Influence of Annealing on NiTi Shape Memory Alloy Subjected to Severe Plastic

8/10/2019 Influence of Annealing on NiTi Shape Memory Alloy Subjected to Severe Plastic

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Inuence of annealing on NiTi shape memory alloy subjected to severe plasticdeformation

Shuyong Jiang a,*, Yanqiu Zhang a, Lihong Zhao a, Yufeng Zheng b

a Industrial Training Centre, Harbin Engineering University, Harbin 150001, Chinab Center for Biomedical Materials and Engineering, Harbin Engineering University, Harbin 150001, China

a r t i c l e i n f o

Article history:

Received 13 February 2012

Received in revised form

30 April 2012

Accepted 19 July 2012

Available online 10 October 2012

Keywords:

A. Nanostructured intermetallics

B. Martensitic transformations

B. Shape-memory effects

C. Heat treatment

C. Plastic forming, cold

a b s t r a c t

The amorphous phase with the retained nanocrystalline phase, the deformation bands and the amor-

phous bands coexist due to inhomogeneous plastic deformation in nickeletitanium shape memory alloy

(NiTi SMA) subjected to severe plastic deformation (SPD) based on local canning compression. Trans-

mission electron microscopy, X-ray diffraction and differential scanning calorimetry are used to inves-

tigate microstructural evolution and phase transformation of NiTi sample subjected to SPD in the case of

annealing for 2 h at 300 C, 450 C and 600 C, respectively. Annealing at 300 C and 450 C leads to

nanocrystallization of amorphous NiTi sample, while annealing at 600 C results in the coarse-grained

NiTi sample, where the (001) martensite compound twins are found. The precipitation phases such as

Ni4Ti3 and Ni3Ti are suppressed in NiTi sample formed as a result of crystallization of the amorphous

structure and thus occur more easily in the deformation bands in the case of annealing. Martensitic

phase transformation is inuenced by the grain size and is suppressed with the decrease in the nano-

crystalline grain size.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Nickeletitanium shape memory alloy (NiTi SMA) is widely

used in engineering elds due to its shape-memory effect as well

as superelasticity [1]. Cold working plays an important role in

engineering application of NiTi SMA and has a signicant inu-

ence on shape-memory effect as well as superelasticity of NiTi

SMA. In general, cold plastic deformation leads to a high density

of dislocations in NiTi SMA, which makes a contribution to

superelasticity of NiTi SMA [2e4]. However, severe plastic

deformation (SPD) based on cold working is able to lead to

nanocrystallization or amorphization of NiTi SMA. So far, plenty

of work with respect to SPD of NiTi SMA in the case of cold

working has been done over the last decade by means of high-pressure torsion (HPT) [5,6], cold rolling [7,8], cold drawing

[9,10] and surface mechanical attrition treatment (SMAT)[11,12].

SPD of NiTi SMA followed by subsequently appropriate heat

treatment contributes to improving the mechanical properties

and the functional properties of NiTi SMA, such as high ultimate

strength and high elongation at the elevated temperatures [13]

and perfect superelasticity [14,15], which is attributed to

nanocrystallization of amorphous NiTi SMA prepared by SPD in

the case of annealing. Many researchers investigated crystalliza-

tion mechanism of amorphous NiTi SMA subjected to SPD and

phase transformation behaviour of NiTi SMA after crystallization.

Waitz et al. [16,17] induced nanocrystallization of an amorphous

NiTi SMA based on HPT in the case of annealing and found that

martensitic phase transformation is suppressed with decreasing

the nanocrystalline grain size and phase transformation behav-

iour is closely related to the nanocrystalline grain size. Peter-

lechner et al. [18] studied relaxation and crystallization kinetics

of amorphous NiTi SMA made by repeated cold rolling and

revealed that nucleation and growth of the crystalline phase is

based on mixed nucleation or nucleation with a decreasing rate

as well as three-dimensional growth. Srivastava et al. [19]investigated the inuence of the different annealing tempera-

tures on crystallization of amorphous bands of cold-rolled

NiTi SMA and demonstrated that annealing for 30 min at or

above 350C leads to crystallization of the amorphous regions

and the grain size increases with the increase in the annealing

temperature.

In the present study, local canning compression was used in

order to lead to SPD of NiTi SMA, and subsequently the inuence of

annealing on microstructural evolution, precipitation and phase

transformation of NiTi samples subjected to SPD was investigated

systematically.* Corresponding author. Tel.: 86 139 36266338; fax: 86 451 82519952.

E-mail address:[email protected](S. Jiang).

Contents lists available at SciVerse ScienceDirect

Intermetallics

j o u r n a l h o m e p a g e : w w w . e l s e v i e r .c o m / l o c a t e / i n t e r m e t

0966-9795/$e see front matter 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.intermet.2012.07.025

Intermetallics 32 (2013) 344e351
mailto:[email protected]://www.sciencedirect.com/science/journal/09669795http://www.elsevier.com/locate/intermethttp://dx.doi.org/10.1016/j.intermet.2012.07.025http://dx.doi.org/10.1016/j.intermet.2012.07.025http://dx.doi.org/10.1016/j.intermet.2012.07.025http://dx.doi.org/10.1016/j.intermet.2012.07.025http://dx.doi.org/10.1016/j.intermet.2012.07.025http://dx.doi.org/10.1016/j.intermet.2012.07.025http://www.elsevier.com/locate/intermethttp://www.sciencedirect.com/science/journal/09669795mailto:[email protected]


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2. Materials and methods

As-received NiTi alloy with a nominal composition of Ni50.9Ti49.1(at.%) was prepared by means of vacuum induction melting

method, and then was rolled at 800 C and nally was drawn to

NiTi bar with the diameter of 12 mm. The transformation temper-

atures of NiTi bar are as follows: Ms 27.2C, Mf41.7

C,

As 17.3C,Af4.1

C. Therefore, microstructure of as-received

NiTi bar possesses a typical B2 austenite structure at room

temperature. NiTi samples with the diameter of 4 mm and the

height of 6 mm were cut from the NiTi bar by means of electro-

discharge machining (EDM) and then were locally inserted into

the low carbon steel cans with the inner diameter of 4 mm, the wall

thickness of 3 mm and the height of 3 mm. The locally canned NiTi

samples were placed between the top anvil and the bottom one of

INSTRON-5500R universal testing machine and then were

compressed at the reduction in height by 75% at the strain rate of

0.05 s1 at room temperature. The compressed NiTi samples were

annealed for 2 h at 300 C, 450C and 600C, respectively, and

subsequently were cooled to room temperature at the atmosphere.

Microstructural evolution of NiTi samples subjected to SPD and

subsequent NiTi samples subjected to annealing was investigated

by means of transmission electron microscope (TEM). Foils for TEMobservation were mechanically ground to 70 mm and then thinned

by twin-jet polishing in an electrolyte consisting of 6% HClO4, 34%

C4H10O and 60% CH3OH by volume fraction. TEM observations were

conducted on a FEI TECNAI G2 F30 microscope with a side-entry

and double-tilt specimen stage with angular range of40 at an

accelerating voltage of 300 kV.

In order to further understand the phase structures of NiTi

samples subjected to annealing, X-ray diffraction (XRD) was carried

out using a Philips XPert Pro diffractometer with CuKaradiation at

ambient temperature. The samples were scanned over 2q ranging

from 10 to 100 by continuous scanning with tube voltage of 40 kV

and tube current of 40 mA.

Differential scanning calorimeter (DSC) was used in order to

obtain the transformation temperatures of NiTi samples subjected

to annealing in the temperature range from 100C to 100C at

the heating and cooling rates of 10 C/min.

3. Results

3.1. NiTi sample subjected to SPD under local canning compression

Fig.1 demonstrates the schematic diagram of deformation mode

of NiTi sample under local canning compression and the corre-

sponding TEM photographs taken from the different deformation

zones of NiTi sample subjected to SPD. Fig. 1(a) shows the sche-

matic diagram of deformation mode of NiTi sample subjected to

SPD based on local canning compression. It can be seen from

Fig. 1(a) that NiTi sample is caused to be in a three-dimensional

compressive stress state during plastic deformation. In order to

better understand deformation mechanism of NiTi sample under

local canning compression, the continuous deformation process

can be divided into two stages, namely intermediate deformation

degree by 50% and nal deformation degree by 75%. When NiTi

sample is subjected to reduction in height by 50%, it is completely

compressed into the steel can, which undergoes radial and

Fig. 1. Schematic diagram of deformation mode of NiTi sample under local canning compression and the corresponding TEM photographs taken from the different deformation

zones. (a) Schematic diagram of deformation mode showing NiTi s ample subjected to SPD; (b) dark-eld image showing nanocrystalline phase embedded in the amorphous matrix;

(c) bright-

eld image showing the presence of amorphous bands; and (d) bright-

eld image showing the presence of deformation bands.

S. Jiang et al. / Intermetallics 32 (2013) 344e351 345


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tangential elongation strain without axial compression strain. Both

NiTi sample and the steel can are subjected to radial and tangential

elongation strain as well as axial compression strain in the case of

reduction in height by 75%. Furthermore, the plastic deformation

zones of NiTi sample under local canning compression consist of

the principal deformation zone, the intermediate deformation zone

and the minimum deformation zone. It can be found that inho-

mogeneous plastic deformation leads to the different microstruc-

tural morphologies in NiTi sample subjected to SPD. TEM bright-

eld image of NiTi sample in the principal deformation zone is

indicative of the presence of an almost complete amorphous phase,

in which the retained nanocrystalline phase is embedded in the

amorphous matrix, as shown inFig. 1(b). However, the amorphous

bands and the deformation bands occur in the plastic deformation

zones other than the principal deformation zone, as shown in

Fig. 1(c) and (d), respectively.

3.2. Nanocrystallization of amorphous NiTi sample subjected to

annealing

Figs. 2 and 3 demonstrate TEM photographs of NiTi samples

subjected to SPD after annealing for 2 h at 300C and 450 C,

respectively. NiTi samples subjected to SPD before annealingbelong to an almost complete amorphous phase, in which a small

amount of nanocrystalline phase is embedded in the amorphous

matrix. It can be seen fromFigs. 2 and 3 that annealing at 300 C

and 450C for 2 h leads to nanocrystallization of amorphous NiTi

samples. Furthermore, the nanocrystalline grain size increases

with the increase in the annealing temperature. The size distri-

bution of the nanocrystalline grains was measured by means of

extra TEM bright and dark images, in which the diameter of the

nanocrystalline grain in the selected area can be obtained by

combining DigitalMicrograph software with equal perimeter

method. The mean grain size in the diameter can be calculated

according to the selected 200 grains. In the case of annealing at

300 C, the mean grain size is about 12 nm in the diameter, but in

the case of annealing at 450C, the mean grain size is about50 nm in the diameter.

3.3. Twinning of amorphous NiTi samples subjected to annealing

Fig. 4 indicates microstructural morphologies of NiTi sample

subjected to SPD after annealing at 600 C for 2 h. It can be found

from Fig. 4 that NiTi sample subjected to SPD is completely

crystallized after annealing at 600C for 2 h and is characterized by

the coarsegrains rather than the nonacrystalline grains. In addition,

there exist plenty of annealing twins in the coarse-grained NiTi

sample. The corresponding selected area electron diffraction

patterns (SAEDPs) reveal that the annealing twins belong to (001)

martensite compound twins.

3.4. Microstructural evolution of amorphous bands after annealing

There exist a lot of amorphousbands in NiTi sample subjectedto

SPD.Fig. 5illustrates microstructural evolution of the amorphous

bands after annealing for 2 h at 300 C and 450C, respectively. It

can be seen fromFig. 5that a lot of nanocrystalline grains arise in

the amorphous bands of NiTi sample subjected to SPD. In terms of

annealing at 300 C, the dislocation defects remain between the

amorphous bands since thermal driving force is insufcient to

eliminate them. However, the amorphous bands in NiTi sample

after annealing at 600C for 2 h is unable to be found.

3.5. Microstructural evolution of deformation bands after annealing

There exist a lot of deformation bands in NiTi sample subjected

to SPD due to inhomogeneous plastic deformation under local

canning compression. The deformation bands are attributed to

a consequence of dislocation slip. Fig. 6 indicates that the micro-

structures of the deformation bands in NiTi sample subjected to

SPD after annealing at 300 C and 450C, respectively. It can be

seen from Fig. 6 that annealing at the twotemperatures is unable to

lead to complete elimination of the deformation bands and a lot of

ne precipitation phases occur in the deformation bands after

annealing. Furthermore, as compared to annealing at 450C, the

deformation bands of NiTi sample in the case of annealing at 300C

are more apparent since thermal driving force is more insufcient.

However, the deformation bands in NiTi sample after annealing at

600 C for 2 h have been completely eliminated by means of

thermal driving force.

3.6. XRD curves of annealed NiTi samples

Fig. 7 indicates XRD curves of NiTi samples subjected to SPD

after annealing at 300 C, 450 and 600 C, respectively. NiTi

sample annealed at 300 C is mainly composed of B2 austenite

phase with a small amount of Ni4Ti3 precipitationphase. In the case

of annealing at 450C, NiTi sample consists of B2 austenite phase,

Fig. 2. TEM photographs of NiTi sample subjected to SPD after annealing at 300

C. (a) Bright-

eld image; and (b) dark-

eld image.

S. Jiang et al. / Intermetallics 32 (2013) 344e351346


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samples for DSC test are located in the principal deformation zone

and thus are obtained by means of crystallization of the amorphousNiTi phase. It can be found fromFig. 8that NiTi sample annealed at

300 C shows no phase transformation, while NiTi samples

annealed at 450C and 600 C exhibit martensitic phase trans-

formation. Furthermore, as compared to as-received NiTi sample,

martensitic phase transformation temperatures are enhanced for

NiTi samples subjected to SPD as well as subsequent annealing.

4. Discussion

Inhomogeneous plastic deformation is unavoidable during SPD of

NiTi alloy under local canning compression at room temperature. An

almost complete amorphous matrix with a small amount of nano-

crystalline phase occurs in the principal deformation zone of NiTi

sample. In the plastic deformation zones other than the principaldeformation zone, there exist a lot of deformation bands in which

there are plenty of crystal defects, such as dislocations, stacking

faults, and twins. At the low annealing temperatures of 300 C and

450C, some crystal defects can be eliminated by means of static

recovery, but the deformation bands are unable to be removed

completely due to insufcient thermal driving force. In the case of

annealing at 600C, the occurrence of static recrysallization leads to

complete elimination of the deformation bands. However, the

amorphous NiTi sample caused by SPD belongs to the metastablephase and possesses the low thermal stability[16]. For example, the

fully amorphous NiTi sample after being subjected to high-pressure

torsion (HPT) exhibits partial nanocrystallization along with the

extremely small grains of 10e20 nm even at room temperature,

while the complete nanocrystallization with homogeneous nano-

crystalline structure of 20e30 nm is performed in the case of

annealing at 200 C for 2 h[13]. In the present study, the complete

crystallization of the amorphous NiTi sample is obtained by

annealing for 2 h at 300 C, 450C and 600 C, respectively, while

the grain size increases with the increase in the annealing temper-

ature. In particular, the complete nanocrystallization is obtained in

the case of annealing at 300 C and 450C, respectively. Physical

mechanism of crystallization with respect to the amorphous NiTi

sample obtained by SPD can be outlined as nucleation and growth ofthe grains in the amorphous matrix, in which the retained nano-

crystalline debriscanrstly act as the heterogeneous nucleation sites

[16]. The nucleation rate and the growth rate during crystallization

play the critical roles in determining the nal grain size. In general,

the fast nucleation rate and the slow growth rate contribute to

obtaining the ne grains. In the present work, the annealing

temperature is considered as the only variable. Therefore, the grain

Fig. 5. TEM photographs of amorphous bands in NiTi samples subjected to SPD after annealing at different temperatures: (a) 300 C; and (b) 450 C.

Fig. 6. TEM photographs of deformation bands in NiTi samples subjected to SPD after annealing at different temperatures: (a) 300

C; and (b) 450

C.



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size after crystallization depends on the growth rate of the grains,

while the elevated annealing temperature leads to increasing the

growth rate and thus results in the occurrence of the coarser grains.

As a result, in the case of annealing at 600 C, the faster growth rate

leads to rapid growth of the grains in a certain time, so the coarse-

grained NiTi sample occurs.

Furthermore, annealing has an inuence on not only micro-

structural evolution, but also precipitation process of NiTi sample

subjected to SPD. It can be seen from XRD curves in Fig. 7 that

annealingat 300C leads toonly Ni4Ti3 precipitate and annealing at

600 C leads to only Ni3Ti precipitate, while Ni4Ti3precipitate and

Ni3Ti precipitate coexist in the case of annealing at 450C. It can be

Fig. 7. XRD curves of NiTi samples subjected to SPD after annealing at different temperatures: (a) 300 C; (b) 450 C; and (c) 600 C.

Fig. 8. DSC curves of NiTi samples subjected to SPD after annealing at different temperatures: (a) 300

C; (b) 450

C; and (c) 600

C.



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found that SPD results in the occurrence of Ni 3Ti precipitate at the

lower temperature and leads to the decrease of the highest

temperature at which Ni4Ti3 precipitate occurs. In general, there are

three precipitation phases including Ni4Ti3, Ni3Ti2and Ni3Ti in Ni-

rich NiTi SMA after annealing, which depends on annealing

temperature and annealing time. Ni4Ti3 phase occurs at the lower

annealing temperature and the shorter annealing time, and Ni3Ti

phase arises at the higher annealing temperature and the longer

annealing time, while Ni3Ti2 phase appears at intermediate

annealing temperature and time [1]. Therefore, both Ni4Ti3 and

Ni3Ti2 are the metastable phases, while Ni3Ti is the equilibrium

phase. In the present work, no Ni3Ti2 phase is found at the three

annealing temperatures. In addition, TEM observations reveal that

no precipitation phases are found in NiTi specimens derived from

crystallization of amorphous phase. Accordingly, the precipitation

phases occur in the deformation bands of NiTi sample after

annealing. The grain size seems to have a considerable effect on

nucleation and growth of Ni4Ti3 precipitate. For instance, Ni4Ti3precipitate shall be suppressed in the nanocrystalline NiTi sample

obtained by crystallization of the amorphous phase after HPT[20].

Therefore, in the present study, annealing at 300 C and 450C

leads to nanocrystallization of amorphous NiTi sample and conse-

quently the precipitation phases shall be suppressed since thenanocrystalline grains impede the formation of the self-

accommodation. However, no precipitation phases are found in

the coarse-grained NiTi sample formed as a result of crystallization

of the amorphousstructure by means of annealing at 600C, which

shall be further investigated in the future work.

It can be seen from the previous experimental results that the

grain size has a considerable inuence on martensitic phase

transformation of NiTi sample. Annealing at 300 C leads to the

nanocrystalline NiTi sample with the smallest mean grain size of

about 12 nm, where martensitic phase transformation is

completely suppressed. Annealing at 450C results in further

increase of the grain size of the nanocrystalline NiTi sample, where

the mean grain size is about 50 nm, while the largest grain size is

about 90 nm. Therefore, in the nanocrystalline NiTi sample ob-tained in the case of annealing at 450 C, martensitic phase trans-

formation takes place in the largernanocrystalline grains, butis still

suppressed in the smaller nanocrystalline grains. However,

annealing at 600C leads to the coarse-grained NiTi sample of

which the grain size is greater than 200 nm. Martensitic phase

transformation occurs more easily in the coarser grains. Accord-

ingly, it can be concluded that martensitic phase transformation is

suppressed with the decrease in the nanocrystalline grain size.

Similar conclusions were reported in the nanocrystalline NiTi

sample derived from crystallization of the amorphous structure in

the case of HPT and subsequent annealing [16]. Suppression of

martensitic phase transformation in the nanocrystalline NiTi

sample is possible to be attributed to the presence of elastic strains,

lattice defects or crystal renement[7,16]. In the present study, theelastic strains and the lattice defects shall be almost eliminated in

the nanocrystalline NiTi sample resulting from crystallization of the

amorphous phase in the case of annealing, so grain renement as

well as the corresponding constraint by the grain boundaries plays

an important role in suppressing martensitic phase transformation

based on self-accommodation of the martensitic variants. As

compared to the nanocrystalline NiTi sample, martensite is able to

nucleate at the dislocation tangles and near the grain boundaries in

the coarse-grained NiTi sample, which contributes to the occur-

rence of martensitic phase transformation.

Furthermore, the (001) compound twins are observed during

transformation of B2 austenite to B190 martensite in the coarse-

grained NiTi sample formed as a consequence of annealing at

600

C. It is well known that f111gtype I twin, h011i type II twin

and (001) compound twin are frequently found in martensitic

phase transformation of NiTi alloy in the case of heat treatment.

f111g type I twin and h011i type II twin comply with the

phenomenological crystallographic theory and thus belong to

a lattice invariant shear[21e23]. However, (001) compound twin

does not give a solution to the phenomenological crystallographic

theory and cannot be a lattice invariant shear, so it is recognized as

a deformation twin[24e26]. In the present study, the other two

twin modes except the (001) compound twin are not found. Only

(001) compound twins are observed in the nanocrystalline NiTi

sample formed as a result of crystallization of the amorphous

structure by HPT and subsequent annealing[16,17]. It is proposed

that in NiTi sample obtained by crystallization of the amorphous

phase, the (001) compound twin occurs more frequently instead of

f111gtype I twin and h011i type II twin. The lattice distortion along

with the dislocation defects frequently occurs in NiTi sample with

the coarse grains derived from crystallization of the amorphous

phase and thus contributes to the occurrence of the (001)

martensite compound twins. In particular, the formation of the

twins should be more susceptible to taking place at the grain

boundaries.

5. Conclusions

(1) SPDof NiTi samplebased on local canning compressionleads to

the complicated microstructural morphologies, including

deformation bands, amorphous bands and amorphous phase in

which the retained nanocrystalline phase is embedded in the

amorphous matrix.

(2) Annealing at 300 C and 450 C leads to nanocrystallization of

amorphous NiTi sample, but is unable to result in complete

elimination of deformation bands and amorphous bands.

Annealing at 600 C results in complete crystallization of NiTi

sample subjected to SPD, where deformation bands and

amorphous bands are not able to be detected. The (001)

compound twins are found during transformation of B2

austenite to B190 martensite in the coarse-grained NiTi sampleformed as a consequence of annealing at 600 C.

(3) The precipitation phases are suppressed in NiTi sample formed

as a result of crystallization of amorphous structure in the case

of annealing at 300 C, 450 C and 600 C, respectively. The

precipitation phases occur more easily in the deformation

bands of NiTi sample after annealing. SPD results in the

occurrence of Ni3Ti precipitate at the lower temperature and

leads to the decrease of the highest temperature at which

Ni4Ti3precipitate occurs.

(4) The grain size has a considerable inuence on martensitic

phase transformation of NiTi sample. Grain renement plays an

important role in suppressing martensitic phase trans-

formation. Martensitic phase transformation is suppressed

with the decrease in the nanocrystalline grain size.

Acknowledgements

The work was nancially supported by National Natural Science

Foundation of China (No. 51071056) and the Fundamental Research

Funds for the Central Universities of China (No. HEUCFR1132 and

No. HEUCF121712).

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Influence of Annealing on NiTi Shape Memory Alloy Subjected to Severe Plastic