CAREER: Development of a Structurally Based Plastic Flow Model to Enhance the Utilization of Bulk Metallic Glasses

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Bulk metallic glasses (BMGs) represent a revolutionary new class of engineering materials with potential applications ranging from automotive and aerospace structures to biomedical devices and sporting goods. These fully metallic systems exhibit extraordinary tensile strengths, large elastic deflections, and fracture toughness values an order of magnitude higher than traditional glasses. Due to their unique, disordered atomic structure, BMGs soften considerably at elevated temperatures prior to melting. This "homogeneous flow" at low stresses permits the use of inexpensive polymer molding and forming techniques, previously unheard of for high strength materials. Such inexpensive manufacturing techniques give BMGs an additional competitive advantage over traditional alloys. Structural reliability of BMG components, particularly in safety critical applications, requires the capability for generalized plastic flow at room temperature. While generalized, homogeneous flow occurs easily at high temperature, at room temperature flow becomes highly localized in shear bands. Rapid propagation of a single shear band can cause catastrophic failure. Mechanisms for distributing flow over multiple shear bands are therefore of interest. This requires a better understanding of the relationship between the glass structure, particularly the local structure of "flow defects", and the flow behavior over a wide range of temperatures and stress states. The mechanical behavior of metallic glasses has been historically difficult to characterize experimentally due to component size limitations. The high cooling rates required to form the early metallic glass ribbons constrained experimental work to specimens that were small in at least one dimension. The advent of bulk forming metallic glasses, with characteristic dimensions on the order of millimeters or centimeters, makes it possible for the first time to characterize both homogeneous and localized flow behavior under the more complex loading conditions expected in service. Flow in metallic glasses is typically understood as a diffusional process involving the "free volume", atomic scale open spaces in the otherwise densely packed structure. Variations in the free volume distribution result in the formation of flow defects. Prior results by the PI and others indicate that the observed softening during flow is associated with an increase in free volume, consistent with model predictions. However, the details of the glass structure, including flow defects, and the rearrangements during flow and shear band formation are not well understood. The ultimate goal of this work is to provide a model for the flow behavior in BMGs based on a realistic understanding of the flow defect structure, similar to the descriptions possible in crystalline materials. The proposed program focuses on three areas of technical merit: (i) characterization of the homogeneous and localized flow behavior as a function of temperature, strain rate, and stress state; (ii) identification of the glass and flow defect atomic structure through novel experimental techniques; (iii) description of the atomic structure evolution during flow through computer simulations. This combination of experimental techniques with computer simulations will provide the feedback framework necessary to predict and validate the structure - property relationship for flow in this unique class of materials.This program will have broad impact (i) as the foundation for the accelerated development and optimization of new BMG alloys, structures, and processing techniques and (ii) through the preparation of a new generation of scientists and engineers. The proposed work will provide insight into glass forming ability by clarifying the local atomic structure of BMGs as well as identify structural changes that occur during flow. This knowledge will in turn accelerate the development of manufacturing processes, including joining techniques, critical for the widespread adoption of these unique materials in structural applications. In terms of educational value, BMGs excite student interest with their novel behavior and sports applications. Educational activities will emphasize how the design of such advanced materials impacts society and everyday life. These efforts are especially necessary for non-engineering students because the discipline of materials science remains underexposed to the general public. In the laboratory, proposed recruiting efforts will focus on encouraging young women, particularly those from women's colleges, to consider graduate work in engineering. Students directly involved in this research will be exposed to cutting-edge science while obtaining the solid foundation in the fundamentals of materials science required for employment in industry and academia.