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Proton-irradiation-induced structural and magnetic property changes in Ni–Mn–Ga high-temperature shape memory films
Materials Science and Engineering B 223 (2017) 76–83
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Proton-irradiation-induced structural and magnetic property changes in Ni–Mn–Ga high-temperature shape memory films Zhai-Ping Yang, Zhi-Yong Gao, Wei Cai ⇑ School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 6 February 2017 Received in revised form 17 May 2017 Accepted 2 June 2017 Available online 13 June 2017 Keywords: Ni–Mn–Ga film Proton irradiation Crystal structure Martensitic transformation Magnetic properties
a b s t r a c t Ni51.90Mn28.86Ga19.24 alloy films were irradiated by 120 keV protons at different fluences to investigate the effect of irradiation on their crystal structure, phase-transition temperature, and magnetic properties. The results show not only that a nanoscale amorphous layer may have been induced on the surface, but also that non-modulated (NM) martensite was produced. Moreover, differential scanning calorimetry peaks of the martensitic and austenitic transition temperatures are shifted to higher temperatures after irradiation; in particular, two endothermic and two exothermic peaks were observed for a proton fluence of 2.0 1016/cm2. Further, the formation of NM martensite could produce an extra shoulder on the M–T curves of the irradiated films. Regarding the magnetic properties, a gradual decrease in the Curie temperature TC can be caused by irradiation at a higher proton fluence. The changes in the saturation magnetization Ms with the proton fluence seem to be related to the reduction in TC. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction In the past few years, magnetic shape memory alloy films have been developed as novel functional materials with potential applications in sensor and actuation devices [1]. Ferromagnetic offstoichiometric Ni2MnGa shape memory alloys have been widely studied owing to their giant strains (up to 10%) and highfrequency response under an external magnetic field [2]. When magnetic shape memory alloy films are used in some extreme environments, such as aerospace, they need to function at high temperature (i.e., above 400 K) with high thermal and magnetic stability [3]. Several investigations have shown that Ni-rich Ni–Mn–Ga alloys are a promising candidate for use as hightemperature shape memory alloys owing to their high and adjustable martensitic phase transition temperature [4]. The martensitic transformation behavior, shape memory effect, and magneticfield-induced strain of Ni–Mn–Ga high-temperature shape memory alloys have been investigated [4–7]. However, it is noteworthy that Ni–Mn–Ga alloys will be irradiated by high-energy ions, especially protons, when they are used in the aerospace environment. In this case, their properties, such as their magnetic, electrical, and mechanical properties, will probably be changed by space radiation [8–10]. It is well known that irra-
diation could create defects and induce phase transformations [11,12], which may influence the martensitic transition temperature and Curie temperature (TC) of Ni–Mn–Ga alloys. To date, there have been various studies on the effect of ion irradiation on metals and alloys. Tolley et al. [12,13] reported an increase in the phase transformation temperature in proton- and electron-irradiated Cu–Zn–Al alloys. It was found that in Ni–Mn–Sn alloy, TC can be changed by increasing the fluence of Ar ions or Au ions [14,15]. Moreover, irradiation could also alter the saturation magnetization (Ms) of alloys [16]. Thus, structural and magnetic investigation of hightemperature Ni–Mn–Ga alloys under proton irradiation is essential. In this work, our goal is to provide information about the changes in the structure, phase transition temperature, and magnetic properties of high-temperature Ni–Mn–Ga films under proton irradiation with different fluences. This work can not only reveal the evolution of the structure and magnetic properties of Ni–Mn–Ga high-temperature shape memory films, but also promote and accelerate their practical application in the aerospace industry. In the following sections, the detailed experimental procedure is first described. Then the main results are presented, and the structural and magnetic changes after proton irradiation are discussed.
2. Experimental ⇑ Corresponding author. E-mail (W. Cai).
addresses:
[email protected] (Z.-P. Yang), [email protected]
http://dx.doi.org/10.1016/j.mseb.2017.06.004 0921-5107/Ó 2017 Elsevier B.V. All rights reserved.
Ni–Mn–Ga films were prepared by direct-current magnetron sputtering with high-purity argon at 0.14 Pa in a high-vacuum
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(8.0 105 Pa) chamber. The films were first deposited onto a Si (1 1 1) substrate whose temperature was maintained at 300 K using a cooling water system during deposition. The sputtering power was 100 W, and the deposition rate was 0.49 nm s1. After the deposition process, the films were mechanically removed from the Si substrate as freestanding samples. Subsequently, the freestanding films were annealed at 873 K in a high-vacuum chamber (<6.0 105 Pa) for about 3600 s. Fracture sections of the annealed films were observed by an FEI Quanta 200FEG scanning electron microscope (SEM, high voltage of 20 kV) to estimate the film thickness, as shown in Fig. 1(a). The thickness of the obtained films was found to be approximately 3.3 lm. Furthermore, the compositions of the annealed films were determined by energy-dispersive spectrometry (EDS, energy resolution of 128 eV) equipment installed on the SEM (Fig. 1(b)). The annealed films were found to have a composition of Ni51.90Mn28.86Ga19.24 (at.%) without any other redundant elements. Proton irradiation was conducted using a proton accelerator attached to the ground-based complex irradiation simulation system at Harbin Institute of Technology, China. An electric scanning unit is used in the X direction and a magnetic scanning unit is used in the Y direction to obtain a homogeneous irradiated proton-beam field of 20 20 mm2. Three square samples with dimension of 20 20 mm2 were cut from the freestanding films and irradiated with 120 keV protons under high-vacuum conditions (1.2 104 Pa). A proton flux of 1.85 1012 protons/cm2 s1 was used in the proton irradiation experiment. For every square sample, the proton fluence was maintained at 0, 1.0 1016, or 2.0 1016 protons/cm2. During irradiation, the proton beam was incident perpendicular to the samples. Moreover, the temperature of all of the samples during irradiation was kept at room temperature using a circulating cooling water system. The Monte Carlo code SRIM was used to calculate the damage distribution [17]. The displacement threshold energy (Ed) of Ni, Mn, and Ga is 25 eV. Depth profiles of the atomic displacement damage and hydrogen concentration are shown in Fig. 2, in which TR corresponds to the track region, CR is the cascade region, and NIR is the non-implanted region [18]. The projected range of protons in the Ni51.90Mn28.86Ga19.24 film is clearly approximately 800 nm. The crystalline structures of unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films were identified by wide-angle X-ray diffraction (WA-XRD) and grazing-angle incidence XRD (GI-XRD) using Cu Ka radiation (k = 0.154 nm) at room temperature. To investigate the microstructural changes after proton irradiation, a transmission electron microscopy (TEM) sample irradiated with 2.0 1016 -
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Fig. 2. Depth profiles of the displacement damage and hydrogen concentration in Ni51.90Mn28.86Ga19.24 films irradiated by 120 keV protons (calculated by SRIM2008 code). TR: track region, CR: cascade region, NIR, non-implanted region.
protons/cm2 was prepared by focused ion beam (FIB, HELIOS NanoLab 600i) milling. Before ion milling by the FIB, a thin protective layer of platinum was deposited on the surface of the Ni51.90Mn28.86Ga19.24 films using ion-beam-assisted chemical vapor deposition. Then the prepared samples were observed using a Tecnai G2 F30 TEM instrument. The phase transformation temperatures of these films were determined using a Perkin-Elmer differential scanning calorimetry (DSC) instrument at cooling and heating rates of 20 °Cmin1. Furthermore, the magnetic properties of these Ni51.90Mn28.86Ga19.24 films were probed with a vibrating sample magnetometer (Squid VSM, Quantum Design PPMS). The temperature range for the temperature-dependent magnetization measurement was 10–400 K, and the magnetic field was uniformly set to 10 mT along the film plane. The magnetic hysteresis loop was measured at 300 and 10 K at magnetic fields ranging from 5 to 5 T. 3. Results and discussion Regarding the crystal structure of the films, Fig. 3 shows the XRD patterns of the unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films at room temperature. These results, as well as the TEM micrographs in Fig. 4, selected-area electron diffraction (SAED) patterns in Fig. 5, and DSC curves in Fig. 6, reveal that the
Fig. 1. Thickness and composition of the annealed Ni–Mn–Ga films: (a) SEM image of the fracture section of a film used for thickness measurement; (b) EDS pattern.
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Fig. 3. XRD patterns of the unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films: (a) GI-XRD; (b) WA-XRD.
Fig. 4. Bright-field TEM micrographs of Ni51.90Mn28.86Ga19.24 films irradiated with 2.0 1016/cm2 proton fluence.
unirradiated film has a seven-layered modulated (7M) orthorhombic martensitic structure (the space group is 58, Pnnm) at room temperature. The three typical peaks at 43.1°, 44.4°, and 45.8°, which are very similar to the reference angles of 43.3°, 44.7°, and 46.0° from [19], are marked by their Miller indices as the {2 2 0}, {2 0 2}, and {0 2 2} planes, respectively. The GI-XRD results in Fig. 3(a) clearly show that the irradiated films have diffraction peaks similar to those of the unirradiated film, indicating that no
new phase is generated on surface of the irradiated films or the quantity of any new phase is too small to be detected. In addition, the WA-XRD patterns of the irradiated films also have diffraction peaks similar to those of the unirradiated films at approximately 42.8°, 44.2°, and 45.6°, as shown Fig. 3(b). However, for the 2.0 1016/cm2 irradiated film, an extra peak appears at 43.2°. In combination with the SAED patterns in Fig. 5, these results suggest that the peak at 43.2° indicates the formation of a new phase of non-modulated (NM) tetragonal martensite (139, I4/mmm) [20]. In addition, the lattice constants of the unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films were calculated from the diffraction peaks at 2h = 42°–46°, as summarized in Table 1. The changes in the lattice constants and cell volume (V) of the irradiated films with increasing proton fluence are not obvious. To further confirm the formation of a new phase in the films irradiated with a proton fluence of 2.0 1016/cm2 (Fig. 3(b)), a TEM sample was prepared by FIB milling along the proton incidence direction to reveal the cross section of the irradiated film, as shown in Fig. 4. The red dashed line separates the irradiated and unirradiated zones according to the depth profile of the displacement damage in Fig. 2. Furthermore, Fig. 5 shows magnified bright-field images and the corresponding SAED patterns of the surface, the areas labeled A in the unirradiated zone and B in the irradiated zone in Fig. 4. First, a distinctive nanoscale layer approximately 16 nm thick is observed on the surface of the irradiated films, as shown in Fig. 5 (a). A high-resolution TEM (HRTEM) image of the blue dashed box shows the nanoscale layer more clearly. A clear boundary appears between the nanoscale layer and irradiated zone, and the corresponding fast Fourier transformation confirms that the nanoscale layer is amorphous. Note that the amorphous layer can stem from the proton irradiation or ion beam damage during TEM sample preparation process. Fig. 5(b,c) and (d,e) display the bright-field TEM images and their corresponding SAED patterns taken from regions A (unirradiated zone) and B (irradiated zone) in Fig. 4, respectively. The A zone is 7 M martensite [19]. However, the B zone is NM martensite [20]. These results suggest that 120 keV proton irradiation may not only induce the formation of the nanoscale amorphous layer on the surface of the Ni51.90Mn28.86Ga19.24 films, but also promote the phase transformation of martensite from 7 M to NM in the irradiated zone. Fig. 6 shows the DSC curves of the free-standing Ni51.90Mn28.86Ga19.24 films irradiated at various proton fluences. For the unirradiated Ni51.90Mn28.86Ga19.24 films, an obvious exothermic peak appears during the cooling process, whereas an endothermic peak
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Fig. 5. Bright-field TEM micrographs and corresponding SAED patterns of Ni51.90Mn28.86Ga19.24 films irradiated with 2.0 1016/cm2 proton fluence: (a) Bright-field image, HRTEM pattern, and SAED pattern of nano-amorphous layer on surface of films; (b) and (c) bright-field image and the corresponding SAED pattern, respectively, of A (unirradiated zone) in Fig. 4; (d) and (e) bright-field image and the corresponding SAED pattern, respectively, of B (irradiated zone) in Fig. 4.
occurs during the heating process. This is the well-known typical characteristic of the one-step thermoelastic martensitic transition [6]. After irradiation with a proton fluence of 1.0 1016/cm2, the
DSC curve of the irradiated film resembles that of the unirradiated film, but this irradiation fluence affects the martensitic transformation temperature (Tm) and austenitic transformation temperature
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Fig. 6. DSC curves of Ni51.90Mn28.86Ga19.24 films irradiated at various proton fluences.
(Ta) of the Ni51.90Mn28.86Ga19.24 films. Fig. 6 shows that the peaks of Tm and Ta for the 1.0 1016/cm2 proton-irradiated films are shifted to higher temperatures. Moreover, the martensitic transformation temperatures of both the unirradiated and 1.0 1016/cm2 proton-irradiated films are above room temperature. It is indicated that they are martensite at room temperature. It is interesting that, when the Ni51.90Mn28.86Ga19.24 films were irradiated with a higher proton fluence of 2.0 1016/cm2, two endothermic peaks and two exothermic peaks were observed on their DSC curves. This is entirely different from the results for the unirradiated and 1.0 1016 protons/cm2 irradiated films. All the phase transformation temperatures of the unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films are listed in Table 2 for clear comparison. The variation in the transformation temperature and the appearance of double peaks for the irradiated Ni51.90Mn28.86Ga19.24 films may be attributed to the migration of lattice defects (such as interstitials and vacancies) and the formation of NM martensite induced by proton irradiation [12]. During the heating process, lattice defects introduced by proton irradiation could hinder the austenitic transformation and lead to the increase in Ta from 468.7 to 483.7 K. As the heating temperature continues to increase, diffusion of vacancies occurs, and the number of lattice defects in the
austenite decreases, which facilitates the martensitic transformation during the cooling process and leads to an increase in Tm. As shown in Fig. 2, the SRIM code was used to calculate displacement cross section as a function of depth for the Ni51.90Mn28.86Ga19.24 films irradiated with protons at an energy of 120 keV. From the calculation, we obtained a projected range of 800 nm, which indicates that all the incident protons will come to rest in the films. When the proton fluence is large enough, for example, 2.0 1016/cm2, NM martensite was formed in the films (Fig. 5(e)). The phase transformation temperatures of the NM martensite may differ from those of the matrix, as indicated in the DSC curves by the double endothermic and exothermic peaks. Fig. 7 shows the magnetization versus temperature (M–T) curves of the unirradiated and irradiated martensitic Ni51.90Mn28.86Ga19.24 films, which were used to determine the effect of the proton fluence on their Curie temperature (TC). For the unirradiated film, TC is determined to be 346.9 K, at which the film shows a transition from the paramagnetic state to the ferromagnetic state during cooling, as shown in Fig. 7(a). As the temperature decreases further to approximately 10 K, we did not observe the martensitic transition, indicating that the martensitic transition temperature is higher than TC, as shown in the DSC results. In addition, the heating curve had a shape similar to that of the cooling curve. These results show that the unirradiated Ni51.90Mn28.86Ga19.24 film is ferromagnetic martensite at room temperature. Furthermore, when the Ni51.90Mn28.86Ga19.24 films were irradiated by 120 keV protons, a gradual decrease in TC was observed with increasing proton fluence, as shown in Fig. 7(b–d). The TC values of the unirradiated and irradiated films are also listed in Table 2, which shows that TC decreases from 346.9 to 312.3 K after irradiation with 2.0 1016 protons/cm2. It is well known that an ordered film could become chemically disordered under irradiation, showing features such as point defects and dense cascades, which would reduce its Curie temperature [21]. In addition, an extra shoulder is observed below TC1 = 287.8 K and TC2 = 245.6 K in the M–T curves of the films irradiated by proton fluences of 1.0 1016/cm2 and 2.0 1016/cm2, respectively. The shoulder is more obvious at the higher fluence. This phenomenon may be attributed to the existence of a magnetic phase with a low Curie temperature [22]. The discussion of the XRD, DSC, and TEM results stated that NM martensite was formed in the irradiated films, and the NM martensite might be just the magnetic precipitate phase, with a lower Curie temperature than the film itself.
Table 1 Lattice constants and cell volume of unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films. Material state
0/cm2 1.0 1016/cm2 2.0 1016/cm2
Lattice constant a (Å)
Error (r)
b (Å)
Error (r)
c (Å)
Error (r)
V (Å3)
Error (r)
6.189 6.163 6.178
1.5E 2 0.4 E 2 1.5E 2
5.770 5.761 5.774
2.6E 2 0.7 E 2 2.5 E 2
5.437 5.480 5.440
1.0E 2 0.3E 2 1.0E 2
194.147 194.552 194.041
6.1E 1 1.7 E 1 5.9 E 1
Table 2 Effect of proton fluence on phase and magnetic transition temperatures and saturation magnetization (Ms) of Ni51.90Mn28.86Ga19.24 films. Proton fluence (protons/ cm2)
T (K)
0
—
Ms (Am2kg1)
Tm
1.0 10
16
—
2.0 10
16
437.7 ± 1.1 (3)
Ta 447.4 ± 0.9 (2) 453.1 ± 0.3 (3) 461.9 ± 2.1 (3)
— — 463.1 ± 1.1 (3)
TC 468.7 ± 1.2 (2) 471.6 ± 2.8 (3) 483.7 ± 1.0 (3)
The number in parentheses represents the number of measurements for each parameter.
— 287.8 ± 1.0 (2) 245.6 ± 2.0 (2)
346.9 ± 0.6 (2) 322.9 ± 0.6 (2) 312.3 ± 1.8 (2)
300 K
10 K
52.08 ± 0.08 (2) 21.04 ± 0.05 (2) 14.01 ± 0.06 (2)
55.84 ± 0.03 (2) 60.67 ± 0.03 (2) 86.77 ± 0.06 (2)
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Fig. 7. M–T curves of the Ni51.90Mn28.86Ga19.24 films at low field (100 Oe): (a) unirradiated film; (b), (c), and (d) films irradiated at different fluences, where the red line corresponds to the first derivative.
Magnetization curves of unirradiated Ni51.90Mn28.86Ga19.24 films were measured at temperatures of 300 and 380 K along a direction parallel to the film plane, as shown in Fig. 8. Fig. 8(a) clearly shows that the magnetization curve at 380 K is nearly linear, which demonstrates that the unirradiated film is in a paramagnetic state at 380 K. However, the unirradiated film is found to be in the ferromagnetic state at room temperature (300 K), at which its saturation magnetization (Ms) is 52.08 A m2 kg1, and its coercivity is 50.79 (103/4p Am1) (Fig. 8(b)).
The effects of the proton fluence on the magnetization curves at 300 K of the Ni51.90Mn28.86Ga19.24 films irradiated by 120 keV protons are shown in Fig. 9(a). The saturation magnetization (Ms) decreased from 52.08 to 14.01 A m2 kg1 when the proton fluence increased from 0 to 2.0 1016/cm2. The Ms values of the unirradiated and irradiated films are listed in Table 2 for clear comparison. The changes in the Ms values of the Ni51.90Mn28.86Ga19.24 films after proton irradiation may be attributed to the formation of NM martensite. The magnetization curves measured at 300 K reveal
Fig. 8. Magnetization curves of Ni51.90Mn28.86Ga19.24 films: (a) magnetization curves of the unirradiated films at 380 and 300 K; (b) magnified view of the central area in (a).
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Fig. 9. Effect of proton fluence on the magnetization curves of irradiated Ni51.90Mn28.86Ga19.24 films at different temperatures: (a) 300 K; (b) 10 K.
that the NM martensite induced by proton irradiation is paramagnetic, as indicated by the reduction of the Curie temperature to TC1 = 287.8 K or TC2 = 245.6 K (Fig. 8). Therefore, a major portion of these changes seems to be directly related to the reduction in TC. To further confirm the above statement, the effects of the proton fluence on the hysteresis loops far from TC, that is, at 10 K, were also measured (Fig. 9(b)). When the magnetic hysteresis loops were measured at 10 K, both the unirradiated and irradiated films were found to be ferromagnetic martensite. Ms increased from 55.84 to 86.77 A m2 kg1 when the proton fluence increased from 0 to 2.0 1016/cm2. It is well known that the electron concentration (e/a, average number of valence electrons per atom) is a critical factor that influences Ms in Heusler compounds. At 10 K, Ms has a maximum value when e/a = 7.53, beyond which Ms decreases linearly with increasing e/a regardless of the type of martensitic structure [23]. The e/a value of the experimental film in this investigation is 7.79 [24]. After the films are irradiated with protons, an interaction between H 1 s and Mn 4 s may result in a decrease in the metallic bond strength [25], which subsequently leads to a decrease in e/a. Therefore, the proton-irradiation-induced decrease of e/a will result in an increase in Ms. 4. Conclusions The effects of 120 keV proton irradiation on the structure, phase transition temperature, and magnetic properties of a ferromagnetic film, i.e., Ni51.90Mn28.86Ga19.24, were studied in detail. Structural analysis by GI-XRD indicates that no new phase is generated on surface of the irradiated films or the quantity of any new phase is too small to be detected, but an extra peak appears at 43.2° in the WA-XRD pattern for the film irradiated at a proton fluence of
2.0 1016/cm2, indicating the formation of a new phase. TEM observations revealed that the cross section of the irradiated film was divided into three layers: a distinctive 16 nm amorphous layer on the surface, an irradiated layer where NM martensite was produced, and an unirradiated layer. The peak appearing at 43.2° indicates the formation of NM tetragonal martensite. Moreover, it was found that the phase transformation temperature and magnetic properties of Ni51.90Mn28.86Ga19.24 films can be changed by using a sufficiently high proton fluence. For example, using a higher proton fluence causes the phase transformation temperature to increase. Of particular interest is that two endothermic peaks and two exothermic peaks were observed on the DSC curves when the proton fluence was 2.0 1016/cm2. In addition, an extra shoulder was observed below TC1 = 287.8 K and TC2 = 245.6 K on the M–T curves of the irradiated films, which may be attributed to the existence of a magnetic phase with a low Curie temperature, such as the NM martensite produced in the irradiated layer. Concerning the magnetic properties, a gradual decrease in the Curie temperature TC is observed with increasing proton fluence. The saturation magnetization Ms decreased from 52.08 to 14.01 A m2 kg1 when the proton fluence increased from 0 to 2.0 1016/cm2 at 300 K; however, Ms increased from 55.84 to 86.77 A m2 kg1 at 10 K, indicating that a large portion of these changes are directly related to the reduction of TC induced by the formation of NM martensite. Acknowledgements This work was financially supported by the National Natural Science Foundation of China [grant number 51471060]. The authors would like to take this opportunity to express their gratitude to the funding body.
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Proton-irradiation-induced structural and magnetic property changes in Ni–Mn–Ga high-temperature shape memory films Zhai-Ping Yang, Zhi-Yong Gao, Wei Cai ⇑ School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 6 February 2017 Received in revised form 17 May 2017 Accepted 2 June 2017 Available online 13 June 2017 Keywords: Ni–Mn–Ga film Proton irradiation Crystal structure Martensitic transformation Magnetic properties
a b s t r a c t Ni51.90Mn28.86Ga19.24 alloy films were irradiated by 120 keV protons at different fluences to investigate the effect of irradiation on their crystal structure, phase-transition temperature, and magnetic properties. The results show not only that a nanoscale amorphous layer may have been induced on the surface, but also that non-modulated (NM) martensite was produced. Moreover, differential scanning calorimetry peaks of the martensitic and austenitic transition temperatures are shifted to higher temperatures after irradiation; in particular, two endothermic and two exothermic peaks were observed for a proton fluence of 2.0 1016/cm2. Further, the formation of NM martensite could produce an extra shoulder on the M–T curves of the irradiated films. Regarding the magnetic properties, a gradual decrease in the Curie temperature TC can be caused by irradiation at a higher proton fluence. The changes in the saturation magnetization Ms with the proton fluence seem to be related to the reduction in TC. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction In the past few years, magnetic shape memory alloy films have been developed as novel functional materials with potential applications in sensor and actuation devices [1]. Ferromagnetic offstoichiometric Ni2MnGa shape memory alloys have been widely studied owing to their giant strains (up to 10%) and highfrequency response under an external magnetic field [2]. When magnetic shape memory alloy films are used in some extreme environments, such as aerospace, they need to function at high temperature (i.e., above 400 K) with high thermal and magnetic stability [3]. Several investigations have shown that Ni-rich Ni–Mn–Ga alloys are a promising candidate for use as hightemperature shape memory alloys owing to their high and adjustable martensitic phase transition temperature [4]. The martensitic transformation behavior, shape memory effect, and magneticfield-induced strain of Ni–Mn–Ga high-temperature shape memory alloys have been investigated [4–7]. However, it is noteworthy that Ni–Mn–Ga alloys will be irradiated by high-energy ions, especially protons, when they are used in the aerospace environment. In this case, their properties, such as their magnetic, electrical, and mechanical properties, will probably be changed by space radiation [8–10]. It is well known that irra-
diation could create defects and induce phase transformations [11,12], which may influence the martensitic transition temperature and Curie temperature (TC) of Ni–Mn–Ga alloys. To date, there have been various studies on the effect of ion irradiation on metals and alloys. Tolley et al. [12,13] reported an increase in the phase transformation temperature in proton- and electron-irradiated Cu–Zn–Al alloys. It was found that in Ni–Mn–Sn alloy, TC can be changed by increasing the fluence of Ar ions or Au ions [14,15]. Moreover, irradiation could also alter the saturation magnetization (Ms) of alloys [16]. Thus, structural and magnetic investigation of hightemperature Ni–Mn–Ga alloys under proton irradiation is essential. In this work, our goal is to provide information about the changes in the structure, phase transition temperature, and magnetic properties of high-temperature Ni–Mn–Ga films under proton irradiation with different fluences. This work can not only reveal the evolution of the structure and magnetic properties of Ni–Mn–Ga high-temperature shape memory films, but also promote and accelerate their practical application in the aerospace industry. In the following sections, the detailed experimental procedure is first described. Then the main results are presented, and the structural and magnetic changes after proton irradiation are discussed.
2. Experimental ⇑ Corresponding author. E-mail (W. Cai).
addresses:
[email protected] (Z.-P. Yang), [email protected]
http://dx.doi.org/10.1016/j.mseb.2017.06.004 0921-5107/Ó 2017 Elsevier B.V. All rights reserved.
Ni–Mn–Ga films were prepared by direct-current magnetron sputtering with high-purity argon at 0.14 Pa in a high-vacuum
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(8.0 105 Pa) chamber. The films were first deposited onto a Si (1 1 1) substrate whose temperature was maintained at 300 K using a cooling water system during deposition. The sputtering power was 100 W, and the deposition rate was 0.49 nm s1. After the deposition process, the films were mechanically removed from the Si substrate as freestanding samples. Subsequently, the freestanding films were annealed at 873 K in a high-vacuum chamber (<6.0 105 Pa) for about 3600 s. Fracture sections of the annealed films were observed by an FEI Quanta 200FEG scanning electron microscope (SEM, high voltage of 20 kV) to estimate the film thickness, as shown in Fig. 1(a). The thickness of the obtained films was found to be approximately 3.3 lm. Furthermore, the compositions of the annealed films were determined by energy-dispersive spectrometry (EDS, energy resolution of 128 eV) equipment installed on the SEM (Fig. 1(b)). The annealed films were found to have a composition of Ni51.90Mn28.86Ga19.24 (at.%) without any other redundant elements. Proton irradiation was conducted using a proton accelerator attached to the ground-based complex irradiation simulation system at Harbin Institute of Technology, China. An electric scanning unit is used in the X direction and a magnetic scanning unit is used in the Y direction to obtain a homogeneous irradiated proton-beam field of 20 20 mm2. Three square samples with dimension of 20 20 mm2 were cut from the freestanding films and irradiated with 120 keV protons under high-vacuum conditions (1.2 104 Pa). A proton flux of 1.85 1012 protons/cm2 s1 was used in the proton irradiation experiment. For every square sample, the proton fluence was maintained at 0, 1.0 1016, or 2.0 1016 protons/cm2. During irradiation, the proton beam was incident perpendicular to the samples. Moreover, the temperature of all of the samples during irradiation was kept at room temperature using a circulating cooling water system. The Monte Carlo code SRIM was used to calculate the damage distribution [17]. The displacement threshold energy (Ed) of Ni, Mn, and Ga is 25 eV. Depth profiles of the atomic displacement damage and hydrogen concentration are shown in Fig. 2, in which TR corresponds to the track region, CR is the cascade region, and NIR is the non-implanted region [18]. The projected range of protons in the Ni51.90Mn28.86Ga19.24 film is clearly approximately 800 nm. The crystalline structures of unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films were identified by wide-angle X-ray diffraction (WA-XRD) and grazing-angle incidence XRD (GI-XRD) using Cu Ka radiation (k = 0.154 nm) at room temperature. To investigate the microstructural changes after proton irradiation, a transmission electron microscopy (TEM) sample irradiated with 2.0 1016 -
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Fig. 2. Depth profiles of the displacement damage and hydrogen concentration in Ni51.90Mn28.86Ga19.24 films irradiated by 120 keV protons (calculated by SRIM2008 code). TR: track region, CR: cascade region, NIR, non-implanted region.
protons/cm2 was prepared by focused ion beam (FIB, HELIOS NanoLab 600i) milling. Before ion milling by the FIB, a thin protective layer of platinum was deposited on the surface of the Ni51.90Mn28.86Ga19.24 films using ion-beam-assisted chemical vapor deposition. Then the prepared samples were observed using a Tecnai G2 F30 TEM instrument. The phase transformation temperatures of these films were determined using a Perkin-Elmer differential scanning calorimetry (DSC) instrument at cooling and heating rates of 20 °Cmin1. Furthermore, the magnetic properties of these Ni51.90Mn28.86Ga19.24 films were probed with a vibrating sample magnetometer (Squid VSM, Quantum Design PPMS). The temperature range for the temperature-dependent magnetization measurement was 10–400 K, and the magnetic field was uniformly set to 10 mT along the film plane. The magnetic hysteresis loop was measured at 300 and 10 K at magnetic fields ranging from 5 to 5 T. 3. Results and discussion Regarding the crystal structure of the films, Fig. 3 shows the XRD patterns of the unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films at room temperature. These results, as well as the TEM micrographs in Fig. 4, selected-area electron diffraction (SAED) patterns in Fig. 5, and DSC curves in Fig. 6, reveal that the
Fig. 1. Thickness and composition of the annealed Ni–Mn–Ga films: (a) SEM image of the fracture section of a film used for thickness measurement; (b) EDS pattern.
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Fig. 3. XRD patterns of the unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films: (a) GI-XRD; (b) WA-XRD.
Fig. 4. Bright-field TEM micrographs of Ni51.90Mn28.86Ga19.24 films irradiated with 2.0 1016/cm2 proton fluence.
unirradiated film has a seven-layered modulated (7M) orthorhombic martensitic structure (the space group is 58, Pnnm) at room temperature. The three typical peaks at 43.1°, 44.4°, and 45.8°, which are very similar to the reference angles of 43.3°, 44.7°, and 46.0° from [19], are marked by their Miller indices as the {2 2 0}, {2 0 2}, and {0 2 2} planes, respectively. The GI-XRD results in Fig. 3(a) clearly show that the irradiated films have diffraction peaks similar to those of the unirradiated film, indicating that no
new phase is generated on surface of the irradiated films or the quantity of any new phase is too small to be detected. In addition, the WA-XRD patterns of the irradiated films also have diffraction peaks similar to those of the unirradiated films at approximately 42.8°, 44.2°, and 45.6°, as shown Fig. 3(b). However, for the 2.0 1016/cm2 irradiated film, an extra peak appears at 43.2°. In combination with the SAED patterns in Fig. 5, these results suggest that the peak at 43.2° indicates the formation of a new phase of non-modulated (NM) tetragonal martensite (139, I4/mmm) [20]. In addition, the lattice constants of the unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films were calculated from the diffraction peaks at 2h = 42°–46°, as summarized in Table 1. The changes in the lattice constants and cell volume (V) of the irradiated films with increasing proton fluence are not obvious. To further confirm the formation of a new phase in the films irradiated with a proton fluence of 2.0 1016/cm2 (Fig. 3(b)), a TEM sample was prepared by FIB milling along the proton incidence direction to reveal the cross section of the irradiated film, as shown in Fig. 4. The red dashed line separates the irradiated and unirradiated zones according to the depth profile of the displacement damage in Fig. 2. Furthermore, Fig. 5 shows magnified bright-field images and the corresponding SAED patterns of the surface, the areas labeled A in the unirradiated zone and B in the irradiated zone in Fig. 4. First, a distinctive nanoscale layer approximately 16 nm thick is observed on the surface of the irradiated films, as shown in Fig. 5 (a). A high-resolution TEM (HRTEM) image of the blue dashed box shows the nanoscale layer more clearly. A clear boundary appears between the nanoscale layer and irradiated zone, and the corresponding fast Fourier transformation confirms that the nanoscale layer is amorphous. Note that the amorphous layer can stem from the proton irradiation or ion beam damage during TEM sample preparation process. Fig. 5(b,c) and (d,e) display the bright-field TEM images and their corresponding SAED patterns taken from regions A (unirradiated zone) and B (irradiated zone) in Fig. 4, respectively. The A zone is 7 M martensite [19]. However, the B zone is NM martensite [20]. These results suggest that 120 keV proton irradiation may not only induce the formation of the nanoscale amorphous layer on the surface of the Ni51.90Mn28.86Ga19.24 films, but also promote the phase transformation of martensite from 7 M to NM in the irradiated zone. Fig. 6 shows the DSC curves of the free-standing Ni51.90Mn28.86Ga19.24 films irradiated at various proton fluences. For the unirradiated Ni51.90Mn28.86Ga19.24 films, an obvious exothermic peak appears during the cooling process, whereas an endothermic peak
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Fig. 5. Bright-field TEM micrographs and corresponding SAED patterns of Ni51.90Mn28.86Ga19.24 films irradiated with 2.0 1016/cm2 proton fluence: (a) Bright-field image, HRTEM pattern, and SAED pattern of nano-amorphous layer on surface of films; (b) and (c) bright-field image and the corresponding SAED pattern, respectively, of A (unirradiated zone) in Fig. 4; (d) and (e) bright-field image and the corresponding SAED pattern, respectively, of B (irradiated zone) in Fig. 4.
occurs during the heating process. This is the well-known typical characteristic of the one-step thermoelastic martensitic transition [6]. After irradiation with a proton fluence of 1.0 1016/cm2, the
DSC curve of the irradiated film resembles that of the unirradiated film, but this irradiation fluence affects the martensitic transformation temperature (Tm) and austenitic transformation temperature
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Fig. 6. DSC curves of Ni51.90Mn28.86Ga19.24 films irradiated at various proton fluences.
(Ta) of the Ni51.90Mn28.86Ga19.24 films. Fig. 6 shows that the peaks of Tm and Ta for the 1.0 1016/cm2 proton-irradiated films are shifted to higher temperatures. Moreover, the martensitic transformation temperatures of both the unirradiated and 1.0 1016/cm2 proton-irradiated films are above room temperature. It is indicated that they are martensite at room temperature. It is interesting that, when the Ni51.90Mn28.86Ga19.24 films were irradiated with a higher proton fluence of 2.0 1016/cm2, two endothermic peaks and two exothermic peaks were observed on their DSC curves. This is entirely different from the results for the unirradiated and 1.0 1016 protons/cm2 irradiated films. All the phase transformation temperatures of the unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films are listed in Table 2 for clear comparison. The variation in the transformation temperature and the appearance of double peaks for the irradiated Ni51.90Mn28.86Ga19.24 films may be attributed to the migration of lattice defects (such as interstitials and vacancies) and the formation of NM martensite induced by proton irradiation [12]. During the heating process, lattice defects introduced by proton irradiation could hinder the austenitic transformation and lead to the increase in Ta from 468.7 to 483.7 K. As the heating temperature continues to increase, diffusion of vacancies occurs, and the number of lattice defects in the
austenite decreases, which facilitates the martensitic transformation during the cooling process and leads to an increase in Tm. As shown in Fig. 2, the SRIM code was used to calculate displacement cross section as a function of depth for the Ni51.90Mn28.86Ga19.24 films irradiated with protons at an energy of 120 keV. From the calculation, we obtained a projected range of 800 nm, which indicates that all the incident protons will come to rest in the films. When the proton fluence is large enough, for example, 2.0 1016/cm2, NM martensite was formed in the films (Fig. 5(e)). The phase transformation temperatures of the NM martensite may differ from those of the matrix, as indicated in the DSC curves by the double endothermic and exothermic peaks. Fig. 7 shows the magnetization versus temperature (M–T) curves of the unirradiated and irradiated martensitic Ni51.90Mn28.86Ga19.24 films, which were used to determine the effect of the proton fluence on their Curie temperature (TC). For the unirradiated film, TC is determined to be 346.9 K, at which the film shows a transition from the paramagnetic state to the ferromagnetic state during cooling, as shown in Fig. 7(a). As the temperature decreases further to approximately 10 K, we did not observe the martensitic transition, indicating that the martensitic transition temperature is higher than TC, as shown in the DSC results. In addition, the heating curve had a shape similar to that of the cooling curve. These results show that the unirradiated Ni51.90Mn28.86Ga19.24 film is ferromagnetic martensite at room temperature. Furthermore, when the Ni51.90Mn28.86Ga19.24 films were irradiated by 120 keV protons, a gradual decrease in TC was observed with increasing proton fluence, as shown in Fig. 7(b–d). The TC values of the unirradiated and irradiated films are also listed in Table 2, which shows that TC decreases from 346.9 to 312.3 K after irradiation with 2.0 1016 protons/cm2. It is well known that an ordered film could become chemically disordered under irradiation, showing features such as point defects and dense cascades, which would reduce its Curie temperature [21]. In addition, an extra shoulder is observed below TC1 = 287.8 K and TC2 = 245.6 K in the M–T curves of the films irradiated by proton fluences of 1.0 1016/cm2 and 2.0 1016/cm2, respectively. The shoulder is more obvious at the higher fluence. This phenomenon may be attributed to the existence of a magnetic phase with a low Curie temperature [22]. The discussion of the XRD, DSC, and TEM results stated that NM martensite was formed in the irradiated films, and the NM martensite might be just the magnetic precipitate phase, with a lower Curie temperature than the film itself.
Table 1 Lattice constants and cell volume of unirradiated and irradiated Ni51.90Mn28.86Ga19.24 films. Material state
0/cm2 1.0 1016/cm2 2.0 1016/cm2
Lattice constant a (Å)
Error (r)
b (Å)
Error (r)
c (Å)
Error (r)
V (Å3)
Error (r)
6.189 6.163 6.178
1.5E 2 0.4 E 2 1.5E 2
5.770 5.761 5.774
2.6E 2 0.7 E 2 2.5 E 2
5.437 5.480 5.440
1.0E 2 0.3E 2 1.0E 2
194.147 194.552 194.041
6.1E 1 1.7 E 1 5.9 E 1
Table 2 Effect of proton fluence on phase and magnetic transition temperatures and saturation magnetization (Ms) of Ni51.90Mn28.86Ga19.24 films. Proton fluence (protons/ cm2)
T (K)
0
—
Ms (Am2kg1)
Tm
1.0 10
16
—
2.0 10
16
437.7 ± 1.1 (3)
Ta 447.4 ± 0.9 (2) 453.1 ± 0.3 (3) 461.9 ± 2.1 (3)
— — 463.1 ± 1.1 (3)
TC 468.7 ± 1.2 (2) 471.6 ± 2.8 (3) 483.7 ± 1.0 (3)
The number in parentheses represents the number of measurements for each parameter.
— 287.8 ± 1.0 (2) 245.6 ± 2.0 (2)
346.9 ± 0.6 (2) 322.9 ± 0.6 (2) 312.3 ± 1.8 (2)
300 K
10 K
52.08 ± 0.08 (2) 21.04 ± 0.05 (2) 14.01 ± 0.06 (2)
55.84 ± 0.03 (2) 60.67 ± 0.03 (2) 86.77 ± 0.06 (2)
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Fig. 7. M–T curves of the Ni51.90Mn28.86Ga19.24 films at low field (100 Oe): (a) unirradiated film; (b), (c), and (d) films irradiated at different fluences, where the red line corresponds to the first derivative.
Magnetization curves of unirradiated Ni51.90Mn28.86Ga19.24 films were measured at temperatures of 300 and 380 K along a direction parallel to the film plane, as shown in Fig. 8. Fig. 8(a) clearly shows that the magnetization curve at 380 K is nearly linear, which demonstrates that the unirradiated film is in a paramagnetic state at 380 K. However, the unirradiated film is found to be in the ferromagnetic state at room temperature (300 K), at which its saturation magnetization (Ms) is 52.08 A m2 kg1, and its coercivity is 50.79 (103/4p Am1) (Fig. 8(b)).
The effects of the proton fluence on the magnetization curves at 300 K of the Ni51.90Mn28.86Ga19.24 films irradiated by 120 keV protons are shown in Fig. 9(a). The saturation magnetization (Ms) decreased from 52.08 to 14.01 A m2 kg1 when the proton fluence increased from 0 to 2.0 1016/cm2. The Ms values of the unirradiated and irradiated films are listed in Table 2 for clear comparison. The changes in the Ms values of the Ni51.90Mn28.86Ga19.24 films after proton irradiation may be attributed to the formation of NM martensite. The magnetization curves measured at 300 K reveal
Fig. 8. Magnetization curves of Ni51.90Mn28.86Ga19.24 films: (a) magnetization curves of the unirradiated films at 380 and 300 K; (b) magnified view of the central area in (a).
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Fig. 9. Effect of proton fluence on the magnetization curves of irradiated Ni51.90Mn28.86Ga19.24 films at different temperatures: (a) 300 K; (b) 10 K.
that the NM martensite induced by proton irradiation is paramagnetic, as indicated by the reduction of the Curie temperature to TC1 = 287.8 K or TC2 = 245.6 K (Fig. 8). Therefore, a major portion of these changes seems to be directly related to the reduction in TC. To further confirm the above statement, the effects of the proton fluence on the hysteresis loops far from TC, that is, at 10 K, were also measured (Fig. 9(b)). When the magnetic hysteresis loops were measured at 10 K, both the unirradiated and irradiated films were found to be ferromagnetic martensite. Ms increased from 55.84 to 86.77 A m2 kg1 when the proton fluence increased from 0 to 2.0 1016/cm2. It is well known that the electron concentration (e/a, average number of valence electrons per atom) is a critical factor that influences Ms in Heusler compounds. At 10 K, Ms has a maximum value when e/a = 7.53, beyond which Ms decreases linearly with increasing e/a regardless of the type of martensitic structure [23]. The e/a value of the experimental film in this investigation is 7.79 [24]. After the films are irradiated with protons, an interaction between H 1 s and Mn 4 s may result in a decrease in the metallic bond strength [25], which subsequently leads to a decrease in e/a. Therefore, the proton-irradiation-induced decrease of e/a will result in an increase in Ms. 4. Conclusions The effects of 120 keV proton irradiation on the structure, phase transition temperature, and magnetic properties of a ferromagnetic film, i.e., Ni51.90Mn28.86Ga19.24, were studied in detail. Structural analysis by GI-XRD indicates that no new phase is generated on surface of the irradiated films or the quantity of any new phase is too small to be detected, but an extra peak appears at 43.2° in the WA-XRD pattern for the film irradiated at a proton fluence of
2.0 1016/cm2, indicating the formation of a new phase. TEM observations revealed that the cross section of the irradiated film was divided into three layers: a distinctive 16 nm amorphous layer on the surface, an irradiated layer where NM martensite was produced, and an unirradiated layer. The peak appearing at 43.2° indicates the formation of NM tetragonal martensite. Moreover, it was found that the phase transformation temperature and magnetic properties of Ni51.90Mn28.86Ga19.24 films can be changed by using a sufficiently high proton fluence. For example, using a higher proton fluence causes the phase transformation temperature to increase. Of particular interest is that two endothermic peaks and two exothermic peaks were observed on the DSC curves when the proton fluence was 2.0 1016/cm2. In addition, an extra shoulder was observed below TC1 = 287.8 K and TC2 = 245.6 K on the M–T curves of the irradiated films, which may be attributed to the existence of a magnetic phase with a low Curie temperature, such as the NM martensite produced in the irradiated layer. Concerning the magnetic properties, a gradual decrease in the Curie temperature TC is observed with increasing proton fluence. The saturation magnetization Ms decreased from 52.08 to 14.01 A m2 kg1 when the proton fluence increased from 0 to 2.0 1016/cm2 at 300 K; however, Ms increased from 55.84 to 86.77 A m2 kg1 at 10 K, indicating that a large portion of these changes are directly related to the reduction of TC induced by the formation of NM martensite. Acknowledgements This work was financially supported by the National Natural Science Foundation of China [grant number 51471060]. The authors would like to take this opportunity to express their gratitude to the funding body.
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