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Thermal stability of Ni54Mn25Ga20.9Gd0.1 high-temperature shape memory alloy with large reversible strain
Materials Letters 123 (2014) 250–253
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Thermal stability of Ni54Mn25Ga20.9Gd0.1 high-temperature shape memory alloy with large reversible strain Xin Zhang, Jiehe Sui n, Zheyi Yang, Xiaohang Zheng, Wei Cai 1 Science and Technology on Materials Performance Evaluation in Space Environment Laboratory, Harbin Institute of Technology, Harbin 150001, China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 December 2013 Accepted 3 February 2014 Available online 7 March 2014
The thermal stability of polycrystalline Ni54Mn25Ga20.9Gd0.1 high-temperature shape memory alloy was systematically investigated by the combination of X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and compression tests. The results show that the non-modulated tetragonal martensite of structure and type I(111)M twin of substructure was unchanged even after 2000 thermal cycles, and some dislocation structures were formed in martensite of thermal-cycled alloy. The martensitic transformation temperatures after different thermal cycles have no difference, and the shape memory effect decreased from 7.5% to 7.4% due to formation of dislocation structures. The excellent thermal stability was ascribed to the single-phase and well self-accommodated martensitic twins. & 2014 Elsevier B.V. All rights reserved.
Keywords: Ni–Mn–Ga alloy Thermal stability Phase transformation Shape memory materials Structural
1. Introduction Recently, Ni–Mn–Ga shape memory alloy has attracted more and more interests for their large magnetic field-induced strain and high response frequency [1–3]. In addition to the magnetic properties, Ni–Mn–Ga also has many other characteristics. The martensitic transformation temperature (Ms) of Ni–Mn–Ga alloys can be changed in the range from 100 to 450 1C by adjusting the composition [4,5]. Xu et al. reported an SME of 6.1% in Ni54Mn25 Ga21 single crystal with martensitic transformation temperature higher than 250 1C [6]. This SME remains stable even after 1000 thermal cycles. These qualities make the Ni–Mn–Ga alloy to be an excellent candidate for HTSMAs; however, such alloy is usually too brittle for practical use. Some approaches were adopted to improve mechanical properties, including grain refinement [7], second phase introduction [8] and fourth element addition [9]. Nevertheless, the mechanical properties and SME of Ni–Mn–Ga polycrystalline alloys were still lower than those of Ni–Mn–Ga single crystal [10,11]. It has been proved that rare earth addition can toughen Ni–Mn–Ga ferromagnetic shape memory alloy. Gao et al. discovered that addition of Gd, Dy and Y substituting Ga significantly improved mechanical properties of Ni50Mn29Ga21 ferromagnetic shape memory alloy [12–14]. Therefore, rare earth addition became an effective method to improve mechanical properties of Ni–Mn–Ga HTSMAs. In previous publication [15],
n
Corresponding author. Tel./fax: þ 86 451 8641 8649. E-mail addresses: [email protected] (J. Sui), [email protected] (W. Cai). 1 Tel./fax: þ 86 451 8641 8649.
http://dx.doi.org/10.1016/j.matlet.2014.02.088 0167-577X & 2014 Elsevier B.V. All rights reserved.
Ni54Mn25Ga20.9Gd0.1 HTSMA with 24.6% of compressive fracture strain and 7.5% of reversible strain had been obtained, and the improvement in mechanical properties and SME are mainly attributed to refinement strengthening and solid solution strengthening. The mechanical properties are similar to Ni54Mn25Ga21 single crystal and reversible strain is even better. However, thermal stability of this alloy is not clear because Xu et al. used a compressive method to study thermal stability of SME on
Fig. 1. XRD patterns of Ni54Mn25Ga20.9Gd0.1 alloy at original state and after different thermal cycles. a) Original state; b) 1000 thermal cycles; c) 2000 thermal cycles.
X. Zhang et al. / Materials Letters 123 (2014) 250–253
Ni54Mn25Ga21 single crystal. In order to compare thermal stability of our alloy with single crystal, SME of Ni54Mn25Ga20.9Gd0.1 HTSMA after 2000 thermal cycles is researched by the compressive method in this paper. At the same time, microstructure and phase transformation temperatures before and after thermal cycles are also investigated.
251
2. Material and experimental methods The nominal composition of the alloy studied was Ni54Mn25Ga20.9Gd0.1. High purity nickel, manganese, gallium and gadolinium, with a purity level of 99.99%, 99.7%, 99.99% and 99.99%, respectively, were melted in a non-consumed vacuum arc furnace
Fig. 2. (a) Bright field image of original Ni54Mn25Ga20.9Gd0.1 alloy; (b) and (c) the magnification and corresponding electron diffraction pattern of [011] zone of region A in (a); (d) bright field image of Ni54Mn25Ga20.9Gd0.1 alloy after 2000 thermal cycles and the corresponding diffraction pattern of [011] zone; (e) the magnification of area B in (d); (f) the magnification of area C in (e).
252
X. Zhang et al. / Materials Letters 123 (2014) 250–253
under argon atmosphere to prepare Ni54Mn25Ga20.9Gd0.1 alloy. The ingot was remelted six times to ensure homogeneity. The sample was annealed in vacuum quartz tubes at 800 1C for 24 h, and then immersed into ice water. X-ray diffraction (XRD) measurements were performed by a Rigaku D/max-rB diffractometer with Cu Kα radiation. The phase transformation temperatures were determined by differential scanning calorimetry (DSC) with heating and cooling rates of 20 1C/min. The microstructures of the alloys were surveyed by a transmission electron microscope (TEM)). Thin-foil specimens for TEM observation were mechanically polished to about 100 mm thickness and twinjet electropolished with an electrolyte of nitric acid and methanol (3:7 in volume) at 20 1C. The compression tests were performed at room temperature on an Instron 5569 testing system at a crosshead displacement speed of 0.2 mm/min, and the size of the sample was Ф3 mm 5 mm. The phase transformation stability was measured by cycling 2000 times between 300 1C and room temperature. Fig. 3. DSC curves of the Ni54Mn25Ga20.9Gd0.1 alloy after different thermal cycles.
3. Results and discussion 300 250
Original 2000 times
Stress (MPa)
200 150 100 50 0
0
1
2
3
4
5
6
7
Strain (%)
700
Original 2000 times
600 500
Stress (MPa)
Fig. 1 shows XRD patterns of Ni54Mn25Ga20.9Gd0.1 alloy at original state and after different thermal cycles. Ni54Mn25Ga20.9Gd0.1 alloy possesses pure martensitic structure as shown in Fig. 1(a). Six reflection peaks can be indexed by the complex tetragonal-structured martensite. XRD patterns of thermal-cycled alloys are shown in Fig. 1(b) and (c) with six reflection peaks of tetragonal-structured martensite. The positions of six diffraction peaks of martensite are almost the same even for 2000 thermal cycles, which demonstrates that thermal cycle does not change the structure of Ni54Mn25Ga20.9Gd0.1 alloy. In order to confirm the thermal stability of structures and substructures in Ni54Mn25Ga20.9Gd0.1 alloy, TEM observation was carried out. Fig. 2 shows TEM images and corresponding diffraction patterns of Ni54Mn25Ga20.9Gd0.1 alloy before and after 2000 thermal cycles. Fig. 2(a) shows the bright field image for original Ni54Mn25Ga20.9Gd0.1 alloy, and the magnified image of region A is shown in Fig. 2(b). Ni54Mn25Ga20.9Gd0.1 alloy displays stripe-like martensite plates with 1–2 mm width and well accommodated morphology, and hair-like stripes can also be seen in the big plate. The boundaries between various plates are distinct and straight. Fig. 2(c) shows corresponding diffraction pattern of area A in Fig. 2(a). The structure of martensite can be indexed as a non-modulated tetragonal structure, and substructure is type I(111)M twin. Fig. 2(d) and (e) shows the bright field images and corresponding SAED pattern for thermalcycled Ni54Mn25Ga20.9Gd0.1 alloy. It can be seen that the structure of martensite is not changed, and the boundaries between various plates remain distinct and straight. The substructure is still type I (111)M twin. Some dislocations are generated through thermal cycle process as shown in Fig. 2(f) which is magnification of region C in Fig. 2(e). The dislocation formation is attributed to the repeated moves of interface between martensite and parent phase. Although dislocations are formed through thermal cycle, the structure and substructures of Ni54Mn25Ga20.9Gd0.1 alloy remain unchanged. Ni54Mn25Ga20.9Gd0.1 alloy exhibits typical one-step martensitic transformation like Ni54Mn25Ga21 alloy, and the peak temperature of martensitic reverse transformation (Ap) and peak temperature of martensitic transformation (Mp) temperatures are 205 1C and 190 1C, respectively. Fig. 3 shows the DSC curves undergoing different thermal cycles. All of thermal-cycled samples still have one-step martensitic transformation, and exhibit Ap and Mp in the range of 205–206 1C and 189–194 1C, respectively. It can be proved that martensitic transformation temperatures after thermal cycle have no difference. Ni54Mn25Ga20.9Gd0.1 alloy exhibits an excellent thermal stability without change in martensitic structure and transformation
400 300 200 100
7.4% 0
0
2
4
6
8
10
Strain (%) Fig. 4. The compressive stress–strain curves of the Ni54Mn25Ga20.9Gd0.1 alloy at original state and after 2000 thermal cycles with (a) 6% and (b) 10% pre-strains.
temperature after 2000 thermal cycles. This phenomenon is due to single-phase structure and well self-accommodated martensitic twins stated above in Ni54Mn25Ga20.9Gd0.1 alloy.
X. Zhang et al. / Materials Letters 123 (2014) 250–253
Table 1 Shape memory behavior of Ni54Mn25Ga20.9Gd0.1 alloy at original state and after 2000 thermal cycles.
Sample
Pre-strain (%)
Strain after unloading (%)
Strain after heating (%)
SME strain (%)
Original state
6 10
4.2 7.5
0 0
4.2 7.5
After 2000 thermal cycles
6 10
4.3 7.6
0 0.2
4.3 7.4
In order to investigate the thermal stability of SME, compression tests were carried out at room temperature. Fig. 4 shows compressive stress–strain curves of Ni54Mn25Ga20.9Gd0.1 alloy with 6% and 10% pre-strains. The dotted lines represent recovery strain after being heated to 300 1C for 1 min. It can be seen that stress– strain curves of thermal-cycled Ni54Mn25Ga20.9Gd0.1 alloy are similar to those of original one. This proves that mechanical properties of Ni54Mn25Ga20.9Gd0.1 alloy do not change after 2000 thermal cycles. The SME and recovery ratio of Ni54Mn25Ga20.9Gd0.1 alloy at original state and after 2000 thermal cycles are listed in Table 1. Complete recovery is observed in both original and thermal-cycled Ni54Mn25Ga20.9Gd0.1 alloys when pre-strain is 6%. When pre-strain is 10%, 7.5% reversible strain and 100% recovery ratios are obtained in original Ni54Mn25Ga20.9Gd0.1 alloy. Nevertheless, the irreversible deformation occurs in thermal-cycled Ni54Mn25Ga20.9Gd0.1 alloy due to dislocation formation, but its reversible strain still reaches to 7.4%. The excellent stability of SME on Ni54Mn25Ga20.9Gd0.1 alloy is attributed to high thermal stability of reversible martensitic transformation.
4. Conclusions The microstructure, phase transformation behavior, and SME of Ni54Mn25Ga20.9Gd0.1 HTSMA with 2000 thermal cycles are investigated. The concrete conclusion is as follows:
253
(1) Ni54Mn25Ga20.9Gd0.1 alloy possesses pure tetragonal martensite of structure and type I(111)M twin of substructure. This structure and substructure have excellent thermal stability. (2) Ni54Mn25Ga20.9Gd0.1 alloy exhibits typical one-step martensitic transformation, and transformation temperature does not change after 2000 thermal cycles due to single-phase structure and well self-accommodated martensitic twins. (3) Ni54Mn25Ga20.9Gd0.1 alloy has outstanding performance on SME stability which is attributed to high thermal stability of reversible martensitic transformation. (4) Thermal stability of Ni54Mn25Ga20.9Gd0.1 alloy is as good as that of Ni54Mn25Ga21 single crystal.
Acknowledgment The study was supported by the Natural Science Foundation of China (Nos. 51271065 and 51271069) and Major State Basic Research Development Program of China (No. 2012CB619403) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20112302130006). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Ullakko K, Huang JK, Kantner C, O’Handley RC. Appl Phys Lett 1996;69:1966–8. Sozinov A, Likhachev AA, Lanska N, Ullakko K. Appl Phys Lett 2002;80:1746. Murray SJ, Marioni M, Allen SM, O’Handley RC. Appl Phys Lett 2000;77:886–8. Ma YQ, Jiang CB, Li Y, Xu HB, Wang CP, Liu XJ. Acta Mater 2007;55:1533–41. Chernenko VA, Cesari E, Kokorin VV, Vitenko IN. Scr Metall Mater 1995;33: 1239–44. Xu HB, Ma YQ, Jiang CB. Appl Phys Lett 2003;82:3206–8. Li Y, Xin Y, Jiang CB, Xu HB. Scr Mater 2004;51:849–52. Xin Y, Li Y, Chai L, Xu HB. Scr Mater 2007;57:599–601. Cai W, Gao L, Liu AL, Sui JH, Gao ZY. Scr Mater 2007;57:659–62. Yang SY, Ma YQ, Jiang HF, Liu XJ. Intermetallics 2011;19:225–8. Ma YQ, Lai SL, Yang SY, Luo Y, Wang CP, Liu XJ. Trans Nonferr Metal Soc 2011;21:96–101. Cai W, Gao L, Gao ZY. J Mater Sci 2007;42:9216–20. Gao L, Sui JH, Cai W, Gao ZY. Solid State Commun 2009;149:257–60. Gao L, Cai W, Liu AL, Zhao LC. J Alloys Compd 2006;425:314–7. Zhang X, Sui JH, Zheng XH, Yang ZY, Tian XH, Cai W. J Alloys Compd 2013;557: 60–6.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Thermal stability of Ni54Mn25Ga20.9Gd0.1 high-temperature shape memory alloy with large reversible strain Xin Zhang, Jiehe Sui n, Zheyi Yang, Xiaohang Zheng, Wei Cai 1 Science and Technology on Materials Performance Evaluation in Space Environment Laboratory, Harbin Institute of Technology, Harbin 150001, China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 December 2013 Accepted 3 February 2014 Available online 7 March 2014
The thermal stability of polycrystalline Ni54Mn25Ga20.9Gd0.1 high-temperature shape memory alloy was systematically investigated by the combination of X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and compression tests. The results show that the non-modulated tetragonal martensite of structure and type I(111)M twin of substructure was unchanged even after 2000 thermal cycles, and some dislocation structures were formed in martensite of thermal-cycled alloy. The martensitic transformation temperatures after different thermal cycles have no difference, and the shape memory effect decreased from 7.5% to 7.4% due to formation of dislocation structures. The excellent thermal stability was ascribed to the single-phase and well self-accommodated martensitic twins. & 2014 Elsevier B.V. All rights reserved.
Keywords: Ni–Mn–Ga alloy Thermal stability Phase transformation Shape memory materials Structural
1. Introduction Recently, Ni–Mn–Ga shape memory alloy has attracted more and more interests for their large magnetic field-induced strain and high response frequency [1–3]. In addition to the magnetic properties, Ni–Mn–Ga also has many other characteristics. The martensitic transformation temperature (Ms) of Ni–Mn–Ga alloys can be changed in the range from 100 to 450 1C by adjusting the composition [4,5]. Xu et al. reported an SME of 6.1% in Ni54Mn25 Ga21 single crystal with martensitic transformation temperature higher than 250 1C [6]. This SME remains stable even after 1000 thermal cycles. These qualities make the Ni–Mn–Ga alloy to be an excellent candidate for HTSMAs; however, such alloy is usually too brittle for practical use. Some approaches were adopted to improve mechanical properties, including grain refinement [7], second phase introduction [8] and fourth element addition [9]. Nevertheless, the mechanical properties and SME of Ni–Mn–Ga polycrystalline alloys were still lower than those of Ni–Mn–Ga single crystal [10,11]. It has been proved that rare earth addition can toughen Ni–Mn–Ga ferromagnetic shape memory alloy. Gao et al. discovered that addition of Gd, Dy and Y substituting Ga significantly improved mechanical properties of Ni50Mn29Ga21 ferromagnetic shape memory alloy [12–14]. Therefore, rare earth addition became an effective method to improve mechanical properties of Ni–Mn–Ga HTSMAs. In previous publication [15],
n
Corresponding author. Tel./fax: þ 86 451 8641 8649. E-mail addresses: [email protected] (J. Sui), [email protected] (W. Cai). 1 Tel./fax: þ 86 451 8641 8649.
http://dx.doi.org/10.1016/j.matlet.2014.02.088 0167-577X & 2014 Elsevier B.V. All rights reserved.
Ni54Mn25Ga20.9Gd0.1 HTSMA with 24.6% of compressive fracture strain and 7.5% of reversible strain had been obtained, and the improvement in mechanical properties and SME are mainly attributed to refinement strengthening and solid solution strengthening. The mechanical properties are similar to Ni54Mn25Ga21 single crystal and reversible strain is even better. However, thermal stability of this alloy is not clear because Xu et al. used a compressive method to study thermal stability of SME on
Fig. 1. XRD patterns of Ni54Mn25Ga20.9Gd0.1 alloy at original state and after different thermal cycles. a) Original state; b) 1000 thermal cycles; c) 2000 thermal cycles.
X. Zhang et al. / Materials Letters 123 (2014) 250–253
Ni54Mn25Ga21 single crystal. In order to compare thermal stability of our alloy with single crystal, SME of Ni54Mn25Ga20.9Gd0.1 HTSMA after 2000 thermal cycles is researched by the compressive method in this paper. At the same time, microstructure and phase transformation temperatures before and after thermal cycles are also investigated.
251
2. Material and experimental methods The nominal composition of the alloy studied was Ni54Mn25Ga20.9Gd0.1. High purity nickel, manganese, gallium and gadolinium, with a purity level of 99.99%, 99.7%, 99.99% and 99.99%, respectively, were melted in a non-consumed vacuum arc furnace
Fig. 2. (a) Bright field image of original Ni54Mn25Ga20.9Gd0.1 alloy; (b) and (c) the magnification and corresponding electron diffraction pattern of [011] zone of region A in (a); (d) bright field image of Ni54Mn25Ga20.9Gd0.1 alloy after 2000 thermal cycles and the corresponding diffraction pattern of [011] zone; (e) the magnification of area B in (d); (f) the magnification of area C in (e).
252
X. Zhang et al. / Materials Letters 123 (2014) 250–253
under argon atmosphere to prepare Ni54Mn25Ga20.9Gd0.1 alloy. The ingot was remelted six times to ensure homogeneity. The sample was annealed in vacuum quartz tubes at 800 1C for 24 h, and then immersed into ice water. X-ray diffraction (XRD) measurements were performed by a Rigaku D/max-rB diffractometer with Cu Kα radiation. The phase transformation temperatures were determined by differential scanning calorimetry (DSC) with heating and cooling rates of 20 1C/min. The microstructures of the alloys were surveyed by a transmission electron microscope (TEM)). Thin-foil specimens for TEM observation were mechanically polished to about 100 mm thickness and twinjet electropolished with an electrolyte of nitric acid and methanol (3:7 in volume) at 20 1C. The compression tests were performed at room temperature on an Instron 5569 testing system at a crosshead displacement speed of 0.2 mm/min, and the size of the sample was Ф3 mm 5 mm. The phase transformation stability was measured by cycling 2000 times between 300 1C and room temperature. Fig. 3. DSC curves of the Ni54Mn25Ga20.9Gd0.1 alloy after different thermal cycles.
3. Results and discussion 300 250
Original 2000 times
Stress (MPa)
200 150 100 50 0
0
1
2
3
4
5
6
7
Strain (%)
700
Original 2000 times
600 500
Stress (MPa)
Fig. 1 shows XRD patterns of Ni54Mn25Ga20.9Gd0.1 alloy at original state and after different thermal cycles. Ni54Mn25Ga20.9Gd0.1 alloy possesses pure martensitic structure as shown in Fig. 1(a). Six reflection peaks can be indexed by the complex tetragonal-structured martensite. XRD patterns of thermal-cycled alloys are shown in Fig. 1(b) and (c) with six reflection peaks of tetragonal-structured martensite. The positions of six diffraction peaks of martensite are almost the same even for 2000 thermal cycles, which demonstrates that thermal cycle does not change the structure of Ni54Mn25Ga20.9Gd0.1 alloy. In order to confirm the thermal stability of structures and substructures in Ni54Mn25Ga20.9Gd0.1 alloy, TEM observation was carried out. Fig. 2 shows TEM images and corresponding diffraction patterns of Ni54Mn25Ga20.9Gd0.1 alloy before and after 2000 thermal cycles. Fig. 2(a) shows the bright field image for original Ni54Mn25Ga20.9Gd0.1 alloy, and the magnified image of region A is shown in Fig. 2(b). Ni54Mn25Ga20.9Gd0.1 alloy displays stripe-like martensite plates with 1–2 mm width and well accommodated morphology, and hair-like stripes can also be seen in the big plate. The boundaries between various plates are distinct and straight. Fig. 2(c) shows corresponding diffraction pattern of area A in Fig. 2(a). The structure of martensite can be indexed as a non-modulated tetragonal structure, and substructure is type I(111)M twin. Fig. 2(d) and (e) shows the bright field images and corresponding SAED pattern for thermalcycled Ni54Mn25Ga20.9Gd0.1 alloy. It can be seen that the structure of martensite is not changed, and the boundaries between various plates remain distinct and straight. The substructure is still type I (111)M twin. Some dislocations are generated through thermal cycle process as shown in Fig. 2(f) which is magnification of region C in Fig. 2(e). The dislocation formation is attributed to the repeated moves of interface between martensite and parent phase. Although dislocations are formed through thermal cycle, the structure and substructures of Ni54Mn25Ga20.9Gd0.1 alloy remain unchanged. Ni54Mn25Ga20.9Gd0.1 alloy exhibits typical one-step martensitic transformation like Ni54Mn25Ga21 alloy, and the peak temperature of martensitic reverse transformation (Ap) and peak temperature of martensitic transformation (Mp) temperatures are 205 1C and 190 1C, respectively. Fig. 3 shows the DSC curves undergoing different thermal cycles. All of thermal-cycled samples still have one-step martensitic transformation, and exhibit Ap and Mp in the range of 205–206 1C and 189–194 1C, respectively. It can be proved that martensitic transformation temperatures after thermal cycle have no difference. Ni54Mn25Ga20.9Gd0.1 alloy exhibits an excellent thermal stability without change in martensitic structure and transformation
400 300 200 100
7.4% 0
0
2
4
6
8
10
Strain (%) Fig. 4. The compressive stress–strain curves of the Ni54Mn25Ga20.9Gd0.1 alloy at original state and after 2000 thermal cycles with (a) 6% and (b) 10% pre-strains.
temperature after 2000 thermal cycles. This phenomenon is due to single-phase structure and well self-accommodated martensitic twins stated above in Ni54Mn25Ga20.9Gd0.1 alloy.
X. Zhang et al. / Materials Letters 123 (2014) 250–253
Table 1 Shape memory behavior of Ni54Mn25Ga20.9Gd0.1 alloy at original state and after 2000 thermal cycles.
Sample
Pre-strain (%)
Strain after unloading (%)
Strain after heating (%)
SME strain (%)
Original state
6 10
4.2 7.5
0 0
4.2 7.5
After 2000 thermal cycles
6 10
4.3 7.6
0 0.2
4.3 7.4
In order to investigate the thermal stability of SME, compression tests were carried out at room temperature. Fig. 4 shows compressive stress–strain curves of Ni54Mn25Ga20.9Gd0.1 alloy with 6% and 10% pre-strains. The dotted lines represent recovery strain after being heated to 300 1C for 1 min. It can be seen that stress– strain curves of thermal-cycled Ni54Mn25Ga20.9Gd0.1 alloy are similar to those of original one. This proves that mechanical properties of Ni54Mn25Ga20.9Gd0.1 alloy do not change after 2000 thermal cycles. The SME and recovery ratio of Ni54Mn25Ga20.9Gd0.1 alloy at original state and after 2000 thermal cycles are listed in Table 1. Complete recovery is observed in both original and thermal-cycled Ni54Mn25Ga20.9Gd0.1 alloys when pre-strain is 6%. When pre-strain is 10%, 7.5% reversible strain and 100% recovery ratios are obtained in original Ni54Mn25Ga20.9Gd0.1 alloy. Nevertheless, the irreversible deformation occurs in thermal-cycled Ni54Mn25Ga20.9Gd0.1 alloy due to dislocation formation, but its reversible strain still reaches to 7.4%. The excellent stability of SME on Ni54Mn25Ga20.9Gd0.1 alloy is attributed to high thermal stability of reversible martensitic transformation.
4. Conclusions The microstructure, phase transformation behavior, and SME of Ni54Mn25Ga20.9Gd0.1 HTSMA with 2000 thermal cycles are investigated. The concrete conclusion is as follows:
253
(1) Ni54Mn25Ga20.9Gd0.1 alloy possesses pure tetragonal martensite of structure and type I(111)M twin of substructure. This structure and substructure have excellent thermal stability. (2) Ni54Mn25Ga20.9Gd0.1 alloy exhibits typical one-step martensitic transformation, and transformation temperature does not change after 2000 thermal cycles due to single-phase structure and well self-accommodated martensitic twins. (3) Ni54Mn25Ga20.9Gd0.1 alloy has outstanding performance on SME stability which is attributed to high thermal stability of reversible martensitic transformation. (4) Thermal stability of Ni54Mn25Ga20.9Gd0.1 alloy is as good as that of Ni54Mn25Ga21 single crystal.
Acknowledgment The study was supported by the Natural Science Foundation of China (Nos. 51271065 and 51271069) and Major State Basic Research Development Program of China (No. 2012CB619403) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20112302130006). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Ullakko K, Huang JK, Kantner C, O’Handley RC. Appl Phys Lett 1996;69:1966–8. Sozinov A, Likhachev AA, Lanska N, Ullakko K. Appl Phys Lett 2002;80:1746. Murray SJ, Marioni M, Allen SM, O’Handley RC. Appl Phys Lett 2000;77:886–8. Ma YQ, Jiang CB, Li Y, Xu HB, Wang CP, Liu XJ. Acta Mater 2007;55:1533–41. Chernenko VA, Cesari E, Kokorin VV, Vitenko IN. Scr Metall Mater 1995;33: 1239–44. Xu HB, Ma YQ, Jiang CB. Appl Phys Lett 2003;82:3206–8. Li Y, Xin Y, Jiang CB, Xu HB. Scr Mater 2004;51:849–52. Xin Y, Li Y, Chai L, Xu HB. Scr Mater 2007;57:599–601. Cai W, Gao L, Liu AL, Sui JH, Gao ZY. Scr Mater 2007;57:659–62. Yang SY, Ma YQ, Jiang HF, Liu XJ. Intermetallics 2011;19:225–8. Ma YQ, Lai SL, Yang SY, Luo Y, Wang CP, Liu XJ. Trans Nonferr Metal Soc 2011;21:96–101. Cai W, Gao L, Gao ZY. J Mater Sci 2007;42:9216–20. Gao L, Sui JH, Cai W, Gao ZY. Solid State Commun 2009;149:257–60. Gao L, Cai W, Liu AL, Zhao LC. J Alloys Compd 2006;425:314–7. Zhang X, Sui JH, Zheng XH, Yang ZY, Tian XH, Cai W. J Alloys Compd 2013;557: 60–6.