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.
, NSF Eager Award 1005334 Principal Investigator, Professor J.H. Perepezko, University of Wisconsin-Madison, Department of Materials Science and Engineering Intellectual Merit The nanocrystalline state is often viewed in terms of isolated nanocrystalline particles, but an equal, if not more important form of 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. For example, after undergoing partial crystallization to yield a distribution of nano-sized crystals, amorphous aluminum-rich alloys have shown strengths up to 1500 MPa. Since the strength depends on the particle size and volume fraction, understanding and controlling the mechanisms of primary aluminum nanocrystal formation are of special interest. It is now established that the primary nanocrystallization reaction in amorphous Al alloys yields single crystal Al nanocrystals with number densities of 1021 – 1023 m-3 that are enveloped in a solute-rich layer that develops during growth. 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. Our recent nanoscale structural analysis has revealed that the atomic arrangements in the amorphous phase are spatially heterogeneous due to the development of medium range order (MRO) that involves the correlation in atomic arrangement of the second and third nearest neighbors. Conventional kinetics analysis methods are based upon a uniform amorphous phase. It is evident that a new kinetic model is required to treat transformation in a spatially heterogeneous phase. In the NSF EAGER project we have developed a model to account for the kinetic effects of spatial heterogeneities in terms of their catalysis and time constants that affect transient reaction rates to provide for an ultra high nucleation density. To evaluate the model we have employed an experimental strategy based upon both calorimetric and microstructure measurements that allows access to measurement of nucleation delay times. The delay times permit direct evaluation of the controlling diffusivity that controls the atomic transport involved in nanocrystal nucleation and is not available in common approaches. Initial results support the model that provides an accurate accounting for the observed nanocrystal particle densities. For a complete validation of the new kinetics model it will be necessary in the future to carry out a systematic study of nanocrystal nucleation and structural characterization in Al-Y-Fe amorphous alloys with minor alloying additions. An important outcome project is the formulation of a new paradigm for the kinetics analysis of nanoscale microstructure synthesis in heterogeneous amorphous phases. Furthermore, the success of the kinetics model can enable the use of nucleation kinetics as a probe of amorphous structure through the influence of structural heterogeneities on the evolving cluster populations during crystallization. Broader Impact The advancement in the understanding and control of nanostructure synthesis resulting from the project research has yielded a deeper fundamental understanding of the undercooled liquid state, the atomic configurations in undercooled liquids and glasses and new insight on transient nucleation reactions that develop very high nanocrystal particle densities. At the same time the nanoscale microstructures that evolve during primary crystallization in bulk volumes are responsible for the attractive structural and functional materials properties that have applications in transportation and communications. The main focus of the work has been on amorphous Al alloys, but 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 is the education of graduate students, the mentoring of a postdoctoral fellow and the research experience for undergraduate and high school students. Further, course lectures and demonstrations on nanostructured materials have been incorporated into campus outreach programs for high school and incoming undergraduate students.
