Shape memory alloys (SMAs) are 'smart' materials which are able to change their shape in response to changes in temperature. This unusual behavior arises from a solid-state phase transformation, which can be utilized to generate force. These extraordinary properties have made them of great interest to the automotive and aerospace industries for potential light-weight solid-state actuator applications. An actuator is any mechanism which converts energy, such as heat or electricity, into motion.

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... MAs outshine traditional actuating systems such as pneumatics, hydraulics, and DC motors due to their remarkably high power-to-weight ratios [1]. By replacing heavy conventional actuating systems, they offer the possibility of higher reliability, lighter weight and increased capability while lowering space and power consumption. This will lead to improved efficiency and reduced emissions, particularly in aircraft. There is currently a drive toward developing SMAs which can be used in high temperature environments, for applications such as fuel control valves within jet engines [2]. Two systems are at the core of this effort: NiTiHf and NiTiAu. The addition of hafnium (Hf) and gold (Au) dramatically increases the viable operating temperature window in these alloys. NiTiHf can be tailored to achieve a highly favorable balance of properties, including high strength, stability, and work output at temperatures approaching 300°C [3]. This behavior is strongly influenced by the formation of nano-scale precipitates, known as H-phase. These precipitates raise transformation temperatures and enhance shape memory behavior, while improving stability by suppressing plasticity during transformation. Less is known about the NiTiAu system. Recent constant-force thermal cycling (CFTC) experiments have demonstrated work output at temperatures above 400 o C, as well as notable compositional insensitivity. Microstructural investigations have shown the presence of two types of secondary phase in these alloys, which may be responsible for the remarkable properties observed. Advanced scanning transmission electron microscopy (STEM) based characterization techniques are being used to explore the mechanisms responsible for the unusual behavior seen in both alloy systems. In order to advance the reliability of these alloys to where commercialization is viable, a comprehensive understanding of the important microstructure-property relationships will need to be developed. Two compositions of NiTiHf (Ni51Ti29Hf20 and Ni50.3Ti29.7Hf20) and four compositions of NiTiAu (Ni11Ti49Au40, Ni10.3Ti49.7Au40, Ni10Ti50Au40, Ni9Ti51Au40) were prepared by vacuum induction melting at NASA Glenn Research Center. Small cylindrical bars were cut for mechanical testing and heat treatments. Nano-scale needles of aged material were trenched and thinned to electron transparency using a focused ion beam (FIB) on a FEI Nova 600. High angle annular dark field (HAADF) STEM was performed using a probe-corrected FEI Titan 80-300 at 300kV. Investigations have highlighted the powerful effects of temperature, composition, and aging on critical alloy properties. Structural analysis of the H-phase precipitate was completed in previous publications 1954