Date

2020-9

Author

Ulucan, Tolga Han

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In this thesis structural and corresponding mechanical properties of marginal metallic glass-forming alloys were studied. Al90Tb10 system exhibits unique devitrification features due to their exceptional primary crystallization products of Al nanocrystals with populations reaching up to 1024 m−3 . A full agreement on how this abnormal nucleation event occurs is still lacking. Our previous studies on Al-RE (RE: rareearth element) have shown that two different amorphous precursors prepared using melt-spinning and magnetron sputtering techniques showed clear differences in phase selection hierarchy upon continuous heating. It is considered that this difference is originated from the medium range ordered structure, exist in as-quenched melt-spun ribbon which is inherited from its molten state. In this study the local structure, chemistry and any possible higher order correlations were investigated by performing critical high energy synchrotron X-ray Diffraction (HEXRD),Extended X-ray Absoprtion Fine Structure (EXAFS), ab-initio and Reverse Monte Carlo (RMC) modelling. RE atoms were found to be highly correlated with Al atoms which results in formation of network dividing the amorphous matrix which in turn result in abnormal crystallization behavior. Afterwards, effects of these nanocrystals on mechanical properties are investigated via micropillar compression tests and transmission electron microscopy (TEM). Embedding nanocrystals on amorphous matrix increased the fracture stress and ductility at the same time where as increasing heat treatment temperature beyond first crystallization event temperature caused formation of intermetallic compound which decreased the ductility while further increasing fracture stress

Subject Keywords

Metallic Glasses, Nanocrystals, Crystallization, Synchrotron XRD, EXAFS, RMC, TEM, Micropillar Compression Testing

URI

https://hdl.handle.net/11511/69211

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Graduate School of Natural and Applied Sciences, Thesis