ABSTRACT

TECHNICAL SUMMARY

This CAREER award supports mesoscale modeling of solid-state interfaces in metals, with a view to predicting interface structure and interface interactions with crystals defects: point defects, dislocations, and cracks. This effort will lead to quantitative structure-property relations for interfaces, which may then be used to design structural composite materials with interfaces tailored to yield desired functionalities, such as high strength or fracture toughness, radiation or wear resistance, reduced corrosion or creep, and others. Such materials would have a major impact on energy applications.

The PI will pursue three specific scientific objectives. The first is to create mesoscale models that quantitatively describe and predict detailed interface structure. The second is to discover the mechanisms of interface-defect interactions, and create quantitative mesoscale models of these mechanisms. Finally, the third objective is to validate structure and defect interaction predictions through numerical uncertainty quantification and hypothesis-driven experiments. Initially, this project will be restricted to a subset of all possible interfaces, namely semicoherent interfaces formed between immiscible single-element metals. The structure of such interfaces may be described using misfit dislocations, which eases development of quantitative structure models and provides a basis for predicting interface-defect interactions. Heterophase interfaces between immiscible metals generally remain stable under a variety of conditions and do not migrate or intermix easily, easing experimental investigation.

Work will focus on flat interfaces in their lowest energy state, but with differing crystallographic characters. Only pairs of metals whose crystal structures may be related to the face-centered-cubic structure by uniform deformations will be studied. Both heterophase interface and grain boundaries fall within this subset. The effect of temperature will be studied, but investigations of the effects of curvature, faceting, large pre-existing extrinsic defect concentrations, or non-equilibrium state will be postponed. This project will consider interface interactions with three types of defects: point defects, dislocations, and cracks. The interactions to be studied are defect trapping, emission, transmission, and motion near and within interfaces. Interactions with other types of defects - such as voids, inclusions, point defect clusters, or other interfaces - will not be studied as part of this project. There are no fundamental physical limitations that prevent broadening the scope of future work to interfaces, interactions, and defect types other than those listed above.

The education component of this project will support the development of a new class on defect physics and the revision of an exiting class on mechanical behavior of materials. All materials as well as videotaped lectures for both classes will be made available worldwide through MIT's OpenCourseWare. This project will enhance the training of future scientists by providing undergraduate research opportunities in materials modeling and integrating their work with international collaborations through the MIT International Science and Technology Initiatives program. Postdoctoral experience is increasingly critical for scientists to gain proficiency in leading research projects that span across and integrate both modeling and experimental results from different fields of study. The PI will establish a postdoc office that will undertake to enhance the postdoc experience at MIT.


NON-TECHNICAL SUMMARY

This CAREER project aims to increase our understanding of interfaces in metals through theory and computer modeling. Interfaces are locations where two different crystals meet and are ubiquitous in engineering alloys such as steel, aluminum, titanium, and many others. Although interfaces typically comprise less than 0.01% of the volume of such materials, they play a decisive role in determining their mechanical, electrical, thermal, and diffusion properties. Textbooks often portray them schematically as two-dimensional and abrupt. This simplification is convenient and often necessary, but fundamentally false: interface structure is inherently three dimensional, often complex, and occasionally quite beautiful. Thus, much remains to be understood about the structure and properties of interfaces.

An improved understanding of interfaces may provide a path to making better engineering alloys. The performance envelope of materials limits much of what technology can accomplish, for example in energy applications: from steam generators and batteries to high-voltage power lines and nuclear reactors, better materials translate into cleaner, safer, and cheaper energy. Materials performance in these applications is often controlled by crystal defects, such as vacancies, dislocations, and cracks. Tailoring the interactions of interfaces with such defects is one path to expanding the performance envelope of materials.

This project has three objectives. The first is to develop models that capture the full complexity of interface structure with enough precision to make quantitative predictions. The second is to discover how interfaces with different structures interact with defects that control materials performance and to develop the capability to predict these interface-defect interactions from interface structure. Finally, the third objective is to validate the interface structure and defect interaction models described above. All models make simplifying assumptions. A major goal of this work is to develop strategies for validating models of interface structure and defects interactions using both theory and experiments.

Because there is an infinite number of possible interfaces, this project has to focus on a selected subset of them. The specific types of interfaces to be studied have therefore been downselected based on criteria that give the highest likelihood of making rapid progress towards achieving the goals of this project. Similarly, interactions of interfaces with only three types of defects will be considered: point defects, dislocations, and cracks. There are, however, no fundamental physical limitations that prevent broadening the scope of future work to interfaces, interactions, and defect types other than those initially downselected.

The defect and interface physics involved in this project will be used to support the development of a new class and the revision of an existing class on mechanical behavior of materials at MIT. Thanks to MIT's commitment to share its educational resources through online programs such as OpenCourseWare, these classes will benefit a worldwide audience. This project will enhance the training of future scientists by providing undergraduate research opportunities both at MIT and internationally through the MIT International Science and Technology Initiatives program. Finally, this project will enhance the experience of scientists at MIT who recently received their PhDs and intend to continue building a career in research.

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