Effect of component substitution on the atomic dynamics in glass-forming binary metallic melts

We investigate the substitution of early transition metals (Zr, Hf, and Nb) in Ni-based binary glass-forming metallic melts and the impact on structural and dynamical properties by using a combination of neutron scattering, electrostatic levitation (ESL), and isotopic substitution. The self-diffusion coefficients measured by quasielastic neutron scattering (QENS) identify a sluggish diffusion as well as an increased activation energy by almost a factor of 2 for ${\mathrm{Hf}}₃₅{\mathrm{Ni}}₆₅$ compared to ${\mathrm{Zr}}₃₆{\mathrm{Ni}}₆₄$. This finding can be explained by the locally higher packing density of Hf atoms in ${\mathrm{Hf}}₃₅{\mathrm{Ni}}₆₅$ compared to Zr atoms in ${\mathrm{Zr}}₃₆{\mathrm{Ni}}₆₄$, which has been derived from interatomic distances by analyzing the measured partial structure factors. Furthermore, QENS measurements of liquid ${\mathrm{Hf}}₃₅{\mathrm{Ni}}₆₅$ prepared with $⁶⁰\mathrm{Ni}$, which has a vanishing incoherent scattering cross section, have demonstrated that self-diffusion of Hf is slowed down compared to the concentration weighted self-diffusion of Hf and Ni. This implies a dynamical decoupling between larger Hf and smaller Ni atoms, which can be related to a saturation effect of unequal atomic nearest-neighbor pairs, that was observed recently for Ni-rich compositions in Zr-Ni metallic melts. In order to establish a structure-dynamics relation, measured partial structure factors have been used as an input for mode-coupling theory (MCT) of the glass transition to calculate self-diffusion coefficients for the different atomic components. Remarkably, MCT can reproduce the increased activation energy for ${\mathrm{Hf}}₃₅{\mathrm{Ni}}₆₅$ as well as the dynamical decoupling between Hf and Ni atoms.