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Materials Letters 91 (2013) 202205Contents lists available at
SciVerse ScienceDirectMaterials Letters0167-57
http://d
n Corr
E-mjournal homepage: www.elsevier.com/locate/matletGrain
refinement of NiTi shape memory alloy thin films by W
additionNavjot Kaur, Davinder Kaur n
Functional Nanomaterials Research Lab, Department of Physics and
Centre of Nanotechnology, Indian Institute of Technology Roorkee,
Roorkee 247667, Uttarakhand, Indiaa r t i c l e i n f o
Article history:
Received 3 September 2012
Accepted 23 September 2012Available online 29 September 2012
Keywords:
Thin films
Sputtering
Grain size
Elastic modulus7X/$ - see front matter & 2012 Elsevier
B.V.
x.doi.org/10.1016/j.matlet.2012.09.073
esponding author. Tel.: 91 1332 2285407;ail address:
[email protected] (D. Kaur).a b s t r a c t
The present research explores the novel approach to achieve
grain refinement in NiTi shape memory
alloy thin films by adding W in the matrix of NiTi. It involves
production of NiTiW shape memory alloy
thin films by Co-sputtering of NiTi and W targets. The grain
size of B2-NiTi decreases with increasing W
content, due to the immiscible W layer obstructing its grain
growth. Moreover addition of W induces
the B2R single step transformation by suppressing thermally
induced martensite phase due to
grain size refinement below 40 nm. With W content ranging from
2.6 at% to 4.5 at%, the films are
strengthened and can reach highest hardness and elastic modulus
of 32.872.7 GPa and 167.8378.64 GPa, respectively. With further
increase in W content, the mechanical properties of films
decrease
gradually. This behavior can be explained in terms HallPetch
theory and lattice distortion of NiTi
crystals with increasing the W content.
& 2012 Elsevier B.V. All rights reserved.1. Introduction
Nanocrystalline materials have been the subject of
considerableresearch in recent years for their novel and enhanced
properties[1,2]. The nanocrystalline NiTi shape memory alloys were
synthe-sized in their bulk form by severe plastic deformation
(SPD),including high pressure torsion (HPT), equal-channel angular
press-ing (ECAP), and multi-step SPD deformations [3]. Grain
refinement ofNiTi enhances its mechanical properties and modify the
phasetransformation path to B2R having smaller hysteresis, low
trans-formation strains and higher work output as compared to
B2RB190
present in coase grained NiTi [4]. Because of the small
hysteresis, aquick response is expected in microactuators using
such an R-phasetransformation. Thus the application of grain
refinement by varioustechniques is a powerful tool to design
microstructures with superiorproperties and performance. However
above mentioned mechanicalmethods of grain refinement are limited
to bulk samples only. Thepresent research explores the novel
approach to achieve grainrefinement in NiTi thin films by adding W
in matrix of NiTi. Effectof grain refinement on structural, phase
transformation and mechan-ical properties of NiTi was
investigated.2. Experimental procedure
NiTiW thin films were deposited on Si substrates by magne-tron
sputtering using Dc magnetron Co-sputtering system (ExcelAll rights
reserved.
fax: 91 1332 273560.Instruments, India), which is equipped with
two magnetron guns.High purity (99.99%) NiTi (50 at% Ni; 50 at% Ti)
and W (tungsten)metal targets of 50 mm diameter and 3 mm thickness
were used.Amount of W added to the matrix of NiTi could be
preciselycontrolled with power to each target. The target power
wastypically set at 112 W for NiTi and varied from 3 W to 50 W forW
(tungsten) target and thus series of NiTiW shape memory alloyfilms
with different W content were deposited.
The structure, surface morphology and chemical compositionof
films were studied using X-Ray diffraction (XRD), atomic
forcemicroscope (AFM), field emission scanning electron
microscope(FESEM) and energy dispersive X-Ray Spectrometry (EDX).
Fourprobe resistivity methods were used to study the shape
memoryeffect. Hardness and elastic modulus of NiTiW thin films
weremeasured by Nanoindenter device (Micromaterials, UK)
usingBerkovich indenter. The results were averaged by 12
independentindentations on each sample.3. Results and
discussion
Fig. 1(c) shows the XRD spectra of pure NiTi and NiTiW
shapememory alloy thin films. Pure NiTi film reveals the formation
offully austenite phase with dominant reflection from (110)
plane.With increasing the W content, the intensity of NiTi (110)
diffrac-tion peak decreases gradually while its width increases
(Table 1).This reveals that grain size of NiTi decreases gradually
with increaseof W content. R phase reflection is also present in
XRD curves of allthe NiTiW thin films but the diffraction peak of
B2 and R-phase arevery close and is not possible to separate them.
Hence they weretreated as same (110) B2/(300) R in XRD pattern.
From XRD results
www.elsevier.com/locate/matletwww.elsevier.com/locate/matletdx.doi.org/10.1016/j.matlet.2012.09.073dx.doi.org/10.1016/j.matlet.2012.09.073dx.doi.org/10.1016/j.matlet.2012.09.073mailto:[email protected]/10.1016/j.matlet.2012.09.073
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Fig. 1. Influence of W content on (a) unit cell volume change
DV/V0, (b) lattice parameter and lattice distortion, (c) X-ray
diffractograms and (d) AFM and FESEM images ofNiTiW sputtered thin
films.
N. Kaur, D. Kaur / Materials Letters 91 (2013) 202205 203it
appears that W solubility in NiTi is limited for W content lessthan
4.5 at% as no peak of b-W phase was observed in the XRDpattern of
NiTiW (2.6) and NiTiW (4.5) films. However withincrease in W
concentration above solubility limit (which isapproximately less
than 5 at% in present case), the b-W phase isclearly evidenced in
XRD curves of NiTiW (9.1), NiTiW (12.8) andNiTiW (33.6) thin films.
Fig. 1(b) shows the variation of latticeconstant and relative
lattice distortion of NiTi thin films withincreasing W content. The
incorporation of W in NiTi lattice up to4.5 at%, gives rise to
lattice contraction also indicated by decrease inlattice constant.
This is because smaller W (0.068 nm) atomsreplace larger Ti (0.076
nm) atoms in NiTi lattice. As the W contentis further increased
lattice constant increases indicating dialation oflattice. This
increase in lattice parameter of B2NiTi is due tointerfacial strain
energy which arises due to lattice misfit betweenb-W (due to its
stabilization at higher W content) and B2NiTilattice. Accordingly
the unit cell volume of NiTiW thin films wasalso found to be
changed relative to NiTi (Fig. 1(a)). For the NiTiWsamples with 2.6
and 4.5 at%W content, the unit cell volume change(DV/V0) is
negative; i.e., the lattice structure is contracted, while
thelattice is dilated or expanded beyond 4.5 at% W addition
indicatedby positive values of DV/V0. Fig. 1(d) shows the AFM and
FESEMimages of pure NiTi and different NiTiW thin films. The
filmswere very dense, smooth and crackfree. The films show
granular
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Table 1Various parameters of NiTi and NiTiW shape memory alloy
thin films.
Sample name Structural properties Mechanical properties
Grain size (nm) Average
roughness (nm)
Total deptha
hmax (nm)
Residual deptha
hr (nm)
Elastic recovery
ratioa, dHardnessa
H (GPa)
Elastic modulusa
E (Gpa)
H3/Er2a (GPa)
XRD FESEM AFM
Pure NiTi 25.2 80.4 87.9 20.5 114.4170.72 88.671.68 0.22570.015
8.0570.29 85.671.98 0.07170.006NiTiW (2.6) 7.16 24.1 24.9 7.5
74.5570.31 51.6171.27 0.30870.017 23.4471.14 162.9775.94
0.48470.096NiTiW (4.5) 6.23 20.1 21.7 6.3 71.5770.56 43.6371.94
0.3970.029 32.872.76 167.8378.64 1.2570.376NiTiW (9.1) 4.16 13.8
14.3 3.53 98.3670.71 70.4172.64 0.28470.026 12.5970.97 100.9473.76
0.19670.053NiTiW (12.8) 3.34 11.1 11.4 3.03 103.9970.92 75.972.13
0.2770.021 10.870.64 93.276.01 0.14570.036NiTiW (33.6) 2.76 9 9.6
2.41 107.491.33 79.5672.15 0.25970.022 9.8670.38 89.475.98
0.11970.028
a The values are mean7standard deviation for n12.
100 150 200 250 300 350 400 0.56
0.60
0.64
0.68
0.72
Res
ista
nce
(ohm
)
Temperature (K)
Res
ista
nce
(ohm
)
Temperature (K)
0.336
0.340
0.344
0.348
70 140 210 280 350 420
Fig. 2. Phase transformation behavior of pure NiTi and NiTiW
sputtered thin films.
N. Kaur, D. Kaur / Materials Letters 91 (2013)
202205204morphology with average granule size and surface
roughnessdecreasing with increasing W content (Table 1).
Fig. 2 shows the RT plots of NiTi and NiTiW thin filmsexplaining
their phase transformation behavior. As, Af, Rs, Rf, Ms,Mf
represent the transformation temperatures where A: austenite(B2);
R: martensite (R-phase); M: martensite (B190); s: starttemperature
and f: finish temperature. It is evident from the RTcurve of NiTi
that, it undergoes B2RB190 transformation exhi-biting wide (28 K)
thermal hysteresis. However addition of W intoNiTi above its solid
solubility limit (W44.5 at%) results in singlestep B2R reversible
phase transformation. Fig. 2 shows the RTcurve of NiTiW (9.1) thin
film exhibiting B2R phase transforma-tion with much reduced thermal
hysteresis of 11 K. The change inphase transformation behavior
observed in this film is correlatedto grain size of B2NiTi phase.
In NiTiW (9.1) thin film, immiscibleb-W phase obstructs grain
growth of the B2NiTi grains. Thisleads to grain size refinement of
B2NiTi resulting in high densityof grain boundaries which act as
obstacles that hinder the growthof martensite and autocatalytic
nucleation potency. Thereforeretarded grain growth can effectively
suppress the B190 marten-site transformation as compared to R-phase
transformation. Thisis according to the reported result that below
a grain size of50 nm, the B190 martensite was completely suppressed
[5]. Thereason for the suppression of martensite as compared to R
phaseis that nanograins contain only small lattice strains which
canaccommodate only R phase because transformation strains of
theR-phase (about 1%) are smaller than those of B190(about 10%)
[5].
Fig. 3(a) shows the hardness (H) and elastic modulus (Er) ofpure
NiTi and NiTiW thin films as function of W content. It can beseen
that the hardness of pure NiTi film is about 8.0570.29 GPa.After
adding some W into NiTi film, the hardness increasesrapidly and
reaches maximum value of 32.872.76 GPa when Wcontent is 4.5 at%.
With further increase in W content, it decreasesfirst rapidly and
then gradually. The change of elastic modulus ofNiTiW films has the
same trend as that of hardness. It reaches amaximum value of
167.8378.64 GPa at 4.5 at% W and thengradually decreases to about
89.475.98 GPa with furtherincrease in W content. The indentation
induced superelastic effectcan be characterized by the elastic
recovery ratio which isobtained from the loaddisplacement curves
as
ER hmaxhrhmax
where hmax is the penetration depth at maximum load, and hr
isthe residual depth when the load returns to zero during
unload-ing [6]. H3/Er
2 is also an important material parameter whichindicates the
resistance of the coating to plastic deformation or tocrack
formation and propagation, when the film is exposed toexternal load
[7]. Fig. 3(b) shows the elastic recovery ER and H3/Er
2
ratio of NiTiW films as a function of W content. ER and H3/Er2
also
increases with increase in W content and reaches maximum at4.5
at% W, indicating better elastic recovery and toughness ofthese
films. Drastic decrease is also observed in these parametersbeyond
4.5 at% W addition. This variation in mechanical proper-ties could
be explained by two possible mechanisms: One is grain size
refinement with increasing the W content.
Other is lattice distortion of NiTi lattice with increasing
the
W content.
Increase in hardness from 8.0570.29 GPa for pure NiTi
to32.872.76 GPa with 4.5 at% W addition is expected to be due
to
-
W content (at.%)
-0.3
0.0
0.3
0.6
0.9
1.2
1.5
0
10
20
30
40
0 5 10 15 20 25 30 35
H3/Er2
ER (%)
Ela
stic
Rec
over
y R
atio
ER
(%)
H3 /
Er2
W content (at.%)
Har
dnes
s (H
) (G
Pa)
Ela
stic
Mod
ulus
(E
r) (
GPa
)
0
40
80
120
160
7
14
21
28
35
0 5 10 15 20 25 30 35
(Er) Hardness
Fig. 3. (a) Hardness and elastic modulus, (b) elastic recovery
ratio (ER%) and toughness(H3/Er
2) vs. W content of various NiTiW thin films.
N. Kaur, D. Kaur / Materials Letters 91 (2013) 202205
205significant grain size refinement (Table 1). It is widely
recognizedthat strengthening with grain size refinement is
attributed to thepropagation of slip across grain boundaries by
pile up of dislocationdescribed by the HallPetch relationship [8].
Grain refinement byWaddition results in large number of grain
boundaries which restrictsthe plastic deformation of the films to
large extent by acting asbarriers to dislocation motion thereby
increasing the elastic recov-ery and toughness of the films.
Drastic decrease in mechanicalproperties with increasing the W
content beyond 4.5 at% is due tograin size refinement below
critical length 15 nm as shown inTable 1. As the grain size is
reduced to critical length scale,dislocation sources and pile-ups
are not expected to exist withinthe individual grains due to
spatial confinement and high volumefraction of grain boundaries. As
can be expected, grain boundarysliding and grain boundary rotation
play dominant role in deforma-tion mechanism, leading to inverse
HallPetch effect [9] asobserved experimentally. This departure from
HallPetch relationbelow 15 nm can be confirmed by calculating the
critical grain sizeof NiTi films with maximum hardness according to
followingrelation [8]:
dc Gb
p1usappwhere G is the shear modulus, b is the Burgers vector,n
is the Poissonratio and sapp is the applied stress. The calculated
value of dc is15 nm, below which hardness and other mechanical
properties areexpected to decrease and agrees well with
experimental results.
Lattice structure of the nm-sized crystallites may play
animportant role in mechanical properties of nanocrystalline
mate-rials. A film under lattice contraction resists the
penetration of theindenter thereby decreasing the indentation depth
(Table 1) andinhabiting the crack propagation. This leads to
increase in hard-ness and toughness of thin films. However, under
the latticeexpansion, opposite trend is observed. As shown in Fig.
1(b),when the lattice contracts, the interatomic spacing is
compressed,and a larger applied force is needed to break bonds; in
contraryunder lattice expansion, the spacing is extended, and a
smallerforce is needed. Thus as aforementioned, the mechanical
proper-ties of the NiTiW films undergoing lattice contraction
weremeasured better than that undergoing lattice expansion [10].4.
Conclusions
The present paper investigates the effect of grain refinementon
structural, phase transformation behavior and mechanicalproperties
of NiTi thin films. This is done by adding W into thematrix of NiTi
by Co-sputtering of NiTi and W targets. Addition ofW into NiTi
above its solid solubility limit induces B2R singlestep
transformation having much less thermal hysteresis. Hardnessand
elastic modulus increase with increasing W reaching maximumvalues
of 32.872.76 GPa and 167.8378.64 GPa respectively. Withfurther
increase in W content mechanical properties are
reducedgradually.Acknowledgment
The financial support provided by Ministry of Communicationsand
Information Technology (MIT), India, under NanotechnologyInitiative
Program with Reference no. 20(11)/2007-VCND ishighly
acknowledged.References
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Grain refinement of NiTi shape memory alloy thin films by W
additionIntroductionExperimental procedureResults and
discussionConclusionsAcknowledgmentReferences