Kumar, Parikshith K. (2009-12). Influence of Inelastic Phenomena on the Actuation Characteristics of High Temperature Shape Memory Alloys. Doctoral Dissertation. Thesis uri icon


  • 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
    microstructural changes.
    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

publication date

  • December 2009