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: dkaurfph@iitr.ernet.in (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:dkaurfph@iitr.ernet.indx.doi.org/10.1016/j.matlet.2012.09.073

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

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