High Temperature Shape Memory Alloys for Useful Devices

Shape memory alloys (SMAs) are used increasingly in aerospace, medicine, instrumentation, and consumer products. The most common SMA, based on titanium-nickel, has an upper transition temperature limit of about 100¿C. Many potential applications requirea higher transition temperature. Higher actuation temperatures have been demonstrated in ternary alloys containing small percentages of palladium or hafnium. These alloys have not been commercialized because of inferior mechanical properties, adeficiency that can be ameliorated by improved knowledge of the variation of thermo-mechanical properties with composition. This proposal outlines a strategy for finding compositions of ternary alloys that are optimized for temperature, stress-strain, andfatigue properties.Ternary alloys, TiNiHf and TiNiPd, will be deposited as thin film on silicon substrates, with graded compositions across the surface. Resistivity measurement will be used to determine transition temperature of samples taken from local areas on the surfaceof the substrate, and stress-strain-temperature isotherms will be used to measure ductility. Compositions within local areas will be measured. Ingots will be formulated and cast using selected compositions. Material sputtered from these ingots will beexamined for improved thermo-mechanical characteristics. These steps may be iterated to optimize shape-memory characteristics at transition temperatures higher than 100¿C.Applications of high-temperature film from this research will be in microactuated valves, microrelays, and fiber optic switches. The same compositions that are used for thin film will be useful for making macroscopic devices for industrial and militaryapplications.