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Martensitic transformation of Ni–Fe–Ga magnetic shape memory alloys
Journal of Alloys and Compounds 385 (2004) 144–147
Martensitic transformation of Ni–Fe–Ga magnetic shape memory alloys H.X. Zheng, M.X. Xia, J. Liu, J.G. Li∗ School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 31 March 2004; received in revised form 26 April 2004; accepted 26 April 2004
Abstract The effects of the Ni and Ga content on the martensitic transformation of Ni–Fe–Ga alloys with composition close to Ni55 Fe17.5 Ga27.5 were investigated. The DSC results show that the martensitic transformation temperatures increase with Ni addition or Ga depletion. A complete thermoelastic intermartensitic transformation and retransformation in Ni58 Fe17.5 Ga27.5 alloy in the vicinity of 370 K was observed, the hysterisis of martensitic and intermartensitic transformation are 10.54 and 5.07 K, respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Ni–Fe–Ga; Magnetic shape memory alloy; Martensitic transformation
1. Introduction Heusler alloys have attracted much attentions for their unique potential as actuators in microelectromechnical systems [1,2], especially Ni–Mn–Ga, in which a magnetic-field-induced strain up to 6% has been reported [3]. Subsequently, some new Heusler alloys, such as Co–Ni–Al, Co–Ni–Ga and Ni–Fe–Ga were explored [4–6]. Similar to Ni2 MnGa alloy, other Heusler alloys with a reversible martensitic transformation also possessed the capability of magnetic-field-induced strains. Although a lot of papers have described the essential magnetic properties and martensitic transformation of Ni–Mn–Ga alloys [7,8], there is still new science to be uncovered if we understand better how the twin boundaries in these shape memory alloys respond to the application of a magnetic field as well as to stress. The most important problem of Ni–Mn–Ga alloys to be used in practical engineering fields is how to increase the martensitic transformation temperatures, that is, the operating temperature must increase. Progress is being made in this direction. The main method is to adjust the alloy compositions [9,10]. For the newly-explored Heusler alloys, few researches have been conducted [4,11] and they are still not sufficient for designing alloys with proper martensitic transformation temperatures. In this article, the effects
of Ni and Ga content on the martensitic transformation of polycrystalline Ni–Fe–Ga magnetic shape memory alloys will be reported.
2. Experimental Polycrystalline samples of about 35 g of Ni55 Fe17.5 Ga27.5 , Ni57 Fe17.5 Ga27.5 , Ni58 Fe17.5 Ga27.5 , Ni55 Fe17.5 Ga27 and Ni55 Fe17.5 Ga26.5 alloys were made by using arc-melting under argon atmosphere (the purity of the elements is higher than 99.99%). The samples were melted four times to ensure homogeneity. The obtained specimens were sealed in a vacuum silica tubes, heated to 773 K and then cooled slowly to room temperature. Microstructure observation was carried out using an optical microscope (Neoplot-1) and a scanning electron microscope (SEM-520) with EDS. The phase transformation temperatures were measured in a modulated differential scanning calorimeter (MDSC2910) with heating and cooling rates of 10 K/min. Phase identification was performed in a D8 ADVANCE X-ray powder diffractmeter with CuK␣ radiation.
3. Results and discussion ∗
Corresponding author. Tel.: +86-21-62932569; fax: +86-21-62933074. E-mail address: [email protected] (J.G. Li). 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.124
Fig. 1 shows the surface relief of Ni55 Fe17.5 Ga26.5 and Ni55 Fe17.5 Ga27.5 alloys, Fig. 2a is the optical image of a
H.X. Zheng et al. / Journal of Alloys and Compounds 385 (2004) 144–147
145
Fig. 1. Surface relief of Ni55 Fe17.5 Ga26.5 alloy (a) and Ni55 Fe17.5 Ga27.5 alloy (b) at room temperature.
single columnar grain of unpolished Ni58 Fe17.5 Ga26.5 alloy and Fig. 2b is the amplified SEM photo at room temperature, which all exhibit a stripe-like morphology of well accommodated martensitic variants. The boundaries between different martensitic variants are straight and distinct. The powder XRD patterns at room temperature are indexed in Fig. 3 and identified well as a 14M martensitic structure. No other extra peaks were observed in the XRD results except a few weak peaks of disordered fcc(␥), and no ␥ phase was observed in the optical images. In most previously studied Heusler alloys, a typical DSC curve (Fig. 4a) usually include an exothermic peak originating from the thermoelastic martensitic transformation and an endothermic peak originating from the thermoelastic martensitic retransformation, and the four characteristic temperatures of martensitic transformation can be determined. For Ni–Mn–Ga alloys having Ms well above room temperature (Ms ∼ 400 K), Chernenko et al. observed a two-step thermally induced martensitic transformation during cooling
Fig. 3. XRD patterns of the powder of Ni–Fe–Ga alloys at room temperature.
Fig. 2. Optical image (a) and the amplified SEM image (b) of unpolished Ni57 Fe17.5 Ga27.5 alloy at room temperature.
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Fig. 5. Martensitic transformation temperatures vs. electron concentration of Ni–Fe–Ga alloys.
Fig. 4. DSC curves of Ni55 Fe17.5 Ga26.5 (a) and Ni58 Fe17.5 Ga27.5 (b) alloys.
and only one-step reverse transformation was observed during heating. These authors proposed that a sequence of martensitic and intermartensitic transformation take place during cooling, whereas the low temperature martensitic phase directly reverts to the parent phase upon heating [12]. Up to date, the intermartensitic retransformation was never detected in Heusler alloys during heating. Fortunately, we firstly observed a complete two-step martensitic transformation in Ni58 Fe17.5 Ga27.5 alloy not only during cooling but also during heating (Fig. 4b). Based on the well known knowledge of Ni–Mn–Ga, we conclude that an intermartensitic transformation occurs and different from Ni–Mn–Ga alloys, the low temperature martensitic phase reverts to the intermartensitic phase firstly and then the intermartensitic phase reverts to the parent phase during heating, instead of reverting to the parent phase directly. In addition, according to the DSC curve, the hysterisis of martensitic transformation (Th = Af − Ms ) and intermartensitic transformation are 10.54 and 5.07 K, respectively. The values of the transformation heat Q corresponding to the two peaks obtained from the heating curve are 1.152 and 3.425 J/g, and the Q values corresponding to the two peaks obtained from the cooling curve are 0.8337 and 3.313 J/g. Apparently, the intermartensitic transformation and martensitic transformation are both typical thermoelastic transformations. Details
of the phase transformation will be studied further. In Fig. 5, the martensitic transformation temperatures are depicted as a function of the electron concentration. The martensitic transformation temperatures increase monotonically with electron concentration. The atomic radius and the number of valence electrons are 0.1246 nm and 10 for Ni, 0.1274 nm and 8 for Fe, 0.1353 nm and 3 for Ga, respectively. So, increasing the Ni and decreasing the Ga content both cause an increase in electron concentration and a slight decrease of the unit-cell volumes. In previous studies of Ni–Mn–Ga alloys, the electron concentration was considered to play an important role and prone to promote the formation of the martensitic structure [10,13]. Jiang et al. suggested that the electrons above the Fermi level move to the corner states of the Brillouin zone, and the system energy increases due to the lattice distortion, which would lead to the martensitic formation [10]. In other words, the martensitic transformation temperatures would increase with the increase of the electron concentration and the slight decrease of the unit-cell volumes. Our results support this point to some extent.
4. Conclusions The martensitic transformation temperatures of Ni–Fe–Ga alloys with composition close to Ni55 Fe17.5 Ga27.5 increase with Ni addition or Ga depletion. A complete thermoelastic intermartensitic transformation and retransformation was clearly observed in Ni58 Fe17.5 Ga27.5 alloy.
Acknowledgements The authors express their appreciation for the financial support of the National Natural Science Foundation and the National Science Fund for Distinguished Scholars of China.
H.X. Zheng et al. / Journal of Alloys and Compounds 385 (2004) 144–147
References [1] O. Heczko, N. Glavatska, V. Gavriljuk, K. Ullakko, Mater. Sci. Forum 373–376 (2001) 341. [2] K. Ullakko, J.K. Huang, V.V. Kokorin, R.C. O’Handley, Scripta Mater. 36 (1997) 1133. [3] K. Ullakko, J. Mater. Eng. Perf. 5 (1996) 405. [4] K. Oikawa, T. Ota, F. Gejima, T. Ohmori, R. Kainuma, K. Ishida, Mater. Trans. 42 (2001) 2472. [5] I. Karaman, H.E. Karaca, D.C. Lagoudas, H.J. Maier, Y.I. Chumlyakov, Scripta Mater. 49 (2003) 831. [6] K. Oikawa, T. Ota, Y. Sutou, T. Ohmori, R. Kainuma, K. Ishida, Mater. Trans. 43 (2002) 2360.
147
[7] V.A. Chernenko, V.A. L’vov, J. Pons, E. Cesari, J. Appl. Phys. 93 (2003) 2394. [8] N.I. Glavatska, A.A. Rudenko, V.A. L’vov, J. Magn. Magn. Mater. 241 (2002) 287. [9] M. Matsumoto, M. Ebisuya, T. Kanomata, R. Note, H. Yoshida, T. Kaneko, J. Magn. Magn. Mater. 239 (2002) 521. [10] C.B. Jiang, F. Gen, S.K. Gong, H.B. Xu, Mater. Sci. Eng. A 342 (2003) 231. [11] K. Oikawa, T. Ota, T. Ohmori, Y. Tanaka, H. Morito, A. Fujita, R. Kainuma, K. Fukamichi, K. Ishida, Appl. Phys. Lett. 81 (2002) 5201. [12] V.A. Chernenko, C. Segui, E. Cesari, J. Pons, V.V. Kokorin, Phys. Rev. B 57 (1995) 2659. [13] P.J. Webster, K.R.A. Ziebeck, S.L. Town, M.S. Peak, Phil. Mag. B 49 (1984) 295.
Martensitic transformation of Ni–Fe–Ga magnetic shape memory alloys H.X. Zheng, M.X. Xia, J. Liu, J.G. Li∗ School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 31 March 2004; received in revised form 26 April 2004; accepted 26 April 2004
Abstract The effects of the Ni and Ga content on the martensitic transformation of Ni–Fe–Ga alloys with composition close to Ni55 Fe17.5 Ga27.5 were investigated. The DSC results show that the martensitic transformation temperatures increase with Ni addition or Ga depletion. A complete thermoelastic intermartensitic transformation and retransformation in Ni58 Fe17.5 Ga27.5 alloy in the vicinity of 370 K was observed, the hysterisis of martensitic and intermartensitic transformation are 10.54 and 5.07 K, respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Ni–Fe–Ga; Magnetic shape memory alloy; Martensitic transformation
1. Introduction Heusler alloys have attracted much attentions for their unique potential as actuators in microelectromechnical systems [1,2], especially Ni–Mn–Ga, in which a magnetic-field-induced strain up to 6% has been reported [3]. Subsequently, some new Heusler alloys, such as Co–Ni–Al, Co–Ni–Ga and Ni–Fe–Ga were explored [4–6]. Similar to Ni2 MnGa alloy, other Heusler alloys with a reversible martensitic transformation also possessed the capability of magnetic-field-induced strains. Although a lot of papers have described the essential magnetic properties and martensitic transformation of Ni–Mn–Ga alloys [7,8], there is still new science to be uncovered if we understand better how the twin boundaries in these shape memory alloys respond to the application of a magnetic field as well as to stress. The most important problem of Ni–Mn–Ga alloys to be used in practical engineering fields is how to increase the martensitic transformation temperatures, that is, the operating temperature must increase. Progress is being made in this direction. The main method is to adjust the alloy compositions [9,10]. For the newly-explored Heusler alloys, few researches have been conducted [4,11] and they are still not sufficient for designing alloys with proper martensitic transformation temperatures. In this article, the effects
of Ni and Ga content on the martensitic transformation of polycrystalline Ni–Fe–Ga magnetic shape memory alloys will be reported.
2. Experimental Polycrystalline samples of about 35 g of Ni55 Fe17.5 Ga27.5 , Ni57 Fe17.5 Ga27.5 , Ni58 Fe17.5 Ga27.5 , Ni55 Fe17.5 Ga27 and Ni55 Fe17.5 Ga26.5 alloys were made by using arc-melting under argon atmosphere (the purity of the elements is higher than 99.99%). The samples were melted four times to ensure homogeneity. The obtained specimens were sealed in a vacuum silica tubes, heated to 773 K and then cooled slowly to room temperature. Microstructure observation was carried out using an optical microscope (Neoplot-1) and a scanning electron microscope (SEM-520) with EDS. The phase transformation temperatures were measured in a modulated differential scanning calorimeter (MDSC2910) with heating and cooling rates of 10 K/min. Phase identification was performed in a D8 ADVANCE X-ray powder diffractmeter with CuK␣ radiation.
3. Results and discussion ∗
Corresponding author. Tel.: +86-21-62932569; fax: +86-21-62933074. E-mail address: [email protected] (J.G. Li). 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.124
Fig. 1 shows the surface relief of Ni55 Fe17.5 Ga26.5 and Ni55 Fe17.5 Ga27.5 alloys, Fig. 2a is the optical image of a
H.X. Zheng et al. / Journal of Alloys and Compounds 385 (2004) 144–147
145
Fig. 1. Surface relief of Ni55 Fe17.5 Ga26.5 alloy (a) and Ni55 Fe17.5 Ga27.5 alloy (b) at room temperature.
single columnar grain of unpolished Ni58 Fe17.5 Ga26.5 alloy and Fig. 2b is the amplified SEM photo at room temperature, which all exhibit a stripe-like morphology of well accommodated martensitic variants. The boundaries between different martensitic variants are straight and distinct. The powder XRD patterns at room temperature are indexed in Fig. 3 and identified well as a 14M martensitic structure. No other extra peaks were observed in the XRD results except a few weak peaks of disordered fcc(␥), and no ␥ phase was observed in the optical images. In most previously studied Heusler alloys, a typical DSC curve (Fig. 4a) usually include an exothermic peak originating from the thermoelastic martensitic transformation and an endothermic peak originating from the thermoelastic martensitic retransformation, and the four characteristic temperatures of martensitic transformation can be determined. For Ni–Mn–Ga alloys having Ms well above room temperature (Ms ∼ 400 K), Chernenko et al. observed a two-step thermally induced martensitic transformation during cooling
Fig. 3. XRD patterns of the powder of Ni–Fe–Ga alloys at room temperature.
Fig. 2. Optical image (a) and the amplified SEM image (b) of unpolished Ni57 Fe17.5 Ga27.5 alloy at room temperature.
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Fig. 5. Martensitic transformation temperatures vs. electron concentration of Ni–Fe–Ga alloys.
Fig. 4. DSC curves of Ni55 Fe17.5 Ga26.5 (a) and Ni58 Fe17.5 Ga27.5 (b) alloys.
and only one-step reverse transformation was observed during heating. These authors proposed that a sequence of martensitic and intermartensitic transformation take place during cooling, whereas the low temperature martensitic phase directly reverts to the parent phase upon heating [12]. Up to date, the intermartensitic retransformation was never detected in Heusler alloys during heating. Fortunately, we firstly observed a complete two-step martensitic transformation in Ni58 Fe17.5 Ga27.5 alloy not only during cooling but also during heating (Fig. 4b). Based on the well known knowledge of Ni–Mn–Ga, we conclude that an intermartensitic transformation occurs and different from Ni–Mn–Ga alloys, the low temperature martensitic phase reverts to the intermartensitic phase firstly and then the intermartensitic phase reverts to the parent phase during heating, instead of reverting to the parent phase directly. In addition, according to the DSC curve, the hysterisis of martensitic transformation (Th = Af − Ms ) and intermartensitic transformation are 10.54 and 5.07 K, respectively. The values of the transformation heat Q corresponding to the two peaks obtained from the heating curve are 1.152 and 3.425 J/g, and the Q values corresponding to the two peaks obtained from the cooling curve are 0.8337 and 3.313 J/g. Apparently, the intermartensitic transformation and martensitic transformation are both typical thermoelastic transformations. Details
of the phase transformation will be studied further. In Fig. 5, the martensitic transformation temperatures are depicted as a function of the electron concentration. The martensitic transformation temperatures increase monotonically with electron concentration. The atomic radius and the number of valence electrons are 0.1246 nm and 10 for Ni, 0.1274 nm and 8 for Fe, 0.1353 nm and 3 for Ga, respectively. So, increasing the Ni and decreasing the Ga content both cause an increase in electron concentration and a slight decrease of the unit-cell volumes. In previous studies of Ni–Mn–Ga alloys, the electron concentration was considered to play an important role and prone to promote the formation of the martensitic structure [10,13]. Jiang et al. suggested that the electrons above the Fermi level move to the corner states of the Brillouin zone, and the system energy increases due to the lattice distortion, which would lead to the martensitic formation [10]. In other words, the martensitic transformation temperatures would increase with the increase of the electron concentration and the slight decrease of the unit-cell volumes. Our results support this point to some extent.
4. Conclusions The martensitic transformation temperatures of Ni–Fe–Ga alloys with composition close to Ni55 Fe17.5 Ga27.5 increase with Ni addition or Ga depletion. A complete thermoelastic intermartensitic transformation and retransformation was clearly observed in Ni58 Fe17.5 Ga27.5 alloy.
Acknowledgements The authors express their appreciation for the financial support of the National Natural Science Foundation and the National Science Fund for Distinguished Scholars of China.
H.X. Zheng et al. / Journal of Alloys and Compounds 385 (2004) 144–147
References [1] O. Heczko, N. Glavatska, V. Gavriljuk, K. Ullakko, Mater. Sci. Forum 373–376 (2001) 341. [2] K. Ullakko, J.K. Huang, V.V. Kokorin, R.C. O’Handley, Scripta Mater. 36 (1997) 1133. [3] K. Ullakko, J. Mater. Eng. Perf. 5 (1996) 405. [4] K. Oikawa, T. Ota, F. Gejima, T. Ohmori, R. Kainuma, K. Ishida, Mater. Trans. 42 (2001) 2472. [5] I. Karaman, H.E. Karaca, D.C. Lagoudas, H.J. Maier, Y.I. Chumlyakov, Scripta Mater. 49 (2003) 831. [6] K. Oikawa, T. Ota, Y. Sutou, T. Ohmori, R. Kainuma, K. Ishida, Mater. Trans. 43 (2002) 2360.
147
[7] V.A. Chernenko, V.A. L’vov, J. Pons, E. Cesari, J. Appl. Phys. 93 (2003) 2394. [8] N.I. Glavatska, A.A. Rudenko, V.A. L’vov, J. Magn. Magn. Mater. 241 (2002) 287. [9] M. Matsumoto, M. Ebisuya, T. Kanomata, R. Note, H. Yoshida, T. Kaneko, J. Magn. Magn. Mater. 239 (2002) 521. [10] C.B. Jiang, F. Gen, S.K. Gong, H.B. Xu, Mater. Sci. Eng. A 342 (2003) 231. [11] K. Oikawa, T. Ota, T. Ohmori, Y. Tanaka, H. Morito, A. Fujita, R. Kainuma, K. Fukamichi, K. Ishida, Appl. Phys. Lett. 81 (2002) 5201. [12] V.A. Chernenko, C. Segui, E. Cesari, J. Pons, V.V. Kokorin, Phys. Rev. B 57 (1995) 2659. [13] P.J. Webster, K.R.A. Ziebeck, S.L. Town, M.S. Peak, Phil. Mag. B 49 (1984) 295.