Date/Time: 04-05-2018 - Thursday - 05:00 PM - 07:00 PM
Paul Rasmussen1 Rohit Sarkar1 Jagannathan Rajagopalan1

1, Arizona State University, Tempe, Arizona, United States

Controlling the micro/nanostructure of thin films would enable us to explicitly tailor their mechanical behavior. Here, we describe a new process to synthesize thin films with precise microstructural control via systematic, in-situ seeding of nanocrystals, and subsequent crystallization of amorphous precursor films. Using this process, we synthesized an austenitic NiTi film with a mean grain size of around 100 nm. We then co-fabricated freestanding samples of the film with MEMS testing stages and performed a series of cyclic tensile load-unload experiments. The film showed a high phase transformation stress (> 700 MPa) during the first cycle that increased even further during subsequent cycles. Furthermore, the film exhibited significant inelastic strain recovery during unloading, which was characterized by a continuous decrease in stress-strain slope rather than the pseudoelastic behavior typically observed in microcrystalline NiTi. Interestingly, the strain recovery continued even after the film was fully unloaded (macroscopically free of stress), and accelerated when the temperature was increased, with full recovery occurring at 60 oC. Preliminary in-situ TEM straining studies suggest that this unusual unloading/post-unloading behavior is caused by the heterogeneous deformation of the nanocrystalline microstructure. While some of the grains accommodate the inelastic deformation during loading by phase transformation, others accommodate it via plasticity. Hence, when the film is unloaded, inelastic strain recovery occurs through a combination of reverse phase transformation and reverse plasticity, leading to a divergence from the conventional pseudoelastic/shape memory behavior.

Meeting Program

5:00 PM–7:00 PM Apr 5, 2018

PCC North, 300 Level, Exhibit Hall C-E