Quantitative Determination of Dislocation Core Structure and Mobility Using Atomic Resolution Microscopy and Multiscale Modeling: Application to High Entropy Alloys

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Non-Technical Description: Dislocations are linear, mobile defects in crystals that control the strength and ductility of metals. Despite tremendous advances to model the structure and movement of dislocations at the atomic scale, the ability to validate these model predictions is significantly lagging. For instance, multiple microscopy methods have not been applied in concert to characterize dislocations. This research will develop innovative analysis techniques that help shape the future of defect analysis and are transportable to other metallic materials, ceramics, and semiconductors. Historically, top-down approaches have been employed whereby macroscopic measurements are used to deduce forces on defects and their mobility. This research enables a bottoms-up approach to determine these fundamental quantities, by leveraging revolutionary advances in electron microscopy with advanced atomic-scale modeling and multi-scale probes. These advances are applied to the high entropy alloys -a new class of materials with attractive and unusual properties, including increased strength and fracture toughness at lower temperatures. This research advances experimental and computational approaches to understand the origin of these remarkable properties at a fundamental defect level. This project synergizes new educational approaches that cross-cut microscopy and computational content. It also provides opportunities for undergraduate students to participate in interdisciplinary senior capstone projects. This research impacts pre-college education, through participation in the Ohio Department of Education Math and Science Program. It also offers professional development for high school science teachers, through an annual 'Materials Camp for Teachers' and an on-line repository of instructional materials targeted for grades 8-12.Technical Description: Atomistic and first principles calculations of dislocation structure and behavior have become an essential part of 'bottoms-up' modeling of the mechanical behavior of metals and alloys, and they are a key component of computational materials design in the Materials Genome Initiative. However, an inherent problem exists: atomic-scale calculations often lack validation at an appropriate length scale. The aim of this project is to transform bottoms-up modeling, by developing a coordinated approach for quantitative, experimentally-informed measurements of dislocation core structures and mobility. This is achieved by coupling recent advances in atomic resolution scanning electron microscopy with atomic-scale computations and multi-scale modeling. The experimental data are analyzed with computational techniques that quantify errors and extract local deformation, local strain energy, and thermodynamic forces on dislocations and other defects. Thermo-mechanical studies are conducted using in-situ heating and nano-drilling of holes in specimens to create non-equilibrium dislocation configurations. This opens up exciting, new possibilities for both static and dynamic study of fundamental dislocation behavior. This innovative approach is applied to a material system of keen, current interest, for which dislocation-level structure and behavior is important but presently unknown - namely the 'high entropy' alloys. Exciting preliminary results for a five-component fcc solid solution alloy have been obtained and are extended during initial studies. The applications are expanded during the program and as new alloy behavior is discovered. Experimental and computational procedures for the proposed dynamic measurements are developed initially using low-angle Al bicrystal structures that offer a simple 'model' system with well-defined dislocation structures. This transformative research establishes robust protocols to guide the emerging aspects for both static and dynamic dislocation analysis. For instance, the proposed microscopy methods are applied in concert to characterize the same type of defect structures. This research, when combined with the proposed innovative analysis techniques, helps to shape the future of defect analysis and is transportable to other metallic materials, ceramics, and semiconductors.