CHAPTER 2 LITERATURE REVIEW

Transcription

1 13 CHAPTER 2 LITERATURE REVIEW 2.1 PROGRESS IN MFIS OF Ni-Mn-Ga ALLOY For more than 40 years, the Ni-Mn-Ga alloys have been studied as one of the Heusler alloys with the chemical formula X 2 YZ. Heusler et al (1903) first reported that in Cu-Mn based Heusler alloys even their constituent elements are not ferromagnetic. A survey of the existing literature shows that Soltys was the first person who started to work on the Ni-Mn-Ga alloy system, as described in (Soltys 1974 and 1975). Thereafter, a systematic study was carried out in the following years. The ferromagnetic transition at 376 K and thermoelastic martensitic transformation at 202 K in Ni 2 MnGa was reported by Webster et al (1984) and by Kokorin et al (1990). Chernenko et al (1995) initiated the investigation on the Ni-Mn-Ga alloys in the early years of As part of their study, Ullakko et al (1996) first described the possibility for a magnetic-fieldinduced strain in the Ni-Mn-Ga alloys. Followed by this, Ullakko et al (1996 a) demonstrated the magnetic field induced strain of 0.2% in a single crystal Ni 2 MnGa by the application of magnetic field of 800 kam -1 at the temperature of 265 K. Consequently, Murray et al (1999) reported a remarkable increase in the magnetic field induced strain of 0.57% in single crystal Ni-Mn-Ga alloy, when the alloy is subjected to a magnetic field of 500 kam -1 at room temperature. Followed by this, Tickle et al (1999) elucidated magnetostrictive strains of nearly 1.3% in the Ni-Mn-Ga system.

2 14 In the year 1999, Heczko et al (2000) observed the giant magnetic-field-induced strain, more than 5% in nearly tetragonal Ni 48 Mn 31 Ga 21. In the same year, Murray et al (2000) demonstrated 6% MFIS in single crystal Ni 49.8 Mn 28.5 Ga 21.7 alloy and in 2002, Sozinov et al (2002) reported the MFIS of nearly 10% in a modulated orthorhombic sample of Ni 48.8 Mn 29.7 Ga 21.5 at ambient temperature in a magnetic field of less than 1T, (Figure 2.1) which is the highest strain reported in Ni-Mn-Ga alloy till date. This 10% strain was observed in the time period of 10 millisecond under the magnetic field of kam -1, through the orientation of martensite structure. Figure 2.1 Giant MFIS in a seven-layered martensitic Ni-Mn-Ga alloy, observed by Sozinov et al (2002) According to Sozinov et al (2002), this 10% of strain produced by the modulated orthorhombic Ni-Mn-Ga alloy is 50 times lager than the strain observed in Terfenol-D. The fast response and maximum strain makes this alloy as a novel material for magnetic actuators, as pointed out by Tellinen (2002). The strain of the Ni-Mn-Ga alloys has been studied in different aspects such as martensitic and magnetic transformation temperatures, magnetocrystalline anisotropy and saturation magnetizations.

3 EXPERIMENTAL EVIDENCE FOR MFIS Figure 2.2 illustrates the experimental setup used by Marioni et al (2003) for the measurement of magnetic-field-induced strain by the application of pulsed magnetic field (17.9 koe, 600 s). A control system monitors the charge of the capacitor arrangement with a high voltage power supply. The capacitors are then discharged through an air-coil (a Helmholtz pair) when the control system fires the silicon controlled rectifier (SCR). Thus it produces a magnetic field. Figure 2.2 Experimental arrangement used by Marioni et al (2003), for the measurement of elongation of the Ni-Mn-Ga crystal. The elongation has been measured in terms of intensity variation of the He-Ne laser beam A Ni-Mn-Ga crystal is fixed in a cantilever inside the coil. A highly monochromatic He-Ne laser and optical fiber cables form the displacement sensor. The elongation of the crystal during the magnetic field is measured with the help of a mirror attached to the free end, which reflects the He-Ne

4 16 laser beam incident at an angle into a photo detector. Elongation of the crystal causes the beam to reflect the ray at different points in the mirror. The photo detector receives the laser beam reflected from the mirror at an angle. Hence, the intensity of the signal reaches the detector which varies with respect to the elongation of the crystal. Using this experimental arrangement they have measured a strain of 0.16 mm, which is only 15% of the theoretical maximum strain. Figure 2.3 Elongation of the Ni-Mn-Ga crystal with magnetic pulse measured by Marioni et al (2003). Magnetic field is marked in the y axis; the change in length of the crystal is shown in right side axis with time Figure 2.3 depicts the curve drawn between applied field Vs. elongation and time of the Ni-Mn-Ga studied by Marioni et al (2003). The applied magnetic field is represented by H and the curve dx represents the elongation of the crystal. The figure shows that the elongation and magnetic pulse doesn t start at same time and the extension of the sample is lagging behind the applied magnetic field by 100 s as the twin boundaries were immobile which reduced the volume of the material.

5 17 The novel Ni-Mn-Ga FSMAs has been studied by means of (i) (ii) (iii) (iv) The crystal structure Structural transformation Magnetic transformation and Industrial applications 2.3 CRYSTAL STRUCTURE OF Ni-Mn-Ga ALLOY Ni-Mn-Ga alloy is an intermetallic compound that displays the Heusler structure. X-ray diffraction, electron diffraction, neutron diffraction, Mossbauer spectroscopy and selected area electron diffraction pattern are some of the techniques available for determination of the crystal structure of Ni-Mn-Ga alloy, studied by Ranjan et al (2006), Banik et al (2006), Zhou et al (2005 b), CGomez-Polo et al (2009) Pons et al (2006) and Richard et al (2006). Sozhinov et al (2001) has reported that the crystal structure of martensite is an important factor that affects both the magnetic anisotropy and mechanical properties of ferromagnetic Ni Mn Ga alloys. Pons et al (2006) has found that at room temperature the stoichiometric Ni-Mn-Ga displays the austenite phase with Fm3m cubic symmetry (L2 1 ordering) and has the Curie temperature (T C ) of around 373 K. They also studied that above 1073 K, Mn and Ga atoms get disordered from the parent austenite phase and transforms to the structure of Pm3m symmetry. Hosoda et al (2004) pointed out that the stoichiometric Ni-Mn-Ga has the magnetic transition temperature (T C ) of around 373 K. Upon cooling, Ni 2 MnGa undergoes a martensitic transition which results in a reduction in symmetry from cubic to tetragonal or orthorhombic depending on the valence electron to atom ratio (e/a). The transformed

6 18 martensite unit cell has a body-centered tetragonal structure (BCT) with 14/mmm symmetry. Figure 2.4 displays the crystal structure of the Ni 2 MnGa alloy in austenite and martensite condition. Figure 2.4 Crystal structure of (a) cubic austenite structure shows L2 1 ordering and (b) tetragonal martensite, identified by Wan and Wang (2005) In the literature, martensite is referred to face-centered tetragonal (FCT) which is well connected with the austenite cubic structure, where the c-axis contracts to form a tetragonal structure (c/a < 1).Thisc/a ratio is useful to determine the maximum MFIS of single crystal Ni-Mn-Ga alloy using the formula 1- (c/a) There is rich evidence for the Ni rich Ni-Mn-Ga alloys having both tetragonal and orthorhombic martensite structures. Different martensitic phases have been reported based on X-ray and neutron diffraction studies. As mentioned by Banik et al (2007), some of the Ni-Mn-Ga alloys have five layered modulated tetragonal (5M) structure, some others have seven layered

7 19 modulated orthogonal (7M) and few non-modulated (NM) tetragonal structures. The layered structures have been interpreted in the literatures, in two different ways; they are modulated structures with shuffling of the atomic planes derivedfrom{110} aust by a function with the corresponding periodicity and stacking of nearly close-packed basal planes derived from {110} aust in Ni Al alloys, as predicted by Kainuma et al (1996) and Morito and Outsuka (1996). Of these two types of layered structure, Brown et al (2002) have identified that 5M modulation is the basic requirement for the giant MSME. The modulated structure consists of a periodic shuffling of atomic planes in the [110] direction of the cubic axes which results giant MSME. The five layered (5M) modulated martensite has very low twinning stress and high magnetic anisotropy. Once it was believed that the MFIS is not possible in NM Ni-Mn-Ga alloy. But, Chernenko et al (2009) proved that the MFIS is possible in NM Ni-Mn-Ga alloy also. They found a large MFIS of 0.17% for a stress-free Ni 53.1 Mn 26.6 Ga 20.3 single crystal, which is ten times larger than values reported so far in NM martensites. Modulation is also connected with the valence electron to atom ratio (e/a) as reported by Ranjan et al (2006). For an alloy with e/a ratio of , both 5M and NM phase could exist. Above the modulation is absent.

8 20 Here, the term modulation is used to describe the periodicity of the atoms found in a particular composition of Ni-Mn-Ga system. This modulation can be easily seen in XRD and in Selected Area Electron Diffraction Pattern (SAEDP). Mogylnyy et al (2003) calculated the atomic shift from the equilibrium positions by using a modulated lattice approach. Displacement of each j plane along the new a axis [110] aust from its regular position is given y function j containing three harmonic terms. Here, j=asin(2πj/l) + B (4πj/L) + C (6πj/L) (2.1) L Represents the modulation period A, B and C are the constants selected for using the least difference between the experimentally measured intensities of the main and extra spots and the calculated intensities. Corresponding constants, calculated for the thermally induced 5M martensite (L = 5) at room temperature are: A = 0.055, B = and C = However, according to Martynov (1995), displacement of the atoms from their regular positions in multilayer structure changes from to PHASE TRANSFORMATION IN Ni-Mn-Ga ALLOY Zasimchuk et al (1990), Kawamura et al (2006), Banik et al (2008) and Dong et al (2008) have characterized the spontaneous phase transition during cooling by means of several martensites with different directions of the tetragonal axis. The distinct weaker reflections on the X-ray patterns in addition to the principal maxima of the BCT structure indicate that the specimens contain one predominant martensite orientation. It showed that they are all arranged regularly along the rows of reflections parallel to one of

9 21 the two [110]* directions of the reciprocal lattice. In this direction, the distance between the two strongest reflections of the BCT lattice is divided into five equal segments by the additional maxima. The principal reflections are indexed as "strong" (S), the four additional reflections are indexed as "moderate" (M), and "weak" (W) (Figure 2.5). The intensity ratio of these reflections in a row passing through a reciprocal lattice site (200) has been estimated to be S: M: W = 100:10:1. In 5M structure, there are four additional spots between two fundamental spots and in 7M structure six additional spots, illustrated in Figure 2.6. Figure 2.5 Reflections on the XRD patterns. The principal reflection is indexed as strong (S) and the four additional reflections are indexed as moderate (M) and weak (W), shown by Zasimchuk et al (1990). In accordance with Pons et al (1999) and Chernenko et al (2002 a), the crystal structure of the Ni Mn Ga martensitic phase strongly depends on composition and temperature. The relation between c/a and e/a was investigated by Tsuchiya et al (2000). They observed that e/a value of 7.7 is the critical value, at which the crystal structure of the martensitic phase changes from 5M to NM. However, Tsuchiya et al (2001) estimated the critical value to be for the transformation from 5M to NM.

10 22 Figure 2.6 Selected area diffraction pattern of 5M structure (left) and 7M structure (right), recorded by Pons et al (2006) 2.5 RELATION BETWEEN MARTENSITIC TRANSITION TEMPERATURE (T m ) AND VALENCE ELECTRON TO ATOM RATIO (e/a) The Ni-Mn-Ga FSMA is well known for its room temperature martensitic transformation. Martensitic transition temperature (T m ) is characterized as a function of valence electron to atom ratio (e/a), which in turn depends on composition. It is reported by Chernenko et al (2002 b) that the Ni-Mn-Ga alloys with the e/a < have T m below T C and the alloys with e/a > have T m in paramagnetic state. Earlier reports show that by altering Ni and Mn concentration in stoichiometric Ni-Mn-Ga alloy, T m and T C can be tuned to match each other and at a particular composition Tm becomes equal to T C (T m =T C ). This significant effect is useful in magnetic refrigeration. Many researchers are working towards the co-occurrence of both martensitic and magnetic transitions(t m =T C ). Chernenko et al (2002) predicted the possibility for the co-occurrence of the T m =T C at e/a = 7.7. However, Pareti et al (2003) reported the same at 7.64 in Ni Mn Ga and Xuehi et al (2004) reported

11 23 at 7.61 in Ni 55.2 Mn 18.6 Ga These values are less than those predicted by Chernenko et al (2002). With reference to Pons et al (2000) the Ni-Mn-Ga alloys with e/a range between 7.35 and 8.10 have the martensitic transformation temperature range from 200 K to 626 K. The general view of shift in transformation temperatures in Ni-Mn-Ga alloys was studied by Wu and Yang (2003) and Chernenko et al (1995) as a function of the element content. They found that at a constant Ni content, Mn addition increases the T m drastically. at a constant Mn content, Ga addition in the place of Ni lowers the T m. at a constant Ga content, substitution of Ni by Mn lowers the T m. The above shift in transformation temperatures is also confirmed by Wirth et al (1997) and Dikhstein et al (1999). A ternary diagram of Ni Mn Ga system is shown in Figure 2.7 to display the relation between e/a and T m. In this Figure, filled squares indicate the samples with T C >T m, empty squares indicate the samples with T C <T m and filled circles indicate the samples with T C T m (i.e. with a magnetostructural transition). The vertical dashed lines on the diagram indicate a constant e/a. Three different regions can be seen in the diagram. The first region is characterized by e/a = In this region T C >T m, and martensitic transformation takes place in the ferromagnetic state.

12 24 Alloys from the second region are characterized by the coupled magnetostructural transition, i.e. T C T m. Ferromagnetic transition in this compositional interval has the characteristic of a first-order phase transition, showing pronounced hysteresis on temperature and field dependencies of magnetization. Such unusual magnetic properties of these alloys have been attributed to the simultaneously occurring martensitic and ferromagnetic transitions, as shown by Khovailo et al (2002), Filippov et al (2003) and Vasil ev (2003). Finally, the third region is characterized by e/a >7.65, here the martensitic transformation takes place in the paramagnetic state. Figure 2.7 Ternary diagram of Ni Mn Ga alloy system drawn by Borisenko et al (2006). The figure depicts the relation between T m,t C and e/a Generally, alloys from this region have a high T m,upto650k,and a low T C ~350K.TheoccurrenceofhighT m from this region makes this alloy attractive for application as high-temperature shape memory materials. The diagram infers that the area of the magnetostructural transition extends along the vertical line characterized by e/a 7.7 independent of substitution.

13 25 This shows a strong interrelation between T m corresponding between 7.6 and 7.7. and e/a in the region 2.6 MAGNETIC PROPERTIES OF Ni-Mn-Ga ALLOY Magnetic properties of Ni-Mn-Ga alloys have been studied earlier by Ullakko et al (1996), Tickle and James (1999), Zhou et al (2005 a), Zhao et al (2007) and Ahuja et al (2007). Of them, Ullakko et al (1996), Tickle and James (1999) made measurements on magnetic anisotropy energy in a single crystal of Ni 2 MnGa. Particularly, Ullakko et al (1996) calculated the magnetic anisotropy energy of 0.12 MJ/m 3 from the magnetization curve studied at 265 K. However, Tickle and James (1999) found it to be MJ/m 3 at the same temperature of 256 K in a single variant state of a mechanically constrained sample. Later, Kokorin et al (1992) reported the T C of 350 K for Ni 2 MnGa. The same T C has been reported by Murray et al (1998) in polycrystalline Ni 41.7 Mn 31.4 Ga The particle size also affects the magnetic property of the magnetic materials, discussed in detail by Dutta et al (2003). Composition dependence on magnetic properties have been examined by Jiang et al (2004) in Ni 50 Mn 25+x Ga 25-x (x = 0-5). They found that the saturation magnetization decreases with an increase in Mn content in austenite and martensite state and that there is a sudden increase at Mn 28, shown in Figure 2.8. They also noticed a change in anisotropy constant in the same tendency as that of saturation magnetization. The magnetic anisotropy constant was measured by them to be 1.01x10 5 J/m 3 for Mn 30 and 0.64 x10 5 J/m 3 for Mn 25.

14 26 Figure 2.8 Saturation magnetizations Vs magnetocrystalline anisotropy constants K 1 of Ni 50 Mn 25+x Ga 25-x (x=2,3,4,5)alloysat 300 K and 5 K. The measurements was carried out by Jiang et al (2004) Vasil ev et al (1999) have reported a decrease in T C of 10% per Mn atom substitution for Ga. However, only 2% reduction in T C per Mn atom substitution in the place of Ga has been recorded by Jiang et al (2004), which is much smaller compared with the earlier report of Vasil ev et al (1999). This clearly shows that T C is influenced by the substitution of the Mn atom. Jiang et al (2004) reported that the ferromagnetic property of the Ni-Mn-Ga alloy mainly depends on the magnetic moment of the Mn atom. It is about 4 B per Mn ion and it is less than 0.3 B per Ni. As the magnetic moment of Ni is negligible, it is understood that the ferromagnetism in Ni-Mn-Ga alloy is only due to the magnetic moment of Mn atom. The magnetic moment of the Mn atom decreases from 4.38 B to 2.93 B when the

15 27 Mn content is increased from 0 to 5 in Ni 50 Mn 25+x Ga 25-x. According to Jiang et al (2004), the variation of magnetic moment is attributed to the negative effect of excess Mn atom. σ s = σ s Mn + (0.04x) σ s Mn a (2.2) where σ s is the overall magnetic moment σ s Mn is the magnetic moment of Mn atom in stoichiometric Ni 2 MnGa σ s Mn a is the average contribution of Mn atoms in excess The influence of the applied magnetic field on the martensitic region has been studied in detail at 273 K and 300 K by Gutierrez et al (2006) for Ni 51 Mn 28 Ga 21 alloy, well below its Tm (337 K). At 300 K, the measured magnetic moment was 1.25 B. This clearly shows that the critical magnetic field to induce variants reorientation is approximately 2 koe at room temperature with some hysteresis, and the critical magnetic field lowers as the temperature increases and becomes zero at Tm. The measurements have shown that the variants of the five and seven-layered martensites re-orient very easily in comparison with the non-modulated tetragonal phase, as observed by Soolshenko et al (2003). The MFIS was studied for Ni 49. Mn 29.1 Ga 21.2 by Heczko et al (2005). The results are shown in Figure 2.9. The much smaller MFIS about 0.06% has been reported and the magnitude of the strain is said to have decreased further to about 0.008% with increasing compressive stress at 60 MPa. Irrespective of the stress, the strain saturates in the same field. This phenomenon suggests that rearrangement of martensite variants may not occur at all under such stress. This makes Ni-Mn-Ga appear to be soft with small blocking stress and energy density, undesirable for actuator applications.

16 28 Figure 2.9 MFIS of Ni 49.7 Mn 29.1 Ga 21.2 studied by Heczko et al (2005) 2.7 MECHANICAL PROPERTIES OF Ni-Mn-Ga ALLOY New results on Ni-Mn-Ga alloy are being reported in the literature, for example Marchenkova et al (2010), Santamarta et al (2010) but most of them are related to single crystals and thin films. The reason is that the SME of single crystal Ni-Mn-Ga alloy is high in the order of 10% and its properties are not much affected by the microstructural phenomenon, as described by Kustov et al (2009). The favourable orientation of the magnetic domains with respect to the crystallographic axes leads to maximum strain. This maximum strain is the result of the orientation of the magnetic field along the [001] crystal axis. But in polycrystalline Ni-Mn-Ga alloy, the [001] direction vary from grain to grain. Therefore, the strain observed in this alloy is always less than that of the single crystal Ni-Mn-Ga alloy. Xu et al (2003), Ma et al (2008) and Ma et al (2009) have analyzed the shape memory effect of the Ni-Mn-Ga alloy by compression study. It is proved that the polycrystalline Ni-Mn-Ga alloys are extremely brittle due to mechanical constraints at the boundaries in randomly textured grain.

17 29 Jeong et al (2003), Li et al (2004), as well as Chernenko et al (2000) have studied in detail the mechanical behaviour and pseudoelasticity of FSMA. A pseudoelasticity of 2.2% for polycrystalline Ni 53.5 Mn 19.5 Ga 27 followed with a MFIS of 0.82% was reported by Jeong et al (2003). Li et al (2004) investigated the stress strain behaviour in the compression mode and SME of polycrystalline Ni 54 Mn 25 Ga 21 alloy with high transformation temperature and they proved the effectiveness of the grain refinements. They examined a sample of rod shape and button shape. The rod shape sample was subjected to a higher cooling rate than the button shape sample. They found that the high cooling rate results in relatively small grain sizes in rod shape sample, shown in Figure Figure 2.10 Microstructures of the polycrystalline Ni 54 Mn 25 Ga 21 alloys observed by Li et al (2004). Button sample shown in left exhibits a coarse grained structure with 200 µm in size containing lamellar twins. The right one is the rod sample which is composed of small grains ranging from 10 to 50 µm in size The compressive stress-strain curve of the polycrystalline Ni 54 Mn 25 Ga 21 alloy was studied by Li et al (2004). It is interesting to note that the initial linear part of both the stress strain curve is identical up to a

18 30 strain of 2% and a stress of 120 MPa. This strain is known as the elastic strain and the stress is known as martensite reorientation stress. It can be seen in Figure 2.11 that in curve b the stress slowly rises from 2% to 7% for the rod shape sample. This slow process indicates the reorientation of the martensitic variants. However, the button sample exhibits a very short reorientation strain of 1%. The compressive strength and compressive strain of the button shape sample are 440 MPa and 10%, respectively, and the corresponding values of the rod sample are 970 MPa and 16% respectively. Figure 2.11 Compressive stress strain curves for (a) button and (b) rod samples of the polycrystalline Ni 54 Mn 25 Ga 21 investigated by Li et al (2004) Mechanical properties of the Ni 54 Mn 25 Ga 21 rod sample can be understood by considering its relatively small grain size and much finer martensitic twin structure as shown in Figure This result shows that grain refinement is an effective method to improve the shape memory properties in polycrystalline Ni-Mn-Ga alloys. The SME of 4.2% and a

19 31 compressive plasticity of 10% lead to recognize this alloy as a hightemperature SMA. Ni Mn Ga single crystal is found to form cracks easily when the crystal is thermally cycled through phase transformation temperatures, as shown in Figure It is believed that the co-existence of several martensite twinned variants is the main reason for the formation of the crack network leading to a fracture. The fracture surface was found to relate to the {1 1 2} twin planes in the martensitic phase. The fracture plane is a slope intersecting the front surface by an angle of around 45. Figure 2.12 Evolution of crack due to result of thermal cycling in polished surface of Ni Mn Ga single crystal specimen observed by Xiong et al (2005)

20 EFFECT OF Fe, Co, Cu SUBSTITUTIONS ON T m AND CURIE TEMPERATURE (T C ) The stoichiometric Ni-Mn-Ga alloy has the T C of 376K and T m of 202 K, as measured by Webster et al (1984). But for actuator application, the T m should be above room temperature in order to avoid cooling of the device. A number of researchers have paid attention to the development of a reliable room temperature Ni-Mn-Ga component with T m above room temperature by changing the composition of Ni, Mn and Ga. Notably, the T m is found to rise to 85 K per at % for Ga replaced by Ni, reported in Jiang et al (2003) and in Chakrabartietal(2005). Changes in composition may raise the T m. On the other hand, composition change may cause instability in T m,t C, saturation magnetization and high output power to weight ratio. In order to meet these emerging thrust in the field of actuator and sensor, the research has been accelerated by doping the 3d transition metal elements and 4f rare earth elements with a single crystalline Ni-Mn-Ga. Tsuchiya et al (2007), Tsuchiya et al (2004) and Shihai et al (2005) have studied the effect of doping of Fe, Co, and Tb in Ni-Mn-Ga alloy. In their study, they reported that the partial substitution of Fe in the place of Ga increases both T m and T C in Ni 50 Mn 27 Ga 23-x Fe x (x = 0, 1, 2). Similarly, partial substitution of Fe in place of Mn in Ni 47 Mn 31 Fe 1 Ga 21 alloy increases T m but decreases T C without affecting the crystal structure of the alloy, as described by Koho et al (2004). It is also reported that the transformation property of Co doping in Ni-Mn-Ga is exactly opposite to that of Fe doping. Co addition leads to

21 33 lowering of T m with a raise in T C. It is considered that this raise in T C is due to the ferromagnetic property of Co. Besides, the rise in T C, Co-stabilizes the T m in Ni-Mn-Ga alloys. Recent investigation also supports that the addition of Co contributes to a sudden decrease of T m in Ni-Mn-Ga, when its content exceeds 6%. Cong et al (2008) reported that the substitution of Co in the place of Ni has proved to be efficient in increasing T C. Recently, Ma et al (2009) have investigated the Ni Mn Ga alloys substituted by Co for high-temperature applications. They found that the substitution of Ni by Co lead to decrease in T m with increasing Co content. This is likely to have resulted from an increase in the unit-cell volume and a decrease in e/a. However, the substitution of Co in the place of either Ni and Mn or Mn resulted in the decrease of T m with decreasing e/a, as discussed in Sui et al (2008). In both cases, the unit-cell volume increases, which indicate that the effect of substituting Co for Mn in martensitic transformation is complex. Tsuchiya et al (2000) found that the effect is minor on lattice parameter but the T C increases considerably with the Cu addition. Ma et al (2009) also investigated the Ni-Mn-Ga alloy system by introducing the Cu atom in the place of Mn. The variations in T m and shape memory effects with Cu contents correlate with changes in size factor, e/a and unit-cell volume. When Mn atoms are replaced by Cu atoms, due to a higher number of valence electrons, an increase in e/a occurs, till x < 2. But when x 2, no more Cu atoms can be accommodated in the tetragonally structured martensite. Therefore, Tm remains almost constant when x 2.

22 SINGLE CRYSTAL V s POLYCRYSTALLINE Ni-Mn-Ga ALLOY Day to day, new results on Ni-Mn-Ga alloys is being reported in literatures, but most of them are related to single crystals and thin films. Only limited attentions have been paid on polycrystalline Ni-Mn-Ga alloy, as studied by Singh et al (2008) and CGomez-Polo et al (2009). The reason is that the SME of single crystal Ni-Mn-Ga alloy is high i.e., of the order of 10% and their properties are not altered by the microstructural phenomenon like, the variation of crystallographic direction from grain to grain. Therefore, the strain observed in polycrystalline Ni-Mn-Ga alloy is always less than that of single crystal. Figure 2.13 Composition distributions along the axis of the single crystal Ni 50 Mn Ga grown by Wang and Jiang (2008). Calculated and experimental values of composition are shown by the solid line and the dots respectively

23 35 In order to meet the industrial requirements, Ni-Mn-Ga FSMAs are prepared in the form of single crystals covering various methods like Bridgman method and zone melting method, and fast-solidification method. Figure 2.13 depicts the compositional variation in single crystal Ni 50 Mn Ga alloy prepared by Wang and Jiang (2008) using fastsolidification method. It is reported that there is about 50 K variation in the T m along the axis of the single crystal Ni-Mn-Ga prepared using the Bridgman technique, as reported by Jiang et al (2005). This composition variation is less than 10K along the 72 mm length of the single crystal Ni-Mn-Ga alloy, prepared by the zone melting method, Liu et al (2005). Compositional variation is an important factor to be considered before selecting a Ni-Mn-Ga alloy for actuator application. Particularly; composition variation may introduce the defects in the structure, which in turn affects the output strain of the actuator. Cost and shaping is still a problem in single crystal Ni-Mn-Ga alloy. Owing to this composition variation, it is necessary to identify a suitable method to prepare the Ni-Mn-Ga alloys with a narrow range of martensitic transformation temperature. In polycrystalline Ni-Mn-Ga, this compositional variation can be greatly minimized. Polycrystalline Ni-Mn-Ga alloy has the intergranular fracture, brittleness and low field-induced strain. Because of its cost-effective, it is preferred for commercial purposes. A study made by Guimaraes (2007) on the transformation property of polycrystalline Ni-Mn-Ga alloy supports 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

24 36 polycrystalline FSMA, austenite to martensite transformation is influenced by the grain size INDUSTRIAL APPLICATIONS OF Ni-Mn-Ga ALLOY Recently, Sarawate and Dapino (2009) focused on the characterization and modeling of a commercial Ni Mn Ga alloy for use of dynamic deformation sensor. The flux density has been experimentally determined as a function of cyclic strain loading at frequencies from 0.2 to 160 Hz. The outcome of their work is that increasing hysteresis in magnetization must be considered when utilizing the material in dynamic sensing applications. Figure 2.14 Schematic representation of microactuator made up of Ni-Mn-Ga thin film, designed by Auernhammer et al (2009), which employs with combined magnetic field induced actuation and magnetostriction. Actuator is designed as double-beam cantilevers with a thickness of 10 μm, and a length and width of 3 and 0.4 mm respectively for each beam

25 37 Auernhammer et al (2009) devised a FSMA microactuator for position sensing as shown in Figure The micro-actuator is designed as a double-beam cantilever made of a polycrystalline Ni Mn Ga thin film, which has both forward and reverse martensitic transformation in the temperature range of 333 K 359 K and a ferromagnetic transition at about 370 K. The microactuator is placed in the inhomogeneous magnetic field of a miniature Nd Fe B magnet causing a mixed thermo-magnetoresistance effect upon actuation. The maximum in-plane magnetic field is about 0.38 T. In this case, the maximum magnetoresistance is 0.19%. Under these conditions, a maximum positioning accuracy of 18 m can be reached within the deflection system. This device demonstrates the feasibility of combined magnetic-fieldinduced actuation and magnetoresistance. Recent breakthrough in polycrystalline Ni-Mn-Ga alloy is the introduction of pores. Dunand et al (2009) have reported a polycrystalline Ni-Mn-Ga foam structure, which is shown in Figure They introduced porosity using a simple and inexpensive casting technique. The polycrystalline Ni-Mn-Ga foams form a network of single crystals. The foam structure permits single crystallinity to extend over longer distances resulting in greater MFIS of 10% over 244,000 magneto-mechanical cycles. The porous alloy has great potential for uses that require light weight such as space and automotive applications, tiny motion control devices, and biomedical pumps with no moving parts.

26 38 Figure 2.15 Ni-Mn-Ga alloy foam structure reported by Dunand et al (2009) contain pores. The strain produced by this foam structure is three orders of magnitude larger than MFIS shown by non-porous, fine grained Ni-Mn-Ga and other FSMAs Barandiaran et al (2009 a), Hosoda et al (2005), Srivastava et al (2006), Bhowmik et al (2005), and Bhowmik et al (2006) have studied in more detail about the applications of magnetic materials in the field of engineering. Commonly, magnetostrictive materials are used as energy absorbers. Now, polymer composites embedded in a Ni-Mn-Ga particle cured by compressive stress have been identified as energy absorbers and this composite is a good alternative for the conventional magnetostrictive materials such as Terfenol-D. The composite is cured under a compressive stress and magnetic field to induce the formation of chains of single-variant particles, enhancing the twin boundary motion. According to thermodynamics, twin boundary motion is described as an irreversible process. It indicates that the energy supplied to a Ni-Mn-Ga energy absorber, moves the twin boundary and the same energy is dissipated

27 39 in the form of heat. This energy dissipation mechanism makes the Ni-Mn-Ga alloys useful material in vibration-damping. For energy dissipation applications, the loss ratio should be preferably greater than 10%. The loss ratio of Ni-Mn-Ga FSMA polymer composite is 67% under a small stress of 1.5 MPa, as shown by Marioni et al (2005), Feuchtwanger et al (2003) and Feuchtwanger et al (2005). A study carried out by Feuchtwanger et al (2009) infers that this outstanding high loss ratio under small stress makes Ni-Mn-Ga FSMA polymer composite as a novel material in energy absorption and actuator technology INFLUENCE OF ANNEALING ON TRANSFORMATION PROPERTIES OF Ni-Mn-Ga ALLOY As mentioned in the previous section 2.9, the preparation of a single crystal consumes a long time, they are highly expensive and there is a compositional variation along the axis of the crystal. Due to these difficulties, polycrystalline Ni-Mn-Ga alloy has gained a lot of attention in the recent years. The fabrication process of polycrystalline Ni-Mn-Ga is easy and inexpensive. Nevertheless, polycrystalline Ni-Mn-Ga exhibits low strain due to the presence of grain boundaries and random orientation of domains. These grain boundaries and random orientation of domains are the limiting agencies of twin boundary motion, discussed in detail by Besseghini et al (2004), Martin et al (2007) and Tian et al (2008). In addition to the chemical composition, martensite transformation is also sensitive to annealing, according to Duan et al (2007 b) and Hosoda et al (2006). Earlier report on the effect of thermal treatment on polycrystalline Ni-Mn-Ga alloy made by Seguı et al (2005) and Chernenko et al (2005) shows that both T C and T m depend on quenching temperature and it is found to increase upon subsequent annealing.

28 40 Gaitzsch et al (2006 b) found the evolution of the microstructure upon annealing in a Ni-Mn-Ga alloy, as shown in the Figure It is worth noting that the appropriate annealing procedure results in an ordered L2 1 structure and makes the martensitic transformation temperature range very narrow, as shown by Tsuchiya et al (2000), Besseghini et al (2001). This behaviour displays a strong dependence of annealing on the transformation properties of Ni-Mn-Ga alloys. Figure 2.16 Evolution of the microstructure upon annealing. As-cast (left) and the annealed polycrystalline samples (right), reported by Gaitzsch et al (2006 b) Another study by Besseghini et al (2004) shows that annealing modifies the grain structure sharpens the transformation peaks and changes the orientation of the crystal planes from (110) to (100). This implies that annealing results in homogenization and stress relaxation. The above studies clearly indicate that annealing has some effect of on transformation properties of Ni-Mn-Ga alloy. As the fabrication process of polycrystalline Ni-Mn-Ga is easy and inexpensive polycrystalline Ni-Mn-Ga alloy has been chosen for this dissertation.