EXAFS and molecular dynamics simulation studies of Cu-Zr metallic glass: Short-to-medium range order and glass forming ability

Atomic structures of Cu100−xZrx (x = 41.18-66.67) metallic glasses were investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy and molecular dynamics (MD) simulations. It is demonstrated that both the fraction and average volume of the typical atomic clusters, characterized by Voronoi cells, are of compositional dependence. With the increasing of Zr content, the fraction of Cu and Zr centered Voronoi clusters with coordination number lower than 11 and 14 respectively increases, while that of clusters with larger coordination numbers reduces. Among the major Voronoi clusters, the Cu-centered icosahedra are distinctive. Their average volume is far smaller than that of the other Cu-centered Voronoi cells with the same coordination number. More importantly, they prefer to interlink into pentagon-rich icosahedral super-clusters. However, compared with their neighbors, the glasses with x = 44 and x = 50 exhibit no other particularity in Cu-centered icosahedra than a slightly enhanced interlink at x = 50 although glass forming ability (GFA) reaches a local maximum at these two compositions, indicating that there are other factors dominating the GFA change with composition. A significant reduction in free volume for the loosely packed regions takes place at x = 44 and x = 50, due to which the corresponding Cu-Zr metallic glasses have the maximum microhardness.