NSF-BSF: Stress-Assisted Structural Phase Transformations and Plasticity in Bicontinuous Nanomaterials

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Nanoporous (NP) metals with tortuous surfaces and large surface-to-volume ratio are promising candidates for various important advanced technologies, from electrochemical sensors/actuators and fuel cell filters to energy conversion and storage systems. However, a hindrance to widespread technological application of NP metals is that they fail suddenly and with minimal uniform macroscopic plastic deformation. This award supports fundamental research to explore new ways of enabling homogeneous deformation in bicontinuous NP materials via alternative phase transformation (PT) mechanisms. The homogenous PT mechanisms would prevent localized deformation and, thus, enhance ductility across the whole system. The insights to be gained in this project will enable design of strong and ductile NP materials by utilizing metastable phases which cannot be obtained in conventional bulk metals and alloys. The fundamental mechanisms elucidated in this project will be applicable to other refractory materials and can also be scaled up for large-scale structural and functional applications. The award will also support undergraduate summer internships, a workshop on exploring nanotechnology for high school students with focus on underrepresented minority participation, and international student exchange.This project seeks to enable mechanisms of uniform (instead of local) PT and achieve significantly improved plastic deformability in Molybdenum (Mo)-based NP metals. Extreme stresses (on the order of tens of gigapascals) are required to thermodynamically drive a PT in Mo from a low-energy body centered cubic phase to a high-energy face centered cubic state instead of activating dislocation nucleation or propagation mechanisms. While this PT has been observed locally and at the atomic scale, it remains totally unclear whether it can occur homogenously to significantly modify mechanical behavior of specimens larger than about hundred nanometers in size. This project will study the combination of microstructure features including 1) small (defect-free) ligaments, 2) tortuosity of the structure, and 3) interfaces between Mo and a secondary material, that will enable a uniform distribution of high internal stresses through the entirety of NP composite structures and promote uniform deformation and better ductility. Molecular dynamics simulations and nanomechanical testing experiments with finite elements analysis will be integrated to study the mechanical response and post mortem microstructures for potential PT-based uniform plastic deformation.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.