.vacuum.2017.10.022
Reference:
VAC 7651
To appear in:
Vacuum
Received Date: 16 September 2017 Revised Date:
14 October 2017
Accepted Date: 16 October 2017
Please cite this article as: Yao J, Cui B, Zheng X, Wu Y, Sui J, Cai W, Thermal stability and hightemperature shape memory effect of Ni55.2Mn24.7Ga19.9Gd0.2 thin film, Vacuum (2017), doi: 10.1016/ j.vacuum.2017.10.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Thermal stability and high-temperature shape memory effect of Ni55.2Mn24.7Ga19.9Gd0.2 thin film Jian Yao1, Bo Cui1, Xiaohang Zheng1,*, Ye Wu, Jiehe Sui1, Wei Cai1,* School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
RI PT
1
*To whom correspondence should be addressed.
E-mail: [email protected] (Wei cai), [email protected] (Xiaohang Zheng)
SC
Abstract
Thermal stability and shape memory effect (SME) of Ni-Mn-Ga-based thin film
M AN U
were firstly investigated. The fluctuation scope of phase transformation temperatures did not exceed 2 °C during 100 thermal cycles. The maximum SME of 0.64% was quantitatively measured under 600 MPa applied stress. SME almost kept constant after
TE D
20 times thermal cycling deformation under 300 MPa applied stress. Keywords: Ni-Mn-Ga alloy; Thin films; Thermal stability; Shape memory effect. With the development of micro-electro-mechanical systems (MEMS), there are
EP
urgent demands for developing high temperature shape memory alloy (HTSMA) thin
AC C
films as microactuator using at elevated temperatures (˃100 °C) for automotive, aerospace and energy industries [1]. Until now, extensive research has been focused on Ti-Ni-X (X=Hf, Zr, Pd, Pt) [2-7], Au-Cu-Al [8] and Ti-Ta [9] HTSMA thin films. Nevertheless, the commercial application of HTSMA thin films is limited due to the high price in Ti-Ni-Pd, Ti-Ni-Pt and Au-Cu-Al systems and the poor thermal stability in Ti-Ni-Hf, Ti-Ni-Zr and Ti-Ta systems [10-12]. Recently, Ni-Mn-Ga based alloys have attracted much attention as a kind of
1
ACCEPTED MANUSCRIPT potential HTSMA since their adjustable martensitic transformation temperature, good thermal stability and relatively low cost [13, 14]. Xu et al. reported a large SME of 6.1% in Ni54Mn25Ga21 single crystal alloy with excellent thermal stability during 1000
RI PT
thermal cycles [15]. However, the fabrication of single crystal is time-consuming and very difficult to perform. Unfortunately, polycrystalline Ni-Mn-Ga alloys are extremely brittle due to the weak grain boundary. Xin et al. proposed that introducing a ductile γ
SC
phase by substitution of Ni for Ga is an effective method to improve the plasticity of
M AN U
Ni-Mn-Ga alloys. But the γ phase is harmful to the thermal stability and SME [16]. Additionally, it is prone to bring plastic deformation during actuation process even under low bias stress, which is not suitable for engineering application. Grain refinement is also a useful way to overcome this problem without reducing the SME
TE D
and thermal stability [17]. It is easy to attain fine-grained Ni-Mn-Ga thin film prepared by magnetron sputtering. The research on Ni-Mn-Ga-based HTSMA thin films has also been reported, which mainly is interested in film preparation, microstructure, phase
EP
transformation and magnetic properties [18-22]. However, the reports on the thermal
AC C
stability and shape memory effect (SME) of Ni-Mn-Ga based HTSMA thin films are not available. In our previous report, nanocrystalline Ni55.2Mn24.7Ga19.9Gd0.2 HTSMA thin films with SME have been successfully prepared by magnetron sputtering [21]. The present study aims to investigate the stability of phase transformation and shape memory behavior during thermal cycles in Ni55.2Mn24.7Ga19.9Gd0.2 HTSMA thin film. Ni55.2Mn24.7Ga19.9Gd0.2 (at. %) thin films were deposited on unheated silicon substrates by DC magnetron sputtering technique. Details of thin film preparation were
2
ACCEPTED MANUSCRIPT given by [21]. After sputtering, thin films were peeled off from the substrates to avoid an interfacial reaction between the thin film and substrate during the crystallization heat treatment. The freestanding films were fully crystallized at 650 °C for 60 s in a rapid
RI PT
thermal annealing furnace under a vacuum condition of 2.2×10-3 Pa. In order to measure the phase transformation temperatures of the film annealed at 650 °C, differential scanning calorimeter (DSC) equipment was employed with a
SC
heating speed of 20 °C/min. The mass of the film used for the DSC measurement was 9
M AN U
mg. After 100 thermal cycles in the DSC equipment, transmission electron microscopy (TEM) was used to investigate the internal structure of the film. A solution of 30% nitric acid in methanol was used to electrolytically polish the film for TEM observation at about -20 °C. Shape memory behavior of annealed film was examined by thermal
TE D
cycling tests under various constant stresses using a dynamical mechanical analyzer (DMA Q800). The size of the test sample was 5 µm×3 mm×12 mm (gauge portion). The sample was loaded a stress at 400 °C, then conducted thermal cycles from 160 to
EP
400 °C at a rate of 10 °C /min.
AC C
DSC curves of the annealed film after 1, 10, 50, 100 thermal cycles were shown in Fig. 1(a). It can be clearly seen that the shape and location of transformation peaks on the DSC curves don’t change significantly with the number of thermal cycles. Furthermore,
the
evolution
of
martensitic
transformation
temperatures
and
transformation latent heat (∆H) with the number of thermal cycles are shown as Fig. 1(b). It shows that the Mp (peak temperature of martensitic transformation) and Ap (peak temperature of austenitic transformation) vary in the range of 227.12~228.81 °C and
3
ACCEPTED MANUSCRIPT 257.18~258.91
°
C, respectively. As for the latent heat releasing from phase
transformation, there is no noticeable variation indicating the good resistance to phase decomposition during the thermal cycles. These findings reveal the excellent thermal
M AN U
SC
RI PT
stability of phase transformation in Ni55.2Mn24.7Ga19.9Gd0.2 thin film.
Fig. 1 Effect of thermal cycles on phase transformation of Ni55.2Mn24.7Ga19.9Gd0.2 thin film: (a) DSC
TE D
curves after different thermal cycles; (b) Mp, Ap transformation peak temperatures and latent heat after different thermal cycles.
In order to clarify the good thermal stability of phase transformation, TEM
EP
observations were carried out to investigate the microstructure of the annealed thin film
AC C
after 100 thermal cycles. The bright-field (BF) image exhibits the microstructure of the annealed film, which is consists of very fine grain with regular martensitic plates, as shown in Fig. 2(a). The microstructure is very similar to that in the annealed film before thermal cycles [21]. The nanoscale grain structure is beneficial to achieve good thermal stability. On the one hand, the small grain size of the film increases the critical stress for slip, which suppresses the dislocation formation and movement during thermal cycling [11, 12]. On the other hand, based on the geometric non-linear theory of martensite
4
ACCEPTED MANUSCRIPT (GNLTM), when λ2 (the middle eigenvalue of the transformation matrix) closes to 1, the SMA can achieve high functional stability [23]. Sun et al. reported that λ2 gradually approaches 1 with the reduction of grain size in Ti-Ni alloy [24]. In the
RI PT
Ni55.2Mn24.7Ga19.9Gd0.2 film, we determined from previous lattice parameters result that, λ2=1.0261 [21]. Compared with the value of λ2=1.0691 of the bulk counterpart in the similar compositions with a grain size of tens of micrometers [25], the film with
SC
nanograin tends to achieve better thermal stability. There are no precipitates observed in
M AN U
the microstructure, which is consistent with the constant transformation latent heat during thermal cycles. Fig. 2(b) is the magnified image from circular region in Fig. 2(a). It displays a strip-like morphology of well accommodated martensite plates with the width of ~20 nm. The adjacent plates connected by a distinct and straight twin boundary
TE D
exhibits (202) type-I twin relationship as shown in Fig. 2(c). Additionally, it can be easily observed that two thick variant plates both consist of thinner needle-like plates. The similar nanoscale internal twin structure was also found by reference [26] in
EP
Ni53Mn25Ga22 alloy. The HRTEM image further displays a clear and straight boundary
AC C
between the parallel thin plates with the width of ~1 nm in the thick plate as shown in Fig. 2(d). The Fast Fourier Transform (FFT) pattern taken from the rectangle region in the HRTEM image displays six extra satellite spots (marked by yellow arrows) with the main diffraction spots along (220) direction. That is the typical diffraction pattern of seven-layered (marked as 7M) martensite structure. In fact, 7M martensite can be considered as an adaptive nano-twinned phase with (52) stacking sequence built up by a non-modulated tetragonal phase unit cells defined as T martensite, according to the
5
ACCEPTED MANUSCRIPT viewpoint of S. Kaufmann [27] and J. Pons [28]. In this concept, every thin plate is nano-twinned structure in nature which is composed of a five atomic (220) planes variant and a two atomic (220) planes variant. Thus, another level of few atomic planes
RI PT
nano-twinned structure is added to the hierarchical twinned martensite. The hierarchical self-accommodated structure is in favor of releasing the stress arising from the geometrical incompatibility between austenite and martensite phase. It can effectively
SC
reduce the accumulation of plastic deformation during phase transformation, thereby
M AN U
improving the thermal stability of phase transformation. In summary, the nanoscale grain, mono-phase structure and hierarchical self-accommodated twinned structure are
AC C
EP
TE D
the main reasons for the excellent thermal stability of phase transformation in the film.
6
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
EP
Fig. 2 TEM micrographs of the annealed film after 100 thermal cycles: (a) BF image under low magnification; (b) amplified image of circular region in (a); (c) corresponding selected area electron
AC C
diffraction pattern of circular region in (b) with electron beam//[111]; (d) HRTEM image of the circular region in (b) and corresponding FFT(insert image) taken from rectangle region.
Shape memory properties of the film annealed at 650 °C were investigated via
strain-temperature curves under various applied stress as shown in Fig. 3(a). The applied stress was promoted by the stepwise of 100 MPa after each cycle from 300 MPa to 600MPa. The martensitic transformation start (Ms) temperatures were measured for each curve by tangent method. The contraction is originated from the recovery strain (εa) 7
ACCEPTED MANUSCRIPT representing the SME under constant stress due to the reverse martensitic transformation during heating. Fully recoverable strains can be obtained in the strain-temperature curves with the applied stresses varied from 300 MPa to 500 MPa.
RI PT
An irrecoverable strain (εp) of 0.1% is observed when the applied stress increased to 600 MPa. The irrecoverable strain is ascribed to a plastic deformation introduced during the transformation, which means that the critical stress for dislocation slip (yield stress)
SC
is about 600 MPa in Ni55.2Mn24.7Ga19.9Gd0.2 thin film. Compared with other HTSMA
M AN U
films such as Ti-Ni-Hf, Ti-Ni-Pd and Ti-Ni-Zr [2-4], Ni55.2Mn24.7Ga19.9Gd0.2 thin film possesses the highest yield stress, which means a high stress actuation performance as a kind of microactuator material. The high yield stress is originated from the intrinsic high strength of Ni-Mn-Ga alloy and the nanocrystalline microstructure in the film.
TE D
Shape memory properties of the film under different applied stresses are further summarized in Table 1. Ms increases from 239 °C to 268 °C with increasing applied stress, which is in accordance with the Clausius-Clapeyron equation. The value of εa
EP
also increases linearly from 0.51% to 0.63%, when the applied stress increases from 300
AC C
MPa to 500 MPa, which results in a greater volume fraction of preferentially oriented martensite variants during cooling. However, the value of εa just increases slightly to 0.64% when the applied stress further increases to 600 MPa since the introduction of plastic deformation in the film. Compared with the SME of 2.1% in dual-phase Ni-Mn-Ga-Co alloy measured by tensile deformation [29], the SME of the Ni55.2Mn24.7Ga19.9Gd0.2 thin film is evident smaller. It may be explained by the increasing volume of grain boundary due to the grain refinement, which restricts the
8
ACCEPTED MANUSCRIPT growth of preferentially oriented martensite variant during cooling. It is noteworthy that the quantitative SME under constant stress is reported firstly in polycrystalline Ni-Mn-Ga-based thin film. The work output, which is an important parameter for
RI PT
actuation performance, is also listed in Table 1. The maximum value of work output is 3.84 J/cm3 under the applied stress of 600 MPa. Although it is relatively lower than that in other HTSMA films [2-4], due to the small SME, the work output is still larger than
SC
other microactuators [30].
M AN U
The stability of shape memory behavior against deformation is of profound importance for SMA thin film as micro-actuator. For characterization of stability of shape memory behavior, the strain versus temperature curves as a function of the number of cycling deformation under a constant stress of 300 MPa was also evaluated,
TE D
as shown in Fig. 3(b). All the curves exhibit the similar SME and the Ms and εa are almost constant after cycling for 20 times. The reason for the high stability of shape memory behavior in the annealed thin film can be ascribe to the high yield stress of 600
AC C
EP
MPa, which blocks the introduction of dislocation during cyclic deformation.
9
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 3 (a) Strain-temperature curves measured under various applied stresses for Ni55.2Mn24.7Ga19.9Gd0.2 film; (b) Effect of the number of thermal cycling deformation on strain-temperature curves for Ni55.2Mn24.7Ga19.9Gd0.2 film under 300 MPa applied stress.
Applied stress (MPa)
Ms
Recovery strain
(oC)
εa (%)
239
EP
300
TE D
Table 1 Shape memory properties of Ni55.2Mn24.7Ga19.9Gd0.2 film under different applied stresses. Irrecoverable
Work output
strain εp (%)
(J/cm3)
0.51
0
1.53
252
0.57
0
2.28
500
258
0.63
0
3.15
268
0.64
0.1
3.84
AC C
400
600
In conclusion, Ni55.2Mn24.7Ga19.9Gd0.2 thin film showed excellent thermal stability of
martensitic transformation during 100 thermal cycles due to the mono-phase structure and hierarchical self-accommodated twinned structure. The quantitative SME under constant stress is reported firstly in polycrystalline Ni-Mn-Ga-based thin film. SME increased from 0.51% to 0.64% with the increasing applied stress from 300 MPa to 600
10
ACCEPTED MANUSCRIPT MPa. The plastic deformation occurred when the applied stress increased to 600 MPa. The SME almost kept constant after 20 times thermal cycling deformation under 300 MPa applied stress due to the high yield stress. With the excellent thermal stability of
thin film can be a competitive candidate for HTSMA thin film. Acknowledgements
RI PT
martensitic transformation and stable shape memory behavior, Ni55.2Mn24.7Ga19.9Gd0.2
SC
The authors of this research gratefully acknowledge the financial support denoted
M AN U
by the NSFC (Grant Nos. 51471060, 51501049 and 51371069). References
Y. Motemani, P.J.S. Buenconsejo, A. Ludwig, Shap. Mem. Superelasticity 1 (2015) 450-459.
[2]
G.K. Rasmussen, F. Chang, J. Zhang, et al., US 6592724B1, US Patent, 2003.
[3]
H.Y. Kim, M. Mizutani, S. Miyazaki, Acta Mater. 57 (2009) 1920-1930.
[4]
T. Sawaguchi, M. Sato, A. Ishida, Mater. Sci. Eng. A 332 (2002) 47-55.
[5]
M.K. Panduranga, D.D. Shin, G.P. Carman, Thin Solid Films 515 (2006) 1938-1941.
[6]
S. Inoue, N. Sawada, T. Namazu, Vacuum 83 (2008) 664-667.
[7]
C. Zhao, Y. Jin, S. Zhao, et al., Vacuum 144 (2017) 261-265.
[8]
P.J.S. Buenconsejo, A. Ludwig, Acta Mater. 85 (2015) 378-386.
[9]
Y. Motemani, P.J.S. Buenconsejo, C. Craciunescu, et al., Adv. Mater. Interfaces
TE D
[1]
(2014) 1400019.
[10] Y. Motemani, P.M. Kadletz, B. Maier, et al., Adv. Eng. Mater. 17 (2015) 1425–1433.
EP
[11] Y. Motemani, P.J. Mccluskey, C. Zhao, et al., Acta Mater. 59 (2011) 7602–7614. [12] P.J. Mccluskey, C. Zhao, O. Kfir, et al., Acta Mater. 59 (2011) 5116-5124. [13] Y. Ma, C. Jiang, Y. Li, et al., Acta Mater. 55 (2007) 1533-1541.
AC C
[14] X. Zhang, J. Sui, Z. Yang, et al., Mater. Lett. 123 (2014) 250-3. [15] H.B. Xu, Y.Q. Ma, C.B. Jiang, Appl. Phys. Lett. 82 (2003) 3206-3208. [16] Y. Xin, Y. Li, Z. Liu, Scr. Mater. 63 (2010) 35-38. [17] Y. Li, Y. Xin, C.B. Jiang, et al., Scr. Mater. 51 (2004) 849-852. [18] C. Liu, W. Cai, X. An, et al., Mater. Sci. Eng. A 438–440 (2006) 986-989. [19] H.B. Wang, C. Liu, Y.C. Lei, et al., J. Alloys Compd. 465 (2008) 458-461. [20] C. Liu, H.W. Mu, L.X. Gao, et al., Appl. Surf. Sci. 256 (2010) 6655-6659. [21] J. Yao, X. Zheng, W. Cai, et al., J. Alloys Compd. 661 (2016) 43-48. [22] A. Sharma, S. Mohan, S. Suwas, Acta Mater. 113 (2016) 259-271. [23] Y. Song, X. Chen, V. Dabade, et al., Nature 502 (2013) 85-88. [24] A. Ahadi, Q. Sun, Acta Mater. 90 (2015) 272-281. [25] X. Zhang, J. Sui, X. Zheng, et al., J. Alloys Compd. 557 (2013) 60-66. [26] D.Y. Cong, Y.D. Zhang, C. Esling, et al., Acta Mater. 59 (2011) 7070-7081.
11
ACCEPTED MANUSCRIPT [27] S. Kaufmann, U. Rößler, O. Heczko, et al., Phys. Rev. Lett. 104 (2010) 145702. [28] J. Pons, R. Santamarta, V.A. Chernenko, et al., J. Appl. Phys. 97 (2005). [29] Y. Ma, S. Yang, Y. Liu, et al., Acta Mater. 57 (2009) 3232-3241.
AC C
EP
TE D
M AN U
SC
RI PT
[30] N. Choudhary, D. Kaur, Sens. Actuators, A 242 (2016) 162-181.
12
ACCEPTED MANUSCRIPT
Highlights Film shows good stability of martensitic transformation after 100 thermal cycles. 0.64% high-temperature SME is obtained under the applied stress of 600 MPa.
AC C
EP
TE D
M AN U
SC
RI PT
SME of the film keeps stable after 20 thermal cycling deformations.
S0042-207X(17)31273-3
DOI:
10.1016/j.vacuum.2017.10.022
Reference:
VAC 7651
To appear in:
Vacuum
Received Date: 16 September 2017 Revised Date:
14 October 2017
Accepted Date: 16 October 2017
Please cite this article as: Yao J, Cui B, Zheng X, Wu Y, Sui J, Cai W, Thermal stability and hightemperature shape memory effect of Ni55.2Mn24.7Ga19.9Gd0.2 thin film, Vacuum (2017), doi: 10.1016/ j.vacuum.2017.10.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Thermal stability and high-temperature shape memory effect of Ni55.2Mn24.7Ga19.9Gd0.2 thin film Jian Yao1, Bo Cui1, Xiaohang Zheng1,*, Ye Wu, Jiehe Sui1, Wei Cai1,* School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
RI PT
1
*To whom correspondence should be addressed.
E-mail: [email protected] (Wei cai), [email protected] (Xiaohang Zheng)
SC
Abstract
Thermal stability and shape memory effect (SME) of Ni-Mn-Ga-based thin film
M AN U
were firstly investigated. The fluctuation scope of phase transformation temperatures did not exceed 2 °C during 100 thermal cycles. The maximum SME of 0.64% was quantitatively measured under 600 MPa applied stress. SME almost kept constant after
TE D
20 times thermal cycling deformation under 300 MPa applied stress. Keywords: Ni-Mn-Ga alloy; Thin films; Thermal stability; Shape memory effect. With the development of micro-electro-mechanical systems (MEMS), there are
EP
urgent demands for developing high temperature shape memory alloy (HTSMA) thin
AC C
films as microactuator using at elevated temperatures (˃100 °C) for automotive, aerospace and energy industries [1]. Until now, extensive research has been focused on Ti-Ni-X (X=Hf, Zr, Pd, Pt) [2-7], Au-Cu-Al [8] and Ti-Ta [9] HTSMA thin films. Nevertheless, the commercial application of HTSMA thin films is limited due to the high price in Ti-Ni-Pd, Ti-Ni-Pt and Au-Cu-Al systems and the poor thermal stability in Ti-Ni-Hf, Ti-Ni-Zr and Ti-Ta systems [10-12]. Recently, Ni-Mn-Ga based alloys have attracted much attention as a kind of
1
ACCEPTED MANUSCRIPT potential HTSMA since their adjustable martensitic transformation temperature, good thermal stability and relatively low cost [13, 14]. Xu et al. reported a large SME of 6.1% in Ni54Mn25Ga21 single crystal alloy with excellent thermal stability during 1000
RI PT
thermal cycles [15]. However, the fabrication of single crystal is time-consuming and very difficult to perform. Unfortunately, polycrystalline Ni-Mn-Ga alloys are extremely brittle due to the weak grain boundary. Xin et al. proposed that introducing a ductile γ
SC
phase by substitution of Ni for Ga is an effective method to improve the plasticity of
M AN U
Ni-Mn-Ga alloys. But the γ phase is harmful to the thermal stability and SME [16]. Additionally, it is prone to bring plastic deformation during actuation process even under low bias stress, which is not suitable for engineering application. Grain refinement is also a useful way to overcome this problem without reducing the SME
TE D
and thermal stability [17]. It is easy to attain fine-grained Ni-Mn-Ga thin film prepared by magnetron sputtering. The research on Ni-Mn-Ga-based HTSMA thin films has also been reported, which mainly is interested in film preparation, microstructure, phase
EP
transformation and magnetic properties [18-22]. However, the reports on the thermal
AC C
stability and shape memory effect (SME) of Ni-Mn-Ga based HTSMA thin films are not available. In our previous report, nanocrystalline Ni55.2Mn24.7Ga19.9Gd0.2 HTSMA thin films with SME have been successfully prepared by magnetron sputtering [21]. The present study aims to investigate the stability of phase transformation and shape memory behavior during thermal cycles in Ni55.2Mn24.7Ga19.9Gd0.2 HTSMA thin film. Ni55.2Mn24.7Ga19.9Gd0.2 (at. %) thin films were deposited on unheated silicon substrates by DC magnetron sputtering technique. Details of thin film preparation were
2
ACCEPTED MANUSCRIPT given by [21]. After sputtering, thin films were peeled off from the substrates to avoid an interfacial reaction between the thin film and substrate during the crystallization heat treatment. The freestanding films were fully crystallized at 650 °C for 60 s in a rapid
RI PT
thermal annealing furnace under a vacuum condition of 2.2×10-3 Pa. In order to measure the phase transformation temperatures of the film annealed at 650 °C, differential scanning calorimeter (DSC) equipment was employed with a
SC
heating speed of 20 °C/min. The mass of the film used for the DSC measurement was 9
M AN U
mg. After 100 thermal cycles in the DSC equipment, transmission electron microscopy (TEM) was used to investigate the internal structure of the film. A solution of 30% nitric acid in methanol was used to electrolytically polish the film for TEM observation at about -20 °C. Shape memory behavior of annealed film was examined by thermal
TE D
cycling tests under various constant stresses using a dynamical mechanical analyzer (DMA Q800). The size of the test sample was 5 µm×3 mm×12 mm (gauge portion). The sample was loaded a stress at 400 °C, then conducted thermal cycles from 160 to
EP
400 °C at a rate of 10 °C /min.
AC C
DSC curves of the annealed film after 1, 10, 50, 100 thermal cycles were shown in Fig. 1(a). It can be clearly seen that the shape and location of transformation peaks on the DSC curves don’t change significantly with the number of thermal cycles. Furthermore,
the
evolution
of
martensitic
transformation
temperatures
and
transformation latent heat (∆H) with the number of thermal cycles are shown as Fig. 1(b). It shows that the Mp (peak temperature of martensitic transformation) and Ap (peak temperature of austenitic transformation) vary in the range of 227.12~228.81 °C and
3
ACCEPTED MANUSCRIPT 257.18~258.91
°
C, respectively. As for the latent heat releasing from phase
transformation, there is no noticeable variation indicating the good resistance to phase decomposition during the thermal cycles. These findings reveal the excellent thermal
M AN U
SC
RI PT
stability of phase transformation in Ni55.2Mn24.7Ga19.9Gd0.2 thin film.
Fig. 1 Effect of thermal cycles on phase transformation of Ni55.2Mn24.7Ga19.9Gd0.2 thin film: (a) DSC
TE D
curves after different thermal cycles; (b) Mp, Ap transformation peak temperatures and latent heat after different thermal cycles.
In order to clarify the good thermal stability of phase transformation, TEM
EP
observations were carried out to investigate the microstructure of the annealed thin film
AC C
after 100 thermal cycles. The bright-field (BF) image exhibits the microstructure of the annealed film, which is consists of very fine grain with regular martensitic plates, as shown in Fig. 2(a). The microstructure is very similar to that in the annealed film before thermal cycles [21]. The nanoscale grain structure is beneficial to achieve good thermal stability. On the one hand, the small grain size of the film increases the critical stress for slip, which suppresses the dislocation formation and movement during thermal cycling [11, 12]. On the other hand, based on the geometric non-linear theory of martensite
4
ACCEPTED MANUSCRIPT (GNLTM), when λ2 (the middle eigenvalue of the transformation matrix) closes to 1, the SMA can achieve high functional stability [23]. Sun et al. reported that λ2 gradually approaches 1 with the reduction of grain size in Ti-Ni alloy [24]. In the
RI PT
Ni55.2Mn24.7Ga19.9Gd0.2 film, we determined from previous lattice parameters result that, λ2=1.0261 [21]. Compared with the value of λ2=1.0691 of the bulk counterpart in the similar compositions with a grain size of tens of micrometers [25], the film with
SC
nanograin tends to achieve better thermal stability. There are no precipitates observed in
M AN U
the microstructure, which is consistent with the constant transformation latent heat during thermal cycles. Fig. 2(b) is the magnified image from circular region in Fig. 2(a). It displays a strip-like morphology of well accommodated martensite plates with the width of ~20 nm. The adjacent plates connected by a distinct and straight twin boundary
TE D
exhibits (202) type-I twin relationship as shown in Fig. 2(c). Additionally, it can be easily observed that two thick variant plates both consist of thinner needle-like plates. The similar nanoscale internal twin structure was also found by reference [26] in
EP
Ni53Mn25Ga22 alloy. The HRTEM image further displays a clear and straight boundary
AC C
between the parallel thin plates with the width of ~1 nm in the thick plate as shown in Fig. 2(d). The Fast Fourier Transform (FFT) pattern taken from the rectangle region in the HRTEM image displays six extra satellite spots (marked by yellow arrows) with the main diffraction spots along (220) direction. That is the typical diffraction pattern of seven-layered (marked as 7M) martensite structure. In fact, 7M martensite can be considered as an adaptive nano-twinned phase with (52) stacking sequence built up by a non-modulated tetragonal phase unit cells defined as T martensite, according to the
5
ACCEPTED MANUSCRIPT viewpoint of S. Kaufmann [27] and J. Pons [28]. In this concept, every thin plate is nano-twinned structure in nature which is composed of a five atomic (220) planes variant and a two atomic (220) planes variant. Thus, another level of few atomic planes
RI PT
nano-twinned structure is added to the hierarchical twinned martensite. The hierarchical self-accommodated structure is in favor of releasing the stress arising from the geometrical incompatibility between austenite and martensite phase. It can effectively
SC
reduce the accumulation of plastic deformation during phase transformation, thereby
M AN U
improving the thermal stability of phase transformation. In summary, the nanoscale grain, mono-phase structure and hierarchical self-accommodated twinned structure are
AC C
EP
TE D
the main reasons for the excellent thermal stability of phase transformation in the film.
6
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
EP
Fig. 2 TEM micrographs of the annealed film after 100 thermal cycles: (a) BF image under low magnification; (b) amplified image of circular region in (a); (c) corresponding selected area electron
AC C
diffraction pattern of circular region in (b) with electron beam//[111]; (d) HRTEM image of the circular region in (b) and corresponding FFT(insert image) taken from rectangle region.
Shape memory properties of the film annealed at 650 °C were investigated via
strain-temperature curves under various applied stress as shown in Fig. 3(a). The applied stress was promoted by the stepwise of 100 MPa after each cycle from 300 MPa to 600MPa. The martensitic transformation start (Ms) temperatures were measured for each curve by tangent method. The contraction is originated from the recovery strain (εa) 7
ACCEPTED MANUSCRIPT representing the SME under constant stress due to the reverse martensitic transformation during heating. Fully recoverable strains can be obtained in the strain-temperature curves with the applied stresses varied from 300 MPa to 500 MPa.
RI PT
An irrecoverable strain (εp) of 0.1% is observed when the applied stress increased to 600 MPa. The irrecoverable strain is ascribed to a plastic deformation introduced during the transformation, which means that the critical stress for dislocation slip (yield stress)
SC
is about 600 MPa in Ni55.2Mn24.7Ga19.9Gd0.2 thin film. Compared with other HTSMA
M AN U
films such as Ti-Ni-Hf, Ti-Ni-Pd and Ti-Ni-Zr [2-4], Ni55.2Mn24.7Ga19.9Gd0.2 thin film possesses the highest yield stress, which means a high stress actuation performance as a kind of microactuator material. The high yield stress is originated from the intrinsic high strength of Ni-Mn-Ga alloy and the nanocrystalline microstructure in the film.
TE D
Shape memory properties of the film under different applied stresses are further summarized in Table 1. Ms increases from 239 °C to 268 °C with increasing applied stress, which is in accordance with the Clausius-Clapeyron equation. The value of εa
EP
also increases linearly from 0.51% to 0.63%, when the applied stress increases from 300
AC C
MPa to 500 MPa, which results in a greater volume fraction of preferentially oriented martensite variants during cooling. However, the value of εa just increases slightly to 0.64% when the applied stress further increases to 600 MPa since the introduction of plastic deformation in the film. Compared with the SME of 2.1% in dual-phase Ni-Mn-Ga-Co alloy measured by tensile deformation [29], the SME of the Ni55.2Mn24.7Ga19.9Gd0.2 thin film is evident smaller. It may be explained by the increasing volume of grain boundary due to the grain refinement, which restricts the
8
ACCEPTED MANUSCRIPT growth of preferentially oriented martensite variant during cooling. It is noteworthy that the quantitative SME under constant stress is reported firstly in polycrystalline Ni-Mn-Ga-based thin film. The work output, which is an important parameter for
RI PT
actuation performance, is also listed in Table 1. The maximum value of work output is 3.84 J/cm3 under the applied stress of 600 MPa. Although it is relatively lower than that in other HTSMA films [2-4], due to the small SME, the work output is still larger than
SC
other microactuators [30].
M AN U
The stability of shape memory behavior against deformation is of profound importance for SMA thin film as micro-actuator. For characterization of stability of shape memory behavior, the strain versus temperature curves as a function of the number of cycling deformation under a constant stress of 300 MPa was also evaluated,
TE D
as shown in Fig. 3(b). All the curves exhibit the similar SME and the Ms and εa are almost constant after cycling for 20 times. The reason for the high stability of shape memory behavior in the annealed thin film can be ascribe to the high yield stress of 600
AC C
EP
MPa, which blocks the introduction of dislocation during cyclic deformation.
9
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 3 (a) Strain-temperature curves measured under various applied stresses for Ni55.2Mn24.7Ga19.9Gd0.2 film; (b) Effect of the number of thermal cycling deformation on strain-temperature curves for Ni55.2Mn24.7Ga19.9Gd0.2 film under 300 MPa applied stress.
Applied stress (MPa)
Ms
Recovery strain
(oC)
εa (%)
239
EP
300
TE D
Table 1 Shape memory properties of Ni55.2Mn24.7Ga19.9Gd0.2 film under different applied stresses. Irrecoverable
Work output
strain εp (%)
(J/cm3)
0.51
0
1.53
252
0.57
0
2.28
500
258
0.63
0
3.15
268
0.64
0.1
3.84
AC C
400
600
In conclusion, Ni55.2Mn24.7Ga19.9Gd0.2 thin film showed excellent thermal stability of
martensitic transformation during 100 thermal cycles due to the mono-phase structure and hierarchical self-accommodated twinned structure. The quantitative SME under constant stress is reported firstly in polycrystalline Ni-Mn-Ga-based thin film. SME increased from 0.51% to 0.64% with the increasing applied stress from 300 MPa to 600
10
ACCEPTED MANUSCRIPT MPa. The plastic deformation occurred when the applied stress increased to 600 MPa. The SME almost kept constant after 20 times thermal cycling deformation under 300 MPa applied stress due to the high yield stress. With the excellent thermal stability of
thin film can be a competitive candidate for HTSMA thin film. Acknowledgements
RI PT
martensitic transformation and stable shape memory behavior, Ni55.2Mn24.7Ga19.9Gd0.2
SC
The authors of this research gratefully acknowledge the financial support denoted
M AN U
by the NSFC (Grant Nos. 51471060, 51501049 and 51371069). References
Y. Motemani, P.J.S. Buenconsejo, A. Ludwig, Shap. Mem. Superelasticity 1 (2015) 450-459.
[2]
G.K. Rasmussen, F. Chang, J. Zhang, et al., US 6592724B1, US Patent, 2003.
[3]
H.Y. Kim, M. Mizutani, S. Miyazaki, Acta Mater. 57 (2009) 1920-1930.
[4]
T. Sawaguchi, M. Sato, A. Ishida, Mater. Sci. Eng. A 332 (2002) 47-55.
[5]
M.K. Panduranga, D.D. Shin, G.P. Carman, Thin Solid Films 515 (2006) 1938-1941.
[6]
S. Inoue, N. Sawada, T. Namazu, Vacuum 83 (2008) 664-667.
[7]
C. Zhao, Y. Jin, S. Zhao, et al., Vacuum 144 (2017) 261-265.
[8]
P.J.S. Buenconsejo, A. Ludwig, Acta Mater. 85 (2015) 378-386.
[9]
Y. Motemani, P.J.S. Buenconsejo, C. Craciunescu, et al., Adv. Mater. Interfaces
TE D
[1]
(2014) 1400019.
[10] Y. Motemani, P.M. Kadletz, B. Maier, et al., Adv. Eng. Mater. 17 (2015) 1425–1433.
EP
[11] Y. Motemani, P.J. Mccluskey, C. Zhao, et al., Acta Mater. 59 (2011) 7602–7614. [12] P.J. Mccluskey, C. Zhao, O. Kfir, et al., Acta Mater. 59 (2011) 5116-5124. [13] Y. Ma, C. Jiang, Y. Li, et al., Acta Mater. 55 (2007) 1533-1541.
AC C
[14] X. Zhang, J. Sui, Z. Yang, et al., Mater. Lett. 123 (2014) 250-3. [15] H.B. Xu, Y.Q. Ma, C.B. Jiang, Appl. Phys. Lett. 82 (2003) 3206-3208. [16] Y. Xin, Y. Li, Z. Liu, Scr. Mater. 63 (2010) 35-38. [17] Y. Li, Y. Xin, C.B. Jiang, et al., Scr. Mater. 51 (2004) 849-852. [18] C. Liu, W. Cai, X. An, et al., Mater. Sci. Eng. A 438–440 (2006) 986-989. [19] H.B. Wang, C. Liu, Y.C. Lei, et al., J. Alloys Compd. 465 (2008) 458-461. [20] C. Liu, H.W. Mu, L.X. Gao, et al., Appl. Surf. Sci. 256 (2010) 6655-6659. [21] J. Yao, X. Zheng, W. Cai, et al., J. Alloys Compd. 661 (2016) 43-48. [22] A. Sharma, S. Mohan, S. Suwas, Acta Mater. 113 (2016) 259-271. [23] Y. Song, X. Chen, V. Dabade, et al., Nature 502 (2013) 85-88. [24] A. Ahadi, Q. Sun, Acta Mater. 90 (2015) 272-281. [25] X. Zhang, J. Sui, X. Zheng, et al., J. Alloys Compd. 557 (2013) 60-66. [26] D.Y. Cong, Y.D. Zhang, C. Esling, et al., Acta Mater. 59 (2011) 7070-7081.
11
ACCEPTED MANUSCRIPT [27] S. Kaufmann, U. Rößler, O. Heczko, et al., Phys. Rev. Lett. 104 (2010) 145702. [28] J. Pons, R. Santamarta, V.A. Chernenko, et al., J. Appl. Phys. 97 (2005). [29] Y. Ma, S. Yang, Y. Liu, et al., Acta Mater. 57 (2009) 3232-3241.
AC C
EP
TE D
M AN U
SC
RI PT
[30] N. Choudhary, D. Kaur, Sens. Actuators, A 242 (2016) 162-181.
12
ACCEPTED MANUSCRIPT
Highlights Film shows good stability of martensitic transformation after 100 thermal cycles. 0.64% high-temperature SME is obtained under the applied stress of 600 MPa.
AC C
EP
TE D
M AN U
SC
RI PT
SME of the film keeps stable after 20 thermal cycling deformations.