Effect of seven-layered martensite in Ni-Mn-Ga magnetic shape memory alloys

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1 Indian Journal of Engineering & Materials Sciences Vol. 24, August 2017, pp Effect of seven-layered martensite in Ni-Mn-Ga magnetic shape memory alloys F A Sajitha Banu, S Vinodh Kumar, S Seenithurai & Manickam Mahendran* Department of Physics, Thiagarajar College of Engineering, Madurai , India Received 30 March 2016; accepted 7 March 2017 The off-stoichiometric Ni50.9Mn26.7Ga22.4 polycrystalline alloy is prepared by using arc-melting furnace. The effect of annealing on the microstructure and the magnetic properties of Ni50.9Mn26.7Ga22.4 alloy has been investigated. It exhibits single phase of seven-layer modulation with good crystallinity of orthorhombic structure. The scanning electron microscopy studies are employed at room temperature to analyze the twin formation in 7M martensitic structure and the stripe like fine martensitic bands observed. The residual stresses have been observed inside the sample while crushing the test sample, which exhibits the broadened peaks. At room temperature, the alloy showed the ferromagnetic hysteresis behavior with soft magnetic nature. Keywords: Smart materials, Ferromagnetic shape memory alloy, Ni-Mn-Ga, Polycrystalline alloy, Structural analysis, Magnetic properties, Phase During the last few decades, ferromagnetic shape memory alloys (FSMAs) have received increasing attention because of their great scientific and technological significance 1-3. To date, a variety of FSMAs such as Ni-Mn-Ga, Ni-Mn-Sn have been found to exhibit shape memory behavior. Among these, Ni-Mn-Ga alloys have been of great concern in recent years because of their unusual properties, such as large magnetic field induced strain (MFIS), high frequency response (below 1 khz) and short response time (less than 1ms). In addition to these, Ni-Mn-Ga system possesses giant magneto-caloric effect due to large entropy change across the martensite transition 4,5 and large negative magneto-resistance 6,7. In Ni-Mn-Ga system the magnetic field control of SME was first observed experimentally in 1996 by Ullakko et al. 1 and produces 0.2% of field induced strain in Ni 2 MnGa single crystal. The MFIS can occur due to the rearrangement of the twin variants in martensite phase by a change in the magnetic field. The stoichiometric Ni-Mn-Ga alloy has the cubic L2 1 structure and it produce the martensite transformation below the room temperature which limits their practical applications. More efforts have been taken to rise the martensitic transformation interval above the room temperature. The martensitic transformation temperature 8, magnetocrystalline anisotropy 9, magnetoresistance 10 and the Curie *Corresponding author: ( manickam-mahendran@tce.edu) temperature 11 are very sensitive to the composition of the alloys. The crystal structure of the alloy is also strongly depends on the composition of the alloy as reported earlier 12. Depending on the composition, the martensite structure of five-layered modulation (5M), seven-layered modulation (7M) and as well as nonmodulation have been reported in off-stoichiometric Ni-Mn-Ga alloys 13,14. Sozinov et al. 15 reported that in seven-layered modulated structure the twinning stress (σ tw ) needed for the martensite variant reorientation is 1 MPa and the magnetically induced stress (Δσ mag ) is 1.5 MPa 15. From these one of the criteria for the shape memory effect Δσ mag σ tw is satisfied in 7M structure. The giant magnetic field induced strain of 6% is observed in five-layered tetragonal martensite 16 and 10% strain is observed in seven-layered orthorhombic structure in a magnetic field of 1 tesla 17. The alloy with seven-layered orthorhombic structure has low twining stress and high magnetic anisotropy produce a large magnetic field induced strain. In view of its interesting properties, we have selected the off stoichiometeric Ni-Mn-Ga polycrystalline alloy to observe the ferromagnetic twinned martensite structure at room temperature measurement. The aim of the present work is to investigate the microstructural and magnetic behavior of 7M crystal structure in Ni-Mn-Ga polycrystalline alloy and also to investigate the role of heat treatment on the microstructure and magnetic properties in Ni 50 Mn 26 Ga 24 polycrystalline alloy.

2 302 INDIAN J. ENG. MATER. SCI., AUGUST 2017 Experimental Procedure The off-stoichiometeric of Ni-Mn-Ga Heusler alloy has been chosen to increase the transformation temperature to a much higher temperature than the room temperature. Ni 50 Mn 26 Ga 24 polycrystalline alloy has been prepared by using the arc-melting technique under Ar atmosphere. High purity Ni (99.9%), Mn (99.9%) and Ga (99.9%) were used as a raw material to prepare the alloy. The mixture of these materials were taken in a Cu crucible and melted in an arc melting furnace. To improve the homogenity, the alloy was reverted and melted again, and the process was repeated for four times. To avoid the oxidation during annealing, the alloy was sealed in a vacuum quartz ampoule. Thereafter, the samples were annealed at 1073 K for 6 h and then quenched in ice water. The crystal structure of both austenite and martensite structure was analyzed thorough the X-ray diffractometer with Cu-Kα radiation (XRD-Philips PW 1320). The microstructure and alloy composition was studied by using scanning electron microscopy (SEM) with ultra-cool energy dispersive X-ray SiLi detector (EDAX).The magnetization measurements were carried out by using vibrating sample magnetometer (VSM-883A) between Oe to Oe. Results and Discussion The X-ray diffraction is an effective technique for analyzing the crystallization behavior of the alloys. The XRD measurements are carried out by using copper rotating anode based powder diffractometer fitted with graphite monochromator in the diffracted beam. The data are recorded in the angle interval of 20 < 2θ < 75 with a step size of 0.05 and a holding time of 2 s for each step at room temperature. Fig. 1, shows the XRD pattern of the as cast Ni 50.9 Mn 26.7 Ga 22.4 alloy which exhibits the disordered structure with low intensity. The peaks splitting at 44 is corresponding to the (220)A and (202)M and such a splitting is attributed to the existence of some martensitic feature. The large amount of residual stress can be produced inside the sample while crushing the alloy, as a result the peaks appeared as broadened. To remove the residual stress the sample was annealed at 1073 K for 6 h. It is interesting to note, that after annealing the intensity of the peaks are improved and the peaks are obtained sharper, which is shown in Fig. 2. The heat treatment process changes the crystal structure of the alloy which was reported by Sozinov et al. 18 In addition to that some more additional peaks are obtained and this annealing effect cause lattice distortion in the structure. It is shown that the additional peaks are due to the presence of long period superstructure. The term superstructure is called as a linear combination of the translations of the parent structure. The peak splitting between the angle of 43 and 45 and marked as (220)A and (202)M which is shown in Fig. 2. This type of splitting is observed in seven layered orthorhombic structure and which are similar to the earlier reported results by Rama Rao et al. 19 and Santanna et al. 20 The seven-layered modulated orthorhombic martensite structure (7M) was first introduced in Ni-Mn-Ga alloy by Martynov and Kokorin 21. According to Martynov concept, the modulated structure can be observed due to the periodic shuffling at the lattice planes. The term modulated can describe lattice distortion starting Fig. 1 XRD pattern of as-cast Ni50.9Mn26.7Ga22.4 polycrystalline alloy Fig. 2 XRD pattern of Ni50.9Mn26.7Ga22.4 alloys annealed at 1073 K for 6 h

3 BANU et al.: Ni-Mn-Ga MAGNETIC SHAPE MEMORY ALLOYS 303 Fig. 3 (a) SEM micrograph Ni50.9Mn26.7Ga22.4 polycrystalline alloy of the annealed sample show martensite structure at room temperature and (b) inset of Fig. 3a, unidirectional stripe twins are obtained from the original parent phase to martensite phase. As a result, the structure can be changed from cubic to orthorhombic or tetragonal. The alloy with sevenlayered orthorhombic structure 17 has low twining stress and high magnetic anisotropy which produces a large magnetic field induced strain up to 10%. The SEM measurements have been carried out at room temperature to clarify the details of martensitic structure in Ni-Mn-Ga alloy. The martensitic variants play a vital role in Ni-Mn-Ga alloy, which exhibit the magnetic shape memory effect and the microstructure of the Ni-Mn-Ga alloy is also a key factor for actuator applications. Thus, the characterization of martensite structure in Ni-Mn-Ga alloy is very important. From the micrograph, we confirm that the structure is built of single phase and there is no secondary phase is present. The typical morphology of stripe like pattern is formed in Ni 50.9 Mn 26.7 Ga 22.4 polycrystalline alloy consisting with well-accumulated unitary martensitic variants, which is shown in Fig. 3. For the thermoelastic martensitic transformation, the twin martensitic microstructure is more important which is present in the sample. From the result, the presence of stripe like martensite pattern indicates that the thermo-elastic martensitic transformation for these alloys occurs above the room temperature. The shape of the variants in the morphology can be distinguished as stripe or lamellar, which align in a unidirectional way. The magnified image (Fig. 3b) clearly shows the welldefined stripe like domain patterns of a martensite phase. Generally, the thickness of the martensite bands can be related to the structure of modulation. The thickness is narrow for 7M and wide for 5M structure. In the SEM image, fine martensite bands are observed which confirms 7M modulated structure. The magnetic property is a directional dependent one which is called as magnetic anisotropy. It controls the coercivity and strongly affects the shape of the hysteresis loop. The magnetic anisotropy is mainly depends on the crystal structure and it is called as magneto-crystalline anisotropy. This anisotropy is independent of the grain size. In order to find out the coercivity (H c ), saturation magnetization (M s ) and the retentivity (M r ), M-H loops are analyzed at room temperature by using vibrating sample magnetometer. The hysteresis loop of the as cast and heat treated Ni 50.9 Mn 26.7 Ga 22.4 polycrystalline alloy were recorded at room temperature which is shown in Fig. 4. The magnetic measurement for the as cast and heat treated alloys has been taken from Oe to Oe. The results revealed that the alloy show the single phase heusler structure and magnetically soft material with a ferromagnetic nature. In Fig. 4b, magnetization starts gradually increasing with the increase of the magnetic field. After 7 koe, it gets saturated and the measured saturation magnetization (M s ) is 61 emu/g. The as cast alloy also showed the typical ferromagnetic behavior, but with low magnetic saturation and high coercivity (Fig. 4a). It is evident from Fig. 4 that the heat treatment of the alloy provokes improving of the ferromagnetic behavior which has attributed to the homogenization of the alloys. The effect of annealing enhances the saturation magnetization due to the Ga vacancies concentration and/or some ordering of the Mn/Ga atoms in the lattice 22. For the annealed sample, the magnetization linearly increases due to the presence of martensitic variants. The martensite variants oriented along the direction of the magnetic field so only the magnetization increases while increasing the magnetic

4 304 INDIAN J. ENG. MATER. SCI., AUGUST 2017 field. The coercivity decreases from 376 Oe to 74 Oe for the heat treated alloy which was also less than the earlier reported values. The low coercivity material is important for potential application which was reported by Giapintzakis et al. 23 Theoretical Explanation for Twin Boundary Motions: A Seminal Approach using the Mean Field Theory Approximation We have kept the FSMA Ni-Mn-Ga sample in the laboratory electromagnet for 48 h to align the particles along the direction of the magnetic field. Experimental arrangement to show the position of the FSMA Ni-Mn-Ga sample is as shown in Fig. 5a. The twin boundary motions are illustrated in Fig. 5b where the exchange interaction between the variant 1 and variant 2 is taking place assuming the mean field theory approximation. As system is a ferromagnetic one, we have assumed that there is a presence of internal magnetic field (H j ) in the test specimen. The effective magnetic moments (μ tb ) associated with the twin boundary motions is given as 24,25 B ( x); x N H / K T (1) tb j j B where μ is the magnetic moment expressed in terms of Bohr magneton number, B j (x) is the Brillouin function, H j is the effective applied magnetic field at each variant j, K B is the Boltzmann constant, T is the room temperature and N is the number of alloy systems used. The experimentally measured magnetic moment 4.36 μ B of Ni-Mn-Ga is in appreciable agreement with that of the theoretically computed value 3.43 μ B using Eq. (1). It is observed that the mean field theory approximation can be used for ferromagnetic alloy systems to determine the magnetic moments. Further, the development of the computational tools to carry out more structured calculations of the magnetic moments for this Ni-Mn-Ga FSMA is in progress. Fig. 4 M vs H curve of Ni-Mn-Ga polycrystalline alloy (a) as-cast Ni50.9Mn26.7Ga22.4 alloy and (b) annealed Ni50.9Mn26.7Ga22.4 alloy Fig. 5 (a) Experimental arrangement to show the position of the FSMA Ni-Mn-Ga sample in the laboratory electromagnet through which the magnetic field is applied, and (b) twin boundary motions are illustrated where the exchange interaction between the variant 1 and variant 2 is taking place assuming the mean field theory approximation

5 BANU et al.: Ni-Mn-Ga MAGNETIC SHAPE MEMORY ALLOYS 305 Conclusions The as-cast Ni 50.9 Mn 26.7 Ga 22.4 polycrystalline alloy exhibiting single phase of seven-layered martensite (7M) with an orthorhombic structure with poor crystalline nature is obtained. After annealing the high crystallinity with the same orthorhombic structure is obtained. The hysteresis loop confirms the ferromagnetic nature of the alloy. The microstructures of the alloy observed at room temperature show the stripe like martensitic bands with fine structures, which confirms 7M orthorhombic structure. As a consequence, we believe this Ni 50.9 Mn 26.7 Ga 22.4 alloy has an ability to produce high magnetic field induced strain and good shape memory effect at room temperature. As a result, this alloy composition is suitable for the applications of sensors and actuators at room temperature. Acknowledgements One of the authors (MM) acknowledges the UGC, New Delhi to support this work under the Grant Nos and UGC- RA (1843). References 1 Ullakko K, Huang J K, Kantner C, O Handley R C & Kokorin V V, Appl Phys Lett, 69 (1996) Chernenko V A, Chmielus M & Mullner P, Appl Phys Lett, 95 (2009) Chernenko V A, Barandiarán J M, L vov V A, Gutiérrez J, Lázpita P & Orue I, J Alloy Compd, 577 (2013) S305-S Murray S J, Marioni M, Kukla A M, Robinson J, O Handley R C & Allen S M, J Appl Phys, 87 (2000) Nie Z H, Cong D Y, Liu D M, Ren Y, Pötschke M, Roth S & Wang Y D, Appl Phys Lett, 99 (2011) Straka L, Lanska N, Ullakko K & Sozinov A, Appl Phys Lett, 96 (2010) Faran E & Shilo D, Appl Phys Lett, 100 (2012) Chun-Mei Li, Hu-Bin Luo, Qing-Miao Hu, Rui Yang, Börje Johansson & Levente Vitos, Phys Rev B, 82 (2010) Oleg Heczko & Ladislav Straka, J Magn Magn Mater, 272 (2004) Banik S, Sanjay Singh, Rawat R, Mukhopadhyay P K, Ahuja B L, Awasthi A M, Barman S R & Sampathkumaran E V, J Appl Phys,106 (2009) Lanska N, Söderberg O, Sozinov A, Ge Y, Ullakko K & Lindroos V K, J Appl Phys, 95 (2004) Richard M, Feuchtwanger J, Schlagel D, Lograsso T, Allen S M & O Handley R C, Scr Mater, 54 (2006) Pons J, Santamarta R, Chernenko V A & Cesari E, Mater Chem Phys, 9806 (2003) Pons J, Santamarta R, Chernenko V A & Cesari E, Mater Sci Eng A, 438 (2006) Sozinov A, Likhachev A A, Lanska N, Söderberg O, Ullakko K & Lindroos V K, Mater Sci Eng A, 378 (2004) Murray S J, Marioni M, Allen S M, O'Handley R C & Lograsso T A, Appl Phys Lett, 77 (2000) Mediha Kok & Yıldırım Aydogdu, Thermochim Acta, 548 (2012) Sozinov A, Likhachev A A, Lanska N, Ullakko K & Lindroos V K, J Phys IV France, 112 (2003) Rama Rao N V, Gopalan R, Manivel Raja M, AroutChelvane J, Majumdar B & Chandrasekaran V, Scr Mater, 56 (2007) desantanna Y V B, de Melo M A C, Santos I A, Coelho A A, Gama S & Cótica L F, Solid State Commun,148 (2008) Martynov V V & Kokorin V V, J Phys III France, 2 (1992) Jan Romberg, Claudia Hürrich, Martin Pötschke, Sandra Kauffmann-Weiss, Uwe Gaitzsch, Stefan Roth, Peter Müllner & Ludwig Schultz, J Alloy Compd, 577 (2013) S344 S Giapintzakis J, Grigorescu C, Klini A, Manousaki A, Zorba V, Androulakis J, Viskadourakis Z & Fotakis C, Appl Phy Lett, 80 (2002) Von Boehm J & Per Bak, Phys Rev Lett, 42 (1979) Liu F, Press M R, Khanna S N & Jena P, Phys Rev B, 39 (1989) 6914.