Utilizing Micro-Mechanisms to Extend Fatigue Life in High Temperature Shape Memory Alloys

Shape memory alloys (SMAs) are multifunctional materials with thermoelastic phase transformation behavior capable of recovering the designed geometry after deformation. These alloys can undergo a solid-to-solid phase transformation in response to external stimuli, such as pressure or temperature, to accommodate or alleviate strain. Such a response can be utilized to design solid-state actuators by converting thermal energy into mechanical energy. Interest in these unique alloys is expanding to conserve energy and reduce weight and mechanical complexity in various engineering fields such as the medical, aerospace, automotive, and refrigeration industries. Aerospace and energy applications need high temperature operating SMAs due to the commercially available binary NiTi SMAs having a limited maximum austenite start temperature of 115 °C, thus encouraging research towards high temperature SMAs (HTSMAs). The challenge with designing HTSMAs is maintaining the functional and actuation response throughout the service life of high cycle fatigue SMA components in aerospace applications. Actuation cycles consequently lead to transformation-induced plasticity (TRIP) and microcracking, effectively evolving after every thermal cycle. The primary focus of this dissertation is linking macroscopic damage evaluation to microscopic mechanisms to improve the thermomechanical response and actuation fatigue. This was achieved by examining a variety of NiTiHf compositions using in situ synchrotron radiation X-ray diffraction, microscopy, tomography, and novel processing techniques after evaluating the fatigue response. Through this research, several paths to improving actuation response and fatigue life are revealed by utilizing temperature to control damage mechanisms and resist functional and structural fatigue through processing and microstructure.