CHAPTER 2 LITERATURE REVIEW - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/10514/7/07... ·...
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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.
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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.
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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
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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.
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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
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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
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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.
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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
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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.
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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
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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.
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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.
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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.
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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
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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.
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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.
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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
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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
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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)
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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
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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.
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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
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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
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.
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.