The creep behavior and the phase transformation of Ti50Pd30Ni20 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.