Shape-memory alloys (SMAs) have wide aerospace and biomedical applications due to the capability to recover from large deformations via solid-to-solid transformations from martensitic to austenitic phases. Recently, Ni-Ti-Hf SMAs have attracted tremendous research interests because their phase transformation temperature is higher than that of traditional Ni-Ti counterparts. This expands their applications in high-temperature environments. Our previous research has indicated that the presence of nano-scale precipitates could further improve the cyclic transformation response in the Ni-Ti-Hf system. However, there lacks a detailed understanding of these precipitates, including crystal structure, chemistry, size, distribution and volume fraction. Moreover, the precipitate evolution as a function of heat treatment temperature and time is also not well understood. Such information is imperative for modeling the phase transformation responses of this class of materials.
The lack of detailed understanding on these nano-precipitates is primarily attributed to the resolution limit of conventional characterization tools such as scanning electron microscopy (SEM). In this work, we propose to combine scanning transmission electron microscopy (STEM, offers up to sub-nanometer spatial resolution) with TEM tomography to construct 3D models for these nano-scale precipitates. The T3 funding will be a valuable resource to initiate this work for generating preliminary results and to subsequently draft an NSF proposal. We also expect broader scientific merits from this work: the ability to engineer nano-precipitates and to predict their attendant responses from micromechanical modeling will enable us to design SMAs with even more consistent actuation responses for high-temperature applications.
