Collaborative Research: Kinetics of Defect Phases Emerging from Nanoscale Interfaces

About This Grant

NON-TECHNICAL SUMMARY The defects present in engineering metals and alloys are pivotal in controlling properties such as mechanical strength and toughness. Properties such as these are crucial in fueling advanced technologies. Yet, alloy design principles and frameworks that target specific materials phases seldom treat the defects as objects for design themselves. Extending the fundamental thermodynamic, kinetic, and structural principles widely used to design bulk materials to the defects within these materials would enable more purposeful engineering of new materials with improved properties. To address this goal, this project will synthesize, characterize, and simulate the thermodynamics and kinetics of nanostructured metallic alloys containing a large quantity of interfacial defects known as grain boundaries. By encouraging specific chemical and structural environments at these defects, local phase transitions will be studied with the goal of promoting excellent thermal stability and ultimately mechanical behavior. This research aims to expand the scientific underpinnings of nucleation from nanoscale confined arrangements for bulk metallic systems where classical nucleation theories are not appropriate. A primary goal is to also use the insights gained from this research to advance the emerging paradigm of defect phases and their intentional design, which requires new theories on the thermodynamics and kinetics of small confined systems, as well as new approaches to defect-aware materials design. This project also supports outreach to K-12 populations in the Santa Barbara, CA and Baltimore, MD communities through local school and art museum engagement in addition to engagement with older adults using STEM-based programming to help combat social isolation and loneliness. TECHNICAL SUMMARY Amorphous complexions, also known as defect phases, are analogous to bulk amorphous phases but restricted to a thin nanoscale film along grain boundaries. Such microstructural features are particularly useful interfacial states since they imbue nanocrystalline materials with thermal stability at very high temperatures. Interestingly, these features are considered to be a major weakness of these otherwise promising materials. A fundamental advantage of amorphous complexions as designer interfaces is that their disordered structure is the preferred local equilibrium state upon undergoing a pre-melting event. However, the kinetics of these phase transitions localized to defects are not well understood, since their signatures are challenging to measure and depend strongly on the nature of the abutting crystals that surround the resulting confined structures. The proposed work will use alloy design of nanocrystalline metals to target these pre-melting transitions, produce thermally-stable, nanoscale-confined disordered interfacial states, and quantify the kinetics of these transitions. These regions are the starting point for solidification pathways with microstructures not accessible through conventional alloy synthesis and processing routes. State-of-the-art experimental approaches including ultrafast calorimetry, materials synthesis in both thin film and bulk forms, and in-situ scanning/transmission electron microscopy will be complemented with computations of phase equilibria and kinetics using atomistic and kinetic simulation approaches based on machine-learned interatomic force fields. The insights gained on the kinetics of nanoscale complexions having varying degrees of order are expected to inform frameworks for predicting the behavior of confined interfacial states of matter and guide alloy design strategies with intentional populations of targeted defects. This project also supports outreach to K-12 populations in the Santa Barbara, CA and Baltimore, MD communities through local school and art museum engagement in addition to engagement with older adults using STEM-based programming to help combat social isolation and loneliness. 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.