Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.4, pp.343-351, 2010 Printed in the USA. All rights reserved
Hot Rolling and Ageing Effect on the Pseudoelasticity Behaviou r of Ti-Rich
TiNi Shape Memory Alloy
Narendranath. S. 1, Vijay Desai1, S. Basavarajappa2*, K.V. Arun3, S. Manjunath Yadav2
1Department of Mechanical Engineering, National Institute of Technology, Srinivasanagar,
Surathkal, Karnataka, India
2Department of Studies in Mechanical Engineering, University B D T College of
Engineering, Davanagere-577 004 India
3Department of Studies in I & P Engineering, University B D T College of Engineering,
Davanagere-577 004 India
* Corresponding Author:
This paper presents the findings of an experimental study of how ageing temperature affects the
pseudoelasticity (PE) and its phases of Shape Memory Alloy (SMA). These have been examined
by means of tensile testing machine and X-ray diffraction (XRD). By increasing the ageing
temperature (constant ageing time) from 350ºC to 550ºC critical stress for martensite formation
and psuedo-elastic elongation increased. However, specimen aged at 650ºC showed a decrease
in the critical stress for martensite formation and psuedo-elastic elongation. After second and
third sequence of loading and unloading, the critical stress shifted towards higher side was
observed. The XRD measurement revealed that phases present in the matrix drastically changes
with ageing temperatures. Transformation was taking place in two-way transformation, i.e.,
during heating martensite (B19') transformed to austenite (B2) while cooling the austenite (B2)
transformed back to martensite (B19').
Key words: Pseudoelasticity; S M A; Critical stress; Elongation
The development of new materials that can be applied in contemporary engineering structures
determines progress in modern technology. The shape memory alloys are the candidate materials
for the actuators of smart structures since they have the ability to change properties and are able
to function in a controlled response to a change in environment or operating conditions. SMAs
344 Narendranath.S., Vijay Desai, S. Basavarajappa, K.V. Arun, S. Manjunath Yadav Vol.9, No.4
are the metal alloys that exhibit the special characteristics of either large recoverable strains or
large induced internal forces under load and/or temperature changes [1,2]. This ability to fully
recover the large inelastic strains is a result of a reversible crystallographic thermoelastic
martensite transformation from the austenite/parent phase to the martensite phase or vice-versa.
Martensitic transformations involve lattice transformation featuring shear deformation and a
coordinated atomic movement, which maintains the one-to-one lattice correspondence between
the lattice point in the parent and transformed phases [3,4].
It has been known that SMAs exhibit a large strain beyond the linear elastic range when an
external stress is applied in the austenitic state. At the temperature above Af (austenite finish),
the extra strain is recoverable almost completely on unloading and as a result, the martensite will
transform spontaneously to undeformed austenite upon release of the stress. On the other hand,
SME is one in which the alloy, once deformed at low temperature will stay deformed until
heated. The SMAs are heated above the martensite to austenite transformation temperature As
and the martensite to austenite transformation will end at Af. This strain recovery phenomenon is
called the SME. These two effects in SMAs, which show thermoelastic martensite
transformation, have been understood using the self-accommodating mechanism and variant
coalescence upon heating. If SMAs encounter any constrain during reverse transformation, they
can generate extremely large recovery stresses. This phenomenon provides a unique mechanism
for the actuation [5]. So far, quite a few researchers on the transformation of TiNi shape memory
alloys have focused on the Ni-rich alloys under the conditions of various compositions, different
heat treatments, precipitates, large deformation by tension, thermal cycling, etc [5-10]. However,
only a few investigations have been carried out for the Ti-rich alloys [11] and thus further
investigations are called for.
In view of the above in the present study, experimental results of thermomechanically treated Ti-
rich Ti52Ni48 SMAs are presented with discussions
A Ti-48Ni (at%) alloy was prepared by tungsten inert gas melting in a water cooled copper
crucible having diameter of 24 cm. 100gms of Ti-48Ni (at %) was melted and remelted at least
six times in a high purity argon atmosphere. The as melted buttons were hot rolled upto 2mm
thick plates at 850°C, keeping a constant temperature. The hot rolled plates were homogenized at
850°C for 8hrs in high purity argon atmosphere and subsequently furnace cooled.
These homogenized plates were again aged at 350°C, 450°C, 550°C and 650°C for 6hrs in argon
atmosphere and subsequently furnace cooled. Specimens for tensile testing were cut by wire-
EDM. The gauge length of the specimens was 25mm long, 5mm width and 1.8mm thick. Tensile
tests were carried out by Hounsfield tensile testing machine. All tests were conducted at room
temperature and the strain rate was 4 x10-4 per second. For XRD characterization, samples were
carefully cut from these plates with a low speed diamond cutter. The specimens were mirror
Vol.9, No.4 Hot Rolling and Ageing Effect 345
polished manually for X-Ray Diffraction analysis radiation Co Kα and the scanning angle range
chosen for such analysis was 35° to 120°.
3.1 XRD Characterization
The thermomechanically treated Ti-rich TiNi specimens were scanned over the range of 35° to
120° in a Philips X-ray diffractometer. Fig 1 shows the XRD results obtained at room
temperature from undeformed specimens. Five specific phases of TiNi were identified.
Martensite TiNi (M), austenite TiNi (A), stable secondary Ti2Ni, TiNi3 and metastable Ti3Ni4
were clearly observed. It can be seen from Fig 1 that on increasing the ageing temperature
(constant ageing time), the major austenite (A) phase peak base width slowly increases.
Figure 1. XRD Results of aged Ti52Ni48.
It can be also seen that martensite (M) peak slowly raises indicating increase in the volume
fraction of martensite. New metastable Ti3Ni4 peak was also observed but in small quantity. At
ageing temperatures upto 550°C, Ti3Ni4 peak increased with ageing temperature, where as the
volume fraction of Ti3Ni4 was very small in specimen aged at 650°C.
346 Narendranath.S., Vijay Desai, S. Basavarajappa, K.V. Arun, S. Manjunath Yadav Vol.9, No.4
This indicates that some growth of already existing martensite has occurred at the expense of
retained high temperature phase [12]. Probably, at different ageing temperatures, this high
temperature phase as well as secondary stable phases slowly precipitate in the form of Ti3Ni4.
This locally creates strain zones in the matrix. However, during tensile test (loading and
unloading) stress applied on the specimen leads to transformation from martensite to austenite.
During unloading partially converted austenite transforms back to martensite.
3.2 Deformation Behavior
Upon loading, a fully annealed near-equiatmoic TiNi shape memory alloy may experience an
apparent yield via any one of the three processes- martensite reorientation (MR), stress induced
martensitic transformation (SIM) and plastic yielding of austenite[13], depending on the testing
temperature[14]. For brevity the stress strain curves for tensile tested specimens were aged at
350ºC, 450ºC are shown in Fig 2, the results were same for other two temperatures.
Figure 2. Stress strain curves of Ti52Ni48 alloy aged at different temperatures.
For each test a virgin specimen was used and a constant strain rate of 4 x10-4 per second was
selected. Three loading and unloading cycles were performed on each specimen with 3000N,
4000N and 5000N loads. Stress-strain curves of aged TiNi specimens that had undergone three
loading and unloading sequences are shown in Fig 2. The first cycle starts and reaches a load of
3000N and unloaded was done immediately. Further 4000N and 5000N cycles were performed
without disturbing the set up on the same sample at same strain rate. These curves indicate the
work hardening of the specimen as the number of cycles increased. A small amount of residual
stress was stored in the matrix. This reveals that, beyond the purely elastic stage (o to a), the
curve may be divided into two stages: a to b and b to c. The nucleation of the martensite may
TiNi 5248 - 3500C
Stress (MPa)
E lo n g a tio n (%)
a) b)
Vol.9, No.4 Hot Rolling and Ageing Effect 347
occur by means of the stacking fault mechanism [15] and SIM (stress induced martensite) will
appear through the B19 to B2 transformation under deformation below the Md temperatures.
Obviously, the deformation behavior during a to b stage (Fig. 2) corresponds to the SIM.
However beyond point b, both SIM and small amount of plastic deformation leading to residual
stress is possibly taking place. This result in permanent change and rise in the stress-strain curve
for the next cycle.
The corresponding points a, b and c in the stress strain curve is shown in Figure 2 (a). After
points a, b and c are fixed on curve A, the three stages of the stress strain curve can be clearly
observed. Projecting points a, b and c to the strain axis produces strain values of a', b' and c'
from which the value of
can be calculated,
= [16],
where ''
b- '
a and ''
ac= '
c- '
a. The value of
quantifies the shape memory effect. In
Figure 2 (a) curves B and C represent second and third loading and unloading sequences.
The effects of ageing after hot rolling on the martensitic phase transformation behavior and
mechanical behavior of Ti-rich TiNi shape memory alloys have been discussed.
The effect of ageing on the critical stresses for stress induced martensitic transformation (σSIM) is
shown in Table 1. Ageing temperature had effect on the σSIM but not very significant. However,
the pseudoelasticity significantly varied with ageing temperature. The phenomenon of TiNi
specimens behaving differently in the first thermal transformation cycle after deformation than in
subsequent transformation cycle has been observed previously. Lin and Wu [17], reported that
the critical temperature for the reverse transformation in a specimen deformed by 5% in rolling
was increased by 28K. These increments in the critical temperature become larger when the
amount of cold deformation increases and reaches 68K for a deformation of 20%. The increase
in the critical temperature for the reverse transformations with increase in ageing temperature is
indicative of stabilization effect acting on the deformed martensite.
Two possible mechanisms exist [13] which explains this stabilization effect. Firstly, in a heavily
deformed specimen, a certain number of structural defects in particular dislocations have been
introduced in the martensite matrix by the deformation. These dislocations are expected to be
evolved and arranged in such a way that they match the reorientation of the oriented martensite
variants. Therefore upon heating, extra resistance has to be overcome for the reverse
transformation to proceed, resulting in an increase in the critical temperature for the reverse
transformation. Once the oriented martensite has been forced to revert back to austenite, the
stabilization effect vanishes in subsequent thermal cycles in which a different, self-
accommodating martensite is formed. However, the dislocations created by the deformation
remain in the matrix and this is expected to cause some relatively minor changes in the
348 Narendranath.S., Vijay Desai, S. Basavarajappa, K.V. Arun, S. Manjunath Yadav Vol.9, No.4
transformation behavior. Secondly, at low levels of deformation, the changes in the dislocation
structure caused by the deformation are negligible. In this case, the martensite stabilization effect
can only relate to changes in variant accommodation morphologies.
Table 1. Martensite start stress and elongation results for different loads of TiNi
Load=4000N Load=5000N
ε (%)
ε (%)
ε (%)
1 350 2754.7
410 5.5 540 6.6
2 450 2855.5400 6.8 510 8.4
3 550 2905.9
400 7.4 520 8.6
4 650 2754.4390 5.8 510 6.6
In the present study martensite is getting more and more stabilized with each deformation cycle
by the mechanism array of dislocation exerting back stress or internal elastic energy which has to
be overcome by more applied stress with each cycle the dislocation density is increasing and thus
the martensite stabilization. The near identical psudeo-elastic behavior indicates that permanent
changes to the matrix are negligible. The stabilization effect in this case is attributed with the
change in the internal elastic energy [13] associated with the change in the structure of
A minimum stress, σSIM is recognized to exist for the stress-induced martensitic transformations
at Ms. This stress is attributed to the mechanical resistance to phase boundary movement during
the stress induced martensitic transformation, since the thermodynamic resistance to the A to M
transformation at this temperature is expected to be practically nil. This stress is believed to
originate from the same source as the resistance to twin boundary movement during martensite
reorientation and to be dependent on the history of thermomechanical treatment of a specimen.
This expectation agrees with the experimental observation that σSIM increased with increasing
ageing temperatures. The reason for the increase in σSI M with ageing temperature with in the
range of 350ºC to 550ºC for 3000N loading cycle. Probably, for the lower ageing temperature,
austenite phase was more in the matrix than compared to 650ºC aged specimen, as confirmed by
XRD results of Fig. 1.
The observed maximum transformation strains, which were measured for first, second, and third
loading and unloading cycles are shown in Table 1. The large difference between them also
corresponds well to the large difference between the lattice distortions for both transformations.
However, the temperature dependence of the transformation strains for both stages is very
Vol.9, No.4 Hot Rolling and Ageing Effect 349
different from each other. The strain associated with the stress-induced martensitic
transformation εm increases with increasing ageing temperature (350ºC-550ºC), while the strain
associated with the 650ºC aged specimen εm decreases as shown in Table 1. Therefore, the
temperature dependence of the former strain does not reflect the lattice distortion for the
martensitic transformation, while the latter strain reflects well the lattice distortion for the M-
phase transition [18].
Ageing after solution treatment or ageing at temperatures below recrystallization temperature
immediately after hot work produces precipitates and/or dislocations which suppress the
martensitic transformation by forming back stress around them against transformation. The
amount of the back stress strongly depends on the amount of strain formed by a transformation.
The transformation strain associated with the martensitic transformation is about ten times larger
than that associated with R-phase transition. Therefore, the Ms point is depressed by the
precipitates and/or dislocations [18]. From XRD results Fig 1 we can confirm the presence of
Ti3Ni4 secondary precipitate although the peak intensity was very small.
Hot rolling processing and ageing effect of the shape memory alloys are important for the use of
SMAs in engineering applications. In some situations, more recoverable pseudoelastic
transformation strain may be needed to achieve better displacement control and some times
larger energy absorbability is needed for SMAs to passively reduce vibration. Therefore, the
evaluation of the behavior of the hot rolled SMAs, such as pseudoelastic strain, transformation
temperatures for loading and unloading cycles is necessary. The hot rolling analysis and
mechanical testing can guide the user to control the microstructure of the material and optimal
use the SMAs according to the practical applications.
The martensitic phase transformation behavior and mechanical properties has been studied by
using XRD and tensile tests at room temperature in Ti-rich Ti52Ni48 shape memory alloys
subjected to different ageing temperature treatments consequently the following results were
A minimum stress is required to induce martensitic transformation at Ms. This stress is
attributed to the mechanical resistance of phase boundary movement during the
transformation and is belived to be related to the critical stress for martensite
reorientation. This stress is dependent on the metallurgical conditions of an alloy.
The stress-induced martensite is observed to be more stable relative to the thermal
martensite. This stabilization effect is believed to be associated with the change in variant
accommodation morphology from self-accommodation for thermal martensite to the
oriented state of the stress-induced martensite. The same stabilization effect is also
expected for martensite deformed by a reorientation process.
350 Narendranath.S., Vijay Desai, S. Basavarajappa, K.V. Arun, S. Manjunath Yadav Vol.9, No.4
The psuedoelastic strain associated with stress induced martensitic transformation εM
increases with increasing ageing temperature (350ºC-550ºC). This is due to increase in
Ti3Ni4 and thus the lattice distortions. With increase in temperature. During 650ºC
ageing, precipitate of Ti3Ni4 is relatively low leading to decrease in psuedo-elasticity.
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