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* corresponding author: erhard.hornbogen@ruhr-uni-bochum.de

Microstructure and Thermo-Mechanical Properties of NiTi Shape Memory Alloys

E. Hornbogen*

Ruhr-Universität, D-44801 Bochum, Germany

Keywords: NiTi, Shape Memory Alloys, Microstructure, Thermo-Mechanical Properties.

Abstract. Shape memory alloys transform at low temperature by a martensitic reaction � � � ,

which implies formation of a domain structure (similar to ferromagnetic, ferroelectric

transformations). This microstructure allows complex pseudo-plastic deformability (high degree of

freedom). Reversion of � into � restores a definite shape, either by heating to T > Af (austenite

finish), or by reduction of the external transformational stress (pseudo-elasticity). It is discussed

how a defined final shape is achieved at temperatures high above Af, at which diffusional processes

can take place. In addition maximum strength is required for many SMA applications,- especially a

high stability at repeated transformations (fatigue, loss of memory). The origin of the microstructure

is discussed, which leads to the desired combination of properties. Special attention is paid to

thermo-mechanical treatments (TMT), by which a wide range of microstructures can be obtained. A

systematic approach is provided for the types of reverse reactions � � �, which are found between,

crystalographically reversible (martensitic) and diffusion controlled (combined reactions). Features

of optimum microstructures for shaping and desired SM-properties are discussed.

Temperature Ranges

There is a wide temperature range in which SMA behave like normal intermetallic compounds with

CsCl-structure: Md < �T1 < Tm (Fig.1) [1]. At and below Md martensitic phase transformation can

take place. It is complete below a temperature designated as Mf. Thus we find an intermediate range

Mf < �T2 < Md, in which the alloy is transforming, and the range 0K < �T3 < Mf in which

transformation is complete. The two upper temperature ranges have to be subdivided further.

Diffusion controlled processes will become considerable at TD > 1/3 Tm .This amounts to ~ 300°C

for NiTi, less than 200°C for CuZn. The martensite transformation temperatures (Ms, Mf, As, Af,

Md) should be situated safely below this temperature, if ageing effects are to be avoided. Above TD

extends the range in which, for example, precipitation or creep is occurring.

The intermediate (transforming) temperature range �T2 is to be subdivided into three parts with

qualitatively different behaviour [2]:

Af < T2A < Md plastic strain in � is preceding the transformation

Ms < T2B < Af � transforms stress-induced into �

Mf < T2C < Ms � is partially transformed, external stress induces reorientation

and additional transformation of residual �

The upper limit for range �T2B is defined by

�*�� = �y� (1)

the transformational stress, is determined by a modified Clausius Clapeiron equation (Fig.1), is

valid at T > Ms:

dT

d ��� =

0T

H ���� (2)

Materials Science Forum Vols. 455-456 (2004) pp 335-341Online available since 2004/May/15 at www.scientific.net© (2004) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.455-456.335

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���� is the upper limit of stresses applicable for pseudo-elasticity PE and pseudo-plasticity PP

(Fig.1).

Fig. 1. Temperature scales for shape memory alloys.

Fig. 2. Thermo-mechanical treatments:

(a) ausforming and (b) marforming and ageing.

Thermo-Mechanical Treatments

Thermo mechanical treatments are of interest in three different aspects: 1. Shaping of semi-finished

products (rolling, wire drawing); 2. Adjustment of bulk properties to desired values; 3. Fundamental

study of the effect of microstructure and lattice defects on the mode of the martensitic

transformation [3,4,5]. We distinguish hot-forming in range 1 (ausforming) and cold forming

(marforming) in range 3 (Fig. 2) [3,4,5].

Ausforming introduces defects into austenite, which are transferred to martensite during

subsequent cooling to �T3 < Mf (Fig. 3). Consequently workhardening in austensite �y� + ��� is

transferred into martensite:

�y� + ��� = �*y� � �y� + ��� = �

*y� (3)

If the chemical composition of the alloy stays constant, workhardening due to defects in

austenite will reduce the martensite temperature (Fig. 3c) [6].

MS = MS (0) � �MS (p) = MS(0) � C�*

y� (4)

Marforming is limited by the inherent brittleness of intermetallic compounds [1,7]. The true plastic

deformability does not amount to more than p < 50%. Deformations of p < 20% render the alloy

(a)

(b)

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untransformable. This is due to disorder and introduction of lattice defects into the structure of

martensite. Consequently in this state the alloy does not show shape memory.

(a)

(b) (c)

Fig. 3. (a) Microstructure of ausformed binary NiTi (50.7 at % Ni- 196°C cooled below Mf).

(b) Comparison of stress strain curves in pseudoelastic state. (c) Effect of ausforming on strength and martensite peak temperature Mm. (50.0 at %Ni, MS I 6°C).

It has to be tempered to reform the �-phase and regain transformability (Fig. 4). This will take

place at T3B > 300°C (see chapter �Ways of � � � reversion�) and leads to a wealth of useful

microstructures. Prerequisite for all TM-treatments is the preservation or restoration of the �-phase

in a transformable condition (Fig. 5), so that shape memory is not reduced [7,8].

The embossing process

The final shape, which the alloy always remembers, is connected with the �-phase (austenite)

[9,10]. It has to be shaped without loss of reversible transformability.

Materials Science Forum Vols. 455-456 337

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Fig. 4. Microstructure of marformed and aged alloy (50.7% at. Ni) . SEM micrographs of marforming and

tempering.

(a) (b)

Fig. 5. (a) Structural changes during the embossment process (stage 3 � heated to diffusion temperatures � 400°C). (b) The four stages in the stress-, strain-, temperature diagram.

The simplest way is by starting with the �-structure, clamping it in a die with the desired

dimensions, and heating it above Af, into the temperature range �T1. The total strain 0 = const., is

transformed into plastic strain p by relaxation of the stresses which were created by clamping (Fig.

6).

starting condition: 0 = e + pe (5a)

after ��� transformation: 0 = e + p (5b)

final, desired shape: 0 = p (5c)

Ni Ti martensite : T < Mf Ni Ti marforming : = 41, 4 %

Ni Ti = 41, 4 % 30'- 200°C / H2O marformed plus tempered

Ni Ti = 41, 4 % 30'- 800°C / H2O rebetatized

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(a) (b)

Fig. 6. (a) Clamping device and specimens in initial and final shape. (b) Typical shapes of the originally

straight test wires (final shape: 180º).

Prerequisite is that the material will sustain the stresses which are developed during heating to

T1 > Af (Eq. 5b). Temperature and time is determined by the creep rate of the �-phase. The stress,

�0 = E� (e + pe) (6)

which had been created during heating of the clamped material (Fig. 7 and 8) will relax until 0 = p

(Eq. 5c) is reached. Due to the creep resistance of the intermetallic NiTi this process is rather slow

and incomplete. 0 is the strain, which leads to the desired final shape.

The initial stress after transformation into the austenite � is �0 (Eq.6).

It will relax by changing elastic strains e into plastic strains until p = 0 at � = 0:

� = �0 exp (- �

t ) (7a)

The materials property is the relaxation time �, the temperature dependence of which is

controlled by an activation energy Q. Its value is controlled by climb of dislocations in the as-

betatized state Qcd. Evidently it is much smaller if stress relaxation takes place in a deformed

microstructure which shows a combined recrystallisation and transformation reaction, Qcr < Qcd

(Eq.7B) during tempering.

� = �0 exp (RT

Q ) (7b)

Therefore a favourable alternative is to include a TMT into the procedure. For this purpose the alloy

is marformed (Fig. 2) for example by rolling or wire drawing to p = 40%. Consequently it is

workhardened, but rendered absolutely untransformable. Heating of the clamped specimen leads to

a result, which is surprising at the first sight. The alloy assumes the desired shape more precisely at

a much shorter time (respectively lower temperature) as compared to the as-betatized state (Fig. 4, 7

and 8).

Materials Science Forum Vols. 455-456 339

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Fig. 7. Course of strength during the constrained

tempering procedure (50.7 at % Ni).

Fig. 8. Comparison of the marformed and as-

betatized specimens, schematic: The cold worked

state relaxes stress and aquired shape much quicker.

Ways of ���� ���� ���� reversion. Evidently certain reactions which restore the high temperature phase �

are decisive for the rapidity of the relaxation process. For the as-betatized state this reversion is

completed at Af � 60°C. For the marformed starting condition a transformation temperature in the

range of 350 - 400°C was found by DSC-analysis (Fig. 4, 7 and 9). Between these two extremes the

following reversion reactions are to be expected, depending on temperature, time, heating rate, and

amount of marforming:

a) � � � (faultless)

b) �d � � (faults are annihilated, effect of first cycle)

c) �d � �d (faults are transferred into � � ghosts)

d) �*d � �r � �r + � � � (non �transformable starting condition)

e) �*d � �r + P � ( �r + P ) + � � � + P � � (alloy prone to precipitation)

�d, �d �, � which contain defects (dislocations)

�d* � is rendered untransformable by high defect concentration (amount of marforming)

�r recovered �

P precipitate particles in � or � (Ni4Ti3)

�r + � microduplex structure (superplastic)

In Fig. 7 evidently the consequences of reaction A and E are shown. The course following

reaction E starts with the initial workhardening. The reverse transformation is not only preceded by

recovery of untransformable �*d, but also by precipitation (P�Ni4Ti3) which causes considerable

hardening. The maximum strength is connected with with the onset of reformation of � by

nucleation and growth. This is a combined phase transformation plus recrystallisation reaction. It

induces in a narrow temperature range an ultrafine ( � + � ) � microstructure which, evidently, leads

to a high creep rate (superplasticity) (Fig. 4 and 9a). At still higher tempering temperatures a

homogeneous �-structure forms, which is connected with reappearance of transformability and

shape memory.

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(a) (b)

Fig. 9. (a) Analysis of the tempering process, Eq. 8. Microstructures, schematic, (see Fig. 4).

(b) DSC-analysis of transformation: diffusion controlled reverse reaction � � � : Af > 400°C.

Depending on the alloy composition the �-grain structure may contain a dispersoid of particles,

which controls grain growth. If particles dissolve at still higher temperature, rapid grain growth and

grain boundary embrittlement was observed.

Favourable Combinations of Properties

The good hot forming ability of the ( � + � ) microduplex structure is not coincident with the

structure which provides optimum shape memory properties, because only part of the volume ( � )

is transformable Fig. 9a). Thus, after embossment the alloy has to be heated until the �-phase has

disappeared. But to preserve an ultrafine grain size it is feasible not to overheat until the dispersoid

of precipitate particles have dissolved. Rather a diameter of the particles in the nano-meter range

should be aspired because not only shape memory, but also high strength is useful. It has to be taken

into account that the particles affect the martensitic transformation in a similar way as it was

discussed for ausforming in equ.4. In this case however the martensite temperature may go down or

up. This is due to the fact that precipitation in � changes the chemical composition and

consequently the metastable equilibrium temperature T0 [6]. This effect may be used if, for

example, for medical applications, a defined transformation temperature is required. The reverse

reactions, starting from highly deformed and therefore defect martensite, have hardly been explored

in a systematic manner [10]. They provide a key for optimisation of SMA for a wide range of

applications.

Acknowledgement

This work was supported by the German Science foundation: DFG SFB 459. Thanks are due to Mr.

Karl Rittner for his help with developing the test method for acquiring the final shape and for

conducting all thermo-mechanical treatments.

References

[1] H. Warlimont: Order- disorder transformations in alloys (Springer, Berlin 1974).

[2] T. Duerig, et.al.: Engineering aspects of shape memory alloys (Butterworth, London 1990).

[3] M. Franz, E. Hornbogen,: Z. Metallkde Vol. 86 (1995), p. 31.

[4] E. Hornbogen, G. Brückner, G. Gottstein: Z. Metallkde Vol. 93 (2002), p. 3.

[5] E. Hornbogen: Mat. Sci. Engg. A273- 275 (1999), p. 630.

[6] E. Hornbogen: Z. Metallkde Vol. 75 (1984), p. 741.

[7] E. Hornbogen, E. Kobus: Z. Metallkde Vol. 87 (1996), p. 442.

[8] D. Wurzel: Mat. Sci. Engg. A 273- 275 (1999), p. 634.

[9] E. Hornbogen, K. Rittner: Prakt. Met. Vol. 36 (1999), p. 539.

[10] E. Hornbogen: Met. Trans. Vol. 10A (1979), p. 947.

Materials Science Forum Vols. 455-456 341

Advanced Materials Forum II 10.4028/www.scientific.net/MSF.455-456 Microstructure and Thermo-Mechanical Properties of NiTi Shape Memory Alloys 10.4028/www.scientific.net/MSF.455-456.335

DOI References

[2] T. Duerig, et.al.: Engineering aspects of shape memory alloys (Butterworth, London 1990).

doi:10.4028/www.scientific.net/MSF.56-58.679 [4] E. Hornbogen, G. Brückner, G. Gottstein: Z. Metallkde Vol. 93 (2002), p. 3.

doi:10.1111/j.1574-6968.2002.tb11123.x [5] E. Hornbogen: Mat. Sci. Engg. A273- 275 (1999), p. 630.

doi:10.1016/S0921-5093(99)00337-8 [6] E. Hornbogen: Z. Metallkde Vol. 75 (1984), p. 741.

doi:10.1016/0001-6160(84)90135-4 [8] D. Wurzel: Mat. Sci. Engg. A 273- 275 (1999), p. 634.

doi:10.1016/S0921-5093(99)00338-X [9] E. Hornbogen, K. Rittner: Prakt. Met. Vol. 36 (1999), p. 539.

doi:10.1023/A:1004563216567 [10] E. Hornbogen: Met. Trans. Vol. 10A (1979), p. 947.

doi:10.1007/BF02811643