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 X2YZ. 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 Ni2MnGa 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 1990. As part of their study,

Ullakko et al (1996) first described the possibility for a magnetic-field-

induced 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 Ni2MnGa 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.

14

In the year 1999, Heczko et al (2000) observed the giant

magnetic-field-induced strain, more than 5% in nearly tetragonal

Ni48Mn31Ga21. In the same year, Murray et al (2000) demonstrated 6% MFIS

in single crystal Ni49.8Mn28.5Ga21.7 alloy and in 2002, Sozinov et al (2002)

reported the MFIS of nearly 10% in a modulated orthorhombic sample of

Ni48.8Mn29.7Ga21.5 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 400-640 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.

15

2.2 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

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.

17

The novel Ni-Mn-Ga FSMAs has been studied by means of

(i) The crystal structure

(ii) Structural transformation

(iii) Magnetic transformation and

(iv) 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 (L21 ordering) and has the Curie

temperature (TC) 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 (TC) of

around 373 K.

Upon cooling, Ni2MnGa 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

18

martensite unit cell has a body-centered tetragonal structure (BCT) with

14/mmm symmetry. Figure 2.4 displays the crystal structure of the Ni2MnGa

alloy in austenite and martensite condition.

Figure 2.4 Crystal structure of (a) cubic austenite structure shows L21

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). This c/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

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

derived from {1 1 0}aust by a function with the corresponding

periodicity and

• stacking of nearly close-packed basal planes derived from

{1 1 0}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 Ni53.1Mn26.6Ga20.3 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 7.61-7.715, both 5M and NM phase could exist.

Above 7.715 the modulation is absent.

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 = A sin (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 = 0.003 and C = 0.006. However, according to Martynov (1995),

displacement of the atoms from their regular positions in multilayer structure

changes from -0.051 to 0.051.

2.4 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

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 7.61-7.62 for the transformation from 5M to NM.

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 (Tm) 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 (Tm) 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 < 7.6 -7.62 have Tm below TC and the alloys

with e/a > 7.62 - 7.7 have Tm in paramagnetic state.

Earlier reports show that by altering Ni and Mn concentration in

stoichiometric Ni-Mn-Ga alloy, Tm and TC can be tuned to match each other

and at a particular composition Tm becomes equal to TC (Tm =TC). This

significant effect is useful in magnetic refrigeration. Many researchers are

working towards the co-occurrence of both martensitic and magnetic

transitions(Tm =TC). Chernenko et al (2002) predicted the possibility for the

co-occurrence of the Tm =TC at e/a = 7.7. However, Pareti et al (2003)

reported the same at 7.64 in Ni54.75Mn20.25Ga and Xuehi et al (2004) reported

23

at 7.61 in Ni55.2Mn18.6Ga26.2. 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 Tm

drastically.

• at a constant Mn content, Ga addition in the place of Ni lowers

the Tm.

• at a constant Ga content, substitution of Ni by Mn lowers the

Tm.

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 Tm. In this Figure, filled squares indicate

the samples with TC > Tm, empty squares indicate the samples with TC< Tm

and filled circles indicate the samples with TC ≈ Tm

(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 = 7.65. In this region

TC > Tm, and martensitic transformation takes place in the ferromagnetic state.

24

Alloys from the second region are characterized by the coupled

magnetostructural transition, i.e. TC ≈ Tm. 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 Tm, TC and e/a

Generally, alloys from this region have a high Tm, up to 650 K, and

a low TC ~ 350 K. The occurrence of high Tm 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.

25

This shows a strong interrelation between Tm and e/a in the region

corresponding between 7.6 and 7.7.

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 Ni2MnGa. Particularly, Ullakko et al (1996) calculated the magnetic

anisotropy energy of 0.12 MJ/m3

from the magnetization curve studied at

265 K. However, Tickle and James (1999) found it to be 0.245 MJ/m3

at the

same temperature of 256 K in a single variant state of a mechanically

constrained sample. Later, Kokorin et al (1992) reported the TC of 350 K for

Ni2MnGa. The same TC has been reported by Murray et al (1998) in

polycrystalline Ni41.7Mn31.4Ga24.2. 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 Ni50Mn25+xGa25-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 Mn28,

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 1.01x105

J/m3

for Mn30 and 0.64 x105

J/m3

for Mn25.

26

Figure 2.8 Saturation magnetizations Vs magnetocrystalline anisotropy

constants K1 of Ni50Mn25+x Ga25-x (x = 2, 3, 4, 5) alloys at

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 TC of 10% per Mn

atom substitution for Ga. However, only 2% reduction in TC 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 TC 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

27

Mn content is increased from 0 to 5 in Ni50Mn25+xGa25-x. According to Jiang et

al (2004), the variation of magnetic moment is attributed to the negative effect

of excess Mn atom.

σs = σsMn

+ (0.04x) σsMn a

(2.2)

where σs is the overall magnetic moment

σsMn

is the magnetic moment of Mn atom in stoichiometric Ni2MnGa

σsMn 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 Ni51Mn28Ga21 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 Ni49.Mn29.1Ga21.2 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.

28

Figure 2.9 MFIS of Ni49.7Mn29.1Ga21.2 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.

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 Ni53.5Mn19.5Ga27 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 Ni54Mn25Ga21 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 2.10.

Figure 2.10 Microstructures of the polycrystalline Ni54Mn25Ga21 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

Ni54Mn25Ga21 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

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 Ni54Mn25Ga21 investigated by

Li et al (2004)

Mechanical properties of the Ni54Mn25Ga21 rod sample can be

understood by considering its relatively small grain size and much finer

martensitic twin structure as shown in Figure 2.10. 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

31

compressive plasticity of 10% lead to recognize this alloy as a high-

temperature 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 2.12. 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)

32

2.8 EFFECT OF Fe, Co, Cu SUBSTITUTIONS ON Tm AND

CURIE TEMPERATURE (TC)

The stoichiometric Ni-Mn-Ga alloy has the TC of 376K and Tm

of 202 K, as measured by Webster et al (1984). But for actuator application,

the Tm 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 Tm above room

temperature by changing the composition of Ni, Mn and Ga. Notably, the Tm

is found to rise to 85 K per at % for Ga replaced by Ni, reported in Jiang et al

(2003) and in Chakrabarti et al (2005).

Changes in composition may raise the Tm. On the other hand,

composition change may cause instability in Tm, TC, 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 Tm and TC in Ni50Mn27Ga23-xFex (x = 0, 1, 2). Similarly, partial

substitution of Fe in place of Mn in Ni47Mn31Fe1Ga21 alloy increases Tm but

decreases TC 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

33

lowering of Tm with a raise in TC. It is considered that this raise in TC is due to

the ferromagnetic property of Co. Besides, the rise in TC,

Co-stabilizes the Tm in Ni-Mn-Ga alloys. Recent investigation also supports

that the addition of Co contributes to a sudden decrease of Tm 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 TC.

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 Tm 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 Tm 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 TC 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 Tm 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.

34

2.9 SINGLE CRYSTAL Vs 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

Ni50Mn28.25Ga21.75 grown by Wang and Jiang (2008).

Calculated and experimental values of composition are

shown by the solid line and the dots respectively

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

Ni50Mn28.25Ga21.75 alloy prepared by Wang and Jiang (2008) using fast-

solidification method.

It is reported that there is about 50 K variation in the Tm 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

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polycrystalline FSMA, austenite to martensite transformation is influenced by

the grain size.

2.10 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

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Auernhammer et al (2009) devised a FSMA microactuator for

position sensing as shown in Figure 2.14. 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-field-

induced 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 2.15. 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.

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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

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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.

2.11 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 TC and Tm depend on quenching temperature and it is found

to increase upon subsequent annealing.

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Gaitzsch et al (2006 b) found the evolution of the microstructure upon

annealing in a Ni-Mn-Ga alloy, as shown in the Figure 2.16. It is worth

noting that the appropriate annealing procedure results in an ordered L21

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.