Magnetic Transformation in Ferromagnetic Ni-Mn-Ga Shape Memory Alloy

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1 Magnetic Transformation in Ferromagnetic Ni-Mn-Ga Shape Memory Alloy K. Pushpanathan, R. Senthur Pandi, R. Chokkalingam, A.Sivakami and M.Mahendran Department of Physics, Thiagarajar College of Engineering, Madurai , India ABSTRACT This article covers the detailed study on the preparation and characterization of off-stiochiometric Ferromagnetic Ni-Mn-Ga shape memory alloy. Annealing the polycrystalline Ni-Mn-Ga alloy at 700 C for 20 hours under organ atmosphere found to reduce the range of transformation temperature. The DSC measurement reveals that the alloy has the martensitic transformation temperature (Tm) at room temperature and it less than (Tc). Magnetic measurement has been performed using VSM. The observed narrow transformation hysteresis (around 3 C) during cooling and heating suggests that annealing the polycrystalline Ni-Mn-Ga at moderate temperature for long time changes the transformation temperatures of the martensite and austenite phases. From the present study it is concluded that the annealed polycrystalline Ni-Mn-Ga alloy undergoes thermoelastic martensitic transformation. Keywords: Ferromagnetic Shape Memory Alloy(FSMA), Actuator, Sensors, Austenite, Martensite, Twin variant 1. INTRODUCTION Owing to the development in science and technology, smart materials have been gaining attention for the past fifteen years. As a result, numbers of new high quality materials have come into market for various engineering applications. In which, smart materials is one of the important high quality engineering materials. The term smart describes self adaptability, self sensing and multiple functionalities of the materials. These materials are capable of acting as both sensor and actuator by changing their mechanical properties by means of temperature, electrical and magnetic field. These characteristics makes number of possible applications of these materials as a structural materials in aerospace, mechanical, electrical, electronics civil and in biomedical engineering systems. Ferromagnetic Shape Memory alloys (FSMA) 1-6 is one kind of smart material and it is recognised by the two stable states: Austenite and martensite. Austenite is the high temperature stable phase. It has the simple cubic structure. Martensite is the low temperature phase. The low temperature martensite can have the tetragonal (or) orthorhombic structure depending on the composition. The incorporated transformations are diffusionless, martensitic phase transitions, for which the crystallographic unit cell undergoes large deformations. During transformation, large force is generated. The force generated so is utilized in SMA based actuators, sensors, micro-gripper 7, an artificial hand of robotic system, and in energy absorber 8. As FSMA wires can fully recover its original state, it is used in vibration dampers and isolators. The energy dissipation property of FSMAs proves that it is a useful constructional material for building against earthquake 9. In recent years ferromagnetic Ni -Mn-Ga shape memory alloys have been extensively studied by various research groups because of their considerable deformation when they are exposed to a magnetic field. This property is of particular interest, since the switching process can be made much faster and better controlled compared to conventional shape memory alloys. The thermo elastic phase transformation is the origin of the shape memory effect in Ni-Mn-Ga. Martensitic phase transformation from cubic L2 1 has been found to occur on cooling Ni 2 MnGa to 204K 15. Though number of alloys undergoes martensitic transformation, Ni-Mn-Ga has a particular interest. On cooling they first become ferromagnetic at Curie temperature (Tc) and below this temperature; it undergoes martensitic transformation i.e. this alloy exhibits both martensitic and ferromagnetic. The Ni-Mn-Ga Heusler alloy shows two types of transformations. Transition from ferromagnetic austenite to ferromagnetic martensite is known as structural transition and the transition from ferromagnetic austenite to paramagnetic austenite is called as magnetic transition. These two phase transitions can be overlapped in this alloy by varying composition. Moreover, microstructure of the alloy plays an important role in determining the phase transformation temperature by means of twin rearrangement. Ullakko 16 and James 17 first described the possibility for the magnetic field induced strain in Ni-Mn-Ga alloys. Following this, Ullakko 3 and Murray 18 demonstrated the 0.2% field induced strain in a single crystal Ni-Mn-Ga in the magnetic field of 800kAm -1 at the temperature of 265K. In Ni-Mn-Ga, the phase transformation can be induced by the application of thermal energy (or) by the magnetic field. After phase transformation, martensites form a multivariant pattern. This multivariant minimises the transformation strain. The Further author information: (Send correspondence to M. Mahendran) M. Mahendran: manickam-mahendran@tce.edu

2 magnetic field induced strain is increased in step by step manner from 0.32% to 6% for five-layered martensite through the different composition of Ni/Mn/Ga and by changing the volume fraction of a particular twin variant 4,19,20,21 and 22,. The giant magnetic field-induced strain of about 10% has been reported at room temperature for a seven-layered orthorhombic Ni 49 Mn 30 Ga 23 21, in the magnetic field of kA/m, which is 50 times lager than the strain observed in Terfenol-D. Ni-Mn-Ga is a complex material and the basic condition for the giant magnetic shape memory effect is the existence of 5-layered modulated martensite, which has very low twinning and high magnetic anisotropy. It is considered that the large strain produced in this material is due to the rearrangement of the twin variant in the modulated martensite state, resulting macroscopic change in dimension. Theoretically it is predicted that the maximum strain of 10% can be produced and this maximum strain range is limited by the martensitic lattice parameter, i. e. (1-c/a). Till now no one reported the magnetic shape memory effect in nonmodulated structure though it is having the ability to produce the strain nearly 20%. The unique property of the FSMA is the fact that the martensitic transformation and structural transformation temperatures are extremely sensitive to the composition. The martensitic transformation and the magnetic properties of the alloys are found to depend strongly on Mn/Ga ratio and thermal treatment 24, 25, 26. From the literatures it is found that the very first reported Ni 2 MnGa showed the martensitic transformation temperature of 204K 15. Now numerous studies is going on around the world to rise both the martensitic and magnetic transition transformation temperatures by varying the Ni/Mn/Ga composition and by substituting the atom like Fe, Co, Tb, C, Si, Bi, Pb, Zn and Sn to meet the demand of the industry. Unfortunately, Ni 2 MnGa alloys are very brittle, which significantly limits their practical applications. To overcome this problem, now a days, FSMA nano particles /polymer composites are being studied 14. In this system, the polymer matrix provides structural integrity, ductility and the FSMA nano particle produces the desired shape changes. Recent study on transformation property of polycrystalline ferromagnetic shape memory alloy evidences that the austenite grain size is the driving force to initiate the martensitic transformation. The controlled propagation of the martensite plate lowers the transformation temperature in fine-grained austenite, and favours for the formation of clusters of partially transformed grains. Therefore, in polycrystalline ferromagnetic shape memory alloy, austenite / martensite transformation is influenced by the grain size 27.The aim of our research group is to investigate the basic things responsible for the shape memory effect in polycrystalline ferromagnetic shape memory alloy and how it can be used in sensors and actuators in the form of thin films. The present work focuses on the investigation of magnetic transformation property of a offstoichiometric Ni-Mn-Ga compound with excess Ni. 2. EXPERIMENTAL PROCEDURE High purity Ni (99.99% pure), Mn (99.99% pure), and Ga (99.99% pure) were taken as the raw materials for the present study. These raw materials were mixed together at room temperature with mortar and pestle. Then the mixture of these materials of weight 8gms were taken in a silica crucible and melted in a tabular furnace under argon atmosphere. Initially, the sample was kept at the temperature of 700 C for 30 min. Then it was heated at 900 C for 40 min and at 1000 C for 60 min and finally the temperature was fixed and allowed to melt at 1025 C for 15 hours towards the through mixing of Ni/Mn/Ga and then allowed to cool at room temperature. The alloy was homogenised at 700 C for 2 days in a sealed quartz ampoule and then quenched into water. In view of minimizing the evaporation of manganese during melting, the furnace has been backfilled with argon gas before keeping the sample in side the chamber. The martensitic transformation and structural transformation temperatures were measured by means of Differential Scanning Calorimeter (MITTER) at cooling and heating rate of 5 K/min. The composition of the alloy Ni Mn Ga was determined by the energy dispersive photoelectron spectrometer. Temperature dependence of magnetization of the sample was carried out using Vibration sample magnetometer (VSM-5, TOEI industries) between 25 C to 105 C 3. RESULTS AND DISCUSSION The Differential Scanning Calorimeter has the enough sensitivity to measure the Phase transition associated with heat transfer 28. Thermoelastic martensitic transformation observed in the off-stoichiometric Ni-Mn-Ga has a

3 particular interest in the sense of considerable change in martensitic microstructure due to thermal process Cooling curve M f =36 C, M s =40 C Tc = 110 C Heat flow in (mw) Temperature in C Heating curve A s = 42 C, A f = 45 C Fig.1. Differential Scanning Calorimetry profile of NiMnGa sample shows the heat flow signals during forward and reverse martensitic transformation temperatures. Hence, monitoring the heat flow during forward and reverse martensitic transformation temperatures process can give us an accurate indication of the phase transition temperature. Twinned martensites are mobile, which is proved in temperature dependant shape memory alloys. In the martensitic phase, the alloy is deformed easily due to the easy mobility of the twin martensites. In this present study, the reversibility of the transformation is confirmed by the DSC run. The figure 1 indicates the heat flow signals during forward and reverse martensitic transformation. The measured transformation temperatures are A s = 42 C, A f = 45 C, M s = 40 C, and M f = 36 C. The recorded DSC data implies that the alloy has martensite structure at room temperature. High temperature peak denoted by Tc =110 C, corresponds to ferromagnetic transition temperature of the austenite state. Commonly the thermal hysteresis for the single crystalline FSMA is narrow, which is attributed to the martensite plates of uniform thickness. But, it is broader for powder sample. The observed narrow thermal hysteresis (around 3 C) of the present study suggests that annealing a polycrystalline FSMA at moderate temperature for long time can change the transformation temperatures of the martensite and austenite phases and reduces the width of the thermal hysteresis. Therefore, from the present study it is concluded that the annealed polycrystalline Ni-Mn-Ga alloy undergo thermoelastic martensitic transformation and one can get the near single crystalline property. Our observation is in close agreement with the already reported one 29. Vibrational Sample Magnetometer is a very powerful tool to characterize the various phase transformation, and has been widely used to analyse the thermoelastic martensitic transformation in shape memory alloys.

4 1.20 Temperature (C) Ms As 1.00 Magentization (emu) (emu) A s = 45 C, A f =42 C M s =40 C, M f =36 C 0.20 A f - M f Fig.2: Typical temperature dependant magnetization curve measured by Vibrational sample Magnetometer (VSM) showing the magnetic shape memory effect Figure 2 represents the temperature dependence of magnetization measured with VSM. The curve shows very sharp abrupt changes at austenitic martensitic and magnetic transition points. Narrow austenite martensite phase transformation indicates the high phase homogeneity. The transformation temperatures measured with VSM are As = 42 C, A f = 45 C, Ms = 40 C, M f = 36 C and Tc= 95 C.The coincidence of DSC and VSM confirms that the sample is a ferromagnetic one. A sharp change of Tc in the above figure is a mark of a co-operative phase transition but the slight difference in Tc measured by VSM and DSC needs further clarification. 4. CONCLUSION The martensitic and magnetic transformation properties of Ni Mn Ga magnetic shape memory alloy have been investigated. The alloy shows the martensitic transformations below Curie temperature (Ms = 40 C, M f = 36 C). From the present study it is concluded that the near single crystalline property can be obtained in a polycrystalline sample, if it subjected to heat treatment at moderate temperature. Further improvement of the materials property of this is composition, in the form of thin film, is expected making this alloy an potential candidate for certain micro-damper and energy dissipation mechanism. ACKNOWLEDGEMENT One of the authors (M.M) would like to thank Robert C.O Handley for his guidance and for introducing him to the subject and providing Ni-Mn-Ga single crystal. DRDO, New Delhi is greatly acknowledged for financial support (ERIP/ER/ /M/01/953)

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