The focus of the present work is to study the effect of stress and temperature on the accumulated residual
strain during the thermomechanical cycling of Shape Memory Alloys (SMAs). NiTi wires were pseudoelastically
trained at different temperature above the austenitic finish temperature, up to different maximum applied stress
levels. The total residual strain recorded during each training experiment was decomposed into the contributing
plastic strain and retained martensite. The quantity of retained martensite in the trained wire was determined
by a flash heating the trained SMA and recording the recovered strain. Preliminary observations from the
thermomechanical test results suggest that the retained martensite formation is dependent on the maximum
applied stress level during the thermomechanical test and is not dependent on the transformation plateau stress
level of the SMA. On the contrary the transformation plateau stress level or consequently the test temperature is
a critical parameter in dictating the irrecoverable plastic strain generated during the thermomechanical cycling
The creep behavior and the phase transformation of Ti<sub>50</sub>Pd<sub>30</sub>Ni<sub>20</sub> High Temperature Shape Memory Alloy
(HTSMA) is investigated by standard creep tests and thermomechanical tests. Ingots of the alloy are induction
melted, extruded at high temperature, from which cylindrical specimens are cut and surface polished. A
custom high temperature test setup is assembled to conduct the thermomechanical tests. Following preliminary
monotonic tests, standard creep tests and thermally induced phase transformation tests are conducted on the
The creep test results suggest that over the operating temperatures and stresses of this alloy, the microstructural
mechanisms responsible for creep change. At lower stresses and temperatures, the primary creep mechanism
is a mixture of dislocation glide and dislocation creep. As the stress and temperature increase, the mechanism
shifts to predominantly dislocation creep. If the operational stress or temperature is raised even further, the
mechanism shifts to diffusion creep.
The thermally induced phase transformation tests show that actuator performance can be affected by rate
independent irrecoverable strain (transformation induced plasticity + retained martensite) as well as creep.
The rate of heating and cooling can adversely impact the actuators performance. While the rate independent
irrecoverable strain is readily apparent early in the actuators life, viscoplastic strain continues to accumulate
over the lifespan of the HTSMA. Thus, in order to get full actuation out of the HTSMA, the heating and cooling
rates must be sufficiently high enough to avoid creep.
The focus of the current effort is to characterize the viscoplastic behavior in high temperature shape memory
alloys and understand the impact of creep on their actuation characteristics. For this a Ti<sub>50</sub>Pd<sub>40</sub>Ni<sub>10</sub> alloy was
cast and hot rolled. Standard creep tests and isobaric thermal cycling tests were conducted on a custom test
setup. The results from the thermomechanical tests indicate large irrecoverable strains due to creep. Varying
thermally induced transformation cycling rates did not impact the transformation behavior in the SMA. From
these observations, it can be suggested that the rate of thermal cycling can alter the impact of viscoplasticity
on the actuators performance. However the creep behavior itself is decoupled from the transformation and does
not impact the transformation or the rate independent irrecoverable strain generated.
The focus of this paper is the study of tensile work characteristics and the transformation behavior of a High
Temperature Shape Memory Alloy (HTSMA) by thermomechanical characterization at temperatures ranging from 200
to 500°C. In order to investigate the above issues, a nominal composition of Ti<sub>50</sub>Pd<sub>40</sub>Ni<sub>10</sub> HTSMA was used. The alloy
was fabricated using a vacuum arc melting technique. The melt was cast and hot rolled followed by cutting of tensile
specimen using Electrode Discharge Machining (EDM). A high temperature experimental setup was developed on a load
frame to test the material at high temperatures under constrained actuation conditions. The stability of the material
response under cyclic actuation was also investigated. The observations from the tests are presented in this paper.
Microprobe analysis was performed on the as-cast and rolled material to study the composition. The material was also
studied by X-ray diffraction (XRD) and optical microscopy before and after testing. Certain key observations about the
material response are discussed specifically, in terms of transformation behavior, recoverable strains under various
applied total strains, and cyclic thermomechanical behavior.
The focus of this paper is the study and thermomechanical characterization of High Temperature Shape Memory Alloys (HTSMAs) at different stress levels and temperatures ranging from 300 to 500°C. The stability of the material response under cyclic actuation is also investigated. The observations deduced from the tests are presented in detail. In order to investigate the above issues a Ti<sub>50</sub>Pd<sub>40</sub>Ni<sub>10</sub> HTSMA was used. The alloy was fabricated by a vacuum arc melting technique, followed by casting and hot rolling. A high temperature experimental setup was developed on a load frame to test the material at high temperatures under constrained actuation conditions. Certain key observations on the material response, in terms of recoverable strains under various applied total strains and actuation stress levels, and cyclic thermomechanical behavior are presented.