The research objective of this award is to develop a fundamental understanding of martensitic phase transformation at nanometer scales and the resulting thermomechanical behavior of shape memory alloy (SMA) nanowires. SMA nickel-titanium (NiTi) is a promising functional material for the nano-biotechnology field, constituting an ideal, biocompatible, nanoscale mechanical transducer. The research will result in methods to synthesize nanostructured NiTi and to characterize their resulting shape memory properties. The research approach includes (1) development of a novel bottom-up approach to synthesize NiTi nanowires and nanopillars, (2) experimental measurement of the thermomechanical deformation and shape memory effects of NiTi nanopillars, and (3) use of novel atomistic simulations to interpret experimental findings and guide experimental test plans. If successful, the results of the research will provide original contributions to elucidate the fundamental deformation mechanisms of shape memory alloy NiTi nanowires. The active coupling of the experimental and modeling approaches will yield a deeper understanding of martensitic phase transformation and dislocation nucleation in nanostructures. Another expected benefit of the research is the development of a new material for nanometer scale actuation. The ability to invoke mechanical transduction at the nanoscale is imperative for many emerging nanosystems. Some innovative applications may include: directed actuation for device self assembly, repeated actuation for locomotion, pressure induced local fluid flow, generation of local force on adhered cells or organic molecules, force delivery from a micro-device. The project will engage graduate researchers, as well as high school students and teachers.
The overarching goal of this NSF grant was to develop a fundamental understanding of stress- and temperature-induced martensitic phase transformation at nanometer scales. A martensitic transformation is a diffusionless (rapid) transformation that is responsible for the shape memory properties (pseudoelasticity, shape memory effects) of shape memory alloys such as nickel-titanium (NiTi). A wide range of modeling tools was employed in this research, including atomistic simulations, to understand the mechanics of martensitic phase transformation at the nanoscale and the resulting thermomechanical behavior of NiTi nanospecimens, which are great candidates for nano-scale actuators in biosystems. One of the main achievements of this research was to develop a new interatomic potential for NiTi that allowed accurate studies of the atomic-level structures of NiTi. Using molecular statics and dynamics simulations, a novel nanotwinned structure was found, forming through the martensitic transformation of sub-lattices, which has also been observed experimentally. Furthermore, the simulations predicted temperatures of phase transformation which are consistent with experimental measurements. These results provided an atomistic basis for further study of nanometer length scale effects on the martensitic phase transformation and shape memory behavior. Specifically, the underlying atomistic mechanisms responsible for pseudoelasticity and shape memory effects in NiTi nanopillars were investigated by using large scale molecular dynamics simulations. It was shown that the irreversible twinning arises owing to the dislocation pinning of twin boundaries, while the hierarchically twinned microstructures facilitate the reversible twinning. Another significant outcome was to develop continuum phase field simulations to bridge atomic-level and continuum simulations, thereby allowing direct correlation with experimental characterization of twinned microstructures. The results reveal the spatial-temporal evolution of twinned microstructures under different thermal-mechanical conditions. Another major research thrust was to develop a technique to measure the mechanical properties of nanospecimens while observing the evolution of their microstructure in a transmission electron microscope (quantitative in situ TEM technique). Within that effort, a micron-scale testing machine was fabricated using microelectromechanical systems (MEMS) technology. Nanospecimens are manipulated and placed onto the machine using a nanomanipulator. The machine applies a displacement based on thermal actuation (the beams of the thermal actuator are heated by passing a current, resulting in an expansion of the beams and therefore a displacement). Part of that displacement goes into the nanospecimen, and part goes into the load sensing beams. If we can measure the displacement going into the nanospecimen and load sensor, we know the resulting deformation of the specimen. This is achieved thanks to two capacitive sensors placed on each sides of the specimen. By measuring tiny changes in capacitance between the two capacitive sensors, we can calculate the applied stress and strain of the nanospecimen. Preliminary results have shown that this technique works inside a scanning or transmission electron microscope.
