Short-range and medium-range order in(Zr(70)Cu(20)Ni(10))(90-x)Ta(x)Al(10) bulk amorphous alloys

We have used x-ray scattering to examine short-range and medium-range order in (Zr 7 0 Cu 2 0 Ni 1 0 ) 9 0 - x Ta x Al 1 0 amorphous alloys. Analysis of the radial distribution functions (RDF's) shows that the addition of 4 at. % Ta enhances the average short-range topological order, as the nearest-neighbor peak in the RDF becomes more sharply defined. The enhanced order due to the Ta addition persists beyond the first few atomic shells, however, out to distances of at least 15 A. From resonant x-ray scattering near the Zr K absorption edge, we are able to extract differential radial distribution functions (DRDF's) which show the atomic environment around Zr atoms only. The DRDF's show that Ta has little effect on the nearest neighbors of Zr atoms, but does significantly enhance the medium-range order (over distances of 5-15 A from an average Zr atom). To explain these observations, we propose that topologically ordered atomic clusters are a significant feature of the structure of Zr-based amorphous alloys and that the influence of Ta is to enhance the order associated with packing of these clusters.