On The Functional Stability Of Tita Shape Memory Alloys

Shape memory alloys attracted a lot of attention in the last years due to their unique properties. However, most traditional shape memory alloys lose their shape recovery ability above temperatures of about 80 °C. Shape memory alloys which exhibit transformation temperatures above this temperature regime have been coined as high temperature shape memory alloys (HTSMA). HTSMAs such as NiTiHf and NiTiPd show transformation temperatures up to 400 °C with fully recoverable transformation strains during thermal cycling. However, expensive alloying elements, poor workability and limited ductility of most actual HTSMAs provide the main challenges to be solved for an effective economical use in industrial applications. In this regard binary and ternary TiTa alloys seem to fulfill these requirements as they show excellent workability and high ductility with deformation degrees up to 90 %.

However, as HTSMAs will typically experience cyclic loads, the aspect of functional fatigue has to be addressed. It is known that thermal cycling at elevated temperatures may lead to degradation of shape memory properties as a result of a shift in transformation temperatures and a decrease in transformation strains due to the formation of additional phases generated during thermal cycling.

In order to shed light on the mechanisms that lead to functional degradation, this study focused on the functional fatigue of Ti-Ta HTSMAs in isostress thermal cycling experiments under tensile loading conditions. In‑Situ optical microscopy was carried out in order to highlight the microstructural evolution as a function of thermal cycles.