Influence of Inelastic Phenomena on the Actuation Characteristics of High Temperature Shape Memory Alloys

Abstract

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 modeling.