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.
Over the past few decades, binary NiTi shape memory alloys have received attention due to their unique mechanical
characteristics, leading to their potential use in low-temperature, solid-state actuator applications. However, prior to
using these materials for such applications, the physical response of these systems to mechanical and thermal stimuli
must be thoroughly understood and modeled to aid designers in developing SMA-enabled systems. Even though shape
memory alloys have been around for almost five decades, very little effort has been made to standardize testing
procedures. Although some standards for measuring the transformation temperatures of SMA's are available, no real
standards exist for determining the various mechanical and thermomechanical properties that govern the usefulness of
these unique materials. Consequently, this study involved testing a 55NiTi alloy using a variety of different test
methodologies. All samples tested were taken from the same heat and batch to remove the influence of sample pedigree
on the observed results. When the material was tested under constant-stress, thermal-cycle conditions, variations in the
characteristic material responses were observed, depending on test methodology. The transformation strain and
irreversible strain were impacted more than the transformation temperatures, which only showed an affect with regard to
applied external stress. In some cases, test methodology altered the transformation strain by 0.005-0.01mm/mm, which
translates into a difference in work output capability of approximately 2 J/cm<sup>3</sup> (290 in•lbf/in<sup>3</sup>). These results indicate the
need for the development of testing standards so that meaningful data can be generated and successfully incorporated
into viable models and hardware. The use of consistent testing procedures is also important when comparing results
from one research organization to another. To this end, differences in the observed responses will be presented,
contrasted and rationalized, in hopes of eventually developing standardized testing procedures for shape memory alloys.
High-temperature shape memory alloys in the NiTiPd system are being investigated as lower cost alternatives to NiTiPt
alloys for use in compact solid-state actuators for the aerospace, automotive, and power generation industries. A range of
ternary NiTiPd alloys containing 15 to 46 at.% Pd has been processed and actuator mimicking tests (thermal cycling
under load) were used to measure transformation temperatures, work behavior, and dimensional stability. With
increasing Pd content, the work output of the material decreased, while the amount of permanent strain resulting from
each load-biased thermal cycle increased. Monotonic isothermal tension testing of the high-temperature austenite and
low temperature martensite phases was used to partially explain these behaviors, where a mismatch in yield strength
between the austenite and martensite phases was observed at high Pd levels. Moreover, to further understand the source
of the permanent strain at lower Pd levels, strain recovery tests were conducted to determine the onset of plastic
deformation in the martensite phase. Consequently, the work behavior and dimensional stability during thermal cycling
under load of the various NiTiPd alloys is discussed in relation to the deformation behavior of the materials as revealed
by the strain recovery and monotonic tension tests.
Potential applications involving high-temperature shape memory alloys have been growing in recent years. Even in those cases where promising new alloys have been identified, the knowledge base for such materials contains gaps crucial to their maturation and implementation in actuator and other applications. We begin to address this issue by characterizing the mechanical behavior of a Ni<sub>19.5</sub>Pd<sub>30</sub>Ti<sub>50.5</sub> high-temperature shape memory alloy in both uniaxial tension and compression at various temperatures. Differences in the isothermal uniaxial deformation behavior were most notable at test temperatures below the martensite finish temperature. The elastic modulus of the material was very dependent on strain level; therefore, dynamic Young's Modulus was determined as a function of temperature by an impulse excitation technique. More importantly, the performance of a thermally activated actuator material is dependent on the work output of the alloy. Consequently, the strain-temperature response of the Ni<sub>19.5</sub>Pd<sub>30</sub>Ti<sub>50.5</sub> alloy under various loads was determined in both tension and compression and the specific work output calculated and compared in both loading conditions. It was found that the transformation strain and thus, the specific work output were similar regardless of the loading condition. Also, in both tension and compression, the strain-temperature loops determined under constant load conditions did not close due to the fact that the transformation strain during cooling was always larger than the transformation strain during heating. This was apparently the result of permanent plastic deformation of the martensite phase with each cycle. Consequently, before this alloy can be used under cyclic actuation conditions, modification of the microstructure or composition would be required to increase the resistance of the alloy to plastic deformation by slip.