Nanobeam diffraction fluctuation electron microscopy technique for structural characterization of disordered materials—Application to Al 88− xY 7Fe 5Ti x metallic glasses

Abstract

A fundamental experimental challenge in understanding the physical properties of non-crystalline (i.e., highly disordered) materials is that of fully characterizing the structural order that is present. Medium-range order is particularly difficult to characterize. Fluctuation electron microscopy (FEM) is a relatively new spatially resolved, diffraction technique that measures statistical fluctuations in the scattering of electrons arising from nanometer-scale ordered regions in a sample. The scattering fluctuations can be measured from dark-field (DF) images (the most common approach) or from a series of nanobeam diffraction (NBD) patterns. Here, the effectiveness of the NBD–FEM method for measuring atomic structure in disordered materials is evaluated. In particular, we show that the NBD–FEM statistical measures of fluctuations in diffraction intensity depend strongly on several instrumental parameters; measurement and analysis methodologies that constrain these parameters to minimize associated artifacts are presented. For illustration, the structure of the Al 88Y 7Fe 5 metallic glass is examined. It has been shown previously that the substitution of 0.5 at% Ti for the Al in this alloy significantly increases the glass forming ability. We provide NBD–FEM evidence indicating that the atomic structure of the glass is modified upon microalloying, in agreement with earlier extended X-ray absorption fine structure and high-energy X-ray diffraction studies.