TRANSFORMATION INDUCED EFFECTS IN TINI SHAPE MEMORY
ALLOY SUBJECTED TO TENSION
E. A. Pieczyska1, S. P. Gadaj1, W. K. Nowacki1 and H. Tobushi2
1
Institute of Fundamental Technological Research PAS,
Swietokrzyska Street 21, 00-049 Warsaw, Poland
Phone: 48 228261281 ext. 260; FAX: 48 228268911; E-mail: [email protected]
2
Department of Mechanical Engineering, AICHI Institute of Technology, 1247 Yachigusa,
Yakusa-cho, Toyota 470-0392, Japan
1. ABSTRACT
Transformation induced effects in shape memory alloy (SMA) subjected to tension test at room temperature are
investigated. The experiments were carried out on belt type specimens of TiNi SMA. The loading was realized on high
quality testing machine, which enables to obtain the stress-strain characteristics with high accuracy. Smart infrared
camera was used to register the temperature distribution on the specimen surface and to find its temperature
changes, with the accuracy up to 0.05K.
Recent study show that during superelastic deformation of TiNi SMA, the phase transformations can be
accompanied by unstable mechanical behavior and localized Lüders-like transformation, resulting in propagating of
phase transformation fronts. The bands of the new phase were characterized by the angle of inclination with the
direction of tension of 48° and the variation in temperature of about 8 K. They were followed by the next generation of
the bands inclined at the same angle but developing in the opposite direction [1-4].
Some effects of the localized phase transformation during stress relaxation for not temperature-controlled test,
i.e. carried out in room conditions were investigated in [5, 6]. A large stress drops, caused by the developing
transformations, were observed there in both the course of loading and unloading, while for the temperature controlled
conditions, in the unloading branch the stress increased under constant strain, due to the stress recovery [7].
Objective of the current work is study of terms of nucleation, developing as well as vanishing of the martensitic
and reverse phase transformations in particular experimental conditions. The goal was to demonstrate that
thermomechanical couplings do operate in TiNi Shape memory alloy during the process of stress-induced phase
transformations. The onset and evolution of the phase transformation fronts developing under specific conditions is
analyzed.
To this end, the specimens of TiNi SMA have been subjected to a particular program of tension test under
stress-controlled conditions, i.e., with a constant stress rate. The investigations have been started from creep-like
tests, i.e., while keeping the constant stress level, the strain and temperature changes have been recorded and
analyzed. The mechanical curves were registered and the specimen surface was observed by infrared camera. Next,
a special program of testing was realized. When the martensite transformation has been found to be advanced and
bands of localized heterogeneous phase transformation accompanied by no-uniform temperature distribution occur,
the stress has been programmed to decrease in proper way in order to stop the phase transformation. It works, since
the temperature decreases and started to be more uniform. For the reverse transformation, the stress has been
programmed to increase, respectively, just when the transformation was found to be advanced and inhomogeneous
temperature distribution was noticed.
2. Experimental procedure
The tension tests were carried out on a belt type specimen of 160 x 10 x 0.4 mm, cut off from a strip of TiNi SMA
of the constitution Ti-55.3wt%Ni and characterized by the austenite finish temperature Af equal to 283 K. All
investigations were performed at room temperature of about 296 K. Since the Af temperature is so low, the SMA
demonstrates a complete loop of pseudoelasticity during the tension tests carried out at room temperature. Before the
testing, the specimen surface was covered with a very thin layer of carbon black powder in order to make its emissivity
higher and more homogeneous. The specimens were subjected to stress-controlled tension tests with of 12.5 MPa/s.
In the course of investigations both the mechanical characteristics and the distribution of the infrared radiation emitted
by the specimen surface were continuously registered. The stress and the strain quantities were related to the current
(instantaneous) value of the specimen cross-section and thickness values, in order to obtain, so called, true stress
and strain values. The temperature distribution was registered by using infrared equipment allowing for infrared
photographs, i.e. thermograms, to be stored in digital form with a maximal frequency of 50Hz. That allows
reproduction of the images at any moment and makes the calculation of temperature as well as its presentation
straightforward. The temperature can be presented as a function of time or other parameters of the deformation
process, if required.
The infrared camera used is long wave type, working in the wave range of 7.5 µm - 13 µm. The matrices size is
320 x 240 pixels. The spatial and the temperature resolutions depend on the camera-specimen distance. In the case
of the measurement presented here, the distance was 10 cm and the spatial resolution was 0.3 mm. The
measurement temperature sensitivity, in the range up to 30K, is below 0.08 K.
For these investigations three kinds of temperature registration were applied:
- temperature distribution on the specimen surface,
- mean temperature taken from the chosen specimen area,
- change in temperature of a chosen point on the specimen surface.
The temperature distribution on the specimen surface immediately reflects the origin and development of the new
phases, both martensite and reverse, due to the significant temperature variations observed between the parent and
the new phase.
The average temperature was calculated over an area of 8 mm × 60 mm, located in the central part of the
specimen. The temperature measured in this way was presented in the graphs and used in the thermomechanical
coupling analysis.
The point temperature was taken from the central part of the specimen surface where the first band of the higher
temperature, related to the new martensite phase, was noticed.
The temperature and the mechanical data enable the analysis of the nucleation process and further development
of both the martensitic and the reverse transformations.
The scheme of the measurement set-up is shown in Fig. 1. The photo of the testing machine and the infrared
camera used was shown in Fig. 2.
GL
σ
T
SMA SPECIMEN
INFRARED
CAMERA
ε
ε
TESTING MACHINE
Fig. 1. Scheme of the experimental set-up
Fig. 2. Photo of the SMA specimen subjected to stress-induced phase transformation in Instron testing machine.
Three kinds of stress-controlled tension tests were carried out:
1.
Complete loading-unloading test, while registering martensite and reverse transformation hysteresis loop
2.
Investigation of transformation-induced creep-like effects, namely 3 min. breaks with constant stress
level, induced in the branch of loading, or unloading, respectively
3.
Investigation of influence of thermomechanical couplings during the martensite and reverse
transformations, namely by introducing in the branch of loading 3 min breaks with decreasing stress, or
by introducing in the branch of unloading 3 min breaks with increasing stress
3. Deformation behavior of TiNi SMA subjected to loading and unloading with constant stress rate
The mechanical characteristics of TiNi SMA subjected to typical tension test with constant stress rate 12.5
MPa/s is presented in Fig. 3. In addition, the change in average temperature measured on the specimen surface is
presented in the figure.
30
.
TiNi-test16
σ = 12.5 MPa/s
True stress (MPa)
600
20
σ
ΔΤ
400
10
200
0
0
Temperature variation (K)
800
-10
0.00
0.02
0.04
0.06
True strain
Fig. 3. Stress and average temperature changes vs. strain curves of TiNi SMA subjected to loading and unloading
with constant stress rate 12.5 MPa/s
Looking at the temperature ΔT vs. strain curve, one can notice that after pure elastic deformation, at the
beginning the average temperature increase up to 2K is observed. This is probably a confirmation of the R-phase
transformation that can occur, before the martensite transformation has started. Next, a stress-strain waving plateau,
related to localized Lüders-like deformation is registered. The waving part of the stress-strain curve is accompanied by
a no uniform temperature distribution, related to inhomogeneous martensite transformation. The start of localized
martensite transformation is accompanied by a jump of the average temperature, up to 10 K.
After the waving stage, the martensite transformation becomes more homogeneous which was confirmed by
more uniform temperature distribution. At this stage, an upswing region of the stress-strain curve is observed,
manifested by not only more homogeneous but also more advanced martensite transformation, accompanied by the
more uniform temperature distribution. However, the temperature change ΔT increases significantly at this stage of
the phase transformation. The higher temperature increase, up to 22.5 K was registered at the end of the martensite
transformation.
During the process of unloading, after passing its elastic stage, the reverse transformation initiates. After it is
completed, the material almost returns to the parent austenite phase. However, the residual strains, related to a small
amount of the residual martensite and irreversible macro-structural changes appear, depending on the TiNi SMA
specimen history. The reverse, austenite transformation is related to the significant temperature decrease. The
temperature distribution, accompanied the reverse transformation, is also inhomogeneous. At the end of the reverse
transformation, the temperature drops below the initial specimen temperature.
4. Investigation of transformation-induced Creep-like effects, induced in upper and lower branch of loading
The investigations have been started from creep-like tests, i.e., while keeping the constant stress level, the
strain and the temperature changes have been recorded and analyzed. During the loading, when the martensite
transformation has been found to be advanced, namely the bands of localized heterogeneous phase transformation
accompanied by no-uniform temperature distribution occur, the stress was kept constant during 3 min. For the next
test, during the unloading, when the austenite transformation has been found to be advanced, namely the bands of
localized heterogeneous phase transformation accompanied by no-uniform temperature distribution occur, the stress
was kept constant during 3 min. In this way the program of investigation was realized as follows:
1. Loading to 2.5mm (2.5%), keeping the constant stress level for about 180s, reloading to 8mm, unloading.
2. Loading to 8mm, unloading to 5mm, keeping the constant stress level for about 180s, unloading.
The obtained stress-strain characteristics are presented in Fig. 4: for the Test 1 - curve 5c, for the Test 2 curve 3c, respectively. The crosses denote the start of the creep process.
1000
True stress (MPa)
800
.
TiNi SMA
σ = 12.5 MPa/s
5c - creep at upper branch
3c - creep at lower branch
3c
600
5c
400
200
0
0.00
0.02
0.04
0.06
0.08
True strain
Fig. 4. Stress-strain curves obtained for TiNi SMA subjected to tension test with stress rate 12.5 MPa/s and breaks
during loading (5c curve) and unloading (3c curve).
In the initial stage of the loading, the both curves are overleaping each other. After introducing the break in the
loading, the stress-strain curves are modified, according to the test program.
(a)
(b)
σ
400
-10
<
200
-20
0
-30
0.02
0.04
True strain
0.06
0.08
40
800
30
600
20
σ
ΔΤ
400
10
200
0
0
Temperature variation (K)
0
.
σ = 12.5 MPa/s
>
>
600
TiNi-test3c
>
10
ΔΤ
0.00
1000
Temperature variation (K)
True stress (MPa)
800
True stress (MPa)
20
.
σ = 12.5 MPa/s
TiNi creep-5c
<
1000
-10
0.00
0.02
0.04
0.06
0.08
True strain
Fig. 5. Stress and average temperature changes curves vs. strain of TiNi SMA subjected to loading and unloading
with constant stress rate 12.5 MPa/s
For both tests, the run of the stress as well as the run of the temperature curves are significantly modified, due
to the 3 min break in the loading (curve 5c) or in the unloading (curve 3c); Figs. 4, 5. However, the phase
transformations are still developing which was also confirmed by the observation of the temperature distributions on
the specimen’s surface during the processes. It turned out that in both cases, the transformations once started is also
developing during keeping constant stress level, however the process is slower.
During the martensite transformation developing in the upper branch of loading, the average specimen
temperature increases up to 10K (Fig. 5a). During unloading, the average specimen temperature decreases for about
35K, which means that the temperature drops below zero.
5. Experimental investigation of thermomechanical couplings operating during the martensite and reverse
transformations
It was experienced by many researchers involved in investigation of thermomechanical couplings of shape
memory alloys that the martensite transformation is accompanied by the temperature increase, while the reverse
transformation is accompanied by the temperature decrease [1-8]. The main task in this approach was to introduce the
factor operating against the effects of the phase transformation, i.e., against the temperature increase during the
martensite transformation, and against the temperature decrease during the reverse transformation. The idea was
suggested by Prof. B. Raniecki to manifest that the thermomechanical couplings do operate during the stress-induced
phase transformations.
To this end, two tests were performed as follows:
1. When the martensite transformation has been found to be advanced and bands of the localized
heterogeneous phase transformation accompanied by no-uniform temperature distribution occur, the stress
has been programmed to decrease by about 5% (0.5 MPa/s) in order to affect, i.e., to stop the phase
transformation; Fig. 6 a, b.
2. When the reverse austenite transformation has been found to be advanced and bands of the localized
heterogeneous phase transformation accompanied by no-uniform temperature distribution occur, the stress
has been programmed to increase by about 5% (0.5 MPa/s) in order to affect, i.e., to stop the phase
transformation; Fig. 7 a, b.
a)
b)
1000
30
1200
20
1000
True stress (MPa)
ar.
800
10
ΔΤ
600
p1
p2
0
σ
400
-10
200
-20
0
-30
0
100
200
Time (s)
300
400
.
30
σ = 12.5 MPa/s
TiNi creep-11e
20
800
10
ΔΤ
σ
>
600
0
400
-10
200
-20
0
-30
0.00
0.02
0.04
0.06
Temperature variation (K)
.
σ = 12.5 MPa/s
TiNi creep-11e
Temperature variation (K)
True stress (MPa)
1200
0.08
True strain
Fig. 6. Stress σ and temperature change a) vs. time during particular program of tension test of TiNi SMA; p1 –
temperature change ΔT at point of 1-st transition band, p2 – point at minimum temperature, ΔT- average temperature,
b) Stress σ and temperature change ΔT vs. strain.
Figures 6 a, b present stress σ and temperature change ΔT vs. time and vs. strain curves, obtained for the
test 1, while Figures 7 a, b present stress σ and temperature change ΔT vs. time and vs. strain curves, obtained for
the test 2. The temperature denoted “ΔT” or “ar” was measured as an average from the chosen area, while the
temperature denoted “p1” or “p2” was measured from 2 different points chosen on the specimen surface.
During investigation of the transformation-induced effects, due to the factors introduced against the phase
transformation direction in the branch of loading (Fig. 6), the thermomechanical coupling do operate - the temperature
decreases, the temperature distribution observed by infrared camera started to be more uniform and finally, the
process of the phase transformation was stopped. During reloading after the break, the stress increased and the
temperature at the beginning also increased, however after a while the temperature drops which manifest that the
martensite transformation was almost completed during the 3min break with decreasing stress level.
During investigation of the transformation-induced effects, due to the factors introduced against the phase
transformation direction, in the branch of unloading (Fig. 7), the austenite transformation has been still developed,
however the transformation process was much slower. At the beginning, the temperature distribution observed by
infrared camera started to be more uniform, but finally tiny bands of the inhomogeneous reverse transformations were
also noticed. The rate of the stress change was probably too small to stop the process of transformation. The proper
rate of the stress decrease was really difficult to realize, even on the high quality, new Instron testing machine.
b)
1000
σ = 12.5 MPa/s
20
ΔΤ
10
ar.
p2
400
0
200
-10
True stress (MPa)
p1
600
σ
800
σ
800
True stress (MPa)
.
TiNi creep-12e
.
σ = 12.5 MPa/s
TiNi creep-4d
30
Temperature variation (K)
1000
40
30
600
20
>
ΔΤ
400
10
200
0
Temperature variation (K)
a)
<
0
-20
0
200
400
600
Time (s)
800
1000
0
-10
0.00
0.02
0.04
0.06
0.08
True strain
Fig. 7. Stress σ and temperature change a) vs. time during particular program of tension test of TiNi SMA; p1 –
temperature change ΔT at point of 1-st transition band, p2 – point at minimum temperature, ΔT- average temperature,
b) Stress σ and temperature change ΔT vs. strain.
6. Concluding remarks
Stress-induced phase transformation in TiNi shape memory alloy under stress-controlled conditions is
inhomogeneous process. It nucleates and develops by narrow bands of significantly higher temperature, related to the
martensitic phase, or lower temperature, related to the reverse one, in similar way to Lüders bands.
The bands of new phase differ in temperature for about 8 K; they are inclined for about 48 degree, according to
the specimen tension, and evolve in 2 almost perpendicular directions towards grips.
After introducing 3 min breaks under keeping constant stress level, the both martensite and reverse transformations
are still developing, however the process is slower.
During investigation of the transformation-induced effects, induced against the phase transformation direction,
in the branch of loading, the thermomechanical coupling do operate - the temperature was decreasing, the
temperature distribution observed by infrared camera started to be more uniform and finally, the phase transformation
was stopped.
During investigation of the transformation-induced effects, induced against the phase transformation direction,
in the branch of unloading, the austenite transformation has been still developing; however the transformation process
was slower. Probably, the rate of the stress change was too small to stop the process of the reverse transformation.
Acknowledgement:
The research has been carried out with the support of the Japan Society for Promotion of Science under Post-doc
P04774, JSPS and PAS: 6612, Cooperation IFTR and AIT. Authors wish to extend theirs thanks to Prof. B. Raniecki
from IFTR (Poland) and Prof. S. Miyazaki from Tsukuba Univ. (Japan) for scientific advice and fruitful discussions.
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