Breakdown of the Stokes-Einstein relationship and rapid structural ordering in CuZrAl metallic glass-forming liquids

The results of a combined structural and dynamical study of Cu-Zr-Al metallic glass forming liquids are presented. Containerless high-energy x-ray scattering experiments made using electrostatic levitation are combined with molecular dynamics simulations to probe the onset of rapid structural ordering as well as the temperature-dependent diffusivity and viscosity in three liquids: Cu₄₉Zr₄₅Al₆, Cu₄₇Zr₄₅Al₈, and Cu₄₃Zr₄₅Al₁₂. These compositions were chosen because they are reported to have dramatically different glass forming-ability. Experimental data show that the first peak in the x-ray static structure factor displays evidence for a Curie-Weiss type behavior, but also a peak in the effective Curie temperature. The evidence provided here for the onset of cooperativity, marked by a crossover temperature, T_A (which is usually above the liquidus temperature), is accompanied by the onset of development of more spatially extended structural order in the liquids. Based on the molecular dynamics simulations, each of the liquids exhibits a clear breakdown of the Stokes-Einstein relation at a temperature near, but below, the crossover temperature, T_A. The breakdown is manifest as a rapid reduction in the relative diffusion coefficients between Cu, Zr, and Al.