COMPOSITE MATERIALS

UDC 621.785.3:669.017.3:669-419.4

EFFECT OF ANNEALING ON MARTENSITIC TRANSFORMATIONS

IN STEEL TiNi ALLOY EXPLOSION WELDED BIMETALLIC COMPOSITE

S. P. Belyaev,1 V. V. Rubanik,2, 3 N. N. Resnina,1 V. V. Rubanik (Jr.),2, 3 and O. E. Rubanik3

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 30 34, September, 2010.

The effect of explosion welding on the kinetics of martensitic transformations in a steel TiNi alloy bime-

tallic composite and the effect of the temperature and duration of annealing on recovery of the characteristics

of the martensitic transformations are studied. It is shown that annealing in the range of 450 600C accom-

panied by retrogression of structure causes full recovery of the transformation kinetics in the alloy.

Key words: bimetallic composite, explosion welding, martensitic transformations, annealing.

INTRODUCTION

Thermomechanical drives are articles produced from

shape memory alloys (SMA). The operating principle of the

drives is based on the capacity of the alloys to restore consid-

erable inelastic deformations in heating and to develop con-

siderable forces simultaneously. A spring fabricated from

SMA deformed preliminarily in a low-temperature marten-

sitic state is a simple example of a thermomechanical drive.

Upon heating in the temperature range of the inverse marten-

sitic transformation the spring recovers its initial shape and is

capable to perform work and overcome the counteracting

force [1]. Such a thermomechanical drive is a single-action

device, because in order to initiate its operation again the

spring should be deformed in a martensitic state again. Ope-

ration of a reusable thermomechanical drive can be based

either on the effect of reversible shape memory or on the use

of an elastic balance body. In the first case the thermoche-

mical drive will be characterized by low displacements and

forces due to the special features of the reversible shape

memory effect [2]. The use of an elastic member in combina-

tion with an SMA member provides multiple operation of the

drive under repeated heat cycles without the need for re-

peated active deformation after each heating cycle. Such a

drive operates in the following manner. The SMA member is

deformed in a low-temperature condition and connected to

the elastic member. In heating, the SMA member regains its

initial shape, deforms the elastic member, and this gives rise

to stresses in the system. In cooling in the range of the tem-

peratures of forward martensitic transformation these stresses

initiate the effect of transformation plasticity in the SMA

member. As a result, it deforms and accumulates inelastic

strain, and the stress in the system is relaxed. In the subse-

quent heat cycles all the actions described are repeated thus

ensuring multiple action of the thermomechanical drive. In

most operating drives the elastic member and the SMA mem-

ber are different bodies connected to each other. At the same

time, another possible solution is when two members, one of

which possess shape memory and the other possesses elastic

properties, constitute one body. This can be implemented in a

bimetallic composite. The advantage of this engineering so-

lution is the possibility of attainment of maximum compact-

ness and even miniaturization of the drive [3].

The shape memory alloy is commonly attached to other

metals by methods of laser or plasma welding [4, 5]. How-

ever, these processes have some disadvantages, the main of

which is the formation of a heat-affected zone due to local

heating of the material in the welded region. In this zone the

phase composition of the alloys [5], the grain size, and the

texture [4] differ considerably from the corresponding cha-

racteristics in the main volume of the joined members. This

causes substantial lowering of the strength of the bimetallic

composite [5] and worsens the operating properties [6]. It is

Metal Science and Heat Treatment Vol. 52, Nos. 9 10, 2010

432

0026-0673/10/0910-0432 2010 Springer Science + Business Media, Inc.

1St. Petersburg State University, St. Petersburg, Russia (e-mail:

spb@smel.math.spbu.ru).2

Vitebsk State Technological Institute, Vitebsk, Belarus.3

Institute for Technical Acoustics of the National Academy of Sci-

ences of Belarus, Vitebsk, Belarus.

natural to expect the best results from the use of cold ex-

plosion welding [7, 8]. In explosion welding the alloys expe-

rience high plastic deformations which change the tempera-

ture, succession, and fullness of implementation of the

martensitic transformations and hence the characteristics of

the shape memory effect. The consequences of plastic strain-

ing can be removed by regenerative tempering or annealing.

However, there are virtually no data on the effect of the tem-

perature and duration of annealing on the recovery of proper-

ties of shape memory alloys in a bimetallic composite. The

aim of the present work consisted in studying the effect ex-

plosion and subsequent annealing on the kinetics of marten-

sitic transformations in a shape-memory TiNi alloy in a bi-

metallic composite.

METHODS OF STUDY

In order to obtain a bimetal we chose plates of steel

Kh18N10T 50 50 1 mm in size and of alloy Ti 50.6 at.%

Ni 50 50 1.68 mm in size. A bimetallic composite was

formed by explosion welding at room temperature as de-

scribed in [8]. The thickness of a bimetallic specimen was

2.3 mm. The bimetallic specimens and TiNi plates were sub-

jected to isochronous annealing. The isochronous annealing

was performed at a temperature of 150 600C at a step of

50C with a hold for 20 min at each temperature. The deve-

lopment of the martensitic transformations was studied using

a Mettler Toledo 822e differential scanning calorimeter and

varying the temperature at a rate of 10 Kmin.

RESULTS

Figure 1 presents the calorimetric curves obtained due to

cooling and heating a TiNi plate in the state as delivered and

a bimetallic specimen. In the case of cooling of the TiNi

plate the calorimetric curve has two peaks of heat emission

in the ranges of 7 % 4C and 26 % 47C; in the case of

heating we observe one peak of heat absorption in the range

of 3 % 18C. Comparison of the calorimetric data with the

results of measurement of electrical conductivity allowed us

to establish that the first peak of heat emission is as result of

transformation of a cubic B2-phase into a rhombohedral

R-phase; the second peak is caused by transformation of the

rhombohedral R-phase into a monoclinic B19-phase. In

heating, we detected a reverse transformation of the mono-

clinic B19-phase directly into a cubic B2-phase.

After explosion welding martensitic transformations in

the TiNi alloy develop in the same succession. In cooling we

observe the B2 R B19 chain of transformations and in

heating we observe a B19 B2 transformation (Fig. 1b ).

However, the temperatures of the structural transformations

increase substantially, the temperature ranges of the transfor-

mations widen, and the energy of the transformations de-

creases. For example, the temperature Ri of the start of the

B2 R transformation is 7C in the initial TiNi plate, while

in the bimetallic composite it increases to 60C; the energy

of the transition decreases from 4.2 to 0.7 Jg. The tempera-

ture Mi of the start of the R B19 transformation increases

from 26 to 47C. The temperature Ai of the start of the

backward transformation increases from 18 to 65C. Growth

in the temperature of phase transformations, widening of

their temperature ranges, and decrease in the energy are typi-

cal for alloys subjected to high plastic deformations. The de-

crease in the transformation energy is obviously caused by

the decrease in the volume of the material undergoing the

structural transformation. This agrees well with the data of

[9], in which it is shown that plastic deformation causes sup-

pression of martensitic transformations.

The consequences of the action of plastic strain can be

removed to a considerable degree by annealing of the bime-

tallic composite. In order to determine the temperature range

of recovery of its properties a bimetallic specimen was sub-

jected to isochronous annealing. The results obtained (Fig. 2)

show that growth in the annealing temperature to 400C does

not virtually change the location of the calorimetric peaks

observed in cooling and heating. Only their intensity in-

creases somewhat. Annealing at 450 550C shifts the ther-

mal peaks to lower temperatures and their intensity increases

substantially. Growth in the temperature to 600C shifts the

second peak on the cooling curve to higher temperatures,

which causes overlapping of the peaks of heat emission

Effect of Annealing on Martensitic Transformations in Steel TiNi Alloy Composite 433

Ex

oE

xo

R B19

R B19

B R2

B R2

B B19 2

B B19 2

b

50 0 50

50 0 50

t, C

t, C

Fig. 1. Calorimetric curves obtained in cooling and heating of a

plate of TiNi alloy (a) and of bimetallic composite (b ).

caused by the B2 R and R B19 transformations. In the

heating process heat is absorbed in many stages.

Figure 3a presents the dependences of the temperatures

of the start and end of the B2 R (Ri and Rf ) and R B19

(Mi and Mf ) transformations on the annealing temperature. It

can be seen that the kinetics of the martensitic transforma-

tions changes starting with annealing temperature of 300C.

At tan > 300C the temperature range of the B2 R transfor-

mation becomes narrower. Growth in the annealing tempera-

ture from 300 to 400C decreases this range from 45 to 16C.

At tan & 400C the transformation temperature decreases

markedly. The B2 R transition is finished fully before the

beginning of the R B19 transformation and therefore the

calorimetric dependences have two well manifested peaks

(see Fig. 2). However, after annealing at 600C the tempera-

tures of the two transformations coincide again.

Growth in the annealing temperature is also accompa-

nied by a change in the transformation energy determined as

the area under the calorimetric peaks. Figure 3b presents the

dependence of the total energy of the B2 R and R B19

transformations on the annealing temperature. It can be seen

that the transformation energy increases upon growth in the

annealing temperature. It seems that this indicates the in-

volvement of larger and larger volumes of the material into

the phase transition.

The changes in the kinetics of martensitic transforma-

tions in annealing may be connected not only with the retro-

gression processes developing in plastically deformed alloy.

Heat treatment of TiNi alloys with excess nickel content and

relatively equiatomic composition is known to cause segre-

gation of secondary phases, which also stimulates changes in

the temperatures and in the succession of the transformations

[1, 10]. It is obvious that similar processes develop in the

studied alloy too. This is proved by the experiments with an

initial undeformed specimen of the alloy fabricated from a

plate not subjected to explosion. The respective calorimetric

curves are presented in Fig. 4. In contrast to the data obtained

after annealing of the bimetallic specimen, the temperature

of the B2 R transformation changes in the initial NiTi

plate after annealing at even 300C. In addition, the course of

the backward transformation, in which the crystal lattice is

restructured from a B19- phase into a B2-phase, changes and

develops through an intermediate R-phase; the calorimetric

curve has two well manifested peaks of heat absorption

(Fig. 4b ). Growth in the annealing temperature to 400C

promotes further increase in the temperature of this transfor-

434 S. P. Belyaev et al.

600

600

550

550

500

500

400

400

300

300

50 0 50

0 50 100

t, C

t, C

Em

issio

no

fheat

Em

issio

no

fheat

b

Without h.t.

Without h.t.

Fig. 2. Calorimetric curves obtained in cooling (a) and heating (b )

of a bimetallic plate subjected to isochronous annealing at various

temperatures (given at the curves in C).

50

0

50

0 200 400 600

0 200 400 600

tan ,

tan ,

t,

Mi

Ri

Mf

Rf

b

10

5

E, J g

Fig. 3. Dependence of the transformation temperatures (a) and of

the emitted energy (b ) on the temperature of isochronous annealing

of a bimetallic plate (Riand R

fare the initial and final temperatures

of the B2 R transition; Miand M

fare the initial and final tempera-

tures of the R B19 transformation).

mation. This is accompanied by some growth in the tempera-

tures of the R B19 transformation too. Annealing at

500C does not affect the course of the forward transforma-

tions and only lowers somewhat the temperatures of the

B2 R transformation. The backward transformation again

develops in one stage, like in the not annealed material. Fur-

ther increase in the annealing temperature results in a sin-

gle-stage forward transition from a B2 structure right to a

monoclinic B19.

The results obtained show that isochronous annealing of

the initial plate not subjected to explosion affects the kinetics

of martensitic transformations in a different manner. The

temperatures of the B2 R transformation are more sensi-

tive to annealing at 300 400C, which has not been de-

tected for the bimetallic plate, and the formation of rhombo-

hedral phase is suppressed fully after heating to tan > 500C.

Such changes in the characteristics of martensitic trans-

formations developing in the TiNi plate agree well with the

available data on changes in the structure of the alloy upon

annealing [1, 10]. For example, annealing at 300 450C

promotes formation of particles of a Ti3Ni4 phase, which cre-

ate rhombohedral distortions in the TiNi matrix, and this ini-

tiates the occurrence of both forward and backward transfor-

mations through an intermediate R-phase [11]. In addition,

the temperatures of the transformations increase because of

the depletion of the matrix of nickel due to formation of

Ti3Ni4 [12]. At 450 500C the particles start to dissolve,

which is accompanied by growth in the concentration of

nickel in the TiNi matrix and hence by an increase in the

temperatures of the transformations. In the case of annealing

at 550C the particles dissolve and the matrix undergoes only

one B2 B19 transformation during cooling, which occurs

at low temperatures.

DISCUSSION

We see that the laws of variation of the kinetics of mar-

tensitic transformations during annealing of a bimetallic

specimen and a specimen of TiNi alloy not subjected to ex-

plosion differ substantially. It is obvious that the processes of

segregation of secondary phases in explosion-deformed

specimens are either suppressed to a considerable degree or

do not affect substantially the changes in the annealing pro-

cess. The main role in the recovery of properties of the de-

formed alloy belongs to retrogression and recrystallization,

which are accompanied by rearrangement of the defects of

the structure and lowering of the density of lattice imperfec-

tions. This is the reason behind the decrease of the ranges of

martensitic transitions and growth in the energy of the trans-

formation (see Fig. 3). The recovery of properties is the most

intense when the annealing is performed at 450 600C. Ex-

periments show that long-term annealing at such tempera-

tures leads to full restoration of the kinetics of martensitic

transformations. Figure 5 shows the variation of the tempera-

ture range of the R B19 transformation upon growth in

the time of annealing at various temperatures. It can be seen,

that the range Mi Mf in the bimetallic specimen is 46C af-

ter 20-min hold at 450C and shortens to 17C after a

120-min hold, just as in the undeformed alloy.

CONCLUSIONS

We may infer that high plastic strain arising in a bimetal-

lic steel TiNi alloy composite changes the kinetics of

Effect of Annealing on Martensitic Transformations in Steel TiNi Alloy Composite 435

Em

issio

no

fheat

Em

issio

no

fheat

600

600

550

550

500

500

400

400

300

300

b

50 0 50

50 0 50

t, C

t, C

Without h.t.

Without h.t.

Fig. 4. Calorimetric curves obtained in cooling (a) and heating (b )

of a plate of TiNi alloy subjected to isochronous annealing at various

temperatures (given at the curves in C).

60

40

20

M Mi f ,

tan , 0 200 400

Fig. 5. Change in the temperature interval of R B19 transforma-

tion as a function of temperature and duration of annealing of bime-

tal plate and TiNi alloy: ) bimetal, an = 20 min; ) bimetal,

an= 120 min; ) TiNi alloy,

an= 20 min.

martensitic transformations in titanium nickelide. The cha-

racteristic temperatures and the temperature ranges of the

transformations grow, and the energy emitted and absorbed

in the transformations decreases considerably. Subsequent

annealing in a temperature range of 450 600C accompa-

nied by retrogression of structure causes full restoration of

the kinetics of the transformations in the alloy.

The work has been performed within a Russia Bela-

rus grant (RFFI No. 08-08-900010 Bel a and BFFI

No. T08R-225) and a grant of the President of the Russian

Federation for supporting young candidates of science

No. MK-466.2010.8.

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