Experimental Determination of and Microstructural Influence on Crack Growth Rates during Thermo-Mechanical Cycling of NiTi and NiTiHf Shape Memory Alloys

There is an increasing demand to compact and lightweight actuators in aerospace and automotive industries. The solid-state actuators are promising candidates for replacing some existing actuators or making it possible to put actuators where it was impossible or impractical with current commercially available methods. Shape memory alloys (SMAs) have the highest work output for a specific volume among all active materials. They can create high forces and large strokes, as well as providing multifunctionality, which is beneficial due to the reduced complexity, weight and volume.

It is important to characterize SMAs for fatigue and fracture. The SMA actuators operate by cycling temperature, thus to get representative fatigue life or crack growth behavior thermal cycling must be considered. In this work binary Ni₅₀Ti₅₀ and ternary Ni_50.3Ti_29.7Hf₂₀ have been tested for thermal actuation crack growth. In the case of NiTiHf, the effect of precipitation on the actuation crack growth rate was also investigated. It is seen that thermal cycling produces faster, but still stable crack growth when compared to mechanical cycling. The strain fields during cycling were mapped with digital image correlation (DIC), and the driving forces behind crack growth were investigated with Finite Element Analysis (FEA). It is hypothesized that the change in modulus between austenite and martensite caused by the global transformation causes a greater driving force than mechanical cycling.