EAGER: Stability and Nanocrystallization Kinetics of Amorphous Al Alloys

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TECHNICAL SUMMARY: An important form of bulk nanostructured alloys is based upon a high density dispersion of nanocrystals in an amorphous matrix. Crystallization from an amorphous matrix is a preferred synthesis method in order to allow the control and tailoring of the microstructure and therefore properties such as high strength. Since the strength depends on the particle size and volume fraction, understanding and controlling the mechanisms of primary aluminum nanocrystal formation with number densities of 1021 ? 1023 m-3 dispersed in an amorphous matrix are of special interest. While the technological significance of this microstructure design is clear, the fundamental understanding of the controlling nucleation and growth kinetics has not been resolved for the early stage evolution of nanocrystals. Moreover, recent nanoscale structural analysis has revealed that the atomic arrangements in the amorphous phase are spatially heterogeneous so that a new model that will be advanced in this study is required to account for the kinetic effects of spatial heterogeneities. In order to provide the database to test the fidelity of the unproven kinetics analysis, nanocrystal nucleation and structural characterization will be studied systematically in Al-Y-Fe amorphous alloys with minor alloying. An important outcome from a successful project will be the formulation of a new paradigm for the kinetics analysis of nanoscale microstructure synthesis in heterogeneous materials.NON-TECHNICAL SUMMARY: The nanoscale microstructures that evolve during primary crystallization of amorphous alloys in bulk volumes are responsible for the attractive structural and functional materials properties that have technological applications in transportation and communications. In order to develop nanoscale microstructures it is essential to understand the controlling reaction kinetics so that the nanocrystal number density is maximized and the growth is limited. Previous kinetics models have treated the amorphous phase as uniform on an atomic scale, but recent structural analysis has revealed atomic scale spatial heterogeneities. A new kinetics analysis will be developed to treat the spatially heterogeneous state and tested with nanocrystal nucleation measurements in amorphous Al-Y-Fe alloys. The results have a broader relevance to the nature of primary crystallization as a common devitrification reaction in many metallic glasses. An important component of the project effort will be the education of graduate students and the research experience for undergraduate and high school students. Further, lecture demonstrations on nanostructured materials will be incorporated into campus outreach programs for high school and incoming undergraduate students.