Processing and Thermomechanical Stability of Low Hysteresis Shape Memory Alloys

Shape memory alloy (SMA) actuators can combine a high work output to a significantly reduced component size. However, without significant training, binary NiTi SMAs suffer from relatively poor functional and structural fatigue, which affects the actuator’s overall thermomechanical stability. Another detrimental problem facing binary NiTi SMAs is attributed to the width of the hysteresis loop between the austenite final and martensite final temperatures, which accounts for the input energy an SMA actuator needs to produce a desired stroke. By reducing the width of the hysteresis loop, the energy needed for each cycle can be significantly decreased, thus increasing the actuator’s overall efficiency. Previous studies have indicated that a narrow hysteresis loop and higher thermomechanical stability of NiTi-based SMAs are both directly linked to a higher degree of coherence between the austenite and martensite interface. It has been shown that small additions of substitutional elements to both Ni and Ti in NiTi-based SMAs can significantly improve the interfacial coherency of the two phases. In this study, elemental additions of Cu and Pd to the NiTi-based SMAs were investigated for their ability to reduce the thermomechanical hysteresis and thermomechanical stability of SMA actuator components. Two quaternary SMAs containing NiTiCuPd were selected based on synchrotron radiation X-ray diffraction combinatorial studies, where the first alloy was targeted at a low M_f temperature (below RT), and the second at a higher M_f temperature (above RT). Upon characterization, both quaternary SMAs have displayed very narrow hysteresis as well as exceptional thermomechanical stability.