UCLA

Amorphous and glassy materials have many technologically useful properties which are dependent on the process by which they are constructed. Obtaining insight into these properties using thermomechanical simulations requires explicit knowledge of atomic positions. However, determining atomic structures of amorphous and glassy materials is impeded by their disordered nature and large number of configurational degrees of freedom. While conventional diffraction provides a measure of average spatial frequency components over a large specimen area, it is most sensitive to short range order among atom positions and is inadequate for distinguishing among specimens by their medium range order. Fluctuation microscopy is an experimental method which, by calculating spatial variance of diffraction, extracts information which is sensitive to medium range order in amorphous and glassy materials. In this work I bring simulated fluctuation microscopy closer to experiment by enabling examination of models of size comparable to their experimental counterparts. Experimental non-idealities are implemented in simulation and their influence on the characteristic measures of medium range order are demonstrated. By evaluating polycrystalline models of controlled grain size, I show that the established fluctuation microscopy measure of pair-pair correlation distance only correlates with trends in grain size for one of the given FTEM measures, and only when the probe size is limited. A strong dependence of the pair-pair correlation distance on spatial frequency is also demonstrated. Imitating physical material processes with molecular dynamics, I construct several categories of amorphous or glassy materials and demonstrate the ability of fluctuation microscopy to distinguish between models for which conventional diffraction measures are unsatisfactory.