Structural and dynamical evolution of nanocrystals

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2013

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Yıldırım, Can

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Among all metallic glasses, marginal glass-forming alloys exhibit unique devitrification features due to their primary crystallization products having intriguing nucleation number density values up to 1024m-3. Remarkable materials properties such as high strength, corrosion resistance and appealing magnetic properties along with the inconsistency of classical nucleation theory predictions with the experimentally observed number density of nanocrystals have recently attracted much of an attention. A full agreement as to how this abnormal nucleation event occurs is still lacking. The present work consists of investigation of amorphous state and devitrification behavior of Al-Tb system using advanced characterization techniques such as synchrotron X-rays and electron beam techniques. Two different amorphization precursors were used to emphasize the effect of topological ordering on devitrification behavior as well as understanding the nucleation mechanism. Samples were prepared using melt-spinning and magnetron sputtering techniques. In-situ XRD experiments showed difference in phase selection hierarchy between ribbons and films. DSC was used to furnish a description regarding the growth of fcc-Al nanocrystals embedded in amorphous matrix using isothermal and isochronal interpretation methods. Unlike melt-spun ribbons, a phase separation was observed in sputtered films. This is attributed to the difference in amorphous solid states in such a way that Al like MRO is decreased in sputtering processes according to XRD and FEM analyses. Approximately three orders of magnitude higher nucleation number density and smaller sized fcc-Al was observed for sputtered films in HRTEM results. A growth model supported by compositional analysis carried out in S/TEM was implemented.

Subject Keywords

Nanocrystals., Synchrotrons., Metallic glasses., Electron microscopy.

URI

http://etd.lib.metu.edu.tr/upload/12616060/index.pdf
https://hdl.handle.net/11511/22678

Collections

Graduate School of Natural and Applied Sciences, Thesis

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Citation Formats

C. Yıldırım, “Structural and dynamical evolution of nanocrystals,” M.S. - Master of Science, Middle East Technical University, 2013.