Collaborative Research: Coordinated In-situ Dynamic Experiments and Atomistic Modeling of Surface Segregation in Alloys

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Non-technical abstract:Surface segregation phenomena - the enrichment of one element at the surface relative to the bulk - have enormous consequences in many areas of materials science because most engineering materials are alloys either by design for improving properties or by impurities remaining after processing or by contamination from the surrounding. Often, minor compositional modifications that occur locally in a material can lead to drastic changes in properties such as corrosion resistance, catalytic function, fracture strength, and interfacial adhesion. This project seeks to uncover the dependence of surface segregation on controllable parameters such as bulk composition, surface orientation and morphology, and external stimuli as well as the microscopic mechanism of the surface segregation process. The fundamental insight will have considerable practical importance for a wide range of material systems, properties, and reactions because segregation not only modifies surface chemistry and composition but also the atomic structure and strain state in the surface and subsurface regions in multicomponent materials. The fundamental knowledge of the prototypes of basic processes controlling surface composition and structure evolution will open up new perspectives of designing alloys with desired surface properties by manipulating bulk properties of the alloy and their interplay with the surroundings. As part of this research program, students at the graduate and undergraduate levels will learn about new microscopy, spectroscopy and computational techniques as well as work on materials issues that are at the forefront of current materials research. The training of students in the broader area of materials science will result in future leaders that are better equipped to solve the complex energy and environmental problems that face society. Results from this project will also be incorporated into undergraduate- and graduate-level courses and high school outreach programs to advance nanomaterials-related education.Technical abstract: Although phase diagrams delineating the thermodynamic conditions for phase/structure selection in bulk alloys are well established, the composition and structure of an alloy surface can be significantly different from those of the bulk due to the surface segregation of the alloying element. A microscopic understanding of many physical and chemical processes taking place at the surface of multicomponent materials requires as a prerequisite atomic-scale understanding of surface segregation induced compositional and structural evolution. Despite this importance, the atomic processes governing the onset, promotion, and termination of surface segregation under practical conditions are largely unknown. This project employs atomistically informed approaches toward a mechanistic understanding of surface segregation phenomena by gaining transformational knowledge of surface compositional and structural dynamics of alloys. The research is based on a combined atomistic experimental and computational program with tightly integrated feedback loops, including quantitative in-situ metrology that uses complementary forefront techniques for dynamically measuring surface composition, structure and chemistry of the alloys under realistic environment conditions and closely coordinated atomistic modeling ranging from first-principles calculations to large-scale molecular dynamics and Monte Carlo simulations. The comprehensive understanding will shed light on many multicomponent systems due to a shared set of prototypical basic processes governing surface segregation, including thermodynamic driving forces, interplay between chemical ordering and elemental segregation, and kinetic obstacles of atomic exchanges.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.