Structural signature in Au-based amorphous alloys

Here we elucidated that the anomalous shoulder at the left side of first main peak in structural factor is resulted from packing of AuAu bonds in the Au55Cu25Si20 amorphous alloy (AA), while the first main peak is largely from contributions of AuAu, AuCu and AuSi bonds. Local atomic packing of this glass is mainly composed of 9, 10 and 11-coordinated polyhedra rather than icosahedral-like clusters, in which the small sized clusters prefer avoiding each other while large ones prefer connecting themselves. This anomalous atomic packing structure limits its glass forming ability (GFA) and results in the anomalous shoulder of the first peak in structural factor. With the replacement of 5 at.% Cu atoms by Ag atoms, the spatial connectivity and the atomic packing density in Au55Cu20Ag5Si20 AA are largely increased, which homogenize atomic structures at least up to the second nearest neighbors and frustrate the geometry of competing crystalline-like nuclei, consequently, enhancing the GFA, as observed in experiments. This work illustrates how the atomic packing in an Au-based glass is altered by element substitution and sheds new insights to understanding of high GFA from the atomic scale and thus developing novel AAs with larger critical sizes.

One X-ray diffraction pattern of a typical Au55Cu25Si20 MG prepared by melt-spun technique, recorded using Cu Kα radiation. The first broad peak is located at two-theta of about 25-50°, which is fitted by using two Gaussian profiles, while the second broad peak is located at two-theta of about 60-80°, which is fitted by using one Gaussian profile.123