Physica B 406 (2011) 8–11

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Mechanical response of proton beam irradiated nitinol

Naveed Afzal, I.M. Ghauri n, F.E. Mubarik, F. Amin

Centre for Advanced Studies in Physics, GC University, Lahore, Pakistan

a r t i c l e i n f o

Article history:

Received 11 February 2010

Received in revised form

30 July 2010

Accepted 28 September 2010

Keywords:

Shape memory alloy

Irradiation

Deformation

Mechanical properties

26/$ - see front matter & 2010 Elsevier B.V. A

016/j.physb.2010.09.040

esponding author.

ail address: ijaz.phys@gmail.com (I.M. Ghauri

a b s t r a c t

The present investigation deals with the study of mechanical behavior of proton beam irradiated nitinol

at room temperature. The specimens in austenitic phase were irradiated over periods of 15, 30, 45

and 60 min at room temperature using 2 MeV proton beam obtained from Pelletron accelerator. The

stress–strain curves of both unirradiated and irradiated specimens were obtained using a universal

testing machine at room temperature. The results of the experiment show that an intermediate

rhombohedral (R) phase has been introduced between austenite and martensite phase, which resulted

in the suppression of direct transformation from austenite to martensite (A–M). Stresses required to

start R-phase (sRS) and martensitic phase (sMS) were observed to decrease with increase in exposure time.

The hardness tests of samples before and after irradiation were also carried out using Vickers hardness

tester. The comparison reveals that the hardness is higher in irradiated specimens than that of the

unirradiated one. The increase in hardness is quite sharp in specimens irradiated for 15 min, which then

increases linearly as the exposure time is increased up to 60 min. The generation of R-phase, variations in

the transformation stresses sRS and sMS and increase in hardness of irradiated nitinol may be attributed to

lattice disorder and associated changes in crystal structure induced by proton beam irradiation.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Shape memory alloys are intelligent materials that have gainedworldwide recognition because of their unique properties, such assuperelasticity and shape memory effect [1,2]. These alloys undergo areversible martensitic phase transformation under the action of eithertemperature or applied stress, where austenite phase is transformedinto a martensite phase. The transformation process is accompaniedby shape memory effect and superelasticity [1]. Nitinol is a wellknown shape memory alloy which is commonly used in biomedical,aerospace and nuclear engineering applications [3–8]. Therefore itsthermo-mechanical response under irradiation environment is ofprincipal importance. In this regard various attempts in the past havebeen made to investigate the irradiation effects of electrons, protons,neutrons and heavy ions on the physical and metallurgical behaviorof nitinol [9–20]. Occurrence of transition from crystalline toamorphous (C–A) structure has been observed in nitinol afterirradiation [9–15]. Cheng and Ardell [14] reported that C–A transitionin nitinol is a function of both irradiation dose and temperature. Moriet al. [15] studied the electron irradiation induced C-A transition innitinol. It was found that electron irradiation induces a localizedamorphization which mainly occurs along dislocation lines and grainboundaries. Effects of irradiation on transformation characteristics ofnitinol were studied by Hoshiya et al. [16] and later on by Konopleva

ll rights reserved.

).

et al. [17]. They observed that neutron irradiation of nitinol decreasedits transformation. Similar studies were also carried out by Al-Aqlet al. [18] using proton beam irradiation. Wang et al. [19] studied theexistence of R-phase in proton beam irradiated TiNi alloy. The alloyspecimens were exposed to 18 MeV proton beam and the irradiationresulted in a decrease in R-phase transformation start temperatureand the reverse martensitic transformation finish temperature withincrease in irradiation dose. Investigating the effects of protonirradiation energy on the martensitic transformation temperature ofnitinol, Dughaish [20] reported a decrease in transformationtemperature with increase in irradiation energy.

The foregoing analysis of previous work reveals that irradia-tion induced C–A transitions and irradiation effects on thetransformation temperatures in nitinol has been the main focusof research. The aim of the present study is to investigatetransformation characteristics of nitinol (NiTi) alloy through itsmechanical response after irradiation for different exposure timeswith 2 MeV proton beam.

2. Experimental work

NiTi alloy in the form of wire with 50.5 at% Ni and 49.5 at% Tiwas obtained from Alfa Aeisar (USA). The specimens, each oflength 60 mm and diameter 1 mm, were cut from the as-receivedwire and then mechanically polished using diamond paste ofgrades 6, 3 and 0.1 mm to remove any distortion from the surface.

00

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1000

1200

1400

54 3

2

1

ED

CB/

B

ED

CB

A

Stre

ss (M

Pa)

Strain (%)

1. Unirradiated Nitinol2. 15 min Irradiated Nitinol3. 30 min Irradiated Nitinol4. 45 min Irradiated Nitinol5. 60 min Irradiated Nitinol

2 4 6 8 10 12 14 16 18 20 22

�RS

�MS

Fig. 2. Comparison between stress–strain curves of unirradiated and 15–60 min

proton beam irradiated nitinol.

N. Afzal et al. / Physica B 406 (2011) 8–11 9

The polished specimens were annealed at 703 K for 1 h in a hightemperature vacuum furnace. The annealed samples were thensubjected to 2 MeV proton beam irradiation for 15, 30, 45 and60 min under vacuum at 298 K to a fluence of 1015/cm2. Theirradiations were carried out using Pelletron accelerator installedat accelerator lab of Government College University, Lahore. Thetemperature of samples during irradiation was kept constant bycontinuous circulation of liquid nitrogen in the irradiationchamber. A digital temperature controller attached to thechamber indicated variations of a maximum of 71 K during theentire duration of irradiation. The XRD analysis of unirradiatedand irradiated specimens was carried out using Phillips Panaly-tical XRD system. Both unirradiated and irradiated specimenswere deformed at room temperature using Universal TestingMachine with a cross-head speed of 0.5 mm/min. The deforma-tion of specimens was recorded in the form of stress–straincurves, which were used for further analysis. Micro-hardness ofspecimens was also measured using Vickers hardness tester at amaximum load of 200 g. The data thus obtained, associated withmicrographs, was utilized to compare the mechanical response ofboth unirradiated and proton beam irradiated specimens.

0

250

300

350

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450

500

550

600

650

Har

dnes

s (H

V)

Time (minutes)

10 20 30 40 50 60

Fig. 3. Variations of micro-hardness of nitinol with irradiation exposure time.

3. Results and discussion

The stress–strain curve of unirradiated nitinol deformed atroom temperature is shown in Fig. 1. Nitinol is in austenitic phasebefore deformation (point A). The austenite phase continues todeform with elastic modulus EA, as the elastic deformationproceeds. At point B, a transformation of phase from austeniticto martensitic begins to take place and the corresponding stress iscalled martensite transformation start stress (sMS). Asdeformation continues, a stress plateau BC appears in thestress–strain curve indicating that a large strain in the materialhas been produced on the application of small stress beyond sMS.

The deformation in region BC is mainly carried out by transfor-mation from austenitic to martensitic crystal structure throughdetwinning. As stress is increased further beyond point C, thecurve again shows a linear stress–strain relationship indicatingthat the martensite is deformed elastically with elastic modulusEM. Beyond point D, the material deforms plastically, leading to itsfailure. Fig. 2 shows the comparison of stress–strain curves ofunirradiated and 15–60 min proton beam irradiated nitinol. It canbe seen that as a result of irradiation, an additional intermediateregion BB/ of rhombohedral (R) phase has appeared betweenaustenitic (A) and martensitic (M) phase. The comparison

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E

EA

EM

D

CB

A

Stre

ss (M

Pa)

Strain (%)2 4 6 8 10 12 14 16 18 20 22

�MS

Fig. 1. Stress–strain behavior of unirradiated nitinol. Fig. 4. Microstructure of unirradiated nitinol (magnification 1000� ).

Fig. 5. Microstructure of 30 min proton beam irradiated nitinol (magnification

1000� ).

Fig. 6. Microstructure of 60 min proton beam irradiated nitinol (magnification

1000� ).

20

0

500

1000

∗

∗∗

∗

∗ ∗(110) ∗

∗

(002) ∗

Inte

nsity

2 T

(422)

30 40 50

Fig. 7. XRD spectra of unirradiated and 15–

N. Afzal et al. / Physica B 406 (2011) 8–1110

between unirradiated and 15 min proton beam irradiated nitinolclearly shows that irradiation has resulted in an increase in thetransformation stress sMS. However the R-phase start stress sRS

and M-phase start stress sMS decrease with increase in exposuretime. The fracture stress for irradiated specimens is much higherthan that of the unirradiated one. The results of micro-hardnesstests of samples reveal that hardness is higher in irradiatedspecimens than that of unirradiated one. The increase in hardnessis quite sharp after 15 min of irradiation, which then increaseslinearly with further increase in exposure time up to 60 min(Fig. 3). The comparison between the microstructures ofunirradiated specimen (Fig. 4) with that of 30 min proton beamirradiated specimen (Fig. 5) shows that irradiation produces wearand tear on the surface and it also causes the formation ofprecipitates, which are non-uniformly distributed over thesurface. The defects/precipitates seem to agglomerate withincrease in irradiation time up to 60 min (Fig. 6).

XRD spectra of unirradiated and irradiated specimens areshown in Fig. 7. The spectrum of unirradiated nitinol shows aprominent peak of cubic NiTi2 at 2y value of 381 corresponding to(4 2 2) plane. The spectrum also indicate a NiTi peak at 431corresponding to (0 0 2) plane and two monoclinic peaks at 551,and 691 corresponding to (1 2 0) and (2 1 0) planes respectively.The monoclinic peaks however disappear and a new peak at 421corresponding to (1 1 0) plane of cubic NiTi appears when thespecimens are irradiated with 2 MeV proton beam for 15–60 min.Similarly two more peaks of Ni3Ti also appear at 2y value of 641and 821 corresponding to planes (3 0 1) and (2 1 5), respectively.The generation of Ni3Ti phases and the re-crystallization ofmonoclinic NiTi into cubic NiTi are clear indications of structuralevolution induced by irradiation. The presence of Ni3Ti peaksshows the formation of precipitates responsible for the internalstresses within the material.

It has been established in the literature that proton irradiationof nitinol produces small displacement cascades and thusgenerates vacancies, interstitials and/or precipitates [19]. Thelattice disorder and stress fields associated with the defects/precipitates produced during irradiation are responsible for thegeneration of R-phase that nucleates around the local stress fieldsand grows through the matrix [19,21]. With increase inirradiation exposure time, the density of defects also increases,resulting in an increase in local stress concentration, whichthen decreases the sRS and sMS. Radiation induced defects in the

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o

o o

o(215)

210 ∗(120) ∗

60 min

45 min

30 min

15 min

Untreated

heta

(301)

NiTi2∗ NiTio

60 70 80 90

Ni3Ti

60 min proton beam irradiated nitinol.

N. Afzal et al. / Physica B 406 (2011) 8–11 11

material enhance the shear movement of atoms at the beginningof martensitic transformation and cause an increase in anti-phaseboundaries [20,22], thus resulting in a decrease in sMS. Once themartensitic phase begins to form, the deformation in the materialis carried out by detwinning under the action of internal stresses.As deformation increases beyond point C, the detwinning processcontinues. The planes that are not favorably oriented with respectto applied stress require higher stresses for their re-orientation.As the stress reaches a critical level, the detwinning process iscompleted and upon any further increase in the stress levels,plastic deformation takes place through the generation ofdislocations, which ultimately leads to the fracture of thespecimens. The observed higher values of fracture stress inirradiated specimens as compared to unirradiated ones may beattributed to the interaction of mobile dislocations with radiationinduced defects.

4. Conclusions

The following conclusions can be drawn from the foregoinganalysis:

1)

Irradiation of nitinol suppresses the direct austenite tomartensite (A–M) phase transformation by introducing anintermediate R-phase between A and M phases.

2)

The transformation stresses sRS and sMS decrease withincrease in exposure time.

3)

The micro-hardness of nitinol increases as the exposure timeincreases.

4)

The generation of R-phase and decrease of sRS and sMS can beattributed to the lattice disorder and stress fields associatedwith the defects produced during irradiation.

Acknowledgement

The authors would like to thank the accelerator group of CASPfor their help during irradiation of the samples.

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