Nanometer-scale Structure and Properties of Amorphous Alloys

1. Nanometer-scale Structure and Properties of Amorphous Alloys Todd Hufnagel, Johns Hopkins University, DMR 0307009 Structural order on length scales of 1-3 nm in metallic glasses is commonly called “medium range order,” or MRO. It is difficult to investigate structure on this length scale in an amorphous material. One of the most promising approaches is fluctuation electron microscopy, in which the variations in intensity of electron scattering are examined for tell-tale signs of MRO. We have developed (in collaboration with scientists from Tohoku University) a new approach to fluctuation microscopy based on parallel-beam nanodiffraction. At right are two diffraction patterns from a metallic glass. The top pattern, taken in a traditional manner (with a large selected area aperture) shows diffraction features that are uniform due to averaging over a large volume of material. Using a very small selected area aperture samples a much smaller region of material (15 nm square), resulting in a diffraction pattern containing much more information about local structure (bottom). Statistical analysis of these kinds of diffraction patterns shows that these glasses have significant structure on a length scale of ~2 nm. Computer simulations to permit interpretation of these data in terms of real-space structures are underway.

2. Nanometer-scale Structure and Properties of Amorphous Alloys Todd Hufnagel, Johns Hopkins University, DMR 0307009 Education: One of the two graduate students supported by this program developed a productive research collaboration with scientists at Tohoku University that led to the development of a new approach to fluctuation electron microscopy (see research highlight). This was made possible by an NSF East Asia Summer Institute fellowship during summer 2007. One other student has graduated, and a third is also working on this project. Broader Activities: Professor Hufnagel was an invited co-author on a broad review of the mechanical behavior of metallic glasses (Acta Materialia 55, 4067 (2007)). The last comprehensive review of this subject was written more than a decade ago, a period which has seen an explosion of knowledge in this field. We expect that this review will be an important reference for new researchers entering the field and that it will play a significant role in defining new research directions.