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In situ TEM observation of two-step martensitic transformationin aged NiTi shape memory alloy

L. Tan a, W.C. Crone b,*

a Materials Science Program, University of Wisconsin Madison, 1509 University Avenue, Madison, WI 53706, USAb Department of Engineering Physics, University of Wisconsin Madison, 1500 Engineering Drive, Madison, WI 53706-1687, USA

Received 10 August 2003; received in revised form 10 December 2003; accepted 15 December 2003

Abstract

Martensitic transformation was investigated in an aged NiTi alloy with DSC and a temperature controllable TEM specimen

stage to observe the influence of Ti11Ni14 precipitates and R-phase on martensitic transformation in situ. The R-phase, conventional

martensitic twins, and a new morphology of interwoven austenite/martensitic structure were observed.

2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Martensitic transformation; Ageing; TEM; DSC

1. Introduction

Shape memory effect, first discovered in binary alloys

of CuZn alloys and CuSn alloys in 1938 [1], was notwidely applied until the discovery in NiTi alloys in

1963 [2]. In addition to the shape memory effect giving

the material the ability to return to a predetermined

shape when heated, NiTi shape memory alloys (SMAs)

are capable of displaying pseudoelasticity giving the

material the ability to transform between phases upon

loading and unloading and recover to its original zero

strain shape after significant deformation. With the

properties such as repeatability, wear resistance, corro-

sion resistance and biocompatibility, NiTi is the most

commercially successful shape memory alloy [35].

The shape memory effect of NiTi is related to a

martensitic transformation between the high-tempera-

ture austenite phase and the low-temperature martensite

phase. Unlike fully annealed near-equiatomic NiTi al-

loys which transform from austenite to martensite di-

rectly, post-aged near-equiatomic NiTi alloys normally

transform in two steps showing two peaks on the dif-

ferential scanning calorimetry (DSC) curve in the cool-

ing direction. The first DSC peak correlates with the

transformation from the austenite in a cubic structure

(B2) to the R-phase in a rhombohedral structure; thesecond DSC peak correlates with the transformation

from the R-phase to the martensite in a monoclinic

structure (B190) [6]. Dlouhy et al. [7] proposed a new

explanation based on their in-situ TEM observation that

the second DSC peak in the cooling direction may in-

clude two transformations: R-phase to B190 in the grain-

boundary regions with precipitates, and B2 directly to

B190 in the precipitate-free grain interiors. Multi-step

(three or more steps) transformation could occur in

appropriately aged near-equiatomic NiTi alloys.

The explanations for the conditions of multi-step

transformation are still debated [710], but it is clear

that Ti11Ni14 precipitates enhance the R-phase for-

mation which is favorable for the shape memory effect

[11].

It is known that Ti11Ni14 precipitate, which helps

martensitic transformation, appears in the aged NiTi

shape memory alloys with nickel compositions greater

than 50.5 at.% [12]. The the R-phase nucleates at

Ti11Ni14 precipitates because they have the same rhom-

bohedral structure with a 0:738 nm and c 0:532 nm[13] for the R-phase and a 1:124 nm and c 0:5077nm [12] for Ti11Ni14. Their same crystal structure and

close lattice parameters makes formation of R-phase at

* Corresponding author. Tel.: +1-608-262-8384; fax: +1-608-263-

7451.

E-mail address: [email protected] (W.C. Crone).

1359-6462/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.scriptamat.2003.12.019

Scripta Materialia 50 (2004) 819823

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the boundary of Ti11Ni14 easier than the formation of

the monoclinic martensite phase. In order to ensure the

presence of the R-phase, an ageing temperature below

600 C was chosen in this study. In addition to the

differential scanning calorimetry (DSC) used to analyze

the behavior of phase transformation, transmission

electron microscopy (TEM) was employed to charac-terize the microstructural evolution during martensitic

transformation in situ. The phase relationships among

Ti11Ni14 precipitate, R-phase and martensite, and a new

martensite/austenite morphology are reported below.

2. Sample preparation and characterization

The material used in this study was a commercial Ti

50.7at.%Ni alloy procured from Nitinol Devices &

Components (NDC), Inc., in the form of as-rolled

polycrystalline sheet (0.81 mm in thickness) and an-

nealed condition. The sheet was cut into 5 mm by 10 mm

flat samples with electro-discharge machining (EDM).

An ageing treatment was performed on the samples at

550 C for 20 min followed by quenching in water at

room temperature. To eliminate the effect of the thick

oxide layer created during thermal treatment, the sam-

ples were chemically etched with an 1HF + 4HNO3

5H2O solution to remove the surface oxides, and

mechanically polished using SiC paper in successive

grades from 240 to 1200 grit prior to DSC analysis.

The phase transformation characteristics of the aged

samples were determined with a Perkin-Elmer DSC 7

instrument under sub-ambient conditions and analyzedaccording to the ASTM standardsF2004 [14] and

F2005 [15]. During the DSC test, a sample weight of

approximately 10 mg was analyzed with an empty alu-

minum pan as the reference. A temperature range from

60 to )100 C was scanned at a rate of 10 C/min during

cooling and heating.

JEOL 200CX-II TEM was used at 200 kV to char-

acterize the microstructural evolution of the aged sam-

ples. Bright-field images and selected area electron

diffraction patterns were obtained from cross-section

specimens. Sections of 0.5 mm in thickness were cut

from the cross-section of an aged sample and sand-

wiched between two silicon wafers with epoxy. The

specimens were polished until the silicon appeared yel-

low, indicating that the specimen thickness is thinner

than 5 lm [16]. Final thinning was done by argon ion

milling after the specimen had been mounted on a

copper grid with epoxy. Since the aged sample is in

austenitic phase at room temperature, the martensitic

transformation was investigated with a special TEM

specimen stage cooled by liquid nitrogen. The flux of the

liquid nitrogen was controlled to obtain desirable tem-

peratures for detecting the stable phases occurring at

different temperatures.

3. Results and discussion

The DSC results of the sample aged at 550 C for 20

min is shown in Fig. 1. Two distinct peaks in the cooling

direction and two superimposed peaks in the heating

direction are observed, indicating a two-step phase

transformation. The two peaks in the cooling directioncorresponding to the R-phase and the martensite phase

(M) were determined to have transformation tempera-

tures Rs )9 C, Rf)36 C, Ms )30 C, Mf)76

C. The subscript s denotes the temperature at which

the phase transformation starts, and the f denotes the

temperature at which the phase transformation finishes.

The peak in the heating direction corresponds to the

superposition of the R0-phase [15] and the austenite (A)

with R0s )23 C, R0

f)1 C, As )16 C and Af)3

C determined by deconvoluting the curve. Thus the

aged samples with Af)3 C is in the austenite phase at

room temperature. The area under each power peak or

valley separating the solid-state phases, austenite (A),

martensite (M), rhombohedral-phase (R and R0), rep-

resents the heat of transformation. As can be seen from

Fig. 1, the heat of transformation for A fi M is

approximately equal to that of M fi A.

Using TEM at room temperature, the material was

shown to be in the austenite phase with selected area

diffraction (SAD) as is shown in the inset of Fig. 2. The

two-step transformation (austenite to R-phase then

martensite) observed by DSC was verified by TEM upon

cooling with liquid nitrogen. At low flux of liquid

nitrogen in the TEM specimen stage, R-phase formed

first generating a stress field which reduced the phasetransformation barrier of martensite. As shown in Fig.

2, the parallel structure of the R-phase grows from the

nucleus sites at the surface boundaries of the Ti11Ni14precipitates, which is in accordance with the findings of

Kainuma et al. [17]. (No preferential distribution of the

-80 -60 -40 -20 0 20 40-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

HeatFlow(W/g)

Temperature (C)

Heating

Cooling

R'+A

R

M

HMA = -18.6 J/g

HAM= 18.4 J/g

Fig. 1. DSC curves of Ti50.7at.%Ni alloy aged at 550 C for 20 min

indicating a two-step phase transformation occurred during cooling

and heating the NiTi alloy.

820 L. Tan, W.C. Crone / Scripta Materialia 50 (2004) 819823


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Ti11Ni14 precipitates was observed in this material in our

previous work [18].) The Ti11Ni14 phase can introduce

internal tensile stress originating from the different

density between the Ti11Ni14 and matrix (B2)

(qTi11Ni14=qmatrix 1:06) and the misfit strain [19]. Thestress state created by the presence of these precipitates

decreases the barrier to R-phase transformation and the

martensitic transformation that follows. The restrictionof the Ti11Ni14 precipitates on the growth of the R-phase

is also indicated in Fig. 2, which confirms the suggestion

that the high density of Ti11Ni14 precipitates in the

matrix of the aged Ni-rich NiTi SMAs are related to the

smaller R-phase domain [20]. A martensite phase do-

main forming in a region of R-phase is shown in the

upper left corner of Fig. 2. Martensite does not appear

around the Ti11Ni14 precipitates due to the larger

nucleation barrier to martensite compared to that with

R-phase [10]. The parallel structure of the R-phase is

magnified in Fig. 3, and its corresponding diffraction

pattern with [11 0]B2 zone is shown in the inset. The

fringe contrast of the R-phase is also called the APB

(antiphase boundary)-like contrast. This contrast was

proposed to be due to a two dimensional crystal defect

(or displacement boundary) that arises when a diffu-

sionless transformation originates in different regions of

the crystal on different sublattices [21]. The diffraction

pattern of R-phase is consistent with the work of Wu

and Wayman [20] who provided the detailed diffraction

information correlating with reciprocal lattice of R-

phase.

At high flux of liquid nitrogen in the TEM specimen

stage, an interwoven austenite/martensite structure

originates at the boundary of the R-phase and parent

phase and grows into both the parent phase and the R-

phase regions. The R-phase fraction diminished with the

growth of the martensite, which may support the argu-

ment of Dlouhy et al. [7] that the R-phase transforms to

B190 in addition to the direct transformation from B2 to

B190. An interwoven austenite/martensite structure,

shown in Fig. 4, with white cells denoting the austenite

phase and black and gray cells denoting the martensite

variants was stabilized at this lower temperature. The

corresponding diffraction pattern of Fig. 4 with zone

axis [111]B2 is shown in the inset of Fig. 4. The large

bright diffraction spots forming an equilateral triangle

Fig. 2. Bright-field image of the aged Ni-rich NiTi SMA demonstrating the correlation of Ti11Ni14 precipitates, R-phase (R), and interwovenaustenite/martensite (M/A) phases. The inset is the select area diffraction pattern taken at the region labelled M/A prior to cooling, which indicates

the parent phase was austenite with [1 1 1]B2 zone.

Fig. 3. Bright-field image of R-phase in the shape of parallel lines with

30 nm width. The inset is the diffraction pattern of the R-phase in

[1 1 0]B2 zone.

L. Tan, W.C. Crone / Scripta Materialia 50 (2004) 819823 821


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result from the austenite phase; and the streaks beside

the austenite diffraction spots result thin lamellae of the

martensite variants. The dim diffraction spots in a hexa-

gonal pattern surrounding the large austenite diffraction

spots correspond to the R-phase in the up-right corner

of the bright-field image (Fig. 4).

This is the first time that this type of interwoven

austenite/martensite structure has been reported to be

observed in NiTi. In contrast to the situation in bulkshape memory alloys where it is not possible to form an

interface between austenite and a single variant of

martensite, in thin films the situation can be quite dif-

ferent [22]. A similar interwoven austenite/martensite

structure to that observed experimentally was predic-

ted analytically for CuAlZn SMAs by Sreekala and

Ananthakrishna [23] with a two-dimensional model.

According to the in situ TEM observations shown here,

this interwoven structure forms preferentially in defect-

free and precipitate-free regions which are surrounded

by the R-phase in the thin planar TEM sample. The

formation of the interwoven austenite/martensite struc-

ture is thought to be a result of the strains surrounding

the R-phase, which favor the growth of the slabs along

the certain directions [23].

In addition to the interesting interwoven austenite/

martensite structure, conventional martensitic twins

were also observed in the material during and after

cooling with liquid nitrogen. As shown in Fig. 5(a),

poorly defined martensitic twin boundaries start to grow

when cooling at low flux of liquid nitrogen. After

cooling at high flux of liquid nitrogen, straight and well-

defined martensitic twin boundaries are stabilized as

shown in Fig. 5(b).

4. Conclusions

A two-step martensitic transformation occurring in

an aged NiTi shape memory alloy was analyzed with

DSC and TEM. A specimen stage cooled with liquid

nitrogen was employed during TEM analysis to char-

acterize the correlation of Ti11Ni14 precipitates, R-phase,

and martensite. It was revealed that the R-phase

nucleates at Ti11Ni14 precipitates and its growth is con-

strained by these precipitates. In addition to the growth

of the conventional martensitic twins, a new morphol-

ogy of interwoven austenite/martensitic twin structurewas observed in this material, which forms preferentially

at the defect-free and precipitate-free regions.

Acknowledgements

Support for this research was provided by Air Force

Office of Scientific Research (award #F49620-01-0146).

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Fig. 4. Bright-field image of martensitic bands in 50 nm width. White

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(a) and close to the edge of the sample.

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