Kumar, Parikshith K. (2009-12). Influence of Inelastic Phenomena on the Actuation Characteristics of High Temperature Shape Memory Alloys. Doctoral Dissertation.
Most e orts on High Temperature Shape Memory Alloys (HTSMAs), have focused
on improving their work characteristics by thermomechanical treatment methods.
However, the in
uence of transformation induced plasticity (TRIP) and viscoplasticity
during actuation has not been studied. The objective of this dissertation
work was to study the in
uence of plasticity and viscoplasticity on the transformation
characteristics that occur during two common actuation-loading paths in TiPdNi
HTSMAs. Thermomechanical tests were conducted along di erent loading paths.
The changes in the transformation temperature, actuation strain and irrecoverable
strain during the tests were monitored. Transmission Electron Microscopy (TEM)
studies were also conducted on select test specimens to understand the underlying
The study revealed that plasticity, which occurs during certain actuation load
paths, alters the transformation temperatures and/or the actuation strain depending
on the loading path chosen. The increase in the transformation temperature and
the irrecoverable strain at the end of the loading path indicated that the rate independent
irrecoverable strain results in the generation of localized internal stresses.
The increased transformation temperatures were mapped with an equivalent stress
(which corresponds to an internal stress) using the as-received material's transformation
phase diagram. A trend for the equivalent internal stress as a function of the applied stress and accumulated plastic strain was established. Such a function can be
implemented into thermomechanical models to more accurately capture the behavior
of HTSMAs during cyclic actuation.
On the contrary, although the viscoplastic strain generated during the course of
constant stress thermal actuation could signi cantly reduce actuation strain depending
on the heating/cooling rate. Additional thermomechanical and microstructural
tests revealed no signi cant change in the transformation behavior after creep tests
on HTSMAs. Comparing the thermomechanical test results and TEM micrographs
from di erent cases, it was concluded that creep does not alter the transformation
behavior in the HTSMAs, and any change in the transformation behavior can be attributed
to the retained martensite which together with TRIP contributes to the rate
independent irrecoverable strain. As a consequence, a decrease in the volume fraction
of the martensite contributing towards the transformation must be considered in the