Collaborative Research: Fatigue Crack Formation and Growth in the Presence of Reversible Martensitic Transformation in High Temperature Shape Memory Alloys

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Newly discovered high-temperature shape memory alloys can recover large deformations at elevated temperatures through a reversible transformation of their crystal structure. This has warranted significant interest in their use as solid-state actuators in aeronautics, automotive, and energy-conversion applications. These actuators could potentially provide ultra-high energy density actuation and would allow multifunctional use, resulting in lower cost and maintenance due to simplified device designs. The aim of this research project is to accelerate the development cycle of these materials by elucidating the life-limiting fatigue mechanisms. The knowledge of fatigue progression in these alloys will facilitate the wider acceptance of materials exhibiting reversible martensitic transformation in engineering applications. Furthermore, through the synergistic combination of experiments and modeling and its multidisciplinary nature, this research will train the next generation of experts in multifunctional, phase transforming materials. It will also support the involvement of underrepresented groups in research and professional experiences via partnership with the Boeing Company, NASA, and international collaborators.This research will involve multiscale thermomechanical testing and microstructural characterization as well as sophisticated modeling to identify defect kinetics as a function of microstructural attributes in the presence of reversible martensitic transformation and associated irreversible processes. While recently discovered Nickel-Titanium-Hafnium (NiTiHf) alloys will be used as a model material system, many of the experimental findings and models are expected to be transferable to other phase transforming materials as well. The experimental work will rely on in-situ thermomechanical testing in scanning electron microscope and ex-situ transmission electron microscopy, as well as digital image correlation and synchrotron X-ray tomography to monitor defect formation/growth and to characterize microstructure, deformation structure, and fracture surfaces. The modeling work will be based on a phase field approach to fatigue crack formation and growth. This approach will provide an advantage over current methods that rely on self-similar crack growth assumptions of conventional fracture mechanics, which break down due to the variability of the underlying microstructure and their complex influence on the mechanical fields close to the crack tip.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.