Interaction of creep with the martensitic transformation in TiPdNi High Temp erature Shape Memory Alloys

Abstract

High Temperature Shape Memory Alloys represent a class of Shape Memory Alloys with transformation temperatures greater than 100◦C. However, as a consequence of their high transformation temperatures, these alloys can be exposed to a temperature regime where creep behavior can occur simultaneously during the phase transformation. Depending on the transformation temperatures of a shape memory alloy, the rate dependent plastic strain generated at the high operating temperatures, can significantly impact the actuators performance. The present effort focuses on understanding the interaction between the phase transformation and creep occurring simultaneously in shape memory alloys. For this purpose two alloys with transformation temperatures that overlap with the creep regime of the material are selected and fabricated by hot rolling/extrusion. Compression specimens are machined from the rolled material and standard isothermal creep tests are conducted at specific stress levels. In addition transformation induced thermal cycling is conducted to study the simultaneous transformation/viscoplastic behavior. Optical microscopy and TEM studies are conducted on the tested specimen to observe the changes in the microstructure. Based on the thermomechanical tests and the microstructural observations suitable conclusions are drawn to explain the interaction between the two mechanisms.